Table Of ContentUltrafast Electron-Lattice Coupling Dynamics in VO and V O
2 2 3
Thin Films
Elsa Abreu,1,2, Stephanie N. Gilbert Corder,1,3 Sun Jin Yun,4,5 Siming
∗
Wang,6,7,8,9 Juan Gabriel Ram´ırez,10 Kevin West,11,6,7 Jingdi Zhang,6,
†
Salinporn Kittiwatanakul,11 Ivan K. Schuller,6,7,8 Jiwei Lu,11 Stuart A.
7
Wolf,11,12 Hyun-Tak Kim,4,5 Mengkun Liu,3, and Richard D. Averitt6,
1 ‡ §
0
2
1Both authors contributed equally to this work.
n
a
J 2Institute for Quantum Electronics, Department of Physics,
9
1 ETH Zurich, 8093 Zurich, Switzerland
]
l 3Department of Physics and Astronomy,
e
-
r Stony Brook University, New York, 11794
t
s
.
t 4Metal-Insulator Transition Lab, ETRI, Daejeon 305-350, South Korea
a
m
- 5School of Advanced Device Technology,
d
n
University of Science and Technology, Daejeon 305-333, South Korea
o
c
[ 6Department of Physics, The University of California
1
at San Diego, La Jolla, California 92093, USA
v
1
3 7Center for Advanced Nanoscience,
5
5
The University of California at San Diego, La Jolla, California 92093, USA
0
.
1
8Materials Science and Engineering Program,
0
7
1 The University of California at San Diego, La Jolla, California 92093, USA
:
v
i 9Materials Sciences Division, Lawrence Berkeley
X
r National Laboratory, Berkeley, California 94720, USA
a
10Department of Physics, Universidad de los Andes, Bogot´a 111711, Colombia
11Department of Materials Science and Engineering,
University of Virginia, Charlottesville VA 22904, USA
12Department of Physics, University of Virginia, Charlottesville VA 22904, USA
(Dated: January 20, 2017; Received)
1
Abstract
Ultrafast optical pump - optical probe and optical pump - terahertz probe spectroscopy were
performed on vanadium dioxide (VO ) and vanadium sesquioxide (V O ) thin films over a wide
2 2 3
temperaturerange. Acomparisonoftheexperimentaldatafromthesetwodifferenttechniquesand
twodifferentvanadiumoxides,inparticularacomparisonoftheelectronicoscillationsgeneratedby
the photoinduced longitudinal acoustic modulation, reveals the strong electron-phonon coupling
that exists in the metallic state of both materials. The low energy Drude response of V O
2 3
appears more susceptible than VO to ultrafast strain control. Additionally, our results provide
2
a measurement of the temperature dependence of the sound velocity in both systems, revealing a
four-tofivefoldincreaseinVO andathree-tofivefoldincreaseinV O acrossthephasetransition.
2 2 3
Our data also confirm observations of strong damping and phonon anharmonicity in the metallic
phase of VO , and suggest that a similar phenomenon might be at play in the metallic phase of
2
V O . More generally, our simple table-top approach provides relevant and detailed information
2 3
about dynamical lattice properties of vanadium oxides, opening the way to similar studies in other
complex materials.
2
I. INTRODUCTION
Vanadium oxides are well known examples of materials where the phases are determined
by strong interactions between different degrees of freedom. Such complex systems, where
charge, lattice, orbital, and spin contributions can be equally strong and are frequently
coupled, exhibit a variety of phenomena including high temperature superconductivity1,
colossal magnetoresistance2, multiferroicity3, and topological surface states4. Given the
complex nature of these materials, the various phases are generally challenging to investi-
gate experimentally. Ultrafast time resolved techniques are a successful route to approach
these problems5,6. In particular, time resolved measurements in different energy ranges
have contributed to the understanding of insulator-to-metal transitions (IMTs) in vanadium
dioxide (VO ) and vanadium sesquioxide (V O ) as a function of temperature, pressure, or
2 2 3
doping7–14. These measurements have also shed light onto the nature of the different insula-
tor phases in both systems8,15–17, and the electron-phonon coupling driven acoustic response
in V O 8,9.
