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.

Impact of isoelectronic substitution and hydrostatic pressure on the quantum critical properties of CeRhSi3.

There is an ongoing dispute in the community about the absence of a magnetic quantum critical point (QCP) in the noncentrosymmetric heavy fermion compound CeRhSi3. In order to explore this question we prepared single crystals of CeRh(Si1-x Ge x )3 with x = 0.05 and 0.15 and determined the temperature-pressure (T-p) phase diagram by means of measurements of the electrical resistivity. The substitution of isoelectronic but large Ge enforces a lattice volume increase resulting in a weakening of the Kondo interaction. As a result, the x = 0.05 and x = 0.15 compound exhibit a transition into the antiferromagnetic (AFM) at higher temperatures being T N = 4.7 K and T N1 = 19.7 K, respectively. Application of pressure suppresses T N (T N1) monotonically and pressure induced superconductivity is observed in both Ge-substituted compounds above p ⩾ 2.16 GPa (x = 0.05) and p ⩾ 2.93 GPa (x = 0.15). Extrapolation of T N(p) → 0 of CeRh(Si0.95Ge0.05)3 yields a critical pressure of p c ≈ 3.4 GPa (in CeRh(Si0.85Ge0.15)3 p c ≈ 3.5 GPa) pointing to the presence of an AFM QCP located deep inside the superconducting state.

Ab initio calculations of the crystal field and phonon dispersions in CePd2Al2 and LaPd2Al2.

CePd2Al2 crystallizes in the CaBe2Ge2-type tetragonal structure (P4/nmm, 129) and undergoes a phase transition to the orthorhombic Cmme structure at around 13 K. Its inelastic neutron spectra reveal an additional magnetic excitation that was ascribed to electron-phonon interaction leading to a formation of a new quantum quasi-bound vibron state. We present the first-principles calculations of the crystal field excitations and lattice dynamics calculations of the phonon dispersions to compare with the experimental data. The calculated crystal field energy splitting in CePd2Al2 agrees well with the model used to describe the experimental neutron scattering spectra. The first excited crystal field level moves to higher energies when undergoing the transformation from tetragonal to orthorhombic structure, in agreement with the experiment. The analysis based on calculated elastic constants and lattice dynamics calculations show that in both tetragonal and orthorhombic structures there are no imaginary modes for any q-wave vector within the Brillouin zone, and therefore the lattice structures are stable. The phonon dispersions and density of states are calculated for both crystal structures of CePd2Al2 and its nonmagnetic counterpart LaPd2Al2. The results generally agree well with the experimental data including the high phonon density of states around 12 meV. The phonon density of states is also used to calculate the mean squared displacement, Debye temperature, lattice heat capacity and compared with similar properties of the available experiment.

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.

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.

Magnetism of PrAl2 nanoparticle investigated with a quantum simulation model.

For many years, micromagnetism and Monte Carlo simulation have served as the two main tools for studying the magnetic structures and physical properties of nanomagnets. However, the two approaches are based on classical physics, and thus lack the flexibility to deal with complex nanosystems, such as those of very tiny size or consisting of ions of different elements. To overcome the difficulty, a quantum simulation model has been proposed and a new computational algorithm developed in the present work. Both have been successfully applied to an assumed PrAl2 nanoparticle to study its magnetic behavior in external magnetic fields exerted along the crystal axes. The theoretical results obtained with the model and the new algorithm are reasonable physically and exhibit strong finite-size effects. The model can be generalized to study the magnetic configurations and physical properties of more complicated nanosystems, such as nanowires, nanotubes, etc.