arXiv subjects

S. Sarangi

Publications and source records attributed to S. Sarangi.

15 recordsLinked to original sources

arXiv2009

Pion correlations in Nuclear Matter

The saturation properties of the nuclear matter taking pion correlations into account is studied. We construct a Bogoliubov transformations for the pion pair operators and calculate the energy associated with the pion pairs. The pion dispersion relation is investigated. We next study the correlation energy due to one pion exchange in nuclear matter and neutron matter at random phase approximation using the generator coordinate method. The techniques of the charged pion correlations are discussed in the neutron matter calculations. We observe that there is no sign of the pion condensation in this model.

nucl-th↗

arXiv2008

Asymmetric nuclear matter : a variational approach

We discuss here a self-consistent method to calculate the properties of the cold asymmetric nuclear matter. In this model, the nuclear matter is dressed with s-wave pion pairs and the nucleon-nucleon (N-N) interaction is mediated by these pion pairs, $ω$ and $ρ$ mesons. The parameters of these interactions are calculated self-consistently to obtain the saturation properties like equilibrium binding energy, pressure, compressibility and symmetry energy. The computed equation of state is then used in the Tolman- Oppenheimer-Volkoff (TOV) equation to study the mass and radius of a neutron star in the pure neutron matter limit.

nucl-th↗

arXiv2007

Asymmetric Nuclear Matter with Pion Dressing

We discuss a self-consistent method to calculate the properties of cold asymmetric nuclear matter which is dressed with isoscalar scalar pion condensates. The nucleon-nucleon interaction is mediated by these pion pairs, omega- and rho- mesons. The parameters of these interactions are evaluated self-consistently using the saturation properties of nuclear matter like binding energy, pressure, compressibility and symmetry energy. The computed equation of state of pure neutron matter (PNM) is used to calculate mass and radius of a pure neutron star.

nucl-th↗

arXiv2007

Non-resonant microwave absorption studies on MgB2 thin film

We report on the presence of weak links in the intermetallic superconductor MgB2 from the non-resonant microwave absorption (NRMA) studies on the MgB2 thin film. The strong low field signal, which is the characteristics of weak links and the presence of hysteresis, which arises due to flux trapping, are observed. The studies are carried out as a function of temperature, microwave power, magnitude of modulation field and the range of field scan. Presence of weak links is discussed in terms of increase in the oxygen content of grain boundaries in the thin film and because of disorder effects arises due to the aging of the sample. The pinning of flux lines with the changes in sweeping direction leads to hysteresis in the NRMA signal and shows the contribution of flux line motion in the power absorption.

cond-mat.supr-con↗

arXiv2006

Equipartition of Current in Parallel Conductors on Cooling Through the Superconducting Transition

Our experiments show that for two or more pieces of a wire, of different lengths in general, combined in parallel and connected to a dc source, the current ratio evolves towards unity as the combination is cooled to the superconducting transition temperature Tc, and remains pinned at that value below it. This re-distribution of the total current towards equipartition without external fine tuning is a surprise. It can be physically understood in terms of a mechanism that involves the flux-flow resistance associated with the transport current in a wire of type-II superconducting material. It is the fact that the flux-flow resistance increases with current that drives the current division towards equipartition.

cond-mat.supr-con↗

arXiv2005

Non-resonant radio-frequency response in superconducting MgB2

Non-resonant radio frequency absorption (NRRA) of superconducting MgB2 pellet is investigated as a function of frequency, temperature, pressure, rf power, magnetic field, sample size and grain size. The NRRA signals of granular MgB2 samples show three separate and distinguish temperature dependent phase reversals, one of them is similar to the phase reversal occurs very often in high Tc superconductors at temperature few degrees below Tc, the other two are anomalous and occur at lower temperatures. Interpretation based on the assumptions in the line shape model indicates that the Josephson junctions are weakly coupled in MgB2 than other high temperature superconductors like YBa2Cu3O7, La1.9Sr0.1CuO4 and Bi2Sr2CaCu2O8. The samples show a significant magnetic field sweep hysteresis, which is a signature of trapped flux. The hysteresis behavior suggests that the vortices in the MgB2 are rather rigid. The effects of passing dc current through the sample on the NRRA signal patterns are discussed. It is seen that the transition temperature Tc measured using the NRRA technique decreases with decreasing sample size, whereas the Tc measured using ac susceptibility and temperature dependent resistivity is independent of the sample size. The results are discussed in terms of resistively shunted junction, flux flow and bond percolation models. Effect of MgO doping is analyzed and discussed. The intergranular coupling energy and the activation energy of MgB2 are calculated and compared with other high temperature superconductors. The effects of sintering on the weak links in MgB2 polycrystalline pellet are discussed.

