If the superconducting order parameter changes its sign under the inversion operation, the superconductor has an odd-parity pairing symmetry, which can be represented by the p- or f-wave in the presence of a rotationally invariant system. An odd-parity superconductor is in general also a spin-triplet superconductor because the two electrons forming a Cooper pair are fermions. Odd-parity, spin-triplet superconductors are rare. An overwhelming majority of known superconducting materials, including high-Tc cuprates, are even-parity, spin-singlet (s- or d-wave) superconductors. Sr2RuO4, the n=1 member of the Ruddelsden-Popper series of Srn+1RunO3n+1, is the only layered perovskite that becomes superconducting without the presence of Cu. It was predicted shortly after superconductivity was discovered in Sr2RuO4 that the pairing symmetry in this material is p-wave as opposed to s- or d-wave, which was supported by a large array of subsequent experiments. Our work on Sr2RuO4 focuses on electronic tunneling and phase-sensitive measurements at very low temperatures. The high-Tc research has shown that phase-sensitive measurements constitute the most stringent test on the pairing symmetry of a new superconductor. We carried out the first SQUID-based phase sensitive measurements on Sr2RuO4 and demonstrated that the phase of the superconducting order parameter in Sr2RuO4 changes by π under inversion and π/2 for a 90-degree rotation. Therefore our work demonstrated unambiguously that Sr2RuO4 is an odd-parity, spin-triplet superconductor. Our current work on Sr2RuO4 focuses on issues related to the exact form of the order parameter for Sr2RuO4 including the kz-dependence of the gap function, existence of chiral pairing state featuring domains and domain walls, and the consequences of odd-parity superconductivity including half-flux quantum vortices. We are also exploring the possibility of using Sr2RuO4 for novel applications, such as topological quantum computing based on the possible existence of a Majorana mode bound to a half-flux-quantum vortex.
Advances in e-beam lithography and other nanofabrication techniques in recent years have made it possible to prepare nanostructures with desired characteristics. In addition to the effective dimension, size, and shape that are traditionally considered important for determining the properties of a nanostructure, we found that the sample topology, namely the connectivity of the nanostructure, is also important. For example, doubly-connected nanoscopic superconductors such as a loop or a hollow cylinder possess properties not present in singly connected samples. We have studied the effects of sample topology in nanoscopic superconductors including rings and cylinders. Our earlier efforts led to the experimental discovery of the destructive regime - superconductivity in a ring with its diameter smaller than the zero-temperature superconducting coherence length is destroyed around half-flux quanta - predicted originally by de Gennes. More recently, we found that this destructive regime can be suppressed by the addition of a side branch. This result was again predicted by de Gennes, and is referred to as Little-Parks-de Gennes effect. We also studied singly connected superconducting nanowires, some of which had a diameter much thinner than the superconducting coherence length but did not show any sign of the destructive regime, underscoring the importance of the sample topology. We are now investigating superconducting fluctuations near the quantum phase transition found at the onset of the destructive regime in doubly connected superconducting nanostructures as well as the general effects of the sample topology.
Atomically sharp interfaces
Unexpected physical phenomena were often found in heterostructures featuring an atomically sharp interface. Superconductivity was observed at such interface systems of Ag/Ge, and more recently, in SrTiO3/LaAlO3. In both cases, the material at the interface is superconducting. In the eutectic phase of Ru-Sr2RuO4 featuring mesoscopic islands of crystalline Ru embedded in bulk Sr2RuO4, the interface between s-wave superconductor Ru and p-wave Sr2RuO4 is also atomically sharp. Here Ru is an s-wave superconductor with a transition temperature (Tc), 0.5 K, and the bulk p-wave superconductor Sr2RuO4 has a Tc of 1.5 K. Therefore this system provides an opportunity to study the proximity effect between a non-s-wave and normal metal above 0.5 K and the Josephson coupling between an s-wave and a p-wave superconductor below 0.5 K. More interestingly, however, the onset superconducting transition temperature of this eutectic phase is nearly 3 K (therefore referred to as the 3-K phase of Sr2RuO4), a fact that may have implications on the mechanism of superconductivity. Most recently, monolayer single-crystal films of FeSe grown on SrTiO3 were found to exhibit very high Tc, possibly above 100 K, suggesting new physics in this interface system. We are also interested in atomically sharp interfaces between two very different materials where electronic reconstruction, correlation, and orbital ordering can be expected, and novel physical phenomena not found in the bulk may emerge.
Following the discovery of graphene, which features a zero effective mass and surprisingly high mobility (>105 cm2/Vs at room temperature, which is more than 10 times better than Si-based heterostructures), two-dimensional (2D) crystals beyond graphene have attracted an increasing amount of attention. These crystals feature a band structure that is typically very different from the bulk and other novel properties. We developed an all-dry, lithography-free process to prepare electric-field-effect devices of graphene and other 2D crystals, measured conductance fluctuations and weak localization effects in few-layer graphene, and observed quantum oscillations in graphene hybrids consisting of a single sheet of both mono- and bilayer graphene. We extended our work to few-layer crystals, such as atomically thin flakes of superconducting NbSe2, and demonstrated superconductivity in bilayer NbSe2 along with its significantly large response to gating. Our current work focuses on the effect of band structure of 2D crystals, including the interplay between superconductivity and charge density waves, electric field tuned superconductor-insulator quantum phase transitions, and the search for ultra high temperature superconductivity. In addition, we study nanostructures made of 2D crystals. We're currently exploring the behavior of Abrikosov vortices confined in superconducting nanostructures prepared on ultrathin flakes of NbSe2, taking advantage of the small pinning force and small vortex normal core that are favorable for achieving coherent motion of vortices.
Discovery of new single-crystal materials exhibiting new physical phenomena has served as a powerful engine driving condensed matter physics. Working with our collaborators, we have been exploring the growth and characterization of novel electronic materials ranging from transition metal oxides to quantum nanocrystals, often by pursuing low-temperature electrical transport and magneto thermoelectrical measurements. In the past few years we have studied ruthenates belonging to the Ruddelsden-Popper series of Srn+1RunO3n+1 (n = 1, 2, 3...) that show remarkably rich electronic and magnetic properties including p-wave superconductivity and ferromagnetism. We have also studied other non-perovskite ruthenates including La4Ru6O19 with metal-metal bonding, La4Ru2O10 exhibiting an orbital ordering transition, and quasi one-dimensional (1D) hollandite BaRu6O12 exhibiting a quantum phase transition. In collaboration with our colleagues, we're also exploring metamaterials formed by inverse opals, aiming at novel effect of strong correlation that might be tuned by designing appropriate "meta-atoms" - the building block of the metamaterial.