The understanding of weak measurements and interaction-free measurements has greatly expanded the conceptual and experimental toolbox to explore the quantum world. Here we demonstrate single-shot variable-strength weak measurements of the electron and nuclear spin states of a ${}^{31}\mathrm{P}$ single-atom donor in silicon. We first show how the partial collapse of the nuclear spin due to measurement can be used to coherently rotate the spin to a desired pure state. We explicitly demonstrate that phase coherence is preserved with high fidelity throughout multiple sequential single-shot weak measurements and that the partial state collapse can be reversed. Second, we use the relation between measurement strength and perturbation of the nuclear state as a physical meter to extract the tunnel rates between the ${}^{31}\mathrm{P}$ donor and a nearby electron reservoir from data conditioned on observing no tunneling events. Our experiments open avenues to measurement-based state preparation, steering and feedback protocols for spin systems in the solid state, and highlight the fundamental connection between information gain and state modification in quantum mechanics.

We compute annihilation rates of metastable magnetic skyrmions using a form of Langer's theory in the intermediate-to-high damping (IHD) regime. For a Néel skyrmion, a Bloch skyrmion, and an antiskyrmion, we look at two possible paths to annihilation: collapse and escape through a boundary. We also study the effects of a curved versus a flat boundary, a second skyrmion, and a nonmagnetic defect. We find that the skyrmion's internal modes play a dominant role in the thermally activated transitions compared to the spin-wave excitations and that the relative contribution of internal modes depends on the nature of the transition process. Our calculations for a small skyrmion stabilized at zero field show that collapse on a defect is the most probable path. In the absence of a defect, the annihilation is largely dominated by escape mechanisms, even though in this case the activation energy is higher than that of collapse processes. Escape through a flat boundary is found more probable than through a curved boundary. The potential source of stability of metastable skyrmions is therefore found not to lie in high activation energies, nor in the dynamics at the transition state, but comes from entropic narrowing in the saddle point region which leads to lowered attempt frequencies. This narrowing effect is found to be primarily associated with the skyrmion's internal modes.

Using the infinite projected entangled pair states algorithm, we study the ground-state properties of the spin-$1/2$ quantum Heisenberg antiferromagnet on the star lattice in the thermodynamic limit. By analyzing the ground-state energy of the two inequivalent bonds of the lattice in different unit-cell structures, we identify two competing valence-bond-solid (VBS) phases for different antiferromagnetic Heisenberg exchange couplings. More precisely, we observe (i) a VBS state which respects the full symmetries of the Hamiltonian, and (ii) a resonating VBS state which, in contrast to previous predictions, has a six-site unit-cell order and breaks $C_3$ symmetry. We also studied the ground-state phase diagram by measuring the ground-state fidelity and energy derivatives, and further confirmed the continuous nature of the quantum phase transition in the system. Moreover, an analysis of the isotropic point shows that its ground state is also a VBS as in (i), which is as well in contrast with previous predictions.

By combining scanning tunneling microscopy/spectroscopy and first-principles calculations, we systematically study the local electronic states of magnetic dopants V and Cr in the topological insulator (TI) ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. Spectroscopic imaging shows diverse local defect states between Cr and V, which agree with our first-principle calculations. The unique spectroscopic features of V and Cr dopants provide electronic fingerprints for the codoped magnetic TI samples with the enhanced quantum anomalous Hall effect. Our results also facilitate the exploration of the underlying mechanism of the enhanced quantum anomalous Hall temperature in Cr/V codoped TIs.

The study of hyperfine interaction by high-resolution inelastic neutron scattering is not very well known compared to the other competing techniques viz. nuclear magnetic resonance, Mössbauer, perturbed angular correlation spectroscopy, etc. Also, studies have been limited mostly to magnetically ordered systems. Here, we report such a study on ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{FeCoO}}_6$ ($x=0,1,2$) of which the first (${\mathrm{Sr}}_2{\mathrm{FeCoO}}_6$ with $x=0$) has a canonical spin-glass state, the second (${\mathrm{SrLaFeCoO}}_6$ with $x=1$) has a so-called magnetic glass state, and the third (${\mathrm{La}}_2{\mathrm{FeCoO}}_6$ with $x=2$) has a magnetically ordered ground state. Our present study revealed a clear inelastic signal for ${\mathrm{SrLaFeCoO}}_6$, a possible inelastic signal for ${\mathrm{Sr}}_2{\mathrm{FeCoO}}_6$ below the spin freezing temperatures $T_{\text{sf}}$, but no inelastic signal at all for the magnetically ordered ${\mathrm{La}}_2{\mathrm{FeCoO}}_6$ in the neutron-scattering spectra. The broadened inelastic signals observed suggest hyperfine field distributions in the two disordered magnetic glassy systems, whereas the absent inelastic signal for the third compound suggests no, or a very small, hyperfine field at the Co nucleus due to Co electronic moment. The hyperfine splitting on the Co nucleus is induced by the electronic spin state of the magnetic sample atom, and our experiments add information concerning the timescale of electronic spin fluctuations by the appearance of quasielastic broadening in the $\mu \mathrm{eV}$ range at low Q and spin freezing on the nanosecond timescale below $T_{\text{sf}}$. Whereas these features are observed at low Q for $x=0$ and 1, they are absent for ${\mathrm{La}}_2{\mathrm{FeCoO}}_6$, which evidences a gradual increase of the elastic intensity only at large Q near an emerging Bragg peak. Thus both electronic magnetic spin freezing and inelastic excitations arising from nuclear hyperfine splitting at the Co site consistently indicate a different behavior for $x=2$.

