arXiv subjects

A. K. Pramanik

Publications and source records attributed to A. K. Pramanik.

30 records · Page 2Linked to original sources

arXiv2014

Temperature evolution of magnetic and transport behavior in 5\textit{d} Mott insulator Sr$_2$IrO$_4$: Significance of magneto-structural coupling

We have investigated the temperature evolution of magnetism and its interrelation with structural parameters in perovskite-based layered compound Sr$_2$IrO$_4$, which is believed to be a $J_{eff}$ = 1/2 Mott insulator. The structural distortion plays an important role in this material which induces a weak ferromagnetism in otherwise antiferromagnetically ordered magnetic state with transition temperature around 240 K. Interestingly, at low temperature below around 100 K, a change in magnetic moment has been observed. Temperature dependent x-ray diffraction measurements show sudden changes in structural parameters around 100 K are responsible for this. Resistivity measurements show insulating behavior throughout the temperature range across the magnetic phase transition. The electronic transport can be described with Mott's two-dimensional variable range hopping (VRH) mechanism, however, three different temperature ranges are found for VRH, which is a result of varying localization length with temperature. A negative magnetoresistance (MR) has been observed at all temperatures in contrast to positive behavior generally observed in strongly spin-orbit coupled materials. The quadratic field dependence of MR implies a relevance of a quantum interference effect.

cond-mat.str-el↗

arXiv2013

Finite-size effect on evolution of Griffiths phase in manganite nanoparticles

The finite-size effect on the evolution of Griffiths phase (GP) is studied using nanoparticles of half-doped manganite Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$ with different average particle sizes but having similar structural parameters. All the samples exhibit pronounced GP behavior. With reducing particle size, the Griffiths temperature remains almost unchanged but the characteristic critical temperature $T_c^R$ decreases and the GP properties are strengthened. It is noteworthy that the shift of $T_c^R$ follows finite-size scaling with the particle size revealing an exotic interplay between the GP properties and the sample dimension. This reinforces an earlier proposal of length-scale related evolution of GP.

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arXiv2013

Evolution of superconducting correlations within magnetic-field-decoupled CuO(2) layers of La(1.905)Ba(0.095)CuO(4)

We explore the evolution of superconductivity in La(2-x)Ba(x)CuO(4) with x=0.095 in magnetic fields of up to 35 T applied perpendicular to the CuO(2) planes. Previous work on this material has shown that perpendicular fields enhance both charge and spin stripe order within the planes. We present measurements of the resistivity parallel and perpendicular to the planes, as well as the Hall effect. Measurements of magnetic susceptibility for fields of up to 15 T applied both parallel and perpendicular to the planes provide complementary measures of the superconductivity. We show that fields sufficient to destroy pair tunneling between the planes do not disrupt the superconducting correlations within the planes. In fact, we observe an onset of large amplitude but phase disordered superconductivity within the planes at approximately 30 K that is remarkably insensitive to field. With further cooling, we observe a phase-transition-like drop in the in-plane resistivity to an apparent state of superconductivity, despite the lack of phase coherence between the layers. These observations raise interesting questions concerning the identification of the upper critical field, where pairing is destroyed, in underdoped cuprates.

cond-mat.supr-con↗

arXiv2012

Flux jumps and vortex pinning in Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ single crystals

In this work we present the results of the bulk magnetization measurements in a superconducting state of single crystals of Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$. The isothermal magnetic field ($H||c$ axis) dependent magnetization ($M$) loops exhibit a second peak (SP) or `fishtail effect', as well as remarkable flux jumps at low temperatures. The critical current density $J_c$ obtained from the $M(H)$ loops is rather high, of the order of 10$^6$ A/cm$^2$. The analysis of the temperature and field dependent $J_c$ implies that high $J_c$ is mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. Pronounced magnetic instabilities in terms of flux jumps depend strongly on temperature as well on the field sweep rate. The field for first flux jump as calculated from an adiabatic model, however, is much lower than the experimentally observed values, and this enhanced stability is attributed to a flux creep phenomenon. The analysis of field dependent magnetic relaxation data additionally supports a collective pinning model. The data further suggests that SP in M(H) is likely related to crossover in creep dynamics from elastic to plastic mechanism. We have constructed the vortex phase diagram on field-temperature plane.

cond-mat.supr-con↗

arXiv2011

Multi-gap superconductivity in single crystals of Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$: A calorimetric investigation

