arXiv subjects

Carsten Timm

Publications and source records attributed to Carsten Timm.

At least 73 records · Page 4Linked to original sources

arXiv2011

Microscopically derived Ginzburg-Landau theory for magnetic order in the iron pnictides

We examine the competition of the observed stripe spin density wave (SDW) with other commensurate and incommensurate SDW phases in a two-band model of the pnictides. Starting from this microscopic model, we rigorously derive an expansion of the free energy in terms of the different order parameters at the mean-field level. We show that three distinct commensurate SDW states are possible and study their appearance as a function of the doping and the electronic structure. We show that the stripe phase is generally present, but its extent in the phase diagram depends strongly upon the number of hole Fermi pockets that are nested with the electron Fermi pockets. Electron pockets competing for the same portion of a hole pocket play a crucial role. We discuss the relevance of our results for the antiferromagnetism of the pnictides.

cond-mat.supr-con↗

arXiv2011

Transport through a quantum spin Hall quantum dot

Quantum spin Hall insulators, recently realized in HgTe/(Hg,Cd)Te quantum wells, support topologically protected, linearly dispersing edge states with spin-momentum locking. A local magnetic exchange field can open a gap for the edge states. A quantum-dot structure consisting of two such magnetic tunneling barriers is proposed and the charge transport through this device is analyzed. The effects of a finite bias voltage beyond linear response, of a gate voltage, and of the charging energy in the quantum dot are studied within a combination of Green-function and master-equation approaches. Among other results, a partial recurrence of non-interacting behavior is found for strong interactions, and the possibility of controlling the edge magnetization by a locally applied gate voltage is proposed.

cond-mat.mes-hall↗

arXiv2011

Effect of intraband Coulomb repulsion on the excitonic spin-density wave

We present a study of the magnetic ground state of a two-band model with nested electron and hole Fermi surfaces and both interband and intraband Coulomb interactions. Our aim is to understand how the excitonic spin-density-wave (ESDW) state induced by the interband Coulomb repulsion is affected by the intraband interactions. We first determine the magnetic instabilities of our model in an unbiased way by employing the random-phase approximation (RPA) to calculate the static spin susceptibility in the paramagnetic state. From this, we construct the mean-field phase diagram, demonstrating the robustness of the ESDW against the intraband interaction. We then calculate the RPA transverse spin susceptibility in the ESDW state and show that the intraband Coulomb repulsion significantly renormalizes the paramagnon line shape and suppresses the spin-wave velocity. We conclude with a discussion of the relevance of this suppression for the commensurate ESDW state of Mn-doped Cr alloys.

cond-mat.supr-con↗

arXiv2011

Magnetic order in orbital models of the iron pnictides

We examine the appearance of the experimentally-observed stripe spin-density-wave magnetic order in five different orbital models of the iron pnictide parent compounds. A restricted mean-field ansatz is used to determine the magnetic phase diagram of each model. Using the random phase approximation, we then check this phase diagram by evaluating the static spin susceptibility in the paramagnetic state close to the mean-field phase boundaries. The momenta for which the susceptibility is peaked indicate in an unbiased way the actual ordering vector of the nearby mean-field state. The dominant orbitally resolved contributions to the spin susceptibility are also examined to determine the origin of the magnetic instability. We find that the observed stripe magnetic order is possible in four of the models, but it is extremely sensitive to the degree of the nesting between the electron and hole Fermi pockets. In the more realistic five-orbital models, this order competes with a strong-coupling incommensurate state which appears to be controlled by details of the electronic structure below the Fermi energy. We conclude by discussing the implications of our work for the origin of the magnetic order in the pnictides.

cond-mat.supr-con↗

arXiv2011

Spin Josephson effect with a single superconductor

A thin ferromagnetic layer on a bulk equal-spin-pairing triplet superconductor is shown to mediate a Josephson coupling between the spin $\uparrow$ and $\downarrow$ condensates of the superconductor. By deriving analytic expressions for the bound states at the triplet superconductor-ferromagnet interface, we show that this spin Josephson effect establishes an effective anisotropy axis in the ferromagnetic layer. The associated Josephson spin current is predicted to cause a measurable precession of the magnetization about the vector order parameter of the triplet superconductor.

cond-mat.supr-con↗

arXiv2011

Topologically protected flat zero-energy surface bands in non-centrosymmetric superconductors

Nodal non-centrosymmetric superconductors (NCS) have recently been shown to be topologically non-trivial. An important consequence is the existence of topologically protected flat zero-energy surface bands, which are related to the topological characteristics of the line nodes of the bulk gap via a bulk-boundary correspondence [Schnyder and Ryu, arXiv:1011.1438]. In this paper we examine these zero-energy surface bands using a quasiclassical theory. We determine their spectrum and derive a general condition for their existence in terms of the sign change of the gap functions. A key experimental signature of the zero-energy surface bands is a zero-bias peak in the tunneling conductance, which depends strongly on the surface orientation. This can be used as a fingerprint of a topologically non-trivial NCS.

