422 research outputs found
Two-dimensional topological order of kinetically constrained quantum particles
We investigate how imposing kinetic restrictions on quantum particles that
would otherwise hop freely on a two-dimensional lattice can lead to
topologically ordered states. The kinetically constrained models introduced
here are derived as a generalization of strongly interacting particle systems
in which hoppings are given by flux-lattice Hamiltonians and may be relevant to
optically driven cold-atom systems. After introducing a broad class of models,
we focus on particular realizations and show numerically that they exhibit
topological order, as witnessed by topological ground-state degeneracies and
the quantization of corresponding invariants. These results demonstrate that
the correlations responsible for fractional quantum Hall states in lattices can
arise in models involving terms other than density-density interactions.This work was supported in part by Engineering and Physical Sciences Research Council Grant No. EP/G049394/1, the Helmholtz Virtual Institute “New States of Matter and Their Excitations” and the EPSRC NetworkPlus on “Emergence and Physics far from Equilibrium”. S.K. acknowledges financial support by the ICAM Branch Contributions. The authors are grateful to M. Bukov, C. Chamon, N. R. Cooper, M. Daghofer, A. G. Grushin, C. Mudry, T. Neupert, and J. K. Pachos for stimulating discussions.This is the author accepted manuscript. The final version is available from the American Physical Society via http://dx.doi.org/10.1103/PhysRevB.91.15513
Free coherent spinons in quantum square ice
We investigate the quantum dynamics of monopole-like excitations in quantum
square ice, as captured by the strongly anisotropic spin-1/2 XXZ model on the
checkerboard lattice. We obtain exact results for excitation dynamics in both
analytically solvable effective models and a fully interacting model of quantum
square ice on finite clusters. We find that the dispersive lower bound of the
dynamic response of freely propagating spinons is recovered in the dynamic
structure factor of the interacting system, yielding a marked fingerprint of
coherent spinon dispersion. Our results provide unbiased evidence for the
formation of coherent quasiparticles propagating freely in the correlated
"vacuum" of quantum square ice
Quantum spin liquid at finite temperature: proximate dynamics and persistent typicality
Quantum spin liquids are long-range entangled states of matter with emergent
gauge fields and fractionalized excitations. While candidate materials, such as
the Kitaev honeycomb ruthenate -RuCl, show magnetic order at low
temperatures , here we demonstrate numerically a dynamical crossover from
magnon-like behavior at low and frequencies to long-lived
fractionalized fermionic quasiparticles at higher and . This
crossover is akin to the presence of spinon continua in quasi-1D spin chains.
It is further shown to go hand in hand with persistent typicality down to very
low . This aspect, which has also been observed in the spin-1/2 kagome
Heisenberg antiferromagnet, is a signature of proximate spin liquidity and
emergent gauge degrees of freedom more generally, and can be the basis for the
numerical study of many finite- properties of putative spin liquids.Comment: 13 pages, 11 figures, accepted versio
Fractional Chern insulator on a triangular lattice of strongly correlated electrons
We discuss the low-energy limit of three-orbital Kondo-lattice and Hubbard
models describing orbitals on a triangular lattice near half-filling.
We analyze how very flat bands with non-trivial topological character, a Chern
number C=1, arise both in the limit of infinite on-site interactions as well as
in more realistic regimes. Exact diagonalization is then used to investigate
fractional filling of an effective one-band spinless-fermion model including
nearest-neighbor interaction ; it reveals signatures of fractional Chern
insulators (FCIs) for several filling fractions. In addition to indications
based on energies, e.g. flux insertion and fractional statistics of quasiholes,
Chern numbers are obtained. It is shown that FCIs are robust against disorder
in the underlying magnetic texture that defines the topological character of
the band. We also investigate competition between FCI states and a charge
density wave (CDW) and discuss particle-hole asymmetry as well as Fermi-surface
nesting. FCI states turn out to be rather robust and do not require very flat
bands, but can also arise when filling or an absence of Fermi-surface nesting
disfavor the competing CDW. Nevertheless, very flat bands allow FCI states to
be induced by weaker interactions than those needed for more dispersive bands.Comment: 14 pages, 13 figure
Quantum spin liquid at finite temperature: Proximate dynamics and persistent typicality
Quantum spin liquids are long-range entangled states of matter with emergent gauge fields and fractionalized excitations. While candidate materials, such as the Kitaev honeycomb ruthenate α-RuCl3, show magnetic order at low temperatures T , here we demonstrate numerically a dynamical crossover from magnonlike behavior at low T and frequencies ω to long-lived fractionalized fermionic quasiparticles at higher T and ω. This crossover is akin to the presence of spinon continua in quasi-1D spin chains. It is further shown to go hand in hand with
persistent typicality down to very low T . This aspect, which has also been observed in the spin-1/2 kagome Heisenberg antiferromagnet, is a signature of proximate spin liquidity and emergent gauge degrees of freedom more generally, and can be the basis for the numerical study of many finite-T properties of putative spin liquids
Open-Loop Control of Flexible Manipulator Systems Using Filtered Inputs
This paper presents an investigation into the development of open-loop control strategies for flexible manipulator systems using filtering techniques. Shaped torque inputs, including lowpass and bandstop filtered torque input functions, are developed and used in an open-loop configuration and their performances within a simulation environment characterising a constrained planar single-link flexible manipulator system studied in comparison to a bang-bang torque input. Simulation results verifying the performance of the developed control strategies are presented and discussed
PNAS plus: plasmodium falciparum responds to amino acid starvation by entering into a hibernatory state
The human malaria parasite Plasmodium falciparum is auxotrophic for most amino acids. Its amino acid needs are met largely through the degradation of host erythrocyte hemoglobin; however the parasite must acquire isoleucine exogenously, because this amino acid is not present in adult human hemoglobin. We report that when isoleucine is withdrawn from the culture medium of intraerythrocytic P. falciparum, the parasite slows its metabolism and progresses through its developmental cycle at a reduced rate. Isoleucine-starved parasites remain viable for 72 h and resume rapid growth upon resupplementation. Protein degradation during starvation is important for maintenance of this hibernatory state. Microarray analysis of starved parasites revealed a 60% decrease in the rate of progression through the normal transcriptional program but no other apparent stress response. Plasmodium parasites do not possess a TOR nutrient-sensing pathway and have only a rudimentary amino acid starvation-sensing eukaryotic initiation factor 2α (eIF2α) stress response. Isoleucine deprivation results in GCN2-mediated phosphorylation of eIF2α, but kinase-knockout clones still are able to hibernate and recover, indicating that this pathway does not directly promote survival during isoleucine starvation. We conclude that P. falciparum, in the absence of canonical eukaryotic nutrient stress-response pathways, can cope with an inconsistent bloodstream amino acid supply by hibernating and waiting for more nutrient to be provided
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