arXiv subjects

H. Keller

Publications and source records attributed to H. Keller.

At least 73 records · Page 4Linked to original sources

arXiv2009

Anomalous asymmetry of the Fermi surface in the YBa2Cu4O8 high temperature superconductor revealed by Angle Resolved Photoemission Spectroscopy

We use microprobe Angle-Resolved Photoemission Spectroscopy to study the Fermi surface and band dispersion of the CuO2 planes in the high temperature superconductor, YBa2Cu4O8. We find a strong in-plane asymmetry of the electronic structure between directions along a and b axes. The saddle point of the antibonding band lies at a significantly higher energy in the a direction (pi,0) than the b direction (0,pi), whereas the bonding band displays the opposite behavior. We demonstrate that the abnormal band shape is due to a strong asymmetry of the bilayer band splitting, likely caused by a non-trivial hybridization between the planes and chains. This asymmetry has an important implication for interpreting key properties of the Y-Ba-Cu-O (YBCO) family, especially the superconducting gap, transport and results of inelastic neutron scattering.

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arXiv2009

Coexistence of incommensurate magnetism and superconductivity in Fe_{1+y}Se_xTe_{1-x}

We report an investigation into the superconducting and magnetic properties of Fe_{1+y}Se_{x}Te_{1-x} single crystals by magnetic susceptibility, muon spin rotation, and neutron diffraction. We find three regimes of behavior in the phase diagram for 0\leq x\leq 0.5: (i) commensurate magnetic order for x< 0.1, (ii) bulk superconductivity for $x\lesssim 0.1$, and (iii) a range \sim 0.25\leq x\leq 0.45 in which superconductivity coexists with static incommensurate magnetic order. The results are qualitatively consistent with a two-band mean-field model in which itinerant magnetism and extended s-wave superconductivity are competing order parameters.

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arXiv2009

Comment on ``Muon-spin rotation studies of the superconducting properties of Mo_3Sb_7'', Phys.Rev.B 78, 172505 (2008)

In a recent article Tran et al. [Phys. Rev.B 78, 172505 (2008)] report on the result of the muon-spin rotation (\muSR) measurements of Mo_3Sb_7 superconductor. Based on the analysis of the temperature and the magnetic field dependence of the Gaussian relaxation rate σ_{sc} they suggest that Mo_3Sb_7 is the superconductor with two isotropic s-wave like gaps. An additional confirmation was obtained from the specific heat data published earlier by partly the same group of 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 Mo_3Sb_7 can not be justified. The analysis of \muSR data does not account for the reduction of σ_{sc} with increasing temperature, and, hence, yields inaccurate information on the magnetic penetration depth. The specific heat data can be satisfactory described within the framework of the one-gap model with the small residual specific heat component. The experimental data of Tran et al., as well as our earlier published \muSR data [Phys. Rev. B 78, 014502 (2008)] all seem to be consistent with is the presence of single isotropic superconducting energy gap in Mo_3Sb_7.

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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.

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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.

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arXiv2009

Muon-spin rotation measurements of the penetration depth of the Mo_3Sb_7 superconductor

Measurements of the magnetic field penetration depth λin superconductor Mo_3Sb_7 (T_c~2.1 K) were carried out by means of muon-spin-rotation. The absolute values of λ, the Ginzburg-Landau parameter κ, the first H_{c1} and the second H_{c2} critical fields at T=0 are λ(0)=720(100)nm, κ(0)=55(9), μ_0H_{c1}(0)=1.8(3)mT, and μ_0H_{c2}(0)=1.9(2)T. The zero temperature value of the superconducting energy gap Δ(0) was found to be 0.35(1)meV corresponding to the ratio 2Δ(0)/k_BT_c=3.83(10). At low temperatures λ^{-2}(T) saturates and becomes constant below T~0.3T_c, in agreement with what is expected for s-wave BCS superconductors. Our results suggest that Mo_3Sb_7 is a BCS superconductor with the isotropic energy gap

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arXiv2009

Superconductivity at 23 K and Low Anisotropy in Rb-Substituted BaFe_2As_2 Single Crystals

Single crystals of Ba_{1-x}Rb_{x}Fe_2As_2 with x=0.05-0.1 have been grown from Sn flux and are bulk superconductors with T_c up to 23 K. The crystal structure was determined by X-ray diffraction analysis, and Sn is found to be incorporated for 9% Ba, shifted by 1.1 Angstroem away from the Ba site towards the (Fe_2As_2)-layers. The upper critical field deduced from resistance measurements is anisotropic with slopes of 7.1(3) T/K (H || ab-plane) and 4.2(2) T/K (H || c-axis), sufficiently far below T_c. The extracted upper critical field anisotropy of 3 close to T_c, is in good agreement with the estimate from magnetic torque measurements. This indicates that the electronic properties in the doped BaFe_2As_2 compound are significantly more isotropic than those in the LnFeAsO family. The in-plane critical current density at 5 K exceeds 10^6 A/cm^2, making Ba_{1-x}Rb_xFe_2As_2 a promising candidate for technical applications.

