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arXiv:2102.00934 · Physical review. B./Physical review. B

Thermoelectric Hall conductivity of fractional quantum Hall systems on a disk

Zi-Yi Fang, Dan Ye, Yuyu Zhang, Zi-Xiang Hu

Published Jun 25, 2021• 3 citations
arXivHTMLPDF

Abstract

For fractional quantum Hall states on a finite disk, we study the thermoelectric transport properties under the influence of the presence of an edge and its reconstruction. In a recent study on a torus [Phys. Rev. B 101, 241101(R) (2020)], Sheng and Fu found a universal non-Fermi liquid power-law scaling of the thermoelectric conductivity ${\ensuremath{\alpha}}_{xy}\ensuremath{\propto}{T}^{\ensuremath{\eta}}$ for the gapless composite Fermi liquid state. The exponent $\ensuremath{\eta}\ensuremath{\sim}0.5$ appears an independence of the filling factors and the details of the interactions. In the presence of an edge, we find the properties of the edge spectrum dominants the low-temperature behaviors and a nonuniversal scaling behavior in low temperature is observed. To consider individually the effect of the edge states, the real space entanglement spectrum of the model wave function, which contains only the spectrum of edge excitation is employed. Its spectrum energies could be tuned by varying the radius of the subsystem. In the non-Abelian Moore-Read state, the Majorana neutral edge mode is found to have more significant effects than that of the charge mode in the low temperature.

