Temperature dependence of single particle excitations in a S=1 chain: exact diagonalization calculations compared to neutron scattering experiments

Exact diagonalization calculations of finite antiferromagnetic spin-1 Heisenberg chains at finite temperatures are presented and compared to a recent inelastic neutron scattering experiment for temperatures T up to 7.5 times the intrachain exchange constant J. The calculations show that the excitations at the antiferromagnetic point q=1 and at q=0.5 remain resonant up to at least T=2J, confirming the recent experimental observation of resonant high-temperature domain wall excitations. The predicted first and second moments are in good agreement with experiment, except at temperatures where three-dimensional spin correlations are most important. The ratio of the structure factors at q=1 and at q=0.5 is well predicted for the paramagnetic infinite-temperature limit. For T=2J, however, we found that the experimentally observed intensity is considerably less than predicted. This suggests that domain wall excitations on different chains interact up to temperatures of the order of the spin band width.Comment: 9 pages revtex, submitted to PR

Teneurins in development and disease

The teneurins are a novel type II transmembrane protein family originally discovered in Drosophila and highly conserved between invertebrates and vertebrates. Studies in invertebrates suggest important functions for the teneurins in many processes during development. However, still very little is known about the biological function and mechanism of action of the vertebrate teneurin family, which consists of four paralogs called teneurin-1 to -4. In the first part of my thesis, I analyzed the expression pattern and signaling mechanism of teneurin-1 during chick development. Teneurin-1 was prominently expressed in specific regions of the brain, and often complementary to teneurin-2. The presence of teneurin-1 and -2 in interconnected regions of the brain implies a role for teneurins in the establishment of appropriate neuronal connectivity. Using a novel antibody recognizing the teneurin-1 intracellular domain (ICD), N-terminal processing products were detected and nuclear staining was observed in specific neurons and tissues. This provides evidence for our working hypothesis, according to which teneurins can be processed by a mechanism called regulated intramembrane proteolysis, resulting in the release and nuclear translocation of the ICD. Similar results were obtained for teneurin-4 during chick development. In the second part of my thesis, I investigated the implication of teneurins in two human diseases. X-linked mental retardation: Teneurin-1 was analyzed as an X-linked mental retardation (XLMR) candidate gene in 23 XLMR patients. No mutation that is likely to cause the disease was found the coding region or splice sites of the teneurin-1 gene in these patients. Brain tumors: Teneurin-4 was found to be upregulated in a microarray analysis of human brain tumors including astrocytomas, oligodendrogliomas and glioblastomas (GBMs). The overexpression of teneurin-4 was confirmed on protein level in brain tumor lysates. Immunohistochemistry (IHC) revealed strong staining around tumors cells in some brain tumors whereas in others teneurin-4 restricted to blood vessels

Substantially enhanced cloning efficiency of SAGE (Serial Analysis of Gene Expression) by adding a heating step to the original protocol

The efficiency of the original SAGE (Serial Analysis of Gene Expression) protocol was limited by a small average size of cloned concatemers. We describe a modification of the technique that overcomes this problem. Ligation of ditags yields concatemers of various sizes. Small concatemers may aggregate and migrate with large ones during gel electrophoresis. A heating step introduced before gel electrophoresis breaks such contaminating aggregates. This modification yields cloned concatemers with an average size of 67 tags as compared to 22 tags by the original protocol. It enhances the length of cloned concatemers substantially and reduces the costs of SAG

Temperature Evolution of the Quantum Gap in CsNiCl3

Neutron scattering measurements on the one-dimensional gapped S=1 antiferromagnet, CsNiCl3, have shown that the excitation corresponding to the Haldane mass gap Delta at low temperatures persists as a resonant feature to high temperatures. We find that the strong upward renormalisation of the gap excitation, by a factor of three between 5 and 70K, is more than enough to overcome its decreasing lifetime. We find that the gap lifetime is substantially shorter than that predicted by the scaling theory of Damle and Sachdev in its low temperature range of validity. The upward gap renormalisation agrees with the non-linear sigma model at low temperatures and even up to T of order 2Delta provided an upper mass cutoff is included.Comment: Latex, 3 figures, accepted by Pysical Review

Candidate Quantum Spin Liquid in the Ce\textsuperscript{3+} Pyrochlore Stannate Ce2_2Sn2_2O7_7

We report the low temperature magnetic properties of Ce$_2$Sn$_2$O$_7$, a rare-earth pyrochlore. Our susceptibility and magnetization measurements show that due to the thermal isolation of a Kramers doublet ground state, Ce$_2$Sn$_2$O$_7$ has Ising-like magnetic moments of $\sim1.18$ $\mu_\mathrm{B}$. The magnetic moments are confined to the local trigonal axes, as in a spin ice, but the exchange interactions are antiferromagnetic. Below 1 K the system enters a regime with antiferromagnetic correlations. In contrast to predictions for classical $\langle 111 \rangle$-Ising spins on the pyrochlore lattice, there is no sign of long-range ordering down to 0.02 K. Our results suggest that Ce$_2$Sn$_2$O$_7$ features an antiferromagnetic liquid ground state with strong quantum fluctuations.Comment: 6 pages, 4 figure

Comment on Ferroelectricity in Spiral Magnets

A Comment on the Letter by Maxim Mostovoy, [Phys. Rev. Lett. 96, 067601 (2006)]. The author of the Letter offers a Reply