arXiv subjects

T. Kernreiter

Publications and source records attributed to T. Kernreiter.

At least 19 recordsLinked to original sources

arXiv2016

Quantum capacitance of an HgTe quantum well as an indicator of the topological phase

Varying the quantum-well width in an HgTe/CdTe heterostructure allows to realize normal and inverted semiconducting band structures, making it a prototypical system to study two-dimensional (2D) topological-insulator behavior. We have calculated the zero-temperature thermodynamic density of states $D_\mathrm{T}$ for the electron-doped situation in both regimes, treating interactions within the Hartree-Fock approximation. A distinctively different behavior for the density dependence of $D_\mathrm{T}$ is revealed in the inverted and normal cases, making it possible to detect the system's topological order through measurement of macroscopic observables such as the quantum capacitance or electronic compressibility. Our results establish the 2D electron system in HgTe quantum wells as unique in terms of its collective electronic properties.

cond-mat.mes-hall↗

arXiv2016

Spin susceptibility of two-dimensional transition metal dichalcogenides

We have obtained analytical expressions for the q-dependent static spin susceptibility of monolayer transition metal dichalcogenides, considering both the electron-doped and hole-doped cases. Our results are applied to calculate spin-related physical observables of monolayer MoS2, focusing especially on in-plane/out-of-plane anisotropies. We find that the hole-mediated RKKY exchange interaction for in-plane impurity-spin components decays with the power law $R^{-5/2}$ as a function of distance $R$, which deviates from the $R^{-2}$ power law normally exhibited by a two-dimensional Fermi liquid. In contrast, the out-of-plane spin response shows the familiar $R^{-2}$ long-range behavior. We also use the spin susceptibility to define a collective g-factor for hole-doped MoS2 systems and discuss its density-dependent anisotropy.

cond-mat.mes-hall↗

arXiv2016

Anomalous Spin Response and Virtual-Carrier-Mediated Magnetism in a Topological Insulator

We present a comprehensive theoretical study of the static spin response in HgTe quantum wells, revealing distinctive behavior for the topologically nontrivial inverted structure. Most strikingly, the q=0 (long-wave-length) spin susceptibility of the undoped topological-insulator system is constant and equal to the value found for the gapless Dirac-like structure, whereas the same quantity shows the typical decrease with increasing band gap in the normal-insulator regime. We discuss ramifications for the ordering of localized magnetic moments present in the quantum well, both in the insulating and electron-doped situations. The spin response of edge states is also considered, and we extract effective Lande g-factors for the bulk and edge electrons. The variety of counter-intuitive spin-response properties revealed in our study arises from the system's versatility in accessing situations where the charge-carrier dynamics can be governed by ordinary Schrodinger-type physics, mimics the behavior of chiral Dirac fermions, or reflects the material's symmetry-protected topological order.

cond-mat.mes-hall↗

arXiv2015

Coulomb-exchange effects in nanowires with spin splitting due to a radial electric field

We present a theoretical study of Coulomb exchange interaction for electrons confined in a cylindrical quantum wire and subject to a Rashba-type spin-orbit coupling with radial electric field. The effect of spin splitting on the single-particle band dispersions, the quasiparticle effective mass, and the system's total exchange energy per particle are discussed. Exchange interaction generally suppresses the quasiparticle effective mass in the lowest nanowire subband, and a finite spin splitting is found to significantly increase the magnitude of the quasiparticle-mass suppression (by upto 15\% in the experimentally relevant parameter regime). In contrast, spin-orbit coupling causes a modest (1\%-level) reduction of the magnitude of the exchange energy per particle. Our results shed new light on the interplay of spin-orbit coupling and Coulomb interaction in quantum-confined systems, including those that are expected to host exotic quasiparticle excitations.

cond-mat.mes-hall↗

arXiv2014

Exporting superconductivity across the gap: Proximity effect for semiconductor valence-band states due to contact with a simple-metal superconductor

The proximity effect refers to the phenomenon whereby superconducting properties are induced in a normal conductor that is in contact with an intrinsically superconducting material. In particular, the combination of nano-structured semiconductors with bulk superconductors is of interest because these systems can host unconventional electronic excitations such as Majorana fermions when the semiconductor's charge carriers are subject to a large spin-orbit coupling. The latter requirement generally favors the use of hole-doped semiconductors. On the other hand, basic symmetry considerations imply that states from typical simple-metal superconductors will predominantly couple to a semiconductor's conduction-band states and, therefore, in the first instance generate a proximity effect for band electrons rather than holes. In this article, we show how the superconducting correlations in the conduction band are transferred also to hole states in the valence band by virtue of inter-band coupling. A general theory of the superconducting proximity effect for bulk and low-dimensional hole systems is presented. The interplay of inter-band coupling and quantum confinement is found to result in unusual wave-vector dependencies of the induced superconducting gap parameters. One particularly appealing consequence is the density tunability of the proximity effect in hole quantum wells and nanowires, which creates new possibilities for manipulating the transition to nontrivial topological phases in these systems.

