Fermi surface topology and low-lying quasiparticle structure of magnetically ordered Fe1+xTe

0.6 MB·English

by  Y. Xia

Fermi surface topology and low-lying quasiparticle structure of magnetically ordered
Fe Te
1+x
Y. Xia,1 D. Qian,1,2 L. Wray,1,3 D. Hsieh,1 G.F. Chen,4 J.L. Luo,4 N.L. Wang,4 and M.Z. Hasan1,5,6
1Joseph Henry Laboratories of Physics, Department of Physics, Princeton University, Princeton, NJ 08544
2Department of Physics, Shanghai Jiao Tong University, Shanghai 200030, China
3Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94305
4Beijing National Laboratory for Condensed Matter Physics,
Institute of Physics, Chinese Academy of Sciences, Beijing, China
9 5Princeton Center for Complex Materials, Princeton University, Princeton, NJ 08544
0 6Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, NJ 08544∗
0 (Dated: January 9, 2009)
2
Wereport thefirstphotoemission studyofFe1+xTe-thehost compoundofthenewlydiscovered
n iron-chalcogenidesuperconductors(maximumTc∼27K).Ourresultsrevealapairofnearlyelectron-
a
hole compensated Fermi pockets, strong Fermi velocity renormalization and an absence of a spin-
J
density-wavegap. Ashadowholepocketisobservedatthe”X”-pointoftheBrillouin zonewhichis
9
consistentwithalong-rangeorderedmagneto-structuralgroundstate. NosignatureofFermisurface

nesting instability associated with Q=(π/2, π/2) is observed. Our results collectively reveal that
]
n the Fe1+xTe series is dramatically different from the undoped phases of the high Tc pnictides and
o likely harbor unusualmechanism for superconductivityand magnetic order.
c
- PACSnumbers:
r
p
u
The discovery of superconductivity in the pnictides
at.s (dFeerAstsa-nbdaisnegdthcoemgepnoeurnadlsin)tehrapslagyenoferqautaendtuinmtemreasgtnientisumn-, ol) 0.009 Fe1+xTe SrFe2As2 FeTe
m
m electronic structure and superconductivity in iron-based mu/ 0.006 H//c
- layered compounds [1, 2]. The recent observation of un- e
d usualsuperconductivityandmagneticorderinthestruc- χ(
n H//a-b
turallysimplercompoundssuchasFeSeandFe Teare 0.003
o 1+x 0 100 200 300 b
the highlights of current research [3, 4, 5, 6, 7]. The ex-
c (a) T(K) (b) (c) a c
[ pectation is that these compounds may provide a way

1 to isolate the key ingredients for superconductivity and FIG. 1: Magneto-structural transition and long-range order
the nature of the parent magnetically order state which
v inFe1+xTe: (a)Temperaturedependenceofthemagneticsus-
9 maypotentiallydifferentiatebetweenvastlydifferentthe- ceptibility [6]. (b) The spins in SrFe2As2 are collinear and
9 oretical models [8, 9, 10]. The crystal structure of these QAF points along the (π,0) direction. (c) The ordering vec-
2 superconductors comprises of a direct stacking of tetra- tor, QAF in FeTe, is rotated by 45◦ and points along the
1 hedral FeTe layers along the c-axis and the layers are (π,π) direction [15].
2 2
. bonded by weak van der Waals coupling. Superconduc-
1
0 tivity with transition temperature up to 15K is achieved
9 in the Fe (Se,Te) series [3, 4] and T increases up to
1+x c
thisdate. Herewereportthefirstangle-resolvedphotoe-
0 27K under a modest application of pressure [5]. Den-
v: sityfunctionaltheories(DFT)predictthattheelectronic mission (ARPES) study of the Fe1+xTe - the host com-
pound of the superconductor series. Our results reveal a
i structure is very similar to the iron-pnictides and mag-
X pair of nearly electron-hole compensated Fermi pockets,
netic order in FeTe originates from very strong Fermi
r surface (FS) nesting leading to the largest SDW gap in strong band renormalization and a remarkable absence
a of the predicted large SDW gap. Although the observed
the series. Consequently, the doped FeTe compounds
Fermi surface topology is broadly consistent with the
are expected to exhibit T much higher than that ob-
c
DFT calculations, no Fermi surface nesting instability
served in the iron-pnictides if superconductivity would
associated with the magnetic order vector was observed.
