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G. F. Ji,
J. S. Zhang,
Long Ma,
P. Fan,
P. S. Wang,
J. Dai,
G. T. Tan,
Y. Song,
C. L. Zhang,
Pengcheng Dai,
B. Normand, Weiqiang Yu
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ABSTRACT: We present a high-pressure NMR study of the overdoped iron pnictide
superconductor NaFe$_{0.94}$Co$_{0.06}$As. The normal-state low-energy spin
fluctuations, manifest as the Curie-Weiss upturn in the spin-lattice relaxation
rate $1/^{75}T_1T$, first increase strongly with pressure but fall again at $p
> p_{\rm opt} =$ 2.2 GPa. Neither antiferromagnetic long-range order nor the
structural phase transition is encountered up to 2.5 GPa. The superconducting
transition temperature $T_c$ shows a pressure-dependence identical to the spin
fluctuations, also rising to a maximum at $p_{\rm opt}$ before decreasing
beyond this. Our observations demonstrate that, when extraneous ordering
processes are avoided, magnetic correlations and superconductivity are
optimized simultaneously as a function of the electronic structure, thereby
supporting very strongly a magnetic origin of superconductivity.
05/2013;
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ABSTRACT: 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.
Physical Review Letters 11/2012; 109(19):197002. · 7.37 Impact Factor
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ABSTRACT: We use $^{75}$As nuclear magnetic resonance (NMR) 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 $T_N \approx$ 60 K and to
superconductivity at $T_C \approx$ 15 K. Below $T_N$, our data show that the
system is fully magnetic, with a commensurate antiferromagnetic structure and a
moment of 0.4 $\mu_B$/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.
05/2012;
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ABSTRACT: We report $^{75}$As NMR studies on single crystals of rare-earth doped iron
pnictides superconductor Ca$_{1-x}$Pr$_{x}$Fe$_{2}$As$_{2}$ ($x$=0.075 and
0.15). The $^{75}$As spectra show a chemical pressure effect with doping and a
first order structure transition to the collapsed tetragonal phase upon
cooling. A sharp drop of the Knight shift is seen below the structural
transition, whereas $1/T_1$ is strongly enhanced at low-temperatures. These
evidences indicate quenching of Fe local magnetism and short-range ordering of
Pr$^{3+}$ moment in the collapsed tetragonal phase. The quenched Fe moment
through structure collapse suggests a strong interplay of structure and
magnetism, which is important for understanding the nature of the collapsed
tetragonal phase.
05/2012;
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ABSTRACT: We report both 23Na and 75As NMR studies on hole-doped Ca1-xNaxFe2As2
superconducting single crystals (x\approx 0.67) with Tc =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 {\mu}m at T=2 K. 1/75T1T shows an anisotropic behavior
and a prominent low-temperature upturn, which indicates strong low-energy
antiferromagnetic spin fluctuations and supports a magnetic origin of
superconductivity.
04/2012;
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ABSTRACT: 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.
Physical Review Letters 05/2011; 106(19):197001. · 7.37 Impact Factor
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ABSTRACT: We report nuclear magnetic resonance (NMR) studies of the intercalated iron
selenide superconductor (Tl, Rb)$_{y}$Fe$_{2-x}$Se$_2$ ($T_c = 32$ K).
Single-crystal measurements up to 480 K on both $^{77}$Se and $^{87}$Rb nuclei
show a superconducting phase with no magnetic order. The Knight shifts $K$ and
relaxation rates $1/T_1T$ increase very strongly with temperature above $T_c$,
before flattening at 400 K. The quadratic $T$-dependence and perfect
proportionality of both $K$ and $1/T_1T$ data demonstrate their origin in
paramagnetic moments. A minimal model for this pseudogap-like response is not a
missing density of states but two additive contributions from the itinerant
electronic and local magnetic components, a framework unifying the $K$ and
$1/T_1 T$ data in many iron-based superconductors.
03/2011;
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ABSTRACT: We report both 77Se and 87Rb NMR studies on Tl0.47Rb0.34Fe1.63Se2
single-crystalline superconductors (T_c about 32 K). Singlet superconductivity
is decisively determined by a sharp drop of the Knight shift K(T) below T_c,
after subtracting the superconducting diamagnetic effect. However, the
Hebel-Slichter coherence peak below T_c is not observed in the spin-lattice
relaxation rate 1/T_1, even with a low in-plane NMR field of 2.6 Tesla. Just
above T_c, no evidence of low-energy spin fluctuation is found in the
spin-lattice relaxation rate on both the 77Se and the 87Rb sites. Upon warming,
however, the Knight shifts and the spin-lattice relaxation rates of both nuclei
increase substantially with temperature. In particular, the Knight shift is
nearly isotropic and follow a function fit of K=a+bT^2 from T_c up to 300 K.
