Bhaskar Kamble - Academia.edu (original) (raw)

Papers by Bhaskar Kamble

Pramana-journal of Physics, 2006

We investigate the energy transfer between various Fourier modes in a low-dimensional model for t... more We investigate the energy transfer between various Fourier modes in a low-dimensional model for thermal convection. We have used the formalism of mode-to-mode energy transfer rate in our calculation. The evolution equations derived using this scheme is the same as those derived using the hydrodynamical equations for thermal convection in Boussinesq fluids. Numerical and analytical studies of this model show that convective rolls appear as the Rayleigh number R is raised above its critical value R c. Further increase of Rayleigh number generates rolls in the perpendicular directions as well, and we obtain a dynamic asymmetric square pattern. This pattern is due to Hopf bifurcation. There are two sets of limit cycles corresponding to the two competing asymmetric square patterns. When the Rayleigh number is increased further, the limit cycles become unstable simultaneously, and chaotic motion sets in. The onset of chaos is via intermittent route. The trajectories wander for quite a long time almost periodically before jumping irregularly to one of the two ghost limit cycles.

Journal of Physics-condensed Matter, 2012

The correlated motion of electrons in multi-orbital metallic ferromagnets is investigated in term... more The correlated motion of electrons in multi-orbital metallic ferromagnets is investigated in terms of a realistic Hubbard model with {\cal N}-fold orbital degeneracy and arbitrary intra- and inter-orbital Coulomb interactions U and J using a Goldstone-mode-preserving non-perturbative scheme. An effective quantum parameter '\hbar'=\frac{U^2+({\cal N}-1)J^2}{(U+({\cal N}-1)J)^2} is obtained which determines, in analogy with 1/S for quantum spin systems and 1/N for the N-orbital Hubbard model, the strength of correlation-induced quantum corrections to magnetic excitations. The rapid suppression of this quantum parameter with Hund's coupling J, especially for large {\cal N}, provides fundamental insight into the phenomenon of strong stabilization of metallic ferromagnetism by orbital degeneracy and Hund's coupling. This approach is illustrated for the case of ferromagnetic iron and the half metallic Heusler alloy Co_2 Mn Si. For realistic values for iron, the calculated spin stiffness and Curie temperature values obtained are in quantitative agreement with measurements. Significantly, the contribution of long wavelength modes is shown to yield a nearly ~25% reduction in the calculated Curie temperature. Finally, an outline is presented for extending the approach to generic multi-band metallic ferromagnets including realistic band-structure features of non-degenerate orbitals and inter-orbital hopping as obtained from LDA calculations.

Physical Review B, 2008

A purely fermionic representation is introduced for the ferromagnetic Kondo lattice model which a... more A purely fermionic representation is introduced for the ferromagnetic Kondo lattice model which allows conventional diagrammatic tools to be employed to study correlation effects. Quantum 1/S corrections to magnon excitations are investigated using a systematic inverse-degeneracy expansion scheme which incorporates correlation effects in the form of self-energy and vertex corrections, while explicitly preserving the continuous spin-rotation symmetry. Magnon self-energy is studied in the full range of interaction strength, and shown to result in strong magnon damping and anomalous softening for zone boundary modes, which accounts for several zone-boundary anomalies observed in recent spin-wave measurements of ferromagnetic manganites.

Journal of Physics-condensed Matter, 2010

Correlation-induced spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnet... more Correlation-induced spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnetic manganites are calculated with realistic parameters in order to provide a quantitative comparison with experimental results for spin stiffness, magnon dispersion, magnon damping, anomalous zone-boundary magnon softening, and Curie temperature. The role of orbital degeneracy, orbital ordering, and orbital correlations on spin dynamics in different doping regimes is highlighted.

Physical Review B, 2010

The correlated motion of electrons in the presence of strong orbital fluctuations and correlation... more The correlated motion of electrons in the presence of strong orbital fluctuations and correlations is investigated with respect to magnetic couplings and excitations in an orbitally degenerate ferromagnet. Introduction of the orbital degree of freedom results in a class of diagrams representing spin-orbital coupling which become particularly important near the orbital ordering instability. Low-energy staggered orbital fluctuation modes, particularly with momentum near (\pi/2,\pi/2,0) (corresponding to CE-type orbital correlations), are shown to generically yield strong intrinsically non-Heisenberg (1-\cos q)^2 magnon self energy correction, resulting in no spin stiffness reduction, but strongly suppressed zone-boundary magnon energies in the Gamma-X direction. The zone-boundary magnon softening is found to be strongly enhanced with increasing hole doping and for narrow-band materials, which provides insight into the origin of zone-boundary anomalies observed in ferromagnetic manganites.

Physical Review B, 2009

An effective quantum parameter is obtained for the band ferromagnet in terms of orbital degenerac... more An effective quantum parameter is obtained for the band ferromagnet in terms of orbital degeneracy and Hund's coupling. This quantum parameter determines, in analogy with 1/N for the generalized Hubbard model and 1/S for quantum spin systems, the strength of quantum corrections to spin stiffness and spin-wave energies. Quantum corrections are obtained by incorporating correlation effects in the form of self-energy and vertex corrections within a spin-rotationally-symmetric approach in which the Goldstone mode is explicitly preserved order by order. It is shown that even a relatively small Hund's coupling is rather efficient in strongly suppressing quantum corrections, especially for large N, resulting in strongly enhanced stability of the ferromagnetic state. This mechanism for the enhancement of ferromagnetism by Hund's coupling implicitly involves a subtle interplay of lattice, dimensionality, band dispersion, spectral distribution, and band filling effects.

