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We report the effects of electron doping on the crystal structure and electrical resistivity of LaOBiS$_{2-x}$F$_x$ (0.05 $\leq$ $x$ $\leq$ 0.2). The $ab$ plane is found to be relatively insensitive to the amount of F, whereas the $c$ axis shrinks continuously with increasing $x$, suggesting that the doped F atoms substitute selectively into the apical sites in the BiS$_2$ layer. At $x$ = 0.10, as the temperature is decreased from room temperature, the electrical resistivity is temperature-independent from room temperature to 285 K, increases linearly with decreasing temperature from 285 K to 205 K and then shows obvious insulating behavior below 205 K, which may be due to strong spin-orbit coupling. LaOBiS$_{1.9}$F$_{0.1}$ shows the significantly weak and temperature-independent diamagnetism without any evident anomalies caused by a phase transition.

In the present work, we report the new findings on the doping level dependence of the phase coherence between superconducting Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi-2212) grains. The experimental results from the strongly underdoped and overdoped regimes deviated from the expectation based on the doping level dependence of the superfluid density at $T$ = 0 K. These findings appear to be governed by interplay between competing orders inside the superconducting dome of cuprate superconductors. Two quantum critical points are likely to exist at the underdoped and overdoped regimes beneath the superconducting dome., Comment: 10 pages, 2 figures

We report the synthesis of a new oxychalcogenide HgOCuSe sample. The resistivity decreases as a function of $T^{1.75}$ with decreasing temperature from room temperature down to around 80 K. There exists a very sharp ferromagnetic-like phase transition at around 60 K under a field of $H$ = 100 Oe. Contrary to the usual ferromagnetic materials, the descending and ascending branches of the magnetic hysteresis curve, at 30 K, are reversed in the whole irreversible field range and the reverse irreversibility decreases at 5 K.

Magnetic properties of MnAs were studied as a function of temperature with a superconducting interference device (5 K to 350 K), and atomic force microscopy/magnetic force microscopy (20 K to 360 K). Structural and magnetic properties of MnAs depend on film thickness both near and far below the Curie temperature. In samples with coexisting ferromagnetic alpha-MnAs and paramagnetic beta-MnAs, the domain structures are affected by the distribution of two phases. The magnetic domain structures below the temperature range of this mixed phase resemble that of a single domain structure with uniform magnetization along the easy axis, except there are regions elongated along the easy axis embedded where the magnetization is along the second easy axis, i.e., normal to the films. The shape of those regions and their temperature dependence are also related to the MnAs layer thickness., Comment: 3 pages

We discuss a new narrow-gap ferromagnetic (FM) semiconductor alloy, In(1-x)Mn(x)Sb, and its growth by low-temperature molecular-beam epitaxy. The magnetic properties were investigated by direct magnetization measurements, electrical transport, magnetic circular dichroism, and the magneto-optical Kerr effect. These data clearly indicate that In(1-x)Mn(x)Sb possesses all the attributes of a system with carrier-mediated FM interactions, including well-defined hysteresis loops, a cusp in the temperature dependence of the resistivity, strong negative magnetoresistance, and a large anomalous Hall effect. The Curie temperatures in samples investigated thus far range up to 8.5 K, which are consistent with a mean-field-theory simulation of the carrier-induced ferromagnetism based on the 8-band effective band-orbital method., Comment: Invited talk at 11th International Conference on Narrow Gap Semiconductors, Buffalo, New York, U.S.A., June 16 - 20, 2003

A narrow-gap ferromagnetic In(1-x)Mn(x)Sb semiconductor alloy was successfully grown by low-temperature molecular beam epitaxy on CdTe/GaAs hybrid substrates. Ferromagnetic order in In(1-x)Mn(x)Sb was unambiguously established by the observation of clear hysteresis loops both in direct magnetization measurements and in the anomalous Hall effect, with Curie temperatures T_C ranging up to 8.5 K. The observed values of T_C agree well with the existing models of carrier-induced ferromagnetism., Comment: 12 pages, 3 figures

Digital alloys of GaSb/Mn have been fabricated by molecular beam epitaxy. Transmission electron micrographs showed good crystal quality with individual Mn-containing layers well resolved; no evidence of 3D MnSb precipitates was seen in as-grown samples. All samples studied exhibited ferromagnetism with temperature dependent hysteresis loops in the magnetization accompanied by metallic p-type conductivity with a strong anomalous Hall effect (AHE) up to 400 K (limited by the experimental setup). The anomalous Hall effect shows hysteresis loops at low temperatures and above room temperature very similar to those seen in the magnetization. The strong AHE with hysteresis indicates that the holes interact with the Mn spins above room temperature. All samples are metallic, which is important for spintronics applications. * To whom correspondence should be addressed. E-mail: luo@buffalo.edu