DOI
10.1103/physrevb.103.235161

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For fractional quantum Hall states on a finite disk, we study the thermoelectric transport properties under the influence of the presence of an edge and its reconstruction. In a recent study on a torus [Phys. Rev. B 101, 241101(R) (2020)], Sheng and Fu found a universal non-Fermi liquid power-law scaling of the thermoelectric conductivity \ensuremathαxy\ensuremath∝T\ensuremathη{\ensuremath{\alpha}}_{xy}\ensuremath{\propto}{T}^{\ensuremath{\eta}}\ensuremathαxy​\ensuremath∝T\ensuremathη for the gapless composite Fermi liquid state. The exponent \ensuremathη\ensuremath∼0.5\ensuremath{\eta}\ensuremath{\sim}0.5\ensuremathη\ensuremath∼0.5 appears an independence of the filling factors and the details of the interactions. In the presence of an edge, we find the properties of the edge spectrum dominants the low-temperature behaviors and a nonuniversal scaling behavior in low temperature is observed. To consider individually the effect of the edge states, the real space entanglement spectrum of the model wave function, which contains only the spectrum of edge excitation is employed. Its spectrum energies could be tuned by varying the radius of the subsystem. In the non-Abelian Moore-Read state, the Majorana neutral edge mode is found to have more significant effects than that of the charge mode in the low temperature.
We study the thermoelectric transport properties of fractional quantum Hall systems based on an exact diagonalization calculation. Based on the relation between the thermoelectric response and thermal entropy, we demonstrate that thermoelectric Hall conductivity ${\ensuremath{\alpha}}_{xy}$ has power-law scaling ${\ensuremath{\alpha}}_{xy}\ensuremath{\propto}{T}^{\ensuremath{\eta}}$ for gapless composite Fermi-liquid states at filling numbers $\ensuremath{\nu}=1/2$ and 1/41/41/4 at low temperatures (TTT), with an exponent $\ensuremath{\eta}\ensuremath{\sim}0.5$ distinctly different from Fermi liquids. The power-law scaling remains unchanged for different forms of interaction including Coulomb and short-range ones, demonstrating the robustness of non-Fermi-liquid behavior of these interacting systems at low TTT. In contrast, for the 1/31/31/3 fractional quantum Hall state, ${\ensuremath{\alpha}}_{xy}$ vanishes at low TTT with an activation gap associated with neutral collective modes rather than charged quasiparticles. Our results establish another manifestation of the non-Fermi-liquid nature of quantum Hall fluids at a finite temperature.
We study the ground state and low-energy excitations of fractional quantum Hall systems on a disk at a filling fraction nu = 5/2, with Coulomb interaction and a background confining potential. We find the Moore-Read ground state is stable within a finite but narrow window in parameter space. The corresponding low-energy excitations contain a fermionic branch and a bosonic branch, with widely different velocities. A short-range repulsive potential can stabilize a charge +e/4 quasihole at the center, leading to a different edge excitation spectrum due to the change of boundary conditions for Majorana fermions, clearly indicating the non-Abelian nature of the quasihole.
The edge of the electronic fractional quantum Hall (FQH) system obeys the law of the chiral Luttinger liquid theory due to its intrinsic topological properties and the relation of bulk-edge correspondence. However, in a realistic experimental system, such as the usual Hall bar setup, the softening of the background confinement potential can induce the reconstruction of the edge spectrum, which breaks the chirality and universality of the FQH edge. The entanglement spectrum (ES) of the FQH ground state has the same counting structure as that of the energy spectrum indicating the topological characters of the quantum state. In this work, we report that the ES can also have an edge reconstruction while sweeping the area of the subsystem in real space cut. Moreover, we found that the critical area of the subsystem matches accurately with the intrinsic building block of the FQH liquids, namely the correlation hole of the FQH liquids. The above results seem to be universal after our study of a series of typical FQH states, such as two Laughlin states at $\ensuremath{\nu}=1/3$ and 1/51/51/5, and the Moore-Read state for $\ensuremath{\nu}=5/2$.
We show that it is possible and rather efficient to compute at non-zero temperature the thermoelectric characteristics of Coulomb blockaded fractional quantum Hall islands, formed by two quantum point contacts inside of a Fabry–Pérot interferometer, using the conformal field theory partition functions for the chiral edge excitations. The oscillations of the thermopower with the variation of the gate voltage as well as the corresponding figure-of-merit and power factors, provide finer spectroscopic tools which are sensitive to the neutral multiplicities in the partition functions and could be used to distinguish experimentally between different universality classes sharing the same electric properties. We also propose a procedure for measuring the ratio r=vn/vc of the Fermi velocities of the neutral and charged edge modes for filling factor νH=5/2 from the power-factor data in the low-temperature limit.
The neutral fermionic edge mode is essential to the non-Abelian topological property and its experimental detection in Zk{Z}_{k}Zk​ fractional quantum Hall (FQH) state for k>1k>1k>1. Usually, the identification of the edge modes in a finite size system is difficult, especially near the region of the edge reconstruction, due to mixing with the bosonic edge mode and the bulk states as well. We study the edge-mode excitations of the Moore-Read (MR) and Read-Rezayi (RR) states by using Jack polynomials in the truncated subspace. It is found that the electron density, as a detector, has marked different behaviors between the bosonic and fermionic edge modes. As an application, it helps us to identify them near the edge reconstruction and in the RR edge spectrum. On the other hand, we systematically study the edge excitations for the RR state, extrapolating the edge velocities and their related coherence length and temperature in the interferometer experiments.
We present a comprehensive numerical study of a microscopic model of the fractional quantum Hall system at filling fraction $\ensuremath{\nu}=5∕2$, based on the disk geometry. Our model includes Coulomb interaction and a semirealistic confining potential. We also mix in a three-body interaction in some cases to help elucidate the physics. We obtain a phase diagram, discuss the conditions under which the ground state can be described by the Moore-Read state, and study its competition with neighboring stripe phases. We also study quasihole excitations and edge excitations in the Moore-Read-like state. From the evolution of the edge spectrum, we obtain the velocities of the charge and neutral edge modes, which turn out to be very different. This separation of velocities is a source of decoherence for a non-Abelian quasihole and/or quasiparticle (with charge $\ifmmode\pm\else\textpm\fi{}e∕4$) when propagating at the edge; using numbers obtained from a specific set of parameters, we estimate the decoherence length to be around $4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. This sets an upper bound for the separation of the two point contacts in a double point-contact interferometer, designed to detect the non-Abelian nature of such quasiparticles. We also find a state that is a potential candidate for the recently proposed anti-Pfaffian state. We find the speculated anti-Pfaffian state is favored in weak confinement (smooth edge), while the Moore-Read Pfaffian state is favored in strong confinement (sharp edge).