Heavy-fermion behavior, driven by the interaction-induced enhancement of electronic mass, underpins exotic states ranging from unconventional superconductivity to quantum criticality. Long restricted to complex three-dimensional f-electron compounds, these phenomena are now predicted to emerge within the flat bands of magic-angle twisted bilayer graphene. Here, we use high-resolution planar tunneling spectroscopy to perform inverse-compressibility and entropy measurements, providing direct evidence for topological heavy-fermion behavior in magic-angle graphene. Our inverse-compressibility data reveal strong, filling-dependent mass renormalization, consistent with the hybridization between localized and itinerant electrons within a Kondo-lattice framework. Furthermore, entropy spectroscopy reveals two plateau structures whose degeneracies directly encode the combinatorial structure of local moment states. This 8-to-4-fold degeneracy reduction is an entropic signature of strain-mediated symmetry breaking, consistent with the topological heavy-fermion model. Magic-angle graphene is known to host flat bands and emergent phenomena. Here, the authors provide thermodynamic evidence for topological heavy-fermion behavior, observing a strain-induced reduction in the combinatorial degeneracy of isospin local moments (from eight-fold to four-fold) alongside pronounced electronic mass renormalization near integer band fillings.

Computation in biological neural circuits arises from the interplay of nonlinear temporal responses and spatially distributed dynamic network interactions. Replicating this richness in hardware has remained challenging, as most neuromorphic devices emulate only isolated neuron- or synapse-like functions. Here we introduce an integrated neuromorphic computing platform in which both nonlinear spatiotemporal processing and programmable memory are realized within a single perovskite nickelate material system. By engineering symmetric and asymmetric hydrogenated NdNiO3 junction devices on the same wafer, we combine ultrafast, proton-mediated transient dynamics with stable multilevel resistance states. Networks of symmetric NdNiO3 junctions exhibit emergent spatial interactions mediated by proton redistribution, while each node simultaneously provides short-term temporal memory, enabling nanosecond-scale operation with an energy cost of ~0.2 nJ per input. When interfaced with asymmetric output units serving as reconfigurable long-term weights, these networks allow both feature transformation and linear classification in the same material system. Leveraging these emergent interactions, the platform enables real-time pattern recognition and achieves high accuracy in spoken digit classification and early seizure detection, outperforming temporal-only or uncoupled architectures. These results position protonic nickelates as a compact, energy-efficient, CMOS-compatible platform that integrates processing and memory for scalable intelligent hardware.

Though research on graphene by itself has waned, the interest in moire materials, materials made with stacked layers of graphene with a rotational twist between the layers, has exploded in popularity. These layered devices show a key feature, flat bands. Flat bands localize electrons, which in turn leads to the expression of correlated states such as Mott insulators, superconductivity, and more. A key property of these devices is that their 2D nature allows us to tune them in situ, effectively allowing us to change the device's electronic properties. This powerful ability greatly reduces the time and money required to study superconductivity. The superconductivity in these systems seems to be similar to high-temperature superconductors such as cuprates, giving us a path towards studying high-temperature superconductivity. The fabrication of these devices is nontrivial, and thus we detail one general way to create these layered devices to give maximal tunability.

Topological heavy fermion models[1-5] describe the flat bands in magic-angle twisted bilayer graphene (MATBG) as arising from the hybridization between localized flat-band orbitals (f-electrons) and nearly-free conduction bands (c-electrons). The interplay between these f-electrons and c-electrons is theorized to give rise to emergent phenomena, including unconventional superconductivity[6-8], non-Fermi liquid behavior[9-11], and topologically nontrivial phases[12-14]. However, the fundamental properties of f- and c-electrons, such as their respective heavy and light effective mass and their properties under strain, need experimental verification. Here we report on the electronic inverse compressibility, effective mass, and entropy of MATBG, obtained from planar tunneling spectroscopy. Our results include the observation of electron mass renormalization, found to be consistent with the topological heavy fermion model prediction of heavy charge-one excitations away from integer fillings. Importantly, we present entropic evidence for 4-fold and 8-fold degenerate isospin local moment states emerging at temperatures of 10K and 20K, respectively, consistent with the entropy of 8 heavy-fermions flavors energetically split by the sample strain.

