arXiv subjects

Ivo Souza

Publications and source records attributed to Ivo Souza.

At least 19 recordsLinked to original sources

arXiv2026

Electronic bounds in magnetic crystals

We present a systematic study of bound relations between different electronic properties of magnetic crystals: electron density, effective mass, orbital magnetization, localization length, Chern invariant, and electric susceptibility. All relations are satisfied for a group of low-lying bands, while some remain valid for upper bands. New results include a lower bound on the electric susceptibility of Chern insulators, and an upper bound on the sum-rule part of the orbital magnetization. In addition, bounds involving the Chern invariant are generalized from two dimensions (Chern number) to three (Chern vector). Bound relations are established for metals as well as insulators, and are illustrated for model systems. The manner in which they approach saturation in a model Chern insulator with tunable flat bands is analyzed in terms of the optical absorption spectrum.

cond-mat.mtrl-sci↗

arXiv2026

Orbital Hall effect from orbital magnetic moments of Bloch states: the role of a new correction term

We present a rigorous derivation of the matrix elements of the orbital magnetic moment (OMM) of Bloch states. Our calculations include the Berry connection term in the k-derivatives of Bloch states, which was omitted in previous works. The resulting formula for the OMM matrix elements applies to any non-degenerate Bloch states within Hilbert space. We identify two new contributions: the first restores gauge covariance for non-degenerate states, while the second, being itself gauge covariant, can provide significant quantitative corrections depending on the system under study. We examine their impact on the orbital Hall effect in two bilayer systems: a 2H transition metal dichalcogenide bilayer and a biased bilayer graphene. In both cases, these new terms reduce the orbital Hall conductivity plateau compared with results that neglect them, suggesting that multi-layered van der Waals materials may be particularly susceptible to the derived OMM corrections. Our findings may contribute to the formal understanding of electronic OMM transport and to the conceptual foundations of the emerging field of orbitronics.

cond-mat.mes-hall↗

arXiv2026

Anomalous Nonlinear Magnetoconductivity in van der Waals Magnet CrSBr

Nonlinear magnetoconductivity (NLMC) is a nonreciprocal transport response arising in non-centrosymmetric materials. However, this ordinary NLMC signal vanishes at zero magnetic field, limiting its potential for applications. Here, we report the observation of an anomalous NLMC controlled by internal order parameters such as the magnetization or Néel vectors. We achieve this response by breaking both inversion and time-reversal symmetry in artificial van der Waals heterostructures based on the magnetic CrSBr and insulating hBN. The nonreciprocal signal can be tuned between two different states in ferromagnetic monolayer CrSBr and among four different states in antiferromagnetic bilayer CrSBr, thanks to its metamagnetic transition. Remarkably, this output signal in the ferromagnetic (antiferromagnetic) state of CrSBr is three (one) orders of magnitude higher than those previously measured. A conductivity scaling analysis reveals the Berry connection polarizability as the origin of the anomalous NLMC. Our results pave the way for high-frequency rectifiers with magnetically switchable output polarity as well as for an efficient electrical readout of the magnetic state of antiferromagnetic materials.

cond-mat.mes-hall↗

arXiv2025

Precise quantum-geometric electronic properties from first principles

The calculation of quantum-geometric properties of Bloch electrons -- Berry curvature, quantum metric, orbital magnetic moment and effective mass -- was implemented in a pseudopotential plane-wave code. The starting point was the first derivative of the periodic part of the wavefunction psi_k with respect to the wavevector k. This was evaluated with perturbation theory by solving a Sternheimer equation. Comparison of effective masses obtained from perturbation theory for silicon and gallium arsenide with carefully-converged numerical second derivatives of band energies confirms the high precision of the method. Calculations of quantum-geometric quantities for gapped graphene were performed by adding a bespoke symmetry-breaking potential to first-principles graphene. As the two bands near the opened gap are reasonably isolated, the results could be compared with those obtained from an analytical two-band model, allowing to assess the strengths and limitations of such widely-used models. The final application was trigonal tellurium, where quantum-geometric quantities flip sign with chirality.

cond-mat.mtrl-sci↗

arXiv2025

Theory of infrared magneto-optical effects from chiral phonons in solids

In crystals with broken time-reversal symmetry, zone-center phonons can acquire a finite angular momentum via velocity-dependent forces on the nuclei. Despite having the same order of magnitude as the electron spin angular momentum, the phonon angular momentum can be hard to detect because the frequency splitting is small. Here, by developing a theory of lattice magneto-optical effects in reflection and transmission, we show that infrared magnetic circular dichroism is a sensitive probe of zone-center phonon chirality. We evaluate the infrared magneto-optical Faraday, Kerr, and circular-dichroism spectra of CrI$_3$ from time-dependent density-functional theory in the adiabatic local-density approximation. We find sizeable circular dichroism from the infrared-active E$_u$ mode at $\approx 214$ cm$^{-1}$, even though the calculated splitting is only 0.22 cm$^{-1}$.

