arXiv subjects

A. Bussmann-Holder

Publications and source records attributed to A. Bussmann-Holder.

36 records · Page 2Linked to original sources

arXiv2009

Unconventional dynamical covalency driven superconductivity in Nb doped SrTiO3

Nb doped SrTiO3, the first discovered two-gap superconductor, is shown to be the most unconventional one of the known multiband superconductors, since the smaller of the two superconducting gaps follows a non BCS temperature dependence. Such a behavior stems from two cooperating effects: an extreme anisotropy in the frequency dependent interactions, involving one very soft mode and an almost vanishing interband interaction. In contrast to all other multiband superconductors, the temperature dependence of the superfluid density of Nb doped SrTiO3 is predicted to exhibit an inflection point close to Tc and not close to T=0.

cond-mat.supr-con↗

arXiv2009

Identifying the pairing mechanism in Fe-As based superconductors: gaps and isotope effects

The temperature dependencies of the coupled superconducting gaps, observed in Fe-As based superconducting compounds is calculated and a universal temperature scaling observed which is only present if the coupled order parameters both have s-wave symmetry. Predictions for possible isotope effects on the transition temperature are made if phonons are involved in the pairing or polaronic effects are of importance. Comparison to experimental data is given where these are available.

cond-mat.supr-con↗

arXiv2009

Superconductivity in Fe and As based compounds: a bridge between MgB2 and cuprates

By interpreting various experimental data for the new high temperature FeAs type superconductors in terms of lattice mediated multi gap superconductivity, it is shown that these systems strongly resemble MgB2, however, with the distinction that local polaronic lattice effects exist. This fact establishes a connection to cuprate high temperature superconductors where polaron formation is essential for the pseudogap phase and the unconventional isotope effects observed there. However, similar to MgB2 and in contrast to cuprates the two superconducting gaps in the Fe-As based materials are isotropic s-wave gaps.

cond-mat.supr-con↗

arXiv2008

Polaron Coherence as Origin of the Pseudogap Phase in High Temperature Superconducting Cuprates

Within a two component approach to high Tc copper oxides including polaronic couplings, we identify the pseudogap phase as the onset of polaron ordering. This ordering persists in the superconducting phase. A huge isotope effect on the pseudogap onset temperature is predicted and in agreement with experimental data. The anomalous temperature dependence of the mean square copper oxygen ion displacement observed above, at and below Tc stems from an s-wave superconducting component of the order parameter, whereas a pure d-wave order parameter alone can be excluded.

cond-mat.supr-con↗

arXiv2007

Oxygen-isotope effect on the superconducting gap in the cuprate superconductor Y_{1-x}Pr_xBa_2Cu_3O_{7-δ}

The oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the zero-temperature superconducting energy gap Δ_0 was studied for a series of Y_{1-x}Pr_xBa_2Cu_3O_{7-δ} samples (0.0\leq x\leq0.45). The OIE on Δ_0 was found to scale with the one on the superconducting transition temperature. These experimental results are in quantitative agreement with predictions from a polaronic model for cuprate high-temperature superconductors and rule out approaches based on purely electronic mechanisms.

cond-mat.supr-con↗

arXiv2007

Universal observation of multiple order parameters in cuprate superconductors

The temperature dependence of the London penetration depth λwas measured for an untwined single crystal of YBa_2Cu_3O_{7-δ} along the three principal crystallographic directions (a, b, and c). Both in-plane components (λ_a and λ_b) show an inflection point in their temperature dependence which is absent in the component along the c-direction (λ_c). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of s+d-wave symmetry whereas it is exclusively s-wave along the c-direction. In conjunction with previous results it is concluded that coupled s+d-order parameters are universal and intrinsic to cuprate superconductors.

cond-mat.supr-con↗

arXiv2007

s-wave symmetry along the c-axis and s+d in-plane superconductivity in bulk YBa_2Cu_4O_8

