arXiv subjects

Dimitrie Culcer

Publications and source records attributed to Dimitrie Culcer.

134 records · Page 8Linked to original sources

arXiv2007

Generation of spin currents and spin densities in systems with reduced symmetry

We show that the spin-current response of a semiconductor crystal to an external electric field is considerably more complex than previously assumed. While in systems of high symmetry only the spin-Hall components are allowed, in systems of lower symmetry other non-spin-Hall components may be present. We argue that, when spin-orbit interactions are present only in the band structure, the distinction between intrinsic and extrinsic contributions to the spin current is not useful. We show that the generation of spin currents and that of spin densities in an electric field are closely related, and that our general theory provides a systematic way to distinguish between them in experiment. We discuss also the meaning of vertex corrections in systems with spin-orbit interactions.

cond-mat.mes-hall↗

arXiv2006

Spin precession and alternating spin polarization in spin-3/2 hole systems

The spin density matrix for spin-3/2 hole systems can be decomposed into a sequence of multipoles which has important higher-order contributions beyond the ones known for electron systems [R. Winkler, Phys. Rev. B \textbf{70}, 125301 (2004)]. We show here that the hole spin polarization and the higher-order multipoles can precess due to the spin-orbit coupling in the valence band, yet in the absence of external or effective magnetic fields. Hole spin precession is important in the context of spin relaxation and offers the possibility of new device applications. We discuss this precession in the context of recent experiments and suggest a related experimental setup in which hole spin precession gives rise to an alternating spin polarization.

cond-mat.other↗

arXiv2006

Geometrical phase effects on the Wigner distribution of Bloch electrons

We investigate the dynamics of Bloch electrons using a density operator method and connect this approach with previous theories based on wave packets. We study non-interacting systems with negligible disorder and strong spin-orbit interactions, which have been at the forefront of recent research on spin-related phenomena. We demonstrate that the requirement of gauge invariance results in a shift in the position at which the Wigner function of Bloch electrons is evaluated. The present formalism also yields the correction to the carrier velocity arising from the Berry phase. The gauge-dependent shift in carrier position and the Berry phase correction to the carrier velocity naturally appear in the charge and current density distributions. In the context of spin transport we show that the spin velocity may be defined in such a way as to enable spin dynamics to be treated on the same footing as charge dynamics. Aside from the gauge-dependent position shift we find additional, gauge-covariant multipole terms in the density distributions of spin, spin current and spin torque.

cond-mat.mes-hall↗

arXiv2005

Coherent wave-packet evolution in coupled bands

We develop a formalism for treating coherent wave-packet dynamics of charge and spin carriers in degenerate and nearly degenerate bands. We consider the two-band case carefully in view of spintronics applications, where transitions between spin-split bands often occur even for relatively weak electromagnetic fields. We demonstrate that much of the semiclassical formalism developed for the single-band case can be generalized to multiple bands, and examine the nontrivial non-Abelian corrections arising from the additional degree of freedom. Along with the center of mass motion in crystal momentum and real space, one must also include a pseudo-spin to characterize the dynamics between the bands. We derive the wave packet energy up to the first order gradient correction and obtain the equations of motion for the real- and $k$-space center of the wave-packet, as well as for the pseudo-spin. These equations include the non-Abelian Berry curvature terms and a non-Abelian correction to the group velocity. As an example, we apply our formalism to describe coherent wave-packet evolution under the action of an electric field, demonstrating that it leads to electrical separation of up and down spins. A sizable separation will be observed, with a large degree of tunability, making this mechanism a practical method of generating a spin polarization. We then turn our attention to a magnetic field, where we recover Larmor precession, which cannot be obtained from a single-band point of view. In this case, the gradient energy correction can be regarded as due to a magnetic moment from the self-rotation of the wave-packet, and we calculate its value for the light holes in the spherical four-band Luttinger model.

cond-mat.other↗

arXiv2005

Electric generation of spin in crystals with reduced symmetry

We propose a simple way of evaluating the bulk spin generation of an arbitrary crystal with a known band structure in the strong spin-orbit coupling limit. We show that, in the presence of an electric field, there exists an intrinsic torque term which gives rise to a nonzero spin generation rate. Using methods similar to those of recent experiments which measure spin polarization in semiconductors, this spin generation rate should be experimentally observable. The wide applicability of this effect is emphasized by explicit consideration of a range of examples: bulk wurtzite and strained zincblende (n-GaAs) lattices, as well as quantum well heterojunction systems.

cond-mat.mes-hall↗

arXiv2004

Semiclassical theory of spin transport in spin-orbit coupled systems

Motivated by recent interest in novel spintronics effects, we develop a semiclassical theory of spin transport that is valid for spin-orbit coupled bands. Aside from the obvious convective term in which the average spin is transported at the wavepacket group velocity, the spin current has additional contributions from the wavepacket's spin and torque dipole moments. Electric field corrections to the group velocity and carrier spin contribute to the convective term. Summing all terms we obtain an expression for the intrinsic spin-Hall conductivity of a hole-doped semiconductor, which agrees with the Kubo formula prediction for the same quantity. We discuss the calculation of spin accumulation, which illustrates the importance of the torque dipole near the boundary of the system.

cond-mat.mes-hall↗

arXiv2004

Universal Intrinsic Spin-Hall Effect

We describe a new effect in semiconductor spintronics that leads to dissipationless spin-currents in paramagnetic spin-orbit coupled systems. We argue that in a high mobility two-dimensional electron system with substantial Rashba spin-orbit coupling, a spin-current that flows perpendicular to the charge current is intrinsic. In the usual case where both spin-orbit split bands are occupied, the spin-Hall conductivity has a universal value.

cond-mat.mes-hall↗

arXiv2003

Anomalous Hall effect in paramagnetic two dimensional systems

We investigate the possibility of observing the anomalous Hall effect (AHE) in two dimensional paramagnetic systems. We apply the semiclassical equations of motion to carriers in the conduction and valence bands of wurtzite and zincblende quantum wells in the exchange field generated by magnetic impurities and we calculate the anomalous Hall conductivity based on the Berry phase corrections to the carrier velocity. We show that under certain circumstances this conductivity approaches one half of the conductance quantum. We consider the effect of an external magnetic field and show that for a small enough field the theory is unaltered.

cond-mat↗