The possibility to achieve entirely frictionless, i.e. superlubric, sliding between solids, holds enormous potential for the operation of mechanical devices. At small length scales, where mechanical contacts are well-defined, Aubry predicted a transition from a superlubric to a pinned state when the mechanical load is increased. Evidence for this intriguing Aubry transition (AT), which should occur in one dimension (1D) and at zero temperature, was recently obtained in few-atom chains. Here, we experimentally and theoretically demonstrate the occurrence of the AT in an extended two-dimensional (2D) system at room temperature using a colloidal monolayer on an optical lattice. Unlike the continuous nature of the AT in 1D, we observe a first-order transition in 2D leading to a coexistence regime of pinned and unpinned areas. Our data demonstrate that the original concept of Aubry does not only survive in 2D but is relevant for the design of nanoscopic machines and devices at ambient temperature.In the expanding fields of nanoscience, where the competition of length scales is of key

We experimentally study the motion of a colloidal monolayer which is driven across a commensurate substrate potential whose amplitude is periodically modulated in time. In addition to a significant reduction of the static friction force compared to an unmodulated substrate, we observe a Shapiro step structure in the force dependence of the mean particle velocity which is explained by the dynamical mode locking between the particle motion and the substrate modulation. In this regime, the entire crystal moves in a stick-slip fashion similar to what is observed when a single point contact is driven across a periodic surface. Contrary to numerical simulations, where typically a large number of Shapiro steps is found, only a single step is observed in our experiments. This is explained by the formation of kinks which weaken the synchronization between adjacent particles.

Articles you may be interested inRandom vs realistic amorphous carbon models for high resolution microscopy and electron diffraction Effects of electron scattering at metal-nonmetal interfaces on electron-phonon equilibration in gold filmsWe discuss the observation of a transient (000)-order attenuation in time-resolved transmission electron diffraction experiments. It is shown that this effect causes a decrease of the diffraction intensity of all higher diffraction orders. This effect is not unique to specific materials as it was observed in thin Au, Ag and Cu films.

Isolated impurity states in epitaxially grown semiconductor systems possess important radiative features such as distinct wavelength emission with a very short radiative lifetime and low inhomogeneous broadening, which make them promising for the generation of indistinguishable single photons. In this study, we investigate chlorine-doped ZnSe/ZnMgSe quantum well (QW) nanopillar (NP) structures as a highly efficient solid-state single-photon source operating at cryogenic temperatures. We show that single photons are generated due to the radiative recombination of excitons bound to neutral Cl atoms in ZnSe QW and the energy of the emitted photon can be tuned from about 2.85 down to 2.82 eV with ZnSe well width increase from 2.7 to 4.7 nm. Following the developed advanced technology, we fabricate NPs with a diameter of about 250 nm using a combination of dry and wet-chemical etching of epitaxially grown ZnSe/ZnMgSe QW structures. The remaining resist mask serves as a spherical- or cylindrical-shaped solid immersion lens on top of NPs and leads to the emission intensity enhancement by up to an order of magnitude in comparison to the pillars without any lenses. NPs with spherical-shaped lenses show the highest emission intensity values. The clear photon-antibunching effect is confirmed by the measured value of the second-order correlation function at a zero time delay of 0.14. The developed single-photon sources are suitable for integration into scalable photonic circuits.

Laser excitation of thin bismuth films leads to a reduction in the diffraction intensity, which exhibits a characteristic angular anisotropy. The anisotropy depends on the polarization of the laser pulse and persists for approximately 150 ps. The effect clearly indicates coherent atomic motion in a preferential direction that we tentatively attribute to a transient shear deformation due to the photoelastic stress induced by the laser pulse.

We show that time-resolved electron diffraction is capable of revealing the ultrafast lattice heating in thin metal films following excitation by a femtosecond laser pulse. The buildup of the lattice temperature leads to a reduction of the diffraction intensity of the various diffraction orders due to the Debye-Waller-effect. We also observed a reduction of the transmitted (000)-signal which exhibits the same temporal evolution as the diffraction signals.

Whenever two adjacent bodies are brought into relative motion, friction occurs. Despite its broad implications to our daily life and technical applications, many aspects of friction are only poorly understood. This particularly applies to conditions, where two atomically smooth surfaces are sliding on top of each other. Using a colloidal monolayer which is driven across a structured surface, we can confirm the dominating role of topological solitons during this process. In addition, we provide experimental evidence for the Aubry transition which predicts a frictionless sliding at incommensurate conditions and for finite substrate corrugations. Our results do not only reveal the subtle structural changes of the monolayer to avoid friction but also a sharp transition from a fully superlubric to a strongly pinned state when the contact strength between the surfaces is increased.

The experimental demonstration of Ge1-xSnx alloys lasers opened group-IV materials towards high-performance electronic and photonic devices that can be easily integrated with the current Si semiconductor technology. In recent years, GeSn-based optoelectronic devices including light-emitting and detectors, modulators, and CMOS have been proven. The major challenges for the Ge1-xSnx epitaxy arise from the low solid solubility of Sn in Ge, the large lattice mismatch, and the reduced thermal stability between Ge and Sn. All these are becoming extremely critical at higher Sn contents. Non-equilibrium conditions offered by molecular beam epitaxy (MBE), chemical vapor deposition (CVD), flash lamp, or laser annealing have been lately investigated. Between them, CVD is to date the preferred growth technique for its current development compatible with the industry offering micron-thick layers with the highest crystal quality. While Tin-tetrachloride (SnCl4) becomes the standard Sn precursor, for Ge different gasses, like germane (GeH4) and digermane (Ge2H6) are used attempting to archive high Sn incorporation and high material quality. While Ge1-xSnx films with the same high Sn content can be obtained regardless of the used precursor, the advantages and disadvantages of each precursor are discussed in this work. The use of Ge2H6 is accompanied by high growth rates, being favorable in applications where relatively thick films are needed, such as laser structures. On the other hand, with a relatively low growth rate, GeH4 provides a greater thickness control, achieving clear and sharp interfaces in heterostructures. For this reason, GeH4 is the appropriate precursor for quantum transport or spintronic. The biggest challenge in heterostructure designs is going up and down in Sn content. The growth of a Ge1-ySny on a Ge1-xSnx, y<x, or SiGeSn layer cannot be performed by increasing the growth temperature. Post-annealing processes lead to strong crystallinity degradation of the already grown layer by strong Sn diffusion or Sn segregation due to the limited thermal stability of Ge1-xSnx alloys. In this work, we address simple methodologies to enhance the gradient or step Sn content without changing the process temperature. Controlling only the carrier gas flow while keeping the standard growth parameters constant, high-quality Ge1-xSnx alloys with uniform Sn content up to 15 at.% are obtained. The proposed method acts as guidance to produce Ge1-xSnx heterostructures that can be extended to any CVD reactor, independently of the used precursor, GeH4 or Ge2H6. Different devices structures are presented proving the applicability of the isothermal multilayer growth. Figure 1