1,021 research outputs found
Frequency dependence of specific heat in supercooled liquid water and emergence of correlated dynamics
Molecular origin of the well-known specific heat anomaly in supercooled
liquid water is investigated here by using extensive computer simulations and
theoretical analyses. A rather sharp increase in the values of isobaric
specific heat with lowering temperature and the weak temperature dependence of
isochoric specific heat in the same range are reproduced in simulations. We
calculated the spatiotemporal correlation among temperature fluctuations and
examined the frequency dependent specific heat. The latter shows a rapid growth
in the low frequency regime as temperature is cooled below 270 K. In order to
understand the microscopic basis of this increase, we have performed a shell
wise decomposition of contributions of distant molecules to the temperature
fluctuations in a central molecule. This decomposition reveals the emergence,
at low temperatures, of temporally slow, spatially long ranged large
temperature fluctuations. The temperature fluctuation time correlation function
(TFCF) can be fitted to a William-Watts stretched exponential form with the
stretching parameter close to 0.6 at low temperatures, indicating highly
non-exponential relaxation. Temperature dependence of the relaxation time of
the correlation function can be fitted to Vogel-Fulcher-Tamermann expression
which provides a quantitative measure of the fragility of the liquid.
Interestingly, we find that the rapid growth in the relaxation time of TFCF
with lowering temperature undergoes a sharp crossover from a markedly fragile
state to a weakly fragile state around 220 K.Comment: 19 pages, 6 figures (including 1 supplemental figure
Multiple time scales hidden in heterogeneous dynamics of glass-forming liquids
A multi-time probing of density fluctuations is introduced to investigate
hidden time scales of heterogeneous dynamics in glass-forming liquids.
Molecular dynamics simulations for simple glass-forming liquids are performed,
and a three-time correlation function is numerically calculated for general
time intervals. It is demonstrated that the three-time correlation function is
sensitive to the heterogeneous dynamics and that it reveals couplings of
correlated motions over a wide range of time scales. Furthermore, the time
scale of the heterogeneous dynamics is determined by the
change in the second time interval in the three-time correlation function. The
present results show that the time scale of the heterogeneous dynamics
becomes larger than the -relaxation time at low
temperatures and large wavelengths. We also find a dynamical scaling relation
between the time scale and the length scale of
dynamical heterogeneity as with .Comment: 4 pages, 5 figures, to appear in Phys. Rev. E (Rapid Communications
Electrical spin injection in p-type Si using Fe/MgO contacts
We report the successful electrical creation of spin polarization in p-type
Si at room temperature by using an epitaxial MgO(001) tunnel barrier and
Fe(001) electrode. Reflection high-energy electron diffraction observations
revealed that epitaxial Fe/MgO(001) tunnel contacts can be grown on a (2 x 1)
reconstructed Si surface whereas tunnel contacts grown on the (1 x 1) Si
surface were polycrystalline. Transmission electron microscopy images showed a
more flat interface for the epitaxial Fe/MgO/Si compared to that of the
polycrystalline structure. For the Fe/MgO/p-Si devices, the Hanle and inverted
Hanle effects were clearly observed at 300 K by using a three-terminal
configuration, proving that spin polarization can be induced in the Si at room
temperature. Effective spin lifetimes deduced from the width of the Hanle curve
were 95 +/- 6 ps and 143 +/- 10 ps for the samples with polycrystalline and
epitaxial MgO tunnel contacts, respectively. The observed difference can be
qualitatively explained by the local magnetic field induced by the larger
roughness of the interface of the polycrystalline sample. The sample with
epitaxial Fe/MgO tunnel contact showed higher magnitude of the spin
accumulation with a nearly symmetric behavior with respect to the bias polarity
whereas that of the polycrystalline MgO sample exhibited a quite asymmetric
evolution. This might be attributed to the higher degree of spin polarization
of the epitaxial Fe/MgO(001) tunnel contact, which acts as a spin filter. Our
experimental results suggest that an epitaxial MgO barrier is beneficial for
creating spins in Si.Comment: Paper presented at SPIE Nanoscience + Engineering, Spintronics V
session in San Diego, US on August 13th, 201
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