Evolution of ferromagnetic and non-Fermi liquid state with doping:the case of Ru doped UCoGe

We have investigated the impact of Ru substitution for Co on the behavior of the ferromagnetic superconductor UCoGe by performing x-ray diffraction, magnetization, specific heat and electrical resistivity measurements on polycrystalline samples of the $\mathrm{UCo}_{1-x}\mathrm{Ru}_{x}\mathrm{Ge}$ series ($0\geq x\leq0.9$). The initial Ru substitution up to $x\approx0.1$ leads to a simultaneous sharp increase of the Curie temperature and spontaneous magnetization up to maximum values of $T_{\mathrm{C}}=8.6 K$ and $M_{\mathrm{S}}=0.1 \mu_{\mathrm{B}}$ per formula unit, respectively, whereas superconductivity vanishes already for $x\approx0.03$. Further increase of the Ru content beyond $x\approx0.1$ leads to a precipitous decrease of both, $T_{\mathrm{C}}$ and $M_{\mathrm{S}}$ towards a ferromagnetic quantum critical point (QCP) at $x_{\mathrm{cr}}=0.31$. Consequently the $T-x$ magnetic phase diagram consists of a well-developed ferromagnetic dome. We discuss the evolution of ferromagnetism with $x$ on the basis of band structure changes due to varying 5$f$-ligand hybridization. This scenario is supported by the results of electronic structure calculations and consideration of the simplified periodic Anderson model. The analysis of the temperature dependencies of the electrical resistivity and heat capacity at low temperatures of the samples in the vicinity of the QCP reveals a non-Fermi liquid behavior and assigns the ferromagnetic quantum phase transition to be most likely of a continuous Hertz-Millis type.Comment: accepted for publication in PR

Gradual localization of 5f states in orthorhombic UTX ferromagnets - polarized neutron diffraction study of Ru substituted UCoGe

We report on a microscopic study of the evolution of ferromagnetism in the Ru substituted ferromagnetic superconductor (FM SC) UCoGe crystallizing in the orthorhombic TiNiSi-type structure. For that purpose, two single crystals with composition UCo0.97Ru0.03Ge and UCo0.88Ru0.12Ge have been prepared and characterized by magnetization, AC susceptibility, specific heat and electrical resistivity measurements. Both compounds have been found to order ferromagnetically below TC = 6.5 K and 7.5 K, respectively, which is considerably higher than the TC = 3 K of the parent compound UCoGe. The higher values of TC are accompanied by enhanced values of the spontaneous moment mspont. = 0.11 mB/f.u. and mspont. = 0.21 mB/f.u., respectively in comparison to the tiny spontaneous moment of UCoGe (about 0.07mB/f.u.). No sign of superconductivity was detected in either compound. The magnetic moments of the samples were investigated on the microscopic scale using polarized neutron diffraction (PND) and for UCo0.88Ru0.12Ge also by soft X-ray magnetic circular dichroism (XMCD). The analysis of the PND results indicates that the observed enhancement of ferromagnetism is mainly due to the growth of the orbital part of the uranium 5f moment mL(U), reflecting a gradual localization of the 5f electrons with Ru substitution. In addition, the parallel orientation of the U and Co moments has been established in both substituted compounds. The results are discussed and compared with related isostructural ferromagnetic UTX compounds (T - transition metals, X - Si, Ge) in the context of a varying degree of the 5f-ligand hybridization

Orbital-selective behavior in cubanite CuFe2S3

Using ab initio band structure calculations we show that mineral cubanite, CuFe2S3, demonstrates an orbital-selective behavior with some of the electrons occupying molecular orbitals of x(2) - y(2) symmetry and others localized at atomic orbitals. This is a rare situation for 3d transition metal compounds that explains the experimentally observed absence of charge disproportionation, anomalous Mossbauer data, and ferromagnetic ordering in between nearest-neighbor Fe ions

Interplay between valence fluctuations and lattice instabilities across the quantum phase transition in EuCu2(Ge1-xSix)(2)

