Showing 1–50 of 100 results for author: Ortix, C

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

Authors: Mathews Benny, Xujia Gong, Amar Fakhredine, Raphaël Salazar, Ashutosh S. Wadge, Juraj Krempaský, Gunther Springholz, Sarath Sasi, Mahdi Hajloui, Martin Heinrich, Dawid Wutke, Rafał Kurleto, Natalia Olszowska, Sahar Izadi Vishkayi, Asiyeh Shokri, Ján Minár, Carmine Ortix, Jeroen van den Brink, Jakub Schusser, Carmine Autieri

Abstract: 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… ▽ More 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. △ Less

Submitted 6 August, 2026; v1 submitted 3 August, 2026; originally announced August 2026.

Spin selectivity induced by non-collinear spins in Rashba wires

Authors: Luciano Jacopo D'Onofrio, Maria Teresa Mercaldo, Mario Cuoco, Carmine Ortix

Abstract: 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 systematica… ▽ More 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. △ Less

Submitted 1 August, 2026; originally announced August 2026.

Comments: 13 pages (with supplemental material), 5 figures

Quantum metric quadrupoles in elemental bismuth thin films

Authors: Rhonald Burgos Atencia, Pavlo Makushko, Gabriele Naselli, Debottam Mandal, Paul Chekhonin, Sergey Kovalev, Steffen Kober, Zhe Wang, Igor Veremchuk, Oleksiy Pashkin, Fabian Ganss, Maria Teresa Mercaldo, Denys Makarov, Carmine Ortix

Abstract: 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,… ▽ More 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. △ Less

Submitted 28 July, 2026; originally announced July 2026.

Transverse Magnetic Response from Orbitally Polarized Cooper Pairs in Elemental Superconductors

Authors: Gabor Csire, Maria Teresa Mercaldo, Balazs Ujfalussy, Carmine Ortix, Mario Cuoco

Abstract: 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, ac… ▽ More 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. △ Less

Submitted 11 May, 2026; originally announced May 2026.

Comments: 9 pages, 4 figures

Curvature-induced nonlinear anomalous Hall effect in thin magnetic shells

Authors: Maria Teresa Mercaldo, Mario Cuoco, Carmine Ortix

Abstract: 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 thickne… ▽ More 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. △ Less

Submitted 30 April, 2026; originally announced April 2026.

Comments: main text: 11 pages, 4 figures; supplementary inf. 6 pages 6 figures

Topological piezomagnetic effect in two-dimensional Dirac quadrupole altermagnets

Authors: H. Radhakrishnan, B. Bell, C. Ortix, J. W. F. Venderbos

Abstract: Altermagnets provide a natural platform for studying and exploiting piezomagnetism. In this paper, we introduce a class of insulating altermagnets in two dimensions (2D) referred to as Dirac quadrupole altermagnets, and show based on microscopic minimal models that the orbital piezomagnetic polarizability of such altermagnets has a topological contribution described by topological response theory.… ▽ More Altermagnets provide a natural platform for studying and exploiting piezomagnetism. In this paper, we introduce a class of insulating altermagnets in two dimensions (2D) referred to as Dirac quadrupole altermagnets, and show based on microscopic minimal models that the orbital piezomagnetic polarizability of such altermagnets has a topological contribution described by topological response theory. The essential low-energy electronic structure of Dirac quadrupole altermagnets can be understood from a gapless parent phase (i.e., the Dirac quadrupole semimetal), which has important implications for their response to external fields. Focusing on the strain-induced response, here we demonstrate that the topological piezomagnetic effect is a consequence of the way in which strain affects the Dirac points forming a quadrupole. We consider two microscopic models: a spinless two-band model describing a band inversion of $s$ and $d$ states, and a Lieb lattice model with collinear Néel order. The latter is a prototypical minimal model for altermagnetism in 2D and is realized in a number of recently proposed material compounds, which are discussed. △ Less

Submitted 21 April, 2026; v1 submitted 5 February, 2026; originally announced February 2026.

Comments: v2, 5 pages, 3 figures; Suppl.: 5 pages, 1 figure

Layer dipole magnetoelectric polarizability of antiferromagnetic bilayers

Authors: H. Radhakrishnan, C. Ortix, J. W. F. Venderbos

Abstract: In this paper we study magnetoelectric effects in two-dimensional magnetic bilayers and introduce the notion of a layer dipole magnetoelectric polarizability. This magnetoelectric polarizability describes the magnetization response to an applied electric field perpendicular to the bilayer. As such, it represents the electric analog of the spin magnetoelectric polarizability, governing the charge p… ▽ More In this paper we study magnetoelectric effects in two-dimensional magnetic bilayers and introduce the notion of a layer dipole magnetoelectric polarizability. This magnetoelectric polarizability describes the magnetization response to an applied electric field perpendicular to the bilayer. As such, it represents the electric analog of the spin magnetoelectric polarizability, governing the charge polarization response to an applied Zeeman field. Starting from the orbital magnetization produced by a perpendicular displacement field, we derive a microscopic expression for the layer dipole magnetoelectric polarizability and apply it to two minimal models for bilayer magnets, i.e., a buckled square lattice model and a magnetic topological insulator model. In the case of the buckled square lattice model we show that the layer dipole magnetoelectric polarizability has a (quasi-)topological contribution, revealing a topological magnetoelectric response of two-dimensional antiferromagnets associated with the layer pseudospin degree of freedom. △ Less

Submitted 5 December, 2025; originally announced December 2025.

Comments: 5 pages; 4 figures; 2 appendices

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

Authors: Jörn W. F. Venderbos, Paola Gentile, Carmine Ortix

Abstract: 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 ferm… ▽ More 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. △ Less

Submitted 5 December, 2025; originally announced December 2025.

Comments: 22 pages; 11 figures; 7 appendices

Probing the quantum metric of 3D topological insulators

Authors: Giacomo Sala, Emanuele Longo, Maria Teresa Mercaldo, Stefano Gariglio, Mario Cuoco, Roberto Mantovan, Carmine Ortix, Andrea D. Caviglia

Abstract: 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… ▽ More 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. △ Less

Submitted 20 January, 2026; v1 submitted 21 September, 2025; originally announced September 2025.

Comments: Revised manuscript

Bending nanoribbon to induce large anisotropic magnetoconductance

Authors: Ponder Liu, Hao-Cheng Hung, You-Ting Huang, Jia-Cheng Li, Carmine Ortix, Ching-Hao Chang

Abstract: 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 s… ▽ More 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. △ Less

Submitted 21 August, 2025; originally announced August 2025.

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

Authors: Rhonald Burgos Atencia, Antonio Vecchione, Denys Makarov, Carmine Ortix

Abstract: 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… ▽ More 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. △ Less

Submitted 5 October, 2025; v1 submitted 9 July, 2025; originally announced July 2025.

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

Authors: Federico Mazzola, Wojciech Brzezicki, Chiara Bigi, Armando Consiglio, Luciano Jacopo D' Onofrio, Maria Teresa Mercaldo, Adam Kłosiński, François Bertran, Patrick Le Fèvre, Oliver J. Clark, Mark T. Edmonds, Manuel Tuniz, Alessandro De Vita, Vincent Polewczyk, Jeppe B. Jacobsen, Henrik Jacobsen, Jill A. Miwa, Justin W. Wells, Anupam Jana, Ivana Vobornik, Jun Fujii, Niccolò Mignani, Narges Samani Tarakameh, Alberto Crepaldi, Giorgio Sangiovanni , et al. (10 additional authors not shown)

Abstract: 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… ▽ More 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. △ Less

Submitted 26 February, 2025; originally announced February 2025.

