arXiv subjects

Alessandro Soncini

Publications and source records attributed to Alessandro Soncini.

13 recordsLinked to original sources

arXiv2026

Finite-Temperature Toroidal Moment Amenable to Direct Observation in an Fe$_{10}$Dy$_{10}$ Molecular Ring

Single-molecule toroics (SMTs) host closed magnetic-vortex configurations that carry toroidal moments $\boldsymbolτ$, whose electric-dipole symmetry enables magnetoelectric spin control. Yet, opposite toroidal chiralities are degenerate in conventional magnetic fields, making direct detection of molecular toroidal polarisation challenging. Current approaches probe molecular toroidal dynamics only indirectly through weak residual magnetism, leaving direct interrogation of toroidal polarisation an open challenge. Moreover, the survival of toroidal polarization at finite temperature, and realistic preparation-and-readout conditions, have not been quantitatively established. Here we investigate the icosanuclear $3d$--$4f$ molecular ring Fe$_{10}$Dy$_{10}$, featuring a $\sim$62-billion-dimensional low-energy manifold with pervasive toroidal character, rendered computationally tractable via an ab initio-informed transfer-matrix framework with perturbative corrections. Our model reproduces magnetic and calorimetric measurements and reveals a maximally toroidal ground doublet with robust finite-temperature toroidal response. We introduce the toroidal susceptibility $ξ$ as a finite-temperature linear-response function to quantify toroidal polarisation induced by magnetic-field curl. We then develop a preparation-and-detection protocol in which a temporally asymmetric near-infrared waveform generates a cumulative toroidal population imbalance, while an ab initio-informed magnetoelectric tensor predicts an electric-field-induced magnetic moment within $μ$SQUID detectability. These results establish Fe$_{10}$Dy$_{10}$ as a molecular platform where toroidal polarisation can be prepared, accumulated and read out under realistic experimental conditions.

cond-mat.mes-hall↗

arXiv2025

Preparation and coherent manipulation of toroidal moments in molecules

Molecules with an odd number of electrons typically display paramagnetic behaviour in a uniform magnetic field. Single-molecule toroics -- a family of open shell lanthanide complexes -- instead display an unprecedented vanishing magnetization. The anomaly is reconciled by degenerate quantum states where electron spins and orbital currents give rise to time-odd and space-odd magnetic vortices known as toroidal moments, that carry a vanishing magnetic dipole. Resilient to stray magnetic fields and susceptible to electric manipulation, toroidal moments have attracted interest for spintronic, magnonic, and photonic applications. While macroscopic toroidal moments feature in some extended systems, molecular toroidal states have yet to be observed, as it remains unclear how existing experimental set-ups could split degenerate states carrying counter-rotating vortices. We propose a realistic pulsed radiation protocol to polarise and observe molecular toroidal moments in a class of MDy$_6$ (M = Al$^{3+}$, Cr$^{3+}$) molecules with coupled Dy$_3$ toroidal moieties. Three resonant MW-pulses -- delivered sequentially or simultaneously -- selectively and coherently transfers population to a long-lived toroidally-polarised state whose ensuing magneto-electric properties provide a read-out mechanism. Our results provide a strategy to measure and coherently manipulate toroidal states in molecular systems, which is expected to trigger applications of molecular toroidal states to quantum technologies.

cond-mat.mes-hall↗

arXiv2020

Lanthanide-radical magnetic coupling in [LnPc$_2$]$^0$: Competing exchange mechanisms captured via ab initio multi-reference calculations

We present a computational investigation of the intramolecular exchange coupling in [LnPc$_2$]$^0$ (Ln = Tb, Dy, Ho, and Er) between the Ln$^{3+}$ 4f electrons and the spin-1/2 radical on the phthalocyanine ligands. A series of ab initio multi-configurational/multi-reference Complete/Restricted Active Space Self-Consistent-Field calculations (CASSCF/RASSCF), including non-perturbative spin--orbit coupling, were performed on [LnPc$_2$]$^0$ and on the smaller model compound [LnPz$_2$]$^0$. We find that the exchange coupling mechanisms are restricted by symmetry, but also dependent on the spin polarization effect triggered by the Pc$_2$ ligands $π$--$π^*$ excitations. The calculated exchange splittings are small, amounting to at most a few cm$^{-1}$, in disagreement with previous literature reports of strong antiferromagnetic coupling, but in good agreement with recent EPR experiments on [TbPc$_2$]$^0$. Furthermore, the coupling strength is found to decrease from [TbPc$_2$]$^0$ to [ErPc$_2$]$^0$, with decreasing number of unpaired electron spins in the lanthanide ground (Hund's rule) Russell--Saunders term.

cond-mat.str-el↗

arXiv2017

Ferrotoroidic Ground State in a Heterometallic Cr$^{\mathrm{III}}$Dy$^{\mathrm{III}}_6$ Complex Displaying Slow Magnetic Relaxation

