Topological Hall effect and Berry phase in magnetic nanostructures

We discuss the anomalous Hall effect in a two-dimensional electron gas subject to a spatially varying magnetization. This topological Hall effect (THE) does not require any spin-orbit coupling, and arises solely from Berry phase acquired by an electron moving in a smoothly varying magnetization. We propose an experiment with a structure containing 2D electrons or holes of diluted magnetic semiconductor subject to the stray field of a lattice of magnetic nanocylinders. The striking behavior predicted for such a system (of which all relevant parameters are well known) allows to observe unambiguously the THE and to distinguish it from other mechanisms.Comment: 5 pages with 4 figure

ac Josephson effect in asymmetric superconducting quantum point contacts

We investigate ac Josephson effects between two superconductors connected by a single-mode quantum point contact, where the gap amplitudes in the two superconductors are unequal. In these systems, it was found in previous studies on the dc effects that, besides the Andreev bound-states, the continuum states can also contribute to the current. Using the quasiclassical formulation, we calculate the current-voltage characteristics for general transmission $D$ of the point contact. To emphasize bound versus continuum states, we examine in detail the low bias, ballistic (D=1) limit. It is shown that in this limit the current-voltage characteristics can be determined from the current-phase relation, if we pay particular attention to the different behaviors of these states under the bias voltage. For unequal gap configurations, the continuum states give rise to non-zero sine components. We also demonstrate that in this limit the temperature dependence of the dc component follows $\tanh(\Delta_s/2T)$, where $\Delta_s$ is the smaller gap, with the contribution coming entirely from the bound state.Comment: To appear in PR

Harmonic lattice behavior of two-dimensional colloidal crystals

Using positional data from video-microscopy and applying the equipartition theorem for harmonic Hamiltonians, we determine the wave-vector-dependent normal mode spring constants of a two-dimensional colloidal model crystal and compare the measured band-structure to predictions of the harmonic lattice theory. We find good agreement for both the transversal and the longitudinal mode. For $q\to 0$, the measured spring constants are consistent with the elastic moduli of the crystal.Comment: 4 pages, 3 figures, submitte

Age-Related Changes in Proximal Humerus Bone Health in White Males

poster abstractThe proximal humerus is a common site for osteoporotic fracture during aging, accounting for up to 5% of fractures to the appendicular skeleton. While falls onto an outstretched hand are usually physically responsible for proximal humerus fractures, the ability of the underlying bone to resist applied loads must also play a role. Few studies have assessed proximal humerus bone health with aging. The aim of the current study was to explore age-related bone changes at the proximal humerus in men. A cross-sectional study design was used to assess peripheral quantitative computed tomography (pQCT)-derived bone properties of the proximal humerus in a cohort of 112 white males (age range = 30-85 yrs). A tomographic slice of the non-dominant upper extremity was acquired at 80% of humeral length proximal from its distal end—a location corresponding to the surgical neck of the humerus. Images were assessed for cortical (Ct.BMC) and trabecular (Tb.BMC) BMC, total (Tt.Ar), cortical (Ct.Ar) and medullary (Me.Ar) area, periosteal (Ps.Pm) and endosteal (Es.Pm) perimeter, cortical thickness (Ct.Th), and bone strength index for compression (BSIc). BSIc was calculated as the product of Tt.Ar and the square of total volumetric BMD. Data were plotted against age and linear regression lines assessed for their slope. Slopes were subsequently converted to percent change in the bone property per year. During aging, the proximal humerus expanded with Tt.Ar and Ps.Pm increasing at rates of 0.40%/yr and 0.19%/yr, respectively. However, Me.Ar (0.62%/yr) and Es.Pm (0.34%/yr) expanded at faster rates such that there was net loss of both Ct.BMC (-0.23%/yr) and Tb.BMC (-1.08%/yr). Also, the more rapid expansion of Me.Ar relative to Tt.Ar meant that Ct.Ar (-0.15%/yr) and Ct.Th (-0.34%/yr) both decreased with age. The net result of these mass and structural changes was progressive loss of bone strength with age, as indicated by a 0.44%/yr decline in BSIc. These data provide a picture of bone changes at the proximal humerus during aging. They suggest that between age 30 and 80 yrs, approximately 54% and 11% of Tb.BMC and Ct.BMC at the proximal humerus is lost, respectively. They also suggest that compressive strength of the proximal humerus declines by 22% between age 30 and 80 years. These declines in proximal humerus bone health have implications for fracture risk at this location during aging

ac Josephson effect in superconducting d-wave junctions

We study theoretically the ac Josephson effect in superconducting planar d-wave junctions. The insulating barrier assumed to be present between the two superconductors may have arbitrary strength. Many properties of this system depend on the orientation of the d-wave superconductor: we calculate the ac components of the Josephson current. In some arrangements there is substantial negative differential conductance due to the presence of mid-gap states. We study how robust these features are to finite temperature and also comment on how the calculated current-voltage curves compare with experiments. For some other configurations (for small barrier strength) we find zero-bias conductance peaks due to multiple Andreev reflections through midgap states. Moreover, the odd ac components are strongly suppressed and even absent in some arrangements. This absence will lead to a doubling of the Josephson frequency. All these features are due to the d-wave order parameter changing sign when rotated $90^{\circ}$. Recently, there have been several theoretical reports on parallel current in the d-wave case for both the stationary Josephson junction and for the normal metal-superconductor junction. Also in our case there may appear current density parallel to the junction, and we present a few examples when this takes place. Finally, we give a fairly complete account of the method used and also discuss how numerical calculations should be performed in order to produce current-voltage curves

