Determination of the Néel temperature from measurements of the thermal conductivity of the Co3O4 antiferromagnet nanostructured in porous glass channels

The Néel temperature T _N(n) of the Co₃O₄ antiferromagnet nanostructured in channels of porous borosilicate glass with channel cross sections of ??7 nm has been determined from thermal conductivity measurements. It has been shown that the Néel temperature T _N(n) of this nanomaterial is approximately equal to 20 K, which is considerably lower than T _N = (30?C40) K for the bulk Co₃O₄ sample.     

Comparative study of the specific heat of superconducting and non-superconducting YBa2Cu3O7−x

We have measured the specific heat, the electrical resistance and the magnetic susceptibility of YBa₂Cu₃O_7–x in a superconducting (sc) and in a non-superconducting (nsc) version. The latter was obtained by inducing a small loss of oxygen. In the sc sample we find a jump of the specific heat with C=3.6 J mole^–1 K^–1. Below the maximum nearT _c the specific heat of the sc version drops too fast for an electronic effect alone: there is a cross-over from excess to deficiency already at 0.9T _c . The specific heat of the nsc version shows a change of slope in the temperature range between 70 and 80 K, which indicates the existence of a second specific heat anomaly, which apparently exists independently of that due to the onset of superconductivity and explains at least partially the premature crossover.Alexander von Humboldt fellow. On leave in absence from Centro Atomico Bariloche—CNEA Argentina     

Thermodynamics of the 3D Hubbard model on approaching the Néel transition

We study the thermodynamic properties of the 3D Hubbard model for temperatures down to the Néel temperature by using cluster dynamical mean-field theory. In particular, we calculate the energy, entropy, density, double occupancy, and nearest-neighbor spin correlations as a function of chemical potential, temperature, and repulsion strength. To make contact with cold-gas experiments, we also compute properties of the system subject to an external trap in the local density approximation. We find that an entropy per particle S/N ≈ 0.65(6) at U/t = 8 is sufficient to achieve a Néel state in the center of the trap, substantially higher than the entropy required in a homogeneous system. Precursors to antiferromagnetism can clearly be observed in nearest-neighbor spin correlators.     

Thermal conductivity and thermal diffusivity of samarium in the temperature range of 293–1773 K

The thermal conductivity and the thermal diffusivity coefficients of samarium have been measured by the laser flash method in the temperature interval of 293–1773 K in solid and liquid states including the regions of phase transitions. The measurement errors of the heat transfer coefficients were ±(3–6)%. The approximation equations and the tables of reference data for the temperature dependence of properties have been obtained. The obtained results have been compared with the available literature data.     

Study of electrical conductivity and phase transition in Bi2O3–V2O5 system

The solid solutions of bismuth–vanadate were prepared by the conventional solid-state reaction. The sample characterization and the study of phase transition were done by using FT-IR, X-ray diffraction (XRD) and DSC measurements. AC impedance measurements proved that the oxide ion conductivity predominantly arises from the grain and grain boundary contributions as two well-defined semicircles are clearly seen along with an inclined spike. The electrical conductivity of Bi₂O₃–V₂O₅ has been studied at different temperatures for various molar ratios. The isothermal conductivity increases with an increase in the concentration of V₂O₅ due to the vacancy migration phenomenon. It has been found that the conductivity of different compositions of Bi₂O₃–V₂O₅ increases and shows a jump in the temperature range 230–260°C due to the phase transition of BiVO₄ from monoclinic scheelite type to that of tetragonal scheelite type. The endothermic peak in DSC at around 260°C reveals the phase transition, which is also confirmed by the XRD and FT-IR analysis. The XRD patterns confirmed the monoclinic structure of BiVO₄.     

Lattice model for the SU(N) Néel to valence-bond solid quantum phase transition at large N

We generalize the SU(N=2) S=1/2 square-lattice quantum magnet with nearest-neighbor antiferromagnetic coupling (J(1)) and next-nearest-neighbor ferromagnetic coupling (J(2)) to arbitrary N. For all N>4, the ground state has valence-bond-solid order for J(2)=0 and Néel order for J(2)/J(1)?1, allowing us access to the transition between these types of states for large N. Using quantum Monte Carlo simulations, we show that both order parameters vanish at a single quantum-critical point, whose universal exponents for large enough N (here up to N=12) approach the values obtained in a 1/N expansion of the noncompact CP(N-1) field theory. These results lend strong support to the deconfined quantum-criticality theory of the Néel-valence-bond-solid transition.     

Novel mechanism of anomalous electron heat conductivity and thermal crashes during Alfvénic activity in the Wendelstein 7-AS stellarator

Enhanced plasma heat conductivity in the presence of kinetic Alfvén waves (KAW) is predicted theoretically. The enhancement is shown to be strongest when the electron collision frequency exceeds the particle transit frequency in the wave field. Alfvén waves (both KAW and ideal MHD Alfvén eigenmodes generating the KAW) are studied in a shot of the Wendelstein 7-AS stellarator. On the basis of these results, strong thermal crashes observed during bursting Alfvénic activity in the mentioned shot are explained.     

