Magnon Heat Conductivity and Mean Free Paths in Two-Leg Spin Ladders: A Model-Independent Determination

The magnon thermal conductivity $\kappa_{\mathrm{mag}}$ of the spin ladders in $\rm Sr_{14}Cu_{24-x}Zn_xO_{41}$ has been investigated at low doping levels $x=0$, 0.125, 0.25, 0.5 and 0.75. The Zn-impurities generate nonmagnetic defects which define an upper limit for $l_{\mathrm{mag}}$ and therefore allow a clear-cut relation between $l_{\mathrm{mag}}$ and $\kappa_{\mathrm{mag}}$ to be established independently of any model. Over a large temperature range we observe a progressive suppression of $\kappa_{\mathrm{mag}}$ with increasing Zn-content and find in particular that with respect to pure $\rm Sr_{14}Cu_{24}O_{41}$ $\kappa_{\mathrm{mag}}$ is strongly suppressed even in the case of tiny impurity densities where $l_{\mathrm{mag}}\lesssim 374$~{\AA}. This shows unambiguously that large $l_{\mathrm{mag}}\approx 3000$~{\AA} which have been reported for $\rm Sr_{14}Cu_{24}O_{41}$ and $\rm La_{5}Ca_9Cu_{24}O_{41}$ on basis of a kinetic model are in the correct order of magnitude

Towards an Intelligent Workflow Designer based on the Reuse of Workflow Patterns

In order to perform process-aware information systems we need sophisticated methods and concepts for designing and modeling processes. Recently, research on workflow patterns has emerged in order to increase the reuse of recurring workflow structures. However, current workflow modeling tools do not provide functionalities that enable users to define, query, and reuse workflow patterns properly. In this paper we gather a suite for both process modeling and normalization based on workflow patterns reuse. This suite must be used in the extension of some workflow design tool. The suite comprises components for the design of processes from both legacy systems and process modeling

Ballistic heat transport of quantum spin excitations as seen in SrCuO2

Fundamental conservation laws predict ballistic, i.e., dissipationless transport behaviour in one-dimensional quantum magnets. Experimental evidence, however, for such anomalous transport has been lacking ever since. Here we provide experimental evidence for ballistic heat transport in a S=1/2 Heisenberg chain. In particular, we investigate high purity samples of the chain cuprate SrCuO2 and observe a huge magnetic heat conductivity $\kappa_{mag}$. An extremely large spinon mean free path of more than a micrometer demonstrates that $\kappa_{mag}$ is only limited by extrinsic scattering processes which is a clear signature of ballistic transport in the underlying spin model

Magnetic heat conductivity in CaCu2O3\rm\bf CaCu_2O_3: linear temperature dependence

We present experimental results for the thermal conductivity $\kappa$ of the pseudo 2-leg ladder material $\rm CaCu_2O_3$. The strong buckling of the ladder rungs renders this material a good approximation to a $S=1/2$ Heisenberg-chain. Despite a strong suppression of the thermal conductivity of this material in all crystal directions due to inherent disorder, we find a dominant magnetic contribution $\kappa_\mathrm{mag}$ along the chain direction. $\kappa_\mathrm{mag}$ is \textit{linear} in temperature, resembling the low-temperature limit of the thermal Drude weight $D_\mathrm{th}$ of the $S=1/2$ Heisenberg chain. The comparison of $\kappa_\mathrm{mag}$ and $D_\mathrm{th}$ yields a magnetic mean free path of $l_\mathrm{mag}\approx 22 \pm 5$ \AA, in good agreement with magnetic measurements.Comment: appears in PR

Realistic interpretation of a superposition state does not imply a mixture

Contrary to previous claims, it is shown that, for an ensemble of either single-particle systems or multi-particle systems, the realistic interpretation of a superposition state that mathematically describes the ensemble does not imply that the ensemble is a mixture. Therefore it cannot be argued that the realistic interpretation is wrong on the basis that some predictions derived from the mixture are different from the corresponding predictions derived from the superposition state

A possible new phase of antagonistic nematogens in a disorienting field

A simple model is proposed for nematogenic molecules that favor perpendicular orientations as well as parallel ones. (Charged rods, for example, show this antagonistic tendency.) When a small disorienting field is applied along $z$, a low density phase $N_-$ of nematic order parameter $S_z<0$ coexists with a dense biaxial nematic $N_b$. (At zero field, $N_-$ becomes isotropic and $N_b$ uniaxial.) But at stronger fields, a new phase $N_{+4}$, invariant under $\pi/2$ rotations around the field axis, appears in between $N_-$ and $N_b$. Prospects for finding the $N_{+4}$ phase experimentally are briefly discussed.Comment: 4 pages, 2 figures. Accepted for publication in PR