The recent discovery of superconductivity in iron arsenide compounds RFeAsO (R= rare earth) or AFe 2 As 2[1~8] (A= alkaline earth) attracts great attention due to the unexpected high T c in the system containing ferromagnetic elements like Fe. Similar to high T c cuprates, the superconductivity in iron arsenide is related to a layered structure. Searching for new superconductors with [FeAs] layer but of simpler structure will be of scientific significance either to build up new multilayered superconductors that may reach higher T c or to study the mysterious underlined superconducting mechanism in iron arsenide compounds. Here we report that a new superconducting iron arsenide system LiFeAs was found. The compound crystallizes into structure containing [FeAs] conducting layer that is interlaced with Li charge reservoir. Superconductivity was observed with T c up to 18 K in the compounds.

Diluted magnetic semiconductors have received much attention due to their potential applications for spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since the 1990s. The simultaneous spin and charge doping via hetero-valent (Ga 3 þ ,Mn 2 þ ) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Here we report the synthesis of a new diluted magnetic semiconductor (Ba 1 À x K x )(Zn 1 À y Mn y ) 2 As 2 , which is isostructural to the 122 iron-based superconductors with the tetragonal ThCr 2 Si 2 (122) structure. Holes are doped via (Ba 2 þ , K 1 þ ) replacements, while spins via isovalent (Zn 2 þ ,Mn 2 þ ) substitutions. Bulk samples with x ¼ 0.1 À 0.3 and y ¼ 0.05 À 0.15 exhibit ferromagnetic order with T C up to 180 K, which is comparable to the highest T C for (Ga,Mn)As and significantly enhanced from T C up to 50 K of the '111'-based Li(Zn,Mn)As. Moreover, ferromagnetic (Ba,K)(Zn,Mn) 2 As 2 shares the same 122 crystal structure with semiconducting BaZn 2 As 2 , antiferromagnetic BaMn 2 As 2 and superconducting (Ba,K)Fe 2 As 2 , which makes them promising for the development of multilayer functional devices.

The cubic perovskite BaRuO3 has been synthesized under 18 GPa at 1,000°C. Rietveld refinement indicates that the new compound has a stretched Ru-O bond. The cubic perovskite BaRuO3 remains metallic to 4 K and exhibits a ferromagnetic transition at Tc ‫؍‬ 60 K, which is significantly lower than the Tc Ϸ 160 K for SrRuO3. The availability of cubic perovskite BaRuO3 not only makes it possible to map out the evolution of magnetism in the whole series of ARuO3 (A ‫؍‬ Ca, Sr, Ba) as a function of the ionic size of the A-site rA, but also completes the polytypes of BaRuO3. Extension of the plot of Tc versus rA in perovskites ARuO3 (A ‫؍‬ Ca, Sr, Ba) shows that Tc does not increase as the cubic structure is approached, but has a maximum for orthorhombic SrRuO3. Suppressing Tc by Ca and Ba doping in SrRuO3 is distinguished by sharply different magnetic susceptibilities (T) of the paramagnetic phase. This distinction has been interpreted in the context of a Griffiths' phase on the (Ca Sr)RuO3 side and bandwidth broadening on the (Sr,Ba)RuO3 side.magnetism ͉ compounds ͉ Ruthenate

Although the mechanism of superconductivity in the cuprates remains elusive, it is generally agreed that at the heart of the problem is the physics of doped Mott insulators. A crucial step for solving the high temperature superconductivity puzzle is to elucidate the electronic structure of the parent compound and the behaviour of doped charge carriers. Here we use scanning tunnelling microscopy to investigate the atomic-scale electronic structure of the Ca 2 CuO 2 Cl 2 parent Mott insulator of the cuprates. The full electronic spectrum across the Mott-Hubbard gap is uncovered for the first time, which reveals the particle-hole symmetric and spatially uniform Hubbard bands. Defect-induced charge carriers are found to create broad in-gap electronic states that are strongly localized in space. We show that the electronic structure of pristine Mott insulator is consistent with the Zhang-Rice singlet model, but the peculiar features of the doped electronic states require further investigations.

Perovskite PbCoO synthesized at 12 GPa was found to have an unusual charge distribution of PbPbCoCoO with charge orderings in both the A and B sites of perovskite ABO. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to PbPbCoCoO quadruple perovskite structure. It is shown that the average valence distribution of PbCoO between PbCrO and PbNiO can be stabilized by tuning the energy levels of Pb 6s and transition metal 3d orbitals.

A new iron pnictide LiFeP superconductor was found. The compound crystallizes into a Cu2Sb structure containing an "FeP" layer showing superconductivity with maximum Tc of 6K. This is the first "111" type iron pnictide superconductor containing no arsenic. The new superconductor is featured with itinerant behavior at normal state that could be helpful to understand the novel superconducting mechanism of iron pnictide compounds.

Abstract:One of the biggest puzzles concerning the cuprate high temperature superconductors is what determines the maximum transition temperature (T c,max ), which varies from less than 30 K to above 130 K in different compounds. Despite this dramatic variation, a robust trend is that within each family, the double-layer compound always has higher T c,max than the single-layer counterpart. Here we use scanning tunneling microscopy to investigate the electronic structure of four cuprate parent compounds belonging to two different families. We find that within each family, the double layer compound has a much smaller charge transfer gap size (∆ CT ), indicating a clear anticorrelation between ∆ CT and T c,max . These results suggest that the charge transfer gap plays a key role in the superconducting physics of cuprates, which shed important new light on the high T c mechanism from doped Mott insulator perspective.

The unique physical property of negative thermal expansion (NTE) is not only interesting for scientific research but also important for practical applications. Chemical modification generally tends to weaken NTE. It remains a challenge to obtain enhanced NTE from currently available materials. Herein, we successfully achieve enhanced NTE in Pb(TiV)O by improving its ferroelectricity. With the chemical substitution of vanadium, lattice tetragonality (c/a) is highly promoted, which is attributed to strong spontaneous polarization, evidenced by the enhanced covalent interaction in the V/Ti-O and Pb-O2 bonds from first-principles calculations. As a consequence, Pb(TiV)O exhibits a nonlinear and much stronger NTE over a wide temperature range with a volumetric coefficient of thermal expansion α = -3.76 × 10/°C (25-550 °C). Interestingly, an intrinsic giant volume contraction (∼3.7%) was obtained at the composition of Pb(TiV)O during the ferroelectric-to-paraelectric phase transition, which represents the highest value ever reported. Such volume contraction is well correlated to the effect of spontaneous volume ferroelectrostriction. The present study extends the scope of the NTE family and provides an effective approach to explore new materials with large NTE, such as through adjusting the NTE-related ferroelectric property in the family of ferroelectrics.