S N Behera
Articles written in Pramana – Journal of Physics
Volume 7 Issue 2 August 1976 pp 102-112 Solids
Impurity pair modes in a diatomic linear chain: nearest neighbour pair
The behaviour of the impurity modes due to a pair of substitutional impurities characterized by both mass as well as force-constant changes and occupying nearest neighbour positions in a diatomic linear chain, is studied. The results are compared with those for the case of impurity pairs occupying next nearest neighbour sites discussed earlier as well as the existing three dimensional calculations of Elliott and Pfeuty. The nearest neighbour impurity pair gap and local modes can be interpreted in terms of two single impurities substituted in the two different sublattices unlike the next nearest neighbour pair modes. The inband resonant modes are totally new features characteristic of the pair. Finally, the predictions of the theory are compared with the available experimental data for Si-impurity-pair-complexes and qualitative agreement is shown.
Volume 8 Issue 3 March 1977 pp 255-265 Solids
A new criterion for the mixed crystal behaviour in the diatomic linear chain model
Lucovsky, Brodsky, Burstein (LBB) have studied the behaviour of mixed crystals by setting up a criterion for the existence of local mode frequencies in real crystals starting from a diatomic linear chain model. This, while successfully predicting the one and two mode behaviour for some systems fails to predict the mixed mode behaviour. We propose a similar criterion for the existence of gap modes, by demanding that the gap mode predicted by the diatomic linear chain model should lie within the gap of the real three dimensional solid for its existence. It is shown that the gap modes for various systems calculated using this criterion are in reasonable agreement with the experimental values. The infrared behaviour of mixed crystals has to be determined by examining the existence of local as well as gap modes for the two end members of the system. This generalized new criterion successfully predicts the mixed mode behaviour of III–V mixed crystals besides predicting the one and two mode behaviour, observed in infrared absorption of mixed alkali halioes and III-V compounds.
Volume 10 Issue 6 June 1978 pp 621-627 Solids
Three impurity cluster in a diatomic linear chain
The modes of vibration of a three impurity cluster in a diatomic linear chain is studied. The configuration of the three impurity atoms is such that two of these go into next nearest neighbour sites in one of the sub-lattices whereas the third occupies the central in between site in the other sub-lattice. New local, gap and inband modes are seen to exist, besides the ones corresponding to the impurity pairs. A new feature which appears is that the frequencies of some of the pair modes remain totally unaffected in the presence of the third impurity atom.
Volume 12 Issue 2 February 1979 pp 121-138 Solids
Lattice dynamics of impurity clusters. Application to pairs
N Chandralekha Devi S N Behera
A general solution is obtained for the lattice dynamics of a cluster of
Volume 14 Issue 5 May 1980 pp 327-341 Solid State Physics
Anharmonic oscillator model for first order structural phase transition
Exact solutions for the motion of a classical anharmonic oscillator in the potential
Volume 15 Issue 3 September 1980 pp 245-269 Statistical Mechanics
Classical φ^{6}-field theory in (1+1) dimensions. A model for structural phase transitions
The classical φ^{6}-field theory in (1+1) dimensions, is considered as a model for the first order structural phase transitions. The equation of motion is solved exactly; and the presence of domain wall (kink) solutions at and below the transition point, in addition to the usual phonon-like oscillatory solutions, is demonstrated. The domain wall solutions are shown to be stable, and their mass and energies are calculated. Above the transition point there exists exotic unstable kink-like solutions which takes the particle from one hill top to the other of the potential. The partition function of the system is calculated exactly using the functional integral method together with the transfer matrix techniques which necessitates the determination of the eigenvalues of a Schrödinger-like equation. Thus the exact free energy is evaluated which in the low temperature limit has a phonon part and a contribution coming from the domain wall excitations. It was shown that this domain wall free energy differs from that calculated by the use of the domain wall phenomenology proposed by Krumhansl and Schrieffer. The exact solutions of the Schrödinger-like equation are also used to evaluate the displacement-displacement, intensity-intensity correlation functions and the probability distribution function. These results are compared with those obtained from the phenomenology as well as the φ^{4}-field theory. A qualitative picture of the central peak observed in structural phase transitions is proposed.
Volume 15 Issue 4 October 1980 pp 375-398 Solid State Physics
Two-phonon bound states in imperfect crystals
The question of the occurrence of two-phonon bound states in imperfect crystals is investigated. It is shown that the anharmonicity mediated two-phonon bound state which is present in perfect crystals gets modified due to the presence of impurities. Moreover, the possibility of the occurrence of a purely impurity mediated two-phonon bound state is demonstrated. The bound state frequencies are calculated using the simple Einstein oscillator model for the host phonons. The two-phonon density of states for the imperfect crystal thus obtained has peaks at the combination and difference frequencies of two host phonons besides the peaks at the bound state frequencies. For a perfect crystal the theory predicts a single peak at the two-phonon bound state frequency in conformity with experimental observations and other theoretical calculations. Experimental data on the two-phonon infrared absorption and Raman scattering from mixed crystals of GA_{1−c} Al_{c} P and Ge_{1−c} Si_{c} are analysed to provide evidence in support of impurity-mediated two-phonon bound states. The relevance of the zero frequency (difference spectrum) peak to the central peak observed in structural phase transitions, is conjectured;
Volume 15 Issue 5 November 1980 pp 501-505 Solid State Physics
The existence of a domain wall-like contribution to the free energy above the first order phase transition point is demonstrated for a system described by the ϕ^{6}-field theory in (1+1) dimensions.
Volume 18 Issue 1 January 1982 pp 73-87 Solid State Physics
Concentration dependence of the mixed crystal modes of vibration
The concentration dependence of the reststrahl absorption in various mixed crystals exhibiting one, two and mixed mode behaviour is investigated using the coherent potential approximation (
Volume 19 Issue 5 November 1982 pp 467-482 Solid State Physics
A coherent potential approximation (
Volume 19 Issue 6 December 1982 pp 645-663 Solid State Physics
The effect of small concentrations of non-magnetic substitutional impurities (characterized by changes in mass, nearest neighbour force constants and local electron-phonon interaction) on the observability of superconducing (
Volume 26 Issue 3 March 1986 pp 239-261 Solid State Physics
Theory of two-phonon modes in layered charge-density-wave systems
A theoretical model with electron-phonon and anharmonic interactions is proposed to explain the two-phonon mode observed in the Raman spectra of layered transition metal dichalcogenides, which exhibit charge density wave (
Volume 54 Issue 2 February 2000 pp 305-315 Research Articles
The electron-phonon interaction in the periodic Anderson model (PAM) is considered. The PAM incorporates the effect of onsite Coulomb interaction (
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