Table Of ContentChemical Substitution and High Pressure Effects on Superconductors in the LnOBiS
2
(Ln = La-Nd) System
Y. Fang1,3, C. T. Wolowiec2,3, D. Yazici2,3, and M. B. Maple1,2,3
1Materials Science and Engineering Program, University of California, San Diego, La Jolla, California 92093, USA
2Department of Physics, University of California,
San Diego, La Jolla, California 92093, USA and
3Center for Advanced Nanoscience, University of California, San Diego, La Jolla, California 92093, USA∗
AlargenumberofcompoundswhichcontainBiS layersexhibitenhancedsuperconductivityupon
2
electrondoping. MuchinterestandresearchefforthasbeenfocusedonBiS -basedcompoundswhich
2
6 providenewopportunitiesforexploringthenatureofsuperconductivity. Importanttothestudyof
1 BiS -basedsuperconductorsistherelationbetweenstructureandsuperconductivity. Bymodifying
2
0 either the superconducting BiS layers or the blocking layers in these layered compounds, one can
2
2
effectively tune the lattice parameters, local atomic environment, electronic structure, and other
n physical properties of these materials. In this article, we will review some of the recent progress
a on research of the effects of chemical substitution in BiS2-based compounds, with special attention
J given to the compounds in the LnOBiS (Ln = La-Nd) system. Strategies which are reported to
2
6 be essential in optimizing superconductivity of these materials will also be discussed.
2
PACSnumbers: 74.25.F-,74.25.Dw,74.62.Bf
]
n
o I. INTRODUCTION conductors, such as Bi O S , Bi O S , LnO F BiS
4 4 3 3 2 3 1−x x 2
c
(Ln = La, Ce, Pr, Nd, Yb), La M OBiS (M = Ti,
- 1−x x 2
pr Following the discovery of superconductivity by Kam- Zr,Hf,Th),Sr1−xLaxFBiS2,EuBiS2F,andEu3Bi2S4F4,
have been developed with T ’s ranging from 2.7 to 10.6
u merlingh Onnes in 1911, efforts in the search for new c
K3,10,17–26.
s superconducting materials during the past century have
t. shown that most of the pure metals and an enormous The superconducting properties of BiS2-based com-
a number of alloys and compounds become superconduct- pounds are also sensitive to the environment in which
m
ing at low temperatures. The highest superconducting they are synthesized or being measured. For example,
d- critical temperature Tc that has been reported is 133 K polycrystalline samples of LaO1−xFxBiS2 subjected to
n for the cuprates at ambient pressure and 203 K in sul- highpressure(HP)annealinghaveTc valuessignificantly
o fur hydride at 155 GPa, although the latter claim has higher than as-grown (AG) samples. Also, with the ap-
c not yet been independently verified1,2. In order to ex- plication of external hydrostatic pressure, a monotonic
[ perimentally realize new superconducting materials with decrease in Tc is observed for both the Bi4O4S3 and
1 propertiessuitableforpracticalapplications,thereisstill SrFBiS2 compounds27,28; in contrast, the compounds
v muchtoaccomplishinboththeoryandexperiment. The LnO1−xFxBiS2,Eu3Bi2S4F4,EuFBiS2,LaO0.5F0.5BiSe2
8 recentlydiscoveredfamilyofBiS2-basedsuperconductors and Sr0.5La0.5FBiS2 were reported to have a sharp jump
2 exhibit a layered crystal structure which is characterized in Tc under applied pressure, indicating a pressure-
1 by an alternating pattern of BiS2 double layers that are induced transition from a low-Tc to high-Tc supercon-
7 separatedbyblockinglayers3. Thislayeredstructurepro- ductor phase27,29–34. It is also observed that supercon-
0
. vides new hope and opportunity to enrich the class of ductivity in the BiS2-based compounds is very sensitive
1 layered superconductors with higher values of T . tochemicalcompositionandtothelevelofelectrondop-
c
0 Chemical substitution may induce significant change ing in particular. As a result, Tc values of the samples
6
inmaterialstotheirlatticeparameters, crystalstructure with the same nominal chemical composition are not al-
1
: and electronic structure. It has been broadly used as a ways the same since there might be slight differences in
v strategy to tune physical properties of materials includ- the actual chemical composition.
i
X ing electronic transport, magnetic, thermal, mechanical, At present, there have been more than one hundred
and luminescent properties. Chemical substitution has researchpaperspublishedregardingsuperconductivityin
r
a been found to induce superconductivity via electron or the BiS -based compounds, which have helped increase
2
hole doping in LnFeAsO compounds by replacing F for ourunderstandingofthesenovelmaterialsandsupercon-
O4–10, Co for Fe11, Sr for La12, Th for Gd13, and also ductivity in general. The peculiar crystal structure of
through the introduction of oxygen vacancies14,15. Simi- theseBiS -basedcompoundsasalandscapeforstudying
2
lartotheLnFeAsOcompounds,ithasbeenreportedthat and optimizing superconductivity has fostered a healthy
electroncarriersareessentialfortheemergenceofsuper- competition within the scientific community to explore
conductivity in the BiS -based materials3,16. By modi- the important critical parameters that affect T as well
2 c
fying the blocking layers of the BiS -based compounds, as to understand the nature of superconductivity itself
2
electron carriers can be generated within the structures. intheselayeredstructures. Inthisarticle, wewillreview
As a result, a large number of novel BiS -based super- someoftherecentprogressinboththestudyofchemical
2
2
substitution and external pressure effects on the BiS - a b
2
based compounds. Discussions about some of the pa-
rametersthatareessentialforsuperconductivityinthese
compounds will also be presented.
S2
II. BI O S
4 4 3
S1
As the first discovered superconductor in the BiS -
2
based compounds, Bi O S is a bulk superconductor
4 4 3
withaT ofabout4.5K3,35. ThestructureofBi O S is
c 4 4 3
derived from the structure of the parent Bi O S with a
6 8 5
50% deficiency in SO units; this results in the presence
4
SO
ofelectroncarriersintheBiS2layerswhichinducethesu- 4
perconductivityinBi O S 3,36. Interestingly, theparent
4 4 3
BiO
phase, Bi6O8S5, which was initially regarded as a band 2 2 BiS
insulator, was soon found to show superconductivity at 2
∼4.2 K when the sample is annealed in vacuum37.
