Long Ma

Na-23 and As-75 NMR study of antiferromagnetism and spin fluctuations in NaFeAs single crystals

We report the Na-23 and As-75 nuclear magnetic resonance (NMR) studies on NaFeAs single crystals. The structure transition temperature T-S (55 K) and the spin density wave (SDW) transition temperature T-SDW (40.5 K) are determined by the NMRline splits. The spin-lattice relaxation rates indicate that the spin fluctuations are strongly enhanced just below T-S and drive a second order SDW transition. A fluctuating feature of the SDW ordering is also seen below the T-SDW. We further performed high-pressure NMR studies on NaFeAs, and found that the T-SDW increases by similar to 7 K and the magnetic moment increases by similar to 30% under 2.5 GPa pressure.

As-75 NMR study of single crystals of the heavily overdoped pnictide superconductors Ba1-xKxFe2As2 (x=0.7 and 1)

We performed As-75 NMR studies on two overdoped high-quality Ba1-xKxFe2As2 (x=0.7 and 1.0) single crystals. In the normal states, we found a dramatic increase in the spin-lattice relaxation (1/T-75(1)) from the x =0.7 to the x=1.0 samples. In KFe2As2, the ratio of 1/(T1TKn2)-T-75, where K-75(n) is the Knight shift, increases as temperature drops. These results indicate the existence of another type of spin fluctuations in KFe2As2 which is accustomed to being treated as a simple Fermi liquid. In the superconducting state, the temperature scalings of 1/T-75(1) below T-c in the overdoped samples are significantly different from those in the under or optimally doped ones. A power-law scaling behavior 1/(T1T)-T-75 similar to T-0.5 is observed, which indicates universal strong low-energy excitations in the overdoped hole-type superconductors.

77Se NMR study of the pairing symmetry and the spin dynamics in K(y)Fe(2-x)Se2.

We present a 77Se NMR study of the newly discovered iron selenide superconductor K(y)Fe(2-x)Se2, in which T(c) = 32  K. Below T(c), the Knight shift 77K drops sharply with temperature, providing strong evidence for singlet pairing. Above T(c), Korringa-type relaxation indicates Fermi-liquid behavior. Our experimental results set strict constraints on the nature of possible theories for the mechanism of high-T(c) superconductivity in this iron selenide system.

Microscopic Coexistence of Superconductivity and Antiferromagnetism in Underdoped Ba(Fe(1-x)Ru(x))2As2.

We use (75)As nuclear magnetic resonance to investigate the local electronic properties of Ba(Fe(1-x)Ru(x))(2)As(2) (x = 0.23). We find two phase transitions: to antiferromagnetism at T(N) ≈ 60 K and to superconductivity at T(C) ≈ 15 K. Below T(N), our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of 0.4μ(B)/Fe. The spin-lattice relaxation rate 1/(75)T(1) is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below T(C), 1/(75)T(1) on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity.

Local spin fluctuations in iron-based superconductors:Se77andRb87NMR measurements on Tl0.47Rb0.34Fe1.63Se2

We report nuclear magnetic resonance (NMR) studies of the intercalated iron selenide superconductor (Tl, Rb)

Microscopic Coexistence of Superconductivity and Antiferromagnetism in Underdoped Ba(Fe1-xRux)(2)As-2

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Quenched Fe moment in the collapsed tetragonal phase of Ca1-xPrxFe2As2

Fe

Microscopic Coexistence of Superconductivity and Antiferromagnetism in UnderdopedBa(Fe1−xRux)2As2

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Experimental Approach to the Thermodynamics of the Pure Two-Dimensional Spin-1/2 Triangular Lattice Antiferromagnet in Ba8CoNb6O24

Se

Mermin-Wagner physics, (H,T) phase diagram, and candidate quantum spin-liquid phase in the spin-1/2 triangular-lattice antiferromagnet Ba 8 CoNb 6 O 24

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