A study on the kinetic arrest of magnetic phases in nanostructured Nd0.6Sr0.4MnO3thin films.

The Nd0.6Sr0.4MnO3(NSMO) manganite system exhibits a phase transition from paramagnetic insulating (PMI) to ferromagnetic metallic (FMM) state around its Curie temperatureTC= 270 K (bulk). The morphology-driven changes in the kinetically arrested magnetic phases in NSMO thin films with granular and crossed-nano-rod-type morphology are studied. The manganite thin films at low temperatures possess a magnetic glassy state arising from the coexistence of the high-temperature PMI and the low-temperature FMM phases. The extent of kinetic arrest and its relaxation was studied using the 'cooling and heating in unequal field (CHUF)' protocol in magnetic and magnetotransport investigations. The sample with rod morphology showed a large extent of phase coexistence compared to the granular sample. Further, with a field-cooling protocol, time-evolution studies were carried out to understand the relaxation of arrested magnetic phases across these morphologically distinct thin films. The results on the devitrification of the arrested magnetic state are interpreted from the point of view of homogeneous and heterogeneous nucleation of the ferromagnetic phase in the paramagnetic matrix with respect to temperature.

Enhancing the efficiency and cost-effectiveness of magnetocardiography by optimal channel selection for cardiac diagnosis.

Background. Magnetocardiography (MCG) is a non-invasive and non-contact technique that measures weak magnetic fields generated by the heart. It is highly effective in the diagnosis of heart abnormalities. Multichannel MCG provides detailed spatio-temporal information of the measured magnetic fields. While multichannel MCG systems are costly, usage of the optimal number of measurement channels to characterize cardiac magnetic fields without any appreciable loss of signal information would be economically beneficial and promote the widespread use of MCG technology.Methods. An optimization method based on the sequential selection approach is used to choose channels containing the maximum signal information while avoiding redundancy. The study comprised 40 healthy individuals, along with two subjects having ischemic heart disease and one subject with premature ventricular contraction. MCG measured using a 37 channel MCG system. After revisiting the existing methods of optimization, the mean error and correlation of the optimal set of measurement channels with those of all 37 channels are evaluated for different sets, and it has been found that 18 channels are adequate.Results. The chosen 18 optimal channels exhibited a strong correlation (0.99 ± 0.006) between the original and reconstructed magnetic field maps for a cardiac cycle in healthy subjects. The root mean square error is 0.295 pT, indicating minimal deviation.Conclusion. This selection method provides an efficient approach for choosing MCG, which could be used for minimizing the number of channels as well as in practical unforeseen measurement conditions where few channels are noisy during the measurement.

The effect of charged particle irradiation on the transport properties of bismuth chalcogenide topological insulators: a brief review.

Topological insulators (TIs) offer a novel platform for achieving exciting applications, such as low-power electronics, spintronics, and quantum computation. Hence, the spin-momentum locked and topologically nontrivial surface state of TIs is highly coveted by the research and development industry. Particle irradiation in TIs is a fast-growing field of research owing to the industrial scalability of the particle irradiation technique. Unfortunately, real three-dimensional TI materials, such as bismuth selenide, invariably host a significant population of charged native defects, which cause the ideally insulating bulk to behave like a metal, masking the relatively weak signatures of metallic topological surface states. Particle irradiation has emerged as an effective technique for Fermi energy tuning to achieve an insulating bulk in TI along with other popularly practiced methods, such as substitution doping and electrical gating. Irradiation methods have been used for many years to enhance the thermoelectric properties of bismuth chalcogenides, predominantly by increasing carrier density. In contrast, uncovering the surface states in bismuth-based TI requires the suppression of carrier density via particle irradiation. Hence, the literature on the effect of irradiation on bismuth chalcogenides extends widely to both ends of the spectrum (thermoelectric and topological properties). This review attempts to collate the available literature on particle irradiation-driven Fermi energy tuning and the modification of topological surface states in TI. Recent studies on particle irradiation in TI have focused on precise local modifications in the TI system to induce magnetic topological ordering and surface selective topological superconductivity. Promising proposals for TI-integrated circuits have also been put forth. The eclectic range of irradiation-based studies on TI has been reviewed in this manuscript.

Interfacial interaction driven enhancement in the colossal magnetoresistance property of ultra-thin heterostructure of Pr0.6Sr0.4MnO3 in proximity with Pr0.5Ca0.5MnO3.

