Spin State of Cobalt and Electrical Transport Mechanism in MgCo<Subscript>2</Subscript>O<Subscript>4</Subscript> System

MgCo₂O₄ samples were synthesized by inverse co-precipitation method. The formation of a single-phase spinel structure was confirmed by X-ray diffraction measurements and Fourier-transform infrared spectroscopy. The samples crystallized in a face-centered cubic structure with Fd-3m space group as revealed from the Rietveld refinement of X-ray diffraction data. Magnetic measurements carried out in a broad temperature range of 5–300 K showed antiferromagnetic to paramagnetic phase transition (Neel temperature) observed at 101 K. Magnetic susceptibility data fitted using the Curie Weiss law and effective Bohr magnetic moment (μ_eff) for Co atoms was determined. Calculated μ_eff comes out to be 3.05 μ_B. These results were correlated to the spin states of Co³⁺ atoms. A small hysteresis in the field-dependent magnetization MH loop taken at 5 K indicates the existence of weak ferromagnetism in this system. The electrical resistivity measurement in the temperature range 77–750 K displayed the semiconducting-like behavior for this system.     

Estimating Bounds on Collisional Relaxation Rates of Spin-Polarized 87Rb Atoms at Ultracold Temperatures

We present quantum scattering calculations for the collisional relaxation rate coefficient of spin-polarized ⁸⁷Rb(f = 2,m = 2) atoms, which determines the loss rate of cold Rb atoms from a magnetic trap. Unlike the lighter alkali atoms, spin-polarized ⁸⁷Rb atoms can undergo dipolar relaxation due to both the normal spin-spin dipole interaction and a second-order spin-orbit interaction with distant electronic states of the dimer. We present ab initio calculations for the second-order spin-orbit terms for both Rb₂ and Cs₂. The corrections lead to a reduction in the relaxation rate for ⁸⁷Rb. Our primary concern is to analyze the sensitivity of the ⁸⁷Rb trap loss to the uncertainties in the ground state molecular potentials. Since the scattering length for the a³Σ⁺_u state is already known, the major uncertainties are associated with the X¹Σ⁺_g potential. After testing the effect of systematically modifying the short-range form of the molecular potentials over a reasonable range, and introducing our best estimate of the second-order spin-orbit interaction, we estimate that in the low temperature limit the rate coefficient for loss of Rb atoms from the f = 2,m = 2 state is between 0.4 × 10⁻¹⁵ cm³/s and 2.4 × 10⁻¹⁵ cm³/s (where this number counts two atoms lost per collision). In a pure condensate the rate coefficient would be reduced by 1/2.     

Trapping Fermionic 40K and Bosonic 87Rb on a Chip

We demonstrate the loading of a Bose–Fermi mixture into a microfabricated magnetic trap. In a single-chamber vacuum system, laser-cooled atoms are transported to the surface of a substrate on which gold wires have been microfabricated. The magnetic field minimum formed near these current-carrying wires is used to confine up to 6 × 10⁴ neutral ⁴⁰K atoms. In addition, we can simultaneously load 2 × 10^{5 87}Rb atoms, demonstrating the confinement of two distinct elements with such a trap. In a sequence optimized for ⁸⁷Rb alone, we observe up to 1 × 10⁷ trapped atoms. We describe in detail the experimental apparatus, and discuss prospects for evaporative cooling towards quantum degeneracy in both species.     

Atomic nanofabrication using an ytterbium atomic beam

Abstract

Highly parallel and periodically narrow lines of ytterbium (Yb) atoms were successfully produced on a substrate using a near resonant laser light and direct-write atomic nanofabrication. Yb atoms are a promising material for nanofabrication using atom optics particularly due to their electric conductivity, the laser wavelength required for their manipulation, and the vapor pressure required for their fabrication. Collimated ¹⁷⁴Yb atoms were channeled into the nodes of an optical standing wave with dipole force and then deposited onto a substrate. We clearly observed a grating pattern of Yb atoms fabricated on a substrate with a line separation of approximately 200 nm after examining the surface of the substrate with an atomic force microscope. This is the first demonstration of nanofabrication using the atom-optical approach with Yb atoms.     

