arXiv subjects

Giovanni Vignale

Publications and source records attributed to Giovanni Vignale.

At least 19 recordsLinked to original sources

arXiv2026

Topological fragility and bilinear magnetoelectric resistance in gapless edge states

In time-reversal symmetric systems such as topological and higher-order topological insulators, 1D spin-momentum locked edge and hinge states are theoretically ``perfectly conducting'', being immune to backscattering by non-magnetic disorder. Here, we reveal a fundamental ``topological fragility'': these states exhibit a bilinear magnetoelectric resistance significantly larger than in 2D systems. This effect requires two ingredients: (i) spin-momentum locking, which maximizes time-reversal symmetry breaking in the non-linear regime, and (ii) random spin-orbit interaction -- the same mechanism behind Elliott - Yafet spin relaxation in heavy elements. Together, these generate a robust backscattering channel when a modest external magnetic field is applied. Our theory requires no gap opening or complex many-body effects, offering a simple and general mechanism that quantitatively explains recent observations in Bismuth hinge states.

cond-mat.mes-hall↗

arXiv2026

Ward identities and orbital magnetization in current density functional theory

We revisit the derivation of the orbital magnetization formula for periodic crystals in current density functional theory (CDFT)[1]. Our new derivation computes the linear response of the energy density to a periodic magnetic field in the long-wavelength limit. We unveil a Ward identity which connects the current vertex to the derivative of the Kohn-Sham self-energy. The result of Ref.[1] is confirmed: the orbital magnetization of the interacting solid can be computed exactly (in principle) from the self-consistent eigenfunctions and eigenvalues of the Kohn-Sham equation of CDFT.

cond-mat.mtrl-sci↗

arXiv2026

Continuous correlated states and dual-flatness in a moir\'e heterostructure

Many-body effects in condensed matter yield novel quantum states when the electronic density of states is enhanced. A vivid example is flat bands, which suppress kinetic energy and let interactions dominate, when they are filled with an integer number of electrons in moire systems. Yet flat bands and commensurate fillings are not the only conditions for correlated phenomena. Situations may occur where the band structure develops locally enhanced density of states, leading to strong correlations even at non-integer fillings, although such cases often yield pseudogaps that make detection elusive. Here we demonstrate that small-angle twisted monolayer-bilayer graphene combines moire-induced global flat band and additional local band flattening. Their coexistence allows direct comparison of correlated effects. The global route stabilizes commensurate states, while the local mechanism produces nearly flat bands, lifting degeneracy and generating symmetry breaking at non-integer fillings, yet without opening a global gap. Because there is no global gapped signature, the system remains metallic, but the effect reveals itself in anomalous Hall responses, signaling time-reversal symmetry breaking and valley polarization. Our results demonstrate dual-flatness as a guiding principle, extending moire physics beyond commensurate fillings and identifying topological transport as a probe of gapless correlated metals.

cond-mat.str-el↗

arXiv2026

Low-Field Metal-Insulator Transition in AB-Stacked Bilayer Graphene

We investigate the interplay of in-plane magnetic and transverse electric fields in AB-stacked bilayer graphene. In prior work neglecting trigonal warping, we demonstrated that this configuration induces an insulator-metal transition purely via orbital effects, albeit requiring impractically large magnetic fields ($>100$ T). Here, we extend the analysis to the ultra-low-energy regime by incorporating interlayer skew couplings. In a restricted region of momentum space, trigonal warping produces a fine splitting of Dirac cones leading to a compensated semimetallic state at zero external field. Application of a transverse electric field above a small threshold ($V_c\sim0.6$ meV) reinstates an insulating gap. When an in-plane magnetic field is applied, the orbital gauge vector immediately breaks the $C_3$ spatial symmetry of the lattice, and we uncover two sequential, field-driven transitions separated by an order of magnitude in scale. First, at fields ($B \approx 1$ T), the Zeeman effect drives an indirect insulator-to-semimetal transition at the ungated charge neutrality point; the cross-spin gap closes, generating distinct electron and hole pockets separated in momentum space. Second, as the field increases to $B \approx 10$--$25$ T, the orbital coupling closes the same-spin gap at an energy well away from the charge-neutral Fermi level. By electrostatically tuning the Fermi level to this specific gap-closing energy, we reveal the emergence of $C_3$-broken same-spin Fermi pockets, accompanied by a distinct step-like onset in the density of states. This dual-transition regime provides a highly sensitive platform for tunable, spin-selective transport.

