Instituto de Fisica de Liquidos y Sistemas Biologicos

Sources of magnetic fields-magnetic monopoles-have so far proven elusive as elementary particles. Condensed-matter physicists have recently proposed several scenarios of emergent quasiparticles resembling monopoles. A particularly simple proposition pertains to spin ice on the highly frustrated pyrochlore lattice. The spin-ice state is argued to be well described by networks of aligned dipoles resembling solenoidal tubes-classical, and observable, versions of a Dirac string. Where these tubes end, the resulting defects look like magnetic monopoles. We demonstrated, by diffuse neutron scattering, the presence of such strings in the spin ice dysprosium titanate (Dy2Ti2O7). This is achieved by applying a symmetry-breaking magnetic field with which we can manipulate the density and orientation of the strings. In turn, heat capacity is described by a gas of magnetic monopoles interacting via a magnetic Coulomb interaction.

In principle, a complex assembly of strongly interacting electrons can self-organize into a wide variety of collective states, but relatively few such states have been identified in practice. We report that, in the close vicinity of a metamagnetic quantum critical point, high-purity strontium ruthenate Sr3Ru2O7 possesses a large magnetoresistive anisotropy, consistent with the existence of an electronic nematic fluid. We discuss a striking phenomenological similarity between our observations and those made in high-purity two-dimensional electron fluids in gallium arsenide devices.

Squeezing into the third dimension Cuprate superconductors are known to harbor charge order in part of their phase diagram. Curiously, the order has a two-dimensional (2D) character at zero magnetic field, whereas a 3D order appears at high fields. Kim et al. now show that in a yttrium-based cuprate, a 3D charge order can be induced even at zero magnetic field. The authors compressed the material along one direction and measured a large inelastic x-ray scattering signal that was consistent with the formation of a 3D order. The measurements suggest that the induced order is associated with an optical lattice mode in the material. Science , this issue p. 1040

Magnetic skyrmions are topological solitons with a nanoscale winding spin texture that hold promise for spintronics applications1–4. Skyrmions have so far been observed in a variety of magnets that exhibit nearly parallel alignment for neighbouring spins, but theoretically skyrmions with anti-parallel neighbouring spins are also possible. Such antiferromagnetic skyrmions may allow more flexible control than conventional ferromagnetic skyrmions5–10. Here, by combining neutron scattering measurements and Monte Carlo simulations, we show that a fractional antiferromagnetic skyrmion lattice is stabilized in MnSc2S4 through anisotropic couplings. The observed lattice is composed of three antiferromagnetically coupled sublattices, and each sublattice is a triangular skyrmion lattice that is fractionalized into two parts with an incipient meron (half-skyrmion) character11,12. Our work demonstrates that the theoretically proposed antiferromagnetic skyrmions can be stabilized in real materials and represents an important step towards their implementation in spintronic devices. Theoretically predicted fractional antiferromagnetic skyrmions are experimentally realized in MnSc2S4 and are found to originate from anisotropic couplings over nearest neighbours in the crystal lattice.

We present an experimental study of jamming in the discharge of grains through an opening in a two-dimensional silo. For a wide range of outlet sizes, we obtain the size distribution of avalanche defined as the number of grains that fall between two consecutive jams. From these distributions, we obtain the probability that the silo jams before N particles pass through the orifice. Then a simple model of arch formation is proposed that predicts the shape of the jamming probability function and reveals that it does not exist a critical size of the orifice above which there is not jamming.

Abstract Magnetic skyrmions are topological solitons with a nanoscale winding spin texture that hold promise for spintronics applications1–4. Skyrmions have so far been observed in a variety of magnets that exhibit nearly parallel alignment for neighbouring spins, but theoretically skyrmions with anti-parallel neighbouring spins are also possible. Such antiferromagnetic skyrmions may allow more flexible control than conventional ferromagnetic skyrmions5–10. Here, by combining neutron scattering measurements and Monte Carlo simulations, we show that a fractional antiferromagnetic skyrmion lattice is stabilized in MnSc2S4 through anisotropic couplings. The observed lattice is composed of three antiferromagnetically coupled sublattices, and each sublattice is a triangular skyrmion lattice that is fractionalized into two parts with an incipient meron (half-skyrmion) character11, 12. Our work demonstrates that the theoretically proposed antiferromagnetic skyrmions can be stabilized in real materials and represents an important step towards their implementation in spintronic devices.

