2,056 research outputs found
Apparent slow dynamics in the ergodic phase of a driven many-body localized system without extensive conserved quantities
We numerically study the dynamics on the ergodic side of the many-body
localization transition in a periodically driven Floquet model with no global
conservation laws. We describe and employ a numerical technique based on the
fast Walsh-Hadamard transform that allows us to perform an exact time evolution
for large systems and long times. As in models with conserved quantities (e.g.,
energy and/or particle number) we observe a slowing down of the dynamics as the
transition into the many-body localized phase is approached. More specifically,
our data is consistent with a subballistic spread of entanglement and a
stretched-exponential decay of an autocorrelation function, with their
associated exponents reflecting slow dynamics near the transition for a fixed
system size. However, with access to larger system sizes, we observe a clear
flow of the exponents towards faster dynamics and can not rule out that the
slow dynamics is a finite-size effect. Furthermore, we observe examples of
non-monotonic dependence of the exponents with time, with dynamics initially
slowing down but accelerating again at even larger times, consistent with the
slow dynamics being a crossover phenomena with a localized critical point.Comment: 9 pages, 8 figures; added details on the level statistics and the
energy absorptio
Long-range and short-range magnetic correlations, and microscopic origin of net magnetization in the spin-1 trimer chain compound CaNi3P4O14
Spin-spin correlations and microscopic origin of net magnetization in the
spin-1 trimer chain compound CaNi3P4O14 have been investigated by powder
neutron diffraction. The present study reveals a 3D long-range magnetic
ordering below 16 K where the magnetic structure consists of ferromagnetic
trimers that are coupled ferromagnetically along the spin-chain. The moment
components along the a and c axes arrange antiferromagnetically. Our study
establishes that the uncompensated moment components along the b axis result in
a net magnetization per unit cell. The magnetic structure, determined in the
present study, is in agreement with the results of recent first principles
calculation; however, it is in contrast to a fascinating experimental
prediction of ferrimagnetic ordering based on the periodicity of the exchange
interactions in CaNi3P4O14. Our study also confirms the presence of broad
diffuse magnetic scattering, due to 1D short-range spin-spin correlations, over
a wide temperature range below ~50 K down to a temperature well below the Tc.
Total neutron scattering analysis by the RMC method reveals that the dominating
spin-spin correlation above Tc is ferromagnetic and along the b axis. The
nearest neighbour spin-spin correlations along the a and c axes are found to be
weakly antiferromagnetic. The nature of the trimer spin structure of the
short-range state is similar to that of the 3D long-range ordered state. The
present investigation of microscopic nature of the magnetic ground state also
explains the condition required for the 1/3 magnetization plateau to be
observed in the trimer spin-chains. In spite of the S=1 trimer chain system,
the present compound CaNi3P4O14 is found to be a good realization of 3D magnet
below the Tc=16 K with full ordered moment values of ~2 mu_B/Ni2+ (1.98 and
1.96 mu_B/Ni2+ for two Ni sites, respectively) at 1.5 K.Comment: 10 pages, 8 figure
Elastic properties of graphene flakes: boundary effects and lattice vibrations
We present a calculation of the free energy, the surface free energy and the
elastic constants ("Lam'e parameters" i.e, Poisson ratio, Young's modulus) of
graphene flakes on the level of the density functional theory employing
different standard functionals. We observe that the Lam'e parameters in small
flakes can differ from the bulk values by 30% for hydrogenated zig-zag edges.
The change results from the edge of the flake that compresses the interior.
