arXiv subjects

S. Takahashi

Publications and source records attributed to S. Takahashi.

At least 73 records · Page 4Linked to original sources

arXiv2012

Initial Results from the Nobeyama Molecular Gas Observations of Distant Bright Galaxies

We present initial results from the CO survey toward high redshift galaxies using the Nobeyama 45m telescope. Using the new wide bandwidth spectrometer equipped with a two-beam SIS receiver, we have robust new detections of three high redshift (z=1.6-3.4) submillimeter galaxies (SXDF 1100.001, SDP9, and SDP17), one tentative detection (SDSS J160705+533558), and one non-detection (COSMOS-AzTEC1). The galaxies observed during the commissioning phase are sources with known spectroscopic redshifts from previous optical or from wide-band submm spectroscopy. The derived molecular gas mass and line widths from Gaussian fits are ~10^11 Msun and 430-530 km/s, which are consistent with previous CO observations of distant submm galaxies and quasars. The spectrometer that allows a maximum of 32 GHz instantaneous bandwidth will provide new science capabilities at the Nobeyama 45m telescope, allowing us to determine redshifts of bright submm selected galaxies without any prior redshift information.

astro-ph.CO↗

arXiv2012

Free-Electron Laser-Powered Electron Paramagnetic Resonance Spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy interrogates unpaired electron spins in solids and liquids to reveal local structure and dynamics; for example, EPR has elucidated parts of the structure of protein complexes that have resisted all other techniques in structural biology. EPR can also probe the interplay of light and electricity in organic solar cells and light-emitting diodes, and the origin of decoherence in condensed matter, which is of fundamental importance to the development of quantum information processors. Like nuclear magnetic resonance (NMR), EPR spectroscopy becomes more powerful at high magnetic fields and frequencies, and with excitation by coherent pulses rather than continuous waves. However, the difficulty of generating sequences of powerful pulses at frequencies above 100 GHz has, until now, confined high-power pulsed EPR to magnetic fields of 3.5 T and below. Here we demonstrate that ~1 kW pulses from a free-electron laser (FEL) can power a pulsed EPR spectrometer at 240 GHz (8.5 T), providing transformative enhancements over the alternative, a state-of-the-art ~30 mW solid state source. Using the UC Santa Barbara FEL as a source, our 240 GHz spectrometer can rotate spin-1/2 electrons through pi/2 in only 6 ns (vs. 300 ns with the solid state source). Fourier transform EPR on nitrogen impurities in diamond demonstrates excitation and detection of EPR lines separated by ~200 MHz. Decoherence times for spin-1/2 systems as short as 63 ns are measured, enabling measurement of the decoherence time in a frozen solution of nitroxide free-radicals at temperatures as high as 190 K. Both FELs and the quasi-optical technology developed for the spectrometer are scalable to frequencies well in excess of 1 THz, opening the possibility of high-power pulsed EPR spectroscopy up to the highest static magnetic fields on earth.

cond-mat.other↗

arXiv2011

Decoherence in Crystals of Quantum Molecular Magnets

Decoherence in Nature has become one of the most pressing problems in physics. Many applications, including quantum information processing, depend on understanding it; and fundamental theories going beyond quantum mechanics have been suggested [1-3], where the breakdown of quantum theory appears as an 'intrinsic decoherence', mimicking environmental decoherence [4]. Such theories cannot be tested until we have a handle on ordinary environmental decoherence processes. Here we show that the theory for insulating electronic spin systems can make accurate predictions for environmental decoherence in molecular-based quantum magnets [5]. Experimental understanding of decoherence in molecular magnets has been limited by short decoherence times, which make coherent spin manipulation extremely difficult [6-9]. Here we reduce the decoherence by applying a strong magnetic field. The theory predicts the contributions to the decoherence from phonons, nuclear spins, and intermolecular dipolar interactions, for a single crystal of the Fe8 molecular magnet. In experiments we find that the decoherence time varies strongly as a function of temperature and magnetic field. The theoretical predictions are fully verified experimentally - there are no other visible decoherence sources. Our investigation suggests that the decoherence time is ultimately limited by nuclear spins, and can be extended up to about 500 microseconds, by optimizing the temperature, magnetic field, and nuclear isotopic concentrations.

