Phase diagram of turbulence in superfluid 3He-B

No Heading In superfluid ³He-B mutual-friction damping of vortex-line motion decreases roughly exponentially with temperature. We record as a function of temperature and pressure the transition from regular vortex motion at high temperatures to turbulence at low temperatures. The measurements are performed with non-invasive NMR techniques, by injecting vortex loops into a long column in vortex-free rotation. The results display the phase diagram of turbulence at high flow velocities where the transition from regular to turbulent dynamics is velocity independent. At the three measured pressures 10.2, 29.0, and 34 bar, the transition is centered at 0.52–0.59 T_c and has a narrow width of 0.06 T_c while at zero pressure turbulence is not observed above 0.45 T_c.PACS numbers: 47.37, 67.40, 67.57     

Systemic Study of Orbital and Spin Nematicity in NaFe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As by NMR

Nematic order, a self-organized state with rotational symmetry broken, has been observed in both copper-oxide and iron-pnictide high temperature superconductors. However, its origin is still a mystery in the iron pnictides although it is considered as a key to understand the mechanism of superconductivity. Here, we report a systemic nuclear magnetic resonance (NMR) study on NaFe_1?x Co _x As (0 ≤x ≤ 0.042) that an orbital order, accompanied by an instant spin nematicity, occurs at at a temperature T ^? far above structural transition temperature T _s in the tetragonal phase. We show that the observed NMR spectra splitting and its evolution is due to an incommensurate orbital order that sets in below T ^? and becomes commensurate below T _s. We show that the electric field gradient asymmetry parameter is a good measure for the orbital order parameter which undergoes a Landau-like 2nd-order phase transition. We further show that the spin nematicity is well accounted for by the observed orbital order.     

Pressure dependence of Tc for the shear-stress-induced tetragonal phase of V3Si

The effect of hydrostatic pressure on the superconducting transition temperature (T_c) of the shear-stress-induced tetragonal phase of V₃Si (T_c ≈ 10 K, c/a ≈ 1.05) is reported for the first time. The T_c of this phase increases linearly with pressure at a rate of (1.0 ± 0.1) × 10^?5 K bar^?1, substantially lower than values of dT_c/dp previously reported for the other two phases of V₃Si.     

Some properties of electron beam evaporated amorphous Mo-N films

A series of amorphous Mo-N films was prepared by electron beam evaporation of molybdenum in varying partial pressures of nitrogen and deposited onto substrates cooled to about 80 K. The alloy films were characterized by X-ray diffraction, the superconducting transition temperature,T _c, and the crystallization temperature,T _x. The maximumT _c (8.3 K) and sharpest transition occurred with the minimum nitrogen pressure necessary to form the amorphous structure, as revealed by X-ray diffraction. After annealing the as-deposited films, both bcc and fcc phases were found with the bcc/fcc ratio decreasing with increasing nitrogen partial pressure. Differential scanning calorimetry (DSC) measurements showed significant differences in the shape of peaks associated with either bcc, bcc+fcc, or fcc phases. The temperature,T _x, associated with the fcc crystallization increased with nitrogen content. Heats of crystallization had an average value of about 63 J g^?1. Changes in position of the first amorphous X-ray diffraction peak showed that the amorphous structure was expanded by increasing nitrogen content.     

Study of rotational temperature of oxygen DC glow discharge in Silica and Pyrex discharge tubes

Active DC glow discharges in oxygen have been studied in Silica and Pyrex discharge tubes for medium pressures up to 550 Pa and for discharge currents up to 40 mA. Electric field strength measured by a double-probe technique was found to increase with the pressure and to decrease with the discharge current, which is typical for DC glow discharges. We have focused on the emitted radiation. The rotational temperature (T_rot) of molecular oxygen was determined from the ^PP and ^PQ branches of the well-resolved atmospheric A-band of molecular oxygen at 760 nm. Good agreement between values of T_rot obtained from particular branches was found. The increase of the rotational temperature with increasing pressure and discharge current has been observed in both discharge tubes, however, the values of rotational temperature were systematically higher in the tube made of Pyrex glass. This difference was explained by the particular thermal conductivity of the tube material.     

