Quantum fluctuations in the cohesive force of metallic nanowires

Based on the recent free electron model for cohesion in narrow metallic constrictions by Stafford et al., we calculate the quantum fluctuations in the cohesive force versus elongation. The fluctuations are dominated by states near the Fermi energy, thus explaining their apparently universal magnitude of order epsilon_F/lambda_F. We present numerical results for the force fluctuations in a simple geometry and show that they are well described by the contributions of a few classical periodic orbits in the Balian-Bloch trace formula for the density of states of transverse motion.Comment: 4 pages, 2 figures, RevTeX, minor changes, to appear in Phys. Rev. B Rapid. Com

Effects of organic farming on weed abundance - long-term results from a site in northern Germany

One of the aims of organic farming systems is to maintain or enhance the agricultural and natural biodiversity in agro-ecosystems. Weeds can contribute to biodiversity but also need to be controlled. Compared to conventional farming the weed density and diversity in organic farming is expected to be enhanced because of reduced fertilisation and low efficacy of control measures. It is also assumed that the diversity of crops, especially alternation of summer and winter crops, results in a greater range of weed species. However, relatively few data are available confirming this benefit of organic farming. Thus, continuous monitoring studies have been started in 1996 on a trial area for organic farming in northern Germany

Critical Fields of mesoscopic superconductors

Recent measurements have shown oscillations in the upper critical field of simply connected mesoscopic superconductors. A quantitative theory of these effects is given here on the basis of a Ginzburg-Landau description. For small fields, the $H-T$ phase boundary exhibits a cusp where the screening currents change sign for the first time thus defining a lower critical field $H_{c1}$. In the limit where many flux quanta are threading the sample, nucleation occurs at the boundary and the upper critical field becomes identical with the surface critical field $H_{c3}$.Comment: 5 pages (Revtex and 2 PostScript figures), to apppear in Z. Phys.

Theory of RF-spectroscopy of strongly interacting Fermions

We show that strong pairing correlations in Fermi gases lead to the appearance of a gap-like structure in the RF-spectrum, both in the balanced superfluid and in the normal phase above the Clogston-Chandrasekhar limit. The average RF-shift of a unitary gas is proportional to the ratio of the Fermi velocity and the scattering length with the final state. In the strongly imbalanced case, the RF-spectrum measures the binding energy of a minority atom to the Fermi sea of majority atoms. Our results provide a qualitative understanding of recent experiments by Schunck et.al.Comment: revised version, 4 pages, 3 figures, RevTex

Higgs Mode and Magnon Interactions in 2D Quantum Antiferromagnets from Raman Scattering

We present a theory for Raman scattering on 2D quantum antiferromagnets. The microscopic Fleury-Loudon Hamiltonian is expressed in terms of an effective $O(3)$ - model. Well within the N\'eel ordered phase, the Raman spectrum contains a two-magnon and a two-Higgs contribution, which are calculated diagramatically. The vertex functions for both the Higgs and magnon contributions are determined from a numerical solution of the corresponding Bethe-Salpeter equation. Due to the momentum dependence of the Raman vertex in the relevant $B_{1g}+E_{2g}$ symmetry, the contribution from the Higgs mode is strongly suppressed. Except for intermediate values of the Higgs mass, it does not show up as separate peak in the spectrum but gives rise to a broad continuum above the dominant contribution from two-magnon excitations. The latter give rise to a broad, asymmetric peak at $\omega\simeq 2.44\, J$, which is a result of magnon-magnon interactions mediated by the Higgs mode. The full Raman spectrum is determined completely by the antiferromagnetic exchange coupling $J$ and a dimensionless Higgs mass. Experimental Raman spectra of undoped cuprates turn out to be in very good agreement with the theory only with inclusion of the Higgs contribution. They thus provide a clear signature of the presence of a Higgs mode in spin one-half 2D quantum antiferromagnets.Comment: 12 pages, 15 figure