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

Magnetic excitations in two-leg S=1/2 ladders are studied both experimentally and theoretically. Experimentally, we report on the reflectivity, the transmission and the optical conductivity sigma(omega) 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_parallel/J_perpendicular <= 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.Comment: 6 pages, 7 eps figures, submitted to SNS 200

Magnetodielectric and magnetoelastic coupling in TbFe3(BO3)4

We have studied the magnetodielectric and magnetoelastic coupling in TbFe3(BO3)4 single crystals by means of capacitance, magnetostriction and Raman spectroscopy measurements. The data reveal strong magnetic field effects on the dielectric constant and on the macroscopic sample length which are associated to long range magnetic ordering and a field-driven metamagnetic transition. We discuss the coupling of the dielectric, structural, and magnetic order parameters and attribute the origin of the magnetodielectric coupling to phonon mode shifts according to the Lyddane-Sachs-Teller (LST) relation.Comment: Accepted for publication in Physical Review

Stripe order of La1.64Eu0.2Sr0.16CuO4 in magnetic fields studied by resonant soft x ray scattering

We present results on the magnetic field dependence of the stripe order in La1.64Eu0.2Sr0.16CuO4 LESCO . Using resonant soft x ray scattering at the oxygen K edge to probe the 0.259,0,0.648 superlattice reflection, which is commonly associated to charge stripes, we found no pronounced difference in the wave vector, peak widths, and integrated intensity for magnetic fields up to B 6 T. This is in strong contrast to the behavior observed for La1.875Sr0.125CuO4, where a stabilization of the charge modulation in high magnetic fields has been demonstrate

Non-Fermi-liquid scattering rates and anomalous band dispersion in ferropnictides

Angle-resolved photoemission spectroscopy (ARPES) is used to study the band dispersion and the quasiparticle scattering rates in two ferropnictides systems. Our ARPES results show linear-in-energy dependent scattering rates which are constant in a wide range of control parameter and which depend on the orbital character of the bands. We demonstrate that the linear energy dependence gives rise to weakly dispersing band with a strong mass enhancement when the band maximum crosses the chemical potential. In the superconducting phase the related small effective Fermi energy favors a Bardeen-Cooper-Schrieffer (BCS)\,\cite{Bardeen1957}-Bose-Einstein (BE)\,\cite{Bose1924} crossover state.Comment: 5 pages, 4 figures Supplement 4 pages, 6 figure

Liquid Limits: The Glass Transition and Liquid-Gas Spinodal Boundaries of Metastable Liquids

The liquid-gas spinodal and the glass transition define ultimate boundaries beyond which substances cannot exist as (stable or metastable) liquids. The relation between these limits is analyzed {\it via} computer simulations of a model liquid. The results obtained indicate that the liquid - gas spinodal and the glass transition lines intersect at a finite temperature, implying a glass - gas mechanical instability locus at low temperatures. The glass transition lines obtained by thermodynamic and dynamic criteria agree very well with each other.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let

Heat conductivity of the spin-Peierls compounds TiOCl and TiOBr

We report experimental results on the heat conductivity \kappa of the S=1/2 spin chain compounds TiOBr and TiOCl for temperatures 5K<T<300K and magnetic fields up to 14. Surprisingly, we find no evidence of a significant magnetic contribution to \kappa, which is in stark contrast to recent results on S=1/2 spin chain cuprates. Despite this unexpected result, the thus predominantly phononic heat conductivity of these spin-Peierls compounds exhibits a very unusual behavior. In particular, we observe strong anomalies at the phase transitions Tc1 and Tc2. Moreover, we find an overall but anisotropic suppression of \kappa in the intermediate phase which extends even to temperatures higher than Tc2. An external magnetic field causes a slight downshift of the transition at Tc1 and enhances the suppression of \kappa up to Tc2. We interprete our findings in terms of strong spin-phonon coupling and phonon scattering arising from spin-driven lattice distortions.Comment: 6 pages, 3 figure

Evidence of d-wave Superconductivity in K_(1-x)Na_xFe_2As_2 (x = 0, 0.1) Single Crystals from Low-Temperature Specific Heat Measurements

From the measurement and analysis of the specific heat of high-quality K_(1-x)Na_xFe_2As_2 single crystals we establish the presence of large T^2 contributions with coefficients alpha_sc ~ 30 mJ/mol K^3 at low-T for both x=0 and 0.1. Together with the observed square root field behavior of the specific heat in the superconducting state both findings evidence d-wave superconductivity on almost all Fermi surface sheets with an average gap amplitude of Delta_0 in the range of 0.4 - 0.8 meV. The derived Delta_0 and the observed T_c agree well with the values calculated within the Eliashberg theory, adopting a spin-fluctuation mediated pairing in the intermediate coupling regime.Comment: 8 pages, 5 figures, field dependence of the specific heat added, slightly changed title, changed sequence of authors, one author added, accepted by Phys. Rev. B Rapid Communication

