Superconductivity and Dirac Fermions in 112-phase Pnictides

5.2 MB·

by  S. J. Ray

Superconductivity and Dirac Fermions in 112-phase Pnictides
S. J. Ray
∗
Department of Physics, Indian Institute of Technology Patna, Bihta 801103, India and
Institute of Materials Science, Technische Universita¨t Darmstadt,
Alarich-Weiss-Straße 2, Darmstadt 64287, Germany
L. Alff
Institute of Materials Science, Technische Universita¨t Darmstadt,
Alarich-Weiss-Straße 2, Darmstadt 64287, Germany
7 This article reviews the status of current research on the 112-phase of pnictides. The 112-phase
1 hasgainedaugmentedattentionduetotherecentdiscoveryofhigh-temperaturesuperconductivity
0 in Ca La FeAs with a maximum critical temperature T 47K upon Sb substitution. The
1 x x 2 c
2 − ∼
structural,magnetic,andelectronicpropertiesofCa La FeAs bearsomesimilaritieswithother
1 x x 2
−
n superconducting pnictide phases, however, the different valence states of the pnictogen and the
a presence of a metallic spacer layer are unique features of the 112-system. Low-temperature su-
J perconductivity which coexists with antiferromagnetic order was observed in transition metal (Ni,

0 Pd) deficient 112-compounds like CeNixBi2, LaPdxBi2, LaPdxSb2, LaNixSb2. Besides supercon-
1 ductivity, the presence of naturally occurring anisotropic Dirac Fermionic states were observed in
thelayered112-compoundsSrMnBi2,CaMnBi2,LaAgBi2 whichareofsignificantinterestforfuture
] nanoelectronics as an alternative to graphene. In these compounds, the linear energy dispersion
n
resulted in a high magnetoresistance that stayed unsaturated even at the highest applied magnetic
o
fields. Here, we describe various 112-type materials systems combining experimental results and
c
theoreticalpredictionstostimulatefurtherresearchonthislesswell-knownmemberofthepnictide
-
r family.
p
u
s
Contents I. Introduction
.
t
a
m Thediscoveryofsuperconductivityinthepnictidefam-
I. Introduction 1
- ilyofsuperconductorsLaOFeP[1]hasfuelledresearchon
d II. Synthesis Techniques 2 the Fe-based superconducting compounds after the ob-
n A. BulkSynthesis 2
servation of a critical temperature of T 26K in the
o c ∼
isostructural compound La(O F )FeAs [2], which was
c III. Crystal Structure 3 1 x x
[ soon increased up to 43K [3]−under the application of
1 IV. EA.leTcthreoonreicticSatlrIuncvteustrigeations 55 hwiagshopbrseesrsvuerde [i4n].SmVeFreyAssoOon, aFn e[v5e,n6]h.igWherithTcdoifffe5r4enKt
v B. ExperimentalResults 6 levels of doping and elemen1t−axl suxbstitutions T values of
0 c
0 V. Magnetic Structure 6 55-58K[7–10]wereobservedinseveralothercompounds
7 in the 1111-family of pnictides. In the next few years,
2 VI. Superconducting Properties 7 superconductivity was found in various other Fe-based
0 A. LowCriticalTemperatureMaterials 7 pnictidesystemslike(Ba,K)Fe As (122-type)[11],FeSe
. B. HighCriticalTemperatureMaterials 7 2 2
1 1. RareEarthSubstitution 7 (11-type) [12], LiFeAs (111-type) [13–15] and, most re-
0 2. TransitionMetalSubstitution 9 cently, Ca1 xLaxFeAs2 (112-type)withaTc of38K[16–
7 3. CriticalCurrentandCriticalField 10 18]. The −fundamental interest in these materials lies
1 4. EffectofPressure 10 in understanding the mechanism behind the coexistence
:
v of superconductivity and magnetism. The high critical
VII. Dirac Fermions 11
i fields and isotropic critical currents [19–21] could be of
X
VIII. Thin Films 12 interest for electrical power and magnetic applications.
r A. GrowthbyMolecularBeamEpitaxyTechnique 13 ApartfromthepresenceofFeinthesecompounds,which
a
B. ThinFilmStructure 13 is believed to be harmful for conventional superconduc-
C. SuperconductingandMagneticPropertiesofThin
tivity, the uniqueness lies in the origin of superconduc-
Films 13
tivity due to the presence of the Fe-3d electrons[4].
IX. Conclusion and outlook 14
Althoughthereareseveralstructurallydifferentphases
References 15 of pnictides, they all contain a common Fe-pnictogen
(Pn)layerinatetrahedralarrangementthatisseparated
byblockinglayers. Thecompositionoftheblockinglayer
is believed to affect the superconducting properties [22].
Correspondingauthor: [email protected],[email protected] The Fe-Pn (or Fe-Ch) layers are tetrahedrally coordi-
∗
2
natedbythePnorCh(chalcogenide)atoms,andhighest bulk 122, 11, 111-type pnictides so far. One interesting
critical temperatures were observed for an ideal tetrahe- fact about the superconductivity in the 112-phase is the
dralarrangement[23]withtheFe-As-Febondangleclos- existenceofmixed-valancestatesofthepnictogen[33,38]
estto109.47 . Similartothehigh-T Cuprates,thepnic- which has different valencies in the Pn square net layer
◦ c
tides are also quasi-two-dimensional with reduced elec- and the metal-pnictogen layers.
tronic coupling along the c-axis, and the appearance of Inthepasttwoyears,researchinterestagainincreased
superconductivityisobserveduponthesuppressionofthe for Ca La FeAs [41–54] due to the availability of sin-
1 x x 2
antiferromagneticorder[24]. IncomparisontoCuprates, gle crys−tals, high-T in the bulk phase, and the possi-
c
however, therangeofsuperconductingmaterialsismuch bility of a range of substitutions. Significant amount
larger in case of the pnictides offering a huge range of of research work was done earlier in other 112-type ma-
chemical substitution possibilities [25]. Due to the lim- terials like LaPd Sb [55], LaPd Bi [55, 56], CeNi Sb
x 2 x 2 x 2
ited availability of high-quality and larger sized single [33, 34, 36], LaNi Sb [57, 58] etc. with low T <
x 2 c
∼
crystals [22], research was focussed on the 1111- and 5K. Although primary research work in the 112-system
122-type compounds. Recently, significant interest has started to look for possible high-T materials, recently
c
shifted to 11-type FeSe thin films[26], where evidence of a large amount of work has also been performed with
straininducedinterfacialsuperconductivitywasobserved respect to anisotropic Dirac fermionic states[29–31, 59–
up to a maximum T 109K [27]. 64] in (Ca/Sr)MnSb [61, 63, 65, 66], LaAgBi [60, 67–
c ∼ 2 2
A prediction of the existence of a 112-type pnictide 69] and understanding their role in the observed large
phase was made by Shim et al. [28] in the hypothetical magnetoresistance[30, 65, 66, 68, 70, 71] and magneto-
compoundsBaFeAs andBaFeSb withmetallicblocking thermopower generation[60, 71]. The purpose of this re-
2 2
layers unlike other pnictide systems where these layers view article is to summarise the work done in the 112-
are insulating. While these compounds remain elusive, systems so far, and to stimulate further research.
the related compound SrMnBi was synthesized [29–31]
2
II. Synthesis Techniques
as single crystals with physical and structural similari-
ties to BaFeAs . Despite having a large N´eel tempera-
2
Similar to the other pnictide systems, primarily three
ture (T 290K), no evidence of superconductivity was
N
∼ techniques are used for bulk sample preparation of 112-
observed in SrMnBi . Band structure calculations sug-
2
pnictides[25]: (a) solid state reaction, (b) high-pressure
gested the presence of anisotropic Dirac fermions in the
synthesis, and (c) self-flux method. The first two meth-
Bi square net layer which was experimentally confirmed
ods are mostly used for polycrystalline and powder sam-
later through the observation of quantum oscillations
ples, while the last one is convenient for single crys-
andangle-resolvedphotoelectronspectroscopy(ARPES)
tal growth. Growth of single crystalline thin films by
measurements [31].
molecular beam epitxy (MBE) is discussed in detail in
Superconductivityinthe112-typepnictidewasfirstre-
Sec. VIIIA.
portedintheCeTMPn (TM =transitionmetal, Pn=
2
pnictogen) family of intermetallic compounds with TN A. Bulk Synthesis
∼
5Kalthoughseveralheavy-fermionsuperconductorswith
layeredtetragonalstructureswereknownearlier[32]. Su- The parent compound CaFeAs has not yet been syn-
2
perconductivityinCeNixBi2 withTc 4.2K[33,34]was thesised, but incorporation of a small amount of La in
∼
claimedtooriginatefromNideficiency,asnoevidenceof place of Ca[16, 17] was found to be essential for stabilis-
bulk superconductivity was observed in the parent com- ingthe112-phaseandinducingsuperconductivity. Single
pound CeNiBi2 [35–37]. However, it has been suggested crystals of Ca1 xLaxFeAs2 were grown using FeAs-self-
from the coexistence of light and heavy carriers that the flux[16–18,43,−49,51]wherealltheconstituentsCa,La,
superconductingchargecarriersarehostedbythepnicto- As, FeAs were mixed in appropriate stoichiometric ratio
gen square net layer [33]. The presence of superconduc- inside an aluminium crucible and sealed inside an evacu-
tivity with low superconducting volume fraction (SVF) ated quartz tube. In order to avoid contamination from
were also observed in LaNixBi2 (SVF = 1-3%) [38, 39], theatmosphere,thewholeprocesswascarriedoutinside
NdNixBi2 (SVF = 14%) [33], and YNixBi2 [33] (SVF = a glove box filled with argon gas. The sealed ampules
17%) [33]. were heated at appropriate temperatures typically at a
Interest in the 112-system has grown considerably af- maximum of 1100 C and kept there for several hours.
◦
ter the discovery of a high T of 34K in Ca La FeAs Finally the furnace was cooled slowly ( 1.25 C/hour)
[16] and in Ca Pr FeAs cwith T 20K1−x[40]x. Sub2- to room temperature before taking the s∼ingle c◦rystals of
1 x x 2 c
sequently, it wa−s reported that addin∼g a small amount maximum size of 2mm [45] out of the furnace. Zhou
of P (0.5%) and Sb (1%) substituting As in the par- et al.[45, 47] suggested that larger amounts of starting
ent compound leads to a drastic enhancement of T materialsarenecessaryforthegrowthoflargesizedcrys-
c
in Ca La FeAs to 41K and 43K, respectively [17]. talsandasmallamountofCaOishelpfulforcrystallisa-
1 x x 2
Later,i−twasrevealedthatalargerlevelofSbdopingcan tion.
further enhance T to 47K in Ca La Fe(As Sb ) Polycrystalline samples were synthesised using solid
c 1 x x 1 y y
[18] which is higher than the maxim−um T obser−ved in state reaction inside a high-pressure cell [40, 48]. For
c
3
a b c d
Bi(1)
Bi(2)
Square Net
LaFeAsO LaNixBi2 Ca1-xLaxFeAs2
FIG.1: Schematiccrystalstructuresof(a)1111-typeLaFeAsO,(b)112-typeLaNi Bi and(c)112-typeCa La FeAs [16]. (d)
x 2 1 x x 2
−
TopviewoftheAs-zigzagchainsinCa La FeAs . Thecolourmaprepresentsthecontourofthechargedistributionaround
1 x x 2
−
As-atoms. Charge accumulation between neighbouring As-atoms are suggestive of the formation of the covalent bonds[4, 16].