Quantity and Distribution of Salmonella Recovered from Three Swine Lairage Pens

The quantity of Salmonella recoverable from three lairage pens in a swine abattoir was determined. Using dry four-ply cotton gauze pads measuring 10 by 10 cm, 100 fecal slurry samples were collected from each of the three pens. Salmonella recovery was expressed as the log CFU per milliliter of sample. Mean values were 2.5 log CFU/ml in pen A, 2.7 log CFU/ ml in pen B, and 0.89 log CFU/ml in pen C. Median values were 2.6 log CFU/ml in pen A, 2.0 log CFU/ml in pen B, and below the detectable limit in pen C. In pen C, Salmonella was not recoverable from a high number of samples. Pen B results suggested spatial dependency, i.e., samples close together were more similar than samples farther apart. These results indicate that Salmonella concentrations vary within and between lairage pens. Because of the limited number of pens assessed, it was not possible to identify factors that were associated with the observed variation in Salmonella concentrations within and between pens. However, this variation suggests that numerous samples are required to adequately describe the concentration of Salmonella in a lairage pen

Conjugacy of one-dimensional one-sided cellular automata is undecidable

Two cellular automata are strongly conjugate if there exists a shift-commuting conjugacy between them. We prove that the following two sets of pairs $(F,G)$ of one-dimensional one-sided cellular automata over a full shift are recursively inseparable: (i) pairs where $F$ has strictly larger topological entropy than $G$, and (ii) pairs that are strongly conjugate and have zero topological entropy. Because there is no factor map from a lower entropy system to a higher entropy one, and there is no embedding of a higher entropy system into a lower entropy system, we also get as corollaries that the following decision problems are undecidable: Given two one-dimensional one-sided cellular automata $F$ and $G$ over a full shift: Are $F$ and $G$ conjugate? Is $F$ a factor of $G$? Is $F$ a subsystem of $G$? All of these are undecidable in both strong and weak variants (whether the homomorphism is required to commute with the shift or not, respectively). It also immediately follows that these results hold for one-dimensional two-sided cellular automata.Comment: 12 pages, 2 figures, accepted for SOFSEM 201

Interplay between carrier and impurity concentrations in annealed Ga1−x_{1-x}Mnx_{x}As intrinsic anomalous Hall Effect

Investigating the scaling behavior of annealed Ga$_{1-x}$Mn$_{x}$As anomalous Hall coefficients, we note a universal crossover regime where the scaling behavior changes from quadratic to linear, attributed to the anomalous Hall Effect intrinsic and extrinsic origins, respectively. Furthermore, measured anomalous Hall conductivities when properly scaled by carrier concentration remain constant, equal to theoretically predicated values, spanning nearly a decade in conductivity as well as over 100 K in T$_{C}$. Both the qualitative and quantitative agreement confirms the validity of new equations of motion including the Berry phase contributions as well as tunablility of the intrinsic anomalous Hall Effect.Comment: 4 pages, 5 figure

Hamiltonian approach to the ac Josephson effect in superconducting-normal hybrid systems

The ac Josephson effect in hybrid systems of a normal mesoscopic conductor coupled to two superconducting (S) leads is investigated theoretically. A general formula of the ac components of time-dependent current is derived which is valid for arbitrary interactions in the normal region. We apply this formula to analyze a S-normal-S system where the normal region is a noninteracting single level quantum dot. We report the physical behavior of time-averaged nonequilibrium distribution of electrons in the quantum dot, the formation of Andreev bound states, and ac components of the time-dependent current. The distribution is found to exhibit a population inversion; and all Andreev bound states between the superconducting gap $\Delta$ carry the same amount of current and in the same flow direction. The ac components of time-dependent current show strong oscillatory behavior in marked contrast to the subharmonic gap structure of the average current.Comment: 23 pages, 10 figures, LaTe

Structural and electronic properties of Pb1-xCdxTe and Pb1-xMnxTe ternary alloys

A systematic theoretical study of two PbTe-based ternary alloys, Pb1-xCdxTe and Pb1-xMnxTe, is reported. First, using ab initio methods we study the stability of the crystal structure of CdTe - PbTe solid solutions, to predict the composition for which rock-salt structure of PbTe changes into zinc-blende structure of CdTe. The dependence of the lattice parameter on Cd (Mn) content x in the mixed crystals is studied by the same methods. The obtained decrease of the lattice constant with x agrees with what is observed in both alloys. The band structures of PbTe-based ternary compounds are calculated within a tight-binding approach. To describe correctly the constituent materials new tight-binding parameterizations for PbTe and MnTe bulk crystals as well as a tight-binding description of rock-salt CdTe are proposed. For both studied ternary alloys, the calculated band gap in the L point increases with x, in qualitative agreement with photoluminescence measurements in the infrared. The results show also that in p-type Pb1-xCdxTe and Pb1-xMnxTe mixed crystals an enhancement of thermoelectrical power can be expected.Comment: 10 pages, 13 figures, submitted to Physical Review