Study of Fractional Quantum Hall Effect in Periodic Geometries

Study of Fractional Quantum Hall Effect in Periodic Geometries
Author: Songyang Pu
Publisher:
Total Pages:
Release: 2021
Genre:
ISBN:

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The fractional quantum Hall effect is one of the most exotic collective phenomena discovered in nature that has triggered the ideas of emergent topological order, fractional statistics, and many other novel concepts. A powerful tool to study the fractional quantum Hall effect is the construction of microscopic trial wave functions, which not only capture the topological features of the physical states in the fractional quantum effect but also allow quantitative calculations of various observables that can be compared to experimental results. A broad class of fractional quantum Hall states is described by the composite fermion theory. Composite fermions are the emergent bound states of electrons and even number of quantized vortices. The microscopic wave functions of composite fermions have been constructed for disk and spherical geometry and have been widely used in explaining experimental results. On the other hand, there are two periodic geometries, torus and cylinder, which are useful for theoretical studies. There are several reasons why people care about these two periodic geometries. First, these geometries allow some freedom to tune the periodic boundary conditions and the geometry itself, making it convenient to calculate some topological quantities, such as Chern number and Hall viscosities. Second, the torus is the natural geometry to study Fermi sea states and crystal states since it can be mapped into a complex plane without defects. Third, the torus is the natural geometry to compare different topological states at the same filling factor because of the absence of "shift". Fourth, the cylinder is the natural geometry to study edge physics. It is also natural to view a cylinder as a quasi-one-dimensional system, which provides convenience to apply the density matrix renormalization group algorithm to the study of the fractional quantum Hall effect. Earlier, only several trial wave functions, such as the Laughlin wave function, the Moore-Read wave function, and the composite fermion Fermi sea wave function are known on a torus. In this thesis, we first construct the composite fermion wave functions for the general Jain states. We introduce a non-trivial projection method to construct wave functions in the lowest Landau level. We further show that the composite fermion wave functions we construct are very accurate descriptions of exact Coulomb eigenstates in the lowest Landau level. They allow the numerical study of large systems, which are not accessible in the exact diagonalization study in the torus geometry. We then apply these composite fermion wave functions to study Berry phase and Hall viscosities. In recent years, the issue of the nature of the composite fermion Fermi sea at $\nu=1/2$ has been of interest. In particular, the Berry phase associated with a loop around the Fermi surface is a criterion to determine whether a composite fermion is a Dirac Fermion. We have applied our lowest Landau level projection approach to the composite fermion Fermi sea to evaluate the Berry phase. We find the $\pi$ Berry phase for the projected composite fermion wave function, which other works have also reported. We further demonstrate that the Berry phase shifts away from $\pi$ with Landau level mixing. More importantly, the rate that the Berry phase rotates away from $\pi$ with the mixing of higher Landau level components increases with the system size. Hall viscosity is a geometric response of Hall fluid. It has been proposed as a topological quantity of quantum Hall fluid by Read. It can be evaluated by deforming the geometry of a torus. We evaluate Hall viscosities for a series of Jain states and showed they are consistent with Read's quantization relation. We show that with some assumption, the Hall viscosity can be derived analytically for the composite fermions and, more generally, the so-called "parton states". We also calculate Hall viscosities of various composite fermion Fermi seas at different fillings and find they cannot be viewed as naive limits of Jain states. Finally, we study the ``composite anyons" which have fractional statistics. They can be viewed as intermediate states between composite fermions when the numbers of vortices attached are fractions. We construct their wave functions on a torus, show the multi-component wave functions satisfy the braiding ground and have expected ground state degeneracy. We also use these anyon wave functions to calculate transport gaps, Chern numbers, and Hall viscosities. We also briefly introduce the composite fermion wave functions on a cylinder and composite fermion crystal wave functions on a torus in this thesis. We also give the outlook for future research at the end.