cond-mat.mes-hall↗

arXiv2011

Giant enhancement of spin accumulation and long-distance spin precession in metallic lateral spin valves

The nonlocal spin injection in lateral spin valves is highly expected to be an effective method to generate a pure spin current for potential spintronic application. However, the spin valve voltage, which decides the magnitude of the spin current flowing into an additional ferromagnetic wire, is typically of the order of 1 μV. Here we show that lateral spin valves with low resistive NiFe/MgO/Ag junctions enable the efficient spin injection with high applied current density, which leads to the spin valve voltage increased hundredfold. Hanle effect measurements demonstrate a long-distance collective 2-pi spin precession along a 6 μm long Ag wire. These results suggest a route to faster and manipulable spin transport for the development of pure spin current based memory, logic and sensing devices.

cond-mat.mes-hall↗

arXiv2011

Composite excitation of Josephson phase and spin waves in Josephson junctions with ferromagnetic insulator

Coupling of Josephson-phase and spin-waves is theoretically studied in a superconductor/ferromagnetic insulator/superconductor (S/FI/S) junction. Electromagnetic (EM) field inside the junction and the Josephson current coupled with spin-waves in FI are calculated by combining Maxwell and Landau-Lifshitz-Gilbert equations. In the S/FI/S junction, it is found that the current-voltage (I-V) characteristic shows two resonant peaks. Voltages at the resonant peaks are obtained as a function of the normal modes of EM field, which indicates a composite excitation of the EM field and spin-waves in the S/FI/S junction. We also examine another type of junction, in which a nonmagnetic insulator (I) is located at one of interfaces between S and FI. In such a S/I/FI/S junction, three resonant peaks appear in the I-V curve, since the Josephson-phase couples to the EM field in the I layer.

cond-mat.supr-con↗

arXiv2011

Field test of quantum key distribution in the Tokyo QKD Network

A novel secure communication network with quantum key distribution in a metropolitan area is reported. Different QKD schemes are integrated to demonstrate secure TV conferencing over a distance of 45km, stable long-term operation, and application to secure mobile phones.

quant-ph↗

arXiv2010

SiO Maser Spectra of V407 Cyg after the 2010 March Nova Outburst

We report on the time variation of SiO maser emission from the symbiotic stellar system V407 Cyg after the classical nova outburst on 2010 March 10. Although both the SiO $J=1$--0 $v=1$ and 2 lines at 43.122 and 42.821 GHz were found previously in the envelope of a mira in the binary system, only weak emission of the $J=1$--0 $v=2$ line has continuously been detectable after the nova outburst. The line profile exhibited a dramatic change several weeks after the burst; the component on the higher-velocity side of the systemic velocity disappeared two weeks after the burst, and a new persistent component appeared on the lower-velocity side later. These observations indicate that the SiO emitting regions are wiped out in a time scale of two weeks by the nova shock, but a part of the masing region is quickly replenished by cool molecular gases expelled by the mira pulsation.

astro-ph.SR↗

arXiv2010

Spin Seebeck insulator

Thermoelectric generation is an essential function of future energy-saving technologies. However, this generation has been an exclusive feature of electric conductors, a situation which inflicts a heavy toll on its application; a conduction electron often becomes a nuisance in thermal design of devices. Here we report electric-voltage generation from heat flowing in an insulator. We reveal that, despite the absence of conduction electrons, a magnetic insulator LaY2Fe5O12 converts a heat flow into spin voltage. Attached Pt films transform this spin voltage into electric voltage by the inverse spin Hall effect. The experimental results require us to introduce thermally activated interface spin exchange between LaY2Fe5O12 and Pt. Our findings extend the range of potential materials for thermoelectric applications and provide a crucial piece of information for understanding the physics of the spin Seebeck effect.

cond-mat.mtrl-sci↗

arXiv2009

Large anisotropy of spin-orbit interaction in a single InAs self-assembled quantum dot

Anisotropy of spin-orbit interaction (SOI) is studied for a single uncapped InAs self-assembled quantum dot (SAQD) holding just a few electrons. The SOI energy is evaluated from anti-crossing or SOI induced hybridization between the ground and excited states with opposite spins. The magnetic angular dependence of the SOI energy falls on an absolute cosine function for azimuthal rotation, and a cosine-like function for tilting rotation. The SOI energy is even quenched at a specific rotation. These angular dependence compare well to calculation of Rashba SOI in a two-dimensional harmonic potential.