indeed be originating from the so called (π,0) spin fluc-
An additional hole pocket is observed in our data which
tuations [10]. These predictions critically base their ori-
can be interpreted to be associated with a local-moment
ginontheFermisurfacetopologyandtheband-structure
magneto-structuralgroundstateinclearcontrasttointer-
details, however, no experimental results on the Fermi-
band nesting. Our measurements reported here collec-
ology and band-structure exist on this sample class to
tivelysuggestthatthe FeTecompoundseriesisdramati-
cally different from the parentcompound of the pnictide
superconductorsandmayharbornovelformsofmagnetic
∗Electronicaddress: [email protected] andsuperconductinginstabilities notpresentin the high
T pnictides.
c
2
0 meV -20 meV -40 meV -60 meV energy evolution of the band features and present the
data in a way to simulate the effect of rigidly lowering
M
of the chemical potential down to 60meV. The FS (at
0 meV) is seen to consist of circular hole pockets cen-
Γ
teredatΓ,andellipticalelectronpocketsattheMpoints.
(a) (b) (c) (d)
An elliptical-like feature at the M point expands as the
LDASchematic binding energy is increased, suggesting that the associ-
E
F M(π, 0) ated band is hole-like. We will subsequently show that
Γ
Γ this band lies below µ and the M-point Fermi pockets
Γ BanMd X X( π2 ,π2) anraentoninlytenelseitcytrpoant-tliekrens(i2n(ep)a)n.elB-(ya-cdo)n,saidpearritnfgrotmheadwoemaik-
Schematic M feature at ”X”-point, the FS topology is similar to that
(e) (f) (g)
expected from the DFT calculations ( 2(g)) [10]. In ad-
dition, the area of the hole-like FS pocket at Γ and the
FIG. 2: Fermi surface topology: (a) Fermi surface topol-
electron-like FS pocket at M are approximately equal in
ogy of FeTe. (a-d) Connection to the underlying band-
size suggesting nearly equal number of electron and hole
structureis revealed byconsidering theevolution of theden-
carrier densities in this material. Therefore, if the ex-
sity of states, n(k), as the chemical potential is rigidly low-
eredby20,40and60meV.TheFermisurfaceconsistsofhole cess Fe atoms are contributing to the carrier density it
pocketscenteredatΓ,andelectronpocketscenteredatM.An is likely to be small and beyond our k-resolution of the
additional hole-like feature is observed at M in (b-d), which experiment.
isattributedtoabandlyingbelowEF (seeschematicin(e)). In order to systematically study the low lying energy
(e) presents a schematic of the band dispersions in the Γ-M
bandstructure,ARPESspectraaretakenalongdifferent
direction forming the observed electron and hole pockets. A
k-spacecutdirectionsinthe2DBrillouinzone(BZ).Two
qualitative agreement is observed between the (f) the exper-
different electron-photon scattering geometries are used
imentally measured FS topology and (g) theLDAcalculated
to ensure that all bands are imaged. In a σ scattering
[10] FS although measured bands are strongly renormalized.
geometry where the polarization vector is parallelto the
k axis, photoelectron signal is predominantly from the
y
Single crystals of Fe1+xTe were grown from a mixture dxy,dxz,anddz2 energybandsduetothedipoleemission
ofgroundedFeandTepowderusingtheBridgemantech- matrix elements [11, 12]. Similarly, when the polariza-
nique. Thepowderwasheatedto920◦Cinanevacuated tion vector is parallelto kx, the π-geometry, the dyz and
tubethenslowlycooled,formingsinglecrystals. Theiron dx2−y2 states are predominantly excited. Figure 3 (a)
concentrationwasmeasuredbyICP(inductively-coupled and (b) present scans along the Γ−M direction in the
plasma)techniqueandavalueofxwasdeterminedtobe σ and π geometries. In both sets of spectra, one finds
less than 0.05. High-resolution ARPES measurements a broad hole-like band centered at Γ. However, near M
were then performed using linearly-polarized 40eV pho- thescanstakenattwodifferentgeometriesaredrastically
tons on beamline 10.0.1 of the Advanced Light Source different. Incut1,twohole-likebands(α2,α3 asmarked
at the Lawrence Berkeley National Laboratory. The en- in panel (a)) are observed to be approaching µ. Under
ergy and momentum resolution was 15meV and 2% of theπ geometrythebandemissionpatternnearMisdra-
thesurfaceBrillouinZone(BZ)usingaScientaanalyzer. matically different (see the β1 band in cut 2), while the
Thein-planecrystalorientationwasdeterminedbyLaue α2 andα3 bandsignalsaresignificantlyweaker. The po-
x-raydiffractionpriortoinsertingintotheultra-highvac- larization dependence suggests that the β1 band should
uum measurement chamber. The magnetic order below have dyz or dx2−y2 orbital character.
