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## Engineering the optical properties of subwavelength devices and materials

Anant, Vikas, 1980-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 154 p.
ENG
Relevância na Pesquisa
65.68%
Many applications demand materials with seemingly incompatible optical characteristics. For example, immersion photolithography is a resolution enhancing technique used to fabricate the ever-shrinking nanostructures in integrated circuits but requires a material that has-at the same time--a large index of refraction and negligible optical loss. Other applications require devices that have optical properties that seem exorbitant given the constraints posed by the geometry, materials, and desired performance of these devices. The superconducting nanowire single-photon detector (SNSPD) is one such device that, on the one hand, needs to absorb and detect single telecom-wavelength photons (A = 1.55 pm) with near-perfect efficiency, but on the other hand, has an absorber that is subwavelength in its thickness (A/390). For both cases, it is simply not enough to look for alternative materials with the desired optical properties, because the materials may not exist in nature. In fact, it has become necessary to engineer the optical properties of these devices and materials using other means. In this thesis, we have investigated how the optical properties of materials and devices can be engineered for specific applications. In the first half of the thesis...

## Correlation of superconducting properties and microstructure in MgB2 using SEM, EPMA and TEM; Korrelation von supraleitenden Eigenschaften und Mikrostruktur in MgB2 mittels REM, Elektronenstrahl-Mikrosnde und TEM

Birajdar, Balaji
Tipo: Dissertação
EN
Relevância na Pesquisa
55.7%
MgB2 is an intermetallic compound, has hexagonal crystal structure and is a clear example of two band superconductivity. Superconducting transition temperature (Tc) of MgB2 is at 39 K, which is twice that of Nb3Sn and four times that of Nb-Ti, the two most common commercial superconductors. The upper critical field (Bc2) of MgB2 is anisotropic but variable and can be enhanced by introducing structural disorder by e.g. C-doping and neutron irradiation. Recently Bc2(0) of 70 T in thin films and 37 T in bulk samples have been reported. The superconducting coherence length of MgB2 lies in the range of about 2-10 nm. Superconducting wires and tapes are hysteretic type II superconductors that are exposed to high magnetic fields and are in the critical (Shubnikov) state. Therefore, the critical current density (Jc (B,T)) is an exclusively important figure of merit for these superconductors. Most of the applied research on MgB2 is aimed at enhancing the Jc which reached about 1x105 Acm-2 (at 4.2 K and 12 T). In MgB2 the correlation of microstructure with the superconducting properties, in particular the critical current density requires powerful analytical tools. Critical current densities and electrical resistivities of different MgB2 superconductors differ by orders of magnitudes and the current limiting mechanisms have not been fully understood. Granularity of MgB2 is one significant reason for reduced critical current density and is introduced intrinsically by the anisotropy of Bc2 but also extrinsically by the microstructure of the material. This thesis can be subdivided as follows: (I) Development of the quantitative electron microscopy and spectroscopy methods essential for the microstructural analysis of MgB2 and thereby assess the performance of two energy-filtered TEMs (Zeiss 912 and Zeiss Libra 200FE). By quantitative electron microscopy and spectroscopy we mean a combined SEM and TEM analysis that covers various length scales from µm to nm. Contamination free sample preparation...

## Superconducting Properties and Nanostructure of YBa2Cu3O7-delta Coated Conductors Prepared By Chemical Solution Deposition; Supraleitende Eigenschaften und Nanostruktur von YBa2Cu3O7-delta Coated Conductors hergestellt durch chemische Tauchbeschichtung

