Noble-transition metal ($\mathrm{\text{noble}}=\mathrm{Pt},\mathrm{Au}$; $\mathrm{\text{transition}}=\mathrm{Co},\mathrm{Ni},\mathrm{Cu}$) alloy particles with sizes of about 5 nm have been studied by in situ high-energy x-ray diffraction while subjected to oxidizing (${\mathrm{O}}_2$) and reducing (${\mathrm{H}}_2$) gas atmospheres at elevated temperatures. The different gas atmospheres do not affect substantially the random alloy, face-centered-cubic structure type of the particles but do affect the way the metal atoms pack together. In an ${\mathrm{O}}_2$ atmosphere, atoms get extra separated from each other, whereas, in an ${\mathrm{H}}_2$ atmosphere, they come closer together. The effect is substantial, amounting to 0.1 Å difference in the first neighbor atomic distances, and concurs with a dramatic change of the particle catalytic properties. It is argued that such reactive gas induced “expansion shrinking” is a common phenomenon that may be employed for the engineering of “smart” nanoparticles responding advantageously to envisaged gas environments.

The evolution of orientation-dependent metastable states during shock-induced solid-liquid phase transitions in crystalline Al is followed using moving window molecular dynamics simulations. The orientation-dependent transition pathways towards an orientation-independent final state Hugoniot include both “cold melting” followed by recrystallization in [110]- and [111]-oriented shock waves and crystal overheating followed by melting in [100] shock waves. The orientation-dependent dynamics take place within a zone that can extend up to hundreds of nanometers behind the shock front.

Peierls distortion and quantum solitons are two hallmarks of 1-dimensional condensed-matter systems. Here we propose a quantum model for a one-dimensional system of nonlinearly interacting electrons and phonons, where the phonons are represented via coherent states. This model permits a unified description of Peierls distortion and quantum solitons. The nonlinear electron-phonon interaction and the resulting deformed symmetry of the Hamiltonian are distinctive features of the model, of which that of Su, Schrieffer, and Heeger can be regarded as a special case.

We provide evidence that the relaxation dynamics of one-dimensional, metallic Fermi systems resulting out of an abrupt amplitude change of the two-particle interaction has aspects which are universal in the Luttinger liquid sense: the leading long-time behavior of certain observables is described by universal functions of the equilibrium Luttinger liquid parameter and the renormalized velocity. We analytically derive those functions for the Tomonaga–Luttinger model and verify our hypothesis of universality by considering spinless lattice fermions within the framework of the density-matrix renormalization group.

We consider electron-phonon coupling in crystalline organic semiconductors, using naphthalene for our case study. Employing a first-principles approach, we compute the changes in the selfconsistent Kohn-Sham potential corresponding to different phonon modes and go on to obtain the carrier-phonon coupling matrix elements (vertex functions). We then evaluate perturbatively the quasiparticle spectral residues for electrons at the bottom of the lowest unoccupied (LUMO), and holes at the top of the highest occupied (HOMO), band, obtaining $Z_{\mathrm{e}}\approx 0.74$ and $Z_{\mathrm{h}}\approx 0.78$, respectively. Along with the widely accepted notion that the carrier-phonon coupling strengths in polyacenes decrease with increasing molecular size, our results provide strong microscopic evidence for the previously conjectured nonpolaronic nature of bandlike carriers in these systems.

We provide a prescription for constructing Hamiltonians representing the low-energy physics of correlated electron materials with dynamically screened Coulomb interactions. The key feature is a renormalization of the hopping and hybridization parameters by the processes that lead to the dynamical screening. The renormalization is shown to be non-negligible for various classes of correlated electron materials. The bandwidth reduction effect is necessary for connecting models to materials behavior and for making quantitative predictions for low-energy properties of solids.

The Coulomb gap in the single-particle density of states (DOS) is a universal consequence of electron-electron interaction in disordered systems with localized electron states. Here we show that in arrays of monodisperse metallic nanocrystals, there is not one but three identical adjacent Coulomb gaps, which together form a structure that we call a “Coulomb gap triptych.” We calculate the DOS and the conductivity in two- and three-dimensional arrays using a computer simulation. Unlike in the conventional Coulomb glass models, in nanocrystal arrays the DOS has a fixed width in the limit of large disorder. The Coulomb gap triptych can be studied via tunneling experiments.

