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Unconventional superconductors have Cooper pairs with lower symmetries than in conventional superconductors. In most unconventional superconductors, the additional symmetry breaking occurs in relation to typical ingredients such as strongly correlated Fermi liquid phases, magnetic fluctuations, or strong spin-orbit coupling in noncentrosymmetric structures. In this article, we show that the time-reversal symmetry breaking in the superconductor LaNiGa$_2$ is enabled by its previously unknown topological electronic band structure. Our single crystal diffraction experiments indicate a nonsymmorphic crystal structure, in contrast to the previously reported symmorphic structure. The nonsymmorphic symmetries transform the $k_z=\pi/c$ plane of the Brillouin zone boundary into a node-surface. Band-structure calculations reveal that distinct Fermi surfaces become degenerate on the node-surface and form Dirac lines and a Dirac loop at the Fermi level. Two symmetry related Dirac points remain degenerate under spin-orbit coupling. ARPES measurements confirm the calculations and provide evidence for the Fermi surface degeneracies on the node-surface. These unique topological features enable an unconventional superconducting gap in which time-reversal symmetry can be broken in the absence of other typical ingredients. LaNiGa$_2$ is therefore a topological crystalline superconductor that breaks time-reversal symmetry without any overlapping magnetic ordering or fluctuations. Our findings will enable future discoveries of additional topological superconductors.

A high sublimation rate of Yb14MnSb11 as a promising thermoelectric material at working temperatures is a critical factor for its space power application. In search of a way to suppress the sublimation, an isothermal thermogravimetric procedure under controlled conditions was developed and ability of Yb14MnSb11 doped by trivalent rare-earth elements to form the protective air-oxidized scale was studied. A series of ceramic rods Yb14−xRExMnSb11 with RE = La–Lu (except Ce and Eu) and x = 0.1, 0.3 and 0.4 that is close to the solubility limit was examined. Thermogravimetric curves as a time function of a progressively increasing mass of the rods in dry air at temperatures 773, 873, 973 K were recorded, and the kinetic and energetic characteristics of the oxidation process were calculated for the whole series of the isostructural Yb14−xRExMnSb11 solid solutions. Changes of the oxidation curves were considered in the context of the classic Wagner’s theory, and the general kinetic description of the process was made based on the parabolic model. This thermoanalytical investigation for the row of the isostructural Yb14−xRExMnSb11 compounds provided new vision of the oxidation mechanism and the role of the RE additions in the scale formation. For all these compounds, the oxidation kinetics followed the parabolic rate law and solid-state diffusion was the rate-limiting step for the reaction of a dense scale formation from the mixed Yb–RE oxides.

The syntheses of YbTe1−x Sb x (x = 0, 0.05, 0.2, 0.5, 0.8, 1) were investigated by solid state reactions and formed into dense pellets by spark plasma sintering. X-ray powder diffraction and microprobe analysis indicated no solubility of Sb in YbTe, and these phases are better described as composite phases (YbTe)1−x (YbSb) x (x = 0, 0.05, 0.2, 0.5, 0.8, 1). Thermal conductivity, electrical resistivity, and Seebeck coefficients were acquired for the larger values of x (x = 0.2, 0.5, 0.8, 1) from room temperature to 773 K, and the figure of merit was calculated. Thermal conductivities for x = 0, 0.05 are also reported; however, measurements of Seebeck coefficients and electrical resistivity were not possible due to large resistivity. The figure of merit for all samples was low, and the maximum zT measured was zT 791K = 0.018 for YbSb. Low figures of merit were primarily the result of very high resistivity in YbTe rich samples, and high thermal conductivity, and a small Seebeck coefficient in all samples.

The enthalpies of oxidative drop solution (ΔHds) for a series of CdSxSe1–xsamples were obtained by calorimetry in molten 3Na2O·4MoO3at 975 K. They become more exothermic linearly with increasing S content. The enthalpies of formation from the elements (ΔHf,el) depend linearly on molar ratio of S/(S + Se). This is the first report of thermodynamic properties of CdSxSe1–xsolid solutions measured by any direct calorimetric method. The enthalpies of formation at 298 K from the binary chalcogenide end-members (ΔHf,CdM) (M= S, Se) for wurtzite CdSxSe1–xare found to be zero within experimental errors. These results strongly suggest that wurtzite CdS and CdSe form an ideal solid solution, despite a substantial difference in molar volume and anion radius. This implies that size difference affects thermodynamics less strongly when larger and more polarizable anions are mixed in chalcogenides than when cations are mixed in oxides.

