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Title: Excitacões de spin em nanofitas de grafeno
Keywords: .
Issue Date: 1-Jul-2012
Abstract: In this work we have studied electronic properties and magnetic responses of zigzag graphene nanoribbons, following a tight-binding approximation, within a Hartree-Fock mean-field approach. We focus our attention to the study of the system responses to a transverse magnetic field, through the calculation of the transverse dynamic susceptibility. From this susceptibility one may get information of the spin wave excitations of the nanosystem. We observe that the dispersion relation for thin nanoribbons is dominated by antiferromagnetic correlations between opposite sites of the edges. It was verified that the spin wave lifetime is too high due to the semiconducting nature of the electric neutral nanoribbons. However, a small amount of gate voltage leads to a discontinuous transition, characterized by a finite lifetime for the spin wave. The results indicate that as the gate voltage intensity increases, the ferromagnetic correlations through the edge become instable, in relation to the transverse spin fluctuations. In the second part of the work we present an investigation of spin wave excitations and electronic transport of devices made of graphene nanoribbon devices with zigzag edges. For the electric transport properties we use the Landauer formalism. The magnetic region of the device (central part) is coupled to two charge reservoir that are non magnetic. Due to the lack of translation symmetry, of the finite system, the magnetic momenta, depend now on the carbon site along the edge extension. These magnetic momenta may be tuned by a manipulation of the length of the magnetic region of the nanoribbon and also by the intensity of the coupling between the central region and the leads. Further variations may be provided by applying different gate voltages at both leads. A variety of spin wave modes are identified and we have studied the behavior of such modes for different nanoribbon lengths. The signature of antiferromagnetic correlations is still present in the predominantly linear relationship between standing mode energy and mode wavevector. The effect of an external doping is also considered and, as in the nanoribbon infinite example, it is found that ferromagnetic order along the ribbons edges become unstable at modest doping levels.
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