Filtering Spin and Orbital Moment in Centrosymmetric Systems

Authors: Luciano Jacopo DOnofrio, Maria Teresa Mercaldo, Wojciech Brzezicki, Adam Klosinski, Federico Mazzola, Carmine Ortix, Mario Cuoco

Abstract: 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 st… ▽ More 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. △ Less

Submitted 23 June, 2025; v1 submitted 15 February, 2025; originally announced February 2025.

Comments: 13 pages, 12 figures

Journal ref: Physical Review B 112, 085428 (2025)

Imaging orbital Rashba induced charge transport anisotropy

Authors: Eylon Persky, Xi Wang, Giacomo Sala, Thierry C. van Thiel, Edouard Lesne, Alexander Lau, Mario Cuoco, Marc Gabay, Carmine Ortix, Andrea D. Caviglia, Beena Kalisky

Abstract: 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 (SQUI… ▽ More 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. △ Less

Submitted 13 February, 2025; originally announced February 2025.

Comments: 22 pages, 15 figures

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

Authors: Francesco Gabriele, Carmine Ortix, Mario Cuoco, Filomena Forte

Abstract: 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 eme… ▽ More 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. △ Less

Submitted 9 October, 2025; v1 submitted 8 January, 2025; originally announced January 2025.

Comments: 6 pages, 2 figures

Journal ref: Phys. Rev. B 112, 155421 (2025)

Room temperature Planar Hall effect in nanostructures of trigonal-PtBi2

Authors: Arthur Veyrat, Klaus Koepernik, Louis Veyrat, Grigory Shipunov, Iryna Kovalchuk, Saicharan Aswartham, Jiang Qu, Ankit Kumar, Michele Ceccardi, Federico Caglieris, Nicolás Pérez Rodríguez, Romain Giraud, Bernd Büchner, Jeroen van den Brink, Carmine Ortix, Joseph Dufouleur

Abstract: 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 H… ▽ More 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. △ Less

Submitted 24 October, 2025; v1 submitted 16 October, 2024; originally announced October 2024.

Dissipationless transport signature of topological nodal lines

Authors: Arthur Veyrat, Klaus Koepernik, Louis Veyrat, Grigory Shipunov, Saicharan Aswartham, Jiang Qu, Ankit Kumar, Michele Ceccardi, Federico Caglieris, Nicolás Pérez Rodríguez, Romain Giraud, Bernd Büchner, Jeroen van den Brink, Carmine Ortix, Joseph Dufouleur

Abstract: Topological materials, such as topological insulators or semimetals, usually not only reveal the nontrivial properties of their electronic wavefunctions through the appearance of stable boundary modes, but also through very specific electromagnetic responses. The anisotropic longitudinal magnetoresistance of Weyl semimetals, for instance, carries the signature of the chiral anomaly of Weyl fermion… ▽ More Topological materials, such as topological insulators or semimetals, usually not only reveal the nontrivial properties of their electronic wavefunctions through the appearance of stable boundary modes, but also through very specific electromagnetic responses. The anisotropic longitudinal magnetoresistance of Weyl semimetals, for instance, carries the signature of the chiral anomaly of Weyl fermions. However for topological nodal line semimetals -- materials where the valence and conduction bands cross each other on one-dimensional curves in the three-dimensional Brillouin zone -- such a characteristic has been lacking. Here we report the discovery of a peculiar charge transport effect generated by topological nodal lines: a dissipationless transverse signal in the presence of coplanar electric and magnetic fields, which originates from a Zeeman-induced conversion of topological nodal lines into Weyl nodes under infinitesimally small magnetic fields. We evidence this dissipationless topological response in trigonal \ce{PtBi2} persisting up to room temperature, and unveil the extensive topological nodal lines in the band structure of this non-magnetic material. These findings provide a new pathway to engineer Weyl nodes by arbitrary small magnetic fields and reveal that bulk topological nodal lines can exhibit non-dissipative transport properties. △ Less

Submitted 3 October, 2024; originally announced October 2024.

Journal ref: Nature Communications 16, 6711 (2025)

Manipulation by magnetic frustration in ferrotoroidal spin chains via curvature and torsion

Authors: Oleksandr V. Pylypovskyi, Enrico Di Benedetto, Carmine Ortix, Denys Makarov

Abstract: Geometric effects in curvilinear nanomagnets can enable chiral, anisotropic and even magnetoelectric responses. Here, we study the effects of magnetic frustration in curvilinear (quasi-)1D magnets represented by spin chains arranged along closed space curves of constant torsion. Considering the cases of easy- and hard-axis anisotropy in ferro- and antiferromagnetic samples, we determine their grou… ▽ More Geometric effects in curvilinear nanomagnets can enable chiral, anisotropic and even magnetoelectric responses. Here, we study the effects of magnetic frustration in curvilinear (quasi-)1D magnets represented by spin chains arranged along closed space curves of constant torsion. Considering the cases of easy- and hard-axis anisotropy in ferro- and antiferromagnetic samples, we determine their ground states and analyze the related magnetoelectric multipoles. A constant torsion along the chain results in alternating regions of high and low curvature, facilitating the spin spiral state perturbed by the (anti)periodic boundary conditions on the magnetic order parameter. While easy-axis ferromagnetic chains develop a purely toroidal configuration with the magnetic toroidal moment oriented along the geometry symmetry axis, hard-axis antiferromagnetic chains support multiple magnetic toroidal domains. Our findings suggest that tailoring curvature and torsion of a spin chain enables a new physical mechanism for magnetic frustration, which can be observed in the inhomogeneity of the magnetic order parameter and in the local ferrotoroidic responses. △ Less

Submitted 30 August, 2024; originally announced August 2024.

Journal ref: Physical Review Research, 7, 013088 (2025)

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

Authors: Maria Teresa Mercaldo, Mario Cuoco, Carmine Ortix

Abstract: 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 dire… ▽ More 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. △ Less

Submitted 22 April, 2025; v1 submitted 18 August, 2024; originally announced August 2024.

Comments: main: 7 pages, 4 figures; supplemental materials: 7 pages, 9 figures

Journal ref: Phys. Rev. B 111, 155442 (2025)

Large positive magnetoconductance in carbon nanoscrolls

Authors: Yu-Jie Zhong, Jia-Cheng Li, Xuan-Fu Huang, Ying-Je Lee, Ting-Zhen Chen, Jia-Ren Zhang, Angus Huang, Hsiu-Chuan Hsu, Carmine Ortix, Ching-Hao Chang

Abstract: 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 po… ▽ More 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. △ Less

Submitted 20 March, 2025; v1 submitted 6 August, 2024; originally announced August 2024.

Comments: 19 pages, 6 figures

The quantum metric of electrons with spin-momentum locking

Authors: Giacomo Sala, Maria Teresa Mercaldo, Klevis Domi, Stefano Gariglio, Mario Cuoco, Carmine Ortix, Andrea D. Caviglia

Abstract: 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 antiferromagnet… ▽ More 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. △ Less

Submitted 25 August, 2025; v1 submitted 9 July, 2024; originally announced July 2024.

