Sinusoidal electromagnon in RMnO3: Indication of anomalous magnetoelectric coupling

The optical spectra in the family of multiferroic manganites RMnO3 is a great puzzle. Current models can not explain the fact that two strong electromagnons are present in the non-collinear spin cycloidal phase, with only one electromagnon surviving the transition into the collinear spin sinusoidal phase. We show that this is a signature of the presence of anomalous magnetoelectric coupling that breaks rotational invariance in spin space and generates oscillatory polarization in the ground state.Comment: 5 pages, 2 figure

Response functions of an artificial Anderson atom in the atomic limit

We consider the spin and pseudospin (charge) response functions of the exactly soluble Anderson atom model. We demonstrate, in particular, that a deviation from the magnetic Curie-law behaviour, appropriate for a free spin one-half, increases with increasing asymmetry and temperature. In general, oscillator strength is transferred from the spin degrees of freedom to the pseudospin modes. We also consider the negative-U Anderson atom and demonstrate that the pseudospin modes are the relevant low-energy excitations in this case. Especially, the roles of the spin and charge excitations are interchanged upon reversal of the intrasite Coulomb repulsion, U.Comment: 23 pages, 12 figures. Accepted for publication in J. Low Temp. Phy

Use of near infrared reflectance spectroscopy to predict nitrogen uptake by winter wheat within fields with high variability in organic matter

In this study, the ability to predict N-uptake in winter wheat crops using NIR-spectroscopy on soil samples was evaluated. Soil samples were taken in unfertilized plots in one winter wheat field during three years (1997-1999) and in another winter wheat field nearby in one year (2000). Soil samples were analyzed for organic C content and their NIR-spectra. N-uptake was measured as total N-content in aboveground plant materials at harvest. Models calibrated to predict N-uptake were internally cross validated and validated across years and across fields. Cross-validated calibrations predicted N-uptake with an average error of 12.1 to 15.4 kg N ha-1. The standard deviation divided by this error (RPD) ranged between 1.9 and 2.5. In comparison, the corresponding calibrations based on organic C alone had an error from 11.7 to 28.2 kg N ha-1 and RPDs from 1.3 to 2.5. In three of four annual calibrations within a field, the NIR-based calibrations worked better than the organic C based calibrations. The prediction of N-uptake across years, but within a field, worked slightly better with an organic C based calibration than with a NIR based one, RPD = 1.9 and 1.7 respectively. Across fields, the corresponding difference was large in favour of the NIR-calibration, RPD = 2.5 for the NIR-calibration and 1.5 for the organic C calibration. It was concluded that NIR-spectroscopy integrates information about organic C with other relevant soil components and therefore has a good potential to predict complex functions of soils such as N-mineralization. A relatively good agreement of spectral relationships to parameters related to the N-mineralization of datasets across the world suggests that more general models can be calibrated

Are pinholes the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic "pinholes" in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlOx/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlOx/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.Comment: 4 pages, 4 figure

Model for twin electromagnons and magnetically induced oscillatory polarization in multiferroic RMnO3_3

We propose a model for the pair of electromagnon excitations observed in the class of multiferroic materials {\it R}MnO$_3$ ({\it R} is a rare-earth ion). The model is based on a harmonic cycloid ground state interacting with a zone-edge magnon and its twin excitation separated in momentum space by two times the cycloid wave vector. The pair of electromagnons is activated by cross coupling between magnetostriction and spin-orbit interactions. Remarkably, the spectral weight of the twin electromagnon is directly related to the presence of a magnetically induced oscillatory polarization in the ground state. This leads to the surprising prediction that TbMnO$_3$ has an oscillatory polarization with amplitude 50 times larger than its uniform polarization.Comment: 4 pages, 3 figure

Higher-order mesoscopic fluctuations in quantum wires: Conductance and current cumulants

We study conductance cumulants $>$ and current cumulants $C_j$ related to heat and electrical transport in coherent mesoscopic quantum wires near the diffusive regime. We consider the asymptotic behavior in the limit where the number of channels and the length of the wire in the units of the mean free path are large but the bare conductance is fixed. A recursion equation unifying the descriptions of the standard and Bogoliubov--de Gennes (BdG) symmetry classes is presented. We give values and come up with a novel scaling form for the higher-order conductance cumulants. In the BdG wires, in the presence of time-reversal symmetry, for the cumulants higher than the second it is found that there may be only contributions which depend nonanalytically on the wire length. This indicates that diagrammatic or semiclassical pictures do not adequately describe higher-order spectral correlations. Moreover, we obtain the weak-localization corrections to $C_j$ with $j\le 10$.Comment: 7 page

Shot Noise in Mesoscopic Diffusive Andreev Wires

We study shot noise in mesoscopic diffusive wires between a normal and a superconducting terminal. We particularly focus on the regime, in which the proximity-induced reentrance effect is important. We will examine the difference between a simple Boltzmann-Langevin description, which neglects induced correlations beyond the simple conductivity correction, and a full quantum calculation. In the latter approach, it turns out that two Andreev pairs propagating coherently into the normal metal are anti-correlated for E<E_c, where E_c=D/L^2 is the Thouless energy. In a fork geometry the flux-sensitive suppression of the effective charge was confirmed experimentally.Comment: 12 pages, proceedings of the NATO ARW MQO, Bled, Sloveni

Nonlinear shot noise in mesoscopic diffusive normal-superconducting systems

We study differential shot noise in mesoscopic diffusive normal-superconducting (NS) heterostructures at finite voltages where nonlinear effects due to the superconducting proximity effect arise. A numerical scattering-matrix approach is adopted. Through an NS contact, we observe that the shot noise shows a reentrant dependence on voltage due to the superconducting proximity effect but the differential Fano factor stays approximately constant. Furthermore, we consider differential shot noise in the structures where an insulating barrier is formed between normal and superconducting regions and calculate the differential Fano factor as a function of barrier height.Comment: 4 pages, 6 figure