arXiv subjects

Darrell G. Schlom

Publications and source records attributed to Darrell G. Schlom.

At least 37 records · Page 2Linked to original sources

arXiv2026

Anisotropic magnon transport in an antiferromagnetic trilayer heterostructure: is BiFeO$_3$ an altermagnet?

Magnons provide a route to ultra-fast transport and non-destructive readout of spin-based information transfer. Here, we report magnon transport and its emergent anisotropic nature in BiFeO$_3$ layers confined between ultrathin layers of the antiferromagnet LaFeO$_3$. Due to the confined state, BiFeO$_3$ serves as an efficient magnon transmission channel as well as a magnetoelectric knob by which to control the stack by means of an electric field. We discuss the mechanism of the anisotropic spin transport based on the interaction between the antiferromagnetic order and the electric field. This allows us to manipulate and amplify the spin transport in such a confined geometry. Furthermore, lower crystal symmetric and suppression of the spin cycloid in ultrathin BiFeO$_3$ stabilizes a non-trivial antiferromagnetic state exhibiting symmetry-protected spin-split bands that provide the non-trivial sign inversion of the spin current, which is a characteristic of an altermagnet. This work provides an understanding of the anisotropic spin transport in complex antiferromagnetic heterostructures where ferroelectricity and altermagnetism coexist, paving the way for a new route to realize electric-field control of a novel state of magnetism.

cond-mat.mes-hall↗

arXiv2025

Preparation and evaluation of alexandrite, forsterite, and topaz substrates for the epitaxial growth of rutile oxides

Metal-insulator transitions and superconductivity in rutile-structured oxides hold promise for advanced electronic applications, yet their thin film synthesis is severely hindered by limited substrate options. Here, we present three single- crystalline substrates, BeAl2O4, Mg2SiO4, and Al2SiO4(F,OH)2, prepared via optimized thermal and chemical treatments to achieve atomically smooth surfaces suitable for epitaxial growth. Atomic force microscopy confirms atomic step-and-terrace surface morphologies, and oxide molecular-beam epitaxy growth on these substrates demonstrates successful heteroepitaxy of rutile TiO2, VO2, NbO2, and RuO2 films. Among these unconventional substrates, BeAl2O4 exhibits exceptional thermal and chemical stability, making it a versatile substrate candidate. These findings introduce new substrate platforms that facilitate strain engineering and exploration of rutile oxide thin films, potentially advancing the study of their strain-dependent physical properties.

cond-mat.mtrl-sci↗

arXiv2025

Control of out-of-plane anti-damping spin torque with a canted ferromagnetic spin source

To achieve efficient anti-damping switching of nanoscale magnetic memories with perpendicular magnetic anisotropy using spin-orbit torque requires that the anti-damping spin-orbit torque have a strong out-of-plane component. The spin anomalous Hall effect and the planar Hall effect spin current produced by a ferromagnetic layer are candidate mechanisms for producing such an out-of-plane anti-damping torque, but both require that the magnetic moment of the spin source layer be canted partly out of the sample plane at zero applied magnetic field. Here we demonstrate such a canted configuration for a ferromagnetic SrRuO3 layer and we characterize all vector components of the torque that it produces, including non-zero out-of-plane anti-damping torques. We verify that the out-of-plane spin component can be tuned by the orientation of magnetic moment, with significant contributions from both the spin anomalous Hall effect and the planar Hall effect spin current.

cond-mat.mes-hall↗

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↗

arXiv2025

Turn-on of Current-Induced Spin Torque upon Noncollinear Antiferromagnetic Ordering in Delafossite PdCrO2

We report measurements of the current-induced spin torque produced by the delafossite antiferromagnet PdCrO2 and acting on an adjacent ferromagnetic permalloy layer. The spin torque increases strongly as the temperature is reduced through the Neel temperature, when the PdCrO2 transitions from a paramagnetic phase to a noncollinear antiferromagnetic state. This result is qualitatively consistent with density functional theory calculations regarding how spin-current generation changes upon antiferromagnetic ordering in PdCrO2.