Comments: Published

Journal ref: Science 389, 822-825, 2025

Supercurrent diode with high winding vortex

Authors: Yuri Fukaya, Maria Teresa Mercaldo, Daniel Margineda, Alessandro Crippa, Elia Strambini, Francesco Giazotto, Carmine Ortix, Mario Cuoco

Abstract: 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 demons… ▽ More 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. △ Less

Submitted 22 October, 2025; v1 submitted 7 March, 2024; originally announced March 2024.

Comments: 20 pages and 12 figures

Journal ref: Commun. Phys. 8, 518 (2025)

Signatures of a surface spin-orbital chiral metal

Authors: Federico Mazzola, Wojciech Brzezicki, Maria Teresa Mercaldo, Anita Guarino, Chiara Bigi, Jill A. Miwa, Domenico De Fazio, Alberto Crepaldi, Jun Fujii, Giorgio Rossi, Pasquale Orgiani, Sandeep Kumar Chaluvadi, Shyni Punathum Chalil, Giancarlo Panaccione, Anupam Jana, Vincent Polewczyk, Ivana Vobornik, Changyoung Kim, Fabio Miletto Granozio, Rosalba Fittipaldi, Carmine Ortix, Mario Cuoco, Antonio Vecchione

Abstract: The relation between crystal symmetries, electron correlations, and electronic structure steers the formation of a large array of unconventional phases of matter, including magneto-electric loop currents and chiral magnetism. Detection of such hidden orders is a major goal in condensed matter physics. However, to date, nonstandard forms of magnetism with chiral electronic ordering have been experi… ▽ More The relation between crystal symmetries, electron correlations, and electronic structure steers the formation of a large array of unconventional phases of matter, including magneto-electric loop currents and chiral magnetism. Detection of such hidden orders is a major goal in condensed matter physics. However, to date, nonstandard forms of magnetism with chiral electronic ordering have been experimentally elusive. Here, we develop a theory for symmetry-broken chiral ground states and propose a methodology based on circularly polarized spin-selective angular-resolved photoelectron spectroscopy to probe them. We exploit the archetypal quantum material Sr2RuO4 and reveal spectroscopic signatures which, even though subtle, may be reconciled with the formation of spin-orbital chiral currents at the material surface. As we shed light on these chiral regimes, our findings pave the way for a deeper understanding of ordering phenomena and unconventional magnetism. △ Less

Submitted 13 February, 2024; originally announced February 2024.

Back-action supercurrent diodes

Authors: Daniel Margineda, Alessandro Crippa, Elia Strambini, Laura Borgongino, Alessandro Paghi, Giorgio de Simoni, Lucia Sorba andYuri Fukaya, Maria Teresa Mercaldo, Carmine Ortix, Mario Cuoco, Francesco Giazotto

Abstract: Back-action refers to a response that retro-acts on a system to tailor its properties with respect to an external stimulus. This effect is at the heart of many electronic devices such as amplifiers, oscillators, and sensors. Here, we demonstrate that back-action can be exploited to achieve non-reciprocal transport in superconducting circuits. In our devices, dissipationless current flows in one di… ▽ More Back-action refers to a response that retro-acts on a system to tailor its properties with respect to an external stimulus. This effect is at the heart of many electronic devices such as amplifiers, oscillators, and sensors. Here, we demonstrate that back-action can be exploited to achieve non-reciprocal transport in superconducting circuits. In our devices, dissipationless current flows in one direction whereas dissipative transport occurs in the opposite direction. Supercurrent diodes presented so far rely on magnetic elements or vortices to mediate charge transport or external magnetic fields to break time-reversal symmetry. Back-action solely turns a conventional reciprocal superconducting weak link with no asymmetry between the current bias directions into a rectifier, where the critical current amplitude depends on the bias sign. The self-interaction of the supercurrent stems from the gate tunability of the critical current in metallic and semiconducting systems, which promotes nearly ideal magnetic field-free rectification with selectable polarity. △ Less

Submitted 3 January, 2025; v1 submitted 24 November, 2023; originally announced November 2023.

Journal ref: Commun Phys 8, 16 (2025)

Spin-orbit interaction driven terahertz nonlinear dynamics in transition metals

Authors: Ruslan Salikhov, Markus Lysne, Philipp Werner, Igor Ilyakov, Michael Schüler, Thales V. A. G. de Oliveira, Alexey Ponomaryov, Atiqa Arshad, Gulloo Lal Prajapati, Jan-Christoph Deinert, Pavlo Makushko, Denys Makarov, Thomas Cowan, Jürgen Fassbender, Jürgen Lindner, Aleksandra Lindner, Carmine Ortix, Sergey Kovalev

Abstract: The interplay of electric charge, spin, and orbital polarizations, coherently driven by picosecond long oscillations of light fields in spin-orbit coupled systems, is the foundation of emerging terahertz spintronics and orbitronics. The essential rules for how terahertz light interacts with these systems in a nonlinear way are still not understood. In this work, we demonstrate a universally applic… ▽ More The interplay of electric charge, spin, and orbital polarizations, coherently driven by picosecond long oscillations of light fields in spin-orbit coupled systems, is the foundation of emerging terahertz spintronics and orbitronics. The essential rules for how terahertz light interacts with these systems in a nonlinear way are still not understood. In this work, we demonstrate a universally applicable electronic nonlinearity originating from spin-orbit interactions in conducting materials, wherein the interplay of light-induced spin and orbital textures manifests. We utilized terahertz harmonic generation spectroscopy to investigate the nonlinear dynamics over picosecond timescales in various transition metal films. We found that the terahertz harmonic generation efficiency scales with the spin Hall conductivity in the studied films, while the phase takes two possible values (shifted by π), depending on the d-shell filling. These findings elucidate the fundamental mechanisms governing non-equilibrium spin and orbital polarization dynamics at terahertz frequencies, which is relevant for potential applications of terahertz spin- and orbital-based devices. △ Less

Submitted 22 November, 2023; originally announced November 2023.

Comments: 11 pages, 4 figures

Tunable room temperature nonlinear Hall effect from the surfaces of elementary bismuth thin films

Authors: Pavlo Makushko, Sergey Kovalev, Yevhen Zabila, Igor Ilyakov, Alexey Ponomaryov, Atiqa Arshad, Gulloo Lal Prajapati, Thales V. A. G. de Oliveira, Jan-Christoph Deinert, Paul Chekhonin, Igor Veremchuk, Tobias Kosub, Yurii Skourski, Fabian Ganss, Denys Makarov, Carmine Ortix

Abstract: The nonlinear Hall effect (NLHE) with time-reversal symmetry constitutes the appearance of a transverse voltage quadratic in the applied electric field. It is a second-order electronic transport phenomenon that induces frequency doubling and occurs in non-centrosymmetric crystals with large Berry curvature -- an emergent magnetic field encoding the geometric properties of electronic wavefunctions.… ▽ More The nonlinear Hall effect (NLHE) with time-reversal symmetry constitutes the appearance of a transverse voltage quadratic in the applied electric field. It is a second-order electronic transport phenomenon that induces frequency doubling and occurs in non-centrosymmetric crystals with large Berry curvature -- an emergent magnetic field encoding the geometric properties of electronic wavefunctions. The design of (opto)electronic devices based on the NLHE is however hindered by the fact that this nonlinear effect typically appears at low temperatures and in complex compounds characterized by Dirac or Weyl electrons. Here, we show a strong room temperature NLHE in the centrosymmetric elemental material bismuth synthesized in the form of technologically relevant polycrystalline thin films. The ($1\,1\,1$) surface electrons of this material are equipped with a Berry curvature triple that activates side jumps and skew scatterings generating nonlinear transverse currents. We also report a boost of the zero field nonlinear transverse voltage in arc-shaped bismuth stripes due to an extrinsic geometric classical counterpart of the NLHE. This electrical frequency doubling in curved geometries is then extended to optical second harmonic generation in the terahertz (THz) spectral range. The strong nonlinear electrodynamical responses of the surface states are further demonstrated by a concomitant highly efficient THz third harmonic generation which we achieve in a broad range of frequencies in Bi and Bi-based heterostructures. Combined with the possibility of growth on CMOS-compatible and mechanically flexible substrates, these results highlight the potential of Bi thin films for THz (opto)electronic applications. △ Less

Submitted 23 October, 2023; originally announced October 2023.

