LOW TEMPERATURE SPECIFIC HEAT ANALYSIS OF LaMnO3+δ MANGANITES

We report a theoretical analysis for the experimental specific heat C(T) data of the perovskite manganites LaMnO 3+δ, with δ=0.11, 0.15 and 0.26, in the temperature domain 4≤T≤10 K . Calculations of C(T) have been made within the two-component scheme: one is the Fermionic and the other is Bosonic (phonon or magnon) contribution. Lattice specific heat is well estimated from the Debye temperature for lanthanum manganites with different δ obtained following an overlap repulsive potential. Fermionic component as the electronic specific heat coefficient is deduced using the band structure calculations. Later on, following double exchange mechanism the role of magnons is assessed toward specific heat and is found that at low temperatures, specific heat shows almost T3/2 dependence on the temperature. We note that the lattice specific heat is smaller for δ=0.11 when compared with that of magnon specific heat below 6 K, while the lattice contribution is larger with the magnon contribution for δ=0.15 and 0.26. It is further noticed that in the ferromagnetic phase, deduced electronic specific heat is smaller in comparison with reported large electronic term in low temperature domain. The present investigations allow us to stress that electron correlations are essential to enhanced density of state over simple Fermi liquid approximation in pure LaMnO 3+δ (δ=0.11, 0.15 and 0.26). These findings express that the large Coulomb interaction U suppresses the double occupancies of eg electrons and enhanced electronic specific heat, while there is a decrease of T3/2-term with δ from 0.26 to 0.11. The present numerical analysis of specific heat shows similar results as those revealed from experiments.

ANALYSIS OF HEAT CAPACITY OF YBa2Cu3O7-δ SUPERCONDUCTORS: ROLE OF LATTICE AND ELECTRON CONTRIBUTION

Observed temperature-dependent heat capacity C (T) behavior of high-Tc YBa 2 Cu 3 O 7-δ cuprate superconductors has been theoretically analyzed in the temperature domain 70 ≤ T ≤ 110 K . Calculations of C (T) have been made within the two component scheme: one is the Fermionic term and the other the Bosonic (phonon) contribution. While estimating the electronic term, we use a mean field step and follow two-fluid model below and above Tc. Later on, the lattice heat capacity is estimated within harmonic approximation for high temperature expansion (T > θ/2π), the model has only one free parameter, the moments of phonon density of states. Within the two-fluid model for electronic specific heat along with reported γ value leads to a sharp discontinuity at Tc. The Coulomb correlations and electron-phonon coupling strength have significant implications on the γ. Henceforth, the present numerical analysis of specific heat from the present model shows similar results as those revealed from experiments. The accurate fitting of the specific heat data reveals that it is possible to decompose the documented specific heat into dominant lattice contribution and electronic channel. However, the specific heat from electronic term is only a fraction of lattice specific heat in YBa 2 Cu 3 O 7-δ high-Tc superconductors.

Student Projects Embracing New Computer Technologies: Opportunities for Student Scholarship on the World Wide Web

In this article, we demonstrate how faculty can integrate World Wide Web projects into course assignments, We explore: (a) implementation of electronic term papers (ETP), (b) pedagogical objectives of the assignment, (c) educational benefits of the ETP, (d) an exemplar ETP, (e) grading of ETPs, (f) resource and copyright issues, and (g) student evaluations of the approach. Our goal is to demonstrate how students and faculty can join together to use new technologies in a collaborative fashion that results in student self-discovery and self-construction of knowledge.

Electronic Structure and Spatial Arrangement of C2v-Coordinated Ferric Iron in Metmyoglobin, Metmyoglobin Fluoride, and Methemoglobin

Electronic Structure, Metmyoglobin, Metmyoglobin Fluoride, Methemoglobin The electronic term scheme of ferric iron in metmyoglobin, metmyoglobin fluoride, and met­ hemoglobin is evaluated by a Hamiltonian which involves the Coulomb repulsion of the 3d electrons, their interaction with the C2v-coordinated ligands, and spin-orbit coupling. The adjustable parameters of the theory were determined by a least squares fit to experimental EPR, susceptibility, and far-infrared data reported in the literature. According to these results, the structural properties of the ferric ion and its neighboring ligands were discussed by means of group theoretical arguments: An increasing out of plane position of the ferric ion is found in the sequence metHb- metMb -MbF which corresponds to an increasing binding strength with the axial ligands.

The Anomalous Thermal Expansion of Chromium

Experimental data on the linear thermal expansion between 20 and 50�C of specimens of high-purity chromium confirm the existence of an anomaly centred at 38 �C. The exact shape of this expansion anomaly between about 37 and 39�C varies markedly from sample to sample, but outside this narrow temperature interval the behaviour is more regular and seemingly related to antiferromagnetic ordering. Data on expansion are compared with other recent observations on heat capacity, magnetic susceptibility, and electrical resistivity of high-purity chromium. It is suggested that an electronic term in the expansion coefficient, observed below 20 oK to be negative, is sensitive to band structure and may be partly responsible for the change in expansion coefficient.

An experiment on the mechanism of superconductivity

The Thomson coefficient of superconductive lead has been determined by a direct experiment and found to be zero (<4x 10 -9 V/deg.). It has been concluded from this result that the electrons engaged in a superconductive current remain energetically at absolute zero. The apparent electronic specific heat of a superconductor is assumed to be due to an excitation of electrons from the lowest state. The magnetic data suggest that this apparent specific heat is proportional to T 3 . A purely empirical model of the electronic term system for a superconductor has been suggested in which a small energy gap ( ~10 -4 eV) separates the upper limit of the Fermi distribution at absolute zero from a continuum of higher states. The frictionless transport of electrons is supposed to be due to metastable states within the gap in which energy cannot be dissipated. In such a model the number of superconductive electrons at absolute zero has been calculated to be of the order of 10 -3 of the number of atoms. Attention has been drawn to a peculiar similarity between the frictionless transport in superconductors and that in liquid helium II. It has been concluded that the cause for both phenomena may be essentially the same—an aggregation of freely mobile particles of zero thermal energy which follows similar rules irrespective of the nature of the particles involved.