cond-mat.mtrl-sci↗

arXiv2025

Colossal Cryogenic Electro-Optic Response Through Metastability in Strained BaTiO$_{3}$ Thin Films

The search for thin film electro-optic (EO) materials that can retain superior performance under cryogenic conditions has become critical for quantum computing. Barium titanate thin films show large linear EO coefficients in the tetragonal phase at room temperature, which is severely degraded down to ~200 pm V$^{-1}$ in the rhombohedral phase at cryogenic temperatures. There is immense interest in manipulating these phase transformations and retaining superior EO properties down to liquid helium temperature. Utilizing the thermodynamic theory of optical properties, a large low-temperature EO response is designed by engineering the energetic competition between different ferroelectric phases, leading to a low-symmetry monoclinic phase with a massive EO response. The existence of this phase is demonstrated in a strain-tuned BaTiO$_{3}$ thin film that exhibits a linear EO coefficient of 2516 +/- 100 pm V$^{-1}$ at 5 K, which is an order of magnitude higher than the best reported performance thus far. Importantly, the EO coefficient increases by 100x during cooling, unlike the conventional films, where it degrades. Further, at the lowest temperature, significant higher order EO responses also emerge. These results represent a new framework for designing materials with property enhancements by stabilizing highly tunable metastable phases with strain. Copyright 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. (A. Suceava, S. Hazra, A. Ross, et al. "Colossal Cryogenic Electro-Optic Response Through Metastability in Strained BaTiO3 Thin Films." Adv. Mater. (2025): e07564. https://doi.org/10.1002/adma.202507564)

cond-mat.mtrl-sci↗

arXiv2025

Large phonon-drag thermopower polarity reversal in Ba-doped KTaO3

This study reports the observation of phonon-drag thermopower polarity reversal in Ba-doped KTaO3 thin films, mediated by electron-phonon Umklapp scattering. Epitaxial films with distinct carrier concentrations (3.7 x 10^20 cm^-3 and 4.9 x 10^19 cm^-3) were grown via molecular-beam epitaxy. In heavily doped samples, where the Fermi surface spans 80% of the Brillouin zone, the Umklapp condition is satisfied, reversing electron momentum. This manifests as a sign-reversal in the thermopower around 80 K upon cooling despite the sample having only n-type carriers. On the other hand, the lightly doped sample (4.9 x 10^19 cm^-3) exhibits only a negative thermopower down to 2 K. These results advance the understanding of Umklapp electron-phonon drag in oxides and highlight KTaO3's potential for engineering unconventional thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2025

Topotactic oxidation of Ruddlesden-Popper nickelates reveals new structural family: oxygen-intercalated layered perovskites

Layered perovskites such as the Dion-Jacobson, Ruddlesden-Popper, and Aurivillius families host a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here we report a new family of layered perovskites, realized through topotactic oxygen intercalation of La_{n+1}Ni_{n}O_{3n+1} (n=1-4) Ruddlesden-Popper nickelate thin films grown by ozone-assisted molecular-beam epitaxy. Post-growth ozone annealing induces a large c-axis expansion - 17.8% for La_{2}NiO_{4} (n=1) - that monotonically decreases with increasing n. Surface X-ray diffraction coupled with Coherent Bragg Rod Analysis reveals that this structural expansion arises from the intercalation of approximately 0.7 oxygen atoms per formula unit into interstitial sites within the rock salt spacer layers. The resulting structures exhibit a spacer layer composition intermediate between that of the Ruddlesden-Popper and Aurivillius phases, defining a new class of layered perovskites. Oxygen-intercalated nickelates exhibit metallicity and significantly enhanced nickel-oxygen hybridization, a feature linked to high-temperature superconductivity. Our work establishes topotactic oxidation as a powerful synthetic approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to tune properties via spacer-layer chemistry.

