Masaya Nakagawa

Laser-Induced Kondo Effect in Ultracold Alkaline-Earth Fermions.

We demonstrate that laser excitations can coherently induce a novel Kondo effect in ultracold atoms in optical lattices. Using a model of alkaline-earth fermions with two orbitals, it is shown that the optically coupled two internal states are dynamically entangled to form the Kondo-singlet state, overcoming the heating effect due to the irradiation. Furthermore, a lack of SU(N) symmetry in the optical coupling provides a peculiar feature in the Kondo effect, which results in spin-selective renormalization of effective masses. We also discuss the effects of interorbital exchange interactions, and reveal that they induce novel crossover or reentrant behavior of the Kondo effect owing to control of the coupling anisotropy. The laser-induced Kondo effect is highly controllable by tuning the laser strength and the frequency, and thus offers a versatile platform to study the Kondo physics using ultracold atoms.

Laser-irradiated Kondo insulators: Controlling the Kondo effect and topological phases

We investigate theoretically the nature of laser-irradiated Kondo insulators. Using Floquet theory and the slave-boson approach, we study a periodic Anderson model and derive an effective model that describes laser-irradiated Kondo insulators. In this model, we find two generic effects induced by laser light. One is dynamical localization, which suppresses hopping and hybridization. The other is laser-induced hopping and hybridization, which can be interpreted as synthetic spin-orbit coupling or a magnetic field. The first effect drastically changes the behavior of the Kondo effect. In particular, the Kondo effect under laser light qualitatively changes its character depending on whether the hybridization is on-site or off-site. The second effect triggers topological phase transitions. In topological Kondo insulators, linearly polarized laser light realizes phase transitions between trivial, weak topological, and strong topological Kondo insulators. Moreover, circularly polarized laser light breaks time-reversal symmetry and induces Weyl semimetallic phases. Our results make it possible to dynamically control the Kondo effect and topological phases in heavy-fermion systems. We also discuss experimental setups to detect the signatures.

Bosonic integer quantum Hall effect as topological pumping

Based on a quasi-one-dimensional limit of quantum Hall states on a thin torus, we construct a model of interaction-induced topological pumping which mimics the Hall response of the bosonic integer quantum Hall (BIQH) state. The quasi-one-dimensional counterpart of the BIQH state is identified as the Haldane phase composed of two-component bosons which form effective spin-

Nonequilibrium topological phase transitions in two-dimensional optical lattices

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Symmetry-protected topological phase transition in one-dimensional Kondo lattice and its realization with ultracold atoms

degrees of freedom. An adiabatic change between the Haldane phase and trivial Mott insulators constitute {\it off-diagonal} topological pumping in which the translation of the lattice potential for one component induces a current in the other. The mechanism of this pumping is interpreted in terms of changes in polarizations between symmetry-protected quantized values.

Laser-induced phase transitions of topological Kondo insulators

Recently, concepts of topological phases of matter are extended to nonequilibrium systems, especially periodically driven systems. In this paper, we construct an example which shows nonequilibrium topological phase transitions using ultracold fermions in optical lattices. We show that the Rabi oscillation has the possibility to induce nonequilibrium topological phases which are classified into time-reversal-invariant topological insulators for a two-orbital model of alkaline-earth-metal atoms. Furthermore, we study the nonequilibrium topological phases using time-dependent Schrieffer\char21{}Wolff-type perturbation theory, and we obtain an analytical expression to describe the topological phase transitions from a high-frequency limit of external driving fields.

Dynamically Induced Topological Properties in Two-Dimensional Optical Lattices

We propose that ultracold alkaline-earth-like atoms confined in one-dimensional optical lattice can realize a Kondo lattice model which hosts a symmetry-protected topological phase and an associated quantum phase transition in a controllable manner. The symmetry protection of the phase transition is discussed from two different viewpoints: topological properties related to spatial patterns of Kondo singlets, and symmetry eigenvalues of the spin states. We uncover the role of various symmetries in the phase diagram of this system by combining a weak-coupling approach by Abelian bosonization and strong-coupling pictures of ground states. Furthermore, in the bosonization language, we elucidate how the charge degrees of freedom of the Kondo lattice change the nature of the symmetry-protected topological phase from bosonic spin systems to interacting fermions.

Spatial-Translation-Induced Discrete Time Crystals

Abstract In this paper, we theoretically investigate how laser fields change the nature of topological Kondo insulators(TKIs). By employing a prototypical model of TKIs, we treat the effect of the laser fields with Floquet theory, which gives effective description under high frequency laser fields. We derive the effective model of TKIs under the laser irradiation and discuss its topological properties. We demonstrate a possible realization of Floquet Chern insulators specified by various values of Chern number and reveal how the topological phase changes with increasing the laser light intensity. Furthermore, it is shown that Floquet Weyl semimetals, which have some pairs of Weyl nodes protected topologically, can emerge in the three-dimensional case. We explain how the Weyl nodes are created with varying the strength of the laser field.

Breakdown of topological Thouless pumping in the strongly interacting regime

Topological properties of matter are one of hot topics in condensed matter physics. Recently, these concepts are extended to non-equilibrium systems, especially periodically driven systems. Here we study such non-equilibrium topological quantum phenomena in cold atomic systems in twodimensional optical lattices. First we show results on non-interacting atoms, and then focus on interaction effects on the non-equilibrium topological phases based on time-dependent Schrieffer-Wolfftype perturbation theory.