Bi-collinear antiferromagnetic order in the tetragonal α\alpha-FeTe

By the first-principles electronic structure calculations, we find that the ground state of PbO-type tetragonal $\alpha$-FeTe is in a bi-collinear antiferromagnetic state, in which the Fe local moments ($\sim2.5\mu_B$) are ordered ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe square lattice. This bi-collinear order results from the interplay among the nearest, next nearest, and next next nearest neighbor superexchange interactions $J_1$, $J_2$, and $J_3$, mediated by Te $5p$-band. In contrast, the ground state of $\alpha$-FeSe is in the collinear antiferromagnetic order, similar as in LaFeAsO and BaFe$_2$As$_2$.Comment: 5 pages and 5 figure

Droplet-like Fermi surfaces in the anti-ferromagnetic phase of EuFe2_2As2_2, an Fe-pnictide superconductor parent compound

Using angle resolved photoemission it is shown that the low lying electronic states of the iron pnictide parent compound EuFe$_2$As$_2$ are strongly modified in the magnetically ordered, low temperature, orthorhombic state compared to the tetragonal, paramagnetic case above the spin density wave transition temperature. Back-folded bands, reflected in the orthorhombic/ anti-ferromagnetic Brillouin zone boundary hybridize strongly with the non-folded states, leading to the opening of energy gaps. As a direct consequence, the large Fermi surfaces of the tetragonal phase fragment, the low temperature Fermi surface being comprised of small droplets, built up of electron and hole-like sections. These high resolution ARPES data are therefore in keeping with quantum oscillation and optical data from other undoped pnictide parent compounds.Comment: 4 figures, 6 page

Pairing symmetry and properties of iron-based high temperature superconductors

Pairing symmetry is important to indentify the pairing mechanism. The analysis becomes particularly timely and important for the newly discovered iron-based multi-orbital superconductors. From group theory point of view we classified all pairing matrices (in the orbital space) that carry irreducible representations of the system. The quasiparticle gap falls into three categories: full, nodal and gapless. The nodal-gap states show conventional Volovik effect even for on-site pairing. The gapless states are odd in orbital space, have a negative superfluid density and are therefore unstable. In connection to experiments we proposed possible pairing states and implications for the pairing mechanism.Comment: 4 pages, 1 table, 2 figures, polished versio

Stabilizing a hydrogen-rich superconductor at 1 GPa by the charge-transfer modulated virtual high-pressure effect

Applying pressure around megabar is indispensable in the synthesis of high-temperature superconducting hydrides, such as SH$_3$ and LaH$_{10}$. Stabilizing the high-pressure phase of hydride around ambient condition is a severe challenge. Based on the density-functional theory calculations, we give the first example that the structure of hydride CaBH$_5$ predicted above 280 GPa, can maintain its dynamical stability with pressure down to 1 GPa, by modulating the charge transfer from metal atoms to hydrogen atoms via the replacement of Ca with alkali metal atoms e.g. Cs, in which the [BH$_5$]$^{2-}$ anion shrinks along $c$ axis and expands in the $ab$ plane, experiencing an anisotropic virtual high pressure. This mechanism, namely charge transfer modulated virtual high pressure effect, plays a vital role in enhancing the structural stability and leading to the reemergence of ambient-pressure-forbidden [BH$_5$]$^{2-}$ anion around 1 GPa in CsBH$_5$. Moreover, we find that CsBH$_5$ is a strongly coupled superconductor, with transition temperature as high as 98 K, well above the liquid-nitrogen temperature. Our findings provide a novel mechanism to reduce the critical pressure required by hydrogen-rich compound without changing its crystal structure, and also shed light on searching ambient-pressure high-temperature superconductivity in metal borohydrides.Comment: accepted for publication as a Letter in Phys. Rev.