To clarify the order parameter symmetry of cuprates, the magnetic penetration depth $λ$ was measured along the crystallographic directions $a$, $b$, and $c$ in single crystals of YBa$_2$Cu$_4$O$_8$ via muon spin rotation. This method is direct, bulk sensitive, and unambiguous. The temperature dependences of $λ_a^{-2}$ and $λ_b^{-2}$ exhibit an inflection point at low temperatures as is typical for two-gap superconductivity (TGS) with $s+d-$wave character in the planes. Perpendicular to the planes a pure s-wave gap is observed thereby highlighting the important role of c-axis effects. We conclude that these are generic and universal features in the bulk of cuprates.

cond-mat.supr-con↗

arXiv2006

Comment on: Role of inelastic tunneling through the insulating barrier in scanning-tunneling-microscope experiments on cuprate superconductors

In a recent letter [1] Pilgram, Rice and Sigrist (PRS) propose to perform oxygen isotope dependent conductivity measurements along the c-axis of the cuprate superconductor BSSCO. In Raman isotope data for vibrations along the c-axis from 1988 [2], and in site selective susceptibility experiments determining Tc, no isotope effects were observed for oxygen ions in the insulating layer [3]. Based on these earlier not by PRS quoted experiments, we argue that the suggestion made in Ref. 1 is not worth to be undertaken, and if it is carried through, no effect will be seen. From the meanwhile 12 years old site selective data [3] the observed isotope effects stem to nearly 100% from oxygen ions in the CuO2-planes. These results evidence polaronic behavior in these planes [4].

cond-mat.supr-con↗

arXiv2006

Coexistence of polar order and local domain dynamics in ferroelectric SrTi18O3

Perovskite oxide ferroelectrics show classical soft mode behaviour typical for the onset of a homogeneous long-range polar state and a displacive phase transition. Besides these long wave length properties, local effects are observed by different probes which reveal that dynamical symmetry breaking already takes place already far above the actual instability. It is shown here that displacive mean-field type dynamics can indeed coexist with local dynamical symmetry breaking.

cond-mat.other↗

arXiv2006

Mixed order parameter symmetries in cuprate superconductors

The recent observation of an inflection point in the temperature dependence of the in-plane magnetic field dependence (lambda_ab) is investigated within a two-band model with coupled order parameters of different symmetries. While the dominant order parameter has d-wave symmetry, the smaller one is of s-wave symmetry. Superconductivity is robust in the d-wave channel and induced via interband interactions in the s-wave subsystem.

cond-mat.supr-con↗

arXiv2006

Real and marginal isotope effects in cuprate superconductors

We critically review recent and earlier results on isotope effects in cuprate superconductors and emphasize that the sample preparation and the isotope exchange and back exchange are crucial in understanding and interpreting the data. Only extremely careful preparation techniques yield reliable results and permit differentiation between real isotope effects and marginal ones. The former are substantial and highlight the lattice vibrational importance in cuprate superconductors.

cond-mat.supr-con↗

arXiv2006

Evidence for complex order parameter in La_{1.83}Sr_{0.17}CuO_4

The in-plane magnetic field penetration depth (λ_{ab}) in single-crystal La_{1.83}Sr_{0.17}CuO_4 was investigated by means of the muon-spin rotation (\muSR) technique. The temperature dependence of λ^{-2}_{ab} has an inflection point around 10-15K, suggesting the presence of two superconducting gaps: a large gap (Δ_1^d) with d-wave and a small gap (Δ_2^s) with s-wave symmetry. The zero-temperature values of the gaps at μ_0H=0.02T were found to be Δ_1^d(0)=8.2(2)meV and Δ_2^s(0)=1.57(8)meV.

cond-mat.supr-con↗

arXiv2004

Polaron formation as origin of unconventional isotope effects in cuprate superconductors

Various unconventional isotope effects have been reported in high-temperature superconducting copper oxides which are beyond the scheme of BCS theory. Their origin is investigated within polaron theory which leads to a renormalization of the single particle energies and introduces a level shift here. It is found that the exponential squeezing of the second nearest neighbour hopping integral carries the correct isotope effect on the superconducting transition temperature Tc, as well as the one on the penetration depth. The average superconducting gap is predicted to have an isotope effect comparable to the one on the penetration depth. The results imply that the coupling of the electronic degrees of freedom to the Jahn-Teller Q2-type mode is the origin of these isotope effects.

