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In transition metal dichalcogenides a plethora of emergent states arise from competing electron-electron and electron-phonon interactions. Among these, the non-volatile metallic 'hidden' state of 1T-TaS2 can be induced from its insulating equilibrium charge-density wave ground state using either optical or electrical pulses. Here we report in-operando micro-beam X-ray diffraction, fluorescence, and concurrent transport measurements, allowing us to spatially image the non-thermal hidden state induced by electrical switching of a 1T-TaS2 device. Our findings reveal that the electrically and optically switched hidden states are structurally equivalent. Additionally, we observe a bulk switching channel extending beyond the intergap space to partially underneath the electrodes, suggesting that the non-equilibrium phase is caused by a combination of charge flow and lattice response. Besides identifying strain propagation as an important factor for non-thermal switching of layered materials, our results illustrate the power of non-destructive, three-dimensional X-ray imaging for studying phase-change materials and devices.

Fabrication of semiconductor heterostructures is now so precise that metrology has become a key challenge for progress in science and applications. It is now relatively straightforward to characterize classic III-V and group IV heterostructures consisting of slabs of different semiconductor alloys with thicknesses of $\sim$5 nm and greater using sophisticated tools such as X-ray diffraction, high energy X-ray photoemission spectroscopy, and secondary ion mass spectrometry. However, profiling thin layers with nm or sub-nm thickness, e.g. atomically thin dopant layers ($\delta$-layers), of impurities required for modulation doping and spin-based quantum and classical information technologies is more challenging. Here, we present theory and experiment showing how resonant-contrast X-ray reflectometry meets this challenge. The technique takes advantage of the change in the scattering factor of atoms as their core level resonances are scanned by varying the X-ray energy. We demonstrate the capability of the resulting element-selective, non-destructive profilometry for single arsenic $\delta$-layers within silicon, and show that the sub-nm electronic thickness of the $\delta$-layers corresponds to sub-nm chemical thickness. In combination with X-ray fluorescence imaging, this enables non-destructive three-dimensional characterization of nano-structured quantum devices. Due to the strong resonances at soft X-ray wavelengths, the technique is also ideally suited to characterize layered quantum materials, such as cuprates or the topical infinite-layer nickelates.

We report on the induction of magnetization in Rydberg systems by means of the inverse Faraday effect, and propose the appearance of the effect in two such systems, Rydberg atoms proper and shallow dopants in semiconductors. Rydberg atoms are characterized by a large orbital radius. This large radius gives such excited states a large angular moment, which when driven with circularly polarized light, translates to a large effective magnetic field. We calculate this effect to generate effective magnetic fields of $O(10\,\text{mT})\times\left( \frac{\omega}{1\,\text{THz}} \right)^{-1} \left( \frac{I}{10\,{W\,cm}^{-2}} \right)$ in Rydberg states of Rb and Cs for a $1\,\text{THz}$ beam of intensity $10\,\text{W}\,\text{cm}^{-2}$. The magnitude of the effective magnetic field scales with the principal quantum number as $n^4$. Additionally, THz spectroscopy of phosphorus doped silicon reveals a large cross-section for excitation of shallow dopants to Rydberg-like states, which even for small $n$ can be driven similarly with circularly polarized light to produce even larger magnetization, with ${B}_{\text{eff}}$ which we estimate as $O(1\,\text{mT})$ for Si:P with the same beam parameters.

In this work, we show the feasibility of extreme ultraviolet (EUV) patterning on an HF-treated Si(100) surface in the absence of a photoresist. EUV lithography is the leading lithography technique in semiconductor manufacturing due to its high resolution and throughput, but future progress in resolution can be hampered because of the inherent limitations of the resists. We show that EUV photons can induce surface reactions on a partially H-terminated Si surface and assist the growth of an oxide layer, which serves as an etch mask. This mechanism is different from the H-desorption in scanning tunneling microscopy-based lithography. We achieve SiO2/Si gratings with 75 nm half-pitch and 31 nm height, demonstrating the efficacy of the method and the feasibility of patterning with EUV lithography without the use of a photoresist. Further development of the resistless EUV lithography method can be a viable approach to nm-scale lithography by overcoming the inherent resolution and roughness limitations of photoresist materials., Comment: 15 pages, 7 figures

Two-dimensional dopant layers ($\delta$-layers) in semiconductors provide the high-mobility electron liquids (2DELs) needed for nanoscale quantum-electronic devices. Key parameters such as carrier densities, effective masses, and confinement thicknesses for 2DELs have traditionally been extracted from quantum magnetotransport. In principle, the parameters are immediately readable from the one-electron spectral function that can be measured by angle-resolved photoemission spectroscopy (ARPES). Here, buried 2DEL $\delta$-layers in silicon are measured with soft X-ray (SX) ARPES to obtain detailed information about their filled conduction bands and extract device-relevant properties. This study takes advantage of the larger probing depth and photon energy range of SX-ARPES relative to vacuum ultraviolet (VUV) ARPES to accurately measure the $\delta$-layer electronic confinement. The measurements are made on ambient-exposed samples and yield extremely thin ($\approx 1$ $nm$) and dense ($\approx$ $10^{14}$ $cm^2$) 2DELs. Critically, this method is used to show that $\delta$-layers of arsenic exhibit better electronic confinement than $\delta$-layers of phosphorus fabricated under identical conditions., Comment: Published in Advanced Science as a Research Article