cond-mat.mtrl-sci↗

arXiv2025

Unconventional polaronic ground state in superconducting LiTi$_2$O$_4$

Geometrically frustrated lattices can display a range of correlated phenomena, ranging from spin frustration and charge order to dispersionless flat bands due to quantum interference. One particularly compelling family of such materials is the half-valence spinel Li$B_2$O$_4$ materials. On the $B$-site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi$_2$O$_4$ and heavy fermion behavior in LiV$_2$O$_4$. To date, however, LiTi$_2$O$_4$ has primarily been understood as a conventional BCS superconductor despite a lattice structure that could host more exotic groundstates. Here, we present a multimodal investigation of LiTi$_2$O$_4$, combining ARPES, RIXS, proximate magnetic probes, and ab-initio many-body theoretical calculations. Our data reveals a novel mobile polaronic ground state with spectroscopic signatures that underlie co-dominant electron-phonon coupling and electron-electron correlations also found in the lightly doped cuprates. The cooperation between the two interaction scales distinguishes LiTi$_2$O$_4$ from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi$_2$O$_4$. Our work deepens our understanding of the rare interplay of electron-electron correlations and electron-phonon coupling in unconventional superconducting systems. In particular, our work identifies the geometrically frustrated, mixed-valence spinel family as an under-explored platform for discovering unconventional, correlated ground states.

cond-mat.supr-con↗

arXiv2025

Low energy electrodynamics and a hidden Fermi liquid in the heavy-fermion CeCoIn$_5$

We present time-domain THz spectroscopy of thin films of the heavy-fermion superconductor CeCoIn$_5$. Below the $\approx$ 40 K Kondo coherence temperature, a narrow Drude-like peak forms, as the result of the $f$ orbital - conduction electron hybridization and the formation of the heavy-fermion state. The complex optical conductivity is analyzed through a Drude model and extended Drude model analysis. Via the extended Drude model analysis, we measure the frequency-dependent scattering rate ($1/ τ$) and effective mass ($m^*/m_b$). This scattering rate shows a linear dependence on temperature, which matches the dependence of the resistivity as expected. Nevertheless, the width of the low-frequency Drude peak itself that is set by the {\it renormalized} quasiparticle scattering rate ($1 / τ^* = m_b/ m^* τ$) shows a $T^2$ dependence. This is the scattering rate that characterizes the relaxation time of the renormalized quasiparticles. This gives evidence for Fermi liquid state, which in conventional transport experiments is hidden by the strong temperature dependent mass.

cond-mat.str-el↗

arXiv2025

Colossal enhancement of spin transmission through magnon confinement in an antiferromagnet

Since Felix Bloch's introduction of the concept of spin waves in 1930, magnons (the quanta of spin waves) have been extensively studied in a range of materials for spintronics, particularly for non-volatile logic-in-memory devices. Controlling magnons in conventional antiferromagnets and harnessing them in practical applications, however, remains a challenge. In this letter, we demonstrate highly efficient magnon transport in an LaFeO$_3$/BiFeO$_3$/LaFeO$_3$ all-antiferromagnetic system which can be controlled electrically, making it highly desirable for energy-efficient computation. Leveraging spin-orbit-driven spin-charge transduction, we demonstrate that this material architecture permits magnon confinement in ultrathin antiferromagnets, enhancing the output voltage generated by magnon transport by several orders of magnitude, which provides a pathway to enable magnetoelectric memory and logic functionalities. Additionally, its non-volatility enables ultralow-power logic-in-memory processing, where magnonic devices can be efficiently reconfigured via electrically controlled magnon spin currents within magnetoelectric channels.

