Pressure-dependent optical investigations of α\alpha-(BEDT-TTF)2_2I3_3: tuning charge order and narrow gap towards a Dirac semimetal

Infrared optical investigations of $\alpha$-(BEDT-TTF)$_2$I$_3$ have been performed in the spectral range from 80 to 8000~cm$^{-1}$ down to temperatures as low as 10~K by applying hydrostatic pressure. In the metallic state, $T > 135$~K, we observe a 50\% increase in the Drude contribution as well as the mid-infrared band due to the growing intermolecular orbital overlap with pressure up to 11~kbar. In the ordered state, $T<T_{\rm CO}$, we extract how the electronic charge per molecule varies with temperature and pressure: Transport and optical studies demonstrate that charge order and metal-insulator transition coincide and consistently yield a linear decrease of the transition temperature $T_{\rm CO}$ by $8-9$~K/kbar. The charge disproportionation $\Delta\rho$ diminishes by $0.017~e$/kbar and the optical gap $\Delta$ between the bands decreases with pressure by -47~cm$^{-1}$/kbar. In our high-pressure and low-temperature experiments, we do observe contributions from the massive charge carriers as well as from massless Dirac electrons to the low-frequency optical conductivity, however, without being able to disentangle them unambiguously.Comment: 13 pages, 17 figures, submitted to Phys. Rev.

RF Characterisation of Laser Treated Copper Surfaces for the Mitigation of Electron Cloud in Accelerators

In accelerator beam chambers and RF waveguides, electron cloud and multipacting can be mitigated effectively by reducing the secondary electron yield (SEY). In recent years, it has been established that laser-engineered surface structuring is a very efficient method to create a copper surface with a SEY maximum close to or even below unity. Different laser pulse durations, from nanoseconds to picoseconds, can be used to change surface morphology. Conversely, the characteristics that minimise the SEY, such as the moderately deep grooves and the redeposited nanoparticles, might have unfavourable consequences, including increased RF surface resistance. In this study, we describe the techniques used to measure the surface resistance of laser-treated copper samples using an enhanced dielectric resonator with 12 cm diameter sample sizes operating in the GHz range. The quantification basis lies in a non-contact measurement of the high-frequency losses, focusing on understanding the variation of surface resistance levels depending on the specifics of the treatment and possible post-treatment cleaning procedures.</p

Role of surface microgeometries on electron escape probability and secondary electron yield of metal surfaces

The influence of microgeometries on the Secondary Electron Yield (SEY) of surfaces is investigated. Laser written structures of different aspect ratio (height to width) on a copper surface tuned the SEY of the surface and reduced its value to less than unity. The aspect ratio of microstructures was methodically controlled by varying the laser parameters. The results obtained corroborate a recent theoretical model of SEY reduction as a function of the aspect ratio of microstructures. Nanostructures - which are formed inside the microstructures during the interaction with the laser beam - provided further reduction in SEY comparable to that obtained in the simulation of structures which were coated with an absorptive layer suppressing secondary electron emission

RF Characterisation of Laser Treated Copper Surfaces for the Mitigation of Electron Cloud in Accelerators

In accelerator beam chambers and RF waveguides, electron cloud and multipacting can be mitigated effectively by reducing the secondary electron yield (SEY). In recent years, it has been established that laser-engineered surface structuring is a very efficient method to create a copper surface with a SEY maximum close to or even below unity. Different laser pulse durations, from nanoseconds to picoseconds, can be used to change surface morphology. Conversely, the characteristics that minimise the SEY, such as the moderately deep grooves and the redeposited nanoparticles, might have unfavourable consequences, including increased RF surface resistance. In this study, we describe the techniques used to measure the surface resistance of laser-treated copper samples using an enhanced dielectric resonator with 12 cm diameter sample sizes operating in the GHz range. The quantification basis lies in a non-contact measurement of the high-frequency losses, focusing on understanding the variation of surface resistance levels depending on the specifics of the treatment and possible post-treatment cleaning procedures.</p

Out of focus ultrafast processing of metals for reduced secondary electron yield

We have demonstrated out-of-focus ultrafast pulsed laser processing of copper with a variable working distance, without the need for mechanical movement. This was achieved by employing a diffractive optical element. The method has been demonstrated in a practical application to reduce the secondary electron yield (SEY) of copper to below 1.3. We show that using an extended focus element not only increases the consistency of processing across a range of working distances, but also changes the topography of the produced structures, reducing the SEY. This presented approach shows promise in facilitating the Large Hadron Collider’s (LHC’s) upcoming high luminosity upgrade by preventing electron clouds

First beam test of Laser Engineered Surface Structures (LESS) at cryogenic temperature in CERN SPS accelerator

Electron cloud mitigation is an essential requirement for accelerators of positive particles with high intensity beams to guarantee beam stability and limited heat load in cryogenic systems. Laser Engineered Surface Structures (LESS) are being considered, within the High Luminosity upgrade of the LHC collider at CERN (HL-LHC), as an option to reduce the Secondary Electron Yield (SEY) of the surfaces facing the beam, thus suppressing the elec-tron cloud phenomenon. As part of this study, a 2.2 m long Beam Screen (BS) with LESS has been tested at cryogenic temperature in the COLD bore EXperiment (COLDEX) facility in the SPS accelerator at CERN. In this paper, we describe the manufacturing procedure of the beam screen, the employed laser treatment technique and discuss our first observations in COLDEX confirming electron cloud suppression.Electron cloud mitigation is an essential requirement for accelerators of positive particles with high intensity beams to guarantee beam stability and limited heat load in cryogenic systems. Laser Engineered Surface Structures (LESS) are being considered, within the High Luminosity upgrade of the LHC collider at CERN (HL-LHC), as an option to reduce the Secondary Electron Yield (SEY) of the surfaces facing the beam, thus suppressing the electron cloud phenomenon. As part of this study, a 2.2 m long Beam Screen (BS) with LESS has been tested at cryogenic temperature in the COLD bore EXperiment (COLDEX) facility in the SPS accelerator at CERN. In this paper, we describe the manufacturing procedure of the beam screen, the employed laser treatment technique and discuss our first observations in COLDEX confirming electron cloud suppression