Elemental mapping using energy-dispersive x-ray spectroscopy in scanning transmission electron microscopy, a well-established technique for precision elemental concentration analysis at submicron resolution, was first demonstrated at atomic resolution in 2010. However, to date atomic resolution elemental maps have only been interpreted qualitatively because the elastic and thermal scattering of the electron probe confounds quantitative analysis. Accounting for this scattering, we present absolute scale quantitative comparisons between experiment and quantum mechanical calculations for both energy dispersive x-ray and electron energy-loss spectroscopy using off-axis reference measurements. The relative merits of removing the scattering effects from the experimental data against comparison with direct simulations are explored.

It has been widely accepted that planar boron structures, composed of triangular and hexagonal motifs are the most stable two-dimensional (2D) phases and likely precursors for boron nanostructures. Here we predict, based on an ab initio evolutionary structure search, a novel 2D boron structure with nonzero thickness, which is considerably, by $50\mathrm{meV}/\mathrm{atom}$, lower in energy than the recently proposed $\alpha$-sheet structure and its analogues. In particular, this phase is identified for the first time to have a distorted Dirac cone, after graphene and silicene the third elemental material with massless Dirac fermions. The buckling and coupling between the two sublattices not only enhance the energetic stability, but also are the key factors for the emergence of the distorted Dirac cone.

The crystal structure of layered metal ${\mathrm{IrTe}}_2$ is determined using single-crystal x-ray diffraction. At $T=220\mathrm{K}$, it exhibits Ir and Te dimers forming a valence-bond crystal. Electronic structure calculations reveal an intriguing quasi-two-dimensional electronic state, with planes of reduced density of states cutting diagonally through the Ir and Te layers. These planes are formed by the dimers exhibiting a signature of covalent bonding character development. Evidence for significant charge disproportionation among the dimerized and nondimerized Ir (charge order) is presented. We argue that the structural transition is driven by the Ir dimerization and bonding, while electronic correlations (dynamical mean field theory corrections to density functional theory) and spin orbit coupling play a secondary role.

The origin of ferromagnetism in strained epitaxial ${\mathrm{LaCoO}}_3$ films has been a long-standing mystery. Here, we combine atomically resolved $Z$-contrast imaging, electron-energy-loss spectroscopy, and density-functional calculations to demonstrate that, in epitaxial ${\mathrm{LaCoO}}_3$ films, oxygen-vacancy superstructures release strain, control the film’s electronic properties, and produce the observed ferromagnetism via the excess electrons in the Co $d$ states. Although oxygen vacancies typically dope a material $n$-type, we find that ordered vacancies induce Peierls-like minigaps which, combined with strain relaxation, trigger a nonlinear rupture of the energy bands, resulting in insulating behavior.

We determine dominant next-to-next-to-leading order QCD corrections to single-inclusive jet production at the LHC and Tevatron, using the established threshold resummation framework. In contrast to previous literature on this topic, our study incorporates all of the following features: (1) It properly accounts for the way a jet is defined in experiment and treated in available full next-to-leading order calculations, (2) it includes the three leading classes of logarithmic terms in the perturbative expansion, and (3) it is adapted to the full kinematics in jet transverse momentum and rapidity relevant for experiments. A recent full next-to-next-to-leading order calculation in the purely gluonic channel allows us to assess the region where our approximate corrections provide an accurate description. We expect our results to be important on the way to precision jet phenomenology at the LHC and as a benchmark for further full next-to-next-to-leading order calculations.

We present the complete next-to-leading-order (NLO) contributions to the $pp\to e^{+}{\nu }_e{\mu }^{-}{\overline{\nu }}_{\mu }b\overline{b}+X$ process in the four-flavor scheme, i.e., with massive $b$ quarks, and its contribution to the $H\to W{W}^{(*)}\to ll\nu \nu$ measurement in the 1-jet bin at the LHC. This background process includes top pair, single top, and non-top-quark resonant contributions. The uncertainty at NLO from the renormalization and factorization scale dependence is about $+30\%-20\%$. We show that the NLO corrections are relatively small, and that separating this background in top pair, $Wt\text{−}$ and $b$-quark-associated $ll\nu \nu$ production is a fair approximation.

