Bridging k- and q- Space in the Cuprates: Comparing ARPES and STM Results

A critical comparison is made between the ARPES-derived spectral function and STM studies of Friedel-like oscillations in Bi_2Sr_2CaCu_2O_{8+delta} (Bi2212). The data can be made approximately consistent, provided that (a) the elastic scattering seen in ARPES is predominantly small-angle scattering and (b) the `peak' feature seen in ARPES is really a dispersive `bright spot', smeared into a line by limited energy resolution; these are the `bright spots' which control the quasiparticle interferences. However, there is no indication of bilayer splitting in the STM data.Comment: 6 eps figures, revte

Remnant Fermi Surfaces in Photoemission

Recent experiments have introduced a new concept for analyzing the photoemission spectra of correlated electrons -- the remnant Fermi surface (rFs), which can be measured even in systems which lack a conventional Fermi surface. Here, we analyze the rFs in a number of interacting electron models, and find that the results fall into two classes. For systems with pairing instabilities, the rFs is an accurate replica of the true Fermi surface. In the presence of nesting instabilities, the rFs is a map of the resulting superlattice Brillouin zone. The results suggest that the gap in Ca_2CuO_2Cl_2 is of nesting origin.Comment: 4 pages LaTex, 3 ps figure

Structural, magnetic, dielectric and mechanical properties of (Ba,Sr)MnO3_3 ceramics

Ceramic samples, produced by conventional sintering method in ambient air, 6H-SrMnO$_3$(6H-SMO), 15R-BaMnO$_3$(15R-BMO), 4H-Ba$_{0.5}$Sr$_{0.5}$MnO$_3$(4H-BSMO) were studied. In the XRD measurements for SMO the new anomalies of the lattice parameters at 600-800 K range and the increasing of thermal expansion coefficients with a clear maximum in a vicinity at 670 K were detected. The N$\acute{e}$el phase transition for BSMO was observed at $T_N$=250 K in magnetic measurements and its trace was detected in dielectric, FTIR, DSC, and DMA experiments. The enthalpy and entropy changes of the phase transition for BSMO at $T_N$ were determined as 17.5 J/mol and 70 mJ/K mol, respectively. The activation energy values and relaxation times characteristic for relaxation processes were determined from the Arrhenius law. Results of ab initio simulations showed that the contribution of the exchange correlation energy to the total energy is about 30%.Comment: 12 pages, 12 figure

Proposal to determine the Fermi-surface topology of a doped iron-based superconductor using bulk-sensitive Fourier-transform Compton scattering

We have carried out first-principles calculations of the Compton scattering spectra to demonstrate that the filling of the hole Fermi surface in LaO$_{1-x}$F$_{x}$FeAs produces a distinct signature in the Fourier transformed Compton spectrum when the momentum transfer vector lies along the [100] direction. We thus show how the critical concentration $x_c$, where hole Fermi surface pieces are filled up and the superconductivity mediated by antiferromagnetic spin fluctuations is expected to be suppressed, can be obtained in a bulk-sensitive manner.Comment: 4 pages, 6 figures, accepted in Physical Review

Stripes, Pseudogaps, and Van Hove Nesting in the Three-band tJ Model

Slave boson calculations have been carried out in the three-band tJ model for the high-T_c cuprates, with the inclusion of coupling to oxygen breathing mode phonons. Phonon-induced Van Hove nesting leads to a phase separation between a hole-doped domain and a (magnetic) domain near half filling, with long-range Coulomb forces limiting the separation to a nanoscopic scale. Strong correlation effects pin the Fermi level close to, but not precisely at the Van Hove singularity (VHS), which can enhance the tendency to phase separation. The resulting dispersions have been calculated, both in the uniform phases and in the phase separated regime. In the latter case, distinctly different dispersions are found for large, random domains and for regular (static) striped arrays, and a hypothetical form is presented for dynamic striped arrays. The doping dependence of the latter is found to provide an excellent description of photoemission and thermodynamic experiments on pseudogap formation in underdoped cuprates. In particular, the multiplicity of observed gaps is explained as a combination of flux phase plus charge density wave (CDW) gaps along with a superconducting gap. The largest gap is associated with VHS nesting. The apparent smooth evolution of this gap with doping masks a crossover from CDW-like effects near optimal doping to magnetic effects (flux phase) near half filling. A crossover from large Fermi surface to hole pockets with increased underdoping is found. In the weakly overdoped regime, the CDW undergoes a quantum phase transition ($T_{CDW}\to 0$), which could be obscured by phase separation.Comment: 15 pages, Latex, 18 PS figures Corrects a sign error: major changes, esp. in Sect. 3, Figs 1-4,6 replace

