Optical Spectroscopy as a Probe of Gaps and Kinetic Electronic Energy in p- and n-type cuprates

The real part of the optical in-plane conductivity of p-- and n--type cuprates thin films at various doping levels was deduced from highly accurate reflectivity measurements. We present here a comprehensive set of optical spectral weight data as a function of the temperature $T (> T_c$), for underdoped and overdoped samples. The temperature dependence of the spectral weight is not universal. Using various cut-off frequencies for the spectral weight, we show that n--type Pr$_{2-x}$Ce$_x$CuO$_4$ and p--type Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ exhibit both similarities and striking differences. The Fermi surface is closed in overdoped metallic samples. In underdoped Pr$_{2-x}$Ce$_x$CuO$_4$ samples, it clearly breaks into arcs, giving rise to a "pseudogap" signature. It is argued that such a signature is subtle in underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$.Comment: Proceedings M2SHTSCVIII, to appear in Physica

Observation of a two-dimensional electron gas at CaTiO3_3 film surfaces

The two-dimensional electron gas at the surface of titanates gathered attention due to its potential to replace conventional silicon based semiconductors in the future. In this study, we investigated films of the parent perovskite CaTiO$_3$, grown by pulsed laser deposition, by means of angular-resolved photoelectron spectroscopy. The films show a c(4x2) surface reconstruction after the growth that is reduced to a p(2x2) reconstruction under UV-light. At the CaTiO$_3$ film surface, a two-dimensional electron gas (2DEG) is found with an occupied band width of 400 meV. With our findings CaTiO$_3$ is added to the group of oxides with a 2DEG at their surface. Our study widens the phase space to investigate strontium and barium doped CaTiO$_3$ and the interplay of ferroelectric properties with the 2DEG at oxide surfaces. This could open up new paths to tailor two-dimensional transport properties of these systems towards possible applications

Temperature dependence of the spectral weight in p- and n-type cuprates: a study of normal state partial gaps and electronic kinetic energy

The optical conductivity of CuO2 (copper-oxygen) planes in p- and n-type cuprates thin films at various doping levels is deduced from highly accurate reflectivity data. The temperature dependence of the real part sigma1(omega) of this optical conductivity and the corresponding spectral weight allow to track the opening of a partial gap in the normal state of n-type Pr{2-x}Ce(x)CuO4 (PCCO), but not of p-type Bi2Sr2CaCu2O(8+delta} (BSCCO) cuprates. This is a clear difference between these two families of cuprates, which we briefly discuss. In BSCCO, the change of the electronic kinetic energy Ekin - deduced from the spectral weight- at the superconducting transition is found to cross over from a conventional BCS behavior (increase of Ekin below Tc to an unconventional behavior (decrease of Ekin below Tc) as the free carrier density decreases. This behavior appears to be linked to the energy scale over which spectral weight is lost and goes into the superfluid condensate, hence may be related to Mott physics

Optical sum rule in metals with a strong interaction

The restricted optical sum rule and its dependence on the temperature, a superconducting gap and the cutoff energy have been investigated. As known this sum rule depends on the cutoff energy and the relaxation rate even for a homogeneous electron gas interacting with impurities or phonons. It is shown here that additional dependence of the spectral weight on a superconducting gap is very small in this model and this effect disappears totally when the relaxation rate is equal zero. The model metal with a single band is considered in details. It is well known that for this model there is the dependence of the sum rule on the temperature and the energy gap even in the case when the relaxation is absent. This dependence exists due to the smearing of the electron distribution function and it is expressed in the terms of Sommerfeld expansion. Here it is shown that these effects are considerably smaller than that of related with the relaxation rate if the band width is larger than the average phonon frequency. It is shown also that the experimental data about the temperature dependence of the spectral weight for the high- materials can be successfully explained in the framework approach based on the temperature dependence of the relaxation rateComment: 13 pages, 7 figures, the talk given on Internatinal coference on theoretical physics, april 11-16,2005, Mosco

Kinetic energy change with doping upon superfluid condensation in high temperature superconductors

In conventional BCS superconductors, the electronic kinetic energy increases upon superfluid condensation (the change DEkin is positive). Here we show that in the high critical temperature superconductor Bi-2212, DEkin crosses over from a fully compatible conventional BCS behavior (DEkin>0) to an unconventional behavior (DEkin<0) as the free carrier density decreases. If a single mechanism is responsible for superconductivity across the whole phase diagram of high critical temperature superconductors, this mechanism should allow for a smooth transition between such two regimes around optimal doping.Comment: 3 pages, 2 figure

Momentum-resolved evolution of the Kondo lattice into 'hidden-order' in URu2Si2

We study, using high-resolution angle-resolved photoemission spectroscopy, the evolution of the electronic structure in URu2Si2 at the Gamma, Z and X high-symmetry points from the high-temperature Kondo-screened regime to the low-temperature `hidden-order' (HO) state. At all temperatures and symmetry points, we find structures resulting from the interaction between heavy and light bands, related to the Kondo lattice formation. At the X point, we directly measure a hybridization gap of 11 meV already open at temperatures above the ordered phase. Strikingly, we find that while the HO induces pronounced changes at Gamma and Z, the hybridization gap at X does not change, indicating that the hidden-order parameter is anisotropic. Furthermore, at the Gamma and Z points, we observe the opening of a gap in momentum in the HO state, and show that the associated electronic structure results from the hybridization of a light electron band with the Kondo-lattice bands characterizing the paramagnetic state.Comment: Updated published version. Mansucript + Supplemental Material (8 pages, 9 figures). Submitted 16 September 201

