Debanand Sa

A generalized Ginzburg-Landau approach to second harmonic generation

Abstract:We develop a generalized Ginzburg-Landau theory for second harmonic generation (SHG) in magnets by expanding the free energy in terms of the order parameter in the magnetic phase and the susceptibility tensor in the corresponding high-temperature phase. The non-zero components of the SHG susceptibility in the ordered phase are derived from the symmetries of the susceptibility tensor in the high-temperature phase and the symmetry of the order parameter. In this derivation, the dependence of the SHG susceptibility on the order parameter follows naturally, and therefore its nonreciprocal optical properties. We examine this phenomenology for the magnetoelectric compound Cr2O3 as well as for the ferroelectromagnet YMnO3.

Anomalous x-ray diffraction peak broadening and lattice strains in Zn1?xCoxO dilute magnetic semiconductors

Nanocrystalline powders of Zn(1-x)Co(x)O synthesized by the coprecipitation technique show anomalous anisotropic broadening for x>0.05. This peak broadening is shown to be not only due to a reduction in the particle size but also due to a significant strain contribution, as confirmed by Williamson-Hall analysis. The presence of grouped Co(2+) ions, revealing the presence of clusters of high spin Co(2+) with antiferromagnetically coupled spins, as indicated by magnetization studies, seems to be responsible for the strain.

Is Li2Pd3B a self-doped hole superconductor?

Abstract We propose that the electrons responsible for superconductivity in Li2Pd3B come from the palladium 4d-electrons. So, its electronic properties are likely to be dominated by strong electronic correlations. The basic unit in this material are Pd6B octahedra which share vertices to form a 3-dimensional network. Due to the highly distorted nature of the Pd6B octahedron, one far stretched Pd atom per octahedra becomes almost inactive for electronic conduction. Thus, the material escapes the fate of becoming a half-filled insulating Mott antiferromagnet by hiding extra charges at these inactive Pd sites and becomes a self-doped correlated metal. We propose a 3-dimensional single band t–J model which could be the correct minimal model for this material.

Novel topological phase due to coexistence of superconductivity and spin-density wave: Application to high Tc superconductors

Abstract We consider a model of a coexistence phase between superconductivity(SC) and spin-density wave(SDW) where the SC is assumed to be of d x 2 − y 2 + id xy ( d 1 + id 2 ) symmetry and the SDW order parameter is of s-wave as well as d xy symmetry. The Hamiltonian having such a symmetry is shown to yield a novel topological coexistence phase in addition to the conventional one. It is shown that the amplitudes of both the order parameters determine a new phase diagram characterizing different topological phases. A possible experimental realization of such topological phases in a recent ARPES data of high T c superconductors in the extreme underdoped regime is analyzed by considering the effective order parameter to be a combination of d x 2 − y 2 SC and dxy SDW.

A phenomenological Landau theory for electromagnons in cubic spinel multiferroic CoCr2O4

Non-anisotropic free energy is considered which under minimization yields two magnetic phases: a conical spin density wave and a low temperature conical cycloid. Using equations of motion, the excitation spectrum is studied. Knowing the nature of these excitations, the dielectric function as well as the fluctuation specific heat is computed and compared with the experimental spectrum. Due to the electromagnon going soft, the dielectric function (imaginary part) as well as the specific heat capacity show peaks at the temperature where ferroelectricity appears in the system.

Phonon-Raman scattering in a correlated superconductor

Abstract The phonon spectral density function is calculated to explain the observed anomalies in the Raman spectrum of the cuprate superconductors. These being strongly correlated systems, their ground state is taken to be the resonating valence bond (RVB) state. In this state, the phonons are shown to couple predominantly to the RVB pair excitations besides the usual interaction with the charge carrier density. It is proposed that the superconductivity in the system appears due to the correlation bag mechanism where the collective excitations of the RVB state mediate an attractive interaction between the charge carriers which leads to pairing. The dependence of the transition temperature on the RVB gap parameter as well as the dopant concentration is demonstrated. The phonon self-energies, which involve the pair correlation function and the density response function, are evaluated at zero temperature for the normal state as well as the superconducting states. It is shown that the pair correlation function strongly renormalises the phonon spectrum. Its spectral density shows a broad peak corresponding to the RVB-amplitude mode, which qualitatively resembles the observed Raman spectrum in the superconducting state.

Hidden quantum critical point in a ferromagnetic superconductor

We consider a coexistence phase of both ferromagnetism and superconductivity and solve the self-consistent mean-field equations at zero temperature. The superconducting gap is shown to vanish at the Stoner point whereas the magnetization does not. This indicates that the paramagnetic-ferromagnetic quantum critical point becomes a hidden critical point. The effective mass in such a phase gets enhanced whereas the spin-wave stiffness is reduced as compared to the pure ferromagnetic phase. The spin-wave stiffness remains finite even at the paramagnetic-ferromagnetic quantum critical point.

Raman scattering in a correlated superconductor II. A finite temperature study

Abstract The temperature dependence of the Raman spectrum of copper oxide superconductors is calculated assuming that the normal state of these systems is a correlated metal and that the superconductivity arises due to the correlation bag mechanism proposed earlier (Debanand Sa, S.N. Behera (1992), here after referred to as I). The correlated metallic state is characterized by the presence of a strong interaction between the doped charge carriers and the quanta of fluctuations of the pre-existing resonating valence bonds in the normal state. It is shown that the presence of such an interaction can give rise to the marginal Fermi liquid behaviour, which follows from the pair polarisability function. The model therefore, successfully explains the constant intensity background observed in the Raman spectrum due to the scattering by the charge carriers. In order to understand the observed temperature dependent shift and change in width of the phonons, their spectral density functions are calculated. Two different kinds of interaction between the charge carriers and phonons were derived in I, i.e. (i) the usual electron-phonon interaction where a phonon decays by exciting an electron-hole pair and (ii) the other where the decay process involves a quasi-particle pair. It was postulated that the localised phonons couple to the charge carrier by the later process whereas the propagating ones interact by the former. The temperature dependence of the phonon self-energies due to the two processes is calculated and the spectral density functions are computed. The calculated temperature dependent shift and change in width of the phonons show qualitative agreement with the observed result.

Pressure-driven reentrant phenomena in liquid crystals: the role of inverse layer spacing

The occurrence of pressure-driven reentrant phenomena observed in high-pressure experimental studies in some achiral mesogenic materials has been explained using a thermodynamic model based on Landau–de Gennes theory. In this approach, the free-energy is expanded in terms of nematic, smectic A order parameters and the couplings (cubic and biquadratic) between them. The basic theme here is that the ‘inverse layer spacing’, which mimics an order parameter, becomes coupled to the nematic and smectic A order parameters. Secondly, in addition to the order parameter couplings the N–SA metastable temperature (which appears due to elimination of inverse layer spacing from the free-energy density expression) becomes pressure dependent. The occurrence of a pressure-driven reentrant nematic phase is explained in terms of these three pressure-dependent parameters. They all show smooth but rapid variation at the critical pressure beyond which nematic reentrance appears.

A mechanism for pairing of RVB excitations

The fluctuations in the RVB order parameter over their meanfield values are considered in the BZA theory which mediate the interaction between the RVB excitations. The condition under which the effective interaction could be attractive and result in super-conductivity is investigated. The dependence of the transition temperature on the dopant concentration and the RVB order parameter is calculated.