Electron dynamics in inhomogeneous magnetic fields

Ballistic Hall Photovoltammetry of Magnetic Resonance in Individual Nanomagnets

We report on ballistic Hall photovoltammetry as a contactless probe of localized spin excitations. Spins resonating in the near field of a two-dimensional electron system are shown to induce a long range electromotive force that we calculate. We use this coupling mechanism to detect the spin wave eigenmodes of a single ferromagnet of sub-100 nm size. The high sensitivity of this detection technique, 380 spins/sqrt[Hz], and its noninvasiveness present advantages for probing magnetization dynamics and spin transport.

Instability induced by exchange forces in a 2D electron gas in a magnetic field with uniform gradient

The exchange interaction is investigated theoretically for electrons confined to a 2D sample placed in a linearly varying magnetic field perpendicular to the plane. Unusual and interesting behavior is predicted: starting from zero electrons, as one adds electrons to the system the maximum distance an electron can travel transverse to the B _z = 0 line (i.e. the system's width) increases continuously but this width will subsequently begin shrinking at some critical number. However this collapse will be reversed as the number crosses another critical value, which we estimate here. For electron parameters typical for two dimensional electron gases, the instability could be observable at sufficiently low electron densities. A Hartree Fock equation is derived. We also show that in an appropriate asymptotic limit this leads to an approximately local potential. One key lesson is that the exchange interaction is large and cannot be reasonably excluded from any valid theoretical investigation.

Confinement of Dirac electrons in graphene magnetic quantum dots

We characterize the confinement of massless Dirac electrons under axially symmetric magnetic fields in graphene, including zero energy modes and higher energy levels. In particular, we analyze in detail the Aharonov-Casher theorem, on the existence of zero modes produced by magnetic fields with finite flux in two dimensions. We apply techniques of supersymmetric quantum mechanics to determine the confined states by means of the quantum number j associated to isospin and angular momentum. We focus on magnetic fields, regular at the origin, whose asymptotic behaviour is [Formula: see text], with α a real number. A confinement of infinite zero-energy modes and excited states is possible as long as [Formula: see text]. When [Formula: see text] the quantum dot is able to trap an infinite number of zero modes but no excited states, while for [Formula: see text] only a finite number of zero modes are confined.

Magnetic Domain Walls as Hosts of Spin Superfluids and Generators of Skyrmions

A domain wall in a magnet with easy-axis anisotropy is shown to harbor spin superfluid associated with its spontaneous breaking of the U(1) spin-rotational symmetry. The spin superfluid is shown to have several topological properties, which are absent in conventional superfluids. First, the associated phase slips create and destroy Skyrmions to obey the conservation of the total Skyrmion charge, which allows us to use a domain wall as a generator and detector of Skyrmions. Second, the domain wall engenders the emergent magnetic flux for magnons along its length, which are proportional to the spin supercurrent flowing through it, and thereby provides a way to manipulate magnons. Third, the spin supercurrent can be driven by the magnon current traveling across it owing to the spin transfer between the domain wall and magnons, leading to the magnonic manipulation of the spin superfluid. The theory for superfluid spin transport within the domain wall is confirmed by numerical simulations.

Driving a Superconductor to Insulator Transition with Random Gauge Fields

Typically the disorder that alters the interference of particle waves to produce Anderson localization is potential scattering from randomly placed impurities. Here we show that disorder in the form of random gauge fields that act directly on particle phases can also drive localization. We present evidence of a superfluid bose glass to insulator transition at a critical level of this gauge field disorder in a nano-patterned array of amorphous Bi islands. This transition shows signs of metallic transport near the critical point characterized by a resistance , indicative of a quantum phase transition. The critical disorder depends on interisland coupling in agreement with recent Quantum Monte Carlo simulations. We discuss how this disorder tuned SIT differs from the common frustration tuned SIT that also occurs in magnetic fields. Its discovery enables new high fidelity comparisons between theoretical and experimental studies of disorder effects on quantum critical systems.

