46 Matching Results

Search Results

Advanced search parameters have been applied.

Electron transfer and physical and chemical processes at low temperatures

Description: We summarize some phenomena that occur at temperatures of the order of 15K, and are dominated by quantum mechanical tunneling. Although electron tunneling dominates many conduction processes at low temperatures, we discuss evidence that phenomena like oxidation, as well as the solution of alkali metals in ammonia, can also be dominated by electron tunneling. Both phenomena demonstrate that the chemical potential of a metastable system can equilibrate at low temperatures by electron tunneling. The case of alkali metal clusters covered with ammonia is contrasted to covering the clusters with Xe. In this case changes in the activated conduction are observed which are consistent with the dielectric constant of the rare gas.
Date: October 1, 1995
Creator: Strongin, M.; Xia, B. & Jacobsen, F.M.
Partner: UNT Libraries Government Documents Department

Weak ferromagnetism in {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Cl

Description: We present magnetization measurements at ambient pressure on {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Cl, an organic salt that is superconducting under pressure ({Tc} = 12.8 K at 0.3 kbar). An antiferromagnetic transition near 45 K and, for the first time in this class of materials, a transition near 22 K to a state displaying weak ferromagnetic hysteresis with a saturation moment of 8 x 10 {sup {minus}4}{mu}{sub B}/formula are observed. This low temperature state is characterized by a sequence of first order magnetization jumps.
Date: October 1, 1992
Creator: Welp, U.; Kwok, W. K.; Crabtree, G. W.; Carlson, K. D.; Wang, H. H.; Geiser, U. et al.
Partner: UNT Libraries Government Documents Department

Development of 0.5-5 W, 10K Reverse Brayton Cycle Cryocoolers - Phase II Final Report

Description: Miniature cryocoolers for the 8-30 K range are needed to provide 0.5-5 w of cooling to high sensitivity detectors (for long-wave-length IR, magnetism, mm-wave, X-ray, dark matter, and possibly y-ray detection) while maintaining low mass, ultra-low vibration, and good efficiency. This project presents a new approach to eliminating the problems normally encountered in efforts to build low-vibration, fieldable, miniature cryocoolers. Using the reverse Brayton Cycle (RBC), the approach applies and expands on existing spinner technology previously used only in Nuclear Magnetic Resonance (NMR) probes.
Date: October 15, 2001
Creator: Doty, F. D.; Boman, A.; Arnold, S.; Spitzmesser, J. B.; Jones, D.; McCree, D. et al.
Partner: UNT Libraries Government Documents Department

Thermal oscillation smoothing of DT solid layers for HAPL and NIF scale targets

Description: Deuterium-Tritium (D-T) solid fuel layers must meet stringent roughness specifications for both the ICF and IFE laser fusion programs and native beta-layering alone is unable to provide sufficient solid layer smoothing to meet these specifications at 18.3 K or below. Consequently, several supplemental smoothing options have been proposed to resolve this issue, including a technique called 'Thermal Breathing'. This technique consists of oscillating the temperature of the solid D-T layer about its equilibration temperature for a period of one to several hours. Recently, thermal oscillations have been used to successfully smooth rough solid D{sub 2} in spherical targets. In order to study this particular smoothing technique, we examined the effects of thermal oscillations on equilibrated D-T solid layers, using both ICF and IFE scale layering cells and layer thicknesses. The D-T solid layers that were Subjected to thermal breathing in these studies were equilibrated at temperatures ranging from 16.0 K to 19.25 K, followed by 1.5 to 2 hours of temperature oscillations. During the HAPL scale experiments the amplitude and period of the oscillations were both varied to examine parametric effects of these variables on final layer roughness. In both sets of experiments, once the oscillations completed we allowed the layers to 'relax' at their initial equilibration temperature for another 1 to 2 hours, to observe any 'rebounding' or re-roughening that might occur. The rCF scale experiments were performed using a 2 mm beryllium torus, for which the layer was free from optical distortions that were observed in our IFE scale cell (a 4 mm dia. sapphire sphere-cylinder). Our results showed a temperature dependent smoothing effect ofthe DT solid layer ranging from 20% to 35% over the temperature range of 17.3 K to 19.0 K for the rCF-scale, 2-mm celL The final RMS roughness for layers grown in this 2-mm ...
Date: January 1, 2009
Creator: Sheliak, John D; Geller, Drew A & Hoffer, James K
Partner: UNT Libraries Government Documents Department

