Magnetoconductance of interacting electrons in quantum wires: Spin density functional theory study

Abstract

We present a systematic quantitative description of the magnetoconductance of split-gate quantum wires focusing on formation and evolution of the odd ͑spin-resolved͒ conductance plateaus. We start from the case of spinless electrons where the calculated magnetoconductance in the Hartree approximation shows the plateaus quantized in units of 2e 2 / h separated by transition regions, whose width grows as the magnetic field is increased. We show that the transition regions are related to the formation of the compressible strips in the middle of the wire occupied by electrons belonging to the highest ͑spin-degenerate͒ subband. Accounting for the exchange and correlation interactions within the spin density functional theory ͑DFT͒ leads to the lifting of the spin degeneracy and formation of the spin-resolved plateaus at odd values of e 2 / h. The most striking feature of the magnetoconductance is that the width of the odd conductance steps in the spin DFT calculations is equal to the width of the transition intervals between the conductance steps in the Hartree calculations. A detailed analysis of the evolution of the Hartree and the spin DFT subband structure provides an explanation of this finding. Our calculations also reveal the effect of the collapse of the odd conductance plateaus for lower fields. We attribute this effect to the reduced screening efficiency in the confined ͑wire͒ geometry when the width of the compressible strip in the center becomes much smaller than the extent of the wave function. A detailed comparison to the experimental data demonstrates that the spin DFT calculations reproduce not only qualitatively but also quantitatively all the features observed in the experiment. This includes the dependence of the width of the odd and even plateaus on the magnetic field as well as the estimation of the subband index corresponding to the last resolved odd plateau in the magnetoconductance.

Spin polarization andg-factor enhancement in graphene nanoribbons in a magnetic field

“…[19][20][21] In this section we therefore outline the electronic and transport properties of armchair and zigzag nanoribbons in the Hartree approximation for spinless electrons [i.e., disregarding the Hubbard and Zeeman interactions, V Z = V U = 0, in Hamiltonian (1)], focusing on the formation of the compressible strips.…”

“…Note that various aspects of electron and spin interactions in the high magnetic field have been extensively studied in conventional semiconducting quantum wires defined in a two-dimensional electron gas (2DEG). [13][14][15][16][17][18][19][20][21] One of the motivations for such studies is related to advances in semiconductor spintronics utilizing the spin degree of freedom for adding new functionalities to electronic devices. 22 Some proposed and investigated devices for spintronics and quantum computation applications operate in the edge-state regime, 23,24 which obviously requires a detailed knowledge of the structure of the states in a quantum wire or at the edge of the 2DEG.…”

Generic suppression of conductance quantization of interacting electrons in graphene nanoribbons in a perpendicular magnetic field

“…This, in turn, leads to a strongly suppressed backscattering and hence to a drastic improvement of the conductance quantization[2-6]. Taking electron interaction and screening in high magnetic fields into account leads to new features such as formation of compressible and incompressible strips [7], which are essential for an interpretation of various magnetotransport phenomena in conventional QPCs and quantum wires defined in two-dimensional electron gases (2DEGs) [7,8].The isolation of graphene[9] has immediately inspired the search for conductance quantization in graphene nanoribbons (GNRs). However, in all experiments reported so far conductance quantization at B = 0 is absent [10] or strongly suppressed [11], which by now is well understood and attributed to the effects of impurity scattering and/or edge disorder [12].…”

Electron interaction, charging, and screening at grain boundaries in graphene

“…This is similar to the spin polarization of compressible strips in quantum wires and graphene nanoribbons in a high magnetic field. 29,33 For the case of the (2,1) GB, the corresponding state is fully occupied even for small applied V g , and therefore the spin-up and spin-down electron densities are the same. As a result, the potential felt by different spin species is the same and the spin polarization for the (2,1) state is completely suppressed.…”