Alex Frano

Understanding and controlling phase transitions is fundamental to physics and central to many technological revolutions. There is currently strong interest in materials with coupled structural and electronic phase transitions, where controlling the transition plasticity naturally yields built-in memory, key for emulating neurons and synapses in neuromorphic technologies. Here, operando coherent Bragg X-ray diffraction is used to study the evolution of the nano-domain distribution at the micron-scale in neuromorphic devices made from the archetypal Mott insulator vanadium dioxide. Electrical switching leads to nano-domain reconfiguration over thousands of seconds and a jamming transition. Repetitive above-threshold currents plastically drive the system into a jammed/glassy state where switching is deterministic. Sub-threshold currents erase the short-term memory defined by the nano-domain distribution, recovering stochastic switching, thus offering a path for in-device learning. These results highlight the importance of nanoscale physics in phase transitions, even for macroscopic devices, and offer guidance for future device operation.

Kitaev iridates are an important class of spin-orbit-entangled quantum materials, where bond-directional exchange generates strong magnetic frustration and unconventional correlated states. The α , β , γ − Li ₂ IrO ₃ family exhibit fragile and highly anisotropic magnetic behavior, including incommensurate counter-rotating order, strong field sensitivity, and pressure-driven electronic reconstruction. Understanding how chemical substitution perturbs these competing interactions provides a route toward probing the underlying frustrated magnetic state. Here, we study single crystals of β − Li ₂ Ru _x Ir _{1− x} O ₃ with dilute Ru substitution, x ≲ 10%. Through magnetometry, resonant elastic X-ray scattering, a c -heat capacity, and muon spin relaxation/rotation, we show that weak magnetic disorder suppresses the incommensurate antiferromagnetic ground state. Instead, the system evolves into a bulk spin glass characterized by slow relaxation and frozen, local magnetic fields. Despite the loss of long-range order, the glassy state retains substantial directional anisotropy, evidenced by enhanced b -axis magnetic susceptibility and direction-dependent thermoremanent relaxation. Structural probes further indicate that Ru substitution leaves the hyperhoneycomb lattice largely undistorted while disrupting magnetic coherence, suggesting that disorder freezes spins within a still-active bond-directional exchange environment. In this picture, dilute Ru substitution provides a controlled pathway into an anisotropic Kitaev spin glass regime that preserves essential fingerprints of the underlying Kitaev exchange network.

Phase transition materials such as NdNiO ₃ (NNO) when coupled with low damping ferromagnets such as La _0.67 Sr _0.33 MnO ₃ (LSMO) hold the promise for developing new multi-functional material systems harnessing the interplay of charge, spin, and orbital degrees of freedom. In this study, we probe the evolution of spin-to-charge conversion in epitaxial all-oxide LSMO (12 nm)/NNO (4, 8, and 16 nm) bilayers. Using spin pumping ferromagnetic resonance, we track the spin-to-charge conversion within the NNO layer through the paramagnetic metal to antiferromagnetic insulator transition and observe a pronounced enhancement of the inverse spin Hall effect signal at the onset of this phase transition. We attribute this enhancement to the electronic and magnetic disorder in NNO at the first-order phase transition, thereby providing insights into the mechanism of spin transport through the phase transition. The tunability of spin-to-charge conversion in this low damping bilayer system offers a pathway for developing reconfigurable, energy-efficient spintronic devices.

In film-substrate systems, the substrate role is often considered to be limited to providing static mechanical constraints. Dynamic film-substrate interactions when a structural change in the film modifies the substrate are generally disregarded. Using combined X-ray and electron microscopies, we observed that the electrically induced filament in a VO ₂ film created strong asymmetric strain in the underlying Al ₂ O ₃ substate. This asymmetric substrate strain fed back into the film and defined the filament expansion direction, revealing the importance of film-substrate dynamic interactions in determining film functionality. Furthermore, the strain imprint propagated at least tens of microns deep into the substrate, exceeding the film thickness more than 200 times, potentially enabling substrate functionalization as an active mechanical coupling media in 3D-integrated microelectronics architectures.