Evidence of Phase Separation in the P-T Phase Diagram of the La0.75Ca0.25MnO3 Manganite by Infrared Measurements

Recent infrared absorption measurements performed at different pressures and temperatures on the La_0.75Ca_0.25MnO₃ manganite provided an estimate of the insulator-to-metal transition temperature T _IM(P), i.e., the P-T phase diagram of a pseudocubic manganite. Here we consider in detail the P- and T-dependence of the absorption spectral weights n ^*. The n ^*(T,P) behavior gives evidence of phase-separation between conducting and insulating domains in the P-T phase diagram, and allows to establish the occurrence of a characteristic temperature T ^* ? 0.6 T _IM. For T > T ^*, when insulating domains decrease on decreasing temperature or on increasing pressure, it is possible to distinguish a metallic region (connected metallic domains) for T ^* < T < T _IM, and an insulating region (disconnected metallic domains) for T > T _IM. Below T ^*, no reduction of the insulating domains, that may still exist, occurs.     

Critical and tricritical dynamics in3He-4He mixtures

A new formalism is presented to study the critical and tricritical dynamics of³He-⁴He mixtures near the superfluid transition for arbitrary concentration. In this fluid two conserved variables, the molar concentrationX and the entropys, are twofold coupled to a complex order parameter ψ first in the dynamic equations reversibly and second in the free energy dissipatively. However, at an intermediate concentrationX=X _D (which is 0.37 at the saturated vapor pressure) a linear combination ofX ands is found to be asymptotically decoupled from ψ both reversibly and dissipatively. There, dynamic renormalization group equations reduce to those of pure⁴He (or those of the F model) and some dynamic properties are common to those of pure⁴He. For example, atX≈X _D , the gradient ?(T?T _λ ) under heat flow goes to zero without³He mass flow, whereT _λ is the critical temperature, dependent onX, and the thermodiffusion ratio loses the singularity with the critical exponent α. Our dynamic renormalization group equations take into account the above two nonlinear couplings and can be used for any concentrations. Furthermore, using a linear response scheme, general relations are obtained among the kinetic coefficients. As a result the thermal conductivity on the λ line is found to be exactly proportional toX ^?1 at smallX. The coefficient in front ofX ^?1 can be expressed in terms of the diffusion constant of an isolated³He molecule in⁴He.     

Magnetic and Electronic Quantum Criticality in YbRh2Si2

The unconventional nature of the quantum criticality in YbRh₂Si₂ is highlighted on the basis of magnetoresistivity and susceptibility measurements. Results obtained under chemical pressure realized by isoelectronic substitution on the rhodium site are presented. These results illustrate the position of the T^?-line associated with a breakdown of the Kondo effect near the antiferromagnetic instability in the low-temperature phase diagram. Whereas at zero temperature the Kondo breakdown and the antiferromagnetic quantum critical point coincide in the proximity of the stoichiometric compound, they are seen to be detached under chemical pressure: For positive chemical pressure the magnetically ordered phase extends beyond the T ^?(B)-line. For sufficiently high negative pressure the T ^?(B)-line is separated from the magnetically ordered phase. From our detailed analysis we infer that the coincidence is retained at small iridium concentrations, i.e., at small negative chemical pressure. We outline further measurements which may help to clarify the detailed behavior of the two instabilities.     

