Thermoelectric transport in the three terminal quantum dot

The thermoelectric transport in the system composed of a quantum dot in contact with superconducting, ferromagnetic and normal metal electrodes has been studied. Such a system can support pure spin current in the normal electrode. In the limit of a large superconducting gap and weak coupling between the dot and the electrodes we investigate the sub-gap charge and spin transport via Andreev mechanism using the standard master equation technique, which is known to be valid in the sequential tunnelling regime. The Zeeman splitting of the dot level induces pure spin current in the ferromagnetic electrode under an appropriate bias. This opens a novel possibility to switch the spin current between two electrodes by electric means. The calculated spin and charge thermopower coefficients attain very large values, of the order of a few hundreds μV K(-1), and show similar dependences on the position of the on-dot energy level and temperature.     

A complex impedance study of Pt∣NaCl and C∣NaCl interfaces

The a.c. impedance of Pt∣NaCl∣Pt and C∣NaCl∣C cells was measured in the frequency range from 10^?3 to 10³ Hz. Two contributions to the interface impedance could be distinguished, hindered charge transfer and an adsorption/desorption or slow reaction contribution, the latter being the rate determining step in the frequency range studied. An approximately symmetric nonlinear dependence of both the capacitance and the conductance on overvoltage was found at the lowest frequencies, indicating that the crystal/electrode interface is in a state which can be characterized as close to equilibrium. It should be ensured by two electrochemical reactions of approximately equal rates. The concept of a temporarily reversible electrode is introduced. It is a result of the ability of ions to discharge at the interface and to produce a finite reservoir of adsorbed or absorbed reaction products. An external voltage increases or reduces this reservoir, and therefore only a transient electric current may be carried by a backward reaction.     

NMR frequency and magnetic dipole moment of (3)He nucleus

We present new gas-phase NMR spectra which relate the resonance frequency of (3)He nucleus to the resonance frequency of the proton in tetramethylsilane (TMS). We discuss the dependence of (3)He resonance frequency on the density of the solvent gas, and we consider in detail the absolute shielding scales of both nuclei. Finally, we analyse the accuracy of the results, using the relationship between the resonance frequencies, absolute shielding constants and magnetic dipole moments of (1)H and (3)He nuclei.     

Thermodynamic properties of a diluted Heisenberg ferromagnet with interaction anisotropy—Magnetocaloric point of view

The thermodynamics of a site-diluted ferromagnetic Heisenberg model for spin S=1/2 with interaction anisotropy in spin space is investigated. The study is aimed at presenting the magnetocaloric properties of such a model, including the entropy and temperature changes in magnetization/demagnetization processes, generalized Grüneisen ratio as well as the quantities characterizing the efficiency of magnetic cooling cycles. The results are obtained using pair approximation (PA) method and extensively compared with the molecular field approximation (MFA) calculations. The importance of interaction anisotropy and site-dilution is discussed. The inadequacy of the MFA approach (even on the qualitative level) is found for selected quantities, while PA provides the results which are consistent with the experimentally observed behavior.     

A comparative molecular dynamics study of crystalline,paracrystalline and amorphous states of cellulose

The quintessential form of cellulose in wood consists of microfibrils that have high aspect ratio crystalline domains embedded within an amorphous cellulose domain. In this study, we apply united-atom molecular dynamics simulations to quantify changes in different morphologies of cellulose. We compare the structure of crystalline cellulose with paracrystalline and amorphous phases that are both obtained by high temperature equilibration followed by quenching at room temperature. Our study reveals that the paracrystalline phase may be an intermediate, kinetically arrested phase formed upon amorphisation of crystalline cellulose. The quenched structures yield isotropic amorphous polymer domains consistent with experimental results, thereby validating a new computational protocol for achieving amorphous cellulose structure. The non-crystalline cellulose compared to crystalline structure is characterized by a dramatic decrease in elastic modulus, thermal expansion coefficient, bond energies, and number of hydrogen bonds. Analysis of the lattice parameters shows that Iβ cellulose undergoes a phase transition into high-temperature phase in the range of 450–550 K. The mechanisms of the phase transition elucidated here present an atomistic view of the temperature dependent dynamic structure and mechanical properties of cellulose. The paracrystalline state of cellulose exhibits intermediate mechanical properties, between crystalline and amorphous phases, that can be assigned to the physical properties of the interphase regions between crystalline and amorphous cellulose in wood microfibrils. Our results suggest an atomistic structural view of amorphous cellulose which is consistent with experimental data available up to date and provide a basis for future multi-scale models for wood microfibrils and all-cellulose nanocomposites.     

Characterization of tri-o-methylcellulose by one- and two-dimensional NMR methods

Tri-O-methylcellulose was prepared from partially O-methylated cellulose and its chemical shifts (¹H and ¹³C), and proton coupling constants were assigned using the following NMR methods: (1) One-dimensional ¹H and ¹³C spectra of the title compound were used to assign functional groups and to compare with literature data; (2) double quantum filtered proton–proton correlation spectroscopy (¹H, ¹H DQF-COSY) was used to assign the chemical shifts of the network of 7 protons in the anhydroglucose portion of the repeat unit; (3) the heteronuclear single-quantum coherence (HSQC) spectrum was used to establish connectivities between the bonded protons and carbons; (4) the heteronuclear multiple-bond correlation (HMBC) spectrum was used to connect the hydrogens of the methyl ethers to their respective sugar carbons; (5) the combination of HSQC and HMBC spectra was used to assign the ¹³C shifts of the methyl ethers; (6) all spectra were used in combination to verify the assigned chemical shifts; (7) first-order proton coupling constants data (J_H,H in Hz) were obtained from the resolution-enhanced proton spectra. The NMR spectra of tri-O-methylcellulose and other cellulose ethers do not resemble the spectra of similarly substituted cellobioses. Although the ¹H and ¹³C shifts and coupling constants of 2,3,6-tri-O-methylcellulose closely resemble those of methyl tetra-O-methyl-β-D -glucoside, there are differences with regard to the chemical shifts and the order of appearances of the resonating nuclei of the methyl ether appendages and the proton at position 4 in the pyranose ring. H4 in tri-O-methylcellulose is deshielded by the acetal system comprising the β-1→4 linkage, and it resonates downfield. H4 in the permethylated glucoside is not as deshielded by the equitorial O-methyl group at C4, and it resonates upfield. The order of appearance of the ¹H and ¹³C resonances in the spectra of the tri-O-methylcellulose repeat unit (from upfield to downfield) are H2 < H3 < H5 < H6a < H3a < H2a < pro R H6B < H4 < pro S H6A ≪ H1 and C6a < C3a < C2a < C6 < C5 < C4 < C2 < C3 ≪ C1, respectively. Close examination of the pyranose ring coupling constants of the repeat unit in tri-O-methylcellulose supports the ⁴C₁ arrangement of the glucopyranose ring. Examination of the proton coupling constants about the C5-C6 bond (J_5,6A and J_5,6B) in the nuclear Overhauser effect difference spectra revealed that the C6 O-methyl group is predominantly in the gauche gauche conformation about the C5-C6 bond for the polymer in solution. © 1999 John Wiley & Sons, Inc.* J Polym Sci A: Polym Chem 37: 4019–4032, 1999     

An investigation of the reliability of the Galerkin-Petrov method

It is shown that the Galerkin-Petrov method if applied in a controlled way yields reliable results for excited states of the same symmetry classifications as the ground state. Computations are performed for the 2¹ S and 3¹ S states of He. The problem of optimizing nonlinear parameters of the basis functions by means of the GP method is discussed. A special optimization scheme is suggested and numerically illustrated for someS states of He.