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arXiv:1904.12450 · Physical review. B./Physical review. B

Multifractality of the entanglement Hamiltonian eigenmodes

Mohammad Pouranvari

Published Apr 11, 2019• 6 citations
arXivHTMLPDF

Abstract

We study the fractal properties of single-particle eigenmodes of an entanglement Hamiltonian in free-fermion models. One of these modes that has the highest entanglement information and is thus called the maximally entangled mode (MEM) is especially considered. In free-fermion models with Anderson localization, the fractality of the MEM is obtained numerically and compared with the fractality of the Hamiltonian eigenmode at the Fermi level. We show that both eigenmodes have similar fractal properties: both have the same single-fractal dimension in the delocalized phase which equals the dimension of the system, and both show multifractality at the phase transition point. Therefore, we conclude that fractal behavior of the MEM, in addition to the fractal behavior of the Hamiltonian eigenmode, can be used as a quantum phase transition characterization.

DOI
10.1103/physrevb.99.155121

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We study the fractal properties of single-particle eigenmodes of an entanglement Hamiltonian in free-fermion models. One of these modes that has the highest entanglement information and is thus called the maximally entangled mode (MEM) is especially considered. In free-fermion models with Anderson localization, the fractality of the MEM is obtained numerically and compared with the fractality of the Hamiltonian eigenmode at the Fermi level. We show that both eigenmodes have similar fractal properties: both have the same single-fractal dimension in the delocalized phase which equals the dimension of the system, and both show multifractality at the phase transition point. Therefore, we conclude that fractal behavior of the MEM, in addition to the fractal behavior of the Hamiltonian eigenmode, can be used as a quantum phase transition characterization.
Single-particle entanglement entropy (SPEE) is calculated for entanglement Hamiltonian eigenmode in a one-dimensional free fermion model that undergoes a delocalized–localized phase transition. In this numerical study, we show that SPEE of entanglement Hamiltonian eigenmode has the same behavior as SPEE of Hamiltonian eigenmode at the Fermi level: as we go from delocalized phase toward localized phase, SPEE of both modes decrease in the same manner. Furthermore, fluctuations of SPEE of entanglement Hamiltonian eigenmode — which can be obtained through the calculation of moments of SPEE — signature very sharply the phase transition point. These two modes are also compared by calculation of single-particle Rényi entropy (SPRE). We show that SPEE and SPRE of entanglement Hamiltonian eigenmode can be used as phase detection parameters.
We study the entanglement Hamiltonian (EH) associated with the reduced density matrix of free fermion models in a delocalized-localized Anderson phase transition. We show numerically that the structure of the EH matrix differentiates the delocalized from the localized phase. In the delocalized phase, EH becomes a long-range Hamiltonian but is short-range in the localized phase, no matter what the configuration of the system's Hamiltonian is (whether it is long- or short-range). With this view, we introduce the entanglement conductance (EC), which quantifies how much EH is long-range, and we propose it as an alternative quantity to measure entanglement in the Anderson phase transition, by which we locate the phase transition point of some one-dimensional free fermion models. In addition, by applying the finite-size method to the EC, we find three-dimensional Anderson phase transition critical disorder strength.
We study in this work the ground-state entanglement properties of two models of noninteracting fermions moving in one dimension, that exhibit metal-insulator transitions. We find that entanglement entropy grows either logarithmically or in a power-law fashion in the metallic phase, thus violating the (one-dimensional version of) entanglement area law. No such violation is found in the insulating phase. We further find that characteristics of single fermion states at the Fermi energy (which can not be obtained from the many-fermion Slater determinant) is captured by the lowest energy single fermion mode of the entanglement Hamiltonian; this is particularly true at the metal-insulator transition point. Our results suggest entanglement is a powerful way to detect metal-insulator transitions, without knowledge of the Hamiltonian of the system.
We numerically investigate measurement-induced phase transitions in monitored free fermions through the spectral and eigenstate properties of the entanglement Hamiltonian. By analyzing entanglement scaling, we identify three non-trivial fixed points, for which the entanglement follows exact analytical scaling forms: chaotic unitary dynamics at infinitesimal monitoring, characterized by a Gaussian Page law; a Fermi-liquid fixed point at moderate monitoring, defining a metallic phase with logarithmic entanglement growth and emergent space-time invariance; and a quantum Lifshitz fixed point marking the measurement-induced phase transition into a localized area-law phase. Adopting a random-matrix perspective on the entanglement Hamiltonian, we show that short-range spectral correlations, such as the adjacent gap ratio ⟨r~⟩\langle \tilde r\rangle⟨r~⟩ and the Kullback-Leibler divergence KL1KL_1KL1​, sharply detect an ergodic to non-ergodic phase transition at the quantum Lifshitz fixed point, and yield precise estimates of the critical point and correlation length exponent. Long-range probes, including the spectral form factor and the associated Thouless time, corroborate this picture, while the variant KL2KL_2KL2​ uncovers signatures of a possible non-ergodic extended (multifractal) regime at intermediate monitoring strengths. Together, these results establish the entanglement Hamiltonian as a powerful framework for diagnosing metallic, localized, and multifractal regimes in monitored quantum dynamics, and highlight unexpected scaling structures, Fermi-liquid and Lifshitz criticality, that lie beyond current field-theoretic approaches.
Abstract For short-ranged disordered quantum models in one dimension, the many-body-localization is analyzed via the adaptation to the many-body context (Serbyn et al 2015 Phys. Rev . X 5 041047) of the Thouless point of view on the Anderson transition: the question is whether a local interaction between two long chains is able to reshuffle completely the eigenstates (delocalized phase with a volume-law entanglement) or whether the hybridization between tensor states remains limited (many-body-localized phase with an area-law entanglement). The central object is thus the level of hybridization induced by the matrix elements of local operators, as compared with the difference of diagonal energies. The multifractal analysis of these matrix elements of local operators is used to analyze the corresponding statistics of resonances. Our main conclusion is that the critical point is characterized by the strong-multifractality spectrum f ( 0 ⩽ α ⩽ 2 ) = α 2 , well known in the context of Anderson localization in spaces of effective infinite dimensionality, where the size of the Hilbert space grows exponentially with the volume. Finally, the possibility of a delocalized non-ergodic phase near criticality is discussed.
The von Neumann entanglement entropy is a useful measure to characterize a quantum phase transition. We investigate the nonanalyticity of this entropy at disorder-dominated quantum phase transitions in noninteracting electronic systems. At these critical points, the von Neumann entropy is determined by the single particle wave function intensity, which exhibits complex scale invariant fluctuations. We find that the concept of multifractality is naturally suited for studying von Neumann entropy of the critical wave functions. Our numerical simulations of the three dimensional Anderson localization transition and the integer quantum Hall plateau transition show that the entanglement at these transitions is well described using multifractal analysis.