Abstract
In a noncentrosymmetric system, an intrinsic electric polarization is allowed and may lead to unusual nonreciprocal charge transport phenomena. As a result, a current-dependent resistance, arising from the magnetoelectric anisotropy term of $k\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{P\vec}{E}\ifmmode\times\else\texttimes\fi{}\stackrel{P\vec}{B}$, appears and acts as a current rectifier with the amount of rectification being linearly proportional to the magnitude of both current and applied magnetic field. In this work, a different type of nonreciprocal transport effect was demonstrated in a graphene-based device, which requires no external magnetic field. Due to the unique pseudospin (valley) degree of freedom in chiral fermions with trigonal warping, a large nonreciprocal transport effect was uncovered in a gapped bilayer graphene, where electric-field tunabilities of the band gap and valley polarization play an important role. The exact cancellation of nonreciprocal effect between two different valleys is effectively removed by breaking the inversion symmetry via electric gatings. The magnitude of the current rectification appears to be at a maximum when the Fermi surface undergoes a Lifshitz transition near the band edges, which is proportional to the current and the displacement field strength. The full electric-field tuning of the nonreciprocal transport effect without a magnetic field opens up a new direction for valleytronics in two-dimensional-based devices.
Highlights
From a symmetry argument, a magnetoelectric anisotropy term in the form of k · E × B can generally exist in all systems [1], which gives rise to additional contributions to various magnetoelectric properties in matter [2,3,4,5]
An unusual current-direction dependence on the charge transport is a direct consequence of the nonreciprocal transport effect (NRTE)
We remark that such a Lifshitz transition is a direct consequence of the trigonal warping effect that becomes significant at lower energy, and it happens for all values near the band edges [25]
Summary
A magnetoelectric anisotropy term in the form of k · E × B can generally exist in all systems [1], which gives rise to additional contributions to various magnetoelectric properties in matter [2,3,4,5]. We note that the |D| opens up a band gap in a BLG and effectively polarizes the valley degree of freedom, and the exact cancellation of the NRTE from two different valleys of K+ and K− is removed to result in a large R2ω response. This effect is fully adjustable by electric gating alone and requires no external magnetic field. Those findings can be understood qualitatively from the velocity asymmetry in the Boltzmann transport equation for massive chiral fermions with trigonal warping
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