Electronic supplementary information
Coexistence of localized and itinerant electronic states in the multiband FeSe0.42Te0.58
superconductor
D. Arčon,1, 2 P. Jeglič,1 A. Zorko,1 A. Potočnik,1 A. Y. Ganin,3 Y. Takabayashi,4 M. J.
Rosseinsky,3 and K. Prassides4
1
2
Institute "Jozef Stefan", Jamova 39, 1000 Ljubljana, Slovenia
Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana,
Slovenia
3
Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK
4
Department of Chemistry, Durham University, Durham DH1 3LE, UK
Sample preparation and characterization
Synthesis
In an Ar glove box, 510 mg of Fe2TeSe precursor prepared as described elsewhere [1] was
loaded into an 8 mm silica ampoule along with a piece of previously degassed amorphous
carbon (the purpose of the amorphous carbon is discussed in [2]) which was kept separately
from the precursor. The ampoule was sealed under dynamic vacuum (10-5 mbar), jacketed into
a 12 mm ampoule, placed into a muffle oven and heated at 5oC/min to 1075oC. After 48 hours
of reaction, it was cooled down to 420oC at 5oC/min, taken from the oven and quenched in
water. The ampoule was opened in the glove box. 400 mg of the reground product was mixed
with 1.8 g of KCl/NaCl = 1:1 and loaded into 8 mm silica ampoule. As before, a piece of
amorphous carbon was placed separately from the reaction mixture into the ampoule. The
ampoule was sealed under dynamic vacuum (10-5 mbar), additionally jacketed into a 12 mm
1
ampoule and placed at 750oC into an oven followed by heating at 10oC/min to 800oC. After
48 hrs of reaction, the oven was cooled down at 10oC/min to 420 oC, the ampoule was taken
out of the oven and quenched into water. The ampoule was opened in air and a large single
crystal (10 mm  5 mm  3 mm) was extracted along with smaller shiny crystallites by
repeated washing with distilled water. The crystal was dried under vacuum and kept under
inert conditions to avoid potential oxidation in air.
Characterization
The quality and composition of the sample were checked prior and after the magnetic
resonance experiments. A part of the single-crystalline sample was investigated by single
crystal X-ray diffraction. The crystal structure was solved in space group P4/nmm with a =
3.7921(12) Å and c = 5.964(2) Å, V = 85.77(5) Å3 at 100(2) K. No additional electronic
density, which would indicate the presence of Fe atom interstitials, Fe(2) on the 2c sites, was
observed. The refined stoichiometry of the sample was Fe1.0Se0.42(2)Te0.58(2).
A part of the single-crystalline sample, which was used in the NMR and EPR experiments,
was also crushed into a powder and subsequently investigated by high-resolution synchrotron
X-ray powder diffraction ( = 0.40004 Å, ambient temperature, beamline ID31, ESRF,
Grenoble). Data analysis was performed with the GSAS suite of Rietveld refinement
programs. The sample was found to consist of a majority (96.423(6) %) tetragonal phase with
composition consistent to that derived by the single-crystal XRD study (space group P4/nmm,
a = 3.80268(2) Å and c = 6.02593(6) Å). Two impurity minority phases were identified: a hcp
FeSe phase (1.27(2) %) with a = 3.7246(3) Å and c = 5.723(1) Å together with a small
fraction of unreacted elemental Se (2.31(4) %) (Fig. S1).
Complementary energy dispersive x-ray analysis (EDX) and topographic STM images of
cleaved surfaces have been taken in order to confirm the stoichiometry of the same single
crystal that was also used in ARPES measurements reported before [3]. The composition of
the sample established in this way is Fe1.001Se0.44(1)Te0.56(1).
The bulk magnetic susceptibility, S, was measured with a commercial Quantum Design
MPMS system on a 0.547 mg single crystal and with the magnetic field applied along the
crystal c-axis. S was determined by taking the difference between measurements performed
in applied fields of 3 and 2 T in order to subtract the contribution from ferromagnetic
impurities.
