arXiv subjects

Fabrizio Cossu

Publications and source records attributed to Fabrizio Cossu.

11 recordsLinked to original sources

arXiv2026

Strain-induced structural transitions in (111)-oriented (LaMnO$_3$)$_{2n}|$(SrMnO$_3$)$_n$ superlattices

By means of first-principles electronic structure calculations, we hereby investigate the structural transitions induced by epitaxial strain in (111)-oriented (LaMnO$_3$)$_{2n}|$(SrMnO$_3$)$_n$ superlattices, with $n=2,4,6$. All superlattices in the explored range of strain are shown to prefer a half-metallic ferromagnetic order where the local magnetic moments are coupled to volume-breathing distortions. More in detail, our results reveal that thickness plays a crucial role in the response to epitaxial strain, which is particularly evident in the resulting tilt pattern of the oxygen octahedra. The thinnest superlattice, for $n=2$, always adopts the $a^-a^-a^-$ tilt pattern and the competing $a^-a^-c^+$ tilt pattern can be stabilized as a metastable state only in presence of compressive strain. Instead, the superlattice with $n=4$ favours the $a^-a^-c^+$ tilt pattern at equilibrium conditions, but the in-phase rotations around the third pseudocubic axis are so fragile that the $a^-a^-a^-$ pattern is recovered under a tiny amount of either compressive or tensile strain. The superlattice with $n=6$ exhibits a more nuanced behaviour: compressive strain drives a transition from $a^-a^-c^+$ to $a^-a^-a^-$, whereas tensile strain preserves the $a^-a^-c^+$ tilt pattern and significantly accentuates the structural differences between the two inequivalent sublattices within this symmetry. In fact, the Jahn-Teller distortions are quenched in one of the sublattices, leading to enhanced volume-breathing distortions and corresponding enhanced charge and spin oscillations. This suggests that Hund's physics may be more relevant in this regime of tensile strain, maximizing the interplay between strong electronic correlations and structural effects.

cond-mat.mtrl-sci↗

arXiv2026

Exchange interactions and finite-temperature magnetism in (111)-oriented (LaMnO$_3$)$_{2n}$|(SrMnO$_3$)$_n$ superlattices

We present a first-principles investigation of magnetic exchange interactions and critical behavior in (111)-oriented (LaMnO$_3$)$_{2n}$|(SrMnO$_3$)$_n$ superlattices for $n=2,4,6$. For all superlattices under investigation, we find robust half-metallic ferromagnetism extending across all the layers of both component regions. Changing octahedral tilt patterns is found to have negligible effects on the magnetic properties, despite determining the presence or absence of small Jahn-Teller distortions. The analysis of the response of the magnetic coupling to a variation of the Coulomb interaction parameters demonstrates that ferromagnetism is driven by a double-exchange mechanism involving itinerant $e_g$ electrons, while its final strength is hampered by antiferromagnetic contributions due to the superexchange of localized $t_{2g}$ electrons. Multi-scale simulations based on atomistic spin dynamics show that the thinnest superlattices, $n=2,4$, possess an ordering temperature that is at least comparable to that of La$_{2/3}$Sr$_{1/3}$MnO$_3$. Conversely, as thickness increases, a two-phase behavior emerges, where the SrMnO$_3$ region loses long-range order faster than the LaMnO$_3$ region. While the global ordering temperature increases together with thickness, we argue that the high-temperature regime for the observed two-phase behavior is not representative of the real physical system, which will undergo a combined electronic, magnetic and structural phase transition as soon as the long-range order is lost inside the SrMnO$_3$ region. This study provides insights into the emergent magnetic phases and transition temperatures relevant to oxide heterostructures.

cond-mat.str-el↗

arXiv2026

Orientation-driven route to an intrinsic insulating ferromagnetic state in manganite superlattices

Increasing precision in the growth of superlattices sparks hope in applications that may arise from engineering layered structures. Heterostructuring and functionalization of magnetic oxides have been very popular due to their versatility and readiness for integration in modern electronics. In this study, we provide yet another example of this phenomenology by predicting that an insulating ferromagnetic state can be realized in superlattices of LaMnO$_3$ and SrTiO$_3$ oriented along the (111) direction. In strike contrast with respect to other orientations, these properties are not of extrinsic origin but arise from the interplay of structural order, strain and quantum confinement. The bandgap is shown to be either direct and indirect, depending on the precise composition, which can be explained in terms of the geometrical properties of (111)-oriented bilayers of LaMnO$_3$. The electronic structure shows narrow bands indicating localized $e_g$ states for all the investigated superlattices. These features and the analysis of the inter-atomic magnetic coupling suggest that the investigated superlattices behave as a Kugel-Khomskii material, at least for the explored compositions. Our results provide not only a new route to an insulating ferromagnet, but also novel insight into the intricate interplay between lattice symmetry, Hubbard physics and Hund's coupling to be exploited in next-generation spintronic applications.

