Quantum Interference and Surface States Effects in Bismuth Nanowires

We report the observation of Aharonov-Bohm (AB) oscillations for single Bi nanowires with diameter d< 80 nm. The single nanowire samples with glass coating were prepared by the Ulitovsky technique; they were cylindrical single crystals with (10 $\bar{1}$ 1) orientation along the wire axis. The surface of Bi nanowire supports surface states which give rise to a significant population of charge carriers with high effective mass that form a highly conducting tube around the nanowire. The oscillations of longitudinal magnetoresistance (MR) of Bi nanowires with two periods ΔB ₁ and ΔB ₂ proportional to Φ₀ and Φ₀/2 were observed, where Φ₀=h/e is the flux quantum. From B～ 8 T down to B = 0 the extremums of Φ₀/2 oscillations are shifted up to 3π at B = 0 which is the manifestation of Berry phase shift due to electron moving in a nonuniform magnetic field. A derivative of MR was measured at various inclined angles. The observed angle variation of the periods is not in agreement with the theoretical dependence Δ(α)=Δ(0)/cos?α of the size effect oscillations of the “flux quantization” type. Moreover, the equidistant oscillations of MR exist in transverse magnetic fields under certain rotation angles. An interpretation of the MR oscillations is presented.     

Quantum Interference of Surface States in Bismuth Nanowires in Transverse Magnetic Fields

We report the results of studies of the magnetoresistance (MR) and electric field effect (EFE) of single-crystal Bi nanowires with diameter d<80 nm at low temperatures. Single-crystal nanowire samples were prepared by the Taylor-Ulitovsky technique; they were cylindrical single crystals with the $(10\bar{1}1)$ orientation along the wire axis where the C ₃ axis was inclined at an angle of 70^° to the wire axis. According to theory of S. Murakami, bismuth bilayers can exhibit the quantum spin Hall effect. A Bi crystal can be viewed as a stacking of bilayers with a honeycomblike lattice structure along the [111] direction. An interpretation of transverse MR oscillations with using this theory is presented.     

Quantum Interference in Bismuth Nanowires: Evidence for Surface Charges

We report the results of studies of the transverse magnetoresistance (MR) of single-crystal Bi nanowires with diameter d<80 nm. The single-crystal nanowire samples were prepared by the Taylor-Ulitovsky technique. Due to the semimetal-to-semiconductor transformation and high density of surface states with strong spin-orbit interactions, the charge carriers are confined to the conducting tube made of surface states. The non monotonic changes of transverse MR that are equidistant in a direct magnetic field were observed at low temperatures in a wide range of magnetic fields up to 14 T. The period of oscillations depends on the wire diameter d as for the case of longitudinal MR. An interpretation of transverse MR oscillations is presented.     

Features of Lifshits Electron Topological Transitions Induced by Anisotropic Deformation in Thin Wires of Doped Bismuth

For the first time, glass-coated single-crystal Bi–0.05Sn wires with d=200 nm ÷ 3 μm with C ₃ oriented along the wire axis were obtained by the recrystallization method of the wires with standard (10 $\bar{1}$ 1) orientation, which made it possible to study the thermopower anisotropy. With the use of ShdH oscillations method, we have determined characteristics of changes in the Fermi surface topology at Lifshits electron topological transitions induced by tension in single-crystal Bi–0.05Sn wires with a standard (10 $\bar{1}$ 1) orientation and with the C ₃ orientation along the wire axis. It is found that the thermopower increase with deformation and thermopower anisotropy of the Bi–0.05Sn wires with trigonal orientation achieves a value of 130–150 μV/K which leads to a considerable increase of thermoelectric efficiency, which is important for practical applications, in particular, for their use as anisotropic thermoelectric energy converters.     

Fabrication of patterned boron carbide nanowires and their electrical, field emission, and flexibility properties

Large-area patterned boron carbide nanowires (B₄C NWs) have been synthesized using chemical vapor deposition (CVD). The average diameter of B₄C NWs is about 50 nm, with a mean length of 20 ??m. The B₄C NWs have a single-crystal structure and conductivities around 5.1 × 10^?2 ??^?1·cm^?1. Field emission measurements of patterned B₄C NWs films show that their turn-on electric field is 2.7 V/??m, lower than that of continuous B₄C NWs films. A single nanowire also exhibits excellent flexibility under high-strain bending cycles without deformation or failure. All together, this suggests that B₄C NWs are a promising candidate for flexible cold cathode materials.      

