40 research outputs found
Quantum fluctuation driven first order phase transition in weak ferromagnetic metals
In a local Fermi liquid (LFL), we show that there is a line of weak first
order phase transitions between the ferromagnetic and paramagnetic phases due
to purely quantum fluctuations. We predict that an instability towards
superconductivity is only possible in the ferromagnetic state. At T=0 we find a
point on the phase diagram where all three phases meet and we call this a
quantum triple point (QTP). A simple application of the Gibbs phase rule shows
that only these three phases can meet at the QTP. This provides a natural
explanation of the absence of superconductivity at this point coming from the
paramagnetic side of the phase diagram, as observed in the recently discovered
ferromagnetic superconductor, .Comment: 5 pages, 5 figure
Mesoscopic theory for inhomogeneous mixtures
Mesoscopic density functional theory for inhomogeneous mixtures of sperical
particles is developed in terms of mesoscopic volume fractions by a systematic
coarse-graining procedure starting form microscopic theory. Approximate
expressions for the correlation functions and for the grand potential are
obtained for weak ordering on mesoscopic length scales. Stability analysis of
the disordered phase is performed in mean-field approximation (MF) and beyond.
MF shows existence of either a spinodal or a -surface on the
volume-fractions - temperature phase diagram. Separation into homogeneous
phases or formation of inhomogeneous distribution of particles occurs on the
low-temperature side of the former or the latter surface respectively,
depending on both the interaction potentials and the size ratios between
particles of different species. Beyond MF the spinodal surface is shifted, and
the instability at the -surface is suppressed by fluctuations. We
interpret the -surface as a borderline between homogeneous and
inhomogeneous (containing clusters or other aggregates) structure of the
disordered phase. For two-component systems explicit expressions for the MF
spinodal and -surfaces are derived. Examples of interaction potentials
of simple form are analyzed in some detail, in order to identify conditions
leading to inhomogeneous structures.Comment: 6 figure
Phase-field crystal modelling of crystal nucleation, heteroepitaxy and patterning
We apply a simple dynamical density functional theory, the
phase-field-crystal (PFC) model, to describe homogeneous and heterogeneous
crystal nucleation in 2d monodisperse colloidal systems and crystal nucleation
in highly compressed Fe liquid. External periodic potentials are used to
approximate inert crystalline substrates in addressing heterogeneous
nucleation. In agreement with experiments in 2d colloids, the PFC model
predicts that in 2d supersaturated liquids, crystalline freezing starts with
homogeneous crystal nucleation without the occurrence of the hexatic phase. At
extreme supersaturations crystal nucleation happens after the appearance of an
amorphous precursor phase both in 2d and 3d. We demonstrate that contrary to
expectations based on the classical nucleation theory, corners are not
necessarily favourable places for crystal nucleation. Finally, we show that
adding external potential terms to the free energy, the PFC theory can be used
to model colloid patterning experiments.Comment: 21 pages, 16 figure
Bianchi Attractors: A Classification of Extremal Black Brane Geometries
Extremal black branes are of interest because they correspond to the ground
states of field theories at finite charge density in gauge/gravity duality. The
geometry of such a brane need not be translationally invariant in the spatial
directions along which it extends. A less restrictive requirement is that of
homogeneity, which still allows points along the spatial directions to be
related to each other by symmetries. In this paper, we find large new classes
of homogeneous but anisotropic extremal black brane horizons, which could
naturally arise in gauge/gravity dual pairs. In 4+1 dimensional spacetime, we
show that such homogeneous black brane solutions are classified by the Bianchi
classification, which is well known in the study of cosmology, and fall into
nine classes. In a system of Einstein gravity with negative cosmological term
coupled to one or two massive Abelian gauge fields, we find solutions with an
additional scaling symmetry, which could correspond to the near-horizon
geometries of such extremal black branes. These solutions realize many of the
Bianchi classes. In one case, we construct the complete extremal solution which
asymptotes to AdS space.Comment: Minor changes and a reference added. 43 Pages, 6 Figure