cond-mat.mes-hall↗

arXiv2013

Suppression of Coulomb exchange energy in quasi-two-dimensional hole systems

We have calculated the exchange-energy contribution to the total energy of quasi-two-dimensional hole systems realized by a hard-wall quantum-well confinement of valence-band states in typical semiconductors. The magnitude of the exchange energy turns out to be suppressed from the value expected for analogous conduction-band systems whenever the mixing between heavy-hole and light-hole components is strong. Our results are obtained using a very general formalism for calculating the exchange energy of many-particle systems where single-particle states are spinors. We have applied this formalism to obtain analytical results for spin-3/2 hole systems in limiting cases.

cond-mat.mes-hall↗

arXiv2013

RKKY interaction induced by two-dimensional hole gases

We analytically compute the RKKY range function as induced by two-dimensional (2D) hole gases. The bulk valence-band includes heavy-hole (HH) and light-hole (LH) states and their dynamics is described by the Luttinger Hamiltonian which we adopt as our framework. We show that even for situations where only the lowest HH-like subband is occupied the resulting form of the RKKY function can be very different as compared to the one of a 2D electron gas. The associated spin susceptibility tensor has entries along the quantum-well directions and perpendicular to it. Our formluae for the spin susceptibility tensor reveal the crucial influence of HH-LH mixing which gives rise to large anisotropies both among the in-plane components as well as among the in-plane components and the component perpendicular to the quantum-well.

cond-mat.mes-hall↗

arXiv2013

Carrier-density-controlled anisotropic spin susceptibility of two-dimensional hole systems

We have studied quantum-well-confined holes based on the Luttinger-model description for the valence band of typical semiconductor materials. Even when only the lowest quasi-two-dimensional (quasi-2D) subband is populated, the static spin susceptibility turns out to be very different from the universal isotropic Lindhard-function lineshape obtained for 2D conduction-electron systems. The strongly anisotropic and peculiarly density-dependent spin-related response of 2D holes at long wavelengths should make it possible to switch between easy-axis and easy-plane magnetization in dilute magnetic quantum wells. An effective g factor for 2D hole systems is proposed.

cond-mat.mes-hall↗

arXiv2011

Charge transport by modulating spin-orbit gauge fields for quasi-onedimensional holes

We present a theoretical study of ac charge transport arising from adiabatic temporal variation of zero-field spin splitting in a quasi-onedimensional hole system (realized, e.g., in a quantum wire or point contact). As in conduction-electron systems, part of the current results from spin-dependent electromotive forces. We find that the magnitude of this current contribution is two orders of magnitude larger for holes and exhibits parametric dependences that make it more easily accessible experimentally. Our results suggest hole structures to be good candidates for realizing devices where spin currents are pumped by time-varying electric fields.

cond-mat.mes-hall↗

arXiv2010

Impact of squark generation mixing on the search for squarks and gluinos at LHC

We study the effect of squark-generation mixing on production and decays of squarks and gluinos at LHC in the Minimal Supersymmetric Standard Model (MSSM). We show that the mixing effects can be very large in a significant range of the squark-generation mixing parameters despite the very strong constraints on quark-flavour violation (QFV) from experimental data on B mesons. We find that under favourable conditions the QFV decay branching ratio B(gluino -> c bar{t} (t bar{c}) neutralino_1) can be as large as about 50%, which may lead to significant QFV signals at LHC. We also find that the squark generation mixing can result in a novel multiple-edge (3- or 4-edge) structure in the charm-top quark invariant mass distribution. Further we show that the two lightest up-type squarks ~u_{1,2} can have very large branching ratios for the decays ~u_i -> c neutralino_1 and ~u_i -> t neutralino_1 simultaneously due to the mixing effect, resulting in QFV signals 'pp -> c bar{t} (t bar{c}) + missing-E_T + X' at a significant rate at LHC. These remarkable signatures could provide a powerful test of supersymmetric QFV at LHC and could have an important impact on the search for squarks and gluinos and the determination of the MSSM parameters at LHC.

hep-ph↗

arXiv2010

Static polarizability of two-dimensional hole gases

We have calculated the density-density (Lindhard) response function of a homogeneous two-dimensional (2D) hole gas in the static (omega=0) limit. The bulk valence-band structure comprising heavy-hole (HH) and light-hole (LH) states is modeled using Luttinger's kdotp approach within the axial approximation. We elucidate how, in contrast to the case of conduction electrons, the Lindhard function of 2D holes exhibits unique features associated with (i) the confinement-induced HH-LH energy splitting and (ii) the HH-LH mixing arising from the charge carriers' in-plane motion. Implications for the dielectric response and related physical observables are discussed.