Comments: 44 pages, 21 figures

Journal ref: Nature Electronics 7, 207 (2024)

High orbital-moment Cooper pairs by crystalline symmetry breaking

Authors: Maria Teresa Mercaldo, Carmine Ortix, Mario Cuoco

Abstract: The pairing structure of superconducting materials is regulated by the point group symmetries of the crystal. Here, we study spin-singlet multiorbital superconductivity in materials with unusually low crystalline symmetry content and unveil the the appearance of even-parity (s-wave) Cooper pairs with high orbital moment. We show that the lack of mirror and rotation symmetries makes pairing states… ▽ More The pairing structure of superconducting materials is regulated by the point group symmetries of the crystal. Here, we study spin-singlet multiorbital superconductivity in materials with unusually low crystalline symmetry content and unveil the the appearance of even-parity (s-wave) Cooper pairs with high orbital moment. We show that the lack of mirror and rotation symmetries makes pairing states with quintet orbital angular momentum symmetry-allowed. A remarkable fingerprint of this type of pairing state is provided by a nontrivial superconducting phase texture in momentum space with $π$-shifted domains and walls with anomalous phase winding. The pattern of the quintet pairing texture is shown to depend on the orientation of the orbital polarization and the strength of the mirror and/or rotation symmetry breaking terms. Such momentum dependent phase makes Cooper pairs with net orbital component suited to design orbitronic Josephson effects. We discuss how an intrinsic orbital dependent phase can set out anomalous Josephson couplings by employing superconducting leads with nonequivalent breaking of crystalline symmetry. △ Less

Submitted 17 May, 2023; originally announced May 2023.

Comments: 11 pages, 9 figures

Journal ref: Adv Quantum Technol. 2023, 2300081

Orbital design of Berry curvature: pinch points and giant dipoles induced by crystal fields

Authors: Maria Teresa Mercaldo, Canio Noce, Andrea D. Caviglia, Mario Cuoco, Carmine Ortix

Abstract: The Berry curvature (BC) - a quantity encoding the geometric properties of the electronic wavefunctions in a solid - is at the heart of different Hall-like transport phenomena, including the anomalous Hall and the non-linear Hall and Nernst effects. In non-magnetic quantum materials with acentric crystalline arrangements, local concentrations of BC are generally linked to single-particle wavefunct… ▽ More The Berry curvature (BC) - a quantity encoding the geometric properties of the electronic wavefunctions in a solid - is at the heart of different Hall-like transport phenomena, including the anomalous Hall and the non-linear Hall and Nernst effects. In non-magnetic quantum materials with acentric crystalline arrangements, local concentrations of BC are generally linked to single-particle wavefunctions that are a quantum superposition of electron and hole excitations. BC-mediated effects are consequently observed in two-dimensional systems with pairs of massive Dirac cones and three-dimensional bulk crystals with quartets of Weyl cones. Here, we demonstrate that in materials equipped with orbital degrees of freedom local BC concentrations can arise even in the complete absence of hole excitations. In these solids, the crystals fields appearing in very low-symmetric structures trigger BCs characterized by hot-spots and singular pinch points. These characteristics naturally yield giant BC dipoles and large non-linear transport responses in time-reversal symmetric conditions. △ Less

Submitted 11 January, 2023; originally announced January 2023.

Comments: 11 pages, 4 figure and supplemental material (6 pages, figures)

Journal ref: npj Quantum Mater. 8, 12 (2023)

Real-space Obstruction in Quantum Spin Hall Insulators

Authors: Philipp Eck, Carmine Ortix, Armando Consiglio, Jonas Erhardt, Maximilian Bauernfeind, Simon Moser, Ralph Claessen, Domenico Di Sante, Giorgio Sangiovanni

Abstract: The recently introduced classification of two-dimensional insulators in terms of topological crystalline invariants has been applied so far to "obstructed" atomic insulators characterized by a mismatch between the centers of the electronic Wannier functions and the ionic positions. We extend this notion to quantum spin Hall insulators in which the ground state cannot be described in terms of time-… ▽ More The recently introduced classification of two-dimensional insulators in terms of topological crystalline invariants has been applied so far to "obstructed" atomic insulators characterized by a mismatch between the centers of the electronic Wannier functions and the ionic positions. We extend this notion to quantum spin Hall insulators in which the ground state cannot be described in terms of time-reversal symmetric localized Wannier functions. A system equivalent to graphene in all its relevant electronic and topological properties except for a real-space obstruction is identified and studied via symmetry analysis as well as with density functional theory. The low-energy model comprises a local spin-orbit coupling and a non-local symmetry breaking potential, which turn out to be the essential ingredients for an obstructed quantum spin Hall insulator. An experimental fingerprint of the obstruction is then measured in a large-gap triangular quantum spin Hall material. △ Less

Submitted 26 September, 2022; originally announced September 2022.

Comments: 9 pages, 6 figures and 1 table. Supplementary material: 5 pages, 4 figures and 2 tables

Magnetoelectricity induced by rippling of magnetic nanomembranes and wires

Authors: Carmine Ortix, Jeroen van den Brink

Abstract: Magnetoelectric crystals have the interesting property that they allow electric fields to induce magnetic polarizations, and vice versa, magnetic fields to generate ferroelectric polarizations. Having such a magnetoelectric coupling usually requires complex types of magnetic textures, e.g., of spiralling type. Here we establish a novel approach to generate a linear magnetoelectric coupling in insu… ▽ More Magnetoelectric crystals have the interesting property that they allow electric fields to induce magnetic polarizations, and vice versa, magnetic fields to generate ferroelectric polarizations. Having such a magnetoelectric coupling usually requires complex types of magnetic textures, e.g., of spiralling type. Here we establish a novel approach to generate a linear magnetoelectric coupling in insulators with a conventional, ferromagnetic ground state. We show that nanoscale curved geometries lead to a reorganization of the magnetic texture that spontaneously breaks inversion symmetry and thereby induces macroscopic magnetoelectric multipoles. Specifically, we prove that structural deformation in the form of controlled ripples activate a magnetoelectric monopole in the recently synthesised two-dimensional magnets. We also demonstrate that in zig-zag shaped ferromagnetic wires in planar architectures, a magnetic toroidal moment triggers a direct linear magnetoelectric coupling. △ Less

Submitted 1 September, 2022; originally announced September 2022.

