Kent Shirer

Long range order and two-fluid behavior in heavy electron materials

The heavy electron Kondo liquid is an emergent state of condensed matter that displays universal behavior independent of material details. Properties of the heavy electron liquid are best probed by NMR Knight shift measurements, which provide a direct measure of the behavior of the heavy electron liquid that emerges below the Kondo lattice coherence temperature as the lattice of local moments hybridizes with the background conduction electrons. Because the transfer of spectral weight between the localized and itinerant electronic degrees of freedom is gradual, the Kondo liquid typically coexists with the local moment component until the material orders at low temperatures. The two-fluid formula captures this behavior in a broad range of materials in the paramagnetic state. In order to investigate two-fluid behavior and the onset and physical origin of different long range ordered ground states in heavy electron materials, we have extended Knight shift measurements to URu2Si2, CeIrIn5, and CeRhIn5. In CeRhIn5 we find that the antiferromagnetic order is preceded by a relocalization of the Kondo liquid, providing independent evidence for a local moment origin of antiferromagnetism. In URu2Si2 the hidden order is shown to emerge directly from the Kondo liquid and so is not associated with local moment physics. Our results imply that the nature of the ground state is strongly coupled with the hybridization in the Kondo lattice in agreement with phase diagram proposed by Yang and Pines.

Coexistence of cluster spin glass and superconductivity in Ba(Fe(1-x)Co(x))2As2 for 0.060≤x≤0.071.

We present 75As nuclear magnetic resonance data from measurements of a series of Ba(Fe(1-x)Co(x))2As2 crystals with 0.00≤x≤0.075 that reveals the coexistence of frozen antiferromagnetic domains and superconductivity for 0.060≤x≤0.071. Although bulk probes reveal no long range antiferromagnetic order beyond x=0.06, we find that the local spin dynamics reveal no qualitative change across this transition. The characteristic domain sizes vary by more than an order of magnitude, reaching a maximum variation at x=0.06. This inhomogeneous glassy dynamics may be an intrinsic response to the competition between superconductivity and antiferromagnetism in this system.

NMR evidence for inhomogeneous nematic fluctuations in BaFe2(As1-xPx)2

We present evidence for nuclear spin-lattice relaxation driven by glassy nematic fluctuations in isovalent P-doped BaFe_{2}As_{2} single crystals. Both the ^{75}As and ^{31}P sites exhibit a stretched-exponential relaxation similar to the electron-doped systems. By comparing the hyperfine fields and the relaxation rates at these sites we find that the As relaxation cannot be explained solely in terms of magnetic spin fluctuations. We demonstrate that nematic fluctuations couple to the As nuclear quadrupolar moment and can explain the excess relaxation. These results suggest that glassy nematic dynamics are a common phenomenon in the iron-based superconductors.

Nuclear magnetic resonance as a probe of electronic states of Bi2Se3

We present magnetotransport and Bi-209 nuclear magnetic resonance (NMR) data on a series of single crystals of Bi2Se3, Bi2Te2Se and Cu_xBi2Se3 with varying carrier concentrations. The Knight shift of the bulk nuclei is strongly correlated with the carrier concentration via a hyperfine coupling of 27 ueV, which may have important consequences for scattering of the protected surface states. Surprisingly we find that the NMR linewidths and the spin lattice relaxation rate appear to be dominated by the presence of localized spins, which may be related to the presence of Se vacancies.

Nuclear magnetic resonance studies of pseudospin fluctuations in URu2Si2

Here, we report 29Si nuclear magnetic resonance measurements in single crystals and aligned powders of URu2Si2 in the hidden order and paramagnetic phases. The spin-lattice relaxation data reveal evidence of pseudospin fluctuations of U moments in the paramagnetic phase. We find evidence for partial suppression of the density of states below 30 K and analyze the data in terms of a two-component spin-fermion model. We propose that this behavior is a realization of a pseudogap between the hidden-order transition THO and 30 K. This behavior is then compared to other materials that demonstrate precursor fluctuations in a pseudogap regime above a ground state with long-range order.

Evolution of hyperfine parameters across a quantum critical point in CeRhIn 5

We report Nuclear Magnetic Resonance (NMR) data for both the In(1) and In(2) sites in the heavy fermion material CeRhIn

NMR evidence for inhomogeneous glassy behavior driven by nematic fluctuations in iron arsenide superconductors

_5

NMR evidence for spin fluctuations in the bilayer nickelate La3Ni2O6

under hydrostatic pressure. The Knight shift data reveal a suppression of the hyperfine coupling to the In(1) site as a function of pressure, and the electric field gradient,

NMR Evidence of anisotropic Kondo liquid behavior in CeIrIn

\nu_{\alpha\alpha}

_5

, at the In(2) site exhibits a change of slope,