It has recently been shown that electronic states in bulk gapless HgCdTe offer another realization of pseudo-relativistic three-dimensional particles in condensed matter systems. These single valley relativistic states, massless Kane fermions, cannot be described by any other relativistic particles. Furthermore, the HgCdTe band structure can be continuously tailored by modifying cadmium content or temperature. At critical concentration or temperature, the bandgap collapses as the system undergoes a semimetal-to-semiconductor topological phase transition between the inverted and normal alignments. Here, using far-infrared magneto-spectroscopy we explore the continuous evolution of band structure of bulk HgCdTe as temperature is tuned across the topological phase transition. We demonstrate that the rest mass of Kane fermions changes sign at critical temperature, whereas their velocity remains constant. The velocity universal value of (1.07±0.05)x1E6 m/s remains valid in a broad range of temperatures and Cd concentrations, indicating a striking universality of the pseudo-relativistic description of the Kane fermions in HgCdTe.

(Al,Ga)N-based quantum dots (QDs) grown on Al0.5Ga0.5N by molecular beam epitaxy have been studied as the active region for the fabrication of ultra-violet (UV) light emitting diodes (LEDs). In the first part, using both “polar” (0001) and “semipolar” (112̄2) surface orientations, the structural and optical prop- erties of different QD structures are investigated and compared. In particular, their propensity to get an emission in the UV range is analyzed in correlation with the influence of the internal electric field on their optical properties. In a second part, (0001) and (112̄2)-oriented LEDs using GaN/Al0.5Ga0.5N QD as active regions have been fabricated. Their main current-voltage characteristics and electroluminescence properties are discussed, with a focus on the LED emission wavelength range reached for both surface orientations: it is shown that a large part of the UV-A region can be covered, with longer wavelengths- from 415 to 360 nm-for the “polar” LEDs, and shorter ones-from 345 to 325 nm-for the “semipolar” LEDs. In addition, the influence of the internal electric field on the QD-LEDs working operation is shown.

We perform molecular dynamics simulations for a silica glass former model proposed by Coslovich and Pastore (CP) over a wide range of densities. The density variation can be mapped onto the change of the potential depth between Si and O interactions of the CP model. By reducing the potential depth (or increasing the density), the anisotropic tetrahedral network structure observed in the original CP model transforms into the isotropic structure with the purely repulsive soft-sphere potential. Correspondingly, the temperature dependence of the relaxation time exhibits the crossover from the Arrhenius to the super-Arrhenius behavior. Being able to control the fragility over a wide range by tuning the potential of a single model system helps us to bridge the gap between the network and isotropic glass formers and to obtain the insight into the underlying mechanism of the fragility. We study the relationship between the fragility and dynamical properties such as the magnitude of the Stokes-Einstein violation and the stretch exponent in the density correlation function. We also demonstrate that the peak of the specific heat systematically shifts as the density increases, hinting that the fragility is correlated with the hidden thermodynamic anomalies of the system.

Systems with short-range attractive and long-range repulsive interactions are able to form mesophases at sufficiently low temperatures. In two dimensions, such mesophases emerge as clusters, stripes or bubbles. Using extensive Monte Carlo simulations we investigate the static and the dynamic properties of such a cluster-forming system over a broad temperature range and for different densities. Via the static properties we analyse how ordering into close packed configurations sets in both at the level of the particles as well as at the level of the clusters. The dynamic properties provide information on how, at low temperature, the motion of individual particles is influenced by the dynamic slowing down of the clusters. Finally, we discuss the different diffusion mechanisms at play at low and intermediate

Microvessel blood flow imaging techniques are widely used in biomedical research and clinical diagnostics where many diseases have a vascular etiology or involvement. For testing purposes, zebrafish embryo provides an ideal animal model to achieve high-resolution imaging of superficial and deeply localized vessels. Moreover, the study of the formation of a closed circulatory system in vertebrates is a topic of recent interest in biophysics. However, most of the existing techniques are invasive due to the use of a contrast agent for imaging purposes. Recent developments in Digital Holography and Laser Doppler Holography techniques can be considered to alleviate this issue. Laser Doppler holography and transmission microscopy can be coupled to analyze blood flow in fish embryos by adapting a laser Doppler holographic setup to a standard bio-microscope: the two beams of the holographic interferometer (illumination of the object and reference), whose frequency offset is controlled, were addressed to the microscope by optical fibers. Multimodal acquisition and analysis of the data is made by acting on the frequency offset of the two beams, and on the location of the Fourier space filtered zone. In this work, we show that it is possible to select the signal of moving scatterers, and to image Red Blood Cells (RBCs) and blood vessels. Individual RBCs are imaged, and movies showing the RBC motion are obtained. Microsc. Res. Tech., 2016. © 2016 Wiley Periodicals, Inc.

We study the interplay of conductive and radiative heat transfer (RHT) in planar geometries and predict that temperature gradients induced by radiation can play a significant role on the behavior of RHT with respect to gap sizes, depending largely on geometric and material parameters and not so crucially on operating temperatures. Our findings exploit rigorous calculations based on a closed-form expression for the heat flux between two plates separated by vacuum gaps d and subject to arbitrary temperature profiles, along with an approximate but accurate analytical treatment of coupled conduction-radiation in this geometry. We find that these effects can be prominent in typical materials (e.g., silica and sapphire) at separations of tens of nanometers, and can play an even larger role in metal oxides, which exhibit moderate conductivities and enhanced radiative properties. Broadly speaking, these predictions suggest that the impact of RHT on thermal conduction, and vice versa, could manifest itself as a limit on the possible magnitude of RHT at the nanoscale, which asymptotes to a constant (the conductive transfer rate when the gap is closed) instead of diverging at short separations.

In recent years indirect (dipolar) excitons have emerged as highly interesting objects for fundamental studies like Bose-Einstein condensation as well as a concept of excitonic devices for development of excitonic signal processing. An indirect exciton (IX) is a bound pair of an electron and a hole (e-h) in spatially separated quantum wells (QWs). Studies of IX have been concentrated almost entirely on (Al,Ga)As/GaAs coupled double QWs (CDQWs). Due to the large exciton binding energy polar heterostructures of Group III nitrides seem to be more suitable for observation of IX. In the present work we have studied IXs formed by a e-h pair in InGaN/GaN CDQWs. Very thin barrier of 1-2 atomic monolayer (ML) separating QWs enables a tunneling of electrons and holes after optical excitation to the adjacent QWs. The samples consisted of DQWs of In0.17Ga0.83N/In0.02Ga0.98N grown by MBE technique. Intended width of QWs was 2.6 nm and the barrier thickness varied between 1- 8 ML (025-2nm). GaN crystals were used as substrates. The XRD and TEM measurements showed that the obtained structures agree well with the intended ones. Modeling of their band structure using the NEXTNANO software revealed that the structures with the thinnest barriers (1-2 ML) formed one electron (hole) state located in the conduction (valence) band of adjacent QWs. The corresponding emission wavelength was λ≈510 nm. Simulations performed for structures with the thicker barriers demonstrated presence of e- and h-states in each QW and the emission at λ≈450nm. PL measured in these samples showed such an emission. For structures with 1-2 ML barrier we found a peak at λ≈505-520nm which we associated with IX in the system of CDQW. To prove strongly indirect character of the long-wavelength PL-band we performed TRPL characterization of the studied structures. At T=4K, the band at λ≈510nm showed the decay time, τD, approaching msec range. Whereas the band at λ≈450nm is characterized by τD of about 100 ns.