arXiv subjects

U. Zuelicke

Publications and source records attributed to U. Zuelicke.

At least 37 records · Page 2Linked to original sources

arXiv2010

AC transport properties of single and bilayer graphene

We have performed a theoretical study of electronic transport in single and bilayer graphene based on the standard linear-response (Kubo) formalism and continuum-model descriptions of the graphene band structure. We are focusing especially on the interband contribution to the optical conductivity. Analytical results are obtained for a variety of situations, which allow clear identification of features in the conductivity that are associated with relevant electronic energy scales. Our work extends previous numerical studies and elucidates ways to infer electronic properties of graphene samples from optical-conductivity measurements.

cond-mat.mes-hall↗

arXiv2009

Features due to spin-orbit coupling in the optical conductivity of single-layer graphene

We have calculated the optical conductivity of a disorder-free single graphene sheet in the presence of spin-orbit coupling, using the Kubo formalism. Both intrinsic and structural-inversion-asymmetry induced types of spin splitting are considered within a low-energy continuum theory. Analytical results are obtained that allow us to identify distinct features arising from spin-orbit couplings. We point out how optical-conductivity measurements could offer a way to determine the strengths of spin splitting due to various origins in graphene.

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arXiv2009

Observation of orientation- and $k$-dependent Zeeman spin-splitting in hole quantum wires on (100)-oriented AlGaAs/GaAs heterostructures

We study the Zeeman spin-splitting in hole quantum wires oriented along the $[011]$ and $[01\bar{1}]$ crystallographic axes of a high mobility undoped (100)-oriented AlGaAs/GaAs heterostructure. Our data shows that the spin-splitting can be switched `on' (finite $g^{*}$) or `off' (zero $g^{*}$) by rotating the field from a parallel to a perpendicular orientation with respect to the wire, and the properties of the wire are identical for the two orientations with respect to the crystallographic axes. We also find that the $g$-factor in the parallel orientation decreases as the wire is narrowed. This is in contrast to electron quantum wires, where the $g$-factor is enhanced by exchange effects as the wire is narrowed. This is evidence for a $k$-dependent Zeeman splitting that arises from the spin-3/2 nature of holes.

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arXiv2008

Spin-3/2 physics of semiconductor hole nanowires: Valence-band mixing and tunable interplay between bulk-material and orbital bound-state spin splittings

We present a detailed theoretical study of the electronic spectrum and Zeeman splitting in hole quantum wires. The spin-3/2 character of the topmost bulk-valence-band states results in a strong variation of subband-edge g factors between different subbands. We elucidate the interplay between quantum confinement and heavy-hole - light-hole mixing and identify a certain robustness displayed by low-lying hole-wire subband edges with respect to changes in the shape or strength of the wire potential. The ability to address individual subband edges in, e.g., transport or optical experiments enables the study of holes states with nonstandard spin polarization, which do not exist in spin-1/2 systems. Changing the aspect ratio of hole wires with rectangular cross-section turns out to strongly affect the g factor of subband edges, providing an opportunity for versatile in-situ tuning of hole-spin properties with possible application in spintronics. The relative importance of cubic crystal symmetry is discussed, as well as the spin splitting away from zone-center subband edges.

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arXiv2008

Tunable Aharonov-Anandan phase in transport through mesoscopic hole rings

We present a theoretical study of spin-3/2 hole transport through mesoscopic rings, based on the spherical Luttinger model. The quasi-one-dimensional ring is created in a symmetric two-dimensional quantum well by a singular-oscillator potential for the radial in-plane coordinate. The quantum-interference contribution to the two-terminal ring conductance exhibits an energy-dependent Aharonov-Anandan phase, even though Rashba and Dresselhaus spin splittings are absent. Instead, confinement-induced heavy-hole - light-hole mixing is found to be the origin of this phase, which has ramifications for magneto-transport measurements in gated hole rings.

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arXiv2007

Tailoring hole spin splitting and polarization in nanowires

Spin splitting in p-type semiconductor nanowires is strongly affected by the interplay between quantum confinement and spin-orbit coupling in the valence band. The latter's particular importance is revealed in our systematic theoretical study presented here, which has mapped the range of spin-orbit coupling strengths realized in typical semiconductors. Large controllable variations of the g-factor with associated characteristic spin polarization are shown to exist for nanowire subband edges, which therefore turn out to be a versatile laboratory for investigating the complex spin properties exhibited by quantum-confined holes.

