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Electrical control of magnetism has been a major technological pursuit of the spintronics community, owing to its far-reaching implications for data storage and transmission. Here, we propose and analyze a new mechanism for electrical switching of isospin, using chiral-stacked graphene multilayers, such as Bernal bilayer graphene or rhombohedral trilayer graphene, encapsulated by transition metal dichalcogenide (TMD) substrates. Leveraging the proximity-induced spin-orbit coupling from the TMD, we demonstrate electrical switching of correlation-induced spin and/or valley polarization, by reversing a perpendicular displacement field or the chemical potential. We substantiate our proposal with both analytical arguments and self-consistent Hartree-Fock numerics. Finally, we illustrate how the relative alignment of the TMDs, together with the top and bottom gate voltages, can be used to selectively switch distinct isospin flavors, putting forward correlated Van der Waals heterostructures as a promising platform for spintronics and valleytronics.

The Hofstadter model successfully describes the behavior of non-interactingquantum particles hopping on a lattice coupled to a gauge field, and hence isubiquitous in many fields of research, including condensed matter, optical, andatomic physics. Motivated by this, we introduce HofstadterTools(https://hofstadter.tools), a Python package that can be used to analyze theenergy spectrum of a generalized Hofstadter model, with any combination ofhoppings on any regular Euclidean lattice. The package can be applied tocompute key properties of the band structure, such as quantum geometry andtopology, as well as plot Hofstadter butterflies and Wannier diagrams that arecolored according to their Chern numbers.

The obstruction to constructing localized degrees of freedom is a signatureof several interesting condensed matter phases. We introduce a localizationrenormalization procedure that harnesses this property, and apply our method todistinguish between topological and trivial phases in quantum Hall and Cherninsulators. By iteratively removing a fraction of maximally-localizedorthogonal basis states, we find that the localization length in the residualHilbert space exhibits a power-law divergence as the fraction of remainingstates approaches zero, with an exponent of $u=0.5$. In sharp contrast, thelocalization length converges to a system-size-independent constant in thetrivial phase. We verify this scaling using a variety of algorithms to truncatethe Hilbert space, and show that it corresponds to a statistically self-similarexpansion of the real-space projector. This result accords with arenormalization group picture and motivates the use of localizationrenormalization as a versatile numerical diagnostic for quantum Hall systems.

The stability of fractional Chern insulators is widely believed to bepredicted by the resemblance of their single-particle spectra to Landau levels.We investigate the scope of this geometric stability hypothesis by analyzingthe stability of a set of fractional Chern insulators that explicitly do nothave a Landau level continuum limit. By computing the many-body spectra ofLaughlin states in a generalized Hofstadter model, we analyze the relationshipbetween single-particle metrics, such as trace inequality saturation, andmany-body metrics, such as the magnitude of the many-body and entanglementgaps. We show numerically that the geometric stability hypothesis holds forChern bands that are not continuously connected to Landau levels, as well asconventional Chern bands, albeit often requiring larger system sizes toconverge for these configurations.

Time-resolved ultrafast EUV magnetic scattering was used to test a recentprediction of >10 km/s domain wall speeds by optically exciting a magneticsample with a nanoscale labyrinthine domain pattern. Ultrafast distortion ofthe diffraction pattern was observed at markedly different timescales comparedto the magnetization quenching. The diffraction pattern distortion shows athreshold-dependence with laser fluence, not seen for magnetization quenching,consistent with a picture of domain wall motion with pinning sites. Supportedby simulations, we show that a speed of $\approx$ 66 km/s for highly curveddomain walls can explain the experimental data. While our data agree with theprediction of extreme, non-equilibrium wall speeds locally, it differs from thedetails of the theory, suggesting that additional mechanisms are required tofully understand these effects.

Ultrafast charge transfer processes provide a facile way to create interlayerexcitons in directly contacted transition metal dichalcogenide (TMD) layers.More sophisticated heterostructures composed of TMD/hBN/TMD enable new ways tocontrol interlayer exciton properties and achieve novel exciton phenomena, suchas exciton insulators and condensates, where longer lifetimes are desired. Inthis work, we experimentally study the charge transfer dynamics in aheterostructure composed of a 1 nm thick hBN spacer between MoSe$_{2}$ andWSe$_{2}$ monolayers. We observe the hole transfer from MoSe$_{2}$ to WSe$_{2}$through the hBN barrier with a time constant of 500 ps, which is over 3 ordersof magnitude slower than that between TMD layers without a spacer. Furthermore,we observe strong competition between the interlayer charge transfer andintralayer exciton-exciton annihilation processes at high excitation densities.Our work opens possibilities to understand charge transfer pathways inTMD/hBN/TMD heterostructures for the efficient generation and control ofinterlayer excitons.

A spontaneous ferromagnetic moment can be induced in Bi$_{2}$Te$_{3}$ thinfilms below a temperature T $\approx$ 16 K by the introduction of Mn dopants.We demonstrate that films grown via molecular beam epitaxy with thestoichiometry Mn$_{0.14}$Bi$_{1.86}$Te$_3$ maintain the crystal structure ofpure Bi$_{2}$Te$_{3}$. The van der Waals nature of inter-layer forces in theMn$_{0.14}$Bi$_{1.86}$Te$_3$ crystal causes lattice mismatch with theunderlayer to have a limited effect on the resulting crystal structure, as wedemonstrate by thin film growth on tetragonal MgF$_{2}$ (110) and NiF$_{2}$(110). Electronic transport and magnetic moment measurements show that theferromagnetic moment of the Mn$_{0.14}$Bi$_{1.86}$Te$_3$ thin films is enhancedas the Fermi level moves from the bulk conduction band and towards the bulkband gap, suggesting that electronic surface states play an important role inmediating the ferromagnetic order. FerromagneticMn$_{0.14}$Bi$_{1.86}$Te$_3$/antiferromagnetic NiF$_{2}$ bilayers show evidencethat the ferromagnetic moment of the Mn$_{0.14}$Bi$_{1.86}$Te$_3$ film issuppressed, suggesting the existence of an interface effect between the twomagnetic layers.

