arXiv subjects

Ken West

Publications and source records attributed to Ken West.

41 records · Page 3Linked to original sources

arXiv2011

Rapid collapse of spin waves in non-uniform phases of the second Landau level

The spin degree of freedom in quantum phases of the second Landau level is probed by resonant light scattering. The long wavelength spin wave, which monitors the degree of spin polarization, is at the Zeeman energy in the fully spin-polarized state at $ν$=3. At lower filling factors the intensity of the Zeeman mode collapses indicating loss of polarization. A novel continuum of low-lying excitations emerges that dominates near $ν$=8/3 and $ν$=5/2. Resonant Rayleigh scattering reveals that quantum fluids for $ν<3$ break up into robust domain structures. While the state at $ν$=5/2 is considered to be fully polarized, these results reveal unprecedented roles for spin degrees of freedom.

cond-mat.str-el↗

arXiv2010

Local Charge of the nu=5/2 Fractional Quantum Hall State

Electrons in two dimensions and strong magnetic fields effectively lose their kinetic energy and display exotic behavior dominated by Coulomb forces. When the ratio of electrons to magnetic flux quanta in the system is near 5/2, the unique correlated phase that emerges is predicted to be gapped with fractionally charged quasiparticles and a ground state degeneracy that grows exponentially as these quasiparticles are introduced. Interestingly, the only way to transform between the many ground states would be to braid the fractional excitations around each other, a property with applications in quantum information processing. Here we present the first observation of localized quasiparticles at nu=5/2, confined to puddles by disorder. Using a local electrometer to compare how quasiparticles at nu=5/2 and nu=7/3 charge these puddles, we are able to extract the ratio of local charges for these states. Averaged over several disorder configurations and samples, we find the ratio to be 4/3, suggesting that the local charges are e/3 at seven thirds and e/4 at five halves, in agreement with theoretical predictions. This confirmation of localized e/4 quasiparticles is necessary for proposed interferometry experiments to test statistics and computational ability of the state at nu=5/2.

cond-mat.mes-hall↗

arXiv2004

Dynamics of in-plane charge separation front in 2D electron-hole gas

We show both experimentally and theoretically that the recently observed optically induced in-plane charge separation in quantum well (QW) structures and the exciton ring emission pattern at this charge separation boundary have an extremely long lifetime. The oppositely charged carriers remain separated and provide a reservoir of excitons at their boundary with a persistent emission which lasts hundreds of microseconds (orders of magnitude longer than their recombination time) after the external excitation is removed. This long lifetime is due to an interplay between the slow in-plane carrier diffusion and slow carrier tunnelling perpendicular to the QW plane.

cond-mat.other↗

arXiv2004

Tunneling between Two Quantum Hall Droplets

We report on tunneling experiment between two quantum Hall droplets separated by a nearly ideal tunnel barrier. The device is produced by cleaved edge overgrowth that laterally juxtaposes two two-dimensional electron systems across a high quality semiconductor barrier. The dramatic evolution of the tunneling characteristics is consistent with the magnetic field-dependent tunneling between the coupled edge states of the quantum Hall droplets. We identify a series of quantum critical points between successive strong and weak tunneling regimes that are reminiscent of the plateau-transitions in quantum Hall effect. Scaling analysis shows that the conductance near the critical magnetic fields $B_{c}$ is a function of a single scaling argument $|B-B_{c}|T^{-κ}$, where the exponent $κ= 0.42$. This puzzling resemblance to a quantum Hall-insulator transition points to the significance of interedge correlation in the lateral tunneling of quantum Hall droplets.

cond-mat.mes-hall↗

arXiv2003

Mechanism of Luminescence Ring Pattern Formation in Quantum Well Structures: Optically-Induced In-Plane Charge Separation

About a year ago, two independent experiments [1,2], imaging indirect exciton luminescence from doped double quantum wells under applied bias and optical excitation, reported a very intriguing observation: under certain experimental conditions, the exciton luminescence exhibits a ring pattern with a dark region in between the center excitation spot and the luminescent ring that can extend more than a millimeter from the center spot. Initial speculations on the origin of this emission pattern included supersonic ballistic transport of excitons due to their dipole-dipole repulsion and Bose superfluidity of excitons. In this paper we show that the ring effect is also observed in single quantum well structures, where only direct excitons exist. More importantly, we find that these experimental results are quantitatively explained by a novel coupled 2D electron-hole plasma dynamics, namely, photoinduced in-plane charge separation. This charge separation explains extremely long luminescence times that may be more than a microsecond for the ring -- orders of magnitude longer than the emission lifetime of the excitons in the center spot. This method of continuously creating excitons may result in a highly dense exciton gas which is also well thermalized with the lattice (since the particles can cool over the very long luminescence time after their hot optical creation), thus opening up opportunities for a detailed study of quantum statistics. The in-plane separation of the charges into positive and negative regions, with a sharp interface between them is an interesting new example of nonequilibrium dynamics and pattern formation.

cond-mat↗