[Fig. 1(c-d) : Reprinted with permission from Katayama et al.[16]. Copyright 2013 by the Physics Society of Japan.]
Ca RE FeAs (RE =rareearthsfromLa Gd)syn- InthestoichiometricallydeficientNi Bilayer(analogous
1 x x 2 x
thes−is[48],amixtureofFeAs,REAs,Caand→Aspowders to the FeAs layer in LaFeAsO), Bi(1) forms a distorted
were mixed and pelletised which was later allowed to re- tetrahedron in a trivalent charge state due to Coulomb
act inside a boron nitride crucible between 1000-1200 C attraction. This forces the other Bi(2) ion in a charge
◦
for 1h under 2GPa pressure [42, 48, 72]. Sala et al. [48] state of -1 to occupy a narrower space with a shorter Bi-
pointed out that high pressure for synthesis is essential Bi bond-length to form a square net layer as illustrated
toincorporatesmallerRE-ionsandatmuchhigherpres- inFig.1(b). Thepresenceofthetwo-dimensionalsquare
sure (> 2GPa) doping of Tb, Dy, Ho and Y into the net layer is the most unique feature of the 112-phase
112-phase could be possible. whichstabilisesduetotheCoulombattractiondrivenre-
Single crystal synthesis of CeNi Bi [37], laxation between the RE-ions. The square net layer can
x 2
CePd Bi [73], LaPd Bi [56], and SrMnBi be considered as the blocking layer of the 112-system,
1 x 2 1 x 2 2
[29] ne−eded excess Bi flu−x and different temperature though it is metallic unlike the insulating blocking lay-
treatments while polycrystalline samples [33, 36, 38] ers present in other pnictide phases. The presence of
were prepared inside an evacuated silica tube through the two different oxidation states of the pnictide was ob-
standardsolidstatereactionatelevatedtemperaturesfor served by XPS in LaPd Sb [55] where two 3d photo-
x 2
RENi Bi (RE = La, Ce, Nd, Y). It was observed that electron lines of Sb are separated by ∆E = 9.4 eV that
x 2
the Bi and Sb-based 112-systems decompose gradually correspondto-1and-3oxidationstatesoftheSb-atoms.
when exposed to ambient atmosphere. Hence, storage of A similar metallic square net layer was also observed
these materials in evacuated atmosphere is essential for in SrMnBi (crystallizes in the SrZnBi -type structure
2 2
achieving longer lifetime[33, 34, 36, 55, 57]. withSGI4/mmm(no. 139)confirmedvianeutronscat-
tering measurements[63] as opposed to LaPd Pn with
x 2
III. Crystal Structure P4/nmm (no. 129) symmetry) which is metallic with
a large T 290K[29]. Multiple Dirac cone like dis-
N
Initialreportson112-typepnictidesystemsweremade persions wer∼e observed close to the Fermi level. In the
on RENixBi2[33] where RE = La, Ce, Nd, Gd and single crystalline RNixBi2 y (for R = La, Ce-Nd, Sm,
other rare earth elements. These 112-compounds crys- Gd-Dy),amonotonicdecre±aseinthelatticeparametersa
tallise in the HfCuSi2-type structure, which can be re- (2.1%)andc(5.3%)wereobservedduetothelanthanide
lated to 1111-type compounds with the ZrCuSiAs-type contraction[39].
structure. Structural similarity between the two phases
can be found from Fig 1(a-b). The 1111-compound Initial reports suggests that the 112-type
LaFeAsO[2] contains two different anions (O/As) while Ca La FeAs crystallises in monoclinic structure with
1 x x 2
the 112-compound RENi Bi has only Bi as anion, but spac−e group P2 (no. 4)[16] or P2 /m (no. 11)[40, 48]
x 2 1 1
in two different valance states, namely Bi(1) and Bi(2). whichisdifferentfromtheotherpnictidesystemshaving
2
4
tetragonal or orthorhombic space groups. However,
(a)
recently Harter et al.[74] observed second harmonic
generation (SHG) in Ca La FeAs that is a signature
1 x x 2
of noncentrosymmetric c−rystal structure, which suggets
that the space group of Ca La FeAs should not
1 x x 2
be centrosymmetric P2 /m, b−ut noncentrosymmetric
1
P2 . The monoclinic structure stays stable up to
1
450K[16]. Alternatively stacked FeAs layers are present
in Ca La FeAs (Fig. 1(c)) separated by zig-zag As
1 x x 2
bondla−yerswithCa/Laplacedinbetweenthem[4]. The
distance between neighbouring FeAs layers is slightly
larger than in the 1111-phase materials. The most
(b)
interesting feature of this material is the presence of 2D
Aslayerswithtwodifferentbondlengthsthatweremea-
sured using synchrotron X-ray diffraction [16] as shown
inFig.1(d). Theshorterone( 2.53˚A)isidenticaltothe
∼
As-As single bond length with As (4p4 configuration)
−
formingaone-dimensionalzig-zagchainalongtheb-axis,
while the larger one ( 3.02˚A) corresponds to the inter-
∼
chain distance between the zig-zag chains. In the FeAs
layer, As is in a As3 valance state (4p6 configuration).
−
The chemical formula of Ca La FeAs can be written
1 x x 2
as(Ca2+ RE3+)(Fe2+As3 )A−s xe withexcesscharge
injecte1d−ixnsidex the FeAs la−yer [4−].· Th−is can be compared
to the 1111-type CaFeAsF whose structure can be
written similarly as (Ca2+ RE3+)(Fe2+As3 )F xe
where F− forms a FsqIuGar1e−.xn1e.twxoCrka. In Ltha−is w−ay·,FtehA−e s :FofIGn(ua.c2)l:eaTTre(me0pm2er0pa)tNuerraeanddtepumernagednenedtcieceop(f1/teh2ne1d(/a2e)1nn/et2u)tMreolnpeeicnattkesrn,is(cibty)resistivity ρ (I//ab) and ρ (I ab). Inset: The top
C11a2F-eCAas1F−,xLalatxhFoeuAgsh2tvhstiereucwhcteumoreifcactlahbnoenbd0eFi.ne7rge3lsaatinneddt0h.tes2o7psp1a1a1cce1er-r 2AS(sρpesciuafibcb)hliaenattCta(iCcpe)LsaanadFtedAe6sr0i.vatTKivhe.e odOfottrtheaednilnign-peelsanbaeraerletlsh:iesttiFvemiety.- Grey ball: /A/sabin the spacer lay⊥earbs. Th⊥e length of the orange
1 x x 2
layers are not the sFamee-Finebbotohncdaseas.ndForblCuaeFeAFseF-,Fepbero(cid:107)atnudresiast w2h.−i7ch6t6he(4st)ru˚Actuaranl rdesp2..m7a6g2ne(t4ic)p˚Ahaseretrsapn-ectively, which are calculated using the data in Table SI and
the CaF layer is made of strong ionic bonds while sitions were observed[54]. Reprinted with permission from
SII. [12] 2L and 2L are depicted in the figure. 2L =2L if γ = 90 . The possibility of SC in iron pnictides in the HfCuSi
CaAs layers in Ca1 xLaxFeAs2 cons1ists of zig-za2g As Jiang et al.[54]. Copyright 2015 by Ame1rican Ph2ysical Soci- ◦ 2
chains of covalent bo−sntdrsuwcetakulyrecowupalesd ttohteheoradejtaiccenatlly eptyu. t forward in Ref.[3]. (b) Temperature dependent χ and χ . Inset: C /T vs. T2. (c) The
//ab ab p
Ca layers. For this reason, the interlayer distance ⊥
between FeAs planestienm11p2-eCraatuLraeFdeAespe(nd10e.3n5t˚A)neutron intensity of the nuclear (0 2 0)N and the magnetic (1/2 1/2 1/2)M peaks. (d) The temperature
1 x x 2
[18] is higher than dinepCaeFneAdseF−nt( C8.6˚Aa)nd[7∼5]t.heThdiserivative of ρ . (e) The neutron intensity of the (1/2 1/2 1/2)M peak at 55 K and 54 K with offset.
∼ p //ab
layered structure was confirmed by high-angle annular
dark field-scanning(tfr)anTsmhisesionneueletcrtoronn imnitceronscsoiptyy ofCatFheeAs(2[0282], C0a)1NxLpaxeFaekAs2at(x2=000.2K7)[5a4n, d74]4i.s5thKe . (g) The temperature dependent ordered volume fraction V
non-superconducti−ng parent compound of this family
(HAADF-STEM) measurements of (Ca,Pr)FeAs [40]
and transverse relaxa2tion tiwmhiechσisinnatZuFralµlySsRtruactsuyramllymdeettwriynnesdpeatctarmab.ienBtlack curve is the mean field fitting. (h) The polarized optical
with an interlayer distance of 10.4˚A. X-ray scattering
pressure and becomes superconducting on electron or [1]
data[76] suggested itmhatagmeicroofscoapiccrmysantiaplulaattion29o0f K (top) and 5 K (bottom). (i) The T x phase diagram of the (Ca La )(Fe Co )As ). T
hole-doping. Neutron diffraction and muon spin rota- 0.27 0.27 1 x x 2 c
the electronically active FeAs layer is more effective − −
and T[2] are inferred from ρtiobny(µtShRe) omneasseurtemaenntds stuhggeesoteffdsaetstroufctturhael pshuaspeerconductivity. Inset: single crystals against the 1 mm scale.
compared to the larger structcural tuning for controlling
transition from a monoclinic to a triclinic phase at 58K
the superconducting properties of Ca La FeAs .
