High-transition temperature (high-$T_c$) superconductivity in the iron pnictides/chalcogenides emerges from the suppression of the static antiferromagnetic order in their parent compounds, similar to copper oxides superconductors. This raises a fundamental question concerning the role of magnetism in the superconductivity of these materials. Neutron scattering, a powerful probe to study the magnetic order and spin dynamics, plays an essential role in determining the relationship between magnetism and superconductivity in high-$T_c$ superconductors. The rapid development of modern neutron time-of-flight spectrometers allows a direct determination of the spin dynamical properties of iron-based superconductors throughout the entire Brillouin zone. In this review, we present an overview of the neutron scattering results on iron-based superconductors, focusing on the evolution of spin excitation spectra as a function of electron/hole-doping and isoelectronic substitution. We compare spin dynamical properties of iron-based superconductors with those of copper oxide and heavy fermion superconductors, and discuss the common features of spin excitations in these three families of unconventional superconductors and their relationship with superconductivity.

Antiferromagnetic order and spin dynamics in iron-based superconductors

The pseudogap regime of high-temperature cuprates harbours diverse manifestations of electronic ordering whose exact nature and universality remain debated. Here, we show that the short-ranged charge order recently reported in the normal state of YBa2Cu3Oy corresponds to a truly static modulation of the charge density. We also show that this modulation impacts on most electronic properties, that it appears jointly with intra-unit-cell nematic, but not magnetic, order, and that it exhibits differences with the charge density wave observed at lower temperatures in high magnetic fields. These observations prove mostly universal, they place new constraints on the origin of the charge density wave and they reveal that the charge modulation is pinned by native defects. Similarities with results in layered metals such as NbSe2, in which defects nucleate halos of incipient charge density wave at temperatures above the ordering transition, raise the possibility that order-parameter fluctuations, but no static order, would be observed in the normal state of most cuprates if disorder were absent.

/pdf/incipient-charge-order-observed-by-nmr-in-the-normal-state-12k849lm1u.pdf

Incipient charge order observed by NMR in the normal state of YBa2Cu3Oy

Fluid Mechanics of Hydraulic Fracturing: a Review

Nearly 70 years old, hydraulic fracturing is a core technique for stimulating hydrocarbon production in a majority of oil and gas reservoirs. Complex fluids are implemented in nearly every step of the fracturing process, most significantly to generate and sustain fractures and transport and distribute proppant particles during and following fluid injection. An extremely wide range of complex fluids are used: naturally occurring polysaccharide and synthetic polymer solutions, aqueous physical and chemical gels, organic gels, micellar surfactant solutions, emulsions, and foams. These fluids are loaded over a wide range of concentrations with particles of varying sizes and aspect ratios and are subjected to extreme mechanical and environmental conditions. We describe the settings of hydraulic fracturing (framed by geology), fracturing mechanics and physics, and the critical role that non-Newtonian fluid dynamics and complex fluids play in the hydraulic fracturing process.

/pdf/complex-fluids-and-hydraulic-fracturing-ctnsfz5suf.pdf

Complex Fluids and Hydraulic Fracturing

Universal scaling laws can guide the understanding of new phenomena, and for cuprate high-temperature superconductivity the influential Uemura relation showed, early on, that the maximum critical temperature of superconductivity correlates with the density of the superfluid measured at low temperatures. Here we show that the charge content of the bonding orbitals of copper and oxygen in the ubiquitous CuO2 plane, measured with nuclear magnetic resonance, reproduces this scaling. The charge transfer of the nominal copper hole to planar oxygen sets the maximum critical temperature. A three-dimensional phase diagram in terms of the charge content at copper as well as oxygen is introduced, which has the different cuprate families sorted with respect to their maximum critical temperature. We suggest that the critical temperature could be raised substantially if one were able to synthesize materials that lead to an increased planar oxygen hole content at the expense of that of planar copper.

/pdf/perspective-on-the-phase-diagram-of-cuprate-high-temperature-39ct1x6i6h.pdf

Perspective on the phase diagram of cuprate high-temperature superconductors.

Abstract Nitrogen-vacancy (NV) centers in diamond have been developed into essential hardware units for a wide range of solid-state-based quantum technology applications. While such applications require the long spin coherence times of the NV centers, they are often limited due to decoherence. In this study, we theoretically investigate the decoherence of NV-spin ensembles induced by nitrogen impurities (P1 centers), which are one of the most dominant and inevitable magnetic field noise sources in diamond. We combined cluster correlation expansion and density functional theory to compute the Hahn-echo spin-coherence time of the NV centers for a broad range of P1 concentrations. Results indicate a clear linear dependence of T 2 on P1 concentrations on a log scale with a slope of −1.06, which is in excellent agreement with previous experimental results. The interplay between the Jahn–Teller effect and the hyperfine interaction in the P1 center plays a critical role in determining the bath dynamics and the resulting NV decoherence. Our results provide a theoretical upper bound for the NV-spin T 2 over a wide range of P1 densities, serving as a key reference for materials optimization and spin bath characterization to develop highly coherent NV-based devices for quantum information technology. 

/pdf/decoherence-of-nitrogen-vacancy-spin-ensembles-in-a-nitrogen-22pk6vf5.pdf

Decoherence of nitrogen-vacancy spin ensembles in a nitrogen electron-nuclear spin bath in diamond

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 YBa2Cu3O7−x, Bi2SrCa2Cu2O8+δ, and Tl2Ba2CuO6+δ. 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 Bi2SrCa2Cu2O8+δ. 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.

Nuclear magnetic resonance studies of vortices in high temperature superconductors

NMR measurements have been performed on single crystals of Ba1 xKxFe2As2 (x = 0, 0.45) and CaFe2As2 grown from Sn ux. The Ba-based pnictide crystals contain signicant amounts of Sn in their structure, 1%, giving rise to magnetic impurity eects 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 eld 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.

Magnetic Impurities in the Pnictide Superconductor

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.

Magnetic Impurities in the Pnictide Superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

The spatially averaged density of states of an unconventional d-wave superconductor is magnetic field dependent, proportional to H-1/2, 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/T-1 proportional to , for an s(perpendicular to) state which will depend on the magnetic field. We have measured the As-75 1/T-1 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/T-1 which increase closer to the vortex core, with a spatial average proportional to H-2, inconsistent with recent theory [Y. Bang, Phys. Rev. B 85, 104524 (2012)].

Magnetic field dependence of spin-lattice relaxation in the s(+/-) state of Ba0.67K0.33Fe2As2

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