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VO2 Does Not Host a Photoinduced Long-lived Monoclinic Metallic Phase

Femtosecond laser pulses can trigger phase transitions and sometimes even achieve transient nonequilibrium states that are not accessible in equilibrium. Vanadium dioxide (VO2) exhibits a first-order insulator-to-metal transition accompanied by a monoclinic-to-rutile structural transition near room temperature. Recently there is extensive debate over whether VO2 hosts a transient long-lived monoclinic metallic phase with femtosecond photoexcitation. We developed a new method to synthesize quasi-single-crystal freestanding VO2 films and reexamined the photoinduced phase transition with mega-electron-volt ultrafast electron diffraction. Benefitting from the well separated diffraction spots and high temporal resolution, we found no evidence for the transient monoclinic metallic phase through the accurate measurements of the photoinduced evolution of the diffraction intensity in the momentum space. Based on the laser fluence thresholds measured at various temperatures, we suggest that the phase transition threshold is thermally driven although the transition occurs within 200 femtoseconds, highlighting the interplay of the electron-electron correlation and the lattice vibrations in driving the phase transition in VO2.

Femtosecond laser pulses can trigger phase transitions and sometimes even achieve transient nonequilibrium states that are not accessible in equilibrium. Vanadium dioxide (VO2) exhibits a first-order insulator-to-metal transition accompanied by a monoclinic-to-rutile structural transition near room temperature. Recently there is extensive debate over whether VO2 hosts a transient long-lived monoclinic metallic phase with femtosecond photoexcitation. We developed a new method to synthesize quasi-single-crystal freestanding VO2 films and reexamined the photoinduced phase transition with mega-electron-volt ultrafast electron diffraction. Benefitting from the well separated diffraction spots and high temporal resolution, we found no evidence for the transient monoclinic metallic phase through the accurate measurements of the photoinduced evolution of the diffraction intensity in the momentum space. Based on the laser fluence thresholds measured at various temperatures, we suggest that the phase transition threshold is thermally driven although the transition occurs within 200 femtoseconds, highlighting the interplay of the electron-electron correlation and the lattice vibrations in driving the phase transition in VO2.
In low temperature monoclinic phase (called M 1 phase), there are V-V dimers that cause the superstruc-ture peaks in X-ray or electron diffraction. When VO 2 becomes the high temperature rutile phase (called R phase), those superstructure diffraction peaks disappear. Intensity of the non-superstructure diffraction peaks either slightly increases or decreases depending on the structural factor [17]. Surprisingly, previous UED experiments on polycrystalline VO 2 samples [18] found that the reduction of the superstructure diffraction peaks and the enhancement of non-superstructure diffraction peaks exhibited two different incident laser fluence thresholds (called F c1 and F c2 , respectively). F c1 is much larger than F c2 . Furthermore, infrared transmissivity measurements suggested that VO 2 reaches a metallic state after photoexcitation with a laser fluence close to F c2 but much lower than F c1 [18]. According to these observations, a transient mM phase has been suggested for VO 2 [18]. The phase transition occurs after a few hundreds of femtoseconds and can last for hundreds of picoseconds. The presence of such mM phase is very interesting. The separation of the IMT and SPT offers new insights to the phase transition mechanism in VO 2 . Low laser energy is also beneficial to the potential application. However, the existence of mM phase is still under strong debate. Recently, it is demonstrated that the heat accumulation in pump-probe experiments could cause unexpected uncertainty in determining the fluence thresholds [30]. Only one threshold was observed in time-resolved XAS and the wavelength dependent optical spectroscopy measure- ments, indicating the absence of mM phase [30]. However, the existence of two thresholds for the diffraction peaks can not be explained with heat accumulation and spectroscopic measurement lacks direct sensitivity to the lattice structure. It should be noted that in previous UED experiments [18], the superstructure peaks are extremely weak in polycrystalline samples, which might introduce considerable uncertainty in determination of the thresholds.
In this work, we developed a new method to fabricate high quality quasi-single crystalline VO 2 freestanding films and revisited the ultrafast phase transition using the state of the art UED with mega-electron-volt (MeV) electrons. Due to the well separation of the superstructure diffraction peaks from other diffraction peaks in the momentum space and the high temporal resolution of MeV UED, the fluence thresholds of the superstructure and non-superstructure diffraction peaks are accurately measured. Both types of the diffraction peaks give the same threshold of ∼ 3.1 mJ/cm 2 at 300 K. Above the threshold, photoinduced SPT occurs within 200 fs. Our findings strongly suggest that VO 2 does not host a photoinduced long-lived mM Phase as found in previous UED experiment. Our threshold results measured at various temperatures imply that the threshold is thermally dominated.