The Fractional Quantum Hall Effect

The Fractional Quantum Hall Effect
Author: Tapash Chakraborty
Publisher: Springer Science & Business Media
Total Pages: 186
Release: 2012-12-06
Genre: Science
ISBN: 3642971016

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The experimental discovery of the fractional quantum Hall effect (FQHE) at the end of 1981 by Tsui, Stormer and Gossard was absolutely unexpected since, at this time, no theoretical work existed that could predict new struc tures in the magnetotransport coefficients under conditions representing the extreme quantum limit. It is more than thirty years since investigations of bulk semiconductors in very strong magnetic fields were begun. Under these conditions, only the lowest Landau level is occupied and the theory predicted a monotonic variation of the resistivity with increasing magnetic field, depending sensitively on the scattering mechanism. However, the ex perimental data could not be analyzed accurately since magnetic freeze-out effects and the transitions from a degenerate to a nondegenerate system complicated the interpretation of the data. For a two-dimensional electron gas, where the positive background charge is well separated from the two dimensional system, magnetic freeze-out effects are barely visible and an analysis of the data in the extreme quantum limit seems to be easier. First measurements in this magnetic field region on silicon field-effect transistors were not successful because the disorder in these devices was so large that all electrons in the lowest Landau level were localized. Consequently, models of a spin glass and finally of a Wigner solid were developed and much effort was put into developing the technology for improving the quality of semi conductor materials and devices, especially in the field of two-dimensional electron systems.

The Quantum Hall Effects

The Quantum Hall Effects
Author: Tapash Chakraborty
Publisher: Springer
Total Pages: 332
Release: 1995
Genre: Science
ISBN:

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The Fractional Quantum Hall Effect presents a general survery of most of the theoretical work on the subject and briefly reviews the experimental results on the excitation gap. Several new topics like anyons, radiative recombinations in the fractional regime, experimental work on the spin-reversed quasi-particles, etc. are added to render the monographic treatment up-to-date. To complete the picture this second edition includes three chapters on the integral quantum Hall effect.

Geometry, Stability and Response in Lattice Quantum Hall Systems

Geometry, Stability and Response in Lattice Quantum Hall Systems
Author: David Bauer
Publisher:
Total Pages: 106
Release: 2018
Genre:
ISBN:

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Integer and fractional quantum Hall phases are prototypical examples of topologically ordered phases of matter. A standard setting for studying these phases is in continuum two-dimensional electron fluids with broken time-reversal symmetry, where the single particle bands are familiar Landau levels. In this thesis, we explore aspects of quantum Hall fluids in the presence of periodic lattice potential. We focus mostly on the perturbative, Harper-Hofstadter regime of low magnetic flux density, where the single-particle bands are effectively mixtures of Landau levels. We study the stability of strongly-correlated fractional quantum Hall fluids in relation to the geometry of single-particle Chern bands in tight-binding models. We first treat the case of the Hofstadter model and show the convergence of the geometry of Hofstadter bands to that of Landau levels in the limit of small flux density. We then introduce and study a new series of tight-binding models which do not necessarily converge to Landau levels but still host fractional phases. In each case, we observe correlations between band geometry and many-body gaps consistent with the "geometric stability hypothesis." Finally, we give a perturbative calculation of the current response of Chern bands to inhomogeneous electric fields. In continuum quantum Hall fluids with galilean symmetry, the O(q2) contribution to this response is related to the Hall viscosity, which measures the linear response to variations in spatial geometry in time-reversal odd two-dimensional fluids.

Quantum Hall Effect

Quantum Hall Effect
Author: A. H. MacDonald
Publisher: Jaca Book
Total Pages: 286
Release: 1989
Genre: Science
ISBN:

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The Quantum Hall Effect

The Quantum Hall Effect
Author: Richard E. Prange
Publisher: Springer Science & Business Media
Total Pages: 487
Release: 2012-12-06
Genre: Technology & Engineering
ISBN: 146123350X

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After a foreword by Klaus von Klitzing, the first chapters of this book discuss the prehistory and the theoretical basis as well as the implications of the discovery of the Quantum Hall effect on superconductivity, superfluidity, and metrology, including experimentation. The second half of this volume is concerned with the theory of and experiments on the many body problem posed by fractional effect. Specific unsolved problems are mentioned throughout the book and a summary is made in the final chapter. The quantum Hall effect was discovered on about the hundredth anniversary of Hall's original work, and the finding was announced in 1980 by von Klitzing, Dorda and Pepper. Klaus von KIitzing was awarded the 1985 Nobel prize in physics for this discovery.

