Nodal gap structure and order parameter symmetry of the unconventional superconductor UPt₃

Abstract

Spanning a broad range of physical systems, complex symmetry breaking is widely recognized as a hallmark of competing interactions. This is exemplified in superfluid 3 He which has multiple thermodynamic phases with spin and orbital quantum numbers S = 1 and L = 1, that emerge on cooling from a nearly ferromagnetic Fermi liquid. The heavy fermion compound UPt 3 exhibits similar behavior clearly manifest in its multiple superconducting phases. However, consensus as to its order parameter symmetry has remained elusive. Our small angle neutron scattering measurements indicate a linear temperature dependence of the London penetration depth characteristic of nodal structure of the order parameter. Our theoretical analysis is consistent with assignment of its symmetry to an L = 3 odd parity state for which one of the three thermodynamic phases in non-zero magnetic field is chiral.

Broken time-reversal symmetry in the topological superconductor UPt3

“…Here λ ab and ξ ab are respectively penetration depth and coherence length in the screening current plane (ab-plane), and c is a constant of order unity (typically between 1 4 and 2)) [1]. Previous SANS measurements yielded a penetration depth λ ab = 680 nm [2], and consequently (λ ab Q) 2 1 for the entire range of applied magnetic fields in this work. As a result the expression for the form factor simplifies to…”

“…Topological properties of materials are of fundamental as well as practical importance, and are currently the focus of intense research [1]. Recently, topological superconductors have received increased attention, as it was realized that vortices in them can host Majorana fermions with potential use in quantum computing [2][3][4]. As a result, some well known unconventional superconductors have received renewed attention [5,6], particularly UPt 3 [7, 8], Sr 2 RuO 4 [9,10] and superfluid 3 He [11,12].…”

“…These include the H-T phase diagram [15,18,21,22], thermodynamic and transport studies [23,24], muon spin rotation experiments [16], phasesensitive Josephson tunneling in the B-phase [7], and the previously mentioned directional tunneling to resolve the nodal structure in the A-phase [25] as well as polar Kerr rotation that shows evidence for broken time-reversal symmetry in the B phase, but not the A phase [8]. Furthermore, the linear temperature dependence of the London penetration depth, obtained from both magnetization [26,27] and SANS [28] measurements, is consistent with quadratic dispersion at the polar nodes of the energy gap structure that is one characteristic of the E 2u model. With the exception of the SANS experiment by Gannon et al [28], all of the experiments listed above that support an odd-parity chiral state for the B phase were performed in the Meissner state [7,8,16,25].…”

Simultaneous evidence for Pauli paramagnetic effects and multiband superconductivity inKFe2As2by small-angle neutron scattering studies of the vortex lattice

“…Superconducting vortices, introduced by an applied magnetic field, can serve as a sensitive probe of the superconducting state in the host material. For more than half a century small-angle neutron scattering (SANS) has been used to study the vortex lattice (VL) in a wide range of materials, [17][18][19][20][21][22] and has provided often unique information about gap nodes and dispersion, 6,[23][24][25] non-local effects, 26,27 multiband superconductivity, 28 Pauli paramagnetic effects, [29][30][31][32] and a direct measure of the intrinsic superconducting anisotropy (Γ ac ). [32][33][34][35][36][37][38] The latter quantity may be directly measured by the field-angledependent distortion of the VL structure (Γ VL ) from a regular hexagonal pattern.…”

γ-U phase in U-Pt system retained to low temperatures by means of rapid cooling

“…The heavy-fermion superconductor UPt 3 is of a particular interest, since it belongs to the most exotic U-compounds due to its unconventional superconductivity coexisting with pronounced spin fluctuations at low temperatures, revealing a double stepstructure at the superconducting transition at T c = 0.54 K [19] and at about 50 mK below T c [20] and a complex phase diagram with three different superconducting phases as a function of temperature and magnetic field [21]. The weak antiferromagnetic order below T N = 5 K with an extremely small uranium moment of only 0.02µ B /U atom is believed to play a crucial role in the existence of multiple superconducting phases [22,23]. The rich-U part (or low-Pt part (with the Pt concentration < 50 at.%) of the U-Pt system has been much less studied.…”