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.     

Effect of Incommensurate Charge-Density Wave Scattering on the Electronic Structure of High-T c 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.     

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.     

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.     

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.     

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.     

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 CuO₆₊ 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-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.     

The Role of In-plane Oxygens in Optimally Doped NCCO

Within the framework of the three-band Hubbard (Emery) model of high-temperature superconductors, we find a new physical regime for the electron-doped compound NCCO at optimal doping. It is characterized by a strong renormalization of the bare copper–oxygen hopping, similar to the one on the hole-doped side, and a low value of the effective copper–oxygen splitting. After a zeroth order fit of the Fermi surface is achieved, ARPES data are reproduced by adding a simple phenomenological weak-coupling antiferromagnetic perturbation of the conduction electrons. The experimental situation corresponds to a pseudogap across the whole Fermi surface. The wide dispersive features in NCCO ARPES spectra are analogous to the hump in BSCCO. The oxygen degree of freedom dominates the band structure of both NCCO and BSCCO, however, in NCCO this cannot be demonstrated without simultaneously accounting for the antiferromagnetic scattering. The difference in T _c of the two compounds may be related in part to the difference in the calculated densities of states.     

Calculation of band structure and superconductivity in the simple cubic phase of phosphorus

We report a calculation of the band structure and superconductivity of phosphorus in the simple cubic phase under pressure. The effect of pressure on the band structure is obtained by means of the linear muffin-tin orbital method. The superconducting transition temperature(T _c) is calculated using the Allen-Dynes formula. It is found that the value ofT _c increases continuously with pressure from 110 kbar up to 210 kbar and then decreases. The change in the slope ofT _c is associated with the appearance of a new piece of Fermi surface. The calculated values ofT _c are compared with the available experimental data.     

Tunneling conductance of break junction with the (100) plane fracture surface of Bi2Sr2CaCu2O8

Using the new method of breaking, the tunnel junctions with the (100) plane fracture surface could be obtained. The observed singularities in conductivity can be explained by tunneling between the different layers (CuO₂→CuO₂, sharp peaks at 2Δ _p−p =64±2mV;CuO ₂→BiO, peaks at 100–140 mV; BiO→BiO, wide maxima at about 250 mV). The tunneling spectra measured at T=4.2–100 K show that a shift of the 2Δ _p−p peak to lower biases is much stronger (from 64 to 37 mV) than the BCS prediction at T/T_c≤0.7. At higher T (up to T_c≈87K) the changes of the spectra look like those expected for gapless superconductivity in agreement with the ARPES data for a-direction.     

Superconductivity in MgB2: Phonon modes and influence of carbon doping

Following a brief overview, results of our investigations on phonon modes in MgB₂, and superconducting transition in carbon doped MgB₂ are presented. The superconducting transition temperature in MgB_{2 x}C_x as obtained from susceptibility and resistivity measurements is observed to decrease systematically from 39-4 K forx = 0 to 26 K forx = 0.5. It is shown the changes in lattice volume, as obtained from x-ray diffraction measurements, can account only partially for the observed decrease inT _c . The observed variation ofT _c with carbon content is seen to correlate with the Debye temperatures, obtained from an analysis of the resistivity data. Investigation of the phonon modes in MgB₂, through infrared absorption measurements indicate three modes at 410,475 and 560 cm^-1. The former two are associated with the infrared active modes, and the third component is associated with the Raman mode, that gets activated due to disorder. A study of the temperature dependence of these modes indicates no changes across the superconducting transition. The mode at 560 cm^-1 shows a significant hardening and a corresponding decrease in linewidth, with the lowering of temperature, that can been accounted in terms of anharmonicity.     

Acoustical Experiments on Superfluid 3He-4He Mixtures in Aerogel

This report discusses our results on the superfluidity of ³ He- ⁴ He mixtures in a 98% porosity silica aerogel. We have used low frequency sound to probe helium mixtures confined to aerogel, and have observed both the slow mode of superfluid ³ He in aerogel, which is manifested only below T_c, and an additional sound mode present only in the mixture. We attribute this novel sound mode to the slow-mode in the ⁴ He rich phase of the dilute ³ He- ⁴ He mixture. This mode exhibits positive frequency shifts below T_c in aerogel, while above T_c the mode is observed at a temperature independent frequency until close to T where it shifts to zero frequency.     

