Home > Press > Magnetic Transistor Could "Dial In" Quantum Effects
Physicists Propose Innovative Probe For Quantum Criticalities
Magnetic Transistor Could "Dial In" Quantum Effects
Houston, TX | December 12, 2005
A team of theoretical and experimental physicists
from Rice University are preparing a unique probe in hopes of "dialing in"
elusive quantum states called "quantum criticalities." The team is using
nanotechnology to create a probe capable of trapping and tuning a single
electron to create the rarified physical state in nearby magnetic
The probe, a transistor thousands of times smaller than a living cell, is
described in research published online this week by the Proceedings of the
National Academy of Sciences.
"The traditional theory of metals, which has held sway for 50 years and has
fostered terrific technological advances in computing and materials science,
breaks down completely in matter that exists in a 'quantum critical state,'"
said Qimiao Si, professor of physics and astronomy at Rice and the lead
theoretician on the project. "Previous experiments indicate that quantum
criticality is characterized by the inherent quantum effect of entanglement,
and the nanoscale magnetic probe we've proposed could provide a controlled
and tunable setting to study entanglement at a quantum critical point."
The term "quantum critical point" refers to a phase transition. Just as
water goes through a phase transition when it turns to ice or steam, all
matter is subject to phase transitions due to fluctuations produced by the
peculiar forces of quantum mechanics.
The probe proposed by Si and colleagues is based on a transistor with an
active channel measuring just a few billionths of meter across. The
transistor also uses a pair of electrodes made of ferromagnetic metal. The
researchers plan to trap a single electron in the active channel between the
electrodes. Then, they will capitalize on a uniquely quantum effect - the
tendency of a trapped electron to "tunnel," or wink out of existence in one
place and appear in another - to establish a quantum critical state in the
metallic electrodes that trap the tiny particle.
"In principle, we can use the gate voltage in this setup to tune the
physical state," said Douglas Natelson, assistant professor of physics and
astronomy and of electrical and computer engineering. "We should be able to
move the system from a quantum critical state and back again, simply by
turning the knob on the voltage. That's a level of precision that's never
been possible in another experimental system, and it's really nanotechnology
- the control of matter at the atom-by-atom level - that will make it
Elementary particles like electrons have an intrinsic angular momentum known
as spin. The probe's design will allow the physicists to confine an electron
with its spin on one molecule inside the transistor. In one quantum state,
the tunneling effect causes the constrained electron spin to become
"entangled" with the spins of electrons in the nearby metal electrodes. The
magnetic nature of the electrodes also dictates the existence of a
collective oscillation among the spins of electrons in the electrodes. These
oscillations known as "spin waves" - will interact with the magnetic
moment of the constrained electron's spin and try to break the entanglement.
The quantum critical point occurs when it is broken and the material
transitions from one quantum phase to the next.
Natelson has already used the technique to study electron spin in similar
molecules while using non-magnetic gold metal electrodes. Results of those
experiments are due to be published shortly in the journal Physical Review
"The usage of the ferromagnetic electrodes in the proposed probe brings in
spin waves, which couple to the local magnetic moment of the molecule as a
fluctuating magnetic field," said theorist and co-author Stefan Kirchner, a
postdoctoral fellow of physics and astronomy at Rice. "It is this coupling
that gives rise to the ability to tune the degree of and even destroy
the magnetic quantum entanglement."
The effect is manifested in the unique way that the electrical conductance
of the transistor depends on temperature and frequency.
Though nano in scale, the new probe serves as a realistic model system to
elucidate physics that cannot be explained by the traditional theory of
metals, including phenomena associated with bulk materials like
rare-earth-based heavy fermion metals and copper-based high temperature
superconductors. For example, the nanoprobe allows the physicists to
introduce competition between two quantum effects - magnetic quantum
entanglement and coupling with spin waves. By accessing the quantum critical
point that lies at the phase change associated with these competing forces,
the researchers can draw a direct linkage between the quantum criticality in
the new probe and quantum criticalities in bulk materials like heavy fermion
In a 2001 paper in Nature, Si and collaborators offered a new theory
regarding a similar destruction of the magnetic quantum entanglement that
appears at the quantum critical point of heavy fermion metals. The new probe
could provide direct experimental evidence of this proposed effect.
"Based on previous experiments and theoretical predications, the new probe
should provide us with much-anticipated evidence about the precise way that
quantum criticality forms in nature," Si said. "With this unique
experimental data, we hope to gain an in-depth understanding of the
phenomena that may well be what engineers need in order to harness the power
for high-temperature superconductivity."
The research was funded by the German Research Foundation (Deutsche
Forschungsgemeinschaft), the Robert A. Welch Foundation, the National
Science Foundation, the Alfred P. Sloan Foundation and the David and Lucille
Packard Foundation. Co-authors on the study include Kirchner, Si, Natelson
and Lijun Zhu of the University of California at Riverside.
About Rice University:
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Rice University is consistently ranked one of America's best teaching and research universities. It is distinguished by its: size: 2,850 undergraduates and 1,950 graduate students; selectivity: 10 applicants for each place in the freshman class; resources: an undergraduate student-to-faculty ratio of 6-to-1, and the fifth largest endowment per student among American universities; residential college system, which builds communities that are both close-knit and diverse; and collaborative culture, which crosses disciplines, integrates teaching and research, and intermingles undergraduate and graduate work. Rice's wooded campus is located in the nation's fourth largest city and on America's South Coast.
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