The Quantum Hall Effect

The Quantum Hall Effect
Author: Daijiro Yoshioka
Publisher: Springer Science & Business Media
Total Pages: 214
Release: 2013-03-09
Genre: Science
ISBN: 3662050161

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The fractional quantum Hall effect has opened up a new paradigm in the study of strongly correlated electrons and it has been shown that new concepts, such as fractional statistics, anyon, chiral Luttinger liquid and composite particles, are realized in two-dimensional electron systems. This book explains the quantum Hall effects together with these new concepts starting from elementary quantum mechanics.

THEORETICAL STUDY OF FRACTIONAL QUANTUM HALL EFFECT UNDER REALISTIC CONDITIONS.

THEORETICAL STUDY OF FRACTIONAL QUANTUM HALL EFFECT UNDER REALISTIC CONDITIONS.
Author: Tongzhou Zhao
Publisher:
Total Pages: 0
Release: 2022
Genre:
ISBN:

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The fractional quantum Hall effect (FQHE) arises when electrons near zero temperature confined in a 2D plane are subjected to a strong perpendicular magnetic field. Since its first discovery [1], the FQHE has always been one of the most important topics in condensed matter physics. The origin of the FQHE can be understood by the composite fermion (CF) theory, according to which emergent particles called composite fermions are formed when an even number of vortices are attached to each electron. Because the vortices partially cancel the Aharonov-Bohm phase generated by the external magnetic field, CFs experience a reduced effective magnetic field, in which Landau-level-like energy bands called the [lambda] levels are formed and filled by CFs at an integer filling. The FQHE of electrons of filling factor [nu] = n 2pn±1 is therefore mapped into the integer quantum Hall effect (IQHE) of CFs. The CF theory successfully predicts many properties of the FQHE, such as filling fractions, collective excitations, spin structures, emergent Fermi sea of CFs with well-defined Fermi wave vectors, and many more. While many great achievements have been made, lots of questions remain to be answered. For example, a typical simplification in FQHE problems is that the electron system is treated as a strict 2D system. While this approximation has been proved to be useful in many cases, it turns out that there are exceptions. In experiments, electrons are usually confined within finite quantum wells. The finite width modifies the effective interaction between electrons. It also changes the nature of the ground state by including the new degree of freedom, as the finite well allows the mixing between different subbands. Another factor of importance is called the Landau level mixing, which is usually neglected in theoretical studies under the approximation that the magnetic field is strong enough to quench electrons to the lowest Landau level. However, under typical experimental conditions at present, the magnetic field is usually not that strong, and higher Landau level components are likely to mix into the system's ground state. The Landau level mixing brings a difference in the effective interaction between electrons, and it also introduces the three-body interaction, which breaks the particle-hole symmetry. The finite width effect and the Landau level mixing effect modify the effective interaction between electrons and may lead to new phases. For example, as the repulsion between electrons is reduced due to the finite width, it is possible that the vortices attached to electrons overscreen the repulsion and make the net interaction between composite fermions attractive. The attraction, therefore, can cause the pairing of composite fermions and lead to the so-called Moore-Read Pfaffian state or its particle-hole conjugation, the anti-Pfaffian state. To quantitatively describe the influence of the finite width and the iii Landau level mixing, we develop the three-dimensional fixed-phase diffusion Monte Carlo method. This method takes care of the finite width effect and the Landau level mixing effect in a single framework, and it is not a perturbative method, which makes it suitable for studying strongly-correlated systems. Equipped with the fixed-phase diffusion Monte Carlo method, especially its 3D version, we systematically study several different systems in this thesis. We find that the finite width together with the Landau level mixing effect can lead to new phases as well as affect the systems' quantitative properties, such as the transport gap. To be explicit, we find that the FQHE at filling factor 1/2 in finite GaAs quantum wells might be the Moore-Read Pfaffian state. We also find that the charge-imbalance in such quantum wells does not favor the Moore-Read Pfaffian state; we find that the Bloch ferromagnetism of composite fermions observed in Ref. [2] might be induced by the change of the Landau level mixing; we find that the Landau level mixing and the finite width cannot fully explain the discrepancy between the theoretical calculation and the experimental measurement of the transport gaps of the FQHEs in the sequence of n 2n+1.