I know of only one program that calls itself a "pre-PhD diploma" program: The pre-PhD program at the Abdus Salam International Centre for Theoretical Physics (ICTP). This seems to likely be the program you are speaking of. This program is at least pretty well regarded from what I can determine. It would likely help formalize some of what you need to learn if you want to study condensed matter physics. If time is not an issue, then the program could be helpful.
This being said, I will add my personal opinion that doing a pre-PhD seems a bit redundant. If you are able to get into a funded PhD program, why not begin your PhD immediately? You might not be in a hurry to finish now, but I'm also not sure that you will still feel that way five years down the road. Having all the time in the world is fine when you are 25. It becomes much harder when you reach your 30s.
The finances are also of consideration here. It seems that the ICTP's program is unfunded unless you are from specific countries. (Maybe this is your case, which is fine).
Moreover, note that a pre-PhD program is probably one of two things:
- So fast that it covers nothing in enough depth to allow you to actually complete a PhD on the subject.
- So fast that you will not be able to absorb the blitzkrieg of information you are being taught.
Simply put, one does not easily learn the principles of advanced condensed matter physics in a rapid-fire program.
Also be aware that a "pre-PhD" sounds a bit like something one would get from an online pay-to-learn website. For those not familiar with the program, it may not be given much weight.
As far as convincing an admissions committee that you will be successful in studying condensed matter physics, you may have a tough time doing such if you have zero courses in the subject. In my opinion, stating "_________ really is interesting to me" never gets an applicant through the door if they have zero experience in studying the subject. (Which you have taken note of above). You could try including an explanation of the personal work you have done in studying the subject. If you can speak somewhat at length to the actual efforts you have given on the topic, that may bode more favorably for your cause. Because this pre-PhD is so niche, it is very hard to know what they are looking for.
I will add some general thoughts on getting into graduate programs on the whole:
- Bad GPA/Test Scores are very hard to overcome. Passion is not enough.
- Letters of recommendation matter. Immensely so, at times.
- Expressing interest in studying a very niche field will usually result in very narrow acceptance parameters.