We show how disorder can be used to guide a broadband ultrasonic wave. The idea is to exploit the transverse localization regime that has been reported for light. Our waveguide consists of a set of parallel cylindrical scatterers randomly distributed in the transverse plane. An ultrasonic beam propagating along the direction of scatterers is found to remain confined in the two other directions on a size smaller than the waveguide diameter and driven by the localization length. Interestingly, the guided wave is also found to propagate with a very limited temporal dispersion.

This work presents ${}^{75}$As NMR spin-echo decay rate ($1/T_2$) measurements in Ba(Fe${}_{1-x}$Rh${}_x$)${}_2$As${}_2$ superconductors, for $0.041⩽x⩽0.094$. It is shown that $1/T_2$ increases upon cooling, in the normal phase, suggesting the onset of an unconventional very low-frequency activated dynamic. The correlation times of the fluctuations and their energy barriers are derived. The motion is favored at large Rh content, while it is hindered by the application of a magnetic field perpendicular to the FeAs layers. The same dynamic is observed in the spin-lattice relaxation rate, in a quantitatively consistent manner. These results are discussed in the light of nematic fluctuations involving domain wall motion. The analogies with the behavior observed in the cuprates are also outlined.

We performed a neutron Brillouin scattering determination of the dynamic structure factor of liquid gold in the wave-vector range $6<Q<16$ nm${}^{-1}$. Despite the experimental difficulties due to the considerable neutron absorption and the high melting temperature of the sample, a non-negligible coherent signal could successfully be extracted and revealed the presence of underdamped ion density-fluctuation modes in the whole $Q$ range of the experiment. The quality of the data further enabled a significant comparison with the results of ab initio simulations for liquid gold. The agreement found between neutron and simulation data not only provides a necessary test of ab initio methods still limited to the use of few hundreds atoms, but also reasonably justifies a thorough analysis of the simulated dynamics in the extended $Q$ range that can be more comfortably accessed by calculations. A viscoelastic modeling of the simulated spectra proves to be appropriate and, surprisingly, the so-derived salient dynamic features of liquid gold show an overall $Q$ behavior globally similar to that already found for insulating liquids. The present and recent results on the collective properties of simple liquids thus lead us to embrace the concept that simple liquid metals and insulators, at the nanometer length scale, are dynamically much less different than expected.

We show that the elliptic parametrization of the coupling constants of the quantum $XYZ$ spin chain can be analytically extended outside of their natural domain, to cover the whole phase diagram of the model, which is composed of 12 adjacent regions, related to one another by a spin rotation. This extension is based on the modular properties of the elliptic functions and we show how rotations in parameter space correspond to the double covering $PGL(2,\mathbb{Z})$ of the modular group, implying that the partition function of the $XYZ$ chain is invariant under this group in parameter space, in the same way as a conformal field theory partition function is invariant under the modular group acting in real space. The encoding of the symmetries of the model into the modular properties of the partition function could shed light on the general structure of integrable models.

Multiferroic BiFeO${}_3$ undergoes a transition from a distorted spiral phase to a G-type antiferromagnet above a critical field $H_c$ that depends on the field orientation $\mathbf{m}$. We show that $H_c\left(\mathbf{m}\right)$ has a maximum when oriented along a cubic diagonal parallel to the electric polarization $\mathbf{P}$ and a minimum in the equatorial plane normal to $\mathbf{P}$ when two magnetic domains with the highest critical fields are degenerate. The orientational dependence of $H_c\left(\mathbf{m}\right)$ is more complex than indicated by earlier work, which did not consider the competition between magnetic domains. Some recent measurements might be explained by a mixture of magnetic domains.

Spin-polarized photoemission is used to study Si${}_{1-x}$Ge${}_x$ alloys epitaxially grown on a Si substrate. Spin-oriented electrons are generated in the conduction band upon excitation with circularly polarized light. We prove that in these structures it is possible to lower the vacuum level of the system below the conduction band minimum at the $\Gamma$ point of the Brillouin zone and show that electron spin polarization values $P=40\pm 3\%$ can be achieved when promoting electrons with direct transitions at $\Gamma$. Such values can be greatly increased, up to $P=72\pm 3\%$, in compressively strained thin epitaxial films, thus obtaining performances very close to those of III-V semiconductor heterostructures.