The feasibility of using silanization as a general tool to functionalize the surface of silicon nanoparticles (NPs) has been investigated in detail. Silicon NPs were prepared from reduction of silicon tetrachloride with sodium naphthalide. The terminal chloride on the surface of as-synthesized particles was substituted by methanol and water, in sequence. The particles were then silanized by octyltrichlorosilane, 11-bromoundecyltrichlorosilane, or 2-(carbomethoxy)ethyltrichlorosilane. These treatments yielded alkyl-, bromo-, or ester-termini on NP surfaces, respectively. The NPs were characterized by TEM, NMR, FTIR, UV–Vis, and PL spectroscopy. Changes of termination groups brought various functionalities to the NPs, without loss of the photophysics of the original NPs.

Synthesis of silicon nanoparticles of various sizes from 3 to 9 nm in diameter was accomplished via a low temperature solution route. These nanoparticles are prepared via reduction of SiCl4 with Na naphthalide in dimethoxyethane and capped with octasiloxane. The resulting nanoparticles were characterized by transmission electron microscopy (TEM), high resolution (HR) TEM, selected area electron diffraction (SAED), energy dispersive X-ray (EDX) spectroscopy, powder X-ray diffraction, UV–vis, photoluminescence, and their quantum yields were determined. TEM micrographs show that the nanoparticles are well dispersed and SAED and lattice fringes are consistent with diamond structured silicon. X-ray powder diffraction provides no diffraction peaks. UV–vis and photoluminescence show characteristic shifts corresponding to size, consistent with quantum confinement. The smallest sized nanoparticles show the largest quantum yield, consistent with an indirect bandgap nanoparticles.

Photoluminescence (PL) from alkyl-terminated silicon nanocrystallites as a function of size has been studied. Ultraviolet–blue luminescence (390–410 nm) is observed from as-prepared silicon nanoclusters with diameters from 3 to 8 nm. After 1 h of annealing at 162°C in 2-methoxyethyl ether (diglyme), the λ max of PL shifts from 360 to 420 nm. High-resolution transmission electron microscopy (HRTEM) images show that individual silicon nanoparticles are fused to form pairs of nanoparticles. FTIR spectra show that the alkyl groups remain on the surface of silicon nanoparticles. As the temperature is raised to 250°C for 1 h, the PL no longer shows any peak in the visible light region. TEM images show that the silicon nanoparticles are aggregated and fused uniformly in one single dimension, to form a strip, and these strips parallel each other. When the temperature is raised to 350°C these silicon nanoparticles form a large piece of silicon textile network, showing that functionalized alkyl surface does not persist above this temperature. A strong Si–O–Si asymmetric stretching vibration appears between 1000 and 1100 cm−1 at the expense of the C–H vibrational modes and there is no more change after 3 h of annealing at 250 or 350°C. These results provide strong evidence that the PL originates from quantum confinement.

We report the chemical synthesis of Fe-core/Au-shell nanoparticles (Fe/Au) by a reverse micelle method, and the investigation of their growth mechanisms and oxidation-resistant characteristics. The core-shell structure and the presence of the Fe and Au phases have been confirmed by transmission electron microscopy, energy dispersive spectroscopy, x-ray diffraction, Mössbauer spectroscopy, and inductively coupled plasma techniques. Additionally, atomic-resolution Z-contrast imaging and electron energy loss spectroscopy in a scanning transmission electron microscope have been used to study details of the growth processes. The Au-shells grow by nucleating on the Fe-core surfaces before coalescing. First-order reversal curves, along with the major hysteresis loops of the Fe/Au nanoparticles have been measured as a function of time in order to investigate the evolution of their magnetic properties. The magnetic moments of such nanoparticles, in the loose powder form, decrease over time due to oxidation. The less than ideal oxidation-resistance of the Au shell may have been caused by the rough Au surfaces. In a small fraction of the particles, off-centered Fe cores have been observed, which are more susceptible to oxidation. However, in the pressed pellet form, electrical transport measurements show that the particles are fairly stable, as the resistance and magnetoresistance of the pellet do not change appreciably over time. Our results demonstrate the complexity involved in the synthesis and properties of these heterostructured nanoparticles.

Single crystals of two new compounds, Eu3InP3 and Eu3In2P4, were prepared by In flux synthesis. Eu3InP3 crystallizes in the Pnma orthorhombic space group with a = 12.6517(15) Å, b = 4.2683(5) Å, c = 13.5643(14) Å, and Z = 4 while Eu3In2P4 belongs to Pnnm space group, a = 6.6999(6) Å, b = 16.1019(13) Å, c = 4.2725(4) Å, and Z = 2. These two compounds are isotypic to the main group compounds Sr3InP3 and Sr3In2P4 respectively. Both structures consist of distorted In-P tetrahedra and isolated Eu2+ ions. In Eu3InP3 these tetrahedra share corners to form one-dimensional chains. The Eu2+ ions occupy three different sites, and form a complicated network containing some triangles. In Eu3In2P4 these tetrahedra form edge-shared dimers, and these dimers form chains. Eu3InP3 shows unusual magnetic properties: there are three magnetic ordering transitions at 14 K, 10.4 K, and around 5 K and the magnetization curve shows steps in the low temperature region. Eu3In2P4 magnetically orders at 14.5 K. Its magnetization curve saturates at around 1T. Eu3InP3 and Eu3In2P4 are both semiconductors.

