2,956 research outputs found

    Low Velocity Granular Drag in Reduced Gravity

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    We probe the dependence of the low velocity drag force in granular materials on the effective gravitational acceleration (geff) through studies of spherical granular materials saturated within fluids of varying density. We vary geff by a factor of 20, and we find that the granular drag is proportional to geff, i.e., that the granular drag follows the expected relation Fprobe = {\eta} {\rho}grain geff dprobe hprobe^2 for the drag force, Fprobe on a vertical cylinder with depth of insertion, hprobe, diameter dprobe, moving through grains of density {\rho}grain, and where {\eta} is a dimensionless constant. This dimensionless constant shows no systematic variation over four orders of magnitude in effective grain weight, demonstrating that the relation holds over that entire range to within the precision of our data

    Quantum-Classical Reentrant Relaxation Crossover in Dy2Ti2O7 Spin-Ice

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    We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through measurements of the a.c. magnetic susceptibility. While the characteristic spin relaxation time is thermally activated at high temperatures, it becomes almost temperature independent below Tcross ~ 13 K, suggesting that quantum tunneling dominates the relaxation process below that temperature. As the low-entropy spin ice state develops below Tice ~ 4 K, the spin relaxation time increases sharply with decreasing temperature, suggesting the emergence of a collective degree of freedom for which thermal relaxation processes again become important as the spins become highly correlated

    Structural, orbital, and magnetic order in vanadium spinels

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    Vanadium spinels (ZnV_2O_4, MgV_2O_4, and CdV_2O_4) exhibit a sequence of structural and magnetic phase transitions, reflecting the interplay of lattice, orbital, and spin degrees of freedom. We offer a theoretical model taking into account the relativistic spin-orbit interaction, collective Jahn-Teller effect, and spin frustration. Below the structural transition, vanadium ions exhibit ferroorbital order and the magnet is best viewed as two sets of antiferromagnetic chains with a single-ion Ising anisotropy. Magnetic order, parametrized by two Ising variables, appears at a tetracritical point.Comment: v3: streamlined introductio

    Low Temperature Spin Freezing in Dy2Ti2O7 Spin Ice

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    We report a study of the low temperature bulk magnetic properties of the spin ice compound Dy2Ti2O7 with particular attention to the (T < 4 K) spin freezing transition. While this transition is superficially similar to that in a spin glass, there are important qualitative differences from spin glass behavior: the freezing temperature increases slightly with applied magnetic field, and the distribution of spin relaxation times remains extremely narrow down to the lowest temperatures. Furthermore, the characteristic spin relaxation time increases faster than exponentially down to the lowest temperatures studied. These results indicate that spin-freezing in spin ice materials represents a novel form of magnetic glassiness associated with the unusual nature of geometrical frustration in these materials.Comment: 24 pages, 8 figure

    Nanoengineered Curie Temperature in Laterally-Patterned Ferromagnetic Semiconductor Heterostructures

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    We demonstrate the manipulation of the Curie temperature of buried layers of the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the effect of annealing. Patterning the GaAs-capped ferromagnetic layers into nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As layers and thus allows the removal of Mn interstitials using annealing. This leads to an enhanced Curie temperature and reduced resistivity compared to unpatterned samples. For a fixed annealing time, the enhancement of the Curie temperature is larger for narrower nanowires.Comment: Submitted to Applied Physics Letters (minor corrections

    Evidence for a Self-Bound Liquid State and the Commensurate-Incommensurate Coexistence in 2D 3^3He on Graphite

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    We made heat-capacity measurements of two dimensional (2D) 3^3He adsorbed on graphite preplated with monolayer 4^4He in a wide temperature range (0.1 T\leq T \leq 80 mK) at densities higher than that for the 4/7 phase (= 6.8 nm2^{-2}). In the density range of 6.8 ρ\leq \rho \leq 8.1 nm2^{-2}, the 4/7 phase is stable against additional 3^3He atoms up to 20% and they are promoted into the third layer. We found evidence that such promoted atoms form a self-bound 2D Fermi liquid with an approximate density of 1 nm2^{-2} from the measured density dependence of the γ\gamma-coefficient of heat capacity. We also show evidence for the first-order transition between the commensurate 4/7 phase and the ferromagnetic incommensurate phase in the second layer in the density range of 8.1 ρ\leq \rho \leq 9.5 nm2^{-2}.Comment: 6 pages, 4 figure

    Thermodynamic Study of Excitations in a 3D Spin Liquid

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    In order to characterize thermal excitations in a frustrated spin liquid, we have examined the magnetothermodynamics of a model geometrically frustrated magnet. Our data demonstrate a crossover in the nature of the spin excitations between the spin liquid phase and the high-temperature paramagnetic state. The temperature dependence of both the specific heat and magnetization in the spin liquid phase can be fit within a simple model which assumes that the spin excitations have a gapped quadratic dispersion relation.Comment: 5 figure

    Experimental determination of superconducting parameters for the intermetallic perovskite superconductor ${\text {MgCNi}}_3

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    We have measured upper-critical-field Hc2H_{\text c2}, specific heat C, and tunneling spectra of the intermetallic perovskite superconductor MgCNi3{}_3 with a superconducting transition temperature Tc7.6T_{\text c}\approx 7.6 K. Based on these measurements and relevant theoretical relations, we have evaluated various superconducting parameters for this material, including the thermodynamic critical field HcH_{\text c}(0), coherence length ξ\xi(0), penetration depth λ\lambda(0), lower-critical-field Hc1H_{\text c1}(0), and Ginsberg-Landau parameter κ\kappa(0). From the specific heat, we obtain the Debye temperature ΘD\it \Theta_{\text D} \approx 280 K. We find a jump of ΔC/γTc\Delta C/\gamma T_{\text c}=2.3 at TcT_{\text c} (where γ\it \gamma is the normal state electronic specific coefficient), which is much larger than the weak coupling BCS value of 1.43. Our tunneling measurements revealed a gap feature in the tunneling spectra at Δ\it \Delta with 2Δ/kBTc2\it {\Delta}/{\text k}_{\text B}T_{\text c}\approx 4.6, again larger than the weak-coupling value of 3.53. Both findings indicate that MgCNi3_3 is a strong-coupling superconductor. In addition, we observed a pronounced zero-bias conductance peak (ZBCP) in the tunneling spectra. We discuss the possible physical origins of the observed ZBCP, especially in the context of the pairing symmetry of the material.Comment: 5 pages, 4 figure

    Performance of a GridPix detector based on the Timepix3 chip

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    A GridPix readout for a TPC based on the Timepix3 chip is developed for future applications at a linear collider. The GridPix detector consists of a gaseous drift volume read out by a single Timepix3 chip with an integrated amplification grid. Its performance is studied in a test beam with 2.5 GeV electrons. The GridPix detector detects single ionization electrons with high efficiency. The Timepix3 chip allowed for high sample rates and time walk corrections. Diffusion is found to be the dominating error on the track position measurement both in the pixel plane and in the drift direction, and systematic distortions in the pixel plane are below 10 μ\mum. Using a truncated sum, an energy loss (dE/dx) resolution of 4.1% is found for an effective track length of 1 m.Comment: To be published in Nuclear Instruments and Methods in Physics Research Section
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