1−x x 2 and a paramagnetic to stripe AFM phase transition at
RecentlyJosephetal.[44]reportedthestructuralevo-
54K (Fig. 2), both of which can be suppressed by Co-
lution of Ca La FeAs (x = 0.18) with temperature
1 x x 2 substitution on the Fe-sites[54]. Additionally, the pres-
(110K - 300K−) from=po3wd4e6redKX-,raywdhiffircahctionismeaal-most twice as much as that for creases below 58 K (Fig. 1(c)). It is likely related to the
ence of two different crystallographic phases in different
surements. The expansion of the c-axis lattice constant
temperature regions were further confirmed from an op-
BaFe As ,[11] pointing to a much stiffer lattice in 112. extinction release which arises from the stress caused by a
goes through a distinct chan2ge aro2und 150K which can tical 2nd harmonic generation study[74] where no signifi-
be correlated with aOchuanrgestinutdheyreosifstiCvitoy ssloupbe.stInitutciaontnmoodnificFateionsoifttehseerleectsruonlitcsstriunctuarenwaeswobservedstructural phase transition. The (1/2 1/2 1/2)M peak is
this temperature range, the in-plane lattice constants go
as a result of the phase transitions. A similar struc-
through a constant tfhaermmaillyexpoanfsisonuwpheicrhciosnandourdcetrors, where bulk SC takes place in absent at 55 K but well develops at 54 K (Fig. 1(e)). All
turalphasetransitionwasrecentlyreportedinsupercon-
of magnitude smaller ( 0.3 10 4˚A/K) than for the
− ducting Ca La FeAs (x = 0.15) around 100K using
a ∼dom×e like region with a max1imx uxm T2 of 20 K (Fig. these evidence a structural phase transition at T =58 K
c-lattice constant. The anisotropic thermal expansion temperature−dependentcX-ray measurements[77] while s
suggestsachangeintheinter-layerinteractionwithtem-
1(i)). Therefore, we denotKeawtahsaekiCetaal.[51]LreaportedFaneAAFsM oarsdertinhgearoundand an AFM phase transition at T =54 K, which agree
perature, although its relation to the resistivity change 0.73 0.27 2 m
T 62K for a sample with bulk T 35K. This
N c
with temperature is“noptaclreaernytet”. compound of the ∼112 FBS. The 41 K SC∼ in well with the two-kink feature in the specific heat and the
suggests a correlation between the structural and AFM
Alternative claims have been made that instead of phase transition in Ca La FeAs occurring in similar
Ca La FeAs actually arises from1txhex hol2e doping derivative of resistivity (Fig. 1(d)). µSR measurements
0.82 0.18 2 −
through the Ca substitution on the La sites. It is in- were performed on 200 pieces of randomly oriented
∼
teresting to see how robust this material is in mediating single crystals (Fig. 1(g)).[12] The relaxation time σ,
high T SC. The As chains make the individual Fe-Fe which is proportional to the size of the local magnetic
c
nearest neighbor distance slightly different although the moment, can be well fitted by the mean field theory us-
2L equals to the 2L at room temperature (inset of Fig. ing M/M = (1 T/T )β (T < T ) with T = 54.3(2)
1 2 0 m m m
−
1(a)). As a result, irregular FeAs tetrahedra appear. K and β = 0.225(10). The ordering fraction grows much
4
Instead of two in other FBS, four characteristic As-Fe- faster with temperature than the local moment, indicat-
As angles exist with α = 109.1(2) , α = 107.1(2) , ing homogeneous samples. No detectable extinction ef-
1 ◦ 2 ◦
β = 110.2(2) and β = 110.1(2) at 4.5 K for the fect exists in the single crystal II measured at HB-3A.[14]
1 ◦ 2 ◦
“parent” one.[12] The pnictogen height is 1.422(5)˚A for This crystal has only one growth domain.[12] Its (0 2
the “parent” compound while it is 1.412(5)˚A for the SC 0)N peak shows no split or broadening at 200 K and 4.5
one.[2] These violate the empirical rule that implies T K (Fig. 1(f)), which is dramatically different from the
c
is enhanced when the α bond angle is near to BaFe As at ambient pressure.[15] Inside it, the (0 2 0)N
As-Fe-As 2 2
109.47 or the pnictogen height is close to 1.38˚A since peak is broadened below T because of the formation of
◦ s
two instead of one α exist in this material.[13] the structural twinning walls (T-walls), across which the
To reveal the nature of the anomalies observed in Fig. spin pattern, spin orientation and crystalline axis rotate
1(a)-(b), neutron and synchrotron x-ray diffraction mea- 90◦.[16–18] Instead, Fig. 1(f) is reminiscent of the fully
surements were performed in ORNL. The neutron inten- detwinned BaFe2As2 under 0.7 MPa,[15] where T-walls
sity of the (0 2 0)N nuclear Bragg peak, measured on the are wiped off. Figure 1(h) visually shows the absence of
single crystal I containing multiple growth domains, in- T-walls via the polarized optical images. At 290 K, this
5
(b)
E
F dxy dxz
V) dyz dxy
E (eF dxz
E-
G
M M
FIG.3: DFTcalculatedFermisurfaceofCaFeAs inthenon-
2
magnetic state[53]. Reprinted with permission from Huang
-1.4 0 1.4
et al.[53]. Copyright 2015 by American Institute of Physics. k (p /a)
//
FIG. 4: Band structure of Ca La FeAs projected to the
1 x x 2
−
in-plane BZ as obtained from ARPES measurements[52].
temperature window and a weak coupling between the
ReprintedwithpermissionfromLiuetal.[52]. Copyright2015
structural and magnetic order.
by American Institute of Physics.
Upon Co-doping into Ca La FeAs (x = 0.2) [49],
1 x x 2
a mixture of 112- and 122-p−hases were observed. For
low Co-content, almost pure 112-phase was found with
monoclinic structure, but with an enhancement of the
Co-doping level a mixture of the 112 and 122-phases three-dimensional (3D) hole pocket at the BZ centre (Γ
were observed which for a higher Co-doping (> 6%) re- point)[16, 53, 78, 79] as illustrated in Fig. 3. The pres-
sulted even in the complete disappearance of the 112- ence of the additional hole pocket at (0,0,0) (likely origi-
phase. OwingtotheslightlysmallerionicradiusofCo2+ nating from the hybridisation between the Fe 3d /3d
xz yz
(74pm) compared to Fe2+ (76pm), the successful sub- andAs(1)4porbitalsfromtheFeAslayer)andfourelec-
stitution could be confirmed from the shift of the out-of- tronconesattheGpoint(contributedbyAs(2)p-states)
plane X-ray reflection. This suggests that the structural have not been found earlier in 1111[80, 81] and 122[82]-
stability of the 112-phase only exists in a narrow win- pnictide phases. The nesting between the Γ point hole
dow of Co-doping level putting strong constraints to the pockets and G point electron cones possibly results into
crystal growth conditions. a AFM spin density wave (SDW) phase which gets sup-
pressed upon RE-doping favoring the superconducting
IV. Electronic Structure state[53, 83].
The band dispersion in Ca La FeAs showed con-
A. Theoretical Investigations 1 x x 2
siderable 2D character contrib−uted mostly by the As
layers. In the non-magnetic calculation, four hole-like
Density functional theory (DFT) band structure ana-
bands(aroundΓ(0,0)point)andtwoelectron-likebands
lysis of fully stoichiometric LaPdBi revealed the almost
2
(around M(π,π) point) were found from the band struc-
equalcontributionofallconstituentatomsneartheFermi
ture analysis (see Fig. 4) which has some similarity
level, except for Pd which has a much higher domi-
with BaFe As [78]. It was predicted that for T < T ,
nance. Changing the Pd content in CePd Bi affects 2 2 c
x 2
Ca La FeAs could work as a natural topological in-
the Fermi surface topology significantly and in the pres- 1 x x 2
sula−tor/superconductor hybrid structure (FeAs layer to
ence of significant Pd vacancies, Fermi surface nesting
beresponsibleforthesuperconductivityandtheAschain
can be avoided. This suppresses any kind of charge den-
layer being the topological insulator) that could be ideal
sitywave(CDW)intheBisquarenetlayerwhichallows
for the realisation of Majorana fermions[84].
the superconducting state to stabilise[56].
The density of states of two hypothetical 112- Nagai et al. [85] investigated the effect of Sb-
structures BaFeAs and BaFeSb [28] showed a consid- substitution on the superconducting properties of
2 2
erable amount of Fe-3d states at E with a small con- CaFe(Sb As ) . Sb-doping in the As zig-zag layer
F x 1 x 2
tribution from the spacer layer. This has been claimed increased the−lattice parameters a,b with an overall in-
to originate from the larger distance of separation be- crease in the unit cell volume leading to an overall sta-
tween the As(1) and FeAs layers with minimal hybridi- bilisation of the structure which is energetically more
sation between them. For CaFeAs , first-principles cal- favourable than substitution in the FeAs layer. How-
2
culationalsosuggestedthestrongpresenceofFe-3delec- ever,thecalculatedbandstructurewithandwithoutSb-
tronsinthedensityofstatesneartheFermilevel[16,53]. dopingisverysimilarexceptasmallshiftalongtheG Γ
−
The Fermi surface of CaFeAs consists of two electron direction. Primary investigations suggested the role of
2
cylinders at the Brillouin zone (BZ) corner (M point), Sb-substitution in the enhancement of T to be possibly
c
four Dirac cone type electron cones at the BZ edge relatedtothestabilisationoftheAs-chainswhichhavea
(G point) and three hole cylinders with an additional crucial role in controlling T .
c
K.KODAMAetal. PHYSICALREVIEWB83,214512(2011) 6
2500 antiferromagnetic orderings of the Ce moment at Ne´el
00sec.)44050000 100 32..3895 KK pteemrcpoenrdautucrteivs,itieTsN a∼pp3ea.0r bKeloawnd 21..52 Kan,dre1s.p7ecKti,vealny.d Sthue-
ntensity 21050000 (a) y (Counts/133500000 54..4354 KK abnneettliiocfwerBrtohrmaegsagegntpeeemtiacpkesorradatepurprieensag.r14anbIdenlothtwh∼eessTueNpceoarcmnodpnodtuhunecditrisv,iitntyhteecnosmeixtaiiegss-t
d I 100 nsit2500 increasewithdecreasingT.However,thedevelopmentsofthe
grate 1000 Inte2000 29.0 30.0 31.0 malmagonsettiflcaBtrtaoggthientetnemsitpieesraatureresubpeplorwesseTdc.1a5n,1d6 tThehyebperceosmenet
n 2θ (deg.)
e T dependence of the magnetic Bragg intensity suggests
Int that the Ce 4f electron is almost independent of the su-
500
perconductivity and this compound is not a heavy fermion
superconductor.
