A review of the electronic structure of CaFe(2)As(2) and FeTe(0.6)Se(0.4)

0.74 MB·English

by  Kalobaran Maiti

A review of the electronic structure of CaFe As and FeTe Se
2 2 0.6 0.4
∗
Kalobaran Maiti
Department of Condensed Matter Physics and Materials Science,
Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai - 400 005, INDIA.
(Dated: January 7, 2015)
Fe-basedsuperconductorshavedrawnmuchattentionduringthelastdecadeduetothefindingof
superconductivity in materials containing the magnetic element, Fe, and the coexistence of super-
conductivity & magnetism. Extensive study of the electronic structure of these systems suggested
5
dominant role of d states in their electronic properties, whereas the cuprate superconductors show
1
majorroleoftheligandderivedstates. Inthisarticle,wereviewsomeofourresultsontheelectronic
0
2 structure of these fascinating systems employing high resolution photoemission spectroscopy. The
combinedeffectofelectroncorrelationandcovalencyrevealaninterestingscenariointheirelectronic
n structure. The ligand p states contribution at theFermi level is found to be much more significant
a
thanthat indicated in earlier studies. Temperatureevolution of theenergy bandsreveals signature
J
of transition akin to Lifshitz transition in thesesystems.
6
PACSnumbers: 74.70.Xa,74.25.Jb,71.20.-b,79.60.-i
]
n
o
INTRODUCTION lier [8–10] and try to bring out common features among
c
- these materials. Fe(TeSe) group of compounds, popu-
r
p larlyknownas‘11’systemsformsinanti-PbO-typecrys-
High temperature superconductivity continued to be
u tal structure (space group P4/nmm) [11], and are be-
one of the thrust area in condensed matter research for
s lieved to be the most correlated ones due to their large
. many decades, where most of the focus has been cen-
t ‘chalcogenheight’[12] (the heightofthe anionsfromthe
a tered around the study of cuprates superconductors [1].
m Fe-plane) [13]. The end members, FeTe exhibits spin
Discovery of superconductivity in Fe-based compounds
densitywave(SDW)-typeantiferromagnetictransitionat
- [2, 3] renewed great attention in the study of high tem-
d 65 K [14, 15] and FeSe is a superconductor below 8 K
perature superconductivity. Fe-based systems are sig-
n [16, 17]. Homovalent substitution of Te at Se-sites in
o nificantly different from the cuprates. The parent com-
FeSe leads to an increase in superconducting transition
c pounds in cuprates are antiferromagnetic Mott insula-
[ tors, where the insulating property arises due to strong temperature, Tc with maximum Tc of 15 K for 60% Te-
substitutions[18],despitethefactthatsuchsubstitutions
1 electron correlation compared to the width of their con-
often introduces disorder in the system that is expected
v duction band. The antiferromagnetism gets suppressed
toreducethesuperconductingtransitiontemperature,T
7 with the charge carrier doping and superconductivity c
9 [19].
emerges beyond some critical doping. The normal phase
0 CaFe2As2 belong to another class of materials known
of these materials exhibit plethora of unusual behavior
1 as ‘122’compounds andcrystallize in the ThCr2Si2 type
0 such as pseudogap phase, strange metallicity etc.
tetragonal structure at room temperature, (space group
.
1 Onthe other hand,the parentcompounds ofFe-based I4/mmm). CaFe2As2 exhibits SDW transition due to
0 compounds (pnictides or chalcogenides) are metals ex-
the long range magnetic ordering of the Fe moments at
5 hibiting spin density wave (SDW) phase in the ground
T = 170 K along with a structural transition to an
1 SDW
state. Charge carrier doping in these systems leads to
: orthorhombicphase. Highpressure[5],substitutionofFe
v superconductivityviasuppressionoflongrangemagnetic
byCo,Ni[20]andotherdopantsinducessuperconductiv-
Xi orderin the parentcompositions[4]. Interestingly,many ityinCaFe2As2. TheSDWtransitionisfoundtoaccom-
of these Fe-based compounds exhibit pressure induced
r pany a nesting of the Fermi surface [21, 22] along with
a superconductivity [5, 6]. Application of pressure usu-
a transition from two dimensional (2D) to three dimen-
ally renormalizes the hopping interaction strengths due
sional (3D) Fermi surface associated with the structural
to the compression/distortion of the real lattice with-
transition [23–25].
outsignificantchangeintheoverallcarrierconcentration.