2
Electron paramagnetic resonance (EPR) experiments
Continuous-wave X-band (9.4 GHz) EPR experiments were conducted with a commercial
Bruker E580 spectrometer. An ESR900 Oxford Cryogenics cryostat and an ITC502
temperature controller were used for cooling the sample with a temperature stability better
than ±0.1 K over the entire temperature range (4–300 K). A Varian dual rectangular cavity (E232) operating in the TE104 mode with a nominal frequency of 9.6 GHz and unloaded Q value
of about 4000 was used in these experiments. In the course of the temperature dependent EPR
measurements, a strong pitch standard was placed in the second coupled cavity and kept at
room temperature. Regularly, spectra of the strong pitch standard were taken and used to
correct the EPR spin susceptibility for the temperature dependence of the Q value of the
resonator. Samples were loaded into a standard 4-mm-diameter silica tube (Wilmad Lab
Glass) and sealed under dynamic vacuum.
Various very thin flakes of the Fe1.0Se0.42Te0.58 single crystal with typical mass of ~0.1
mg were exfoliated from the bulk crystal in an Ar-filled glove box. Their EPR spectra were
practically unchanged from that of the bigger samples (~1-5 mg) exposed to air. The EPR
spectra were fitted with the Dysonian lineshape taking into account both resonances at +Bres
and - Bres,
dI
d  B  2r B  Bres  B  2r B  Bres  

.


d B d B  4B  Bres 2  B 2 4B  Bres 2  B 2 
The magnitude of dispersion admixed into the absorption spectrum was angularly dependent
and amounted to r = 0 – 0.07.
The signal intensity, which is a measure of the spin susceptibility even in the metallic
phase [4], was calibrated at room temperature with the CuSO4·5H2O standard. In principle,
the estimated EPR susceptibility at room temperature, EPR = 1.3(5)·10-3 emu/mol could be
either intrinsic or extrinsic. We can exclude the following extrinsic cases:
a) Isolated Fe impurities: These could explain the strong temperature dependence of the
EPR intensity. However, assuming that the room-temperature susceptibility value,
EPR = 1.3(5)·10-3 emu/mol was of Curie origin, the required amount of the impurity
phase would be at least 9% in the case of Fe2+ (S=5/2) spins. This is significantly
higher than the amount of detected Fe interstitials (<<0.1%), or any other extrinsic Fephases.
3
b) Fe-oxide ferromagnetic impurities: Fe-oxide ferromagnetic impurities are believed to
be very effectively removed by adding amorphous carbon into the ampoule during the
synthesis procedure [2]. Evaluation of bulk magnetic properties of the investigated
sample confirms this. Namely, typical values of the saturation magnetization in Feoxides range between 1-100 emu/g [5], the former value corresponding to goethite and
the latter value corresponding to the most magnetic magnetite. The magnetization of
our sample in the EPR measurements was EPRBres= 0.028 emu/g. To explain this
magnetization the required amount of impurity Fe-oxide would range between 0.03%3%. No such phase was experimentally observed. Furthermore, even for the case of
magnetite, we do not expect any appreciable temperature dependence of the EPR
intensity since the magnetization would be saturated in the field of Bres=3400 G.
c) Hexagonal Fe1-xSe phases - detected in the XRD analysis of the present sample - can
be ferromagnetic with Curie temperature above room temperature for a narrow range
of x close to 0.5 [6]. However, since the minority phase here is close to stoichiometric
FeSe and the magnetization of any FM impurities in this phase field is expected to be
already saturated in the temperature and field range of our experiments, we conclude
that they are unlikely to be the origin of the EPR signal which shows such strong lowtemperature dependence and scales so well with the NMR results.
Nuclear magnetic resonance (NMR) experiments
125
Te and
77
Se (both nuclei with I = 1/2) NMR experiments were conducted in a magnetic
field 9.4 T. The reference frequencies of (125Te) = 126.193 MHz and (77Se) = 76.282 MHz
were determined from Me2Te and Me2Se standards, respectively. The
125
Te frequency-swept
spectra (Fig. 2a) were measured with Hahn echo pulse sequence  –  – 2 –  – echo, a pulse
length  = 4 s, interpulse delay  = 50 s and repetition time 50 ms at room temperature.
