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### Structural and electronic properties of SrZrO3 and Sr(Ti,Zr) O3 alloys

Using hybrid density functional calculations, we study the electronic and structural properties of SrZrO3 and ordered Sr(Ti,Zr)O3 alloys. Calculations were performed for the ground-state orthorhombic (Pnma) and high-temperature cubic (Pm3m) phases of SrZrO3. The variation of the lattice parameters and band gaps with Ti addition was studied using ordered SrTixZr1-xO3 structures with x=0, 0.25, 0.5, 0.75, and 1. As Ti is added to SrZrO3, the lattice parameter is reduced and closely follows Vegard's law. On the other hand, the band gap shows a large bowing and is highly sensitive to the Ti distribution. For x=0.5, we find that arranging the Ti and Zr atoms into a 1Ă—1SrZrO3/SrTiO3 superlattice along the [001] direction leads to interesting properties, including a highly dispersive single band at the conduction-band minimum (CBM), which is absent in both parent compounds, and a band gap close to that of pure SrTiO3. These features are explained by the splitting of the lowest three conduction-band states due to the reduced symmetry of the superlattice, lowering the band originating from the in-plane Ti 3dxy orbitals. The lifting of the t2g orbital degeneracy around the CBM suppresses scattering due to electron-phonon interactions. Our results demonstrate how short-period SrZrO3/SrTiO3 superlattices could be exploited to engineer the band structure and improve carrier mobility compared to bulk SrTiO3

### Hubbard-corrected density functional perturbation theory with ultrasoft pseudopotentials

We present in full detail a newly developed formalism enabling density functional perturbation theory (DFPT) calculations from a DFT+U ground state. The implementation includes ultrasoft pseudopotentials and is valid for both insulating and metallic systems. It aims at fully exploiting the versatility of DFPT combined with the low-cost
DFT+U functional. This allows us to avoid computationally intensive frozen-phonon calculations when
DFT+U is used to eliminate the residual electronic self-interaction from approximate functionals and to capture the localization of valence electrons, e.g., on d or f states. In this way, the effects of electronic localization (possibly due to correlations) are consistently taken into account in the calculation of specific phonon modes, Born effective charges, dielectric tensors, and in quantities requiring well converged sums over many phonon frequencies, as phonon density of states and free energies. The new computational tool is applied to two representative systems, namely CoO, a prototypical transition metal monoxide and LiCoO2, a material employed for the cathode of Li-ion batteries. The results show the effectiveness of our formalism to capture in a quantitatively reliable way the vibrational properties of systems with localized valence electrons

### Anisotropic Power-law Inflation

We study an inflationary scenario in supergravity model with a gauge kinetic
function. We find exact anisotropic power-law inflationary solutions when both
the potential function for an inflaton and the gauge kinetic function are
exponential type. The dynamical system analysis tells us that the anisotropic
power-law inflation is an attractor for a large parameter region.Comment: 14 pages, 1 figure. References added, minor corrections include

### Anisotropic Inflation with Non-Abelian Gauge Kinetic Function

We study an anisotropic inflation model with a gauge kinetic function for a
non-abelian gauge field. We find that, in contrast to abelian models, the
anisotropy can be either a prolate or an oblate type, which could lead to a
different prediction from abelian models for the statistical anisotropy in the
power spectrum of cosmological fluctuations. During a reheating phase, we find
chaotic behaviour of the non-abelian gauge field which is caused by the
nonlinear self-coupling of the gauge field. We compute a Lyapunov exponent of
the chaos which turns out to be uncorrelated with the anisotropy.Comment: 16 pages, 4 figure

### Inflation with stable anisotropic hair: is it cosmologically viable?

Recently an inflationary model with a vector field coupled to the inflaton
was proposed and the phenomenology studied for the Bianchi type I spacetime. It
was found that the model demonstrates a counter-example to the cosmic no-hair
theorem since there exists a stable anisotropically inflationary fix-point. One
of the great triumphs of inflation, however, is that it explains the observed
flatness and isotropy of the universe today without requiring special initial
conditions. Any acceptable model for inflation should thus explain these
observations in a satisfactory way. To check whether the model meets this
requirement, we introduce curvature to the background geometry and consider
axisymmetric spacetimes of Bianchi type II,III and the Kantowski-Sachs metric.
We show that the anisotropic Bianchi type I fix-point is an attractor for the
entire family of such spacetimes. The model is predictive in the sense that the
universe gets close to this fix-point after a few e-folds for a wide range of
initial conditions. If inflation lasts for N e-folds, the curvature at the end
of inflation is typically of order exp(-2N). The anisotropy in the expansion
rate at the end of inflation, on the other hand, while being small on the
one-percent level, is highly significant. We show that after the end of
inflation there will be a period of isotropization lasting for about 2N/3
e-folds. After that the shear scales as the curvature and becomes dominant
around N e-folds after the end of inflation. For plausible bounds on the reheat
temperature the minimum number of e-folds during inflation, required for
consistency with the isotropy of the supernova Ia data, lays in the interval
(21,48). Thus the results obtained for our restricted class of spacetimes
indicates that inflation with anisotropic hair is cosmologically viable.Comment: 25 pages, 3 figures; v2: Minor changes, refs added; v3: JHEP version
(proof-reading corrections

