Nicola Manca

Multistate memory devices based on free-standing VO2/TiO2 microstructures driven by Joule self-heating.

Two-terminal multistate memory elements based on VO(2)/TiO(2) thin film microcantilevers are reported. Volatile and non-volatile multiple resistance states are programmed by current pulses at temperatures within the hysteretic region of the metal-insulator transition of VO(2). The memory mechanism is based on current-induced creation of metallic clusters by self-heating of micrometric suspended regions and resistive reading via percolation.

Programmable Mechanical Resonances in MEMS by Localized Joule Heating of Phase Change Materials

A programmable micromechanical resonator based on a VO2 thin film is reported. Multiple mechanical eigenfrequency states are programmed using Joule heating as local power source, gradually driving the phase transition of VO2 around its Metal-Insulator transition temperature. Phase coexistence of domains is used to tune the stiffness of the device via local control of internal stresses and mechanical properties. This study opens perspectives for developing mechanically configurable nanostructure arrays.

Epitaxial growth and thermodynamic stability of SrIrO3/SrTiO3 heterostructures

Obtaining high-quality thin films of 5d transition metal oxides is essential to explore the exotic semimetallic and topological phases predicted to arise from the combination of strong electron correlations and spin-orbit coupling. Here, we show that the transport properties of SrIrO3 thin films, grown by pulsed laser deposition, can be optimized by considering the effect of laser-induced modification of the SrIrO3 target surface. We further demonstrate that bare SrIrO3 thin films are subject to degradation in air and are highly sensitive to lithographic processing. A crystalline SrTiO3 cap layer deposited in-situ is effective in preserving the film quality, allowing us to measure metallic transport behavior in films with thicknesses down to 4 unit cells. In addition, the SrTiO3 encapsulation enables the fabrication of devices such as Hall bars without altering the film properties, allowing precise (magneto)transport measurements on micro- and nanoscale devices.

Striped nanoscale phase separation at the metal-insulator transition of heteroepitaxial nickelates

Nucleation processes of mixed-phase states are an intrinsic characteristic of first-order phase transitions, typically related to local symmetry breaking. Direct observation of emerging mixed-phase regions in materials showing a first-order metal–insulator transition (MIT) offers unique opportunities to uncover their driving mechanism. Using photoemission electron microscopy, we image the nanoscale formation and growth of insulating domains across the temperature-driven MIT in NdNiO3 epitaxial thin films. Heteroepitaxy is found to strongly determine the nanoscale nature of the phase transition, inducing preferential formation of striped domains along the terraces of atomically flat stepped surfaces. We show that the distribution of transition temperatures is a local property, set by surface morphology and stable across multiple temperature cycles. Our data provide new insights into the MIT of heteroepitaxial nickelates and point to a rich, nanoscale phenomenology in this strongly correlated material.

Spin-Orbit Semimetal SrIrO3 in the Two-Dimensional Limit

We investigate the thickness-dependent electronic properties of ultrathin SrIrO_{3} and discover a transition from a semimetallic to a correlated insulating state below 4 unit cells. Low-temperature magnetoconductance measurements show that spin fluctuations in the semimetallic state are significantly enhanced while approaching the transition point. The electronic properties are further studied by scanning tunneling spectroscopy, showing that 4 unit cell SrIrO_{3} is on the verge of a gap opening. Our density functional theory calculations reproduce the critical thickness of the transition and show that the opening of a gap in ultrathin SrIrO_{3} requires antiferromagnetic order.

Side Gate Tunable Josephson Junctions at the LaAlO3/SrTiO3 Interface

Novel physical phenomena arising at the interface of complex oxide heterostructures offer exciting opportunities for the development of future electronic devices. Using the prototypical LaAlO3/SrTiO3 interface as a model system, we employ a single-step lithographic process to realize gate-tunable Josephson junctions through a combination of lateral confinement and local side gating. The action of the side gates is found to be comparable to that of a local back gate, constituting a robust and efficient way to control the properties of the interface at the nanoscale. We demonstrate that the side gates enable reliable tuning of both the normal-state resistance and the critical (Josephson) current of the constrictions. The conductance and Josephson current show mesoscopic fluctuations as a function of the applied side gate voltage, and the analysis of their amplitude enables the extraction of the phase coherence and thermal lengths. Finally, we realize a superconducting quantum interference device in which the critical currents of each of the constriction-type Josephson junctions can be controlled independently via the side gates.

Reversible oxygen vacancies doping in (La0.7,Sr0.3)MnO3 microbridges by combined self-heating and electromigration

Combination of electric fields and Joule self-heating is used to change the oxygen stoichiometry and promote oxygen vacancy drift in a freestanding (La,Sr)MnO3 thin film microbridge placed in controlled atmosphere. By controlling the local oxygen vacancies concentration, we can reversibly switch our (La,Sr)MnO3-based microbridges from metallic to insulating behavior on timescales lower than 1 s and with small applied voltages (<5 V). The strong temperature gradients given by the microbridge geometry strongly confine the motion of oxygen vacancies, limiting the modified region within the free-standing area. Multiple resistive states can be set by selected current pulses that determine different oxygen vacancies profiles within the device. Qualitative analysis of device operation is also provided with the support of finite element analysis.

Electro-thermal bistability in (La0.7Sr0.3)MnO3 suspended microbridges: Thermal characterization and transient analysis

Bistability of the electrical resistance in free-standing (La0.7Sr0.3)MnO3 conducting microbridges under Joule self-heating conditions is reported and modeled by Finite Element Analysis. We show that a low (LRS) and a high (HRS) resistance state can be selected below room temperature, where the typical non-linearity of ρ(T) relationship of manganites determines multiple thermal equilibrium conditions. We analyze bistability in microbridges in terms of temperature and heat dissipation conditions. Thanks to the small thermal coupling of the suspended geometry, switching between LRS/HRS can be driven with small amount of power (∼250 μW). Finally, temporal evolution of the transition between the LRS and HRS is discussed in the framework of the heating dynamics.

Metal–insulator transition in free-standing VO2/TiO2 microstructures through low-power Joule heating

We investigated voltage bias-driven electronic phase switching from insulating to metallic states in the VO2 thin films having freestanding structures (FSS) and non-freestanding structures (N-FSS). By measuring the electrical power during switching under different thermal conditions, we found that the thermal coupling of the microstructures determined the spatial temperature distribution on the device and strongly affected the efficiency of the insulator–metal switching induced by the Joule effect. The power required for switching in the FSS was two orders lower than that for the N-FSS. This indicates that an appropriate design of the thermal flow is a fundamental issue for developing efficient switching and memristive devices.

Strain response of La0.7Sr0.3CoO3 epitaxial thin films probed by SrTiO3 crystalline microcantilevers

We investigate the effect of tensile strain on the resistivity of La0.7Sr0.3CoO3 thin films grown on SrTiO3 microcantilevers. Strain is applied by bending the microcantilevers both mechanically through an atomic force microscope tip and electrostatically. A strain gauge factor of 12 is observed, similar to the value measured for manganite thin films. Such low value evidences the role of extrinsic effects in the cobaltites giant gauge factor observed by other authors.