cond-mat.supr-con↗

arXiv2011

Cotunneling through a magnetic single-molecule transistor based on N\atC60

We present an experimental and theoretical study of a magnetic single-molecule transistor based on N@C60 connected to gold electrodes. Particular attention is paid to the regime of intermediate molecule-lead coupling, where cotunneling effects manifest themselves in the Coulomb-blockade regime. The experimental results for the differential conductance as a function of bias, gate voltage, and external magnetic field are in agreement with our analysis of the tunneling rates and provide evidence of magnetic signatures in single-N@C60 devices arising from an antiferromagnetic exchange interaction between the C60 spin and the nitrogen spin.

cond-mat.mes-hall↗

arXiv2010

Andreev spectroscopy and surface density of states for a three-dimensional time-reversal invariant topological superconductor

A topological superconductor is a fully gapped superconductor that exhibits exotic zero-energy Andreev surface states at interfaces with a normal metal. In this paper we investigate the properties of a three-dimensional time reversal invariant topological superconductor by means of a two-band model with unconventional pairing in both the inter- and intraband channels. Due to the bulk-boundary correspondence the presence of Andreev surface states in this system is directly related to the topological structure of the bulk wavefunctions, which is characterized by a winding number. Using quasiclassical scattering theory we construct the spectrum of the Andreev bound states that appear near the surface and compute the surface density of states for various surface orientations. Furthermore, we consider the effects of band splitting, i.e., the breaking of an inversion-type symmetry, and demonstrate that in the absence of band splitting there is a direct transition between the fully gapped topologically trivial phase and the nontrivial phase, whereas in the presence of band splitting there exists a finite region of a gapless nodal superconducting phase between the fully gapped topologically trivial and nontrivial phases.

cond-mat.supr-con↗

arXiv2010

Time-convolutionless master equation for quantum dots: Perturbative expansion to arbitrary order

The master equation describing the non-equilibrium dynamics of a quantum dot coupled to metallic leads is considered. Employing a superoperator approach, we derive an exact time-convolutionless master equation for the probabilities of dot states, i.e., a time-convolutionless Pauli master equation. The generator of this master equation is derived order by order in the hybridization between dot and leads. Although the generator turns out to be closely related to the T-matrix expressions for the transition rates, which are plagued by divergences, in the time-convolutionless generator all divergences cancel order by order. The time-convolutionless and T-matrix master equations are contrasted to the Nakajima-Zwanzig version. The absence of divergences in the Nakajima-Zwanzig master equation due to the nonexistence of secular reducible contributions becomes rather transparent in our approach, which explicitly projects out these contributions. We also show that the time-convolutionless generator contains the generator of the Nakajima-Zwanzig master equation in the Markov approximation plus corrections, which we make explicit. Furthermore, it is shown that the stationary solutions of the time-convolutionless and the Nakajima-Zwanzig master equations are identical. However, this identity neither extends to perturbative expansions truncated at finite order nor to dynamical solutions. We discuss the conditions under which the Nakajima-Zwanzig-Markov master equation nevertheless yields good results.

cond-mat.mes-hall↗

arXiv2010

Hysteresis in the magneto-transport of Manganese-doped Germanium: evidence for carrier-mediated ferromagnetism

We report the fabrication of Ge:Mn ferromagnetic semiconductors by Mn-ion implantation into Ge followed by pulsed laser annealing. Benefiting from the short time annealing, the hole concentration in Mn-implanted Ge has been increased by two orders of magnitude from 10$^{18}$ to over 10$^{20}$ cm$^{-3}$. Likely due to the high hole concentration, we observe that the longitudinal and Hall resistances exhibit the same hysteresis as the magnetization, which is usually considered as a sign of carrier-mediated ferromagnetism.

cond-mat.mtrl-sci↗

arXiv2010

Resonant and Kondo tunneling through molecular magnets

Transport through molecular magnets is studied in the regime of strong coupling to the leads. We consider a resonant-tunneling model where the electron spin in a quantum dot or molecule is coupled to an additional local, anisotropic spin via exchange interaction. The two opposite regimes dominated by resonant tunneling and by Kondo transport, respectively, are considered. In the resonant-tunneling regime, the stationary state of the impurity spin is calculated for arbitrarily strong molecule-lead coupling using a master-equation approach, which treats the exchange interaction perturbatively. We find that the characteristic fine structure in the differential conductance persists even if the hybridization energy exceeds thermal energies. Transport in the Kondo regime is studied within a diagrammatic approach. We show that magnetic anisotropy gives rise to a splitting of the Kondo peak at low bias voltages.