When including the vibrational zero point motion, we detect a decrease in the
bending rigidity by ~26%. This correction is depending on the flake size, N,
because the vibrational frequencies flow with growing N due to the release of
the edge induced compression. We calculate Grueneisen parameters and find good
agreement with previous authors.Comment: 11 pages, 12 figure
Chromium (VI) Biosorption by Immobilized Biomass of Bacillus ceres M116
Biosorption is potentially an attractive technology for treatment of wastewater for retaining heavy metals from dilute solutions. This study investigated the feasibility of Bacillus cereus M116 immobilized in different carriers as a biosorbent for chromium removal from aqueous solutions in batch mode; optimum conditions were determined. Experimental results showed the bacterial strain immobilized in calcium alginate gel matrix was most effective in removing Cr(VI) ion from solution. The uptake of metal was very fast initially, and equilibrium was attained within 80 mins. The overall biosorption process was best described by the pseudo second-order kinetics. Intraparticle diffusion was not the only rate-determining step. The sorption data conformed well to the Fruendlich isotherm model. The highest value of Cr(VI) uptake by Bacillus cereus M116 (6.0g/L ,dry basis) immobilized in 3% calcium alginate was 92.5% at 25°C, when initial chromium concentration was 50 mg /L
ARC: Alignment-based Redirection Controller for Redirected Walking in Complex Environments
We present a novel redirected walking controller based on alignment that
allows the user to explore large and complex virtual environments, while
minimizing the number of collisions with obstacles in the physical environment.
Our alignment-based redirection controller, ARC, steers the user such that
their proximity to obstacles in the physical environment matches the proximity
to obstacles in the virtual environment as closely as possible. To quantify a
controller's performance in complex environments, we introduce a new metric,
Complexity Ratio (CR), to measure the relative environment complexity and
characterize the difference in navigational complexity between the physical and
virtual environments. Through extensive simulation-based experiments, we show
that ARC significantly outperforms current state-of-the-art controllers in its
ability to steer the user on a collision-free path. We also show through
quantitative and qualitative measures of performance that our controller is
robust in complex environments with many obstacles. Our method is applicable to
arbitrary environments and operates without any user input or parameter
tweaking, aside from the layout of the environments. We have implemented our
algorithm on the Oculus Quest head-mounted display and evaluated its
performance in environments with varying complexity. Our project website is
available at https://gamma.umd.edu/arc/
Investigation of the magnetic structure and crystal field states of pyrochlore antiferromagnet Nd2Zr2O7
We present synchrotron x-ray diffraction, neutron powder diffraction and
time-of-flight inelastic neutron scattering measurements on the rare earth
pyrochlore oxide Nd2Zr2O7 to study the ordered state magnetic structure and
cystal field states. The structural characterization by high-resolution
synchrotron x-ray diffraction confirms that the pyrochlore structure has no
detectable O vacancies or Nd/Zr site mixing. The neutron diffraction reveals
long range all-in/all-out antiferromagnetic order below T_N ~ 0.4 K with
propagation vector k = (0 0 0) and an ordered moment of 1.26(2) \mu_B/Nd at 0.1
K. The ordered moment is much smaller than the estimated moment of 2.65
\mu_B/Nd for the local Ising ground state of Nd3+ (J=9/2) suggesting that
the ordering is partially suppressed by quantum fluctuations. The strong Ising
anisotropy is further confirmed by the inelastic neutron scattering data which
reveals a well-isolated dipolar-octupolar type Kramers doublet ground state.
