Exact Solution for the Distribution of Transmission Eigenvalues in a Disordered Wire and Comparison with Random-Matrix Theory

An exact solution is presented of the Fokker-Planck equation which governs the evolution of an ensemble of disordered metal wires of increasing length, in a magnetic field. By a mapping onto a free-fermion problem, the complete probability distribution function of the transmission eigenvalues is obtained. The logarithmic eigenvalue repulsion of random-matrix theory is shown to break down for transmission eigenvalues which are not close to unity. ***Submitted to Physical Review B.****Comment: 20 pages, REVTeX-3.0, INLO-PUB-931028

Electronic Phase Separation Transition as the Origin of the Superconductivity and the Pseudogap Phase of Cuprates

We propose a new phase of matter, an electronic phase separation transition that starts near the upper pseudogap and segregates the holes into high and low density domains. The Cahn-Hilliard approach is used to follow quantitatively this second order transition. The resulting grain boundary potential confines the charge in domains and favors the development of intragrain superconducting amplitudes. The zero resistivity transition arises only when the intergrain Josephson coupling $E_J$ is of the order of the thermal energy and phase locking among the superconducting grains takes place. We show that this approach explains the pseudogap and superconducting phases in a natural way and reproduces some recent scanning tunneling microscopy dataComment: 4 pages and 5 eps fig

SAMplus: adaptive optics at optical wavelengths for SOAR

Adaptive Optics (AO) is an innovative technique that substantially improves the optical performance of ground-based telescopes. The SOAR Adaptive Module (SAM) is a laser-assisted AO instrument, designed to compensate ground-layer atmospheric turbulence in near-IR and visible wavelengths over a large Field of View. Here we detail our proposal to upgrade SAM, dubbed SAMplus, that is focused on enhancing its performance in visible wavelengths and increasing the instrument reliability. As an illustration, for a seeing of 0.62 arcsec at 500 nm and a typical turbulence profile, current SAM improves the PSF FWHM to 0.40 arcsec, and with the upgrade we expect to deliver images with a FWHM of $\approx0.34$ arcsec -- up to 0.23 arcsec FWHM PSF under good seeing conditions. Such capabilities will be fully integrated with the latest SAM instruments, putting SOAR in an unique position as observatory facility.Comment: To appear in Proc. SPIE 10703 (Ground-based and Airborne Instrumentation for Astronomy VII; SPIEastro18

High-resolution abundance analysis of HD 140283

HD 140283 is a reference subgiant that is metal poor and confirmed to be a very old star. The abundances of this type of old star can constrain the nature and nucleosynthesis processes that occurred in its (even older) progenitors. The present study may shed light on nucleosynthesis processes yielding heavy elements early in the Galaxy. A detailed abundance analysis of a high-quality spectrum is carried out, with the intent of providing a reference on stellar lines and abundances of a very old, metal-poor subgiant. We aim to derive abundances from most available and measurable spectral lines. The analysis is carried out using high-resolution (R = 81 000) and high signal-to-noise ratio (800 < S/N/pixel < 3400) spectrum, in the wavelength range 3700 - 10475, obtained with a seven-hour exposure time, using the ESPaDOnS at the CFHT. The calculations in LTE were performed with the OSMARCS 1D atmospheric model and the spectrum synthesis code Turbospectrum, while the analysis in NLTE is based on the MULTI code. We present LTE abundances for 26 elements, and NLTE calculations for the species C I, O I, Na I, Mg I, Al I, K I, Ca I, Sr II, and Ba II lines. The abundance analysis provided an extensive line list suitable for metal-poor subgiant stars. The results for Li, CNO, alpha-, and iron peak elements are in good agreement with literature. The newly NLTE Ba abundance, along with a NLTE Eu correction and a 3D Ba correction from literature, leads to [Eu/Ba] = +0.59 +/- 0.18. This result confirms a dominant r-process contribution, possibly together with a very small contribution from the main s-process, to the neutron-capture elements in HD 140283. Overabundances of the lighter heavy elements and the high abundances derived for Ba, La, and Ce favour the operation of the weak r-process in HD 140283.Comment: 34 pages, 27 figure