We establish in a combination of ab initio theory and experiments that the tunneling process in scanning tunneling microscopy or spectroscopy on the $A$-122 iron pnictide superconductors—in this case ${\mathrm{BaFe}}_{2\mathrm{\text{−}}x}{\mathrm{Co}}_x{\mathrm{As}}_2$—involves a strong adatom filtering of the differential conductance from the near-$E_F$ $\mathrm{Fe}\mathrm{\text{−}}3d$ states, which in turn originates from the topmost subsurface Fe layer of the crystal. The calculations show that the dominance of surface Ba-related tunneling pathways leaves fingerprints found in the experimental differential conductance data, including large particle-hole asymmetry and energy-dependent contrast inversion in conductance maps.

In this Letter, we explore the phase diagram and excitations of a distorted triangular lattice antiferromagnet. The unique two-dimensional distortion considered here is very different from the “isosceles”-type distortion that has been extensively investigated. We show that it is able to stabilize a $120°$ spin structure for a large range of exchange interaction values, while new structures are found for extreme distortions. A physical realization of this model is $\alpha \mathrm{\text{−}}{\mathrm{CaCr}}_2{\mathrm{O}}_4$, which has a $120°$ structure but lies very close to the phase boundary. This is verified by inelastic neutron scattering which reveals unusual rotonlike minima at reciprocal space points different from those corresponding to the magnetic order.

We present a theoretical discovery of an unconventional mechanism of inverse Faraday effect which acts selectively on topological magnetic structures. The effect, topological inverse Faraday effect, is induced by the spin Berry’s phase of the magnetic structure when a circularly polarized light is applied. Thus a spin-orbit interaction is not necessary unlike that in the conventional inverse Faraday effect. We demonstrate by numerical simulation that topological inverse Faraday effect realizes ultrafast switching of a magnetic vortex within a switching time of 150 ps without magnetic field.

${\mathrm{CuFeO}}_2$ is one of the multiferroic materials and is the first case that the electric polarization is not explained by the magnetostriction model or the spin-current model. We have studied this material using soft x-ray resonant diffraction and found that superlattice reflection 0 $1-2q$ 0 appears in the ferroelectric and incommensurate magnetic ordered phase at the Fe $L_{2,3}$ absorption edges and moreover that the rotation of the x-ray polarization such as from $\sigma$ to $\pi$ or from $\pi$ to $\sigma$ is allowed at this reflection. These findings definitely provide direct evidence that the $3d$ $t_{2g\downarrow }$ orbital state of Fe ions has a long-range order in the ferroelectric state. The spin-orbit interaction in Fe ions plays a crucial role to the ferroelectricity in ${\mathrm{CuFeO}}_2$, coupling two nontrivial spin and orbital orders, both of which break the crystal symmetry.

It is widely believed that magnetic excitations become increasingly incoherent as the temperature is raised due to random collisions which limit their lifetime. This picture is based on spin-wave calculations for gapless magnets in 2 and 3 dimensions and is observed experimentally as a symmetric Lorentzian broadening in energy. Here, we investigate a three-dimensional dimer antiferromagnet and find unexpectedly that the broadening is asymmetric—indicating that far from thermal decoherence, the excitations behave collectively like a strongly correlated gas. This result suggests that a temperature activated coherent state of quasiparticles is not confined to special cases like the highly dimerized spin-$1/2$ chain but is found generally in dimerized antiferromagnets of all dimensionalities and perhaps gapped magnets in general.

We interpret recent measurements of the zero field muon relaxation rate in the magnetic pyrochlore ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ as resulting from the quantum diffusion of muons in the material. In this scenario, the plateau observed at low temperature ($<7\mathrm{K}$) in the relaxation rate is due to coherent tunneling of muons through a spatially disordered spin state and not to any magnetic fluctuations persisting at low temperature. Two further regimes either side of a maximum relaxation rate at $T^{*}=50\mathrm{K}$ correspond to a crossover between tunneling and incoherent activated hopping motion of the muon. Our fit of the experimental data is compared with the case of muonium diffusion in KCl.