Polarization orientation dependence of the far infrared spectra of oriented single crystals of 1,3,5,-trinitro-s-triazine (RDX) using terahertz-time-domain spectroscopy

Description: The far infrared spectra of (100), (010), and (001)-oriented RDX single crystals were measured as the crystal was rotated about the axis perpendicular to the polarization plane of the incident radiation. Absorption measurements were taken at temperatures of both 20 K and 295 K for all rotations using terahertz time-domain spectroscopy. A number of discrete absorptions were found ranging from 10-100 cm(-1) (0.3-3 THz). The absorptions are highly dependent on the orientation of the terahertz polarization with respect to crystallographic axes.
Date: January 1, 2008
Creator: Whitley, Von H; Hooks, Dan E; Ramos, Kyle J; O' Hara, John F; Azad, A K; Taylor, A J et al.
Partner: UNT Libraries Government Documents Department

Onset of bulk pinning in BSCCO single crystals

Description: The long growth defects often found in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} single crystals effectively weaken the geometrical barrier and lower the field of first flux penetration. This means that the intrinsic (bulk) magnetic properties can be more easily accessed using magnetic measurements. Thus, the onset of strong bulk flux pinning in the sample bulk is determined to lie at T {approximately} 40 K, independent of whether the field strength is above or below the field of the second peak in the magnetization.
Date: December 1, 1996
Creator: Beek, C.J. van der; Indenbom, M.V.; Berseth, V.; Benoit, W. & Li, T.W.
Partner: UNT Libraries Government Documents Department

Surface roughness statistics and temperature step stress effects for D-T solid layers equilibrated inside a 2 mm beryllium torus

Description: Solid D-T layers are equilibrated inside a 2 mm diameter beryllium toroidal cell at temperatures ranging from 19.0 K to 19.6 K, using the beta-layering process. The experimental runs consists of multiple cycles of rapid- or slow-freezing of the initially liquid D-T charge, followed by a lengthy period of beta-layering equilibration, terminated by melting the layer. The temperature was changed in discrete steps at the end of some equilibration cycles in an attempt to simulate actual ICF target conditions. High-precision images of the D-T solid-vapor interface were analyzed to yield the surface roughness {sigma}{sub mns} as a sum of modal contributions. Results show an overage {sigma}{sub mns} of 1.3 {+-} 0.3 {micro}m for layers equilibrated at 19.0 K and show an inverse dependence of {sigma}{sub mns} on equilibration temperature up to 19.525 K. Inducing sudden temperature perturbations lowered {sigma}{sub mns} to 1.0 {+-} 0.05 {micro}m.
Date: December 31, 1998
Creator: Sheliak, J.D. & Hoffer, J.K.
Partner: UNT Libraries Government Documents Department

Magnetoresistance behavior of UNiGe

Description: The authors have measured the temperature dependences of the magnetoresistance of single-crystalline UNiGe for both parallel (i//B//c-axis) and perpendicular configurations (i//a-axis, B//c-axis) in magnetic fields up to 18 T. Both configurations yield similar magnetoresistance behavior, which emphasizes the strong magnetic contribution to the resistivity in all directions. Crossing magnetic-phase boundaries causes anomalies in the magnetoresistance, which allowed a completion of the previously proposed magnetic phase diagram.
Date: May 1, 1995
Creator: Nakotte, H.; Lacerda, A.; Purwanto, A.; Havela, L.; Sechovsky, V.; Prokes, K. et al.
Partner: UNT Libraries Government Documents Department

