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arXiv:2006.11862 · Physical Review Research

Interaction effects on quantum Hall transitions: Dynamical scaling laws and superuniversality

Prashant Kumar, P. A. Nosov, S. Raghu

Published Aug 24, 2022• 11 citations
arXivHTMLPDF

Abstract

We study the role of electron-electron interactions near integer and abelian fractional quantum Hall (QH) transitions using composite fermion (CF) representations. Interaction effects are encapsulated in CF theories as gauge fluctuations. Without gauge fluctuations, the CF system realizes a `dual' representation of the noninteracting QH transition. With gauge fluctuations, the system is governed by a gauged nonlinear sigma model (NLSM) with a $\ensuremath{\theta}\phantom{\rule{0.28em}{0ex}}\mathrm{term}$. While the transition is described by a strong-coupling fixed point of the NLSM, we are nevertheless able to deduce two of its properties. With $1/r$ interactions, (1) the transition has a dynamical exponent $z=1$, and (2) all transitions are ``superuniversal'': fractional and integer QH transitions are in the same universality class. With short-range interactions, $z=2$ and the fate of superuniversality remains unclear.

DOI
10.1103/physrevresearch.4.033146

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We study the role of electron-electron interactions near integer and abelian fractional quantum Hall (QH) transitions using composite fermion (CF) representations. Interaction effects are encapsulated in CF theories as gauge fluctuations. Without gauge fluctuations, the CF system realizes a `dual' representation of the noninteracting QH transition. With gauge fluctuations, the system is governed by a gauged nonlinear sigma model (NLSM) with a \ensuremathθterm\ensuremath{\theta}\phantom{\rule{0.28em}{0ex}}\mathrm{term}\ensuremathθterm. While the transition is described by a strong-coupling fixed point of the NLSM, we are nevertheless able to deduce two of its properties. With 1/r1/r1/r interactions, (1) the transition has a dynamical exponent z=1z=1z=1, and (2) all transitions are ``superuniversal'': fractional and integer QH transitions are in the same universality class. With short-range interactions, z=2z=2z=2 and the fate of superuniversality remains unclear.
Quantum Hall inter-plateau transitions are physical exemplars of quantum phase transitions. Near each of these transitions, the measured electrical conductivity scales with the same correlation length and dynamical critical exponents, i.e., the critical points are superuniversal. In apparent contradiction to these experiments, prior theoretical studies of quantum Hall phase transitions within the framework of Abelian Chern-Simons theory coupled to matter found correlation length exponents that depend on the value of the quantum critical Hall conductivity. Here, we use non-Abelian bosonization and modular transformations to theoretically study the phenomenon of superuniversality. Specifically, we introduce a new effective theory that has an emergent U(N)U(N)U(N) gauge symmetry with any N>1N>1N>1 for a quantum phase transition between an integer quantum Hall state and an insulator. We then use modular transformations to generate from this theory effective descriptions for transitions between a large class of fractional quantum Hall states whose quasiparticle excitations have Abelian statistics. We find the correlation length and dynamical critical exponents are independent of the particular transition within a controlled 't Hooft large NNN expansion, i.e., superuniversal! We argue that this superuniversality could survive away from this controlled large NNN limit using recent duality conjectures.
We study the influence of short-range electron-electron interactions on scaling behavior near the integer quantum Hall plateau transitions. Short-range interactions are known to be irrelevant at the renormalization group fixed point which represents the transition in the noninteracting system. We find, nevertheless, that transport properties change discontinuously when interactions are introduced. Most importantly, in the thermodynamic limit the conductivity at finite temperature is zero without interactions, but nonzero in the presence of arbitrarily weak interactions. In addition, scaling as a function of frequency $\ensuremath{\omega}$ and temperature T is determined by the scaling variable $\ensuremath{\omega}{/T}^{p}$ (where p is the exponent for the temperature dependence of the inelastic scattering rate) and not by $\ensuremath{\omega}/T,$ as it would be at a conventional quantum phase transition described by an interacting fixed point. We express the inelastic exponent p and the thermal exponent zT{z}_{T}zT​ in terms of the scaling dimension $\ensuremath{-}\ensuremath{\alpha}<0$ of the interaction strength and the dynamical exponent z (which has the value z=2),z=2),z=2), obtaining $p=1+2\ensuremath{\alpha}/z$ and zT=2/p.{z}_{T}=2/p.zT​=2/p.
Superuniversality and non-abelian bosonization in 2+12+12+1 dimensions
