arXiv subjects

Peter Thalmeier

Publications and source records attributed to Peter Thalmeier.

At least 73 records · Page 4Linked to original sources

arXiv2010

Nonadiabatic effects of rattling phonons and 4f excitations in Pr(Os{1-x}Ru{x})4Sb12

In the skutterudite compounds the anharmonic 'rattling' oscillations of 4f-host ions in the surrounding Sb12 cages are found to have significant influence on the low temperature properties. Recently specific heat analysis of Pr(Os{1-x}Ru{x})4Sb12 has shown that the energy of crystalline electric field (CEF) singlet-triplet excitations increases strongly with Ru-concentration x and crosses the almost constant rattling mode frequency $ω_0$ at about x ~ 0.65. Due to magnetoelastic interactions this may entail prominent nonadiabatic effects in inelastic neutron scattering (INS) intensity and quadrupolar susceptibility. Furthermore the Ru- concentration dependence of the superconducting Tc, notably the minimum at intermediate x is explained as a crossover effect from pairforming aspherical Coulomb scattering to pairbreaking exchange scattering.

cond-mat.str-el↗

arXiv2010

Paramagnetic Acoustic Faraday Rotation in Tb3Ga5O12

The acoustic Faraday rotation in the 4f paramagnet Tb3Ga5O12 has recently been observed by Sytcheva et al (arXiv:1006.0141). As in earlier examples the rotation angle per unit length of transverse acoustic modes was found to depend linearly on sound frequency. Existing theories for this effect consistently require that it should vary with the square of the frequency. In the present work a solution for this long-standing problem is provided. We propose a model based on magnetoelastic interactions with 4f quadrupole moments that includes both acoustic and optical phonons. The symmetry allows a direct and induced coupling between the latter. This leads to an indirect acoustic Faraday rotation via the field induced splitting of doubly degenerate optical phonon modes. It varies linearly with frequency in accordance with experiment and dominates the rotation angle. It also explains the observed resonance of the rotation angle in the field range between 17-20 T. The mechanism is of general validity for non-Bravais lattices and applies to previous examples of the acoustic Faraday effect.

cond-mat.str-el↗

arXiv2010

Frustrated local moment models for Fe-pnictide magnetism

The low energy spin excitations of the Fe pnictide parent compounds have been determined by inelastic neutron scattering and interpreted within the local moment J_1a,b-J_2 Heisenberg model with orthorhombic symmetry. This has led to alternative exchange models that strongly differ in the size of anisotropy. Although the compounds are itinerant the localised spin model can explain basic features of the excitations. The inherent frustration of this model leads to quantum fluctuations and possible moment reduction. We investigate this question in detail using spin wave approximation and partly exact diagonalisation Lanczos calculations for finite clusters. We find that the orthorhombic anisotropy stabilizes the columnar AF phase and its moment. For the exchange models proposed from inelastic neutron scattering we can exclude a strong influcence of frustration on the moment size. We also investigate dependence of magnetisation and susceptibility on field and temperature.

cond-mat.str-el↗

arXiv2009

Magnetic impurity resonance states and symmetry of the superconducting order parameter in iron-based superconductors

We investigate the effect of magnetic impurities on the local quasiparticle density of states (LDOS) in iron-based superconductors. Employing the two-orbital model where 3$d$ electron and hole conduction bands are hybridizing with the localized $f$-orbital of the impurity spin, we investigate how various symmetries of the superconducting gap and its nodal structure influence the quasiparticle excitations and impurity bound states. We show that the bound states behave qualitatively different for each symmetry. Most importantly we find that the impurity-induced bound states can be used to identify the nodal structure of the extended s-wave symmetry ($S^{\pm}$) that is actively discussed in ferropnictides.

cond-mat.supr-con↗

arXiv2009

Surface acoustic wave propagation in monolayer graphene

Surface acoustic wave (SAW) propagation is a powerful method to investigate 2D electron systems. We show how SAW observables are influenced by coupling to the 2D massless Dirac electrons of graphene and argue that Landau oscillations can be observed as function of gate voltage for constant field. Contrary to other transport measurements, the zero-field SAW propagation gives the wave vector dependence of graphene conductivity for small wave numbers. We predict a crossover from Schroedinger to Dirac like behaviour as a function of gate voltage, with no attenuation in the latter for clean samples.

cond-mat.mes-hall↗

arXiv2009

Triplet Cooper Pair Formation by Anomalous Spin Fluctuations in Non-centrosymmetric Superconductors

A microscopic theory for the spin triplet Cooper pairing in non-centrosymmetric superconductors like CePt_3Si and CeTSi_3 (T=Rh, Ir) is presented. The lack of inversion symmetry leads to new anomalous spin fluctuations which stabilize the triplet part in addition to the singlet part originating from the centrosymmetric spin fluctuations. It is shown that both parts have similar nontrivial momentum dependence of A_1 type. Therefore the mixed singlet-triplet gap function has accidental line nodes on both Fermi surface sheets which are stable as function of temperature. This gap function explains the salient features of CePt_3Si and CeTSi_3 superconductors.