Angle-resolved photoemission spectroscopy (ARPES) is one of the most ubiquitous characterization techniques utilized in the field of condensed matter physics. The resulting spectral intensity consists of a coherent and incoherent part, whose relative contribution is governed by atomic disorder, where thermal contribution is expressed in terms of the Debye-Waller factor (DWF). In this work, we present a soft-X-ray study on the sputter-induced disorder of InAs(110) surface. We define a new quantity, referred to as the coherence factor FC, which is the analogue of the DWF, extended to static disorder. We show that FC alone can be used to quantify the depletion of coherent intensity with increasing disorder, and, in combination with the DWF, allows considerations of thermal and static disorder effects on the same footing. Our study also unveils an intriguing finding: as disorder increases, the ARPES intensity of quantum well states originating from the conduction band depletes more rapidly compared to the valence bands. This difference can be attributed to the predominance of quasi-elastic defect scattering and the difference in phase space available for such scattering for conduction-band (CB) and valence-band (VB) initial states. Specifically, the absence of empty states well below the Fermi energy (EF) hinders the quasi-elastic scattering of the VB states, while their abundance in vicinity of EF enhances the scattering rate of the CB states. Additionally, we observe no noticeable increase in broadening of the VB dispersions as the sputter-induced disorder increases. This observation aligns with the notion that valence initial states are less likely to experience the quasi-elastic defect scattering, which would shorten their lifetime, and with the random uncorrelated nature of the defects introduced by the ion sputtering.

Special arrangements of atoms with more than one atom per unit cell, including honeycomb or kagome (woven bamboo mat) lattices, can host propagating excitations with non-trivial topology as defined by their evolution along closed paths in momentum space. Excitations on such lattices can also be momentum-independent, meaning that they are localized notwithstanding strong hopping of the underlying disturbances between neighbouring sites. The associated flat bands are interesting because the interactions between the heavy quasiparticles inhabiting them will become much more important than for strong dispersion, resulting in novel quantum solid and liquid states. Different stackings of two-dimensional lattices, for example twisted graphene bilayers, provide routes to further engineer topology and many-body effects. Here, we report the discovery, using circularly polarized x-rays for the unambiguous isolation of magnetic signals, of a nearly flat spin wave band and large (compared to elemental iron) orbital moment for the metallic ferromagnet Fe3Sn2, built from compact AB-stacked kagome bilayers and which has a topologically non-trivial electronic band structure controllable by modest external magnetic fields. As a function of out-of-plane momentum, the nearly flat optical mode and the global rotation symmetry-restoring acoustic mode are out of phase, consistent with a bilayer exchange coupling that is larger than the already large in-plane couplings. The defining units of this topological metal are therefore a triangular lattice of octahedral iron clusters rather than weakly coupled kagome planes. The spin waves are strongly damped when compared to elemental iron, opening the topic of interactions of topological bosons (spin waves) and fermions (electrons) with the very specific target of explaining boson lifetimes., Comment: 19 pages, 5 figures

Recent development in angle-resolved photoemission spectroscopy (ARPES) technique involves spatially resolving samples while maintaining the high-resolution feature of momentum space. This development easily expands the data size and its complexity for data analysis, where one of it is to label similar dispersion cuts and map them spatially. In this work, we demonstrate that the recent development in representational learning (self-supervised learning) model combined with k-means clustering can help automate that part of data analysis and save precious time, albeit with low performance. Finally, we introduce a few-shot learning (k-nearest neighbour or kNN) in representational space where we selectively choose one (k=1) image reference for each known label and subsequently label the rest of the data with respect to the nearest reference image. This last approach demonstrates the strength of the self-supervised learning to automate the image analysis in ARPES in particular and can be generalized into any science data analysis that heavily involves image data.

Dynamical perturbations modify the states of classical systems in surprising ways and give rise to important applications in science and technology. For example, Floquet engineering exploits the possibility of band formation in the frequency domain when a strong, periodic variation is imposed on parameters such as spring constants. We describe here Kapitza engineering, where a drive field oscillating at a frequency much higher than the characteristic frequencies for the linear response of a system changes the potential energy surface so much that maxima found at equilibrium become local minima, in precise analogy to the celebrated Kapitza pendulum where the unstable inverted configuration, with the mass above rather than below the fulcrum, actually becomes stable. Our starting point is a quantum field theory of the Ginzburg-Devonshire type, suitable for many condensed matter systems, including particularly ferroelectrics and quantum paralectrics. We show that an off-resonance oscillatory electric field generated by a laser-driven THz source can induce ferroelectric order in the quantum-critical limit. Heating effects are estimated to be manageable using pulsed radiation; ``hidden" radiation-induced order can persist to low temperatures without further pumping due to stabilization by strain. We estimate the Ginzburg-Devonshire free-energy coefficients in SrTiO${}_{3}$ using density-functional theory (DFT) and the stochastic self-consistent harmonic approximation accelerated by a machine-learned force field. Although we find that SrTiO${}_{3}$ is not an optimal choice for Kapitza stabilization, we show that scanning for further candidate materials can be performed at the computationally convenient density-functional theory level. We suggest second-harmonic-generation, soft-mode-spectroscopy, and X-ray-diffraction experiments to characterize the induced order., Comment: 9+4 pages, 5+2 figures, submitted to Physical Review X