65K was confirmed by DC susceptibility measurements In order to fully resolve the broad band feature cen-
(Fig-1). The samples were cleaved in situ at 10K under tered at Γ, high resolution scans are performed along
pressures of less than 5×10−11 torr, resulting in shiny different k-cut directions through the zone center. Fig-
flat surfaces. Cleavage properties were characterized by ure 3(c) presents one cut in the Γ−M direction inside
STMtopographyandbyexaminingtheopticalreflection the first zone, together with the corresponding energy
properties. distribution curve (EDC). Two hole-like bands are re-
Figure 2 presents the momentum dependence of the solved,labeled α and α . Since there are traces of mul-
2 3
photoemission intensity n(k) from the sample at 10K at tiple bands near M, one might wonder whether the α
3
severaldifferent binding energies (0, 20, 40 and 60 meV) bandcrossesµneartheM point,thusformingahole-like
integratedoverafiniteenergywindow(±5meV)ateach Fermipocket. To systematicallyinvestigatethis, aseries
binding energy. The non-zero spectral intensity at the of spectra are taken along the Γ−M direction with π-
Fermilevel(µ)confirmedthatthelowtemperaturestate geometry(3(d)-(f)) throughmultiple k-cutsintersecting
of Fe Te is metallic-like. At the Fermi level, electrons the M-point Fermi pocket. At M, cut-4 shows a strong
1+x
aremostly distributed inone broadhole-likepocketatΓ electron-likebandformingtheFSpocket. Thisbandcan
and another similar size electron-like pocket around M. beattributedtotheβ band(alsoobservedincut-2). As
1
To revealthe bandshapes we presenta gradualbinding- one moves away from M, the band intensity becomes in-
3
FIG. 3: Low-lying band topology: ARPES spectra along the Γ−M direction in the (a)σ and (b)π-scattering geometries.
vF of thequasiparticle band forming the Γ hole pocket is calculated from thedashed line. (c) The EDC of a high momentum
resolutionscanthroughΓshowstwohole-likebandscrossingtheEF. (d)-(f)CutsalongtheM-Γdirectionthroughtheelectron
FS pocket show that the hole band near M remains at least 10meV below EF. For each scan direction the corresponding
EDC is also presented, within the momentum range marked by green dashed lines. Comparison with (g) a band dispersion
schematic of the LDA result [10] shows a good agreement between experiment and theory. (h) The directions of the six scans
are summarized and identified in the first BZ. The direction of theelectric field is shown.
creasinglymorehole-like,indicatingtheemergenceofthe layers and the crystal cleaves at a weak van der Waals
α band. Nevertheless,theholebandliescompletelyand bond bewteen two adjacent layers no strong 2×1 long-
2
consistently below the Fermi level, with some weak elec- range ordered A-type reconstruction is expected except
tron quasiparticle intensity above the band maximum. for a weak bulk-like structural orthorhombicity tied to
The β intensity becomes the weakest near the edge of the magnetic order (magneto-structural effect). Recent
1
theMpocket(cut-6). Butevenatthatlocation,thehole neutron[15,16]andx-raydiffraction[17,18]studieshave
pocket lies at least 10meV below the chemical potential. shown that FeTe undergoes a bulk structural distortion
The result shows that there are no hole pocket features from the tetragonal to weakly-monoclinic or orthorhom-
intheFSnearM whichsupportsthe interpretationthat bicphasenear65K,accompaniedbylong-rangemagnetic
this material is nearly electron-hole compensated. Ad- order Q =(π,π).
