947 research outputs found
Significance of interface anisotropy in laser induced magnetization precession in ferromagnetic metal films
Laser induced ultrafast demagnetization in ferromagnetic metals was
discovered almost 20 years ago, but currently there is still lack of consensus
on the microscopic mechanism responsible for the corresponding transfer of
angular momentum and energy between electron, lattice and spin subsystems. A
distinct, but intrinsically correlated phenomenon occurring on a longer
timescale is the magnetization precession after the ultrafast demagnetization
process, if a magnetic field is applied to tilt the magnetization vector away
from its easy direction, which can be attributed to the change of anisotropy
after laser heating. In an in-plane magnetized Pt/Co/Pt thin film with
perpendicular interface anisotropy, we found excellent agreement between
theoretical prediction with plausible parameters and experimental data measured
using time resolved magneto-optical Kerr effect. This agreement confirms that
the time evolution of the anisotropy field, which is driven by the interaction
between electrons and phonons, determines the magnetization precession
completely. A detailed analysis shows that, even though the whole sample is
magnetized in-plane, the dynamic interface anisotropy field dictates the
initial phase of the magnetization precession, highlighting the significance of
the interface anisotropy field in laser induced magnetization precession.Comment: 11 pages, 2 figure
Poincare Polynomials and Level Rank Dualities in the Coset Construction
We review the coset construction of conformal field theories; the emphasis is
on the construction of the Hilbert spaces for these models, especially if fixed
points occur. This is applied to the superconformal cosets constructed by
Kazama and Suzuki. To calculate heterotic string spectra we reformulate the
Gepner con- struction in terms of simple currents and introduce the so-called
extended Poincar\'e polynomial. We finally comment on the various equivalences
arising between models of this class, which can be expressed as level rank
dualities. (Invited talk given at the III. International Conference on
Mathematical Physics, String Theory and Quantum Gravity, Alushta, Ukraine, June
1993. To appear in Theor. Math. Phys.)Comment: 14 pages in LaTeX, HD-THEP-93-4
Observation of atom pairs in spontaneous four wave mixing of two colliding Bose-Einstein Condensates
We study atom scattering from two colliding Bose-Einstein condensates using a
position sensitive, time resolved, single atom detector. In analogy to quantum
optics, the process can also be thought of as spontaneous, degenerate four wave
mixing of de Broglie waves. We find a clear correlation between atoms with
opposite momenta, demonstrating pair production in the scattering process. We
also observe a Hanbury Brown and Twiss correlation for collinear momenta, which
permits an independent measurement of the size of the pair production source
and thus the size of the spatial mode. The back to back pairs occupy very
nearly two oppositely directed spatial modes, a promising feature for future
quantum optics experiments.Comment: A few typos have been correcte
Hanbury Brown Twiss effect for ultracold quantum gases
We have studied 2-body correlations of atoms in an expanding cloud above and
below the Bose-Einstein condensation threshold. The observed correlation
function for a thermal cloud shows a bunching behavior, while the correlation
is flat for a coherent sample. These quantum correlations are the atomic
analogue of the Hanbury Brown Twiss effect. We observe the effect in three
dimensions and study its dependence on cloud size.Comment: Figure 1 availabl
Automorphism Modular Invariants of Current Algebras
We consider those two-dimensional rational conformal field theories (RCFTs)
whose chiral algebras, when maximally extended, are isomorphic to the current
algebra formed from some affine non-twisted Kac--Moody algebra at fixed level.
In this case the partition function is specified by an automorphism of the
fusion ring and corresponding symmetry of the Kac--Peterson modular matrices.
We classify all such partition functions when the underlying finite-dimensional
Lie algebra is simple. This gives all possible spectra for this class of RCFTs.