ChemicalsubstitutionshavebeenfoundtosuppressT
c
inthesuperconductingcompoundBi O S . Substitution
4 4 3
BiS
ofSeattheSsiteresultsinaslightincreaseinthelattice 2
parameter c and a remarkable enhancement of the nor- O S
malstateresistivity38. TheT oftheBi O S Se com-
c 4 4 3−x x
pound decreases with increasing Se concentration from
4.4Katx=0to3.5Katx=0.15,whiletheuppercrit-
Ln Bi
icalfieldat0K,H (0),determinedbyfittingtheconven-
c2
tionalone-bandWerthamer–Helfand–Hohenberg(WHH)
equation to the H (T) data, is also suppressed from 2.3 FIG.1: (Coloronline)Representativecrystalstructureof(a)
c2
to1.8T39. Similarly,Tc oftheBi4−xAgxO4S3 compound Bi4O4S3 and (b) LnOBiS2. O2− ions in the LnO blocking
layercanbepartiallyreplacedbyF−. S1andS2indicatethe
graduallydecreaseswithincreasingAgconcentrationand
in-plane and out-of-plane positions of S, respectively, in the
a subtle shrinkage of both the a and c lattice parame-
superconducting BiS layers of the two structures.
ters were observed up to x = 0.240. It should be noted 2
that the chemical pressure generated by Ag substitution
is negligible in Bi O S and the suppression of T most
4 4 3 c
likelyarisesfromthesubstitutionoftheAgionsintothe
Currently, superconductivity has been induced in five
superconducting BiS layers and the resultant change in
2 different parent compounds Ln(O,F)BiS (Ln = La, Ce,
2
the electronic structure of the compound. Pr, Nd, Yb), with the substitution of F for O21. Poly-
crystalline samples of these compounds can be synthe-
sized by using solid state reactions in sealed quartz
III. LNO1−xFxBIS2 (LN = LA, CE, PR, ND, YB) tubes. However, along with the main superconduct-
ing phase, small amounts of secondary phases including
The parent compounds in the LnOBiS family are LnF andBiF mayalsobeformedinthepolycrystalline
2 3 3
band insulators. The substitution of F for O induces samples10,19–21,41,42. As a result, the actual chemical
superconductivity via electron doping into the BiS lay- composition of these compounds may be different from
2
ers. By applying relatively small magnetic fields (∼1 T) the nominal chemical composition, particularly regard-
to polycrysatlline specimens of these superconductors, ing the concentration of F, resulting in an overestimated
the superconducting state can be destroyed21. Due to level of electron doping into the BiS layers. This con-
2
the chemical similarity of the rare earth elements, there cernhasalsobeenraisedinthestudyofsingle-crystalline
is considerable flexibility in modifications that can be samplesofNdO F BiS ,inwhichcase,theactualelec-
1−x x 2
made to the blocking layers of these compounds which tron doping level was reported to be much lower than
facilitates the investigation of the superconducting and anticipated43. Hence,T valuesofsampleswiththesame
c
normal state properties of BiS -based compounds. The nominal chemical composition reported by different au-
2
LnOBiS compoundscrystallizeinatetragonalstructure thorsmaydifferfromoneanother21,22. Althoughconclu-
2
with space group P4/nmm20. Between each LnO block- siveevidenceofinhomogeneityinpolycrystallinesamples
ing layer, there is a double BiS superconducting layer oftheBiS -basedsuperconductorshasnotbeenreported,
2 2
in which there are two distinct positions for the S ions: measurements of electrical resistivity (ρ), magnetic sus-
in-plane position (S1) and out-of-plane position (S2), as ceptibility (χ), and specific heat (C), etc. may still be
indicated in Fig. 1. partially affected by possible inhomogeneity of the sam-
3
ples,andthuscompromisereportsofsomeoftheintrinsic 4.10 13.9
propertiesofthesecompounds. Inaddition,theexistence
of defects and disorder in these materials may also com- 13.8
plicate the analysis of experiments and comparison with 4.08
theoretical models. As mentioned by Kuroki, establish- 13.7
ing the intrinsic phase diagram of the BiS -based super-
2
conductors is still a significant challenge in research on (Å)4.06 13.6 (Å)
BiS -based superconductors44. a c
2
13.5
4.04 L a O 1-xF xB iS 2
13.4
A. Electronic and crystal structures, and their
correlation with T
c 4.02 13.3
0.0 0.2 0.4 0.6 0.8
F c o n c e n tra tio n
6 FIG. 3: (Color online) Nominal F concentration depen-
dence of the lattice constants a (red) and c (blue) of AG
L n O 1-xF xB iS 2 LaO1−xFxBiS2. The data are obtained from Ref.24.
4
eV51, which is consistent with experimental evidence
revealing significantly high values of ρ and negative
)
T(Kc L n = L a tseammppelersa5t2u.reAcoseffiFciiesntssub(dstρi/tudtTed<in0t)ointhpeolOycrsyistteal(lsineee
2 L n = C e Fig. 1), it is expected that electrons will be doped into
L n = P r the conduction bands which consist mainly of Bi-6p or-
L n = N d
bitals, resulting in an increase in charge carrier density
in the BiS layers3,50. The possibility of conventional
2
strong coupling s-wave superconductivity for Bi O S ,
0 4 4 3
0 0 .2 0 .4 0 .6 0 .8 1 .0 LaO F BiS , and NdO F BiS has been empha-
F c o n c e n tra tio n 0.5 0.5 2 1−x x 2
sized by magnetic penetration depth measurements by
FIG. 2: (Color online) Dependence of T on nominal F con- means of muon-spin spectroscopy or tunnel diode res-
c
centration for LnO F BiS (Ln = La-Nd). The T values onatortechniques53–56, andsometheoreticalstudiesalso
1−x x 2 c
for Ln = La, Ce, Pr, Nd are obtained from Ref.45–48, re- suggest that BiS2-based compounds are strong electron-
spectively. For interpretation of the definition of T of each phonon coupled superconductors16,51,57. However, other
c
sample, the reader is referred to the corresponding articles. unconventional pairing mechanisms including extend s-
or d-wave pairing, g-wave pairing, and triplet spin pair-
The T vs F concentration plots for LnO F BiS , ing were also proposed58–62. While evidence for both
c 1−x x 2
displayed in Fig. 2, show a broad range of F concentra- electron-phonon coupling16,51,53–57 and strong electron-
tion in which superconductivity can be induced; there is electron interactions58–64 has been reported, the con-
also an apparent enhancement of Tc with increasing Ln centration of charge carriers in the BiS2 layers is con-
atomic number. The optimal F concentration for super- sidered to be essential for the superconductivity of the
conductivity in these compounds falls within the range LnO1−xFxBiS2 compounds18,20,21,24,27,51,52,54,65.