AF 2 2
ditionally, one can map the observed bands to the band
In the parent compound of the pnictide superconduc-
structure estimated by DFT calculations (schematic in
tors such as the SrFe As or BaFe As , the SDW vector
2 2 2 2
3(g) [10]). The calculation finds three bands (α -α )
1 3 Q =(π, 0) coincides with a Fermi surface nesting of
SDW
crossing Fermi level near Γ and two near (β , β ) M,
1 2 vector connecting the hole-pocket at Γ and the electron
forming the electron and hole pockets. While the DFT
pocket at M. Currently, it is believed that this nesting
calculated bands agree fairly well with our data, within
is responsible for opening a gap in the low temperature
our resolution or the sample surface roughness, we have
physicalproperties [6, 11, 19]. In the case of FeTe, while
notsucceededinfullyresolvingtheα bandnearΓ. The
1 a nesting vector can indeed be drawn along Q =(π,
SDW
β band, which forms the second M electron pocket, is
2 0) between a pair of electron and hole pockets, all avail-
observed in the Fermi surface topology (Fig. 2). The
ableneutronscatteringmeasurementsreportthatthean-
overall band narrowing is about factor of 2 compared to ◦
tiferromagnetic ordering vector is 45 away from that in
theDFTcalculationshighlightingtheimportanceofcor- SrFe As ,namely,inFe Te,Q =(π,π). Theorder-
relationeffects. ThisisalsoconsistentwithasmallFermi 2 2 1+x AF 2 2
ingshowsacommensuratetoincommensuratecross-over
velocity (v ∼ 0.7 eV·˚A) observed (Fig.3(a)).
Fermi if the concentration of excess iron, x, is increased. An-
We now revisit the details of the FS map (Fig.-2) and other remarkable difference is that the magnetic suscep-
discuss a weak feature observed at X=(π,π) between tibility is Curie-Weiss like in FeTe suggesting that the
2 2
two Γ points (Fig. 4(e)). Such a feature is not expected magnetism is of local-moment origin. Within a local
fromthe DFT calculations. Asimilarfeature,whoseori- moment-like AFM long-range ordered state which also
gin is debated, has been observed in AFe As (A=Ba, couples to a weak structural distortion one should ex-
2 2
Sr,Ca),whichisattributedtoa2×1surfacereconstruc- pect relatively intense shadow Fermi surfaces along the
tion of the A atom layer [13, 14], in addition to a weak Neel vector Q . The X-point Fermi surface we ob-
AF
bulk structural distortion. However, in FeTe there are serve thus can be related to the vector observed in neu-
no additional atoms (such as Sr or Ba) between the Fe tron scattering X = Γ+Q . The weak shadow-like X
AF
4
SrFe2As2 M FeTe sistent with our observation of a Fermi surface in the
correlatedmagneto-structurallyorderedstate. To exam-
ine the band dispersioncharacterof the X pocket, figure
Γ X X Γ ? 4 (f) presents a spectra along the Γ−X direction. Re-
Q sults show two bands dispersing towards µ, forming the
SDW
Q hole pocket in the ”X”-FS whose shapes are indeed very
SDW
similar to the bands that form the central Γ-pocket FS.
M
(a) (b)
Furtherevidenceagainstnestingcomesfromtheabsence
2´1 Sr Order (c) No Sr Order Gold of a large gap at the overall low temperature electron
y distributions presented in Fig-2 and 3 ( 4(d)). Our re-
nsit M sults seem to suggest that the groundstate is a nearly
e
m. Int Γ eisleccotrnosnis-theonltewcoitmhpreencseanttedbuslekmoimpteitcaall.cAonbdseunccteivoitfy,asgpaep-
or
N cificheatandHallmeasurementsinFeTe[6](whilemost
(d)
measurements do report a gap in SrFe As [19]).
-0.05 0.00 2 2
SrFeAs FeTe EB(eV) A recent density functional calculation [21] suggests
2 2 Fe Sr
Γ X that the excess Fe is in a valence state near Fe+ and
1
therefore donates electron to the system. Due to the in-
0.0
M teraction of the magnetic moment of excess Fe with the
V) -0.1 itinerant electrons of FeTe layer a complex magnetic or-
e
Γ1 E(B-0.2 QAF dering pattern is realized. In this scenario, excess Fe
Q
AF X (f) would lead to an enlargement of the electron pocket FS,
-0.3
(e) 0.0 0.5 1.0 1.5 however, within our resolution electron and hole Fermi
k(Å-1) surfacepocketsaremeasuredtobe ofverysimilarinsize
suggestingalackofsubstantialelectrondopingduetoex-
cessFe. Finally,wenotethatagaplessyetlong-rangeor-
FIG.4: Electronic structure and Magnetism: While(a)
deredlocal-momentmagneto-structuralgroundstatecon-
the electron and hole pockets in SrFe2As2 can be kinemat-
sistent with ARPES and neutron data taken together is
ically nested by QAF=(π,0), there exists (b) no FS pocket
which can be nested by QAF = (π2,π2) in the FeTe BZ. A captured in both first-principles electronic structure [8]
FS pocket observed at X in SrFe2As2 has been attributed to andmany-bodyspinmodel[9]calculations. However,the
the(c)2×1surfaceorderoftheSratomiclayer(red),which broad agreement of DFT calculations with experimental
occursinadditiontoaweakbulkdistortion(blue)acrossthe band-structure data except for about a factor 2 renor-
SDW transition. However, there are no additional Sr atoms malization is also remarkable. A complete understand-
between the Fe layers in FeTe, so no Sr order is possible, al- ing of electron correlation, moment localization and the
thoughaweakbulkdistortionisnotexcluded. (d)TheEDCs
true nature of the gapless antiferromagnetic state would
measuredatneartheΓandMpocketsexhibitnoevidenceof
require further systematic ARPES study [22].
