We have used laser pulses with a temporally shaped polarization to demonstrate the multiphoton excitation of the xenon $5g$ state within a subcycle of a laser pulse. Our polarization gated laser pulses are composed of circularly polarized sections at the leading and trailing edges of the pulse and of an experimentally defined linearly polarized central part. Only the linear part (the gate) of the pulse can excite neutral xenon in the $5g$ state. The transition cannot be driven with circularly polarized light because the number of photons needed would cause a violation of selection rules for the change of the magnetic quantum number. We show that the linearly polarized central part can be reduced to a subcycle pulse. This allows us to study excitation with an effective pulse as short as 2.3 fs at 800 nm. Electron imaging spectroscopy has been used to visualize the presence of excited states as a function of the pulse duration of the gate.

There is no consensus on the magnitude and shape of the charge transfer cross section in low-energy ${\mathrm{H}}^{+}+{\mathrm{H}}_2$ collisions, in spite of the fundamental importance of these collisions. Experiments have thus been carried out in the energy range $15\le E\le 5000\mathrm{eV}$. The measurements invalidate previous recommended data for $E\le 200\mathrm{eV}$ and confirm the existence of a local maximum around 45 eV, which was predicted theoretically. Additionally, vibrationally resolved cross sections allow us to investigate the evolution of the underlying charge transfer mechanism as a function of $E$.

A homogeneous linewidth of $85.6\pm 4.4\mathrm{kHz}$ is reported in 60 nm ${\mathrm{Eu}}^{3+}$ doped ${\mathrm{Y}}_2{\mathrm{O}}_3$ nanocrystals at 1.3 K. This linewidth was measured by two-pulse photon echoes on highly scattering powders using heterodyne detection. Spectral diffusion was also investigated by three-pulse photon echoes and resulted in a limited broadening of the homogenous linewidth of about 250 kHz over $120\mu \mathrm{s}$. Compared to achievable Rabi frequencies, in the range of several MHz, these values show that rare earth doped nanocrystals can be useful for applications in optical quantum information processing.

We demonstrate that the surface second-harmonic generation can lead to the formation of nonlinear plasmonic whispering-gallery modes (WGMs) in microcavities made of metallic nanowires. Since these WGMs are excited by induced surface nonlinear dipoles, they can be generated even when they are not coupled to the radiation continuum. Consequently, the quality factor of these nonlinear modes can be as large as the theoretical limit imposed by the optical losses in the metal. Remarkably, our theoretical analysis shows that nonlinear plasmonic WGMs are characterized by fractional azimuthal modal numbers. This suggests that the plasmonic cavities investigated here can be used to generate multicolor optical fields with fractional angular momentum. Applications to plasmonic sensors are also discussed.

Blood flow speed is an important functional parameter. Doppler ultrasound flowmetry lacks sufficient sensitivity to slow blood flow (several to tens of millimeters per second) in deep tissue. To address this challenge, we developed ultrasonically encoded photoacoustic flowgraphy combining ultrasonic thermal tagging with photoacoustic imaging. Focused ultrasound generates a confined heat source in acoustically absorptive fluid. Thermal waves propagate with the flow and are directly visualized in pseudo color using photoacoustic computed tomography. The Doppler shift is employed to calculate the flow speed. This method requires only acoustic and optical absorption, and thus is applicable to continuous fluid. A blood flow speed as low as $0.24\mathrm{mm}·{\mathrm{s}}^{-1}$ was successfully measured. Deep blood flow imaging was experimentally demonstrated under 5-mm-thick chicken breast tissue.

The critical strength of a magnetic field required for the suppression of the Richtmyer-Meshkov instability (RMI) is investigated numerically by using a two-dimensional single-mode analysis. For the cases of magnetohydrodynamic parallel shocks, the RMI can be stabilized as a result of the extraction of vorticity from the interface. A useful formula describing a critical condition for magnetohydrodynamic RMI is introduced and is successfully confirmed by direct numerical simulations. The critical field strength is found to be largely dependent on the Mach number of the incident shock. If the shock is strong enough, even low-$\beta$ plasmas can be subject to the growth of the RMI.

An experimental study of a nanosecond pulsed regime in a THz gyrotron oscillator operating in a self-consistent $Q$-switch regime has been carried out. The gyrotron is operated in the ${\mathrm{TE}}_{7,2}$ transverse mode radiating at a frequency of 260.5 GHz. The 5 W nanosecond pulses are obtained in a self-consistent $Q$-switch regime in which the cavity diffraction quality factor dynamically varies by nearly 2 orders of magnitude on a subnanosecond time scale via the nonlinear interaction of different mode-locked frequency-equidistant sidebands. The experimental results are in good agreement with numerical simulations performed with the TWANG code based on a slow time scale formulation of the self-consistent time-dependent nonlinear wave particle interaction equations.

