Condensed Matter — Open Access Journal

In this paper, we analyzed the ultrafast transient transport in n-doped ZnS in wurtzite and zincblende phases, driven far away from equilibrium by an electric field. Through the numerical solution of associate quantum transport equations based on the Non-Equilibrium Statistical Operator Method, the time evolution of the electron drift velocity and the non-equilibrium temperature of electrons and phonons were obtained, analyzing the dependence on the electric field strength. Full article

Black phosphorous (BP) is one of the important emerging two-dimensional systems. We have undertaken a structural investigation of BP in the temperature range of 320 K to 85 K using synchrotron X-ray diffraction (XRD) studies. The XRD pattern of BP is heavily influenced by the preferred orientation effects. Collection of the diffraction pattern in a standard capillary geometry with controlled capillary rotations perpendicular to the X-ray direction permitted us to provide insights to the effects of the preferred orientation. In the range of 320 K to 85 K, BP remains in the so-called “A17” orthorhombic structure. Lattice parameters show a regular shrinkage with the lowering of the temperature as expected for any elemental metallic system. Dense temperature sampling permitted us to observe a small but clear deviation from the linear behavior in of one of the in-plane lattice parameters. This temperature-dependent structural evolution seems to provide some insights into the temperature dependence of the macroscopic properties of BP such as the Hall coefficient, thermal conductivity, etc. Full article

VO₂ is a transition metal oxide in which complex electronic phases appear near the metal-to-insulator transition due to electron correlation and electron–lattice interactions. This system is characterized by a metal-to-insulator transition (MIT) at around 341 K. The metal (high T) phase is tetragonal while the insulator (low T) phase is monoclinic and the resistivity changes at the MIT by about five orders of magnitude. Here, we report investigations of the MIT in a thin VO₂ film deposited on a sapphire substrate showing hysteresis. The MIT has been characterized by resistance measurements versus temperature and a DC magnetic field. The thin sample shows different final resistance values in both the insulating and metallic state after different temperature cycles. Moreover, some cycles do not close in the insulating phase. An unexpected magnetic dependence of the temperature cycle in the sample was also observed. The results show that the MIT of VO₂ can be controlled by reducing the thickness below 40 nm in micron-sized ribbons since MIT is associated with the emergence of coexisting metastable conformations controlled by the thickness-dependent misfit strain and stress distributions induced by the mismatch between thin ribbon film and the substrate. Full article

Deposition of an SnO₂ thin film was carried out by sol–gel-dip-coating and doped with Ce³⁺ or Eu³⁺, and a GaAs layer was deposited by resistive evaporation or sputtering. This investigation combines the emission properties of these rare-earth ions with the unique transport properties generated by the heterostructure assembly. Illumination with light with energy above the GaAs bandgap and below the SnO₂ bandgap drastically increases the GaAs/SnO₂ heterostructure conductance, which becomes practically temperature-independent. This was associated with the presence of interface conduction, possibly a two-dimensional electron gas at the GaAs/SnO₂ interface. This feature takes place only for the sample where the GaAs bottom layer is deposited via sputtering. Irradiation with energies above the SnO₂ bandgap only excites the top oxide layer. The heterostructure assembly GaAs/SnO₂:Eu leads to emission from Eu³⁺, unlike SnO₂ deposition directly on a glass substrate, where the Eu³⁺ transitions are absent. Eu emission comes along a broad band, located at a higher energy compared to Eu³⁺ transitions, which are blue-shifted as the thermal annealing temperature increases. Luminescence from Ce³⁺ ions in the heterostructure can be detected, but the ions overlap with emission from the matrix, and a cleaning procedure helps to identify Ce³⁺ transitions. Full article

A spaser consists of a plasmonic noble-metal nanostructure that acts as nanocavity, when incorporated or surface-coupled two-level emitters constitute the nanoscale gain medium. Suited two-level emitters are, for instance, laser dyes. Optical pumping may provide efficient excitation energy transfer between the two-level emitters in the gain medium and the surface plasmons sustained in the nanocavity. Strong resonant coupling of the surface plasmon modes to the gain medium may establish an inherent feedback amplification mechanism which finally drives the spaser action. In this contribution, we demonstrate that spaser emission can be generated by amplifying longitudinal surface plasmon modes in gold nanorods by optically pumping surface-attached resonantly-coupled laser dyes. Therefore, we synthesized gold nanorods whose longitudinal surface plasmon resonance peak was adjusted between 680 and 700 nm. The gain medium was realized by electrostatically attaching the laser dye phthalocyanine tetrasulfonate via the positively-charged CTAB (cetyltrimethylammonium bromide) bilayer to the gold-nanorod surface. Phthalocyanine tetrasulfonate exhibits fluorescence at 700 nm. Fluorescence quenching experiments unambiguously gave indication of resonant excitation energy transfer. The fluorescence intensity ratio $I_F^0/I_F$ follows the Stern–Volmer relationship, and the Stern–Volmer coefficient was determined as K_SV = 1.22 × 10⁶ M⁻¹. The spaser emission was observed in fs transient absorption spectra as an ultrafast decaying narrow emission peak around 716 nm. Full article

