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 SrTiO₃ 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 SrTiO₃ 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 LaVO₃ 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 LaVO₃ 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 LaVO₃ bulk single crystals. Photoconductivity measurements revealed a significant photoresponsivity increase as high as tenfold of stoichiometric LaVO₃ 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 Bi₂Se₃ 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 Bi₂Se₃ 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>_D^* = 0.18 ± 0.01 m_<i>e</i>. 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_1–<i>x</i>In_<i>x</i>)₂Se₃

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_1–<i>x</i>In_<i>x</i>)₂Se₃ 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