24,126 research outputs found
Modern calorimetry: going beyond tradition
Calorimetry has been a traditional tool for obtaining invaluable
thermodynamic information of matter, the free energy. We describe recent
efforts to go beyond this traditional calorimetry: After introducing dynamic
heat capacity, we present the various experimental methods to measure it.
Applications and future prospects are also given.Comment: 13 pages, 1 figur
Robustness of the in-degree exponent for the world-wide web
We consider a stochastic model for directed scale-free networks following
power-laws in the degree distributions in both incoming and outgoing
directions. In our model, the number of vertices grow geometrically with time
with growth rate p. At each time step, (i) each newly introduced vertex is
connected to a constant number of already existing vertices with the
probability linearly proportional to the in-degree of a selected vertex, and
(ii) each existing vertex updates its outgoing edges through a stochastic
multiplicative process with mean growth rate of outgoing edges g and variance
. Using both analytic treatment and numerical simulations, we show
that while the out-degree exponent depends on the
parameters, the in-degree exponent has two distinct values,
for and 1 for , independent of different
parameters values. The latter case has logarithmic correction to the power-law.
Since the vertex growth rate p is larger than the degree growth rate g for the
world-wide web (www) nowadays, the in-degree exponent appears robust as
for the www
Graphene nanosystems and low-dimensional Chern-Simons topological insulators
A graphene nanoribbon is a good candidate for a Chern-Simons
topological insulator since it obeys particle-hole symmetry. We show that in a
finite semiconducting armchair ribbon, which has two zigzag edges and two
armchair edges, a Chern-Simons topological insulator is indeed realized
as the length of the armchair edges becomes large in comparison to that of the
zigzag edges. But only a quasi-topological insulator is formed in a metallic
armchair ribbon with a pseudogap. In such systems a zigzag edge acts like a
domain wall, through which the polarization changes from to , forming
a fractional charge of one-half. When the lengths of the zigzag edges and the
armchair edges are comparable a rectangular graphene sheet (RGS) is realized,
which also possess particle-hole symmetry. We show that it is a
Chern-Simons topological insulator. We find that the cyclic Berry phase of
states of a RGS is quantized as or (mod ), and that the Berry
phases of the particle-hole conjugate states are equal each other. By applying
the Atiyah-Singer index theorem to a rectangular ribbon and a RGS we find that
the lower bound on the number of nearly zero energy end states is approximately
proportional to the length of the zigzag edges. However, there is a correction
to this index theorem due to the effects beyond the effective mass
approximation.Comment: 7 pages, 9figure
Topological end states and Zak phase of rectangular armchair ribbon
We consider the end states of a half-filled rectangular armchair graphene
ribbon (RAGR) in a staggered potential. Taking electron-electron interactions
into account we find that, as the strength of the staggered potential varies,
three types of couplings between the end states can occur: antiferromagnetic
without or with spin splitting, and paramagnetic without spin-splitting. We
find that a spin-splitting is present only in the staggered potential region
. The transition from the antiferromagnetic state at
to the paramagnetic state goes through an intermediate spin-split
antiferromagnetic state, and this spin-splitting disappears suddenly at
. For small and large values of the end charge of a RAGR can
be connected to the Zak phase of the periodic armchair graphene ribbon (PARG)
with the same width, and it varies continuously as the strength of the
potential changes
Gap states and edge properties of rectangular graphene quantum dot in staggered potential
We investigate edge properties of a gapful rectangular graphene quantum dot
in a staggered potential. In such a system gap states with discrete and closely
spaced energy levels exist that are spatially located on the left or right
zigzag edge. We find that, although the bulk states outside the energy gap are
nearly unaffected, spin degeneracy of each gap state is lifted by the staggered
potential. We have computed the occupation numbers of spin-up and -down gap
states at various values of the strength of the staggered potential. The
electronic and magnetic properties of the zigzag edges depend sensitively on
these numbers. We discuss the possibility of applying this system as a single
electron spintronic device
Transverse Acoustic Phonon Transistor Based on Asymmetric Potential Distribution
We experimentally demonstrate a transverse acoustic (TA) phonon transistor.
Phonons are coherently initiated by femtosecond photocarrier screening on
potential gradients. Although translational symmetry within the isotropic plane
normally prohibits optical generation of TA phonons, we show that the combined
application of an external bias in the vertical and lateral directions can
break the selection rules, generating the forbidden TA mode. The amplitude and
on-state time of the TA mode can be modulated by the external field strength
and size of the laterally biased region. The observed frequency shift with an
external bias as well as the strong geometrical dependence confirm the role of
the asymmetric potential distribution in electrically manipulating the crystal
symmetry to control and activate the transistor.Comment: 5 pages, 5 figure
The Commercial Stainless Steel Tube Enveloping Technique for MgB2
A commercial stainless steel tube was employed to synthesize MgB2. The
specimen was prepared by a stoichiometric mixture of Mg and B. The specimen
that had been enveloped in the commercial stainless steel tube was synthesized
for 2 hours at 1193 K. X-ray spectra showed there were no second phases like
MgO. The transition temperature of the specimen was 37.5 K with a sharp
transition width within 1K. The specimen showed a good connection between
grains and critical current density as calculated with the Bean model is more
than 100,000 A/cm2 at 20 K and in zero field.Comment: 6 pages, 5 figure
Electroweak phase transition in an extension of the standard model with a real Higgs singlet
The Higgs potential of the standard model with an additional real Higgs
singlet is studied in order to examine if it may allow the strongly first order
electroweak phase transition. It is found that there are parameter values for
which this model at the one-loop level with finite temperature effect may allow
the desired phase transition. Those parameter values also predict that the
masses of the neutral scalar Higgs bosons of the model are consistent with the
present experimental bound, and that their productions in collisions
may be searched at the proposed ILC with = 500 GeV in the near
future.Comment: 25 pages, 12 figure
Radiative CP violation in the Higgs sector of the Next-to-minimal supersymmetric model
We investigate the neutral Higgs sector in the next-to-minimal supersymmetric
standard model (NMSSM) with explicit CP violation at the one-loop level by
using the effective potential method. In general, explicit CP violation is
possible at the tree-level in the Higgs potential of the NMSSM, which may
possess a complex phase. The tree-level Higgs potential can be made CP
conserving by assuming that all the relevant parameters are real. However, the
CP-conserving Higgs potential at the tree level may still develop complex
phases at the one-loop level through radiative corrections. These complex
phases exhibit explicit CP violation through the mixings between the scalar and
pseudoscalar Higgs bosons. One of the complex phases arise in the neutralino
sector. For a particular choice of the relevant parameter values, the
scalar-pseudoscalar mixings are found to strongly dependent on this phase.
Meanwhile, for a wide region in the parameter space, the ratios of CP mixing
elements of the neutral Higgs boson mass matrix to CP non-mixing ones increase
weakly against this phase. Also, the neutral Higgs boson masses are roughly
stable against the variation of this phase.Comment: Latex, 48 pages, 1 table, 17 figures, Changed conten
Peltier ac calorimeter
A new ac calorimeter, utilizing the Peltier effect of a thermocouple junction
as an ac power source, is described. This Peltier ac calorimeter allows to
measure the absolute value of heat capacity of small solid samples with
sub-milligrams of mass. The calorimeter can also be used as a dynamic one with
a dynamic range of several decades at low frequencies.Comment: 12 pages, 4 figure
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