Weyl fermions with tilted linear dispersions characterized by several different velocities appear in some systems including the quasi-two-dimensional organic semiconductor $\alpha \text{−}(\mathrm{BEDT}\text{−}\mathrm{TTF}{)}_2{\mathrm{I}}_3$ and three-dimensional ${\mathrm{WTe}}_2$. The Coulomb interaction between electrons modifies the velocities in an essential way in the low-energy limit, where the logarithmic corrections dominate. Taking into account the coupling to both the transverse and longitudinal electromagnetic fields, we derive the renormalization group equations for the velocities of the tilted Weyl fermions in two dimensions, and found that they increase as the energy decreases and eventually hit the speed of light $c$ to result in the Cherenkov radiation. Especially, the system restores the isotropic Weyl cone even when the bare Weyl cone is strongly tilted and the velocity of electrons becomes negative in certain directions.

We show that an electrically soft ferroelectric host can be used to tune the photoluminescence (PL) response of rare-earth emitter ions by external electric field. The proof of this concept is demonstrated by changing the PL response of the ${\mathrm{Eu}}^{3+}$ ion by electric field on a model system Eu-doped $0.94({\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_3)\text{−}0.06({\mathrm{BaTiO}}_3)$. We also show that new channels of radiative transitions, forbidden otherwise, open up due to positional disorder in the system, which can as well be tuned by electric field.

A wave-number-dependent dissipative term to magnetization dynamics, mirroring the conservative term associated with exchange, has been proposed recently for ferromagnetic metals. We present measurements of wave-number-($k$-)dependent Gilbert damping in three metallic ferromagnets, NiFe, Co, and CoFeB, using perpendicular spin wave resonance up to 26 GHz. In the thinnest films accessible, where classical eddy-current damping is negligible, size effects of Gilbert damping for the lowest and first excited modes support the existence of a $k^2$ term. The new term is clearly separable from interfacial damping typically attributed to spin pumping. Higher-order modes in thicker films do not show evidence of enhanced damping, attributed to a complicating role of conductivity and inhomogeneous broadening. Our extracted magnitude of the $k^2$ term, $\mathrm{\Delta }{\alpha }_{kE}^{*}=\mathrm{\Delta }{\alpha }_0^{*}+A_k^{*}{k}^2$, where $A_k^{*}=0.08–0.1{\mathrm{nm}}^2$ in the three materials, is an order of magnitude lower than that identified in prior experiments on patterned elements.

A first-principles-based effective Hamiltonian technique is used to investigate the interplay between geometrical frustration and the ordering of topological defects in a ferroelectric nanocomposite consisting of a square array of ${\mathrm{BaTiO}}_3$ nanowires embedded in a ${\mathrm{Ba}}_{0.15}{\mathrm{Sr}}_{0.85}{\mathrm{TiO}}_3$ matrix. Different arrangements of the wires’ chiralities geometrically frustrate the matrix, which in response exhibits point topological defects featuring self-assembled ordered structures spatially fluctuating down to the lowest temperatures. These fluctuations thereby endow the system with residual configurational entropy from which many properties characteristic of geometric frustration, such as the ground state degeneracy and the broadness of the dielectric response, are further found to originate.

In quantum physics, measurements give random results and yield a corresponding random backaction on the state of the system subject to measurement. If a quantum system is probed continuously over time, its state evolves along a stochastic quantum trajectory. To investigate the characteristic properties of such dynamics, we perform weak continuous measurements on a superconducting qubit that is driven to undergo Rabi oscillations. From the data we observe a number of striking temporal correlations within the time dependent signals and the quantum trajectories of the qubit, and we discuss their explanation in terms of quantum measurement and photodetection theory.