cond-mat.mes-hall↗

arXiv2026

Two-Dimensional Twisted Ferromagnetic Domain Wall as a Spin-Wave Diffraction Grating

We present a theoretical study of spin-wave scattering by a twisted domain wall (DW) in a two-dimensional ferromagnet with easy-axis anisotropy. While the twisted DW generates an effective gauge field for spin waves, leading to a deflection of their trajectories, our main focus is on a distinct effect that arises when a hard-axis anisotropy is present in addition to the easy-axis anisotropy. In this case, the translational symmetry of the spin-wave Hamiltonian along the DW is broken, resulting in a periodic modulation of the Hamiltonian. This periodicity leads to the formation of multiple diffracted spin wave modes on both sides of the DW, engendering a DW-induced magnonic diffraction pattern. The interplay between the emergent gauge field and the anisotropy-induced periodicity reveals rich spin-wave dynamics and suggests potential applications for manipulating magnon flow in two-dimensional magnetic textures.

cond-mat.mes-hall↗

arXiv2025

Nonequilibrium Exchange Nonlinear Hall Effect

Quantum geometric electronic responses are often viewed through a non-interacting lens: independent quasiparticles accumulate Berry phases as they move through a static crystal and background potential. Here we argue that the combined action of electron-electron interactions and an out-of-equilibrium many-body state can produce striking departures from this familiar picture. We demonstrate how nonequilibrium exchange interactions produce a nonequilibrium collective quantum geometry distinct from that of its equilibrium ground state. We find this manifests as an exchange induced nonlinear Hall effect with nonlinear Hall current signals competitive with that of well-known non-interacting mechanisms. This highlights the critical role electron interactions and nonequilibrium states can play in the nonlinear response of quantum matter.

cond-mat.mes-hall↗

arXiv2025

Altermagnetism, Kagome Flat Band, and Weyl Fermion States in Magnetically Intercalated Transition Metal Dichalcogenides

Altermagnetic (AM) compounds have recently emerged as a promising platform for realizing unconventional quantum phases, enabled by their unique spin-split band structure at zero net magnetization. Here, we present a first-principles investigation of magnetically intercalated transition metal dichalcogenides (TMDs) of the form XY$_4$Z$_8$ (X $=$ Mn, Fe, Co, Ni, Cr, or V; Y $=$ Nb or Ta; and Z $=$ Se or S), identifying a subset of new versatile AM candidates. Our results establish a systematic correlation between interatomic geometry, quantified by the ratio of interlayer to intralayer spacing, and the magnetic ground states. Systems with A-type antiferromagnetic order exhibit momentum-dependent spin splitting consistent with AM behavior. The combination of the AM spin-splitting and the spin-orbit coupling leads to the emergence of Weyl nodes together with the corresponding topological Fermi arc surface states. Moreover, we identify flat bands near the Fermi level that originate from the intercalant-induced formation of an effective kagome-like sublattice in the TMD layer. These results collectively establish magnetically intercalated TMDs as a promising platform for engineering altermagnetism, flat bands, and Weyl fermions within a single material family, facilitating the development of topological and spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Physical spin torques from exactly constrained exchange-correlation torques