Interactions are essential for the creation of correlated quantum many-body states. Although two-body interactions underlie most natural phenomena, three- and four-body interactions are important for the physics of nuclei1, exotic few-body states in ultracold quantum gases2, the fractional quantum Hall effect3, quantum error correction4 and holography5,6. Recently, a number of artificial quantum systems have emerged as simulators for many-body physics, featuring the ability to engineer strong interactions. However, the interactions in these systems have largely been limited to the two-body paradigm and require building up multibody interactions by combining two-body forces. Here we implement a scheme to create a higher-order interaction between photons stored in multiple electromagnetic modes of a microwave cavity. The system is dressed such that there is collectively no interaction until a target total photon number is reached across multiple distinct modes, at which point the photons interact strongly. In our demonstration, we create interactions involving up to three bodies and across up to five modes. We harness the interaction to prepare single-mode Fock states and multimode W states, which we verify by introducing a multimode Wigner tomography method.

Spectral response functions are central quantities in the analysis of quantummany-body states, since they describe the response of many-body systems toexternal perturbations and hence directly correspond to observables inexperiments. In this paper, we evaluate a momentum-averaged dynamical densitystructure factor for the fermionic $u=1/3$ fractional quantum Hall state on atorus, using the continued fraction method to compute the dynamical correlationfunction. We establish the scaling behavior of the screened Coulomb structurefactor with respect to interaction range, and expose an inherentself-similarity of structure factors in the frequency domain. These resultshighlight the statistical properties of spectral response functions forfractional quantum Hall states and show how they can be efficientlyapproximated in numerical models.

Recent developments in fractional quantum Hall (FQH) physics highlight theimportance of studying FQH phases of particles partially occupying energy bandsthat are not Landau levels. FQH phases in the regime of strong lattice effects,called fractional Chern insulators, provide one setting for such studies. Asthe strength of lattice effects vanishes, the bands of generic lattice modelsasymptotically approach Landau levels. In this article, we construct latticemodels for single-particle bands that are distinct from Landau levels even inthis continuum limit. We describe how the distinction between such bands andLandau levels is quantified by band geometry over the magnetic Brillouin zoneand reflected in the electromagnetic response. We analyze thelocalization-delocalization transition in one such model and compute alocalization length exponent of 2.57(3). Moreover, we study interactionsprojected to these bands and find signatures of bosonic and fermionic Laughlinstates. Most pertinently, our models allow us to isolate conditions for optimalband geometry and gain further insight into the stability of FQH phases onlattices.

Atomic-scale characteristics of surfaces dictate the principles governingnumerous scientific phenomena ranging from catalysis to friction. Despite thisfact, our ability to visualize and alter surfaces on the atomic scale isseverely hampered by the strict conditions under which the related methods areoperated to achieve high spatial resolution. In particular, the two prominentmethods utilized to achieve atomic-resolution imaging - scanning tunnelingmicroscopy (STM) and noncontact atomic force microscopy (NC-AFM) - aretypically performed under ultrahigh vacuum (UHV) and often at low temperatures.Perhaps more importantly, results obtained under such well-controlled, pristineconditions bear little relevance for the great majority of processes andapplications that often occur under ambient conditions. As such, a method thatcan robustly image surfaces on the atomic scale under ambient conditions haslong been thought of as a "holy grail" of surface science. Here, by way of aproof-of-principle measurement on molybdenum disulfide (MoS2), we report thatthe method of conductive atomic force microscopy (C-AFM) can be utilized toachieve true atomic-resolution imaging under ambient conditions as proven bythe imaging of a single atomic vacancy, without any control over theoperational environment or elaborate sample preparation. While the physicalmechanisms behind this remarkable observation are not elucidated yet, ourapproach overcomes many of the classical limitations associated with STM andNC-AFM, and the findings herald the potential emergence of C-AFM as a powerfultool for atomic-resolution imaging under ambient conditions.

van der Waals materials exhibit naturally passivated surfaces and an ability to form versatile heterostructures to enable an examination of carrier transport mechanisms not seen in traditional materials. Here, we report a new type of homojunction termed a "band-bending junction" whose potential landscape depends solely on the difference in thickness between the two sides of the junction. Using MoS2 on Au as a prototypical example, we find that surface potential differences can arise from the degree of vertical band bending in thin and thick regions. Furthermore, by using scanning ultrafast electron microscopy, we examine the spatiotemporal dynamics of charge carriers generated at this junction and find that lateral carrier separation is enabled by differences in the band bending in the vertical direction, which we verify with simulations. Band-bending junctions may therefore enable new optoelectronic devices that rely solely on band bending arising from thickness variations to separate charge carriers.

