Youiti Ootuka

Anomalous magnetoresistance in heavily antimony doped germanium

Abstract Anomalous magnetoresistance in heavily antimony doped germanium is studied down to 5 mK. The experimental results is reasonably interpreted within a framework of s-d interaction with a negative exchange constant. Electrons localized in a sense of Anderson are suggested to play a role of d electrons when Coulomb correlation is introduced.

Magneto-Coulomb oscillation in ferromagnetic single electron transistors

The mechanism of the magneto-Coulomb oscillation in ferromagnetic single electron transistors (SETs) is theoretically considered. Variations in the chemical potentials of the conduction electrons in the ferromagnetic island electrode and the ferromagnetic lead electrodes in magnetic fields cause changes in the free energy of the island electrode of the SET. This is a plausible origin of the conductance oscillation of the SET in sweeping an applied magnetic field.

Transmission Spectra of Third Sound in a Fibonacci Lattice

Transmission spectra were experimentally studied in third sound that was propagated through a Fibonacci lattice. Thin aluminum films were used to configure the Fibonacci lattice on a glass substrat...

Electrical properties of isotopically enriched neutron-transmutation-doped 70 Ge:Ga near the metal-insulator transition

metal-insulator transition. The nine samples closest to Nc have Ga concentrations N in the range 0.99Nc < N < 1.01Nc. The electrical conductivity σ has been measured in the temperature range T = 0.02 − 1 K. On the metallic side of the transition the standard σ(T) = a + bT q with q = 1/2 was observed for all the samples except for the two that are closest to Nc with N between Nc and 1.0015Nc. These samples clearly show q = 1/3. An extrapolation technique has been developed in order to obtain the zero-temperature conductivity σ(0) from σ(T) with different dependence on T. Based on the analysis, ν ≈ 0.5 in the familiar form of σ(0) ∝ (N/Nc − 1) � has been found. On the insulating side of the transition, variable range hopping resistivity ρ(T) ∝ exp(T0/T) p with p = 1/2 has been observed for all the samples having N < 0.991Nc. In this regime T0 ∝ (1 − N/Nc) � with α ≈ 1 as N → Nc. The values of T0 agree very well with theoretical estimates based on the modified Efros and Shklovskii relation kBT0 ≈ (2.8e 2 /4πǫ0κ0ξ0)(1 − N/Nc) � , where κ0 and ξ0 are the dielectric constant and the Bohr radius, respectively. The insulating samples very close to the transition (0.991Nc < N < Nc) exhibit quite a different behavior. In this range 1/p increases rapidly as N changes from 0.991Nc to Nc. The relevance of our findings to the collapsing of the Coulomb gap is discussed.

Driving the single-electron device with a magnetic field (invited)

The single-electron devices some of whose electrodes are composed of ferromagnetic metals can be driven with a magnetic field. It is based on the fact that a ferromagnetic electrode has a function as a magnetochemical or magnetoelectric coupling component in the device through the Zeeman effect on the electron spins in it. Double- and triple-small-junction devices composed of Ni and Co electrodes showed conductance oscillations typical to the single-electron device when the applied magnetic field was swept, illustrating the magnetic-field control of the device operation. A single-electron box and a single-electron pump driven with a magnetic field are described based on the functions of the ferromagnetic electrodes.

SENSITIVE GERMANIUM THERMISTORS FOR CRYOGENIC THERMAL DETECTOR OF TOKYO DARK MATTER SEARCH PROGRAMME

Abstract Sensitive n-type and p-type germanium thermistors were fabricated by the melt doping technique and by the neutron transmutation doping (NTD) technique, respectively, aiming at the use for cryogenic thermal detector, or bolometer of Tokyo dark matter search programme. We report on the measurements of the sensitivities of these thermistors. In particular, the p-type thermistors are sensitive enough to scale up our existing prototype LiF bolometer and realize a multiple array of the bolometers with the total absorber mass of about 1 kg.

Cryogenic thermal detector with LiF absorber for direct dark matter search experiment

Abstract A cryogenic thermal detector with a 2.8 g LiF crystal is constructed. A photoelectric peak of 60 keV γ-rays is clearly seen with LiF crystal with an RMS energy resolution of 3.8 keV. LiF is suitable for the detection of possible particle dark matter with a spin-dependent interaction like neutralinos since both Li and F are expected to have high interaction rates with such particles.

Variable range hopping in Si:P at very low temperature

Abstract The resistivity of doped semiconductor Si: P in the intermidiate concentration range was measured in low temperature region down to 10 mK. In the lowest temperatures Motts formula for the variable range hopping is shown to be not applicable and T -n -like temperature dependence of resistivity is confirmed. The Importance of the level correlation in the random system is discussed.

Comment on “variable range hopping in Si:P at very low temperature” surface conduction in Si:P

Abstract The electrical resistivity of doped semiconductor Si:P in the intermediate concentration range was measured under various surface conditions. The surface treatments are shown to have a great influence upon the low temperature electrical conduction, and T -2 -like temperature dependence of resistivity is attributed to the surface layer electrical conduction.

Development of the bolometer for the β+β+ decay experiment

Abstract We developed a bolometer with a 0.5 g CdTe absorber using a high-sensitivity NTD germanium thermistor. An energy resolution of 24 keV (FWHM) is obtained for 0.66 to 1.8 MeV γ-rays. It shows a good linearity of the response in the energy range between 0.66 and 5.5 MeV. The gain is found to be stable for a period of 72 days of continuous operation and relatively insensitive to temperature changes of the refrigerator. The temperature change of the refrigerator for this period is within ±1 mK. Since this material contains double positron emission ( β + β + ) nuclides 106 Cd, 108 Cd and 120 Te, a new series of double beta decay experiments may become possible with this new method.