Varun Harbola

While mechanical instabilities enable engineered nonlinear responses, their controlled application in functional crystalline oxides has remained largely unexplored. Recent advances in freestanding oxide membranes now allow their integration into nanoscale, bistable architectures. Here, we demonstrate that freestanding SrTiO3 and BaTiO3 membranes relax into well-defined metastable buckling states when they are transferred onto lithographically defined cavities. This deformation, driven by the interplay of residual strain, bending stiffness, and cavity geometry, produces reproducible bistable states with distinct strain distribution. Using atomic force microscopy, Kelvin probe measurements, and finite element modeling, we reveal that these mechanically locked states directly govern the electromechanical potential landscape of ferroelectric BaTiO3. We further demonstrate reversible snap-through transitions between degenerate states, establishing complex oxides as deterministic, geometry-tunable building blocks for nonlinear nanoelectromechanical architectures. These results provide a general strategy for exploiting mechanical instabilities to manipulate functional responses in ultrathin, crystalline membranes.

Mechanical instabilities in thin solids offer a powerful route to engineer nonlinear responses, yet their controlled use in functional crystalline oxides has remained largely unexplored. Notably, by changing the aspect ratio of solids, the energy landscape around equilibrium can be modified to induce non-linearities under lateral stresses through non-lateral deformations. These nonlinear systems can develop multiple local energy minima where the system can settle and switch between states through the application of a driving force. Crucially, recent advances in oxide thin film growth have enabled the fabrication of freestanding oxide membranes, paving a viable path for their use in bistable architecture, particularly at the nanoscale. Here, we demonstrate that freestanding oxide membranes, such as SrTiO3 (STO) and BaTiO3 (BTO), relax into well-defined metastable buckling states when transferred onto lithographically defined cavities. The membrane deformation is determined by the interplay between built-in residual strain, bending stiffness, and cavity geometry, resulting in reproducible bistable states with distinct strain distributions. Using a combination of atomic force microscopy, in-contact Kelvin probe measurements, and finite-element modelling, we reveal that these mechanically locked states directly shape the electromechanical potential landscape of ferroelectric BaTiO3. We further demonstrate reversible snapthrough transitions between mechanically degenerate states, establishing complex oxides as deterministic, geometry-tunable building blocks for nonlinear nanoelectromechanical architectures. Our results illustrate a general strategy for exploiting mechanical instabilities to encode and manipulate functional responses in ultrathin crystalline membranes.

The growth of a wide range of binary nitride films with excellent properties is reported. These films are deposited by thermal laser epitaxy (TLE). If equipped with CO2–laser substrate heating, TLE allows growth temperatures ≫ 1000 °C combined with strong nitriding conditions, which are established by using ammonia (NH3) gas as a nitrogen precursor. Films deposited on c-plane sapphire substrates include semiconducting nitrides (BN, AlN, ScN, and YN), superconducting nitrides (TiN, VN, ZrN, and NbN), and magnetic nitrides (CrN and GdN). The superconducting transition temperatures of TiN, VN, ZrN, and NbN films are 5.7, 8.9, 8.9, and 16.9 K, respectively. The Néel temperature of CrN films is ≃280 K, and the Curie temperature of GdN films is 66 K; both lie within the reported ranges. The controlled fabrication of different phases of individual nitride compounds is explored, revealing that subnitride phases can be grown at lower NH3 pressures. This study also demonstrates the heteroepitaxial growth of multilayer nitride films on c-plane sapphire.

Superconducting films of α-Ta are promising candidates for the fabrication of advanced superconducting qubits. However, α-Ta films suffer from many growth-induced structural inadequacies that negatively affect their performance. We have therefore explored a new synthesis method for α-Ta films, which allows for the growth of these films with an unprecedented quality. Using this method, high quality α-Ta films are deposited at a comparably high substrate temperature of 1150 °C. They are single-phase α-Ta and have a single out-of-plane (110) orientation. They consist of grains ≥2 μm that have one of three possible in-plane orientations. As shown by scanning transmission electron microscopy and electron energy loss studies, the substrate–film interfaces are sharp with no observable intermixing. The obtained insights into the epitaxial growth of body-centered-cubic films on quasi-hexagonal substrates lay the basis for harnessing the high structural coherence of such films in various applications.

We study the temperature dependent elastic properties of Ba_0.8Sr_0.2TiO₃ freestanding membranes across the ferroelectric-to-paraelectric phase transition using an atomic force microscope. The bending rigidity of thin membranes can be stiffer compared to stretching due to strain gradient elasticity (SGE). We measure the Young's modulus of freestanding Ba_0.8Sr_0.2TiO₃ drumheads in bending and stretching dominated deformation regimes on a variable temperature platform, finding a peak in the difference between the two Young's moduli obtained at the phase transition. This demonstrates a dependence of SGE on the dielectric properties of a material and alludes to a flexoelectric origin of an effective SGE.

We present a way to fabricate bicrystal Josephson junctions of high-Tc cuprate superconductors that are not grown on bulk bicrystalline substrates. Based on vector substrate technology, this approach makes use of a few tens-of-nanometer-thick bicrystalline membranes transferred onto conventional substrates. We demonstrate 24° [001]-tilt YBa2Cu3O7−x Josephson junctions fabricated on sapphire single crystals by utilizing 10-nm-thick bicrystalline SrTiO3 membranes. This technique allows one to manufacture bicrystalline Josephson junctions of high-Tc superconductors on a large variety of bulk substrate materials, providing distinctive degrees of freedom in designing the junctions and their electronic properties. Furthermore, it offers the capability to replace the fabrication of bulk bicrystalline substrates with thin-film growth methods.

We present a way to We present a way to fabricate bicrystal Josephson junctions of high-Tc cuprate superconductors that does not require bulk bicrystalline substrates. Based on vector substrate technology, this novel approach makes use of a few tens-of-nanometers-thick bicrystalline membranes transferred onto conventional substrates.We demonstrate 24° YBa2Cu3O7-x Josephson junctions fabricated on sapphire single crystals by utilizing 10-nm-thick bicrystalline membranes of SrTiO3. This technique allows one to manufacture bicrystalline Josephson junctions of high-Tc superconductors on a large variety of bulk substrate materials, providing novel degrees of freedom in designing the junctions and their electronic properties. It furthermore offers the capability to replace the fabrication of bulk bicrystalline substrates with thin-film growth methods

Substrates are essential in thin‐film deposition, yet they do not always meet the requirements of a given application. For example, they might be prohibitively expensive or not even available with the necessary crystal lattice structures. This paper introduces the innovative concept of “vector substrates,” where the template layer for thin‐film growth, referred to as vector, is chemically and crystallographically independent of the bulk of the substrate. This approach reduces material costs and offers unparalleled flexibility. The fabrication of vector substrates leverages thin‐film membrane technology. The process begins by growing a template layer on a parent substrate. The template layer is then transferred onto a carrier substrate, thereby generating the vector substrate. Vector substrate technology enables researchers and manufacturers with thin‐film deposition facilities to fabricate optimized substrates that would otherwise be difficult to obtain using bulk single‐crystal growth technology. An in‐depth discussion of the benefits and limitations of vector substrates compared with traditional substrates is provided. Furthermore, the practical viability of the concept is demonstrated by fabricating sets of vector substrates. Although this concept is still in its infancy, it has a significant potential to complement conventional substrates for future advancements in substrate technology and thin‐film deposition.