cond-mat.mtrl-sci↗

arXiv2025

Improper Ferroelectricity at the Monolayer Limit

Ultrathin ferroelectric films with out-of-plane polarization and high Curie temperatures are key to miniaturizing electronic devices. Most ferroelectrics employed in devices are proper ferroelectrics, where spontaneous polarization is the primary order parameter. Unfortunately, the Curie temperature of proper ferroelectrics is drastically reduced as the ferroelectric becomes thin; nearly all proper ferroelectrics need to be thicker than several unit cells. The absence of an ultrathin limit has been predicted, but not verified for improper ferroelectrics. These are ferroelectrics where the polarization emerges secondary to the primary order parameter, such as a structural distortion. Here we report improper ferroelectricity with an undiminished Curie temperature in a 0.75-unit-cell-thick hexagonal LuFeO3 (h-LuFeO3) film grown on a SrCo2Ru4O11 bottom electrode with an atomically engineered monolayer bridging layer. Our results demonstrate the absence of a critical thickness for improper ferroelectricity and provide a methodology for creating ultrathin improper ferroelectrics by stabilizing their primary order parameters.

cond-mat.mtrl-sci↗

arXiv2025

Unveiling the Interfacial Reconstruction Mechanism Enabling Stable Growth of the Delafossite PdCoO2 on Al2O3 and LaAlO3

Delafossites, comprised of noble metal (A+) and strongly correlated sublayers (BO2-), form natural superlattices with highly anisotropic properties. These properties hold significant promise for various applications, but their exploitation hinges on the successful growth of high-quality thin films on suitable substrates. Unfortunately, the unique lattice geometry of delafossites presents a significant challenge to thin-film fabrication. Different delafossites grow differently, even when deposited on the same substrate, ranging from successful epitaxy to complete growth suppression. These variations often lack a clear correlation to obvious causes like lattice mismatch. Unidentified stabilization mechanisms appear to enable growth in certain cases, allowing these materials to form stable thin films or act as buffer layers for subsequent delafossite growth. This study employs advanced scanning transmission electron microscopy techniques to investigate the nucleation mechanism underlying the stable growth of PdCoO2 films on Al2O3 and LaAlO3 substrates, grown via molecular-beam epitaxy. Our findings reveal the presence of a secondary phase within the substrate surface that stabilizes the films. This mechanism deviates from the conventional understanding of strain relief mechanisms at oxide heterostructure interfaces and differs significantly from those observed for Cu-based delafossites.

cond-mat.mtrl-sci↗

arXiv2024

Picosecond expansion in LaAlO3 resonantly driven by infrared-active phonons

We investigate the ultrafast structural dynamics of LaAlO3 thin films driven by short mid-infrared laser pulses at 20 THz. Time-resolved X-ray diffraction reveals an immediate lattice expansion and an acoustic breathing mode of the film. First-principles theory and a spring-mass model identify the direct coupling between coherently driven infrared-active phonons and strain as the underlying mechanism. Time-resolved optical birefringence measurements confirm that the amplitude of this acoustic mode scales linearly with the pump fluence, which agrees with the theory. Furthermore, time-resolved X-ray diffuse scattering indicates that THz excitation enhances crystallinity by inducing a non-thermal increase in structural symmetry originating from preexisting defects. These findings highlight the potential of a multimodal approach-combining time-resolved X-ray and optical measurements and first-principles theory-to elucidate and control structural dynamics in nanoscale materials.

cond-mat.mtrl-sci↗

arXiv2024

In situ etching of \b{eta}-Ga2O3 using tert-butyl chloride in an MOCVD system

In this study, we investigate in situ etching of \b{eta}-Ga2O3 in a metal-organic chemical vapor deposition (MOCVD) system using tert-Butyl chloride (TBCl). We report the successful etching of both heteroepitaxial (-201)-oriented and homoepitaxial (010)-oriented \b{eta}-Ga2O3 films over a wide range of substrate temperature, TBCl molar flows, and reactor pressures. We identify that the likely etchant is HCl (g) formed by the pyrolysis of TBCl in the hydrodynamic boundary layer above the substrate. The temperature dependence of the etch rate reveals two distinct regimes characterized by markedly different apparent activation energies. The extracted apparent activation energies suggest that at temperatures below ~800 °C the etch rate is likely limited by desorption of etch products. The relative etch rates of heteroepitaxial (-201) and homoepitaxial (010) \b{eta}-Ga2O3 were observed to scale by the ratio of the surface energies indicating an anisotropic etch. For (010) homoepitaxial films, relatively smooth post-etch surface morphology was achieved by tuning the etching parameters.

