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arXiv:2110.00591 · Physical Review Research

Criticality and rigidity of dissipative discrete time crystals in solids

Koki Chinzei, Tatsuhiko N. Ikeda

Published Apr 8, 2022• 9 citations
arXivHTMLPDF

Abstract

We consider a dissipative quantum Ising model periodically driven by a train of $\ensuremath{\pi}$ pulses and investigate dissipative discrete time crystals (DTCs) in solids. In this model, the interaction between the spins spontaneously breaks the discrete time translation symmetry, giving rise to a dissipative DTC, where two ferromagnetic states are switched alternately by each pulse. We microscopically describe the generic dissipation due to thermal contact to an equilibrium heat bath using the Bloch-Redfield equation. In contrast to other DTC studies, this dissipation stabilizes, rather than destroys, the DTC order without fine-tuning as long as the temperature is low enough. Invoking the time-dependent mean-field theory and solving self-consistently the periodic drive, dissipation, and DTC order parameter, we investigate the nonequilibrium DTC phase transition and determine the critical exponents, including a dynamical one. We also find phase transitions without an equilibrium counterpart: a nontrivial interplay of the periodic drive and dissipation gives rise to reentrant DTC transition when changing the pulse interval at a fixed temperature. Also, to demonstrate the rigidity of the DTC, we consider imperfect $\ensuremath{\pi}$ pulses, showing that the DTC is robust against the small imperfections and finding that discrete time quasicrystals can appear for the larger imperfections. Together with experimental proposals in magnetic materials, our results pave the way for realizing the DTC and for uncovering nonequilibrium critical phenomena in real solid-state materials.

