Subnanosecond Single Electron Source in the Time-Domain

We describe here the realization of a single electron source similar to single photon sources in optics. On-demand single electron injection is obtained using a quantum dot connected to the conductor via a tunnel barrier of variable transmission (quantum point contact). Electron emission is triggered by a sudden change of the dot potential which brings a single energy level above the Fermi energy in the conductor. A single charge is emitted on an average time ranging from 100 ps to 10 ns ultimately determined by the barrier transparency and the dot charging energy. The average single electron emission process is recorded with a 0.5 ns time resolution using a real-time fast acquisition card. Single electron signals are compared to simulation based on scattering theory approach adapted for finite excitation energies.     

Homogeneous Turbulence in Superfluid 4He in the Low-Temperature Limit: Experimental Progress

Selected advances in the research on the dynamics of tangles of quantized vortices in superfluid helium with little normal component during the last 50 years are briefly reviewed. The main emphasis is on the experimental techniques of generating and probing homogeneous one-component superfluid turbulence of various energy spectra in superfluid ⁴He in the low-temperature limit. The most recent experimental progress, modern theoretical concepts and future outlook are summarized.     

Theory of Decay of Superfluid Turbulence in the Low-Temperature Limit

We review the theory of relaxational kinetics of superfluid turbulence—a tangle of quantized vortex lines—in the limit of very low temperatures when the motion of vortices is conservative. While certain important aspects of the decay kinetics depend on whether the tangle is non-structured, like the one corresponding to the Kibble-Zurek picture, or essentially polarized, like the one that emulates the Richardson-Kolmogorov regime of classical turbulence, there are common fundamental features. In both cases, there exists an asymptotic range in the wavenumber space where the energy flux is supported by the cascade of Kelvin waves (kelvons)—precessing distortions propagating along the vortex filaments. At large enough wavenumbers, the Kelvin-wave cascade is supported by three-kelvon elastic scattering. At zero temperature, the dissipative cutoff of the Kelvin-wave cascade is due to the emission of phonons, in which an elementary process converts two kelvons with almost opposite momenta into one bulk phonon. Along with the standard set of conservation laws, a crucial role in the theory of low-temperature vortex dynamics is played by the fact of integrability of the local induction approximation (LIA) controlled by the parameter Λ=ln?(λ/a ₀), with λ the characteristic kelvon wavelength and a ₀ the vortex core radius. While excluding a straightforward onset of the pure three-kelvon cascade, the integrability of LIA does not plug the cascade because of the natural availability of the kinetic channels associated with vortex line reconnections. We argue that the crossover from Richardson-Kolmogorov to the Kelvin-wave cascade is due to eventual dominance of local induction of a single line over the collective induction of polarized eddies, which causes the breakdown of classical-fluid regime and gives rise to a reconnection-driven inertial range.     

Pseudogap in Fermionic Density of States in the BCS-BEC Crossover of Atomic Fermi Gases

We study pseudogap behaviors of ultracold Fermi gases in the BCS-BEC crossover region. We calculate the density of states (DOS), as well as the single-particle spectral weight, above the superfluid transition temperature T _c including pairing fluctuations within a T-matrix approximation. We find that DOS exhibits a pseudogap structure in the BCS-BEC crossover region, which is most remarkable near the unitarity limit. We determine the pseudogap temperature T ^* at which the pseudogap structure in DOS disappears. We also introduce another temperature T ^** at which the BCS-like double-peak structure disappears in the spectral weight. While one finds T ^*>T ^** in the BCS regime, T ^** becomes higher than T ^* in the crossover and BEC regime. We also determine the pseudogap region in the phase diagram in terms of temperature and pairing interaction.     

