Quantum and classical transport in two dimensional systems

The quantum and classical transport in two-dimensional systems are investigated theoretically, with an emphasis on the scaling and the universality at the integer quantum Hall plateau transitions. The two-terminal zero-temperature conductances of two-dimensional disordered electron systems are calculated numerically with the multichannel Landauer formula by employing the transfer matrix technique within the tight-binding model. By applying the one-parameter scaling idea, all data for system sizes from 16 x 16 to very large sizes 192 x 192 and for energies within the first Landau band were scaled beautifully to one universal curve. The localization length was found to diverge at the critical point with an exponent [nu] =2.37, in accord with previous calculations and in support of the idea of universality. The critical conductance, extrapolated to macroscopic sizes through the analysis of finite size corrections from irrelevant operators, was found to be \u3c Gc\u3e =0.506e2[over]h, in excellent agreement with the value [sigma]xx=0.5e2[over]h predicted by an analytical theory, but in disagreement with a recent calculation based on the network model, \u3c Gc\u3e =0.58e2[over]h. This small but significant difference may indicate that the two-terminal critical conductance at the localization-delocalization transition is non-universal, unlike the Hall conductances which are precisely quantized due to topological invariance. Scaling of conductance is also extended to systems with anisotropic hopping integrals with the same critical exponent. However, the geometrical mean of the averaged critical conductances in the two directions does not seem to stay as a constant independent of the anisotropy, casting further doubt on the universality of the critical conductance. The statistical distribution of the conductance at the critical point showed a marked difference between the two directions in aniostropic systems. The distributions has a large weight in small G in the weak coupling direction while the distribution for the strong coupling direction shows a large dip at small G. Monte Carlo simulations on the classical adatom diffusion reveals the influence of the STM tip field on the diffusion process in the early non-equilibrium stage as seen by the change of the shape and the decay constant of the density-density correlation functions

Reading Without Speech Sounds: VWFA and its Connectivity in the Congenitally Deaf

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What is the right form of the probability distribution of the conductance at the mobility edge?

The probability distribution of the conductance Pc(g) at the Anderson critical point is calculated. It is find that Pc(g) has a dip at small g in agreement with epsilon expansion results. The Pc(g) for the 3d system is quite different from the 2d quantum critical point of the integer quantum Hall effect. The universality or not of these distributions is of central importance to the field of disordered systems.Comment: 1 page, 1 figure submitted to Phys. Rev. Lett. (Comment

Scaling Properties of Conductance at Integer Quantum Hall Plateau Transitions

We investigate the scaling properties of zero temperature conductances at integer quantum Hall plateau transitions in the lowest Landau band of a two-dimensional tight-binding model. Scaling is obeyed for all energy and system sizes with critical exponent nu =7/3 . The arithmetic average of the conductance at the localization-delocalization critical point is found to be _c = 0.506 e^2 / h, in agreement with the universal longitudinal conductance predicted by an analytical theory. The probability distribution of the conductance at the critical point is broad with a dip at small G.Comment: 4 pages, 3 postscript figures, Submitted to PR

Quantum and classical transport in two dimensional systems

The quantum and classical transport in two-dimensional systems are investigated theoretically, with an emphasis on the scaling and the universality at the integer quantum Hall plateau transitions. The two-terminal zero-temperature conductances of two-dimensional disordered electron systems are calculated numerically with the multichannel Landauer formula by employing the transfer matrix technique within the tight-binding model. By applying the one-parameter scaling idea, all data for system sizes from 16 x 16 to very large sizes 192 x 192 and for energies within the first Landau band were scaled beautifully to one universal curve. The localization length was found to diverge at the critical point with an exponent [nu] =2.37, in accord with previous calculations and in support of the idea of universality. The critical conductance, extrapolated to macroscopic sizes through the analysis of finite size corrections from irrelevant operators, was found to be =0.506e2[over]h, in excellent agreement with the value [sigma]xx=0.5e2[over]h predicted by an analytical theory, but in disagreement with a recent calculation based on the network model, =0.58e2[over]h. This small but significant difference may indicate that the two-terminal critical conductance at the localization-delocalization transition is non-universal, unlike the Hall conductances which are precisely quantized due to topological invariance. Scaling of conductance is also extended to systems with anisotropic hopping integrals with the same critical exponent. However, the geometrical mean of the averaged critical conductances in the two directions does not seem to stay as a constant independent of the anisotropy, casting further doubt on the universality of the critical conductance. The statistical distribution of the conductance at the critical point showed a marked difference between the two directions in aniostropic systems. The distributions has a large weight in small G in the weak coupling direction while the distribution for the strong coupling direction shows a large dip at small G. Monte Carlo simulations on the classical adatom diffusion reveals the influence of the STM tip field on the diffusion process in the early non-equilibrium stage as seen by the change of the shape and the decay constant of the density-density correlation functions.</p

word_ISC

intersubject consistency of word representations from behavioral and neuroimaging dat

Early language exposure affects neural mechanisms of semantic representations

One signature of the human brain is its ability to derive knowledge from language inputs, in addition to nonlinguistic sensory channels such as vision and touch. How does human language experience modulate the mechanism by which semantic knowledge is stored in the human brain? We investigated this question using a unique human model with varying amounts and qualities of early language exposure: early deaf adults who were born to hearing parents and had reduced early exposure and delayed acquisition of any natural human language (speech or sign), with early deaf adults who acquired sign language from birth as the control group that matches on nonlinguistic sensory experiences. Neural responses in a semantic judgment task with 90 written words that were familiar to both groups were measured using fMRI. The deaf group with reduced early language exposure, compared with the deaf control group, showed reduced semantic sensitivity, in both multivariate pattern (semantic structure encoding) and univariate (abstractness effect) analyses, in the left dorsal anterior temporal lobe (dATL). These results provide positive, causal evidence that language experience drives the neural semantic representation in the dATL, highlighting the roles of language in forming human neural semantic structures beyond nonverbal sensory experiences

Resting-state functional connectivity patterns predict Chinese word reading competency.

Resting-state functional connectivity (RSFC) offers a novel approach to reveal the temporal synchronization of functionally related brain regions. Recent studies have identified several RSFCs whose strength was associated with reading competence in alphabetic languages. In the present study, we examined the role of intrinsic functional relations for reading a non-alphabetic language--Chinese--by correlating RSFC maps of nine Chinese reading-related seed regions and reaction time in the single-character reading task. We found that Chinese reading efficiency was positively correlated with the connection between left inferior occipital gyrus and left superior parietal lobule, between right posterior fusiform gyrus and right superior parietal lobule, and between left inferior temporal gyrus and left inferior parietal lobule. These results could not be attributed to inter-individual differences arising from the peripheral processes of the reading task such as visual input detection and articulation. The observed RSFC-reading correlation relationships are discussed in the framework of Chinese character reading, including visuospatial analyses and semantic/phonological processes