Experimental studies on quantum magnets in the presence of disorder

The rare-earth material ${\mathrm{LiHoF}}_{4}$ is believed to be an experimental realization of the celebrated (dipolar) Ising model and, upon the inclusion of a transverse field ${B}_{x}$ , an archetypal quantum Ising model. Moreover, by substituting the magnetic Ho ions by nonmagnetic Y ions, disorder can be introduced into the system, giving rise to a dipolar disordered magnet and at high disorders to a spin glass. Indeed, this material has been scrutinized experimentally, numerically, and theoretically over many decades with the aim of understanding various collective magnetic phenomena. One of the to-date open questions is the discrepancy between the experimental and theoretical ${B}_{x}\ensuremath{-}T$ phase diagram at low fields and high temperatures. Here we propose a mechanism, backed by numerical results, that highlights the importance of quantum fluctuations induced by the off-diagonal dipolar terms, in determining the critical temperature of anisotropic dipolar magnets in the presence and in the absence of a transverse field. We thus show that the description as a simple Ising system is insufficient to quantitatively describe the full phase diagram of ${\mathrm{LiHoF}}_{4}$ , for the pure as well as for the dilute system.The rare-earth material ${\mathrm{LiHoF}}_{4}$ is believed to be an experimental realization of the celebrated (dipolar) Ising model and, upon the inclusion of a transverse field ${B}_{x}$, an archetypal quantum Ising model. Moreover, by substituting the magnetic Ho ions by nonmagnetic Y ions, disorder can be introduced into the system, giving rise to a dipolar disordered magnet and at high disorders to a spin glass. Indeed, this material has been scrutinized experimentally, numerically, and theoretically over many decades with the aim of understanding various collective magnetic phenomena. One of the to-date open questions is the discrepancy between the experimental and theoretical ${B}_{x}\ensuremath{-}T$ phase diagram at low fields and high temperatures. Here we propose a mechanism, backed by numerical results, that highlights the importance of quantum fluctuations induced by the off-diagonal dipolar terms, in determining the critical temperature of anisotropic dipolar magnets in the presence and in the absence of a transverse field. We thus show that the description as a simple Ising system is insufficient to quantitatively describe the full phase diagram of ${\mathrm{LiHoF}}_{4}$, for the pure as well as for the dilute system.

arXiv:2203.04359Physical review. B./Physical review. BMay 25, 20228 citations

Disorder correlations at smeared phase transitions

C. Svoboda, D. Nozadze, Fawaz Y. Hrahsheh +1 more

We investigate the influence of spatial disorder correlations on smeared phase transitions, taking the magnetic quantum phase transition in an itinerant magnet as an example. We find that even short-range correlations can have a dramatic effect and qualitatively change the behavior of observable quantities compared to the uncorrelated case. This is in marked contrast to conventional critical points, at which short-range correlated disorder and uncorrelated disorder lead to the same critical behavior. We develop an optimal fluctuation theory of the quantum phase transition in the presence of correlated disorder, and we illustrate the results by computer simulations. As an experimental application, we discuss the ferromagnetic quantum phase transition in Sr 1− x Ca x RuO 3 .We investigate the influence of spatial disorder correlations on smeared phase transitions, taking the magnetic quantum phase transition in an itinerant magnet as an example. We find that even short-range correlations can have a dramatic effect and qualitatively change the behavior of observable quantities compared to the uncorrelated case. This is in marked contrast to conventional critical points, at which short-range correlated disorder and uncorrelated disorder lead to the same critical behavior. We develop an optimal fluctuation theory of the quantum phase transition in the presence of correlated disorder, and we illustrate the results by computer simulations. As an experimental application, we discuss the ferromagnetic quantum phase transition in Sr 1− x Ca x RuO 3 .

arXiv:1109.4290Europhysics Letters (EPL)Jan 1, 20128 citations

Random quantum magnets with broad disorder distribution

Dragi Karevski, Yu‐Cheng Lin, Heiko Rieger +2 more

arXiv:cond-mat/0009144The European Physical Journal BMar 1, 200153 citations

QUANTUM FRUSTRATED MAGNETS: FROM THEORY TO EXPERIMENTS

Frédéric Mila

Recent Progress in Many-Body TheoriesDec 1, 20020 citations

The rare-earth material

{\mathrm{LiHoF}}_{4}

is believed to be an experimental realization of the celebrated (dipolar) Ising model and, upon the inclusion of a transverse field

{B}_{x}

, an archetypal quantum Ising model. Moreover, by substituting the magnetic Ho ions by nonmagnetic Y ions, disorder can be introduced into the system, giving rise to a dipolar disordered magnet and at high disorders to a spin glass. Indeed, this material has been scrutinized experimentally, numerically, and theoretically over many decades with the aim of understanding various collective magnetic phenomena. One of the to-date open questions is the discrepancy between the experimental and theoretical ${B}_{x}\ensuremath{-}T$ phase diagram at low fields and high temperatures. Here we propose a mechanism, backed by numerical results, that highlights the importance of quantum fluctuations induced by the off-diagonal dipolar terms, in determining the critical temperature of anisotropic dipolar magnets in the presence and in the absence of a transverse field. We thus show that the description as a simple Ising system is insufficient to quantitatively describe the full phase diagram of

{\mathrm{LiHoF}}_{4}

, for the pure as well as for the dilute system.