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arXiv:2004.08360 · Physical review. E

Scaling properties of the dynamics at first-order quantum transitions when boundary conditions favor one of the two phases

Andrea Pelissetto C, Davide Rossini, Ettore Vicari

Published Jul 22, 2020• 27 citations
arXivHTMLPDF

Abstract

We address the out-of-equilibrium dynamics of a many-body system when one of its Hamiltonian parameters is driven across a first-order quantum transition (FOQT). In particular, we consider systems subject to boundary conditions favoring one of the two phases separated by the FOQT. These issues are investigated within the paradigmatic one-dimensional quantum Ising model, at the FOQTs driven by the longitudinal magnetic field h, with boundary conditions that favor the same magnetized phase (EFBC) or opposite magnetized phases (OFBC). We study the dynamic behavior for an instantaneous quench and for a protocol in which h is slowly varied across the FOQT. We develop a dynamic finite-size scaling theory for both EFBC and OFBC, which displays some remarkable differences with respect to the case of neutral boundary conditions. The corresponding relevant timescale shows a qualitative different size dependence in the two cases: it increases exponentially with the size in the case of EFBC, and as a power of the size in the case of OFBC.

DOI
10.1103/physreve.102.012143

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We address the out-of-equilibrium dynamics of a many-body system when one of its Hamiltonian parameters is driven across a first-order quantum transition (FOQT). In particular, we consider systems subject to boundary conditions favoring one of the two phases separated by the FOQT. These issues are investigated within the paradigmatic one-dimensional quantum Ising model, at the FOQTs driven by the longitudinal magnetic field h, with boundary conditions that favor the same magnetized phase (EFBC) or opposite magnetized phases (OFBC). We study the dynamic behavior for an instantaneous quench and for a protocol in which h is slowly varied across the FOQT. We develop a dynamic finite-size scaling theory for both EFBC and OFBC, which displays some remarkable differences with respect to the case of neutral boundary conditions. The corresponding relevant timescale shows a qualitative different size dependence in the two cases: it increases exponentially with the size in the case of EFBC, and as a power of the size in the case of OFBC.
We investigate scaling phenomena at first-order quantum transitions, when the boundary conditions favor one of the two phases. We show that the corresponding finite-size scaling behavior, arising from the interplay between the driving parameter and the finite size of the system, is more complex than that emerging when boundary conditions do not favor any phase. We discuss this issue in the framework of the paradigmatic one-dimensional quantum Ising model, along its first-order quantum transition line driven by an external longitudinal field. Specifically, three regions with distinct scaling behaviors emerge, which correspond to different values of the field (small, intermediate, and large field), according to its capability to modify the phase favored by the boundary conditions.
We investigate the quantum dynamics generated by quantum quenches (QQs) of the Hamiltonian parameters in many-body systems, focusing on protocols that cross first-order and continuous quantum transitions, both in finite-size systems and in the thermodynamic limit. As a paradigmatic example, we consider the quantum Ising chain in the presence of homogeneous transverse (ggg) and longitudinal (hhh) magnetic fields. This model exhibits a continuous quantum transition (CQT) at g=gcg=g_cg=gc​ and h=0h=0h=0, and first-order quantum transitions (FOQTs) driven by hhh along the line h=0h=0h=0 (g<gcg<g_cg<gc​). In the integrable limit h=0h=0h=0, the system can be mapped onto a quadratic fermionic theory; however, any nonvanishing longitudinal field breaks integrability and the spectrum of the resulting Hamiltonian is generally expected to enter a chaotic regime. We analyze QQs in which the longitudinal field is suddenly changed from a negative value hi<0h_i < 0hi​<0 to a positive value hf>0h_f>0hf​>0. We focus on values of hfh_fhf​ such that the spectrum of the post-QQ Hamiltonian H(g,hf)H(g,h_f)H(g,hf​) lies in the chaotic regime, where thermalization may emerge at asymptotically long times. We study the out-of-equilibrium dynamics for different values of ggg, finding qualitatively distinct behaviors for g>gcg > g_cg>gc​ (where the chain is in the disordered phase), for g=gcg = g_cg=gc​ (QQ across the CQT), and for g<gcg<g_cg<gc​ (QQ across the FOQT line).
