Effects of Orthorhombic Distortion on Magnetic Excitation in t-J Model

Neutron scattering experiments on La_2–x Sr _x CuO₄ (LSCO) have revealed the incommensurate antiferromagnetic peaks do not lie exactly on the symmetry axes (q _x=± and q _y=±), but, are slightly shifted from them. In this paper, a scenario is presented for such shift in terms of the anisotropy of t (next-nearest-neighbor hopping integral on the square lattice) in the slave-boson scheme of the two-dimensional t-J model. Since the predictions of the present theory are different from those based on the spin-charge stripes hypothesis, further studies of the shift may clarify the factor responsible for the incommensurate antifcrromagnetic fluctuations in LSCO systems.     

Relation of Spin Susceptibility and a.c. Conductivity in Nearly Antiferromagnetic Fermi Liquid

Coupling of the charge carriers through the magnetic spin susceptibility which is strongly peaked at (, ) in the two dimensional CuO ₂ Brillouin zone leads to strong momentum anisotropies and energy dependencies in the inelastic optical scattering rates. The a.c. conductivity is isotropic and can be used to extract an effective, isotropic spin-charge carrier spectral density which reproduces almost exactly the optical scattering rates. The technique is then applied to data in superconducting, optimally doped YBCO and used to measure the charge carrier coupling to the 41 meV peak observed in spin polarized neutron scattering.     

A Critical Examination of the Spin Dynamics in High-Tc Cuprates

A critical examination of the spin dynamics in high-T _c cuprates is made in the light of recent inelastic neutron scattering results obtained by different groups. The neutron data show that incommensurate magnetic peaks in YBCO belong to the same excitation as the resonance peak observed at (/a, /a). Being observed only in the superconducting state, the incommensurability is rather difficult to reconcile with a stripe picture. We also discuss the link between the resonance peak spectral weight and the superconducting condensation energy.     

A dilatometric method to measure the thermal diffusivity of nonmetallic liquids

A new method to measure the thermal diffusivity of liquids is presented. It requires determination of the time dependence of the thermal expansion of the liquid when it is subjected to a heat source at the top of the cell containing the liquid. The high accuracy of the method (about 3%) is due to an essential reduction of convective currents and also to the absence of temperature detectors, which generally introduce unwanted perturbations on the thermal Field.Nomenclature Thermal conductivity - c Specific heat - Density - c = specific heat x density - h Newton coefficient - Thermal diffusivity - T, 0 Temperature - tV Electric signal - Calibration coefficient - _exp, _th Volume change of the liquid     

In-Plane Fourfold Symmetry of the Upper Critical Field Observed in La(Sr)214 Single Crystals

In order to establish the existence of in-plane anisotropy of the upper critical field H _c2 (), the out-of-plane resistivity _c measurements were performed on La(Sr)214 single crystals with rectangular and cylindrical shape under rotating magnetic field applied within the ab-plane. Although observed _c shows non-sinusoidal twofold symmetry, clear fourfold symmetry was obtained after subtracting twofold sinusoidal component in _c which is due to the unavoidable misalignment of the magnetic field with respect to ab-plane. H _c2 () is estimated from the extracted fourfold component of _c with the flux flow theory. Angular dependence of H _c2 () was well fitted by cos(4). Since the fourfold component of H _c2 () was largest at = n/2, which corresponds to the a-axis direction, the present results strongly suggest the type symmetry in La(Sr)214. In addition, the difference in fourfold component of H _c2 at = n/2 and n/2+/4 was found to increase with decreasing temperature.     

Conventional Electronic Structure of MgB2 and ZrB2: LDA vs. de Haas-v. Alphen & ARPES Data

We compare full potential LDA band calculations of the Fermi surfaces areas and band masses of MgB₂ and ZrB₂ previously reported and new dHvA data. Discrepancies in areas in MgB₂ can be removed by a small shift of bands relative to bands. Comparison of effective masses lead to orbit averaged el-ph coupling constants =1.3 and =0.5, whereas for ZrB₂ only weak el-ph coupling with <0.3 is found. The ARPES data can be also well described by the LDA showing the presence of surface states.     