The quasi-single-crystal VO 2 films are epitaxially grown on SrTiO 3 (111) (STO) substrates by pulsed laser deposition. We grow 20 nm Sr 3 Al 2 O 6 (SAO) as a sacrificial layer followed by a 2 nm STO(111) film as the buffer layer to improve the lattice match. The layout of the sample is shown in Fig. 1(a). Consistent with VO 2 films directly grown on STO(111) [33], the normal direction of our films is 010 , which nicely matches the UED experiments since V-V dimers exist in the (010) plane. At growth temperature (450 o ) where VO 2 is in R-phase, there are three in-plane domains. Each domain becomes twin domains when the film is cooled down to room temperature where VO 2 is in M 1 -phase. The SAO layer is water soluble [34]. After removing SAO, we succeeded in obtaining relatively large freestanding films (40 nm VO 2 /2 nm STO). The free-standing films are transferred to a transmission electron microscope (TEM) grid with ultrathin amorphous carbon film for MeV UED measurements. The amorphous carbon film effectively reduces heat accumulation from the optical pump. The electron beam has a kinetic energy of about 3 MeV and is compressed with a double-bend achromatic magnet system [35]. The repetition rate is 100 Hz or 50 Hz. The overall temporal resolution is ∼ 50 fs (FWHM) for the dynamics measurements and ∼ 100 fs for the threshold measure-  The static electron diffraction pattern at 300 K measured by 200 keV TEM is shown in Fig. 1(b) with Miller indexes shown for several representative diffraction spots. Superstructure spots in M 1 -phase are marked with a "M" subscript and non-superstructure spots existing in both M 1 -and R-phase are marked with a "R" subscript. All Miller indexes use M 1 -phase's representation. Diffraction spots from 2 nm STO buffer layer were also observed as indicated by "STO". Both the pattern and the elongated shape of the diffraction spots from VO 2 can be nicely understood based on three 120 o in-plane domains (plus twinning in each domain) and the anisotropic shape of the crystal grain (see details in SM Figs. S1 and S2). STO spots are close to 102 M and 202 R /204 R spots, but completely away from 100 M , 102 M /302 M and 002 R /200 R /202 R spots. Figure 2(a) presents the static diffraction pattern measured by the 3 MeV electrons in MeV UED system below and above the phase transition temperature. The momentum resolution in Fig. 2(a) is not as good as that in Fig. 1(a). There are two reasons. First, unlike the TEM, the electron beam is not focused (beam size ∼ 150 µm) in UED to achieve high temporal resolution. Second, the higher kinetic energy in MeV UED also slightly lowers the momentum resolution. As a result, diffraction spots close to each other in Fig. 1(b) merge together in Fig.  2(a). For instance, 102 M and 302 M overlap. For simplicity, we use 102 M to index this big spot. Similarly, 200 R , 002 R and 202 R spots also overlap, and we use 200 R as the simple index. As expected, the pure superstructure spots 102 M completely disappear above the transition temperature ( Fig. 2(a)-right). The advantage of the quasi-single-crystal samples for UED measurements is demonstrated in Fig. 2(b). To simulate polycrystalline sample to some extent, we plot the azimuthal averaged diffraction intensity in Fig. 2(b). Clearly, because of the huge intensity of the 200 R peak, the superstructure peaks (100 M , 102 M and 102 M ) are only a small fraction of the background, rendering accurate determination of the threshold difficult in polycrystalline samples [18]. In our quasi-singlecrystal samples, superstructure spots are well separated from the non-superstructure spots, which allows us to accurately analyze the evolution of the diffraction intensity. Following the temperature dependence of the intensity of 102 M peak, well defined SPT with temperature hysteresis loop is obtained. The SPT temperature of the sample in Fig. 2(a) is determined as ∼ 320 K (see details in SM Fig. S3). VO 2 sample shows ultrafast response to the laser pump. Figures 3(a),(b) present the normalized intensity differences between the diffraction pattern at t=1.5 ps after photoexcitation and that at t=0 (without photoexcitaion) under the low (5 mJ/cm 2 ) and high (13 mJ/cm 2 ) laser fluence, respectively. Upon photoexcitation, the intensity of all the superstructure spots decrease while non-superstructure spots are enhanced. Figure 3(c) shows the transient intensity change of the 102 M peak extracted from Figs. 3(a),(b). Under low fluence, intensity drops only by about 13% and oscillations of 6 THz related to a coherent phonon of the zigzag motion of V atoms [19,29,36] are clearly observed at 240 K. In contrast, under high fluence, intensity drops by more than 70% within 200 fs and no oscillation was detected. The absence of oscillation implies a fast M 1 -to-R phase transition, in agreement with previous UED and ultrafast Xray free-electron laser experiments on VO 2 [17][18][19]. The 102 M peak shows similar behavior in Fig. 3(d). Because 102 M spot overlaps with the STO spot, the change of the intensity is much smaller than that of 102 M due to the contribution from STO which has negligible response to 800 nm laser. Qualitatively consistent with previous UED experiments [18], the intensity of 200 R increases after photoexcitation, shown in Fig. 3(e). The peak width of 200 R along the azimuthal direction keeps nearly constant under low fluence, while it decreases under high fluence as shown in Fig. 3(f). This leads to differential intensity decrease at both sides of 200 R peaks, as shown in Fig. 3(b). The decrease of the peak width results from the disappearance of twin domains (see details in SM Fig. S3). Therefore, the width of 200 R peak can be considered as another signature for SPT beside the suppression of the superstructure diffraction spots.