We investigate the electronic properties and the superconducting gap characteristics of a single crystal of hole-doped 122 Fe-pnictide Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ by means of specific heat measurements. The specific heat exhibits a pronounced anomaly around the superconducting transition temperature $T_c$ = 29.4 K, and a small residual part at low temperature. In a magnetic field of 90 kOe, the transition is broadened and $T_c$ is lowered insignificantly by an amount $\sim$ 1.5 K. We estimate a high electronic coefficient in the normal state with a value 57.5 mJ mol$^{-1}$ K$^2$, being consistent with hole-doped 122 compounds. The temperature-dependent superconducting electronic specific heat cannot be described with single-gap BCS theory under weak coupling approach. Instead, our analysis implies a presence of two s-wave like gaps with magnitudes $Δ_1(0)/k_BT_c$ = 1.06 and $Δ_2(0)/k_BT_c$ = 2.08 with their respective weights of 48% and 52%. While our results have qualitative similarities with other hole-doped 122 materials, the gap's magnitude and their ratio are quite different.

cond-mat.supr-con↗

arXiv2010

The dynamics of magnetization in phase separated manganite around half doping: a case study for Pr$_{0.5}$Sr$_{0.5}$Mn$_{0.925}$Ga$_{0.075}$O$_{3}$

We investigate the dynamics of magnetization in the phase separated (PS) state after introducing the quenched disorder at the Mn-site of a manganite around half doping. The compound, Pr$_{0.5}$Sr$_{0.5}$Mn$_{0.925}$Ga$_{0.075}$O$_{3}$, exhibits PS with the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) clusters where the size of the FM clusters is substantially reduced due to the disorder introduced by nonmagnetic Ga substitution. At low temperature, the system develops a new magnetic anomaly, which is marked by a peak in the zero field cooled magnetization. Detailed study of linear as well as nonlinear ac susceptibilities coupled with dc magnetization indicates that this peak arises due to the thermal blocking of nanometer size FM clusters demonstrating superparamagnetic behavior. The system, however, exhibits slow magnetic relaxation, aging effect, memory effect in both field cooled and zero field cooled magnetization below the blocking temperature. These imply the presence of collective behavior induced by the interaction between the clusters. Moreover, the magnetic relaxation measured with positive and negative temperature excursions exhibits asymmetric response suggesting that the dynamics in this phase separated system is accounted by the hierarchical model rather than the droplet model which are commonly used to describe the similar collective dynamics in glassy system.

cond-mat.str-el↗

arXiv2010

Fishtail effect and vortex dynamics in LiFeAs single crystals

We investigate the fishtail effect, critical current density ($J_c$) and vortex dynamics in LiFeAs single crystals. The sample exhibits a second peak (SP) in the magnetization loop only with the field $||$ c-axis. We calculate a reasonably high $J_c$, however, values are lower than in 'Ba-122' and '1111'-type FeAs-compounds. Magnetic relaxation data imply a strong pinning which appears not to be due to conventional defects. Instead, its behavior is similar to that of the triplet superconductor Sr$_2$RuO$_4$. Our data suggest that the origin of the SP may be related to a vortex lattice phase transition. We have constructed the vortex phase diagram for LiFeAs on the field-temperature plane.

cond-mat.supr-con↗

arXiv2010

The interparticle interaction and crossover in critical lines on field-temperature plane in Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$ nanoparticles

The magnetic properties and the effects of interparticle interaction on it have been studied in nanoparticles of half doped Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$. Three samples consisting of nanoparticles of different average particle sizes are synthesized to render the variation in interparticle interaction. Though all the samples crystallize in the same structure to that of their bulk compound, the low temperature ferromagnetic-antiferromagnetic transition, which is present in bulk compound, is not evident in the nanoparticles. Linear as well as nonlinear ac susceptibility coupled with dc magnetic measurements have shown the superparamagnetic behavior of these nanoparticles where the blocking temperature increases with the increasing particle size. Presence of interparticle interaction is confirmed from the temperature variation of coercive field and the analysis of frequency dependent ac susceptibility. We have identified the nature of this interaction to be of dipolar type, and show that its strength decreases with the increasing particle size. The effect of this dipolar interaction on magnetic properties is intriguing as the compounds exhibit crossover from de Almeida-Thouless to Gabay-Toulouse like critical lines on field-temperature plane above their respective interaction field. In agreement with theoretical prediction, we infer that this crossover is induced by the unidirectional anisotropy arising from interparticle interaction, and this is confirmed from the presence of exchange bias phenomenon.

cond-mat.str-el↗

arXiv2010

Critical current and vortex dynamics in single crystals of Ca(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$