2 3
Despite many ultrafast and static measurements, the exact mechanisms responsible for
the IMTs in vanadium oxides and especially in VO remain widely debated18–26, namely
2
the contribution of electronic correlations (Mott-Hubbard picture) and of electron-lattice
mediated effects (Peierls model)16,22,27–29. It is clear, however, that multiple pathways are
possible to initiate pressure and temperature dependent transitions, and that electronic and
lattice effects are strongly coupled16,17,27,30. Measurements that dynamically investigate this
coupling are therefore essential to assist in understanding the nature of IMTs.
Bulk VO undergoes an IMT at 340 K, along with a monoclinic-to-rutile structural
2
transition18,31 (cf. phase diagram in Fig. S1a32). In the low temperature phase, the vana-
dium ions dimerize and tilt to form a nonmagnetic insulator22. Above-bandgap photoex-
citation of the monoclinic insulator can promote electrons to anti-bonding states, causing
repulsion between the dimerized vanadium ions and lattice expansion, followed by long-
range shear rearrangements at the speed of sound17,21. Above a critical fluence threshold the
metallic phase forms via nucleation and growth16,29,33,34. The acoustic and optical phonon
landscape changes drastically across the IMT, and phonons have recently been seen to ac-
count for 2/3 of the entropy increase at the IMT and to stabilize the metallic phase25,27.
These observations suggest that lattice effects play a significant role in driving the IMT in
3
VO .
2
V O is a low temperature monoclinic antiferromagnetic insulator which transitions to a
2 3
paramagneticrhombohedralmetalabove155K18,35 (cf. phasediagraminFig. S1b32). Initial
reports stated that the system exhibits a classical Mott-Hubbard transition where electrons
localize to form the insulating phase19,20. However, more recent work points to electron-
latticeeffectscontributingstronglytothefirstorderIMTintheundopedcompound23,27, and
questions the equivalence of temperature and pressure routes in driving the phase transition
in doped V O 10. The strong strain-dependence of metallic V O manifests via the large
2 3 2 3
influence of ultrafast acoustic modulations on the spectral weight redistribution dynamics,
particularly in the far infrared region of the spectra corresponding to the low energy Drude
weight9.
In this work, we present a comparative ultrafast pump-probe study of photoinduced
acoustic effects in VO and V O , present in both optical reflectivity and terahertz (THz)
2 2 3
conductivity dynamics. Transient reflectivity measurements at 1.55 eV are sensitive to inter-
band transitions in VO and V O , with insulating gaps of 0.6 eV and 0.5 eV, respectively22,
2 2 3
while THz probes the quasiparticle dynamics, effectively yielding a dynamical measurement
of the dc conductivity.
Our results show that, similar to V O 8,9, VO exhibits a modulation of the electronic
2 3 2
response due to photoinduced acoustic effects. This influence of small structural variations
on the electronic behavior attests to significant electron-phonon coupling. Comparing both
materials, we observe that V O is significantly more amenable than VO to Drude weight
2 3 2
modulation via acoustic wave propagation. Also, thin film samples with varying defect den-
sity exhibit different static and dynamic electronic responses. Thin films with fewer defects
have properties closer to bulk and are therefore characterized by a larger THz conductivity
in the metallic state and by larger photoinduced conductivity variations. In contrast to this
electronic behavior, the acoustic signatures we observe appear to be quite robust against
varying defect density in the thin films. Differences in the response to photoinduced acous-
tic excitation between VO and V O , and between nominally equivalent samples, could be
2 2 3
related to the strong and distinct influence of defects on the electronic response of these
materials30.
In addition, our data provide a temperature dependent measure of the sound velocity in
bothVO andV O , revealinganincreaseinthesoundvelocityacrosstheinsulator-to-metal
2 2 3
4
transition, four- to fivefold for VO and three- to fivefold for V O .
