353 research outputs found
Surface melting of the vortex lattice
We discuss the effect of an (ab)-surface on the melting transition of the
pancake-vortex lattice in a layered superconductor within a density functional
theory approach. Both discontinuous and continuous surface melting are
predicted for this system, although the latter scenario occupies the major part
of the low-field phase diagram. The formation of a quasi-liquid layer below the
bulk melting temperature inhibits the appearance of a superheated solid phase,
yielding an asymmetric hysteretic behavior which has been seen in experiments.Comment: 4 pages, 3 figure
Free energies, vacancy concentrations and density distribution anisotropies in hard--sphere crystals: A combined density functional and simulation study
We perform a comparative study of the free energies and the density
distributions in hard sphere crystals using Monte Carlo simulations and density
functional theory (employing Fundamental Measure functionals). Using a recently
introduced technique (Schilling and Schmid, J. Chem. Phys 131, 231102 (2009))
we obtain crystal free energies to a high precision. The free energies from
Fundamental Measure theory are in good agreement with the simulation results
and demonstrate the applicability of these functionals to the treatment of
other problems involving crystallization. The agreement between FMT and
simulations on the level of the free energies is also reflected in the density
distributions around single lattice sites. Overall, the peak widths and
anisotropy signs for different lattice directions agree, however, it is found
that Fundamental Measure theory gives slightly narrower peaks with more
anisotropy than seen in the simulations. Among the three types of Fundamental
Measure functionals studied, only the White Bear II functional (Hansen-Goos and
Roth, J. Phys.: Condens. Matter 18, 8413 (2006)) exhibits sensible results for
the equilibrium vacancy concentration and a physical behavior of the chemical
potential in crystals constrained by a fixed vacancy concentration.Comment: 17 pages, submitted to Phys. Rev.
Surface Melting of the Vortex Lattice in Layered Superconductors: Density Functional Theory
We study the effects of an -surface on the vortex-solid to vortex-liquid
transition in layered superconductors in the limit of vanishing inter-layer
Josephson coupling. We derive the interaction between pancake vortices in a
semi-infinite sample and adapt the density functional theory of freezing to
this system. We obtain an effective one-component order-parameter theory which
can be used to describe the effects of the surface on vortex-lattice melting.
Due to the absence of protecting layers in the neighbourhood of the surface,
the vortex lattice formed near the surface is more susceptible to thermal
fluctuations. Depending on the value of the magnetic field, we predict either a
continuous or a discontinuous surface melting transition. For intermediate
values of the magnetic field, the surface melts continuously, assisting the
formation of the liquid phase and suppressing hysteresis above the melting
transition, a prediction consistent with experimental results. For very low and
very high magnetic fields, the surface melts discontinuously. The two different
surface melting scenarios are separated by two surface multicritical points,
which we locate on the melting line.Comment: 16 pages, 12 figure
The electronic transport properties and microstructure of carbon nanofiber/epoxy composites
Carbon nanofibres (CNF) were dispersed into an epoxy resin using a
combination of ultrasonication and mechanical mixing. The electronic transport
properties of the resulting composites were investigated by means of impedance
spectroscopy. It was found that a very low critical weight fraction (pc = 0.064
wt %) which may be taken to correspond to the formation of a tunneling
conductive network inside the matrix. The insulator-to-conductor transition
region spanned about one order of magnitude from 0.1 to 1 wt %. Far from the
transition, the conductivity increased by two orders of magnitude. This
increase and the low value of the conductivity were explained in terms of the
presence of an epoxy film at the contact between CNF. A simple model based on
the CNF-CNF contact network inside the matrix was proposed in order to evaluate
the thickness of that film.Comment: 7 page
Visualization of wave function of quantum dot at fermi-edge singularity regime
