Delta Ln Research Articles

Pressure dependence of the magnetisation of YFe2and ZrFe2: computation and experiment

Values of delta ln sigma / delta P, where sigma is the magnetisation per unit mass, for YFe2 (ZrFe2) of -8.4+or-0.4 (-6.3+or-0.3)*10-4 kbar-1 were obtained from forced magnetostriction measurements on polycrystalline samples at 4.2K in fields of up to 12 T. These values are shown to be in reasonable agreement with ab initio computer calculations of delta ln sigma / delta P but contradict earlier experimental work. The agreement between experiment and theory for delta ln sigma / delta P provides further evidence that YFe2 and ZrFe2 are ferrimagnetic.

Magnetovolume and magnetoelastic effects in dilute Cr-Ga alloys

The thermal expansion and elastic constants of a series of dilute Cr-Ga alloys are reported between 77 and 500K. In addition to magnetic anomalies at the Neel temperature, TN, well defined anomalies in both properties have been observed for the first time at the incommensurate-commensurate transition temperatures of this system which has a triple point near 0.5 at.% Ga and 280K where the paramagnetic, incommensurate (I) and commensurate (C) spin density wave (SDW) states coexist. The temperature dependence of the magnetovolume, Delta omega , of alloys showing I-C transitions is analysed to yield the ratio (m2(0))CSDW/(m2(0))ISDW=1.5+or-0.2, where (m2(0)) is the mean squared value of the effective local moment at 0K. Using this result the magnetic contribution to the bulk modulus at 0K in a Cr+1.00 at.% Ga alloy is found to be mod Delta B(0) mod CSDW=(0.20+0.05)*1011 N m-2. Also for this alloy, for example, delta ln TN/ delta omega ISDW=20+or-4, in good agreement with independent measurements. For some alloys the calculated pressure dependence of the Neel temperatures agrees fairly well with direct measurements. Above TN the magnetovolume due to antiferromagnetic spin fluctuations decays roughly exponentially with temperature.

An impedance cardiographic study of the mechanism of blood pressure fall after moderate dietary sodium restriction.

Impedance cardiography was performed on 13 free-living subjects to determine the mechanism which ties modest sodium restriction to a reduction in blood pressure. During salt restriction intervention, significant decreases in systolic and diastolic blood pressure (6 and 7 mmHg) were observed associated with mild reduction in urinary sodium excretion from 176 to 85 mmol/day. The impedance cardiogram demonstrated a significant decrease in stroke volume (12 ml), which was related to a fall in diastolic pressure (r = 0.59, p less than 0.05). In the further analysis of individual parameters in Kubicek's formula calculating stroke volume, thoracic impedance (Zo/L) showed a significant increase during salt restriction and a change (delta) in ln L/Zo related significantly to delta ln diastolic pressure in linear multiple regression analysis. As thoracic impedance has been reported to reflect intrathoracic fluid content, a decrease in extracellular fluid volume was suggested as a possible mechanism of the fall in blood pressure even with modest sodium restriction.

On the gauge equivalence of the equation of dynamics for uniaxial ferromagnets

The equations of dynamics for an easy-plane anisotropy uniaxial ferromagnet St-S*Sxx+ beta (S.n)S*n=0 S2=1 n=(0,0,1) beta >0 is shown to be gauge equivalent to the equation (vt+vxx-2/v/2v+2/vx/2v/( beta -4/v/2)-(vx/2) delta ln( beta -4/v)2)/ delta x=0. The formulae that relate the solutions of these equations for natural boundary conditions are derived.

Pressure and temperature dependence of the hyperfine fields of YFe2, YFe3and YIG

The hyperfine field (HT) of 89Y in YFe2 and YFe3 and of 57Fe in single-crystal and sintered polycrystalline YIG has been measured as a function of pressure to 15 bar at room temperature and for YIG also to 7 kbar at 77K. The value of ( delta ln HT/ delta P)T for YIG at 77K is zero to experimental accuracy but is positive at room temperature due to the pressure dependence of the Neel temperature. Little difference was found between the values for single-crystal and sintered YIG. The values of ( delta ln HT/ delta P)T for 89Y in YFe2 and YFe3 were found to be in general agreement with those to be expected if the Y hyperfine field (HFF) is produced by the polarisation of s like conduction electrons but a small contribution from the d band is probably required to explain the results in detail. The spin-wave stiffness coefficient of YIG was found to be (0.568+or-0.011) eV AA2 from an analysis of the temperature dependence of the HFF below 20K. This value is in good agreement with those found less directly by earlier HFF and magnetisation measurements and confirms that there is a large discrepancy with the value found by neutron scattering experiments. The reduced HFF at the two Y sites in YFe3 was found to have the same temperature dependence below 35K (T3/2 law) and in this region decreases far more slowly than was predicted from analysis of Mossbauer measurements made up to room temperature. Above 100K the difference in the reduced HFF at the two Y sites in YFe3 appeared to be proportional to T3/2 which, in view of the low-temperature result, must arise from a partial cancellation of higher order spin-wave terms.

