Effect of Impurities and Effective Masses on Spin-Dependent Electrical Transport in Ferromagnet-Normal Metal-Ferromagnet Hybrid Junctions

The effect of nonmagnetic impurities and the effective masses on the spin-dependent transport in a ferromagnet-normal metal-ferromagnet junction is investigated on the basis of a two-band model. Our results show that impurities and the effective masses of electrons in two ferromagnetic electrodes have remarkable effects on the behaviors of the conductance, namely, both affect the oscillating amplitudes, periods, as well as the positions of the resonant peaks of the conductance considerably. The impurity tends to suppress the amplitudes of the conductance, and makes the spin-valve effect less obvious, but under certain conditions the phenomenon of the so-called impurity-induced resonant tunneling is clearly observed. The impurity and the effective mass both can lead to nonmonotonous oscillation of the junction magnetoresistance (JMR) with the incident energy and the thickness of the normal metal. It is also observed that a smaller difference of the effective masses of electrons in two ferromagnetic electrodes would give rise to a larger amplitude of the JMR.Comment: Revtex, 10 figure

Power loss and electromagnetic energy density in a dispersive metamaterial medium

The power loss and electromagnetic energy density of a metamaterial consisting of arrays of wires and split-ring resonators (SRRs) are investigated. We show that a field energy density formula can be derived consistently from both the electrodynamic (ED) approach and the equivalent circuit (EC) approach. The derivations are based on the knowledge of the dynamical equations of the electric and magnetic dipoles in the medium and the correct form of the power loss. We discuss the role of power loss in determining the form of energy density and explain why the power loss should be identified first in the ED derivation. When the power loss is negligible and the field is harmonic, our energy density formula reduces to the result of Landau's classical formula. For the general case with finite power loss, our investigation resolves the apparent contradiction between the previous results derived by the EC and ED approaches.Comment: 10 pages, 1 figure, Submitted to Phys. Rev.

Tunneling spectra of layered strongly correlated d-wave superconductors

Tunneling conductance experiments on cuprate superconductors exhibit a large diversity of spectra that appear in different nano-sized regions of inhomogeneous samples. In this letter, we use a mean-field approach to the tt't''J model in order to address the features in these spectra that deviate from the BCS paradigm, namely, the bias sign asymmetry at high bias, the generic lack of evidence for the Van Hove singularity, and the occasional absence of coherence peaks. We conclude that these features can be reproduced in homogeneous layered d-wave superconductors solely due to a proximate Mott insulating transition. We also establish the connection between the above tunneling spectral features and the strong renormalization of the electron dispersion around (0,pi) and (pi,0) and the momentum space anisotropy of electronic states observed in ARPES experiments.Comment: 4 pages, 3 figures. Added comment on the role of sample inhomogeneity. Published version. Homepage http://dao.mit.edu/~wen

Magnitude of Magnetic Field Dependence of a Possible Selective Spin Filter in ZnSe/Zn_{1-x}Mn_{x}Se Multilayer Heterostructure

Spin-polarized transport through a band-gap-matched ZnSe/Zn_{1-x}Mn_{x} Se/ZnSe/Zn_{1-x}Mn_{x}Se/ZnSe multilayer structure is investigated. The resonant transport is shown to occur at different energies for different spins owing to the split of spin subbands in the paramagnetic layers. It is found that the polarization of current density can be reversed in a certain range of magnetic field, with the peak of polarization moving towards a stronger magnetic field for increasing the width of central ZnSe layer while shifting towards an opposite direction for increasing the width of paramagnetic layer. The reversal is limited in a small-size system. A strong suppression of the spin up component of the current density is present at high magnetic field. It is expected that such a reversal of the polarization could act as a possible mechanism for a selective spin filter device

Effective Field Theory and Projective Construction for the Z_k Parafermion Fractional Quantum Hall States

The projective construction is a powerful approach to deriving the bulk and edge field theories of non-Abelian fractional quantum Hall (FQH) states and yields an understanding of non-Abelian FQH states in terms of the simpler integer quantum Hall states. Here we show how to apply the projective construction to the Z_k parafermion (Laughlin/Moore-Read/Read-Rezayi) FQH states, which occur at filling fraction \nu = k/(kM+2). This allows us to derive the bulk low energy effective field theory for these topological phases, which is found to be a Chern-Simons theory at level 1 with a U(M) \times Sp(2k) gauge field. This approach also helps us understand the non-Abelian quasiholes in terms of holes of the integer quantum Hall states.Comment: 7 page

Detection and characterization of Sp1 binding activity in human chondrocytes and its alterations during chondrocyte dedifferentiation.

We have detected DNA binding activity for a synthetic oligonucleotide containing an Sp1 consensus sequence in nuclear extracts from human chondrocytes. Changes in the levels of Sp1 oligonucleotide binding activity were examined in nuclear extracts from freshly isolated human chondrocytes, from chondrocytes that had been cultured under conditions that allowed the maintenance of a chondrocyte-specific phenotype on plastic dishes coated with the hydrogel poly(2-hydroxyethyl methacrylate), and from chondrocytes induced to dedifferentiate into fibroblast-like cells by passage in monolayer culture on plastic substrata. It was observed that Sp1 binding was 2-3-fold greater in nuclear extracts from dedifferentiated chondrocytes than in nuclear extracts from either freshly isolated chondrocytes or from cells cultured in suspension. The Sp1 binding activity was specific, since it was competed by unlabeled Sp1 but not by AP1 or AP2. The addition of a polyclonal antibody against Sp1 to nuclear extracts from freshly isolated chondrocytes or to extracts isolated from chondrocytes cultured in monolayer decreased the binding of Sp1 by approximately 85%. However, when the same experiment was carried out with nuclear extracts prepared from cells cultured on poly(2-hydroxyethyl methacrylate)-coated plates, only a very slight inhibition of Sp1 binding was observed. When fragments of the COL2A1 promoter containing putative Sp1 binding sites amplified by polymerase chain reaction were examined, it was found that the amounts of DNA-protein complex formed with nuclear extracts from dedifferentiated chondrocytes were 2-3-fold greater than the amounts formed with nuclear extracts from freshly isolated chondrocytes or from cells cultured in suspension. Quantitation of DNA binding activity by titration experiments demonstrated that nuclear extracts from fibroblast-like cells contained approximately 2-fold greater Sp-1 specific binding activity than nuclear extracts from chondrocytes. The direct role of Sp1 in type II collagen gene transcription was demonstrated by co-transfection experiments of COL2A1 promoter-CAT constructs in Drosophila Schneider line L2 cells that lack Sp1 homologs. This is the first demonstration of Sp1 binding activity in human chondrocytes and of differences in Sp1 DNA binding activity between differentiated and dedifferentiated chondrocytes

Thermal rectifier from deformed carbon nanohorns

We study thermal rectification in single-walled carbon nanohorns (SWNHs) by using non-equilibrium molecular dynamics (MD) method. It is found that the horns with the bigger top angles show larger asymmetric heat transport due to the larger structural gradient distribution. This kind of gradient behavior can be further adjusted by applying external strain on the SWNHs. After being carefully elongated along the axial direction, the thermal rectification in the elongated SWNHs can become more obvious than that in undeformed ones. The maximum rectification efficiency of SWNHs is much bigger than that of carbon nanotube intramolecular junctions.Comment: 3 figure