Effect of Incommensurate Charge-Density Wave Scattering on the Electronic Structure of High-Tc Cuprates

We argue that the collective mode as observed in angle resolved photoemission spectroslopy (ARPES) on a large class of cuprates can be associated with dynamic incommensurate CDW fluctuations present in these materials. This scenario is substantiated by a comparison of calculated spectra with experimental ARPES data where we obtain a mode frequency that decreases toward optimal doping, thus strongly supporting the existence of a quantum critical point around this concentration. Moreover, we extract the temperature dependence of the associated bosonic spectrum from ARPES data, where it turns out that there is a continuous evolution from mode-type behavior below T _c to a marginal Fermi liquid structure well above T _c.     

Effect of Incommensurate Charge-Density Wave Scattering on the Electronic Structure of High-Tc Cuprates

We argue that the collective mode as observed in angle resolved photoemission spectroslopy (ARPES) on a large class of cuprates can be associated with dynamic incommensurate CDW fluctuations present in these materials. This scenario is substantiated by a comparison of calculated spectra with experimental ARPES data where we obtain a mode frequency that decreases toward optimal doping, thus strongly supporting the existence of a quantum critical point around this concentration. Moreover, we extract the temperature dependence of the associated bosonic spectrum from ARPES data, where it turns out that there is a continuous evolution from mode-type behavior below T _c to a marginal Fermi liquid structure well above T _c.     

Effect of Incommensurate Charge-Density Wave Scattering on the Electronic Structure of High-Tc Cuprates

We argue that the collective mode as observed in angle resolved photoemission spectroscopy (ARPES) on a large class of cuprates can be associated with dynamic incommensurate CDW fluctuations present in these materials. This scenario is substantiated by a comparison of calculated spectra with experimental ARPES data where we obtain a mode frequency which decreases towards optimal doping thus strongly supporting the existence of a quantum critical point around this concentration. Moreover we extract the temperature dependence of the associated bosonic spectrum from ARPES data where it turns out that there is a continuous evolution from mode-type behavior below T _c to a marginal Fermi liquid structure well above T _c.     

High-T c superconductors: New insights from angle-resolved photoemission

Recent angle-resolved photoemission (ARPES) studies of the high-T _c superconductors by the Argonne group are briefly reviewed. First we discuss sum rules to establish a spectral function interpretation of the data, and the use of ARPES to obtain the momentum distribution. We then apply these ideas to the normal and superconducting state spectra for B₂Sr₂Cu₂O₈. Among the topics discussed are the Fermi surface, polarization selection rules, bilayer splitting, and the superconducting gap.     

Electron Spectroscopy of Single-Layer (n = 1) Bi2Sr2−xLaxCuO6+δ Crystals at Optimal Doping

On single crystals of the single-layer (n = 1) high-T _c superconductor Bi₂Sr_2?x La _x CuO_6+δ at optimal doping (x = 0.4), the electron spectroscopies x-ray absorption (XAS) and high-resolution angle-resolved photoemission (ARPES) were performed. The XAS gives the intensity of the so-called prepeak of the O 1s line what is due to the unoccupied part of the Zhang–Rice (ZR) singlet band. For ARPES, the advantages of single-layer material are the absence of bilayer effects and the possibility to study the electronic properties of the normal state at a sample temperature where the thermal broadening is extremely small (<10 meV). The controlled variation of the polarization vector of the synchrotron radiation made it possible to resolve a distinct fine-structure of the occupied part of the ZR singlet band at the Fermi level. These observations have enormous consequences for line shape analyses and the determination of pseudogaps, and thus the mechanism of high-T _c superconductivity.     

High-Tc superconductors: New insights from angle-resolved photoemission

Recent angle-resolved photoemission (ARPES) studies of the high-T _c superconductors by the Argonne group are briefly reviewed. First we discuss sum rules to establish a spectral function interpretation of the data, and the use of ARPES to obtain the momentum distribution. We then apply these ideas to the normal and superconducting state spectra for B₂Sr₂Cu₂O₈. Among the topics discussed are the Fermi surface, polarization selection rules, bilayer splitting, and the superconducting gap.     

