511 research outputs found
Substitution effects on the temperature vs. magnetic-field phase diagrams of the quasi-1D effective Ising spin-1/2 chain system BaCoVO
BaCoVO is a one-dimensional antiferromagnetic spin-1/2 chain
system with pronounced Ising anisotropy of the magnetic exchange. Due to finite
interchain interactions long-range antiferromagnetic order develops below
K, which is accompanied by a structural distortion in
order to lift magnetic frustration effects. The corresponding temperature magnetic-field phase diagram is highly anisotropic with respect to the
magnetic-field direction and various details are still under vivid discussion.
Here, we report the influence of several substitutions on the magnetic
properties and the phase diagrams of BaCoVO. We investigate the
substitution series
BaSrCoVO
over the full range as well as the influence of a partial
substitution of the magnetic Co by small amounts of other magnetic
transition metals or by non-magnetic magnesium. In all cases, the phase
diagrams were obtained on single crystals from magnetization data and/or
high-resolution studies of the thermal expansion and magnetostriction.Comment: 10 pages, 10 figure
Large Fourier transforms never exactly realized by braiding conformal blocks
Fourier transform is an essential ingredient in Shor's factoring algorithm.
In the standard quantum circuit model with the gate set \{\U(2),
\textrm{CNOT}\}, the discrete Fourier transforms , can be realized exactly by
quantum circuits of size , and so can the discrete
sine/cosine transforms. In topological quantum computing, the simplest
universal topological quantum computer is based on the Fibonacci
(2+1)-topological quantum field theory (TQFT), where the standard quantum
circuits are replaced by unitary transformations realized by braiding conformal
blocks. We report here that the large Fourier transforms and the discrete
sine/cosine transforms can never be realized exactly by braiding conformal
blocks for a fixed TQFT. It follows that approximation is unavoidable to
implement the Fourier transforms by braiding conformal blocks
Quantum Criticality of an Ising-like Spin-1/2 Antiferromagnetic Chain in Transverse Magnetic Field
We report on magnetization, sound velocity, and magnetocaloric-effect
measurements of the Ising-like spin-1/2 antiferromagnetic chain system
BaCoVO as a function of temperature down to 1.3 K and applied
transverse magnetic field up to 60 T. While across the N\'{e}el temperature of
K anomalies in magnetization and sound velocity confirm the
antiferromagnetic ordering transition, at the lowest temperature the
field-dependent measurements reveal a sharp softening of sound velocity
and a clear minimum of temperature at T,
indicating the suppression of the antiferromagnetic order. At higher fields,
the curve shows a broad minimum at T, accompanied by a
broad minimum in the sound velocity and a saturation-like magnetization. These
features signal a quantum phase transition which is further characterized by
the divergent behavior of the Gr\"{u}neisen parameter . By contrast, around the critical field, the
Gr\"{u}neisen parameter converges as temperature decreases, pointing to a
quantum critical point of the one-dimensional transverse-field Ising model.Comment: Phys. Rev. Lett., to appea
Thermodynamic properties of the Shastry-Sutherland model from quantum Monte Carlo simulations
We investigate the minus-sign problem that afflicts quantum Monte Carlo (QMC)
simulations of frustrated quantum spin systems, focusing on spin S=1/2, two
spatial dimensions, and the extended Shastry-Sutherland model. We show that
formulating the Hamiltonian in the diagonal dimer basis leads to a sign problem
that becomes negligible at low temperatures for small and intermediate values
of the ratio of the inter- and intradimer couplings. This is a consequence of
the fact that the product state of dimer singlets is the exact ground state
both of the extended Shastry-Sutherland model and of a corresponding
"sign-problem-free" model, obtained by changing the signs of all positive
off-diagonal matrix elements in the dimer basis. By exploiting this insight, we
map the sign problem throughout the extended parameter space from the
Shastry-Sutherland to the fully frustrated bilayer model and compare it with
the phase diagram computed by tensor-network methods. We use QMC to compute
with high accuracy the temperature dependence of the magnetic specific heat and
susceptibility of the Shastry-Sutherland model for large systems up to a
coupling ratio of 0.526(1) and down to zero temperature. For larger coupling
ratios, our QMC results assist us in benchmarking the evolution of the
thermodynamic properties by systematic comparison with exact diagonalization
calculations and interpolated high-temperature series expansions.Comment: 13 pages including 10 figures; published version with minor changes
and correction
