摘要:

The interaction mechanism involving fermions and lochons (local charged bosons) located at polarizable dielectric regions in the layered cuprate systems is discussed in the context of the pairing symmetry, the appearance of the resonance peak and the pseudogap which is seen aboveTc.It is found that the mechanism allows both s and d waves and the dominance of one or the other depends on the relative values of the Fermi energy and the nearest neighbor hopping integral. The model accounts for the pseudogap and the resonance peak is identified with the local pair fluctuation state. The mechanism, along with a weak phonon contribution, emerges as an acceptable model for HTSC. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59609

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We test the canonical BCS wave functions for fixed number of electrons for the attractive Hubbard model. We present results in one dimension for various chain lengths, electron densities, and coupling strengths. The ground-state energy and energy gap to the first excited state are compared with the exact solutions obtained by the Bethe ansatz as well as with the results from the conventional grand-canonical BCS approximations. While the canonical and grand-canonical BCS results are both in very good agreement with the exact results for the ground state energies, improvements due to conserving the electron number in finite systems are manifest in the energy gap. As the system size is increased, the canonical results converge to the grand-canonical ones. The “parity” effect that arises from the number parity (even or odd) of electrons are studied with our canonical scheme. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59610

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We present our theoretical study of the single-particle spectral function in two-dimensional cuprates by using numerically exact diagonalization method on small clusters of the extendedt-Jmodel containing long-range hopping matrix elements. For a set of the parameter values, the calculated spectral function systematically explains the angle-resolved photoemission spectroscopy (ARPES) data forBi2Sr2CaCu2O8+&dgr;.ForLa2−xSrxCuO4(LSCO), it is found that slightly smaller values of long-range hoppings are necessary for understanding an observed change of the Fermi surface topology withx. In addition, the dramatic suppression of spectral weight along (0,0)-(&pgr;,&pgr;) direction observed in under-doped LSCO is found to be explained by introducing an additional potential for holes which stabilizes vertical charge stripes. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59612

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We note that the zero bias conductance peak observed in tunneling into the [110] surface of adx2−y2superconductor has a width that is independent of disorder either in the bulk or at the surface. This remarkable property is accounted for qualitatively in terms of the tunneling into surface bound states in the superconductor which have widths that depend strongly on the momentum of the surface bound state. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59587

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We review the reasons that make superconductivity unlikely to arise in a ferromagnet. Then, in light of the report by Tallon and collaborators thatRuSr2GdCu2O8becomes superconducting at ∼35 K which is well below the Curie temperature of 132 K, we consider whether the objections really apply to this compound. Our considerations are supported by local spin density calculations for this compound, which indeed indicate a ferromagneticRuO2layer. The Ru moment resides int2gorbitals but is characteristic of itinerant magnetism (and is sensitive to choice of exchange-correlation potential and to the atomic positions). Based on the small exchange splitting that is induced in the Cu-O layers, the system seems capable of supporting singlet superconductivity an FFLO-type order parameter and possibly a &pgr;-phase alternation between layers. If instead the pairing is triplet in theRuO2layers, it can be distinguished by a spin-polarized supercurrent. Either type of superconductivity seems to imply a spontaneous vortex phase if the magnetization is rotated out of the plane. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59586

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

It is natural for a Mott antiferromagnetism in RVB description to become a superconductor in doped metallic regime. But the issue of superconducting transition temperature is highly nontrivial, as the AF fluctuations in the form of RVB pair-breaking are crucial in determining the phase coherence of the superconductivity. Underestimated AF fluctuations in a fermionic RVB state are the essential reason causing an overestimate ofTcin the same system. We point out that by starting with abosonicRVB description where both the long-range and short-range AF correlations can be accurately described, the AF fluctuations can effectively reduceTcto a reasonable value through the phase string effect, by controlling the phase coherence of the superconducting order parameter. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59588

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

An anomalous diamagnetic moment can be observed aboveTcandHc2in inhomogeneous superconductors. Such inhomogeneity can be caused by a non-uniform distribution of magnetic impurities. The model describes recent experimental data on overdoped cuprates. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59589

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Substitutional impurities in theCuO2planes of the cuprates allow us to probe the electronic properties of the host material. The pseudo-gap in the underdoped regime is unmodified far from the impurities even thoughTcis greatly reduced. The spin polarization induced by magnetic impurities has an oscillatory behavior reflecting the existing AF correlations between the Cu spins. Its influence on the NMR spectra opens a way to determine the q dependence of the static spin susceptibility and theTdependence of the AF correlation length. NMR measurements demonstrate that non-magnetic impurities such as Zn induce a local moment behavior on the neighboring Cu sites. This magnetism revealed by spin-less sites can be understood on theoretical grounds in the case of undoped quantum spin systems, while here the carriers greatly complicate the situation. Susceptibility data show that the magnitude of the local moment decreases with increasing hole doping. This experimental evidence directly reflects the influence of AF correlations and the interference between the carriers and the Cu hole spins in the cuprates. The anomalously large scattering of the carriers on spinless defects is another indication of the originality of the electronic properties of the cuprates, which apparently extends even to the overdoped regime. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59613

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

According to the recent neutron scattering data onYBa2Cu3O6+x,it has been concluded that the incommensurability of the spin dynamics is a common feature among oxide high-Tc superconductors, which cannot be explainable by Fermi surface anomaly. In this paper, we summarize neutron data on the incommensurability and show a strong correlation between it and superconductivity, for which spin singlet formation in the spin stripe could play an important role. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59590

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We present the results on structure, superconductingTc,and magnetic ordering temperature of Pr moments of several Pr substituted rare earth (RE) oxide systems such asY1−xPrxBaSrCu3O7(Y/PrBaSr:123),La1−xPrxBaCaCu3O7(La/PrBaCa:123) andNd1−xPrxBaCaCu3O7(Nd/PrBaCa:123). We observe that the suppression of superconductivity due to Pr in all these systems is less in comparison to that in their RE:123 counterparts, i.e., in Pr substituted Y:123, La:123 and Nd:123, respectively. Interestingly with full Pr substitution in RE:123 compounds, the Pr moments order antiferromagnetically at 17K, while in the presently studied three systems, the same happens at around 10K only. Antiferromagnetic ordering temperature of the Pr moments of the end member of the series, which may be taken to be a measure of the strength of the hybridization of Pr-4f orbitals with Cu-O conduction band, appears to have a direct correlation with suppression of superconductivity in the corresponding series. Our results suggest that by various on-site substitutions or by modifying the parent unit cell of Pr:123 compound, one may be able to isolate the Pr-4f magnetic moments or decrease their interaction with the Cu-O conduction band, and achieve superconductivity in the presence of Pr. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59591

出版商:AIP    

年代:1999

数据来源: AIP