2,067 research outputs found
Lectures on Supersymmetry Breaking
We review the subject of spontaneous supersymmetry breaking. First we
consider supersymmetry breaking in a semiclassical theory. We illustrate it
with several examples, demonstrating different phenomena, including metastable
supersymmetry breaking. Then we give a brief review of the dynamics of
supersymmetric gauge theories. Finally, we use this dynamics to present various
mechanisms for dynamical supersymmetry breaking. These notes are based on
lectures given by the authors in 2007, at various schools.Comment: 47 pages. v2: minor correction
Do thermodynamically stable rigid solids exist?
Customarily, crystalline solids are defined to be {\em rigid} since they
resist changes of shape determined by their boundaries. However, rigid solids
cannot exist in the thermodynamic limit where boundaries become irrelevant.
Particles in the solid may rearrange to adjust to shape changes eliminating
stress without destroying crystalline order. Rigidity is therefore valid only
in the {\em metastable} state that emerges because these particle
rearrangements in response to a deformation, or strain, are associated with
slow collective processes. Here, we show that a thermodynamic collective
variable may be used to quantify particle rearrangements that occur as a solid
is deformed at zero strain rate. Advanced Monte Carlo simulation techniques are
then employed to obtain the equilibrium free energy as a function of this
variable. Our results lead to a new view on rigidity: While at zero strain a
rigid crystal coexists with one that responds to infinitesimal strain by
rearranging particles and expelling stress, at finite strain the rigid crystal
is metastable, associated with a free energy barrier that decreases with
increasing strain. The rigid phase becomes thermodynamically stable by
switching on an external field, which penalises particle rearrangements. This
produces a line of first-order phase transitions in the field - strain plane
that intersects the origin. Failure of a solid once strained beyond its elastic
limit is associated with kinetic decay processes of the metastable rigid
crystal deformed with a finite strain rate. These processes can be understood
in quantitative detail using our computed phase diagram as reference.Comment: 11 pages, 7 figure
Exploring the high-pressure materials genome
A thorough in situ characterization of materials at extreme conditions is
challenging, and computational tools such as crystal structural search methods
in combination with ab initio calculations are widely used to guide experiments
by predicting the composition, structure, and properties of high-pressure
compounds. However, such techniques are usually computationally expensive and
not suitable for large-scale combinatorial exploration. On the other hand,
data-driven computational approaches using large materials databases are useful
for the analysis of energetics and stability of hundreds of thousands of
compounds, but their utility for materials discovery is largely limited to
idealized conditions of zero temperature and pressure. Here, we present a novel
framework combining the two computational approaches, using a simple linear
approximation to the enthalpy of a compound in conjunction with
ambient-conditions data currently available in high-throughput databases of
calculated materials properties. We demonstrate its utility by explaining the
occurrence of phases in nature that are not ground states at ambient conditions
and estimating the pressures at which such ambient-metastable phases become
thermodynamically accessible, as well as guiding the exploration of
ambient-immiscible binary systems via sophisticated structural search methods
to discover new stable high-pressure phases.Comment: 14 pages, 6 figure
Thermodynamic phase-field model for microstructure with multiple components and phases: the possibility of metastable phases
A diffuse-interface model for microstructure with an arbitrary number of
components and phases was developed from basic thermodynamic and kinetic
principles and formalized within a variational framework. The model includes a
composition gradient energy to capture solute trapping, and is therefore suited
for studying phenomena where the width of the interface plays an important
role. Derivation of the inhomogeneous free energy functional from a Taylor
expansion of homogeneous free energy reveals how the interfacial properties of
each component and phase may be specified under a mass constraint. A diffusion
potential for components was defined away from the dilute solution limit, and a
multi-obstacle barrier function was used to constrain phase fractions. The
model was used to simulate solidification via nucleation, premelting at phase
boundaries and triple junctions, the intrinsic instability of small particles,
and solutal melting resulting from differing diffusivities in solid and liquid.
The shape of metastable free energy surfaces is found to play an important role
in microstructure evolution and may explain why some systems premelt at phase
boundaries and phase triple junctions while others do not.Comment: 14 pages, 8 figure
Vevacious: A Tool For Finding The Global Minima Of One-Loop Effective Potentials With Many Scalars
Several extensions of the Standard Model of particle physics contain
additional scalars implying a more complex scalar potential compared to that of
the Standard Model. In general these potentials allow for charge and/or color
breaking minima besides the desired one with correctly broken SU(2)_L times
U(1)_Y . Even if one assumes that a metastable local minimum is realized, one
has to ensure that its lifetime exceeds that of our universe. We introduce a
new program called Vevacious which takes a generic expression for a one-loop
effective potential energy function and finds all the tree-level extrema, which
are then used as the starting points for gradient-based minimization of the
one-loop effective potential. The tunneling time from a given input vacuum to
the deepest minimum, if different from the input vacuum, can be calculated. The
parameter points are given as files in the SLHA format (though is not
restricted to supersymmetric models), and new model files can be easily
generated automatically by the Mathematica package SARAH. This code uses
HOM4PS2 to find all the minima of the tree-level potential, PyMinuit to follow
gradients to the minima of the one-loop potential, and CosmoTransitions to
calculate tunneling times.Comment: 44 pages, 1 figure, manual for publicly available software, v2
corresponds to version accepted for publication in EPJC [clearer explanation
of scale dependence and region of validity, explicit mention that SLHA files
should have blocks matching those expected by model files, updated
references
Generation of Coherent Structures After Cosmic Inflation
We investigate the nonlinear dynamics of hybrid inflation models, which are
characterized by two real scalar fields interacting quadratically. We start by
solving numerically the coupled Klein-Gordon equations in static Minkowski
spacetime, searching for possible coherent structures. We find long-lived,
localized configurations, which we identify as a new kind of oscillon. We
demonstrate that these two-field oscillons allow for "excited" states with much
longer lifetimes than those found in previous studies of single-field
oscillons. We then solve the coupled field equations in an expanding
Friedmann-Robertson-Walker spacetime, finding that as the field responsible for
inflating the Universe rolls down to oscillate about its minimum, it triggers
the formation of long-lived two-field oscillons, which can contribute up to 20%
of the total energy density of the Universe. We show that these oscillons
emerge for a wide range of parameters consistent with WMAP 7-year data. These
objects contain total energy of about 25*10^20 GeV, localized in a region of
approximate radius 6*10^-26 cm. We argue that these structures could have
played a key role during the reheating of the Universe.Comment: 12 pages, 10 .pdf figures, uses RevTex4; v2: expanded discussion in
section IV, accepted for publication in Phys.Rev. D. Results remain the sam
Chiral liquid crystal colloids
Colloidal particles disturb the alignment of rod-like molecules of liquid
crystals, giving rise to long-range interactions that minimize the free energy
of distorted regions. Particle shape and topology are known to guide this
self-assembly process. However, how chirality of colloidal inclusions affects
these long-range interactions is unclear. Here we study the effects of
distortions caused by chiral springs and helices on the colloidal
self-organization in a nematic liquid crystal using laser tweezers, particle
tracking and optical imaging. We show that chirality of colloidal particles
interacts with the nematic elasticity to predefine chiral or racemic colloidal
superstructures in nematic colloids. These findings are consistent with
numerical modelling based on the minimization of Landau-de Gennes free energy.
Our study uncovers the role of chirality in defining the mesoscopic order of
liquid crystal colloids, suggesting that this feature may be a potential tool
to modulate the global orientated self-organization of these systems
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