Showing papers in "Journal of Low Temperature Physics in 2005"

Magnetic Quantum Tunneling in the Single-Molecule Magnet Mn12-Acetate

Abstract: The symmetry of magnetic quantum tunneling (MQT) in the single molecule magnet Mn2-acetate has been determined by sensitive low-temperature magnetic measurements in the pure quantum tunneling regime and high frequency EPR spectroscopy in the presence of large transverse magnetic fields. The combined data set definitely establishes the transverse anisotropy terms responsible for the low temperature quantum dynamics. MQT is due to a disorder induced locally varying quadratic transverse anisotropy associated with rhombic distortions in the molecular environment (2nd order in the spin-operators). This is superimposed on a 4th order transverse magnetic anisotropy consistent with the global (average) S4 molecule site symmetry. These forms of the transverse anisotropy are incommensurate, leading to a complex interplay between local and global symmetries, the consequences of which are analyzed in detail. The resulting model explains: (1) the observation of a twofold symmetry of MQT as a function of the angle of the transverse magnetic field when a subset of molecules in a single crystal are studied; (2) the non-monotonic dependence of the tunneling probability on the magnitude of the transverse magnetic field, which is ascribed to an interference (Berry phase)effect; and (3) the angular dependence of EPR absorption peaks, including the fine structure in the peaks, among many other phenomena. This work also establishes the magnitude of the 2nd and 4th order transverse anisotropy terms for Mn12-acetate single crystals and the angle between the hard magnetic anisotropy axes of these terms. EPR as a function of the angle of the field with respect to the easy axes (close to the hard-medium plane) confirms that there are discrete tilts of the molecular magnetic easy axis from the global (average) easy axis of a crystal, also associated with solvent disorder. The latter observation provides a very plausible explanation for the lack of MQT selection rules, which has been a puzzle for many years.

Superfluid to Mott insulator transition in one, two, and three dimensions

Abstract: We have created one-, two-, and three-dimensional quantum gases and study the superfluid to Mott insulator transition. Measurements of the transition using Bragg spectroscopy show that the excitation spectra of the low-dimensional superfluids differ significantly from the three-dimensional case.

The motion of micron-sized particles in He II counterflow as observed by the PIV technique

Abstract: The motion of micro-spheres in He II thermal counterflow has been measured using the Particle Image Velocimetry (PIV) technique. Although the tracer particles are able to track the motion of the normal fluid component, a significant discrepancy between the measured particle velocity and theoretical normal fluid velocity is observed. Further analysis of this velocity discrepancy suggests that it may be caused by the interaction between particles and vortex lines in the superfluid component. A semi-empirical correlation for the interaction force is developed and compared to the experimental results, from which new dynamic behavior of particles in He II is presented in the form of an effective kinematic viscosity of the superfluid.

Permutation Sampling in Path Integral Monte Carlo

Abstract: A simple algorithm is described to sample permutations of identical particles in Path Integral Monte Carlo (PIMC) simulations of continuum many-body systems. The sampling strategy illustrated here is fairly general, and can be easily incorporated in any PIMC implementation based on the staging algorithm. Although it is similar in spirit to an existing prescription, it differs from it in some key aspects. It allows one to sample permutations efficiently, even if long paths (e.g., hundreds, or thousands of slices) are needed. We illustrate its effectiveness by presenting results of a PIMC calculation of thermodynamic properties of superfluid 4He, in which a very simple approximation for the high-temperature density matrix was utilized.

Quantum Phase Transition in the BCS-to-BEC Evolution of p-wave Fermi Gases

Abstract: We discuss the possibility of a quantum phase transition in ultra-cold spin-polarized Fermi gases which exhibit a p-wave Feshbach resonance. We show that when fermionic atoms form a condensate that can be externally tuned between the BCS and BEC limits, the zero temperature compressibility and the spin susceptibility of the fermionic gas are non-analytic functions of the two-body bound state energy. This non-analyticity is due to a massive rearrangement of the momentum distribution in the ground state of the system. Furthermore, we show that the low temperature superfluid density is also non-analytic and exhibits a dramatic change in behavior when the critical value of the bound state energy is crossed.

