Superconductivity, magnetism and nematicity
in thin films of Fe chalcogenides
A. Maeda, and F. Nabeshima
Department of Basic Science, University of Tokyo, Tokyo 153-8902, JapanFe chalcogenide is unique in the sense that it shows at least three different superconductivities(SCs) in the same material; (1) 10-40 K SC with both of hole and electron Fermi surfaces (FSs),
which is observed for the broadest cases, including physically pressurized cases, (2) 40-50 K
SC with electron FS alone, often realized by the electric field doping and various intercalation
etc., (3) 65 K (or higher) SC in ultrathin films on some oxide substrates. In particular, as forthe 1st category, Te substituted FeSe, FeSe1-xTex, in thin film form shows a characteristic Tc
behavior as a function of Te content x. Introducing Te suppresses the nematic transition
temperature T0, which vanishes at around a critical concentration xc, dependent on the degreeof strain, and brings a succeeding sudden increase of Tc (for instance 13 K just below xc to 23
K just above xc), followed by a gradual decrease of Tc for further increase of x[1]. This Tc
behavior is in contrast to that of 2nd category[2], where Tc of Te substituted samples
monotonously decreases, suggesting the presence of novel important physics. Since the strain
dependences of Tc and T0[3] are completely reproduced in bulk crystals[4,5], data in film
samples should be understood on the same ground in a continuous manner with those of bulk
crystals. On the other hand, we recently found that the nematic transition is purely electronic
without any lattice distortion in film samples[6]. This means that FeSe1-xTex films provide a
unique valuable playground to observe the pure responses of electrons unaffected by lattice.
We have studied FeSe1-xTex (and also FeSe1-ySy) PLD grown films by dc transport[7],ARPES[8], optical properties[9], magnetic properties by µ-SR[10] and DFT calculation[11]above Tc, and superfluid density and quasi-particle dynamics below Tc[11]. Based on all ofthese studies, and keeping the pure electronic aspect of the responses in these epitaxial film
samples in mind, we find that (1) the sudden increase of Tc just after the disappearance of thenematic transition at xc is caused by the change of the carrier density/density of state of Fermi
level at xc, (2) succeeding gradual decease of Tc with further increasing x is due to the increaseof electron correlation, (3) the so-called Uemura relation between Tc and the superfluid density
is valid irrespective with the presence/absence of the nematic transition, suggesting that thesuperconducting mechanism is common in a broad sense, (4) superconducting gap structure isdifferent between SC below xc and above xc, which might be related to the orbital switch at theFermi level caused by the upward shift of dxy orbital with increasing x. These suggest that thecharacteristic Tc behavior as a function of x is not in line with a scenario which assumes aquantum critical point at some x value, and rather consistent with a more conventional scenario.
We also studied SCs in categories 2 and 3, and realized the highest zero resistivity
temperature[2] and the interface superconductivity as the first successful case among PLD
grown films[12].
We will introduce detailed data on these issues in the talk.
[1] Y. Imai et al., Proc. Natl. Acad. Sci. USA 112, 1937 (2015), ibid, Sci. Rep. 7, 46653 (2017) .
[2] N. Shikama et al., Phys. Rev. B 104, 094512 (2021).
[3] F. Nabeshima et al., Jpn. J. Appl. Phys. 57, 120314 (2018).
[4] M. Ghini et al., Phys. Rev. B 103, 205139 (2021).
[5] M. Nakajima et al., Phys. Rev. Mat. 5, 044801 (2021).
[6] Y. Kubota et al., unpublished.
[7] F. Nabeshima et al., Phys. Rev. B 101, 184517 (2020).
[8] K. Nakayama et al., Phys. Rev. Res. 3, L012007 (2021).

[9] M. Nakajima et al., Phys. Rev. B 104, 024512 (2021).
[10] F. Nabeshima et al., Phys. Rev. B 103, 184504 (2021).
[11] H. Kurokawa et al., Phys. Rev. B 104, 014505 (2021).
[12] T. Kobayashi et al., SUST, in press (arXiv. 2203.04534.)