2 2 3
We also verify that acoustic damping increases in the metallic phase of VO , compared
2
to the insulating phase, in agreement with previous reports that present phonon entropy
as a stabilization mechanism for the metallic phase27. V O exhibits a similarly increased
2 3
damping in the metallic phase, hinting that phonon anharmonicity might also be much
stronger than in the insulating phase, and that phonon effects might play a significant role
in the IMT of this material as well.
II. EXPERIMENTAL METHODS
Transient optical reflectivity measurements were performed using 35 fs pulses at 800 nm
(1.55 eV) from a 1 KHz repetition rate, 3 W average power Ti:sapphire regenerative ampli-
fier. Pump and probe fluences were set at 1-4 mJ/cm2 and < 10 µJ/cm2, respectively, and
both beams were set at approximately normal incidence to the sample. THz pulses 1 ps in
duration were generated via optical rectification and detected by electro-optic sampling in
ZnTe. In contrast to the optical measurements at 1.55 eV, the THz pulses (0.1 - 2.5 THz)
effectively probe the low energy Drude response22. Transient optical reflectivity measure-
ments at 800 nm, yielding ∆R/R, and THz conductivity measurements, yielding ∆σ5, were
performed with the same pump conditions.
Of particular relevance to this work is the ability to generate coherent acoustic phonons
using ultrafast optical excitation. The ultrafast above bandgap optical pulse is absorbed
near the surface causing a localized temperature increase. Thermal expansion leads to a
transient stress which launches a strain wave in the material. The propagating acoustic
phonon modifies the refractive index and can therefore be detected optically via modulation
of ∆R/R, at 800 nm, and of ∆σ, around 1 THz. When the film thickness is on the order
of the longitudinal acoustic phonon wavelength an acoustic standing wave is generated and
detected instead36. Analytical37,38 and conceptual36 aspects of the generation and detection
of coherent acoustic modulations can be found in the literature36–38.
The samples consist of 75 nm and 50 nm thick films of VO on c-cut Al O 39, 75 nm thick
2 2 3
films of V O on c-cut Al O 40, and 95 nm thick films of V O on r-cut Al O 30. The VO
2 3 2 3 2 3 2 3 2
films have a well defined out of plane rutile c-axis, and three preferred in-plane orientations
due to the hexagonal symmetry of the c-cut Al O substrate41. The 75 nm V O film has a
2 3 2 3
5
well-defined rhombohedral out of plane [110] axis but is polycrystalline in plane, while the
95 nm film is nearly single crystalline throughout. Details of the fabrication can be found
elsewhere30,39,40. Unless otherwise specified, the results presented in this paper correspond
to the 75 nm films.
The sample temperature was set using a continuous flow cryostat, pumped down to 10 6
−
mbar and equipped with a heating stage.
III. RESULTS
(a) 5000
)4000
-1m VO2
1c3000
-Ω
(
Hz2000
T
σ
1000
T increase
T decrease
0
250 300 350 400 450 500
Temperature (K)
(b)1000
1) 800
-m
c
1 600
-Ω
V O
( 2 3
Hz 400
T
σ
200
T increase
T decrease
0
100 150 200 250 300 350
Temperature (K)
FIG. 1. Temperature dependence of the static far-infrared conductivity for the 75 nm thick (a)
VO and(b)V O thinfilms. Openblack(solidred)symbolsindicateincreasing(decreasing)tem-
2 2 3
perature. Both samples display hysteresis, characteristic of first order phase transitions happening
at 360 K in VO and at 160 K in V O . The fully metallic region is shaded gray.