The crystal field level scheme and ground state wavefunction have been
determined.Comment: 12 pages, 15 figures, 2 table
Spinon confinement in a quasi one dimensional anisotropic Heisenberg magnet
Confinement is a process by which particles with fractional quantum numbers
bind together to form quasiparticles with integer quantum numbers. The
constituent particles are confined by an attractive interaction whose strength
increases with increasing particle separation and as a consequence, individual
particles are not found in isolation. This phenomenon is well known in particle
physics where quarks are confined in baryons and mesons. An analogous
phenomenon occurs in certain magnetic insulators; weakly coupled chains of
spins S=1/2. The collective excitations in these systems is spinons (S=1/2). At
low temperatures weak coupling between chains can induce an attractive
interaction between pairs of spinons that increases with their separation and
thus leads to confinement. In this paper, we employ inelastic neutron
scattering to investigate the spinon confinement in the quasi-1D S=1/2 XXZ
antiferromagnet SrCo2V2O8. Spinon excitations are observed above TN in
quantitative agreement with established theory. Below TN the pairs of spinons
are confined and two sequences of meson-like bound states with longitudinal and
transverse polarizations are observed. Several theoretical approaches are used
to explain the data. A new theoretical technique based on Tangent-space Matrix
Product States gives a very complete description of the data and provides good
agreement not only with the energies of the bound modes but also with their
intensities. We also successfully explained the effect of temperature on the
excitations including the experimentally observed thermally induced resonance
between longitudinal modes below TN ,and the transitions between thermally
excited spinon states above TN. In summary, our work establishes SrCo2V2O8 as a
beautiful paradigm for spinon confinement in a quasi-1D quantum magnet and
provides a comprehensive picture of this process.Comment: 17 pages, 18 figures, submitted to PR
Bioaccumulation of Pb(II) From Aqueous Solutions by Bacillus cereus M1 16
Because of the severity of heavy metal contamination and potential adverse health impact on the public, tremendous efforts have been made to purify waters containing toxic metal ions. Biosorption is presented as an alternative to traditional physicochemical means for removing toxic metals from groundwaters and wastewaters. Removal of lead from solution was studied using growing cells and washed cells of Bacillus cereus M116. The removal of Pb(II) ions with growing cells was maximum (85%) when initial lead concentration was 50 mg/L. Other process conditions were optimized. These were volume of medium: 40 ml in a 250-ml Erlenmeyer flask, temperature: 30° C, pH: 6.0, fermentation time: 30 hours, and inoculum concentration (24-hour cell growth): 4%. Biosorption of Pb(II) on washed biomass of the selected strain was investigated in batch mode and optimum conditions were determined. The uptake of metal was very fast, and equilibrium was attained within 30 minutes. It was found that the overall adsorption process was best described by pseudo second-order kinetics. Both Langmuir and Freundlich isotherms were tested, and it was found that the latter had a better fit with the data. The adsorption continuously increased in the pH range of 3.6 – 6.0, beyond which the adsorption could not be carried out due to precipitation of metal. The highest value of lead uptake was 96%, with 1.8 g/L washed biomass (dry basis) at 20°C and 92% at 30°C
Biosorption of Pb(II) by Bacillus cereus M1 16 Immobilized in Calcium Alginate Gel
Biosorption is an alternative to traditional physicochemical means for removing toxic metals from ground and wastewaters. Biosorption of Pb(II) ion from solution was studied using Bacillus cereus M116 immobilized in calcium alginate gel in batch mode, and optimum conditions were determined. The experimental results showed that the immobilized bacterial strain was effective in removing Pb(II) ion from solution. The uptake of metal was very fast initially, and equilibrium was attained within 270 min. It was found that the overall adsorption process was best described by pseudo second-order kinetics. Intra-particle diffusion was not the only rate determining step. The sorption data conformed well to the Freundlich isotherm model. Adsorption increased with an increase in pH in the pH range 3.6 – 6.0, beyond which the adsorption could not be carried out due to precipitation of metal. The highest value of lead uptake was 99%, with 5.4% biomass in 500 mg of adsorbent using 50-ml solution containing 50 mgL-1 Pb(II) ion in a 250-ml Erlenmeyer flask at 300 C and 120 rpm
Perturbative and non-perturbative studies with the delta function potential
We show that the delta function potential can be exploited along with
perturbation theory to yield the result of certain infinite series. The idea is
that any exactly soluble potential if coupled with a delta function potential
remains exactly soluble. We use the strength of the delta function as an
expansion parameter and express the second-order energy shift as an infinite
sum in perturbation theory. The analytical solution is used to determine the
second-order energy shift and hence the sum of an infinite series. By an
appropriate choice of the unperturbed system, we can show the importance of the
continuum in the energy shift of bound states.Comment: 19 pages, 2 table
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