J. Prakash

Potassium fluoride doped LaOFeAs multi-band superconductor: Evidence of extremely high upper critical field

The recently discovered superconductors based on oxypnictides have rekindled the search for new non-copper based high-Tc superconductors. After the initial report by Kamihara et al. of superconductivity in (La-O/F-Fe-As)-based compounds, there have been several reports on these new superconductors but the synthesis of pure phases of these oxypnictides has remained a challenge. Here we describe a new methodology of synthesizing these oxypnictide superconductors with the commonly available potassium fluoride (KF) as a source of fluorine instead of the expensive LaF3. This route also allows the substitution of potassium at lanthanum sites which leads to an increase in the upper critical field as would be expected for a multi-band superconductor. We also report the highest-Tc (onset) of 28.50 K and highest upper critical field at ambient pressure in the family of La-based oxypnictides.

Enhancement in transition temperature and upper critical field of CeO0.8F0.2FeAs by yttrium doping

We report significant enhancement in superconducting properties of yttrium substituted Ce1−xYxOFFeAs superconductors. The polycrystalline samples were prepared by two step solid state reaction technique. X-ray diffraction confirmed tetragonal ZrCuSiAs structure with decrease in both a and c lattice parameters on increasing yttrium substitution (with fixed F content). With smaller ion Y in place of Ce, the transition temperature increased by 6 K. Yttrium doping also lead to higher critical fields as well as broader magnetization loops, particularly at elevated temperature.

Enhancement of the superconducting transition temperature and upper critical field of LaO0.8F0.2FeAs with antimony doping

We report the synthesis and characterization of the antimony-doped oxypnictide superconductor, LaO0.8F0.2FeAs1?xSbx (x = 0.05 and 0.10). The parent compound LaOFeAs with fluorine doping exhibits superconductivity at the maximum transition temperature ~28.5?K (at ambient pressure). Here we partially substitute As by Sb (LaO0.8F0.2FeAs1?xSbx) and observe enhancement of the transition temperature to 30.1?K. This is one rare case where Tc increases with doping in the conducting layer (FeAs) and this leads to the highest transition temperature in any La-based oxypnictide. X-ray diffraction and energy-dispersive x-ray analysis measurements confirm the phase purity of the samples and the presence of Sb. The magneto-resistance measurements show that the value of the upper critical field Hc2(0) is about 73?T, corresponding to a coherence length (?GL) of 22??. The Seebeck coefficient measurements indicate electron transport with a strong contribution from electron?electron correlation. These results provide interesting insight into the origin of superconductivity in this novel series of compounds.

Pressure induced structural transition and enhancement of superconductivity in Co doped CeFeAsO

The superconducting CeFe1−xCoxAsO (Co=0.1) oxyarsenide with a transition temperature (Tc) 11.4 K has been investigated by in situ high pressure synchrotron x-ray diffraction, magnetization, and resistivity measurements. The experiments performed at 10 K up to 6 GPa and at room temperature (RT) up to 55 GPa indicate large anisotropic lattice compression. A pressure induced structural change to a collapsed tetragonal structure is observed above 10 GPa at RT. We report here the enhancement of Tc from 11.4 to 12.3 K with a small increase in pressure up to 0.4 GPa and is first observed in an electron doped Ce-1111 system.

Temperature-dependent Raman study of a CeFeAsO 0.9 F 0.1 superconductor: crystal field excitations, phonons and their coupling

We report temperature-dependent Raman spectra of CeFeAsO(0.9)F(0.1) from 4 to 300 K in the spectral range of 60-1800 cm(-1) and interpret them using estimates of phonon frequencies obtained from first-principles density functional calculations. We find evidence for strong coupling between the phonons and crystal field excitations; in particular the Ce(3 + ) crystal field excitation at 432 cm(-1) couples strongly with the E(g) oxygen vibration at 389 cm(-1). Below the superconducting transition temperature, the phonon mode near 280 cm(-1) shows softening, signaling its coupling with the superconducting gap. The ratio of the superconducting gap to T(c), thus estimated to be ~10, suggests CeFeAsO(0.9)F(0.1) to be a strong coupling superconductor. In addition, two high frequency modes observed at 1342 and 1600 cm(-1) are attributed to electronic Raman scattering from the (x(2)-y(2)) to xz /yz d-orbitals of Fe.