The Hofstadter model is a popular choice for theorists investigating thefractional quantum Hall effect on lattices, due to its simplicity, infiniteselection of topological flat bands, and increasing applicability to realmaterials. In particular, fractional Chern insulators in bands with Chernnumber $|C|>1$ can demonstrate richer physical properties than continuum Landaulevel states and have recently been detected in experiments. Motivated by this,we examine the stability of fractional Chern insulators with higher Chernnumber in the Hofstadter model, using large-scale infinite density matrixrenormalization group simulations on a thin cylinder. We confirm the existenceof fractional states in bands with Chern numbers $C=1,2,3,4,5$ at the fillingfractions predicted by the generalized Jain series [Phys. Rev. Lett. 115,126401 (2015)]. Moreover, we discuss their metal-to-insulator phasetransitions, as well as the subtleties in distinguishing between physical andnumerical stability. Finally, we comment on the relative suitability offractional Chern insulators in higher Chern number bands for proposed modernapplications.

Van der Waals heteroepitaxy allows deterministic control over lattice mismatch or azimuthal orientation between atomic layers to produce long-wavelength superlattices. The resulting electronic phases depend critically on the superlattice periodicity and localized structural deformations that introduce disorder and strain. In this study we used Bragg interferometry to capture atomic displacement fields in twisted bilayer graphene with twist angles

AbstractWe give a nonequilibrium Green’s function (NEGF) perspective on thermodynamics formulations for open quantum systems that are strongly coupled to baths. A scattering approach implying thermodynamic consideration of a supersystem (system plus baths) that is weakly coupled to external superbaths is compared with the consideration of thermodynamics of a system that is strongly coupled to its baths. We analyze both approaches from the NEGF perspective and argue that the latter yields a possibility of thermodynamic formulation consistent with a dynamical (quantum transport) description.

AbstractWe give a nonequilibrium Green’s function (NEGF) perspective on thermodynamics formulations for open quantum systems that are strongly coupled to baths. A scattering approach implying thermodynamic consideration of a supersystem (system plus baths) that is weakly coupled to external superbaths is compared with the consideration of thermodynamics of a system that is strongly coupled to its baths. We analyze both approaches from the NEGF perspective and argue that the latter yields a possibility of thermodynamic formulation consistent with a dynamical (quantum transport) description.

Moiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a moiré system with tunable flat bands. Here, we use gate-tuned scanning tunneling spectroscopy to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of electronic correlations and topology for its flat band. Our spectroscopic experiments are in agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands with a large effective g-factor.

Topological solitons, a class of stable nonlinear excitations, appear in diverse domains as in the Skyrme model of nuclear forces. Here, we argue that similar excitations play an important role in a remarkable material obtained on stacking and twisting two sheets of graphene. Close to a magic twist angle, insulating behavior is observed, which gives way to superconductivity on doping. Here, we propose a unifying description of both observations. A symmetry breaking condensate leads to the ordered insulator, while topological solitons in the condensate-skyrmions-are shown to be charge 2e bosons. Condensation of skyrmions leads to a superconductor, whose physical properties we calculate. More generally, we show how topological textures can mitigate Coulomb repulsion and provide a previously unexplored route to superconductivity. Our mechanism not only clarifies why several other moiré materials do not show superconductivity but also points to unexplored platforms where robust superconductivity is anticipated.

The discovery of interaction-driven insulating and superconducting phases in moiré van der Waals heterostructures has sparked considerable interest in understanding the novel correlated physics of these systems. While a significant number of studies have focused on twisted bilayer graphene, correlated insulating states and a superconductivity-like transition up to 12 K have been reported in recent transport measurements of twisted double bilayer graphene. Here we present a scanning tunneling microscopy and spectroscopy study of gate-tunable twisted double bilayer graphene devices. We observe splitting of the van Hove singularity peak by ~20 meV at half-filling of the conduction flat band, with a corresponding reduction of the local density of states at the Fermi level. By mapping the tunneling differential conductance we show that this correlated system exhibits energetically split states that are spatially delocalized throughout the different regions in the moiré unit cell, inconsistent with order originating solely from onsite Coulomb repulsion within strongly-localized orbitals. We have performed self-consistent Hartree-Fock calculations that suggest exchange-driven spontaneous symmetry breaking in the degenerate conduction flat band is the origin of the observed correlated state. Our results provide new insight into the nature of electron-electron interactions in twisted double bilayer graphene and related moiré systems.

We study how the intrinsic anomalous Hall conductivity is modified in two-dimensional crystals with broken time-reversal symmetry due to weak inhomogeneity of the applied electric field. Focusing on a clean noninteracting two-band system without band crossings, we derive the general expression for the Hall conductivity at small finite wave vector q to order q^{2}, which governs the Hall response to the second gradient of the electric field. Using the Kubo formula, we show that the answer can be expressed through the Berry curvature, Fubini-Study quantum metric, and the rank-3 symmetric tensor which is related to the quantum geometric connection and physically corresponds to the gauge-invariant part of the third cumulant of the position operator. We further compare our results with the predictions made within the semiclassical approach. By deriving the semiclassical equations of motion, we reproduce the result obtained from the Kubo formula in some limits. We also find, however, that the conventional semiclassical description in terms of the definite position and momentum of the electron is not fully consistent because of singular terms originating from the Heisenberg uncertainty principle. We thus present a clear example of a case when the semiclassical approach inherently suffers from the uncertainty principle, implying that it should be applied to systems in nonuniform fields with extra care.

