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Superconductor-insulator (S-I) transitions tuned by normal tunneling resistance or magnetic field in two-dimensional granular films of tin are investigated, with emphasis on the effect of grain size. We found that for 280 A grains, the sheet resistance at the S-I boundary in a magnetic field-tunneling resistance plane is close to R Q (≡ h /4 e 2 ), while that for 1200 A grains is much higher.

The temperature and magnetic field dependence of sheet resistance was measured in aluminum granular films which consisted of 10 A grains and thin oxides on grain surfaces. For the samples with normal resistance less than 6 kohm, a superconducting transition was observed, while the others behaved as insulators. The transition between superconductor and insulator was induced also by a magnetic field, and the normal sheet resistance at the boundary was 6.7 kohm. The results are discussed in connection with the theories for the universal critical resistance R Q (=h/4e 2 ), and with the results of previous experiments for films of larger grains.

We studied the charge and vortex Kosterlitz-Thouless-Berezinskii transitions in random networks of small Josephson junctions. The networks were composed of small tin particles with controlled size and surface oxidation. Competition between the two transitions was clearly seen. The observed phase boundaries agree qualitatively with a theoretical phase diagram.

We measured the low-temperature resistance of two-dimensional random networks of very small tunneling junctions. The networks consist of slightly oxidized tin particles with linear size of about 0.1 µ m. The temperature dependence of the resistance was found to be stronger than that of the Arrhenius type within a certain temperature range. We interpret this behavior as a precursor of the charge Kosterlitz-Thouless-Berezinskii transition. The real transition is not attainable because the system is not ideally two-dimensional.

By measuring the Knight shifts of Cu NMR in small particles of Cu–Mn alloys with mean diameter of 60 A, we showed that the anomaly in the spin susceptibility due to the level quantization of conduction electrons can be suppressed by the spin scattering. The condition for sufficient suppression was τ s ≪ h /δ where τ s is the spin scattering time and δ the mean level spacing.

We studied the charge and vortex K-T transitions in two-dimensional random network of small Josephson junctions. The networks were composed of small tin particles with controlled size (mean diameter: 890A) and surface oxidation. Competition between the two transitions was clearly observed.

We measured the internal field at oxygen sites in a single crystal of the magnetic compound YBa2Cu3O6.25 with the time differential perturbed angular distribution method using 19F isomer. The asymmetry of the rotation pattern in 1.37 T decreases to about half at low temperature from the value at 300 K. At the same time, the rotation amplitude decays considerably while the frequency is uncharged. The present results suggest that the magnetic interaction between Cu ions in the plane layers is the exchange via oxygen while the oxygens in the barium and chain layers feel the weak dipole field.

We studied the gate voltage dependence of the resistance of a two-dimensional array of Al/AlOx/Al small tunnel junctions. The resistance exhibited periodic oscillations similar to those in a single electron transistor in both superconducting and normal states. The amplitude of the oscillation in the superconducting state was much smaller than that in the normal state.

The Knight shift of 27 Al in small Al particles is measured in wide ranges of temperature and magnetic field. The samples with diameters smaller than 240 A are investigated. We observe the change in the hamiltonian symmetry with increasing field as a recovery of spin paramagnetic susceptibility. We also observe the critical magnetic field of superconductivity due to orbital pair-breaking effect.

An anomalous nuclear spin depolarization is observed in field cycling experiments of aluminum small particles at 30 mK. The depolarization time is of the order of several minutes and depends on the size of the particles. The origin of the depolarization is assigned to the large quadrupole interaction of 27 Al nucleus in random electric field gradient of the small particles.

Current-voltage characteristics are studied in highly disordered Josephson networks consisting of small tin particles. For the specimens with a certain range of resistance, notable structures are observed such as negative resistance when the voltage is swept, and voltage steps when the current is swept. The structures are similar to what is expected for resistance quantization. The origin of the structure should be clarified in future.

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.

