The effect of uniaxial stress along the c axis on the metal-insulator transition of VO 2 has been studied in the form of epitaxial thin films grown on TiO 2 ͑001͒ and ͑110͒ substrates. A large reduction in the transition temperature T MI from 341 K for a single crystal to 300 K has been observed in the film on TiO 2 ͑001͒ where the c-axis length is compressed owing to an epitaxial stress, while the T MI has been increased to 369 K in the film on TiO 2 ͑110͒ where the c-axis length is expanded. The correlation between the c-axis length and T MI is suggested: the shorter c-axis length results in the lower T MI .Vanadium dioxide, VO 2 , undergoes a metal-to-insulator ͑MI͒ transition at 341 K which is a first-order phase transition accompanied by a structural change from a hightemperature tetragonal form to a low-temperature monoclinic form. 1 Dramatic changes in electrical resistivity and infrared transmission occur across the phase transition, thus making the material useful for potential applications as switching devices. [2][3][4][5] There has been an enduring interest in modifying the MI transition temperature T MI of VO 2 by applying pressure or doping it with elements like Nb and W. 6,7 Hydrostatic pressure affected the T MI only slightly (dT MI /dP ϭ0.6 K/GPa), while a relatively large pressure dependence was reported under uniaxial stress along the c axis (dT MI /dPϭϪ12 K/GPa). 8 On the other hand, a reduction in T MI down to 318 K was reported for reactively sputtered VO 2 films deposited on Al 2 O 3 ͑0001͒ substrates. 9,10 These results suggest that in the form of a thin film the T MI of VO 2 can be modified through the control of stress along the c axis induced by the lattice mismatch between the film and substrate.In this work we prepared high-quality VO 2 thin films on TiO 2 ͑001͒ and ͑110͒ substrates. Table I shows the lattice parameters a and c, and interplanar spacing of ͑110͒, d 110 , of VO 2 and TiO 2 in a tetragonal form, together with the corresponding lattice mismatch. Since both the lattice parameters of VO 2 are smaller than those of TiO 2 , the c-axis length should decrease for a VO 2 film epitaxially grown on TiO 2 ͑001͒ because of an in-plane tensile stress at the interface ͑lattice mismatch: 0.86%͒, while increase for a VO 2 film grown on TiO 2 ͑110͒ ͑mismatch: 3.6%͒. Therefore, a reduction or an increase in T MI is expected there, respectively.Thin films of VO 2 were prepared using a pulsed laser deposition technique on TiO 2 ͑001͒ and ͑110͒ planes. A V 2 O 3 pellet was used as a target, which was obtained by reducing V 2 O 5 under H 2 atmosphere at 1173 K. During the deposition, the substrate temperature T s was kept at certain temperature between 523 and 743 K, and oxygen pressure was maintained at 1.0 Pa. After deposition, the films were cooled down to 300 K in 30 min under the same oxygen pressure. A deposition rate was about 0.2 nm/min. The thickness of the grown films measured by a surface profilometer ͑Tencor, Alpha sensor 500͒ was 10-15 nm. Figure 1͑a͒ shows a typical x-ray diffra...

We report the first pyrochlore oxide superconductor Cd 2 Re 2 O 7 . Resistivity, magnetic susceptibility, and specific heat measurements on single crystals evidence a bulk superconductivity at 1 K. Another phase transition found at 200 K suggests that a peculiar electronic structure lies behind the superconductivity.

We report the discovery of a new superconductor KOs 2 O 6. The compound crystallizes in a defect pyrochlore structure, where Os atoms form a cornersharing tetrahedral network called the pyrochlore lattice. Resistivity and magnetic susceptibility measurements on a polycrystalline sample provide evidence of bulk superconductivity with T c = 9.6 K.

We report the single-crystal X-ray analysis of the structure of the pyrochlore oxide superconductor KOs 2 O 6 . The structure was identified as the b-pyrochlore structure with space group Fd3m and lattice constant a ¼ 10:089ð2ÞÅ at 300 K: the K atom is located at the 8b site, not at the 16d site as in conventional pyrochlore oxides. We found an anomalously large atomic displacement parameter U iso ¼ 0:0735ð8ÞÅ 2 at 300 K for the K cation, which suggests that the K cation weakly bound to an oversized Os 12 O 18 cage exhibits intensive rattling, as recently observed for clathrate compounds. The rattling of A cations is a common feature in the series of bpyrochlore oxide superconductors AOs 2 O 6 (A ¼ Cs, Rb and K), and is greatest for the smallest K cation. r 2005 Elsevier Inc. All rights reserved.

The detail of electronic structures near the Fermi level in URu 2 Si 2 has been investigated employing stateof-art laser angle-resolved photoemission spectroscopy. The observation of a narrow dispersive band near the Fermi level in the ordered state as well as its absence in a Rh-substituted sample strongly suggest that the emergence of the narrow band is a clear signature of the hidden-order transition. The temperature dependence of the narrow band, which appears at the onset of the hidden-order transition, invokes the occurrence of periodicity modification in the ordered state, which is shown for the first time by any spectroscopic probe. We compare our data to other previous studies and discuss possible implications.

We report the discovery of a new pyrochlore oxide superconductor RbOs2O6. The compound crystallizes in the same beta-pyrochlore structure as the recently discovered superconductor KOs2O6, where Os atoms form a corner-sharing tetrahedral network called the pyrochlore lattice with Rb or K atoms in the cage. Resistivity, magnetic susceptibility and specific heat measurements on polycrystalline samples evidence a bulk superconductivity with Tc = 6.3 K

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The discovery of a new β-pyrochlore oxide superconductor CsOs 2 O 6 with T c = 3.3 K is reported. It is the third superconductor in the family of β-pyrochlore oxides, following KOs 2 O 6 with T c = 9.6 K and RbOs 2 O 6 with T c = 6.3 K. The T c of this series decreases with increasing the ionic radius of alkaline metal ions, imposing negative 2-4)Recently, we found another type of pyrochlore oxides with the general formula AB 2 O 6 called the β-pyrochlore oxide, 5) where A is a large monovalent alkaline metal cation. Two osmates, RbOs 2 O 6 and KOs 2 O 6 , have been prepared in this family, which exhibit superconductivity at higher T c 's of 6.3 K and 9.6 K, respectively. 5,6)They crystallize in a modified pyrochlore structure, where Os atoms form a corner-sharing tetrahedral network called the pyrochlore lattice, as in A 2 B 2 O 7 -type pyrochlore oxides which is now called α-pyrochlore, while alkaline metal atoms occupy the 8b site which is the O' site in the α-pyrochlore. Here we report on the discovery of another new β-pyrochlore oxide superconductor CsOs 2 O 6 .A polycrystalline sample was prepared using conventional solid-state reaction method in an evacuated silica tube. To control the oxygen partial pressure, a certain amount of AgO was added separately from the pellet in the tube: AgO decomposes into silver and oxygen above 370 K, and thus generates an oxidizing atmosphere. The 1