Possible origin of the 0.5 plateau in the ballistic conductance of quantum point contacts

Abstract

A non-equilibrium Green function formalism (NEGF) is used to study the conductance of a side-gated quantum point contact (QPC) in the presence of lateral spin-orbit coupling (LSOC). A small difference of bias voltage between the two side gates (SGs) leads to an inversion asymmetry in the LSOC between the opposite edges of the channel. In single electron modeling of transport, this triggers a spontaneous but insignificant spin polarization in the QPC. However, the spin polarization of the QPC is enhanced substantially when the effect of electron-electron interaction is included. The spin polarization is strong enough to result in the occurrence of a conductance plateau at 0.5G 0 (G 0 = 2e 2 /h) in the absence of any external magnetic field. In our simulations of a model QPC device, the 0.5 plateau is found to be quite robust and survives up to a temperature of 40K. The spontaneous spin polarization and the resulting magnetization of the QPC can be reversed by flipping the polarity of the source to drain bias or the potential difference between the two SGs. These numerical simulations are in good agreement with recent experimental results for side-gated QPCs made from the low band gap semiconductor InAs.

Hysteresis in the conductance of asymmetrically biased GaAs quantum point contacts with in-plane side gates

“…A similar grading was also used along the walls going from the wider part of the channel to the central constriction of the QPC (Fig.4). This gradual change in the conduction band at the QPC channel/vacuum interface is responsible for the LSOC that triggers the spin polarization of the QPC in the presence of an asymmetry in V sg1 and V sg2 [24]. The parameter d used in the potential profile modeling the conduction band variation at the channel/vacuum interface was set equal to 1.6 nm.…”

“…The conductance of the QPC was calculated using a NEGF method under the assumption of ballistic transport [24,25]. We used a Hartree-Fock approximation following Lassl et al [30] to include the effects of electron-electron interaction in the QPC.…”

“…(1). At the interface between the rectangular region of size w 2 ×l 2 and vacuum, the conduction band discontinuities at the bottom and the top interface were modeled using a smooth conductance band change spread over a range d selected to be in the nm range to represent a gradual variation of the conduction band profile from the inside of the quantum wire to the vacuum region [23][24][25]. A similar grading was also used along the walls going from the wider part of the channel to the central constriction of the QPC (Fig.4).…”

Intrinsic bistability in quantum point contacts with in-plane side gates

“…The conductance through the QPC was calculated using a NEGF method under the assumption of ballistic transport [11,12]. We used a Hartree-Fock approximation following Lassl et al [13] to include the effects of electron-electron interaction.…”

“…The use of a Non Equilibrium Green's Function (NEGF) approach to calculate the conductance of QPCs shows that the onset of spin polarization in devices with in-plane side gates required three ingredients: (1) an asymmetric lateral confinement, (2) a LSOC induced by lateral confining potential of the QPC, and (3) a strong electron-electron interaction [11,12]. The NEGF approach was used to study the ballistic conductance of asymmetrically biased side-gated QPCs in the presence of LSOC and electronelectron interaction for a wide range of QPC dimensions and gate bias voltage [12].…”