energygaps. Nevertheless,(e)aweakFSpocketisobservedat
In conclusion, we have presented the first ARPES
X,(f)correspondingtotwohole-likebandsdispersingtowards
the chemical potential which is consistent with a long-range study of Fermi surface topology and band structure of
local moment magneto-structural order. Fe1+xTe. Our results reveal a pair of nearly electron-
hole compensated Fermipockets, strongrenormalization
and the remarkable absence of a spin-density-wave gap.
pocket FS might therefore arise from a band folding due The observed shadow hole pocket is consistent with a
tolong-rangemagneticorder. However,unlikeSrFe Se , long-rangeorderedlocalmoment-likemagneto-structural
2 2
where the Γ electron pocketnests with the M hole pock- groundstatewhereas,most remarkably,no Fermi surface
etsviaQ =(π,0)[20],ananalogousnestingchannel nesting instability associated with the antiferromagnetic
SDW
is unavailableat (π,π) in the FeTe ( 4(b)) clearly ruling orderwasobserved. Contrarytobandtheorysuggestions
2 2
out FS nesting as the origin of magnetic order. In the [23], results collectively suggest that the Fe Te series
1+x
absence of nesting FS gapping is not expected in FeTe is dramatically different from the undoped phases of the
which is consistent with our results in Fig-2 and 3. A high T pnictides and likely harbor a novel mechanism
c
large low temperature specific heat value [6] is thus con- for superconductivity and quantum magnetism.
[1] Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, X. H.Chen et al.,Nature 453, 761 (2008).
J. Am. Chem. Soc. 130, 3296 (2008); Z.A. Ren et al., [3] F.C.Hsuetal.,Proc.Natl.Acad.Sci.105,14262(2008).
Chin. Phys.Lett. 25, 2215 (2008). [4] M.H. Fanget al.,Phys.Rev. B 78, 224503 (2008).
[2] G.F. Chen et al., Phys. Rev. Lett. 100, 247002 (2008); [5] Y.Mizuguchietal.,Appl.Phys.Lett.93,152505(2008).
5
[6] G.F. Chen et al.,arXiv:0811.1489 (2008). [16] W. Bao et al., arXiv:0809.2058 (2008).
[7] C. Xu and S. Sachdev,NaturePhys. 4, 898 (2008). [17] K. Yeh et al.,arXiv:0808.0474 (2008).
[8] F.J. Ma et al.,arXiv:0809.4732 (2008). [18] Y. Mizuguchi et al., arXiv:0810.5191 (2008).
[9] C. Fang et al.,arXiv:0811.1294 (2008). [19] W.Z. Huet al.,Phys.Rev. Lett.101, 257005 (2008).
[10] A.Subediet al., Phys.Rev.B 78, 134514 (2008). [20] J. Zhao et al.,Phys. Rev.B 78, 140504 (2008).
[11] D.Hsieh et al.,arXiv:0812.2289 (2008). [21] L. Zhang et al.,arXiv:0810.3274 (2008).
[12] D.Qian et al.,Phys. Rev.Lett. 97, 186405 (2006). [22] Y. Xia, M.Z. Hasan et al.,.
[13] M.C. Boyer et al.,arXiv:0806.4400 (2008). [23] M. Johannes, Physics 1, 28 (2008)
[14] Y.Yin et al.,arXiv:0810.1048 (2008).
[15] S.Li et al., arXiv:0811.0195 (2008).