7,535 research outputs found
Emergent Phenomena in Spatially Confined Manganites
Rare earth manganites exhibit colossal magnetoresistance (CMR). There is evidence that alloyed single crystal materials in this class can display electronic inhomogeneity in which areas with vastly different electronic and magnetic properties can form and coexist in phase separated domains ranging in size from a few nanometers to micrometers. This phase separation (PS) is of particular interest, as it has been suggested that it is the central feature that leads to CMR in manganites, the Mott transition in VO2 and may play a role in high-TC superconductivity in cuprates. However there is debate as to its precise role.
The purpose of my research is to answer fundamental questions about the specific role of PS in complex oxides. I reduce single crystal thin films of an electronically phase separated manganite to the scale of their inherent electronic phase domains near the metal-insulator transition. Unlike transport measurements done on bulk or thin films where the electrons follow only the metallic path of least resistance, this configuration forces electrons to travel through both the metallic and insulating regions residing in the material. This has led to observations of several new phenomena such as a reemergent metal-insulator transition, ultra-sharp jumps in resistivity at the metal-insulator transition, and the first high resolution observation of single domain electronic phase transitions in the time domain.
While the manganites will be the primary focus throughout this dissertation, the spatial confinement techniques presented here are not limited to only these materials. They can be applied to any phase separated system to probe regions resistively hidden to transport measurements
Inflation from Geometrical Tachyons
We propose an alternative formulation of tachyon inflation using the
geometrical tachyon arising from the time dependent motion of a BPS -brane
in the background geometry due to parallel 5-branes arranged around a
ring of radius . Due to the fact that the mass of this geometrical tachyon
field is times smaller than the corresponding open-string tachyon
mass, we find that the slow roll conditions for inflation and the number of
e-foldings can be satisfied in a manner that is consistent with an effective
4-dimensional model and with a perturbative string coupling. We also show that
the metric perturbations produced at the end of inflation can be sufficiently
small and do not lead to the inconsistencies that plague the open string
tachyon models. Finally we argue for the existence of a minimum of the
geometrical tachyon potential which could give rise to a traditional reheating
mechanism.Comment: Latex, 20 pages, 4 figures; correction of algebraic errors in section
5 concerning the tachyon potential near its minimum. Conclusions unchange
First-order melting of a weak spin-orbit Mott insulator into a correlated metal
The electronic phase diagram of the weak spin-orbit Mott insulator
(Sr(1-x)Lax)3Ir2O7 is determined via an exhaustive experimental study. Upon
doping electrons via La substitution, an immediate collapse in resistivity
occurs along with a narrow regime of nanoscale phase separation comprised of
antiferromagnetic, insulating regions and paramagnetic, metallic puddles
persisting until x~0.04. Continued electron doping results in an abrupt,
first-order phase boundary where the Neel state is suppressed and a homogenous,
correlated, metallic state appears with an enhanced spin susceptibility and
local moments. As the metallic state is stabilized, a weak structural
distortion develops and suggests a competing instability with the parent
spin-orbit Mott state.Comment: 5 pages, 4 figure
Growth diagram of La0.7Sr0.3MnO3 thin films using pulsed laser deposition
An experimental study was conducted on controlling the growth mode of
La0.7Sr0.3MnO3 thin films on SrTiO3 substrates using pulsed laser deposition
(PLD) by tuning growth temperature, pressure and laser fluence. Different thin
film morphology, crystallinity and stoichiometry have been observed depending
on growth parameters. To understand the microscopic origin, the adatom
nucleation, step advance processes and their relationship to film growth were
theoretically analyzed and a growth diagram was constructed. Three boundaries
between highly and poorly crystallized growth, 2D and 3D growth, stoichiometric
and non-stoichiometric growth were identified in the growth diagram. A good fit
of our experimental observation with the growth diagram was found. This case
study demonstrates that a more comprehensive understanding of the growth mode
in PLD is possible
Structural and electronic origin of the magnetic structures in hexagonal LuFeO
Using combined theoretical and experimental approaches, we studied the
structural and electronic origin of the magnetic structure in hexagonal
LuFeO. Besides showing the strong exchange coupling that is consistent with
the high magnetic ordering temperature, the previously observed spin
reorientation transition is explained by the theoretically calculated magnetic
phase diagram. The structural origin of this spin reorientation that is
responsible for the appearance of spontaneous magnetization, is identified by
theory and verified by x-ray diffraction and absorption experiments.Comment: 5 pages, 2 tables and 4 figures, Please contact us for the
supplementary material. Accepted in Phys. Rev. B, in productio
Tuning Structural, Transport and Magnetic Properties of Epitaxial SrRuO3 through Ba-Substitution
The perovskite ruthenates (ARuO3, A = Ca, Ba, or Sr) exhibit unique
properties owing to a subtle interplay of crystal structure and electronic-spin
degrees of freedom. Here, we demonstrate an intriguing continuous tuning of
crystal symmetry from orthorhombic to tetragonal (no octahedral rotations)
phases in epitaxial SrRuO3 achieved via Ba-substitution (Sr1-xBaxRuO3 with 0 <
x < 0.7). An initial Ba-substitution to SrRuO3 not only changes the
ferromagnetic properties, but also tunes the perpendicular magnetic anisotropy
via flattening the Ru-O-Ru bond angle (to 180{\deg}), resulting in the maximum
Curie temperature and an extinction of RuO6 rotational distortions at x = 0.20.
For x > 0.2, the suppression of RuO6 octahedral rotational distortion
dominantly enhances the ferromagnetism in the system, though competing with the
impact of the RuO6 tetragonal distortion. Further increasing x > 0.2 gradually
enhances the tetragonal-type distortion, resulting in the tuning of Ru-4d
orbital occupancy and suppression of ferromagnetism. Our results demonstrate
that isovalent substitution of the A-site cations significantly and
controllably impacts both electronic and magnetic properties of perovskite
oxides
Anisotropy of thermal conductivity oscillations in relation to the Kitaev spin liquid phase
In the presence of external magnetic field, the Kitaev model could either
hosts gapped topological anyon or gapless Majorana fermions. In
-RuCl, the gapped and gapless cases are only separated by a
thirty-degree rotation of the in-plane magnetic field vector. The
presence/absence of the spectral gap is key for understanding the thermal
transport behavior in -RuCl. Here, we study the anisotropy of the
oscillatory features of thermal conductivity in -RuCl. We examine
the oscillatory features of thermal conductivities (k//a, k//b) with fixed
external fields and found distinct behavior for the gapped (B//a) and gapless
(B//b) scenarios. Furthermore, we track the evolution of thermal resistivity
() and its oscillatory features with the rotation of in-plane
magnetic fields from B//b to B//a. The thermal resistivity
display distinct rotational symmetries before and after the emergence of the
field induced Kitaev spin liquid phase. These experiment data suggest close
correlations between the oscillatory features of thermal conductivity, the
underlying Kitaev spin liquid phase and the fermionic excitation it holds
Room-temperature multiferroic hexagonal LuFeO films
The crystal and magnetic structures of single-crystalline hexagonal LuFeO
films have been studied using x-ray, electron and neutron diffraction methods.
The polar structure of these films are found to persist up to 1050 K; and the
switchability of the polar behavior is observed at room temperature, indicating
ferroelectricity. An antiferromagnetic order was shown to occur below 440 K,
followed by a spin reorientation resulting in a weak ferromagnetic order below
130 K. This observation of coexisting multiple ferroic orders demonstrates that
hexagonal LuFeO films are room-temperature multiferroics
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