Boron fullerenes, nanotubes and sheets: built from a unified scheme
Carbon has exciting nanostructures such as fullerenes, nanotubes and graphenes. In 1985, Robert Curl, Harold Kroto and Richard Smalley at the University of Sussex and Rice University discovered that 60 carbon atoms can form a stable hollow spherical molecule with the shape like a geodesic dome, the building popularized by a noted architect Richard Buckminster Fuller. C60 was thus named as buckminsterfullerene, or fullerene for short. Curl, Kroto and Smalley were awarded Nobel prize in chemistry in 1996 for the discovery of carbon fullerenes. In 2001, Sumio Iijima of NEC synthesized a long cylindrical carbon molecule, soon became known as carbon nanotubes (CNTs), which can be formed by rolling a single layer of graphite in certain ways. It has been shown that CNTs have many unusual characteristics such as extraordinary strength, unique electrical properties, and efficient heat conduction, etc. In 2008, Sumio Iijima shared the Kavli prize for nanoscience with Louis E. Brus for!
the respective discoveries of car
bon nanotubes and quantum dots. In 2004, the single layer of graphite, or graphene, was successfully fabricated by Andre Geim and Kostya Noveselov from the University of Manchester, which is comprised of solely carbon atoms, and is the thinnest film in the world up to date. Graphenes are shown to exhibit intriguing properties, and have potential wide applications, thereby attracting worldwide extensive explorations, and rapidly becoming one of the hottest topics in condensed matter physics. In 2008, Geim and Noveselov were awarded the Europhysics Prize for discovering graphene. It is crystal clear that each of the three amazing nanostructures of carbon really makes a great contribution in science and technology.
How about boron? As boron and carbon are neighbors in the periodic table, and possess many structural analogies, people may ask naturally if boron can form nanostructures similar to carbon that may also lead to crucial breakthroughs in science and technology. Efforts have already been attempted. In 1976, William N. Lipscomb of Harvard University had predicted that a molecule with 32 boron atoms would exhibit an icosahedral structure not unlike the buckyball's soccerball shape. However, this single nearly spherical polyhedron is not comprised of pure boron but boranes. Some less spherical multicage structures of boranes were also discussed. In 2007, I. Yakobson and co-workers [PRL 98, 166804 (2007)] of Rice University predicted that a boron fullerene B80 can be formed, whose geometrical shape is very similar to carbon buckyball C60 except that an additional boron atom sits in the center of each hexagon. Experimentally, the boron nanotubular structures have been fabricated. In!
addition, several two-dimensional
boron sheets (BS) have been theoretically proposed, showing that the hexagonal graphitic BS is unstable, while the most stable structures consist of buckled triangular motifs. Recently, a new type of boron sheet (NBS) [PRL 99, 115501 (2007)] and related boron nanotubes (BNTs) [PRB 77, 041402R (2008)] were predicted by research groups of Yale University and Tsinghua University (China), respectively, to be remarkably more stable than the boron sheet and BNTs with triangular structures. Several other fullerene-like boron nanostructures are also predicted in literature.
Now, a group from the Graduate University of Chinese Academy of Sciences (GUCAS), led by Prof. Gang Su, in their Rapid Communications in Physical Review B to be printed soon, proposed a generic constructing scheme that not only shows how to generate a great family of novel boron monoelemental, hollow fullerenes (B32+8k, k is positive integer, which were coined as S-fullerenes due to their basic building blocks being the snowdrop-like motifs) with remarkable stabilities and other quasi-planar boron nanostructures (nanotubes, sheets, etc) including B80 buckyball and newly predicted boron sheet, NBS, as special examples, but also proposes an electron counting rule and an isolated hollow rule to show why the predicted boron S-fullerenes are stable and how the electrons in these fullerenes are bonded, thereby establishing the relationship between the geometrical and electronic structures of the boron fullerenes, which is of great interest in the field of boron chemistry. In addit!
ion, the work by GUCAS group shows
that the apparently independent structures like B80 and NBS are essentially connected, and their stabilities could be understood in a unified framework. The proposal in this paper may arouse follow-up investigations on boron nanostructures both theoretically and experimentally in near future, and could also present a practical route for finding other fullerenes even other than boron.
***
LT11752DR
Black Hole Bounce
While the popular image of a black hole is that it is completely and
utterly black, in reality things are actually a little more subtle
than that. Stephen Hawking has predicted that black holes will slowly
evaporate due to subtle quantum effects, but over and above this
quantum instability of the horizon, the gravitational field of the
black hole can lead to a certain amount of "bounce" and
"reflection". The so-called "greybody" factors modify the Hawking
radiation that reaches infinity, so that it is not exactly the naive
Planck blackbody spectrum originally predicted by Hawking. These
"greybody" factors also imply that if you drop a quantum object into
a black hole, there is a small chance it will "bounce".
In this article we derive some rigorous bounds on just how big this
"bounce" effect can be. At high energies the "bounce" is guaranteed
to be tiny, at low energies the effect can become very large. The
rigorous bounds are important because they verify our intuition
(developed from various approximation techniques) as to the size of
the effect, and also demonstrate that what is going on inside the
black hole is not relevant to the "greybody factors" --- to do the
calculation we only need to know what is going on outside the horizon.