The design of nanoantennas has so far been mainly inspired by radio-frequency technology. However, the material properties and experimental settings need to be reconsidered at optical frequencies, which would entail the need for alternative optimal antenna designs. Here we subject a checkerboard-type, initially random array of gold cubes to evolutionary optimization. To illustrate the power of the approach, we demonstrate that by optimizing the near-field intensity enhancement, the evolutionary algorithm finds a new antenna geometry, essentially a split-ring–two-wire antenna hybrid that surpasses by far the performance of a conventional gap antenna by shifting the $n=1$ split-ring resonance into the optical regime.

We report a numerical investigation of the structural properties of very large three-dimensional heaps of particles produced by ballistic deposition from extended circular dropping areas. Very large heaps are found to contain three new geometrical characteristics not observed before: they may have two external angles of repose, an internal angle of repose, and four distinct packing fraction (density) regions. Such characteristics are shown to be directly correlated with the size of the dropping zone. In addition, we also describe how noise during the deposition affects the final heap structure.

We present measurements of the stress as a function of vertical position in a column of granular material sheared in a cylindrical Couette device. All three components of the stress tensor on the outer cylinder were measured as a function of distance from the free surface at shear rates low enough that the material was in the dense, slow flow regime. We find that the stress profile differs fundamentally from that of fluids, from the predictions of plasticity theories, and from intuitive expectation. We argue that the anomalous stress profile is due to an anisotropic fabric caused by the combined action of gravity and shear.

Neutron scattering reveals a complex dynamics in polypeptide chains, with two main onsets of anharmonicity whose physical origin and biological role are still debated. In this study the dynamics of strategically selected homomeric polypeptides is investigated with elastic neutron scattering using different energy resolutions and compared with that of a real protein. Our data spotlight the dependence of anharmonic transition temperatures and fluctuation amplitudes on energy resolution, which we quantitatively explain in terms of a two-site model for the protein-hydration water energy landscape. Experimental data strongly suggest that the protein dynamical transition is not a mere resolution effect but is due to a real physical effect. Activation barriers and free energy values obtained for the protein dynamical transition allow us to make a connection with the two-well interaction potential of supercooled-confined water proposed to explain a $\mathrm{\text{low}}\mathrm{\text{−}}\mathrm{\text{density}}\to \mathrm{\text{high}}\mathrm{\text{−}}\mathrm{\text{density}}$ liquid-liquid transition.

The molecular and cellular basis of left-right asymmetry in plant morphogenesis is a fundamental issue in biology. A rapidly elongating root or hypocotyl of twisting mutants of Arabidopsis thaliana exhibits a helical growth with a handedness opposite to that of the underlying cortical microtubule arrays in epidermal cells. However, how such a hierarchical helical order emerges is currently unknown. We propose a model for investigating macroscopic chiral asymmetry in Arabidopsis mutants. Our elastic model suggests that the helical pattern observed is a direct consequence of the simultaneous presence of anisotropic growth and tilting of cortical microtubule arrays. We predict that the root helical pitch angle is a function of the microtubule helical angle and elastic moduli of the tissues. The proposed model is versatile and is potentially important for other biological systems ranging from protein fibrous structures to tree trunks.

We study the efficiency of self-propelled swimmers at low Reynolds numbers, assuming that the local energetic cost of maintaining a propulsive surface slip velocity is proportional to the square of that velocity. We determine numerically the optimal shape of a swimmer such that the total power is minimal while maintaining the volume and the swimming speed. The resulting shape depends strongly on the allowed maximum curvature. When sufficient curvature is allowed the optimal swimmer exhibits two protrusions along the symmetry axis. The results show that prolate swimmers such as Paramecium have an efficiency that is $\sim 20\%$ higher than that of a spherical body, whereas some microorganisms have shapes that allow even higher efficiency.

The dynamics of the multilamellar vesicle (MLV) is analyzed theoretically, where membrane interaction squeezes the solvent to flow between the neighboring membranes. With the applied affine shear, the dynamic free energy density of the MLV develops a minima, which selects the MLV size. The model predicts a terminal shear rate, below which the metastable MLV exists. The scaling relations for the MLV size and the terminal shear are both consistent with the experiments.