Electronic Structure of VO 2 near Phase Transition by Tunneling Spectroscopy
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Electronic Structure of VO2 near Phase Transition by Tunneling Spectroscopy Changman Kim, Yasushi Oikawa, Takashi Tamura, Jae-Soo Shin1 and Hajime Ozaki Electrical Engineering and Bioscience, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan 1 Advanced Materials Engineering, Daejeon University, 96-3 Yongun, Dong-gu, Daejeon 300-716, Korea ABSTRACT Tunneling spectroscopy was carried out on W doped VO2 single crystal in the temperature region across the phase transition. Double pseudo gap structures were observed across the Fermi level at temperatures below the phase transition. When the temperature was increased across the phase transition, the outer gap structure disappeared and the inner gap structure of about 0.36 eV became sharp. A precursor of phase transition to low temperature phase was observed in the tunneling density of states near the Fermi level, when the temperature approached 0.2 K to the threshold of transition in the electrical resistivity.
INTRODUCTION The vanadium dioxide (VO2) undergoes a Metal-Insulator Transition at about 340 K which is a first-order phase transition accompanied by a structural change from a high-temperature rutile type to a low-temperature monoclinic type [1, 2]. Dramatic changes of VO2 in electrical resistivity and infrared transmission have attracted physical interest and attention for useful applications, e.g. temperature sensing devices, optical switching devices, optical data storage medium, etc. [3]. Particularly, the discovery of the high-Tc cuprate superconductors have attracted renewed physical interest in the metal-insulator transitions in 3d transition metal compounds [4]. Tunneling studies have shown interesting results on the phase transitions of superconductivity and transition-metal dichalcogenides [5, 6]. Recently, the electronic structures of VO2 were reported by Ultraviolet Photoemission Spectroscopy (UPS) [4, 7] and Scanning Tunneling Spectroscopy (STS) [8, 9]. However, the study on the behavior of change of electronic structure in the phase transition has not been made so far. In the present study, tunneling measurements were carried out on tungsten doped VO2 (WxV1-xO2), as a function of temperature near the phase transition, in order to investigate the change of the electronic structure through the phase transition.
EXPERIMENTAL DETAILS Crystal growth The crystal growth of VO2 was performed using VO2 and V2O5 powders and with WO3 powder for W doping. The well mixed powders were sealed in quartz tube with 90 mm (long) ×
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Figure 1. Schematic planar contact tunnel junction employed in this study. 10 mm (diameter) under 1 × 10-3 Pa. The sealed quartz tube was placed vertically in a furnace. The temperature of the furnace was kept at 1000°C for 5 hours and then decreased at a rate of 2.67 °C/hr to 800 °C. At 800 °C, the quartz tube was inverted in the furnace so as to separate the useless solution from the crystals which were grown in the melt at the bottom of the quartz tube. The crystals were annealed for 2 hou
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