Comments: 5 pages, 4 figures

Electronic materials with nanoscale curved geometries

Authors: Paola Gentile, Mario Cuoco, Oleksii M. Volkov, Zu-Jian Ying, Ivan J. Vera-Marun, Denys Makarov, Carmine Ortix

Abstract: Research into electronic nanomaterials has recently seen a growing focus into the synthesis of structures with unconventional curved geometries including bent wires in planar systems and three-dimensional architectures obtained by rolling up nanomembranes. The inclusion of these geometries has led to the prediction and observation of a series of novel effects that either result from shape-driven m… ▽ More Research into electronic nanomaterials has recently seen a growing focus into the synthesis of structures with unconventional curved geometries including bent wires in planar systems and three-dimensional architectures obtained by rolling up nanomembranes. The inclusion of these geometries has led to the prediction and observation of a series of novel effects that either result from shape-driven modifications of the electronic motion or from an intrinsic change of electronic and magnetic properties due to peculiar confinement effects. Moreover, local strains often generated by curvature also trigger the appearance of new phenomena due to the essential role played by electromechanical coupling in solids. Here we review the recent developments in the discovery of these shape-, confinement- and strain-induced curvature effects at the nanoscale, and discuss their potential use in electronic and spintronic devices. △ Less

Submitted 14 September, 2022; v1 submitted 19 July, 2022; originally announced July 2022.

Comments: 17 pages, 4 figures. Review article

Journal ref: Nature Electronics (2022)

Designing spin and orbital sources of Berry curvature at oxide interfaces

Authors: Edouard Lesne, Yildiz G. Saǧlam, Raffaele Battilomo, Maria Teresa Mercaldo, Thierry C. van Thiel, Ulderico Filippozzi, Canio Noce, Mario Cuoco, Gary A. Steele, Carmine Ortix, Andrea D. Caviglia

Abstract: Quantum materials can display physical phenomena rooted in the geometry of electronic wavefunctions. The corresponding geometric tensor is characterized by an emergent field known as Berry curvature (BC). Large BCs typically arise when electronic states with different spin, orbital or sublattice quantum numbers hybridize at finite crystal momentum. In all materials known to date, the BC is trigger… ▽ More Quantum materials can display physical phenomena rooted in the geometry of electronic wavefunctions. The corresponding geometric tensor is characterized by an emergent field known as Berry curvature (BC). Large BCs typically arise when electronic states with different spin, orbital or sublattice quantum numbers hybridize at finite crystal momentum. In all materials known to date, the BC is triggered by the hybridization of a single type of quantum number. Here, we report the discovery of the first material system having both spin and orbital-sourced BC: LaAlO$_3$/SrTiO$_3$ interfaces grown along the [111] direction. We detect independently these two sources and directly probe the BC associated to the spin quantum number through measurements of an anomalous planar Hall effect. The observation of a nonlinear Hall effect with time-reversal symmetry signals large orbital-mediated BC dipoles. The coexistence of different forms of BC enables the combination of spintronic and optoelectronic functionalities in a single material. △ Less

Submitted 3 February, 2023; v1 submitted 28 January, 2022; originally announced January 2022.

Comments: 32 pages, 25 figures, accepted in Nature Materials

Journal ref: Nature Materials 22, 576 (2023)

Zero magnetic-field orbital vortices in s-wave spin-singlet superconductors

Authors: Maria Teresa Mercaldo, Carmine Ortix, Francesco Giazotto, Mario Cuoco

Abstract: Breaking of time-reversal and point-group spatial symmetries can have a profound impact on superconductivity. One of the most extraordinary effects, due to the application of a magnetic field, is represented by the Abrikosov vortices with charged supercurrents circulating around their cores. Whether a similar phenomenon can be obtained by exploiting spatial symmetry breaking, e.g. through electric… ▽ More Breaking of time-reversal and point-group spatial symmetries can have a profound impact on superconductivity. One of the most extraordinary effects, due to the application of a magnetic field, is represented by the Abrikosov vortices with charged supercurrents circulating around their cores. Whether a similar phenomenon can be obtained by exploiting spatial symmetry breaking, e.g. through electric fields or mechanical strain, is a fundamentally relevant but not yet fully settled problem. Here, we show that in two-dimensional spin-singlet superconductors with unusually low degree of spatial symmetry content, vortices with supercurrents carrying angular momentum around the core can form and be energetically stable. The vortex has zero net magnetic flux since it is made up of counter-propagating Cooper pairs with opposite orbital moments. By solving self-consistently the Bogoliubov - de Gennes equations in real space, we demonstrate that the orbital vortex is stable and we unveil the spatial distribution of the superconducting order parameter around its core. The resulting amplitude has a characteristic pattern with a pronounced angular anisotropy that deviates from the profile of conventional magnetic vortices. These hallmarks guide predictions and proposals for the experimental detection. △ Less

Submitted 29 November, 2021; v1 submitted 19 May, 2021; originally announced May 2021.

Comments: (main: 6 pages, 3 panels of figures; supplemental material: 5 pages 5 panels of figures)

Journal ref: Phys. Rev. B 105, L140507 (2022)

Magnetoconductance modulations due to interlayer tunneling in radial superlattices

Authors: Yu-Jie Zhong, Angus Huang, Hui Liu, Xuan-Fu Huang, Horng-Tay Jeng, Jhih-Shih You, Carmine Ortix, Ching-Hao Chang

Abstract: Radial superlattices are nanostructured materials obtained by rolling-up thin solid films into spiral-like tubular structures. The formation of these "high-order" superlattices from two-dimensional crystals or ultrathin films is expected to result in a transition of transport characteristics from two-dimensional to one-dimensional. Here, we show that a transport hallmark of radial superlattices is… ▽ More Radial superlattices are nanostructured materials obtained by rolling-up thin solid films into spiral-like tubular structures. The formation of these "high-order" superlattices from two-dimensional crystals or ultrathin films is expected to result in a transition of transport characteristics from two-dimensional to one-dimensional. Here, we show that a transport hallmark of radial superlattices is the appearance of magnetoconductance modulations in the presence of externally applied axial magnetic fields. This phenomenon critically relies on electronic interlayer tunneling processes that activates an unconventional Aharonov-Bohm-like effect. Using a combination of density functional theory calculations and low-energy continuum models, we determine the electronic states of a paradigmatic single-material radial superlattice -- a two-winding carbon nanoscroll -- and indeed show momentum-dependent oscillations of the magnetic states in axial configuration, which we demonstrate to be entirely due to hopping between the two windings of the spiral-shaped scroll. △ Less

Submitted 9 December, 2021; v1 submitted 24 April, 2021; originally announced April 2021.

Comments: 21 pages, 4 figures. To appear in Nanoscale Horizons

Nonlinear Hall effect with time-reversal symmetry: Theory and material realizations

Authors: Carmine Ortix

Abstract: The appearance of a Hall conductance necessarily requires breaking of time-reversal symmetry, either by an external magnetic field or by the internal magnetization of a material. However, as a second response, Hall dissipationless transverse currents can appear even in time-reversal symmetric conditions in non-centrosymmetric materials. Moreover, this non-linear effect has a quantum origin: it is… ▽ More The appearance of a Hall conductance necessarily requires breaking of time-reversal symmetry, either by an external magnetic field or by the internal magnetization of a material. However, as a second response, Hall dissipationless transverse currents can appear even in time-reversal symmetric conditions in non-centrosymmetric materials. Moreover, this non-linear effect has a quantum origin: it is related to the geometric properties of the electronic wavefunctions and encoded in the dipole moment of the Berry curvature. Here we review the general theory underpinning this effect and discuss various material platforms where non-linear Hall transverse responses have been theoretically proposed and experimentally verified. On the theoretical front, the link between the non-linear Hall effect and the Berry curvature dipole is discussed using Boltzmann transport theory. On the material front, different platforms, including topological crystalline insulators, transition metal dichalcogenides, graphene, and Weyl semimetals are reviewed. △ Less

Submitted 22 July, 2021; v1 submitted 14 April, 2021; originally announced April 2021.

