65 research outputs found
Hopping Transport in SrTiO3/Nd1-xTiO3/SrTiO3 Heterostructures
Electronic transport near the insulator-metal transition is investigated in
the molecular beam epitaxy-grown SrTiO3/Nd1-xTiO3/SrTiO3 heterostructures using
temperature dependent magnetotransport measurements. It was found that
Nd-vacancies introduce localized electronic states resulting in the variable
range hopping transport at low temperatures. At a fixed Nd-vacancies
concentration, a crossover from Mott to Efros-Shklovskii (ES) variable range
hopping transport was observed with decreasing temperature. With increasing
disorder, a sign reversal of magnetoresistance from positive to negative was
observed revealing interplay between intra-state interaction and the energy
dependence of the localization length as a function of disorder. These findings
highlight the important role of stoichiometry when exploring intrinsic effect
using heterostructure and interfaces in addition to offering broad opportunity
to tailor low temperature transport using non-stoichiometry defects
Band alignment of epitaxial SrTiO3 thin films with (LaAlO3)0.3-(Sr2AlTaO6)0.7 (001)
SrTiO (STO) epitaxial thin films and heterostructures are of
considerable interest due to the wide range of functionalities they exhibit.
The alloy perovskite (LaAlO)-(SrAlTaO) (LSAT)
is commonly used as a substrate for these material structures due to its
structural compatibility with STO and the strain-induced ferroelectric response
in STO films grown on LSAT. However, surprisingly little is known about the
electronic properties of the STO/LSAT interface despite its potentially
important role in affecting the overall electronic structure of system. We
examine the band alignment of STO/LSAT heterostructures using x-ray
photoelectron spectroscopy for epitaxial STO films deposited using two
different molecular beam epitaxy approaches. We find that the valence band
offset ranges from +0.2(1) eV to -0.2(1) eV depending on the film surface
termination. From these results we extract a conduction band offset from
-2.4(1) eV to -2.8(1) eV, indicating that the conduction band edge is more
deeply bound in STO and that LSAT will not act as a sink or trap for electrons
in the supported film or multilayer.Comment: 14 pages, 3 figures; Accepted in Applied Physics Letter
Unraveling the Effect of Electron-Electron Interaction on Electronic Transport in High-Mobility Stannate Films
Contrary to the common belief that electron-electron interaction (EEI) should
be negligible in s-orbital-based conductors, we demonstrated that the EEI
effect could play a significant role on electronic transport leading to the
misinterpretation of the Hall data. We show that the EEI effect is primarily
responsible for an increase in the Hall coefficient in the La-doped SrSnO3
films below 50 K accompanied by an increase in the sheet resistance. The
quantitative analysis of the magnetoresistance (MR) data yielded a large phase
coherence length of electrons exceeding 450 nm at 1.8 K and revealed the
electron-electron interaction being accountable for breaking of electron phase
coherency in La-doped SrSnO3 films. These results while providing critical
insights into the fundamental transport behavior in doped stannates also
indicate the potential applications of stannates in quantum coherent electronic
devices owing to their large phase coherence length.Comment: 3 figure
Ferromagnetism and spin-dependent transport at a complex oxide interface
Complex oxide interfaces are a promising platform for studying a wide array
of correlated electron phenomena in low-dimensions, including magnetism and
superconductivity. The microscopic origin of these phenomena in complex oxide
interfaces remains an open question. Here we investigate for the first time the
magnetic properties of semi-insulating NdTiO/SrTiO (NTO/STO) interfaces
and present the first milli-Kelvin study of NTO/STO. The magnetoresistance (MR)
reveals signatures of local ferromagnetic order and of spin-dependent
thermally-activated transport, which are described quantitatively by a simple
phenomenological model. We discuss possible origins of the interfacial
ferromagnetism. In addition, the MR also shows transient hysteretic features on
a timescale of ~10-100 seconds. We demonstrate that these are consistent with
an extrinsic magneto-thermal origin, which may have been misinterpreted in
previous reports of magnetism in STO-based oxide interfaces. The existence of
these two MR regimes (steady-state and transient) highlights the importance of
time-dependent measurements for distinguishing signatures of ferromagnetism
from other effects that can produce hysteresis at low temperatures
A New Line Defect in NdTiO3 Perovskite
Perovskite oxides form an eclectic class of materials owing to their
structural flexibility in accommodating cations of different sizes and
valences. They host well known point and planar defects, but so far no line
defect has been identified other than dislocations. Using analytical scanning
transmission electron microscopy (STEM) and ab initio calculations we have
detected and characterized the atomic and electronic structures of a novel line
defect in NdTiO3 perovskite. It appears in STEM images as a perovskite cell
rotated by 45 degrees. It consists of self-organized Ti-O vacancy lines
replaced by Nd columns surrounding a central Ti-O octahedral chain containing
Ti4+ ions, as opposed to Ti3+ in the host. The distinct Ti valence in this line
