Damage in graphene due to electronic excitation induced by highly charged ions

Graphene is expected to be rather insensitive to ionizing particle radiation. We demonstrate that single layers of exfoliated graphene sustain significant damage from irradiation with slow highly charged ions. We have investigated the ion induced changes of graphene after irradiation with highly charged ions of different charge states (q = 28-42) and kinetic energies E_kin = 150-450 keV. Atomic force microscopy images reveal that the ion induced defects are not topographic in nature but are related to a significant change in friction. To create these defects, a minimum charge state is needed. In addition to this threshold behaviour, the required minimum charge state as well as the defect diameter show a strong dependency on the kinetic energy of the projectiles. From the linear dependency of the defect diameter on the projectile velocity we infer that electronic excitations triggered by the incoming ion in the above-surface phase play a dominant role for this unexpected defect creation in graphene

Temperature dependence of the energy dissipation in dynamic force microscopy

The dissipation of energy in dynamic force microscopy is usually described in terms of an adhesion hysteresis mechanism. This mechanism should become less efficient with increasing temperature. To verify this prediction we have measured topography and dissipation data with dynamic force microscopy in the temperature range from 100 K up to 300 K. We used 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) grown on KBr(001), both materials exhibiting a strong dissipation signal at large frequency shifts. At room temperature, the energy dissipated into the sample (or tip) is 1.9 eV/cycle for PTCDA and 2.7 eV/cycle for KBr, respectively, and is in good agreement with an adhesion hysteresis mechanism. The energy dissipation over the PTCDA surface decreases with increasing temperature yielding a negative temperature coefficient. For the KBr substrate, we find the opposite behaviour: an increase of dissipated energy with increasing temperature. While the negative temperature coefficient in case of PTCDA agrees rather well with the adhesion hysteresis model, the positive slope found for KBr points to a hitherto unknown dissipation mechanism

Isolating the Nonlinear Optical Response of a MoS2_2 Monolayer under Extreme Screening of a Metal Substrate

Transition metal dichalcogenides (TMDCs) monolayers, as two-dimensional (2D) direct bandgap semiconductors, hold promise for advanced optoelectronic and photocatalytic devices. Interaction with three-dimensional (3D) metals, like Au, profoundly affects their optical properties, posing challenges in characterizing the monolayer's optical responses within the semiconductor-metal junction. In this study, using precise polarization-controlled final-state sum frequency generation (FS-SFG), we successfully isolated the optical responses of a MoS$_2$ monolayer from a MoS$_2$/Au junction. The resulting SFG spectra exhibit a linear lineshape, devoid of A or B exciton features, attributed to the strong dielectric screening and substrate induced doping. The linear lineshape illustrates the expected constant density of states (DOS) at the band edge of the 2D semiconductor, a feature often obscured by excitonic interactions in week-screening conditions such as in a free-standing monolayer. Extrapolation yields the onset of a direct quasiparticle bandgap of about $1.65\pm0.20$ eV, indicating a strong bandgap renormalization. This study not only enriches our understanding of the optical responses of a 2D semiconductor in extreme screening conditions but also provides a critical reference for advancing 2D semiconductor-based photocatalytic applications.Comment: 14 pages, 4 figures + supplemental materia

TThermodynamics of the thermoelectric working fluid close to the superconducting phase transition

The bottleneck in state-of-the-art thermoelectric power generation and cooling is the low performance of thermoelectric materials. While the adverse effects of lattice phonons on performance can be mitigated, the main difficulty remains to obtain a large thermoelectric power factor as the Seebeck coefficient and the electrical conductivity cannot be increased independently. Here, relating the thermoelastic properties of the electron gas that performs the thermoelectric energy conversion, to its transport properties, we analyze theoretically whether an electronic phase transition can enhance thermoelectric conversion and at what cost. More precisely, we consider the metal-to-superconductor phase transition in a model two-dimensional system, and we seek to quantify the contribution of the 2D fluctuating Cooper pairs to the power factor in the close vicinity of the critical temperature $T_{\rm c}$. In addition, we provide experimental evidence of the rapid increase of the Seebeck coefficient without decreasing the electrical conductivity near $T_{\rm c}$ in a 100-nm Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ thin film with high structural quality resulting in a power factor enhancement of approximately 300. This level of performance cannot be achieved in a system with low structural quality as shown experimentally with our sample degraded by ion bombardment as defects preclude the strong enhancement of the Seebeck coefficient near the phase transition. Finally, we theoretically discuss the ideal thermoelectric conversion efficiency (i.e. disregarding adverse phonon effects) and show that driving the electronic system to the vicinity of a phase transition may be an innovative path towards a strong performance increase but at the cost of a narrow temperature range of use of such materials.Comment: Submission to SciPos

Creation of multiple nanodots by single ions

In the challenging search for tools that are able to modify surfaces on the nanometer scale, heavy ions with energies of several 10 MeV are becoming more and more attractive. In contrast to slow ions where nuclear stopping is important and the energy is dissipated into a large volume in the crystal, in the high energy regime the stopping is due to electronic excitations only. Because of the extremely local (< 1 nm) energy deposition with densities of up to 10E19 W/cm^2, nanoscaled hillocks can be created under normal incidence. Usually, each nanodot is due to the impact of a single ion and the dots are randomly distributed. We demonstrate that multiple periodically spaced dots separated by a few 10 nanometers can be created by a single ion if the sample is irradiated under grazing angles of incidence. By varying this angle the number of dots can be controlled.Comment: 12 pages, 6 figure

Pulmonary vein reconnection and repeat ablation characteristics following cryoballoon‐compared to radiofrequency‐based pulmonary vein isolation

