arXiv subjects

Carsten Timm

Publications and source records attributed to Carsten Timm.

At least 55 records · Page 3Linked to original sources

arXiv2015

Microscopic coexistence of superconductivity and magnetism in Ca1-xNaxFe2As2

We present a detailed investigation of the magnetic and superconducting properties of Ca1-xNaxFe2As2 single crystals with x = 0.00, 0.35, 0.50, and 0.67 by means of the local probe techniques Moessbauer spectroscopy and muon spin relaxation experiments. With increasing Na substitution level, the magnetic order parameter as well as the magneto-structural phase transition are suppressed. For x = 0.50 we find a microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature Tc. A systematic comparison with other 122 pnictides reveals a square-root correlation between the reduction of the magnetic order parameter and the ratio of the transition temperatures, Tc/TN, which can be understood in the framework of a Landau theory. In the optimally doped sample with Tc = 34 K, diluted magnetism is found and the temperature dependence of the penetration depth and superfluid density are obtained, proving the presence of two superconducting s-wave gaps

cond-mat.supr-con↗

arXiv2015

Interaction-Induced Renormalization of Tunneling into Multiple Majorana End States of a Topological Superconducting Wire

We investigate a system of multiple Majorana states at the end of a topological superconducting wire coupled to a normal lead. For a minimum of three Majorana fermions at the interface, we find nontrivial renormalization physics. Interface tunneling processes can be classified in terms of spin-1/2 and spin-3/2 irreducible representations of the SU(2) group. We show that the renormalization of the tunneling amplitudes belonging to different representations is completely different in that one type is suppressed, whereas the other is enhanced, depending on the sign of the interaction coupling. This results in distinct temperature dependencies of the tunneling current through the interface and different spin polarizations of this current.

cond-mat.mes-hall↗

arXiv2015

What is the probability that a large random matrix has no real eigenvalues?

We study the large-$n$ limit of the probability $p_{2n,2k}$ that a random $2n\times 2n$ matrix sampled from the real Ginibre ensemble has $2k$ real eigenvalues. We prove that, $$\lim_{n\rightarrow \infty}\frac {1}{\sqrt{2n}} \log p_{2n,2k}=\lim_{n\rightarrow \infty}\frac {1}{\sqrt{2n}} \log p_{2n,0}= -\frac{1}{\sqrt{2π}}ζ\left(\frac{3}{2}\right),$$ where $ζ$ is the Riemann zeta-function. Moreover, for any sequence of non-negative integers $(k_n)_{n\geq 1}$, $$\lim_{n\rightarrow \infty}\frac {1}{\sqrt{2n}} \log p_{2n,2k_n}=-\frac{1}{\sqrt{2π}}ζ\left(\frac{3}{2}\right),$$ provided $\lim_{n\rightarrow \infty} \left(n^{-1/2}\log(n)\right) k_{n}=0$.

math.PR↗

arXiv2014

Application of a time-convolutionless stochastic Schrödinger equation to energy transport and thermal relaxation

Quantum stochastic methods based on effective wave functions form a framework for investigating the generally non-Markovian dynamics of a quantum-mechanical system coupled to a bath. They promise to be computationally superior to the master-equation approach, which is numerically expensive for large dimensions of the Hilbert space. Here, we numerically investigate the suitability of a known stochastic Schrödinger equation that is local in time to give a description of thermal relaxation and energy transport. This stochastic Schrödinger equation can be solved with a moderate numerical cost, indeed comparable to that of a Markovian system, and reproduces the dynamics of a system evolving according to a general non-Markovian master equation. After verifying that it describes thermal relaxation correctly, we apply it for the first time to the energy transport in a spin chain. We also discuss a portable algorithm for the generation of the coloured noise associated with the numerical solution of the non-Markovian dynamics.

cond-mat.stat-mech↗

arXiv2014

Resistive anisotropy due to spin-fluctuation scattering in the nematic phase of iron pnictides

The large in-plane anisotropy of the resistivity is a hallmark of the nematic state of the iron pnictides. Solving the Boltzmann transport equation, we show that the prominent doping dependence as well as the large values of the anisotropy can be well explained by momentum-dependent spin-fluctuation scattering without assuming anisotropic impurity states. Due to the forward-scattering corrections, the hot spots contribute to the resistive anisotropy even in the case of strong spin fluctuations, which makes large values of the anisotropy possible. The ellipticity of the electron pockets plays an important role in explaining the dominance of positive values of the anisotropy, i.e., larger resistivity in the direction with weaker spin fluctuations, throughout the doping range.

cond-mat.str-el↗

arXiv2014

Topological Surface States in Paramagnetic and Antiferromagnetic Iron Pnictides

The electronic structure of iron pnictides is topologically nontrivial, leading to the appearance of Dirac cones in the band structure for the antiferromagnetic phase. Motivated by the analogy with Dirac cones in graphene, we explore the possible existence of topologically protected surface states. Surprisingly, bands of surface states exist even in the paramagnetic state. A realistic five-orbital model predicts two such bands. In the antiferromagnetic phase, these surface bands survive but split. We obtain the bulk and surface dispersion from exact diagonalization of two- and five-orbital models in a strip geometry and discuss the results based on topology.

