Recent studies of holographic tensor network models defined on regular tessellations of hyperbolic space have not yet addressed the underlying discrete geometry of the boundary. We show that the boundary degrees of freedom naturally live on a novel structure, a “conformal quasicrystal,” that provides a discrete model of conformal geometry. We introduce and construct a class of one-dimensional conformal quasicrystals and discuss a higher-dimensional example (related to the Penrose tiling). Our construction permits discretizations of conformal field theories that preserve an infinite discrete subgroup of the global conformal group at the cost of lattice periodicity.

We show that frustrated quasidoublets without time-reversal symmetry can host highly unconventional magnetic structures with continuously distributed order parameters even in a single-phase crystal. Our study comprises a comprehensive thermodynamic and neutron diffraction investigation on the single crystal of ${\mathrm{TmMgGaO}}_4$, which entails non-Kramers ${\mathrm{Tm}}^{3+}$ ions arranged on a geometrically perfect triangular lattice. The crystal electric field randomness caused by the site-mixing disorder of the nonmagnetic ${\mathrm{Mg}}^{2+}$ and ${\mathrm{Ga}}^{3+}$ ions merges two lowest-lying crystal electric field singlets of ${\mathrm{Tm}}^{3+}$ into a ground-state quasidoublet. Well below $T_c\sim 0.7\mathrm{K}$, a small fraction of the antiferromagnetically coupled ${\mathrm{Tm}}^{3+}$ Ising quasidoublets with small inner gaps condense into two-dimensional up-up-down magnetic structures with continuously distributed order parameters, and give rise to the columnar magnetic neutron reflections below ${\mu }_0{H}_c\sim 2.6\mathrm{T}$, with highly anisotropic correlation lengths, ${\xi }_{ab}\ge 250a$ in the triangular plane and ${\xi }_c<c/12$ between the planes. The remaining fraction of the ${\mathrm{Tm}}^{3+}$ ions remain nonmagnetic at 0 T and become uniformly polarized by the applied longitudinal field at low temperatures. We argue that the similar model can be generally applied to other compounds of non-Kramers rare-earth ions with correlated ground-state quasidoublets.

We study the classical dimer model on rhombic Penrose tilings, whose edges and vertices may be identified as those of a bipartite graph. We find that Penrose tilings do not admit perfect matchings (defect-free dimer coverings). Instead, their maximum matchings have a monomer density of $81-50\phi \approx 0.098$ in the thermodynamic limit, with $\phi =(1+\sqrt{5})/2$ the golden ratio. Maximum matchings divide the tiling into a fractal of nested closed regions bounded by loops that cannot be crossed by monomers. These loops connect second-nearest-neighbor even-valence vertices, each of which lies on such a loop. Assigning a charge to each monomer with a sign fixed by its bipartite sublattice, we find that each bounded region has an excess of one charge, and a corresponding set of monomers, with adjacent regions having opposite net charge. The infinite tiling is charge neutral. We devise a simple algorithm for generating maximum matchings and demonstrate that maximum matchings form a connected manifold under local monomer-dimer rearrangements. We show that dart-kite Penrose tilings feature an imbalance of charge between bipartite sublattices, leading to a minimum monomer density of $(7-4\phi )/5\approx 0.106$ all of one charge.

Photocurrents are central to understanding and harnessing the interaction of light with matter. Here, we introduce a contact-free method to spatially resolve photocurrent distributions using proximal quantum magnetometers. We interface monolayer ${\mathrm{MoS}}_2$ with a near-surface ensemble of nitrogen-vacancy centers in diamond and map the generated photothermal current distribution through its magnetic-field profile. By synchronizing pulsed photoexcitation with dynamical decoupling of the sensor spin, we extend the sensor’s quantum coherence and resolve time-dependent, two-dimensional current densities as small as $20\mathrm{nA}/\mu \mathrm{m}$, with a projected sensitivity of $200\mathrm{nA}/(\mu \mathrm{m}\sqrt{\mathrm{Hz}})$. Our spatially resolved measurements reveal that optical excitation can generate micron-sized photocurrent vortices in ${\mathrm{MoS}}_2$, manifesting a photo-Nernst effect exceeding that of gate-tuned graphene at comparable magnetic fields. We further probe the rise time of the photocurrents and show that thermal diffusion determines its spatial variation. These spatiotemporal capabilities establish an optically accessed, local probe for optoelectronic phenomena, ideally suited to the emerging class of two-dimensional materials, for which making contacts is challenging and can alter the intrinsic material properties.

