Transient currents and universal timescales for a fully time-dependent quantum dot in the Kondo regime

Using the time-dependent non-crossing approximation, we calculate the transient response of the current through a quantum dot subject to a finite bias when the dot level is moved suddenly into a regime where the Kondo effect is present. After an initial small but rapid response, the time-dependent conductance is a universal function of the temperature, bias, and inverse time, all expressed in units of the Kondo temperature. Two timescales emerge: the first is the time to reach a quasi-metastable point where the Kondo resonance is formed as a broad structure of half-width of the order of the bias; the second is the longer time required for the narrower split peak structure to emerge from the previous structure and to become fully formed. The first time can be measured by the gross rise time of the conductance, which does not substantially change later while the split peaks are forming. The second time characterizes the decay rate of the small split Kondo peak (SKP) oscillations in the conductance, which may provide a method of experimental access to it. This latter timescale is accessible via linear response from the steady stateand appears to be related to the scale identified in that manner [A. Rosch, J. Kroha, and P. Wolfle, Phys. Rev. Lett. 87, 156802 (2001)].Comment: Revtex with 15 eps figures. Compiles to 11 page

Kondo time scales for quantum dots - response to pulsed bias potentials

The response of a quantum dot in the Kondo regime to rectangular pulsed bias potentials of various strengths and durations is studied theoretically. It is found that the rise time is faster than the fall time, and also faster than time scales normally associated with the Kondo problem. For larger values of the pulsed bias, one can induce dramatic oscillations in the induced current with a frequency approximating the splitting between the Kondo peaks that would be present in steady state. The effect persists in the total charge transported per pulse, which should facilitate the experimental observation of the phenomenon.Comment: 5 pages with 4 encapsulated figures which come in separate postscript files: latex file: text.tex figures: fig1.eps, fig2.eps, fig3.eps, fig4.ep

Kondo effect in coupled quantum dots: a Non-crossing approximation study

The out-of-equilibrium transport properties of a double quantum dot system in the Kondo regime are studied theoretically by means of a two-impurity Anderson Hamiltonian with inter-impurity hopping. The Hamiltonian, formulated in slave-boson language, is solved by means of a generalization of the non-crossing approximation (NCA) to the present problem. We provide benchmark calculations of the predictions of the NCA for the linear and nonlinear transport properties of coupled quantum dots in the Kondo regime. We give a series of predictions that can be observed experimentally in linear and nonlinear transport measurements through coupled quantum dots. Importantly, it is demonstrated that measurements of the differential conductance ${\cal G}=dI/dV$, for the appropriate values of voltages and inter-dot tunneling couplings, can give a direct observation of the coherent superposition between the many-body Kondo states of each dot. This coherence can be also detected in the linear transport through the system: the curve linear conductance vs temperature is non-monotonic, with a maximum at a temperature $T^*$ characterizing quantum coherence between both Kondo states.Comment: 20 pages, 17 figure

Green function techniques in the treatment of quantum transport at the molecular scale

The theoretical investigation of charge (and spin) transport at nanometer length scales requires the use of advanced and powerful techniques able to deal with the dynamical properties of the relevant physical systems, to explicitly include out-of-equilibrium situations typical for electrical/heat transport as well as to take into account interaction effects in a systematic way. Equilibrium Green function techniques and their extension to non-equilibrium situations via the Keldysh formalism build one of the pillars of current state-of-the-art approaches to quantum transport which have been implemented in both model Hamiltonian formulations and first-principle methodologies. We offer a tutorial overview of the applications of Green functions to deal with some fundamental aspects of charge transport at the nanoscale, mainly focusing on applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references, submitted to Springer series "Lecture Notes in Physics

Sleep-Related Hippocampo-Cortical Interplay during Emotional Memory Recollection

Emotional events are usually better remembered than neutral ones. This effect is mediated in part by a modulation of the hippocampus by the amygdala. Sleep plays a role in the consolidation of declarative memory. We examined the impact of sleep and lack of sleep on the consolidation of emotional (negative and positive) memories at the macroscopic systems level. Using functional MRI (fMRI), we compared the neural correlates of successful recollection by humans of emotional and neutral stimuli, 72 h after encoding, with or without total sleep deprivation during the first post-encoding night. In contrast to recollection of neutral and positive stimuli, which was deteriorated by sleep deprivation, similar recollection levels were achieved for negative stimuli in both groups. Successful recollection of emotional stimuli elicited larger responses in the hippocampus and various cortical areas, including the medial prefrontal cortex, in the sleep group than in the sleep deprived group. This effect was consistent across subjects for negative items but depended linearly on individual memory performance for positive items. In addition, the hippocampus and medial prefrontal cortex were functionally more connected during recollection of either negative or positive than neutral items, and more so in sleeping than in sleep-deprived subjects. In the sleep-deprived group, recollection of negative items elicited larger responses in the amygdala and an occipital area than in the sleep group. In contrast, no such difference in brain responses between groups was associated with recollection of positive stimuli. The results suggest that the emotional significance of memories influences their sleep-dependent systems-level consolidation. The recruitment of hippocampo-neocortical networks during recollection is enhanced after sleep and is hindered by sleep deprivation. After sleep deprivation, recollection of negative, potentially dangerous, memories recruits an alternate amygdalo-cortical network, which would keep track of emotional information despite sleep deprivation

Changes in recognition memory over time: an ERP investigation into vocabulary learning

Although it seems intuitive to assume that recognition memory fades over time when information is not reinforced, some aspects of word learning may benefit from a period of consolidation. In the present study, event-related potentials (ERP) were used to examine changes in recognition memory responses to familiar and newly learned (novel) words over time. Native English speakers were taught novel words associated with English translations, and subsequently performed a Recognition Memory task in which they made old/new decisions in response to both words (trained word vs. untrained word), and novel words (trained novel word vs. untrained novel word). The Recognition task was performed 45 minutes after training (Day 1) and then repeated the following day (Day 2) with no additional training session in between. For familiar words, the late parietal old/new effect distinguished old from new items on both Day 1 and Day 2, although response to trained items was significantly weaker on Day 2. For novel words, the LPC again distinguished old from new items on both days, but the effect became significantly larger on Day 2. These data suggest that while recognition memory for familiar items may fade over time, recognition of novel items, conscious recollection in particular may benefit from a period of consolidation

Are spatial memories strengthened in the human hippocampus during slow wave sleep ?

In rats, the firing sequences observed in hippocampal ensembles during spatial learning are replayed during subsequent sleep, suggesting a role for posttraining sleep periods in the offline processing of spatial memories. Here, using regional cerebral blood flow measurements, we show that, in humans, hippocampal areas that are activated during route learning in a virtual town are likewise activated during subsequent slow wave sleep. Most importantly, we found that the amount of hippocampal activity expressed during slow wave sleep positively correlates with the improvement of performance in route retrieval on the next day. These findings suggest that learning-dependent modulation in hippocampal activity during human sleep reflects the offline processing of recent episodic and spatial memory traces, which eventually leads to the plastic changes underlying the subsequent improvement in performance.Comparative StudyJournal ArticleResearch Support, Non-U.S. Gov'tinfo:eu-repo/semantics/publishe