cond-mat.mtrl-sci↗

arXiv2025

Non-linear Transport in Non-centrosymmetric Systems: From Fundamentals to Applications

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, non-linear transport effects are permitted with quadratic scaling between voltages and currents. Here, we review the main demonstrations of non-linear transport in non-centrosymmetric systems, analyzing the connection between non-linear behavior and the system's symmetry. Additionally, we delve into the microscopic mechanisms driving these effects, such as Berry curvature dipole and Berry connection polarizability. Finally, we highlight potential applications of non-linear transport in spintronics and energy harvesting.

cond-mat.mes-hall↗

arXiv2025

Optical bounds on many-electron localization

We establish rigorous inequalities between different electronic properties linked to optical sum rules, and organize them into weak and strong bounds on three characteristic properties of insulators: electron localization length $\ell$ (the quantum fluctuations in polarization), electric susceptibility $χ$, and optical gap $E_{\rm G}$. All-electron and valence-only versions of the bounds are given, and the latter are found to be more informative. The bounds on $\ell$ are particularly interesting, as they provide reasonably tight estimates for an ellusive ground-state property - the average localization length of valence electrons - from tabulated experimental data: electron density, high-frequency dielectric constant, and optical gap. The localization lengths estimated in this way for several materials follow simple chemical trends, especially for the alkali halides. We also illustrate our findings via analytically solvable harmonic oscillator models, which reveal an intriguing connection to the physics of long-ranged van der Waals forces.

cond-mat.mtrl-sci↗

arXiv2025

Optical spatial dispersion via Wannier interpolation

We present a numerical implementation, based on Wannier interpolation, of a Kubo-Greenwood formalism for computing the spatially dispersive optical conductivity in crystals at first order in the wave vector of light. This approach is more efficient than direct $\textit{ab initio}$ methods because, with less computational cost, it allows for a much finer sampling of reciprocal space, resulting in better resolved spectra. Moreover, Wannier interpolation avoids errors arising from truncation of the sums over conduction bands when evaluating the spatially dispersive optical matrix elements. We validate our method by computing the optical activity spectrum of selected crystals, both polar (GaN) and chiral (trigonal Te, trigonal Se, and $α$-quartz), and comparing with existing literature.

cond-mat.mtrl-sci↗

arXiv2025

Symmetry Analysis of the Non-Hermitian Electro-Optic Effect in Crystals

Here, we investigate how crystal symmetry tailors the non-Hermitian electro-optic effect arising from the Berry curvature dipole. Specifically, we demonstrate the critical influence of the material's point group symmetry and external electric biases in shaping this effect, leading to current-induced optical gain and non-reciprocal optical responses. Through a symmetry-based analysis of the crystallographic point groups, we identify how different symmetries affect the electro-optic response, enabling the engineering of polarization-dependent optical gain without the need for gyrotropic effects. In particular, we demonstrate that the non-Hermitian electro-optic response in a broad class of crystals is characterized by linear dichroic gain. In this type of response, the eigenpolarizations that activate the gain or dissipation are linearly polarized. Depending on the point group symmetry, it is possible to achieve gain (or dissipation) for all eigenpolarizations or to observe polarization-dependent gain and dissipation. Weyl semimetals emerge as promising candidates for realizing significant non-Hermitian electro-optic effects and linear dichroic gain. We further examine practical applications by studying the reflectance of biased materials in setups involving mirrors, demonstrating how optical gain and attenuation can be controlled via symmetry and bias configurations.

cond-mat.mes-hall↗

arXiv2025

Symmetry Origin and Microscopic Mechanism of Electrical Magnetochiral Anisotropy in Tellurium

Non-linear transport effects in response to external magnetic fields, i.e. electrical magnetochiral anisotropy (eMChA), have attracted much attention for their importance to study quantum and spin-related phenomena. Indeed, they have permitted the exploration of topological surface states and charge-to-spin conversion processes in low-symmetry systems. Nevertheless, despite the inherent correlation between the symmetry of the material under examination and its non-linear transport characteristics, there is a lack of experimental demonstration to delve into this relationship and to unveil their microscopic mechanisms. Here, we study eMChA in chiral elemental Tellurium (Te) along different crystallographic directions, establishing the connection between the different eMChA components and the crystal symmetry of Te. We observed different longitudinal eMChA components with collinear current and magnetic field, demonstrating experimentally the radial angular momentum texture of Te. We also measured a transverse non-linear resistance which, as the longitudinal counterpart, scales bilinearly with current and magnetic fields, illustrating that they are different manifestations of the same effect. Finally, we study the scaling law of the eMChA, evidencing that extrinsic scattering from dynamic sources is the dominant microscopic mechanism. These findings underscore the efficacy of symmetry-based investigations in understanding and predicting non-linear transport phenomena, with potential applications in spintronics and energy harvesting.