Structural properties of Cd–Co ferrites

Ferrite samples with composition, Cd $_{\emph{x}}$ Co $_{1-\emph{x}}$ Fe₂O₄ (x = 0·80, 0·85, 0·90, 0·95 and 1·0), were prepared by standard ceramic method and characterized by XRD, IR and SEM techniques. X-ray analysis confirms the formation of single phase cubic spinel structure. Lattice constant and grain size of the samples increase with increase in cadmium content. Bond length (A–O) and ionic radii ( ${\emph{r}}_{\rm {A}})$ on A-sites increase, whereas bond length (B–O) and ionic radii ( ${\emph{r}}_{\rm{B}})$ on B-site decrease. The crystallite sites of the samples lie in the range of 29·1–42·8 nm. IR study shows two absorption bands around 400 cm^???1 and 600 cm^???1 corresponding to tetrahedral and octahedral sites, respectively.     

Correlation Between Band Structure and Magneto- Transport Properties in HgTe/CdTe Two-Dimensional Far-Infrared Detector Superlattice

Theoretical calculations of the electronic properties of n-type HgTe/CdTe superlattices (SLs) have provided an agreement with the experimental data on the magneto-transport behaviour. We have measured the conductivity, Hall mobility, Seebeck and Shubnikov-de Haas effects and angular dependence of the magneto-resistance. Our sample, grown by MBE, had a period d=d ₁+d ₂ (124 layers) of $d_{1}=8.6~\mathrm{nm}~\mathrm{(HgTe)} /d_{2}=3.2~\mathrm{nm}~\mathrm{(CdTe)}$ . Calculations of the spectras of energy E(d ₂), E(k _z ) and E(k _p ), respectively, in the direction of growth and in plane of the superlattice; were performed in the envelope function formalism. The energy E(d ₂,Γ,4.2 K), shown that when d ₂ increase the gap E _g decrease to zero at the transition semiconductor to semimetal conductivity behaviour and become negative accusing a semimetallic conduction. At 4.2 K, the sample exhibits n type conductivity, confirmed by Hall and Seebeck effects, with a Hall mobility of $2.5 \times 10^{5}~\mathrm{cm}^{2}/ \mathrm{V\,s}$ . This allowed us to observe the Shubnikov-de Haas effect with n=3.20×10¹² cm^?2. Using the calculated effective mass ( $m^{*}_{E1}(E_{F}) = 0.05 m_{0}$ ) of the degenerated electrons gas, the Fermi energy (2D) was E _F =88 meV in agreement with 91 meV of thermoelectric power α. In intrinsic regime, α～T ^?3/2 and R _H T ^3/2 indicates a gap E _g =E ₁?HH ₁=101 meV in agreement with calculated E _g (Γ,300 K)=105 meV. The formalism used here predicts that the system is semiconductor for d ₁/d ₂=2.69 and d ₂<100 nm. Here, d ₂=3.2 nm and E _g (Γ,4.2 K)=48 meV so this sample is a two-dimensional modulated nano-semiconductor and far-infrared detector (12 μm<λ _c <28 μm).     

Structural, Magnetic, and Magnetocaloric Properties of La0.5M0.1Sr0.4MnO3 (M=Bi, Eu, Gd, and Dy) Perovskite Manganites

Structural, magnetic, and magnetocaloric properties of La_0.5M_0.1Sr_0.4MnO₃ (M=Bi, Eu, Gd, and Dy) powder samples, synthesized using the solid-state reaction at high temperature, have been experimentally investigated. X-ray diffraction analysis using the Rietveld refinement show that La_0.5Bi_0.1Sr_0.4MnO₃ sample is single phase and crystallizes in the rhombohedral system with $R\overline{3}c$ space group whereas a mixture of orthorhombic (Pbnm) and rhombohedral ( $R\overline{3}c$ ) phases is observed for M=Eu, Gd, and Dy compounds. The Curie temperature, T _C , shifts to lower temperature with decreasing the average A-site ionic radius 〈r _A 〉, which is consistent with large cationic disorder. Arrott plots show that all our samples exhibit a second order magnetic phase transition. From the measured magnetization data of La_0.5M_0.1Sr_0.4MnO₃ (M=Bi, Eu, Gd, and Dy) samples as a function of magnetic applied field, the associated magnetic entropy change |ΔS _M | has been determined. In the vicinity of T _C , |ΔS _M | reached, in a magnetic applied field of 1 T, maximum values of 0.98 J/kg?K, 1.01 J/kg?K, 0.81 J/kg?K, and 0.77 J/kg?K for M=Bi, Eu, Gd, and Dy, respectively.     

