Sources of negative tunneling magnetoresistance in multilevel quantum dots with ferromagnetic contacts

Abstract

Document VersionPublisher's PDF, also known as Version of record Citation for published version (APA):Koller, S., Grifoni, M., & Paaske, J. (2012 We analyze distinct sources of spin-dependent energy level shifts and their impact on the tunneling magnetoresistance (TMR) of interacting quantum dots coupled to collinearly polarized ferromagnetic leads. Level shifts due to virtual charge fluctuations can be quantitatively evaluated within a diagrammatic representation of our transport theory. The theory is valid for multilevel quantum dot systems and we exemplarily apply it to carbon nanotube quantum dots, where we show that the presence of many levels, among them of excited states, can qualitatively influence the TMR effect.

Optimized fabrication and characterization of carbon nanotube spin valves

“…More elaborate models 26,27 predict major contributions to the MR from shifts of the CB resonances in an effective magnetic field, caused either by spin dependent electron scattering at the QD-contact interfaces or by a spindependent renormalization of the QD energy levels. Characteristic for both mechanisms is that the sign of the shifts depends on the spin state of the CB resonance.…”

Cotunneling renormalization in carbon nanotube quantum dots

“…In this case, for symmetric couplings to the left and right leads we can find the eigenspectrum around the middle of the diamond by linearizing the logarithms in Eq. (19), which yields the following energy difference between ground and excited states…”

Transport across a carbon nanotube quantum dot contacted with ferromagnetic leads: Experiment and nonperturbative modeling

“…In terms of an effective energy shift in a multi-level quantum dot the renormalization due to excited states was also discussed in Ref. 21. To illustrate the effect of such a modification we plot the real and imaginary parts of the self energy Σ in the vicinity of the transition |(K ↓), n |·, n + 1 for different sets of charge fluctuations within energy ranges of γ 0 , 0 , 2 0 and 3 0 in Fig.…”

“…Noticeably, the commonly used perturbative description of the Coulomb resonances predicts temperature broadened peaks and maxima whose positions are solely determined by the isolated quantum dot spectrum implying a constant, positive TMR [21]. A transport theory accounting for charge fluctuations non-perturbatively was shown to shift the quantum dot energy levels depending on the magnetization configuration of the leads [21].…”

“…In most cases, a spatially confined quantum dot is coupled to ferromagnetic electrodes. Electronic transport across CNT quantum dots can take place in different regimes: Depending on the relative magnitude of coupling strength, temperature and charging energy, this ranges from an opaque Coulomb-blockade regime [16][17][18][19], to an intermediate coupling regime with lead induced energy level shifts [20][21][22], to a strongly correlated Kondo regime [23][24][25][26]. For highly transparent contacts, in contrast, the dot behaves essentially like an electronic wave guide [27,28].…”