We present angle-resolved photoemission studies on the rare-earth-hexaboride ${\mathrm{YbB}}_6$, which has recently been predicted to be a topological Kondo insulator. Our data do not agree with the prediction and instead show that ${\mathrm{YbB}}_6$ exhibits a novel topological insulator state in the absence of a Kondo mechanism. We find that the Fermi level electronic structure of ${\mathrm{YbB}}_6$ has three 2D Dirac cone like surface states enclosing the Kramers’s points, while the $f$ orbital that would be relevant for the Kondo mechanism is $\sim 1\mathrm{eV}$ below the Fermi level. Our first-principles calculation shows that the topological state that we observe in ${\mathrm{YbB}}_6$ is due to an inversion between Yb $d$ and B $p$ bands. These experimental and theoretical results provide a new approach for realizing novel correlated topological insulator states in rare-earth materials.

We have developed methods for acquiring temporally and spatially resolved spectrograms of the velocity of sliding charge-density waves (CDWs), allowing unprecedented access to CDW dynamics. Complex transients arising from the interplay between elastic and plastic processes occur when the driving field direction is reversed. A transient spectral component due to shear elasticity can be unambiguously identified, and allows the most direct determination to date of the CDW’s shear elastic modulus. Near current contacts, initially elastic displacements are followed by an elastic-to-plastic transition. A simple model provides a semiquantitative account of many aspects of these transients.

We report accurate energetics of defects introduced in GaN on doping with divalent metals, focusing on the technologically important case of Mg doping, using a model that takes into consideration both the effect of hole localization and dipolar polarization of the host material, and includes a well-defined reference level. Defect formation and ionization energies show that divalent dopants are counterbalanced in GaN by nitrogen vacancies and not by holes, which explains both the difficulty in achieving $p$-type conductivity in GaN and the associated major spectroscopic features, including the ubiquitous 3.46 eV photoluminescence line, a characteristic of all lightly divalent-metal-doped GaN materials that has also been shown to occur in pure GaN samples. Our results give a comprehensive explanation for the observed behavior of GaN doped with low concentrations of divalent metals in good agreement with relevant experiment.

Scanning tunneling microscopy has developed into a powerful tool for the characterization of conductive surfaces, for which the overlap of tip and sample wave functions determines the image contrast. On insulating layers, as the CaO thin film grown on Mo(001) investigated here, direct overlap between initial and final states is not enabled anymore and electrons are transported via hopping through the conduction-band states of the oxide. Carrier transport is accompanied by strong phonon excitations in this case, imprinting an oscillatory signature on the differential conductance spectra of the system. The phonons show a characteristic spatial dependence and become softer around lattice irregularities in the oxide film, such as dislocation lines.

Majorana fermions have been intensively studied in recent years for their importance to both fundamental science and potential applications in topological quantum computing. They are predicted to exist in a vortex core of superconducting topological insulators. However, it is extremely difficult to distinguish them experimentally from other quasiparticle states for the tiny energy difference between Majorana fermions and these states, which is beyond the energy resolution of most available techniques. Here, we circumvent the problem by systematically investigating the spatial profile of the Majorana mode and the bound quasiparticle states within a vortex in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ films grown on a superconductor ${\mathrm{NbSe}}_2$. While the zero bias peak in local conductance splits right off the vortex center in conventional superconductors, it splits off at a finite distance $\sim 20\mathrm{nm}$ away from the vortex center in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. This unusual splitting behavior has never been observed before and could be possibly due to the Majorana fermion zero mode. While the Majorana mode is destroyed by the interaction between vortices, the zero bias peak splits as a conventional superconductor again. This work provides self-consistent evidences of Majorana fermions and also suggests a possible route to manipulating them.

We use ultrafast optical spectroscopy to observe binding of charged single-particle excitations (SE) in the magnetically frustrated Mott insulator ${\mathrm{Na}}_2{\mathrm{IrO}}_3$. Above the antiferromagnetic ordering temperature ($T_N$) the system response is due to both Hubbard excitons (HE) and their constituent unpaired SE. The SE response becomes strongly suppressed immediately below $T_N$. We argue that this increase in binding energy is due to a unique interplay between the frustrated Kitaev and the weak Heisenberg-type ordering term in the Hamiltonian, mediating an effective interaction between the spin-singlet SE. This interaction grows with distance causing the SE to become trapped in the HE, similar to quark confinement inside hadrons. This binding of charged particles, induced by magnetic ordering, is a result of a confinement-deconfinement transition of spin excitations. This observation provides evidence for spin liquid type behavior which is expected in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$.

We propose and demonstrate an approach to measuring the biphoton temporal wave function with polarization-dependent and time-resolved two-photon interference. Through six sets of two-photon interference measurements projected onto different polarization subspaces, we can reconstruct the amplitude and phase functions of the biphoton temporal waveform. For the first time, we apply this technique to experimentally determine the temporal quantum states of the narrow-band biphotons generated from the spontaneous four-wave mixing in cold atoms.

We present measurements of coherence and successive decay dynamics of higher energy levels of a superconducting transmon qubit. By applying consecutive $\pi$ pulses for each sequential transition frequency, we excite the qubit from the ground state up to its fourth excited level and characterize the decay and coherence of each state. We find the decay to proceed mainly sequentially, with relaxation times in excess of $20\mu \mathrm{s}$ for all transitions. We also provide a direct measurement of the charge dispersion of these levels by analyzing beating patterns in Ramsey fringes. The results demonstrate the feasibility of using higher levels in transmon qubits for encoding quantum information.

