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K. Nakamura

Publications and source records attributed to K. Nakamura.

At least 145 records · Page 8Linked to original sources
arXiv2006

Generalised Ornstein-Uhlenbeck processes

We solve a physically significant extension of a classic problem in the theory of diffusion, namely the Ornstein-Uhlenbeck process [G. E. Ornstein and L. S. Uhlenbeck, Phys. Rev. 36, 823, (1930)]. Our generalised Ornstein-Uhlenbeck systems include a force which depends upon the position of the particle, as well as upon time. They exhibit anomalous diffusion at short times, and non-Maxwellian velocity distributions in equilibrium. Two approaches are used. Some statistics are obtained from a closed-form expression for the propagator of the Fokker-Planck equation for the case where the particle is initially at rest. In the general case we use spectral decomposition of a Fokker-Planck equation, employing nonlinear creation and annihilation operators to generate the spectrum which consists of two staggered ladders.

cond-mat.stat-mech↗
arXiv2006

Decay to the nonequilibrium steady state of the thermal diffusion in a tilted periodic potential

We investigate asymptotic decay phenomenon towards the nonequilibrium steady state of the thermal diffusion in the presence of a tilted periodic potential. The parameter dependence of the decay rate is revealed by investigating the Fokker-Planck (FP) equation in the low temperature case under the spatially periodic boundary condition (PBC). We apply the WKB method to the associated Schrödinger equation. While eigenvalues of the non-Hermitian FP operator are complex in general, in a small tilting case the imaginary parts of the eigenvalues are almost vanishing. Then the Schrödinger equation is solved with PBC. The decay rate is analyzed in the context of quantum tunneling through a triple-well effective periodic potential. In a large tilting case, the imaginary parts of the eigenvalues of FP operator are crucial. We apply the complex-valued WKB method to the Schrödinger equation with the absorbing boundary condition, finding that the decay rate saturates and depends only on the temperature, the potential periodicity and the viscous constant. The intermediate tilting case is also explored. The analytic results agree well with the numerical data for a wide range of tilting.

cond-mat.stat-mech↗
arXiv2005

Staggered Ladder Spectra

We exactly solve a Fokker-Planck equation by determining its eigenvalues and eigenfunctions: we construct nonlinear second-order differential operators which act as raising and lowering operators, generating ladder spectra for the odd and even parity states. These are staggered: the odd-even separation differs from even-odd. The Fokker-Planck equation describes, in the limit of weak damping, a generalised Ornstein-Uhlenbeck process where the random force depends upon position as well as time. Our exact solution exhibits anomalous diffusion at short times and a stationary non-Maxwellian momentum distribution.

cond-mat.soft↗
arXiv2005

Sensitivity of r-Process Nucleosynthesis to Light-Element Nuclear Reactions

We study the efficiency and sensitivity of r-process nucleosynthesis to 18 light-element nuclear reaction rates. We adopt empirical power-law relations to parameterize the reaction sensitivities. We utilize two different hydrodynamic models for the neutrino-driven winds in order to study the dependence of our result on supernova wind models. We also utilize an exponential model to approximate a wide variety of other plausible conditions for the r-process. We identify several specific nuclear reactions among light neutron-rich nuclei that play a critical role in determining the final r-process nucleosynthesis yields. As an illustration, we examine ``semi-waiting'' points among the carbon isotopes. We show that not only neutron capture and $β$-decay, but also $(α, \mathrm{n})$ reactions are important in determining waiting points along the r-process path. Our numerical results from this sensitivity analysis serve foremost to clarify which light nuclear reactions are most influential in determining the final r-process abundances. We also quantify the effects of present nuclear uncertainties on the final r-process abundances. This study thus emphasizes and motivates which future determinations of nuclear reaction rates will most strongly impact our understanding of r-process nucleosynthesis.

astro-ph↗
arXiv2005

Equatorial and related non-equilibrium states in magnetization dynamics of ferromagnets: Generalization of Suhl's spin-wave instabilities

We investigate the nonlinear dynamics underlying the evolution of a 2-D nanoscale ferromagnetic film with uniaxial anisotropy in the presence of perpendicular pumping. Considering the associated Landau-Lifshitz spin evolution equation with Gilbert damping together with Maxwell equation for the demagnetization field, we study the dynamics in terms of the stereographic variable. We identify several new fixed points for suitable choice of external field in a rotating frame of reference. In particular, we identify explicit equatorial and related fixed points of the spin vector in the plane transverse to the anisotropy axis when the pumping frequency coincides with the amplitude of the static parallel field. We then study the linear stability of these novel fixed points under homogeneous and spin wave perturbations and obtain a generalized Suhl's instability criterion, giving the condition for exponential growth of P-modes under spin wave perturbations. Two parameter phase diagrams (in terms of amplitudes of static parallel and oscillatory perpendicular magnetic fields) for stability are obtained, which differ qualitatively from those for the conventional ferromagnetic resonance near thermal equilibrium and are amenable to experimental tests.

