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Nawazish A. Khan

Bio: Nawazish A. Khan is an academic researcher from Quaid-i-Azam University. The author has contributed to research in topics: Electrical resistivity and conductivity & Superconductivity. The author has an hindex of 22, co-authored 234 publications receiving 1940 citations. Previous affiliations of Nawazish A. Khan include NED University of Engineering and Technology & University of Agriculture, Peshawar.


Papers
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Journal ArticleDOI
TL;DR: In this paper, the interplane couplings in Cu 0.5Tl0.5Ba2Ca2Cu3O10-δ superconductor have been enhanced by Mg doping and the improved interplane coupling a- and c-axis lengths are decreased while critical current density [Jc(H=0)] is enhanced by two orders of magnitude and the zero resistivity critical temperature [Tc(R=0] is increased by 10 K.
Abstract: In the present studies, the interplane couplings in Cu0.5Tl0.5Ba2Ca2Cu3O10-δ superconductor have been enhanced by Mg doping. With the improved interplane coupling a- and c-axis lengths are decreased while critical current density [Jc(H=0)] is enhanced by two orders of magnitude and the zero resistivity critical temperature [Tc(R=0)] is increased by 10 K. Predominant single phase of Cu1-xTlxBa2Ca2-yMgyCu3O10-δ (y=0, 0.5, 1.0, and 1.5) was prepared by the solid state reaction of Ba(NO3)2, Ca(NO3)2, MgO, and Cu(CN). The material was checked by x-ray diffraction for crystallinity and was found to be the predominant single phase of Cu1-xTlxBa2Ca2-yMgyCu3O10-δ (y=0, 0.5, 1.0, and 1.5) with a- and c-axes lengths 3.840 and 14.20 A, respectively. The c-axis length decreases with the increased concentration of Mg in the compound. Fourier transform infrared absorption measurements have shown softening of apical oxygen modes with increased Mg doping and improved interplane coupling.

60 citations

Journal ArticleDOI
TL;DR: In this article, the amorphous phase was deposited by sputtering on the substrate and annealed in a thallium atmosphere at /spl sim/885/spl deg/C for 50 min.
Abstract: The bi-axially-oriented thin films of Cu/sub 1-x/Tl/sub x/(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10-y/(Cu/sub 1-x/Tl/sub x/-1223: x=0.2-1.0) were prepared by an amorphous phase epitaxy (APE) technique on SrTiO/sub 3/ substrates. The amorphous phase was deposited by sputtering on the substrate and annealed in a thallium atmosphere at /spl sim/885/spl deg/C for /spl sim/50 min. Their T/sub c/ values were 100-113 K and higher than those of the YBa/sub 2/Cu/sub 3/O/sub 7-y/ (Y-123, that is, Cu-1212) system. Their J/sub c/ values were 2/spl times/10/sup 7/ A/cm/sup 2/ (77 K, 0 T) and 4/spl times/10/sup 5/ A/cm/sup 2/ (77 K, 10 T) at their maximum for x/spl sim/0.5. The J/sub c/ values were higher than the maximum values of Y-123 thin films by a factor of 2. The high J/sub c/ values are consistent with the low superconducting anisotropy (/spl gamma/=/spl xi//sub ab///spl xi//sub c/=2.5-4) of Cu/sub 1-x/Tl/sub x/-1223 and its high T/sub c/.

48 citations

Journal ArticleDOI
TL;DR: In this paper, the effects of post-annealing conditions on the apical oxygen phonon modes have been investigated and their correlations with the critical temperature of the Cu 1− x Tl x Ba 2 Ca 2 Cu 3 O 10− δ (Cu 1 − x Tls x -1223) superconductor thin films have been developed.
Abstract: The effects of post-annealing conditions on the apical oxygen phonon modes have been investigated and their correlations with the critical temperature of the Cu 1− x Tl x Ba 2 Ca 2 Cu 3 O 10− δ (Cu 1− x Tl x -1223) superconductor thin films have been developed. The samples were annealed in different ambient environments, such as air, N 2 , O 2 and vacuum and their FTIR absorption spectrum has been taken. Three absorption peaks observed between 455–440, 665–648 and 1240–1246 cm −1 are assigned to apical oxygen vibrations, the chain axis O(3) vibrations and to C–O single-bond, respectively. The frequency of the mode at 655 cm −1 is sensitive to the concentration of O(3) in the charge reservoir layer, which in turn controls increase or decrease of the bond length of apical oxygen. The softening of the apical oxygen mode is possibly linked with the reduction of thallium, which in turn is associated with the amount of oxygen [O(3)] in the charge reservoir layer. In as-prepared material and annealed under oxygen, the state of thallium is not reduced from Tl +3 to Tl +1 and sufficient number of carriers are not supplied to CuO 2 planes, the material remains in the region of carrier under-doping. The softening of apical oxygen mode positioned at 455 cm −1 is associated with the formation of low T c phases. The C–O related IR mode is softened with the post-annealing in air at 780 °C and annealing in N 2 ambient at 400 °C. The critical transition temperature is found to decrease with the post-annealing at 400 °C in N 2 , which is possibly due to the under-doped CuO 2 planes of the material. However, the post-annealing at 780 and 650 °C increases T c (mid-point) to 107 and 111 K respectively.