Influence of the Drying Temperature on the Performance and Binder Distribution of Sulfurized Poly(acrylonitrile) Cathodes
Improving membrane protein expression by optimizing integration efficiency
The heterologous overexpression of integral membrane proteins in Escherichia coli often yields insufficient quantities of purifiable protein for applications of interest. The current study leverages a recently demonstrated link between co-translational membrane integration efficiency and protein expression levels to predict protein sequence modifications that improve expression. Membrane integration efficiencies, obtained using a coarse-grained simulation approach, robustly predicted effects on expression of the integral membrane protein TatC for a set of 140 sequence modifications, including loop-swap chimeras and single-residue mutations distributed throughout the protein sequence. Mutations that improve simulated integration efficiency were 4-fold enriched with respect to improved experimentally observed expression levels. Furthermore, the effects of double mutations on both simulated integration efficiency and experimentally observed expression levels were cumulative and largely independent, suggesting that multiple mutations can be introduced to yield higher levels of purifiable protein. This work provides a foundation for a general method for the rational overexpression of integral membrane proteins based on computationally simulated membrane integration efficiencies
Competition between intermediate plaquette phases in SrCu(BO) under pressure
Building on the growing evidence based on NMR, magnetization, neutron
scattering, ESR, and specific heat that, under pressure, SrCu(BO)
has an intermediate phase between the dimer and the N\'eel phase, we study the
competition between two candidate phases in the context of a minimal model that
includes two types of intra- and inter-dimer interactions without enlarging the
unit cell. We show that the empty plaquette phase of the Shastry-Sutherland
model is quickly replaced by a quasi-1D full plaquette phase when intra- and/or
inter-dimer couplings take different values, and that this full plaquette phase
is in much better agreement with available experimental data than the empty
plaquette one.Comment: 19 page
Convergence of the Magnus series
The Magnus series is an infinite series which arises in the study of linear
ordinary differential equations. If the series converges, then the matrix
exponential of the sum equals the fundamental solution of the differential
equation. The question considered in this paper is: When does the series
converge? The main result establishes a sufficient condition for convergence,
which improves on several earlier results.Comment: 11 pages; v2: added justification for conjecture, minor
clarifications and correction
New applications for the Boris Spectral Deferred Correction algorithm for plasma simulations
The paper investigates two new use cases for the Boris Spectral Deferred Corrections (Boris-SDC) time integrator for plasma simulations. First, we show that using Boris-SDC as a particle pusher in an electrostatic particle-in-cell (PIC) code can, at least in the linear regime, improve simulation accuracy compared with the standard second order Boris method. In some instances, the higher order of Boris-SDC even allows a much larger time step, leading to modest computational gains. Second, we propose a modification of Boris-SDC for the relativistic regime. Based on an implementation of Boris-SDC in the RUNKO PIC code, we demonstrate for a relativistic Penning trap that Boris-SDC retains its high order of convergence for velocities ranging from 0.5c to >0.99c
Near-field interactions between metal nanoparticle surface plasmons and molecular excitons in thin-films: part I: absorption
In this and the following paper (parts I and II, respectively), we systematically study the interactions between surface plasmons of metal nanoparticles (NPs) with excitons in thin-films of organic media. In an effort to exclusively probe near-field interactions, we utilize spherical Ag NPs in a size-regime where far-field light scattering is negligibly small compared to absorption. In part I, we discuss the effect of the presence of these Ag NPs on the absorption of the embedding medium by means of experiment, numerical simulations, and analytical calculations, all shown to be in good agreement. We observe absorption enhancement in the embedding medium due to the Ag NPs with a strong dependence on the medium permittivity, the spectral position relative to the surface plasmon resonance frequency, and the thickness of the organic layer. By introducing a low index spacer layer between the NPs and the organic medium, this absorption enhancement is experimentally confirmed to be a near field effect In part II, we probe the impact of the Ag NPs on the emission of organic molecules by time-resolved and steady-state photoluminescence measurements
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