cond-mat.mes-hall↗

arXiv2010

Impact of squark generation mixing on the search for gluinos at LHC

We study gluino decays in the Minimal Supersymmetric Standard Model (MSSM) with squark generation mixing. We show that the effect of this mixing on the gluino decay branching ratios can be very large in a significant part of the MSSM parameter space despite the very strong experimental constraints on quark flavour violation (QFV) from B meson observables. Especially we find that under favourable conditions the branching ratio of the the QFV gluino decay gluino -> c bar{t} (bar{c} t) + neutralino_1 can be as large as about 50%. We also find that the squark generation mixing can result in a multiple-edge (3- or 4-edge) structure in the charm-top quark invariant mass distribution. The appearance of this remarkable structure provides an additional powerful test of supersymmetric QFV at LHC. These could have an important impact on the search for gluinos and the determination of the MSSM parameters at LHC.

hep-ph↗

arXiv2010

Minimal Supersymmetric Inverse Seesaw: Neutrino masses, lepton flavour violation and LHC phenomenology

We study neutrino masses in the framework of the supersymmetric inverse seesaw model. Different from the non-supersymmetric version a minimal realization with just one pair of singlets is sufficient to explain all neutrino data. We compute the neutrino mass matrix up to 1-loop order and show how neutrino data can be described in terms of the model parameters. We then calculate rates for lepton flavour violating (LFV) processes, such as $μ\to e γ$, and chargino decays to singlet scalar neutrinos. The latter decays are potentially observable at the LHC and show a characteristic decay pattern dictated by the same parameters which generate the observed large neutrino angles.

hep-ph↗

arXiv2009

CP-sensitive spin-spin correlations in neutralino production at the ILC

We study the CP-violating terms of the spin-spin correlations in neutralino production and their subsequent two-body decays into sleptons plus leptons at the ILC. We analyze CP-sensitive observables with the help of T-odd products of the spin-spin terms. These terms depend on the polarizations of both neutralinos, with one polarization perpendicular to the production plane. We present a detailed numerical study of the CP-sensitive observables, cross sections, and neutralino branching ratios in the Minimal Supersymmetric Standard Model with complex parameters.

hep-ph↗

arXiv2008

CP observables with spin-spin correlations in chargino production

We study the CP-violating terms of the spin-spin correlations in chargino production e+e- -> chi^+/-_1 chi^-/+_2, and their subsequent two-body decays into sneutrinos plus leptons. We propose novel CP-sensitive observables with the help of T-odd products of the spin-spin terms. These terms depend on the polarizations of both charginos, with one polarization perpendicular to the production plane. We identify two classes of CP-sensitive observables; one requires the reconstruction of the production plane, the other not. Our framework is the Minimal Supersymmetric Standard Model with complex parameters.

hep-ph↗

arXiv2008

Impact of slepton generation mixing on the search for sneutrinos

We perform a systematic study of sneutrino production and decays in the Minimal Supersymmetric Standard Model (MSSM) with slepton generation mixing. We study both fermionic decays like sneutrino \to l^- chargino^+, nu neutralino and bosonic decays such as sneutrino \to slepton^- H^+, slepton^- W^+. We show that the effect of slepton generation mixing on the sneutrino production cross sections and its decay branching ratios can be quite large in a significant part of the MSSM parameter space despite the very strong experimental limits on lepton flavour violating processes. This could have an important impact on the search for sneutrinos and the determination of the MSSM parameters at future colliders, such as LHC, ILC, CLIC and muon collider.

hep-ph↗

arXiv2008

Collider aspects of flavour physics at high Q

This review presents flavour related issues in the production and decays of heavy states at LHC, both from the experimental side and from the theoretical side. We review top quark physics and discuss flavour aspects of several extensions of the Standard Model, such as supersymmetry, little Higgs model or models with extra dimensions. This includes discovery aspects as well as measurement of several properties of these heavy states. We also present public available computational tools related to this topic.

hep-ph↗

arXiv2007

CP asymmetries with Longitudinal and Transverse Beam Polarizations in Neutralino Production and Decay into the Z^0 Boson at the ILC

We study neutralino production at the linear collider with the subsequent two-body decays ~chi^0_i -> ~chi^0_n Z^0 and Z^0 -> l^+ l^-, with l=e, mu, tau, or Z^0 -> q q- with q=c,b. We show that transverse electron and positron beam polarizations allow the definition of unique CP observables. These are azimuthal asymmetries in the distributions of the final leptons or quarks. We calculate these CP asymmetries and the cross sections in the Minimal Supersymmetric Standard Model with complex higgsino and gaugino parameters mu and M_1. For final quark pairs, we find CP asymmetries as large as 30%. We discuss the significances for observing the CP asymmetries at the International Linear Collider (ILC). Finally we compare the CP asymmetries with those asymmetries which require unpolarized and/or longitudinally polarized beams only.

hep-ph↗