Magnetic and Structural Studies of the Quasi-Two-Dimensional Spin-Gap System (CuCl)LaNb2O7

We report magnetization, nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and transmission electron microscopy (TEM) studies on the quasi-two-dimensional spin-gap system (CuCl)LaNb2O7, a possible candidate for the J1-J2 model on a square lattice. A sharp single NQR line is observed at the Cu and Cl sites, indicating that both Cu and Cl atoms occupy a unique site. However, the electric field gradient tensors at the Cu, Cl, and La sites do not have axial symmetry. This is incompatible with the reported crystal structure. Thus the J1-J2 model has to be modified. We propose alternative two-dimensional dimer models based on the NMR, NQR, and TEM results. The value of the hyperfine coupling constant at the Cu sites indicates that the spin density is mainly on the d(3z2-r2) orbital (z parallel c). At 1.5 K, Cu- and Nb-NMR signals disappear above the critical field Bc1 = 10.3 T determined from the onset of the magnetization, indicating a field-induced magnetic phase transition at Bc1.Comment: 9 pages, 16 figure

Collective Singlet Excitations and Evolution of Raman Spectral Weights in the 2D Spin Dimer Compound SrCu2(BO3)2

We present a Raman light scattering study of the two-dimensional quantum spin system SrCu2(BO3)2 and show that the magnetic excitation spectrum has a rich structure, including several well-defined bound state modes at low temperature, and a scattering continuum and quasielastic light scattering contributions at high temperature. The key to the understanding of the unique features of SrCu2(BO3)2 is the presence of strong interactions between well-localized triplet excitations in the network of orthogonal spin dimers realized in this compound. Based on our analysis of the Heisenberg model relevant for this material, we argue that the collective excitations involving two and three-particle singlet bound states have large binding energies and are observed as well-defined peaks in the Raman spectrum.Comment: 5 pages, 2 figures. Revised version, to appear in Phys. Rev. Lett. (2000

Strong damping of phononic heat current by magnetic excitations in SrCu_2(BO_3)_2

Measurements of the thermal conductivity as a function of temperature and magnetic field in the 2D dimer spin system SrCu$_2$(BO$_3$)$_2$ are presented. In zero magnetic field the thermal conductivity along and perpendicular to the magnetic planes shows a pronounced double-peak structure as a function of temperature. The low-temperature maximum is drastically suppressed with increasing magnetic field. Our quantitative analysis reveals that the heat current is due to phonons and that the double-peak structure arises from pronounced resonant scattering of phonons by magnetic excitations.Comment: a bit more than 4 pages, 2 figures included; minor changes to improve the clarity of the presentatio

Geodynamo and mantle convection simulations on the Earth Simulator using the Yin-Yang grid

We have developed finite difference codes based on the Yin-Yang grid for the geodynamo simulation and the mantle convection simulation. The Yin-Yang grid is a kind of spherical overset grid that is composed of two identical component grids. The intrinsic simplicity of the mesh configuration of the Yin-Yang grid enables us to develop highly optimized simulation codes on massively parallel supercomputers. The Yin-Yang geodynamo code has achieved 15.2 Tflops with 4096 processors on the Earth Simulator. This represents 46% of the theoretical peak performance. The Yin-Yang mantle code has enabled us to carry out mantle convection simulations in realistic regimes with a Rayleigh number of $10^7$ including strongly temperature-dependent viscosity with spatial contrast up to $10^6$.Comment: Plenary talk at SciDAC 200

Promotive effects of hyperthermia on the сytostatic activity to ehrlich ascites tumor cells by diverse delta-alkyllactones

