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arXiv:2411.04089 · Physical review. B./Physical review. B

Disorder-order interface propagating over the ferromagnetic ground state in the transverse field Ising chain

Vanja Marić, Florent Ferro, Maurizio Fagotti

Published May 14, 2025• 6 citations
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Abstract

When matter in different phases is brought into contact, an interface with unexpected properties can emerge. In the transverse-field Ising chain, the authors investigate the interface formed by joining a disordered state with a ferromagnetically ordered one. They show that, within this emergent region, order parameters and entanglement asymmetries of subsystems exhibit universal behavior. Correlations extend across the entire region, and the Wigner--Yanase skew information of large subsystems reveals macroscopic multipartite entanglement.

DOI
10.1103/physrevb.111.205118

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When matter in different phases is brought into contact, an interface with unexpected properties can emerge. In the transverse-field Ising chain, the authors investigate the interface formed by joining a disordered state with a ferromagnetically ordered one. They show that, within this emergent region, order parameters and entanglement asymmetries of subsystems exhibit universal behavior. Correlations extend across the entire region, and the Wigner--Yanase skew information of large subsystems reveals macroscopic multipartite entanglement.
We consider translationally invariant quantum spin-1/2 chains with local interactions and a discrete symmetry that is spontaneously broken at zero temperature. We envision experimenters switching off the couplings between two parts of the system and preparing them in independent equilibrium states. One side of the chain is prepared in a disordered phase, and the other in a symmetry-breaking ground state. When the couplings are switched back on, time evolution ensues. We argue that in integrable systems the front separating the ordered region recedes at the maximal velocity of quasiparticle excitations over the ground state. We infer that, generically, the order parameters should vary on a subdiffusive scale of order t^{1/3}, where t is time, and their fluctuations should exhibit the same scaling. This interfacial region exhibits full range correlations, indicating that it cannot be decomposed into nearly uncorrelated subsystems. Using the transverse-field Ising chain as a case study, we demonstrate that all order parameters follow the same universal scaling functions. Through an analysis of the skew information, we uncover that the breakdown of cluster decomposition has a quantum contribution: each subsystem within the interfacial region, with extent comparable to the region, exists in a macroscopic quantum state.
We investigate the ground-state property of the one-dimensional spin-1/2 antiferromagnetic X X Z chain in a symmetry-breaking field, by means of numerical diagonalization method for finite-size spin systems combined with the phenomenological renormalization group method. It is found that, with increasing field, there occurs the ground-state phase transition from an ordered phase to a disordered phase, whose critical property belongs to the universality class of the transverse Ising model. The ground-state phase diagram is determined in two parameter regions; (i) an Ising-like region and (ii) an X Y -like region. Our inspection of the field dependence of the energy gap and the static spin structure factors reveals that the ordered phase in both regions exhibits the characteristics of Néel order. Quantum nature of the ground state is also briefly discussed.
We study the melting of domain walls in the ferromagnetic phase of the transverse Ising chain, created by flipping the order-parameter spins along one-half of the chain. If the initial state is excited by a local operator in terms of Jordan-Wigner fermions, the resulting longitudinal magnetization profiles have a universal character. Namely, after proper rescalings, the profiles in the noncritical Ising chain become identical to those obtained for a critical free-fermion chain starting from a step-like initial state. The relation holds exactly in the entire ferromagnetic phase of the Ising chain and can even be extended to the zero-field XY model by a duality argument. In contrast, for domain-wall excitations that are highly non-local in the fermionic variables, the universality of the magnetization profiles is lost. Nevertheless, for both cases we observe that the entanglement entropy asymptotically saturates at the ground-state value, suggesting a simple form of the steady state.