Following significant progress in the visualization and characterization of Majorana end modes in hybrid systems of semiconducting nanowires and superconducting islands, much attention is devoted to the investigation of the electronic structure at the buried interface between the semiconductor and the superconductor. The properties of that interface and the structure of the electronic wave functions that occupy it determine the functionality and the topological nature of the superconducting state induced therein. Here we study this buried interface by performing spectroscopic mappings of superconducting aluminum islands epitaxially grown in situ on indium arsenide nanowires. We find unexpected robustness of the hybrid system as the direct contact with the aluminum islands does not lead to any change in the chemical potential of the nanowires, nor does it induce a significant band bending in their vicinity. We attribute this to the presence of surface states bound to the facets of the nanowire. Such surface states, which are present also in bare nanowires prior to aluminum deposition, pin the Fermi level, thus rendering the nanowires resilient to surface perturbations. The aluminum islands further display Coulomb blockade gaps and peaks that signify the formation of a resistive tunneling barrier at the InAs-Al interface. The extracted interface resistivity, $\rho \approx 1.3\times {10}^{-6}\mathrm{\Omega }{\mathrm{cm}}^2$, will allow us to proximity induce superconductivity with negligible Coulomb blockade effects by islands with interface area as small as $0.01\mu {\mathrm{m}}^2$. At low energies we identify a potential energy barrier that further suppresses the transmittance through the interface. A corresponding barrier exists in bare semiconductors between surface states and the accumulation layer, induced to maintain charge neutrality. Our observations elucidate the delicate interplay between the resistive nature of the InAs-Al interface and the ability to proximitize superconductivity and tune the chemical potential in semiconductor-superconductor hybrid nanowires.

Combinations of nontrivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances, material candidates are emerging. Yet, so far a compound that combines a band-inverted electronic structure with an intrinsic net magnetization remains unrealized. ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ has been established as the first antiferromagnetic topological insulator and constitutes the progenitor of a modular $({\mathrm{Bi}}_2{\mathrm{Te}}_3{)}_n({\mathrm{MnBi}}_2{\mathrm{Te}}_4)$ series. Here, for $n=1$, we confirm a nonstoichiometric composition proximate to ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$. We establish an antiferromagnetic state below 13 K followed by a state with a net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission experiments and density-functional calculations reveal a topologically nontrivial surface state on the ${\mathrm{MnBi}}_4{\mathrm{Te}}_7(0001)$ surface, analogous to the nonmagnetic parent compound ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Our results establish ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ as the first band-inverted compound with intrinsic net magnetization providing a versatile platform for the realization of magnetic topological states of matter.

We develop a quantum theory of magnetic skyrmions and antiskyrmions in a spin-$1/2$ Heisenberg magnet with frustrating next-nearest-neighbor interactions. Using exact diagonalization we show numerically that a quantum skyrmion exists as a stable many-magnon bound state and investigate its quantum numbers. We then derive a phenomenological Schrödinger equation for the quantum skyrmion and its internal degrees of freedom. We find that quantum skyrmions have highly unusual properties. Their bandwidth is exponentially small and arises from tunneling processes between skyrmion and antiskyrmion. The band structure changes both qualitatively and quantitatively when a single spin is added or removed from the quantum skyrmion, reflecting a locking of angular momentum and spin quantum numbers characteristic for skyrmions. Additionally, while for weak forces the quantum skyrmion is accelerated parallel to the force, it moves in a perpendicular direction for stronger fields.

