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Title:

Critical Collapse of the Exchange Enhanced Spin Splitting in 2-D Systems

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The critical filling factor # c where Shubnikov-de Haas oscillations become spin split is investigated for a set of GaAs-GaAlAs heterojunctions. Finite temperature magnetoresistance measurements are used to extract the value of # c at zero temperature. The critically point is where the disorder potential has the same magnitude as the exchange en...

The critical filling factor # c where Shubnikov-de Haas oscillations become spin split is investigated for a set of GaAs-GaAlAs heterojunctions. Finite temperature magnetoresistance measurements are used to extract the value of # c at zero temperature. The critically point is where the disorder potential has the same magnitude as the exchange energy, leading to the empirical relationship # c = g # n e # s h/2m 0 . This is valid for all the samples studied, where the density n e and single particle lifetime # s both vary by more than an order of magnitude and g # the exchange enhanced g-factor has a weak dependence on density. For each sample the spin gap energy shows a linear increase with magnetic field. Experiments in tilted magnetic field show the spin gap is the sum of the bare Zeeman energy and an exchange term. This explains why measurements of the enhanced g-factor from activation energy studies in perpendicular field and the coincidence method in tilted fields have . Minimize

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The Pennsylvania State University CiteSeerX Archives

Year of Publication:

2009-04-14

Source:

http://www.warwick.ac.uk/~phsbm/papers/spinspli.pdf

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text

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en

DDC:

530 Physics *(computed)*

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Title:

Critical Collapse of the Exchange Enhanced Spin Splitting in 2-D Systems

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The critical filling factor νc where Shubnikov-de Haas oscillations become spin split is investigated for a set of GaAs-GaAlAs heterojunctions. Finite temperature magnetoresistance measurements are used to extract the value of νc at zero temperature. The critically point is where the disorder potential has the same magnitude as the exchange ener...

The critical filling factor νc where Shubnikov-de Haas oscillations become spin split is investigated for a set of GaAs-GaAlAs heterojunctions. Finite temperature magnetoresistance measurements are used to extract the value of νc at zero temperature. The critically point is where the disorder potential has the same magnitude as the exchange energy, leading to the empirical relationship νc = g ∗ neτsh/2m0. This is valid for all the samples studied, where the density ne and single particle lifetime τs both vary by more than an order of magnitude and g ∗ the exchange enhanced g-factor has a weak dependence on density. For each sample the spin gap energy shows a linear increase with magnetic field. Experiments in tilted magnetic field show the spin gap is the sum of the bare Zeeman energy and an exchange term. This explains why measurements of the enhanced g-factor from activation energy studies in perpendicular field and the coincidence method in tilted fields have previously disagreed. Minimize

Contributors:

The Pennsylvania State University CiteSeerX Archives

Year of Publication:

2012-11-19

Source:

http://arxiv.org/pdf/cond-mat/9805332v1.pdf

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Document Type:

text

Language:

en

DDC:

530 Physics *(computed)*

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Title:

Skyrmions and composite fermions in the limit of vanishing Zeeman energy

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Abstract. We describe the properties of a strongly interacting two-dimensional (2D) electron gas in high magnetic fields whose properties can be described in terms of the formation of composite fermions at fractional Landau level occupancy. For a Landau level occupancy ν = 1 the excitations correspond to spin waves for high g-factors and magneti...

