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Multiple Magnetorotons and Spectral Sum Rules in Fractional Quantum Hall Systems
No full textWe study numerically the the charge neutral excitations (magnetorotons) in fractional quantum Hall
systems, concentrating on the two Jain states near quarter filling, ν = 2/7 and ν = 2/9, and the ν = 1/4
Fermi-liquid state itself. In contrast to the ν = 1/3 states and the Jain states near half filling, on each
of the two Jain states ν = 2/7 and ν = 2/9 the graviton spectral densities show two, instead of one,
magnetoroton peaks. The magnetorotons have spin 2 and have opposite chiralities in the ν = 2/7 state
and the same chirality in the ν = 2/9 state. We also provide a numerical verification of a sum rule relating the guiding center spin ¯s with the spectral densities of the stress tensor
Neutral excitations of quantum Hall states: A density matrix renormalization group study
No full textWe use the dynamical structure factors of the quantum Hall states at ν = 1/3 and ν = 1/2 in the
lowest Landau level to study their excitation spectrum. Using the density matrix renormalization
group in combination with the time-dependent variational principle on an infinite cylinder geometry,
we extract the low energy properties. At ν = 1/3, a sharp magnetoroton mode and the two-roton
continuum are present and the finite-size effects can be understood using the fractional charge of the
quasi-particle. At ν = 1/2, we find low energy modes with linear dispersion and the static structure
factor ¯s(q) ∼ (q`)3 in the limit q` → 0. The properties of these modes agree quantitatively with the
predictions of the composite-fermion theory placed on the infinite cylinder
On the dimensionality of behavior
No full textHow do we characterize animal behavior? Psychophysics started with human behavior in the laboratory, and focused on simple contexts, such as the decision among just a few alternative actions in response to sensory inputs. In contrast, ethology focused on animal behavior in the natural environment, emphasizing that evolution selects potentially complex behaviors that are useful in specific contexts. New experimental methods now make it possible to monitor animal and human behaviors in vastly greater detail. This “physics of behavior” holds the promise of combining the psychophysicist’s quantitative approach with the ethologist’s appreciation of natural context. One question surrounding this growing body of data concerns the dimensionality of behavior. Here I try to give this concept a precise definition.There is a growing effort in the “physics of behavior” that aims at complete quantitative
characterization of animal movements under more complex, naturalistic conditions.
One reaction to the resulting explosion of high-dimensional data is the search for
low-dimensional structure. Here I try to define more clearly what we mean by the
dimensionality of behavior, where observable behavior may consist of either continuous
trajectories or sequences of discrete states. This discussion also serves to isolate situations
in which the dimensionality of behavior is effectively infinite
Observation of Fermi Arc Surface States in a Topological Metal: A New Type of 2D Electron Gas beyond Z2 Topological Insulators
No full textIn a topological insulator, it is the electrons on the surface or edge that carry the signature of topology. Recently, a novel topological state has been proposed in metals or semimetals (gapless) whose band-structure is similar to that of a three-dimensional analog of graphene. However, to this date the signature of its topology remains an open question. We report the experimental discovery of a pair of polarized Fermi arc surface state modes in the form of a new type of two-dimensional polarized electron gas on the surfaces of Dirac semimetals. These Fermi arc surface states (FASS) are observed to connect across an even number of bulk band gapless nodes and found to have their spin uniquely locked to their momentum. We show that these states are distinctly different from the topological surface states (TSS) seen in all known topological insulators. Our observed exotic two-dimensional states not only uncover the novel topology of gapless Dirac metals (such as sodium tribismuth NaBi) but also opens new research frontiers for the utilization of topological Fermi arc electron gases for a wide range of fundamental physics and spintronic studies
A Maximum Parsimony Principle for Multichromosomal Complex Genome Rearrangements
No full textMotivation. Complex genome rearrangements, such as chromothripsis and chromoplexy, are common in cancer and have also been reported in individuals with various developmental and neurological disorders. These mutations are proposed to involve simultaneous breakage of the genome at many loci and rejoining of these breaks that produce highly rearranged genomes. Since genome sequencing measures only the novel adjacencies present at the time of sequencing, determining whether a collection of novel adjacencies resulted from a complex rearrangement is a complicated and ill-posed problem. Current heuristics for this problem often result in the inference of complex rearrangements that affect many chromosomes.