High-Field ESR and Magnetization Study of a Novel Macrocyclic Chelate Trinuclear Ni(II) Complex

We have investigated magnetic properties of a novel macrocyclic chelate Ni(II) complex [Ni₃(L)(OAc)₂], by means of the static magnetization M and high field frequency tunable electron spin resonance (HF-ESR). Magnetic field B dependencies of M reveal the magnetic ground state with a total spin S $^{\mathit{tot}}_{0}=1$ and a strong antiferromagnetic coupling between three Ni(II) ions. HF-ESR measurements at frequencies ν=80–350 GHz and B up to 15 T yield a magnetic anisotropy gap of the order of 60 GHz (2.9 K) and a g-factor of 2.2. In addition, the modelling reveals a positive single ion anisotropy (D>0) corresponding to an easy plane situation for the Ni₃ complex.     

Barrier inhomogeneities in titanium Schottky contacts formed on argon plasma etched p-type Si0.95Ge0.05

Electrical properties of Ti Schottky barrier diode fabricated on argon plasma etched p-type Si_0.95Ge_0.05 were studied using current–voltage (I–V) over a wide temperature range (100–300 K). The transport properties of the junction were analyzed by investigating the temperature dependence of both the effective Schottky barrier height (Φ _0bp) and the ideality factor (n). It is shown that the ideality factor increases and the Schottky barrier height (SBH) decreases with decreasing temperature. This abnormal temperature dependence of the Φ _0bp and n is explained on the basis of a thermionic emission conduction mechanism with Gaussian distributed barrier heights due to the barrier height inhomogeneities at the metal-p-Si_0.95Ge_0.05 interface. From the linear plot of the experimental SBH versus 1/T, a homogeneous SBH ( $\overline{\varPhi }_{0bp}$ ) and a zero-bias standard deviation (σ_0s) values of approximately 0.55 eV and 67 mV, respectively were computed. Furthermore the modified Richardson plot according to the Gaussian distribution model resulted in a homogeneous SBH ( $\overline{\varPhi }_{0bp}$ ) and a Richardson constant (A*) of 0.55 eV and 35 A/cm² K², respectively. The A* value obtained from this plot is in very close agreement with the theoretical value of 32 A/cm² K² for p-type Si_0.95Ge_0.05. Furthermore, the SBH is found to decrease linearly as the interface states density (N _ss) increases. It is proposed that the lateral inhomogeneities of the SBH are actually attributed to the distribution of the interface states which are in turns resulting from the plasma etching induced defects beneath the Si_0.95Ge_0.05 surface.     

Dissociated \frac{1}{3}\langle 0\bar{1}11\rangle dislocations in Bi2Te3 and their relationship to seven-layer Bi3Te4 defects

We investigate the structure of $ \frac{1}{3}\langle 0\bar{1}11\rangle $ dislocations observed in Bi₂Te₃ nanowires. This particular type of dislocation is interesting because it has a large Burgers vector (b = 1.048 nm) with a component normal to the basal planes equal to the thickness of one full Bi₂Te₃ quintuple unit (i.e., c/3). Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy observations show that the dislocations form with a complex dissociated core structure. This structure consists of two partial dislocations that separate a defected region consisting of a seven-plane-thick septuple unit, consistent with a local patch of Bi₃Te₄, rather than the normal Bi₂Te₃ quintuple layer structure. As we discuss, details of the core structure can be understood from an analysis of the crystallographic parameters of the observed partial dislocations. This analysis suggests a mechanism to accommodate the loss of tellurium through the heterogeneous nucleation and growth of seven-layer defects at $ \frac{1}{3}\langle 0\bar{1}11\rangle $ —type dislocations.     

