arXiv subjects

Alessandro Stroppa

Publications and source records attributed to Alessandro Stroppa.

At least 19 recordsLinked to original sources

arXiv2026

Switchable Altermagnetism via Spin-Induced Improper Polarization

Enabling reversible spin-splitting switching in stray-field-free altermagnets is promising for spintronic applications, but currently limited to a narrow class of polar materials. We propose a broader approach based on spin-induced improper polarization in nonpolar dual-sublattice magnets. We demonstrate this mechanism in DyFeO3, where the product of nonpolar Fe and Dy spin modes transforms as an induced polar mode. Density functional theory shows that the relative Dy--Fe spin alignment selects the polarization, while the Fe sublattice controls nonrelativistic spin splitting, thus enabling reversible switching. These results establish spin-induced improper polarization as a route to switchable altermagnetism in nonpolar bulk systems.

cond-mat.mtrl-sci↗

arXiv2026

Coupling Chirality, Polar Order, and Altermagnetic Spin Splitting in a Hybrid Manganese Chloride

Hybrid manganese halides enable the coexistence of molecular chirality, polar order, and magnetic exchange within a single lattice. Here, we combine first-principles calculations with spin-space-group analysis to investigate the synthesized enantiomeric pair [(R)/(S)-MPA]2[MnCl4(H2O)] (MPA = beta-methylphenethylammonium). We predict that its compensated magnetic state hosts altermagnetic spin splitting in the nonrelativistic limit, and that the coupled chiral, polar, and magnetic degrees of freedom define a symmetry-related manifold. From this manifold, we derive simple sign rules for the electronic and magneto-optical response: reversing both chirality and polarity, or reversing the magnetic domain alone, inverts the spin splitting throughout the Brillouin zone, whereas reversing chirality alone or polarity alone changes the spin-splitting sign only in symmetry-selected regions. With spin-orbit coupling, reversing chirality or magnetic order flips the Kerr rotation angle, while changing the polar variant leaves it unchanged. These results reveal a chemically accessible route to translate molecular handedness into symmetry-controlled spin splitting and magneto-optical readout in hybrid manganese halides. Critically, we show that the sign and momentum pattern of the splitting are governed by the interplay of the chiral, polar, and magnetic degrees of freedom. This interplay opens the possibility to control the spin splitting through a judicious design of the organic cations, by modulating their chirality and polarity.

cond-mat.mtrl-sci↗

arXiv2026

Weak Polar Optical Phonon Scattering Decouples Electron and Phonon Transport in Layered Thermoelectric Materials

High-performance thermoelectric (TE) materials are crucial for efficient waste-heat recovery and solid-state cooling technologies. A persistent challenge in TE materials design arises from the strong interdependence among the electrical conductivity ($\sigma$), Seebeck coefficient ($S$), and lattice thermal conductivity ($\kappa_{\mathrm{L}}$). Layered compounds can effectively suppress $\kappa_{\mathrm{L}}$ along the cross-plane direction owing to weak interlayer interactions; however, they often suffer from low carrier mobility ($\mu$) caused by limited band dispersion and strong polar optical phonon (POP) scattering. Here, we perform high-throughput density functional theory calculations to screen 236 layered semiconductors and identify candidates with low effective mass ($m^{*}$) and weak POP scattering. We identify 23 compounds with high cross-plane $\mu$, among which 14 exhibit large power factors ($S^{2}\sigma$). Notably, GaGe$_{2}$Te stands out with exceptionally high cross-plane $\sigma$ and power factor, enabled by a favorable combination of small $m^{*}$ and a small ionic dielectric constant. Simultaneously, GaGe$_{2}$Te exhibits an ultralow cross-plane $\kappa_{\mathrm{L}}$ of 0.57~W~m$^{-1}$~K$^{-1}$ at 300~K, originating from weak interlayer bonding and pronounced phonon anharmonicity. These results demonstrate an effective strategy to decouple electron and phonon transport in layered materials by mitigating POP scattering, thereby providing a promising pathway toward high-performance thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2026

A Relationship between the Molecular Parity-Violation Energy and the Electronic Chirality Measure