Abstract. We describe the properties of a strongly interacting two-dimensional (2D) electron gas in high magnetic fields whose properties can be described in terms of the formation of composite fermions at fractional Landau level occupancy. For a Landau level occupancy ν = 1 the excitations correspond to spin waves for high g-factors and magnetic fields, but when the g-factor is reduced close to zero by the use of hydrostatic pressure there is strong evidence for the formation of skyrmions. Studies of composite fermions in the limit of vanishing Zeeman energy also suggest that skyrmionic excitations can occur for composite fermions. The quantum Hall effect and fractional quantum Hall effect (FQHE) which are observed for a two-dimensional system in high magnetic fields are two of the most fascinating examples of the importance of electron–electron interactions in condensed matter physics [1, 2]. In the last few years it has been realized that a good way to understand the FQHE is through the introduction of composite fermion (CF) particles [3], where the strong electron–electron interaction causes new excitations of the system in which an electron becomes bound to a pair of flux quanta. The basic idea is to use a Chern–Simons gauge transformation to describe the electron–electron interactions with mean field, BM = 2ne�0, which corresponds Minimize

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The Pennsylvania State University CiteSeerX Archives

Year of Publication:

2008-07-01

Source:

http://www.warwick.ac.uk/~phsbm/papers/skyrjpcm.pdf

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text

Language:

en

DDC:

539 Modern physics *(computed)*

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Title:

FRACTIONAL QUANTUM HALL EFFECT MEASURED IN A SPIN DEGENERATE ELECTRON GAS

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The influence of spin on the Fractional Quantum Hall Effect is investigated in GaAs-GaAlAs heterojunctions in a filling factor region where both spin states of the lowest Landau level are occupied. When the single particle spin splitting is reduced by hydrostatic pressure features at even numerator fractions are enhanced while those with odd num...

The influence of spin on the Fractional Quantum Hall Effect is investigated in GaAs-GaAlAs heterojunctions in a filling factor region where both spin states of the lowest Landau level are occupied. When the single particle spin splitting is reduced by hydrostatic pressure features at even numerator fractions are enhanced while those with odd numerators are diminished. This is due to the existence of spin- and Landau gaps in the composite Fermion energy spectrum. In this paper the influence of spin splitting on the Fractional Quantum Hall Effect (FQHE) is studied experimentally by reducing the energy difference between electron states of spin up and spin down with hydrostatic pressure. 1 The results are interpreted in the composite Fermion (CF) framework. Usually only one spin state is considered, i.e. the electrons are treated as spinless particles or in the limit of infinite spin splitting. Then the FQHE can be interpreted as an integer QHE of spinless CFs and all the features seen at high magnetic fields for filling factors ν<1 can be explained by forming CF Landau levels (LL) in an effective field centred on ν=1/2. 2 However, for 2>ν>1 and at low fields both spin states must be considered. By using hydrostatic pressure to tune the Landé g-factor we can investigate the Minimize

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The Pennsylvania State University CiteSeerX Archives

Year of Publication:

2008-07-01

Source:

http://www.warwick.ac.uk/~phsbm/papers/icps23.pdf

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text

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en

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Title:

Measurements of composite Skyrmions at filling factor 1/3

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Measurements of the fractional quantum Hall effect energy gaps at high pressures are presented that provide evidence for the existence of composite Skyrmions. Just as charged spin textures known as Skyrmions are thought to be the lowest lying excitations for electrons at ν=1, we show that composite Skyrmions can be formed at ν=1/3 when the elect...

Measurements of the fractional quantum Hall effect energy gaps at high pressures are presented that provide evidence for the existence of composite Skyrmions. Just as charged spin textures known as Skyrmions are thought to be the lowest lying excitations for electrons at ν=1, we show that composite Skyrmions can be formed at ν=1/3 when the electron g-factor is sufficiently small. Keywords: Fractional quantum Hall effect, Skyrmions, composite Fermions. The fully polarised fractional quantum Hall effect (FQHE) state at filling factor ν=1/3 consists of exactly one full composite Fermion Landau level (CF LL) and as such is completely analogous to the integer quantum Hall state at ν=1. Indeed the fact that Laughlin’s wavefunction for ν=1/3 contains the ν=1 wavefunction as a factor is one of the basis for the composite Fermion model [1]. Thus, as at all odd ν, the ground state at ν=1/3 should be seen as an itinerant ferromagnet. Excitations from these ferromagnetic ground states, which determine the conductivity, are dominated by the Coulomb exchange energy, Ec=e 2 /4πε lB (lB = h eB is the magnetic length) which is much larger than the single particle Zeeman energy gμBB. Single spin flips generate spin waves [2], but, if the Zeeman energy is not too large, there may be spin-textured chiral solitons Minimize

