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arXiv:2006.15567

Geometry and topology in many-body physics

Raffaele Resta

Published Jun 28, 2020• 0 citations
arXivHTMLPDF

Abstract

Some intensive observables of the electronic ground state in condensed matter have a geometrical or even topological nature. In this Review I present the geometrical observables whose expression is known in a full many-body framework, beyond band-structure theory. The formalism allows dealing with the general case of disordered and/or correlated many-electron systems.

DOI
10.48550/arxiv.2006.15567

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Some intensive observables of the electronic ground state in condensed matter have a geometrical or even topological nature. In this Review I present the geometrical observables whose expression is known in a full many-body framework, beyond band-structure theory. The formalism allows dealing with the general case of disordered and/or correlated many-electron systems.
We develop a conditionally closed theoretical architecture for condensed-matter and solid-state physics from three physical axioms: microscopic ontic definiteness and finite localization, the physical reality of the spacetime-vacuum substrate, and persistent causal source–response coupling. Electrons, nuclei, atoms, and molecular units enter as finite topological carriers, whereas crystals, energy bands, phonons, quasiparticles, order parameters, defects, and topological phases are derived as spectral, variational, statistical, or homotopy properties of the resulting many-body system. We construct a sorted first-order language, a model-theoretic conservative extension of the underlying physical theory, and a constrained Legendre descent from a unified matter–substrate action to a self-adjoint, lower-bounded many-body Hamiltonian. A signed ownership ledger prevents double counting among self-energy, screening, correlation, electron–phonon, electromagnetic, historical-memory, and environmental contributions. For neutral stable branches, we establish a thermodynamic limit under translation covariance, two-sided almost-additivity, controlled Coulomb surface terms, and passive auxiliary realizations of causal memory. The framework derives crystal symmetry, Bloch and Wannier representations, lattice dynamics, phonon quantization, electronic bands, semiconductor statistics, Green functions, conserving Kadanoff–Baym dynamics, strong-correlation reductions, magnetism, polarons, superconductivity, disorder, localization, transport, critical phenomena, dielectric response, defects, interfaces, and low-dimensional systems. Conventional Bloch, tight-binding, nearly-free-electron, Debye, Landau, Fermi-liquid, Hubbard, Heisenberg, BCS/BdG, Boltzmann, and Kubo theories are recovered only as controlled limits with explicit validity domains and failure conditions. Fundamental-carrier topology, band topology, and interacting many-body topology are distinguished rigorously. Stability criteria are given for spectral and mobility gaps, Green-function invariants, Floquet and non-Hermitian phases, completely positive open dynamics, Liouvillian steady-state bundles, and finite-memory auxiliary systems. We also specify when thermodynamic, zero-frequency, zero-wave-number, long-time, symmetry-breaking, and regulator limits commute and when their ordering defines distinct physical observables. A terminal completion theorem identifies the conditions for pure-theory closure: semantic conservativity, action traceability, self-adjoint lower-bounded dynamics, thermodynamic-limit control, causality, positivity, conservation laws, controlled standard limits, valid topological domains, and falsifiable observable maps. This theoretical closure does not imply that the spectrum, phase diagram, or transport coefficients of any particular material have been calculated or experimentally confirmed; such tasks belong to separate material-application programs. Keywords **global realism; topological condensed matter; solid-state physics; many-body theory; causal memory; historical disturbance field; Green functions; strong correlations; superconductivity; topological phases; quantum transport; thermodynamic limit; open quantum systems; Liouvillian topology; controlled effective theories**
There have been significant recent advances in realizing band structures with geometrical and topological features in experiments on cold atomic gases. This review summarizes these developments, beginning with a summary of the key concepts of geometry and topology for Bloch bands. Descriptions are given of the different methods that have been used to generate these novel band structures for cold atoms and of the physical observables that have allowed their characterization. The focus is on the physical principles that underlie the different experimental approaches, providing a conceptual framework within which to view these developments. Also described is how specific experimental implementations can influence physical properties. Moving beyond single-particle effects, descriptions are given of the forms of interparticle interactions that emerge when atoms are subjected to these energy bands and of some of the many-body phases that may be sought in future experiments.
T HE BERRY PHASE, A CENTRAL CONCEPT IN QUANTUM physics, is rooted in the state space structure of quantum theory and has a profound observable impact within a wide range of phenomena. The remarkably elegant interpretation of this geometric phase as the holonomy of a fiber bundle provides it with a mathematical framework capable of accounting for a broad spectrum of classical and quantum phenomena, such as, for example, the precession of a Foucault pendulum, the parallel transport of a vector on a sphere, the Aharonov-Bohm effect, the gauge theory of molecular physics, the electric polarization and orbital magnetization of solids, and the integer and fractional quantum Hall effects. The Berry phase is the central concept in topological band theory and provides the mechanism responsible for anyonic statistics in correlated many-body systems, a key element that could be used to achieve fault-tolerant quantum computation. In other words, the Berry phase is not just some fashionable idea useful for explaining a few exotic phenomena, but a generic feature of quantum mechanics and an essential ingredient necessary for a coherent understanding of basic quantum phenomena, such as, for example, the electronic properties of materials. It is therefore rather surprising that the full significance of geometric phases was practically ignored for more than fifty years prior to the publication of Berry’s work in 1984 [50]. In this chapter we discuss the basic physics that gives rise to these geometric phases, point out the elements that allow us to distinguish different types of holonomies, summarize the basic concepts relevant to understanding the mathematical structure of geometric phases, and mention a few relevant applications. The concepts and the mathematical framework introduced here will be used in subsequent chapters as the basis for describing the topological properties of condensed matter systems.