When the weak-forces producing parity-violating effects are taken into account, there is a tiny energy difference between the total electronic energies of two enantiomers ($\Delta E_{PV}$), which might be the key to understand the evolution of the biological homochirality. We focus on the electronic chirality measure ($ECM$), a powerful descriptor based on the electronic charge density, for quantifying the chirality degree of a molecule, for a representative set of chiral molecules, together with their E$_{PV}$ energies. Our results show a novel, strong and \textit{positive} correlation between $\Delta E_{PV}$ and $ECM$, supporting a subtle interplay between the weak-forces acting within the nuclei of a given molecule and its chirality. These findings suggest that experimental investigations for molecular parity violation detection should consider molecules with as large $ECM$ values as possible, and may support that a chiral signature is imprinted on life by fundamental physics via the parity-violating weak interactions.

physics.chem-ph↗

arXiv2026

Mixed Organic Cation in Chiral Two-Dimensional Organic-Inorganic Hybrid Metal Halides: An ab-initio Study of Nonlinear Optical (NLO) Properties

The mixing of organic cations represents yet another direction to explore in the field of chiral organic-inorganic hybrid metal halides (OIHMH). Here, we perform structural optimizations, electronic structures, and non-linear optical (NLO) studies using the density functional theory of two recently synthesized chiral OIHMHs, [R-MePEA][C3A]PbBr4, and [R-MePEA][C4A]PbBr4, with mixed chiral arylammonium and achiral alkylammonium cations. We find that the noncovalent weak interactions (e.g. Br...NH interactions) play an important role in the formation of these OIHMHs. Our study further indicates that the two non-centrosymmetric compounds exhibit relative wide bandgaps (~3.5 eV), strong second harmonic generation (SHG) responses (~0.5-1.5*KDP), and moderate birefringence (~0.088), indicating possible applications NLO materials. Atom response theory analysis reveals that the SHG responses are determined mainly by the occupied Br 3p non-bonding orbitals as well as by the unoccupied Pb 5p orbitals which shows the important contribution of the inorganic PbBr4 layer to the nonlinear optical properties.

cond-mat.mtrl-sci↗

arXiv2025

Optical Spin Effects Induced by Phase Conjugation at a Space-Time Interface

Electromagnetic temporal boundaries, emerging when the constitutive parameters of a medium undergo abrupt temporal variations, have garnered significant interest for their role in facilitating unconventional wave phenomena and enabling sophisticated field manipulations. A key manifestation is temporal reflection in an unbounded spatial domain, where a sudden temporal discontinuity induces phase-conjugated backward waves alongside anomalous spin conversion. This study explores distinctive spin-conversion dynamics at a time-dependent spatial interface governed by Lorentz-type dispersion, in which the plasma frequency undergoes rapid modulation over time. The interaction of a circularly polarized wave with a space-time interface excites electromagnetic signals at the system's natural resonance, allowing precise control over polarization states. The scattered field stems from the combined influence of temporal and spatial boundaries, yielding a superposition of the original incident wave's polarization and its phase-conjugated counterpart.

physics.optics↗

arXiv2025

Generating ferro-spinetic polarizations in altermagnetic insulators

Altermagnets are a novel class of fully spin-compensated magnetic materials that nevertheless have spin-split electronic bands, offering novel perspectives for spintronics applications. Based on a rigorous analysis of altermagnetic many-body models and their symmetry we establish the important role of two fundamental types of polarizations in altermagnetic insulators: the charge and the spinetic one, where the latter corresponds to a macroscopic spin-displacement field. First principles calculations confirm and quantify their presence in real materials. The two polarizations are directly coupled and emerge in orthogonal directions when inversion symmetry is broken, either by the system developing a spontaneously ferroelectric polarization (in ferroelectric altermagnets), or by a charge displacement induced by an external electric field (for inversion invariant altermagnetic insulators). This presence of large and switchable spin accumulations constitute an attractive fundamental feature of altermagnetic insulators.

cond-mat.str-el↗

arXiv2025

A route to fully-compensated ferrimagnetic metal: electric-field annihilation of the bilayer bandgap

