Ming Lei

Monocrystal elastic constants of orthotropic Y1Ba2Cu3O7 : an estimate

For Y{sub 1}Ba{sub 2}Cu{sub 3}O{sub 7}, using only reported monocrystal measurements and some analysis--theory, we estimated the complete nine-component orthotropic-symmetry elastic-stiffness matrix, the Voigt {ital C}{sub {ital ij}} matrix. Comparison with very-high-frequency tetragonal-symmetry phonon-dispersion results shows good agreement (9% on average), except for {ital C}{sub 12}.

Elastic constants, debye temperatures, and electron-phonon parameters of superconducting cuprates and related oxides

Abstract Using both measurements and modeling, we studied the cohesive and related properties of several oxides. Superconducting oxides include La1.85Sr0.15CuO4, Y1Ba2Cu3O7, and (Bi-Pb)2Sr2Ca2Cu3O10. Related nonsuperconducting oxides include SrTiO3, BaTiO3, and La2CuO4. For these materials, we give the complete quasiisotropic elastic constants corrected to the void-free state. From elastic constants and atomic volume, we calculated Debye characteristic temperatures, Θ D . Using Kresins model, valid for all values of the electron-phonon parameter, λ, we estimated I from Tc and Θ D . For the superconducting cuprates, λ ranges from 1.3 (La-O) to 10.0 (Tl-O). Except perhaps for Tl-O, these parameters fall within a range predicted by theory. For all the above materials, we show the 295–5-K variation of Θ D . We support our elastic-constant measurements with Born-model calculations of the bulk modulus.

Is Y 1 Ba 2 Cu 3 O 7 stiff or soft

Using several measured and calculated physical properties, we argue that the high- T c metal-oxide superconductor Y 1 Ba 2 Cu 3 O 7 is elastically soft compared with BaTiO 3 or SrTiO 3 . We conclude that the bulk modulus equals approximately 107 GPa, despite several high-pressure x-ray diffraction studies that report values up to approximately 200 GPa. Part of the argument uses an ionic-crystal-model calculation of the bulk modulus.

Torsion modulus and internal friction of a fiber‐reinforced composite

By a kilohertz‐frequency resonance method, we determined the torsion modulus and internal friction of a uniaxially fiber‐reinforced composite. The composite was composed of glass fibers in an epoxy‐resin matrix. We studied three fiber contents: 0, 41, and 49 volu2009%. The internal friction failed to fit a classical free‐damped‐oscillator model where one assumes a linear rule‐of‐mixture for three quantities: oscillator mass, force constant, and mechanical‐resistance constant. The torsion modulus approximately fits a plane‐wave‐scattering ensemble‐average model. The microstructure showed strong fiber‐distribution nonhomogeneity. Considering this nonhomogeneity yielded a better agreement between model and observation. Thus, torsion‐modulus measurements provide a method to detect and quantify fiber‐distribution nonhomogeneity.

Madelung potentials and valences in the Y1Ba2Cu3O7 superconductor

Abstract Using Ewalds method, we calculated the ion-site potentials and Madelung energy of Y 1 Ba 2 Cu 3 O 7 (orthorhombic, Pmmm, No. 47). We considered the effects of copper-ion and oxygen-ion valences. Among seven suggested copper-oxygen ion-charge configurations, only two give a low electrostatic bonding energy to agree with oxygen vacancies on O( 1 ) sites. Only one configuration gives a realistic bulk modulus. We perturbed this configuration by equalizing the Cu( 1 )-Cu( 2 ) valences and introducing a hole into the CuO 2 plane at the oxygen sites. This configuration also agrees well with observation.

Thermoelastic coefficient and its pressure derivative: Derivation from a Mie-Grüneisen interatomic potential

Abstract Using a Mie-Gruneisen interatomic potential, we derived a relationship for the thermoelastic coefficient K = -( 1 T )( ∂T ∂σ ) , the temperature change caused by stress, and for the thermoelastic-coefficient pressure derivative, ( 1 K )( ∂K ∂P ) . The latter, related to third-order and fourth-order elastic constants, relates simply and approximately to both the bulk-modulus temperature derivative, ( 1 B )( ∂B ∂T ) , and the bulk-modulus pressure derivative, ( 1 B )( ∂B ∂P ) .

Elastic constants of fiber-reinforced composites: a fresh measurement approach

Composites reinforced with uniaxial fibers possess five independent elastic constants: C/sub 11/, C/sub 33/, C/sub 44/, C/sub 66/, and C/sub 13/. Measurements on a single specimen along principal axes give all the C/sub ij/ except C/sub 13/. To get C/sub 13/, a new approach is proposed that requires measurements only along principal directions: <100>. The approach also avoids the thin-plate problem, where measuring C/sub 22/ and C/sub 33/ may be difficult. A pulse-echo method is used to measure C/sub 11/, C/sub 44/, and C/sub 66/, and a resonance (kilohertz-frequency) method is used to measure two elastic compliances: S/sub 11/ and S/sub 33/. S/sub 44/ and S/sub 12/ come from simple formulas containing C/sub 44/ and C/sub 66/. S/sub 13/ comes from a relatively simple formula containing S/sub 11/, S/sub 12/, S/sub 33/, and C/sub 11/. The approach is applied to a graphite-magnesium composite.<<ETX>>

Estimated dTc⧸dP and dTc⧸dσij for the Y1Ba2Cu3O7 superconductor

Abstract For Y 1 Ba 2 Cu 3 O 7 , we estimated the critical-temperature pressure derivative d T c /d P and the three principal uniaxial compressive-stress derivatives d T c /dσ a , d T c /dσ b , d T c /dσ c . These estimates are useful because the measured values, especially d T c /dσ ij , show such strong disagreement. To estimate these thermodynamic quantities, we extended a model of Ohta and coworkers that relates T c to Δ V A , the difference in Madelung energy at the two oxygen sites, apical and planar. We found that d T c /d P is positive, but small: 0.09 K/GPa. For uniaxial-stress, we found that d T c /dσ c : 0.09 K/GPa. Thus, uniaxial compression along the c -axis (bringing together the CuO 2 planes) produces no remarkable change in T c . We found a negative d T c /dσ b : -0.06 K/GPa. This negative value agrees with results derived by Millis and Rabe, and it agrees with T c increasing with an increasing b unit-cell dimension. Plane compressive stress in the a – c plane produces the largest effect: 0.17 K/GPa, twice that for hydrostatic compression.