We study the Hubbard model on the frustrated honeycomb lattice with nearest-neighbor hopping $t_1$ and second nearest-neighbor hopping $t_2$, which is isomorphic to the bilayer triangle lattice, using the SU(2)-invariant slave boson theory. We show that the Coulomb interaction $U$ induces antiferromagnetic (AF) chiral spin density wave ($\chi \mathrm{SDW}$) order in a wide range of $\kappa =t_2/t_1$ where both the two-sublattice AF order at small $\kappa$ and the decoupled three-sublattice 120° order at large $\kappa$ are strongly frustrated, leading to three distinct phases with different anomalous Hall responses. We find a continuous transition from a $\chi \mathrm{SDW}$ semimetal with the anomalous Hall effect to a topological chiral Chern insulator exhibiting the quantum anomalous Hall effect, followed by a discontinuous transition to a $\chi \mathrm{SDW}$ insulator with a zero total Chern number but an anomalous ac Hall effect. The $\chi \mathrm{SDW}$ is likely a generic phase of strongly correlated and highly frustrated hexagonal lattice electrons.

We report on the dramatic slowing down of the charge carrier dynamics in a quasi-two-dimensional organic conductor, which can be reversibly tuned through the Mott metal-insulator transition (MIT). At the finite-temperature critical end point, we observe a divergent increase of the resistance fluctuations accompanied by a drastic shift of spectral weight to low frequencies, demonstrating the critical slowing down of the order parameter (doublon density) fluctuations. The slow dynamics is accompanied by non-Gaussian fluctuations, indicative of correlated charge carrier dynamics. A possible explanation is a glassy freezing of the electronic system as a precursor of the Mott MIT.

We derive a general scaling relation for the anomalous Hall effect in ferromagnetic metals involving multiple competing scattering mechanisms, described by a quadratic hypersurface in the space spanned by the partial resistivities. We also present experimental findings, which show strong deviation from previously found scaling forms when different scattering mechanisms compete in strength but can be nicely explained by our theory.

Entanglement between a single photon and a quantum memory forms the building blocks for a quantum repeater and quantum network. Previous entanglement sources are typically with low retrieval efficiency, which limits future larger-scale applications. Here, we report a source of highly retrievable spin-wave–photon entanglement. Polarization entanglement is created through interaction of a single photon with an ensemble of atoms inside a low-finesse ring cavity. The cavity is engineered to be resonant for dual spin-wave modes, which thus enables efficient retrieval of the spin-wave qubit. An intrinsic retrieval efficiency up to 76(4)% has been observed. Such a highly retrievable atom-photon entanglement source will be very useful in future larger-scale quantum repeater and quantum network applications.

The Galactic 1.809-MeV $\gamma$-ray signature from the $\beta$ decay of ${}^{26g}\mathrm{Al}$ is a dominant target of $\gamma$-ray astronomy, of which a significant component is understood to originate from massive stars. The ${}^{26g}\mathrm{Al}(p,\gamma ){}^{27}\mathrm{Si}$ reaction is a major destruction pathway for ${}^{26g}\mathrm{Al}$ at stellar temperatures, but the reaction rate is poorly constrained due to uncertainties in the strengths of low-lying resonances in ${}^{27}\mathrm{Si}$. The ${}^{26g}\mathrm{Al}(d,p){}^{27}\mathrm{Al}$ reaction has been employed in inverse kinematics to determine the spectroscopic factors, and hence resonance strengths, of proton resonances in ${}^{27}\mathrm{Si}$ via mirror symmetry. The strength of the 127-keV resonance is found to be a factor of 4 higher than the previously adopted upper limit, and the upper limit for the 68-keV resonance has been reduced by an order of magnitude, considerably constraining the ${}^{26g}\mathrm{Al}$ destruction rate at stellar temperatures.

We observe distinct regimes of orbital angular momentum (OAM) transfer from two-dimensional Bessel-shaped acoustic vortices to matter. In a homogeneous diphasic mixture of microparticles and water, slow swirling about the vortex axis is seen. This effect is driven by the absorption of OAM across the mixture, the motion following the OAM density distribution. Larger particles are formed into clusters by the acoustic radiation force, making the mixture nonhomogeneous. Here, the OAM transfer to the microparticle clusters dominates and they spin at high speeds entraining the surrounding fluid.

Cu substrates are highly preferred for the potential mass production of high-quality graphene, yet many of the important aspects of the atomistic growth mechanisms involved remain to be explored. Using multiscale modeling, we identify C-C dimers as the dominant feeding species in the epitaxial growth of graphene on both Cu(111) and Cu(100) substrates. By contrasting the different activation energies involved in C-C dimer diffusion on terraces and its attachment at graphene island edges, we further reveal why graphene growth is diffusion limited on Cu(111), but attachment limited on Cu(100). We also find even higher potential energy barriers against dimer diffusion along the island edges; consequently, a dendritic-to-compact transition is predicted to take place during graphene enlargement on either substrate, but at different growth temperatures. These findings serve as new insights for better control of epitaxial graphene growth.