cond-mat.mes-hall↗

arXiv2024

Unveiling intrinsic bulk photovoltaic effect in atomically thin ReS2

The bulk photovoltaic effect (BPVE) offers a promising avenue to surpass the efficiency limitations of current solar cell technology. However, disentangling intrinsic and extrinsic contributions to photocurrent remains a significant challenge. Here, we fabricate high-quality, lateral devices based on atomically thin ReS2 with minimal contact resistance, providing an optimal platform for distinguishing intrinsic bulk photovoltaic signals from other extrinsic photocurrent contributions originating from interfacial effects. Our devices exhibit large bulk photovoltaic performance with intrinsic responsivities of 1 mA/W in the visible range, without the need for external tuning knobs such as strain engineering. Our experimental findings are supported by theoretical calculations. Furthermore, our approach can be extrapolated to investigate the intrinsic BPVE in other non-centrosymmetric van der Waals materials, paving the way for a new generation of efficient light-harvesting devices.

cond-mat.mes-hall↗

arXiv2024

The Wannier Function Software Ecosystem for Materials Simulations

Over the last two decades, following the early developments on maximally localized Wannier functions, an ecosystem of electronic-structure simulation techniques and software packages leveraging the Wannier representation has flourished. This environment includes codes to obtain Wannier functions and interfaces with first-principles simulation software, as well as an increasing number of related post-processing packages. Wannier functions can be obtained for isolated or extended systems (both crystalline and disordered), and can be used to understand chemical bonding, to characterize electric polarization, magnetization, and topology, or as an optimal basis set, providing very accurate interpolations in reciprocal space or large-scale Hamiltonians in real space. In this review, we summarize the current landscape of techniques, materials properties and simulation codes based on Wannier functions that have been made accessible to the research community, and that are now well integrated into what we term a \emph{Wannier function software ecosystem}. First, we introduce the theory and practicalities of Wannier functions, starting from their broad domains of applicability to advanced minimization methods using alternative approaches beyond maximal localization. Then we define the concept of a Wannier ecosystem and its interactions and interoperability with many quantum simulations engines and post-processing packages. We focus on some of the key properties and capabilities that are empowered by such ecosystem\textemdash from band interpolations and large-scale simulations to electronic transport, Berryology, topology, electron-phonon couplings, dynamical mean-field theory, embedding, and Koopmans functionals\textemdash concluding with the current status of interoperability and automation. [...]

cond-mat.mtrl-sci↗

arXiv2024

Electrical magnetochiral anisotropy in trigonal tellurium from first principles

Structural chirality gives rise to characteristic responses that change sign with the handedness of the crystal structure. One example is electrical magnetochiral anisotropy (eMChA), a change in resistivity that depends linearly on the applied current and on the magnetic field. Motivated by recent measurements of a strong eMChA in $p$-doped trigonal tellurium, we carry out an \textit{ab initio} study of the eMChA response in this material as a function of temperature and doping concentration. We use the semiclassical Boltzmann transport formalism within the constant relaxation-time approximation to express the bulk eMChA response tensor in terms of the energy dispersion, intrinsic magnetic moment, and Berry curvature of the conduction Bloch states. We find that the orbital Zeeman coupling dominates the calculated response, with smaller contributions coming from the spin Zeeman coupling and from the Berry curvature, and that the effect is maximal when both the current and magnetic field are along the trigonal axis. The calculated data shows a reasonable agreement with the experiments. We provide the open-source code to facilitate further \textit{ab initio} studies of eMChA in other materials.

cond-mat.mtrl-sci↗

arXiv2024

Computing Floquet quasienergies in finite and extended systems: Role of electromagnetic and quantum-geometric gauges

We present an approach to compute the Floquet quasienergy spectrum of time-periodic systems. The method allows to characterize the light-matter interaction in finite and extended structures by carefully addressing the resolution of the position operator. In periodic systems we discuss the role of the quantum-geometric gauge freedom of Bloch states and employ a Wannier-based scheme to compute the required matrix elements. As a consequence, the method is accurate and applicable to a broad range of systems, from atoms and molecules to cold atomic gases and materials described by density functional theory, as well as model systems. We demonstrate the applicability of the approach by studying two cases: a particle trapped in a one-dimensional box and the semiconducting material BC$_2$N. We employ the first example to provide a numerical proof of the invariance of the Floquet quasienergy spectrum with respect to the choice of electromagnetic gauge. The analysis of BC$_2$N then serves to illustrate the physical effects described by the quasienergies, such as multiphoton resonances, and their expected range of occurrence in real materials in terms of external electric field and frequency of the drive pulse.

