arXiv subjects

Federico Pisani

Publications and source records attributed to Federico Pisani.

4 recordsLinked to original sources

arXiv2026

Probing the real-space density of spin-entangled electrons

On the textbook example of an isolated antiferromagnetic Heisenberg dimer, we demonstrate that the magnetic form factor and the magnetic electron density distribution can be extracted from the momentum-dependence of the inelastic neutron scattering (INS) intensity of a magnetic excitation. We measure the three-dimensional (3D) magnetic structure factor of the singlet-to-triplet excitation in Cu(II) acetate monohydrate with INS. Using a minimal parametrization of the magnetic electron density, we deduce the real-space density of the spin-entangled electrons and the transfer of magnetic electron density between metal and ligand atoms from the experimental data. Density functional theory (DFT) calculations reproduce the measured structure factor quantitatively, providing a direct validation of DFT broken-symmetry spin densities against full 3D INS data. The quantitative agreement between experiment, parametrization, and theory establishes a robust framework for determining magnetic form factors and the magnetic electron density in a broad range of magnetic materials and demonstrates INS as a probe of the envelope of spatial electronic wavefunctions.

cond-mat.str-el↗

arXiv2026

Coupling between CaWO$_4$ phonons and Er$^{3+}$ dopants

We investigate the lattice dynamics of CaWO$_4$, a promising host crystal for erbium-based quantum memories, using inelastic neutron scattering together with density-functional perturbation theory. The measured phonon dispersion along the (100), (001), and (101) reciprocal space direction reveals phonon bands extending up to 130 meV, with a gap between 60 and 80 meV, in good agreement with our calculations. From a symmetry analysis of the phonon eigenmodes, we identify eight Raman-active modes that can couple directly to the Er$^{3+}$ crystal-field operators, including a low-energy $B_g$ mode at 9.1 meV that is expected to play a dominant role in phonon-assisted spin-lattice relaxation. These results provide a microscopic description of the phonon bath in CaWO$_4$ and establish a basis for engineering phononic environments to mitigate the loss of stored quantum states and optimize Er-doped CaWO$_4$ for quantum-memory applications.

cond-mat.mtrl-sci↗

arXiv2024

Persistent quantum vibronic dynamics in a $5d^1$ double perovskite oxide

Quantum entanglement between the spin, orbital, and lattice degrees of freedom in condensed matter systems can emerge due to an interplay between spin-orbit and vibronic interactions. Heavy transition metal ions decorated on a face-centered cubic lattice, for example, in $5d^1$ double perovskites, are particularly suited to support these quantum entangled states, but direct evidence has not yet been presented. In this work, we report additional peaks in the low-energy spectra of a $5d^1$ double perovskite, Ba$_2$CaReO$_6$, which cannot be explained by adopting a purely classical description of lattice vibrations. Instead, our theoretical analysis demonstrates that these spectroscopic signatures are characteristic of orbital-lattice entangled states in Ba$_2$CaReO$_6$. Crucially, both theory and experiment demonstrate that these quantum-entangled states persist to low temperatures, despite the onset of multipolar order.

cond-mat.str-el↗

arXiv2024

Dynamic Jahn-Teller effect in the strong spin-orbit coupling regime

Exotic quantum phases, arising from a complex interplay of charge, spin, lattice and orbital degrees of freedom, are of immense interest to a wide research community. A well-known example of such an entangled behavior is the Jahn-Teller effect, where the lifting of orbital degeneracy proceeds through lattice distortions, often accompanied by ordering of spins and metal-insulator transitions. Static distortions, including cooperative behavior, have been associated with colossal magneto-resistance, multiferroicity, high-$T_\mathrm{C}$ superconductivity and other correlated phenomena. Realizations of the dynamic Jahn-Teller effect, on the other hand, are scarce since the preservation of vibronic symmetries requires subtle tuning of the local environment. Here we demonstrate that a highly-symmetrical 5d$^1$ double perovskite Ba$_2$MgReO$_6$, comprising of a 3D array of isolated ReO$_6$ octahedra, fulfils these requirements, resulting in a unique case of a dynamic Jahn-Teller system with strong spin-orbit coupling. Thermodynamic and resonant inelastic x-ray scattering experiments undoubtedly show that the Jahn-Teller instability leads to a ground-state doublet, invoking a paradigm shift for this family of compounds. The restoration of vibronic degrees of freedom arises from a quantum-mechanical zero-point motion, as revealed by detailed quantum chemistry calculations. The dynamic state of ReO$_6$ octahedra persists down to the lowest temperatures, where a multipolar order sets in, allowing for investigations of the interplay between a dynamic JT effect and strongly correlated electron behavior.

cond-mat.str-el↗