cond-mat.mes-hall↗

arXiv2009

Random transition-rate matrices for the master equation

Random-matrix theory is applied to transition-rate matrices in the Pauli master equation. We study the distribution and correlations of eigenvalues, which govern the dynamics of complex stochastic systems. Both the cases of identical and of independent rates of forward and backward transitions are considered. The first case leads to symmetric transition-rate matrices, whereas the second corresponds to general, asymmetric matrices. The resulting matrix ensembles are different from the standard ensembles and show different eigenvalue distributions. For example, the fraction of real eigenvalues scales anomalously with matrix dimension in the asymmetric case.

cond-mat.stat-mech↗

arXiv2009

Excitonic spin density wave state in iron pnictides

We examine the appearance of a spin density wave in the FeAs parent compounds due to an excitonic instability. Using a realistic four-band model, we show that the magnetic state depends very sensitively upon the details of the band structure. We demonstrate that an orthorhombic distortion of the crystal enhances the stability of the antiferromagnetic order.

cond-mat.supr-con↗

arXiv2008

Tunneling Spectra of Individual Magnetic Endofullerene Molecules

The manipulation of single magnetic molecules may enable new strategies for high-density information storage and quantum-state control. However, progress in these areas depends on developing techniques for addressing individual molecules and controlling their spin. Here we report success in making electrical contact to individual magnetic N@C60 molecules and measuring spin excitations in their electron tunneling spectra. We verify that the molecules remain magnetic by observing a transition as a function of magnetic field which changes the spin quantum number and also the existence of nonequilibrium tunneling originating from low-energy excited states. From the tunneling spectra, we identify the charge and spin states of the molecule. The measured spectra can be reproduced theoretically by accounting for the exchange interaction between the nitrogen spin and electron(s) on the C60 cage.

cond-mat.mes-hall↗

arXiv2008

Reentrant magnetic ordering and percolation in a spin-crossover system

Spin-crossover compounds, which are characterized by magnetic ions showing low-spin and high-spin states at thermally accessible energies, are ubiquitous in nature. We here focus on the effect of an exchange interaction on the collective properties for the case of non-magnetic low-spin ions, which applies to Fe(II) compounds. Monte Carlo simulations are used to study a three-dimensional spin-crossover model for the full parameter range from essentially pure high spin to essentially pure low spin. We find that as the low-spin state becomes more favorable, the Curie temperature drops, the universality class deviates from the three-dimensional Heisenberg class, and the transition eventually changes to first order. A heat-bath algorithm that grows or shrinks low-spin and high-spin domains is developed to handle the first-order transition. When the ground state has low spin, a reentrant magnetic transition is found in a broad parameter range. We also observe a percolation transition of the high spins, which branches off the first-order magnetic transition.

cond-mat.stat-mech↗

arXiv2008

Tunneling through molecules and quantum dots: master-equation approaches

An important class of approaches to the description of electronic transport through molecules and quantum dots is based on the master equation. We discuss various formalisms for deriving a master equation and their interrelations. It is shown that the master equations derived by Wangsness, Bloch, and Redfield and by Koenig et al. are equivalent. The roles of the large-reservoir and Markov approximations are clarified. The Markov approximation is traced back to nonzero bias voltage and temperature, whereas interactions and the corresponding rapid relaxation in the leads are shown to be irrelevant for the transport under certain conditions. It is explained why the T-matrix formalism gives incomplete results except for diagonal density operators and to second order in the tunneling amplitudes. The time-convolutionless master equation is adapted to tunneling problems and a diagrammatic scheme for generating arbitrary orders in the tunneling amplitudes is developed.

cond-mat.mes-hall↗

arXiv2007

Cotunneling and non-equilibrium magnetization in magnetic molecular monolayers

Transport and non-equilibrium magnetization in monolayers of magnetic molecules subject to a bias voltage are considered. We apply a master-equation approach going beyond the sequential-tunneling approximation to study the Coulomb-blockade regime. While the current is very small in this case, the magnetization shows changes of the order of the saturation magnetization for small variations of the bias voltage. Inelastic cotunneling processes manifest themselves as differential-conductance steps, which are accompanied by much larger changes in the magnetization. In addition, the magnetization in the Coulomb-blockade regime exhibits strong signatures of sequential tunneling processes de-exciting molecular states populated by inelastic cotunneling. We also consider the case of a single molecule, finding that cotunneling processes lead to the occurrence of magnetic sidebands below the Coulomb-blockade threshold. In the context of molecular electronics, we study how additional spin relaxation suppresses the fine structure in transport and magnetization.

cond-mat.mes-hall↗

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↗