of 0.4 (cid:54) x (cid:54) 0.7. As the concentration of F is increased, The substitution of F for O in LnOBiS may also in-
2
thesuperconductivityofthesecompoundswasalsofound duce a change in lattice parameters due to the smaller
to be more resistant to the destructive effects of an ex- size of the F− ions compared with O2− ions. For
ternal magnetic field up to the F solubility limit47,48. As LnO F BiS , thelengthofthea-axisdoesnotchange
1−x x 2
anexample,theuppercriticalfieldofNdO F BiS in- significantlyuponFsubstitution; however,thec-axisde-
1−x x 2
creases from 0.9 T at x = 0.3 to 3.3 T at x = 0.748. For creases monotonically until the F concentration reaches
LaO F BiS , first-principles calculations suggest that the solubility limit (typically around 50%) as illus-
1−x x 2
the density of states at the Fermi level increases with in- trated by the example of LaO F BiS shown in
1−x x 2
creasing F substitution and attains its maximum value Fig.310,19,20,47,48. The highest value of T was reported
c
atx=0.516. Hence,alargenumberofstudieshavebeen when c attains its minimum value, and the evolution of
made on compounds with a nominal composition given T seems consistent with the change in lattice parameter
c
by LnO F BiS 10,19,20. c20,47,48. However, it should be noted that these results
0.5 0.5 2
Electronic structure calculations for LnO F BiS , donotnecessarilyindicateanexclusiveroleofthelattice
1−x x 2
and, in particular, for the compound LaO F BiS , parameter c in affecting T , since it is observed that the
0.5 0.5 2 c
have been widely reported49,50. It was suggested that level of electron doping also reaches a maximum when
LaOBiS is an insulator with an energy gap of ∼0.8 the c parameter is smallest.
2
4
F S
2
S
O 1
Bi
162.1°
Bi S1
2.66 Å
3.1 Å
S
2
FIG. 4: (Color online) (left) Crystal structure of
LaO F BiS . (right) The side view of the BiS layers for
0.5 0.5 2 2
x = 0 and x = 0.5 is displayed in the top and bottom panel,
respectively. Adapted with permission from Ref.16. Copy-
righted by the American Physical Society.
Local distortion of the lattice may also be induced by
Fsubstitution. Asymmetricbroadeningofthediffraction
peaks with increasing F concentration in LaO F BiS
1−x x 2
was observed in neutron scattering measurements, sug-
gesting that strain may be induced along the c-axis due FIG. 5: (Color online) Near-neighbor distances from Bi (up-
to a random substitution of the F ions at O sites52. It is per) and Ce (lower) as a function of F concentration. Blue
also possible for a distortion to occur in the BiS plane circlesandreddiamondsrepresentthedatatakenfromtheas-
2
grownandhighpressureannealedsamples,respectively. The
as a result of F substitution. For the LaO F BiS
1−x x 2
insets show structural cartoons with near-neighbour atomic
compounds, first-principles calculations show that the z
clusters (encircled) around Bi and Ce. Reprinted with per-
coordinateofthein-planeSandBiatomsintheunitcell
mission from Ref.66: Determination of local atomic displace-
decreases with doping, yielding a nearly perfect planar
ments in CeO F BiS system, J. Phys.: Condens. Matter
1−x x 2
structure at x = 0.5 (see Fig. 4). In fact, by using single
26 (2014) 435701. Copyright (2014) by the IOP Publishing.
crystal x-ray diffraction, nearly flat in-plane S-Bi-S an-
gles were observed in LaO F BiS at x = 0.46, which
1−x x 2
is close to the optimal F concentration value (x = 0.5)
thatyieldsthehighestvalueofT 67,68. AflatBi–Splane been suggested that the valence of Ce, i.e., the coupling
c
between the Ce 4f and Bi 6p states, is not beneficial to
would result in an increase in hybridization of the p /p
x y
the superconductivity.
orbitals of Bi and S and, in turn, enhanced supercon-
ductivity. Investigationofextendedx-rayabsorptionfine Considering the contraction in radius for the Ln ions,
structure(EXAFS)oftheCeO F BiS compoundsre- onemayexpectchangesinbothlatticeparametersaand
1−x x 2
veals that the local structure of both the BiS supercon- caswellaschangesinT withincreasingatomicnumber
2 c
ductinglayerandtheCe(O,F)blockinglayerchangessys- of Ln. Figure 6 displays typical values of T , defined as
c
tematically with increasing F concentration66. The Bi- the temperature at which ρ falls to 50% of its normal-
S2 distance increases with F concentration while the Ce- state value, for the compounds in the Ln(O,F)BiS sys-
2
S2 distance decreases with F concentration (see Fig. 5), temasafunctionoflatticeconstanta. ExceptfortheLn
leadingtothebreakingoftheCe-S-Bicouplingchannel69. =YbandNdcases,theT valuesofthesamplesafterHP
c
Consequently, the hybridization between the Ce 4f or- annealing are higher than the values of the AG samples.
bital and the Bi 6p conduction band decreases with F Inaddition,thedependenceofT onthelatticeparamter
c
concentration and the Ce3+/Ce4+ valence fluctuations a for the AG samples are completely different from the
are suppressed, resulting in a completely localized 4f1 HP annealed samples: for the AG samples, T increases
c
(Ce3+) state at x > 0.4, where the samples exhibit su- with decreasing a (heavier Ln); however, the value of T
c
perconductivity and ferromagnetism69. Therefore, it has for LaO F BiS after HP annealing (a larger value of
0.5 0.5 2
5
Y b N d P r C e L a L a 3 .0 1 8
L a O F B iS
1-x x 2
8 2 .5 x = 0 .4 6 1 5
x = 0 .2 3
L n (O ,F )B iS
2
7
2 .0 1 2
) )
6 -cm1 .5 9 -cm
) W W
(Kc5 r (m1 .0 6 r (m
T
4
0 .5 3
3
0 0
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0
2 T (K )
3 .9 8 4 .0 0 4 .0 2 4 .0 4 4 .0 6 4 .0 8
a (Å ) FIG. 7: (Color online) Temperature dependence of the elec-
tricalresistivityforLaO F BiS (x=0.23and0.46)single
1−x x 2
FIG. 6: (Color online) Correlation between Tc and a-axis of crystals. The data are obtained from Ref.67.
