Hysteresis in the de Haas-van Alphen Effect

A hysteresis loop is observed for the first time in the de Haas-van Alphen (dHvA) effect of beryllium at low temperatures and quantizing magnetic field applied parallel to the hexagonal axis of the single crystal. The irreversible behavior of the magnetization occurs at the paramagnetic part of the dHvA period in conditions of Condon domain formation arising by strong enough dHvA amplitude. The resulting extremely nonlinear response to a very small modulation field offers the possibility to find in a simple way the Condon domain phase diagram. From a harmonic analysis, the shape and size of the hysteresis loop is constructed.Comment: 4 pages, 5 figures, submitted to PR

Atomic quasi-Bragg diffraction in a magnetic field

We report on a new technique to split an atomic beam coherently with an easily adjustable splitting angle. In our experiment metastable helium atoms in the |{1s2s}^3S_1 M=1> state diffract from a polarization gradient light field formed by counterpropagating \sigma^+ and \sigma^- polarized laser beams in the presence of a homogeneous magnetic field. In the near-adiabatic regime, energy conservation allows the resonant exchange between magnetic energy and kinetic energy. As a consequence, symmetric diffraction of |M=0> or |M=-1> atoms in a single order is achieved, where the order can be chosen freely by tuning the magnetic field. We present experimental results up to 6th order diffraction (24 \hbar k momentum splitting, i.e., 2.21 m/s in transverse velocity) and present a simple theoretical model that stresses the similarity with conventional Bragg scattering. The resulting device constitutes a flexible, adjustable, large-angle, three-way coherent atomic beam splitter with many potential applications in atom optics and atom interferometry.Comment: 4 pages, 5 figure

Helicoidal magnetic order in a clean copper oxide spin chain compound

We report susceptibility, specific heat, and neutron diffraction measurements on NaCu$_2$O$_2$, a spin-1/2 chain compound isostructural to LiCu$_2$O$_2$, which has been extensively investigated. Below 13 K, we find a long-range ordered, incommensurate magnetic helix state with a propagation vector similar to that of LiCu$_2$O$_2$. In contrast to the Li analogue, substitutional disorder is negligible in NaCu$_2$O$_2$. We can thus rule out that the helix is induced by impurities, as was claimed on the basis of prior work on LiCu$_2$O$_2$. A spin Hamiltonian with frustrated longer-range exchange interactions provides a good description of both the ordered state and the paramagnetic susceptibility.Comment: 4 pages, 4 figures Improved Fig.1 and 4. Minor rephrasing. Reference adde

Direct Observation of Condon Domains in Silver by Hall Probes

Using a set of micro Hall probes for the detection of the local induction, the inhomogeneous Condon domain structure has been directly observed at the surface of a pure silver single crystal under strong Landau quantization in magnetic fields up to 10 T. The inhomogeneous induction occurs in the theoretically predicted part of the H-T Condon domain phase diagram. Information about size, shape and orientation of the domains is obtained by analyzing Hall probes placed along and across the long sample axis and by tilting the sample. On a beryllium surface the induction inhomogeneity is almost absent although the expected induction splitting here is at least ten times higher than in silver.Comment: 4 pages, 6 figures, submitted to PR

An optimized chiral nucleon-nucleon interaction at next-to-next-to-leading order

We optimize the nucleon-nucleon interaction from chiral effective field theory at next-to-next- to-leading order. The resulting new chiral force NNLOopt yields \chi^2 \approx 1 per degree of freedom for laboratory energies below approximately 125 MeV. In the A = 3, 4 nucleon systems, the contributions of three-nucleon forces are smaller than for previous parametrizations of chiral interactions. We use NNLOopt to study properties of key nuclei and neutron matter, and demonstrate that many aspects of nuclear structure can be understood in terms of this nucleon-nucleon interaction, without explicitly invoking three-nucleon forces.Comment: 6 pages, 4 figure

Anomalous temperature evolution of the internal magnetic field distribution in the charge-ordered triangular antiferromagnet AgNiO2

Zero-field muon-spin relaxation measurements of the frustrated triangular quantum magnet AgNiO2 are consistent with a model of charge disproportionation that has been advanced to explain the structural and magnetic properties of this compound. Below an ordering temperature of T_N=19.9(2) K we observe six distinct muon precession frequencies, due to the magnetic order, which can be accounted for with a model describing the probable muon sites. The precession frequencies show an unusual temperature evolution which is suggestive of the separate evolution of two opposing magnetic sublattices.Comment: 4 pages, 3 figure

