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The Raman effect, inelastic scattering of light by lattice vibrations (phonons), produces an optical response closely tied to a material’s crystal structure. Here we show that resonant optical excitation of IR and Raman phonons gives rise to a Raman-scattering effect that can induce giant shifts to the refractive index and induce new optical constants that are forbidden in the equilibrium crystal structure. We complete the description of light-matter interactions mediated by coupled IR and Raman phonons in crystalline insulators—currently the focus of numerous experiments aiming to dynamically control material properties—by including a forgotten pathway through the nonlinear lattice polarizability. Our work expands the toolset for control and development of new optical technologies by revealing that the absorption of light within the terahertz gap can enable control of optical properties of materials over a broad frequency range.

Epitaxial strain and chemical substitution have been the workhorses of functional materials design. These static techniques have shown immense success in controlling properties in complex oxides through the tuning of subtle structural distortions. Recently, an approach based on the excitation of an infrared active phonon with intense midinfrared light has created an opportunity for dynamical control of structure through special nonlinear coupling to Raman phonons. We use first-principles techniques to show that this approach can dynamically induce a magnetic phase transition from the ferromagnetic ground state to a hidden antiferromagnetic phase in the rare earth titanate GdTiO 3 for realistic experimental parameters. We show that a combination of a Jahn–Teller distortion, Gd displacement, and infrared phonon motion dominate this phase transition with little effect from the octahedral rotations, contrary to conventional wisdom. Perovskite titanates: Stabilizing hidden phases First-principles calculations reveal that the antiferromagnetic phase in GdTiO 3 can be stabilized through the optical excitation of particular phonon modes, combined also with epitaxial strain. Oxide perovskites provide a rich playground for studying the coupling between structural, electronic and magnetic degrees of freedom. Although their properties can be tuned through strain or doping, certain phases, like the antiferromagetic phase in GdTiO3, have not been achieved experimentally. Guru Khalsa and Nicole A. Benedek demonstrate through first-principles calculations that the excitation of phonon modes through ultrafast optical pulses can induce dynamically this phase, at the cost however of high electric fields. Nevertheless, their amplitudes can be significantly lowered when optical excitation is combined with modest epitaxial strain. The model that they put forward, provides significant light into the competing structural dynamics at play in perovskites out of equilibrium.

When an electronic system is subjected to a sufficiently strong magnetic field that the cyclotron energy is much larger than the Fermi energy, the system enters the extreme quantum limit (EQL) and becomes susceptible to a number of instabilities. Bringing a three-dimensional electronic system deeply into the EQL can be difficult however, since it requires a small Fermi energy, large magnetic field, and low disorder. Here we present an experimental study of the EQL in lightly-doped single crystals of strontium titanate. Our experiments probe deeply into the regime where theory has long predicted an interaction-driven charge density wave or Wigner crystal state. A number of interesting features arise in the transport in this regime, including a striking re-entrant nonlinearity in the current–voltage characteristics. We discuss these features in the context of possible correlated electron states, and present an alternative picture based on magnetic-field induced puddling of electrons. At sufficiently strong magnetic fields and low temperatures, electrons assume a quasi-one-dimensional quantum state that is challenging to observe. Here, Bhattacharya et al . report on electron transport in lightly-doped single crystals of SrTiO 3 deep in this extreme quantum limit.

Experiments involving resonant optical excitation of infrared-active phonons in crystals have emerged as a powerful new way to tune materials properties. A puzzling and so far unexplained aspect of some so-called nonlinear phononics experiments is that the observed lifetimes of the optically created metastable phases are sometimes orders of magnitude longer than expected based on the nonlinear phononics mechanism assumed in most works. We use a combination of phenomenological theory and first-principles calculations to demonstrate that strong coupling between different lattice degrees of freedom (strains and Raman-active phonons) can give rise to a long-lived metastable phase recently observed in experiments on perovskite LaAlO 3 [Hortensius et al. npj Quantum Mater . 5 95 (2020)]. We show that the long-timescale oscillatory response in the experimental optical reflectivity data is not due solely to shear strains, as originally suggested, but arises from a “hybrid” mode involving displacements of Raman-active phonons of the same symmetry. Our work suggests that strong coupling between different order parameters can provide a mechanism for long-lived optically created metastable phases and points towards strategies, such as strain engineering, for modifying or increasing the lifetime of light-induced phases in ultrafast optical experiments.