We describe an experimental technique to measure the chemical potential μ in atomically thin layered materials with high sensitivity and in the static limit. We apply the technique to a high quality graphene monolayer to map out the evolution of μ with carrier density throughout the N=0 and N=1 Landau levels at high magnetic field. By integrating μ over filling factor ν, we obtain the ground state energy per particle, which can be directly compared to numerical calculations. In the N=0 Landau level, our data show exceptional agreement with numerical calculations over the whole Landau level without adjustable parameters as long as the screening of the Coulomb interaction by the filled Landau levels is accounted for. In the N=1 Landau level, a comparison between experimental and numerical data suggests the importance of valley anisotropic interactions and reveals a possible presence of valley-textured electron solids near odd filling.

Controlling electronic properties via band structure engineering is at the heart of modern semiconductor devices. Here, we extend this concept to semimetals where, using LuSb as a model system, we show that quantum confinement lifts carrier compensation and differentially affects the mobility of the electron and hole-like carriers resulting in a strong modification in its large, nonsaturating magnetoresistance behavior. Bonding mismatch at the heteroepitaxial interface of a semimetal (LuSb) and a semiconductor (GaSb) leads to the emergence of a two-dimensional, interfacial hole gas. This is accompanied by a charge transfer across the interface that provides another avenue to modify the electronic structure and magnetotransport properties in the ultrathin limit. Our work lays out a general strategy of using confined thin-film geometries and heteroepitaxial interfaces to engineer electronic structure in semimetallic systems, which allows control over their magnetoresistance behavior and simultaneously provides insights into its origin.

The Dzyaloshinskii-Moriya interaction (DMI) has drawn much attention, as it stabilizes magnetic chirality, with important implications in fundamental and applied research. This antisymmetric exchange interaction is induced by the broken inversion symmetry at interfaces or in noncentrosymmetric lattices. Significant interfacial DMIs are often found at magnetic/heavy-metal interfaces with large spin-orbit coupling. Recent studies have shown promise for induced DMI at interfaces involving light elements such as carbon (graphene) or oxygen. Here, we report direct observation of induced DMI by chemisorption of the lightest element, hydrogen, on a ferromagnetic layer at room temperature, which is supported by density functional theory calculations. We further demonstrate a reversible chirality transition of the magnetic domain walls due to the induced DMI via hydrogen chemisorption and desorption. These results shed new light on the understanding of DMI in low atomic number materials and the design of novel chiral spintronics and magneto-ionic devices.

Imposing additional confinement in two-dimensional (2D) materials yields further control over their electronic, optical, and topological properties. However, synthesis of ultranarrow nanoribbons (NRs) remains challenging, particularly for transition metal dichalcogenides (TMDs), and synthesizing TMD NRs narrower than 50 nm has remained elusive. Here, we report the vapor-phase synthesis of ultranarrow TaS2 NRs. The NRs are grown within carbon nanotubes, limiting their width and layer number, while stabilizing them against the environment. The NRs reach monolayer thickness and exhibit widths down to 2.5 nm. Atomic-resolution scanning transmission electron microscopy reveals the detailed atomic structure of the ultranarrow NRs and we observe a hitherto unseen atomic structure supermodulation of ordered defect arrays within the NRs. Density functional theory calculations show the presence of flat bands and boundary-localized states, and help identify the atomic configuration of the supermodulation. Nanotube-templated synthesis represents a unique, transferable, and broadly deployable route toward ultranarrow TMD NR growth.

Scanning transmission electron microscopy (STEM) is an extremely versatilemethod for studying materials on the atomic scale. Many STEM experiments aresupported or validated with electron scattering simulations. However, using theconventional multislice algorithm to perform these simulations can requireextremely large calculation times, particularly for experiments with millionsof probe positions as each probe position must be simulated independently.Recently, the PRISM algorithm was developed to reduce calculation times forlarge STEM simulations. Here, we introduce a new method for STEM simulation:partitioning of the STEM probe into "beamlets," given by a natural neighborinterpolation of the parent beams. This idea is compatible with PRISMsimulations and can lead to even larger improvements in simulation time, aswell requiring significantly less computer RAM. We have performed varioussimulations to demonstrate the advantages and disadvantages of partitionedPRISM STEM simulations. We find that this new algorithm is particularly usefulfor 4D-STEM simulations of large fields of view. We also provide a referenceimplementation of the multislice, PRISM and partitioned PRISM algorithms.

The search for topological phases of matter is evolving towards strongly interacting systems, including magnets and superconductors, where exotic effects emerge from the quantum-level interplay between geometry, correlation and topology. Over the past decade or so, scanning tunnelling microscopy has become a powerful tool to probe and discover emergent topological matter, because of its unprecedented spatial resolution, high-precision electronic detection and magnetic tunability. Scanning tunnelling microscopy can be used to probe various topological phenomena, as well as complement results from other techniques. We discuss some of these proof-of-principle methodologies applied to probe topology, with particular attention to studies performed under a tunable vector magnetic field, which is a relatively new direction of recent focus. We then project the future possibilities for atomic-resolution tunnelling methods in providing new insights into topological matter.

The structure of MX3 transition metal trichalcogenides (TMTs, with M a transition metal and X a chalcogen) is typified by one-dimensional (1D) chains weakly bound together via van der Waals interactions. This structural motif is common across a range of M and X atoms (e.g., NbSe3, HfTe3,TaS3), but not all M and X combinations are stable. We report here that three new members of the MX3 family which are not stable in bulk, specifically NbTe3, VTe3, and TiTe3, can be synthesized in the few- (2-4) to single-chain limit via nanoconfined growth within the stabilizing cavity of multiwalled carbon nanotubes. Transmission electron microscopy (TEM) and atomic-resolution scanning transmission electron microscopy (STEM) reveal the chain-like nature and the detailed atomic structure. The synthesized materials exhibit behavior unique to few-chain quasi-1D structures, such as few-chain spiraling and a trigonal antiprismatic rocking distortion in the single-chain limit. Density functional theory (DFT) calculations provide insight into the crystal structure and stability of the materials, as well as their electronic structure.

Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Nevertheless, the dynamic structural origins of nonradiative relaxations in such materials are not understood. Here, femtosecond electron diffraction measurements corroborated by atomistic simulations uncover transient lattice deformations accompanying radiationless electronic processes in colloidal semiconductor nanocrystals. Investigation of the excitation energy dependence in a core/shell system shows that hot carriers created by a photon energy considerably larger than the bandgap induce structural distortions at nanocrystal surfaces on few picosecond timescales associated with the localization of trapped holes. On the other hand, carriers created by a photon energy close to the bandgap of the core in the same system result in transient lattice heating that occurs on a much longer 200 picosecond timescale, dominated by an Auger heating mechanism. Elucidation of the structural deformations associated with the surface trapping of hot holes provides atomic-scale insights into the mechanisms deteriorating optoelectronic performance and a pathway towards minimizing these losses in nanocrystal devices.

The scientific and technological exploration of artificially designedthree-dimensional magnetic nanostructures opens the path to exciting novelphysical phenomena, originating from the increased complexity in spin textures,topology, and frustration in three dimensions. Theory predicts that theequilibrium magnetic ground state of two-dimensional systems which reflects thecompetition between symmetric (Heisenberg) and antisymmetric(Dzyaloshinskii-Moriya interaction (DMI)) exchange interaction is significantlymodified on curved surfaces when the radius of local curvature becomescomparable to fundamental magnetic length scales. Here, we present anexperimental study of the spin texture in an 8 nm thin magnetic multilayer withgrowth-induced in-plane anisotropy and DMI deposited onto the curved surface ofa 1.8 {\mu}m long non-magnetic carbon nanowire with a 67 nm radius. Usingmagnetic soft x-ray tomography the three-dimensional spin configuration in thisnanotube was retrieved with about 30nm spatial resolution. The transitionbetween two vortex configurations on the two ends of the nanotube with oppositecirculation occurs through a domain wall that is aligned at an inclined anglerelative to the wire axis. Three-dimensional micromagnetic simulations supportthe experimental observations and represent a visualization of thecurvature-mediated DMI. They also allow a quantitative estimate of the DMIvalue for the magnetic multilayered nanotube.

Artificial spin ices (ASI) have been widely investigated as magnetic metamaterials with exotic properties governed by their geometries. In parallel, interest in x-ray photon orbital angular momentum (OAM) has been rapidly growing. Here we show that a square ASI with a patterned topological defect, a double edge dislocation, imparts OAM to scattered x rays. Unlike single dislocations, a double dislocation does not introduce magnetic frustration, and the ASI equilibrates to its antiferromagnetic (AFM) ground state. The topological charge of the defect differs with respect to the structural and magnetic order; thus, x-ray diffraction from the ASI produces photons with even and odd OAM quantum numbers at the structural and AFM Bragg conditions, respectively. The magnetic transitions of the ASI allow the AFM OAM beams to be switched on and off by modest variations of temperature and applied magnetic field. These results demonstrate ASIs can serve as metasurfaces for reconfigurable x-ray optics that could enable selective probes of electronic and magnetic properties.

Anomalous heat transport in one-dimensional nanostructures, such as nanotubesand nanowires, is a widely debated problem in condensed matter and statisticalphysics, with contradicting pieces of evidence from experiments andsimulations. Using a comprehensive modeling approach, comprised of latticedynamics and molecular dynamics simulations, we proved that the infinite lengthlimit of the thermal conductivity of a (10,0) single-wall carbon nanotube isfinite but this limit is reached only for macroscopic lengths due to thermalphonon mean free path of several millimeters. Our calculations showed that theextremely high thermal conductivity of this system at room temperature isdictated by quantum effects. Modal analysis showed that the divergent nature ofthermal conductivity, observed in one-dimensional model systems, is suppressedin carbon nanotubes by anharmonic scattering channels provided by the flexuraland optical modes with polarization in the plane orthogonal to the transportdirection.

Metal-organic species can be designed to self-assemble in large-scale,atomically defined, supramolecular architectures. Hybrid quantum wells, whereinorganic two-dimensional (2D) planes are separated by organic ligands, are aparticular example. The ligands effectively provide an intralayer confinementfor charge carriers resulting in a 2D electronic structure, even inmultilayered assemblies. Air-stable metal organic chalcogenides hybrid quantumwells have recently been found to host tightly bound 2D excitons with strongoptical anisotropy in a bulk matrix. Here, we investigate the excited carrierdynamics in the prototypical metal organic chalcogenide [AgSePh], disentanglingthree excitonic resonances by low temperature transient absorptionspectroscopy. Our analysis suggests a complex relaxation cascade comprisingultrafast screening and renormalization, inter-exciton relaxation, andself-trapping of excitons within few picoseconds. The ps-decay provided by theself-trapping mechanism may be leveraged to unlock the material's potential forultrafast optoelectronic applications.

