Daisuke Nakamura

Terahertz conductivity measurement of FeSe0.5Te0.5 and Co-doped BaFe2As2 thin films

We have performed high-resolution angle-resolved photoemission spectroscopy of iron-chalcogenide superconductor Fe1.03Te0.7Se0.3 (Tc = 13 K) to investigate the electronic structure relevant to superconductivity. We observed a hole- and an electron-like Fermi surfaces at the Brillouin zone center and corner, respectively, which are nearly nested by the Q~(pi, pi) wave vector. We do not find evidence for the nesting instability with Q~(pi+delta, 0) reminiscent of the antiferromagnetic order in the parent compound Fe1+yTe. We have observed an isotropic superconducting gap along the hole-like Fermi surface with the gap size Delta of ~4 meV (2Delta/kBTc~7), demonstrating the strong-coupling nature of the superconductivity. The observed similarity of low-energy electronic excitations between iron-chalcogenide and iron-arsenide superconductors strongly suggests that common interactions which involve Q~(pi, pi) scattering are responsible for the superconducting pairing.

Superconducting Fluctuation Investigated by THz Conductivity of La2-xSrxCuO4 Thin Films

The frequency-dependent terahertz conductivities of La 2- x Sr x CuO 4 thin films with various carrier concentrations were investigated. The imaginary part of the complex conductivity considerably increased from a temperature far above the zero-resistance superconducting transition temperature T c zero , because of the existence of fluctuation in superfluid density with a short lifetime. The onset temperature of the superconducting fluctuation is at most ∼2 T c zero for underdoped samples, which is consistent with a previous report on the analysis of microwave conductivity. The superconducting fluctuation was not enhanced under a 0.5 T magnetic field. We also found that the temperature dependence of the superconducting fluctuation was sensitive to the carrier concentration of La 2- x Sr x CuO 4 , which reflects the difference in the nature of the critical dynamics near the superconducting transition temperature. Our results suggest that the onset temperature of the Nernst signal is not related to the supe...

Band-edge exciton states in a single-walled carbon nanotube revealed by magneto-optical spectroscopy in ultrahigh magnetic fields

We report high field magneto-optical study on the first and second sub-band transitions of single-chirality single-walled carbon nanotubes. The ordering and relative energy splitting between bright and dark excitonic states were found to be inverse between the first and second subbands. We verified that the zero-momentum dark singlet exciton lies below the bright exciton for the first subband transitions, while for the second sub-band transitions, it was found to have higher energy than the bright excitonic state. Effect of this peculiar excitonic structure was found to manifest itself in distinctive Aharonov-Bohm splitting in ultra-high magnetic fields up to 190 T.

On-line and hierarchical design methods of dynamics based information processing system

In this paper, we develop the on-line design method and the hierarchical design method of dynamics based information processing system for the robot intelligence. By using the forgetting parameter, dynamics memorizes a new robot motion forgetting an old motion, which means the plasticity of the system. The hierarchical structure enables information processing for complex and continuous environment. We implement the proposed method to a humanoid robot and realize the motion generation and transition.

Anomalous diamagnetic shifts in InP-GaP lateral quantum-wires.

Linearly polarized photoluminescence (PL) measurements were carried out on InP-GaP lateral nanowires grown using a lateral composition modulation method in pulsed magnetic fields up to ∼ 50 T. In these structures, the energy band alignment becomes type-I and type-II in In-rich wire and Ga-rich barrier regions, respectively. It is revealed that the polarization of the type-I PL is oriented along the [11̄0] crystal direction, whereas that of the type-II PL is along the [110] direction in the absence of magnetic field. These two different PL peaks exhibit anomalous energy shifts with respect to the direction of the magnetic field due to the variation of the confined energy in the exciton center of mass potential.

Note: An approach to 1000 T using the electro-magnetic flux compression

The maximum magnetic field obtained by the electro-magnetic flux compression technique was investigated with respect to the initial seed magnetic field. It was found that the reduction in the seed magnetic field from 3.8 T to 3.0 T led to a substantial increase in the final peak magnetic field. The optical Faraday rotation method with a minimal size probe evades disturbances from electromagnetic noise and shockwave effects to detect such final peak fields in a reduced space of an inner wall of the imploding liner. The Faraday rotation signal recorded the maximum magnetic field increased significantly to the highest magnetic field of 985 T approaching 1000 T, ever achieved by the electro-magnetic flux compression technique as an indoor experiment.