Strong spin-orbit coupling (SOC) in combination with a lack of inversion symmetry and exchange magnetic interaction proves to be a sophisticated instrument allowing efficient control of the spin orientation, energy and trajectories of two-dimensional (2D) electrons and holes trapped at surfaces or interfaces. Exploiting Kondo-related phenomena and crystal-electric-field effects at reduced dimensionalities opens new opportunities to handle their spin-dependent properties offering novel functionalities. We consider here a 2D Kondo lattice represented by a Si-Ir-Si-Yb (SISY) surface block of the heavy-fermion material ${\mathrm{YbIr}}_2{\mathrm{Si}}_2$. We show that the Kondo interaction with $4f$ moments allows finely tuning the group velocities of the strongly spin-polarized carriers in 2D itinerant states of this noncentrosymmetric system. To unveil the peculiarities of this interaction, we used angle-resolved photoemission measurements complemented by first-principles calculations. We established that the strong SOC of the Ir atoms induces spin polarization of the 2D states in SISY block, while the 2D lattice of Yb $4f$ moments acts as a source for coherent $f-d$ interplay. The strong SOC and lack of inversion symmetry turn out to lead not only to the anticipated Rashba-like splitting of the 2D states, but also to spin splitting of the $4f$ Kramers doublets. They couple temperature-dependently to the spin-polarized 2D states and thereby guide the properties of the latter.

We investigate excitonic absorption and emission in monolayer (1L) ${\mathrm{WSe}}_2$ under tensile strain. We observe a redshift of 100 meV in the $A$ exciton energy and a decrease of 25 meV in its estimated binding energy under 2.1% strain. Surprisingly, the linewidth of the $A$ exciton decreases by almost a factor of two under strain, from 42 to 24 meV at room temperature. We explain this effect as the result of suppression of phonon-mediated exciton scattering channels. We show that such a suppression results from the relative shift under strain of a secondary valley in the conduction band that is nearly degenerate with the $K$ valley where the $A$ exciton is formed.

The Kitaev quantum spin liquid (KSL) is a theoretically predicted state of matter whose fractionalized quasiparticles are distinct from bosonic magnons, the fundamental excitation in ordered magnets. The layered honeycomb antiferromagnet $\alpha \text{−}{\mathrm{RuCl}}_3$ is a KSL candidate material, as it can be driven to a magnetically disordered phase by application of an in-plane magnetic field, with $H_c\sim 7\mathrm{T}$. Here, we report a detailed characterization of the magnetic excitation spectrum of this material by high-resolution time-domain terahertz spectroscopy. We observe two sharp magnon resonances whose frequencies and amplitudes exhibit a discontinuity as a function of applied magnetic field, as well as two broader peaks at higher energy. Below the Néel temperature, we find that linear spin wave theory can account for all of these essential features of the spectra when a $C_3$-breaking distortion of the honeycomb lattice and the presence of structural domains are taken into account.

