Single unit cell films of iron selenide (1UC FeSe) grown on SrTiO3 (STO) substrates have recently shown superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 K), a record for iron-based superconductors.
Towards understanding why Cooper pairs form at such high temperatures, a primary question to address is the role, if any, of the STO substrate. Here, we report high resolution angle resolved photoemission spectroscopy (ARPES) results which reveal an unexpected and unique characteristic of the 1UC FeSe/STO system: shake-off bands suggesting the presence of bosonic modes, most likely oxygen optical phonons in STO, which couple to the FeSe electrons with only small momentum transfer. Such coupling has the unusual benefit of helping superconductivity in most channels, including those mediated by spin fluctuations. Our calculations suggest such coupling is responsible for raising the superconducting gap opening temperature in 1UC FeSe/STO. This discovery suggests a pathway to engineer high temperature superconductors.
High-temperature superconductivity (HTSC) mysteriously emerges upon doping holes or electrons into insulating copper oxides with antiferromagnetic (AFM) order. It has been thought that the large energy scale of magnetic excitations, compared to phono
n energies for example, lies at the heart of an electronically-driven superconducting phase at high temperatures. However, despite extensive studies, little information is available for comparison of high-energy magnetic excitations of hole- and electron-doped superconductors to assess a possible correlation with the respective superconducting transition temperatures. Here, we use resonant inelastic x-ray scattering (RIXS) at the Cu L3-edge to reveal high-energy collective excitations in the archetype electron-doped cuprate Nd2-xCexCuO4 (NCCO). Surprisingly, despite the fact that the spin stiffness is zero and the AFM correlations are short-ranged, magnetic excitations harden significantly across the AFM-HTSC phase boundary, in stark contrast with the hole-doped cuprates. Furthermore, we find an unexpected and highly dispersive mode in superconducting NCCO that is undetected in the hole-doped compounds, which emanates from the zone center with a characteristic energy comparable to the pseudogap, and may signal a quantum phase distinct from superconductivity. The uncovered asymmetry in the high-energy collective excitations with respect to hole and electron doping provides additional constraints for modeling the HTSC cuprates.
We report on transport measurement performed on a room-temperature-operating ultra-small Coulomb blockade devices with a silicon island of sub-5nm. The charge stability at 300K exhibits a substantial change in slopes and diagonal size of each success
ive Coulomb diamond, but remarkably its main feature persists even at low temperature down to 5.3K except for additional Coulomb peak splitting. This key feature of charge stability with additional fine structures of Coulomb peaks are successfully modeled by including the interplay between Coulomb interaction, valley splitting, and strong quantum confinement, which leads to several low-energy many-body excited states for each dot occupancy. These excited states become enhanced in the sub-5nm ultra-small scale and persist even at 300K in the form of cluster, leading to the substantial modulation of charge stability.
We present a time-dependent cosmic-ray modified shock model for which the calculated H-alpha emissivity profile agrees well with the H-alpha flux increase ahead of the Balmer-dominated shock at knot g in Tychos supernova remnant, observed by Lee et a
l (2007). The backreaction of the cosmic ray component on the thermal component is treated in the two-fluid approximation, and we include thermal particle injection and energy transfer due to the acoustic instability in the precursor. The transient state of our model that describes the current state of the shock at knot g, occurs during the evolution from a thermal gas dominated shock to a smooth cosmic-ray dominated shock. Assuming a distance of 2.3 kpc to Tychos remnant we obtain values for the cosmic ray diffusion coefficient, the injection parameter, and the time scale for the energy transfer of 10^{24} cm^{2} s^{-1}, 4.2x10^{-3}, and 426 y, respectively. We have also studied the parameter space for fast (300 km s^{-1} - 3000 km s^{-1}), time-asymptotically steady shocks and have identified a branch of solutions, for which the temperature in the cosmic ray precursor typically reaches 2-6x10^{4} K and the bulk acceleration of the flow through the precursor is less than 10 km s^{-1}. These solutions fall into the low cosmic ray acceleration efficiency regime and are relatively insensitive to shock parameters. This low cosmic ray acceleration efficiency branch of solutions may provide a natural explanation for the line broadening of the H-alpha narrow component observed in non-radiative shocks in many supernova remnants.
