Jeff M. Byers

Spin diffusion in semiconductors

The behavior of spin diffusion in doped semiconductors is shown to be qualitatively different than in undoped (intrinsic) ones. Whereas a spin packet in an intrinsic semiconductor must be a multiple-band disturbance, involving inhomogeneous distributions of both electrons and holes, in a doped semiconductor a single-band disturbance is possible. For n-doped nonmagnetic semiconductors the enhancement of diffusion due to a degenerate electron sea in the conduction band is much larger for these single-band spin packets than for charge packets-this explains the anomalously large spin diffusion recently observed in n-doped GaAs at 1.6 K. In n-doped ferromagnetic and semimagnetic semiconductors the motion of spin packets polarized antiparallel to the equilibrium carrier spin polarization is predicted to be an order of magnitude faster than for parallel polarized spin packets. These results are reversed for p-doped semiconductors.

Probing spatial correlations with nanoscale two-contact tunneling.

Interference effects on the transport through two localized tunnel junctions on the surface of a well-grounded sample reveal intrinsic spatial correlations characteristic of the uncoupled sample. Differential conductances of the two-junction probe are related to the spatial correlations of both normal and superconducting samples. For a superconducting sample the gap anisotropy strongly affects the results. This may serve as a sensitive probe of the order parameter in high-temperature superconductors.

Effects of defects on magnetoresistivity in La0.7Sr0.3MnO3

The effects of structural and oxygen site defects on the ferromagnetic phase transition and magnetotransport in doped lanthanum manganite films have been examined. Oxygen defects were introduced through a vacuum annealing process while structural defects were introduced using ion irradiation. The introduction of both defect types strongly suppressed the Curie temperature Tc while increasing the peak resistivity, activation energy, and magnetoresistance ratio. For defect types leading to similar reductions in Tc, structural defects lead to a broader transition regime and a smaller MRR than are found for oxygen defects. Well above Tc both defect types lead to nearly identical resistivity curves. Structural defects are argued to primarily affect the carrier mobility, hence these data provide a clue that the carrier mobility is dominating the transport properties in this regime. The effects of structural and oxygen site defects on the ferromagnetic phase transition and magnetotransport in doped lanthanum manganite films have been examined. Oxygen defects were introduced through a vacuum annealing process while structural defects were introduced using ion irradiation. The introduction of both defect types strongly suppressed the Curie temperature Tc while increasing the peak resistivity, activation energy, and magnetoresistance ratio. For defect types leading to similar reductions in Tc, structural defects lead to a broader transition regime and a smaller MRR than are found for oxygen defects. Well above Tc both defect types lead to nearly identical resistivity curves. Structural defects are argued to primarily affect the carrier mobility, hence these data provide a clue that the carrier mobility is dominating the transport properties in this regime.

A New Methodology for Quantitative LSPR Biosensing and Imaging

A new quantitative analysis methodology for localized surface plasmon resonance (LSPR) biosensing which determines surface-receptor fractional occupancy, as well as an LSPR imaging technique for the spatiotemporal mapping of binding events, is presented. Electron beam nanolithography was used to fabricate 20 × 20 arrays of gold nanostructures atop glass coverslips. A single biotinylated array was used to measure the association kinetics of neutravidin to the surface by spectroscopically determining the fractional occupancy as a function of time. By regenerating the same array, a reliable comparison of the kinetics could be made between control samples and neutravidin concentrations ranging from 1 μM to 50 nM. CCD-based imagery of the array, taken simultaneously with the spectroscopic measurements, reveals the binding of neutravidin to the surface as manifested by enhanced scattering over the majority of the resonance peak. The temporal resolution of the LSPR imaging technique was 200 ms and the spatial resolution was 8 μm(2).

SUPPRESSION OF SUPERCONDUCTIVITY BY INJECTION OF SPIN-POLARIZED CURRENT

A spin polarized current has been injected into high Tc superconductors resulting in a significant reduction in the superconductor’s critical current. Such injection may serve as the basis of a new class of superconducting devices for control, switching and amplification. Preliminary results using both Permalloy and CMR materials as injectors are presented.

