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Writing and erasing. On the basis of the
experimental finding that nanoscale conducting
regions can be created and erased using voltages
applied by a conducting AFM probe (24), vari-
ous multiterminal devices have been constructed.
The structure investigated here consists of nom-
inally 3.3 unit cell thick LaAlO3 films grown on
SrTiO3 [see (27) for fabrication and measurement
details]. A conducting AFM tip is scanned along
a programmed trajectory x(t), y(t) with a voltage
Vtip(t) applied to the tip. Positive tip voltages above
a threshold Vtip > Vt ~ 2 to 3 V produce conducting
regions at the LaAlO3-SrTiO3 interface directly
below the area of contact. The lateral size dx of this
conducting nanoregion increases monotonically
with tip bias. Typical values are dx =2.1nmand
12 nm at Vtip = +3 Vand +10 V, respectively (fig.
S2, A and B). Subsequent erasure of the structures
can be induced by scanning with a negative volt-
age or by illuminating with light of photon energy
E > Eg (band gap of SrTiO3 ~3.2eV) (17, 18).
Structures can be written and erased hundreds of
times without observable degradation (fig. S2C).
All of the structures described here are written
within the same working area; similar structures
have been created and measured for other electrode
sets, with consistent results.
Designer potential barriers. The writing
and erasing process allows for a remarkable ver-
satility in producing quantum mechanical tun-
neling barriers (Fig. 1A). The transport properties
of these tunnel barriers are investigated in two
different experiments. Both begin with nano-
wires (width w ~ 12 nm) written with a positive
tip voltage Vtip = +10 V. In the first study, a four-
terminal transport measurement is performed. A
current (I) is sourced from two leads, while a
second pair of sense leads is used to measure the
voltage (V) across a section L =2 mmatthe
middle of the nanowire (Fig. 1B). As prepared,
the nanowire is well-conducting (resistance R0 =
147 kilohms, corresponding to a conductivity s =
6.8 mS) (Fig. 1D, upper inset). This conductivity
together with the nanowire’s aspect ratio (length/
width = 160) yield a sheet conductance sS =
1.1 × 10−3
S,whichis~200timesthatofthe
unstructured sample with LaAlO3 film thickness
exceeding dc [sfilm
S ≈ 2×10−5
S(17)].
A negatively biased tip (Vtip < 0 V) is then
scanned across the wire. I-V curves are acquired
after each pass of the tip. Scanning with a nega-
tive bias restores the insulating state, presumably
by shifting the local density of states in the
SrTiO3 upwardinenergy(24), thus providing a
barrier to conduction (Fig. 1A, inset). The tip bias
starts at Vtip = –0.5 Vand then increases linearly
in absolute numbers (–1V, –2V, –3V,…, –10 V).
All these I-V characteristics are highly nonlinear
(Fig. 1D), showing vanishing conductance at zero
bias, and a turn-on voltage Von (defined as the
voltage for which the current exceeds 10 nA) that
increases monotonically with tip voltage (Fig. 1D,
lower inset). A small residual conductance (4.1
nS) is observed, which is independent of Vtip and
hence is associated not with the nanowire and
tunnel barrier but with an overall parallel back-
ground conductance of the heterostructure.
In the second study, an AFM tip is scanned
repeatedly across a nanowire with relatively small
fixed bias Vtip = −50 mV (Fig. 1A). An alternating
voltage (Vac = 1mV) is applied across the nanowire
(Fig. 1C) and the resulting in-phase ac current Iac is
detected with a lock-in amplifier.With each pass of
the AFM tip, conductance G = Iac /Vac decreases
monotonically, exhibiting three qualitatively dis-
tinct regimes (Fig. 1E). For Ncut < 10, we observe
that the conductance reduces only slightly with each
pass. For 10 < Ncut < 25, the behavior transitions to
one in which the conductance decays approximately
exponentially with Ncut.For Ncut > 25, we observe a
clear deviation from this straight exponential fall-
off. We propose that the AFM probe is gradually
increasing the potential barrier between the nano-
wire leads (24). Although this process must even-
tually saturate for largeNcut, for the regime explored
the potential appears to scale linearly with Ncut,as
suggested by the observed dependence of the
conductance with Ncut over many experiments
(Fig. 1E). Along the center of the wire, the induced
potential after Ncut passes is therefore described by
an effective potential:VN(x)= V0 +NcutVb(x), where
Vb(x) is a sharply peaked (~2 nm wide) function of
position. The conductance of the nanowire mea-
sured as a function ofNcut (Fig. 1E) shows evidence
for a crossover from a highly conducting regime
(Ncut < 10) to an exponential thermal hopping
regime (10 < Ncut < 25) to one dominated by quan-
tummechanical tunneling through the barrier (Ncut >
25). The latter nonexponential form is consistent
with a tunneling probability tºexp[–A´(V – EF)
1/2
]