arXiv subjects

B. Joseph

Publications and source records attributed to B. Joseph.

29 records · Page 2Linked to original sources

arXiv2011

Local structural studies of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ using atomic pair distribution function analysis

Systematic local structural studies of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ system are undertaken at room temperature using atomic pair distribution function (PDF) analysis. The local structure of the Ba$_{1-x}$K$_x$Fe$_2$As$_2$ is found to be well described by the long-range structure extracted from the diffraction experiments, but with anisotropic atomic vibrations of the constituent atoms ($U_{11}$ = $U_{22} \ne U_{33}$). The crystal unit cell parameters, the FeAs$_4$ tetrahedral angle and the pnictogen height above the Fe-plane are seen to show systematic evolution with K doping, underlining the importance of the structural changes, in addition to the charge doping, in determining the properties of Ba$_{1-x}$K$_x$Fe$_2$As$_2$.

cond-mat.supr-con↗

arXiv2010

A study of electronic structure of FeSe$_{1-x}$Te$_{x}$ chalcogenides by Fe and Se K-edge x-ray absorption near edge structure measurements

Fe K-edge and Se K-edge x-ray absorption near edge structure (XANES) measurements are used to study FeSe$_{1-x}$Te$_{x}$ electronic structure of chalcogenides. An intense Fe K-edge pre-edge peak due to Fe 1s$\to$3d (and admixed Se/Te $p$ states) is observed, showing substantial change with the Te substitution and X-ray polarization. The main white line peak in the Se K-edge XANES due to Se 1s $\to$ 4p transition appear similar to the one expected for Se$^{2-}$ systems and changes with the Te substitution. Polarization dependence reveals that unoccupied Se orbitals near the Fermi level have predominant $p_{x,y}$ character. The results provide key information on the hybridization of Fe $3d$ and chalcogen $p$ states in the Fe-based chalcogenide superconductors.

cond-mat.supr-con↗

arXiv2010

Structural phase transition and superlattice misfit strain of RFeAsO (R = La, Pr, Nd and Sm)

The tetragonal-to-orthorhombic structural phase transition (SPT) in LaFeAsO (La-1111) and SmFeAsO (Sm-1111) single crystals measured by high resolution x-ray diffraction is found to be sharp while the RFeAsO (R=La, Nd, Pr, Sm) polycrystalline samples show a broad continuous SPT. Comparing the polycrystalline and the single crystal 1111 samples, the critical exponents of the SPT are found to be the same while the correlation length critical exponents are found to be very different. These results imply that the lattice fluctuations in 1111 systems change in samples with different surface to volume ratio that is assigned to the relieve of the temperature dependent superlattice misfit strain between active iron layers and the spacer layers in 1111 systems. This phenomenon that is missing in the AFe2As2 (A=Ca, Sr, Ba) "122" systems, with the same electronic structure but different for the thickness and the elastic constant of the spacer layers, is related with the different maximum superconducting transition temperature in the 1111 (55 K) versus 122 (35 K) systems and implies the surface reconstruction in 1111 single crystals.

cond-mat.supr-con↗

arXiv2010

Evidence of local structural inhomogeneity in FeSe(1-x)Te(x) from extended x-ray absorption fine structure

Local structure of FeSe(1-x)Te(x) has been studied by extended x-ray absorption fine-structure (EXAFS) measurements as a function of temperature. Combination of Se and Fe K edge EXAFS has permitted to quantify the local interatomic distances and their mean-square relative displacements. The Fe-Se and Fe-Te bond lengths in the ternary system are found to be very different from the average crystallographic Fe-Se/Te distance, and almost identical to the Fe-Se and Fe-Te distances for the binary FeSe and FeTe systems, indicating distinct site occupation by the Se and Te atoms. The results provide a clear evidence of local inhomogeneities and coexisting electronic components in the FeSe1-xTex, characterized by different local structural configurations, with direct implication on the fundamental electronic structure of these superconductors.

cond-mat.supr-con↗

arXiv2009

Observation of a Universal Aggregation Mechanism and a Possible Phase Transition in Au Sputtered by Swift Heavy Ions

Two exponents, $δ$, for size distribution of $n$-atom clusters, $Y(n)\sim n^{-δ}$, have been found in Au clusters sputtered from embedded Au nanoparticles under swift heavy ion irradiation. For small clusters, below 12.5 nm in size, $δ$ has been found to be 3/2, which can be rationalized as occurring from a steady state aggregation process with size independent aggregation. For larger clusters, a $δ$ value of 7/2 is suggested, which might come from a dynamical transition to another steady state where aggregation and evaporation rates are size dependent. In the present case, the observed decay exponents do not support any possibility of a thermodynamic liquid-gas type phase transition taking place, resulting in cluster formation.

cond-mat.mtrl-sci↗

arXiv2009

Local structure of REFeAsO (RE=La, Pr, Nd, Sm) oxypnictides studied by Fe K-edge EXAFS

