566 research outputs found
Steps and facets at the surface of soft crystals
We consider the shape of crystals which are soft in the sense that their
elastic modulus is small compared to their surface tension , more
precisely where is the lattice spacing. We show that
their surface steps penetrate inside the crystal as edge dislocations. As a
consequence, these steps are broad with a small energy which we calculate. We
also calculate the elastic interaction between steps a distance apart,
which is a repulsion. We finally calculate the roughening temperatures
of successive facets in order to compare with the remarkable shapes of
lyotropic crystals recently observed by P. Pieranski et al. Good agreement is
found.Comment: 8 Pages, 1 Figure. To appear on Eur. Phys. Journal.
Non-linear effects and shock formation in the focusing of a spherical acoustic wave : Numerical simulations and experiments in liquid helium
The focusing of acoustic waves is used to study nucleation phenomena in
liquids. At large amplitude, non-linear effects are important so that the
magnitude of pressure or density oscillations is difficult to predict. We
present a calculation of these oscillations in a spherical geometry.
We show that the main source of non-linearities is the shape of the equation
of state of the liquid, enhanced by the spherical geometry. We also show that
the formation of shocks cannot be ignored beyond a certain oscillation
amplitude. The shock length is estimated by an analytic calculation based on
the characteristics method. In our numerical simulations, we have treated the
shocks with a WENO scheme. We obtain a very good agreement with experimental
measurements which were recently performed in liquid helium. The comparison
between numerical and experimental results allows in particular to calibrate
the vibration of the ceramics used to produce the wave, as a function of the
applied voltage.Comment: 20 pages, 26 figures. Submitted to The European Physical Journal
Dislocation networks in helium-4 crystals
The mechanical behavior of crystals is dominated by dislocation networks,
their structure and their interactions with impurities or thermal phonons.
However, in classical crystals, networks are usually random with impurities
often forming non-equilibrium clusters when their motion freezes at low
temperature. Helium provides unique advantages for the study of dislocations:
crystals are free of all but isotopic impurities, the concentration of these
can be reduced to the ppb level, and the impurities are mobile at all
temperatures and therefore remain in equilibrium with the dislocations. We have
achieved a comprehensive study of the mechanical response of 4He crystals to a
driving strain as a function of temperature, frequency and strain amplitude.
The quality of our fits to the complete set of data strongly supports our
assumption of string-like vibrating dislocations. It leads to a precise
determination of the distribution of dislocation network lengths and to
detailed information about the interaction between dislocations and both
thermal phonons and 3He impurities. The width of the dissipation peak
associated with impurity binding is larger than predicted by a simple Debye
model, and much of this broadening is due to the distribution of network
lengths.Comment: accepted by Phys. Rev.
Thermally assisted quantum cavitation in solutions of 3He in 4He
We have investigated the quantum-to-thermal crossover temperature T* for
cavitation in liquid helium mixtures up to 0.05 3He concentrations. With
respect to the pure 4He case, T* is sizeably reduced, to a value below 50 mK
for 3He concentrations above 0.02. As in pure 4He, the homogeneous cavitation
pressure is systematically found close to the spinodal pressure.Comment: Typeset using Revtex, 9 pages and 4 figure
Cavitation pressure in liquid helium
Recent experiments have suggested that, at low enough temperature, the
homogeneous nucleation of bubbles occurs in liquid helium near the calculated
spinodal limit. This was done in pure superfluid helium 4 and in pure normal
liquid helium 3. However, in such experiments, where the negative pressure is
produced by focusing an acoustic wave in the bulk liquid, the local amplitude
of the instantaneous pressure or density is not directly measurable. In this
article, we present a series of measurements as a function of the static
pressure in the experimental cell. They allowed us to obtain an upper bound for
the cavitation pressure P_cav (at low temperature, P_cav < -2.4 bar in helium
3, P_cav < -8.0 bar in helium 4). From a more precise study of the acoustic
transducer characteristics, we also obtained a lower bound (at low temperature,
P_cav > -3.0 bar in helium 3, P_cav > - 10.4 bar in helium 4). In this article
we thus present quantitative evidence that cavitation occurs at low temperature
near the calculated spinodal limit (-3.1 bar in helium 3 and -9.5 bar in helium
4). Further information is also obtained on the comparison between the two
helium isotopes. We finally discuss the magnitude of nonlinear effects in the
focusing of a sound wave in liquid helium, where the pressure dependence of the
compressibility is large.Comment: 11 pages, 9 figure
Citizens without nations
To broach the question of whether citizenship could exist without (or beyond) community, this paper discusses genealogies of citizenship as membership that binds an individual to the community of birth (of the self or a parent). It is birthright as fraternity that blurs the boundary between citizenship and nationality. After briefly discussing recent critical studies on birthright citizenship (whether it is civic or ethnic or blood or soil) by Ayelet Shachar and Jacqueline Stevens, the paper discusses three critical genealogies of the relationship between birthright and citizenship by Max Weber, Hannah Arendt, and Michel Foucault. Although each provides a critical perspective into the question, Weber reduces citizenship to fraternity with nation and Arendt reduces citizenship to fraternity with the state. It is Foucault who illustrates racialization of fraternity as the connection between citizenship and nationality. Yet, since Foucault limits his genealogical investigations to the 18th and 19th centuries, a genealogy of fraternity of what he calls an immense biblical and Greek tradition remains for Derrida to articulate as a question of citizenship
Unraveling the Landau's consistence criterion and the meaning of interpenetration in the "Two-Fluid" Model
In this letter we show that it is possible to unravel both the physical
origin of the Landau's consistence criterion and the specific and subtle
meaning of interpenetration of the "two fluids" if one takes into account that
in the hydrodynamic regime one needs a coarse-graining in time to bring the
system into local equilibrium. That is, the fuzziness in time is relevant for
the phenomenological Landau's consistency criterion and the meaning of
interpenetration. Note also that we are not questioning the validity of the
"Two-Fluid" Model.Comment: 8 pages, affiliation added, typos corrected, final version published
in Eur. Phys. J.
Quantum dislocations: the fate of multiple vacancies in two dimensional solid 4He
Defects are believed to play a fundamental role in the supersolid state of
4He. We have studied solid 4He in two dimensions (2D) as function of the number
of vacancies n_v, up to 30, inserted in the initial configuration at rho =
0.0765 A^-2, close to the melting density, with the exact zero temperature
Shadow Path Integral Ground State method. The crystalline order is found to be
stable also in presence of many vacancies and we observe two completely
different regimes. For small n_v, up to about 6, vacancies form a bound state
and cause a decrease of the crystalline order. At larger n_v, the formation
energy of an extra vacancy at fixed density decreases by one order of magnitude
to about 0.6 K. In the equilibrated state it is no more possible to recognize
vacancies because they mainly transform into quantum dislocations and
crystalline order is found almost independent on how many vacancies have been
inserted in the initial configuration. The one--body density matrix in this
latter regime shows a non decaying large distance tail: dislocations, that in
2D are point defects, turn out to be mobile, their number is fluctuating, and
they are able to induce exchanges of particles across the system mainly
triggered by the dislocation cores. These results indicate that the notion of
incommensurate versus commensurate state loses meaning for solid 4He in 2D,
because the number of lattice sites becomes ill defined when the system is not
commensurate. Crystalline order is found to be stable also in 3D in presence of
up to 100 vacancies
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