2022 Abstracts Orbitronics

Marcio COSTA

In this talk, we are going to discuss the results of the spin and orbital Hall effect in 2D materials. We performed high throughput calculations to compute the spin (SHC) and orbital (OHC) Hall conductivities for hundreds of 2D materials[1]. The calculations were performed using the C2DB materials database [2] following a workflow based on DFT + an effective local Hamiltonian (PAOFLOW Hamiltonians)[3]. Initially, we discuss the OHC plateau in the insulating gap of
2D transition metal dichalcogenides(TMD). These TMDs are identified as higher-order topological insulators (HOTI) by a Z4 topological invariant. Using general symmetry arguments, we
establish a connection between the two phenomena with potential implications for spin-orbitronics[4]. A more general picture is formed using the SHC and OHC results for the C2DB database.

[1] npj Computational Materials 7, 49 (2021).
[2] Phys. Rev. Lett. 126, 056601, (2021).
[3] Computational Materials Science 200, 110828 (2021).
[4] arXiv:2205.00997, (2022).

Paul M. HANEY

Berry curvature plays an essential role in determining material properties. In time-reversal symmetric systems which break inversion symmetry, the total Berry curvature vanishes while the crystal momentum-resolved Berry curvature may be nonzero. In this talk, we consider two nonmagnetic systems in which the compensated Berry curvature in different valleys leads to unique observable effects. We first consider MoS2, which exhibits an orbital Hall effect. We show that an applied electric field induces a change in the charge density distribution, and utilize first principles calculations to show that the distorted charge density profile may be used to determine to the energy-resolved orbital Hall conductivity [1]. We next consider twisted double bilayer graphene. This system develops valley contrasting Berry curvature and orbital magnetization as a function of vertical displacement field [2]. We use semi-classical analysis together with calculations of the Hofstadter spectrum to show how the valley-dependent orbital magnetization is reflected in the magnetic field-dependent spectrum [3]. We compare our results on the twisted double bilayer graphene system with scanning tunneling microscopy data.

[1] F. Xue, V. Amin, and P. M. Haney, Phys. Rev. B 102, 161103(R) (2020).
[2] N. R. Chebrolu, B. L. Chittari, and J. Jung, Phys. Rev. B 99, 235417 (2019).
[3] Y. Gao and C. Niu, Proc. Natl. Acad. Sci., 114, 7295 (2017).

Sergio VALENZUELA

The large variety of 2D materials and their co-integration in van der Waals heterostructures enable innovative device engineering. In particular, their atomically thin nature promotes the design of artificial quantum materials by proximity effects that originate from short-range interactions [1]. This designer approach is especially compelling for spintronic devices, which usually harness their functionalities from thin layers of magnetic and non-magnetic materials and the interfaces between them [2]. In this talk, I will highlight recent advances in this rapidly evolving field. I will further introduce novel ways to investigate proximity phenomena by means of spin transport dynamics, as reflected in spin relaxation anisotropy [3] and charge to spin interconversion [4] experiments. I will also discuss the relevance of crystal symmetry and the emergence of unconventional charge to spin conversion components when crystal symmetries are broken [5].

[1] J. F. Sierra et al., Nature Nano. 16, 856-868 (2021)
[2] H. Yang, S O. Valenzuela et al., Nature 606, 663-673 (2022)
[3] L. A. Benítez et al., Nature Phys. 14, 303-308 (2018); APL Materials 7, 120701 (2019)
[4] L. A. Benítez et al., Nature Mater. 19, 170-175 (2020)
[5] L. Camosi et al., 2D Mater. 9, 035014 (2022)

Laurent VILA

While spintronics has traditionally relied on ferromagnetic metals as spin generators and detectors, efficient spin-charge interconversion enabled by spin-orbit coupling in non-magnetic systems has drawn considerable interest in recent years. We report a new approach to generate and detect spin currents by exploiting the interplay between spin-orbit effects and ferroelectricity, in two classes of materials: two-Dimensional Electron Gases (2DEGS) appearing at oxides surfaces or interfaces [1], ferroelectric Rashba semiconductors [2] and at surfaces of Topological Insulators [3].
Our results demonstrate that the spin-to-charge conversion, due to the spin-orbit coupling, can be controlled in sign in a remanent way, through the ferroelectric polarization. Such a control can lead to the emergence of a ferroelectric spintronics, which could result in a reduction of the power consumption of non-volatile spintronic devices by a factor of one thousand. It also provides a way for a non-destructive readout of ferroelectric states.
This opens new opportunities for creating spin-based devices, such as the MESO transistor proposed recently by Intel [4], which relies on the writing of a magnetic information through magnetoelectric coupling, and of its reading by spin-charge conversion. Indeed, by controlling directly the sign of the interconversion by a ferroelectric state, it is possible to merge in a single device its writing and reading blocks, and the need for reversing a magnetic state.
In this presentation we will present the basic principle of the control of this spin-to-charge conversion, report our results obtained on SrTiO3 2D gases, the ferroelectric Rashba semiconductors GeTe and the gate control of the bilinear MR of the TI HgTe [3]. We will discuss the perspective of this work.

[1] P. Noel et al., Nature 580, 483–86 (2020).
[2] S. Varotto et al., Nature Electronics 4, 740 (2021).
[3] Y. Fu et al., arXiv:2111.15594 (2021).
[4] S. Manipatruni et al., Nature 565, 35–42 (2019).

