Sound velocity in single crystals of synthetic opals

The [111] longitudinal sound velocity (v _L) in a single-crystal synthetic opal has been measured at a frequency of 10 MHz in the temperature range 4.2–300 K. At 300 K, v _L=2.1×10⁵ cm/s. The quantity dv _L/v _{300 K}(T) (where v _T,K?v_{300 K}) in the ranges 4.2–200 and 200–300 K behaves in the way typical of amorphous and crystalline solids, respectively.     

Spin correlations in YBa2(Cu1−x Fx)3O7+y ceramic

To detect scattering by magnetic correlations and to estimate their characteristic space scale, YBa₂(Cu_1?x Fe_x)₃O_7+y ceramic with x=0.13 and y=0.4 is investigated by the small-angle scattering of polarized neutrons. The measurements are carried out in the range of temperatures 15 K?T?315 K and magnetic fields 0<H?4500 Oe. Anomalies in the temperature curves of the intensity I(T,q) (where q is the momentum transfer) and the polarization P(T,q) are observed in the temperature range T<40 K. Interference between nuclear and magnetic scattering is also observed in this temperature range. The observed phenomena are interpreted as scattering by magnetic correlations having a scale 70 Å<R<370 Å. Irreversible effects and the type of magnetic ordering are discussed.     

EPR and optical absorption studies of Fe ions in sodium borophosphate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectral investigations have been carried out on Fe³⁺ ions doped sodium borophosphate glasses (NaH₂PO₄-B₂O₃-Fe₂O₃). The EPR spectra exhibit resonance signals with effective g values at g=2.02, g=4.2 and g=6.4. The resonance signal at g=4.2 is due to isolated Fe³⁺ ions in site with rhombic symmetry whereas the g=2.02 resonance is due to Fe³⁺ ions coupled by exchange interaction in a distorted octahedral environment. The EPR spectra at different temperatures (123-295 K) have also been studied. The intensity of the resonance signals decreases with increase in temperature whereas linewidth is found to be independent of temperature. The paramagnetic susceptibility (χ) was calculated from the EPR data at various temperatures and the Curie constant (C) and paramagnetic Curie temperature (θ_p) have been evaluated from the 1/χ versus T graph. The optical absorption spectrum exhibits bands characteristic of Fe³⁺ ions in octahedral symmetry. The crystal field parameter (Dq) and the Racah interelectronic repulsion parameters (B and C) have also been evaluated and discussed.     

Electron-phonon scattering in potassium at high magnetic fields

We have measured the temperature dependence of both the zero-field resistivity and the transverse magnetoresistance of polycrystalline potassium wires (?(300 K)/?(4.2 K)=140 to 6000) in fieldsH?35 kG and at temperaturesT?4.2 K. Our principal findings are: 1) The presence of a large magnetic fieldH=35 kG does not alter the temperature dependence of ? from that observed atH=0; below 4.2 K theT-dependent part of the resistivities,_?T (H=0) and_?T (H=35 kG), fit well to the function exp (?Θ*/T) with the same Θ*=23K. 2) Deviations from Matthiessen's rule are significantly reduced in a strong field so that the magnitude of_?T (H=0) approaches that of_?T (H=35 kG) as sample purity decreases. 3) The slope of the high-field linear magnetoresistance increases slightly (?8%) from 1.5 K to 4.2 K. We attribute the exponential temperature dependence of_?T (H) to the freezing out of electron-phonon umklapp processes as has been shown for the zero-field resistivity. The reduction in deviations from Matthiessen's rule at high fields can be understood within semiclassical theory, but the latter cannot explain the failure of_?T (H) to saturate at high fields. A proposal by Young that electron-phonon umklapp scattering may contribute aT-dependent high-field linear magnetoresistance in potassium is considered.     

Ultrasonic studies of the high Tc superconductor (La1−xSrx)2CuO4−δ

We synthesized (La_1−xSr_x)₂CuO_4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature T_c ≲ 38 K and narrow transition width ΔT_c. For the sample with the maximum T_c among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔV_s(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏω_op/k_B ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high T_c of this material.     

Heavy fermion behaviour of the nonmagnetic CeCu4Al compound

We present measurements of the temperature dependence of the electrical resistivity, the thermopower and the specific heat of the hexagonal compound CeCu₄Al. At high temperatures, the electrical resistivity is characterized by a nearly temperature independent behaviour, followed by a continuous increase below 100K. No maximum has been found down to 1.7 K. The thermopower shows a positive maximum at about 30 K. As in CeCu₆ no negative values are observable in the range from 4.2 K up to a room temperature. The specific heat data between 7 and 15 K reveal a γ value around 280 mJ mol^-1 K^-2. Below this temperature range the specific heat c_p/T shows a rapid rise and crosses the value of 1 J mol^-1 K^-2 at about 1.45 K.     