Two-Fermi-surface superconducting state and a nodal d-wave gap in the electron-doped Sm(1.85)Ce(0.15)CuO(4-d) cuprate superconductor

We report on laser-excited angle-resolved photoemission spectroscopy (ARPES) in the electron-doped cuprate Sm(1.85)Ce(0.15)CuO(4-d). The data show the existence of a nodal hole-pocket Fermi-surface both in the normal and superconducting states. We prove that its origin is long-range antiferromagnetism by an analysis of the coherence factors in the main and folded bands. This coexistence of long-range antiferromagnetism and superconductivity implies that electron-doped cuprates are two-Fermi-surface superconductors. The measured superconducting gap in the nodal hole-pocket is compatible with a d-wave symmetry.Comment: 4 pages, 3 figures, accepted to Phys. Rev. Let

Estimativa do comprimento m?dio de tra?os de descontinuidades em maci?os rochosos utilizando m?todos estat?sticos e fotogrametria digital terrestre.

Programa de P?s-Gradua??o em Engenharia Mineral. Departamento de Engenharia de Minas, Escola de Minas, Universidade Federal de Ouro Preto.O maci?o rochoso ? material de trabalho para muitas obras de engenharia, tais como taludes rodovi?rios e de minera??o, escava??es subterr?neas de t?neis e galerias minera??o. O comportamento mec?nico e as propriedades hidr?ulicas deste maci?o s?o fortemente influenciados pela presen?a das descontinuidades que o divide em por??es de rocha intacta. A caracteriza??o quantitativa dos par?metros geomec?nicos das descontinuidades ? uma tarefa muito complicada e demorada. O tamanho, ou persist?ncia, das descontinuidades, ? um dos par?metros mais importantes para esta caracteriza??o, mas ? tamb?m o mais dif?cil de se avaliar. Ao longo dos anos, foram desenvolvidos m?todos de amostragem areal para se estimar o comprimento m?dio das descontinuidades. No entanto, os m?todos de amostragem carregam vieses, principalmente de censura e de tamanho que perturbam as estimativas. Para reduzir estes vieses, algumas considera??es podem ser feitas em rela??o ? forma e distribui??o espacial dos centr?ides de descontinuidades no maci?o rochoso. A partir disso, os m?todos de amostragem podem definir, ou n?o, fun??es de distribui??o para se estimar a m?dia e o desvio padr?o do comprimento de tra?os a partir de interpreta??es estereol?gicas. Os m?todos de amostragem utilizando janelas circulares n?o assumem nenhuma fun??o de distribui??o de probabilidade espec?fica para o tamanho ou a orienta??o das descontinuidades. Mais recentemente, o uso de m?todos de fotogrametria tem ganhado relev?ncia para o mapeamento geomec?nico devido ? facilidade e agilidade dos procedimentos, mas tamb?m a bons resultados representativos. Este estudo utiliza m?todos de fotogrametria e mapeamento geomec?nico para estimar a m?dia e o desvio padr?o do di?metro das descontinuidades. Esse comprimento m?dio de tra?o ? medido diretamente, obtendo um valor num?rico e ? comparado com o valor obtido por meio de m?todos de amostragem utilizando janelas circulares. As exposi??es de rochas mostram uma distribui??o Log-Normal dos discos de descontinuidades. No entanto, o comprimento m?dio dos tra?os de descontinuidade obtido por janelas de amostragem ? decididamente menor do que o comprimento m?dio dos tra?os medido diretamente com o aux?lio da fotogrametria.Rock mass is the environment for many engineering works, such as road and mining slopes or underground tunnel and mining excavations. The mechanical behavior and hydraulic characteristics of rock mass are strongly influenced by the discontinuities that divide it into intact rock portions. The quantitative characterization of the geomechanical parameters of discontinuities is a very complicated and time-consuming task. The size, or persistence, of discontinuities, is one of the most critical parameters for this characterization, but it is also the most difficult to evaluate. Over the years, areal sampling methods are developed to estimate the mean trace length of discontinuities. However, areal sampling methods carry on a bias, mainly of censorship and the size that disturb this estimation. For diminishing this bias, some assumptions can be made regarding the shape and spatial distribution of centroids of discontinuities in the rock mass. From this, the sampling methods could define, or not, distribution functions to estimate the average and standard deviation of trace length from stereological interpretations. Sampling methods using circular windows do not assume the size or orientation of discontinuities to follow an specific probability density function. More recently, the use of photogrammetry methods has been gaining relevance for geomechanical mapping due to the ease and agility of the procedures, but also to good representative results. This study uses photogrammetry and geomechanical mapping methods to estimate the mean and standard deviation of the diameter of the discontinuities. This mean trace length is directly measured obtaining a numeric value and is compared with the value obtained through sampling methods using circular windows. Rock exposures show a Log-Normal distribution of the disc of discontinuities. However, the mean trace length obtained by sampling windows are decidedly smaller than the mean trace length directly measured with the aid of photogrammetry