Magnetic Polarons and Bipolarons in Quantum Dots

Magnetically doped (typically by Mn) semiconductor quantum dots (QDs) allow a control of magnetic ordering in ways not available in the bulk. For example, onset of magnetism can be realized by adding a single carrier or changing symmetry of the quantum confinement, even at a fixed carrier number [1]. Recent experiments revisit the concept of magnetic polaron [2], formed when a single carrier added to a QD aligns the Mn spins through exchange interaction. The experiments [3,4] show that the induced magnetization persists at relatively high temperatures. First, we discuss a QD system, in which the experimental magnetic polaron energy, in addition to its relatively high value, shows a surprisingly weak temperature dependence [4]. We explain this effect by magnetic anisotropy of the QD. Next, we turn to the case where a magnetic QD contains two carriers. We find theoretically that Mn spins align, forming a magnetic 'bipolaron', even when the ground state has zero carrier-spin [5]. The corresponding state breaks spatial symmetry, unlike in the case of a single magnetic polaron. We propose experimental tests of our prediction. We also explore the stability of the broken-symmetry state with zero net magnetization versus other patterns of magnetization [6]. Finally, we show some interesting consequences of diffusive coupling of a magnetic QD to a reservoir of carriers [7]. This work was done in collaboration with P. Stano, J. Pientka, A. Petukhov, and I. Zutic.\\[4pt] [1] R. M. Abolfath, et al. Phys. Rev. Lett. 101, 207202 (2008).\\[0pt] [2] Maksimov, A. et al. Phys. Rev. B 62, R7767 (2000), J. Seufert et al., Phys. Rev. Lett. 88, 027402 (2002).\\[0pt] [3] R. Beaulac et al., Science 325, 973 (2009).\\[0pt] [4] I. R. Sellers, et al., Phys. Rev. B 82, 195320 (2010).\\[0pt] [5] R. Oszwaldowski, et al., Phys. Rev. Lett. 106, 177201 (2011)\\[0pt] [6] P. Stano, R. Oszwaldowski, A. G. Petukhov, and I. Zutic, preprint.\\[0pt] [7] J. M. Pientka, R. Oszwaldowski, A. G. Petukhov, J. E. Han and I. Zutic, Reentrant Magnetic Polaron Formation in Quantum Dots, preprint.