U. Ammerahl

Inhomogeneous Low Frequency Spin Dynamics in La{sub 1.65}Eu{sub 0.2}Sr{sub 0.15}CuO{sub 4}

We report Cu and La nuclear magnetic resonance measurements in the title compound that reveal an inhomogeneous glassy behavior of the spin dynamics. A low temperature peak in the La spin lattice relaxation rate and the wipeout of Cu intensity both arise from these slow electronic spin fluctuations that reveal a distribution of activation energies. Inhomogeneous slowing of spin fluctuations appears to be a general feature of doped lanthanum cuprate.

Crystal growth and characterization of the spin-ladder compound (Sr, Ca)14Cu24O41

Abstract High-quality centimetric size single crystals of Sr 14− x Ca x Cu 24 O 41 (0⩽ x ⩽12) have been successfully grown using the travelling solvent floating zone (TSFZ) method associated with an image furnace. We present the growth conditions together with a characterization of the single crystals by means of optical and electron microscopy, EDX, X-ray powder diffraction, Laue X-ray back-reflection and neutron diffraction.

Crystal growth of the spin-ladder compound (Ca, La)14Cu24O41 and observation of one-dimensional disorder

Abstract Centimeter size single crystals of Ca 14− x La x Cu 24 O 41 have been successfully grown by the travelling solvent floating zone method using an image furnace. Crystals grown under oxygen pressure of several bar are single phase for 2⩽ x ⩽5. Single crystal X-ray studies on Ca 8.4 La 5.6 Cu 24 O 41 and Ca 9 La 5 Cu 24 O 41 samples, using precession and Weissenberg methods, show diffraction streaks from the CuO 2 chains of the structure which are interpreted as an indication of a one-dimensional initial phase disorder along the a -direction.

Temperature and magnetic-field dependence of the lattice constant in the spin-Peierls cuprate CuGeO 3 sstudied by capacitance dilatometry in fields up to 16 T

We present high-resolution measurements of the thermal-expansion coefficient and the magnetostriction along the a axis of

Magnon hole scattering in (Sr, Ca, La)14Cu24O41

{mathrm{CuGeO}}_{3}

Magnetic excitations in two-leg spin 1/2 ladders: experiment and theory

in magnetic fields up to 16 T. From the pronounced anomalies of the lattice constant a, occurring for both temperature- and field-induced phase transitions, clear structural differences between the uniform, dimerized, and incommensurate phases are established. A precise field temperature phase diagram is derived and compared in detail with existing theories. Although there is fair agreement with the calculations within the Cross-Fisher theory, some significant and systematic deviations are present. In addition, our data yield a high-resolution measurement of the field and temperature dependence of the spontaneous strain scaling with the spin-Peierls order parameter. Both the zero-temperature values as well as the critical behavior of the order parameter are nearly field independent in the dimerized phase. A spontaneous strain is also found in the incommensurate high-field phase, which is significantly smaller and shows a different critical behavior than that in the low-field phase. The analysis of the temperature dependence of the spontaneous strain yields a pronounced field dependence within the dimerized phase, whereas the temperature dependence of the incommensurate lattice modulation compares well with that of the dimerization in zero magnetic field.

Thermodynamic properties of the incommensurate phase of CuGeO3.

Abstract We have investigated the one-dimensional magnon heat transport of the spin ladder system (Sr,Ca,La) 14 Cu 24 O 41 . Along the ladder direction in all compounds heat conduction is dominated by magnon heat conduction which in some cases exceeds the phonon heat conduction by almost two orders of magnitude. Isovalent as well as trivalent doping strongly affect the magnon heat transport: Large magnon heat conduction is observed as long as charges are localized whereas it is effectively suppressed if charges become mobile as a function of temperature and/or doping.

ESR study of (Sr,La,Ca)14Cu24O41

Abstract Magnetic excitations in two-leg S =1/2 ladders are studied both experimentally and theoretically. Experimentally, we report on the reflectivity, the transmittance and the optical conductivity σ ( ω ) of undoped La x Ca 14− x Cu 24 O 41 for x =4, 5, and 5.2. Using two different theoretical approaches (Jordan–Wigner fermions and perturbation theory), we calculate the dispersion of the elementary triplets, the optical conductivity and the momentum-resolved spectral density of two-triplet excitations for 0.2≤ J ∥ / J ⊥ ≤1.2. We discuss phonon-assisted two-triplet absorption, the existence of two-triplet bound states, the two-triplet continuum, and the size of the exchange parameters.

Crystal growth of Na(V,Ti)2O5

We present high resolution measurements of the specific heat and the thermal expansion of the inorganic spin--Peierls cuprate CuGeO_3 in a magnetic field of 16 Tesla. At the transition from the incommensurate to the uniform phase both quantities show pronounced anomalies, which allow to derive the uniaxial pressure dependencies of the transition temperature. In high magnetic fields the specific heat is dominated by magnetic excitations and follows a T^3 law at low temperatures. The thermal expansion measurements show the occurrence of spontaneous strains along all three lattice constants and yield high resolution measurements of the temperature dependence of the incommensurate structural distortion. The sizes of the spontaneous strains in the incommensurate phase are significantly reduced, but both their anisotropy as well as their temperature dependencies are very similar to those in zero field.

Charge density waves in Sr14-x Cax Cu24 O41: Electron correlations versus structural effects

Abstract We report an electron spin resonance (ESR) study of single crystals of the spin-chain spin-ladder compound (Sr,La,Ca) 14 Cu 24 O 41 . The data suggest that in intrinsically hole doped Sr 14− x Ca x Cu 24 O 41 , only a small amount of holes is transferred from the chains to the ladders with increasing x , resulting in a crossover from spin dimerized to uniform spin chains. In the samples of La 14− x Ca x Cu 24 O 41 with reduced hole content a very broad signal is observed in the paramagnetic state, indicative of a surprisingly strong anisotropy of the nearest neighbor exchange in the chains.