Thermal Conductivity of Methane-Hydrate

Abstract: The thermal conductivity of the methane hydrate CH 4 (5.75H 2 O) was measured in the interval 2–140 K using the steady-state technique. The thermal conductivity corresponding to a homogeneous substance was calculated from the measured effective thermal conductivity obtained in the experiment. The temperature dependence of the thermal conductivity is typical for the thermal conductivity of amorphous solids. It is shown that after separation of the hydrate into ice and methane, at 240 K, the thermal conductivity of the ice exhibits a dependence typical of heavily deformed fine-grain polycrystal. The reason for the glass-like behavior in the thermal conductivity of clathrate compounds has been discussed. The experimental results can be interpreted within the phenomenological soft-potential model with two fitting parameters.

Decay of quantised vorticity by sound emission

Abstract: It is thought that in a quantum fluid sound generation is the sink of turbulent kinetic energy in the absence of any other dissipation mechanism near absolute zero. We show that a suitably trapped Bose-Einstein condensate provides a model system to study the sound emitted by accelerating vortices in a controlled way.

Observation of Laminar and Turbulent Flow in Superfluid 4He using a Vibrating Wire

Abstract: We have investigated the laminar and the turbulent flow in superfluid 4He using a vibrating wire made of thin NbTi (ϕ 2.5 μm). The wire velocity as a function of applied force has shown a large hysteresis at the first cooling from normal fluid to the superfluid state. But after a couple of increasing and decreasing wire velocity we have found that the hysteresis vanished and the laminar and the turbulent flow are clearly separated at a critical velocity. The wire moving just after the first cooling must be influenced by remnant vortices nucleated through the superfluid transition. The appearance of the laminar flow below the critical velocity suggests that vortex strings on the wire seem to be selected as suitable sizes by a vibrating flow at higher velocities. We also measured the velocity dependence after immersing the wire directly into the superfluid and found that the laminar region expands up to a velocity much higher than the critical velocity observed above. This result indicates that remnant vortices are considerably reduced by the immersing method.

Observation of Non-Classical Rotational Inertia in Solid 4He Confined in Porous Gold

Abstract: Recent torsional oscillator measurements on solid 4He confined in Vycor glass at 62 bars show supersolid response, an abrupt drop in rotational moment of inertia, at 175 mK1 We have investigated the pore-size dependence of the supersolid behavior by confining solid 4He in a different porous host, porous gold, of a considerably larger pore diameter When solid 4He in porous gold is cooled below 02 K a sharp drop in the resonant period is found The supersolid response exhibits a strong dependence on the amplitude of oscillation

Low Temperature Properties and Superconductivity of LuB12

Abstract: We have investigated the low temperature properties of LuB12 by measuring its magnetic susceptibility, heat capacity, and electrical resistivity, as well as by point-contact spectroscopy using both the spear-anvil type technique and mechanically controllable break junctions. Our specific heat measurements and point - contact spectroscopy results indicate that LuB12 is a simple weak-coupling BCS-type superconductor with TC ≈ 0.4 K, a superconducting energy gap of 2 Δ ≈ 0.12 meV, and a very small critical field BC ≈ 1 mT. From the dU/dI(U) characteristics in the superconducting state, the energy gap 2Δ, the critical current IC and the Andreev-reflection excess current Iex as a function of normal-state point contact resistance RN have been determined. At low RN all three parameters are strongly suppressed, possibly due to the self-magnetic field. At contacts with large RN the model of resistively and capacitively shunted Josephson junctions (RCSJ) allowed us to estimate the superconducting current plasma frequency and the Josephson coupling energy. Moreover, from the d2U/d I2(U) spectra in the normal state the (point-contact) electron-phonon interaction function and the characteristic phonon energies of LuB12 have been determined.

Brute force polarization of 129Xe

Abstract: Hyperpolarized 129Xe has numerous applications in Nuclear magnetic resonance (NMR) and magnetic resonance imaging(MRI). As an alternative to the laser optical pumping method of production of hyperpolarized 129Xe gas, we have investigated the brute force technique which uses a very high magnetic field and millikelvin temperatures. One obstacle to this technique is the extremely long spin lattice relaxation times of nuclei in solids at very low temperature. We exploit the fact that liquid 3He can be used as an effective relaxant to enable 129Xe to be spin polarized on a high surface area substrate in a few hours. We are able to identify the contributions of different atomic layers to the magnetization by analyzing the 129Xe NMR spectrum. Furthermore, the addition of 4He allows us to turn off the relaxation mechanism, thereby preserving the polarization. The technique could be applied to nuclei other than 129Xe.