Strain engineering in SiGe nanostructures is fundamental for the design of optoelectronic devices at the nanoscale. Here we explore a new strategy, where SiGe structures are laterally confined by the Si substrate, to obtain high tensile strain yet avoid the use of external stressors, thus improving the scalability. Spectromicroscopy techniques, finite element method simulations, and ab initio calculations are used to investigate the strain state of laterally confined Ge-rich SiGe nanostripes. Strain information is obtained by tip-enhanced Raman spectroscopy with an unprecedented lateral resolution of ∼30 nm. The nanostripes exhibit a large tensile hydrostatic strain component, which is maximal at the center of the top free surface and becomes very small at the edges. The maximum lattice deformation is larger than the typical values of thermally relaxed Ge/Si(001) layers. This strain enhancement originates from a frustrated relaxation in the out-of-plane direction, resulting from the combination of the lateral confinement induced by the substrate side walls and the plastic relaxation of the misfit strain in the (001) plane at the SiGe/Si interface. The effect of this tensile lattice deformation at the stripe surface is probed by work function mapping, which is performed with a spatial resolution better than 100 nm using x-ray photoelectron emission microscopy. The nanostripes exhibit a positive work function shift with respect to a bulk SiGe alloy, quantitatively confirmed by electronic structure calculations of tensile-strained configurations. The present results have a potential impact on the design of optoelectronic devices at a nanometer-length scale.

We examine two factors that could limit the doping of $n$-type III-V semiconductor nanowires, namely, the solubility of the dopants and the possible formation of $DX$-like defect centers. We find that it is preferable to dope zinc-blende nanowires via anion substitution as opposed to cation substitution. The comparison with previous work on $n$-type III-V semiconductor nanocrystals also allows us to determine the role of dimensionality and quantum confinement on doping characteristics of materials. Our work is based on quantum calculations of InP nanowires using real-space pseudopotentials constructed within density functional theory.

During molecular beam epitaxial growth of Ge islands on Si(001) substrates containing pit-patterned areas, an efficient Ge surface diffusion from unpatterned to pit-patterned substrate regions results in position-dependent island growth rates. We exploit this effect to map out the evolution of strictly ordered Ge islands from prepyramids via pyramids and transition domes into domes by atomic force microscopy. A one-dimensional diffusion model is established which allows a quantitative determination of the Ge surface diffusion constant and the rates at which Ge atoms are incorporated into the growing islands. We find evidence that during the shape transitions from nonfaceted islands to $\left\{105\right\}$-faceted pyramids and from pyramids to domes, Ge is incorporated at higher rates than during the growth of pure pyramids or domes. From these data it follows that a Ge atom passes on average several thousand pits before it is incorporated into one. Within our model we show that secondary-island nucleation in the substrate regions between the pits is a consequence of a mismatch between Ge incorporation and deposition rates, which results in a growing wetting layer between the ordered island sites, the thickness of which eventually exceeds the critical one for spontaneous island nucleation. We experimentally demonstrate that by lowering the Ge deposition rate, perfectly ordered islands with large pit periods of around 1 $\mu$m can be grown, in agreement with the model predictions.

We propose an intrinsic spin scattering mechanism in graphene originated by the interplay of atomic spin-orbit interaction and the local curvature induced by flexural distortions of the atomic lattice. Starting from a multiorbital tight-binding Hamiltonian with spin-orbit coupling considered nonperturbatively, we derive an effective Hamiltonian for the spin scattering of the Dirac electrons due to flexural distortions. We compute the spin lifetime due to both flexural phonons and ripples and we find values in the microsecond range at room temperature. Interestingly, this mechanism is anisotropic on two counts. First, the relaxation rate is different for off-plane and in-plane spin quantization axis. Second, the spin relaxation rate depends on the angle formed by the crystal momentum with the carbon-carbon bond. In addition, the spin lifetime is also valley dependent. The proposed mechanism sets an upper limit for spin lifetimes in graphene and will be relevant when samples of high quality can be fabricated free of extrinsic sources of spin relaxation.

We report on the measurement of first-order spatial correlations on a one-dimensional $97\%$ excitonic polariton condensate, excited in cooled ZnO microwires by nonresonant optical pulses. We find that, thanks to the tiny $3\%$ photonic fraction, a coherence length as large as 10 $\mu$m can build up, i.e., orders of magnitude larger than any reported so far for an ultracold exciton gas. Based on a driven-dissipative mean-field model we show that the decay of correlations is mainly due to a shallow disorder and driven-dissipative conditions.

