T. Y. Chen

A BCS-like gap in the superconductor SmFeAsO0.85F0.15

Since the discovery of superconductivity in the high-transition-temperature (high-Tc) copper oxides two decades ago, it has been firmly established that the CuO2 plane is essential for superconductivity and gives rise to a host of other very unusual properties. A new family of superconductors with the general composition of LaFeAsO1-xFx has recently been discovered and the conspicuous lack of the CuO2 planes raises the tantalizing question of a different pairing mechanism in these oxypnictides. The superconducting gap (its magnitude, structure, and temperature dependence) is intimately related to pairing. Here we report the observation of a single gap in the superconductor SmFeAsO0.85F0.15 with Tc = 42 K as measured by Andreev spectroscopy. The gap value of 2Δ = 13.34 ± 0.3 meV gives 2Δ/kBTc = 3.68 (where kB is the Boltzmann constant), close to the Bardeen–Cooper–Schrieffer (BCS) prediction of 3.53. The gap decreases with temperature and vanishes at Tc in a manner consistent with the BCS prediction, but dramatically different from that of the pseudogap behaviour in the copper oxide superconductors. Our results clearly indicate a nodeless gap order parameter, which is nearly isotropic in size across different sections of the Fermi surface, and are not compatible with models involving antiferromagnetic fluctuations, strong correlations, the t-J model, and the like, originally designed for the high-Tc copper oxides.

Spin polarization of amorphous CoFeB determined by point-contact Andreev reflection

Point-contact Andreev reflection measurements reveal that amorphous CoxFe80−xB20 (x=20, 40, and 60) alloys possess spin polarization of as much as 65%, much higher than the values of 43%–45% for Co and Fe. This accounts for the high magnetoresistance values in magnetic tunnel junctions incorporating amorphous CoFeB as the ferromagnetic electrodes. The crystallization of the amorphous alloys substantially reduces the spin polarization.

Optical and electrical properties of Zn1−xCdxO films grown on Si substrates by reactive radio-frequency magnetron sputtering

Zn1−xCdxO films (0⩽x⩽0.179) were grown on Si (001) substrates at 750°C with a radio-frequency reactive magnetron sputtering method. Difference between the photoluminescence (PL) spectra taken at room temperature (RT) and at 12K is reported and is deduced to be the result of PL emission from the ZnCdO phases with wurtzite and zinc blende structures. It is also found that the RT PL intensity is in inverse proportion to the carrier concentration in the films. Cd incorporation results in the transform of conductivity from p type to n type and a decrease of carrier mobility.

Switching by point-contact spin injection in a continuous film

Spin-polarized currents injected through a point-contact into a continuous Co/Cu/Co trilayer film can reversibly switch the magnetization of small magnetic bits in the top Co layer. The magnetic states written depend on the polarity of the injection currents, and remain stable at room temperature. The reversible writing can be achieved for a wide range of contact resistances with a well-defined voltage for the reversal.

Enhanced Curie temperature and spin polarization in Mn4FeGe3

Intermetallic ferromagnetic compound Mn5Ge3, with Curie temperature TC=296K, a spin polarization of P=42%, and a good lattice match to semiconductors, is a potential spin injector for spintronics. We report the enhanced magnetic properties and spin polarization of Mn4FeGe3. By replacing one Mn atom in Mn5Ge3 with Fe, the TC of Mn4FeGe3 has been enhanced to 319.50K, while still maintaining the same crystal structure, high electrical conductivity, and a high residual resistivity ratio of 5.33. More importantly, the spin polarization in excess of 60% of Mn4FeGe3, much higher than that of Mn5Ge3, has been realized.

