Search for: All records

Crystal symmetry plays an important role in the Hall effects. Unconventional spin Hall effect (USHE), characterized by Dresselhaus and out-of-plane spins, has been observed in materials with low crystal symmetry. Recently, antisymmetric planar Hall effect (APHE) was discovered in rutile RuO2 and IrO2 (101) thin films, which also exhibit low crystal symmetry. In this study, we report the observation of both USHE and APHE in IrO2 (111) films, using spin-torque ferromagnetic resonance and harmonic Hall measurements, respectively. Notably, the unconventional spin-torque efficiency from Dresselhaus spin was more than double that of a previous report. Additionally, the temperature dependence of APHE suggests that it arises from the Lorentz force, constrained by crystal symmetry. Symmetry analysis supports the coexistence of USHE and APHE and demonstrates that both originate from the crystal symmetry of IrO2 (111), paving the way for a deeper understanding of Hall effects and related physical phenomena. 

Exploration and advancements in ultrawide bandgap (UWBG) semiconductors are pivotal for next-generation high-power electronics and deep-ultraviolet (DUV) optoelectronics. Here, we used a thin heterostructure design to facilitate high conductivity due to the low electron mass and relatively weak electron-phonon coupling, while the atomically thin films ensured high transparency. We used a heterostructure comprising SrSnO₃/La:SrSnO₃/GdScO₃(110), and applied electrostatic gating, which allow us to effectively separate charge carriers in SrSnO₃from dopants and achieve phonon-limited transport behavior in strain-stabilized tetragonal SrSnO₃. This led to a modulation of carrier density from 10¹⁸to 10²⁰cm⁻³, with room temperature mobilities ranging from 40 to 140 cm²V⁻¹s⁻¹. The phonon-limited mobility, calculated from first principles, closely matched experimental results, suggesting that room temperature mobility could be further increased with higher electron density. In addition, the sample exhibited 85% optical transparency at a 300-nm wavelength. These findings highlight the potential of heterostructure design for transparent UWBG semiconductor applications, especially in DUV regime. 

Charge transfer or redistribution at oxide heterointerfaces is a critical phenomenon, often leading to remarkable properties such as two-dimensional electron gas and interfacial ferromagnetism. Despite studies on LaNiO₃/LaFeO₃superlattices and heterostructures, the direction and magnitude of the charge transfer remain debated, with some suggesting no charge transfer due to the high stability of Fe³⁺(3d⁵). Here, we synthesized a series of epitaxial LaNiO₃/LaFeO₃superlattices and demonstrated partial (up to ~0.5 e⁻/interface unit cell) charge transfer from Fe to Ni near the interface, supported by density functional theory simulations and spectroscopic evidence of changes in Ni and Fe oxidation states. The electron transfer from LaFeO₃to LaNiO₃and the subsequent rearrangement of the Fe 3d band create an unexpected metallic ground state within the LaFeO₃layer, strongly influencing the in-plane transport properties across the superlattice. Moreover, we establish a direct correlation between interfacial charge transfer and in-plane electrical transport properties, providing insights for designing functional oxide heterostructures with emerging properties. 

The oxides of platinum group metals are promising for future electronics and spintronics due to the delicate interplay of spin-orbit coupling and electron correlation energies. However, their synthesis as thin films remains challenging due to their low vapour pressures and low oxidation potentials. Here we show how epitaxial strain can be used as a control knob to enhance metal oxidation. Using Ir as an example, we demonstrate the use of epitaxial strain in engineering its oxidation chemistry, enabling phase-pure Ir or IrO2 films despite using identical growth conditions. The observations are explained using a density-functional-theory-based modified formation enthalpy framework, which highlights the important role of metal-substrate epitaxial strain in governing the oxide formation enthalpy. We also validate the generality of this principle by demonstrating epitaxial strain effect on Ru oxidation. The IrO2 films studied in our work further revealed quantum oscillations, attesting to the excellent film quality. The epitaxial strain approach we present could enable growth of oxide films of hard-to-oxidize elements using strain engineering. 

Perovskite SrIrO3 films and its heterostructures are very promising, yet less researched, avenues to explore interesting physics originating from the interplay between strong spin–orbit coupling and electron correlations. Elemental iridium is a commonly used source for molecular beam epitaxy (MBE) synthesis of SrIrO3 films. However, elemental iridium is extremely difficult to oxidize and evaporate while maintaining an ultra-high vacuum and a long mean free path. Here, we calculated a thermodynamic phase diagram to highlight these synthesis challenges for phase-pure SrIrO3 and other iridium-based oxides. We addressed these challenges using a novel solid-source metal-organic MBE approach that rests on the idea of modifying the metal-source chemistry. Phase-pure, single-crystalline, coherent, epitaxial (001)pc SrIrO3 films on (001) SrTiO3 substrate were grown. Films demonstrated semi-metallic behavior, Kondo scattering, and weak antilocalization. Our synthesis approach has the potential to facilitate research involving iridate heterostructures by enabling their atomically precise syntheses. 

SrTiO 3 (STO) is an incipient ferroelectric perovskite oxide for which the onset of ferroelectric order is suppressed by quantum fluctuations. This property results in a very large increase in static dielectric constant from ∼300 at room temperature to ∼20,000 at liquid He temperature in bulk single crystals. However, the low-temperature dielectric constant of epitaxial STO films is typically a few hundred to a few thousand. Here, we use all-epitaxial capacitors of the form n -STO/undoped STO/ n -STO (001) prepared by hybrid molecular beam epitaxy, to demonstrate intrinsic dielectric constants of an unstrained STO (001) film exceeding 25,000. We show that the n -STO/undoped STO interface plays a critically important role not previously considered in determining the dielectric properties that must be properly accounted for to determine the intrinsic dielectric constant. 

The historical motivation for serverless comes from internet-of-things, smartphone client server, and the objective of simplifying programming (no provisioning) and scale-down (pay-for-use). These applications are generally low-performance best-effort. However, the serverless model enables flexible software architectures suitable for a wide range of applications that demand high-performance and guaranteed performance. We have studied three such applications - scientific data streaming, virtual/augmented reality, and document annotation. We describe how each can be cast in a serverless software architecture and how the application performance requirements translate into high performance requirements (invocation rate, low and predictable latency) for the underlying serverless system implementation. These applications can require invocations rates as high as millions per second (40 MHz) and latency deadlines below a microsecond (300 ns), and furthermore require performance predictability. All of these capabilities are far in excess of today's commercial serverless offerings and represent interesting research challenges.