Anomalously Weak Cooper Pair-breaking by Exchange Energy in Ferromagnet/Superconductor Bilayers

We report the superconducting transition temperature T _c versus thickness d _F of Ferromagnet/Superconductor (F/S) bilayers, where F is a strong 3d ferromagnet (Ni, Ni _0.81 Fe _0.19 (Permalloy), Co _0.5Fe _0.5) and S = Nb, taken from superfluid density measurements rather than resistivity. By regrouping the many physical parameters that appear in theory, we show that the effective exchange energy is determined from the F film thickness d _F where T _c versus d _F begins to flatten out. Using this rearranged theory, we conclude 1) the effective exchange energy, E _{e x} , is about 15 times smaller than measured by ARPES and five times smaller than deduced in previous studies similar to ours; 2) the dirty-limit coherence length, ξ _F , for Cooper pairs in F is larger than the electron mean free path, ? _F ; and 3) the 3d-F/Nb interface is enough of a barrier that Cooper pairs typically must hit it several times before getting through. The Py/Nb and CoFe/Nb interfaces are more transparent than the Ni/Nb interface.     

Multiband Superconductors Close to a 3D–2D Electronic Topological Transition

Within the two-band model of superconductivity, we study the dependence of the critical temperature T _c and of the isotope exponent α in the proximity to an electronic topological transition (ETT). The ETT is associated with a 3D–2D crossover of the Fermi surface of one of the two bands: the σ subband of the diborides. Our results agree with the observed dependence of T _c on Mg content in A $_{1-x}{\rm Mg}_x{\rm B}_2$ (A?=?Al or Sc), where an enhancement of T _c can be interpreted as due to the proximity to a ‘shape resonance.’ Moreover we have calculated a possible variation of the isotope effect on the superconducting critical temperature by tuning the chemical potential.     

On the Interplay of Jahn–Teller Physics and Mott Physics Leading to the Occurrence of Fermi Pockets Without Pseudogap Hypothesis and d-Wave High <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> Superconductivity in Underdoped Cuprate Superconductors

In this Festschrift paper celebrating Prof. Jacques Friedel 90 years’ birthday, we review the latest developments of the model proposed by Kamimura and Suwa, which bears important characteristics born from the interplay of Jahn–Teller physics and Mott physics. First it is shown that the feature of Fermi surfaces is the Fermi pockets constructed by doped holes under the coexistence of a metallic state and of local antiferromagnetic order. Then the phonon-involved mechanism based on the Kamimura–Suwa model is discussed; it leads to d-wave superconductivity. Further it is shown that T _c is higher in the cuprates with CuO₅ pyramid than those with CuO₆ octahedron. Finally a recent theoretical result on the energy distribution curves (EDCs) of angle-resolved photoemission spectroscopy (ARPES) below T _c is presented, and a remark is made on the phase diagram for underdoped cuprates.     

Study of Hole Pair Condensation based on the SU(2) Slave-Boson Approach to the t-J Hamiltonian; Temperature, Momentum and Doping Dependences of Spectral Functions

Based on the U(1) and SU(2) slave-boson approaches to the t-J Hamil-tonian, we evaluate the one electron spectral functions for the hole doped high T _c cuprates for comparison with the angle resolved photoemission spectroscopy(ARPES) data. We find that the observed quasiparticle peak in the superconducting state is correlated with the hump which exists in the normal state. We find that the spectral weight of the quasiparticle peak increases as doping rate increases, which is consistent with observation. As a conse-quence of the phase fluctuation effects of the spinon and holon pairing order parameters the spectral weight of the predicted peak obtained from the SU(2) theory is found to be smaller than the one predicted from U(1) mean field theory.     

