Van der Waals-Zeeman Institute

This review discusses disordered materials that only flow if the imposed stress is beyond a certain threshold value. These so-called yield stress materials cannot be described as elastic solids nor as simple Newtonian fluids. Examples include whipped cream, toothpaste, cement, ketchup, and mayonnaise. The physical origin of the yield stress, the nonlinear flow behavior, and experimental techniques to investigate these materials are discussed. Also an overview of the microscopic theoretical descriptions of the nonlinear flow dynamics is presented.

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T(C)=3 K and a small ordered moment m(0)=0.03 micro(B). Superconductivity is observed with a resistive transition temperature T(s)=0.8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T(s), and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.

We study droplet coalescence in a molecular system with a variable viscosity and a colloid-polymer mixture with an ultralow surface tension. When either the viscosity is large or the surface tension is small enough, we observe that the opening of the liquid bridge initially proceeds at a constant speed set by the capillary velocity. In the first system we show that inertial effects become dominant at a Reynolds number of about 1.5+/- 0.5 and the neck then grows as the square root of time. In the second system we show that decreasing the surface tension by a factor of 10(5) opens the way to a more complete understanding of the hydrodynamics involved.

We show that the expansion of an initially confined interacting 1D Bose-Einstein condensate can exhibit Anderson localization in a weak random potential with correlation length ${\ensuremath{\sigma}}_{R}$. For speckle potentials the Fourier transform of the correlation function vanishes for momenta $k>2/{\ensuremath{\sigma}}_{R}$ so that the Lyapunov exponent vanishes in the Born approximation for $k>1/{\ensuremath{\sigma}}_{R}$. Then, for the initial healing length of the condensate ${\ensuremath{\xi}}_{\mathrm{in}}>{\ensuremath{\sigma}}_{R}$ the localization is exponential, and for ${\ensuremath{\xi}}_{\mathrm{in}}<{\ensuremath{\sigma}}_{R}$ it changes to algebraic.

The propagation of light in nonperiodic quasicrystals is studied by ultrashort pulse interferometry. Samples consist of multilayer dielectric structures of the Fibonacci type and are realized from porous silicon. We observe mode beating and strong pulse stretching in the light transport through these systems, and a strongly suppressed group velocity for frequencies close to a Fibonacci band gap. A theoretical description based on transfer matrix theory allows us to interpret the results in terms of Fibonacci band-edge resonances.

The study of the behavior of sessile droplets on solid substrates is not only associated with common everyday phenomena, such as the coffee stain effect, limescale deposits on our bathroom walls , but also very important in many applications such as purification of pharmaceuticals, de-icing of airplanes, inkjet printing and coating applications. In many of these processes, a phase change happens within the drop because of solvent evaporation, temperature changes or chemical reactions, which consequently lead to liquid to solid transitions in the droplets. Here we show that crystallization patterns of evaporating of water drops containing dissolved salts are different from the stains reported for evaporating colloidal suspensions. This happens because during the solvent evaporation, the salts crystallize and grow during the drying. Our results show that the patterns of the resulting salt crystal stains are mainly governed by wetting properties of the emerging crystal as well as the pathway of nucleation and growth, and are independent of the evaporation rate and thermal conductivity of the substrates.

The magnetic properties of ${\mathrm{YVO}}_{3}$ single crystals have been studied in the temperature range from 350 to 4.2 K and in magnetic fields up to 7 T. It is found that in an applied field less than 4 kOe remarkable magnetization reversals occur at two distinct temperatures: an abrupt switch at ${T}_{s}=77$ K associated with a first-order structure phase transition and a gradual reversal at ${T}^{*}\ensuremath{\approx}95$ K without a structural anomaly. Most interestingly, the magnetization always switches to the opposite direction if the crystal is cooled or warmed through ${T}_{s}$ and ${T}^{*}$ in a field less than \ensuremath{\sim}500 Oe. In higher magnetic fields the magnetization does not change sign but has a minimum at ${T}^{*}$ and a sudden change at ${T}_{s}.$ A possible mechanism for the observed peculiar magnetic behavior is discussed, related to the competition of the single-ion magnetic anisotropy and the antisymmetric Dzyaloshinsky-Moriya interaction accompanied by a change of orbital ordering.

