1,026 research outputs found
Ordered Loop Current States in Bilayer Graphene
While single-layer graphene shows extraordinary phenomena which are stable
against electronic interactions, the non-interacting state of bilayer graphene
is unstable to infinitesimal interactions leading to one of many possible
exotic states. Indeed a gapped state is found in experiments but none of the
states proposed so far can provide full accounts of its properties. Here we
show that a magnetoelectric (ME) state is consistent with the experimental
observations. This state breaks time-reversal symmetry through a pair of
spontaneously generated current loops in each layer, and has odd-parity with
respect to the two layers. We also suggest further experiments to check whether
the ME state is indeed the gapped state found in experiments.Comment: 8 pages, 10 figure
CampProf: A Visual Performance Analysis Tool for Memory Bound GPU Kernels
Current GPU tools and performance models provide some common architectural insights that guide the programmers to write optimal code. We challenge these performance models, by modeling and analyzing a lesser known, but very severe performance pitfall, called 'Partition Camping', in NVIDIA GPUs. Partition Camping is caused by memory accesses that are skewed towards a subset of the available memory partitions, which may degrade the performance of memory-bound CUDA kernels by up to seven-times. No existing tool can detect the partition camping effect in CUDA kernels.
We complement the existing tools by developing 'CampProf', a spreadsheet based, visual analysis tool, that detects the degree to which any memory-bound kernel suffers from partition camping. In addition, CampProf also predicts the kernel's performance at all execution configurations, if its performance parameters are known at any one of them. To demonstrate the utility of CampProf, we analyze three different applications using our tool, and demonstrate how it can be used to discover partition camping. We also demonstrate how CampProf can be used to monitor the performance improvements in the kernels, as the partition camping effect is being removed.
The performance model that drives CampProf was developed by applying multiple linear regression techniques over a set of specific micro-benchmarks that simulated the partition camping behavior. Our results show that the geometric mean of errors in our prediction model is within 12% of the actual execution times. In summary, CampProf is a new, accurate, and easy-to-use tool that can be used in conjunction with the existing tools to analyze and improve the overall performance of memory-bound CUDA kernels
Topological Excitations near the Local Critical Point in the Dissipative 2D XY model
The dissipative XY model in two spatial dimensions belongs to a new
universality class of quantum critical phenomena with the remarkable property
of the decoupling of the critical fluctuations in space and time. We have shown
earlier that the quantum critical point is driven by proliferation in time of
topological configurations that we termed warps. We show here that a warp may
be regarded as a configuration of a monopoles surrounded symmetrically by
anti-monopoles so that the total charge of the configuration is zero. Therefore
the interaction with other warps is local in space. They however interact with
other warps at the same spatial point logarithmically in time. As a function of
dissipation warps unbind leading to a quantum phase transition. The critical
fluctuations are momentum independent but have power law correlations in time
Theory of Superconductivity in the Cuprates
The quantum critical fluctuations of the time-reversal breaking order
parameter which is observed in the pseudogap regime of the Cuprates are shown
to couple to the lattice equivalent of the local angular momentum of the
fermions. Such a coupling favors scattering of fermions through angles close to
which is unambiguously shown to promote d-wave pairing. The right
order of magnitude of both and the normalized zero temperature gap
are calculated using the same fluctuations which give the
temperature, frequency and momentum dependence of the the anomalous normal
state properties for dopings near the quantum-critical value and with two
parameters extracted from fit to such experiments.Comment: Accepted for publication in PRB with the title "Theory of the
coupling of quantum-critical fluctuations to fermions and d-wave
superconductivity in the cuprates
Natural Regulation of Energy Flow in a Green Quantum Photocell
Manipulating the flow of energy in nanoscale and molecular photonic devices
is of both fundamental interest and central importance for applications in
light harvesting optoelectronics. Under erratic solar irradiance conditions,
unregulated power fluctuations in a light harvesting photocell lead to
inefficient energy storage in conventional solar cells and potentially fatal
oxidative damage in photosynthesis. Here, we show that regulation against these
fluctuations arises naturally within a two-channel quantum heat engine
photocell, thus enabling the efficient conversion of varying incident solar
spectrum at Earth's surface. Remarkably, absorption in the green portion of the
spectrum is avoided, as it provides no inherent regulatory benefit. Our
findings illuminate a quantum structural origin of regulation, provide a novel
optoelectronic design strategy, and may elucidate the link between
photoprotection in photosynthesis and the predominance of green plants on
Earth.Comment: 17 pages, 4 figure
Theory of the Quantum Critical Fluctuations in Cuprates
The statistical mechanics of the time-reversal and inversion symmetry
breaking order parameter, possibly observed in the pseudogap region of the
phase diagram of the Cuprates, can be represented by the Ashkin-Teller model.
We add kinetic energy and dissipation to the model for a quantum generalization
and show that the correlations are determined by two sets of charges, one
interacting locally in time and logarithmically in space and the other locally
in space and logarithmically in time. The quantum critical fluctuations are
derived and shown to be of the form postulated in 1989 to give the marginal
fermi-liquid properties. The model solved and the methods devised are likely to
be of interest also to other quantum phase transitions
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