arXiv subjects

Yorick A. Birkholzer

Publications and source records attributed to Yorick A. Birkholzer.

2 recordsLinked to original sources

arXiv2026

Growth and Interface Engineering of Superconducting TiN on Sapphire by Thermal-Laser Epitaxy

Thermal-laser epitaxy (TLE) extends the accessible pressure, temperature, and growth-rate regimes of conventional molecular-beam epitaxy, enabling in situ laser annealing, high-purity buffer-layer growth, and efficient evaporation of refractory elements. These capabilities make TLE a promising platform for engineering low-loss superconducting resonators and Josephson-junction heterostructures. Here, we study the TLE growth of TiN on sapphire and observe improved transport properties with increasing growth temperature up to 1150 C. Unfortunately, we observe that the high ammonia pressure and elevated substrate temperatures required for optimal TiN properties promote reactions at the sapphire surface, resulting in voids at the TiN-substrate interface. These defects increase interfacial surface area, introduce dangling bonds, and could compromise tunnel-barrier heterostructures. We mitigate this degradation using an initial TiN seed layer grown at 850 C. With the seed layer, we achieve a superconducting transition temperature of 5.8 K, a residual resistivity ratio of 12.2, and a resistivity of 1.19 micro-ohm cm at 10 K, which, to our knowledge, is the lowest reported for TiN grown on sapphire.

cond-mat.supr-con↗

arXiv2020

Giant strain gradient elasticity in SrTiO3 membranes: bending versus stretching

Young's modulus determines the mechanical loads required to elastically stretch a material, and also, the loads required to bend it, given that bending stretches one surface while compressing the opposite one. Flexoelectric materials have the additional property of becoming electrically polarized when bent. While numerous studies have characterized this flexoelectric coupling, its impact on the mechanical response, due to the energy cost of polarization upon bending, is largely unexplored. This intriguing contribution of strain gradient elasticity is expected to become visible at small length scales where strain gradients are geometrically enhanced, especially in high permittivity insulators. Here we present nano-mechanical measurements of freely suspended SrTiO3 membrane drumheads. We observe a striking non-monotonic thickness dependence of Young's modulus upon small deflections. Furthermore, the modulus inferred from a predominantly bending deformation is three times larger than that of a predominantly stretching deformation for membranes thinner than 20 nm. In this regime we extract a giant strain gradient elastic coupling of ~2.2e-6 N, which could be used in new operational regimes of nano-electro-mechanics.

cond-mat.mtrl-sci↗