A precursor superconductivity approach to magnetic field effects in the pseudogap phase

We demonstrate that the observed dependences of T_c and T^∗ on small magnetic fields can be readily understood in a precursor superconductivity approach to the pseudogap phase. In this approach, the presence of a pseudogap at T_c (but not at T^∗) and the associated suppression of the density of states lead to very different sensitivities to pair-breaking perturbations for the two temperatures. Our semi-quantitative results address the puzzling experimental observation that the coherence length ξ is weakly dependent on hole concentration x throughout most of the phase diagram. We present our results in a form which can be compared with the recent experiments of Shibauchi et al. and argue that orbital effects contribute in an important way to the H dependence of T^∗.     

Amplitudes of thermal vibrations in molecular solids

The mean-square nuclear displacements 〈U²〉 _T have been evaluated for the molecular (rare-gas) solids by a lattice dynamical rigid-atom Model. The model derives the inter-molecular interaction from a “two-piece four parameter” pair-potential, includes the contribution of zero-point vibration through potential parameters by a self-consistent method, and accounts for the cubic and quartic anharmonic potential terms as perturbations to the harmonic Hamiltonian. The effect of many body interactions has been included on the basis of Axilrod-Teller approximation. The effect of including three-body forces as well as anharmonicity is found to decrease the values of 〈U²〉_T at all temperatures for all the solids. However, the ratio of the root mean-square nuclear displacements at melting to the nearest-neighbour distance, i.e. the Lindemann parameter (δ) is approximately the same for all the solids under study, which shows that the Lindemann parameter is structure as well as interaction dependent. The results are consistent with the other conventional calculations.     

Carrier concentration dependence and temperature dependence of mobility in silicon (100) N-channel inversion layers at low temperatures

The carrier concentration (N_s) dependence of electron mobility in Si (100) inversion layers has been measured at temperatures T = 1.5?70K for high- and low-mobility MOSFETs. An extrinsic term is observed in the T-dependent part of the scattering probability, τ^?1_T. At T = 4.4 K, τ^?1_T depends on N_s as N^?1.9_s in low mobility samples. In high-mobility samples, τ^?1_T increases with increasing N_s in high N_s region while τ^?1_T ∝ N^?1.6_s in low N_s region. The N_s-dependence of τ^?1_T becomes weaker with increasing T in both of low- and high-mobility samples. At N_s = 3 × 10¹² cm^?2, τ^?1_T depends on T as T^1.8 in low-mobility samples and τ^?1_T ∝ T^2.0 in high- mobility samples at T 5 K.     

NMR investigations of dynamical processes in orientationally ordered and disordered NaCN,RbCN and CsCN

The nuclear spin lattice relaxation timeT ₁ of the²³Na,⁸⁵Rb,⁸⁷Rb,¹³³Cs,¹⁴N nuclei is measured in NaCN, RbCN and CsCN as a function of temperature below and above the ferroelastic phase transition temperatureT _c. BelowT _c the behaviour ofT ₁ of the alkali nuclei renders possible to determine the flip frequency of the CN molecules and its temperature dependence. AboveT _c from the¹⁴NT ₁ the correlation time τ_c of the rotational motions of the CN molecules and its temperature dependence is determined. An empirical rule is verified demonstrating that atT _c the correlation times take nearly the same values for all cyanides. For the high and low temperature phases one obtains atT _c about τ_c=5·10^?13s and τ_c=5·10^?11s, respectively. The results are discussed with respect to the mechanism of the phase transition.     

Temperature dependence of the diffuse streak system in K2SnCl6

We have investigated the system of diffuse streaks in K₂SnCl₆ single crystals in the temperature interval from 4 to 640 K by the photographic registration of the scattered neutron intensity. The streak intensity decreases from T_{c¹ = 261 K} with increasing as well as with In the region of the phase transition at T_{c¹ = 261 K} and T_{c² = 255 K} the intensity decreases with falling temperature to one third. The high temperature behaviour is explained by a soft A_2g librational mode. Below T_{c² = 255 K} the intensity is proportional to ΔT. The streaks are detectable down to 230 K.     

