Hanbury Brown --Twiss type exchange effects in a four terminal diffusive conductor

We have investigated current-current correlations in a mesoscopic four-terminal diffusive conductor, and performed an electronic equivalent of Hanbury Brown -Twiss (HBT) type of experiment. In the experiment, cross -spectrum noise between two terminals of a cross is measured in three different bias configurations and exchange correction factor $\Delta $S $=$ \textbar SC\textbar - \textbar SA\textbar - \textbar SB\textbar [1] is calculated from these measurements. In the non-interacting regime, we find an increase in $\Delta $S with bias from the thermal limit and with further increase in bias the $\Delta $S becomes bias independent [2]. In contrast to the diffusive cross, our measurements on metallic islands yielded strong HBT-type of correlations that agree with the expected intrinsic correlations in a four-terminal cavity coupled to reservoirs by weak tunneling contacts. Moreover, we find non-classical HBT contributions for the case of interacting electrons (hot electrons), where $\Delta $S is found to be negative.\\[4pt] [1] Ya. M. Blanter, M. B\"{u}ttiker, Phys. Rev. B \textbf{56}, 2127 (1997).\\[0pt] [2] E. V. Shukhorukov, D. Loss, Phys. Rev. B \textbf{59}, 13054 (1999).