Molina-Luna, Leopoldo
Tipo: Dissertação
EN
Relevância na Pesquisa
55.78%
Wire fabrication is the enabling technology for superconducting devices in the magnet and energy technology. Up to now no adequate superconducting material for operation at 77 K and magnetic fields larger than 1 T is available. Therefore, there is a fast-growing worldwide interest in the second generation (2G) of high-temperature superconductor (HTS) wires for applications such as fault current limiters and motors. These materials are also known as YBa2Cu3O7-x (YBCO) - coated conductors and are prepared by thin film technologies. Among the various deposition methods, Chemical Solution Deposition (CSD) has specific advantages with respect to Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) such as flexibility in composition and easy up-scaling, allowing the coating of long-length substrates and since it is a non-vacuum method, less expensive equipment is required. Due to these advantages there is a considerable interest in the development of all-solution based processes. A CSD prepared YBCO coated conductor consists of a biaxially textured nickel tungsten substrate (Rolling-Assisted-Biaxially-Textured Substrates - RABiTS) which is dip-coated in a precursor solution, in this way buffer layers and a superconducting YBCO film are deposited on top of the substrate. The biaxial texture is transferred from the substrate up to the superconducting layer forming a small-angle grain boundary network in the YBCO film...

## Bulk Magnesium Diboride, Mechanical and Superconducting Properties

Nesterenko, Vitali F.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.62%
Hot Isostatic Pressing was applied to synthesize bulk samples with diameter up to 30 mm starting with Alfa Aesar magnesium diboride powder. HIPing at 2 kbar, 1000 C with cooling under pressure (DMCUP cycle) resulted in a fully dense (or low porosity) material with a sharp superconducting transition and critical current comparable with the best reported. Elastic constants were measured using method of resonant ultrasound spectroscopy. Their values for most dense samples (bulk modulus 142.5 GPa and Young's modulus 272.5 GPa) are close to theoretically predicted values for solid magnesium diboride. Porosity is resposible for lower values of elastic constants. Fracture toughness is evaluated based on measurements of microhardness and elastic constants. Upper critical field and irreversibility field are comparable with data for samples sintered at 3 GPa. Microstructural propeties of HIPed smaples are responsible for enhanced flux pinning. Method allows scaling of the size of the samples and manufacture of complex shapes.; Comment: 15 pages, 10 figures, 1 table. Invited talk at Eleventh International Symposium on Processing and Fabrication of Advanced Materials, ASM Materials Solutions, 2002, October 7-10, Columbus, Ohio. Conference Proceedings...

## Superconducting properties and c-axis superstructure of Mg1-xAlxB2

Xiang, J. Y.; Zheng, D. N.; Li, J. Q.; Li, L.; Lang, P. L.; Chen, H.; Dong, C.; Che, G. C.; Ren, Z. A.; Qi, H. H.; Tian, H. Y.; Ni, Y. M.; Zhao, Z. X.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.62%
The superconducting and structural properties of a series of Mg1-xAlxB2 samples have been investigated. X-ray diffraction results confirmed the existence of a structural transition associated with the significant change in inter-boron layer distance as reported previously by Slusky et al. Moreover,transmission-electron-microscopy observations revealed the existence of a superstructure with doubled lattice constant along the c-axis direction. We propose that this superstructure is essentially related to the structural transition. The modifications of superconducting transition temperature Tc, the normal state resistivity, and the upper critical field Bc2 by Al doping are discussed in terms of Al-substitution induced changes in the electronic structure at the Fermi energy.; Comment: 15 pages, 7 figures

## Superconducting properties of nanocrystalline MgB$_2$ thin films made by an in situ annealing process

Zeng, X. H.; Sukiasyan, A.; Xi, X. X.; Hu, Y. F.; Wertz, E.; Li, Qi; Tian, W.; Sun, H. P.; Pan, X. Q.; Lettieri, J.; Schlom, D. G.; Brubaker, C. O.; Liu, Zi-Kui; Li, Qiang
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.64%
We have studied the structural and superconducting properties of MgB$_2$ thin films made by pulsed laser deposition followed by in situ annealing. The cross-sectional transmission electron microscopy reveals a nanocrystalline mixture of textured MgO and MgB$_2$ with very small grain sizes. A zero-resistance transition temperature ($T_{c0}$) of 34 K and a zero-field critical current density ($J_c$) of $1.3 \times 10^6$ A/cm$^2$ were obtained. The irreversibility field was $\sim$ 8 T at low temperatures, although severe pinning instability was observed. These bulk-like superconducting properties show that the in situ deposition process can be a viable candidate for MgB$_2$ Josephson junction technologies.