The progress of miniaturisation in integrated electronics has led to atomic and nanometre-sized dopant devices in silicon. Such structures can be fabricated routinely by hydrogen resist lithography, using various dopants such as phosphorous and arsenic. However, the ability to non-destructively obtain atomic-species-specific images of the final structure, which would be an indispensable tool for building more complex nano-scale devices, such as quantum co-processors, remains an unresolved challenge. Here we exploit X-ray fluorescence to create an element-specific image of As dopants in silicon, with dopant densities in absolute units and a resolution limited by the beam focal size (here $\sim1~\mu$m), without affecting the device's low temperature electronic properties. The As densities provided by the X-ray data are compared to those derived from Hall effect measurements as well as the standard non-repeatable, scanning tunnelling microscopy and secondary ion mass spectroscopy, techniques. Before and after the X-ray experiments, we also measured the magneto-conductance, dominated by weak localisation, a quantum interference effect extremely sensitive to sample dimensions and disorder. Notwithstanding the $1.5\times10^{10}$ Sv ($1.5\times10^{16}$ Rad/cm$^{-2}$) exposure of the device to X-rays, all transport data were unchanged to within experimental errors, corresponding to upper bounds of 0.2 Angstroms for the radiation-induced motion of the typical As atom and 3$\%$ for the loss of activated, carrier-contributing dopants. With next generation synchrotron radiation sources and more advanced optics, we foresee that it will be possible to obtain X-ray images of single dopant atoms within resolved radii of 5 nm.

One material containing kagome bilayers and featuring both exceptional magnetism and electron transport is the ferromagnetic metal Fe3Sn2. Notwithstanding the widespread interest in Fe3Sn2, crystal twinning, difficulties in distinguishing surface from bulk states, and a large unit cell have until now prevented the synchrotron-based spectroscopic observation of sharply resolved quasiparticles near the Fermi surface which could be responsible for the anomalous properties appearing at low temperatures for the material. Here we report microfocused laser-based angle-resolved photoemission spectroscopy (micro-ARPES), which offers the first look at such quasiparticles. The high spatial resolution allows individual crystal twin domains to be examined in isolation, resulting in the discovery of three-fold symmetric electron pockets at the Brillouin zone (BZ) center, not predicted by early tight-binding descriptions but in agreement with density functional theory (DFT) calculations, which also feature Weyl nodes. The quasiparticles in these pockets have remarkably long mean free paths, and their Fermi surface area is consistent with reported quantum oscillations. At the same time, though, the best-defined Fermi surface is reduced at low temperature, and the quasiparticles generally are marginal in the sense that their wavelength uncertainty is of order the deviation of the quasiparticle wavelength from the Fermi vector. We attribute these manifestations of strong electron-electron interactions to a flat band predicted by our DFT to lie just above the dispersive bands seen in this experiment. Thus, beyond demonstrating the impact of twin averaging for ARPES measurements of band structures, our experiments reveal many-body physics unaccounted for by current theories of metallic kagome ferromagnets.

Superfluidity and superconductivity are macroscopic manifestations of quantum mechanics, which have fascinated scientists since their discoveries roughly a century ago. Ever since the initial theories of such quantum fluids were formulated, there has been speculation as to the possibility of multi-component quantum order. A particularly simple multi-component condensate is built from particles occupying different quantum states, or bands, prior to condensation. The particles in one or both bands may undergo condensation, as seen for certain solids and anticipated for certain cold atom systems. For bulk solids, the different bands always order simultaneously, with conventional pairing characterized by complex order parameters describing the condensates in each band. Another type of condensate, notably occurring at room temperature, has been identified for magnons, the magnetic analogue of lattice vibrations, injected by microwaves into yttrium iron garnet. Here we show that magnon quantization for thin samples results in a new multi-band magnon condensate. We establish a phase diagram, as a function of microwave drive power and frequency relative to the magnon bands, revealing both single and multi-band condensation. The most stable multi-band condensate is found in a narrow regime favoured on account of a resonance in the scattering between two bands. Our discovery introduces a flexible non-equilibrium platform operating at room temperature for a well-characterised material, exploiting a Feshbach-like resonance, for examining multi-band phenomena. It points to qualitatively new ways to engineer and control condensates and superconducting states in multiband systems and potential devices containing multiple interacting condensates.

In condensed matter systems, the atoms, electrons or spins can sometimes arrange themselves in ways that result in unexpected properties but that cannot be detected by conventional experimental probes. Several historical and contemporary examples of such hidden orders are known and more are awaiting discovery, perhaps in the form of more complex composite, entangled or dynamical hidden orders., Comment: Author Accepted Manuscript

The electronic structure of surfaces plays a key role in the properties of quantum devices. However, surfaces are also the most challenging to simulate and engineer. Here, we study the electronic structure of InAs(001), InAs(111), and InSb(110) surfaces using a combination of density functional theory (DFT) and angle-resolved photoemission spectroscopy (ARPES). We were able to perform large-scale first principles simulations and capture effects of different surface reconstructions by using DFT calculations with a machine-learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. To facilitate direct comparison with ARPES results, we implemented a "bulk unfolding" scheme by projecting the calculated band structure of a supercell surface slab model onto the bulk primitive cell. For all three surfaces, we find a good agreement between DFT calculations and ARPES. For InAs(001), the simulations clarify the effect of the surface reconstruction. Different reconstructions are found to produce distinctive surface states. For InAs(111) and InSb(110), the simulations help elucidate the effect of oxidation. Owing to larger charge transfer from As to O than from Sb to O, oxidation of InAs(111) leads to significant band bending and produces an electron pocket, whereas oxidation of InSb(110) does not. Our combined theoretical and experimental results may inform the design of quantum devices based on InAs and InSb semiconductors, e.g., topological qubits utilizing the Majorana zero modes.