It is believed that Fe 3d states play dominant role in
Thus, the finding of pressure induced superconductivity
the electronic properties of these systems in contrast to
expands the domain of unresolved puzzles significantly.
cuprates,wherethedopedholespossessdominantligand
Mostinterestingly,some Fe-basedcompounds exhibit an
2p orbital character [1]. Thus, the physics of unconven-
unusual coexistence of magnetic order and superconduc-
tional superconductors is complex due to the significant
tivity [6, 7].
differencesamongdifferentclassesofmaterials. Here,we
Here, we review the experimental results of two Fe- show that the ligand p electrons play much more impor-
based superconductors, Fe(TeSe) and CaFe2As2 belong- tant role than what was anticipated. The temperature
ing to two difference class of materials published ear- evolution of the electronic structure reveals interesting
2
scenario in these materials.
A
units) FAes dp Expt (a)
EXPERIMENTAL nsity (arb. As p x 10 D C B
fluTxhaendsinthgelescinryglsetaclrsyostfaCllianFee2saAms2plweeorfeFgerToew0n.6Sues0in.4g[2S6n] Inte 6 Bindin4g e nergy (2eV) 0
was grownby self flux method. The growncrustals were nits) (b) (c)
characterized by x-ray diffraction, Laue, Mo¨ussbauer b. u
and tunneling electron microscopic measurements estab- y (ar He I 1 1000 KK
lishing stoichiometric and homogeneous composition of nsit He II 300 K
the sample with no trace of additional Fe in the ma- Inte 0.8 0.4 0.0 2 1 0
Binding energy (eV)
terial. The grown crystals are flat platelet like, which
canbe cleavedeasilyandthe cleavedsurfacelookedmir-
rorshiny. Photoemissionmeasurementswerecarriedout
FIG. 1. (a) XP valence band spectrum of CaFe2As2 at 300
using a Gammadata Scienta analyzer, R4000 WAL and
K (symbols). Fe 3d and As 4p contributions obtained from
monochromaticphotonsources,AlKα(hν =1486.6eV), ab initio calculations are shown by dashed and solid lines.
He I (hν = 21.2 eV) and He II (hν = 40.8 eV) sources. The solid circles represent rescaled As 4p contributions. (b)
The energy resolution and angle resolution were set to 2 Near Fermi level feature from HeI and HeII excitations. (c)
meV and 0.3o respectively for ultraviolet photoemission Evolution of theXP valence band spectra near ǫF with tem-
peratures.
(UP) studies and the energy resolution was fixed to 350
meV for x-ray photoemission (XP) measurements. The
sample was cleaved in situ (base pressure < 3× 10−11 line), while the As 4p states (solid line) contribute at
Torr) at each temperature several times to have a clean higher binding energies.
well ordered surface for the photoemission studies. Re- In order to learn this better, we critically investigate
producibility ofthe data in both cooling and heating cy- the electronic states close to ǫ . It appears that the hy-
F
clewasobserved. TheenergybandstructureofCaFe2As2 bridizationbetweenAs4pandFe3dstatesisquitestrong
and FeTe0.5Se0.5 was calculated using full potential lin- with significant contribution coming from As 4p PDOS
earized augmented plane wave method within the lo- near ǫ as shown by solid circles in the figure. This is
F
caldensity approximation(LDA) using Wien2k software furtherexaminedexperimentallyinFig. 1(b)bycompar-
[27]. The energy convergence was achieved using 512 k- ingthespectraobtainedusingHeIandHeIIexcitations.
points within the first Brillouin zone. Theatomicphotoemissioncross-sectionforFe3dandAs
4p at He I energy excitation are 4.833 & 3.856, and at
HeIIare8.761&0.2949,respectively[30]. Thus,relative
RESULTS AND DISCUSSIONS
intensity corresponding to As 4p states will increase sig-
nificantly at He I energy compared to He II energy. The
As discussed above, the major difference of Fe-based comparison of the He I and He II spectra having same
superconductors with the cuprates is believed to be the resolution broadening suggests that the contribution of
character of the conduction electrons. In cuprates, the As 4p states at the Fermi level is indeed large compared
charge transfer energy (= the energy required to trans- to the band structure results shown in Fig. 1(a).
