Anisotropic thermopower of the organic metal, β-(BEDT-TTF)2I3

Thermopower of the ambient pressure organic superconductor β-(BEDT-TTF)₂I₃ has been studied. Measurements performed on, respectively, crystals of needle formed morphology and on flake-like crystals with hexagon shape showed equal thermopower results. S was measured along the a-axis as well as along the b′-axis. Marked anisotropy is observed in the entire temperature region studied. The temperature dependence, as represented by dS/dT, is, however, nearly isotropic. On the basis of an analysis of anisotropic thermopower we attribute the isotropic part of S to a term depending on the bond-properties and the anisotropic part of S to the scattering mechanism. The analysis yields transfer integrals of the order of 0.12 eV along both a- and b′-axes.     

The experimental and theoretical investigation of vibration spectra in ferroelectric semiconductor SbSBrxI1−x crystals

The vapor grown SbSBr_xI_1−x (x=0.1; 0.5; 0.9) crystals with clear mirror surfaces have been used for infrared reflection measurements with Fourier spectrometer. The vibration frequencies along c(z)-axis have been derived from Kramers–Kroning and optical parameters fitting analysis of the experimental reflectivity spectra at T=300 K. The theoretical vibration spectra of SbSBr_xS_1−x (x=0.1; 0.5; 0.9) crystals in paraelectric phase (T=300 K) along c(z)-axis have been determined in quasiharmonic approximation by diagonalization of dynamical matrix. The theoretical vibration spectra of these crystals in a–b(x−y) plane have been determined in harmonic approximation. In this work we discuss the nature of anharmonism in SbSBr_xI_1−x crystals along the c(z)-axis.     

Direct observation of a large London penetration depth in an organic superconductor

Careful observation of the diamagnetic shielding signal in small fields applied along the a-axis of deuterated (TMTSF)₂ClO₄ crystals reveal a systematic dependence of the diamagnetic susceptibility on the thickness (b- and c-dimensions) of the crystals, which we explain by a macroscopically large value of 0.004 cm for the London penetration depth.     

Effects of uniaxial pressure and annealing on the resistivity of Ba(Fe1−xCox)2As2

Single crystals of underdoped Ba(Fe_1−xCo_x)₂As₂ were detwinned by applying uniaxial pressure. The anisotropic in-plane resistivity was measured using the Montgomery method without releasing pressure. The resistivity along the a-axis shows metallic behavior down to 5 K, while the resistivity along the b-axis shows an insulator-like behavior in some temperature range. Annealing the sample radically reduces the residual resistivity for x=0, and at the same time the anisotropy becomes much smaller at low temperatures.     

One-dimensional electronic order in underdoped surface of YBa2Cu3Oy studied by STM

We have performed low-temperature scanning tunneling microscopy (STM) experiments on the cold-cleaved surface of YBa₂Cu₃O_y single crystals to study the nanoscale electronic order in high-T_c superconductors. STM images measured at low-bias voltage below ∼50 meV show the one-dimensional (1D) electronic modulation along the Cu-O bonds (parallel to the b-axis). The 1D electronic modulation does not have long-range order and the periodicity along the a-axis varies within the range ∼2a-4a depending on the position on the surface, indicating the glassy electronic order in the underdoped CuO₂ plane.     

The nature of anharmonicity and anomalous piezoelectric properties of ferroelectric SbSBr crystal

The contribution of soft mode at Sb atom's sites, to the temperature dependences of Sb atom's equilibrium position's difference Δz(T) has been studied theoretically, when SbSBr crystal is deformed along a(x), b(y) and c(z)-axis in paraelectric phase and is deformed along c(z)-axis in ferroelectric phase. The largest change of Δz₃₃(T) occurs in the ferroelectric phase near the phase transition temperature in the range from 16 K to 21 K. The temperature dependence of Sb atom's equilibrium position's displacements Δz₃₃ is very similar to the temperature dependence of experimental piezoelectric modulus, when SbSBr crystal is deformed in the direction of c(z)-axis in ferroelectric phase.     

Anisotropic thermopower of (TMTSF)2PF6: 1D–2D cross over and SDW ordering

The thermopower of (TMTSF)₂PF₆ has been measured along the a-axis as well as along the b-axis. Marked anisotropy is seen in the whole temperature region studied. Close to 100 K, an anomaly is attributed to cross over from dominating one-dimensionality to dominating two-dimensionality. Precursor effects seen in S_a near T_c are attributed to SDW fluctuations. Well below T_c, both thermopower components exhibit typical semiconducting properties.     

