1,268 research outputs found
Experimental evidence of enhancement without the influence of spin fluctuations: NMR study on LaFeAsO_{1-x}H_x under a pressure of 3.0 GPa
The electron-doped high-transition-temperature (T_c) iron-based pnictide
superconductor LaFeAsO_{1-x}H_x has a unique phase diagram: superconducting
(SC) double domes are sandwiched by antiferromagnetic phases at ambient
pressure and they turn to a single dome with a maximum T_c that exceeds 45K at
a pressure of 3.0 GPa. We studied whether spin fluctuations are involved in
increasing T_c under a pressure of 3.0 GPa by using ^{75}As nuclear magnetic
resonance (NMR) technique. The ^{75}As-NMR results for the powder samples show
that T_c increases up to 48 K without the influence of spin fluctuations. The
fact indicates that spin fluctuations are not involved in raising T_c, which
implies that other factors, such as orbital degrees of freedom, may be
important for achieving a high T_c of almost 50 K.Comment: Correponding Author: Naoki Fujiwar
^{75}As NMR study of the growth of paramagnetic-metal domains due to electron doping near the superconducting phase in LaFeAsO_{1-x}F_{x}
We studied the electric and magnetic behavior near the phase boundary between
antiferromagnetic (AF) and superconducting (SC) phases for a prototype of
high-T_c pnictides LaFeAsO_{1-x}F_{x} by using nuclear magnetic resonance, and
found that paramagnetic-metal (PM) domains segregate from AF domains. PM
domains grow in size with increasing electron doping level and are accompanied
by the onset of superconductivity, and thus application of pressure or
increasing the doping level causes superconductivity. The existence of PM
domains cannot be explained by the existing paradigm that focuses only on the
relationship between superconductivity and antiferromagnetism. Based on orbital
fluctuation theory, the existence of PM domains is evidence of the
ferroquadrupole state.Comment: 5 figure
Quantum critical behavior in heavily doped LaFeAsOH pnictide superconductors analyzed using nuclear magnetic resonance
We studied the quantum critical behavior of the second antiferromagnetic (AF)
phase in the heavily electron-doped high- pnictide, LaFeAsOH
by using As and H nuclear-magnetic-resonance (NMR) technique. In
the second AF phase, we observed a spatially modulated spin-density-wave-like
state up to =0.6 from the NMR spectral lineshape and detected a low-energy
excitation gap from the nuclear relaxation time of As. The
excitation gap closes at the AF quantum critical point (QCP) at . The superconducting (SC) phase in a lower-doping regime contacts the
second AF phase only at the AF QCP, and both phases are segregated from each
other. The absence of AF critical fluctuations and the enhancement of the
in-plane electric anisotropy are key factors for the development of
superconductivity.Comment: accepted in Phys. Rev.
Seiberg-Witten prepotential for E-string theory and random partitions
We find a Nekrasov-type expression for the Seiberg-Witten prepotential for
the six-dimensional non-critical E_8 string theory toroidally compactified down
to four dimensions. The prepotential represents the BPS partition function of
the E_8 strings wound around one of the circles of the toroidal
compactification with general winding numbers and momenta. We show that our
expression exhibits expected modular properties. In particular, we prove that
it obeys the modular anomaly equation known to be satisfied by the
prepotential.Comment: 14 page
Detection of antiferromagnetic ordering in heavily doped LaFeAsO1-xHx pnictide superconductors using nuclear-magnetic-resonance techniques
We studied double superconducting (SC) domes in LaFeAsO1-xHx by using 75As-
and 1H-nuclear magnetic resonance techniques, and unexpectedly discovered that
a new antiferromagnetic (AF) phase follows the double SC domes on further H
doping, forming a symmetric alignment of AF and SC phases in the electronic
phase diagram. We demonstrated that the new AF ordering originates from the
nesting between electron pockets, unlike the nesting between electron and hole
pockets as seen in the majority of undoped pnictides. The new AF ordering is
derived from the features common to high-Tc pnictides: however, it has not been
reported so far for other high-Tc pnictides because of their poor electron
doping capability.Comment: 5 figures, in press in PR
Origin of critical-temperature enhancement of an iron-based high-T_c superconductor, LaFeAsO_{1-x}F_{x} : NMR study under high pressure
Nuclear magnetic resonance (NMR) measurements of an iron (Fe)-based
superconductor LaFeAsO_{1-x}F_x (x = 0.08 and 0.14) were performed at ambient
pressure and under pressure. The relaxation rate 1/T_1 for the overdoped
samples (x = 0.14) shows T-linear behavior just above T_c, and pressure
application enhances 1/T_1T similar to the behavior of T_c. This implies that
1/T_1T = constant originates from the Korringa relation, and an increase in the
density of states at the Fermi energy D(E_F) leads to the enhancement of T_c.
