arXiv subjects

Ulrich Eckern

Publications and source records attributed to Ulrich Eckern.

34 records · Page 2Linked to original sources

arXiv2015

Non-equilibrium transport through a model quantum dot: Hartree-Fock approximation and beyond

The finite-temperature transport properties of the spinless interacting fermion model coupled to non-interacting leads are investigated. Employing the unrestricted time-dependent Hartree-Fock (HF) approximation, the transmission probability and the non-linear $I$-$V$ characteristics are calculated, and compared with available analytical results and with numerical data obtained from a Hubbard-Stratonovich decoupling of the interaction. In the weak interaction regime, the HF approximation reproduces the gross features of the exact $I$-$V$ characteristics but fails to account for subtle properties like the particular power law for the reflected current in the interacting resonant level model.

cond-mat.mes-hall↗

arXiv2014

Room temperature spin thermoelectrics in metallic films

Considering metallic films at room temperature, we present the first theoretical study of the spin Nernst and thermal Edelstein effects which takes into account dynamical spin-orbit coupling, i.e., direct spin-orbit coupling with the vibrating lattice (phonons) and impurities. This gives rise to two novel processes, namely a dynamical Elliott-Yafet spin relaxation and a dynamical side-jump mechanism. Both are the high-temperature counterparts of the well-known $T = 0$ Elliott-Yafet and side-jump, central to the current understanding of the spin Hall, spin Nernst and Edelstein effects at low $T$. We consider the experimentally relevant regime $T > T_D$, with $T_D$ the Debye temperature, as the latter is lower than room temperature in transition metals such as Pt, Au and Ta typically employed in spin injection/extraction experiments. We show that the interplay between intrinsic (Bychkov-Rashba type) and extrinsic (dynamical) spin-orbit coupling yields a nonlinear $T$- dependence of the spin Nernst and spin Hall conductivities.

cond-mat.mes-hall↗

arXiv2014

Driving spin and charge in quantum wells by surface acoustic waves

Recent experiments have shown the potential of surface acoustic waves as a mean for transporting charge and spin in quantum wells. In particular, they have proven highly effective for the coherent transport of spin-polarized wave packets, suggesting their potential in spintronics applications. Motivated by these experimental observations, we have theoretically studied the spin and charge dynamics in a quantum well under surface acoustic waves. We show that the dynamics acquires a simple and transparent form in a reference frame co-moving with the surface acoustic wave. Our results, e.g., the calculated spin relaxation and precession lengths, are in excellent agreement with recent experimental observations.

cond-mat.mes-hall↗

arXiv2014

Effect of Gd doping and O deficiency on the Curie temperature of EuO

The effect of Gd doping and O deficiency on the electronic structure, exchange interaction, and Curie temperature of EuO in the cubic and tetragonal phases is studied by means of density functional theory. For both defects, the Curie temperature is found to exhibit a distinct maximum as a function of the defect concentration. The existence of optimal defect concentrations is explained by the interplay of the on-site, RKKY, and superexchange contributions to the magnetism.

cond-mat.str-el↗

arXiv2014

The fate of half-metallicity near interfaces: the case of NiMnSb/MgO and NiMnSi/MgO

The electronic and magnetic properties of the interfaces between the half-metallic Heusler alloys NiMnSb, NiMnSi and MgO have been investigated using first-principles density-functional calculations with projector augmented wave potentials generated in the generalized gradient approximation. In the case of the NiMnSb/MgO (100) interface the half-metallicity is lost, whereas the MnSb/MgO contact in the NiMnSb/MgO (100) interface maintains a substantial degree of spin polarization at the Fermi level ($\sim 60$%). Remarkably, the NiMnSi/MgO (111) interface shows 100% spin polarization at the Fermi level, despite considerable distortions at the interface, as well as rather short Si/O bonds after full structural optimization. This behavior markedly distinguishes NiMnSi/MgO (111) from the corresponding NiMnSb/CdS and NiMnSb/InP interfaces.

