Magnetoconductance of Independently Tunable Tunnel-Coupled Double Quantum Wires Page: 4 of 6
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gates by electron beam lithography on each
side of the DQW heterostructure, whose
thickness is only 3000 A. Not only does this
process allow sub-tenth-micron alignment of
top and back gates, but has no deleterious
effect on sample mobility. The resulting
sample has both top and back split gates,
each placed only -1500 A from the DQW.
Fig. 1(a) and (b) show schematics of the
sample, and Fig. 1(c) shows a scanning
electron micrograph of a cross sectioned
dual split-gate test structure of similar
geometry, but a thickness of -1 pm.
By controlling the relative values of top
and back split-gate voltages VT and VB, the
sample can be tuned into five different
regimes depending on the widths of the
individual QWs. These regimes are (i) 2D-
2D; (ii) 2D-1D; (iii) 1D-1D; (iv) 1D-
pinched-off; and (v) both pinched-off. Fig.
2 shows the sample resistance as a function
of VT, with VB = 0. A number of features
appear which correspond to transitions
between the different regimes. At VT = 0,
both channels are 2D. As VT is made
increasingly negative, the channel resistance
shows a clear shoulder at -0.3 V followed
by a sharp increase at -0.7 V. These two
features are due to the sequential depletion
of first the top and then the bottom QW, in
Fig. 2 Resistance of device as VT is swept with VB
grounded. Inset at bottom shows transition from
regime (i) to (ii) at V-T = -0.3 V. Transition from (ii)
to (iii) can be seen at V-T = -0.7 V while distinct
increase in slope at VT = -2.6 V marks transition
from (iii) to (iv). Bottom figure is magnified 8x to
the regions directly beneath the top split
gate. Thus, at VT = -0.7 V, where electrons
beneath the top split gate become
completely depleted from both QWs,
coupled 1D channels are formed in both
QWs. Hence this marks the transition from
regime (ii), the 1D-2D case, to regime (iii),
the 1D-1D case. A simple capacitance
calculation predicts the complete depletion
of electrons beneath the split gates at VT = -
0.9 V, close to the experimental value.
As VT is swept further, we expect the
QPC widths to narrow. Due to the fact that
the split gate separation is comparable to the
electron layer depth, the top channel width
narrows more rapidly than the bottom width.
At VT = -2.6 V, the top QPC completely
pinches off, leaving only the bottom QPC,
corresponding to the transition between
regimes (iii) and (iv). A weak plateau is
observed at this point, as well as a change in
the slope of the resistance. As VT is made
yet more negative, the familiar steps in
resistance occurring at h/2ne2 (n = 1, 2, ...)
are observed due to the narrowing of the
bottom QPC. Finally, at VT = -3.8 V, after
the last step at R = h/2e2 occurs, both
channels are completely pinched off.
Similar results are obtained when instead VT
= 0 and VB is swept negative.
These different regimes are evident in a
waterfall plot of the conductance G of the
device as a function of both VT and VB,
shown in Fig. 3. Regimes (iii) and (iv) are
now easily identified by their markedly
different behavior. When only one wire is
occupied (regime iv), uniform steps in the
conductance quantized in units of 2e2/h
appear. This can be seen clearly at the base
of the plot, near VT = -1.0 V, VB = -2.0 V,
and VT = -3.0 V, VB = -1.0 V. However,
when both wires are occupied (regime iii),
quantized steps are again present, but form a
complicated interference pattern as VT and
VB are varied. By tracking the position of
individual conductance steps as a function of
both VT and VB, each step can be assigned to
one quantum wire or the other. As expected,
the total conductance of the VCQPC
throughout regime (iii) and (iv) agrees with
a count of the total number of quantized
steps multiplied by 2e2/h, indicating that the
transport is ballistic. Clearly, each QPC
width--and thus the number of occupied
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BLOUNT,MARK A.; MOON,J.S.; SIMMONS,JERRY A.; LYO,SUNGKWUN K.; WENDT,JOEL R. & RENO,JOHN L. Magnetoconductance of Independently Tunable Tunnel-Coupled Double Quantum Wires, article, July 13, 2000; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc703427/m1/4/: accessed December 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.