Atomic nanofabrication using an ytterbium atomic beam

Highly parallel and periodically narrow lines of ytterbium (Yb) atoms were successfully produced on a substrate using a near resonant laser light and direct-write atomic nanofabrication. Yb atoms are a promising material for nanofabrication using atom optics particularly due to their electric conductivity, the laser wavelength required for their manipulation, and the vapor pressure required for their fabrication. Collimated ¹⁷⁴Yb atoms were channeled into the nodes of an optical standing wave with dipole force and then deposited onto a substrate. We clearly observed a grating pattern of Yb atoms fabricated on a substrate with a line separation of approximately 200 nm after examining the surface of the substrate with an atomic force microscope. This is the first demonstration of nanofabrication using the atom-optical approach with Yb atoms.     

Realization of a mirror magneto-optical trap

Abstract

We present details for construction and the operation of a mirror magneto-optical trap for cooling and trapping rubidium atoms. For trap operation, only four input laser beams are needed in contrast to the normal six for a standard trap. In excess of 10⁸ atoms have been trapped with this arrangement, with the atomic ensemble only ～1mm from the surface of a reflective mirror. This trap is highly suited to studies of magnetic guiding and magnetic manipulation of cold atomic ensembles.     

Structure and Some Properties of a Double Neptunyl(V) Cesium Molybdate,Cs[NpO<Subscript>2</Subscript>MoO<Subscript>4</Subscript>(H<Subscript>2</Subscript>O)]

A double neptunyl(V) cesium molybdate, Cs[NpO₂MoO₄(H₂O)], was studied by single crystal X-ray diffraction. Crystal data: rhombic system, a = 9.478(2), b = 7.900(1), c = 10.499(2) Å; space group Pnna, Z = 4, d = 5.05 g cm^?3, R = 0.030. The compound has a framework structure; the coordination polyhedron of the Np atom is a distorted pentagonal bipyramid with the equatorial positions occupied by four O atoms of four molybdate groups and an O atom of the coordinated water molecule. The IR and visible absorption spectra of this compound and of Cs₂[(NpO₂)₂Mo₂O₈] whose structure had been determined previously were measured. The NpO ₂ ⁺ stretching vibration frequencies in the IR spectra of these compounds virtually coincide. Incorporation of the O atom of the Mo-O-Mo bridge into the first coordination sphere of the neptunyl(V) ion in Cs₂[(NpO₂)₂Mo₂O₈] exerts the same disturbing effect on the electronic absorption spectrum as does the cation-cation interaction with one of the O atoms acting as a bridge between two Np atoms.     

Vortices in a stirred Bose-Einstein condensate

Abstract

We stir with a focused laser beam a Bose-Einstein condensate of ⁸⁷Rb atoms confined in a magnetic trap. We observe the formation of a single vortex for a stirring frequency exceeding a critical value. At larger rotation frequencies we produce states of the condensate for which up to eleven vortices are simultaneously present. We present measurements of the decay of a vortex array once the stirring laser beam is removed.     

Crystal and molecular structures of tricarbonyltechnetium acetylacetonate and its adduct with diethylamine

The crystal and molecular structures of tricarbonyltechnetium acetylacetonate and its adduct with diethylamine were studied by single crystal X-ray diffraction. Tricarbonyltechnetium acetylacetonate crystallizes in the triclinic system, space group \(P\bar 1\), a = 8.1154(1), b = 8.1961(2), c = 8.2695(1) Å, α = 64.5618(9)°, β = 69.7899(8)°, γ = 83.6925(9)°, V = 465.652(14) ų (at 100 K); Z = 2. The adduct also crystallizes in the triclinic system, space group P{ie145-2}, a = 7.9338(3), b = 8.5421(3), c = 12.1506(7) Å, α = 96.961(3)°, β = 99.952(3)°, γ = 112.001(2)°, V = 736.25(6) ų (at 100 K); Z = 2. Tricarbonyltechnetium acetylacetonate has a centrosymmetrical dimeric structure. The dimerization occurs through the formation of a bridging bond with the methine carbon atom of the acetylacetonate ligand which is thus coordinated in the tridentate chelating-bridging fashion. The adduct has a monomeric structure with chelating coordination of the acetylacetonate ligand and coordination of diethylamine to the technetium atom via N atom. In both complexes, the carbonyl groups are in the fac position. Differences between the structures of tricarbonyltechnetium acetylacetonate in the crystal and in solution are discussed.     

