arXiv subjects

Michael Wimmer

Publications and source records attributed to Michael Wimmer.

85 records · Page 5Linked to original sources

arXiv2016

Giant spin-orbit splitting in inverted InAs/GaSb double quantum wells

Transport measurements in inverted InAs/GaSb quantum wells reveal a giant spin-orbit splitting of the energy bands close to the hybridization gap. The splitting results from the interplay of electron-hole mixing and spin-orbit coupling, and can exceed the hybridization gap. We experimentally investigate the band splitting as a function of top gate voltage for both electron-like and hole-like states. Unlike conventional, noninverted two-dimensional electron gases, the Fermi energy in InAs/GaSb can cross a single spin-resolved band, resulting in full spin-orbit polarization. In the fully polarized regime we observe exotic transport phenomena such as quantum Hall plateaus evolving in $e^2/h$ steps and a non-trivial Berry phase.

cond-mat.mes-hall↗

arXiv2016

Quantized conductance and large g-factor anisotropy in InSb quantum point contacts

Due to a strong spin-orbit interaction and a large Landé g-factor, InSb plays an important role in research on Majorana fermions. To further explore novel properties of Majorana fermions, hybrid devices based on quantum wells are conceived as an alternative approach to nanowires. In this work, we report a pronounced conductance quantization of quantum point contact devices in InSb/InAlSb quantum wells. Using a rotating magnetic field, we observe a large in-plane (|g1|=26) and out-of-plane (|g1|=52) g-factor anisotropy. Additionally, we investigate crossings of subbands with opposite spins and extract the electron effective mass from magnetic depopulation of one-dimensional subbands.

cond-mat.mes-hall↗

arXiv2015

Electric and Magnetic Tuning Between the Trivial and Topological Phases in InAs/GaSb Double Quantum Wells

Among the theoretically predicted two-dimensional topological insulators, InAs/GaSb double quantum wells (DQWs) have a unique double-layered structure with electron and hole gases separated in two layers, which enables tuning of the band alignment via electric and magnetic fields. However, the rich trivial-topological phase diagram has yet to be experimentally explored. We present an in situ and continuous tuning between the trivial and topological insulating phases in InAs/GaSb DQWs through electrical dual-gating. Furthermore, we show that an in-plane magnetic field shifts the electron and hole bands relatively to each other in momentum space, functioning as a powerful tool to discriminate between the topologically distinct states.

cond-mat.mes-hall↗

arXiv2014

Kwant: a software package for quantum transport

Kwant is a Python package for numerical quantum transport calculations. It aims to be an user-friendly, universal, and high-performance toolbox for the simulation of physical systems of any dimensionality and geometry that can be described by a tight-binding model. Kwant has been designed such that the natural concepts of the theory of quantum transport (lattices, symmetries, electrodes, orbital/spin/electron-hole degrees of freedom) are exposed in a simple and transparent way: Defining a new simulation setup is very close to describing the corresponding mathematical model. Kwant offers direct support for calculations of transport properties (conductance, noise, scattering matrix), dispersion relations, modes, wave functions, various Green's functions, and out-of-equilibrium local quantities. Other computations involving tight-binding Hamiltonians can be implemented easily thanks to its extensible and modular nature. Kwant is free software available at http://kwant-project.org/.

cond-mat.mes-hall↗

arXiv2014

Electric control of tunneling energy in graphene double dots

We theoretically investigate the spectrum of a single electron double quantum dot, defined by top gates in a graphene with a substrate induced gap. We examine the effects of electric and magnetic fields on the spectrum of localized states, focusing on the tunability of the inter-dot coupling. We find that the substrate induced gap allows for electrostatic control, with some limitations that for a fixed inter-dot distance, the inter-dot coupling can not be made arbitrarily small due to the Klein tunneling. On the other hand, the proximity of the valence band in graphene allows for new regimes, such as an $npn$ double dot, which have no counterparts in GaAs.

