arXiv subjects

A. C. Garcia-Castro

Publications and source records attributed to A. C. Garcia-Castro.

At least 19 recordsLinked to original sources

arXiv2025

Chiral symmetry and magnetism in a 3D Kagome lattice: RPt$_2$B (R = La and Nd) prototype crystals

Chirality in crystals arises from the exclusive presence of proper symmetry operations, such as rotations and screw axes, while lacking improper operations like inversion, mirror planes, and roto-inversions. Crystallographic chirality is expected to be coupled with magnetic responses in magnetically active chiral compounds. Therefore, this study investigates the interplay between structural chirality and magnetic ordering in the rare-earth platinum boride family, RPt$_2$B, where R denotes lanthanide elements. Our results show that the R sites structurally form a chiral three-dimensional Kagome lattice, which can lead to magnetic frustration resolved through chiral antiferromagnetic orderings in conjunction with chiral symmetry. Symmetry analysis reveals that these chiral antiferromagnetic states are low-energy states, competing with higher-in-energy (001) ferromagnetic configuration. We also identified Kramers-type Weyl points in the electronic structure without magnetic response. In the magnetically active chiral compound NdPt$_2$B, Zeeman splitting lifts degeneracies at the high-symmetry points; however, Weyl points persist due to the breaking of time-reversal (T) and inversion (P) symmetries. We also estimate the anomalous Hall conductivity, a measurable observable of the allowed topological features finding a value of $σ_{xy} = 293$ S$\cdot$cm$^{-1}$ comparable with another Kagome magnetic materials like Mn$_3$PtN and FeSn, for example. This study elucidates the intricate interplay among chirality, magnetism, and topology in rare-earth Kagome materials.

cond-mat.str-el↗

arXiv2025

Efficiency of band edge optical transitions of 2D monolayer materials: A high-throughput computational study

We performed high-throughput density functional theory calculations of optical matrix elements between band edges across a diverse set of non-magnetic two-dimensional monolayers with direct band gaps. Materials were ranked as potential optical emitters, leading to the identification of transition-metal nitrogen halides (ZrNCl, TiNBr, TiNCl) and bismuth chalcohalides (BiTeCl) with optical coupling comparable to or exceeding MoS$_2$. Despite strong in-plane dipole transitions, most two-dimensional materials underperform bulk semiconductors due to the absence of out-of-plane components. To elucidate interband transitions, we introduced the orbital overlap tensor and established a correlation between anomalous Born effective charges and optical coupling, linking charge redistribution to transition strength. We also identified chalcogen-mediated $d$-$d$ transition as a key mechanism enabling optical responses in transition-metal dichalcogenides. We derived an analytical radiative recombination model incorporating multi-valley effects and found that excitonic corrections are essential for accurate lifetime predictions. Some direct-gap materials exhibit dark excitons as their lowest-energy states, classifying them as quasi-direct band gap semiconductors, which is critical for tuning excitonic recombination dynamics.

cond-mat.mtrl-sci↗

arXiv2024

Polar phonons and magnetic excitations in the antiferromagnet CoF$_2$

The coupling between antiferromagnetic spins and infrared-active phonons in solids is responsible for many intriguing phenomena and is a field of intense research with extensive potential applications in the modern devices based on antiferromagnetic spintronics and phononics. Insulating rutile antiferromagnetic crystal CoF$_2$ is one of the model materials for studying nonlinear magnetophononics due to the strong spin-lattice coupling as a result of the orbitally degenerate ground state of Co$^{2+}$ ions manifested in the plethora of static and induced piezomagnetic effects. Here we report results on the complete infrared spectroscopy study of lattice and magnetic dynamics in CoF$_2$ in a wide temperature range and their careful analysis. We observed that infrared-active phonons demonstrate frequency shifts at the antiferromagnetic ordering. Furthermore, using first-principles calculations, we examined the lattice dynamics and disclosed that these frequency shifts are rather due to the spin-phonon coupling than geometrical lattice effects. Next we found that the low-frequency dielectric permittivity demonstrates distinct changes at the antiferromagnetic ordering due to the spontaneous magnetodielectric effect caused by the behavior of infrared-active phonons. In addition, we have observed magnetic excitations in the infrared spectra and identified their magnetodipole origin. To strengthen our conclusions, we analyze the theoretical phonon-magnon coupling overall phonons at the $Γ$ point. We conclude that the largest effect comes from the $A_{1g}$ and $B_{2g}$ Raman-active modes. As such, our results establish a solid basis for further investigations and more deeper understanding of the coupling of phonons with spins and magnetic excitations in antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2024