cond-mat.supr-con↗

arXiv2009

The two-dimensional frustrated Heisenberg model on the orthorhombic lattice

We discuss new high-field magnetization data recently obtained by Tsirlin et al. for layered vanadium phosphates in the framework of the square-lattice model. Our predictions for the saturation fields compare exceptionally well to the experimental findings, and the strong bending of the curves below saturation agrees very well with the experimental field dependence. Furthermore we discuss the remarkably good agreement of the frustrated Heisenberg model on the square lattice in spite of the fact that the compounds described with this model actually have a lower crystallographic symmetry. We present results from our calculations on the thermodynamics of the model on the orthorhombic (i.e., rectangular) lattice, in particular the temperature dependence of the magnetic susceptibility. This analysis also sheds light on the discussion of magnetic frustration and anisotropy of a class of iron pnictide parent compounds, where several alternative suggestions for the magnetic exchange models were proposed.

cond-mat.str-el↗

arXiv2009

Multiorbital Effects on Antiferromagnetism in Fe Pnictides

We apply a Hartree-Fock approximation to a two-orbital model proposed for Fe pnictide superconductors. It is found that the antiferromagnetic (AFM) order with the ordering vector Q=(pi,0) is realized. The AFM order appears simultaneously with ferro-orbital order, the latter leads to a secondary lattice distortion. We also investigate the influence of doping on the AFM order. The size of the AFM moment changes continuously for lightly doped cases, but when the amount of doped carriers exceeds a certain value the AFM state is suddenly destroyed. We also show that Fermi surfaces remain and change significantly on doping even in the AFM state. This behaviour is explained by considering the nesting due to the multi-sheet Fermi-surface structure and multiorbital nature of the electronic bands characteristic to Fe pnictides.

cond-mat.supr-con↗

arXiv2009

Bound state of 4$f$-excitation and magnetic resonance in unconventional superconductors

We analyze the influence of unconventional superconductivity on crystalline electric field (CEF) excitations of rare earth ions in novel superconductors. We show that resonant magnetic excitations of the conduction electrons that have been observed in these systems below T$_c$ may result in the formation of the bound state in the 4$f$-electron susceptibility. This occurs at energies well below the CEF excitation energy. The effect is discussed as a function of temperature and the strength of the coupling between the 4$f$ and $d$-electrons. We argue that these effects may be present in the layered cuprates and ferropnictides which contain rare-earth ions.

cond-mat.supr-con↗

arXiv2009

Theory of Spin Exciton in the Kondo Semiconductor $Yb B_{12}$

The Kondo semiconductor $Yb B_{12}$ exhibits a spin and charge gap of approximately 15 meV. Close to the gap energy narrow dispersive collective excitations were identified by previous inelastic neutron scattering experiments. We present a theoretical analysis of these excitations. Starting from a periodic Anderson model for crystalline electric field (CEF) split 4f states we derive the hybridized quasiparticle bands in slave boson mean-field approximation and calculate the momentum dependent dynamical susceptibility in random phase approximation (RPA). We show that a small difference in the hybridization of the two CEF (quasi-) quartets leads to the appearance of two dispersive spin resonance excitations at the continuum threshold. Their intensity is largest at the antiferromagnetic (AF) zone boundary point and they have an upward dispersion which merges with the continuum less than halfway into the Brillouin zone. Our theoretical analysis explains the most salient features of previously unexplained experiments on the magnetic excitations of $Yb B_{12}$.

cond-mat.str-el↗

arXiv2008

Local density of states in disordered graphene

We study two lattice models, the honeycomb lattice (HCL) and a special square lattice (SQL), both reducing to the Dirac equation in the continuum limit. In the presence of disorder (gaussian potential disorder and random vector potential), we investigate the behaviour of the density of states (DOS) numerically and analytically. While an upper bound can be derived for the DOS on the SQL at the Dirac point, which is also confirmed by numerical calculations, no such upper limit exists for the HCL in the presence of random vector potential. A careful investigation of the lowest eigenvalues indeed indicate, that the DOS can possibly be divergent at the Dirac point on the HCL. In spite of sharing a common continuum limit, these lattice models exhibit different behaviour.

cond-mat.mes-hall↗

arXiv2008

Superconductivity in $β$-pyrochlore superconductor KOs$_{2}$O$_{6}$: treatment within strong-coupling Eliashberg theory

We study the influence of the rattling phonons on superconductivity in $β$-pyrochlore KOs$_{2}$O$_{6}$ compound based on the strong-coupling Eliashberg approach. In particular, analyzing the specific heat data we find that the rattling phonon frequency changes discontinuously at the critical temperature of the first order phase transition. Solving the strong-coupling Eliashberg equations with temperature dependent $α^{2}F(ω)$, we investigate the consequence of this first order phase transition for the anomalous temperature dependence of the superconducting gap. We discuss our results in context of the recent experimental data.