These normal state properties may be an indication of thermally activated spin
fluctuations. Our observations should put a strong constraint to the theory of
magnetism and superconductivity in the newly discovered iron-based
superconductors.
02/2011;
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ABSTRACT: We report the 77Se NMR Knight shift and spin-lattice relaxation studies on
the superconducting state of the ternary iron selenide K0.8Fe2-xSe2 with Tc~30
K. Just below Tc, the Knight shift 77Kn shows an immediate drop, indicating a
singlet pairing. The spin-lattice relaxation rate 1/77T1 decreases rapidly in
the temperature range from TC to Tc/2, which can be fit with an isotropic gap
of \Delta~3.8\pm 0.5 k_BTc. The Hebel-Slichter coherence peak is not observed.
These data give bulk evidence for a strongly coupled superconductivity with
isotropic gaps in K0.8Fe2-xSe2, which is similar to other iron-based high
temperature superconductors. Below Tc/2, the spin-part of the Knight shift
levels off to a constant value (~0.09%), and the spin-lattice relaxation
follows a 1/T1 T^2 behavior, which are not well understood.
01/2011;
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ABSTRACT: We present a 77Se NMR study of the newly discovered iron selenide
superconductor KyFe2-xSe2, in which Tc = 32 K. Below Tc, the Knight shift 77K
drops sharply with temperature, providing strong evidence for singlet pairing.
Above Tc, Korringa-type relaxation indicates Fermi-liquid behavior. Our
experimental results set strict constraints on the nature of possible theories
for the mechanism of high-Tc superconductivity in this iron selenide system.
01/2011;
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ABSTRACT: In this letter, we reported the results of NMR study on LiFeAs single
crystals. We find a strong evidence of the low temperature spin fluctuations;
by changing sample preparation conditions, the system can be tuned toward an
spin-density-wave (SDW) quantum-critical point. The detection of an
interstitial Li(2) ion, possibly locating in the tetrahedral hole, suggests
that the off-stoichiometry and/or lattice defect can probably account for the
absence of the SDW ordering in LiFeAs. These facts show that LiFeAs is a
strongly correlated system and the superconductivity is likely originated from
the SDW fluctuations.
08/2010;
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ABSTRACT: We report our $^{23}$Na and $^{75}$As NMR studies on isovalent phosphorus-doped NaFeAs$_{0.8}$P$_{0.2}$ ($T_c=$ 33 K) single crystals. Our data suggest a dramatic enhancement of the electron density of states on the Fermi surface, and a large residual electron density of state below $T_c$. However, evidence of antiferromagnetic spin fluctuations is shown by the spin-lattice relaxation rate. The penetration depth, revealed from $^{23}$Na NMR linewidth, follows the universal Uemura relation in iron pnictides. These observations suggest that the correlation effects are still important for the superconductivity in the phosphorus doped sample, although the Fermi surface is probably significantly changed upon doping. Comment: This paper has been withdrawn by the author
05/2010;
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ABSTRACT: We report the $^{23}$Na and $^{75}$As 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 NMR line
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 $\sim$7 K and the magnetic moment increases by 30% under