cond-mat.mtrl-sci↗

arXiv2024

Morphogenesis of Spin Cycloids in a Non-collinear Antiferromagnet

Pattern formation in spin systems with continuous-rotational symmetry (CRS) provides a powerful platform to study emergent complex magnetic phases and topological defects in condensed-matter physics. However, its understanding and correlation with unconventional magnetic order along with high-resolution nanoscale imaging is challenging. Here, we employ scanning NV magnetometry to unveil the morphogenesis of spin cycloids at both the local and global scales within a single ferroelectric domain of (111)-oriented BiFeO$_3$ (which is a non-collinear antiferromagnet), resulting in formation of a glassy labyrinthine pattern. We find that the domains of locally oriented cycloids are interconnected by an array of topological defects and exhibit isotropic energy landscape predicted by first-principles calculations. We propose that the CRS of spin-cycloid propagation directions within the (111) drives the formation of the labyrinthine pattern and the associated topological defects such as antiferromagnetic skyrmions. Unexpectedly, reversing the as-grown ferroelectric polarization from [$\bar{1}$$\bar{1}$$\bar{1}$] to [111] induces a magnetic phase transition, destroying the labyrinthine pattern and producing a deterministic non-volatile non cycloidal, uniformly magnetized state. These findings highlight that (111)-oriented BiFeO$_3$ is not only important for studying the fascinating subject of pattern formation but could also be utilized as an ideal platform for integrating novel topological defects in the field of antiferromagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2024

Atomistic understanding of hydrogen coverage on RuO2(110) surface under electrochemical conditions from ab initio statistical thermodynamics

Understanding the dehydrogenation of transition metal oxide surfaces under electrochemical potential is critical to the control of important chemical processes such as the oxygen evolution reaction (OER). Using first principles computations, we model the thermodynamic dehydrogenation process on RuO$_2$(110) and compare the results to experimental cyclic voltammetry (CV) on single crystal. We use a cluster expansion model trained on *ab initio* energy data coupled with Monte Carlo (MC) sampling to derive the macroscopic electrochemical observables, i.e., experimental CV, from the energetics of different hydrogen coverage microstates on well-defined RuO$_2$(110). Our model reproduces the unique "two-peaks" cyclic voltammogram observed experimentally with current density peak positions and shapes in good qualitative agreement. We show that RuO$_2$(110) starts as a water-covered surface with hydrogen on bridge (BRG) and coordination-unsaturated sites (CUS) at low potential (less than 0.4 V vs. reversible hydrogen electrode, RHE). As the potential increases, the hydrogens on BRG desorb, becoming the main contributor to the first CV peak with smaller contributions from CUS. When all BRG hydrogens are desorbed (before 1.2 V vs. RHE), the remaining CUS hydrogens desorb abruptly in a very small potential window leading to the sharp second peak observed during CV. Our work shows that above 1.23 V, the OER proceeds on a fully dehydrogenated RuO$_2$(110) surface.

cond-mat.mtrl-sci↗

arXiv2024

Adsorption-controlled Growth of Homoepitaxial c-plane Sapphire Films

Sapphire is a technologically highly relevant material, but it poses many challenges to performing epitaxial thin-film deposition. We have identified and applied the conditions for adsorption-controlled homoepitaxial growth of c-plane sapphire. The films thus grown are atomically smooth, have a controlled termination, and are of outstanding crystallinity. Their chemical purity exceeds that of the substrates. The films exhibit exceptional optical properties such as a single-crystal-like bandgap and a low density of F+ centers.

cond-mat.mtrl-sci↗