61 research outputs found
Experimental confirmation of the low B isotope coefficient in MgB2
Recent investigations have shown that the first proposed explanations of the
disagreement between experimental and theoretical value of isotope coefficient
in MgB2 need to be reconsidered. Considering that in samples with residual
resistivity of few mu-Ohm cm critical temperature variations produced by
disorder effects can be comparable with variations due to the isotopic effect,
we adopt a procedure in evaluating the B isotope coefficient which take account
of these effects, obtaining a value which is in agreement with previous results
and then confirming that there is something still unclear in the physics of
MgB2.Comment: 8 pages, 3 figures Title has been changed A statement has been added
in page 7 of the pdf file "Finally we would..." Reference 21 has been added
Figure 1 anf Figure 2 have been change
Seebeck effect in the conducting LaAlO_{3}/SrTiO_{3} interface
The observation of metallic behavior at the interface between insulating
oxides has triggered worldwide efforts to shed light on the physics of these
systems and clarify some still open issues, among which the dimensional
character of the conducting system. In order to address this issue, we measure
electrical transport (Seebeck effect, Hall effect and conductivity) in
LaAlO_{3}/SrTiO_{3} interfaces and, for comparison, in a doped SrTiO_{3} bulk
single crystal. In these experiments, the carrier concentration is tuned, using
the field effect in a back gate geometry. The combined analysis of all
experimental data at 77 K indicates that the thickness of the conducting layer
is ~7 nm and that the Seebeck effect data are well described by a
two-dimensional (2D) density of states. We find that the back gate voltage is
effective in varying not only the charge density, but also the thickness of the
conducting layer, which is found to change by a factor of ~2, using an electric
field between -4 and +4MV/m at 77K. No enhancement of the Seebeck effect due to
the electronic confinement and no evidence for two-dimensional quantization
steps are observed at the interfaces.Comment: 15 pages, 5 figure
Stress transmission in granular matter
The transmission of forces through a disordered granular system is studied by
means of a geometrical-topological approach that reduces the granular packing
into a set of layers. This layered structure constitutes the skeleton through
which the force chains set up. Given the granular packing, and the region where
the force is applied, such a skeleton is uniquely defined. Within this
framework, we write an equation for the transmission of the vertical forces
that can be solved recursively layer by layer. We find that a special class of
analytical solutions for this equation are L\'evi-stable distributions. We
discuss the link between criticality and fragility and we show how the
disordered packing naturally induces the formation of force-chains and arches.
We point out that critical regimes, with power law distributions, are
associated with the roughness of the topological layers. Whereas, fragility is
associated with local changes in the force network induced by local granular
rearrangements or by changes in the applied force. The results are compared
with recent experimental observations in particulate matter and with computer
simulations.Comment: 14 pages, Latex, 5 EPS figure
Thermal conductivity of MgB in the superconducting state
We present thermal conductivity measurements on very pure and dense bulk
samples, as indicated by residual resistivity values as low as 0.5 mW cm and
thermal conductivity values higher than 200 W/mK. In the normal state we found
that the Wiedemann Franz law, in its generalized form, works well suggesting
that phonons do not contribute to the heat transport. The thermal conductivity
in the superconducting state has been analysed by using a two-gap model. Thank
to the large gap anisotropy we were able to evaluate quantitatively intraband
scattering relaxation times of and bands, which depend on the
disorder in different way; namely, as the disorder increases, it reduces more
effectively the relaxation times of than of bands, as
suggested by a recent calculation [1].Comment: 12 pages, 5 figure
Biotransformation of β-Hydroxypyruvate and Glycolaldehyde to L-Erythrulose by Pichia pastoris strain GS115 overexpressing native Transketolase
Transketolase is a proven biocatalytic tool for asymmetric carbon-carbon bond formation,
both as a purified enzyme and within bacterial whole-cell biocatalysts. The performance of
Pichia pastoris as a host for transketolase whole-cell biocatalysis was investigated using a
transketolase-overexpressing strain to catalyse formation of L-erythrulose from β-
hydroxypyruvic acid and glycolaldehyde substrates. Pichia pastoris transketolase coding
sequence from the locus PAS_chr1-4_0150 was subcloned downstream of the methanolinducible
AOX1 promoter in a plasmid for transformation of strain GS115, generating strain
TK150. Whole and disrupted TK150 cells from shake flasks achieved 62% and 65%
conversion, respectively, under optimal pH and methanol induction conditions. In a 300µL
reaction, TK150 samples from a 1L fed-batch fermentation achieved a maximum Lerythrulose
space time yield (STY) of 46.58 g L
-1 hr-1, specific activity of 155 U gCDW
-1
,
product yield on substrate (Yp/s) of 0.52 mol mol-1 and product yield on catalyst (Yp/x) of
2.23g gCDW
-1. We have successfully exploited the rapid growth and high biomass
characteristics of Pichia pastoris in whole cell biocatalysis. At high cell density, the
engineered TK150 Pichia pastoris strain tolerated high concentrations of substrate and
product to achieve high STY of the chiral sugar L-erythrulose
Intraband versus interband scattering rate effects in neutron irradiated MgB2
One of the most important predictions of the two-gap theory of
superconductivity concerns the role of interband scattering (IBS) by
impurities. IBS is expected to decrease the critical temperature, Tc, of MgB2
to a saturation value of about 20 K, where the two gaps merge to a single one.