***
EQ10515
Towards attacking HIV by suppressing its budding and assembly
Human immunodeficiency virus (HIV) is notorious for its ability to induce
AIDS. Understanding the life cycle of this virus is therefore crucial to
cure this vital disease. One important step of the HIV life cycle is its
budding and assembly. It is known that they happen concurrently on the
membrane of the host cell. This is different from most regular viruses and
provides a unique two dimensional self-assembly mechanism which involves
membrane elastic deformation. In this work, methods in theoretical soft
condensed matter physics have be applied to study HIV's budding and
assembly. The result shows that complete assembled virus capsids indeed
correspond to a global minimum in free energy. However, it is fairly easy to
tune the membrane elasticity and attraction between HIV composite proteins
to create a local minimum, at which the budding and assembly is trapped and
the capsid is only partially formed. This explains related experimental
observations on partial budding.
***
LW11105
A New Solid State Qubit
The spin of an electron can be prepared and measured (non-destructively) with lasers in an artificial atom (quantum dot), but so far both could not be done simultaneously. This was a major hurdle in using quantum dots for quantum computing. Physicists at the Naval Research Laboratory in Washington D.C. now show that this is possible, where instead of using a single dot they use two. One of the dots holds the electron while the other dot interacts with the laser. Depending on the wavelength of the laser they can either force the electron to be in one of two spin states, and measure this spin projection without destroying it. Quantum dots are an attractive implementation for quantum computing since they are scalable and can interface with photons in the telecommunications band.
***
LP11026E
THE BINARY CODE OF LIFE
Computers work by coding information in the most simple mathematical representation: binary numbers composed only by 0’s and 1’s. Is it possible that life manages biological information on the same ground than that of man made computers? In this paper it is shown that, indeed, life uses the binary representation for coding genetic information. The starting point is a recently developed mathematical theory of the genetic code [1-3].The theory allows the uncovering of a dichotomic (or binary) class associated to codons, the parity class (a codon is a sequence of three consecutive mRNA nucleotides or bases that specify an amino acid in protein synthesis). The value of this parity class can be obtained by using chemical information of the last two bases of a codon. On the same ground the model describes both the known Rumer’s dichotomic class, and a newly defined one, the Hidden class. Moreover, the three classes are associated to a perfect symmetry described through the discrete Klein V group structure. A statistical study of the cross-entropy between these dichotomic classes, reveals strong short-range correlations for particular base positions. These correlations give new information and provide a fresh insight about both, the extant structure of genetic information, and the origin and evolution of primordial genetic codes, one of the most compelling problems in Evolutionary Molecular Biology.
***
LT11904
Spin echo in cold atoms
Spins in a cold atomic gas can come back coherently after they once diffuse. The unique phenomenon is well known as "spin echo" in nuclear magnetic resonance (NMR), also having been used in magnetic resonance imaging (MRI) in hospitals. In this paper, we demonstrate spin echo in a cold gas system for the first time. The study opens a new field of magnetic resonance in cold atoms, which has never been addressed. Quantum vortices and solitons have been visualized in the system; magnetic resonance will be visualized after this study.
***
LW11111
Kondo resonance
A mixture of electron donor and electron acceptor molecules ordered on a
metal surface exhibit a key fingerprint of magnetic systems: a Kondo
resonance. The Kondo resonance reflects the presence of an extra
electron in the acceptor molecule, donated by the surrounding
environment. Traditional metal-based magnetism stems from the
localization of a single electron in an atomic orbital. In our work, we
find that the extra electron lies in an extended ? molecular orbital
(?-orbital magnetism), being therefore delocalized all around the
molecular structure. This gives rise to interesting phenomena, because
the electron and its spin are coupled to atomic vibrations of the
molecule. We demonstrate this by measuring with a scanning tunnelling
microscope the conductance spectrum of one of these molecules, at a
temperature of 5 Kelvin. A Kondo resonance represents spin-flip events
excited by electrons tunnelling through the molecule. Here, we find two
additional Kondo-like resonances, whose origin is due to the excitation
of both a spin-flip and a molecular vibration. Our results then suggest
that organic magnetic materials, besides being cheaper and lighter than
their metallic counterparts, also may exhibit new functionality because
of the interaction of atomic motion with magnetic moments of electrons
in molecular orbitals.
***
BU10752
Electrically driven nuclear spins operate electron spin
An ensemble of loosely bound nuclear spins can drive coherent oscillations
of electron spin. Electron spin is traditionally operated by resonant
magnetic fields acting on its magnetic moment. But for nanotechnology,
operating electron spins by resonant electric fields has tremendous
advantage because they can be applied to individual quantum dots, tiny
artificial atoms of the size about 100 nm. Recently a Harvard team lead by
Charles Marcus (PRL 99, 246601, 2007) achieved such electrical control of
electron spins in GaAs quantum dots that is efficient at weak magnetic
fields of only about 100 mT. The underlying mechanism was identified as the
relativistic interaction between the electron and nuclear spins known as
hyperfine coupling. In my paper BU10752, I developed a theory revealing in
which way a million of loosely coupled nuclear spins act on electron spin
like a macroscopic field. It also explains a striking anomaly: why instead
of regular Rabi oscillations of the electron spin the signal saturates. And
it predicts new effects at surprisingly early times, of only about a few
Rabi periods. Theory is critical for manipulating spins in quantum dots
envisioned as basic elements of quantum computers.
