Superconductivity above 100 K in single-layer FeSe films on doped SrTiO3

Abstract

Search for superconductors with a T c above the liquid nitrogen temperature (77 K) led to the discovery of a high-T c cuprate with a T c above 130 K over two decades ago 16 . Even though the value of T c is only 26 K in the first Fe-based superconductor, LaFeAsO (ref. 17) with the earlier work on superconducting oxides interfaces [3][4][5][6] , demonstrates that interface between two different materials provide not only a rich system for studying two-dimensional (2D) superconductivity, but also a potential pathway to high-T c superconductivity [7][8][9][10][11][12] .Indeed, recent angle-resolved photoemission spectroscopy (ARPES) experiments on the FeSe/STO system revealed different electronic structure from those of bulk FeSe and possible occurrence of superconductivity around 65 K (refs. 13,14). An ex situ transport measurements performed on FeSe/STO protected by multiple layers of FeTe and amorphous Si overlay revealed a zero-resistance T c of 23.5 K and an onset T c > 40 K (ref. 15). Evidently the addition of protection layers suppresses superconductivity in single-layer FeSe. In this work, we report electrical transport measurements on single-layer films of FeSe grown on Nb-doped SrTiO 3 substrate using an in situ 4-point probe (4PP) technique. We found that superconductivity could be obtained even at a temperature as high as 109 K.Single-layer films of FeSe were grown on Nb-doped SrTiO 3 (001) surface by the same method as reported previously 1 , employing extra Se flux in an MBE system equipped with STM/STS and 4PP capabilities. The growth process was monitored by reflection high-energy electron diffraction (RHEED) (Fig. 1a), which allows the precise control of film growth needed to achieve one unit-cell thickness as exactly as possible. The crystal nature of the films was confirmed by STM imaging at both large and atomic scales, as shown in Figs grown on a conducting substrate, it does mean zero resistance of the film as the film shorts the conducting substrate. However, when the 4PP detects a finite voltage, it may not necessarily mean that the sample is not superconducting. Indeed, the 4PP technique is a powerful tool for investigating superconductivity in films that cannot be taken out of a UHV system or an interface that is not accessible by surface probes.Two typical I-V curves collected at 3 K in C1423 and C1234 are shown in Figs. 2b and 2c, respectively. The data demonstrate explicitly that the film is superconducting, with the critical current (I c ) defined by the current value for which the superconducting top layer can no longer short the conducting substrate with a finite resistance. Even though interpretation of the finite voltage in the superconducting I-V curves are complicated, the essentially zero voltage seen at low currents cannot be resulted from artificial effects of the contact, since all four tips of the 4PP have Ohmic contacts with the sample individually (Supplementary Information). It is also interesting to note that the extracted I c have similar values for both measurement con...

Interface induced high temperature superconductivity in single unit-cell FeSe on SrTiO3(110)

“…In contrast to irregular bumps observed in Fig. 1(b), the 1-UC FeSe films on insulating STO exhibit nearly ordered stripes with periods consistent with the 6 × 1 and 5 × 1 reconstructions of STO (110) [1][2][3][4][5][6][7][8][9][10] direction, whereas an one-to-one correspondence between FeSe and STO exists along the [001] direction. It is worth noting that either irregular bumps ( Fig.…”

“…The single unit cell (UC) films of FeSe on perovskite SrTiO3(STO)(001) substrate (referred as FeSe/STO(001) hereafter) exhibits probably the highest superconducting transition temperature TC [1][2][3][4][5][6][7][8] among all heterostructure systems discovered so far. A superconducting gap of 20 meV 1 , almost one order of magnitude larger than that of bulk FeSe 9 , was observed by scanning tunneling microscopy (STM).…”

New Superconducting Phase of Lix(C6H16N2)yFe2−zSe2 with Tc = 41 K Obtained through the Post-Annealing

“…6 as a function of d. 1,7,8,10,11,[13][14][15]17,19,20,[23][24][25][26][27] As for the origin of the decrease in T c at d > 9 Å, it may be due to the decrease of the interlayer coupling of cooper pairs. Considering the superconductivity above 100 K in single-layer FeSe films as mentioned above, 6) however, this may be incorrect. According to the prediction by Guterding et al, 18) …”

“…Moreover, a superconducting transition has been detected at a high temperature above 100 K in the electrical transport measurements of the single-layer FeSe film. 6) These results suggest that T c of FeSe is very sensitive to the lattice and that FeSe has a great potential in achieving high T c 's. Through the intercalation of potassium 7) and sodium 8) into FeSe, in fact, the interlayer spacing between neighboring Fe layers, d, has increased from 5.52 Å to 7.02 Å and 6.83 Å, so that T c has increased up to ~ 31 K and ~ 37 K, respectively, while T c of lithium-intercalated FeSe without increase in d has remained 8 K. 9) Further increase in d has been attained through the co-intercalation of alkali or alkaline-earth metals and ammonia, so that d and T c have increased to 7 -11 Å and 31 -46 K in A x (NH 3 ) y Fe 2-z Se 2 (A: alkali or alkaline-earth metals), respectively.…”

Interlayer electron–phonon coupling in WSe2/hBN heterostructures

“…Exploiting interactions between electrons in one layered material and phonons in an adjacent material could enable new ways to control electron-phonon coupling and realize novel quantum behaviour that has not previously been possible. For example, it has been recently shown that electrons in monolayer FeSe can couple strongly with phonons in the adjacent SrTiO 3 substrate, which may play an important role in the anomalously high critical temperature for superconductivity in the system 21,22 . However, the unusual interlayer electron-phonon interactions in the van der Waals heterostructures have been little explored so far, although there have been indications that interlayer interactions between graphene electrons and the substrate phonons is a limiting factor for higher graphene electron mobility at room temperature 23,24 .…”