arXiv subjects

Dávid Beke

Publications and source records attributed to Dávid Beke.

6 recordsLinked to original sources

arXiv2026

Solution-phase fluorination of nanodiamond: near-surface NV$^-$ activation and spin relaxation

Nanodiamonds hosting luminescent point defects, known as fluorescent nanodiamonds (FND), are a leading platform for quantum technology. The nitrogen-vacancy (NV) centre is the most intensively studied of these; its negatively charged state (NV$^-$) can serve as a qubit at room temperature, and its stability is governed by surface functional groups. We present two solution-phase fluorination routes for stabilising NV$^-$: direct C-F bond formation by decarboxylation with xenon difluoride via a radical mechanism, and the Balz-Schiemann reaction, which replaces surface amino groups with fluorine. The two routes were compared by infrared, X-ray photoelectron, energy-dispersive X-ray, Raman and photoluminescence spectroscopy. Both gave a high NV$^-$ fraction, up to $\sim$90% on average and approaching 100% in fluorine-rich regions, which to our knowledge is among the highest reported for surface-terminated nanodiamonds of this size and, in particular, for fluorine termination. Frequency-domain relaxometry shows that the fluorinated particles retain a long spin-lattice relaxation time, $733\pm56$ and $712\pm20$ $μ$s for the XeF$_2$ and Balz-Schiemann routes, several times the values reported for commercial HPHT nanodiamonds, although shorter than the $1173\pm123$ $μ$s of the as-received material. Charge-state stability and spin lifetime therefore do not improve together: fluorination activates near-surface NV$^-$ centres, which are the most exposed to surface noise but also the ones that dominate relaxometric sensing.

quant-ph↗

arXiv2026

Transient Plastic Spin Labeling with Chlorine Dioxide

Plastic waste, being one of the most important problems for humankind, poses severe threats to ecosystems, wildlife, and human health. Tracing, quantifying, and identifying types of plastic waste is of crucial importance to understand its environmental pathways and develop targeted strategies for reduction, recycling, and remediation. To contribute to addressing this global issue, we investigated the spin-labeling capabilities of aqueous chlorine dioxide (ClO$_2$) radicals upon introduction into poly(ethylene terephthalate) and utilized electron spin resonance spectroscopy for detection. The technique is capable of identifying plastic species as the unpaired electron of the radical molecule is strongly sensitive to its local environment through its coupling parameters. Temperature-dependent measurements revealed that the molecules are immobilized at low temperatures and exhibit well-resolved anisotropic and hyperfine spectra that are quantitatively described by a model spin Hamiltonian. Even above the melting point of water, some degrees of freedom remain blocked as a result of the polymer matrix. Furthermore, employing a time-series measurement at room temperature enabled us to determine the diffusion coefficient of the molecule in the polymer.

cond-mat.other↗

arXiv2025

Highly Efficient Functionalization of hBN with Lithium Oxalate: A Multifunctional Platform for Composites, Ion Transport, and Spin Labeling

The development of multifunctional solid-state materials is key to advancing lithium-ion batteries with enhanced safety and simplified architectures. Here, we report a scalable, highly efficient (near $100\%$), solvent-free mechanochemical synthesis of hexagonal boron nitride (hBN) functionalized with lithium oxalate (Li$_2$C$_2$O$_4$), yielding a novel lamellar composite that functions both as a lithium-ion conductor and separator. The high-energy milling process promotes exfoliation of hBN and covalent attachment of oxalate groups at edge and defect sites, forming a brown, nanocrystalline material with uniform lithium distribution. The composite exhibits room-temperature ionic and negligible electronic conductivity, thermal stability at least up to $350~^{\circ}$C, and hosts stable free radicals enabling its use as a spin label. The synthesis produces no byproducts and can be extended towards lithium doping via secondary mechanochemical steps, creating highly doped, chemically stable phases that host additional Li for ionic conduction. These results introduce a new class of lithium-rich, boron nitride-based solids for solid-state batteries, combining ion conduction, mechanical robustness, and thermal resilience in a single material platform.

cond-mat.mtrl-sci↗

arXiv2025

Unprecedented Spin-Lifetime of Itinerant Electrons in Natural Graphite Crystals

A long spin-lifetime of electrons is the holy grail of spintronics, a field exploiting the electron angular momentum as information carrier and storage unit. Previous reports indicated a spin lifetime, $τ_{\text{s}}$ near $10$ ns at best in graphene-based devices at low temperatures. We detail the observation of $τ_{\text{s}}$ approaching the ultralong $1{,}000$~ns at room temperature in natural graphite crystals using magnetic resonance spectroscopy. The relaxation time shows a giant anisotropy: the lifetime of spins, polarized perpendicular to the graphite plane, is more than $50$ times longer than for the in-plane polarization. The temperature dependence of $τ_{\text{s}}$ proves that diffusion of spins to the crystallite edges, where relaxation occurs, limits the lifetime. This suggests that graphite is an excellent candidate for spintronic applications, seamlessly integrating with emerging 2D van der Waals technologies.

cond-mat.mes-hall↗

arXiv2025

Surface Chemistry-Driven Oxidation Mechanisms in Ti$_{\text{3}}$C$_{\text{2}}$T$_{\textit{x}}$ MXenes

Ti$_3$C$_2$T$_x$ is a leading compound within the MXenes family and can find host in widespread applications. It is synthesized by selectively etching layers from the Ti$_3$AlC$_2$ precursor, and this process typically introduces surface terminations, T$_x$, such as $-$OH, $=$O, or $-$F. However, the aggressive chemical conditions required for its preparation, as well as exposure to air, humidity, and heat, can lead to impurity phases that potentially compromise its desirable properties. We reveal a two-step oxidation process during heat treatment, where initial oxidation occurs between layers without altering the integrity of the Ti$_3$C$_2$ layered structure, followed by the formation of anatase TiO$_2$ at elevated temperatures. The process was carefully monitored using \emph{in situ} Raman spectroscopy and \emph{in situ} microwave conductivity measurements, employed to Ti$_3$C$_2$T$_x$ prepared using various etching techniques involving concentrated HF, LiF + HCl, and HF + HCl mixtures. The oxidation process is heavily influenced by the synthesis route and surface chemistry of Ti$_3$C$_2$T$_x$, with fluoride and oxyfluoride groups playing a pivotal role in stabilizing the anatase phase. The absence of these groups, in contrast, can lead to the formation of rutile TiO$_2$.

cond-mat.mtrl-sci↗

arXiv2024

Unexpected Tuning of the Anomalous Hall Effect in Altermagnetic MnTe Thin Films

The discovery of an anomalous Hall effect (AHE) sensitive to the magnetic state of antiferromagnets can trigger a new era of spintronics, if materials that host a tunable and strong AHE are identified. Altermagnets are a new class of materials that can under certain conditions manifest a strong AHE, without having a net magnetization. But the ability to control their AHE is still lacking. In this study, we demonstrate that the AHE in altermagnetic α-MnTe grown on GaAs(111) substrates can be "written on-demand" by cooling the material under an in-plane magnetic field. The magnetic field controls the strength and the coercivity of the AHE. Remarkably, this control is unique to α-MnTe grown on GaAs and is absent in α-MnTe grown on SrF2. The tunability that we reveal challenges our current understanding of the symmetry-allowed AHE in this material and opens new possibilities for the design of altermagnetic spintronic devices.

cond-mat.mtrl-sci↗