We measure the temperature dependence of the velocity of third soundC 3 in the vicinity of the critical temperatureT c . It is found thatC 3 2 is well fitted by the experimental formulaC 3(T c )2[1+1/2(−Ct)1/2], wheret=(T−T c )/T c andC is a constant, equal to about 5.4. This (−t)1/2 cusp is consistent with the theory of the Kosterlitz-Thouless (KT) transition in two dimensions. The chemical potential μ0 for the vortex pair excitation is estimated as μ0=3.03k B T c with the aid of the static theory of the KT transition. The consistency between the experimentally obtained value of μ0 and the KT transition theory is discussed.

Detailed study of NMR line shape and spin-lattice relaxation time has been carried out on phosphorus doped silicon with a purpose of making clear the nature of the electronic state in samples which show intermediate or metallic impurity conduction. On the basis of these observations previously published pictures for metallic transition were reexamined. A picture that the Anderson localized states with “intra” state correlation exist over a wide range of donor concentration, including the critical concentration of metallic transition, is shown to explain the observed properties in a consistent way.

Magnetization in phosphorus doped silicon was measured with magnetic fields up to 95 kG and temperatures down to 1.5 K. The donor concentration of measured samples ranged from 0.53 to 6.5 times 10 18 cm -3 . The most heavily doped sample shows diamagnetic susceptibility which is consistent with a simplified picture of degenerate semiconductor. Other samples, including two metallic ones, show a temperature dependence of magnetization which can be attributed to free spins, and the number densities of free spins estimated from the magnetization curve are shown consistent with those estimated from the specific heat anomaly. The origin of these free spins is discussed with taking into account the intrastate Coulomb energy of localized states.

Anomalously long nuclear spin-lattice relaxation time was observed at low temperatures below 0.5 K in small metallic particles of copper with diameters smaller than 100 A by a new method for setting the nuclear magnetization to zero. The observed anomaly is in accordance with what is expected for very small metals in which the spacing of energy levels of conduction electrons is much larger than the life time broadening of the levels. The new method was indispensable to observe the intrinsic relaxation time in copper particles where the electric quadrupole broadening of the nuclear magnetic resonance was huge.

Spin echo spectra, Knight shift and spin-lattice relaxation time are measured on 31 P in silicon crystals with phosphorus concentrations ranging from 4.5×10 18 cm -3 to 9.6×10 19 cm -3 over a temperature region from 4.2 K to 0.4 K. The shift and width of 31 P line is atrributed to the inhomogeneous electron contact field. The results together with those of 29 Si (J. Phys. Soc. Japan 36 (1974) 1377) support the Mott transition mechanism of metal-non metal transition and allow to interpret various properties of the metallic samples in terms of an electron gas with correlation.

The conductivity σ LT of disordered copper films without and with magnetic impurities was measured at low temperature in order to study the metal-insulator transition. In both films, a continuous transition was observed with the increase of disorder and σ LT was fitted to a scaling form with the same exponent of unity using high temperature conductivity as a parameter. These results are discussed along the recent scaling theory for interacting electrons.

The specific heat of phosphorus doped silicon was measured at temperatures 0.1 T K in external magnetic fields 0 ≦ H ext ≦ 38 kOe . The phosphorus concentration of the samples ranges from 5.3 × 10 17 to 8.9 × 10 18 cm −3 . The magnetic field dependence of the specific heat was observed in the just metallic samples as well as the non-metallic ones. The metal—non-metal transition is discussed on the basis of the Anderson localized states with correlations.

The Hall coefficient (HC) of the persistent photoconductor Al0.3Ga0.7As:Si has been measured near the metal-insulator transition (MIT) critical point. The temperature dependence of the HC was weak even in the vicinity of the MIT critical point and no divergence of the HC was observed at the transition tuned by both the effective electron concentration and the magnetic field.

The inelastic scattering time of electrons and the metal-insulator transition are studied in disordered thick bismuth films. In weakly disordered films, the anti-localization effect is observed, and dimensional crossover of the inelastic scattering between two and three is demonstrated by changing the thickness of the films. The critical exponent of the metal-insulator transition in this system which represents a disordered metal with strong spin-orbit interaction is examined and fixed to be unity. This indicates that the transition is governed by the electron-electron interaction.

