LF11730B
Hidden One-Dimensional Fermi Surface in Hexagonal Potassium
Tungsten Bronze
Summary :
The electronic structure of hexagonal potassium tungsten bronze K_(x)WO_(3)
has been investigated by high-resolution angle-resolved photoemission
spectroscopy (ARPES). The experimentally determined band structure resolves
the long-standing puzzles concerning the anomalous transport properties in
this hexagonal bronze. It has been found that K_(x)WO_(3) for 0.18 < *x* <
0.32 shows superconductivity at low temperature, while exhibiting
characteristic anomalies in electrical resistivity suggestive of charge
density wave (CDW) formation at high temperature. This characteristic
temperature highly depends on the alkali metal (K) concentration, exhibiting
a maximum at 350 K for x = 0.25. We have performed ARPES measurements on
potassium tungsten bronze, for x = 0.25 (K_(0.25)WO_(3)) by a
high-resolution spectrometer at the Synchrotron Radiation Center in
Wisconsin, USA. We have also performed ab initio band-structure calculations
to compare our experimental results. From our ARPES experiments, we found
that the Fermi surface (FS) (see Figure) of K_(0.25)WO_(3) is the
consequence of one-dimensional (1D) bands, which can be well connected by
common vectors called nesting vectors. The nesting vectors, *q1* and q2
derived from our experiments matches well with the commensurate
super-lattice reflection vectors obtained from other experiments like
electron diffraction. We conclude that the CDW formation, which reduces the
conductivity in the system, is due to the presence of quasi 1D-like bands
and/or FSs and is responsible for the high temperature anomaly in transport
properties.
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LL10932.
First quantitative evidence for evolutionary control
In a famous paper presented before the Linnean Society in 1858, Alfred Russel Wallace - often considered the co-discoverer of natural selection - proposed that evolution exploits the principles of feedback control in order to generate biological complexity. To our knowledge, no direct evidence for this contention has ever been reported. In "Mutagenic evidence for the optimal control of evolutionary dynamics" (Phys. Rev. Lett., to appear), we present what is ostensibly the first quantitative experimental evidence, since Wallace's original proposal, that nature employs evolutionary control strategies to maximize the fitness of biological networks. Protein mutagenesis experiments carried out in our laboratory demonstrate that the electron transport chain - the fundamental metabolic pathway of most forms of life - has evolved hierarchically, with evolution on long time scales steering the later stages of evolution toward optimal refinement of the network's function. Based on this observation, we develop a general theoretical framework - the optimal control theory for evolutionary dynamics - that can be used to interrogate the role of evolutionary control phenomena in other biological systems.
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LL11107
A SUPERFLUID AT ROOM TEMPERATURE
For many years, one of the 'holy grails' of physics has been to make a
superfluid at room temperature in the lab - until now superfluidity has
only been found at temperatures close to absolute zero. This paper has shown theoretically how one can stabilise a
high-density superfluid up to room temperature in a magnet. The magnet
is the well-known 'Yttrium Iron Garnet' system - usually called "YIG"
- which has been used for over 50 years in magnetic technology. The
superfluid is made from 'magnons', ie, from the quantum-mechanical version
of the oscillations of magnetisation in the magnet. In the superfluid all
these magnons go into the same quantum state - called a 'Bose condensate'.
The paper of Tupitsyn et al. was a reaction to experiments by Demokritov
et al in Germany, who saw hints of this superfluidity in thin films of
YIG. Tupitsyn et al show that these hints must have been produced by a
very low-density unstable precursor to the superfluid, and that to
stabilise a high-density superfluid, all one has to do is apply an
external magnetic field perpendicular to the film. Now the experimental
race will be on to find this superfluid, and see how it behaves!
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BQ10924
Remarkable electronic properties in Mg_(2)Al_(3)
The magnetoresistance of a conductor is the change of its
electrical resistance in magnetic field. With current through a
conductor in a magnetic field, a voltage (the Hall voltage)
occurs, which is directed perpendicularly to both current and
magnetic field. For simple metal alloys the magnetoresistances is
usually small (order of ppm in fields of a few Teslas), and the
Hall voltage depends linearly on magnetic field and is nearly
independent of temperature.
In this paper we have studied two closely related phases of
Mg_(2)Al_(3), a metal alloy with the large number of 1170 atoms in
the unit cell. In one phase the Hall voltage and
magnetoresistance have values as outlined above. In the second
phase, the magnetoresistance increases strongly with decreasing
temperature below 100 K and exceeds 6 % at 4 K. The Hall field
becomes temperature dependent below 100 K. At 4 K it displays a
maximum as a function of magnetic field. These results open
challenging questions about the relation between structure and
physical properties for these alloys.
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EQ10334
Random migrations of non-Brownian neutrally buoyant particles across
the streamlines of a carrier flow in a dilute sheared suspension are
simulated on the basis of a dipole model. Both passing and reversing
trajectories of isolated particle pairs predicted earlier for
wall-bounded shear flows are obtained within the model. A diffusivity
is due to far-field collective hydrodynamic interactions rather than
long-term pair interactions on reversing trajectories. The calculated
self-diffusivity coefficient is linear in particle volume content and
agrees well with the experimental data. The self-diffusivity in a
homogeneous suspension is linear in the particle volume content. Even
small concentration inhomogeneity across the cell width results in a
significant diffusivity growth near the walls.
