arXiv subjects

S. -R. Eric Yang

Publications and source records attributed to S. -R. Eric Yang.

At least 55 records · Page 3Linked to original sources

arXiv2005

Reconstruction of condensed magnetoexciton droplet in a trap in strong magnetic fields

We investigate theoretically the Bose-Einstein condensation of trapped magnetoexcitons in a two-layered system with one layer containing electrons and the other layer containing holes. We have studied the spatial variations of the condensate density in the droplet of electrons and holes. We find that the shape of the electron and hole densities may change due to the competition between repulsive electron-electron/hole-hole interaction and confinement potential of the trap. Our mean field calculations show that when the confinement strength is strong enough the condensate density is peaked at the edge of the droplet, and as the confinement strength weakens the condensate density displays one inner peak and another peak at the edge. For much weaker confinement potential the condensate density may display one broad peak.

cond-mat.mes-hall↗

arXiv2004

Spin singlet-triplet transition in a Si-based two-electron double quantum dot molecule

We report a successful measurement of the magnetic field-induced spin singlet-triplet transition in silicon-based coupled dot systems. Our specific experimental scheme incorporates a lateral gate-controlled Coulomb-blockaded structure in Si to meet the proposed scheme of Loss and DiVincenzo [1], and a non-equilibrium single-electron tunneling technique to probe the fine energy splitting between the spin singlet and triplet, which varies as a function of applying magnetic fields and interdot coupling constant. Our results, exhibiting the singlet-triplet crossing at a magnetic field for various interdot coupling constants, are in agreement with the theoretical predictions, and give the first experimental demonstration of the possible spin swapping occurring in the coupled double dot systems with magnetic field. *Electronic address: jungchoi@chungbuk.ac.kr [1] D. Loss and D. P. DiVincenzo, Phys. Rev. A 57, 120 (1998).

cond-mat.mes-hall↗

arXiv2004

Hole maximum density droplets of an antidot in strong magnetic fields

We investigate a quantum antidot in the integer quantum Hall regime (the filling factor is two) by using a Hartree-Fock approach and by transforming the electron antidot into a system which confines holes via an electron-hole transformation. We find that its ground state is the maximum density droplet of holes in certain parameter ranges. The competition between electron-electron interactions and the confinement potential governs the properties of the hole droplet such as its spin configuration. The ground-state transitions between the droplets with different spin configurations occur as magnetic field varies. For a bell-shape antidot containing about 300 holes, the features of the transitions are in good agreement with the predictions of a recently proposed capacitive interaction model for antidots as well as recent experimental observations. We show this agreement by obtaining the parameters of the capacitive interaction model from the Hartree-Fock results. An inverse parabolic antidot is also studied. Its ground-state transitions, however, display different magnetic-field dependence from that of a bell-shape antidot. Our study demonstrates that the shape of antidot potential affects its physical properties significantly.

cond-mat.mes-hall↗

arXiv2003

Bose-Einstein condensate of two-dimensional excitons in a ring: Necklace-like modulation of order parameter

We have studied theoretically the Bose-Einstein condensation (BEC) of two-dimensional excitons in a ring with small random width variation. We derive a nonlinear Gross-Pitaevkii equation (GPE) for such a condensate. Our numerical solution of the ground state of the GPE displays a necklace-like structure in the presence of small random variation of ring width. This is a consequence of the interplay between random potential and the nonlinear repulsive term of the GPE. Our result suggests that the formation of ring and necklace-like structures observed recently in the photoluminescence of quantum wells may be stable even in the BEC phase.

cond-mat.str-el↗

arXiv2003

Kondo Effect of Quantum Dots in the Quantum Hall Regime

Quantum dots in the quantum Hall regime can have pairs of single Slater determinant states that are degenerate in energy. We argue that these pairs of many body states may give rise to a Kondo effect which can be mapped into an ordinary Kondo effect in a fictitious magnetic field. We report on several properties of this Kondo effect using scaling and numerical renormalization group analysis. We suggest an experiment to investigate this Kondo effect.

