摘要:

In the first part of this article (section 2 to 10) a review is given on the first-principles studies of the many-electron electronic structures of superconducting cuprates performed by Kamimura and his co-workers. In this review, unusual electronic states in the underdoped cuprates are clarified. This characteristic state is called the “Kamimura-Suwa model”. In section 11 in the latter part we describe the very interesting results recently obtained by Kamimura, Hamada and Ushio, based on the Kamimura-Suwa model. According to them the microscopic origins of the “high energy” pseudogap and of the peculiar hole-concentration dependence of the electronic entropy correspond to a phase change between a phase consisting of the small Fermi surfaces and a phase with a large Fermi surface, although the phase change is not sharp due to the finite spin-correlation length for local antiferromagnetic ordering. Through this calculation we will show that the hole carriers in cuprates behave like heavy fermions. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1363056

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Near the metal-insulator transition competing forces to localize and delocalize charges are in delicate balance, and small perturbations can produce drastic effects. For the manganites that show colossal magnetoresistivity (CMR) the electron-lattice interaction plays a key role in producing and altering the balance through formation of polarons. Our recent measurements of high-energy LO phonons by inelastic neutron scattering demonstrate that polaronic charge localization tendency exists also in the cuprates that exhibit high-temperature superconductivity (HTSC). It is suggested that the lattice is an integral part of the HTSC mechanism, and thatmarginalstability of polaron is important for the CMR effect in manganites as ell as for the HTSC in cuprates. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1363057

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

In the last decade, we have witnessed a number of cases where a small deviation from crystallographic or average structure strongly modifies electron states, physical and chemical properties and functions. The crucial role of breaking local symmetry, stabilizing a certain electronic (spin-state) configuration, has been recognized recently, by x-ray absorption spectroscopy (XAS). In biological systems such as haeme proteins, having3dtransition metal ions at active sites in a large molecule, XAS studies clarified the intimate relation between the local structure and the spin state. A number of highTcsuperconducting oxides have been investigated and a generic behavior of local lattice as an intrinsic tendency of highly correlated electrons is revealed. We find anomalous local lattice distortions inYBa2Cu3Oyemerge at characteristic temperaturesT*andT0associated with changes of electron states, indicatinga strong electron-lattice coupling. We demonstrate thatin situXAS can probe the local lattice distortions during photo-excitation. XAS showed that photo-induced nonthermal melting occurs in chalcogenide glasses and the frozen-in local lattice distortions cause Coulomb repulsion observed as photo-structural change. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1363058

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

By using ion channeling, an ultrafast real space probe of small (sub picometer range) incoherent atomic displacements, direct experimental evidence for electronic phase separation inYBa2Cu3O7−xas a function of oxygen doping has been obtained. The stripe dynamics in YBCO has been predicted to undergo three phase crossovers/transitions as a function of temperature, at the stripe phase(T1),spingap formation temperature(T2)and the global superconducting transition temperatureTc.Three similar temperatures are seen in ion channeling investigations, in the form of non analycities in the incoherent lattice fluctuations. Their observed weak doping dependence conforms to the suggested behavior of the pseudo-gap formation temperatureT2,whileT1has shown a clear difference in the spin and charge dynamics. Similar incoherent atomic displacements have been observed in CMR manganites at the phase transition from paramagnetic insulator to ferromagnetic metal. Replacing16Oby18Oin high quality single crystallineNd0.67Sr0.33MnO3films (pulse laser deposited), a clear change in the atomic displacement is seen in the two cases, close to the respective phase transitions. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1363059

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The dynamical stripes of separated spin and charge densities were investigated inLa2−xSrxCuO4utilizing the high frequency response of Raman scattering. The two-magnon peak splits into double peaks below about 300–350 K at0.035⩽x⩽0.06andx=0.115due to the formation of the stripe structure. The spectra recover metallic electronic response below about 1100 cm−1. It suggests that one observes the instantaneous picture of magnetic excitations above this characteristic frequency of the stripe motion and the averaged picture of the fluctuating charge stripes below it. The threshold frequency is assigned to the two-phonon energy of the breathing mode in which a hole is migrating synchronously with the vibration between the antiphaseCuO6pair. At high temperature or at the carrier concentration less relating to the stripe structure, the influence of this phonon mode is reserved in theB2gspectra. On the other hand theB1gspectra include the influence of only magnetic excitations. TheB1gandB2gspectra correspond to the electronic excitations along the (0,0)–(&pgr;,0) and the (0,0)–(&pgr;,&pgr;) directions in thek-space, respectively. The anisotropic spectra in thek-space can be interpreted as the anisotropic charge propagation, that is, the charge transfer along the Cu–O–Cu direction excites magnons, while the charge transfer along the diagonal direction excites phonons. This anisotropic charge transfer mechanism gives fundamental properties to the highTcsuperconductors. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1363060

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Different classes of physical systems with sizeable electron-phonon coupling and lattice distortions present anomalous resistivity behaviors versus temperature. We study a molecular lattice Hamiltonian in which polaronic charge carriers interact with non linear potentials provided by local atomic fluctuations between two equilibrium sites. A path integral model is developed to select the class of atomic oscillations which mainly contributes to the partition function and the electrical resistivity is computed in a number of representative cases. We argue that the common origin of the observed resistivity anomalies lies in the time retarded nature of the polaronic interactions in the local structural instabilities. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1363061

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Green’s function method is used to calculate destabilization criteria for local perturbations causing structural distortions in cuprate superconductors. The build-up of soft low-frequency dynamical resonances is traced. Chain O(1)a-axis and apex O(4) ⟨110⟩-instabilities inYBa2Cu3O7have been investigated. The Cu out-of-planec-axis destabilization have been considered also forLa2CuO4.©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1363062

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The effect of the dynamic vibrations of the in-plane oxygen atoms on the hole pairing and charge ordering in the high-Tccuprates is investigated based on generalized 2Dt-JHolstein models. In-plane (breathing) and out-of-plane (buckling) vibrational modes are considered. The exact diagonalization of the finite clusters revealed that the buckling vibrations stabilize thed-wave hole pairing, while the breathing ones suppress it. It is also found that the breathing modes possibly play an important role in some static order formations, while the buckling ones yield no effect. In addition we discuss on the polaron states with some phonon-dressed hole operators. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1363063

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

In a strong covalent bond network system, tilt order ofMO6octahedra (M; Transition metals), produced by twin order or microscopic phase mixture, is proposed as an origin of charge order or stripe formation in the vicinity of insulator-metal transition. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1363064

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Nonlinear feedback effects due to dispersive bosons are studied in fermion-boson coupled systems by using an extension of dynamical mean-field theory. In the nonperturbative regime, strong coupling and nonadiabaticity reduce effective interaction between fermions as the width of boson dispersion increases. The boson field is completely softened at the crossover to this regime. The softening is significantly enhanced by the boson dispersion. We elucidate the controlling parameters of this nonperturbative regime where fluctuations of the dispersive bosons accelerate the delocalization of fermions. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1363065

出版商:AIP    

年代:1901

数据来源: AIP