The study of the production of two forward jets with a large interval of rapidity at hadron colliders was proposed by Mueller and Navelet as a possible test of the high energy dynamics of QCD. We analyze this process within a complete next-to-leading logarithm framework, supplemented by the use of the Brodsky-Lepage-Mackenzie procedure extended to the perturbative Regge dynamics, to find the optimal renormalization scale. This leads to a very good description of the recent CMS data at LHC for the azimuthal correlations of the jets.

We show that the statistics of fluctuation-driven initial-state anisotropies in proton-proton, proton nucleus and nucleus-nucleus collisions is to a large extent universal. We propose a simple parametrization for the probability distribution of the Fourier coefficient ${ϵ}_n$ in harmonic $n$, which is in good agreement with Monte Carlo simulations. Our results provide a simple explanation for the 4-particle cumulant of triangular flow measured in Pb-Pb collisions and for the 4-particle cumulant of elliptic flow recently measured in $p\text{−}\mathrm{Pb}$ collisions. Both arise as natural consequences of the condition that initial anisotropies are bounded by unity. We argue that the initial rms anisotropy in harmonic $n$ can be directly extracted from the measured ratio $v_n\{4\}/v_n\{2\}$: this gives direct access to a property of the initial density profile from experimental data. We also make quantitative predictions for the small lifting of degeneracy between $v_n\{4\}$, $v_n\{6\}$, and $v_n\{8\}$. If confirmed by future experiments, they will support the picture that long-range correlations observed in $p\text{−}\mathrm{Pb}$ collisions at the LHC originate from collective flow proportional to the initial anisotropy.

We report on electric-field and temperature-dependent transport measurements in exfoliated thin crystals of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ topological insulator. At low temperatures ($<50\mathrm{K}$) and when the chemical potential lies inside the bulk gap, the crystal resistivity is strongly temperature dependent, reflecting inelastic scattering due to the thermal activation of optical phonons. A linear increase of the current with voltage is obtained up to a threshold value at which current saturation takes place. We show that the activated behavior, the voltage threshold, and the saturation current can all be quantitatively explained by considering a single optical-phonon mode with energy $\hslash \mathrm{\Omega }\approx 8\mathrm{meV}$. This phonon mode strongly interacts with the surface states of the material and represents the dominant source of scattering at the surface at high electric fields.

Resonant photoemission spectroscopy has been used to investigate the spin-dependent electron transport from core-excited argon monolayers to ferromagnetic substrates with subfemtosecond precision. Charge transfer times as a function of the spin of the resonantly excited electron and the magnetization of the substrate have been studied for $\mathrm{Ar}/\mathrm{Fe}(110)$, $\mathrm{Ar}/\mathrm{Co}(0001)$, and $\mathrm{Ar}/\mathrm{Ni}(111)$. For argon adsorbed on iron and cobalt, a significantly faster charge transfer is found for minority electrons, whereas no such spin dependence is obtained for nickel. The results are explained considering the different empty densities of states of the metals for minority and majority electrons.

We show that the Rashba effect at the polar perovskite surfaces and interfaces can be tuned by manipulating the two-dimensional electron gas by an applied electric field, using it to draw the two-dimensional electron gas out to the surface or push it deeper into the bulk, thereby controlling the surface-sensitive phenomenon. These ideas are illustrated by a comprehensive density-functional study of the recently discovered polar ${\mathrm{KTaO}}_3$ surface. Analytical results obtained with a tight-binding model unravel the interplay between the various factors affecting the Rashba effect such as the strengths of the spin-orbit interaction and the surface-induced asymmetry. Our work helps interpret the recent experiments on the ${\mathrm{KTaO}}_3$ surface as well as the ${\mathrm{SrTiO}}_3/{\mathrm{LaAlO}}_3$ interface.

