W.J. Gannon

Observation of Broken Time-Reversal Symmetry in the Heavy Fermion Superconductor UPt3

Optically probed superconductor The exotic superconductor UPt3 has two superconducting phases that appear at different temperatures, but their nature remains unclear. Schemm et al. shone circularly polarized light on a crystal of UPt3 and studied its reflection (see the Perspective by van der Marel and Sawatzky). In the low-temperature phase, the pairs of electrons that make the material superconducting have a handedness to them. The finding narrows down the possible descriptions of the electron-pair wave function. Science, this issue p. 190; see also p. 138 Optical measurements elucidate the nature of superconductivity in an exotic compound. [Also see Perspective by van der Marel and Sawatzky] Models of superconductivity in unconventional materials can be experimentally differentiated by the predictions they make for the symmetries of the superconducting order parameter. In the case of the heavy-fermion superconductor UPt3, a key question is whether its multiple superconducting phases preserve or break time-reversal symmetry (TRS). We tested for asymmetry in the phase shift between left and right circularly polarized light reflected from a single crystal of UPt3 at normal incidence and found that this so-called polar Kerr effect appears only below the lower of the two zero-field superconducting transition temperatures. Our results provide evidence for broken TRS in the low-temperature superconducting phase of UPt3, implying a complex two-component order parameter for superconductivity in this system.

The Transition Between Real and Complex Superconducting Order Parameter Phases in UPt3

Mind the Gap The symmetry of the pairing gap is one of the most important properties of a superconductor. Whereas conventional superconductors such as lead have a gap that is uniform in momentum space, the enigmatic cuprates have point nodes and a characteristic d-wave symmetry. The heavy fermion compound UPt3 has long been known to exhibit unconventional pairing. Now, Strand et al. (p. 1368) have measured the momentum space dependence of UPt3s superconducting gap as a function of temperature. A real gap with line nodes was observed to develop first; then, at a lower critical temperature, a complex component appeared, with the total gap expected to become fully uniform at absolute zero. The temperature evolution of two superconducting phases has been observed in a heavy fermion superconductor. Order parameter symmetry is one of the basic characteristics of a superconductor. The heavy fermion compound UPt3 provides a rich system for studying the competition between superconductivity and other forms of electronic order and exhibits two distinct superconducting phases that are characterized by different symmetries. We fabricated a series of Josephson tunnel junctions on the as-grown surfaces of UPt3 single crystals spanning the a-b plane. By measuring their critical current, we mapped out the magnitude of the superconducting order parameter as a function of the momentum-space direction and temperature. In the high-temperature phase, we observed a sharp node in the superconducting gap at 45° with respect to the a axis; an out-of-phase component appeared in the low-temperature phase, creating a complex order parameter.

New chiral phases of superfluid 3He stabilized by anisotropic silica aerogel

A rich variety of Fermi systems condense by forming bound pairs, including high-temperature and heavy-fermion superconductors, Sr_2RuO_4 (ref. 3), cold atomic gases and superfluid ^3He (ref. 5). Some of these form exotic quantum states with non-zero orbital angular momentum. We have discovered, in the case of ^3He, that anisotropic disorder, engineered from highly porous silica aerogel, stabilizes a chiral superfluid state that otherwise would not exist. Furthermore, we find that the chiral axis of this state can be uniquely oriented with the application of a magnetic field perpendicular to the aerogel anisotropy axis. At sufficiently low temperature we observe a sharp transition from a uniformly oriented chiral state to a disordered structure consistent with locally ordered domains, contrary to expectations for a superfluid glass phase.

Identification of superfluid phases of He3 in uniformly isotropic 98.2% aerogel

Superfluid ^{3}He confined to high porosity silica aerogel is the paradigm system for understanding impurity effects in unconventional superconductors. However, a crucial first step has been elusive: exact identification of the microscopic states of the superfluid in the presence of quenched disorder. Using a new class of highly uniform aerogel materials, we report pulsed nuclear magnetic resonance experiments that demonstrate definitively that the two observed superfluid states in aerogel are impure versions of the isotropic and axial p-wave states. The theoretically predicted destruction of long-range orbital order (Larkin-Imry-Ma effect) in the impure axial state is not observed.

Anisotropy of the superconducting state in Sr2RuO4.

Despite intense studies the exact nature of the order parameter in superconducting Sr2RuO4 remains unresolved. We have used small-angle neutron scattering to study the vortex lattice in Sr2RuO4 with the field applied close to the basal plane, taking advantage of the transverse magnetization. We measured the intrinsic superconducting anisotropy between the c axis and the Ru-O basal plane (~60), which greatly exceeds the upper critical field anisotropy (~20). Our result imposes significant constraints on possible models of triplet pairing in Sr2RuO4 and raises questions concerning the direction of the zero spin projection axis.

Anisotropy of the Superconducting State inSr2RuO4

Despite intense studies the exact nature of the order parameter in superconducting Sr2RuO4 remains unresolved. We have used small-angle neutron scattering to study the vortex lattice in Sr2RuO4 with the field applied close to the basal plane, taking advantage of the transverse magnetization. We measured the intrinsic superconducting anisotropy between the c axis and the Ru-O basal plane (~60), which greatly exceeds the upper critical field anisotropy (~20). Our result imposes significant constraints on possible models of triplet pairing in Sr2RuO4 and raises questions concerning the direction of the zero spin projection axis.

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

Spanning a broad range of physical systems, complex symmetry breaking is widely recognized as a hallmark of competing interactions. This is exemplified in superfluid 3He 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 UPt3 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.

The superfluid glass phase of 3He-A

Confined within a porous aerogel, superfluid 3He loses its long-range order owing to random microscopic disorder, and becomes a glassy superfluid. Intriguingly, this effect can be switched off and the superfluidity restored.

Zeeman Splitting and Nonlinear Field-Dependence in Superfluid 3He

We have studied the acoustic Faraday effect in superfluid 3He up to significantly larger magnetic fields than in previous experiments achieving rotations of the polarization of transverse sound as large as 1710°. We report nonlinear field effects, and use the linear results to determine the Zeeman splitting of the imaginary squashing mode (ISQ) frequency in 3He-B.

Magnetization in the superconducting state of UPt3 from polarized neutron diffraction

The heavy fermion superconductor UPt