Josephson Vortex Flow and Plasma Amplification in High-T c Superconductors

We investigate Josephson vortex flow states in two device arrangements considered to amplify the excited Josephson plasma. First, we study an intrinsic Josephson junction embedded inside a wave guide. The simulation result in the system reveals that the amplitude of the excited electric field almost linearly grows along the junction plane at the resonant voltage with the Josephson plasma. Second, we simulate a system composed of two separated Josephson junctions embedded inside a long wave guide and confirm that the excited electric field is amplified by the synchronization between two junction sites.     

Physical Picture of High-T c Superconductivity in Cuprates

A simple physical picture of high-T _c superconductivity of CuO₂ planes is proposed. It possesses all characteristic features of HTS, such as a high superconducting transition temperature, the $d_{x^{2}-y^{2}}$ symmetry of order parameter, and the coexistence of a single-electron Fermi surface and a pseudogap in the normal state. Values of pseudogap are calculated for different doping levels.     

Optical Sum Rule Anomalies in the High-T c Cuprates

We provide a brief summary of the observed sum rule anomalies in the high-T _c cuprate materials. A recent issue has been the impact of a non-infinite frequency cutoff in the experiment. In the normal state, the observed anomalously high temperature dependence can be explained as a ‘cutoff effect’. The anomalous rise in the optical spectral weight below the superconducting transition, however, remains as a solid experimental observation, even with the use of a cutoff frequency.     

Pressure Effect on Hall Coefficient in Multilayered High-T c Cuprates

Temperature dependence of the Hall coefficient and the resistivity in Tl₂Ba₂CaCu₂O _y (TL-2212), Tl₂Ba₂Ca₂Cu₃O _y (Tl-2223) and Cu_0.7C_0.3Ba₂Ca₃Cu₄O _y (Cu-1234) have been measured under high pressure up to 6 GPa. The values of the intrinsic T _c enhancement not related to charge transfer by applied pressure were estimated to be 0.8K/GPa for the Tl system and 1.1K/GPa for Cu-1234.     

Correlation between Superconducting Gap and Pseudogap in High-T c Cuprates

We studied the low-temperature energy gap 2 ₀ on Bi ₂ Sr ₂ CaCu ₂ o ₈₊ (Bi2212) and La _2–x Sr _x CuCO ₄ (La214) systematically over a wide range of doping level p using STS, break junction tunneling spectroscopy, Raman scattering and low-T electronic specific heat data. We have also studied the electronic specific heat of La214 in the normal state at T > T _c , and confirmed that pseudogap behavior appears at around T*, below which the in-plane resistivity and magnetic susceptibility tend to be slightly suppressed. Similar suppression appears in and of Bi2212 below the onset temperature of pseudogap T*. It is pointed out in the present study that 2 ₀ is closely related to T* in both Bi2212 and La214 systems; T* 2 ₀ /4.3k _B . It is also pointed out that 2 ₀ is in almost linear proportion to k _B T _max ( T*), where T _max is the temperature exhibiting a broad peak in –T curves and k _B T _max can be considered to give a measure of the effective antiferromagnetic exchange energy J _eff. The factors in 2 ₀ k _B T _max J _eff are 1 for La214 and 2 for Bi2212, respectively. We also report that in both Bi22l2 and La214 systems T _c roughly scales with p ₀ except in highly doped samples, where T _c 2 ₀ .     

The Mechanism of d-Wave Superconductivity of Underdoped High-T c Cuprates

We have worked out the theory of d-wave superconductivity of the doped cuprate that is consistent with up-to-date data on the basis of the assumption of two-channel Kondo fixed point of the latter. Strong local Coulomb scattering between carriers combined with strong and weak involvement, respectively, of charge (spin) fluctuations and phonons in pairing justify the assumption of two-channel Kondo effect in the doped cuprate. This is true for diverse other data as well. The assumption explains not only the d-wave superconductivity but also all the relevant physics of this material.     