Surface Superconducting State of Heavy-Fermion Superconductor CeIrIn5 Probed by CeIrIn5-Ag Junctions

The CeIrIn₅-Ag junctions of about 2×10^–9 cm^–2 area have been made using microfabrication techniques, and the surface superconducting state of CeIrIn₅, which has two characteristic temperatures T ₀ and T _c, has been investigated, where T ₀ and T _c are the transition temperature to zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. The temperature, below which superconducting anomalies are observed, varies from junction to junction, and yet it is always well above T _c=0.4 K. This result, together with no indication of transition at T _c, suggests that at least the surface of CeIrIn₅ is in the superconducting state above T _c. The data on the critical current I _c in superconducting anomalies point to the possibility to define a local transition temperature for each junction.     

Evidence for Dielectric (e–h) and Superconducting (e–e) Pairing in Cuprates

It is demonstrated that anomalous behavior of cuprates can be described in a natural way by the model with partial dielectrization of conduction electron energy spectra (EES). In such a model, near the Fermi surface (FS), there are formed the fours of charged particles (FCP) rather than independent electron–hole (e–h)- and electron–electron (e–e)-pairs. Since the bonding energies are the same for Cooper (e–e)- and dielectric (e–h)-pairs, then the temperature of SC transition T _c due to the Bose-condensation of FCP can be much more than the temperature of Bose-condensation of Cooper pairs only. The dielectric (e–h)-transition is connected with structural phase transformation in the cuprate system, formation of stripe structure, etc. The model successfully describes the presence of maximum at T _c (x)-curve, T-dependence of pseudogap (PG) and SC-gap, effect of nonmagnetic impurities to T _c , STM and ARPES spectra and other properties of cuprates.     

Organic superconductors with an incommensurate anion structure

Abstract

Superconducting incommensurate organic composite crystals based on the methylenedithio-tetraselenafulvalene (MDT-TSF) series donors, where the energy band filling deviates from the usual 3/4-filled, are reviewed. The incommensurate anion potential reconstructs the Fermi surface for both (MDT-TSF)(AuI₂)_0.436 and (MDT-ST)(I₃)_0.417 neither by the fundamental anion periodicity q nor by 2q, but by 3q, where MDT-ST is 5H-2-(1,3-dithiol-2-ylidene)-1,3-diselena-4,6-dithiapentalene, and q is the reciprocal lattice vector of the anion lattice. The selection rule of the reconstructing vectors is associated with the magnitude of the incommensurate potential. The considerably large interlayer transfer integral and three-dimensional superconducting properties are due to the direct donor–donor interactions coming from the characteristic corrugated conducting sheet structure. The materials with high superconducting transition temperature, T_c, have large ratios of the observed cyclotron masses to the bare ones, which indicates that the strength of the many-body effect is the major determinant of T_c. (MDT-TS)(AuI₂)_0.441 shows a metal–insulator transition at T_MI=50 K, where MDT-TS is 5H-2-(1,3-diselenol-2-ylidene)-1,3,4,6-tetrathiapentalene, and the insulating phase is an antiferromagnet with a high Néel temperature (T_N=50 K) and a high spin–flop field (B_sf=6.9 T). There is a possibility that this material is an incommensurate Mott insulator. Hydrostatic pressure suppresses the insulating state and induces superconductivity at T_c=3.2 K above 1.05 GPa, where T_c rises to the maximum, T_c^max=4.9 K at 1.27 GPa. This compound shows a usual temperature–pressure phase diagram, in which the superconducting phase borders on the antiferromagnetic insulating phase, despite the unusual band filling.     