Strontium ruthenate (${\mathrm{Sr}}_2{\mathrm{RuO}}_4$) is a multiband superconductor that displays evidence of topological superconductivity, although a model of the order parameter that is consistent with all experiments remains elusive. We integrated a piezoelectric-based strain apparatus with a scanning superconducting quantum interference device microscope to map the diamagnetic response of single-crystal ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ as a function of temperature, uniaxial pressure, and position with micron-scale spatial resolution. We thereby obtained local measurements of the superconducting transition temperature $T_c$ vs anisotropic strain $\epsilon$ with sufficient sensitivity for comparison to theoretical models that assume a uniform $p_x\pm i{p}_y$ order parameter. We found that $T_c$ varies with position and that the locally measured $T_c$ vs $\epsilon$ curves are quadratic ($T_c\propto {\epsilon }^2$), as allowed by the $C_4$ symmetry of the crystal lattice. We did not observe the low-strain linear cusp ($T_c\propto \left|\epsilon \right|$) that would be expected for a two-component order parameter such as $p_x\pm i{p}_y$. These results provide new input for models of the order parameter of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

We propose a mechanism for the generation of pure superconducting spin-current carried by equal-spin triplet Cooper pairs in a superconductor (S) sandwiched between a ferromagnet (F) and a normal metal (${\mathrm{N}}_{\mathrm{so}}$) with intrinsic spin-orbit coupling. We show that in the presence of Landau Fermi liquid interactions the superconducting proximity effect can induce nonlocally a ferromagnetic exchange field in the normal layer, which disappears above the superconducting transition temperature of the structure. The internal Landau Fermi liquid exchange field leads to the onset of a spin supercurrent associated with the generation of long-range spin-triplet superconducting correlations in the trilayer. We demonstrate that the magnitude of the spin supercurrent as well as the induced magnetic order in the ${\mathrm{N}}_{\mathrm{so}}$ layer depends critically on the superconducting proximity effect between the S layer and the F and ${\mathrm{N}}_{\mathrm{so}}$ layers and the magnitude of the relevant Landau Fermi liquid interaction parameter. We investigate the effect of spin flip processes on this mechanism. Our results demonstrate the crucial role of Landau Fermi liquid interaction in combination with spin-orbit coupling for the creation of spin supercurrent in superconducting spintronics, and give a possible explanation of a recent experiment utilizing spin-pumping via ferromagnetic resonance [Jeon et al., Nat. Mater. 17, 499 (2018)].

Recent successes in Monte Carlo methods for simulating fermionic quantum impurity models have been based on diagrammatic resummation techniques, but they are restricted by the need to sum over factorially large classes of diagrams individually. We present a fast algorithm for summing over the diagrams appearing in Inchworm hybridization expansions. The method relies on the inclusion-exclusion principle to reduce the scaling from factorial to exponential. We analyze the growth rate and compare with related algorithms for expansions in the many-body interaction. An implementation demonstrates that for a simulation of a concrete physical model at reasonable parameters and accuracy within the Inchworm hybridization expansion, our algorithm not only scales better asymptotically, but also provides performance gains of approximately two orders of magnitude in practice over the previous state of the art.

The magnetic field dependence of electronic transport, magnetic, and elastic properties in single crystals of PbTe were investigated in high magnetic fields up to 55 T. The magnetoresistance, magnetization, ultrasonic attenuation, and sound velocity showed clear quantum oscillations with pronounced Zeeman splitting, which causes a large second harmonic in the frequency spectra. The ratio of the Zeeman to the cyclotron energy, which is regarded as an index of “Diracness” [H. Hayasaka and Y. Fuseya, J. Phys.: Condens. Matter 28, 31LT01 (2016)], was determined to be 0.52 and 0.57 in samples with different carrier densities. We also pointed out that the effect of Zeeman splitting seriously affects the Landau-level fan diagram analysis, which is widely used to extract the nontrivial Berry's phase from the quantum oscillations.

Synthesizing a polarizable environment surrounding a low-dimensional metal to generate superconductivity is a simple theoretical idea that still awaits a convincing experimental realization. The challenging requirements are satisfied in a metallic bilayer when the ratio between the Fermi velocities is small and both metals have a similar, low carrier density. In this case, the slower electron gas acts as a retarded polarizable medium (a “dielectric” environment) for the faster metal. Here we show that this concept is naturally optimized for the case of an atomically thin bilayer consisting of a Dirac semimetal (e.g., graphene) placed in atomic-scale proximity to a doped semiconducting transition metal dichalcogenide (e.g., ${\mathrm{WSe}}_2$). The superconducting transition temperature that arises from the dynamically screened Coulomb repulsion is computed using the linearized Eliashberg equation. In the case of graphene on ${\mathrm{WSe}}_2$, we find that $T_c$ can exceed 100 mK, and it increases further when the Dirac valley degeneracy is reduced. Thus, we argue that suspended van der Waals bilayers are in a unique position to realize experimentally this long-anticipated theoretical concept.