Spin Injection in High-T c Cuprates: An STM Spectroscopy Study

To study how the high-T _c order parameter (OP) evolves under the injection of spin-polarized quasiparticles, STM spectroscopy has been performed on superconductor/ferromagnet thin-film heterostructures comprising YBa₂Cu₃O_7–x (YBCO) and La_0.7Ca_0.3MnO₃ (LCMO) at 4.2 K. Quasiparticle-tunneling and Andreev-reflection characteristics measured on the YBCO under spin-injection from the LCMO were analyzed with the d-wave Blonder-Tinkham-Klapwijk theory, to reveal a spectral evolution which provides direct evidence for dynamic magnetic pair-breaking. The spectral analysis also shows the d-wave OP to remain time-reversal invariant as it is suppressed by the spin-injection. These results are discussed in terms of the general search for quantum-critical points in the high-T _c phase diagram.     

Influence of Structural Anisotropy on the Irreversibility Line of High-T c Cuprates

From zero-field-cooled and field-cooled magnetic response, we have determined the irreversibility lines for a number of high-T _c oxides, viz., La(Sr)-214, Nd-223, Dy(Tb)-124, (Tl, Pb)-1212, and Tl-11112, in the H–T plane, which fit the relation H=A(1 –T/T _c ) ⁿ . Our results are consistent with a correlation between the anisotropy of the structure and the value of n, is in agreement with the Josephson decoupling model [1].     

Correlation-induced Suppression of Bilayer Splitting in High-T c Cuprates: A Variational Cluster Approach

We carry out a theoretical study of the bilayer single-band Hubbard model in the undoped and in the superconducting phases by means of the variational cluster approach. In particular, we focus on the splitting between the ??bonding?? and ??antibonding?? bands induced by the interlayer hopping, as well as its interplay with strong correlation effects. We find that the splitting is considerably suppressed in both the normal and superconducting phases, in qualitative agreement with experiments on Bi₂Sr₂CaCu₂O_8+?? . In addition, in the superconducting phase, the shape of the splitting in k space is modified by correlations.     

Reduced-Gap Ratio of High-T c Cuprates Within the d-Wave Two-Dimensional Van Hove Scenario

The gap-to-T _c ratio (R) of high-temperature superconductors is calculated in the context of d symmetry of the superconducting order parameter and a two-dimensional Van Hove singularity in the density of states, using the BCS theory for weak coupling. Exact numerical calculation and an analytic formula for R are given. The ratios are found to be substantially larger than the BCS weak coupling limit of 3.53. The overall dependence of R on ω _D /T _c, where ω _D is the cutoff frequency, is given.     

Advantages of High-T c Cuprates over Semiconductors for Making Room Temperature Electronic Devices

This paper demonstrates that the high-T _c cuprates in the pseudogap states, which is called pseudogapbody, have many advantages over the semiconductors for making room temperature electronic devices. This paper predicts that the pseudogapbody will replace the semiconductors in many applications, and pseudogapbody electronics will appear as a new subject.     

Effect of Normal-State Pseudogap on Physical Quantities in Hubbard Model for Underdoped High-T c Cuprates

We develop the strong-coupling theory of coexisting charge-density-wave (CDW) and superconductivityd-wave gaps within the framework of the FLEX (fluctuation exchange) approximation for the two-dimensional Hubbard model. For nested sections of the Fermi surface these equations reduce to the previous FLEX equations for superconductivity where the squared energy gap _s ² in the denominator of the Green's function is replaced by ( _s ² + _c ² ) (here _s is the superconductivity and _c the CDW gap). We solve these equations by taking for _c a phenomenologicald-wave gap. The resulting neutron scattering intensity, spin-lattice relaxation rate 1/T₁ , Knight shift, resistivity, and photoemission intensity are in qualitative agreement with the data on underdoped high-T_c cuprates. TheT_c for superconductivity decreases and the crossover temperature T_* for 1/T₁Tincreases with increasing gap amplitude of _c which is in qualitative agreement with the phase diagram for underdoped cuprates.     

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.     

