[en] Nuclear magnetic resonance has emerged as a vital technique for investigating strongly correlated electron systems, and is particularly important for studying superconductivity. In this paper the basic features of NMR as a technique for probing the superconducting state are reviewed. Topics include spin relaxation processes, studies of vortex lattices and phenomena associated with unconventional pairing symmetries. Recent experimental work is reviewed, with a particular emphasis on the heavy fermion superconductors

[en] In recent years there has been increasing interest in materials with strong spin–orbit coupling (SOC). Nuclear magnetic resonance is a valuable microscopic probe of such systems because of the hyperfine interactions between the nuclear spins and the electron degrees of freedom. In materials with weak SOC the NMR Knight shift contains two contributions: one from the electron orbital susceptibility and the other from the electron spin susceptibility. These contributions can be separated by plotting the Knight shift versus the bulk susceptibility and extracting the slope and intercept. Here we examine the case where the SOC is non-negligible, in which case the slope and intercept are no longer simply related to these two contributions. These results have important implications for NMR studies of heavy fermions, as well as 4d and 5d systems. (paper)

[en] We report ¹¹⁵In NMR measurements in the heavy fermion superconductor CeIrIn₅. The Knight shift for the symmetric In(1) site displays the same behaviour as the related material CeCoIn₅. Our results support the phenomenological two-fluid model proposed by Nakatsuji et al. Comparison to recent DMFT calculations for CeIrIn₅ further strengthens our conclusions.

[en] We present ⁷⁵As nuclear magnetic resonance data in the paramagnetic and magnetic states of single-crystal CaFe₂As₂. The electric field gradient and the internal magnetic field at the As sites change discontinuously below the first-order structural transition at T ₀=169 K. In the magnetic state, we find a single value of the internal hyperfine field consistent with commensurate antiferromagnetic order of Fe moments pointing in the ab plane. The spin lattice relaxation rate shows Korringa behavior for T∼< T ₀/3, reflecting the metallic nature of the ordered state. Surprisingly, T ₁^-1 exhibits a small peak at 10 K, revealing the presence of slow spin fluctuations that may be associated with domain wall motion.

[en] New ¹¹⁵In NQR data is presented for the 9/2 ↔ 7/2 of the In(2) site transition in the heavy fermion compound CePt₂In₇. We extract the sub-lattice magnetization in the antiferromagnetic state. The spectra reveal that roughly half of the In(2) sites experience no static hyperfine field.

[en] The electric field gradient (EFG) at the As nuclei in the BaFe₂As₂ and SrFe₂As₂ iron arsenide compounds becomes axially asymmetric in the low temperature orthorhombic phase, but little has been reported about the EFG in the CaFe₂As₂ analog. We present As nuclear magnetic resonance (NMR) spectra as a function of orientation in this phase and extract the EFG tensor. The EFG asymmetry is significantly less in this material, which is a result of the unusually strong c-axis hybridization. (paper)

[en] We report ⁵¹V zero-field NMR of the manganese vanadate spinel compound, MnV₂O₄, together with both ac and dc magnetization measurements. The field and temperature dependences of ac susceptibilities show a re-entrant-spin-glass-like behavior below the ferrimagnetic (FEM) ordering temperature. The zero-field NMR spectrum consists of multiple lines ranging from 240 to 320 MHz. Its temperature and field dependences are discussed in terms of the persistence of a small fraction of the cubic phase within the FEM ordered ground state. Due to strong spin-orbit couplings the disordered phase induces an anomalous structural and electronic state in an external field. This suggests a close correlation between magnetism and structure

[en] We propose a predictive standard model for heavy electron systems based on a detailed phenomenological two-fluid description of existing experimental data. It leads to a new phase diagram that replaces the Doniach picture, describes the emergent anomalous scaling behavior of the heavy electron (Kondo) liquid measured below the lattice coherence temperature, T^*, seen by many different experimental probes, that marks the onset of collective hybridization, and enables one to obtain important information on quantum criticality and the superconducting/antiferromagnetic states at low temperatures. Because T* is ∼ J²_ρ/2, the nearest neighbor RKKY interaction, a knowledge of the single-ion Kondo coupling, J, to the background conduction electron density of states, ρ, makes it possible to predict Kondo liquid behavior, and to estimate its maximum superconducting transition temperature in both existing and newly discovered heavy electron families.

[en] We present field-swept ⁷⁵As Nuclear Magnetic Resonance (NMR) measurements in the spin-density wave (SDW) state of Ba(Fe_1-xNi_x)₂As₂ for x = 0.0072. The large single crystals were aligned with the external field H₀ perpendicular to the c axis. The spectra are increasingly broadened as a function of doping, and are well fit by a model of a commensurate SDW with local impurities.

[en] We report an ¹¹B NMR study of heavily boron-doped diamond. By comparing ¹¹B spectra with those from ¹³C, we find that substitutional boron dopants are limited to 0.26 at.% in an isolated form. This observation reveals that, above the metal-insulator transition, boron dopants are incorporated into carbon sites mostly in the form of boron aggregates and suggests that, in addition to the holes in the valence band, an impurity band formed by boron aggregates plays an important role in the superconductivity in the heavily B-doped regime. A strong disorder effect deduced from Knight shift and nuclear spin-lattice relaxation rate measurements is attributed to the overlap between the intrinsic valence band and a boron impurity band.