The world of 2D materials is rapidly expanding with new discoveries of stackable and twistable layered systems composed of lattices of different symmetries, orbital character, and structural motifs. Often, however, it is not clear a priori whether a pair of monolayers twisted at a small angle will exhibit correlated or interaction-driven phenomena. The computational cost to make accurate predictions of the single particle states is significant, as small twists require very large unit cells, easily encompassing 10,000 atoms, and therefore implementing a high throughput prediction has been out of reach. Here we show a path to overcome this challenge by introducing a machine learning (ML) based methodology that efficiently estimates the twisted interlayer tunneling at arbitrarily low twist angles through the local-configuration based approach that enables interpolating the local stacking for a range of twist angles using a random forest regression algorithm. We leverage the kernel polynomial method to compute the density of states (DOS) on large real space graphs by reconstructing a lattice model of the twisted bilayer with the ML fitted hoppings. For twisted bilayer graphene (TBG), we show the ability of the method to resolve the magic angle DOS at a substantial improvement in computational time. We use this new technique to scan through the database of stable 2D monolayers (MC2D) and reveal new twistable candidates across the five possible points groups in two-dimensions with a large DOS near the Fermi energy, with potentially exciting interacting physics to be probed in future experiments.

When dimensionality is reduced, enhanced quantum fluctuations can destroy long-range phase coherence, driving a superconductor insulator transition, SIT, where disorder and electronic correlations give rise to novel many-body states. Here, we report the first observation of a magnetic field tuned SIT in mirrorsymmetric twisted trilayer graphene, TTG. Remarkably, signatures of quantum criticality persist over an exceptionally broad range of magnetic fields and are well described by the formation of a quantum Griffiths phase, a regime in which rare spatially extended regions develop local order within a globally disordered phase. This leads to a quantum phase transition governed by an infinite-randomness fixed point and characterized by ultraslow relaxation dynamics. Near the quantum critical region, transport measurements reveal strongly nonlinear electrical behavior, including a current-driven reentrant transition from insulating to superconducting transport, providing direct evidence of local superconducting order. By tilting the magnetic field, we are able to collapse the broad Griffiths regime into a single quantum critical point, QCP, demonstrating a striking level of control over disorder induced quantum dynamics. Our results further show that TTG strongly violates the Pauli limit and establishes twisted trilayer graphene as a tunable platform for exploring quantum phase fluctuations, Cooper pair localization, and unconventional superconductivity.

Stacking two atomic crystals with a twist between their crystal axes produces moiré potentials that modify the electronic properties. Here we show that double moiré potentials generated by superposing three atomic crystals create a new class of tunable quasiperiodic structures that alter the symmetry and spatial distribution of the electronic wavefunctions. By using scanning tunneling microscopy and spectroscopy to study twisted bilayer graphene on hexagonal boron nitride (hBN), we unveil a moiré phase diagram defined by the lattice constants of the two moiré lattices (graphene-on-graphene and graphene-on-hBN), comprising both commensurate periodic and incommensurate quasiperiodic crystals. Remarkably, the 1:1 commensurate crystal, which should theoretically exist at only one point on this phase diagram, is observed over a wide range, demonstrating an unexpected self-alignment mechanism. The incommensurate crystals include quasicrystals, which are quasiperiodic and feature a Bravais-forbidden dodecagonal symmetry, and intercrystals, which are also quasiperiodic but lack forbidden symmetries. This rich variety of tunable double moiré structures offers a synthetic platform for exploring the unique electronic properties of quasiperiodic crystals, which are rarely found in nature.