We propose a scheme for universal quantum computing based on Kramers rare-earth ions. Their nuclear spins in the presence of a Zeeman-split electronic crystal field ground state act as 'passive' qubits which store quantum information. The qubits can be activated optically by fast coherent transitions to excited crystal field states with a magnetic moment. The dipole interaction between these states is used to implement CNOT gates. We compare our proposal with a similar one based on phosphorus donor atoms in silicon and discuss the significantly improved CNOT gate time as compared to rare-earth implementations via the slower dipole blockade., Comment: 19 pages, 10 figures

A finite length ferromagnetic chain with opposite spin polarisation imposed at its two ends is one of the simplest frustrated spin models. In the clean classical limit the domain wall inserted on account of the boundary conditions resides with equal probability on any one of the bonds, and the degeneracy is precisely equal to the number of bonds. If quantum mechanics is introduced via a transverse field, the domain wall will behave as a particle in a box, and prefer to be nearer the middle of the chain rather than the ends. A simple characteristic of a real quantum annealer is therefore which of these limits obtains in practice. Here we have used the ferromagnetic chain with antiparallel boundary spins to test a real flux qubit quantum annealer and discover that contrary to both expectations, the domain walls found are non-uniformly distributed on account of effective random longitudinal fields present notwithstanding tuning carried out to zero out such fields when the couplings between qubits are nominally zero. We present a simple derivation of the form of the distribution function for the domain walls, and show also how the effect we have discovered can be used to determine the strength of the effective random fields (noise) characterising the annealer. The noise measured in this fashion is smaller than what is seen during the single qubit tuning process, but nonetheless qualitatively affects the outcome of the simulation performed by the annealer., Comment: 14 pages, 9 figures plus supplemental material, title change in v2 and v3, accepted in npj Quantum Information, arXiv matches accepted version

Lineshape analysis is a recurrent and often computationally intensive task in optics, even more so for multiple peaks in the presence of noise. We demonstrate an algorithm which takes advantage of peak multiplicity (N) to retrieve line shape information. The method is exemplified via analysis of Lorentzian and Gaussian contributions to individual lineshapes for a practical spectroscopic measurement and benefits from a linear increase in sensitivity with the number N. The robustness of the method and its benefits in terms of noise reduction and order of magnitude improvement in run-time performance are discussed., Comment: 12 pages, 6 figures

We argue that frequent sampling of the fraction of infected people (either by random testing or by analysis of sewage water), is central to managing the COVID-19 pandemic because it both measures in real time the key variable controlled by restrictive measures, and anticipates the load on the healthcare system due to progression of the disease. Knowledge of random testing outcomes will (i) significantly improve the predictability of the pandemic, (ii) allow informed and optimized decisions on how to modify restrictive measures, with much shorter delay times than the present ones, and (iii) enable the real-time assessment of the efficiency of new means to reduce transmission rates. Here we suggest, irrespective of the size of a suitably homogeneous population, a conservative estimate of 15000 for the number of randomly tested people per day which will suffice to obtain reliable data about the current fraction of infections and its evolution in time, thus enabling close to real-time assessment of the quantitative effect of restrictive measures. Still higher testing capacity permits detection of geographical differences in spreading rates. Furthermore and most importantly, with daily sampling in place, a reboot could be attempted while the fraction of infected people is still an order of magnitude higher than the level required for a relaxation of restrictions with testing focused on symptomatic individuals. This is demonstrated by considering a feedback and control model of mitigation where the feed-back is derived from noisy sampling data., Comment: 18 pages, 6 figures, 2 appendices. Phys. Biol. (2020)

Iron phthalocyanine (FePc) is a molecular semiconductor whose building blocks are one-dimensional ferromagnetic chains. We show that its optical and magnetic properties are controlled by the growth strategy, obtaining extremely high coercivities of over 1 T and modulating the exchange constant between 15 and 29 K through tuning the crystal phase by switching from organic molecular beam deposition, producing continuous thin films of nanocrystals with controlled orientations, to organic vapour phase deposition, producing ultralong nanowires. Magnetisation measurements are analysed using a suite of concepts and simply stated formulas with broad applicability to all one-dimensional ferromagnetic chains. They show that FePc is best described by a Heisenberg model with a preference for the moments to lie in the molecular planes, where the chain Hamiltonian is very similar to that for the classic inorganic magnet CsNiF3, but with ferromagnetic rather than antiferromagnetic interchain interactions. The data at large magnetic fields are well-described by the soliton picture, where the dominant (and topologically non-trivial) degrees of freedom are moving one-dimensional magnetic domain walls, which was successful for CsNiF3, and at low temperatures and fields by the super-Curie-Weiss law of 1/(T^2+theta^2) characteristic of nearly one-dimensional xy and Heisenberg ferromagnets. The ability to control the molecular orientation and ferromagnetism of FePc systems, and produce them on flexible substrates as thin films or nanowires, taken together with excellent transistor characteristics reported previously for nanowires of copper and cobalt analogues, makes them potentially useful for magneto-optical and spintronic devices.

