Raman scattering in high temperature superconductors : An integrated view

The common features in the Raman data of high temperature superconductors: (the cuprates, bismathates, alkali doped fullerides and some organic superconductors), are analyzed. It was shown that qualitative understanding of the data can be achieved in terms of non-Fermi liquid models for their normal state, with appropiate bag mechanisms for the superconducting state.Comment: To appear in Physica B (1996). Invited talk presented by S. N. Behera, Latex file in revtex style, six figures available on request to first author (e-mail : [email protected]

Interplay of spin density wave and superconductivity with different pairing symmetry

A model study for the coexistence of the spin density wave and superconductivity is presented. With reference to the recent angle resolved photo emmission experimental data in high T_c cuprates, presence of the nested pieces of bands is assumed. The single band Hubbard model, therefore, when treated within the Hatree-Fock mean field theory leads to a spin density wave (SDW) ground state. The superconductivity (SC) is assumed to be due to a generalised attractive potential with a separable form without specifying to any particular origin. It therefore allows a comparative study of the coexistence of superconductivity of different order parameter symmetry with the spin density wave state. We find that the phase diagram, comprising of the amplitudes of the respective gaps (SC and SDW) Vs. band filling resembles to that of the high T_c cuprates only when the order parameter of the superconducting phase has d-wave symmetry. Thermal variation of different order parameters (e.g, SC and SDW) also show interesting coexistence and reentrance behaviors that are consistent with experimental observations, specially for the borocarbides.Comment: 8 pages, 6 figures (postscript attached), Physica C (in press

Can the "brick wall" model present the same results in different coordinate representations?

By using the 't Hooft's "brick wall" model and the Pauli-Villars regularization scheme we calculate the statistical-mechanical entropies arising from the quantum scalar field in different coordinate settings, such as the Painlev\'{e} and Lemaitre coordinates. At first glance, it seems that the entropies would be different from that in the standard Schwarzschild coordinate since the metrics in both the Painlev\'{e} and Lemaitre coordinates do not possess the singularity at the event horizon as that in the Schwarzschild-like coordinate. However, after an exact calculation we find that, up to the subleading correction, the statistical-mechanical entropies in these coordinates are equivalent to that in the Schwarzschild-like coordinate. The result is not only valid for black holes and de Sitter spaces, but also for the case that the quantum field exerts back reaction on the gravitational field provided that the back reaction does not alter the symmetry of the spacetime.Comment: 8 pages, Phys. Rev. D in pres

Mutual influence of structural distortion and superconductivity in systems with degenerate bands

The interplay between the band Jahn-Teller distortion and the superconductivity is studied for the system whose Fermi level lies in two-fold degenerate band. Assuming that the lattice distortion is coupled to the orbital electron density and the superconductivity arises due to BCS pairing mechanism between the electrons, the phase diagram is obtained for different doping with respect to half-filled band situation. The coexistence phase of superconductivity and distortion occurs within limited range of doping and the distortion lowers the superconducting transition temperature $T_c$. In presence of strong electron-lattice interaction the lattice strain is found to be maximum at half-filling and superconductivity does not appear for low doping. The maximum value of $T_c$ obtainable for an optimum doping is limited by the structural transition temperature $T_s$. The growth of distortion is arrested with the onset of superconductivity and the distortion is found to disappear at lower temperature for some hole density. Such arresting of the growth of distortion at $T_c$ produces discontinuous jump in thermal expansion coefficient. The variation of strain with temperature as well as with doping, thermal expansion coefficient, the $T_c$ vs $\delta$ behaviour are in qualitative agreement with recent experimental observations on interplay of distortion and superconductivity in cuprates.Comment: 15 pages Revtex style, 9 figures available on request to first Autho

Scintillation and charge extraction from the tracks of energetic electrons in superfluid helium-4

An energetic electron passing through liquid helium causes ionization along its track. The ionized electrons quickly recombine with the resulting positive ions, which leads to the production of prompt scintillation light. By applying appropriate electric fields, some of the ionized electrons can be separated from their parent ions. The fraction of the ionized electrons extracted in a given applied field depends on the separation distance between the electrons and the ions. We report the determination of the mean electron-ion separation distance for charge pairs produced along the tracks of beta particles in superfluid helium at 1.5 K by studying the quenching of the scintillation light under applied electric fields. Knowledge of this mean separation parameter will aid in the design of particle detectors that use superfluid helium as a target material.Comment: 10 pages, 8 figure

