arXiv subjects

Zishen Tian

Publications and source records attributed to Zishen Tian.

3 recordsLinked to original sources

arXiv2026

Breaking symmetry to create a parallel-plate varactor dielectric with unparalleled microwave performance

Voltage-tunable capacitors (varactors) are key to microwave circuits. Tunable dielectric varactors outperform competing technologies in almost every relevant metric but usually suffer from high dielectric loss. In contrast, Ruddlesden-Popper (RPs) dielectric thin films have remarkably low microwave loss. Unfortunately, their crystallographic symmetry has until recently dictated an in-plane device structure, precluding the favorable out-of-plane parallel-plate varactor design for minimized size and maximized electric field in the tunable dielectric. Guided by theory, we report RPs akin to the widely studied tunable microwave dielectric BaxSr1-xTiO3. Assembling these same atoms into the first RP phase with broken out-of-plane symmetry, we achieve a low-loss, out-of-plane tunable dielectric thin film. The highest performing film, (ATiO3)nAO film with A = Ba0.45Sr0.55 and n = 8, unlocks a tenfold improvement in the figure of merit for out-of-plane tunable dielectrics at 10 GHz, paving the way for a new generation of tunable monolithic microwave integrated circuits.

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

Operando Electron Microscopy of Nanoscale Electronic Devices on Non-Conductive Substrates

Achieving operating conditions comparable to ``bulk'' electronic devices, such as thin film capacitors, during \textit{operando} electron microscopy remains challenging, particularly when devices are grown on non-conductive substrates. Limited precision of focused ion beam milling for sample preparation often necessitates the use of conductive substrates or artificially thick layers that differ from actual device architectures. These modifications can alter native strain, electrostatic boundary conditions, and ultimately device response. Here, we present a generic and versatile workflow for \textit{operando} biasing of thin-film capacitors in the (scanning) transmission electron microscope, including sample fabrication and device operation. By introducing a patterned insulating barrier adjacent to the bulk-characterized capacitors, our approach enables sample preparation without altering the original film structure. As a case study, we apply the method to a piezoelectric thin-film capacitor grown on an insulating substrate, and demonstrate that it preserves the boundary-condition-sensitive domain switching at the atomic scale under applied electric fields. Overall, the process can help to establish a foundation for systematic \textit{operando} studies of complex thin-film systems under representative bulk testing geometries.

cond-mat.mtrl-sci↗