The solid line in Fig. 4(a) shows the fitting of data below
0
0 2 4 6 8 5.0 K to the function that the intensity is proportional to,
T (K) 1 (T/T )2, and the extrapolation of the fitting function to
0F−KIG..T6h:eNPahmapsleitduidaegroafmthoefmCao1m−txeLnatxaFte0AsK2 ofobrtavianreidoubsydtohpeing
levels of La[51]. Reprinted with permission from Kawasaki
FIG. 5: Tem(pb)erature dependence of the integrated1in0t2ensity fietttinagl.[i5s11]..7C1oµpBy.rIignhtthe20e1x5pebryimAemntaerliaccacnurPahcyy,siacnayl sStorucicettuyr.al
of the80(01000) Bragg peak as obtained from neutron diffrac- change is not observed and the tetragonal symmery remains
tyion measurements of CeNixBi2. The solid line describes downto2.9K.Inthetetragonalsymmetry,energylevelsofthe
ansit(1−(T/TN)2) dependence of the intensity below 5K[36]. Ce4f1 statesplitintothreedoubletstates,!7(1),!7(2),and!6,
RrInteigehptrin20te1d1 bwyitAhmpeerrimcaisnsiPonhyfsriocmal SKoocdieatmy.a et al.[36]. Copy- bcWoynteshiiesstscorlyfatwlailnlieχnaer=ecloemcCtbr/ii(cnTafitei−oldn,θowcf)hJerreevethaele3tdw/2oaafnoCdrmuJreiredTouebm5l/ep2ser-
ed 7500 staattuesr,eanθdc t=hel−at1te.5rKconasnisdtsaonfJeffze=ct±i1v/e2mstaagten.eItnzic=thme±coamseent
grat ofµfreeCffeeA=Cge2S3.+b826iµwoBnh/iciCnhdeaiaclsatootimhnagswtahhieZchrzlCoi=ucsSao±liiAfssesi-dmtyniplaeatrsutvrraeulcuotuefretah,setfhoCerea-
Inten B. Experimental Results gthmereoxouicsmnrtdyeesnnstacttalesltienioneifsetAClheeeFcPtM!rdi6cx-soBfitraeid2tlede.,reirnxeTvcgheitaeaoltfeinodtenhbg.eyaTtthiChveeeea-mamθtecpoalmsistuuusrpdeepmatooernfhttstihgoehtfheer
antiferromagneticorderedmomentisingoodagreementwith
7000 temperatures[56]. TheHall-coefficientR ofCePd Bi is
a bAaRnPdEsStrumcteuarseurseimmeilnatrstoonthCea1o−txhLeraxpFneiActsi2deinsdyisctaetmeds ginncJoteµhrgBiraseJtcsziopv∼moenpw0do.i4ust3nhtdµoaiBnsa.m1ac7vuaOecrrhrnailegtahreregcveooartnlhtucheeearnnhotatfrhna1edt.m7,iot×honem10ooeH−frnd4t3eo.cr7femCd31em/A0Co2gm2xSw/ebcnh2m2t,ic3h.
consisting0 of three2hole like b4ands (dxz6,dyz,dxy c8harac- indicating that the ground state of this compound×is not the
R stays temperature independent suggesting a single-
ter) at the Γ point and two eTle (cKtr)on like bands (dxz,dxy !6Hstate. Such differences of the amplitude of the ordered
band character of the carriers [56]. The strong interplay
types) at the M point of the BZ[50, 52, 54, 78], with momentand/orthegroundstatearecausedbythedifferences
reasFoIGna.b4.leTenmepsetriantugremdeopsetnldyenocersigoifntahteiinngtegfrraotemdinttehnesitFiees-o3fd obftehtewseteruncttuhrealKpaornamdoetearnsdanCdEcoFnsitnittueernatcetlieomnsenltesa.dAscttuoalaly,re-
(a) 100 and (b) 102 reflections, respectively. The solid line shows constructionoftheFermisurfacetopologywhichismost
electrons (see Fig. 4). No evidence of the As-p or Ca- other compounds with a ZrCuSiAs-type structure, CeCuBi
2
dthestafittteinsgwtheeredaftoaubnedlowin5thKetoFetrhme ifusnuctrifoancethamt athpe sinctaennsniteyd alnidkeClyeArgesBpio,nesxibhliebitfoarnttifheerroambsaegnnceeticoofrsduerpinergcownitdhucetaisvyity
oivseprropaorctioomnapllteot1e−3(DT/mTNo)2maenndtuthme exstpraapcoelatwiohniocfhthiesfisttiimngi- aixnesCepPardaxllBel2i2toastihseo-sctrduicrteuctriaolnL. aTPhdeixrBsi2atiusrastuepderocrodnerdeudct-
lfaurncttioontthoezearobkseelnvcine.Ionfthienitnesrestt:itteiamlpebralotucrkesdeopnendtehneceFoferthmei minomgeanntsdwbhoicthh ahraeveestsimimaitleadrbFyermmaignseutirzfaatcieosn acubrovvees a7r5eK.
peakprofileof100reflection.
surface in 1038-phase (CaFeAs)10Pt3.58As8 and 1048- 1E.7l6ecatnrdon2ic.1sµtrBu,ctreusrpeecctaivlceulyl.atTiohnesesubgegheasvtieodrstahraetsitmheilaPrd-
phase (CaFe0.95Pt0.05As)10Pt3As8[86] and 1111-phase tovatchaenpcrieessenintdcuocmepsoturnodngwhscilaettineriCnegCeuffBei2ctasnidnCtehAegPBdi2x,Bi
Somf Fe200iCnotheAcsaOlc[u8l7a]tisoyns,ttehmeso,bbseurtveindcinotnetnrsaitsitestocatnhneotpbree- thlaeyseurpethrcaotnednuhctaivnictyesisthnoetCoEbsFerevffedecatndtothqeuirenmcahgntehteizCateiomno-
deisctt∼imioa1n−tsexdmdaudxeetofrtohmeexepleecrtimroennitcalsetrrruocr,tuasremceanltciounlaetdioanbos[v8e3.]. cmurvenestsshaotwlomwetteammapgenreattiucrbee.hTavhioerFwehrmichisiusrnfoatcoebrseecrovnedstirnuc-
HowTehveeirn,sLetieotfaFli.g[.504](am) aisntahgeedtemtopererastoulrvee(tTh)edperpeesnednecnecoef thteiopnreasnendtRcoumdpeorumnadn.-Kittel-Kasuya-Yoshida (RKKY) in-
aonfatdheditpioeankalp3rDofihleoleoflik1e00barnedfleacttiotnh.eAzosneshcoewnntreinantdhea teCraecNtii0o.n8Biin2dhuacsedthmeasganmeeticcroyrsdtaelrisntgrucctaunreinadsuctheesusop-er-
fiansstetd,is1a0p0pereaflriencgtiobnanadppneeaarsr tbheeloXw-p5o.4in5tKatatnhde zthoenepecaokr- ccaollneddu1c1ti1v1itysyisntemthewhheicahvyh-faesrmaiomnaxsyimstuemm TCceCouf2aSbio2[u8t8],
ngerradinuaCllyadevLelaopFsewAisth d(xec=re0a.1si)nagnTd.aFlisgourfeosr4x(a=) a0n.2d74[5(b4)]. 5w5hKicahmiosnugnilriokne-lbyatsoedohcicgohu-TrcinsutpheerccoansdeuocftoCrse.PTdhxeBorie2tiwcailth-
1 x x 2
Tshhoewprtehseen−TcedoefpaencdoennscideseraobflethAesi-n4tpegroartebditailntfeonrstithieeshoolfe stouudtieesxotnerannailroinn-flbuaesendcehsig[7h3-]T.csuperconductorpointoutthat
z
l1ik0e0baanndd1a0n2drAeflse-c4tpions,orrebsipteaclstivfoerlyt.hTehneairnrtoewnsibtyanodfw10e2re magTnheteicgflrouuctnudatisotnataerisoifngthferoCme-abanseesdtinigntbeertmweeetanllFicercmoim-
rreesfloelcvteidonwhhailseacolnasrigdxee,yrearbrolrehbyabrriindisthaitsiosncableetwbeeceanutsheethAiss- suprofuacneds aCte!NiaxnBdi2MispgooinvtesrninedanbyunftohledeidntBerripllliaoyunbeztowneeen
4rpefleacntidonFein-3cldudoersbitthaelsnuwcelreearfoscuantdteriinngt.hTehheoilne-telinkseitibeasnodf. isRaKnKoYrigainndoKf othnedosuipnetrecroancdtuiocntisv.ityT.1h8–e21KEonxpdeoriimnteenrtaalclyti,on
bozth reflections increase with decreasing T below about stsrtornegngcothrreilsatdioentserbmetiwneeednbthyethhigeh-leTvceslupoefrchoynbdruidctiisvaittyioanndbe-
5 K, consistentVw.ithMtheagtenmetpiecraStutrreucattuwrheich the jump of thtewesepninthfleucCtuea-4tifonanadrecoonbsdeurvcetidoninelaenctirroonns-bfaasveodrihniggha-nTcon-
the specific heat is observed.6 As shown in the main panel sumpaegrcnoentdicucgtroor.unTdhestianteelawsthicilentehuetrolonngscraattnegriengmamgenaestuircea-lly
ofTFhieg.la4y(ae)r,etdhecotemmppoeurantudreCdeePvdeloBpimeinstaofKthoendmoa-glantettiicce moerndtesroendLstaaFteeAissOp1rexfFexrrwedithbxy the0.0R5K7,K0Y.08i2n,tearnadct0i.o1n5[73,9].
ABFraMggwinhteicnhsitiysismmetoanlolitconaobuosvdeow7n5xtKo,22.a9nKdaenxdhaibniatsnobmealolyw shTohwetphaartetnhtecsopminpflo−uucntudaotifonCeoNbs=ierBveidiisnCtheeNisBami pwlehsiwchithisa
x 2 2
7i5sKnotaosbtsreornvegdinattethreplsauypebrectowndeuenctiKngontrdanosatinondtcermypstearaltufireel,d xmod0e.r0a5t7elyanhdea0v.0y8-f2e,rmwhioonseaTnctifaerreroamboaugtne3t0wKit,halamomsatg-
=
et4fhffe.a2ermtcKtits.ohIn(teCissEmuqFpuaei)ttr[eec7rod3iani]f.ldfeuMirscetanoagtrnsnfiresoiotmnitcrtwoshpuehisibcccehehdpauttvhieibeoitrlosmitoyatbghdsneeaerttCviaecEdsuFoingrdgeheffeersaeitvnceygtds dhnriigseeahptp-ioTpcrectoaserruddspeetirrnhicnaogttnhdetTuemcstaipvmineitprylaCeitseuwaNrleimithToBsNixtsou=fp5pirK0ne.cs1[3sr5ee67da]..s2ie2RnsOowwnsahtihtiehcethoaathtnlh.e[er3in2-]
wanitdhtahne sAuFpeMrcoonrdduecrtiinvgitytecmoepxeirsat.tuFroerTexam=p6leK, C[5d6-]d.opTehde hcarneda,sethiensxu,pwerhcoicnNhduaclstiovietynhmaxnayce2sa−ptypheearmwagitnheotuitcmanaigsnoettriocpy
hCigehC-otIenm5,pCeerCatou(Irne1 sxuCsdcxe)p5twibiitlhitxy fi0t.t1eadnNdu0s.i0n7g5,tehxehibCiuttrhiee- floufctCuaet3i+onaitnlCoweNtie0m.8Bpie2r,abteucraeu.seBNelioiwsn5oKnm,tahgeneCtiecamnodmtheents
− =
214512-4
week ending
PRL106,057002(2011) PHYSICAL REVIEW LETTERS 4FEBRUARY2011
TABLE II. Obtained parameters for superconductivity of electric field of a Ce3 ion, and (3) if the heavy electrons
þ
RNixBi2. give superconductivetransitions, themagnitude ofthe C
p
Compound Tc=K Hc1=Oea Volumefractiona jump should be !2 Jmol"1K"1 ("Tc); however, the ob-
served jump was 4 Jmol 1K 1. This AFM transition
" "
LaNi Bi 4.0 90 0.01 !