Stochastic resonance and dynamic first-order pseudo-phase-transitions in the irreversible growth of thin films under spatially periodic magnetic fields

We study the irreversible growth of magnetic thin films under the influence of spatially periodic fields by means of extensive Monte Carlo simulations. We find first-order pseudo-phase-transitions that separate a dynamically disordered phase from a dynamically ordered phase. By analogy with time-dependent oscillating fields applied to Ising-type models, we qualitatively associate this dynamic transition with the localization-delocalization transition of spatial hysteresis loops. Depending on the relative width of the magnetic film L compared to the wavelength of the external field λ, different transition regimes are observed. For small systems (L < λ), the transition is associated with the standard stochastic resonance regime, while for large systems (L > λ), the transition is driven by anomalous stochastic resonance. The origin of the latter is identified as due to the emergence of an additional relevant length scale, namely, the roughness of the spin domain switching interface. The distinction between different stochastic resonance regimes is discussed at length both qualitatively by means of snapshot configurations and quantitatively via residence-length and order-parameter probability distributions.

Large local Hall effect in pin-hole dominated multigraphene spin-valves

We report local and non-local measurements in pin-hole dominated mesoscopic multigraphene spin-valves. Local spin-valve measurements show spurious switching behavior in resistance during magnetic field sweeping similar to the signal observed due to spin injection into multigraphene. The switching behavior has been explained in terms of a local Hall effect due to a thickness irregularity of the tunnel barrier. The local Hall effect appears due to a large local magnetostatic field produced near the roughness in the AlO(x) tunnel barrier. In our samples the resistance change due to the local Hall effect remains negligibly small above 75 K. A strong local Hall effect might hinder spin injection into multigraphene, resulting in no spin signal in non-local measurements.

Valley filtering in gapped graphene modulated by an antisymmetric magnetic field and an electric barrier

We investigate valley-dependent electron transport properties of a gapped graphene film modulated by a ferromagnetic metal (FM) stripe with magnetization along the current direction. The antisymmetric stray field of the FM stripe alone does not generate a valley-polarized current due to an intrinsic symmetry. The inclusion of an electric barrier breaks this symmetry. It is shown that highly valley-polarized electron transport can be achieved in this magnetic-electric barrier structure, which results from a valley-dependent phase mechanism. The valley polarization can be tuned by the barrier parameters.

Magnetic Kronig-Penney-type graphene superlattices: finite energy Dirac points with anisotropic velocity renormalization

We study the energy band structure of magnetic graphene superlattices with delta-function magnetic barriers and zero average magnetic field. The dispersion relation obtained using the T-matrix approach shows the emergence of an infinite number of Dirac-like points at finite energies, while the original Dirac point is still located at the same place as that for pristine graphene. The carrier group velocity at the original Dirac point is isotropically renormalized, but at finite energy Dirac points it is generally anisotropic. An asymmetry in the width between the wells and the barriers of the periodic potential induces a shift of the original Dirac point in the zero-energy plane, keeping the velocity renormalization isotropic.

Graphene in inhomogeneous magnetic fields: bound, quasi-bound and scattering states

The electron states in graphene-based magnetic dot and magnetic ring structures and combinations of both are investigated. The corresponding spectra are studied as a function of the radii, the strengths of the inhomogeneous magnetic field and of a uniform background field, the strength of an electrostatic barrier and the angular momentum quantum number. In the absence of an external magnetic field we have only long-lived quasi-bound and scattering states and we assess their influence on the density of states. In addition, we consider elastic electron scattering by a magnetic dot, whose average B vanishes, and show that the Hall and longitudinal resistivities, as a function of the Fermi energy, exhibit a pronounced oscillatory structure due to the presence of quasi-bound states. Depending on the dot parameters this oscillatory structure differs substantially for energies below and above the first Landau level.

Kronig-Penney model of scalar and vector potentials in graphene

We consider a one-dimensional (1D) superlattice (SL) on graphene consisting of very high and very thin (δ-function) magnetic and potential barriers with zero average potential and zero magnetic field. We calculate the energy spectrum analytically, study it in different limiting cases, and determine the condition under which an electron beam incident on an SL is highly collimated along its direction. In the absence of the magnetic SL the collimation is very sensitive to the value of W/W(s) and is optimal for W/W(s) = 1, where W is the distance between the positive and negative barriers and L = W + W(s) is the size of the unit cell. In the presence of only the magnetic SL the collimation decreases and the symmetry of the spectrum around k(y) is broken for W/W(s) ≠ 1. In addition, a gap opens which depends on the strength of the magnetic field. We also investigate the effect of spatially separated potential and magnetic δ-function barriers and predict a better collimation in specific cases.