LnO F BiS ( x = 0.5 for Ln = La, Pr, Nd, Yb, x = 0.3
1−x x 2
for Ln = Ce) superconductors. The open squares and filled
circlesrepresentthedataobtainedfromas-grownsamplesand
high pressure annealed samples, respectively. Dashed lines B. Normal state electrical resistivity, magnetic
are guides to the eye. The T and a values for Ln = Nd, Yb susceptibility, and specific heat
c
samples are obtained from Ref.21 and the data for Ln = La,
Ce, Pr samples are taken from Refs.65,70,71, respectively.
Given that F substitution enhances the charge carrier
density of BiS -based parent compounds, one would ex-
2
pect that F-substitution may induce more metallic-like
behavior in these materials. The temperature depen-
dence of ρ(T) in the normal state for single crystals of
LaO F BiS (x=0.23, 0.46)isshowninFig. 7; while
1−x x 2
the crystal with x = 0.23 exhibits semiconducting-like
behavior (dρ/dT < 0), the crystal with x = 0.46 shows
a) is significantly higher than the value of other HP an- metallicbehavior(dρ/dT >0)withsignificantlyreduced
nealed samples (smaller values of a). While the values values of resistivity67. These results are consistent with
of T for both AG and HP annealed samples appear to electronicbandstructurecalculationswhichindicatethat
c
becorrelated,althoughdifferently,withthelengthofthe the parent compounds in this system should behave as
a-axis36, no structural phase transition was observed by band insulators but become metallic upon doping elec-
HP annealing and the lattice parameters a and c for the tron carriers into the BiS2 layers50. However, the theo-
AG and HP annealed samples are similar; this suggests retical calculations do not fit well with measurements of
that there should be other factors which have a signif- ρ(T) of polycrystalline samples of LnOBiS2 which tend
icant effect on T . The markedly different values in T to exhibit strong sample dependent results. Some of the
c c
between the AG and HP annealed samples currently has LnOBiS2parentcompounds,whichareinpolycrystalline
nosatisfactoryexplanation. However,evidenceofalocal form, have been reported to show semimetallic behav-
distortion in the lattice has been reported, and this dis- iorincontrasttothesemiconducting-likebehaviorofthe
tortionislikelytobeasourceofthedifferenceinT . For correspondingF-dopedpolycrystallinesamples10,25,46–48.
c
theHPannealedsamplesofCeO F BiS ,thein-plane Thisbehaviormayberelatedtothequalityofthesepoly-
1−x x 2
Bi-S1distanceisshorter;however,theout-of-planeBi-S2 crystalline samples, since complete semiconducting-like
distanceandthesizingoftheblockinglayersappeartobe behavior is also reported in the case of LaOBiS2 and in
marginally effected by the HP annealing (see Fig. 5)66. the case of polycrystalline CeOBiS2, the resistivity can
It was also suggested that the crystal structure within be significantly increased together with the appearance
theabplaneoftheCeO F BiS compoundevolvedto of semiconducting-like behavior after HP annealing46,52.
0.3 0.7 2
ahighersymmetryafterHPannealing,resultinginabet- SubstitutionofFintothelatticedoesnotalwaysresult
ter tetragonal phase70. Finally, the uniaxial strain along inasuppressionofsemiconducting-likebehaviorinpoly-
the c-axis, which was found in annealed samples of the crystalline samples. In fact, F substitution can enhance
LaO F BiS and PrO F BiS compounds, was re- semiconducting-likebehaviorwhichhasbeenobservedin
0.5 0.5 2 0.5 0.5 2
ported to be positively correlated with the enhancement AGsamplesofCeO F BiS andHPannealedsamples
1−x x 2
ofsuperconductivity65,71. Theseresultsindicatethatthe of LaO F BiS 10,24; this is in contrast to observations
1−x x 2
optimization of the local structure is important for real- of suppressed semiconducting-like behavior by F substi-
izinghigherT valuesintheBiS -basedsuperconductors. tutioninsingle-crystallinesamplesofCeO F BiS and
c 2 1−x x 2
6
LaO F BiS 67,72. Although the cause of such unusual
1−x x 2
behaviorisstillunclear,theexistenceofgrainboundaries
0
aswellaspoorintergraincontactsinpolycrystallinesam-
ples may contribute significantly to the semiconducting- H = 5 O e
like behavior of the F-substituted compounds. In ad-
dition to the semiconducting-metallic transition induced 0 .4
by F substitution in single crystals of LaO F BiS ,
1−x x 2
research on single crystals of NdO1−xFxBiS2 reveals pc-4
metallic behavior in the normal state along the ab-
plane of the crystal; this is different from the semi- 0 .8
conducting behavior observed in polycrystalline samples L a O F B iS
0.5 0.5 2
of NdO1−xFxBiS243,48,64,72,73. However, reports of the N d O 0.5F 0.5B iS 2
electrical transport properties in the normal state of P rO 0.5F 0.5B iS 2
1 .2
single crystals of CeO1−xFxBiS2 reveal semiconducting- 2 3 4 5
like behavior in the ab-plane which is consistent with T (K )
measurements on polycrystalline samples of the same
compound68. Hence, the intrinsic electrical transport FIG.8: Temperaturedependenceofthemagneticsusceptibil-
properties of the LnO1−xFxBiS2 compounds seem to be ityχforLnO0.5F0.5BiS2 (Ln =La,Pr,Nd)measuredinthe
field-cooled(FC)(opensymbols)andzero-field-cooled(ZFC)
dependentonacombinationoflanthanideelementLn,F
(filled symbols) processes. The data are taken from Ref.21.
concentration x, and quality of the samples.