Low Temperature Heat Capacity of Layered Superconductors SrNi2Ge2 and SrPd2Ge2

Low-temperature heat capacity C(T) of the weakly electron-correlated SrNi₂Ge₂ 122-layer compound undergoes a superconducting transition with onset at 1.4 K and a bulk T _c =0.75 K, where heat-capacity jump ratio ΔC(T _c )/γT _c =0.88–1.05. A small average superconducting energy gap E _g (ave)=2.21 kT _c =0.14 meV is derived for this multi-gap superconductor. Similar results for isostructural SrPd₂Ge₂ include T _c (onset)=3.5 K, bulk T _c of 2.92 K, ΔC(T _c )/γT _c =0.70 and E _g (ave)=2.54 kT _c =0.64 meV. The higher T _c onset could be associated with stoichiometric 1:2:2 grains in the polycrystalline samples. In addition, deviations of E _g /kT _c from the BCS ratio of 3.5 suggest that, just like their iron-based counterpart, these 122-layer germanides may also exhibit an unconventional, fully-opened multi-gap s-wave superconductivity.     

Weakly coupled highT c superconductors—An overview

A short review of the main achievements in the development of high-T _c Josephson junction investigation is given. The six years of history of high-T _c superconductivity have produced in this field a variety of interesting results providing exciting perspectives and, at the same time, raising challenging questions. We shall attempt here to give a brief sketch of this topic, suitable for a nonspecialized audience, which, although oversimplified as well as incomplete, could supply some ideas about where we stand and where we are headed in this field.     

Study of Flux-Creep in Bulk Melt-Textured YBa2Cu3Ox in the Regime of Individual Pinning

Magnetic relaxation, with magnetic fields applied parallel to the c axis, was measured in a high-quality melt-textured YBa₂Cu₃O_x bar. Zero-field-cooled magnetization-versus-time data were obtained within a regime of field and temperatures dominated by single vortex pinning. The activation pinning energy U = U _M was calculated from experimental data and compared to U = U ₀ ln(J _c /J), where U, is a constant, J _c is the critical current density, and J the current density. This expression for U obtains within the individual pinning regime. A logarithmic dependence of U _M on J was found, and the data indicated that a scaling function of U _M , g(T/T _c ) = (1?T/T _c )^1/2, was appropriate for describing the single-vortex pinning regime.     

Ion-channeling studies of lattice anomalies well aboveT c in YBa2CU3O7−y

Backscattering yields in the <001> axial channeling mode using D⁺ ions have been measured for YBa₂Cu₃O_7–y single crystals with T_c=59K. 68K and 91K at temperatures between 40K and 295K. In 60 K-class YBa₂Cu₃O_7–y with significant anomalies associated with the spin gap, it is found that the channeling anomalies are observed at 130–140 K(T_pa) in addition to anomalies at T_c. The channeling anomalies at T_c follow to the shift of T_c, suggesting the phonon anomalies induced by the superconducting-gap opening. On the other hand, T_pa is found to be almost unchanged for varying T_c, although T_pa appears near the temperature where the spin gap is opened. This leads a question whether the channeling anomalies at T_pa is directly related to the phonon anomalies induced by the spin-gap opening. The existence of lattice instability is suggested as one of possible explanations for the anomalies at T_pa.     

Vortex Formation and Annihilation in Rotating Superfluid 3He-B at Low Temperatures

In superfluid ³He-B the damping of vortex motion varies many orders of magnitude in the currently accessible temperature range from T _c to below 0.2?T _c. The large variation in the dynamics switches on sequentially different processes of vortex formation as a function of temperature. These can be examined in a smooth-walled rotating cylinder in the absence of surface pinning. We provide an overview of vortex formation in spin-up and of annihilation in spin-down measurements at temperatures below the hydrodynamic transition at 0.6?T _c where turbulence in the bulk volume becomes possible. Of central current interest are the very lowest temperatures at 0.2?T _c where the dynamic processes reflect on the properties of the T→0 limit.     

