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Physical review. B./Physical review. B

FFLO state driven by quasiperiodic Zeeman field and its transition to localized states

Xingbo Wei, Wei Kang Zhu

Published Mar 14, 2022• 1 citation
Publisher

Abstract

We numerically study the ground-state properties of a one-dimensional lattice model in the presence of a quasiperiodic Zeeman field, by means of the density matrix renormalization group algorithm. We map out a global phase diagram, which encloses a Bardeen-Cooper-Schrieffer (BCS) phase, a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase where the Cooper pairs obtain a finite center-of-mass momentum, and a localized phase. The FFLO phase is promoted by the quasiperiodic Zeeman field, which demonstrates a power-law pairing correlation with a critical exponent ${\ensuremath{\eta}}_{\mathrm{FFLO}}<1$ decaying much slower than the charge density and spin density correlations. By tuning the interaction and the filling factor, we find the FFLO phase is suppressed in the strong interaction and low filling regions. A large Zeeman field destroys the FFLO state and drives a superconductor-insulator transition. Furthermore, we propose this FFLO phase and the superconductor-insulator transition could be observed in the optical lattice experiment.

DOI
10.1103/physrevb.105.094203

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We numerically study the ground-state properties of a one-dimensional lattice model in the presence of a quasiperiodic Zeeman field, by means of the density matrix renormalization group algorithm. We map out a global phase diagram, which encloses a Bardeen-Cooper-Schrieffer (BCS) phase, a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase where the Cooper pairs obtain a finite center-of-mass momentum, and a localized phase. The FFLO phase is promoted by the quasiperiodic Zeeman field, which demonstrates a power-law pairing correlation with a critical exponent \ensuremathηFFLO<1{\ensuremath{\eta}}_{\mathrm{FFLO}}<1\ensuremathηFFLO​<1 decaying much slower than the charge density and spin density correlations. By tuning the interaction and the filling factor, we find the FFLO phase is suppressed in the strong interaction and low filling regions. A large Zeeman field destroys the FFLO state and drives a superconductor-insulator transition. Furthermore, we propose this FFLO phase and the superconductor-insulator transition could be observed in the optical lattice experiment.
The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase is an unconventional superconducting state found under the influence of a strong Zeeman field. This phase is identified by finite center-of-mass momenta in the Cooper pairs, causing the pairing amplitude to oscillate in real space. Repulsive correlations, on the other hand, smear out spatial inhomogeneities in ddd-wave superconductors. We investigate the FFLO state in a strongly correlated ddd-wave superconductor within a consolidated framework of Hartree-Fock-Bogoliubov theory and the Gutzwiller approximation. We find that the crucial effects of strong correlations lie in shifting the BCS-FFLO phase boundary towards a lower Zeeman field and thereby enlarging the window of the FFLO phase. In the FFLO state, our calculation features a sharp midgap peak in the density of states, indicating the formation of strongly localized Andreev bound states. We also find that the signatures of the FFLO phase survive even in the presence of an additional translational-symmetry-breaking competing order in the ground state. This is demonstrated by considering a broken-symmetry ground state with a simultaneous presence of the ddd-wave superconducting order and a spin-density wave order, often found in unconventional superconductors.
Determining the center-of-mass (COM) momentum of Cooper pairs in unconventional superconductors or superfluids is a topic of great interest in condensed matter physics and ultracold atomic gases. Theoretically, we investigate the dynamical excitations of a two-dimensional spin-polarized attractive Hubbard model on a square optical lattice under an effective Zeeman field by computing the density and spin dynamical structure factors, focusing on phase transition from a Bardeen-Cooper-Schrieffer (BCS) superfluid to a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid. In the FFLO superfluid, besides the phonon mode in the density channel, a low-energy bogolon mode emerges in the spin channel, which is associated with Bogoliubov quasiparticles on a Bogoliubov Fermi surface. Moreover, the dynamical excitations exhibit pronounced anisotropy in momentum space due to the finite COM momentum. At half filling, the roton mode around [ π , π ] evolves from a point-like minimum into a ring structure shifted by the COM momentum across the BCS-FFLO transition, providing a roton-based protocol to extract the COM momentum. These predictions provide key insights for confirming the existence of FFLO superfluids and understanding their dynamical excitation spectra.