Superuniversality is the sharing of the same critical exponents among distinct critical points. This surprising behavior is observed at phase transitions between various quantum Hall states found in the two-dimensional electron gas. Motivated by the theoretical challenge posed by superuniversality, we introduce a new effective description that has an emergent U(N)U(N)U(N) gauge symmetry with N>1N > 1N>1 for a quantum phase transition between an integer quantum Hall state and an insulator. We use modular transformations to generate from this theory new effective descriptions for quantum phase transitions between a large class of fractional quantum Hall states whose quasiparticle excitations have abelian statistics. In the 't Hooft large NNN limit, the correlation length and dynamical critical exponents are independent of the particular transition in the absence of disorder. We argue that this conclusion survives away from the large NNN limit using recent duality conjectures and, thereby, provide theoretical support for the phenomenon of superuniversality found at distinct quantum Hall critical points.
The composite fermion (CF) theory gives both a phenomenological description for many fractional quantum Hall (FQH) states, as well as a microscopic construction for large scale numerical calculation of these topological phases. The fundamental postulate of mapping FQH states of electrons to integer quantum Hall (IQH) states of CFs, however, was not formally established. The Landau level (LL) projection needed for the microscopic calculations is in some sense uncontrolled and unpredictable. We rigorously derive the unitary relationship between electrons and the CFs, showing the latter naturally emerge from special interactions within a single LL, without resorting to any projection by hand. In this framework, all FQH states topologically equivalent to those described by the conventional CF theory (e.g., the Jain series) have exact model Hamiltonians that can be explicitly derived, and we can easily generalize to FQH states from interacting CFs. Our derivations reveal fundamental connections between the CF theory and the pseudopotential/Jack polynomial constructions, and argue that all Abelian CF states are physically equivalent to the IQH states, while a plethora of non-Abelian CF states can be systematically constructed and classified. We also discuss about implications to experiments and effective field theory descriptions based on the descriptions with CFs as elementary particles.
The integer quantum Hall to insulator transition (IQHIT) is a paradigmatic quantum critical point. Key aspects of this transition, however, remain mysterious, due to the simultaneous effects of quenched disorder and strong interactions. We study this transition using a composite fermion (CF) representation, which incorporates some of the effects of interactions. As we describe, the transition also marks an IQHIT of CFs: this suggests that the transition may exhibit ``self-duality.'' We show the explicit equivalence of the electron and CF Lagrangians at the critical point via the corresponding nonlinear sigma models, revealing the self-dual nature of the transition. We show analytically that the resistivity tensor at the critical point is ${\ensuremath{\rho}}_{xx}^{c}={\ensuremath{\rho}}_{xy}^{c}=\frac{h}{{e}^{2}}$, which are consistent with the expectations of self-duality, and in rough agreement with experiments.
We study particle-hole symmetry at the integer quantum Hall plateau transition using composite fermion mean-field theory. Because this theory implicitly includes some electron-electron interactions, it also has applications to certain fractional quantum Hall plateau transitions. Previous work [P. Kumar et al., Phys. Rev. B 100, 235124 (2019)] using this approach showed that the diffusive quantum criticality of this transition is described by a nonlinear sigma model with topological $\ensuremath{\theta}=\ensuremath{\pi}$ term. This result, which holds for both the Dirac and Halperin, Lee, and Read composite fermion theories, signifies an emergent particle-hole (reflection) symmetry of the integer (fractional) quantum Hall transition. Here we consider the stability of this result to various particle-hole symmetry-violating perturbations. In the presence of quenched disorder that preserves particle-hole symmetry, we find that finite longitudinal conductivity at this transition requires the vanishing of a symmetry-violating composite fermion effective mass, which if present would generally lead to $\ensuremath{\theta}\ensuremath{\ne}\ensuremath{\pi}$ and a corresponding violation of particle-hole symmetric electrical transport ${\ensuremath{\sigma}}_{xy}\ensuremath{\ne}\frac{1}{2}\frac{{e}^{2}}{h}$. When the disorder does not preserve particle-hole symmetry, we find that $\ensuremath{\theta}$ can vary continuously within the diffusive regime. Our results call for further study of the universality of the quantum Hall plateau transition.