Pressure dependence and non-universal effects of microscopic couplings on the spin-Peierls transition in CuGeO_3

The theory by Cross and Fisher (CF) is by now commonly accepted for the description of the spin-Peierls transition within an adiabatic approach. The dimerization susceptibility as the essential quantity, however, is approximated by means of a continuum description. Several important experimental observations can not be understood within this scope. Using density matrix renormalization group (DMRG) techniques we are able to treat the spin system exactly up to numerical inaccuracies. Thus we find the correct dependence of the equation of state on the spin-spin interaction constant J, still in an adiabatic approach. We focus on the pressure dependence of the critical temperature which is absent in the CF theory as the only energy scale with considerable pressure dependence is J which drops out completely. Comparing the theoretical findings to the experimentally measured pressure dependence of the spin-Peierls temperature we obtain information on the variation of the frustration parameter with pressure. Furthermore, the ratio of the spectral gap and the transition temperature is analyzed.Comment: 5 pages, 5 figures and 1 table include

Thyroid hormone-induced expression of the ADP/ATP carrier and its effect on fatty acid-induced uncoupling of oxidative phosphorylation

AbstractLiver mitochondria from rats made hypothyroid by administration of 2-mercapto-1-methylimidazole were less sensitive to the uncoupling effect of myristic acid, as measured by the increase of resting state respiration, than mitochondria from euthyroid animals, whereas subsequent administration to the animals of triiodothyronine (`hyperthyroidism') resulted in an increased uncoupling action of myristate. `Hyperthyroidism' also resulted in doubling of the carboxyatractyloside-sensitive portion of the myristate-stimulated respiration. Parallel to this was a twofold increase of the mitochondrial content of the ADP/ATP carrier protein and an over threefold increase of its activity. The uncoupling effect of phytanic acid was less sensitive to carboxyatractyloside and was increased in the hyperthyroid state to a smaller extent than in the case of myristate. These results provide further support to the thesis [Skulachev, V.P., FEBS Lett. 294 (1991) 158–162] that the ADP/ATP carrier is involved in the mechanism of the uncoupling effect of long-chain fatty acids

Response of benthic foraminifera to ocean acidification in their natural sediment environment: a long-term culturing experiment

Calcifying foraminifera are expected to be endangered by ocean acidification, However, the response of a complete community kept in natural sediment and over multiple generations under controlled laboratory conditions has not been constrained to date. During 5 six month incubation, foraminiferal assemblages were treated with pCO2 enriched seawater of 430, 907, 1865 and 3247 μatm pCO2. The fauna was dominated by Ammonia aomoriensis and Elphidium species, whereas agglutinated species were rare. After 6 months incubation, pore water alkalinity was much higher in comparison to the overlying seawater. Consequently, the saturation state of Òcalc was much higher in the sedi10 ment than in the water column in all pCO2 treatments and remained close to saturation. As a result, the life cycle of living assemblages was largely unaffected by the tested pCO2 treatments. Growth rates, reproduction and mortality, and therefore population densities and size-frequency distribution of Ammonia aomoriensis varied markedly during the experimental period. Growth rates varied between 25 and 50 μm per month, 15 which corresponds to an addition of 1 or 2 new chambers per month. According to the size-frequency distribution, foraminifera start reproduction at a diameter of 250 μm. Mortality of large foraminifera was recognized, commencing at a test size of 285 μm at a pCO2 ranging from 430 to 1865 μatm, and of 258 μm at 3247 μatm. The total organic content of living Ammonia aomoriensis has been determined to be 4.3% of dry 20 weight. Living individuals had a calcium carbonate production rate of 0.47 gm−2 yr−1, whereas dead empty tests accumulated at a rate of 0.27 gm−2a−1. Although Òcalc was close to 1, some empty tests of Ammonia aomoriensis showed dissolution features at the end of incubation. In contrast, tests of the subdominant species, Elphidium incertum, stayed intact. This species specific response could be explained by differences in 25 the elemental test composition, in particular the higher Mg-concentrations in Ammonia aomoriensis tests. Our results emphasize that the sensitivity to ocean acidification of endobenthic foraminifera in their natural sediment habitat is much lower compared to the experimental response of specimens isolated from the sediment

