Number statistics of molecules formed from ultra-cold atoms

We calculate the number statistics of a single-mode molecular field excited by photoassociation or via a Feshbach resonance from an atomic Bose-Einstein condensate (BEC), a normal atomic Fermi gas and a Fermi system with pair correlations (BCS state). We find that the molecule formation from a BEC is a collective process that leads for short times to a coherent molecular state in the quantum optical sense. Atoms in a normal Fermi gas, on the other hand, are converted into molecules independently of each other and result for short times in a molecular state analogous to that of a classical chaotic light source. The BCS situation is intermediate between the two and goes from producing an incoherent to a coherent molecular field with increasing gap parameter.Comment: 5 pages, 4 figure

Bose Einstein Condensation of incommensurate solid 4He

It is pointed out that simulation computation of energy performed so far cannot be used to decide if the ground state of solid 4He has the number of lattice sites equal to the number of atoms (commensurate state) or if it is different (incommensurate state). The best variational wave function, a shadow wave function, gives an incommensurate state but the equilibrium concentration of vacancies remains to be determined. In order to investigate the presence of a supersolid phase we have computed the one--body density matrix in solid 4He for the incommensurate state by means of the exact Shadow Path Integral Ground State projector method. We find a vacancy induced Bose Einstein condensation of about 0.23 atoms per vacancy at a pressure of 54 bar. This means that bulk solid 4He is supersolid at low enough temperature if the exact ground state is incommensurate.Comment: 5 pages, 2 figure

Comment on ``Analytical and numerical verification of the Nernst heat theorem for metals''

Recently, H{\o}ye, Brevik, Ellingsen and Aarseth (quant-ph/0703174) claimed that the use of the Drude dielectric function leads to zero Casimir entropy at zero temperature in accordance with Nernst's theorem. We demonstrate that their proof is not applicable to metals with perfect crystal lattices having no impurities. Thus there is no any contradiction with previous results in the literature proving that the Drude dielectric function violates the Nernst theorem for the Casimir entropy in the case of perfect crystal lattices. We also indicate mistakes in the coefficients of their asymptotic expressions for metals with impurities.Comment: 6 page

A helium-3 refrigerator employing capillary confinement of liquid cryogen

A condensation refrigerator suitable for operation in a zero gravity space environment was constructed. The condensed liquid refrigerant is confined by surface tension inside a porous metal matrix. Helium-4 and helium-3 gases were condensed and held in a copper matrix. Evaporative cooling of confined liquid helium-4 resulted in a temperature of 1.4K. Using a zeolite adsorption pump external to the cryostat, a temperature of 0.6 K was achieved through evaporative cooling of liquid helium-3. The amount of time required for complete evaporation of a controlled mass of liquid helium-4 contained in the copper matrix was measured as a function of the applied background power. For heating powers below 18 mW the measured times are consistent with the normal boiling of the confined volume of liquid refrigerant. At background powers above 18 mW the rapid rise in the temperature of the copper matrix the signature of the absence of confined liquid occurs in a time a factor of two shorter than that expected on the basis of an extrapolation of the low power data

Dynamic Kerr effect responses in the Terahertz-range

Dynamic Kerr effect measurements provide a simple realization of a nonlinear experiment. We propose a field-off experiment where an electric field of one or several sinusoidal cycles is applied to a sample in thermal equilibrium. Afterwards, the evolution of the polarizability is measured. If such an experiment is performed in the Terahertz-range it might provide valuable information about the low-frequency dynamics in disordered systems. We treat these dynamics in terms of a Brownian oscillator model and calculate the Kerr effect response. It is shown that frequency-selective behaviour can be expected. In the interesting case of underdamped vibrational motion we find that the frequency-dependence of the phonon-damping can be determined from the experiment. Also the behaviour of overdamped relaxational modes is discussed. For typical glassy materials we estimate the magnitude of all relevant quantities, which we believe to be helpful in experimental realizations.Comment: 26 pages incl. 5 figure

Universal low-temperature tricritical point in metallic ferromagnets and ferrimagnets