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The Pennsylvania State University CiteSeerX Archives

Year of Publication:

2008-07-01

Source:

http://www.warwick.ac.uk/~phsbm/papers/ep2ds97.pdf

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text

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en

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Title:

Studies of Skyrmions at ν=1 in the Limit of Zero g-factor.

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In a two-dimensional electron gas at ν=1 (where the filling factor ν=neh/eB measures how many Landau levels (LL) are filled) the ground state should be regarded as a ferromagnet since all the spin down states in the lowest LL are occupied while all the spin up states are empty. In GaAs the single particle (SP) Zeeman energy (ZE) gμBB is very sma...

In a two-dimensional electron gas at ν=1 (where the filling factor ν=neh/eB measures how many Landau levels (LL) are filled) the ground state should be regarded as a ferromagnet since all the spin down states in the lowest LL are occupied while all the spin up states are empty. In GaAs the single particle (SP) Zeeman energy (ZE) gμBB is very small 0.3 K/T, and Coulomb interactions are very significant. This has led several authors to suggest that novel charged excitations with number of reversed spin R> 1, and with non-trivial spin order, known as charged spin-texture excitations or Skyrmions, may occur [1,2]. This type of excitation is based on a spin texture that consists of a central reversed spin surrounded by rings of spin that gradually cant in a radial direction until at the edge they are aligned with the external magnetic field. The balance between the SPZE and the Coulomb energy is Minimize

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The Pennsylvania State University CiteSeerX Archives

Year of Publication:

2008-07-01

Source:

http://www.warwick.ac.uk/~phsbm/papers/skyrus.pdf

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text

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en

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Searches for skyrmions in the limit of zero g-factor

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Abstract. Energy gaps have been measured for the ferromagnetic quantum Hall effect states at ν = 1 and 3 in GaAs/Ga0.7Al0.3As heterojunctions as a function of Zeeman energy, which is reduced to zero by applying hydrostatic pressures of up to 20 kbar. At large Zeeman energy the gaps are consistent with spin-wave excitations. For a low-density sam...

Abstract. Energy gaps have been measured for the ferromagnetic quantum Hall effect states at ν = 1 and 3 in GaAs/Ga0.7Al0.3As heterojunctions as a function of Zeeman energy, which is reduced to zero by applying hydrostatic pressures of up to 20 kbar. At large Zeeman energy the gaps are consistent with spin-wave excitations. For a low-density sample the gap at ν = 1 decreases with increasing pressure and reaches a minimum when the g-factor vanishes. At small Zeeman energy the excitation appears to consist of a large number of reversed spins and may be interpreted as a skyrmion. The data also suggest that skyrmionic excitations take place at ν = 3. The width of the minimum at ν = 1 is found to decrease as the g-factor is reduced in a similar way for all samples. 1. Minimize

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The Pennsylvania State University CiteSeerX Archives

Year of Publication:

2008-07-01

Source:

http://www.warwick.ac.uk/~phsbm/papers/skyrm1.pdf

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text

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en

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Title:

Fractional Quantum Hall Effect Measurements at Zero g-Factor

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Fractional quantum Hall effect energy gaps have been measured in GaAs/Ga0.7Al0.3As heterojunctions as a function of Zeeman energy, which is varied by applying hydrostatic pressure up to 20 kbar. The gap at ν = 1/3 decreases with pressure until the g-factor changes sign when it again increases. The behavior is similar to that seen at ν = 1 and sh...

