Wyniki wyszukiwania - Biblioteka Nauki

1

Composite Fermions and the Fractional Quantum Hall Effect

100%

Wójs A. , Quinn J.

Acta Physica Polonica A

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1999

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tom 96

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nr 5

593-602

EN

The mean field composite fermion picture successfully predicts low lying states of fractional quantum Hall systems. This success cannot be attributed to a cancellation between the Coulomb and Chern-Simons interactions beyond the mean field and solely depends on the short-range of the Coulomb pseudopotential in the lowest Landau level. The class of pseudopotentials for which the mean field composite fermion picture can be applied is defined. The success or failure of the mean field composite fermion picture in various systems (electrons in excited Landau levels, Laughlin quasiparticles, charged magnetoexcitons) is explained.

2

Calculation of Exciton and Trion Absorption Spectra in High Magnetic Fields

100%

Gładysiewicz A. , Wójs A.

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tom 110

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nr 2

183-188

EN

Density of states and absorption spectrum of narrow quantum wells containing a small number of free electrons and subject to a high magnetic field are calculated numerically. The effect of an additional, second electron on the photoexcited electron-hole pair is analyzed. In density of states, the exciton-electron interaction fills the gaps between the Landau levels and yields additional discrete peaks corresponding to bound trions. In absorption, interaction with the additional free electron has no effect on the position or intensity of the main sequence of excitonic peaks. However, it gives rise to additional weaker trion peaks, both in the lowest and higher Landau levels.

3

Mathematical Structure of Bosonic and Fermionic Jack States and Their Application in Fractional Quantum Hall Effect

80%

Kuśmierz B. , Wu Y. , Wójs A.

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nr 5

1134-1136

EN

Fractional quantum Hall effect is a remarkable behaviour of correlated electrons, observed exclusively in two dimensions, at low temperatures, and in strong magnetic fields. The most prominent fractional quantum Hall state occurs at Landau level filling factor ν = 1/3 and it is described by the famous Laughlin wave function, which (apart from the trivial Gaussian factor) is an example of Jack polynomial. Fermionic Jack polynomials also describe another pair of candidate fractional quantum Hall states: Moore-Read and Read-Rezayi states, believed to form at the ν = 1/2 and 3/5 fillings of the second Landau level, respectively. Bosonic Jacks on the other hand are candidates for certain correlated states of cold atoms. We examine here a continuous family of fermionic Jack polynomials whose special case is the Laughlin state as approximate wave functions for the 1/3 fractional quantum Hall effect.

4

Fractional Quantum Hall States with Non-Abelian Statistics

80%

Möller G. , Wójs A. , Cooper N.

Acta Physica Polonica A

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2009

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tom 116

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nr 5

847-848

EN

Using exact numerical diagonalization we have studied correlated many-electron ground states in a partially filled second Landau level. We consider filling fractions ν = 1/2 and 2/5, for which incompressible quantum liquids with non-Abelian anion statistics have been proposed. Our calculations include finite layer width, Landau level mixing and arbitrary deformation of the interaction pseudopotential. Computed energies, gaps, and correlation functions support the non-Abelian ground states at both ν = 1/2 ("Pfaffian") and ν = 2/5 ("parafermion" state).

5

Spin Phase Transition in a Correlated Composite Fermion Liquid

80%

Wójs A. , Simion G. , Wodziński D. , Quinn J.

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nr 2

153-156

EN

We study spin polarization of the ν_e=4/11 fractional quantum Hall state corresponding to the ν=1/3 filling of the second composite fermion Landau level, and predict a spin phase transition in realistic systems.

6

Energy and Optical Absorption Spectra of Multiply Charged Anisotropic Quantum Boxes

80%

Trojnar A. , Małachowski T. , Wójs A.

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nr 2

173-176

EN

Using a simple model of a two-dimensional rectangular quantum box we study the effects of size and anisotropy on the energy and photoluminescence spectra of neutral and charged quantum dots. The competition of symmetries and energy/length scales of the free exciton or trion and of the confining potential is analyzed. The numerical calculations consisted of the diagonalization of the few-electron-hole Hamiltonian matrices in the full configuration-interaction basis, with the simultaneous resolution of the conserved orbital and spin quantum numbers.

7

Composite Fermion Dynamics in Half-Filled Landau Levels of Graphene

80%

Wójs A. , Möller G. , Cooper N.

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nr 5

592-594

EN

We report on exact-diagonalization studies of correlated many-electron states in the half-filled Landau levels of graphene, including pseudospin (valley) degeneracy. We demonstrate that the polarized Fermi sea of non-interacting composite fermions remains stable against a pairing transition in the lowest two Landau levels. However, it undergoes spontaneous depolarization, which is unprotected owing to the lack of single-particle pseudospin splitting. These results suggest the absence of the Pfaffian phase in graphene.