Fully-compensated ferrimagnet has garnered widespread attention due to its zero-net total magnetic moment and non-relativistic global spin splitting. In general, for a fully-compensated ferrimagnet, at least one spin channel should be gapped to ensure a zero-net total magnetic moment, which would lead to a fully-compensated ferrimagnetic (FC-FIM) semiconductor or half-metal, and appears to limit the existence of an FC-FIM metal. Here we propose that an FC-FIM metal can be achieved by electrically closing the gap of a bilayer system. Using two-dimensional (2D) ferromagnetic (FM) semiconductor as building block, we examine both FM and antiferromagnetic (AFM) interlayer couplings and distinguish unipolar magnetic semiconductor (UMS) and bipolar magnetic semiconductor (BMS) monolayers. It is concluded that an electric field can annihilate the bilayer gap and realize the FC-FIM metal only when the interlayer coupling is AFM and the building block is a UMS. Our scheme for realizing an FC-FIM metal can be generalized to electrically tuned 2D spin-degenerate metal with spin-layer locking. Using first-principles calculations, we have validated our proposal by taking bilayer MnOF, bilayer $\mathrm{ScI_2}$ and monolayer $\mathrm{Hf_2S}$ as examples. Our work offers an alternative route to realize the originally forbidden FC-FIM metal, paving the way for further exploration of FC-FIM metal.

cond-mat.mtrl-sci↗

arXiv2025

Unconventional Magnetism, Sliding Ferroelectricity, and Magneto-Optical Kerr Effects in a Multiferroic Bilayer

Antiferromagnetic (AFM) materials offer a promising platform for exploring novel couplings between altermagnetic (AM) spin-splitting and magneto-optical Kerr effect (MOKE), with potential applications in next-generation quantum technologies. In this work, first-principles calculations, symmetry analysis, and kp modeling are employed to demonstrate how interlayer sliding in AFM multiferroic bilayers enables engineering of the electronic, magnetic, and magneto-optical properties. This study reveals an unprecedented dimension-driven AM crossover, where the 2D paraelectric (PE) bilayer exhibits spin-degenerate bands protected by the [C2||Mc] spin-space symmetry, while the 3D counterpart manifests AM spin-splitting along kz not equal to 0 paths. Furthermore, interlayer sliding breaks the Mc symmetry and stabilizes a ferroelectric (FE) state characterized by compensated ferrimagnetism and a Zeeman effect, which produces non-relativistic spin-split bands. In the FE phase, the inclusion of spin-orbit coupling (SOC) lifts accidental degeneracies, creating `alternating' spin-polarized bands due to the interplay of Zeeman and Rashba effects. Crucially, the spin polarization, ferro-valley polarization, and Kerr angle are simultaneously reversible by switching either interlayer sliding or the Neel vector. These findings highlight the rich coupling between electronic, magnetic, and optical orders in sliding multiferroics, thereby paving the way for ultra-low-power spintronics and optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

First-principles prediction of altermagnetism in transition metal graphite intercalation compounds

We report the emergence of altermagnetism, a magnetic phase characterized by the coexistence of compensated spin ordering and momentum-dependent spin splitting, in graphite intercalation compounds (GICs), a prototypical material system long investigated for its tunable electronic and structural properties. Through first-principles calculations, we demonstrate that vanadium-intercalated stage-1 graphite compounds, exhibit inherent altermagnetic properties. The hexagonal crystal system and antiferromagnetic ordering of V atoms generate a magnetic space group that enforces alternating spin polarization in momentum space while maintaining zero net magnetization. The calculated band structure reveals robust altermagnetic signatures: along the high-symmetry direction, we observe a pronounced spin splitting of ~270 meV with alternating spin polarization. Crucially, the spin splitting exhibits minimal sensitivity to spin-orbit coupling (SOC) effect, highlighting the dominance of exchange interactions over relativistic effects. From Monte Carlo simulations, we predict a magnetic transition temperature ($T_m$ ) of ~228 K, indicating stable magnetic ordering above liquid nitrogen temperatures. The combination of symmetry-protected spin textures, SOC-independent splitting, and elevated $T_m$ temperature makes V-GICs as a promising candidate for spintronic applications, particularly for zero-field spin-polarized current generation and topologically robust spin transport. As the first demonstration of carbon-based alternating magnetic systems, this work offers a design paradigm for engineering spin-polarized quantum states governed by crystalline symmetry constraints.

cond-mat.mtrl-sci↗

arXiv2025

Multiferroic nematic d-wave altermagnetism driven by orbital-order on the honeycomb lattice

