We report the observation of subpicosecond terahertz ($T$-ray) pulses with energies $\ge 460\mu \mathrm{J}$ from a laser-driven ion accelerator, thus rendering the peak power of the source higher even than that of state-of-the-art synchrotrons. Experiments were performed with intense laser pulses (up to $5\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$) to irradiate thin metal foil targets. Ion spectra measured simultaneously showed a square law dependence of the $T$-ray yield on particle number. Two-dimensional particle-in-cell simulations show the presence of transient currents at the target rear surface which could be responsible for the strong $T$-ray emission.

Using density functional theory calculations, ultrathin films of ${\mathrm{SrVO}}_3(d^1)$ and ${\mathrm{SrCrO}}_3(d^2)$ on ${\mathrm{SrTiO}}_3$ substrates have been studied as possible multiferroics. Although both are metallic in the bulk limit, they are found to be insulating as a result of orbital ordering driven by lattice distortions at the ultrathin limit. While the distortions in ${\mathrm{SrVO}}_3$ have a first-order Jahn-Teller origin, those in ${\mathrm{SrCrO}}_3$ are ferroelectric in nature. This route to ferroelectricity results in polarizations comparable with conventional ferroelectrics.

We prepare a superposition of two motional states by addressing lithium atoms immersed in a Bose-Einstein condensate of sodium with a species-selective potential. The evolution of the superposition state is characterized by the populations of the constituent states as well as their coherence. The latter we extract employing a novel scheme analogous to the spin-echo technique. Comparing the results directly to measurements on freely evolving fermions allows us to isolate the decoherence effects induced by the bath. In our system, the decoherence time is close to the maximal possible value since the decoherence is dominated by population relaxation processes. The measured data are in good agreement with a theoretical model based on Fermi’s golden rule.

I present arguments indicating the impossibility of spontaneously rotating “quantum time crystals,” as recently proposed by Frank Wilczek. In particular, I prove a “no-go theorem,” rigorously ruling out the possibility of spontaneous ground-state (or thermal equilibrium) rotation for a broad class of systems.

We consider a class of large superposed states, obtained from dimer coverings on spin-$1/2$ isotropic lattices, whose potential usefulness ranges from organic molecules to quantum computation. We show that they are genuinely multiparty entangled, irrespective of the geometry and dimension of the isotropic lattice. We then present an efficient method to characterize the genuine multisite entanglement in the case of isotropic square spin-$1/2$ lattices, with short-range dimer coverings. We use this iterative analytical method to calculate the multisite entanglement of finite-sized lattices, which through finite-size scaling, enables us to obtain the estimate of the multisite entanglement of the infinite square lattice. The method can be a useful tool to investigate other single- and multisite properties of such states.

We investigate the statistics of the gap $G_n$ between the two rightmost positions of a Markovian one-dimensional random walker (RW) after $n$ time steps and of the duration $L_n$ which separates the occurrence of these two extremal positions. The distribution of the jumps ${\eta }_i$’s of the RW, $f(\eta )$, is symmetric and its Fourier transform has the small $k$ behavior $1-\widehat{f}(k)\sim |k{|}^{\mu }$, with $0<\mu \le 2$. For $\mu =2$, the RW converges, for large $n$, to Brownian motion, while for $0<\mu <2$ it corresponds to a Lévy flight of index $\mu$. We compute the joint probability density function (PDF) $P_n(g,l)$ of $G_n$ and $L_n$ and show that, when $n\to \infty$, it approaches a limiting PDF $p(g,l)$. The corresponding marginal PDFs of the gap, $p_{\mathrm{gap}}(g)$, and of $L_n$, $p_{\mathrm{time}}(l)$, are found to behave like $p_{\mathrm{gap}}(g)\sim g^{-1-\mu }$ for $g\gg 1$ and $0<\mu <2$, and $p_{\mathrm{time}}(l)\sim l^{-\gamma (\mu )}$ for $l\gg 1$ with $\gamma (1<\mu \le 2)=1+1/\mu$ and $\gamma (0<\mu <1)=2$. For $l$, $g\gg 1$ with fixed $l{g}^{-\mu }$, $p(g,l)$ takes the scaling form $p(g,l)\sim g^{-1-2\mu }{\tilde{p}}_{\mu }(l{g}^{-\mu })$, where ${\tilde{p}}_{\mu }(y)$ is a ($\mu$-dependent) scaling function. We also present numerical simulations which verify our analytic results.