We demonstrate how the spin-$1/2$ $XYZ$ quantum Heisenberg model can be realized with bosonic atoms loaded in the $p$ band of an optical lattice in the Mott regime. The combination of Bose statistics and the symmetry of the $p$-orbital wave functions leads to a nonintegrable Heisenberg model with antiferromagnetic couplings. Moreover, the sign and relative strength of the couplings characterizing the model are shown to be experimentally tunable. We display the rich phase diagram in the one-dimensional case and discuss finite size effects relevant for trapped systems. Finally, experimental issues related to preparation, manipulation, detection, and imperfections are considered.

The role of surface and subsurface O vacancies for gold adsorption on crystalline ${\mathrm{CeO}}_2(111)$ films has been investigated by scanning tunneling microscopy and density functional theory. Whereas surface vacancies serve as deep traps for the Au atoms, subsurface defects promote the formation of characteristic Au pairs with a mean atom distance of two ceria lattice constants (7.6 Å). Hybrid density functional theory calculations reveal that the pair formation arises from a titration of the two ${\mathrm{Ce}}^{3+}$ ions generated by a single O vacancy. The $\mathrm{Au}\mathrm{\text{−}}{\mathrm{Ce}}^{3+}$ bond forms due to a strain effect, as the associated charge transfer from the spacious ${\mathrm{Ce}}^{3+}$ into the adgold enables a substantial relaxation of the ceria lattice. Also the experimentally determined Au-pair length is reproduced in the calculations, as we find a ${\mathrm{Ce}}^{3+}\mathrm{\text{−}}{\mathrm{Ce}}^{3+}$ spacing of two ceria lattice parameters to be energetically preferred. Single Au atoms can thus be taken as position markers for ${\mathrm{Ce}}^{3+}$ ion pairs in the surface, providing unique information on electron-localization phenomena in reduced ceria.

We argue that a class of strongly spin-orbit-coupled materials, including some pyrochlore iridates and the inverted band gap semiconductor HgTe, may be described by a minimal model consisting of the Luttinger Hamiltonian supplemented by Coulomb interactions, a problem studied by Abrikosov and collaborators. It contains twofold degenerate conduction and valence bands touching quadratically at the zone center. Using modern renormalization group methods, we update and extend Abrikosov’s classic work and show that interactions induce a quantum critical non-Fermi-liquid phase, stable provided time-reversal and cubic symmetries are maintained. We determine the universal power-law exponents describing various observables in this Luttinger-Abrikosov-Beneslavskii state, which include conductivity, specific heat, nonlinear susceptibility, and the magnetic Gruneisen number. Furthermore, we determine the phase diagram in the presence of cubic and/or time-reversal symmetry breaking perturbations, which includes a topological insulator and Weyl semimetal phases. Many of these phases possess an extraordinarily large anomalous Hall effect, with the Hall conductivity scaling sublinearly with magnetization ${\sigma }_{xy}\sim M^{0.51}$.

Most physical systems known to date tend to resist entering the topological phase and must be fine-tuned to reach that phase. Here, we introduce a system in which a key dynamical parameter adjusts itself in response to the changing external conditions so that the ground state naturally favors the topological phase. The system consists of a quantum wire formed of individual magnetic atoms placed on the surface of an ordinary $s$-wave superconductor. It realizes the Kitaev paradigm of topological superconductivity when the wave vector characterizing the emergent spin helix dynamically self-tunes to support the topological phase. We call this phenomenon a self-organized topological state.

A topological surface state that is protected physically under the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$-like five-layer block has been revealed on the Pb-based topological insulator (TI) ${\mathrm{PbBi}}_4{\mathrm{Te}}_7$ by bulk sensitive angle-resolved photoelectron spectroscopy (ARPES). Furthermore, conservation of the spin polarization of the hidden topological surface states is directly confirmed by bulk-sensitive spin ARPES observation. This finding paves the way to realize the real spintronics devices by TIs that are operable in the real environment.

We have made the first observation of superconductivity in ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$ at $T_C=3.7\mathrm{K}$, and it appears to involve heavy electrons with an effective mass $m^{*}=(14–20)m_b$, as inferred from the normal-state electronic specific heat and the upper critical field, $H_{C2}(T)$. We found that the zero-field electronic specific-heat data, $C_{\mathrm{es}}(T)$ ($0.5\mathrm{K}\le T<3.7\mathrm{K}$) in the superconducting state can be fitted with a two-gap BCS model, indicating that ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$ seems to be a multiband superconductor, which is consistent with the band calculation for the isostructural ${\mathrm{KNi}}_2{\mathrm{S}}_2$. It is also found that the electronic specific-heat coefficient in the mixed state ${\gamma }_N(H)$ exhibits a $H^{1/2}$ behavior, which is considered as a common feature of the $d$-wave superconductors. ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$, as a $d$-electron system with heavy electron superconductivity, may be a bridge between cuprate- or iron-based and conventional heavy-fermion superconductors.