Altermagnets provide promising platforms for unconventional magnetism, whose controllability would enable a whole new generation of spintronic devices. While a variety of bulk altermagnets have been discovered, altermagnetism in two-dimensional van der Waals materials has remained elusive. Here we demonstrate that the strained honeycomb monolayer VCl$_{3}$ is an orbital-order-driven ferroelectric altermagnet, exhibiting a significant and switchable spin-splitting. By using low-energy Hamiltonian and first-principles methods in combination with symmetry analysis, we reveal a unique anti-ferro-orbital-antiferromagnetic phase characterized by a 2D nematic $d$-wave altermagnetic spin splitting, tightly coupled with an orbital-ordered induced ferroelectric polarization. Finally, through symmetry mode analysis, we investigate how structural distortions favor the intricate interplay between orbital, altermagnetic, and ferroelectric degrees of freedom. Our study identifies VCl$_3$ as a prototypical 2D orbital-order-driven multiferroic altermagnet on the honeycomb lattice, establishing a van der Waals monolayer featuring altermagnetic ferroelectricity.

cond-mat.mtrl-sci↗

arXiv2024

Ferroelectric switchable altermagnetism

We propose a novel ferroelectric switchable altermagnetism effect, the reversal of ferroelectric polarization is coupled to the switching of altermagnetic spin splitting. We demonstrate the design principles for the ferroelectric altermagnets and the additional symmetry constraints necessary for switching the altermagnetic spin splitting through flipping the electric polarization based on the state-of-the-art spin-group symmetry techniques. 22 ferroelectric altermagnets are found by screening through the 2001 experimental reported magnetic structures in the MAGNDATA database and 2 of them are identified as ferroelectric switchable altermagnets. Using the hybrid improper ferroelectric material [C(NH2)3]Cr(HCOO)3 as an example, we show how the altermagnetic spin splitting is tightly coupled to the ferroelectric polarization, providing an ideal platform for designing electric-field-controllable multiferroic devices. Finally, we find that such manipulation of altermagnetism can be detected by monitoring the physical quantities that are related to the non-vanishing Berry curvature dipole, such as the linearly polarized photogalvanic spin current.

cond-mat.mtrl-sci↗

arXiv2024

Ge-based Clinopyroxene series: first principles and experimental local probe study

The structural and electronic properties of the CaMnGe$_2$O$_6$ and SrMnGe$_2$O$_6$ clinopyroxene systems have been investigated by means of perturbed angular correlation (PAC) measurements, performed at ISOLDE, combined with $ab-initio$ electronic structure calculations within the density functional theory (DFT) framework. The partial density of states (PDOS) of the CaMnGe$_2$O$_6$ and SrMnGe$_2$O$_6$ stable compounds has been determined, and it has been observed that the requirement of including an on-site Hubbard-$U$ potential was necessary in order to describe the highly correlated Mn $3d$-states. By considering $U_{eff}$=4 eV, we obtained a band gap width of 1.82 eV and 1.70 eV, for the CaMnGe$_2$O$_6$ and SrMnGe$_2$O$_6$, respectively. Combining electric field gradient (EFG) first principles calculations, using a supercell scheme, with experimental PAC results, we were able to infer that the Cd probe can replace either the $A$ (Ca, Sr) or the Mn sites in the crystalline structures. We also showed that Cd substitution is expected to lead to a reduction in the width of the band gap in these systems, evidencing opportunities for potential band-gap engineering.

cond-mat.mtrl-sci↗

arXiv2023

Magneto-optical Kerr effect in ferroelectric antiferromagnetic two-dimensional heterostructures

We study the magneto-optical Kerr effect (MOKE) of the two-dimensional heterostructure CrI3/In2Se3/CrI3 by using density functional theory calculations and symmetry analysis. The spontaneous polarization in the In2Se3 ferroelectric layer and the antiferromagnetic ordering in CrI3 layers break the mirror symmetry and the timereversal symmetry, thus activating MOKE. We show that the Kerr angle can be switched by either the polarization or the antiferromagnetic order parameter. Our results suggest that ferroelectric and antiferromagnetic 2D heterostructures could be exploited for ultra-compact information storage devices, where the information is encoded by the two ferroelectric or the two time-reversed antiferromagnetic states, and the read-out performed optically by MOKE.

cond-mat.mtrl-sci↗

arXiv2022

Activating magnetoelectric optical properties by twisting antiferromagnetic bilayers