8

Analysis of Optical Properties of MoS₂ Monolayer using Minimal-Basis Tight-Binding Models

71%

Nouri N. , Potasz P. , Zia Borujeni S. , Wójs A. , Rashedi G.

Acta Physica Polonica A

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2017

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tom 132

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nr 2

313-315

EN

Optical properties of transition metal dichalcogenides monolayer of MoS₂ are analyzed using multi-orbital tight-binding models with only Mo d-orbitals (three-band model) and with an inclusion of S p-orbitals (six-band model). We look at band structures, momentum matrix elements between valence and conduction band, and joint optical density of states. Good agreement between the two models is shown in a vicinity of K point of the Brillouin zone. On line connecting K and Γp points, a local conduction band minimum at Q point is recovered only by six-band model in agreement with density functional theory and experimental results. We show that optical transitions at this point are active for both light polarizations. A peak in joint optical density of states is also seen at this point suggesting its potentially important role in a proper description of excitonic effects.

9

Skyrmions in a Hole Gas with Large Spin Gap and Strong Disorder

71%

Bryja L. , Ryczko K. , Wójs A. , Misiewicz J. , Potemski M.

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tom 110

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nr 2

163-168

EN

In the photoluminescence excitation spectra of two-dimensional valence holes with large spin gap and strong disorder we find evidence for quantum Hall ferromagnetism and small skyrmions around the Landau level filling factorν=1. This interpretation is supported by numerical calculations.

10

Effect of Disorder on Spin and Charge Excitations in the Fractional Quantum Hall Effect

71%

Výborný K. , Müller C. , Dethlefsen A. , Haug R. , Wójs A.

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nr 2

249-254

EN

A simple model of disorder in fractional quantum Hall systems is combined with the standard exact diagonalisation technique. Electron-density-dependent gaps at filling factors 1/3,2/3,2/5, and 3/5 measured by activated transport can then be fitted with a single reasonable value of d which has the meaning of the separation of ionized donors from the quasi-2D electron gas.

11

Combined Exciton-Cyclotron Resonance in Photoluminescence of a Two-Dimensional Hole Gas

61%

Jadczak J. , Bryja L. , Płochocka P. , Wójs A. , Misiewicz J. , Maude D. , Potemski M.

Acta Physica Polonica A

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2009

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tom 116

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nr 5

852-853

EN

A two-dimensional hole gas in an asymmetric GaAs/Ga_{1-x}Al_{x}As quantum well is studied by polarization-resolved photoluminescence in high magnetic fields (up to B = 20 T) and at low temperatures (down to T = 50 mK). In addition to the previously reported dominant emission channels of various free and acceptor-bound trions, the high-energy hole cyclotron replicas of the bound states are now also observed, corresponding to the combined exciton-cyclotron resonance. Identification of different transitions in the rich, multi-peak spectra was possible by the analysis of optical selection rules and comparison of the experimental spectra with realistic numerical calculations.

12

Time Resolved Photoluminescence Study of the Wide (Cd,Mn)Te/(Cd,Mg)Te Quantum Well

51%

Kutrowska J. , Bugajny P. , Baranowski M. , Bryja L. , Syperek M. , Wójs A. , Misiewicz J. , Wiater M. , Karczewski G.

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nr 5

895-897

EN

The static and dynamic properties of excitons and trions in a 80 nm wide Cd_{1-x}Mn_xTe/Cd_{0.7}Mg_{0.3}Te quantum well with extremely small Mn content (x=0.00027) have been studied by means of time-integrated and time-resolved photoluminescence experiment at low and elevated temperatures. The trion binding energy has been estimated to be 2.6 ± 0.8 meV. The exciton and trion lifetimes have been measured to be ≈ 150 ps, and ≈ 200 ps, respectively. The temperature dependence of both lifetimes together with the multicomponent character of the PL decay process suggest a spatial localization of excitons and trions in the investigated quantum well.

13

Exciton Exchange between Nearly-Free and Acceptor-Bound States of a Positive Trion Assisted by Cyclotron Excitation

51%

Jadczak J. , Bryja L. , Wójs A. , Bartsch G. , Yakovlev D. , Bayer M. , Plochocka P. , Potemski M. , Reuter D. , Wieck A.

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nr 5

600-601

EN

In magneto-photoluminescence spectra of a two-dimensional hole gas in a GaAs quantum well we observe coupling of two different radiative states. The pair of coupled states are an acceptor-bound trion AX^{+} and an essentially free (only weakly localized by a shallow lateral potential) trion X^{+}, brought into resonance by an additional cyclotron excitation controlled by the magnetic field. The coupling mechanism is the exciton transfer, and the optical signature is a clear anticrossing of the emission lines of an X^{+} and a cyclotron replica of the AX^{+}.