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arXiv:1611.10267 · Physical Review A

Unconventional fermionic pairing states in a monochromatically tilted optical lattice

Alberto Nocera, Anatoli Polkovnikov, Adrian E. Feiguin

Published Feb 1, 2017• 11 citations
arXivHTMLPDF

Abstract

We study the one-dimensional attractive fermionic Hubbard model under the influence of periodic driving with the time-dependent density matrix renormalization group method. We show that the system can be driven into an unconventional pairing state characterized by a condensate made of Cooper pairs with a finite center-of-mass momentum similar to a Fulde-Ferrell state. We obtain results both in the laboratory and the rotating reference frames demonstrating that the momentum of the condensate can be finely tuned by changing the ratio between the amplitude and the frequency of the driving. In particular, by quenching this ratio to the value corresponding to suppression of the tunneling and the Coulomb interaction strength to zero, we are able to ``freeze'' the condensate. We finally study the effects of different initial conditions and compare our numerical results to those obtained from a time-independent Floquet theory in the large frequency regime. Our work offers the possibility of engineering and controlling unconventional pairing states in fermionic condensates.

DOI
10.1103/physreva.95.023601

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We study the one-dimensional attractive fermionic Hubbard model under the influence of periodic driving with the time-dependent density matrix renormalization group method. We show that the system can be driven into an unconventional pairing state characterized by a condensate made of Cooper pairs with a finite center-of-mass momentum similar to a Fulde-Ferrell state. We obtain results both in the laboratory and the rotating reference frames demonstrating that the momentum of the condensate can be finely tuned by changing the ratio between the amplitude and the frequency of the driving. In particular, by quenching this ratio to the value corresponding to suppression of the tunneling and the Coulomb interaction strength to zero, we are able to ``freeze'' the condensate. We finally study the effects of different initial conditions and compare our numerical results to those obtained from a time-independent Floquet theory in the large frequency regime. Our work offers the possibility of engineering and controlling unconventional pairing states in fermionic condensates.
We theoretically investigate the behavior of a system of fermionic atoms loaded in a bipartite one-dimensional optical lattice that is under the action of an external time-periodic driving force. By using Floquet theory, an effective model is derived. The bare hopping coefficients are renormalized by zeroth-order Bessel functions of the first kind with different arguments for the nearest-neighbor and next-nearest-neighbor hopping. The insulating behavior characterizing the system at half filling in the absence of driving is dynamically suppressed, and for particular values of the driving parameter the system becomes either a standard metal or an unconventional metal with four Fermi points. The existence of the four-Fermi-point metal relies on the fact that, as a consequence of the shaking procedure, the next-nearest-neighbor hopping coefficients become significant compared to the nearest-neighbor ones. We use the bosonization technique to investigate the effect of on-site Hubbard interactions on the four-Fermi-point metal-insulator phase transition. Attractive interactions are expected to enlarge the regime of parameters where the unconventional metallic phase arises, whereas repulsive interactions reduce it. This metallic phase is known to be a Luther-Emery liquid (spin-gapped metal) for both repulsive and attractive interactions, contrary to the usual Hubbard model, which exhibits a Mott-insulator phase for repulsive interactions. Ultracold fermions in driven one-dimensional bipartite optical lattices provide an interesting platform for the realization of this long-studied four-Fermi-point unconventional metal.
In the strongly interacting limit of the Hubbard model localized double occupancies form effective hard-core bosonic excitations, called doublons, which are long lived due to energy conservation. Using time-dependent density-matrix renormalization group methods we investigate numerically the dynamics of doublons arising from the sudden expansion of a spatially confined band-insulating state in one spatial dimension. By analyzing the occupation scaling of the natural orbitals within the many-body state, we show that doublons dynamically quasicondense at the band edges, consistent with the spontaneous emergence of an $\ensuremath{\eta}$ quasicondensate. Building on this, we study the effect of periodically driving the system during the expansion. Floquet analysis reveals that doublon hopping and doublon repulsion are strongly renormalized by the drive, breaking the $\ensuremath{\eta}\text{\ensuremath{-}}\mathrm{SU}(2)$ symmetry of the Hubbard model. Numerical simulation of the driven expansion dynamics demonstrates that the momentum in which doublons quasicondense can be controlled by the driving amplitude. These results point to new pathways for engineering nonequilibrium condensates in fermionic cold-atom experiments and are potentially relevant to driven solid-state systems.
Abstract We here present an experimentally feasible proposal for manipulating Cooper pairs in degenerate Fermi gases trapped by an optical lattice. Upon introducing an in situ periodically driven field, the system may be described by an effective time-independent Hamiltonian, in which the Cooper pairs, generated by the bound molecule state in Feshbach resonance, host a nonzero center-of-mass momentum. The system thus processes a crossover from a Bardeen–Cooper–Schrieffer superfluid phase to a Fulde–Ferrell (FF) one. Furthermore, the magnitude and direction of the Cooper pairs in the synthetic FF superfluids are both directly controllable via the periodically driven field. Our proposal offers a reliable and feasible scenario for manipulating the Cooper pairs in cold atoms, serving as a tunable as well as powerful platform for quantum-emulating and exploring the FF superfluid phase.
We propose a novel possibility of dynamically changing the pairing of superconductors from sss wave to $\ensuremath{\eta}$ pairing (where the pairs condense at the Brillouin-zone corner momenta) by driving the system with ac fields. We consider a periodically-driven attractive Hubbard model in the strong-coupling regime and show that the pair-hopping and pair-repulsion terms in the effective Hamiltonian in the Floquet formalism are drastically renormalized in different manners between the two terms, which can change the ground states from an sss-wave superconductivity to an $\ensuremath{\eta}$-pairing superconductivity or a charge-ordered phase. While in isolated systems such as cold atoms a simple quench scheme would not realize the dynamical phase transition into $\ensuremath{\eta}$ pairing, we show that there are pathways that realize the dynamical transition, where the field amplitude is varied via a charge-ordered phase as an intermediate state.
We study the ground-state properties of a two-component one-dimensional system of a few ultra-cold fermions with attractive interactions. We show that, by ramping up an external potential barrier felt by one of the components, it is possible to induce regions of exotic superfluid phases, characterized by a tunable finite net momentum of the Cooper pair, without changing the overall spin populations. We show that these phases, which are the few-body analogs of the celebrated Fulde-Ferrell-Larkin-Ovchinnikov state, can be distinguished by analyzing a specific two-particle correlation encoded in the noise correlation function. Our theoretical results can be addressed in current experiments with cold atoms confined in spin-selective optical traps.
We study the properties of a one-dimensional (1D) gas of fermions trapped in a lattice by means of the density matrix renormalization group method, focusing on the case of unequal spin populations, and strong attractive interaction. In the low-density regime, the system phase separates into a well-defined superconducting core and a fully polarized metallic cloud surrounding it. We argue that the superconducting phase corresponds to a 1D analog of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, with a quasicondensate of tightly bound bosonic pairs with a finite center-of-mass momentum that scales linearly with the magnetization. In the large density limit, the system allows for four phases: in the core, we either find a Fock state of localized pairs or a metallic shell with free spin-down fermions moving in a fully filled background of spin-up fermions. As the magnetization increases, the Fock state disappears to give room for a metallic phase, with a partially polarized superconducting FFLO shell and a fully polarized metallic cloud surrounding the core.