arXiv subjects

Pablo S. Cornaglia

Publications and source records attributed to Pablo S. Cornaglia.

At least 19 recordsLinked to original sources

arXiv2026

Tunable Emergent Gauge Fields from Skyrmions in a Quasicrystalline Lattice

We study magnetic skyrmions in a two-dimensional quasicrystalline lattice using a classical Heisenberg model with Dzyaloshinskii-Moriya interactions and an external magnetic field. The competition between the skyrmion-skyrmion repulsion and an emergent quasiperiodic pinning landscape gives rise to a sequence of distinct skyrmion lattice configurations as a function of field. The resulting hierarchy of quasiperiodic pinning potentials, characterized by closely spaced quasi-degenerate minima, enables a quasi-continuous suppression of the skyrmion density as the saturation field is approached, in sharp contrast to the strongly first-order collapse of skyrmion crystals on periodic lattices. This provides a direct mechanism for controlling the topological charge and, consequently, the emergent gauge field for itinerant electrons. As a consequence, the Hall conductivity can be strongly modified with small changes in the magnetic field and driven smoothly to zero near saturation. This field-controlled tunability, rooted in the underlying multistability, identifies quasicrystalline magnets as a platform for tunable topological textures, with potential applications in magnetic memory and magnetoelectronic response.

cond-mat.str-el↗

arXiv2026

A minimal model for the Weyl nodes and Fermi arcs of PtBi$_2$

Weyl semimetals host topologically protected Fermi arcs on their surfaces, originating from the Chern number of the bulk Weyl nodes. In trigonal PtBi$_2$, superconducting signatures have been associated with the Fermi arcs. Theoretical descriptions of this surface superconductivity have so far relied on effective models that are not directly tied to the microscopic electronic structure of the material. In this work we develop a minimal description of the low-energy bands guided by density functional theory calculations and fully constrained by the relevant crystalline and time-reversal symmetries. The model captures the evolution of the Weyl nodes with spin-orbit coupling, including node annihilations, and is able to describe the spin-momentum locking of the surface Fermi arcs. It reproduces the orbital content and the band topology of PtBi$_2$ and provides a starting point for further studies.

cond-mat.supr-con↗

arXiv2025

Interplay between inversion and translation symmetries in trigonal PtBi$_2$

The trigonal Weyl semimetal PtBi$_2$ presents an intriguing superconducting phase, previously reported to be confined to its topological Fermi arcs within a certain temperature range. This observation highlights the importance of a thorough understanding of its normal phase, particularly the roles that spin-orbit coupling (SOC) and inversion-symmetry breaking play in shaping its band structure. Our density-functional theory calculations reveal that the semimetallic nature of trigonal PtBi$_2$ can be interpreted as stemming from a noncentrosymmetric crystal distortion of a parent structure that drives a metal-to-semimetal transition. This distortion breaks inversion symmetry and, crucially, reduces translational symmetry. Due to its interplay with translational symmetry, inversion-symmetry breaking emerges as the dominant energy scale producing substantial asymmetries ($\sim$ 0.6\,eV) in certain short-range hopping amplitudes, superseding the effects of SOC, whose primary role is to define the characteristics of the low-energy nodal structure and of the topological Fermi arcs. This also applies to the formation of the Weyl nodes closest to the Fermi energy, which are found to exist even in the absence of SOC as a result of the orbital physics associated with the reduced translational symmetry.

cond-mat.supr-con↗

arXiv2025

Gapless Topological Peierls-like instabilities in more than one dimension

A periodic lattice distortion that reduces the translational symmetry folds electron bands into a reduced Brillouin zone, leading to band mixing and a tendency to gap formation, as in the Peierls transition in one-dimensional systems. However, in higher dimensions, the resulting phase can present topological obstructions preventing a complete gap opening. We discuss two different mechanisms for such obstructions, emergent Weyl nodes and symmetry protected band crossings. Based on density-functional calculations, we show these mechanisms are at play in trigonal PtBi$_2$.

