- 2/18/2009 (Wed.) 2:20 ~ 3:20 P.M., Dr. Ching-Yuan Huang (Department of Physics and Astronomy, Iowa State University)
Place : Rm. 833, CCMS & New Physics Building, NTU
Title : Gamma-rays produced in cosmic-ray interactions and the TeV-band spectrum of RX J1713.7-3946
abstract, slides, pic
In this talk, the individual contribution to diffuse gamma-ray emission from
the secondary products in hadronic interactions generated by cosmic rays
(CRs) is discussed, in addition to the contribution of pi0 decay. For this
purpose, the Monte Carlo particle collision code DPMJET3.04 is employed to
determine the multiplicity spectra of various secondary particles with
gammas as the final decay state, that result from inelastic collisions
between cosmic-ray protons and helium nuclei and the ISM with standard
composition. An easy-to-use gamma-ray production matrix for cosmic rays with
energies up to about 10 PeV is derived.
This production matrix is applied to the GeV excess in diffuse galactic
gamma-rays observed by EGRET. Although the non-pi0 decay components have
contributed to the total emission with a different spectrum from the
pi0-decay component, they are insufficient to explain the GeV excess.
Finally, we test the hypothesis that the TeV-band gamma-ray emission of the
shell-type SNR RX J1713.7-3946 observed with HESS is caused by
shock-accelerated hadronic cosmic rays. This scenario implies a very high
efficacy of particle acceleration, so the particle spectrum is expected to
continuously harden toward high energies on account of cosmic-ray
modification of the shock. Using the chi2 statistics we find that a
continuously softening spectrum is strongly preferred, in contrast to
expectations. A hardening spectrum has about 1% probability
to explain the HESS data, but then only if a hard cut-off at 50-100 TeV is
imposed on the particle spectrum.
- 2/25/2009 (Wed.) 2:20 ~ 3:20 P.M., 溫文鈺 博士 Dr. Wen-Yu Wen (台灣大學物理系 NTU)
Place : Rm. 833, CCMS & New Physics Building, NTU
Title : Entropy for 5D Black Holes and Black Rings
abstract, slides,
Black objects in higher dimensions have richer horizon topology other than the
sphere. In this talk, I will review the string theory construction of supersymmetric
black holes and black rings in 5D and discuss the recent Kerr/CFT correspondence inmicrostates counting.
- 3/4/2009 (Wed.) 2:20 ~ 3:20 P.M., Dr. Jeng-Da Chai (蔡政達) (Department of Chemistry, Berkeley)
Place : Rm. 833, CCMS & New Physics Building, NTU
Title : Advances in Density Functionals
abstract, slides,
In the last two decades, density functional theory (DFT) based on the Kohn-Sham
approach has been the most popular quantum-mechanical method for calculations on
large-scale systems. As a result, developing an accurate exchange-correlation energy
functional continues being the subject of intense current interest. In this talk, I
will describe recent developments of new functionals that are free of self-interaction
error at long range [1,2,3,4]. These new functionals are shown to be accurate in
thermochemistry, kinetics, and noncovalent interactions, when compared with common
hybrid density functionals. The qualitative failures of the commonly used hybrid
density functionals in some "difficult problems", such as dissociation of symmetric
radical cations and long-range charge-transfer excitations, are significantly reduced
by these new functionals. Prospects for further improvements are discussed.
References:
[1] J.-D. Chai and M. Head-Gordon, J. Chem. Phys. 128, 084106 (2008).
[2] J.-D. Chai and M. Head-Gordon, Phys. Chem. Chem. Phys. 10, 6615 (2008).
[3] J.-D. Chai and M. Head-Gordon, Chem. Phys. Lett. 467, 176 (2008).
[4] J.-D. Chai and M. Head-Gordon, (manuscript to be submitted).
