Enrico Di Lucente

The interactions among lattice, charge, and spin degrees of freedom fundamentally shape material properties. In FeSe, symmetry-breaking perturbations serve as highly sensitive probes of these couplings. Previous work has shown that defects and isoelectronic substitution can substantially alter these interactions, giving rise to additional phonon modes. In this study, uniaxial strain is employed as a tunable symmetry-breaking control parameter to probe the intrinsic lattice response in the absence of disorder. The temperature evolution of phonon excitations was examined with a temperature resolution of 3 K in the vicinity of the nemato-structural transition temperature T s , under strain applied along the 〈 110 〉 and 〈 100 〉 crystallographic directions. A subtle asymmetry of the A 1 g p h mode appears in the unstrained crystal within a narrow temperature window around T s , originating from the emergence of an additional mode in the fully symmetric channel. With applied strain, this feature becomes more distinctly resolved. The anomaly is attributed to modifications of the coupling between lattice and electronic degrees of freedom driven by the ordering fluctuations right above the nematic transition. These fluctuations may enhance the susceptibility of the electron-lattice system and enable two-phonon Raman scattering involving the acoustic mode near the X and R points. The presence of this two-phonon scattering depends on both the strength and the direction of the applied strain, indicating a high sensitivity of FeSe to local symmetry breaking.

Among all materials, monocrystalline diamond has one of the highest measured thermal conductivities, with values above 2000 W/m/K at room temperature. Diamond nanostructures are increasingly used in photonics, electronics, and quantum technologies, where heat dissipation is critical. However, predicting nanoscale heat flow is made difficult by the predominance of momentum-conserving "normal" phonon-phonon scattering processes. Here, we use dilute nitrogen-vacancy color centers as in situ, highly precise spin defect thermometers to image temperature inhomogeneities in single-crystal diamond microstructures heated from ambient conditions. We analyze cantilevers with cross sections in the range from about 0.2 to 2.6 μm^{2}, observing a strong reduction in the cantilevers' conductivity as width decreases. We use first-principles simulations based on the linearized phonon Boltzmann transport equation and viscous heat equations to quantitatively predict the cantilevers' thermal transport properties, rationalizing how the interplay between intrinsic and extrinsic phonon scattering mechanisms determines the observed nondiffusive behavior. Our temperature-imaging method paves the way for the exploration of unconventional, nondiffusive heat-transport phenomena in devices and nanostructures of arbitrary geometries.

Skutterudites are promising materials for thermoelectric and spintronics applications. Here, we explore spin fluctuations in the FeSb 3 skutterudite and their effect on its electronic structure using Hubbard-corrected density-functional theory calculations. We identify multiple magnetic- and charge-disproportionated configurations, with an antiferromagnetic metallic ground state. Paramagnetic fluctuations modeled through a special quasirandom spin structure open a 61 meV gap, consistent with experiments. This state features nondegenerate spin channels and band-avoided crossings, resembling a Luttinger-compensated ferrimagnet. Mapping the electronic structure to a Heisenberg Hamiltonian fails to explain the low Néel temperature ( ≲ 10 K ), suggesting that factors such as stoichiometry and magnetic exchange frustration may play an important role, calling for more detailed experimental investigations.

Recent experiments have provided compelling and renewed interest in phonon hydrodynamics. At variance with ordinary diffusive heat transport, this regime is primarily governed by momentum-conserving phonon collisions. At the mesoscopic scale it can be described by the viscous heat equations (VHEs), that resemble the Navier-Stokes equations (NSEs) in the laminar regime. Here, we show that the VHEs can be separated and recast as modified biharmonic equations in the velocity potential and stream function-solvable analytically. These two can be merged into a complex potential defining the flow streamlines, and give rise to two distinct temperature contributions, ultimately related to thermal compressibility and vorticity. The irrotational and incompressible limits of the phonon VHEs are analyzed, showing how the latter mirror the NSEs for the electron fluid. This Letter also extends to the electron compressible regime that arises when drift velocities can be higher than plasmonic velocities. Finally, by examining thermal flow within a 2D graphite strip device, we explore the boundary conditions and transport coefficients needed to observe thermal vortices and negative thermal resistance, or heat backflow from cooler to warmer regions. This Letter provides analytical tools to design hydrodynamic phonon and electron flows, offering a foundational framework to understand and guide experiments on nonstandard transport, plasmonics, and collective excitations in microscale systems.

Magnetic materials can display many solutions to the electronic-structure problem, corresponding to different local or global minima of the energy functional. In Hartree-Fock or density-functional theory different single-determinant solutions lead to different magnetizations, ionic oxidation states, hybridizations, and inter-site magnetic couplings. The vast majority of these states can be fingerprinted through their projection on the atomic orbitals of the magnetic ions. We have devised an approach that provides an effective control over these occupation matrices, allowing us to systematically explore the landscape of the potential energy surface. We showcase the emergence of a complex zoology of self-consistent states; even more so when semi-local density-functional theory is augmented - and typically made more accurate - by Hubbard corrections. Such extensive explorations allow to robustly identify the ground state of magnetic systems, and to assess the accuracy (or not) of current functionals and approximations.

Magnetic materials can display many solutions to the electronic-structure problem, corresponding to different local or global minima of the energy functional. In Hartree-Fock or density-functional theory different single-determinant solutions lead to different magnetizations, ionic oxidation states, hybridizations, and inter-site magnetic couplings. The vast majority of these states can be fingerprinted through their projection on the atomic orbitals of the magnetic ions. We have devised an approach that provides an effective control over these occupation matrices, allowing us to systematically explore the landscape of the potential energy surface. We showcase the emergence of a complex zoology of self-consistent states; even more so when semi-local density-functional theory is augmented - and typically made more accurate - by Hubbard corrections. Such extensive explorations allow to robustly identify the ground state of magnetic systems, and to assess the accuracy (or not) of current functionals and approximations.

Skutterudites are crystals with a cagelike structure that can be augmented with filler atoms (“rattlers”), usually leading to a reduction in thermal conductivity that can be exploited for thermoelectric applications. Here, we leverage the recently introduced Wigner formulation of thermal transport to elucidate the microscopic physics underlying heat conduction in skutterudites, showing that filler atoms can drive a crossover from the Boltzmann to the Wigner regimes of thermal transport, i.e., from particlelike conduction to wavelike tunneling. At temperatures where the thermoelectric efficiency of skutterudites is largest, wavelike tunneling can become comparable to particlelike propagation. We define a Boltzmann deviation descriptor able to differentiate the two regimes and relate the competition between the two mechanisms to the materials' chemistry, providing a design strategy to select rattlers and identify optimal compositions for thermoelectric applications.