Microscopic Scenario for x = 1/8 Anomaly in La2 ? xSrxCuO4

We adopt a t ₁-t ₂-t ₃-J-G model for explanation of x = 1/8 anomaly in La_{2 – x} Sr _x CuO₄ family compound. The calculated charge susceptibility shows a maximum near Q = (, ) at intermediate temperatures and near (, /2) as temperature approaches zero, in agreement with neutron scattering experiments. Coulomb repulsion G between the first neighbors turns out to be the source of Charge Density Waves (CDW) in narrow band t ^eff ₁, t ^eff ₂, t ^eff ₃ < G. For physically realistic hopping values we obtain the CDW amplitude e _Q = x. The in-phase domain structure as a candidate for stripe picture is proposed.     

Dynamical Charge and Spin Susceptibilities in a Frame of t-J-G Model

We have calculated the dynamical charge and spin susceptibilities using the new analytical expression obtained beyond a conventional random phase approximation scheme. Both susceptibilities are strongly peaked along a contour around wave vector Q = (, ). We have analyzed the dispersions of the collective excitations near Q = (, ) corresponding to a spin density wave and charge density wave modes, respectively. In addition we have calculated the momentum dependence of the imaginary part of the charge and spin susceptibilities along the instability contour and show that both susceptibilities display a maximum around the points (, ±q ₀), (±q ₀, ) in Brillouine zone with decreasing temperature that indicates that the stripe-like instability may become preferable.     

Analytical Theory of DC SQUIDS Operating in the Presence of Thermal Fluctuations

A comprehensive analytical theory of symmetric DC SQUIDs is presented taking into account the effects of thermal fluctuations. The SQUID has a reduced inductance < 1/ where = 2LI_c/₀, L is the loop inductance, ₀ is the flux quantum, and I_c is the critical current of the identical Josephson junctions which are assumed to be overdamped. The analysis, based on the two dimensional Fokker–Planck equation, has been successfully performed in first order approximation with considered a small parameter. All important SQUID characteristics (circulating current, current-voltage curves, transfer function, and energy sensitivity) are obtained. In the limit 1( = 2k_BT/I_c0 is the noise parameter, k_B is the Boltzmann constant, and T is the absolute temperature) the theory reproduces the results of numerical simulations performed for the case of small thermal fluctuations. It was found that for < 1 the SQUID energy sensitivity is optimum when is higher than 1/, i.e., outside the range for which the present analysis is valid. However, for 1 the energy sensitivity has a minimum at L = L_F , where L_F = ( ₀ /2) ²/k_B , and therefore, in this case, the optimal reduced DC SQUID inductance is _opt = 1/, i.e., within the range for which the present analysis is valid. In contrast to the case of an RF SQUID, for a DC SQUID the transfer function decreases not only with increasing L/L_F but also with increasing (as 1/). As a consequence, the energy sensitivity of a DC SQUID with < 1/ degrades more rapidly (as ⁴ ) with the increase of than that of an RF SQUID does (as ² ).     

The piezoresistance coefficients of copper and copper-nickel alloys

This paper attempts to further a better understanding of the piezoresistance coefficients by studying the piezoresistive effects in copper and copper-nickel alloys. The experimental evidence of isotropic piezoresistance coefficients (₁₁=₁₂) has been obtained for the annealed copper and copper-nickel alloys. The piezoresistance coefficients of the cold-worked copper and Cu₆₀Ni₄₀ alloy are of the tensor character (₁₁₁₂). A physical explanation has been given to the change of the ( _ij ) tensor.     

Some electrical characteristics of lithium and yttrium sialons

Some electrical properties of hot-pressed lithium sialons, Li _x/8Si_6–3x/4Al_5x/8O _x N_8–x havingx<5 and an yttrium sialon were measured between 291 and 775 K; the former consisted essentially of a single crystalline phase whereas the latter contained 98% glassy phase. For lithium sialons, the charging and discharging current followed al(t) t ^–nlaw withn=0.8 at room temperature. The d.c. conductivities were about 10^–13 ohm^–1 cm^–1 at 291 K and rose to 5×10^–7 ohm^–1 cm^–1 at 775 K. At high temperatures electrode polarization effects were observed in d.c. measurements. The variation of the conductivity over the frequency range 200 Hz to 9.3 GHz followed the () ⁿ law. The data also fitted the Universal dielectric law,() ^n–1 well, and approximately fitted the Kramers-Kronig relation()/()– =cot (n/2) withn decreasing from 0.95 at 291 K to 0.4 at 775 K. The temperature variations of conductivities did not fit linearly in Arrhenius plots. Very similar behaviour was observed for yttrium sialon except that no electrode polarization was observed. The results have been compared with those obtained previously for pure sialon; the most striking feature revealed being that _d.c. for lithium sialon can be at least 10³ times higher than that of pure sialon. Interpretation of the data in terms of hopping conduction suggests that very similar processes are involved in all three classes of sialon.     

