Monte Carlo Investigation of the Magnetization Distribution in Fe/Cu Multilayer

By applying Monte Carlo simulations in the canonical ensemble, investigation of the magnetic properties of Fe/Cu multilayers is carried out in comparison with Mössbauer experimental results. From differing relaxation rates for spins with few near-neighbor spins and particularly those located at the interface, the Mössbauer result can be simulated. Our model consists of an alternate stacking of iron and copper layers (Fe _nFe/Cu _nCu) with disordered interface. The simulation results confirm that the concentration of interface alloys (Fe_1?x Cu _x ), measurement temperature and magnetic layer thickness modify systemically the magnetization distribution in the Fe/Cu multilayers. The result is compared with Mössbauer experimental analysis. Meaningful results are obtained by analysing the dynamical microscopic behaviour of individual spins.     

Quantum Transport and Cyclotron Resonance Study of Ge/SiGe Quantum Wells in High Magnetic Fields

Shubnikov-de Haas oscillation and cyclotron resonance were studied for high mobility p-type Ge channels in strained Ge/Si_1?x Ge _x quantum wells, using pulsed high magnetic fields up to 50 T. Fine quantum oscillations were observed in ρ _xx . Reflecting the complex Landau level structure in the nearly degenerate valence bands, the Fourier transform of the oscillatory spectra consists of several peaks. Cyclotron resonance was measured at photon energies between 10 and 17 meV. Two well-defined resonance peaks were observed in two samples with different x, resulting in different strains. A large non-parabolicity and large strain dependence of the effective masses were observed.     

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.     

An Idea for Development of Intelligent Materials and Devices

本文基于铁电体的自发极化P     

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.     

Development of the High-Field Imaging System in Pulsed-High Magnetic Fields and Its Application to Manganites

We developed a high-speed polarizing microscope imaging system combined with a 37 T pulse magnet. With utilizing this system, we can visualize some kinds of structural transitions induced by high magnetic fields. The efficiency of this system was tested through observation of the field-induced collapse of charge-orbital ordering (COO) in mixed-valent manganites. In perovskite-type Nd_1/2Sr_1/2MnO₃, quantitative analyses of the obtained polarizing microscope images clearly show the discontinuous change in birefringence accompanied with hysteresis, which are characteristic of the first order transitions. This observation of field-induced melting of the COO was also successful in a layered manganite La_1/2Sr_3/2MnO₄ in which changes in magnetization and magnetoresistance at the transition are less clear. The present success in observation of the melting of the COO demonstrates the potential applicability of this system to visualize other kind of structural transitions even in a tiny piece of crystal.     

THz-Photoconductivity of Quantum Hall Systems in Quasi-Corbino-Geometry

The THz-photoresponse (PR) between two separately contacted edge-channels of a two-dimensional electron gas in the quantum Hall regime is investigated. We use a not-simply-connected sample geometry, which is topologically equivalent to a ring shape (Corbino-geometry). At filling factors ν<2, spectrally resolved PR-measurements show a Lorentzian resonance, centered at the cyclotron-frequency, whereas above the integer filling factor, an asymmetric broadening is observed. Two independent contributions to the PR-signal can be resolved. One contribution clearly results from bolometric heating inside the bulk and the other one is caused by a non-bolometric mechanism.     

Materials Development for Auxiliary Components for Large Compact Mo/Au TES Arrays

Pursuing the feasibility of scaling conventionally-micromachined transition-edge-sensor (TES) arrays, we have undertaken a study of materials suitable for array integration. A potential limitation of increased pixel count is adequate heatsinking of each detector element to its base temperature. We describe technical approaches for heat sinking large compact TES microcalorimeter arrays and calculate the achievable heatsinking based on measured material parameters. Techniques include backside-deposited thick film copper on arrays with deep-etched wells in the substrates and electroplated gold and copper-filled micro-trenches on the substrate surface. Another limitation is the sensitivity of the thin film circuit elements to applied stress, which can arise in fabrication and mounting of arrays of increasing size. We have explored stress and deposition temperature sensitivity in our molybdenum-based bilayers. Such process parameters can impact options for array heat sinking and electrical interconnects.      

Investigation of superconductivity in electrochemically fabricated AuSn nanowires

We have fabricated intermetallic AuSn nanowires by electrochemical deposition in porous polycarbonate membranes. By controlling the deposition parameters, nanowires of a single intermetallic phase, namely AuSn, can be fabricated. AuSn nanowires are found to be crystalline and fairly resistant to oxidation. Electrical transport measurements on arrays of nanowires showed a superconducting transition temperature, T(c)～1.5?K. In addition, four-probe measurements were made on individual freestanding nanowires with electrodes formed by a focused ion beam (FIB). Results from the two sets of measurements are found to be in close agreement.     