Both zero-field-cooled (ZFC) and field-cooled
(FC) measurements of χ(T) were performed for
theotherLnO F BiS compoundswithLn =Ce, Pr,
LnO F BiS samples with different Ln and F con- 0.5 0.5 2
1−x x 2 Nd, and Yb, the significant upturns in the C/T vs T
centrations. As shown in Fig. 8, ZFC measurements on
data observed at low temperature are likely due to the
LnO F BiS (Ln = La, Pr, Nd) yield a significant
0.5 0.5 2 presenceofaSchottky-likeanomalyforLn =PrandNd
shielding volume fraction with T onset values that are
c or magnetic ordering for Ln = Ce and Yb21,74. No clear
consistent with the ρ(T) data, while FC measurements
C(T)/T jumprelatedtotheappearanceofsuperconduc-
reveal that χ(T) remains nearly unchanged in the super-
tivity was observed in these samples21. The jumps in
conducting state relative to the normal state, indicating
C(T)/T in these compounds may not be readily observ-
strong vortex pinning21. For the case of samples with
able because they may be reduced by the pairbreaking
Ln = Ce and Yb and nominal F concentrations of x (cid:62)
interactionduetothemagneticions;thejumpsmayalso
0.7 and x = 0.5, respectively, magnetic phase transitions
be obscured by the broadening of the superconducting
were also observed, apart from diamagnetic signals
transitions by sample inhomogeneity or by the large up-
arising from the appearance of superconductivity21,70,72.
turns in C(T)/T. More rigorous experimental and the-
In addition to the induction of superconductivity and
oretical studies on higher quality samples are needed to
enhancement of T as a result of F substitution, the
c establish the nature of superconductivity in these com-
shielding volume fraction may be significantly enhanced
pounds.
at those temperatures where it has begun to saturate
as a result of the increase in F concentration; a typical
example of this behavior was found for NdO F BiS
1−x x 2
reported in Ref.47. IV. LA M OBIS (M = TH, HF, ZR, AND TI)
1−x x 2
Figure 9 displays specific heat C(T) data for the poly-
crystallineLaO F BiS compoundinthetemperature As an alternative to F substitution, electron doping
0.5 0.5 2
range 1.8 K (cid:54) T (cid:54) 50 K21. The expression C(T)/T = in the LaOBiS system is possible by partially replacing
2
γ +βT2, where γ is the electronic specific heat coeffi- trivalent La with tetravalent M (M = Th, Hf, Zr, and
cient and β is the coefficient of the lattice contribution, Ti)25. It has been reported that the La M OBiS
1−x x 2
was used in to fit the C/T vs T2 data in the normal system exhibits superconductivity with values of T up
c
state which is displayed in the inset in the lower right to 2.85 K. A comparison of T values observed in the
c
hand side of Fig. 9. Shown in the inset in the upper various compounds is displayed in the inset of Fig. 10 in
left hand side of Fig. 9 are C /T vs T data, where C which20%(x=0.2)ofM =Th,Hf,Zr,andTiwassub-
e e
is the electronic contribution to the specific heat. Bulk stituted for La in La M OBiS ; there appears to be
1−x x 2
superconductivity in polycyrstalline LaO F BiS was no clear correlation between T and the atomic radius of
0.5 0.5 2 c
confirmed with the appearance of a clear jump in the the tetravalent atom that replaces the La ion. The low-
C /T vs T data at T . The ratio of the observed jump est value of T is observed for the La Zr OBiS com-
e c c 0.8 0.2 2
to the electronic contribution to the specific heat at T pound; while sharper superconducting transitions are
c
is estimated to be ∆C/γT = 0.94, which is lower than observed for the La Hf OBiS and La Ti OBiS
c 0.8 0.2 2 0.8 0.2 2
the weak coupling BCS value of 1.43, but large enough compounds, broader transitions are observed for the
toconfirmbulksuperconductivityinthiscompound. For La Zr OBiS and La Th OBiS compounds. X-
0.8 0.2 2 0.8 0.2 2
7
-1-1C (J mol K)1122334405050505 -1-2C/T (mJ mol K)e2233445.......05050501.5 DC /(2g.T0c) = 02..95T4 ( K)3T.0c = 2.39.35 K 4.0 -1-2J mol K)23450000 g = 2.53 mLJa mO o0l.-51 FK0-2.5B iS 2 2C /T (J / mol-K)00000.....23456 2C / T (mJ / mol-K)12300000 5 10 TgbQ 2=D1 =( 5 = K00 .2.259)2810 2 mm0 KJJ//mm o2o5ll--KK24 30 2J / mol-K)L a23 0.85TT ~ h0.09 .K15 ODTCBc/ g=T ic2 S=.5 0 2K.91
5 C/T (m1000 10 20 3Q0D = 24201 K 50 0.1 C /T (me1
T 2 (K2) 00 1 2 3 4
0 0 10 20 30 40 50 0 T (K)
T (K ) 5 10 15 20 25 30
T (K )
FIG. 9: (Color online) Specific heat C vs temperature T for FIG. 11: (Color online) Specific heat C divided by temper-
LaO F BiS . AplotofC/T vsT2 isshownintheinsetin ature T, C/T, vs T for La Th OBiS . C/T vs T2 is
0.5 0.5 2 0.85 0.15 2
the lower right hand part of the figure. The red line is a fit shown in the inset in the upper left hand part of the figure.
of the expression C(T)/T = γ + βT2. A plot of C /T vs T, The red line is a fit of the expression C(T)/T = γ + βT2 to
e
where C is the electronic contribution to the specific heat, thedata. Theinsetinthelowerrightpartofthefigureshows
e
in the vicinity of the superconducting transition is shown in a plot of C (T) vs T in the vicinity of the superconducting
e
the inset in the upper left hand side of the figure. The data transition. ReprintedwithpermissionfromRef.25. Copyright
are taken from Ref.21. (2013) by the American Physical Society.