0:65 2 hasalsobeenpointedoutbyseveralgroupsonthebasisof
CeNi Bi 4.2 65 0.96
0:80 2 the magnetic susceptibility data on CeNiBi [18–20]. A
NdNi Bi 4.1 55 0.14 2
0:89 2 similarsmall humpnear7K wasshownintheright-hand
YNi Bi 4.1 67 0.17
0:85 2 insetofFig.2(b).Theevolutionofthemagneticpeaksfor
aThesewereestimatedfromanM-H curveat2K. our sample was confirmed below 5 K by powder neutron
diffraction [14]. The AFM ordering of the Ce 4f1 spin in
which the magnetic moment is parallel to the c axis was
takes place at 4.2 K. A small hump is also observed at
confirmed by the appearance of new magnetic peaks due
7 K,asshownintheright-handinset.TheM-Hcurveat
! tothedisappearanceofn-glideplane.
2Kintheleft-handinsetofFig.2(b)showsatypicalprofile
Mass enhancement presumably results from strong in-
foratype-IIsuperconductorwithalowersuperconducting
teraction of the Ce 4f electron with the carrier electrons,
critical magnetic field (H ) of 65 Oe. Table II summa-
c1 ! which may come from Ni 3d electrons mixed with Bi(1)
rizes T , H , and the superconducting volume fraction of
c c 6pelectrons.Thepeakduetosuperconductivetransitionin
fourkindsofRNi Bi compounds.AlthoughT issimilar
x 2 c thespecificheatwasnotobserved.TheBi6pbandinthe
for thefourcompounds,thevolumefractionofthesuper-
metallic Bi(2) square net contains a positive hole, which
conductivityphasewasfoundtobesignificantlyenhanced
mayleadtothesmallblockingeffectagainsttheNi Bi 1
intheCecompound. x ð Þ
conductinglayer.ThemassofcarrierintheBi(2)6pband
Figure 3 shows the heat capacity of CeNi Bi as a
0:8 2 is not heavy; that is, the "value is smaller by an order of
function of temperature. A distinct ! peak is observed
magnitudethantheobservedvalueof0:4 JK 2molCe 1.
only for CeNi Bi . Application of the magnetic field " "
0:8 2 The"valuefortheBisquarenethasnotpreviouslybeen
suppressed the ! peak. The Sommerfeld coefficient (")
reported. Thus, we estimated the " value from the data
at 5.0 K was estimated to be 0:4 JK 2molCe 1 (inset of
" " on similar compounds. For example, the " value for the
Fig. 3), which in turn led to an observable large mass
Ni Bi superconductor with T 4 K is reported to be
enhancementoftheconductingcarrier,becausethe"value 1=3 c ¼
4:3 mJK 2mol 1,andthespecificheatjump!C should
is proportionalto thedensityofstates at E .Such a large " " p
F
be "T , 20 mJK 1mol 1 [21]. The magnitude of this
mass enhancement was not seen for the La or Y systems. c " "
!
Wethusattributedtheobserved!peaktoAFMorderingof jumprelativeto theobservedAFMpeakwas toosmall to
the Ce 4f electron spin, rather than the superconductive be observed. When the light carriers coexisting with the
transition on the basis of the following reasons: (1) The heavy carriers cause superconductivity, the peak originat-
peaksuppressionbehaviorbyHisratherdifferent,(2)the ing from the superconductivity is hidden by overlap of
entropy obtained from the integration of C =T is Rln2 the strong peak due to the AFM transition. Therefore, the
p ! specific heat data indicated that there are two kinds of
corresponding to a doublet ground state of the crystal 7
carriers with noticeably different effective masses. The
heavy carrier is due to magnetic interaction between the
ity sets in at T = 35K [51]. With an increase in the
conductionelectrocnsandtheCe4f electronwhichcauses
doping concentration, AFM order gets enhanced as T
the AFM ordering at 5 K, and the light carriers cause N
rising up to 70K[51]!for x=0.24 (see Fig. 6) which pos-
1 superconductivityat 4 K.Itisproposedthattheoriginof
−K 5 thsieblliyghotriegliencatrtoenssfrcoom!mtehsefrnoemstiBnig(2o)f6tpheaFnedrsmuipseurcrofancdeuscd-ue
−1C / J molCe p 0124 TTTT 1−2− / T / J molCeC Kp010 20 40 60 tefL[wi3otothtHalvaeeoau,rrgeiN2oacidrtaanw2yktaaieviCrPe]endlroey,denOosdvnpNateiceisdfnlhtdacroRitoin0,uahrcop:CEd8sroobnttiseBuhnamyhaCdg1iegleit2ee−ohntre1sphebxaluft1nieiaLetnhrsnhp1shcosaegee1temimaxt.rdsr-tnFiosicweBoAgmcyenotNnoeinaAsnads-ltiiddslfisdxsiewimsoqB2umiccT-mpauaceiiNhsðlaintnBeLan1irathnnraeÞagliosaysgnFbpinlbbnodbeshacalyeenAh,yeaeonrtaieems,cisvgLdlnnreOkehsewaeidd.eAn1bn[flh5ri−oetgoTaFe0acusxtgrhMt,butnhHerrlhepaooda7aaxeyvcnsm8on[Traeei]9trcc.crstod1woheTt,esnhseTa(abNtr9,d!ornsshe2[uushda5ae]cecc4e4lrttowassou,AtoKotvehobrhrrxyF9ees)fouuei2Mbrssc]ett--.
7 T T2 massenhancementbytheCe3 ionshouldbedisadvanta-
inCa La FeAs microscoþpically[51],whilebothorders
0 gaeroeuscof1mo−rxptlheetxeelmyesregg2ernecgeatoefdsuinpeLrcaoFnedAuscOtivityHin[t9h3e].Bi(2)
0 10 20 1 x x
squarenet. −
T / K
This worVkI.wasSuspupeprcoortneddubctyintghePrFoupnedrintigesProgram
for World-Leading Innovative R&D on Science and
FIG. 3 (color online). Magnetic field dependence of the heat
FcIaGpa.c7i:tyToemf pCeerNaiturBeidebpeelnodwen1c5eKof.tThheespinecseifitcshhoewats(tChpe)Tfo2r TechnolAog.y (LFoIwRSCTr),itJicaaplanT.eWmepetrhaatnukreDrM. Sa.teSrhiaalmsoto,
different applied m0:8agn2etic fields with distinct λ peak around
5dKepeinndCenecNei Bofi .theInsheeta:tCca/pTacityTp2eartu0nOite[t3e3m].peRraetuprreintoefd Dr. K. Kodama, and Dr. S. Wakimoto (JAER) for powder
wCitehNip0e:8rBmi2isasxtio0n2Ofreo.mMizogpuchi−etal.[33]. Copyright2011by neuCtreoNnidxiBffir2acistioantmypeea-sIuIresumpeenrtcso.nductor (Tc =4.2K)[33]
with a SVF of 96%. Below 10K, the resistivity shows
American Physical Society.
a ρ(T) T2 dependence and increases linearly with
057002-3 ∼
temperature up to 100K. For other RE-containing
RENi Bi systems with RE = La, Nd, Y, also T val-
x 2 c
ues around 4K were measured, but the SVF was signif-
order antiferromagnetically with the easy magnetisation
icantly less than the Ce-containing phase. The specific
axisalongthec-directionwithasaturatedmagneticmo-
heat data for CeNi Bi (see Fig. 7) suggested the pres-
ment of 1.71µ as T 0K. Superconductivity is in- x 2
B → ence of two different types of carries with different effec-
ducedinCeNiBi viaNi-deficiencysimilartotheoxygen
2 tive masses. The AFM ordering around 5K was caused
deficiency in the 1111-system[89]. From powder neutron
bytheheavycarriersoriginatingfromthemagneticinter-
diffraction measurements (see Fig. 5) clear Bragg peaks
actionbetweenconductionelectronsandCe-4f electrons
are observed at q = (0,0,0) below the magnetic order-
in the Ni Bi plane, while the light carriers from the Bi
ing temperature T 5.45K and the intensity of the x
N ∼ square net layer were responsible for superconductivity.
(100) Bragg peak increases down to the lowest temper-
The entropy calculation suggested that the sharp jump
ature. However, no anomaly or jump was observed in
in the specific heat around T corresponds to the mag-
the peak intensity around T which is unlike the features c
c neticorderingofCe-4f moments, whicharenotinvolved
observed in heavy-fermion superconductors where Bragg
in the occurrence of superconductivity. Lin et al.[39]
intensities are suppressed below T [90]. This suggested
c claimed that the observed superconducting properties of
that Ce-4f electrons do not contribute to the supercon-
LaNi Bi and CeNi Bi are most likely due to the pres-
ductivity and the material cannot be considered to be a x 2 x 2
ence of Bi and Ni-Bi binary impurities as very little evi-
heavy-fermion superconductor. Although CeNi Bi has
x 2 denceofbulksuperconductivitywasfoundfromtheirsin-
a similar crystal structure like 1111-pnictide, the T is
c gle crystalline samples. However, highly crystalline thin
much lower than in the 1111-system which can be due
films also showed superconductivity around 4K[34, 55].
to the absence of magnetic fluctuations in CeNi Bi [36].
x 2 The zero-resistance state in LaPd Bi [56] was observed
Anisotropicmagneticbehaviorwasobservedinthesingle x 2
around 2K with a bulk superconducting phase around
crystalline RENi Bi compounds (RE = Ce-Nd, Sm,
x 2 y the same temperature which is likely s-wave in nature.
Gd-Dy) which order±antiferromagnetically at low tem-
In the normal state, it is metallic with a residual resis-
peratures between 3.3K (Sm) and 10.2K (Tb)[39].
tivity ratio (RRR) of 2.45 indicating strong scattering in
From first-principles based investigation, the ground the conducting layers.
stateofCaFeAs waspredictedtobeaSDWtypestriped
2
AFM phase driven by Fermi surface nesting with the B. High Critical Temperature Materials
magnetic moment of each Fe atom to be 2.1µ [53],
B
significantly smaller in value than the LDA calculated 1. Rare Earth Substitution
value for 1111-LaFeAsO [16] and the hypothetical 112-
compounds BaFeAs and BaFeSb [28]. Electron doping The predicted mother compound of the (Ca,RE)-112
2 2
using rare-earth elements can help suppressing the SDW family CaFeAs has not been synthesised yet and super-
2
state and stabilising the superconducting state. Nuclear conductivitywasonlyobservedintheRE-dopedsystems
magnetic resonance (NMR) measurements revealed that within a limited doping range. Doping of RE-elements
the AFM ordering sets in below T = 62K for highly also improved the T value of 122-type (Ca,RE)Fe As
N c 2 2
doped Ca La FeAs (x=0.15) while superconductiv- with T onset values : Ca Pr Fe As (47K)[94],
1 x x 2 c 1 x x 2 2
− −
8
70 4.00
(a) Ca LaFe(As Sb) (c)
(a) Ca1-xLaxFe(As1-ySby)2 60 1-x x 1-y y2 a
0.0 50 3.95
Å)
3m) (K)Tc 4300 (a, b 3.90 b
mu/c 20 y0 .=1 0
H (e -0.5 30 Oe 10 00..0010 3.85
M/π H // ab 0 10.5
4 (d)
161 (b) Å) 10.4
-1.0 x = 0.16, y = 0.00 (c 10.3
x = 0.15, y = 0.01
x = 0.12, y = 0.10 3Å)160 10.2
me ( 94
0 10 T2e0mpera3t0ure ( 4K0 ) 50 60 Volu115598 ( ° )β 92 (e)
90
(b) Ca LaFe(As Sb) 0.05 0.10 0.15 0.20 0.25 0.05 0.10 0.15 0.20 0.25
0.6 1-x x 1-y y2
x x
x = 0.16, y = 0.00
x = 0.15, y = 0.01
m) x = 0.12, y = 0.10
cΩ 0.4 FIG. 9: Dependence of (a) Tc, (b) unit cell volume, (c) a,
m b lattice constants, (d) c-lattice constant, and (e) β angle of
(ρab Ca1 xLaxFe(As1 ySby)2 for various La-doping levels x.[18].