Relative power loss of misalignment based on electro-optical printed circuit board

In this paper, the power loss values between 45° total internal reflecting waveguide mirror (TIRWM) and the polymer optical waveguide layer are obtained by vector finite element method. There are some misalignments during an actual fabrication of electro-optical printed circuit board (EOPCB). And, b, a, c correspond to the error value of alignment in the x-axis, y-axis and z-axis direction, respectively. Another, four effective refractive indices of the 45° TIRWM and polymer optical waveguide layer are calculated, separately. And, n_eff1 = 1.425211 is uniquely chosen. Next, these relative power errors Δ are calculated, when a, b, c separately change. Moreover, these error values are plotted into some curved surface figures. By these figures, it is easy to find the relationship between a, b, c and Δ. Furthermore, it is beneficial for us to avoid the region of larger power loss during an actual fabrication of EOPCB.     

CDW-induced negative thermal expansion in two-dimensional conductor η-Mo4O11

Two-dimensional oxide η-Mo₄O₁₁ exhibits almost isotropic thermal expansion in the normal phase, while anisotropic negative thermal expansion (NTE) in the charge density wave (CDW) phase below T_c1=105 K, where a remarkable anomalous expansion occurs along the a-axis. The incommensurate nesting vector along the b-axis is independent of temperature, q₁=(0.000(1) 0.2335(1) 0.0000(5)). We propose that the CDW-induced NTE occurs as a result of the structural relaxation of the MoO₆ octahedra layers along the stacking direction with the aid of the open spaces around the MoO₄ tetrahedra.     

Pyroelectric properties of a ferroelectric single crystal [NH2(CH3)2]3Sb2Cl9 (DMACA)

The pyroelectric properties of DMACA single crystals have been measured in the range 135–293 K, revealing the existence of ferroelectric second order phase transition at T_c = 243 K. The saturation value of spontaneous polarization P_s along a-axis amounts to 6.8 × 10⁻³ Cm⁻² at about 203 K. Critical exponent β = 0.5 has been found in the region 0.5–10 K away from T_c.     

Optical absorption in tin monoselenide single crystal

Optical absorption in single crystals of tin monoselenide has been measured at room temperature with plane polarized light near the fundamental absorption edge. The electric field of the incident light was oriented parallel to the a- and b-crystallographic axes in the plane of cleavage. Results have been analyzed on basis of two- and three-dimensional models. Absorption near the fundamental edge was found to be due to indirect forbidden transition with two phonons involved in the process. The energy gap and phonon energies were found to be 0.948 eV and 54 and 21 meV for the a-axis and 0.902 eV and 41 and 20meV for the b-axis. The two-dimensional model gave results similar to those obtained from the three-dimensional model and could thus be used to describe the main optical properties of SnSe single crystal.     

Theory of microwave losses of one-dimensional conductors in the skin effect regime: Application of TTF-TCNQ

Solutions for the microwave cavity losses due to a long isotropic or anisotropic conducting crystal oriented parallel to the maximum cavity E field are reported. The results are used to study the temperature dependent microwave conductivity of TTF-TCNQ along the principal conducting b-axis.     

Optical anisotropy of orthorhombic InS

The optical anisotropy of InS single crystals in the range of photon energy from 1.8 to 3.5 eV has been studied by absorption, electroreflectance and wavelength-derivative reflectance measurements. These systematic optical measurements for the polarizations, E//a and E//b, have revealed that the transition at the fundamental absorption edge of InS is allowed for E//b, and there exist three distinct doublet transitions having different selection rules in the photon energy region from 2 to 3.5 eV; Bo and B^'₀doublet allowed only for E//b, A₀ and A^'₀ allowed only for E//a, and E₁ and E^'₁ allowed for both polarizations. The observed results are discussed based on the anisotropic nature of two chemical bonds in InS, cation-cation and cation-anion.     

Morphological Changes in Flat-on and Edge-on Lamellae of Poly(Ethylene Succinate) Crystallized from Molten Thin Films

Corrugated crystal morphologies of poly(ethylene succinate) (PESU) with a bird-wing shape grew from the thin-film melt with the film thickness less than 1 μm. The center of the bird-wing-shaped crystal consisted of a lozenge crystal. Corrugated steps were observed in the direction of the short axis of the lozenge crystal (b-axis). Fibrous textures extended in the direction of the long axis of the lozenge crystal (a-axis), and they wrapped around the corrugated crystals. The corrugated crystals and the fibrous textures consisted of flat-on and edge-on lamellae, respectively. The wing-shaped morphology was constructed by combining the crystal growth with the flat-on and edge-on lamellae.     