Turbulence generated by vibrating wire resonators in superfluid 4He at low temperatures

Abstract: We have measured responses of vibrating wire resonators in superfluid 4He at millikelvin temperatures. We find evidence for turbulence generation above a critical velocity on the order of a few cm/s. At the critical velocity for the onset of turbulence, the resonator velocity abruptly decreases and shows hysteretic behavior. Surprisingly we find that the resonant frequencies of the resonators increase in the turbulent regime. We also find that the critical velocity may be reduced by the presence of turbulence generated by a neighboring vibrating wire resonator, allowing the detection of existing turbulence in the low temperature regime.

Trapping Fermionic 40K and Bosonic 87Rb on a Chip

Abstract: 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 × 105 87Rb 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.

Hydrogen–Helium Mixtures at High Pressure

Abstract: The properties of hydrogen–helium mixtures at high pressure are crucial to address important questions about the interior of Giant planets e.g. whether Jupiter has a rocky core and did it emerge via core accretion? Using path integral Monte Carlo simulations, we study the properties of these mixtures as a function of temperature, density and composition. The equation of state is calculated and compared to chemical models. We probe the accuracy of the ideal mixing approximation commonly used in such models. Finally, we discuss the structure of the liquid in terms of pair correlation functions.

Specific Heat of Solid Helium

Abstract: Recent torsional oscillator measurements of solid 4He in Vycor glass suggest the existence of a supersolid state of matter below 175mK. We have obtained preliminary data of the specific heat of solid helium down to 80mK. No sharp feature at the onset temperature is observable to within 1%. We are currently working towards extending our measurement to lower temperatures.

A New Method for Computation of Long Ranged Coulomb Forces in Computer Simulation of Disordered Systems

Abstract: Applications of a new method for computation of Coulomb forces in Monte Carlo or molecular dynamics simulation of a wide class of disordered systems including plasmas, ionic fluids and amorphous solids is discussed. This method, based on angular averaging of Ewald sums over all orientations of the reciprocal lattice under conditions of computer simulation, eliminates periodicity artifacts imposed by conventional Ewald scheme and provides much faster computation of electrostatic energy in computer simulations of disordered condensed systems.

Grid Generated Turbulence in Helium II

Abstract: We present experimental data on decaying turbulence generated by towing a grid of crossed tines through a stationary sample of He II. The present data were obtained using a grid of substantially improved design from that used in previous investigations of grid generated superfluid turbulence. An important preliminary observation is that both the magnitude and temperature dependence of the effective kinematic viscosity of the turbulence—deduced from measurements of the attenuation of second sound as a function of time—are not substantially altered by details of the grid used to generate the turbulent flow.

Magnetic Properties of a Superconductor with no Inversion Symmetry

Abstract: We study the magnetic properties of a superconductor in a crystal without $$z \rightarrow -z$$ symmetry, in particular how the lack of this symmetry exhibits itself. We show that, though the penetration depth itself shows no such effect, for suitable orientation of magnetic field, there is a magnetic field discontinuity at the interface which shows this absence of symmetry. The magnetic field profile of a vortex in the x − y plane is shown to be identical to that of an ordinary anisotropic superconductor to second order in a small parameter $$\tilde{k}$$ . For a vortex along z, there is an induced magnetization along the radial direction.

Full Biot-Savart Numerical Simulation of Vortices in He II

Abstract: We present the results of numerical simulation for stochastic dynamics of quantized vortex filaments in HeII. Unlike many previous similar investigations, we performed calculations on base of the full Biot-Savart law. We also use a new algorithm for reconnection processes, which is based on considerations of crossing lines. In addition we introduce the random forces stirring the system. This Langevin statement of problem enables to control various types of random action. In the present simulations we take while noise as a random force. We observe that the stationary state of vortex tangle is strongly nonuniform and fluctuating, with knots suddenly appearing and disappearing, this pattern resembles the famous intermittency effect. We also present calculations of some properties of a vortex tangle (VT) such as the total length, the distribution of the length of loops, and the energy spectrum.

Raman Spectroscopy under Extreme Conditions

Abstract: We report the results of Raman measurements of various materials under simultaneous conditions of high temperature and high pressure in the diamond anvil cell (DAC). High temperatures are generated by laser heating or internal resistive (ohmic) heating or a combination of both. We present Raman spectra of cubic boron nitride (cBN) to 40 GPa and up to 2300 K that show a continuous pressure and temperature shift of the frequency of the transverse optical mode. We have also obtained high-pressure Raman spectra from a new noble metal nitride, which we synthesized at approximately 50 GPa and 2000 K. We have obtained high-temperature spectra from pure nitrogen to 39 GPa and up to 2000 K, which show the presence of a hot band that has previously been observed in CARS measurements. These data have also allowed us to constrain the melting curve and to examine changes in the intramolecular potential with pressure.

