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Spin Hall effect
1 October 2006
D.Awschalom and his colleagues from the University of California
and Pennsylvania State University have detected the spin Hall
effect at room temperature for the first time. The effect was
observed in a nonmagnetic material in the absence of an external
magnetic field and consisted in the fact that an electron spin flow
developed perpendicular to the electric field direction - leading
to a concentration difference between opposite spin electrons on
the side faces of the sample. The reason for this phenomenon is
that the scattering direction of an electron depends on precisely
how the electron's spin is directed when participating in the spin-
orbit interaction. The spin Hall effect was predicted by M.I.Dyakonov
and V.I.Perel in 1971 and first discovered by D.Awschalom
and his colleagues in their 2004 study on GaAs at 20K. In the new
experiment, 1.5mk thick semiconducting films of chlorine-doped
ZnSe were investigated using Kerr rotation spectroscopy. As the
temperature increased from 10 to 295K, the size of the spin Hall
effect (i. e., side face spin polarization) decreased by about ten
times while still remaining measurable. The task for future
research is to increase the spin coherence time and the fraction of
polarized electrons at elevated temperatures. The spin Hall effect
can find applications in designing spin current sources for
spintronic devices. Source: Phys. Rev. Lett. 97 126603 (2006)
Quantum cooling
1 October 2006
Measuring the quantum state of a system always exerts a perturbing
effect on the system. Now an experiment on the practical use of
this effect for cooling a microscopic beam has been performed at
the University of Maryland. In the experiment, use was made of the
fact the mechanical vibration amplitude of the beam could be put
into correspondence with a certain effective temperature. Close to
the beam, the team placed a superconducting single-electron
transistor, the electromagnetic field in which depended on (and
thus allowed measurement of) the level of vibrations in the beam.
This quantum measurement led in some cases to smaller vibration
amplitudes and thus to the cooling of the beam. The cooling
obtained in the experiment was from 550 to 300K. The reason for
the cooling lies in the asymmetric spectrum of the transistor's
quantum noises that exert back-action on the beam being measured.
It is believed that this method of cooling may in the future be
useful for nanoelectronics device applications. Source: Nature 443 123 (2006)
Superconducting qubits
1 October 2006
M.Steffen and his colleagues from the University of California at
Santa Barbara have for the first time obtained a quantum-correlated
(entangled) state of two superconducting Josephson tunnelling
junctions. The researchers used the method of `quantum tomography'
when performing the measurement of the quantum state of the system
that confirmed the appearance of an entangled state. Because
superconducting elements can store quantum bits (qubits) of
information, creating coherent systems of superconducting elements
is promising for the development of quantum computers. In one of
the alternative approaches, as many as eight ions in an atomic trap
were brought into an entangled state. Thus far, no difficulties of
principle are known which would prevent creating similar entangled
states in systems of more than two superconducting elements. Source:
http://physicsweb.org/articles/news/10/9/3/1
Superhigh frequency nanotube resonators
1 October 2006
Researchers from Lawrence Berkeley National Laboratory and the
University of California report creating a nanoelectromechanical
resonator based on a carbon nanotube. The nanotube is attached to
two metallic contacts 300nm apart, by means of which a high
frequency alternating current is passed through the nanotube, and
at a distance of 200nm from the tube, a third (gate) contact is
placed, to which a signal of a different frequency is applied.
While the electric field of the gate exerts a force on the
nanotube, the vibrations of the nanotube, in turn, change the
capacity between itself and the gate. All the contacts are
connected to a radio scheme, which measures the amplitude and phase
of the vibrations. At room temperature and an air pressure of 1atm., a
resonance occurs between the mechanical vibrations of the
tube and the electromagnetic oscillations at about 1.3GHz. That
this frequency is somewhat lower than in vacuum is due to the air
molecules depositing themselves on the nanotube surface. The
dependence of the resonant frequency on the deposited mass allowed
supersmall masses (10-18g) to be measured. The effective quality of
the resonator reached Q=440. Source: Phys. Rev. Lett. 97 087203 (2006)
Testing general relativity
1 October 2006
Of all currently known systems for probing the effects of general
relativity, there are a number of reasons why the double pulsar PSR
J0737-3039A/B is the most promising: both neutron stars (A and B)
are observed as radio pulsars, the pair is relatively close to the
Sun (500pc), and the orbital period is only 2.4 hours - leading
to large orbital velocities and orbital accelerations and allowing
an independent determination of the mass ratio of the stars. PSR
J0737-3039A/B has been studied by several telescopes since its
discovery 2.5 years ago. By observing the shape and spacing of the
radio pulses from the pulsar, it proved possible to study such
effects as orbit shrinking due to gravitational wave emission by
the system; non-Keplerian corrections due to spacetime curvature;
and the effect of gravitational field on signal propagation. All in
all, a test of general relativity theory was carried out at 105 times
solar system's gravitational field to within 0.005% - a record
high accuracy for the high field range. Source:
astro-ph/0609417
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The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics. Permanent editor is Yu.N. Eroshenko. It is compiled from a multitude of Internet sources.
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