1,025 research outputs found
Topological Defects and Morphology of Graphitic Carbon Materials: An Approach Based on Differential Geometry
It has been known that pentagons and heptagons in hexagonal graphitic network
give rise to a certain amount of curvature in the three dimensional structure
of graphitic carbon materials. The amount of curvature is quantized due to the
symmetry of graphite and, as a result, the structure formed by the network is
also restricted. We clarify the effects of curvature quantization on the forms
of graphitic carbon materials, employing the knowledge of differential
geometry, especially the Gauss-Bonnet theorem.Comment: 8 pages, 9 figure
A Numerical Formulation to Calculate the Conductance of Mesoscopic Conductors Using Singular Value Decomposition
We present a new formulation to calculate the electric conductance of
mesoscopic conductors by utilizing the singular value decomposition, which is a
mathematical technique to manipulate matrices. Our formulation is useful in
treating conductors with rather complicated atomic structures, for which naive
recursion formula is cumbersome. It also has an advantage in scaling up the
calculation by using parallel computation, which potentially allows us the
real-scale calculations at the atomic level. On the other hands, the effects of
electron-electron interactions are hard to be treated within this framework,
since it depends crucially on the linearity of the system. In this paper, we
study graphene nanoribbons with external leads for a simple example.Comment: 9 pages, 5 figures; Some changes have been made in DIscussion and
Appendices. Several errors have been correcte
Spectral Flow and Quantum Theory of Dissipation in the Vortex Core of BCS superconductors
The dissipation process in two dimensional BCS superconductor due to the
quasiparticles in the core of moving vortex is studied from a quantum
mechanical point of view, especially paying attention to the spectral flow of
the bound state energies in the vortex core. In order to clarify the nature of
the spectral flow, we performed a numerical study of a finite system and, by
extending the analysis to include the effect of impurities, we discuss the
quantum mechanical origin of the dissipation in the vortex core.Comment: 4 pages, 5 figures. Some grammatical changes were made and new
references were adde
Spectral Flow and the Dynamics of Dislocations in Charge Density Waves
The spectral flow in the cores of moving dislocations is found to play an
important role in the dynamics and transport of charge density wave (CDW) by
significantly modifying the hydrodynamic effective action of the condensate.
The analogy of spectral flow in CDW with the baryogenesis in the early universe
is pointed out.Comment: 5 pages, 2 figure
On the Ginzburg-Landau Free Energy of Charge Density Waves with a Three-Dimensional Order
The effective free energy of a charge density wave (CDW) with a
three-dimensional order is derived from a microscopic model (Fr\"olich model)
based on the path integral method. Electron hoping and Coulomb interaction
between chains are taken into account perturbatively, leading to an elastic
interchain coupling of the CDW ordered state.Comment: 8 pages, Revtex format, no figure
Superconductivity on a M\"obius strip: numerical studies on order parameter and quasiparticles
Superconducting states of an anisortopic s-wave superconductor on a M\"obius
strip are studied numerically based on the Ginzburg-Landau theory and the
Bogoliubov-de Gennes theory. In both, the equations are solved numerically on
discitized lattice and the nonlinearity and the self-consistency are fully
taken into account. First, we study the superconducting states on the M\"obius
strip in the presence of the Aharonov-Bohm flux threading the ring by employing
the Ginzburg-Landau theory, and confirm the phase diagram previously proposed
by Hayashi and Ebisawa [J. Phys. Soc. Jpn. {\bf 70}, 3495 (2002)]. The
metastable states as well as the equilibrium energy state are studied and the
nonequiriblium processes when the magnetic field is varied at a fixed
temperature are discussed. Next, we study the microscopic superconducting
states on the M\"obius strip based on the Bogoliubov-de Gennes theory,
especially focusing on the state with a real-space node in the superconducting
gap, which is expected to appear when the flux threading the ring is half the
superconducting flux quantum. The local density of states in this {\it nodal
state} is calculated in detail and the existence of the zero-energy bound
states is shown.Comment: 7 pages, 8 figure
Time-dependent Ginzburg-Landau equation and Boltzmann transport equation for charge-density-wave conductors
The time-dependent Ginzburg-Landau equation and the Boltzmann transport
