arXiv subjects

Carmine Ortix

Publications and source records attributed to Carmine Ortix.

At least 19 recordsLinked to original sources

arXiv2026

Magnetically tunable symmetry-enforced nodal lines producing huge anomalous Hall conductivity in altermagnetic $α$-MnTe

Altermagnetic $α$-MnTe exhibits huge anomalous Hall conductivity (AHC) up to room-temperature together with weak ferromagnetism arising from spin and orbital polarizations. We clarify the origin of the large value of the AHC by identifying two sets of distinct symmetry-enforced nodal lines in the valence bands with Mn character, located at $k_z=0$ and $k_z=\fracπ{c}$, protected by mirror symmetry $M_z$ and glide symmetry $G_z = \{M_z\,|\,0,0,\tfrac{c}{2}\}$, respectively. Both nodal lines are energy-dependent with an approximate C$_6$ symmetry, which is reduced to an exact C$_2$ symmetry due to the presence of the Néel vector. The highest valence band exhibits a Mexican-hat dispersion, whereas the second-highest valence band exhibits an inverted Mexican-hat dispersion, with nodal lines at the crossing between the two bands. Within first-principles accuracy, we demonstrate that these nodal lines give rise to the large AHC observed experimentally and exhibit a strong interplay with the weak ferromagnetism. We further show that even a small spin canting strongly modifies the nodal lines and the AHC, making them both magnetically tunable. By disentangling the altermagnetic and ferromagnetic contributions to the AHC, the altermagnetic contribution dominates at small canting angles, while the ferromagnetic contribution becomes sizeable for larger values. Using linear dichroism in angle-resolved photoemission spectroscopy, we show a signature of the nodal line at the border of the Brillouin zone.

cond-mat.mtrl-sci↗

arXiv2026

Spin selectivity induced by non-collinear spins in Rashba wires

We report a previously overlooked general mechanism to obtain highly efficient spin selectivity in conventional time-reversal symmetric one-dimensional systems without invoking phase decoherence. We reveal that Rashba quantum wires featuring non-collinear spin states at the Fermi level inherently possess spin-selective transport properties. We show that this spin noncollinearity can be systematically designed and engineered by introducing an additional pseudospin degree of freedom - such as valley, sublattice, or orbital angular momentum - into spin-orbit coupled systems. By applying this framework to multi-subband semiconducting quantum wires and oxide nanowires, we establish a generalized route toward quantum-coherent spin selectivity up to 10 %. Our findings offer practical design principles for spin-selective transport devices.

cond-mat.mes-hall↗

arXiv2026

Quantum metric quadrupoles in elemental bismuth thin films

The nonlinear transport properties of solids are deeply rooted in the quantum geometry of their electronic wavefunctions, which is encoded in the quantum geometric tensor. Its real part, known as the quantum metric, has been recently identified as a primary origin of nonlinear transport in quantum materials where time-reversal and inversion symmetries are not simultaneously present. Consequently, the influence of the quantum metric on the largest class of materials -- non-magnetic and centrosymmetric systems -- has remained entirely elusive. Here, we demonstrate that third-order transport in centrosymmetric materials hosting relativistic fermions is governed by quantum metric quadrupoles (QMQs). We show that these QMQs can originate from both the non-Abelian quantum geometry of bulk three-dimensional Dirac fermions and the Abelian quantum geometry of spin-orbit-coupled surface states. In stark contrast to all zero-field nonlinear transport signatures known to date, the current driven by these QMQs persists as a robust, non-vanishing observable even in highly scalable polycrystalline thin films. We experimentally validate this quantum metric footprint by measuring nonlinear transport in thin films of elemental bismuth, observing a robust, surface-dominated, and broadband third-harmonic generation that persists up to room temperature. Our findings uncover a hidden role of the quantum metric in polycrystalline systems, establishing third-order nonlinear transport as a high-precision diagnostic tool of wavefunction geometry under ambient conditions.

