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arXiv:1403.5515

Timescales, dynamical phase transitions and 3rd order phase transitions in the 1d anisotropic XY model

James M. Hickey

Published Mar 21, 2014• 1 citation
arXivHTMLPDF

Abstract

Recent research into time-integrated observables has revealed a special class of states which cap- ture the singular features of the generating functions of those observables, as estimated by full counting statistics (FCS). In this work we extend the results of [Phys. Rev. B 87 184303 (2013)] to the 1d anisotropic XY -model and find a set of FCS critical points associated with the time-integrated transverse magnetization and anisotropy. We show dynamical phase transitions (DPTs) as defined in [Phys. Rev. Lett. 110 135704 (2013)] do not emerge on quenching the states associated with the time-integrated anisotropy across FCS critical points. We also study the timescales required to prepare the associated special states of the transverse magnetization using appropriate Markovian baths and find they are independent of the number of spins in the chain. However the probability to evolve to such a state decreases drastically with increasing system size due to dissipation. Thus when preparing such states it is preferable to use few body systems and to reach the thermodynamic limit it is necessary to use an ancillary system in conjunction with the Markovian baths.

DOI
10.48550/arxiv.1403.5515

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Recent research into time-integrated observables has revealed a special class of states which cap- ture the singular features of the generating functions of those observables, as estimated by full counting statistics (FCS). In this work we extend the results of [Phys. Rev. B 87 184303 (2013)] to the 1d anisotropic XY -model and find a set of FCS critical points associated with the time-integrated transverse magnetization and anisotropy. We show dynamical phase transitions (DPTs) as defined in [Phys. Rev. Lett. 110 135704 (2013)] do not emerge on quenching the states associated with the time-integrated anisotropy across FCS critical points. We also study the timescales required to prepare the associated special states of the transverse magnetization using appropriate Markovian baths and find they are independent of the number of spins in the chain. However the probability to evolve to such a state decreases drastically with increasing system size due to dissipation. Thus when preparing such states it is preferable to use few body systems and to reach the thermodynamic limit it is necessary to use an ancillary system in conjunction with the Markovian baths.
We show that there exist dynamical phase transitions (DPTs), as defined by Heyl et al. [Phys. Rev. Lett. 110, 135704 (2013)], in the transverse-field Ising model (TFIM) away from the static quantum critical points. We study a class of special states associated with singularities in the generating functions of time-integrated observables as found by Hickey et al. [Phys. Rev. B 87, 184303 (2013)]. Studying the dynamics of these special states under the evolution of the TFIM Hamiltonian, we find temporal nonanalyticities in the initial-state return probability associated with dynamical phase transitions. By calculating the Berry phase and Chern number we show the set of special states have interesting geometric features similar to those associated with static quantum critical points.
The dynamical behavior of many-body systems is often richer than what can be anticipated from their static properties. Here we show that in closed quantum systems this becomes evident by considering the statistics of time-integrated observables. In particular, the analytic properties of their generating functions, as estimated by full counting statistics (FCS), allow one to identify FCS phases, i.e., phases with specific fluctuation properties of time-integrated observables, and to locate transitions between these phases. We discuss in detail the case of the quantum Ising chain in a transverse field. We show that this model displays a continuum of full counting statistics transitions, of which the static transition is just an end point. These singularities are not a consequence of particular choices of initial conditions or other external nonequilibrium protocols such as quenches in coupling constants. They can be probed generically through quantum jump statistics of an associated open problem, and for the case of the quantum Ising chain we outline a possible experimental realization of this scheme by digital quantum simulation with cold ions.
We obtain the steady-state phase diagram of a transverse-field XY spin chain coupled at its ends to magnetic reservoirs held at different magnetic potentials. In the long-time limit, the magnetization bias across the system generates a current-carrying nonequilibrium steady state. We characterize the different nonequilibrium phases as functions of the chain's parameters and magnetic potentials, in terms of their correlation functions and entanglement content. The mixed-order transition, previously observed for the case of a transverse-field Ising chain, is established to emerge as a generic feature of a wider class of out-of-equilibrium problems. The critical exponents associated with this universality class are determined analytically. Results are also contrasted with those obtained in the limit of Markovian reservoirs. Our findings should prove helpful in establishing the properties of nonequilibrium phases and phase transitions of extended open quantum systems.
Anisotropic quantum XY spin chains in a quasiperiodic (QP) transverse field provide a very interesting and a rich class of one-dimensional models exhibiting extended (E), localized (L), and critical (C) phases in addition to a magnetic transition to long-range order. The interesting feature in this class of models is the fact that the intermediate C phase exists in a finite window of size determined by the magnitude of spin-space anisotropy. Furthermore, even a small amount of anisotropy in the spin space destroys the fractal phase boundary between the E and L phases of the isotropic model.
Dynamical phase transitions (DPTs) occur after quenching some global parameters in quantum systems, and are signalled by the nonanalytical time evolution of the dynamical free energy, which is calculated from the Loschmidt overlap between the initial and time evolved states. In a recent Letter [M. Heyl et al., Phys. Rev. Lett. 110, 135704 (2013)], it was suggested that DPTs are closely related to equilibrium phase transitions (EPTs) for the transverse field Ising model. By studying a minimal model, the XY chain in a transverse magnetic field, we show analytically that this connection does not hold generally. We present examples where DPT occurs without crossing any equilibrium critical lines by the quench, and a nontrivial example with no DPT but crossing a critical line by the quench. Although the nonanalyticities of the dynamical free energy on the real time axis do not indicate the presence or absence of an EPT, the structure of Fisher lines for complex times reveals a qualitative difference.
We study an integrable spin chain with three spin interactions and the staggered field $(\ensuremath{\lambda})$ while the latter is quenched either slowly [in a linear fashion in time (t)(t)(t) as $t/\ensuremath{\tau}$, where ttt goes from a large negative value to a large positive value and $\ensuremath{\tau}$ is the inverse rate of quenching] or suddenly. In the process, the system crosses quantum critical points and gapless phases. We address the question whether there exist nonanalyticities [known as dynamical phase transitions (DPTs)] in the subsequent real-time evolution of the state (reached following the quench) governed by the final time-independent Hamiltonian. In the case of sufficiently slow quenching (when $\ensuremath{\tau}$ exceeds a critical value ${\ensuremath{\tau}}_{1})$, we show that DPTs, of the form similar to those occurring for quenching across an isolated critical point, can occur even when the system is slowly driven across more than one critical point and gapless phases. More interestingly, in the anisotropic situation we show that DPTs can completely disappear for some values of the anisotropy term $(\ensuremath{\gamma})$ and $\ensuremath{\tau}$, thereby establishing the existence of boundaries in the $(\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\tau})$ plane between the DPT and no-DPT regions in both isotropic and anisotropic cases. Our study therefore leads to a unique situation when DPTs may not occur even when an integrable model is slowly ramped across a QCP. On the other hand, considering sudden quenches from an initial value ${\ensuremath{\lambda}}_{i}$ to a final value ${\ensuremath{\lambda}}_{f}$, we show that the condition for the presence of DPTs is governed by relations involving ${\ensuremath{\lambda}}_{i},{\ensuremath{\lambda}}_{f}$, and $\ensuremath{\gamma}$, and the spin chain must be swept across $\ensuremath{\lambda}=0$ for DPTs to occur.