A Comprehensive Study of Dilute Magnetic ... - OPUS Würzburg

7.2. Quantum dot coupled to both baths 87
0.75
0.70
Γ
0.65
0.60
35 40 45 50 55
VmV
Figure 7.4: Bias voltage dependence of the Fermi-edge exponents for spin-up (red) and spindown
(blue) electrons as used in the model. The dashed gray lines depict the curve for T→0
K, where each vertical steps is the amplitude of the phase loss dγ per additional conducting
transport channel. The intrinsic line width Γ i is used as the temperature scale for the broadening
of the red and blue curve.
comprised of more than one tunneling contribution. Plotting dI/dV in fig. 7.2b+c gives
further support to this assumption, as it clearly reveals the fine structure on the higher
bias voltage tails of the resonance-peaks.
This fine structure however is of different character than the one seen for the FES in
the non-magnetic quantum dot (see fig. 6.6) and was also never seen in such a magnetic
quantum dot RTD without an apparent FES enhancement.
Furthermore, this fine structure is clearly observed at zero magnetic field and cannot
be caused by Landau level quantization effects. We thus argue that it originates from the
simultaneous observation of both many body correlation effects (FES and BMP) on the
same quantum dot. Figure 7.3c shows a further magnification of feature A at B=0 T and
T=20 mK while 7.3d again plots its derivative. Even though it exhibits a complex fine
structure, it still has some resemblance with the I-V characteristics of an FES enhanced
resonance-peak. The sharp onset as well as the characteristic high bias voltage tail are
still clearly observed, and the overall shape suggests that feature A is comprised of a
superposition of several FES-like contributions. In fig. 7.3a, we present a modeled I-
V characteristic employing the aforementioned observations. The red (blue) curves are
contributions from various FES enhanced spin-up (spin-down) transport channels. The
I-V characteristics of each transport channel are given by equation (6.1) with the above
determined lever arm α − and an intrinsic quantum dot width of Γ i =0.4 meV. Each of the
additional resonance-peaks occurring at increased bias voltages has a reduced amplitude,
stemming from a reduction of the Fermi-edge exponent γ.
The dI/dV plot of the experimental data in fig. 7.3d suggests a total of ten transport
channels. We thus assume four additional excited states on top of the spin-up and spindown
states. The apparent fine structure is labeled from 1-10 in order to compare to the
dI/dV plot of the modeled data in fig. 7.3b.