Alberto de la Torre

MnSb is a metallic ferromagnet with a high Curie temperature (∼587 K) and strong spin–orbit coupling, making it attractive for room-temperature spintronic and magnetic sensing applications. We report heteroepitaxial growth of MnSb on GaAs (111) by molecular beam epitaxy, yielding locally epitaxial, strain-relaxed island structures with multiple dominant crystallographic orientations. Structural analysis reveals the coexistence of basal-plane- and pyramidal-plane-oriented domains, enabling investigation of magneto-crystalline anisotropy in an orientationally inhomogeneous ferromagnetic system. Room-temperature magnetic characterization using magnetic force microscopy, scanning nitrogen-vacancy center magnetometry, and angle-dependent ferromagnetic resonance demonstrates a strongly anisotropic, orientation-dependent magnetic response across multiple length scales. Correlating these magnetic responses with crystal structure and local stray fields reveals the role of structural inhomogeneity in governing magnetic anisotropy and spin dynamics in MnSb thin film. These results establish MnSb thin films as a promising platform for vector magnetic-field sensing and orientation-encoded spintronic device concepts.

Geometrically frustrated lattices can display a range of correlated phenomena, ranging from spin frustration and charge order to dispersionless flat bands due to quantum interference. One particularly compelling family of such materials is the half-valence spinel LiB₂O₄ materials. On the B-site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi₂O₄ and heavy fermion behavior in LiV₂O₄. To date, however, LiTi₂O₄ has primarily been understood as a conventional BCS superconductor despite a lattice structure that could host more exotic ground states. Here, we present a multimodal investigation of LiTi₂O₄, combining ARPES, RIXS, proximate magnetic probes, and ab-initio many-body theoretical calculations. Our data reveals a novel mobile polaronic ground state with spectroscopic signatures that underlie co-dominant electron-phonon coupling and electron-electron correlations also found in the lightly doped cuprates. The cooperation between the two interaction scales distinguishes LiTi₂O₄ from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi₂O₄. Our work deepens our understanding of the rare interplay of electron-electron correlations and electron-phonon coupling in unconventional superconducting systems. In particular, our work identifies the geometrically frustrated, mixed-valence spinel family as an under-explored platform for discovering unconventional, correlated ground states.

Quantum spin liquids, where the frustrated magnetic ground state hosts highly entangled spins resisting long-range order to 0 K, are exotic quantum magnets proximate to unconventional superconductivity and candidate platforms for topological quantum computing. Although several quantum spin liquid material candidates have been identified, thin films crucial for device fabrication and further tuning of properties remain elusive. Recently, hexagonal TbInO ₃ has emerged as a quantum spin liquid candidate which also hosts improper ferroelectricity and exotic high-temperature carrier transport. Here, we synthesize thin films of TbInO ₃ and characterize their magnetic and electronic properties. Our films present a highly frustrated magnetic ground state without long-range order to 0.4 K, consistent with bulk crystals. We further reveal a rich ferroelectric domain structure and unconventional non-local transport near room temperature, suggesting hexagonal TbInO ₃ as a promising candidate for realizing exotic magnetic and transport phenomena in epitaxial heterostructures.

The fate of the Fermi surface in bulk electron-doped Sr₂IrO₄ remains elusive, as does the origin and extension of its pseudogap phase. Here, we use high-resolution angle-resolved photoelectron spectroscopy (ARPES) to investigate the electronic structure of Sr_2−x La _x IrO₄ up to x = 0.2, a factor of two higher than in previous work. We find that the antinodal pseudogap persists up to the highest doping level, and thus beyond the sharp increase in Hall carrier density to ≃ 1 + x recently observed above x* ≃ 0.16¹. This suggests that doped iridates host a unique phase of matter in which a large Hall density coexists with an anisotropic pseudogap, breaking up the Fermi surface into disconnected arcs. The temperature boundary of the pseudogap is T* ≃ 200 K for x = 0.2, comparable to cuprates and to the energy scale of short range antiferromagnetic correlations in cuprates and iridates.

The dynamical phase transition of a system with two coexisting competing order parameters is studied using the time-dependent-Ginzburg-Landau framework. The dynamics are induced by parameters capturing the physics of driving the system with an ultrafast laser pulse. A remarkable enhancement of the order parameter with a smaller mean-field value following the pump and the emergence of an induced metastable state is investigated through analytical and numerical studies. The effect of order parameter fluctuations on the exploration of the nonequilibrium free energy landscape reveals important information about the impact of both thermal and nonthermal fluctuations on the dynamics of the metastable state. Our results provide an interpretation of previously unexplained ultrafast experiments on superconductors with competing charge density wave order. Our formalism is relevant across broad classes of out-of-equilibrium systems beyond the condensed matter context, such as the Kibble-Zurek cosmological model.

We present the first circularly polarized non-resonant pump time-resolved resonant inelastic X-ray scattering (tr-RIXS) experiment in H₃LiIr₂O₆, an iridium-based Kitaev system. Our calculations and experimental results are consistent with the modification of the low-energy magnetic excitations in H₃LiIr₂O₆ only during illumination by the laser pulse. We discuss these results in a cooperative framework between the Floquet engineering of the exchange interactions and the dynamic renormalization of electron-electron correlations. However, the penetration length mismatch between the X-ray probe and laser pump and the intrinsic complexity of Kitaev magnets prevents us from unequivocally extracting towards which ground state H₃LiIr₂O₆ is driven. We outline possible solutions to these challenges for light-driven stabilization and observation of the Kitaev quantum spin liquid limit by RIXS.

Competing interactions in frustrated magnets can give rise to highly degenerate ground states from which correlated liquidlike states of matter often emerge. The scaling of this degeneracy influences the ultimate ground state, with extensive degeneracies potentially yielding quantum spin liquids, while subextensive or smaller degeneracies yield static orders. A long-standing problem is to understand how ordered states precipitate from this degenerate manifold and what echoes of the degeneracy survive ordering. Here, we use neutron scattering to experimentally demonstrate a new “nodal-line” spin liquid, where spins collectively fluctuate within a subextensive manifold spanning one-dimensional lines in reciprocal space. Realized in the spin-orbit-coupled, face-centered-cubic iridate K 2 IrCl 6 , we show that the subextensive degeneracy is robust, but remains susceptible to fluctuations or longer-range interactions which cooperate to select a magnetic order at low temperatures. Proximity to the nodal-line spin liquid influences the ordered state, enhancing the effects of quantum fluctuations that in turn act to stabilize the sublattice magnetization through the self-consistent opening of a large spin-wave gap. Our results demonstrate how quantum fluctuations can act counterintuitively in frustrated materials: Even in a case where fluctuations are ineffective at selecting an ordered state from a degenerate manifold, at the brink of the nodal spin liquid, they can act to protect the ordered state and dictate its low-energy physics.