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A. M. Mounce,
Sangwon Oh,
Jeongseop A. Lee,
W. P. Halperin,
A. P. Reyes,
P. L. Kuhns,
M. K. Chan,
C. Dorow,
L. Ji,
D. Xia,
X. Zhao,
M. Greven
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ABSTRACT: The simple structure of HgBa$_2$CuO$_{4+\delta}$ (Hg1201) is ideal among
cuprates for study of the pseudogap phase as a broken symmetry state. We have
performed $^{17}$O nuclear magnetic resonance (NMR) on an underdoped Hg1201
crystal with transition temperature of 74 K to look for circulating orbital
currents proposed theoretically and inferred from neutron scattering. The
narrow spectra preclude static local fields in the pseudogap phase at the
apical site, suggesting that the moments observed with neutrons are
fluctuating. The NMR frequency shifts are consistent with a dipolar field from
the Cu$^{+2}$ site.
04/2013;
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ABSTRACT: Based on ARPES and STM work, Co doped NaFeAs has been reported to be a
strongly coupled superconductor, similar to cuprates. However, a recent ARPES
result, claims otherwise, with $2\Delta/k_BT_c$ slightly beyond the BCS limit,
3.53. We have performed $^{75}$As and $^{11}$Na nuclear magnetic resonance(NMR)
on high-quality, single crystals of NaFe$_{0.975}$Co$_{0.025}$As. The
temperature dependence of the $^{75}$As Knight shift, fitted to a Yosida
function, suggests moderately weak-coupling superconductivity in a singlet
state with $2\Delta/k_BT_c = 4.0\pm0.3$. The temperature dependence of the
penetration depth obtained from $^{11}$Na NMR was found to be consistent with
the Knight shift results and $\lambda_{ab}(0)$ was found to be $5,327 \pm 78
\AA$.
02/2013;
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ABSTRACT: The distinct distribution of local magnetic fields due to superconducting
vortices can be detected with nuclear magnetic resonance (NMR) and used to
investigate vortices and related physical properties of extreme type II
superconductivity. This review summarizes work on high temperature
superconductors (HTS) including cuprates and pnictide materials. Recent
experimental results are presented which reveal the nature of vortex matter and
novel electronic states. For example, the NMR spectrum has been found to
provide a sharp indication of the vortex melting transition. In the vortex
solid a frequency dependent spin-lattice relaxation has been reported in
cuprates, including YBa$_2$Cu$_3$O$_{7-x}$, Bi$_2$SrCa$_2$Cu$_2$O$_{8+\delta}$,
and Tl$_2$Ba$_2$CuO$_{6+\delta}$. These results have initiated a new
spectroscopy via Doppler shifted nodal quasiparticles for the investigation of
vortices. At very high magnetic fields this approach is a promising method for
the study of vortex core excitations. These measurements have been used to
quantify an induced spin density wave near the vortex cores in
Bi$_2$SrCa$_2$Cu$_2$O$_{8+\delta}$. Although the cuprates have a different
superconducting order parameter than the iron arsenide superconductors there
are, nonetheless, some striking similarities between them regarding vortex
dynamics and frequency dependent relaxation.
12/2011;
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ABSTRACT: The spatially averaged density of states 〈N(0)〉 of an unconventional d-wave superconductor is magnetic field dependent, proportional to H1/2, owing to the Doppler shift of quasiparticle excitations in a background of vortex supercurrents [ G. E. Volovik J. Phys. C 21 L221 (1988); JETP Lett. 58 469 (1993)]. This phenomenon, called the Volovik effect, has been predicted to exist for a sign-changing s± state [ Y. Bang Phys. Rev. Lett. 104 217001 (2010)], although it is absent in a single-band s-wave superconductor. Consequently, we expect there to be Doppler contributions to the NMR spin-lattice relaxation rate, 1/T1∝〈N(0)2〉, for an s± state which will depend on the magnetic field. We have measured the 75As 1/T1 in a high-quality single crystal of Ba0.67K0.33Fe2As2 over a wide range of field up to 28 T. Our spatially resolved measurements show that indeed there are Doppler contributions to 1/T1 which increase closer to the vortex core, with a spatial average proportional to H2, inconsistent with recent theory [ Y. Bang Phys. Rev. B 85 104524 (2012)].
Phys. Rev. B. 09/2011; 85(17).