0.2 Rep−rinted with p−ermission from Kudo et al.[18]. Copyright
2014 by Physics Society of Japan.
0.0
0 100 200 300
39K, but the SVF increased to 77%. For Sb-doped
Temperature (K)
Ca La Fe(As Sb ) (x = 0.16), the bulk T in-
1 x x 0.99 0.01 2 c
crea−sed to 43K with a significant SVF of 78%. From
FIG. 8: Temperature dependence of the (a) magnetisation
the trend in the T vs. x phase diagram in Fig. 9(a),
at H( ab) = 30G in ZFC and FC conditions and (b) in- c
(cid:107) the highest T was found for all materials at the lowest
plane resistivity (ρ ab) for different La and Sb-doping c
in Ca1 xLaxFe(As1 y(cid:107)Sby)2[18]. Reprinted with permission x value. This suggests that a much higher Tc could be
from K−udo et al.[18−]. Copyright 2014 by Physics Society of expected if x can be reduced further below 0.15 which
Japan. needs development of chemical synthesis techniques.
Further enhancement of T was possible with a higher
c
levelofSbsubstitutioninCa La Fe(As Sb ) (x=
1 x x 1 y y 2
0.12,y =0.1)to47K[18]with−aSVFof100−%at2K(see
Ca1 xLaxFe2(As1 yPy)2 (45K)[95], Ca1 xPrxFe2As2 Fig. 8) indicating the appearance of complete bulk su-
(49K−)[96]. Howev−er, complete appearance−of bulk su- perconductivity. This enhanced T could originate from
c
perconductivity was not observed in some of these cases, two different effects: (1) simultaneous Sb doping, (2) de-
possibly relating to the filamentary nature of the super- crease in La content resulting in an increase in cell vol-
conducting phase. ume(Fig.9(b)). ThedecreaseinLacontentsuggeststhe
The in-plane resistivity of Ca La FeAs with x = reduction of the number of charge carriers as the ionic
1 x x 2
0.16 goes through a superconduct−ing transition at 36K radii of La3+ and Ca2+ are comparable. Secondly, ad-
[16] with a transition width of 2.4K and bulk super- ditional Sb substitution can induce a negative chemical
conductivity was observed from magnetisation measure- pressureduetoanincreaseincellvolumeastheionicra-
ments at 34K with a SVF of 66%. For x = 0.21, dius of Sb3 (Sb ) is larger than As3 (As ) which was
− − − −
Tonset increased to 45K [16], but the zero-resistance observed from the increase in the a,b lattice constants
c
state was only observed at 25K which is consistent (see Fig. 9(c-d)) and the higher level of localisation of
with the T determined from the magnetisation data. the d-electrons with a larger sized Pn atom. However,
c
In (Ca Pr )Fe As O , a small level of O-doping thismechanismisnotvalidfortheT enhancementinthe
0.9 0.1 1.3 1.8 0.2 c
helpedenhancingtheSVFasbulksuperconductivitywas P-dopedsystem[17]asneithertheLacontentgotreduced
observed with a T of 20K[40]. nor the lattice parameters changed in that case.
c
Replacing As by a small amount of isovalent substi- In Sb-substituted Ca RE Fe(As Sb ) , there is
1 x x 1 y y 2
tutional elements like P and Sb enhanced T further a general trend of T enh−ancement and−improvement of
c c
[17]. For 5% P doping in Ca La Fe(As P ) the superconducting properties[18]. In the absence of
1 x x 0.995 0.005 2
(x = 0.16) bulk T was enh−anced to 41K with a Sb(y =0),fortheCe-dopedsystem,noevidenceofbulk
c
SVF of 44% at 5K. For x = 0.18, T got reduced to superconductivitywasobserved,whilePr-andNd-doped
c
9
H.Yakitaetal./PhysicaC518(2015)14–17 17

ab, c
Jc
5
10
)
2
m
c
A/ mean
( 4 Jc , H || ab
c 10 c, ab
J Jc
H.Yakitaetal./PhysicaC518(2015)14–17 17
3
Fig.4. (a)Tcand(b)dFe–Feforthehighpressuresynthesizedsamplesasafunctionofionicradiiofthe1RE03+0ionsina10coordin2a0tionnu3m0ber(C.4N0.)of8.50 60
H (kOe)
FIG.11: Magneticfielddependenceofcriticalcurrentdensity
samples [9]. For TM-free compounds, a trend that T decrease trend. On the other hand, exceptionally low T and long d
c J of Ca La FeAs in the in-pclane (H aFbe)–Feand out-of-
according to decrease of ionic radii of RE3+ was found except for wasfoundinc(Ca,Eu)111−2x,suxggestin2gtheco-existenceofEu(cid:107)2+and
plane (H c) configuration[47]. Reprinted with permission
CeandEudopedsamplesshowingexceptionallylowTc.Sameten- Eu3+.SlightdfreocmreaZsehoinu(cid:107)deFte–aFel.b[4y7C]o.-Ccoo-pdyorpiignhgtw2a0s1a4lsboycoJnafipramneesde. Society of
dencywasobservedforCoorNico-dopedsamples.Ontheother
Applied Physics.
hand, T of the Co-co-doped (Ca Ce )112 was close to that Acknowledgements
c 0.925 0.075
ofCo-co-doped(Ca La )112and(Ca Pr )112.Fig.4(b)shows
0.9 0.1 0.9 0.1
relationshipbetweeninterlayerdistancebetweenFeplanesd This work was partly supported by the JSPS KAKENHI Grant
Fe–Fe of T with decrease of the ionic radii of the RE atoms,
valuesanalyzedfromXRDresultsandionicradiiofRE3+ofTM-free Number 263900c45, Izumi Science and Technology Foundation,
a similar behavior was observed for d (interlayer
and Co 3% co-doped (Ca,RE)112 samples. The values of d andEuropeanUnion-JapanprojectSUPER-IRON(grantagFreeemFeent
decreasedfollowingdecreaseofionicradiiofRE3+.However,thFeo–sFee No.283204)d.istancebetweentheneighbouringFeplan−es). Notethat
ofEudopedsampleswererelativelylargeconsideringionicradiiof none of these trends are clearly established due to the
Fig.4. (a)Tcand(b)dFe–FeforthehighpressuEreus3y+n(t1h.e0s6iz6edÅFs:aImGCp.Nl.es.1=a0s:8a)f.(uanE)cxticToenpotafioinondnaicl(lrybad)iliadorfgtehedRFEe3–+Foieofnis(nCina(aC,Rcao,oEErdu)i)Fn1ae1tAio2nsnuamsRbeefruf(enCr.cNe-.n)cofe8s.inhomogeneity of the phases. Compositional analysis re-
wexaisstaelnscoeroefptEoiuor2tne+(do1f.i2n5thRÅEe:1Cio5.Nn%.iccd=o8rpa)edadniidsaoEmfu3pv+aFle(res1i−o.b0Fuy6se6SRaÅl)Eac3ea+tnabiloe.n[c9so]n.wsCiidot--h2coor[1d]inY.aK-amihavdraeu,aTe.lWetdoatattnhhaebaet,sMitm.hHieilraaanrcoi,toHun.aHilcoslroenavod,eiJl.iAo[m4f.2CC]hweemhi.niScohct.h1c3eo0uC(2l0ad08bs)ieteairseahsigohn
samples [9]. For TM-free compounds, a treenrdedthaastthTec trdeieoacnsroennausfmeorbererlteariteginvhedtl.[y4O2lon].wtRhTeepaorntinhdteetrhdehwaenintdlh,arpegxeecrdempditsisoinoanlolfyfrolomwYTack3iat2na9d6–3lo2n97g. dFe–Fe
according to decrease of ionic radii of RE3+ was found execteaplt.[f4o2r]. Cowpaysrifgohutnd20in15c(Cbay,EEu)l1se1v2i,esru.ggestiFneg–Fteheco-exi[2st]eFn.Cc.eHosuf,EJbu.Ye2.+hLuainon,ddK.Wth.eYeahb,Ts.eKn. Ccheeno,fTs.Wu.pHeuracnog,nPd.Mu.cWtiuv,iYt.yC.iLnee,thY.Le.Ce-doped
CeandEudopedsamplesshowingexceptiona(lClya,lEouw)1T1.2S.aSmtüerzteenr-et alE.uh3a+v.eSlirgehptodrteecdreRasEe(iRnEd=Y, Lbay–CNod-,coSm-d–opingwasHaulasnog,coY.nYsfi.yrCsmhtuee,mdD...C. Yan,M.K.Wu,Proc. Natl.Acad.Sci. U.S.A. 23 (2008)
Lu) dopincg effect for Ca (Pt As )(Fe As ) (10-3F-e8–F)e[14], which 14262–14264.
dencywasobservedforCoorNico-dopedsamples.Ontheother 10 3 8 2 25 [3] M.J.Pitcher,D.R.Parker,P.Adamson,S.J.C.Herkelrath,A.T.Boothroyd,R.M.
havesimilarcrystalstructureto(Ca,RE)112.Inthesecompounds,
hofanCdo,-cToc-odfoptheedC(Coa-0c.o9L-da0o.1p)e1d12(Caan0d.92(5CCae00.9.0P7r50).11lc)1a1o2r1ng2wde.uraFcsuitgnicv.ilti4ots(ycbseeyw)ltlssoetthrhetoeahmwnoassbtosteehxrevhreAidRbcEkiitnndeoEodwupeTldedcod1pga0eet-dm311-e080n-Kta3sn-d8a.naTdbhsee1sn1ecKearoerfessisumppieelcar-rtively[4w]iIJBb.tHb7h.e8Tras(2op0np0,,8ZM)..T0Ba6rn0ug5n,0eB5l.l.iL,vS,.KJ..CSlaas2rmk.ea,l,CBTh.erLmaorn.eCsnoizmt,iPmo.Cnu.Wn.M.(C2eh0tu0a,8Al).5MS9u.1Gb8u–sl5toi9yt2,u0Pt.hiyosn.Rev.