Twisting in bilayers introduces structural chirality with two enantiomers, i.e., left- and right-hand bilayers, depending on the oriented twist angle. The interplay between this global chirality and additional degrees of freedom, such as magnetic ordering and the local octahedral chirality arising from the geometry of the bonds, can yield striking phenomena. In this work, we focus on collinear antiferromagnetic CrI$_3$ twisted homo-bilayers, which are characterized by a staggered octahedral chirality in each monolayer. Using symmetry analysis, density functional theory and tight-binding model calculations we show that layers twisting can lower the structural and magnetic point-group symmetries, thus activating pyroelectricity and the magneto-optical Kerr effect, which would otherwise be absent in untwisted antiferromagnetic homo-bilayers. Interestingly, both electric polarization and Kerr angle are controllable by the twist angle and their sign is reversed when switching from left- to right-twisted bilayers. We further unveil the occurrence of unconventional vortices with spin textures that alternate opposite chiralities in momentum space. These findings demonstrate that the interplay between twisting and octahedral chirality in magnetic bilayers and related van der Waals heterostructures represents an extraordinary resource for tailoring their physical properties for spintronic and optoelectronic applications.

cond-mat.mes-hall↗

arXiv2022

Ferroelectricity and topological vortices from molecular ordering in metal-organic frameworks

Metal-organic frameworks comprehend a wide class of hybrid organic-inorganic materials with general structure A$_m$BX$_n$, with $A$ and $X$ being organic molecules and B a metal cation. This often results in enhanced structural flexibility and new functionalities. Hybrid perovskites ABX$_3$ are a well-known example.} In an Iron-based perovskites, (DMA)Fe^{II-III}(COOH)_3, dimethylammonium (DMA) molecules are organized in a hexagonal structure. They are orientationally disordered at high temperatures, but order at around $T=100$~K in a peculiar toroidal pattern. Recent experimental and theoretical study suggest the appearance of ferroelectric polarization in this phase, although the measured polarization is small, and the mechanism of ferroelectricity is still debated. We formulate a Landau-type theory that clarifies the connection between the electric polarization, molecular pattern, and distortive modes of the inorganic lattice. We find a remarkable mechanism of improper ferroelectricity, analogue to the trimerization process in inorganic hexagonal ferrites and manganites, but here driven by the ordering of organic molecules in a metal-organic framework. Our study reveals an extremely rich phase diagram with the prediction of topological domain walls, where the ferroelectricity arise from tripling the unit cells due to molecular ordering. Wide domain walls with inner structure are predicted.

cond-mat.mtrl-sci↗

arXiv2021

Whirling interlayer fields as a source of stable topological order in moir\'e CrI3

The moir\'e engineering of two-dimensional magnets opens unprecedented opportunities to design novel magnetic states with promises for spintronic device applications. The possibility of stabilizing skyrmions in these materials without chiral spin-orbit couplings or dipolar interactions is yet to be explored. Here, we investigate the formation and control of ground state topological spin textures (TSTs) in moir\'e CrI3 using stochastic Landau-Lifshitz-Gilbert simulations. We unveil the emergence of interlayer vortex and antivortex Heisenberg exchange fields, stabilizing spontaneous and field-assisted ground state TSTs with various topologies. The developed study accounts for the full bilayer spin dynamics, thermal fluctuations, and intrinsic spin-orbit couplings. By examining the effect of the Kitaev interaction and the next nearest-neighbor Dzyaloshinskii-Moriya interaction, we propose the latter as the unique spin-orbit coupling mechanism compatible with experiments on monolayer and twisted CrI3. Our findings contribute to the current knowledge about moir\'e skyrmionics and uncover the nature of spin-orbit coupling in CrI3.

cond-mat.mtrl-sci↗

arXiv2021

2D hybrid CrCl2(N2C4H4)2 with tunable ferromagnetic half-metallicity

Two-dimensional ferromagnetic (2D FM) half-metal holds great potential for quantum magnetoelectronics and spintronic devices. Here, using density functional calculations and magnetic pictures, we study the electronic structure and magnetic properties of the novel van der Waals (vdW) metal-organic framework (MOF), CrCl2(N2C4H4)2, i.e. CrCl2(pyrazine)2. Our results show that CrCl2(pyrazine)2 is a 2D FM half-metal, having a strong intralayer FM coupling but a much weak interlayer one due to the vdW spacing. Its spin-polarized conduction bands are formed by the pyrazine molecular orbitals and are polarized by the robust Cr3+ local spin = 3/2. These results agree with the recent experiments [Pedersen et al., Nature Chemistry, 2018, 10, 1056]. More interestingly, CrCl2(pyrazine)2 monolayer has a strong doping tunability of the FM half-metallicity, and the FM coupling would be significantly enhanced by electron doping. Our work highlights a vital role of the organic ligand and suggests that vdW MOF is also worth exploration for new 2D magnetic materials.

cond-mat.mtrl-sci↗