Metal atom-induced diffuse scattering from short-range order among deuterium atoms in single crystal α-phase V2D

X-ray scattering has been studied above the critical ordering temperature, T_c (133°C), in b.c.c. α-phase V₂D. At 140 and 200°C quite weak diffuse intensity was observed centered on the reciprocal lattice points of the ordered superlattice. This intensity is due to short-range order among deuterium atoms which induces a local or short-range modulation of the parent vanadium lattice and suggests the possibility of local octahedral D—D fluctuations building up well above T_c.     

Influence of interchain coupling on the one-dimensional magnon raman spectrum of CsCoBr3

Light scattering from magnons in CsCoBr₃ has been measured for temperatures well below the upper antiferromagnetic 3D ordering temperature of T^l_N = 28 K. These experiments reveal multiple magnon features of energies in the range 90 to 170 cm^-1 similar to those found in CsCoCl₃ but previously unobserved for the bromide. Prominent features in the spectrum and their temperature dependence are described in terms of a recent theory by Shiba. Other, weaker features are explained by a simple extension of the theory to include fluctuations. A new band is observed at 178 cm^-1, whose intensity drops sharply prior to the lower 3D ordering transition at T^∥_N = 10 K. This band is assigned to magnon-phonon combination scattering.     

Influence of interchain coupling on the one-dimensional magnon Raman spectrum of CsCoBr3

Light scattering from magnons in CsCoBr₃ has been measured for temperatures well below the upper antiferromagnetic 3D ordering temperature of T^l_N = 28 K. These experiments reveal multiple magnon features of energies in the range 90 to 170 cm^-1 similar to those found in CsCoCl₃ but previously unobserved for the bromide. Prominent features in the spectrum and their temperature dependence are described in terms of a recent theory by Shiba. Other, weaker features are explained by a simple extension of the theory to include fluctuations. A new band is observed at 178 cm^-1, whose intensity drops sharply prior to the lower 3D ordering transition at T^l_N = 10 K. This band is assigned to magnon-phonon combination scattering.     

Electron hopping and optic phonons in Eu3S4

Raman scattering on single crystals of Eu₃S₄ does not show the allowed q=o phonon modes in the cubic phase and exhibits no new modes in the distorted low temperature phase (T<186 K). Above the Curie temperature T_c=3.8 K the scattering is dominated by a spin-disorder induced one-phonon density of states allowing for the observation of the zone boundary phonon breathing mode of the S^2?ions. This mode does not show any anomaly near the charge order -disorder phase transition T_t=186 K. Temperature tunable spin fluctuations associated with the temperature activated Eu²⁺→Eu³⁺ electron hopping are detected in the scattering intensity, superimposed on the usual thermal spin disorder.     

Spinodal ordering in Cu3Au

Results from, X-ray diffuse scattering and inelastic neutron scattering demonstrate the presence of long-lived fluctuations in order up to 5.0°C above the first-order phase transition, T_c = 383.2°C. The temperature for continuous ordering has been determined to be 358.2°C. The coherent phase boundary is 3–13°C below T_c.     

H+ irradiation effect in Co-doped BaFe2As2 single crystals

We report the suppression of the critical temperature T_c in single crystalline Ba(Fe_1?xCo_x)₂As₂ at under-, optimal-, and over-doping levels by 3 MeV proton irradiation. T_c decreases and residual resistivity increases monotonically with increasing the dose. The low-temperature resistivity does not show the upturn in contrast with the α-particle irradiated NdFeAs(O,F), which suggests that proton irradiation introduces nonmagnetic scattering centers. Critical scattering rates for all samples obtained by three different ways are much higher than that expected in s±-pairing scenario based on inter-band scattering due to antiferro-magnetic spin fluctuations.     