Increasing Si concentrations in the EuCu2(Ge1-xSix)(2) series tunes the divalent Eu antiferromagnetic (AF) compound EuCu2Ge2(T-N = 14 K) to a nonmagnetic intermediate valence (IV) system EuCu2Si2. There is a collapse of the magnetic state and heavy quasiparticles occur at x similar to 0.7 corresponding to a quantum critical point (QCP). We have systematically investigated the Eu-valence and magnetic states as well as the coupling to the lattice through the QCP in EuCu2(Ge1-xSix)(2). This involved the Eu-151 Mossbauer effect spectroscopy and angle-resolved x-ray diffraction measurements as a function of Si concentration (0 0.6, which is associated with a simultaneous sharp change of the valence state towards a nonmagnetic IV state. The crossover from the AF ordered state to the nonmagnetic IV state is found at a QCP corresponding to x similar to 0.7, at which the nonmagnetic IV state is inhomogeneous and exhibits an enhanced Eu nu+ mean valence (nu similar to 2.5). We believe that the emergence of such an unusual valence state is related to the observed heavy quasiparticles at low temperatures near the QCP. Magnetic order and a nonmagnetic inhomogeneous IV state coexist in a narrow region 0.6 <= x < 0.7, which evolves to a homogeneous IV state above x - 0.8. The ThCrSi2-type tetragonal structure is maintained throughout the series, although there is a precipitous increase in the c/a ratio at 10 K when the valence fluctuations become enhanced at the critical concentration x = 0.7. X-ray diffraction temperature scans at the critical concentration indicate conspicuous changes to steep temperature dependences of decreasing (increasing) values of a (c) lattice parameters and decreasing unit-cell volume at T < 100 K, as the IV ground state become preferentially populated at low temperatures. Thus there is a clear manifestation of strong coupling between the lattice and the valence fluctuation process. A corresponding detailed phase diagram is constructed and compared with that obtained from recent external pressure studies on the system

Quantum ferromagnet in the proximity of the tricritical point

Echoes of quantum phase transitions at finite temperatures are theoretically and experimentally challenging and unexplored topics. Particularly in metallic quantum ferromagnets the experimental investigations are hampered by an intricate preparation of sufficiently pure samples and the access to the proper coordinates in parameter space. The present study shows that it is possible to tune a specific system at easily accessible conditions to the vicinity of its quantum phase transition. The physics is demonstrated on Ru-doped UCoAl, driven by pressure or substitution to and across the tricritical point and follows the first-order transition line to the theoretically presumed quantum phase transition. These findings open the possibilities for further in-depth studies of classical and quantum critical phenomena at easily reachable conditions

Phase diagram of the heavy fermion system YbFe/sub 2/Ge/sub 2/ under pressure

The phase diagram of the heavy fermion compound YbFe2Ge2 under high pressures P ≤ 18.2 GPa was obtained by electrical resistivity measurements. Pressure drives the system from a paramagnetic Fermi liquid state to a magnetically ordered state, with a quantum critical point at PC ≈ 9.4 GPa. In the vicinity of PC a non-Fermi-liquid behavior ascribed to two-dimensional antiferromagnetic fluctuations is observed. In the magnetic side, the resistivity shows the existence of spin-wave excitations characteristic of an antiferromagnet

Phase diagram of the heavy fermion system YbFe/sub 2/Ge/sub 2/ under pressure

The phase diagram of the heavy fermion compound YbFe2Ge2 under high pressures P ≤ 18.2 GPa was obtained by electrical resistivity measurements. Pressure drives the system from a paramagnetic Fermi liquid state to a magnetically ordered state, with a quantum critical point at PC ≈ 9.4 GPa. In the vicinity of PC a non-Fermi-liquid behavior ascribed to two-dimensional antiferromagnetic fluctuations is observed. In the magnetic side, the resistivity shows the existence of spin-wave excitations characteristic of an antiferromagnet