Superconductivity emerges in proximity to a nematic phase in most iron-based superconductors. It is therefore important to understand the impact of nematicity on the electronic structure. Orbital assignment and tracking across the nematic phase transition prove to be challenging due to the multiband nature of iron-based superconductors and twinning effects. Here, we report a detailed study of the electronic structure of fully detwinned FeSe across the nematic phase transition using angle-resolved photoemission spectroscopy. We clearly observe a nematicity-driven band reconstruction involving $d_{xz}$, $d_{yz}$, and $d_{xy}$ orbitals. The nematic energy scale between $d_{xz}$ and $d_{yz}$ bands reaches a maximum of 50 meV at the Brillouin zone corner. We are also able to track the $d_{xz}$ electron pocket across the nematic transition and explain its absence in the nematic state. Our comprehensive data of the electronic structure provide an accurate basis for theoretical models of the superconducting pairing in FeSe.

We report that dark excitons can have a large contribution to the emission intensity in carbon nanotubes due to an efficient exciton conversion from a dark state to a bright state. Time-resolved photoluminescence measurements are used to investigate decay dynamics and diffusion properties of excitons, and we obtain intrinsic lifetimes and diffusion lengths of bright excitons as well as diffusion coefficients for both bright and dark excitons. We find that the dark-to-bright transition rates can be considerably high and that more than half of the dark excitons can be converted into the bright excitons. The state-transition rates have a large chirality dependence with a family pattern, and the conversion efficiency is found to be significantly enhanced by adsorbed air molecules on the surface of the nanotubes. By understanding the importance of the dark exciton diffusion and the surface condition on the conversion process, performance improvement of nanotube light emitters is expected. Our findings show the nontrivial significance of the dark excitons on the emission kinetics in low-dimensional materials and demonstrate the potential for engineering the dark-to-bright conversion process by using surface interactions.

Topological defects play a prominent role in the physics of two-dimensional materials. When driven out of equilibrium in active nematics, disclinations can acquire spontaneous self-propulsion and drive self-sustained flows upon proliferation. Here, we construct a general hydrodynamic theory for a two-dimensional active nematic interrupted by a large number of such defects. Our equations describe the flows and spatiotemporal defect chaos characterizing active turbulence, even close to the defect-unbinding transition. At high activity, nonequilibrium torques combined with many-body screening cause the active disclinations to spontaneously break rotational symmetry, forming a collectively moving defect-ordered polar liquid. By recognizing defects as the relevant quasiparticle excitations, we construct a comprehensive phase diagram for two-dimensional active nematics. Using our hydrodynamic approach, we additionally show that activity gradients can act like “electric fields,” driving the sorting of topological charge. This result demonstrates the versatility of our continuum model and its relevance for quantifying the use of spatially inhomogeneous activity for controlling active flows and for the fabrication of active devices with targeted transport capabilities.

Despite recent progress in our understanding of complex dynamic networks, it remains challenging to devise sufficiently accurate models to observe, control, or predict the state of real systems in biology, economics, or other fields. A largely overlooked fact is that these systems are typically open and receive unknown inputs from their environment. A further fundamental obstacle is structural model errors caused by insufficient or inaccurate knowledge about the quantitative interactions in the real system. Here, we show that unknown inputs to open systems and model errors can be treated under the common framework of invertibility, which is a requirement for reconstructing these disturbances from output measurements. By exploiting the fact that invertibility can be decided from the influence graph of the system, we analyze the relationship between structural network properties and invertibility under different realistic scenarios. We show that sparsely connected scale-free networks are the most difficult to invert. We introduce a new sensor node placement algorithm to select a minimum set of measurement positions in the network required for invertibility. This algorithm facilitates optimal experimental design for the reconstruction of inputs or model errors from output measurements. Our results have both fundamental and practical implications for nonlinear systems analysis, modeling, and design.

Understanding the time evolution of contact strength in silica nanocontacts is of great fundamental and practical relevance in diverse areas like earthquake dynamics, wafer bonding mechanisms, as well as MEMS applications. The logarithmic increase of contact strength with hold time, termed contact aging, can be “quantitative” due to deformation creep in plastic contacts. An alternative mechanism, termed “qualitative aging,” is the gradual change in interfacial chemistry, which so far was only observed in the presence of humidity. Here we present nanoscale friction experiments of dry silica contacts in ultrahigh vacuum that show a doubling of shear strength with time following a logarithmic law. We find that the aging rate scales linearly with temperature, and that shear stress shifts the relevant energy barriers. All-atom MD simulations provide a live picture of the bond formation dynamics occurring at the interface. Our experiments link contact aging to thermally activated bond formation, show that it exists even in the absence of water molecules, and demonstrate that this atomic aging mechanism can stretch over timescales up to several seconds. Qualitative contact aging is thus highly relevant for a broad variety of material combinations and conditions.