cond-mat.mes-hall↗

arXiv2011

Universal spatial correlations in random spinor fields

We identify universal spatial fluctuations in systems with non trivial spin dynamics. To this end we calculate by exact numerical diagonalization a variety of experimentally relevant correlations between spinor amplitudes, spin polarizations and spin currents both in the bulk and near the boundary of a confined two-dimensional clean electron gas in the presence of spin-orbit interaction and a single magnetic impurity. We support or claim of universality with the excellent agreement between the numerical results and system-independent spatial correlations of a random field defined on both the spatial and spin degrees of freedom. A rigorous identity relating our universal predictions with response functions provides a direct physical interpretation of our results in the framework of linear response theory.

cond-mat.mes-hall↗

arXiv2010

Weak localization in mesoscopic hole transport: Berry phases and classical correlations

We consider phase-coherent transport through ballistic and diffusive two-dimensional hole systems based on the Kohn-Luttinger Hamiltonian. We show that intrinsic heavy-hole light-hole coupling gives rise to clear-cut signatures of an associated Berry phase in the weak localization which renders the magneto-conductance profile distinctly different from electron transport. Non-universal classical correlations determine the strength of these Berry phase effects and the effective symmetry class, leading even to antilocalization-type features for circular quantum dots and Aharonov-Bohm rings in the absence of additional spin-orbit interaction. Our semiclassical predictions are quantitatively confirmed by numerical transport calculations.

cond-mat.mes-hall↗

arXiv2009

Conformal mapping and shot noise in graphene

Ballistic transport through a collection of quantum billiards in undoped graphene is studied analytically within the conformal mapping technique. The billiards show pseudodiffusive behavior, with the conductance equal to that of a classical conductor characterized by the conductivity $σ_0=4e^2/πh$, and the Fano factor $F=1/3$. By shrinking at least one of the billiard openings, we observe a tunneling behavior, where the conductance shows a power-law decay with the system size, and the shot noise is Poissonian (F=1). In the crossover region between tunneling and pseudodiffusive regimes, the conductance $G\approx (1-F)\times se^2/h$. The degeneracy $s=8$ for the Corbino disk, which preserves the full symmetry of the Dirac equation, $s=4$ for billiards bounded with smooth edges which break the symplectic symmetry, and $s=2$ when abrupt edges lead to strong intervalley scattering. An alternative, analytical or numerical technique, is utilized for each of the billiards to confirm the applicability of the conformal mapping for various boundary conditions.

cond-mat.mes-hall↗

arXiv2008

Optimal block-tridiagonalization of matrices for coherent charge transport

Numerical quantum transport calculations are commonly based on a tight-binding formulation. A wide class of quantum transport algorithms requires the tight-binding Hamiltonian to be in the form of a block-tridiagonal matrix. Here, we develop a matrix reordering algorithm based on graph partitioning techniques that yields the optimal block-tridiagonal form for quantum transport. The reordered Hamiltonian can lead to significant performance gains in transport calculations, and allows to apply conventional two-terminal algorithms to arbitrary complex geometries, including multi-terminal structures. The block-tridiagonalization algorithm can thus be the foundation for a generic quantum transport code, applicable to arbitrary tight-binding systems. We demonstrate the power of this approach by applying the block-tridiagonalization algorithm together with the recursive Green's function algorithm to various examples of mesoscopic transport in two-dimensional electron gases in semiconductors and graphene.

cond-mat.mes-hall↗

arXiv2008

Spin-polarized Quantum Transport in Mesoscopic Conductors: Computational Concepts and Physical Phenomena

