Structural transition,electric transport,and electronic structures in the compressed trilayer nickelate La_(4)Ni_(3)O_(10)

Atomic structure and electronic band structure are fundamental properties for understanding the mechanism of superconductivity. Motivated by the discovery of pressure-induced high-temperature superconductivity at 80 K in the bilayer Ruddlesden-Popper nickelate La_(3)Ni_(2)O_(7), the atomic structure and electronic band structure of the trilayer nickelate La_(4)Ni_(3)O_(10) under pressure up to 44.3 GPa are investigated. A structural transition from the monoclinic P2_(1)/a space group to the tetragonal I4/mmm around 12.6-13.4 GPa is identified, accompanied by a drop of resistance below 7 K. Density functional theory calculations suggest that the bonding state of Ni 3d_(z^(2)) orbital rises and crosses the Fermi level at high pressures, which may give rise to possible superconductivity observed in resistance under pressure in La_(4)Ni_(3)O_(10). The trilayer nickelate La_(4)Ni_(3)O_(10) shows some similarities with the bilayer La_(3)Ni_(2)O_(7) and has unique properties, providing a new platform to investigate the underlying mechanism of superconductivity in nickelates.

Extremely strong coupling s-wave superconductivity in the medium-entropy alloy TiHfNbTa

Here we report a TiHfNbTa bulk medium-entropy alloy(MEA)superconductor crystallized in the body-centered cubic structure with the unit cell parameter a=3.35925?,which is synthesized by an arc melting method.Superconducting properties of the TiHfNbTa are studied by employing magnetic susceptibility,resistivity,and specific heat measurements.Experimental results show a bulk superconducting transition temperature(Tc)of around 6.75 K.The lower and upper critical fields for TiHfNbTa are45.8 m T and 10.46 T,respectively.First-principles calculations show that the d electrons of Ti,Hf,Nb,and Ta are the main contribution to the total density of states near the Fermi level.Our results indicate that the superconductivity is a conventional swave type with extremely strong coupling(△C_(el)/γ_(n)T_(c)=2.88,2△_(0)/k_(B)T_(c)=5.02,and λ_(ep)=2.77).The extremely strong coupling behavior in the s-wave type Ti Hf Nb Ta MEA superconductor is unusual because it generally happens in cuprates,pnictides,and other unconventional superconductors.

Structure and magnetic properties of the S=3/2 zigzag spin chain antiferromagnet BaCoTe_(2)O_(7)

We report a study of the structure and magnetic properties of the S=3/2 zigzag spin chain compound BaCoTe_(2)O_(7).Neutron diffraction measurements show that it crystallizes in the noncentrosymmetric space group Ama2 with a canted↑↑↓↓spin structure along the quasi-one-dimensional zigzag chain and a moment size of 1.89(2)μBat 2 K.Both magnetic susceptibility and specific heat measurements yield an antiferromagnetic phase transition at TN=6.2 K.A negative Curie-Weiss temperature,ΘCW=-74.7(2) K,and an empirical frustration parameter,f=|ΘCW|/TN≈12,are obtained by fitting the magnetic susceptibility,indicating antiferromagnetic interactions and strong magnetic frustration.From ultraviolet-visible absorption spectroscopy and first-principles calculations,an indirect band gap of 2.68(2) eV is determined.We propose that the canted zigzag spin chain of BaCoTe_(2)O_(7) may produce a change in the polarization via the exchange-striction mechanism.