Charge ordering and elastic constants in Fe3-xZnx xO4

We present the temperature dependence of elastic modes of Fe_3-xZn_xO₄ with x = 0, x = 0.02 and x = 0.032. The c₄₄ shear modes show a pronounced softening which can be normalized to a common behavior for all these substances. We can explain the softening with a bilinear coupling of the elastic strain to an order parameter linked to charge ordering processes. The other elastic modes (c₁₁, c₁₂ and the bulk modulus) do not show any softening. We present a symmetry analysis for the charge ordering model. Received 27 April 1999     

Average entropy dissipation in irreversible mesoscopic processes

The Clausius inequality is rectified to obtain expressions for the entropy produced as the result of irreversible work and/or heat exchanges. These are combined with the Jarzynski equality and its generalizations to explicitly relate mean (as opposed to transient) entropy dissipation to experimental or simulation observables. The results are extended to nonisothermal processes which begin and end at nonequilibrium steady states, leading to a new exact expression and positive lower bound for the resulting average entropy dissipation. The Carnot engine emerges as a limiting case of a family of irreversible processes arising from an interface between materials at different temperatures, whose mapping onto reversible and irreversible molecular extension experiments is discussed.     

The rectified second law of thermodynamics

Equilibrium thermodynamics is combined with Jarzynski's irreversible work theorem to quantify the excess entropy produced by irreversible processes. The resulting rectified form of the second law parallels the first law, in the sense that it facilitates the experimental measurement of excess entropy changes resulting from irreversible work and heat exchanges, just as the first law quantifies energy changes produced by either reversible or irreversible work and heat exchanges. The general form of the rectified second law is further applied to a broad class of quasi-static irreverisble (QSI) processes, for which all of the thermodynamic functions of both the system and surroundings remain continuously well-defined, thus facilitating excess entropy measurements by integrating exact differential functions along QSI paths. The results are illustrated by calculating the mechanical and thermal excess entropy produced by the irreversible unfolding of an RNA molecule.     

An X-ray investigation of the high temperature anomaly in pure V2O3

An X-ray investigation of pure V₂O₃ in the temperature region 300 to 700 K has revealed the presence of more than one phase coexisting from ≈450 to at least 700 K. This observation can be correlated with the high temperature resistivity anomaly and with recent nmr studies on this material.     

Magnetic energy states of shallow acceptors in silicon from spin-dependent luminescence

Quenching of the shallow donor-acceptor radiative transition probability in silicon as a function of the product of electron and hole spin-polarizations enables one to determine neutral acceptor static magnetic susceptibility functions. From these, acceptor g_A-factors for $\text{m}{}_{\mathrm{J}}\text{= ±}\text{3}\text{2}$ levels, with [100]∥H, denoted as g₀ [100] (the outer level g_A factor), have been determined for B, Al, Ga and In acceptors to be 1.10 ± 0.02,0.77 ± 0.02, 0.89 ± 0.02 and 0.56 ± 0.03. The method is insensitive to the inner level g_A factors, g_i, which correspond to the $\text{m}{}_{\mathrm{J}}\text{= ±}\text{1}\text{2}$ levels. The susceptibility measurements were anisotropic and for the [111]∥H orientation could be fit either with an independent set of g₀ [111] values, which turned out to be very different from g_o [100] for B and In, or by assuming g₀ [100] =g₀[111] and introducing a local strain term along 〈111〉 directions. In view of other reports indicating near equality of g₀ [100] and g_o[111], we believe the invoked strain energies are applicable. Their respective values for B, Al, Ga and In are ?0.10, ?0.03, 0.00 and 0.13 meV, consistent with internal strains along 〈111〉 which have as origin the size misfit of the respective acceptors in the silicon lattice. At very high value of H/T, where electron and hole spin-polarizations approach 1, the luminescence does not go to zero as would be expected from an s -envelope function for the acceptor, but approaches a residual luminescence intensity. This fractional residual luminescence is shown to arise from d-function mixing with the predominantly s-function acceptor envelope state, which mixing was first calculated by Schechter. The residual luminescence parameters, which are a measure of the amount of d-function admixture, are respectively for B, Al, Ga and In equal to 0.23, 0.18, 0.16 and 0.08. The progressive decrease of these values with increasing acceptor ionization energies are suggested to be related to s-envelope function stabilization with respect to increased core potentials. Spin-resonance of the donor electrons was optically detected via the unquenching of the luminescence signal, and donor spin-lattice relaxation rates, arising from a neutral donor-neutral acceptor interaction, were determined as a function of D⁰?A⁰ pair separation.     

Progress in Switching Technology for Mets Systems

Three distinct sets of switching requirements have emerged from design optimization studies of large superconducting magnetic energy storage systems, such as the METS system to power the adiabatic plasma compression field in the proposed ?-pinch SFTR. Extremely low joule loss cryogenic disconnects are required between storage coils in the liquid helium environment to allow charging the coils in series over a prolonged time, then to isolate the coils for parallel fast discharging into the load. Another switch must break the current in the series charging loop and absorb the energy from the stray inductance. This action will allow the subsequent opening of the cryogenic disconnects under near zero current condition. The current now has been transferred to the many paralled circuits, each containing a high current, high voltage interrupter. The opening and arc commutation of the interrupter starts the energy transfer into the load. The primary activities associated with the cryogenic disconnect have been testing and development of contact materials, configurations, and closing forces for carrying 26 kA with a resistance less than 40 n?, and development of an actuating system that is both reliable and fast acting in a liquid helium environment. The charging loop switch will include a continuous duty switch and a vacuum interrupter. The continuous duty switch resistance can be an order of magnitude larger than that of the cryogenic disconnect because it does not present a refrigeration load.     

Calculating the electrostatic potential of molecules in solution: Method and error assessment

We present a numerical method for calculating the electrostatic potential of molecules in solution, using the linearized Poisson-Boltzmann equation. The emphasis in this work is on applications to biological macromolecules. The accuracy of the method is assessed by comparisons with analytic solutions for the case of a single charge in a dielectric sphere (Tanford-Kirkwood theory), which serves as a model for a macromolecule. We find that the solutions are generally accurate to within 5%. Larger errors occur close to the charge and the dielectric boundary, but the maximum error found at ion-bonding distance (3 Å) from a charge close to the boundary (1 Å deep) is only ～15%. Several algorithmic improvements, described here, contribute to the accuracy of the method. The programs involved compose a coherent software package, called Del Phi, which goes from a Brookhaven Protein Data Bank format file to calculated electrostatic fields.