Superconducting and normal state properties of the systems La1−xMxPt4Ge12 (M=Ce,Th)

Electrical resistivity, magnetization, and specific heat measurements were performed on polycrystalline samples of the filled-skutterudite systems La1−xMxPt4Ge12 (M = Ce and Th). Superconductivity in LaPt4Ge12 was quickly suppressed with Ce substitution and no evidence for superconductivity was found down to 1.1 K for x \u3e 0.2. Temperature-dependent specific heat data at low temperatures for La1−xCexPt4Ge12 show a change from powerlaw to exponential behavior, which may be an indication for multiband superconductivity in LaPt4Ge12. A similar crossover was observed in the Pr1−xCexPt4Ge12 system. However, the suppression rates of the superconducting transition temperatures Tc(x) in the two systems are quite disparate, indicating a difference in the nature of superconductivity, which is conventional in LaPt4Ge12 and unconventional in PrPt4Ge12. In comparison, a nearly linear and smooth evolution of Tc with increasing Th was observed in the La1−xThxPt4Ge12 system, with no change of the superconducting energy gap in the temperature dependence of the specific heat, suggesting similar types of superconductivity in both the LaPt4Ge12 and ThPt4Ge12 compounds

Quantum criticality in Ce1−x_{1-x}Smx_xCoIn5_5

Motivated by the possibility of observing the co-existence between magnetism and unconventional superconductivity in heavy-fermion Ce$_{1-x}$Sm$_x$CoIn$_5$ alloys, we studied how the samarium substitution on the cerium site affects the magnetic field-tuned-quantum criticality of stoicheometric CeCoIn$_5$ by performing specific heat and resistivity measurements. By applying an external magnetic field, we have observed Fermi-liquid to non-Fermi-liquid crossovers in the temperature dependence of the electronic specific heat normalized by temperature and of the resistivity. We obtained the magnetic-field-induced quantum critical point (QCP) by extrapolating to zero temperature the temperature - magnetic field dependence at which the crossovers take place. Furthermore, a scaling analysis of the electronic specific heat is used to confirm the existence of the QCP. We have found that the magnitude of the magnetic-field-induced QCP decreases with increasing samarium concentration. Our analysis of heat capacity and resistivity data reveals a zero-field QCP for $x_\textrm{cr} \approx 0.15$, which falls inside the region where Sm ions antiferromagnetism and superconductivity co-exist

Investigation of superconducting and normal-state properties of the filled-skutterudite system PrPt4Ge12−xSbx

We report a study of the superconducting and normal-state properties of the filled-skutterudite system PrPt4Ge12−xSbx . Polycrystalline samples with Sb concentrations up to x = 5 were synthesized and investigated by means of x-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements. We observed a suppression of superconductivity with increasing Sb substitution up to x = 4, above which no signature of superconductivity was observed down to 140 mK. The Sommerfeld coefficient, γ , of superconducting specimens decreases with increasing x up to x = 3, suggesting that superconductivity may depend on the density of electronic states in this system. The specific heat for x = 0.5 exhibits an exponential temperature dependence in the superconducting state, reminiscent of a nodeless superconducting energy gap. We observed evidence for a weak “rattling” mode associated with the Pr ions, characterized by an Einstein temperature ΘE ∼ 60 K for 0 ≤ x ≤ 5; however, the rattling mode may not play any role in suppressing superconductivity

Tuning the magnetic ground state of Ce1−xYbxRhIn5 by Yb valence fluctuations

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 \u3c 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

Temperature vs. Sm concentration phase diagram and quantum criticality in the correlated electron system Ce1-xSmxCoIn5

We report electrical resistivity, magnetization, and specific heat measurements on the correlated electron system Ce1−xSmxCoIn5 (0≤x≤1). Superconductivity (SC) in the heavy-fermion compound CeCoIn5, which is suppressed with increasing Sm concentration x, and antiferromagnetic (AFM) order of SmCoIn5, which is suppressed with decreasing x, converge near a quantum critical point at xQCP≈0.15, with no indication of coexistence of SC and AFM in the vicinity of the QCP. Non-Fermi-liquid (NFL) behavior is observed in the normal-state electrical resistivity, ρ(T), and specific heat, C(T), in the vicinity of the QCP; e.g., the coefficient and the exponent of the power-law T dependence of ρ(T) exhibit pronounced maxima and minima, respectively, at xQCP, while C(T)/T exhibits a logarithmic divergence in T at xQCP. A low-temperature upturn in ρ(T) develops in the range 0.70≤x≤0.85 which is reminiscent of a single impurity Kondo effect, suggesting that Sm substitution tunes the relative strength of competing Kondo and Ruderman-Kittel-Kasuya-Yosida energy scales. The suppression of SC with increasing x is probably associated with the exchange interaction between the Ce quasiparticles involved in the superconductivity and the magnetic moments of the Sm ions

From hidden order to antiferromagnetism Electronic structure changes in Fe doped URu2Si2

The transition of URu2Si2 to an ordered state below 17.5 K has been a puzzle of condensed matter physics for over 30 y, earning it the soubriquet of the hidden order HO state. Intriguingly, pressure or doping can transform the HO into an antiferromagnetic AFM state, of well known symmetry. Here, by angle resolved photoemission spectroscopy, the electronic structure of URu2Si2 in the HO phase is directly compared with its AFM counterpart. This reveals topographically identical Fermi surfaces; however, they differ by the size of some of their pockets. The overall nonrigid change of the electronic structure across the AFM HO phase boundary indicates that a change in the interaction strength between states near the Fermi level is essential to stabilize the HO stat