arXiv subjects

Javier Sivianes

Publications and source records attributed to Javier Sivianes.

5 recordsLinked to original sources

arXiv2026

Optical Response Beyond Magnetic Symmetries

The optical response of magnetic materials is conventionally classified through magnetic space groups (MSGs), where spin and lattice are locked by the relativistic spin-orbit interaction. However, most optical observables are governed primarily by nonrelativistic physics, and thus a purely MSG-based description can overlook important insights. Here we systematically show that spin-space groups (SSGs), which operate at the nonrelativistic level, provide a broader and more predictive framework for analyzing a variety of optical responses of magnets. Focusing on linear optical absorption, we derive the transformation rules imposed by SSGs and show that they generate effective real-space point groups, which can enforce relations among charge response coefficients that are absent from conventional MSG analysis. We illustrate the basic principle in a Lieb-lattice altermagnet model with tunable spin-orbit coupling, where SSG predictions on the linear dichroism remain remarkably accurate even when relativistic band splittings become sizable. We further establish the predictive power of this framework through first-principles calculations on two altermagnetic candidates: the actinide UCr2Si2C, where the optical absorption remains nearly isotropic despite its strong spin-orbit coupling and the pronounced anisotropy apparent from magnetic symmetries, and the transition-metal fluoride RbMnF4, where birefringence is confined to a single plane by symmetries emerging exclusively from SSGs. Finally, we extend the concept to the spin Hall response of the coplanar noncollinear antiferromagnet ScMnO3, where SSGs explain the hierarchy of calculated spin Hall coefficients, demonstrating their direct relevance to spintronics as well.

cond-mat.mtrl-sci↗

arXiv2026

Linear response of the Chern insulator MnBi$_2$Te$_4$: A Wannier function approach

Recent work demonstrated that in the long wavelength limit the linear response of a Chern insulator to finite-frequency electric fields is the sum of two terms: A general frequency-dependent Kubo contribution that is present irrespective of band topology, and a topological Hall term that vanishes for topologically trivial insulators. Motivated by recent experiments and theoretical predictions, we use these expressions to calculate the optical conductivity and susceptibility of intrinsically magnetic MnBi$_2$Te$_4$ thin films with one, four, five, and eleven septuple layers by combining density functional theory with "single-shot" Wannier functions. To characterize the underlying topology of these systems, we compute the two-dimensional Chern number of these films using recently derived global expressions formulated in terms of Bloch energies and velocity matrix elements; the use of these expressions allows us to circumvent numerical issues at band crossings. Films with eleven septuple layers are of particular interest. We find that they have the same Chern number as five septuple layer films, in contrast to the reported "higher Chern-number phase" of these systems in other studies; we discuss a few possible reasons for the discrepancy. We also identify spin-orbit coupling-driven band inversions as a possible indicator of these topological phases.

cond-mat.mes-hall↗

arXiv2025

Surface-state engineering for nonlinear charge and spin photocurrent generation

We systematically explore a pathway for generating nonlinear charge and spin photocurrents using spin-orbit-split surface states. This mechanism enables net charge and spin flow along the surface plane even in centrosymmetric bulk environments like the Rashba prototype Au(111), where we establish the key principles by combining model predictions with density functional calculations. We further identify the Tl/Si(111) surface, characterized by strong non-Rashba spin-orbit coupling, as a prime candidate for experimental validation; with slight doping, it develops metallic spin-orbit-split states featuring remarkable relativistic properties while the bulk remains semiconducting. Our non-linear simulations reveal distinct angular signatures and magnitudes comparable to bulk ferroelectrics, highlighting the potential of surface-state photocurrents for low-bias optoelectronic applications. Moreover, the intricate spin polarization of surface states opens new possibilities as a nonlinear spin filter, providing a far more versatile platform than the spin Hall effect.

cond-mat.mes-hall↗

arXiv2025

Optical signatures of spin symmetries in unconventional magnets

The concept of spin symmetries has gained renewed interest as a valuable tool for classifying unconventional magnetic phases, including altermagnets and recently identified p-wave magnets. In this work, we show that in compounds with weak spin-orbit coupling, the dominant spin and charge photoresponse is determined by spin group rather than the conventional magnetic group symmetry. As a concrete realization we consider the nonlinear shift photocurrent in Mn$_5$Si$_3$, a material that features the two possible classes of unconventional p-wave magnetism in the form of two competing spin structures, a coplanar and non-coplanar one. While both are predicted to generate shift currents based on magnetic symmetry considerations, only the non-coplanar configuration survives the spin symmetry requirements. This is numerically confirmed by our $\textit{ab-initio}$ calculations, providing a protocol to experimentally identify the spin configuration of this promising material in photogalvanic or transport measurements.

cond-mat.mes-hall↗

arXiv2023

Shift photoconductivity in the Haldane model

The shift current is part of the second-order optical response of materials with a close connection to topology. Here we report a sign inversion in the band-edge shift photoconductivity of the Haldane model when the system undergoes a topological phase transition. This result is obtained following two complementary schemes. On one hand, we derive an analytical expression for the band-edge shift current in a two-band tight-binding model showing that the sign reversal is driven by the mass term. On the other hand, we perform a numerical evaluation on a continuum version of the Haldane model. This approach allows us to include off-diagonal matrix elements of the position operator, which are discarded in tight-binding models but can contribute significantly to the shift current. Explicit evaluation of the shift current shows that while the model predictions remain accurate in the deep tight-binding regime, significant deviations arise for shallow potential landscapes. Notably, the sign reversal across the topological phase transition is observed in all regimes, implying it is a robust effect that could be observable in a wide range of topological insulators.

cond-mat.mes-hall↗