Path Integral Approach to the Scattering Theory of Quantum Transport

The scattering theory of quantum transport relates transport properties of disordered mesoscopic conductors to their transfer matrix \bbox{T}. We introduce a novel approach to the statistics of transport quantities which expresses the probability distribution of \bbox{T} as a path integral. The path integal is derived for a model of conductors with broken time reversal invariance in arbitrary dimensions. It is applied to the Dorokhov-Mello-Pereyra-Kumar (DMPK) equation which describes quasi-one-dimensional wires. We use the equivalent channel model whose probability distribution for the eigenvalues of \bbox{TT}^{\dagger} is equivalent to the DMPK equation independent of the values of the forward scattering mean free paths. We find that infinitely strong forward scattering corresponds to diffusion on the coset space of the transfer matrix group. It is shown that the saddle point of the path integral corresponds to ballistic conductors with large conductances. We solve the saddle point equation and recover random matrix theory from the saddle point approximation to the path integral.Comment: REVTEX, 9 pages, no figure

Mesoscopic Transport Through Ballistic Cavities: A Random S-Matrix Theory Approach

We deduce the effects of quantum interference on the conductance of chaotic cavities by using a statistical ansatz for the S matrix. Assuming that the circular ensembles describe the S matrix of a chaotic cavity, we find that the conductance fluctuation and weak-localization magnitudes are universal: they are independent of the size and shape of the cavity if the number of incoming modes, N, is large. The limit of small N is more relevant experimentally; here we calculate the full distribution of the conductance and find striking differences as N changes or a magnetic field is applied.Comment: 4 pages revtex 3.0 (2-column) plus 2 postscript figures (appended), hub.pam.94.

Generalized Fokker-Planck Equation For Multichannel Disordered Quantum Conductors

The Dorokhov-Mello-Pereyra-Kumar (DMPK) equation, which describes the distribution of transmission eigenvalues of multichannel disordered conductors, has been enormously successful in describing a variety of detailed transport properties of mesoscopic wires. However, it is limited to the regime of quasi one dimension only. We derive a one parameter generalization of the DMPK equation, which should broaden the scope of the equation beyond the limit of quasi one dimension.Comment: 8 pages, abstract, introduction and summary rewritten for broader readership. To be published in Phys. Rev. Let

Equivalence of Fokker-Planck approach and non-linear σ\sigma-model for disordered wires in the unitary symmetry class

The exact solution of the Dorokhov-Mello-Pereyra-Kumar-equation for quasi one-dimensional disordered conductors in the unitary symmetry class is employed to calculate all $m$-point correlation functions by a generalization of the method of orthogonal polynomials. We obtain closed expressions for the first two conductance moments which are valid for the whole range of length scales from the metallic regime ($L\ll Nl$) to the insulating regime ($L\gg Nl$) and for arbitrary channel number. In the limit $N\to\infty$ (with $L/(Nl)=const.$) our expressions agree exactly with those of the non-linear $\sigma$-model derived from microscopic Hamiltonians.Comment: 9 pages, Revtex, one postscript figur

Intensity correlations in electronic wave propagation in a disordered medium: the influence of spin-orbit scattering

We obtain explicit expressions for the correlation functions of transmission and reflection coefficients of coherent electronic waves propagating through a disordered quasi-one-dimensional medium with purely elastic diffusive scattering in the presence of spin-orbit interactions. We find in the metallic regime both large local intensity fluctuations and long-range correlations which ultimately lead to universal conductance fluctuations. We show that the main effect of spin-orbit scattering is to suppress both local and long-range intensity fluctuations by a universal symmetry factor 4. We use a scattering approach based on random transfer matrices.Comment: 15 pages, written in plain TeX, Preprint OUTP-93-42S (University of Oxford), to appear in Phys. Rev.

Ballistic Transport Through Chaotic Cavities: Can Parametric Correlations and the Weak Localization Peak be Described by a Brownian Motion Model?

A Brownian motion model is devised on the manifold of S-matrices, and applied to the calculation of conductance-conductance correlations and of the weak localization peak. The model predicts that (i) the correlation function in $B$ has the same shape and width as the weak localization peak; (ii) the functions behave as $\propto 1-{\cal O}(B^2)$, thus excluding a linear line shape; and (iii) their width increases as the square root of the number of channels in the leads. Some of these predictions agree with experiment and with other calculations only in the limit of small $B$ and a large number of channels.Comment: 5 pages revtex (twocolumn