Transistor Doping Experiments on Superconducting CaCuO2 and C60 Interpreted on a Bond Ordering Model

The curve shape of T _c vs. holes injected by fields (transistor doping) of CaCuO₂ and C₆₀ are interpreted on a charge or bond ordering [BO] model. For CaCuO₂ both the magnitude of optimal T _c (89 K vs. a calculated 83 K) and the linear portions around a “sharp” T _c peak at h ～ 0.17 = 1/6 are in accord with a universal algorithm for cuprates, in which T _c scales linearly with radical bond density to an optimal BO. These linear regions extrapolate to h = 0 and 1/3 respectively. Small trends to electronic freezing are observed at h = 1/8. A second linear region extends from h ～ 1/12 to 1/16 with 4 times the slope of the first linear region, indicating a second mode of BO filling. A surprisingly similar behavior is observed for transistor doped C₆₀ type materials with characteristic sequence of BO dictated hole concentrations for T _c onset, rise, a linear region extrapolating to T _c = 0 at h = 0, indications for electronic freezing, a sharp peak and linear decrease thereafter. This suggests that a common phenomenology obtains for all high T _c materials, representing generally an ordering phenomenon of doped covalent radical bonds.     

Thermal conductivity of high-T c superconductors

This paper reviews existing data on the thermal conductivity of high-T _c superconductors. Included are discussions of pristine polycrystalline high-T _c ceramics, single crystal specimens, and high-T _c materials structurally modified by substitution or by radiation damage. The thermal conductivity of high-T _c superconductors is compared with that of conventional superconductors, and dramatic differences are found between the two families. Mechanisms of thermal conductivity applicable to high-T _c perovskites are discussed and implications for theories of high-T _c superconductivity are noted.     

Lattice Effects Across the Phase Diagram of Pnictides

We have studied by Raman spectroscopy the effect of doping, temperature, and hydrostatic pressure on selected Fe pnictides of the 1111 series. Two sets of RFeAsO_1?x F _x compounds have been examined (R=Sm and Nd) with a varying amount of doping and transition temperature. The doping dependence of the Raman active modes reveals that the rare earth phonon is correlated with the transition temperature (T _c) and not with the amount of doping. As in the case of several other pnictides, the low temperature measurements indicate phonon modifications at much higher temperatures than T _c even in the superconducting compounds. The application of hydrostatic pressure indicates a nonlinear behavior of the rare earth phonon, which increases with doping and in the superconducting compounds correlates with modifications in T _c. The results are similar with those of the cuprates, where hydrostatic pressure has induced phonon and structure modifications at characteristic pressures where the T _c dependence on pressure is also modified. All results point to some role of the lattice for superconductivity in the pnictides.     

The possibility of increasingT c in oxide high-T c superconductor films: Interaction with elastic fields of defects

The influence onT _c of dilatation centers (DC) as elastic defects near the surface or in thin films of oxide high-T_c superconductors is considered. The distribution function ofT _c in this case was obtained. This is the Lorentzian with center shifted to largerT _c values. The half width of theT _c distribution βn (n is the concentration of DC) is larger than its center shift and corresponds to the increase ofT _c fluctuations near the surface. As a result, a continuum percolation behavior with increasing critical temperature percolation levelT ^(c) near the surface appears. The inequalityβn>T ^(c)>T _c ^initial is fulfilled. ForT>T ^(c) the quasi-2D Josephson media takes place where a finite superconductive domain withT _c ^local >T ^(c) exists. The influence of DC considerably increases for strong DC such as off-center impurity ions.     

Effect of f Electron Excitations in Heavy Fermion and?Unconventional Superconductors

The low energy crystalline electric field (CEF) excitations of f electrons can play a twofold role in heavy fermion and unconventional superconductors. They may act as a glue for the formation of Cooper pairs and simultaneously as a probe to investigate the symmetry of the order parameter. The former has been found in the skutterudite heavy fermion compound PrOs₄Sb₁₂, where a singlet?Ctriplet CEF excitation ?? contributes to the pair formation and enhances T _c with respect to L aOs₄Sb₁₂. In substituted P r(Os_1?x Ru _x )₄Sb₁₂,?a continuous increase of ?? leads to a crossing with the local Pr rattling phonon mode and vibronic mode formation caused by magnetoelastic interaction. Furthermore, the T _c enhancement turns into a reduction. It is proposed that this signifies a crossover from mainly pair-forming aspherical Coulomb to pair-breaking dipolar exchange scattering with increasing Ru content. In the Ce-based Fe pnictides CEF transitions show anomalous temperature dependence of the line width due to a pronounced feedback effect, which gives direct evidence for the unconventional 3d superconductivity.