We use inelastic neutron scattering to show that superconductivity in electron-underdoped ${\mathrm{NaFe}}_{0.985}{\mathrm{Co}}_{0.015}\mathrm{As}$ induces a dispersive sharp resonance near $E_{r1}=3.25\mathrm{meV}$ and a broad dispersionless mode at $E_{r2}=6\mathrm{meV}$. However, similar measurements on overdoped superconducting ${\mathrm{NaFe}}_{0.935}{\mathrm{Co}}_{0.045}\mathrm{As}$ find only a single sharp resonance at $E_r=7\mathrm{meV}$. We connect these results with the observations of angle-resolved photoemission spectroscopy that the superconducting gaps in the electron Fermi pockets are anisotropic in the underdoped material but become isotropic in the overdoped case. Our analysis indicates that both the double neutron spin resonances and gap anisotropy originate from the orbital dependence of the superconducting pairing in the iron pnictides. Our discovery also shows the importance of the inelastic neutron scattering in detecting the multiorbital superconducting gap structures of iron pnictides.

We propose a scheme to distinguish zero-energy peaks due to Majorana from those due to other effects at finite temperature by simply replacing the normal metallic lead with a resistive lead (large $R\sim k\Omega$) in the tunneling spectroscopy. The dissipation effects due to the large resistance change the tunneling conductance significantly in different ways. The Majorana peak remains increase as temperature decreases $G\sim T^{2r-1}$ for $r=e^{2}R/h<1/2$. The zero-energy peak due to other effects splits into two peaks at finite temperature and the conductance at zero voltage bias varies with temperature by a power law. The dissipative tunneling with a Majorana mode belongs to the same universal class as the unstable critical point of the case with a non-Majorana mode.

We present ${}^{75}\mathrm{As}$ nuclear magnetic resonance data from measurements of a series of $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ crystals with $0.00\le x\le 0.075$ that reveals the coexistence of frozen antiferromagnetic domains and superconductivity for $0.060\le x\le 0.071$. Although bulk probes reveal no long range antiferromagnetic order beyond $x=0.06$, we find that the local spin dynamics reveal no qualitative change across this transition. The characteristic domain sizes vary by more than an order of magnitude, reaching a maximum variation at $x=0.06$. This inhomogeneous glassy dynamics may be an intrinsic response to the competition between superconductivity and antiferromagnetism in this system.

The nonequilibrium response of a quantum many-body system defines its fundamental transport properties and how initially localized quantum information spreads. However, for long-range-interacting quantum systems little is known. We address this issue by analyzing a local quantum quench in the long-range Ising model in a transverse field, where interactions decay as a variable power law with distance $\propto r^{-\alpha }$, $\alpha >0$. Using complementary numerical and analytical techniques, we identify three dynamical regimes: short-range-like with an emerging light cone for $\alpha >2$, weakly long range for $1<\alpha <2$ without a clear light cone but with a finite propagation speed of almost all excitations, and fully nonlocal for $\alpha <1$ with instantaneous transmission of correlations. This last regime breaks generalized Lieb-Robinson bounds and thus locality. Numerical calculation of the entanglement spectrum demonstrates that the usual picture of propagating quasiparticles remains valid, allowing an intuitive interpretation of our findings via divergences of quasiparticle velocities. Our results may be tested in state-of-the-art trapped-ion experiments.

We show, using a large new data set, that the temporally resolved speed of active cargo transport in living cells follows a scaling law over several decades of time and length. The statistical regularities display a time-averaged shape that we interpret to reflect stress buildup, followed by rapid release. The scaling power law agrees quantitatively with those reported in inanimate systems (jammed colloids and granular media, and magnetic Barkhausen noise), suggesting a common origin in pushing through a crowded environment in a weak force regime. The implied regulation of the speed of active cellular transport due to environmental obstruction results in bursts of speed and acceleration. These findings extend the classical notion of molecular crowding.

We propose an efficient protocol for braiding Majorana fermions realized as edge states in atomic wire networks, and demonstrate its robustness against experimentally relevant errors. The braiding of two Majorana fermions located on one side of two adjacent wires requires only a few local operations on this side which can be implemented using local site addressing available in current experiments with cold atoms and molecules. Based on this protocol we provide an experimentally feasible implementation of the Deutsch-Jozsa algorithm for two qubits in a topologically protected way.

We show that the effective gauge field for photons provides a versatile platform for controlling the flow of light. As an example we consider a photonic resonator lattice where the coupling strength between nearest neighbor resonators are harmonically modulated. By choosing different spatial distributions of the modulation phases, and hence imposing different inhomogeneous effective magnetic field configurations, we numerically demonstrate a wide variety of propagation effects including negative refraction, one-way mirror, and on- and off-axis focusing. Since the effective gauge field is imposed dynamically after a structure is constructed, our work points to the importance of the temporal degree of freedom for controlling the spatial flow of light.