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Register a new userEvolution of ferromagnetic order in URhGe alloyed with Ru, Co and Si
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We have investigated the evolution of ferromagnetic order in the correlated metal URhGe (Curie temperature $T_{rm C} = $9.5 K) by chemical substitution of Ru, Co and Si. Polycrystalline samples URh$_{1-x}$Ru$_x$Ge ($x leq $0.6), URh$_{1-x}$Co$_x$Ge ($x leq $0.9) and URhGe$_{1-x}$Si$_x$ ($x leq $0.2) have been prepared and the magnetic properties have been investigated by magnetization and transport experiments. In the case of Ru doping, $T_{rm C}$ initially increases, but then decreases linearly as a function of $x$ and is completely suppressed for $x_{rm cr} approx 0.38$. The Curie temperature in the URh$_{1-x}$Co$_x$Ge series has a broad maximum $T_{rm C} = 20$ K near $x=0.6$ and then drops to 8 K for $x=0.9$. In the case of Si doping $T_{rm C}$ stays roughly constant. We conclude that the alloy systems URh$_{1-x}$Ru$_x$Ge and URh$_{1-x}$Co$_x$Ge are interesting candidates to study the ferromagnetic instability.
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In most unconventional superconductors, like the high-Tc cuprates, iron pnictides, or heavy fermion systems, superconductivity emerges in the proximity of an electronic instability. Identifying unambiguously the pairing mechanism remains nevertheless an enormous challenge. Among these systems, the orthorhombic uranium ferromagnetic superconductors have a unique position, notably because magnetic fields couple directly to ferromagnetic order, leading to the fascinating discovery of the re-emergence of superconductivity in URhGe at high field. Here we show that uniaxial stress is a remarkable tool allowing fine-tuning of the pairing strength. With a relatively small stress, the superconducting phase diagram is spectacularly modified, with a merging of the low and high field superconducting states and a significant enhancement of superconductivity. The superconducting critical temperature increases both at zero field and under field, reaching 1K, more than twice higher than at ambient pressure. The enhancement of superconductivity is directly related to a change of the magnetic dimensionality with an increase of the transverse magnetic susceptibility, demonstrating that in addition to the Ising-type longitudinal ferromagnetic fluctuations, transverse magnetic fluctuations also play an important role in the superconducting pairing.
The field-reentrant (field-reinforced) superconductivity on ferromagnetic superconductors is one of the most interesting topics in unconventional superconductivity. The enhancement of effective mass and the induced ferromagnetic fluctuations play key roles for reentrant superconductivity. However, the associated change of the Fermi surface, which is often observed at (pseudo-) metamagnetic transition, can also be a key ingredient. In order to study the Fermi surface instability, we performed Hall effect measurements in the ferromagnetic superconductor URhGe. The Hall effect of URhGe is well explained by two contributions, namely by the normal Hall effect and by the large anomalous Hall effect due to skew scattering. The large change in the Hall coefficient is observed at low fields between the paramagnetic and ferromagnetic states for H // c-axis (easy-magnetization axis) in the orthorhombic structure, indicating that the Fermi surface is reconstructed in the ferromagnetic state below the Curie temperature (T_Curie=9.5K). At low temperatures (T << T_Curie), when the field is applied along the b-axis, the reentrant superconductivity was observed in both the Hall resistivity and the magnetoresistance below 0.4K. Above 0.4K, a large jump with the first-order nature was detected in the Hall resistivity at a spin-reorientation field H_R ~ 12.5T, demonstrating that the marked change of the Fermi surface occurs between the ferromagnetic state and the polarized state above H_R. The results can be understood by the Lifshitz-type transition, induced by the magnetic field or by the change of the effective magnetic field.
