32 research outputs found
Effect of structural defects on anomalous ultrasound propagation in solids during second-order phase transitions
The effect of structural defects on the critical ultrasound attenuation and
ultrasound velocity dispersion in Ising-like three-dimensional systems is
studied. A field-theoretical description of the dynamic effects of
acoustic-wave propagation in solids during phase transitions is performed with
allowance for both fluctuation and relaxation attenuation mechanisms. The
temperature and frequency dependences of the scaling functions of the
attenuation coefficient and the ultrasound velocity dispersion are calculated
in a two-loop approximation for pure and structurally disordered systems, and
their asymptotic behavior in hydrodynamic and critical regions is separated. As
compared to a pure system, the presence of structural defects in it is shown to
cause a stronger increase in the sound attenuation coefficient and the sound
velocity dispersion even in the hydrodynamic region as the critical temperature
is reached. As compared to pure analogs, structurally disordered systems should
exhibit stronger temperature and frequency dependences of the acoustic
characteristics in the critical region.Comment: 7 RevTeX pages, 4 figure
Calculations of the dynamical critical exponent using the asymptotic series summation method
We consider how the Pad'e-Borel, Pad'e-Borel-Leroy, and conformal mapping
summation methods for asymptotic series can be used to calculate the dynamical
critical exponent for homogeneous and disordered Ising-like systems.Comment: 21 RevTeX pages, 2 figure
Ordering in the dilute weakly-anisotropic antiferromagnet Mn(0.35)Zn(0.65)F2
The highly diluted antiferromagnet Mn(0.35)Zn(0.65)F2 has been investigated
by neutron scattering in zero field. The Bragg peaks observed below the Neel
temperature TN (approximately 10.9 K) indicate stable antiferromagnetic
long-range ordering at low temperature. The critical behavior is governed by
random-exchange Ising model critical exponents (nu approximately 0.69 and gamma
approximately 1.31), as reported for Mn(x)Zn(1-x)F2 with higher x and for the
isostructural compound Fe(x)Zn(1-x)F2. However, in addition to the Bragg peaks,
unusual scattering behavior appears for |q|>0 below a glassy temperature Tg
approximately 7.0 K. The glassy region T<Tg corresponds to that of noticeable
frequency dependence in earlier zero-field ac susceptibility measurements on
this sample. These results indicate that long-range order coexists with
short-range nonequilibrium clusters in this highly diluted magnet.Comment: 7 pages, 5 figure
Effective and Asymptotic Critical Exponents of Weakly Diluted Quenched Ising Model: 3d Approach Versus -Expansion
We present a field-theoretical treatment of the critical behavior of
three-dimensional weakly diluted quenched Ising model. To this end we analyse
in a replica limit n=0 5-loop renormalization group functions of the
-theory with O(n)-symmetric and cubic interactions (H.Kleinert and
V.Schulte-Frohlinde, Phys.Lett. B342, 284 (1995)). The minimal subtraction
scheme allows to develop either the -expansion series or to
proceed in the 3d approach, performing expansions in terms of renormalized
couplings. Doing so, we compare both perturbation approaches and discuss their
convergence and possible Borel summability. To study the crossover effect we
calculate the effective critical exponents providing a local measure for the
degree of singularity of different physical quantities in the critical region.
We report resummed numerical values for the effective and asymptotic critical
exponents. Obtained within the 3d approach results agree pretty well with
recent Monte Carlo simulations. -expansion does not allow
reliable estimates for d=3.Comment: 35 pages, Latex, 9 eps-figures included. The reference list is
refreshed and typos are corrected in the 2nd versio
Unveiling genomic regions that underlie differences between Afec-Assaf sheep and its parental Awassi breed
Neutron-powder-diffraction study of the nuclear and magnetic structures of YBa2Fe3O8 at room temperature
The nuclear and magnetic structures of YBa 2 Fe 3 O 8 have been investigated by powder neutron diffraction at room temperature. The nuclear structure of the compound has the symmetry of space group P4/mmm and lattice parameters a=3.9170(1) and c=11.8252(4) Ă
. The configuration of the atoms in the unit cell is very similar to that of the superconductor YBa 2 Cu 3 O 7 , with the exception that the iron ions corresponding to the Cu-chain ions have octahedral coordination, rather than square planar; the octahedra thus are arranged in layers rather than in chains. This significant difference is a consequence of the fact that all possible oxygen sites in these layers are fully occupied, resulting in an oxygen content of eight rather than seven atoms per formula unit. A second consequence of the full occupancy of the oxygen sites is that the Ba ions have a twelvefold cuboctahedral coordination in the iron compound, rather than tenfold. The magnetic structure is based on a unit cell related to that of the nuclear structure by an axis transformation of matrix (11ÂŻ0/110/002). The magnetic origin of the extra intensities and the basic spin configuration were determined by polarized-neutron-diffraction measurements. The iron moments are coupled antiferromagnetically within each FeO 2 layer, as well as along the c axis. The magnetic moments of all the iron ions are the same, with a value of 3.49(2) ÎŒ B . This configuration results in the magnetic symmetry Icmmâm.
© 1992 American Physical Societ
Neutron-powder-diffraction study of the nuclear and magnetic structures of the substitution compound (Y1âxCax)Ba2Fe3O8+d (x = 0.05, 0.10 and 0.20)
The nuclear and magnetic structures of ( Y 1 â x Ca x ) Ba 2 Fe 3 O 8 + ÎŽ with x=0.05, 0.10, and 0.20 have been investigated by neutron-powder diffraction at room temperature. The compound crystallizes with the symmetry of space group P4/mmm and has a structure similar to that of the superconductor YBa 2 Cu 3 O 7 (1:2:3). There are two types of Fe atoms in the unit cell, one having fivefold pyramidal coordination and the other octahedral coordination. The Ca and Y atoms are randomly distributed over the 1/2,1/2,1/2 position, which are exclusively occupied by Y in the 1:2:3 superconductor. Since the oxygen sites on the basal plane of the structure are fully occupied to achieve octahedral coordination for the iron atoms on this plane, the oxygen content is eight atoms per formula unit, the barium atoms are twelvefold coordinated, and the coordination polyhedron is a cuboctahedron. Extra oxygen atoms corresponding to ÎŽ=0.08 and 0.05 are located in the Y/Ca layer. The presence of these atoms produces disorder in the structure resulting in a shift of the oxygen atoms located near the plane of the pyramidal iron atoms. The charge compensation required by the substitution of Y3+ by Ca 2 + is achieved in these materials by elimination of the extra oxygen, rather than oxidation of the iron atoms. The magnetic structure is based on a unit cell related to that of the nuclear structure by the transformation of axes (1,-1,0/1,1,0/0,0,2).
The magnetic origin of some of the observed diffraction peaks was established by polarized neutron diffraction measurements. As in YBa 2 Fe 3 O 8 , the iron moments are coupled antiferromagnetically within each ( FeO 2 ) layer as well as along the c axis of the structure, and they lie in the planes perpendicular to c. The magnetic moments of the two iron atoms are practically identical and have values at room temperature of ăÎŒă=3.50(3) ÎŒ B , 3.52(2) ÎŒ B , and 3.62(2) ÎŒ B for the three compositions x=0.05, 0.10, and 0.20, respectively.
© 1994 American Physical Societ