arXiv subjects

Jonas Okkels Birk

Publications and source records attributed to Jonas Okkels Birk.

3 recordsLinked to original sources

arXiv2026

Initial Characterisation of a Prototype TMR Assembly for an Electron-Driven CANS at CERN's CLEAR Facility

The Versatile ULtra-Compact Accelerator-based Neutron source (VULCAN) project is developing a compact accelerator-driven neutron source (CANS) optimised for neutron diffractometry in industrial and university settings. Central to VULCAN is a novel target-moderator-reflector (TMR) assembly optimised to convert a 35 MeV pulsed electron beam into short neutron pulses (FWHM < 20 μs) in the 1.5-3.5 Å wavelength range. To validate the simulation-driven design process, a prototype TMR was developed for testing at CERN's CLEAR facility, and this paper presents the design, installation, and results of the first experimental campaign. While moderated neutron pulses were successfully detected, significant discrepancies were observed between the experimental and simulated energy spectra. Potential causes are discussed and recommendations for follow-up measurements are provided.

physics.ins-det↗

arXiv2022

Nanocriticality in the magnetic phase transition of CoO nanoparticles

The universal theory of critical phase transitions describes the critical behavior at second-order phase transitions in infinitely large systems. With the increased contemporary interest in nanoscale materials, we investigated CoO nanoparticles by means of neutron scattering and found how the theory of critical phenomena breaks down in the nanoscale regime. Using CoO as a model system, we have identified a size-dependent nanocritical temperature region close to the antiferromagnetic phase transition where the magnetic correlation length of the nanoparticles converges to a constant value, which is significantly smaller than that of the saturated state found at low temperatures. This is in clear contrast to the divergence around $T_{\rm N}$ observed for bulk systems. Our findings of nanocriticality in the magnetic phase transition is of great importance for the understanding of phase transitions at the nanoscale.

cond-mat.mtrl-sci↗

arXiv2017

Multi-Q mesoscale magnetism in CeAuSb$_2$

We report the discovery of a field driven transition from a striped to woven Spin Density Wave (SDW) in the tetragonal heavy fermion compound CeAuSb$_2$. Polarized along $\bf c$, the sinusoidal SDW amplitude is 1.8(2) $μ_B$/Ce for $T \ll T_N$=6.25(10) K with wavevector ${\bf q}_{1}=( η, η, \frac{1}{2} )$ ($η=0.136(2)$). For ${\bf H}\parallel{\bf c}$, harmonics appearing at $2{\bf q}_{1}$ evidence a striped magnetic texture below $μ_\circ H_{c1}=2.78(1)$ T. Above $H_{c1}$, these are replaced by woven harmonics at ${\bf q}_{1}+{\bf q}_2=(2η, 0, 0)+{\bf c}^*$ until $μ_\circ H_{c2}=5.42(5)$ T, where satellites vanish and magnetization non-linearly approaches saturation at 1.64(2) $μ_B$/Ce for $μ_\circ H\approx 7$ T.

cond-mat.str-el↗