Unconventional dc Transport in Rashba Electron Gases

Abstract

We discuss the transport properties of a disordered two-dimensional electron gas with strong Rashba spin-orbit coupling. We show that in the high-density regime where the Fermi energy overcomes the energy associated with spin-orbit coupling, dc transport is accurately described by a standard Drude's law, due to a nontrivial compensation between the suppression of backscattering and the relativistic correction to the quasiparticle velocity. On the contrary, when the system enters the opposite dominant spin-orbit regime, Drude's paradigm breaks down and the dc conductivity becomes strongly sensitive to the spin-orbit coupling strength, providing a suitable tool to test the entanglement between spin and charge degrees of freedom in these systems. DOI: 10.1103/PhysRevLett.116.166602 Spin-orbit (SO) coupling is a fundamental ingredient in spintronics [1], as it provides an advantageous locking between spin and electron orbital momentum. Recently, intense research efforts [2] have been devoted to twodimensional materials with broken inversion symmetry, where the SO strength, parametrized by a characteristic energy scale E 0 , can be tuned by means of external conditions (electric fields, gating, doping, pressure, strain, etc.). In most of these systems (for example, surface alloys [3][4][5][6][7][8][9], layered bismuth tellurohalides [10][11][12][13][14][15][16], HgTe quantum wells [17], and interfaces between complex oxides [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]) the total charge carrier density n can be tuned down to very small concentrations, implying very small Fermi energies E F . Although the high-density (HD) regime E F ≳ E 0 has been widely investigated [2,[33][34][35][36][37][38], relatively less attention has been paid to the opposite regime of dominant SO (DSO), E 0 ≳ E F .In this Letter we provide a detailed investigation of the dc conductivity of a 2D electron gas (2DEG) with Rashba [39] SO coupling in the different density regimes. Using a Boltzmann approach and a fully quantum analysis based on the Kubo formula, we show that in the high-density regime E F ≳ E 0 dc transport is independent of the SO strength, and the dc conductivity σ dc of electrons having effective mass m and scattering time τ 0 follows the conventional Drude law for 2DEGs,which results from a nontrivial cancellation of the SO coupling effects on the quasiparticle velocity and transport scattering time. Remarkably, as soon as the system enters the DSO regime E 0 ≳ E F , Drude's paradigm Eq. (1) breaks down and the dc conductivity accurately follows the analytical formula:where n 0 ¼ 2mE 0 =ðπℏ 2 Þ is the density at E F ¼ E 0 . In contrast to the linear dependence of σ dc on the charge density found in the HD regime, n ≥ n 0 , Eq. (2) predicts an unconventional nonlinear behavior of σ dc with n that is controlled by the SO interaction encoded in n 0 . The relevance of this result is twofold: demonstrating that dc transport is strongly sensitive to Rashba SO coupling, not only does it suggest that SO coupling cou...

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