Science - USA (2021-07-16)

In addition to electrical excitation, we also
separately pumped the samples using syn-
chronized ~100-fs, 1.55-eV optical pulses to
study the dynamics during P-IMT (Fig. 1B,
bottom). The devices display typical threshold-
switching behavior, as shown by the current-
voltage curve in Fig. 1C. When biased above a
threshold voltage, the current increases abrupt-
ly as a result of the formation of an electrically
conducting state (figs. S2 and S3). Figure 1D
shows the resistance of the VO 2 channel (RVO2)
asafunctionoftimeafterastepvoltageisap-
plied. The three curves correspond to voltage
magnitudes of 4 V (red), 4.8 V (blue), and 5.6 V
(green).RVO2decreases strongly after a finite
incubation time ( 26 ); this time scale is smaller
at higher voltages. The transient character-
istics are repeatable over millions of cycles,
which is a crucial requirement for strobosco-
pic measurements. This is shown in Fig. 1E,
where we plot the resistance ratio between
the insulating and metallic states (top) and
the incubation time (bottom) versus cycle
number.
The diffraction data were analyzed by azi-
muthally integrating the polycrystalline dif-
fraction pattern and computing changes in
intensity relative to the unexcitedM1phase
( 25 ). Figure 2A displays a two-dimensional
(2D) color map of the normalized intensity
change,dI(Q,t)=[I(Q,t)−I 0 (Q)]/I 0 (Q), as a
function of momentum transferQand delay
timet.I 0 (Q) corresponds to the unexcited state
att<0.Alineoutatadelayof500ms is shown
in Fig. 2B, where three peaks of interest are
indexed, namely 30ðÞ 2 , 31ðÞ 3 , and (220). To
compare the structural dynamics during E-IMT
with P-IMT, we excited the same device with
~100-fs optical pulses at 1.55 eV. As seen in
Fig. 2C, photoexcitation induces ultrafast struc-
tural dynamics on the picosecond time scale.
Figure 2D plots a lineout at a delay of 5 ps.
To resolve the time scales of the structural
transformation after electrical excitation, we
plot time-dependentdI 30  2 ,dI 31  3 , anddI 220 at
different voltagesV= 4, 4.8, and 5.6 V (Fig. 3,
A to C). Three distinct regimes are identified.
(i) As the voltage is turned on, the intensities
of the 30ðÞ 2 and 31ðÞ 3 peaks decrease, whereas
the intensity of the (220) peak increases. As
shown by structure factor calculations (fig.
S4A), the 30ðÞ 2 and 31ðÞ 3 peaks are present
in theM1but absent in the high-symmetry
Rphase. Therefore,dI 30  2 anddI 31  3 capture
theM1→Rstructural phase transformation
(SPT)—i.e., the nucleation and growth of the
metallicRphase under electrical excitation.
As we show below, the dynamics of the (220)
peak encode information about the nonequili-
brium structural changes occurring during the
E-IMT. (ii) After the voltage is turned off, the
structure persists for a finite duration, possibly
owing to hysteresis in the phase transition as
thedevicecoolsandtheassociatedbarrierfor

the reverseR→M1transformation ( 12 ). (iii) The

structure returns to theM1phase in a quasi-

exponential manner within ~2 ms, consist-

ent with time scales for lateral heat transport

along the membrane into the Si substrate.

In Fig. 3, D to F, we zoom in to the rising

edge of the voltage pulse and probe structural

dynamics and transport with higher time reso-

lution (see fig. S5 for the corresponding de-

vice characteristics). A delay is observed in the

SCIENCEsciencemag.org 16 JULY 2021•VOL 373 ISSUE 6552 353

e-

Delay (ps)

ph

time

Optical

Pump

e-

V

Delay (μs-ms)

Electrical

Pump

time

180 Hz

VVO2 (V)

0

0.2

0.4

0.6

I (mA)

024

0 200 400

1

10

100

Time delay (μs)

R

VO2

(k

)

ABC

D

012 3

#Cycles (×10^6 )

50

150

250

tinc

(μs)

15

20

25

R

high

/R

low

E

4 V

4.8 V

5.6 V

4 V

4.8 V

5.6 V

Voltage pulses

Electron pulses

~100 fs, 2.4 MeV

Optical pulses

~100 fs, 1.55 eV

RS

Rscope

Vscope

I

Si

VO 2

Pt

Diffraction pattern

Probe Pump

Pump

Fig. 1. Stroboscopic pumpÐprobe setup.(A) Schematic of the experimental setup showing the VO 2 membrane

device excited by voltage and laser pulses and probed by megaÐelectron volt electron pulses in transmission

mode. The scale bar in the zoomed-in optical micrograph is 50mm. (B) Stroboscopic measurement scheme

showing voltage and laser pulses synchronized to the 180-Hz electron probe pulses with tunable delay.

(C) Quasistatic current versus voltage characteristics of a representative VO 2 device, measured using a

voltage source. (D) Transient electrical characteristics showing VO 2 resistance as a function of time after the

application of a step voltage with magnitude 4 V (red), 4.8 V (blue), and 5.6 V (green) for device 1 (channel width =

40 mm, length = 20mm). (E) Resistance-switching ratio between the insulating and metallic states (top) and

incubation time (bottom) plotted versus cycle number. All the data were taken at a stage temperature of 300 K.

-0.5 0 0.5 1 1.5 2 2.5

2

3

4

5

6

Q

(Å

-1)

2

3

4

5

6

Q

(Å

-1)

Time delay (ms) Time delay (ps)

-2 0 2 4 6 8 10 12

0

2

4

-2

-4

0

1

2

3

-1

-2

-3

static static

Electrical pump I (%) Optical pump I (%)

Intensity (a.u.)

-4

0

4

Q (Å-1)

2 3 456

I (%)

2

-2 Intensity (a.u.)

-6

0

6

Q (Å-1)

2 3 456

I (%)

3

-3

t = 500 μs t = 5 ps

220

302 313

220

302 313

A

B

C

D

Fig. 2. Similarity of transient structures formed under electrical and optical excitation.(A) Normalized

intensity changes during E-IMT with a 5.6-V, 500-ms pulse. The static diffraction pattern is shown by the

black curve. (B) Lineout at a delay of 500ms showing the static pattern (purple; intensity is multiplied byQ1.4

to aid visualization) and the normalized intensity change (green). a.u., arbitrary units. (C) Normalized intensity

changes during P-IMT with a ~100-fs pulse at a fluence of ~49 mJ cm−^2 .(D) Lineout at a delay of 5 ps. In (B) and

(D), three important peaks are indexed, 30 2

, 31 3

, and (220). Both E-IMT and P-IMT measurements were

made on device 1 (channel width = 40mm, length = 20mm) at a stage temperature of 300 K.

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