345 research outputs found
Optical evidence for a magnetically driven structural transition in the spin web
is a modest frustrated spin system, which undergoes an
anti-ferromagnetic transition at . The anti-ferromagnetic spin
alignment in below is supposed to induce a magneto-elastic
strain of the lattice. The complete absorption spectrum of is
obtained through Kramers-Kronig transformation of the optical reflectivity,
measured from the far-infrared up to the ultraviolet spectral range as a
function of temperature (). Below , we find a new mode at
208 . The spectral weight associated to this additional mode increases
as with decreasing below . The implication of
the optical findings will be discussed in relation to the magnetic phase
transition at .Comment: 9 pages, 3 figure
Interference and Interaction in Multiwall Carbon Nanotubes
We report equilibrium electric resistance R and tunneling spectroscopy dI/dV
measurements obtained on single multiwall nanotubes contacted by four metallic
Au fingers from above. At low temperature quantum interference phenomena
dominate the magnetoresistance. The phase-coherence and elastic-scattering
lengths are deduced. Because the latter is of order of the circumference of the
nanotubes, transport is quasi-ballistic. This result is supported by a dI/dV
spectrum which is in good agreement with the density-of-states (DOS) due to the
one-dimensional subbands expected for a perfect single-wall tube. As a function
of temperature T the resistance increases on decreasing T and saturates at
approx. 1-10 K for all measured nanotubes. R(T) cannot be related to the
energy-dependent DOS of graphene but is mainly caused by interaction and
interference effects. On a relatively small voltage scale of order 10 meV, a
pseudogap is observed in dI/dV which agrees with Luttinger-Liquid theories for
nanotubes. Because we have used quantum diffusion based on Fermi-Liquid as well
as Luttinger-Liquid theory in trying to understand our results, a large
fraction of this paper is devoted to a careful discussion of all our results.Comment: 14 pages (twocolumn), 8 figure
Femtosecond data storage, processing and search using collective excitations of a macroscopic quantum state
An ultrafast paralell data processor is described in which amplitude mode
excitations of a charge density wave (CDW) are used to encode data on the
surface of a 1-T TaS_2 crystal. The data are written, manipulated and read
using parallel femtosecond laser pulse beams, and the operation of a database
search algorithm is demonstrated on a 2-element array.Comment: To be published in App. Phys. Let
Signatures of quantum criticality in the thermopower of Ba(Fe(1-x)Co(x))2As2
We demonstrate that the thermopower (S) can be used to probe the spin
fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based
superconductors. The sensitivity of S to the entropy of charge carriers allows
us to observe an increase of S/T in Ba(Fe(1-x)Co(x))2As2 close to the
spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy
conduction electrons to two-dimensional SFs, similar to heavy-fermion systems.
The low-temperature enhancement of S/T in the Co substitution range 0.02 < x <
0.1 is bordered by two Lifshitz transitions, and it corresponds to the
superconducting region, where a similarity between the electron and
non-reconstructed hole pockets exists. The maximal S/T is observed in proximity
to the commensurate-to-incommensurate SDW transition, for critical x_c ~ 0.05,
close to the highest superconducting T_c. This analysis indicates that low-T
thermopower is influenced by critical spin fluctuations which are important for
the superconducting mechanism
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