Vortex Dynamics and Pinning in CaKFe4As4 Single Crystals from DC Magnetization Relaxation and AC Susceptibility

Among various "families" of iron-based superconductors, the quite recently discovered AeAFe(4)As(4) (where Ae is an alkali-earth metal and A is an alkali metal) has high critical current density, a very high upper critical field, and a low anisotropy, and has recently received much interest for the possibility of high magnetic field applications at the liquid hydrogen temperature. We have performed DC magnetization relaxation and frequency-dependent AC susceptibility measurements on high-quality single crystals of CaKFe4As4 with the aim of determining the pinning potential U*. The temperature dependence of U* displays a clear crossover between elastic creep and plastic creep. At temperatures around 27-28 K, U* has a very high value, up to 1200 K, resulting in an infinitesimally small probability of thermally activated flux jumps. From the dependence of the normalized pinning potential on irreversible magnetization, we have determined the creep exponents in the two creep regimes, which are in complete agreement with theoretical models. The estimation of the pinning potential from multifrequency AC susceptibility measurements was possible only near the critical temperature due to equipment limitations, and the resulting value is very close to the one that resulted from the magnetization relaxation data. Magnetic hysteresis loops revealed a second magnetization peak and very high values of the critical current density

Thermal and magnetic properties of the low-temperature antiferromagnet Ce4 Pt12 Sn25

We report specific heat (C) and magnetization (M) of single crystalline Ce4 Pt12 Sn25 at temperature down to ∼50 mK and in fields up to 3 T. C/T exhibits a sharp anomaly at 180 mK, with a large ΔC/T∼30 J/molCe K2, which, together with the corresponding cusplike magnetization anomaly, indicates an antiferromagnetic (AFM) ground state with a Néel temperature TN =180 mK. Numerical calculations based on a Heisenberg model reproduce both zero-field C and M data, thus placing Ce 4 Pt12 Sn25 in the weak exchange coupling J< Jc limit of the Doniach diagram, with a very small Kondo scale TK TN. Magnetic field suppresses the AFM state at H⊃ ≈0.7 T, much more effectively than expected from the Heisenberg model, indicating additional effects possibly due to frustration or residual Kondo screening. © 2010 The American Physical Society

Thermal and magnetic properties of a low-temperature antiferromagnet Ce4Pt12Sn25

We report specific heat (C) and magnetization (M) of single crystalline Ce4Pt12Sn25 at temperature down to 50 mK and in fields up to 3 T. C/T exhibits a sharp anomaly at 180 mK, with a large jump in a Sommerfeld coefficient γ C/T of Δγ 30 J/molK2-Ce, which, together with corresponding cusp-like magnetization anomaly, indicate antiferromagnetic (AFM) ground state with Nel temperature TN 0.18 K. Numerical calculations based on Heisenberg model reproduce well zero field specific heat data, and point to a very small Kondo scale TK, clearly placing Ce4Pt12Sn25 in the weak exchange coupling J < Jc limit of the Doniac diagram. Magnetic field suppresses AFM state at H* 0.7 T, much more rapidly than indicated by theoretical calculations. © Published under licence by IOP Publishing Ltd

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

Electronic inhomogeneity in a Kondo lattice

Inhomogeneous electronic states resulting from entangled spin, charge, and lattice degrees of freedom are hallmarks of strongly correlated electron materials; such behavior has been observed in many classes of d-electron materials, including the high-Tc copper-oxide superconductors, manganites, and most recently the iron-pnictide superconductors. The complexity generated by competing phases in these materials constitutes a considerable theoretical challenge - one that still defies a complete description. Here, we report a manifestation of electronic inhomogeneity in a strongly correlated f-electron system, using CeCoIn5 as an example. A thermodynamic analysis of its superconductivity, combined with nuclear quadrupole resonance measurements, shows that nonmagnetic impurities (Y, La, Yb, Th, Hg, and Sn) locally suppress unconventional superconductivity, generating an inhomogeneous electronic "Swiss cheese" due to disrupted periodicity of the Kondo lattice. Our analysis may be generalized to include related systems, suggesting that electronic inhomogeneity should be considered broadly in Kondo lattice materials