Microwave Conductivity Spectra in High Landau Levels under Tilted Magnetic Fields

We present the results of a systematic study of the microwave conductivity in high Landau levels (LLs) of very clean two- dimensional electron systems (2DES), in the 20 MHz to 6 GHz range. When magnetic field ($B$) is applied perpendicular to the 2DES, a resonance that appears between LL fillings ($\nu$) of 4.2 and 4.4 is interpreted as due to the pinning mode of bubble phase of electrons [1]. We observe that the frequency of this resonance increases as a function of {\it in-plane} $B$ ($B_{|| }$). A resonance associated with a Wigner crystal composed of top LL electrons or holes [2] in the ranges 1.85$<\nu<$1.95 and 2.05$<\nu<$2.15 similarly increases with $B_{||}$. However, the peak frequency increases asymmetrically with $B_{||}$ on either side of $\nu$=2.\\ \noindent [1] R.M. Lewis {{\it et al.}}, PRL {\bf 89}, 136804 (2002)\\ \noindent [2] Y. Chen {\it {et al.}}, PRL {\bf 91}, 016801 (2003)