Dielectric properties of chemically vapour-deposited Si3N4

The dielectric properties of chemically vapour-deposited (CVD) amorphous and crystalline Si₃N₄ were measured in the temperature range from room temperature to 800° C. The a.c. conductivity ( _a.c.) of the amorphous CVD-Si₃N₄ was found to be less than that of the crystalline CVD-Si₃N₄ below 500° C, but became greater than that of the crystalline CVD-Si₃N₄ over 500° C due to the contribution of d.c. conductivity ( _d.c.). The measured loss factor () and dielectric constant () of the amorphous CVD-Si₃N₄ are smaller than those of the crystalline CVD-Si₃N₄ in all of the temperature and frequency ranges examined. The relationships of ^n-1, (- ) ^n-1 and/(- ) = cot (n/2) (were observed for the amorphous and crystalline specimens, where is angular frequency andn is a constant. The values ofn of amorphous and crystalline CVD-Si₃N₄ were 0.8 to 0.9 and 0.6 to 0.8, respectively. These results may indicate that the a.c. conduction observed for both of the above specimens is caused by hopping carriers. The values of loss tangent (tan) increased with increasing temperature. The relationship of log (tan) T was observed. The value of tan for the amorphous CVD-Si₃N₄ was smaller than that of the crystalline CVD-Si₃N₄.     

Torsional load transfer from a rigid shaft to an elastic plane with a slit

A plane elastostatic problem for an elastic wedge loaded by a concentrated moment at its apex provides an example of violation of the Saint-Venant principle for apex angles 2 larger than . Considering the problem for a truncated wedge, Neuber demonstrated the method of construction of an applicable solution for any apex angles in the range 22, despite the failure of the Saint-Venant principle. In the present paper the particularly important case of the truncated-wedge problem is examined. The truncated wedge degenerates into a slitted elastic plane, while a rigid circular shaft, acted upon by a torsional moment, is inserted into the plane. The analytical solution of the mixed boundary-value problem is obtained. Numerical results turn out to be in complete agreement with Neuber's results for the slitted elastic plane.     

Simulation of the Process of Water Freezing in a Round Pipe

The problem of freezing of pure water in a round pipe is treated with due regard for convection under asymmetric thermal boundary conditions in the absence of motion along the pipe. The problem is solved numerically using the control volume approach, SIMPLER algorithm, and the enthalpy method. Results are obtained for three Grashof (Gr) and six Biot (Bi) numbers: Gr = 1.55 × 10⁶, Bi = 0.305 (0 < ), Bi = 0.044 ( < 2); Gr = 1.24 × 10⁷, Bi = 0.610 (0 < ), Bi = 0.087 ( < 2); Gr = 9.89 × 10⁷, Bi = 1.220 (0 < ), Bi = 0.174 ( < 2). The correctness of calculation of the problem disregarding free-convection flows is analyzed.     

On the dispersion of gases under the action of the medium

Some general regularities of dispersion of a gas emerging from a nozzle submerged in a liquid are considered. A condition for establishment of the so-called maximum dispersion state is formulated.Notation ₀ coefficient of surface tension at the liquidgas boundary - contact angle of wetting of the nozzle material surface by the liquid - p_at atmospheric pressure - p air pressure - density of the liquid - g gravitational acceleration - h height of the liquid column - ₁, and _g dynamic viscosity coefficients of the liquid and gas, respectively - R and r radii of the bubble and nozzle, respectively - Q and F dimensionless criteria - , , , , and undetermined coefficients - ratio of the circumference of a circle to its diameter     

Deformation approach to the evaluation of the period of initiation and growth of fatigue macrocracks

We performed experimental investigation of the opening displacements of the contours of stress concentrators (notches and cracks) for various amplitudes of cyclic loading. On the basis of experimental results, we propose a new deformation parameter _t which is a function of the notch (crack) tip opening displacement , namely, _t /(+d*), where is the radius of the tip of the notch andd* is the characteristic size of the prefracture zone. It is shown that this parameter uniquely determines the number of cyclesN _l to the initiation of a fatigue macrocrack independently of the geometry of the specimens and stress concentrators in elastic and elastoplastic materials, i.e., the dependence of _t onN ₁ is a characteristic of the material. It is experimentally demonstrated that this dependence enables one to quantitatively describe the process of fatigue fracture both in the stage of initiation of macrocracks and their propagation.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 5, pp. 7–21, September – October, 1995.     

