33 research outputs found
Quantum FFLO state in clean layered superconductors
We investigate the influence of Landau quantization on the superconducting
instability for a pure layered superconductor in the magnetic field directed
perpendicular to the layers. We demonstrate that the quantization corrections
to the Cooper-pairing kernel with finite Zeeman spin splitting promote the
formation of the nonuniform state in which the order parameter is periodically
modulated along the magnetic field, i.e., between the layers
(Fulde-Ferrell-Larkin-Ovchinnikov [FFLO] state). The conventional uniform state
experiences such a quantization-induced FFLO instability at low temperatures
even in a common case of predominantly orbital suppression of superconductivity
when the Zeeman spin splitting is expected to have a relatively weak effect.
The maximum relative FFLO temperature is given by the ratio of the
superconducting transition temperature and the Fermi energy. This maximum is
realized when the ratio of the spin-spitting energy and the Landau-level
separation is half-integer. These results imply that the FFLO states may exist
not only in the Pauli-limited superconductors but also in very clean materials
with small Zeeman spin-splitting energy. We expect that the described
quantization-promoted FFLO instability is a general phenomenon, which may be
found in materials with different electronic spectra and order-parameter
symmetries.Comment: 18 pages, 10 figures, minor correction
Interplay between orbital-quantization effects and the Fulde-Ferrell-Larkin-Ovchinnikov instability in multiple-band layered superconductors
We explore superconducting instability for a clean two-band layered
superconductor with deep and shallow bands in the magnetic field applied
perpendicular to the layers. In the shallow band, the quasiclassical
approximation is not applicable, and Landau quantization has to be accounted
for exactly. The electronic spectrum of this band in the magnetic field is
composed of the one-dimensional Landau-level minibands. With increasing
magnetic field the system experiences series of Lifshitz transitions when the
chemical potential enters and exits the minibands. These transitions profoundly
influence the shape of the upper critical field at low temperatures. In
addition, the Zeeman spin splitting may cause the nonuniform state with
interlayer modulation of the superconducting order parameter
(Fulde-Ferrell-Larkin-Ovchinnikov state). Typically, the quantization effects
in the shallow band strongly promote the formation of this state. The uniform
state remains favorable only in the exceptional resonance cases when the
spin-splitting energy exactly matches the Landau-level spacing. Furthermore,
for specific relations between electronic spectrum parameters, the alternating
FFLO state may realize, in which the order parameter changes sign between the
neighboring layers. For all above cases, the reentrant high-field
superconducting states may emerge at low temperatures if the shallow band has
significant contribution to the Cooper pairing.Comment: 25 pages, 14 figures, minor revisions and more references adde
Strong Landau-quantization effects in high-magnetic-field superconductivity of a two-dimensional multiple-band metal near the Lifshitz transition
We investigate the onset of superconductivity in magnetic field for a clean
two-dimensional multiple-band superconductor in the vicinity of the Lifshitz
transition when one of the bands is very shallow. Due to small number of
carriers in this band, the quasiclassical Werthamer-Helfand approximation
breaks down and Landau quantization has to be taken into account. We found that
the transition temperature TC2(H) has giant oscillations and is resonantly
enhanced at the magnetic fields corresponding to full occupancy of the Landau
levels in the shallow band. This enhancement is especially pronounced for the
lowest Landau level. As a consequence, the reentrant superconducting regions in
the temperature-field phase diagram emerge at low temperatures near the
magnetic fields at which the chemical potential matches the Landau levels. The
specific behavior depends on the relative strength of the intraband and
interband pairing interactions and the reentrance is most pronounced in the
purely interband coupling scenario. The reentrant behavior is suppressed by the
Zeeman spin splitting in the shallow band, 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 separation between the Landau levels. The predicted
behavior may realize in the gate-tuned FeSe monolayer.Comment: 23 pages, 9 figures, more references added and one figure adde
Vortex matter in layered superconductors without Josephson coupling: numerical simulations within a mean-field approach
We study vortex matter in layered superconductors in the limit of zero Josephson coupling. The long range of the interaction between pancake vortices in the c direction allows us to employ a mean-field method: all attractive interlayer interactions are reduced to an effective substrate potential, which pancakes experience in addition to the same-layer pancake repulsion. We perform numerical simulations of this mean-field model using two independent numerical implementations with different simulation methods (Monte Carlo sampling and Langevin molecular dynamics). The substrate potential is updated self-consistently from the averaged pancake density. Depending on temperature, this potential converges to a periodic profile (crystal) or vanishes (liquid). We compute thermodynamic properties of the system, such as the melting line, the instability line of the crystal, and the entropy jump across the melting transition. The simulation results are in good agreement with approximate analytical calculations