Connective tissues-skeleton, dermis, pericytes, fascia-are a key cell source for regenerating the patterned skeleton during axolotl appendage regeneration. This complexity has made it difficult to identify the cells that regenerate skeletal tissue. Inability to identify these cells has impeded a mechanistic understanding of blastema formation. By tracing cells during digit tip regeneration using brainbow transgenic axolotls, we show that cells from each connective tissue compartment have distinct spatial and temporal profiles of proliferation, migration, and differentiation. Chondrocytes proliferate but do not migrate into the regenerate. In contrast, pericytes proliferate, then migrate into the blastema and give rise solely to pericytes. Periskeletal cells and fibroblasts contribute the bulk of digit blastema cells and acquire diverse fates according to successive waves of migration that choreograph their proximal-distal and tissue contributions. We further show that platelet-derived growth factor signaling is a potent inducer of fibroblast migration, which is required to form the blastema.

A generalization of the van der Waals equation of state is presented for a confined fluid in a nanopore. The pressure in the fluid, confined in a narrow pore of infinite length, has tensorial character. From this hypothesis, the Helmholtz free energy is constructed and expressions for the axial and transversal components of the pressure tensor are obtained. The equations predict liquid-vapor equilibria, and a shift of the critical point with respect to that obtained from the van der Waals bulk equation. The results are in good agreement with recent experiments.

Low-temperature phase transitions and the associated quantum critical points are a major field of research, but one in which experimental information about thermodynamics is sparse. Thermodynamic information is vital for the understanding of quantum many-body problems. We show that combining measurements of the magnetocaloric effect and specific heat allows a comprehensive study of the entropy of a system. We present a quantitative measurement of the entropic landscape of Sr3Ru2O7, a quantum critical system in which magnetic field is used as a tuning parameter. This allows us to track the development of the entropy as the quantum critical point is approached and to study the thermodynamic consequences of the formation of a novel electronic liquid crystalline phase in its vicinity.

Interatomic potentials are determined in the framework of a shell model used to simulate the structural instabilities, dynamical properties, and phase transition sequence of BaTiO 3 . The model is developed from first-principles calculations by mapping the potential energy surface for various ferroelectric distortions. The parameters are obtained by performing a fit of interatomic potentials to this energy surface. Several zero-temperature properties of BaTiO 3 , which are of central importance, are correctly simulated in the framework of our model. The phase diagram as a function of temperature is obtained through constant-pressure molecular dynamics simulations, showing that the non-trivial phase transition sequence of BaTiO 3 is correctly reproduced. The lattice parameters and expansion coefficients for the different phases are in good agreement with experimental data, while the theoretically determined transition temperatures tend to be too small.

The static and dynamic properties of \ensuremath{\beta}-Sn, SnO, and ${\mathrm{SnO}}_{2}$ are studied using the full-potential linear-muffin-tin-orbital method within the local-density approximation (LDA). Equilibrium lattice parameters and bulk moduli (including pressure variations) are in excellent agreement with experimental values. The cohesive energies are calculated too large, in accordance with the usual overbinding found in the LDA. Optical \ensuremath{\Gamma}-point phonon frequencies are obtained using the frozen phonon approach. For those phonon modes which have been measured, experimental identifications are confirmed, with the single exception of the ${\mathit{B}}_{1\mathit{g}}$ mode of SnO, for which we find a frequency that is three times larger than the measured value. It is argued that the assignment of the observed mode is wrong.