Nuclear magnetic resonance studies of CePd2In and LaPd2In at low temperatures

We report on In nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) measurements on the new heavy-electron compound CePdZIn over a wide temperature range, from 45 mK up to 30 K. CePd₂In undergoes an antiferromagnetic (AF) phase transition at T _N = 1.23 K involving small localised Ce moments of 0.11 μ_B. In zero field, the spin-lattice relaxation rate T ₁ ^?1 (T) shows remarkable changes in its temperature dependence. Above 3 K, T ₁ ^?1 is constant and 850 sec^?1. Between T _N and 2T _N, (T ₁ T)^?1 = 330 (Ksec)^?1, but rapidly decreases below T _N. A Korringatype relaxation, characteristic for simple metals at low temperatures, with (T ₂ T)^?1 = 17(Ksec)¹ is resumed below 0.6 K. This value is an order of magnitude larger than (T ₁ T)^?1 for LaPd₂In and therefore is associated with low-energy excitations of the itinerant charge carriers with 4f symmetry. The T ₁ ^?1 data at various non-zero magnetic fields fall on a single curve when plotted as a function of (T/H) if H exceeds 3.5 T. Thus the AF ordering, the 4f moment fluctuations and the Kondo screening are drastically suppressed by the application of fields H of the order of 3.5 T.     

Lattice thermal conductivity of compounds with inhomogeneous intermediate rare-earth ion valence

The thermal conductivity κ (within the range 4–300 K) and electrical conductivity σ (from 80 to 300 K) of polycrystalline Sm₃S₄ with the lattice parameter a=8.505 Å (with a slight off-stoichiometry toward Sm₂S₃) are measured. For T>95 K, charge transfer is shown to occur, as in stoichiometric Sm₃S₄ samples, by the hopping mechanism (σ ～ exp(?ΔE/kT) with ΔE ～ 0.13 eV). At low temperatures [up to the maximum in the lattice thermal conductivity κ_ph(T)], κ_ph ～ T ^2.6; in the range 20–50 K, κ_ph ～ T ^?1.2; and for T>95 K, where the hopping charge-transfer mechanism sets in, κ_ph ～ T ^?0.3 and a noticeable residual thermal resistivity is observed. It is concluded that in compounds with inhomogeneous intermediate rare-earthion valence, to which Sm₃S₄ belongs, electron hopping from Sm²⁺ (ion with a larger radius) to Sm³⁺ (ion with a smaller radius) and back generates local stresses in the crystal lattice which bring about a change in the thermal conductivity scaling of κ_ph from T ^?1.2 to T ^?0.3 and the formation of an appreciable residual thermal resistivity.     

Hyperfine interactions of 57Fe impurity nuclei in TmNiO3 and YbNiO3 nickelates in the range of magnetic and structure phase transitions

Hyperfine interactions of ⁵⁷Fe impurity nuclei are studied by probe Mössbauer spectroscopy in TmNiO₃ and YbNiO₃ perovskite-like nickelates in the ranges of temperature transitions of an insulator (T < T _IM ) ? metal (T > T _IM ) and antiferromagnetic (T < T _N ) ? paramagnetic (T > T _N ). The changing behavior of hyperfine interaction parameters of ⁵⁷Fe nuclei in the ranges of phase transition temperatures (T _IM and T _N ) is analyzed. The results are interpreted in the context of the charge disproportionation of Ni³⁺ cations associated with the electronic localization in monoclinic-distorted nickelates at T < T _IM .     

Spin gap, phase separation and d-wave pairing as revealed by electron spin-lattice relaxation in 123 and 124 YBaCuO systems doped with Gd3+

With an original modulation technique, the Gd³⁺ electron spin-lattice relaxation has been investigated in normal and superconducting states of YBa₂Cu₃O_6+x (123) and YBa₂Cu₄O₈ (124) compounds doped with 1% Gd. In the 123 sample withx = 0.9T _c = 90 K), theT ₁ behavior within 50 <T< 200 K reveals the [1 ? tanh²(Δ/2kT)]/T dependence typical of a spin gap opening with Δ ≈ 240 K. Below 50 K, the exponential slowing down ofT ₁ is limited by the Korringa-like behaviorT ₁ T = const); the same Korringa-like law is found in the 123 sample withx = 0.59 (T _c = 56 K) within the total 4.2–200 K temperature range. This is interpreted in terms of microscopic separation of the normal and superconducting phases allowing for the electron spin cross-relaxation between them. In the 124 sample (T _c = 82 K), the Gd³⁺ relaxation rate below 60 K is found to obey a power lawT _n with an exponentn ≈ 3. Such a behavior (previously reported for nuclear spin relaxation) is indicative of the d-wave superconducting pairing. Additional paramagnetic centers characterized by relatively slow spin-lattice relaxation are found in both 123 and 124 systems. A well-pronounced change in theT ₁ temperature dependence atTT* ≈ 180–200 K is observed for these slowly relaxing centers as well as for the conventional, fast-relaxing Gd³⁺ ions, suggesting microscopic phase separation and a change in the relaxation mechanism due to electronic crossover related with the opening of the spin gap. This hypothesis is supported by some “180 K anomalies” previously reported by other authors.     

