M. K. Wu

Superconductivity in the PbO-type structure α-FeSe

The recent discovery of superconductivity with relatively high transition temperature (Tc) in the layered iron-based quaternary oxypnictides La[O1−xFx] FeAs by Kamihara et al. [Kamihara Y, Watanabe T, Hirano M, Hosono H (2008) Iron-based layered superconductor La[O1-xFx] FeAs (x = 0.05–0.12) with Tc = 26 K. J Am Chem Soc 130:3296–3297.] was a real surprise and has generated tremendous interest. Although superconductivity exists in alloy that contains the element Fe, LaOMPn (with M = Fe, Ni; and Pn = P and As) is the first system where Fe plays the key role to the occurrence of superconductivity. LaOMPn has a layered crystal structure with an Fe-based plane. It is quite natural to search whether there exists other Fe based planar compounds that exhibit superconductivity. Here, we report the observation of superconductivity with zero-resistance transition temperature at 8 K in the PbO-type α-FeSe compound. A key observation is that the clean superconducting phase exists only in those samples prepared with intentional Se deficiency. FeSe, compared with LaOFeAs, is less toxic and much easier to handle. What is truly striking is that this compound has the same, perhaps simpler, planar crystal sublattice as the layered oxypnictides. Therefore, this result provides an opportunity to better understand the underlying mechanism of superconductivity in this class of unconventional superconductors.

Tellurium substitution effect on superconductivity of the α-phase iron selenide

We have carried out a systematic study of the PbO-type compound FeSe1−xTex (x=0–1), where the Te substitution effect on superconductivity is investigated. It is found that the superconducting transition temperature reaches a maximum of Tc=15.2 K at about 50% Te substitution. The pressure-enhanced Tc of FeSe0.5Te0.5 is more than 10 times larger than that of FeSe. Interestingly, FeTe is no longer superconducting. A low-temperature structural distortion changes FeTe from triclinic symmetry to orthorhombic symmetry. We believe that this structural change breaks the magnetic symmetry and suppresses superconductivity in FeTe.

First-order magnetic and structural phase transitions in Fe1+ySexTe1-x

We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in Fe1+ySexTe1-x. Fe1.068Te exhibits a first-order phase transition near 67 K with a tetragonal-to-monoclinic structural transition and simultaneously develops a collinear antiferromagnetic (AF) order responsible for the entropy change across the transition. Systematic studies of the FeSe1-xTex system reveal that the AF structure and lattice distortion in these materials are different from those of FeAs-based pnictides. These results call into question the conclusions of present density-functional calculations, where FeSe1-xTex and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density wave AF order.

Observation of enhanced properties in samples of silver oxide doped YBa2Cu3Ox

Samples of YBa2 Cu3Ox doped with silver oxide (Tc ∼93 K) have exhibited attractive forces in gradient magnetic fields, both normal and tangential to the surfaces, which are more than twice the sample weight. This allows sample suspension below a rare earth magnet. Critical current density was increased by ∼102 at 77 K. Persistent fields, flux pinning, magnetization, and modeling are described.

Crystal orientation and thickness dependence of the superconducting transition temperature of tetragonal FeSe1-x thin films.

Superconductivity was recently found in the tetragonal phase FeSe. A structural transformation from tetragonal to orthorhombic (or monoclinic, depending on point of view) was observed at low temperature, but was not accompanied by a magnetic ordering as commonly occurs in the parent compounds of FeAs-based superconductors. Here, we report the correlation between structural distortion and superconductivity in FeSe(1-x) thin films with different preferred growth orientations. The films with preferred growth along the c axis show a strong thickness dependent suppression of superconductivity and low temperature structural distortion. In contrast, both properties are less affected in the films with (101) preferred orientation. These results suggest that the low temperature structural distortion is closely associated with the superconductivity of this material.

k=0 Magnetic Structure and Absence of Ferroelectricity in SmFeO3

SmFeO3 has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of ∼675  K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

MAGNETIC HYSTERESIS OF HIGH-TEMPERATURE YBa2Cu3Ox-AgO SUPERCONDUCTORS: EXPLANATION OF MAGNETIC SUSPENSION

We have measured the magnetization M of superconducting YBa2Cu3Ox-AgO composites with Tc approximately equal to 92K as a function of an applied magnetic field H at 77 and 87K. A very pronounced M-H hysteresis loop occurs even at 87K, indicating the presence of extremely strong pinning centers. The results of these measurements, together with a simple model, explain quantitatively why these superconductors could be suspended below a magnet.

Se and Te Doping Study of the FeSe Superconductors

Here we present a systematic study of correlation between superconductivity with Se-content and Te-doping to the �. -FeSe system. Detailed structural and specific heat measurements at low temperature suggest that the occurrence of superconductivity is closely related to the presence of P-1 (triclinic) symmetry at low temperature. Magnetic field dependence of the resistive transition in polycrystalline �. -FeSe1-x and FeSe1-xTex suggest that Te-doping tends to induce the system to become more 2D-like and enhances substantially the upper critical field value, whereas the FeSe system behaves more like 3D superconductor.

CuBr2--a new multiferroic material with high critical temperature.

A new multiferroic material, CuBr(2) , is reported for the first time. CuBr(2) has not only a high transition temperature (close to liquid nitrogen temperature) but also low dielectric loss and strong magnetoelectric coupling. These findings reveal the importance of anion effects, in the search for the high temperature multiferroics materials among these low-dimensional spin systems.

Enhanced magnetoresistance and Griffiths phase induced by Mo substitution in La0.7Ca0.15Sr0.15Mn1−xMoxO3 (0 ⩽ x ⩽ 0.05)

Structural, magnetic and electrical properties of the La0.7Ca0.15Sr0.15Mn1−xMoxO3 (0 x 0.05) compounds have been investigated. Powder x-ray analysis reveals that the sample with x = 0 crystallizes in the rhombohedral (R3c) structure, whereas in the Mo doped samples the structure can be indexed by an orthorhombic (Pbnm) structure. The important observations of the magnetic and transport properties are: (i) the Mo substitution induces a distinct suppression of the metal‐insulator (TMI) and ferromagnetic (FM)—paramagnetic transition (TC) and the temperature of TMI was found to be higher than TC in the Mo-doped samples, (ii) the substitution of Mo enhances the magnetoresistance at room temperature, (iii) a large deviation from the Curie‐Weiss law well above TC in the Mo substituted samples indicates the existence of a Griffiths phase and (iv) long-range FM order persists in all samples with a linear decrease of saturation moment as x increases. These results are discussed in terms of the Mn-site disorder and opening of strong FM coupling between Mn 2+ ‐O‐Mn 3+ , due to the Mn 2+ ions induced by Mo 6+ at the expense of Mn 4+ ions in the La0.7Ca0.15Sr0.15Mn1−xMoxO3 system. (Some figures in this article are in colour only in the electronic version)