An earlier theory of the quantum phase transition in metallic ferromagnets is revisited and generalized in three ways. It is shown that the mechanism that leads to a fluctuation-induced first-order transition in metallic ferromagnets with a low Curie temperature is valid, (1) irrespective of whether the magnetic moments are supplied by the conduction electrons or by electrons in another band, (2) for ferromagnets in the XY and Ising universality classes as well as for Heisenberg ferromagnets, and (3) for ferrimagnets as well as for ferromagnets. This vastly expands the class of materials for which a first-order transition at low temperatures is expected, and it explains why strongly anisotropic ferromagnets, such as UGe2, display a first-order transition as well as Heisenberg magnets.Comment: 11pp, 2 fig

A Hybrid model for the origin of photoluminescence from Ge nanocrystals in SiO2_2 matrix

In spite of several articles, the origin of visible luminescence from germanium nanocrystals in SiO$_2$ matrix is controversial even today. Some authors attribute the luminescence to quantum confinement of charge carriers in these nanocrystals. On the other hand, surface or defect states formed during the growth process, have also been proposed as the source of luminescence in this system. We have addressed this long standing query by simultaneous photoluminescence and Raman measurements on germanium nanocrystals embedded in SiO$_2$ matrix, grown by two different techniques: (i) low energy ion-implantation and (ii) atom beam sputtering. Along with our own experimental observations, we have summarized relevant information available in the literature and proposed a \emph{Hybrid Model} to explain the visible photoluminescence from nanocrystalline germanium in SiO$_2$ matrix.Comment: 23 pages, 8 figure

Renormalization of the spin-wave spectrum in three-dimentional ferromagnets with dipolar interaction

Renormalization of the spin-wave spectrum is discussed in a cubic ferromagnet with dipolar forces at $T_C\gg T\ge0$. First 1/S-corrections are considered in detail to the bare spectrum $\epsilon_{\bf k} = \sqrt{Dk^2 (Dk^2 + S\omega_0\sin^2\theta_{\bf k})}$, where $D$ is the spin-wave stiffness, $\theta_{\bf k}$ is the angle between $\bf k$ and the magnetization and $\omega_0$ is the characteristic dipolar energy. In accordance with previous results we obtain the thermal renormalization of constants $D$ and $\omega_0$ in the expression for the bare spectrum. Besides, a number of previously unknown features are revealed. We observe terms which depend on azimuthal angle of the momentum $\bf k$. It is obtained an isotropic term proportional to $k$ which makes the spectrum linear rather than quadratic when $\sin\theta_{\bf k}=0$ and $k \ll \omega_0/D$. In particular a spin-wave gap proportional to $\sin\theta_{\bf k}$ is observed. Essentially, thermal contribution from the Hartree-Fock diagram to the isotropic correction as well as to the spin-wave gap are proportional to the demagnetizing factor in the direction of domain magnetization. This nontrivial behavior is attributed to the long-range nature of the dipolar interaction. It is shown that the gap screens infrared singularities of the first 1/S-corrections to the spin-wave stiffness and longitudinal dynamical spin susceptibility (LDSS) obtained before. We demonstrate that higher order 1/S-corrections to these quantities are small at $T\ll\omega_0$. However the analysis of the entire perturbation series is still required to derive the spectrum and LDSS when $T\gg\omega_0$.Comment: 11 pages, 1 figur

Unconventional Hall effect in pnictides from interband interactions

We calculate the Hall transport in a multiband systems with a dominant interband interaction between carriers having electron and hole character. We show that this situation gives rise to an unconventional scenario, beyond the Boltzmann theory, where the quasiparticle currents dressed by vertex corrections acquire the character of the majority carriers. This leads to a larger (positive or negative) Hall coefficient than what expected on the basis of the carrier balance, with a marked temperature dependence. Our results explain the puzzling measurements in pnictides and they provide a more general framework for transport properties in multiband materials.Comment: 5 pages, 2 figure

Landau-Zener Tunnelling in Waveguide Arrays

Landau-Zener tunnelling is discussed in connection with optical waveguide arrays. Light injected in a specific band of the Bloch spectrum in the propagation constant can be transmitted to another band, changing its physical properties. This is achieved using two waveguide arrays with different refractive indices, which amounts to consider a Schr\"odinger equation in a periodic potential with a step. The step causes wave "acceleration" and thus induces Landau-Zener tunnelling. The region of physical parameters where this phenomenon can occur is analytically determined and a realistic experimental setup is suggested. Its application could allow the realization of light filters.Comment: 4 pages, 6 figure