We classify topological insulators and superconductors in the presence of additional symmetries such as reflection or mirror symmetries. For each member of the 10 Altland-Zirnbauer symmetry classes, we have a Clifford algebra defined by operators of the generic (time-reversal, particle-hole, or chiral) symmetries and additional symmetries, together with gamma matrices in Dirac Hamiltonians representing topological insulators and superconductors. Following Kitaev's approach, we classify gapped phases of noninteracting fermions under additional symmetries by examining all possible distinct Dirac mass terms which can be added to the set of generators of the Clifford algebra. We find that imposing additional symmetries in effect changes symmetry classes and causes shifts in the periodic table of topological insulators and superconductors. Our results are in agreement with the classification under reflection symmetry recently reported by Chiu, Yao, and Ryu [Phys. Rev. B 88, 075142 (2013)]. Several examples are discussed including a topological crystalline insulator with mirror Chern numbers and mirror superconductors.

We study heating and dissipation effects in granular nanosystems in the regime of weak coupling between the grains. We focus on the cotunneling regime and solve the heat-dissipation problem in an array of grains exactly. We show that the power to heat ratio has a universal quantized value, which is geometrically protected: It depends only on the number of grains.

We have calculated the exchange-energy contribution to the total energy of quasi-two-dimensional hole systems realized by a hard-wall quantum-well confinement of valence-band states in typical semiconductors. The magnitude of the exchange energy turns out to be suppressed from the value expected for analogous conduction-band systems whenever the mixing between heavy-hole and light-hole components is strong. Our results are obtained using a very general formalism for calculating the exchange energy of many-particle systems where single-particle states are spinors. We have applied this formalism to obtain analytical results for spin-3/2 hole systems in limiting cases.

Energy versus magnetic field (Hofstadter butterfly diagram) in twisted bilayer graphene is studied theoretically. If we take the usual Landau gauge, we cannot take a finite periodicity even when the magnetic flux through a supercell is a rational number. We show that the periodic Landau gauge, which has the periodicity in one direction, makes it possible to obtain the Hofstadter butterfly diagram. Since a supercell can be large, magnetic flux through a supercell normalized by the flux quantum can be a fractional number with a small denominator, even when a magnetic field is not extremely strong. As a result, quantized Hall conductance can be a solution of the Diophantine equation which cannot be obtained by the approximation of the linearized energy dispersion near the Dirac points.

Nonlinear optical properties of a regular array of triangular-shaped vortex magnetic nanoparticles is studied using the optical second harmonic generation (SHG) technique. We demonstrate that the SHG azimuthal anisotropy is consistent with the 3$m$ symmetry of individual Co nanodots placed in a square surface lattice. Qualitatively different SHG magnetic hysteresis loops are obtained for circular and linear polarizations of the fundamental radiation. In the first case, a wide SHG hysteresis at zero DC magnetic field $H$ is observed, which is attributed to a macroscopic magnetic toroid moment in Co nanodots induced by a noncentrosymmetric distribution of the magnetization. On the contrary, for the linear pump polarization the SHG loop is similar to observed commonly in linear magnetooptics for vortex magnetic structures and reveals a rather narrow width at $H=0$. A phenomenological SHG description based on the introduction of the SHG polarization induced by a magnetic toroid moment in vortex magnetic nanostructures is presented.

Coherent magnons and acoustic phonons were impulsively excited and probed in thin films of the room temperature multiferroic Bi${}_{1-x-y}$Dy${}_x$La${}_y$FeO${}_3$ using femtosecond laser pulses. The elastic moduli of rhombohedral, tetragonal, and rare-earth doped BiFeO${}_3$ were determined from acoustic-mode frequencies in conjunction with spectroscopic ellipsometry. A weak ferromagnetic order, induced alternately by magnetization in the growth direction or by tetragonality, created a magnon oscillation at 75 GHz, indicative of a Dzyaloshinskii-Moriya interaction energy of 0.31 meV.

The possibility of thermal excitation of intrinsic localized modes (ILMs) arising from anharmonicity in ionic perfect crystals is studied numerically for realistic model systems in one and three dimensions. Implications are discussed for an interesting high-temperature feature seen in earlier inelastic neutron scattering experiments on single crystal NaI. The general conclusion is that ILM formation energies are far too large for thermal excitation of ILMs to account for the observed feature in a pure crystal.