The problem of capturing physical spin torques in non-collinear magnetic systems has dominated the scene of spin-density functional theory (SDFT) in the last two decades. Progress has been hindered by the fact that the spin torque is directly connected to the divergence of the spin current, a quantity that is {\em extraneous} to SDFT -- thus leading to {\em spurious} exchange-correlation (xc) torques in the spin dynamics. Moreover, SDFT cannot rigorously include vector potentials and spin-orbit couplings. Here, we propose a solution that exploits the U(1)$\times$SU(2)-invariance of the xc energy of SpinCurrent-DFT (SCDFT) -- an exact constraint that is not accessible to SDFT. Non-vanishing xc torques obtained on non-collinear solutions are constrained by the aforementioned exact internal symmetry and do not enter the propagation of the spin magnetization -- i.e., the spin dynamics involve {\em only} the physical currents and physical spin-torques.

cond-mat.mtrl-sci↗

arXiv2025

Orbital-Free Density Functional Theory for Periodic Solids: Construction of the Pauli Potential

The practical success of density functional theory (DFT) is largely credited to the Kohn-Sham approach, which enables the exact calculation of the non-interacting electron kinetic energy via an auxiliary noninteracting system. Yet, the realization of DFT's full potential awaits the discovery of a direct link between the electron density, $n$, and the non-interacting kinetic energy, $T_{S}[n]$. In this work, we address two key challenges towards this objective. First, we introduce a new algorithm for directly solving the constrained minimization problem yielding $T_{S}[n]$ for periodic densities -- a class of densities that, in spite of its central importance for materials science, has received limited attention in the literature. Second, we present a numerical procedure that allows us to calculate the functional derivative of $T_{S}[n]$ with respect to the density at constant electron number, also known as the Kohn-Sham potential $V_{S}[n](\rv)$. Lastly, the algorithm is augmented with a subroutine that computes the ``derivative discontinuity", i.e., the spatially uniform jump in $V_{S}[n](\rv)$ which occurs upon increasing or decreasing the total number of electrons. This feature allows us to distinguish between ``insulating" and ``conducting" densities for non interacting electrons. The code integrates key methodological innovations, such as the use of an adaptive basis set (``equidensity orbitals") for wave function expansion and the QR decomposition to accelerate the implementation of the orthogonality constraint. Notably, we derive a closed-form expression for the Pauli potential in one dimension, expressed solely in terms of the input density, without relying on Kohn-Sham eigenvalues and eigenfunctions. We validate this method on one-dimensional periodic densities, achieving results within ``chemical accuracy".

physics.comp-ph↗

arXiv2025

Many-body perturbation theory for moir\'{e} systems

Moir\'{e} systems such as magic-angle twisted bilayer graphene have attracted significant attention due to their ability to host correlated phenomena including superconductivity and strongly correlated insulating states. By defining the single-particle Green's function in the band basis, we systematically develop a many-body perturbation theory framework to address correlations beyond the usual mean-field Hartree-Fock approaches. As a specific example, we first analyze twisted bilayer graphene within the Hartree-Fock approximation. We derive analytical solutions for symmetry-breaking states at integer fillings and the finite-temperature metal-insulator transition that closely match previously known numerical results in the literature. Moving beyond Hartree-Fock, we incorporate self-consistent GW corrections demonstrating that first-order diagrams significantly overestimate the filling-dependent fluctuations in the electronic compressibility. This framework provides a comprehensive pathway for exploring strong electronic correlations in moir\'{e} systems beyond mean-field, giving new insights into the interplay of symmetry breaking and electron correlations.

cond-mat.str-el↗

arXiv2024

Thermal magnetoresistance from magnon scattering from a domain wall in an antiferromagnetic insulator

We theoretically investigate magnon heat transport in an antiferromagnetic (AFM) insulator containing a domain wall (DW) in the presence of a magnetic field applied along the easy axis. We show that the intrinsic spin of the DW couples to the external magnetic field which modifies the transmission of spin wave (SW) through the DW. Applying the magnetic field lifts the degeneracy between two AFM magnon modes and results in different occupation numbers for the two magnon modes. Combined with the finite reflection of a narrow domain wall, this is found to have a significant impact on the magnon heat transport, giving rise to thermal magnetoresistance. Our findings suggest that an AFM DW can be used as a controllable element for regulating the magnon heat current in magnonic devices through the application of a magnetic field.