A normal metal exhibits a valence plasmon, which is a sound wave in its conduction electron density. The mysterious strange metal is characterized by non-Boltzmann transport and violates most fundamental Fermi-liquid scaling laws. A fundamental question is, do strange metals have plasmons? Using momentum-resolved inelastic electron scattering, we recently showed that, rather than a plasmon, optimally doped ${\mathrm{Bi}}_{2.1}{\mathrm{Sr}}_{1.9}{\mathrm{Ca}}_{1.0}{\mathrm{Cu}}_{2.0}{\mathrm{O}}_{8+x}$ (Bi-2212) exhibits a featureless, temperature-independent continuum with a power-law form over most energy and momentum scales [M. Mitrano et al., Proc. Natl. Acad. Sci. U.S.A. 115, 5392 (2018)]. Here, we show that this continuum is present throughout the fan-shaped, strange metal region of the phase diagram. Outside this region, dramatic changes in spectral weight are observed: In underdoped samples, spectral weight up to 0.5 eV is enhanced at low temperature, biasing the system toward a charge order instability. The situation is reversed in the overdoped case, where spectral weight is strongly suppressed at low temperature, increasing quasiparticle coherence in this regime. Optimal doping corresponds to the boundary between these two opposite behaviors at which the response is temperature independent. Our study suggests that plasmons do not exist as well-defined excitations in Bi-2212 and that a featureless continuum is a defining property of the strange metal, which is connected to a peculiar crossover where the spectral weight change undergoes a sign reversal.

Intrinsic anomalous Nernst effect, like its Hall counterpart, is generated by Berry curvature of electrons in solids. Little is known about its response to disorder. In contrast, the link between the amplitude of the ordinary Nernst coefficient and the mean-free path is extensively documented. Here, by studying ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, a topological half-metallic semimetal hosting sizable and recognizable ordinary and anomalous Nernst responses, we demonstrate an anticorrelation between the amplitudes of carrier mobility and the anomalous $S_{xy}^A$ (the ratio of transverse electric field to the longitudinal temperature gradient in the absence of magnetic field). We argue that the observation, paradoxically, establishes the intrinsic origin of the anomalous Nernst effect in this system. We conclude that various intrinsic off-diagonal coefficients are set by the way the Berry curvature is averaged on a grid involving the mean-free path, the Fermi wavelength, and the de Broglie thermal length.

We report an experimental study of the emergence of nontrivial topological winding and long-range order across the paramagnetic-to-skyrmion lattice transition in the transition metal helimagnet MnSi. Combining measurements of the susceptibility with small-angle neutron scattering, neutron-resonance spin-echo spectroscopy, and all-electrical microwave spectroscopy, we find evidence of skyrmion textures in the paramagnetic state exceeding ${10}^3\AA$, with lifetimes above several ${10}^{-9}\mathrm{s}$. Our experimental findings establish that the paramagnetic-to-skyrmion lattice transition in MnSi is well described by the Landau soft-mode mechanism of weak crystallization, originally proposed in the context of the liquid-to-crystal transition. As a key aspect of this theoretical model, the modulation vectors of periodic small-amplitude components of the magnetization form triangles that add to zero. In excellent agreement with our experimental findings, these triangles of the modulation vectors entail the presence of the nontrivial topological winding of skyrmions already in the paramagnetic state of MnSi when approaching the skyrmion lattice transition.

Although strong electronic correlations are known to be responsible for some highly unusual behaviors of solids such as metal-insulator transitions, magnetism, and even high-temperature superconductivity, their interplay with recently discovered topological states of matter awaits a full exploration. Here, we use a modern electronic structure method, combining the density functional theory of band electrons with dynamical self-energies of strongly correlated states, to predict that two well-known phases of actinide compound UNiSn, a paramagnetic semiconducting and antiferromagnetic metallic, correspond to topological insulator (TI) and Weyl semimetal (WSM) phases of topological quantum matter. Thus, the famous unconventional insulator-metal transition observed in UNiSn is also a TI-to-WSM transition. Driven by a strong hybridization between U $f$-electron multiplet transitions and band electrons, multiple energy gaps open up in the single-particle spectrum whose topological physics is revealed using the calculation of $Z_2$ invariants in the strongly correlated regime. A simplified physical picture of these phenomena is provided based on a periodic Anderson model of strong correlations and multiple band inversions that occur in this fascinating compound. Studying the topology of interacting electrons reveals interesting opportunities for finding exotic phase transitions in strongly correlated systems.

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.

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 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.