Fractional quantum Hall effect energy gaps have been measured in GaAs/Ga0.7Al0.3As heterojunctions as a function of Zeeman energy, which is varied by applying hydrostatic pressure up to 20 kbar. The gap at ν = 1/3 decreases with pressure until the g-factor changes sign when it again increases. The behavior is similar to that seen at ν = 1 and shows that excitations from the 1/3 ground state can be spin-like in character. At small Zeeman energy, the excitation appears to consist of 3 spins and may be interpreted as a small composite skyrmion. 73.40.Hm, 73.20.Dx, 72.20.Jv Typeset using REVTEX 1 The two dimensional electron gas in a high magnetic field is an excellent test bed for studying electron-electron interactions. In recent years our understanding has been greatly simplified by the composite Fermion (CF) model, which maps the fractional quantum Hall effect (FQHE) of electrons onto an integer quantum Hall effect (IQHE) of CFs [1–3]. Thus Minimize

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The Pennsylvania State University CiteSeerX Archives

Year of Publication:

2012-11-19

Source:

http://arxiv.org/pdf/cond-mat/9706157v1.pdf

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text

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en

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Title:

Searches for Skyrmions in the Limit of Zero g-Factor

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Energy gaps have been measured for the ferromagnetic quantum Hall effect states at ν = 1 and 3 in GaAs/Ga0.7Al0.3As heterojunctions as a function of Zeeman energy, which is reduced to zero by applying hydrostatic pressures of up to 20 kbar. At large Zeeman energy the gaps are consistent with spin wave excitations. For a low density sample the ga...

Energy gaps have been measured for the ferromagnetic quantum Hall effect states at ν = 1 and 3 in GaAs/Ga0.7Al0.3As heterojunctions as a function of Zeeman energy, which is reduced to zero by applying hydrostatic pressures of up to 20 kbar. At large Zeeman energy the gaps are consistent with spin wave excitations. For a low density sample the gap at ν = 1 decreases with increasing pressure and reaches a minimum when the g-factor vanishes. At small Zeeman energy the excitation appears to consist of a large number of reversed spins and may be interpreted as a Skyrmion. The data also suggest Skyrmionic excitations take place at ν = 3. The width of the minimum at ν = 1 is found to decrease as the g-factor is reduced in a similar way for all samples. Minimize

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The Pennsylvania State University CiteSeerX Archives

Year of Publication:

2012-11-19

Source:

http://arxiv.org/pdf/cond-mat/9805357v1.pdf

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text

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en

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Title:

Two-dimensional magneto-phonon resonance in GaInAs-InP and GaInAs-AlInAs heterojunctions and superlattices

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2030161010173 ; 電機工程學系 ; We report the observation of magnetophonon resonance in GaInAs-InP heterojunctions and measurements of the temperature dependence of the oscillations. A single series of oscillations due to scattering by the “GaAs-like” mode of GaInAs is seen, in contrast to GaInAs-InP superlattices, where scattering from InP phonons is ...

2030161010173 ; 電機工程學系 ; We report the observation of magnetophonon resonance in GaInAs-InP heterojunctions and measurements of the temperature dependence of the oscillations. A single series of oscillations due to scattering by the “GaAs-like” mode of GaInAs is seen, in contrast to GaInAs-InP superlattices, where scattering from InP phonons is also observed, and GaInAs-AlInAs heterojunctions, where coupling to “InAs-like” modes only is seen. This behaviour is discussed in terms of long-range phonon interactions and interface phonons. Minimize

Publisher:

Elsevier

Contributors:

鄭克勇

Year of Publication:

1984

Document Type:

Journal

Language:

EN

Subjects:

Two-dimensional;magneto-phonon;GaInAs-InP;GaInAs-AlInAs;heterojunctions;superlattices ; 45

Two-dimensional;magneto-phonon;GaInAs-InP;GaInAs-AlInAs;heterojunctions;superlattices ; 45 Minimize

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Surface Science, Elsevier, Volume 142, Issues 1-3, 1 July 1984, Pages 368-374

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