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Chinese Physics B

Many-body multipole indices revealed by real-space dynamical mean-field theory

Guoao Yang, Jianhui Zhou, Tao Qin

Published Feb 23, 2026• 0 citations
Publisher

Abstract

Abstract Multipole moments, fundamental characteristics of insulating materials, have garnered significant interest with the recent emergence of higher-order topological insulators. However, a practical method to explore them in correlated insulators is still lacking. Here, we introduce a systematic approach, which combines the general Green’s function formula for multipoles with real-space dynamical mean-field theory, to calculate multipole moments in correlated materials. Our demonstration calculations for the correlated two-dimensional Benalcazar–Bernevig–Hughes model are consistent with symmetry analysis. This method opens a new avenue to study topological phase transitions in correlated multipole insulators and other crucial physical quantities closely related to multipole moments.

DOI
10.1088/1674-1056/ae48c3

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Abstract Multipole moments, fundamental characteristics of insulating materials, have garnered significant interest with the recent emergence of higher-order topological insulators. However, a practical method to explore them in correlated insulators is still lacking. Here, we introduce a systematic approach, which combines the general Green’s function formula for multipoles with real-space dynamical mean-field theory, to calculate multipole moments in correlated materials. Our demonstration calculations for the correlated two-dimensional Benalcazar–Bernevig–Hughes model are consistent with symmetry analysis. This method opens a new avenue to study topological phase transitions in correlated multipole insulators and other crucial physical quantities closely related to multipole moments.
We study interaction-induced topological phase transition in Bernevig-Hughes-Zhang model. The topological nature of the phase transition is revealed by directly calculating the Z 2 index of the interacting system from the single-particle Green's function. The interacting Z 2 index is also consistently checked through the edge spectra. Combined with ab initio methods, the present approach is a useful tool for searching for correlated topological insulating materials from the first-principle point of view.
The one-body picture underlies our understanding of weakly interacting solids but breaks down in strongly correlated systems. We develop a general framework, based on the single-particle Green's function and the one-body reduced density matrix (1RDM), to characterize correlated electronic phases. Applying it to the Hubbard diamond chain, we combine density matrix renormalization group and cellular dynamical mean-field theory to construct symmetry-resolved effective orbitals and track their evolution across its Mott transitions, while the 1RDM purity provides a scalar indicator of the phase boundaries. These tools offer a general route to extend one-body concepts to correlated materials.
Dynamical Mean-Field Theory (DMFT) has opened new perspectives for the investigation of strongly correlated electron systems and greatly improved our understanding of correlation effects in models and materials. In contrast to Hartree-Fock-type approximations the mean field of DMFT is dynamical, whereby local quantum fluctuations are fully taken into account. DMFT becomes exact in the limit of high spatial dimensions or coordination number. Using DMFT the dynamics of correlated electron systems can be investigated non-perturbatively at all interaction strengths, electron densities and temperatures. By merging density functional theory with DMFT a powerful method for the calculation of the properties of correlated electron materials has become available, which is applicable to bulk systems and heterostructures, including topological states of matter. The inclusion of non-local correlations into DMFT makes it possible to explore unconventional superconductivity and the critical behavior at thermal or quantum phase transitions. By generalizing DMFT to non-equilibrium states also the real-time dynamics of correlated systems can be investigated. In this brief review the foundations and current status of DMFT are discussed along with characteristic physical insights obtained with this approach.
We propose many-body order parameters for bulk multipoles in crystalline systems, whenever the bulk multipole moments can be defined in terms of the nontrivial topology of the nested Wilson loop. The many-body order parameters are designed to measure multipolar charge distribution in a crystalline unit cell, and they match the localized corner charge originating from the multipoles when the symmetries quantizing the multipoles are protected. Our many-body order parameters provide a complementary view to the nested Wilson loop approaches and even go beyond, as our many-body order parameters are readily applicable to interacting quantum many-body systems. Furthermore, even when the symmetries quantizing multipoles are lost so that the nested Wilson loop spectrum does not exactly reproduce the physical multipole moments, our many-body order parameters faithfully measure the physical multipole moments, which is confirmed numerically during Thouless pumping processes. We provide analytic arguments and numerical demonstration of the order parameters for various higher-order insulators having bulk multipole moments. Finally, we discuss the applicability of our many-body order parameters in the cases where the Wannier gap is absent so that the bulk multiple moments cannot be defined in terms of the nested Wilson loop.
We review the dynamical mean-field theory of strongly correlated electron systems which is based on a mapping of lattice models onto quantum impurity models subject to a self-consistency condition. This mapping is exact for models of correlated electrons in the limit of large lattice coordination (or infinite spatial dimensions). It extends the standard mean-field construction from classical statistical mechanics to quantum problems. We discuss the physical ideas underlying this theory and its mathematical derivation. Various analytic and numerical techniques that have been developed recently in order to analyze and solve the dynamical mean-field equations are reviewed and compared to each other. The method can be used for the determination of phase diagrams (by comparing the stability of various types of long-range order), and the calculation of thermodynamic properties, one-particle Green's functions, and response functions. We review in detail the recent progress in understanding the Hubbard model and the Mott metal-insulator transition within this approach, including some comparison to experiments on three-dimensional transition-metal oxides. We present an overview of the rapidly developing field of applications of this method to other systems. The present limitations of the approach, and possible extensions of the formalism are finally discussed. Computer programs for the numerical implementation of this method are also provided with this article.
We investigate the effects of electronic correlations on the Bernevig-Hughes-Zhang model using the real-space density matrix renormalization group (DMRG) algorithm. We introduce a method to probe topological phase transitions in systems with strong correlations using DMRG, substantiated by an unsupervised machine learning methodology that analyzes the orbital structure of the real-space edges. Including the full multi-orbital Hubbard interaction term, we construct a phase diagram as a function of a gap parameter (mmm) and the Hubbard interaction strength (UUU) via exact DMRG simulations on $N\ifmmode\times\else\texttimes\fi{}4$ cylinders. Our analysis confirms that the topological phase persists in the presence of interactions, consistent with previous studies, but it also reveals an intriguing phase transition from a paramagnetic to a stripey antiferromagnetic topological insulator. The combination of the magnetic structure factor, strength of magnetic moments, and the orbitally resolved density, provides real-space information on both topology and magnetism in a strongly correlated system.