In the underdoped samples (x = 0.08), 1/T_1T measured at ambient pressure also
shows T-independent behavior in a wide temperature range above T_c. However, it
shows Curie-Weiss-like T dependence at 3.0 GPa accompanied by a small increase
in T_c, suggesting that predominant antiferromagnetic fluctuation suppresses
development of superconductivity or remarkable enhancement of T_c. The
qualitatively different features between underdoped and overdoped samples are
systematically explained by a band calculation with hole and electron pockets
Spin density wave and superconductivity in CaFe_{1-x}Co_{x}AsF studied by nuclear magnetic resonance
We performed nuclear magnetic resonance (NMR) measurements to investigate the
evolution of spin-density-wave (SDW) and superconducting (SC) states upon
electron doping in CaFe_{1-x}Co_{x}AsF, which exhibits an intermediate phase
diagram between those of LaFeAsO_{1-x}F_x and Ba(Fe_{1-x}Co_x)_2As_2. We found
that homogeneous coexistence of the incommensurate SDW and SC states occurs
only in a narrow doping region around the crossover regime, which supports
S_{+-}-wave symmetry. However, only the structural phase transition survives
upon further doping, which agrees with predictions from orbital fluctuation
theory. The transitional features upon electron doping imply that both spin and
orbital fluctuations are involved in the superconducting mechanism
Magnetic order in CaFe1-xCoxAsF (x = 0, 0.06, 0.12) superconductor compounds
A Neutron Powder Diffraction (NPD) experiment has been performed to
investigate the structural phase transition and magnetic order in CaFe1-xCoxAsF
superconductor compounds (x = 0, 0.06, 0.12). The parent compound CaFeAsF
undergoes a tetragonal to orthorhombic phase transition at 134(3) K, while the
magnetic order in form of a spin-density wave (SDW) sets in at 114(3) K. The
antiferromagnetic structure of the parent compound has been determined with a
unique propagation vector k = (1,0,1) and the Fe saturation moment of 0.49(5)uB
aligned along the long a-axis. With increasing Co doping, the long range
antiferromagnetic order has been observed to coexist with superconductivity in
the orthorhombic phase of the underdoped CaFe0.94Co0.06AsF with a reduced Fe
moment (0.15(5)uB). Magnetic order is completely suppressed in optimally doped
CaFe0.88Co0.12AsF. We argue that the coexistence of SDW and superconductivity
might be related to mesoscopic phase separation.Comment: 4pages, 4figure
Homogeneous coexistence of SDW and SC states in CaFe(1-x)Co(x)AsF studied by nuclear magnetic resonance
We investigated the homogeneous coexistence of spin-density-wave (SDW) and
superconducting (SC) states via 75As-nuclear magnetic resonance (NMR) in
CaFe(1-x)Co(x)AsF and found that the electronic and magnetic properties of this
compound are intermediate between those of LaFeAsO(1-x)F(x) and
Ba(Fe(1-x)Co(x))2As2. For 6% Co-doped samples, the paramagnetic spectral weight
completely disappears in the crossover regime between the SDW and SC phases
followed by the anomalous behavior of relaxation rate (1/T1), implying that the
two phases are not segregated. The 59Co-NMR spectra show that spin moments are
not commensurate but spatially modulated. These experimental results suggest
that incommensurate SDW (IC-SDW) and SC states are compatible in this compound.Comment: 5 pages, 4 figure
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