Ion drift in nanoscale electronically inhomogeneous semiconductors is among the most important mechanisms being studied for designing neuromorphic computing hardware. However, nondestructive imaging of the ion drift in operando devices directly responsible for multiresistance states and synaptic memory represents a formidable challenge. Here, we present Kelvin probe force microscopy imaging of hydrogen-doped perovskite nickelate device channels subject to high-speed electric field pulses to directly visualize proton distribution by monitoring surface potential changes spatially, which is also supported with finite element-based electric field distribution studies. First-principles calculations provide mechanistic insights into the origin of surface potential changes as a function of hydrogen donor doping that serves as the contrast mechanism. We demonstrate 128 (7-bit) nonvolatile conductance levels in such devices relevant to in-memory computing applications. The synaptic plasticity measurements are implemented in spiking neural networks and show promising results for classification (SciKit Learn's Iris and Wine data sets) and control (OpenAI's CartPole-v1 and BipedalWalker-v3) simulation tasks.

We present a flexible mechanism for vertically transferring scanning probe microscope (SPM) heads in ultra-high vacuum. The microscope head is transferred from a room temperature site down to an SPM pluggable connector at low temperatures within the bore of a superconducting magnet located in a top-loading cryostat. Insertion and extraction of the microscope head to and from the connector are accomplished with gravity-assisted tools. Contrasted with typical rigid designs, the low-profile mechanism described here has no external mounting structure, which reduces sensitivity to mechanical disturbances, decreases vacuum volume, and enables installation in standard laboratory spaces.

Atomically thin crystals have changed our understanding of materials. With all their atoms exposed at the surface, it has become possible to tune their properties without altering their chemical composition, by means such as strain, electric-fields, or substrate engineering, resulting in new states of matter [1,2].

One of the daunting challenges in modern low temperature scanning tunneling microscopy (STM) is the difficulty of combining atomic resolution with cryogen free cooling. Further functionality needs, such as ultra-high vacuum (UHV), high magnetic field, and compatibility with {\mu}m-sized samples, pose additional challenges to an already ambitious build. We present the design, construction, and performance of a cryogen free, UHV, low temperature, and high magnetic field system for modular STM operation. An internal vibration isolator reduces vibrations in this system allowing atomic resolution STM imaging while maintaining a low base temperature of ~4K and magnetic fields up to 9T. Samples and tips can be conditioned in-situ utilizing a heating stage, an ion sputtering gun, an e-beam evaporator, a tip treater, and sample exfoliation. In-situ sample and tip exchange and alignment are performed in a connected UHV room temperature stage with optical access. Multisite operation without breaking vacuum is enabled by a unique quick-connect STM head design. A novel low-profile vertical transfer mechanism permits transferring the STM between room temperature and the low temperature cryostat.

The proximity-effect, whereby materials in contact appropriate each others electronic-properties, is widely used to induce correlated states, such as superconductivity or magnetism, at heterostructure interfaces. Thus far however, demonstrating the existence of proximity-induced charge-density-waves (PI-CDW) proved challenging. This is due to competing effects, such as screening or co-tunneling into the parent material, that obscured its presence. Here we report the observation of a PI-CDW in a graphene layer contacted by a 1T-TaS2 substrate. Using scanning tunneling microscopy (STM) and spectroscopy (STS) together with theoretical-modeling, we show that the coexistence of a CDW with a Mott gap in 1T-TaS2 coupled with the Dirac-dispersion of electrons in graphene, makes it possible to unambiguously demonstrate the PI-CDW by ruling out alternative interpretations. Furthermore, we find that the PI-CDW is accompanied by a reduction of the Mott gap in 1T-TaS2 and show that the mechanism underlying the PI-CDW is well-described by short-range exchange-interactions that are distinctly different from previously observed proximity effects.