The transverse (Hall) voltage in thin films of the high-temperature superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ with an artificial periodic pinning array is investigated. Columnar defect regions along the crystallographic $c$ axis, in which superconductivity is suppressed, are created by irradiation with ${\mathrm{He}}^{+}$ ions through a silicon stencil mask. The commensurate arrangement of magnetic flux quanta with the artificial defect lattice is confirmed by maxima of the critical current and minima of the resistance, respectively. The magnetic field dependence of the transverse voltage reveals a commensurability effect characterized by a narrow peak of the Hall coefficient with reversed polarity compared to the background signal. This signature of vortex matching disappears at larger vortex velocities substantiating its close connection with enhanced pinning of vortices at the periodic pinning landscape.

We report a comprehensive study on the optical properties of cubic boron nitride ($c$-BN) and its optically active defects. Using electron energy-loss spectroscopy (EELS) within a monochromated scanning transmission electron microscope (STEM) on the highest-quality crystals available, we demonstrate unequivocally that the optical-gap energy of $c$-BN slightly exceeds 10 eV. Further theoretical analysis in the framework of the Bethe-Salpeter equation of many-body perturbation theory supports this result. The spatial localization of defect-related emissions has been investigated using nanometric resolved cathodoluminescence (nano-CL) in a STEM. By high-temperature annealing a $c$-BN powder, we have promoted phase transitions in nanometric domains which have been detected by the appearance of specific hexagonal-phase signatures in both EELS and CL spectra. A high number of intragap optically active centers are known in $c$-BN, but the literature is rather scattered and hence has been summarized here. For several emission lines we have obtained nano-CL maps which show emission spot sizes as small as few tens of nanometers. Finally, by coupling nano-CL to a Hanbury-Brown-Twiss intensity interferometer, we have addressed individual spots in order to identify the possible presence of single-photon sources. The observed CL bunching effect is compatible with a limited set of single-photon emitters and it permits obtaining emission lifetimes of the order of the nanosecond.

This is a joint publication with the paper by Huang and Liu [Phys. Rev. Lett. 121, 126401 (2018)]. In this work, we propose the spin Bott index to identify the quantum spin Hall (QSH) state in both crystalline and nonperiodic systems. The applicability of the spin Bott index is confirmed by analyzing the periodic and disorder Kane-Mele models. As an example of nonperiodic systems, we systematically investigate the QSH effect in a Penrose-type quasicrystal lattice (QL). We characterize the nontrivial electronic topology of the QL by directly calculating the spin Bott index. In addition, the topological edge states, the localization of wave functions, and quantized transport signatures are also studied in detail.

Surface plasmons dominate the optical response of metal surfaces, and their nature is controlled by surface geometry. Here we study metasurfaces containing singularities in the form of sharp edges and characterized by three quantum numbers despite the two-dimensional nature of the surface. We explore the nature of the plasmonic excitations, their ability to generate large concentrations of optical energy, and the transition from the discrete excitation spectrum of a nonsingular surface to the continuous spectrum of a singular metasurface.

Using large-scale quantum Monte Carlo simulations, we exactly solve a model of fermions hopping on the honeycomb lattice with cluster charge interactions, which has been proposed as an effective model with possible application to twisted bilayer graphene near half-filling. We find an interaction driven semimetal to insulator transition to an insulating phase consisting of a valence bond solid with Kekulé pattern. Finite size scaling reveals that the phase transition of the semimetal to Kekulé valence bond solid phase is continuous and belongs to chiral $XY$ universality class.

We present a simple model of a domain wall in a thin-film ferromagnet. A domain wall is represented as a nonreciprocal string, on which transverse waves propagate with different speeds in opposite directions. The model has three parameters: mass density, tension, and a gyroscopic constant quantifying the nonreciprocity. We discuss the unusual dynamics of a nonreciprocal string in finite geometry. It agrees well with numerically simulated motion of a ferromagnetic domain wall in a strip of constant width.

We consider (2+1)-dimensional lattice models of interacting bosons or spins, with both magnetic flux and fractional spin in the unit cell. We propose and prove a modified Lieb-Schultz Mattis theorem in this setting, which applies even when the spin in the enlarged magnetic unit cell is integral. There are two nontrivial outcomes for gapped ground states that preserve all symmetries. In the first case, exotic bulk excitations, i.e., topological order, is necessarily present even though the enlarged unit cell contains integer spin. In the second case, topological order can be avoided but then a symmetry protected topological (SPT) phase is necessarily realized. The resulting SPTs display a dyonic character in that they associate spin/charge with symmetry flux, allowing the flux in the unit cell to screen the fractional spin/charge on the sites. We provide an explicit formula that encapsulates this physics, which identifies a specific set of allowed SPT phases. This also provides a route to constructing models of SPT states by decorating dimer models of Mott insulators, which should be useful in their physical realization.