Despina Louca

The layered transition metal dichalcogenide, TaSe2, exhibits rich electronic phenomena across its polymorphs, 1T, 2H, and 3R, largely driven by differences in atomic coordination and c-axis stacking. In the 1T phase, octahedral coordination and AA stacking promote strong interlayer coupling and stabilize a commensurate charge density wave (CDW) with star-of-David clusters that set in at high temperatures. The 2H phase exhibits trigonal prismatic coordination with AB stacking, and hosts both incommensurate and commensurate CDW phases and weak superconductivity at very low temperatures. The 3R phase, characterized by ABC stacking and trigonal prismatic coordination, exhibits enhanced superconductivity along with CDW order, attributed to modified interlayer hybridization and reduced CDW competition. These stacking-dependent variations in interlayer coupling are critical in tuning correlated states in the dichalcogenides.

Phase change materials (PCMs) are well-known for their reversible and rapid switching between crystalline and amorphous phases through thermal excitations mediated by strong electrical or laser pulses. This crystal-to-amorphous transition is accompanied by a remarkable contrast in optical and electronic properties, making PCMs useful in nonvolatile data storage applications. Here, we combine electrical transport and angle resolved photoemission spectroscopy (ARPES) measurements to study the electronic structure of bulk Ge2Sb2Te5−5xSe5x (GSST) for 0≤x≤0.8, where x represents the amount of Se substituting Te in Ge2Sb2Te5—a prototypical PCM. The single-particle density of states (SDOS) derived from the integrated ARPES data display metallic behavior for all x, as evidenced by the presence of a finite density of states in the vicinity of the chemical potential. Transport measurements also display clear signatures of metallic transport, consistent with the SDOS data. The temperature dependence of the resistance indicates the onset of moderate electron–electron Coulomb interaction effects at low temperatures for x≥0.6. At the same time, the magnetoresistance data show signatures of weak antilocalization for x≥0.6. An analysis of the temperature dependence of the phase coherence length suggests that electron dephasing is primarily due to inelastic electron–electron scattering. We find that these effects are enhanced with increasing x, portraying GSST as a PCM with electronic interactions that can be tuned via chemical doping.

Time-resolved elastic neutron scattering combined with rapid laser heating was used to probe the charge density wave (CDW) state in 1T-TiSe$_2$, capturing both the melting and reformation of the CDW on long timescales and providing clues on the roles of phonons and excitons. With the laser source on, superlattice Bragg peaks such as (-1.5, -1.5, 1.5) observed below the CDW transition due to the new lattice periodicity, dissipate within 5 seconds, at a rate that is much slower than the sample's thermal response to the heat wave propagation. Whereas the electronic ordering associated with the CDW phase is disrupted rapidly by the laser-induced heating, the periodic lattice distortion (PLD) exhibits a markedly slower evolution during the melting process. This delayed suppression of the PLD relative to the thermal response indicates that CDW melting proceeds through a nonthermal pathway, likely linked to the loss of superlattice phonons such as the soft mode at q = (0.5 ,0, 0.5 ).

The ilmenite NiTiO3 consists of a buckled honeycomb lattice, with the Ni spins aligned ferromagnetically in-plane and antiferromagnetically out-of-plane. Using neutron spectroscopy, the magnetic structure and the dynamics were investigated as a function of temperature. Dispersive acoustic bands and nearly dispersionless optical bands at ~3.7 meV are described by a highly anisotropy Heisenberg model with stronger antiferromagnetic (AFM) out-of-plane, weaker ferromagnetic (FM) in-plane interactions and an anisotropy gap of 0.95 meV. The order parameter yields a critical exponent between the Heisenberg and two-dimensional Ising models, consistent with highly anisotropic Heisenberg systems. The frustration parameter ~ 2 supports a weakly frustrated system.