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arXiv:1901.10368 · Physical review. B./Physical review. B

Construction of many-body eigenstates with displacement transformations in disordered systems

M. Ortuño, A. M. Somoza, Louk Rademaker

Published Aug 6, 2019• 5 citations

Abstract

Many-body eigenstates beyond the Gaussian approximation can be constructed in terms of local integrals of motion (IOMs), although their actual computation has been, until now, a daunting task. We present a practical computation of IOMs based on displacement transformations and apply it to the problem of many-body localization in disordered one-dimensional systems. It represents a general and systematic way to extend Hartree-Fock and configuration interaction theories to higher order. Our method combines minimization of energy and energy variance of a reference state with exact diagonalization. We show that our implementation for one-dimensional disordered fermions is able to perform ground-state calculations with high precision for relatively large systems. Since it keeps track of the IMOs forming a reference state, our method is particularly efficient dealing with excited states, both in accuracy and in the number of different states that can be constructed.

DOI: 10.1103/physrevb.100.085115

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Many-body eigenstates beyond the Gaussian approximation can be constructed in terms of local integrals of motion (IOMs), although their actual computation has been, until now, a daunting task. We present a practical computation of IOMs based on displacement transformations and apply it to the problem of many-body localization in disordered one-dimensional systems. It represents a general and systematic way to extend Hartree-Fock and configuration interaction theories to higher order. Our method combines minimization of energy and energy variance of a reference state with exact diagonalization. We show that our implementation for one-dimensional disordered fermions is able to perform ground-state calculations with high precision for relatively large systems. Since it keeps track of the IMOs forming a reference state, our method is particularly efficient dealing with excited states, both in accuracy and in the number of different states that can be constructed.

We obtain eigenstates of interacting disorder Hamiltonians using unitary displacement transformations that rotate the state of the system. The method generates excited states if the strength of these transformations is chosen to optimize the energy, while decreasing the energy variance. We apply the method to analyse the localization properties of one-dimensional spinless fermions with short range interactions, reaching relatively large system sizes. We quantify the degree of localization through the size and disorder dependence of the inverse participation ratio.

Many-body localization (MBL) addresses the absence of thermalization in interacting quantum systems, with nonergodic high-energy eigenstates behaving as ground states, only area-law entangled. However, computing highly excited many-body eigenstates using exact methods is very challenging. Instead, we show that one can address high-energy MBL physics using ground-state methods, which are much more amenable to many efficient algorithms. We find that a localized many-body ground state of a given interacting disordered Hamiltonian

{\mathcal{H}}_{0}

is a very good approximation for a high-energy eigenstate of a parent Hamiltonian, close to

{\mathcal{H}}_{0}

but more disordered. This construction relies on computing the covariance matrix, easily achieved using density-matrix renormalization group for disordered Heisenberg chains up to

L=256

sites.

We study many‐body localization (MBL) from the perspective of integrals of motion (IOMs). MBL can be understood phenomenologically through the existence of macroscopically many localized IOMs. We develop a systematic procedure based on IOM to calculate many‐body quantities. Displacement transformations made clear that any operator can be expanded in 1‐,2‐ ... n ‐particles terms. We use this property to develop a systematic procedure to approximately calculate IOMs and many‐body quantities. We characterize the decay with distance of the IOM's and their interactions through effective localization lengths. For all values of disorder the typical IOMs are localized, suggesting the importance of rare fluctuations in understanding the MBL‐to‐ergodic transition. image

We show how the thermodynamic properties of large many-body localized systems can be studied using quantum Monte Carlo simulations. We devise a heuristic way of constructing local integrals of motion of high quality, which are added to the Hamiltonian in conjunction with Lagrange multipliers. The ground state simulation of the shifted Hamiltonian corresponds to a high-energy state of the original Hamiltonian in the case of exactly known local integrals of motion. The inevitable mixing between eigenstates as a consequence of nonperfect integrals of motion is weak enough such that the characteristics of many-body localized systems are not averaged out, unlike the standard ensembles of statistical mechanics. Our method paves the way to study higher dimensions and indicates that a fully many-body localized phase in 2D, where (nearly) all eigenstates are localized, is likely to exist.

This paper focuses on developing a new algorithm (En-DMRG) for obtaining highly excited states of disordered and strongly interacting spin systems. This line of research is stimulated by the growing interest in understanding the many-body localized (MBL) state, which emerges in disordered interacting systems at high energy densities through a disorder-driven dynamic phase transition. By studying the one-dimensional Heisenberg spin chain (with up to 72 spins) in a random field, the authors demonstrate the accuracy of the method in obtaining energy eigenstates and the corresponding statistical results of the quantum system in the MBL phase. The method presented in this work has the potential to be applicable in a variety of quantum statistical physics problems, possibly including the MBL phase in higher dimensions.

Recently, it has been suggested that the many-body localized phase can be characterized by local integrals of motion. Here we introduce a Hilbert-space-preserving renormalization scheme that iteratively finds such integrals of motion exactly. Our method is based on the consecutive action of a similarity transformation using displacement operators. We show, as a proof of principle, localization and the delocalization transition in interacting fermion chains with random on-site potentials. Our scheme of consecutive displacement transformations can be used to study many-body localization in any dimension, as well as disorder-free Hamiltonians.