Dielectric properties of BiFeO3 ceramics obtained from mechanochemically synthesized nanopowders

Dielectric behaviour of BiFeO3 ceramics,obtained by hot-pressing of nanopowders produced by mechanochemical synthesis from Bi2O3 and Fe2O3 oxides (weight ratio 2:1), was studied in the temperature range 125–575 K. The ceramics was found to exhibit step-like dielectric response ε*(T) with high permittivity values, similar to the behaviour of materials with giant dielectric permittivity. Three overlapping relaxation processes contribute to the dielectric response: i) relaxation in the lowtemperature range (220–420 K), characterized by activation energy of 0.4 eV, ii) relaxation in the temperature range 320–520 K with activation energy of 1.0 eV and iii) broad dielectric anomaly in the vicinity of 420 K, which disappears after 1 h annealing at 775 K. The lowtemperature relaxation is ascribed to the carrier hopping process between Fe2+ and Fe3+ ions. The presence of mixed valence of the Fe ions was proved by X-ray photoelectron spectroscopy. Dielectric relaxation in the middle-temperature range is considered as a result of grain boundary effect and internal barrier layers related to Bi25FeO40 phase as verified by X-ray diffraction. The high-temperature dielectric anomaly we relate to short-range hopping of ordered oxygen vacancies

Dielectric properties of BiFeO3 ceramics obtained from mechanochemically synthesized nanopowders

Dielectric behaviour of BiFeO3 ceramics,obtained by hot-pressing of nanopowders produced by mechanochemical synthesis from Bi2O3 and Fe2O3 oxides (weight ratio 2:1), was studied in the temperature range 125–575 K. The ceramics was found to exhibit step-like dielectric response ε*(T) with high permittivity values, similar to the behaviour of materials with giant dielectric permittivity. Three overlapping relaxation processes contribute to the dielectric response: i) relaxation in the lowtemperature range (220–420 K), characterized by activation energy of 0.4 eV, ii) relaxation in the temperature range 320–520 K with activation energy of 1.0 eV and iii) broad dielectric anomaly in the vicinity of 420 K, which disappears after 1 h annealing at 775 K. The lowtemperature relaxation is ascribed to the carrier hopping process between Fe2+ and Fe3+ ions. The presence of mixed valence of the Fe ions was proved by X-ray photoelectron spectroscopy. Dielectric relaxation in the middle-temperature range is considered as a result of grain boundary effect and internal barrier layers related to Bi25FeO40 phase as verified by X-ray diffraction. The high-temperature dielectric anomaly we relate to short-range hopping of ordered oxygen vacancies

Flux Phase as a Dynamic Jahn-Teller Phase: Berryonic Matter in the Cuprates?

There is considerable evidence for some form of charge ordering on the hole-doped stripes in the cuprates, mainly associated with the low-temperature tetragonal phase, but with some evidence for either charge density waves or a flux phase, which is a form of dynamic charge-density wave. These three states form a pseudospin triplet, demonstrating a close connection with the E X e dynamic Jahn-Teller effect, suggesting that the cuprates constitute a form of Berryonic matter. This in turn suggests a new model for the dynamic Jahn-Teller effect as a form of flux phase. A simple model of the Cu-O bond stretching phonons allows an estimate of electron-phonon coupling for these modes, explaining why the half breathing mode softens so much more than the full oxygen breathing mode. The anomalous properties of $O^{2-}$ provide a coupling (correlated hopping) which acts to stabilize density wave phases.Comment: Major Revisions: includes comparisons with specific cuprate phonon modes, 16 eps figures, revte