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arXiv:1504.08365

Environment tensor as order parameter for symmetry breaking and (symmetry-protected) topological orders

Fangzhou Liu, Xiao-Gang Wen

Published Apr 30, 2015• 4 citations
arXivHTMLPDF

Abstract

Spontaneous symmetry breaking is well understood through the classical "Mexican Hat" picture, which describes many quantum phases of matter. Recently, several new classes of quantum phases of matter, such as topological orders and symmetry protected topological (SPT) orders, were discovered. In an attempt to address the transitions between all those phases of quantum matter under the same framework, we introduced an analogous yet very simple picture for phase transitions in the context of tensor-networks. Using a very simple iteration process, we found that both symmetry breaking and some topological phase transitions (for topological orders described by gauge theory and 1D SPT orders) could be marked by a sudden change in the symmetry structure of the so-called "environment matrix". In this process, the environment matrix serves as an "order parameter" that captures patterns of entanglement in topological phases. The symmetry change in the environment matrix is very much like the symmetry breaking of conventional order parameters. We applied this method to both the transverse Ising model (1D and 2D honeycomb), spin-1 model (1D), and the Toric Code model in a magnetic field (2D honeycomb), and explored the corresponding symmetry structure changes in their environment matrices in details. With just a few variational parameters and a few minutes' run time on a laptop, we could get the corresponding phase transition points within a few percent error compared with the Quantum Monte Carlo results.