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ABSTRACT: Competition with magnetism is at the heart of high-temperature superconductivity, most intensely felt near a vortex core. To investigate vortex magnetism we have developed a spatially resolved probe based upon NMR spin-lattice-relaxation spectroscopy. With this approach we have found a spin-density wave associated with the vortex core in Bi(2)Sr(2)CaCu(2)O(8+y), similar to checkerboard patterns in the local density of electronic states reported from scanning tunneling microscope experiments. We have determined both the spin-modulation amplitude and decay length from the vortex core in fields up to H=30 T.
Physical Review Letters 02/2011; 106(5):057003. · 7.37 Impact Factor
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ABSTRACT: The superconducting state of an optimally doped single crystal of
Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ was investigated by $^{75}$As NMR in high
magnetic fields from 6.4 T to 28 T. It was found that the Knight shift is least
affected by vortex supercurrents in high magnetic fields, $H>11$ T, revealing
slow, possibly higher order than linear, increase with temperature at $T
\lesssim 0.5 \, T_c$, with $T_c \approx 23 \, K$. This is consistent with the
extended s-wave state with $A_{1g}$ symmetry but the precise details of the gap
structure are harder to resolve. Measurements of the NMR spin-spin relaxation
time, $T_2$, indicate a strong indirect exchange interaction at all
temperatures. Below the superconducting transition temperature vortex dynamics
lead to an anomalous dip in $T_2$ at the vortex freezing transition from which
we obtain the vortex phase diagram up to $H = 28$ T.
Phys. Rev. B. 01/2011; 83(21).
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ABSTRACT: Competition with magnetism is at the heart of high temperature superconductivity, most intensely felt near a vortex core. To investigate vortex magnetism we have developed a spatially resolved probe using nuclear magnetic resonance. Our spin-lattice-relaxation spectroscopy is spatially resolved both within a conduction plane as well as from one plane to another. With this approach we have found a spin-density wave associated with the vortex core in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, which is expected from scanning tunneling microscope observations of "checkerboard" patterns in the local density of electronic states.[1] We determine both the spin-modulation amplitude and decay length from the vortex core in fields up to H=30 T. Comment: 7 pages, 5 figures
11/2010;
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ABSTRACT: It has been predicted that superconducting vortices should be electrically charged and that this effect is particularly enhanced for, high temperature superconductors.\cite{kho95,bla96} Hall effect\cite{hag91} and nuclear magnetic resonance (NMR) experiments\cite{kum01} suggest the existence of vortex charging, but the effects are small and the interpretation controversial. Here we show that the Abrikosov vortex lattice, characteristic of the mixed state of superconductors, will become unstable at sufficiently high magnetic field if there is charge trapped on the vortex core. Our NMR measurements of the magnetic fields generated by vortices in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+y}$ single crystals\cite{che07} provide evidence for an electrostatically driven vortex lattice reconstruction with the magnitude of charge on each vortex pancake of $\mathbf{\sim 2}$x$\mathbf{10^{-3} e}$, depending on doping, in line with theoretical estimates.\cite{kho95,kna05} Comment: to appear in Nature Physics; 6 pages, 7 figures
09/2010;
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ABSTRACT: Nuclear magnetic resonance (NMR) measurements have been performed on single crystals of Ba1−xKxFe2As2 (x=0, 0.45) and CaFe2As2 grown from Sn flux. The Ba-based pnictide crystals contain significant amounts of Sn in their structure, ~1%, giving rise to magnetic impurity effects evident in the NMR spectrum and in the magnetization. Our experiments show that the large impurity magnetization is broadly distributed on a microscopic scale, generating substantial magnetic field gradients. There is a concomitant 20% reduction in the transition temperature, which is most likely due to magnetic electron scattering. We suggest that the relative robustness of superconductivity (x=0.45) in the presence of severe magnetic inhomogeneity might be accounted for by strong spatial correlations between impurities, such as clustering on the coherence length scale.
New Journal of Physics 05/2009; 11(5):055002. · 4.18 Impact Factor
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ABSTRACT: NMR measurements have been performed on single crystals of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ (x = 0, 0.45) and CaFe$_2$As$_2$ grown from Sn flux. The Ba-based pnictide crystals contain significant amounts of Sn in their structure, $\sim 1$%, giving rise to magnetic impurity effects evident in the NMR spectrum and in the magnetization. Our experiments show that the large impurity magnetization is broadly distributed on a microscopic scale, generating substantial magnetic field gradients. There is a concomitant 20% reduction in the transition temperature which is most likely due to magnetic electron scattering. We suggest that the relative robustness of superconductivity ($x=0.45$) in the presence of severe magnetic inhomogeneity might be accounted for by strong spatial correlations between impurities on the coherence length scale.
04/2009;