The pressure dependence of the spin equilibrium in camphor-bound ferric cytochrome P-450.

The spin equilibrium of camphor-bound ferric cytochrome P-450 has been measured between 1-1000 bar (10(5)-10(8) Pa). Increasing pressure shifts the absorption spectrum from the high-spin form at 392 nm to the low-spin form at 417 nm. The molar volume change for the spin states delta V = -RT delta ln Ke/ delta P and the equilibrium coefficient Ke = [high spin]/[low spin] depend on the solvent conditions. At pH 5.6 the equilibrium coefficient at 1 bar, K1 = 0.5 and delta V = 312 cm3/mol. A sample with 10 mM KCl at pH 7 has K1 = 7.0 amd delta V = 52 cm3/mol. Solvent changes producing a larger K1 also result in a larger delta V which ranged over 16-74 cm3/mol. The correlation can be approximated as delta V = 36 + 18 log K1, which implies that there is a pressure, 3000 bar, for camphor-bound ferric cytochrome P-450 at 4 degrees C, at which the changes in delta V are compensated by the other thermodynamic parameters leaving Ke independent of the solvent conditions. Although the protein is not stable about 1000 bar for most sample conditions, the extrapolated log Ke versus pressure curves for all sample conditions intersect near 3000 bar. Camphor-bound cytochrome P-450 appears to be a rather flexible protein, having a low denaturing pressure, a large volume change, and a high sensitivity to the protein environment.

Multiphoton resonance effects in very high laser fields: twelve-photon ionisation of krypton at 1013W cm-2

Resonance effects in the multiphoton ionisation of atoms are described in terms of the characteristic times of the resonant and off-resonant processes. This description explains qualitatively why resonance effects in multiphoton ionisation of atoms appear differently in caesium atoms at a laser intensity I=108 W cm-2 and in rare gases at I=1013 W cm-2. Resonance effects in twelve-photon ionisation of krypton are investigated at 1013 W cm-2 by the use of a tunable-wavelength bandwidth-limited 15 ps laser pulse at 1.06 mu m. For the laser intensity used, the experimental results give no evidence of a significant enhancement in the number of ions Ni, while resonance effects appear through the variation of the slope K= delta ln Ni/ delta ln I which acquires values much larger than 12 (up to 40) within a frequency range less than 1 cm-1. New theoretical data on resonance effects in multiphoton ionisation of rare gases at 1013 W cm-2 are needed to explain the present experimental results quantitatively. In the absence of such theoretical data a simple model is used to fit the experimental results qualitatively.

Effects of Ischemia on the Removal of Extracellular Potassium in Cat Cortex during Pentylenététrazol Seizures

Changes in cortical extracellular potassium activity ([K+]0), NADH fluorescence, and oxygen consumption were studied in anesthetized cats during pentylenetetrazol seizures. The effects of partial ischemia induced by either hypotension or intermittent carotid artery occlusion on these parameters were investigated. Nonischemic seizures were characterized by gradual generalized decreases in cortical NADH fluorescence and increases in O2 consumption, along with rapid increases in [K+]0, which then usually fell slightly as the ictal discharge continued. Ischemic seizures, on the other hand, were accompanied by complex changes in NADH fluorescence, by smaller delayed maximal increases in O2 consumption that lasted beyond the end of ictal activity, and by more sustained increases in [K+]0. The decay of [K+]0 after the termination of seizures in both nonischemic and moderately ischemic animals was not a monoexponential function: plots of ln delta [K+]0 versus time showed an initial linear decline (of slope M1) that rather abruptly slowed (to slope M2) after 2 to 5 sec and then often increased to the original rate. Both M1 and M2 were proportionately decreased by ischemia. In addition, the rate of [K+]0 removal could be slowed by acute ischemia induced either during or after the end of ictal activity. The initial rate of postictal [K+]0 removal (M1) was found to be linearly and inversely related to the level of cortical NADH fluorescence at the time of seizure termination. The results of this study suggest that an O2-dependent transport mechanism plays a major role in the removal of [K+]0 during and following the termination of generalized pentylenetetrazol seizures in the cat.