Large geometric polarization and magnetic behavior in the multiferroic quasi-2D SrNiF$_4$ fluoride

In the last decades, multifunctional single-crystals that show, for example, multiferroic and magnetoelectric responses have attracted considerable attention due to the potential applications and physical phenomena involved in the entanglement of the ferroic orders. This paper investigates the structural, ferroelectric, and magnetic properties of the unexplored layered SrNiF$_4$ fluoride compound. We show that, in terms of the vibrational phonon modes, this fluoride compound shows a tangible ferroelectric behavior with a spontaneous polarization as large as $P_s$ = 14.8 $μ$C$\cdot$cm$^{-2}$ standing as one of the largest among its family. Besides, based on our findings such ferroelectric polarization presents a geometric origin that is strongly entangled with the octahedral rotations within the $Γ_2^-$ phonon mode and enhanced by the structure's $Γ_1^+$ mode. We also observed that the spontaneous polarization is coupled to the noncollinear $G$-type antiferromagnetic ordering in the structure allowing a switching of the weak antiferromagnetic component when the polarization is reversed.

cond-mat.mtrl-sci↗

arXiv2024

Promises and technological prospects of two-dimensional Rashba materials

The Rashba spin-orbit coupling effect, primarily arising from structural-inversion asymmetry in periodic crystals, has garnered considerable attention due to its tunability and potential applications in spintronics. Its capability to manipulate electron spin without an external magnetic field opens new avenues for spintronic device design, particularly in semiconductor technology. Within this framework, 2D Rashba materials hold special interest due to their inherent characteristics, which facilitate miniaturization and engineering capabilities. In this Perspective article, we provide an overview of recent advancements in the research of 2D Rashba materials, aiming to offer a comprehensive understanding of the diverse manifestations and multifaceted implications of the Rashba effect in material science. Rather than merely presenting a list of materials, our approach involves synthesizing various viewpoints, assessing current trends, and addressing challenges within the field. Our objective is to bridge the gap between fundamental research and practical applications by correlating each material with the necessary advancements required to translate theoretical concepts into tangible technologies. Furthermore, we highlight promising avenues for future research and development, drawing from insights gleaned from the current state of the field.

cond-mat.mtrl-sci↗

arXiv2023

Spin-phonon coupling suppressing the structural transition in perovskite-like oxide

Multifunctional properties in quantum systems require the interaction between different degrees of freedom. As such, spin-phonon coupling emerges as an ideal mechanism to tune multiferroicity, magnetism, and magnetoelectric response. In this letter, we demonstrate and explain, based on theoretical and experimental analyses, an unusual manifestation of spin-phonon coupling, i.e., prevention of a ferroelastic structural transition, and locking of high-temperature R-3m phase in a magnetically frustrated perovskite-like oxide Ba2NiTeO6. We present Ba2NiTeO6 as a prototype example among its family where long-range antiferromagnetic structure couples with a low-frequency Eg mode (at 55 cm-1) that exhibits a large anharmonicity. Our findings establish that spin-phonon coupling clearly suppresses the phonon anharmonicity preventing the structural phase transition from the R-3m to the C2/m phase in Ba2NiTeO6.

cond-mat.mtrl-sci↗

arXiv2023

Chiral magnetism, lattice dynamics, and anomalous Hall conductivity in the novel V$_3$AuN antiferromagnetic antiperovskite

Antiferromagnetic antiperovskites, where magnetically active 3$d$ metal cations are placed in the octahedral corners of a perovskite structure, are in the spotlight due to their intertwined magnetic structure and topological properties. Especially their anomalous Hall conductivity, which can be controlled by applied strain and/or electric field, makes them highly attractive in different electronic applications. Here, we present the study and theoretical understanding of a new antiperovskite compound that can offer enormous opportunities in a broad set of applications. Using first-principles calculations, we investigated the structure, lattice dynamics, noncollinear magnetic ordering, and electronic behavior in the Vanadium-based antiperovskite V$_3$AuN. We found an antiperovskite structure centered on N similar to the Mn$_3A$N family as the structural ground state. In such a phase, a \emph{Pm$\bar{3}$m} ground state was found in contrast to the \emph{Cmcm} post-antiperovskite layered structure, as in the V$_3A$N, $A$ = Ga, Ge, As, and P. We studied the lattice dynamics and electronic properties, demonstrating its vibrational stability in the cubic structure and a chiral antiferromagnetic noncollinear ordering as a magnetic ground state. Finally, we found that the anomalous Hall conductivity, associated with the topological features induced by the magnetic symmetry, is $σ_{xy}$ = $-$291 S$\cdot$cm$^{-1}$ ($σ_{111}$ = $-$504 S$\cdot$cm$^{-1}$). The latter is the largest reported in the antiferromagnetic antiperovskite family of compounds.