The band alignment of semiconductor-metal interfaces plays a vital role in modern electronics, but remains difficult to predict theoretically and measure experimentally. For interfaces with strong band bending a main difficulty originates from the in-built potentials which lead to broadened and shifted band spectra in spectroscopy measurements. In this work we present a method to resolve the band alignment of buried semiconductor-metal interfaces using core level photoemission spectroscopy and self-consistent electronic structure simulations. As a proof of principle we apply the method to a clean in-situ grown InAs(100)/Al interface, a system with a strong in-built band bending. Due to the high signal-to-noise ratio of the core level spectra the proposed methodology can be used on previously inaccessible semiconductor-metal interfaces and support targeted design of novel hybrid devices and form the foundation for a interface parameter database for specified synthesis processes of semiconductor-metal systems., Comment: 12 pages, 5 figures

First order phase transitions, where one phase replaces another by virtue of a simple crossing of free energies, are best known between solids, liquids and vapours, but they also occur in a wide range of other contexts, including even elemental magnets. The key challenges are to establish whether a phase transition is indeed first order, and then to determine how the new phase emerges because this will determine thermodynamic and electronic properties. Here we meet both challenges for the spin reorientation transition in the topological metallic ferromagnet Fe3Sn2. Our magnetometry and variable temperature magnetic force microscopy experiments reveal that, analogous to the liquid-gas transition in the temperature-pressure plane, this transition is centred on a first order line terminating in a critical end point in the field-temperature plane. We directly image the nucleation and growth associated with the transition and show that the new phase emerges at the most convoluted magnetic domain walls for the high temperature phase and then moves to self organize at the domain centres of the high temperature phase. The dense domain patterns and phase coexistence imply a complex inhomogeneous electronic structure, which will yield anomalous contributions to the magnetic field- and temperature-dependent electrical conductivity., Comment: 14 pages, 8 figures

High resolution (900 Hz full-width at half maximum) frequency domain spectroscopy near 0.2Terahertz is achieved using an exact frequency spacing comb-source in the optical communications band, filtering and photo-mixing in a custom Uni-Travelling-Carrier Photodiode (UTC-PD) for THz signal generation and coherent down-conversion for detection. Via time domain modulation of one of the comb lines, a fully controllable spectrometer can be defined in software, and this principle is demonstrated for magnetic field-free readout of the electronuclear spectrum for the Ho ions in \lihof, a material often used for demonstration experiments in quantum science. In particular, homogeneous and inhomogeneous contributions to the spectrum are readily separated.

Hybrid semiconductor-ferromagnetic insulator heterostructures are interesting due to their tunable electronic transport, self-sustained stray field and local proximitized magnetic exchange. In this work, we present lattice matched hybrid epitaxy of semiconductor - ferromagnetic insulator InAs/EuS heterostructures and analyze the atomic-scale structure as well as their electronic and magnetic characteristics. The Fermi level at the InAs/EuS interface is found to be close to the InAs conduction band and in the bandgap of EuS, thus preserving the semiconducting properties. Both neutron and X-ray reflectivity measurements show that the ferromagnetic component is mainly localized in the EuS thin film with a suppression of the Eu moment in the EuS layer nearest the InAs. Induced moments in the adjacent InAs layers were not detected although our ab initio calculations indicate a small exchange field in the InAs layer. This work presents a step towards realizing high quality semiconductor - ferromagnetic insulator hybrids, which is a critical requirement for development of various quantum and spintronic applications without external magnetic fields.

An X-ray free-electron laser oscillator (XFELO) is a new type of hard X-ray source that would produce fully coherent pulses with meV bandwidth and stable intensity. The XFELO complements existing sources based on self-amplified spontaneous emission (SASE) from high-gain X-ray free-electron lasers (XFEL) that produce ultra-short pulses with broad-band chaotic spectra. This report is based on discussions of scientific opportunities enabled by an XFELO during a workshop held at SLAC on June 29 - July 1, 2016, Comment: 21 pages, 12 figures

Doping of silicon via phosphene exposures alternating with molecular beam epitaxy overgrowth is a path to Si:P substrates for conventional microelectronics and quantum information technologies. The technique also provides a new and well-controlled material for systematic studies of two-dimensional lattices with a half-filled band. We show here that for a dense ($n_s=2.8\times 10^{14}$\,cm$^{-2}$) disordered two-dimensional array of P atoms, the full field angle-dependent magnetostransport is remarkably well described by classic weak localization theory with no corrections due to interaction effects. The two- to three-dimensional cross-over seen upon warming can also be interpreted using scaling concepts, developed for anistropic three-dimensional materials, which work remarkably except when the applied fields are nearly parallel to the conducting planes., Comment: 9 pages, 4 figures, supplementary information

