A minimal model is exactly solved for electron spin transport on a helix. Electron transport is assumed to be supported by well oriented p(z) type orbitals on base molecules forming a staircase of definite chirality. In a tight binding interpretation, the spin-orbit coupling (SOC) opens up an effective π(z) - π(z) coupling via interbase p(x,y) - p(z) hopping, introducing spin coupled transport. The resulting continuum model spectrum shows two Kramers doublet transport channels with a gap proportional to the SOC. Each doubly degenerate channel satisfies time reversal symmetry; nevertheless, a bias chooses a transport direction and thus selects for spin orientation. The model predicts (i) which spin orientation is selected depending on chirality and bias, (ii) changes in spin preference as a function of input Fermi level and (iii) back-scattering suppression protected by the SO gap. We compute the spin current with a definite helicity and find it to be proportional to the torsion of the chiral structure and the non-adiabatic Aharonov-Anandan phase. To describe room temperature transport, we assume that the total transmission is the result of a product of coherent steps.

Twisted graphene bilayers develop highly localized states around AA-stacked regions for small twist angles. We show that interaction effects may induce either an antiferromagnetic or a ferromagnetic (FM) polarization of said regions, depending on the electrical bias between layers. Remarkably, FM-polarized AA regions under bias develop spiral magnetic ordering, with a relative 120°misalignment between neighboring regions due to a frustrated antiferromagnetic exchange. This remarkable spiral magnetism emerges naturally without the need of spin-orbit coupling, and competes with the more conventional latticeantiferromagnetic instability, which interestingly develops at smaller bias under weaker interactions than in monolayer graphene, due to Fermi velocity suppression. This rich and electrically controllable magnetism could turn twisted bilayer graphene into an ideal system to study frustrated magnetism in two dimensions. DOI: 10.1103/PhysRevLett.119.107201 Magnetism in 2D electronic systems is known to present a very different phenomenology from its three-dimensional counterpart due to the reduced dimensionality and the increased importance of fluctuations. Striking examples are the impossibility of establishing long range magnetic order in a 2D system without magnetic anisotropy [1] or the emergence of unique finite-temperature phase transitions that are controlled by the proliferation of topological magnetic defects [2]. In the presence of magnetic frustration, in, e.g., Kagome [3,4] or triangular lattices [5][6][7][8], 2D magnetism may also lead to the formation of remarkable quantum spin-liquid phases [3,9,10]. The properties of these states remain under active investigation, and have recently been shown to develop exotic properties, such as fractionalized excitations [11], long-range quantum entanglement of their ground state [12,13], topologically protected transport channels [14], or even high-T C superconductivity upon doping [4,15,16].The importance of 2D magnetism extends also beyond fundamental physics into applied fields. One notable example is data storage technologies. Recent advances in this field are putting great pressure on the magnetic memory industry to develop solutions that may remain competitive in speed and data densities against new emerging platforms. Magnetic 2D materials are thus in demand as a possible way forward [17]. Of particular interest for applications in general are 2D crystals and van der Waals heterostructures. These materials have already demonstrated a great potential for a wide variety of applications, most notably nanoelectronics and optoelectronics [18][19][20]. Some of them have been shown to exhibit considerable tunability through doping, gating, stacking, and strain. Unfortunately, very few 2D crystals have been found to exhibit intrinsic magnetism [21,22], let alone magnetic frustration and potential spin-liquid phases.In this work we predict that twisted graphene bilayers could be a notable exception, realizing a peculiar magnetism on an effective triangular supe...

Abstract. We calculate the tunneling process of a Dirac particle across two square barriers separated a distance d, as well as the scattering by a double cusp barrier where the centers of the cusps are separated a distance larger than their screening lengths. Using the scattering matrix formalism, we obtain the transmission and reflection amplitudes for the scattering processes of both configurations. We show that, the presence of transmission resonances modifies the Lorentizian shape of the energy resonances and induces the appearance of additional maxima in the transmission coefficient in the range of energies where transmission resonances occur. We calculate the Wigner time-delay and show how their maxima depend on the position of the transmission resonance.

Intervalley scattering of carriers in graphene at "top" adatoms may give rise to a hidden Kekulé ordering pattern in the adatom positions. This ordering is the result of a rapid modulation in the electron-mediated interaction between adatoms at the wave vector K − K , which has been shown experimentally and theoretically to dominate their spatial distribution. Here we show that the adatom interaction is extremely sensitive to strain in the supporting graphene, which leads to a characteristic spatial modulation of the Kekulé order as a function of adatom distance. Our results suggest that the spatial distributions of adatoms could provide a way to measure the type and magnitude of strain in graphene and the associated pseudogauge field with high accuracy.

We study the spin polarization of mixed and entangled electron states in a four probe/beam splitter geometry with local Rashba and Dresselhaus interactions. A pair of maximally entangled electrons collides with the beam splitter and enters into two perpendicular branches of length L, composed of spin-orbit active materials (gate confined 2D electron gas). One of the branches is connected to an electron reservoir that acts as a source of decoherence by either behaving as a voltage probe or as a controlled source or sink of current at fixed voltage. Such decoherence source is used to modify the entropy of an unpolarized incoming state in order to generate electron polarization at one or both output branches. The degree of entanglement of the global state and the spin polarization is computed for the outgoing electrons as a function of the coupling to the electron reservoir. Experimentally available spin-orbit strength at the beam splitter arms, for arm lengths of a few micrometers, is able to modulate spin polarization up to 80% in particular spin axes. The Dresselhaus and Rashba coefficients play a symmetric role in modulating the polarization. Significantly less polarization is achieved for incoming mixed states due to the local operation of the reservoir.