Quantum Dynamics of Atom-Molecule BECs in a Double-Well Potential

We investigate the dynamics of two-component Bose-Josephson junction composed of atom-molecule BECs. Within the semiclassical approximation, the multi-degree of freedom of this system permits chaotic dynamics, which does not occur in single-component Bose-Josephson junctions. By investigating the level statistics of the energy spectra using the exact diagonalization method, we evaluate whether the dynamics of the system is periodic or non-periodic within the semiclassical approximation. Additionally, we compare the semiclassical and full-quantum dynamics.     

Properties of Superconducting Rhenium as an Absorber for Magnetic Calorimeters

A still puzzling problem in the development of low temperature micro-calorimeters for the measurement of the ¹⁸⁷rhenium β-spectrum is the understanding of the thermalization of energetic electrons in the superconducting rhenium absorber. We studied metallic magnetic calorimeters (MMC) with single crystal rhenium absorbers and paramagnetic Au:Er temperature sensors. The energy released into the detector leads to a change of magnetization of the paramagnetic sensor located in a weak magnetic field. A SQUID with meander shaped inductance is used to read out this change. This setup allows the study of several properties of the superconducting absorber. The transition to the superconducting state is studied by measuring the magnetic flux expelled by the rhenium sample. The resistivity of rhenium above T _c can be estimated from the measurement of the spectral power density of the Johnson noise. Furthermore the quasiparticle lifetime can be investigated through the analysis of the shape of detector signals caused by intrinsic β-decays and the absorption of X-rays. We present the data obtained in these experiments and discuss the physical quantities which can be derived from these.      

Effect of Charge State in Nearby Quantum Dots on Quantum Hall Systems

Transport properties have been measured on two-dimensional electron system (2DES) with a nearby InAs self-assembled quantum dot layer, in order to clarify the anomalous behavior of magnetoresistance, ρ _xx , and Hall resistance, ρ _xy at $\nu=\frac{1}{3}$ , which was an unsolved problem in our previous paper (Takehana et al.: J. Phys. Soc. Jpn. 75, 114713, 2006). The significant suppression of both ρ _xx and ρ _xy was found to be reduced in the vicinity of $\nu=\frac{1}{3}$ , which indicates that the fractional quantum Hall effect prevent the spin-flip process between localized electrons and that extended 2DES, according to the discussion of the previous paper.     

Development of Superconducting Coaxial Cables for Cryogenic Detectors

Fast readout with low noise is essential to apply superconducting detectors to such experiments as time-of-flight mass spectrometry or X-ray spectroscopy, where high counting efficiency is required. We have developed a thin seamless superconducting coaxial cable employing NbTi alloy in both of the outer and inner electrical conductors. The outer diameter is 1.60 mm, and both conductors are separated by dielectric material of PTFE. The coaxial cable revealed to become superconductive below about 10 K. The thermal conductance was measured between 0.4 and 6 K and consistent with literature. Performance at high frequency was also measured at 4.2 K. The attenuation was very small and less than 1 dB up to about 7 GHz. The effect of mechanical treatment to thermal and electrical properties of NbTi alloy seems to be small in the present forming process of coaxial cable.      

Quantum Tunneling of D(H) Atoms and 2 – in Solid Hydrogen

This paper contains three results. First, the rate constants for the tunneling reaction HD + D H + D ₂ in solid HD increase steeply with increasing temperature above 5 K, while they are almost independent of temperature below 5 K. A mechanism of a vacancy–assisted tunneling reaction is proposed to account for this temperature dependence. Second, a hydrogen atom and a hydrogen molecule form a van der Waals complex in the Ar matrix at 20 K, where the tunneling reaction HD + D H + D ₂ takes place in this complex. The analysis of well–resolved ESR spectra of the complex determined the distance between a hydrogen atom and a hydrogen molecule as 2.3 – 2.5 Å. Third, the decay rate constants of anions in solid parahydrogen decrease with decreasing temperature below 6.6 K, attain the minimum value at 5 K, and then increase with decreasing temperature in the range of 5 2.7 K. The abnormal temperature dependence of the decay rate constants below 5 K is ascribed to a phonon–scattering process of quantum diffusion.     