cond-mat.supr-con↗

arXiv2004

Spin and lattice effects in the Kondo lattice model

The magnetic properties of a system of coexisting localized spins and conduction electrons are investigated within an extended version of the one dimensional Kondo lattice model in which effects stemming from the electron-lattice and on-site Coulomb interactions are explicitly included. After bosonizing the conduction electrons, is it observed that intrinsic inhomogeneities with the statistical scaling properties of a Griffiths phase appear, and determine the spin structure of the localized impurities. The appearance of the inhomogeneities is enhanced by appropriate phonons and acts destructively on the spin ordering. The inhomogeneities appear on well defined length scales and can be compared to the formation of intrinsic mesoscopic metastable patterns which are found in two-fluid phenomenologies.

cond-mat.str-el↗

arXiv2003

Superconductivity in a two-component model with local electron pairs

Superconductivity in the two component model of coexisting local electron pairs (hard-core charged bosons) and itinerant fermions coupled via charge exchange mechanism is discussed. The cases of isotropic s-wave and anisotropic pairing of extended s-wave and d_{x^2-y^2} symmetries are analyzed for a 2D square lattice within the BCS-mean field approximation and the Kosterlitz-Thouless theory. The phase diagrams and superconducting characteristics of this induced pairing model as a function of the position of the local pair (LP) level and the total carrier concentration are determined. The model exhibits several types of interesting crossovers between the BCS like behavior and that of LP's. In addition, the Uemura plots are obtained for extended s and d_{x^2-y^2} pairing symmetries. Finally, we analyze the pairing fluctuation effects (in 3D) within a generalized T-matrix approach. Some of our results are discussed in connection with a two-component scenario of preformed pairs and unpaired electrons for high temperature superconductors.

cond-mat.supr-con↗

arXiv2003

Competing interactions of spin and lattice in the Kondo lattice model

The magnetic properties of a system of coexisting localized spins and conduction electrons are investigated within an extended version of the one dimensional Kondo lattice model in which effects stemming from the electron-lattice and on-site Coulomb interactions are explicitly included. After bosonizing the conduction electrons, is it observed that intrinsic inhomogeneities with the statistical scaling properties of a Griffiths phase appear, and determine the spin structure of the localized impurities. The appearance of the inhomogeneities is enhanced by appropriate phonons and acts destructively on the spin ordering. The inhomogeneities appear on well defined length scales, can be compared to the formation of intrinsic mesoscopic metastable patterns which are found in two-fluid systems.

cond-mat.str-el↗

arXiv2002

On the Superconductivity in the Induced Pairing Model

The two component model of coexisting local electron pairs and itinerant fermions coupled via charge exchange mechanism, which mutually induces superconductivity in both subsystems, is discussed. The cases of isotropic s-wave and anisotropic pairing of extended s and d_{x^2-y^2} -wave symmetries are analyzed for a 2D square lattice within the BCS-mean field approximation and the Kosterlitz-Thouless theory. We determined the phase diagrams and superconducting characteristics as a function of the position of the local pair (LP) level and the total electron concentration. The model exhibits several types of interesting crossovers from BCS like behavior to that of LP's. Some of our results are discussed in connection with a two-component scenario of preformed pairs and unpaired electrons for exotic superconductors.

cond-mat.supr-con↗

arXiv2000

Interplay between proton ordering and ferroelectric polarization in H-bonded KDP-type crystals

The origin of ferroelectricity in KH_2PO_4 (KDP) is studied by first-principles electronic structure calculations. In the low-temperature phase, the collective off-center ordering of the protons is accompanied by an electronic charge delocalization from the "near" and localization at the "far" oxygen within the O-H...O bonds. Electrostatic forces, then, push the K+ ions towards off-center positions, and induce a macroscopic polarization. The analysis of the correlation between different geometrical and electronic quantities, in connection with experimental data, supports the idea that the role of tunnelling in isotopic effects is irrelevant. Instead, geometrical quantum effects appear to play a central role.

cond-mat.mtrl-sci↗