A. M. Chang

Scanning Hall probe microscopy

We describe the implementation of a scanning Hall probe microscope of outstanding magnetic field sensitivity (∼0.1 G) and unprecedented spatial resolution (∼0.35 μm) to detect surface magnetic fields at close proximity to a sample. Our microscope combines the advantages of a submicron Hall probe fabricated on a GaAs/Al0.3Ga0.7As heterostructure chip and the scanning tunneling microscopy technique for precise positioning. We demonstrate its usefulness by imaging individual vortices in high Tc La1.85Sr0.15CuO4 films and superconducting networks, and magnetic bubble domains.

Weak localization in chaotic versus nonchaotic cavities: A striking difference in the line shape.

We report experimental evidence that chaotic and non-chaotic scattering through ballistic cavities display distinct signatures in quantum transport. In the case of non-chaotic cavities, we observe a linear decrease in the average resistance with magnetic field which contrasts markedly with a Lorentzian behavior for a chaotic cavity. This difference in line-shape of the weak-localization peak is related to the differing distribution of areas enclosed by electron trajectories. In addition, periodic oscillations are observed which are probably associated with the Aharonov-Bohm effect through a periodic orbit within the cavities.

Transition between Quantum States in a Parallel-Coupled Double Quantum Dot

Strong electron and spin correlations in a double quantum dot (DQD) can give rise to different quantum states. We observe a continuous transition from a Kondo state exhibiting a single-peak Kondo resonance to another exhibiting a double peak by increasing the interdot coupling (t) in a parallel-coupled DQD. The transition into the double-peak state provides evidence for spin entanglement between the excess electrons on each dot. Toward the transition, the peak splitting merges and becomes substantially smaller than t because of strong Coulomb effects. Our device tunability bodes well for future quantum computation applications.

Crystal orientation and thickness dependence of the superconducting transition temperature of tetragonal FeSe1-x thin films.

Superconductivity was recently found in the tetragonal phase FeSe. A structural transformation from tetragonal to orthorhombic (or monoclinic, depending on point of view) was observed at low temperature, but was not accompanied by a magnetic ordering as commonly occurs in the parent compounds of FeAs-based superconductors. Here, we report the correlation between structural distortion and superconductivity in FeSe(1-x) thin films with different preferred growth orientations. The films with preferred growth along the c axis show a strong thickness dependent suppression of superconductivity and low temperature structural distortion. In contrast, both properties are less affected in the films with (101) preferred orientation. These results suggest that the low temperature structural distortion is closely associated with the superconductivity of this material.

Evidence for Macroscopic Quantum Tunneling of Phase Slips in Long One-Dimensional Superconducting Al Wires

Quantum phase slips have received much attention due to their relevance to superfluids in reduced dimensions and to models of cosmic string production in the early universe. Their establishment in one-dimensional superconductors has remained controversial. Here we study the nonlinear current-voltage characteristics and linear resistance in long superconducting Al wires with lateral dimensions approximately 5 nm. We find that, in a magnetic field and at temperatures well below the superconducting transition, the observed behaviors can be described by the nonclassical, macroscopic quantum tunneling of phase slips, and are inconsistent with the thermal activation of phase slips.

A unified transport theory for the integral and fractional quantum hall effects: Phase boundaries, edge currents, and transmission/reflection probabilities

Abstract We propose a unified transport theory for the fractional (FQHE) and integral quantum Hall effect (IQHE) of the Buttiker-Landauer type in the presence of smooth confinement and impurity potentials, where resistances are expressed in terms of transmission and reflection probabilities of edge channels in the incompressible fluid states. Electron-electron interaction is taken into account in the FQHE by writing down a Laughlin type wave function for a pure phase, and in the compressible fluids at non-special filling factors by a self-consistent screening of the background potential. Our theory is able to explain recent experimental observations.

Tunneling current noise in thin gate oxides

We have examined fluctuations in the tunneling current of 3.5 nm SiO2 barriers for voltages in the direct tunneling regime. We find a 1/f power law for the spectral density of the fluctuations where f is the frequency. This 1/f noise can be attributed to fluctuations of a trap assisted tunneling current through the oxide that causes current noise but is not evident in the I–V curves. We suggest that this noise may be a more sensitive probe of trap assisted tunneling and degradation in thin oxides than other measures. At voltages above a threshold of 2.5 V, we observe the reversible onset of non‐Gaussian current transients in the noise. The onset of these current transients can be related to a transition in the spacial uniformity of the tunneling current density that may result in eventual breakdown of the oxide.

Electrophysiological Study of Recovery of Peripheral Nerves Regenerated by a Collagen-Glycosaminoglycan Copolymer Matrix

We have studied the effects of variations in the structure of a collagen-glycosaminoglycan (CG) copolymer matrix on the regeneration of transected rat sciatic nerves. Silicone tubes ensheathing 10-mm lengths of CG copolymer were grafted between the transected sciatic nerve stumps. Empty and saline-filled silicone tubes, as well as autografts, were studied as controls. The mean pore diameter and the degradation rate of the copolymer in collagenase were independently varied to investigate how each affects regeneration. Electrophysiological properties of the regenerating motor nerve fibers innervating the plantar flexor muscles, were serially monitored over about 40 weeks following surgery. Rapidly degrading CG copolymers with pore channels oriented predominantly along the axes of the tubes induced regeneration with a success rate of 100% (n = 35). Although CG copolymers with axially-oriented pore channels that degraded slowly had a success rate as high as 96% (n = 23), the long-term electrophysiological results were markedly inferior to those obtained with the rapidly degrading grafts. In another study of axially-oriented pore structures, the level of recovery of long-term electrophysiological results was observed to increase monotonously as preliminary results showed that CG copolymers with pore channels predominantly oriented along the radial direction of the tubes had a success rate of only 50% (n = 6). Control groups of empty and saline-filled tubes had an aggregated success rate of 29% (n = 21). The ongoing study has shown that systematic physiochemical manipulation of simple chemical analogs of the extracellular matrix can be used to define substrate features which encourage regeneration.

Transmission and reflection probabilities between ν=1 and ν=23 quantum Hall effects and between ν=23 and ν=13 effects

Abstract We perform transport experiments in the quantum Hall regime on transmission through a barrier region exhibiting a fractional quantum Hall effect. For transmission from the ν=1 effect through a barrier at the 2 3 effect, the transmission probability is 2 3 (0.998±.005) and the reflection is 1 3 (.983±.010). Interpreted in terms of conduction via edge states, our results yield a quasi-particle charge of −e 3 ±2% in the 2 3 effect in one picture, and a reduction in the single-particle density of states in another. Data on transmission from the 2 3 effect through a barrier at 1 3 are consistent with these interpretations.

Scanning Hall-Probe Microscopy of a Vortex and Field Fluctuations in La1.85Sr0.15CuO4 Films

A high-resolution scanning Hall-probe microscope is used to spatially resolve vortices in high-temperature superconducting La1.85Sr0.15CuO4 films. At low magnetic fields, a disordered vortex arrangement is observed. A fit to the surface field of an individual vortex is consistent with one flux quantum, and is used to determine the local penetration depth and its temperature dependence. At higher fields, magnetic fluctuations are observed and compared to a collective pinning model. For films grown with the c-axis tilted from the surface normal, oval vortices are observed.