2,185 research outputs found
Electrostatic analogy for integrable pairing force Hamiltonians
For the exactly solved reduced BCS model an electrostatic analogy exists; in
particular it served to obtain the exact thermodynamic limit of the model from
the Richardson Bethe ansatz equations. We present an electrostatic analogy for
a wider class of integrable Hamiltonians with pairing force interactions. We
apply it to obtain the exact thermodynamic limit of this class of models. To
verify the analytical results, we compare them with numerical solutions of the
Bethe ansatz equations for finite systems at half-filling for the ground state.Comment: 14 pages, 6 figures, revtex4. Minor change
Topology induced anomalous defect production by crossing a quantum critical point
We study the influence of topology on the quench dynamics of a system driven
across a quantum critical point. We show how the appearance of certain edge
states, which fully characterise the topology of the system, dramatically
modifies the process of defect production during the crossing of the critical
point. Interestingly enough, the density of defects is no longer described by
the Kibble-Zurek scaling, but determined instead by the non-universal
topological features of the system. Edge states are shown to be robust against
defect production, which highlights their topological nature.Comment: Phys. Rev. Lett. (to be published
Optimal correlations in many-body quantum systems
Information and correlations in a quantum system are closely related through
the process of measurement. We explore such relation in a many-body quantum
setting, effectively bridging between quantum metrology and condensed matter
physics. To this aim we adopt the information-theory view of correlations, and
study the amount of correlations after certain classes of
Positive-Operator-Valued Measurements are locally performed. As many-body
system we consider a one-dimensional array of interacting two-level systems (a
spin chain) at zero temperature, where quantum effects are most pronounced. We
demonstrate how the optimal strategy to extract the correlations depends on the
quantum phase through a subtle interplay between local interactions and
coherence.Comment: 5 pages, 5 figures + supplementary material. To be published in PR
Topology induced anomalous defect production by crossing a quantum critical point
We study the influence of topology on the quench dynamics of a system driven
across a quantum critical point. We show how the appearance of certain edge
states, which fully characterise the topology of the system, dramatically
modifies the process of defect production during the crossing of the critical
point. Interestingly enough, the density of defects is no longer described by
the Kibble-Zurek scaling, but determined instead by the non-universal
topological features of the system. Edge states are shown to be robust against
defect production, which highlights their topological nature.Comment: Phys. Rev. Lett. (to be published
Exploring the ferromagnetic behaviour of a repulsive Fermi gas via spin dynamics
Ferromagnetism is a manifestation of strong repulsive interactions between
itinerant fermions in condensed matter. Whether short-ranged repulsion alone is
sufficient to stabilize ferromagnetic correlations in the absence of other
effects, like peculiar band dispersions or orbital couplings, is however
unclear. Here, we investigate ferromagnetism in the minimal framework of an
ultracold Fermi gas with short-range repulsive interactions tuned via a
Feshbach resonance. While fermion pairing characterises the ground state, our
experiments provide signatures suggestive of a metastable Stoner-like
ferromagnetic phase supported by strong repulsion in excited scattering states.
We probe the collective spin response of a two-spin mixture engineered in a
magnetic domain-wall-like configuration, and reveal a substantial increase of
spin susceptibility while approaching a critical repulsion strength. Beyond
this value, we observe the emergence of a time-window of domain immiscibility,
indicating the metastability of the initial ferromagnetic state. Our findings
establish an important connection between dynamical and equilibrium properties
of strongly-correlated Fermi gases, pointing to the existence of a
ferromagnetic instability.Comment: 8 + 17 pages, 4 + 8 figures, 44 + 19 reference
Connecting dissipation and phase slips in a Josephson junction between fermionic superfluids
We study the emergence of dissipation in an atomic Josephson junction between
weakly-coupled superfluid Fermi gases. We find that vortex-induced phase
slippage is the dominant microscopic source of dissipation across the BEC-BCS
crossover. We explore different dynamical regimes by tuning the bias chemical
potential between the two superfluid reservoirs. For small excitations, we
observe dissipation and phase coherence to coexist, with a resistive current
followed by well-defined Josephson oscillations. We link the junction transport
properties to the phase-slippage mechanism, finding that vortex nucleation is
primarily responsible for the observed trends of conductance and critical
current. For large excitations, we observe the irreversible loss of coherence
between the two superfluids, and transport cannot be described only within an
uncorrelated phase-slip picture. Our findings open new directions for
investigating the interplay between dissipative and superfluid transport in
strongly correlated Fermi systems, and general concepts in out-of-equlibrium
quantum systems.Comment: 6 pages, 4 figures + Supplemental Materia
Bethe Ansatz solution of a new class of Hubbard-type models
We define one-dimensional particles with generalized exchange statistics. The
exact solution of a Hubbard-type Hamiltonian constructed with such particles is
achieved using the Coordinate Bethe Ansatz. The chosen deformation of the
statistics is equivalent to the presence of a magnetic field produced by the
particles themselves, which is present also in a ``free gas'' of these
particles.Comment: 4 pages, revtex. Essentially modified versio
Networked volunteering during the 2013 Sardinian floods
The article describes how ordinary citizens used Twitter as an emergency-management tool during the heavy floods that occurred in Sardinia, Italy, in November 2013. The case study constitutes an example of digital volunteering in the aftermath of a disaster event. The article applies the connective action framework (Bennet & Segerberg, 2012) for a deeper understanding of the dynamics of self-organized disaster communication activities on social media. Utilizing a dataset of 93,091 tweets that used the hashtag #allertameteoSAR (weather alert in Sardinia), the analysis focuses on: 1) the roles and patterns of influence among the main actors; and 2) the strategies for a peer ‘curation’ and sharing of a disaster-recovery oriented communication. The article highlights the role of Twitter celebrities and engaged ordinary users as digital volunteers and explains how they succeeded in activating bottom-up disaster-relief oriented communication
Out of equilibrium correlation functions of quantum anisotropic XY models: one-particle excitations
We calculate exactly matrix elements between states that are not eigenstates
of the quantum XY model for general anisotropy. Such quantities therefore
describe non equilibrium properties of the system; the Hamiltonian does not
contain any time dependence. These matrix elements are expressed as a sum of
Pfaffians. For single particle excitations on the ground state the Pfaffians in
the sum simplify to determinants.Comment: 11 pages, no figures; revtex. Minor changes in the text; list of
refs. modifie
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