LOCAL ELECTRONIC STRUCTURE OF A SINGLE MAGNETIC IMPURITY IN A SUPERCONDUCTOR

We present the first three-dimensional, self-consistent calculation of the electronic structure near a strong classical magnetic impurity in a superconductor. Localized excited states are found within the energy gap which are half electron and half hole. The spatial structure of the positive-frequency (electronlike) spectral weight (or local density of states) can differ strongly from that of the negative frequency (holelike) spectral weight. The effect of the impurity on the continuum states above the energy gap is calculated with good spectral resolution for the first time. For sufficiently strong impurity potentials, the order parameter may change sign at the impurity site. [S0031-9007(97)03084-6] PACS numbers: 74.80. ‐ g, 74.25.Ha, 61.16.Ch Magnetic impurities have a dramatic effect on superconductivity. Most works on the experimental and theoretical effects of magnetic impurities have focused on bulk thermodynamic quantities, such as the reduction of the superconducting transition temperature Tc with increasing magnetic impurity concentration [1]. Theoretical approaches to treat the thermodynamic effects of impurities include Born scattering (Abrikosov-Gor’kov theory) [2] and approximate solutions to all orders in the impurity potential for classical [3 ‐ 5] and quantum [6,7] spins. A key issue addressed in Ref. [3] was the evolution of localized excited states (not obtainable in Born theory) around impurities into an impurity band, eventually leading to gapless superconductivity at high enough impurity concentrations. The effects of impurities on bulk properties have also been treated within a strong-coupling formalism (e.g., Ref. [8]), but only very recently self-consistently and beyond the Born approximation [7]. Concern about bulk properties in the above and related work did not extend to properties very near the impurity. Among the first properties calculated in the vicinity of an impurity were the structures of screening clouds around a charged impurity [9,10] and a magnetic impurity [10,11] in a superconductor (characterized by exponentially decaying Friedel-like oscillations). The oscillation of the order parameter around a magnetic impurity was first evaluated without self-consistency [12,13]. A self-consistent calculation of the order parameter at the impurity and very far away for weak impurity potentials was done by Schlottmann [14]. Interest in local properties near impurities has been revived by advances in scanning tunneling microscopy (STM) near impurities embedded in a metallic medium [15]. The differential conductivity measured by a STM at a point x, voltage V , and temperature T can be related to the local density of states (LDOS) at the tip location as follows: dIsx, V , T d

Quantitative imaging of protein secretions from single cells in real time.

Protein secretions from individual cells create spatially and temporally varying concentration profiles in the extracellular environment, which guide a wide range of biological processes such as wound healing and angiogenesis. Fluorescent and colorimetric probes for the detection of single cell secretions have time resolutions that range from hours to days, and as a result, little is known about how individual cells may alter their protein secretion rates on the timescale of minutes or seconds. Here, we present a label-free technique based upon nanoplasmonic imaging, which enabled the measurement of individual cell secretions in real time. When applied to the detection of antibody secretions from single hybridoma cells, the enhanced time resolution revealed two modes of secretion: one in which the cell secreted continuously and another in which antibodies were released in concentrated bursts that coincided with minute-long morphological contractions of the cell. From the continuous secretion measurements we determined the local concentration of antibodies at the sensing array closest to the cell and from the bursts we estimated the diffusion constant of the secreted antibodies through the extracellular media. The design also incorporates transmitted light and fluorescence microscopy capabilities for monitoring cellular morphological changes and intracellular fluorescent labels. We anticipate that this technique can be adapted as a general tool for the quantitative study of paracrine signaling in both adherent and nonadherent cell lines.

Optimizing the two-coil mutual inductance measurement of the superconducting penetration depth in thin films

When a pair of coils is positioned on opposite sides of a superconducting thin film, measurement of their mutual inductance may in principle be used to infer the penetration depth λ in the superconductor. We have studied how to optimize this measurement with respect to coil design, and have found that the approach that has been generally used is far from the optimum. Useful simplifications to the expression relating mutual inductance to penetration depth are derived. An analysis of the sources of uncertainty in determining λ is presented. For an optimized coil set, the major source of uncertainty often is uncertainty of the thickness of the film. The sensitivity to changes in λ is also studied; it is shown that this can approach 1 pm for a typical high temperature superconductor sample. Finally, it is shown that the analysis may be extended to normal metal films, with the skin depth playing a role similar to that of the penetration depth in superconductors. Measurement of a high conductivity normal metal ...

Magnetic and transport properties of radiation damaged La0.7Ca0.3MnO3.0 thin films

Magnetization and resistivity measurements are reported for a series of radiation damaged La0.7Ca0.3MnO3 pulsed laser deposited thin films. When plotted as a function of activation energy, trends in the electrical transport properties are similar to those exhibited in the magnetic properties. A sharp drop in both Tc and Tp in samples with activation energies greater than ∼110 meV suggests a “decoupling” of the magnetic and transport properties. The results suggest the magnetic order is no longer sufficient to delocalize the system of the extra disorder induced by the radiation damage.

Local electronic structure of defects in superconductors

The electronic structure near defects (such as impurities) in superconductors is explored using a fully self-consistent technique. This technique exploits the short-range nature of the impurity potential and of the induced change in the superconducting order parameter to calculate features in the electronic structure down to the atomic scale with unprecedented spectral resolution. Magnetic and nonmagnetic static impurity potentials are considered, as well as local alterations in the pairing interaction. Extensions to strong-coupling superconductors and superconductors with anisotropic order parameters are formulated. {copyright} {ital 1997} {ital The American Physical Society}