cond-mat.supr-con↗

arXiv2005

AC loss in granular superconducting MgB2 at radio frequencies

AC losses of granular superconducting MgB2 were measured using non-resonant rf power absorption techniques. The presence of two band-gaps makes the temperature dependent ac loss pattern of MgB2 different from other high Tc superconductors like YBa2Cu3O and BSCCO-2223. Josephson junction decoupling plays a major role in MgB2 polycrystalline samples, both in the presence and absence of magnetic field. We studied the ac loss of MgB2 samples with different crystalline properties, grain sizes and pressures and sintered under different physical and chemical conditions. The effects of frequency and amplitude of ac current on the ac loss are discussed in detail.

cond-mat.supr-con↗

arXiv2005

Magnetic field dependence of kinetic inductance in Bi2Sr2Ca2Cu3O10 superconducting strip and its feasible applications

The kinetic inductance properties of a thin superconducting sample of BSCCO-2223 compound is studied. A strong dependence of kinetic inductance with the variation of applied magnetic field is found at low temperature in the superconducting state. The kinetic inductance of a thin superconductor increases linearly with increasing magnetic field. This behavior is used for designing a superconducting modulator circuit. This behavior can be useful for device applications and designing of superconducting electronic circuits like superconducting power amplifier, superconducting transmitter and superconducting ac current controller.

cond-mat.supr-con↗

arXiv2005

Angular dependence of direct rf power absorption studies in Bi2Sr2CaCu2O8 single crystals

We have studied rf power dissipation in Bi2Sr2CaCu2O8 single crystals as a function of angle between the applied field and c-axis of the crystal by using a novel technique of measuring direct rf power absorption. We observed two peaks in power absorption as a function of temperature. The amplitude and position of the first peak (peak A) change with orientation and attributed to Josephson junctions decoupling. The second peak (peak B) appears just below Tc has been attributed to the vortex motion and its amplitude depends on the angle of orientation.

cond-mat.supr-con↗

arXiv2005

Experimental Evidence for Zero DC Resistance of Superconductors

Even after nearly a century of discovery of superconductivity, there has been no direct experimental proof of the expected zero resistance of superconductors. Indeed, it has been believed that it is impossible to experimentally show that the resistance has fallen exactly to zero. In this work we demonstrate that the dc resistivity of a superconducting material below the transition temperature has to be exactly zero.

cond-mat.supr-con↗

arXiv2005

Charge order suppression and antiferromagnetic to ferromagnetic switch over in Pr_0.5Ca_0.5MnO_3 nanowires

We have prepared crystalline nanowires (diameter ~ 50 nm, length ~ a few microns) of the charge ordering manganite Pr_0.5Ca_0.5Mn_O3 using a low reaction temperature hydrothermal method and characterized them using X-ray diffraction, transmission electron microscopy, SQUID magnetometry and electron magnetic resonance measurements. While the bulk sample shows a charge ordering transition at 245 K and an antiferromagnetic transition at 175 K, SQUID magnetometry and electron magnetic resonance experiments reveal that in the nanowires phase, a ferromagnetic transition occurs at ~ 105 K. Further, the antiferromagnetic transition disappears and the charge ordering transition is suppressed. This result is particularly significant since the charge order in Pr_0.5Ca_0.5MnO_3 is known to be very robust, magnetic fields as high as 27 T being needed to melt it.

cond-mat.str-el↗

arXiv2005

Magnetic field dependent direct rf power absorption studies in Bi_2 Sr_2 CaCu_2 O_8 single crystals

Using a novel technique of measuring direct rf power dissipation, the temperature and magnetic field dependence of rf absorption in superconducting single crystals of Bi_2Sr_2CaCu_2O_8 is studied. An unexpected, larger than normal state, dissipation is observed below Tc, which exhibits peaks at magnetic field dependent temperatures. An explanation is provided for this observation in terms of the energy required to decouple the intrinsic Josephson junctions in the crystals.

