Seiichiro Ikehata

Specific heat study of heavily P doped Si

Electronic specific heat of phosphorous doped silicon is measured at tempraatures 0.5 <T<4.2 K. Phosphorus concentration ranges from 3.4 x 1017 to 9.6 x 1019 cm−3, covering the intermediate and metallic regions of impurity conduction. The result for metallic samples is interpreted in terms of correlated electron gas. The electronic specific heat coefficient γ or the density of states at the Fermi level is shown to vary continuously over the metal-non metal transition region.

Magnetic field dependence of the specific heat of heavily phosphorus doped silicon

Abstract 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.

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.

NMR Study on Electronic States in Phosphorus Doped Silicon

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

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.

NMR Study on Heavily Doped Silicon. II

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.

Electric Conductivity and Proton Motion below Superionic Transition on Rb3H(SeO4)2

We have studied the proton motion below the ferroelastic-paraelastic phase transition (superionic transition) temperature T c =449 K from the NMR absorption lines. From the analysis of the NMR absorption lines, it was found that the second moment changes drastically in the temperature region from 340 K to 420 K. Moreover, we obtained the result that even below T c the correlation time of proton τ c obeys the Arrhenius relation τ c = τ 0 exp ( Δ E / k T ) with τ 0 =4.3 ×10 -11 s and Δ E = 0.47 eV. This activation energy is close to that of proton above T c . Furthermore, the electric conductivity calculated with τ c , which is obtained from the second moment of the NMR absorption lines, was compared with the experimental one. The result indicated that proton motion plays an important role in the electric conductivity even below T c .

Absolute spin susceptibility of K-graphite intercalation compounds and C24SbCl5: Orbital paramagnetism

Abstract We have measured the Pauli susceptibility of the stages 1, 2 and 3 K GICs and of the stage 2 SbCl5-GIC. Pauli susceptibility per mole of intercalant χp is almost independent of the stage number in K-GICs. The orbital susceptibility χorb estimated from the experimental χp is large and paramagnetic, and it increases rapidly with the increase of the stage number. Pauli susceptibility of C24SbCl5 is less than one half of that of C24K suggesting that the excess charge per intercalant is smaller in acceptor compounds than in donor compounds.

Electrical properties and memory effect in the field effect transistor based on organic ferroelectric insulator and pentacene

We have fabricated a field effect transistor (FET) based on an organic ferroelectric insulator and molecular conductor, and investigated the electrical properties and memory effects on the PEN-FET. We have observed a drastic change in the drain current at around the coercive electric fieldEc of the organic ferroelectric insulator in not only a FET (PEN-FET) based on a pentacene (PEN) film but also a FET (IPEN-FET) based on an iodine doped PEN film. The magnitude of the change of the drain current for the IPEN-FET is 200 times larger than that for the PEN-FET. It is expected from these results that the PEN-FET (especially the IPEN-FET) is an improvement in such devices, since it operates at a low gate electric field accompanied by the appearance of the spontaneous polarization in the organic ferroelectric insulator. In addition, we have found that the drain current for the PEN-FET does not return to the initial drain current ofEG=0 V/cm for more than one week, even if the gate electric field is changed to 0 V/cm from 500 V/cm(>Ec). From these results, it is suggested that the PEN-FET becomes a memory device.

Dependence of Coulomb Blockade in Ultrasmall Single Tunnel Junctions on Tunnel Resistance

We studied small-capacitance single tunnel junctions with various tunnel resistances. In order to reduce the environmental effect and to observe the Coulomb blockade clearly, we have adopted high-impedance leads which consist of multiple junctions to isolate the single junction. We observed the gradual weakening and disappearance of the Coulomb blockade as the tunnel resistance in the absence of charging effects is lowered below the resistance quantum. As the tunnel resistance increases above the resistance quantum, the zero-bias resistance divided by the tunnel resistance tends to flatten off. The observed dependence of the zero-bias resistance on the tunnel resistance is in good agreement with the theory of Brown and Simanek.