H. Keller

Jahn–Teller physics and high-Tc superconductivity

The discovery of high-temperature superconductivity in copper oxides was not accidental, but was based on the knowledge that the divalent copper ion, Cu2+, is one of the strongest Jahn–Teller ions. The Jahn–Teller effect is a consequence of the interplay between electronic degeneracy and coupling to the lattice, i.e. unconventional local electron–lattice interactions. The search for superconductivity in copper oxides was motivated by the idea that Jahn–Teller polaron formation could be a novel and much stronger glue for electron pairing than conventional Bardeen–Cooper–Schrieffer electron–phonon coupling. The consequences of these ideas are unconventional isotope effects and complex pairing symmetries related to multiband superconductivity, which are reviewed here.

Detection of charge transfer processes in Cr-doped SrTIO3 single crystals

An insulator-to-metal transition is observed in Cr-doped SrTiO3 single crystals upon extended exposure to a high electric field, namely, electroconditioning (EC). Electron paramagnetic resonance (EPR) and transport measurements under laser irradiation show anticorrelation between the Cr3+ EPR signal and the electrical current. This proves that the Cr3+ ions are responsible for the photocurrent that initiates the EC process. We observe the presence of Cr3+∕Cr4+ mixed valencies in the bulk in the conducting state. The EPR characterization of the spectra in the conducting state excludes the possibility of a Cr3+-oxygen vacancy complex in the bulk as a result of the EC.

Comparison of different methods for analyzing μSR line shapes in the vortex state of type-II superconductors

A detailed analysis of muon-spin rotation (μSR) spectra in the vortex state of type-II superconductors using different theoretical models is presented. Analytical approximations of the London and Ginzburg-Landau (GL) models, as well as an exact solution of the GL model were used. The limits of the validity of these models and the reliability for extracting parameters such as the magnetic penetration depth λ and the coherence length ξ from the experimental μSR spectra were investigated. The analysis of the simulated μSR spectra showed that at high magnetic fields there is a strong correlation between λ and ξ obtained for any value of the Ginzburg-Landau parameter κ = λ/ξ. The smaller the applied magnetic field, the smaller the possibility of finding the correct value of ξ. A simultaneous determination of λ and ξ without any restrictions is very problematic, regardless of the model used to describe the vortex state. It was found that for extreme type-II superconductors and low magnetic fields, the fitted value of λ is practically independent of ξ. The second-moment method frequently used to analyze μSR spectra by means of a multi-component Gaussian fit generally yields reliable values of λ over the whole range of applied fields [Formula: see text] (H(c1) and H(c2) are the first and second critical fields, respectively). These results are also relevant for the interpretation of small-angle neutron scattering experiments on the vortex state in type-II superconductors.

Hybrid paramagnon phonon modes at elevated temperatures in EuTiO3

EuTiO3 (ETO) has recently experienced a significant revival of interest because of its possible multiferroic properties, which are currently the focus of much research. Unfortunately, ETO is an unlikely candidate for enlarged multifunctionality since the mode softening—typical of ferroelectrics—remains incomplete, and the antiferromagnetic properties appear only at 5.5xa0K. However, a strong coupling between lattice and Eu spins exists and this leads to the appearance of a magnon–phonon-hybrid mode at elevated temperatures as evidenced by EPR, muon spin relaxation experiments and model predictions based on a coupled spin-polarizability Hamiltonian. This novel finding supports the notion of strong magneto-dielectric effects being realized in ETO and opens up new strategies in material design and technological applications.

Tuning the static spin-stripe phase and superconductivity in La2−xBaxCuO4 (x = 1/8) by hydrostatic pressure

Magnetization and muon spin rotation experiments were performed in La2−xBaxCuO4 (xxa0=xa01/8) as a function of hydrostatic pressure up to pxa0≃xa02.2xa0GPa. It was found that the magnetic volume fraction of the static stripe phase strongly decreases linearly with pressure, while the superconducting volume fraction increases by the same amount. This demonstrates competition between bulk superconductivity and static magnetic order in the stripe phase of La1.875Ba0.125CuO4 and that these phenomena occur in mutually exclusive spatial regions. The present results also reveal that the static spin-stripe phase still exists at pressures, where the long-range low-temperature tetragonal (LTT) structure is completely suppressed. This indicates that the long-range LTT structure is not necessary for stabilizing the static spin order in La1.875Ba0.125CuO4.

Correlation between the transition temperature and the superfluid density in BCS superconductor NbB2+x

The results of the muon-spin rotation experiments on BCS superconductors NbB2+x (x=0.2 and 0.34) are reported. Both samples, studied in the present work, exhibit rather broad transitions to the superconducting state, suggesting a distribution of the volume fractions with different transition temperatures (Tcs). By taking these distributions into account, the dependence of the inverse squared zero-temperature magnetic penetration depth (lambda 0 -2 ) on Tc was reconstructed for temperatures in the range 1.5 K 0 -2 was found to obey the power law dependence lambda 0 -2 [proportional]T c 3.1(1) which appears to be common for some families of BCS superconductors as, e.g., Al doped MgB2 and high-temperature cuprate superconductors as underdoped YBa2Cu3O7−delta.

Comment on “Muon-spin-rotation study of the superconducting properties of Mo_{3}Sb_{7}”

In a recent article, Tran et al. [Phys. Rev. B 78, 172505 (2008)] reported results from muon-spin-rotation (μSR) measurements of an Mo3Sb7 superconductor. Based on the analysis of the temperature and the magnetic field dependences of the Gaussian relaxation rate, σsc, it was suggested that Mo3Sb7 is a superconductor with two isotropic s-wavelike energy gaps. This relates to results found previously in the specific-heat measurements by several of the same authors in Acta Mater. 56, 5694 (2008). The purpose of this Comment is to point out that from the analysis made by Tran et al., the presence of two superconducting energy gaps in Mo3Sb7 cannot be justified. The analysis of μSR data does not account for the reduction in σsc with increasing temperature. The specific-heat data can be satisfactorily described within the framework of the one-gap model by assuming the presence of a small residual specific-heat component. The experimental data of Tran et al., as well as our earlier published μSR results [Phys. Rev. B 78, 014502 (2008)], all seem to be consistent with the presence of single isotropic superconducting energy gap in Mo3Sb7. n n© 2010 The American Physical Society