Recent breakthrough in fabrication of graphene (monoatomic graphite
layer) and subsequent transport measurements revealed remarkable
electronic properties of this material. One of the most striking
observations is the minimal conductivity of undoped samples, which is
of the order of conductance quantum and stays constant from room
temperature down to 1K, showing no trace of quantum localization by
disorder. In this paper, we develop a theory of electron transport in
graphene with impurities that explain the experimental findings. The
following properties of graphene are crucial for our theory. First,
its low-energy states are split into two "valleys" in the
energy-momentum landscape. Second, the electrons in each valley
behave as massless relativistic particles. Third, experimentally
relevant scatterers in graphene -- charged impurities or so-called
ripples -- are of long-range character. As a result, the dominant
scattering cannot transfer the particle from one valley into the
other. We derive a quantum field theory describing electrons in this
situation and show that, due to the valley decoupling, the electrons
are characterized by very peculiar topological properties. As a result
the theory is at a novel quantum critical point, implying, in
partiuclar, that the conductivity is temperature-independent. This
criticality bears similarity with the famous quantum Hall transition
but occurs without any magnetic field! The non-trivial topology of
our theory explains also another striking experimental observation --
the anomalous, half-integer, quantum Hall effect in graphene in strong
magnetic field.
***
Enabling a Mother Qubit to deliver three lovely qubits
In quantum computers, interacting qubits perform computations.
In topological quantum computers, on the other hand, their offspirings
called topological qubits perform computations merrily. Their topological
character make them ghost like and oblivious to the material environment.
Quantum computation for them becomes a `merry go round', getting entangled
and picking up non-Abelian Berry phases, without ever losing their
coherence, even in an unfriendly and decohering environment.
Kitaev model, a remarkable, non trivial and exactly solvable 2 dimensional
quantum spin model exemplifies key ides of this topological quantum
computation. In our theoretical work on Kitaev model, where we present
certain exact results for dynamical properties, birth of a
Topological Qubit, from a `mother qubit' is made visible. We show that
changing the quantum state of one qubit results in the birth of a
triplet of topological qubits: an immobile `Siamese twin' of
`pi-fluxes' and a very dynamic `Majorana fermion' which frees itself
away from the mother qubit and is ever ready for quantum computation,
from the time of its birth.
Our finding will have a role in creating and organizing quantum computations
in the much awaited future quantum computers.
***
A Paul trap for neutral atoms
This paper presents the first trapping of ground-state neutral atoms in a macroscopic AC electric trap. Similarly to ions in a Paul trap (Nobel Prize, 1989), trapping is achieved here by alternating between two electric fields that result in a swinging motion of the atoms. For the first time, this motion in the trap is directly visualized using 2D images of the atom cloud.
The special feature of an AC electric trap is the ability to trap atoms or molecules in their ground state. The ground state is always attracted towards high electric fields, but it cannot be trapped in a static field, as static fields cannot possess a maximum in free space. Trapping in the ground state is therefore only possible when AC fields are used.
In any AC trap, an electric field is created with a saddle point at the trap center, resulting in attractive forces in one direction and repulsive forces along the other two directions. The electric field configuration is then switched to a second configuration in which the roles of the forces are reversed. Cycling between these two configurations leads to stable dynamic confinement of the particles.
***
Speeding up microfluidicsThe term lab-on-a-chip summarizes the effort to miniaturize chemical
production lines into millimeter-sized devices. Due to the down scaling
of the dimensions, the fluid dynamics is altered and viscous forces now
take over inertia. This circumstance hampers rapid mixing in the
microchannels of lab-on-a-chip devices because now diffusion is the
dominant mechanism. A second challenge in microfluidic application is
the actuation of liquids. Currently the lab-on-a-chip devices rely on
additional fabrication steps and an elaborate integration of connections
and wiring, e.g. for pneumatic or electrical controls. An
interdisciplinary research team from the University of Twente (The
Netherlands) and Shimadzu Corporation (Germany) came up with a
revolutionary approach to overcome both limits. The team lead by
Claus-Dieter Ohl sped up microfluidics with single vapor bubbles thus
they surpassed the viscous constrains inherently found in lab-on-a-chip
devices. These short lived vapor bubbles are created with a focused
laser pulse of a few nanosecond duration. The miniature vapor explosions
accelerate the flow to 20m/s and more. Additionally, when the bubble
grows close to a channel wall liquid flow becomes focused and
accelerated to even higher speeds. This allows liquid pumping at fast
pace. And third advantage of the new technique implementing lasers
pulses is the easy applicability: on transparent lab-on-a-chip systems
any spot can be addressed just by positioning the laser spot thus
external connection are obsolete.
***
Emergence of collectivity in a few-electron system
The existence and nature of end and central plasmon resonances in a linear
atomic chain, the 1-dimensional analog to ¿surface¿ and ¿bulk¿ plasmons in
2-dimensional metals, has first been predicted by time-dependent density
functional theory. It is well known that electronic energies and
wavefunctions of such atomic chains are quantized, and can be understood
by the ¿particle-in-a-box¿ model. Whether these quantized states can
support collective electron oscillations has, however, not been
investigated. The calculation reported here shows the emergence and
development of collective excitations, as the chain length increases atom
by atom. In the long-chain limit, these collective modes converge to a
single longitudinal resonance and two transverse ones, which are localized
at the ends and center of the chains. These collective modes bridge the
gaps, in concept and scale, between the collective excitation in atomic
physics and nanoplasmonics. It also outlines a route to atomic-scale
engineering of collective excitations via atomic manipulation.
***
Dancing With the Spins
Using a novel device called the spin dynamo, researchers demonstrated that the
electron spins dance in a small stage with more than 20 different steps, rather
than the few steps we know before.
Watching the magnificent ball dancing in the fabulous Imperial Hofburg Palace in
Vienna, Austrian, is a very different experience as watching the ABC¿s TV show
of ¿Dancing with the Stars¿, where pairs dance in a compact studio stage. So is
for scientists studying the dance of electron spins in samples with different
dimensions. The dance of spins plays the most important role in operating your
computer hard disks and memory cells. Whenever you click on the mouse to save
your data, millions of electron spins in your computer disk dance collectively
to change their patterns, and thereby save the information.
Because everyone wants to have a computer that stores more data and faster,
scientists are given the task to study how electron spins dance in
nanostructures that are invisible to the naked eye. The fundamental questions
are how do electron spins influence each other (the collective phenomenon), and
how are they influenced by the finite stage where they are dancing (the boundary
condition problem). Find the answer is the major challenge of two emerging
disciplines known as the ¿spintronics¿ and ¿nanomagnetism¿.
Now Can-Ming Hu and his colleagues at the University of Manitoba in Canada has
created a novel device called ¿spin dynamo¿ [1], which generates current from
the dance of electron spins. Using this device, they have made a break through
in studying how electron spins dance in a small piece of magnet, where they
have observed more than 20 different ¿steps¿ of dancing for the electron spins
[2]! Previously, only a few steps of them have been seen. The observed new
phenomena allow the team to precisely explain how the size of a finite stage
influences the dance of spins, which is a long standing problem in the field of
magnetism, and it¿s solution is pivotal for the architecture of the next
generation computer memories.