cond-mat.mes-hall↗

arXiv2003

Strongly correlated quantum dots in weak confinement potentials and magnetic fields

We explore a strongly correlated quantum dot in the presence of a weak confinement potential and a weak magnetic field. Our exact diagonalization studies show that the groundstate property of such a quantum dot is rather sensitive to the magnetic field and the strength of the confinement potential. We have determined rich phase diagrams of these quantum dots. Some experimental consequences of the obtained phase diagrams are discussed.

cond-mat.mes-hall↗

arXiv2002

Momentum Distribution Function of a Narrow Hall Bar in the FQHE Regime

The momentum distribution function ($n(k)$) of a narrow Hall bar in the fractional quantum Hall effect regime is investigated using Luttinger liquid and microscopic many-particle wavefunction approaches. For wide Hall bars with filling factor $ν=1/M$, where $M$ is an odd integer, $n(k)$ has singularities at $\pm M k_F$. We find that for narrow Hall bars additional singularities occur at smaller odd integral multiples of $k_F$: $ n(k) \sim A_p \mid k\pm pk_{F} \mid ^{2Δ_{p}-1}$ near $k=\pm pk_{F}$, where $p$ is an odd integer $M,M-2,M-4,...,1$. If inter-edge interactions can be neglected, the exponent $2 Δ_{p}= (1/ ν+p^{2} ν)/2$ is independent of the width ($w$) of the Hall bar but the amplitude of the singularity $A_p$ vanishes exponentially with $w$ for $p\not=M$.

cond-mat.mes-hall↗

arXiv2002

Non-Drude Optical Conductivity of (III,Mn)V Ferromagnetic Semiconductors

We present a numerical model study of the zero-temperature infrared optical properties of (III,Mn)V diluted magnetic semiconductors. Our calculations demonstrate the importance of treating disorder and interaction effects simultaneously in modelling these materials. We find that the conductivity has no clear Drude peak, that it has a broadened inter-band peak near 220 meV, and that oscillator weight is shifted to higher frequencies by stronger disorder. These results are in good qualitative agreement with recent thin film absorption measurements. We use our numerical findings to discuss the use of f-sum rules evaluated by integrating optical absorption data for accurate carrier-density estimates.

cond-mat.mtrl-sci↗

arXiv2002

Quantum-Hall Quantum-Bits

Bilayer quantum Hall systems can form collective states in which electrons exhibit spontaneous interlayer phase coherence. We discuss the possibility of using bilayer quantum dot many-electron states with this property to create two-level systems that have potential advantages as quantum bits.

cond-mat.mes-hall↗

arXiv2002

Coupling between Edge and Bulk in Strong-Field Quantum Dots

The maximum-density-droplet (MDD) state of quantum-dot electrons becomes unstable at strong magnetic fields to the addition of interior holes. Using exact diagonalization, we demonstrate that the first hole is located at the center of the dot when the number of electrons $N$ is smaller than $\sim 14$ and is located away from the center for larger dots. The separation between field strengths at which additional holes are introduced becomes small for large dots, explaining recent observations of a rapid increase in dot area when the magnetic field is increased beyond the MDD stability limit. We comment on correlations between interior hole and collective edge fluctuations, and on the implications of these correlations for edge excitation models in bulk systems.

cond-mat.mes-hall↗

arXiv2002

Metal-Insulator Transition and Ferromagnetism in Diluted Magnetic Semiconductors

We have investigated the interplay between the metal-insulator transition and ferromagnetism in $({\rm III}_{1-x},{\rm Mn}_x){\rm V}$ ferromagnetic semiconductors. Our study is based on a model in which $S=5/2$ Mn local moments are exchange-coupled to band electrons that interact via Coulomb interactions which each other, with ionized Mn acceptors, and with the antisite defects present in these materials. We find quasiparticle participation ratios that are consistent with a metal-insulator transition that occurs in the ferromagnetic state near $x \sim 0.01$. By evaluating the distribution of mean-field exchange coupling strengths at Mn moment sites, we provide evidence in favor of the applicability of small polaron and hole gas models on insulating and metallic sides of the phase transition respectively.

