A time-resolution study with a plastic scintillator read out by a Geiger-mode Avalanche Photodiode

In this work we attempt to establish the best time resolution attainable with a scintillation counter consisting of a plastic scintillator read out by a Geiger-mode Avalanche Photodiode. The measured time resolution is inversely proportional to the square root of the energy deposited in the scintillator, and scales to 18ps (sigma) at 1MeV. This result competes with the best ones reported for photomultiplier tubes.Comment: 8 pages, 8 figure

Direct evidence of superconductivity and determination of the superfluid density in buried ultrathin FeSe grown on SrTiO3_3

Bulk FeSe is superconducting with a critical temperature $T_c$ $\cong$ 8 K and SrTiO$_3$ is insulating in nature, yet high-temperature superconductivity has been reported at the interface between a single-layer FeSe and SrTiO$_3$. Angle resolved photoemission spectroscopy and scanning tunneling microscopy measurements observe a gap opening at the Fermi surface below $\approx$ 60 K. Elucidating the microscopic properties and understanding the pairing mechanism of single-layer FeSe is of utmost importance as it is a basic building block of iron-based superconductors. Here, we use the low-energy muon spin rotation/relaxation technique (LE-$\mu$SR) to detect and quantify the supercarrier density and determine the gap symmetry in FeSe grown on SrTiO$_3$ (100). Measurements in applied field show a temperature dependent broadening of the field distribution below $\sim$ 60 K, reflecting the superconducting transition and formation of a vortex state. Zero field measurements rule out the presence of magnetism of static or fluctuating origin. From the inhomogeneous field distribution, we determine an effective sheet supercarrier density $n_s^{2D} \simeq 6 \times 10^{14}$ cm$^{-2}$ at $T \rightarrow 0$ K, which is a factor of 4 larger than expected from ARPES measurements of the excess electron count per Fe of 1 monolayer (ML) FeSe. The temperature dependence of the superfluid density $n_s(T)$ can be well described down to $\sim$ 10 K by simple s-wave BCS, indicating a rather clean superconducting phase with a gap of 10.2(1.1) meV. The result is a clear indication of the gradual formation of a two dimensional vortex lattice existing over the entire large FeSe/STO interface and provides unambiguous evidence for robust superconductivity below 60 K in ultrathin FeSe.Comment: 9 pages, 8 figure

Spatial confinement of muonium atoms

We report the achievement of spatial confinement of muonium atoms (the bound state of a positive muon and an electron). Muonium emitted into vacuum from mesoporous silica reflects between two SiO$_2$ confining surfaces separated by 1 mm. From the data, one can extract that the reflection probability on the confining surfaces kept at 100 K is about 90% and the reflection process is well described by a cosine law. This technique enables new experiments with this exotic atomic system and is a very important step towards a measurement of the 1S-2S transition frequency using continuous wave laser spectroscopy.Comment: 5 pages, 6 figure