publications

Technical Publications

Neutral velocity distribution at a negatively biased electrode in a collisional ion sheath

S. Mukherjee

Abstract:

This paper reports on the planar one-dimensional theoretical estimation of neutral velocity distribution at a negatively biased electrode. The pressure is chosen to be high (~ few Torr) so that the ion sheath is collisional with charge exchange being the dominant collision mechanism. This collision leads to a distribution of ion velocities at the electrode and also generates a large number of energetic neutrals inside the ion sheath that bombards the electrode. Depending on the place of their generation inside the ion sheath, the neutrals have a distribution of velocities at the electrode. The theory indicates that the peak of the neutral velocity distribution shifts to lower velocities as pressure increases. It also indicates that in such collisional ion sheaths, the neutrals deposit more momentum and kinetic energy on the negatively biased electrode than the ions.

A study of martensitic stainless steel AISI 420 modified using plasma nitriding

I. Alphonsa, A. Chainani, P.M. Raole, B. Ganguli and P.I. John

Surface and Coatings Technology , 150 (2002) Pg: 263 -268.

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We studied martensitic stainless steel AISI 420, modified using glow discharge plasma nitriding. Microhardness measurements, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) are used to investigate the surface microhardness, crystal structure, microstructure and chemical bonding in the modified surfaces. High surface microhardness (~1300 HV) over a case depth of about 60 microns is obtained. Glancing Incidence X-ray Diffraction (GIXRD) indicates the presence of a predominantly Fe3N phase with dispersed CrN within 2-5 microns on the surface. In addition, using Bragg-Brentano geometry, we measure the presence of a minor phase of Fe4N in the case depth. SEM confirms that the microstructure within 2-5 microns of the surface is different from that of the bulk. XPS shows nitride phase formation on the surface. AES measured over the cross-section of the case depth shows a direct relation of the increased surface microhardness to the high nitrogen content.

Electron-phonon coupling induced pseudogap and the superconducting transition in Ba_0.67K_0.33BiO₃

A. Chainani,^1,2T. Yokoya,¹ T. Kiss,¹ S. Shin,^1,3 T. Nishio,⁴ and H. Uwe⁴

Physical Review B 64 (2001) Pg :180509-1 : 180509-4

1Institute for Solid State Physics, University of Tokyo, Kashiwa,Chiba 277-8581, Japan ²Institute for Plasma Research, Gandhinagar 382 428, India ³The Institute of Physical and Chemical Research(RIKEN), Sayo-gun, Hyogo 679-5143, Japan ⁴Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan

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We study the single particle density of states (DOS) across the superconducting transition (T_c = 31 K) in single-crystal Ba_0.67K_0.33BiO₃using ultrahigh resolution angle-integrated photoemission spectroscopy. The superconducting gap opens with a pile-up in the DOS,delta(5.3 k)=meV and 2 delta k_BT_c = 3.9. In addition, we observe a pseudogap below and above T_c, occurring as a suppression in intensity over an energy scale up to the breathing mode phonon(~ 70 meV). The results indicate electron-phonon coupling induces a pseudogap in Ba_0.67K_0.33BiO₃.

Electronic structure of Carbon-free Silicon Oxynitride films grown using an organic precursor Hexamethyl-Disilazane

A. Chainani, S. K. Nema, P. Kikani and P. I. John
Institute for Plasma Research, Gandhinagar - 382 428 ,Gujarat, India

Submitted to Journal of Physics D Applied Physics

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Silicon Oxynitride films are grown by Plasma-enhanced chemical vapor deposition(PECVD) on single crystal Si(100) and textured Si-solar cells, using a safe organic precursor, Hexamethyl-Disilazane. Using the Lucovsky-Phillips criterion of bond co-ordination constraints, we grow high-quality thin (~20 Aº) and thick (up to 2700 Aº) films which are Carbon free (<1.0 % ) as characterized by X-ray photoemission spectroscopy(XPS) and Auger electron spectroscopy(AES) depth profiles. Core-level and valence band XPS is used to conclusively identify oxy-nitride bonding and band gap reduction in SiO_xN_y. For a /4 ‘blue’ anti-reflection coating on the solar cells with uniform thickness (870+15 Aº) and composition (SiO_1.6+0.1N_0.3+0.05), an efficiency(AM 1) increase of 1% is obtained.

