29 research outputs found
Cooperative localization-delocalization in the high Tc cuprates
The intrinsic metastable crystal structure of the cuprates results in local
dynamical lattice instabilities, strongly coupled to the density fluctuations
of the charge carriers. They acquire in this way simultaneously both,
delocalized and localized features. It is responsible for a partial fractioning
of the Fermi surface, i.e., the Fermi surface gets hidden in a region around
the anti-nodal points, because of the opening of a pseudogap in the normal
state, arising from a partial charge localization. The high energy localized
single-particle features are a result of a segregation of the homogeneous
crystal structure into checker-board local nano-size structures, which breaks
the local translational and rotational symmetry. The pairing in such a system
is dynamical rather than static, whereby charge carriers get momentarily
trapped into pairs in a deformable dynamically fluctuating ligand environment.
We conclude that the intrinsically heterogeneous structure of the cuprates must
play an important role in this type of superconductivity.Comment: 14 pages, 8 figures, Proceedings of the "International Conference on
Condensed Matter Theories", Quito, 2009 Int. J. Mod. Phys. B 2010 (Accepted
Spin singlet small bipolarons in Nb-doped BaTiO3
The magnetic susceptibility and electrical resistivity of n-type
BaTi{1-x}Nb{x}O3 have been measured over a wide temperature range. It is found
that, for 0 < x < 0.2, dopant electrons form immobile spin singlet small
bipolarons with binding energy around 110 meV. For x = 0.2, a maximum in the
electrical resistivity around 15 K indicates a crossover from band to hopping
transport of the charge carriers, a phenomenon expected but rarely observed in
real polaronic systems.Comment: 5 pages, 4 figure
Improved Thermoelectric Cooling Based on the Thomson Effect
Traditional thermoelectric Peltier coolers exhibit a cooling limit which is
primarily determined by the figure of merit, zT. Rather than a fundamental
thermodynamic limit, this bound can be traced to the difficulty of maintaining
thermoelectric compatibility. Self-compatibility locally maximizes the cooler's
coefficient of performance for a given zT and can be achieved by adjusting the
relative ratio of the thermoelectric transport properties that make up zT. In
this study, we investigate the theoretical performance of thermoelectric
coolers that maintain self-compatibility across the device. We find such a
device behaves very differently from a Peltier cooler, and term self-compatible
coolers "Thomson coolers" when the Fourier heat divergence is dominated by the
Thomson, as opposed to the Joule, term. A Thomson cooler requires an
exponentially rising Seebeck coefficient with increasing temperature, while
traditional Peltier coolers, such as those used commercially, have
comparatively minimal change in Seebeck coefficient with temperature. When
reasonable material property bounds are placed on the thermoelectric leg, the
Thomson cooler is predicted to achieve approximately twice the maximum
temperature drop of a traditional Peltier cooler with equivalent figure of
merit (zT). We anticipate the development of Thomson coolers will ultimately
lead to solid state cooling to cryogenic temperatures.Comment: The Manuscript has been revised for publication in PR
Enhanced superconducting pairing interaction in indium-doped tin telluride
The ferroelectric degenerate semiconductor SnTe exhibits
superconductivity with critical temperatures, , of up to 0.3 K for hole
densities of order 10 cm. When doped on the tin site with greater
than indium atoms, however, superconductivity is observed up
to 2 K, though the carrier density does not change significantly. We present
specific heat data showing that a stronger pairing interaction is present for
than for . By examining the effect of In dopant atoms on
both and the temperature of the ferroelectric structural phase
transition, , we show that phonon modes related to this transition are
not responsible for this enhancement, and discuss a plausible candidate
based on the unique properties of the indium impurities.Comment: 7 page
Dependence of the current gain on the surface-recombination rate in a drift-free transistor
Effective diffusivity of a Brownian particle in a two-dimensional periodic channel of abruptly alternating width
Determination of the Predominant Ionization and Loss Mechanisms for the Low‐Voltage Arc Mode in a Neon Plasma Diode
A reaction‐rate analysis is used to determine the relative importance of the predominant ionization and loss mechanisms in a neon low‐voltage arc. Experimental data are derived from various experiments to determine density, cross section, etc. It is found that the resonance and metastable state atoms are primarily generated in a region near the cathode which corresponds to the ``cathode ball‐of‐fire'' region of the low‐voltage arc. The predominant ion‐generation process is found to be a result of collisions between excited atoms which cause the formation of a molecular ion. Direct ionization of ground‐state atoms is of secondary but non‐negligible, importance. Consideration of quasi‐equilibrium multistage ionization shows that, unlike the cesium low‐voltage arc, it is unimportant in the neon low‐voltage arc. The escape of resonance radiation accounts for approximately one‐fifth of the total power loss while ionization accounts for approximately one‐eighth. Most of the remaining power loss appears as power dissipation at the anode.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69978/2/JAPIAU-39-9-4299-1.pd