cond-mat.mes-hall↗

arXiv2024

Odd non-linear conductivity under spatial inversion in chiral Tellurium

Electrical transport in non-centrosymmetric materials departs from the well-established phenomenological Ohm's law. Instead of a linear relation between current and electric field, a non-linear conductivity emerges along specific crystallographic directions. This non-linear transport is fundamentally related to the lack of spatial inversion symmetry. However, the experimental implications of an inversion symmetry operation on the non-linear conductivity remain to be explored. Here, we report on a large, non-linear conductivity in chiral Tellurium. By measuring samples with opposite handedness, we demonstrate that the non-linear transport is odd under spatial inversion. Furthermore, by applying an electrostatic gate, we modulate the non-linear output by a factor of 300, reaching the highest reported value excluding engineered heterostructures. Our results establish chiral Te as an ideal compound not just to study the fundamental interplay between crystal structure, symmetry operations and non-linear transport, but also to develop wireless rectifiers and energy-harvesting chiral devices.

cond-mat.mes-hall↗

arXiv2024

Non-Hermitian Linear Electrooptic Effect in 3D materials

Here, we present an in-depth theoretical analysis of the linear electrooptic effect in low-symmetry three-dimensional (3D) conductive materials with large Berry curvature dipoles. Our study identifies two distinct kinetic contributions to the linear electrooptic effect: a gyrotropic Hermitian (conservative) piece and a non-Hermitian term that can originate optical gain. We concentrate on the study of 3D materials belonging to the 32 ($D_3$) point group subject to a static electric bias along the trigonal axis. Our investigation shows that doped trigonal tellurium has promising properties, with its gyrotropic electrooptic response offering the potential for realizing electrically-biased electromagnetic isolators and inducing significant optical dichroism. Most notably, it is demonstrated that under sufficiently large static electric bias, tellurium's non-Hermitian electrooptic response may lead to optical gain. Using first-principles calculations, it is shown that n-doped tellurium is particularly promising, as it can host significantly larger Berry curvature dipoles than the more common p-doped tellurium.

physics.optics↗

arXiv2023

Multipole theory of optical spatial dispersion in crystals

Natural optical activity is the paradigmatic example of an effect originating in the weak spatial inhomogeneity of the electromagnetic field on the atomic scale. In molecules, such effects are well described by the multipole theory of electromagnetism, where the coupling to light is treated semiclassically beyond the electric-dipole approximation. That theory has two shortcomings: it is limited to bounded systems, and its building blocks - the multipole transition moments - are origin dependent. In this work, we recast the multipole theory in a translationally-invariant form that remains valid for crystals. Working in the independent-particle approximation, we introduce "intrinsic" multipole transition moments that are origin independent and transform covariantly under gauge transformations of the Bloch eigenstates. Electric-dipole transitions are given by the interband Berry connection, while magnetic-dipole and electric-quadrupole transitions are described by matrix generalizations of the intrinsic magnetic moment and quantum metric. In addition to multipole-like terms, the response of crystals at first order in the wavevector of light contains band-dispersion terms that have no counterpart in molecular theories. The full response is broken down into magnetoelectric and quadrupolar parts, which can be isolated in the static limit where electric and magnetic fields become decoupled. The rotatory-strength sum rule for crystals is found to be equivalent to the topological constraint for a vanishing chiral magnetic effect in equilibrium, and the formalism is validated by numerical tight-binding calculations.

cond-mat.mtrl-sci↗

arXiv2023

Ab initio study of the nonlinear optical properties and d.c. photocurrent of the Weyl semimetal TaIrTe$_4$

We present a first principles theoretical study employing nonlinear response theory to investigate the d.c. photocurrent generated by linearly polarized light in the type-II Weyl semimetal TaIrTe4. We report the low energy spectrum of several nonlinear optical effects. At second-order, we consider the shift and injection currents. Assuming the presence of a built-in static electric field, at third-order we study the current-induced shift and injection currents, as well as the jerk current. We discuss our results in the context of a recent experiment measuring an exceptionally large photoconductivity in this material [J. Ma et at., Nat. Mater. 18, 476 (2019)]. According to our results, the jerk current is the most likely origin of the large response. Finally, we propose means to discern the importance of the various mechanisms involved in a time-resolved experiment.

cond-mat.mes-hall↗