Role of hyperfine coupling in magnetic and quadrupolar ordering of Pr3Pd20Si6.

We study the ternary clathrate Pr3Pd20Si6 in specific heat and ac susceptibility measurements on a high-quality single crystal, distinguishing antiferromagnetic and antiferroquadrupolar ordering, as well as a hitherto unknown magnetic low-temperature transition. The specific heat shows the direct involvement of nuclear spin degrees of freedom in the antiferromagnetic ordering, which is well supported by our calculation of the hyperfine level scheme without adjustable parameters. Pr3Pd20Si6 is, therefore, one of the rare materials where the nuclear moments are involved in the formation of the magnetic ground state.

Controllable chirality-induced geometrical Hall effect in a frustrated highly correlated metal.

A current of electrons traversing a landscape of localized spins possessing non-coplanar magnetic order gains a geometrical (Berry) phase, which can lead to a Hall voltage independent of the spin-orbit coupling within the material-a geometrical Hall effect. Here we show that the highly correlated metal UCu(5) possesses an unusually large controllable geometrical Hall effect at T<1.2 K due to its frustration-induced magnetic order. The magnitude of the Hall response exceeds 20% of the ν=1 quantum Hall effect per atomic layer, which translates into an effective magnetic field of several hundred Tesla acting on the electrons. The existence of such a large geometric Hall response in UCu(5) opens a new field of enquiry into the importance of the role of frustration in highly correlated electron materials.

Pressure dependence of the Fulde-Ferrell-Larkin-Ovchinnikov state in CeCoIn5.

Pressure studies of the thermodynamics of CeCoIn5 under magnetic fields H parallel to c and H parallel to ab have been made up to P = 1.34 GPa. We recorded the signature of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state for all pressures when H parallel to ab. Also remarkably, the FFLO regime suddenly expands for P = 1.34 GPa. With the help of a microscopic theory for d-wave superconductivity, we have extracted the gyromagnetic ratio g and the Fermi velocities nu(a) and nu(c). Our study is the first evidence for the existence of the FFLO state away from the influence of the antiferromagnetic fluctuations. We find a close parallel between the T-P phase diagram of CeCoIn5 and the T-x phase diagram of the high-Tc cuprates, where x is the hole concentration.