Bumsung Lee

Realization of giant magnetoelectricity in helimagnets.

We show that low field magnetoelectric (ME) properties of helimagnets Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22 can be efficiently tailored by the Al-substitution level. As x increases, the critical magnetic field for switching electric polarization is systematically reduced from approximately 1 T down to approximately 1 mT, and the ME susceptibility is greatly enhanced to reach a giant value of 2.0x10{4} ps/m at an optimum x=0.08. We find that control of the nontrivial orbital moment in the octahedral Fe sites through the Al substitution is crucial for fine-tuning the magnetic anisotropy and obtaining the conspicuously improved ME characteristics.

Pauli-limiting effects in the upper critical fields of a clean LiFeAs single crystal

We have investigated the temperature-dependence of the upper critical field Hc2(T) in a LiFeAs single crystal by direct measurements of resistivity under static magnetic fields up to 36 T. We find in the case of a magnetic field H along the ab-plane that Hc2ab(0) = 30 T is clearly lower than the orbital limiting field Hc2orb,ab (0) = 39.6 T estimated by the |dHc2ab / dT|Tc, suggesting the presence of both Pauli- and orbital-limiting effects in the pair breaking process. The best fit of Hc2ab(T) to the Werthamer-Helfand-Hohenberg formula results in the Maki parameter {\alpha} = 0.9 and negligible spin-orbit scattering constant ({\lambda}so = 0.0). On the other hand, for H along the c-axis, Hc2c(T) increases linearly down to our lowest temperature of 0.8 K, which can be explained by the multi-band effects. The anisotropy ratio Hc2ab(T) / Hc2c(T) is 3 near Tc and systematically decreases upon lowering temperature to become 1.3 at zero temperature. A comparative overview of the behavior of Hc2ab(T) in various Fe-based superconductors shows that, similar to LiFeAs, the calculated Hc2orb,ab (0) is generally much larger than the measured Hc2ab(0) and thus finite {\alpha} values ranging from ~ 0.4 to 3 are necessary to describe the low temperature Hc2ab(T) behaviors. Moreover, LiFeAs is found to have the smallest |dHc2ab / dT|Tc values, indicating that LiFeAs is one of the cleanest Fe-based superconductors with a finite Maki parameter. We also discuss the implications of multi-band effects and spin-orbit scattering based on the finding that the estimated Pauli-limiting field is generally much larger than the BCS prediction in the Fe-based superconductors.

Orbital selective Fermi surface shifts and mechanism of high Tc superconductivity in correlated AFeAs (A=Li,Na)

Based on the dynamical mean field theory and angle resolved photoemission spectroscopy, we have investigated the mechanism of high T(c) superconductivity in stoichiometric LiFeAs. The calculated spectrum is in excellent agreement with the measured angle resolved photoemission spectroscopy. The Fermi surface (FS) nesting, which is predicted in the conventional density functional theory method, is suppressed due to the orbital-dependent correlation effect within the dynamical mean field theory method. We have shown that such marginal breakdown of the FS nesting is an essential condition to the spin-fluctuation mediated superconductivity, while the good FS nesting in NaFeAs induces a spin density wave ground state. Our results indicate that a fully charge self-consistent description of the correlation effect is crucial in the description of the FS nesting-driven instabilities.

Single-crystal growth and superconducting properties of LiFeAs

We report the successful growth of high-quality single crystals of LiFeAs with lateral sizes up to 5×5 mm2 by the Sn-flux method. Electrical resistivity studies reveal that the superconducting onset temperature is 18.2 K with a transition width less than 1.1 K and the ratio of room temperature to residual resistivity is about 24. Bulk superconductivity is supported by perfect shielding in the magnetic susceptibility and a clear jump in the specific heat Cp, resulting in ΔCp/T ≈ 20.0 mJ/mol K2. Upper-critical-field slopes of dHc2c/dT ≈−1.39 and dHc2ab/dT ≈−2.99 T/K near Tc predict zero-temperature upper critical fields of Hc2c(0) ≈ 17.2 and Hc2ab(0) ≈ 36.9 T and coherence lengths of ξab=4.4 and ξc=2.0 nm in a single-band model. This result points to a modest superconducting anisotropy about 2.3 in LiFeAs.

Probing the order parameter of superconducting LiFeAs using Pb/LiFeAs and Au/LiFeAs point-contact spectroscopy

We have fabricated c-axis point contact junctions between high-quality LiFeAs single crystals and Pb or Au tips in order to study the nature of the superconducting order parameter of LiFeAs, one of the few stoichiometric iron-based superconductors. The observation of the Josephson current in c-axis junctions with a conventional s-wave superconductor as the counterelectrode indicates that the pairing symmetry in LiFeAs is not pure d-wave or pure spin-triplet p-wave. A superconducting gap is clearly observed in point contact Andreev reflection measurements performed on both Pb/LiFeAs and Au/LiFeAs junctions. The conductance spectra can be well described by the Blonder-Tinkham-Klapwijk model with a lifetime broadening term, resulting in a gap value of \approx 1.6 meV (2{\Delta}/kBTC \approx 2.2).

Magnetic-field-dependent pinning potential in LiFeAs superconductor from its Campbell penetration depth

A true critical current density,

Temperature- and Magnetic-Field-Induced Change of Electric Polarization in a Multiferroic Mn0.93Co0.07W0.93O4-delta Single Crystal

j_{c}

Tuning the interplay between nematicity and spin fluctuations in Na 1−x Li x FeAs superconductors

, as opposite to commonly measured relaxed persistent (Bean) current,

Enhanced upper critical fields in a new quasi-one-dimensional superconductor Nb2PdxSe5

j_{B}

Electronic Structure of Detwinned BaFe(2) As(2) from Photoemission and First Principles

, was extracted from the Campbell penetration depth,