-
3/11/2009 (Wed.)
the special seminar
-
3/18/2009 (Wed.) 2:20 ~ 3:20 P.M., Prof. Ying-Jer Kao (高英哲) (Department of Physics, NTU)
Place : Rm. 833, CCMS & New Physics Building, NTU
Title : Variational approaches for quantum spin systems based on tensor networks
abstract, slides,
We present variational schemes based on tensor networks in one and two
dimensions. First, variational QMC studies based on matrix product
states of one-dimensional antiferromagnetic Heisenberg and J1-J2
models with PBC is discussed. We show the matrices can be optimized
not just for the ground state, but also, simultaneously, for the
lowest states in several different lattice and spin symmetry sectors.
Second, we will present a variational scheme of contracting a
square-lattice tensor network in two dimensions, based on auxiliary
tensors accomplishing successive truncations of 8-index tensors for
2x2 plaquettes into 4-index tensors. Application to the transverse
Ising model is discussed.
-
3/25/2009 (Wed.) 2:20 ~ 3:20 P.M., Prof. Fa-Yueh Wu (Northeastern U.)
Place : Rm. 833, CCMS & New Physics Building, NTU
Title : Knot theory and statistical mechanics
abstract, slides,
A knot is a loop of line, which generally is “knotted”, in the
3-dimensional space. A knot invariant is an algebraic entity which is
intrinsic to a knot. Mathematicians have been interested in finding knot
invariants since the early 20-th century, but progress was slow and limited
due to the complexity of knots. A breakthrough occurred in 1985 when Jones
discovered a new invariants now known as the Jones polynomial (for which
Jones was awarded the Fields medal, the “Nobel prize” in mathematics).
Subsequently, it was discovered that the Jones polynomial is related to the
Potts model, and that it is a special case of a host of new knot invariants
that can be constructed from soluble models in statistical mechanics.
This talk explores the connection of knot invariants with statistical
mechanics. The talk begins with a brief review of elements of knot theory
and relevant results in statistical mechanics. It is followed by a
discussion on how knot invariants can be constructed by applying techniques
used in solving lattice models. The talk assumes no prior knowledge of knot
theory.
-
4/1/2009 (Wed.) 2:20 ~ 3:20 P.M.
particle spring school 3/31-4/3
no seminar scheduled
-
4/8/2009 (Wed.) 2:20 ~ 3:20 P.M., PeiJen Lin (NCTS)
Place : Rm. 833, CCMS & New Physics Building, NTU
Title : Superconductor Vortex Fluctuations
abstract, slides,
Type-II superconductors are characterized by their confinement of
applied magnetic fields into vortices. This vortex matter can be
modeled using Ginzburg-Landau theory, in order to describe its
fluctuations, phases and dynamics. To begin with, an introduction to
the empirical properties of vortex matter and a review of the methods
of Ginzburg-Landau theory will be presented, along with a discussion
of fluctuations and phases. Liquid vortex phases in layered
superconductors will then be discussed, in particular the effective
2d-3d nature of such systems and properties of the melting transition.
Next, solid phases in the vortex matter of an anisotropic
superconductor and the influence of defects will be treated. Finally,
some dynamical properties of vortex matter will be introduced in the
context of time-dependent Ginzburg-Landau theory, including the Nernst
effect and response to oscillatory applied fields. Type-II superconductors are characterized by their confinement of
applied magnetic fields into vortices. This vortex matter can be
modeled using Ginzburg-Landau theory, in order to describe its
fluctuations, phases and dynamics. To begin with, an introduction to
the empirical properties of vortex matter and a review of the methods
of Ginzburg-Landau theory will be presented, along with a discussion
of fluctuations and phases. Liquid vortex phases in layered
superconductors will then be discussed, in particular the effective
2d-3d nature of such systems and properties of the melting transition.
Next, solid phases in the vortex matter of an anisotropic
superconductor and the influence of defects will be treated. Finally,
some dynamical properties of vortex matter will be introduced in the
context of time-dependent Ginzburg-Landau theory, including the Nernst
effect and response to oscillatory applied fields.
