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.     

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 .     

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.     

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.     

Dielectricly Enhanced T c in Underdoped Cuprates

It is proposed to create a multilayer structure in which an underdoped copper-oxide high-temperature superconductor is sandwiched between high-dielectric-constant insulator layers such as ferro- or ferri-electrics, thereby reducing the Coulomb repulsion between the intrinsically present clusters or stripes in the CuO₂ layers of the pseudogap phase. This should lead to an increase in the size of such clusters, resulting in smaller distances between them and coherence at higher temperature, i.e., a higher T _c with a smaller pseudogap (T ^??T _c ).     

Electronic Structure in Underdoped Cuprates Due to the Emergence of a Pseudogap

The phenomenological Green’s function developed in the works of Yang, Rice, and Zhang has been very successful in understanding many of the anomalous superconducting properties of the deeply underdoped cuprates. It is based on considerations of the resonating valence bond spin liquid approximation and is designed to describe the underdoped regime of the cuprates. Here, we emphasize the region of doping, x, just below the quantum critical point at which the pseudogap develops. In addition to Luttinger hole pockets centered around the nodal direction, there are electron pockets near the antinodes which are connected to the hole pockets by gapped bridging contours. We determine the contours of nearest approach as would be measured in angular resolved photoemission experiments and emphasize signatures of the Fermi surface reconstruction from the large Fermi contour of Fermi liquid theory (which contains 1+x hole states) to the Luttinger pocket (which contains x hole states). We find that the quasiparticle effective mass renormalization increases strongly toward the edge of the Luttinger pockets beyond which it diverges.     

Isotope Effect of Underdoped Cuprates in the Yang-Rice-Zhang Model

The underdoped region of the cuprates phase diagram displays many novel electronic phenomena both in the normal and the superconducting state. Many of these anomalous properties have found a natural explanation within the resonating valence bond spin liquid phenomenological model of Yang-Rice-Zhang (YRZ) which includes the rise of a pseudogap. This leads to Fermi surface reconstruction and profoundly changes the electronic structure. Here, we extend the previous work to consider the shift in critical temperature on ¹⁶O to ¹⁸O substitution. The isotope effect has been found experimentally to be very small at optimal doping yet to rapidly increase to very large values with underdoping. The YRZ model provides a natural explanation of this behavior and supports the idea of a pairing mechanism which is mainly spin fluctuations with a subdominant (～10%) phonon contribution.     

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.     

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.     

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.     

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.     

A New Model of Pseudogap Physics in the Cuprates

We discuss a short-range order mechanism for understanding pseudogap physics of cuprates in which the competition between a variety of magnetic orders frustrates the development of the long-range order. We show in particular that the competition between the effects of Van Hove singularity nesting and conventional Fermi surface nesting leads to a material-dependent transition between Mott and Slater physics, and the emergence of a spin-frustrated state in the crossover region.     

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].     

The Origin of the Pseudogap in Underdoped HTSC

Here, the origin of the pseudogap in HTSC is attributed to the modulated antiferromagnetic (AFM) phase, whose preliminary version has been sketched recently by the present author (in J. Super. Nov. Mag. 22:517, 2009). Starting from the t-J Hamiltonian, I show that the formal failure of the perturbation theory leads to a transformation to the pseudogap phase. This phase is characterized by the aggregation of the holes into rows and columns, which in turn results in two internal fields. The first is the modulated AFM field, whose main evidence comes from Neutron scattering experiments. The second internal field is made up by the checkerboard charge density waves that have been observed by scanning tunneling measurements. The present paper deals mainly with the internal field of the first type, and discusses the second type only tentatively. Formalism is derived that yields the ground state, the internal field, the Hamiltonian, and the propagators of the condensed phase. Our results resolve the presumably inherent self-contradictory concept of pseudogap. It is shown that the excitation energy spectrum is gapless despite the order parameter that is inherent to the condensed system. In addition, it is shown qualitatively that our model predicts “Fermi surface” that is in agreement with the experiment.     

Polarized Electronic Raman Scattering Studies of Underdoped and Overdoped High-T c Superconductors

We report on polarized Raman scattering from high-T _c superconductors in the Bi ₂ Sr ₂ CaCu ₂ O ₈₊ (Bi2212) and HgBa ₂ CuO ₄₊ (Hg1201) classes, with an emphasis on the effect of superconductivity on the low frequency electronic continuum. The Hg-based materials confirm earlier observations from Bi2212 that the superconducting energy gap 2 peak is not present in underdoped samples, while its Raman shift is maximum in B _1g symmetry at optimal doping, and decreases dramatically with overdoping. These similarities despite the different numbers of bands crossing the Fermi surface argues against a multiple-band explanation for the anomalous doping dependence of the gap. We also discuss an anomalous B _1g feature observed at 600 cm ^–1 in Y underdoped Bi2212. This pseudogap-like feature has several properties which suggest it is of electronic - rather than phononic - origin.     

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.     

Two Domains of the Parameter Space of the 2D Hubbard Model for High-T c Superconductors

High-T _c superconducting cuprates have two types of Fermi surfaces: simple-2D-tight-binding-band type (LSCO type) and the much deformed one (Bi2212 type). The difference is attributed to that of band parameter values, i.e., t′ ～ ?0.1 and t″ ～ 0 versus t′ ～ ?0.3 and t″ ～ 0.2 in terms of the second- and third-neighbor transfer energies t′ and t″, respectively (energy unit is the nearest-neighbor transfer energy t). Assuming a moderate value of on-site Coulomb energy U ～ 6 and performing the variational Monte Carlo computation, we found that the two superconducting parameter domains exist in fact around these parameter sets, respectively, in which superconductivity predominates over spin density wave (SDW) due to the latter being at the brink of vanishing. Stripes were obtained in the first domain but tend to disappear in the second. In the latter domain there seems to exist parameter sets for which superconductivity appears without doping.