Modifying the surface charge of single track-etched conical nanopores in polyimide

Chemical modification of nanopore surfaces is of great interest as it means that the surface composition is no longer fixed by the choice of substrate material, even to the point where large biomolecules can be attached to the pore walls. Controlling nanopore transport characteristics is one important application of surface modification which is very relevant given the significant interest in sensors based on the transport of ions and molecules through nanopores. Reported here is a method to change the surface charge polarity of single track-etched conical nanopores in polyimide, which also has the potential to attach more complex molecules to the carboxyl groups on the nanopore walls. These carboxyl groups were converted into terminal amino groups, first by activation with N-(3-dimethylaminopropyl)-N-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS) followed by the covalent coupling of ethylenediamine. This results in a changed surface charge polarity. Regeneration of a carboxyl-terminated surface was also possible, by reaction of the amino groups with succinic anhydride. The success of these reactions was confirmed by measurements of the pore's pH sensitive current-voltage (I-V) characteristics before and after the chemical modification, which depend on surface charge. The?permselectivity of the pores also changed accordingly with the modification.     

Characterizing the Superconducting-to-Normal Transition in Mo/Au Transition-Edge Sensor Bilayers

We are developing arrays of Mo/Au bilayer transition-edge sensors (TES’s) for future X-ray astronomy missions such as NASA’s Constellation-X. The physical properties of the superconducting-to-normal transition in our TES bilayers, while often reproducible and characterized, are not well understood. The addition of normal metal features on top of the bilayer are found to change the shape and temperature of the transition, and they typically reduce the unexplained ‘excess’ noise. In order to understand and potentially optimize the properties of the transition, we have been studying the temperature, widths and current dependence of these transitions. We report on the characterization of devices both deposited on silicon substrates and suspended on thin silicon nitride membranes. This includes key device parameters such as the logarithmic resistance sensitivity with temperature α, and the logarithmic resistance sensitivity with current β, and their correlation with excess noise.      

Dispersion-Induced Splitting of the Collective Mode Spectrum in Axial and Planar Phases of Superfluid 3He

The whole collective mode spectrum in axial and planar phases of superfluid ³He with dispersion corrections is calculated for the first time. In axial A-phase the degeneracy of clapping modes depends on the direction of the collective mode momentum k with respect to the vector l (mutual orbital moment of Cooper pairs), namely: the mode degeneracy remains the same as in case of zero momentum k for k‖l only. For any other directions there is a threefold splitting of these modes, which reaches maximum for k ⊥ l. In planar 2D-phase, which exists in the magnetic field (at H>H _C ) we find that for clapping modes the degeneracy depends on the direction of the collective mode momentum k with respect to the external magnetic field H, namely: the mode degeneracy remains the same as in case of zero momentum k for k‖H only. For any other directions different from this one (for example, for k ⊥ H) there is twofold splitting of these modes. The obtained results imply that new interesting features can be observed in ultrasound experiments in axial and planar phases: the change of the number of peaks in ultrasound absorption into clapping mode. One peak, observed for these modes by Ling et al. (J. Low Temp. Phys. 78:187, 1990), will split into two peaks in a planar phase and into three peaks in an axial phase under the change of ultrasound direction with respect to the external magnetic field H in a planar phase and with respect to the vector l in an axial phase. In planar phase, some Goldstone modes in the magnetic field become massive (quasi-Goldstone) and have a similar twofold splitting under the change of ultrasound direction with respect to the external magnetic field H. The obtained results as well will be useful under interpretation of the ultrasound experiments in axial and planar phases of superfluid ³He.     

Direct Measurement of the Critical Velocity Above Which a Tuning Fork Generates Turbulence in Superfluid Helium

The dynamics of an electrically-driven 8 kHz quartz tuning fork has been studied experimentally in liquid helium-4 in the temperature range 1.3<T<4.2 K under the saturated vapour pressure. The fork has relatively large dimensions compared to standard 32 kHz fork used in recent investigations. The velocity of the tip of the fork prong is measured by the indirect electromechanical equivalent method and is compared with the velocity of another 8 kHz fork (from the same batch) determined by direct optical measurement of the oscillation amplitude through Michelson interferometry. A comparison of these results has provided absolute values for the critical velocity of the transition to the turbulent state.     

The predominant role of collagen in the nucleation, growth, structure and orientation of bone apatite

The involvement of collagen in bone biomineralization is commonly admitted, yet its role remains unclear. Here we show that type I collagen in?vitro can initiate and orientate the growth of carbonated apatite mineral in the absence of any other vertebrate extracellular matrix molecules of calcifying tissues. We also show that the collagen matrix influences the structural characteristics on the atomic scale, and controls the size and the three-dimensional distribution of apatite at larger length scales. These results call into question recent consensus in the literature on the need for Ca-rich non-collagenous proteins for collagen mineralization to occur in vivo. Our model is based on a collagen/apatite self-assembly process that combines the ability to mimic the in vivo extracellular fluid with three major features inherent to living bone tissue, that is, high fibrillar density, monodispersed fibrils and long-range hierarchical organization.     

