Cong Ren

Ferromagnetism and magnetoresistance of Co–ZnO inhomogeneous magnetic semiconductors

Co–ZnO inhomogeneous magnetic semiconductor thin films were synthesized on the subnanometer scale by sputtering. Room temperature ferromagnetism with high magnetization was found. Large negative magnetoresistance of 11% was found at room temperature, and its value increased with a decrease in temperature up to 36% at 4.8 K. The mechanism for large negative magnetoresistance is discussed.

Modulation of noise in submicron GaAs/AlGaAs hall devices by gating.

We present a systematic characterization of fluctuations in submicron Hall devices based on GaAs/AlGaAs two-dimensional electron gas heterostructures at temperatures between 1.5 to 60 K. A large variety of noise spectra, from 1/f to Lorentzian, are obtained by gating the Hall devices. The noise level can be reduced by up to several orders of magnitude with a moderate gate voltage of 0.2 V, whereas the carrier density increases less than 60% in the same range. The significant dependence of the Hall noise spectra on temperature and gate voltage is explained in terms of the switching processes related to impurities in n-AlGaAs.

Evidence for Half-Metallicity in n-type HgCr2Se4.

High quality HgCr2Se4 single crystals have been investigated by magnetization, electron transport, and Andreev reflection spectroscopy. In the ferromagnetic ground state, the saturation magnetic moment of each unit cell corresponds to an integer number of electron spins (3u2009u2009μB/Cr3+), and the Hall effect measurements suggest n-type charge carriers. Spin polarizations as high as 97% were obtained from fits of the differential conductance spectra of HgCr2Se4/Pb junctions with the modified Blonder-Tinkham-Klapwijk theory. The temperature and bias-voltage dependencies of the subgap conductance are consistent with recent theoretical calculations based on spin active scatterings at a superconductor-half-metal interface. Our results suggest that n-HgCr2Se4 is a half-metal, in agreement with theoretical calculations that also predict undoped HgCr2Se4 is a magnetic Weyl semimetal.

Weak-coupling Bardeen-Cooper-Schrieffer superconductivity in the electron-doped cuprate superconductors

We use in-plane tunneling spectroscopy to study the temperature dependence of the local superconducting gap Delta(T) in electron-doped copper oxides with various T(c)s and Ce-doping concentrations. We show that the temperature dependence of Delta(T) follows the expectation of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, where Delta(0)/k(B)T(c)approximate to 1.72 +/- 0.15 and Delta(0) is the average superconducting gap across the Fermi surface, for all the doping levels investigated. These results suggest that the electron-doped superconducting copper oxides are weak-coupling BCS superconductors.

Zeeman splitting in ferromagnetic Schottky barrier contacts based on doped EuS

Schottky barrier contacts of nonstoichiometric EuS, a ferromagnetic semiconductor, have been fabricated on Si (100) substrate and the current–voltage (I–V) characteristics are investigated at temperatures 150–5.0K. The electrical transport across such Schottky contacts is found to be dominated by thermionic emission at high temperatures, while at low temperatures and low biases, electron tunneling is dominant. The lower bound estimate of the Schottky barrier heights was obtained by analyzing the high-bias current–voltage characteristics. A decrease in barrier height of 0.26±0.06eV was deduced from the I–V characteristics as the temperature decreases below the ferromagnetic ordering temperature (TC) of the EuS. The variation of the barrier height below TC is the result of a spontaneous Zeeman splitting of the conduction band, and its temperature dependence resembles that of the spontaneous moment in EuS. The results point to the plausibility of using doped EuS as a spin injector and detector.

Measurement of the spin polarization of the magnetic semiconductor EuS with zero-field and Zeeman-split Andreev reflection spectroscopy

We report measurements of the spin polarization

Spontaneous spin-filter effect across EuS∕GaAs heterojunction

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Single s-band Half-Metallicity in n-type HgCr2Se4

of the concentrated magnetic semiconductor

High-pressure magnetotransport measurements of the semimetallic ferromagnet EuB

mathrm{EuS}

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using both zero-field and Zeeman-split Andreev reflection spectroscopy (ARS) with