fer an electron from ligand to the copper site) is smaller The temperature evolutionof the feature A in the XP
than the electron correlation strength [28]. Therefore, valence band spectra is shown in Fig. 1(c) after normal-
the Fermi level, ǫ lies at the top of the O 2p band and izing by the spectral intensities in the energy range be-
F
the electrons close to the Fermi level possess dominant yond1eVbindingenergy. TheintensityofthefeatureA
O 2p character. The electron correlation strength in Fe- exhibits gradual enhancement with the decrease in tem-
pnictidesisexpectedtoberelativelysmaller[29]andthe perature. Valence band spectra of a correlated system
electrons close to ǫ were described to be dominated by exhibitsignatureofupperandlowerHubbardbandscon-
F
Fe 3d character. This appears to be the case in the va- stituted by the correlated electronic states (often called
lence band spectrum of CaFe2As2 shown in Fig. 1(a). incoherentfeature)andaKondoresonancefeaturecalled
The x-ray photoemission (XP) spectrum at 300 K ex- coherent feature appears at the Fermi level represent-
hibitsfourdistinctfeatures,A,B,CandD[9]. Thecalcu- ing the itinerant electrons. The decrease in temperature
lated spectral functions obtained by convoluting the the leads to an increase in the coherent feature intensity at
bandstructureresultswiththeFermi-Diracfunctionand the cost of incoherent features [31, 32]. While such in-
resolution broadening function exhibit good representa- creasein spectralintensity canhaveother origin[33], we
tion of the experimental spectra. It is clear that the fea- strongly feel, the enhancement of the feature A with the
ture A possesses dominant Fe 3d contributions (dashed decrease in temperature shown in Fig. 1(c) can be at-
3

units) (aS)e 4s Te 5s
y (arb. 10 K 2 (a) FeTe0.5Se0.5 Fe 3d
nsit 300 K 1
Inte 15 10 5 0 -1m) 0
Binding energy (eV) -1V.ato 0.5 (b) UU == 04 eeVV Se 4p
e
nsity (arb. units) (b) HHXeeP SIII 12310500000 K KKK (c) PDOS (states. 00..04 (c) Te 5p
Inte 1.2 0.8 0.4 0.0 1.2 0.8 0.4 0.0
Binding en ergy (eV) 0.0
units) (d) 10 K 6 B4inding e2nergy (eV0) -2
y (arb. 213000000 KKK
nsit
nte 716 712 708 704
I
Binding energy (eV)
FIG.3. Calculated (a) Fe3d, (b)Se4p and(c) Te5p partial
densityofstatesforuncorrelated(thinlineforU =0eV)and
correlated system (thick line for U = 4.0 eV).
FIG.2. (a)XPvalencebandspectrumofFeTe0.6Se0.4 at300
K (line) and 10 K (symbols). (b) Near Fermi level spectra
at 10 K obtained using He I, He II and Al Kα lines at 10 K.
feature intensity [31], the core hole is expected to be
(c)TemperatureevolutionofthenearFermilevelXPfeature.
(d)Fe2pcorelevelspectra. Insetshowsexpandedviewofthe more efficiently screened at lower temperatures as more
well screened feature. number of mobile electrons are available at low temper-
atures. Therefore, the temperature evolution of the core
tributed to correlation induced effects as justified later level spectra observed here can also be attributed to the
in the text. correlationinducedeffectdiscussedforthevalenceband.
In Fig. 2(a), we show the valence band spectra of In order to investigate the dominance of p character
FeTe0.6Se0.4 obtained using Al Kα x-ray source at 10 K near ǫF in the experimental spectra in contrast to the
and300K[8]. Thevalencebandexhibitmultiplefeatures predictionofthe dominance ofFe 3dstates, we showthe
- the feature close to ǫ possess dominant Fe 3d charac- calculated partial density of states corresponding to the
F
ter and Te/Se p related features primarily contribute in correlatedanduncorrelatedgroundstates[8]. Finiteelec-
the energy range 2 - 7 eV as also appeared in the case tron correlation leads to a spectral weight transfer from
of CaFe2As2. The higher binding energy features corre- ǫF to higher binding energiesleadingto anenhancement
spondtoSe/Tesstatesexcitations. Acomparisonofthe of the intensity around 2 eV (incoherent feature). Elec-
near Fermi level feature at 10 K obtained using different troncorrelationaffectstheelectronicstateswithdifferent
photonenergiesexhibitinteresting scenario. The feature orbitalcharacterdifferently depending ontheir degreeof
around 100 meV is most intense in the He I spectra in- itineracyinthe uncorrelatedsystem[36]. This isevident
dicating again large chalcogenp contributions. Decrease in Fig. 3 exhibiting significant transfer of the Fe 3d par-
intemperature leadsto agradualincreasein intensityof tial density of states (PDOS) to higher binding energies.
the nearǫ featureasshowninFig. 2(c)consistentwith However, the Se 4p / Te 5p contributions increase near
F
the scenario in correlated electron systems. ǫF. Thus, the relative intensity of the p-states near ǫF
The correlation induced effect can be verified further will be enhanced significantly with respect to the Fe d
by inspecting the Fe 2p core level spectra shown in Fig. states. This explains the presence of dominant p charac-
2(d). The spectra exhibit an interesting evolution with ter near ǫF in the experiment.
temperature. The intensity of the peak around 707 eV Beinginvestigatedthecharacteroftheelectronicstates
binding energy increases gradually with the decrease in near ǫ , we now turn to the energy band structure of a
F
temperature (see inset). This feature is often referred typical pnictide, CaFe2As2 - all the samples in this class
as the well screened feature, where the core hole created of materials exhibit essentially similar electronic struc-
by photoemission is screened by a conduction electron ture. The calculated energy bands are shown in Fig.
in the final state [34, 35]. Since the decrease in temper- 4(a) exhibiting t2g bands close to the Fermi level, and
ature leads to an enhancement of the coherent feature bothbonding&anti-bondinge bandsappearawayfrom
g
intensity with a consequent decrease in the incoherent ǫF. Three energy bands having t2g symmetry and de-
4
(a) HeII (a)X HeII (b) HeI (c)
HeI (b)
Energy (eV) --042 αβ,γ Intensity (arb. units) 0.0 αα 0β.,5γ (c) Intensity (arb. units) !"# X !!" ! !