Origin of Anomalous Bump at Low Temperatures in the c-Axis Optical Conductivity of High-T c Cuprates

The optical spectra of some high-Tc cuprates show anomalous bump around 450 cm ^–1 and dramatic phonon changes for E parallel to c axis, which grows at low temperatures. This phenomenon has been observed in Yba ₂ Cu ₃ O _6.6, YBa ₂ Cu ₄ O ₈ and Pb ₂ Sr ₂ RCu ₃ O ₈ , but not in (La,Sr) ₂ CuO ₄. In this study, we report the first observation of a similar anomaly in underdoped Bi ₂ Sr ₂ CaCu ₂ O ₈. The spectra were measured from two mosaics of Ba ₂ Sr ₂ CaCu ₂ O ₈ with T _c 80 K and 60 K. Considering that these spectral features seem to be common for all high-T _c cuprates with double CuO pyramids, we propose one plausible explanation that it is attributed to the second Josephson plasmon for intracell coupling in the Josephson coupled layer model.     

Correction to: Reduced-Gap Ratio of High-T c Cuprates Within the d-Wave Two-Dimensional Van Hove Scenario

Journal of Superconductivity and Novel Magnetism -     

Effects of Excess or Deficiency of Oxygen Content on the Electronic Structure of High-T c Cuprates

Band structure calculations are presented for large supercells of Ba₂CuO₄ (BCO) with O-vacancies in planar or apical positions, and of superoxygenated La₂CuO₄ (LCO) with oxygen interstitials in the La₂O₂ layers. It is found that apical oxygen vacancies in BCO act as electron dopants and makes the electronic structure similar to that of hole doped LCO. Excess oxygen interstitials forming wires in the La₂O₂ layers of LCO are shown to yield a much larger density-of-states at the Fermi energy than for the stoichiometric compound related with a segmentation of the Fermi surface. Antiferromagnetic (AFM) spin fluctuations are strengthened by O-vacancies in BCO as well as by oxygen interstitials in LCO, but are strongly suppressed in O-deficient LCO. Our results indicate the complexity of doping by O-vacancies, and by ordered defects that are a significant factor controlling the electronic properties of cuprates.     

Influence of Long-Range Coulomb Interaction and On-Site Hubbard Repulsion on the Formation of d-Wave Copper-Pairing in High-T c Cuprates

We develop a diagram technique for the self-consistent treatment of the long-range Coulomb interaction and on-site Hubbard repulsion in the normal and superconducting state of high-T _c cuprates. The resultant analytical expression for the “screened” matrix elements taking into account long-range and on-site repulsion has been derived. In particular, it accounts for processes with and without spin-flip due to an exchange of spin and charge density fluctuations. Furthermore, we derive the expressions for the normal and anomalous self-energy parts near the superconducting transition temperature T _c that takes into account the vertex corrections including crossing diagrams. The contribution of the crossing parts is taken within the ladder approximation (similar to Fluctuation-Exchange approximation) where the role of Hubbard on-site interaction is replaced by the Coulomb matrix element with a spin-flip averaged over the momentum. Finally, the developed scheme allows to analyze the formation of d-wave superconductivity and its stability in presence of the long-range Coulomb repulsion within a self-consistent anisotropic Eliashberg-like approach.     

Multiband Responses in High-T c Cuprate Superconductors

We report on the interplay of localized and extended degrees of freedom in the metallic state of high-temperature superconductors in a multiband setting. Various ways in which the bare magnetic response may become incommensurate are measured against both phenomenological and theoretical requirements. In particular, the pseudogap temperature is typically much higher than the incommensurability temperature. When microscopic strong-coupling effects with real-time dynamics between copper and oxygen sites are included, they tend to restore commensurability. Quantum transport equations for low-dimensional multiband electronic systems are used to explain the linear doping dependence of the dc conductivity and the doping and temperature dependence of the Hall number in the underdoped LSCO compounds. Coulomb effects of dopands are inferred from the doping evolution of the Hartree–Fock model parameters.     

High-T c superconductivity in thed-p electron system

The relaxation time with spin flipτ _s and the parametersξ, δ, χ of superconducting phase have been calculated on the basis of the kinematical mechanism of superconductivity in strongly correlated oxide models. An inter-relation between the superconducting gap Δ₀ and the specific heat jump Δ _c allowing the experimental verification was obtained and the Ginsburg-Landau equation derived.