The LaAlO 3 =SrTiO 3 interface hosts a two-dimensional electron system that is unusually sensitive to the application of an in-plane magnetic field. Low-temperature experiments have revealed a giant negative magnetoresistance (dropping by 70%), attributed to a magnetic-field induced transition between interacting phases of conduction electrons with Kondo-screened magnetic impurities. Here we report on experiments over a broad temperature range, showing the persistence of the magnetoresistance up to the 20 K rangeindicative of a single-particle mechanism. Motivated by a striking correspondence between the temperature and carrier density dependence of our magnetoresistance measurements we propose an alternative explanation. Working in the framework of semiclassical Boltzmann transport theory we demonstrate that the combination of spin-orbit coupling and scattering from finite-range impurities can explain the observed magnitude of the negative magnetoresistance, as well as the temperature and electron density dependence. The mobile electrons at the LaAlO 3 =SrTiO 3 (LAO=STO) interface [1] display an exotic combination of superconductivity [2,3] and magnetic order [4][5][6][7]. The onset of superconductivity at sub-Kelvin temperatures appears in an interval of electron densities where the effect of Rashba spin-orbit coupling on the band structure at the Fermi level is strongest [8,9], but whether this correlation implies causation remains unclear.Transport experiments above the superconducting transition temperature have revealed a very large ("giant") drop in the sheet resistance of the LAO=STO interface upon application of a parallel magnetic field [10][11][12][13]. An explanation has been proposed [13,14] in terms of the Kondo effect: Variation of the electron density or magnetic field drives a quantum phase transition between a highresistance correlated electronic phase with screened magnetic impurities and a low-resistance phase of polarized impurity moments. The relevance of spin-orbit coupling for magnetotransport is widely appreciated [10,[14][15][16][17][18][19], but it was generally believed to be too weak an effect to provide a single-particle explanation of the giant magnetoresistance.In this work we provide experimental data (combining magnetic field, gate voltage, and temperature profiles for the resistance of the LAO=STO interface) and theoretical calculations that support an explanation fully within the single-particle context of Boltzmann transport. The key ingredients are the combination of spin-orbit coupling, band anisotropy, and finite-range electrostatic impurity scattering.The thermal insensitivity of the giant magnetoresistance [10,11], in combination with a striking correspondence that we have observed between the gate voltage and temperature dependence of the effect, are features that are difficult to reconcile with the thermally fragile Kondo interpretationbut fit naturally in the semiclassical Boltzmann description.We first present the experimental data and then turn to the theoretical de...

A Weyl semimetal with broken time-reversal symmetry has a minimum of two species of Weyl fermions, distinguished by their opposite chirality, in a pair of Weyl cones at opposite momenta ±K that are displaced in the direction of the magnetization. Andreev reflection at the interface between a Weyl semimetal in the normal state (N) and a superconductor (S) that pairs ±K must involve a switch of chirality, otherwise it is blocked. We show that this "chirality blockade" suppresses the superconducting proximity effect when the magnetization lies in the plane of the NS interface. A Zeeman field at the interface can provide the necessary chirality switch and activate Andreev reflection.

The Fermi surface of a conventional two-dimensional electron gas is equivalent to a circle, up to smooth deformations that preserve the orientation of the equi-energy contour. Here we show that a Weyl semimetal confined to a thin film with an in-plane magnetization and broken spatial inversion symmetry can have a topologically distinct Fermi surface that is twisted into a figure-8-opposite orientations are coupled at a crossing which is protected up to an exponentially small gap. The twisted spectral response to a perpendicular magnetic field B is distinct from that of a deformed Fermi circle, because the two lobes of a figure-8 cyclotron orbit give opposite contributions to the Aharonov-Bohm phase. The magnetic edge channels come in two counterpropagating types, a wide channel of width b µ l B 1 m 2 and a narrow channel of width µ l B 1 m (with  = l eB m the magnetic length and β the momentum separation of the Weyl points). Only one of the two is transmitted into a metallic contact, providing unique magnetotransport signatures.

LaAlO3/SrTiO3 and LaTiO3/SrTiO3 (LXO / STO) interfaces are known to host a strongly inhomogeneous (nearly) two-dimensional electron gas (2DEG). In this work we present three unconventional electronic mechanisms of electronic phase separation (EPS) in a 2DEG as a possible source of inhomogeneity in oxide interfaces. Common to all three mechanisms is the dependence of some (interaction) potential on the 2DEGs density. We first consider a mechanism resulting from a sizable density-dependent Rashba spin-orbit coupling. Next, we point that an EPS may also occur in the case of a density-dependent superconducting pairing interaction. Finally, we show that the confinement of the 2DEG to interface by a density-dependent, self-consistent electrostatic potential can by itself cause an EPS.

The unconventional magnetotransport at the interface between transition-metal oxides LaAlO 3 (LAO) and SrTiO 3 (STO) is frequently related to mobile electrons interacting with localized magnetic moments. However nature and properties of magnetism at this interface are not well understood so far. In this paper, we focus on transport effects driven by spin-orbit coupling and intentionally neglect possible strong correlations. The electrical resistivity tensor is calculated as a function of the magnitude and orientation of an external magnetic field parallel to the interface. The semiclassical Boltzmann equation is solved numerically for the two-dimensional system of spin-orbit coupled electrons accelerated by an electric field and scattered by spatially-correlated impurities. At temperatures of a few Kelvin and densities such that the chemical potential crosses the second pair of spin-orbit split bands, we find a strongly anisotropic modulation of the (negative) magnetoresistance above 10 T, characterized by multiple maxima and minima away from the crystalline axes. Along with the drop of the magnetoresistance, an abrupt enhancement of the transverse resistivity occurs. The angular modulation of the latter considerably deviates from a (low-field) sinusoidal dependence to a (high-field) step-like behaviour. These peculiar features are the consequences of the anisotropy of both (intra-band and inter-band ) scattering-amplitudes in the Brillouin zone when the relevant energy scales in the system -chemical potential, spin-orbit interaction and Zeeman energy -are all comparable to each other. The theory provides good qualitative agreement with experimental data in the literature. arXiv:1609.00663v3 [cond-mat.mes-hall]