Photon Factory Activity Report 2014 #32 (2015) B

BL-3A, 4C/2009S2-003, 2012G649

X-ray study metal-insulator transition in W-doped VO2 thin film
Daisuke Okuyama*1, Keisuke Shibuya1,2, Reiji Kumai3, Takehito Suzuki1, Yuuichi Yamasaki1,3,4,
Hironori Nakao3, Youichi Murakami3, Masashi Kawasaki1,4, Yasujiro Taguchi1, Yoshinori Tokura1,4,
Taka-hisa Arima1,51
RIKEN CEMS, Wako 351-0198, Japan
2
AIST, Tsukuba 305-8562, Japan
3
CMRC and PF, Institute of Materials Structure Science, KEK, Tsukuba 305-0801, Japan4
Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
5
Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8561, JapanAn x-ray study in V1-xWxO2 films has been carried out. The insulating phase for x < 0.07 exhibits cell-doubling with theV dimerization similar to bulk VO2, while the insulating phase for x > 0.11 does not. The temperature and x dependenceof superlattice reflection casts doubt about the direct relationship between the dimerization of V ions and metal-insulatortransition. X-ray-induced persistent phase transitions are observed at low temperatures in each insulating phase in thevicinity of the boundary to the metallic phase regardless of the difference in the electronic structure.
1 Introduction
VO2 is known to exhibit a metal-insulator transitions(MIT) accompanying structural deformation. At thetransition temperature, the crystal structure changes fromtetragonal to monoclinic with V dimerization. Shibuya etal. fabricated single-crystalline thin films of V1-xWxO2 andobtained the phase diagram [1]. The MIT temperature
decreases with increasing W for x < 0.07. For x > 0.11,
the MIT temperature increases with increasing W. If theorigins of the MIT for x < 0.07 and for x > 0.11 aredifferent, some difference in the structural property isexpected. VO2-related materials have been attracting
interest also from the viewpoint of photo-induced effects.
Recently, x-ray-induced persistent insulator-to-metaltransition was observed in an x = 0.065 film [2].

The transition temperature increases with increasing x to
0.17. The (0 -1/2 3/2) reflection is not observed in the x =0.11 sample. These results suggest that the MIT andstructural transitions without V dimerization take place.
We have investigated in detail the x-ray-induced phasetransition in V1-xWxO2 films. Figures 1 (d – i) show thetemperature dependence of the diffraction profiles of (0 02) reflection before and after the irradiation of x-ray beamat 7 K. The shrinkage of the c-axis length is commonly
observed, namely, the x-ray-induced phase transition
generally takes place not only for x < 0.07 but also for x >0.11. Detailed information is found in [3].

2 Experiment
V1-xWxO2 films with thicknesses 80 nm were fabricatedon rutile TiO2 (001)-substrates by a PLD method [1].
Note that two electrons are doped by substituting the W6+ion for V4+ ion. Synchrotron x-ray diffraction experiments
were performed at BL-3A and 4C. Symmetry lowering
was monitored by the emergence of (0 -1/2 3/2) and (1 02). The lattice spacing c was estimated from the peak
position of (0 0 2) reflection.
3 Results and Discussion
Temperature dependence of the intensities of thesuperlattice reflection is compared with that of resistivity
and c-axis length in Fig. 1 (a - c). For x < 0.07, the (0 -1/23/2) reflections appear at low temperatures, indicatingthat V ions are dimerized. The modulation vector of Vdimerization remains to be (0 1/2 1/2). This result would
contradict a naive picture of Peierls-type MIT. Moreover,for x = 0.065, the cell doubling appears below room
temperature, which is far above the MIT temperature,
below which the c-axis length begins to increase. For x =0.11, a MIT with a change of the c-axis length is observed.

Fig. 1: Temperature dependences of resistivity (a), c-axis
length (b), intensity of (0 -1/2 3/2) superlattice reflectionin V1-xWxO2. Temperature dependence of c-axis lengthand (0 0 2) profile for (d, g) x = 0.05, (e, h) 0.065, and (f,i) 0.11 before/after irradiation of strong x-ray.
References[1] K. Shibuya et al., Appl. Phys. Lett. 96, 022102 (2010).
[2] K. Shibuya et al., Phys. Rev. B 84, 165108 (2011).
[3] D. Okuyama et al., Phys. Rev. B 91, 064101 (2015).
*okudaisu@tagen.tohoku.ac.jp