Magnetic Grüneisen ratio of the random transverse‐field Ising chain

The magnetic analog of the Grüneisen parameter, that is, 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 low‐temperature behavior at the exotic infinite‐randomness quantum critical point and in the associated quantum Griffiths phases. We present extensive numerical simulations showing that the magnetic Grüneisen parameter diverges logarithmically with decreasing temperature in the quantum Griffiths phase. It changes sign right at criticality. These results confirm a recent strong‐disorder renormalization group theory. We also compare our findings to the behavior of the clean transverse‐field Ising chain.