arXiv subjects

Francesco Martinelli

Publications and source records attributed to Francesco Martinelli.

3 recordsLinked to original sources

arXiv2026

Quadrupole formation and coupling to magnetic and structural degrees of freedom in the $5d^1$ double perovskites Ba$_2$MgReO$_6$ and Ba$_2$NaOsO$_6$

We investigate the interplay between charge, magnetic, and structural degrees of freedom in the isostructural and isoelectronic $5d^1$ double-perovskites Ba$_2$MgReO$_6$ and Ba$_2$NaOsO$_6$. Using first-principles-based electronic structure calculations, we show that both materials exhibit a tendency toward spontaneous quadrupolar order in the cubic paramagnetic phase, which is slightly weaker in Ba$_2$NaOsO$_6$ than in Ba$_2$MgReO$_6$. Our analysis further reveals an intimate coupling between the local magnetic moments and charge quadrupoles, mediated by the strong spin-orbit interaction, that leads to the unusual canted configuration of magnetic moments observed in these systems. When structural degrees of freedom are included, the two materials exhibit pronounced differences. In Ba$_2$MgReO$_6$ the strong coupling to Jahn-Teller distortions stabilizes the antiferroic $\mathcal{Q}_{x^2-y^2}$ order, yielding excellent agreement with available experimental data. In contrast, the Jahn-Teller coupling is significantly weaker in Ba$_2$NaOsO$_6$ and appears insufficient to stabilize the antiferroic quadrupolar order. While this is consistent with the absence of any measurable long-range structural distortion above the magnetic transition temperature, it contrasts with experimental results indicating a strong canting of the magnetic moments. Our analysis thus successfully describes the mechanisms shaping the properties of the Re-compound while a full quantitative description of the magnetic ground state of Ba$_2$NaOsO$_6$ is still elusive.

cond-mat.mtrl-sci↗

arXiv2025

Multipoles as quantitative order parameters for altermagnetic spin splitting

We establish a quantitative relation between the altermagnetic spin-splitting and different higher order multipoles of the charge and magnetization density around the magnetic atoms. Magnetic multipoles such as octupoles or triakontadipoles have been suggested as potential ferroic order parameters for d- and g-wave altermagnetism, respectively, based mainly on qualitative symmetry arguments. We use first-principles-based electronic structure calculations to establish a clear quantitative relation between the strength of the altermagnetic spin splitting and the magnitude of certain local multipoles. We vary the magnitude of these multipoles either by applying an appropriate constraint on the charge density or by varying a corresponding structural distortion mode, using two simple perovskite materials, SrCrO3 and LaVO3, as model systems. Our analysis indicates that in general the altermagnetic spin splitting is not exclusively determined by the lowest order nonzero magnetic multipole, but results from a superposition of contributions from different multipoles with comparable strength, suggesting the need for a multi-component order parameter to describe altermagnetism. We also discuss different measures to quantify the overall spin-splitting of a material, without relying on features that might be specific to only individual bands.

cond-mat.mtrl-sci↗

arXiv2024

Designing Optically Addressable Nitrogen-Vacancy Centers in Ultra-Small Nanodiamonds: Insights from First-Principles Calculations

Ultrasmall nanodiamonds (USNDs) are promising platforms for fluorescent and quantum sensing applications. Here we present first-principles electronic structure calculations of color centers in USNDs, specifically the nitrogen-vacancy (NV-) and we investigate their optical addressability as a function of the surface termination. We consider both isolated nanoparticles and arrays of USNDs with different degrees of packing, and we include quantum vibronic effects in our analysis, using stochastic methods. We find that the NV in USNDs can be stabilized in a negative charge state if the nanoparticles are terminated by fluorine, hydroxyl, and ether. While fluorine terminations can be used for fluorescent bio-tags, we suggest that hydroxyl and ether terminations are beneficial for quantum sensing applications. We also find that the NV- can be stabilized in arrays of USNDs when inter-particle separations are larger than the diameter of the nanoparticle. Interestingly, the phonon renormalizations of single-particle energy levels found in arrays contribute to the charge stability of negatively charged NV centers.

physics.comp-ph↗