Anderson localization, the absence of diffusive transport in disordered systems, has been manifested as hopping transport in numerous electronic systems, whereas in recently discovered topological insulators it has not been directly observed. Here, we report experimental demonstration of a crossover from diffusive transport in the weak antilocalization regime to variable range hopping transport in the Anderson localization regime with ultrathin $({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x{)}_2{\mathrm{Te}}_3$ films. As disorder becomes stronger, negative magnetoconductivity due to the weak antilocalization is gradually suppressed, and eventually, positive magnetoconductivity emerges when the electron system becomes strongly localized. This work reveals the critical role of disorder in the quantum transport properties of ultrathin topological insulator films, in which theories have predicted rich physics related to topological phase transitions.

We used resonant inelastic x-ray scattering (RIXS) with and without analysis of the scattered photon polarization, to study dispersive spin excitations in the high temperature superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ over a wide range of doping levels ($0.1\le x\le 1$). The excitation profiles were carefully monitored as the incident photon energy was detuned from the resonant condition, and the spin excitation energy was found to be independent of detuning for all $x$. These findings demonstrate that the largest fraction of the spin-flip RIXS profiles in doped cuprates arises from magnetic collective modes, rather than from incoherent particle-hole excitations as recently suggested theoretically [Benjamin et al. Phys. Rev. Lett. 112, 247002 (2014)]. Implications for the theoretical description of the electron system in the cuprates are discussed.

We analyze under which dynamical conditions the coherence of an open quantum system is totally unaffected by noise. For a single qubit, specific measures of coherence are found to freeze under different conditions, with no general agreement between them. Conversely, for an $N$-qubit system with even $N$, we identify universal conditions in terms of initial states and local incoherent channels such that all bona fide distance-based coherence monotones are left invariant during the entire evolution. This finding also provides an insightful physical interpretation for the freezing phenomenon of quantum correlations beyond entanglement. We further obtain analytical results for distance-based measures of coherence in two-qubit states with maximally mixed marginals.

The Wigner little group for massless particles is isomorphic to the Euclidean group SE(2). Applied to momentum eigenstates, or to infinite plane waves, the Euclidean “Wigner translations” act as the identity. We show that when applied to finite wave packets, the translation generators move the packet trajectory parallel to itself through a distance proportional to the particle’s helicity. We relate this effect to the spin Hall effect of light and to the Lorentz-frame dependence of the position of a massless spinning particle.

Weak values and measurements have been proposed as a means to achieve dramatic enhancements in metrology based on the greatly increased range of possible measurement outcomes. Unfortunately, the very large values of measurement outcomes occur with highly suppressed probabilities. This raises three vital questions in weak-measurement-based metrology. Namely, (Q1) Does postselection enhance the measurement precision? (Q2) Does weak measurement offer better precision than strong measurement? (Q3) Is it possible to beat the standard quantum limit or to achieve the Heisenberg limit with weak measurement using only classical resources? We analyze these questions for two prototypical, and generic, measurement protocols and show that while the answers to the first two questions are negative for both protocols, the answer to the last is affirmative for measurements with phase-space interactions, and negative for configuration space interactions. Our results, particularly the ability of weak measurements to perform at par with strong measurements in some cases, are instructive for the design of weak-measurement-based protocols for quantum metrology.

We introduce a new class of scalar-tensor theories of gravity that extend Horndeski, or “generalized Galileon,” models. Despite possessing equations of motion of higher order in derivatives, we show that the true propagating degrees of freedom obey well-behaved second-order equations and are thus free from Ostrogradski instabilities, in contrast to standard lore. Remarkably, the covariant versions of the original Galileon Lagrangians—obtained by direct replacement of derivatives with covariant derivatives—belong to this class of theories. These extensions of Horndeski theories exhibit an uncommon, interesting phenomenology: The scalar degree of freedom affects the speed of sound of matter, even when the latter is minimally coupled to gravity.

We develop a formalism that allows one to systematically calculate the weakly interacting massive particle (WIMP) annihilation rate into gamma rays whose energy far exceeds the weak scale. A factorization theorem is presented which separates the radiative corrections stemming from initial-state potential interactions from loops involving the final state. This separation allows us to go beyond the fixed order calculation, which is polluted by large infrared logarithms. For the case of Majorana WIMPs transforming in the adjoint representation of SU(2), we present the result for the resummed rate at leading double-log accuracy in terms of two initial-state partial-wave matrix elements and one hard matching coefficient. For a given model, one may calculate the cross section by finding the tree level matching coefficient and determining the value of a local four-fermion operator. The effects of resummation can be as large as 100% for a 20 TeV WIMP. However, for lighter WIMP masses relevant for the thermal relic scenario, leading-log resummation modifies the Sudakov factors only at the 10% level. Furthermore, given comparably sized Sommerfeld factors, the total effect of radiative corrections on the semi-inclusive photon annihilation rate is found to be percent level. The generalization of the formalism to other types of WIMPs is discussed.

We formulate an effective field theory description for $\mathrm{SU}(2{)}_L$ triplet fermionic dark matter by combining nonrelativistic dark matter with gauge bosons in the soft-collinear effective theory. For a given dark matter mass, the annihilation cross section to line photons is obtained with 5% precision by simultaneously including Sommerfeld enhancement and the resummation of electroweak Sudakov logarithms at next-to-leading logarithmic order. Using these results, we present more accurate and precise predictions for the gamma-ray line signal from annihilation, updating both existing constraints and the reach of future experiments.