Eleven new compounds, R Ni2Cd20 (R=Y, La–Nd, Sm, Gd, Tb) and R Pd2Cd20 (R=Ce, Pr, Sm), were grown as single crystals in high temperature cadmium-rich solutions. They crystallize in the cubic ...CeCr2Al20-type structure (Fd3¯m, Z=8) as characterized by measurements of powder X-ray diffraction. Electrical resistivity, magnetization, and specific heat measurements were performed on R Ni2Cd20 (R=Y, La–Nd, Sm, Gd, Tb) single crystals. Whereas YNi2Cd20 and LaNi2Cd20 exhibit unremarkable metallic behavior, when magnetic moments from localized 4f electron states (Gd3+–Tb3+) are embedded into this host, they exhibit ferromagnetic order with values of the Curie temperature TC for R Ni2Cd20 (R=Gd, and Tb) which scale with the de Gennes factor.
Specific heat divided by temperature C/T vs. T for single crystals of R Ni2Cd20 (R=Y, La–Nd, Gd, and Tb). Left inset: Low temperature C/T vs. T2 for LaNi2Cd20. The solid line represents a linear fit of the data. Right inset: Low-temperature C/T data vs. T for R=Ce–Nd, Gd, and Tb; magnetic ordering temperatures are indicated by arrows. Display omitted
•R Ni2Cd20 (R=Y, La–Nd, Sm, Gd, Tb) single crystals synthesized for the first time.•R Pd2Cd20 (R=Ce, Pr, Sm) single crystals synthesized for the first time.•Single crystals are of good metallurgical quality (large RRR values).•NdNi2Cd20 orders antiferromagnetically at TN=1.5K.•R Ni2Cd20 (R=Sm, Gd, Tb) order ferromagnetically.
We characterize the properties of Ce1−xYbxRhIn5 single crystals with 0≤x≤1 using measurements of powder x-ray diffraction, energy dispersive x-ray spectroscopy, electrical resistivity, magnetic ...susceptibility, specific heat, x-ray absorption near edge structure (XANES), and neutron diffraction. The Yb valence vYb, calculated from the magnetic susceptibility and measured using XANES, decreases from 3+ at x=0 to ∼2.1+ at xact=0.2, where xact is the measured Yb concentration. A transition from incommensurate to commensurate antiferromagnetism is observed in neutron diffraction measurements along Q=(0.5,0.5,l) between 0.2≤xact≤0.27; this narrative is supported by specific-heat measurements in which a second robust feature appears at a temperature TI(TI<TN) for the same concentration range. Magnetic susceptibility measurements also reveal features which provide additional evidence of magnetic ordering. The results of this study suggest that the evolution of the Yb valence plays a critical role in tuning the magnetic ground state of Ce1−xYbxRhIn5.
We have carried out a careful magnetic neutron scattering study of the heavy fermion compound URu2Si2 to probe the possible existence of a small magnetic moment parallel to tetragonal basal plane in ...the 'hidden-order' phase. This small in-plane component of the magnetic moment on the uranium sites S has been postulated by two recent models (rank-5 superspin hastatic order) aiming to explain the hidden-order phase, in addition to the well-known out-of-plane component S 0.01-0.04 μB U. In order to separate S and S , we take advantage of the condition that for magnetic neutron scattering only the components of the magnetic structure that are perpendicular to the scattering vector Q contribute to the magnetic scattering. We find no evidence for an in-plane magnetic moment S . Based on the statistics of our measurement, we establish that the upper experimental limit for the size of any possible in-plane component is Smax 1 × 10−3 μB U.
One of the most notorious non-Fermi-liquid properties of both archetypal heavy-fermion systems and the high-Tc copper oxide superconductors is an electrical resistivity that evolves linearly (rather ...than quadratically) with temperature, T. In the heavy-fermion superconductor CeCoIn5 (ref. 6), this linear behaviour was one of the first indications of the presence of a zero-temperature instability, or quantum critical point. Here, we report the observation of a unique control parameter of T-linear scattering in CeCoIn5, found through systematic chemical substitutions of both magnetic and non-magnetic rare-earth, R, ions into the Ce sublattice. We find that the evolution of inelastic scattering in Ce1−xRxCoIn5 is strongly dependent on the f-electron configuration of the R ion, whereas two other key properties-Cooper-pair breaking and Kondo-lattice coherence-are not. Thus, T-linear resistivity in CeCoIn5 is intimately related to the nature of incoherent scattering centres in the Kondo lattice, which provides insight into the anomalous scattering rate synonymous with quantum criticality.
•Quasiparticles in heavy-fermion compounds are much heavier than free electrons.•Superconductivity involves pairing of these massive quasiparticles.•Quasiparticle pairing mediated by magnetic or ...quadrupolar fluctuations.•We review the properties of superconductivity in heavy-fermion compounds.