Mesoscopic conductors are electronic systems of sizes in between nano- and micrometers, and often of reduced dimensionality. In the phase-coherent regime at low temperatures, the conductance of these devices is governed by quantum interference effects, such as the Aharonov-Bohm effect and conductance fluctuations as prominent examples. While first measurements of quantum charge transport date back to the 1980s, spin phenomena in mesoscopic transport have moved only recently into the focus of attention, as one branch of the field of spintronics. The interplay between quantum coherence with confinement-, disorder- or interaction-effects gives rise to a variety of unexpected spin phenomena in mesoscopic conductors and allows moreover to control and engineer the spin of the charge carriers: spin interference is often the basis for spin-valves, -filters, -switches or -pumps. Their underlying mechanisms may gain relevance on the way to possible future semiconductor-based spin devices. A quantitative theoretical understanding of spin-dependent mesoscopic transport calls for developing efficient and flexible numerical algorithms, including matrix-reordering techniques within Green function approaches, which we will explain, review and employ.

cond-mat.mes-hall↗

arXiv2006

Zeeman ratchets for ballistic spin currents

We investigate the possiblity of creating directed spin-polarized currents in a two-dimensional electron gas (2DEG) subject to an asymmetric magnetic field and an external adiabatic driving. We thereby generalize concepts of quantum charge ratchets to the case with spin. Due to the Zeeman term in the Hamiltonian, spin-up and -down electrons experience different effective potentials, which can be tailored to achieve net spin currents without corresponding charge currents. We consider ballistic, coherent transport in waveguides defined in a 2DEG, where the magnetic field modulation is induced by ferromagnetic stripes on top of the 2DEG.

cond-mat.mes-hall↗

arXiv2005

Biexciton recombination rates in self-assembled quantum dots

The radiative recombination rates of interacting electron-hole pairs in a quantum dot are strongly affected by quantum correlations among electrons and holes in the dot. Recent measurements of the biexciton recombination rate in single self-assembled quantum dots have found values spanning from two times the single exciton recombination rate to values well below the exciton decay rate. In this paper, a Feynman path-integral formulation is developed to calculate recombination rates including thermal and many-body effects. Using real-space Monte Carlo integration, the path-integral expressions for realistic three-dimensional models of InGaAs/GaAs, CdSe/ZnSe, and InP/InGaP dots are evaluated, including anisotropic effective masses. Depending on size, radiative rates of typical dots lie in the regime between strong and intermediate confinement. The results compare favorably to recent experiments and calculations on related dot systems. Configuration interaction calculations using uncorrelated basis sets are found to be severely limited in calculating decay rates.

cond-mat.mes-hall↗

arXiv1996

Top Quark and Higgs Boson Masses: Interplay between Infrared and Ultraviolet Physics

We review recent efforts to explore the information on masses of heavy matter particles, notably of the top quark and the Higgs boson, as encoded at the quantum level in the renormalization group (RG) equations. The Standard Model (SM) and the Minimal Supersymmetric Standard Model (MSSM) are considered in parallel. First, the question is addressed to which extent the infrared (IR) physics of the ``top-down'' RG flow is independent of the ultraviolet (UV) physics. The central issues are i) IR attractive fixed point values for the top and the Higgs mass, the most outstanding one being m_t=O(190 GeV)sin(beta) in the MSSM, ii) IR attractive relations between parameters, the most prominent ones being an IR fixed top-Higgs mass relation in the SM, leading to m_H=O(156) GeV for the experimental top mass, and an IR fixed relation between the top mass and tan(beta) in the MSSM, and iii) an analytical assessment of their respective strengths of attraction. The triviality and vacuum stability bounds on the Higgs and top masses in the SM and the upper bound on the lightest Higgs boson mass in the MSSM are reviewed. The mathematical backbone, the rich structure of IR attractive fixed points, lines, surfaces,... in the multiparameter space, is made transparent. Interesting hierarchies emerge, most remarkably: IR attraction in the MSSM is systematically stronger than in the SM. Tau-bottom-(top) Yukawa coupling unification in supersymmetric grand unified theories and its power to focus the ``top-down'' RG flow into the IR top mass fixed point resp. onto the IR fixed line in the m_t-tan(beta) plane is reviewed. The program of reduction of parameters, a search for RG invariant relations between couplings, guided by the requirement of asymptotically free couplings in the UV limit,is summarized; its interrelations with the search for

hep-ph↗