1 .2 ity was observed in La Sr OBiS down to ∼1 K in
L a T h O B iS 1−x x 2
0.8 0.2 2 the range 0.1 (cid:54) x (cid:54) 0.3, which is in agreement with
1 .0 L a 0.8H f0.2O B iS 2 the behavior reported for La Mg OBiS (x = 0-0.2)
L a Z r O B iS 1−x x 2
0 .8 L a 00..88T i00..22O B iS 22 samples25,75. These results suggest that hole doping is
) 3 notsufficienttoinducesuperconductivityandrevealsthe
/ (T) (5K(cid:2)(cid:1)(cid:1)(cid:2)00 ..46 T (K)c12 itmivMpitoyeratisnaunrBceeimSo2ef-nbetalsescoetdfroρcn(oTmd)oppfooiurnngtdhisne. aLcah0i.e85vTinhg0.s1u5pOeBrciSon2dauncd-
L a M O B iS La Hf OBiS compounds were performed in various
1-x x 2 0.8 0.2 2
0 .2 (b ) (x= 0 .2 ) magnetic fields down to 0.36 K25. As magnetic field is
0
T i Z r H f T h
(a ) M increased,thesuperconductingtransitionwidthbroadens
0
0 1 2 3 4 5 and the onset of superconductivity gradually shifts to
T (K )
lower temperatures. The orbital critical fields H (0) for
(cid:2)(cid:1)(T ) /(cid:1)(cid:2) (5 K ) c2
the La Th OBiS and La Hf OBiS compounds
0.85 0.15 2 0.8 0.2 2
FIG.10: Resistivesuperconductingtransitionsand(inset)su-
were inferred from their initial slopes of H with respect
c
perconductingtransitiontemperaturesT forLa M OBiS
c 1−x x 2 to T using the conventional one-band WHH theory39,
compounds with M = Th, Hf, Zr, Ti and x = 0.2. T is de-
c yielding values of 1.09 and 1.12 T, respectively. These
fined as the temperature where ρ drops to 50% of its normal
valuesofH (0)areclosetothevaluesof1.9TforH (0)
statevalue,andthewidthofthetransitionisdeterminedfrom c2 c2
observed in LaO F BiS 18,21, suggesting that there is
thetemperatureswheretheresistivitydropsto90%and10% 0.5 0.5 2
of its normal state value. Reprinted with permission from a common superconducting phase characteristic of BiS2-
Ref.25. Copyright (2013) by the American Physical Society. based superconductors.
Specific heat measurements were performed for the
La Th OBiS and La Hf OBiS compounds in
0.85 0.15 2 0.8 0.2 2
ray diffraction experiments show that both the a and c the temperature range of 0.36 K (cid:54) T (cid:54) 30 K25.
lattice parameters do not change significantly with dif- A clear feature is observed between 1 and 3 K for
ferent M4+ substitutions at the x = 0.2 concentration. La Th OBiS as shown in the inset at the lower
0.85 0.15 2
However, with increasing M concentration, both values right hand side of Fig. 11. A value of T = 2.5 K was
c
of a and c for La M OBiS were observed to decrease estimatedfromanidealizedentropyconservingconstruc-
1−x x 2
gradually. For La Th OBiS , T first decreases with tion,whichisclosetotheT obtainedfromρ(T)measure-
1−x x 2 c c
x from 2.85 K at x = 0.15 to 2.05 K at x = 0.20 and ment(T =2.85K).Thepresenceofthefeaturesuggests
c
then remains roughly constant at higher concentration. thatsuperconductivityisabulkphenomenoninthiscom-
A similar evolution of T with increasing M was also ob- pound. The value of ∆C/γT for La Th OBiS is
c c 0.85 0.15 2
served for La M OBiS (M = Hf, Ti); however, after estimated to be ∼0.91, which is less than the value of
1−x x 2
partially replacing La3+ with Sr2+, no superconductiv- 1.43 predicted by the BCS theory, but similar to that
8
seen in LaO F BiS 21. However, only a small feature
0.5 0.5 2
inC /T dataforLa Hf OBiS isobservedaroundthe
e 0.8 0.2 2
T (2.36 K) obtained from ρ(T) measurements. The ab-
c
sence of a well-defined superconducting jump at T for
c
La Hf OBiS is likely to be a consequence of the su-
0.8 0.2 2
perconducting transition being spread out in tempera-
ture due to sample inhomogeneity21.
V. CHEMICAL SUBSTITUTION EFFECTS ON
LN(O,F)BIS
2
Ithasbeendemonstratedthatthechargecarrier(elec-
tron) density is important for the superconductivity of
BiS2-based compounds. In addition, values of Tc for 0 0.05 0.10 0.15 0.20 0.25
AG samples of LnO F BiS increase with increasing Y concentration
0.5 0.5 2
atomic number of the Ln component as shown in Fig.
621,22. (The ionic size of the Ln3+ ion decreases with FIG.12: (Coloronline)(a)La(Y)-O(F)-La(Y)bondanglevs
increasing atomic number.) This size contraction has nominal yttrium concentration. (b) Dependence of Tc on Y
the effect of reducing the dimensions of the blocking lay- concentration for the La1−xYxO0.5F0.5BiS2 system. The or-
ers which contain the Ln3+ ions, thereby squeezing the angestaristheTc ofLaO0.5F0.5BiS2 reportedinRef.21. The
inset shows the behavior of T under hydrostatic pressure30.
neighboringBiS layers,i.e.,effectivelygeneratingchem- c
2 Reprinted with permission from Ref.42. Copyright (2014) by
icalpressureonthesuperconductingBiS layers. Asdis-
2 the American Physical Society.
cussedinsection3.1,itappearsthatthereisacorrelation
between T and the decrease in the size of the a-axis in
c
the Ln(O,F)BiS system. Hence, substitution of the el-
2
ement La with heavier lanthanide elements such as Sm, valueofc. Unfortunately,duetothesolubilitylimitofY
Eu, and Gd holds promise as a strategy for further en- in La1−xYxO0.5F0.5BiS2, further investigation of the re-
hancementofTc intheBiS2-basedsuperconductingcom- lationbetweenlatticeparameters,bondangle,andTc by
pounds. Recently, the compound with nominal chemi- the method of substitution is not possible and the high-
cal composition SmO0.5F0.5BiS2 was synthesized and re- est chemical pressure obtained in La0.8Y0.2O0.5F0.5BiS2
ported to be nonsuperconducting76; attempts have been is still lower than 0.7 GPa, above which a structural
unsuccessful in synthesizing other LnO F BiS com- phase transition from tetragonal (P4/nmm) to mono-
0.5 0.5 2
pounds with Ln = Eu-Tm. However, there has been clinic (P21/m) has been observed in LaO0.5F0.5BiS2 un-
recent success in synthesizing compounds in which there der applied external hydrostatic pressure30,80.