We present general expressions for the magnetic transition rates in electron paramagnetic resonance (EPR) experiments of anisotropic spin systems in the solid state. The expressions apply to general spin centers and arbitrary excitation geometry (Voigt, Faraday, and intermediate). They work for linear and circular polarized as well as unpolarized excitation, and for crystals and powders. The expressions are based on the concept of the (complex) magnetic transition dipole moment vector. Using the new theory, we determine the parities of ground and excited spin states of high-spin ($S=5/2$) ${\mathrm{Fe}}^{\mathrm{III}}$ in hemin from the polarization dependence of experimental EPR line intensities.

We advocate a search for an extended scalar sector at the LHC via $hh$ production, where $h$ is the 125 GeV Higgs boson. A resonance feature in the $hh$ invariant mass is a smoking gun of an $s$-channel heavy Higgs resonance, $H$. With one $h$ decaying to two photons and the other decaying to $b$ quarks, the resonant signal may be discoverable above the $hh$ continuum background for $M_H<1\mathrm{TeV}$. The product of the scalar and top Yukawa couplings can be measured to better than 10%–20% accuracy, and its sign can be inferred from the $hh$ line shape via interference effects.

Within the framework of the unrestricted time-dependent density functional theory, we present for the first time an analysis of the relativistic Coulomb excitation of the heavy deformed open shell nucleus ${}^{238}\mathrm{U}$. The approach is based on the superfluid local density approximation formulated on a spatial lattice that can take into account coupling to the continuum, enabling self-consistent studies of superfluid dynamics of any nuclear shape. We compute the energy deposited in the target nucleus as a function of the impact parameter, finding it to be significantly larger than the estimate using the Goldhaber-Teller model. The isovector giant dipole resonance, the dipole pygmy resonance, and giant quadrupole modes are excited during the process. The one-body dissipation of collective dipole modes is shown to lead a damping width ${\mathrm{\Gamma }}_{\downarrow }\approx 0.4\mathrm{MeV}$ and the number of preequilibrium neutrons emitted has been quantified.

We report the first experimental evidence of enhancement of self-amplified spontaneous emission, due to the use of an optical klystron. In this free-electron laser scheme, a relativistic electron beam passes through two undulators, separated by a dispersive section. The latter converts the electron-beam energy modulation produced in the first undulator in density modulation, thus enhancing the free-electron laser gain. The experiment has been carried out at the FERMI facility in Trieste. Powerful radiation has been produced in the extreme ultraviolet range, with an intensity a few orders of magnitude larger than in pure self-amplified spontaneous emission mode. Data have been benchmarked with an existing theoretical model.

The flat-lens concept based on negative refraction proposed by Veselago in 1968 has been mostly investigated in the monochromatic regime. It was recently recognized that time development of the superlensing effect discovered in 2000 by Pendry is yet to be assessed and may spring surprises: Time-dependent illumination could improve the spatial resolution of the focusing. We investigate dynamics of flexural wave focusing by a 45°-tilted square lattice of circular holes drilled in a duralumin plate. Time-resolved experiments reveal that the focused image shrinks with time below the diffraction limit, with a lateral resolution increasing from $0.8\lambda$ to $0.35\lambda$, whereas focusing under harmonic excitation remains diffraction limited. Modal analysis reveals the role in pulse reconstruction of radiating lens resonances, which repeatedly self-synchronize at the focal spot to shape a superoscillating field.

We study the indentation of a thin elastic film floating at the surface of a liquid. We focus on the onset of radial wrinkles at a threshold indentation depth and the evolution of the wrinkle pattern as indentation progresses far beyond this threshold. Comparison between experiments on thin polymer films and theoretical calculations shows that the system very quickly reaches the far from threshold regime, in which wrinkles lead to the relaxation of azimuthal compression. Furthermore, when the indentation depth is sufficiently large that the wrinkles cover most of the film, we recognize a novel mechanical response in which the work of indentation is transmitted almost solely to the liquid, rather than to the floating film. We attribute this unique response to a nontrivial isometry attained by the deformed film, and we discuss the scaling laws and the relevance of similar isometries to other systems in which a confined sheet is subjected to weak tensile loads.

We demonstrate a reduction in the measured inner wall shear stress in moderately turbulent Taylor-Couette flows by depositing sprayable superhydrophobic microstructures on the inner rotor surface. The magnitude of reduction becomes progressively larger as the Reynolds number increases up to a value of 22% at $\mathrm{Re}=8.0\times 10^4$. We show that the mean skin friction coefficient $C_f$ in the presence of the superhydrophobic coating can be fitted to a modified Prandtl–von Kármán–type relationship of the form $(C_f/2{)}^{-1/2}=M\ln (\mathrm{Re}(C_f/2{)}^{1/2})+N+(b/\mathrm{\Delta }r)\mathrm{Re}(C_f/2{)}^{1/2}$ from which we extract an effective slip length of $b\approx 19\mu \mathrm{m}$. The dimensionless effective slip length $b^{+}=b/{\delta }_{\nu }$, where ${\delta }_{\nu }$ is the viscous length scale, is the key parameter that governs the drag reduction and is shown to scale as $b^{+}\sim {\mathrm{Re}}^{1/2}$ in the limit of high Re.