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In a recent paper Castro-Neto and Jones argue that because the observability of quantum Griffiths-McCoy effects in metals is controlled by non-universal quantities, the quantum Griffiths-McCoy scenario may be a viable explanation for the non-fermi-liquid behavior observed in heavy fermion compounds. In this Comment we point out that the important non-universal quantity is the damping of the spin dynamics by the metallic electrons; quantum Griffiths-McCoy effects occur only if this is parametrically weak relative to other scales in the problem, i.e. if the spins are decoupled from the carriers. This suggests that in heavy fermion materials, where the Kondo effect leads to a strong carrier-spin coupling, quantum Griffiths-McCoy effects are unlikely to occur.
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Magnetoconductance (MC) in a parallel magnetic field B has been measured in a two-dimensional electron system in Si, in the regime where the conductivity decreases as sigma (n_s,T,B=0)=sigma (n_s,T=0) + A(n_s)T^2 (n_s -- carrier density) to a non-zero value as temperature T->0. Very near the B=0 metal-insulator transition, there is a large initial drop in sigma with increasing B, followed by a much weaker sigma (B). At higher n_s, the initial drop of MC is less pronounced.
We report the observation of a metal-insulator transition in a two-dimensional electron gas in silicon. By applying substrate bias, we have varied the mobility of our samples, and observed the creation of the metallic phase when the mobility was high enough ($mu ~> 1 m^2/Vs$), consistent with the assertion that this transition is driven by electron-electron interactions. In a perpendicular magnetic field, the magnetoconductance is positive in the vicinity of the transition, but negative elsewhere. Our experiment suggests that such behavior results from a decrease of the spin-dependent part of the interaction in the vicinity of the transition.
The temperature dependence of conductivity $sigma (T)$ of a two-dimensional electron system in silicon has been studied in parallel magnetic fields B. At B=0, the system displays a metal-insulator transition at a critical electron density $n_c(0)$, and $dsigma/dT >0$ in the metallic phase. At low fields ($Blesssim 2$ T), $n_c$ increases as $n_c(B) - n_c(0) propto B^{beta}$ ($betasim 1$), and the zero-temperature conductivity scales as $sigma (n_s,B,T=0)/sigma (n_s,0,0)=f(B^{beta}/delta_n)$ (where $delta_n=(n_s-n_c(0))/n_c(0)$, and $n_s$ is electron density) as expected for a quantum phase transition. The metallic phase persists in fields of up to 18 T, consistent with the saturation of $n_c$ at high fields.
The relaxations of conductivity have been studied in the glassy regime of a strongly disordered two-dimensional electron system in Si after a temporary change of carrier density during the waiting time t_w. Two types of response have been observed: a) monotonic, where relaxations exhibit aging, i.e. dependence on history, determined by t_w and temperature; b) nonmonotonic, where a memory of the sample history is lost. The conditions that separate the two regimes have been also determined.
We study the ferromagnetic phase transition in a randomly layered Heisenberg model. A recent strong-disorder renormalization group approach [Phys. Rev. B 81, 144407 (2010)] predicted that the critical point in this system is of exotic infinite-randomness type and is accompanied by strong power-law Griffiths singularities. Here, we report results of Monte-Carlo simulations that provide numerical evidence in support of these predictions. Specifically, we investigate the finite-size scaling behavior of the magnetic susceptibility which is characterized by a non-universal power-law divergence in the Griffiths phase. In addition, we calculate the time autocorrelation function of the spins. It features a very slow decay in the Griffiths phase, following a non-universal power law in time.
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