cond-mat.mes-hall↗

arXiv2026

Transverse Magnetic Response from Orbitally Polarized Cooper Pairs in Elemental Superconductors

We demonstrate how crystalline symmetry lowering, as for instance through strain, allows elemental superconductors such as vanadium and niobium to realize spin-singlet orbitally polarized Cooper pairs composed of electrons with identical orbital moments. Using superconducting density functional theory, we show that lowering of trigonal symmetry to $C_s$, thus keeping only a single mirror plane, activates interorbital pairing in bulk and (111) surfaces, with a pronounced surface enhancement. In a magnetic field, the resulting orbitally polarized superconducting state leads to a novel transverse magnetic response. For in--plane field orientations that break the remaining mirror symmetry, a sizable orbital magnetization emerges perpendicular to the applied field. We show that this effect is a direct consequence of equal--orbital-moment Cooper pairing, providing an experimentally accessible signature of this state. Our results establish strained elemental superconductors as a minimal material platform for superconducting orbitronics.

cond-mat.supr-con↗

arXiv2026

Curvature-induced nonlinear anomalous Hall effect in thin magnetic shells

Optoelectronic and nonlinear transport experiments probe the quantum geometric tensor of Bloch states, whose real and imaginary components -- the quantum metric and the Berry curvature -- are typically constrained by symmetry. Here, we show that geometric bending provides a route to engineer such responses in centrosymmetric ferromagnets. Curvature-induced strain gradients across the shell thickness break inversion symmetry and activate an orbital Rashba coupling. In the presence of in-plane magnetization and spin-orbit coupling, this generates spin textures with a nontrivial quantum geometry, leading to an intrinsic nonlinear anomalous Hall effect (NAHE) governed by the quantum metric and maximized when the magnetization aligns with the applied electric field. When geometric deformations further break twofold rotational symmetry around the out-of-plane axis, an additional NAHE emerges, maximal for magnetization perpendicular to the driving electric field and governed by the Berry curvature dipole, thus giving access to the imaginary component of the quantum geometric tensor. These results establish curved ferromagnetic shells as a platform for engineering anisotropic nonlinear transport and for selectively probing both components of the quantum geometric tensor.

cond-mat.mes-hall↗

arXiv2026

Probing the quantum metric of 3D topological insulators

The surface states of 3D topological insulators possess geometric structures that imprint distinctive signatures on electronic transport. A prime example is the Berry curvature, which controls, for instance, electric frequency doubling via its higher order moments. In addition to the Berry curvature, topological surface states are expected to exhibit a nontrivial quantum metric, which plays a key role in governing nonlinear magnetotransport. However, its manifestation has yet to be experimentally observed and controlled in 3D topological insulators. Here, we provide evidence for a nonlinear response activated by the quantum metric of the topological surface states of Sb$_2$Te$_3$. We measure a time-reversal odd, nonlinear magnetoresistance that is independent from the temperature and the scattering time below 30 K, and is thus of intrinsic geometrical origin. This quantum metric magnetoresistance can be controlled by tuning the contributions of the top and bottom topological surface states by voltage gating. Our measurements thus demonstrate the existence and tunability of quantum geometry-induced transport in topological phases of matter and provide strategies for designing novel functionalities in topological devices.

cond-mat.mes-hall↗

arXiv2025

Topological spin multipolization and linear magnetoelectric coupling in two-dimensional antiferromagnets

In this paper we predict that the magnetoelectric response of two-dimensional (2D) antiferromagnets is determined by the topology of the ground state. This topological magnetoelectric response, encoded in the spin magnetoelectric polarizability and its closely related spin multipolization, occurs when the electronic structure of the antiferromagnetic insulator is described by massive 2D Dirac fermions, and is therefore native to 2D, unlike the topological magnetoelectric effect of three-dimensional topological insulators. To demonstrate the topological contribution to the (spin) magnetoelectric polarizability, we compute the magnetoelectric polarizability microscopically for two distinct minimal lattice models: a spin-orbit coupled Néel antiferromagnet and a spin-orbit-free noncollinear antiferromagnet with double-$Q$ spin order. We show that the topological origin of the revealed magnetoelectric effect can be traced back to the electromagnetic response of topological semimetals in two dimensions, and hence is ultimately governed by a strong topological invariant in one dimension. Given this dimensional hierarchy, we further consider two minimal lattice models in one dimension, both one-dimensional variants of the 2D lattice models, and show that the magnetoelectric polarizability exhibits a clear signature of nontrivial crystalline topology. Possible material realizations are discussed.

