4,948 research outputs found
Finite-temperature Wigner solid and other phases of ripplonic polarons on a helium film
Electrons on liquid helium can form different phases depending on density,
and temperature. Also the electron-ripplon coupling strength influences the
phase diagram, through the formation of so-called "ripplonic polarons", that
change how electrons are localized, and that shifts the transition between the
Wigner solid and the liquid phase. We use an all-coupling, finite-temperature
variational method to study the formation of a ripplopolaron Wigner solid on a
liquid helium film for different regimes of the electron-ripplon coupling
strength. In addition to the three known phases of the ripplopolaron system
(electron Wigner solid, polaron Wigner solid, and electron fluid), we define
and identify a fourth distinct phase, the ripplopolaron liquid. We analyse the
transitions between these four phases and calculate the corresponding phase
diagrams. This reveals a reentrant melting of the electron solid as a function
of temperature. The calculated regions of existence of the Wigner solid are in
agreement with recent experimental data.Comment: 12 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1012.4576, arXiv:0709.4140 by other author
Superfluid to Bose-glass transition in a 1D weakly interacting Bose gas
We study the one-dimensional Bose gas in spatially correlated disorder at
zero temperature, using an extended density-phase Bogoliubov method. We analyze
in particular the decay of the one-body density matrix and the behaviour of the
Bogoliubov excitations across the phase boundary. We observe that the
transition to the Bose glass phase is marked by a power-law divergence of the
density of states at low energy. A measure of the localization length displays
a power-law energy dependence in both regions, with the exponent equal to -1 at
the boundary. We draw the phase diagram of the superfluid-insulator transition
in the limit of small interaction strength.Comment: 4 pages, 4 figure
The role of the European Union in the Human Rights Council
The European Union has and continues to place human rights and democracy at the heart of its external relations, namely through its activities in the UN human rights system. With the ongoing criticism about the inadequacies of the UN Commission on Human Rights Kofi Annan recommended that this ailing body be replaced with a new and more effective Human Rights Council. In light of the European Union’s desire to further human rights and democracy across the globe it rapidly got involved in playing an active and visible role in the overarching UN human rights reform process. Negotiations for establishing the Human Rights Council in addition to the deliberations of the mandated working groups outlining the details and modalities of the Council involved making difficult and demanding decisions. The different opinions and positions not only led to a delayed inception but also generated concerns that the new Council would not be any more efficient or credible than its predecessor. Many have claimed that its first operational year has produced disappointing results by not taking more concrete action regarding countries facing dire human rights crises such as Zimbabwe. In contrast, others argue that in its first year the Council did achieve a number of successes including the modalities of the Universal Periodic Review process, the review of UN Special Procedures, the adoption of the codes of conduct for mandate holders in addition to the adoption of a number of resolutions of great importance in the Council’s plenary.
The European Union actively participated in each phase and in all areas of the Human Rights Council from the setting up process to engaging in the sometimes arduous negotiations of both procedural and substantive matters. While it has indeed been successful in the workings of some areas the EU’s shortcomings have recently overshadowed its achievements. The following study explores the role of the EU in the Human Rights Council from the setting-up process to its contributions in the review of the UN Special Procedures, the UPR process and the plenary sessions since the Council’s inception. Following the highlighting of both the EU’s achievements and shortcomings the study concludes with five core recommendations that have the potential to help rectify the challenges currently faced by the European Union in the
Human Rights Council
Micellar dye shuttle between water and an ionic liquid
The reversible transfer of poly(2-ethyl-2-oxazoline)-block-poly(2-nonyl-2-oxazoline) nanocarriers comprising encapsulated dyes is demonstrated between water and an ionic liquid. This dye transfer concept is shown to be applicable for loading and delivery of dye molecules as well as to provide a protective environment for the encapsulated dye
Mean-field phase diagram of the 1-D Bose gas in a disorder potential
We study the quantum phase transition of the 1D weakly interacting Bose gas
in the presence of disorder. We characterize the phase transition as a function
of disorder and interaction strengths, by inspecting the long-range behavior of
the one-body density matrix as well as the drop in the superfluid fraction. We
focus on the properties of the low-energy Bogoliubov excitations that drive the
phase transition, and find that the transition to the insulator state is marked
by a diverging density of states and a localization length that diverges as a
power-law with power 1. We draw the phase diagram and we observe that the
boundary between the superfluid and the Bose glass phase is characterized by
two different algebraic relations. These can be explained analytically by
considering the limiting cases of zero and infinite disorder correlation
length.Comment: 10 pages, 10 figure
Energy-weighted density matrix embedding of open correlated chemical fragments
We present a multi-scale approach to efficiently embed an ab initio
correlated chemical fragment described by its energy-weighted density matrices,
and entangled with a wider mean-field many-electron system. This approach,
first presented in Phys. Rev. B, 98, 235132 (2018), is here extended to account
for realistic long-range interactions and broken symmetry states. The scheme
allows for a systematically improvable description in the range of correlated
fluctuations out of the fragment into the system, via a self-consistent
optimization of a coupled auxiliary mean-field system. It is discussed that the
method has rigorous limits equivalent to existing quantum embedding approaches
of both dynamical mean-field theory, as well as density matrix embedding
theory, to which this method is compared, and the importance of these
correlated fluctuations is demonstrated. We derive a self-consistent local
energy functional within the scheme, and demonstrate the approach for Hydrogen
rings, where quantitative accuracy is achieved despite only a single atom being
explicitly treated.Comment: 14 pages, 8 figure
Cavity-enhanced photoionization of an ultracold rubidium beam for application in focused ion beams
A two-step photoionization strategy of an ultracold rubidium beam for
application in a focused ion beam instrument is analyzed and implemented. In
this strategy the atomic beam is partly selected with an aperture after which
the transmitted atoms are ionized in the overlap of a tightly cylindrically
focused excitation laser beam and an ionization laser beam whose power is
enhanced in a build-up cavity. The advantage of this strategy, as compared to
without the use of a build-up cavity, is that higher ionization degrees can be
reached at higher currents. Optical Bloch equations including the
photoionization process are used to calculate what ionization degree and
ionization position distribution can be reached. Furthermore, the ionization
strategy is tested on an ultracold beam of Rb atoms. The beam current is
measured as a function of the excitation and ionization laser beam intensity
and the selection aperture size. Although details are different, the global
trends of the measurements agree well with the calculation. With a selection
aperture diameter of 52 m, a current of pA is
measured, which according to calculations is 63% of the current equivalent of
the transmitted atomic flux. Taking into account the ionization degree the ion
beam peak reduced brightness is estimated at A/(msreV).Comment: 13 pages, 9 figure
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