arXiv subjects

Julen Ibañez-Azpiroz

Publications and source records attributed to Julen Ibañez-Azpiroz.

At least 19 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

Many-Body effects beyond excitons in second-harmonic generation of monolayer MoS$_{2}$

We present a quantitative study of many-body effects including the three-particle level on second-harmonic generation in monolayer MoS$_2$. Our approach combines many-body perturbation theory with time-dependent current-density-functional theory within an \textit{ab initio} framework in the optical limit. The inclusion of two-particle excitonic effects \textit{via} a dynamical long-range linear exchange-correlation kernel reproduces the qualitative features of the second-harmonic response, but underestimates the experimentally reported magnitude by nearly a factor of two. By incorporating three-particle trionic correlations through a static long-range quadratic exchange-correlation kernel, we achieve significantly improved quantitative agreement with experiment. These findings highlight the role of many-body interactions beyond the excitonic level in accurately describing second-order optical responses in two-dimensional semiconductors.

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↗

arXiv2026

Control of the MoTe$_2$ Fermi Surface by Nb Doping

Ab initio calculations and angle-resolved photoemission experiments show that the bulk and surface electronic structure of Weyl semimetal candidate MoTe$_2$ changes significantly by tuning the chemical potential by less than 0.4 eV. Calculations show that several Lifshitz transitions can occur among multiple electron and hole Fermi pockets of differing orbital character. Experiments show that 18% Nb-Mo substitution reduces the occupation of bulk and (001) surface bands, effectively producing a chemical potential shift of $\approx 0.3$ eV. Orbital character and dimensionality of the bulk bands is examined by soft X-ray angle resolved photoemission with control of the excitation light polarization. The band filling at the surface is shown to increase upon deposition of alkali atoms. The results indicate that multiple regimes of electronic properties can be easily accessed in this versatile, layered material.

cond-mat.mtrl-sci↗

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↗

arXiv2025

Giant non-reciprocal band structure effect in a multiferroic material

Multiferroic materials, characterized by the coexistence of ferroelectricity and ferromagnetism, may unveil band structures suggestive of complex phenomena and new functionalities. In this Letter, we analyze the band structure of EuO in its multiferroic phase. Using density functional theory calculations and detailed symmetry analysis, we reveal a previously overlooked non-reciprocal band structure effect, where the electronic energy bands exhibit asymmetry along opposite directions with respect to the special points in the Brillouin zone. This effect, which is enabled by spin-orbit coupling, is giant for the top valence Eu $4f$ bands, and can be switched by external electric or magnetic fields. Furthermore, this results in an enhanced bulk photovoltaic effect. Specifically, our predictions indicate the emergence of a large injection current response to linearly polarized light, resulting in a photoconductivity value several orders of magnitude higher than that reported in any other oxide material. Ultimately, this non-reciprocal band structure effect and the associated large bulk photovoltaic response may be general phenomena emerging not just in EuO but also in other multiferroics or magnetoelectrics, potentially providing new cross-functionalities.

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↗

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↗

arXiv2023

Understanding the large shift photocurrent of WS$_{2}$ nanotubes: A comparative analysis with monolayers

We study the similarities and differences in the shift photocurrent contribution to the bulk photovoltaic effect between transition-metal dichalcogenide monolayers and nanotubes. Our analysis is based on density functional theory in combination with the Wannier interpolation technique for the calculation of the shift photoconductivity tensor. Our results show that for nanotube radii of practical interest $r>60$~Å, the shift photoconductivity of a single-wall nanotube is well described by that of the monolayer. Additionally, we quantify the shift photocurrent generated under realistic experimental conditions like device geometry and absorption capabilities. We show that a typical nanotube can generate a photocurrent of around 10 nA, while the monolayer only attains a maximum of 1 nA. This enhancement is mainly due to the larger conducting cross section of a nanotube in comparison to a monolayer. Finally, we discuss our results in the context of recent experimental measurements on WS$_{2}$ monolayer and nanotubes[Zhang et al., Nature 570, 349 (2019)].

