P. Vargas

Flat bands in slightly twisted bilayer graphene: Tight-binding calculations

The presence of flat bands near Fermi level has been proposed as an explanation for high transition temperature superconductors. The bands of graphite are extremely sensitive to topological defects which modify the electronic structure. In this Rapid Communication, we found nondispersive flat bands no farther than 10 meV of the Fermi energy in slightly twisted bilayer graphene as a signature of a transition from a parabolic dispersion of bilayer graphene to the characteristic linear dispersion of graphene. This transition occurs for relative rotation angles of layers around 1.5° and is related to a process of layer decoupling. We have performed ab initio calculations to develop a tight-binding model with an interaction Hamiltonian between layers that include the orbitals of all atoms and takes into account interactions up to third nearest neighbors within a layer.

Chirality switching and propagation control of a vortex domain wall in ferromagnetic nanotubes

We propose a procedure to manipulate the chirality and propagation of a vortex domain wall in ferromagnetic nanotubes by applying magnetic field pulses. It is found that the chiral state of the vortex wall can be switched, provided that (1) the field amplitude is between two critical values, the so-called chiral field and the well-known Walker field, and (2) the pulse length is longer than a critical time, which is the time needed by the wall to overcome a local energy barrier. These key parameters are estimated for Permalloy nanotubes and range between a few miliTesla and some nanoseconds.

Metastable states in the triangular-lattice Ising model studied by Monte Carlo simulations: Application to the spin-chain compound Ca 3 Co 2 O 6

It is known that the spin-chain compound Ca3Co2O6 exhibits very interesting plateaus in the magnetization as a function of the magnetic field at low temperatures. The origin of them is still controversial. In this paper we study the thermal behavior of this compound with a single-flip Monte Carlo simulation on a triangular lattice and demonstrate the decisive influence of metastable states in the splitting of the ferrimagnetic 1/3 plateau below 10 K. We consider the [Co2O6]n chains as giant magnetic moments described by large Ising spins on planar clusters with open boundary conditions. With this simple frozen-moment model we obtain stepped magnetization curves which agree quite well with the experimental results for different sweeping rates. We describe particularly the out-ofequilibrium states that split the low-temperature 1/3 plateau into three steps. They relax thermally to the 1/3 plateau, which has long-range order at the equilibrium. Such states are further analyzed with snapshots unveiling a domain-wall structure that is responsible for the observed behavior of the 1/3 plateau. A comparison is also given of the exact results in small triangular clusters with our Monte Carlo results, providing further support for our thermal description of this compound.

Asymmetrical giant magnetoimpedance in exchange-biased NiFe

Linear asymmetrical magnetoimpedance (MI) has been obtained in thin multilayer strips of exchange-coupled FeNi(5 nm)/IrMn(15 nm)/FeNi(40 nm)/Cu(100 nm)/FeNi(40 nm)/IrMn(15 nm)/FeNi(5 nm), in which the MI response is enhanced by including a Cu layer between the two 40 nm NiFe layers. The antiferromagnetic layer creates an exchange bias that shifts both the hysteresis loop and the MI response. The asymmetrical MI peak positions are shifted to higher magnetic field as the probe frequency increases. Linear MI behavior can be tuned around zero external field without the need for external biasing fields or additional coils, by changing the frequency. The impedance of a planar magnetic conductor was modeled based on a modified Stoner–Wohlfarth energy density.

A detailed analysis of dipolar interactions in arrays of bi-stable magnetic nanowires

The investigation of the role of interactions in magnetic wire arrays is complex and often involves substantial simplifications. In this paper analytical expressions taking into consideration the geometry of the wires and dipolar interactions between them have been obtained. An expansion of these terms, at first order, can be easily evaluated and shows a good agreement with the total expression for the energy. The extent of the interwire magnetostatic coupling has also been investigated, and it is shown that the number of wires required to reach a size independent magnetic state in the array strongly depends on the relative magnetic orientation of the wires.

