Controlling the parameters of a laser plasma accelerated electron beam is a topic of intense research with a particular focus placed on controlling the injection phase of electrons into the accelerating structure from the background plasma. An essential prerequisite for high-quality beams is dark-current free acceleration (i.e., no electrons accelerated beyond those deliberately injected). We show that small-scale density ripples in the background plasma are sufficient to cause the uncontrolled (self-)injection of electrons. Such ripples can be as short as $\sim 50\mu \mathrm{m}$ and can therefore not be resolved by standard interferometry. Background free injection with substantially improved beam characteristics (divergence and pointing) is demonstrated in a gas cell designed for a controlled gas flow. The results are supported by an analytical theory as well as 3D particle in cell simulations.

Valley polarized topological kink states, existing broadly in the domain wall of hexagonal lattice systems, are identified in experiments. Unfortunately, only very limited physical properties are given. Using an Aharanov-Bohm interferometer composed of domain walls in graphene systems, we study the periodical modulation of a pure valley current in a large range by tuning the magnetic field or the Fermi level. For a monolayer graphene device, there exists one topological kink state, and the oscillation of the transmission coefficients has a single period. The $\pi$ Berry phase and the linear dispersion relation of kink states can be extracted from the transmission data. For a bilayer graphene device, there are two topological kink states with two oscillation periods. Our proposal provides an experimentally feasible route to manipulate and characterize the valley-polarized topological kink states in classical wave and electronic graphene-type crystalline systems.

We study $S=1$ quantum spin systems on the infinite chain with short ranged Hamiltonians that have a certain rotational and discrete symmetry. We define a ${\mathbb{Z}}_2$ index for any gapped unique ground state, and prove that it is invariant under a smooth deformation. By using the index, we provide the first rigorous proof of the existence of a “topological” phase transition, which cannot be characterized by any conventional order parameters, between the Affleck-Kennedy-Lieb-Tasaki (AKLT) model and trivial models. This rigorously establishes that the AKLT model is in a nontrivial symmetry protected topological phase.

This Letter presents the results of a search for pair-produced particles of masses above 100 GeV that each decay into at least four quarks. Using data collected by the CMS experiment at the LHC in 2015–2016, corresponding to an integrated luminosity of $38.2{\mathrm{fb}}^{-1}$, reconstructed particles are clustered into two large jets of similar mass, each consistent with four-parton substructure. No statistically significant excess of data over the background prediction is observed in the distribution of average jet mass. Pair-produced squarks with dominant hadronic $R$-parity-violating decays into four quarks and with masses between 0.10 and 0.72 TeV are excluded at 95% confidence level. Similarly, pair-produced gluinos that decay into five quarks are also excluded with masses between 0.10 and 1.41 TeV at 95% confidence level. These are the first constraints that have been placed on pair-produced particles with masses below 400 GeV that decay into four or five quarks, bridging a significant gap in the coverage of $R$-parity-violating supersymmetry parameter space.

We investigate the harmonic generation induced by the interaction of a laser field with a solid target. The harmonic spectra is composed of the contribution of two processes interpreted as interband and intraband transitions. The interband process corresponds to the recombination from an upper band, populated during the laser interaction, to a lower band. The intraband process originates from nonlinear processes of the current in individual bands. In this Letter, we develop a theory based on Wannier states and reveal in depth the underlying physics of intraband dynamics. In particular, this approach highlights the determinant role of transitions between different lattice wells. Furthermore, our approach provides quantitative predictions concerning high-order harmonic energy cutoffs, harmonic yields, and emission times.

We consider the deformation of a thin elastic sheet which is stiff in traction but very soft in compression, as happens in the presence of wrinkling. We use the tension-field material model and explore numerically the response of a thin sheet containing multiple cracks of different geometries, when subjected to applied tension. With a single crack, the stress concentrates along a St. Andrew’s cross-shaped pattern, whose branches extend from the crack tips to the corners of the domain; at a (small) distance $r$ from the crack tip, the stress displays the usual $r^{-1/2}$ stress singularity but with an unusual and nonuniversal angular dependence. A strong interaction between multiple cracks is reported and discussed: in particular, for certain configurations of the cracks, the tensile stiffness of a cracked sheet can be zero even though the sheet is made up of a single component.

We present a new magnetic field generation mechanism in underdense plasmas driven by the beating of two, copropagating, Laguerre-Gaussian orbital angular momentum laser pulses with different frequencies and also different twist indices. The resulting twisted ponderomotive force drives up an electron plasma wave with a helical rotating structure. To second order, there is a nonlinear rotating current leading to the onset of an intense, static axial magnetic field, which persists over a long time in the plasma (ps scale) after the laser pulses have passed by. The results are confirmed in three-dimensional particle-in-cell simulations and also theoretical analysis. For the case of 300 fs duration, $3.8\times {10}^{17}\mathrm{W}/{\mathrm{cm}}^2$ peak laser intensity we observe magnetic field of up to 0.4 MG. This new method of magnetic field creation may find applications in charged beam collimation and microscale pinch.

