The discovery of cuprate high TC superconductors has inspired the search for unconventional superconductors in magnetic materials.A successful recipe has been to suppress long−range order in a magnetic parent compound...The discovery of cuprate high TC superconductors has inspired the search for unconventional superconductors in magnetic materials.A successful recipe has been to suppress long−range order in a magnetic parent compound by doping or high pressure to drive the material towards a quantum critical point.We report an exception to this rule in the recently discovered potassium iron selenide.The superconducting composition is identified as the iron vacancy ordered K_(0.83)(2)Fe_(1.64)(1)Se_(2) with T_(C) above 30 K.A novel large moment 3.31μB/Fe antiferromagnetic order that conforms to the tetragonal crystal symmetry has an unprecedentedly high ordering temperature TN≈559 K for a bulk superconductor.Staggeringly polarized electronic density of states is thus suspected,which would stimulate further investigation into superconductivity in a strong spin-exchange field under new circumstances.展开更多
Combining in-depth neutron diffraction and systematic bulk studies,we discover that the√5×√5 Fe vacancy order,with its associated block antiferromagnetic order,is the ground state with varying occupancy ratios ...Combining in-depth neutron diffraction and systematic bulk studies,we discover that the√5×√5 Fe vacancy order,with its associated block antiferromagnetic order,is the ground state with varying occupancy ratios of the iron 16i and vacancy 4d sites across the phase-diagram of K_(x)Fe_(2)-ySe_(2).The orthorhombic order,with one of the four Fe sites vacant,appears only at intermediate temperatures as a competing phase.The material experiences an insulator to metal crossover when the√5×√5 order is highly developed.Superconductivity occurs in such a metallic phase.展开更多
We report both 23Na and 75As nuclear magnetic resonance (NMR ) studies on hole-doped Ca_(1-x)Na_(x)Fe_(2)As_(2) superconducting single crystals (x ≈ 0.67) with the superconducting temperature Tt =32 K.Singlet superco...We report both 23Na and 75As nuclear magnetic resonance (NMR ) studies on hole-doped Ca_(1-x)Na_(x)Fe_(2)As_(2) superconducting single crystals (x ≈ 0.67) with the superconducting temperature Tt =32 K.Singlet superconductivity is suggested by a sharp drop of the Knight shift 75K below Tc.The spin-lattice relaxation rate 1/T1 does not show the Slichter-Hebel coherence peak,which suggests an unconventional pairing.The penetration depth is estimated to be 0.24 μm at temperature T =2K.Here 1/75T1T shows an anisotropic behavior and a prominent low-temperature upturn,with 75T1 denoting the 75As spin-lattice relaxation time and T the temperature,which indicates strong low-energy antiferromagnetic spin fluctuations and supports a magnetic origin of superconductivity.展开更多
基金by the National Natural Science Foundation of China under Grant Nos 11034012,10834013 and 10974254the National Basic Research Program of China under Grant Nos 2011CBA00112,2010CB923000 and 2009CB009100.
文摘The discovery of cuprate high TC superconductors has inspired the search for unconventional superconductors in magnetic materials.A successful recipe has been to suppress long−range order in a magnetic parent compound by doping or high pressure to drive the material towards a quantum critical point.We report an exception to this rule in the recently discovered potassium iron selenide.The superconducting composition is identified as the iron vacancy ordered K_(0.83)(2)Fe_(1.64)(1)Se_(2) with T_(C) above 30 K.A novel large moment 3.31μB/Fe antiferromagnetic order that conforms to the tetragonal crystal symmetry has an unprecedentedly high ordering temperature TN≈559 K for a bulk superconductor.Staggeringly polarized electronic density of states is thus suspected,which would stimulate further investigation into superconductivity in a strong spin-exchange field under new circumstances.
基金Supported by the National Basic Research Program of China under Grant Nos 2012CB921700,2011CBA00112,2011CBA00103 and 2009CB929104National Natural Science Foundation of China under Grant Nos 11034012,11190024,10974175 and 10934005.
文摘Combining in-depth neutron diffraction and systematic bulk studies,we discover that the√5×√5 Fe vacancy order,with its associated block antiferromagnetic order,is the ground state with varying occupancy ratios of the iron 16i and vacancy 4d sites across the phase-diagram of K_(x)Fe_(2)-ySe_(2).The orthorhombic order,with one of the four Fe sites vacant,appears only at intermediate temperatures as a competing phase.The material experiences an insulator to metal crossover when the√5×√5 order is highly developed.Superconductivity occurs in such a metallic phase.
基金Supported by the National Basic Research Program of China under Grant Nos 2010CB923004 and 2011CBA00112the National Natural Science Foundation of China under Grant Nos 11074304,11104070,11004245 and 10974254+1 种基金the Program for New Century Excellent Talents in University under Grant No NCET-08-0546the Fundamental Research Funds for the Central Universities of China.
文摘We report both 23Na and 75As nuclear magnetic resonance (NMR ) studies on hole-doped Ca_(1-x)Na_(x)Fe_(2)As_(2) superconducting single crystals (x ≈ 0.67) with the superconducting temperature Tt =32 K.Singlet superconductivity is suggested by a sharp drop of the Knight shift 75K below Tc.The spin-lattice relaxation rate 1/T1 does not show the Slichter-Hebel coherence peak,which suggests an unconventional pairing.The penetration depth is estimated to be 0.24 μm at temperature T =2K.Here 1/75T1T shows an anisotropic behavior and a prominent low-temperature upturn,with 75T1 denoting the 75As spin-lattice relaxation time and T the temperature,which indicates strong low-energy antiferromagnetic spin fluctuations and supports a magnetic origin of superconductivity.