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arXiv2007

Magnetic focusing of charge carriers from spin-split bands: Semiclassics of a Zitterbewegung effect

We present a theoretical study of the interplay between cyclotron motion and spin splitting of charge carriers in solids. While many of our results apply more generally, we focus especially on discussing the Rashba model describing electrons in the conduction band of asymmetric semiconductor heterostructures. Appropriate semiclassical limits are distinguished that describe various situations of experimental interest. Our analytical fomulae, which take full account of Zeeman splitting, are used to analyse recent magnetic-focusing data. Surprisingly, it turns out that the Rashba effect can dominate the splitting of cyclotron orbits even when the Rashba and Zeeman spin-splitting energies are of the same order. We also find that the origin of spin-dependent cyclotron motion can be traced back to Zitterbewegung-like oscillatory dynamics of charge carriers from spin-split bands. The relation between the two phenomena is discussed, and we estimate the effect of Zitterbewegung-related corrections to the charge carriers' canonical position.

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arXiv2007

Large variations in the hole spin splitting of quantum-wire subband edges

We study Zeeman splitting of zone-center subband edges in a cylindrical hole wire subject to a magnetic field parallel to its axis. The g-factor turns out to fluctuate strongly as a function of wire-subband index, assuming values that differ substantially from those found in higher-dimensional systems. We analyze the spin properties of hole-wire states using invariants of the spin-3/2 density matrix and find a strong correlation between g-factor value and the profile of hole-spin polarization density. Our results suggest possibilities for confinement engineering of hole spin splittings.

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arXiv2007

Universal characteristics of resonant-tunneling field emission from nanostructured surfaces

We have performed theoretical and experimental studies of field emission from nanostructured semiconductor cathodes. Resonant tunneling through electric-field-induced interface bound states is found to strongly affect the field-emission characteristics. Our analytical theory predicts power-law and Lorentzian-shaped current-voltage curves for resonant-tunneling field emission from three-dimensional substrates and two-dimensional accumulation layers, respectively. These predicted line shapes are observed in field emission characteristics from self-assembled silicon nanostructures. A simple model describes formation of an accumulation layer and of the resonant level in these systems.

cond-mat.mes-hall↗

arXiv2007

Rashba interferometers: Spin-dependent single and two-electron interference

Quantum transport in semiconductor nanostructures can be described theoretically in terms of the propagation and scattering of electron probability waves. Within this approach, elements of a phase-coherent electric circuit play a role similar to quantum-optical devices that can be characterised by scattering matrices. Electronic analogues of well-know optical interferometers have been fabricated and used to study special features of charge carriers in solids. We present results from our theoretical investigation into the interplay between spin precession and quantum interference in an electronic Mach-Zehnder interferometer with spin-orbit coupling of the Rashba type. Intriguing spin-dependent transport effects occur, which can be the basis for novel spintronic devices such as a magnet-less spin-controlled field-effect transistor and a variety of single-qubit gates. Their functionality arises entirely from spin-dependent interference of each single input electron with itself. We have also studied two-electron interference effects for the spin-dependent Mach-Zehnder interferometer, obtaining analytical expressions for its two-fermion-state scattering matrix. Using this result, we consider ways to generate two-electron output states for which the Rashba spin-subband quantum number and the output-arm index are entangled. Combining spin-dependent interference in our proposed Mach-Zehnder interferometer with a projective charge measurement at the output enables entanglement generation. As our particular scheme involves tuneable spin precession, electric-field control of entanglement production can be achieved.

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arXiv2006

Electronic and spin properties of hole point contacts

We have studied theoretically the effect of a tuneable lateral confinement on two-dimensional hole systems realised in III-V semiconductor heterostructures. Based on the 4x4 Luttinger description of the valence band, we have calculated quasi-onedimensional (quasi-1D) hole subband energies and anisotropic Lande g-factors. Confinement-induced band mixing results in the possibility to manipulate electronic and spin properties of quasi-1D hole states over a much wider range than is typically possible for confined conduction-band electrons. Our results are relevant for recent experiments where source-drain-bias spectroscopy was used to measure Zeeman splitting of holes in p-type quantum point contacts.

