We present results on the leveling of polymer microdroplets on thin films prepared from the same material. In particular, we explore the crossover from a droplet spreading on an infinitesimally thin film (Tanner’s law regime) to that of a droplet leveling on a film thicker than the droplet itself. In both regimes, the droplet’s excess surface area decreases towards the equilibrium configuration of a flat liquid film, but with a different power law in time. Additionally, the characteristic leveling time depends on molecular properties, the size of the droplet, and the thickness of the underlying film. Flow within the film makes this system fundamentally different from a droplet spreading on a solid surface. We thus develop a theoretical model based on thin film hydrodynamics that quantitatively describes the observed crossover between the two leveling regimes.

Dynamic mode interaction between fundamental and second-harmonic modes has been observed in high-power sub-terahertz gyrotrons [T. Notake et al., Phys. Rev. Lett. 103, 225002 (2009); T. Saito et al. Phys. Plasmas 19, 063106 (2012)]. Interaction takes place between a parasitic fundamental or first-harmonic (FH) mode and an operating second-harmonic (SH) mode, as well as among SH modes. In particular, nonlinear excitation of the parasitic FH mode in the hard self-excitation regime with assistance of a SH mode in the soft self-excitation regime was clearly observed. Moreover, both cases of stable two-mode oscillation and oscillation of the FH mode only were observed. These observations and theoretical analyses of the dynamic behavior of the mode interaction verify the nonlinear hard self-excitation of the FH mode.

Using solid, machined $\mathsf{X}$-pinch targets driven by currents rising from 0 to 5–6 MA in 60 ns, we observed bright spots of 5–9-keV continuum radiation from $5\pm 2\mathrm{\text{−}}\mu \mathrm{m}$ diameter regions. The $>6\mathrm{\text{−}}\mathrm{keV}$ radiation is emitted in about 0.4 ns, and the bright spots are roughly 75 times brighter than the bright spots measured at 1 MA. A total x-ray power of 10 TW peak and yields of $165\pm 20\mathrm{kJ}$ were emitted from a 3-mm height. The 3–5-keV continuum radiation had a 50–90-GW peak power and 0.15–0.35-kJ yield. The continuum is plausibly from a $1275\pm 75\mathrm{\text{−}}\mathrm{eV}$ blackbody or alternatively from a $3500\pm 500\mathrm{\text{−}}\mathrm{eV}$ bremsstrahlung source.

We measure the excitation spectrum of a dipolar chromium Bose-Einstein condensate with Raman-Bragg spectroscopy. The energy spectrum depends on the orientation of the dipoles with respect to the excitation momentum, demonstrating an anisotropy that originates from the dipole-dipole interactions between the atoms. We compare our results with the Bogoliubov theory based on the local density approximation and, at large excitation wavelengths, with the numerical simulations of the time-dependent Gross-Pitaevskii equation. Our results show an anisotropy of the speed of sound.

Quantum nuclear zero-point motions in solid ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ under pressure are investigated at 80 K up to 160 GPa by first-principles path-integral molecular dynamics calculations. Molecular orientations are well defined in phase II of ${\mathrm{D}}_2$, while solid ${\mathrm{H}}_2$ exhibits large and very asymmetric angular quantum fluctuations in this phase, with possible rotation in the ($bc$) plane, making it difficult to associate a well-identified single classical structure. The mechanism for the transition to phase III is also described. Existing structural data support this microscopic interpretation.

We show that antiparallel triplet pairing correlations are generated in superfluids with purely $s$-wave interactions whenever population imbalance enforces anisotropic Fulde-Ferrell (FF) or inhomogeneous Larkin-Ovchinikov (LO) states. These triplet correlations appear in the Cooper pair wave function, while the triplet part of the gap remains zero. The same set of quasiparticle states contributes to the triplet component and to the polarization, thus spatially correlating them. In the LO case, this set forms a narrow band of Andreev states centered on the nodes of the $s$-wave order parameter. This picture naturally provides a unifying explanation of previous findings that attractive $p$-wave interaction stabilizes FFLO states. We also study a similar triplet mixing which occurs when a balanced two-component system displays FFLO-type oscillations due to a spin-dependent optical lattice. We discuss how this triplet component can be measured in systems of ultracold atoms using a rapid ramp across a $p$-wave Feshbach resonance. This should provide a smoking gun signature of FFLO states.