We examine the existence and characteristics of the exotic Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase in a one-dimensional Fermi gas with attractive Hubbard interactions in the presence of spin-orbit coupling (SOC) and Zeeman field. We show that a robust FFLO phase can be created in the presence of attractive on-site interactions and Zeeman field, and that the addition of SOC suppresses the FFLO order and enhances the pair formation. In the absence of SOC, the system shows four phases: Bardeen-Cooper-Schrieffer (BCS), FFLO, multimode pairing, and fully polarized phases by tuning the Zeeman field hhh. The quantum transition between these phases is discontinuous with respect to hhh. In the presence of SOC, the transition from the BCS to FFLO phase becomes continuous. We present a complete phase diagram of this model both in the presence and in the absence of SOC at a quarter electron filling and also explore the effect of SOC on the FFLO phase.
We examine the existence and characteristics of the exotic Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase in a one-dimensional Fermi gas with attractive Hubbard interactions, in the presence of spin-orbit coupling (SOC) and Zeeman field. We show that a robust FFLO phase can be created in the presence of attractive on-site interactions and Zeeman field, and that the addition of SOC suppresses the FFLO order and enhances the pair formation. In absence of SOC, the system shows four phases: Bardeen-Cooper-Schrieffer (BCS), FFLO, multi- mode pairing and fully polarized phases by tuning the Zeeman field h, and the quantum transition between these phases is discontinuous with respect to h. In the presence of SOC, the transition from the BCS to FFLO phase becomes continuous. We present a complete phase diagram of this model both in the presence and in the absence of SOC at quarter electron filling and also explore the effect of SOC on the FFLO phase.
ddd-wave FFLO state and charge-2e supersolidity in the ttt-t′t't′-JJJ model under Zeeman fields
Unconventional superconductivity under strong Zeeman fields--particularly beyond the Pauli paramagnetic limit--remains a central challenge in condensed matter physics. The exotic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in particular, remains in need of definitive study within fundamental electronic models. Here we employ state-of-the-art finite-temperature and ground-state tensor network approaches to systematically explore the superconducting (SC) phase diagram of the ttt-t′t't′-JJJ model subjected to Zeeman fields. We find that zero-momentum ddd-wave superconductivity persists until the spin gap closes, coexisting with charge density waves. A novel ddd-wave FFLO phase emerges under a higher Zeeman field even above the Pauli limit, concomitant with a field-enhanced spin density waves. We identify these phases, characterized by the simultaneous presence of pairing condensate and density wave orders, as charge-2e supersolids. Analysis of Matsubara Green's function reveals that the FFLO pairing momentum is locked to the underlying Fermi surface. Our results provide microscopic insights into field-induced unconventional pairing mechanisms and reveal the long-sought FFLO state in a fundamental correlated electron model, offering a promising route for its realization in ultracold atom optical lattice.
We study the interplay between the long- and short-range interaction of a one-dimensional optical lattice system of two-component dipolar fermions by using the density matrix renormalization group method. The atomic density profile, pairing-pairing correlation function, and compressibility are calculated in the ground state, from which we identify the parameter region of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing state, half-metal (HM) state, FFLO-HM state, and the normal polarized state, and thus the phase diagram in the coordinates of the long- and short-range interaction strength. The effect of the long-range dipolar interaction on the FFLO state is discussed in detail. We find that the long-range part of the dipole-dipole interaction does not sweep away the FFLO superconducting region that is driven by the short-range interaction in the Hubbard model, and thus the FFLO state survives in the wide parameter space of the long-range interaction, polarization, and filling.