To evaluate promotive effects of hyperthermia on antitumor activity of new delta-alkyllactones (DALs) of low molecular weight (184–254 Da), chemically synthesized, which are different from natural macrocyclic lactones of high molecular weight (348–439 Da), such as camptothecin and sultriecin. Methods: A suspension of Ehrlich ascites tumor (EAT) cells was mixed with a DAL in a glass tube, heated at 37 or 42 °C for 30 min in a water bath, and cultured at 37 °C for 20 or 72 h. Cell viability was measured by the mitochondrial dehydrogenase- based WST-1 assay. DALs incorporated into EAT cells was extracted and measured by gas-liquid chromatography. Results: The reduction of cell viability by DALs was markedly enhanced upon the treatment at 42 °C compared to that at 37 oC. At 37 oC, delta-hexadecalactone (DH16 : 0) and delta-tetradecalactone (DTe14 : 0) displayed cytostatic activity (at 100 µM survival level: 20.7%, 66.1%; at 50 µM — 41.2%, 82.4%, respectively). Their activity was more marked at 42 °C (at 100 µM 10.6%, 27.6%; at 50 µM 30.6, 37.5 %, ibid). The other DALs, delta-undecalactone (DU11 : 0), delta-dodecalactone (DD12 : 0), and delta-tridecanolactone (DTr13 : 0) were almost ineffective. Evaluation of survival rate in the cells treated for 30 min by DALs with the next culturing of EAT cells for 72 h resulted in the enhanced carcinostatic activity of DH16:0 and DTe14:0 even at concentrations as low as 25 µM at either 37 °C (18.5%, 78.5%, ibid) or 42 °C (5.0%, 42.0%, ibid), but the others exhibited slight activity or none. DH16 : 0 was effective at either 37 °C (36.0%) or 42 °C (23.0%) even at a lower dose of 10 µM. At the same time only the most cytostatic DH16 : 0 was incorporated into EAT cells and the rate of incorporation was more at 42 °C than at 37 °C. Conclusion: Delta-hexadecanolactone (DH16 : 0) exhibited the most cytostatic effect that was significantly enhanced by hyperthermia. It allows to consider it as a potent antitumor agent, especially in combination with hyperthermia.Цель: оценить промоторный эффект гипертермии на противоопухолевую активность новых низкомолекулярных (184–254 Да) дельта-алкиллактонов (DALs), химически синтезированных из разных макроциклических высокомолекулярных (348–439Да) лактонов естественного происхождения, таких как камптотецин и салтриецин. Методы: суспензию клеток асцитной опухоли Эрлиха (EAT) смешивали с DAL в стеклянной пробирке, нагревали до 37 °C или 42 °C в течение 30 мин на водяной бане и далее культивировали при 37 °C в течение 20 или 72 ч. Оценку жизнеспособности клеток проводили с помощью WST-1 анализа, основанного на определении митохондриальной дегидрогеназы. Инкорпорированные в EAT-клетки DALs экстрагировали, их уровень измеряли с помощью газо-жидкостной хроматографии. Результаты: DALs значительно снижали жизнеспособность клеток после предварительной обработки при 42 °C по сравнению с 37 °C. При 37 °C были эффективными дельта-гексадекалактон (DH16 : 0) и дельта-тетрадекалактон (DTe14 : 0) (при 100 μM уровень выживаемости: 20,7; 66,1%; при 50 μM — 41,2; 82,4% соответственно). Этот эффект был более выраженным при 42 °C (при 100 μM 10,6; 27,6%; при 50μM 30,6; 37,5% соответственно). Другие DALs, а именно дельта-ундекалактон (DU11 : 0), дельта-додекалактон (DD12 : 0) и дельта-тридекалактон (DTr13 : 0) были практически не эффективны. Оценка уровня выживаемости EAT-клеток, 30 мин обработанных DALs с последующим культивированием в течение 72 ч, показала повышенную канцеростатичсекую активность DH16 : 0 и DTe14 :0 даже при 25 μM концентрации, как при 37 °C (18,5; 78,5% соответсвенно), так и при 42 °C (5,0; 42,0% соответственно). Для других DALs данный эффект был незначительный либо отсутствовал. DH16 : 0 оставался эффективным как при 37 °C (36,0%), так и при 42 °C (23,0%) в 10 μM концентрации. В то же время только наиболее эффективный DAL — DH16 : 0 инкорпорировался в клетки EAT, и уровень инкорпорирования был выше при 42 °C, чем при 37 °C. Выводы: дельта-гексадеканолактон (DH16 : 0) показал наибольшую цитостатическую активность, которая значительно усиливалась в комбинации с гипертермией. Этот DAL можно рассматривать как потенциальный цитостатик, действие которого усиливается при гипертермии

BiCu2_2VO6_6: a new narrow-band spin-gap material

A new spin-ladder family material BiCu$_2$VO$_6$ is studied by means of the magnetic susceptibility, heat capacity and neutron inelastic scattering measurements on powder sample. Singlet ground state and a finite spin gap are confirmed by thermal-activated type susceptibility and by distinct peak at 16 meV in spin excitation. Triple narrow band structure in spin excitation spectrum, probably due to complex crystal structure, is observed and the possibility of weakly-interacting spin-cluster system is discussed