Attribution is separate from permission. You of course need to cite your sources, but this is separate from the question of whether you need a rights holder's permission to include excerpts from or copies of those sources to begin with.
As a copyright holder, the author has exclusive rights to (among other things) reproducing the work. If you want to reproduce still-in-copyright work in your dissertation, you'll need to decide whether it's fair use or get the copyright holder's permission. See the Understand Copyright Basics tab for more info.
Publication online implies nothing about whether the work is in the public domain.
Content that appears online--and thus is publicly accessible--may very well be copyrighted, and thus you must comply with copyright law when using it. "Public domain" instead refers specifically to work that no longer is entitled to copyright protection (i.e. the copyright protection expired), or works for which copyright protection was never available (e.g. U.S. Federal Government works, facts/ideas, etc.). See the discussion of Public Domain in Step 1 for more information.
No! It is not an infringement to link to content that has been uploaded lawfully.
If, however, you have reason to believe that the content you're linking to was uploaded in violation of copyright, then you should not link to it. Doing so could be construed as contributory infringement. In those circumstances, work through the Step 1 questions with respect to use of legitimate copies of the content, rather than linking to infringing online reproductions.
Maybe. If the work that you want to use is something you previously wrote, you may no longer hold copyright over it if you assigned copyright to a publisher--making it important to go through Step 1 questions.
As copyright holder of your scholarship, you're entitled to make derivative works and adapt or rearrange your work as you see fit. Though, a journal may want you to edit your work a bit to make it something different for the journal iteration. Every version or adaptation of your work is a separate work in which you hold copyright.
Keep in mind, though: If you publish journal articles that are merely excerpted from your digital project without modification, you should be careful about assigning copyright to the journals. If you later wish to reuse the same language in, say, a manuscript for a book, you don't want to have transferred your rights to that iteration.
For instance, one of the fair use factors takes into consideration whether your use is for non-profit educational purposes. Once you begin publishing a book and earning royalties, the commercial nature of the endeavor may weigh against fair use for that factor. You'll need to undertake Step 1 analysis again in its entirety when it comes time to publish your manuscript commercially, and determine whether you need permissions.
Keep in mind, too, that your publisher may--as a matter of policy, to protect itself--want you to obtain permissions irrespective of whether you believe use would have been fair without permission.
If they contained authored, original expressions, they were, and maybe still are, protected by copyright. But like any other copyrighted work, they may have entered the public domain.
Unpublished works subject to copyright protection. However, the duration of copyright for unpublished works can differ based on whether they are signed, anonymous, etc. For more on copyright length for unpublished works, consult the discussion of Unpublished Works (Ch. 3.2.1) in Peter Hirtle's Copyright & Cultural Institutions book; see also 17 USC §§ 302, and 303.
Keep in mind, too, that while unpublished works are not excluded from your use as fair use, what constitutes fair use of unpublished works may construed more narrowly by a court.
They were at some point. Whether they still are depends.
The length of protection in the U.S. for unpublished material is the same regardless of where the work was created, or what nationality the author was (17 USC § 104). If the copyright term for the unpublished work has expired, it's in the public domain for purposes of publishing your dissertation in the U.S.
If you're looking to use foreign works in your dissertation being published in the U.S., the general rule of thumb is that anything first published in a foreign country prior to 1923 has entered the public domain, and most everything else published abroad since then remains protected by copyright.
The more complex answer is that, for foreign works: Based on the nationality of the author and place of publication, one can calculate whether the foreign material has entered the public domain. Though, you don't have to--you can use the wonderful Cornell University Public Domain chart prepared by Peter Hirtle. Check out the section "Works First Published Outside the U.S. by Foreign Nationals or U.S. Citizens Living Abroad."
Ultimately, the determination of whether something is fair use must be made by you, rather than the Library. But if you are a UC Berkeley graduate student, we are here to help talk through how fair use works, and answer questions about resources as you work through the four factors. We would be delighted to meet or speak with UCB students, so please contact the Scholarly Communication Officer.
For additional assistance making fair use determinations, check out: