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F. L. Ning

Publications and source records attributed to F. L. Ning.

36 records · Page 2Linked to original sources
arXiv2011

Spin Ice: Magnetic Excitations without Monopole Signatures using Muon Spin Rotation

Theory predicts the low-temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (muSR) experiment [S T Bramwell et al, Nature 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation - the so-called second Wien effect. We demonstrate via a new series of muSR experiments in Dy_2Ti_2O_7 that such an effect is not observable in a TF muSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.

cond-mat.str-el↗
arXiv2011

Magnetism in superconducting EuFe$_2$As$_{1.4}$P$_{0.6}$ single crystals studied by local probes

We have studied the magnetism in superconducting single crystals of EuFe2 As1.4 P0.6 by using the local probe techniques of zero-field muon spin rotation/relaxation and 151 Eu/57 Fe Mössbauer spec- troscopy. All of these measurements reveal magnetic hyperfine fields below the magnetic ordering temperature TM = 18 K of the Eu2+ moments. The analysis of the data shows that there is a coexistence of ferromagnetism, resulting from Eu2+ moments ordered along the crystallographic c-axis, and superconductivity below TSC \approx 15 K. We find indications for a change in the dynamics of the small Fe magnetic moments (\sim 0.07 μB) at the onset of superconductivity: below TSC the Fe magnetic moments seem to be "frozen" within the ab-plane.

cond-mat.str-el↗
arXiv2011

Absence of static magnetic order in lightly-doped Ti1-xScxOCl down to 1.7 K

Impurity-induced magnetic order has been observed in many quasi-1D systems including doped variants of the spin-Peierls system CuGeO3. TiOCl is another quasi-1D quantum magnet with a spin-Peierls ground state, and the magnetic Ti sites of this system can be doped with non-magnetic Sc. To investigate the role of non-magnetic impurities in this system, we have performed both zero field and longitudinal field muSR experiments on polycrystalline Ti1-xScxOCl samples with x = 0, 0.01, and 0.03. We verified that TiOCl has a non-magnetic ground state, and we found no evidence for spin freezing or magnetic ordering in the lightly-doped Sc samples down to 1.7 K. Our results instead suggest that these systems remain non-magnetic up to the x = 0.03 Sc doping level.

cond-mat.str-el↗
arXiv2010

Contrasting Spin Dynamics Between Underdoped and Overdoped Ba(Fe[1-x]Co[x])2As2

We report the first NMR investigation of spin dynamics in the overdoped non-superconducting regime of Ba(Fe[1-x]Co[x])2As2 up to x =0.26. We demonstrate that the absence of inter-band transitions with large momentum transfer Q(AF) ~ (pi, 0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x > 0.15. Our experimental results provide direct evidence for a correlation between Tc and the strength of Q(AF) antiferromagnetic spin fluctuations.

cond-mat.supr-con↗
arXiv2009

59Co and 75As NMR Investigation of the Lightly Doped Ba[Fe(1-x)Co(x)]2As2 (x =0.02, 0.04)

We investigate the electronic properties of Ba[Fe(1-x)Co(x)]2As2 (x =0.02, 0.04) in the lightly electron-doped regime by 59Co and 75As NMR. We demonstrate that Co doping significantly suppresses the magnetic ordering temperature to the SDW state, T_{SDW}. Furthermore, ordered moments below T_{SDW} exhibit large distribution. Strong spin fluctuations remain even below T_{SDW}, persisting all the way down to 4.2 K. We find no signature of additional freezing of spin degrees of freedom unlike the case of the lightly hole-doped stripe phase of the cuprates.

cond-mat.supr-con↗
arXiv2009

The Electronic Phase Diagram of the Iron-based High Tc Superconductor Ba(Fe(1-x)Co(x))2As2 Under Hydrostatic Pressure (0 < x <0.099)

We report comprehensive resistivity measurements of single crystalline samples of the Ba(Fe(1-x)Co(x))2As2 high Tc superconductor under hydrostatic pressure up to 2.75 GPa and over a broad concentration range, 0 < x < 0.099. We show that application of pressure progressively suppresses the SDW transition temperature, T_SDW, in the underdoped regime. There is no sign of pressure-induced superconductivity in the undoped BaFe2As2 down to 1.8 K, but applied pressure dramatically enhances Tc in the underdoped regime. The effect of pressure on Tc is very small in the optimally and overdoped regimes. As a consequence, the dome of the superconducting phase extends to x 0.02 under pressure. We discuss the implications of our findings in the context of a possible quantum phase transition between the SDW and superconducting phases.

cond-mat.supr-con↗
arXiv2009

Why Does Undoped FeSe Become A High Tc Superconductor Under Pressure?

