arXiv subjects

Peter Thalmeier

Publications and source records attributed to Peter Thalmeier.

At least 55 records · Page 3Linked to original sources

arXiv2013

Full t-matrix approach to quasiparticle interference in non-centrosymmetric superconductors

We develop the full t-matrix theory of quasiparticle interference (QPI) for non-centrosymmetric (NCS) superconductors with Rashba spin-orbit coupling. We give a closed solution for the QPI spectrum for arbitrary combination and strength of nonmagnetic (Vc) and magnetic (Vm) impurity scattering potentials in terms of integrated normal and anomalous Green's functions. The theory is applied to a realistic 2D model of the Ce-based 131- type heavy fermion superconductors. We discuss the QPI dependence on frequency, composition and strength of scattering and compare with Born approximation results. We show that the QPI pattern is remarkably stable against changes in the scattering model and can therefore give reliable information on the properties of Rashba-split Fermi surface sheets and in particular on the accidental nodal position of the mixed singlet-triplet gap function in NCS superconductors.

cond-mat.supr-con↗

arXiv2013

Multiorbital and hybridization effects in the quasiparticle interference of triplet superconductor Sr2RuO4

The tetragonal compound Sr2RuO4 exhibits a chiral p-wave superconducting (SC) state of its three t2g-type conduction bands. The characteristics of unconventional gap structure are known from experiment, in particular field-angle resolved specific heat measurements and from microscopic theories. A rotated extremal structure on the main active SC band with respect to the nodal gaps on the passive bands was concluded. We propose that this gap structure can be further specified by applying the STM- quasiparticle interference (QPI) method. We calculate the QPI spectrum within a three band and chiral three gap model and give closed analytical expressions. We show that as function of bias voltage the chiral three gap model will lead to characteristic changes in QPI that may be identified and may be used for more quantitative gap determination of the chiral gap structure.

cond-mat.supr-con↗

arXiv2013

Rashba spin-orbit coupling effects in quasiparticle interference of non-centrosymmetric superconductors

The theory of quasiparticle interference (QPI) for non-centrosymmetric (NCS) superconductors with Rashba spin-orbit coupling is developed using T-matrix theory in Born approximation. We show that qualitatively new effects in the QPI pattern originate from the Rashba spin-orbit coupling: The resulting spin coherence factors lead to a distinct difference of charge- and spin- QPI and to an induced spin anisotropy in the latter even for isotropic magnetic impurity scattering. In particular a cross - QPI appears describing the spin oscillation pattern due to nonmagnetic impurity scattering which is directly related to the Rashba vector. We apply our theory to a 2D model for the NCS heavy fermion unconventional superconductor CePt$_3$Si and discuss the new QPI features for a gap model with accidental node lines due to its composite singlet-triplet nature.

cond-mat.supr-con↗

arXiv2013

The role of hyperfine coupling in magnetic and quadrupolar ordering of Pr3Pd20Si6

We study the ternary clathrate Pr3Pd20Si6 in specific heat and AC-susceptibility measurements on a high-quality single crystal, distinguishing antiferromagnetic (AFM) and antiferroquadrupolar (AFQ) ordering on two sublattices of inequivalent Pr sites. The specific heat shows the direct involvement of nuclear spin degrees of freedom in the AFM ordering, which is well supported by our calculation of the hyperfine level scheme without adjustable parameters. Pr3Pd20Si6 is therefore one of the rare materials where the nuclear moments are involved in the formation of the magnetic ground state.

cond-mat.str-el↗

arXiv2013

Evolution of the multiband RKKY interaction: Application to iron pnictides and chalcogenides

The indirect RKKY interaction in iron pnictide and chalcogenide metals is calculated for a simplified four bands Fermi surface (FS) model. We investigate the specific multi-band features and show that distinct length scales of the RKKY oscillations appear. For the regular lattice of the local moments, the generalized RKKY interaction is defined in momentum space. We consider its momentum dependence in paramagnetic and spin density wave (SDW) phases, discuss its implications for the possible type of magnetic order and compare it to the results obtained from more realistic tight-binding type Fermi surface model. Our finding can give important clues on the magnetic ordering of the 4f- iron based superconductors.

cond-mat.supr-con↗

arXiv2013

Inelastic magnetic scattering effect on LDOS of topological insulators

Magnetic ions such as Fe, Mn and Co with localized spins may be adsorbed on the surface of topological insulators like \Bi. They form scattering centers for the helical surface states which have a Dirac cone dispersion as long as the local spins are disordered. However, the local density of states (LDOS) may be severely modified by the formation of bound states. Commonly only elastic scattering due to normal and exchange potentials of the adatom is assumed. Magnetization measurements show, however, that considerable magnetic single ion anisotropies exist which lead to a splitting of the local impurity spin states resulting in a singlet ground state. Therefore inelastic scattering processes of helical Dirac electrons become possible as described by a dynamical local self energy of second order in the exchange interaction. The self energy influences bound state formation and leads to significant new anomalies in the LDOS at low energies and low temperatures which we calculate within T-matrix approach. We propose that they may be used for spectroscopy of local impurity spin states by appropriate tuning of chemical potential and magnetic field.

