Ab-plane Direction Research Articles

Anisotropic thermoelectric power in single crystalline Mg1-xAlxB2 compounds

The thermoelectric power of single crystalline Mg1−xAlxB2 compounds is measured and analyzed along the main crystallographic directions (ab-plane and c-axis direction). It is shown that the classical model of thermoelectric power with phonon drag and diffusion terms provides satisfactory explanation of the temperature dependence of the Seebeck parameter S(T). Agreement of S(T) with the study of resistivity is achieved with some variation of the Debye temperature being a model-sensitive parameter. Presence of positive and negative charge carriers is observed and competing contributions of the π and σ bands are discussed.

Rutile TiO2 Creates Advanced Na-Storage Materials

Rutile-type titanium oxide (TiO2) is a resource-rich and inexpensive material with a one-dimensional ion-diffusion path along the c-axis. However, it has received no attention as an anode material for Na-ion batteries because of its low electronic conductivity and limited ion diffusion in the ab-plane direction. We have revealed for the first time the reversible Na insertion–extraction reaction of rutile TiO2 and have recently reported the improvement in the anode performances by doping with various impurity elements such as Nb5+, Ta5+, and In3+. In this Review, we introduce a material chemistry methodology to enhance the anode properties of the rutile TiO2 electrode by impurity doping, introduction of oxygen vacancies, formation of single-crystalline particles, and reduction of their aspect ratio. The knowledge is remarkably effective for creating advanced high-performance anode materials with not only using rutile TiO2 but also using anatase TiO2 and other titanium-based oxides.

Anisotropic thermoelectric transport in textured Sb1.5Bi0.5Te3 nanomaterial synthesized by facile bottom-up physical process

High anisotropy in thermal conductivity (κ) coupled with anisotropy in Seebeck coefficient (S) has resulted in peak zT value of 0.85 at 150 °C in the textured Sb1.5Bi0.5Te3 nanomaterial in perpendicular to the preferential ab-plane direction. Further, strong and opposite temperature dependences in electrical conductivity (σ) and thermal conductivity, in the ranges of 1.6–1.8 and 1.4–1.8 respectively, have resulted in better average zT value of 0.72 in the 50°C–250 °C temperature range along this direction, where zT = (S2σ/κ)T. The high anisotropy in Seebeck coefficient in the range of 0.82–0.84 is peculiar, which may be attributed to differential scattering for holes and electrons by oxide interface on the ab-planes. Sample temperature and laser power dependent Raman spectroscopy have revealed that Eg(2) mode is dominant phonon transfer mode in this material and therefore, scattering of Eg(2) mode phonons may be critical in obtaining thermal conductivity reduction. The nanomaterial has been synthesized by a facile bottom-up physical synthesis process and consolidated by direct current hot pressing. Our synthesis process requires controlled melting of ingredient metals at just above their melting points-rocking and air quenching. The synthesized nanomaterial hardly experiences its melting point in this process. This energy efficient process does not require any kind of milling and produces single phase nanomaterial in unique plate-like morphology, which easily results in high texturing. This texturing has been studied by XRD analysis as well as SEM images and is also correlated with texture factor in the thermoelectric measurements. HRTEM image has shown high grain boundary density within the plates, but these have not been able to scatter phonons for thermal conductivity reduction within ab-plane. This is possibly due to smaller than optimum size of these randomly oriented grains. Such features also offer possibility of zT enhancement along preferential ab-plane direction in the textured specimen. Further, the study of anisotropy in power output density and thereby, engineered power factor under practical temperature gradients is also presented. The process also offers simplicity in obtaining further thermal conductivity reduction in perpendicular to the preferential ab-plane direction by use of tellurium or semiconductor interface layer or by use of metallic cluster in the ab-plane direction.