nlin.CD↗
arXiv2004

Ion acceleration during internal magnetic reconnection events in TST-2

Characteristics of ion acceleration in the internal magnetic reconnection events (IRE) have been studied by means of a neutral particle energy analyzer (NPA) in Tokyo Spherical Tokamak (TST-2). The major and minor radii are 0.38 m and 0.25m, respectively. The magnetic field strength is 0.3T and the maximum plasma current is up to 140 kA. The electron and ion temperatures are 0.4-0.5 keV and 0.1 keV, respectively and the electron density is ~1x1019 m-3. The NPA can be scanned toroidally from q = 74&deg; (cw) to q = 114&deg; (ccw), where q = 90&deg; corresponds to the perpendicular sightline. The direction of the plasma current is cw. The NPA signals are digitized at every 50 ms. The NPA is calibrated in the energy range of 0.1 keV < E < 8.4 keV. When the IRE occurs, it is observed that the plasma current increases by ~ 20% and the loop voltage drops from 0.6 V to-5 V for ~ 0.1 ms. The enhanced charge exchange flux is observed by more than one order of magnitude at ~ 1 keV for this reconnection phase. The ion temperature increases by 80 eV at IREs. The angle q dependence of increment of Ti shows that DTi (q = 74&deg;) is higher than that for q = 114&deg;. This observation suggests that an ion is accelerated initially in the direction of magnetic field lines. The time evolution of the ion distribution function is simulated with a Fokker-Planck code taking into account the electric field effects.

physics.plasm-ph↗
arXiv2004

Estimation of Power Balance in Steady State LHCD Discharges on TRIAM-1M

On TRIAM-1M, a long duration discharge for more than 5 h was achieved successfully by fully non-inductive lower hybrid current drive. Heat load distribution to the plasma facing components (PFCs) in the 5 h discharge was investigated by using calorimetric measurements. The injected RF power was coincident with the total amount of heat load to PFCs estimated by calorimetric measurement. The power balance including the portion of direct loss power of fast electron, heat flux due to the charge exchange process was able to be estimated in this long duration discharge.

physics.plasm-ph↗
arXiv2004

Transport barrier formation by LHCD on TRIAM-1M

Internal transport barrier (ITB) has been obtained in full lower hybrid current driven (LHCD) plasmas on a superconducting tokamak, TRIMA-1M (R=0.84m, a x b=0.12mx0.18m, BT<8T). The formation of ITB depends on the current density profile, j(r), varied by the power deposition of the lower hybrid (LH). The plasma with ITB can be maintained by the LH power deposited around the foot point of ITB up to 25 sec, which corresponds to more than 100 times of current diffusion time, $τ$L/R. ITB is terminated by the reduction of current drive efficiency caused by metal impurities accumulation. In some condition, self-organized slow sawtooth oscillations (SSSO) of plasma current, density, temperature, and so on with the period comparable to the current diffusion time have been also observed during ITB discharge. The oscillation has the capability of particle exhaust, as the result, it may play an role in the avoidance of the impurity accumulation and the dilution in the future steady state fusion plasma with ITB, as the edge-localized mode in H-mode.

physics.plasm-ph↗
arXiv2004

Search for Dark Matter WIMPs using Upward Through-going Muons in Super-Kamiokande

We present the results of indirect searches for Weakly Interacting Massive Particles (WIMPs) with 1679.6 live days of data from the Super-Kamiokande detector using neutrino-induced upward through-going muons. The search is performed by looking for an excess of high energy muon neutrinos from WIMP annihilations in the Sun, the core of the Earth, and the Galactic Center, as compared to the number expected from the atmospheric neutrino background. No statistically significant excess was seen. We calculate flux limits in various angular cones around each of the above celestial objects. We obtain conservative model-independent upper limits on WIMP-nucleon cross-section as a function of WIMP mass and compare these results with the corresponding results from direct dark matter detection experiments.

hep-ex↗
arXiv2003

Signature of Quantum Chaos in SQUID's

Spectral statistics in band structures is studied in a realistic model describing superconducting quantum interference devices (SQUID's). By controlling an external magnetic flux, the level statistics may show a crossover from the GUE to the GOE. Effects of secondary discrete symmetries seen in specific regions of the first Brillouin zone are also discussed.

cond-mat.mes-hall↗
arXiv2003

Two interacting hard disks within a circular cavity: towards a quantal equation of states

We investigate a circular cavity billiard within which a pair of identical hard disks of smaller but finite size is confined. Each disk shows a free motion except when bouncing elastically with its partner and with the boundary wall. Despite its circular symmetry, this system is nonintegrable and almost chaotic because of the (short-range) interaction between the disks. We quantize the system by incorporating the excluded volume effect for the wavefunction. Eigenvalues and eigenfunctions are obtained by tuning the relative size between the disks and the billiard. We define the volume V of the cavity and the pressure P, i.e., the derivative of each eigenvalue with respect to V. Reflecting the fact that the energy spectra of eigenvalues versus the disk size show a multitude of level repulsions, P-V characteristics shows the anomalous fluctuations accompanied by many van der Waals-like peaks in each of individual excited eigenstates taken as a quasi-equilibrium. For each eigenstate, we calculate the expectation values of the square distance between two disks, and point out their relationship with the pressure fluctuations.