We report on an experiment on Grover's quantum search algorithm showing that classical waves can search a N-item database as efficiently as quantum mechanics can. The transverse beam profile of a short laser pulse is processed iteratively as the pulse bounces back and forth between two mirrors. We directly observe the sought item being found in approximately square root[N] iterations, in the form of a growing intensity peak on this profile. Although the lack of quantum entanglement limits the size of our database, our results show that entanglement is neither necessary for the algorithm itself, nor for its efficiency.

Glasses behave as solids on experimental time scales due to their slow relaxation. Growing dynamic length scales due to cooperative motion of particles are believed to be central to this slow response. For quiescent glasses, however, the size of the cooperatively rearranging regions has never been observed to exceed a few particle diameters, and the observation of long-range correlations has remained elusive. Here, we provide direct experimental evidence of long-range correlations during the deformation of a dense colloidal glass. By imposing an external stress, we force structural rearrangements, and we identify long-range correlations in the fluctuations of microscopic strain and elucidate their scaling and spatial symmetry. The applied shear induces a transition from homogeneous to inhomogeneous flow at a critical shear rate, and we investigate the role of strain correlations in this transition.

Salt crystallization is a major cause of weathering of rocks, artworks and monuments. Damage can only occur if crystals continue to grow in confinement, i.e. within the pore space of these materials, thus generating mechanical stress. We report the direct measurement, at the microscale, of the force exerted by growing alkali halide salt crystals while visualizing their spontaneous nucleation and growth. The experiments reveal the crucial role of the wetting films between the growing crystal and the confining walls for the development of the pressure. Our results suggest that the measured force originates from repulsion between the similarly charged confining wall and the salt crystal separated by a ~1.5 nm liquid film. Indeed, if the walls are made hydrophobic, no film is observed and no repulsive forces are detected. We also show that the magnitude of the induced pressure is system specific explaining why different salts lead to different amounts of damage to porous materials.

We have observed resonant energy transfer between cold Rydberg atoms in spatially separated cylinders. Resonant dipole-dipole coupling excites the 49s atoms in one cylinder to the 49p state while the 41d atoms in the second cylinder are transferred down to the 42p state. We have measured the production of the 49p state as a function of separation of the cylinders (0-80 microm) and the interaction time (0-25 micros). In addition, we measured the width of the electric field resonances. A full many-body quantum calculation reproduces the main features of the experiments.

Sensitization of Er emission by Si nanoclusters (Si-nc) is investigated with pulsed and continuous optical pumping, in and off resonance with excited states of ${\text{Er}}^{3+}$ ion. We show that under high-power pulsed pumping, the excitation process is limited by the finite energy transfer time from Si-nc to ${\text{Er}}^{3+}$ ions. By comparison between pulsed and steady-state excitation, the concentration of sensitizers and average number of ${\text{Er}}^{3+}$ ions coupled to a single nanocluster are independently determined in an experiment. The results clarify conditions needed for efficient sensitization of ${\text{SiO}}_{2}:\text{Er}$ by Si-nc.

The quantization of the electromagnetic field in a three-dimensional inhomogeneous dielectric medium with losses is carried out in the framework of a damped-polariton model with an arbitrary spatial dependence of its parameters. The equations of motion for the canonical variables are solved explicitly by means of Laplace transformations for both positive and negative time. The dielectric susceptibility and the quantum noise-current density are identified in terms of the dynamical variables and parameters of the model. The operators that diagonalize the Hamiltonian are found as linear combinations of the canonical variables, with coefficients depending on the electric susceptibility and the dielectric Green function. The complete time dependence of the electromagnetic field and of the dielectric polarization is determined. Our results provide a microscopic justification of the phenomenological quantization scheme for the electromagnetic field in inhomogeneous dielectrics.