Temperature dependence of the two-photon absorption spectrum of poly(di-n-hexylsilane)

Description: We have measured the temperature dependence of the intensity of the lowest energy two-photon absorption line in poly(di-n-hexylsilane) and find that it does not change between ambient temperature and 14 K. The line width decreases by about a factor of three.
Date: April 1, 1994
Creator: Kepler, R. G. & Soos, Z. G.
Partner: UNT Libraries Government Documents Department

Polarized neutron reflection study of the unidirectional magnetic anisotropy of permalloy on Ni{sub 0.5}Co{sub 0.5}O

Description: Couples of ferromagnetic permalloy/antiferromagnetic metal oxide exhibit unidirectional magnetic anisotropy along a magnetic field applied during cooling. Polarized neutron reflection has been used to check if the effect is due to a bias of the antiferromagnetic configuration in the oxide, where the layer immediately adjacent to the permalloy is polarized parallel to the easy direction of magnetization. The measurements were made on a sample consisting of permalloy/500{Angstrom} Ni{sub 0.05}Co{sub 0.5}O. The polarized neutron reflectivities taken at the four corners of the magnetic hysteresis loop at 20K. A faint magnetic signal consistent with the unidirectional bias of the F/AF structure is also observed. This behavior is discussed in the light of the current theories.
Date: August 1, 1992
Creator: Felcher, G. P.; Huang, Y. Y.; Carey, M. & Berkowitz, A.
Partner: UNT Libraries Government Documents Department

Relaxation of hysteresis loop in a single crystal of YBa{sub 2}Cu{sub 4}O{sub 8}

Description: Magnetic relaxation has been studied for Y{sub 1}Ba{sub 2}Cu{sub 4}O{sub 8} in order to look for changes in the relaxation rates that arise because different pinning mechanisms dominate in different magnetic field regimes. In the low field regime where the Bean-Livingston surface barrier dominates, the vortices exit on a filed decreasing branch with a relaxation rate about ten times faster than they enter on a corresponding field increasing branch. In high magnetic fields where bulk pinning dominates, the relaxation rates for field increasing and field increasing and field decreasing are essentially the same.
Date: April 1, 1993
Creator: Xu, Ming; Finnemore, D. K.; Crabtree, G. W.; Vinokur, V. M.; Hinks, D. G.; Zhang, K. et al.
Partner: UNT Libraries Government Documents Department

Magnetic hysteresis and flux creep of YBa{sub 2}Cu{sub 3}O{sub x} grown by the melt-powder-melt-growth (MPMG) process

Description: Magnetic hysteresis and flux creep of melt-powder-melt-growth (MPMG) YBa{sub 2}Cu{sub 3}O{sub x} containing nominal 0, 25 and 40 mole% Y{sub 2}BaCuO{sub 5} (21 1) were measured in the temperature range of 5 to 80 K and in magnetic fields up to 5 T. With the introduced fine dispersion of second phase 211 particles, the critical magnetization current density J{sub c} shows a weak field dependence over a wide range of temperature, and the effective pinning energy U{sub eff} is much enhanced. From these results, a functional expression U{sub eff}(J,T) = {minus} U{sub o} G(T) (J {vert_bar}J{sub i}){sup n} is obtained, where G(T) = [1 {minus} (T{vert_bar}T{sub x}){sup 2}]{sup 2} with Tx = 82.5 K near the irreversibility temperature. The observed power-law relationship of U{sub eff}(J, T) clearly demonstrates two of three regimes as predicted by the theory of collective flux creep, namely n = 3/2 and 7/9 for J < J{sub c} and J {much_lt} J{sub c}, respectively. In addition, the divergence of U{sub eff} at low current densities also suggests the existence of a vortex-glass state.
Date: August 24, 1992
Creator: Kung, P. J.; McHenry, M. E.; Maley, M. P.; Willis, J. O.; Murakami, M. & Tanaka, S.
Partner: UNT Libraries Government Documents Department