Fermi surface sheet-dependent superconductivity in 2H-NbSe2

Takayoshi Yokoya*, Takayuki Kiss*, Ashish Chainani*,@, Shik Shin*,% MinoruNohara# & Hidenori Takagi#

*Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan @Institute for Plasma Research, Bhat, Gandhinagar-382 428, India %The Institute of Physical and Chemical Research (RIKEN), Sayo-gun, Hyogo 67 9-5143, Japan #Department of Advanced Materials Science, University of Tokyo, Tokyo 113-0033, Japan

Science 294 (2001) 2518 -2520 .

Abstract:

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The energy gap in the single-particle excitation spectrum of the superconducting state makes the superconducting properties of a material qualitatively different from its normal state properties. BCS theory assumes electron-electron pairing due to phonons and approximates the pairing strength as a function of momentum to be constant, leading to an isotropic s-wave gap. This is not the case for high-temperature superconductors (high-Tc¥æs), where a highly anisotropic dx2-y2 gap has been confirmed and unconventional pairing mechanisms other than electron-phonon interaction are actively discussed. On the other hand, even for phonon-mediated s-wave superconductors, existence of several Fermi surface (FS) sheets possessing different electron-phonon coupling constants and/or differing density of states (DOS) at the Fermi level (EF) can give rise to a momentum-dependent superconducting gap in real materials. Here we show that the electronic structure of 2H-NbSe2, a quasi-two-dimensional incommensurate charge density wave (CDW) system (TCDW ~ 35 K)which is also a phonon-mediated superconductor below Tc = 7.2 K, exhibits FS sheet dependent superconductivity. The result highlights the importance of FS sheet dependence of electronic structure and electron-phonon interactions in low-Tc superconductors.

Electronic structure Ce₁-_x SrTio ₃ : Comparison between substitutional and vacancy doping

A.Chainani^1,2, T.Yokiya²,T.Sato²,H.Fujisawa², and T.Takahashi²

¹Department of Physics, Tohoku University, Sendai980-8578,Japan. ²Institute for plasma Research, Gandhinagar 382428,India

Physical Review Letter 59 No-3 (1999) 1815 -1818

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The change of the electronic structure across the metal -insulator transition in a Mott-Hubbard system Ce1-xSrxTio3 has been investigated by x-ray absorption and photoemission spectroscopy. The result are compared with CeTi+Delta, where hole doping is achieved by excess oxygen instead of Sr substitution.It is found that addition doped-hole states are created in the insulator gap for both cases, while the desity of states at the fermi level is CeTiO3+delta, is nearly half of that in Ce1-xSrTiO3 at the same nomial doping .This suggest a strong reduction in the mobility of carriers due to cation vacancies produced by excess oxygen.

Photoemission Spectroscopy of the Strong-Coupling Super conducting Transitions in Lead and Niobium

A.Chainani,^1,2 T.Yokoya ², T.Kiss,²,and S. Shin ^2,3

¹ Institute for Plasma Research Bhat ,Gandhinagar –382 428 India ²Institute for Solid State Physics(ISSP),University of Tokyo, Kashiwa, Chiba 277-8581,Japan. ³The Institute for Physical and Chemical Research (RIKEN), Sayo- gun, Hyogo 679-5142 , Japan

Physics Review Letter; Vol 85 No:9 (2000) 1966-1968

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We study the changes in the electronic structure associated with low- Te strong –coupling super conducting transition in Pb and Nb using Ultrahigh –resolution (2.3 mev) temperature-dependent (5.3-12.0 K) photoemission spectroscopy. We observe peaks in the density of states on entering the super-conducting phase accompanying gap formation and spectacular distribution of spectral weight at low energy scales as a function of temperature. The well known peak –dip feature of the high –Tc cuprates is seen in Pb, making it a characteristic of strong-coupling superconductivity.

Electronic structure of black SmS. I. 4d-4f resonance and angle-integrated valence-band photoemission spectroscopy

A. Chainani^1,2,H. Kumigashira², T. Ito², T. Sato², T. Takahashi²,T. Yokoya,³ T. Higuchi³, and T. Takeuchi³S. Shin^4,5N. K. Sato,⁶

¹Institute for Plasma Research, Gandhinagar 382 428, India,²Department of Physics, Tohoku University, Sendai 980-8578, Japan,³Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan,⁴Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan⁵The Institute of Physical and Chemical Research (RIKEN), Sayo-gun, Hyogo 679-5143, Japan⁶Department of Physics, Nagoya University, Chikusa-ku, Nagoya 602-4648, Japan.

