Ben-Li Young

Microscopic evidence for field-induced magnetism in CeCoIn5

We present NMR data in the normal and superconducting states of CeCoIn5 for fields close to H(c2)(0)=11.8 T in the ab plane. Recent experiments identified a first-order transition from the normal to superconducting state for H>10.5 T, and a new thermodynamic phase below 290 mK within the superconducting state. We find that the Knight shifts of the In(1), In(2), and the Co are discontinuous across the first-order transition and the magnetic linewidths increase dramatically. The broadening differs for the three sites, unlike the expectation for an Abrikosov vortex lattice, and suggests the presence of static spin moments in the vortex cores. In the low-temperature and high-field phase, the broad NMR lineshapes suggest ordered local moments, rather than a long-wavelength quasiparticle spin density modulation expected for an FFLO phase.

Interacting antiferromagnetic droplets in quantum critical CeCoIn5.

The heavy fermion superconductor CeCoIn5 can be tuned between superconducting and antiferromagnetic ground states by hole doping with Cd. Nuclear magnetic resonance data indicate that these two orders coexist microscopically with an ordered moment approximately 0.7 microB. As the ground state evolves, there is no change in the low-frequency spin dynamics in the disordered state. These results suggest that the magnetism emerges locally in the vicinity of the Cd dopants.

μSR and NMR in f-electron non-Fermi liquid materials

Abstract Magnetic resonance (μSR and NMR) studies of f-electron non-Fermi-liquid (NFL) materials give clear evidence that structural disorder is a major factor in NFL behavior. Longitudinal-field μSR relaxation measurements at low fields reveal a wide distribution of muon relaxation rates and divergences in the frequency dependence of spin correlation functions in the NFL systems UCu 5− x Pd x and CePtSi 1− x Ge x . These divergences seem to be due to slow dynamics associated with quantum spin-glass behavior, rather than quantum criticality as in a uniform system, for two reasons: the observed strong inhomogeneity in the muon relaxation rate, and the strong and frequency-dependent low-frequency fluctuation observed in U(Cu,Pd) 5 and CePt(Si,Ge). In the NFL materials CeCu 5.9 Au 0.1 , Ce(Ru 0.5 Rh 0.5 ) 2 Si 2 , CeNi 2 Ge 2 , and YbRh 2 Si 2 the low-frequency weight of the spin fluctuation spectrum is much weaker than in the disordered NFL systems.

μSR in Ce1-xLaxAl3: anisotropic Kondo effect?

Zero-field μ + spin relaxation experiments in the heavy-fermion alloys Ce 1-x La x Al 3 , x = 0 and 0.2, examine a recent proposal that the system exhibits a strong anisotropic Kondo effect. We resolve a damped oscillatory component for both La concentrations, indicative of disordered antiferromagnetism. For x = 0.2 the oscillation frequency decreases smoothly with increasing temperature, and vanishes at the specific heat anomaly temperature T*2.2 K. Our results are consistent with the view that T* is due to a magnetic transition rather than anisotropic Kondo behavior.

Nuclear magnetic resonance method for determining the magnetic easy direction in anisotropic paramagnetic powders

We have developed a method for determining the magnetic easy direction (crystalline direction of maximum magnetic susceptibility) in powdered samples of paramagnetic crystals using nuclear magnetic resonance (NMR) spectroscopy. Single-crystal powder grains are aligned using a magnetic field both with and without rotation of the sample about an axis perpendicular to the alignment field. Depending on the magnetic easy direction of the material, only one method produces a good crystalline alignment. By qualitatively comparing the direction-dependent powder-pattern NMR spectra of samples aligned using the two alignment methods, we have successfully determined the magnetic easy directions in CePtSi and Y0.85U0.15Pd2Al3 compounds.