cond-mat.mes-hall↗

arXiv2023

Ab initio study of the structural, vibrational and optical properties of potential parent structures of nitrogen-doped lutetium hydride

The recent report of near-ambient conditions superconductivity in a nitrogen-doped lutetium hydride has inspired a large number of experimental studies with contradictory results. We model from first principles the physical properties of the possible parent structures of the reported superconductor, LuH$_2$ and LuH$_3$. We show that only the phonon band structure of LuH$_3$ can explain the reported Raman spectra due to the presence of hydrogens at the interstitial octahedral sites. However, this structure is stabilized by anharmonicity only above 6 GPa. We find that the intriguing color change with pressure in the reported superconductor is consistent with the optical properties of LuH$_2$, which are determined by the presence of an undamped interband plasmon. The plasmon blue-shifts with pressure and modifies the color of the sample without requiring any structural phase transition. Our findings suggest that the main component in the experiments is LuH$_2$ with some extra hydrogen atoms at octahedral sites. None of LuH$_2$ and LuH$_3$ superconduct at high temperatures.

cond-mat.supr-con↗

arXiv2023

Including many-body effects into the Wannier-interpolated quadratic photoresponse tensor

We present a first-principles scheme for incorporating many-body interactions into the unified description of the quadratic optical response to light of noncentrosymmetric crystals. The proposed method is based on time-dependent current-density response theory and includes the electron-hole attraction \textit{via} a tensorial long-range exchange-correlation kernel, which we calculate self-consistently using the bootstrap method. By bridging with the Wannier-interpolation of the independent-particle transition matrix elements, the resulting numerical scheme is very general and allows resolving narrow many-body spectral features at low computational cost. We showcase its potential by inspecting the second-harmonic generation in the benchmark zinc-blende semiconductor GaAs, the layered graphitic semiconductor BC$_{2}$N and the Weyl semimetal TaAs. Our results show that excitonic effects can give rise to large and sharply localized one- and two-photon resonances that are absent in the independent-particle approximation. We find overall good agreement with available experimental measurements, capturing the magnitude and peak-structure of the spectrum as well as the angular dependence at fixed photon energy. The implementation of the method in Wannier-based code packages can serve as a basis for performing accurate theoretical predictions of quadratic optical properties in a vast pool of materials.

cond-mat.mes-hall↗

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↗

arXiv2022

Microscopic theory of spin-relaxation of a single Fe adatom coupled to substrate vibrations

Understanding the spin-relaxation mechanism of single adatoms is an essential step towards creating atomic magnetic memory bits or even qubits. Here we present an essentially parameter-free theory by combining \textit{ab-initio} electronic and vibrational properties with the many-body nature of atomic states. Our calculations account for the millisecond spin lifetime measured recently on Fe adatoms on MgO/Ag(100) and reproduce the dependence on the number of decoupling layers and the external magnetic field. We show how the atomic interaction with the environment should be tuned in order to enhance the magnetic stability, and propose a clear fingerprint for experimentally detecting a localized spin-phonon excitation.

cond-mat.mes-hall↗

arXiv2022

Assessing the role of interatomic position matrix elements in tight-binding calculations of optical properties

We study the role of hopping matrix elements of the position operator $\mathbf{\hat{r}}$ in tight-binding calculations of linear and nonlinear optical properties of solids. Our analysis relies on a Wannier-interpolation scheme based on \textit{ab initio} calculations, which automatically includes matrix elements of $\mathbf{\hat{r}}$ between different Wannier orbitals. A common approximation, both in empirical tight-binding and in Wannier-interpolation calculations, is to discard those matrix elements, in which case the optical response only depends on the on-site energies, Hamiltonian hoppings, and orbital centers. We find that interatomic $\mathbf{\hat{r}}$-hopping terms make a sizeable contribution to the shift photocurrent in monolayer BC$_2$N, a covalent acentric crystal. If a minimal basis of $p_z$ orbitals on the carbon atoms is used to model the band-edge response, even the dielectric function becomes strongly dependent on those terms.

