Controlling Spin and Magnetism in Quantum Dots – Rafal Oszwaldowski

Date: Mon. April 19th, 2010, 12:30 pm-1:30 pm
Location: Rockefeller 221

A promising approach for the next generation of applications for information storage and processing comes from the field of spintronics (spin-electronics) that seeks to use spin of carriers, rather than just their charge [1]. Commercial spintronic devices, such as computer hard drives, are based on metallic magnetic multilayers, and utilize the magnetic moment associated with spin to read magnetically stored information. Unfortunately, for advanced functions such as signal processing and digital logic, these structures are of limited use, and it would be more desirable to control spin and magnetism in semiconductors. Carrier-mediated magnetism in such semiconductors as (In,Mn)As, (Ga,Mn)As or (Cd,Mn)Te [2], shows important differences from ferromagnetic metals. For example, the increase in carrier density induced by light or bias [1,3,4], could be sufficient to turn the ferromagnetism on and off.

In this talk I focus on magnetic (Mn-doped) semiconductor quantum dots (“artificial atoms”), which offer new opportunities to control magnetism at the nanoscale. First, I will discuss our recent experiments [5] on Mn-doped ZnSe/(Zn,Mn)Te quantum dots. Photoluminescence reveals a surprisingly robust magnetic behavior in this system. We attribute it to formation of magnetic polarons – “clouds” of Mn-spins, aligned by single photo-generated carriers. Next, I will discuss our proposal of a new broken-symmetry ground state of magnetic quantum dots [6]. We show that the intuitive assumption, that a closed-shell quantum dot is inherently “nonmagnetic”, may be incorrect. Using the example of two carriers, we predict magnetism in closed-shell quantum dots, and suggest how it can be realized in experiment.

[1]I. Zutic , J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004).
[2] T. Dietl at al., Science 287, 1019 (2000).
[3] H. Ohno, et al., Nature 408, 944 (2000).
[4] D. Chiba al., Nature 455, 515 (2008).
[5] I. R. Sellers, R. Oszwaldowski, V. R. Whiteside, M. Eginligil, A. Petrou, I. Zutic, W-C. Chou, A. Petukhov and B. D. McCombe, arXiv: 0912.0138, preprint. [6] R. Oszwaldowski, Igor Zutic, and A. G. Petukhov, preprint.