The Structure of the [Zn_In - V_P] Defect Complex in Zn Doped InP

We study the structure, the formation and binding energies and the transfer levels of the zinc-phosphorus vacancy complex [Zn_In - V_P] in Zn doped p-type InP, as a function of the charge, using plane wave ab initio DFT-LDA calculations in a 64 atom supercell. We find a binding energy of 0.39 eV for the complex, which is neutral in p-type material, the 0/-1 transfer level lying 0.50 eV above the valence band edge, all in agreement with recent positron annihilation experiments. This indicates that, whilst the formation of phosphorus vacancies (V_P) may be involved in carrier compensation in heavily Zn doped material, the formation of Zn-vacancy complexes is not. Regarding the structure: for charge states Q=+6 to -4 the Zn atom is in an sp^2 bonded DX position and electrons added/removed go to/come from the remaining dangling bonds on the triangle of In atoms. This reduces the effective vacancy volume monatonically as electrons are added to the complex, also in agreement with experiment. The reduction occurs through a combination of increased In-In bonding and increased Zn-In electrostatic attraction. In addition, for certain charge states we find complex Jahn-Teller behaviour in which up to three different structures, (with the In triangle dimerised, antidimerised or symmetric) are stable and are close to degenerate. We are able to predict and successfully explain the structural behaviour of this complex using a simple tight binding model.Comment: 10 pages text (postscript) plus 8 figures (jpeg). Submitted to Phys. Rev.

On One-Loop Gap Equations for the Magnetic Mass in d=3 Gauge Theory

Recently several workers have attempted determinations of the so-called magnetic mass of d=3 non-Abelian gauge theories through a one-loop gap equation, using a free massive propagator as input. Self-consistency is attained only on-shell, because the usual Feynman-graph construction is gauge-dependent off-shell. We examine two previous studies of the pinch technique proper self-energy, which is gauge-invariant at all momenta, using a free propagator as input, and show that it leads to inconsistent and unphysical result. In one case the residue of the pole has the wrong sign (necessarily implying the presence of a tachyonic pole); in the second case the residue is positive, but two orders of magnitude larger than the input residue, which shows that the residue is on the verge of becoming ghostlike. This happens because of the infrared instability of d=3 gauge theory. A possible alternative one-loop determination via the effective action also fails. The lesson is that gap equations must be considered at least at two-loop level.Comment: 21 pages, LaTex, 2 .eps figure

A background-priority discrete boundary triangulation method

Discrete boundary triangulation methods generate triangular meshes through the centers of the boundary voxels of a volumetric object. At some voxel configurations it may be arbitrary whether a part of the volume should be included in the object or could be classified as background. Consequently, important details such as concave and convex edges and corners are not consistently preserved in the describing geometry. We present a "background priority" version of an existing "object priority" algorithm [6]. We show that the ad hoc configurations of the well-known Discretized Marching Cubes algorithm [13] can be derived from our method and that a combined triangulation with "object priority" and "background priority" better would preserve object details

Balancing Bounded Treewidth Circuits

Algorithmic tools for graphs of small treewidth are used to address questions in complexity theory. For both arithmetic and Boolean circuits, it is shown that any circuit of size $n^{O(1)}$ and treewidth $O(\log^i n)$ can be simulated by a circuit of width $O(\log^{i+1} n)$ and size $n^c$, where $c = O(1)$, if $i=0$, and $c=O(\log \log n)$ otherwise. For our main construction, we prove that multiplicatively disjoint arithmetic circuits of size $n^{O(1)}$ and treewidth $k$ can be simulated by bounded fan-in arithmetic formulas of depth $O(k^2\log n)$. From this we derive the analogous statement for syntactically multilinear arithmetic circuits, which strengthens a theorem of Mahajan and Rao. As another application, we derive that constant width arithmetic circuits of size $n^{O(1)}$ can be balanced to depth $O(\log n)$, provided certain restrictions are made on the use of iterated multiplication. Also from our main construction, we derive that Boolean bounded fan-in circuits of size $n^{O(1)}$ and treewidth $k$ can be simulated by bounded fan-in formulas of depth $O(k^2\log n)$. This strengthens in the non-uniform setting the known inclusion that $SC^0 \subseteq NC^1$. Finally, we apply our construction to show that {\sc reachability} for directed graphs of bounded treewidth is in $LogDCFL$