cond-mat.str-el↗

arXiv2025

Room temperature Planar Hall effect in nanostructures of trigonal-PtBi2

Trigonal-PtBi2 has recently garnered significant interest as it exhibits unique superconducting topological surface states due to electron pairing on Fermi arcs connecting bulk Weyl nodes. Furthermore, topological nodal lines have been predicted in trigonal-PtBi2, and their signature was measured in magnetotransport as a dissipationless, i.e. odd under a magnetic field reversal, anomalous planar Hall effect. Understanding the topological superconducting surface state in trigonal-PtBi2 requires unravelling the intrinsic geometric properties of the normal state electronic wavefunctions and further studies of their hallmarks in charge transport characteristics are needed. In this work, we reveal the presence of a strong dissipative, i.e. even under a magnetic field reversal, planar Hall effect in PtBi2 at low magnetic fields and up to room temperature. This robust response can be attributed to the presence of Weyl nodes close to the Fermi energy. While this effect generally follows the theoretical prediction for a planar Hall effect in a Weyl semimetal, we show that it deviates from theoretical expectations at both low fields and high temperatures. We also discuss the origin of the PHE in our material, and the contributions of both the topological features in PtBi2 and its possible trivial origin. Our results strengthen the topological nature of PtBi2 and the strong influence of quantum geometric effects on the electronic transport properties of the low energy normal state.

cond-mat.mes-hall↗

arXiv2025

Supercurrent diode with high winding vortex

Nonreciprocal supercurrent refers to the phenomenon where the maximum dissipationless current in a superconductor depends on its direction of flow. This asymmetry underlies the operation of superconducting diodes and is often associated with the presence of vortices. Here, we investigate supercurrent nonreciprocal effects in a superconducting weak-link hosting distinct types of vortices. We demonstrate how the winding number of the vortex, its spatial configuration, and the shape of the superconducting lead can steer the sign and amplitude of the supercurrent rectification. We identify a general criterion for optimizing the rectification amplitude based on vortex patterns, focusing on configurations where the first harmonic of the supercurrent vanishes. We prove that supercurrent nonreciprocal effects can be used to diagnose high-winding vortex and to distinguish between different types of vorticity. Our results provide a toolkit for controlling supercurrent rectification through vortex phase textures and detecting unconventional vortex states.

cond-mat.supr-con↗

arXiv2025

Hallmarks of spin textures for high-harmonic generation in two-dimensional materials

Spin-orbit coupling and quantum geometry are fundamental aspects in modern condensed matter physics, with their primary manifestations in momentum space being spin textures and Berry curvature. In this work, we investigate their interplay with high-harmonic generation (HHG) in two-dimensional non-centrosymmetric materials, with an emphasis on even-order harmonics. Our analysis reveals that the emergence of finite even-order harmonics necessarily requires a broken twofold rotational symmetry in the spin texture, as well as a non-trivial Berry curvature in systems with time-reversal invariance. This symmetry breaking can arise across various degrees of freedom and impact both spin textures and optical response via spin-orbit interactions. We also show that HHG is particularly sensitive to dynamical rotational-symmetry breaking, as even high-order components can be modulated by a time-dependent symmetry breaking. These findings underscore the potential of HHG as a tool for exploring electronic phases with broken rotational symmetry, as well as the associated phase transitions in two-dimensional materials, and provide novel perspectives for designing symmetry-dependent nonlinear optical phenomena.