Magnetic structure of KFeS2 — A linear chain antiferromagnet and a spin analog of active sites of two iron ferredoxins — By neutron diffraction

Magnetic structure of KFeS₂ — a spin analog of the oxidized two iron ferredoxins — was determined by the neutron diffraction method. Spins are antiferromagnetically coupled along the c-axis chain, and they make ferromagnetic sheets in the planes containing a and b axes below the transition temperature of 250.0 ± 0.5 K. The iron moment of 2.43 ± 0.03 μ_B lies in the a?c plane and is canted by 125.3 ± 0.7 degrees from a-axis at 4.2 K.     

Structural and physical–chemical properties of the CuGa5Se8, CuGa3Se5 and CuIn3Se5 compounds

Large-block (8×4×3 mm³) CuGa₅Se₈ crystals were obtained by the horizontal Bridgman method. Homogeneous CuGa₃Se₅ and CuIn₃Se₅ single crystals 12 mm in diameter and up to 40 mm in length were grown by directed crystallization of the melt. All three compounds were found to have the chalcopyrite-related structure. The melting points of these compounds were defined by means of the differential thermal analysis. The lattice parameters a and c as well as the axial thermal expansion coefficients α_a and α_c were determined as a function of temperature in the range from 90 to 650 K by the X-ray diffraction method. It is revealed that for all three compounds the coefficients of expansion along the a-axis are larger than those along the c-axis over the entire temperature range studied.     

Anomalous shear ultrasonic attenuation peak at 95.5°K in holmium2

An anomalous attenuation peak at 95.5°K has been observed for shear ultrasonic waves transmitted along the c-axis of two holmium single crystals. The velocity change across this temperature is on the order of 10^?4. In the presence of an external magnetic field, the peak moves toward high temperatures for both H∥a and H∥c. A rotation of the field in the basal plane produces a two-fold symmetry pattern in the attenuation. A double peak structure centered at the c-plane has also been observed when the field is rotated away from the a-axis. A considerable fraction of the attenuation peak appears to go as sin² θ where θ is the angle between the shear wave polarization and the external field.     

Ferroelectric phase transitions in NH4HSeO4 crystals

Results are presented of measurements of spontaneous polarization, electric permittivity and electrical conductance of NH₄HSeO₄ crystals in three principal crystallographic directions from liquid nitrogen temperature to 320 K. NH₄HSeO₄ crystals exhibit ferroelectric properties in the a-axis direction between 106–250 K, whereas in the b-direction they are pyroelectrie over the entire temperature range. The calculated activation energy (1.42 eV) is indicative of semiconducting behavior.     

Low-temperature heat transport in α-HgI2 single crystals

The thermal conductivity of several samples from α-HgI₂ crystals grown by two different methods has been measured from 50 mK to 200 K. The thermal conductivity is found to be intrinsic but anisotropic above 15 K: it is smaller along c-axis than along a-axis, the anisotropy ratio being about 5 between 15 and 200 K. Below 15 K, the thermal conductivity is sample dependent and the calculated Casimir limit is not reached at the lowest temperatures. The results have been interpreted considering phonon scattering by structural defects. A simple quantitative analysis of the curves suggests that phonons are scattered mainly by large clusters of interstitial defects due to the lack of stoichiometry of the crystals; the typical dimensions of these clusters are not smaller than 1 μm perpendicular to c-axis and 0.3 /gmm along c-axis. The presence of plane defects is also detected. Point defect scattering is relatively small and explained by residual metallic impurities and carbon at interstitial sites. The intrinsic anisotropy is briefly discussed.     

Irreversible magnetovolume effect in Nd7Rh3 single crystal

Magnetovolume effect in Nd₇Rh₃ single crystal has been studied by measuring the magnetostriction as a function of external magnetic field at 4.2 K. An irreversible magnetovolume effect having a negative remanent volume magnetostriction was observed when the external magnetic field was applied along the b-axis. The irreversible magnetostrictive effect takes place in the longitudinal magnetostriction along the b-axis. The remanent magnetostriction along the b-axis relaxes after removing external magnetic field for several hours and equilibrium state is stabilized.