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The family of the superconducting quasiskutterudites (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) features a structural quantum critical point at x(c)=0.9, around which a dome-shaped variation of the superconducting transition temperature T(c) is found. Using specific heat, we probe the normal and the superconducting states of the entire series straddling the quantum critical point. Our analysis indicates a significant lowering of the effective Debye temperature on approaching x(c), which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of 2Δ/k(B)T(c) and ΔC/γT(c) beyond the Bardeen-Cooper-Schrieffer values is found in the vicinity of the structural quantum critical point. The phase diagram of (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) thus provides a model system to study the interplay between structural quantum criticality and strong electron-phonon coupling superconductivity.

The quasi-skutterudite superconductor Sr3Rh4Sn13 features a pronounced anomaly in electrical resistivity at T * ∼138 K. We show that the anomaly is caused by a second-order structural transition, which can be tuned to 0 K by applying physical pressure and chemical pressure via the substitution of Ca for Sr. A broad superconducting dome is centred around the structural quantum critical point. Detailed analysis of the tuning parameter dependence of T * as well as insights from lattice dynamics calculations strongly support the existence of a structural quantum critical point at ambient pressure when the fraction of Ca is 0.9 (i.e., xc = 0.9). This establishes (CaxSr1−x)3Rh4Sn13 series as an important system for exploring the physics of structural quantum criticality without the need of applying high pressures.Quantum criticality associated with a second-order phase transition at zero temperature has been a fruitful concept in the search for superconductivity. Particularly clear examples include the heavy fermion [1, 2] and iron-based systems [3][4][5][6], where superconductivity is stabilized in the vicinity of an antiferromagnetic quantum critical point (QCP). In addition to magnetic instabilities, structural instabilities also have an intricate influence on superconductivity. However, clean systems with continuous structural transition tunable by external parameters are relatively rare, and for the few cases studied in recent years, the structural order is found in the vicinity of magnetic order [7], making the notion of structural quantum criticality and its influence on the superconductivity less advanced than the magnetic counterpart.Superconducting stannides [8,9], including A 3 T 4 Sn 13 systems where A=La,Sr,Ca and T=Ir,Rh, have recently been reexamined using a wide range of probes [10][11][12][13][14][15][16][17][18][19][20][21][22]. Sr 3 Rh 4 Sn 13 , a member of this stoichiometry family, is a strong-coupling superconductor with a critical temperature T c ≈4.7 K. The superconducting gap function ∆ is nodeless, with 2∆/k B

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We synthesized the novel S = 1/2Mo 3 cluster magnet Li 2 ScMo 3 O 8 as well as the isomorphic compound Li 2 InMo 3 O 8 , and investigated their macroscopic and microscopic physical properties. Li 2 InMo 3 O 8 showed magnetic ordering at T N ∼ 12 K with a 120 • structure, while no magnetic ordering is observed down to 0.5 K in Li 2 ScMo 3 O 8 despite the strong antiferromagnetic interaction among clusters probed by the Weiss temperature. Results of the 7 Li nuclear magnetic resonance spectra and the nuclear spin-lattice relaxation rate (1/T 1 ) measurement show direct microscopic proof of the spin-disordered ground state. We discuss the difference in the ground state between Li 2 ScMo 3 O 8 and Li 2 InMo 3 O 8 using the description of localized magnetism based on spin frustration within the triangular lattice and that of inter-and intracluster charge fluctuations.