Dielectric permittivity of ultrathin PbTiO3 nanowires from first principles

We propose an efficient method to compute the dielectric permittivity of nanostructures by combining first principles density functional perturbation theory with effective medium theory. Specifically, ultrathin axially symmetric ferroelectric PbTiO₃ nanowires are considered. As established previously by Pilania and Ramprasad (Phys Rev B 82:155442, 2010), (4 × 4) PbO-terminated nanowire and (4 × 4) TiO₂-terminated nanowire display, respectively, a uniform axial and a vortex polarization in their ground state configurations (the latter with a non-zero axial toroidal moment). Both nanowires, regardless of the lateral surface termination, display a significantly larger dielectric constant value along the axial direction, and diminished values along the off-axis directions, as compared to the corresponding bulk values. Our results further suggest that the nanowires with unconventional vortex-type polarization states are expected to have an increased dielectric response as compared to those with conventional uniform axial polarization. The method proposed here is quite general and readily extendable to other zero-, one-, and two-dimensional nanostructures.     

Multiple Andreev Reflections Spectroscopy of Two-Gap 1111- and 11 Fe-Based Superconductors

Using the “break-junction” technique, we prepared and studied superconductor–constriction–superconductor (ScS) nanocontacts in polycrystalline samples of Fe-based superconductors CeO_0.88F_0.12FeAs (Ce-1111; $T_{C}^{\mathrm{bulk}} = 41 \pm1~\mathrm{K}$ ), LaO_0.9F_0.1FeAs (La-1111; $T_{C}^{\mathrm{bulk}} = 28 \pm1~\mathrm {K}$ ), and FeSe ( $T_{C}^{\mathrm{bulk}} = 12 \pm1~\mathrm{K}$ ). We detected two subharmonic gap structures related with multiple Andreev reflections, indicating the presence of two superconducting gaps with the BCS-ratios 2Δ _L /k _B T _C =4.2÷5.9 and 2Δ _S /k _B T _C ～1?3.52, respectively. Temperature dependences of the two gaps Δ _L,S (T) in FeSe indicate a k-space proximity effect between two superconducting condensates. For the studied iron-based superconductors, we found a linear relation between the gap Δ _L and magnetic resonance energy, E _res≈2Δ _L .     

Ultraviolet photoconductance of a single hexagonal WO3 nanowire

The ultraviolet (UV) photoconductance properties of a single hexagonal WO₃ nanowire have been studied systematically. The conductance of WO₃ nanowires is very sensitive to ultraviolet B light and a field-effect transistor (FET) nanodevice incorporating a single WO₃ nanowire exhibits excellent sensitivity, reversibility, and wavelength selectivity. A high photoconductivity gain suggests that WO₃ nanowires can be used as the sensing element for UV photodetectors. Measurements under UV light in vacuum show that the adsorption and desorption of oxygen molecules on the surface of the WO₃ nanowire can significantly influence its photoelectrical properties. The WO₃ nanowires have potential applications in biological sensors, optoelectronic devices, optical memory, and other areas.      

Effects of Bi doping on dielectric and ferroelectric properties of PLBZT ferroelectric thin films synthesized by sol–gel processing

[Pb _0·95(La_1???y Bi _y ) _0·05][Zr_0·53Ti_0·47]O₃ (PLBZT) ferroelectric thin films have been synthesized on indium tin oxide (ITO)-coated glass by sol–gel processing. PLBZT thin films were annealed at a relatively low temperature of 550 °C in oxygen ambient. Effects of Bi doping on structure, dielectric and ferroelectric properties of PLBZT were investigated. Bi doping is useful in crystallization of PLBZT films and promoting grain growth. When the Bi-doping content ${\mathit{y}}$ is not more than 0·4, an obvious improvement in dielectric properties and leakage current of PLBZT was confirmed. However, when the Bi-doping content is more than 0·6, the pyrochlore phase appears and the remnant polarization P _r of PLBZT thin films is smaller than that of $\left({Pb}_{{1-x}} {\bf La}_{x}\right)\!\!\left({Zr}_{{1-y}} {Ti}_{y}\right){O}_{3}$ (PLZT) thin films without Bi doping. PLBZT thin films with excessive Bi-doping content are easier to fatigue than PLZT thin films.     

Observation of the Semiconductor-Semimetal and Semimetal-Semiconductor Transitions in Bi Quantum Wires Induced by Anisotropic Deformation and Magnetic Field

The semimetal-semiconductor transition is observed in glass-coated quantum single-crystal bismuth wires with diameters less than 70 nm due to the quantum size effect. It is found that elastic deformation of Bi nanowires (10 $\bar{1}$ 1) oriented along the wire axis with the semiconductor dependence R(T) leads to the approaching of L and T bands and to the semiconductor-semimetal transition; as a result, Shubnikov-de Haas oscillations appear on the magnetoresistance dependences R(H). It is shown that strong magnetic field and elastic deformation are the tools to control gap size in quantum bismuth wires, which is principal for their practical use in particular in thermoelectricity.     