The discovery of the Dirac electron dispersion in graphene led to the question of the Dirac cone stability with respect to interactions. Coulomb interactions between electrons were shown to induce a logarithmic renormalization of the Dirac dispersion. With a rapid expansion of the list of compounds and quasiparticle bands with linear band touching, the concept of bosonic Dirac materials has emerged. We consider a specific case of ferromagnets consisting of the Van der Waals-bonded stacks of honeycomb layers, e.g chromium trihalides CrX3 (X = F, Cl, Br and I), that display two spin wave modes with energy dispersion similar to that for the electrons in graphene. At the single particle level, these materials resemble their fermionic counterparts. However, how different particle statistics and interactions affect the stability of Dirac cones has yet to be determined. To address the role of interacting Dirac magnons, we expand the theory of ferromagnets beyond the standard Dyson theory to a case of non-Bravais honeycomb layers. We demonstrate that magnon-magnon interactions lead to a significant momentum-dependent renormalization of the bare band structure in addition to strongly momentum-dependent magnon lifetimes. We show that our theory qualitatively accounts for hitherto unexplained anomalies in a nearly half century old magnetic neutron scattering data for CrBr3. We also show that honeycomb ferromagnets display dispersive surface and edge states, unlike their electronic analogs.

Clustered quantum materials provide a new platform for the experimental study of many-body entanglement. Here we address a simple model of a single-molecule nano-magnet featuring N interacting spins in a transverse field. The field can induce an entanglement transition (ET). We calculate the magnetisation, low-energy gap and neutron-scattering cross-section and find that the ET has distinct signatures, detectable at temperatures as high as 5% of the interaction strength. The signatures are stronger for smaller clusters., Comment: 14 pages, 11 Figures; minor changes; Phys. Rev. B (accepted)

Diffusion across surfaces generally involves motion on a vibrating but otherwise stationary substrate. Here, using molecular dynamics, we show that a layered material such as graphene opens up a new mechanism for surface diffusion whereby adsorbates are carried by propagating ripples via a motion similar to surfing. For water nanodroplets, we demonstrate that the mechanism leads to exceedingly fast diffusion that is 2-3 orders of magnitude faster than the self-diffusion of water molecules in liquid water. We also reveal the underlying principles that regulate this new mechanism for diffusion and show how it also applies to adsorbates other than water, thus opening up the prospect of achieving fast and controllable motion of adsorbates across material surfaces more generally., Comment: 3 figures

For randomly selected couplers and fields, the D-Wave device typically yields a highly Boltzmann like distribution [ indicating equilibration. These equilibrated data however do not contain much useful information about the dynamics which lead to equilibration. To illuminate the dynamics, special Hamiltonians can be chosen which contain large energy barriers. In this paper we generalize this approach by considering a class of Hamiltonians which map clusters of spin-like qubits into 'superspins', thereby creating an energy landscape where local minima are separated by large energy barriers. These large energy barriers allow us to observe signatures of the transverse field frozen. To study these systems, we assume that the these frozen spins are describes by the Kibble-Zurek mechanism which was originally developed to describe formation of topological defects in the early universe. It was soon realized that it also has applications in analogous superconductor systems and later realized to also be important for the transverse field Ising model . We demonstrate that these barriers block equilibration and yield a non-trivial distribution of qubit states in a regime where quantum effects are expected to be strong, suggesting that these data should contain signatures of whether the dynamics are fundamentally classical or quantum. We exhaustively study a class of 3x3 square lattice superspin Hamiltonians and compare the experimental results with those obtained by exact diagonalisation. We find that the best fit to the data occurs at finite transverse field. We further demonstrate that under the right conditions, the superspins can be relaxed to equilibrium, erasing the signature of the transverse field. These results are interesting for a number of reasons. They suggest a route to detect signatures of quantum mechanics on the device on a statistical level., Comment: 10 pages, 9 figures. Data from classical simulations to follow in later versions

Optimisation problems in science and engineering typically involve finding the ground state (i.e. the minimum energy configuration) of a cost function with respect to many variables. If the variables are corrupted by noise then this approach maximises the likelihood that the solution found is correct. An alternative approach is to make use of prior statistical information about the noise in conjunction with Bayes's theorem. The maximum entropy solution to the problem then takes the form of a Boltzmann distribution over the ground and excited states of the cost function. Here we use a programmable Josephson junction array for the information decoding problem which we simulate as a random Ising model in a field. We show experimentally that maximum entropy decoding at finite temperature can in certain cases give competitive and even slightly better bit-error-rates than the maximum likelihood approach at zero temperature, confirming that useful information can be extracted from the excited states of the annealing device. Furthermore we introduce a microscopic bit-by-bit analytical method which is agnostic to the specific application and use it to show that the annealing device samples from a highly Boltzmann-like distribution. Machines of this kind are therefore candidates for use in a wide variety of machine learning applications which exploit maximum entropy inference, including natural language processing and image recognition. We further show that the limiting factor for performance in our experiments is likely to be control errors rather than failure to reach equilibrium. Our work also provides a method for determining if a system is in equilibrium which can be easily generalized. We discuss possible applications of this method to spin glasses and probing the performance of the quantum annealing algorithm., Comment: 9 figures in main text 9 figures in supplemental material. Significant amount of new Monte Carlo data added in v2 at referees request. Accepted for Scientific Reports

