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IRG 1: Interplay Of Magnetism And Transport In Correlated Electronic
Materials
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Main photo, left to right: Zahid Hasan, Duncan Haldane, Shivaji Sondhi, David Huse, Bob Cava (co-leader), Ali Yazdani, Ravin Bhatt, Dan Tsui, Phuan Ong (co-leader)
Below: Mansour Shayegan
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We focus on how magnetism affects charge conduction in materials
with unusual magnetization. The presence of strong interaction between
electrons leads to a variety of magnetically ordered states ranging
from ferromagnetism, ferrimagnetism, antiferromagnetism, spin density
waves, and the spin-Peierls state.
Strong interest in how magnetism affects charge transport has come
from the growing importance of magnetism in electronics applications.
For example, the rapid decrease in the size of hard drives is driven
by modern micro-sensors based on the giant magnetoresistance (GMR)
effect in transition-metal multi-layers. Spin valves in magneto-tunneling
junctions based on similar technology are also highly promising
for the next generation of non-volatile memory. Advances in materials
synthesis technology, including molecular beam epitaxy, optical
furnaces, and high-pressure furnaces, have greatly improved our
ability to engineer new materials to address specific problems.
Insights gained in understanding how magnetism affects transport
have led to new concepts that may solve other problems of long standing.
One example is the old problem of the anomalous Hall-effect in ferromagnets
(a magnetization-dependent Hall resistance). The “Berry-phase”
formulation, originally used to describe hole propagation in a fluctuating
spin background in underdoped cuprates, has recently been shown
to be deeply relevant to the anomalous Hall effect in ferromagnetic
pyrochlores.
The interesting effect of geometric frustration on a triangular
lattice and its effect on transport is being investigated in the
layered cobalt oxides doped with Na. We have found that, as the
Na content is varied, a series of electronic states involving the
interplay between frustrated antiferromagnetism and charge transport
occurs.
Comparisons of experiments with theory are often most incisive
in very pure materials. MBE-grown semiconductor heterostructures
and quantum wells are used to test the theory of magnetism in the
limit of very low electron density and high purity. The flexibility
of MBE for engineering 2D materials with highly specific properties
also allows us to investigate magnetic transitions between quantum
Hall states of different magnetizations. Such experiments open yet
another regime for exploring Berry-phase effects on transport.
Selected Publications:
- Y. Wang, L. Li, M.J. Naughton, G.D. Gu, S. Uchida, and N.P. Ong, “Field-Enhanced
Diamagnetism in the Pseudogap State of the Cuprate Bi2
Sr2CaCu2O
8+δ Superconductor in an Intense Magnetic Field,”
Phys. Rev. Lett., 95, 247002 (2005).
- S. Melinte, M. Berciu, C. G. Zhou, E. Tutuc, S. J. Papadakis, C. Harrison, E. P. De
Poortere, M. Wu, P. M. Chaikin, M. Shayegan, R. N. Bhatt, and R. A. Register, “A
Laterally Modulated 2D Electron System in the Extreme Quantum Limit,” Phys. Rev.
Lett., 92, 036802 (2004).
- Y. Wang, N.S. Rogado, R.J. Cava, N.P. Ong, “Spin Entropy as the Likely Source
of Enhanced Thermopower in NaxCo2
O4,” Nature, 423, 425 (2003).
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