arXiv subjects

Benjamin Z. Gregory

Publications and source records attributed to Benjamin Z. Gregory.

4 recordsLinked to original sources

arXiv2026

Crystallography of periodic nanotextures in a strained Mott insulator

Here we investigate stripes of alternating structural phases spontaneously forming in epitaxially strained $Ca_2RuO_4$ thin films below the metal-insulator transition. Using large-volume X-ray reciprocal-space mapping, we show that satellite-pattern intensities across 24 symmetry-inequivalent Bragg reflections collapse onto a single parameter-free curve. The collapse identifies a coherent martensitic laminate of few-nm-wide domains separated by ${012}$ interfaces, with displacements along $\left\langle01\bar{2}\right\rangle$. Satellite-extinction analysis demonstrates that both coexisting phases retain the bulk orthorhombic space group despite the pseudocubic $LaAlO_3$ substrate, biaxial epitaxial strain, and intrinsic strain at the interfaces. Classical invariant-plane-strain crystallography thus governs the nanoscale domain geometry of a Mott insulator with intertwined magnetic, electronic, and lattice order.

cond-mat.mtrl-sci↗

arXiv2026

Synthesis of epitaxial TaO$_2$ thin films on Al$_2$O$_3$ by suboxide molecular-beam epitaxy and thermal laser epitaxy

Tantalum dioxide (TaO2) is a metastable tantalum compound. Here, we report the epitaxial stabilization of TaO2 on Al2O3 (1-102) (r-plane sapphire) substrates using suboxide molecular-beam epitaxy (MBE) and thermal laser epitaxy (TLE), demonstrating single-oriented, monodomain growth of anisotropically strained thin films. Microstructural investigation is performed using synchrotron X-ray diffraction and scanning transmission electron microscopy. The tetravalent oxidation state of tantalum is confirmed using X-ray absorption and photoemission spectroscopy as well as electron energy-loss spectroscopy. Optical properties are investigated via spectroscopic ellipsometry and reveal a 0.3 eV Mott gap of the tantalum 5d electrons. Density-functional theory and group theoretical arguments are used to evaluate the limited stability of the rutile phase and reveal the potential to unlock a hidden metal-insulator transition concomitant with a structural phase transition to a distorted rutile phase, akin to NbO2. Our work expands the understanding of tantalum oxides and paves the way for their integration into next-generation electronic and photonic devices.

cond-mat.mtrl-sci↗

arXiv2025

Resonant diffraction and photoemission inconsistent with altermagnetism in epitaxial RuO$_2$ films

Excitement about the magnetic and electronic properties of RuO$_2$ is growing, fueled by reports of antiferromagnetism, strain-induced superconductivity, and its recent classification as a member of a newly proposed magnetic class, altermagnets, with RuO$_2$ widely regarded as the paradigmatic example. Nevertheless, the magnetic ground state of RuO$_2$ remains contentious, as several recent experiments report no evidence of magnetic order. To address this discrepancy, we performed resonant elastic scattering measurements on a series of epitaxial RuO$_2$ thin films grown on the (100)-plane of TiO$_2$ substrates across a range of strain states. Leveraging full polarization control and azimuthal scans of the structurally forbidden 100 Bragg reflection, we systematically tested for signatures of colinear antiferromagnetic order. We found that the resonant elastic scattering signal in RuO$_2$ thin films likely originates from anisotropic charge scattering, not long-range antiferromagnetic order. Using angle-resolved photoemission spectroscopy we uncover a band structure without altermagnetic band splitting that is consistent with a nonmagnetic phase. Similarly, anisotropic magnetoresistance results show no evidence of magnetism. The combination of three independent measurements suggests the absence of altermagnetism in RuO$_2$.

cond-mat.mtrl-sci↗

arXiv2024

Unraveling the nature of quasi van der Waals Epitaxy of magnetic topological insulators Cr: (BixSb1-x)2Te3 on a GaAs (111) substrate through coherently strained interface

Quasi van der Waals Epitaxy (qvdWE) has been realized for decades at the interfaces between 3D and 2D materials or van der Waals materials. The growth of magnetic topological insulators (MTI) Cr: (BixSb1-x)2Te3 (CBST) on GaAs (111) substrates for Quantum Anomalous Hall Effect (QAH) is actually one of the examples of qvdWE, which is not well noticed despite the fact that its advantages have been used in growth of various MTI materials. This is distinguished from the growth of MTIs on other substrates. Although the qvdWE mode has been used in many 2D growth on III-V substrates, the specific features and mechanisms are not well demonstrated and summarized yet. Here in this work, we have for the first time shown the features of both coherent interfaces and the existence of strain originating from qvdWE at the same time.

cond-mat.mtrl-sci↗