NMR Study of 3He Adsorbed in Zeolite Pores

The NMR properties of ³He adsorbed in the pores of MCM-41 zeolite have been studied in the temperature range 1.4 to 15 K, at pulsed NMR frequencies of 1.66 and 3.26 MHz. At a coverage x=0.84 monolayer, the linewidth 1/T ^* ₂ scales approximately linearly with the magnitude of the static magnetic field, and T ^* ₂ increases linearly with increasing temperature with an extrapolated low temperature limit of order 80 µs. However T ₂ is significantly longer, increasing from 0.7 µs at the lowest temperatures investigated in a way suggesting thermally activated motion. We attribute T ^* ₂ to static field variations between pores arising from paramagnetism of the zeolite and the random orientation of the pores. On the other hand T ₁ increases monotonically with decreasing temperature to around 80 ms at 1.6 K, indicating that the correlation time of the local magnetic field fluctuations τ _c>ω ^?1 ₀. A detailed temperature dependence of T ₁ and T ₂ has been carried out at a coverage of 0.32 monolayer. Here a minimum in T ₁ is observed at 11 K for a Larmor frequency of 3.26 MHz corresponding to a correlation time τ _c of 5×10^?8 s. T ₁ increases by around three orders of magnitude on cooling to 1.8 K. At this temperature T ₁ decreases significantly with increasing coverage while T ₂ shows a very much weaker coverage dependence.     

Superfluid 3He Confined to a Single 0.6 Micron Slab Stability and Properties of the A-Like Phase Near the Weak Coupling Limit

We present the first study of the phase diagram of a thick film of superfluid ³He confined within a nanofabricated slab geometry. This cryogenic microfluidic chamber provides a well-defined environment for the superfluid, in which both the regular geometry and surface roughness may be fully characterised. The chamber is designed with a slab thickness d=0.6 μm and 3 mm thick walls to allow pressure tuning of the effective confinement between 0 and 5.5 bar. Over this range the zero temperature superfluid coherence length, ξ₀, decreases by approximately a factor of two from 77 to 40 nm. Samples have so far been cooled to 350 μK. We use nuclear magnetic resonance (NMR) to ‘finger-print’ the superfluid order parameter, with the static field applied perpendicular to the slab. To enable us to resolve high quality NMR signals from the tiny amount of superfluid ³He in the slab, we have developed a spectrometer using a two stage SQUID amplifier with unprecedented sensitivity. Simple NMR zeugmatography allows the slab signal to be unambiguously distinguished from that of a small bulk liquid region near the fill line. The measured slab transition temperature, T _c ^slab , shows a suppression proportional to ξ ₀ ² , as expected theoretically, but the absolute suppression is less than expected. Below T _c ^slab , an A-like phase is stable over a significant temperature range. A transition temperature, T _AX , is measured on warming from a so far unidentified phase, occurring at lower temperatures, into the A-phase. At the pressures investigated (3 to 5.5 bar) the transition appears to occur at an approximately fixed value of the effective confinement d/ξ(T _AX ). In this geometry we predict that the A-phase will be stable to T=0 at zero pressure.     

Role of diffusion-annealing temperature on the microstructural and superconducting properties of Cu-doped MgB2 superconductors

This study deals with not only investigate the effect of the copper diffusion on the microstructural and superconducting properties of MgB₂ superconducting samples employing dc resistivity as a function of temperature, scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements but also calculate the diffusion coefficient and the activation energy of copper for the first time. Electrical-resistivity measurements indicate that both the room-temperature resistivity value and zero resistivity transition temperatures (T _c ) increase with increasing the diffusion-annealing temperature from 650 to 850?°C. SEM measurements show that not only the surface morphology and grain connectivity improve but also the grain size of the samples increases with the increase in the diffusion-annealing temperature up to 850?°C. As for the XRD results, all the samples contain the MgB₂ phase only and exhibit the polycrystalline superconducting phase with more intensity of diffraction lines, leading to the increasement in the lattice parameter a and c. Additionally, the diffusion coefficient is observed to increase from 6.81?×?10^?8 to 4.69?×?10^?7?cm²?s^?1 as the diffusion-annealing temperature increases, confirming that the Cu diffusion at lower temperatures is much less significant. Temperature dependence of the Cu diffusion coefficient is described with the aid of the Arrhenius relation D?=?3.75?×?10^?3 exp (?1.15?±?0.10?eV/k _B T) and the corresponding activation energy of copper in MgB₂ system is found to be about 1.15?eV. The possible reasons for the observed improvement in microstructural and superconducting properties of the samples due to Cu diffusion are also discussed.     

