Study of solid 4He in two dimensions. The issue of zero-point defects and study of confined crystal

Defects are believed to play a fundamental role in the supersolid state of 4He. We report on studies by exact Quantum Monte Carlo (QMC) simulations at zero temperature of the properties of solid 4He in presence of many vacancies, up to 30 in two dimensions (2D). In all studied cases the crystalline order is stable at least as long as the concentration of vacancies is below 2.5%. In the 2D system for a small number, n_v, of vacancies such defects can be identified in the crystalline lattice and are strongly correlated with an attractive interaction. On the contrary when n_v~10 vacancies in the relaxed system disappear and in their place one finds dislocations and a revival of the Bose-Einstein condensation. Thus, should zero-point motion defects be present in solid 4He, such defects would be dislocations and not vacancies, at least in 2D. In order to avoid using periodic boundary conditions we have studied the exact ground state of solid 4He confined in a circular region by an external potential. We find that defects tend to be localized in an interfacial region of width of about 15 A. Our computation allows to put as upper bound limit to zero--point defects the concentration 0.003 in the 2D system close to melting density.Comment: 17 pages, accepted for publication in J. Low Temp. Phys., Special Issue on Supersolid

First-Principles Study of Electronic and Vibrational Properties of BaHfN2_2

The transition metal nitride BaHfN$_2$, which consists of weakly bonded neutral slabs of closed shell ions, has structural and chemical similarities to other layered nitrides which have impressive superconducting T$_c$ when electron doped: A$_x$HfNCl, A$_x$ZrNCl, A$_x$TiNCl, with $T_c= 25.5$, $15.2$ and $16.5$ K, respectively for appropriate donor (A) concentrations $x$. These similarities suggest the possibility of BaHfN$_2$ being another relatively high T$_c$ nitride upon doping, with effects of structure and the role of specific transition metal ions yet to be understood. We report first-principles electronic structure calculations for stoichiometric BaHfN$_2$ using density functional theory with plane-wave basis sets and separable dual-space Gaussian pseudopotentials. An indirect band gap of 0.8 eV was obtained and the lowest conduction band is primarily of Hf 5$d_{xy}$ character, similar to $\beta$-ZrNCl and $\alpha$-TiNCl. The two N sites, one in the Hf layer and another one in the Ba layer, were found to have very anisotropic Born effective charges (BEC):deviations from the formal charge (-3) are opposite for the two sites, and opposite for the two orientations (in-plane, out of plane). LO-TO splittings and comparison of BECs and dielectric constant tensors to those of related compounds are discussed, and the effect of electron doping on the zone-center phonons is reported.Comment: 11 pages, 5 figure

The power spectrum of systematics in cosmic shear tomography and the bias on cosmological parameters

Cosmic shear tomography has emerged as one of the most promising tools to both investigate the nature of dark energy and discriminate between General Relativity and modified gravity theories. In order to successfully achieve these goals, systematics in shear measurements have to be taken into account; their impact on the weak lensing power spectrum has to be carefully investigated in order to estimate the bias induced on the inferred cosmological parameters. To this end, we develop here an efficient tool to compute the power spectrum of systematics by propagating, in a realistic way, shear measurement, source properties and survey setup uncertainties. Starting from analytical results for unweighted moments and general assumptions on the relation between measured and actual shear, we derive analytical expressions for the multiplicative and additive bias, showing how these terms depend not only on the shape measurement errors, but also on the properties of the source galaxies (namely, size, magnitude and spectral energy distribution). We are then able to compute the amplitude of the systematics power spectrum and its scaling with redshift, while we propose a multigaussian expansion to model in a non-parametric way its angular scale dependence. Our method allows to self-consistently propagate the systematics uncertainties to the finally observed shear power spectrum, thus allowing us to quantify the departures from the actual spectrum. We show that even a modest level of systematics can induce non-negligible deviations, thus leading to a significant bias on the recovered cosmological parameters.Comment: 19 pages, 5 tables, 4 figure

Order-N Density-Matrix Electronic-Structure Method for General Potentials

A new order-N method for calculating the electronic structure of general (non-tight-binding) potentials is presented. The method uses a combination of the ``purification''-based approaches used by Li, Nunes and Vanderbilt, and Daw, and a representation of the density matrix based on ``travelling basis orbitals''. The method is applied to several one-dimensional examples, including the free electron gas, the ``Morse'' bound-state potential, a discontinuous potential that mimics an interface, and an oscillatory potential that mimics a semiconductor. The method is found to contain Friedel oscillations, quantization of charge in bound states, and band gap formation. Quantitatively accurate agreement with exact results is found in most cases. Possible advantages with regard to treating electron-electron interactions and arbitrary boundary conditions are discussed.Comment: 13 pages, REVTEX, 7 postscript figures (not quite perfect