45 citations

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TL;DR: In this paper, the effects of Zn atoms and specifically their role in modifying dielectric properties, such as dielectrics loss (tan δ) and ac-conductivity (σac), were investigated in the normal state of the samples at room temperature and in their superconducting state.
Abstract: We have studied the dielectric properties of Cu0.5Tl0.5Ba2Ca2Cu3−yZnyO10−δ (y=0, 1.0, 1.5, 2.0, 2.5) superconductors to investigate the effects of Zn atoms and specifically their role in modifying dielectric properties, such as dielectric loss (tan δ) and ac-conductivity (σac). These dielectric properties have been investigated in the normal state of the samples at room temperature and in their superconducting state. From these analyses, we have determined the excess conductivity in the superconducting state (lower energy state) of the system and suggested its possible role in the mechanism of superconductivity. The lower thermal agitation at 79 K may enhance the polarizability of atoms and hence their dielectric constants (e′, e″). A comparison with the dielectric properties of Tl2Ba2Ca2Cu2Ox has shown that our Cu0.5Tl0.5Ba2Ca2Cu3−yZnyO10−δ material has lower losses and higher ac-conductivity at 290 and 79 K. The decreased dielectric loss with increased Zn doping strongly suggested that the polarization ...

40 citations

Journal ArticleDOI
TL;DR: In this paper, the substitution of Be at the Ca site in normal pressure synthesized Cu 0.5 Tl 0.7, 1.5, 2.0 superconductors, and the crystal structure of the material was determined by X-ray diffraction.
Abstract: The potential of Cu 0.5 Tl 0.5 Ba 2 Ca n −1− y Be y Cu n O 2 n +4− δ superconductor have been realized by the substitution of Be at Ca site in our normal pressure synthesized Cu 0.5 Tl 0.5 Ba 2 Ca n −1 Cu n O 2 n +4− δ ( n = 3, 4) superconductors. The primary goal of synthesizing Be substituted samples was to achieve improve inter-plane couplings. Predominant single phases of Cu 0.5 Tl 0.5 Ba 2 Ca 2− y Be y Cu 3 O 2 n +4− δ ( y = 0.7, 1.7) and Cu 0.5 Tl 0.5 Ba 2 Ca 3− y Be y Cu 4 O 12− δ ( y = 1.5, 2.0) superconductors were synthesized by the solid-state reaction method. The samples have shown a zero resistivity critical temperature around 95 K. The bulk superconductivity was measured by ac-susceptibility. The crystal structure of the material was determined by X-ray diffraction. The cell parameters have shown a decrease in the c -axis length with Be substitution. The decreased c -axis length suggests the improved inter-plane coupling in these superconductors. The increased inter-plane coupling is also evidenced by the softening of FTIR absorption modes of apical oxygen around 500 cm −1 and the CuO 2 planner mode at 575 cm −1 .

39 citations


Cited by
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TL;DR: The fundamentals, recent research progress, present status, and views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices are described.
Abstract: The photovoltaics of organic–inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regardi...

1,720 citations

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TL;DR: It is proposed from the current studies that engineering of perovskite film near the electron transporting layer and the hole transporting layer is of vital importance for achieving hysteresis-free PSCs and extremely high efficiency.
Abstract: Current-density-voltage (J-V) hysteresis in perovskite solar cells (PSCs) is a critical issue because it is related to power conversion efficiency and stability. Although parameters affecting the hysteresis have been already reported and reviewed, little investigation is reported on scan-direction-dependent J-V curves depending on perovskite composition. This review investigates J-V hysteric behaviors depending on perovskite composition in normal mesoscopic and planar structure. In addition, methodologies toward hysteresis-free PSCs are proposed. There is a specific trend in hysteresis in terms of J-V curve shape depending on composition. Ion migration combined with nonradiative recombination near interfaces plays a critical role in generating hysteresis. Interfacial engineering is found to be an effective method to reduce the hysteresis; however, bulk defect engineering is the most promising method to remove the hysteresis. Among the studied methods, KI doping is proved to be a universal approach toward hysteresis-free PSCs regardless of perovskite composition. It is proposed from the current studies that engineering of perovskite film near the electron transporting layer (ETL) and the hole transporting layer (HTL) is of vital importance for achieving hysteresis-free PSCs and extremely high efficiency.

311 citations

Journal ArticleDOI
TL;DR: The growth and properties of electronic oxide thin films are reviewed in this paper, in particular the synthesis of superconducting, insulating, conducting, magnetic, and semiconducting epitaxial oxide structures.
Abstract: The growth and properties of electronic oxide thin films are reviewed. In particular, the synthesis and properties of superconducting, insulating, conducting, magnetic, and semiconducting epitaxial oxide structures are discussed. Crystal structures and functional properties common to many oxide materials are briefly reviewed. A description of film-growth techniques follows. Finally, an extensive overview of the epitaxial growth for specific oxide material systems is given. This includes the epitaxial growth of electronic oxide thin films on oxide and non-oxide substrates.

306 citations