cond-mat.other↗

arXiv2024

Quasicrystalline chiral soliton lattices in a Fibonacci helimagnet

We investigate the ground state magnetic configurations of a Fibonacci chain of classical spins with nearest-neighbor ferromagnetic and monoaxial Dzyaloshinskii-Moriya exchange interactions. Our analysis reveals a diverse array of magnetic textures induced by an external magnetic field perpendicular to the Dzyaloshinskii-Moriya vector. These textures exhibit a spectrum ranging from a quasi-fully polarized non-collinear state under high magnetic fields, capable of maintaining metastable chiral soliton topological defects, to a variety of quasicrystalline chiral soliton lattices below a critical field $H_c$. For a range of magnetic fields below $H_c$, the ground state spin textures result from the interplay between an effective quasiperiodic potential influencing the solitons and their repulsive interactions. At lower magnetic fields, the system experiences a commensurate-incommensurate transition, signified by the appearance of discommensurations in the quasicrystalline soliton lattice. In the absence of an external magnetic field, the ground state assumes a helical configuration with a quasiperiodic pitch angle.

cond-mat.str-el↗

arXiv2024

Magnetic structure and component-separated transitions of HoNiSi$_{3}$

HoNiSi$_{3}$ is an intermetallic compound characterized by two successive antiferromagnetic transitions at $T_{N1} = 6.3$ K and $T_{N2} = 10.4$ K. Here, its zero-field microscopic magnetic structure is inferred from resonant x-ray magnetic diffraction experiments on a single crystalline sample that complement previous bulk magnetic susceptibility data. For $T < T_{N2}$, the primitive magnetic unit cell matches the chemical cell. The magnetic structure features ferromagnetic {\it ac} planes stacked in an antiferromagnetic $\uparrow \downarrow \uparrow \downarrow$ pattern. For $T_{N1} < T < T_{N2}$, the ordered magnetic moment points along $\vec{a}$, and for $T < T_{N1}$ a component along $\vec{c}$ also orders. A symmetry analysis indicates that the magnetic structure for $T<T_{N1}$ is not compatible with the presumed orthorhombic $Cmmm$ space group of the chemical structure, and therefore a slight lattice distortion is implied. Mean-field calculations using a simplified magnetic Hamiltonian, including a reduced set of three independent exchange coupling parameters determined by density functional theory calculations and two crystal electric field terms taken as free-fitting parameters, are able to reproduce the main experimental observations. An alternative approach using a more complete model including seven exchange coupling and nine crystal electric field terms is also explored, where the search of the ground state magnetic structure compatible with the available anisotropic magnetic susceptibility and magnetization data is carried out with the help of an unsupervised machine learning algorithm. The possible magnetic configurations are grouped into five clusters, and the cluster that yields the best comparison with the experimental macroscopic data contains the parameters previously found with the simplified model and also predicts the correct ground-state magnetic structure.

cond-mat.str-el↗

arXiv2023

Unveiling Exotic Magnetic Phases in Fibonacci Quasicrystalline Stacking of Ferromagnetic Layers through Machine Learning

In this study, we conduct a comprehensive theoretical analysis of a Fibonacci quasicrystalline stacking of ferromagnetic layers, potentially realizable using van der Waals magnetic materials. We construct a model of this magnetic heterostructure, which includes up to second neighbor interlayer magnetic interactions, that displays a complex relationship between geometric frustration and magnetic order in this quasicrystalline system. To navigate the parameter space and identify distinct magnetic phases, we employ a machine learning approach, which proves to be a powerful tool in revealing the complex magnetic behavior of this system. We offer a thorough description of the magnetic phase diagram as a function of the model parameters. Notably, we discover among other collinear and non-collinear phases, a unique ferromagnetic alternating helical phase. In this non-collinear quasiperiodic ferromagnetic configuration the magnetization decreases logarithmically with the stack height.