Due to the low NMR sensitivity of
77
Se the Carr-Purcell spin-echo technique with interpulse
delay  = 60 s was used to measure the temperature dependence of the
77
Se NMR spectra
(Fig. 2b). This technique also enabled us to extract frequency-dependent spin-spin relaxation
time T2 (Fig. S2). In addition we measured also T2 for 125Te at room temperature and found it
to be
125
T2=300 s, i.e. nearly identical to
125
T1 values. Both
77
T2 and
125
T2 are over entire
temperature range longer than used interpulse delay  and thus cannot account for reduced
line width at high temperatures (Fig. 2a). We also stress, that both
125
Te and
77
Se NMR
4
spectra become narrower at low temperatures (Fig. 2a,b) and T2 data (Fig. S2) imply that
surprising lineshape narrowing below TC is intrinsic and not simply a result of T2-weighted
contrast effect.
The spin lattice relaxation rates 1/125T1 and 1/77T1 were measured with inversion-recovery
technique at the NMR lineshape peak positions (Fig. 2a,b), which also corresponded to the
extracted Knight shifts (Fig. 2c). Additionally, using the same technique we measured spin
lattice relaxation rate 1/77T1 at different positions of the 77Se NMR lineshape (Fig. 4b) in order
to demonstrate its strong frequency dependence.
The above experiments enable us to simultaneously compare the temperature dependence of
bulk magnetic susceptibility, S, EPR spin susceptibility, EPR, EPR g-factor measured
relative to the electron free value g0=2.0023 and the
77
Se NMR Knight shift (Fig. S3). It is
evident that all these parameters do not scale with each other thus supporting the picture with
coexisting localized and itinerant states in FeSe0.42Te0.58 sample.
5
FIG. S1: Final observed (red) and calculated (blue line) synchrotron X-ray powder diffraction
profile ( = 0.40004 Å) for the crushed Fe1.0Se0.42(2)Te0.58(2) single crystal at ambient
temperature. The lower green line shows the difference profile, and the ticks show the
reflection positions of the tetragonal FeSe0.42Te0.58 (top), hcp FeSe (middle, 1.27(2)%), and
elemental Se (bottom, 2.31(4)%) phases. Inset: expanded view of the diffraction profile in the
angular range 5-12.5°.
6
FIG S2: (a)
77
Se NMR spectra at selected temperatures and (b) frequency dependence of
1/77T2 measured at different points of the line.
7
FIG S3: Overplot of the temperature dependence of bulk magnetic susceptibility (solid black
line), EPR spin susceptibility (solid circles), EPR g-factor measured relative to the electron
free value g0=2.0023 (open blue circles) and the 77Se NMR Knight shift (red solid diamonds).
References:
[1] N.C. Gresty, Y. Takabayashi, A. Y. Ganin, M.T. McDonald, J.B. Claridge, D. Giap, Y.
Mizuguchi, Y. Takano, T Kagayama, Y. Ohishi, M. Takata, M. J. Rosseinsky, S.
Margadonna, and K. Prassides, J. Am. Chem. Soc. 131, 16944 (2009).
[2] T. M.McQueen, Q.Huang, V. Ksenofontov,C. Felser, Q. Xu, H. Zandbergen, Y.S. Hor, J.
Allred, A.J. Williams, D. Qu, J. Checkelsky, N.P. Ong, R.J. Cava, Physical Review B79,
014522 (2009).
[3] A. Tamai et al., Phys. Rev. Lett. 104, 097002 (2010).
[4] See for instance D. Arčon and K. Prassides, Structure and Bonding, Vol. 100, Editor J.
Veciana, Springer-Verlag, Berlin-Heidelberg (2001).
[5] R. M. Cornel and U. Schwertmann, The Iron Oxides (Wiley-VCH, Weinheim, 2003).
[6] See for instance T. Kamimura et al., J. Phy. Soc. Jpn. 45, 1769 (1978).
8