## Superconducting properties of sulfur-doped iron selenide

Abdel-Hafiez, Mahmoud; Zhang, Yuan-Yuan; Cao, Zi-Yu; Duan, Chun-Gang; Karapetrov, G.; Pudalov, V. M.; Vlasenko, V. A.; Chareev, D. A.; Volkova, O. S.; Vasiliev, A. N.; Chen, Xiao-Jia
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.67%
The recent discovery of high-temperature superconductivity in single-layer iron selenide has generated significant experimental interest for optimizing the superconducting properties of iron-based superconductors through the lattice modification. For simulating the similar effect by changing the chemical composition due to S doping, we investigate the superconducting properties of high-quality single crystals of FeSe$_{1-x}$S$_{x}$ ($x$=0, 0.04, 0.09, and 0.11) using magnetization, resistivity, the London penetration depth, and low temperature specific heat measurements. We show that the introduction of S to FeSe enhances the superconducting transition temperature $T_{c}$, anisotropy, upper critical field $H_{c2}$, and critical current density $J_{c}$. The upper critical field $H_{c2}(T)$ and its anisotropy are strongly temperature dependent, indicating a multiband superconductivity in this system. Through the measurements and analysis of the London penetration depth $\lambda _{ab}(T)$ and specific heat, we show clear evidence for strong coupling two-gap $s$-wave superconductivity. The temperature-dependence of $\lambda _{ab}(T)$ calculated from the lower critical field and electronic specific heat can be well described by using a two-band model with $s$-wave-like gaps. We find that a $d$-wave and single-gap BCS theory under the weak-coupling approach can not describe our experiments. The change of specific heat induced by the magnetic field can be understood only in terms of multiband superconductivity.; Comment: 13 pages...

## Electronic, dynamical and superconducting properties of CaBeSi

Bersier, C.; Floris, A.; Sanna, A.; Profeta, G.; Continenza, A.; Gross, E. K. U.; Massidda, S.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.63%
We report first-principles calculations on the normal and superconducting state of CaBe(x)Si(2-x) (x=1), in the framework of density functional theory for superconductors (SCDFT). CaBeSi is isostructural and isoelectronic to MgB2 and this makes possible a direct comparison of the electronic and vibrational properties and the electron-phonon interaction of the two materials. Despite the many similarities with MgB2 (e.g. sigma bands at the Fermi level and a larger Fermi surface nesting), according to our calculations CaBeSi has a very low critical temperature (Tc ~ 0.4 K, consistent with the experiment). CaBeSi exhibits a complex gap structure, with three gaps at Fermi level: besides the two sigma and pi gaps, present also in MgB2, the appearance of a third gap is related to the anisotropy of the Coulomb repulsion, acting in different way on the bonding and antibonding electronic pi states.; Comment: 6 pages, 5 figures

## Effect of high oxygen pressure annealing on superconducting Nd1.85Ce0.15CuO4 thin films by pulsed laser deposition from Cu-enriched targets

Hoek, M.; Coneri, F.; Leusink, D. P.; Eerkes, P. D.; Wang, X. Renshaw; Hilgenkamp, H.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.62%
We show that the quality of Nd1.85Ce0.15CuO4 films grown by pulsed laser deposition can be enhanced by using a non-stoichiometric target with extra copper added to suppress the formation of a parasitic (Nd, Ce)2O3 phase. The properties of these films are less dependent on the exact annealing procedure after deposition as compared to films grown from a stoichiometric target. Film growth can be followed by a 1 bar oxygen annealing, after an initial vacuum annealing, while retaining the superconducting properties and quality. This enables the integration of electron-doped cuprates with their hole-doped counterparts on a single chip, to create, for example, superconducting pn-junctions.; Comment: This is an author-created, un-copyedited version of an article accepted for publication in Superconductor Science and Technology. The publisher is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at http://dx.doi.org/10.1088/0953-2048/27/4/044017

