'have since demonstrated' is correct and usable in written English.
You can use it when you are talking about something that has been shown to be true over a period of time. For example: "The effectiveness of online learning has been questioned in the past, but numerous studies have since demonstrated its success."
Those politicians have since demonstrated a keen desire to leave behind the pageantry and dive straight into the process of consolidating power and delivering speeches laden with standard party language.
One of the first studies on the matter was led by Sir Christopher Andrewes, of the Common Cold Research Unit, in Salisbury, England, a half century ago....Experiments have since demonstrated that being chilled has no effect on either the likelihood of catching a cold or the severity of a cold once you've caught it.
Security researchers have since demonstrated that this information could be used to expose at least some of the Adobe account holders' details.
The enormous human toll of this tragedy spurred the development of the concept of inherently safer design (ISD), and several published studies have since demonstrated the application of ISD concepts to the Bhopal process.
However, Meriki et al. have since demonstrated similar reproducibility of left and right MPI [ 54].
Several investigators have since demonstrated that RUSG-guided PDT (RUSG-PDT) is feasible and potentially useful [ 22- 29].
Dr. Painter has since demonstrated in the laboratory that ionic liquids have the potential for greatly reducing the amount of water used in the oil sands industry.
But she has since demonstrated that traditional Italian family-run firms can be compatible with modern, global capitalism and need not be afraid of it.
He has since demonstrated the necessary responsibility in the devotion to his children and also in the rebuilding of his private life with skier Lindsey Vonn.
Yarnold, who had barely heard of skeleton five years ago when she was picked out by a UK Sport talent search as having the right physical attributes to succeed, has since demonstrated the focus and sheer bloodymindedness that mark out the very best.
If what he wanted was to clear the record, then Taylor -- who said he was no longer taking drugs -- should be given credit for the resolve he has since demonstrated in changing his life.