Background: Despite advances in efficacy and safety of pulmonary vein isolation (PVI), atrial fibrillation (AF) recurrence after PVI remains common. PV‐reconnection is the main finding during repeat PVI procedures performed to treat recurrent AF. Objective: To analyze pulmonary vein (PV) reconnection patterns during repeat ablation procedures in a large cohort of consecutive patients undergoing radio frequency or cryoballoon‐based PVI. Methods: Retrospective analysis of PV‐reconnection patterns and analysis of re‐ablation strategies in consecutive index RF‐ and CB‐based PVI and their respective re‐ablation procedures during concomitant usage of both energy sources at a single high‐volume center in Germany. Results: A total of 610 first (06/2015–10/2022) and 133 s (01/2016–11/2022) repeat ablation procedures after 363 (60%) RF‐ and 247 (40%) CB‐based index PVIs between 01/2015 and 12/2021 were analyzed. PV‐reconnection was found in 509/610 (83%) patients at first and 74/133 (56%) patients at second repeat procedure. 465 of 968 (48%) initially via CB isolated PVs were reconnected at first re‐ablation but 796 of 1422 initially RF‐isolated PV (56%) were reconnected (OR: 0.73 [95% CI: 0.62–0.86]; p &lt; .001). This was driven by fewer reconnections of the left PVs (LSPV: OR: 0.60 [95% CI: 0.42–0.86]; p = .005 and LSPV: 0.67 [0.47–0.95]; p = .026). PV‐reconnection was more likely after longer, RF‐based index PVI and in older females. Repeat procedures were shorter after CB‐compared to after RF‐PVI. Conclusions: Reconnection remains the most common reason for repeat AF ablation procedures after PVI. Our data suggest to preferentially use of the cryoballoon during index PVI, especially in older women

Pulmonary vein reconnection and repeat ablation characteristics following cryoballoon‐compared to radiofrequency‐based pulmonary vein isolation

Background: Despite advances in efficacy and safety of pulmonary vein isolation (PVI), atrial fibrillation (AF) recurrence after PVI remains common. PV‐reconnection is the main finding during repeat PVI procedures performed to treat recurrent AF. Objective: To analyze pulmonary vein (PV) reconnection patterns during repeat ablation procedures in a large cohort of consecutive patients undergoing radio frequency or cryoballoon‐based PVI. Methods: Retrospective analysis of PV‐reconnection patterns and analysis of re‐ablation strategies in consecutive index RF‐ and CB‐based PVI and their respective re‐ablation procedures during concomitant usage of both energy sources at a single high‐volume center in Germany. Results: A total of 610 first (06/2015–10/2022) and 133 s (01/2016–11/2022) repeat ablation procedures after 363 (60%) RF‐ and 247 (40%) CB‐based index PVIs between 01/2015 and 12/2021 were analyzed. PV‐reconnection was found in 509/610 (83%) patients at first and 74/133 (56%) patients at second repeat procedure. 465 of 968 (48%) initially via CB isolated PVs were reconnected at first re‐ablation but 796 of 1422 initially RF‐isolated PV (56%) were reconnected (OR: 0.73 [95% CI: 0.62–0.86]; p &lt; .001). This was driven by fewer reconnections of the left PVs (LSPV: OR: 0.60 [95% CI: 0.42–0.86]; p = .005 and LSPV: 0.67 [0.47–0.95]; p = .026). PV‐reconnection was more likely after longer, RF‐based index PVI and in older females. Repeat procedures were shorter after CB‐compared to after RF‐PVI. Conclusions: Reconnection remains the most common reason for repeat AF ablation procedures after PVI. Our data suggest to preferentially use of the cryoballoon during index PVI, especially in older women

Response of GaN to energetic ion irradiation: conditions for ion track formation

We investigated the response of wurzite GaN thin films to energetic ion irradiation. Both swift heavy ions (92 MeV Xe23+, 23 MeV I6+) and highly charged ions (100 keV Xe40+) were used. After irradiation, the samples were investigated using atomic force microscopy, grazing incidence small angle X-ray scattering, Rutherford backscattering spectroscopy in channelling orientation and time of flight elastic recoil detection analysis. Only grazing incidence swift heavy ion irradiation induced changes on the surface of the GaN, when the appearance of nanoholes is accompanied by a notable loss of nitrogen. The results are discussed in the framework of the thermal spike model

Mechanism of Disruption of the Amt-GlnK Complex by PII-Mediated Sensing of 2-Oxoglutarate

GlnK proteins regulate the active uptake of ammonium by Amt transport proteins by inserting their regulatory T-loops into the transport channels of the Amt trimer and physically blocking substrate passage. They sense the cellular nitrogen status through 2-oxoglutarate, and the energy level of the cell by binding both ATP and ADP with different affinities. The hyperthermophilic euryarchaeon Archaeoglobus fulgidus possesses three Amt proteins, each encoded in an operon with a GlnK ortholog. One of these proteins, GlnK2 was recently found to be incapable of binding 2-OG, and in order to understand the implications of this finding we conducted a detailed structural and functional analysis of a second GlnK protein from A. fulgidus, GlnK3. Contrary to Af-GlnK2 this protein was able to bind both ATP/2-OG and ADP to yield inactive and functional states, respectively. Due to the thermostable nature of the protein we could observe the exact positioning of the notoriously flexible T-loops and explain the binding behavior of GlnK proteins to their interaction partner, the Amt proteins. A thermodynamic analysis of these binding events using microcalorimetry evaluated by microstate modeling revealed significant differences in binding cooperativity compared to other characterized PII proteins, underlining the diversity and adaptability of this class of regulatory signaling proteins