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arXiv:cond-mat/0612538 · Journal of Statistical Mechanics Theory and Experiment

Quasi-adiabatic continuation in gapped spin and fermion systems: Goldstone’s theorem and flux periodicity

M. B. Hastings

Published May 14, 2007• 19 citations
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Abstract

We apply the technique of quasi-adiabatic continuation to study systems with continuous symmetries. We first derive a general form of Goldstone's theorem applicable to gapped nonrelativistic systems with continuous symmetries. We then show that, for a fermionic system with a spin gap, it is possible to insert π flux into a cylinder with only exponentially small change in the energy of the system, a scenario which covers several physically interesting cases such as an s-wave superconductor or a resonating valence bond state.

DOI
10.1088/1742-5468/2007/05/p05010

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We apply the technique of quasi-adiabatic continuation to study systems with continuous symmetries. We first derive a general form of Goldstone's theorem applicable to gapped nonrelativistic systems with continuous symmetries. We then show that, for a fermionic system with a spin gap, it is possible to insert π flux into a cylinder with only exponentially small change in the energy of the system, a scenario which covers several physically interesting cases such as an s-wave superconductor or a resonating valence bond state.
The first proof of the quantum adiabatic theorem was given as early as 1928. Today, this theorem is increasingly applied in a many-body context, e.g., in quantum annealing and in studies of topological properties of matter. In this setup, the rate of variation ϵ of local terms is indeed small compared to the gap, but the rate of variation of the total, extensive Hamiltonian, is not. Therefore, applications to many-body systems are not covered by the proofs and arguments in the literature. In this Letter, we prove a version of the adiabatic theorem for gapped ground states of interacting quantum spin systems, under assumptions that remain valid in the thermodynamic limit. As an application, we give a mathematical proof of Kubo's linear response formula for a broad class of gapped interacting systems. We predict that the density of nonadiabatic excitations is exponentially small in the driving rate and the scaling of the exponent depends on the dimension.
This work provides an overview of gapped quantum spin systems, including concepts, techniques, properties, and results. The basic framework and objects of interest for quantum spin systems are introduced, and the main ideas behind methods for proving spectral gaps for frustration-free models are outlined. After reviewing recent progress on several spectral gap conjectures, we discuss quasi-locality of the Heisenberg dynamics and its utility in proving properties of gapped quantum spin systems. Lieb-Robinson bounds have played a central role in establishing exponential decay of ground state correlations, an area law for one-dimensional systems, a many-body adiabatic theorem, and spectral gap stability. They also aided in the development of the quasi-adiabatic continuation, which is a useful for investigating gapped ground state phases, both of which are also discussed.
We present a possible definition of a mobility gap for a many-body quantum system, in analogy to definitions of dynamical localization for single particle systems. Using this definition, we construct "corrected" quasi-adiabatic continuation operators. Under an appropriate definition of a unique ground state, we show how to introduce virtual fluxes. Armed with these results, we can directly carry over previous results in the case of a spectral gap. We present a proof of decay of correlation functions and we present a proof of Hall conductance quantization under very mild density-of-states assumptions defined later. We also generalize these definitions to the case of a "bulk mobility gap", in the case of a system with boundaries, and present a proof of Hall conductance quantization on an annulus under appropriate assumptions. Further, we present a new "optimized" quasi-adiabatic continuation operator which simplifies previous estimates and tightens bounds in certain cases. This is presented in an appendix which can be read independently of the rest of the paper as it also improves estimates in the case of systems with a spectral gap. This filter function used decays in time at least as fast as O(exp⁡(−tα)){\cal O}(\exp(-t^α))O(exp(−tα)) for all α<1α<1α<1, a class of decay called subexponential (a tighter description of what is possible is below). Using this function it is possible to tighten recent estimates of the Hall conductance quantization for gapped systems\cite{hall} to a decay which is subexponential in system size.
In accord with Nambu-Goldstone theorem spontaneous breaking of a global continuous symmetry of the system by a mean-field parameter causes appearance of massless Goldstone bosons, called also gapless Goldstone modes. It is demonstrated below that the theorem can be violated, i.e. the Goldstone modes possess finite gap, in the case of a novel (quantum ordered) state of interacting fermions, that was proposed recently {mukhin 2011}. This result may lead to the important consequences when considered in combination with another peculiar properties of the quantum parameter (QOP) that where recently found: QOP's Green's function is finite and periodic along the Matsubara's axis, but Wick-rotated to the axis of real frequencies, possesses infinite periodic chain of \it{second poles} along this axis {mukhin 2011}. This property of QOP leads to a remarkable physical consequence: \it{QOP does not scatter anything in Minkowski world} ! Hence, the present theory, we believe, may naturally indicate that two effects: hidden order revealed in ARPES spectra of fermi-excitations and neutron resonance feature at finite frequency in the magnetic (bosonic) excitations spectrum in lightly hole-doped copper oxides, could be emerging fingerprints of the Euclidian crystal state. A peculiar manifestation of the QOP self-consistent solution in the form of non-classical phonon states is described for an Euclidian predicted recently {mukhin,fistul 2013} in the array of Josephson junctions coupled via electromagnetic field in the superconducting resonator. It is demonstrated that it may constitute a stable state of the system, that, for a particular quantum disordering model, exists inside the stability domain of the classical photonic condensate delimited by the critical value of the quantum disordering strength.
When global continuous symmetries are spontaneously broken, there appear gapless collective excitations called Nambu-Goldstone modes (NGMs) that govern the low-energy property of the system. The application of this famous theorem ranges from high-energy, particle physics to condensed matter and atomic physics. When a symmetry breaking occurs in systems that lack the Lorentz invariance to start with, as is usually the case in condensed matter systems, the number of resulting NGMs can be fewer than that of broken symmetry generators, and the dispersion of NGMs is not necessarily linear. In this article, we review recently established formulas for NGMs associated with broken internal symmetries that work equally for relativistic and nonrelativistic systems. We also discuss complexities of NGMs originating from space-time symmetry breaking. In the process we cover many illuminating examples from various context. We also present a complementary point of view from the Lieb-Schultz-Mattis theorem.