cond-mat.mes-hall↗

arXiv2021

Electron-phonon coupling of Fe-adatom electron states on MgO/Ag(100)

We study the strength of the electron-phonon interaction on Fe single adatoms on MgO/Ag(100) based on many-body \textit{ab-initio} spin collinear calculations. In particular, we analyze the relative importance of the substrate and, among other results, we conclude that the interface electron state of Ag(100) plays a prominent role in determining the electron-phonon coupling of localized Fe electron states. The analysis of the hybridization of the adatom with the substrate reveals qualitative differences for even or odd coverages of MgO, affecting significantly the spectral structure and strength of the electron-phonon coupling. Our calculations indicate that the electron-phonon interaction is very strong for $\le$~1 layers of MgO, while it is sharply suppressed for larger coverages, a trend that is consistent with recent experimental findings.

cond-mat.mes-hall↗

arXiv2020

Zero-point quantum swing of magnetic couples

Quantum fluctuations are ubiquitous in physics. Ranging from conventional examples like the harmonic oscillator to intricate theories on the origin of the universe, they alter virtually all aspects of matter -- including superconductivity, phase transitions and nanoscale processes. As a rule of thumb, the smaller the object, the larger their impact. This poses a serious challenge to modern nanotechnology, which aims total control via atom-by-atom engineered devices. In magnetic nanostructures, high stability of the magnetic signal is crucial when targeting realistic applications in information technology, e.g. miniaturized bits. Here, we demonstrate that zero-point spin-fluctuations are paramount in determining the fundamental magnetic exchange interactions that dictate the nature and stability of the magnetic state. Hinging on the fluctuation-dissipation theorem, we establish that quantum fluctuations correctly account for the large overestimation of the interactions as obtained from conventional static first-principles frameworks, filling in a crucial gap between theory and experiment [1,2]. Our analysis further reveals that zero-point spin-fluctuations tend to promote the non-collinearity and stability of chiral magnetic textures such as skyrmions -- a counter-intuitive quantum effect that inspires practical guidelines for designing disruptive nanodevices.

cond-mat.mtrl-sci↗

arXiv2019

Spontaneous and externally driven quantum spin fluctuations of 3d and 4d single atoms adsorbed on graphene

At the heart of current information nanotechnology lies the search for ideal platforms hosting the smallest possible magnets, i.e. single atoms with magnetic moments pointing out-of-plane, as requested in a binary-type of memory. For this purpose, a 2D material such as graphene would be an ideal substrate thanks to its intrinsic low electron and phonon densities, as well as its 6-fold symmetry. Here we investigate, from first-principles, a fundamental mechanism detrimental for the magnetic stability: the zero-point spin-fluctuations modifying the effective energy landscape perceived by the local spin moments of 3$d$ and 4$d$ transition metal atoms deposited on a free standing graphene. Utilizing time-dependent density functional theory and by virtue of the fluctuation-dissipation theorem, these spontaneous quantum fluctuations are found to be negligible for most of the 3$d$ elements, in strong contrast to the 4$d$ atoms. Surprisingly, we find that such fluctuations can promote the magnetic stability by switching the easy direction of the magnetic moment of Tc from being initially in-plane to out-of-plane. The adatom-graphene complex gives rise to impurity states settling in some cases the magnetocrystalline anisotropy energy --- the quantity that defines the energy barrier protecting the magnetic moments and, consequently, the spin-excitation behavior detectable with inelastic scanning tunneling spectroscopy. A detailed analysis is provided on the impact of electron-hole excitations, damping and lifetime of the spin-excitations on the dynamical behavior of the adsorbed magnetic moments on graphene.

cond-mat.mtrl-sci↗