Superfluidity versus localization in bulk 4He at zero temperature

We present a zero-temperature quantum Monte Carlo calculation of liquid $^4$He immersed in an array of confining potentials. These external potentials are centered in the lattice sites of a fcc solid geometry and, by modifying their well depth and range, the system evolves from a liquid phase towards a progressively localized system which mimics a solid phase. The superfluid density decreases with increasing order, reaching a value $\rho_{\rm s}/\rho = 0.079(16)$ when the Lindemann's ratio of the model equals the experimental value for solid $^4$He.Comment: 5 pages,5 figure

Magnetic Properties of a Two-Dimensional Mixed-Spin System

Using a Langmuir-Blodgett (LB) synthesis method, novel two-dimensional (2D) mixed-spin magnetic systems, in which each magnetic layer is both structurally and magnetically isolated, have been generated. Specifically, a 2D Fe-Ni cyanide-bridged network with a face-centered square grid structure has been magnetically and structurally characterized. The results indicate the presence of ferromagnetic exchange interactions between the Fe$^{3+}$ ($S=1/2$) and Ni$^{2+}$ (S=1) centers.Comment: 2 pages, 3 figs., submitted 23rd International Conference on Low Temperature Physics (LT-23), Aug. 200

Ultrasound attenuation and a P-B-T phase diagram of superfluid 3He in 98% aerogel

Longitudinal sound attenuation measurements in superfluid 3He in 98% aerogel were conducted at pressures between 14 and 33 bar and in magnetic fields up to 4.44 kG. The temperature dependence of the ultrasound attenuation in the A-like phase was determined for the entire superfluid region exploiting the field induced meta-stable A-like phase at the highest field. In the lower field, the A-B transition in aerogel was identified by a smooth jump in attenuation on both cooling and warming. Based on the transitions observed on warming, a phase diagram as a function of pressure (P), temperature (T) and magnetic field (B) is constructed. We find that the A-B phase boundary in aerogel recedes in a drastically different manner than in bulk in response to an increasing magnetic field. The implications of the observed phase diagram are discussed.Comment: 9 pages, 13 figures, accepted to PR

Magnetodielectric coupling of infrared phonons in single crystal Cu2_{2}OSeO3_{3}

Reflection and transmission as a function of temperature have been measured on a single crystal of the magnetoelectric ferrimagnetic compound Cu$_{2}$OSeO$_{3}$ utilizing light spanning the far infrared to the visible portions of the electromagnetic spectrum. The complex dielectric function and optical properties were obtained via Kramers-Kronig analysis and by fits to a Drude-Lortentz model. The fits of the infrared phonons show a magnetodielectric effect near the transition temperature ($T_{c}\sim 60$~K). Assignments to strong far infrared phonon modes have been made, especially those exhibiting anomalous behavior around the transition temperature

ESR Study of (C_5H_{12}N)_2CuBr_4

ESR studies at 9.27, 95.4, and 289.7 GHz have been performed on (C$_5$H$_{12}$N)$_2$CuBr$_4$ down to 3.7 K. The 9.27 GHz data were acquired with a single crystal and do not indicate the presence of any structural transitions. The high frequency data were collected with a polycrystalline sample and resolved two absorbances, consistent with two crystallographic orientations of the magnetic sites and with earlier ESR studies performed at 300 K. Below $B_{C1}=6.6$ T, our data confirm the presence of a spin singlet ground state.Comment: 2 pages, 4 figs., submitted 23rd International Conference on Low Temperature Physics (LT-23), Aug. 200

Size dependence of the photoinduced magnetism and long-range ordering in Prussian blue analog nanoparticles of rubidium cobalt hexacyanoferrate

Nanoparticles of rubidium cobalt hexacyanoferrate (Rb$_j$Co$_k$[Fe(CN)$_6$]$_l \cdot n$H$_2$O) were synthesized using different concentrations of the polyvinylpyrrolidone (PVP) to produce four different batches of particles with characteristic diameters ranging from 3 to 13 nm. Upon illumination with white light at 5 K, the magnetization of these particles increases. The long-range ferrimagnetic ordering temperatures and the coercive fields evolve with nanoparticle size. At 2 K, particles with diameters less than approximately 10 nm provide a Curie-like magnetic signal.Comment: 10 pages, 6 figures in text, expanded text and dat