We demonstrate via a muon spin rotation experiment that the electronic ground state of the iridium spinel compound ${\mathrm{CuIr}}_2{\mathrm{S}}_4$ is not the presumed spin-singlet state but a novel paramagnetic state, showing a quasistatic spin glasslike magnetism below $\sim 100\mathrm{K}$. Considering the earlier indication that ${\mathrm{IrS}}_6$ octahedra exhibit dimerization associated with the metal-to-insulator transition below 230 K, the present result suggests that a strong spin-orbit interaction may be playing an important role in determining the ground state that accompanies magnetic frustration.

The quantum spin-liquid compound $({\mathrm{C}}_4{\mathrm{H}}_{12}{\mathrm{N}}_2){\mathrm{Cu}}_2{\mathrm{Cl}}_6$ is studied by muon spin relaxation under hydrostatic pressures up to 23.6 kbar. At low temperatures, pressure-induced incommensurate magnetic order is detected beyond a quantum critical point at $P_c\sim 4.3\mathrm{kbar}$. An additional phase transition to a different ordered phase is observed at $P_1\sim 13.4\mathrm{kbar}$. The data indicate that the high-pressure phase may be a commensurate one. The established ($P\text{−}T$) phase diagram reveals the corresponding pressure-induced multicritical point at $P_1$, $T_1=2.0\mathrm{K}$.

We investigate the consequences of broken translational symmetry in the superconductor ${\mathrm{FeSe}}_x{\mathrm{Te}}_{1-x}$ using angle-resolved photoemission spectroscopy. We find that the intensity does not follow the periodicity dictated by the crystal structure, owing to the form of the perturbing potential and the symmetries of the Fe $d$ orbitals. Their interplay leads to substantial differences in the orbital character and spectral features observed at nominally equivalent locations in the reciprocal space. Such differences cannot be accounted for by the usual dipole matrix element effects and are due instead to the structure factor, which must be explicitly considered whenever more than one atom is present in the unit cell.

The bulk electronic structure of ${\mathrm{NiS}}_{2-x}{\mathrm{Se}}_x$ has been studied across the bandwidth-control Mott transition (BCMT) using soft x-ray angle-resolved photoemission spectroscopy. We show that Se doping does not alter the Fermi surface volume. When approaching the insulating phase with decreasing Se concentration, we observed that the Fermi velocity continuously decreases. Meanwhile, the weight of the coherent quasiparticle, which sits on a large incoherent spectrum, continuously decreases and is transferred to higher binding energies, until it suddenly disappears across the Mott transition. In the insulating phase, there is still finite spectral weight at the Fermi energy, but it is incoherent and dispersionless due to strong correlations. Our results provide a direct observation of the distinct characters of BCMT in a multiband non-half-filled system.

High-resolution x-ray imaging techniques offer a variety of possibilities for studying the nanoscale structure of biological cells. A challenging task remains the study of cells by x rays in their natural, aqueous environment. Here, we overcome this limitation by presenting scanning x-ray diffraction measurements with beam sizes in the range of a few hundred nm on living and fixed-hydrated eukaryotic cells in microfluidic devices which mimic a native environment. The direct comparison between fixed-hydrated and living cells shows distinct differences in the scattering signal, pointing to structural changes on the order of 30 to 50 nm.

The viscoelastic response of complex fluids is length- and time-scale dependent, encoding information on intrinsic dynamic correlations and mesoscopic structure. We study the length scale above which bulk viscoelasticity sets in, and the material response that precedes it at shorter distances. We show that the crossover between these two regimes may appear at a surprisingly large distance. We generalize the framework of microrheology to include both regimes and apply it to F-actin networks, thereby extracting their dynamic correlation length from their bulk and local viscoelastic properties.

We study a new production mechanism for heavy neutrinos at the LHC, which dominates over the usually considered $s$-channel $W$-exchange diagram for heavy-neutrino masses larger than 100–200 GeV. The new mechanism is infrared enhanced by $t$-channel $W\gamma$-fusion processes. This has important implications for experimental tests of the seesaw mechanism of neutrino masses and, in particular, for the ongoing heavy neutrino searches at the LHC. We find that the direct collider limits on the light-to-heavy neutrino mixing can be significantly improved when this new production channel is properly taken into account. The scope of this new mechanism can be equally well extended to other exotic searches at the LHC.