The electronic structure and frequency dependent dielectric function $\ensuremath{\varepsilon}(\ensuremath{\omega})$ of rocksalt semiconductors PbSe and PbTe are investigated using the local density approximation (LDA) and the generalized gradient approximation as two different exchange and correlation approximations, within the full-potential linearized augmented plane-wave approach. Spin-orbit coupling has been incorporated in the study. The results are presented and compared with other recent calculations and experimental data. Structural properties are also obtained by means of calculations of total energy as a function of lattice parameters. The bulk structural parameters are sensitive to the choice of exchange and correlation approximation. The essential features of the band structure and density of states of PbSe and PbTe are reproduced by our calculations and agree quite well with available experimental results. The position of the minimum energy gap is correctly predicted, although the value of the gap is as usual, underestimated by the local density approximation with respect to the experimental data. This gap value is improved by the inclusion of the generalized gradient approximation. Also, we have calculated the real $[{\ensuremath{\varepsilon}}_{1}(\ensuremath{\omega})]$ and imaginary $[{\ensuremath{\varepsilon}}_{2}(\ensuremath{\omega})]$ parts of $\ensuremath{\varepsilon}(\ensuremath{\omega})$ for both compounds, in the framework of the LDA scheme for exchange and correlation. The inclusion of spin-orbit coupling leads to a richer structure in both ${\ensuremath{\varepsilon}}_{1}(\ensuremath{\omega})$ and ${\ensuremath{\varepsilon}}_{2}(\ensuremath{\omega}).$ The agreement with experimental results is satisfactory.

In an externally applied magnetic field, ultrapure crystals of the bilayer compound ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ undergo a metamagnetic transition below a critical temperature, ${T}^{\ensuremath{\ast}}$, which varies as a function of the angle between the magnetic field $H$ and the Ru-O planes. Moreover, ${T}^{\ensuremath{\ast}}$ approaches zero when $H$ is perpendicular to the planes. This putative ``metamagnetic quantum critical point,'' however, is pre-empted by a nematic fluid phase with order one resistive anisotropy in the $ab$ plane. In a ``realistic'' bilayer model with moderate strength local Coulomb interactions, the existence of a sharp divergence of the electronic density of states near a van Hove singularity of the quasi-one-dimensional bands, and the presence of spin-orbit coupling results in a mean-field phase diagram which accounts for many of these experimentally observed phenomena. Although the spin-orbit coupling is not overly strong, it destroys the otherwise near-perfect Fermi-surface nesting and hence suppresses spin-density-wave ordering.

In this paper, we present the results of electronic structure, ab initio calculations performed on ReO 3, WO 3, and the stoichiometric tungsten bronze NaWO 3 . We examine the relation between the structural and the electronic properties of the three materials and comment on the solid state chemistry governing the interaction between the transition metal and its oxygen ligands. We show that off-center displacements of the W ion in WO 3 are driven by the onset of covalent interactions with the nearest oxygen, while the metallic materials ReO 3 and NaWO 3 are stable when cubic. In the latter case, antibonding contributions due to the occupation of the conduction band oppose the deformation. The different behavior is justified by examining the band structure of the compounds. The effect of the different number of valence electrons and of the different nature of the transition metal on the electronic distribution in the solid are analyzed. Finally, by comparing the mechanical properties of the three oxides, we show that the antibonding conduction electron makes ReO 3 very rigid and can suggest an explanation for the pressure-induced phase transition observed for this material.

We present experimental results of the jamming of noncohesive particles discharged from a flat bottomed silo subjected to vertical vibration. When the exit orifice is only a few grain diameters wide, the flow can be arrested due to the formation of blocking arches. Hence, an external excitation is needed to resume the flow. The use of a continuous gentle vibration is a usual technique to ease the flow in such situations. Even though jamming is less frequent, it is still an issue in vibrated silos. There are, in principle, two possible mechanisms through which vibrations may facilitate the flow: (i) a decrease in the probability of the formation of blocking arches and (ii) the breakage of blocking arches once they have been formed. By measuring the time intervals inside an avalanche during which no particles flow through the outlet, we are able to estimate the probability of breaking a blocking arch by vibrations. The result agrees with the prediction of a bivariate probabilistic model in which the formation of blocking arches is equally probable in vibrated and nonvibrated silos. This indicates that the second aforementioned mechanism is mainly responsible for improving the flowability in gently vibrated silos.