The proximity-effect, whereby materials in contact appropriate each other's electronic-properties, is widely used to induce correlated states, such as superconductivity or magnetism, at heterostructure interfaces. Thus far however, demonstrating the existence of proximity-induced charge-density-waves (PI-CDW) proved challenging. This is due to competing effects, such as screening or co-tunneling into the parent material, that obscured its presence. Here we report the observation of a PI-CDW in a graphene layer contacted by a 1T-TaS2 substrate. Using scanning tunneling microscopy (STM) and spectroscopy (STS) together with theoretical-modeling, we show that the coexistence of a CDW with a Mott-gap in 1T-TaS2 coupled with the Dirac-dispersion of electrons in graphene, makes it possible to unambiguously demonstrate the PI-CDW by ruling out alternative interpretations. Furthermore, we find that the PI-CDW is accompanied by a reduction of the Mott gap in 1T-TaS2 and show that the mechanism underlying the PI-CDW is well-described by short-range exchange-interactions that are distinctly different from previously observed proximity effects.

To meet changing research demands, new scanning tunneling microscope (STM) features must constantly evolve. We describe the design, development, and performance of a modular plug-in STM, which is compact and stable. The STM head is equipped with a quick-connect socket that is matched to a universal connector plug, enabling it to be transferred between systems. This head can be introduced into a vacuum system via a load-lock and transferred to various sites equipped with the connector plug, permitting multi-site STM operation. Its design allows for reliable operation in a variety of experimental conditions, including a broad temperature range, ultra-high vacuum, high magnetic fields, and closed-cycle pulse-tube cooling. The STM's compact size is achieved by a novel nested piezoelectric coarse walker design, which allows for large orthogonal travel in the X, Y, and Z directions, ideal for studying both bulk and thin film samples ranging in size from mm to μm. Its stability and noise tolerance are demonstrated by achieving atomic resolution under ambient conditions on a laboratory desktop with no vibrational or acoustic isolation. The operation of the nested coarse walkers is demonstrated by successful navigation to a μm-sized 2D sample.

The proximity-effect, a phenomenon whereby materials in close contact appropriate each other’s electronic-properties, is widely used in nano-scale devices to induce electron-correlations at heterostructure interfaces. Commonly observed proximity-induced correlation-effects include superconductivity, magnetism, and spin-orbit interactions. Thus far, however, proximity induced charge density waves (CDW) have not been rigorously explored, primarily because of screening in 3D metals and defect scattering at interfaces. Here, we report the observation of a CDW proximity effect between graphene and the commensurate CDW in 1T-TaS2 (henceforth called TaS2 for brevity). Using scanning tunneling microscopy (STM) and spectroscopy (STS) together with theoretical modeling to probe the interface between graphene and a TaS2 crystal, we demonstrate the existence of a proximity induced CDW within graphene. Furthermore, we observe that graphene modifies the band structure at the surface of TaS2, by providing mid-gap carriers and reducing the strength of electron correlations there. We show that the mechanism underlying the proximity induced CDW is well-described by short-range exchange interactions that are distinctly different from previously observed proximity effects.

In single sheets of graphene, vacancy-induced states have been shown to host an effective spin-1/2 hole that can be Kondo screened at low temperatures. Here, we show how these vacancy-induced impurity states survive in twisted bilayer graphene (TBG), which thus provides a tunable system to probe the critical destruction of the Kondo effect in pseudogap hosts. Ab initio calculations and atomic-scale modeling are used to determine the nature of the vacancy states in the vicinity of the magic angle in TBG, demonstrating that the vacancy can be treated as a quantum impurity. Utilizing this insight, we construct an Anderson impurity model with a TBG host that we solve using the numerical renormalization group combined with the kernel polynomial method. We determine the phase diagram of the model and show how there is a strict dichotomy between vacancies in the AA/BB versus AB/BA tunneling regions. In AB/BA vacancies, the Kondo temperature at the magic angle develops a broad distribution with a tail to vanishing temperatures due to multifractal wave functions at the magic angle. We argue that scanning tunneling microscopy in the vicinity of the vacancy can act as a probe of both the critical single-particle states and the underlying many- body ground state in magic-angle TBG.

