Wednesday 29 July 2015 - Jinfeng Jia -Superconductivity in single-layer films of FeSe with a transition temperature above 100 K

Dr. Jia presented very interesting result on high temperature superconductivity in FeSe monolayer thin films grown on Nb doped metallic SrTiO3 grown through molecular beam epitaxy. He performed in-situ 4-probe transport measurements (critical current from I-V characteristics and temperature/magnetic field dependence of resistance) on the FeSe thin films using a specially designed spring loaded 4-probe contact. The separation between the contacts were 10 micron. He setup allowed him to perform measurements on different areas on the surface by moving his contacts around. Many points on the sample (though not all) showed a clear existence of critical current. The Tc was estimated in two different ways: The first was by plotting Ic (not all at the same location) as a function of temperature. The second way was by measuring the resistance versus temperature at a single point. Both measurements showed a very high Tc ~ 110 K, which is an order of magnitude larger than bulk FeSe. Similar measurements in magnetic field also revealed a very high upper critical field. At 96 K the measured Hc2 ~ 8 T. The estimated Hc2 from the slope of Hc2-T close to Tc, was ~115 T.

The results triggered a lot of discussion on what could be origin of such a high critical temperature. It was suggested by the speaker that doping from Nb-doped SrTiO3 could be playing a role. It was also noted that monolayer FeSe films on graphene did not show superconductivity. The other surprising aspect of the result was the  sharpness of the transition in R-T measurement. It was pointed out that for a monolayer films the transition in zero field is expected of the BKT type, and BKT fluctuations should broaden the transition. However, in this meeting similar sharp transition was also shown for interfacial superconductivity in LAO/STO interface (Mannhart). This seeems to be an outstanding puzzle and direct superfluid density measurements could shed light on this issue.

In the last part of the talk Dr.Jia presented scanning tunneling spectroscopy data on Bi2Se3 islands deposited on superconducting NbSe2 crystals. He pointed out some differences of the spacial and field dependence of the zero bias conductance peak observed inside the vortex core, in bare NbSe2 and Bi2Se3-NbSe2. He claimed that this difference was an evidence of Majorana Fermion in the vortex core. However, there is no theoretical model to associate these differences to Majorana Fermion. To this blogger these results looked very tentative at the moment.

Blogged by Pratap Raychaudhuri