G. M. Pang

Fermi surface reconstruction and multiple quantum phase transitions in the antiferromagnet CeRhIn5

L. Jiao, H. Q. Yuan, ∗ Y. Kohama, E. D. Bauer, J. -X. Zhu, J. Singleton, T. Shang, J. L. Zhang, Y. Chen, H. O. Lee, T. Park, M. Jaime, J. D. Thompson, F. Steglich, and Q. Si † Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou, Zhejiang 310027, China Los Alamos National Laboratory, Los Alamos, NM 87545 Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany Department of Physics and Astronomy, Rice University, Houston, TX 77005 (Dated: May 11, 2014)Significance Conventional, thermally driven continuous phase transitions are described by universal critical behavior that is independent of microscopic details of a specific material. An analogous description is lacking for phase transitions that are driven at absolute zero temperature by a nonthermal control parameter. Classification of quantum-driven phase transitions is a fundamental but open problem that arises in diverse contexts and multiple classes of materials. Here we report the first observation, to our knowledge, of a sharp Fermi surface reconstruction while applying a strong magnetic field to suppress an antiferromagnetic transition to zero temperature. These experiments demonstrate that direct measurements of the Fermi surface can distinguish theoretically proposed models of quantum criticality and point to a universal description of quantum phase transitions. Conventional, thermally driven continuous phase transitions are described by universal critical behavior that is independent of the specific microscopic details of a material. However, many current studies focus on materials that exhibit quantum-driven continuous phase transitions (quantum critical points, or QCPs) at absolute zero temperature. The classification of such QCPs and the question of whether they show universal behavior remain open issues. Here we report measurements of heat capacity and de Haas–van Alphen (dHvA) oscillations at low temperatures across a field-induced antiferromagnetic QCP (Bc0 ≈ 50 T) in the heavy-fermion metal CeRhIn5. A sharp, magnetic-field-induced change in Fermi surface is detected both in the dHvA effect and Hall resistivity at B0* ≈ 30 T, well inside the antiferromagnetic phase. Comparisons with band-structure calculations and properties of isostructural CeCoIn5 suggest that the Fermi-surface change at B0* is associated with a localized-to-itinerant transition of the Ce-4f electrons in CeRhIn5. Taken in conjunction with pressure experiments, our results demonstrate that at least two distinct classes of QCP are observable in CeRhIn5, a significant step toward the derivation of a universal phase diagram for QCPs.

Evidence for nodeless superconductivity in NdO1−xFxBiS2 (x = 0.3 and 0.5) single crystals

We study the superconducting pairing states of NdO(1-x)F(x)BiS2 (x = 0.3 and 0.5) by measuring the magnetic penetration depth Δλ(T) using the tunnel-diode-oscillator (TDO) technique. An upturn is observed in Δλ(T) as well as the magnetic susceptibility χ(T) in the low-temperature limit, which is attributed to the paramagnetism of Nd ions. After subtracting the paramagnetic contributions, the penetration depth Δλ(T) follows an exponential-type temperature dependence at T ≪ T(c), providing evidence of nodeless superconductivity for NdO(1-x)F(x)BiS2. This is further supported by the analyses of superfluid density ρ(s)(T), which can be well described by a BCS model with an energy gap of Δ(0) ∼ 2.15 k(B)T(c).

Two-Gap Superconductivity in LaNiGa2 with Nonunitary Triplet Pairing and Even Parity Gap Symmetry

The nature of the pairing states of superconducting LaNiC_{2} and LaNiGa_{2} has to date remained a puzzling question. Broken time reversal symmetry has been observed in both compounds and a group theoretical analysis implies a nonunitary triplet pairing state. However, all the allowed nonunitary triplet states have nodal gap functions but most thermodynamic and NMR measurements indicate fully gapped superconductivity in LaNiC_{2}. Here we probe the gap symmetry of LaNiGa_{2} by measuring the London penetration depth, specific heat, and upper critical field. These measurements demonstrate two-gap nodeless superconductivity in LaNiGa_{2}, suggesting that this is a common feature of both compounds. These results allow us to propose a novel triplet superconducting state, where the pairing occurs between electrons of the same spin, but on different orbitals. In this case the superconducting wave function has a triplet spin component but isotropic even parity gap symmetry, yet the overall wave function remains antisymmetric under particle exchange. This model leads to a nodeless two-gap superconducting state which breaks time reversal symmetry, and therefore accounts well for the seemingly contradictory experimental results.

Weak interband-coupling superconductivity in the filled skutterudite LaPt4Ge12

The superconducting pairing state of LaPt

Nodeless superconductivity in noncentrosymmetric PbTaSe

_{4}

_2

Ge

single crystals

_{12}

Fully gapped d-wave superconductivity in CeCu2Si2

is studied by measuring the magnetic penetration depth

Nodeless superconductivity and time-reversal symmetry breaking in the noncentrosymmetric superconductor Re24Ti5

\lambda(T,B)

Probing the superconducting gap structure of (Li1−xFex)OHFeSe

and the superfluid density