After 600 fs, the intensity of 102 M , 102 M and 200 R peaks starts to recover under low fluence, but stays nearly constant under high fluence (see details in SM Fig. S5).
To determine the fluence threshold for these peaks, we measured the diffraction intensity at the time delay of 10 ps when the electron and lattice largely reaches a quasi-equilibrium condition. The previously suggested mM phase would last for hundreds of picoseconds [18], therefore measuring the threshold at 10 ps is reasonable for checking the signature of mM phase. Figure 4(a) shows the fluence dependence of 102 M peak measured at 300 K. A distinct kink is observed at ∼ 3.1 mJ/cm 2 . The intensity decreases slowly below this value while a fast decrease is observed above this value. Similar behavior and kink are observed for 102 M peak, as shown in Fig. 4(b). Because 102 M peak and 102 M peak are directly associated with the V-V dimerization, the presence of a kink indicates a fluence threshold for SPT, e.g. a critical energy is needed to melt the V-V dimers for the transition to the R-phase. Therefore, at 300 K, F c1 ∼ 3.1 mJ/cm 2 . For 200 R peak, a kink is also observed at ∼ 3.1 mJ/cm 2 , shown in Fig. 4(c). The width of the 200 R peak as a function of fluences is shown in Fig. 4(d). Again, a threshold of ∼ 3.1 mJ/cm 2 is observed. Therefore, we obtained F c2 ∼ 3.1 mJ/cm 2 . This is in stark contrast to previous work where F c1 is much larger than F c2 [18]. In a separate experiment, the repetition rate is reduced to 50 Hz and similar fluence threshold is observed, indicating that the heat accumulation effect in our measurement is negligible. Furthermore, we repeated the measurements at several temperatures (280 K, 260 K, 240 K, and 220 K) and consistently the threshold for 102 M and 200 R peaks are very similar(see details in SM Fig. S6). Our results strongly suggest that there is no difference between F c2 and F c1 .
The thresholds measured at different temperatures are shown in Fig. 4(e). Assuming photoinduced SPT in VO 2 is a thermally dominated process, the total thermal energy required to drive the phase transition can be described by E th = C v (T)dT + H L , where C v is heat capacity and H L is latent heat. According to the previous report [37], the heat capacity of VO 2 increases approximately linearly from 220 K to 300 K, which means that the thermal energy needed to drive the phase transition should decrease quadratically. The experimental data can be nicely fitted by a quadratic curve, yielding a latent heat of ∼ 2 mJ/cm 2 at T c =320 K. Although the obtained threshold at T c in our sample is in good agreement with that obtained by THz spectroscopy [25], it must be noted that we should be very careful to compare the absolute threshold values among different experiments with different samples because the threshold could be affected by strain and defect in the sample [38]. Nevertheless, in our experiments, the fact that the thresholds obtained from the same sample at different temperatures fit quite well with a quadratic curve suggests that the threshold is likely thermally driven. This together with the ultrafast measurements in Fig. 3 imply that heat can be very quickly transferred from the photoexcited electrons to the lattice and SPT occurs once the thermal barrier is crossed.
In conclusion, we have revisited the photoinduced phase transition in quasi-single crystalline VO 2 sample with MeV UED. The highly improved signal-to-noise ratio and temporal resolution allow us to not only observe the coherent phonon related to the zigzag motion of V atoms connecting the two phases, but also to accurately determine the fluence threshold for the phase transition. The fact that only one threshold is observed for both the superstructure and non-superstructure peaks strongly imply the absence of a photoinduced long-lived monoclinic metallic phase. It should be noted that our results do not rule out the existence of monoclinic metallic phase in the first 100 fs following photoexcitation. It has been reported in several publications that the IMT may occur well below 50 fs [39,40], faster than half the period of the phonons related to the zigzag motion of the V atoms. With further improvement in temporal resolution, the single crystal sample together with a large momentum space accessed by UED may allow one to quantitatively track the atomic pathway immediately after photoexcitation, providing valuable information to understand the nature of ultrafast photoinduced phase transition in VO 2 .