DOI
10.48550/arxiv.1504.08365

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Spontaneous symmetry breaking is well understood through the classical "Mexican Hat" picture, which describes many quantum phases of matter. Recently, several new classes of quantum phases of matter, such as topological orders and symmetry protected topological (SPT) orders, were discovered. In an attempt to address the transitions between all those phases of quantum matter under the same framework, we introduced an analogous yet very simple picture for phase transitions in the context of tensor-networks. Using a very simple iteration process, we found that both symmetry breaking and some topological phase transitions (for topological orders described by gauge theory and 1D SPT orders) could be marked by a sudden change in the symmetry structure of the so-called "environment matrix". In this process, the environment matrix serves as an "order parameter" that captures patterns of entanglement in topological phases. The symmetry change in the environment matrix is very much like the symmetry breaking of conventional order parameters. We applied this method to both the transverse Ising model (1D and 2D honeycomb), spin-1 model (1D), and the Toric Code model in a magnetic field (2D honeycomb), and explored the corresponding symmetry structure changes in their environment matrices in details. With just a few variational parameters and a few minutes' run time on a laptop, we could get the corresponding phase transition points within a few percent error compared with the Quantum Monte Carlo results.
Symmetry-protected topological (SPT) phases exhibit nontrivial order if symmetry is respected but are adiabatically connected to the trivial product phase if symmetry is not respected. However, unlike the symmetry-breaking phase, there is no local order parameter for SPT phases. Here we employ a tensor-network method to compute the topological invariants characterized by the simulated modular SSS and TTT matrices to study transitions in a few families of two-dimensional (2D) wave functions which are ZN{\mathbb{Z}}_{N}ZN​ (N=2and3)(N=2\phantom{\rule{4.pt}{0ex}}\text{and}\phantom{\rule{4.pt}{0ex}}3)(N=2and3) symmetric. We find that in addition to the topologically ordered phases, the modular matrices can be used to identify nontrivial SPT phases and detect transitions between different SPT phases as well as between symmetric and symmetry-breaking phases. Therefore modular matrices can be used to characterize various types of gapped phases in a unifying way.
We study the renormalization group flow of the Lagrangian for statistical and quantum systems by representing their path integral in terms of a tensor network. Using a tensor-entanglement-filtering renormalization approach that removes local entanglement and produces a coarse-grained lattice, we show that the resulting renormalization flow of the tensors in the tensor network has a nice fixed-point structure. The isolated fixed-point tensors Tinv{T}_{\text{inv}}Tinv​ plus the symmetry group Gsym{G}_{\text{sym}}Gsym​ of the tensors (i.e., the symmetry group of the Lagrangian) characterize various phases of the system. Such a characterization can describe both the symmetry breaking phases and topological phases, as illustrated by two-dimensional (2D) statistical Ising model, 2D statistical loop-gas model, and 1+1D1+1\text{D}1+1D quantum spin-1/2 and spin-1 models. In particular, using such a (Gsym,Tinv)({G}_{\text{sym}},{T}_{\text{inv}})(Gsym​,Tinv​) characterization, we show that the Haldane phase for a spin-1 chain is a phase protected by the time-reversal, parity, and translation symmetries. Thus the Haldane phase is a symmetry-protected topological phase. The (Gsym,Tinv)({G}_{\text{sym}},{T}_{\text{inv}})(Gsym​,Tinv​) characterization is more general than the characterizations based on the boundary spins and string order parameters. The tensor renormalization approach also allows us to study continuous phase transitions between symmetry breaking phases and/or topological phases. The scaling dimensions and the central charges for the critical points that describe those continuous phase transitions can be calculated from the fixed-point tensors at those critical points.
Symmetry-protected topological (SPT) phases are short-range entangled phases of matter with a nonlocal order parameter which are preserved under a local symmetry group. Here, by using an unsupervised learning algorithm, namely, diffusion maps, we demonstrate that we can detect phase transitions between symmetry-broken and topologically ordered phases, and between nontrivial topological phases in different classes. In particular, we show that the phase transitions associated with these phases can be detected in different bosonic and fermionic models in one dimension. This includes the interacting Su-Schrieffer-Heeger model, the Affleck-Kennedy-Lieb-Tasaki model and its variants, and weakly interacting fermionic models. Our approach serves as an inexpensive computational method for detecting topological phase transitions associated with SPT systems which can also be applied to experimental data obtained from quantum simulators.
Symmetry-protected topological (SPT) phases with gapless edge excitations have been shown to exist in strongly interacting bosonic/fermionic systems and it is highly desirable to identify practical systems realizing such phases through numerical simulation. A central question to be addressed is how to determine the SPT order in the system given the simulation result, while no local order parameter can be measured to distinguish the phases from a trivial one. In the tensor network approach to simulate strongly interacting systems, the quantum state renormalization algorithm has been shown to be effective in identifying intrinsic topological orders. Here, we show that a modified algorithm can identify SPT orders by extracting the symmetry-protected fixed point entanglement pattern in the ground-state wave function, which is essential for the existence of SPT order. The key to this approach is to add proper symmetry protection to the renormalization process. We demonstrate the effectiveness of this algorithm with examples of nontrivial SPT phases with internal symmetry in 1D and internal and translation symmetry in 2D.
Since the discovery of phase transitions driven by topological defects, the classification of phases of matter has been significantly extended beyond Ginzburg and Landau's paradigm of spontaneous symmetry breaking (SSB). In particular, intrinsic and symmetry-protected topological (SPT) orders have been discovered in (mostly gapped) quantum many-body ground states. However, these are commonly viewed as zero-temperature phenomena, and their robustness in a gapless ground state or against thermal fluctuations remains challenging to tackle. Here we introduce an explicit construction for SPT-type states with hidden order associated with SSB: They feature (quasi-)long-range correlations along appropriate edges but short-range order in the bulk, ground state degeneracy associated with SSB, and nonlocal string order in the bulk. We apply our construction to predict two types of finite-temperature SPT transitions protected by 1-form symmetries in the Ising and BKT class, respectively, where the usual signs of criticality appear despite the absence of a diverging correlation length in the bulk. While the state featuring hidden Ising order is gapped, the other SPT state associated with the BKT-SPT transition has hidden U ( 1 ) or XY order and constitutes an intrinsically gapless SPT state, associated with a gapless Goldstone mode. Specifically, in this work we discuss spins with global Z 2 or U ( 1 ) symmetry coupled to link variables constituting a loop gas model with a 1-form symmetry. By mapping this system to an Ising-gauge theory, we demonstrate that one of the SPT phases we construct corresponds to the Higgs-SPT phase at T = 0 – which we show here to remain stable at finite temperature. Our work paves the way for a more systematic search for hidden order SPT phases, including in gapless systems, and raises the question if a natural (finite- T ) spin liquid candidate exists that realizes hidden order in the Higgs-SPT class.
We establish an intriguing connection between quantum phase transitions and bifurcations in the reduced fidelity between two different reduced density matrices for quantum lattice many-body systems with symmetry-breaking order. Our finding is based on the observation that in the conventional Landau–Ginzburg–Wilson paradigm a quantum system undergoing a phase transition is characterized in terms of spontaneous symmetry breaking that is captured by a local-order parameter, which in turn results in an essential change of the reduced density matrix in the symmetry-broken phase. Two quantum systems on an infinite lattice in one spatial dimension, i.e. a quantum Ising model in a transverse magnetic field and a quantum spin-1/2 XYX model in an external magnetic field, are considered in the context of the tensor network algorithm based on the matrix product state representation.