We investigate the possibility of fitting the putative spectrum of gamma rays from annihilating dark matter at the galactic center by a superexponential template. By generating and analyzing mock data for several ongoing and future experiments (HESS, CTA, and DMA), we find that the preference for a superexponential template (over an exponential) is especially strong for dark matter annihilating to ${\tau }^{+}{\tau }^{-}$ and ${\mu }^{+}{\mu }^{-}$ relative to other annihilation channels.

The control of light-matter interaction at the quantum level usually requires coherent laser fields. But already an exchange of virtual photons with the electromagnetic vacuum field alone can lead to quantum coherences, which subsequently suppress spontaneous emission. We demonstrate such spontaneously generated coherences (SGC) in a large ensemble of nuclei operating in the x-ray regime, resonantly coupled to a common cavity environment. The observed SGC originates from two fundamentally different mechanisms related to cooperative emission and magnetically controlled anisotropy of the cavity vacuum. This approach opens new perspectives for quantum control, quantum state engineering and simulation of quantum many-body physics in an essentially decoherence-free setting.

The present study experimentally examines how an electron vortex beam with orbital angular momentum (OAM) undergoes diffraction through a forked grating. The $n$th-order diffracted electron vortex beam after passing through a forked grating with a Burgers vector of 1 shows an OAM transfer of $n\hslash$. Hence, the diffraction patterns become mirror asymmetric owing to the size difference between the electron beams. Such a forked grating, when used in combination with a pinhole located at the diffraction plane, could act as an analyzer to measure the OAM of input electrons.

Turbulence is generally associated with universal power-law spectra in scale ranges without significant drive or damping. Although many examples of turbulent systems do not exhibit such an inertial range, power-law spectra may still be observed. As a simple model for such situations, a modified version of the Kuramoto-Sivashinsky equation is studied. By means of semianalytical and numerical studies, one finds power laws with nonuniversal exponents in the spectral range for which the ratio of nonlinear and linear time scales is (roughly) scale independent.

We propose a procedure to rank the most interesting solutions from high-throughput materials design studies. Such a tool is becoming indispensable due to the growing size of computational screening studies and the large number of criteria involved in realistic materials design. As a proof of principle, the binary tungsten alloys are screened for both large-weight and high-impact materials, as well as for fusion reactor applications. Moreover, the concept is generally applicable to any design problem where multiple competing criteria have to be optimized.

We study the ternary clathrate ${\mathrm{Pr}}_3{\mathrm{Pd}}_{20}{\mathrm{Si}}_6$ in specific heat and ac susceptibility measurements on a high-quality single crystal, distinguishing antiferromagnetic and antiferroquadrupolar ordering, as well as a hitherto unknown magnetic low-temperature transition. The specific heat shows the direct involvement of nuclear spin degrees of freedom in the antiferromagnetic ordering, which is well supported by our calculation of the hyperfine level scheme without adjustable parameters. ${\mathrm{Pr}}_3{\mathrm{Pd}}_{20}{\mathrm{Si}}_6$ is, therefore, one of the rare materials where the nuclear moments are involved in the formation of the magnetic ground state.

We report on the polarization selection rules of inter-Landau-level transitions using reflection-type optical Hall effect measurements from 600 to $4000{\mathrm{cm}}^{-1}$ on epitaxial graphene grown by thermal decomposition of silicon carbide. We observe symmetric and antisymmetric signatures in our data due to polarization preserving and polarization mixing inter-Landau-level transitions, respectively. From field-dependent measurements, we identify that transitions in coupled graphene monolayers are governed by polarization mixing selection rules, whereas transitions in decoupled graphene monolayers are governed by polarization preserving selection rules. The selection rules may find explanation by different coupling mechanisms of inter-Landau-level transitions with free charge carrier magneto-optic plasma oscillations.