Specific Heat Measurements of TiB(2)[subscript] and (6)[superscript]LiF from 0.5 to 30K

Description: The specific heats of TiB{sub 2} and {sup 6}LiF have been measured from 0.5 to 30 K as part of a larger project in the construction of a neutron spectrometer. For this application, the measured specific heats were used to extrapolate the specific heats down to 0.1 K with lattice, electronic, and Schottky equations for the respective samples. The resultant specific heat values at 0.1 K for TiB{sub 2} and {sup 6}LiF are 4.08 x 10{sup -4} {+-} 0.27 x 10{sup -4} J/K/mol and 9.19 x 10{sup -9} {+-} 0.15 x 10{sup -9} J/K/mol, respectively.
Date: March 28, 2005
Creator: Lang, B.E.; Marcus, H.D.; Woodfield, B.F.; Burger, A; Utupal, N.R; Lamberti, V. et al.
Partner: UNT Libraries Government Documents Department

YBa{sub 2}Cu{sub 3}O{sub 7-x} 45{degree} [001] tilt grain boundaries induced by controlled low-energy sputtering of MgO substrates: Transport properties and atomic-scale structure

Description: Grain boundaries can act as weak links in the high {Tc} materials. If properly controlled, these grain boundaries can be used in various device applications. We have been able to reproducibly form 45{degree} [001] tilt grain boundary junctions in YBa{sub 2}Cu{sub 3}0{sub 7-x} thin films. The films were grown on MgO substrates using a pre-growth substrate treatment. A low energy broad beam Argon ion source was used to irradiate a select region of (100) MgO substrates. The film on the milled portion of the substrate grows predominantly with a grain orientation rotated 45{degree} about the c-axis with respect to the grain on the unmilled portion. Backscattered electron Kikuchi patterns have been used to confirm that the rotation occurs across the entire milled portion of the substrate. Transport properties of these films are discussed and related to high resolution electron microstructural and microchemical analyses of the grain boundaries. This technique has potential use in device applications as a method for controlled grain boundary engineering.
Date: October 1, 1994
Creator: Vuchic, B.V.; Merkle, K.L.; Funkhouser, J.W.; Bucholz, D.B.; Dean, K.A.; Chang, R.P.H. et al.
Partner: UNT Libraries Government Documents Department

Weak ferromagnetism in [kappa]-(ET)[sub 2]Cu[N(CN)[sub 2]]Cl

Description: We present magnetization measurements at ambient pressure on [kappa]-(ET)[sub 2]Cu[N(CN)[sub 2]]Cl, an organic salt that is superconducting under pressure ([Tc] = 12.8 K at 0.3 kbar). An antiferromagnetic transition near 45 K and, for the first time in this class of materials, a transition near 22 K to a state displaying weak ferromagnetic hysteresis with a saturation moment of 8 x 10 [sup [minus]4][mu][sub B]/formula are observed. This low temperature state is characterized by a sequence of first order magnetization jumps.
Date: October 1, 1992
Creator: Welp, U.; Kwok, W.K.; Crabtree, G.W.; Carlson, K.D.; Wang, H.H.; Geiser, U. et al.
Partner: UNT Libraries Government Documents Department

Transport properties of Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8} meander lines

Description: The authors have measured the electrical transport properties of 10 cm long Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8} thin film meander lines in magnetic fields up to 18T. The large resistance of the meander line effectively increased the electric field, or equivalently, resistivity sensitivity by two orders of magnitude. In moderate magnetic fields the critical current density is found to depend only on the component of the magnetic field normal to the ab planes owing to the highly anisotropic nature of the system. The resistivity versus temperature was measured as a function of magnetic field up to 18T. The linear resistivity is found to vanish in a manner consistent with the occurrence of a second order phase transition. The critical exponent v(z-1) is virtually field independent with a value 5.3 {+-}1 consistent with previous measurements of a three dimensional vortex glass transition in YBa{sub 2}Cu{sub 3}O{sub 7} and Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}. The critical regime is 10-15K wide in agreement with values for Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} at high magnetic field.
Date: December 31, 1995
Creator: Fleshler, S.; Maley, M. P.; Jia, Quanxi; Wu, Xin Di & Lacerda, A.
Partner: UNT Libraries Government Documents Department