The superposition principle is one of the landmarks of quantum mechanics. The importance of quantum superpositions provokes questions about the limitations that quantum mechanics itself imposes on the possibility of their generation. In this work, we systematically study the problem of the creation of superpositions of unknown quantum states. First, we prove a no-go theorem that forbids the existence of a universal probabilistic quantum protocol producing a superposition of two unknown quantum states. Second, we provide an explicit probabilistic protocol generating a superposition of two unknown states, each having a fixed overlap with the known referential pure state. The protocol can be applied to generate coherent superposition of results of independent runs of subroutines in a quantum computer. Moreover, in the context of quantum optics it can be used to efficiently generate highly nonclassical states or non-Gaussian states.

We experimentally demonstrate a universal strategy for producing a quantum state that is orthogonal to an arbitrary, infinite-dimensional, pure input one, even if only a limited amount of information about the latter is available. Arbitrary coherent superpositions of the two mutually orthogonal states are then produced by a simple change in the experimental parameters. We use input coherent states of light to illustrate two variations of the method. However, we show that the scheme works equally well for arbitrary input fields and constitutes a universal procedure, which may thus prove a useful building block for quantum state engineering and quantum information processing with continuous-variable qubits.

We have studied the photodissociation of ${\mathrm{CH}}^{+}$ in the Cryogenic Storage Ring at ambient temperatures below 10 K. Owing to the extremely high vacuum of the cryogenic environment, we were able to store ${\mathrm{CH}}^{+}$ beams with a kinetic energy of $\sim 60\mathrm{keV}$ for several minutes. Using a pulsed laser, we observed Feshbach-type near-threshold photodissociation resonances for the rotational levels $J=0–2$ of ${\mathrm{CH}}^{+}$, exclusively. In comparison to updated, state-of-the-art calculations, we find excellent agreement in the relative intensities of the resonances for a given $J$, and we can extract time-dependent level populations. Thus, we can monitor the spontaneous relaxation of ${\mathrm{CH}}^{+}$ to its lowest rotational states and demonstrate the preparation of an internally cold beam of molecular ions.

We present a combined experimental and theoretical low temperature kinetic study of water cluster formation. Water cluster growth takes place in low temperature (23–69 K) supersonic flows. The observed kinetics of formation of water clusters are reproduced with a kinetic model based on theoretical predictions for the first steps of clusterization. The temperature- and pressure-dependent association and dissociation rate coefficients are predicted with an ab initio transition state theory based master equation approach over a wide range of temperatures (20–100 K) and pressures (${10}^{-6}-10\mathrm{bar}$).

Electroluminescence, the emission of light in the presence of an electric current, provides information on the allowed electronic transitions of a given system. It is commonly used to investigate the physics of strongly coupled light-matter systems, whose eigenfrequencies are split by the strong coupling with the photonic field of a cavity. Here we show that, together with the usual electroluminescence, systems in the ultrastrong light-matter coupling regime emit a uniquely quantum radiation when a flow of current is driven through them. While standard electroluminescence relies on the population of excited states followed by spontaneous emission, the process we describe herein extracts bound photons from the dressed ground state and it has peculiar features that unequivocally distinguish it from usual electroluminescence.

Orbital-free molecular dynamics simulations are used to benchmark two popular models for hot dense plasmas: the one component plasma (OCP) and the Yukawa model. A unified concept emerges where an effective OCP (EOCP) is constructed from the short-range structure of the plasma. An unambiguous ionization and the screening length can be defined and used for a Yukawa system, which reproduces the long-range structure with finite compressibility. Similarly, the dispersion relation of longitudinal waves is consistent with the screened model at vanishing wave number but merges with the OCP at high wave number. Additionally, the EOCP reproduces the overall relaxation time scales of the correlation functions associated with ionic motion. In the hot dense regime, this unified concept of EOCP can be fruitfully applied to deduce properties such as the equation of state, ionic transport coefficients, and the ion feature in x-ray Thomson scattering experiments.