Electronic Structure and Magnetic Interactions in Ti-Doped and Ti-V<Subscript>O</Subscript>-Co-Doped <Emphasis Type="Italic">β</Emphasis>-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> from First-Principles Calculations

Based on first-principles calculation method, the electronic structure and magnetic interactions of Ti-doped β-Ga₂O₃ and Ti-V_O-co-doped in β-Ga₂O₃ have been investigated. The calculated results indicate that Ti atom tends to substitute the Ga atom at the octahedral coordinated site and the doped system prefers spin-polarized state. The magnetic moment of Ti-doped β-Ga₂O₃ is 0.546 μ _B per cell, which mainly arises from Ti 0.48 μ _B. The origin of ferromagnetism in Ti-doped β-Ga₂O₃ can be explained by a double-exchange mechanism. When a V_O defect is introduced into Ti-doped β-Ga₂O₃, the magnetic moment increases and the ferromagnetic coupling between Ti atoms is strengthened. The influence of V_O defect on ferromagnetic properties of Ti-doped β-Ga₂O₃ comes from the enhancement of hybridization between Ti-3d and O-2p states.     

The Effect of Boron on the Structure and Conductivity of Thin Films Obtained by Laser Ablation of Diamond with Deposition at 700°C

Structural features of CB_x films obtained by pulsed laser ablation of targets made of pressed diamond powder with boron-powder additions at B/C atomic ratio of x = 0.33 have been studied. The films were deposited on heated substrates, so that diffusion processes involving C and B atoms on the surface and in the volume of films were possible. Selected conditions of film deposition ensured their effective doping with boron (0.4 ≤ x ≤ 0.6). The incorporation of B atoms was accompanied by the formation of B–C chemical bonds, whereas the formation of sp² graphite bonds and their ordering in clusters with laminar packing was suppressed. The films possessed very low resistivity (~1.4 mΩ cm) at room temperature and exhibited metallic type of conductance on decreasing the temperature to 77 K.     

The Annular Shell Atom Trap (ASAT)

In the course of exploring some aspects of atom guiding in a hollow, optical fibre, a small negative potential energy well was found just in front of the repulsive or guiding barrier. This results from the optical dipole and the van der Waals potentials. The ground state for atoms bound in this negative potential well was determined by numerically solving the Schrödinger equation and it was found that this negative well could serve as an atom trap. This trap is referred to as the Annular Shell Atom Trap or ASAT because of the geometry of the trapped atoms which are located in the locus of points defining a very thin annular shell just in front of the guiding barrier. A unique feature of the ASAT is the compression of the atoms from the entire volume to the volume of the annular shell resulting in a very high density of atoms in this trap. This trap may have applications to very low temperatures using evaporative cooling and possibly the formation of BEC. Finally, a scheme is discussed for taking advantage of the de Broglie wavelength to store atoms in a bottle trap based on the inability of long de Broglie wavelengths to escape through a selective de Broglie wavelength filter in the atom bottle trap.     

A two species trap for chromium and rubidium atoms

Abstract

We realize a combined trap for bosonic chromium (⁵²Cr) and rubidium (⁸⁷Rb) atoms. Initial experiments focus on exploring a suitable loading scheme for the combined trap and on studies of new trap loss mechanisms originating from simultaneous trapping of two species. By comparing the trap loss from the ⁸⁷Rb magneto-optical trap (MOT) in the absence and presence of magnetically trapped ground state ⁵²Cr atoms we determine the scattering cross-section of σ _inel,RbCr = 5.0 ± 4.0 × 10^?18 m² for light-induced inelastic collisions between the two species. Studying the trap loss from the Rb magneto-optical trap induced by the Cr cooling-laser light, the photoionization cross-section of the excited 5P_3/1 state at an ionizing wavelength of 426 nm is measured to be σ _P = 1.1 ± 0.3 × 10^?21 m².     