On Decaying Counterflow Turbulence in He II

Abstract: We review available experimental and numerical data on decaying counterflow turbulence in He II and present our current understanding of the underlying physics. We take into account the temperature gradient present in the steady-state counterflow turbulence, the fact that in the early stage of the decay the turbulence is still thermally driven, and the fact that at the beginning of the decay the vortex tangle is strongly polarized. When the heater that generates the counterflow turbulence is switched off, the vortex tangle decays, the vortex lines randomize their spatial orientation and the tangle’s polarization decreases. The process of depolarization slows down the recovery of the transverse second sound signal which measures the vortex line density; at some values of parameters it even leads to a net decrease of the amplitude of the transverse second sound prior to reaching the universal −3/2 power temporal law decay typical of classical homogeneous isotropic turbulence in a finite-sized channel.

Effect of exchange on the rotational dynamics of doped helium clusters

Abstract: We study the importance of exchange effects on the rotational dynamics of small doped helium clusters by comparing the result of path integral Monte Carlo simulations for Boltzmann and Bose-Einstein statistics. The variation of the effective rotational constant with cluster size and temperature is discussed for both statistics. We rationalize the discrepancy between the Boltzmann and the Bose-Einstein results in terms of the superfluid response of the complex defined in the molecule frame of the rotating impurity.

Laser Irradiation of Polymer-Doped Cryogenic Matrices

Abstract: Interactions between intense (1019 photons/cm 2 per shot) UV laser light and doped ice matrices were studied by the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) deposition technique. Water, isopropanol, acetone and toluene ice matrices have been used as hosts for the biotechnologically important polymer – polyethylene glycol (PEG). The polymer–matrix system was irradiated under vacuum conditions with high-intensity laser beam at 355 nm in the fluence range 2–10 J/cm 2 . We have explored the ejection of material from the ices in terms of light absorption by the matrix and photochemical interactions between matrix and polymer molecules during the irradiation process. The transfer of polymer from the matrix to a substrate has been studied on the basis of deposition rates measured with a quartz crystal microbalance (QCM) and Fourier-Transform Infrared (FTIR) spectra recorded from the deposits.

Alkali Atoms attached to 3He Nanodroplets

Abstract: We have experimentally studied the electronic 3p ← 3s excitation of Na atoms attached to 3 He droplets by means of laser-induced fluorescence as well as beam depletion spectroscopy. From the similarities of the spectra (width/shift of absorption lines) with these of Na on 4 He droplets, we conclude that sodium atoms reside in a “dimple” on the droplet surface and that superfluid-related effects are negligible. The experimental results are supported by Density Functional calculations at zero temperature, which confirm the surface location of Na, K and Rb atoms on 3 He droplets. In the case of Na, the calculated shift of the excitation spectra for the two isotopes is in good agreement with the experimental data.

The Weak Li2 — He Interaction Revisited: a Combined Ab-initio and Empirical Modelling.

Abstract: The potential energy surfaces (PES) for the interaction between Li2 (1∑+g) and 4He has been computed earlier by using an accurate, post-Hartree-Fock quantum calculation for its ground electronic state and was presented in previous papers1,2. Here we report a revised version of the same interaction that we have generated using instead an empirical approach that replaces the two-body contributions in the ab-initio potential with external estimates. We have repeated Diffusion Monte Carlo calculations to obtain the quantum ground states for the Li2Hen clusters already discussed in our previous paper1 and we compare the two sets of fundings.

Variational Monte Carlo for Interacting Electrons in Quantum Dots

Abstract: We use a variational Monte Carlo algorithm to solve the electronic structure of two-dimensional semiconductor quantum dots in external magnetic field. We present accurate many-body wave functions for the system in various magnetic field regimes. We show the importance of symmetry, and demonstrate how it can be used to simplify the variational wave functions. We present in detail the algorithm for e_cient wave function optimization. We also present a Monte Carlo -based diagonalization technique to solve the quantum dot problem in the strong magnetic _eld limit where the system is of a multiconfiguration nature.