cond-mat.mes-hall↗

arXiv2024

Diagonalization without Diagonalization: A Direct Optimization Approach for Solid-State Density Functional Theory

We present a novel approach to address the challenges of variable occupation numbers in direct optimization of density functional theory (DFT). By parameterizing both the eigenfunctions and the occupation matrix, our method minimizes the free energy with respect to these parameters. As the stationary conditions require the occupation matrix and the Kohn-Sham Hamiltonian to be simultaneously diagonalizable, this leads to the concept of ``self-diagonalization,'' where, by assuming a diagonal occupation matrix without loss of generality, the Hamiltonian matrix naturally becomes diagonal at stationary points. Our method incorporates physical constraints on both the eigenfunctions and the occupations into the parameterization, transforming the constrained optimization into an fully differentiable unconstrained problem, which is solvable via gradient descent. Implemented in JAX, our method was tested on aluminum and silicon, confirming that it achieves efficient self-diagonalization, produces the correct Fermi-Dirac distribution of the occupation numbers and yields band structures consistent with those obtained with SCF methods in Quantum Espresso.

physics.chem-ph↗

arXiv2024

Nonconserved Density Accumulations in Orbital Hall Transport: Insights from Linear Response Theory

We present a linear response theory for stationary density accumulations in anomalous transport phenomena, such as the orbital Hall effect, where the transported density is odd under time reversal and the underlying charge is not conserved. Our framework applies to both metals and insulators, topologically trivial or nontrivial, and distinguishes between contributions from bulk and edge states, as well as undergap and dissipative currents. In time-reversal invariant systems, we prove a microscopic reciprocity theorem showing that only dissipative currents at the Fermi level contribute to density accumulation, while undergap currents do not. In contrast, in non-time-reversal invariant systems, non-dissipative density accumulations, such as magnetoelectric polarization, can appear in both the bulk and edges. Importantly, we find that the net density accumulation does not always vanish, pointing to a global non-conservation that implies the existence of a non-vanishing integrated ``net torque'' in addition to a ``distributed torque'', which has zero spatial average. We show that the distributed torque can be absorbed in the divergence of a redefined current that satisfies Onsager reciprocity, while the net torque must be explicitly accounted for. Finally, we apply our theory to two-dimensional models with edge terminations.

cond-mat.mes-hall↗

arXiv2024

Superconducting Berry Curvature Dipole

Superconductivity and Bloch band Berry curvature responses represent two distinct paradigms of quantum coherent phenomena. The former relies on the collective motion of Cooper pairs while the latter proceeds from the momentum-space winding of Bloch wave functions. Here we reveal a superconducting Berry curvature dipole (BCD) that arises as a collective phenomenon in noncentrosymmetric superconductors. Strikingly, we find the superconducting BCD is sensitive to the phase of the order parameter and depends on the noncentrosymmetric structure of its pairing. This unusual property enables a BCD proximity effect in hybrid quantum materials that induces nonreciprocity even in a target centrosymmetric metal. We find a superconducting BCD naturally produces nonreciprocal electromagnetic responses that include dissipationless supercurrent-induced dynamical Hall conductivity as well as a giant second-order nonlinearity. This renders noncentrosymmetric superconductors an exciting platform for realizing unconventional dissipationless responses and their BCD responses a novel diagnostic of the structure of the superconducting gap.

cond-mat.supr-con↗

arXiv2024

Meta-Generalized-Gradient Approximation made Magnetic

The Jacob's ladder of density functional theory (DFT) proposes the compelling view that by extending the form of successful approximations -- being guided by exact conditions and selected (least empirical) norms -- upper rungs will do better than the lower, thus allowing to balance accuracy and computational effort. Meta-generalized-gradient-approximations (MGGAs) belong to the last rung of the semi-local approximations before hybridization with non-local wave function theories. Among the MGGAs, the Strongly Constrained and Appropriately Normed Approximation (SCAN) greatly improves upon GGAs from the lower rung. But the over magnetized solutions of SCAN make GGAs more reliable for magnetism. Here, we provide a solution that satisfies the most pressing {\em desiderata} for density functional approximations for ferromagnetic, antiferromagnetic and non-collinear states. The approach is available in an implementation in the \textsc{Crystal} electronic structure package.

