A. C. Graham

Imaging fractal conductance fluctuations and scarred wave functions in a quantum billiard.

We present scanning-probe images and magnetic-field plots which reveal fractal conductance fluctuations in a quantum billiard. The quantum billiard is drawn and tuned using erasable electrostatic lithography, where the scanning probe draws patterns of surface charge in the same environment used for measurements. A periodicity in magnetic field, which is observed in both the images and plots, suggests the presence of classical orbits. Subsequent high-pass filtered high-resolution images resemble the predicted probability density of scarred wave functions, which describe the classical orbits.

Fano Factor Reduction on the 0.7 Conductance Structure of a Ballistic One-Dimensional Wire

We have measured the nonequilibrium current noise in a ballistic one-dimensional wire which exhibits an additional conductance plateau at 0.7x2e(2)/h. The Fano factor shows a clear reduction on the 0.7 structure, and eventually vanishes upon applying a strong parallel magnetic field. These results provide experimental evidence that the 0.7 structure is associated with two conduction channels that have different transmission probabilities.

Bias-controlled spin polarization in quantum wires

We demonstrate that a source-drain bias creates a fully spin-polarized current as the 0.25(2e2∕h) plateau in quantum wires even in zero magnetic field. When a source-drain bias lifts the momentum degeneracy, the dc measurements show that it is possible to achieve a unidirectional ferromagnetic order and this ordered spin array is destroyed once transport in both directions commences. The spin polarization of currents, between full spin polarization and partial spin polarization (or spin degeneracy), is thus simply controlled by source-drain bias and split-gate voltage, something of considerable value for spintronics.

Energy-level pinning and the 0.7 spin state in one dimension: GaAs quantum wires studied using finite-bias spectroscopy

We study the effects of electron-electron interactions on the energy levels of GaAs quantum wires (QWs) using finite-bias spectroscopy. We probe the energy spectrum at zero magnetic field, and at crossings of opposite-spin-levels in high in-plane magnetic field B. Our results constitute direct evidence that spin-up (higher energy) levels pin to the chemical potential mu as they populate. We also show that spin-up and spin-down levels abruptly rearrange at the crossing in a manner resembling the magnetic phase transitions predicted to occur at crossings of Landau levels. This rearranging and pinning of subbands provides a phenomenological explanation for the 0.7 structure, a one-dimensional (1D) nanomagnetic state, and its high-B variants.

Non-Kondo zero-bias anomaly in quantum wires

It has been suggested that a zero-bias conductance peak in quantum wires signifies the presence of Kondo spin-correlations, which might also relate to an intriguing one-dimensional (1D) spin effect known as the 0.7 structure. These zero-bias anomalies (ZBA) are strongly temperature dependent, and have been observed to split into two peaks in magnetic field, both signatures of Kondo correlations in quantum dots. We present data in which ZBAs in general do not split as magnetic field is increased up to 10 T. A few of our ZBAs split in magnetic field but by significantly less than the Kondo splitting value, and evolve back to a single peak upon moving the 1D constriction laterally. The ZBA therefore does not appear to have a Kondo origin, and instead we propose a simple phenomenological model to reproduce the ZBA which is in agreement mostly with observed characteristics.

Spin effects in one-dimensional systems

We review the progress in the study of the 0.7 structure, a many-body spin effect observed in GaAs quantum wires. Electrical and thermal measurements show that the 0.7 structure is characterized by a spontaneous spin splitting which causes the spin-up subband to stay near the electrochemical potential as the channel is populated. The 0.7 structure is not an isolated phenomenon but is the first in a series of effects that occur whenever opposite spin subbands are degenerate in energy. Bias spectroscopy shows how the levels move with increasing source–drain bias as the 0.7 structure evolves into a structure near 0.85(2e2/h). Our measurements do not support a Kondo effect from a single bound state in the channel as a possible explanation of the 0.7 structure.

Direct observation of nonequilibrium spin population in quasi-one-dimensional nanostructures.

Observation of the interplay between interacting energy levels of two spin species is limited by the difficulties in continuously tracking energy levels and thus leaves spin transport in quantum wires still not well understood. We present a dc conductance feature in the nonequilibrium transport regime, a direct indication that the first one-dimensional subband is filled mostly by one spin species only. How this anomalous spin population changes with magnetic field and source-drain bias is directly measured. We show the source-drain bias changes spin polarization in semiconductor nanowires, providing a fully electrical method for the creation and manipulation of spin polarization as well as spin-polarized currents.

Anomalous spin-dependent behavior of one-dimensional subbands

We report an electron interaction effect in GaAs/AlGaAs quantum wires. Using dc-bias spectroscopy, we show that large and abrupt changes occur to the energies of spin-down (lower energy) states as they populate. The effect is not observed for spin-up energy states. At B=0, interactions have a pronounced effect, in the form of the well-known 0.7 structure. However, our results show that interactions strongly affect the energy spectrum at all magnetic fields, from 0 to 16 T, not just in the vicinity of the 0.7 structure.

Momentum effects on focusing in one‐dimensional systems

We present a study of the effect of a dc source‐drain bias on the transverse electron focusing spectrum of two parallel quantum wires (QWs) at conductances less than 2e2/h. Focusing spectra exhibit peaks periodic in field. Application of a source‐drain bias lifts momentum degeneracy in the 1D channel and reveals additional features at low conductances. We observe a change in the position of focusing peaks as the emitter conductance is increased through the high bias features. It is demonstrated that there is a shift in electron momentum direction directly caused by population of the 1D subband responsible for the low conductance feature in the emitter.

Fano factor reduction on the 0.7 structure

We have measured the non‐equilibrium current noise in a ballistic one‐dimensional wire which exhibits an additional conductance plateau at 0.7 × 2e2/h. The Fano factor shows a clear reduction on the 0.7 structure, and eventually vanishes upon applying a strong parallel magnetic field. These results provide the first experimental evidence that the 0.7 structure is associated with two conduction channels which have different transmission probabilities.