2 2 3
Figure 1 shows the static THz conductivity of the 75 nm VO and V O films measured
2 2 3
using THz spectroscopy. Fig. 1a shows that the IMT in the VO film occurs at T = 360
2 IMT
K, and that a maximum conductivity of 4800 (Ωcm) 1 is obtained at 380 K. V O is seen
− 2 3
6
from Fig. 1b to have T = 160 K, with a peak conductivity of 900 (Ωcm) 1 at 200
IMT −
K. These measurements demonstrate that our VO and V O films behave similarly to
2 2 3
bulk single crystals, which attests to the generality of our conclusions. Above 200 K the
V O metallic state conductivity decreases dramatically (Fig. S132), beyond what would be
2 3
expected from an increased electron-phonon scattering rate, due to the strongly correlated
natureinthis“bad”metalregion22,42. TherelativeconductivitydecreaseintheVO metallic
2
state above 380 K is smaller compared to V O , with a 6% conductivity drop per 50 K for
2 3
VO and an 11 % conductivity drop per 50 K for V O . This is consistent with suggestions
2 2 3
that electronic correlations have a smaller impact on the metallic phase of VO compared
2
to metallic V O 43,44, which enters a crossover region to a pure Mott insulator phase above
2 3
450 K (Fig. S132). Corresponding temperature-dependent static conductivity data for the
95 nm V O film are shown in Fig. S232.
2 3
Figures 2a, 3a and 3b show the transient conductivity (∆σ) and transient reflectivity at
800 nm (∆R/R) of VO at low (Fig. 2a) and high (Figs. 3a and 3b) initial temperatures
2
for a pump fluence of 3.8 mJ/cm2. From the conductivity dynamics at T < T , shown
IMT
in Fig. 2a, it is clear that the maximum value achieved for the transient conductivity
∆σ(t) increases with initial temperature. For T = 77 K, ∆σ(t) recovers in less than 10 ps,
whereas for T > 160 K the deposited energy is sufficient to thermally stabilize the metallic
phase beyond our 350 ps measurement window32. As previously observed, the tens of ps
timescale for the conductivity increase is considerably larger than the <1 ps electron-phonon
thermalization time (Fig. S932) due to the nucleation and growth process that accompanies
the IMT33,45.
For T > T the metallic phase dominates the transient photothermal response of VO .
IMT 2
∆σ, shown in Fig. 3a, decreases following photoexcitation in agreement with Fig. 1a, and
so does ∆R/R (Fig. 3b). Most significantly, the high temperature dynamics of both ∆σ and
∆R/R exhibit oscillatory components (clearly isolated below, in Figs. 4a and 5b) which are
directsignaturesofacousticwavepropagation, similartothosereportedinV O 8,9. Budaiet
2 3
al.27 have observed that strongly anharmonic phonons rather than electronic effects stabilize
the metallic phase of VO . The results shown here confirm the strength of electron-phonon
2
coupling in metallic VO since acoustic signatures are seen not only in ∆R/R, as expected,
2
but also in ∆σ, a clear indication that lattice dynamics modulate the Drude response of the
system above T .
IMT
7
(a)
2000 VO2 330K
)
1
-m1500
c
1
-Ω 320K
( z1000
H
T
σ 280K
Δ
500
160K
77K
0
0 10 20 30 40 50 60
Time delay (ps)
(b)
V O
400 2 3
)
-1m 300 140K
c
1
-Ω
( 200
z
H
T
σ
Δ 100 160K
170K
0
0 50 100 150 200 250
Time delay (ps)
(c) 1
)
z
H
T 75nm
σ
Δ
( 95nm
x
a
m0.5
/ z
H
σT V2O3
Δ 100K, 3mJ/cm2
0
0 50 100 150
Time delay (ps)
FIG. 2. Transient ∆σ of 75 nm thick VO (a), 75nm thick V O (b), and 75 nm and 95 nm thick
2 2 3
V O (c) for temperatures below T at a pump fluence of 3.8 mJ/cm2 (a), 1 mJ/cm2 (b) and 3
2 3 IMT
mJ/cm2 (c). Startingintheinsulatingphase, theTHzresponseconsistsinatransientconductivity
increase which corresponds to the IMT. (c) compares the normalized ∆σ(t) for the 95 nm and 75
nm V O films at 100 K, revealing a significant difference in IMT dynamics at short time delays
2 3
which arises from different defect densities in the films.
