La2Pd2In: superconductivity and lattice properties at ambient and elevated pressures.

Lattice and electronic properties of La2Pd2In were studied at ambient and elevated pressures so as to determine features related to a specific atomic coordination without any influence of magnetism. We describe temperature dependences of lattice parameters, heat capacity and electrical resistivity of single-crystalline La2Pd2In (s.g.P4/mbm) in a broad temperature range 0.09-300 K. Together with the anisotropic effect of hydrostatic pressure, showing that the lattice is more compressible in the basal plane, we can conclude that the lattice is affected by degrees of freedom of the La atoms with positions not imposed by symmetry. The lattice anisotropy is smaller than that found for isostructural ferromagnet Ce2Pd2In. The equilibrium bulk modulusB0= (48 ± 3) GPa was determined on the basis of individual linear compressibilities. Measurement of electrical resistivity indicated a superconducting state belowT= 0.59 K with a low critical field 0.005 T atT= 380 mK. The onset of superconducting state as a bulk property of La2Pd2In was confirmed by measurements of specific heat and AC magnetic susceptibility. Experimental data can be accounted by first-principles electronic-structure calculations based on density-functional theory. The measured Sommerfeld coefficientγ= 10.6 mJ mol-1 K-2, only marginally exceeding the calculatedγ= 9.34 mJ mol-1 K-2, indicates only weak electronic correlations.

Extended stability range of the non-Fermi liquid phase in UCoAl.

High pressure was used to investigate the stability of the non-Fermi liquid (NFL) state, observed in electrical resistivity of uranium-based band metamagnet UCoAl in a pure form (paramagnet) or with Fe substitution (ferromagnetic ground state), both in a single-crystal form. By combining the pressure variations of magnetization and resitivity in these materials the phase diagram for UCoAl had been constructed. The band metamagnet transforms into the ferromagnetic state as the critical metamagnetic field is reduced to zero by the lattice expansion analogous to the negative pressure. Within the same diagram, the increasing hydrostatic pressure drives the critical metamagnetic field upwards while reducing the magnetization increment at the transition. The NFL state persists to about 4-5 GPa. Although spin fluctuations play an important role in the character of UCoAl, they do not exhibit any criticality in the sense of divergence of parameters describing the resistivity around the Ferro-NFL phase transition, which is of the first order type.

Electronic properties of Pu19Os simulating ß-Pu: the strongly correlated Pu phase.

We established the basic electronic properties of ζ-Pu19Os, which is a close analogue to ß-Pu, and its low-temperature variety, η-Pu19Os. Their magnetic susceptibility is 15% higher than for δ-Pu. A specific heat study of ζ-Pu19Os shows a soft lattice similar to δ-Pu, leading to a low Debye temperature Θ D = 101 K. The linear electronic coefficient γ related to the quasiparticle density of states at the Fermi level points to a higher value, 55 ± 2 mJ (mol Pu K2)-1, compared to 40 mJ (mol K2)-1 for δ-Pu. The results confirm that ß-Pu is probably the most strongly correlated Pu phase, as had been indicated by resistivity measurements. The volume and related Pu-Pu spacing is clearly not the primary tuning parameter for Pu metal, as the ß-Pu density stands close to the ground-state α-phase and is much higher than that for δ-Pu. The η-Pu19Os phase has a record γ-value of 74 ± 2 mJ (mol Pu K2)-1. The enhancement is not reproduced by LDA+DMFT calculations in the fcc structure, which suggests that multiple diverse sites can be the key to the understanding of ß-Pu.

ThMn12-type phases for magnets with low rare-earth content: Crystal-field analysis of the full magnetization process.

Rare-earth (R)-iron alloys are a backbone of permanent magnets. Recent increase in price of rare earths has pushed the industry to seek ways to reduce the R-content in the hard magnetic materials. For this reason strong magnets with the ThMn12 type of structure came into focus. Functional properties of R(Fe,T)12 (T-element stabilizes the structure) compounds or their interstitially modified derivatives, R(Fe,T)12-X (X is an atom of hydrogen or nitrogen) are determined by the crystal-electric-field (CEF) and exchange interaction (EI) parameters. We have calculated the parameters using high-field magnetization data. We choose the ferrimagnetic Tm-containing compounds, which are most sensitive to magnetic field and demonstrate that TmFe11Ti-H reaches the ferromagnetic state in the magnetic field of 52 T. Knowledge of exact CEF and EI parameters and their variation in the compounds modified by the interstitial atoms is a cornerstone of the quest for hard magnetic materials with low rare-earth content.