We report the direct observation of switching of the Néel vector of antiferromagnetic (AFM) domains in response to electrical pulses in micron-scale Pt/α-Fe2O3 Hall bars using photoemission electron microscopy. Current pulses lead to reversible and repeatable switching with the current direction determining the final state, consistent with Hall effect experiments that probe only the spatially averaged response. Current pulses also produce irreversible changes in domain structure, in and even outside the current path. In both cases only a fraction of the domains switch in response to pulses. Furthermore, the analysis of images taken with different x-ray polarizations shows that the AFM Néel order has an out-of-plane component in equilibrium that is important to consider in analyzing the switching data. These results show that - in addition to effects associated with spin-orbit torques from the Pt layer, which can produce reversible switching - changes in AFM order can be induced by purely thermal effects.

Among topological solitons, magnetic skyrmions are two-dimensional particle-like objects with a continuous winding of the magnetization, and magnetic Hopfions are three-dimensional objects that can be formed from a closed loop of twisted skyrmion strings. Theoretical models suggest that magnetic Hopfions can be stabilized in frustrated or chiral magnetic systems, and target skymions can be transformed into Hopfions by adapting their perpendicular magnetic anisotropy, but their experimental verification has been elusive so far. Here, we present an experimental study of magnetic Hopfions that are created in Ir/Co/Pt multilayers shaped into nanoscale disks, known to host target skyrmions. To characterize three-dimensional spin textures that distinguish Hopfions from target skyrmions magnetic images are recorded with surface-sensitive X-ray photoemission electron microscopy and bulk-sensitive soft X-ray transmission microscopy using element-specific X-ray magnetic circular dichroism effects as magnetic contrast. These results could stimulate further investigations of Hopfions and their potential application in three-dimensional spintronics devices.

Novel computational paradigms may provide the blueprint to help solving the time and energy limitations that we face with our modern computers, and provide solutions to complex problems more efficiently (with reduced time, power consumption and/or less device footprint) than is currently possible with standard approaches. Spintronics offers a promising basis for the development of efficient devices and unconventional operations for at least three main reasons: (i) the low-power requirements of spin-based devices, i.e., requiring no standby power for operation and the possibility to write information with small dynamic energy dissipation, (ii) the strong nonlinearity, time nonlocality, and/or stochasticity that spintronic devices can exhibit, and (iii) their compatibility with CMOS logic manufacturing processes. At the same time, the high endurance and speed of spintronic devices means that they can be rewritten or reconfigured frequently over the lifetime of a circuit, a feature that is essential in many emerging computing concepts. In this perspective, we will discuss how spintronics may aid in the realization of efficient devices, primarily focusing on magnetic tunnel junctions. We then provide a perspective on how these devices can impact the development of three unconventional computing paradigms, namely, reservoir computing, probabilistic computing and memcomputing. These paradigms may be used to address some limitations of modern computers, providing a realistic path to intelligent hybrid CMOS-spintronic systems.

Novel computational paradigms may provide the blueprint to help solving the time and energy limitations that we face with our modern computers, and provide solutions to complex problems more efficiently (with reduced time, power consumption and/or less device footprint) than is currently possible with standard approaches. Spintronics offers a promising basis for the development of efficient devices and unconventional operations for at least three main reasons: (i) the low-power requirements of spin-based devices, i.e., requiring no standby power for operation and the possibility to write information with small dynamic energy dissipation, (ii) the strong nonlinearity, time nonlocality, and/or stochasticity that spintronic devices can exhibit, and (iii) their compatibility with CMOS logic manufacturing processes. At the same time, the high endurance and speed of spintronic devices means that they can be rewritten or reconfigured frequently over the lifetime of a circuit, a feature that is essential in many emerging computing concepts. In this perspective, we will discuss how spintronics may aid in the realization of efficient devices, primarily focusing on magnetic tunnel junctions. We then provide a perspective on how these devices can impact the development of three unconventional computing paradigms, namely, reservoir computing, probabilistic computing and memcomputing. These paradigms may be used to address some limitations of modern computers, providing a realistic path to intelligent hybrid CMOS-spintronic systems.

We report the direct observation of switching of the Néel vector of antiferromagnetic (AFM) domains in response to electrical pulses in micron-scale Pt/α-Fe2O3 Hall bars using photoemission electron microscopy. Current pulses lead to reversible and repeatable switching with the current direction determining the final state, consistent with Hall effect experiments that probe only the spatially averaged response. Current pulses also produce irreversible changes in domain structure, in and even outside the current path. In both cases only a fraction of the domains switch in response to pulses. Furthermore, the analysis of images taken with different x-ray polarizations shows that the AFM Néel order has an out-of-plane component in equilibrium that is important to consider in analyzing the switching data. These results show that - in addition to effects associated with spin-orbit torques from the Pt layer, which can produce reversible switching - changes in AFM order can be induced by purely thermal effects.

The first order reversal curve (FORC) method is a magnetometry based technique used to capture nanoscale magnetic phase separation and interactions with macroscopic measurements using minor hysteresis loop analysis. This makes the FORC technique a powerful tool in the analysis of complex systems which cannot be effectively probed using localized techniques. However, recovering quantitative details about the identified phases which can be compared to traditionally measured metrics remains an enigmatic challenge. We demonstrate a technique to reconstruct phase-resolved magnetic hysteresis loops by selectively integrating the measured FORC distribution. From these minor loops, the traditional metrics-including the coercivity and saturation field, and the remanent and saturation magnetization-can be determined. In order to perform this analysis, special consideration must be paid to the accurate quantitative management of the so-called reversible features. This technique is demonstrated on three representative materials systems, high anisotropy FeCuPt thin-films, Fe nanodots, and SmCo/Fe exchange spring magnet films, and shows excellent agreement with the direct measured major loop, as well as the phase separated loops.