High-pT pion production in ππ interactions

Charged pion production in high energy π^-π^- interactions is studied in the p_T region of 1–2 GeV/c. The characteristics of pion production in π^-π^- interactions are compared with those in π^-p and pp interactions. The p_T dependence of pion production in these reactions follows a systematic trend which indicates that high-p_T pion production for ππ interactions as well as for πp and pp interactions proceeds via quark-quark scattering as predicted by QCD.     

The spectrum of hot hadronic matter and finite-temperature QCD sum rules

The sum rules method, which is widely used for the investigation of the resonance physics within the QCD framework, is generalized to the case of finite temperatures and densities. Conditions are formulated under which this method is quite efficient for the determination of the spectrum of hadronic matter at T ≠ 0. The finite-temperature QCD sum rules are analysed in the vector channel J^PC = 1⁻⁻. Sharp and qualitative changes in the spectrum are found in the temperature interval T = 130–150 MeV. It is naturally explained as a consequence of the disappearance of confinement at the temperature T_c = 140±10 MeV. The finite-temperature QCD sum rules also show that the restoration of chiral symmetry at some temperature T_F cannot precede deconfinement. In the case T_F⪢T_c the sum rules indicate that the intermediate phase at T_c<T<T_F is dominated by quasi-free quarks with nonzero dynamical mass m_q^T ≈ 300 MeV.     

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.     

Phase transition and relaxation processes by 87Rb and 39K nuclear magnetic measurements of RbKSO4 single crystals

⁸⁷Rb and ³⁹K nuclear magnetic resonance (NMR) spectra of RbKSO₄ single crystals were measured at room temperature. ⁸⁷Rb central line has the angular dependences of second-order quadrupolar shifts. From these results, the quadrupole coupling constant and the asymmetry parameter were determined at room temperature. In addition, the spin–lattice relaxation rate, 1/T₁, and the spin–spin relaxation rate, 1/T₂, were measured as a function of temperature. The values of 1/T₁ for the ⁸⁷Rb and ³⁹K nuclei were found to increase with increasing temperature, and 1/T₁ was determined to be proportional to Tⁿ. Therefore, for the ⁸⁷Rb and ³⁹K nuclei, Raman processes with n=2 are more significantly in nuclear quadrupole relaxation than direct processes.     

NMR relaxation rate in metallic carbon nanotubes

NMR relaxation rate, T₁⁻¹, of the metallic carbon nanotube is discussed based on Tomonaga–Luttinger-liquid theory. It is found that the Coulomb interaction leads to increase of (T₁T)⁻¹ by a power law with decreasing temperature, T. The dependence on temperature of (T₁T)⁻¹ in the multi-wall nanotube (MWNT) is shown to be strongly suppressed by existence of the metallic shells in the MWNTs.     

Electrical resistivity of iron-vanadium alloys between 78 and 1200 K

Electrical resistivity (?) of FeV alloys containing 0.5, 0.9, 2.7, and 6.1 at% V has been measured as a function of temperature (T) between 78 and 1200 K. The ? vs. T curves exhibit a change in the slope at the ferromagnetic Currie temperature (T_c). The d?/dT vs. T curves in the neibhorhood of T_c are similar to the corresponding plot for pure Fe. Our studies confirm the previously observed anomalous effect of V on T_c of Fe, i.e., that T_c increase with small additions of V to Fe. The critical index λ⁺ associated with the power law of d?/dT just above T_c has been determined as a function of V concentration.     

