12 research outputs found
Thermoelectric instability induced by a single pulse and alternating current in superconducting tapes of second generation
We have studied the instability of the current flow in a superconducting tape of the second generation and the transition of the tape into the resistive state. Contrary to usually studied quasisteady regimes of the instability development, we consider here the adiabatic case of fast sample heating. Two kinds of measurements of the current-voltage characteristics (CVC) have been performed, specifically, using the tape excitation by a single sineshaped current pulse I(t)=I₀sin(ωt) with different amplitudes I₀ and by a continuous ac current flow. The main results were obtained for the current amplitudes I₀ exceeding the critical current value Ic . We have found that the dynamic CVC are practically reversible for low amplitudes, whereas they become irreversible and assume the N -shaped form for higher current amplitudes. The dynamic CVC are found to change radically if the dissipated energy attains some threshold value Wth which is equal to about 5 mJ/cm for our tapes. Once achieving this energy, the tape transits to the resistive state due to a normal domain formation. The development of instability for a continuous ac current flow was studied for a relatively small amplitude when the energy dissipated per one half-cycle is much lower than Wth. Even in this case, the tape transition to the resistive state occurs owing to an effect of energy accumulation (heat pumping). Due to this pumping, the transition takes place after a definite number of ac current periods when the total accumulated energy reaches the same threshold value Wth. The specific features of dynamic CVC are qualitatively interpreted within an approach where the appearance of the resistive domain is taken into account. Estimations performed on the basis of the CVC agree well with our experimental data. The results obtained can be useful for the design of superconducting fault current limiters
A Real Space Description of Field Induced Melting in the Charge Ordered Manganites: II. the Disordered Case
We study the effect of A site disorder on magnetic field induced melting of
charge order (CO) in half doped manganites using a Monte-Carlo technique.
Strong A-site disorder destroys CO even without an applied field. At moderate
disorder, the zero field CO state survives but has several intriguing features
in its field response. Our spatially resolved results track the broadening of
the field melting transition due to disorder and explain the unusual dependence
of the melting scales on bandwidth and disorder. In combination with our
companion paper on field melting of charge order in clean systems we provide an
unified understanding of CO melting across all half doped manganites.Comment: 9 pages, pdflatex, 10 embedded png fig
A Real Space Description of Magnetic Field Induced Melting in the Charge Ordered Manganites: I. The Clean Limit
We study the melting of charge order in the half doped manganites using a
model that incorporates double exchange, antiferromagnetic superexchange, and
Jahn-Teller coupling between electrons and phonons. We primarily use a real
space Monte Carlo technique to study the phase diagram in terms of applied
field and temperature , exploring the melting of charge order with
increasing and its recovery on decreasing . We observe hysteresis in
this response, and discover that the `field melted' high conductance state can
be spatially inhomogeneous even without extrinsic disorder. The hysteretic
response plays out in the background of field driven equilibrium phase
separation. Our results, exploring , , and the electronic parameter
space, are backed up by analysis of simpler limiting cases and a Landau
framework for the field response. This paper focuses on our results in the
`clean' systems, a companion paper studies the effect of cation disorder on the
melting phenomena.Comment: 16 pages, pdflatex, 11 png fig
Possible spin-glass state in SmSr-manganites as the origin of the magnetization jumps
Magnetic field-induced step-like changes in magnetization of Sm1-xSrxMnO3 manganites were studied. A strong dependence of these features on the magnetic-field sweep rate was observed. The notable overheating of the sample, starting exactly at the start of the magnetic transition, was observed upon the transition. We suggest that quenched disorder leads to the formation of an inhomogeneous (spin-glass-like) state and to subsequent magnetization jumps driven by a release of latent heat.
Inhomogeneous ferromagnetic insulating state and isotope effect in Pr1-xCaxMnO3
The magnetic and transport properties of the Pr1−xCaxMnO3 ceramic samples were studied with the main emphasis on the ferromagnetic insulating (FI) state. A non-monotonic dependence of the Curie temperature TC on x with the maximum corresponding to x=0.25 was observed. A pronounced effect of 16O→18O isotope substitution on TC was found. The samples exhibited a noticeable drop in the activation energy for resistivity at TC and negative magnetoresistance. A special attention was paid to possible manifestations of inhomogeneity in the FI state