Bulk physical properties of layered pnictide-oxides, Na2Ti2Pn2O (Pn = As, Sb) have been characterized. Both As and Sb compounds exhibit anomalous transitions in temperature dependent magnetic susceptibility and electrical resistivity. The powder neutron diffraction data show no superlattice reflections indicative of long-range order. The origin of the physical property anomalies and structure-property relationship between As and Sb compounds are described in terms of a charge-density-wave/spin-density-wave mechanism.

Interest in the synthesis of semiconductor nanoparticles has been generated by their unusual optical and electronic properties arising from quantum confinement effects. We have synthesized silicon and germanium nanoclusters by reacting Zintl phase precursors with either silicon or germanium tetrachloride in various solvents. Strategies have been investigated to stabilize the surface, including reactions with RLi and MgBrR (R = alkyl). This synthetic method produces group IV nanocrystals with passivated surfaces. These nanoparticle emit over a very large range in the visible region. These particles have been characterized using HRTEM, FTIR, UV-Vis, solid state NMR, and fluorescence. The synthesis and characterization of these nanoclusters will be presented.

NaCuO2 has been prepared via electrodeposition from a sodium hydroxide/potassium hydroxide melt and characterized by single crystal and powder X-ray diffraction, magnetic susceptibility, and resistivity. Clusters of blue-black plate-like and needle crystals are grown at positive potentials on a platinum anode with a platinum or a zirconium crucible serving as the cathode and a platinum reference electrode. Details of the electrochemical procedure are described, along with a cyclic voltammogram. NaCuO2 exhibits temperature independent paramagnetism over the temperature range 30 – 300 K. Its room temperature resistivity is greater than 106 Ω-cm.

We studied the optical properties of silicon nanocrystals incorporated into colloidal and solgel matrices. The silicon nanocrystals are produced by ultrasonic dispersion of porous silicon layers. We report results on the dependence of the photoluminescence (PL) spectra with excitation intensity. The PL shows a blue peak (at ∼ 415-460 nm.) and a red peak (at ∼ 680 nm). This PL spectrum shows a remarkable dependence on the excitation intensity. As the intensity is increased, the blue peak grows at the expense of the red. A model is suggested for this behavior. We also report on the excitation intensity dependence and the emission wavelength dependence of the PL decay at low (1 kHz) and high (82 MHz) repetition rates of optical excitation. When low repetition rate excitation is used, the PL decay times are all exponential, short (ns), and appear to vary little with emission wavelength. This sharply contrasts with what is observed in porous silicon. With high repetition rate excitation, both red and blue peaks show long (100's ns) and short (ps-ns) lifetime components. We contrast the different optical properties of these silicon nanocrystals with that observed in porous silicon.

Nanometer-sized silicon particles have been produced by ultrasonic dispersion of thin sections of porous silicon (PS) in organic solvents. The particles have been characterized by HRTEM (high-resolution transmission electron microscopy), FTIR (Fourier transform infrared) spectroscopy, and fluorescence spectroscopy. HRTEM shows both aggregates of and monodispersed crystallite particles of Si. The larger aggregates range in size from20 to 50 nm and are made up of small crystallites with diameters of 2-10 nm. Monodispersed crystallites ranged in size from 2-10 nm. All the particles have an amorphous layer of SiO{sub 2}. The photoluminescence (PL) spectrum shows two peaks: a red peak at around 680 nm, which is typical for PS and a blue peak between 415 and 446 nm, which is not typical for as-prepared PS. As a function of increased excitation intensity, the blue peak grows at the expense of the red. This is discussed in light of photoluminescence lifetime data. The red luminescence is attributed to quantum confinement. The blue luminescence is attributed to either extremely small Si crystallites or (Si(II)){sup 0} defects embedded in the oxide matrix. 28 refs., 9 figs., 1 tab.

The compounds La1−xBaxTiO3 (0 ≤ × ≤ 1) have been prepared by arc melting stoichiometric amounts of LaTiO3 and BaTiO3. Single phase samples can be made for the entire stoichiometry range. The polycrystalline samples have been characterized by thermal gravimetric analysis, X-ray powder diffraction, and temperature dependent magnetic susceptibility. This series of compounds has been studied as a possible candidate for an early transition metal superconductor.