In contrast to bulk materials, nanoscale crystal growth is critically influenced by size- and shape-dependent properties. However, it is challenging to decipher how stoichiometry, in the realm of mixed-valence elements, can act to control physical properties, especially when complex bonding is implicated by short- and long-range ordering of structural defects. Here, solution-grown iron-oxide nanocrystals (NCs) of the pilot wüstite system are found to convert into iron-deficient rock-salt and ferro-spinel subdomains but attain a surprising tetragonally distorted local structure. Cationic vacancies within chemically uniform NCs are portrayed as the parameter to tweak the underlying properties. These lattice imperfections are shown to produce local exchange-anisotropy fields that reinforce the nanoparticles’ magnetization and overcome the influence of finite-size effects. The concept of atomic-scale defect control in subcritical-size NCs aspires to become a pathway to tailor-made properties with improved performance for hyperthermia heating over defect-free NCs.

The process of frictional rupture, i.e., the failure of frictional systems, abounds in the technological and natural world around us, ranging from squealing car brake pads to earthquakes along geological faults. A general framework for understanding and interpreting frictional rupture commonly involves an analogy to ordinary crack propagation, with far-reaching implications for various disciplines from engineering tribology to geophysics. An important feature of the analogy to cracks is the existence of a reduction in the stress-bearing capacity of the ruptured interface, i.e., of a drop from the applied stress, realized far ahead of a propagating rupture, to the residual stress left behind it. Yet, how and under what conditions such finite and well-defined stress drops emerge from basic physics are not well understood. Here, we show that for a rapid rupture a stress drop is directly related to wave radiation from the frictional interface to the bodies surrounding it and to long-range bulk elastodynamics and not exclusively to the physics of the contact interface. Furthermore, we show that the emergence of a stress drop is a transient effect, affected by the wave travel time in finite systems and by the decay of long-range elastic interactions. Finally, we supplement our results for rapid rupture with predictions for a slow rupture. All of the theoretical predictions are supported by available experimental data and by extensive computations. Our findings elucidate the origin of stress drops in frictional rupture; i.e., they offer a comprehensive and fundamental understanding of why, how, and to what extent frictional rupture might be viewed as an ordinary fracture process.

Noise can be considered the natural enemy of quantum information. An often implied benefit of high-dimensional entanglement is its increased resilience to noise. However, manifesting this potential in an experimentally meaningful fashion is challenging and has never been done before. In infinite dimensional spaces, discretization is inevitable and renders the effective dimension of quantum states a tunable parameter. Owing to advances in experimental techniques and theoretical tools, we demonstrate an increased resistance to noise by identifying two pathways to exploit high-dimensional entangled states. Our study is based on two separate experiments utilizing canonical spatiotemporal properties of entangled photon pairs. Following these different pathways to noise resilience, we are able to certify entanglement in the photonic orbital-angular-momentum and energy-time degrees of freedom up to noise conditions corresponding to a noise fraction of 72% and 92%, respectively. Our work paves the way toward practical quantum communication systems that are able to surpass current noise and distance limitations, while not compromising on potential device independence.

We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying $T_2>100\mu \mathrm{s}$ ($T_2>400\mu \mathrm{s}$ in one device). Moreover, given the state-of-the-art values of the surface loss tangent and the $1/f$ flux-noise amplitude, the coherence time can be further improved beyond 1 ms. Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit—over $10^2$ here—must be minimized for best qubit coherence. We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.

The intrinsic magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ exhibits rich topological effects such as quantum anomalous Hall effect and axion electrodynamics. Here, by combining the use of synchrotron and laser light sources, we carry out comprehensive and high-resolution angle-resolved photoemission spectroscopy studies on ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and clearly identify its topological electronic structure. In contrast to theoretical predictions and previous studies, we observe topological surface states with diminished gap forming a characteristic Dirac cone. We argue that the topological surface states are mediated by multidomains of different magnetization orientations. In addition, the temperature evolution of the energy bands clearly reveals their interplay with the magnetic phase transition by showing interesting differences between the bulk and surface states, respectively. The investigation of the detailed electronic structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and its temperature evolution provides important insight into not only the exotic properties of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, but also the generic understanding of the interplay between magnetism and topological electronic structure in magnetic topological quantum materials.

