arXiv subjects

Shyam Sundar

Publications and source records attributed to Shyam Sundar.

23 records · Page 2Linked to original sources

arXiv2018

Magnetic irreversibility and pinning force density in the Mo$_{100-x}$Re$_x$ alloy superconductors

We have measured the isothermal field dependence of magnetization of the Mo$_{100-x}$Re$_x$ (15 $\leq$ x $\leq$ 48) alloys, and have estimated the critical current and pinning force density from these measurements. We have performed structural characterization of the above alloys using standard techniques, and analyzed the field dependence of critical current and pinning force density using existing theories. Our results indicate that dislocation networks and point defects like voids and interstitial imperfections are the main flux line pinning centres in the Mo$_{100-x}$Re$_x$ alloys in the intermediate fields, i.e., in the small bundle flux line pinning regime. In this regime, the critical current density is also quite robust against increasing magnetic field. In still higher fields, the critical current density is affected by flux creep. In the low field regime, on the other hand, the pinning of the flux lines seems to be influenced by the presence of two superconducting energy gaps in the Mo$_{100-x}$Re$_x$ alloys. This modifies the field dependence of critical current density, and also seems to contribute to the asymmetry in the magnetic irreversibility exhibited by the isothermal field dependence of magnetization

cond-mat.supr-con↗

arXiv2018

Vortex-glass transformation within the surface superconducting state of $β$-phase Mo$_{1-x}$Re$_x$ alloys

We have performed an experimental study on the temperature dependence of electrical resistivity $ρ$($T$) and heat capacity $C$($T$) of the Mo$_{1-x}$Re$_x$ $(x = 0.20, 0.25)$ alloy superconductors in different magnetic fields. In the presence of applied magnetic field, the electrical resistivity of these alloys go to zero at a temperature well above the bulk superconducting transition temperature obtained with the help of heat capacity measurements in the same magnetic field. Our study indicates the presence of surface superconducting state in these alloys, where the flux lines are pinned in the surface sheath of the superconductor. The configuration of the flux-lines (2d pancake-like) in the surface sheath is understood in the realm of the flux-spot model. Experimental evidence in support of the surface mixed-state state or "Kulik vortex-state" and the occurrence of a vortex-liquid to vortex-glass transition is presented.

cond-mat.supr-con↗

arXiv2018

Electronic structure of the Mo$_{1-x}$Re$_x$ alloys studied through resonant photoemission spectroscopy

We have studied the electronic structure of Mo rich Mo$_{1-x}$Re$_x$ alloys (0$\leq$ x $\leq$0.4) using valence band photoemission spectroscopy in the photon energy range 23-70 eV and density of states calculations. Comparison of the photoemission spectra with the density of states calculations suggests that with respect to the Fermi level $E_F$, the $d$ states lie mostly in the range 0 to -6 eV binding energy whereas $s$ states lie in the range -4 to -10 eV binding energy. We have observed two resonances in the photoemission spectra of each sample, one at about 35 eV photon energy and other at about 45 eV photon energy. Our analysis suggest that the resonance at 35 eV photon energy is related to the Mo $4p$-$5s$ transition and the resonance at 45 eV photon energy is related to the contribution from both the Mo $4p$-$4d$ transition (threshold: 42 eV) and Re $5p$-$5d$ transition (threshold: 46 eV). In the CIS plot, the resonance at 35 eV incident photon energy for binding energy features in the range of $E_F$ (B.E. = 0) to -5 eV becomes progressively less prominent with the increasing Re concentration $x$ and vanishes for $x>$ 0.2. The difference plots obtained by subtracting the valence band photoemission spectrum of Mo from that of Mo$_{1-x}$Re$_x$ alloys, measured at 47 eV photon energy, reveal that the Re $d$ like states appear near $E_F$ when Re is alloyed with Mo. These results indicate that interband $s$-$d$ interaction, which is weak in Mo, increases with the increasing $x$ and influences the nature of superconductivity in the alloys with higher $x$.

cond-mat.supr-con↗

arXiv2018

Study of the second magnetization peak and the pinning behaviour in Ba(Fe$_{0.935}$Co$_{0.065}$)$_2$As$_2

Isothermal magnetic field dependence of magnetization and the magnetic relaxation measurements were performed for $H$$\parallel$c axis on single crystal of Ba(Fe$_{0.935}$Co$_{0.065}$)$_2$As$_2$ pnictide superconductor having $T_c$ = 21.7 K. The second magnetization peak (SMP) for each isothermal $M(H)$ was observed in a wide temperature range from $T_c$ to the lowest temperature of measurement (2 K). Magnetic field dependence of relaxation rate $R(H)$, shows a peak (H$_{spt}$) between H$_{on}$ (onset of SMP in $M(H)$) and H$_p$ (peak field of SMP in $M(H)$), which is likely to be related with a vortex-lattice structural phase transition, as suggested in literature for similar sample. In addition, the magnetic relaxation measured for magnetic fields near H$_{spt}$ show some noise which might be the signature of the structural phase transition of the vortex lattice. Analysis of the magnetic relaxation data using Maley's criterion and the collective pinning theory suggests that the second magnetization peak (SMP) in the sample is due to the collective (elastic) to plastic creep crossover, which is also accompanied with a rhombic to square vortex lattice phase transition. Analysis of the pinning force density suggests single dominating pinning mechanism in the sample and is not showing the usual $δ$l and $δT_c$ nature of pinning. The critical current density ($J_c$) estimated using the Bean's critical state model is found to be 5 $\times$ 10$^5$ A/cm$^2$ at 2 K in the zero magnetic field limit. Surprisingly, the maximum in the pinning force density is not responsible for the maximum value of the critical current density in the sample.

cond-mat.supr-con↗

arXiv2018

Plastic pinning replaces collective pinning as the second magnetization peak disappears in the pnictide superconductor Ba-KFe$_2$As$_2$

We report a detailed study of isofield magnetic relaxation and isothermal magnetization measurements with $H$$\parallel$c on an underdoped Ba$_{0.75}$K$_{0.25}$Fe$_2$As$_2$ pnictide single crystal, with superconducting transition temperature $T_c$ = 28 K. The second magnetization peak (SMP) has been observed at temperatures below $T_c$/2 and vanished at higher temperatures. The observed behaviour of the SMP has been studied by measuring the magnetic field dependence of relaxation rate, $R(H)$ and by performing the Maley's analysis. The results suggest that the crossover from collective to plastic pinning observed in the SMP disappears above 12 K with plastic pinning replacing collective pinning. An interesting $H$-$T$ phase diagram is obtained. The critical current density ($J_c$) was estimated using Bean's model and found to be $\sim$ $3.4 \times 10^9$ A/m$^2$ at 10 K in the SMP region, which is comparable to an optimally doped Ba-KFe$_2$As$_2$ superconductor and may be exploited for potential technological applications. The pinning mechanism is found to be unconventional and does not follow the usual $δl$ and $δT_c$ pinning models, which suggest the intrinsic nature of pinning in the compound.

cond-mat.supr-con↗