HeII β,γ 300 K 10 K 10 K
160 80 0 160 80 0 160 80 0
Γ ∆ H XΣΓΛP 0.0 0.5 Binding energy (meV)
k|| (Γ X)
300 K (d) 300 K α (e)
Intensity (arb. units) β, γα Γ Intensity (arb. units) βσ, γ λ Γ F3Kre0I.s0Gp.Ko5n.adnEidnnge(rbtg)oy1dγ0isKatrn.idb(cu)λtiTobnhaecnuEdrDsveCassr(eEinDnteChsste)eHdinelteIhasedpHiencegtrIaItoaattt(h1a0e)
X SDW transition in these materials [21, 22].
σλ
HeI HeII With the decrease in temperature below the SDW
X
800 400 0 800 400 0 transition temperature, the γ band hole pocket and the
Binding energy (meV) Binding energy (meV) λ bandelectronpocketvanishes opening upa gapat the
Fermi level [24, 25]. Subsequently, the crystal structure
also changes from tetragonal to orthorhombic that leads
toachangeintheFermisurfacetopology-theFermisur-
FIG. 4. (a) Calculated energy band structure of CaFe2As2
face corresponding to the α band exhibit k -dependence
showing three energy bands α, β and Γ making three hole z
indicating its transition to three dimensionality [23]. It
pockets around Γ point. Momentum distribution curves at
140 meV and 300 K in the (b) He I and (c) He II spectra. is observed that in the orthorhombic phase, the α-band
The lines show a typical fit exhibiting signature α, β and Γ hole pocket is centered around kz ∼ 2(2n+1)π/c and
bands. Energy distribution curves as (d) He I and (e) He II it is absent around 4nπ/c in the k-plane containing k
z
photon energies and 300 K. axis. Thus, α band is expected to cross ǫF at He I en-
ergy, while it will appear below ǫF at He II energy. We
show the He II EDCs at 300 K and 10 K in Figs. 5(a)
noted by α, β and γ in the figure cross ǫF near Γ point and 5(b), respectively exhibiting exactly the same sce-
forming three hole-pockets. Here, Γ and X points are nario. Interestingly, the α band in the He I spectra also
defined as (0,0) and (π,π) in the xy-plane. The kz val- appears below ǫF at 10 K as shown in Fig. 5(c) indicat-
ues corresponding to He I and He II photon energies are ing the vanishing of the Fermi surface corresponding to
(kz ∼ 9.5π/c and ∼ 12.5π/c, respectively. theαbandat10K-largerintensityoftheαbandinHe
Various angle resolved photoemission spectroscopic II spectra compared to that in He I spectra indicate its
measurements[23–25]showthattheFermisurfacecorre- dominant Fe 3d character.
spondingtoallthesethreebandsexhibittwo-dimensional It is to note here that many of the unconventional
topology at room temperature, where the sample has superconductors exhibit signature of Lifshitz transition
tetragonal structure. The signature of these three [37]. If the Fermi level is in proximity to a point sep-
bandsareobservedinthemomentumdistributioncurves arating hole- and electron-type Fermi surfaces, a small
(MDCs) at 300 K in the He I and He II spectra shown change in a tuning parameter such as doping, pressure
in Figs. 2(b) and 2(c), respectively. The β and γ bands wouldlead to a transitionfrom an electron-type to hole-
possessing d ,d symmetry appear almost degenerate, typeFermisurfaceorviseversa. ThisisknownasLifshitz
xz yz
while the αbandhavingd symmetrydistinctly appear transition. Proximity to Lifshitz transitionindicates sig-
xy
atslightlyhigherbinding energies[9]. The energydistri- nificant quantum fluctuation in the system. The signa-
bution curves (EDCs) in Figs. 2(d) and 2(e) show that tureofLifshitztransitionhasbeenobservedduetosubtle
alltheseenergybandscross,ǫ inthe vicinityofΓpoint changeinchargecarrierconcentrationincuprates[37]as
F
indicating presence of three hole-pockets at room tem- well as in electron doped Ba(Fe1−xCox)2As2 [38]. The
perature. An energy band λ is also observed forming an vanishing of the hole Fermi surface corresponding to the
electronpocketaroundX-point. TheFermisurfacescor- α in CaFe2As2 as a function of temperature is interest-
5
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