Melting at the absolute zero of temperature: Quantum phase transitions in condensed matter

We dedicate this article to our father, Prof. Dr. Guenter Vojta, on the occasion of his 80th birthday. While colleagues appreciate him as an excellent scientist with widespread interests and an amazingly broad knowledge, we see in him the father, teacher, friend, and much more. Thanks to him, we grew up immersed in the fascinating world of physics from an early age. His example and encouragement have crucially influenced our own lives.