Free energy of activation. Definition, properties, and dependent variables with special reference to "linear" free energy relations

The reaction rate constant is expressed as Z exp(-G/sub a//RT). Z is the binary collision frequency. G/sub a/, the free energy of activation, is shown to be the difference between the free energy of the reactive reactants and the free energy of all reactants. The results are derived from both a statistical mechanical and a collision theoretic point of view. While the later is more suitable for an ab-initio computation of the reaction rate, it is the former that lends itself to the search of systematics and of correlations and to compaction of data. Different thermodynamic-like routes to the characterization of G/sub a/ are thus explored. The two most promising ones appear to be the use of thermodynamic type cycles and the changes of dependent variables using the Legendre transform technique. The dependence of G/sub a/ on ..delta..G/sup 0/, the standard free energy change in the reaction, is examined from the later point of view. It is shown that one can rigorously express this dependence as G/sub a/ = ..cap alpha delta..G/sup 0/ + G/sub a//sup 0/M(..cap alpha..). Here ..cap alpha.. is the Bronsted slope, ..cap alpha.. = -par. delta ln k(T)/par. delta(..delta..G/sup 0//RT), G/sub a//sup 0/ is independent ofmore » ..delta..G/sup 0/ and M(..cap alpha..), the Legendre transform of G/sub a/, is a function only of ..cap alpha... For small changes in ..delta..G/sup 0/, the general result reduces to the familiar ''linear'' free energy relation delta G/sub a/ = ..cap alpha.. delta ..delta..G/sup 0/. It is concluded from general considerations that M(..cap alpha..) is a symmetric, convex function of ..cap alpha.. and hence that ..cap alpha.. is a monotonically increasing function of ..delta..G/sup 0/. Experimental data appear to conform well to the form ..cap alpha.. = 1/(1 + exp(-..delta..G/sup 0//G/sub s//sup 0/)). A simple interpretation of the ..delta..G/sup 0/ dependence of G/sub a/, based on an interpolation of the free energy from that of the reagents to that of the products, is offered. 4 figures, 69 references.« less

The distribution of hyperfine fields in two alloys of iron as a function of temperature and pressure

The distribution of hyperfine fields at iron atoms in FeCo and FeNi alloys containing 1/2% of added impurity has been measured as a function of hydrostatic pressure up to 16 kbar. The frequency ( nu ) of the main and satellite lines of FeCo was found to change such that ( delta ln nu / delta P)T=-1.75*10-7 kg-1 cm2. In FeNi the value for the main line was -1.64*10-7 kg-1 cm2 and -2.02*10-7 kg cm-2 for the satellite line. The temperature dependence of the difference between the normalised main and satellite lines was found to be proportional to T3/2 at temperature T, even after the measurements had been reduced to constant volume, which is in disagreement with the prediction of spin wave theory.

Bicarbonate ions in active sodium transport across toad bladder

The conductance, ga, and electromotive force, E, of active ion transport across toad bladders mounted as sacs were estimated from electrical measurements made before and after addition of sufficient ouabain (1.89 X 10(-3) M) to eliminate spontaneous potential. The ratio of net sodium transport (estimated from bidirectional fluxes) to external current in bladders voltage clamped to 0 mV was significantly less than unity in a normal medium containing HCO3- and Cl- ions, and also when Cl- was replaced with SO42-. However, when acetazolamide was added or when HCO3- was replaced by phosphate, short-circuit current and net sodium transport became equal. Spontaneous potential, E, and ga were all reduced about 20% by these maneuvers. The response of bidirectional sodium fluxes to voltage clamping at 0 mV or 120 mV in a bicarbonate-free medium was otherwise similar to that observed in a normal medium: net flux varied linearly with potential and calculated fluxes in the active transport path indicated a value significantly greater than unity for the empirical constant Q in the equation for change in the flux ratio, f, with change in potential, psi, viz., delta ln f = Q(ZF/RT)delta psi, similar to the high value for this constant that we have found in a bicarbonate-containing medium. We conclude that bicarbonate ions facilitate active sodium transport and also may be actively transported from serosa to mucosa in Dominican toads. However, coupling between bicarbonate and sodium fluxes does not account for the high value for Q for sodium in the active transport path.

Calculation of the phonon-limited ideal resistivity of potassium and sodium

The resistivities at constant volume and constant pressure and the volume derivative ( delta ln rho / delta lnV) were calculated over a range of temperatures (20K to melting) employing a fundamental calculation of the form factor and phonon spectrum. No parameters were adjusted to fit experimental data yet the agreement obtained with experiment was very good. The phonon frequencies were calculated using the recently-developed model potentials of Dagens et al. (1975) and the Geldart and Taylor (1970) screening. The form factors were calculated using the same model potential and screening but also incorporating the non-local electron-electron vertex correction of Rasolt (1975).