Upcoming Colloquia

Semiconductor Nanocrystals and High Magnetic Fields

Scott Crooker

National High Magnetic Field Laboratory,
Los Alamos National Laboratory,
Los Alamos, New Mexico 87545, USA

Monday, February 22, 10am

Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103


The strong zero-dimensional quantum confinement realized in colloidal semiconductor nanocrystals leads to very large spin-exchange interactions between electrons and holes (ie, excitons). Whereas typical exchange energies in bulk semiconductors are on the order of tens of micro-eV, those in nanocrystals can be 100-1000 times larger, leading to sizable milli-eV scale "fine structure" splittings of the lowest 1S exciton that manifest at modest temperatures (1 meV == 12 Kelvin). For example, exchange interactions in CdSe nanocrystals lead to a well-separated lowest exciton state that has angular momentum projection J=2 and is therefore optically forbidden, or "dark", resulting in very long photoluminescence lifetimes at cryogenic temperatures.

High magnetic fields couple to the spin degrees of freedom of electrons and holes, and are therefore a useful tool for addressing the large spin-exchange interactions that exist in nanocrystals (1 meV == ~10 Tesla, or 10^5 Gauss). This talk will discuss recent PL, fluorescence-line-narrowing, and magnetic-circular-dichroism studies of nanocrystals in both dc (to 33 Tesla) and pulsed magnetic fields (to 60 Tesla). High fields have been used to reveal, for example, i) dark excitons in both CdSe and PbSe nanocrystals, ii) anisotropic exchange in CdSe nanocrystals, iii) tunable sp-d exchange in magnetically-doped nanocrystals, and iv) Zeeman splittings of single excitons in individual nanocrystals.

Some fraction of this talk will survey the various methods utilized at the National High Magnetic Field Laboratory for generating the world's highest magnetic fields using both nondestructive (to 100 Tesla) as well as destructive (>1000 Tesla) techniques... amusing videos will be featured.