arXiv subjects

Tamara S. Nunner

Publications and source records attributed to Tamara S. Nunner.

17 recordsLinked to original sources

arXiv2018

Boltzmann approach to the longitudinal spin Seebeck effect

We develop a Boltzmann transport theory of coupled magnon-phonon transport in ferromagnetic insulators. The explicit treatment of the magnon-phonon coupling within the Boltzmann approach allows us to calculate the low-temperature magnetic-field dependence of the spin-Seebeck voltage. Within the Boltzmann theory we find that this magnetic field dependence shows similar features as found by Flebus et al. [Phys. Rev. B 95, 144420 (2017)] for a strongly coupled magnon phonon system that forms magnon-polarons, and consistent with experimental findings in yttrium iron garnet by Kikkawa et al. [Phys. Rev. Lett. 117, 207203 (2016)]. In addition to the anomalous magnetic-field dependence of the spin Seebeck effect, we also predict a dependence on the system size.

cond-mat.mes-hall↗

arXiv2013

Evolution of the persistent spin helix in the presence of Hartree-Fock fields

We derive a spin diffusion equation for a spin-orbit coupled two-dimensional electron gas including the Hartree-Fock field resulting from 1st order electron-electron interactions. We find that the lifetime of the persistent spin helix, which emerges for equal linear Rashba- and Dresselhaus spin-orbit interactions, can be enhanced considerably for large initial spin polarizations due to the Hartree-Fock field. The reason is a reduction of the symmetry-breaking cubic Dresselhaus scattering rate by the Hartree-Fock field. Also higher harmonics are generated and the polarization of the persistent spin helix rotates out of the (Sy,Sz)-plane acquiring a finite Sx-component. This effect becomes more pronounced, when the cubic Dresselhaus spin-orbit interaction is large.

cond-mat.mes-hall↗

arXiv2012

Current-induced switching in transport through anisotropic magnetic molecules

Anisotropic single-molecule magnets may be thought of as molecular switches, with possible applications to molecular spintronics. In this paper, we consider current-induced switching in single-molecule junctions containing an anisotropic magnetic molecule. We assume that the carriers interact with the magnetic molecule through the exchange interaction and focus on the regime of high currents in which the molecular spin dynamics is slow compared to the time which the electrons spend on the molecule. In this limit, the molecular spin obeys a non-equilibrium Langevin equation which takes the form of a generalized Landau-Lifshitz-Gilbert equation and which we derive microscopically by means of a non-equilibrium Born-Oppenheimer approximation. We exploit this Langevin equation to identify the relevant switching mechanisms and to derive the current-induced switching rates. As a byproduct, we also derive S-matrix expressions for the various torques entering into the Landau-Lifshitz-Gilbert equation which generalize previous expressions in the literature to non-equilibrium situations.

cond-mat.mes-hall↗

arXiv2011

Relaxation mechanisms of the persistent spin helix

We study the lifetime of the persistent spin helix in semiconductor quantum wells with equal Rashba- and linear Dresselhaus spin-orbit interactions. In order to address the temperature dependence of the relevant spin relaxation mechanisms we derive and solve semiclassical spin diffusion equations taking into account spin-dependent impurity scattering, cubic Dresselhaus spin-orbit interactions and the effect of electron-electron interactions. For the experimentally relevant regime we find that the lifetime of the persistent spin helix is mainly determined by the interplay of cubic Dresselhaus spin-orbit interaction and electron-electron interactions. We propose that even longer lifetimes can be achieved by generating a spatially damped spin profile instead of the persistent spin helix state.

cond-mat.mes-hall↗

arXiv2011

Quasilinear spin voltage profiles in spin thermoelectrics

Recent experiments show that spin thermoelectrics is a promising approach to generate spin voltages. While spin chemical potentials are often limited to a surface layer of the order of the spin diffusion length, we show that thermoelectrically induced spin chemical potentials can extend much further in itinerant ferromagnets with paramagnetic impurities. In some cases, conservation laws, e.g., for a combination of spin and heat currents, give rise to a linear spin voltage profile. More generally, we find quasilinear profiles involving a spin thermoelectric length scale which far exceeds the spin diffusion length.

