1,849 research outputs found
Stable Frank-Kasper phases of self-assembled, soft matter spheres
Single molecular species can self-assemble into Frank Kasper (FK) phases,
finite approximants of dodecagonal quasicrystals, defying intuitive notions
that thermodynamic ground states are maximally symmetric. FK phases are
speculated to emerge as the minimal-distortional packings of space-filling
spherical domains, but a precise quantitation of this distortion and how it
affects assembly thermodynamics remains ambiguous. We use two complementary
approaches to demonstrate that the principles driving FK lattice formation in
diblock copolymers emerge directly from the strong-stretching theory of
spherical domains, in which minimal inter-block area competes with minimal
stretching of space-filling chains. The relative stability of FK lattices is
studied first using a diblock foam model with unconstrained particle volumes
and shapes, which correctly predicts not only the equilibrium {\sigma} lattice,
but also the unequal volumes of the equilibrium domains. We then provide a
molecular interpretation for these results via self-consistent field theory,
illuminating how molecular stiffness regulates the coupling between
intra-domain chain configurations and the asymmetry of local packing. These
findings shed new light on the role of volume exchange on the formation of
distinct FK phases in copolymers, and suggest a paradigm for formation of FK
phases in soft matter systems in which unequal domain volumes are selected by
the thermodynamic competition between distinct measures of shape asymmetry.Comment: 40 pages, 22 figure
Magneto-elastic effects and magnetization plateaus in two dimensional systems
We show the importance of both strong frustration and spin-lattice coupling
for the stabilization of magnetization plateaus in translationally invariant
two-dimensional systems. We consider a frustrated spin-1/2 Heisenberg model
coupled to adiabatic phonons under an external magnetic field. At zero
magnetization, simple structures with two or at most four spins per unit cell
are stabilized, forming dimers or plaquettes, respectively. A much
richer scenario is found in the case of magnetization , where larger
unit cells are formed with non-trivial spin textures and an analogy with the
corresponding classical Ising model is detectable. Specific predictions on
lattice distortions and local spin values can be directly measured by X-rays
and Nuclear Magnetic Resonance experiments.Comment: 4 pages and 4 figure
Competing magnetic interactions in spin-1/2 square lattice: hidden order in SrVO
With decreasing temperature SrVO undergoes two structural phase
transitions, tetragonal-to-orthorhombic-to-tetragonal, without long-range
magnetic order. Recent experiments suggest, that only at very low temperature
SrVO might enter some, yet unknown, phase with long-range magnetic
order, but without orthorhombic distortion. By combining relativistic density
functional theory with an extended spin-1/2 compass-Heisenberg model we find an
antiferromagnetic single-stripe ground state with highly competing exchange
interactions, involving a non negligible inter-layer coupling, which places the
system at the crossover between between the XY and Heisenberg picture. Most
strikingly, we find a strong two-site "spin-compass" exchange anisotropy which
is relieved by the orthorhombic distortion induced by the spin stripe order.
Based on these results we discuss the origin of the hidden order phase and the
possible formation of a spin-liquid at low temperatures
Third order dielectric susceptibility in a model quantum paraelectric
In the context of perovskite quantum paraelectrics, we study the effects of a
quadrupolar interaction , in addition to the standard dipolar one .
We concentrate here on the nonlinear dielectric response , as
the main response function sensitive to quadrupolar (in our case
antiquadrupolar) interactions. We employ a 3D quantum four-state lattice model
and mean-field theory. The results show that inclusion of quadrupolar coupling
of moderate strength () is clearly accompanied by a
double change of sign of from negative to positive, near the
quantum temperature where the quantum paraelectric behaviour sets in. We
fit our to recent experimental data for SrTiO, where the
sign change is identified close to .Comment: 22 page
New class of 3D topological insulator in double perovskite
We predict a new class of three-dimensional topological insulators (TIs) in
which the spin-orbit coupling (SOC) can more effectively generate a large band
gap at point. The band gap of conventional TI such as BiSe is
mainly limited by two factors, the strength of SOC and, from electronic
structure perspective, the band gap when SOC is absent. While the former is an
atomic property, we find that the latter can be minimized in a generic
rock-salt lattice model in which a stable crossing of bands {\it at} the Fermi
level along with band character inversion occurs for a range of parameters in
the absence of SOC. Thus, large-gap TI's or TI's comprised of lighter elements
can be expected. In fact, we find by performing first-principle calculations
that the model applies to a class of double perovskites ABiXO (A = Ca,
Sr, Ba; X = Br, I) and the band gap is predicted up to 0.55 eV. Besides, more
detailed calculations considering realistic surface structure indicate that the
Dirac cones are robust against the presence of dangling bond at the boundary
with a specific termination.Comment: submitted; title changed and new references added; see DOI for
published versio
Towards Mott design by -doping of strongly correlated titanates
Doping the distorted-perovskite Mott insulators LaTiO and GdTiO with
a single SrO layer along the [001] direction gives rise to a rich correlated
electronic structure. A realistic superlattice study by means of the charge
self-consistent combination of density functional theory with dynamical
mean-field theory reveals layer- and temperature-dependent multi-orbital
metal-insulator transitions. An orbital-selective metallic layer at the
interface dissolves via an orbital-polarized doped-Mott state into an
orbital-ordered insulating regime beyond the two conducting TiO layers. We
find large differences in the scattering behavior within the latter. Breaking
the spin symmetry in -doped GdTiO results in blocks of
ferromagnetic itinerant and ferromagnetic Mott-insulating layers which are
coupled antiferromagnetically.Comment: 17 pages, 9 figures, final versio
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