Micromechanical transducers such as cantilevers for AFM often rely on optical readout methods that require illumination of a specific region of the microstructure. Here we explore and exploit the diffraction effects that have been previously neglected when modeling cantilever bending measurement techniques. The illumination of a cantilever end causes an asymmetric diffraction pattern at the photodetector that significantly affects the calibration of the signal in the popular optical beam deflection technique (OBDT). Conditions for optimized linear signals that avoid detection artifacts conflict with small numerical aperture illumination and narrow cantilevers which are softer and therefore more sensitive. Embracing diffraction patterns as a physical measurable allows a richer detection technique that decouples measurements of tilt and curvature and simultaneously relaxes the requirements on the alignment of illumination and detector. We show analytical results, numerical simulations and physiologically relevant experimental data demonstrating the usefulness of these diffraction features. We offer experimental design guidelines and identify and quantify possible sources of systematic error of up to 10% in OBDT. We demonstrate a new nanometre resolution detection method that can replace OBDT, where Frauenhofer and Bragg diffraction effects from finite sized and patterned cantilevers are exploited. Such effects are readily generalized to arrays, and allow transmission detection of mechanical curvature, enabling in-line instruments. In particular, a cantilever with a periodic array of slots produces Bragg peaks which can be analyzed to deduce the cantilever curvature. We highlight the comparative advantages over OBDT by detecting molecular activity of antibiotic Vancomycin, with an RMS noise equivalent to less than $2.5 \mu M$ (1.5 nm), as example of possible multi-maker bio-assays., Comment: 9 pages, 8 figures

The crystal structures of LaAlO3 films grown by pulsed laser deposition on SrTiO3 substrates at oxygen pressure of 10-3 mbar or 10-5 mbar, where kinetics of ablated species hardly depend on oxygen background pressure, are compared. Our results show that the interface between LaAlO3 and SrTiO3 is sharper when the oxygen pressure is lower. Over time, the formation of various crystalline phases is observed while the crystalline thickness of the LaAlO3 layer remains unchanged. X-ray scattering as well as atomic force microscopy measurements indicate three-dimensional growth of such phases, which appear to be fed from an amorphous capping layer present in as-grown samples.

We analytically derive and experimentally demonstrate a method for the simultaneous measurement of deflection for large arrays of cantilevers. The Fresnel diffraction patterns of a cantilever independently reveals tilt, curvature, cubic and higher order bending of the cantilever. It provides a calibrated absolute measurement of the polynomial coefficients describing the cantilever shape, without careful alignment and could be applied to several cantilevers simultaneously with no added complexity. We show that the method is easily implemented, works in both liquid mediums and in air, for a broad range of displacements and is especially suited to the requirements for multi-marker biosensors., Comment: 5 Pages, 4 figures, letter format

Electronic functionalities in materials from silicon to transition metal oxides are to a large extent controlled by defects and their relative arrangement. Outstanding examples are the oxides of copper, where defect order is correlated with their high superconducting transition temperatures. The oxygen defect order can be highly inhomogeneous, even in "optimal" superconducting samples, which raises the question of the nature of the sample regions where the order does not exist but which nonetheless form the "glue" binding the ordered regions together. Here we use scanning X-ray microdiffraction (with beam 300 nm in diameter) to show that for La2CuO4+y, the "glue" regions contain incommensurate modulated local lattice distortions, whose spatial extent is most pronounced for the best superconducting samples. For an underdoped single crystal with mobile oxygen interstitials in the spacer La2O2+y layers intercalated between the CuO2 layers, the incommensurate modulated local lattice distortions form droplets anticorrelated with the ordered oxygen interstitials, and whose spatial extent is most pronounced for the best superconducting samples. In this simplest of high temperature superconductors, there are therefore not one, but two networks of ordered defects which can be tuned to achieve optimal superconductivity. For a given stoichiometry, the highest transition temperature is obtained when both the ordered oxygen and lattice defects form fractal patterns, as opposed to appearing in isolated spots. We speculate that the relationship between material complexity and superconducting transition temperature Tc is actually underpinned by a fundamental relation between Tc and the distribution of ordered defect networks supported by the materials., Comment: 20 pages, 5 figures

Manganites are technologically important materials, used widely as solid oxide fuel cell cathodes: they have also been shown to exhibit electroresistance. Oxygen bulk diffusion and surface exchange processes are critical for catalytic action, and numerous studies of manganites have linked electroresistance to electrochemical oxygen migration. Direct imaging of individual oxygen defects is needed to underpin understanding of these important processes. It is not currently possible to collect the required images in the bulk, but scanning tunnelling microscopy could provide such data for surfaces. Here we show the first atomic resolution images of oxygen defects at a manganite surface. Our experiments also reveal defect dynamics, including oxygen adatom migration, vacancy-adatom recombination and adatom bistability. Beyond providing an experimental basis for testing models describing the microscopics of oxygen migration at transition metal oxide interfaces, our work resolves the long-standing puzzle of why scanning tunnelling microscopy is more challenging for layered manganites than for cuprates., Comment: 7 figures

Pulsed magnetic resonance is a wide-reaching technology allowing the quantum state of electronic and nuclear spins to be controlled on the timescale of nanoseconds and microseconds respectively. The time required to flip either dilute electronic or nuclear spins is orders of magnitude shorter than their decoherence times, leading to several schemes for quantum information processing with spin qubits. We investigate instead the novel regime where the eigenstates approximate 50:50 superpositions of the electronic and nuclear spin states forming "hybrid nuclear-electronic" qubits. Here we demonstrate quantum control of these states for the first time, using bismuth-doped silicon, in just 32 ns: this is orders of magnitude faster than previous experiments where pure nuclear states were used. The coherence times of our states are five orders of magnitude longer, reaching 4 ms, and are limited by the naturally-occurring 29Si nuclear spin impurities. There is quantitative agreement between our experiments and no-free-parameter analytical theory for the resonance positions, as well as their relative intensities and relative Rabi oscillation frequencies. In experiments where the slow manipulation of some of the qubits is the rate limiting step, quantum computations would benefit from faster operation in the hybrid regime., Comment: 20 pages, 8 figures, new data and simulations

