No Arabic abstract
We study the fate of interacting quantum systems which are periodically driven by switching back and forth between two integrable Hamiltonians. This provides an unconventional and tunable way of breaking integrability, in the sense that the stroboscopic time evolution will generally be described by a Floquet Hamiltonian which progressively becomes less integrable as the driving frequency is reduced. Here, we exemplify this idea in spin chains subjected to periodic switching between two integrable anisotropic Heisenberg Hamiltonians. We distinguish the integrability-breaking effects of resonant interactions and perturbative (local) interactions, and illustrate these by contrasting different measures of energy in Floquet states and through a study of level spacing statistics. This scenario is argued to be representative for general driven interacting integrable systems.
We present a variational method for approximating the ground state of spin models close to (Richardson-Gaudin) integrability. This is done by variationally optimizing eigenstates of integrable Richardson-Gaudin models, where the toolbox of integrability allows for an efficient evaluation and minimization of the energy functional. The method is shown to return exact results for integrable models and improve substantially on perturbation theory for models close to integrability. For large integrability-breaking interactions, it is shown how (avoided) level crossings necessitate the use of excited states of integrable Hamiltonians in order to accurately describe the ground states of general non-integrable models.
We investigate the heat conductivity $kappa$ of the Heisenberg spin-1/2 ladder at finite temperature covering the entire range of inter-chain coupling $J_perp$, by using several numerical methods and perturbation theory within the framework of linear response. We unveil that a perturbative prediction $kappa propto J_perp^{-2}$, based on simple golden-rule arguments and valid in the strict limit $J_perp to 0$, applies to a remarkably wide range of $J_perp$, qualitatively and quantitatively. In the large $J_perp$-limit, we show power-law scaling of opposite nature, namely, $kappa propto J_perp^2$. Moreover, we demonstrate the weak and strong coupling regimes to be connected by a broad minimum, slightly below the isotropic point at $J_perp = J_parallel$. As a function of temperature $T$, this minimum scales as $kappa propto T^{-2}$ down to $T$ on the order of the exchange coupling constant. These results provide for a comprehensive picture of $kappa(J_perp,T)$ of spin ladders.
Second-order topological semimetals (SOTSMs) is featured with the presence of hinge Fermi arc. How to generate SOTSMs in different systems has attracted much attention. We here propose a scheme to create exotic SOTSMs by periodic driving. It is found that novel Dirac SOTSMs with a widely tunable number of nodes and hinge Fermi arcs, the adjacent nodes with same chirality, and the coexisting nodal points and nodal loops can be generated at ease by the periodic driving. When the time-reversal symmetry is broken, our scheme also permits us to realize an exotic hybrid-order Weyl semimetals with the coexisting hinge and surface Fermi arcs. The multiplicity of the zero- and $pi/T$-mode Weyl points endows our system more colorful 2D sliced topological phases, which can be any combination of normal insulator, Chern insulator, and SOTI, than the static case. Enriching the family of topological semimetals, our scheme supplies a convenient way to artificially synthesize and control exotic topological phases by periodic driving.
We consider quantum quenches in an integrable quantum chain with tuneable-integrability-breaking interactions. In the case where these interactions are weak, we demonstrate that at intermediate times after the quench local observables relax to a prethermalized regime, which can be described by a density matrix that can be viewed as a deformation of a generalized Gibbs ensemble. We present explicit expressions for the approximately conserved charges characterizing this ensemble. We do not find evidence for a crossover from the prethermalized to a thermalized regime on the time scales accessible to us. Increasing the integrability-breaking interactions leads to a behaviour that is compatible with eventual thermalization.
Starting with Carnot engine, the ideal efficiency of a heat engine has been associated with quasi-static transformations and vanishingly small output power. Here, we exactly calculate the thermodynamic properties of a isothermal heat engine, in which the working medium is a periodically driven underdamped harmonic oscillator, focusing instead on the opposite, anti-adiabatic limit, where the period of a cycle is the fastest time scale in the problem. We show that in that limit it is possible to approach the ideal energy conversion efficiency $eta=1$, with finite output power and vanishingly small relative power fluctuations. The simultaneous realization of all the three desiderata of a heat engine is possible thanks to the breaking of time-reversal symmetry. We also show that non-Markovian dynamics can further improve the power-efficiency trade-off.