cond-mat.str-el↗

arXiv2024

Spin-orbit entangled moments and magnetic exchange interactions in cobalt-based honeycomb magnets BaCo$_2$($X$O$_4$)$_2$ ($X$ = P, As, Sb)

Co-based honeycomb magnets have been actively studied recently for the potential realization of emergent quantum magnetism therein such as the Kitaev spin liquid. Here we employ density functional and dynamical mean-field theory methods to examine a family of the Kitaev magnet candidates BaCo$_2$($X$O$_4$)$_2$ ($X$ = P, As, Sb), where the compound with $X$ = Sb being not synthesized yet. Our study confirms the formation of Mott insulating phase and the $J_{\rm eff}$ = 1/2 spin moments at Co$^{2+}$ sites despite the presence of a sizable amount of trigonal crystal field in all three compounds. The pnictogen substitution from phosphorus to antimony significantly changes the in-plane lattice parameters and direct overlap integral between the neighboring Co ions, leading to the suppression of the Heisenberg interaction. More interestingly, the marginal antiferromagnetic nearest-neighbor Kitaev term changes sign into a ferromagnetic one and becomes sizable at the $X$ = Sb limit. Our study suggests that the pnictogen substitution can be a viable route to continuously tune magnetic exchange interactions and to promote magnetic frustration for the realization of potential spin liquid phases in BaCo$_2$($X$O$_4$)$_2$.

cond-mat.str-el↗

arXiv2024

Stacking of charge-density waves in 2H-NbSe$_2$ bilayers

We employ ab-initio electronic structure calculations to investigate the charge-density waves and periodic lattice distortions in bilayer 2H-NbSe$_2$. We demonstrate that the vertical stacking can give rise to a variety of patterns that may lower the symmetry of the charge-density waves exhibited separately by the two composing 1H-NbSe$_2$ monolayers. The general tendency to a spontaneous symmetry breaking observed in the ground state and the first excited states is shown to originate from a non-negligible inter-layer coupling. Simulated images for scanning tunnelling microscopy (STM) as well as diffraction/scattering patterns show signatures of the different stacking orders. This may not only be useful to reinterpret past experiments on surfaces and thin films, but may also be exploited to devise ad-hoc experiments for the investigation of the stacking order in 2H-NbSe$_2$. We anticipate that our analysis does not only apply to the 2H-NbSe$_2$ bilayer, but is also relevant for thin films and bulk, whose smallest centro-symmetric component is indeed the bilayer. Finally, our results illustrate clearly that the vertical stacking is not only important for 1T structures, as exemplified by the metal-to-insulator transition observed in 1T-TaS$_2$, but seems to be a general feature of metallic layered transition metal dichalcogenides as well.

cond-mat.str-el↗

arXiv2024

Theory of momentum-resolved magnon electron energy loss spectra: The case of Yttrium Iron Garnet

We explore the inelastic spectra of electrons impinging in a magnetic system. The methodology here presented is intended to highlight the charge-dependent interaction of the electron beam in a STEM-EELS experiment, and the local vector potential generated by the magnetic lattice. This interaction shows an intensity $10^{-2}$ smaller than the purely spin interaction, which is taken to be functionally the same as in the inelastic neutron experiment. On the other hand, it shows a strong scattering vector dependence ($κ^{-4}$) and a dependence with the relative orientation between the probe wavevector and the local magnetic moments of the solid. We present YIG as a case study due to its high interest by the community.

cond-mat.mtrl-sci↗

arXiv2023

Emergent half-metal with mixed structural order in (111)-oriented (LaMnO$_3$)$_{2n}$|(SrMnO$_3$)$_n$ superlattices

Using first-principles techniques, we study the structural, magnetic and electronic properties of (111)-oriented (LaMnO$_3$)$_{2n}$$\vert$(SrMnO$_3$)$_{n}$ superlattices of varying thickness ($n=2,4,6$). We find that the properties of the thinnest superlattice ($n=2$) are similar to the celebrated half-metallic ferromagnetic alloy La$_{2/3}$Sr$_{1/3}$MnO$_3$, with quenched Jahn-Teller distortions. At intermediate thickness ($n=4$), the $a^{-}a^{-}a^{-}$ tilting pattern transitions to the $a^{-}a^{-}c^{+}$ tilting pattern, driven by the lattice degrees of freedom in the LaMnO$_3$ region. The emergence of the Jahn-Teller modes and the spatial extent needed for their development play a key role in this structural transition. For the largest thickness considered ($n=6$), we unveil an emergent separation of Jahn-Teller and volume-breathing orders in the ground-state structure with the $a^{-}a^{-}c^{+}$ tilting pattern, whereas it vanishes in the antiferromagnetic configurations. The ground state of all superlattices is half-metallic ferromagnetic, not affected by the underlying series of structural transitions. Overall, these results outline a thickness-induced crossover between the physical properties of bulk La$_{2/3}$Sr$_{1/3}$MnO$_3$ and bulk LaMnO$_3$.