Morphology, Structural, Magnetic, and Magnetocaloric Properties of Pr0.7Ca0.3MnO3 Nanopowder Prepared by Mechanical Ball Milling Method

A sample of Pr_0.7Ca_0.3MnO₃ nanopowder was prepared by the ball milling method. The crystal structure examined by X-ray powder diffraction indicates that the sample is single phase and crystallizes in the orthorhombic perovskite system with Pnma space group at room temperature. The average crystallite size of 29 nm was obtained by X-ray diffraction. Magnetic measurements showed that the sample exhibits a ferromagnetic-to-paramagnetic transition at a Curie temperature close to 120 K. The magnetic entropy change |ΔS _M | has been deduced by the Maxwell relation method. The maximum value of the magnetic entropy change $\vert \Delta {S}_{M}^{\max} \vert$ obtained from the M(H) plot data is found to be 0.86 J/kg?K for an applied magnetic field of 2 T. At this value of magnetic field the relative cooling power (RCP) is 44.05 J/kg. At low temperature, large change in magnetic entropy has been observed in the sample. Our result on magnetocaloric properties suggests that Pr_0.7Ca_0.3MnO₃ nanopowder is attractive as a possible refrigerant for low-temperature magnetic refrigeration.     

Electrical and Magnetic Behaviour of PrFeAsO_{\mathbf{0.8}}F_{\mathbf{0.2}} Superconductor

The superconducting and ground state samples of PrFeAsO_0.8F_0.2 and PrFeAsO have been synthesised via the easy and versatile single step solid state reaction route. X-ray and Reitveld refine parameters of the synthesised samples are in good agreement to the earlier reported value of the structure. The ground state of the pristine compound (PrFeAsO) exhibited a metallic-like step in resistivity below 150 K followed by another step at 12 K. The former is associated with the spin density wave (SDW)-like ordering of Fe spins and later to the anomalous magnetic ordering for Pr moments. Both the resistivity anomalies are absent in case of the superconducting PrFeAsO_0.8F_0.2 sample. Detailed high field (up to 12 Tesla) electrical and magnetization measurements are carried out for the superconducting PrFeAsO_0.8F_0.2 sample. The PrFeAsO_0.8F_0.2 exhibited superconducting onset ( $T_{c}^{\mathrm{onset}}$ ) at around 47 K with T _c (ρ=0) at 38 K. Though the $T_{c}^{\mathrm{onset}}$ remains nearly invariant, the T _c (ρ=0) is decreased with applied field, and the same is around 23 K under an applied field of 12 Tesla. The upper critical field (H _c2) is estimated from the Ginzburg–Landau equation (GL) fitting, which is found to be ～182 Tesla. Critical current density (J _c ), being calculated from high field isothermal magnetization (MH) loops with the help of Beans critical state model, is found to be of the order of 10³ A/cm². Summarily, the superconductivity characterization of the single step synthesised PrFeAsO_0.8F_0.2 superconductor is presented.     

The High Pressure Superconductivity of CaLi2 Compound: The Thermodynamic Properties

The thermodynamic properties of the superconducting state in CaLi₂ at 60 GPa have been described in the paper. The numerical analysis has been carried out in the framework of the Eliashberg formalism. It has been shown that: (i) the critical value of the Coulomb pseudopotential is equal to 0.20, which corresponds to the value of 1795 meV for the Coulomb potential; (ii) the critical temperature (T _C ) cannot be correctly calculated by using the Allen-Dynes (AD) formula; (iii) the dimensionless ratios: $T_{C}C^{N} (T_{C} )/H^{2}_{C} (0 )$ , (C ^S (T _C )?C ^N (T _C ))/C ^N (T _C ) and 2Δ(0)/k _B T _C take the non-BCS values: 0.157, 1.78 and 3.85, respectively. The symbol C ^N represents the specific heat in the normal state, C ^S denotes the specific heat in the superconducting state, H _C (0) is the thermodynamic critical field near the temperature of zero Kelvin, and Δ(0) is the order parameter; (iv) the ratio of the electron effective mass ( $m^{\star}_{e}$ ) to the electron band mass (m _e ) assumes a high value, in the whole range of the temperature, where the superconducting state exists. The maximum of $m^{\star}_{e}/m_{e}$ is equal to 2.15 for T=T _C .