Finite Temperature DMRG Investigation of the Spin-Peierls Transition in CuGeO3_3

We present a numerical study of thermodynamical properties of dimerized frustrated Heisenberg chains down to extremely low temperatures with applications to CuGeO$_3$. A variant of the finite temperature density matrix renormalization group (DMRG) allows the study of the dimerized phase previously unaccessible to ab initio calculations. We investigate static dimerized systems as well as the instability of the quantum chain towards lattice dimerization. The crossover from a quadratic response in the free energy to the distortion field at finite temperature to nonanalytic behavior at zero temperature is studied quantitatively. Various physical quantities are derived and compared with experimental data for CuGeO$_3$ such as magnetic dimerization, critical temperature, susceptibility and entropy.Comment: LaTeX, 5 pages, 5 eps figures include

Soliton Lattices in the Incommensurate Spin-Peierls Phase: Local Distortions and Magnetizations

It is shown that nonadiabatic fluctuations of the soliton lattice in the spin-Peierls system CuGeO_3 lead to an important reduction of the NMR line widths. These fluctuations are the zero-point motion of the massless phasonic excitations. Furthermore, we show that the discrepancy of X-ray and NMR soliton widths can be understood as the difference between a distortive and a magnetic width. Their ratio is controlled by the frustration of the spin system. By this work, theoretical and experimental results can be reconciled in two important points.Comment: 9 pages, 5 figures included, Revtex submitted to Physical Review

Spin-Peierls transition of the first order in S=1 antiferromagnetic Heisenberg chains

We investigate a one-dimensional S=1 antiferromagnetic Heisenberg model coupled to a lattice distortion by a quantum Monte Carlo method. Investigating the ground state energy of the static bond-alternating chain, we find that the instability to a dimerized chain depends on the value of the spin-phonon coupling, unlike the case of S=1/2. The spin state is the dimer state or the uniform Haldane state depending on whether the lattice distorts or not, respectively. At an intermediate value of the spin-phonon coupling, we find the first-order transition between the two states. We also find the coexistence of the two states.Comment: 7 pages, 12 eps figures embedded in the text; corrected typos, replaced figure

Two-magnon Raman scattering in insulating cuprates: Modifications of the effective Raman operator

Calculations of Raman scattering intensities in spin 1/2 square-lattice Heisenberg model, using the Fleury-Loudon-Elliott theory, have so far been unable to describe the broad line shape and asymmetry of the two magnon peak found experimentally in the cuprate materials. Even more notably, the polarization selection rules are violated with respect to the Fleury-Loudon-Elliott theory. There is comparable scattering in $B_{1g}$ and $A_{1g}$ geometries, whereas the theory would predict scattering in only $B_{1g}$ geometry. We review various suggestions for this discrepency and suggest that at least part of the problem can be addressed by modifying the effective Raman Hamiltonian, allowing for two-magnon states with arbitrary total momentum. Such an approach based on the Sawatzsky-Lorenzana theory of optical absorption assumes an important role of phonons as momentum sinks. It leaves the low energy physics of the Heisenberg model unchanged but substantially alters the Raman line-shape and selection rules, bringing the results closer to experiments.Comment: 7 pages, 6 figures, revtex. Contains some minor revisions from previous versio

Excitations of the field-induced soliton lattice in CuGeO3

Here we report the first inelastic neutron scattering study of the magnetic excitations in the incommensurate phase of a spin-Peierls material. The results on CuGeO3 provide direct evidence of a finite excitation gap, two sharp magnetic excitation branches and a very low-lying excitation which is identified as a phason mode, the Goldstone mode of the incommensurate soliton lattice.Comment: 5 pages, revtex, 4 figures (*.eps), win-zippe