cond-mat.mes-hall↗

arXiv2012

Field induced spin exciton doublet splitting in d_{x^2-y^2}-wave 115-heavy electron superconductors

We investigate the spin-exciton modes in the superconducting d_{x^2-y^2} state of CeMIn_5 heavy fermion compounds found at the antiferromagnetic wave vector by inelastic neutron scattering. We present a theoretical model that explains the field dependence for both field directions. We show that the recently observed splitting of the spin exciton doublet in CeCoIn_5 into two non-degenerate modes for in-plane field appears naturally in this model. This is due to the spin anisotropy of g- factors and quasiparticle interactions which lead to different resonant conditions for the dynamic susceptibility components. We predict that the splitting of the spin resonance doublet becomes strongly nonlinear for larger fields when the energy of both split components decreases. For field along the tetragonal axis no splitting but only a broadening of the resonance is found in agreement with experiment.

cond-mat.str-el↗

arXiv2012

Spin Exciton Formation inside the Hidden Order Phase of CeB6

The heavy fermion metal CeB6 exhibits hidden order of antiferroquadrupolar (AFQ) type below T_Q=3.2K and subsequent antiferromagnetic (AFM) order at T_N=2.3K. It was interpreted as ordering of the quadrupole and dipole moments of a $Γ_8$ quartet of localised Ce $4f^1$ electrons. This established picture has been profoundly shaken by recent inelastic neutron scattering (G. Friemel et al., arXiv:1111.4151) that found the evolution of a feedback spin exciton resonance within the hidden order phase at the AFQ wave vector which is stabilized by the AFM order. We develop an alternative theory based on a fourfold degenerate Anderson lattice model, including both order parameters as particle-hole condensates of itinerant heavy quasiparticles. This explains in a natural way the appearance of the spin exciton resonance and the momentum dependence of its spectral weight, in particular around the AFQ vector and its rapid disappearance in the disordered phase. Analogies to the feedback effect in unconventional heavy fermion superconductors are pointed out.

cond-mat.str-el↗

arXiv2011

Topological insulator ribbon: Surface states and dynamical response

We study theoretically the distributions of charge and spin polarization of a topological insulator ribbon, with a realistic rectangular cross section. Due to constriction in two lateral directions, the surface states discretize into a series of subbands inside of the bulk band gap. The charge and spin distribution show interesting characters which are different from an ideal topological surface state. The effect of merging of four different surface states into the new one as an entity are analyzed. Optical conductivity and dynamical spin susceptibility of the ribbon are studied. Different from a single ideal surface, the optical response and dynamical spin susceptibility of a ribbon do not have a clear correspondence. The dynamical spin susceptibility could be used to identify the more adequate model for Bi$_2$Se$_3$.

cond-mat.mtrl-sci↗

arXiv2011

Magnetic torque oscillations from warped helical surface states in topological insulators

A magnetic torque method is proposed that probes the warping and mass gap of Dirac cone surface states in topological insulators like Bi2X3 (X=Se,Te). A rotating field parallel to the surface induces a paramagnetic moment in the helical surface states for nonzero warping. It is non-collinear with the applied field and therefore produces torque oscillations as function of the field angle which are a direct signature of the surface states. The torque dependence on field strength and angle, the chemical potential and the Dirac cone parameters like warping strength and mass gap is calculated. It is shown that the latter leads to a symmetry reduction in the fourfold torque oscillations.

cond-mat.mtrl-sci↗

arXiv2011

RKKY interaction in SDW phase of iron-based superconductors

Using the multiband model we analyze the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the magnetic impurities in layered ferropnictide superconductors. In the normal state the interaction is spin isotropic and is dominated by the nesting features of the electron and hole bands separated by the antiferromagnetic momentum, ${\bf Q}_{AF}$. In the AF state the RKKY interaction maps into an effective anisotropic XXZ-type Heisenberg exchange model. The anisotropy originates from the breaking of the spin-rotational symmetry induced by the AF order and its strength depends on the size of the AF gap and the structure of the folded Fermi surface. We discuss our results in connection to the recent experiments.

cond-mat.str-el↗

arXiv2011

Quasiparticle interference in heavy Fermion superconductor CeCoIn_5

We investigate the quasiparticle interference in the heavy Fermion superconductor CeCoIn_5 as direct method to confirm the d-wave gap symmetry. The ambiguity between d_{xy} and d_{x^2-y^2} symmetry remaining from earlier specific heat and thermal transport investigations has been resolved in favor of the latter by the observation of a spin resonance that can occur only in d_{x^2-y^2} symmetry. However these methods are all indirect and depend considerably on theoretical interpretation. Here we propose that quasiparticle interference (QPI) spectroscopy by scanning tunneling microscopy (STM) can give a direct fingerprint of the superconducting gap in real space which may lead to a definite conclusion on its symmetry for CeCoIn_5 and related 115 compounds. The QPI pattern for both magnetic and nonmagnetic impurities is calculated for the possible d-wave symmetries and characteristic differences are found that may be identified by STM method.