cond-mat.str-el↗

arXiv2022

Large magnetocaloric effect in the frustrated antiferromagnet EuIr$_2$P$_2$

We present a theoretical analysis of the magnetocaloric effect in the frustrated antiferromagnet \Eu. Monte Carlo simulations indicate a large magnetic entropy change $ΔS_M^{\text{max}} \sim 5.14$ J kg$^{-1}$K$^{-1}$ at $T=2.45$K for a moderate change in the external magnetic field $ΔB=1.2$ Tesla. The highest value of $ΔS_M$ seems to be associated with the presence of a multicritical point in the magnetic field versus temperature phase diagram (at $B_m\sim 1.2$ Tesla and $T_m\sim 2.2$ K) but it persists with similar large values for a broad range of temperatures $T\lesssim T_m$. Single crystal \Eu~magnetization vs magnetic field experiments allowed us to obtain the magnetocaloric effect for $T>2$ K. The results are consistent with the theoretical analysis and show a large magnetocaloric effect for $T\sim 2.5$ K. Finally, we present a phenomenological Landau functional analysis that provides a simplified description of the phase diagram and of the magnetocaloric effect. Although the magnetic frustration in this system leads to a rich phase diagram, we found no clear indication that it plays a significant role enhancing the magneticaloric effect, other than by reducing the transition temperatures and fields.

cond-mat.str-el↗

arXiv2021

Magnetic properties of chiral EuIr$_2$P$_2$

We present a minimal model that provides a description of the magnetic and thermodynamic properties of \Eu. The model contains two exchange coupling parameters, which are calculated using Density Functional Theory, and a local easy axis magnetic anisotropy term. The classical ground state of the system is a generalization of the well known 120$^\circ$ structure observed in triangular antiferromagnets. Monte Carlo simulations show two phase transitions as a function of the temperature. With increasing temperature, the system transitions from the ground state into a high-entropy collinear antiferromagnet, which in turn at higher temperatures presents a second order transition to a paramagnetic state. A high enough external magnetic field parallel to the anisotropy axis produces a spin-flop transition at low temperatures. The field also reduces the temperature range of stability of the collinear antiferromagnet phase and leads to a single phase transition as a function of the temperature. The reported behavior of the specific heat, the magnetization, and the magnetic susceptibility is in agreement with the available experimental data. Finally, we present the magnetic phase diagrams for magnetic fields parallel and perpendicular to the easy axis.

cond-mat.other↗

arXiv2020

Quasiparticle mass enhancement as a measure of entanglement in the Kondo problem

We analyze the quantum entanglement between opposite spin projection electrons in the ground state of the Anderson impurity model. In this model, a single level impurity with intralevel repulsion U is tunnel coupled to a free electron gas. The Anderson model presents a strongly correlated many body ground state with mass enhanced quasiparticle excitations. We find, using both analytical and numerical tools, that the quantum entanglement between opposite spin projection electrons is a monotonic universal function of the quasiparticle mass enhancement Z. The mass enhancement, which is used to quantify the correlations in quantum many body systems, could therefore be used to quantify spin entanglement.

cond-mat.str-el↗

arXiv2019

Magnetic couplings and magnetocaloric effect in the GdTX (T=Sc, Ti, Co, Fe; X=Si, Ge) compounds

We compute the magnetocaloric effect (MCE) in the GdTX (T=Sc, Ti, Co, Fe; X=Si, Ge) compounds as a function of the temperature and the external magnetic field. To this end we use a density functional theory approach to calculate the exchange-coupling interactions between Gd$^{3+}$ ions on each compound. We consider a simplified magnetic Hamiltonian and analyze the dependence of the exchange couplings on the transition metal T, the p-block element X, and the crystal structure (CeFeSi-type or CeScSi-type). The most significant effects are observed for the replacements Ti $\to$ Sc or Fe $\to$ Co which have an associated change in the parity of the electron number in the 3d level. These replacements lead to an antiferromagnetic contribution to the magnetic couplings that reduces the Curie temperature and can even lead to an antiferromagnetic ground state. We solve the magnetic models through mean field and Monte Carlo calculations and find large variations among compounds in the magnetic transition temperature and in the magnetocaloric effect, in agreement with the available experimental data. The magnetocaloric effect shows a universal behavior as a function of temperature and magnetic field in the ferromagnetic compounds after a scaling of the relevant energy scales by the Curie temperature $T_C$.

