Construction of a Versatile Ultra-Low Temperature Scanning Tunneling Microscope

We constructed a dilution-refrigerator (DR) based ultra-low temperature scanning tunneling microscope (ULT-STM) which works at temperatures down to 30 mK, in magnetic fields up to 6 T and in ultrahigh vacuum (UHV). Besides these extreme operation conditions, this STM has several unique features not available in other DR based ULT-STMs. One can load STM tips as well as samples with clean surfaces prepared in a UHV environment to an STM head keeping low temperature and UHV conditions. After then, the system can be cooled back to near the base temperature within 3 hours. Due to these capabilities, it has a variety of applications not only for cleavable materials but also for almost all conducting materials. The present ULT-STM has also an exceptionally high stability in the presence of magnetic field and even during field sweep. We describe details of its design, performance and applications for low temperature physics.Comment: 6 pages, 9 figures. accepted for publication in Rev. Sci. Instru

Strongly Correlated Electrons and Neutron Scattering

Various aspects of close and interesting relationship between antiferromagnetism and singlet ground states are introduced for which neutron scattering have been playing vital roles. The special emphasis is on the disorder-induced antiferromagnetism in spin-Peierls systems, which can be viewed as a nucleation process of classical magnetic order in the background of singlet state whose origin is purely quantum-mechanical. It is then pointed out that similar features will be found in Ce_xCu_2Si_2 and high T_c cuprates. Finally the possible charge ordering process in NaV_2O_5 is discussed which leads to the quenching of localized spins.Comment: 8 pages, 3 figures, Proceedings of 7th ISSP International Symposium "Frontiers in Neutron Scattering Research" (To appear in J. Phys. Chem. Solids (1999)

Can The Majorana neutrino CP-violating phases be restricted ?

We reanalyze the constraints in neutrino masses and MNS lepton mixing parameters using the new data from the terrestrial (KamLAND) and astrophysical (WMAP) observations together with the HEIDELBERG-MOSCOW double beta decay experiment. It leads us to the almost degenerate or inverse hierarchy neutrino mass scenario. We discuss the possibility of getting the bound for the Majorana CP violating phase.Comment: 6 pages, 2 figure

Study on successive superconducting transitions in Ta2_{2}S2_{2}C from electrical resistivity and nonlinear AC magnetic susceptibility

Ta$_{2}$S$_{2}$C compound undergoes superconducting transitions at $T_{cl} = 3.60 \pm 0.02$ K and $T_{cu} = 9.0 \pm 0.2$ K. The nature of successive superconducting transitions has been studied from electrical resistivity, linear and nonlinear AC magnetic susceptibilities. The resistivity $\rho$ at $H$ = 0 shows a local maximum near $T_{cu}$, a kink-like behavior around $T_{cl}$, and reduces to zero at below $T_{0}$ = 2.1 K. The $\ln T$ dependence of $\rho$ is observed at $H$ = 50 kOe at low temperatures, which is due to two-dimensional weak-localization effect. Below $T_{cu}$ a two-dimensional superconducting phase occurs in each TaC layer. The linear and nonlinear susceptibilities $\chi_{1}^{\prime\prime}$, $\chi_{3}^{\prime}$, $\chi_{5}^{\prime}$, and $\chi_{7}^{\prime}$ as well as the difference $\delta\chi$ ($= \chi_{FC} - \chi_{ZFC}$) between the FC and ZFC susceptibilities, start to appear below 6.0 K, the onset temperature of irreversibility. A drastic growth of the in-plane superconducting coherence length below 6.0 K gives rise to a three-dimensional superconducting phase below $T_{cl}$, through interplanar Josephson couplings between adjacent TaC layers. The oscillatory behavior of $\chi_{3}^{\prime\prime}$, $\chi_{5}^{\prime\prime}$, and $\chi_{7}^{\prime\prime}$ below $T_{cl}$ is related to the nonlinear behavior arising from the thermally activated flux flow.Comment: 11 pages, 10 figures, Physical Review B (accepted for publication

Determination of the mosaic angle distribution of Grafoil platelets using continuous-wave NMR spectra

We described details of a method to estimate with good accuracy the mosaic angle distributions of microcrystallites (platelets) in exfoliated graphite like Grafoil which is commonly used as an adsorption substrate for helium thin films. The method is based on analysis of resonance field shifts in continuous-wave (CW) NMR spectra of $^{3}$He ferromagnetic monolayers making use of the large nuclear polarization of the adsorbate itself. The mosaic angle distribution of a Grafoil substrate analyzed in this way can be well fitted to a gaussian form with a $27.5\pm2.5$ deg spread. This distribution is quite different from the previous estimation based on neutron scattering data which showed an unrealistically large isotropic powder-like component.Comment: 6 pages, 5 figure