As a counterpart to the data on VO , Figs. 2b, 3c and 3d show ∆σ and ∆R/R for V O
2 2 3
at temperatures below (Fig. 2b) and above (Figs. 3c and 3d) T for a pump fluence of
IMT
1 mJ/cm2. V O also exhibits a tens of ps ∆σ increase characteristic of a percolative IMT
2 3
(Fig. 2b). The decrease in ∆σ with increasing initial temperature stems solely from the
8
(a) 0
360K
VO
1)-100 2
-m
c
-1Ω-200
( Hz
T-300
σ
Δ 380K
-400
400K
-500
0 10 20 30 40 50 60
Time delay (ps)
(b)
VO
2
0
400K
3)
-0-2
1
R (x 380K
R/-4
Δ
360K
-6
0 10 20 30 40 50 60
Time delay (ps)
(c) 0
V O
2 3
-20
1)
-m -40
c 400K
-1Ω -60 360K
( Hz -80 320K
T
σ
Δ-100 280K
-120 240K
-140
0 50 100 150 200 250
Time delay (ps)
(d)
0
V O
2 3
-2
3) 260K 240K280K
-10-4
x
R (-6
R/ 300K
Δ
-8 320K
-10
0 50 100 150 200 250
Time delay (ps)
FIG. 3. Transient ∆σ (a, c) and ∆R/R (b, d) of 75 nm thick VO (a, b) and V O (c, d) for
2 2 3
temperatures above T at a pump fluence of 3.8 mJ/cm2 (a, b) and 1 mJ/cm2 (c, d). Shown are
IMT
the metallic reflectivity responses, dominated by acoustic signatures, and the metallic conductivity
responses, also with clear acoustic signatures.
9
fact that temperatures closer to T are used compared to VO , so that the initial state
IMT 2
already has a finite conductivity (Fig. 1) and the ∆σ response saturates as the full metallic
state is reached. Photoexcitation of the system in the metallic phase at T > T produces
IMT
a decrease of both ∆σ and ∆R/R in addition to clearly defined temperature dependent
acoustic signatures, as previously reported8,9.
Figure 2c compares the normalized ∆σ(t) for the 95 nm and 75 nm V O films at 100
2 3
K for a pump fluence of 3 mJ/cm2. The 95 nm film has a metallic conductivity more than
twice as large as the 75 nm film (Fig. 1b and Fig. S232), which is likely due to its single
crystalline rather than polycrystalline nature and to its consequent smaller defect density.
As discussed in an earlier publication45, the defect density has a strong influence not only
on the static properties but also on the transition dynamics of V O thin films since it
2 3
affects the nucleation and growth of metallic domains in the insulating phase. For V O and
2 3
VO , it is important to distinguish between ultrafast transition dynamics that occur at the
2
microscopic level, independent of the nucleation and growth process, and those that occur at
themesoscopiclevelwheretheresponseisdominatedbynucleationandgrowth. Microscopic
effects have been extensively discussed, particularly in the case of VO 22,46–48. Using a THz
2
probe, which is sensitive to the mesoscale dynamics, we observe fast ∆σ(t) transients that
start during photoexcitation by the pump pulse for the 75 nm film, whereas the 95 nm film
exhibits slower variations and a delayed onset relative to the pump arrival time (Fig. 2c).
These significant differences in conductivity dynamics cannot be explained by the minimal
difference in film thickness. Rather, they occur due to the 95 nm film containing fewer
defects than the 75 nm film, and hence fewer defect-induced preferential nucleation sites,
which slows down the photoinduced IMT in the thicker sample45. Such mesoscopic effects
occur independently in addition to any microscopic modifications, and their effect on the
dynamics must be taken into account in studies of both V O and VO .
2 3 2
DespitedifferencesinIMTtimescales(Figs. 2candS632), hightemperaturedynamicdata
for the 50 nm VO film and the 95 nm V O film are comparable to the 75 nm films of each
2 2 3
material and consequently are shown in Figs. S4 and S7 of the Supplemental Material32.
Finally, ∆R/R dynamics for VO and V O at T < T also exhibit acoustic oscil-
2 2 3 IMT
lations characteristic of the insulating phase. The full dynamic insulating responses have
many contributing factors and are therefore less straightforward to analyze than the metallic
response. As the present work focuses on the acoustic component, the full low temperature
10
