Non-gapped Fermi surfaces, quasiparticles and the anomalous temperature dependence of the near-EFE_F electronic states in the CMR oxide La2−2x_{2-2x}Sr1+2x_{1+2x}Mn2_2O7_7 with x=0.36x=0.36

After years of research into colossal magnetoresistant (CMR) manganites using bulk techniques, there has been a recent upsurge in experiments directly probing the electronic states at or near the surface of the bilayer CMR materials La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ using angle-resolved photoemission or scanning probe microscopy. Here we report new, temperature dependent, angle resolved photoemission data from single crystals with a doping level of $x=0.36$. The first important result is that there is no sign of a pseudogap in the charge channel of this material for temperatures below the Curie temperature $T_C$. The second important result concerns the temperature dependence of the electronic states. The temperature dependent changes in the Fermi surface spectra both at the zone face and zone diagonal regions in $k$-space indicate that the coherent quasiparticle weight disappears for temperatures significantly above $T_C$, and that the $k$-dependence of the T-induced changes in the spectra invalidate an interpretation of these data in terms of the superposition of a `universal' metallic spectrum and an insulating spectrum whose relative weight changes with temperature. In this sense, our data are not compatible with a phase separation scenario.Comment: 6 pages, 4 figure

Spontaneous exciton condensation in 1T-TiSe2: a BCS-like approach

Recently strong evidence has been found in favor of a BCS-like condensation of excitons in 1\textit{T}-TiSe$_2$. Theoretical photoemission intensity maps have been generated by the spectral function calculated within the excitonic condensate phase model and set against experimental angle-resolved photoemission spectroscopy data. Here, the calculations in the framework of this model are presented in detail. They represent an extension of the original excitonic insulator phase model of J\'erome \textit{et al.} [Phys. Rev. {\bf 158}, 462 (1967)] to three dimensional and anisotropic band dispersions. A detailed analysis of its properties and further comparison with experiment are also discussedComment: Submitted to PRB, 11 pages, 7 figure

Disentangling surface and bulk photoemission using circularly polarized light

We show that in the angle resolved photoemission spectroscopy (ARPES) near-surface induced fields can be useful for disentangling the surface and bulk related emission. The jump of the dielectric function at the interface results in a nonzero term $\operatorname{div}\textbf{A}$ in the photoemission matrix element. The term happens to be significant approximately within the first unit cell and leads to the circular dichroism for the states localized therein. As an example we use ARPES spectra of an YBa$_2$Cu$_3$O$_{7-\delta}$ crystal to distinguish between the overdoped surface related component and its bulk counterparts.Comment: 4 pages, 2 figure

Non-monotonic pseudo-gap in high-Tc cuprates

The mechanism of high temperature superconductivity is not resolved for so long because the normal state of cuprates is not yet understood. Here we show that the normal state pseudo-gap exhibits an unexpected non-monotonic temperature dependence, which rules out the possibility to describe it by a single mechanism such as superconducting phase fluctuations. Moreover, this behaviour, being remarkably similar to the behaviour of the charge ordering gap in the transition-metal dichalcogenides, completes the correspondence between these two classes of compounds: the cuprates in the PG state and the dichalcogenides in the incommensurate charge ordering state reveal virtually identical spectra of one-particle excitations as function of energy, momentum and temperature. These results suggest that the normal state pseudo-gap, which was considered to be very peculiar to cuprates, seems to be a general complex phenomenon for 2D metals. This may not only help to clarify the normal state electronic structure of 2D metals but also provide new insight into electronic properties of 2D solids where the metal-insulator and metal-superconductor transitions are considered on similar basis as instabilities of particle-hole and particle-particle interaction, respectively