cond-mat.mes-hall↗

arXiv2012

Modified Li chains as atomic switches

We present electronic structure and transport calculations for hydrogen and lithium chains, using density functional theory and scattering theory on the Green's function level, to systematically study impurity effects on the transmission coefficient. To this end we address various impurity configurations. Tight-binding results allow us to interpret our the findings. We analyze under which circumstances impurities lead to level splitting and/or can be used to switch between metallic and insulating states. We also address the effects of strongly electronegative impurities.

cond-mat.mes-hall↗

arXiv2012

Electronic Transport Through EuO Spin Filter Tunnel Junctions

Epitaxial spin filter tunnel junctions based on the ferromagnetic semiconductor europium monoxide, EuO, are investigated by means of density functional theory. In particular, we focus on the spin transport properties of Cu(100)/EuO(100)/Cu(100) junctions. The dependence of the transmission coefficient and the current-voltage curves on the interface spacing and on the EuO thickness is explained in terms of the EuO density of states and the complex band structure. Furthermore we also discuss the relation between the spin transport properties and the Cu-EuO interface geometry. The level alignment of the junction is sensitively affected by the interface spacing, since this determines the charge transfer between EuO and the Cu electrodes. Our calculations indicate that EuO epitaxially grown on Cu can act as a perfect spin filter, with a spin polarization of the current close to 100%, and with both the Eu-5d conduction band and the Eu-4f valence band states contributing to the coherent transport. For epitaxial EuO on Cu a symmetry filtering is observed, with the Δ_1 states dominating the transmission. This leads to a transport gap larger than the fundamental EuO band gap. Importantly the high spin polarization of the current is preserved up to large bias voltages.

cond-mat.mes-hall↗

arXiv2008

Current Density Functional Theory for one-dimensional fermions

The frequency-dependent response of a one-dimensional fermion system is investigated using Current Density Functional Theory (CDFT) within the local approximation (LDA). DFT-LDA, and in particular CDFT-LDA, reproduces very well the dispersion of the collective excitations. Unsurprisingly, however, the approximation fails for details of the dynamic response for large wavevectors. In particular, we introduce CDFT for the one-dimensional spinless fermion model with nearest-neighbor interaction, and use CDFT-LDA plus exact (Bethe ansatz) results for the groundstate energy as function of particle density and boundary phase to determine the linear response. The successes and failures of this approach are discussed in detail.

cond-mat.str-el↗

arXiv2008

Successes and failures of Bethe Ansatz Density Functional Theory

The response of a one-dimensional fermion system is investigated using Density Functional Theory (DFT) within the Local Density Approximation (LDA), and compared with exact results. It is shown that DFT-LDA reproduces surprisingly well some of the characteristic features of the Luttinger liquid, namely the vanishing spectral weight of low energy particle-hole excitations, as well as the dispersion of the collective charge excitations. On the other hand, the approximation fails, even qualitatively, for quantities for which backscattering is important, i.e., those quantities which are crucial for an accurate description of transport. In particular, the Drude weight in the presence of a single impurity is discussed.

cond-mat.str-el↗

arXiv2001

Two component interacting bosons: 1-d exact results

Motivated by the experiment of two-component Bose-Einstein condensates produced in magnetically trapped $^{87}Rb$, we study one dimensional Boson systems with repulsive $δ$-function interaction in the presence of SU(2) intrinsic degree of freedom by means of coordinate Bethe ansatz. The ground state and low-lying excitations are solved by both numerical and analytical methods. It is shown that the ground state is an isospin-ferromagnetic state and the excitations are composed of three elementary particles: holons, antiholons and isospinons. The isospinon is a triplet coupled to the ``ferromagnetic'' background antiparallelly.

cond-mat.str-el↗

arXiv2000

Fermionic long range correlations realized by particles obeying deformed statistics