Application of an Electro-Magnetic Induction Technique for the Magnetization up to 100 T in a Vertical Single-turn Coil System

The system was developed for the magnetization measurement in the vertical single-turn coil (V-STC) system at ISSP, which can generate magnetic fields over 100 T in a semi-destructive manner. We have adjusted the electro-magnetic induction method to our V-STC. The new system was applied to the manganite with the perovskite-type structure Bi_1/2Ca_1/2MnO₃. The total magnetization process was obtained up to 105 T in excellent quality comparable to those obtained by the non-destructive long pulse magnet.     

Possibility of Inverse Energy Cascade in Two-Dimensional Quantum Turbulence

We numerically study two-dimensional quantum turbulence by using the Gross-Pitaevskii model and the Vortex-Point model. Two-dimensional classical turbulence has long been investigated as an ideal system of geophysical phenomena. The amazing character of this turbulence is inverse energy cascade which carries energy toward low wavenumbers and excites large-scale motion. We expect these phenomena in two-dimensional quantum turbulence because in three-dimensional turbulence we know classical and quantum analogue. However, we have not yet confirmed inverse cascade in two-dimensional quantum turbulence. In this work, we show numerical results and discuss why inverse cascade does not occur in two-dimensional quantum turbulence by referring to the mechanism of two-dimensional classical turbulence.     

Quantum States of Helium Atoms Confined in Nanocage in Na-Y Zeolite

We have studied the heat capacities of heliums adsorbed in Na-Y zeolite down to 70 mK. The Y-type zeolite has void cages 1.3 nm in diameter connected through narrow apertures 0.8 nm in diameter. In the nanopores at low temperatures, the second-layer He adatoms can be confined in each cage by the first solid layer. In the quantum fluid region above the second-layer promotion, where the heat capacities show qualitative differences between ⁴He and ³He, the heat capacities of ³He adatoms and the thermal relaxation processes show large and non-monotonic changes when one ³He atom is added in each cage. These results strongly suggest that the quantum fluid formed in Na-Y zeolite is not fluid film spread over the entire substrate but quantum clusters confined in each cage.     

Perspective in Development of Shape Memory Materials Associated with Martensitic Transformation

By consideration of the characteristics of martensitic transformation and the derivation from the application of the group theory to martensitic transformation, it may be concluded that the shape memory effect (SME) can be attained in materials through a martensitic transformation and its reverse transformation. only when there forms single or nearly single variant of martensite, with an absence of the factors causing the generation of the resistance against SME. on this principle, various shape memory materials including nonferrous alloys. iron-based alloys and ceramics containjng zirconia are expected to be further developed. A criterion for thermoelastic martensitic transformation is presented, Factors which may act as the resistance against SME in various materials are briefly described     

Development of Multi-extreme ESR Measurement System in Kobe

Development of multi-extreme electron spin resonance (ESR) measurement system is in progress in Kobe. We have installed the pulsed magnetic field up to 55 T and our high field ESR system can cover the frequency region from 30 GHz to 7 THz. Our aim is to extend our high field ESR system to the multi-extreme ESR measurement system by combining low temperature, high pressure and nano techniques, and the combination with the high pressure is focused in the paper. High pressure can be achieved up to 1.4 GPa by using the transmission type piston cylinder pressure cell. We have succeeded in combining the high field and the high pressure. As an example, the pressure dependence of the spin gap in SrCu₂(BO₃)₂, S=1/2 Shustally-Sutherland dimer system, observed by our multi-extreme ESR measurement is presented. This measurement is the direct and precise measurement of the spin gap, and the spin gap decreased from 723 GHz (34.7 K) to 581 GHz (27.9 K) at 1.2 GPa.     

Development of Ultra-Low Noise HEMTs for Cryoelectronics at ≤4.2 K

We report on DC and noise characteristics at 4.2 K of High Electron Mobility Transistors (HEMTs) which have been realized at LPN/CNRS. This work is aimed to develop high performance, low-power, low-frequency noise and low-temperature field-effect transistors for the future cryoelectronics. A high quality two-dimensional electron gas (2DEG) based on AlGaAs/GaAs heterostructure has been used to realize appropriately designed HEMTs with different gate configurations. With these transistors, we have obtained, for example, a transconductance of about 100 mS and a voltage gain of 26 with a power dissipation of less than 100 μW at 4.2 K; and a corresponded equivalent input voltage noise of 1.2 nV/Hz^1/2 at 1 kHz and as low as 0.12 nV/Hz^1/2 at 100 kHz.