The rutile ${\mathrm{TiO}}_2(110)$ surface is the most studied surface of titania and considered as a prototype of transition metal oxide surfaces. Reactions on flat ${\mathrm{TiO}}_2(110)\mathrm{\text{−}}(1\mathbf{\times }1)$ surfaces are well studied, but the processes occurring on the step edges have barely been considered. Based on scanning tunneling microscopy studies, we here present experimental evidence for the existence of O vacancies along the $⟨1\overline{1}1{⟩}_{\mathrm{R}}$ step edges (${\mathrm{O}}_{\mathrm{S}}$ vac.’s) on rutile ${\mathrm{TiO}}_2(110)$. Both the distribution of bridging O vacancies on the terraces and temperature-programed reaction experiments of ethanol-covered ${\mathrm{TiO}}_2(110)$ point to the existence of the ${\mathrm{O}}_{\mathrm{S}}$ vac.’s. Based on experiments and density functional theory calculations, we show that ${\mathrm{O}}_{\mathrm{S}}$ vac.’s are reactive sites for ethanol dissociation via O-H bond scission. Implications of these findings are discussed.

An intriguing result of statistical mechanics is that a first-order phase transition can be rounded by disorder coupled to energylike variables. In fact, even more intriguing is that the rounding may manifest itself as a critical point, quantum or classical. In general, it is not known, however, what universality classes, if any, such criticalities belong to. In order to shed light on this question we examine in detail the disordered three-color Ashkin-Teller model by Monte Carlo methods. Extensive analyses indicate that the critical exponents define a new universality class. We show that the rounding of the first-order transition of the pure model due to the impurities is manifested as criticality. However, the magnetization critical exponent, $\beta$, and the correlation length critical exponent, $\nu$, are found to vary with disorder and the four-spin coupling strength, and we conclusively rule out that the model belongs to the universality class of the two-dimensional Ising model.

We show that on suitably pit-patterned Si(001), deposition of just a few atomic layers of Ge can trigger a far larger flow of Si into the pits. This surprising effect results in anomalous smoothing of the substrate preceding island formation in the pits. We show that the effect naturally arises in continuum simulations of growth, and we identify its physical origin in the composition dependence of the surface diffusivity. Our interpretation suggests that anomalous smoothing is likely to also occur in other technologically relevant heteroepitaxial systems.

We investigate the structure of mixed thin films composed of pentacene and diindenoperylene using x-ray reflectivity and grazing incidence x-ray diffraction. For equimolar mixtures we observe vanishing in-plane order coexisting with an excellent out-of-plane order, a yet unreported disordering behavior in binary mixtures of organic semiconductors, which are crystalline in their pure form. One approach to rationalize our findings is to introduce an anisotropic interaction parameter in the framework of a mean field model. By comparing the structural properties with those of other mixed systems, we discuss the effects of sterical compatibility and chemical composition on the mixing behavior, which adds to the general understanding of interactions in molecular mixtures.

We present a comprehensive theoretical investigation of the light absorption rate at a $\mathrm{Pb}/\mathrm{Ge}(111)-\beta \sqrt{3}\times \sqrt{3}R30°$ surface with strong spin-orbit coupling. Our calculations show that electron spin-flip transitions cause as much as 6% of the total light absorption, representing 1 order of magnitude enhancement over Rashba-like systems. Thus, we demonstrate that a substantial part of the light irradiating this nominally nonmagnetic surface is attenuated in spin-flip processes. Remarkably, the spin-flip transition probability is structured in well-defined hot spots within the Brillouin zone, where the electron spin experiences a sudden 90° rotation. This mechanism offers the possibility of an experimental approach to the spin-orbit phenomena by optical means.

A combination of density functional and dynamical mean field theory calculations are used to show that the remarkable metal-insulator transition in the rare-earth-element nickelate perovskites arises from a site-selective Mott phase, in which the $d$ electrons on half of the Ni ions are localized to form a fluctuating moment while the $d$ electrons on other Ni ions form a singlet with holes on the surrounding oxygen ions. The calculation reproduces key features observed in the nickelate materials, including an insulating gap in the paramagnetic state, a strong variation of static magnetic moments among Ni sites and an absence of charge order. A connection between structure and insulating behavior is documented. The site-selective Mott transition may be a more broadly applicable concept in the description of correlated materials.

We study the conductivity of a three-dimensional disordered metal close to antiferromagnetic instability within the framework of the spin-fermion model using the diagrammatic technique. We calculate the interaction correction $\delta \sigma (\omega ,T)$ to the conductivity, assuming that the latter is dominated by the disorder scattering, and the interaction is weak. Although the fermionic scattering rate shows critical behavior on the entire Fermi surface, the interaction correction is dominated by the processes near the hot spots, narrow regions of the Fermi surface corresponding to the strongest spin-fermion scattering. Exactly at the critical point $\delta \sigma \propto [\max (\omega ,T){]}^{3/2}$. At sufficiently large frequencies $\omega$ the conductivity is independent of the temperature, and $\delta \sigma \propto ({\tau }^{-1}-i\omega {)}^{-2}$, $\tau$ being the elastic scattering time. In a certain intermediate frequency range $\delta \sigma (\omega )\propto i\omega ({\tau }^{-1}-i\omega {)}^{-2}$.