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arXiv2009

Towards the Theory of Isotope Effect of the London Penetration Depth in Cuprates

The expressions for the superfluid density have been discussed in both weak and strong coupling approaches. The numerical calculations of temperature and isotope composition dependencies have been performed for a number of High-Tc compounds. The tight-binding parameters and corresponding Fermi surfaces are taken in accord to the available photoemission data.

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arXiv2009

Evidence for two distinct anisotropies in the oxypnictide superconductors SmFeAsO_(0.8)F_(0.2) and NdFeAsO_(0.8)F_(0.2)

Single crystals of the oxypnictide superconductors SmFeAsO_(0.8)F_(0.2) and NdFeAsO_(0.8)F_(0.2) with T_c in the range of 44 K to 48 K were investigated by torque magnetometry. An analysis of the data in terms of a recently proposed model for the anisotropic magnetization in the superconducting state, treating the penetration depth anisotropy differently than the upper critical field anisotropy, provides evidence that in the oxypnictide superconductors two distinct anisotropies are present. As a result the penetration depth anisotropy differs significantly in magnitude and in temperature dependence from the upper critical field anisotropy, analogous to MgB_2 but with a reversed sign of slope. This scenario strongly suggests a new multi-band mechanism in the novel class of oxypnictide high-temperature superconductors.

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arXiv2009

Single crystals of LnFeAsO1-xFx (Ln=La, Pr, Nd, Sm, Gd) and Ba1-xRbxFe2As2: growth, structure and superconducting properties

A review of our investigations on single crystals of LnFeAsO1-xFx (Ln=La, Pr, Nd, Sm, Gd) and Ba1-xRbxFe2As2 is presented. A high pressure technique has been applied for the growth of LnFeAsO1-xFx crystals, while Ba1-xRbxFe2As2 crystals were grown using quartz ampoule method. Single crystals were used for electrical transport, structure, magnetic torque and spectroscopic studies. Investigations of the crystal structure confirmed high structural perfection and show less than full occupation of the (O, F) position in superconducting LnFeAsO1-xFx crystals. Resistivity measurements on LnFeAsO1-xFx crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba1 xRbxFe2As2 simply shifts to lower temperature. Critical current density for both compounds is relatively high and exceeds 2x109 A/m2 at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO1-xFx crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba1-xRbxFe2As2 crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO1-xFx. Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy (~20 meV).

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arXiv2009

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

A detailed analysis of muon-spin rotation ($μ$SR) 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 to extract parameters such as the magnetic penetration depth $λ$ and the coherence length $ξ$ from the experimental $μ$SR spectra were investigated. The analysis of the simulated $μ$SR spectra showed that at high magnetic fields there is a strong correlation between obtained $λ$ and $ξ$ for any value of the Ginzburg-Landau parameter $κ= λ/ξ$. The smaller the applied magnetic field is, the smaller is the possibility to find the correct value of $ξ$. A simultaneous determination of $λ$ and $ξ$ without any restrictions is very problematic, independent 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 $μ$SR spectra by means of a multi-component Gaussian fit, generally yields reliable values of $λ$ in the whole range of applied fields $ H_{c1} \ll H \lesssim H_{c2}$ ($H_{c1}$ and $H_{c2}$ are the first and second critical fields, respectively). These results are also relevant for the interpretation of small-angle neutron scattering (SANS) experiments of the vortex state in type-II superconductors.

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arXiv2009

Probing the Yb$^{3+}$ spin relaxation in Y$_{0.98}$Yb$_{0.02}$Ba$_{2}$Cu$_{3}$O$_{x}$ by Electron Paramagnetic Resonance

The relaxation of Yb$^{3+}$ in YBa$_{2}$Cu$_{3}$O$_{x}$ ($6<x<7$) was studied using Electron Paramagnetic Resonance (EPR). It was found that both electronic and phononic processes contribute to the Yb$^{3+}$ relaxation. The phononic part of the relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy ($\sim$500 K) optical phonons or an Orbach-like process via the excited vibronic levels of the Cu$^{2+}$ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping $x$=6.98, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below $T_{c}$. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible for the sharp decrease of Yb$^{3+}$ relaxation in YBa$_{2}$Cu$_{3}$O$_{6.98}$. It was shown that the electronic part of the Yb$^{3+}$ relaxation in the superconducting state follows the same temperature dependence as $^{63}$Cu and $^{17}$O nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates.