equation for one-dimensional charge-density-wave (CDW) conductors are derived
from a microscopic model by applying the Keldysh Green's function approach
under a quasiclassical approximation. The effects of external electric field
and impurity pinning of the CDW are fully taken into account without relying on
a phenomenological argument. These equations simultaneously describe
spatiotemporal dynamics of both the CDW and quasiparticles, so they serve as a
powerful tool to analyze various nonequilibrium phenomena, such as the current
conversion between the CDW condensate and quasiparticles mediated by phase
slips. It is shown that, in typical situations, the equations correctly
describe the nonlinear behavior of electric conductivity in a simpler manner.Comment: 8 page
Theory of Antiferromagnetic Order in High-Tc Oxides: An Approach Based on Ginzburg-Landau Expansion
The mean-field phase diagram of antiferromagnetic order in t-J model has been
examined, using the free energy obtained by Ginzburg-Landau (GL) expansion. We
extended the usual GL theory in two ways: First, we have included higher order
terms with respect to the spatial derivative (or wave number) to incorporate
the incommensurate antiferromagnetic order. Second, we have also included
higher order terms with respect to the order parameter amplitude, in order to
treat the first order phase transition between paramagnetic and
antiferromagnetic phase, which appears at some doping rates. We found the
possibility of tricritical point and critical endpoint in the magnetic phase
diagram of the high-Tc oxides associated with the commensurate and
incommensurate antiferromagnetic order. The possible effects of thermal
fluctuations and randomness (spin glass) are also discussed qualitatively based
on the GL free energy.Comment: 7 pages, 3 figure
Planetary System Formation in Protoplanetary Disk around HL Tauri
We re-process the Atacama Large Millimeter/Submillimeter Array (ALMA)
long-baseline science verification data taken toward HL Tauri. As shown by the
previous work, we confirm that the high spatial resolution (~ 0."019,
corresponding to ~ 2.7 AU) dust continuum images at \lambda = 0.87, 1.3, and
2.9 mm exhibit a multiple ring-like gap structure in the circumstellar disk.
Assuming that the observed gaps are opened up by currently forming, unseen
bodies, we estimate the mass of such hypothetical bodies based on following two
approaches; the Hill radius analysis and a more elaborated approach developed
from the angular momentum transfer analysis in gas disks. For the former, the
measured gap widths are used for calibrating the mass of the bodies, while for
the latter, the measured gap depths are utilized. We show that their masses are
likely comparable to or less than the mass of Jovian planets, and then discuss
an origin of the observed gap structure. By evaluating Toomre's gravitational
instability (GI) condition and cooling effect, we find that the GI might be a
possible mechanism to form the bodies in the outer region of the disk. As the
disk might be gravitationally unstable only in the outer region of the disk,
inward planetary migration would be needed to construct the current
architecture of the hypothetical bodies. We estimate the gap-opening mass and
show that type II migration might be able to play such a role. Combining GIs
with inward migration, we conjecture that all of the observed gaps may be a
consequence of bodies that might have originally formed at the outer part of
the disk, and have subsequently migrated to the current locations. While ALMA's
unprecedented high spatial resolution observations can revolutionize our
picture of planet formation, more dedicated observational and theoretical
studies are needed in order to fully understand the HL Tau images.Comment: 12 pages, 5 figure
Phase-Sensitive Flux-Flow resistivity in Unconventional Superconductors
We theoretically investigate the magnetic-field-angle dependence of the
flux-flow resistivity in unconventional superconductors. Two
contributions to are considered: one is the quasiparticle (QP)
relaxation time and the other is , which is a counterpart to the interlevel spacing of the QP bound states
in the quasiclassical approach. Here, denotes the position on
a Fermi surface. Numerical calculations are conducted for a line-node s-wave
and a d-wave pair potential with the same anisotropy of their amplitudes, but
with a sign change only for a d-wave one. We show that the field-angle
dependence of differs prominently between s-wave and d-wave
pairs, reflecting the phase of the pair potentials. We also discuss the case
where is constant and compare it with the more general case where
depends on .Comment: 4 pages, 3 figures; Proc. of LT2
- β¦