UH3-based ferromagnets: new look at an old material.

UH3 is the first discovered material with ferromagnetism based purely on the 5f electronic states, known for more than half century. Although the U metal is Pauli paramagnet, the reduced 5f-5f overlap in compounds allows for moment formation and ordering, typically if the U-U spacing exceeds the Hill limit, i.e. about 340 pm. The stable form of UH3, known as ß-UH3, has rather high TC ≈ 170 K. Such high value is rather unusual, considering dU-U = 331 pm. Properties of metastable α-UH3 with dU-U = 360 pm could be never well established. Using the fact that α-UH3 is in fact bcc U with interstitials filled by H, we attempted to synthesize α-UH3 starting from the γ-U alloys, with the bcc structure retained to room temperature by doping combined with ultrafast cooling. While up to 15% Zr a contamination by ß-UH3 was obtained, 20% Zr yielded single phase α-UH3. The TC value remains high and very similar to ß-UH3. One can see an increase up to 187 K for 15% Zr, followed by a weak decrease. Magnetic moments remain close to 1 µB/U atom. An insight is provided by ab-initio calculations, revealing a a charge transfer towards H-1s states, depopulating the U-6d and 7s states, leaving almost pure 5f character around the Fermi level. The 5f magnetism exhibits a high coercivity (µ0Hc up to 5.5 T) and large spontaneous volume magnetostriction of 3.2*10-3. Even higher increase of TC, reaching up to 203 K, can be achieved in analogous Mo stabilized hydrides, which yield an amorphous structure. The compounds represent, together with known hydrides of U6Fe and U6Co, a new group of robust 5f ferromagnets with small dU-U but high TC. Although common hydrides are fine powders, some of the new hydrides described as (UH3)(1-x)T x (T = Zr or Mo) remain monolithic, which allows to study transport and thermodynamic properties.

Racah materials: role of atomic multiplets in intermediate valence systems.

We address the long-standing mystery of the nonmagnetic insulating state of the intermediate valence compound SmB6. Within a combination of the local density approximation (LDA) and an exact diagonalization (ED) of an effective discrete Anderson impurity model, the intermediate valence ground state with the f-shell occupation 〈n4f〉 = 5.6 is found for the Sm atom in SmB6. This ground state is a singlet, and the first excited triplet state ~3 meV higher in the energy. SmB6 is a narrow band insulator already in LDA, with the direct band gap of ~10 meV. The electron correlations increase the band gap which now becomes indirect. Thus, the many-body effects are relevant to form the indirect band gap, crucial for the idea of "topological Kondo insulator" in SmB6. Also, an actinide analog PuB6 is considered, and the intermediate valence singlet ground state is found for the Pu atom. We propose that [Sm, Pu]B6 belong to a new class of the intermediate valence materials with the multi-orbital "Kondo-like" singlet ground-state. Crucial role of complex spin-orbital f( n)-f ( n+1) multiplet structure differently hybridized with ligand states in such Racah materials is discussed.

Electronic properties of α-UH3 stabilized by Zr.

Pure hydride of the α-UH3 type without any ß-UH3 admixture was prepared by high-pressure hydrogenation of bcc U stabilized by Zr. Such material, characterized by a general formula (UH3)1-x Zr x , is stable in air at ambient and elevated temperatures. H release is observed between 400-450 °C similar to ß-UH3. Its stability allowed to measure magnetic properties, specific heat, and electrical resistivity in a wide temperature range. Despite rather different crystal structure and inter-U spacing, the electronic properties are almost identical to ß-UH3. Its ferromagnetic ground state with Curie temperature TC ≈ 180 K (weakly and non-monotonously dependent on Zr concentration) and U moments of 1.0 µB indicate why mixtures of α- and ß-UH3 exhibited only one transition. Magnetic ordering leads to a large spontaneous magnetostriction ωs = 3.2*10-3, which can be explained by the increase of the spin moment between the paramagnetic (Disordered Local Moment) and the ferromagnetic state. The role of orbital moments in magnetism is indicated by fully relativistic electronic structure calculations.

Unusual 5f magnetism in the U2Fe3Ge ternary Laves phase: a single crystal study.