We perform an accurate time-dependent numerical study of an out-of-equilibrium response of a bound state within $t\mathrm{\text{−}}J$ systems on a two-leg ladder and a square lattice. We show that the bound hole pair decays with the onset of finite steady current if both mechanisms for binding and the dissipation share matching degrees of freedom. Moreover, by investigating the mechanism of decay on the square lattice we find that the dynamics is governed by the decay in the direction perpendicular to the electric field, leading to much shorter decay times in comparison to the ladder where such dynamics is topologically restricted.

The thermoelectric coefficients have been measured down to a very low temperature for the Yb-based heavy-fermion compounds $\beta \mathrm{\text{−}}{\mathrm{YbAlB}}_4$ and ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$, often considered as model systems for the local quantum criticality case. We observe a striking difference in the behavior of the Seebeck coefficient $S$ in the vicinity of their respective quantum critical point (QCP). Approaching the critical field, $S/T$ is enhanced in $\beta \mathrm{\text{−}}{\mathrm{YbAlB}}_4$, but drastically reduced in ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$. The ratio of thermopower to specific heat remains constant for $\beta \mathrm{\text{−}}{\mathrm{YbAlB}}_4$, but it is significantly reduced near the QCP in ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$. In both systems, on the other hand, the Nernst coefficient shows a diverging behavior near the QCP. The interplay between valence and magnetic quantum criticality and the additional possibility of a Lifshitz transition crossing the critical field under magnetic field are discussed as the origin of the different behaviors of these compounds.

Resonant scatterers such as hydrogen adatoms can strongly enhance the low-energy density of states in graphene. Here, we study the impact of these impurities on electronic screening. We find a two-faced behavior: Kubo formula calculations reveal an increased dielectric function $\epsilon$ upon creation of midgap states but no metallic divergence of the static $\epsilon$ at small momentum transfer $q\to 0$. This bad metal behavior manifests also in the dynamic polarization function and can be directly measured by means of electron energy loss spectroscopy. A new length scale $l_c$ beyond which screening is suppressed emerges, which we identify with the Anderson localization length.

We demonstrate that a giant spin Hall effect (SHE) can be induced by introducing a small amount of Bi impurities in Cu. Our analysis, based on a new three-dimensional finite element treatment of spin transport, shows that the sign of the SHE induced by the Bi impurities is negative and its spin Hall (SH) angle amounts to $-0.24$. Such a negative large SH angle in CuBi alloys can be explained by applying the resonant scattering model proposed by Fert and Levy [Phys. Rev. Lett. 106, 157208 (2011)] to $6p$ impurities.

Gapless electronic systems containing topologically nontrivial Fermi points are sources of various topological insulators. Whereas, most of these special band-crossing points are built in the electronic structure of the noninteracting lattice models, we show that a quadratic Fermi point characterized by a nonzero winding number emerges with a collinear triple-$\mathbf{Q}$ spin-density-wave state that arises from a perfectly nested but topologically trivial Fermi surface. We obtain a universal low-energy Hamiltonian for the quadratic Fermi point and show that such collinear orderings are unstable against the onset of scalar spin chirality that opens a gap and induces a spontaneous quantum Hall insulator as the temperature tends to zero.

We study ring-shaped (Corbino) devices made of bilayer two-dimensional electron gases in the total filling factor one quantized Hall phase, which is considered to be a coherent Bardeen-Cooper-Schrieffer-like state of interlayer excitons. Identical Josephson currents are observed at the two edges while only a negligible conductance between them is found. The maximum Josephson current observed at either edge can be controlled by passing a second interlayer Josephson current at the other edge. Because of the large electric resistance between the two edges, the interaction between them can only be mediated by the neutral interlayer excitonic ground state.

The Kondo effect is a striking consequence of the coupling of itinerant electrons to a quantum spin with degenerate energy levels. While degeneracies are commonly thought to arise from symmetries or fine-tuning of parameters, the recent emergence of Majorana fermions has brought to the fore an entirely different possibility: a topological degeneracy that arises from the nonlocal character of Majorana fermions. Here we show that nonlocal quantum spins formed from these degrees of freedom give rise to a novel topological Kondo effect. This leads to a robust non-Fermi liquid behavior, known to be difficult to achieve in the conventional Kondo context. Focusing on mesoscopic superconductor devices, we predict several unique transport signatures of this Kondo effect, which would demonstrate the nonlocal quantum dynamics of Majorana fermions and validate their potential for topological quantum computation.