1,216 research outputs found
How to Improve The Accuracy of Equilibrium Molecular Dynamics For Computation of Thermal Conductivity?
Equilibrium molecular dynamics (EMD) simulations through Green-Kubo formula
(GKF) have been widely used in the study of thermal conductivity of various
materials. However, there exist controversial simulation results which have
huge discrepancies with experimental ones in literatures. In this paper, we
demonstrate that the fluctuation in calculated thermal conductivity is due to
the uncertainty in determination of the truncation time, which is related to
the ensemble and size dependent phonon relaxation time. We thus propose a new
scheme in the direct integration of heat current autocorrelation function
(HCACF) and a nonzero correction in the double-exponential-fitting of HCACF to
describe correctly the contribution to thermal conductivity from low frequency
phonons. By using crystalline Silicon (Si) and Germanium (Ge) as examples, we
demonstrate that our method can give rise to the values of thermal conductivity
in an excellent agreement with experimental ones.Comment: 21 pages, 3 figure
Thermal conduction of carbon nanotubes using molecular dynamics
The heat flux autocorrelation functions of carbon nanotubes (CNTs) with
different radius and lengths is calculated using equilibrium molecular
dynamics. The thermal conductance of CNTs is also calculated using the
Green-Kubo relation from the linear response theory. By pointing out the
ambiguity in the cross section definition of single wall CNTs, we use the
thermal conductance instead of conductivity in calculations and discussions. We
find that the thermal conductance of CNTs diverges with the CNT length. After
the analysis of vibrational density of states, it can be concluded that more
low frequency vibration modes exist in longer CNTs, and they effectively
contribute to the divergence of thermal conductance.Comment: 15 pages, 6 figures, submitted to Physical Review
Neutrino masses and CDM in a non-supersymmetric model
We propose a model for neutrino mass generation based on both the tree-level
seesaw mechanism with a single right-handed neutrino and one-loop radiative
effects in a non-supersymmetric framework. The generated mass matrix is
composed of two parts which have the same texture and produce neutrino mass
eigenvalues and mixing suitable for the explanation of neutrino oscillations.
The model has a good CDM candidate which contributes to the radiative neutrino
mass generation. The stability of the CDM candidate is ensured by which
is the residual symmetry of a spontaneously broken U(1). We discuss
the values of and also estimate the masses of the relevant fields to
realize an appropriate abundance of the CDM.Comment: 13 pages, 2 figures, published versio
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Nanometre-scale thermometry in a living cell
Sensitive probing of temperature variations on nanometre scales is an outstanding challenge in many areas of modern science and technology. In particular, a thermometer capable of subdegree temperature resolution over a large range of temperatures as well as integration within a living system could provide a powerful new tool in many areas of biological, physical and chemical research. Possibilities range from the temperature-induced control of gene expression and tumour metabolism to the cell-selective treatment of disease and the study of heat dissipation in integrated circuits. By combining local light-induced heat sources with sensitive nanoscale thermometry, it may also be possible to engineer biological processes at the subcellular level. Here we demonstrate a new approach to nanoscale thermometry that uses coherent manipulation of the electronic spin associated with nitrogen–vacancy colour centres in diamond. Our technique makes it possible to detect temperature variations as small as 1.8 mK (a sensitivity of in an ultrapure bulk diamond sample. Using nitrogen–vacancy centres in diamond nanocrystals (nanodiamonds), we directly measure the local thermal environment on length scales as short as 200 nanometres. Finally, by introducing both nanodiamonds and gold nanoparticles into a single human embryonic fibroblast, we demonstrate temperature-gradient control and mapping at the subcellular level, enabling unique potential applications in life sciences.Physic
Electroweak Symmetry Breaking induced by Dark Matter
The mechanism behind Electroweak Symmetry Breaking (EWSB) and the nature of
dark matter (DM) are currently among the most important issues in high energy
physics. Since a natural dark matter candidate is a weakly interacting massive
particle or WIMP, with mass around the electroweak scale, it is clearly of
interest to investigate the possibility that DM and EWSB are closely related.