Spoken-digit recognition using a nanoscale spintronic oscillator that mimics the behaviour of neurons demonstrates the potential of such oscillators for realizing large-scale neural networks in future hardware. Computerized brain network recognizes voices Neuromorphic computing takes the exceptional information processing capabilities of the biological brain as inspiration and attempts to build artificial neurons, synapses and networks for tackling specific tasks that are challenging or energy-intensive for regular computers, such as recognizing images and patterns in sensory signals. Julie Grollier and colleagues use magnetic nanoscale oscillators to mimic the nonlinear oscillating behaviour of neurons and test the capability of such devices to recognize audio signals. The system was trained to recognize spoken digits from five different voices from a benchmark database and could do so with accuracy comparable to state-of-the-art machine learning. The work opens a new direction for chip-based, low-power, brain-like information processing. Neurons in the brain behave as nonlinear oscillators, which develop rhythmic activity and interact to process information 1 . Taking inspiration from this behaviour to realize high-density, low-power neuromorphic computing will require very large numbers of nanoscale nonlinear oscillators. A simple estimation indicates that to fit 10 8 oscillators organized in a two-dimensional array inside a chip the size of a thumb, the lateral dimension of each oscillator must be smaller than one micrometre. However, nanoscale devices tend to be noisy and to lack the stability that is required to process data in a reliable way. For this reason, despite multiple theoretical proposals 2 , 3 , 4 , 5 and several candidates, including memristive 6 and superconducting 7 oscillators, a proof of concept of neuromorphic computing using nanoscale oscillators has yet to be demonstrated. Here we show experimentally that a nanoscale spintronic oscillator (a magnetic tunnel junction) 8 , 9 can be used to achieve spoken-digit recognition with an accuracy similar to that of state-of-the-art neural networks. We also determine the regime of magnetization dynamics that leads to the greatest performance. These results, combined with the ability of the spintronic oscillators to interact with each other, and their long lifetime and low energy consumption, open up a path to fast, parallel, on-chip computation based on networks of oscillators.

Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors—silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor—an electronic gain element—to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance—a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family. Group III/nitride semiconductors have been grown epitaxially on the superconductor niobium nitride, allowing the superconductor’s macroscopic quantum effects to be combined with the semiconductors’ electronic, photonic and piezoelectric properties. Mix and match The perfect epitaxial growth of one crystalline semiconductor on another is a fundamental feature of many high-performance electronic and optoelectronic devices. Rusen Yan and colleagues demonstrate that a similar level of epitaxial integration can be achieved between the group III nitride semiconductors and the superconducting nitride metal NbN x . This ability to grow highly ordered, high-quality semiconducting structures directly on a crystalline superconductor provides a route for exploring a host of new device possibilities that combine the properties of the two subsystems.

There is significant technological interest in developing ever faster switching between different electronic and magnetic states of matter. Manipulating properties at terahertz rates requires accessing the intrinsic timescales of both electrons and associated phonons, which is possible with short-pulse photoexcitation. However, in many Mott insulators, the electronic transition is accompanied by the nucleation and growth of percolating domains of the changed lattice structure, leading to empirical timescales dominated by slowly coarsening dynamics. Here we use time-resolved X-ray diffraction and reflectivity measurements to show that the photoinduced insulator-to-metal transition in an epitaxially strained Mott insulating thin film occurs without observable domain formation and coarsening effects, allowing the study of the intrinsic electronic and lattice dynamics. Above a fluence threshold, the initial electronic excitation drives a fast lattice rearrangement, which is followed by a slower electronic evolution into a metastable nonequilibrium state. Microscopic model calculations based on time-dependent dynamical mean-field theory and semiclassical lattice dynamics explain the threshold behaviour and elucidate the delayed onset of the electronic phase transition. This work highlights the importance of combined electronic and structural studies in unravelling the physics of dynamic transitions and the timescales of photoinduced processes. During a photoinduced phase transition, electronic rearrangements are usually faster than lattice ones. Time-resolved measurements now show that the insulator-to-metal transition in a thin-film Mott insulator is preceded by lattice reconfiguration.

Pulse profile modelling (PPM) is a comprehensive relativistic ray-tracing technique employed to determine the properties of neutron stars. In this study, we apply this technique to the Type I X-ray burster and accretion-powered millisecond pulsar XTE J1814-338, extracting its fundamental properties using PPM of its thermonuclear burst oscillations. Using data from its 2003 outburst, and a single uniform temperature hot spot model, we infer XTE J1814-338 to be located at a distance of \\(7.2^+0.3_-0.4\\) kpc, with a mass of \\(1.21^+0.05_-0.05\\) M\\(_\\) and an equatorial radius of \\(7.0^+0.4_-0.4\\) km. Our results also offer insight into the time evolution of the hot spot but point to some potential shortcomings of the single uniform temperature hot spot model. We explore the implications of this result, including what we can learn about thermonuclear burst oscillation mechanisms and the importance of modelling the accretion contribution to the emission during the burst.

Problem/Aim. Since ancient times, in both West and East, the number eight is present in many cultural expressions, having important functions, both symbolic and concrete. Concerning the human bodily experience, the aim is to strive not to an abstract summary, an idealistic archetype, but to the identification of the specificities of the different historical processes that have produced different body techniques. Material/Method. Moving from the presence of the number eight in martial arts and traditional dances coming from different backgrounds and using the comparative historical method, article traces the similarities among various disciplines. Results/Conclusions. Dance and martial arts are activities well established already in archaic societies as expressions of universal efficiency of rhythmic alternation between opposing, but complementary and inseparable, forces. Through observation of the nature we can perceive this alternation.