150 research outputs found
Theory of quantum paraelectrics and the metaelectric transition
We present a microscopic model of the quantum paraelectric-ferroelectric
phase transition with a focus on the influence of coupled fluctuating phonon
modes. These may drive the continuous phase transition first order through a
metaelectric transition and furthermore stimulate the emergence of a textured
phase that preempts the transition. We discuss two further consequences of
fluctuations, firstly for the heat capacity, and secondly we show that the
inverse paraelectric susceptibility displays T^2 quantum critical behavior, and
can also adopt a characteristic minimum with temperature. Finally, we discuss
the observable consequences of our results.Comment: 5 pages, 2 figure
A repulsive atomic gas in a harmonic trap on the border of itinerant ferromagnetism
Alongside superfluidity, itinerant (Stoner) ferromagnetism remains one of the
most well-characterized phases of correlated Fermi systems. A recent experiment
has reported the first evidence for novel phase behavior on the repulsive side
of the Feshbach resonance in a two-component ultracold Fermi gas. By adapting
recent theoretical studies to the atomic trap geometry, we show that an
adiabatic ferromagnetic transition would take place at a weaker interaction
strength than is observed in experiment. This discrepancy motivates a simple
non-equilibrium theory that takes account of the dynamics of magnetic defects
and three-body losses. The formalism developed displays good quantitative
agreement with experiment.Comment: 4 pages, 2 figure
Dynamical spin-flip susceptibility for a strongly interacting ultracold Fermi gas
The Stoner model predicts that a two-component Fermi gas at increasing
repulsive interactions undergoes a ferromagnetic transition. Using the
random-phase approximation we study the dynamical properties of the interacting
Fermi gas. For an atomic Fermi gas under harmonic confinement we show that the
transverse (spin-flip) dynamical susceptibility displays a clear signature of
the ferromagnetic phase in a magnon peak emerging from the Stoner particle-hole
continuum. The dynamical spin susceptibilities could be experimentally explored
via spin-dependent Bragg spectroscopy.Comment: 4 pages, 3 figure
Itinerant ferromagnetism in an atomic Fermi gas: Influence of population imbalance
We investigate ferromagnetic ordering in an itinerant ultracold atomic Fermi
gas with repulsive interactions and population imbalance. In a spatially
uniform system, we show that at zero temperature the transition to the
itinerant magnetic phase transforms from first to second order with increasing
population imbalance. Drawing on these results, we elucidate the phases present
in a trapped geometry, finding three characteristic types of behavior with
changing population imbalance. Finally, we outline the potential experimental
implications of the findings.Comment: 10 pages, 4 figures, typos added, references adde
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Imputation versus prediction: applications in machine learning for drug discovery
Imputation is a powerful statistical method that is distinct from the predictive modelling techniques more commonly used in drug discovery. Imputation uses sparse experimental data in an incomplete dataset to predict missing values by leveraging correlations between experimental assays. This contrasts with quantitative structure–activity relationship methods that use only descriptor – assay correlations. We summarize three recent imputation strategies – heterogeneous deep imputation, assay profile methods and matrix factorization – and compare these with quantitative structure–activity relationship methods, including deep learning, in drug discovery settings. We comment on the value added by imputation methods when used in an ongoing project and find that imputation produces stronger models, earlier in the project, over activity and absorption, distribution, metabolism and elimination end points. </jats:p
Superfluidity at the BEC-BCS crossover in two-dimensional Fermi gases with population and mass imbalance
We explore the zero temperature phase behavior of a two-dimensional
two-component atomic Fermi gas with population and mass imbalance in the regime
of the BEC-BCS crossover. Working in the mean-field approximation, we show that
the normal and homogeneous balanced superfluid phases are separated by an
inhomogeneous superfluid phase of Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) type.
We obtain an analytical expression for the line of continuous transitions
separating the normal and inhomogeneous FFLO phases. We further show that the
transition from the FFLO phase to the homogeneous balanced superfluid is
discontinuous leading to phase separation. If the species have different
masses, the superfluid phase is favored when the lighter species is in excess.
We explore the implications of these findings for the properties of the
two-component Fermi gas in the atomic trap geometry. Finally, we compare and
contrast our findings with the predicted phase behavior of the electron-hole
bilayer system.Comment: 11 pages, 6 figures. Accepted by Phys. Rev.
Many-flavor electron gas approach to electron-hole drops
A many-flavor electron gas (MFEG) is analyzed, such as could be found in a
multi-valley semiconductor or semimetal. Using the re-derived polarizability
for the MFEG an exact expression for the total energy of a uniform MFEG in the
many-flavor approximation is found; the interacting energy per particle is
shown to be -0.574447E_h a_0^3/4 m*^3/4 n^1/4 with E_h being the Hartree
energy, a_0 Bohr radius, and m^* particle effective mass. The short
characteristic length-scale of the MFEG motivates a local density
approximation, allowing a gradient expansion in the energy density, and the
expansion scheme is applied to electron-hole drops, finding a new form for the
density profile and its surface scaling properties.Comment: 11 pages, 5 figure
Upper critical field in superconductors near ferromagnetic quantum critical points; UCoGe
We study the strong-coupling superconductivity near ferromagnetic quantum
critical points, mainly focusing on the upper critical fields . Based
on our simple model calculations, we discuss experimentally observed unusual
behaviors of in a recently discovered ferromagnetic superconductor
UCoGe. Especially, the large anisotropy between -axis and
-axis, and the strong-coupling behaviors in
are investigated. We also examine effects of
non-analytic corrections in the spin susceptibility on the superconductivity,
which can arise from effective long range interactions due to particle-hole
excitations.Comment: Proceedings of ICHE2010, Toky
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