Remarks on 't Hooft's Brick Wall Model

A semi-classical reasoning leads to the non-commutativity of the space and time coordinates near the horizon of Schwarzschild black hole. This non-commutativity in turn provides a mechanism to interpret the brick wall thickness hypothesis in 't Hooft's brick wall model as well as the boundary condition imposed for the field considered. For concreteness, we consider a noncommutative scalar field model near the horizon and derive the effective metric via the equation of motion of noncommutative scalar field. This metric displays a new horizon in addition to the original one associated with the Schwarzschild black hole. The infinite red-shifting of the scalar field on the new horizon determines the range of the noncommutativ space and explains the relevant boundary condition for the field. This range enables us to calculate the entropy of black hole as proportional to the area of its original horizon along the same line as in 't Hooft's model, and the thickness of the brick wall is found to be proportional to the thermal average of the noncommutative space-time range. The Hawking temperature has been derived in this formalism. The study here represents an attempt to reveal some physics beyond the brick wall model.Comment: RevTeX, 5 pages, no figure

On "Non-Geometric" Contribution To The Entropy Of Black Hole Due To Quantum Corrections

The quantum corrections to the entropy of charged black holes are calculated. The Reissner-Nordstrem and dilaton black holes are considered. The appearance of logarithmically divergent terms not proportional to the horizon area is demonstrated. It is shown that the complete entropy which is sum of classical Bekenstein-Hawking entropy and the quantum correction is proportional to the area of quantum-corrected horizon.Comment: Latex, 9 page

Theory of nonlinear optical properties of phenyl-substituted polyacetylenes

In this paper we present a theoretical study of the third-order nonlinear optical properties of poly(diphenyl)polyacetylene (PDPA) pertaining to the third-harmonic-generation (THG) process. We study the aforesaid process in PDPA's using both the independent electron Hueckel model, as well as correlated-electron Pariser-Parr-Pople (P-P-P) model. The P-P-P model based calculations were performed using various configuration interaction (CI) methods such as the the multi-reference-singles-doubles CI (MRSDCI), and the quadruples-CI (QCI) methods, and the both longitudinal and the transverse components of third-order susceptibilities were computed. The Hueckel model calculations were performed on oligo-PDPA's containing up to fifty repeat units, while correlated calculations were performed for oligomers containing up to ten unit cells. At all levels of theory, the material exhibits highly anisotropic nonlinear optical response, in keeping with its structural anisotropy. We argue that the aforesaid anisotropy can be divided over two natural energy scales: (a) the low-energy response is predominantly longitudinal and is qualitatively similar to that of polyenes, while (b) the high-energy response is mainly transverse, and is qualitatively similar to that of trans-stilbene.Comment: 13 pages, 7 figures (included), to appear in Physical Review B (April 15, 2004

One-loop Quantum Corrections to the Entropy for an Extremal Reissner-Nordstr\"om Black Hole

The first quantum corrections to the entropy for an eternal 4-dimensional extremal Reissner-Nordstr\"om black hole is investigated at one-loop level, in the large mass limit of the black hole, making use of the conformal techniques related to the optical metric. A leading cubic horizon divergence is found and other divergences appear due to the singular nature of the optical manifold. The area law is shown to be violated.Comment: 10 pages, LaTe

Theory of excited state absorptions in phenylene-based π\pi-conjugated polymers

Within a rigid-band correlated electron model for oligomers of poly-(paraphenylene) (PPP) and poly-(paraphenylenevinylene) (PPV), we show that there exist two fundamentally different classes of two-photon A$_g$ states in these systems to which photoinduced absorption (PA) can occur. At relatively lower energies there occur A$_g$ states which are superpositions of one electron - one hole (1e--1h) and two electron -- two hole (2e--2h) excitations, that are both comprised of the highest delocalized valence band and the lowest delocalized conduction band states only. The dominant PA is to one specific member of this class of states (the mA$_g$). In addition to the above class of A$_g$ states, PA can also occur to a higher energy kA$_g$ state whose 2e--2h component is {\em different} and has significant contributions from excitations involving both delocalized and localized bands. Our calculated scaled energies of the mA$_g$ and the kA$_g$ agree reasonably well to the experimentally observed low and high energy PAs in PPV. The calculated relative intensities of the two PAs are also in qualitative agreement with experiment. In the case of ladder-type PPP and its oligomers, we predict from our theoretical work a new intense PA at an energy considerably lower than the region where PA have been observed currently. Based on earlier work that showed that efficient charge--carrier generation occurs upon excitation to odd--parity states that involve both delocalized and localized bands, we speculate that it is the characteristic electronic nature of the kA$_g$ that leads to charge generation subsequent to excitation to this state, as found experimentally.Comment: Revtex4 style, 2 figures inserted in the text, three tables, 10 page