Dr. Martin Klanjšek: Quantum criticality in quantum spin systems

Just as thermal fluctuations drive the classical phase transitions, quantum fluctuations drive the quantum phase transitions. These are continuous phase transition that take place at absolute zero temperature, but as a function of an external tuning parameter, like the magnetic field or pressure. The influence of a quantum-critical point, where the transition occurs at zero temperature, extends to a broad V-shaped region of quantum criticality at nonzero temperatures. A comprehensive theory of quantum criticality does not exist yet due to its complexity. However, the corresponding physics is supposed to be universal, thus insensitive to the microscopic details describing the system, and scale invariant, where the temperature sets the only energy scale. This type of behavior is found in such diverse systems as metals, magnetic systems, cold atoms and black holes. I will present an experimental insight into the quantum criticality of one of the simplest type of quantum systems: one-dimensional antiferromagnets with the magnetic field as a tuning parameter. Nuclear magnetic resonance as an excellent probe of low-energy excitations allowed us to demonstrate the universality and scale invariance of the quantum-critical behavior.