2.5 GPa pressure.
04/2010;
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ABSTRACT: The excitation spectrum of the one-dimensional spin-orbital model in a magnetic field is studied, using a recently developed dynamical density matrix renormalization group technique. The method is employed on chains with up to 80 sites, and examined for test cases such as the spin-1/2 antiferromagnetic Heisenberg chain, where the excitation spectrum is known exactly from the Bethe Ansatz. In the spin-orbital chain, the characteristic dynamical response depends strongly on the model parameters and the applied magnetic field. The coupling between the spin and orbital degrees of freedom is found to influence the incommensuration at finite magnetizations. In the regions of the phase diagram with only massive spin and orbital excitations, a finite field is required to overcome the spin gap. An incommensurate orbital mode is found to become massless in this partially spin-polarized regime, indicating a strong coupling between the two degrees of freedom. In the critical region with three elementary gapless excitations, a prominent particle-hole excitation is observed at higher energies, promoted by the biquadratic term in the model Hamiltonian of the spin-orbital chain. Comment: RevTex, 20 pages with 18 eps figures
05/2000;
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ABSTRACT: The c-axis charge dynamics of copper oxide materials in the underdoped and optimally doped regimes has been studied by considering the incoherent interlayer hopping. It is shown that the c-axis charge dynamics for the chain copper oxide materials is mainly governed by the scattering from the in-plane fluctuation, and the c-axis charge dynamics for the no-chain copper oxide materials is dominated by the scattering from the in-plane fluctuation incorporating with the interlayer disorder, which would be suppressed when the holon pseudogap opens at low temperatures and lower doping levels, leading to the crossovers to the semiconducting-like range in the c-axis resistivity and the temperature linear to the nonlinear range in the in-plane resistivity. Comment: 21 pages, Revtex, Six figures are included
01/2000;
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ABSTRACT: The magnetic properties of the spin liquid state of the antiferromagnetic Heisenberg model on the kagomé lattice are investigated
within the self-consistent mean-field theory. The results show that the spin liquid ground-state energy per site is , which is in very good agreement with the best numerical estimates. The spin structure factor and spin susceptibility are
also discussed.
Physics of Condensed Matter 12/1999; 13(2):265-269. · 1.53 Impact Factor
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ABSTRACT: The dynamical spin structure factor and the Raman response are calculated for structurally dimerized and spin-Peierls chains in a magnetic field, using exact diagonalization techniques. In both cases there is a spin liquid phase composed of interacting singlet dimers at small fields h < h_c1, an incommensurate regime (h_c1 < h < h_c2) in which the modulation of the triplet excitation spectra adapts to the applied field, and a fully spin polarized phase above an upper critical field h_c2. For structurally dimerized chains, the spin gap closes in the incommensurate phase, whereas spin-Peierls chains remain gapped. In the spin liquid regimes, the dominant feature of the triplet spectra is a one-magnon bound state, separated from a continuum of states at higher energies. There are also indications of a singlet bound state above the one-magnon triplet. Comment: RevTex, 10 pages with 8 eps figures
09/1999;
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ABSTRACT: Within the t-J model, we study the electronic properties of a doped antiferromagnet on a kagomé lattice based on the framework of the self-consistent mean-field theory. At the half-filling, the spin-liquid ground-state energy per site of the kagomé antiferromagnet is Eg/NsJ=-0.859, which is in very good agreement with the numerical estimates. Away from the half-filling, the electron photoemission spectroscopy and density of states are discussed, and the results indicate that there is a gap in the normal state of the system.
Phys. Rev. B. 05/1999; 59(21).
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ABSTRACT: We report both 77Se and 87Rb nuclear magnetic resonance (NMR) studies on Tl0.47Rb0.34Fe1.63Se2 single-crystalline superconductors (Tc≈ 32 K). Singlet superconductivity is suggested by a sharp drop of the Knight shift K(T) below Tc, after subtracting the superconducting diamagnetic effect. However, the Hebel-Slichter coherence peak below Tc is not observed in the spin-lattice relaxation rate 1/T1, even with a low in-plane NMR field of 2.6 T. Just above Tc, no evidence of low-energy spin fluctuation is found in the spin-lattice relaxation rate on both the 77Se and the 87Rb sites. Upon warming, however, the Knight shifts and the spin-lattice relaxation rates of both nuclei increase substantially with temperature. In particular, the Knight shift is nearly isotropic and follow a function fit of K=a+bT2 from Tc up to 300 K. Our observations should put a strong constraint to the theory of magnetism and superconductivity in the recently discovered iron selenide superconductors.
Phys. Rev. B. 83(17).
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ABSTRACT: In this Rapid Communication, we reported the results of NMR study on LiFeAs single crystals. We find a strong evidence of the low-temperature spin fluctuations; by changing sample preparation conditions, the system can be tuned toward a spin-density-wave (SDW) quantum-critical point. The detection of an interstitial Li(2) ion, possibly locating in the tetrahedral hole, suggests that the off-stoichiometry and/or lattice defect can probably account for the absence of the SDW ordering in LiFeAs. These facts show that LiFeAs is a strongly correlated system and the superconductivity is likely originated from the SDW fluctuations.
Phys. Rev. B. 82(18).