cond-mat.str-el↗

arXiv2023

Kagome KMn$_3$Sb$_5$ metal: Magnetism, lattice dynamics, and anomalous Hall conductivity

Kagome metals are reported to exhibit remarkable properties, including superconductivity, charge density wave order, and a large anomalous Hall conductivity, which facilitate the implementation of spintronic devices. In this work, we study a novel kagome metal based on Mn magnetic sites in a KMn$_3$Sb$_5$ stoichiometry. By means of first-principles density functional theory calculations, we demonstrate that the studied compound is dynamically stable, locking the ferromagnetic order as the ground state configuration, thus preventing the charge-density-wave state as reported in its vanadium-based counterpart KV$_3$Sb$_5$. Our calculations predict that KMn$_3$Sb$_5$ exhibits an out-of-plane (001) ferromagnetic response as the ground state, allowing for the emergence of topologically protected Weyl nodes near the Fermi level and nonzero anomalous Hall conductivity ($σ_{ij}$) in this centrosymmetric system. We obtain a tangible $σ_{xy} = 314$ S$\cdot$cm$^{-1}$ component, which is comparable to that of other kagome metals. Finally, we explore the effect of the on-site Coulomb repulsion ($+U$) on the structural and electronic properties and find that, although the lattice parameters and $σ_{xy}$ moderately vary with increasing $+U$, KMn$_3$Sb$_5$ stands as an ideal stable ferromagnetic kagome metal with a large anomalous Hall conductivity response.

cond-mat.str-el↗

arXiv2023

Experimental observation of metallic states with different dimensionality in a quasi-1D charge density wave compound

TaTe$_4$ is a quasi-1D tetrachalcogenide that exhibits a CDW instability caused by a periodic lattice distortion. Recently, pressure-induced superconductivity has been achieved in this compound, revealing a competition between these different ground states and making TaTe$_4$ very interesting for fundamental studies. Although TaTe$_4$ exhibits CDW ordering below 475 K, transport experiments have reported metallic behavior with a resistivity plateau at temperatures lower than 10 K. In this paper, we study the electronic structure of TaTe$_4$ using a combination of high-resolution angle-resolved photoemission spectroscopy and density functional calculations. Our results reveal the existence of the long-sought metallic states. These states exhibit mixed dimensionality, while some of them might have potential topological properties.

cond-mat.str-el↗

arXiv2023

Anionic nickel and nitrogen effects in the chiral antiferromagnetic antiperovskite Mn$_3$NiN

Magnetic antiperovskites, holding chiral noncollinear antiferromagnetic ordering, have shown remarkable properties that cover from negative thermal expansion to anomalous Hall effect. Nevertheless, details on the electronic structure related to the oxidation states and the octahedral center's site effect are still scarce. Here, we show a theoretical study, based on first-principles calculations in the framework of the density-functional theory, DFT, on the electronic details associated with the nitrogen site effect into the structural, electronic, magnetic, and topological degrees of freedom. Thus, we show that the nitrogen-vacancy increases the values of the anomalous Hall conductivity and retains the chiral $Γ_{4g}$ antiferromagnetic ordering. Moreover, we reveal, based on the Bader charges and the electronic structure analysis, the negative and positive oxidation states in the Ni and Mn sites, respectively. The latter is in agreement with the expected $A_3^{α+}B^{β-}X^{δ-}$ oxidation states to satisfy the charge neutrality in the antiperovskites, but rare for transition metals. Finally, we extrapolate our findings on the oxidation states to several Mn$_3B$N compounds showing that the antiperovskite structure is an ideal platform to encounter negative oxidation states in metals sitting at the corner $B$-site.

cond-mat.mtrl-sci↗

arXiv2022

Lattice dynamics across the ferroelastic phase transition in Ba2ZnTeO6: A Raman and first-principles study

Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. With this note, we have investigated the structural transition of perovskite-type trigonal Ba2ZnTeO6, across Tc ~ 150 K, which is also accompanied by a para- to ferroelastic transition. With the help of Raman spectroscopy and density-functional theory (DFT)-based calculations, here we report new intriguing observations associated with the phase transition in Ba2ZnTeO6 elucidating the lattice dynamics across the Tc. We have observed the presence of a central peak (quasi-elastic Rayleigh profile), huge softening in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of a central peak in Ba2ZnTeO6 is manifested by a sharp rise in the intensity of the Rayleigh profile in concomitant with the damping of the soft mode near Tc, shedding light on the lattice dynamics during the phase transition. While most of the phonon bands split below Tc confirming the phase transition, we have observed thermal hysteretic behavior of phonon modes that signifies the first-order nature of the transition and presence of coexisting phases, which are corroborated by our temperature-dependent x-ray diffraction and specific heat measurements. Further, an evidence of the concomitant structural transition appears in the form of huge softening in the thermal response of the soft phonon mode at ~ 31 cm-1 which is remarkable compared to the hitherto known behavior of soft modes in well-known ferroelectrics. This is further corroborated by our phonon calculations that show an unstable Eg-mode in the high-symmetry structure involving TeO6 octahedral rotation (with Ba and Zn translation) which later condenses into the C2/m low-symmetry phase.

cond-mat.mtrl-sci↗

arXiv2022

Anomalous Hall conductivity control in Mn$_3$NiN antiperovskite by epitaxial strain along the kagome plane

Antiferromagnetic manganese-based nitride antiperovskites, such as Mn$_3$NiN, hold a triangular frustrated magnetic ordering over their kagome lattice formed by the Mn atoms along the (111)-plane. As such, frustration imposes a non-trivial interplay between the symmetric and asymmetric magnetic interactions, which can only reach equilibrium in a noncollinear magnetic configuration. Consequently, the associated electronic interactions and their possible tuning by external constraints, such as applied epitaxial strain, play a crucial role in defining the microscopic and macroscopic properties of such topological condensed matter systems. Thus, in the present work, we explored and explained the effect of the epitaxial strain imposed within the (111)-plane, in which the magnetic and crystallographic symmetry operations are kept fixed, and only the magnitude of the ionic and electronic interactions are tuned. We found a linear shifting in the energy of the band structure and a linear increase/decrease of the available states near the Fermi level with the applied strain. Concretely, the compression strain reduces the Mn-Mn distances in the (111) kagome plane but linearly increases the separation between the stacked kagome lattices and the available states near the Fermi level. Despite the linear controlling of the available states across the Fermi energy, the anomalous Hall conductivity shows a non-linear behavior where the $σ_{111}$ conductivity nearly vanishes for tensile strain. On the other hand, $σ_{111}$ fetches a maximum increase of 26\% about the unstrained structure for a compression value close to $-$1.5\%.This behavior found an explanation in the non-divergent Berry curvature within the kagome plane, which is increased for constraining but significantly reduced for expansion strain values...

cond-mat.str-el↗

arXiv2022

Direct observation of discommensurate charge density wave modulation in the quasi-1D Weyl semimetal candidate NbTe$_4$

The transition-metal tetrachalcogenides are a model system to explore the conjunction of correlated electronic states such as charge density waves (CDW), with topological phases of matter. Understanding the connection between these phases requires a thorough understanding of the individual states, which for the case of the CDW in this system, is still missing. In this paper we combine phonon-structure calculations and scanning tunneling microscopy measurements of NbTe$_4$ in order to provide a full characterization of the CDW state. We find that, at short range, the superstructure formed by the CDW is fully commensurate with the lattice parameters. Moreover, our data reveals the presence of phase-slip domain-walls separating regions of commensurate-CDW in the nanoscale, indicating that the CDW in this compound is discommensurate at long-range. Our results solve a long-standing discussion about the nature of the CDW in these materials, and provide a strong basis for the study of the interplay between this state and other novel quantum electronic states.

cond-mat.mtrl-sci↗

arXiv2022

Exploration of the spin-phonon coupling in the noncollinear antiferromagnetic antiperovskite Mn$_3$NiN

Antiferromagnetic antiperovskites, of the form Mn$_3B$N ($B$ = Ni, Cu, Zn, Sn, Ir, and Pt), have shown an outstanding behavior in which, giant negative thermal expansion and chiral magnetic structures are intertwined. As such, aiming to shed light on the magnetostructural behavior, related to the magnetic ordering and structure of this type of materials, we studied by theoretical first-principles calculations the spin-phonon coupling in the Manganese-based antiperovskite Mn$_3$NiN, as a prototype in the Mn$_3B$N family. We found a strong spin-lattice coupling by means of the understanding of the phonon-dispersion curves obtained when including the chiral noncollinear magnetic structure. To do so, we highlight the importance of the exchange-correlation scheme selected and its influence on the electronic, structural, and vibrational degrees of freedom. Finally, we found two unstable vibrational modes at the $M$- and $R$-points when the magnetic ordering is switched to ferromagnetic from the chiral $Γ_{4g}$ and $Γ_{5g}$. The latter coupling was observed as a key signature of the strong spin-lattice interaction.