cond-mat.mes-hall↗

arXiv2007

Berry-phase effects in transport through single Jahn-Teller molecules

The vibrational modes of Jahn-Teller molecules are affected by a Berry phase that is associated with a conical intersection of the adiabatic potentials. We investigate theoretically how this Berry phase affects transport through a single $E \otimes e$ Jahn-Teller molecule when the tunneling electrons continually switch the molecule between a symmetric and a Jahn-Teller distorted charge state. We find that the Berry phase in conjunction with a spectral trapping mechanism leads to a current blockade even in regions outside the Coulomb blockade. The blockade is strongly asymmetric in the gate voltage and induces pronounced negative differential conductance.

cond-mat.mes-hall↗

arXiv2007

Anomalous Hall effect in a two dimensional electron gas with magnetic impurities

Magnetic impurities play an important role in many spintronics-related materials. Motivated by this fact, we study the anomalous Hall effect in the presence of magnetic impurities, focusing on two-dimensional electron systems with Rashba spin-orbit coupling. We find a highly nonlinear dependence on the impurity polarization, including possible sign changes. At small impurity magnetizations, this is a consequence of the remarkable result that the linear term is independent of the spin-orbit coupling strength. Near saturation of the impurity spins, the anomalous Hall conductivity can be resonantly enhanced, due to interference between potential and magnetic scattering.

cond-mat.mtrl-sci↗

arXiv2007

Anomalous Hall effect in a two-dimensional electron gas

The anomalous Hall effect in a magnetic two-dimensional electron gas with Rashba spin-orbit coupling is studied within the Kubo-Streda formalism in the presence of pointlike potential impurities. We find that all contributions to the anomalous Hall conductivity vanish to leading order in disorder strength when both chiral subbands are occupied. In the situation that only the majority subband is occupied, all terms are finite in the weak scattering limit and the total anomalous Hall conductivity is dominated by skew scattering. We compare our results to previous treatments and resolve some of the discrepancies present in the literature.

cond-mat.mtrl-sci↗

arXiv2007

Absence of skew scattering in two-dimensional systems: Testing the origins of the anomalous Hall effect

We study the anomalous Hall conductivity in spin-polarized, asymmetrically confined two-dimensional electron and hole systems, focusing on skew-scattering contributions to the transport. We find that the skew scattering, principally responsible for the extrinsic contribution to the anomalous Hall effect, vanishes for the two-dimensional electron system if both chiral Rashba subbands are partially occupied, and vanishes always for the two-dimensional hole gas studied here, regardless of the band filling. Our prediction can be tested with the proposed coplanar two-dimensional electron/hole gas device and can be used as a benchmark to understand the crossover from the intrisic to the extrinsic anomalous Hall effect.

cond-mat.mtrl-sci↗

arXiv2006

Signatures of modulated pair interaction in cuprate superconductors

Recent low-temperature scanning tunnelling spectroscopy experiments on the surface of BSCCO-2212 have revealed a strong positive correlation between the position of localized resonances at -960 meV identified with interstitial oxygen dopants and the size of the local spectral gap. We review efforts to understand these correlations within a model where the dopants modulate the pair interaction on an atomic scale. We provide further evidence for this model by comparing the correlations between the dopants and the local density of states with experimental results.

cond-mat.supr-con↗

arXiv2006

Thermodynamic transitions in inhomogeneous d-wave superconductors

We study the spectral and thermodynamic properties of inhomogeneous d-wave superconductors within a model where the inhomogeneity originates from atomic scale pair disorder. This assumption has been shown to be consistent with the small charge and large gap modulations observed by scanning tunnelling spectroscopy (STS) on BSCCO. Here we calculate the specific heat within the same model, and show that it gives a semi-quantitative description of the transition width in this material. This model therefore provides a consistent picture of both surface sensitive spectroscopy and bulk thermodynamic properties.

cond-mat.supr-con↗

arXiv2005

Fourier transform spectroscopy of d-wave quasiparticles in the presence of atomic scale pairing disorder

The local density of states power spectrum of optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (BSCCO) has been interpreted in terms of quasiparticle interference peaks corresponding to an "octet'' of scattering wave vectors connecting k-points where the density of states is maximal. Until now, theoretical treatments have not been able to reproduce the experimentally observed weights and widths of these "octet'' peaks; in particular, the predominance of the dispersing "q$_1$'' peak parallel to the Cu-O bond directions has remained a mystery. In addition, such theories predict "background'' features which are not observed experimentally. Here, we show that most of the discrepancies can be resolved when a realistic model for the out-of-plane disorder in BSCCO is used. Weak extended potential scatterers, which are assumed to represent cation disorder, suppress large-momentum features and broaden the low-energy "q$_7$''-peaks, whereas scattering at order parameter variations, possibly caused by a dopant-modulated pair interaction around interstitial oxygens, strongly enhances the dispersing "q$_1$''-peaks.