The Effect of Gd Concentration on the Physical and Magnetic Properties of Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+y Superconductors

Abstract: Bi–Pb–Gd–Sr–Ca–Cu–O bulk samples with nominal composition Bi1.7Pb 0.3-xGd x Sr2Ca3Cu4O12+y (x=0.01, 0.05, 0.075, 0.10) were prepared by the melt-quenching method. The effects of different Gd doping on the structure have been investigated by electrical resistance, scanning electron micrographs, XRD, magnetization and magnetic hysteresis loop measurements. The magnetization measurements have been carried out as a function of magnetic field for fields up to 5 kOe at temperatures well below the zero resistance temperatures of the annealed samples. It has been found that the high-Tc superconducting phase, (2 2 2 3), is formed in the sample A with concentration x = 0.01, annealed at 840°C for 120 h. However, with increasing Gd3+ doping for Pb2+ the (2 2 2 3) phase gradually transforms into the (2 2 1 2) phase. The magnitudes of magnetization and initial susceptibility, | M | and | dM/dH|, and the hysteresis loop areas decrease with increasing Gd concentration x and/or temperature T. The fast decreases in | M|, | dM/dH |, and the hysteresis loop areas related to the superconducting volume, with increasing x and/or T seem to imply an existence of flux pinning centres in our samples. In order to support this implication the critical current densities Jc, of the samples, have been estimated at two fixed temperatures, 9 and 30 K. Our data have indicated that Jc decreases with increasing temperature and/or Gd concentration, as expected.

Ellipsometry of liquid helium films on gold, cesium, and graphite

Abstract: We have developed a modulated null ellipsometer with sub-monolayer resolution to measure adsorbed liquid helium thin films at temperatures below 4 K. Adsorption isotherms of 4 He on gold, cesium, and graphite are presented. For Au and Cs substrates, the reflecting surface for our ellipsometric measurements is the metallic electrode of a quartz crystal microbalance(QCM). Performing both types of measurements simultaneously on the same substrate provides a direct method of converting the ellipsometric signal into an absolute film thickness without constructing a detailed model of the refractive index of the substrate. Isotherms on gold above and below T λ ¸ show that the ellipsometric signal is unaffected by the superfluid transition; the ellipsometer measures the total film thickness independent of the superfluid fraction. Isotherms on cesium above the wetting temperature show a prewetting step. Isotherms on clean graphite show steps due to layering.

Persistent Signal; Coherent NMR state trapped by orbital texture

Abstract: The so-called persistent NMR signal, that was found in superfluid 3He-B at very low temperature corresponds to new states of coherent spin precession. They are distinct from the previously known homogeneously precessing domain (HPD), whose stability is supported by the specific profile of the spin-orbital potential. The new states are stabilized by the orbital texture which provides the required feedback mechanism supporting the precession with a single frequency. New precessing states are obtained by pumping the energy into the spin wave modes localized in the potential well formed by the orbital texture. When the intensity of a given spin wave increases, it influences the orbital degrees of freedom. As a result the potential well is modified and hence modifies the eigen-frequency of the spin-wave mode, which in turn affects the orbital texture. The frequency of this self-consistent precessing state thus depends on the longitudinal magnetization, ω (Sz). If dω(Sz)/dSz < 0 this coherent mode spontaneously arises in pulsed NMR and persists with a continuous drift of the frequency. This state of magnetic superfluidity is similar to the Bose condensate formed the ground state of the potential well, in the case when the well self-consistently depends on the population of the condensate.

Heat capacities of the anomalous fluid phase in two-dimensional 3He

Abstract: We measured heat capacities of two dimensional 3He adsorbed on graphite preplated with a 4He monolayer (3He/4He/gr) in a wide temperature (0.3 ≤ T ≤ 80 mK) and density range (1.3 ≤ ρ ≤ 7.3 nm−2). We found that the system behaves as a normal Fermi fluid at low densities between 1.3 and 5.6 nm−2 where the quasiparticle effective mass seems to diverge at a density for the 4/7 phase (ρ4/7 = 6.8 nm−2). At higher densities but below ρ4/7, we observed anomalous temperature dependencies of heat capacity with two round maxima near 1 and 30 mk. With increasing density in this region, a high temperature weight of heat capacity decreases selectively leaving the 30 mK peak, while the 1 mK peak develops. This unexpected behavior can not be explained by the conventional two-phase co-existence model. Instead, we propose here that the 2D normal Fermi fluid is continuously transformed to the 4/7 Mott localized phase through a new quantum phase where a hopping of the zero-point vacancy plays an important role.