Pramana-journal of Physics, 2006

We investigate the energy transfer between various Fourier modes in a low-dimensional model for t... more We investigate the energy transfer between various Fourier modes in a low-dimensional model for thermal convection. We have used the formalism of mode-to-mode energy transfer rate in our calculation. The evolution equations derived using this scheme is the same as those derived using the hydrodynamical equations for thermal convection in Boussinesq fluids. Numerical and analytical studies of this model show that convective rolls appear as the Rayleigh number R is raised above its critical value R c. Further increase of Rayleigh number generates rolls in the perpendicular directions as well, and we obtain a dynamic asymmetric square pattern. This pattern is due to Hopf bifurcation. There are two sets of limit cycles corresponding to the two competing asymmetric square patterns. When the Rayleigh number is increased further, the limit cycles become unstable simultaneously, and chaotic motion sets in. The onset of chaos is via intermittent route. The trajectories wander for quite a long time almost periodically before jumping irregularly to one of the two ghost limit cycles.

Journal of Physics-condensed Matter, 2012

The correlated motion of electrons in multi-orbital metallic ferromagnets is investigated in term... more The correlated motion of electrons in multi-orbital metallic ferromagnets is investigated in terms of a realistic Hubbard model with {\cal N}-fold orbital degeneracy and arbitrary intra- and inter-orbital Coulomb interactions U and J using a Goldstone-mode-preserving non-perturbative scheme. An effective quantum parameter '\hbar'=\frac{U^2+({\cal N}-1)J^2}{(U+({\cal N}-1)J)^2} is obtained which determines, in analogy with 1/S for quantum spin systems and 1/N for the N-orbital Hubbard model, the strength of correlation-induced quantum corrections to magnetic excitations. The rapid suppression of this quantum parameter with Hund's coupling J, especially for large {\cal N}, provides fundamental insight into the phenomenon of strong stabilization of metallic ferromagnetism by orbital degeneracy and Hund's coupling. This approach is illustrated for the case of ferromagnetic iron and the half metallic Heusler alloy Co_2 Mn Si. For realistic values for iron, the calculated spin stiffness and Curie temperature values obtained are in quantitative agreement with measurements. Significantly, the contribution of long wavelength modes is shown to yield a nearly ~25% reduction in the calculated Curie temperature. Finally, an outline is presented for extending the approach to generic multi-band metallic ferromagnets including realistic band-structure features of non-degenerate orbitals and inter-orbital hopping as obtained from LDA calculations.

Physical Review B, 2008

A purely fermionic representation is introduced for the ferromagnetic Kondo lattice model which a... more A purely fermionic representation is introduced for the ferromagnetic Kondo lattice model which allows conventional diagrammatic tools to be employed to study correlation effects. Quantum 1/S corrections to magnon excitations are investigated using a systematic inverse-degeneracy expansion scheme which incorporates correlation effects in the form of self-energy and vertex corrections, while explicitly preserving the continuous spin-rotation symmetry. Magnon self-energy is studied in the full range of interaction strength, and shown to result in strong magnon damping and anomalous softening for zone boundary modes, which accounts for several zone-boundary anomalies observed in recent spin-wave measurements of ferromagnetic manganites.

Journal of Physics-condensed Matter, 2010

Correlation-induced spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnet... more Correlation-induced spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnetic manganites are calculated with realistic parameters in order to provide a quantitative comparison with experimental results for spin stiffness, magnon dispersion, magnon damping, anomalous zone-boundary magnon softening, and Curie temperature. The role of orbital degeneracy, orbital ordering, and orbital correlations on spin dynamics in different doping regimes is highlighted.

Physical Review B, 2010

The correlated motion of electrons in the presence of strong orbital fluctuations and correlation... more The correlated motion of electrons in the presence of strong orbital fluctuations and correlations is investigated with respect to magnetic couplings and excitations in an orbitally degenerate ferromagnet. Introduction of the orbital degree of freedom results in a class of diagrams representing spin-orbital coupling which become particularly important near the orbital ordering instability. Low-energy staggered orbital fluctuation modes, particularly with momentum near (\pi/2,\pi/2,0) (corresponding to CE-type orbital correlations), are shown to generically yield strong intrinsically non-Heisenberg (1-\cos q)^2 magnon self energy correction, resulting in no spin stiffness reduction, but strongly suppressed zone-boundary magnon energies in the Gamma-X direction. The zone-boundary magnon softening is found to be strongly enhanced with increasing hole doping and for narrow-band materials, which provides insight into the origin of zone-boundary anomalies observed in ferromagnetic manganites.

Physical Review B, 2009

An effective quantum parameter is obtained for the band ferromagnet in terms of orbital degenerac... more An effective quantum parameter is obtained for the band ferromagnet in terms of orbital degeneracy and Hund's coupling. This quantum parameter determines, in analogy with 1/N for the generalized Hubbard model and 1/S for quantum spin systems, the strength of quantum corrections to spin stiffness and spin-wave energies. Quantum corrections are obtained by incorporating correlation effects in the form of self-energy and vertex corrections within a spin-rotationally-symmetric approach in which the Goldstone mode is explicitly preserved order by order. It is shown that even a relatively small Hund's coupling is rather efficient in strongly suppressing quantum corrections, especially for large N, resulting in strongly enhanced stability of the ferromagnetic state. This mechanism for the enhancement of ferromagnetism by Hund's coupling implicitly involves a subtle interplay of lattice, dimensionality, band dispersion, spectral distribution, and band filling effects.