## Hot isostatic pressing of bulk magnesium diboride: mechanical and superconducting properties

Indrakanti, S. S.; Nesterenko, V. F.; Maple, M. B.; Frederick, N. A.; Yuhasz, W. M.; Li, Shi
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.64%
Two different hot isostatic pressing cycles (HIPing) were investigated to synthesize bulk MgB2 samples: a "standard" cycle where a low vessel pressure is maintained while heating to the process temperature with a subsequent simultaneous pressure and temperature decrease and a new method - dense material cooling under pressure (DMCUP). The latter method allowed the synthesis of dense samples with diameters up to 20 mm and thicknesses up to 10 mm from commercial MgB2 powder. Optimal conditions for the DMCUP method with glass encapsulation (maximum pressure 200 MPa, maximum temperature 1000 C over 200 min, and cooling under pressure) resulted in a dense material with a sharp superconducting transition at 38.5 K. This method employs a pressure which is one order of magnitude less than previously reported for pressure assisted sintering of dense material and can be scaled to larger sample sizes and complex shapes. The data for density, microhardness, fracture toughness and sound speed as well as superconducting properties for bulk magnesium diboride are presented. Ball milling the powder enhances sintering and results in a more homogeneous final microstructure.; Comment: 13 text pages, 8 figures. Added investigations of the "standard" HIPing cycle II in comparison to the new DMCUP HIPing cycle...

## Enhanced superconducting properties in FeCr$_x$Se

Yadav, Anil K; Thakur, Ajay D.; Tomy, C V
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.65%
We report an enhancement of superconducting transition temperature ($T_{\rm c}$) when Chromium (Cr) is substituted in excess at the Iron (Fe) site (FeCr$_x$Se, $x=$0.01, 0.02 and 0.03). There is a corresponding increase in the superconducting volume fraction with $T_{\rm c}$ attaining a value of 11.2 K on 2 $%$ Cr substitution when compared to a $T_{\rm c}$ of 8.5 K for the conventional tetragonal Fe-excess sample Fe$_{1+x}$Se. The results point to the role of chemical pressure (introduced via ionic size variation at the Fe site upon Cr substitution in excess) on superconducting properties.; Comment: 6 figures

## New ternary ThCr2Si2-type iron-selenide superconducting materials: synthesis, properties and simulations

Ivanovskii, A. L.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.77%
Very recently (November, 2010, PRB, 82, 180520R), the first ThCr2Si2-type ternary superconductor K0.8Fe2Se2 with enhanced TC ~ 31 K has been discovered. This finding has stimulated much activity in search for related materials and triggered intense studies of their properties. Indeed, very soon superconductivity (TC ~ 28-32 K) was also found in the series of related ternary systems (so-called 122 phases) such as CsxFe2-ySe2, RbxFe2-ySe2, (TlK)xFeySe2, (TlRb)xFeySe2 etc., which formed a new group of superconducting iron-based materials without toxic As. In this paper the recent progress in synthesis of 122-like iron-selenide systems and in experimental research of their properties is reviewed. Available theoretical data on electronic, magnetic, and elastic properties of this newest group of superconducting materials are also discussed.; Comment: 44 pages, 27 figures

