Natbib giving single character instead of author-year

Question

I am using MiKTeX 2.9/TeXmaker. When I cite(\citep{}) with the natbib package, I get random characters e.g [a],[x] etc, instead of e.g [John1998]. I ran PdfLatex->Bibtex->Pdflatex->Pdflatex, but still getting the same result. I am writing my first document in LaTeX and its my thesis, so please forgive if I overlooked any minor error or method.

\documentclass[a4paper,12pt]{book}
\usepackage[square,authoryear]{natbib}
\usepackage{hyperref}

\begin{document}
Photonic crystals are a class of materials 
that are either man made\citep{Yablonovitch1993} or 
naturally found\citep{Zi2003} 
.
.
.
.
.
\backmatter
\bibliographystyle{plainnat}
\bibliography{references}
\end{document}

Output:

Photonic crystals are a class of materials that are either man made[b, a] or naturally found [e, i].

Within BibTeX file

@ARTICLE{Yablonovitch1993,
  author = {Yablonovitch, E.},
  title = {Photonic band-gap structures},
  journal = {J. Opt. Soc. Am. B},
  year = {1993},
  volume = {10},
  pages = {283-295},
  number = {2}
}

@ARTICLE{Zi2003,
  author = {Zi, Jian and Yu, Xindi and Li, Yizhou and Hu, Xinhua and Xu, Chun
    and Wang, Xingjun and Liu, Xiaohan and Fu, Rongtang},
  title = {Coloration strategies in peacock feathers},
  journal = {Proceedings of the National Academy of Sciences},
  year = {2003},
  volume = {100},
  pages = {12576-12578},
  number = {22},
  abstract = {We report the mechanism of color production in peacock feathers. We
    find that the cortex in differently colored barbules, which contains
    a 2D photonic-crystal structure, is responsible for coloration. Simulations
    reveal that the photonic-crystal structure possesses a partial photonic
    bandgap along the direction normal to the cortex surface, for frequencies
    within which light is strongly reflected. Coloration strategies in
    peacock feathers are very ingenious and simple: controlling the lattice
    constant and the number of periods in the photonic-crystal structure.
    Varying the lattice constant produces diversified colors. The reduction
    of the number of periods brings additional colors, causing mixed
    coloration.}
}

.bbl file contents (Only the above two references are shown. I deleted the references from other parts of my document for convenience):

\begin{thebibliography}{29}
\providecommand{\natexlab}[1]{#1}
\providecommand{\url}[1]{\texttt{#1}}
\expandafter\ifx\csname urlstyle\endcsname\relax
  \providecommand{\doi}[1]{doi: #1}\else
  \providecommand{\doi}{doi: \begingroup \urlstyle{rm}\Url}\fi

\bibitem[Yablonovitch(1993)]{Yablonovitch1993}
E.~Yablonovitch.
\newblock Photonic band-gap structures.
\newblock \emph{J. Opt. Soc. Am. B}, 10\penalty0 (2):\penalty0 283--295, 1993.

\bibitem[Zi et~al.(2003)Zi, Yu, Li, Hu, Xu, Wang, Liu, and Fu]{Zi2003}
Jian Zi, Xindi Yu, Yizhou Li, Xinhua Hu, Chun Xu, Xingjun Wang, Xiaohan Liu,
  and Rongtang Fu.
\newblock Coloration strategies in peacock feathers.
\newblock \emph{Proceedings of the National Academy of Sciences}, 100\penalty0
  (22):\penalty0 12576--12578, 2003.

\end{thebibliography}

Answer 1

I carefully sifted through the code, removing bits and pieces and checked for the output. It seems \usepackage{newclude} was the culprit. Without newclude, the citation is displayed perfectly as desired i.e. Author-year.