arXiv subjects

Alexander McLeod

Publications and source records attributed to Alexander McLeod.

6 recordsLinked to original sources

arXiv2026

Mechanically Exfoliated Metallic Delafossite PdCoO2 Nanomembranes: Quantum Transport and Electrical Evaluation Toward Interconnect Applications

Metallic delafossite oxides have drawn attention for their ultralow resistivity and coherent electronic transport. However, mesoscopic transport studies are hindered by the limited access to high quality nanoscale devices. Here, we report single crystalline PdCoO2 nanomembranes, enabling exploration of quasi two dimensional (2D) transport. Shubnikov de Haas and Aharonov Bohm oscillations under different magnetic field orientations are observed at low temperatures, from which the electron effective mass and electron phase coherence length are extracted. Beyond quantum transport, the electrical performance of PdCoO2 toward interconnect applications is evaluated. A nearly thickness independent room temperature resistivity is observed for flakes down to 40 nm thickness. The nanomembranes exhibit a breakdown current density up to 113 MA cm-2, with excellent thermal stability and electromigration resistance. These results demonstrate that mechanically exfoliated PdCoO2 flakes preserve the high crystalline and electronic quality of bulk crystals in the quasi-2D limit, providing a useful platform for mesoscopic transport studies and interconnect applications.

cond-mat.mes-hall↗

arXiv2025

Machine Learning-Assisted Nano-imaging and Spectroscopy of Phase Coexistence in a Wide-Bandgap Semiconductor

Wide bandgap semiconductors with high room temperature mobilities are promising materials for high-power electronics. Stannate films provide wide bandgaps and optical transparency, although electron-phonon scattering can limit mobilities. In SrSnO3, epitaxial strain engineering stabilizes a high-mobility tetragonal phase at room temperature, resulting in a threefold increase in electron mobility among doped films. However, strain relaxation in thicker films leads to nanotextured coexistence of tetragonal and orthorhombic phases with unclear implications for optoelectronic performance. The observed nanoscale phase coexistence demands nano-spectroscopy to supply spatial resolution beyond conventional, diffraction-limited microscopy. With nano-infrared spectroscopy, we provide a comprehensive analysis of phase coexistence in SrSnO3 over a broad energy range, distinguishing inhomogeneous phonon and plasma responses arising from structural and electronic domains. We establish Nanoscale Imaging and Spectroscopy with Machine-learning Assistance (NISMA) to map nanotextured phases and quantify their distinct optical responses through a robust quantitative analysis, which can be applied to a broad array of complex oxide materials.

cond-mat.mtrl-sci↗

arXiv2025

Dual Optical Hyperbolicity of PdCoO$_2$ and PdCrO$_2$ Delafossite Single Crystals

Hyperbolic materials exhibit a very peculiar optical anisotropy with simultaneously different signs of the dielectric tensor components. This anisotropy allows the propagation of exotic surface-wave excitations like hyperbolic phonons and plasmon polaritons. While hyperbolic materials hold promise for applications in subwavelength photonics and enhanced light-matter interactions, their natural occurrence is limited to few materials, often accompanied by significant dielectric losses and limited hyperbolic spectral bandwidth. Focusing on PdCoO$_2$ and PdCrO$_2$ delafossite transition-metal oxides, in this paper we demonstrate their unique dual hyperbolic regimes: one localized around a phonon absorption in the mid-infrared spectral region, and the other extending into the visible range. Both hyperbolic regimes show exceptional properties including low dissipation and high hyperbolic quality factors. These results pave the way for innovative applications of delafossite layered metals in subwavelength photonics, imaging, and sensing.

cond-mat.mtrl-sci↗

arXiv2025

Interfacial strong coupling and negative dispersion of propagating polaritons in freestanding oxide membranes

Membranes of complex oxides like perovskite SrTiO3 extend the multi-functional promise of oxide electronics into the nanoscale regime of two-dimensional materials. Here we demonstrate that free-standing oxide membranes supply a reconfigurable platform for nano-photonics based on propagating surface phonon polaritons. We apply infrared near-field imaging and -spectroscopy enabled by a tunable ultrafast laser to study pristine nano-thick SrTiO3 membranes prepared by hybrid molecular beam epitaxy. As predicted by coupled mode theory, we find that strong coupling of interfacial polaritons realizes symmetric and antisymmetric hybridized modes with simultaneously tunable negative and positive group velocities. By resolving reflection of these propagating modes from membrane edges, defects, and substrate structures, we quantify their dispersion with position-resolved nano-spectroscopy. Remarkably, we find polariton negative dispersion is both robust and tunable through choice of membrane dielectric environment and thickness and propose a novel design for in-plane Veselago lensing harnessing this control. Our work lays the foundation for tunable transformation optics at the nanoscale using polaritons in a wide range of freestanding complex oxide membranes.

cond-mat.mes-hall↗

arXiv2023

Fast and Facile Synthesis Route to Epitaxial Oxide Membrane Using a Sacrificial Layer

The advancement in thin-film exfoliation for synthesizing oxide membranes has opened up new possibilities for creating artificially-assembled heterostructures with structurally and chemically incompatible materials. The sacrificial layer method is a promising approach to exfoliate as-grown films from a compatible material system, allowing their integration with dissimilar materials. Nonetheless, the conventional sacrificial layers often possess intricate stoichiometry, thereby constraining their practicality and adaptability, particularly when considering techniques like Molecular Beam Epitaxy (MBE). This is where easy-to-grow binary alkaline earth metal oxides with a rock salt crystal structure are useful. These oxides, which include (Mg, Ca, Sr, Ba)O, can be used as a sacrificial layer covering a much broader range of lattice parameters compared to conventional sacrificial layers and are easily dissolvable in deionized water. In this study, we show the epitaxial growth of single-crystalline perovskite SrTiO3 (STO) on sacrificial layers consisting of crystalline SrO, BaO, and Ba1-xCaxO films, employing a hybrid MBE method. Our results highlight the rapid (< 5 minutes) dissolution of the sacrificial layer when immersed in deionized water, facilitating the fabrication of millimeter-sized STO membranes. Using high-resolution x-ray diffraction, atomic-force microscopy, scanning transmission electron microscopy, impedance spectroscopy, and scattering-type near-field optical microscopy (SNOM), we demonstrate epitaxial STO membranes with bulk-like intrinsic dielectric properties. The employment of alkaline earth metal oxides as sacrificial layers is likely to simplify membrane synthesis, particularly with MBE, thus expanding research possibilities.

physics.app-ph↗

arXiv2015

Phase transition in bulk single crystals and thin films of VO2 by nano-infrared spectroscopy and imaging

We have systematically studied a variety of vanadium dioxide (VO2) crystalline forms, including bulk single crystals and oriented thin films, using infrared (IR) near-field spectroscopic imaging techniques. By measuring the IR spectroscopic responses of electrons and phonons in VO2 with sub-grain-size spatial resolution (~20 nm), we show that epitaxial strain in VO2 thin films not only triggers spontaneous local phase separations but also leads to intermediate electronic and lattice states that are intrinsically different from those found in bulk. Generalized rules of strain and symmetry dependent mesoscopic phase inhomogeneity are also discussed. These results set the stage for a comprehensive understanding of complex energy landscapes that may not be readily determined by macroscopic approaches.

cond-mat.str-el↗