The ultra-thin heterostructure of Pr0.6Sr0.4MnO3(15 nm)/Pr0.5Ca0.5MnO3(15 nm)/SrTiO3 fabricated using pulsed laser deposition technique exhibits the phase-segregated nature wherein the ferromagnetism of Pr0.6Sr0.4MnO3, and the antiferromagnetic state of Pr0.5Ca0.5MnO3 coexist in proximity. The observation of two exciting phenomena in the grown ultra-thin heterostructure, namely, the kinetic arrest and training effect, confirms its phase-segregated nature. The melting of the antiferromagnetic state in Pr0.5Ca0.5MnO3 into a ferromagnetic state due to the interfacial interaction arising from the magnetic proximity of the ferromagnetic clusters of Pr0.6Sr0.4MnO3 have been observed. A metal-insulator transition (TMIT) found at 215 K, close to its Curie temperature (TCurie) observed at 230 K, reveals a strong correlation between the electrical transport and the magnetization of the ultra-thin heterostructure. The electrical conduction in the high-temperature regime is explained in terms of the adiabatic small polaron hopping model. While the resistance in the metallic regime for temperatures above 100 K is contributed by the inelastic scattering due to the two-magnons, in the metallic regime below 100 K, the one-magnon inelastic scattering contribution is prevalent. An enhanced colossal magnetoresistance property near room temperature is obtained in the ultra-thin heterostructure arising from the proximity-driven interfacial interaction, making it a suitable candidate for technological applications near room temperature.

Effects of Pb assisted cation chemistry on the superconductivity of BSCCO thin films.

Thin films of Bi-based superconductors, highly c-axis oriented, were deposited on single crystalline substrates of SrTiO3, LaAlO3, and MgO using a pulsed laser deposition technique with a Bi-2223 target of nominal composition Bi1.75Pb0.25Sr2Ca2Cu3O10±Î´ prepared by the solid state reaction method. The effect of different deposition parameters on the evolution of the requisite properties in the thin films has been studied. These films have been characterized by X-ray diffraction to investigate their structural properties, scanning electron microscopy to understand the effect of ex situ annealing on the grain growth, and DC resistivity measurements to quantify their superconducting critical temperature. Furthermore, the chemical states of the constituent elements Bi, Pb, Sr, Ca, Cu and O were confirmed using X-ray photoelectron spectroscopy. This information has helped in deciphering the empirical stoichiometry of the films on each of the chosen substrates. We have also been able to comment on the influence made by the choice of the substrates on the mechanism of evolution of superconductivity based on the interplay of the cation chemistry between the substituent and the constituent elements. Thin films with superior superconducting properties were obtained on SrTiO3 substrates with 58% of Bi-2223 phase fraction yielding a superconducting transition temperature (TC,offset) of 107 K. Magnetotransport studies were performed on these films to quantify their superconducting upper critical field and to comprehend the pinning mechanism.

Arduino-Based Novel Hardware Design for Liquid Helium Level Measurement.

Liquid helium (LHe) is used as a cryogen in a variety of applications involving superconductivity and is routinely monitored for conducting low-temperature experiments. Thermoacoustic oscillations, which are inevitably present inside closed LHe containers, are utilized for level detection by sensing the vibrations at the warm end of a thin capillary tube inserted into the Dewar. The position of the capillary tube at which a sudden change occurs in these oscillations is manually sensed to identify the liquid level. The present work proposes a novel hardware design to identify the thermoacoustic oscillations in a reliable way using an accelerometer driven by an Arduino microcontroller. Further, an automated approach has been devised to quantify the rate of change of these helium oscillations to measure the LHe level. The proposed method has been tested during several trials on a 120 L and 100 L capacity Dewar using the proposed hardware, and the mean error in measuring the LHe level was calculated to be less than 1 cm in comparison with the gold standard niobium-titanium level sensor. The results encourage the use of the proposed method to evolve as a cost-effective alternative to the widely used superconducting level sensors in measuring LHe level.

Design and Implementation of a Discrete-Time Proportional Integral (PI) Controller for the Temperature Control of a Heating Pad.

Contact heat evoked potentials (CHEPs) are recorded from the brain by giving thermal stimulations through heating pads kept on the surface of the skin. CHEP signals have crucial diagnostic implications in human pain activation studies. This work proposes a novel design of a digital proportional integral (PI) controller based on Arduino microcontroller with a view to explore the suitability of an electric heating pad for use as a thermode in a custom-made, cost-effective CHEP stimulator. The purpose of PI controller is to set, regulate, and deliver desired temperatures on the surface of the heating pad in a user-defined pattern. The transfer function of the heating system has been deduced using the parametric system identification method, and the design parameters of the controller have been identified using the root locus technique. The efficiency of the proposed PI controller in circumventing the well-known integrator windup problem (error in the integral term builds excessively, leading to large transients in the controller output) in tracking the reference input and the controller effort (CE) in rejecting output disturbances to maintain the set temperature of the heating pad have been found to be superior compared with the conventional PI controller and two of the existing anti-windup models.

Quantum coherence phenomenon in disordered Bi2SeTe2 topological single crystal: effect of annealing.