In 2012, we discovered new layered superconductors whose superconducting states emerge in the BiS2 layers. Since their crystal structure, composed of alternate stacks of BiS2 conduction layers and electrically insulating (blocking) layers, is similar to those of cuprate and Fe-based superconductors, many researchers have explored new BiS2-based superconductors and have studied the physical and chemical properties of the BiS2-based superconductors. In this paper, we present the histories of the discovery of the first BiS2-based superconductor, Bi4O4S3, and the second one, LaO1−xFxBiS2. The structural variation of the BiS2-based superconductor family is briefly introduced. Then, we show the material design concept for the emergence of bulk superconductivity in BiS2-based compounds. At the end, a possible strategy for the enhancement of the transition temperature in the BiS2-based superconductors is proposed. Full article

Motivated by diffraction experiments on the (2√3 x √3) R30◦ reconstructed Si(111) surface due to deposition of rare earth elements (Dy, Tb) and silicide formation, we analyse the splitting and non-splitting of superstructure diffraction spots. For this purpose, we model diffraction patterns for one-dimensional structures generated by the binary surface technique and use supercell models to keep the analysis simple. Diffraction patterns are calculated in the framework of the kinematical diffraction theory, and they are analyzed as a function of the domains and domain boundaries. Basic properties of the diffraction pattern are analyzed for model systems of a two-fold and a three-fold periodicity. The rules derived from these calculations are applied to the “real-world” system of Si(111)-(2√3 × √3) R30◦-RESi_x (RE = Dy or Tb). Depending on the combination of domains and domain boundaries of different types, a plethora of different features are observed in the diffraction patterns. These are analyzed to determine the sizes of both domain boundaries and domains from experimentally observed splitting of specific superstructure spots. Full article

In 2012, a novel superconductor BaTi₂Sb₂O was found in the layered titanium pnictide oxides ATi₂Pn₂O. A related superconductor BaTi₂Bi₂O was subsequently discovered in 2013. The structure of these materials consists of alternate stacking of superconducting Ti₂Pn₂O layers and Ba blocking layers, which is somewhat similar to high-T_c cuprates since the Ti₂Pn₂O layer contains an anti-CuO₂-type Ti₂O square lattice. In addition to the structural similarity to the well-known high-T_c superconductors, BaTi₂Pn₂O shows unique physical properties: two superconducting domes appear in the electronic phase diagram for solid solutions of BaTi₂(Sb_1‒xBi_x)₂O and a unique density-wave instability which coexists with superconductivity. In this short review, the early studies of titanium pnictide oxides, the discovery of novel superconductors BaTi₂Pn₂O, and recent progress are summarized. Full article

It has previously been reported that epitaxial growth of ZnO can be obtained at low temperatures by atomic layer deposition (ALD) onto a GaN (0001-Ga) surface, corresponding to a ~2.3% compressive lattice mismatch of the deposited ZnO. The question addressed here is the atomic ordering of deposited ZnO as a function of the lattice mismatch between ZnO and several single-crystal seeding surfaces. We have deposited ZnO using ALD onto either the (111) cubic or (0001) hexagonal surfaces of a set of available single-crystal substrates (GaAs, InP, GaN, SiC), for which the lattice mismatch varies over a wide range of values, positive and negative. It is found that deposition onto surfaces with very high extensive lattice mismatch (GaAs, InP) leads to polycrystalline ZnO, similar to the configuration obtained on an amorphous SiO₂ surface. In contrast, ZnO ALD deposition onto both 2H-GaN (0001-Ga) and 4H-SiC (0001-Si) surfaces with lower and compressive mismatch leads to epitaxial ordering over the whole substrate temperature range of 180–250 °C. Full article

To find the status of multiple collisions in transitional granular flow, moderate to densely packed monosized plastic disks were sheared in experimental 2D shear flow apparatus by applying shear strain rate in the range of 14.8 s⁻¹ to 34.6 s⁻¹. Application of high speed video camera and subsequent image processing techniques precisely measure the spatial positions of the particles involved in making the flow. Collision detection and contact duration finding algorithms were formulated to detect inter particle collision in each time step. The proportion of binary and multiple collisions was quantified in each time step. It is found that the contribution of multi particle collision is 13% for the lowest normalized solid fraction (${\nu }^{*}$ = 0.50) and 68% for the highest normalized solid fraction (${\nu }^{*}$ = 0.85) under consideration. The inter particle collision time is also found greater than binary collision time in all the flows under consideration. The group size is determined for each time step as an additional length scale associated with multi-particle interactions. Number of groups was made of more than two particles in all the flows under consideration, and the amount of such groups increased on increasing the solid fraction irrespective of the shear rate. In the dense cases (solid fraction > 0.60), rapid increase in the group size and the formation of occasional force chains should be attributed for the change in the stress generation pattern. Full article

The effects of strain, charge doping, and external electric field on the electronic structure of a free-standing silicene layer decorated by hydrogen atoms are studied by first-principles density functional theory. Various phases, including insulating, metallic, spin-polarized, and half-metallic have been found, depending on these external factors. The most efficient way of switching the system between these phases is charge doping. The character of the energy gap of the H/silicene system can also be modified, and for charged or for strained systems, the originally indirect gap can be tuned to become direct. The obtained results are very promising in view of the silicene functionalization and potential applications of silicene in the fields of spintronics and optoelectronics. Full article