defect introduces the possibility of engineering exotic conducting properties
in a single preferred direction and tailoring novel desirable functionalities
in this Mott insulator.Comment: 24 pages, 5 figure
Dopant Solubility, and Charge Compensation in La-doped SrSnO3 Films
We investigate lanthanum (La) as an n-type dopant in the strain-stabilized
tetragonal phase of SrSnO3 grown on GdScO3 (110) using a radical-based hybrid
molecular beam epitaxy approach. Fully coherent, epitaxial films with
atomically smooth film surface were obtained irrespective of doping density. By
combining secondary ion mass spectroscopy and Hall measurements, we demonstrate
that each La atom contributes to one electron to the film confirming it
occupies Sr-site in SrSnO3 and that it is completely activated. Carrier density
exceeding 1 x 10^20 cm-3 was achieved in LSSO films, which is in excellent
agreement with the dopant-solubility limit predicted by the density functional
theory calculations. A record-high room-temperature mobility of 70 cm2V-1s-1 at
1 x 10^20 cm-3 was obtained in 12 nm La-doped SrSnO3 film making this the
thinnest perovskite oxide semiconductor with a reasonably high electron
mobility at room temperature. We discuss the structure-dopant-transport
property relationships providing essential knowledge for the design of
electronic devices using these materials.Comment: 3 figure
Effects of paramagnetic pair-breaking and spin-orbital coupling on multi-band superconductivity
The BCS picture of superconductivity describes pairing between electrons
originating from a single band. A generalization of this picture occurs in
multi-band superconductors, where electrons from two or more bands contribute
to superconductivity. The contributions of the different bands can result in an
overall enhancement of the critical field and can lead to qualitative changes
in the temperature dependence of the upper critical field when compared to the
single-band case. While the role of orbital pair-breaking on the critical field
of multi-band superconductors has been explored extensively, paramagnetic and
spin-orbital scattering effects have received comparatively little attention.
Here we investigate this problem using thin films of Nd-doped SrTiO. We
furthermore propose a model for analyzing the temperature-dependence of the
critical field in the presence of orbital, paramagnetic and spin-orbital
effects, and find a very good agreement with our data. Interestingly, we also
observe a dramatic enhancement in the out-of-plane critical field to values
well in excess of the Chandrasekhar-Clogston (Pauli) paramagnetic limit, which
can be understood as a consequence of multi-band effects in the presence of
spin-orbital scattering.Comment: 32 page
Direct observation and consequences of dopant segregation inside and outside dislocation cores in perovskite BaSnO3
Distinct dopant behaviors inside and outside dislocation cores are identified
by atomic-resolution electron microscopy in perovskite BaSnO3 with considerable
consequences on local atomic and electronic structures. Driven by elastic
strain, when A-site designated La dopants segregate near a dislocation core,
the dopant atoms accumulate at the Ba sites in compressively strained regions.
This triggers formation of Ba-vacancies adjacent to the core atomic sites
resulting in reconstruction of the core. Notwithstanding the presence of
extremely large tensile strain fields, when La atoms segregate inside the
dislocation core, they become B-site dopants, replacing Sn atoms and
compensating the positive charge of the core oxygen vacancies. Electron
energy-loss spectroscopy shows that the local electronic structure of these
dislocations changes dramatically due to the segregation of the dopants inside
and around the core ranging from formation of strong La-O hybridized electronic
states near the conduction band minimum to insulator-to-metal transition
Shubnikov-de Haas effect in low electron density SrTiO3: Conduction band edge of SrTiO3 redux
The Shubnikov-de Haas effect is used to explore the conduction band edge of
high mobility SrTiO3 films doped with La. The results largely confirm the
earlier measurements by Uwe et al. [Jap. J. Appl. Phys. 24, Suppl. 24-2, 335
(1985)]. The band edge dispersion differs significantly from the predictions of
ab initio electronic structure theory
STEM beam channeling in BaSnO3/LaAlO3 perovskite bilayers and visualization of 2D misfit dislocation network
A study of the STEM probe channeling in a heterostructures crystalline
specimen is presented here with a goal to guide appropriate STEM-based
characterization for complex structures. STEM analysis of perovskite
BaSnO3/LaAlO3 bilayers is performed and the dominating effects of beam
channeling on HAADF- and LAADF-STEM are illustrated. To study the electron beam
propagating through BaSnO3/LaAlO3 bilayers, probe intensity depth profiles are
calculated, and the effects of probe defocus and atomic column alignment are
discussed. Characteristics of the beam channeling are correlated to resulting
ADF-STEM images, which is then tested by comparing focal series of plan-view
HAADF-STEM images to those recorded experimentally. Additionally, discussions
on how to visualize the misfit dislocation network at the BaSnO3/LaAlO3
interface using HAADF- and LAADF-STEM images are provided.Comment: 23 pages, 8 figure
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