arXiv subjects

L. W. Engel

Publications and source records attributed to L. W. Engel.

44 records · Page 3Linked to original sources

arXiv2003

Wigner crystalization about $ν$=3

We measure a resonance in the frequency dependence of the real diagonal conductivity, Re[$σ_{xx}$], near integer filling factor, $ν=3$. This resonance depends strongly on $ν$, with peak frequency $f_{pk} \approx 1.7$ GHz at $ν=3.04$ or 2.92 close to integer $ν$, but $f_{pk} \approx$ 600 MHz at $ν=3.19$ or 2.82, the extremes of where the resonance is visible. The dependence of $f_{pk}$ upon $n^*$, the density of electrons in the partially filled level, is discussed and compared with similar measurments by Chen {\it et al.}\cite{yong} about $ν=1$ and 2. We interpret the resonance as due to a pinned Wigner crystal phase with density $n^*$ about the $ν=3$ state.

cond-mat.mes-hall↗

arXiv2003

Microwave Resonance of 2D Wigner Crystal around integer Landau fillings

We have observed a resonance in the real part of the finite frequency diagonal conductivity using microwave absorption measurements in high quality 2D electron systems near {\em integer fillings}. The resonance exists in some neighborhood of filling factor around corresponding integers and is qualitatively similar to previously observed resonance of weakly pinned Wigner crystal in high $B$ and very small filling factor regime. Data measured around both $ν=1$ and $ν=2$ are presented. We interpret the resonance as the signature of Wigner crystal state around integer Landau levels.

cond-mat.mes-hall↗

arXiv2002

Microwave resonances of the bubble phases in 1/4 and 3/4 filled higher Landau levels

We have measured the diagonal conductivity in the microwave regime of an ultrahigh mobility two dimensional electron system. We find a sharp resonance in Re[sigma_{xx}] versus frequency when nu > 4 and the partial filling of the highest Landau level, nu^*, is ~ 1/4 or 3/4 and temperatures < 0.1 K. The resonance appears for a range of nu^* from 0.20 to 0.37 and again from 0.62 to 0.82. the peak frequency, f_{pk} changes from ~ 500 to ~ 150 as nu^* = 1/2 is approached. This range of f_{pk} shows no dependence on nu where the resonance is observed. The quality factor, Q, of the resonance is maximum at ~ nu^* = 0.25 and 0.74. We interpret the resonance as due to a pinning mode of the bubble phase crystal.

cond-mat.mes-hall↗

arXiv2002

Correlation lengths of Wigner crystal order in two dimensional electron system at high magnetic field

The insulator terminating the fractional quantum Hall series at low Landau level filling νis generally taken to be a pinned Wigner crystal (WC), and exhibits a microwave resonance that is interpreted as a WC pinning mode. Systematically studying the resonance in a high quality sample for carrier densities, n, between 1.8 and 5.7 x 10^{10} cm^-2, we find maxima in resonance peak frequency, f_pk, vs magnetic field, B. L, the domain size, or correlation length of Wigner crystalline order, is calculated from f_pk. For each n, L vs νtends at low νtoward a straight line with intercept; the linear fit is accurate over as much as a factor of 5 range of ν. We interpret the striking linear behavior as due to B compressing the electron wavefunctions, to alter the effective electron-impurity interaction.

cond-mat.mes-hall↗

arXiv2001

Electrical control of superposed quantum states evolution in quantum dot molecule by pulsed field

Quantum computing (or generally, quantum information processing) is of prime interest for it potentially has a significant impact on present electronics and computations1-5. Essence of quantum computing is a direct usage of the superposition and entanglement of quantum mechanical states that are pronounced in nanoscopic particles such as atoms and molecules6, 7. On the other hand, many proposals regarding the realization of semiconductor quantum dot (QD) qubits8,9 and QD quantum gates10 were reported and several experimental clues were successfully found11-16. While these pioneering experiments opened up the basis of semiconductor QD quantum computing, direct electrical control of superposed states in time domain17 has not been realized yet. Here, we demonstrate the coherent control of the superposed quantum states evolution in the QD molecule. Short electrical pulses are shown to inject single electron into the QD molecule and the current collected at the substrate exhibits oscillations as a function of the pulse width. The oscillations are originated from different decay rates of the symmetric (S) and the anti-symmetric (AS) state of the QD molecule and the evolution of the occupation probabilities controlled by the injection pulse width

quant-ph↗

arXiv2001

Giant microwave photoresistance of two-dimensional electron gas

We measure microwave frequency (4-40 GHz) photoresistance at low magnetic field B, in high mobility 2D electron gas samples, excited by signals applied to a transmission line fabricated on the sample surface. Oscillatory photoresistance vs B is observed. For excitation at the cyclotron resonance frequency, we find an unprecedented, giant relative photoresistance (ΔR)/R of up to 250 percent. The photoresistance is apparently proportional to the square root of applied power, and disappears as the temperature is increased.

cond-mat.mes-hall↗

arXiv2001

High Magnetic Field Microwave Conductivity of 2D Electrons in an Array of Antidots

We measure the high magnetic field ($B$) microwave conductivity, Re$σ_{xx}$, of a high mobility 2D electron system containing an antidot array. Re$σ_{xx}$ vs frequency ($f$) increases strongly in the regime of the fractional quantum Hall effect series, with Landau filling $1/3<ν<2/3$. At microwave $f$, Re$σ_{xx}$ vs $B$ exhibits a broad peak centered around $ν=1/2$. On the peak, the 10 GHz Re$σ_{xx}$ can exceed its dc-limit value by a factor of 5. This enhanced microwave conductivity is unobservable for temperature $T \gtrsim 0.5$ K, and grows more pronounced as $T$ is decreased. The effect may be due to excitations supported by the antidot edges, but different from the well-known edge magnetoplasmons.

cond-mat.mes-hall↗

arXiv1998

Microwave Resonance and Carrier-Carrier Interaction in Two-Dimensional Hole Systems at High Magnetic Field

Microwave frequency conductivity, Re($σ_{xx}$), of high quality two-dimensional hole systems (2DHS) in a large perpendicular magnetic field (B) is measured with the carrier density ($n_s$) of the 2DHS controlled by a backgate bias. The high B insulating phase of the 2DHS exhibits a microwave resonance that remains well-defined, but shifts to higher peak frequency ($f_{pk}$) as $n_s$ is reduced. Over a wide range of $n_s, f_{pk} \propto n_s^{-1/2}$ is observed for the two samples we measured. The data clearly indicate that both carrier-carrier interactions and disorder are indispensable in determining the dynamics of the insulator. The $n_s$ dependence of $f_{pk}$ is consistent with a weakly pinned Wigner crystal in which domain size increases with $n_s$, due to larger carrier-carrier interaction.

cond-mat.mes-hall↗