Quantized vortices in two dimensional solid 4He

Diagonal and off-diagonal properties of 2D solid 4He systems doped with a quantized vortex have been investigated via the Shadow Path Integral Ground State method using the fixed-phase approach. The chosen approximate phase induces the standard Onsager-Feynman flow field. In this approximation the vortex acts as a static external potential and the resulting Hamiltonian can be treated exactly with Quantum Monte Carlo methods. The vortex core is found to sit in an interstitial site and a very weak relaxation of the lattice positions away from the vortex core position has been observed. Also other properties like Bragg peaks in the static structure factor or the behavior of vacancies are very little affected by the presence of the vortex. We have computed also the one-body density matrix in perfect and defected 4He crystals finding that the vortex has no sensible effect on the off-diagonal long range tail of the density matrix. Within the assumed Onsager Feynman phase, we find that a quantized vortex cannot auto-sustain itself unless a condensate is already present like when dislocations are present. It remains to be investigated if backflow can change this conclusion.Comment: 4 pages, 3 figures, LT26 proceedings, accepted for publication in Journal of Physics: Conference Serie

Optimizing Observational Strategy for Future Fgas Constraints

The Planck cluster catalog is expected to contain of order a thousand galaxy clusters, both newly discovered and previously known, detected through the Sunyaev-Zeldovich effect over the redshift range 0 < z < 1. Follow-up X-ray observations of a dynamically relaxed sub-sample of newly discovered Planck clusters will improve constraints on the dark energy equation-of-state found through measurement of the cluster gas mass fraction fgas. In view of follow-up campaigns with XMM-Newton and Chandra, we determine the optimal redshift distribution of a cluster sample to most tightly constrain the dark energy equation of state. The distribution is non-trivial even for the standard w0-wa parameterization. We then determine how much the combination of expected data from the Planck satellite and fgas data will be able to constrain the dark energy equation-of-state. Our analysis employs a Markov Chain Monte Carlo method as well as a Fisher Matrix analysis. We find that these upcoming data will be able to improve the figure-of-merit by at least a factor two.Comment: 11 pages, 8 figure

Mixing Effects in the Crystallization of Supercooled Quantum Binary Liquids

By means of Raman spectroscopy of liquid microjets we have investigated the crystallization process of supercooled quantum liquid mixtures composed of parahydrogen (pH$_2$) diluted with small amounts of up to 5\% of either neon or orthodeuterium (oD$_2$), and of oD$_2$ diluted with either Ne or pH$_2$. We show that the introduction of Ne impurities affects the crystallization kinetics in both the pH$_2$-Ne and oD$_2$-Ne mixtures in terms of a significant reduction of the crystal growth rate, similarly to what found in our previous work on supercooled pH$_2$-oD$_2$ liquid mixtures [M. K\"uhnel et {\it al.}, Phys. Rev. B \textbf{89}, 180506(R) (2014)]. Our experimental results, in combination with path-integral simulations of the supercooled liquid mixtures, suggest in particular a correlation between the measured growth rates and the ratio of the effective particle sizes originating from quantum delocalization effects. We further show that the crystalline structure of the mixture is also affected to a large extent by the presence of the Ne impurities, which likely initiate the freezing process through the formation of Ne crystallites.Comment: 19 pages, 7 figures, submitted to J. Chem. Phy

BeppoSAX observations of the black hole candidates LMC X-1 and LMC X-3

We describe BeppoSAX observations of the black hole candidates LMC X--1 and LMC X--3 performed in Oct. 1997. Both sources can be modelled by a multicolor accretion disk spectrum, with temperature $\sim 1$ keV. However, there is some evidence that a thin emitting component coexists with the thick disk at these temperatures. In the direction of LMC X--1, we detected a significant emission above 10 keV, which we suspect originates from the nearby source PSR 0540-69. For LMC X--1, we estimate an absorbing column density of $\simeq 6\times 10^{21}$ cm$^{-2}$, which is almost ten times larger than that found for LMC X--3. In both sources, we find no indication of emission or absorption features whatsoever.Comment: 4 pages, 2 figures. Accepted for pubblication in the Proc. of 32nd Cospar scientific assembly, Nagoya, 13-15 July 199

Bose-Einstein Condensation at a Helium Surface

Path Integral Monte Carlo was used to calculate the Bose-Einstein condensate fraction at the surface of a helium film at $T=0.77 K$, as a function of density. Moving from the center of the slab to the surface, the condensate fraction was found to initially increase with decreasing density to a maximum value of 0.9 before decreasing. Long wavelength density correlations were observed in the static structure factor at the surface of the slab. Finally, a surface dispersion relation was calculated from imaginary-time density-density correlations.Comment: 8 pages, 5 figure