Unlike the parent phases of the iron-arsenide high Tc superconductors, undoped FeSe is not magnetically ordered and exhibits superconductivity with Tc~9K. Equally surprising is the fact that applied pressure dramatically enhances the modest Tc to ~37K. We investigate the electronic properties of FeSe using 77Se NMR to search for the key to the superconducting mechanism. We demonstrate that the electronic properties of FeSe are very similar to those of electron-doped FeAs superconductors, and that antiferromagnetic spin fluctuations are strongly enhanced near Tc. Furthermore, applied pressure enhances spin fluctuations. Our findings suggest a link between spin fluctuations and the superconducting mechanism in FeSe.

cond-mat.supr-con↗
arXiv2008

Spin susceptibility, phase diagram, and quantum criticality in the electron-doped high Tc Superconductor Ba[Fe(1-x)Co(x)]2As2

We report a systematic investigation of Ba[Fe(1-x)Co(x)]2As2 based on transport and 75-As NMR measurements, and establish the electronic phase diagram. We demonstrate that doping progressively suppresses the uniform spin susceptibility and low frequency spin fluctuations. The optimum superconducting phase emerges at x_c~0.08 when the tendency toward spin ordering completely diminishes. Our findings point toward the presence of a quantum critical point near x_c between the SDW (spin density wave) and superconducting phases.

cond-mat.supr-con↗
arXiv2008

Pressure effects on the electron-doped high Tc superconductor BaFe(2-x)Co(x)As(2)

Application of pressures or electron-doping through Co substitution into Fe sites transforms the itinerant antiferromagnet BaFe(2)As(2) into a superconductor with the Tc exceeding 20K. We carried out systematic transport measurements of BaFe(2-x)Co(x)As(2) superconductors in pressures up to 2.5GPa, and elucidate the interplay between the effects of electron-doping and pressures. For the underdoped sample with nominal composition x = 0.08, application of pressure strongly suppresses a magnetic instability while enhancing Tc by nearly a factor of two from 11K to 21K. In contrast, the optimally doped x=0.20 sample shows very little enhancement of Tc=22K under applied pressure. Our results strongly suggest that the proximity to a magnetic instability is the key to the mechanism of superconductivity in iron-pnictides.

cond-mat.supr-con↗
arXiv2008

NMR measurements of intrinsic spin susceptibility in LaFeAsO(0.9)F(0.1)

We will probe the intrinsic behavior of spin susceptibility chi_(spin) in the LaFeAsO(1-x)F(x) superconductor (x ~ 0.1, Tc ~ 27K) using 19-F and 75-As NMR techniques. Our new results firmly establish the pseudo-gap behavior with Delta_(PG)/kB ~ 140K. The estimated magnitude of chi_(spin) at 290K, ~1.8x10^(-4) [emu/mol-Fe], is approximately twice larger than that in high Tc cuprates. We also show that chi_(spin) levels off below ~50K down to Tc.

cond-mat.supr-con↗
arXiv2008

59-Co and 75-As NMR Investigation of Electron-Doped High Tc Superconductor BaFe(1.8)Co(0.2)As(2) (Tc = 22K)

We report an NMR investigation of the superconductivity in BaFe(2)As(2) induced by Co doping (Tc=22K). We demonstrate that Co atoms form an alloy with Fe atoms and donate carriers without creating localized moments. Our finding strongly suggests that the underlying physics of iron-pnictide superconductors is quite different from the widely accepted physical picture of high Tc cuprates as doped Mott insulators. We also show a crossover of electronic properties into a low temperature pseudo-gap phase with a pseudo-gap Delta 560K, where chi(spin) constant and resisitivty T. The NMR Knight shift below Tc decreases for both along the c-axis and ab-plane, and is consistent with the singlet pairing scenario.

cond-mat.supr-con↗
arXiv2008

^{59}Co NMR evidence for charge ordering below T_{CO}\sim 51 K in Na_{0.5}CoO_2

The CoO$_{2}$ layers in sodium-cobaltates Na$_{x}$CoO$_{2}$ may be viewed as a spin $S=1/2$ triangular-lattice doped with charge carriers. The underlying physics of the cobaltates is very similar to that of the high $T_{c}$ cuprates. We will present unequivocal $^{59}$Co NMR evidence that below $T_{CO}\sim51 K$, the insulating ground state of the itinerant antiferromagnet Na$_{0.5}$CoO$_{2}$ ($T_{N}\sim 86 K$) is induced by charge ordering.

cond-mat.str-el↗
arXiv2007

Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2

The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hampered by the difficulty of controlling pure phases. We discovered that certain Na concentrations are specially stable and are associated with superlattice ordering of the Na clusters. This leads naturally to a picture of co-existence of localized spins and itinerant charge carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our picture brings coherence to a variety of measurements ranging from NMR to optical to thermal transport. Our results also allow us to take the first step towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We suggest the local moments may form a quantum spin liquid state and we propose experimental test of our hypothesis.

cond-mat.str-el↗
arXiv2005

17-O NMR studies of a triangular-lattice superconductor Na{x}CoO{2}yH{2}O

We report the first 17-O NMR studies of a triangular-lattice superconductor Na{1/3}CoO{2}-[4/3]H{2}O and the host material Na{x}CoO{2} (x=0.35 and 0.72). Knight shift measurements reveal that p-d hybridization induces sizable spin polarization in the O triangular-lattice layers. Water-intercalation makes CoO{2} planes homogeneous, and enhances low frequency spin fluctuations near T{c}=4.5 K at some finite wave-vectors different from both the ferromagnetic and ``120 degree'' modes.

cond-mat.str-el↗
arXiv2004

Spin-Dynamics in the Carrier-Doped $S=1/2$ Triangular-Lattice of Na$_{x}$CoO$_{2}$-yH$_{2}$O

We probed the local electronic properties of the mixed-valent Co(+4-x) triangular-lattice in Na{x}CoO{2}-yH{2}O by 59-Co NMR. We observed two distinct types of Co sites for x>=1/2, but the valence seems averaged out for x~1/3. Local spin fluctuations exhibit qualitatively the same trend down to ~100 K regardless of the carrier-concentration x, and hence the nature of the electronic ground state. A canonical Fermi-liquid behavior emerges below ~100 K only for x~1/3.

cond-mat.str-el↗