cond-mat.mtrl-sci↗

arXiv2019

Minimal model for the magnetic phase diagram of CeTi$_{1-x}$Sc$_{x}$Ge, GdFe$_{1-x}$Co$_{x}$Si, and related materials

We present a theoretical analysis of the magnetic phase diagram of CeTi$_{1-x}$Sc$_{x}$Ge and GdFe$_{1-x}$Co$_{x}$Si as a function of the temperature and the Sc and Co concentration $x$, respectively. CeScGe and GdCoSi, as many other RTX (R=rare earth, T=transition metal, X=p-block element) compounds, present a tetragonal crystal structure where bilayers of R are separated by layers of T and X. While GdFeSi and CeTi$_{0.75}$Sc$_{0.25}$Ge are ferromagnetic, CeScGe and GdCoSi order antiferromagnetically with the R 4f magnetic moments on the same bilayer aligned ferromagnetically and magnetic moments in nearest neighbouring bilayers aligned antiferromagnetically. The antiferromagnetic transition temperature $T_N$ decreases with decreasing concentration $x$ in both compounds and for low enough values of $x$ the compounds show a ferromagnetic behavior. Based on these observations we construct a simplified model Hamiltonian that we solve numerically for the specific heat and the magnetization. We find a good qualitative agreement between the model and the experimental data. Our results show that the main magnetic effect of the Sc $\to$ Ti and Co $\to$ Fe substitution in these compounds is consistent with a change in the sign of the exchange coupling between magnetic moments in neighbouring bilayers. We expect a similar phenomenology for other magnetic RTX compounds with the same type of crystal structure.

cond-mat.mtrl-sci↗

arXiv2018

Hund's metal regimes and orbital selective Mott transitions in three band systems

We analyze the electronic properties of interacting crystal field split three band systems. Using a rotationally invariant slave boson approach we analyze the behavior of the electronic mass renormalization as a function of the intralevel repulsion $U$, the Hund's coupling $J$, the crystal field splitting, and the number of electrons per site $n$. We first focus on the case in which two of the bands are identical and the levels of the third one are shifted by $Δ>0$ with respect to the former. We find an increasing quasiparticle mass differentiation between the bands, for system away from half-filling ($n=3$), as the Hubbard interaction $U$ is increased. This leads to orbital selective Mott transitions where either the higher energy band (for $4>n>3$) or the lower energy degenerate bands ($2<n<3$) become insulating for $U$ larger than a critical interaction $U_{c}(n)$. Away from the half-filled case $|n-3|\gtrsim 0.3$ there is a wide range of parameters for $U<U_c(n)$ where the system presents a Hund's metal phase with the physics dominated by the local high spin multiplets. Finally, we study the fate of the $n=2$ Hund's metal as the energy splitting between orbitals is increased for different possible crystal distortions. We find a strong sensitivity of the Hund's metal regime to crystal fields due to the opposing effects of $J$ and the crystal field splittings on the charge distribution between the bands.

cond-mat.str-el↗

arXiv2018

Magnetostriction Reveals Orthorhombic Distortion in Tetrahedral Gd-compounds

We report detailed thermal expansion and magnetostriction experiments on GdCoIn$_5$ and GdRhIn$_5$ single crystal samples that show a sudden change in the dilation at a field B$^\ast$ for temperatures below the Néel transition temperature TN. We present a first-principles model including crystal-field effects, dipolar and exchange interactions, and the dependence of the exchange couplings with lattice distortions in order to fully account for the magnetostriction and magnetic susceptibility data. The mean-field solution of the model shows that a transition between metastable states occurs at the field B$^\ast$. It also indicates that two degenerate phases coexist in the sample at temperatures below TN. This allows to explain the lack of observation, in high resolution x-ray experiments, of an orthorhombic distortion at the Néeel transition even though the magnetic structure breaks the tetragonal symmetry and the magnetoelastic coupling is significant. These conclusions could be extended to other tetragonal Gd-based compounds that present the same phenomenology.

