Browsing by Author "Paglione, Johnpierre"
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Item Beyond triplet: Unconventional superconductivity in a spin-3/2 topological semimetal(American Association for the Advancement of Science, 2018-04-06) Kim, Hyunsoo; Wang, Kefeng; Nakajima, Yasuyuki; Hu, Rongwei; Ziemak, Steven; Syers, Paul; Wang, Limin; Hodovanets, Halyna; Denlinger, Jonathan D.; Brydon, Philip M. R.; Agterberg, Daniel F.; Tanatar, Makariy A.; Prozorov, Ruslan; Paglione, JohnpierreIn all known fermionic superfluids, Cooper pairs are composed of spin-1/2 quasi-particles that pair to form either spin-singlet or spin-triplet bound states. The “spin” of a Bloch electron, however, is fixed by the symmetries of the crystal and the atomic orbitals from which it is derived and, in some cases, can behave as if it were a spin-3/2 particle. The superconducting state of such a system allows pairing beyond spin-triplet, with higher spin quasi-particles combining to form quintet or septet pairs. We report evidence of unconventional superconductivity emerging from a spin-3/2 quasi-particle electronic structure in the half-Heusler semimetal YPtBi, a low-carrier density noncentrosymmetric cubic material with a high symmetry that preserves the p-like j = 3/2 manifold in the Bi-based Γ8 band in the presence of strong spin-orbit coupling. With a striking linear temperature dependence of the London penetration depth, the existence of line nodes in the superconducting order parameter Δ is directly explained by a mixed-parity Cooper pairing model with high total angular momentum, consistent with a high-spin fermionic superfluid state. We propose a k ⋅ p model of the j = 3/2 fermions to explain how a dominant J = 3 septet pairing state is the simplest solution that naturally produces nodes in the mixed even-odd parity gap. Together with the underlying topologically nontrivial band structure, the unconventional pairing in this system represents a truly novel form of superfluidity that has strong potential for leading the development of a new series of topological superconductors.Item Quantum-critical scale invariance in a transition metal alloy(Springer Nature, 2020-10-15) Nakajima, Yasuyuki; Metz, Tristin; Eckberg, Christopher; Kirshenbaum, Kevin; Hughes, Alex; Wang, Renxiong; Wang, Limin; Saha, Shanta R.; Liu, I-Lin; Butch, Nicholas P.; Campbell, Daniel; Eo, Yun Suk; Graf, David; Liu, Zhonghao; Borisenko, Sergey V.; Zavalij, Peter Y.; Paglione, JohnpierreQuantum-mechanical fluctuations between competing phases induce exotic collective excitations that exhibit anomalous behavior in transport and thermodynamic properties, and are often intimately linked to the appearance of unconventional Cooper pairing. High-temperature superconductivity, however, makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties. Here we report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe1/3Co1/3Ni1/3)2As2, indicative of quantum criticality at zero temperature and applied magnetic field. Beyond a linear-in-temperature resistivity, the hallmark signature of strong quasiparticle scattering, we find a scattering rate that obeys a universal scaling relation between temperature and applied magnetic fields down to the lowest energy scales. Together with the dominance of hole-like carriers close to the zero-temperature and zero-field limits, the scale invariance, isotropic field response, and lack of applied pressure sensitivity suggests a unique quantum critical system unhindered by a pairing instability.