cond-mat.mtrl-sci↗

arXiv2024

Insulator-Metal Transition and Magnetic Crossover in Bilayer Graphene

In-plane magnetic fields offer a relatively unexplored opportunity to alter the band structure of stacks of 2D materials so that they exhibit desired physical properties. Here we show that an in-plane magnetic field combined with a transverse electric field can induce an insulator-metal (IM) transition in bilayer graphene. Our study of the magnetic response reveals that the orbital magnetic susceptibility changes from diamagnetic to paramagnetic around the transition point. We discuss several strategies to observe the IM transition, switch the diamagnetism, and more generally control the band structure of stacked 2D materials at experimentally accessible magnetic fields.

cond-mat.mes-hall↗

arXiv2024

Orbital Magnetic Moment Dynamics and Hanle Magnetoresistance in Multilayered 2D Materials

The orbital Hall effect (OHE), resulting from non-trivial quantum geometry of 2D materials, has several potential advantages over the spin Hall effect (SHE), the latter being well known for its many applications in spintronics. Like the spin Hall effect, the OHE occurs in nonmagnetic materials without stringent symmetry requirements, but unlike the SHE it does no rely on relatively weak spin-orbit interaction. In 2D materials, these advantages risk to be nullified by the difficulty of turning the orbital moment away from the out-of-plane direction. Multilayered 2D materials offer a way out of this difficulty because the fluctuating in-plane component of the orbital moment, due to motion of electrons between the layers, can latch to a magnetic field. To describe this effect we have derived a semi-phenomenological equation of motion for the density of orbital magnetic moment in stacked 2D materials subjected to a magnetic field. Unlike the equations of motion for the spin, these equations produce a strongly anisotropic dynamics, which is governed by an inverse effective mass tensor for which we provide a fully microscopic expression. As a first application, we combine our equation of motion with phenomenological drift-diffusion equations to obtain a theory of orbital Hanle magnetoresistance in multilayered 2D materials.

cond-mat.mes-hall↗

arXiv2024

Control of spin-wave polarity and velocity using a ferrimagnetic domain wall

We present a theoretical study of the scattering of spin waves by a domain wall (DW) in a ferrimagnetic (FiM) spin chain in which two sublattices carry spins of unequal magnitudes. We find that a narrow, but atomically smooth FiM DW exhibits a different behavior in comparison with similarly smooth ferromagnetic and antiferromagnetic DWs due to the inequivalence of the two sublattices. Specifically, for sufficiently weak anisotropy, the smaller spin at the center of the DW is found to become precisely normal to the easy-axis, selecting an arbitrary direction in the $xy$-plane and thereby breaking the U(1) spin-rotational symmetry spontaneously. This particular form of a FiM DW does not occur in antiferromagnetic systems and is shown to lead to a strong dependence of spin wave scattering pattern on the state of polarization of the spin wave, which can be either right-handed or left-handed, suggesting the utilization of such a narrow DW as a spin-wave filter. Moreover, we find that in the case of an atomically sharp DW, where all the spins point either up or down due to strong easy-axis anisotropy and therefore the polarization of the spin wave is conserved upon transmission, the wave vector of the spin wave changes after passing through the DW leading to a change in the group velocity of the spin wave. This change of the wave vector indicates the acceleration or deceleration of the spin waves and thus a sharp FiM DW could serve as a spin wave accelerator or decelerator in spintronics devices, offering a functionality absent in a ferromagnetic and an antiferromagnetic counterpart. Our results indicate that FiM spin textures can interact with spin waves distinctly from ferromagnetic and antiferromagnetic counterparts, suggesting that they may offer spin-wave functionalities that are absent in more conventional magnets.

cond-mat.mes-hall↗