Microscopic interactions in CuGeO 3 and organic Spin-Peierls systems deduced from their pretransitional lattice fluctuations

CuGeO₃ exhibits a Spin-Peierls (SP) transition, at T _SP = 14.3 K, which is announced above 19 K by an important regime of one-dimensional (1D) pretransitional lattice fluctuations which can be detected until about 40 K using X-ray diffuse scattering investigations. A quantitative analysis of this scattering shows that in this 1D direction the correlation length follows the “universal” behaviour expected for the thermal fluctuations of a real order parameter which characterizes the lattice dimerization. This allows to define a 1D mean-field temperature, T _SP ^MF , of about 60 K and invalidates any mean field scenario for the SP transition of CuGeO₃. As T _SP ^MF is as high as 4 T _SP we propose that the 3D-SP order is achieved by the interchain coupling between 1D solitons which form below about 16-20 K. CuGeO₃ being in the non-adiabatic regime, it is also suggested that the observed pretransitional fluctuations of CuGeO₃ originate from the X-ray scattering on a very broad damped critical response of lower frequency than the “critical” phonon modes. From the quantitative analysis of the 1D fluctuations we also estimate the microscopic parameters of the SP chain. These parameters allow to locate CuGeO₃ close to the quantum critical boundary separating the gapped SP ground state to the ungapped anti-ferromagnetic ground state. The vicinity of a quantum critical point emphasizes the role of the quantum and non-adiabatic fluctuations and the importance of the interchain coupling in the physics of CuGeO₃. Finally we compare these findings with those obtained for the organic SP systems (BCPTTF)₂PF₆, (TMTTF)₂PF₆ and MEM(TCNQ)₂. From a similar analysis of the pretransitional lattice fluctuations it is found that (BCPTTF)₂PF₆ and (TMTTF)₂PF₆ are located on the SP gapped classical-quantum boundary and are in the adiabatic regime where the fluctuations lead to the formation of a pseudo-gap in the spin degrees of freedom. Differently, we place MEM(TCNQ)₂ inside the SP quantum phase around the crossover line between the adiabatic and non-adiabatic regimes. Received 13 September 2000 and Received in final form 6 February 2001     

Thermal conductivity and specific heat of SrCu2(BO3)2: A quasi-two-dimensional metal oxide compound with a spin gap

The thermal conductivity and specific heat of SrCu₂(BO₃)₂, a quasi-two-dimensional metal oxide compound with a spin gap, were studied at low temperatures. In the temperature interval 0.4<T<3.2 K, the thermal conductivity of a single crystal sample in the ab plane varies according to the power law κ∝T ^2.73. As the temperature increases further, a deep minimum is observed in the region of T _min≈9.8 K. This behavior is explained by the scattering of phonons—the major heat carriers—on the fluctuations of the spin subsystem.     

Notes on number density fluctuations and the scattering cross section for electromagnetic waves scattered from a thermal nonequilibrium plasma

The scattering cross section and the Doppler spectrum for electromagnetic waves scattered by the electron density fluctuations of a plasma, where the mean kinetic temperature of the electronsT _e may differ from that of the ionsT _i , has been obtained among others byFejer, Buneman, Renau, Camnitz andFlood, andSalpeter. These authors use different methods of approach to calculate the autocovariance of the electron number-density fluctuations (from the mean) and then obtain the scattering cross section. Because of the differing results, the methods, concepts, and derivations of the scattering cross section are carefully examined in this paper. It is shown that the short-time dynamical considerations incorporated in the formulation of the statistical theory of the electron number-density fluctuations of the plasma as used by several authors (for instanceFejer, Buneman, Salpeter,) leads to results of limited validity. In addition, a fundamental error in calculating the electron density fluctuations leads these latter authors to an incorrect scattering cross section. The theory of scattering of electromagnetic waves from a plasma, where the electrons arenot in thermal equilibrium with the ions but statistical equilibrium exists, is developed in a general way. The covariance of the number-density fluctuations from the mean of the charged species of the plasma and the scattering cross sectionσ(q) are obtained. In particular it is shown that for a wavelength λ much greater than the effective Debye lengthd, the backscattering cross section increases and approaches complete incoherent scattering asT _e /T _i increases. This result is explained by noting that in the case of thermal equilibrium, the predicted value of the back-scattering cross section is smaller than that of the backscattering cross section from completely uncorrelated electron density fluctuations because the electrostatic interaction between the charged particles of the plasma, which is a function ofT _e andT _i , introduces a certain amount of organization in otherwise completely uncorrelated electron density fluctuations. When the mean temperature of the electrons increases relative to that of the ions, the organization introduced in the fluctuations diminishes because of the increasing thermal agitation of the electrons relative to that of the ions, and the backscattering process approaches that of incoherent backscattering (Thomson-type scattering). The spectrum function of incoherent scattering of electromagnetic waves from a nonequilibrium plasma is obtained and some cases of current interest are plotted.     

