69,392 research outputs found
Can re-entrance be observed in force induced transitions?
A large conformational change in the reaction co-ordinate and the role of the
solvent in the formation of base-pairing are combined to settle a long standing
issue {\it i.e.} prediction of re-entrance in the force induced transition of
DNA. A direct way to observe the re-entrance, i.e a strand goes to the closed
state from the open state and again to the open state with temperature, appears
difficult to be achieved in the laboratory. An experimental protocol (in direct
way) in the constant force ensemble is being proposed for the first time that
will enable the observation of the re-entrance behavior in the
force-temperature plane. Our exact results for small oligonucleotide that forms
a hairpin structure provide the evidence that re-entrance can be observed.Comment: 12 pages and 5 figures (RevTex4). Accepted in Europhys Lett. (2009
Conversion of glassy antiferromagnetic-insulating phase to equilibrium ferromagnetic-metallic phase by devitrification and recrystallization in Al substituted PrCaMnO
We show that PrCaMnO with 2.5% Al substitution and
LaCaMnO (LCMO) exhibit qualitatively similar and
visibly anomalous M-H curves at low temperature. Magnetic field causes a broad
first-order but irreversible antiferromagnetic (AF)-insulating (I) to
ferromagnetic (FM)-metallic (M) transition in both and gives rise to soft FM
state. However, the low temperature equilibrium state of
PrCaMnAlO (PCMAO) is FM-M whereas that
of LCMO is AF-I. In both the systems the respective equilibrium phase coexists
with the other phase with contrasting order, which is not in equilibrium, and
the cooling field can tune the fractions of the coexisting phases. It is shown
earlier that the coexisting FM-M phase behaves like `magnetic glass' in LCMO.
Here we show from specially designed measurement protocols that the AF-I phase
of PCMAO has all the characteristics of magnetic glassy states. It devitrifies
on heating and also recrystallizes to equilibrium FM-M phase after annealing.
This glass-like AF-I phase also shows similar intriguing feature observed in
FM-M magnetic glassy state of LCMO that when the starting coexisting fraction
of glass is larger, successive annealing results in larger fraction of
equilibrium phase. This similarity between two manganite systems with
contrasting magnetic orders of respective glassy and equilibrium phases points
toward a possible universality.Comment: Highlights potential of CHUF (Cooling and Heating in Unequal Fields),
a new measurement protoco
Comparative experimental and Density Functional Theory (DFT) study of the physical properties of MgB2 and AlB2
In present study, we report an inter-comparison of various physical and
electronic properties of MgB2 and AlB2. Interestingly, the sign of S(T) is +ve
for MgB2 the same is -ve for AlB2. This is consistent our band structure plots.
We fitted the experimental specific heat of MgB2 to Debye Einstein model and
estimated the value of Debye temperature (theta) and Sommerfeld constant
(gamma) for electronic specific heat. Further, from gamma the electronic
density of states (DOS) at Fermi level N(EF) is calculated. From the ratio of
experimental N (EF) and the one being calculated from DFT, we obtained value of
Lembda to be 1.84, thus placing MgB2 in the strong coupling BCS category. The
electronic specific heat of MgB2 is also fitted below Tc using pi-model and
found that it is a two gap superconductor. The calculated values of two gaps
are in good agreement with earlier reports. Our results clearly demonstrate
that the superconductivity of MgB2 is due to very large phonon contribution
from its stretched lattice. The same two effects are obviously missing in AlB2
and hence it is not superconducting. DFT calculations demonstrated that for
MgB2 the majority of states come from Sigma and Pi 2p states of boron on the
other hand Sigma band at Fermi level for AlB2 is absent. This leads to a weak
electron phonon coupling and also to hole deficiency as Pi bands are known to
be of electron type and hence obviously the AlB2 is not superconducting. The
DFT calculations are consistent with the measured physical properties of the
studied borides, i.e., MgB2 and AlB2Comment: 16 pages Text + Figs: comments/suggestions welcome
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