Phys. Rev. B 65, (2002) 155201 -155208

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We study the electronic structure of single-crystal "black" semiconducting SmS using x-ray absorption and resonance photoemission spectroscopy across the 4d-4f threshold and high-resolution temperature-dependent valence-band photoemission spectroscopy. The 4d-4f on-resonance spectra show mixed valency of Sm²⁺ and Sm³⁺ states in semiconducting SmS at low temperature (T=30 K). The high-resolution spectra show a pseudogap within 20 meV of the Fermi level at low temperatures. This pseudogap is gradually filled up accompanied by a redistribution of spectral weight over a larger energy scale of ~200 meV on increasing temperature, resulting in an incipient metallic phase at room temperature. The two energy scales can be associated with the coherence temperature T_c and the Kondo temperature T_K, respectively. The changes in the single-particle density of states at and about the Fermi level are compatible with recent theoretical results on "exhaustion physics" in the periodic Anderson model. The present study indicates a Kondo lattice picture for SmS and suggests a relation between the mixed valency and the temperature dependence of the pseudogap for "black" semiconducting SmS.

Electronic structure of black SmS. II. Angle-resolved photoemission spectroscopy

T. Ito¹A. Chainani^1,2H. Kumigashira and T. Takahashi¹N. K. Sato³

¹Department of Physics, Tohoku University, Sendai 980-8578, Japan. ²Institute for Plasma Research, Gandhinagar 382 428, India. ³Department of Physics, Nagoya University, Chikusa-ku, Nagoya 602-4648, Japan.

Phys. Rev. B, 65 (2002) 155202 - 155206

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We have studied the electronic band structure of semiconducting black SmS with high-resolution angle-resolved photoemission spectroscopy (ARPES). The valence band consists of two well-separated groups of bands: almost nondispersive bands near E_F and highly dispersive bands at higher binding energy. The former is ascribed to the Sm²⁺ (4f⁶-->4f⁵) multiplet and the latter to mainly the S 3p states. We found a small but distinct energy dispersion in the Sm 4f-derived bands near E_F. This indicates a strong hybridization between the "localized" Sm 4f electrons and the "itinerant" conduction electrons, leading to the "delocalized" Sm 4f states in mixed-valent SmS. We have compared the present ARPES results with a recent periodic Anderson model calculation

Oscillatory Discharge of a Negatively Charged Electrode in Plasma

S. K. Karkari and P. I. John Facilitation Centre for Industrial Plasma Technologies,

Institute for Plasma Research, Gandhinagar,

India, Pin: 382 044

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We report on an oscillatory discharge of an electrode in plasma when it is negatively charged by a pulsed electron beam source. The sustenance of oscillations depends on the discharging rate and the discharge duration, which is self-consistently taking place as the plasma ions neutralise the negative charge on the electrode surface.The experimental observations are simulated using Particle simulation of one Dimensional bounded Plasma code,PDP-1 (William S. Lawson, University of California, Berkeley). We also present a qualitative model for the observations in which the ions in the matrix sheath undergo oscillations giving rise to change in the associated capacitance of the ion matrix sheath and coupling a capacitive voltage on the charged electrode when the electrode is discharging at a certain rate.

Manganese Concentration and low temperature annealing dependence of Ga_1-xMn_xAs by X-ray absorbtion spectrocopy

Y. Ishiwata,¹ M. Watanabe,2 R. Eguchi,¹ T. Takeuchi,³ Y. Harada,² A. Chainani,^1,4 S. Shin,^1,2 T. Hayashi,1 Y. Hashimoto,¹ S. Katsumoto,^1,5 and Y. Iye ^1,5

¹Institute for Solid State Physics, University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
²RIKEN/SPring-8, Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5143, Japan
³Department of Applied Physics, Tokyo University of Science, Kagurazaka, Shinjyuku-ku, Tokyo 162-8601, Japan
⁴Institute for Plasma Research, Bhat, Gandhinagar-382 428, India
⁵CREST, Japan Science and Technology Corporation, Mejiro, Tokyo 171-0031, Japan

PHYSICAL REVIEW B, VOLUME 65, 233201

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The Mn-site-projected electronic structure of the diluted magnetic semiconductors Ga12xMnxAs (x 50.032, 0.038, 0.047, 0.052, 0.058! of as-grown and low-temperature ~LT! annealed samples are systematically studied using high-resolution Mn 2p absorption spectroscopy. The study exhibits coexistence of the ferromagnetic Mn21 ion and the paramagnetic Mn-As complex that transforms into the ferromagnetic component with LT annealing. The ratio of ferromagnetic to paramagnetic components is directly related to the x dependence of the hole density and ferromagnetic critical temperature.