One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional "striped" spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La(1.675)Eu(0.2)Sr(0.125)CuO(4), into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.

https://science.sciencemag.org/content/sci/331/6014/189.full.pdf

Light-Induced Superconductivity in a Stripe-Ordered Cuprate

Quantum many-body states of up to four excitons are studied in small parallelepipedal quantum dots with only two nondegenerate electronic levels in the conduction and in the valence bands. Only one type of hole is considered. We find the set of good quantum numbers (including spin variables for the electrons and holes) which block-diagonalizes the Coulomb and the optical dipole interactions. This provides a convenient labeling scheme for the excited states which allows us to diagonalize nearly entirely analytically the 256 x 256 Hamiltonian. The magnitude of the Coulomb level shifts and of the oscillator strength of the optical transitions are shown for a typical case.

Quantum many-body states of excitons in a small quantum dot.

Nanoscale superconductors connected to normal metallic electrodes provide a potential source of entangled electron pairs1,2,3,4,5. Such states would arise from the splitting of Cooper pairs in the superconductor into two electrons with opposite spins, which then tunnel into different leads by means of a process known as crossed Andreev reflection (refs 6, 7, 8). In an actual system, the detection of these processes is hindered by the elastic transmission of individual electrons between the leads, which yields an opposite contribution to the non-local conductance. Here we demonstrate that low-energy collective excitations, which appear in superconducting structures of reduced dimensionality9, can have a significant influence on the transport properties of this type of hybrid nanostructure. When an electron tunnels into the superconductor it can excite such low-energy excitations that alter the balance between the different electronic processes, leading to a dominance of one over the other depending on the spatial symmetry of these excitations. These findings help to clarify some intriguing experimental results and provide future strategies for the detection of entangled electron pairs in solid-state devices for quantum computation.

/pdf/entangled-andreev-pairs-and-collective-excitations-in-1oi9puxoz2.pdf

Entangled Andreev pairs and collective excitations in nanoscale superconductors

We demonstrate a superconductor array of subwavelength holes with active thermal control over the resonant transmission induced by surface plasmon polaritons. The array was lithographically fabricated on a high-temperature yttrium barium copper oxide superconductor and characterized by terahertz time-domain spectroscopy. We observe a clear transition from a virtual excitation of the surface plasmon mode to a real surface plasmon mode. The highly controllable superconducting plasmonic crystals may find promising applications in the design of low-loss, large- dynamic-range amplitude modulation and surface-plasmon-based terahertz devices.

http://utol.okstate.edu/sites/default/files/publications/pubs/175_Paper161.pdf

Terahertz superconducting plasmonic hole array

In the plasmonic regime, an electromagnetic wave bounded to the surface of a conductor can be confined to a region much smaller than its wavelength in free space. A major problem of plasmonic technology, however, is associated with large losses that these surface modes exhibit, intimately linked to Ohmic resistance of metals. In this work, we show that due to their dominant kinetic inductance, superconductors are intriguing yet natural plasmonic media capable of supporting low-loss plasmon waves with extreme confinement and the potential to serve as information carriers in compact terahertz data processing circuits.

/pdf/low-loss-terahertz-superconducting-plasmonics-1qxqn6p41k.pdf

Low-loss terahertz superconducting plasmonics

We calculate the collective-mode spectrum for a layered superconductor structure. We find that for wave vectors directed close to the direction of the superlattice axis, the plasmon mode remains {ital below} the superconducting gap edge (i.e., the Anderson-Higgs mechanism does not give rise to a massive collective mode as it does in the bulk three-dimensional system). We also find a clear signature of superconductivity in the form of a unique line splitting in the collective-mode spectrum as the plasmon crosses the superconducting gap. Experimental implications are discussed.

H.A. Fertig

Papers

Collective modes in layered superconductors

Behavior of electrons at soft edges in the fractional quantum Hall and Wigner crystal states