Quantum Hall skyrmions in a hole gas with large spin gap and strong disorder L. Bryja*, K. Ryczko*, A. Wojs*, J. Misiewicz* and M. Potemsk^ * Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspiahskiego 27, 50-370 Wroclaw, Poland 1Grenoble High Magnetic Field Laboratory, CNRS, F-38042 Grenoble Cedex 9, France Abstract. Photoluminescence excitation spectra of two-dimensional holes are investigated in high magnetic fields. Despite large spin gap and strong disorder, quantum Hall ferromagnetism and small skyrmions are identified around the Landau level filling factor v = 1, hased on the field dependence of polarized emission. Lhis interpretation is supported hy realistic numerics. Keywords: Skyrmion, quantum Hall effect, hole gas PACS: 73.20.Mf, 73.21.Fg, 78.20.Ls, 78.67.De INTRODUCTION Concentration p of a two-dimensional electron (or hole) gas in a high magnetic field B is conveniently expressed by the Landau level (LL) filling factor v = 2npX 2 , where X = \JhcjeB is the magnetic length. Even in the ab- sence of Zeeman spin splitting Ez, exchange interaction between the carriers causes their complete spin polariza- tion at v = 1 [1]. Even small deviation from v = 1 makes such "quantum Hall ferromagnet" unstable [2]. Each ad- ditional vacancy or reversed-spinparticle in the majority- or minority-spin LL, respectively, induces and binds K spin flips to become a skyrmion [3]. The skyrmion size K depends on the ratio of Zeeman and Coulomb energies, g = Ez/{e 2 X~ l ). At g —> 0, K diverges and skyrmions carry macroscopic spin per unit charge [4]. Though skyrmions were demonstrated in several ex- periments [5], the effect has been believed to be relatively subtle, observable only in the superior quality structures. In this work we demonstrate skyrmions in a hole gas with both significant disorder and large spin gap. This proves that, contrary to earlier expectation, skyrmions are a ro- bust feature of quantum Hall systems, requiring neither spin degeneracy nor translational symmetry. RESULTS AND DISCUSSION We report low-temperature polarization-resolved pho- toluminescence excitation (PLE) studies of a j9-doped w = 8 nm GaAs/AlojGaojAs quantum well grown by MBE on the (001) semi-insulating GaAs substrate and modulation C-doped in the barrier on one side. The low- temperature concentration and mobility of the holes were p = 3 • 10 1 cm -2 (yielding v = 1 at B = 13 T) and 1.60 1.61 Energy (eV) 1.60 1.61 1.62 Energy (eV) fi = 3.3 • 10 3 cm 2 /Vs. The spectra were recorded at low temperatures (T = 1.8 K), in magnetic field B < 23 T, in FIGURE 1. Quasi-continuous field evolution of PLE spectra in hoth <T + - and <7~ polarizations. Thick lines: v = 1 and 2. Faraday configuration, for different a + and a helicities of both the excited and emitted light. The polarization-resolved PLE is an indirect measure of the absorption of circularly polarized light ( c ± ) . Due to the simple spin/polarization selection rules combined with Pauli phase space blocking, polarized PLE is a probe of the occupation of LL's by the electrons with a given spin, i.e., to the spin polarization. In Fig. 1 the spectra for both polarizations are com- pared. Their field evolution is completely different, espe- cially in the lowest-energy signals, corresponding to the interband optical excitation from the highest LL in the valence band (lowest heavy-hole LL) to the lowest elec- tron LL (i.e., to the n = 0 —> 0 transitions). The "0 —> 0" line appears in the ff+ PLE spectra at B = 6.5 T and then gradually gains intensity with the increase of B. The same feature in the a PLE also appears around B = 6.5 T, but it gains intensity with increasing field only up to B = 9 T. When the field grows further, the a line first gradually weakens to disappear completely at B = 13 T, and then reappears and regains intensity. This behavior is consistent with the skyrmion picture. CP893, Physics of Semiconductors, 28' International Conference edited by W. Jantsch and F. Schaffler © 2007 American Institute of Physics 978-0-7354-0397-0/07/S23.00 671