We review our recent studies on ferromagnetic superconductors, UGe2, URhGe and UCoGe, where the spin-triplet state with the so-called equal spin pairing is realized. We focus on experimental results of URhGe and UCoGe in which the superconductivity occurs already at ambient pressure. The huge upper critical field Hc2 on UCoGe for the field along the hard magnetization axis (b-axis) is confirmed by the AC susceptibility measurements by the fine tuning of field angle. Contrary to the huge Hc2 along the hard-magnetization axis, Hc2 along the easy-magnetization axis (c-axis) is relatively small in value. However, the initial slope of Hc2, namely dHc2/dT (H -> 0) both in UCoGe and in URhGe indicates the large value, which can be explained by the magnetic domain effect detected in the magnetization measurements. The specific heat measurements using a high quality single crystal of UCoGe demonstrate the bulk superconductivity, which is extended under magnetic field for the field along c-axis.
The discovery in 2000 that the ferromagnetic (FM) compound UGe2 (T_Curie = 52 K at ambient pressure) becomes superconducting under a pressure of P = 1.1 GPa until it enters the paramagnetic (PM) phase above Pc = 1.6 GPa was a surprise. Successive searches for new materials (URhGe and UCoGe) led to the discovery of the coexistence of superconductivity (SC) and ferromagnetism at ambient pressure. Furthermore in UCoGe, it was found that SC survives in the PM regime from P_c = 1.1 to 4 GPa. Focus has been on low-temperature experiments under extreme conditions of magnetic field (H), pressure, and uniaxial stress. In UGe2, strong interplay exists between Fermi surface (FS) reconstructions in the cascade of different FM and PM ground states and their magnetic fluctuations. Similar phenomena occur in URhGe and UCoGe but, at first glance, the SC seems to be driven by the FM fluctuations. In UCoGe, a longitudinal field scan leads to a drastic decrease in the FM fluctuations, while a transverse field scan leads to suppression of the Curie temperature, T_Curie; the consequence is a boost in FM fluctuations, leading to a reinforcement of SC. The singularity in URhGe is the weakness of the anisotropy between c- and b-axes; the most noteworthy feature is the detection of reentrant SC near H_R. All the experimental results give evidence that the SC in these three materials originates from the FM fluctuations, which are amplitude modes of magnetic excitations in the FM state. Spin-triplet pairing has been anticipated in the FM superconductors and was actually observed by Knight-shift measurements in the SC state of UCoGe. Their fascinating (p, T, H) phase diagrams are now well established. Discussion is presented on how different theoretical approaches can describe the various phenomena discovered by experimentalists.
$^{59}$Co NMR experiments have been performed on single crystals of the layered cobaltate Na$_{x}$CoO$_{2}$ with x=0.77 which is an antiferromagnet with Neel temperature $T_{N}=22$~K. In this metallic phase six Co sites are resolved in the NMR spectra, with distinct quadrupole frequencies $ u _{Q}$, magnetic shifts $K_{ZZ}$ and nuclear spin lattice relaxation rates $% 1/T_{1}$. Contrary to the $x=1/2$ or $x=2/3$ phases the 3D stacking of the Na planes is not perfect for $x=0.77$ but this does not influence markedly the electronic properties. We evidence that the magnetic and charge properties of the Co sites are highly correlated with each other as $K_{ZZ}$ and $(1/T_{1})^{1/2}$ scale linearly with $ u _{Q}$. The data analysis allows us to separate the contribution $ u_{Q}^{latt}$ of the ionic charges to $ u _{Q}$ from that $ u _{Q}^{el}$ due to the hole orbitals on the Co sites. We could extend coherently this analysis to all the known phases in the Na cobaltate phase diagram. The variation with $x$ of $ u _{Q}^{latt}$ is found to fit rather well numerical computations done in a point charge model. The second term $ u _{Q}^{el}$ allowed us to deduce the hole concentration on the cobalts. These detailed experimental results should stimulate theoretical calculations of the electronic structure involving both the Co orbital configurations and DMFT approaches to take into account the electronic correlations.
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