We examine the Hall conductivity of a macroscopic two-dimensional quantum system and show that the observed quantities can sometimes violate the fluctuation dissipation theorem (FDT), even in the linear response (LR) regime infinitesimally close to equilibrium. At low temperature and in strong magnetic field, which are experimentally accessible, the violation can be by an order of magnitude larger than the Hall conductivity itself. We further generalize the results and obtain a necessary condition for such large-scale violation to happen. This violation is a genuine quantum phenomenon that appears on a macroscopic scale when the time-reversal symmetry is broken (by, e.g., a magnetic field). Our results are not only bound to the development of the fundamental issues of nonequilibrium physics, but the idea is also meaningful for practical applications, since the FDT is widely used for the estimation of noises from the LRs. We give an alternative formula evaluating precisely the fluctuations from the LRs even in the quantum systems with large FDT violation.
For macroscopic quantum systems, we study what is measured when equilibrium fluctuations of macrovariables are measured in an ideal way that mimics classical ideal measurements as closely as possible. We find that the symmetrized time correlation is always obtained for such measurements. As an important consequence, we show that the fluctuation-dissipation theorem is partially violated as a relation between observed quantities in macroscopic quantum systems even if measurements are made in such an ideal way.
Abstract We consider an infinitely extended system of fermions on a d-dimensional lattice with (magnetic) translation-invariant short-range interactions. We further assume that the system has a gapped ground state. Physically, this is a model for the bulk of a generic topological insulator at zero temperature, and we are interested in the current response of such a system to a constant external electric field. Using the non-equilibrium almost-stationary states approach, we prove that the longitudinal current density induced by a constant electric field of strength
ε ε is of order
O(ε∞) O ( ε ∞ ) , i.e. the system is an insulator in the usual sense. For the Hall current density we show instead that it is linear in
ε ε up to terms of order
O(ε∞) O ( ε ∞ ) . The proportionality factor
σH σ H is by definition the Hall conductivity, and we show that it is given by a generalization of the well known double commutator formula to interacting systems. As a by-product of our results, we find that the Hall conductivity is constant within gapped phases, and that for
d=2 d = 2 the relevant observable that “measures” the Hall conductivity in experiments, the Hall conductance, not only agrees with
σH σ H in expectation up to
O(ε∞) O ( ε ∞ ) , but also has vanishing variance. A notable difference to several existing results on the current response in interacting fermion systems is that we consider a macroscopic system exposed to a small constant electric field, rather than to a small voltage drop.
The fate of the current carrying states of a quantum Hall system is considered in the situation when the disorder strength is increased and the transition from the quantum Hall liquid to the Hall insulator takes place. We investigate a two-dimensional lattice model with spatially correlated disorder potentials and calculate the density of states and the localization length either by using a recursive Green-function method or by direct diagonalization in connection with the procedure of level statistics. From the knowledge of the energy and disorder dependence of the localization length and the density of states (DOS) of the corresponding Landau bands, the movement of the current carrying states in the disorder-energy and disorder-filling-factor planes can be traced by tuning the disorder strength. We show results for all subbands, particularly the traces of the Chern and anti-Chern states as well as the peak positions of the DOS. For small disorder strength W we recover the well-known weak levitation of the critical states, but we also reveal, for larger W, the strong levitation of these states across the Landau gaps without merging. We find the behavior to be similar for exponentially, Gaussian, and Lorentzian correlated disorder potentials. Our study resolves the discrepancies of previously published work in demonstrating the conflicting results to be only special cases of a general lattice model with spatially correlated disorder potentials. To test whether the mixing between consecutive Landau bands is the origin of the observed floating, we truncate the Hilbert space of our model Hamiltonian and calculate the behavior of the current carrying states under these restricted conditions.
The electron distribution over Landau levels is calculated in the regime of the quantum Hall effect at strong Coulomb interaction. For this purpose, a modified scheme of exact diagonalization with a significantly reduced basis of many-particle configurations has been developed, which makes it possible to adequately take into account the mixing of states in several Landau levels. The behavior of the electron distribution function over Landau levels is studied as a function of the filling factor and the Wigner–Seitz radius rs. It is shown that the Landau quantization at a fixed electron density significantly suppresses the smearing of the electron distribution function as compared to the case of zero magnetic field. For example, for rs < 1 and low filling factors, the quasiparticle contribution to the Migdal jump 1 − Z in the distribution function is an approximately linear function of ν and a quadratic function of rs. Simultaneously, as ν decreases, the tails of the distribution function become longer. The mechanism of rearrangement of the distribution function in the regime of the quantum Hall effect is described in terms of the generation of magnetoplasmon fluctuations involved in the structure of the ground state of the system.
The electron distribution over Landau levels is calculated in the regime of the quantum Hall effect at strong Coulomb interaction. For this purpose, a modified scheme of exact diagonalization with a significantly reduced basis of many-particle configurations has been developed, which makes it possible to adequately take into account the mixing of states in several Landau levels. The behavior of the electron distribution function over Landau levels is studied as a function of the filling factor and the Wigner–Seitz radius rs. It is shown that the Landau quantization at a fixed electron density significantly suppresses the smearing of the electron distribution function as compared to the case of zero magnetic field. For example, for rs 1 and low filling factors, the quasiparticle contribution to the Migdal jump 1 − Z in the distribution function is an approximately linear function of ν and a quadratic function of rs. Simultaneously, as ν decreases, the tails of the distribution function become longer. The mechanism of rearrangement of the distribution function in the regime of the quantum Hall effect is described in terms of the generation of magnetoplasmon fluctuations involved in the structure of the ground state of the system.