78 research outputs found
Quantum effects in the diffusion of hydrogen on Ru(0001)
An understanding of hydrogen diffusion on metal surfaces is important, not
just for its role in heterogeneous catalysis and hydrogen fuel cell technology,
but also because it provides model systems where tunneling can be studied under
well-defined conditions. Here we report helium spin-echo measurements of the
atomic-scale motion of hydrogen on the Ru(0001) surface between 75 and 250 K.
Quantum effects are evident at temperatures as high as 200 K, while below 120 K
we observe a tunneling-dominated temperature independent jump rate of
1.910 s, many orders of magnitude faster than previously
seen. Quantum transition state theory calculations based on ab initio
path-integral simulations reproduce the temperature dependence of the rate at
higher temperatures and predict a crossover to tunneling-dominated diffusion at
low temperatures, although the tunneling rate is under-estimated, highlighting
the need for future experimental and theoretical studies of hydrogen diffusion
on well-defined surfaces.Comment: 15 pages, 3 figure
The Boson peak in supercooled water
We perform extensive molecular dynamics simulations of the TIP4P/2005 model of water to investigate the origin of the Boson peak reported in experiments on supercooled water in nanoconfined pores and in hydration water around proteins. We find that the onset of the Boson peak in supercooled bulk water coincides with the crossover to a predominantly low-density-like liquid below the Widom line TW. The frequency and onset temperature of the Boson peak in our simulations of bulk water agree well with the results from experiments on nanoconfined water. Our results suggest that the Boson peak in water is not an exclusive effect of confinement. We further find that, similar to other glass-forming liquids, the vibrational modes corresponding to the Boson peak are spatially extended and are related to transverse phonons found in the parent crystal, here ice Ih.We thank S. V. Buldyrev and S. Sastry for helpful discussions. The simulations were in part performed using resources provided by the Swedish National Infrastructure for Computing (SNIC) at the NSC and HPC2N centers. LGMP, KTW and DS were supported by the Swedish Research Council. KTW is also supported by the Icelandic Research Fund through the START programme. PK acknowledges the support of National Academies Keck Future Initiatives award. HES thanks NSF Grants No. CHE0911389, No. CHE0908218, and No. CHE-1213217. (Swedish Research Council; Icelandic Research Fund through the START programme; National Academies Keck Future Initiatives award; CHE0911389 - NSF; CHE0908218 - NSF; CHE-1213217 - NSF)Published versio
The Boson peak in supercooled water
We perform extensive molecular dynamics simulations of the TIP4P/2005 model
of water to investigate the origin of the Boson peak reported in experiments on
supercooled water in nanoconfined pores, and in hydration water around
proteins. We find that the onset of the Boson peak in supercooled bulk water
coincides with the crossover to a predominantly low-density-like liquid below
the Widom line . The frequency and onset temperature of the Boson peak in
our simulations of bulk water agree well with the results from experiments on
nanoconfined water. Our results suggest that the Boson peak in water is not an
exclusive effect of confinement. We further find that, similar to other
glass-forming liquids, the vibrational modes corresponding to the Boson peak
are spatially extended and are related to transverse phonons found in the
parent crystal, here ice Ih.Comment: 25 pages, 9 figure
X-ray Diffraction and Molecular Dynamics Study of Medium-range Order in Ambient and Hot Water
We have developed x-ray diffraction measurements with high energy-resolution
and accuracy to study water structure at three different temperatures (7, 25
and 66 C) under normal pressure. Using a spherically curved Ge crystal an
energy resolution better than 15 eV has been achieved which eliminates
influence from Compton scattering. The high quality of the data allows a
precise oxygen-oxygen pair correlation function (PCF) to be directly derived
from the Fourier transform of the experimental data resolving shell structure
out to ~12 {\AA}, i.e. 5 hydration shells. Large-scale molecular dynamics (MD)
simulations using the TIP4P/2005 force-field reproduce excellently the
experimental shell-structure in the range 4-12 {\AA} although less agreement is
seen for the first peak in the PCF. The Local Structure Index [J. Chem. Phys.
104, 7671 (1996)] identifies a tetrahedral minority giving the
intermediate-range oscillations in the PCF and a disordered majority providing
a more featureless background in this range. The current study supports the
proposal that the structure of liquid water, even at high temperatures, can be
described in terms of a two-state fluctuation model involving local structures
related to the high-density and low-density forms of liquid water postulated in
the liquid-liquid phase transition hypothesis.Comment: Submitted to Phys. Chem. Chem. Phy
Tuning skyrmions in B20 compounds by 4d and 5d doping
Skyrmion stabilization in novel magnetic systems with the B20 crystal
structure is reported here, primarily based on theoretical results. The focus
is on the effect of alloying on the 3d sublattice of the B20 structure by
substitution of heavier 4d and 5d elements, with the ambition to tune the
spin-orbit coupling and its influence on magnetic interactions.
State-of-the-art methods based on density functional theory are used to
calculate both isotropic and anisotropic exchange interactions. Significant
enhancement of the Dzyaloshinskii-Moriya interaction is reported for 5d-doped
FeSi and CoSi, accompanied by a large modification of the spin stiffness and
spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and
ab initio magnetic interactions reveal a helical ground state and field-induced
skyrmions for all these systems. Especially small skyrmions 50 nm are
predicted for CoOsSi, compared to 148 nm for
FeCoSi. Convex-hull analysis suggests that all B20 compounds
considered here are structurally stable at elevated temperatures and should be
possible to synthesize. This prediction is confirmed experimentally by
synthesis and structural analysis of the Ru-doped CoSi systems discussed here,
both in powder and in single-crystal forms.Comment: 18 pages, 21 figures, 9 table
Ab initio van der Waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like
The structure of liquid water at ambient conditions is studied in ab initio
molecular dynamics simulations using van der Waals (vdW) density-functional
theory, i.e. using the new exchange-correlation functionals optPBE-vdW and
vdW-DF2. Inclusion of the more isotropic vdW interactions counteracts highly
directional hydrogen-bonds, which are enhanced by standard functionals. This
brings about a softening of the microscopic structure of water, as seen from
the broadening of angular distribution functions and, in particular, from the
much lower and broader first peak in the oxygen-oxygen pair-correlation
function (PCF), indicating loss of structure in the outer solvation shells. In
combination with softer non-local correlation terms, as in the new
parameterization of vdW-DF, inclusion of vdW interactions is shown to shift the
balance of resulting structures from open tetrahedral to more close-packed. The
resulting O-O PCF shows some resemblance with experiment for high-density water
(A. K. Soper and M. A. Ricci, Phys. Rev. Lett., 84:2881, 2000), but not
directly with experiment for ambient water. However, an O-O PCF consisting of a
linear combination of 70% from vdW-DF2 and 30% from experiment on low-density
liquid water reproduces near-quantitatively the experimental O-O PCF for
ambient water, indicating consistency with a two-liquid model with fluctuations
between high- and low-density regions
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