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.     

Josephson Plasma in Various High-T c Cuprates

Josephson plasma in various high-T _c cuprates with and without magnetic field is studied by using the sphere resonance method. For Bi ₂ Sr ₂ CaCu ₂ O ₈₊, the plasma in a zero magnetic field exists at 5 cm ^–1 for a slightly overdoped sample (T _c = 85 K) and shifts to 11 cm ^–1 as the doping increases (T _c = 71 K). For SmLa _1–x Sr _x CuO _3.95 (T* phase), two peaks appear at 11 and 30 cm ^–1 in a zero magnetic field, and both peaks shift to lower frequencies as the magnetic field increases. These peaks are identified as the Josephson plasma of the intrinsic Josephson junction at the fluorite-type Sm ₂ O ₂ block layer and the rocksalt-type (La,Sr) ₂ O _2– block layer, respectively. This indicates that the T* phase can be regarded as the ···S/I/S/I/S/I/S/I/S··· (···superconductor/insulator1/superconductor/insulator2/superconductor···) -type Josephson junction array.     

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.     

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.     

Polaron Coherence as Origin of the Pseudogap Phase in High Temperature Superconducting Cuprates

Within a two-component approach to high T _c 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 T ^* 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 T _c , stems from an s-wave superconducting component of the order parameter, whereas a pure d-wave order parameter alone can be excluded.     

Pseudogap in Boron-Doped Diamond and Cuprates

The Marcus model, well known as a general model for electron transfer, is applied to electron pair formation and transfer in systems characterized by neighboring superconducting and local phases, particularly boron-doped diamond and cuprates. Hubbard-U is identified with the adiabatic free energy difference between charged and spin-coupled configurations. Typical for the Marcus model is the coupling of electronic and nuclear coordinates due to structural changes with number of electrons during electron transfer or excitation.     

Magnetic Levitation With High-T c Superconducting Thin Films

The levitation force between a permanent magnet and a superconducting thin film was investigated experimentally. The configuration consisted of a cylindrical NdFeB permanent magnet placed above a circular YBa₂Cu₃O_7?δ disk with common cylinder axis. Precise measurements were made of the vertical force F _z and the magnetic stiffness κ _z as a function of the magnet–superconductor separation at 77 K. Several features contrasting the levitation force produced using bulk superconductors were observed. Thin films produced very high values for F _z and κ _z per unit volume of superconducting material. The hysteretic behavior of F _z during decreasing and increasing separation resulted in loops of nearly symmetrical shape, which also contain a peak in the repulsive force branch. The observations are analyzed and explained with good quantitative agreement using recent theories for flux penetration in thin superconductors in transverse magnetic fields.     

Correlation Between Superconducting Transition Temperature and Bond Length Ba–O in High-T c Cuprate Superconductors

A well-known correlation of the maximum superconducting transition temperature (T _c,max) with ionic radius of rare earths and Y (R) in the series RBa₂Cu₃O_{7 ? δ} is converted to the intrinsic correlation of T _c,max with bond length between Ba and oxygen in CuO₂ plane, being valid not only for RBa₂Cu₃O_{7 ? δ} but also for other cuprate superconductors containing BaO plane such as HgBa₂CaCu₂O_{6 + δ}. It is pointed out that this correlation places a constraint on possible mechanisms inducing the high-T _c superconductivity.     

Phonon Origin of High T c in Superconducting Cuprates

Eliashberg theory (ET), generalized for the account of the peculiar properties of the finite zone width electron-phonon (EP) systems with the non-constant electron density of states, the electron-hole nonequivalence, chemical potential renormalization with doping and frequency, and electron correlations in the vertex function, is used for the study of T _c in cuprates. The phonon contribution to the nodal anomalous electron Green function (GF) in cuprates is considered. The pairing on the full width of the electron zone was taken into account, not just on the Fermi surface. It is found that the finite zone width phenomenon in the newly derived Eliashberg equations for the finite zone width EP system together with the abrupt fall of the density of states above the Fermi surface are the crucial factors for the appearance of the high temperature superconductivity phenomenon. It is shown that near the optimal doping in the hole-doped cuprates high T _c value is reproduced with the EP interaction constant obtained from tunnel experiments.     

Pseudogap and Quantum-Transition Phenomenology in HTS Cuprates

The low-energy excitation spectrum of HTS cuprates is examined in the light of thermodynamic, transport, quasiparticle and spin properties. Changes in the thermodynamic spectrum associated with the normal-state pseudogap disappear abruptly at a critical doping state, p _crit=0.19 holes per Cu. Moreover, ARPES data at 100K show that heavily damped quasiparticles (QP) near (,0) suddenly recover long lifetimes at p _crit, reflecting an abrupt loss of scattering from AF spin fluctuations. This picture is confirmed by SR zero-field relaxation measurements which indicate the presence of a novel quantum glass transition at pcrit. Consistent with this picture resistivity studies on thin films of Y_0.7Ca_0.3Ba₂Cu₃O_7– reveal linear behavior confined to a V-shaped domain focussed on pcrit at T=0. The generic phase behavior of the cuprates may be governed by quantum critical fluctuations above p _crit and the pseudogap appears to be caused by short-range AF correlations.     

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.     

Pseudogap in the Normal State of Cuprates

We consider underdoped cuprates as disordered conductors. The diffusion coefficient D can be as low as 10^–5 m² s^–1. In these conditions, Coulomb interaction between electrons must be taken into account. The main effect is to open a dip and even a gap in the density of state (DOS) near the Fermi level (FL). We show that this model explains most of the observed features of the so-called pseudogap in the normal state and in particular its value, anisotropy, and variation with doping.     

Study of Superconducting Properties in Bismuth-Based Cerium Doped High-T c Superconductors

Superconductivity in bismuth-based high-T _c superconducting materials attracts the researchers for their unique properties. Bismuth-based superconductors commonly called BSCCO have great importance among the superconducting family. These are divided into three phases among them 2223 phase is highly studied in order to investigate its superconducting properties by substitution of different elements. We have studied the substitution of cerium (Ce) on the calcium site of bismuth-based Bi(Pb)Sr(Ba)-2223 high-T _c superconductor. The nominal compositions of Bi_1.6Pb_0.4Sr_1.6Ba_0.4(Ca_1?x Ce _x )₂Cu₃O _x ceramic superconductor were prepared by the sol–gel method. X-ray diffraction (XRD) was done at room temperature for structural analysis and different parameters were calculated. Surface morphology was done by scanning electron microscopy (SEM). DC resistivity measurements for the transition temperature of synthesized superconducting samples were taken by the standard four-probe method, apparatus for which was developed in our laboratory. Current density measurements were also taken by the same apparatus. The synthesized superconducting samples were also characterized by thermogravimetric analysis (TG) and Fourier transformation infrared radiations (FTIR). It is observed that the substitution of cerium on the calcium site favors the formation of single high-T _c 2223 phase.     

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.     

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.