The disposition of defects in metal oxides is a key attribute exploited for applications from fuel cells and catalysts to superconducting devices and memristors. The most typical defects are mobile excess oxygens and oxygen vacancies, and can be manipulated by a variety of thermal protocols as well as optical and dc electric fields. Here we report the X-ray writing of high-qualitysuperconducting regions, derived from defect ordering, in the superoxygenated layered cuprate, La2CuO4+y. Irradiation of a poor superconductor prepared by rapid thermal quenching results first in growth of ordered regions, with an enhancement of superconductivity becoming visible only after a waiting time, as is characteristic of other systems such as ferroelectrics, where strain must be accommodated for order to become extended. However, in La2CuO4+y, we are able to resolve all aspects of the growth of (oxygen) intercalant order, including an extraordinary excursion from low to high and back to low anisotropy of the ordered regions. We can also clearly associate the onset of high quality superconductivity with defect ordering in two dimensions. Additional experiments with small beams demonstrate a photoresist-free, single-step strategy for writing functional materials., Comment: 12 pages, 3 figures

The development of molecular nanostructures plays a major role in emerging organic electronic applications, as it leads to improved performance and is compatible with our increasing need for miniaturisation. In particular, nanowires have been obtained from solution or vapour phase and have displayed high conductivity, or large interfacial areas in solar cells. In all cases however, the crystal structure remains as in films or bulk, and the exploitation of wires requires extensive post-growth manipulation as their orientations are random. Here we report copper phthalocyanine (CuPc) nanowires with diameters of 10-100 nm, high directionality and unprecedented aspect ratios. We demonstrate that they adopt a new crystal phase, designated eta-CuPc, where the molecules stack along the long axis. The resulting high electronic overlap along the centimetre length stacks achieved in our wires mediates antiferromagnetic couplings and broadens the optical absorption spectrum. The ability to fabricate ultralong, flexible metal phthalocyanine nanowires opens new possibilities for applications of these simple molecules.

We review quantum phase transitions of spin systems in transverse magnetic fields taking the examples of the spin-1/2 Ising and XY models in a transverse field. Beginning with an overview of quantum phase transitions, we introduce a number of model Hamiltonians. We provide exact solutions in one spatial dimension connecting them to conformal field theoretical studies. We also discuss Kitaev models and some other exactly solvable spin systems. Studies of quantum phase transitions in the presence of quenched randomness and with frustrating interactions are presented in detail. We discuss novel phenomena like Griffiths-McCoy singularities. We then turn to more recent topics like information theoretic measures of the quantum phase transitions in these models such as concurrence, entanglement entropy, quantum discord and quantum fidelity. We then focus on non-equilibrium dynamics of a variety of transverse field systems across quantum critical points and lines. After mentioning rapid quenching studies, we dwell on slow dynamics and discuss the Kibble-Zurek scaling for the defect density following a quench across critical points and its modifications for quenching across critical lines, gapless regions and multicritical points. Topics like the role of different quenching schemes, local quenching, quenching of models with random interactions and quenching of a spin chain coupled to a heat bath are touched upon. The connection between non-equilibrium dynamics and quantum information theoretic measures is presented at some length. We indicate the connection between Kibble-Zurek scaling and adiabatic evolution of a state as well as the application of adiabatic dynamics as a tool of a quantum optimization technique known as quantum annealing. The final section is dedicated to a detailed discussion on recent experimental studies of transverse Ising-like systems., Comment: 106 pages, 38 figures; an expanded version has been published as a book (330 pages, 72 figures, 874 references) as A. Dutta, G. Aeppli, B. K. Chakrabarti, U. Divakaran, T. F. Rosenbaum and D. Sen, Quantum Phase Transitions in Transverse Field Spin Models: From Statistical Physics to Quantum Information (Cambridge University Press, Cambridge, 2015)

It is well known that the microstructures of the transition-metal oxides (refs 1-3), including the high-transition-temperature (high-Tc) copper oxide superconductors (refs 4-7), are complex. This is particularly so when there are oxygen interstitials or vacancies (ref.8), which influence the bulk properties. For example, the oxygen interstitials in the spacer layers separating the superconducting CuO2 planes undergo ordering phenomena in Sr(2)O(1+y)CuO(2) (ref. 9), YBa(2)Cu(3)O(6+y) (ref. 10) and La(2)CuO(4+y) (refs 11-15) that induce enhancements in the transition temperatures with no changes in hole concentrations. It is also known that complex systems often have a scale-invariant structural organization16, but hitherto none had been found in high-Tc materials. Here we report that the ordering of oxygen interstitials in the La(2)O(2+y) spacer layers of La(2)CuO(4+y) high-Tc superconductors is characterized by a fractal distribution up to a maximum limiting size of 400 microns. Intriguingly, these fractal distributions of dopants seem to enhance superconductivity at high temperature., Comment: 10 pages, 5 figures