quant-ph↗
arXiv2003

Analytic calculation of nonadiabatic transition probabilities from monodromy of differential equations

The nonadiabatic transition probabilities in the two-level systems are calculated analytically by using the monodromy matrix determining the global feature of the underlying differential equation. We study the time-dependent 2x2 Hamiltonian with the tanh-type plus sech-type energy difference and with constant off-diagonal elements as an example to show the efficiency of the monodromy approach. The application of this method to multi-level systems is also discussed.

quant-ph↗
arXiv2003

Cu Nuclear Quadrupole Resonance Study of La_{2-x}Sr_xCu_{1-y}Zn_yO_4 (x=0.10, 0.15 and 0.20): Zn-induced Wipeout Effect near the Magnetic and Electric Instability

We studied Zn-substitution effect on the high-T_c superconductors, underdoped La_{2-x}Sr_xCu_{1-y}Zn_yO_4 (x=0.10; y=0, 0.01, 0.02), optimally doped (x=0.15; y=0, 0.02), and overdoped (x=0.20; y=0, 0.03, 0.06) in a temperature range of T=4.2-300 K, using Cu nuclear quadrupole resonance (NQR) spin-echo technique. We found full disappearance of the Cu NQR signals for the Zn-substituted, Sr-underdoped x=0.10 samples below about 40 K, partial disappearance for the Sr-optimally doped ones below about 50 K, but not for the overdoped x=0.20 ones. From the Zn-doping, the Sr-doping and the temperature dependence of the wipeout effect, we associate the wipeout effect with Zn-induced Curie magnetism or its extended glassy charge-spin stripe formation.

cond-mat.supr-con↗
arXiv2002

Zn-induced wipeout effect on Cu NQR spectra in La$_{2-x}$Sr$_x$Cu$_{1-y}$Zn$_y$O$_4$

We report a systematic study of Zn-substitution effect on Cu NQR spectrum for high $T_c$ superconductors La$_{2-x}$Sr$_x$Cu$_{1-y}$Zn$_y$O$_4$ from carrier-underdoped to -overdoped regimes (polycrystalline samples, $x$ =0.10, 0.15, and 0.20). We observed no appreciable wipeout effect for the overdoped samples, a gradual and partial wipeout effect below about 80 K for the optimally doped ones, and very abrupt and full wipeout effect below about 40 K for the underdoped ones. The wipeout effect indicates a highly enhanced spectral weight of Cu spin fluctuations at a low frequency. We associate the wipeout effect with a Zn-induced local magnetism far above 40 K and with a localization effect below 40 K.

cond-mat.supr-con↗
arXiv2002

Canted ferromagnetism in RuSr$_2$GdCu$_2$O$_8$

First principles calculations using the full-potential linearized augmented plane wave (FLAPW) method including intra-atomic noncollinear magnetism have been performed to determine the magnetic structures of RuSr$_{2}$GdCu$_{2}$O$_{8}$. The magnetism clearly arises from the RuO$_{6}$ octahedra where the moments on neighboring Ru sites order antiferromagnetically but cant perpendicular to the AFM axis - and so induce a weak ferromagnetism. The projected Ru moments along the AFM and FM axes result in magnetic moments of 1.16 and 0.99 $μ_{B}$ respectively. The results are consistent with the possible coexistence of canted ferromagnetism and superconductivity in the RuSr$_{2}$GdCu$_{2}$O$_{8}$ - inferred from experiments.

cond-mat.supr-con↗
arXiv2002

Generation of polarization-entangled photon pairs in a cascade of two type-I crystals pumped by femtosecond pulses

We report the generation of polarization-entangled photons by femtosecond-pulse-pumped spontaneous parametric down-conversion in a cascade of two type-I crystals. Highly entangled pulsed states were obtained by introducing a temporal delay between the two orthogonal polarization components of the pump field. They exhibited high-visibility quantum interference and a large concurrence value, without the need of post-selection using narrow-bandwidth-spectral filters. The results are well explained by the theory which incorporates the space-time dependence of interfering two-photon amplitudes if dispersion and birefringence in the crystals are appropriately taken into account. Such a pulsed entangled photon well localized in time domain is useful for various quantum communication experiments, such as quantum cryptography and quantum teleportation.

quant-ph↗
arXiv2001

Restoration of Entanglement by Spectral Filters

We experimentally demonstrate that entanglement of bi-photon polarization state can be restored by spectral filters. This restoration procedure can be viewed as a new class of quantum eraser, which retrieves entanglement rather than just interference by manipulating an ancillary degree of freedom.

quant-ph↗