We report a comparative magnetotransport study on pristine and annealed Bi2SeTe2 single crystals. The pristine sample shows a metallic trend from 300 to 180 K, and an insulating behavior for T < 180 K, whereas the annealed sample exhibits an insulating nature in the entire 4.2-300 K temperature range. Magnetoresistance (MR) of pristine and annealed samples reveals contrasting behaviour as a function of temperature (T) and magnetic field (B). At 4.2 K, the pristine sample shows weak antilocalization (WAL) behavior at low fields and transforms to weak localization (WL) behavior (negative MR) for B > 2.5 T. Further, the quantum MR behaviours seen at low temperature gradually transform to classical B 2 dependent upon increasing the temperatures. In contrast, the annealed sample shows a WAL at small field superimposed on a parabolic feature for B > ±4 T at low temperatures (T < 20 K). It shows a linear MR at intermediate temperatures (40 K < T < 100 K) and a parabolic MR at temperatures T > 100 K. Hall measurements on both samples exhibit a nonlinear behavior at 4.2 K pointing to the existence of two types of carriers with different mobility. The annealed sample also shows a drastic decrease in mobility by one order of magnitude and a reduction in Ioffe-Regel parameter (k F l) by a factor of ~3. Disorder-induced localization of bulk carriers and its coexistence with localization-immune surface carriers at low T leads to WAL and WL. MR observed in the annealed sample can be attributed to the presence of both quantum-classical contribution and has been analysed using the Hikami-Larkin-Nagaoka (HLN) equation.

Magneto-transport behaviour of Bi2Se3-xTex : role of disorder.

Magneto-resistance and Hall resistance measurements have been carried out in fast-cooled single crystals of Bi2Se3-xTex (x = 0 to 2) in 4-300 K temperature range, under magnetic fields up to 15 T. The variation of resistivity with temperature that points to a metallic behaviour in Bi2Se3, shows an up-turn at low temperatures in the Te doped samples. Magneto-resistance measurements in Bi2Se3 show clear signatures of Shubnikov-de Hass (SdH) oscillations that gets suppressed in the Te doped samples. In the Bi2SeTe2 sample, the magneto-resistance shows a cusp like positive magneto-resistance at low magnetic fields and low temperatures, a feature associated with weak anti-localisation (WAL), that crosses over to negative magneto-resistance at higher fields. The qualitatively different magneto-transport behaviour seen in Bi2SeTe2 as compared to Bi2Se3 is rationalised in terms of the disorder, through an estimate of the carrier density, carrier mobility and an analysis in terms of the Ioffe-Regel criterion with support from Hall Effect measurements. We demonstrate that by introducing Te, in the strongly disordered samples a smooth crossover of SdH and WAL can be seen in the Bi2Se3-xTex series, both of which provide signatures for the presence of topological surface states.

Thickness controlled proximity effects in C-type antiferromagnet/superconductor heterostructure.

Modulation of the superconducting state possessing a C-type antiferromagnetic phase in the Nd0.35Sr0.65MnO3/YBa2Cu3O7 heterostructure is investigated, with the Nd0.35Sr0.65MnO3 thickness (t) varying from 40 to 200 nm. Both the superconducting transition temperature and the upper critical field along the c-axis decrease with increasing t; while the in-plane coherence length increases from 2.0 up to 3.6 nm. Meanwhile, the critical current density exhibits a field-independent behavior, indicating an enhanced flux pinning effect. Furthermore, low-temperature spin canting induces a breakdown and re-entrance of the superconductivity, demonstrating a dynamic completion between the superconducting pairing and the exchange field. An unexpected colossal magnetoresistance is observed below the superconducting re-entrance temperature at t = 200 nm, which is attributed to the dominant influence of the exchange field over the pairing energy.

The pressure induced insulator to metal transition in FeSb2.

The evolution of the ground state properties of FeSb(2) has been investigated via temperature (4.2-300 K), magnetic field (0-12 T) and pressure (0-8.8 GPa) dependent electrical resistivity studies. The temperature dependence of the resistivity follows activated behavior in the high temperature (HT) regime (T > 60 K), while variable range hopping (VRH) dictates the transport in the intermediate temperature (IT) regime (10 K > T > 45 K) and power law behavior is observed in the low temperature (LT) regime (T < 10 K). The pressure profoundly affects the resistivity in all the temperature regimes. The energy gap (Δ) extracted in the HT regime initially increases with pressure and then decreases, while the VRH parameter T(0) deduced in the IT regime is seen to decrease monotonically and vanish beyond 5 GPa leading to an insulator to metal transition (MIT) on account of delocalization of the electronic states in the gap. The analysis of the logarithmic derivative of the conductivity indicates the MIT to occur at ~6 GPa. The magnetoresistivity is found to be positive. The analysis of the resistivity behavior under pressure and magnetic field indicates that the former induces delocalization, while the latter tends to assist localization of the defect states inside the gap of FeSb(2).

High pressure studies on RuIn3 single crystal.

Temperature- and pressure-dependent electrical resistivity studies have been carried out on RuIn(3) single crystal in the 4-300 K range at various pressures between 0 and 5 GPa. While intrinsic semiconducting behaviour is inferred at higher temperatures above 275 K, the low temperature resistivity is primarily dictated by impurity effects. An insulator to metal transition is observed in the low temperature regime around ∼ 1.2 GPa pressure. Band structure calculations show a monotonic decrease of energy gap from a value of 0.222 eV at 0 GPa to 0.167 eV at 8 GPa with increasing pressure, consistent with the experimental findings.