A simple analytic model based on the equations of the propagation matrices theory has been developed in order to evaluate the effective skin depth of coated metallic surfaces. With particular attention to the R&D of highly-performing accelerating structures, different thick coatings with excellent mechanical and electrical properties have been considered such as molybdenum and its oxides, p-doped SiC, and TiN. Calculations show that copper coated with a p-type SiC may exhibit an improved hardness and a higher thermal resistance. Combined with experimental tests, this study may support the identification of reliable multilayers capable of improving the higher power performance of radio-frequency (RF) structures in terms of the accelerating gradient in order to increase the resistance to the high thermal stress of structures made with copper. Full article

The evolution of the electron band structure upon the reduction of Sb film on a Si(111)-(6 × 6)Au substrate, relevant to topological insulator properties, is experimentally systematically investigated by the reflection high-energy electron diffraction (RHEED), in situ surface electron transport and angular resolved photoemission spectroscopy methods. The experiments reveal that a bilayer (BL) of Sb is crystalline but the subsequent three BLs on top of it form amorphous layers. The five-BL-thick film transforms back to the crystalline form. The bilayer as well as 1.2- and 3.8-BL-thick films show the electron band structure with a relatively large energy gap at the Γ point of the Brillouin zone. The theoretically predicted band structure is observed at 4.8 BL coverage. Full article

A theory is derived for plasmons in two-dimensional (2D) materials by using three-dimensional (3D) plasmon theory, which was reported previously in the random phase approximation under high frequency conditions. When the 3D local electron density is expressed by the 2D local electron density $n_{2D}$ multiplied by the delta function in the thickness direction, a self-consistent integral equation for the scalar potential is derived using only $n_{2D}$ and the 2D Coulomb potential. The integral equation consists of the edge and planar plasmon terms which give their resonant frequencies. These frequencies are analytically calculated for uniform 2D atomic layers and nanodisks with step function-like electron densities at their edges. The light emission intensities from the nanodisks are also calculated. These frequencies are compared with those for the 2D and 3D Weyl fermions, i.e., massless Dirac fermions. Full article

The structure of a hydrogel consisting of diamond nanoparticles formed by the explosion method has been studied. Small angle neutron scattering has been used as a method for characterization of the gel. Joint approaches for data analysis in reciprocal and direct space have been developed to restore a multilevel structure. The pristine hydrogel of positively charged diamond particles (~5 nm in size, concentration ~5 wt %), even by four-fold dilution below its formation critical point, (C* ~ 4 wt %) retains practically the original structure where single particles are joined into small groups integrated into chain fractal-type aggregates creating a network. This indicates a local stability of the gel and means a transformation of continuous gel into a system of micro-domains suspended in water. A perfection of the diamond crystals’ facets was revealed that is of principal importance for the configuration of potentials, inducing the diamonds’ electrostatic attraction due to different electric charges of facets. It is distinguished from the results for the suspensions of diamonds in graphene shells that showed a deviation of scattering from Porod’s law. Full article

We report on the initial stage of growing of silicon nanostructures on Pb-induced $\sqrt{3}\times \sqrt{3}$ and $\sqrt{3}\times \sqrt{7}$ reconstructed Si(111) surfaces. The deposition of 0.75 monolayer of Si at a temperature of around 200 K results in Si nanoribbons a few-nanometers in length running in three equivalent high symmetry directions of Si(111) surface, as revealed by low temperature scanning tunneling microscopy measurements. The nanoribbons are predominantly 1.6 nm wide and show local $\sqrt{3}\times \sqrt{3}$ reconstruction. These findings are interpreted within the framework of silicene nanoribbons grown on a bare Si(111) surface. Full article

Nano-carbon materials are promising canidates for applications in high performance devices, including highly sensitive biosensors. We have developed a self-alignment process for nano-carbon field effect transistors (FETs), using a carbon nanowall (CNW)—a nano-carbon materials—to fabricate CNW-FETs. We measured the pH dependence of the device properties. The binding molecules are known to be key components for biosensors. We are concentrating on the development of an influenza virus sensor, because the influenza virus is a major public health problem and a highly sensitive sensor is urgently required. We estimated the size of detected molecules of glycan for influenza viruses using atomic force microscopy. The typical molecule size is around 1 nm, and this may be suitable for electronic detection using a FET structure. Full article

The photon–ion merged-beams technique for photoabsorption studies of ionized nanoparticles with synchrotron radiation is introduced. As an example, recent results from photoionization and photofragmentation of the endohedral fullerene ions Lu₃${\mathrm{N}@\mathrm{C}}_{80}^{+}$, Lu₃${\mathrm{N}@\mathrm{C}}_{80}^{2+}$, and Lu₃${\mathrm{N}@\mathrm{C}}_{80}^{3+}$ are briefly discussed, highlighting the sensitivity and versatility of the experimental technique. Full article