arXiv subjects

Mateusz Kempiński

Publications and source records attributed to Mateusz Kempiński.

3 recordsLinked to original sources

arXiv2026

Crystal-Field--Driven Magnetoelectric Coupling in the Non-Kramers Hexaaluminate PrMgAl11O19

We report broadband dielectric spectra of the non-Kramers hexaaluminate PrMgAl\textsubscript{11}O\textsubscript{19}, revealing a pronounced interplay between permittivity and magnetization at cryogenic temperatures. The zero-field dielectric response follows a Barrett-type quantum-paraelectric form, while a broad dielectric anomaly near \SI{5}{K} shows a complex field dependence that mirrors the multi-hump behavior of the magnetic specific heat, evidencing robust magnetoelectric coupling. The inverse permittivity $\varepsilon'^{-1}(T,H)$ scales linearly with $M^2$, consistent with a biquadratic $P^2M^2$ term in a Landau framework. Fits yield temperature-dependent coupling constant $λ(T)$ that decreases with heating from ($1.07\pm0.01)\times10^{-4}\,μ_{\mathrm{B}}^{-2}$ (at 5\,K) to $(4.77\pm0.02)\times10^{-5}\,μ_{\mathrm{B}}^{-2}$ (at 10\,K), reflecting the thermal population of low-lying energy levels of Pr$^{3+}$. Consistently, the uniaxial thermal expansion develops an additional low-temperature hump below $\sim\SI{30}{K}$ that is progressively suppressed by magnetic field, recovering an approximately saturated response by \SI{9}{T}. These results identify PrMgAl\textsubscript{11}O\textsubscript{19} as a paradigmatic non-Kramers hexaaluminate where quantum paraelectricity and magnetoelectric interactions are intrinsically entangled, establishing hexaaluminates as a tunable platform for magnetoelectric physics in frustrated quantum materials.

cond-mat.str-el↗

arXiv2026

Absence of Long-Range Order and Magnetic Anisotropy in the Triangular Magnet NdMgAl$_{11}$O$_{19}$

We investigated the rare-earth triangular-lattice antiferromagnet NdMgAl$_{11}$O$_{19}$ using single-crystal magnetization (1.8~K $\leq T \leq$ 300~K, $μ_0 H \leq 7$~T) and specific-heat measurements down to 45~mK. The dc susceptibility confirms a well-isolated Kramers doublet ground state with pronounced Ising-type anisotropy, with $g_c \approx 3.7$ and $g_{ab} \approx 1.45$. Curie--Weiss fits yield weak, anisotropic antiferromagnetic exchange, with $θ_c = -0.54$~K and $θ_{ab} = -0.87$~K. Heat-capacity measurements show no long-range magnetic order down to 40~mK, corresponding to a frustration index $f \gtrsim 20$. Instead, $C_m/T$ exhibits a broad maximum near 0.081~K whose magnitude and field evolution are consistent with short-range correlations in an anisotropic triangular lattice. Applied magnetic fields open a Zeeman gap where the specific-heat anomaly follows $Δ= g μ_B μ_0 H$, and $M(H,T)$ is well described by a Brillouin function for an effective $J = 1/2$ moment. The field tuning of the low-temperature entropy manifold allows self-cooling from 1.8~K to 53~mK by adiabatic demagnetisation from a 9~T field. These results identify NdMgAl$_{11}$O$_{19}$ as a nearly ideal weak-exchange triangular magnet with a field-tunable correlated ground state, where two-dimensional crossover effects may emerge from frustrated XXZ interactions.

cond-mat.str-el↗

arXiv2017

Strain- and Adsorption-Dependent Electronic States and Transport or Localization in Graphene

The chapter generalizes results on influence of uniaxial strain and adsorption on the electron states and charge transport or localization in graphene with different configurations of imperfections (point defects): resonant (neutral) adsorbed atoms either oxygen- or hydrogen-containing molecules or functional groups, vacancies or substitutional atoms, charged impurity atoms or molecules, and distortions. To observe electronic properties of graphene-admolecules system, we applied electron paramagnetic resonance technique in a broad temperature range for graphene oxides as a good basis for understanding the electrotransport properties of other active carbons. Applied technique allowed observation of possible metal-insulator transition and sorption pumping effect as well as discussion of results in relation to the granular metal model. The electronic and transport properties are calculated within the framework of the tight-binding model along with the Kubo-Greenwood quantum-mechanical formalism. Depending on electron density and type of the sites, the conductivity for correlated and ordered adsorbates is found to be enhanced in dozens of times as compared to the cases of their random distribution. In case of the uniaxially strained graphene, the presence of point defects counteracts against or contributes to the band-gap opening according to their configurations. The band-gap behaviour is found to be nonmonotonic with strain in case of a simultaneous action of defect ordering and zigzag deformation. The amount of localized charge carriers (spins) is found to be correlated with the content of adsorbed centres responsible for the formation of potential barriers and, in turn, for the localization effects. Physical and chemical states of graphene edges, especially at a uniaxial strain along one of them, play a crucial role in electrical transport phenomena in graphene-based materials.

cond-mat.mes-hall↗