Magnetic properties of CuGa<Subscript>0.94</Subscript>Mn<Subscript>0.06</Subscript>Te<Subscript>2</Subscript>

As part of a search for new spintronic materials, we have studied the magnetic properties of the CuGa_0.94Mn_0.06Te₂ chalcopyrite solid solution in the range 2–400 K in weak and strong magnetic fields. Magnetization isotherms, σ(H), were obtained in magnetic fields of up to 3980 kA/m. σ(T) data were collected in two ways: the sample was cooled in a magnetic field or in zero field. The experimental data were analyzed by fitting to the Langevin function. The data are adequately represented by this relation in the case when the magnetic moment of the clusters is μ_cl = 23.4μ_B and the concentrations of magnetic clusters and noninteracting Mn²⁺ ions are n _cl = 2.4 × 10²⁵ m^?3 and n _pm = 5.7 × 10²⁵ m^?3, respectively. The calculated average cluster size is d _cl = 33 Å, the number of Mn²⁺ ions per cluster is z = 21 atoms per cluster, and the magnetic moment per Mn²⁺ ion in the clusters is μ_Mn = 1.1μ_B. This μ_Mn value is far below the theoretical magnetic moment of the Mn²⁺ ion in the electronic configuration d ⁵(5.9μ_B), suggesting antiferromagnetic exchange interaction.     

Quantum non-demolition measurements using cold atoms in an optical cavity

Abstract

In this paper we present a theoretical analysis of a recent quantum non-demolition experiment in optics using cold atoms in a magneto-optical trap as a nonlinear medium. A signal beam and a meter beam from two independent lasers are coupled within a A-type three-level scheme in the D1 line of ⁸⁷Rb atoms. The experimental results for the relevant quantum correlations of the fields represent up to now the best achievement for a single back-action evading measurement. Moreover, they are found to be in remarkably good agreement with the theoretical predictions from a fully quantum model for three-level atoms in a doubly resonant cavity.     

Effect of preparation conditions on the optical and photoluminescent properties of glassy As<Subscript>2</Subscript>S<Subscript>3</Subscript>

We have studied the 77-K photoluminescent properties of As₂S₃ semiconducting glass prepared at different temperatures (T ₁ = 870 K; T ₂ = 1120 K; T ₃ = 1370 K) and cooling rates (v ₁ = 10^?2 K/s, v ₂ = 1.5 K/s, v ₃ = 1.5 × 10² K/s). The results demonstrate that the structural, optical, and photoluminescent properties of semiconducting chalcogenide glasses can be tuned over a considerable range by varying the preparation conditions.     

Improved Superconducting Properties of Thick YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Film by Adding Amino-Rich Polyethyleneimine in Precursor Solution

Polyethyleneimine (PEI) with an amount of –NH₂ groups used in precursor solution could effectively reduce Cu²⁺ volatilization during the pyrolysis process. Thermogravimetric analysis shows that the temperature window of low-temperature pyrolysis for precursor solution with PEI (PEI-YBCO) is widened significantly. The slower pyrolysis process can enrich Cu²⁺ and improve critical current density (J_c) of PEI-YBCO films. The highest J_c is 3.03 MA/cm² at 77 K when the amount of PEI is 0.5 g/10 mL and the film thickness is 400 nm. Then the thickness increases from 0.4 to 2.0 μm by changing the coating times. The J_c values of PEI-YBCO films decrease gradually with the thickness increase. However, the critical current (I_c) can be up to 197 A/cm (at 77 K, self-field) and J_c can still keep 1.68 MA/cm² at 1.2 μm.