Several targets, covering the spherical, deformed and transition regions of heavy nuclei, were bombarded with 55 MeV protons and angular distributions of the elastically scattered protons were measured. The diffraction structure of them are strongly damped for the deformed nuclei, as compared with that for the spherical nuclei and this feature changes gradually for nuclei of the transition region. The optical model analysis was also performed to discuss the details of the deformation effect.

We measured the magnetic field dependence of the Knight shift of 63 Cu NMR in two samples of small copper particles with average diameters of 60 A and 100 A in fields of up to 4.9 T. When the magnetic field is so high that the Zeeman energy is larger than the average spacing of the energy levels of conduction electrons, the correlation between the levels of opposite spins is lost, and the spin susceptibility anomalies inherent in small particles vanishes. The results of the present experiment are in agreement with this expectation.

Temperature and magnetic field dependence of conductivity in resistive thin films of Zn was measured. In zero field, conductivity decreased as ln T as temperature was lowered and showed an enhanced paraconductivity which diverged at T c . When field was applied, T c decreased, ln T dependence extended to lower temperature and the coefficent of ln T increased. The origin of these results was discussed from the view point of electron localization and interaction effects in two dimensional disordered metal.

Anomalously long nuclear spin-lattice relaxation time is expected in small metallic particles when the life time width of the conduction electron energy levels is smaller than their mean spacing. A new method for setting nuclear magnetization to zero is developed in order to measure the relaxation time in Cu small particles. The anomaly is observed in the relaxation of 63Cu in the particles with the diameters smaller than 100 A at temperatures below 0.5 K.

Temperature and magnetic field dependence of resistivity of two-dimensional Josephson coupled random array of tin particles is measured. Sample consists of Sn particles of 300A in diameter which are randomly distributed on substrate and coupled to each other through thin oxide layers. Variety of temperature and magnetic field dependence is observed, depending on the degree of oxidation.

Anomalously fast nuclear spin depolarization in low magnetic field is observed in tin small particles immersed in liquid 3 He– 4 He mixture at 30 mK. This is ascribed to the cross relaxation due to the dipole interaction between tin and 3 He nuclear spins.

The electrical transport properties of highly resistive films of copper were studied. Logarithmic variations of the conductivity with temperature and magnetic field were observed. Magnetoconductivity was strongly anisotropic. The results are discussed with the recent theories of electron localization in a two dimensional metal.

We have measured the absolute spin susceptibility of Si:P around the critical concentration for the metal-non metal transition. The spin susceptibility reveals an anomalous increase at very low temperatures on both metallic and non metallic regimes.

Temperature dependence of resistivity of two-dimensional Josephson coupled random array of tin fine particles is measured. Sample consists of Sn particles of 300 A in diameter which are randomly distributed on substrate and coupled to each other through oxide layers. Depending on the degree of oxidation, variety of temperature dependence is observed. Results are discussed in terms of superconductive transition of individual particles, phase coherence among particles, the Kosterlitz-Thouless transition and phase-number relation.

Anomalously fast nuclear spin depolarization in low magnetic field is observed in tin small particles immersed in liquid 3He–4He mixture at 30mk. In order to specify its origin, the measurement in pure 4He is performed. The survey is extended to fluorocarbon particles.

Experimental observation of the “Bloch oscillations” which are predicted theoretically in small Josephson junctions, resulting from their macroscopic quantum nature, has been made in superconducting granular tin films, the sheet resistance of which ranges from 3 to 12 kΩ at 4.2 K. We show that the values of 2ef obtained from the experimental V - I characteristics and a relationship I = 2ef are in agreement to within 0.3 % with those calculated using the frequency f of the microwave fields applied. It is one of the important conditions to observe the “Bloch oscillations” in the junction network that the number of parallel paths connecting two current electrodes is a few hundreds or less.