Comments: Review article; 26 pages, 9 figures

Journal ref: Advanced Quantum Technologies 2100056 (2021)

Bulk-corner correspondence of time-reversal symmetric insulators: deduplicating real-space invariants

Authors: Sander H. Kooi, Guido van Miert, Carmine Ortix

Abstract: The topology of insulators is usually revealed through the presence of gapless boundary modes: this is the so-called bulk-boundary correspondence. However, the many-body wavefunction of a crystalline insulator is endowed with additional topological properties that do not yield surface spectral features, but manifest themselves as (fractional) quantized electronic charges localized at the crystal b… ▽ More The topology of insulators is usually revealed through the presence of gapless boundary modes: this is the so-called bulk-boundary correspondence. However, the many-body wavefunction of a crystalline insulator is endowed with additional topological properties that do not yield surface spectral features, but manifest themselves as (fractional) quantized electronic charges localized at the crystal boundaries. Here, we formulate such bulk-corner correspondence for the physical relevant case of materials with time-reversal symmetry and spin-orbit coupling. To so do we develop "partial" real-space invariants that can be neither expressed in terms of Berry phases nor using symmetry-based indicators. These new crystalline invariants govern the (fractional) quantized corner charges both of isolated material structures and of heterostructures without gapless interface modes. We also show that the partial real-space invariants are able to detect all time-reversal symmetric topological phases of the recently discovered fragile type. △ Less

Submitted 17 August, 2020; originally announced August 2020.

Comments: 20 pages, 7 figures, 4 tables

Journal ref: npj Quantum Mater. 6, 1 (2021)

Magnetic impurities along the edge of a quantum spin Hall insulator: Realizing a one-dimensional AIII insulator

Authors: G. A. R. van Dalum, C. Ortix, L. Fritz

Abstract: In this paper we construct a one-dimensional insulator with an approximate chiral symmetry belonging to the AIII class and discuss its properties. The construction principle is the intentional pollution of the edge of a two-dimensional quantum spin Hall insulator with magnetic impurities. The resulting bound states hybridize and disperse along the edge. We discuss under which circumstances this ch… ▽ More In this paper we construct a one-dimensional insulator with an approximate chiral symmetry belonging to the AIII class and discuss its properties. The construction principle is the intentional pollution of the edge of a two-dimensional quantum spin Hall insulator with magnetic impurities. The resulting bound states hybridize and disperse along the edge. We discuss under which circumstances this chain possesses zero-dimensional boundary modes on the level of an effective low-energy theory. The main appeal of our construction is the independence on details of the impurity lattice: the zero modes are stable against disorder and random lattice configurations. We also show that in the presence of Rashba coupling, which changes the symmetry class to A, one can still expect localized half-integer boundary excess charges protected by mirror symmetry although there is no nontrivial topological index. All of the results are confirmed numerically in a microscopic model. △ Less

Submitted 5 February, 2021; v1 submitted 9 July, 2020; originally announced July 2020.

Comments: 10 pages, 9 figures

Journal ref: Phys. Rev. B 103, 075115 (2021)

Anomalous Planar Hall Effect in Two-Dimensional Trigonal Crystals

Authors: Raffaele Battilomo, Niccoló Scopigno, Carmine Ortix

Abstract: The planar Hall effect (PHE) is the appearance of an in-plane transverse voltage in the presence of coplanar electric and magnetic fields. Its hallmark is a characteristic $π$-periodic, i.e. even under a magnetic field reversal, angular dependence with the transverse voltage that exactly vanishes when the electric and magnetic fields are aligned. Here we demonstrate that in two-dimensional trigona… ▽ More The planar Hall effect (PHE) is the appearance of an in-plane transverse voltage in the presence of coplanar electric and magnetic fields. Its hallmark is a characteristic $π$-periodic, i.e. even under a magnetic field reversal, angular dependence with the transverse voltage that exactly vanishes when the electric and magnetic fields are aligned. Here we demonstrate that in two-dimensional trigonal crystals Zeeman-induced non-trivial Berry curvature effects yield a previously unknown anomalous PHE that is odd in the magnetic field and independent of the relative angle with the driving electric field. We further show that when an additional mirror symmetry forces the transverse voltage to vanish in the linear response regime, the anomalous PHE can occur as a second-order response at both zero and twice the frequency of the applied electric field. We demonstrate that this non-linear PHE possesses an antisymmetric quantum contribution that originates from a Zeeman-induced Berry curvature dipole. △ Less

Submitted 21 January, 2021; v1 submitted 4 June, 2020; originally announced June 2020.

Comments: 6 pages, 3 figures

Journal ref: Phys. Rev. Research 3, 012006 (2021)

On the topological immunity of corner states in two-dimensional crystalline insulators

Authors: Guido van Miert, Carmine Ortix

Abstract: A higher-order topological insulator (HOTI) in two dimensions is an insulator without metallic edge states but with robust zero-dimensional topological boundary modes localized at its corners. Yet, these corner modes do not carry a clear signature of their topology as they lack the anomalous nature of helical or chiral boundary states. Here, we demonstrate using immunity tests that the corner mode… ▽ More A higher-order topological insulator (HOTI) in two dimensions is an insulator without metallic edge states but with robust zero-dimensional topological boundary modes localized at its corners. Yet, these corner modes do not carry a clear signature of their topology as they lack the anomalous nature of helical or chiral boundary states. Here, we demonstrate using immunity tests that the corner modes found in the breathing kagome lattice represent a prime example of a mistaken identity. Contrary to previous theoretical and experimental claims, we show that these corner modes are inherently fragile: the kagome lattice does not realize a higher-order topological insulator. We support this finding by introducing a criterion based on a corner charge-mode correspondence for the presence of topological midgap corner modes in n-fold rotational symmetric chiral insulators that explicitly precludes the existence of a HOTI protected by a threefold rotational symmetry. △ Less

Submitted 4 September, 2020; v1 submitted 1 May, 2020; originally announced May 2020.

Comments: 10 pages, 5 figures. Accepted for publication in NPJ Quantum Materials

Journal ref: npj Quantum Materials 5, 63 (2020)

Engineering Topological Nodal Line Semimetals in Rashba Spin-Orbit Coupled Atomic Chains

Authors: Paola Gentile, Vittorio Benvenuto, Carmine Ortix, Canio Noce, Mario Cuoco

Abstract: We study an atomic chain in the presence of modulated charge potential and modulated Rashba spin-orbit coupling (RSOC) of equal period. We show that for commensurate periodicities $λ=4 n$ with integer $n$, the three-dimensional synthetic space obtained by sliding the two phases of the charge potential and RSOC features a topological nodal line semimetal protected by an antiunitary particle-hole sy… ▽ More We study an atomic chain in the presence of modulated charge potential and modulated Rashba spin-orbit coupling (RSOC) of equal period. We show that for commensurate periodicities $λ=4 n$ with integer $n$, the three-dimensional synthetic space obtained by sliding the two phases of the charge potential and RSOC features a topological nodal line semimetal protected by an antiunitary particle-hole symmetry. The location and shape of the nodal lines strongly depend on the relative amplitude between the charge potential and RSOC. △ Less

Submitted 9 December, 2019; originally announced December 2019.

