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Register a new userAging Effects Across the Metal-Insulator Transition in Two Dimensions
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Aging effects in the relaxations of conductivity of a two-dimensional electron system in Si have been studied as a function of carrier density. They reveal an abrupt change in the nature of the glassy phase at the metal-insulator transition (MIT): (a) while full aging is observed in the insulating regime, there are significant departures from full aging on the metallic side of the MIT, before the glassy phase disappears completely at a higher density $n_g$; (b) the amplitude of the relaxations peaks just below the MIT, and it is strongly suppressed in the insulating phase. Other aspects of aging, including large non-Gaussian noise and similarities to spin glasses, also have been discussed.
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A wide range of disordered materials, including disordered correlated systems, show Universal Dielectric Response (UDR), followed by a superlinear power-law increase in their optical responses over exceptionally broad frequency regimes. While extensively used in various contexts over the years, the microscopics underpinning UDR remains controversial. Here, we investigate the optical response of the simplest model of correlated fermions, Falicov-Kimball model (FKM), across the continuous metal-insulator transition (MIT) and analyze the associated quantum criticality in detail using cluster extension of dynamical mean field theory (CDMFT). Surprisingly, we find that UDR naturally emerges in the quantum critical region associated with the continuous MIT. We tie the emergence of these novel features to a many-body orthogonality catastrophe accompanying the onset of strongly correlated electronic glassy dynamics close to the MIT, providing a microscopic realization of Jonschers time-honored proposal as well as a rationale for similarities in optical responses between correlated electronic matter and canonical glass formers.
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.
It is well-known that magnetic impurities can change the symmetry class of disordered metallic systems by breaking spin and time-reversal symmetry. At low temperature these symmetries can be restored by Kondo screening. It is also known that at the Anderson metal-insulator transition, wave functions develop multifractal fluctuations with power law correlations. Here, we consider the interplay of these two effects. We show that multifractal correlations open local pseudogaps at the Fermi energy at some random positions in space. When dilute magnetic impurities are at these locations, Kondo screening is strongly suppressed. We find that when the exchange coupling J is smaller than a certain value J*, the metal-insulator transition point extends to a critical region in the disorder strength parameter and to a band of critical states. The width of this critical region increases with a power of the concentration of magnetic impurities.
We present results from an experimental study of the equilibrium and non-equilibrium transport properties of vanadium oxide nanobeams near the metal-insulator transition (MIT). Application of a large electric field in the insulating phase across the nanobeams produces an abrupt MIT and the individual roles of thermal and non-thermal effects in driving the transition are studied. Transport measurements at temperatures ($T$) far below the critical temperature ($T_c$) of MIT, in several nanoscale vanadium oxide devices, show that both $T$ and electric field play distinctly separate, but critical roles in inducing the MIT. Specifically, at $T << T_c$ electric field dominates the MIT through an avalanche-type process, whereas thermal effects become progressively critical as $T$ approaches $T_c$.
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