cond-mat.supr-con↗

arXiv2005

Andreev states near short-ranged pairing potential impurities

We study Andreev states near atomic scale modulations in the pairing potential in both $s$- and d-wave superconductors with short coherence lengths. For a moderate reduction of the local gap, the states exist only close to the gap edge. If one allows for local sign changes of the order parameter, however, resonances can occur at energies close to the Fermi level. The local density of states (LDOS) around such pairing potential defects strongly resembles the patterns observed by tunneling measurements around Zn impurities in Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (BSCCO). We discuss how this phase impurity model of the Zn LDOS pattern can be distinguished from other proposals experimentally.

cond-mat.supr-con↗

arXiv2005

Dopant-modulated pair interaction in cuprate superconductors

Comparison of recent experimental STM data with single-impurity and many-impurity Bogoliubov-de Gennes calculations strongly suggests that random out-of-plane dopant atoms in cuprates modulate the pair interaction locally. This type of disorder is crucial to understanding the nanoscale electronic structure inhomogeneity observed in BSCCO-2212, and can reproduce observed correlations between the positions of impurity atoms and various aspects of the local density of states such as the gap magnitude and the height of the coherence peaks. Our results imply that each dopant atom modulates the pair interaction on a length scale of order one lattice constant.

cond-mat.supr-con↗

arXiv2004

Microwave conductivity of d-wave superconductors with extended impurities

We investigate the influence of extended scatterers on the finite temperature and finite frequency microwave conductivity of d-wave superconductors. For this purpose we generalize a previous treatment by Durst and Lee, which is based on a nodal approximation of the quasiparticle excitations and scattering processes, and apply it to the analysis of experimental spectra of YBCO-123 and BSCCO-2212. For YBCO, we find that accounting for a slight spatial extension of the strong scattering in-plane defects improves the fit of the low temperature microwave conductivity to experiment. With respect to BSCCO we conclude that it is necessary to include a large concentration of weak-to-intermediate strength extended scatterers, which we attribute to the out-of plane disorder introduced by doping. These findings for BSCCO are consistent with similar analyses of the normal state ARPES spectra and of STM spectra in the superconducting state, where an enhanced forward scattering has been inferred as well.

cond-mat.supr-con↗

arXiv2002

Jordan-Wigner approach to dynamic correlations in spin-ladders

We present a method for studying the excitations of low-dimensional quantum spin systems based on the Jordan-Wigner transformation. Using an extended RPA-scheme we calculate the correlation function of neighboring spin flips which well approximates the optical conductivity of ${\rm Sr_2CuO_3}$. We extend this approach to the two-leg $S=1/2$--ladder by numbering the spin operators in a meander-like sequence. We obtain good agreement with the optical conductivity of the spin ladder compound (La,Ca)$_{14}$Cu$_{24}$O$_{41}$ for polarization along the rungs. For polarization along the legs higher order correlations are important to explain the weight of high-energy continuum excitations and we estimate the contribution of 4-- and 6--fermion processes.

cond-mat.str-el↗

arXiv2002

Cyclic Spin Exchange in Cuprate Spin Ladders

We investigate the influence of a cyclic spin exchange J_{cyc} on the one- and two-triplet excitations of an undoped two-leg S=1/2 ladder, using the density matrix renormalization group (DMRG). The dispersion of the S=0 two-triplet bound state is dramatically reduced by J_{cyc} due to a repulsion between triplets on neighboring rungs. In (La,Ca)_{14}Cu_{24}O_{41} a consistent description of both the spin gap and the S=0 bound state requires $J_{cyc}/J_\perp \approx 0.20-0.27$ and $J/J_{\perp} \approx 1.25-1.35$. With these coupling ratios the recently developed dynamical DMRG yields an excellent description of the entire S=0 excitation spectrum observed in the optical conductivity, including the continuum contribution.

cond-mat.str-el↗