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arXiv2009

Probing superconductivity in MgB2 confined to magnetic field tuned cylinders by means of critical fluctuations

We report and analyze reversible magnetization measurements on a high quality MgB2 single crystal in the vicinity of the zero field transition temperature, T_c=38.83 K, at several magnetic fields up to 300 Oe, applied along the c-axis. Though MgB2 is a two gap superconductor our scaling analysis uncovers remarkable consistency with 3D-xy critical behavior, revealing that close to criticality the order parameter is a single complex scalar as in 4He. This opens up the window onto the exploration of the magnetic field induced finite size effect, whereupon the correlation length transverse to the applied magnetic field H_i applied along the i-axis cannot grow beyond the limiting magnetic length L_Hi, related to the average distance between vortex lines. We find unambiguous evidence for this finite size effect. It implies that in type II superconductors, such as MgB2, there is the 3D to 1D crossover line H_pi and xi denotes the critical amplitudes of the correlation lengths above and below T_c along the respective axis. Consequently, above H_pi(T) and T<T_c superconductivity is confined to cylinders with diameter L_Hi (1D). In contrast, above T_c the uncondensed pairs are confined to cylinders. Accordingly, there is no continuous phase transition in the (H,T)-plane along the H_c2-lines as predicted by the mean-field treatment.

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arXiv2009

Anisotropy of superconducting single crystal SmFeAsO_(0.8)F_(0.2) studied by torque magnetometry

Single crystals of the oxypnictide superconductor SmFeAsO_(0.8)F_(0.2) with T_c=45(1) K were investigated by torque magnetometry. The crystals of mass~0.1 mug were grown by a high-pressure cubic anvil technique. The use of a high-sensitive piezoresistive torque sensor made it possible to study the anisotropic magnetic properties of these tiny crystals. The anisotropy parameter was found to be field independent, but varies strongly with temperature ranging from 8 at T=T_c to 23 at T=0.4T_c. This unusual behavior signals unconventional superconductivity.

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arXiv2008

Zero-field superfluid density in d-wave superconductor evaluated from the results of muon-spin-rotation experiments in the mixed state

We report on measurements of the in-plane magnetic penetration λ_{ab} in the optimally doped cuprate superconductor (BiPb)_2(SrLa)_2CuO_6+δ(OP Bi2201) by means of muon-spin rotation (\muSR). We show that in unconventional $d-$wave superconductors (like OP Bi2201), \muSR experiments conducted in various magnetic fields allow to evaluate the zero-field magnetic penetration depth λ_0, which relates to the zero-field superfluid density in terms of ρ_s\proptoλ_0^-2.

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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.

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arXiv2008

3D-xy critical properties of YBa2Cu4O8 and magnetic field induced 3D to 1D crossover

We present reversible magnetization data of a YBa2Cu4O8 single crystal and analyze the evidence for 3D-xy critical behavior and a magnetic field induced 3D to 1D crossover. Remarkable consistency with these phenomena is observed in agreement with a magnetic field induced finite size effect, whereupon the correlation length transverse to the applied magnetic field cannot grow beyond the limiting magnetic length scale L_H. By applying the appropriate scaling form we obtain the zero-field critical temperature, the 3D to 1D crossover, the vortex melting line and the universal ratios of the related scaling variables. Accordingly there is no continuous phase transition in the (H,T)-plane along the H_c2-lines as predicted by the mean-field treatment.

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arXiv2008

Evidence for competition between the superconducting and the pseudogap state in (BiPb)_2(SrLa)_2CuO_{6+δ} from muon-spin rotation experiments

The in-plane magnetic penetration depth λ_{ab} in optimally doped (BiPb)_2(SrLa)_2CuO_{6+δ} (OP Bi2201) was studied by means of muon-spin rotation. The measurements of λ_{ab}^{-2}(T) are inconsistent with a simple model of a d-wave order parameter and a uniform quasiparticle weight around the Fermi surface. The data are well described assuming the angular gap symmetry obtained in ARPES experiments [Phys. Rev. Lett {\bf 98}, 267004 (2007)], where it was shown that the superconducting gap in OP Bi2201 exists only in segments of the Fermi surface near the nodes. We find that the remaining parts of the Fermi surface, which are strongly affected by the pseudogap state, do not contribute significantly to the superconducting condensate. Our data provide evidence that high temperature superconductivity and pseudogap behavior in cuprates are competing phenomena.

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