Sunseng Pyon

Light-Induced Superconductivity in a Stripe-Ordered Cuprate

Laser pulses are used to enable coherent transport between the copper oxide planes of a cuprate superconductor. 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 La1.675Eu0.2Sr0.125CuO4, 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.

Fermi-surface reconstruction by stripe order in cuprate superconductors

The origin of pairing in a superconductor resides in the underlying normal state. In the cuprate high-temperature superconductor YBa2Cu3Oy (YBCO), application of a magnetic field to suppress superconductivity reveals a ground state that appears to break the translational symmetry of the lattice, pointing to some density-wave order. Here we use a comparative study of thermoelectric transport in the cuprates YBCO and La1.8−xEu0.2SrxCuO4 (Eu-LSCO) to show that the two materials exhibit the same process of Fermi-surface reconstruction as a function of temperature and doping. The fact that in Eu-LSCO this reconstruction coexists with spin and charge modulations that break translational symmetry shows that stripe order is the generic non-superconducting ground state of hole-doped cuprates.

Superconductivity Induced by Bond Breaking in the Triangular Lattice of IrTe2

IrTe 2 , a layered compound with a triangular iridium lattice, exhibits a structural phase transition at approximately 250 K. This transition is characterized by the formation of Ir–Ir bonds along the b -axis. We found that the breaking of Ir–Ir bonds that occurs in Ir 1- x Pt x Te 2 results in the appearance of a structural critical point in the T = 0 limit at x c ≃0.035. Although both IrTe 2 and PtTe 2 are paramagnetic metals, superconductivity at T c = 3.1 K is induced by the bond breaking in a narrow range of x ≥ x c in Ir 1- x Pt x Te 2 . This result indicates that structural fluctuations can be involved in the emergence of superconductivity.

Bi-directional ultrafast electric-field gating of interlayer charge transport in a cuprate superconductor

Scientists demonstrate that strong single-cycle terahertz pulses can switch off interlayer superconductivity in a cuprate superconductor while leaving in-plane superconductivity unaltered. The effect may prove useful for studying and controlling the behaviour of future ultrafast nanoelectronics.

Enhancement of the Nernst effect by stripe order in a high-Tc superconductor

The Nernst effect in metals is highly sensitive to two kinds of phase transition: superconductivity and density-wave order. The large, positive Nernst signal observed in hole-doped high-Tc superconductors above their transition temperature (Tc) has so far been attributed to fluctuating superconductivity. Here we report that in some of these materials the large Nernst signal is in fact the result of stripe order, a form of spin/charge modulation that causes a reconstruction of the Fermi surface. In La2-xSrxCuO4 (LSCO) doped with Nd or Eu, the onset of stripe order causes the Nernst signal to change from being small and negative to being large and positive, as revealed either by lowering the hole concentration across the quantum critical point in Nd-doped LSCO (refs 6–8) or by lowering the temperature across the ordering temperature in Eu-doped LSCO (refs 9, 10). In the second case, two separate peaks are resolved, respectively associated with the onset of stripe order at high temperature and superconductivity near Tc.

Decrease of upper critical field with underdoping in cuprate superconductors

Decreasing the doping of a cuprate superconductor below a certain critical value causes its critical temperature to fall, however the reason for this has been unclear. Sensitive measurements of the Nernst effect in yttrium barium copper oxide suggest it is the result of competition with an emerging stripe phase.

Optical excitation of Josephson plasma solitons in a cuprate superconductor

Josephson plasma waves are linear electromagnetic modes that propagate along the planes of cuprate superconductors, sustained by interlayer tunnelling supercurrents. For strong electromagnetic fields, as the supercurrents approach the critical value, the electrodynamics become highly nonlinear. Josephson plasma solitons (JPSs) are breather excitations predicted in this regime, bound vortex-antivortex pairs that propagate coherently without dispersion. We experimentally demonstrate the excitation of a JPS in La1.84Sr0.16CuO4, using intense narrowband radiation from an infrared free-electron laser tuned to the 2-THz Josephson plasma resonance. The JPS becomes observable as it causes a transparency window in the opaque spectral region immediately below the plasma resonance. Optical control of magnetic-flux-carrying solitons may lead to new applications in terahertz-frequency plasmonics, in information storage and transport and in the manipulation of high-Tc superconductivity.

Dynamics and mechanism of oxygen annealing in Fe1+yTe0.6Se0.4 single crystal

Iron chalcogenide Fe(Te,Se) attracted much attention due to its simple structure, which is favorable for probing the superconducting mechanism. Its less toxic nature compared with iron arsenides is also advantageous for applications of iron-based superconductors. By intercalating spacer layers, superconducting transition temperature has been raised over 40 K. On the other hand, the presence of excess Fe is almost unavoidable in Fe(Te,Se) single crystals, which hinders the appearance of bulk superconductivity and causes strong controversies over its fundamental properties. Here we report a Systematical study of O2-annealing dynamics in Fe1+yTe1−xSex by controlling the amount of O2, annealing temperature, and time. Bulk superconductivity can be gradually induced by increasing the amount of O2 and annealing time at suitable temperatures. The optimally annealed crystals can be easily obtained by annealing with ~1.5% molar ratio of oxygen at 400°C for more than 1 hour. Superconductivity was witnessed to evolve mainly from the edge of the crystal to the central part. After the optimal annealing, the complete removal of excess Fe was demonstrated via STM measurements. Some fundamental properties were recharacterized and compared with those of as-grown crystals to discuss the influence of excess Fe.

Orbital degeneracy and Peierls instability in the triangular-lattice superconductor Ir 1-xPt xTe 2

We have studied the electronic structure of the triangular lattice Ir

Electronic Structure Reconstruction by Orbital Symmetry Breaking in IrTe2

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