***
ESR1039
SIMPLE OBJECTS MAY ROTATE IN STRANGE WAYS
Unexpected motions arising from simple mechanical systems have
been fascinating human minds through the centuries. One classical
example is the retrograde motion of Mars (from time to time the
planet seems to go backwards on its trajectory). The present work
concerns the fact that ordinary objects can also move contrary to
our intuitive expectations, in particularly simple situations.
More specifically, we take a rigid body, for definiteness you may
think of a disk, and make a narrow hole at an arbitrary point P on
it, through which we pass a lubricated rod. Next, we force the rod
to rotate in a circular path, say, in the clockwise direction. Of
course the whole body will follow the circular motion, but, in
addition, it can rotate around itself with the moving rod as the
axis. In this paper we establish, in a precise mathematical way,
what are the conditions for this intrinsic rotation to be
counterclockwise, i.e., opposite to the external rotation. One
nice point is that in these mathematical conditions the same
number always appears, no matter the mass of the object, its size,
its shape or even how fast the rod is being rotated. This
universal number is 2.523.
***
BT10885
Berry phase enhancement
A theoretical effect, not all that new but still very much in fashion,
and a recent technological breakthrough have been merged in this paper
to predict a phenomenal enhancement of the effect and to outline its
observation . The "effect" is the Berry phase, which is the phase that
is added to the complex-valued wave function of the electron as this
performs a cyclic trajectory; the breakthrough is the availability of
clean graphene (a single carbon sheet separated off the familiar
graphite), whose electronic energy-band have features of a "diabolo" ( a
double-conical rubber toy to be tossed to the air with a string). While
the so far familiar values of the Berry-phase are in the range of one or
very few radians, the authors of the paper show that one can achieve
Berry-phases that are hundreds of times larger. The prescription is to
induce (with the aid of a suitable designed AC electric field) a cyclic
motion of the electron that starts very near to the tip of the diabolo
and goes around this in a non-concentric way. Accompanying the phase
acquisition, the electronic current shows a tremor motion, similar to
the "Ziiterbewegung" predicted by Dirac in his relativistic theory of
electrons.
***
BU11029
Kondo physics in break junctions?
Quantum electron transport trough impurities with internal degrees of
freedom is one of the central points in modern nano-electronics.
An impurity with an internal dichotomic degree of freedom (a position,
a spin or whatever else) is, for sure, the simplest object with an
internal structure. The two states could well be the position of an
atom, tunneling quantum mechanically between two double well minima.
We consider that this situation might be realized in a mechanical
break junction. The very instant the atomic contact breaks, there is
an atom which is "contended" between the two leads. Its dynamics is
that of a particle in a double well potential and we have shown that
electrons, hopping through it, may experience a low temperature
Kondo phase of the two-channel sort. Conductance values below the
unitary limit before the breaking of the contact and zero bias
anomalies with non Fermi liquid power laws (T^1/2, V^1/2) are
predicted, which could be sought out as the fingerprints of this
phenomenon. Our theoretical approach resorts to a Numerical
Renormalization Group (NRG) analysis problem and a combined use of
Conformal Field Theory and Fermi Liquid Theory to describe the fixed
points physics.
On the experimental side, a similar or related physics might be
realized by H-atoms captured at nano-contacts, or by conduction
through molecules, such as malonaldehyde, possessing an internal
quantum degree of freedom, such as a
***
LT12007
Empirical Tests of Zipf's Power Law Mechanism in Open Source Linux
Distribution
Zipf's law describes the remarkably universal inverse proportionality
between sizes and rank distributions in natural and socio-economical
systems. For almost a century, various efforts have been underway to
discover the mechanism(s) of Zipf's law, the most famous one being
"proportional growth" introduced in the 1950s by Economic Nobel Laureate
Herbert Simon, recently adapted by R. Albert and L. Barabasi to networks
under the name "preferential attachment." In spite of lacking empirical
verification, the model has been widely used to explain stochastic growth in
complex systems, such as dynamics of cities' or firm sizes, and networks
characterised by a Power-Law distribution of a node's connectivity.
For the first time, the proportional growth mechanism underlying Zipf's law
has been verified in details along three empirical properties in a "real
world" complex system, namely in the dynamical evolution of an open source
software distribution. The result is highly relevant for researchers in a
variety of fields, ranging from physics and biology to economics, sociology
and the Internet. This work emphasizes the role of interactions between
human developers/users and computers, with important applications to the
understanding of upside and downside risks, such as in the virtual world
economy and cyber-risks.
This publication opens new venues in understanding the emergence of Zipf's
law as well as explaining potential success in public goods contributions,
such as open source software. The paper describes a general mechanism as a
benchmark of normal connectivity growth among software packages in an open
source software distribution.