Electron spin-lattice relaxation of Yb3+ and Gd3+ ions in glasses

The electron spin-lattice relaxation rate (T ₁ ^?1) was measured in two glass samples: (i) a phosphate glass doped with 1 wt% Yb₂O₃ and (ii) a Li₂Si₄O₉ glass sample doped with 0.2 wt% Gd₂O₃. In the Yb³⁺-doped glass sample,T ₁ was measured by an electron-spin-echo technique from 4.2 to 6 K, by the modulation method from 10 to 26 K and by the EPR linewidth from 30 to 100 K. It was found thatT ₁ ^?1 αT ⁿ withn=9 in the range 4.2–6 K.n decreased gradually as the temperature was increased and tended towards 2 above 40 K. Over the entire temperature range 4.2–100 K,T ₁ ^?1 was fitted toAT+BT ⁹ J ₈ (Θ _D/T) (whereA andB are two temperature-independent constants,J ₈ is the well-known Van Vleck integral andΘ _D is the Debye temperature). The value ofΘ _D (=46.3±0.9 K) so determined is in good agreement with that of Stevens and Stapleton from theirT ₁ measurements in the range 1.5 to 7 K. In the Gd³⁺-doped glass, it was observed thatT ₁ ^?1 αT over the range 50–150 K. The data for Ye³⁺-doped glass sample were accounted for by assuming that the phonon modulation of the ligand field is the dominant mechanism, associated with a low Debye temperature in accordance with the published data obtained by using other techniques to study lattice dynamics. On the other hand, the data on the Gd³⁺-doped glass sample were explained to be predominantly due to a mechanism involving Two-Level-Systems (TLS)     

Magnetostriction of the NdCo5 uniaxial permanent magnet

Magnetostriction measurements, between ≈5 and 300 K, of powder magnetically aligned samples of the hexagonal compound NdCo₅, in strong pulsed magnetic fields up to 15 T are reported. The measurement have been performed parallel, perpendicular and at 45° to the alignment c-axis, for the field also applied in those directions. This set of measurements allows us to determine the six irreducible magnetoelastic modes: λ^α₁₁, λ^α₂₁, related to the strain dependence of the isotropic exchange, and λ^α₁₂, λ^α₂₂, λ^γ, λ^ϵ, related to the strain dependence of the crystalline field anisotropy energy. Anomalies on the strains, associated with the spin reorientation (SR) regime are observed, in particular at the temperatures of beginning, T_SR1, and end, T_SR2, of the SR. The thermal variation of the strains is explained by the standard magnetostriction model, including an ingredients: exchange striction (varying as m²_Nd), single-ion anisotropy (varying as m³_Nd), (where m_Nd is the reduc ed Nd sublattice magnetization), as well as an angular dependence of the form {sin²θ(H, T)−sin²θ(0, T)}, where θ(H, T) and θ(0, T), respectively, are the SR angles under field and spontaneous. The irreducible magnetoelastic coupling constants at 0 K have been estimated, although the point charge model is far from agreement with those results.     

Structure-mediated transition in the behavior of elastic and inelastic properties of beach tree bio-carbon

Microstructural characteristics and amplitude dependences of the Young modulus E and of internal friction (logarithmic decrement δ) of bio-carbon matrices prepared from beech tree wood at different carbonization temperatures T _carb ranging from 600 to 1600°C have been studied. The dependences E(T _carb) and δ(T _carb) thus obtained revealed two linear regions of increase of the Young modulus and of decrease of the decrement with increasing carbonization temperature, namely, ΔE ～ AΔT _carb and Δδ ～ BΔT _carb, with A ≈ 13.4 MPa/K and B ≈ ?2.2 × 10^?6 K^?1 for T _carb < 1000°C and A ≈ 2.5 MPa/K and B ≈ ?3.0 × 10^?7 K^?1 for T _carb > 1000°C. The transition observed in the behavior of E(T _carb) and δ(T _carb) at T _carb = 900–1000°C can be assigned to a change of sample microstructure, more specifically, a change in the ratio of the fractions of the amorphous matrix and of the nanocrystalline phase. For T _carb < 1000°C, the elastic properties are governed primarily by the amorphous matrix, whereas for T _carb > 1000°C the nanocrystalline phase plays the dominant part. The structurally induced transition in the behavior of the elastic and microplastic characteristics at a temperature close to 1000°C correlates with the variation of the physical properties, such as electrical conductivity, thermal conductivity, and thermopower, reported in the literature.     

Nuclear spin relaxation in itinerant electron antiferromagnetic Cr1−xVx system

Nuclear spin relaxation rate T^?1₁ for ⁵¹V in an incommensurate antiferromagnetic Cr_1?xV_x system has been measured in a temperature range between 1.3 and 4.2 K and in a range of magnetic field from 0 to 13.3 kOe by using a field-cycling nuclear magnetic resonance technique. In the (T₁T)^?1 vs x curve a pronounced maximum was observed near the critical concentration (x_c～0.040). Furthermore for alloys with x = 0.038 and 0.040 a deviation from the Korringa relation, T₁T = constant, was observed. The experimental results of (T₁T)^?1 are interpreted in terms of the spin-fluctuation and d-orbital contributions.