cond-mat.str-el↗

arXiv2018

Spin-orbit and anisotropic strain effects on the electronic correlations of Sr$_2$RuO$_4$

We present an implementation of the rotationally invariant slave boson technique as an impurity solver for density functional theory plus dynamical mean field theory (DFT+DMFT). Our approach provides explicit relations between quantities in the local correlated subspace treated with DMFT and the Bloch basis used to solve the DFT equations. In particular, we present an expression for the mass enhancement of the quasiparticle states in reciprocal space. We apply the method to the study of the electronic correlations in Sr$_2$RuO$_4$ under anisotropic strain. We find that the spin-orbit coupling plays a crucial role in the mass enhancement differentiation between the quasi-one-dimensional $α$ and $β$ bands, and on its momentum dependence over the Fermi surface. The mass enhancement, however, is only weakly affected by either uniaxial or biaxial strain, even across the Lifshitz transition induced by the strain.

cond-mat.str-el↗

arXiv2018

Landau theory for magnetic and structural transitions in CeCo$_{0.85}$Fe$_{0.15}$Si

We present a phenomenological analysis of the magnetoelastic properties of CeCo$_{0.85}$Fe$_{0.15}$Si at temperatures close to the Néel transition temperature $T_N$. Using a Landau functional we provide a qualitative description of the thermal expansion, magnetostriction, magnetization and specific heat data. We show that the available experimental results [Journal of Physics: Condensed Matter 28 346003 (2016)] are consistent with the presence of a structural transition at $T_s\gtrsim T_N$ and a strong magnetoelastic coupling. The magnetoelastic coupling presents a Janus-faced effect: while the structural transition is shifted to higher temperatures as the magnetic field is increased, the resulting striction at low temperatures decreases. The strong magnetoelastic coupling and the proximity of the structural transition to the onset temperature for magnetic fluctuations, suggest that the transition could be an analogue of the tetragonal to orthorhombic observed in Fe-based pcnictides.

cond-mat.str-el↗

arXiv2017

SU(4) Kondo entanglement in double quantum dot devices

We analyze, from a quantum information theory perspective, the possibility of realizing a SU(4) entangled Kondo regime in semiconductor double quantum dot devices. We focus our analysis on the ground state properties and consider the general experimental situation where the coupling parameters of the two quantum dots differ. We model each quantum dot with an Anderson type Hamiltonian including an interdot Coulomb repulsion and tunnel couplings for each quantum dot to independent fermionic baths. We find that the spin and pseudospin entanglements can be made equal, and the SU(4) symmetry recovered, if the gate voltages are chosen in such a way that the average charge occupancies of the two quantum dots are equal, and the double occupancy on the double quantum dot is suppressed. We present density matrix renormalization group numerical results for the spin and pseudospin entanglement entropies, and analytical results for a simplified model that captures the main physics of the problem.

cond-mat.mes-hall↗

arXiv2017

On the nature of the Mott transition in multiorbital systems

We analyze the nature of Mott metal-insulator transition in multiorbital systems using dynamical mean-field theory (DMFT). The auxiliary multiorbital quantum impurity problem is solved using continuous time quantum Monte Carlo (CTQMC) and the rotationally invariant slave-boson (RISB) mean field approximation. We focus our analysis on the Kanamori Hamiltonian and find that there are two markedly different regimes determined by the nature of the lowest energy excitations of the atomic Hamiltonian. The RISB results at $T\to0$ suggest the following rule of thumb for the order of the transition at zero temperature: a second order transition is to be expected if the lowest lying excitations of the atomic Hamiltonian are charge excitations, while the transition tends to be first order if the lowest lying excitations are in the same charge sector as the atomic ground state. At finite temperatures the transition is first order and its strength, as measured e.g. by the jump in the quasiparticle weight at the transition, is stronger in the parameter regime where the RISB method predicts a first order transition at zero temperature. Interestingly, these results seem to apply to a wide variety of models and parameter regimes.

cond-mat.str-el↗