Yuichi Akahama

Electrical Properties of Black Phosphorus Single Crystals

Large single crystals of black phosphorus have been grown under high pressure, and by using the crystals, the Hall measurements have been done in a range from 4.2 K to 550 K. All the undoped samples have exhibited p -type conduction, while we have succeeded in obtaining n -type crystals by doping Te impurity. The effective acceptor concentrations N A -N D of the p -type samples and those of donor in n -type samples N D -N A have been in the range of 2∼5×10 15 cm -3 and 2∼3×10 16 cm -3 , respectively. From the intrinsic range of the conductivity, the energy gap has been estimated to be 0.335 eV, The acceptor and donor activation energies have been determined to be ∼18 meV and ∼39 meV, respectively. The Hall mobility of the hole reaches its maximum of 6.5×10 4 cm 2 /V·sec around 20 K. The anisotropies of the conductivity and the mobility along the three crystal axes have been investigated.

Infrared Investigation of Lattice Vibration in Black Phosphorus

The lattice vibrations in black phosphorus single crystals prepared under high pressure have been studied. The transmission spectra at various temperatures in a frequency range of 100–1000 cm -1 have been measured. We have found in the spectra at 1.5 K sharp absorption peaks at 110, 132 and 474 cm -1 . By measuring the temperature, magnetic field and light-polarization dependences of the spectra and also by examining the correspondence of the observed several peaks in the spectra with the combinations of the single phonon modes, we can determine the energies of the two single infrared active phonon modes: B 1u and B 2u .

Pressure-Induced Localization of 4f Electrons in the Intermediate Valence Compound SmB6

We report high-pressure studies of X-ray diffraction and 11 B-nuclear magnetic resonance (NMR) in the intermediate- valence compound SmB6. The pressure dependence of the lattice constant was precisely determined and no anomaly was observed up to 9.1 GPa. The temperature dependence of the nuclear quadrupole resonance frequencyQ, obtained from the 11 B-NMR measurements, is predominantly contributed by on-site charge distribution. Using the relationship betweenQ and Sm valence at ambient pressure reported previously, we estimate the pressure dependence of the Sm valence up to 6 GPa as well. The increase in the Sm valence accelerates with pressure and reaches an increase of about 10% at 6 GPa. The pressure-induced localization of Sm 4f -holes may be responsible for the long-range magnetic order under pressure.

Anomalous Quantum Transport Properties in Semimetallic Black Phosphorus

Magnetoresistance in single crystals of black phosphorus is studied at ambient and hydrostatic pressures. In the semiconducting states at pressures below 0.71 GPa, the magnetoresistance shows periodic oscillations, which can be ascribed to the magneto-phonon resonance that is characteristic of high mobility semiconductors. In the metallic state above 1.64 GPa, the both transverse and longitudinal magnetoresistance show titanic increase with exhibiting superposed Shubnikov-de Haas oscillations. The observed small Fermi surfaces, high mobilities and light effective masses of carriers in semimetallic black phosphorus are comparable to those in the representative elemental semimetals of bismuth and graphite.

Bcc-fcc structure transition of Te

Using the synchrotron radiation x-ray powder diffraction technique, the structure phase transition of Te has been investigated at pressure up to 330 GPa and at 298 K. The phase transition from the bcc (Te-V) to a new high-pressure phase (Te-VI) was found at 100 GPa and the structure of the new phase was suggested to be a superlattice of the fcc structure. The Te-VI phase further transformed to the fcc phase (Te-VII) at 255 GPa with disappearance of satellite peaks. Since for group 16 elements, it has been considered that the structure of the highest pressure phase is bcc, the present results provided new information about the high-pressure behaviour of these elements.

Equation of state of bcc-Mo by static volume compression to 410 GPa

Unit cell volumes of Mo and Pt have been measured simultaneously to ≈400 GPa by x-ray powder diffraction using a diamond anvil cell and synchrotron radiation source. The body-centered cubic (bcc) phase of Mo was found to be stable up to 410 GPa. The equation of state (EOS) of bcc-Mo was determined on the basis of Pt pressure scale. A fit of Vinet EOS to the volume compression data gave K0 = 262.3(4.6) GPa, K0′ = 4.55(16) with one atmosphere atomic volume V0 = 31.155(24) A3. The EOS was in good agreement with the previous ultrasonic data within pressure difference of 2.5%–3.3% in the multimegabar range, though the EOS of Mo proposed from a shock compression experiment gave lower pressure by 7.2%–11.3% than the present EOS. The agreement would suggest that the Pt pressure scale provides an accurate pressure value in an ultra-high pressure range.

55Mn NQR study in βMn alloys

Abstract The electronic state of Mn at site II in βMn alloys was studied by 55Mn NQR. The nuclear spin-lattice relaxation rate, 1/T1, of βMn varies proportional to √T, where T is temperature, indicating βMn to be an itinerant-type near antiferromagnet. With doping of In and Sn, 1/T1 becomes T-independent in the paramagnetic state, indicating Mn moments to be localized. On the contrary, such a marked change in T-dependence of 1/T1 was not observed in samples with doping of Fe, Cr, Si, Al and Ge.

NMR Investigation of Pressure Effect on Intermediate Valence Compound SmB 6

Kohei Nishiyama1, Takeshi Mito1, Gabriel Pristas1,2, Yukiko Hara1, Takehide Koyama1, Koichi Ueda1, Takao Kohara1, Yuichi Akahama1, Slavomir Gabani2, Marian Reiffers2, Karol Flachbart2, Hideto Fukazawa3, Yoh Kohori3, Nao Takeshita4, and Natalia Shitsevalova5 1Graduate School of Material Science, University of Hyogo, Hyogo 678-1297, Japan 2Institute of Experimental Physics, Slovak Academy of Science, 04001 Kosice, Slovakia 3Graduate School of Science, Chiba University, Chiba 263-8522, Japan 4National Institute of Advanced Industrial Science and Technology, Ibaraki 305-8562, Japan 5Institute for Problem of Material Science, National Academy of Science of Ukraine, 03680 Kiev, Ukraine E-mail: mito@sci.u-hyogo.ac.jp

High-pressure phase diagram of O2 and N2 binary system: formation of kagome-lattice of O2

High-pressure phase diagram of O2 and N2 binary system has been investigated using an x-ray diffraction method and the pressure-composition phase relation has been proposed up to 12 GPa. From the result, a hexagonal alloy phase reported previously was stable in relatively wide pressure and composition ranges. The structure of this phase was also determined to be a hexagonal lattice (space group: P6/mmm) with seven molecules in the unit cell from the Rietveld refinement. In this phase, formation of a new magnetic lattice of O2: the kagome-lattice has also been proposed.