4,572 research outputs found
Enhancement of the Fractional Quantum Hall State in a Small In-Plane Magnetic Field
Using a 50-nm width, ultra-clean GaAs/AlGaAs quantum well, we have studied
the Landau level filling factor fractional quantum Hall effect in a
perpendicular magnetic field 1.7 T and determined its dependence on
tilted magnetic fields. Contrary to all previous results, the 5/2 resistance
minimum and the Hall plateau are found to strengthen continuously under an
increasing tilt angle (corresponding to an in-plane
magnetic field 0 T). In the same range of
the activation gaps of both the 7/3 and the 8/3 states are found to increase
with tilt. The 5/2 state transforms into a compressible Fermi liquid upon tilt
angle , and the composite fermion series [2+],
1, 2 can be identified. Based on our results, we discuss the relevance of
a Skyrmion spin texture at associated with small Zeeman energy in
wide quantum wells, as proposed by Wjs ., Phys. Rev.
Lett. 104, 086801 (2010).Comment: 5+ pages, 3 figures, accepted for by Phy. Rev. Let
Neumann Heat kernel monotonicity
We prove that the diagonal of the transition probabilities for the
d-dimensional Bessel processes on (0, 1], reflected at 1, which we denote by
, is an increasing function of r for d>2 and that this is false
for d=2
Cobordism of Morse functions on surfaces, the universal complex of singular fibers and their application to map germs
We give a new and simple proof for the computation of the oriented and the
unoriented fold cobordism groups of Morse functions on surfaces. We also
compute similar cobordism groups of Morse functions based on simple stable maps
of 3-manifolds into the plane. Furthermore, we show that certain cohomology
classes associated with the universal complexes of singular fibers give
complete invariants for all these cobordism groups. We also discuss invariants
derived from hypercohomologies of the universal homology complexes of singular
fibers. Finally, as an application of the theory of universal complexes of
singular fibers, we show that for generic smooth map germs g: (R^3, 0) -->
(R^2, 0) with R^2 being oriented, the algebraic number of cusps appearing in a
stable perturbation of g is a local topological invariant of g.Comment: This is the version published by Algebraic & Geometric Topology on 7
April 200
Training Diffusion Models with Reinforcement Learning
Diffusion models are a class of flexible generative models trained with an
approximation to the log-likelihood objective. However, most use cases of
diffusion models are not concerned with likelihoods, but instead with
downstream objectives such as human-perceived image quality or drug
effectiveness. In this paper, we investigate reinforcement learning methods for
directly optimizing diffusion models for such objectives. We describe how
posing denoising as a multi-step decision-making problem enables a class of
policy gradient algorithms, which we refer to as denoising diffusion policy
optimization (DDPO), that are more effective than alternative reward-weighted
likelihood approaches. Empirically, DDPO is able to adapt text-to-image
diffusion models to objectives that are difficult to express via prompting,
such as image compressibility, and those derived from human feedback, such as
aesthetic quality. Finally, we show that DDPO can improve prompt-image
alignment using feedback from a vision-language model without the need for
additional data collection or human annotation.Comment: 20 pages, 12 figure
The use of hydrothermal carbonization to recycle nutrients in algal biofuel production
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/100324/1/ep11812.pd
Two State Behavior in a Solvable Model of -hairpin folding
Understanding the mechanism of protein secondary structure formation is an
essential part of protein-folding puzzle. Here we describe a simple model for
the formation of the -hairpin, motivated by the fact that folding of a
-hairpin captures much of the basic physics of protein folding. We argue
that the coupling of ``primary'' backbone stiffness and ``secondary'' contact
formation (similar to the coupling between the ``secondary'' and ``tertiary''
structure in globular proteins), caused for example by side-chain packing
regularities, is responsible for producing an all-or-none 2-state
-hairpin formation. We also develop a recursive relation to compute the
phase diagram and single exponential folding/unfolding rate arising via a
dominant transition state.Comment: Revised versio
Sandpile groups and spanning trees of directed line graphs
We generalize a theorem of Knuth relating the oriented spanning trees of a
directed graph G and its directed line graph LG. The sandpile group is an
abelian group associated to a directed graph, whose order is the number of
