arXiv subjects

Katsuhiro Nakamura

Publications and source records attributed to Katsuhiro Nakamura.

31 records · Page 2Linked to original sources

arXiv2006

Magnetic Domain Patterns Depending on the Sweeping Rate of Magnetic Fields

The domain patterns in a thin ferromagnetic film are investigated in both experiments and numerical simulations. Magnetic domain patterns under a zero field are usually observed after an external magnetic field is removed. It is demonstrated that the characteristics of the domain patterns depend on the decreasing rate of the external field, although it can also depend on other factors. Our numerical simulations and experiments show the following properties of domain patterns: a sea-island structure appears when the field decreases rapidly from the saturating field to the zero field, while a labyrinth structure is observed for a slowly decreasing field. The mechanism of the dependence on the field sweeping rate is discussed in terms of the concepts of crystallization.

cond-mat.other↗

arXiv2006

Role of an intermediate state in homogeneous nucleation

We explore the role of an intermediate state (phase) in homogeneous nucleation phenomenon by examining the decay process through a doubly-humped potential barrier. As a generic model we use the fourth- and sixth-order Landau potentials and analyze the Fokker-Planck equation for the one-dimensional thermal diffusion in the system characterized by a triple-well potential. In the low temperature case we apply the WKB method to the decay process and obtain the decay rate which is accurate for a wide range of depth and curvature of the middle well. In the case of a deep middle well, it reduces to a doubly-humped-barrier counterpart of the Kramers escape rate: the barrier height and the curvature of an initial well in the Kramers rate are replaced by the arithmetic mean of higher(or outer) and lower(or inner) partial barriers and the geometric mean of curvatures of the initial and intermediate wells, respectively. It seems to be a universal formula. In the case of a shallow-enough middle well, Kramers escape rate is alternatively evaluated within the standard framework of the mean-first-passage time problem, which certainly supports the WKB result. The criteria whether or not the existence of an intermediate state can enhance the decay rate are revealed.

cond-mat.stat-mech↗

arXiv2006

Can the Landau-Lifshitz equation explain the spin-wave instability in ferromagnetic thin films for parallel pumping?

Spin-wave instability is studied analytically in the case of parallel pumping for thin films under external field perpendicular to the film plane. It is examined whether the instability threshold derived from only the Landau-Lifshitz (LL) equation can explain experimental instability threshold without using the microscopically-derived spin-wave line width, which is conventionally used. It is revealed that the butterfly curve cannot be explained from only the LL equation at least in an analytical way. By contrast, for the case of perpendicular pumping, the Suhl instability was well explained from the LL equation. The difference between the two cases comes from the nonlinear terms describing the relaxation of spin waves. It is suggested how the nonlinear terms in the LL equation should be related to the spin-wave line width for parallel pumping.

cond-mat.other↗

arXiv2006

Levitation of Bose-Einstein condensates induced by macroscopic non-adiabatic quantum tunneling

We study the dynamics of two-component Bose-Einstein condensates trapped in different vertical positions in the presence of an oscillating magnetic field. It is shown here how tuning appropriately the oscillation frequency of the magnetic field leads to the levitation of the system against gravity. This phenomenon is a manifestation of a macroscopic non-adiabatic tunneling in a system with internal degrees of freedom.

cond-mat.other↗

arXiv2006

Spin-wave instability for parallel pumping in ferromagnetic thin films under oblique field

Spin-wave instability for parallel pumping is studied theoretically. The spin-wave instability threshold is calculated in ferromagnetic thin films under oblique field which has an oblique angle to the film plane. The butterfly curve of the threshold usually has a cusp at a certain value of the static external field. While the static field value of the cusp point varies as the oblique angle changes, the general properties of the butterfly curve show little noteworthy change. For very thin films, however, multiple cusps of the butterfly curve can appear due to different standing spin-wave modes, which indicate a novel feature for thin films under oblique field.

cond-mat.other↗

arXiv2005

Dynamics of Macroscopic Wave Packet Passing through Double Slits: Role of Gravity and Nonlinearity

Using the nonlinear Schroedinger equation (Gross-Pitaevskii equation), the dynamics of a macroscopic wave packet for Bose-Einstein condensates falling through double slits is analyzed. This problem is identified with a search for the fate of a soliton showing a head-on collision with a hard-walled obstacle of finite size. We explore the splitting of the wave packet and its reorganization to form an interference pattern. Particular attention is paid to the role of gravity (g) and repulsive nonlinearity (u_0) in the fringe pattern. The peak-to-peak distance in the fringe pattern and the number of interference peaks are found to be proportional to g^(-1/2) and u_0^(1/2)g^(1/4), respectively. We suggest a way of designing an experiment under controlled gravity and nonlinearity.

cond-mat.other↗

arXiv2005

A model for conservative chaos constructed from multi-component Bose-Einstein condensates with a trap in 2 dimensions

To show a mechanism leading to the breakdown of a particle picture for the multi-component Bose-Einstein condensates(BECs) with a harmonic trap in high dimensions, we investigate the corresponding 2-$d$ nonlinear Schr{ö}dinger equation (Gross-Pitaevskii equation) with use of a modified variational principle. A molecule of two identical Gaussian wavepackets has two degrees of freedom(DFs), the separation of center-of-masses and the wavepacket width. Without the inter-component interaction(ICI) these DFs show independent regular oscillations with the degenerate eigen-frequencies. The inclusion of ICI strongly mixes these DFs, generating a fat mode that breaks a particle picture, which however can be recovered by introducing a time-periodic ICI with zero average. In case of the molecule of three wavepackets for a three-component BEC, the increase of amplitude of ICI yields a transition from regular to chaotic oscillations in the wavepacket breathing.

