摘要:

The dispersion of the in‐plane CuO bond stretching LO phonon mode in the high‐TCcuprate superconductors shows strong softening with the optimum doped samples near the zone boundary, but the overdoped samples with zeroTCdoes not show the anomalous softening. This fact suggests that the phonon is closely related to the mechanism of the high‐TCsuperconductivity. The softening can be described with a negative electronic dielectric function that results in overscreening of the inter‐site Coulomb interaction in the highly correlated cuprate systems. We propose that such a strong electron‐phonon coupling of specific modes can form basis for phonon mechanism of high‐temperature superconductivity. On the basis of the model, with the Eliashberg theory using the experimentally determined electron dispersion and dielectric function, we demonstrate the possibility of superconductivity with the order parameter of the symmetry and the transition temperature well in excess of 100K. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639571

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Recently many people discuss unconventional density waves (i.e. unconventional charge density waves (UCDW) and unconventional spin density waves (USDW)). Unlike in conventional density waves, the quasiparticle spectrum in these systems is gapless. Also these systems remain metallic. Indeed it appears that there are many candidates for UDW. The low temperature phase of &agr;‐(BEDT‐TTF)2KHg(SCN)4, the antiferromagnetic phase in URu2Si2, the CDW in transition metal dichalcogenite NbSe2, the pseudogap phase in highTccuprate superconductors, the glassy phase in organic superconductor &kgr;‐(BEDT‐TTF)2Cu[N(CN)2]Br. After a brief introduction on UCDW and USDW, we shall discuss some of the above systems, where we believe we have evidence for unconventional density waves. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639572

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

When the Fermi level is near the top of a band the carriers (holes) are maximally dressed by electron‐ion and electron‐electron interactions. The theory of hole superconductivity predicts that only in that case can superconductivity occur, and that it is driven byundressingof the carriers at the Fermi energy upon pairing. Indeed, experiments show that dressed hole carriers in the normal state become undressed electron carriers in the superconducting state. This leads to a description of superconductors as giant atoms, where undressed time‐reversed electrons are paired and propagate freely in a uniform positive background. The pairing gap provides rigidity to the wavefunction, and electrons in the giant atom respond to magnetic fields the same way as electrons in diamagnetic atoms. We predict that there is an electric field in the interior of superconductors and that the charge distribution is inhomogeneous, with higher concentration of negative charge near the surface; that the ground state of superconductors has broken parity and possesses macroscopic spin currents, and that negative charge spills out when a body becomes superconducting. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639573

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We review recent work on the properties of a d‐wave superconducting state in which strong Coulomb interactions suppress double occupancy. We find that pairing and phase coherence show qualitatively different trends as a function of doping: the pairing scale decreases monotonically with hole dopingxwhile the order parameter shows a non‐monotonic “dome”. We obtain detailed results for the doping‐dependences of various quantities including the momentum distribution, nodal quasiparticle weights and dispersions, optical spectral weight, and superfluid density. Our results are in remarkable agreement with existing data on the highTccuprates and some of our predictions have been recently verified. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639574

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The extension of the BCS theory to the strong‐coupling regime with small bipolarons numerically accounts for high superconducting critical temperatures and isotope effects, of many high‐Tcsuperconductors. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639575

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

In this presentation three recent contributions to the theory of continuum (or “Fro¨hlich”‐) polarons are discussed. (i) Using a generalization of the Jensen‐Feynman variational principle within the path‐integral formalism for identical particles, the ground‐state energy of a confinedN‐polaron system is studied as a function ofNand of the electron‐phonon coupling strength. (ii) Cyclotron‐resonance (CR) spectra of a gas of interacting polarons in a GaAs/AlAs quantum well are theoretically investigated taking into account the magnetoplasmon‐phonon mixing and the band non‐parabolicity. The calculated CR spectra are in a good agreement with experimental data. The theory explains that, for a high‐density polaron gas, anticrossing of the CR spectra occurs near the GaAs TO‐phonon frequency rather than near the GaAs LO‐frequency. (iii) A theoretical investigation of the optical properties of stacked quantum dots is presented, which is based on the non‐adiabatic approach. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639576