Note on the magnetic properties of amorphous rare-earth nickel alloys

The magnetic properties of several amorphous rare-earth nickel alloys close to 30 at% Ni have been studied in the temperature range 4.2–300 K. The Curie temperatures range from 2 K in Pr₆₉Ni₃₁. Alloys in which the rare-earth component has an orbital moment show large intrinsic coercive forces below T_c. These coercive forces vary considerably with temperature, the temperature dependence of the alloys Dy₆₉Ni₃₁ and Tb₆₉Ni₃₁ is satisfactorily described by means of an exponential law of the form H_c(_T) = H_c(0) exp(-αT).     

Thermal conductivity of the amorphous and nanocrystalline phases of the beech wood biocarbon nanocomposite

Natural composites (biocarbons) obtained by carbonization of beech wood at different carbonization temperatures T _carb in the range of 800–2400°C have been studied using X-ray diffraction. The composites consist of an amorphous matrix and nanocrystallites of graphite and graphene. The volume fractions of the amorphous and nanocrystalline phases as functions of T _carb have been determined. Temperature dependences of the phonon thermal conductivity κ(T) of the biocarbons with different temperatures T _carb (1000 and 2400°C) have been analyzed in the range of 5–300 K. It has been shown that the behavior of κ(T) of the biocarbon with T _carb = 1000°C is controlled by the amorphous phase in the range of 5–50 K and by the nanocrystalline phase in the range of 100–300 K. The character of κ(T) of the biocarbon with T _carb = 2400°C is determined by the heat transfer (scattering) in the nanocrystalline phase over the entire temperature range of 5–300 K.     

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.     

Lokale Messung von Driftgeschwindigkeit und Temperatur der Ionen einer Argon-Niederdruckentladung mit resonanter Laserstreuung

The radial ion drift velocity and temperature in an argon low-pressure discharge have been measured by a resonant laser scattering method with high spatial resolution. The detection limit for the drift velocities was 2 · 10⁴ cm · s^?1. In the vicinity of the discharge axis the drift velocity grows linear with the distance from the axis. The measurements have shown the existence of a radial dependence of the ion temperature T¹(r). Comparisons of axial ion temperature T_? with T_‖(r = 0) are an experimental demonstration of the anisotropy of the ion velocity distribution in this kind of discharges.     

Pre-resonant Raman scattering in antracene crystals

The pre-resonant Raman scattering (PRRS) of anthracene crystals (T = 4.2?30 K) has been studied. A tunable dye laser (v_exc = 25000?25100 cm^-1 was used to produce an excitation close to the lowest exciton band bottom (v₀ = 25097 cm^-1). A sharp increase of the PRRS intensity when approaching the excitation light frequency v_exc to v₀ and then the not less sharp drop of the PRRS intensity close to resonance (v₀ ? v_exc ≈ 35?15 cm^-1) where observed. An extremely high PRRS sensitivity to temperature and the excitation light intensity was found. The results are explained within the framework of stimulated PRRS in anthracene crystals.     

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.     

Manifestation of an intermediate phase in a ferroelastic K3Na(CrO4)2:MnO 4 2− in raman and IR absorption spectra

In a broad temperature range of 4–300 K, we have performed a complex combined investigation of phase transitions in crystals of a ferroelastic K₃Na(CrO₄)₂:MnO ₄ ^2? using methods of Raman light scattering and IR light absorption. Considerable changes that we have observed in both Raman and IR spectra in the range of T ≈ 150 K testify to the occurrence of another phase transition that has not been observed before at this temperature. We have performed a group-theoretical analysis and compared its results with experimental spectra, which has allowed us to conclude that there are two phase transitions in this crystal, \(P\bar 3m1 \to C2/m \to C2/c\) , which occur at temperatures T _c1 ≈ 230 K and T _c2 ≈ 150 K, respectively.     

Impurity spin dynamics in the classical Kondo system Cu: Fe. NMR relaxation of impurity neighbour nuclei

The correlation time τ_i of the impurity electronic spin in the Kondo system Cu:Fe has been measured over the temperature range 4.2 K?T?300 K (T_K=27.6 K) by means of NMR relaxation of impurity neighbour nuclei. τ_i being in the sub-picosecond region varies with temperatures as predicted by the model calculation of Götze and Schlottmann.     