cond-mat↗

arXiv2001

Thermodynamic and Tunneling Density of States of the Integer Quantum Hall Critical State

We examine the long wave length limit of the self-consistent Hartree-Fock approximation irreducible static density-density response function by evaluating the charge induced by an external charge. Our results are consistent with the compressibility sum rule and inconsistent with earlier work that did not account for consistency between the exchange-local-field and the disorder potential. We conclude that the thermodynamic density of states is finite, in spite of the vanishing tunneling density of states at the critical energy of the integer quantum Hall transition.

cond-mat.mes-hall↗

arXiv2001

Bosonization Theory of Excitons in One-dimensional Narrow Gap Semiconductors

Excitons in one-dimensional narrow gap semiconductors of anti-crossing quantum Hall edge states are investigated using a bosonization method. The excitonic states are studied by mapping the problem into a non-integrable sine-Gordon type model. We also find that many-body interactions lead to a strong enhancement of the band gap. We have estimated when an exciton instability may occur.

cond-mat.mes-hall↗

arXiv2000

Numerical Test of Disk Trial Wave function for Half-Filled Landau Level

The analyticity of the lowest Landau level wave functions and the relation between filling factor and the total angular momentum severely limits the possible forms of trial wave functions of a disk of electrons subject to a strong perpendicular magnetic field. For N, the number of electrons, up to 12 we have tested these disk trial wave functions for the half filled Landau level using Monte Carlo and exact diagonalization methods. The agreement between the results for the occupation numbers and ground state energies obtained from these two methods is excellent. We have also compared the profile of the occupation number near the edge with that obtained from a field-theoretical method. The results give qualitatively identical edge profiles. Experimental consequences are briefly discussed.

cond-mat.mes-hall↗

arXiv2000

The effect of Hund's Coupling on one-dimensional Luttinger Liquids

Two one-dimensional Luttinger liquid systems coupled by Hund's coupling are studied by the renormalization group and the non-abelian bosonization methods. It is found that the Hund's coupling is always relevant irrespective of the repulsive interaction between electrons within each chain. The properties of the resulting strong coupling fixed point are discussed.

cond-mat.str-el↗

arXiv1999

Level Fluctuations and Many-Body Effects in Disorder-Free Quantum Dots

We have investigated whether many-body effects can induce significant level fluctuations in a disorder-free quantum dot. The closed energy shell structures and relaxation of the Hartree-Fock potentials are found to play a significant role. The level degeneracy consistent with the rotational symmetry of the confining potential determines the structure of the energy shells. A closed shell state of a dot can give rise to large fluctuations. When a strong magnetic field is present the shell structure is absent and fluctuations are significantly reduced.

cond-mat.mes-hall↗

arXiv1999

Two-component theory of a droplet of electrons in half-filled Landau level

We have investigated low energy excitations of a disk of electrons in half-filled Landau level using trail wave function and small-size exact diagonalization approaches. We have constructed a set of many-body basis states that describe correctly the low energy excitations. In this theory a droplet consists of two types of composite fermion liquids, and suggests that a droplet can support an edge magnetoplasmon and low energy droplet excitations. A possibility of measuring these excitations in a quantum dot is discussed.

cond-mat↗

arXiv1998

Occupation Numbers of a Half-Filled Landau Level

We demonstrate that a theory of the edge of a half-filled Landau level recently proposed by Lee and Wen predicts results for the edge occupation number similar to those of a variational trial wave function proposed previously by us. We treat Lee and Wen's edge action of a half-filled Landau level within the framework of bosonization theory, and show that the momentum occupation numbers are determined by a product of two Green's functions, one charged and one neutral. In the bulk region ($k<0$) we find a linear occupation profile, $n_k \propto A+Bk$, while in the tail region ($k>0$) it is exponentially decaying over the range $k\sim\Ln$, the momentum cutoff for neutral mode. We find a good fit with the numerical results for occupation numbers.

cond-mat.mes-hall↗