cond-mat.supr-con↗

arXiv2008

Coexistence of CDW with staggered superconductivity in a ferromagnetic material

In usual superconductivity (SC), the pairs have zero total momentum irrespective of their symmetry. Staggered SC would involve, instead, pairs with a finite commensurate total momentum, but such exotic states have never been proven to be realized in nature. Here we study for the first time the influence of particle-hole asymmetry on the competition of staggered SC with Charge Density Waves (CDW) in a ferromagnetic medium. We obtain unprecedented situations in which CDW and staggered SC coexist. We also obtain cases of a SC dome near the collapse of a CDW state as well as cascades of transitions that exhibit remarkable similarities with the pressure phase diagram in UGe$_2$ suggesting that SC in this material may be staggered coexisting and competing with a CDW state.

cond-mat.supr-con↗

arXiv2008

Theory of induced quadrupolar order in tetragonal YbRu_{2}Ge_{2}

The tetragonal compound YbRu$_{2}$Ge$_{2}$ exhibits a non-magnetic transition at $T_0$=10.2K and a magnetic transition at $T_1$=6.5K in zero magnetic field. We present a model for this material based on a quasi-quartet of Yb$^{3+}$ crystalline electric field (CEF) states and discuss its mean field solution. Taking into account the broadening of the specific heat jump at $T_0$ for magnetic field perpendicular to [001] and the decrease of $T_0$ with magnetic field parallel to [001], it is shown that ferro-quadrupole order of either O$_{2}^{2}$ or O$_{\rm xy}$ - type are prime candidates for the non-magnetic transition. Considering the matrix element of these quadrupole moments, we show that the lower CEF states of the level scheme consist of a $Γ_{6}$ and a $Γ_{7}$ doublet. This leads to induced type of O$_{2}^{2}$ and O$_{\rm xy}$ quadrupolar order parameters. The quadrupolar order introduces exchange anisotropy for planar magnetic moments. This causes a spin flop transition at low fields perpendicular [001] which explains the observed metamagnetism. We also obtain a good explanation for the temperature dependence of magnetic susceptibility and specific heat for fields both parallel and perpendicular to the [001] direction.

cond-mat.str-el↗

arXiv2008

Eliashberg theory of superconductivity and inelastic rare-earth impurity scattering in filled skutterudite La$_{1-x}$Pr$_{x}$Os$_{4}$Sb$_{12}$

We study the influence of inelastic rare-earth impurity scattering on electron-phonon mediated superconductivity and mass renormalization in (La$_{1-x}$Pr$_{x}$)Os$_{4}$Sb$_{12}$ compounds. Solving the strong coupling Eliashberg equations we find that the dominant quadrupolar component of the inelastic scattering on Pr impurities yields an enhancement of the superconducting transition temperature T$_c$ in LaOs$_{4}$Sb$_{12}$ and increases monotonically as a function of Pr concentration. The calculated results are in good agreement with the experimentally observed T$_c (x)$ dependence. Our analysis suggests that phonons and quadrupolar excitations cause the attractive electron interaction which results in the formation of Cooper pairs and singlet superconductivity in PrOs$_{4}$Sb$_{12}$.

cond-mat.supr-con↗

arXiv2007

Reentrant Kondo effect in Landau quantized graphene

We have studied the interplay of an Anderson impurity in Landau quantized graphene, with special emphasis on the influence of the chemical potential. Within the slave-boson mean-field theory, we found reentrant Kondo behaviour by varying the chemical potential or gate voltage. Between Landau levels, the density of states is suppressed, and by changing the graphene's Fermi energy, we cross from metallic to semiconducting regions. Hence, the corresponding Kondo behaviour is also influenced. The f-level spectral function reveals both the presence of Landau levels in the conduction band and the Kondo resonance.

cond-mat.str-el↗

arXiv2007

Magnetotransport and thermoelectricity in disordered graphene

We have studied the electric and thermal response of two-dimensional Dirac-fermions in a quantizing magnetic field in the presence of localized disorder. The electric and heat current operators in the presence of magnetic field are derived. The self-energy due to impurities is calculated self-consistently, and depends strongly on the frequency and field strength, resulting in asymmetric peaks in the density of states at the Landau level energies, and small islands connecting them. The Shubnikov-de Haas oscillations remain periodic in 1/B, in spite of the distinct quantization of quasiparticle orbits compared to normal metals. The Seebeck coefficient depends strongly on the field strength and orientation. For finite field and chemical potential, the Wiedemann-Franz law can be violated.

cond-mat.str-el↗

arXiv2006

Theory of tunneling spectroscopy in UPd2Al3

There is still significant debate about the symmetry of the order parameter in the heavy-fermion superconductor UPd$_{2}$Al$_{3}$, with proposals for cos(k3), cos(2k3), sin(k3),and exp(iϕ) sin(k3). Here we analyze the tunneling spectroscopy of this compound and demonstrate that the experimental results by Jourdan et al are inconsistent with the last two order parameters, which are expected to show zero-bias conductance peaks. We propose a definitive tunneling experiment to distinguish between the first two order parameters.

cond-mat.supr-con↗