Raman resonance in the strained Ge quantum dot array

We study the Raman resonance of a Ge quantum dot (QD) array grown pseudomorphically to a Si matrix using low-temperature molecular-beam epitaxy. A change of the resonance energy and the shape of the resonance curve in comparison with bulk Ge are observed. These features are shown to be explained by taking into account QD strain and the quasistationary character of the electronic states responsible for the observed resonance. Application of a model of the two-dimensional critical point of the interband density of states allows us to estimate the damping parameter and localization size of these states. It is shown that the observed enhancement of the resonance amplitude in a QD array as compared to the bulk case is related to transformation of the interband density of states into the δ-function due to quantization of the electron-hole spectrum.     

Non-linear Localized Lattice Mode Coupling Mechanism and the Pseudogap in High-temperature Superconducting Cuprates

The pseudo-gap phenomena in high-temperature cuprate superconductors is an outstanding puzzle with no consensus yet on its physical origin. A previous suggestion on the role of non-linear local lattice instability modes on the microscopic pairing mechanism in high temperature cuprate superconductors is re-examined to investigate whether unusual lattice mechanisms could cause a pseudo-gap. By assuming an electron predominantly interacting with a non-linear Q ₂ mode of the oxygen clusters in the CuO₂ planes, we show that the interaction has explicit d-wave symmetry and leads to an indirect coupling of d-wave symmetry between electrons. We show that the polaron formation by the non-linear mode can cause a pseudo-gap of d-wave symmetry before the onset of coherence in the superconducting pairing. We suggest a simple phenomenological explanation of the pseudo-gap crossover temperature and the Fermi arcs. The discussion may be relevant for the pseudo-gap in non-superconducting giant magnetoresistive manganites.     

Effect of Boron Substitution for Bi on Superconductivity of the Bi-2212 Phase in the Bi-Sr-Ca-Cu-O System

Possibility of boron substitution for Bi and the substitution effect on superconductivity is investigated for the Bi-2212 phase of Bi-Sr-Ca-Cu-O. From X-ray diffraction study, it is found that samples in the (Bi_2?x B _x )Sr₂CaCu₂O _z system are mainly of the single 2212 phase in a composition range of 0.0≤x≤0.6, and both of the lattice parameters a and c change with increasing x up to 0.6. From measurements of the magnetic susceptibility and the electrical resistivity, the superconducting transition temperature is found to increase up to 0.6 with increasing x. These results are considered to show that boron is substitutable for Bi up to x=0.6 in the (Bi_2?x B _x )Sr₂CaCu₂O _z system and that the boron substitution causes the number of hole-carriers to decrease in this system.     

Observation of the Semiconductor-Semimetal and Semimetal-Semiconductor Transitions in Bi Quantum Wires Induced by Anisotropic Deformation and Magnetic Field

The semimetal-semiconductor transition is observed in glass-coated quantum single-crystal bismuth wires with diameters less than 70 nm due to the quantum size effect. It is found that elastic deformation of Bi nanowires (10 $\bar{1}$ 1) oriented along the wire axis with the semiconductor dependence R(T) leads to the approaching of L and T bands and to the semiconductor-semimetal transition; as a result, Shubnikov-de Haas oscillations appear on the magnetoresistance dependences R(H). It is shown that strong magnetic field and elastic deformation are the tools to control gap size in quantum bismuth wires, which is principal for their practical use in particular in thermoelectricity.     

Transverse Magnetic Field Distribution in the Vortex State of Noncentrosymmetric Superconductor with O Symmetry

We investigate the magnetic field distribution inside a Type II superconductor which has point group symmetry O such as Li₂Pt₃B. The absence of inversion symmetry as a departure from perfect cubic group O _h causes a magnetization collinear with the phase gradient associated with the order parameter, and a component of current collinear with the local magnetic field. In the vortex state, we predict, by solving the Maxwell equation, a local magnetic field transverse to the vortex lines. The probability distribution of this transverse field is also obtained for the prospective muon-spin-rotation measurements.     

