4 research outputs found
Continuously Tuning Epitaxial Strains by Thermal Mismatch
Strain engineering
of thin films is a conventionally employed approach
to enhance material properties and to energetically prefer ground
states that would otherwise not be attainable. Controlling strain
states in perovskite oxide thin films is usually accomplished through
coherent epitaxy by using lattice-mismatched substrates with similar
crystal structures. However, the limited choice of suitable oxide
substrates makes certain strain states experimentally inaccessible
and a continuous tuning impossible. Here, we report a strategy to
continuously tune epitaxial strains in perovskite films grown on Si(001)
by utilizing the large difference of thermal expansion coefficients
between the film and the substrate. By establishing an adsorption-controlled
growth window for SrTiO<sub>3</sub> thin films on Si using hybrid
molecular beam epitaxy, the magnitude of strain can be solely attributed
to thermal expansion mismatch, which only depends on the difference
between growth and room temperature. Second-harmonic generation measurements
revealed that structure properties of SrTiO<sub>3</sub> films could
be tuned by this method using films with different strain states.
Our work provides a strategy to generate continuous strain states
in oxide/semiconductor pseudomorphic buffer structures that could
help achieve desired material functionalities
High-Quality LaVO<sub>3</sub> Films as Solar Energy Conversion Material
Mott
insulating oxides and their heterostructures have recently been identified
as potential photovoltaic materials with favorable absorption properties
and an intrinsic built-in electric field that can efficiently separate
excited electron–hole pairs. At the same time, they are predicted
to overcome the Shockley-Queisser limit due to strong electron–electron
interaction present. Despite these premises a high concentration of
defects commonly observed in Mott insulating films acting as recombination
centers can derogate the photovoltaic conversion efficiency. With
use of the self-regulated growth kinetics in hybrid molecular beam
epitaxy, this obstacle can be overcome. High-quality, stoichiometric
LaVO<sub>3</sub> films were grown with defect densities of in-gap
states up to 2 orders of magnitude lower compared to the films in
the literature, and a factor of 3 lower than LaVO<sub>3</sub> bulk
single crystals. Photoconductivity measurements revealed a significant
photoresponsivity increase as high as tenfold of stoichiometric LaVO<sub>3</sub> films compared to their nonstoichiometric counterparts. This
work marks a critical step toward the realization of high-performance
Mott insulator solar cells beyond conventional semiconductors
Observation of Magnetoplasmons in Bi<sub>2</sub>Se<sub>3</sub> Topological Insulator
Both the collective (plasmon) and
the single particle (Drude) excitations
of an electron gas can be controlled and modified by an external magnetic
field <i>B</i>. At finite <i>B</i>, plasmon gives
rise to a magnetoplasmon mode and the Drude term to a cyclotron resonance.
These magnetic effects are expected to be extremely strong for Dirac
electrons with a linear energy-momentum dispersion, like those present
in graphene and topological insulators (TIs). Here, we investigate
both the plasmon and the Drude response versus <i>B</i> in
Bi<sub>2</sub>Se<sub>3</sub> topological insulator. At low <i>B</i>, the cyclotron resonance is still well separated in energy
from the magnetoplasmon mode; meanwhile, both excitations asymptotically
converge at the same energy for increasing <i>B</i>, consistently
with a dynamical mass for Dirac carriers of <i>m</i><sub>D</sub><sup>*</sup> = 0.18 ±
0.01 m<sub><i>e</i></sub>. In TIs, one then achieves an
excellent magnetic control of plasmonic excitations and this could
open the way toward plasmon controlled terahertz magneto-optics
Composition Control of Plasmon–Phonon Interaction Using Topological Quantum-Phase Transition in Photoexcited (Bi<sub>1–<i>x</i></sub>In<sub><i>x</i></sub>)<sub>2</sub>Se<sub>3</sub>
Plasmonics is a technology aiming
at light modulation via collective
charge oscillations. Topological insulators, where Dirac-like metallic
surfaces coexist with normal insulating bulk, have recently attracted
great attention in plasmonics due to their topology-originated outstanding
properties. Here, we introduce a new methodology for controlling the
interaction of a plasmon with a phonon in topological insulators,
which is a key for utilizing the unique spectral profiles for photonic
applications. By using both static and ultrafast terahertz spectroscopy,
we show that the interaction can be tuned by controlling the chemical
composition of (Bi<sub>1–<i>x</i></sub>In<sub><i>x</i></sub>)<sub>2</sub>Se<sub>3</sub> microribbon arrays. The
topological quantum-phase transition induced by varying the composition
drives a dramatic change in the strength of the plasmon–phonon
interaction. This was possible due to the availability of manipulating
the spatial overlap between topological surface plasmonic states and
underlying bulk phonons. Especially, we control the laser-induced
ultrafast evolution of the transient spectral peaks arising from the
plasmon–phonon interaction by varying the spatial overlap across
the topological phase transition. This study may provide a new platform
for realizing topological insulator-based ultrafast plasmonic devices