Diego Frustaglia

Spin interference effects in ring conductors subject to Rashba coupling

Quantum interference effects in rings provide suitable means for controlling spin at mesoscopic scales. Here we apply such control mechanisms to coherent spin-dependent transport in one- and two-dimensional rings subject to Rashba spin-orbit coupling. We first study the spin-induced modulation of unpolarized currents as a function of the Rashba coupling strength. The results suggest the possibility of all-electrical spintronic devices. Moreover, we find signatures of Berry phases in the conductance previously unnoticed. Second, we show that the polarization direction of initially polarized, transmitted spins can be tuned via an additional small magnetic control flux. In particular, this enables to precisely reverse the polarization direction at half a flux quantum. We present full numerical calculations for realistic two-dimensional ballistic microstructures and explain our findings in a simple analytical model for one-dimensional rings.

Quantum Transport in Nonuniform Magnetic Fields: Aharonov-Bohm Ring as a Spin Switch

We study spin-dependent magnetoconductance in mesoscopic rings subject to an inhomogeneous in-plane magnetic field. We show that the polarization direction of transmitted spin-polarized electrons can be controlled via an additional magnetic flux such that spin flips are induced at half a flux quantum. This quantum interference effect is independent of the strength of the nonuniform field applied. We give an analytical explanation for one-dimensional rings and numerical results for corresponding ballistic microstructures.

Aharonov-Bohm physics with spin. II. Spin-flip effects in two-dimensional ballistic systems

We study spin effects in the magnetoconductance of ballistic mesoscopic systems subject to inhomogeneous magnetic fields. We present a numerical approach to the spin-dependent Landauer conductance which generalizes recursive Green-function techniques to the case with spin. Based on this method we address spin-flip effects in quantum transport of spin-polarized and spin-unpolarized electrons through quantum wires and various two-dimensional Aharonov-Bohm geometries. In particular, we investigate the range of validity of a spin-switch mechanism recently found which allows for controlling spins indirectly via Aharonov-Bohm fluxes. Our numerical results are compared to a transfer-matrix model for one-dimensional ring structures presented in the first paper [Hentschel et al., Phys. Rev. B, preceding paper, Phys. Rev. B 69, 155326 (2004)] of this series.

Spin filter effects in mesoscopic ring structures

We study the spin-dependent conductance of ballistic mesoscopic ring systems in the presence of an inhomogeneous magnetic field. We show that, for the set-up proposed, even a small Zeeman splitting can lead to a considerable spin polarization of the current. Making use of a spin-switch effect (Frustaglia et al2001 Phys. Rev. Lett. 87 256602) we propose a device of two rings connected in series that in principle allows for both creating and coherently controlling spin polarized currents at low temperatures.

Control of the spin geometric phase in semiconductor quantum rings

Since the formulation of the geometric phase by Berry, its relevance has been demonstrated in a large variety of physical systems. However, a geometric phase of the most fundamental spin-1/2 system, the electron spin, has not been observed directly and controlled independently from dynamical phases. Here we report experimental evidence on the manipulation of an electron spin through a purely geometric effect in an InGaAs-based quantum ring with Rashba spin-orbit coupling. By applying an in-plane magnetic field, a phase shift of the Aharonov–Casher interference pattern towards the small spin-orbit-coupling regions is observed. A perturbation theory for a one-dimensional Rashba ring under small in-plane fields reveals that the phase shift originates exclusively from the modulation of a pure geometric-phase component of the electron spin beyond the adiabatic limit, independently from dynamical phases. The phase shift is well reproduced by implementing two independent approaches, that is, perturbation theory and non-perturbative transport simulations.

Role of Orbital Dynamics in Spin Relaxation and Weak Antilocalization in Quantum Dots

We develop a semiclassical theory for spin-dependent quantum transport to describe weak (anti)localization in quantum dots with spin-orbit coupling. This allows us to distinguish different types of spin relaxation in systems with chaotic, regular, and diffusive orbital classical dynamics. We find, in particular, that for typical Rashba spin-orbit coupling strengths, integrable ballistic systems can exhibit weak localization, while corresponding chaotic systems show weak antilocalization. We further calculate the magnetoconductance and analyze how the weak antilocalization is suppressed with decreasing quantum dot size and increasing additional in-plane magnetic field.

Aharonov-Bohm physics with spin. I. Geometric phases in one-dimensional ballistic rings

We analytically calculate the spin-dependent electronic conductance through a one-dimensional ballistic ring in the presence of an inhomogeneous magnetic field and identify signatures of geometric and Berry phases in the general nonadiabatic situation. For an in-plane magnetic field, we rigorously prove the spin-flip effect presented by Frustaglia et al. [Phys. Rev. Lett. 87, 256602 (2001)], which allows us to control and switch the polarization of outgoing electrons by means of an Aharonov-Bohm flux, and derive analytical expressions for the energy-averaged magnetoconductance. Our results support numerical calculations for two-dimensional ballistic rings presented in the second paper [Frustaglia et al., following paper, Phys. Rev. B 69, 155327 (2004)] of this series.

Electronic Hong-Ou-Mandel interferometer for multimode entanglement detection

We show that multimode entanglement of electrons in a mesoscopic conductor can be detected by a measurement of the zero-frequency current correlations in an electronic Hong-Ou-Mandel interferometer. By this means, one can further establish a lower bound to the entanglement of formation of two-electron input states. Our results extend the work of Burkard and Loss [Phys. Rev. Lett. 91, 087903 (2003)] to many channels and provide a way to test the existence of entangled states involving both orbital and spin degrees of freedom.

Theoretical study of the conductance of ferromagnetic atomic-sized contacts

In this work, we study theoretically the conductance of atomic contacts of the ferromagnetic 3d materials Fe, Co, and Ni. For this purpose, we employ a tight-binding model and we focus on the analysis of ideal contact geometries. In agreement with previous theoretical results, the 3d bands of these transition metals play the key role in the electrical conduction of atomic contacts. As a consequence, in the contact regime, there are partially open conductance channels and the conductance of the last plateau is typically above G0 =2 e 2 /h. Furthermore, in this regime, there is no complete spin polarization of the current i.e., both spin bands contribute to transport and the amplitude of the conductance as well as its spin polarization are very sensitive to disorder in the contact geometry. Finally, we find that in the tunneling regime, a high spin polarization of the current can be achieved.

Signatures of spin-related phases in transport through regular polygons

We address the subject of transport in one-dimensional ballistic polygon loops subject to Rashba spin-orbit coupling. We identify the role played by the polygon vertices in the accumulation of spin-related phases by studying interference effects as a function of the spin-orbit coupling strength. We find that the vertices act as strong spin-scattering centers that hinder the developing of Aharovov-Casher and Berry phases. In particular, we show that the oscillation frequency of interference pattern can be doubled by modifying the shape of the loop from a square to a circle.