64 research outputs found
The influence of boundaries on high pressure melting experiments
At low pressure, free surfaces play a crucial role in the melting transition.
Under pressure, the surface of the sample is acted upon by some pressure
transmitting medium. To examine the effect of this medium on melting, we
performed Monte Carlo simulations of a system of argon atoms in the form of a
slab with two boundaries. We examined two cases, one with a soft and the other
with a rigid medium at the boundaries. We found that in the presence of a rigid
medium, melting resembles the mechanical lattice instability found in a
surface-free solid. With a soft medium at the boundary, melting begins at the
surface and at a lower temperature. The relevance of these results to
experiment is discussed.Comment: 4 pages, 5 figure
Quantum vortex tunneling in thin films
Cuprate films offer a unique opportunity to observe vortex tunneling effects,
due to their unusually low superfluid density and short coherence length. Here,
we measure the magnetoresistance (\textit{MR}) due to vortex motion of a long
meander line of a superconducting film made of underdoped
. At low temperatures (\textit{T}), the \textit{MR}
shows a significant deviation from Arrhenius activation. The data is consistent
with two dimensional Variable Range Hopping (VRH) of single vortices, i.e.
. The VRH temperature scale depends on the
vortex tunneling rates between pinning sites. We discuss its magnitude with
respect to estimated parameters of the meander thin film.Comment: 5 figure
A study of the ferromagnetic transition of in nanometer thick bilayers with , , Au and Cr: Signature of injected carriers in the pseudogap regime
The hypothesis regarding the existence of uncorrelated pre-formed pairs in
the pseudogap regime of superconducting is tested experimentally
using bilayers of and the itinerant ferromagnet . In
our study, we monitor the influence of on , the
ferromagnetic ordering temperature of . Here, is the temperature
of maximum dM/dT or dR/dT where M and R are the magnetization and resistance of
, respectively. We compare the results with similar measurements
carried out on bilayers of , and with
. We find that in bilayers made of underdoped 10 nm /5
nm , the values are shifted to lower temperatures by up to 6-8 K
as compared to K of the 5 nm thick reference film.
In contrast, in the other type of bilayers, which are not in the pseudogap
regime near , only a smaller shift of up to 2 K is observed. These
differences are discussed in terms of a proximity effect, where carriers from
the layer are injected into the layer and vice versa.
We suggest that correlated electrons in the pseudogap regime of
are responsible for the observed large shifts.Comment: 9 figure
Experimental search for anisotropic flux flow resistivity in the a-b plane of optimally doped epitaxial thin films of YBCO
Transport measurements along the node and anti-node directions in the a-b
plane of optimally doped and epitaxial thin films of YBCO are reported. Low
bias magnetoresistance measurements near and below T_c show that the flux flow
resistivity along the node and anti-node directions versus magnetic field are
indistinguishable. This result suggests that within the experimental error of
our measurements, no correspondence is found between the flux pinning
properties in YBCO and the d-wave nature of the order parameter.Comment: 5 figure
Path Integral Monte Carlo study of phonons in the bcc phase of He
Using Path Integral Monte Carlo and the Maximum Entropy method, we calculate
the dynamic structure factor of solid He in the bcc phase at a finite
temperature of T = 1.6 K and a molar volume of 21 cm. Both the
single-phonon contribution to the dynamic structure factor and the total
dynamic structure factor are evaluated. From the dynamic structure factor, we
obtain the phonon dispersion relations along the main crystalline directions,
[001], [011] and [111]. We calculate both the longitudinal and transverse
phonon branches. For the latter, no previous simulations exist. We discuss the
differences between dispersion relations resulting from the single-phonon part
vs. the total dynamic structure factor. In addition, we evaluate the formation
energy of a vacancy.Comment: 10 figure
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