Carbon (C) doping is essential for producing semi-insulating GaN for power electronics. However, to date the nature of C doped GaN, especially the lattice site occupation, is not yet well understood. In this work, we clarify the lattice site of C in GaN using polarized Fourier-transform infrared and Raman spectroscopies, in combination with first-principles calculations. Two local vibrational modes (LVMs) at 766 and $774{\mathrm{cm}}^{-1}$ in C doped GaN are observed. The $766{\mathrm{cm}}^{-1}$ mode is assigned to the nondegenerate $A_1$ mode vibrating along the $\mathit{c}$ axis, whereas the $774{\mathrm{cm}}^{-1}$ mode is ascribed to the doubly degenerate $E$ mode confined in the plane perpendicular to the $\mathit{c}$ axis. The two LVMs are identified to originate from isolated ${\mathrm{C}}_{\mathrm{N}}^{-}$ with local $C_{3v}$ symmetry. Experimental data and calculations are in outstanding agreement both for the positions and the intensity ratios of the LVMs. We thus provide unambiguous evidence of the substitutional C atoms occupying the N site with a $-1$ charge state in GaN and therefore bring essential information to a long-standing controversy.

We performed terahertz time-domain spectroscopy for methylammonium (MA) lead halide perovskite single crystals and characterized the longitudinal optical (LO) phonons directly. We found that the effective LO phonon wave number does not change in the wide temperature range between 10 and 300 K. However, the coupling between MA cation modes and the LO phonon mode derived from lead halide cages induces a mode splitting at low temperatures and a damping of the LO phonon mode at high temperatures. These results influence the interpretation of electron-LO phonon interactions in perovskite semiconductors, as well as the interpretations of mobility, carrier diffusion, and polaron formation.

Electrolyte gating is a powerful means for tuning the carrier density and exploring the resultant modulation of novel properties on solid surfaces. However, the mechanism, especially its effect on the oxygen migration and electrostatic charging at the oxide heterostructures, is still unclear. Here we explore the electrolyte gating on oxygen-deficient interfaces between ${\mathrm{SrTiO}}_3$ (STO) crystals and ${\mathrm{LaAlO}}_3$ (LAO) overlayer through the measurements of electrical transport, x-ray absorption spectroscopy, and photoluminescence spectra. We found that oxygen vacancies (${\mathrm{O}}_{\mathrm{vac}}$) were filled selectively and irreversibly after gating due to oxygen electromigration at the amorphous LAO/STO interface, resulting in a reconstruction of its interfacial band structure. Because of the filling of ${\mathrm{O}}_{\mathrm{vac}}$, the amorphous interface also showed an enhanced electron mobility and quantum oscillation of the conductance. Further, the filling effect could be controlled by the degree of the crystallinity of the LAO overlayer by varying the growth temperatures. Our results reveal the different effects induced by electrolyte gating, providing further clues to understand the mechanism of electrolyte gating on buried interfaces and also opening a new avenue for constructing high-mobility oxide interfaces.

The numerical renormalization group is used to study quantum entanglement in the Kondo impurity model with a density of states $\rho (ϵ)\propto |ϵ{|}^r$ ($0<r<\frac{1}{2}$) that vanishes at the Fermi energy $ϵ=0$. This nonintegrable model features a Kondo-destruction quantum critical point (QCP) separating a partially screened phase from a local-moment phase. The impurity contribution $S_e^{\mathrm{imp}}$ to the entanglement entropy between a region of radius $R$ around the magnetic impurity and the rest of the system reveals a length scale $R^{*}$ that distinguishes a region $R\ll R^{*}$ of strong critical entanglement from one $R\gg R^{*}$ of weak entanglement. Within each phase, $S_e^{\mathrm{imp}}$ is a universal function of $R/R^{*}$ with a power-law decay for $R/R^{*}\gg 1$. The entanglement length $R^{*}$ diverges on approach to the interacting QCP, showing that the critical Kondo screening cloud subsumes the entire system as the impurity becomes maximally entangled with the conduction band. This work has implications for entanglement calculations in other models and for the nature of heavy-fermion quantum criticality.

This Letter presents the observation of the rare $Z$ boson decay $Z\to \psi {\ell }^{+}{\ell }^{-}$. Here, $\psi$ represents contributions from direct $J/\psi$ and $\psi (2S)\to J/\psi X$, ${\ell }^{+}{\ell }^{-}$ is a pair of electrons or muons, and the $J/\psi$ meson is detected via its decay to ${\mu }^{+}{\mu }^{-}$. The sample of proton-proton collision data, collected by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, corresponds to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. The signal is observed with a significance in excess of 5 standard deviations. After subtraction of the $\psi (2S)\to J/\psi X$ contribution, the ratio of the branching fraction of the exclusive decay $Z\to J/\psi {\ell }^{+}{\ell }^{-}$ to the decay $Z\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ within a fiducial phase space is measured to be $\mathcal{B}(Z\to J/\psi {\ell }^{+}{\ell }^{-})/\mathcal{B}(Z\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-})=0.67\pm 0.18(\text{stat})\pm 0.05(\text{syst})$.

