Correlation between ground state and orbital anisotropy in heavy fermion materials.

The interplay of structural, orbital, charge, and spin degrees of freedom is at the heart of many emergent phenomena, including superconductivity. Unraveling the underlying forces of such novel phases is a great challenge because it not only requires understanding each of these degrees of freedom, it also involves accounting for the interplay between them. Cerium-based heavy fermion compounds are an ideal playground for investigating these interdependencies, and we present evidence for a correlation between orbital anisotropy and the ground states in a representative family of materials.

Synthesis, structural characterization, and physical properties of the early rare-earth metal digermanides REGe(2-x) (x ≈ 1/4) [RE = La-Nd, Sm]. A case study of commensurately and incommensurately modulated structures.

Rare-earth metal germanides with the general formula RE(4)Ge(7) (RE = La, Ce, Pr, Nd, Sm) have been synthesized using the In-flux technique. Their structures have been established from single-crystal and powder X-ray diffraction, and the structural elucidation has been aided by electron diffraction. These compounds represent superstructures of the α-ThSi(2) structure type through the long- and/or short-range vacancy ordering.

Magnesium substitutions in rare-earth metal germanides with the orthorhombic Gd5Si4-type structure. Synthesis, crystal chemistry, and magnetic properties of RE(5-x)Mg(x)Ge4 (RE = Gd-Tm, Lu, and Y).

A series of magnesium-substituted rare-earth metal germanides with a general formula RE(5-x)Mg(x)Ge(4) (x approximately = 1.0-2.3; RE = Gd-Tm, Lu, Y) have been synthesized by high-temperature reactions and structurally characterized by single-crystal X-ray diffraction.

Superconducting vortices in CeCoIn5: toward the Pauli-limiting field.

Many superconducting materials allow the penetration of magnetic fields in a mixed state in which the superfluid is threaded by a regular lattice of Abrikosov vortices, each carrying one quantum of magnetic flux. The phenomenological Ginzburg-Landau theory, based on the concept of characteristic length scales, has generally provided a good description of the Abrikosov vortex lattice state. We conducted neutron-scattering measurements of the vortex lattice form factor in the heavy-fermion superconductor cerium-cobalt-indium (CeCoIn5) and found that this form factor increases with increasing field-opposite to the expectations within the Abrikosov-Ginzburg-Landau paradigm.

Structure and properties of Gd3Ge4: the orthorhombic RE3Ge4 structures revisited (RE = Y, Tb-Tm).

The new binary compound Gd(3)Ge(4) has been synthesized and its structure has been determined from single-crystal X-ray diffraction. Gd(3)Ge(4) crystallizes in the orthorhombic space group Cmcm (No. 63) with unit cell parameters a = 4.

Vacancy ordering in SmGe2-x and GdGe2-x (x = 0.33): structure and properties of two Sm3Ge5 polymorphs and of Gd3Ge5.

The crystal structures and the magnetic properties of three new binary rare-earth intermetallic phases are reported. alpha-Sm3Ge5 and beta-Sm3Ge5 and Gd3Ge5 have been prepared from the corresponding elements through high-temperature reactions using the flux-growth method. The structures of the three compounds have been established using single-crystal X-ray diffraction: alpha-Sm3Ge5 crystallizes with its own type in the hexagonal space group P2c (No.

Unusual Mn-Mn spin coupling in the polar intermetallic compounds CaMn2Sb2 and SrMn2Sb2.

Large single-crystals of two polar intermetallic phases, CaMn2Sb2 and SrMn2Sb2, have been grown using In or Sn as metal fluxes and characterized by single-crystal X-ray diffraction. The two compounds are isostructural and crystallize with the CaAl2Si2 structure (space group P3m1, No. 164) with unit cell parameters determined at 120(2) K of a = 4.

Dynamics of photoexcited quasiparticles in heavy electron compounds.

Femtosecond real-time spectroscopy is an emerging new tool for studying low energy electronic structure in correlated electron systems. Motivated by recent advances in understanding the nature of relaxation phenomena in various correlated electron systems (superconductors, density wave systems) the technique has been applied to heavy electron compounds in comparison with their non-magnetic counterparts. While the dynamics in their non-magnetic analogues are similar to the dynamics observed in noble metals (only weak temperature dependences are observed) and can be treated with a simple two-temperature model, the photoexcited carrier dynamics in heavy electron systems show dramatic changes as a function of temperature and excitation level.

Photoexcited electron dynamics in Kondo insulators and heavy fermions.

We have studied the photoexcited carrier relaxation dynamics in the Kondo insulator SmB6 and the heavy fermion metal YbAgCu4 as a function of temperature and excitation level. The dynamic response is found to be both strongly temperature dependent and nonlinear. The data are analyzed with a Rothwarf-Taylor bottleneck model, where the dynamics are governed by the presence of a narrow gap in the density of states near the Fermi level.

Probing the limits of the Zintl concept: structure and bonding in rare-earth and alkaline-earth zinc-antimonides Yb9Zn4+xSb9 and Ca9Zn4.5Sb9.

A new transition metal Zintl phase, Yb(9)Zn(4+x)Sb(9), was prepared by high-temperature flux syntheses as large single crystals, or by direct fusion of the corresponding elements in polycrystalline form. Its crystal structure was determined by single-crystal X-ray diffraction. Its Ca-counterpart, hitherto known as Ca(9)Zn(4)Sb(9), and the presence of nonstoichiometry in it were also studied.

Kondo scaling in the optical response of YbIn1-xAgxCu4.

Theoretical work on Kondo systems predicts universality in the scaling of observable quantities with the Kondo temperature, T(K). Here we report infrared-frequency optical response measurements of the correlated system YbIn(1-x)AgxCu4. We observe that x-dependent variations in the frequency and strength of a low-energy excitation are related to the x-dependent Kondo temperature.

Electrochemical and spectroscopic characterization of the novel charge-transfer ground state in diimine complexes of ytterbocene.

The novel charge-transfer ground state found in alpha,alpha'-diimine adducts of ytterbocene (C(5)Me(5))(2)Yb(L) [L = 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen)] in which an electron is spontaneously transferred from the f(14) metal center into the lowest unoccupied (pi*) molecular orbital (LUMO) of the diimine ligand to give an f(13)-L(*)(-) ground-state electronic configuration has been characterized by cyclic voltammetry, UV-vis-near-IR electronic absorption, and resonance Raman spectroscopies. The voltammetric data demonstrate that the diimine ligand LUMO is stabilized and the metal f orbital is destabilized by approximately 1.0 V each upon complexation for both bpy and phen adducts.

Truly bulk-sensitive spectroscopic measurements of valence in heavy fermion materials.

Intermediate valence is one of the typical phenomena of systems with strong electronic correlation. The Anderson impurity model predicts a scaling of the valence with the reduced temperature T/T(K), which is difficult to observe by traditional surface-sensitive electronic spectroscopies. This paper presents results obtained by resonant inelastic X-ray scattering (RIXS), a bulk-sensitive configuration- and chemical-specific technique.