It is generally recommended that PCA be applied to time series that are stationary. Plot the first column of yield Dat. (You can look at other columns as well. You will see that they are fairly similar.) Does the plot appear stationary? Why or why not? Include your plot with your work.
Another way to check for stationarity is to run the augmented Dickey-Fuller test. You can do that with the following code:
library("tseries")
adf.test(yieldDat[,1])