Session B3: Invited Session: Superconducting Coherence, Fluctuations & Inhomogeneity in Mesoscopic & Low-Dimensions

Emergence of h/e-period oscillations in the critical temperature of small superconducting rings and critical velocity in one-dimensional superconductors

When a large ring of superconductor is thread by a magnetic flux, the resistance and the critical temperature exhibit oscillations in a flux quantum of h/2e. The flux quantum of an electron circling a thread flux on a clean metallic ring is on the contrary h/e. When the radius starts to shrink, electrons that compose of Cooper pairs may be able to roam around the ring individually without costing too much energy. An h/e period should thus arise. We discuss the emergence of h/e-period oscillations in the critical temperature of small superconducting rings and a few scenarios of superconducting-normal metal transitions. Interestingly, a threading flux is equivalent to a momentum boost in the circumferential direction of the ring. We also discuss a related issue as to how high a flow velocity one-dimensional superconductors can sustain before superconductivity gives way to this instability and the system becomes normal.   

Manipulating superconducting fluctuations in ultrasmall loops and quasi one-dimensional wires of Al

Superconducting fluctuations and the control of these fluctuations have been a problem of long-standing interest, with recent impetus provided by its relevance to the pursuit of very high temperature superconductivity through the engineering of global phase coherence. In quasi one-dimensional superconductors, fluctuations due to thermal or quantum processes lead to phase slips, and the appearance of a finite electrical resistance. We found that the critical current in mesoscopic quasi one-dimensional wires of Al is influenced by the bulk measurement electrodes, and in fact increases with magnetic field at low fields, suggesting that the phase slips are suppressed by the loss of superconductivity in the bulk electrodes. Manipulation of superconducting fluctuations is also possible in ultrasmall loops, where the strength of the fluctuations is controlled by the loop's size in comparison with $\xi$ and the enclosed flux. For ultrasmall loops with a circumference $\sim$ $\pi$$\xi$(0), de Gennes predicted more than three decades ago that superconductivity could be completely destroyed near half-integer-flux quanta even to zero temperature. Furthermore, the resulting destructive regime, likely dominated by quantum fluctuations at low temperatures, was predicted to be suppressed with the addition of a superconducting side branch. We observed this Little-Parks-de Gennes effect in ultrasmall Al loops prepared by e-beam lithography and we found that the addition of a superconducting side branch restores the lost phase coherence. We will present our most recent data and discuss the implications of our experimental observations.   

Magnetic-field-induced superconductivity and damping of phase slips in Zn nanowires

We report an observation that Zn nanowires connected with Zn electrodes, after being driven resistive by the current, re-entered their superconducting state upon the application of a small magnetic field [1, 2]. A detailed experimental investigation was carried out, with variation of parameters such as magnetic field orientation, wire length, etc.. The results provide solid evidence that this is a nonequilibrium effect associated the coupling with the boundary electrodes. There are two characteristic length scales involved, approaching either of which weakens the effect. Most importantly, we demonstrated that it is more appropriate to consider the effect to be a stabilization of superconductivity that has been suppressed by an applied current. Although we do not present a formal theory to explain all of our results here, the effect is most likely a consequence of the dampening of phase fluctuations by quasiparticles which are created in the electrodes by small magnetic fields.\\[4pt] [1] Yu Chen, S. D. Snyder, and A. M. Goldman, Phys. Rev. Lett. 103, 127002(2009).\\[0pt] [2] Yu Chen, Yen-Hsiang Lin, S. D. Snyder, and A. M. Goldman, Phys. Rev. B 83, 054505(2011)   

Quantum liquid crystals: spontaneously modulated Fermi superfluids

What happens to a BCS superconductor when the numbers of up and down fermions are unequal? This is a fascinating and difficult problem in many-body physics. The most exciting possibility is that pairing and polarization compete to produce a modulated superfluid state, known as an Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. This is a quantum liquid crystal exhibiting microscale phase separation, in which the excess fermions self-organize into domain walls where the pairing amplitude changes sign. There is a growing body of indirect evidence for the existence of FFLO states. As an example, I present our recent work on Al thin film superconductors in parallel magnetic fields [1]. Just below the Chandrasekhar-Clogston spin paramagnetic transition, our tunneling density of states measurements reveal a significant population of subgap states. These excess states cannot be explained in terms of conventional superconductivity, but they are a natural consequence of disordered FFLO physics. There is also hope for realizing FFLO in ultracold gases of neutral fermionic atoms. Continuum FFLO states are very fragile, but our calculations suggest that FFLO states are greatly stabilized by an optical lattice. For a cubic lattice with suitable parameters, up to 80\% of the fermions participate in the FFLO phase [2]. Furthermore, we propose an interferometric technique to detect pairing amplitude modulations, which may provide the first \emph{direct} evidence of FFLO [3]. \\[4pt] [1] Y. L. Loh, N. Trivedi, Y. M. Xiong, P. W. Adams, and G. Catelani, ``Origin of Excess Low-Energy States in a Disordered Superconductor in a Zeeman Field,'' PRL 107, 067003 (2011) \\[0pt] [2] Y. L. Loh and N. Trivedi, ``Detecting the Elusive Larkin-Ovchinnikov Modulated Superfluid Phases in Imbalanced Fermi Gases in Optical Lattices,'' PRL 165302 (2010) \\[0pt] [3] M. Swanson, Y. L. Loh, and N. Trivedi, ``Proposal for interferometric detection of topological defects in modulated superfluids,'' arXiv:1106.3908