In this Letter, the tensorial nature of the nonequilibrium dynamics in nonlinear mesoscopic elastic materials is evidenced via multimode resonance experiments. In these experiments the dynamic response, including the spatial variations of velocities and strains, is carefully monitored while the sample is vibrated in a purely longitudinal or a purely torsional mode. By analogy with the fact that such experiments can decouple the elements of the linear elastic tensor, we demonstrate that the parameters quantifying the nonequilibrium dynamics of the material differ substantially for a compressional wave and for a shear wave. This result could lead to further understanding of the nonlinear mechanical phenomena that arise in natural systems as well as to the design and engineering of nonlinear acoustic metamaterials.

Under the assumption of local atomic scattering, elastic electron scattering at finite scattering angles implies a small but finite energy loss. This energy loss, which under conventional imaging conditions in high-resolution transmission electron microscopy is of the order of 0.1 meV and thus negligible, increases by more than 2 orders of magnitude if light elements are investigated at sub-Ångström resolution. For a microscope of finite chromatic aberration, the energy loss leads to an element-specific chromatic effect which increases with the instrument resolution and with decreasing mass of the scattering atom. Despite that this effect is small, it can degrade the achievable image contrast. However, the effect can be considered in the optimization of the phase-contrast imaging conditions and even be beneficial to enhance the relative image contrast of light atoms in the presence of heavy atoms.

To finally resolve the controversial issue of whether or not the electronic structure of ${\mathrm{YbB}}_6$ is nontrivially topological, we have made a combined study using angle-resolved photoemission spectroscopy (ARPES) of the nonpolar (110) surface and density functional theory (DFT). The flat-band conditions of the (110) ARPES avoid the strong band bending effects of the polar (001) surface and definitively show that ${\mathrm{YbB}}_6$ has a topologically trivial B $2p\text{−}\mathrm{Yb}$ $5d$ semiconductor band gap of $\sim 0.3\mathrm{eV}$. Accurate determination of the low energy band topology in DFT requires the use of a modified Becke-Johnson exchange potential incorporating spin-orbit coupling and an on-site Yb $4f$ Coulomb interaction $U$ as large as 7 eV. The DFT result, confirmed by a more precise $GW$ band calculation, is similar to that of a small gap non-Kondo nontopological semiconductor. Additionally, the pressure-dependent electronic structure of ${\mathrm{YbB}}_6$ is investigated theoretically and found to transform into a $p\text{−}d$ overlap semimetal with small Yb mixed valency.

We investigate the effect of coupling Anderson localized particles in one dimension to a system of marginally localized phonons having a symmetry protected delocalized mode at zero frequency. This situation is naturally realized for electrons coupled to phonons in a disordered nanowire as well as for ultracold fermions coupled to phonons of a superfluid in a one-dimensional disordered trap. To determine if the coupled system can be many-body localized we analyze the phonon-mediated hopping transport for both the weak and strong coupling regimes. We show that the usual variable-range hopping mechanism involving a low-order phonon process is ineffective at low temperature due to discreteness of the bath at the required energy. Instead, the system thermalizes through a many-body process involving exchange of a diverging number $n\propto -\log T$ of phonons in the low temperature limit. This effect leads to a highly singular prefactor to Mott’s well-known formula and strongly suppresses the variable range hopping rate. Finally, we comment on possible implications of this physics in higher dimensional electron-phonon coupled systems.

We observe interaction-induced broadening of the two-photon $5s\text{−}18s$ transition in ${}^{87}\mathrm{Rb}$ atoms trapped in a 3D optical lattice. The measured linewidth increases by nearly 2 orders of magnitude with increasing atomic density and excitation strength, with corresponding suppression of resonant scattering and enhancement of off-resonant scattering. We attribute the increased linewidth to resonant dipole-dipole interactions of $18s$ atoms with blackbody induced population in nearby $np$ states. Over a range of initial atomic densities and excitation strengths, the transition width is described by a single function of the steady-state density of Rydberg atoms, and the observed resonant excitation rate corresponds to that of a two-level system with the measured, rather than natural, linewidth. The broadening mechanism observed here is likely to have negative implications for many proposals with coherently interacting Rydberg atoms.