Specific heat anomaly of single phase YBa2Cu3O7-δ at superconducting transition temperature

The specific heat of single phase YBa₂Cu₃O_7-δ has been measured using non-adiabatic method between 4.2K and 120K. There is a specific heat anomaly Δc at 90K (about 3.2% of total specific heat) approximately, due to superconducting transition. From the measured value of ΔC and transition temperature T_c, the electronic density of state at Fermi level N(E_F) and Sommerfeld parameter γ calculated are 2.55±0.30states/eV.Cu-atom and 2.77±0.30 mJ/mole.K², respectively. The experimental result of N(E_F) is consistent with that of the band calculation by Mattheiss. The Debye temperature above T_c in this material deduced from Debye function is about 340K. Below 20K, the relation C=γ'T+βT³ is satisfied. But the value of γ' is smaller. That means, most of the electrons have formed superconducting Cooper pairs which give no contribution to specific heat below 20K.     

A quadratic configuration interaction study of N2O and N2O·−

The geometries and vibrational frequencies of both N₂O and N₂O^·– were calculated at the QCISD and QCISD(T) levels of theory using aug-cc-pVDZ and aug-cc-pVTZ basis sets. The electron affinity of N₂O was determined to be ?0.15 eV. This work corroborates an earlier G2 study and suggests that the currently accepted value for the electron affinity, 0.22eV, is in error. This study represents the best calculation to date for the geometry and vibrational frequencies of N₂O^·?     

Electrical conductivity,diffusion of iron and the defect structure of α-Al2O3: Fe

A detailed defect model is developed for α-Al₂O₃:Fe accounting for conduction and diffusion at high temperatures as a function of iron concentration, oxygen pressure and temperature. This model involves single native and iron defects as well as triplets of (Fe_AlAl_iFe_Al), the latter dominating the defect structure at high Fe concentrations. Diffusion of Fe in A1₂O₃ is attributed to Fe_i^3.. The position of the Fe_Al' level is estimated.     

Measurements of the line strengths,N2− and O2-broadened half-widths in the ν3, ν1+2ν2 and 2ν1 bands of 14N2 16O at room temperature

The line strengths, N₂? and O₂-broadened half-widths in the ν₃, ν₁+2ν₂ and 2ν₁ bands of ¹⁴N₂ ¹⁶O were determined from spectra obtained by a high-resolution Fourier transform spectrometer at room temperature. The squared vibrational transition dipole moments and the coefficients of the Herman–Wallis factor were also determined for these bands. The squared vibrational transition dipole moments for these bands agreed with the values of HITRAN and high-resolution experiments within 6%. The N₂? and O₂-broadened half-widths were in agreement with the results of recent high-resolution experiments. The air-broadened half-widths were calculated using the smoothed N₂? and O₂-broadened half-widths and compared with the compiled values in the HITRAN database.     

A Mössbauer study of FeC2O4·2D2O below the néel temperature

From low-temperature Mössbauer measurement on FeC₂O₄·2D₂O the reported difference in quadrupole splitting from the simple dihydrate is inferred to be due to lattice effects. The Zeeman split spectrum has been analyzed taking into account the ambiguity problem and the hyperfine parameters were determined to be I.S.=1.22 mm/sec; Q.S.=1.93 mm/sec; =0.65 to 0.72; =90 to 83.1^o and =0 to 11.8^o. The principal electric field gradient axis lies along the crystal a-axis with V_YY and lying along the crystal b-axis. The crystal field parameters 10Dq, Ds and Dt have been determined to be 10500, 185 and 211 cm^–1, respectively.     

Theoretical study on structures of Ga3N, GaN3, Ga3N2 and Ga2N3 clusters

The structures of Ga_3N, GaN_3, Ga_3N_2 and Ga_2N_3 clusters are studied using the full-potential linear-muffin-tin-orbital molecular dynamics (FP-LMTO MD) method. Four structures for Ga_3 N, five structures for GaN_3, nine structures for Ga_3N_2 and nine structures for Ga_2N_3 have been obtained. The most stable structures of these clusters are planar ones. A strong dominance of the N－-N bond over the Ga－-N and Ga－-Ga bonds appears to control the structural skeletons, supporting the previous result obtained by Kandalam and co-workers. The most stable structures of these small GaN clusters displayed semiconductor-like properties through the calculation of the HOMO－LUMO gaps.     

Fermi surface of CeIn3 above the Néel critical field

We report measurements of the de Haas-van Alphen effect in CeIn(3) in magnetic fields extending to approximately 90 T, well above the Néel critical field of mu(0)H(c) approximately 61 T. The unreconstructed Fermi surface a sheet is observed in the high magnetic field polarized paramagnetic limit, but with its effective mass and Fermi surface volume strongly reduced in size compared to that observed in the low magnetic field paramagnetic regime under pressure. The spheroidal topology of this sheet provides an ideal realization of the transformation from a "large Fermi surface" accommodating f electrons to a "small Fermi surface" when the f-electron moments become polarized.     

Order–disorder transition and thermal conductivity of (Yb x Nd1− x )2Zr2O7 solid solutions

X-ray diffraction, transmission electron microscopy and a laser-flash method were used to investigate the order–disorder transition and thermal conductivity of (Yb _x Nd_{1? x} )₂Zr₂O₇ (0 ≤ x ≤ 1.00) solid solutions. The structures were found to be pyrochlore-type for 0 ≤ x ≤ 0.25, defect fluorite for 0.45 ≤ x ≤ 1.00 and a mixture of these at 0.30 ≤ x ≤ 0.40. The thermal conductivities of (Yb _x Nd_{1? x} )₂Zr₂O₇ first gradually decrease with increasing temperature, and then increase slightly above 800°C due to the increased radiation contribution. YbNdZr₂O₇ has the lowest thermal conductivity due to the reduced cation mean free path at the compositional combination of equal molar Yb³⁺ and Nd³⁺ cations.