Electron and phonon transport anisotropy of ZnO at and above room temperature

Due to the lack of experimental evidence, it is not clearly known if the charge and heat transport of ZnO exhibit noticeable anisotropy at and above room temperature. Here, we measure the charge and heat transport properties of strongly crystallographically textured ZnO polycrystals at and above room temperature, up to 750 K. Our observations reveal a remarkable difference of the electrical conductivity, Hall coefficient, Seebeck coefficient, and electron mobility between the ZnO c-axis and ab-plane directions. The origin of electron transport anisotropy is discussed. We experimentally show that the lattice thermal conductivity for the c-axis direction can be at least 20% larger than that for the ab-plane direction, and the anisotropy ratio is only weakly dependent on temperature, which qualitatively agrees well with our first-principles density functional theory (DFT) calculations. Our DFT calculations also reveal that the heat transport difference between the c-axis and ab-plane directions is due to the anisotropy in phonon group velocities and Umklapp phonon scattering rates. The present work fills in the knowledge gap about ZnO. These findings can provide important implications for designing ZnO crystals to optimize the material or device performance in numerous applications where charge and/or heat transport properties are important.

Crystal Growth and Magnetic Properties of Topological Nodal-Line Semimetal GdSbTe with Antiferromagnetic Spin Ordering

We report crystal growth, AC and DC magnetic susceptibilities [χ(T, H)], magnetization [M(T, H)], and heat capacity [CP(T, H)] measurement results of GdSbTe single crystal. GdSbTe is isostructural to the confirmed nonmagnetic nodal-line semimetal ZrSiS of noncentrosymmetric tetragonal crystal structure in space group P4/nmm (No. 129), but it shows additional long-range antiferromagnetic spin ordering for the Gd spins of S = 7/2 below TN. Both χ(T, H) and CP(T, H) measurements confirm the existence of a long-range antiferromagnetic (AFM) spin ordering of Gd spins below TN ∼ 12 K, and an additional spin reorientation/recovery (sr) behavior is identified from the change of on-site spin anisotropy between Tsr1 ∼ 7 and Tsr2 ∼ 4 K. The anisotropic magnetic susceptibilities of χ(T, H) below TN clearly demonstrate that the AFM long-range spin ordering of GdSbTe has an easy axis parallel to the ab-plane direction. The field- and orientation-dependent magnetization of M(T, H) at 2 K shows two plateaus to indicate the spin-flop transition for H||ab near ∼2.1 T and a metamagnetic state near ∼5.9 T having ∼1/3 of the fully polarized magnetization by the applied field. The heat capacity measurement results yield Sommerfeld coefficient of γ ∼ 7.6(4) mJ/mol K2 and θD ∼ 195(2) K being less than half of that for the nonmagnetic ZrSiS. A three-dimensional (3D) AFM spin structure is supported by the ab initio calculations for Gd having magnetic moment of 7.1 μB and the calculated AFM band structure indicates that GdSbTe is a semimetal with bare density of states (0.36 states/eV fu) at the Fermi level, which is 10 times smaller than the measured one to suggest strong spin-fluctuation.

Synergetic pinning centres in BaZrO3-doped YBa2Cu3O7-x films induced by SrTiO3 nano-layers

We report on the enhancement of critical current density (Jc) and the unusual behaviour of its dependence on field orientation in YBa2Cu3O7−x (YBCO) nanostructured films by a combination of substrate decoration with Ag nano-dots, of the incorporation of BaZrO3 (BZO) nano-particles and nano-rods, and of multilayer architecture (a thin SrTiO3 layer separating two 1.5 μm-thick YBCO layers). SrTiO3 insulating layers were 15, 30 or 45 nm thick. The highest improvement of Jc in applied magnetic fields along the c-axis and smaller than 1 T occurs in the bi-layer with 30 nm-thick STO, but the influence of STO thickness is small. Our thick nanostructured films show significant improvement of Jc in the magnetic field along the ab-plane direction. The presence of BZO nano-rods, ab-plane defects and nano particles of BZO and Y2O3 was observed in transmission electron microscopy (TEM) images of the film. The peculiarities of artificial pinning centres revealed in the TEM images of the nanostructured films are used to explain an unusual split of the peak in the Jc dependence on the magnetic field along the ab-plane of YBCO. Effective pinning potentials in high magnetic fields have rather high values for such thick films.

Polynanocrystalline Graphite: A New Carbon Anode with Superior Cycling Performance for K-Ion Batteries.