-
4/15/2009 (Wed.) 2:20 ~ 3:20 P.M., Prof. TC Yuan (AS)
Place : Rm. 833, CCMS & New Physics Building, NTU
Title : Double Action Dark Matter, PAMELA and ATIC
abstract, slides,
-
4/22/2009 (Wed.) 2:20 ~ 3:20 P.M.
Place : Rm. 833, CCMS & New Physics Building, NTU
Student Fellowship Award Ceremony and Best Theory Paper Presentation
-
4/29/2009 (Wed.) 2:20 ~ 3:20 P.M., Prof. Pik-Yin Lai (NCU)
Place : Rm. 833, CCMS & New Physics Building, NTU
Title : Nonlinear dynamics of Granular Clock and Temperature oscillations
abstract,slides,
Although both granular gases (GG) and molecular gases (MG) are characterized by random
motions of
their constituents, phenomena not possible for MG, such as clustering and Maxwell's
demon and granular oscillations are reported in GG. The origin of these intriguing
phenomena is the dissipative collisions in GG which are coupled to the local density
of the GG in a spatially extended or compartmentalized system. Systems with two or
more types of grains are especially interesting because the asymmetry in the
dissipative collisions between different types of grains can lead to oscillations and
even more interesting dynamics. Flux models with different granular temperatures for
different types of grains are employed together with nonlinear dynamics theory to
understand and explore these phenomena.
-
5/6/2009 (Wed.) 2:20 ~ 3:20 P.M., Dr. So Takei (Max Planck Institute, Stuttgart, Germany)
Place : Rm. 833, CCMS & New Physics Building, NTU
Title : Order parameter dynamics near nonequilibrium quantum criticality
abstract,slides,
A theory is presented of quantum criticality in open (coupled to reservoirs)
itinerant electron magnets, with nonequilibrium drive provided
by current flow across the system. Both departures from equilibrium
at conventional (equilibrium) quantum critical points and the physics
of phase transitions induced by the nonequilibrium drive are treated.
Nonequilibrium-induced phase transitions are found to have the same
leading critical behavior as conventional thermal phase transitions.
The theory is also extended to the case of a coupled bilayer system
of itinerant electron magnets where coupled critical dynamics between
two order parameters becomes possible.
-
5/13/2009 (Wed.) 2:20 ~ 3:20 P.M., Prof. Seokcheon Lee (AS)
Place : Rm. 833, CCMS & New Physics Building, NTU
Title : Scalar-Tensor theories of gravity and its application
abstract,slides,
The current observations confirm that the Universe is under
accelertaing phase. Dark energy and Modified gravity are two major
candidates to explain this observations. As a viable model, the modified
gravity theories should be consistent with solar system test and can
produce current accelerating Universe. In addition to this background
evolution, modified gravity imprints some different behaviors in
perturbed observables. These will give us chances to find the origin of
the acceleration of the Universe.
-
5/20/2009 (Wed.)
ppp8, NCKU, 5/20-23
no seminar scheduled
-
5/13/2009 (Wed.) 2:20 ~ 3:20 P.M., Dr. Je-An Gu (NTU)
Place : Rm. 833, CCMS & New Physics Building, NTU
Title : An Approach to Testing Dark Energy by Observations
abstract,slides,pic
An approach to the consistency test of dark energy models with
observations will be presented. To test a dark energy model, we suggest
introducing a characteristic Q(z) that in general varies with the
redshift z but in that model plays the role of a constant distinct
parameter. Then, by reconstructing dQ(z)/dz from observational data and
comparing it with zero we can assess the consistency between data and
the model under consideration. For reconstructing dQ(z)/dz from data, a
parametrization of the relevant physical quantity is required. This
method of the consistency test is efficient because for all models we
invoke the constraint of only a single parameter space that by choice
can be easily access. The general principle of our approach is not
limited to dark energy. It may also be applied to the testing of various
cosmological models and even the models in other fields beyond the scope
of cosmology.
-
6/3/2009 (Wed.)
SSS, 6/2-5
no seminar scheduled