Considering von Neumann expression for reduced density matrix asthermodynamic entropy of a system strongly coupled to baths, we usenonequilibrium Green's function (NEGF) techniques to derive bath and energyresolved expressions for entropy, entropy production, and information flows.The consideration is consistent with dynamic (quantum transport) descriptionand expressions reduce to expected forms in limiting cases of weak coupling orsteady-state. Formulation of the flows in terms of only system degrees freedomis convenient for simulation of thermodynamic characteristics of opennonequilibrium quantum systems. We utilize standard NEGF for derivations innoninteracting systems, Hubbard NEGF is used for interacting systems.Theoretical derivations are illustrated with numerical simulations withingeneric junction models.

Measuring local temperatures of open systems out of equilibrium is emergingas a novel approach to study the local thermodynamic properties of nanosystems.An operational protocol has been proposed to determine the local temperature bycoupling a probe to the system and then minimizing the perturbation to acertain local observable of the probed system. In this paper, we first showthat such a local temperature is unique for a single quantum impurity and thegiven local observable. We then extend this protocol to open systems consistingof multiple quantum impurities by proposing a local minimal perturbationcondition (LMPC). The influence of quantum resonances on the local temperatureis elucidated by both analytic and numerical results. In particular, wedemonstrate that quantum resonances may give rise to strong oscillations of thelocal temperature along a multiimpurity chain under a thermal bias.

Bilayer graphene has been predicted to host a moiré miniband with flat dispersion if the layers are stacked at specific twist angles known as the ’magic angles’1,2. Recently, twisted bilayer graphene (tBLG) with a magic angle twist was reported to exhibit a correlated insulating state and superconductivity3,4, where the presence of the flat miniband in the system is thought to be essential for the emergence of these ordered phases in the transport measurements. Although tunnelling spectroscopy5–9 and electronic compressibility measurements10 in tBLG have found a van Hove singularity that is consistent with the presence of the flat miniband, a direct observation of the flat dispersion in the momentum space of such a moiré miniband in tBLG is still lacking. Here, we report the visualization of this flat moiré miniband by using angle-resolved photoemission spectroscopy with nanoscale resolution. The high spatial resolution of this technique enabled the measurement of the local electronic structure of the tBLG. The measurements demonstrate the existence of the flat moiré band near the charge neutrality for tBLG close to the magic angle at room temperature.

The topological transitions that occur to the grain boundary network duringgrain growth in a material with uniform grain boundary energies are believed tobe known. The same is not true for more realistic materials, since more generalgrain boundary energies in principle allow many more viable grain boundaryconfigurations. A simulation of grain growth in such a material thereforerequires a procedure to enumerate all possible topological transitions andselect the most energetically favorable one. Such a procedure is developed andimplemented here for a microstructure represented by a volumetric finiteelement mesh. As a specific example, all possible transitions for a typicalconfiguration with five grains around a junction point are enumerated, and someexceptional transitions are found to be energetically similar to theconventional ones even for a uniform boundary energy. A general discreteformulation to calculate grain boundary velocities is used to simulate graingrowth for an example microstructure. The method is implemented as a C++library based on SCOREC, an open source massively parallelizable library forfinite element simulations with adaptive meshing.

Excitonic coupling, electronic coupling, and cooperative interactions in self-assembled lead halide perovskite nanocrystals were reported to give rise to a red-shifted collective emission peak with accelerated dynamics. Here we report that similar spectroscopic features could appear as a result of the nanocrystal reactivity within the self-assembled superlattices. This is demonstrated by studying CsPbBr3 nanocrystal superlattices over time with room-temperature and cryogenic micro-photoluminescence spectroscopy, X-ray diffraction, and electron microscopy. It is shown that a gradual contraction of the superlattices and subsequent coalescence of the nanocrystals occurs over several days of keeping such structures under vacuum. As a result, a narrow, low-energy emission peak is observed at 4 K with a concomitant shortening of the photoluminescence lifetime due to the energy transfer between nanocrystals. When exposed to air, self-assembled CsPbBr3 nanocrystals develop bulk-like CsPbBr3 particles on top of the superlattices. At 4 K, these particles produce a distribution of narrow, low-energy emission peaks with short lifetimes and excitation fluence-dependent, oscillatory decays. Overall, the aging of CsPbBr3 nanocrystal assemblies dramatically alters their emission properties and that should not be overlooked when studying collective optoelectronic phenomena nor confused with superfluorescence effects.

Avalanche phenomena use steeply nonlinear dynamics to generate disproportionately large responses from small perturbations, and are found in a multitude of events and materials1. Photon avalanching enables technologies such as optical phase-conjugate imaging2, infrared quantum counting3 and efficient upconverted lasing4-6. However, the photon-avalanching mechanism underlying these optical applications has been observed only in bulk materials and aggregates6,7, limiting its utility and impact. Here we report the realization of photon avalanching at room temperature in single nanostructures-small, Tm3+-doped upconverting nanocrystals-and demonstrate their use in super-resolution imaging in near-infrared spectral windows of maximal biological transparency. Avalanching nanoparticles (ANPs) can be pumped by continuous-wave lasers, and exhibit all of the defining features of photon avalanching, including clear excitation-power thresholds, exceptionally long rise time at threshold, and a dominant excited-state absorption that is more than 10,000 times larger than ground-state absorption. Beyond the avalanching threshold, ANP emission scales nonlinearly with the 26th power of the pump intensity, owing to induced positive optical feedback in each nanocrystal. This enables the experimental realization of photon-avalanche single-beam super-resolution imaging7 with sub-70-nanometre spatial resolution, achieved by using only simple scanning confocal microscopy and without any computational analysis. Pairing their steep nonlinearity with existing super-resolution techniques and computational methods8-10, ANPs enable imaging with higher resolution and at excitation intensities about 100 times lower than other probes. The low photon-avalanching threshold and excellent photostability of ANPs also suggest their utility in a diverse array of applications, including sub-wavelength imaging7,11,12 and optical and environmental sensing13-15.