cond-mat.other↗

arXiv2005

Energy diffusion in frustrated quantum spin chains exhibiting Gaussian orthogonal ensemble level statistics

Frustrated quantum $XXZ$ spin chains with the next-nearest-neighbor (NNN) couplings are typically deterministic many-body systems exhibiting Gaussian orthogonal ensemble (GOE) spectral statistics. We investigate energy diffusion for these spin chains in the presence of a periodically oscillating magnetic field. Diffusion coefficients are found to obey the power law with respect to both the field strength and driving frequency with its power varying depending on the linear response and non-perturbative regimes. The widths of the linear response and the non-perturbative regimes depend on the strength of frustrations. We have also elucidated a mechanism for oscillation of energy diffusion in the case of weakened frustrations.

cond-mat.stat-mech↗

arXiv2003

Frustrated quantum-spin system on a triangle coupled with $e_g$ lattice vibrations - Correspondence to Longuet-Higgins et al.'s Jahn-Teller model -

We investigate the quantum three spin model $({\bf S_1},{\bf S_2},{\bf S_3})$ of spin$=1/2$ on a triangle, in which spins are coupled with lattice-vibrational modes through the exchange interaction depending on distances between spin sites. The present model corresponds to the dynamic Jahn-Teller system $E_g\otimes e_g$ proposed by Longuet-Higgins {\it et al.}, Proc.R.Soc.A.{\bf 244},1(1958). This correspondence is revealed by using the transformation to Nakamura-Bishop's bases proposed in Phys.Rev.Lett.{\bf 54},861(1985). Furthermore, we elucidate the relationship between the behavior of a chiral order parameter ${\hat χ}={\bf S_1\cdot(S_2\times S_3)}$ and that of the electronic orbital angular momentum ${\hat \ell_z}$ in $E_g\otimes e_g$ vibronic model: The regular oscillatory behavior of the expectation value $<{\hat \ell_z}>$ for vibronic structures with increasing energy can also be found in that of $<{\hat χ}>$. The increase of the additional anharmonicity(chaoticity) is found to yield a rapidly decaying irregular oscillation of $<{\hat χ}>$.

nlin.CD↗

arXiv2003

Chaos-induced breaking of the Franck-Condon approximation

We investigate the vibrational structure of electronic spectra for the transition from the non-degenerate $A$ state to $E$ states in $E_g\otimes e_g$ Jahn-Teller systems with the trigonal field included. In connection with the underlying chaotic behavior for vibronic energy levels in this model reported by the present authors in Phys.Rev.{\bf E68}(2003)046201, we extend the analysis by Longuet-Higgins et al. to the classically chaotic system. In particular, the triple-humped structure is manifest with increasing the anharmonicity. Such structure is completely inconsistent with the shape obtained from Franck-Condon(FC) approximation, and is caused by the chaos-induced beaking of the FC principle.

nlin.CD↗

arXiv2003

Chaos and its quantization in dynamical Jahn-Teller systems

We investigate the $E_g \otimes e_g$ Jahn-Teller system for the purpose to reveal the nature of quantum chaos in crystals. This system simulates the interaction between the nuclear vibrational modes and the electronic motion in non-Kramers doublets for multiplets of transition-metal ions. Inclusion of the anharmonic potential due to the trigonal symmetry in crystals makes the system nonintegrable and chaotic. Besides the quantal analysis of the transition from Poisson to Wigner level statistics with increasing the strength of anharmonicity, we study the effect of chaos on the electronic orbital angular momentum and explore the magnetic $g$-factor as a function of the system's energy. The regular oscillation of this factor changes to a rapidly-decaying irregular oscillation by increasing the anharmonicity (chaoticity).

cond-mat.mtrl-sci↗

arXiv2001

Negative Magneto-Resistance Beyond Weak Localization in Three-Dimensional Billiards: Effect of Arnold Diffusion

We investigate a semiclassical conductance for ballistic open three-dimensional (3-d) billiards. For partially or completely broken-ergodic 3-d billiards such as SO(2) symmetric billiards, the dependence of the conductance on the Fermi wavenumber is dramatically changed by the lead orientation. Application of a symmetry-breaking weak magnetic field brings about mixed phase-space structures of 3-d billiards which ensures a novel Arnold diffusion that cannot be seen in 2-d billiards. In contrast to the 2-d case, the anomalous increment of the conductance should inevitably include a contribution arising from Arnold diffusion as well as a weak localization correction. Discussions are devoted to the physical condition for observing this phenomenon.

cond-mat.mes-hall↗

arXiv2000

h/2e oscillations and quantum chaos in ballistic Aharonov-Bohm billiards

We study the quantum interference effect for the single ballistic Aharonov-Bohm billiard in the presence of a weak magnetic field B. The diagonal part of the wave-number averaged reflection coefficient $δ{\cal R}_D$ is calculated by use of semi-classical scattering theory. In addition to the appearance of "h/2e oscillation" that are caused by interference between time-reversed coherent backscattering classical trajectories, B in the conducting region leads to negative magnetoresistance and dampening of the h/2e oscillation amplitude. The B dependence of the results reflects the underlying classical (chaotic and regular) dynamics.

cond-mat.mes-hall↗