The superconducting transition temperature and its high-pressure behavior of tetragonal Nb3Sn

The superconducting transition temperatureT _C of the tetragonal phase of a Nb₃Sn single crystal was determined calorimetrically to be 17.78±0.02 K. It is suppressed by the application of hydrostatic pressure at a rate of ?(1.40±0.05)×10^?5 K bar^?1. The previous observations of double superconducting transitions in Nb₃Sn and other A15 compounds are discussed.     

The generation of particles to observe quantized vortex dynamics in superfluid helium

We describe a method to prepare a sample of superfluid helium-4 with hydrogen particles suspended within it. The method is to dilute hydrogen gas with helium at room temperature, and bubble the mixture through liquid helium at a temperature above the superfluid phase transition temperature, T_λ ≈ 2.17 K. The procedure yields a suspension of micron-sized particles whose total volume is about 10⁵ times smaller than the fluid volume. The fluid and suspension are then cooled to a temperature below T_λ. We show that the particles, so prepared in superfluid helium, are useful for studying superfluid flows and, in particular, the dynamics of quantized vortices. In addition, the particle-superfluid helium system is rich in not yet fully explained interactions. We review preliminary investigations that include observing the vortex lattice in rotating helium, vortex reconnection in quantized vortex turbulence, and vortex ring decay. These data illustrate the basic mechanisms of dissipation in superfluid turbulence.     

Superfluid 3He Confined in a Single 0.6 Micron Slab

We present the preliminary results of our studies of superfluid ³He in a 0.6 μm thick slab using NMR. Below T _c the A phase is observed, and at low pressures the region of stability of the A phase extends down to the lowest temperatures reached, as described elsewhere. At pressures above 3.2 bar another, so far unidentified phase is observed at low temperatures. In this article we focus on the behavior of this phase and the transition between this phase and the A phase, all studied at 5.5 bar. The NMR response at low temperatures exhibits two possible frequency shifts and the transition is hysteretic in temperature.     

Phase Transition of Superfluid 3He in Aerogel

We have studied phase transition of superfluid ³He in 97.5% porosity aerogel by NMR method. Above 1.0 MPa, superfluid phase transition has been observed. The transition temperature T _c ^a is strongly suppressed from its bulk value. The Pressure-Temperature diagram suggests that superfluid phase will not appear below near 0.8 MPa. The A-B phase transition has been observed above 1.3 MPa, below which a state of superfluid phases remains to be identified. The temperature dependence of NMR frequency shifts Δf in the A-like and the B-like phases are almost linear at pressures below 2.4 MPa. We obtained the differential coefficient of NMR frequency shifts ∂(Δf)/∂(T/T _c ^a ) at 0.9T _c ^a as a function of pressure, and it suggests that superfluid phase will not appear below near 0.8 MPa which is the same pressure estimated by P-T diagram.     

Magnetic Transport Properties in GdBa2Cu3?x Ru x O7?δ Superconducting Phase

Bulk superconducting samples of type GdBa₂Cu_3?x Ru _x O_7?δ phase, Gd-123, with?x ranging from 0.0 to 0.15 were prepared by the conventional solid-state reaction technique. X-ray powder diffraction (XRD) and the electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution on Gd-123 phase. Enhancement of the phase formation and the superconducting transition temperature T _c for GdBa₂Cu_3?x Ru _x O_7?δ phase up to x=0.05 was observed. The effect of magnetic field up to 4.4?kG on the electrical resistivity behavior of the prepared samples was studied to investigate the flux motion of this phase. The derived flux pinning energy?U, based on the thermally activated flux creep TAFC model, decreased with increasing the magnetic field?B. The flux pinning energy followed the exponent behavior as U(B)～B ^?β . The superconducting transition width ΔT increased as the magnetic field increased, showing the scaling relation as ΔT～B ⁿ . Using Ambegaokar and Halperin AH theory, the magnetic field and temperature dependence of U was found to be U(B,T)～ΔTB ^?η , η=β+n. The critical current density J _c (0) enhanced up to x=0.05, beyond which it decreased with further increase in Ru-content.     

