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Register a new userStrong antiferromagnetic proximity coupling in a heterostructured superconductor Sr$_2$VO$_3$FeAs
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We report observation of strong magnetic proximity coupling in a heterostructured superconductor Sr$_2$VO$_3$FeAs, determined by the upper critical fields $H_{c2}(T)$ measurements up to 65 T. Using the resistivity and the radio-frequency measurements for both $H parallel ab$ and $H parallel c$, we found a strong upward curvature of $H_{c2}^c(T)$, together with a steep increase of $H_{c2}^{ab}(T)$ near $T_c$, yielding the anisotropic factor $gamma_H=H_{c2}^{ab}/H_{c2}^c$ up to $sim$ 20, the largest value among iron-based superconductors. These are attributed to the Jaccarino-Peter effect, rather than to the multiband effect, due to strong exchange interaction between itinerant Fe spins of the FeAs layers and localized V spins of Mott-insulating SrVO$_3$ layers. These findings provide evidence for strong antiferromagnetic proximity coupling, comparable with the intralayer superexchange interaction of SrVO$_3$ layer and sufficient to induce magnetic frustration in Sr$_2$VO$_3$FeAs.
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We report the physical properties and electronic structure calculations of a layered chromium oxypnictide, Sr$_2$Cr$_3$As$_2$O$_2$, which crystallizes in a Sr$_2$Mn$_3$As$_2$O$_2$-type structure containing both CrO$_2$ planes and Cr$_2$As$_2$ layers. The newly synthesized material exhibits a metallic conduction with a dominant electron-magnon scattering. Magnetic and specific-heat measurements indicate at least two intrinsic magnetic transitions below room temperature. One is an antiferromagnetic transition at 291 K, probably associated with a spin ordering in the Cr$_2$As$_2$ layers. Another transition is broad, occurring at around 38 K, and possibly due to a short-range spin order in the CrO$_2$ planes. Our first-principles calculations indicate predominant two-dimensional antiferromagnetic exchange couplings, and suggest a KG-type (i.e. K$_2$NiF$_4$ type for CrO$_2$ planes and G type for Cr$_2$As$_2$ layers) magnetic structure, with reduced moments for both Cr sublattices. The corresponding electronic states near the Fermi energy are mostly contributed from Cr-3$d$ orbitals which weakly (modestly) hybridize with the O-2$p$ (As-4$p$) orbitals in the CrO$_2$ (Cr$_2$As$_2$) layers. The bare bandstructure density of states at the Fermi level is only $sim$1/4 of the experimental value derived from the low-temperature specific-heat data, consistent with the remarkable electron-magnon coupling. The title compound is argued to be a possible candidate to host superconductivity.
We report an investigation of the structural and electronic properties of hybrid superconductor/ferromagnet (S/F) bilayers of composition Nb/Cu$_{60}$Ni$_{40}$ prepared by magnetron sputtering. X-ray and neutron reflectometry show that both the overall interfacial roughness and vertical correlations of the roughness of different interfaces are lower for heterostructures deposited on Al$_2$O$_3$(1$bar{1}$02) substrates than for those deposited on Si(111). Mutual inductance experiments were then used to study the influence of the interfacial roughness on the superconducting transition temperature, $T_C$. These measurements revealed a $sim$ 4% higher $T_C$ in heterostructures deposited on Al$_2$O$_3$, compared to those on Si. We attribute this effect to a higher mean-free path of electrons in the S layer, caused by a suppression of diffusive scattering at the interfaces. However, the dependence of the $T_C$ on the thickness of the ferromagnetic layer is not significantly different in the two systems, indicating a weak influence of the interfacial roughness on the transparency for Cooper pairs.
The new two-dimensional (2D) kagome superconductor CsV$_3$Sb$_5$ has attracted much recent attention due to the coexistence of superconductivity, charge order, topology and kagome physics. A key issue in this field is to unveil the unique reconstructed electronic structure, which successfully accommodates different orders and interactions to form a fertile ground for emergent phenomena. Here, we report angle-resolved photoemission spectroscopy (ARPES) evidence for two distinct band reconstructions in CsV$_3$Sb$_5$. The first one is characterized by the appearance of new electron energy band at low temperature. The new band is theoretically reproduced when the three dimensionality of the charge order is considered for a band-folding along the out-of-plane direction. The second reconstruction is identified as a surface induced orbital-selective shift of the electron energy band. Our results provide the first evidence for the three dimensionality of the charge order in single-particle spectral function, highlighting the importance of long-range out-of-plane electronic correlations in this layered kagome superconductor. They also point to the feasibility of orbital-selective control of the band structure via surface modification, which would open a new avenue for manipulating exotic phenomena in this system, including superconductivity.
Strongly correlated electrons in layered perovskite structures have been the birthplace of high-temperature superconductivity, spin liquid, and quantum criticality. Specifically, the cuprate materials with layered structures made of corner sharing square planar CuO$_4$ units have been intensely studied due to their Mott insulating grounds state which leads to high-temperature superconductivity upon doping. Identifying new compounds with similar lattice and electronic structures has become a challenge in solid state chemistry. Here, we report the hydrothermal crystal growth of a new copper tellurite sulfate Cu$_3$(TeO$_4$)(SO$_4$)$cdot$H$_2$O, a promising alternative to layered perovskites. The orthorhombic phase (space group $Pnma$) is made of corrugated layers of corner-sharing CuO$_4$ square-planar units that are edge-shared with TeO$_4$ units. The layers are linked by slabs of corner-sharing CuO$_4$ and SO$_4$. Using both the bond valence sum analysis and magnetization data, we find purely Cu$^{2+}$ ions within the layers, but a mixed valence of Cu$^{2+}$/Cu${^+}$ between the layers. Cu$_3$(TeO$_4$)(SO$_4$)$cdot$H$_2$O undergoes an antiferromagnetic transition at $T_N$=67 K marked by a peak in the magnetic susceptibility. Upon further cooling, a spin-canting transition occurs at $T^{star}$=12 K evidenced by a kink in the heat capacity. The spin-canting transition is explained based on a $J_1$-$J_2$ model of magnetic interactions, which is consistent with the slightly different in-plane super-exchange paths. We present Cu$_3$(TeO$_4$)(SO$_4$)$cdot$H$_2$O as a promising platform for the future doping and strain experiments that could tune the Mott insulating ground state into superconducting or spin liquid states.
La$_2$O$_3$Fe$_2$Se$_2$ can be explained in terms of Mott localization in sharp contrast with the metallic behavior of FeSe and other parent parent compounds of iron superconductors. We demonstrate that the key ingredient that makes La$_2$O$_3$Fe$_2$Se$_2$ a Mott insulator, rather than a correlated metal dominated by the Hunds coupling is the enhanced crystal-field splitting, accompanied by a smaller orbital-resolved kinetic energy. The strong deviation from orbital degeneracy introduced by the crystal-field splitting also pushes this materials close to an orbital-selective Mott transition. We predict that either doping or uniaxial external pressure can drive the material into an orbital-selective Mott state, where only one or few orbitals are metallized while the others remain insulating.
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