arXiv subjects

H. Keller

Publications and source records attributed to H. Keller.

At least 91 records · Page 5Linked to original sources

arXiv2008

Nodeless superconductivity in the infinite-layer electron-doped Sr_0.9La_0.1CuO_2 cuprate superconductor

We report on measurements of the in-plane magnetic penetration depth λ_{ab} in the infinite-layer electron-doped high-temperature cuprate superconductor Sr_0.9La_0.1CuO_2 by means of muon-spin rotation. The observed temperature and magnetic field dependences of λ_{ab} are consistent with the presence of a substantial s-wave component in the superconducting order parameter in good agreement with the results of tunneling, specific heat, and small-angle neutron scattering experiments.

cond-mat.supr-con↗

arXiv2007

Synthesis and Bulk Properties of Oxychloride Superconductor Ca2-xNaxCuO2Cl2

Polycrystalline samples and submillimeter size single crystals of Na-doped Ca2CuO2Cl2 have been synthesized under high pressure. A series of experiments showed that the Na content depends not only on the pressure during the synthesis but also on the synthesis temperature and time. From a comparison of the Na-CCOC data with those of structurally related La214 cuprate superconductors we concluded that chlorine at the apical site is less effective that oxygen in supplying charge carriers to the CuO2 plans. As a result, the coupling between the CuO2 planes is weakened, the transition temperature Tc is reduced and the anisotropic nature is enhanced.

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arXiv2007

Correlation between the transition temperature and the superfluid density in BCS superconductor NbB_2+x

The results of the muon-spin rotation experiments on BCS superconductors NbB_2+x (x = 0.2, 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 (T_c)'s. By taking these distributions into account, the dependence of the inverse squared zero-temperature magnetic penetration depth (λ_0^{-2}) on T_c was reconstructed for temperatures in the range 1.5K<T_c<8.0K. λ_0^{-2} was found to obey the power law dependence λ_0^{-2}\propto T_c^{3.1(1)} which appears to be common for some families of BCS superconductors as, {\it e.g.}, Al doped MgB_2 and high-temperature cuprate superconductors as underdoped YBa_2Cu_3O_{7-δ}.

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arXiv2007

Oxygen isotope effect on superconductivity and magnetism in Y_1-xPr_xBa_2Cu_3O_7-δ

Oxygen isotope (^{16}O/^{18}O) effects (OIE's) on the superconducting transition (T_c), the spin-glass ordering (T_g), and the antiferromagnetic ordering (T_N) temperatures were studied for Y_1-xPr_xBa_2Cu_3O_7-δas a function of Pr content (0.0\leq x \leq 1.0). The OIE on T_c increases with increasing x up to x\approx0.55, where superconductivity disappears. For decreasing x the OIE's on T_N and T_g increase down to x\approx 0.7 where antiferromagnetic order and down to x\approx0.3 where spin-glass behavior vanish, respectively. The OIE's on T_g and T_N are found to have {\it opposite signs} as compared to the OIE on T_c. All OIE's are suggested to arise from the isotope dependent mobility (kinetic energy) of the charge carriers.

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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.

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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.

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arXiv2007

On the interpretation of muon-spin-rotation experiments in the mixed state of type-II superconductors

We argue that claims about magnetic field dependence of the magnetic field penetration depth lambda, which were made on the basis of moun-spin-rotation studies of some superconductors, originate from insufficient accuracy of theoretical models employed for the data analysis. We also reanalyze some of already published experimental data and demonstrate that numerical calculations of Brandt [E.H. Brandt, Phys. Rev. B 68, 54506 (2003)] may serve as a reliable and powerful tool for the analysis of the data collected in experiments with conventional superconductors. Furthermore, one can use this approach in order to distinguish between conventional and unconventional superconductors. It is unfortunate that these calculations have practically never been employed for such analyses.

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arXiv2007

Spontaneous ferromagnetic spin ordering at the surface of La$_2$CuO$_4$

Magnetic properties of high purity stoichiometric La$_2$CuO$_4$ nanoparticles are systematically investigated as a function of particle size. Ferromagnetic single-domain spin clusters are shown to spontaneously form at the surface of fine grains as well as paramagnetic defects. Hysteresis loops and thermomagnetic irreversibility are observed in a wide temperature range $5 - 350$ K with the remnant moment and coercivity gradually decreasing with increasing temperature. Possible origins of the spontaneous surface ferromagnetic clusters and the relation of our data to the appearance of unusual magnetic phenomena and phase separation of doped cuprates are discussed.

cond-mat.str-el↗

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.