## Stability and superconductivity of Ca-B phases at ambient and high pressure

Shah, Sheena; Kolmogorov, Aleksey N.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.63%
In the search for MgB2-like phonon-mediated superconductors we have carried out a systematic density functional theory study of the Ca-B system, isoelectronic to Mg-B, at ambient and gigapascal pressures. A remarkable variety of candidate high-pressure ground states have been identified with an evolutionary crystal structure search, including a stable alkaline-earth monoboride oI8-CaB, a superconductor with an expected critical temperature (Tc) of 5.5 K. We have extended our previous study of CaB6 [Phys. Rev. Lett. 108, 102501 (2012)] to nearby stoichiometries of CaB[6+x], finding that extra boron further stabilizes the proposed B24 units. Here an explanation is given for the transformation of cP7-CaB6 into the more complex oS56 and tI56 polymorphs at high pressure. The stability of the known metallic tP20 phase of CaB4 at ambient pressure is explained from a crystal structure and chemical bonding point of view. The tP20 structure is shown to destabilize at 19 GPa relative to a semiconducting MgB4-like structure due to chemical pressure from the metal ion. The hypothetical AlB2-type structure of CaB2, previously shown to have favorable superconducting features, is demonstrated here to be unstable at all pressures; two new metallic CaB2 polymorphs with unusual boron networks stabilize at elevated pressures above 8 GPa but are found to have very low critical temperatures (Tc ~1 K). The stability of all structures has been rationalized through comparison with alkaline-earth analogs...

## Studies of the superconducting properties of Sn1-xInxTe (x=0.38 to 0.45) using muon-spin spectroscopy

Saghir, M.; Barker, J. A. T.; Balakrishnan, G.; Hillier, A. D.; Lees, M. R.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.65%
The superconducting properties of Sn1-xInxTe (x = 0.38 to 0.45) have been studied using magnetization and muon-spin rotation or relaxation (muSR) measurements. These measurements show that the superconducting critical temperature Tc of Sn1-xInxTe increases with increasing x, reaching a maximum at around 4.8 K for x = 0.45. Zero-field muSR results indicate that time-reversal symmetry is preserved in this material. Transverse-field muon-spin rotation has been used to study the temperature dependence of the magnetic penetration depth lambda(T) in the mixed state. For all the compositions studied, lambda(T) can be well described using a single-gap s-wave BCS model. The magnetic penetration depth at zero temperature lambda(0) ranges from 500 to 580 nm. Both the superconducting gap Delta(0) at 0 K and the gap ratio Delta(0)/kBTc indicate that Sn1-xInxTe (x = 0.38 to 0.45) should be considered as a superconductor with intermediate to strong coupling.; Comment: 7 pages, 6 figures, 3 tables

## Transport and superconducting properties of Fe-based superconductors: SmFeAs(O1-x Fx) versus Fe1+y (Te1-x, Sex)

Tropeano, M.; Pallecchi, I.; Cimberle, M. R.; Ferdeghini, C.; Lamura, G.; Vignolo, M.; Martinelli, A.; Palenzona, A.; Putti, M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.64%
We present transport and superconducting properties - namely resistivity, magnetoresistivity, Hall effect, Seebeck effect, thermal conductivity, upper critical field - of two different families of Fe-based superconductors, which can be viewed in many respects as end members: SmFeAs(O1-xFx) with the largest Tc and the largest anisotropy and Fe1+y(Te1-x,Sex), with the largest Hc2, the lowest Tc and the lowest anisotropy. In the case of the SmFeAs(O1-xFx) series, we find that a single band description allows to extract an approximated estimation of band parameters such as carrier density and mobility from experimental data, although the behaviour of Seebeck effect as a function of doping demonstrates that a multiband description would be more appropriate. On the contrary, experimental data of the Fe1+y(Te1-x,Sex) series exhibit a strongly compensated behaviour, which can be described only within a multiband model. In the Fe1+y(Te1-x,Sex) series, the role of the excess Fe, tuned by Se stoichiometry, is found to be twofold: it dopes electrons in the system and it introduces localized magnetic moments, responsible for Kondo like scattering and likely pair-breaking of Cooper pairs. Hence, excess Fe plays a crucial role also in determining superconducting properties such as the Tc and the upper critical field Bc2. The huge Bc2 values of the Fe1+y(Te1-x...