The Schrödinger equation is nearly a century old, yet we are still in the midst of uncovering the remarkable phenomena emerging in many-body quantum systems. From superconductivity to anyonic quasiparticles, nature consistently surprises with its rich self-organization. To elucidate and grasp this variety, it is paramount to understand the phases of matter that can occur in many-body ground states, as well as their emergent collective excitations. Of particular interest are topological phases of matter, characterized by exotic excitations or edge phenomena. There exist by now several universal frameworks for gapped systems, i.e., those with an energy gap above the ground state. However, in the last decade, a multitude of gapless quantum wires---effectively one-dimensional systems---have been reported to be topologically non-trivial. A framework for their understanding and classification is missing. In addition to ground state order---topological or otherwise---a more complete picture involves the properties of excitations above the ground state. Alas, little is known about excitations beyond the universal low-energy regime. In part, this is due to a lack of analytical and numerical methods able to describe excitations at finite energies, especially in strongly-interacting systems beyond one dimension. In this thesis, we address these issues: firstly, we build a general understanding of topological phases in one dimension, including both gapped and gapless cases. In particular, we unify previously studied examples into a single framework. Secondly, we develop a novel numerical method for obtaining spectral functions in two dimensions---these give direct insight into the properties of excitations and are moreover experimentally measurable. Using this numerical method, we uncover a variety of robust properties of excitations at finite energies. Part I of this thesis concerns gapped and gapless topological phases in one dimension. In Chapter 2, we first treat the case of non-interacting fermions. Therein, we review the known classification of gapped phases before extending it to the gapless case, showing that exponentially-localized Majorana zero modes can still emerge at the edge when the bulk is gapless. Interacting gapped phases are discussed in Chapter 3, with a focus on symmetry-protected topological order. These have already been classified; our contribution is to provide a non-technical review of this classification as well as showing that many paradigmatic model Hamiltonians can be related to one another. Finally, Chapter 4 introduces the notion of symmetry-enriched quantum criticality, which we propose as a framework for classifying gapless phases. The key message is that in the presence of symmetries, a universality class can divide into distinct phases, characterized by the symmetry action on the low-energy scaling operators. This includes gapless topological phases, with examples hiding in plain sight; we clarify their stability and reinterpret previously studied examples. Part II studies the excitations above the ground states of two-dimensional quantum spin models. The main object of our study is the dynamic spin structure factor; this type of spectral function is reviewed in the first part of Chapter 5. The second part of this chapter introduces a novel matrix-product-state-based algorithm to efficiently compute it, opening a new window on the dynamics of two-dimensional quantum systems. We benchmark this numerical method in Chapter 6 on the exactly-solvable Kitaev model---a paradigmatic topological model realizing a quantum spin liquid. By adding non-integrable Heisenberg perturbations, we identify the first unequivocal theoretical realization of a proximate spin liquid: the ground state becomes conventionally ordered, yet the high-energy spectral properties are structurally similar to those of the nearby Kitaev spin liquid. The latter agrees with aspects of recent inelastic neutron scattering experiments on alpha-RuCl3. In Chapter 7, we turn to one of the oldest models in many-body quantum physics: the spin-1/2 Heisenberg antiferromagnet on the square lattice. Despite its venerable history, there is still disagreement about the physical origin of high-energy spectral features which low-order spin wave theory cannot account for. We provide a simple picture for this strongly-interacting-magnon feature by connecting it to a simple Ising limit. Lastly, Chapter 8 discusses the stability of quasiparticles---collective excitations behaving like a single emergent entity, of which magnons are a prime example. These are often known to be stable at the lowest energies and are presumed to decay whenever this is seemingly allowed by energy and momentum conservation. However, we show that strong interactions can prevent this from happening. We numerically confirm this principle of avoided decay in the (slightly-detuned) Heisenberg antiferromagnet on the triangular lattice. Moreover, we can even identify its fingerprints in existing experimental data on Ba3CoSb2O9 and superfluid helium. In this thesis, we thus enlarge our understanding of quantum phases and their excitations. The identification of the key principles of gapless topological phases in one dimension calls for direct analogues in higher dimensions, waiting to be uncovered. With regard to the robust properties of the excitations identified in this thesis, we are hopeful that these can be extended into a theory of quasiparticle properties away from the universal low-energy regime.
The Mott transition is a classic paradigm in quantum many-body physics, in which electronic interactions can produce an insulating state. By contrast, a nontrivial band-structure topology in condensed matter systems leads to a topological insulator. Here, the authors make a connection between the seemingly disparate physics of these systems, showing that the Mott transition can be understood as a topological transition, and the Mott insulating state manifests a hitherto hidden topology in the interaction self-energy.
This paper presents a novel geometric quantum field theory (GQFT) framework for analyzing quantum field theories within condensed matter systems. We introduce a new approach that explicitly incorporates geometric constraints and topological properties, aiming to provide a deeper understanding of emergent phenomena observed in these systems. The core of this work involves extending standard field theory techniques to accommodate these geometric elements, thereby offering a fundamentally different perspective on the behavior of quantum fields within condensed matter. We detail the key mathematical components and demonstrate the framework's ability to successfully model a range of complex quantum phenomena, including the emergence of quantized vortices and topological phase transitions. The resulting theory provides a robust and mathematically sound tool for investigating the intricate interplay between quantum mechanics and geometric structures in condensed matter physics. This research will significantly advance the field by offering a more intuitive and powerful method for tackling problems previously intractable with traditional approaches.