Evidence of the Ettingshausen effect in the breakdown regime of the integer quantum Hall effect has been observed in a GaAs/AlGaAs two-dimensional electron system. Resistance of micro Hall bars attached to both edges of a current channel shows remarkable asymmetric behaviors which indicate an electron temperature difference between the edges. The sign of the difference depends on the direction of the electric current and the polarity of the magnetic field. The results are consistent with the recent theory of Akera.Comment: 4 pages, 6 figures, submitted to Phys. Rev.

A new method for local dynamic nuclear polarization is demonstrated in a GaAs/AlGaAs heterostructure at the Landau level filling factor $\nu=3$. Using a narrow channel sample, where the width varies stepwise along the electron flow, we find that electron cooling (heating) causes the polarization of nuclear spins against (toward) the external magnetic field at liquid helium temperatures. The longitudinal nuclear spin relaxation rate varies exponentially with inverse temperature.Comment: 3 pages, 3 figure

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...

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