is a partial substitution at the Ln site with relatively Figure 13 shows the Sm concentration depen-
smaller atoms as a method for enhancing T and further dence of T for La Sm O F BiS . An en-
c c 1−x x 0.5 0.5 2
determining which parameters are relevant in affecting hancement of superconductivity is observed in the
superconductivity41,42,75–79. La Sm O F BiS compound in which T increases
1−x x 0.5 0.5 2 c
As an example, partial substitution of La with the from ∼2.8 K at x = 0.1 to 5.4 K at the solubility limit
smaller Y ions in the La Y O F BiS compound of x = 0.841. A linear extrapolation of the T (x) data
1−x x 0.5 0.5 2 c
allows one to study of the effects of chemical pres- beyond the x = 0.8 solubility limit to x = 1.0 allows for
sure on superconductivity42. Similar to La, Y has a an estimate of the value of T for the parent compound
c
trivalent electronic configuration and no magnetic mo- SmO F BiS to be as high as ∼6.2 K; this value of
0.5 0.5 2
ment, making Y3+ a suitable replacement for the La3+ T is significantly higher than values of T reported for
c c
ions. As a result of the substitution of Y for La in other AG samples of the LnO F BiS compound and
0.5 0.5 2
La Y O F BiS , T was found to gradually de- isthereforeconsistentwiththetrendthathighervaluesof
1−x x 0.5 0.5 2 c
crease from 2.8 K at x = 0 to 1.8 K at x = 0.1 and T areobservedinLnO F BiS compoundscontaining
c 0.5 0.5 2
then remain roughly constant at ∼3.0 K for x (cid:62) 0.125 a Ln element having a larger atomic number (see the in-
as shown in Fig. 12. The lattice parameter a and the setofFig. 13). AstheSmconcentrationincreasesfromx
unit cell volume of La Y O F BiS monotonically =0.1to0.8,thelatticeconstantcincreaseswhilethelat-
1−x x 0.5 0.5 2
decrease with increasing Y concentration until the solu- tice constant a gradually decreases, resulting in an over-
bility limit x = 0.20. Interestingly, the evolution of the all suppression of the unit cell volume of ∼3%, however,
La-O-Labondanglewithincreasingxcloselyfollowsthe no structural phase transition is observed. In compari-
evolution of T (x) as shown in Fig. 12. It should be son, there is only a ∼2% decrease in the unit cell volume
c
mentioned that the lattice constant c changes in the op- of LaO F BiS before the pressure-induced structure
0.5 0.5 2
posite manner (decreasing with increasing Y concentra- phase transition80. Hence, the effects of Sm substitution
tion), whichsuggeststhatT mayalsobeaffectedbythe forLaandappliedhydrostaticpressureonthecompound
c
9
6
4 .0 6 L a 1-xS m xO 0.5F 0.5B iS 2 1 3 .4 4 (a ) L n O 1-yF yB iS 2 y = 0 .5
4 .0 4 1 3 .4 1 y = 0 .3
5
)4 .0 2 )
(Å 1 3 .3 8 (Å
a c )
4 .0 0 1 3 .3 5 (KC4
T
3 .9 8
y = 0 .7
1 3 .3 2 3
6 6
D . Y a zici e t a l.
L n = S m 5
T
5 4c (K 2 C e 0 .2 0 .4 0 .6 0 .8 N d 0 .2 0 .4 0 .6 0 .8 S m
L n O 0.5F 0.5B iS 2 3) 6 C e 1-xN d x N d 1-zS m z
) 2 (b )
(Kc 4 La C e P r N d P m S m L n O 1-yF yB iS 2
T
r
5
c y = 0 .7
3 c dc
dc
0 0 .2 0 .4 0 .6 0 .8 1 .0 T(K)c 4
x
y = 0 .5
3
FIG.13: (Coloronline)Dependenceofthelatticeconstantsa
y = 0 .3
andc(upperpanel)andT (lowerpanel)onSmconcentration
c
x of La Sm O F BiS . Red, blue, and purple symbols
1−x x 0.5 0.5 2 2
in the lower panel represent results for samples annealed at 3 .2 8 3 .3 0 3 .3 2 3 .3 4 3 .3 6 3 .3 8 3 .4 0 3 .4 2
800◦C,750◦C,and710◦C,respectively. Theinsetshowsrep- c /a
resentative values of T of Ln(O,F)BiS reported in Ref.21.
c 2 FIG. 14: (Color online) (Color online) Superconducting crit-
The other data are obtained from Ref.41.
icaltemperatureT ofLnO F BiS (y =0.7,0.5,and0.3)
c 1−y y 2
asafunctionofLn concentration. Thecrosssymbolsindicate
thatasuperconducting transitionis notobservedabove2K.
LaO F BiS seem somewhat different. Reprinted with permission from Ref.78 J. Kajitani, T. Hiroi,
0.5 0.5 2
A. Omachi, O. Miura, Y. Mizuguchi, J. Phys. Soc. Jpn. 84
Effects of substitutions at the Ln site in the
(2015)044712. CopyrightedbythePhysicalSocietyofJapan.
LnO .5F .5BiS compounds were also investigated at
0 0 2
variousFconcentrationswhichallowsforthestudyofthe
tuning of the lattice constant c and its effects on super-
conductivity. PhasediagramsofT vsLn substitutionfor
c
the Ce Nd O F BiS and Nd Sm O F BiS
1−x x 1−y y 2 1−z z 1−y y 2
systems were determined from measurements of magne- seems to be of less importance in comparison to the lat-
tization at nominal F concentrations of y = 0.7, 0.5, and tice parameter a and the c/a ratio.
