Modern Physics Letters B

We explore the consequences for macroscopic effective Lagrangians of assuming that the momentum density is proportional to the flow of a conserved current. Such a relation holds in models that microscopically contain only one type of charge carrier. We show that such a relation leads to certain universal corrections to the usual effective Lagrangians considered for the description of superfluids. A fully consistent derivation of the superconducting Hall effect predicted by London is given. Some related issues that arise in anyon superconductivity are discussed: the linear Hall effect is derived in a transparent way, and the vanishing of the b term (under appropriate assumptions) is demonstrated formally. Some other possible applications are sketched.

It is shown that at low temperatures, T≪T_F, a general form of short-range s-d type interaction between two species of fermions leads to a resistivity law ρ∝T^d−1 where d=2, 3 is the dimension of the system. In energy units T_F is of the order of the bandwidths of the charge carriers which are assumed to be quasi-free. The calculation is based on a Boltzmann transition rate deduced from the two-body s-d type scattering matrix element. The consequences of making one of the fermions very heavy and of replacing it by a boson are also discussed.

A systematic representation of the resonating-valence-bond state of the 2D spin-1/2 Heisenberg antiferromagnet in terms of anticommuting Grassmann variables is derived. This representation is extended to the dilute case and the holon statistics are considered. Independent phase fluctuations on each valence-bond yield a U(1) gauge theory with low-lying gapless excitations in accordance with other approaches.

A new technique for detecting superconductivity, based on observing the effect of magnetic field modulation on the dc resistance of superconductors, is presented. The phase detected response to the magnetic modulation shows a peak at T_c for both metallic and ceramic superconductors, analogous to that observed in magnetically modulated microwave absorption. The results further demonstrate that the magnetically modulated microwave absorption peak at T_c is not equivalent to the non-resonant low field microwave absorption observed in the ceramic superconductors.

Samples with compositions Bi_1.8Pb_0.2Sr₂(Ca_2−xNa_x)Cu₃O_y, BiSr(Ca_1−xCd_x)Cu₂O_y and BiSr(Ca_1−xBi_x)Cu₂O_y were prepared through solid state reaction. The X-ray diffraction results were analyzed and their resistivity behavior was studied. In general, all such substitution adversely affected their superconductive properties. All samples are of multiphase. The normal state resistivity behaves quite differently with the substitution of electropositive Na as compared with the replacement of electronegative Bi.

The continuous and localized states of electrons in antiferromagnets (AFM) are studied in the presence of the magnetic field or the intrinsic magnetization. Peculiar features of both the Pauli and Curie susceptibilities are found. They may be tested in all intrinsic AFM with residual carriers like spin density wave (SDW) compounds, superconducting cuprates and also in s-f systems if their relevant electronic states are close to the reduced magnetic zone boundary (MZB).

Solid reaction, oxygen relief and absorption in Bi-Sr-Ca-Cu-O system have been investigated by TGA technique. Oxygen contents (x=5.5 and 5.64) for BiSrCaCu₂O_x samples quenched and furnace-cooled were determined, respectively. No oxygen absorption and relief happens below 800°C. Amorphous state can be obtained by heating the sample up to 1000°C and then cooling it quickly. Magnetic susceptibility measurement and oxygen content determination suggest that Cu⁺ and Cu³⁺ exist in the quenched and furnace-cooled samples.

High-T_c superconductors with nominal composition of YBa₂Cu₃O_y and Y_1−xL_xBa₂Cu₃O_y (L=Li, Ce, Eu, V) were synthesis by the solid state reaction of appropriate amount of Y₂O₃, BaO or BaCO₃, Cu₂O, CuO, and LO_x. The Cu³⁺/Cu²⁺ ratio was determined by Iodometric titration and oxygen content in the oxides calculated from the ratio. The crystal structure was determined by electron and powder X-ray diffraction analysis. It shows that that ratio of Cu³⁺/Cu²⁺ and the crystal structure could be changed as dopping appropriate amount of metal in the Y-Ba-Cu-O system.

Raman scattering has been performed on a-Si:H/a-SiN_x:H superlattices in this letter. We have observed the zone-folding effects on longitudinal acoustical phonons in a-Si:H/a-SiN_x:H superlattices. The experimental results confirm the theoretical expectation. Additionally, we have observed the modes of interface between the air and samples.

The influence of a possible and as yet undiscovered narrow resonance in ⁴He on the d-d fusion rate near threshold is examined. A qualitative discussion of the structure of the lowest four 0⁺ states and its impact on the partial widths of decay channels is presented.