### Bulk Axions, Brane Back-reaction and Fluxes

Extra-dimensional models can involve bulk pseudo-Goldstone bosons (pGBs)
whose shift symmetry is explicitly broken only by physics localized on branes.
Reliable calculation of their low-energy potential is often difficult because
it requires details of the stabilization of the extra dimensions. In rugby ball
solutions, for which two compact extra dimensions are stabilized in the
presence of only positive-tension brane sources, the effects of brane
back-reaction can be computed explicitly. This allows the calculation of the
shape of the low-energy pGB potential and response of the extra dimensional
geometry as a function of the perturbing brane properties. If the
pGB-dependence is a small part of the total brane tension a very general
analysis is possible, permitting an exploration of how the system responds to
frustration when the two branes disagree on what the proper scalar vacuum
should be. We show how the low-energy potential is given by the sum of brane
tensions (in agreement with common lore) when only the brane tensions couple to
the pGB. We also show how a direct brane coupling to the flux stabilizing the
extra dimensions corrects this result in a way that does not simply amount to
the contribution of the flux to the brane tensions. We calculate the mass of
the would-be zero mode, and briefly describe several potential applications,
including a brane realization of `natural inflation,' and a dynamical mechanism
for suppressing the couplings of the pGB to matter localized on the branes.
Since the scalar can be light enough to be relevant to precision tests of
gravity (in a technically natural way) this mechanism can be relevant to
evading phenomenological bounds.Comment: 36 pages, JHEP styl

### Hybrid compactifications and brane gravity in six dimensions

We consider a six-dimensional axisymmetric Einstein-Maxwell model of warped
braneworlds. The bulk is bounded by two branes, one of which is a conical
3-brane and the other is a 4-brane wrapped around the axis of symmetry. The
latter brane is assumed to be our universe. If the tension of the 3-brane is
fine-tuned, it folds the internal two-dimensional space in a narrow cone,
making sufficiently small the Kaluza-Klein circle of the 4-brane. An arbitrary
energy-momentum tensor can be accommodated on this ring-like 4-brane. We study
linear perturbations sourced by matter on the brane, and show that weak gravity
is apparently described by a four-dimensional scalar-tensor theory. The extra
scalar degree of freedom can be interpreted as the fluctuation of the internal
space volume (or that of the circumference of the ring), the effect of which
turns out to be suppressed at long distances. Consequently, four-dimensional
Einstein gravity is reproduced on the brane. We point out that as in the
Randall-Sundrum model, the brane bending mode is crucial for recovering the
four-dimensional tensor structure in this setup.Comment: 15 pages, 2 figures; v2: references added; v3: accepted for
publication in Class. Quant. Gra

### Effective Theory Approach to the Spontaneous Breakdown of Lorentz Invariance

We generalize the coset construction of Callan, Coleman, Wess and Zumino to
theories in which the Lorentz group is spontaneously broken down to one of its
subgroups. This allows us to write down the most general low-energy effective
Lagrangian in which Lorentz invariance is non-linearly realized, and to explore
the consequences of broken Lorentz symmetry without having to make any
assumptions about the mechanism that triggers the breaking. We carry out the
construction both in flat space, in which the Lorentz group is a global
spacetime symmetry, and in a generally covariant theory, in which the Lorentz
group can be treated as a local internal symmetry. As an illustration of this
formalism, we construct the most general effective field theory in which the
rotation group remains unbroken, and show that the latter is just the
Einstein-aether theory.Comment: 45 pages, no figures

### Observation by resonant angle-resolved photoemission of a critical thickness for 2-dimensional electron gas formation in SrTiO$_3$ embedded in GdTiO$_3$

For certain conditions of layer thickness, the interface between GdTiO$_3$
(GTO) and SrTiO$_3$ (STO) in multilayer samples has been found to form a
two-dimensional electron gas (2DEG) with very interesting properties including
high mobilities and ferromagnetism. We have here studied two trilayer samples
of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001)
(LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ (LSAT), with the STO layer
thicknesses being at what has been suggested is the critical thickness for 2DEG
formation. We have studied these with Ti-resonant angle-resolved (ARPES) and
angle-integrated photoemission and find that the spectral feature in the
spectra associated with the 2DEG is present in the 1.5 unit cell sample, but
not in the 1.0 unit cell sample. We also observe through core-level spectra
additional states in Ti and Sr, with the strength of a low-binding-energy state
for Sr being associated with the appearance of the 2DEG, and we suggest it to
have an origin in final-state core-hole screening.Comment: 12 pages, 4 figure

### Brane cosmological solutions in six-dimensional warped flux compactifications

We study cosmology on a conical brane in the six-dimensional
Einstein-Maxwell-dilaton system, where the extra dimensions are compactified by
a magnetic flux. We systematically construct exact cosmological solutions using
the fact that the system is equivalently described by (6+n)-dimensional pure
Einstein-Maxwell theory via dimensional reduction. In particular, we find a
power-law inflationary solution for a general dilatonic coupling. When the
dilatonic coupling is given by that of Nishino-Sezgin chiral supergravity, this
reduces to the known solution which is not inflating. The power-law solution is
shown to be the late-time attractor. We also investigate cosmological tensor
perturbations in this model using the (6+n)-dimensional description. We obtain
the separable equation of motion and find that there always exist a zero mode,
while tachyonic modes are absent in the spectrum. The mass spectrum of
Kaluza-Klein modes is obtained numerically.Comment: 12 pages, 2 figures; v2: references added; v3: version published in
JCA

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