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arXiv2007

Study of the in-plane magnetic penetration depth in the cuprate superconductor Ca_2-xNa_xCuO_2Cl_2: role of the apical sites

A study of the in-plane magnetic penetration depth λ_ab in a series of the cuprate superconductors Ca_2-xNa_xCuO_2Cl_2 (Na-CCOC) with Na content x=0.11, 0.12, 0.15, 0.18, and 0.19 is reported. The zero temperature values of λ_ab(0) were obtained by means of the muon-spin rotation technique, as well as from measurements of the intrinsic susceptibility χ^int(0) by using the procedure developed by Kanigel et al. [Phys.Rev.B 71, 224511 (2005)]. λ_ab at T=0K was found to increase with decreasing doping from λ_ab(0)=316(19)nm for the x=0.19 sample to λ_ab(0)=430(26)nm for the x=0.11 one. From a comparison of the present Na-CCOC data with those of Bi2201 and La214 cuprate superconductors it is concluded that substitution of the apical oxygen by chlorine decreases the coupling between the superconducting CuO_2 planes, leading to an enhancement of the two-dimensional properties of Na-CCOC.

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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].

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arXiv2006

Two types of Hc2(T) dependences in Bi_2Sr_2Ca_(1-x)Y_xCu_2O_(8+delta) with different Yttrium content

We reanalyze the magnetization data collected on Bi_2Sr_2Ca_(1-x)Y_xCu_2O_(8+y) samples (Kim at al, Phys. Rev. B 72, 64525 (2005)) and argue that the method, which was used for the analysis of equilibrium magnetization data, is not adequate to the experimental situation. As a result, the temperature dependencies of the upper critical field Hc2(T) and the magnetic field penetration depth lambda (T), obtained in this work, are misinterpreted. Using a different approach to analysis, we demonstrate that the normalizedHc2(T) curves are rather different from those presented in the original publication and do not follow predictions of the Werthamer-Helfand-Hohenberg theory. Another important observation is that the Hc2(T) dependencies for two samples with different levels of doping are qualitatively different.

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arXiv2006

Temperature dependences of the upper critical field and the Ginzburg-Landau parameter of Li2Pd3B from magnetization measurements

We present temperature dependences of the upper critical magnetic field and the Ginzburg-Landau parameter for a ternary boride superconductor Li_2Pd_3B obtained from magnetization measurements. A specially developed scaling approach was used for the data analysis. The resulting H_c2(T) curve turns out to be surprisingly close to predictions of the BCS theory. The magnetic field penetration depth, evaluated in this work, is in excellent agreement with recent muon-spin-rotation experiments. We consider this agreement as an important proof of the validity of our approach.

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arXiv2006

Magnetic field dependence of the oxygen isotope effect on the magnetic penetration depth in hole-doped cuprate superconductors

The magnetic field dependence of the oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth λ_{ab} was studied in the hole-doped high-temperature cuprate superconductors YBa_2Cu_4O_8, Y_0.8Pr_0.2Ba_2Cu_3O_7-δ, and Y_0.7Pr_0.3Ba_2Cu_3O_7-δ. It was found that λ_ab for the ^{16}O substituted samples increases stronger with increasing magnetic field than for the ^{18}O ones. The OIE on λ_ab decreases by more than a factor of two with increasing magnetic field from μ_0H=0.2 T to μ_0H=0.6 T. This effect can be explained by the isotope dependence of the in-plane charge carrier mass m^\ast_{ab}.

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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.

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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.

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arXiv2006

Coexistence of superconductivity and incoherence in quasi 1D chains

The dimensionality of a correlated many-body system has a large impact on its electronic properties. When electrons are confined to one-dimensional chains of atoms their behavior is very different than in higher dimensional systems because they become strongly correlated, even in the case of vanishingly small interactions. The chains consisting of copper and oxygen atoms are particularly interesting, because the CuO orbitals are constituents of all known high temperature superconductors. Most of previous spectroscopic studies of CuO chain systems indicated insulating behavior[23-25]. Here we report the discovery of momentum dependent superconducting gap and hump-peak-dip structure in the spectra of the CuO chains. We demonstrate that superconductivity in the chains arises due to proximity effects and the peculiar momentum dependence of the superconducting gap shows how three dimensional coherence emerges in a layered superconductor. The presence of the hump-dip-peak structure in the spectra of the CuO chains is very unexpected as it was thought to only occur in the CuO2 planes and is frequently considered to be a signature of the d-wave pairing and the pairing boson itself.

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arXiv2006

Dual character of the electronic structure in YBa2Cu4O8: conduction bands of CuO2 planes and CuO chains

We use microprobe Angle-Resolved Photoemission Spectroscopy (muARPES) to separately investigate the electronic properties of CuO2 planes and CuO chains in the high temperature superconductor, YBa2Cu4O8. In the CuO2 planes, a two dimensional (2D) electronic structure with nearly momentum independent bilayer splitting is observed. The splitting energy is 150 meV at (pi,0), almost 50% larger than in Bi2Sr2CaCu2O(8+d) and the electron scattering at the Fermi level in the bonding band is about 1.5 times stronger than in the antibonding band. The CuO chains have a quasi one dimensional (1D) electronic structure. We observe two 1D bands separated by ~ 550meV: a conducting band and an insulating band with an energy gap of ~ 240meV. We find that the conduction electrons are well confined within the planes and chains with a non-trivial hybridization.

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