Magnetic properties of the intermetallic compound U(2)Fe(3)Ge were studied on a single crystal. The compound crystallizes in the hexagonal Mg(2)Cu(3)Si structure, an ordered variant of the MgZn(2) Laves structure (C14). U(2)Fe(3)Ge displays ferromagnetic order below the Curie temperature T(C) = 55 K and presents an exception to the Hill rule, as the nearest inter-uranium distances do not exceed 3.2 Å. Magnetic moments lie in the basal plane of the hexagonal lattice, with the spontaneous magnetic moment M(s) = 1.0 µ(B)/f.u. at T = 2 K. No anisotropy within the basal plane is detected. In contrast to typical U-based intermetallics, U(2)Fe(3)Ge exhibits very low magnetic anisotropy, whose field does not exceed 10 T. The dominance of U in the magnetism of U(2)Fe(3)Ge is suggested by the (57)Fe Mössbauer spectroscopy study, which indicates very low or even zero Fe moments. Electronic structure calculations are in agreement with the observed easy-plane anisotropy but fail to explain the lack of an Fe contribution to the magnetism of U(2)Fe(3)Ge.

Hydrogen induced changes in the crystal structure and magnetic properties of UCoGe.

Hydrogen pressure of 0.5-140 bar has been applied to synthesize hydrides of UCoGe. Besides an α hydride crystallizing in the structure type of the parent compound, which loses the weak ferromagnetism found in pure UCoGe, two distinctly different ß hydrides were identified. The almost pure ß hydride (UCoGeH(1.7)) is a ferromagnet below T(C) = 50 K. The highest H(2) pressures (> 130 bar) produce admixture of another hydride called ß' hydride, with less H/f.u. and T(C) = 8 K, obtained presumably as a decay product of a full hydride UCoGeH(2.0) unstable at ambient conditions. The value of the Sommerfeld coefficient of electronic specific heat γ increases over 100 mJ mol(-1) K(-2) for the magnetic hydrides.

Pressure effect on the crystal lattice of unconventional superconductor UCoGe.

High-pressure techniques were used to determine the structural behaviour of the superconducting ferromagnet UCoGe up to 30 GPa enabling us to determine the link between the effect of pressure on the material magnetic properties and crystal structure. The TiNiSi type structure of UCoGe was preserved up to the highest pressure. The a direction, equivalent to the shortest U-U links, was identified as the critical soft direction. The data are compared with the structural variations in UCoGe α-hydride, which becomes non-magnetic and non-superconducting despite a volume expansion. We show that at least in this case, but probably more generally, the structure impact of hydrogenation is definitely not equivalent to negative pressure.

The effect of hydrogenation on magnetic interactions in CeNi.

The crystal structure and magnetic properties of CeNiH(3.7) were studied by means of powder x-ray diffraction, specific heat, and dc and ac magnetization techniques. It was established that hydrogenation stabilizes the 4f(1) state of Ce and turns CeNi-H into a dilute Kondo system with T(K) = 3.7 K. The Kondo screening in CeNiH(3.7) is suppressed by the applied magnetic field, although it still affects the properties of CeNiH(3.7) at 14 T, as indicated by the enhanced γ-coefficient of electronic specific heat, which remains more than twice as large as in the precursor compound CeNi. Its zero-field value is as high as 1890 mJ (mol K(2))(-1). Hydrogenation acts primarily as the negative pressure agent in CeNiH(3.7), while the role of H-metal bonding is secondary.

Specific heat of delta-Pu stabilized by Am.

Detailed specific heat C(p) measurements of delta-Pu stabilized by Am (8%-20%) were performed in the temperature range 4.5-300 K. The coefficient of the electronic specific heat gamma, which reflects the quasiparticle density of states at the Fermi level E(F), is smaller than originally assumed and, depending on the estimate of phonon contributions, a value between 35 and 55 mJ/mol K2 can be deduced for Pu-8% Am. For higher Am concentrations, which expand the lattice, gamma decreases slightly with the Am content. An applied magnetic field of 9 T had no effect on C(p). The results strongly suggest that itinerant 5f states at E(F) are not appropriate for describing delta-Pu.

5f-electron localization in PuSe and PuSb

Thin layers of PuSb and PuSe were studied by photoelectron spectroscopy. X-ray photoelectron spectroscopy and high-resolution valence-band ultraviolet photoelectron spectroscopy spectra show localization of the 5f states and a low density of states at E(F) in PuSb. In PuSe, which can be classified as a heavy fermion system with low carrier density, we observed three narrow peaks in the valence band, which can be related to the 5f emission. These three features are very sensitive to stoichiometry deviations and disappear for PuSe prepared at T = 77 K.