Effect on C-axis transport properties of a critical state in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}

Description: At low temperatures, the c-axis transport properties of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} single crystals depend strongly on the magnetic field history for fields applied along the c-axis, indicating the effect of a critical state in the ab direction. In this study, the authors report zero field cooled (ZFC), field cooled warming (FCW), and field cooled cooling (FCC) c-axis transport properties. In addition, they also report magnetic relaxation measurements on the same samples to identify the c-axis dissipation mechanism. They observe a relaxation of the c-axis resistivity that follows the relaxation of the flux profile across the ab plane. By identifying the irreversible temperature vs. magnetic field for H // c, they construct a phase diagram of vortex states. Finally, they will discuss the implication of the phase diagram and the features related to the transport properties of the Josephson-coupled system and the role of pancake vortices in highly two-dimensional superconductors.
Date: December 31, 1995
Creator: Cho, J. H.; Maley, M. P. & Bulaevskii, L. N.
Partner: UNT Libraries Government Documents Department

Specific heat of La{sub 1.875-x}Nd{sub x}Sr{sub 0.125}CuO{sub 4}: Magnetic and structural transitions

Description: Near 70K (La,Ba){sub 2}CuO{sub 4} undergoes a structural transition from orthorhombic to tetragonal symmetry (LTO {yields} LTT). Partial substitution of La by Nd produces an additional transition that has been observed in structural studies. The specific heats of La{sub l.475}Nd{sub 0.4}Sr{sub 0.125}CuO{sub 4} and La{sub l.275}Nd{sub 0.6}Sr{sub 0.125}CuO{sub 4} samples exhibit anomalies near 75K, which are associated with these structural transitions, and additional smaller anomalies near 30K, which may be associated with a magnetic transition detected by {mu}SR.
Date: May 1, 1993
Creator: Wright, D. A.; Fisher, R. A.; Phillips, N. E.; Crawford, M. K. & McCarron, E. M. III
Partner: UNT Libraries Government Documents Department

Pulsed field magnets at the United States National High Magnetic Field Laboratory

Description: The pulsed field facility of the National High Magnetic Field Laboratory (NHMFL) consists of four components. Now available are (1) explosive driven flux compression, (2) capacitor-driven magnets, and (3) a 20 T superconducting magnet. The fourth component, a 60 T quasi-continuous magnet, has been designed and is scheduled for installation in early 1995. All magnets have He-4 cryostats giving temperatures from room temperature (RT) to 2.2--1.5 K. Dilution refrigerators for the superconducting 20 T magnet and the 50 T pulsed magnet will be installed by early 1994. A wide range of experiments has been completed within the past year.
Date: November 1, 1993
Creator: Campbell, L. J.; Parkin, D. M.; Crow, J. E.; Schneider-Muntau, H. J. & Sullivan, N. S.
Partner: UNT Libraries Government Documents Department

Non-fermi liquid and spin-glass behavior of the Sc{sub 1-x}U{sub x}Pd{sub 3} system

Description: Previous electrical resistivity {rho}T, magnetic susceptibility {chi}T, and specific heat C(T) measurements on the Y{sub 1-x}U{sub x}Pd{sub 3} system have revealed Kondo behavior for 0 < {times} < 0.2 and spin-glass-like behavior for 0.3 < {times} < 0.5. The Kondo behavior is unusual and characterized by non-Fermi liquid behavior at low temperatures T {much_lt} T{sub K}, where T{sub K} is the Kondo temperature: {rho}T/{rho}(0) {approximately} 1 {minus} T/(aT{sub K}), and C(T)/T {approximately} 1 {minus} (1/T{sub K})1nT with evidence for a finite T = 0 residual entropy S(0) = (R/2)1n(2). We report measurements of {rho}T, {chi}T, and C(T) on the Sc{sub 1-x}U{sub x}Pd{sub 3} system which reveal similar Kondo, non-Fermi liquid, and spin-glass behaviors.
Date: September 1, 1993
Creator: Gajewski, D. A.; Allenspach, P.; Seaman, C. L. & Maple, M. B.
Partner: UNT Libraries Government Documents Department