These predictions have been partially contradicted by experiments. In fact, Tc
does not saturate in irradiated samples, but decreases linearly with residual
resistivity and the merging of the gaps has been observed at a much lower Tc
(11 K). In this paper we argue that, while at low level of disorder IBS is the
leading mechanism that suppresses superconductivity, at higher disorder the
experimental results can only be understood if the smearing of the density of
states due to intraband electron lifetime effects is considered.Comment: 3 figure
The ac stark shift and space-borne rubidium atomic clocks
open7sìDue to its small size, low weight, and low power consumption, the Rb atomic frequency standard (RAFS) is routinely the first choice for atomic timekeeping in space. Consequently, though the device has very good frequency stability (rivaling passive hydrogen masers), there is interest in uncovering the fundamental processes limiting its long-term performance, with the goal of improving the device for future space systems and missions. The ac Stark shift (i. e., light shift) is one of the more likely processes limiting the RAFS' long-term timekeeping ability, yet its manifestation in the RAFS remains poorly understood. In part, this comes from the fact that light-shift induced frequency fluctuations must be quantified in terms of the RAFS' light-shift coefficient and the output variations in the RAFS' rf-discharge lamp, which is a nonlinear inductively-couple plasma (ICP). Here, we analyze the light-shift effect for a family of 10 on-orbit Block-IIR GPS RAFS, examining decade-long records of their on-orbit frequency and rf-discharge lamp fluctuations. We find that the ICP's light intensity variations can take several forms: deterministic aging, jumps, ramps, and non-stationary noise, each of which affects the RAFS' frequency via the light shift. Correlating these light intensity changes with RAFS frequency changes, we estimate the light-shift coefficient, K-LS, for the family of RAFS: K-LS = -(1.9 +/- 0.3) x 10(-12) /%. The 16% family-wide variation in K-LS indicates that while each RAFS may have its own individual K-LS, the variance of K-LS among similarly designed RAFS can be relatively small. Combining K-LS with our estimate of the ICP light intensity's non-stationary noise, we find evidence that random-walk frequency noise in high-quality space-borne RAFS is strongly influenced by the RAFS' rf-discharge lamp via the light shift effect. Published by AIP Publishing.openFormichella, V.; Camparo, J.; Sesia, I.; Signorile, G.; Galleani, L.; Huang, M.; Tavella, P.Formichella, V.; Camparo, J.; Sesia, Ilaria; Signorile, Giovanna; Galleani, L.; Huang, M.; Tavella, Patrizi
Probing the electron-phonon coupling in MgB2 through magnetoresistance measurements in neutron irradiated thin films
We report magnetoresistance (MR) measurements on MgB2 and the corresponding
full account from ab-initio calculations; we suggest that this combination can
be a useful tool to probe electron- phonon coupling. We obtain good
quantitative agreement between high field measurements on neutron irradiated
epitaxial thin films and calculations within Bloch-Boltzmann transport theory
over a wide range of magnetic fields (0-28 T) and temperatures (40-300 K), and
as a function of the field orientation. The crossovers between in-plane and
out-of-plane MR, experimentally observed as a function of either disorder or
temperature are well reproduced indicating that disorder and interaction with
phonons strongly affect the scattering rate of s-carriers.Comment: 13 pages, 4 figures, 1 tabl
Two-band effects in transport properties of MgB2
We present resistivity and thermal conductivity measurements on bulk samples,
prepared either by a standard method or by a one-step technique. The latter
samples, due to their high density and purity, show residual resistivity values
as low as 0.5 mW cm and thermal conductivity values as high as 215 W/mK, higher
than the single crystal ones. Thermal and electrical data of all the samples
are analysed in the framework of the Bloch-Gruneisen equation giving reliable
parameter values. In particular the temperature resitivity coefficient,
obtained both from resistivity and thermal conductivity, in the dirty sample
comes out ten time larger than in the clean ones. This result supports the
hypothesis of ref. [1] that p and s bands conduct in parallel, prevailing p
conduction in clean samples and s conduction in dirty samples .Comment: 8 pages, 5 figures, Presented at the BOROMAG workshop, June 17-19
2002, Genoa, Ital
Magnetoresistivity in MgB2 as a probe of disorder in p- and s-bands
In this paper we present normal state magnetoresistivity data of magnesium
diboride epitaxial thin films with different levels of disorder, measured at
42K in magnetic fields up to 45 Tesla. Disorder was introduced in a controlled
way either by means of neutron irradiation or by carbon doping. From a
quantitative analysis of the magnetoresistivity curves with the magnetic field
either parallel or perpendicular to the plane of the film, we extract the ratio
of the scattering times in p- and s-bands. We demonstrate that the undoped
unirradiated thin film has p scattering times smaller than s ones; upon
irradiation, both bands become increasingly more disordered; eventually the
highly irradiated sample (neutron fluence 7.7X1017 cm-2) and the C-doped sample
have comparable scattering times in the two types of bands. This description of
the effect of disorder in the two kinds of bands on transport is consistent
with the residual resistivity values and with the temperature dependence of the
resistivity.Comment: 19 pages, 3 tables, 2 figure
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