In magnetic topological insulators (TIs), the interplay between magnetic order and nontrivial topology can induce fascinating topological quantum phenomena, such as the quantum anomalous Hall effect, chiral Majorana fermions, and axion electrodynamics. Recently, a great deal of attention has been focused on the intrinsic magnetic TIs, where disorder effects can be eliminated to a large extent, which is expected to facilitate the emergence of topological quantum phenomena. Despite intensive efforts, experimental evidence of the topological surface states (SSs) remains elusive. Here, by combining first-principles calculations and angle-resolved photoemission spectroscopy (ARPES) experiments, we reveal that ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ is an antiferromagnetic TI with the observation of Dirac SSs consistent with our prediction. We also observe nearly gapless Dirac SSs in antiferromagnetic TIs ${\mathrm{MnBi}}_{2n}{\mathrm{Te}}_{3n+1}$ ($n=1$ and 2), which are absent in previous ARPES results. These results provide clear evidence for nontrivial topology of these intrinsic magnetic TIs. Furthermore, we find that the topological SSs show no observable changes across the magnetic transition within the experimental resolution, indicating that the magnetic order has a quite small effect on the topological SSs, which can be attributed to weak hybridization between the localized magnetic moments, from either $4f$ or $3d$ orbitals, and the topological electronic states. This finding provides insights for further research that the correlations between magnetism and topological states need to be strengthened to induce larger gaps in the topological SSs, which will facilitate the realization of topological quantum phenomena at higher temperatures.

The recently discovered antiferromagnetic topological insulators in the Mn-Bi-Te family with intrinsic magnetic ordering have rapidly drawn broad interest since its cleaved surface state is believed to be gapped, hosting the unprecedented axion states with a half-integer quantum Hall effect. Here, however, we show unambiguously by using high-resolution angle resolved photoemission spectroscopy that a gapless Dirac cone at the (0001) surface of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ exists inside the bulk band gap. Such an unexpected surface state remains unchanged across the bulk Néel temperature, and is even robust against severe surface degradation, indicating additional topological protection. Through symmetry analysis and ab initio calculations we consider different types of surface reconstruction of the magnetic moments as possible origins giving rise to such linear dispersion. Our results unveil the experimental topological properties of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, revealing that the intrinsic magnetic topological insulator hosts a rich platform to realize various topological phases by tuning the magnetic or structural configurations, and thus push forward the comprehensive understanding of magnetic topological materials.

We study a possible deconfined quantum phase transition in a realistic model of a two-dimensional Shastry-Sutherland quantum magnet, using both numerical and field theoretic techniques. Using the infinite density matrix renormalization group (IDMRG) method, we verify the existence of an intermediate plaquette valence bond solid (PVBS) order, with twofold degeneracy, between the dimer and Néel ordered phases. We argue that the quantum phase transition between the Néel and PVBS orders may be described by a deconfined quantum critical point (DQCP) with an emergent O(4) symmetry. By analyzing the correlation length spectrum obtained from IDMRG, we provide evidence for the DQCP and emergent O(4) symmetry in the lattice model. Such a phase transition has been reported in the recent pressure-tuned experiments in the Shastry-Sutherland lattice material ${\mathrm{SrCu}}_2({\mathrm{BO}}_3{)}_2$ [Nat. Phys. 13, 962 (2017)]. The nonsymmorphic lattice structure of the Shastry-Sutherland compound leads to extinction points in the scattering, where we predict sharp signatures of a DQCP in both the phonon and magnon spectra associated with the spinon continuum. The effect of weak interlayer couplings present in the three-dimensional material is also discussed. Our results should help guide the experimental study of DQCP in quantum magnets.

Strongly out-of-equilibrium regimes in magnetic nanostructures exhibit novel properties, linked to the nonlinear nature of magnetization dynamics, which are of great fundamental and practical interest. Here, we demonstrate that ferromagnetic resonance driven by microwave magnetic fields can occur with substantial spatial coherency at an unprecedented large angle of magnetization precessions, which is normally prevented by the onset of spin-wave instabilities and magnetization turbulent dynamics. Our results show that this limitation can be overcome in nanomagnets, where the geometric confinement drastically reduces the density of spin-wave modes. When the obtained deeply nonlinear ferromagnetic resonance regime is perturbed, the magnetization undergoes eigenoscillations around the steady state due to torques tending to restore the stable large-angle periodic trajectory. These eigenoscillations are substantially different from the usual spin-wave modes around the ground state because their existence is connected to the presence of a large coherent precession. They are experimentally investigated by a new spectroscopic technique based on the application of a second microwave excitation field that is tuned to resonantly drive them. This two-tone spectroscopy enables us to show that they consist in slow coherent magnetization nutations around the large-angle steady precession, whose frequencies are set by the balance of restoring torques. Our experimental findings are well accounted for by an analytical model derived for systems with uniaxial symmetry. They also provide a new means for controlling highly nonlinear magnetization dynamics in nanostructures, opening interesting applicative opportunities in the context of magnetic nanotechnologies.