Magnetic excitations in triangular lattice NaCrO2

We report the first inelastic neutron scattering measurements on the rhombohedrally stacked triangular antiferromagnet NaCrO₂ which has recently been shown via μSR and NMR measurements to exhibit an unusually broad fluctuating cross-over regime extending far below the onset of spin freezing at T_c. Our results show that strong spin fluctuations persist far below T_c and that dispersive spin wave excitations only appear near the cross-over temperature 0.75T_c.     

Effect of ion irradiation induced defects on the excess conductivity of Cu1−xTlxBa2Ca1Cu2O8−δ superconductor thin films

The Cu_1?xTl_xBa₂Ca₁Cu₂O_8?δ superconductor thin film samples were bombarded with protons, Si and Au ions of energies 6, 20 and 20 MeV respectively using 5MV tandem pelletron accelerator at Experimental Physics Labs. Each un-irradiated sample had different values of normal state resistivity and the zero resistance critical temperature. The zero resistivity critical temperature has been increased after the irradiation by Si and Au ions. The fluctuation induced conductivity (FIC) analysis of the as-prepared and the ion irradiated samples were performed in the light of Aslamasov–Larkin (AL) theory. The FIC analysis has shown three dimensional (3D) fluctuations in the order parameter in all the samples along with a cross-over to two dimensional (2D) fluctuations at higher temperature. The 3D–2D cross-over temperature has been shifted to higher values after the ion irradiation. Moreover, a direct correlation between the zero resistivity critical temperature, 2D–3D cross-over temperature (T_LD) and superconductivity fluctuation temperature (T_scf) was observed. These studies have shown that the fluctuation induced conductivity (excess conductivity) depends on the density of defects and is independent of their nature.     

Critical hyper-Raman scattering in ferroelectrics

It is shown, that the critical hyper Raman scattering (CHRS) due to the polarization fluctuations in ferroelectrics must increase sharply in the vicinity of the phase transition point T_c. The spectral intensity of the CHRS is expressed explicitely in terms of the imaginary part of low-frequency dielectric permeability ε(ω, T) which permits to use the CHRS for investigations of the critical dynamics near T_c, in particular, in centrosymmetric phase, where the ordinary Raman scattering (of the first order) is forbidden. The calculated CHRS intensity in the BaTiO₃ type crystals is found to be larger by 4–5 orders of magnitude than that in the media investigated earlier.     

Observation of critical slowing down of ortho-positron quenching rate near gas-liquid transition

Thermalized ortho-positronium life time spectra have been measured in xenon gas along three different isochores. The quenching rate as a function of T-T₀ shows a striking similarity to the critical slowing down of density fluctuations as measured by the Rayleigh scattering of light.     

Effect of granularity and inhomogeneity in excess conductivity of YBa2Cu3O7−δ+xBaTiO3 superconductor

Synthesis of polycrystalline YBCO+xBaTiO₃ (x=1.0, 2.5, 5.0) superconductor has been done and the effects of granularity and inhomogeneities due to inclusions of nano-BaTiO₃ in excess conductivity are reported in this work. The phase formation, texture and grain alignments were analyzed through XRD and SEM techniques. SEM results reveal that the grain size is reduced and morphology is improved with the incorporation of nano-BaTiO₃ particles. Superconducting order parameter fluctuation (SCOPF) studies on the electrical conductivity were investigated from the resistivity vs. temperature data in the experimental domain relatively above T_c. Log(Δσ) vs. log(ε) plots show that the 2D to 3D crossover temperature (T_LD) that demarcates dimensional nature of fluctuation inside the grains is influenced by BaTiO₃ incorporation in YBCO matrix. An upward shift of T_LD in the mean field region has been observed as a consequent dominance of 3D region with increase in 1 wt% BaTiO₃ in the composites as compared to higher inclusions. It has been analyzed that microscopic inhomogeneities produced as a result of diffusion of a fraction of Ti ions into the grains affect fluctuations in the excess conductive region. The interplay of microscopic inhomogeneities produced inside the grains and mesoscopic inhomogeneities in the grain boundaries on the excess conductivity has been explained in terms of thermal fluctuations for the composites.     