We consider electron tunneling spectroscopy through an InAs quantum dot in a magnetic field applied perpendicular to the tunneling direction. We examine in details the anisotropic behavior of the amplitude and shape of the resonant peaks of I-V curves and concluded that (i) magnetotunneling spectroscopy at FES regime allows establishing position of resonant level in QD with high accuracy. (ii) The distinguishable shape of FES peak allows extracting the amplitude with much better accuracy. (iii) FES exponent dependence on magnetic field gives additional information about potential distribution outside QD.Foundation for Science and Technology (FCT
Temperature Evolution of Sodium Nitrite Structure in a Restricted Geometry
The NaNO nanocomposite ferroelectric material in porous glass was
studied by neutron diffraction. For the first time the details of the crystal
structure including positions and anisotropic thermal parameters were
determined for the solid material, embedded in a porous matrix, in ferro- and
paraelectric phases. It is demonstrated that in the ferroelectric phase the
structure is consistent with bulk data but above transition temperature the
giant growth of amplitudes of thermal vibrations is observed, resulting in the
formation of a "premelted state". Such a conclusion is in a good agreement with
the results of dielectric measurements published earlier.Comment: 4 pages, 4 figure
Melting of Hard Cubes
The melting transition of a system of hard cubes is studied numerically both
in the case of freely rotating cubes and when there is a fixed orientation of
the particles (parallel cubes). It is shown that freelly rotating cubes melt
through a first-order transition, whereas parallel cubes have a continuous
transition in which positional order is lost but bond-orientational order
remains finite. This is interpreted in terms of a defect-mediated theory of
meltingComment: 5 pages, 3 figures included. To appear in Phys. Rev.
Giga-Hertz quantized charge pumping in bottom gate defined InAs nanowire quantum dots
Semiconducting nanowires (NWs) are a versatile, highly tunable material
platform at the heart of many new developments in nanoscale and quantum
physics. Here, we demonstrate charge pumping, i.e., the controlled transport of
individual electrons through an InAs NW quantum dot (QD) device at frequencies
up to GHz. The QD is induced electrostatically in the NW by a series of
local bottom gates in a state of the art device geometry. A periodic modulation
of a single gate is enough to obtain a dc current proportional to the frequency
of the modulation. The dc bias, the modulation amplitude and the gate voltages
on the local gates can be used to control the number of charges conveyed per
cycle. Charge pumping in InAs NWs is relevant not only in metrology as a
current standard, but also opens up the opportunity to investigate a variety of
exotic states of matter, e.g. Majorana modes, by single electron spectroscopy
and correlation experiments.Comment: 21 page
Isomorphs, hidden scale invariance, and quasiuniversality
This paper first establishes an approximate scaling property of the
potential-energy function of a classical liquid with good isomorphs (a
Roskilde-simple liquid). This "pseudohomogeneous" property makes explicit that
- and in which sense - such a system has a hidden scale invariance. The second
part gives a potential-energy formulation of the quasiuniversality of monatomic
Roskilde-simple liquids, which was recently rationalized in terms of the
existence of a quasiuniversal single-parameter family of reduced-coordinate
constant-potential-energy hypersurfaces [J. C. Dyre, Phys. Rev. E 87, 022106
(2013)]. The new formulation involves a quasiuniversal reduced-coordinate
potential-energy function. A few consequences of this are discussed
Simplicity of condensed matter at its core: Generic definition of a Roskilde-simple system
The theory of isomorphs is reformulated by defining Roskilde-simple systems
(those with isomorphs) by the property that the order of the potential energies
of configurations at one density is maintained when these are scaled uniformly
to a different density. Isomorphs remain curves in the thermodynamic phase
diagram along which structure, dynamics, and excess entropy are invariant,
implying that the phase diagram is effectively one-dimensional with respect to
many reduced-unit properties. In contrast to the original formulation of the
isomorph theory, however, the density-scaling exponent is not exclusively a
function of density and the isochoric heat capacity is not an exact isomorph
invariant. A prediction is given for the latter quantity's variation along the
isomorphs. Molecular dynamics simulations of the Lennard-Jones and
Lennard-Jones Gaussian systems validate the new approach
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