Survival of itinerant excitations and quantum spin state transitions in YbMgGaO4 with chemical disorder

Abstract

A recent focus of quantum spin liquid (QSL) studies is how disorder/randomness in a QSL candidate affects its true magnetic ground state. The ultimate question is whether the QSL survives disorder or the disorder leads to a “spin-liquid-like” state, such as the proposed random-singlet (RS) state. Since disorder is a standard feature of most QSL candidates, this question represents a major challenge for QSL candidates. YbMgGaO4, a triangular lattice antiferromagnet with effective spin-1/2 Yb3+ions, is an ideal system to address this question, since it shows no long-range magnetic ordering with Mg/Ga site disorder. Despite the intensive study, it remains unresolved as to whether YbMgGaO4 is a QSL or in the RS state. Here, through ultralow-temperature thermal conductivity and magnetic torque measurements, plus specific heat and DC magnetization data, we observed a residual κ0/T term and series of quantum spin state transitions in the zero temperature limit for YbMgGaO4. These observations strongly suggest that a QSL state with itinerant excitations and quantum spin fluctuations survives disorder in YbMgGaO4.

Disorder-induced broadening of the spin waves in a triangular-lattice quantum-spin-liquid candidate YbZnGaO$_4$

“…Third, besides the INS results, this proposal also properly interprets the power-law behavior of the specific heat 58 . Finally, the system can freeze into a spin glass as well in some cases, consistent with the ac susceptibility measurements 49,50 .…”

“…It shows zero residual term for κ measured along the c-axis, consistent with previous conclusions on YbMgGaO 4 and YbZnGaO 4 (Refs. [48][49][50]. On the other hand, κ a /T (κ a is the thermal conductivity along the a-axis) shows a finite linear term of κ a0 /T = 0.0058 or 0.0016 W m −1 K 2 depending on the fitting range, indicative of the survival of magnetic excitations with mean-free path a few times of the spin-spin distance, even in the presence of disorder in this material 50 .…”

“…[48][49][50]. On the other hand, κ a /T (κ a is the thermal conductivity along the a-axis) shows a finite linear term of κ a0 /T = 0.0058 or 0.0016 W m −1 K 2 depending on the fitting range, indicative of the survival of magnetic excitations with mean-free path a few times of the spin-spin distance, even in the presence of disorder in this material 50 . This is in contrast to earlier reports where the in-plane κ 0 /T is essentially zero for both materials 48,49 .…”

“…On the other hand, our further ultralow-temperature thermal conductivity measurements revealed that there was no linear term contributed by fermions such as spinons. Subsequent ultralow-temperature ac susceptibility measurements captured a broad peak with frequency dependence both in YbMgGaO 4 and YbZnGaO 4 49,50 . Furthermore, nonmagnetic Mg 2+ /Zn 2+ and Ga 3+ ions are randomly distributed on the equivalent sites on the triangular lattice with a 1:1 occupancy 41,49 .…”

“…The central issue of this debate is whether the system is susceptible to disorder due to the random distribution of the nonmagnetic ions 4,12 . Although many efforts have been made to elucidate this point 41,[46][47][48]50,53,[55][56][57][58][59][61][62][63][64][65][66][67][68] , the role of disorder on the spin dynamics is still controversial. Especially, because of the limit on the sample availability for YbZnGaO 4 , research on this compound is rather scarce 49,68 .…”