Photon Factory Activity Report 2014 #32 (2015)

BL-2A/Commissioning

Band structure of the weakly ferromagnetic superconductor Sr2VFeAsO3-
Masafumi Horio1,*, Takao Katagiri2, Hiromitsu Namiki2, Kozo Okazaki1, Hakuto
Suzuki1, Jian Xu1, Keisuke Koshiishi1, Masaki Kobayashi3, Makoto Minohara3, Enju
Sakai3, Koji Horiba3, Hiroshi Kumigashira3, Takao Sasagawa2 and Atsushi Fujimori1
1
Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan2
Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama,Kanagawa 226-8503, Japan
3Photon Factory, KEK, Tsukuba, Ibaraki 305-0801, Japan
1 Introduction
The iron-based superconductor Sr2VFeAsO3- shows arelatively high superconducting transition temperature(Tc) of 37 K for  = 0 [1]. While the Fe 3d electrons in theFeAs layer are certainly responsible for thesuperconductivity, a Mott-insulator-like behavior has
been proposed for the V 3d electrons in the Sr2VO3 layer[2]. When oxygen deficiencies of  = 0.5 are introduced,
Tc decreases to 20 K and a weak ferromagnetism appears
with a Curie temperature of TC ~ 240 K and a magnetic
moment of ~ 0.1 B/V [3]. Although the weakferromagnetism has been attributed to the canted
magnetic moment of the V atom, the origin of theferromagnetism is still unknown. Also, the oxygendeficiency-induced changes in the electronic structure
have not been studied so far.
In this work, we have performed angle-resolved
photoemission spectroscopy (ARPES) measurements on
single crystals of ferromagnetic Sr2VFeAsO3- ( = 0.5)
by use of photons with different polarizations to elucidatethe band structure including its orbital character.
2 ExperimentSingle crystals of Sr2VFeAsO3- (= 0.5) were grownby the self-flux method. The superconducting transition
temperature Tc, which was determined by magnetic
susceptibility measurements, was 14 K, and the Curie
temperature TC was 240 K.
ARPES measurements were carried out at BL-2A ofPhoton Factory using linearly polarized light parallel (ppolarization) and perpendicular (s polarization) to themirror plane with h = 72 eV. A SCIENTA SES-2002
analyzer was used with the total energy resolution of 35
meV. Single crystals were cleaved in-situ in the vacuumof ~ 1 × 10-10 Torr at T = 22 K. Samples were oriented asdescribed in Fig. 1. Here, X and Y are parallel to the
crystal axis, and x and y are parallel to the nearestneighbor Fe-Fe direction.
3 Results and Discussion
Fig. 2 shows Fermi surfaces and intensity plot inenergy-momentum space along the -M line. Clear

Fig. 1 Experimental setup for the ARPES
measurements. X and Y are parallel to thecrystallographic a and b axes, and x and y areparallel to the nearest-neighbor Fe-Fe directions.
differences in the intensity distribution are observed
between the intensity plots taken with s and ppolarization reflecting the selection rule: photons with spolarization excite electrons from dyz and dxy orbitals,
while excitation occurs from dxz, dx2-y2, and d3z2-r2
orbitals with p polarization. By extracting peak
positions in energy distribution curves (EDCs) andmomentum distribution curves (MDCs), band structure
has been deduced as shown in Fig. 3(a).

Fig. 2 ARPES spectra of Sr2VFeAsO3- ( = 0.5). (a)Fermi surfaces. (b), (c) Intensity plots in energymomentum space taken with s and p polarizations,
respectively along the -M line shown in (a).

Photon Factory Activity Report 2014 #32 (2015) B

Experimentally derived band dispersions were further
compared to those calculated within density functional
theory (DFT) using a Wien2k program as shown in Figs.
3(b) and (c). Although in the previous study [2] it has
been claimed that the near-EF band structure can beexplained only by Fe 3d orbitals in the paramagnetic statein the stoichiometric sample Sr2VFeAsO3- ( = 0), it is
not the case for the present  = 0.5 sample. Some of thebands can be attributed to Fe 3d orbitals, but the otherbands around the zone center still remain unassigned. One
explanation for the observed complicated band structureis the antiferromagnetic ordering of Fe. According to an
NMR study it has indeed been claimed that Fe is in theantiferromagnetic state below 165 K [4]. However, this isless likely in the present sample because the band
dispersion around the M point, which could be folded in acomplicated way with antiferromagnetic order, can bedescribed by the calculation for the paramagnetic state.
Another possibility is that the V 3d bands were observed
around the zone center. DFT calculation actually supports
the existence of V dYZ/XZ and dXY bands near the zone
center as shown in Fig. 3(c), and the experimentallyobtained bands in the region |k//| < 0.5 (/a) arereasonably reproduced by considering the V dYZ/XZ
orbitals. Two bands which still cannot be assigned ( andin Fig. 3(d)) would be dXY and dYZ/XZ bands, respectively,
which were scattered by magnetic order found in the Vatom [5,6] or oxygen vacancies if they are periodicallyordered.
In conclusion, we have performed ARPES
measurements
ontheweakly
ferromagnetic
superconductor Sr2VFeAsO3- (= 0.5) using different
photon polarizations and observed band dispersions
orbital-selectively. Obtained band structures werereproduced by considering not only Fe 3d but also V 3dbands.
References[1] X. Zhu et al., Phys. Rev. B. 79, 220512(R) (2009).
[2] T. Qian et al., Phys. Rev. B. 83, 140513 (2011).
[3] T. Katagiri et al., 25pEC-2, JPS autumn meeting(2013).
[4] K. Ueshima et al., Phys. Rev. B. 89, 184506 (2014).
[5] M. Tegel et al., Phys. Rev. B 82, 140507(R) (2010).
[6] F. Hummel et al., Phys. Rev. B 88, 144517(R) (2013).
* horio@wyvern.phys.s.u-tokyo.ac.jp

Fig. 3 Comparison between experimental and calculatedband structure. (a) Peak positions in MDCs and EDCs
extracted from Fig. 1(b) and (c). (b), (c) Band dispersion
near EF calculated using a Wien2k program. The
thickness of the band corresponds to the weight of theindicated Fe or V 3d orbital character. Red and blue
thick bands can be observed with s and p polarization,respectively. (d) Schematically drawn experimental
band structure with orbital character assignment.