We present experimental results on the shape of arches that block the outlet of a two-dimensional silo. For a range of outlet sizes, we measure some properties of the arches such as the number of particles involved, the span, the aspect ratio, and the angles between mutually stabilizing particles. These measurements shed light on the role of frictional tangential forces in arching. In addition, we find that arches tend to adopt an aspect ratio (the quotient between height and half the span) close to 1, suggesting an isotropic load. The comparison of the experimental results with data from numerical models of the arches formed in the bulk of a granular column reveals the similarities of both, as well as some limitations in the few existing models.

The electronic structure of ${\mathrm{SrBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ is investigated from first-principles, within the local-density approximation, using the full-potential linearized augmented plane-wave method. The results show that, besides the large $\mathrm{Ta}(5d)\ensuremath{-}\mathrm{O}(2p)$ hybridization which is a common feature of the ferroelectric perovskites, there is an important hybridization between bismuth and oxygen states. The underlying static potential for the ferroelectric distortion and the primary source for ferroelectricity is investigated by a lattice-dynamics study using the frozen-phonon approach.

The Perdew-Burke-Ernzerhof generalized gradient approximation to the density functional theory is tested with respect to sensitivity to the choice of the value of the parameter \ensuremath{\kappa}, which is associated with the degree of localization of the exchange-correlation hole. A study of structural and dynamical properties of four selected ferroelectric perovskites is presented. The originally proposed value of $\ensuremath{\kappa}=0.804$ works well for some solids, whereas for the $\mathrm{AB}{\mathrm{O}}_{3}$ perovskites it must be decreased in order to predict equilibrium lattice parameters in good agreement with experiments. The effects on the structural instabilities and zone center phonon modes are examined. The need of varying \ensuremath{\kappa} from one system to another reflects the fact that the localization of the exchange-correlation hole is system dependent, and the sensitivity of the structural properties to its actual value illustrates the necessity of finding a universal function for \ensuremath{\kappa}.

Abstract By reviewing the experimental and theoretical literature on γ′‐Fe 4 N, and by a systematic survey of predictions by the LDA, PBE, WC, LDA + U (2×), PBE + U (2×) and B3PW91 exchange‐correlation functionals, the structural, magnetic and hyperfine properties of this material as well as their pressure dependencies are interpreted. The hypothesis is put forward that γ′‐Fe 4 N as found in Nature is exactly at a steep transition between low‐spin and high‐spin behaviour. PBE + U ( U = 0.4 eV) is identified as the most accurate exchange‐correlation functional for this material, although it is needed to fix the magnetization at the experimental value to obtain a satisfying description. Remaining disagreement between theory and experiment is pointed out. A recent experimental claim for a giant magnetic moment in γ′‐Fe 4 N is discussed, and is not reproduced by our calculations. We expect that the new insight obtained in the present work can lead to a consistent ab initio modeling of other materials in the iron‐nitrogen binary system. In an accompanying didactic section, the physics behind some common exchange‐correlation functionals is outlined. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We experimentally demonstrate that the flow rate of granular material through an aperture is controlled by the exit velocity imposed on the particles and not by the pressure at the base, contrary to what is often assumed in previous work. This result is achieved by studying the discharge process of a dense packing of monosized disks through an orifice. The flow is driven by a conveyor belt. This two-dimensional horizontal setup allows us to independently control the velocity at which the disks escape the horizontal silo and the pressure in the vicinity of the aperture. The flow rate is found to be proportional to the belt velocity, independent of the amount of disks in the container and, thus, independent of the pressure in the outlet region. In addition, this specific configuration makes it possible to get information on the system dynamics from a single image of the disks that rest on the conveyor belt after the discharge.