P. Debray

All-electric quantum point contact spin-polarizer

The controlled creation, manipulation and detection of spin-polarized currents by purely electrical means remains a central challenge of spintronics. Efforts to meet this challenge by exploiting the coupling of the electron orbital motion to its spin, in particular Rashba spin-orbit coupling, have so far been unsuccessful. Recently, it has been shown theoretically that the confining potential of a small current-carrying wire with high intrinsic spin-orbit coupling leads to the accumulation of opposite spins at opposite edges of the wire, though not to a spin-polarized current. Here, we present experimental evidence that a quantum point contact -- a short wire -- made from a semiconductor with high intrinsic spin-orbit coupling can generate a completely spin-polarized current when its lateral confinement is made highly asymmetric. By avoiding the use of ferromagnetic contacts or external magnetic fields, such quantum point contacts may make feasible the development of a variety of semiconductor spintronic devices.

Possible origin of the 0.5 plateau in the ballistic conductance of quantum point contacts

A non-equilibrium Green function formalism (NEGF) is used to study the conductance of a side-gated quantum point contact (QPC) in the presence of lateral spin-orbit coupling (LSOC). A small difference of bias voltage between the two side gates (SGs) leads to an inversion asymmetry in the LSOC between the opposite edges of the channel. In single electron modeling of transport, this triggers a spontaneous but insignificant spin polarization in the QPC. However, the spin polarization of the QPC is enhanced substantially when the effect of electron-electron interaction is included. The spin polarization is strong enough to result in the occurrence of a conductance plateau at 0.5G0 (G0 = 2e2/h) in the absence of any external magnetic field. In our simulations of a model QPC device, the 0.5 plateau is found to be quite robust and survives up to a temperature of 40K. The spontaneous spin polarization and the resulting magnetization of the QPC can be reversed by flipping the polarity of the source to drain bias or the potential difference between the two SGs. These numerical simulations are in good agreement with recent experimental results for side-gated QPCs made from the low band gap semiconductor InAs.

Experimental studies of Coulomb drag between ballistic quantum wires

The Coulomb drag between two spatially separated one-dimensional (1D) electron systems in lithographically fabricated 2??m long quantum wires is studied experimentally. The drag voltage VD shows peaks as a function of a gate voltage which shifts the position of the Fermi level relative to the 1D subbands. The maximum in VD and the drag resistance RD occurs when the 1D subbands of the wires are aligned and the Fermi wave vector is small. The drag resistance is found to decrease exponentially with interwire separation. In the temperature region 0.2?K?T?1?K, RD decreases with increasing temperature in a power-law fashion RDTx with x ranging from -0.6 to -0.77 depending on the gate voltage. We interpret our data in terms of the Tomonaga-Luttinger liquid theory.

Coulomb drag between ballistic one-dimensional electron systems

The presence of pronounced electronic correlations in one-dimensional systems strongly enhances Coulomb coupling and is expected to result in distinctive features in the Coulomb drag between them that are absent in the drag between two-dimensional systems. In this review, we review recent Fermi and Luttinger liquid theories of Coulomb drag between ballistic one-dimensional electron systems, also known as quantum wires, in the absence of inter-wire tunnelling, to focus on these features and give a brief summary of the experimental work reported so far on one-dimensional drag. Both the Fermi liquid (FL) and the Luttinger liquid (LL) theory predict a maximum drag resistance RD when the one-dimensional subbands of the two quantum wires are aligned and the Fermi wave vector kF is small, and also an exponential decay of RD with increasing inter-wire separation, both features confirmed by experimental observations. A crucial difference between the two theoretical models emerges in the temperature dependence of the drag effect. Although the FL theory predicts a linear temperature dependence, the LL theory promises a rich and varied dependence on temperature depending on the relative magnitudes of the energy and length scales of the systems. At very low temperatures, the drag resistance may diverge due to the formation of locked charge density waves. At higher temperatures, it should show a power-law dependence on temperature, RD Tx, experimentally confirmed in a narrow temperature range, where x is determined by the Luttinger liquid parameters. The spin degree of freedom plays an important role in the LL theory in predicting the features of the drag effect and is crucial for the interpretation of experimental results. Substantial experimental and theoretical work remains to be done for a comprehensive understanding of one-dimensional Coulomb drag.

All-electric spintronics with quantum point contacts

We review our recent study of the conductance of side gated quantum point contacts (QPCs) in the presence of lateral spin-orbit scattering (LSOC). As shown in Phys. Rev. B 80, 155440 (2009), a small difference of bias between the two side gates of a QPC leads to an inversion asymmetry in the LSOC on opposite edges of the channel. This triggers a small spin imbalance in the QPC. The latter increases substantially when the effects of electron-electron interaction are included in the QPC. This is accompanied by a wide variety of anomalous conductance plateaus as the dimensions of the QPC are varied. In this paper, the spin polarization of the conductance a is studied as a function of the device geometry. A simple model of two QPCs in series predict an ON/OFF ratio equal to 1/(1 − α2) which is around 86 at T = 4.2K for the device parameters investigated. A tandem of QPCs could therefore act as an efficient all-electric spin valve. Design issues for operation at higher temperature are discussed.