arXiv subjects

S. Roh

Publications and source records attributed to S. Roh.

3 recordsLinked to original sources

arXiv2024

Lattice dynamics of the frustrated kagome compound Y-kapellasite

Studying the magnetic ground states of frustrated antiferromagnets provides unique insight into the stability of quantum spin liquids, even if the anticipated state is not realized towards T = 0. Particularly relevant are structural modifications setting in at temperatures where the magnetic correlations come into play. Here we explore the lattice dynamics of Y-kapellasite (Y3Cu9(OH)19Cl8) single crystals by infrared spectroscopy in combination with ab initio calculations. We observe significant changes in the phonon spectra at Ts = 32 K, that gradually evolve down to low temperatures. The increase in the number of phonon modes provides evidence for a lowering of symmetry and we discuss several possibilities of crystal structure modifications. Our analysis also reveals that the structural variation involves exclusively H and O atoms, while the other atoms remain rather unaffected. An 8% red shift of the lowest-lying phonon mode upon cooling indicates strong magneto-elastic effects upon decoupling Cu-6f hexagons through the lattice vibrations.

cond-mat.str-el↗

arXiv2019

Revealing excess protons in the infrared spectrum of liquid water

The most common species in liquid water, next to neutral H$_2$O molecules, are the H$_3$O$^+$ and OH$^-$ ions. In a dynamic picture, their exact concentrations depend on the time scale at which these are probed. Here, using a spectral-weight analysis, we experimentally resolve the fingerprints of the elusive fluctuations-born short-living H$_3$O$^+$, DH$_2$O$^+$, HD$_2$O$^+$, and D$_3$O$^+$ ions in the IR spectra of light (H$_2$O), heavy (D$_2$O), and semi-heavy (HDO) water. We find that short-living ions, with concentrations reaching $\sim 2\%$ of the content of water molecules, coexist with long-living pH-active ions on the picosecond timescale, thus making liquid water an effective ionic liquid in femtochemistry.

cond-mat.mtrl-sci↗

arXiv2019

Shock acceleration efficiency in radio relics

Radio relics in galaxy clusters are giant diffuse synchrotron sources powered in cluster outskirts by merger shocks. Although the relic-shock connection has been consolidated in recent years by a number of observations, the details of the mechanisms leading to the formation of relativistic particles in this environment are still not well understood. The diffusive shock acceleration (DSA) theory is a commonly adopted scenario to explain the origin of cosmic rays at astrophysical shocks, including those in radio relics in galaxy clusters. However, in a few specific cases it has been shown that the energy dissipated by cluster shocks is not enough to reproduce the luminosity of the relics via DSA of thermal particles. Studies based on samples of radio relics are required to further address this limitation of the mechanism. In this paper, we focus on ten well-studied radio relics with underlying shocks observed in the X-rays and calculate the electron acceleration efficiency of these shocks that is necessary to reproduce the observed radio luminosity of the relics. We find that in general the standard DSA cannot explain the origin of the relics if electrons are accelerated from the thermal pool with an efficiency significantly smaller than 10%. Our results show that other mechanisms, such as shock re-acceleration of supra-thermal seed electrons or a modification of standard DSA, are required to explain the formation of radio relics.

astro-ph.HE↗