Spin-Phonon Interaction and the EPR Linewidth in La2CuO4 and Related Cuprates

The spin-phonon interaction is derived for Cu²⁺ ions stronglycoupled by the isotropic exchange interaction. It is shown that in a two-dimensionalquantum Heisenberg antiferromagnetic this interaction due to its antisymmetricstructure couples the phonon modes near the wave vectors (±/a, ±/a)to the staggered magnetization. A contribution of the spin-phonon interactionto the EPR linewidth is calculated and its temperature dependence compared with othersources of the broadening.     

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.     

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.     

Isochoric (p-π-T) measurements on liquid and gaseous air from 67 to 400 K at pressures to 35 MPa

Comprehensive isochoricp--T measurements have been carried out on liquid and gaseous air along 16 isochores at densities ranging from 2 to 32 mol · dm^–3. The air mixture has a nominal composition of 0.7813 N₂ + 0.2096 O₂ + 0.0092 Ar. Thep--T data cover a temperature range from 67 to 400 K at pressures up to 35 MPa. Comparisons with experimental results from independent sources are presented using a fundamental equation of state based. in part, on thep--T data from this study.     

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.     

The electrical conductivity of some azine compounds

Correlation of the molecular structure and thermal study of the electrical conductivity of benzaldazine (BA) and its NO₂-derivatives were carried out. The values of the activation energies, obtained from the electrical conductivity measurements, as well as the energy gaps calculated from ultra violet (u.v.) and visible spectra obtained either in liquid or in solid forms indicate that all compounds behave like a semiconducting material. Both n*s and *s were found to be the main sources participatiog in the cooduction processes.     

Anisotropic Upper Critical Field in Single Crystal MgB2

Recent experiments on MgB₂ single crystals appear to indicate that superconductivity in this material is described not only by two superconducting order parameters attached to the band and the band, respectively, but also these two order parameters have different anisotropy. More explicitly the anisotropy of H _c2(t) requires an oblate order parameter in momentum space attached to the a band while H _c1(t) is described by a prolate order parameter, attached to the band. Therefore the vortex state in MgB₂ should be described by an interplay of these two superconducting order parameters.     

The Tc Amplification by “Shape Resonance” in Doped MgB2

We have applied two channels ( and ) superconductivity model to the Al_1–x Mg _x B₂. Using the experimental data, we have calculated the strength of the interchannel pairing due to quantum interference effects, probed by the interband coupling parameter, and the two gaps as a function of the x. While in MgB₂ the quantum interference effects gives an amplification of T _c by factor 1.5 in comparison with the dominant intra band single channel pairing, in AlMgB₄ the amplification is about 100, in comparison with the dominant intra band single channel pairing.     

Scattering Rate Anisotropy in the Pseudogap State of BSCCO

We study quasi-particle lifetime anisotropy in BSCCO as a function of temperature and doping by relaxational analysis of symmetrized Raman response functions. For moderately underdoped BSCCO the pseudogap behavior manifests itself in anisotropic suppression of the low-frequency scattering rates, sharper in the vicinity of (, 0) and equivalent points of the Brillouin zone, or the hot spots, and milder in the vicinity of (/2, /2) points, or the cold spots. With stronger underdoping the scattering rate suppression becomes more pronounced and isotropic.     

Suppression of π/2−π Spin Echo in Solid 3He in High Fields

High demagnetizing field in solid ³ He leads to rich interesting phenomena such as multiple spin echoes. On the other hand, it has a severe influence on the appearance of the ordinary spin echo using /2– RF pulse sequence. We have observed spin echoes in solid ³ He in 7.4 Tesla, which appeared to decay on a much shorter time scale than T ₂ estimated from theories. This can be explained as follows. The angle made by the magnetization with the field after the -pulse differs a little from that before the pulse, because the -pulse employed in the experiment is inevitably not ideal. Since due to the demagnetizing field, the frequency of the precession motion depends on the angle between the magnetization and the field, the frequencies before and after the -pulse are different, which smeared out the spin echo. We have computed the damping time in various conditions, and found good agreement with experimental results.     