Free-standing, interconnected metallic nanowire networks with densities as low as 40 mg/cm3 have been achieved over centimeter-scale areas, using electrodeposition into polycarbonate membranes that have been ion-tracked at multiple angles. Networks of interconnected magnetic nanowires further provide an exciting platform to explore 3-dimensional nanomagnetism, where their structure, topology, and frustration may be used as additional degrees of freedom to tailor the materials properties. New magnetization reversal mechanisms in cobalt networks are captured by the first-order reversal curve method, which demonstrate the evolution from strong demagnetizing dipolar interactions to intersection-mediated domain wall pinning and propagation, and eventually to shape-anisotropy dominated magnetization reversal. These findings open up new possibilities for 3-dimensional integrated magnetic devices for memory, complex computation, and neuromorphics.

The theory behind the electrical switching of antiferromagnets is premised on the existence of a well-defined broken symmetry state that can be rotated to encode information. A spin glass is, in many ways, the antithesis of this state, characterized by an ergodic landscape of nearly degenerate magnetic configurations, choosing to freeze into a distribution of these in a manner that is seemingly bereft of information. Here, we show that the coexistence of spin glass and antiferromagnetic order allows a novel mechanism to facilitate the switching of the antiferromagnet Fe1/3 + δNbS2, rooted in the electrically stimulated collective winding of the spin glass. The local texture of the spin glass opens an anisotropic channel of interaction that can be used to rotate the equilibrium orientation of the antiferromagnetic state. Manipulating antiferromagnetic spin textures using a spin glass' collective dynamics opens the field of antiferromagnetic spintronics to new material platforms with complex magnetic textures.

Quasi-periodic moiré patterns and their effect on electronic properties of twisted bilayer graphene have been intensely studied. At small twist angle θ, due to atomic reconstruction, the moiré superlattice morphs into a network of narrow domain walls separating micron-scale AB and BA stacking regions. We use scanning probe photocurrent imaging to resolve nanoscale variations of the Seebeck coefficient occurring at these domain walls. The observed features become enhanced in a range of mid-infrared frequencies where the hexagonal boron nitride substrate is optically hyperbolic. Our results illustrate the capabilities of the nano-photocurrent technique for probing nanoscale electronic inhomogeneities in two-dimensional materials.

Ti$_3$C$_2$T$_X$ nano-sheets (MXenes) are an emerging class oftwo-dimensional materials with outstanding potential to be employed in energystorage, catalysis, and triboelectric applications, on length scales rangingfrom the nano- to macroscopic. Despite rapidly accelerating interest in thisnew class of materials, their nanoscale frictional properties and inparticular, their potential for solid lubrication on the nanoscale, have notbeen explored in detail yet. In this short communication, we present the firstresults on the nanoscale frictional characteristics of MXenes, in the form of afriction map obtained on an isolated Ti$_3$C$_2$T$_x$ nano-sheet deposited on asilicon dioxide substrate via friction force microscopy. Our experiments revealthat few-layer Ti$_3$C$_2$T$_x$ nano-sheets indeed act as solid lubricants onthe nanoscale, reducing friction on the silicon dioxide substrates, althoughnot as effectively as few-layer graphene. The results reported here pave theway for further studies focusing on nanoscale solid lubrication achieved byTi$_3$C$_2$T$_X$ (MXene) nano-sheets.

We experimentally demonstrate dressing of the excited exciton states by a continuously tunable Fermi sea of free charge carriers in a monolayer semiconductor. It represents an unusual scenario of two-particle excitations of charged excitons previously inaccessible in conventional material systems. We identify excited state trions, accurately determine their binding energies in the zero-density limit for both electron- and hole-doped regimes, and observe emerging many-body phenomena at elevated doping. Combining experiment and theory we gain access to the intra-exciton coupling facilitated by the interaction with free charge carriers. We provide evidence for a process of autoionization for quasiparticles, a unique scattering pathway available for excited states in atomic systems. Finally, we demonstrate a complete transfer of the optical transition strength from the excited excitons to dressed Fermi-polaron states as well as the associated light emission from their nonequilibrium populations.

Spin-orbit torque (SOT) driven deterministic control of the magnetizationstate of a magnet with perpendicular magnetic anisotropy (PMA) is key to nextgeneration spintronic applications including non-volatile, ultrafast, andenergy efficient data storage devices. But, field-free deterministic switchingof perpendicular magnetization remains a challenge because it requires anout-of-plane anti-damping torque, which is not allowed in conventional spinsource materials such as heavy metals (HM) and topological insulators due tothe system's symmetry. The exploitation of low-crystal symmetries in emergentquantum materials offers a unique approach to achieve SOTs with unconventionalforms. Here, we report the first experimental realization of field-freedeterministic magnetic switching of a perpendicularly polarized van der Waals(vdW) magnet employing an out-of-plane anti-damping SOT generated in layeredWTe2 which is a low-crystal symmetry quantum material. The numericalsimulations confirm that out-of-plane antidamping torque in WTe2 is responsiblefor the observed magnetization switching in the perpendicular direction.