Inside the cell, proteins fold and perform complex functions through global structural rearrangements. For proper function, they need to be at the brink of instability to be susceptible to small environmental fluctuations yet stable enough to maintain structural integrity. These apparently conflicting properties are exhibited by systems near a critical point, where distinct phases merge. This concept goes beyond previous studies that propose proteins have a well-defined folded and unfolded phase boundary in the pressure-temperature plane. Here, by modeling the protein phosphoglycerate kinase (PGK) on the temperature ($T$), pressure ($P$), and crowding volume-fraction ($\varphi$) phase diagram, we demonstrate a critical transition where phases merge, and PGK exhibits large structural fluctuations. Above the critical temperature ($T_c$), the difference between the intermediate and unfolded phases disappears. When $\varphi$ increases, the $T_c$ moves to a lower $T$. With experiments mapping the $T\text{−}P\text{−}\varphi$ space, we verify the calculations and reveal a critical point at 305 K and 170 MPa that moves to a lower $T$ as $\varphi$ increases. Crowding shifts PGK closer to a critical line in its natural parameter space, where large conformational changes can occur without costly free-energy barriers. Specific structures are proposed for each phase based on the simulation.

Laser-cooled atoms that are trapped and optically interfaced with light in nanophotonic waveguides are a powerful platform for fundamental research in quantum optics as well as for applications in quantum communication and quantum-information processing. Ever since the first realization of such a hybrid quantum-nanophotonic system about a decade ago, heating rates of the atomic motion observed in various experimental settings have typically been exceeding those in comparable free-space optical microtraps by about 3 orders of magnitude. This excessive heating is a roadblock for the implementation of certain protocols and devices. Still, its origin has so far remained elusive and, at the typical atom-surface separations of less than an optical wavelength encountered in nanophotonic traps, numerous effects may potentially contribute to atom heating. Here, we theoretically describe the effect of mechanical vibrations of waveguides on guided light fields and provide a general theory of particle-phonon interaction in nanophotonic traps. We test our theory by applying it to the case of laser-cooled cesium atoms in nanofiber-based two-color optical traps. We find excellent quantitative agreement between the predicted heating rates and experimentally measured values. Our theory predicts that, in this setting, the dominant heating process stems from the optomechanical coupling of the optically trapped atoms to the continuum of thermally occupied flexural mechanical modes of the waveguide structure. Surprisingly, the effect of the high-$Q$ mechanical resonances which have previously been observed in this system can be neglected, even if they coincide with the trap frequencies. Beyond unraveling the long-standing riddle of excessive heating in nanofiber-based atom traps, we also study the dependence of the heating rates on the relevant system parameters and find a strong $R^{-5/2}$ scaling with the inverse waveguide radius. Our findings allow us to propose several strategies for minimizing the heating which also provide guidelines for the design of next-generation nanophotonic cold-atom systems. Finally, given that the predicted heating rate is proportional to the mass of the trapped particle, our findings are also highly relevant for optomechanics experiments with dielectric nanoparticles that are optically trapped close to nanophotonic waveguides.

Realization of long-distance quantum communication through the quantum repeater network requires a combination of some key elements, including a multiplexed quantum memory for storage of entanglement and a telecom photon for propagation of information through the fiber. Although impressive experimental advances have demonstrated the individual elements, combining these key capabilities together and realizing them in a single experimental system remains a significant challenge. Here, we report an experimental realization of long-distance entanglement between a multiplexed quantum memory with 49 individually accessible memory cells and a telecom photon after transmission in a 10-km optical fiber. Excitation of an atomic ensemble generates narrow-band polarization entanglement between a telecom photon of 1530-nm wavelength and another photon of 780-nm wavelength, which is then stored into a memory cell of a multiplexed atomic quantum memory and read out after a controllable delay. The entanglement is verified through quantum-state tomography after quantum storage in the atomic memory and fiber transmission of the telecom photon. This experiment demonstrates an important step towards realization of long-distance quantum communication networks.

Because of its nonequilibrium character, active matter in a steady state can drive engines that autonomously deliver work against a constant mechanical force or torque. As a generic model for such an engine, we consider systems that contain one or several active components and a single passive one that is asymmetric in its geometrical shape or its interactions. Generally, one expects that such an asymmetry leads to a persistent, directed current in the passive component, which can be used for the extraction of work. We validate this expectation for a minimal model consisting of an active and a passive particle on a one-dimensional lattice. It leads us to identify thermodynamically consistent measures for the efficiency of the conversion of isotropic activity to directed work. For systems with continuous degrees of freedom, work cannot be extracted using a one-dimensional geometry under quite general conditions. In contrast, we put forward two-dimensional shapes of a movable passive obstacle that are best suited for the extraction of work, which we compare with analytical results for an idealized work-extraction mechanism. For a setting with many noninteracting active particles, we use a mean-field approach to calculate the power and the efficiency, which we validate by simulations. Surprisingly, this approach reveals that the interaction with the passive obstacle can mediate cooperativity between otherwise noninteracting active particles, which enhances the extracted power per active particle significantly.