We synthesized a new type of carbon-polynanocrystalline graphite-by chemical vapor deposition on a nanoporous graphenic carbon as an epitaxial template. This carbon is composed of nanodomains being highly graphitic along c-axis and very graphenic along ab plane directions, where the nanodomains are randomly packed to form micron-sized particles, thus forming a polynanocrystalline structure. The polynanocrystalline graphite is very unique, structurally different from low-dimensional nanocrystalline carbon materials, e.g., fullerenes, carbon nanotubes, and graphene, nanoporous carbon, amorphous carbon and graphite, where it has a relatively low specific surface area of 91 m2/g as well as a low Archimedes density of 0.92 g/cm3. The structure is essentially hollow to a certain extent with randomly arranged nanosized graphite building blocks. This novel structure with disorder at nanometric scales but strict order at atomic scales enables substantially superior long-term cycling life for K-ion storage as an anode, where it exhibits 50% capacity retention over 240 cycles, whereas for graphite, it is only 6% retention over 140 cycles.

High-pressure polymorphism and structural transitions of norsethite, BaMg(CO3)2

In situ high-pressure investigations on norsethite, BaMg(CO3)2, have been performed in sequence of diamond-anvil cell experiments by means of single-crystal X-ray and synchrotron diffraction and Raman spectroscopy. Isothermal hydrostatic compression at room temperature yields a high-pressure phase transition at P c ≈ 2.32 ± 0.04 GPa, which is weakly first order in character and reveals significant elastic softening of the high-pressure form of norsethite. X-ray structure determination reveals C2/c symmetry (Z = 4; a = 8.6522(14) A, b = 4.9774(13) A, c = 11.1542(9) A, β = 104.928(8)°, V = 464.20(12) A3 at 3.00 GPa), and the structure refinement (R 1 = 0.0763) confirms a distorted, but topologically similar crystal structure of the so-called γ-norsethite, with Ba in 12-fold and Mg in octahedral coordination. The CO3 groups were found to get tilted off the ab-plane direction by ~16.5°. Positional shifts, in particular of the Ba atoms and the three crystallographically independent oxygen sites, give a higher flexibility for atomic displacements, from which both the relatively higher compressibility and the remarkable softening originate. The corresponding bulk moduli are K 0 = 66.2 ± 2.3 GPa and dK/dP = 2.0 ± 1.8 for α-norsethite and K 0 = 41.9 ± 0.4 GPa and dK/dP = 6.1 ± 0.3 for γ-norsethite, displaying a pronounced directional anisotropy (α: β −1 = 444(53) GPa, β −1 = 76(2) GPa; γ: β −1 = 5.1(1.3) × 103 GPa, β −1 = 193(6) GPa β −1 = 53.4(0.4) GPa). High-pressure Raman spectra show a significant splitting of several modes, which were used to identify the transformation in high-pressure high-temperature experiments in the range up to 4 GPa and 542 K. Based on the experimental series of data points determined by XRD and Raman measurements, the phase boundary of the α-to-γ-transition was determined with a Clausius–Clapeyron slope of 9.8(7) × 10−3 GPa K−1. An in situ measurement of the X-ray intensities was taken at 1.5 GPa and 411 K in order to identify the nature of the structural variation on increased temperatures corresponding to the previously reported transformation from α- to β-norsethite at 343 K and 1 bar. The investigations revealed, in contrast to all X-ray diffraction data recorded at 298 K, the disappearance of the superstructure reflections and the observed reflection conditions confirm the anticipated $$R\bar{3}m$$ space-group symmetry. The same superstructure reflections, which disappear as temperature increases, were found to gain in intensity due to the positional shift of the Ba atoms in the γ-phase.

Potentials of iron-based superconductors for practical future materials

Since the discovery of high-Tc superconductivity in the REFeAs(O, F) system in 2008, studies on the development of superconducting materials using iron-based superconductors has been undertaken because of their high Hc2 and relatively high Tc. Although the cuprate superconductors exhibit much higher Tc and similar high Hc2, the small degree of electromagnetic anisotropy between the c-axis and ab-plane directions confirmed in 11, 122 and 1111 systems encouraged us to develop more versatile conductors. Single crystals and thin films deposited on single-crystalline and metal substrates have proved that the potentials of the iron-based superconductors are high enough for designing superconducting materials for high field generation. In addition, critical current properties of powder-in-tube processed tapes have been greatly improved in the past two years and are reaching the application level at 4.2 K in high magnetic field. However, the pinning mechanism and determining factors of the critical current properties of the iron-based superconductors have not been well understood. Characteristics and potentials of iron-based superconductors are discussed from various viewpoints in this paper in an effort to understand the current status and future prospects.