We investigate the critical current density and vortex dynamics in single crystals of Ca(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ ($x$ = 0.051, 0.056, 0.065, and 0.073). The samples exhibit different critical temperatures and superconducting phase fractions. We show that in contrast to their Ba-based counterpart, the crystals do not exhibit a second peak in the field dependence of magnetization. The calculated composition-dependent critical current density ($j_{\rm c}$) increases initially with Co doping, maximizing at $x$ = 0.065, and then decreases. This variation in $j_{\rm c}$ follows the superconducting phase fractions in this series. The calculated $j_{\rm c}$ shows strong temperature dependence, decreasing rapidly upon heating. Magnetic relaxation measurements imply a nonlogarithmic dependence on time. We find that the relaxation rate is large, reflecting weak characteristic pinning energy. The analysis of temperature- and field-dependent magnetic relaxation data suggests that vortex dynamics in these compounds is consistent with plastic creeping rather than the collective creep model, unlike other 122 pnictide superconductors. This difference may cause the absence of the second peak in the field dependent magnetization of Ca(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$.

cond-mat.supr-con↗

arXiv2008

Phase separation and the effect of quenched disorder in $Pr_{0.5}Sr_{0.5}MnO_3$

The nature of phase separation in $Pr_{0.5}Sr_{0.5}MnO_3$ has been probed by linear as well as nonlinear magnetic susceptibilities and resistivity measurements across the 2nd order paramagnetic to ferromagnetic transition ($T_C$) and 1st order ferromagnetic to antiferromagnetic transition ($T_N$). We found that the ferromagnetic (metallic) clusters, which form with the onset of long-range order in the system at $T_C$, continuously decrease their size with the decrease in temperature and coexist with non-ferromagnetic (insulating) clusters. These non-ferromagnetic clusters are identified to be antiferromagnetic. Significantly, it is shown that they do not arise because of the superheating effect of the lower temperature 1st order transition. Thus reveals unique phase coexistence in a manganite around half-doping encompassing two long-range order transitions. Both the ferromagnetic and antiferromagnetic clusters form at $T_C$ and persist much below $T_N$. Substitution of quenched disorder (Ga) at Mn-site promotes antiferromagnetism at the cost of ferromagnetism without adding any magnetic interaction or introducing any significant lattice distortion. Moreover, increase in disorder decreases the ferromagnetic cluster size and with 7.5% Ga substitution clusters size reduces to the single domain limit. Yet, all the samples show significant short-range ferromagnetic interaction much above $T_C$. Resistivity measurements also reveal the novel phase coexistence identified from the magnetic measurements. It is significant that, increase in disorder up to 7.5% increases the resistivity of the low temperature antiferromagnetic phase by about four orders.

cond-mat.str-el↗

arXiv2006

Coexisting tuneable fractions of glassy and equilibrium long-range-order phases in manganites

Antiferromagnetic-insulating(AF-I) and the ferromagnetic-metallic(FM-M) phases coexist in various half-doped manganites over a range of temperature and magnetic field, and this is often believed to be an essential ingredient to their colossal magnetoresistence. We present magnetization and resistivity measurements on Pr(0.5)Ca(0.5)Mn(0.975)Al(0.025)O(3) and Pr(0.5)Sr(0.5)MnO(3) showing that the fraction of the two coexisting phases at low-temperature in any specified measuring field H, can be continuously controlled by following designed protocols traversing field-temperature space; for both materials the FM-M fraction rises under similar cooling paths. Constant-field temperature variations however show that the former sample undergoes a 1st order transition from AF-I to FM-M with decreasing T, while the latter undergoes the reverse transition. We suggest that the observed path-dependent phase-separated states result from the low-T equilibrium phase coexisting with supercooled glass-like high temperature phase, where the low-T equilibrium phases are actually homogeneous FM-M and AF-I phases respectively for the two materials.

cond-mat.str-el↗

arXiv2006

Relating supercooling and glass-like arrest of kinetics for phase separated systems: studies on doped CeFe$_2$ and (La,Pr,Ca)MnO$_3$

Coexisting ferromagnetic and antiferromagnetic phases over a range of temperature as well as magnetic field have been reported in many materials of current interest, showing disorder-broadened 1st order transitions. Anomalous history effects observed in magnetization and resistivity are being explained invoking the concepts of kinetic arrest akin to glass transitions. From magnetization measurements traversing novel paths in field-temperature space, we obtain the intriguing result that the regions of the sample which can be supercooled to lower temperatures undergo kinetic-arrest at higher temperatures, and vice versa. Our results are for two diverse systems viz. the inter-metallic doped CeFe$_2$ which has an antiferromagnetic ground state, and the oxide La-Pr-Ca-Mn-O which has a ferromagnetic ground state, indicating the possible universality of this effect of disorder on the widely encountered phenomenon of glass-like arrest of kinetics.

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