Hidden dynamics in a metal-insulator transition based device

VO2copyright|PDI

The drive towards non-von Neumann device architectures has led to an intense focus on insulator-to-metal (IMT) and the converse (MIT) transitions. Studies of electric field-driven IMT in the prototypical VO2 thin-film channel devices are largely focused on the electrical and elastic responses of the films, but the response of the corresponding TiO2 substrate is often overlooked since it is nominally expected to be electrically passive and elastically rigid. Our recent work on in-operando spatiotemporal imaging of the coupled elastodynamics using x-ray diffraction microscopy of a VO2 film channel device on TiO2 substrate reveals two new surprises. First, the film channel bulges during the IMT, the opposite of the expected shrinking in the film undergoing IMT. Secondly, a microns thick proximal layer in the substrate also coherently bulges accompanying the IMT in the film, which is completely unexpected. Phase-field simulations of coupled IMT, oxygen vacancy electronic dynamics, and electronic carrier diffusion incorporating thermal and strain effects suggest that the observed elastodynamics can be explained by the known naturally occurring oxygen vacancies that rapidly ionize (and deionize) in concert with the IMT (MIT). Fast electrical-triggering of the IMT via ionizing defects and an active “IMT-like” substrate layer are critical aspects to consider in device applications. This collaborative work was led by Greg Stone, then a postdoctoral scholar in our group, and Vladimir Stoica, a research associate professor at Penn State.  The work involved close collaboration with Chen group on phase-field modeling, Haidan Wen and Zhonghou Cai on synchrotron x-ray,  Schlom  and Engel-Herbert groups on film growth, and  Datta group on device fabrication.  A news release is found here.

 

Strong Electron-Phonon Coupling in a Correlated Polar Metal

There is tremendous interest in employing collective excitations of lattice, spin, charge, and orbitals to tune strongly correlated electronic phenomena. Hugo Wang, Vincent Xiong and others report in Nature Communications (Sept, 2023) such an effect in a ruthenate, Ca3Ru2O7, where two phonons with strong electron-phonon coupling modulate the electronic pseudogap as well as mediate charge and spin density wave fluctuations. Combining temperature-dependent Raman spectroscopy with density functional theory reveals two phonons one that dominates in the pseudogap phase, and another in the metallic phases; the former opens the pseudogap, while the latter closes it. Moreover, the B2 phonons mediate incoherent charge and spin density wave fluctuations, as evidenced by changes in the background electronic Raman scattering .  The polar order breaks inversion symmetry, enabling infrared activity of these phonons, paving the way for coherent light-driven control of electronic transport. The crystals were grown in Prof. Mao’s group.  The density functional theory was conducted by Prof. Dabo’s group. The optical studies and analysis were conducted in Gopalan group.

A Promising Infrared Nonlinear Optical Crystal

MgSiP2

Superior infrared nonlinear optical (NLO) crystals are in urgent demand in the development of lasers and optical technologies for communications and computing. The critical challenge is to find a crystal with large non-resonant phase-matchable NLO coefficients and high laser damage threshold (LDTs) simultaneously, which however scale inversely. In a recent paper in Advanced Optical Materials (July 2023), by Jingyang He et. al. from our group, we have discovered that MgSiP2, exhibits a large second harmonic generation (SHG) coefficient of d14~d36= 89±5 pm V-1 at 1550 nm fundamental wavelength, surpassing the commercial NLO crystals, AgGaS2, AgGaSe2 and ZnGeP2. First principles theory reveals the polarizability and geometric arrangement of the [SiP4] tetrahedral units as the origin of this large nonlinear response. Remarkably, it also exhibits a high LDT value of 684 GW cm-2, which is six times larger than ZnGeP2 and three times larger than CdSiP2. It has a wide transparency window of 0.53-10.35 μm, allowing broadband tunability. Further, it is Type I and Type II phase-matchable with large effective SHG coefficients of deff,I ≈80.2 pm V–1 and deff,II ≈73.4 pm V–1. The outstanding properties of MgSiP2 make it a highly attractive candidate for optical frequency conversion in the infrared.  The work was performed in close collaboration with Prof. Zhiqiang Mao’s group at Penn State and Prof. Gian-Marco Rigonese’s  group at U. Catholique Louvain, in Belgium.

 

Multimodal Probe of Exchange Coupling in a Magnetic Topological Insulator

Hari Padmanabhan, in collaboration with collaborations from Mao, Chen and Rondinelli groups, as well as groups from SLAC and Argonne Photon Source has quantified for the first time the magnetic exchange coupling between localized spine and itinerant bands in a magnetic topological insulator, MnBi2Te4.  This is crucial for the design paradigm of a magnetic topological insulator that assumes that magnetic exchange gap can be opened in the conduction bands by localized spins. The work is published in Advanced Materials.