Sulfur stoichiometry effects in highly spin polarized CoS2 single crystals

Recent experiments on polycrystalline Co1−xFexS2 demonstrated composition control over the spin polarization by Fermi level manipulation. We report here the growth and characterization of CoS2 single crystals with fine control over the stoichiometry by chemical vapor transport. At the ideal Co:S atomic ratio we observe a minimum in the low temperature resistivity and the x-ray rocking curve width, coincident with a maximum in the residual resistivity ratio and the low temperature magnetoresistance. Point contact Andreev reflection on stoichiometric crystals indicates a spin polarization at the Fermi energy of 64%, a significant increase over the 56% observed in polycrystals.

Above 400-K robust perpendicular ferromagnetic phase in a topological insulator

Topological surface states reveal proximity-induced ferromagnetism with perpendicular anisotropy persisting above 400 K. The quantum anomalous Hall effect (QAHE) that emerges under broken time-reversal symmetry in topological insulators (TIs) exhibits many fascinating physical properties for potential applications in nanoelectronics and spintronics. However, in transition metal–doped TIs, the only experimentally demonstrated QAHE system to date, the QAHE is lost at practically relevant temperatures. This constraint is imposed by the relatively low Curie temperature (Tc) and inherent spin disorder associated with the random magnetic dopants. We demonstrate drastically enhanced Tc by exchange coupling TIs to Tm3Fe5O12, a high-Tc magnetic insulator with perpendicular magnetic anisotropy. Signatures showing that the TI surface states acquire robust ferromagnetism are revealed by distinct squared anomalous Hall hysteresis loops at 400 K. Point-contact Andreev reflection spectroscopy confirms that the TI surface is spin-polarized. The greatly enhanced Tc, absence of spin disorder, and perpendicular anisotropy are all essential to the occurrence of the QAHE at high temperatures.

NMMI: A Mass Compactness Measure for Spatial Pattern Analysis of Areal Features

Spatial pattern analysis plays an important role in geography for understanding geographical phenomena, identifying causes, and predicting future trends. Traditional pattern analysis tools assess cluster or dispersed patterns of geographical features based on the distribution of nonspatial attributes. These metrics ignore the shape of spatial objects—a critical consideration. The study of shape analysis, on the other hand, measures the compactness, elongation, or convexity of an areal feature based merely on geometry, without considering patterns of its attribute distribution. This article reports our efforts in developing a new pattern analysis method called the normalized mass moment of inertia (NMMI) that integrates both shape and nonspatial attributes into the analysis of compactness patterns. The NMMI is based on a well-known concept in physics—the mass moment of inertia—and is capable of detecting the degree of concentration or diffusion of some continuous attribute on an areal feature. We termed this the mass compactness. This measure can be reduced to a shape compactness measure when the attribute is evenly distributed on the feature. We first describe the theoretical model of the NMMI and its computation and then demonstrate its good performance through a series of experiments. We further discuss potentially broad applications of this approach in the contexts of urban expansion and political districting. In the political districting context, higher NMMI of a congressional district suggests a lower degree of gerrymander and vice versa. This work makes an original and unique contribution to spatial pattern and shape analysis by introducing this new, effective, and efficient measure of mass compactness that accounts for both geometric and spatial distribution.

Current-induced switching in a single exchange-biased ferromagnetic layer

We demonstrate current-induced switching effects in a single exchanged-biased ferromagnetic layer. A nanodomain can be switched within the ferromagnetic layer by a spin-polarized current injected through a point contact. The high resistance of the hysteretic switching is due to the formation of a domain wall between the nanodomain and the rest of the layer. The switching behavior observed in a single layer is a type of spin-transfer torque effect which is the inverse effect of domain-wall magnetoresistance. At room temperature, nonhysteretic switching behavior with a broad switching current density range is observed.

Fabrication of highly spin-polarized Co2FeAl0.5Si0.5 thin-films

Ferromagnetic Heusler Co2FeAl0.5Si0.5 epitaxial thin-films have been fabricated in the L21 structure with saturation magnetizations over 1200 emu/cm3. Andreev reflection measurements show that the spin polarization is as high as 80% in samples sputtered on unheated MgO (100) substrates and annealed at high temperatures. However, the spin polarization is considerably smaller in samples deposited on heated substrates.