A Deviation Function Used for the Secondary Realization of the ITS-90 in the Subrange from 83.8033 K to 273.16 K

This article proposes a correlation relation between the resistance ratios of the triple points of argon and mercury. By this relation, the resistance ratio of the triple point of argon can be extrapolated from that of mercury, and a deviation function which is defined in the range from 83.8058 K to 273.16 K can be determined from only the calibration values at the triple points of water and mercury. It is a close approximation to the ITS-90 deviation function in the subrange. Using it, the calibration at the triple point of argon can be saved. Twenty-five standard platinum resistance thermometers are used to check the function. The errors are less than 5 mK. It is sufficient for secondary measurements.     

Effect of Interfacial Roughness Configuration on the Exchange-Bias Field in NiO Based Spin Valves

Two series of NiO-Co-Cu-Co based spin valves with the same nominal structure were grown by the magneton sputtering technique with different deposition parameters. Texture, surface and interfacial microstructures and the magnetic properties have been investigated in detail. X-ray diffraction, grazing incident X-ray reflectivity and transverse X-ray diffuse scattering were combined to analyze the microstructure of the multilayers. The results show that the texture of the multilayers is not a prerequisite factor for an exchange-bias (EB) field in NiO/Co/Cu/Co spin valves. And the EB effect is also independent of the average roughness at the interfaces. Large correlation lengths and two dimensions of the roughness at the interface are more advantageous for the EB effect.     

The effect of residual stresses in the ZnO buffer layer on the density of a ZnO nanowire array

Density control is a valuable concern in the research of ZnO nanowire arrays. In this study, unannealed and annealed ZnO thin films were used as substrates to fabricate ZnO nanowire arrays. In the unannealed thin film, an inhomogeneous distribution of the nanowire array was found: the density of nanowires decreases with the increase of distance to the edge. In the annealed thin film, the density of nanowire array becomes larger and more homogeneous. Moreover, nanowires are found in high density along microcracks. It is proposed that the residual stresses in the thin film and the density of the nanowire array are in inverse proportion, leading to the results mentioned above. The relationship between residual stresses and the density of nanowires will have potential applications in modifying the density of ZnO nanowire arrays.     

Design of the Cryogen-Free Cryogenic System for the CUORE Experiment

We present here the final design of the cryogenic system where the CUORE detector will be installed in 2010. It is a large cryogen-free cryostat cooled by pulse tubes and by a high-power dilution refrigerator. To avoid radioactive background, about 15000 kg of lead will be cooled to below 1 K and only few construction materials are acceptable. The detector assembly will have a total mass of about 1500 kg and must be cooled to less than 10 mK in a vibration-free environment. We discuss the adopted technical solutions, the results of the preliminary thermal analysis of the system, and its expected performance.      

Soliton-Like Spin State in the A-Like Phase of 3He in Anisotropic Aerogel

We have found a new stable spin state in the A-like phase of superfluid ³He confined to intrinsically anisotropic aerogel. The state can be formed by radiofrequency excitation applied while cooling through the superfluid transition temperature and its NMR properties are different from the standard A-like phase obtained in the limit of very small excitation. It is possible that this new state is formed by textural domain walls pinned by aerogel.      

Characterisation of a TES-Based X-ray Microcalorimeter in the Energy Range from 150 to 1800 eV Using an Adiabatic Demagnetisation Refrigerator

We characterised a TES-based X-ray microcalorimeter in an adiabatic demagnetisation refrigerator (ADR) using synchrotron radiation. The detector response and energy resolution was measured at the beam-line in the PTB radiometry laboratory at the electron storage ring BESSY II in the range from 200 to 1800 eV. We present and discuss the results of the energy resolution measurements as a function of energy, beam intensity and detector working point. The measured energy resolution ranges between 1.5 to 2.1 eV in the investigated energy range and is weakly dependent on the detector set point. A first analysis shows a count-rate capability, without considerable loss of performance, of about 500 counts per second.