Muon spin relaxation and neutron scattering investigations of the noncentrosymmetric heavy-fermion antiferromagnet CeRhGe3

The magnetic ground state of CeRhGe${}_{3}$ has been investigated using magnetic susceptibility, heat capacity, neutron diffraction, muon spin relaxation (\ensuremath{\mu}SR), and inelastic neutron scattering (INS) techniques. Our \ensuremath{\mu}SR study clearly reveals the presence of two frequencies below T${}_{\mathrm{N}2}$ $=$ 7 K and three frequencies between 7 K and T${}_{\mathrm{N}1}$ $=$ 14.5 K, indicating long-range magnetic ordering of the Ce${}^{3+}$ moment. The temperature dependence of the highest frequency follows a mean-field order parameter. Our powder neutron diffraction study at 1.5 K reveals the presence of magnetic Bragg peaks, indexed by the propagation vector k $=$ (0, 0, 3/4) with the Ce${}^{3+}$ magnetic moment \ensuremath{\sim}0.45(9) ${\ensuremath{\mu}}_{\mathrm{B}}$ along the $c$ axis. INS studies at 18 K (i.e., above T${}_{\mathrm{N}1}$) show the presence of two well-defined crystal-field (CEF) excitations at 7.5 and 18 meV. At 10 and 4.5 K, a very small increase has been observed in the CEF excitation energies. At 100 K, both CEF excitations broaden and a broad quasielastic component has also been observed. Further, the low-energy INS study reveals the presence of a nearly temperature-independent quasielastic linewidth between 16 and 60 K, which indicates a Kondo temperature T${}_{\mathrm{K}}$ $=$ 12.6(3) K. The presence of well-defined CEF excitations in CeRhGe${}_{3}$ suggests local moment magnetism and may explain the absence of pressure-induced superconductivity. Analyzing the INS data based on a CEF model, we have evaluated the CEF ground-state wave functions and ground-state moment. The observed small value of the ordered moment along the $c$ axis, deduced from the neutron diffraction data, contrasts with the ab-plane moment direction predicted by the single-ion CEF anisotropy and indicates the presence of two-ion anisotropic magnetic exchange interactions, which govern the direction of the moment.

Effect of crystallographic anisotropy on the resistance switching phenomenon in perovskites

Resistance switching effects in metal/perovskite contacts based on epitaxial c-axis oriented YBa2Cu3O6+c (YBCO) thin films with different crystallographic orientation have been studied. Three types of Ag/YBCO junctions with the contact restricted to (i) c-axis direction, (ii) ab-plane direction, and (iii) both were designed and fabricated, and their current-voltage characteristics have been measured. The type (i) junctions exhibited conventional bipolar resistance switching behavior, whereas in other two types the low-resistance state was unsteady and their resistance quickly relaxed to the initial high-resistance state. Physical mechanism based on the oxygen diffusion scenario, explaining such behavior, is discussed.

Evidence of a universal and isotropic2Δ/kBTCratio in 122-type iron pnictide superconductors over a wide doping range

We have systematically investigated the doping and the directional dependence of the gap structure in the 122-type iron pnictide superconductors by point contact Andreev reflection spectroscopy. The studies were performed on single crystals of Ba1-xKxFe2As2 (x = 0.29, 0.49, and 0.77) and SrFe1.74Co0.26As2 with a sharp tip of Pb or Au pressed along the c-axis or the ab-plane direction. The conductance spectra obtained on highly transparent contacts clearly show evidence of a robust superconducting gap. The normalized curves can be well described by the Blonder-Tinkham-Klapwijk model with a lifetime broadening. The determined gap value scales very well with the transition temperature, giving the 2{\Delta}/kBTC value of ~ 3.1. The results suggest the presence of a universal coupling behavior in this class of iron pnictides over a broad doping range and independent of the sign of the doping. Moreover, conductance spectra obtained on c-axis junctions and ab-plane junctions indicate that the observed gap is isotropic in these superconductors.

