● Problem addressed: Mn-doped GaAs is the leading material for spintronics applications. How does the ferromagnetism arise?

● Scanning Tunneling Microscopy allows visualization of electronic states in Ga1-

xMnxAs samples close to the metal-insulator transition.

● Doping-induced disorder produces strong spatial variations in the local tunneling conductance.

● Discovered sharp divergence of correlation length at the Fermi Energy near the metal-insulator transition.

Visualizing Critical Correlations in Ga1-xMnxAsPrinceton Univ., Univ. Illinois and UC Santa Barbara

A. Richardella et al., Science 327, 665 (2010) Princeton Center for Complex Materials

● Conductance maps af Fermi energy, become multifractal.

● At Fermi energy, where signatures of electron-electron interaction are the most prominent, a diverging spatial correlation length was observed. (right)

● Proximity to the metal-insulator transition

plays a more important role in the underlying mechanism of magnetism of Ga1-xMnxAs than previously anticipated.

● experimental approach provides a direct

method to examine critical correlations for other material systems near a quantum phase transition.

Above: dI/dV maps over areas of 700Å at Fermi energy for three different dopings.

Princeton Center for Complex Materials

J. R. Petta, H. Lu, and A. C. Gossard, Science, 327, 669 (2010).

The probability Ps of observing final singlet state plotted as a function of the maximum well detuning es and waiting time ts (scale bar for Ps at right). Bright fringes indicate high probability that electron pair ends up in a triplet state. A direct analogy with optical beam splitter is shown in inset.

Ultra-Fast Electrically Driven Single Spin Rotations (DMR-0819860) Jason Petta1, Hong Lu2, Art Gossard2

1 Department of Physics, Princeton University2 Materials Department, University of California at Santa Barbara

tS (ns)0 5 10 15 20 25

eS (

mV

)

0.6

PS

1.0

0.8

BE= 100 mT

-1.7

-0.2

fU1 U2

U3Det.

Mirror

Mirror

Ultrafast method (ns) to flip individual spins using gate voltage only without affecting neighboring spin.

Separate electrons rapidly, allow states to evolve for ts (5-25 ns), then slowly recombine in right well.

Quantum interference between triplet and singlet states visible as fringes in the probability Ps of obtaining final singlet state (see figure).

gQ2 electrons trapped in quantum wells

Princeton Center for Complex Materials