cond-mat.mtrl-sci↗

arXiv2022

Persistent half-metallic ferromagnetism in a (111)-oriented manganite superlattice

Heterostructures of mixed-valence manganites are still under intense scrutiny, due to the occurrence of exotic quantum phenomena linked to electronic correlation and interfacial composition. For instance, if two anti-ferromagnetic insulators as LaMnO$_3$ and SrMnO$_3$ are grown in a (001)-oriented superlattice, a half-metallic ferromagnet may form, provided that the thickness is sufficiently small to allow tunneling across interfaces. In this article, we employ electronic structure calculations to show that all the layers of a (111)-oriented LaMnO$_3$|SrMnO$_3$ superlattice retain a half-metallic ferromagnetic character for a much larger thickness than in its (001) counterpart. This behavior is shown to be linked to the charge transfer across the interface, favored by the octahedral connectivity between the layers. This also results in a symmetry-induced quenching of the Jahn-Teller distortions, which are replaced by breathing modes. The latter are coupled to charge and spin oscillations, whose components have a pure e g character. Most interestingly, the magnetization reaches its maximum value inside the LaMnO$_3$ region and not at the interface, which is fundamentally different from what observed for the (001) orientation. The analysis of the inter-atomic exchange coupling shows that the magnetic order arises from the double-exchange mechanism, despite competing interactions inside the SrMnO$_3$ region. Finally, the van Vleck distortions and the spin oscillations are found to be crucially affected by the variation of Hund's exchange and charge doping, which allows us to speculate that our system behaves as a Hund's metal, creating an interesting connection between manganites and nickelates.

cond-mat.str-el↗

arXiv2020

Strain-induced stripe phase in charge ordered single layer NbSe$_2$

Charge density waves are ubiquitous phenomena in metallic transition metal dichalcogenides. In NbSe$_2$, a triangular $3\times3$ structural modulation is coupled to a charge modulation. Recent experiments reported evidence for a triangular-stripe transition at the surface, due to strain or accidental doping and associated to a $4\times4$ modulation. We employ \textit{ab-initio} calculations to investigate the strain-induced structural instabilities in a pristine single layer and analyse the energy hierarchy of the structural and charge modulations. Our results support the observation of phase separation between triangular and stripe phases in 1H-NbSe$_2$, relating the stripe phase to compressive isotropic strain, favouring the $4\times4$ modulation. The observed wavelength of the charge modulation is also reproduced with good accuracy.

cond-mat.mtrl-sci↗

arXiv2018

Unveiling hidden charge density waves in single-layer NbSe$_2$ by impurities

We employ {\it{ab-initio}} calculations to investigate the charge density waves in single-layer NbSe$_2$, and we explore how they are affected by transition metal atoms. Our calculations reproduce the observed orthorhombic phase in single-layer NbSe$_2$ in the clean limit, establishing the energy order between three different distorted structures, two consisting of triangular Nb-Nb clusters and a third, energetically unfavoured, consisting of hexagonal Nb-Nb clusters. Such energy order, in agreement with known experimental work, is reversed by the adsorption of Co and Mn, which favour the formation of hexagonal Nb-Nb clusters; this CDW structure is indeed allowed from symmetry point of view but hidden in pure single layers because it is at a higher energy. The other adsorbates, K and Ga, still favour one of the triangular Nb-Nb cluster, while suppressing the other. We report how the energy difference between such distorted structure varies with these adsorbates. Furthermore, transition metals induce magnetism and favour the reduction of the symmetry of the charge density distribution.

cond-mat.mtrl-sci↗

arXiv2014

Fe clusters (Fe$_n$, n=1-6) chemisorbed on vacancy defects in graphene: Stability, spin-dipole moment and magnetic anisotropy

In this work, we have studied the chemical and magnetic interactions of Fe$_n$ ; n=1-6 clusters with vacancy defects (monovacancy to correlated vacancies with six missing C atoms) in a graphene sheet by ab-initio density functional calculations combined with Hubbard U corrections for correlated Fe- d electrons. It is found that the vacancy formation energies are lowered in the presence of Fe, indicating an easier destruction of the graphene sheet. Due to strong chemical interactions between Fe clusters and vacancies, a complex distribution of magnetic moments appear on the distorted Fe clusters which results in reduced averaged magnetic moments compared to the free clusters. In addition to that, we have calculated spin-dipole moments and magnetic anisotropy energies. The calculated spin-dipole moments arising from anisotropic spin density distributions, vary between positive and negative values, yielding increased or decreased effective moments. Depending on the cluster geometry, the easy axis of magnetization of the Fe clusters shows in-plane or out-of-plane behavior.

cond-mat.mtrl-sci↗