A key aspect of how the brain learns and enables decision-making processes is through synaptic interactions. Electrical transmission and communication in a network of synapses are modulated by extracellular fields generated by ionic chemical gradients. Emulating such spatial interactions in synthetic networks can be of potential use for neuromorphic learning and the hardware implementation of artificial intelligence. Here, we demonstrate that in a network of hydrogen-doped perovskite nickelate devices, electric bias across a single junction can tune the coupling strength between the neighboring cells. Electrical transport measurements and spatially resolved diffraction and nanoprobe X-ray and scanning microwave impedance spectroscopic studies suggest that graded proton distribution in the inhomogeneous medium of hydrogen-doped nickelate film enables this behavior. We further demonstrate signal integration through the coupling of various junctions.

Using scanning-tunneling-microscopy and theoretical modeling on heterostructures of twisted bilayer graphene and hexagonal Boron-Nitride, we show that the emergent super-moire structures display a rich landscape of moire-crystals and quasicrystals. We reveal a phase-diagram comprised of commensurate moire-crystals embedded in swaths of moire quasicrystals. The 1:1 commensurate crystal, expected to be a Chern insulator, should only exist at one point on the phase-diagram, implying that it ought to be practically undetectable. Surprisingly we find that the commensurate crystals exist over a much wider than predicted range, providing evidence of an unexpected self-alignment mechanism that is explained using an elastic-network model. The remainder of the phase-diagram, where we observe tunable quasicrystals, affords a new platform for exploring the unique electronic-properties of these rarely found in nature structures.

Artificially twisted heterostructures of semiconducting transition-metal dichalcogenides (TMDs) offer unprecedented control over their electronic and optical properties via the spatial modulation of interlayer interactions and structural reconstruction. Here we study twisted MoS2 bilayers in a wide range of twist angles near 0° using scanning tunneling microscopy/spectroscopy. We investigate the twist angle dependence of the moiré pattern, which is dominated by lattice reconstruction for small angles (<2°), leading to large triangular domains with rhombohedral stacking. Local spectroscopy measurements reveal a large moiré-potential strength of 100-200 meV for angles <3°. In reconstructed regions, we see a bias-dependent asymmetry between neighboring triangular domains, which we relate to the vertical polarization that is intrinsic to rhombohedral stacked TMDs. This viewpoint is further supported by spectroscopy maps and ambient piezoresponse measurements. Our results provide a microscopic perspective of this new class of interfacial ferroelectrics and can offer clues for designing novel heterostructures that harness this effect.

The first experimental realization of the intrinsic (not dominated by defects) charge conduction regime in lead‐halide perovskite field‐effect transistors (FETs) is reported. The advance is enabled by: i) a new vapor‐phase epitaxy technique that results in large‐area single‐crystalline cesium lead bromide (CsPbBr 3 ) films with excellent structural and surface properties, including atomically flat surface morphology, essentially free from defects and traps at the level relevant to device operation; ii) an extensive materials analysis of these films using a variety of thin‐film and surface probes certifying the chemical and structural quality of the material; and iii) the fabrication of nearly ideal (trap‐free) FETs with characteristics superior to any reported to date. These devices allow the investigation of the intrinsic FET and (gated) Hall‐effect carrier mobilities as functions of temperature. The intrinsic mobility is found to increase on cooling from ≈30 cm 2 V −1 s −1 at room temperature to ≈250 cm 2 V −1 s −1 at 50 K, revealing a band transport limited by phonon scattering. Establishing the intrinsic (phonon‐limited) mobility provides a solid test for theoretical descriptions of carrier transport in perovskites, reveals basic limits to the technology, and points to a path for future high‐performance perovskite electronic devices.