Deformed exchange statistics is realized in terms of electronic operators. This is employed to rewrite Hubbard type lattice models for particles obeying deformed statistics (we refer to them as deformed models) as lattice models for electrons. The resulting models show up gauge--like modulations in the hopping processes, which induce long-range correlations in the lattice. The conditions for the Bethe ansatz solvability of the latter are interpreted as restrictions imposed on the statistics to be compatible with the Bethe ansatz solvability of the deformed models. It is found that solvable deformed models are not unitarily equivalent to fermionic models if the exchange of particles with the same spin-orientations is deformed. Statistics deformations, where the exchange relation of two particles is influenced by the presence of other particles, cannot be realized by fermionic operators.

cond-mat.str-el↗

arXiv2000

One dimensional model for doubly degenerate electrons

A Hubbard-like model with SU(4) symmetry for electrons with two-fold orbital degeneracy is studied extensively. Exact solution in one dimension is derived by means of Bethe ansatz, where the sites are supposed to be occupied by at most two electrons. The features of ground state and excited states for repulsive coupling are shown. For finite N number of electrons, the configurations of quantum numbers are given explicitly and the spectra of excitations are obtained by solving the Bethe-ansatz equation numerically. For infinite N, the ground state and various kinds of low-lying excitations are obtained on the basis of thermodynamics limit.

cond-mat.str-el↗

arXiv1999

Enhancement of pair correlation in a one-dimensional hybridization model

We propose an integrable model of one-dimensional (1D) interacting electrons coupled with the local orbitals arrayed periodically in the chain. Since the local orbitals are introduced in a way that double occupation is forbidden, the model keeps the main feature of the periodic Anderson model with an interacting host. For the attractive interaction, it is found that the local orbitals enhance the effective mass of the Cooper-pair-like singlets and also the pair correlation in the ground state. However, the persistent current is depressed in this case. For the repulsive interaction case, the Hamiltonian is non-Hermitian but allows Cooper pair solutions with small momenta, which are induced by the hybridization between the extended state and the local orbitals.

cond-mat.str-el↗

arXiv1998

Metal-insulator transition in VO$_{2}$: a Peierls-Mott-Hubbard mechanism

The electronic structure of VO$_2$ is studied in the frameworks of local density approximation (LDA) and LDA+$U$ to give a quantitative description of the metal-insulator (MI) transition in this system. It is found that, both structural distortion and the local Coulomb interaction, play important roles in the transition. An optical gap, comparable to the experimental value has been obtained in the monoclinic structure by using the LDA+$U$ method. Based on our results, we believe that both, the Peierls and the Mott-Hubbard mechanism, are essential for a description of the MI transition in this system.

cond-mat.mtrl-sci↗

arXiv1998

One-Dimensional XXZ Model for Particles Obeying Fractional Statistics

We define one-dimensional particles as non-abelian representations of the symmetric group $S_N$. The exact solution of an $XXZ$ type Hamiltonian built up with such particles is achieved using the coordinate Bethe Ansatz. The Bethe equations show that fractional statistics, effectively, accounts for coupling an external gauge field to an integer statistics' system.

cond-mat.str-el↗

arXiv1998

Interaction versus dimerization in one-dimensional Fermi systems

In order to study the effect of interaction and lattice distortion on quantum coherence in one-dimensional Fermi systems, we calculate the ground state energy and the phase sensitivity of a ring of interacting spinless fermions on a dimerized lattice. Our numerical DMRG (Density Matrix Renormalization Group) studies, in which we keep up to 1000 states for systems of about 100 sites, are supplemented by analytical considerations using bosonization techniques. We find a delocalized phase for an attractive interaction, which differs from that obtained for random lattice distortions. The extension of this delocalized phase depends strongly on the dimerization induced modification of the interaction. Taking into account the harmonic lattice energy, we find a dimerized ground state for a repulsive interaction only. The dimerization is suppressed at half filling, when the correlation gap becomes large.

cond-mat.str-el↗