The superconducting transition in coupled linear chain systems

The superconducting transition and the critical fluctuations of a model system that can pass continuously from one-dimension to three-dimensions are investigated. The transition is brought about by a variable coupling between families of linear chains. First, a Bose lattice-gas is considered, and the Bose-Einstein transition temperature T_BE is calculated as a function of the coupling strength. Second, the temperature T_oz at which the fluctuations in the gap parameter equal the average gap parameter is calculated as a function of the coupling, and is found to behave in a similar way to T_BE. Both these temperatures go continuously to zero as the system becomes one-dimensional while T_c calculated in mean-field theory does not vanish in this limit.It is found that for coupling parameters believed to be characteristic of some superconductors possessing the A-15 crystal structure, such as Nb₃Sn, the system is essentially three-dimensional (3D) as far as superconducting properties are concerned; but critical fluctuations may be somewhat enhanced, in particular when the electronic density of states is not very large.     

Dynamic critical scattering near the smectic-A—nematic phase transition in CBOOA

High resolution neutron quasielastic scattering has been applied to a study on the critical dynamics near the smectic-A—nematic phase transition in CBOOA. A line narrowing is observed for a certain value of the momentum transfer. We interpret this narrowing as a critical slowing down of the smectic order parameter fluctuations near the transition temperature T_SN. The relaxation times vary between 10^-8 and 10^-7 sec for a temperature interval of 2 K below T_SN.     

Manifestation of multiband optical properties of MgB2

The optical conductivity of MgB₂ has been determined on a dense polycrystalline sample in the spectral range 6 meV–4.6 eV using a combination of ellipsometric and normal incidence reflectivity measurements. σ₁(ω) features a narrow Drude peak with anomalously small plasma frequency (1.4 eV) and a very broad ‘dome’ structure, which comprises the bulk of the low-energy spectral weight. This fact can be reconciled with the results of band structure calculations by assuming that charge carriers from the 2D σ-bands and the 3D π-bands have principally different impurity scattering rates and negligible interband scattering. This also explains a surprisingly small correlation between the defect concentration and T_c, expected for a two-gap superconductor. The large 3D carrier scattering rate suggests their proximity to the localization limit.     

Theoretical analysis of light scattering from super-fluid helium close to the λ phase transition

We calculate the contribution from order parameter fluctuations to the light scattering cross section in ⁴He near T_λ including effects from order parameter relaxation. The existence of a central dip in the critical region is discussed.     

Electron relaxation mechanisms in n-Bi-Sb semiconducting alloys

The magnetic field dependence of diffusion thermal electromotive force α₂₂(H) (?T ∥ C ₁) in degenerate n-Bi-Sb semiconducting alloys, in which only L electrons participate in transfer phenomena, had a maximum at H ∥ C ₃. The electron relaxation time was determined from the magnetic field value corresponding to this maximum. The dependences of the electron relaxation time on temperature and the concentration of alloy components and the dopant (on the concentration of electrons) were used to separate electron relaxation time components corresponding to scattering by phonons, ionized impurities, and component concentration fluctuations. The latter (“alloy”) mechanism of electron scattering by concentration fluctuations was for the first time considered for Bi-Sb alloys; its contribution was found to be comparable with those of the other scattering mechanisms. The obtained relaxation times were used to calculate theoretical magnetic field dependences of thermal electromotive force and the Nernst-Ettingshausen coefficient. The calculation results were in satisfactory agreement with experiment.     

Study ofT > fluctuations in94,96Zr(P,P) in the energy region from 11.2 to 13.4 MeV

Pronounced fluctuations are exhibited by the measured cross section excitation functions of⁹⁴Zr(p, p) elastic and inelastic scattering at backward angles. Fluctuations in the analyzing power were found to be relatively small, in contrast to theoretical predictions and previous observations of strong analyzing power fluctuations in the presence of cross section fluctuations. The measured fluctuations are analyzed in the framework of the statistical model and the observed structures with a coherence width ofγ=(28±3) keV are attributed to overlappingT ^> states. No significant fluctuations are seen in⁹⁶Zr( \(\vec p\) , p₀).