cond-mat.str-el↗

arXiv2011

Staggered moment dependence on field-tuned quantum fluctuations in 2D frustrated antiferromagnets

We propose an efficient method to identify the degree of frustration in quasi-2D antiferromagnets described by the J_1-J_2 Heisenberg model. The frustration ratio J_2/J_1 is usually obtained from analysis of susceptibility, specific heat and saturation field. We show that the non-monotonic field dependence of the staggered moment caused by the suppression of quantum fluctuations in a field depends strongly on the frustration ratio. This gives a powerful criterion to determine J_2/J_1 using a combination of exact diagonalization (ED) method for finite clusters and spin wave analysis. We apply this method to the quasi-2D compound Cu(pz)_2(ClO_4)_2 and show that it leads to an intermediate ratio J_2/J_1 ~ 0.2 for the frustration. We also explain the observed anomalous increase of transition temperature in applied fields as an effect of reduced quantum fluctuations.

cond-mat.str-el↗

arXiv2011

Ordered moment in the anisotropic and frustrated square lattice Heisenberg model

The two-dimensional frustrated next nearest neighbor Heisenberg model on the square lattice is a prime example for a spin system where quantum fluctuations can either destroy or stabilize magnetic order. The phase boundaries and staggered moment dependence on the frustration ratio J_2/J_1 of the exchange constants are fairly well understood both from approximate analytical and numerical methods. In this work we use exact diagonalization for finite clusters for an extensive investigation of the more general J_1a,b-J_2 model which includes a spatial exchange anisotropy between next-neighbor spins. We introduce a systematic way of tiling the square lattice and, for this low symmetry model, define a controlled procedure for the finite size scaling that is compatible with the possible magnetic phases. We obtain ground state energies, structure factors and ordered moments and compare with the results of spin wave calculations. We conclude that J_1a,b exchange anisotropy strongly stabilizes the columnar antiferromagnetic phase for all frustration parameters, in particular in the region of the spin nematic phase of the isotropic model.

cond-mat.str-el↗

arXiv2011

Signatures of hidden order symmetry in torque oscillations, elastic constant anomalies and field induced moments in URu2Si2

We discuss the conclusions on the symmetry of hidden order (HO) in URu2Si2 that may be drawn from recent torque experiments in rotating magnetic field by Okazaki et al. [1]. They are very sensitive to changes in the magnetic susceptibility induced by HO. We show that the observed twofold angular torque oscillations give evidence that hidden order has degenerate E- type (yz,zx) symmetry where both components are realised. The oscillations have the wrong characteristics or are absent for the 1D nontrivial representations like quadrupolar B1 (x^2-y^2) and B2 (xy) type HO or hexadecapolar A2(xy(x^2-y^2)) type HO. Therefore they may be excluded as candidates for hidden order. We also predict the field-angular variation of possible field-induced Bragg peaks based on underlying E-type order parameter and discuss the expected elastic constant anomalies.

cond-mat.str-el↗

arXiv2011

Surface phonon propagation in topological insulators

The effect of helical Dirac states on surface phonons in a topological insulators is investigated. Their coupling is derived in the continuum limit by assuming displacement dependent Dirac cones. The resulting renormalisation of sound velocity and attenuation and its dependence on chemical potential and wave vector is calculated. At finite wave vectors a Kohn anomaly in the renormalized phonon frequency is caused by intraband-transitions. It appears at wave vectors q<2k_F due to a lack of backscattering for helical Dirac electrons. The wave vector and chemical potential dependence of this anomaly is calculated.

cond-mat.mtrl-sci↗

arXiv2010

Correlation Effects on Antiferromagnetism in Fe Pnictides

To investigate correlation effects on antiferromagnetic order in Fe pnictides, we apply a variational Monte Carlo method to a two-orbital model. We obtain a small ordered moment consistent with experimental observations even for a Coulomb interaction comparable to the band width. Studies of estimation of the Coulomb interaction for Fe pnictides suggest values comparable to or slightly smaller than the band width, and much larger ordered moments have been obtained by the Hartree-Fock approximation for such a large Coulomb interaction. Thus, the correlation effect is important for Fe pnictides at least quantitatively.

cond-mat.supr-con↗

arXiv2010

Low-temperature electrical resistivity in paramagnetic spinel LiV2O4

The 3d electron spinel compound LiV2O4 exhibits heavy fermion behaviour below 30K which is related to antiferromagnetic spin fluctuations strongly enhanced in an extended region of momentum space. This mechanism explains enhanced thermodynamic quantities and nearly critical NMR relaxation in the framework of the selfconsistent renormalization (SCR) theory. Here we show that the low-T Fermi liquid behaviour of the resistivity and a deviation from this behavior for higher T may also be understood within that context. We calculate the temperature dependence of the electrical resistivity ρ(T) assuming that two basic mechanisms of the quasiparticle scattering, resulting from impurities and spin-fluctuations, operate simultaneously at low temperature. The calculation is based on the variational principle in the form of a perturbative series expansion for ρ(T). A peculiar behavior of ρ(T) in LiV2O4 is related to properties of low-energy spin fluctuations whose T-dependence is obtained from SCR theory.

cond-mat.str-el↗