Chiral adsorption studied by field emission techniques: the case of alanine on platinum

Chirality at surfaces has become an active research area targeting possible applications of enantioselective separation or detection. Here, we propose a promising route for obtaining fundamental understanding of the enantiospecific interaction of chiral molecules on metal surfaces using field emission techniques, i.e. field ion microscopy (FIM) and field electron microscopy (FEM). These techniques have been chosen for their particular advantages in exposing a wide range of structurally different facets in one atomically resolved picture. This diversity allows the study of interactions between a chemical species and a number of facets during the adsorption process on the same sample. In the present study, we focused on the adsorption of alanine on platinum surfaces modelled as sharp tips and imaged by FIM and FEM. Our results show a clear preference of the alanine to adsorb on chiral facets. Although the 20 A resolution of the FEM does not allow the edges of the facets of interest to be unraveled, the net images after exposure to one enantiomer of alanine show the occurrence of enantioselective adsorption over the sector of the same chiral symmetry. The results show that L-alanine has a strong tendency to adsorb onto R facets. Conversely, D-alanine adsorbs onto S facets.

Nanoscale Chiral Recognition Using Field Ion and Field Emission Microscopy

Chirality at surfaces has become an active research area targeting possible applications of enantioselective separation or detection. In this context, significant success has been achieved these past decades by developing new methods for a better understanding of enantiospecific interactions of chiral adsorbate with surfaces. Here, we propose a promising route for obtaining fundamental understanding of enantiospecific interaction of chiral molecules on metal surfaces using field emission based techniques. This technique has been chosen for its particular advantage to expose a wide range of structurally different facets in one atomically resolved picture. This diversity allows us to screen with one sample the interactions between a chemical species and a number of facets during the adsorption process.In the present study, we envisage the adsorption of alanine on platinum surfaces modelled as sharp tip using field emission and field microscopy along with theoretical studies using density functional theory.In order to observe the adsorption pattern of the adsorption, the Pt surface is kept at temperatures between 150 and 300K, and then exposed to vapors of D or L-alanine. The in-situ FEM is filmed with a high-speed camera. The whole process is also performed in absence of alanine molecules to perform reference experiments. The subtraction of the results before and after the adsorption gives us a net image of the adsorption sites. Our results show a clear preference of the alanine to adsorb on chiral facets. Although the 20 A resolution of the FEM does not allow to unravel the edges of the facets of interest, the net images after exposures to one enantiomer of alanine show the occurrence of an enantioselective adsorption over sector of the same chiral symmetry. The results show that L-alanine has a strong tendency to adsorb onto R facets. Conversely, D-alanine adsorbs onto the S facets.

Study of upper critical field in 1111-ferropnictide superconductors

In this paper we have shown the doping effects of elements Y, F and Sb in place of RE (rare earth elements), O and As respectively in 1111-ferropnictides. We study the behavior of upper critical field and superconducting transition temperatures as a function of doping. In Ce and La based compounds maximum upper critical field was found to be 147 T and 122 T respectively. Correspondingly, the maximum transition temperature was found to be 48.6 K and 34.8 K.

High Upper Critical Field in Potassium Fluoride Doped LaOFeAs Superconductor

We report a new methodology for synthesizing oxypnictide superconductors with the commonly available potassium fluoride (KF) as a source of fluorine instead of expensive LaF3. This results in simultaneous doping of potassium at lanthanum sites and leads to a three-fold increase in the upper critical field. We report the Tc (onset) of 28.50 K by F-doping and highest upper critical field (~122 T) at ambient pressure in the family of La-based oxypnictides. To study the contribution from inter and intra-granular current density, we compare remanent magnetization measurements on La1.03O0.9F0.2FeAs and La0.8K0.2O0.8F0.2FeAs superconductors that show only one major peak indicating substantial electromagnetic granularity. The Seebeck effect (S) with respect to temperature has negative sign and exhibits unconventional temperature dependence. Analysis of resistivity transition broadening under magnetic field reveals the signatures of Arrhenius behavior and the calculated activation energy follows power-law dependence with respect to magnetic field.

Understanding Human Driving Behavior through Computational Cognitive Modeling

As per an article in The Economist, someone, somewhere, dies in a road crash every 30 seconds, and about 10 people are seriously injured. Currently, there are about 1.3 million global deaths per year due to road accidents. Most of these deaths and injuries are caused by either factors that are internal to the driver (e.g., driving experience), or due to factors that are external to the driver (e.g., track complexity). However, currently little is known on how these factors influence human driving behavior. In this research, we investigate the role of an external factor (track complexity) on human driving behavior through computational cognitive modeling. Eighteen human participants were asked to drive on two tracks of the same length: simple (4 curves; N=9) and complex (20 curves; N=9). Later, we used two computational models to fit the human steering control data: an existing near-far-point model and a new heuristic model involving tangent and car-axis angles and a position-correction term. Our modeling results show that the fit of the heuristic model to human data on the simple and complex tracks was superior compared to that by the near-far-point model. We highlight the implications of our model results on human driving behavior.