Rare earth magnetism in CeFeAsO: A single crystal study

Single crystals of CeFeAsO, large enough to study the anisotropy of the magnetic properties, were grown by an optimized Sn-flux technique. The high quality of our single crystals is apparent from the highest residual resistivity ratio, RRR = 12, reported among undoped RFeAsO compounds (R=rare earth) as well as sharp anomalies in resistivity, specific heat, C(T), and thermal expansion at the different phase transitions. The magnetic susceptibility chi(T) presents a large easy-plane anisotropy consistent with the lowest crystal electric field doublet having a dominant Gamma_6 character. Curie-Weiss like susceptibilities for magnetic field parallel and perpendicular to the crystallographic c-axis do not reveal an influence of a staggered field on the Ce site induced by magnetic ordering of the Fe. Furthermore, the standard signatures for antiferromagnetic order of Ce at T_N = 3.7 K observed in chi(T) and C(T) are incompatible with a Zeeman splitting Delta = 10 K of the CEF ground state doublet at low temperature due to the Fe-magnetic order as previously proposed. Our results can be reconciled with the earlier observation by assuming a comparatively stronger effect of the Ce-Ce exchange leading to a reduction of this Zeeman splitting below 15 K.Comment: 15 pages, 6 figures, added section on magn. susceptibilit

Experimental study of nonlinear interaction of plasma flow with charged thin current sheets: 1. Boundary structure and motion

We study plasma transport at a thin magnetopause (MP), described hereafter as a thin current sheet (TCS), observed by Cluster at the southern cusp on 13 February 2001 around 20:01 UT. The Cluster observations generally agree with the predictions of the Gas Dynamic Convection Field (GDCF) model in the magnetosheath (MSH) up to the MSH boundary layer, where significant differences are seen. We find for the MP a normal roughly along the GSE x-axis, which implies a clear departure from the local average MP normal, a ~90 km thickness and an outward speed of 35 km/s. Two populations are identified in the MSH boundary layer: the first one roughly perpendicular to the MSH magnetic field, which we interpret as the &quot;incident&quot; MSH plasma, the second one mostly parallel to <b>B</b>. Just after the MP crossing a velocity jet is observed with a peak speed of 240 km/s, perpendicular to <b>B</b>, with <i>M_A</i>=3 and &beta;>10 (peak value 23). The magnetic field clock angle rotates by 70&deg; across the MP. <i>E_x</i> is the main electric field component on both sides of the MP, displaying a bipolar signature, positive on the MSH side and negative on the opposite side, corresponding to a ~300 V electric potential jump across the TCS. The <i>E</i>&times;<i>B</i> velocity generally coincides with the perpendicular velocity measured by CIS; however, in the speed jet a difference between the two is observed, which suggests the need for an extra flow source. We propose that the MP TCS can act locally as an obstacle for low-energy ions (&lt;350 eV), being transparent for ions with larger gyroradius. As a result, the penetration of plasma by finite gyroradius is considered as a possible source for the jet. The role of reconnection is briefly discussed. The electrodynamics of the TCS along with mass and momentum transfer across it are further discussed in the companion paper by Savin et al. (2006)

Neural development features: Spatio-temporal development of the Caenorhabditis elegans neuronal network

The nematode Caenorhabditis elegans, with information on neural connectivity, three-dimensional position and cell linage provides a unique system for understanding the development of neural networks. Although C. elegans has been widely studied in the past, we present the first statistical study from a developmental perspective, with findings that raise interesting suggestions on the establishment of long-distance connections and network hubs. Here, we analyze the neuro-development for temporal and spatial features, using birth times of neurons and their three-dimensional positions. Comparisons of growth in C. elegans with random spatial network growth highlight two findings relevant to neural network development. First, most neurons which are linked by long-distance connections are born around the same time and early on, suggesting the possibility of early contact or interaction between connected neurons during development. Second, early-born neurons are more highly connected (tendency to form hubs) than later born neurons. This indicates that the longer time frame available to them might underlie high connectivity. Both outcomes are not observed for random connection formation. The study finds that around one-third of electrically coupled long-range connections are late forming, raising the question of what mechanisms are involved in ensuring their accuracy, particularly in light of the extremely invariant connectivity observed in C. elegans. In conclusion, the sequence of neural network development highlights the possibility of early contact or interaction in securing long-distance and high-degree connectivity