Quantum Degenerate States Research Articles

DFT study of structural, electronic, thermo-elastic properties and plausible origin of superconductivity due to quantum degenerate states in LaTiO3

In this paper, computations based on generalized gradient approximations were carried out to investigate the structural, electronic and thermo-elastic properties of LaTiO3 within the frame work of Density Functional Theory. In structural properties, the ground state structural parameters have been found to be in good agreement with those cited in recent literature. For electronic properties, in-depth analysis of quantum degenerate electronic states of LaTiO3 have been explained on the grounds of Projected Density of States. Elastic properties corresponds to anisotropy, elastic moduli’s, phase stability, elastic wave velocities, thermal stability and Debye temperature were calculated and elaborated that has not yet been found in literature. In this observation, LaTiO3 exhibited ductile nature and physically stable indirect bandgap semiconductor behavior with quasi metallic nature near Fermi level due to La-Ti degenerate states. Moreover, longitudinal mode of vibration is observed to be maximum along [100] direction than transverse mode of vibration. A plausible reason of superconductivity may arise in LaTiO3 below Debye temperature.

How not to boil

Engineering the interactions between atoms allows a fragile system in its quantum degenerate state to persist for much longer than equilibrium statistical physics would otherwise allow.

From polariton condensates to highly photonic quantum degenerate states of bosonic matter.

Bose-Einstein condensation (BEC) is a thermodynamic phase transition of an interacting Bose gas. Its key signatures are remarkable quantum effects like superfluidity and a phonon-like Bogoliubov excitation spectrum, which have been verified for atomic BECs. In the solid state, BEC of exciton-polaritons has been reported. Polaritons are strongly coupled light-matter quasiparticles in semiconductor microcavities and composite bosons. However, they are subject to dephasing and decay and need external pumping to reach a steady state. Accordingly the polariton BEC is a nonequilibrium process of a degenerate polariton gas in self-equilibrium, but out of equilibrium with the baths it is coupled to and therefore deviates from the thermodynamic phase transition seen in atomic BECs. Here we show that key signatures of BEC can even be observed without fulfilling the self-equilibrium condition in a highly photonic quantum degenerate nonequilibrium system.

The enzyme aromatic amine dehydrogenase induces a substrate conformation crucial for promoting vibration that significantly reduces the effective potential energy barrier to proton transfer

The role of promoting vibrations in enzymic reactions involving hydrogen tunnelling is contentious. While models incorporating such promoting vibrations have successfully reproduced and explained experimental observations, it has also been argued that such vibrations are not part of the catalytic effect. In this study, we have employed combined quantum mechanical/molecular mechanical methods with molecular dynamics and potential energy surface calculations to investigate how enzyme and substrate motion affects the energy barrier to proton transfer for the rate-limiting H-transfer step in aromatic amine dehydrogenase (AADH) with tryptamine as substrate. In particular, the conformation of the iminoquinone adduct induced by AADH was found to be essential for a promoting vibration identified previously-this lowers significantly the 'effective' potential energy barrier, that is the barrier which remains to be surmounted following collective, thermally equilibrated motion attaining a quantum degenerate state of reactants and products. When the substrate adopts a conformation similar to that in the free iminoquinone, this barrier was found to increase markedly. This is consistent with AADH facilitating the H-transfer event by holding the substrate in a conformation that induces a promoting vibration.

Quantum optics, excitons, and degenerate-state excitation in semiconductor nanostructures

Using a fully microscopic theory it is shown that quantum-optical excitation can be used to generate excitons in quantum-degenerate population states. For the example of a thermal source it is shown that resonant excitation directly converts photons into excitons with effectively no center of mass momentum. The generated quantum-degenerate state possesses long-range order, is very stable against perturbations, and should be observable via its unusual directional and density dependence in luminescence measurements.

Semiconductor excitons in new light

Excitons are quasi-particles that form when Coulomb-interacting electrons and holes in semiconductors are bound into pair states. They have many features analogous to those of atomic hydrogen. Because of this, researchers are interested in exploring excitonic phenomena, from optical, quantum-optical and thermodynamic transitions to the possible condensation of excitons into a quantum-degenerate state. Excitonic signatures commonly appear in the optical absorption and emission of direct-gap semiconductor systems. However, the precise properties of incoherent exciton populations in such systems are difficult to determine and are the subject of intense debate. We review recent contributions to this discussion, and argue that to obtain detailed information about exciton populations, conventional experimental techniques should be supplemented by direct quasi-particle spectroscopy using the relatively newly available terahertz light sources. Finally, we propose a scheme of quantum-optical excitation to generate quantum-degenerate exciton states directly.

Phase separation of atomic Bose-Fermi mixtures in an optical lattice

We study a two-dimensional atomic mixture of bosons and fermions cooled into their quantum degenerate states and subject to an optical lattice. The optical lattice provides van Hove singularities in the fermionic density of states. We find that these van Hove singularities produce new and interesting features for the transition towards phase separation: an arbitrary weak interaction between the bosons and the fermions is sufficient to drive the phase separation at low temperatures. The phase separated state turns stable for attractive and repulsive interaction between the bosons and fermions and can be cast into the standard form of a `liquid--gas' transition.

Supersolid versus phase separation in atomic bose-fermi mixtures.

We show that a two-dimensional atomic mixture of bosons and fermions cooled into their quantum degenerate states and subject to an optical lattice develops a supersolid phase characterized by the simultaneous presence of a nontrivial crystalline order and phase order. This transition is in competition with phase separation. We determine the phase diagram of the system and propose an experiment allowing for the observation of the supersolid phase.

The Fermionic Cousin of Bose–Einstein Condensate Makes its Debut

Experimenters have been romping in the BEC playground ever since a gaseous Bose–Einstein condensate was first produced four years ago. It was almost inevitable, then, that researchers would also want to explore its fermionic cousin—the quantum degenerate state one expects to get by cooling a gas of atoms with half-integer spins. Whereas the integer-spin atoms (bosons) in a Bose–Einstein condensate all pile into the lowest energy state, the half-integer spin atoms (fermions) are prevented from doing so by the Pauli exclusion principle, which prohibits two identical fermions from occupying the same quantum state. Instead, the atoms fill all the lowest energy levels up to the Fermi energy, just as electrons do in a solid. The crossover into this quantum degenerate state is gradual, compared to the abrupt phase transition into a Bose condensate. But that crossover has now been convincingly detected by Deborah Jin, a physicist at the National Institute of Standards and Technology, and Brian DeMarco, a physics graduate student at the University of Colorado at Boulder, both working at JILA.