Khandker Quader

Charge response in 12C and 40Ca

Abstract The charge response for 12 C and 40 Ca in the quasi-elastic region is calculated by means of non-relativistic many-body theory. Within the continuum RPA a finite-range interaction is used which gives a proper description of the low-energy spectrum and the giant resonances. 2p2h effects are included in a phenomenological way utilizing information from the empirical optical potential. The results indicate that the main corrections to the free response are given by the mean field non-locality and the 2p2h effects. Comparisons with nuclear matter indicate the importance of finite size effects. Good agreement with experiment is found in 12 C while the theory overestimates the data in 40 Ca.

Effective Interactions and Elementary Excitations in Nuclear Matter

A polarization potential theory for nuclear matter is presented. Starting from a realistic two-body interaction we construct pseudopotentials which describe the effective quasiparticle interaction. Using these the linear response in various spin-isospin channels is evaluated.

First-principles study of CaFe2As2under pressure

We perform first principles calculations on CaFe2As2 under hydrostatic pressure. Our total energy calculations show that though the striped antiferromagnetic (AFM) orthorhombic (OR) phase is favored at P=0, a non-magnetic collapsed tetragonal (cT) phase with diminished c-parameter is favored for P > 0.36 GPa, in agreement with experiments. Rather than a mechanical instability, this is an enthalpically driven transition from the higher volume OR phase to the lower volume cT phase. Calculations of electronic density of states reveal pseudogaps in both OR and cT phases, though As(p) hybridization with Fe(d) is more pronounced in the OR phase. We provide an estimate for the inter-planar magnetic coupling. Phonon entropy considerations provide an interpretation of the finite temperature phase boundaries of the cT phase.

Spin suceptibility and specific heat bi-layer high-Tc cuprates: on the origin of scaling and universality

Abstract The anomalies in the spin susceptibility X s ( T ) and specific heat C V ( T ) in the high- T c cuprates are shown to have origin in a non-degenerate electron component. This arises when the chemical potential μ is tuned near the top of a 2D band submerged just below the Fermi surface. In bi-layer cuprates the model predicts a scaling of the form: X s (T, x) = Xs (W(x)/T) and C V(T, x)/T = γ(W(x)/T) , where doping-dependent scaling parameter W is determined by the location of μ relative to the top of the non-degenerate band. We compare the results of this model with experiments and find the predicted type of scaling in the susceptibility of under, optimally, and overdoped (TlSr 2 (Lu 1− x Ca x )Cu 2 O y samples. We also argue that the scaling curves X s ( y ) and γ ( y ) may be universal for a certain class of bi-layer cuprates.

Origin and consequences of the ‘gap’ in the cuprate normal state

Abstract We present an alternative to the spin gap as an explanation for the origin of the gap feature observed in the normal state of the high-Tc cuprates. We consider a model that has a non-degenerate as well as a degenerate component of charge carriers. The properties are governed by the location of the chemical potential relative to the top of the non-degenerate band. In the underdoped regime this translates into an energy gap for the non-degenerate carriers; this decreases with doping, vanishing at optimal doping. In the overdoped regime there emerges a small Fermi energy that governs the temperature dependence of properties. As illustrations we consider several spin and charge properties, namely spin susceptibility, entropy, in-plane resistivity, Hall effect and Hall angle.

Lifshitz Transitions in 122-Pnictides Under Pressure

We demonstrate, using T = 0 first principles total energy calculations, that observed pressuredriven anomalies in the entire 122-pnictides family (AFe2As2; A = alkali earth element Ca, Sr, Ba) can be understood as consequences of Lifshitz transitions (LTs) [1]. Our results for energy band dispersions and spectra, lattice parameters, enthalpies, magnetism, and elastic constants over a wide range of hydrostatic pressure provide a coherent understanding of multiple transitions in these compounds, namely, enthalpic, magnetic and tetragonal (T) collapsed tetragonal (cT) transitions. In particular, the T-cT transition and anomalies in lattice parameters and elastic properties, observed at finite temperatures, are interpreted as arising from proximity to T = 0 Lifshitz transitions, wherein pressure causes non-trivial changes in the Fermi surface topology in these materials.

p-wave Pairing in a Two-Component Fermi System with Unequal Population: Weak Coupling BCS to Strong Coupling BEC Regimes

We explore p-wave pairing in a single-channel two-component Fermi system with unequal population near Feshbach resonance. Our analytical and numerical study reveal a rich superfluid (SF) ground state structure as a function of imbalance. In addition to the state ∆±1 ∝ Y1±1, a multitude of “mixed” SF states formed of linear combinations of Y1m’s give global energy minimum under a phase stability condition; these states exhibit variation in energy with the relative phase between the constituent gap amplitudes. States with local energy minimum are also obtained. We provide a geometric representation of the states. A T=0 polarization vs. p-wave coupling phase diagram is constructed across the BEC-BCS regimes. With increased polarization, the global minimum SF state may undergo a quantum phase transition to the local minimum SF state.

Thermal conduction contribution to the ultrasonic attenuation in solids

Abstract We estimate the thermal conduction contribution to the ultrasonic attenuation of crystals. For UPt3, the calculated attenuation exhibits a peak at the temperature where one was observed by Muller et al., but its magnitude, while significant, is too small to account for the data.

p-WAVE PAIRING IN FERMI SYSTEMS WITH UNEQUAL POPULATION NEAR FESHBACH RESONANCE

We explore p-wave pairing in a single-channel two-component Fermi system with unequal population near Feshbach resonance. Our analytical and numerical study reveal a rich superfluid (SF) ground state structure as a function of imbalance. In addition to the state Δ±1 ∝ Y1±1, a multitude of “mixed” SF states formed of linear combinations of Y1ms give global energy minimum under a phase stability condition; these states exhibit variation in energy with the relative phase between the constituent gap amplitudes. States with local energy minimum are also obtained. We provide a geometric representation of the states. A T = 0 polarization vs. p-wave coupling phase diagram is constructed across the BEC-BCS regimes. With increased polarization, the global minimum SF state may undergo a quantum phase transition to the local minimum SF state.

Ginzburg-Landau theory, strong coupling corrections and exchange enhancements: Can spin fluctuations give high Tc's?

Abstract Ginzburg-Landau theory is used to explore the thermodynamic and electrodynamic properties of YBa 2 Cu 3 O 7-δ , and to determine γ, m ∗ /m and the exchange enhancement. This material is found to be in a moderately strong coupling regime, intermediate between dirty and clean limits; strong coupling corrections are estimated. It is shown that, irrespective of the choice of the carrier density, spin fluctuations are unable to give a sufficiently large T c . An upper bound is given for the T c due spin-fluctuation-mediated pairing.