Synthesis and Microstructural Control of In<sub>2</sub>O<sub>3</sub>(ZnO)<sub>3</sub> Layered Compound by Microwave-Heating

In2O3(ZnO)3 layered compound was synthesized by 2.45 GHz microwave heating in a solid state reaction. Microwave-processed samples were obtained at low temperature by the enhancement of solid state diffusion and sublimation of the powder bed. Plate-like grain microstructures formed on the bottom part of the pellet, and vicinity of the surface was dense. The plate-like grain was oriented in the ab-plane direction. Compared with a conventional heated sample, the electrical conductivity increased and the band gap energy decreased. In the case of deposition on a silica substrate by microwave heating, the plate-like grain film was synthesized.

Flux pinning properties of YBCO/DyBCO multilayers

Multilayered structures of YBa2Cu3O7−δ (YBCO) interlayered with thin layers of DyBa2Cu3O7-δ (DyBCO) were prepared on CeO2-buffered r-cut sapphire (Al2O3) substrates by pulsed laser deposition. Evaluation of the magnetic-field angular dependence of the critical current density [Jc(H,θ)] revealed that the flux pinning properties of multilayered YBCO/DyBCO films were significantly enhanced in comparison to single-layer YBCO films (monolayers) prepared using the same experimental parameters. The YBCO/DyBCO multilayers are highly anisotropic, i.e., the angular-dependent Jc exhibits a very prominent peak when the applied magnetic field (H) is oriented parallel to the ab-plane direction (H∥ab). Analysis of the Jc(H,θ) data revealed an enhanced random pinning for the multilayers for the entire range of field investigated. In the angular-dependent Jc data, correlated pinning along the c-axis crystallographic direction was also evidenced at low applied fields by a less prominent peak at H∥c. This result was further corroborated by the presence of defect microstructures comprised of linear and planar defects which were considered as strong sources of c-axis-correlated pinning. However, for higher applied fields the contribution of c-axis-correlated pinning is highly diminished and the ratio of Jc(H∥ab) to Jc(H∥c) is significantly enhanced. In addition to enhanced random pinning, it is considered that improved pinning along the H∥ab direction occurs due to ab-correlated pinning, arising from intrinsic pinning and possibly extended planar defects.

Model for the angular dependence of critical currents in technical superconductors

We propose a model for the effect of anisotropic pinning on the criticalcurrents in superconductor wires as a function of the direction of an externalmagnetic field. The model emphasizes the statistical distribution of the pinningencountered by a vortex as determining the functional forms observed. Ifwe assume anisotropic defect populations with normal distributions of pinningthen we can derive exact functions for the critical current as a function of angle:Jc(θ). The model predicts that the combination of extended defects in thec-axisand ab-plane directions can lead to peaks at intermediate angles. The predicted forms agree wellwith experiments on superconductors with layered structures including cases wherepeaks occur at intermediate angles. The results suggest that common features ofJc(θ) can be identified with combinations of defects. We discuss some implications of thestatistical model including a possible sample thickness dependence of criticalcurrents.

Zero-bias conductance peak reduction by diffusive normal metal in high- Tc superconductor tunnel junctions

We report investigations of the smearing origin about a zero-bias conductance peak (ZBCP) observed in high- T c superconductor (HTSC) tunnel junctions from the point of view of the circuit theory analysis for d-wave superconductors. The ZBCP enhancement was measured in Bi 2Sr 2CaCu 2O 8+ δ –SiO–Ag planar tunnel junctions only for ab-plane directions. On the other hand, the circuit theory has been recently generalized from conventional superconductors to unconventional superconductors. The analysis in terms of the circuit theory reveals that the tunneling spectral behaviors of the ZBCP were well explained even when the Dynes's broadening factor was excluded from a tunneling spectral formula. Thus, the smearing effect in the ZBCP is understood by the influence of a diffusive normal metal resistance in the systems of HTSC tunnel junctions.