While accomplishing this, we also find the primary fields with second smallest
quantum dimension.Comment: 32 pages, plain Te
Setting Up an Industrial Control Systems Laboratory
With the evolution of Industrial Control Systems, many solutions from vendors are offered for industries. But sadly, most of those solutions are close-sourced, delivering lack of support for third parties who aim to develop Industrial Control Systems further. A start-up company named SecurityMatters needs an industrial instrument to simulate industrial environment to have a better idea how a particular protocol works. The application made in this project was developed using Java programming language to have compatibilities across platforms. An Object-Oriented-Programming and Model-View-Controller pattern are used as well to ensure maintainability. This application can be used to demonstrate capabilities of Modbus protocol and test industrial devices for vulnerabilities
The non-compact elliptic genus: mock or modular
We analyze various perspectives on the elliptic genus of non-compact
supersymmetric coset conformal field theories with central charge larger than
three. We calculate the holomorphic part of the elliptic genus via a free field
description of the model, and show that it agrees with algebraic expectations.
The holomorphic part of the elliptic genus is directly related to an
Appell-Lerch sum and behaves anomalously under modular transformation
properties. We analyze the origin of the anomaly by calculating the elliptic
genus through a path integral in a coset conformal field theory. The path
integral codes both the holomorphic part of the elliptic genus, and a
non-holomorphic remainder that finds its origin in the continuous spectrum of
the non-compact model. The remainder term can be shown to agree with a function
that mathematicians introduced to parameterize the difference between mock
theta functions and Jacobi forms. The holomorphic part of the elliptic genus
thus has a path integral completion which renders it non-holomorphic and
modular.Comment: 13 page
Lattice Analogues of Superconformal Models via Quantum Group Truncation
We obtain lattice models whose continuum limits correspond to
superconformal coset models. This is done by taking the well known vertex model
whose continuum limit is the conformal field theory, and
twisting the transfer matrix and modifying the quantum group truncation. We
find that the natural order parameters of the new models are precisely the
chiral primary fields. The integrable perturbations of the conformal field
theory limit also have natural counterparts in the lattice formulation, and
these can be incorporated into an affine quantum group structure. The
topological, twisted superconformal models also have lattice analogues,
and these emerge as an intermediate part of our analysis.Comment: 25 pages and 2 figure
The Emperor's Last Clothes?
We are in the middle of a remarkable paradigm shift in particle physics, a
shift of opinion that occurred so slowly that some even try to deny that they
changed their minds at all. It concerns a very basic question: can we expect to
derive the laws of particle physics from a fundamental theory? The Standard
Model of particle physics as well as the 1984 string theory revolution provided
ample food for thought about this. The reason this was ignored for so long can
be traced back to an old fallacy: a misguided idea about our own importance.Comment: 88 pages, Review intended for readers with an education in physics.
Preprint Nr. added, some statements corrected, some references added. A short
version (about 20 pages) was published in Rept. Prog. Phys. 71:072201,2008.
July 2010: Footnote added to correct a remark on nuclear bindin
A Minimal Superstring Standard Model I: Flat Directions
Three family SU(3)_C x SU(2)_L x U(1)_Y string models in several
constructions generically possess two features: (i) an extra local anomalous
U(1)_A and (ii) numerous (often fractionally charged) exotic particles beyond
those in the minimal supersymmetric model (MSSM). Recently, we demonstrated
that the observable sector effective field theory of such a free fermionic
string model can reduce to that of the MSSM, with the standard observable gauge
group being just SU(3)_C x SU(2)_L x U(1)_Y and the SU(3)_C x SU(2)_L x
U(1)_Y-charged spectrum of the observable sector consisting solely of the MSSM
spectrum. An example of a model with this property was shown. We continue our
investigation of this model by presenting a large set of different flat
directions of the same model that all produce the MSSM spectrum. Our results
suggest that even after imposing the conditions for the decoupling of exotic
states, there may remain sufficient freedom to satisfy the remaining
phenomenological constraints imposed by the observed data.Comment: 64 pages. Latex. Revisions to match version in Int. J. Mod. Physics
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