vdraenaellcdaurteeiCosaonsaesnd3ha%ilfpyozlcbleooed-wtdwfiorneopgemenddXeinc(RtCrDeeara,rlsRaeeEys)ueo1rlft1isd2oiansntsicadanmricaopednliiebicseo.rtfawtobTRdobyheEiiese3tCeon+h.rofvavFH-RateecloEoduow3p-ef+ldiesano(ovnCfazcpoeea(eoTrfiCs,SnMr,nEaotddgVud-,hFF-Ef)ueewoFrr1u––eescFF1i)eeseetot12hiif1s,vt205tiah.t.%0ynoNaNI1nunc.uo–aedg.m0TFash2.sdhEo0bn8tduis2resa3ouirrt2toÅtpwiceh0po2elco4neer6eworac),a3k.moCnad9rnsF0wpeUdde0-aa–naudir4Ftasiceeoo5ttmdrnp,pivvia-ataebIiJrzoltadgutuyuplnTeytmasaMseeynbisdt-asuoipfStrcprtvreceeopeoismeeojdifie1ngres,c2tlcnacteatemraKdhmSelUpaaewbenwslPyeneodaEasdrtsR.tsiahT-grIeeoiRycnbOhJSssNnoPe[[[u567ofSrp]]](lvgtoeXSMNKehrgt.r.aAaC.dye-tKnR.KeWaotFEtCtatoaaNoeyngumraHg,rnmstm,eMIrudQaeua.Ga,.bmInQTtnKnr.se.iae.1osgLtnn4eKiiniutl8ttut,t,,iudDh(Yo2ot..enX0,iJo0.oSh8L.nvmr)ROe,5nnWe3Eoad8tr.tfBi–,-a,.5PslTGth4u.ha0ybMo(s.e.,TsiRLzt.euMFXivkt.e.auYLm)aet-naitec,tgt.doK,o1R.-0m.dSC1um.os(gY2paau0wib,0tnaF8eyr.g)Yra1.,iCLa0Yii7l,.no0CSd0-.uQ6aCig..cityJaaioan1tmm,−eSaossx,liYRd.wtEhhexicFhdeiArhesac2st,
ThisresultisatsimTcil.arStbo-tdhoepdiencgreiamsepirnocv-eadxissulepnegrthcobnydCuocdtiovpiitnygwith aKTitcahama,bKe.Iebna,Ks.tFuudjimieudra,fNo.rNiTshMimot=o,MC.Noo,haNrai,,HM.Sanw[a4,2J.,Ph4y9s.,So7c2.]. Mn-co-
ofEudopedsampleswererelativelylargeconsideringionicradiiof Jpn.82(2013)123702.
Eu3+(1.066Å: C.N.=8). Exceptionally large idn LaFeiAnsO(Coa[1,fE5u2])11aK1n2d(CyaF=eRAe0sfeH.0re1[n1)6cea],snidnd4ic3atKing(yth=at0C.o1-)cof-odroptihnegCe-d[8o]pHe.dYakita,dHo.Opgiinnog,Ts.uOkpapdar,eAs.sYeadmasmuoptoe,rKc.Koinshdiou,cT.tTivohietiy,Y.cIokumhapral,eYt.ely. How-
wasalsoreportedinRE15%dopedsamplesbaylFseo–SFaealacceotmapls.ay[n9si]te.esCmeol-,ec2t6roKn d(yop=ing0.i0n1()Caa,nRdE)14132K. Fu(yrth=er0c.a0r5e)fuflor the PGrot-oh,H.eFuvjeihris,a,Kit.Kawtaaoksa,Hfo.Euisnakdi,J.tShhaimtoyaamas,mJ.Aamll.Chleemv.eSloc.(133%6 ) of Co-
existenceofEu2+(1.25Å:C.N.=8)andEu3+(1.0st6u6dÅie)scaanrebdenoeceopdneesdidd-stoysctleamr[1if],yY2.tK4haeKmrihe(aarysao,=Tn.Wo0af.ta0rne1al)baet,aiMvne.dlHyir4ahn3iogK,hH.TH(cyosbo=yno,0J..A0m5[.9)C]hf(Ae2o.m0Sr1.aS4lao),c8H.41.6aY3–a0n8k(d4i2t9a0.,0HN8.)iO-gdinoop,Ti.nOgkadiam,Ap.Yraomvaemdotot,Kh.eKisshuiop,Se.Irschiodna,dA.uIycot,iHn.g proper-
lcthe(LaoCourarean)vgtd,deheEduraausoctsi)upmt1noiti1vnhifilt2igeat(.ycrCreeSaewcflt,alfrüEeeystrucroshzett)naea1norfl1fobos2oersrt,tterhCrutraeehavcllr1oe.ta0uduRth(irgPEvaiehtnevd3tleAEooysusupr(l8peeoC)ddpe(waoFro,1pceRrTo20etEcAen-d)d3sa1d12-n1u)8R0d2c5E-t.a3ti(sda4nIvh(1-nili.Rid8zer0rtSEee.eyt-IaeuhnadTc3=bndatemh-sslYb8sfyeaeuoTem,o)srnMremevgLcc[laeeaeo1Remard–cdm4Ery1oitSiRptaNeo]nnrd2=hfp,rVapodsEFygoseLKiwrenia,,nuFum–aaltSFhCNgp–nowlmeehimioieGvdlcldntaaotereeseh–dhoso-rf-d,rmtdSredieaFcsoobeNrneeappndiitcetsepnciod[[[[i,dtno2345htchne]]]]-(graaedtCse3FH1MIJBXsib.ewyot.aH24a.rCuTebC7.nh92J,ps..ashe..8sR66csTtnteerHeWP.i–2auEs(eg.cdid2sop3–,t)aaapmhuT0cnp211nDY(b,peh0,,94wg1C.hes8rZJYMi72po,u.2nrca)Y...6hQi,.vTwoe,.ecs04C.BD[iRaQe6rna.Lhc1rni.seEu0t.Ruudrthg1u5oaLo).4n,a,hu1i],0seBPguD0.nhc5eK1lSa.,l.ga.tCriL.sce2KYW,ibkve.n.elSeX,sepsY.h.grKJ.suotla,.Ycae.LeunsCPefbSvavpdc,l.ha,baebnrtstMs,AWrrostmhekuTdob.pta.Ken.abaBanKlei,e.nml.khtt.,eCerWGtBushnCctasioa.iiehautoranoLneumes,rior,,bslncoyrprPLe.Si,etin.rnCi.esXewnnoJTano.tpr.,rzCc.hmgiWecY.,c.netPreiarmNoH.nche.Tne-eCaenHutgd.eatcrWn,iuldk,.vsR.aeb.uAe(b.nelbCC2-rcgcyah.l0u[[[[ai,oatY1111ut0dnhPt0123u,f8it.t,.vA]]]],)MtSuAttt.F5E(iicMW(KNKJH.hhh..2pctn2iT9iY...i.sW0.yn0.eeee.1s.K-aYGU1hK.1Z8LBkuau48ui4.iuh–oiSkdm,,,)3l)do5o.iCooAYtM0uoo9t0y,(.ah..Q7t,2,26CY,,.or2J30.0tC3Tft3PH..oT.,Fo31.J1h1ihKyL.0Z.Yiue0aMn40yreidOhj.e(tM2ss),2ieK,21uagHo..,S.N0h0−RiaikRionzCY90nar.aoeluMosaxfi.3m8ig,vL-dkfooo,u7),R...YdAaak0Fr-b.n,a..td5oEKSi,TsYRh.,pao.MatuxlFkeYpaeiaE.ua.(t,nNdjneTFui,K(moo.dX=eiCt,hOstu.ia1aMykLroahrp−aaiLsa.a,nd,,h,tymRXPTaaJeua...,Ca,oAMmtEXPs,t.honCiiein,DYzy)sysguaJs.e1.,)t.mkw,1MYSAhSsaah.om2PuiiemiSctstm-sig.ur2ho,usJnFo,gHtpoyo,yn.ekesN,ZsahO.al-sK.mt,i8tsatdS.ega3aiKhKis,m,t,hnii.M(,seA2tSghAIs.s0pablipmN[o1y.pan4p,a4ol,.lA,2)hl.nKTP.oa0P,ie.Ihr2hywnhayno5Foy4,sueis,0hvJ.s9s..Hj0rieEpaPEm1].xhrx.eEn.iTpupydtiffsrcsrcreeaoae.eesk,SsCpcsodcsio,Notcef77oJ...md-tTichspcoseat-sdaridametobiderpioelevatcodoertf
observed in (Ca,Eu)112. In addition, dFe–Fe vinalvueesstigaareteddc.e-cCvroeaaolusreeNdsitcaoy-de[od6]paiSMntla.mgtReoiomCttsoepmtr,rmMuouvn.nTec.edh1g4easl8u,nD(pg2.e0eJro0dch8or)efn5no3ddr8tu–,cx5Pth4i0y=ns.g.R0pe.vr1.o5Lpe-−tt.100.12(520(0s8Se)he1im07o0y0a6m.tah,SeupNeric-odndo.pSceid.Tescyhnsotle.(msusb.mitOtednfo3r%publCicaoti-ocno)-doping, the Tc of
bThyisCore-csou-ltdoispsinimgiwlairthto0t.h0e1–d0e.c0r2eaÅsecoinmcp-aarxeidsienltreotCnieTogsMt-hco-offb-raydeleolCp(iFosCneaiaddcmg,orR.(pepCEl9aia)en1(s,sgRce1.E-d2e)1)sba1)e.m2[y7p(o]KRlnNKJeEpuis.dnt=.daK.hT8aLo5act2amas0–y(eae2aKSt,0mxmK1caa.3ei),I)lfbeKi.1and.t2,[iKh3Ksn17u.peg80dFi2ou]t3b.,eji0-smSlou.KaufOgrtdanwgt,aiieNrerciesr.,ecetNTte.iocsdTMhboMiismnzteusohdrkttvooaaaem,ptnMd-i,tT.KNc.boShecua[[cgar11oaa45nw,m]]HaT(Cb.r2e..aS0eSWs,a1twYu5a.ran)zS,g3euJ,r6.gY,t(–PiG.yKThh3.a9y.eDmLMs.i.e,a(rSZ,oCo-.nYWcfd..ar.ee,ZaPehuu,rcE,)S.aM-.s1J.iea1Bn)e2grtt,ssXocy.hL3slient6r,e,YDKm..KJ[.o7Lihun2roec],n,rSd.ewtC,ahShsoi,ielLildd.eJ.SLtasiai,ltgZse.onCRoietfimnh,cmMeau.nnsH.tue2l,p0yH1e.frrcoomnd2u3cKt-
in LaFeAsO [15] and CaFeAsH [16], indicatinigngthtaotFCeos-citoee-q.duTocaplwintagos faouwn[8idt]htHo.aYdaeksciitrmae,aHus.leOtagwinnioet,hoTu.aOskdaredecad,rAeu.acYstaeimoiannmioootonf,iKtc.hKeishcio-,vTa.lTCuohheeeni,,YY..TI.kiWnuhgaanrgta,,rQYa..nTsaoit,iGo.Hn.Cbaoe,cZa.Am.Xeu,sPhhaysr.pReevr.Bsu79g(g2e0s09t)in05g45i2m1.provedgrain
astlsuodiaecscaormepnaeneiedsedelteoctcrolanrifdyopthinegreinas(oCna,oRfE)r(r1Cae1dlaa2,itiC.iveoFe)fu1lyrR1tE2hh3,ei+tgrTohecxaaiTcnrdeecpjfbruuteylsaftosertdhCeebyaAnG(d2sdoe0-tc1FoEr4huee),-aH8Ads4.io6Fsn–pug8jebi4hd9nois.onas,madKm.inaKpaanltleagsoCl.keeaI.,nHcot.hEmiespackoias,siJ.teiSohonifmoyam[a1,6J].APSum.pC.ehCrehcnoegnm,dc..ZoSS.oJcn.ci..XnT1ieae3ncc6hgtn,iovGl..iJt.2y7Ye(2,w0X1i.tF4.h)L0u6s,5i0Bg1.n2L.iefii,cAa.Fn.tWeanngh, aF.nCcheemn,eXn.Gt. Liuno, the Tczero
dalirreeacdtyTMelecdtorpoinngovteordFoepesdite(Cwa,hRiEc)h11i2nc[r1e1a]s.easnedleTcctroofnCcoiat-rccyrIoine-wcdroapipnsoeldoybc(Crsyae[90rs].9vt2AE(ae25i.lsd0CSla1aiek4nli0aw,).,e0M0H7i7.t5.3CY)hF11au0ak1j2ii1ato2.a−k,aHaxT,l.RYmcO.gEoTionasxkofta,FTnf2o.eoO2,lAlkM.oa7sw.d2KaPe,,uAd,tt.siY,tu2ahJm.4peS.aeh6mTrimcKcotooo,yn,aKmd1.Kua7i,.scA9htpioiKpv,lS.-,.PIshhyisd.avvE,Axaap.llIruuyeosees,.H7(.fWromith14anKi→ncr3ea0sKe)inantdhenoCoc-hdaonpgienginletvheel,Taconlisnet-
23.2K, 13.2[1K0] Wan.Zdhou2,2J..Z8huKangf,oF.rYuLana,-X,.LPi,Xr.-X,inNg,dY.-S,unS,mZ.S-h,i,EApupl-.Phys.eEaxprresssu7ppression of T zero and T onset was observed at an
(2014)063102. c c
4.Summary and Gd-dop[1e1d] Ks.yKsutdeom, Ts. Mreizsupkaenci,tYiv.eKliyta,haemxac,eDp.tMiftosuroktah, Ke. CIbae,-K. Fujaimvuerra,aNg.e rate of 1.65K/Co%[49].