Over the past 35years, research on unconventional superconductivity in heavy-fermion systems has evolved from the surprising observations of unprecedented superconducting properties in compounds that convention dictated should not superconduct at all to performing explorations of rich phase spaces in which the delicate interplay between competing ground states appears to support emergent superconducting states. In this article, we review the current understanding of superconductivity in heavy-fermion compounds and identify a set of characteristics that is common to their unconventional superconducting states. These core properties are compared with those of other classes of unconventional superconductors such as the cuprates and iron-based superconductors. We conclude by speculating on the prospects for future research in this field and how new advances might contribute towards resolving the long-standing mystery of how unconventional superconductivity works.
We investigate the effect of enhancement of superconducting transition temperature Tc by nonmagnetic atom disorder in the filled skutterudite-related tetragonal Y5Rh6Sn18 compound doped with Ca. We ...documented experimentally that Y5Rh6Sn18 and its Ca-doped alloys are electronically inhomogeneous at the nanoscale, when Ca content is smaller than ∼1.5 in the Y5−xCaxRh6Sn18 system, while for x≥1.5 much stronger chemical phase inhomogeneity is observed with an about 20% volume fraction of the second 3:4:13 cubic phase, which we interpret as a fluctuation of stoichiometry within the bulk sample. Then the enhancement of Tc vs x could be modeled by a mechanism proposed in recent theoretical reports for increasing the mean-field transition temperature Tc in the presence of nonmagnetic disorder. The increase in disorder with doping increases the sample inhomogeneity and causes a systematic increase in Tc, while for two-phase x≥1.5 samples the critical temperature Tc★≈2×Tc rapidly increases. Based on band structure calculations performed under pressure, we demonstrate how the change in density of states would affect Tc of Y5Rh6Sn18. We obtained the Grüneisen parameter γG larger for the inhomogeneous phase with respect to γG of the bulk Tc phase and attribute the enhancement of Tc to larger stiffening of the high-temperature Tc★ phase.
By performing a series of thermodynamic measurements in an applied magnetic field Hext, we investigated the effects of Eu substitution on the Pr sites in the filled skutterudite compound ...Pr1−xEuxPt4Ge12 (0≤x≤1). A heat capacity Schottky anomaly is present over the whole doping range. For the samples with x>0.5, the temperature of the maximum, Tmax, shifts to lower temperature with increasing Hext. We argue that this behavior reflects the antiferromagnetic (AFM) ordering of the Eu moments, as the AFM transition is suppressed by Hext. The Schottky anomaly in the samples with x≤0.5 shifts to higher temperatures with increasing magnetic field, signaling the presence of an internal magnetic field due to short-range AFM correlations induced by magnetic moments of neighboring Eu sites. In low Hext, the Schottky gaps show a nonlinear relationship with Hext as the magnetic moments become weakly magnetized. In high Hext, the magnetic moments of Eu sites become completely aligned with Hext. Thus, increasing Hext does not further increase the magnetization; hence the Schottky gaps increase linearly with Hext.
Here, we present data on the optical conductivity of URu2-x(Fe,Os)xSi2. While the parent material URu2Si2 enters the enigmatic hidden order (HO) phase below 17.5 K, an antiferromagnetic (AFM) phase ...is induced by the substitution of Fe or Os onto the Ru sites. We find that both the HO and the AFM phases exhibit an identical gap structure that is characterized by a loss of conductivity below the gap energy with spectral weight transferred to a narrow frequency region just above the gap, the typical optical signature of a density wave. The AFM phase is marked by strong increases in both transition temperature and the energy of the gap associated with the transition. In the normal phase just above the transition the optical scattering rate varies as ω2. We find that in both the HO and the AFM phases, our data are consistent with elastic resonant scattering of a Fermi liquid. This indicates that the appearance of a coherent state is a necessary condition for either ordered phase to emerge. Our measurements favor models in which the HO and the AFM phases are driven by the common physics of a nesting-induced density wave gap.
Thermal conductivity measurements were performed on single crystal samples of the superconducting filled-skutterudite compounds PrOs4Sb12 and PrRu4Sb12 both as a function of temperature and ...transverse magnetic field. In a zero magnetic field, the low temperature electronic thermal conductivity of PrRu4Sb12 is consistent with a fully gapped Fermi surface. For PrOs4Sb12, residual electronic conduction in the zero-temperature limit is consistent with the presence of nodes in the superconducting energy gap. The electronic thermal conductivity for both compounds shows a rapid rise at low magnetic fields. In PrRu4Sb12, this is interpreted in terms of multiband effects. In PrOs4Sb12, we consider the Doppler shift of nodal quasiparticles and multiband effects.
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