arXiv subjects

Sergio Di Matteo

Publications and source records attributed to Sergio Di Matteo.

4 recordsLinked to original sources

arXiv2026

Electric-field-induced X-ray Nonreciprocal Dichroism in Hematite

Hematite (alpha-Fe2O3) is a prototypical room temperature antiferromagnet whose time-reversal-odd magnetic structure has recently attracted renewed attention. While such magnetic symmetry can be characterized in terms of higher-order multipoles beyond the magnetic dipole, their manifestation in measurable physical phenomena has remained largely elusive. In this work, we investigate x-ray absorption near the Fe K-edge of hematite under an applied electric field, which explicitly breaks space-inversion symmetry. We observe an electric-field-induced x-ray nonreciprocal linear dichroism (E-induced XNLD) that reflects the time-reversal-odd nature of the magnetic order. Numerical simulations based on ab-initio density functional theory reproduce the observed spectra, including their dependence on the antiferromagnetic domain and x-ray polarization. Furthermore, a symmetry-resolved multipole analysis reveals that this response originates from the magnetic quadrupole and the magnetic toroidal octupole induced by the applied electric field. These results demonstrate that electric-field-modulated x-ray absorption provides direct access to the antiferroic order of higher-order multipoles in time-reversal-odd antiferromagnets, thereby establishing a general framework to uncover hidden symmetry properties in magnetic materials.

cond-mat.mtrl-sci↗

arXiv2023

On the relation between thermodynamical and statistical entropy: The origin of the N!

The division by N! in the expression of statistical entropy is usually justified to students by the statement that classical particles should be counted as indistinguishable. Sometimes, quantum indistinguishability is invoked to explain it. In this paper, we try to clarify the issue starting from Clausius thermodynamical entropy and deriving from it Boltzmann statistical entropy for the ideal gas. This approach appears interesting for two reasons: Firstly, it provides a direct heuristic link between thermodynamical and statistical expressions of entropy that is missing in the usual approach. In second place, it explicitly reminds that also statistical entropy is defined with respect to a reference state, chosen to be the quantum-mechanical (T=0) ground state of the N-particles in a box. Both factors $h^{3N}$ and $N!$ at the denominator of the statistical entropy are a consequence of the quantum nature of the reference state: in particular, the $N!$ is not related to the particle identity, but to the identity of the boundary conditions on the quantum-mechanical ground-state momenta, and to a general statistical maximization principle. The introduction of a reference state also allows to reinterpret the standard case of two mixing gases.

cond-mat.stat-mech↗

arXiv2014

Mean-field solution of the Hubbard model: the magnetic phase diagram

The present paper is based on our graduate lectures in condensed-matter physics. We found that the mean-field solution of the Hubbard model is an excellent tool to stimulate students' reflections towards the treatment of realistic magnetic interactions. We show by detailed analytical and numerical calculations how to find the mean-field solution of the model on a square lattice. We then interpret the physical implications of the ground-state magnetic phase diagram in terms of the electron density and the ratio between the Coulomb repulsion and the electron-structure bandwidth.

cond-mat.str-el↗

arXiv2007

Study of the Electronic Structure in Oxides Using Absorption and Resonant X-Ray Scattering

Resonant X-ray scattering (RXS) is a spectroscopy where both the power of site selective diffraction and the power of local absorption spectroscopy regarding atomic species are combined. By virtue of the dependence on the core level state energy and the three dimensional electronic structure of the intermediate state, this technique is specially suited to study charge, orbital or spin orderings and associated crystal distortions. In the case of charge ordering, we exploit the fact that atoms with closely related site symmetries but with small charge differences exhibit resonances at slightly different energies. The sensitivity of this effect allows for quantitative estimations of the charge disproportion. Opposite to fluorescence or absorption measurements, the power of diffraction relies on the capability of detecting differences that are smaller than the inverse lifetime of the core hole level. To account for the uncertainty of the crystallographic structure and the fact that the charge ordering must be disentangled from the associated atomic displacements, a complete methodology is proposed and applied to the low temperature phase of magnetite. Relative sensitivity on spin, toroidal and orbital ordering is also shown and compared in different transition metal oxide compounds, like V2O3 and GaFeO3.

cond-mat.mtrl-sci↗