Photoreaction of a ZnO gel film chemically modified with β-diketones

Chelate formation is confirmed by a red shift of the n * absorption peak of benzoylacetone (BzAcH) from 309 to 336 nm with its addition to a sol containing Zn²⁺ ions. The chelate bonds between Zn²⁺ and BzAc^– are mostly maintained in the gel film prepared from the sol. Irradiation of the gel film by a Xe lamp with a cut filter (_ex > 300 nm) in the presence of H₂O leads to decomposition of the chelate ring. As a result of the photolysis, ZnO–H groups, CH₃CHO and other carbonyl compounds are generated with the lost of CH₃ groups of the BzAc^– ligand and H₂O involved in the film. INDO/S calculations on a model complex (Zn(BzAc)(OEt)) assign the n * absorption to the electronic transition from a non-bonding molecular orbital (MO) distributed mainly at the phenyl group to an anti-bonding MO localized at the CO bonds. On the basis of these results, a photo-induced hydrolysis mechanism was presented to explain the formation of positive-type patterned ZnO films.     

Interaction among a row of ellipsoidal inclusions

In this paper the interaction among a row of N ellipsoidal inclusions of revolution is considered. Inclusions in a body under both (A) asymmetric uniaxial tension in the x-direction and (B) axisymmetric uniaxial tension in the z-direction are treated in terms of singular integral equations resulting from the body force method. These problems are formulated as a system of singular integral equations with Cauchy-type or logarithmic-type singularities, where unknowns are densities of body forces distributed in the r,,z directions. In order to satisfy the boundary conditions along the ellipsoidal boundaries, the unknown functions are approximated by a linear combination of fundamental density functions and polynomials. The present method is found to yield rapidly converging numerical results for interface stresses. When the elastic ratio E ₁E _I/E _M>1, the primary feature of the interaction is a large compressive or tensile stress _n on the interface =0. When E ₁E _I/E _M<1, a large tensile stress or _t on the interface =1/2 is of interest. If the spacing b/d and the elastic ratio E _I/E _M are fixed, the interaction effects are dominant when the shape ratio a/b is large. For any fixed shape and spacing of inclusions, the maximum stress is shown to be linear with the reciprocal of the squared number of inclusions.     

Superconducting Gap in the Multi-band System MgB2

Based on the angle-resolved photoemission spectra of single crystals, it is demonstrated that a couple of bands cross the Fermi energy in MgB₂, which is in good agreement with band theory. The superconducting gap in this multiband system is carefully examined by Raman scattering spectroscopy with various polarizations. It has been revealed that the large gap (24k _B T _c) that is typical for a clean limit s-wave superconductor is restricted to the -bands, while the gap on the -bands is much smaller (21.1k _B T _c) and strongly affected by the impurity scattering, which gives a dirty limit behavior. This unusual two-gap behavior might be caused by the lack of interband scattering due to special separation of the - and -bands, as predicted by Mazin et al.     

Polarization-dependent angle-resolved photoemission of the layered cuprate Ba2Cu3O4Cl2

We report polarization-dependent angle-resolved photoemission studies of single crystalline Ba₂Cu₃O₄Cl₂. This antiferromagnet adopts a layered body-centered tetragonal structure related to that of the cuprates, but with anextra copper site resulting in a plane with Cu₃O₄ stoichiometry. For the -M direction of the Brillouin zone we observe a dispersive feature with a bandwidth of 400 ±80 meV, which approaches to within 0.9 eV of the Fermi level at (/2,/2). The magnitude of the observed dispersion and its polarization dependence are consistent with its origin as a purely antibonding combination of O2p_x,y and atomic orbitals from the cuprate-like CuO₂ sub-system, i.e. Zhang-Rice singlets.