We investigate a nonsingular initial state of the Universe which leads to inflation naturally. The model is described by a scalar field with a quadratic potential in Eddington-inspired Born-Infeld gravity. The curvature of this initial state is given by the mass scale of the scalar field, which is much smaller than the Planck scale. Therefore, in this model, quantum gravity is not necessary in understanding this preinflationary stage, no matter how large the energy density becomes. The initial state in this model evolves eventually to a long inflationary period which is similar to the usual chaotic inflation.

We propose a simple method to spectrally resolve an array of identical two-level systems coupled to an inhomogeneous oscillating field. The addressing protocol uses a dressing field with a spatially dependent coupling to the atoms. We validate this scheme experimentally by realizing single-spin addressing of a linear chain of trapped ions that are separated by $\sim 3\mu \mathrm{m}$, dressed by a laser field that is resonant with the micromotion sideband of a narrow optical transition.

When exposed to ultraintense x-radiation sources such as free electron lasers (FELs) the innermost electronic shell can efficiently be emptied, creating a transient hollow atom or molecule. Understanding the femtosecond dynamics of such systems is fundamental to achieving atomic resolution in flash diffraction imaging of noncrystallized complex biological samples. We demonstrate the capacity of a correlation method called “partial covariance mapping” to probe the electron dynamics of neon atoms exposed to intense 8 fs pulses of 1062 eV photons. A complete picture of ionization processes competing in hollow atom formation and decay is visualized with unprecedented ease and the map reveals hitherto unobserved nonlinear sequences of photoionization and Auger events. The technique is particularly well suited to the high counting rate inherent in FEL experiments.

We theoretically investigate electron energy loss spectroscopy (EELS) of metallic nanoparticles in the optical frequency domain. Using a quasistatic approximation scheme together with a plasmon eigenmode expansion, we show that EELS can be rephrased in terms of a tomography problem. For selected single and coupled nanoparticles we extract the three-dimensional plasmon fields from a collection of rotated EELS maps. Our results pave the way for a fully three-dimensional plasmon-field tomography and establish EELS as a quantitative measurement device for plasmonics.

We study nondegenerate flatbands at the surfaces of noncentrosymmetric topological superconductors by exact diagonalization of Bogoliubov–de Gennes Hamiltonians. We show that these states are strongly spin polarized and acquire a chiral dispersion when placed in contact with a ferromagnetic insulator. This chiral mode carries a large edge current which displays a singular dependence on the exchange-field strength. The contribution of other edge states to the current is comparably weak. We hence propose that the observation of the edge current can serve as a test of the presence of nondegenerate flatbands.

We consider an active Ising model in which spins both diffuse and align on lattice in one and two dimensions. The diffusion is biased so that plus or minus spins hop preferably to the left or to the right, which generates a flocking transition at low temperature and high density. We construct a coarse-grained description of the model that predicts this transition to be a first-order liquid-gas transition in the temperature-density ensemble, with a critical density sent to infinity. In this first-order phase transition, the magnetization is proportional to the liquid fraction and thus varies continuously throughout the phase diagram. Using microscopic simulations, we show that this theoretical prediction holds in 2D whereas the fluctuations alter the transition in 1D, preventing, for instance, any spontaneous symmetry breaking.

We study the wavelength scaling of high harmonic generation efficiency with visible driver wavelengths in the transition between the tunneling and the multiphoton ionization regimes where the Keldysh parameter is around unity. Our experiment shows a less dramatic wavelength scaling of efficiency than the conventional case for near- and mid-IR driver wavelengths, and it is well explained by a generalized three-step model for increased Keldysh parameters that employs complex ionization times in addition to the nonadiabatic ionization. The complex ionization time is critical to avoid the divergence when replacing the quasistatic ionization model by the more general nonadiabatic ionization model. Together, the two modifications present a consistent description of the influence of the atomic potential on the rescattering process in the intermediate Keldysh regime.