We report on the measurements of the superconducting order parameter in the nonmagnetic borocarbides LuNi2B2C and YNi2B2C. Andreev conductance spectra are obtained from nanoscale metallic junctions on single crystal surfaces prepared along three major crystallographic orientations: These observations are robust and reproducible among all the measurements on two different sets of LuNi2B2C crystals and one set of YNi2B2C crystals. We suggest that the possible gap nodes in the [100] direction may be masked by two effects: different gap anisotropy across multiple Fermi surfaces, as reported in the recent photoemission spectroscopy, and the large tunneling cone. Our results provide a consistent picture of the superconducting gap structure in these materials, addressing the controversy particularly in the reported results of point-contact Andreev reflection spectroscopy.

The La dilution of the Kondo lattice CeCoIn5 is studied. The scaling laws found for the magnetic susceptibility and the specific heat reveal two well-separated energy scales, corresponding to the single-impurity Kondo temperature T(K) and an intersite spin-liquid temperature T(*). The Ce-dilute alloy has the expected Fermi liquid ground state, while the specific heat and resistivity in the dense Kondo regime exhibit non-Fermi-liquid behavior, which scales with T(*). These observations indicate that the screening of the magnetic moments in the lattice involves antiferromagnetic intersite correlations with a larger energy scale in comparison with the Kondo impurity case.

The phase diagram of FeSi(1-x)Ge(x), obtained from magnetic, thermal, and transport measurements on single crystals, shows a discontinuous transition from Kondo insulator to ferromagnetic metal with x at a critical concentration, x(c) approximately 0.25. The gap of the insulating phase strongly decreases with x. The specific heat gamma coefficient appears to track the density of states of a Kondo insulator. The phase diagram is consistent with an insulator-metal transition induced by a reduction of the hybridization with x in conjunction with disorder on the Si/Ge ligand site.

The authors introduce an emergent method to fabricate a few-nanometer-size columnar superlattice with a checkerboard pattern in inorganic spinels by harnessing the Jahn-Teller structural distortion. Transmission electron microscope images reveal that the fundamental building blocks are two types of long nanorods with the ∼4×4×70nm3 size, which are alternatively stacked in a way that the cross sectional and side views show checkerboard and herringbone patterns, respectively. The authors discuss that the strain induced by the Jahn-Teller distortion causes this peculiar self-assembled nanostructure in the coherent mixture of two spinel phases. This pure solid state self-assembly can be implemented to fabricate heterogeneous nanostructures with practical functionalities.

Formation of magnetically ordered array of two types of rectangular nanorods, ∼300nm in length and a few nanometers in size, is achieved in the Mn-doped CoFe2O4 spinel through chemical phase separation mediated by cooperative Jahn-Teller distortions. At room temperature, the magnetic nanorods, with composition close to CoFe2O4, interlace with the paramagnetic counterparts and form a highly organized checkerboard pattern in the cross section. The checkerboard size in the range of ∼13.8×7.9–∼17.3×14.0nm2 is tunable with composition and, particularly, with the isothermal annealing time. These three-dimensional nanocheckerboards exhibit a nearly ideal configuration for the patterned perpendicular recording medium.

When Fe ions in the ferrimagnetic cubic MgFe2O4 are replaced by Jahn-Teller (JT)-active Mn ions, the structure evolves with two-step processes. For example, the quenched cubic MgMn1.5Fe0.5O4 becomes tetragonal and JT distorted with slow cooling. However, with further slow cooling, the clustering tendency of JT-distorted Mn ions induces the formation of a checkerboard nano-self-assembly consisting of Mn-rich (tetragonal, paramagnetic) and -poor (cubic, ferrimagnetic) rods. This morphological evolution accompanies a drastic modification of ferrimagnetic properties, e.g., the magnetic coercivity changes by ∼25. The nanocheckerboard assembly with ferrimagnetic nanorods with large shape anisotropy can be a platform for ultra high-density memory devices.

We present soft x-ray absorption spectroscopy (XAS) on spinel manganites and show it can be used as a powerful tool to directly probe the Jahn-Teller (JT) splitting energy in transition metal oxides. The MnL2,3-edge XAS spectra of the spinel ZnMnxGa2−xO4(x=0.5,1.0,1.4,and2.0) and AMn2O4(A=MgandCd) change very systematically with the structural distortion of JT active octahedral sublattices. The XAS spectra are well reproduced by the configuration interaction cluster model including full ionic multiplet structure, and the spectral evolution is explained by JT energy splitting due to the elongation of the MnO6 octahedra in the model. In the OK-edge XAS spectra, the JT distortion produces a weak effect, less distinctive than in L2,3-edge spectra.

The various phases in Sm 1−x Eu x B 6 are investigated based on magnetic susceptibility, resistivity, and Hall effect measurements. The end compounds are a Kondo insulator (SmB 6 ) and a polaronic ferromagnet (EuB 6 ). For x ≈ 0.2, the ground state undergoes a transition from a Kondo insulator to an antiferromagnetic (AF) insulator phase. Further doping induces a transition to an AF metal at x ≈ 0.4. The spin gap is reduced rapidly with the Eu substitution, while there is a charge gap up to x ≈ 0.4. The Hall effect indicates a dramatic decrease in the carrier density at low temperatures (T ) for the Kondo insulator regime, whereas, the carrier density is almost independent of T in the AF metallic phase.