Ettingshausen Effect around a Landau Level Filling Factorν=3Studied by Dynamic Nuclear Polarization

Abstract

A spin current perpendicular to the electric current is investigated around a Landau level filling factor ν = 3 in a GaAs/AlGaAs two-dimensional electron system. Measurements of dynamic nuclear polarization in the vicinity of the edge of a specially designed Hall bar sample indicate that the direction of the spin current with respect to the Hall electric field reverses its polarity at ν = 3, where the dissipative current carried by holes in the spin up Landau level is replaced with that by electrons in the spin down Landau level.PACS numbers: 72.25.Pn, 73.50.Jt, A two-dimensional electron system (2DES) at low temperatures and in a strong magnetic field shows the quantum Hall (QH) effect, in which the longitudinal resistance vanishes and the Hall resistance is quantized as R H = h/ie 2 for an integer i [1,2]. While the electric current does not involve energy dissipation in the QH state, it can cause heat flow in the transition region between QH states and in the current-induced breakdown regime. Heat flow parallel to the electric current has been studied in the current-induced breakdown regime by measuring the longitudinal resistance with a set of voltage probes along the current channel [3,4,5]. On the other hand, Akera predicted that the heat flow in the current-induced breakdown regime can have a component perpendicular to the electric current [6]. In general, this phenomenon is known as the Ettingshausen effect. His calculation shows that the heat flow across the current channel causes an increase (decrease) of the electron temperature T e in the vicinity of one edge (the other edge) while T e is approximately uniform in the middle of the current channel. Furthermore a recent calculation for the transition region shows that the sign of the electron temperature gradient perpendicular to the electric current exhibits quantum oscillations as a function of the position of the chemical potential µ with respect to the Landau levels (LLs) [7]. Evidence of the Ettingshausen effect is observed in the current-induced breakdown regime at Landau level filling factor ν = 2 by Komori and Okamoto, who used microHall bars attached to both edges of the current channel as electron temperature indicators [8]. However, they were not able to extend their work to the transition region owing to the strong current-dependence of the background at ν = integer.In a strong magnetic field, the spin degeneracy is lifted due to the Zeeman energy and the many body effect. When µ lies between spin-split Landau levels, electron spin polarization strongly depends on T e and the heat flow is dominated by the spin current. One of the advantages of using the spin degree of freedom in the study of heat flow is that the flip of electron spin can be memorized by dynamic nuclear polarization (DNP) owing to the contact hyperfine interaction [9,10,11,12,13,14,15,16],where A (> 0) is the hyperfine constant, and I and S are the nuclear spin and electron spin, respectively. The first term causes the electron-nucleus flip-flop process and the second term...

Doped Two-dimensional Semiconductor Superlattice: Photo-stimulated Quantum Thermo-magnetoelectric Effects under the Influence of a Confined Phonon

“…In Ref. 6, magnetophonon oscillations in both transverse thermopower and the longitudinal thermopower of high-mobility GaAs quantum wells are visible for temperatures above 2 K. Research results on Ettingshausen effect around a Landau level filling factor υ = 3 indicated that a positive (negative) resistance change corresponds to an enhancement (reduction) of the edge-bulk coupling [7]. In DSSs, auxiliary extremum peaks of the Ettingshausen coefficient (EC) are created in a high magnetic field due to the presence of electromagnetic wave [8].…”

“…The quantum Ettingshausen effect is a thermomagnetoelectric effect detected when studying the Hall effect in Bismuth. Many papers related to this issue have been published [5][6][7][8][9]. At low temperatures, the transverse Nernst-Ettingshausen effect in a quantum well monotonically increases when the temperature gradient is along the direction of free motion [5].…”

Antiparallel spin Hall current in a bilayer with skew scattering

“…Such a DNP, generated by the current-induced electron spin polarization in an InGaAs epilayer [71], was successfully observed, in addition to that created by spin-polarized electrons injected from a metallic ferromagnet into a GaAs quantum well [72]. In order to detect the electron spin accumulation induced by the antiparallel spin Hall current, which appears at a sample edge and is opposite in direction between two electron layers, we first deplete one electron layer by applying a gate voltage and then observe the DNP in the other layer (occupied by electrons) through the local electrical detection [73][74][75]. The temporal decay of the DNP is negligible during such a procedure with the gate-voltage application and the electrical detection since the nuclear spin has a long relaxation time of the order of 1000 s. Now we discuss a measurement procedure to confirm the competition between the pseudospin precession ω and the momentum relaxation τ −1 p .…”