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arXiv2006

Anisotropic Zeeman splitting in ballistic one-dimensional hole systems

We have studied the effect of an in-plane magnetic field B on a one-dimensional hole system in the ballistic regime created by surface gate confinement. We observed clearly the lifting of the spin degeneracy due to the Zeeman effect on the one dimensional subbands for B applied parallel to the channel. In contrast, no Zeeman splitting is detected for B applied perpendicular to the channel, revealing an extreme anisotropy of the effective Lande g-factor g*. We demonstrate that this anisotropy is a direct consequence of the one-dimensional confinement on a system with strong spin-orbit coupling.

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arXiv2006

Zeeman splitting in ballistic hole quantum wires

We have studied the Zeeman splitting in ballistic hole quantum wires formed in a (311)A quantum well by surface gate confinement. Transport measurements clearly show lifting of the spin degeneracy and crossings of the subbands when an in-plane magnetic field B is applied parallel to the wire. When B is oriented perpendicular to the wire, no spin-splitting is discernible up to B = 8.8 T. The observed large Zeeman splitting anisotropy in our hole quantum wires demonstrates the importance of quantum-confinement for spin-splitting in nanostructures with strong spin-orbit coupling.

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arXiv2006

Reply to Comment on "Strongly Correlated Fractional Quantum Hall Line Junctions"

In two recent articles [PRL 90, 026802 (2003); PRB 69, 085307 (2004)], we developed a transport theory for an extended tunnel junction between two interacting fractional-quantum-Hall edge channels, obtaining analytical results for the conductance. Ponomarenko and Averin (PA) have expressed disagreement with our theoretical approach and question the validity of our results (cond-mat/0602532). Here we show why PA's critique is unwarranted.

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arXiv2005

Tunable entanglement generation for mobile-electron spin qubits

Recent studies have shown that linear electron optics can be used to generate entangled two-particle states from nonentangled ones if additional measurements of charge or parity are performed. We have investigated such nondeterministic entanglement production in electronic versions of the Mach-Zehnder interferometer, where spin-dependent interference occurs due to the presence of electric-field tunable Rashba spin splitting. Adjustment of the spin-precession length turns out to switch the entangler on and off, as well as control the detailed form of entangled output states.

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arXiv2005

Andreev reflection and cyclotron motion at superconductor -- normal-metal interfaces

We investigate Andreev reflection at the interface between a superconductor and a two--dimensional electron system (2DES) in an external magnetic field such that cyclotron motion is important in the latter. A finite Zeeman splitting in the 2DES and the presence of diamagnetic screening currents in the superconductor are incorporated into a microscopic theory of Andreev edge states, which is based on the Bogoliubov--de Gennes formalism. The Andreev--reflection contribution to the interface conductance is calculated. The effect of Zeeman splitting is most visible as a double--step feature in the conductance through clean interfaces. Due to a screening current, conductance steps are shifted to larger filling factors and the formation of Andreev edge states is suppressed below a critical filling factor.

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arXiv2004

Spin interferometry with electrons in nanostructures: A road to spintronic devices

The wave nature of electrons in semiconductor nanostructures results in spatial interference effects similar to those exhibited by coherent light. The presence of spin-orbit coupling renders interference in spin space and in real space interdependent, making it possible to manipulate the electron's spin state by addressing its orbital degree of freedom. This suggests the utility of electronic analogs of optical interferometers as blueprints for new spintronics devices. We demonstrate the usefulness of this concept using the Mach-Zehnder interferometer as an example. Its spin-dependent analog realizes a spin-controlled field-effect transistor without magnetic contacts and may be used as a quantum logical gate.

cond-mat.mes-hall↗

arXiv2004

Rashba spin splitting in quantum wires

This article presents an overview of results pertaining to electronic structure, transport properties, and interaction effects in ballistic quantum wires with Rashba spin splitting. Limits of weak and strong spin--orbit coupling are distinguished, and spin properties of the electronic states elucidated. The case of strong Rashba spin splitting where the spin--precession length is comparable to the wire width turns out to be particularly interesting. Hybridization of spin--split quantum--wire subbands leads to an unusual spin structure where the direction of motion for electrons can fix their spin state. This peculiar property has important ramifications for linear transport in the quantum wire, giving rise to spin accumulation without magnetic fields or ferromagnetic contacts. A description for interacting Rashba--split quantum wires is developed, which is based on a generalization of the Tomonaga--Luttinger model.

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