The influence of light- and heavy-ion irradiation on the structure,resistivity, and superconducting transition temperature of V3Si. A comparative study

Channeling and x-ray diffraction measurements on Kr- and He-irradiated V₃Si single crystals and films reveal different damage levels for fluences in cases where the superconducting transition temperature T _chas been reduced by the same amount. This indicates that only special defect structures are responsible for the T _c-reduction mechanism. In the fluence region where T _cis decreasing, T _ccorrelates with residual resistivity _o, independent of the kind of irradiation. However, at particle fluences where T _csaturation occurs, different saturation values of _o are observed. The exponential decrease and the saturation of T _cwith fluence are explained by a similar behavior of g9_o versus fluence in the damage production and saturation processes. The increase of the lattice parameter is not uniquely dependent on the decrease of T _c, but also on the amount of damage present.     

Relevance of the Phonon-Coupling Mode on the Superconducting Pairing Interaction of La<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>4</Subscript>

Despite the intensive efforts for determining the mechanism that causes high-temperature superconductivity in copper oxide materials (cuprates), no consensus on the pairing mechanism has been reached. Recent advances in angle-resolved photoemission spectroscopies (ARPES) have suggested that a sizeable electron–phonon coupling exists as the principal cause for kinks in the dispersion relations (energy versus wavevector) of the electronic states. Here, we report on a systematic study of the influence of the electron–phonon-coupling parameter “λ” in the electronic quasiparticle dispersions along the nodal direction for La_2−x Sr _x CuO₄, covering the entire doping range over which the electron transport properties vary from insulating (0≲x≲0.03) to superconducting (0.05≲x≲0.25) and eventually non-superconducting metal (x>0.25). This includes our recently introduced theoretical model to adjust the experimental data on the fermionic band dispersion. The coupling constant λ, calculated consistently with the nodal kink dispersions, reproduces the observed critical temperatures T _c , the gap ratio 2Δ ₀/k _B T _c , and other parameters which have been studied from several equations. Our results suggest that, at least in La_2−x Sr _x CuO₄, electron–phonon coupling is the most relevant boson-coupling mode to influence the electron dynamics, and must therefore be included in any microscopic theory of superconductivity.     

The Attractive Hubbard Model in 2-D: Is It Capable of Describing a Pseudogap and Preformed Pairs?

Deviations from Fermi liquid behavior are well documented in the normal state ofthe cuprate superconductors, and some of these differences seem to be related topretransitional features appearing at temperatures above T _c. The observationof a pseudogap, e.g., in ARPES experiments, is a familiar example of this physics. Onepotential explanation for this behavior involves preformed pairs with finite lifetimesexisting in the normal state above T _c. In this way, two characteristictemperatures can be established. A higher one T* at which pairs begin toform and the actual T _c at which a phase-coherent superconducting phaseis established. In order to test these ideas we have investigated the negative UHubbard model in two dimensions in the fully self-consistent ladder approximation atlow electron densities. In the non-self-consistent version of this theory the systemalways shows an instability toward Bose-condensation of infinite lifetime pairs. In contrastto this, pairs obtain a finite lifetime due to pair–pair interaction and thesharp two-particle bound state is strongly lifetime broadened when self-consistency isapplied. A quasiparticle scattering rate which varies linearly with temperature is alsofound. The fully self-consistent calculation we were able to perform employed ak -averaged approximation in which the self-energyloses its k -dispersion due to ak -average. This approximation is found to preservethe essential physics.     

Correlation between T c and a calculated force constant in homologous crystal structures

Correlations have been discovered which suggest that phases of the crystal structures A2, A15, C15, and D8_b, that have Nb or Mo atoms occupying the chain sites in these structures, belong to a class having a common rigid band shape and having a common electron-phonon coupling mode responsible for T _c. The latter is identified as associated with transverse vibrations of atoms situated on sites along close-packed chains in these structures. On the basis of these correlations and known interatomic potentials, optimum T _c values may be predicted for hypothetical substances. NbMo₂Be is predicted to form an A15 structure and to have an optimum T _c = 30K.Supported financially by the National Science Foundation through Grant DMR 74-22493.