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The density response of a doped Mott‐Hubbard insulator is discussed starting from thet‐Jmodel in a slave boson 1/Nrepresentation. In leading orderO(1) the density fluctuation spectraN(q, &ohgr;) are determined by an undamped collective mode at large momentum transfer, in striking disagreement with results obtained by exact diagonalization, which reveal a very broad dispersive peak, reminescent of strong spin‐charge coupling. The 1/Ncorrections introduce the polaron character of the bosonic holes moving in a uniform RVB background. The resultingN(q, &ohgr;) captures all features observed in diagonalization studies, fulfills the appropriate sum rules, and apart from the broadening of the collective mode shows a new low energy feature at the energy &khgr;J+ &dgr;trelated to the polaron motion in the spinon background. It is further shown that the low energy structure, which is particularly pronounced in (&pgr;, 0) direction, describes the strong renormalization and anomalous damping of the highest bond‐stretching phonons in La2−xSrxCuO4. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639577

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We review recent studies on developing tools for quantum complex phenomena. The tools have been applied for clarifying the perspective of the Mott transitions and the phase diagram of metals, Mott insulators and magnetically ordered phases in the two‐dimensional Hubbard model. The path‐integral renormalization‐group (PIRG) method has made it possible to numerically study correlated electrons even with geometrical frustration effects without biases . It has numerically clarified the phase diagram at zero temperature,T= 0, in the parameter space of the onsite Coulomb repulsion, the geometrical frustration amplitude and the chemical potential. When the bandwidth is controlled at half filling, the first‐order transition between insulating and metallic phases is evidenced. In contrast, the filling‐control transition shows diverging critical fluctuations for spin and charge responses with decreasing doping concentration. Near the Mott transition, a nonmagnetic spin‐liquid phase appears in a region with large frustration effects. The phase is characterized remarkably by gapless spin excitations and the vanishing dispersion of spin excitations. Magnetic orders quantum mechanically melt through diverging magnon mass. The correlator projection method (CPM) is formulated as an extension of the operator projection theory. This method also allows an extension of the dynamical mean‐field theory (DMFT) with systematic inclusion of the momentum dependence in the self‐energy. It has enabled determining the phase diagram atT> 0, where the boundary surface of the first‐order metal‐insulator transition at half filling terminates on the critical end curve atT=Tc. The critical end curve is characterized by the diverging compressibility. The single particle spectra show strong renormalization of low‐energy spectra, generating largely momentum dependent and flat dispersion. The results of two tools consistently suggest that the strong competitions of various phases with underlying diverging compressibility can induce an emergence near a new type of quantum criticality distinguished from the conventional and simple quantum critical phenomena. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639578

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Superconducting pairing mediated by antiferromagnetic (AFM) exchange and spin‐fluctuations is considered within the effective two‐band Hubbard model for a copper‐oxide plane. It is proved that retardation effects for the AFM exchange induced by interband hopping are unimportant that results in pairing of all the carries in the conduction band and highTcproportional to the Fermi energy. The kinematic interaction induced by intraband hopping gives an additional spin‐fluctuation contribution to thed‐wave pairing.Tcdependence on pressure and oxygen isotope effect are explained. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639579

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A theory of the dynamical spin susceptibility is presented, relevant for the normal‐state magnetic response and the resonant magnetic peak in superconducting (SC) cuprates. The analysis is based on the equations of motion for spins within thet‐Jmodel and on the memory‐function representation of magnetic response. An evidence from numerical studies of the model confirms that the damping of the spin collective model is large in the normal state being due to the decay into fermionic degrees of freedom. Assuming the saturation of equal‐time correlations at lowTthis leads to the anomalous &ohgr;/Tscaling, explaining neutron scattering experiments on cuprates at low doping. In the SC phase a d‐wave SC gap leads to a sharp resonant peak with reduced intensity and downward dispersion. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639580

出版商:AIP    

年代:1903

数据来源: AIP