Comparative Investigation of Elastic Diffuse Neutron Scattering in C60 Powder

Abstract

C₆₀ powder samples of varying impurity content have been investigated by neutron scattering techniques. For Q > 2.5 Å^?1 (Q = transferred momentum) strong elastic diffuse scattering is found below T_c ~ 260 K providing evidence of static disorder in the low temperature phase. The elastic scattering increases on cooling from T_c to 150 K but remains virtually constant below this temperature. There is no marked dependence on the impurity level. For Q > 2.5 Å^?1 elastic diffuse scattering obeying a sin(QR)/(QR) law is observed both above and below the phase transition (R= radius of C₆₀ molecule). This scattering is nearly independent of temperature, however, depends strongly on the amount of impurities in the sample. In order to account for the unusually strong elastic diffuse scattering intensity model calculations have been performed which relate orientational disorder in C₆₀ to the elastic scattering observed in powder samples.     

Magnetoresistance Study of Y3Ba5Cu8O18 Superconducting Phase Substituted by Nd3+ and Ca2+ Ions

Superconducting samples of type Y_3?x Nd _x Ba_5?x Ca _x Cu₈O₁₈ with 0.0 ≤ x ≤ 0.4 have been prepared via the solid-state reaction technique. The prepared samples were characterized using X-ray powder diffraction (XRD) technique for phase analysis. The elemental content of the prepared samples was determined using particle-induced X-ray emission (PIXE). In addition, the oxygen content of these samples was obtained using non-Rutherford backscattering spectroscopy (RBS) at 3 MeV proton beam. The results indicate that these substitutions do not affect the orthorhombic structure, while they decrease the oxygen content of Y-358 phase. The electrical resistivity of the prepared samples was measured by the conventional four-probe technique from room temperature down to the zero superconducting transition temperature (T ₀). A slight change in the superconducting transition temperature (T _c) is observed for 0 ≤ x ≤ 0.1, and then it decreases linearly with further increase in x. The linear decrease in T _c is attributed mainly to the partial substitution of Ba²⁺ ions by Ca²⁺ ions rather than the partial substitution of Y³⁺ ions by Nd³⁺ ions. The effect of magnetic fields up to 4.44 kG on the electrical resistivity has been studied to investigate the vortex dynamics for the prepared samples. The experimental data, in the second stage of superconducting transition, fit well with the thermally activated flux creep (TAFC) model, and the activation energy U(B) shows a power law dependence on magnetic field as B ^?β . Also, the transition width is related to the magnetic field according to the relation ΔT α B ⁿ . The values of β and n are strongly dependent on the Nd³⁺ and Ca²⁺ ion substitution. The magnetic field and temperature dependence of the activation energy U(B, T) is found to be U(B, T)? ΔT B ^?η , where η = β + n. Furthermore, the critical current density at zero temperature, J _c(0), as a function of the applied field was calculated for all the prepared samples. The result shows an enhancement in J _c(0) of Y-358 phase at x equals 0.4 at different applied fields.     

Flow of superfluid3He through micrometer-size channels

Measurements of the critical mass currentJ _c through 0.8-µm-diameter channels in the superfluid phases of³He are reported. Experiments were made at pressures from 0 to 27.4 bar in zero external magnetic field. The pressure difference ΔP along the flow channels is immeasurable within our resolution of ±0.1 µbar for sufficiently low currents in both the A and B phases, implying small or zero dissipation. In the B phase ΔP grows rapidly with increasing current aboveJ _c. At low pressuresJ _c behaves like (1?T/T _cyl)^3/2, whereT _cyl is interpreted as the reduced superfluid transition temperature inside the flow channels;T _cyl/T _c=0.935 atP=0. If the liquid in the channels is in the A phase, the behavior of ΔP vs. the mass currentJ _s depends on the phase A or B outside the channels. During warming a drop of about30% inJ _c is found both atT _BA(cyl) and at;T _BA(cyl) is the reduced B→A transition temperature in the channels. AboveT _BA a second dissipation mechanism, with a smallerd(ΔP)/dJ _s, is observed at lower currents. These features indicate that in the A phase the ends of the channels have an important effect on the flow.