In the context of a very simple extension of the Standard Model, the Inert
Doublet Model, we show that dark matter could play a crucial role in the
breaking of the electroweak symmetry. In this model, dark matter is the
lightest component of an inert scalar doublet. The coupling of the latter with
the Standard Model Higgs doublet breaks the electroweak symmetry at one-loop,
"a la Coleman-Weinberg". The abundance of dark matter, the breaking of the
electroweak symmetry and the constraints from electroweak precision
measurements can all be accommodated by imposing an (exact or approximate)
custodial symmetry.Comment: 4 pages, no figure, one tabl
S_3-flavour symmetry as realized in lepton flavour violating processes
A variety of lepton flavour violating effects related to the recent discovery
of neutrino oscillations and mixings is here systematically discussed in terms
of an S_3-flavour permutational symmetry. After a brief review of some relevant
results on lepton masses and mixings, that had been derived in the framework of
a Minimal S_3-Invariant Extension of the Standard Model, we derive explicit
analytical expressions for the matrices of the Yukawa couplings and compute the
branching ratios of some selected flavour changing neutral current (FCNC)
processes, as well as, the contribution of the exchange of neutral flavour
changing scalars to the anomaly of the muon's magnetic moment as functions of
the masses of the charged leptons and the neutral Higgs bosons. We find that
the S_3 x Z_2 flavour symmetry and the strong mass hierarchy of the charged
leptons strongly suppress the FCNC processes in the leptonic sector well below
the present experimental upper bounds by many orders of magnitude. The
contribution of FCNC to the anomaly of the muon's magnetic moment is small but
non-negligible.Comment: 23 pages, one figure. To appear in J. Phys A: Mathematical and
Theoretical (SPE QTS5
The flavour symmetry: Neutrino masses and mixings
We discuss the neutrino masses and mixings as the realization of an
flavour permutational symmetry in two models, namely the Standard Model and an
extension of the Standard Model with three Higgs doublets. In the
Standard Model, mass matrices of the same generic form are obtained for the
neutrinos and charged leptons when the flavour symmetry is broken
sequentially. In the minimal -symmetric extension of the Standard Model,
the symmetry is left unbroken, and the concept of flavour is extended to
the Higgs sector by introducing in the theory three Higgs fields which are
SU(2) doublets. In both models, the mass matrices of the neutrino and charged
leptons are reparametrized in terms of their eigenvalues, and exact, explicit
analytical expressions for the neutrino mixing angles as functions of the
masses of neutrinos and charged leptons are obtained. In the case of the
Standard Model, from a fit of the theoretical expressions of the
lepton mixing matrix to the values extracted from experiment, the numerical
values of the neutrino mixing angles are obtained in excellent agreement with
experimental data. In the extension of the Standard Model, if two of the
right handed neutrinos masses are degenerate, the reactor and atmospheric
mixing angles are determined by the masses of the charged leptons, yielding
in excellent agreement with experimental data, and
different from zero but very small. If the masses of the three right handed
neutrinos are assumed to be different, then it is possible to get
also in very good agreement with experimental data. We also show the branching
ratios of some selected flavour changing neutral currents (FCNC) process as
well as the contribution of the exchange of a neutral flavour changing scalar
to the anomaly of the magnetic moment of the muon.Comment: Accepted for publication in Fortschritte der Physik. Some typos was
corrected. arXiv admin note: text overlap with arXiv:1004.3781,
arXiv:0712.179
Large Thermoelectric Power Factor in TiS2 Crystal with Nearly Stoichiometric Composition
A TiS crystal with a layered structure was found to have a large
thermoelectric power factor.The in-plane power factor at 300 K is
37.1~W/Kcm with resistivity () of 1.7 mcm and
thermopower () of -251~V/K, and this value is comparable to that of the
best thermoelectric material, BiTe alloy. The electrical
resistivity shows both metallic and highly anisotropic behaviors, suggesting
that the electronic structure of this TiS crystal has a
quasi-two-dimensional nature. The large thermoelectric response can be ascribed
to the large density of state just above the Fermi energy and inter-valley
scattering. In spite of the large power factor, the figure of merit, of
TiS is 0.16 at 300 K, because of relatively large thermal conductivity,
68~mW/Kcm. However, most of this value comes from reducible lattice
contribution. Thus, can be improved by reducing lattice thermal
conductivity, e.g., by introducing a rattling unit into the inter-layer sites.Comment: 11 pages, 4 figures, to be published in Physical Review
Anomalous Hall Effect in Ferromagnetic Metals: Role of Phonons at Finite Temperature
The anomalous Hall effect in a multiband tight-binding model is numerically
studied taking into account both elastic scattering by disorder and inelastic
scattering by the electron-phonon interaction. The Hall conductivity is
obtained as a function of temperature , inelastic scattering rate ,
chemical potential , and impurity concentration . We find
that the new scaling law holds over a wide range of these parameters;
, with () being the
conductivity tensor (with only elastic scattering), which corresponds to the
recent experimental observation [Phys. Rev. Lett. {\bf 103} (2009) 087206]. The
condition of this scaling is examined. Also, it is found that the intrinsic
mechanism depends on temperature under a resonance condition.Comment: 5 figure
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