Results. We introduce a model for complex rearrangements that builds upon the methods developed for analyzing simple genome rearrangements such as inversions and translocations. While nearly all of these existing methods use a maximum parsimony assumption of minimizing the number of rearrangements, we propose an alternative maximum parsimony principle based on minimizing the number of chromosomes involved in a rearrangement scenario. We show that our model leads to inference of more plausible sequences of rearrangements that better explain a complex congenital rearrangement in a human genome and chromothripsis events in 22 cancer genomes
When Is Partially Observable Reinforcement Learning Not Scary?
No full textApplications of Reinforcement Learning (RL), in which agents learn to make a sequence of decisions despite lacking complete information about the latent states of the controlled system, that
is, they act under partial observability of the states, are ubiquitous. Partially observable RL can
be notoriously difficult—well-known information-theoretic results show that learning partially observable Markov decision processes (POMDPs) requires an exponential number of samples in the
worst case. Yet, this does not rule out the existence of large subclasses of POMDPs over which
learning is tractable.
In this paper we identify such a subclass, which we call weakly revealing POMDPs. This
family rules out the pathological instances of POMDPs where observations are uninformative to a
degree that makes learning hard. We prove that for weakly revealing POMDPs, a simple algorithm
combining optimism and Maximum Likelihood Estimation (MLE) is sufficient to guarantee polynomial sample complexity. To the best of our knowledge, this is the first provably sample-efficient
result for learning from interactions in overcomplete POMDPs, where the number of latent states
can be larger than the number of observations
Line-graph-lattice crystal structures of stoichiometric materials
No full textThe origin of many quantum-material phenomena is intimately related to the presence of flat electronic bands. In quantum simulation, such bands have been realized through line-graph lattices, a class of lattices
known to exhibit flat bands. Based on that work, we conduct a high-throughput screening for line-graph lattices among the crystalline structures of the Materials Flatband Database and report on new candidates for line-graph materials and lattice models. In particular, we find materials with line-graph-lattice structures beyond the two most commonly known examples, the kagome and pyrochlore lattices. We also identify materials that may exhibit flat topological bands. Finally, we examine the various line-graph lattices detected and highlight those with gapped flat bands and those most frequently represented among this set of materials. With the identification of real stoichiometric materials and theoretical lattice geometries, the results of this work may inform future studies of flat-band many-body physics in both condensed matter experiment and theory
An Information-Theoretic View of Mixed-Delay Traffic in 5G and 6G
No full textFifth generation mobile communication systems (5G) have to accommodate both Ultra-Reliable Low-Latency Communication (URLLC) and enhanced Mobile Broadband (eMBB) services. While eMBB applications support high data rates, URLLC services aim at guaranteeing low-latencies and high-reliabilities. eMBB and URLLC services are scheduled on the same frequency band, where the different latency requirements of the communications render their coexistence challenging. In this survey, we review, from an information theoretic perspective, coding schemes that simultaneously accommodate URLLC and eMBB transmissions and show that they outperform traditional scheduling approaches. Various communication scenarios are considered, including point-to-point channels, broadcast channels, interference networks, cellular models, and cloud radio access networks (C-RANs). The main focus is on the set of rate pairs that can simultaneously be achieved for URLLC and eMBB messages, which captures well the tension between the two types of communications. We also discuss finite-blocklength results where the measure of interest is the set of error probability pairs that can simultaneously be achieved in the two communication regimes
The Simons Observatory 220 and 280 GHz Focal-Plane Module: Design and Initial Characterization
No full textThe Simons Observatory (SO) will detect and map the temperature and polarization of the millimeter-wavelength sky from Cerro Toco, Chile, across a range of angular scales, providing rich data sets for cosmological and astrophysical analysis. The SO focal planes will be tiled with compact hexagonal packages, called universal focal-plane modules (UFMs), in which the transition-edge sensor (TES) detectors are coupled to 100 mK microwave-multiplexing electronics. Three different types of dichroic TES detector arrays with bands centered at 30/40, 90/150, and 220/280 GHz will be implemented across the 49 planned UFMs. The 90/150 GHz and 220/280 GHz arrays each contain 1764 TESes, which are read out with two 910x multiplexer circuits. The modules contain a series of routed silicon chips, which are packaged together in a controlled electromagnetic environment and operated at 100 mK. Following an overview of the module design, we report on early results from the first 220/280 GHz UFM, including detector yield, as well as readout and detector noise levels