doped system[4N8is]h.imoBtor,oYa.Hdiraroeksa,isMt.iNvoehatrar,aJ.nPshiytsi.oSoncs.Jpwn.e8r3e(2o01b4-)025001.Large diamagnetic screening was observed in the Co-
Insummary,CoorNico-doped(Ca,RE)112havebeensesryvnethde-due t[1o2]thKJp.enK.ui8dn3oh,(2Yo0.m1K4it)oa0hg9ae3mn7a0e,5Ko..uFusjimduisrat,rTi.bMuiztuikoanmi,oHf.Othtae,MR.NEohara,J.cPhoy-sd.Soopc.ed (Ca, La)-112-system indicating the presence
sized for RE=La–Gd and their superconducting propertaietsomwesreresul[t1i3n]gH.iYnakiptao,Ho.rOgginroa,Ain.Saclao,Tn.nOkeacdtai,vA.iYtaymainmottoh,Ke.Kpisohiloy,A-.Iyo,Ho.fEisbakui,lJk. superconductivity[42], although it was not clear
investigated.CoorNico-dopingimprovedsuperconductingprop- Shimoyama,Supercond.Sci.Technol.(submittedforpublication)
ertiesofall(Ca,RE)112samples.Tsexceeding30Kwereocbrysesrtvaeldline p[1h4a]sTe.Sst.urzTer,hGe.Dreeroinsdeaaun,Ei.Mn.dBiecrtascthiloenr,Do.Jfoharenddte,cSorleidaSsteateComwmhuny.20a1direct substitution of Co for Fe would result in
c (2015)36–39.
inCo-co-doped(Ca,RE)112(RE=La–Sm)inspiteofdirectTMdop- [15] C.Wang,Y.K.Li,Z.W.Zhu,S.Jiang,X.Lin,Y.K.Luo,S.Chi,L.J.Li,Z.Ren,M.He,H.
ing to Fe site. Tc was found to decrease with a decrease in ionic Chen,Y.T.Wang,Q.Tao,G.H.Cao,Z.A.Xu,Phys.Rev.B79(2009)054521.
radii of RE3+ except for Ce and Eu doped samples. In the case of [16] P. Cheng, Z.J. Xiang, G.J. Ye, X.F. Lu, B. Lei, A.F. Wang, F. Chen, X.G. Luo,
Supercond.Sci.Technol.27(2014)065012.
(Ca,Ce)112, T increased by decreasing nominal Ce composition
c
and T of Co-co-doped (Ca Ce )112 almost followed the T
c 0.925 0.075 c
(E (eV) a) (b) tMotnal SBri 102100DOS (states/u.c.)
10
0
-5 -4 -3 -2 -1 0 1 2
E (eV)
((db))
45 (V) a) (c) (E (meV)b) tMotnal 2100ates/u.c.)
)40 E (e V) kx kySBri 10 DOS (st
T (K E (e -5((ecV)))10-04 -3 -2 -1 0 1 20
35 me50 E (eV)
E (
(d)
0
30 meV) -2 δ 0k (0.021x-22π/a)0 2
0.0 0.5 1.0 1.5 2.0 2.5
(c) E (
P (GPa) FIG. 13: (a) Electronic band structure of SrMnBi . (b)
2
FIG. 12: Pressure dependence of T1 and Tczero of Anisotropic energy surfaces around the Dirac point k0 =
Cwait1h−xpLeramxFisesAiosn2 wfroitmh xZh=ou0.e1t8asli.n[4g5le].cCryosptyarlsig[4h5t].20R1e5pbriyntIend- (d0V)is.2p0e8rs,i0o.n2s08n)eaforrthteheD(irSarcBpi)o+inltaayleornignpSarrkMaxllneBlai2n.d(pce)rpEennedrigcy- ky
stitute of Physics. u(elar directions to the Γ−M sym(em)e1t0r0y line in SrMnBi2[31].
RE eprinted with permission from Park et al.[31]. Copyright
2011 by American Physics Society.V)
such behavior which is also contrary to doping effects in me 50
122- and 1111-phases [97, 98]. However, the improved E (
superconductingpropertiescouldarisefromthedecrease
ing a transition from single-vortex dominated pinning
inLa-contentintheCo-dopedphases,similartoSbdop- 0
to a small bundle pinning. This in-d2icate0s a r2ela-t2ively0 2
ing in (Ca,La)-112[18] which resulted in an optimisation 2D nature of the superconducting stateδa ks (t0h.0a1txin2πt/ha)e
of the As-Fe-As bond angle.
122-system[47]. The critical current density J for 112-
In Ca RE (Fe TM )As (TM = Co, Ni), T in- c
1 x x 1 y y 2 c Ca La FeAs isabout 105A/cm2 (seeFig.11), only
creased w−ith increas−ing ionic radii for all RE elements wea1−klxy dxepend2ent on the∼direction and strength of the
except for Ce and Eu. d went through a similar
Fe Fe applied magnetic field[47, 77]. However, in Sb-doped
trend with ionic radii which−is comparable to the trend
Ca La Fe(As Sb ) , significant improvement
observed earlier in the TM-free (Ca,RE)112-system[48] 0.85 0.15 0.92 0.08 2
in the J value[101] was measured ( 2.2 106A/cm2),
(see Fig. 10). Co-co-doping resulted in slight reduction c ∼ ×
which upon high energy proton irradiation can be en-
ofd forall(Ca,RE)112-systems,keepingthed
Fe Fe Fe Fe haced upto a value of 6.2 106A/cm2. The J value is
vs ion−ic radii trend the same for Co-free and Co-dop−ed × c
comparabletothatof11-typeFe (Te,Se)[102]suggest-
cases. For the (Ca,Ce)112-system, the reduction of the 1+y
ingtheimportanceofstrongbulk-dominatedorartificial
Ce-content increased the T and with Co-co-doping fol-
c defect induced pinning in enhancing the value of J .
lowed the same trend. For the Eu-doped system, an ex- c
ceptionally low T and large d was measured which
c Fe Fe
most likely indicates the coexist−ence of Eu2+ and Eu3+. 4. Effect of Pressure
3. Critical Current and Critical Field Application of external pressure can be useful for the
suppressionoftheAFMgroundstateandstabilisationof
Thelowercriticalfields(H )forCeNi Bi ,LaNi Bi , superconductivity in Fe-based superconductors without
c1 x 2 x 2
NdNi Bi , YNi Bi are 65G, 90G, 55G and 67G re- introducing substitutional elements or impurities. For
x 2 x 2
spectively [33]. The upper critical fields (H ) at zero- Ca La FeAs , Zhou et al. [45] observed that the re-
c2 1 x x 2
temperatureforLaPd Bi was3T[56]whichisrelatively sisti−vity went through a two-step decrease with temper-
x 2
large for a T 2K suggesting type-II superconductiv- ature towards the superconducting phase while no effect
c
∼
ity. For Ca La FeAs , critical fields near T = 0 are of pressure was observed on the normal state resistivity.
1 x x 2
as high as H−c (0) = 39.4T (for H c) and Hab(0) = Thetemperature(T )atwhichthehigh-T resistancede-
c2 (cid:107) c2 1
166.2T (for H ab) which corresponds to coherence crease took place was found to be pressure independent.
lengths[47, 77] o(cid:107)f ξ (0) = 6.9˚A and ξ (0) = 28.9˚A. BoththeTonset andTzero indicatedadome-shapedpres-
c ab c c
The anisotropy γ(0) near T is 2.8 which lies in between sure dependence with its maximum at around 1.19GPa
c
1111 (5 < γ < 9.2)[99]- and 122 (1 < γ < 2)[100]- as illustrated in Fig. 12. The pressure coefficients were
type pnictides. γ is a measure of the interlayer cou- comparable to that of other pnictide phases [103, 104].
pling strength between the FeAs and charge reservoir The maximum Tzero was 38.5K at 1.19GPa which is
c
layers suggesting the presence of moderate anisotropy much higher than the zero-pressure value of 34K, sug-
in 112-system. The anisotropic pinning potential in gesting that the enhancement of T is possible further
c
Ca La FeAs showed a field dependence for H ab in the doped 112-compounds via tuning of the As-Fe-As
1 x x 2
simi−lar to the cuprates and the 1111-system[99] sugg(cid:107)est- bond angle as result of the Sb substitution.