cond-mat.mes-hall↗

arXiv2009

High Field Phenomena of Qubits

Electron and nuclear spins are very promising candidates to serve as quantum bits (qubits) for proposed quantum computers, as the spin degrees of freedom are relatively isolated from their surroundings, and can be coherently manipulated e.g. through pulsed EPR and NMR. For solid state spin systems, impurities in crystals based on carbon and silicon in various forms have been suggested as qubits, and very long relaxation rates have been observed in such systems. We have investigated a variety of these systems at high magnetic fields in our multi-frequency pulsed EPR/ENDOR spectrometer. A high magnetic field leads to large electron spin polarizations at helium temperatures giving rise to various phenomena that are of interest with respect to quantum computing. For example, it allows the initialization of the both the electron spin as well as hyperfine-coupled nuclear spins in a well defined state by combining millimeter and RF radiation; it can increase the T2 relaxation times by eliminating decoherence due to dipolar interaction; and it can lead to new mechanisms for the coherent electrical readout of electron spins. We will show some examples of these and other effects in Si:P, SiC:N, and nitrogen-related centers in diamond.

quant-ph↗

arXiv2009

Sign reversal of ac Josephson current in a ferromagnetic Josephson junction

The ac Josephson effect in a ferromagnetic Josephson junction, which is composed of two superconductors separated by a ferromagnetic metal (FM), is studied by a tunneling Hamiltonian and Green's function method. We obtain two types of superconducting phase dependent current, i.e., Josephson current and quasiparticle-pair-interference current (QPIC). These currents change their signs with thickness of the FM layer due to the 0-$π$ transition characteristic to the ferromagnetic Josephson junction. As a function of applied voltage, the Josephson critical current shows a logarithmic divergence called the Riedel peak at the gap voltage, while the QPIC shows a discontinuous jump. The Riedel peak reverses due to the 0-$π$ transition and disappears near the 0-$π$ transition point. The discontinuous jump in the QPIC also represents similar behaviors to the Riedel peak. These results are in contrast to the conventional ones.

cond-mat.mes-hall↗

arXiv2008

FMR induced Josephson Current in a Superconductor/Ferromagnet/Superconductor Junction

We propose the phase dynamics induced by spin waves in a superconductor/ferromagnet/superconductor (SC/FM/SC) Josephson junction. The resistively shunted junction (RSJ) model, which describes the dynamics of superconducting phase difference, is extended to include the spin wave excitation by ferromagnetic resonance (FMR) using the gauge invariant phase difference between two s-wave superconductors. The current-voltage characteristics show step structures when the magnetization in FM is driven by tuning the microwave frequency to FMR in the SC/FM/SC junction. The result presents a new route to observe the spin wave excitation using the Josephson effect.

cond-mat.mes-hall↗

arXiv2007

Candidate Rotating Toroids around High-Mass (Proto)Stars

Using the OVRO, Nobeyama, and IRAM mm-arrays, we searched for ``disk''-outflow systems in three high-mass (proto)star forming regions: G16.59-0.05, G23.01-0.41, and G28.87+0.07. These were selected from a sample of NH3 cores associated with OH and H2O maser emission and with no or very faint continuum emission. Our imaging of molecular line (including rotational transitions of CH3CN and 3mm dust continuum emission revealed that these are compact, massive, and hot molecular cores (HMCs), that is likely sites of high-mass star formation prior to the appearance of UCHII regions. All three sources turn out to be associated with molecular outflows from CO and/or HCO+ J=1--0 line imaging. In addition, velocity gradients of 10 -- 100 km/s per pc in the innermost densest regions of the G23.01 and G28.87 HMCs are identified along directions roughly perpendicular to the axes of the corresponding outflows. All the results suggest that these cores might be rotating about the outflow axis, although the contribution of rotation to gravitational equilibrium of the HMCs appears to be negligible. Our analysis indicates that the 3 HMCs are close to virial equilibrium due to turbulent pressure support. Comparison with other similar objects where rotating toroids have been identified so far shows that in our case rotation appears to be much less prominent; this can be explained by the combined effect of unfavorable projection, large distance, and limited angular resolution with the current interferometers.