0.3 as shown in Fig. 1478. Superconductivity is induced
and noticeably enhanced for those compounds contain- The role of the charge carrier density in the enhance-
ing higher concentrations of Ln elements with relatively ment of superconductivity is emphasized in the study of
smalleratomicsize(orlargeratomicnumber),i.e.,higher the La Mg O F BiS system in which Mg2+ and
1−x x 1−2x 2x 2
concentrations of Nd and Sm atoms. The shift in the F− aresubstitutedforLa3+ andO2−,respectively75. Su-
phase boundary to the right for decreasing F concentra- perconductivity is observed for nominal x (cid:62) 0.2 where
tion suggests the importance of F concentration in the T continues to increase up to ∼3 K at x = 0.3 which
c
induction of superconductivity in these BiS -based com- is slightly beyond the solubility limit of x = 0.25. For
2
pounds. For a given F concentration, it appears that su- a fixed F concentration at 2x = 0.4, the introduction
perconductivity is enhanced in compounds with Ln ions of Mg2+ ions into the LaO F BiS compound gener-
0.6 0.4 2
of smaller ionic size; however, there is no appreciable atesholesintheBiS superconductinglayerswhichhave
2
increase in T for greater concentrations of F. It is inter- the effect of suppressing superconductivity and thereby
c
estingtonotethatsampleswithdifferentnominalvalues reducing T despite the reduction in lattice parameters.
c
of F concentration that share similar values of T also The research also indicates that a reduction in lattice
c
exhibit similar values of the ratio of the length of the c parameters leads to an enhancement of T . For in-
c
axis to that of the a axis (see Fig. 14(b)); this implies stance,areductioninthevaluesofthelatticeparameters
that the superconductivity observed in the BiS -based appears to enhance superconductivity when comparing
2
compounds is closely related with the lattice constant the La Ca O F BiS and La Mg O F BiS
0.8 0.2 0.6 0.4 2 0.8 0.2 0.6 0.4 2
ratio c/a. The effect of charge carrier density on the en- compounds which have T values of 2.07 K and 2.54 K,
c
hancementofsuperconductivityintheseBiS compounds respectively.
2
10
VI. SUPERCONDUCTIVITY OF LNO F BIS
0.5 0.5 2 (a)
UNDER APPLIED PRESSURE
Inadditiontochemicalsubstitution,theapplicationof
external pressure is another means to induce fundamen-
talchangetoboththecrystallineandelectronicstructure
whichcanhaveasignificantimpactonthesuperconduct-
ingpropertiesofthelayeredBiS -basedcompounds. The
2
first report of a dramatic and abrupt increase in T from
c
∼3 to 10.7 K for the LaO F BiS compound under
0.5 0.5 2
pressure is shown in the upper panel of Fig. 15(a). In
the T (P) phase diagram, there is an obvious transition
c
fromalow-T superconductingphasetoahigh-T super-
c c
conductingphase30. Thephenomenonwassoonreported
tobeassociatedwithapressure-inducedfirstorderphase
transitionfromatetragonaltoamonocliniccrystalstruc-
ture in which there is a relative shift or sliding of the
two neighboring BiS layers along the a axis80. Similar
2
behavior in the evolution of T with pressure has also
c
been observed in other LnO F BiS (Ln = Ce, Pr,
0.5 0.5 2
Nd) compounds as shown in the T (P) phase diagrams
c
of Fig. 1629.
Interestingly, the critical transition pressure, P , that
t
marks the low-T to high-T phase transition in the
c c
superconducting state is coincident with changes in
the normal state electrical transport properties in the
LnO F BiS system;thepressure-inducedphasetran-
0.5 0.5 2
sition corresponds with a saturation in the rate of sup-
pression of semiconducting-like behavior in the normal
state30. In the case of the NdO F BiS compound,
0.5 0.5 2
a semiconductor-metal transition occurs at P as shown
t FIG. 15: (Color online) T (P) phase diagrams for (a)
in Fig. 15(b)29. Such changes in the behavior of the LaO F BiS and (b) NdOc F BiS . The red color gra-
0.5 0.5 2 0.5 0.5 2
normal state ρ(T) indicate that there may be increases dient represents the suppression of the semiconducting be-
in the density of charge carriers that correspond to the havior with pressure as manifested in the decrease of the
rapid increase in T . semiconducting energy gap ∆ /k whose values are indi-
c 1 B
AG samples from the series of LnO F BiS com- cated in the legend. The green region to the right of
0.5 0.5 2
the crosshatching in (b) corresponds to the metallization of
pounds with Ln elements of smaller atomic radius (i.e.,
NdO F BiS . (a), (b) Reprinted with permission from
larger atomic number) exhibit higher T values when 0.5 0.5 2
c Ref.30 (copyright (2013) American Physical Society) and
compared with compounds containing Ln elements of
Ref.29 (Enhancementofsuperconductivitynearthepressure-
larger atomic radius suggesting that T correlates with
c inducedsemiconductor-metaltransitionintheBiS -basedsu-
2
thelatticeparameters. GiventhecorrelationbetweenTc perconductors LnO0.5F0.5BiS2 (Ln = La, Ce, Pr, Nd), J.
and the lattice parameters, one might expect that the Phys.: Condens. Matter 25 (2013) 422201. Copyright (2013)
pressure-induced phase transitions observed for the var- by the IOP Publishing.), respectively. For interpretation of
ious LnO F BiS compounds would occur at a char- the definition of T , vertical bars, and ∆ /k , the reader is
0.5 0.5 2 c 1 B
acteristic and consistent value of the lattice parameter referred to the web versions of the corresponding articles.
a across the LnO F BiS series. Furthermore, con-
0.5 0.5 2
sidering the results from substitution studies discussed
earlier in this article, one would expect a smaller ap- thecriticaltransitionpressurePt increasesastheatomic
plied pressure to induce the phase transition for those size of the Ln ions decreases and that the jump in Tc
LnO0.5F0.5BiS2 compoundswithLn elementsexhibiting during the phase transition (∆Tc) is smaller for smaller
smaller values of the lattice parameter a. To the con- Ln ionsinLnO0.5F0.5BiS229,30. HeavierLn ionsseemto
trary,thea-axislatticeparametersatPt arenotconstant helpstabilizethestructureoftheLnO0.5F0.5BiS2against
across the series LnO F BiS . The lattice parameter external pressure.
0.5 0.5 2
afortheLaO F BiS compoundjustbeforethetran- Figure 17 shows the evolution of T for the
0.5 0.5 2 c
sition pressure, P ∼ 0.7 GPa, is estimated to be 4.08 ˚A La Sm O F BiS compound as a function of both
t 1−x x 0.5 0.5 2
which is larger than the values of a for other members of pressure and Sm concentration x81. Similar to the
the LnO F BiS series at ambient pressure21,80. The LnO F BiS compounds, the La Sm O F BiS
0.5 0.5 2 0.5 0.5 2 1−x x 0.5 0.5 2
T (P) phase diagrams depicted in Fig. 16 indicate that system exhibits a reversible pressure-induced transition
c