Doping-induced temperature evolution of a helicoidal spin structure in the MnGe compound

The helicoidal magnetic structure of a MnGe compound doped with 25% Fe is studied by means of small-angle neutron scattering in a wide temperature range of 10–300 K. Analysis of the scattering-function profile demonstrates that magnetic structures inherent to both pure MnGe and its doped compounds are unstable. The doping of manganese monogermanide is revealed to lead to higher destabilization of the magnetic system. In passing from MnGe to Mn_0.75Fe_0.25Ge, the magnetic-ordering temperature T _N decreases from 130 to 95 K, respectively. It is demonstrated that, at temperatures close to 0 K, the intensity of the contribution to scattering from stable spin helices decreases and the intensity of scattering by spin helix fluctuations increases with increasing impurity-metal concentration. An increased intensity of anomalous scattering caused by spin excitations existing in the system is observed. Helicoidal fluctuations and spin excitations corresponding to low temperatures indicate the quantum nature of the instability in the doped compound. However, MnGe doping with Fe atoms has no influence on the compound’s magnetic properties at temperatures of higher than T _N. The temperature range of short-range ferromagnetic correlations is independent of concentrations and is restricted by temperatures T ranging from 175 to 300 K.     

Thermoelectric power of potassium doped lanthanum manganites at low temperatures

The temperature-dependent resistivity and thermoelectric power of monovalent (K) doped La_1−xK_xMnO₃ polycrystalline pellets (x=0.05, 0.10 and 0.15) between 50 and 300 K are reported. K substitution enhances the conductivity of this system. Curie temperature (T_C) also increases from 260 to 309 K with increasing K content. In the paramagnetic region (T>T_C), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region (T<T_C), the resistivity data show a nearly perfect fit for all the samples to an expression containing, the residual resistivity, spin-wave and two-magnon scattering and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Small polaron hopping mechanism is found to fit well to the thermoelectric power (S) data for T>T_C whereas at low temperatures (T<T_C) in ferromagnetic region (S_FM), S_FM is well explained with the spin-wave fluctuation and electron–magnon scattering. Both, resistivity and thermopower data over the entire temperature range (50–300 K) are also examined in light of a two-phase model based on an effective medium approximation.     

Structural, magnetic and magnetotransport behavior of some Nd-based perovskite manganites

A systematic investigation of Neodymium based colossal magnetoresistive manganites with general formula Nd_0.67A_0.33MnO₃, (A=Ca, Sr, Pb and Ba) has been undertaken mainly to understand their structural, magnetic as well as electrical behavior. The materials were prepared by the sol-gel route sintering at 900 °C. After usual characterization of the materials structurally by XRD, their metal-insulator transition (T_P) as well as magnetic transition (T_C) temperatures were determined and the reasons for the occurrence of ΔT_T(T_C-T_P) values have been explained. X-ray data have been analyzed by using Rietveld analysis and the variations of various parameters are explained. It has been concluded that not only A-site cation radius, 〈r_A〉 but also the size variance factor (σ²) influence both the metal-insulator as well as ferro to para magnetic transition temperatures. A systematic study of electrical conductivity of all the four materials was undertaken as a function of magnetic field upto 7 T mainly to understand the conduction mechanism in the presence of magnetic field. On analyzing the electrical resistivity data, it has been concluded that the metallic (ferromagnetic) part of the resistivity (ρ) (below T_P) can be explained by electron-electron scattering processes (∼T²) and two magnon scattering processes (∼T^4.5), while in the high temperature (T>T_P) paramagnetic insulating regime, the adiabatic small polaron and variable range hopping models are found to fit well.     

μSR investigation of the magnetic ordering in EuAs3

We have investigated the magnetic ordering and the incommensurate-commensurate phase transition in EuAs₃ by zero-field (ZF) and longitudinal-field μSR. In the commensurate phase, stable at temperatures below T_L=10.3 K, the ZF muon signal exhibits oscillations corresponding to four muon precession frequencies the lowest of which behaves anomalously. The muon signal shows no oscillation but exponential decay in the incommensurate phase stable in temperature range from T_L≈10.3 K up to T_N≈11 K. The temperature dependence of the fitted relaxation rate shows divergence-like behaviour at the ordering temperature T_N≈11 K and also at the lock-in transition T_L≈10.3 K. The results are in qualitative agreement with those previously obtained by neutron and X-ray magnetic scattering investigations except for the anomalous temperature dependence of the lowest frequency in the commensurate phase. We propose a model for this anomalous behaviour.     

Polymorphism of anhydrous ferrous sulfate

The Mössbauer spectra of α-FeSO₄ and β-FeSO₄ have been measured in the temperature range 4.2 K ? T ? 30 K. The high temperature (β) phase is accompanied by a weak fraction of a third component. In analogy to CoSO₄, the third component of the three-fold polymorphism in anhydrous ferrous-sulfate is named the γ phase. X-ray powder measurements confirm this phase to be isomorphous to the corresponding γ-CoSO₄ structure. The parameters of the hyperfine interaction at 4.2 K of the three phases are evaluated. The magnetic ordering temperatures are T_N (α) = 24.0 K, T_N(β) = 17.0 ± 1.5 K and T_N(γ) = 19.5 ± 1.0 K.