DOI
10.1103/physrevresearch.4.023025

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We consider a dissipative quantum Ising model periodically driven by a train of \ensuremathπ\ensuremath{\pi}\ensuremathπ pulses and investigate dissipative discrete time crystals (DTCs) in solids. In this model, the interaction between the spins spontaneously breaks the discrete time translation symmetry, giving rise to a dissipative DTC, where two ferromagnetic states are switched alternately by each pulse. We microscopically describe the generic dissipation due to thermal contact to an equilibrium heat bath using the Bloch-Redfield equation. In contrast to other DTC studies, this dissipation stabilizes, rather than destroys, the DTC order without fine-tuning as long as the temperature is low enough. Invoking the time-dependent mean-field theory and solving self-consistently the periodic drive, dissipation, and DTC order parameter, we investigate the nonequilibrium DTC phase transition and determine the critical exponents, including a dynamical one. We also find phase transitions without an equilibrium counterpart: a nontrivial interplay of the periodic drive and dissipation gives rise to reentrant DTC transition when changing the pulse interval at a fixed temperature. Also, to demonstrate the rigidity of the DTC, we consider imperfect \ensuremathπ\ensuremath{\pi}\ensuremathπ pulses, showing that the DTC is robust against the small imperfections and finding that discrete time quasicrystals can appear for the larger imperfections. Together with experimental proposals in magnetic materials, our results pave the way for realizing the DTC and for uncovering nonequilibrium critical phenomena in real solid-state materials.
A discrete time crystal (DTC) is an essentially nonequilibrium phase that breaks discrete time-translation symmetry in periodically driven (Floquet) systems.In ideal isolated quantum systems without dissipation, many kinds of DTCs have been theoretically proposed and experimentally verified in various artificial quantum systems, such as trapped ions, NV centers in diamond, and quantum computers.However, in real physical systems, we cannot avoid dissipation stemming from coupling to an external environment.Nevertheless, the effects of dissipation on DTCs have not been fully explored yet.This thesis reports the author's two original studies on dissipative DTCs.First, we formulate a symmetry-based theory of dissipative DTCs.To this end, we introduce Floquet dynamical symmetry (FDS) governing the long-time dynamics of dissipative Floquet systems.The FDS protects the quantum coherence of the system from dissipation and dephasing and leads to two types of time crystals: the discrete time crystal (DTC) and discrete time quasicrystal (DTQC), which are different in periodicity.We show that these time crystals appear in spin-S Bose-and Fermi-Hubbard models driven by an ac field and that their periodicity can be tuned by adjusting the field strength.Since these time crystals rely on the FDS alone, they appear even in the presence of perturbations as long as the FDS exists.We also elucidate that the FDS explains the symmetry structure behind the conventional DTCs proposed in the previous studies.Second, we investigate dissipative DTCs in solid-state materials by considering a dissipative quantum Ising model periodically driven by π-pulses.While the previous studies on dissipative DTCs have focused on well-controllable artificial quantum systems, we show that realistic dissipation in solids does not destroy but can stabilize the DTC order.Using the time-dependent mean-field theory, we analyze the DTC phase transition and determine critical exponents, including a dynamical one.We also find a novel reentrant phase transition in varying the pulse interval.This transition is due to a nontrivial interplay of the periodic drive and dissipation and has no equilibrium counterparts in the corresponding undriven system.Furthermore, to demonstrate the rigidity (stability) of the DTC, we consider imperfect πpulses, showing that the DTC is robust against minor imperfections and finding that DTQC appears for major imperfections.These results uncover a general symmetry aspect of dissipative DTCs and lay the foundation for realizing DTCs in material science.
Periodically driven quantum systems host a range of non-equilibrium phenomena which are unrealizable at equilibrium. Discrete time-translational symmetry in a periodically driven many-body system can be spontaneously broken to form a discrete time crystal, a putative quantum phase of matter. We present the observation of discrete time crystalline order in a driven system of paramagnetic PPP -donor impurities in isotopically enriched 28Si^{28}Si28Si cooled below 101010 K. The observations exhibit a stable subharmonic peak at half the drive frequency which remains pinned even in the presence of pulse error, a signature of DTC order. We propose a theoretical model based on the paradigmatic central spin model which is in good agreement with experimental observations, and investigate the role of dissipation in the stabilisation of the DTC. Both experiment and theory indicate that the order in this system is primarily a dissipative effect, and which persists in the presence of spin-spin interactions. We present a theoretical phase diagram as a function of interactions and dissipation for the central spin model which is consistent with the experiments. This opens up questions about the interplay of coherent interaction and dissipation for time-translation symmetry breaking in many-body Floquet systems.
Discrete time crystals (DTCs) exhibit a special nonequilibrium phase of matter in periodically driven many-body systems with spontaneous breaking of time translational symmetry. The presence of decoherence generally enhances thermalization and destroys the coherence required for the existence of DTCs. In this paper, we devise a mechanism for establishing a stable DTC with period-doubling oscillations in an open quantum system that is governed by a properly tailored non-Hermitian Hamiltonian. We find a specific class of nonreciprocal couplings in our non-Hermitian dynamics which prevents thermalization through eigenstate ordering. This choice of non-Hermitian dynamics significantly enhances the stability of the DTC against imperfect pulses. Through a comprehensive analysis, we determine the phase diagram of the system in terms of pulse imperfection.
We describe a general and simple paradigm for discrete time crystals (DTCs), systems with a stable subharmonic response to an external driving field, in a classical thermal setting. We consider, specifically, an Ising model in two dimensions, as a prototypical system with a phase transition into stable phases distinguished by a local order parameter, driven by thermal dynamics and periodically kicked with a noisy protocol. By means of extensive numerical simulations for large sizes-allowed by the classical nature of our model-we show that the system features a true disorder-DTC order phase transition as a function of the noise strength, with a robust DTC phase extending over a wide parameter range. We demonstrate that, when the dynamics is observed stroboscopically, the phase transition to the DTC state appears to be in the equilibrium two-dimensional Ising universality class. However, we explicitly show that the DTC is a genuine nonequilibrium state. More generally, we speculate that systems with thermal phase transitions to multiple competing phases can give rise to DTCs when appropriately driven.
Discrete-Time Crystals (DTC) are a non-equilibrium phase of matter characterized by the breaking of time-translation symmetry in periodically driven quantum systems. In this work, we present a detailed thermodynamic analysis of a DTC in a one-dimensional spin-1/2 chain coupled to a thermal bath. We derive a master equation from the microscopic model, and we explore key thermodynamic quantities, such as work, heat, and entropy production. Our results reveal that the DTC signature inevitably decays in the presence of environmental noise, but we show that a periodic measurement scheme can mitigate the effects of decoherence, stabilizing the subharmonic oscillations of the DTC for extended periods. These findings provide insights into the robustness of time-crystalline phases and potential strategies for protecting them in experimental settings.
Discrete time crystals (DTCs) are non-equilibrium phases of matter that break the discrete time-translation symmetry and is characterized by a robust subharmonic response in periodically driven quantum systems. Here, we explore the DTC in a disorder-free, periodically kicked XXZ spin chain, which is stabilized by the Floquet strong Hilbert space fragmentation. We numerically show the period-doubling response of the conventional DTC order, and uncover a multiple-period response with beating dynamics due to the coherent interplay of multiple πππ-pairs in the Floquet spectrum of small-size systems. The lifetime of the DTC order exhibits independence of the driving frequency and a power-law dependence on the ZZ interaction strength. It also grows exponentially with the system size, as a hallmark of the strong fragmentation inherent to the Floquet model. We analytically reveal the approximate conservation of the magnetization and domain-wall number in the Floquet operator for the emergent strong fragmentation, which is consistent with numerical results of the dimensionality ratio of symmetry subspaces. The rigidity and phase regime of the DTC order are identified through finite-size scaling of the Floquet-spectrum-averaged mutual information, as well as via dynamical probes. Our work establishes the Floquet Hilbert space fragmentation as a disorder-free mechanism for sustaining nontrivial temporal orders in out-of-equilibrium quantum many-body systems.