Local structure of REOFeAs (RE=La, Pr, Nd, Sm) system has been studied as a function of chemical pressure varied due to different rare-earth size. Fe K-edge extended X-ray absorption fine structure (EXAFS) measurements in the fluorescence mode has permitted to compare systematically the inter-atomic distances and their mean square relative displacements (MSRD). We find that the Fe-As bond length and the corresponding MSRD hardly show any change, suggesting the strongly covalent nature of this bond, while the Fe-Fe and Fe-RE bond lengths decrease with decreasing rare earth size. The results provide important information on the atomic correlations that could have direct implication on the superconductivity and magnetism of REOFeAs system, with the chemical pressure being a key ingredient.

cond-mat.supr-con↗

arXiv2009

RE L$_{3}$ X-ray absorption study of REO$_{1-x}$F$_{x}$FeAs (RE=La, Pr, Nd, Sm) oxypnictides

Rare-earth L$_{3}$-edge X-ray absorption near edge structure (XANES) spectroscopy has been used to study REOFeAs (RE=La, Pr, Nd, Sm) oxypnictides. The Nd L$_{3}$ XANES due to 2p$_{3/2}\to5ε$d transition shows a substantial change in both white line (WL) spectral weight and the higher energy multiple scattering resonances with the partial substitution of O by F. A systematic change in the XANES features is seen due to varying lattice parameters with ionic radius of the rare earth. On the other hand, we hardly see any change across the structural phase transition. The results provide timely information on the local atomic correlations showing importance of local structural chemistry of the REO spacer layer and interlayer coupling in the competing superconductivity and itinerant striped magnetic phase of the oxypnictides.

cond-mat.supr-con↗

arXiv2009

The mechanism of ion induced amorphization in Si

Some results on damage build up in, and amorphization of, Si, induced by 25-30 keV Al$_5^-$, Si$_5^-$ and Cs$^-$ ions, at room temperature, are reported. We show that at low energy, amorphization is a nucleation and growth process, based on the direct impact mechanism. With an Avrami exponent $\sim 1.6$, the growth towards amorphization seems to be diffusion limited. A transition to a completely amorphized state is indicated at a dose exceeding 17 eV/atom, which is higher than 6-12 eV/atom as predicted by simulations. The observed higher threshold could be due to temperature effects although an underestimation of keV-energy recoils, in simulation, may not be ruled out.

cond-mat.mtrl-sci↗

arXiv2008

Study of low energy Si$_5^-$ and Cs$^-$ implantation induced amorphization effects in Si(100)

The damage growth and surface modifications in Si(100), induced by 25 keV Si$_5^-$ cluster ions, as a function of fluence, $ϕ$, has been studied using atomic force microscopy (AFM) and channeling Rutherford backscattering spectrometry (CRBS). CRBS results indicate a nonlinear growth in damage from which it has been possible to get a threshold fluence, $ϕ_0$, for amorphization as $2.5\times 10^{13}$ ions-cm$^{-2}$. For $ϕ$ below $ϕ_0$, a growth in damage as well as surface roughness has been observed. At a $ϕ$ of $1\times 10^{14}$ ions-cm$^{-2}$, damage saturation coupled with a much reduced surface roughness has been found. In this case a power spectrum analysis of AFM data showed a significant drop, in spectral density, as compared to the same obtained for a fluence, $ϕ< ϕ_0$. This drop, together with damage saturation, can be correlated with a transition to a stress relaxed amorphous phase. Irradiation with similar mass Cs$^-$ ions, at the same energy and fluence, has been found to result in a reduced accumulation of defects in the near surface region leading to reduced surface features.

cond-mat.mtrl-sci↗

arXiv2005

Magnetism in C60 Films Induced by Proton Irradiation

It is shown that polycrystalline fullerene thin films on hydrogen passivated Si(111) substrates irradiated by 2 MeV protons displays ferromagnetic-like behavior at 5 K. At 300 K both the pristine and the irradiated film show diamagnetic behavior. Magnetization data in the temperature range 2 - 300 K, in 1 Tesla applied field, for the irradiated film shows much stronger temperature dependence compared to the pristine film. Possible origin of ferromagnetic-like signals in the irradiated films are discussed.

cond-mat.mtrl-sci↗

arXiv2005

Energy dependent sputtering of nano-clusters from a nanodisperse target and embedding of nanoparticles into a substrate

Au nanoparticles, prepared by thermal evaporation under high vacuum condition on Si substrate, are irradiated with Au ions at different ion energies. During ion irradiation, embedding of nanoparticles as well as ejection of nano-clusters is observed. Ejected particles (usually smaller than those on the Si substrate) due to sputtering are collected on carbon-coated transmission electron microscopy (TEM) grids. Both the TEM grids and the ion-irradiated samples are analyzed with TEM. Unirradiated as well as irradiated samples are also analyzed by Rutherford backscattering spectrometry (RBS). In the case of low energy (32 keV) ions, where the nuclear energy loss is dominant, both sputtering and embedding are less compared to medium energy (1.5 MeV). In the high energy regime (100 MeV), where the electronic energy loss is dominant, sputtering is maximum but practically there is no embedding. Ion bombardment of surfaces at an angle with respect to the surface-normal produces enhanced embedding compared to normal-incidence bombardment. The depth of embedding increases with larger angle of incidence. Au nanoparticles after ion irradiation form embedded gold-silicide. Size distribution of the sputtered Au clusters on the TEM grids for different ion energy regimes are presented.

cond-mat.mtrl-sci↗