We propose a method to subtract a photon from a double sideband mode of continuous-wave light. The central idea is to use phase modulation as a frequency sideband beam splitter in the heralding photon subtraction scheme, where a small portion of the sideband mode is down-converted to 0 Hz to provide a trigger photon. An optical cat state is created by applying the proposed method to a squeezed state at 500 MHz sideband, which is generated by an optical parametric oscillator. The Wigner function of the cat state reconstructed from a direct homodyne measurement of the 500 MHz sideband modes shows the negativity of $W(0,0)=-0.088\pm 0.001$ without any loss corrections.

The recent observation of the reverse Hall-Petch relation in nanocrystalline ceramics offers a possible pathway to achieve enhanced ductility for traditional brittle ceramics via the nanosize effect, just as nanocrystalline metals and alloys. However, the underlying deformation mechanisms of nanocrystalline ceramics have not been well established. Here we combine reactive molecular dynamics (RMD) simulations and experimental transmission electron microscopy to determine the atomic level deformation mechanisms of nanocrystalline boron carbide (${\mathrm{B}}_4\mathrm{C}$). We performed large-scale (up to $\sim 3700000\mathrm{atoms}$) ReaxFF RMD simulations on finite shear deformation of three models of grain boundaries with grain sizes from 4.84 (135 050 atoms) to 14.64 nm (3 702 861 atoms). We found a reverse Hall-Petch relationship in nanocrystalline ${\mathrm{B}}_4\mathrm{C}$ in which the deformation mechanism is dominated by the grain boundary (GB) sliding. This GB sliding leads to the amorphous band formation at predistorted icosahedral GB regions with initiation of cavitation within the amorphous bands. Our simulation results are validated by the experimental observations of an intergranular amorphous GB phase due to GBs sliding under indentation experiments. These theoretical and experimental results provide an atomistic explanation for the influence of GBs on the deformation behavior of nanocrystalline ceramics, explaining the reverse Hall-Petch relation.

We report experimental results on the composition and crystallography of oxides formed on NiCrMo alloys during both high-temperature oxidation and aqueous corrosion experiments. Detailed characterization using transmission electron microscopy and diffraction, aberration-corrected chemical analysis, and atom probe tomography shows unexpected combinations of composition and crystallography, far outside thermodynamic solubility limits. The results are explained using a theory for nonequilibrium solute capture that combines thermodynamic, kinetic, and density functional theory analyses. In this predictive nonequilibrium framework, the composition and crystallography are controlled by the rapidly moving interface. The theoretical framework explains the unusual combinations of composition and crystallography, which we predict will be common for many other systems in oxidation and corrosion, and other solid-state processes involving nonequilibrium moving interfaces.

For a class of 2D hybrid organic-inorganic perovskite semiconductors based on $\pi$-conjugated organic cations, we predict quantitatively how varying the organic and inorganic component allows control over the nature, energy, and localization of carrier states in a quantum-well-like fashion. Our first-principles predictions, based on large-scale hybrid density-functional theory with spin-orbit coupling, show that the interface between the organic and inorganic parts within a single hybrid can be modulated systematically, enabling us to select between different type-I and type-II energy level alignments. Energy levels, recombination properties, and transport behavior of electrons and holes thus become tunable by choosing specific organic functionalizations and juxtaposing them with suitable inorganic components.

We address the problem of the minus sign sampling for two-electron systems using the path integral approach. We show that this problem can be reexpressed as one of computing free energy differences and sampling the tails of statistical distributions. Using metadynamics, a realistic problem like that of two electrons confined in a quantum dot can be solved. We believe that this is a strategy that can possibly be extended to more complex systems.

The emergence of nonequilibrium phenomena in individual complex wave systems has long been of fundamental interest. Its analytic studies remain notoriously difficult. Using the mathematical tool of the concentration of measure, we develop a theory for structures and fluctuations of waves in individual disordered media. We find that, for both diffusive and localized waves, fluctuations associated with the change in incoming waves (“wave-to-wave” fluctuations) exhibit a new kind of universality, which does not exist in conventional mesoscopic fluctuations associated with the change in disorder realizations (“sample-to-sample” fluctuations), and originates from the coherence between the natural channels of waves—the transmission eigenchannels. Using the results obtained for wave-to-wave fluctuations, we find the criterion for almost all stationary scattering states to exhibit the same spatial structure such as the diffusive steady state. We further show that the expectations of observables at stationary scattering states are independent of incoming waves and are given by their averages with respect to eigenchannels. This suggests the possibility of extending the studies of thermalization of closed systems to open systems, which provides new perspectives for the emergence of nonequilibrium statistical phenomena.