cond-mat.mtrl-sci↗

arXiv2021

Incipient geometric lattice instability of cubic fluoroperovskites

Inorganic metal halide perovskites are promising materials for next-generation technologies due to a plethora of unique physical properties, many of which cannot be observed in the oxide perovskites. On the other hand, the search for ferroelectricity and multiferroicity in lead-free inorganic halide perovskites remains a challenging research topic. Here, we experimentally show that cubic fluoroperovskites exhibit proximity to incipient ferroelectrics, which manifested in the softening of the low-frequency polar phonons in the Brillouin zone center at cooling. Furthermore, we reveal the coupling between harmonic and anharmonic force constants of the softening phonons and their correlation with the perovskite tolerance factor. Next, using first-principles calculations, we examine the lattice dynamics of the cubic fluoroperovskites and disclose the incipient lattice instability at which the harmonic force constants of low-lying phonons tend to decrease with a reduction of tolerance factor at all high-symmetry points of the Brillouin zone. The correlations with the tolerance factor indicate the geometric origin of observed incipient lattice instability in the cubic fluoroperovskites caused by the steric effect due to the volume filling of the unit cell by different radius ions. These results provide insights into the lattice dynamics and potential ferroelectric properties of inorganic lead-free metal halide perovskites, relevant to further design and synthesis of new multifunctional materials.

cond-mat.mtrl-sci↗

arXiv2021

Engineering of Ferroic Orders in Thin Films by Anionic Substitution

Multiferroics are a unique class of materials where magnetic and ferroelectric orders coexist. The research on multiferroics contributes significantly to the fundamental understanding of the strong correlations between different material degrees of freedom and provides an energy-efficient route toward the electrical control of magnetism. While multiple ABO3 oxide perovskites have been identified as being multiferroic, their magnetoelectric coupling strength is often weak, necessitating the material search in different compounds. Here, we report the observation of room-temperature multiferroic orders in multi-anion SrNbO3-xNx thin films. In these samples, the multi-anion state enables the room-temperature ferromagnetic ordering of the Nb d-electrons. Simultaneously, we find ferroelectric responses that originate from the structural symmetry breaking associated with both the off-center displacements of Nb and the geometric displacements of Sr, depending on the relative O-N arrangements within the Nb-centered octahedra. Our findings not only diversify the available multiferroic material pool but also demonstrate a new multiferroism design strategy via multi-anion engineering.

cond-mat.mtrl-sci↗

arXiv2019

Structural and electronic phenomena at oxyfluoride KTaO$_3$/K$M$F$_3$ ($M$ = Zn and Ni) superlattices: Rashba splitting and 2DEG

Here, we present the theoretical analysis of the structural and electronic degrees of freedom of two different oxide/fluoride perovskite superlattices, KTaO$_3$/KZnF$_3$ and KTaO$_3$/KNiF$_3$. Using first-principles calculations, we found the appearance of a two-dimensional electron, 2DEG, and hole, 2DHG, gases as a function of the number of layers of the different pristine materials. We demonstrate that the phonon-dynamics at the KTO/K$M$F superlattices play a crucial role in the appearance of these effects. Additionally, our results reveal a rather sizeable Rashba-type spin-splitting at these interfaces in comparison to another oxide/oxide counterparts.

cond-mat.mtrl-sci↗

arXiv2018

Direct magnetization-polarization coupling in BaCuF$_4$ fluoride

Despite oxides and some fluorides perovskites have emerged as prototypes of multiferroic and magnetoelectric materials, they have not impacted real devices. Unfortunately, their working temperatures are very low and the magnetoelectric coupling has been reported to be rather small. Herewith, we report from first-principles calculations an ideal magnetization reversal through polarization switching in BaCuF$_4$ which, according to our results, could be achieved close to room temperature. We also show that this ideal coupling is driven by a soft mode that combines both, polarization and octahedral rotation. The later being directly coupled to the weak ferromagnetism of BaCuF$_4$. This, added to its strong Jahn-Teller distortion and its orbital ordering, makes this material as a very appealing prototype for crystals in the $ABX_4$ family in multifunctional applications.

cond-mat.mtrl-sci↗