cond-mat.supr-con↗

arXiv2005

A new method for direct rf power absorption studies in CMR materials and high T_c superconductors

The design, fabrication and performance of an apparatus for the measurement of direct rf power absorption in colossal magnetoresistive (CMR) and superconducting samples are described. The system consists of a self-resonant LC tank circuit of an oscillator driven by a NOT logic gate. The samples under investigation are placed in the core of the coil forming the inductance L and the absorbed power is determined from the measured change in the current supplied to the oscillator circuit. A customized low temperature insert is used to integrate the experiment with a commercial Oxford Instruments cryostat and temperature controller. The oscillator working in the rf range between 1 MHz to 25 MHz is built around an IC 74LS04. The temperature can be varied from 4.2 to 400 K and the magnetic field from 0 to 1.4 T. The apparatus is capable of measuring direct power absorption in CMR and superconducting samples of volume as small as 1/1000 cm^3 with a signal to noise ratio of 10:1. Further increase in the sensitivity can be obtained by summing the results of repeated measurements obtained at a given temperature. The system performance is evaluated by measuring the absorbed power in La_0.7 Sr_0.3 MnO_3 (LSMO) CMR manganite samples and superconducting Y Ba_2 Cu_3 O_7 (YBCO) samples at different rf frequencies. All operations during the measurements are automated using a computer with a menu-driven software system, user input being required only for the initiation of the measurement sequence.

cond-mat.supr-con↗

arXiv2005

Frequent JJ decoupling is the main origin of AC losses in the superconducting state

The origins of AC losses in the high Tc superconductors are not addressed adequately in literature. We found out, frequent Josephson Junction (JJ) decoupling (both intergranular and the interlayer) due to the flow of AC current is one of the main origins of the AC losses in high Tc superconductors. We have determined the AC losses in superconductors in the rf range by measuring the absolute value of non-resonant rf power absorbed by the samples. Our data shows that under certain conditions when both the number density of JJs present in the sample and the JJ critical current cross a threshold value, AC losses in the superconducting state keeps on increasing with decreasing temperature below Tc. The underlying mechanism is an interesting interplay of JJ coupling energy and the amplitude of rf voltage applied to the sample. The effect of an applied magnetic field, variation of rf frequency and temperature were studied in detail. To find out the exact relation between the JJ coupling energy, JJ number density, applied AC frequency, the amplitude of AC current and the AC losses in superconductors, we have studied samples of different crystalline properties, different grain sizes, pressurized with different pressure and sintered at different physical and chemical situations. The implementations of these results are discussed. These results have important implications for the understanding of the origin of AC losses and characterization of superconducting samples. In this paper we also extend the capability of the AC losses studies in superconductors for the characterization of materials for device applications.

cond-mat.supr-con↗

arXiv2005

Josephson junction decoupling is the main origin of AC resistivity in the superconducting state

The origin of AC resistivity in the high Tc superconductors is not addressed adequately in literature. We found out, Josephson Junction (JJ) decoupling is the main origin of the AC resistivity in high Tc superconductors. We have measured the AC resistivity in the superconductors in the low frequency range by measuring the complex AC impedance of superconducting YBa2Cu3O7 (YBCO) polycrystalline samples. Our data shows that under certain conditions when the number density of Josephson Junctions (JJ) present in the sample and the JJ critical current crosses a threshold value, AC resistivity in the superconducting state keeps on increasing with lowering temperature. The underlying mechanism is an interesting interplay of JJ coupling energy, amplitude of the supply AC voltage and the current applied to the superconducting sample. The effect of the applied AC current of different frequencies and the variation of temperature were studied in detail. To find out the exact relation between the JJ coupling energy, JJ number density, applied AC frequency, the amplitude of AC current and the AC resistivity in the superconductors, we have studied samples of different grain sizes, pressurized with different pressure and sintered at different physical situations. These results have important implications for the understanding of the origin of AC resistivity and characterization of superconducting samples. In this paper we also extend the capability of the AC impedance studies in superconductors for the characterization of materials for device applications.

cond-mat.supr-con↗