arXiv subjects

M. Y. Simmons

Publications and source records attributed to M. Y. Simmons.

80 records · Page 5Linked to original sources

arXiv1998

Interaction Effects in a One-Dimensional Constriction

We have investigated the transport properties of one-dimensional (1D) constrictions defined by split-gates in high quality GaAs/AlGaAs heterostructures. In addition to the usual quantized conductance plateaus, the equilibrium conductance shows a structure close to $0.7(2e^2/h)$, and in consolidating our previous work [K.~J. Thomas et al., Phys. Rev. Lett. 77, 135 (1996)] this 0.7 structure has been investigated in a wide range of samples as a function of temperature, carrier density, in-plane magnetic field $B_{\parallel}$ and source-drain voltage $V_{sd}$. We show that the 0.7 structure is not due to transmission or resonance effects, nor does it arise from the asymmetry of the heterojunction in the growth direction. All the 1D subbands show Zeeman splitting at high $B_{\parallel}$, and in the wide channel limit the $g$-factor is $\mid g \mid \approx 0.4$, close to that of bulk GaAs. As the channel is progressively narrowed we measure an exchange-enhanced $g$-factor. The measurements establish that the 0.7 structure is related to spin, and that electron-electron interactions become important for the last few conducting 1D subbands.

cond-mat.mes-hall↗

arXiv1997

Metal-insulator transition at B=0 in an ultra-low density ($r_{s}=23$) two dimensional GaAs/AlGaAs hole gas

We have observed a metal-insulator transition in an ultra-low density two dimensional hole gas formed in a high quality GaAs-AlGaAs heterostructure at B=0. At the highest carrier density studied ($p_{s}=2.2x10^{10} cm^{-2}, r_{s}=16$) the hole gas is strongly metallic, with an exceptional mobility of $425,000 cm^{2}V^{-1}s^{-1}$. The low disorder and strength of the many-body interactions in this sample are highlighted by the observation of re-entrant metal insulator transitions in both the fractional ($ν< 1/3$) and integer ($2 > ν> 1$) quantum Hall regimes. On reducing the carrier density the temperature and electric field dependence of the resistivity show that the sample is still metallic at $p_{s}=1.3x10^{10} cm^{-2}$ ($r_{s}=21$), becoming insulating at $p_{s}{\simeq}1x10^{10} cm^{-2}$. Our results indicate that electron-electron interactions are dominant at these low densities, pointing to the many body origins of this metal-insulator transition. We note that the value of $r_{s}$ at the transition ($r_{s}=23 +/- 2$) is large enough to allow the formation of a weakly pinned Wigner crystal, and is approaching the value calculated for the condensation of a pure Wigner crystal.

cond-mat.str-el↗

arXiv1997

Metal-insulator transition at B=0 in a dilute two dimensional GaAs-AlGaAs hole gas

We report the observation of a metal insulator transition at B=0 in a high mobility two dimensional hole gas in a GaAs-AlGaAs heterostructure. A clear critical point separates the insulating phase from the metallic phase, demonstrating the existence of a well defined minimum metallic conductivity sigma(min)=2e/h. The sigma(T) data either side of the transition can be `scaled' on to one curve with a single parameter (To). The application of a parallel magnetic field increases sigma(min) and broadens the transition. We argue that strong electron-electron interactions (rs = 10) destroy phase coherence, removing quantum intereference corrections to the conductivity.

cond-mat.str-el↗

arXiv1997

Experimental evidence of a metal-insulator transition in a half-filled Landau level

We have measured the low-temperature transport properties of a high-mobility front-gated GaAs/Al_{0.33}Ga_{0.67}As heterostructure. By changing the applied gate voltage, we can vary the amount of disorder within the system. At a Landau level filling factor $ν=1/2$, where the system can be described by the composite fermion picture, we observe a crossover from metallic to insulating behaviour as the disorder is increased. Experimental results and theoretical prediction are compared.

cond-mat.mes-hall↗

arXiv1996

Fractional Quantum Hall Effect in Bilayer Two Dimensional Hole Gas Systems

We have studied the fractional and integer quantum Hall effect in high mobility double layer 2D hole gas systems. The large hole effective mass inhibits tunneling, allowing us to investigate the regime in which the interlayer and intralayer interactions are comparable without significant interlayer tunneling occurring. As the interlayer separation is reduced we observe the formation of bilayer correlated quantum Hall states at total filling factor $ν$=3/2 and $ν$=1. We find that the bilayer $ν$=3/2 state is rapidly destroyed by small carrier density imbalances between the layers, whereas the bilayer $ν$=1 state evolves continuously into the single layer $ν$=1 state.

cond-mat↗

arXiv1996

Possible Spin Polarization in a One-Dimensional Electron Gas

In zero magnetic field, conductance measurements of clean one-dimensional (1D) constrictions defined in GaAs/AlGaAs heterostructures show twenty-six quantized ballistic plateaux, as well as a structure close to $0.7(2e^2/h)$. In an in-plane magnetic field all the 1D subbands show Zeeman splitting and in the wide channel limit the $g$-factor is $\mid g \mid = 0.4$, close to that of bulk GaAs. For the last subband spin-splitting originates from the structure at $0.7(2e^2/h)$, indicating spin polarization at $B=0$. The measured enhancement of the $g$-factor as the subbands are depopulated suggests that the ``0.7 structure'' is induced by electron-electron interactions.

cond-mat↗

arXiv1996

Ballistic Composite Fermions in Semiconductor Nanostructures

We report the results of two fundamental transport measurements at a Landau level filling factor $ν$ of 1/2. The well known ballistic electron transport phenomena of quenching of the Hall effect in a mesoscopic cross-junction and negative magnetoresistance of a constriction are observed close to B~=~0 and $ν~=~ 1/2$. The experimental results demonstrate semi-classical charge transport by composite fermions, which consist of electrons bound to an even number of flux quanta.

cond-mat↗

arXiv1995

The Aharonov-Bohm Effect in the Fractional Quantum Hall Regime

We have investigated experimentally resonant tunnelling through single-particle states formed around an antidot by a magnetic field, in the fractional quantum Hall regime. For 1/3 filling factor around the antidot, Aharonov-Bohm oscillations are observed with the same magnetic field period as in the integer quantum Hall regime. All our measurements are consistent with quasiparticles of fractional charge e*. However, the results are also consistent with particles of any charge (>= e*) as the system must rearrange every time the flux enclosed increases by h/e.

cond-mat↗