astro-ph↗

arXiv2007

A diffraction-compensating 0-25 ns free space terahertz delay line for coherent quantum control

Free space delay lines provide pulses of variable time spacing for optical experiments such as pump-probe spectroscopy and coherent quantum control, including spin and photon echo techniques. However, in the terahertz (THz) region of the spectrum, beam divergence due to diffraction limits the useful length of traditional free space delay lines. We present a novel double-folded variable delay line for light in the frequency range 0.24-1.2 THz, which incorporates a symmetric arrangement of lenses whose spacing can be adjusted to compensate for diffraction at each delay. Scalable for use in other wavelength regimes, the design relays an input Gaussian beam waist to the output with up to 25 ns (~8 m) total delay and is enclosed in a desiccated volume of <0.5 m^3. The delay line can deliver two or three pulses with relative amplitudes controlled via variable spacing silicon etalon beam splitters. Beam profiles of a 0.24 THz beam show good agreement with calculations at long delays, with insertion loss per delay stage of~3 dB.

physics.optics↗

arXiv2007

Role of magnetic scattering in 0-pi transitions in a superconductor/ferromagnetic metal/superconductor junction

We study the influence of magnetic scattering on the Josephson critical current, Ic, in a superconductor/ferromagnetic metal/superconductor (SFS) junction by a tunneling Hamiltonian approach. An analytical formula of Ic is given in the fourth order perturbation theory as regards the tunneling matrix element. The Ic exhibits the damped oscillatory dependence on the thickness of the ferromagnetic metal, d, and shows the transition between 0- and pi-states with d. When the superconducting transition temperature is comparable to the ferromagnetic Curie temperature, the period of oscillation is obviously changed by increasing temperature, T, due to the magnetic scattering, which induces the 0-pi transition with T. The magnetic scattering provides rich variety of Josephson effect in the SFS junction. Our results present an appropriate condition of a superconductor and a ferromagnetic metal to control the 0- and the pi-states.

cond-mat.mes-hall↗

arXiv2007

Measurements of total and partial charge-changing cross sections for 200-400 MeV/nucleon 12C in water and polycarbonate

We have studied charged nuclear fragments produced by 200 - 400 MeV/nucleon carbon ions, interacting with water and polycarbonate, using a newly developed emulsion detector. Total and partial charge-changing cross sections for the production of B, Be, and Li fragments were measured and compared with both previously published measurements, and model predictions. This study is of importance for validating and improving carbon ion therapy treatment planning systems, and for estimating the radiological risks for personnel on space missions, since carbon is a significant component of the Galactic Cosmic Rays.

nucl-ex↗

arXiv2007

Magnetic Impurity States and Ferromagnetic Interaction in Diluted Magnetic Semiconductors

The electronic structure of diluted magnetic semiconductors is studied, especially focusing on the hole character. The Haldane-Anderson model is extend to a magnetic impurity, and is analyzed in the Hartree-Fock approximation. Due to the strong hybridization of an impurity d-states with host p-states, it is shown that holes are created in the valence band, at the same time the localized magnetic moments are formed. These holes are weakly bound around the impurity site with the characteristic length of several lattice constants. For the case of the two impurities, it is found that when the separation of two impurities is of the order of the length of holes, the overlap of these holes favors the ferromagnetic interaction. The spatially extended holes play an essential role for the ferromagnetic exchange interaction.

cond-mat.mes-hall↗

arXiv2006

Long-range ferromagnetic correlations between Anderson impurities in a semiconductor host

We study the two-impurity Anderson model for a semiconductor host using the quantum Monte Carlo technique. We find that the impurity spins exhibit ferromagnetic correlations with a range which can be much more enhanced than in a half-filled metallic band. In particular, the range is longest when the Fermi level is located above the top of the valence band and decreases as the impurity bound state becomes occupied. Comparisons with the photoemission and optical absorption experiments suggest that this model captures the basic electronic structure of Ga_{1-x}Mn_xAs, the prototypical dilute magnetic semiconductor (DMS). These numerical results might also be useful for synthesizing DMS or dilute-oxide ferromagnets with higher Curie temperatures.

cond-mat.str-el↗