Fernando de Juan

Signatures of unconventional superconductivity have been recently observed in certain transition metal dichalcogenides (TMDs), including 4 H b − TaS 2 and monolayer 2H- NbSe 2 . While the pairing channel remains unknown, it has been argued that spin fluctuations can stabilize pairing in the two-component E ′ channel, a p -wave spin-triplet state which could be consistent with some of the reported signatures. Exploiting the particular multiorbital character of the Fermi surface and the presence of Ising spin-orbit coupling, which enable finite optical conductivity in the clean limit, in this work we predict clear-cut optical signatures to detect and distinguish the chiral and nematic ground states of the E ′ pairing. We quantify how nematic E ′ states produce a diagonal anisotropy σ x x ≠ σ y y due to the broken threefold symmetry ( C 3 ), while chiral E ′ states yield a finite optical Hall conductivity σ x y H due to broken time-reversal symmetry, and find both signals could be detected in current experiments. For instance, for realistic gaps in the meV range, we predict a relative anisotropy Δ σ / σ ∼ 10 − 5 in the nematic states, and a polar Kerr rotation of θ K ∼ 10 − 5 rad in the chiral states. These symmetry fingerprints provide a practical route to distinguish nematic and chiral superconducting order in TMD superconductors.

When the period of an incommensurate charge density wave (ICDW) approaches a multiple of a lattice vector, the energy gain from locking the period can drive a transition into an intermediate near-commensurate (NC) state made of locally commensurate regions separated by phase slips of a complex CDW. In TiSe₂, a paradigmatic CDW system, incommensuration is believed to be induced by carrier doping, yet its putative NC state has never been imaged. Here we observe a striking NC state in ultraclean, doped monolayers of TiSe₂, displaying an intricate network of unidirectional domain walls. Detailed analysis reveals that these are not phase slips of a complex CDW, but rather sign-changing Ising-type domain walls of two coupled real CDWs of different symmetry. In addition, these CDWs are coupled to an unexpected nematic modulation at the lattice Bragg peaks. TiSe₂ is thus a rare example of a NC-CDW of two intertwined real modulations with nematicity.

Recent work suggests that crystal structures with two sublattice pairs per primitive cell can host “dark states”, electronic states that barely interact with light due to destructive interference, which makes them invisible in photoemission. In practice, however, dark states are only approximately dark, arising from near but imperfect translation symmetries. Here, we demonstrate a practical consequence of this in the semiconductor (NbSe ₄ ) ₃ I: Although its band structure indicates an almost direct gap, the material behaves optically like an indirect-gap semiconductor. Angle-resolved photoemission spectroscopy uncovers weak spectral-weight bands folded from a larger Brillouin zone, reflecting approximate intra-unit-cell symmetry. These states form a small direct band gap consistent with transport data but exhibit very low optical transition probability. Instead, optical absorption is dominated by higher-energy transitions involving bands with stronger spectral weight, effectively enlarging the observed optical gap. Our results show that dark states are approximate phenomena with significant consequences for optoelectronic properties.

We report measurements of anisotropic triple-q charge density wave (CDW) fluctuations in the transition metal dichalcogenide 1T-TiSe_{2} over a large volume of reciprocal space with x-ray diffuse scattering. Above the transition temperature, T_{CDW}, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. In addition, the out-of-plane diffuse scattering is characterized by rodlike structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations with a hierarchy of length scales, leading to intricate fluctuation patterns that go beyond the conventional 2D-to-3D crossover picture.

Ohm’s law has been a cornerstone of electronics since its experimental discovery. This law establishes that, in a conductive system, the voltage is directly proportional to the current. Even when time-reversal symmetry is disrupted, leading to the emergence of magnetoresistance and Hall effects, the linear relationship between voltage and current remains intact. However, recent experiments have demonstrated a breakdown of Ohm’s law in non-centrosymmetric structures. In these systems, nonlinear transport effects are permitted with quadratic scaling between voltages and currents. Here we review the main demonstrations of nonlinear transport in non-centrosymmetric systems, analysing the connection between nonlinear behaviour and the system’s symmetry. We also investigate the microscopic mechanisms driving these effects, such as Berry curvature dipole and Berry connection polarizability. Finally, we highlight potential applications of nonlinear transport in spintronics and energy harvesting.