Alloying studies in U{sub 1-x}Th{sub x}Pd{sub 2}Al{sub 3} (0 {le} x {le} 1)

Description: Measurements of temperature and concentration dependences of magnetic susceptibility and electrical resistivity for U{sub 1-x}Th{sub x}Pd{sub 2}Al{sub 3} are presented. The magnetic susceptibility data scale with the concentration of U ions over large regions of x and T. The electrical resistivity shows a smooth evolution from the Kondo lattice regime to the single ion Kondo behavior as Th concentration x is increased. A lack of saturation in resistivity in the samples with x = 0.8 and 0.6, and a logarithmic divergence in {chi}(T) at low temperatures makes the U{sub 1-x}Th{sub x}Pd{sub 2}Al{sub 3} system a good candidate for a two-channel Kondo effect.
Date: October 1, 1993
Creator: Dalichaouch, Y. & Maple, M. B.
Partner: UNT Libraries Government Documents Department

Optical determination of conduction and valence-band dispersion curves in strained single-quantum wells

Description: Conduction and valence-band dispersion curves determined by magnetoluminescence are presented for n-type InGaAs/GaAs strained-single-quantum well structures. The magnetic field range was 0 to 30 tesla, and the temperature varied between 4.2 and 77.4 K.
Date: September 1, 1994
Creator: Jones, E. D.; Dawson, L. R.; Klem, J. F.; Lyo, S. K.; Heiman, D. & Liu, X. C.
Partner: UNT Libraries Government Documents Department

Magnetic phases in UNiGe

Description: Original measurements of 4.2 K magnetization curves and temperature dependence of magnetic susceptibility were completed by an extended study of magnetization, specific heat, and electric resistivity as a function of temperature and magnetic field. Neutron diffraction experiments were also performed on powder and single crystal. The specific heat shows a sharp peak at 41.5 K, corresponding to the first-order phase transition, and a broad anomaly around 50 K. The magnetic structures in UNiGe confirm that the strong bonding of 5f orbitals along the a axis leads to a huge magnetic anisotropy with U magnetic moments perpendicular to this direction.
Date: October 1, 1993
Creator: Sechovsky, V.; Havela, L. & Purwanto, A.
Partner: UNT Libraries Government Documents Department

Flux pinning by 211 precipitates in melt-processed YBa{sub 2}Cu{sub 3}O{sub x}

Description: Magnetic hysteresis data have been taken at 4.2 and 50 K over a wide range of magnetic fields on melt-textured (MT) and quench-melt-growth-processed (QMGP) YBa{sub 2}Cu{sub 3}O{sub x} samples with 6 vol.% Of Y{sub 2}BaCuO{sub x} (211). In order to obtain accurate comparisons of the intragranular critical current densities, the magnetization measurements were performed on finely powdered samples, the average particle size of which was less than the grain size of the original sample. The QMGP samples exhibited substantially larger hysteresis, indicating significantly enhanced flux pinning at these temperatures. At 4.2 K and 5 T, J{sub c} increase from 4 {times} 10{sup 4} A/cm{sub 2} in the MT sample to 5.5 {times} 10{sup 5} A/cm{sup 2} in the QMGP sample. A possible pinning mechanism related to the observed hysteretic behavior is discussed.
Date: August 1, 1992
Creator: Shi, D.; Sengupta, S.; Smith, M.; Wang, Z.; Biondo, A. C.; Balachandran, U. et al.
Partner: UNT Libraries Government Documents Department