Angular Dependence of Critical Current Density in Y-Ba-Cu-O Thin Films

c-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin films for coated conductor applications have anisotropic behaviors of critical current density (J/sub c/) in magnetic fields. The anisotropic vortex pinning in YBCO films fabricated on SrTiO/sub 3/(100) single crystal substrates by pulsed laser deposition was investigated based on the field angular dependencies of J/sub c/, which were measured using the constant Lorentz force configuration (CLFC) and a variable Lorentz force configuration (VLFC). J/sub c/(/spl theta/) as a function of angle /spl theta/ between the ab-plane direction and the magnetic field direction had a sharp peak at /spl theta/=0/spl deg/, which was attributed to the intrinsic pinning. Besides, J/sub c/(/spl theta/) did not depend on the intensity of the Lorentz force when the magnetic field was applied near /spl theta/=90/spl deg/. This indicates that J/sub c/(/spl theta/) in this region is determined by the depinning of the vortex segment trapped by the pinning centers parallel to c-axis of the film. A transition from the intrinsic pinning to the extrinsic pinning is also discussed.

Superconducting properties of single-crystallineCa(Al0.5,Si0.5)2:A ternary silicide with theAlB2-type structure

Superconducting properties of single-crystalline $\mathrm{Ca}({\mathrm{Al}}_{0.5},{\mathrm{Si}}_{0.5}{)}_{2},$ which is isostructural to ${\mathrm{MgB}}_{2},$ were studied by measurements of electrical resistivity, Hall coefficient, and magnetization. $\mathrm{Ca}({\mathrm{Al}}_{0.5},{\mathrm{Si}}_{0.5}{)}_{2}$ is a type-II superconductor with a carrier density of $8.6\ifmmode\times\else\texttimes\fi{}{10}^{26}{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{m}}^{3}$ at 290 K. The lower critical field ${H}_{C1}$ has a parabolic temperature dependence typical for conventional superconductors. The upper critical field ${(H}_{C2})$ anisotropy ratio has a temperature-independent value of 2.3. The Ginzburg-Landau (GL) parameter \ensuremath{\kappa}(0) is equal to 12 and 5.2 in the ab-plane and c-axis directions, respectively. The anisotropy of the critical fields can be well described by the anisotropic GL theory.

Synthesis of c-axis-oriented MgB2 thin films and the Hall effect

We have synthesized c-axis-oriented MgB2 thin films on (1 1̄ 0 2) Al2O3 substrates by using a pulsed laser deposition technique and have measured the longitudinal and the Hall resistivites in the ab-plane direction. The as-grown thin films show Tc of 39.2 K with a sharp transition width of ∼0.15 K. In the normal state, the Hall coefficient (RH) behaves as RH ∼ T with increasing temperature (T) up to 130 K and then deviates from that linear T-dependence at higher temperatures. The T2 dependence of the cotangent of the Hall angle is only observed above 130 K. We estimate the absolute value of the hole density to be about two orders of magnitude higher than that of YBa2Cu3O7.

Do the stripes explain the c-axis transport in cuprates?

We propose that the stripes running differently in different CuO 2-planes cause large impurity and diffusion terms to the c-axis scattering rate τ c −1 in addition to the quadratic Fermi-liquid term. The main assumption made here is that the stripes of localized charge are formed by the bosons localized into rows of tilted CuO 6 octahedra in CuO 2-planes (D-stripes). They can scatter the 3D holes traversing into c-direction which experience the stripes as impurities giving rise to a constant scattering rate and the disorder gives linear temperature dependence. With the Fermi-liquid term added the scattering rate becomes τ c −1( t)= a c + b c t+ d c t 2, with t= T/ T *, where T * is the scaling temperature. In the ab-plane direction the situation is different since the holes and the bosons coexist within the realm of the chemical equilibrium reaction B ++ ⇌ 2h + and the localized bosons do not scatter the holes. Some disorder due to the vacancies in the 2D boson lattice causes still a small linear diffusion term and the rate becomes τ ab −1( t)= bt+ dt 2. Assuming the same carrier system (the holes), the ratio of resistivities for T> T c becomes ρ c / ρ ab ∼( m c / m ab )( a c / b) t −1, where m c and m ab are the effective masses. Since a c can be large and b small, the coefficient of the leading temperature dependence t −1 can be much larger than the mass ratio. This result is in agreement with experiments. Since the carrier density of holes obtained from the chemical equilibrium has linear temperature dependence we obtain linear planar resistivity ρ ab ( t)= A+ Bt and in the c-direction the same singular behavior ρ c ∼ t −1.