We study the out-of-equilibrium Kibble-Zurek (KZ) dynamics in quantum Ising chains in a transverse field, driven by a time-dependent longitudinal field h(t)=t/tsh(t)=t/t_sh(t)=t/ts​ (tst_sts​ is the time scale of the protocol), across their first-order quantum transitions (FOQTs) at h=0h=0h=0. The KZ protocol starts at time ti<0t_i<0ti​<0 from the negatively magnetized ground state for hi=ti/ts<0h_i = t_i/t_s<0hi​=ti​/ts​<0. Then, the system evolves unitarily up to a time tf>0t_f > 0tf​>0, such that the magnetization of the state at time tft_ftf​ is positive. In finite-size systems, the KZ dynamics develops out-of-equilibrium finite-size scaling (OFSS) behaviors. Their scaling variables depend either exponentially or with a power law on the size, depending on the boundary conditions (BC). The OFSS functions can be computed in effective models restricted to appropriate low-energy (magnetized and/or kink) states. The KZ scaling behavior drastically changes in the thermodynamic limit (TL), defined as the infinite-size limit keeping ttt and tst_sts​ fixed, which appears substantially unrelated with the OFSS regime, because it involves higher-energy multi-kink states, which are irrelevant in the OFSS limit. The numerical analyses of the KZ dynamics in the TL show the emergence of a quantum spinodal-like scaling behavior at the FOQTs for all considered BC, which is independent of the BC. The longitudinal magnetization changes sign at h(t)=h∗>0h(t)=h*>0h(t)=h∗>0, where h∗h*h∗ decreases with increasing tst_sts​, as h∗∼1/ln⁡tsh*\sim 1/\ln t_sh∗∼1/lnts​. Moreover, in the large-tst_sts​ limit, the time-dependence of the magnetization is described by a universal function of Ω=t/τsΩ= t/τ_sΩ=t/τs​, with τs=ts/ln⁡tsτ_s = t_s/\ln t_sτs​=ts​/lnts​.
We study the out-of-equilibrium dynamics of one-dimensional quantum Ising models in a transverse field ggg, driven by a time-dependent longitudinal field hhh across their {\em magnetic} first-order quantum transition at h=0h=0h=0, for sufficiently small values of ∣g∣|g|∣g∣. We consider nearest-neighbor Ising chains of size LLL with periodic boundary conditions. We focus on the out-of-equilibrium behavior arising from Kibble-Zurek protocols, in which hhh is varied linearly in time with time scale tst_sts​, i.e., h(t)=t/tsh(t)=t/t_sh(t)=t/ts​. The system starts from the ground state at hi≡h(ti)<0h_i\equiv h(t_i)<0hi​≡h(ti​)<0, where the longitudinal magnetization MMM is negative. Then it evolves unitarily up to positive values of h(t)h(t)h(t), where M(t)M(t)M(t) becomes eventually positive. We identify several scaling regimes characterized by a nontrivial interplay between the size LLL and the time scale tst_sts​, which can be observed when the system is close to one of the many avoided level crossings that occur for h≥0h\ge 0h≥0. In the L→∞L\to\inftyL→∞ limit, all these crossings approach h=0+h=0^+h=0+, making the study of the thermodynamic limit, defined as the limit L→∞L\to\inftyL→∞ keeping ttt and tst_sts​ constant, problematic. We study such limit numerically, by first determining the large-LLL quantum evolution at fixed tst_sts​, and then analyzing its behavior with increasing tst_sts​. Our analysis shows that the system switches from the initial state with M<0M<0M<0 to a positively magnetized state at h=h∗(ts)>0h = h_*(t_s)>0h=h∗​(ts​)>0, where h∗(ts)h_*(t_s)h∗​(ts​) decreases with increasing tst_sts​, apparently as h∗∼1/ln⁡tsh_*\sim 1/\ln t_sh∗​∼1/lnts​. This suggests the existence of a scaling behavior in terms of the rescaled time Ω=tln⁡ts/tsΩ= t \ln t_s/t_sΩ=tlnts​/ts​. The numerical results also show that the system converges to a nontrivial stationary state in the large-ttt limit, characterized by an energy significantly larger than that of the corresponding homogeneously magnetized ground state.
We investigate the effects of dissipation on the quantum dynamics of many-body systems at quantum transitions, especially considering those of the first order. This issue is studied within the paradigmatic one-dimensional quantum Ising model. We analyze the out-of-equilibrium dynamics arising from quenches of the Hamiltonian parameters and dissipative mechanisms modeled by a Lindblad master equation, with either local or global spin operators acting as dissipative operators. Analogously to what happens at continuous quantum transitions, we observe a regime where the system develops a nontrivial dynamic scaling behavior, which is realized when the dissipation parameter uuu (globally controlling the decay rate of the dissipation within the Lindblad framework) scales as the energy difference $\mathrm{\ensuremath{\Delta}}$ of the lowest levels of the Hamiltonian, i.e., $u\ensuremath{\sim}\mathrm{\ensuremath{\Delta}}$. However, unlike continuous quantum transitions where $\mathrm{\ensuremath{\Delta}}$ is power-law suppressed, at first-order quantum transitions $\mathrm{\ensuremath{\Delta}}$ is exponentially suppressed with increasing the system size (provided the boundary conditions do not favor any particular phase).
Abstract We investigate how the scaling behavior of finite systems at magnetic first-order transitions (FOTs) with relaxational dynamics changes in correspondence of various boundary conditions. As a theoretical laboratory, we consider the two-dimensional Ising model in the low-temperature phase. When the boundary conditions do not favor any specific phase of the system, we show that a dynamic finite-size scaling (DFSS) theory can be developed to describe the dynamic behavior in the coexistence region, where different phases coexist. When the boundary conditions at two opposite sides of the system generate a planar interface separating the phases, we show that the autocorrelation times are characterized by a power-law behavior, related to the dynamics enforced by the interface. Numerical results for a purely relaxational dynamics confirm the general picture.