cond-mat.str-el↗

arXiv2025

Zeeman-activated Berry curvature magnetotransport from the bulk of non-magnetic metals with inversion symmetry

The Berry curvature (BC), a quantity encoding the geometry of electronic wavefunctions, governs various electronic transport effects in quantum materials. In magnetic systems, the BC is reponsible for the intrinsic part of the anomalous Hall conductivity. Local concentrations of BC in non-centrosymmetric materials can lead instead to the quantum nonlinear Hall effect. Here, we argue that the bulk of non-magnetic metals with inversion symmetry, systems where the BC is forced to vanish at any momentum, can be endowed with substantial concentrations of BC even with an infinitesimally small Zeeman coupling. This Zeeman-activated BC, independent of the magnetic field strength and instead related to the degree of non-parabolicity of the electronic bands, couples to the electronic orbital motion to generate a negative longitudinal magnetoresistance that scales with the relaxation time as the Drude resistivity. We show that the Zeeman-actived BC and the related intrinsic negative magnetoresistivity are generic: they appear in all centrosymmetric point groups and can occur both in topological and conventional conductors.

cond-mat.mes-hall↗

arXiv2025

The quantum metric of electrons with spin-momentum locking

Quantum materials are characterized by electromagnetic responses intrinsically linked to the geometry and topology of electronic wavefunctions, encoded in the quantum metric and Berry curvature. Whereas Berry curvature-mediated transport effects have been identified in several magnetic and nonmagnetic systems, quantum metric-induced transport phenomena remain limited to topological antiferromagnets. Here we show that spin-momentum locking -- a general characteristic of the electronic states at surfaces and interfaces of spin-orbit coupled materials -- leads to a finite quantum metric. This metric activates a nonlinear in-plane magnetoresistance that we measure and electrically control in 111-oriented LaAlO$_3$/SrTiO$_3$ interfaces. These findings demonstrate the existence of quantum metric effects in a vast class of materials and enable previously unexplored strategies to design functionalities based on quantum geometry.

cond-mat.mes-hall↗

arXiv2025

Bending nanoribbon to induce large anisotropic magnetoconductance

When a nanoribbon is bent under a homogeneous external magnetic field, the effective magnetic field inside becomes either homogeneous or inhomogeneous, depending on the direction of the field. This enables the selective creation of bulk, interface, and edge magnetic states in the bent structure, for a magnetic field with a strength. We establish theoretically that these tuneable states lead to a strong geometry-induced anisotropic magnetoconductance (GAMC) in perpendicularly bent nanoribbon, which can reach up to 100\%. Moreover, the GAMC can be further enhanced to 200\%, 300\%, or even higher by either further bending or tuning the bending angle. The potential of this phenomenon for practical applications is demonstrated by its stable anisotropy, which remains consistent across a wide range of Fermi energies, can be observed even at weak magnetic fields and room temperature, and occurs in various systems such as two-dimensional electron gas (2DEG) and graphene.

cond-mat.mes-hall↗

arXiv2025

Filtering Spin and Orbital Moment in Centrosymmetric Systems

The control of spin and orbital angular momentum without relying on magnetic materials is commonly accomplished by breaking of inversion symmetry, which enables charge-to-spin conversion and spin selectivity in electron transfer processes occurring in chiral media. In contrast to this perspective, we show that orbital moment filtering can be accomplished in centrosymmetric systems: the electron states can be selectively manipulated allowing for the preferential transfer of electrons with a particular orbital momentum orientation. We find that orbital moment filtering is indeed efficiently controlled through orbital couplings that break both mirror and rotational symmetries. We provide the symmetry conditions required for the electron transmission to achieve orbital filtering and relate them to the orientation of the orbital moment. The presence of atomic spin-orbit interaction in the centrosymmetric transmission medium leads to the selective filtering of spin and orbital moments. Our findings allow to identify optimal regimes for having highly efficient simultaneous spin and orbital moment filtering.