Comments: 10 pages, 5 figures

Journal ref: Condens. Matter 2019, 4(1), 25

Berry Curvature Dipole in Strained Graphene: a Fermi Surface Warping Effect

Authors: Raffaele Battilomo, Niccolo' Scopigno, Carmine Ortix

Abstract: It has been recently established that optoelectronic and non-linear transport experiments can give direct access to the dipole moment of the Berry curvature in non-magnetic and non-centrosymmetric materials. Thus far, non-vanishing Berry curvature dipoles have been shown to exist in materials with substantial spin-orbit coupling where low-energy Dirac quasiparticles form tilted cones. Here, we pro… ▽ More It has been recently established that optoelectronic and non-linear transport experiments can give direct access to the dipole moment of the Berry curvature in non-magnetic and non-centrosymmetric materials. Thus far, non-vanishing Berry curvature dipoles have been shown to exist in materials with substantial spin-orbit coupling where low-energy Dirac quasiparticles form tilted cones. Here, we prove that this topological effect does emerge in two-dimensional Dirac materials even in the complete absence of spin-orbit coupling. In these systems, it is the warping of the Fermi surface that triggers sizeable Berry dipoles. We show indeed that uniaxially strained monolayer and bilayer graphene, with substrate-induced and gate-induced band gaps respectively, are characterized by Berry curvature dipoles comparable in strength to those observed in monolayer and bilayer transition metal dichalcogenides. △ Less

Submitted 22 October, 2019; originally announced October 2019.

Comments: 5 pages, 4 figures. Accepted for publication in Physical Review Letters

Journal ref: Phys. Rev. Lett. 123, 196403 (2019)

Zero-magnetic-field Hall effects in artificially corrugated bilayer graphene

Authors: Sheng-Chin Ho, Ching-Hao Chang, Yu-Chiang Hsieh, Shun-Tsung Lo, Botsz Huang, Thi-Hai-Yen Vu, Carmine Ortix, Tse-Ming Chen

Abstract: The ability to engineer the electronic band structure and, more strikingly, to access new exotic phase of matter has been the cornerstone of the advance of science and technology. Twisting van der Waals materials to form moiré superlattice is a powerful paradigm and can drive graphene from a normal metallic state into an insulating, superconducting, or ferromagnetic states. Here, we present a new… ▽ More The ability to engineer the electronic band structure and, more strikingly, to access new exotic phase of matter has been the cornerstone of the advance of science and technology. Twisting van der Waals materials to form moiré superlattice is a powerful paradigm and can drive graphene from a normal metallic state into an insulating, superconducting, or ferromagnetic states. Here, we present a new route to create non-trivial band structure and consequently an exotic phase of matter via lithographically patterned strain (lattice deformation). This method is used to realize an artificially corrugated bilayer graphene wherein the real-space and momentum-space pseudo-magnetic fields (Berry curvatures) coexist and have nontrivial properties, namely, the Berry curvature dipole. This new class of condensed-matter systems enables us to observe the so-called nonlinear anomalous Hall effect and a new type of Hall effect without breaking the time-reversal symmetry. Such artificial material system and our approach to unconventional electronic states may open an avenue of geometrical and/or topological quantum phenomena as well as that of band engineering in van der Waals crystals. △ Less

Submitted 5 March, 2021; v1 submitted 16 October, 2019; originally announced October 2019.

Comments: the submitted (preprint) version

Journal ref: Nature Electronics 4,116-125 (2021)

Geometric driving of two-level quantum systems

Authors: Zu-Jian Ying, Paola Gentile, José Pablo Baltanàs, Diego Frustaglia, Carmine Ortix, Mario Cuoco

Abstract: We investigate a class of cyclic evolutions for %the cyclic evolution of driven two-level quantum systems (effective spin-1/2) with a particular focus on the geometric characteristics of the driving and their specific imprints on the quantum dynamics. By introducing the concept of geometric field curvature for any field trajectory in the parameter space we are able to unveil underlying patterns in… ▽ More We investigate a class of cyclic evolutions for %the cyclic evolution of driven two-level quantum systems (effective spin-1/2) with a particular focus on the geometric characteristics of the driving and their specific imprints on the quantum dynamics. By introducing the concept of geometric field curvature for any field trajectory in the parameter space we are able to unveil underlying patterns in the overall quantum behavior: the knowledge of the field curvature provides a non-standard and fresh access to the interrelation between field and spin trajectories, and the corresponding quantum phases acquired in non-adiabatic cyclic evolutions. In this context, we single out setups in which the driving field curvature can be employed to demonstrate a pure geometric control of the quantum phases. Furthermore, the driving field curvature can be naturally exploited to introduce the geometrical torque and derive a general expression for the total quantum phase acquired in a cycle. Remarkably, such relation allows to access the mechanisms controlling the changeover of the quantum phase across a topological transition and to disentangle the role of the spin and field topological windings. As for implementations, we discuss a series of physical systems and platforms to demonstrate how the geometric control of the quantum phases can be realized for pendular field drivings. This includes setups based on superconducting islands coupled to a Josephson junction and inversion asymmetric nanochannels with suitably tailored geometric shapes. △ Less

Submitted 10 September, 2019; originally announced September 2019.

Comments: 13 pages, 5 figures

Journal ref: Phys. Rev. Research 2, 023167 (2020)

The hybrid-order topology of weak topological insulators

Authors: Sander H. Kooi, Guido van Miert, Carmine Ortix

Abstract: We consider weak topological insulators with a twofold rotation symmetry around the dark direction, and show that these systems can be endowed with the topological crystalline structure of a higher-order topological insulator protected by rotation symmetry. These hybrid-order weak topological insulators display surface Dirac cones on all surfaces. Translational symmetry breaking perturbations gap… ▽ More We consider weak topological insulators with a twofold rotation symmetry around the dark direction, and show that these systems can be endowed with the topological crystalline structure of a higher-order topological insulator protected by rotation symmetry. These hybrid-order weak topological insulators display surface Dirac cones on all surfaces. Translational symmetry breaking perturbations gap the Dirac cones on the side surfaces leaving anomalous helical hinge modes behind. We also prove that the existence of this topological phase comes about due to a hidden crystalline topological invariant of quantum spin-Hall insulators that can neither be revealed by symmetry indicators nor using Wilson loop invariants. Considering the minimal symmetry requirements, we anticipate that our findings could apply to a large number of weak topological insulators. △ Less

Submitted 3 August, 2020; v1 submitted 2 August, 2019; originally announced August 2019.