Incommensurate Dynamical Properties of Optimally Doped YBa2Cu3O6.9

The resonance and incommensurate peaks in optimally doped high-T _c superconductor YBa₂Cu₃O_6.9 are observed by means of TOF neutron scattering technique. Obtained overall dynamical properties in a wide Q-E space show distinct incommensurate structure around antiferromagnetic (AF) zone center (, ). The incommensurability of =0.1 at 36 meV corresponds to the previous triple-axis neutron measurement. Variation of temperature dependence suggests the spectral weight of antiferromagnetic fluctuation observed around 25 meV at (, ) shifts to resonance peak position below T _c. Moreover obtained results with E _i=80 meV neutron energy indicate new feature of dynamical incommensurate structure at higher frequency region up to 53 meV.     

Flux-creep activation energies at the grain boundaries of Y-Ba-Cu-O superconductors in the characteristic frequency band

Measurements on the temperature and frequency dependence of the complex ac susceptibility of sintered Y_1.01Ba_1.95Cu_2.97O _x are reported. The data are used to obtain the flux-creep activation energies at the grain boundaries in the characteristic frequency band. The found nonlinear frequency dependence of these energies can be approximated differently in the low, characteristic, and high frequency bands. The relaxation frequency of thermal fluctuation of the vortex lattice (2 3×10⁴ Hz) and the surface current density (J _s=0–10 A/cm²) have been estimated.     

63Cu NMR in the normal state of HgBa2Ca2Cu3O8+δ

We present a⁶³Cu NMR study of HgBa₂Ca₂Cu₃O₈₊ underdoped single crystals with T_c 115 K. While the uniform spin susceptibility decreases below T_o 370 K, relaxation rate measurements demonstrate the opening of a spingap at Q = (, ) below T* 230 K, the highest temperature reported so far. The characteristic energy of spin fluctuations is shown to be higher than in underdoped YBa₂Cu₃O_7–, and the analysis of the quadrupole and hyperfine couplings suggests that the in-plane Cu-O hybridization is also stronger. The T-dependence of T₁ is the same in the three CuO₂ planes which seems hardly compatible with the pure interlayer spin-pairing picture.     

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.     

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.     

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.     

The transverse acoustic impedance of He II

The complex shear acoustic impedance of liquid He II has been measured at frequenciesf(=/2) of 20.5, 34.1, and 47.8 MHz from 30 mK to the -point T (2.176 K). The impedanceZ was found from the temperature dependence of the quality factor and the resonant frequency of a thickness shear mode quartz crystal resonator immersed in the liquid. The relationship for a hydrodynamic viscous liquidZ(T)=(1–i)(f _n )^1/2 was used to measure the temperature dependence of the viscosity (T) using tabulated values of the normal fluid density _n (T). Deviations from hydrodynamic behavior occurred when the viscous penetration depth was less than the superfluid healing length, the phonon mean free path, and the roton mean free path. Near the -point,Z(T)/Z(T) was frequency dependent and a value for the superfluid healing lengtha=(0.10±0.01)^–2/3 nm was found, where =(T–T)/T. The effects of van der Waals forces near the crystal surface were also observed and a layer model was used to interpret the measurements. Below 1.8 K only rotons contribute significantly toZ and we determined the roton relaxation time as _r =8.5×10^–14 T ^–1/3 exp (8.65/T) sec. Below 1.2 K, _r >1 and we investigated the breakdown of hydrodynamics in this region. ForT<0.6 K the resonant frequency of the crystals decreased by f/f=2×10^–7, but the origin of this effect is not yet known.Financial support provided by the SERC, Bedford College, and the Central Research Fund, University of London.     

Thermohydraulic stability of helium flow at supercritical pressure under conditions of forced and mixed convection in a vertical heated channel

The excitation of thermally induced oscillations in low-temperature helium flow in channels with different orientation are studied and the boundaries of stability are constructed.Notation c heat capacity - D diameter - F area of the transverse cross section - G flow rate - Gr_A Grashof number - g acceleration of gravity - h enthalpy - p pressure - perimeter - Re Reynolds number - s complex Laplace transform variable - T temperature - t time - x distance along the channel - thermal conductivity - circular frequency of the pulsations - p specific density - in at the inlet to the channel - out at the outlet from the channel - liq the flow - O in front of the throttle at the channel input - m the pseudocritical temperature - N in front of the throttle at the channel output - w and c the pipe wall Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 4, pp. 538–545, October, 1988.     

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.     

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 ).