## MgB2 single crystals substituted with Li and with Li-C: Structural and superconducting properties

Karpinski, J.; Zhigadlo, N.; Katrych, S.; Batlogg, B.; Tortello, M.; Rogacki, K.; Puzniak, R.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
65.64%
The effect of Li substitution for Mg and of Li-C co-substitution on the superconducting properties and crystal structure of MgB2 single crystals has been investigated. It has been found that hole doping with Li decreases the superconducting transition temperature Tc, but at a slower rate than electron doping with C or Al. Tc of MgB2 crystals with simultaneously substituted Li for Mg and C for B decreases more than in the case where C is substituted alone. This means that holes introduced by Li cannot counterbalance the effect of decrease of Tc caused by introduction of electrons coming from C. The possible reason of it can be that holes coming from Li occupy the pi band while electrons coming from C fill the sigma band. The temperature dependences of the upper critical field Hc2 for Al and Li substituted crystals with the same Tc show a similar dHc2/dT slope at Tc and a similar Hc2(T) behavior, despite of much different substitution level. This indicates that the mechanism controlling Hc2 and Tc is similar in both hole and electron doped crystals. Electrical transport measurements show an increase of resistivity both in Li substituted crystals and in Li and C co-substituted crystals. This indicates enhanced scattering due to defects introduced by substitutions including distortion of the lattice. The observed behavior can be explained as a result of two effects...

## Superconducting properties of MgB2 thin films prepared on flexible plastic substrates

Kus, P.; Plecenik, A.; Satrapinsky, L.; Xu, Y.; Sobolewski, R.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.64%
Superconducting MgB2 thin films were prepared on 50-micrometer-thick, flexible polyamide Kapton-E foils by vacuum co-deposition of Mg and B precursors with nominal thickness of about 100 nm and a special ex-situ rapid annealing process in an Ar or vacuum atmosphere. In the optimal annealing process, the Mg-B films were heated to approximately 600 C, but at the same time, the backside of the structures was attached to a water-cooled radiator to avoid overheating of the plastic substrate. The resulting MgB2 films were amorphous with the onset of the superconducting transition at T_(c,on) about 33 K and the transition width of approximately 3 K. The critical current density was > 7x10^5 A/cm^2 at 4.2 K, and its temperature dependence indicated a granular film composition with a network of intergranular weak links. The films could be deposited on large-area foils (up to 400 cm^2) and, after processing, cut into any shapes (e.g., stripes) with scissors or bent multiple times, without any observed degradation of their superconducting properties.; Comment: 3 figures

## Crystal growth and characterization of MgB2: Relation between structure and superconducting properties

Lee, S.; Masui, T.; Mori, H.; Eltsev, Yu.; Yamamoto, A.; Tajima, S.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.63%
We discuss the important aspects of synthesis and crystal growth of MgB2 under high pressure (P) and temperature (T) in Mg-B-N system, including the optimisation of P-T conditions for reproducible crystal growth, the role of liquid phases in this process, the temperature dependence of crystal size and the effect of growing instabilities on single crystals morphology. Extensive experiments have been carried out on single crystals with slightly different lattice constants and defects concentration, which revealed and possible effects of Mg-deficiency and lattice strain on the superconducting properties of MgB2 (Tc, Jc, residual resistivity ratio, anisotropy etc.).; Comment: 29 pages, 9 figures, 1 table, Proc.of International Workshop on Superconductivity in Magnesium Diboride and Related Materials "BOROMAG", 17-19 June 2002, Genoa, Italy (submitted to Supercond.Sci and Technol.)

## Structural, magnetic and superconducting properties of pulsed-laser-deposition-grown $\rm{La_{1.85}Sr_{0.15}CuO_{4}/La_{2/3}Ca_{1/3}MnO_{3}}$ superlattices on $\rm{(001)}$-oriented $\rm{LaSrAlO_{4}}$ substrates

Das, S.; Sen, K.; Marozau, I.; Uribe-Laverde, M. A.; Biskup, N.; Varela, M.; Khaydukov, Y.; Soltwedel, O.; Keller, T.; Dobeli, M.; Schneider, C. W.; Bernhard, C.
Tipo: Artigo de Revista Científica