Toroidal quantum states are most promising for building quantum computing and information storage devices as they are insensitive to homogeneous magnetic fields, but interact with charge and spin currents, allowing this moment to be manipulated purely by electrical means. Coupling molecular toroids into larger toroidal moments via ferrotoroidic interactions can be pivotal not only to enhance ground state toroidicity, but also to develop materials displaying ferrotoroidic ordered phases, which sustain linear magneto-electric coupling and multiferroic behaviour. However, engineering ferrotoroidic coupling is known to be a challenging task. Here we have isolated a Cr$^{\mathrm{III}}$Dy$^{\mathrm{III}}_6$ complex, which exhibits the much sought-after ferrotoroidic ground state with an enhanced toroidal moment, solely arising from intramolecular dipolar interactions. Moreover, a theoretical analysis of the observed sub-Kelvin zero-field hysteretic spin dynamics of Cr$^{\mathrm{III}}$Dy$^{\mathrm{III}}_6$ reveals the pivotal role played by ferrotoroidic states in slowing down the magnetic relaxation, in spite of large calculated single-ion quantum tunnelling rates.

physics.chem-ph↗

arXiv2017

CERES: An ab initio code dedicated to the calculation of the electronic structure and magnetic properties of lanthanide complexes

We have developed and implemented a new ab initio code, CERES (Computational Emulator of Rare Earth Systems), completely written in C++11, which is dedicated to the efficient calculation of the electronic structure and magnetic properties of the crystal field states arising from the splitting of the ground state spin-orbit multiplet in lanthanide complexes. The new code gains efficiency via an optimised implementation of a direct configurational averaged Hartree-Fock (CAHF) algorithm for the determination of $4f$ quasi-atomic active orbitals common to all multi-electron spin manifolds contributing to the ground spin-orbit multiplet of the lanthanide ion. The new CAHF implementation is based on quasi-Newton convergence acceleration techniques coupled to an efficient library for the direct evaluation of molecular integrals, and problem-specific density matrix guess strategies. After describing the main features of the new code, we compare its efficiency with the current state--of--the--art ab initio strategy to determine crystal field levels and properties, and show that our methodology, as implemented in CERES, represents a more time-efficient computational strategy for the evaluation of the magnetic properties of lanthanide complexes, also allowing a full representation of non-perturbative spin-orbit coupling effects.

cond-mat.str-el↗

arXiv2016

Configuration-averaged 4f orbitals in ab initio calculations of low-lying crystal field levels in lanthanide(III) complexes

A successful and commonly used ab initio method for the calculation of crystal field levels and magnetic anisotropy of lanthanide complexes consists of spin-adapted state-averaged CASSCF calculations followed by state interaction with spin-orbit coupling (SI-SO). Based on two observations valid for Ln(III) complexes, namely: (i) CASSCF 4f orbitals are expected to change very little when optimized for different states belonging to the 4f electronic configuration, (ii) due to strong spin-orbit coupling the total spin is not a good quantum number, we show here via a straightforward analysis and direct calculation that the CASSCF/SI-SO method can be simplified to a single configuration-averaged HF calculation and one complete active space CI diagonalization, including spin-orbit coupling, on determinant basis. Besides its conceptual simplicity, this approach has the advantage that all spin states of the 4f$^n$ configuration are automatically included in the SO coupling, thereby overcoming one of the computational limitations of the existing CASSCF/SI-SO approach. As an example, we consider three isostructural complexes [Ln(acac)$_3$(H$_2$O)$_2$], Ln = Dy$^{3+}$, Ho$^{3+}$, Er$^{3+}$, and find that the proposed simplified method yields crystal field levels and magnetic g-tensors that are in very good agreement with those obtained with CASSCF/SI-SO.

physics.chem-ph↗

arXiv2016

Room temperature chiral discrimination in paramagnetic NMR spectroscopy

A recently proposed theory of chiral discrimination in NMR spectroscopy based on the detection of a molecular electric polarization $\mathbf{P}$ rotating in a plane perpendicular to the NMR magnetic field [A. D. Buckingham, J. Chem. Phys. $\mathbf{140}$, 011103 (2014)], is here generalized to paramagnetic systems. Our theory predicts new contributions to $\mathbf{P}$, varying as the square of the inverse temperature. Ab initio calculations for ten Dy$^{3+}$ complexes, at 293K, show that in strongly anisotropic paramagnetic molecules $\mathbf{P}$ can be more than 1000 times larger than in diamagnetic molecules, making paramagnetic NMR chiral discrimination amenable to room temperature detection.

physics.chem-ph↗

arXiv2014

Dirac cones in the spectrum of bond-decorated graphenes

We present a two-band model based on periodic Hückel theory, which is capable of predicting the existence and position of Dirac cones in the first Brillouin zone of an infinite class of two-dimensional periodic carbon networks, obtained by systematic perturbation of the graphene connectivity by bond decoration, that is by inclusion of arbitrary $π$-electron Hückel networks into each of the three carbon-carbon $π$-bonds within the graphene unit cell. The bond decoration process can fundamentally modify the graphene unit cell and honeycomb connectivity, representing a simple and general way to describe many cases of graphene chemical functionalization of experimental interest, such as graphyne, janusgraphenes and chlorographenes. Exact mathematical conditions for the presence of Dirac cones in the spectrum of the resulting two-dimensional $π$-networks are formulated in terms of the spectral properties of the decorating graphs. Our method predicts the existence of Dirac cones in experimentally characterized janusgraphenes and chlorographenes, recently speculated on the basis of DFT calculations. For these cases, our approach provides a proof of the existence of Dirac cones, and can be carried out at the cost of a back of the envelope calculation, bypassing any diagonalization step, even within Hückel theory.

