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, Japan

Fe 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 for the 1^st category, Te substituted FeSe, FeSe_1-xTe_x, in thin film form shows a characteristic T_c behavior as a function of Te content x.  Introducing Te suppresses the nematic transition temperature T₀, which vanishes at around a critical concentration x_c, dependent on the degree of strain, and brings a succeeding sudden increase of T_c (for instance 13 K just below x_c to 23 K just above x_c), followed by a gradual decrease of T_c for further increase of x[1].  This T_c behavior is in contrast to that of 2^nd category[2], where T_c of Te substituted samples monotonously decreases, suggesting the presence of novel important physics.  Since the strain dependences of T_c 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 FeSe_1-xTe_x films provide a unique valuable playground to observe the pure responses of electrons unaffected by lattice.  We have studied FeSe_1-xTe_x (and also FeSe_1-yS_y) PLD grown films by dc transport[7], ARPES[8], optical properties[9], magnetic properties by m-SR[10] and DFT calculation[11] above T_c, and superfluid density and quasi-particle dynamics below T_c[11]. Based on all of these 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 T_c just after the disappearance of the nematic transition at x_c is caused by the change of the carrier density/density of state of Fermi level at x_c, (2) succeeding gradual decease of T_c with further increasing x is due to the increase of electron correlation, (3) the so-called Uemura relation between T_c and the superfluid density is valid irrespective with the presence/absence of the nematic transition, suggesting that the superconducting mechanism is common in a broad sense, (4) superconducting gap structure is different between SC below x_c and above x_c, which might be related to the orbital switch at the Fermi level caused by the upward shift of d_xy orbital with increasing x.  These suggest that the characteristic T_c behavior as a function of x is not in line with a scenario which assumes a quantum 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.)

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