Microscopy techniques can uncover the physical properties and dynamic behaviours of materials, driving the discovery of emergent phenomena and guiding the design of next-generation computing hardware. As artificial intelligence becomes pervasive, the demand for high-performance materials to support sustainable information technologies is growing. This Review highlights state-of-the-art imaging from electron and X-ray to optical techniques to probe the dynamics of neuromorphic materials, including operando characterization of devices. We examine design principles for neuromorphic materials, along with obstacles that hinder their development. Emphasis is placed on spatially and temporally resolved approaches that capture state changes including phase transitions, ferroic switching and spin-wave propagation that emulate biological components such as neurons, synapses and their connectivity. We discuss challenges in operando characterization and the integration of artificial intelligence-driven analysis for feedback-guided material discovery. Finally, we outline opportunities for real-time imaging of neuromorphic systems, paving the way towards adaptive, brain-inspired hardware.

Among the family of intercalated transition-metal dichalcogenides (TMDs), Fe_{x}NbS_{2} is found to possess unique current-induced resistive switching behaviors, tunable antiferromagnetic states, and a commensurate charge order, all of which are tied to a critical Fe doping of x_{c}=1/3. However, the electronic origin of such extreme stoichiometry sensitivities remains unclear. Combining angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations, we identify and characterize a dramatic eV-scale electronic restructuring that occurs across the x_{c}. Moment-carrying Fe 3d_{z^{2}} electrons manifest as narrow bands within 200 meV of the Fermi level, distinct from other transition metal intercalated TMD magnets. These states strongly hybridize with itinerant electrons in the TMD layer and rapidly lose coherence above x_{c} due to correlation-driven effects. This sudden quasiparticle decoherence collapses the Fe-Nb hybridization, which explicitly suppresses the out-of-plane effective Fe-Fe exchange interaction, driving the transformation of the magnetic ground state from an antiferromagnetic stripe phase to a zigzag phase. These observations resemble the exceptional electronic and magnetic sensitivity of strongly correlated systems, and demonstrate that quantifying orbital-specific hybridization via ARPES offers an alternative pathway to evaluate effective magnetic exchange in metallic magnets, complementing inelastic neutron and resonant x-ray scattering probes.

The mechanism behind superconductivity suppression induced by Pr substitutions in YBa ₂ Cu ₃ O _{7− δ} (YBCO) has been a mystery since its discovery: in spite of being isovalent to Y ³⁺ with a small magnetic moment, it is the only rare-earth element that has a dramatic impact on YBCO’s superconducting properties. Using angle-resolved photoemission spectroscopy (ARPES) and DFT+ U calculations, we uncover how Pr substitution modifies the low-energy electronic structure of YBCO. Contrary to the prevailing Fehrenbacher–Rice (FR) and Liechtenstein–Mazin (LM) models, the low-energy electronic structure contains no signature of any f -electron hybridization or additional f -state Fermi surface sheets. Yet, strong electron doping is observed primarily on the antibonding Fermi surface. Meanwhile, we reveal major electronic structure modifications to Cu-derived states with increasing Pr substitution: a pronounced CuO ₂ bilayer decoupling and enhanced hopping along the CuO chain, implying indirect electron-release pathways beyond simple 4 f state ionization. Our results challenge the long-standing FR/LM mechanism, and establish Pr substituted YBCO as a potential platform for exploring correlation-driven phenomena in coupled 1D–2D systems.

Organic surface ligands are integral components of nanocrystals and nanoparticles that have a strong influence on their physicochemical properties, their interaction with the environment, and their ability to self-assemble and order into higher-order structures. These hybrid nanomaterials are tunable with applications in catalysis, directed self-assembly, next-generation optoelectronics, and chemical and quantum sensing. Critically, future advances depend on our ability to rationally engineer their surface chemistry. However, fundamental knowledge of ligand-nanoparticle behavior is limited by uncertainty in where and how these ligands bind to surfaces. For nanoparticles, in particular, few characterization techniques offer both the high spatial resolution and the precise chemical mapping needed to identify specific ligand binding sites. In this study, we utilized synchrotron infrared nanospectroscopy (SINS), atomic force microscopy (AFM), and scanning tunneling microscopy (STM) together with first-principles computer simulations to validate the site-selective adsorption of organic ligands on a shaped nanocrystal surface. Specifically, we demonstrate that the sterically encumbered isocyanide ligands (CNAr^Mes2) preferentially bind to the high curvature features of Ag nanocubes (NCs), where low-coordinate Ag atoms are present. In contrast, isocyanide ligands that do not exhibit these steric properties show no surface selectivity. SINS serves as an effective tool to validate these surface binding interactions at the near-single molecule level. These results indicate that steric effects can be successfully harnessed to design bespoke organic ligands for fine-tuning nanocrystal surface chemistry and the properties of the nanocrystal ligand shell.