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Wednesday, March 14, 2018
2:00 PM - 3:00 PM
Sig Hecker Conference Room, TA-3, Bldg 32, Rm 134

CMS Colloquium

Superconducting instabilities in extreme quantum limit in multiple-band materials near Lifshitz transition

Dr. Alexei Koshelev
Argonne National Laboratory

We investigate the superconducting instability in the magnetic field for a clean multiple-band superconductor in the vicinity of the Lifshitz transition when one of the bands is very shallow. Such a situation is realized in several iron-based superconductors. Due to a small number of carriers in the shallow band, the quasiclassical approximation breaks down, and Landau quantization has to be taken into account. In the two-dimensional case [1], the transition temperature is resonantly enhanced at the magnetic fields corresponding to the matching of the chemical potential with the Landau levels (LLs) in the shallow band. As a consequence, the reentrant superconducting regions emerge near these magnetic fields at low temperatures. The reentrant-region size is the largest for the lowest LL and strongly depends on the shallow-band contribution to the pairing. The Zeeman spin splitting in the shallow band suppresses the reentrant behavior; the separated regions disappear already for very small spin-splitting factors. On the other hand, the reentrance is restored in the resonance cases when the spin-splitting energy exactly matches the LL separation. The behavior is even richer for two-band layered materials [2]. The interlayer tunneling transforms LLs into dispersive minibands along the out-of-plane momentum with two van Hove singularities at the miniband edges. In special situations when either the same or opposite van Hove points simultaneously cross the Fermi level, the pairing is enhanced. Moreover, the latter case may yield the alternating Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in which the order-parameter sign changes from layer to layer. These pairing enhancements also may lead to the reentrant high-field superconductivity. In a generic situation, when the Fermi level crosses the spin-up and spin-down minibands, the FFLO states with a longer modulation period are formed, which also may be reentrant. In general, the quantum effects strongly promote the formation of these FFLO states.

Host: Shizeng Lin