cond-mat.other↗

arXiv2025

Nonlinear planar magnetotransport as a probe of the topology of surface states

It has been recently established that transport measurements in the nonlinear regime can give direct access to the quantum metric (QM): the real part of the quantum geometric tensor characterizing the geometry of the electronic wavefunctions in a solid. In topological materials, the QM has been so far revealed in thin films of the topological antiferromagnet MnBi$_2$Te$_4$ where it provides a direct contribution to longitudinal currents quadratic in the driving electric field. Here we show that the Dirac surface states of strong three-dimensional topological insulators have a QM that can be accessed from the nonlinear transport characteristics in the presence of an externally applied planar magnetic field. A previously unknown intrinsic part of the longitudinal magnetoconductivity carries the signature of the QM while coexisting with the extrinsic part responsible for the so-called bilinear magnetoelectric resistance. We prove that the QM-induced nonlinear magnetotransport carries specific signatures of single Dirac cones. This allows to use it as an efficient diagnostic tool of the bulk topology of three-dimensional non-magnetic insulators.

cond-mat.mes-hall↗

arXiv2025

Large positive magnetoconductance in carbon nanoscrolls

We theoretically demonstrate that carbon nanoscrolls -- spirally wrapped graphene layers with open endpoints -- can be characterized by a large positive magnetoconductance. We show that when a carbon nanoscroll is subject to an axial magnetic field of several Tesla, the ballistic conductance at low carrier densities of the nanoscroll has an increase of about 200%. Importantly, we find that this positive magnetoconductance is not only preserved in an imperfect nanoscroll (with disorder or mild inter-turn misalignment) but can even be enhanced in the presence of on-site disorder. We prove that the positive magnetoconductance comes about the emergence of magnetic field-induced zero energy modes, specific of rolled-up geometries. Our results establish curved graphene systems as a new material platform displaying sizable magnetoresistive phenomena.

cond-mat.mes-hall↗

arXiv2025

Anomalous spin-optical helical effect in Ti-based kagome metal

The kagome lattice stands as a rich platform for hosting a wide array of correlated quantum phenomena, ranging from charge density waves and superconductivity to electron nematicity and loop current states. Direct detection of loop currents in kagome systems has remained a formidable challenge due to their intricate spatial arrangements and the weak magnetic field signatures they produce. This has left their existence and underlying mechanisms a topic of intense debate. In this work, we uncover a hallmark reconcilable with loop currents: spin handedness-selective signals that surpass conventional dichroic, spin, and spin-dichroic responses. We observe this phenomenon in the kagome metal CsTi$_3$Bi$_5$ and we call it the anomalous spin-optical helical effect. This effect arises from the coupling of light' s helicity with spin-orbital electron correlations, providing a groundbreaking method to visualize loop currents in quantum materials. Our discovery not only enriches the debate surrounding loop currents but also paves the way for new strategies to exploit the electronic phases of quantum materials via light-matter interaction.

cond-mat.str-el↗

arXiv2025

Imaging orbital Rashba induced charge transport anisotropy

Identifying orbital textures and their effects on the electronic properties of quantum materials is a critical element in developing orbitronic devices. However, orbital effects are often entangled with the spin degree of freedom, making it difficult to uniquely identify them in charge transport phenomena. Here, we present a combination of scanning superconducting quantum interference device (SQUID) current imaging, global transport measurements, and theoretical analysis, that reveals a direct contribution of orbital textures to the linear charge transport of 2D systems. Specifically, we show that in the LaAlO$_3$/SrTiO$_3$ interface, which lacks both rotation and inversion symmetries, an anisotropic orbital Rashba coupling leads to conductivity anisotropy in zero magnetic field. We experimentally demonstrate this result by locally measuring the conductivity anisotropy, and correlating its appearance to the non-linear Hall effect, showing that the two phenomena have a common origin. Our results lay the foundations for an all--electrical probing of orbital currents in two-dimensional systems.

cond-mat.mes-hall↗