cond-mat.str-el↗

arXiv2014

Topological surface states and Andreev bound states in superconducting iron pnictides

The nontrivial topology of the electronic structure of iron pnictides can lead to the appearance of surface states. We study such states in various strip geometries with a focus on the superconducting phase. In the presence of unconventional superconducting pairing with $s_\pm$-wave gap structure, the topological states are quite robust and partly remain in the superconducting gap. Furthermore, Andreev bound states appear, which coexist with the topological states for small superconducting gaps and merge with them for larger gap values. The bulk and surface dispersions are obtained from exact diagonalization for two-orbital and five-orbital models in strip geometries.

cond-mat.str-el↗

arXiv2014

Transport in multiband systems with hot spots on the Fermi surface: Forward-scattering corrections

Multiband models with hot spots are of current interest partly because of their relevance for the iron-based superconductors. In these materials, the momentum-dependent scattering off spin fluctuations and the ellipticity of the electron Fermi pockets are responsible for anisotropy of the lifetimes of excitations around the Fermi surface. The deep minima of the lifetimes---the so-called hot spots---have been assumed to contribute little to the transport as is indeed predicted by a simple relaxation-time approach. Calculating forward-scattering corrections to this approximation, we find that the effective transport times are much more isotropic than the lifetimes and that, therefore, the hot spots contribute to the transport even in the case of strong spin-fluctuation scattering. We discuss this effect on the basis of an analytical solution of the Boltzmann equation and calculate numerically the temperature and doping dependence of the resistivity and the Hall, Seebeck, and Nernst coefficients.

cond-mat.mes-hall↗

arXiv2014

Helical spin texture of surface states in topological superconductors

Surface states of topological noncentrosymmetric superconductors exhibit intricate helical spin textures, i.e., the spin orientation of the surface quasiparticles is coupled to their momentum. Using quasiclassical theory, we study the spin polarization of the surface states as a function of the spin-orbit interaction and superconducting pairing symmetry. We focus on two- and three-dimensional fully gapped and nodal noncentrosymmetric superconductors. For the case of nodal systems, we show that the spin polarization of the topological flat bands is controlled by the spin polarization of the bulk normal states at the bounding gap nodes. We demonstrate that the zero-bias conductance in a magnetic tunnel junction can be used as an experimental test of the surface-state spin polarization.

cond-mat.supr-con↗

arXiv2014

Superconducting pairing in the spin-density-wave phase of iron pnictides

Some of the iron pnictides show coexisting superconductivity and spin-density-wave order. We study the superconducting pairing instability in the spin-density-wave phase. Assuming that the pairing interaction is due to spin fluctuations, we calculate the effective pairing interactions in the singlet and triplet channels by summing the bubble and ladder diagrams taking the reconstructed band structure into account. The leading pairing instabilities and the corresponding superconducting gap structures are then obtained from the superconducting gap equation. We illustrate this approach for a minimal two-band model of the pnictides. Analytical and numerical results show that the existence of propagating magnons in the spin-density-wave phase strongly enhances the pairing in both the singlet and the spin s_z=0 triplet channel. Over a limited parameter range, a spin s_z=0 triplet p_x-wave state is the dominant instability. It competes with various singlet states, which have mostly s^\pm-type structures. We analyze the effect of various symmetry-allowed interactions on the pairing in some detail.

cond-mat.supr-con↗

arXiv2013

Interface currents in topological superconductor-ferromagnet heterostructures

We propose the existence of a substantial charge current parallel to the interface between a noncentrosymmetric superconductor and a metallic ferromagnet. Our analysis focuses upon two complementary orbital-angular-momentum pairing states of the superconductor, exemplifying topologically nontrivial states which are gapped and gapless in the bulk, respectively. Utilizing a quasiclassical scattering theory, we derive an expression for the interface current in terms of Andreev reflection coefficients. Performing a systematic study of the current, we find stark qualitative differences between the gapped and gapless superconductors, which reflect the very different underlying topological properties. For the fully gapped superconductor, there is a sharp drop in the zero-temperature current as the system is tuned from a topologically nontrivial to a trivial phase. We explain this in terms of the sudden disappearance of the contribution to the current from the subgap edge states at the topological transition. The current in the gapless superconductor is characterized by a dramatic enhancement at low temperatures, and exhibits a singular dependence on the exchange-field strength in the ferromagnetic metal at zero temperature. This is caused by the energy shift of the strongly spin-polarized nondegenerate zero-energy flat bands due to their coupling to the exchange field. We argue that the interface current provides a novel test of the topology of the superconductor, and discuss prospects for the experimental verification of our predictions.