Yuriy MOKROUSOV

In modern spintronics properties of non-equilibrium orbital polarization and orbital currents start to attract significant attention. In this talk we will review the theory of orbital magnetism in low-symmetric crystals and corresponding current-induced orbital magnetization. We will in particular show that applied electrical currents and optical pulses can drive non-equilibrium orbital magnetism and currents of orbital angular momentum. These orbital currents can be used to transmit angular momentum over large distances in solids, and can be utilized to exert sizeable orbital torques on magnetization thus enabling magnetic switching even in light materials with weak spin-orbit interaction. Moreover, we will underline that in fluctuating magnets spin excitations can mediate a significant orbital response which can be coupled to temperature gradients so as to ignite thermal orbital currents. We will thus attempt to promote a paradigm that unleashing non-equilibrium orbital physics and entanglement of spin and orbital degrees of freedom in diverse classes of materials can lead to much richer physics than previously expected, and might provide a key to realization of novel properties of matter out of equilibrium as well as energy-efficient applications.

Maria Teresa Mercaldo

The breaking of point-group spatial symmetries can have a profound impact on superconductivity. We consider a multi-orbital spin-singlet superconductor without inversion symmetry, e.g. due to crystalline asymmetry as well as to electric field or mechanical strain. The lack of inversion symmetry yields orbital-Rashba couplings that can turn the superconductor into normal metal, or induce 0−π transition, with the π phase being marked by a sign change of the superconducting order parameter between different bands [1-2]. The occurrence of orbital dependent phase frustration can account for the observation of the suppression of the critical supercurrent without change in the critical temperature, observed in recent experiments [3-4]. Furthermore, in superconductors that lack inversion symmetry, the flow of supercurrent can induce a non-vanishing magnetization, a phenomenon which is at the heart of non-dissipative magneto-electric effects, also known as Edelstein effects. For electrons carrying spin and orbital moments a question of fundamental relevance deals with the orbital nature of magneto-electric effects in conventional spin-singlet superconductors with Rashba coupling. Remarkably, we find that the supercurrent-induced orbital magnetization is more than one order of magnitude greater than that due to the spin, giving rise to a colossal magneto-electric effect [5]. The induced orbital magnetization is shown to be sign tunable, with the sign change occurring for the Fermi level lying in proximity of avoiding crossing points in the Brillouin zone and in the presence of superconducting phase inhomogeneities, yielding domains with opposite orbital moment orientation. Finally, we show that in two-dimensional spin-singlet superconductors with very low degree of spatial symmetry content, vortices with neutral supercurrents carrying angular momentum around the core can form and be energetically stable [6]. The vortex has zero net magnetic flux since it is made up of counterpropagating Cooper pairs with opposite orbital moments. The overall findings unveil a rich scenario to design heterostructures with superconducting orbitronics effects for the achievement, for instance, of all-electric superconducting devices.

[1] M. T. Mercaldo, P. Solinas, F. Giazotto, and M. Cuoco, Phys. Rev. Applied 14, 034041 (2020)
[2] M. T. Mercaldo, F. Giazotto, and M. Cuoco, Phys. Rev. Research 3, 043042 (2021).
[3] G. De Simoni, et al., Nat. Nanotechnol. 13, 802 (2018).
[4] L. Bours, M. T. Mercaldo, M. Cuoco, E. Strambini, and F. Giazotto, Phys. Rev. Research 2, 033353 (2020).
[5] L. Chirolli, M.T. Mercaldo, C. Guarcello, F. Giazotto, and M. Cuoco, arXiv:2107.07476, Phys. Rev. Lett. (2022), to be published.
[6] M. T. Mercaldo, C. Ortix, F. Giazotto, and M. Cuoco, Phys. Rev. B 105, L140507 (2022).

Jagoda Sławińska

Chiral molecules and crystals, similarly to human hands, have distinguishable right-handed and left-handed enantiomers which may manifest dissimilar physical and chemical properties and behave differently in response to external stimuli. In this talk, I will discuss the chirality-induced spin selectivity (CISS) that allows the conversion of charge current to collinear spin current and vice versa in chiral crystals. Based on the first-principles calculations, the CISS effect in solids has been classified as an analog of the Rashba-Edelstein effect, occurring for systems with Weyl-type spin-orbit coupling and a radial spin texture around specific high-symmetry points in the reciprocal space. Importantly, the induced spin accumulation is intrinsically protected by the quasi-persistent spin helix arising as a consequence of crystal symmetries in Weyl systems; the spin transport can be therefore protected over large (even micrometer-scale) distances, as recently observed in elemental tellurium and chiral disilicides. Long-range spin accumulation in chiral crystals opens novel routes for the design of solid-state electronic devices; its orbitronics analog represents another extremely interesting perspective that remains to be explored.

Junyeon KIM

Utilization of the orbital angular momentum (OAM) opens a new way for an alternative method of the spin manipulation. Possibility of using wide range of materials, which comes from the fact that an electrical generation of the OAM does not require a large spin-orbit coupling, is the most remarkable virtue. In this talk, we introduce Ferromagnet (FM)/Nonmagnet (NM)/Al2O3 stacks as a highly efficient platform for current-induced torques and spin-orbitronic devices [1]. A striking feature we find is that the torque efficiency for the FM/Cu/Al2O3 system is highly affected by the annealing temperature and the choice of the FM. Moreover, we find that some samples exhibit gigantic torque efficiency of ~0.25, indicating that the OAM plays a significant role in current-induced torques. We also present our recent results on the variation of the torque efficiency for FM/Ru/Al2O3 [2] and various types of oxide materials in FM/Cu/oxide stacks [3]. Further discussion will be given during the workshop.

[1] J. Kim et al., Phys. Rev. B 103, L020407 (2021)
[2] L. Liao et al., Phys. Rev. B 105, 104434 (2022)
[3] J. Kim et al. in preparation