Natbib giving single character instead of author-year

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}

-
Welcome to TeX.sx! Can you give an example of an entry in the references.bib file? –  egreg Feb 18 '13 at 13:37
Thanks for the quick reply. I use endnote for managing references and then later imported them to Jabref, then used automatic key generation of jabref before exporting the library as references.bib –  Rajaraja Chola Feb 18 '13 at 13:42
Can you add the contents of the .bbl file you get when running BibTeX on the minimal example above? –  egreg Feb 18 '13 at 13:45
You wrote " I deleted the references from other parts of my document for convenience):". This indicates that you didn't test the minimal example but some larger document. Do you get the same result if you really run the minimal example? –  Ulrike Fischer Feb 18 '13 at 13:59
Well create a minimal example - that is not an arbitrary small example, but a small example that demonstrates the problem. So start with your problematic document and strip it down. –  Ulrike Fischer Feb 18 '13 at 14:11

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.