Optical Detection of Magnetic Orders in HgCr2O4 Frustrated Spin Magnet under Pulsed High Magnetic Fields

A magneto-optical survey was conducted for HgCr2O4 powder samples under pulsed high magnetic fields of up to 55 T. Intensity changes in magnetic fields observed for the exciton–magnon–phonon optical transition spectra coincide well with those of magnetization, lattice distortion from X-ray diffraction, and electron-magnetic resonances. The last-ordered phase was detected prior to the fully polarized magnetic phase, similarly to the other chromium spinel oxide, ZnCr2O4 and CdCr2O4.

Ultralow-Dissipative Conductivity by Dirac Fermions in BaFe2As2

We report on the anomalous behavior of the complex conductivity of BaFe2As2, which is considered to be related to the Dirac cone, in the terahertz (THz)-frequency region. Above the spin-density-wave (SDW) transition temperature, the conductivity spectra follow the Drude formula. In the SDW state, the imaginary part of the conductivity, σ2, is suppressed in comparison with that expected according to the Drude model, whereas the real part, σ1, exhibits nearly Drude-like behavior. This behavior (i.e., almost no changes in σ1 and the depression of σ2) can be regarded as reflecting the addition of extra conductivity without any dissipations in the Drude-type conductivity. We argue that the origin of this ultralow-dissipative conductivity is found to be the contribution from quasiparticles within the Dirac cone.

An SoC platform with on-chip web interface for in-field monitoring

As the VLSI technologies scale down to the nanometer regime, the circuit design and verification processes have become more and more complex and a reliable operation of VLSI becomes sensitive to the PVT (Process, Voltage, and Temperature) variations. Therefore, the LSI test only before the shipment to screen out the initial failures has been insufficient to ensure the reliable in-field operation of LSI. In this paper, we propose an SoC platform with on-chip web interface to realize an in-field LSI testing and an easy access to the on-chip LSI monitoring circuits such as scan registers, temperature sensors, and so on. We can control the on-chip monitoring systems through the web interface, and can monitor the LSI correct operations from remote locations using the proposed platform. Therefore, the proposed SoC platform realizes the LSI functionality monitoring even after the shipment and can test the in-field operation of LSI. This platform consists of 16-bit CPU, 64K words of instruction/data memory, and 10Base-T ethernet interface. A preliminary version of the proposed platform was implemented on 0.18µm standard CMOS process. The area overhead is 8.44mm2 on 0.18µm process, and is estimated to scale down to about 1mm2 on 65nm process.

Study of friction using driven vortices of superconductor as a model system

Dynamics of driven vortices in high-Tc superconductors was investigated in terms of the elementary process of the motion and the microscopic friction. The I-V characteristics and the transient response of driven vortices were measured in La2-xSrxCuO4 thin films and in Bi2Sr2CaCu2Oy bulk crystals. First, with an aid of a recently proposed scaling relation between the driving force and the velocity, we found that the non-Arrhenius process did exist for interacting vortices at low temperatures. With increasing magnetic field, Arrhenius process revived suddenly at the vortex-glass vs Bragg-glass transition, which shows that this approach is suitable as a new method to investigate the equilibrium phase diagram of vortices. After constructing an explicit formula connecting the motion of driven vortices to the physics of friction, the dependence of the kinetic friction force on the sliding velocity was investigated. Based on the experimental results, we propose that the non-Amontons-Coulomb behavior is regarded as the consequence of the broadened dynamic phase transition. We also found the remarkable dependence of the maximum static friction force on the waiting time at low temperatures. Such a strong dependence changed into a weak logarithmic dependence at higher temperatures. The strong time dependence suggests that a characteristic time scale to stabilize vortices exists at low temperatures. This relaxation phenomenon is very similar to the so-called boundary lubrication. These results imply that the dynamics of vortices can be used for a model not only of the dry friction but also of the lubricated friction.