cond-mat.supr-con↗

arXiv2013

Edge currents as a signature of flat bands in topological superconductors

We study nondegenerate flat bands at the surfaces of noncentrosymmetric topological superconductors by exact diagonalization of Bogoliubov-de Gennes Hamiltonians. We show that these states are strongly spin polarized, and acquire a chiral dispersion when placed in contact with a ferromagnetic insulator. This chiral mode carries a large edge current which displays a singular dependence on the exchange-field strength. The contribution of other edge states to the current is comparably weak. We hence propose that the observation of the edge current can serve as a test of the presence of nondegenerate flat bands.

cond-mat.supr-con↗

arXiv2013

Transport anomalies due to anisotropic interband scattering

Unexpected transport behavior can arise due to anisotropic single-particle scattering in multiband systems. Specifically, we show within a semiclassical Boltzmann approach beyond the relaxation-time approximation that anisotropic scattering between electronlike and holelike Fermi surfaces generically leads to negative transport times, which in turn cause negative magnetoresistance, an extremum in the Hall coefficient, and a reduction of the resistivity. The anisotropy required for this to occur decreases with increasing mismatch between the Fermi-surface radii.

cond-mat.mes-hall↗

arXiv2012

Spectroscopy of the transition-rate matrix for molecular junctions: dynamics in the Franck-Condon regime

The quantum master equation applied to electronic transport through nanoscopic devices provides information not only on the stationary state but also on the dynamics. The dynamics is characterized by the eigenvalues of the transition-rate matrix, or generator, of the master equation. We propose to use the spectrum of these eigenvalues as a tool for the study of nanoscopic transport. We illustrate this idea by analyzing a molecular quantum dot with an electronic orbital coupled to a vibrational mode, which shows the Franck-Condon blockade if the coupling is strong. Our approach provides complementary information compared to the study of observables in the stationary state.

cond-mat.mes-hall↗

arXiv2012

Memristive properties of single-molecule magnets

Single-molecule magnets weakly coupled to two ferromagnetic leads act as memory devices in electronic circuits---their response depends on history, not just on the instantaneous applied voltage. We show that magnetic anisotropy introduces a wide separation of timescales between fast and slow relaxation processes in the system, which leads to a pronounced memory dependence in a wide intermediate time regime. We study the response to a harmonically varying bias voltage from slow to rapid driving within a master-equation approach. The system is not purely memristive but shows a partially capacitive response on short timescales. In the intermediate time regime the molecular spin can be used as the state variable in a two-terminal molecular memory device.

cond-mat.mes-hall↗

arXiv2012

Doping dependence of antiferromagnetism in models of the pnictides

We study the doping dependence of the spin-density-wave (SDW) state in four models of the 1111 pnictides. The random-phase approximation is used to determine the ordering temperature and the ordering vector as functions of doping, and to evaluate the contribution of the various orbitals to the SDW instability. In addition to the usual assumption of orbitally rotation-invariant interactions, we consider the effect of reduced interactions involving the xy orbital, which are anticipated by crystal-structure considerations. We find that changing the relative strength of the interaction in the xy orbital tunes the system between different nesting instabilities leading to similar SDW order. We identify two models as showing reasonable agreement with experiments, while the other two display significant discrepancies, and discuss the underlying differences between the models.

cond-mat.supr-con↗

arXiv2012

Types of topological surface states in nodal noncentrosymmetric superconductors

Nodal noncentrosymmetric superconductors have topologically nontrivial properties manifested by protected zero-energy surface states. Specifically, it was recently found that zero-energy surface flat bands of topological origin appear at their surface. We show that the presence of certain inversion-type lattice symmetries can give rise to additional topological features of the gap nodes, resulting in surface states forming one-dimensional arcs connecting the projections of two nodal rings. In addition, we demonstrate that Majorana surface states can appear at time-reversal-invariant momenta of the surface Brillouin zone, even when the system is not fully gapped in the bulk. Within a continuum theory we derive the topological invariants that protect these different types of zero-energy surface states. We independently derive general conditions for the existence of zero-energy surface bound states using the complementary quasiclassical scattering theory, explicitly taking into account the effects of spin-orbit splitting of the bands. We compute surface bound-state spectra for various crystal point-group symmetries and orbital-angular-momentum pairing states. Finally, we examine the signatures of the arc surface states and of the zero-energy surface flat bands in tunneling-conductance spectra and discuss how topological phase transitions in noncentrosymmetric superconductors could be observed in experiments.

cond-mat.supr-con↗

arXiv2012

Josephson effect between triplet superconductors through a ferromagnetic barrier of finite width

Charge and spin transport in a junction involving two triplet superconductors and a ferromagnetic barrier are studied. We use Bogoliubov-de Gennes wavefunctions to construct the Green's function, from which we obtain the Josephson currents in terms of the Andreev reflection coefficients. We focus on the consequences of a finite barrier width for the occurrence of 0-πtransitions and for the spin currents, and examine the appropriateness of the common δ-function approximation for the tunneling region.

cond-mat.supr-con↗