Comments: 10 pages, 5 figures

Journal ref: Phys. Rev. B 102, 041122 (2020)

Classification of crystalline insulators without symmetry indicators: atomic and fragile topological phases in twofold rotation symmetric systems

Authors: Sander H. Kooi, Guido van Miert, Carmine Ortix

Abstract: Topological crystalline phases in electronic structures can be generally classified using the spatial symmetry characters of the valence bands and mapping them onto appropriate symmetry indicators. These mappings have been recently applied to identify thousands of topological electronic materials. There can exist, however, topological crystalline non-trivial phases that go beyond this paradigm: th… ▽ More Topological crystalline phases in electronic structures can be generally classified using the spatial symmetry characters of the valence bands and mapping them onto appropriate symmetry indicators. These mappings have been recently applied to identify thousands of topological electronic materials. There can exist, however, topological crystalline non-trivial phases that go beyond this paradigm: they cannot be identified using spatial symmetry labels and consequently lack any classification. In this work, we achieve the first of such classifications showcasing the paradigmatic example of two-dimensional crystals with twofold rotation symmetry. We classify the gapped phases in time-reversal invariant systems with strong spin-orbit coupling identifying a set of three $\mathbb{Z}_2$ topological invariants, which correspond to nested quantized partial Berry phases. By further isolating the set of atomic insulators representable in terms of exponentially localized symmetric Wannier functions, we infer the existence of topological crystalline phases of the fragile type that would be diagnosed as topologically trivial using symmetry indicators, and construct a number of microscopic models exhibiting this phase. Our work is expected to have important consequences given the central role fragile topological phases are expected to play in novel two-dimensional materials such as twisted bilayer graphene. △ Less

Submitted 20 June, 2019; originally announced June 2019.

Comments: 15 pages, 8 figures

Journal ref: Phys. Rev. B 100, 115160 (2019)

Tuning topology in thin films of topological insulators by strain gradients

Authors: Raffaele Battilomo, Niccoló Scopigno, Carmine Ortix

Abstract: We theoretically show that the coupling of inhomogeneous strains to the Dirac fermions of three-dimensional topological insulators (3DTI) in thin film geometries results in the occurrence of phase transitions between topologically distinct insulating phases. By means of minimal k dot p models for strong 3DTI in the Bi 2 Se 3 materials class, we find that in thin films of stoichiometric materials a… ▽ More We theoretically show that the coupling of inhomogeneous strains to the Dirac fermions of three-dimensional topological insulators (3DTI) in thin film geometries results in the occurrence of phase transitions between topologically distinct insulating phases. By means of minimal k dot p models for strong 3DTI in the Bi 2 Se 3 materials class, we find that in thin films of stoichiometric materials a strain-gradient induced structure inversion asymmetry drives a phase transition from a quantum spin-Hall phase to a topologically trivial insulating phase. Interestingly, in alloys with strongly reduced bulk band gaps strain gradients have an opposite effect and promote a topologically non-trivial phase from a parent normal band insulator. These strain-gradient assisted switchings between topologically distinct phases are expected to yield a flexomagnetic coupling in magnetic topological insulator thin films. △ Less

Submitted 14 May, 2019; originally announced May 2019.

Journal ref: Phys. Rev. B 100, 115131 (2019)

Independent geometrical control of spin and charge resistances in curved spintronics

Authors: Kumar Sourav Das, Denys Makarov, Paola Gentile, Mario Cuoco, Bart J. van Wees, Carmine Ortix, Ivan J. Vera-Marun

Abstract: Spintronic devices operating with pure spin currents represent a new paradigm in nanoelectronics, with higher energy efficiency and lower dissipation as compared to charge currents. This technology, however, will be viable only if the amount of spin current diffusing in a nanochannel can be tuned on demand while guaranteeing electrical compatibility with other device elements, to which it should b… ▽ More Spintronic devices operating with pure spin currents represent a new paradigm in nanoelectronics, with higher energy efficiency and lower dissipation as compared to charge currents. This technology, however, will be viable only if the amount of spin current diffusing in a nanochannel can be tuned on demand while guaranteeing electrical compatibility with other device elements, to which it should be integrated in high-density three-dimensional architectures. Here, we address these two crucial milestones and demonstrate that pure spin currents can effectively propagate in metallic nanochannels with a three-dimensional curved geometry. Remarkably, the geometric design of the nanochannels can be used to reach an independent tuning of spin transport and charge transport characteristics. These results put the foundation for the design of efficient pure spin current based electronics, which can be integrated in complex three-dimensional architectures. △ Less

Submitted 5 October, 2019; v1 submitted 5 November, 2018; originally announced November 2018.

Comments: 3 figures, plus Supporting Information

Journal ref: Nano Letters 2019

Novel topological insulators from crystalline symmetries

Authors: Alexander Lau, Carmine Ortix

Abstract: We discuss recent advances in the study of topological insulators protected by spatial symmetries by reviewing three representative, theoretical examples. In three dimensions, these states of matter are generally characterized by the presence of gapless boundary states at surfaces that respect the protecting spatial symmetry. We discuss the appearance of these topological states both in crystals w… ▽ More We discuss recent advances in the study of topological insulators protected by spatial symmetries by reviewing three representative, theoretical examples. In three dimensions, these states of matter are generally characterized by the presence of gapless boundary states at surfaces that respect the protecting spatial symmetry. We discuss the appearance of these topological states both in crystals with negligible spin-orbit coupling and a fourfold rotational symmetry, as well as in mirror-symmetric crystals with sizable spin-orbit interaction characterized by the so-called mirror Chern number. Finally, we also discuss similar topological crystalline states in one-dimensional insulators, such as nanowires or atomic chains, with mirror symmetry. There, the prime physical consequence of the non-trivial topology is the presence of quantized end charges. △ Less

Submitted 30 October, 2018; originally announced October 2018.

Comments: Mini-review, to appear in European Physical Journal Special Topics

Journal ref: Eur. Phys. J. Spec. Top. 227, 1309 (2018)

Spin interference effects in Rashba quantum rings

Authors: Carmine Ortix

Abstract: Quantum interference effects in rings provide suitable means to control spins at the mesoscopic scale. In this chapter we present the theory underlying spin-induced modulations of unpolarized currents in quantum rings subject to the Rashba spin-orbit interaction. We discuss explicitly the connection between the conductance modulations and the geometric phase acquired by the spin during transport,… ▽ More Quantum interference effects in rings provide suitable means to control spins at the mesoscopic scale. In this chapter we present the theory underlying spin-induced modulations of unpolarized currents in quantum rings subject to the Rashba spin-orbit interaction. We discuss explicitly the connection between the conductance modulations and the geometric phase acquired by the spin during transport, as well as pathways to directly control them. △ Less

Submitted 9 October, 2018; originally announced October 2018.

Comments: 20 pages, 8 figures

Journal ref: in Fomin V. (eds) Physics of Quantum Rings. Nanoscience and Technology. Spinger. (2018)

Spin field-effect transistor in a quantum spin-Hall device

Authors: R. Battilomo, N. Scopigno, C. Ortix

Abstract: We discuss the transport properties of a quantum spin-Hall insulator with sizable Rashba spin-orbit coupling in a disk geometry. The presence of topologically protected helical edge states allows for the control and manipulation of spin polarized currents: when ferromagnetic leads are coupled to the quantum spin-Hall device, the ballistic conductance is modulated by the Rashba strength. Therefore,… ▽ More We discuss the transport properties of a quantum spin-Hall insulator with sizable Rashba spin-orbit coupling in a disk geometry. The presence of topologically protected helical edge states allows for the control and manipulation of spin polarized currents: when ferromagnetic leads are coupled to the quantum spin-Hall device, the ballistic conductance is modulated by the Rashba strength. Therefore, by tuning the Rashba interaction via an all-electric gating, it is possible to control the spin polarization of injected electrons. △ Less

Submitted 23 August, 2018; originally announced August 2018.

Journal ref: Phys. Rev. B 98, 075147 (2018)