Quantum phase transitions in random magnets

R. N. Bhatt

Yu, Stephan Haas, Roscilde

Quantum magnets represent an ideal playground for the controlled realization of novel quantum phases and of quantum phase transitions.The Hamiltonian of the system can be indeed manipulated by applying a magnetic field or pressure on the sample.When doping the system with non-magnetic impurities, novel inhomogeneous phases emerge from the interplay between geometric randomness and quantum fluctuations.In this paper we review our recent work on quantum phase transitions and novel quantum phases realized in disordered quantum magnets.The system inhomogeneity is found to strongly affect phase transitions by changing their universality class, giving the transition a novel, quantum percolative nature.Such transitions connect conventionally ordered phases to unconventional, quantum disordered ones -quantum Griffiths phases, magnetic Bose glass phases -exhibiting gapless spectra associated with low-energy localized excitations.

José A. Hoyos, Thomas Vojta

We study the effects of dissipation on a randomly diluted transverse-field Ising magnet close to the percolation threshold. For weak transverse fields, a percolation quantum phase transition separates a superparamagnetic cluster phase from an inhomogeneously ordered ferromagnetic phase. The properties of this transition are dominated by large frozen and slowly fluctuating percolation clusters. This leads to a discontinuous magnetization-field curve and exotic hysteresis phenomena as well as highly singular behavior of magnetic susceptibility and specific heat. We compare our results to the smeared transition in generic dissipative random quantum Ising magnets. We also discuss the relation to metallic quantum magnets and other experimental realizations.

Omid Nohadani

Thomas Vojta, José A. Hoyos

The magnetic analog of the Grüneisen parameter, i.e., the magnetocaloric effect, is a valuable tool for studying field-tuned quantum phase transitions. We determine the magnetic Grüneisen parameter of the one-dimensional random transverse-field Ising model, focusing on its lowtemperature behavior at the exotic infinite-randomness quantum critical point and in the associated quantum Griffiths phases. 1 Introduction Zero-temperature quantum phase transitions are characterized by a competition between different quantum ground states. The resulting quantum fluctuations often cause unconventional behavior of thermodynamic and transport properties in the vicinity of the

Muyu Guo