We demonstrate the experimental feasibility of incompressible fractional quantum Hall-like states in ultracold two-dimensional rapidly rotating dipolar Fermi gases. In particular, we argue that the state of the system at filling fraction nu = 1/3 is well described by the Laughlin wave function and find a substantial energy gap in the quasiparticle excitation spectrum. Dipolar gases, therefore, appear as natural candidates of systems that allow us to realize these very interesting highly correlated states in future experiments.

Evidence has been found for a change in the ordered occupation of the vanadium d orbitals at the 77 K phase transition in YVO3, manifested by a change in the type of Jahn-Teller distortion. The orbital ordering above 77 K is not destroyed at the magnetic ordering temperature of 116 K, but is present as far as a second structural phase transition at 200 K. The transition between orbital orderings is caused by an increase in octahedral tilting with decreasing temperature.

We demonstrate spatially resolved, coherent excitation of Rydberg atoms on an atom chip. Electromagnetically induced transparency (EIT) is used to investigate the properties of the Rydberg atoms near the gold-coated chip surface. We measure distance-dependent shifts ($~$$10$ MHz) of the Rydberg energy levels caused by a spatially inhomogeneous electric field. The measured field strength and distance dependence is in agreement with a simple model for the electric field produced by a localized patch of Rb adsorbates deposited on the chip surface during experiments. The EIT resonances remain narrow ($<4$ MHz) and the observed widths are independent of atom-surface distance down to $~$$20$ $\ensuremath{\mu}$m, indicating relatively long lifetime of the Rydberg states. Our results open the way to studies of dipolar physics, collective excitations, quantum metrology, and quantum information processing involving interacting Rydberg excited atoms on atom chips.

We consider two-dimensional bosonic dipoles oriented perpendicularly to the plane. On top of the usual two-body contact and long-range dipolar interactions we add a contact three-body repulsion as expected, in particular, for dipoles in the bilayer geometry with tunneling. The three-body repulsion is crucial for stabilizing the system, and we show that our model allows for stable continuous space supersolid states in the dilute regime and calculate the zero-temperature phase diagram.

We have made a multimode waveguide of x rays having an air gap as the guiding medium. Individual transverse electric modes were found to propagate through the planar waveguide with essentially no attenuation and with negligible scattering losses to other modes. If different modes are excited simultaneously at the waveguide entrance, then the phase relation between these modes as given by their propagation constants is found to be preserved over the entire length of the waveguide.

Optical analog computing using metasurfaces has been the subject of numerous studies, aimed at implementing highly efficient and ultrafast image processing in a compact device. The proposed approaches to date have shown limitations in terms of spatial resolution, overall efficiency, polarization and azimuthal angular dependence. Here, we present the design of a polarization-insensitive metasurface with tailored nonlocality based on a Fano resonant response, enabling both odd- and even-order analog mathematical operations on an incoming image. The metasurface is formed by a single-layered triangular lattice of holes in a suspended silicon membrane, which induces a strong nonlocal response in the transverse spatial frequency spectrum. Our paper provides a path to realize highly efficient optical metasurfaces performing isotropic and polarization-insensitive edge detection on an arbitrary 2D optical image.

Neutron diffraction, synchrotron x-ray diffraction, and specific heat studies have been carried out to investigate the nature of the ordered occupation of the vanadium d orbitals in perovskite ${\mathrm{YVO}}_{3}.$ Evidence has been found for a change in the type of orbital ordering at the 77-K phase transition in this material, manifested by a change in the type of Jahn-Teller distortion. This transition between orbital orderings is caused by an increase in octahedral tilting with decreasing temperature. The orbital ordering above 77 K is not destroyed at the magnetic ordering temperature of 116 K, but is present as far as a second structural phase transition at 200 K. The entropy changes at the onset of both spin and orbital ordering are much lower than the smallest semiclassical value of $R\mathrm{ln}2$ J/(mole K).