oriented spanning trees rooted at a fixed vertex. In the case when G is regular
of degree k, we show that the sandpile group of G is isomorphic to the quotient
of the sandpile group of LG by its k-torsion subgroup. As a corollary we
compute the sandpile groups of two families of graphs widely studied in
computer science, the de Bruijn graphs and Kautz graphs.Comment: v2 has an expanded section on deletion/contraction for directed
graphs, and a more detailed proof of Theorem 2.3. To appear in Journal of
Combinatorial Theory A
Quantum Relaxation of Magnetisation in Magnetic Particles
At temperatures below the magnetic anisotropy energy, monodomain magnetic
systems (small particles, nanomagnetic devices, etc.) must relax quantum
mechanically. This quantum relaxation must be mediated by the coupling to both
nuclear spins and phonons (and electrons if either particle or substrate is
conducting. We analyze the effect of each of these couplings, and then combine
them. Conducting systems can be modelled by a "giant Kondo" Hamiltonian, with
nuclear spins added in as well. At low temperatures, even microscopic particles
on a conducting substrate (containing only spins) will have their
magnetisation frozen over millenia by a combination of electronic dissipation
and the "degeneracy blocking" caused by nuclear spins. Raising the temperature
leads to a sudden unblocking of the spin dynamics at a well defined
temperature. Insulating systems are quite different. The relaxation is strongly
enhanced by the coupling to nuclear spins. At short times the magnetisation of
an ensemble of particles relaxes logarithmically in time, after an initial very
fast decay; this relaxation proceeds entirely via the nuclear spins. At longer
times phonons take over, but the decay rate is still governed by the
temperature-dependent nuclear bias field acting on the particles - decay may be
exponential or power-law depending on the temperature. The most surprising
feature of the results is the pivotal role played by the nuclear spins. The
results are relevant to any experiments on magnetic particles in which
interparticle dipolar interactions are unimportant. They are also relevant to
future magnetic device technology.Comment: 30 pages, RevTex, e:mail , Submitted to J.Low
Temp.Phys. on 1 Nov. 199
The atomic structure of large-angle grain boundaries and in and their transport properties
We present the results of a computer simulation of the atomic structures of
large-angle symmetrical tilt grain boundaries (GBs) (misorientation
angles \q{36.87}{^{\circ}} and \q{53.13}{^{\circ}}),
(misorientation angles \q{22.62}{^{\circ}} and \q{67.38}{^{\circ}}). The
critical strain level criterion (phenomenological criterion)
of Chisholm and Pennycook is applied to the computer simulation data to
estimate the thickness of the nonsuperconducting layer enveloping
the grain boundaries. The is estimated also by a bond-valence-sum
analysis. We propose that the phenomenological criterion is caused by the
change of the bond lengths and valence of atoms in the GB structure on the
atomic level. The macro- and micro- approaches become consistent if the
is greater than in earlier papers. It is predicted that the
symmetrical tilt GB \theta = \q{53.13}{^{\circ}} should demonstrate
a largest critical current across the boundary.Comment: 10 pages, 2 figure
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Raman-guided subcellular pharmaco-metabolomics for metastatic melanoma cells
Non-invasively probing metabolites within single live cells is highly desired but challenging. Here we utilize Raman spectro-microscopy for spatial mapping of metabolites within single cells, with the specific goal of identifying druggable metabolic susceptibilities from a series of patient-derived melanoma cell lines. Each cell line represents a different characteristic level of cancer cell de-differentiation. First, with Raman spectroscopy, followed by stimulated Raman scattering (SRS) microscopy and transcriptomics analysis, we identify the fatty acid synthesis pathway as a druggable susceptibility for differentiated melanocytic cells. We then utilize hyperspectral-SRS imaging of intracellular lipid droplets to identify a previously unknown susceptibility of lipid mono-unsaturation within de-differentiated mesenchymal cells with innate resistance to BRAF inhibition. Drugging this target leads to cellular apoptosis accompanied by the formation of phase-separated intracellular membrane domains. The integration of subcellular Raman spectro-microscopy with lipidomics and transcriptomics suggests possible lipid regulatory mechanisms underlying this pharmacological treatment. Our method should provide a general approach in spatially-resolved single cell metabolomics studies
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