cond-mat.mes-hall↗

arXiv2014

Complete spectrum of the infinite-$U$ Hubbard ring using group theory

We present a full analytical solution of the multiconfigurational strongly-correlated mixed-valence problem corresponding to the $N$-Hubbard ring filled with $N-1$ electrons, and infinite on-site repulsion. While the eigenvalues and the eigenstates of the model are known already, analytical determination of their degeneracy is presented here for the first time. The full solution, including degeneracy count, is achieved for each spin configuration by mapping the Hubbard model into a set of Huckel-annulene problems for rings of variable size. The number and size of these effective Huckel annulenes, both crucial to obtain Hubbard states and their degeneracy, are determined by solving a well-known combinatorial enumeration problem, the necklace problem for $N-1$ beads and two colors, within each subgroup of the $C_{N-1}$ permutation group. Symmetry-adapted solution of the necklace enumeration problem is finally achieved by means of the subduction of coset representation technique [S. Fujita, Theor. Chim. Acta 76, 247 (1989)], which provides a general and elegant strategy to solve the one-hole infinite-$U$ Hubbard problem, including degeneracy count, for any ring size. The proposed group theoretical strategy to solve the infinite-$U$ Hubbard problem for $N-1$ electrons, is easily generalized to the case of arbitrary electron count $L$, by analyzing the permutation group $C_L$ and all its subgroups.

cond-mat.str-el↗

arXiv2012

Paramagnetic NMR chemical shift in a spin state subject to zero-field splitting

We derive a general formula for the paramagnetic NMR nuclear shielding tensor of an open-shell molecule in a pure spin state, subject to a zero-field splitting (ZFS). Our findings are in contradiction with a previous proposal. We present a simple application of the newly derived formula to the case of a triplet ground state split by an easy-plane ZFS spin Hamiltonian. When $kT$ is much smaller than the ZFS gap, thus a single non-degenerate level is thermally populated, our approach correctly predicts a temperature-independent paramagnetic shift, while the previous theory leads to a Curie temperature dependence.

physics.chem-ph↗

arXiv2012

Theory of NMR chemical shift in an electronic state with arbitrary degeneracy

We present a theory of nuclear magnetic resonance (NMR) shielding tensors for electronic states with arbitrary degeneracy. The shieldings are here expressed in terms of generalized Zeeman ($g^{(k)}$) and hyperfine ($A^{(k)}$) tensors, of all ranks $k$ allowed by the size of degeneracy. Contrary to recent proposals [T. O. Pennanen and J. Vaara, Phys. Rev. Lett. 100, 133002 (2008)], our theory is valid in the strong spin-orbit coupling limit. Ab initio calculations for the 4-fold degenerate $Γ_8$ ground state of lanthanide-doped fluorite crystals CaF$_2$:Ln (Ln = Pr$^{2+}$, Nd$^{3+}$, Sm$^{3+}$, and Dy$^{3+}$) show that previously neglected contributions can account for more than 50% of the paramagnetic shift.

physics.chem-ph↗

arXiv2012

NMR chemical shift as analytical derivative of the Helmholtz free energy

We present a theory for the temperature-dependent nuclear magnetic shielding tensor of molecules with arbitrary electronic structure. The theory is a generalization of Ramsey's theory for closed-shell molecules. The shielding tensor is defined as a second derivative of the Helmholtz free energy of the electron system in equilibrium with the applied magnetic field and the nuclear magnetic moments. This derivative is analytically evaluated and expressed as a sum over states formula. Special consideration is given to a system with an isolated degenerate ground state for which the size of the degeneracy and the composition of the wave functions are arbitrary. In this case the paramagnetic part of the shielding tensor is expressed in terms of the $g$ and $A$ tensors of the EPR spin Hamiltonian of the degenerate state. As an illustration of the proposed theory, we provide an explicit formula for the paramagnetic shift of the central lanthanide ion in endofullerenes Ln@C$_{60}$, with Ln=Ce$^{3+}$, Nd$^{3+}$, Sm$^{3+}$, Dy$^{3+}$, Er$^{3+}$ and Yb$^{3+}$, where the ground state can be a strongly spin-orbit coupled icosahedral sextet for which the paramagnetic shift cannot be described by previous theories.

physics.chem-ph↗

arXiv2009

Molecular spintronics using noncollinear magnetic molecules

We investigate the spin transport through strongly anisotropic noncollinear magnetic molecules and find that the noncollinear magnetization acts as a spin-switching device for the current. Moreover, spin currents are shown to offer a viable route to selectively prepare the molecular device in one of two degenerate noncollinear magnetic states. Spin-currents can be also used to create a non-zero density of toroidal magnetization in a recently characterized Dy_3 noncollinear magnet.

cond-mat.mes-hall↗