John Schliemann

Nonballistic spin-field-effect transistor

We propose a spin-field-effect transistor based on spin-orbit coupling of both the Rashba and the Dresselhaus types. Different from earlier proposals, spin transport through our device is tolerant against spin-independent scattering processes. Hence the requirement of strictly ballistic transport can be relaxed. This follows from a unique interplay between the Dresselhaus and the Rashba coupling; these can be tuned to have equal strengths, leading to k-independent eigenspinors even in two dimensions. We discuss two-dimensional devices as well as quantum wires. In the latter, our setup presents strictly parabolic dispersions which avoids complications from anticrossings of different bands.

Quantum correlations in two-fermion systems

We characterize and classify quantum correlations in two-fermion systems having 2K single-particle states. For pure states we introduce the Slater decomposition and rank (in analogy to Schmidt decomposition and rank): i.e., we decompose the state into a combination of elementary Slater determinants formed by pairs of mutually orthogonal single-particle states. Mixed states can be characterized by their Slater number which is the minimal Slater rank required to generate them. For K = 2 we gi ve a necessary and sufficient condition for a state to have a Slater number 1. We introduce a correlation measure for mixed states which can be evaluated analytically for K = 2. For higher K, we provide a method of constructing and optimizing Slater number witnesses, i.e., operators that detect Slater numbers for some states.

Quantum Correlations in Systems of Indistinguishable Particles

We discuss quantum correlations in systems of indistinguishable particles in relation to entanglement in composite quantum systems consisting of well separated subsystems. Our studies are motivated by recent experiments and theoretical investigations on quantum dots and neutral atoms in microtraps as tools for quantum information processing. We present analogies between distinguishable particles, bosons, and fermions in low-dimensional Hilbert spaces. We introduce the notion of Slater rank for pure states of pairs of fermions and bosons in analogy to the Schmidt rank for pairs of distinguishable particles. This concept is generalized to mixed states and provides a correlation measure for indistinguishable particles. Then we generalize these notions to pure fermionic and bosonic states in higher-dimensional Hilbert spaces and also to the multi-particle case. We review the results on quantum correlations in mixed fermionic states and discuss the concept of fermionic Slater witnesses. Then the theory of quantum correlations in mixed bosonic states and of bosonic Slater witnesses is formulated. In both cases we provide methods of constructing optimal Slater witnesses that detect the degree of quantum correlations in mixed fermionic and bosonic states.

Double-occupancy errors, adiabaticity, and entanglement of spin qubits in quantum dots

Quantum gates that temporarily increase singlet-triplet splitting in order to swap electronic spins in coupled quantum dots lead inevitably to a finite double-occupancy probability for both dots. By solving the time-dependent Schrodinger equation for a coupled dot model, we demonstrate that this does not necessarily lead to quantum computation errors, Instead. the coupled dot ground state evolves quasiadiabatically for typical system parameters so that the double-occupancy probability at the completion of swapping is negligibly small. We introduce a measure of entanglement that explicitly takes into account the possibilty of double occupancies and provides a necessary and sufficient criterion for entangled states.

Zitterbewegung of Electronic Wave Packets in III-V Zinc-Blende Semiconductor Quantum Wells

We study the zitterbewegung of electronic wave packets in III-V zinc-blende semiconductor quantum wells due to spin-orbit coupling. Our results suggest a direct experimental proof of this fundamental effect, confirming a long-standing theoretical prediction. For electron motion in a harmonic quantum wire, we numerically and analytically find a resonance condition maximizing the zitterbewegung.

Photothermoelectric and Photoelectric Contributions to Light Detection in Metal−Graphene−Metal Photodetectors

Graphenes high mobility and Fermi velocity, combined with its constant light absorption in the visible to far-infrared range, make it an ideal material to fabricate high-speed and ultrabroadband photodetectors. However, the precise mechanism of photodetection is still debated. Here, we report wavelength and polarization-dependent measurements of metal-graphene-metal photodetectors. This allows us to quantify and control the relative contributions of both photothermo- and photoelectric effects, both adding to the overall photoresponse. This paves the way for a more efficient photodetector design for ultrafast operating speeds.

Anisotropic transport in a two-dimensional electron gas in the presence of spin-orbit coupling

In a two-dimensional electron gas as realized by a semiconductor quantum well, the presence of spin-orbit coupling of both the Rashba and Dresselhaus type leads to anisotropic dispersion relations and Fermi contours. We study the effect of this anisotropy on the electrical conductivity in the presence of fixed impurity scatterers. The conductivity also shows in general an anisotropy which can be tuned by varying the Rashba coefficient. This effect provides a method of detecting and investigating spin-orbit coupling by measuring spin-unpolarized electrical currents in the diffusive regime. Our approach is based on an exact solution of the two-dimensional Boltzmann equation and provides also a natural framework for investigating other transport effects including the anomalous Hall effect.

Minimum electrical and thermal conductivity of graphene: a quasiclassical approach.

We investigate the minimum conductivity of graphene within a quasiclassical approach taking into account electron-hole coherence effects which stem from the chiral nature of low energy excitations. Relying on an analytical solution of the kinetic equation in the electron-hole coherent and incoherent cases, we study both the electrical and the thermal conductivity whose relation satisfies the Wiedemann-Franz law. We find that most of the previous findings based on the Boltzmann equation are restricted to only high mobility samples where electron-hole coherence effects are not sufficient.

Monte Carlo study of ferromagnetism in (III,Mn)V semiconductors

We report on Monte Carlo studies of the kinetic exchange model for (III,Mn)V ferromagnetic semiconductors in which S=5/2 local moments, representing Mn2+ ions, are exchange coupled to band electrons. We treat the Mn2+ spin orientations as classical degrees of freedom and use the hybrid Monte Carlo algorithm to explore thermodynamically important Mn spin configurations. The critical temperature Tc of the model is unambiguously signalled in our finite-size simulations by pronounced peaks in fluctuations of both Mn and band carrier total spins. The Tc’s we obtain are, over much of the model’s parameter space, substantially smaller than those estimated using mean-field theory. When mean-field theory fails, short-range magnetic order and finite local carrier spin polarization are present for temperatures substantially larger than Tc. For the simplest version of the model, which has a single parabolic band with effective mass m*, the dependence of Tc on m* is sublinear at large masses, in disagreement with the mean-field theory result Tc∝m*.

Electron spin dynamics in quantum dots and related nanostructures due to hyperfine interaction with nuclei

We review and summarize recent theoretical and experimental work on electron spin dynamics in quantum dots and related nanostructures due to hyperfine interaction with surrounding nuclear spins. This topic is of particular interest with respect to several proposals for quantum information processing in solid state systems. Specifically, we investigate the hyperfine interaction of an electron spin confined in a quantum dot in an s-type conduction band with the nuclear spins in the dot. This interaction is proportional to the square modulus of the electron wavefunction at the location of each nucleus leading to an inhomogeneous coupling, i.e. nuclei in different locations are coupled with different strengths. In the case of an initially fully polarized nuclear spin system an exact analytical solution for the spin dynamics can be found. For not completely polarized nuclei, approximation-free results can only be obtained numerically in sufficiently small systems. We compare these exact results with findings from several approximation strategies.