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We discuss the nature of pressure induced phase transitions in standard Quantum Paraelectrics near quantum critical point. From a microscopic theory we first show that near the critical point the transition temperature $T_c(p)$ varies as $ (1-p/p_c)^\xi$ with $\xi={1/2}$. Within the Landau scheme, it is shown that for $\xi \leq 1$, a condition satisfied by most quantum paraelectrics the transition is inevitably first order in nature. The present analysis is valid for non-polar impurity induced transitions in these materials and is supported by some recent experiment., Comment: The paper has been withdrawn

The temperature dependence of static dielectric susceptibility of a system with strongly coupled fluctuating dipoles is calculated within a self consistent mean fluctuation field approximation. Results are qualitatively in good agreement with, a quantum paraelectric, SrTiO$_3$ in the low temperature regime. We identify this system as a {\it gaped quantum paraelectric} and suggest a possible experimental realization of a {\it quantum critical paraelectric} through the application of hydrostatic pressure or doping by impurity., Comment: 9 pages, 3 figures, Published version

Pair fluctuation theory has been used to study the crossover from the weak coupling BCS theory to the strong coupling Bose Einstein Condensation. The effect of fluctuations has been studied over the whole crossover regime. It has been shown that the pair fluctuations are enhanced considerably in both two and three dimensions and hence mean field theory is inadequate to study the physical properties in this regime. A self consistent scheme for calculating the pair susceptibility is given., Comment: File Revtex, 6 Pages, 1 Figure included

Specific heat versus temperature curves for various pressures, or magnetic fields (or some other external control parameter) have been seen to cross at a point or in a very small range of temperatures in many correlated fermion systems. We show that this behavior is related to the vicinity of a quantum critical point in these systems which leads to a crossover at some temperature T^* from quantum to classical fluctuation regime. The temperature at which the curves cross turns out to be near T^*. We have discussed the case of the normal phase of liquid Helium three and the heavy fermion systems CeAl3 and UBe13 in detail within the spin fluctuation theory. When the crossover scale is any homogeneous function of these control parameters there is always crossing at a point., Comment: File Revtex, 4 pages, 4 eps figures included

Specific heat curves for various pressures, in many correlated electron systems, have been seen to cross at a point. We analyze this behavior using the spin fluctuation theory. It is found that when the system is considered near a ferromagnetic instability, the curves cross at one point and for systems near an antiferromagnetic instability, they cross at two points. The crossing behavior is related to the crossover of these systems from quantum to classical fluctuation regimes., Comment: 4 Pages, Revtex file, 3 Figures included

The signature for a non-Fermi liquid behavior near a quantum phase transition has been observed in thermal and transport properties of many metallic systems at low temperatures. In the present work we consider specific examples of itinerant ferromagnet as well as antiferromagnet in the limit of vanishing transition temperature. The temperature variation of spin susceptibility, electrical resistivity, specific heat, and NMR relaxation rates at low temperatures is calculated in the limit of infinite exchange enhancement within the frame work of a self consistent spin fluctuation theory. The resulting non-Fermi liquid behavior is due to the presence of the low lying critically damped spin fluctuations in these systems. The theory presented here gives the leading low temperature behavior, as it turns out that the fluctuation correlation term is always smaller than the mean fluctuation field term in three as well as in two space dimensions. A comparison with illustrative experimental results of these properties in some typical systems has been done. Finally we make some remarks on the effect of disorder in these systems., Comment: File RevTex, 7 Figures available on request, Abstract and text modified, To appear in Phys. Rev. B

We study the existence of ferromagnetism in one dimensional Hubbard Model with on site interaction and nearest and next nearest neighbor hopping. Using the Hubbard I approximation, a self consistent equation for $$, the particle density with spin $\sigma$ is obtained. We find that ferromagnetism exists over a wide range of values of the next nearest neighbor hopping element t' when on-site interaction U is large. The phase diagram as a function of t'/t and the electron density $$ has been obtained by numerically solving the self consistent equation. It is found that the maximum density at which ferromagnetism can occur, peaks around $t'/t \approx -0.53$. We show that this phenomenon is a result of the peculiar way in which the t' term modifies the density of states., Comment: File Latex, 5 eps figures included