Quantifying entanglement of multiple subsystems is a challenging open problem in interacting quantum systems. Here, we focus on two subsystems of length $\ensuremath{\ell}$ separated by a distance $r=\ensuremath{\alpha}\ensuremath{\ell}$ and quantify their entanglement negativity (E\mathcal{E}E) and mutual information (I\mathcal{I}I) in critical random Ising chains. We find universal constant $\mathcal{E}(\ensuremath{\alpha})$ and $\mathcal{I}(\ensuremath{\alpha})$ over any distances, using the asymptotically exact strong disorder renormalization group method. Our results are qualitatively different from both those in the clean Ising model and random spin chains of a singlet ground state, like the spin-12\frac{1}{2}21​ random Heisenberg chain and the random XX chain. While for random singlet states $\mathcal{I}(\ensuremath{\alpha})/\mathcal{E}(\ensuremath{\alpha})=2$, in the random Ising chain this universal ratio is strongly $\ensuremath{\alpha}$ dependent. This deviation between systems contrasts with the behavior of the entanglement entropy of a single subsystem, for which the various random critical chains and clean models give the same qualitative behavior. The reason is that E\mathcal{E}E and I\mathcal{I}I are sensitive to higher order correlations in the ground-state structure. Therefore, studying multipartite entanglement provides additional universal information in random quantum systems, beyond what we can learn from a single subsystem.
We investigate how symmetry-breaking boundary magnetic fields affect magnetization and Rényi-2 entanglement asymmetry (EA) in the transverse-field Ising model. For the ground state, magnetization exhibits qualitatively distinct behaviors in the ordered and disordered phases. EA also exhibits phase-dependent behavior, which is well captured by the lower and upper bounds derived in this work. In the critical regime, both magnetization and EA exhibit behavior qualitatively consistent with the field-theoretical predictions for a related setup. We also study boundary-quench dynamics and find that magnetization propagates from the boundary into the bulk with a velocity consistent with the maximum quasiparticle velocity. In contrast, because EA is a subsystem-integrated quantity, its spatial structure is not directly resolved. We therefore introduce the asymmetry contour, a contour-like finite-difference quantity that provides a spatially resolved characterization of the symmetry-breaking information encoded in the reduced density matrix. The resulting contour is complementary to the local magnetization, retaining sensitivity to nonlocal many-body correlations while its dynamics also reflects the underlying quasiparticle propagation. These results show how EA and its contour can provide complementary information-theoretic characterizations of symmetry breaking and its spatial structure.
The entanglement in quantum XY spin chains of arbitrary length is investigated via the geometric measure of entanglement. The emergence of entanglement is explained intuitively from the perspective of perturbations. The model is solved exactly and the energy spectrum is determined and analyzed in particular for the lowest two levels for both finite and infinite systems. The overlaps for these two levels are calculated analytically for arbitrary number of spins. The entanglement is hence obtained by maximizing over a single parameter. The corresponding ground-state entanglement surface is then determined over the entire phase diagram, and its behavior can be used to delineate the boundaries in the phase diagram. For example, the field-derivative of the entanglement becomes singular along the critical line. The form of the divergence is derived analytically and it turns out to be dictated by the universality class controlling the quantum phase transition. The behavior of the entanglement near criticality can be understood via a scaling hypothesis, analogous to that for free energies. The entanglement density vanishes along the so-called disorder line in the phase diagram, the ground space is doubly degenerate and spanned by two product states. The entanglement for the superposition of the lowest two states is also calculated. The exact value of the entanglement depends on the specific form of superposition. However, in the thermodynamic limit the entanglement density turns out to be independent of the superposition. This proves that the entanglement density is insensitive to whether the ground state is chosen to be the spontaneously Z2Z_2Z2​ symmetry broken one or not. The finite-size scaling of entanglement at critical points is also investigated from two different view points. First, the maximum in the field-derivative of the entanglement density is computed and fitted to a logarithmic dependence of the system size, thereby deducing the correlation length exponent for the Ising class using only the behavior of entanglement. Second, the entanglement density itself is shown to possess a correction term inversely proportional to the system size, with the coefficient being universal (but with different values for the ground state and the first excited state, respectively).