Moiré patterns on STM images of graphite induced by rotations of surface and subsurface layers

M. Flores, E. Cisternas, J. D. Correa, and P. Vargas4∗ Departamento de F́ısica, FCFM, Universidad de Chile, Santiago, Chile, Departamento de Ciencias F́ısicas, Universidad de La Frontera, Casilla 54-D, Temuco, Chile, Departamento de Ciencias F́ısicas, Universidad Andrés Bello, Av. República 220, 837-0134, Santiago, Chile, and Departamento de F́ısica, Universidad Técnica Federico Santa Maŕıa, Casilla 110-V, Valparáıso, Chile,

Effect of the exchange bias coupling strength on the magnetoimpedance of IrMn/NiFe films

The asymmetrical magnetoimpedance (MI) response has been analyzed in thin multilayer strips of exchange-coupled [NiFe (t nm)/IrMn (35 nm)] × n(t), where t and n take values of 20, 30, 40, and 60 nm and n = 15, 10, 8, and 5, respectively. The antiferromagnetic layer creates an exchange bias that shifts both the hysteresis loop and the MI response, and the magnitude of this shift depends on the ferromagnetic (FM) layer thickness. The MI peak positions are also shifted to a higher magnetic field as the probe frequency increases, and the MI ratio increases with increasing thickness of the FM layer. The impedance of a sandwich-like layered system was modeled based on an exchange coupled Stoner–Wohlfarth energy density. The model reproduces the principal features of the experimental results, describing the effect of the strength of the exchange bias on the MI ratio.

Magnetic entropy change plateau in a geometrically frustrated layered system: FeCrAs-like iron-pnictide structure as a magnetocaloric prototype

Monte Carlo modeling suggests that the magnetothermal features of the Fe2P-structured FeCrAs-like compound offer a promising route for the design of magnetocaloric materials. The prototype structure is modeled as antiferromagnetically coupled layered Heisenberg systems mimicking the distorted Kagome/triangular stacked architecture of FeCrAs iron-pnictide. The magnetic entropy change ΔSm(T) presents a plateau-like behavior which can be tailored by tuning either the JCr-Fe/JCr-Cr exchange energy ratio or the magnetic field. The plateau is defined by cooperative spin ordering within a ferrimagnetic region which exists between two critical temperatures separating at the lower bound (Tac) a canted antiferromagnetic phase and at the upper bound (Tdc) the thermally disordered phase. The refrigerant capacity and adiabatic change of temperature are A(H)(Tdc - Tac) and A(H)Tp/Cm respectively, with Tac < Tp < Tdc, A(H) an increasing positive function of the field defining the height of the plateau and Cm the magnetic specific heat, whose critical behavior is related to the T(a,d)(c) values.

Trigonal distortion of topologically confined channels in bilayer graphene

In this work, we show that the trigonal warping of the electronic bands in bilayer graphene dramatically modifies the behavior of the topologically confined one-dimensional modes due to an inhomogeneous bias that changes sign across a channel. Up to four zero modes are present, depending on the orientation of the channel, these zero modes are related to a fractionalization of the topological charge due to the trigonal warping.

Surface channelling and energy losses of 4 keV hydrogen and fluorine ions in grazing scattering on Au(111) and missing row reconstructed Au(110) surfaces

We present the results of an experimental and theoretical study of surface channelling and energy loss of 4 keV hydrogen and fluorine ions in grazing scattering on a missing row reconstructed Au(110) surface and a Au(111) surface. We performed measurements of energy losses for grazing angle scattering in surface channelling conditions for various azimuthal orientations of the crystal. Experimental results are discussed in the light of trajectory calculations of hydrogen and fluorine ions scattered under grazing incidence conditions on the surface. A nonlinear model is used in order to calculate the ion energy loss in these crystalline systems. Ab initio electronic crystal structure calculations and semi-classical simulations are performed and allow us to delineate various trajectory classes that correspond to different contributions in the energy loss spectra for various azimuthal orientations of the surface.