We found that the high-pressure-synthesized material LiOsO 3 (see Supplementary Information) shows a structural transition at a temperature T s = 140 K. The room-temperature crystal structure of LiOsO 3 was initially examined using powder X-ray diffraction (XRD). The Goldschmidt diagram predicts that LiOsO 3 crystallizes into a LiNbO 3 -type structure 3,10 , and a preliminary refinement of the structure was carried out in the R-3c space group with Os at the 6b site 0,0,0 and O at the 18e siteTo investigate the position of the Li ion we turned to neutron diffraction, which is much more sensitive to Li than XRD. The neutron diffraction patterns collected above T s could be successfully described in the R-3c space group, in agreement with the XRD refinement, with the Li ion at the 6a position 0,0,1/4. Atomic absorption spectrometry (see Supplementary Information) indicated that the average Li mass was 2.77%, which corresponds to the composition Li 0.98 OsO 3 . We have used the stoichiometric composition throughout the structural analysis. The refinement indicated highly anisotropic thermal displacements of the Li ions with considerable extension along the c-axis (Table 1 and Fig. 1), which might indicate that the Li ions are distributed equally among equivalent 12c sites 0,0,z and 0,0,1/2-z either side of the oxygen layer at z = 1/4, as reported for LiNbO 3 and LiTaO 3 (refs 3, 11).The thermal variation of the structure of LiOsO 3 was studied by neutron diffraction for temperatures between 10 and 300 K. Figure 1a-d shows structural data obtained from refinements in the R-3c space group. The lattice parameters ( Fig. 1a) decrease uniformly from 300 K until T s = 140 K, below which the parameter c increases and a decreases with only a small variation in the unit-cell volume. Just below T s , the non-symmetry-breaking strain components e xx + e yy and e zz vary almost linearly (Fig. 1b). These 4 results show that the phase transition is continuous and the strain components behave like a secondary order parameter coupled to a primary one via a linear-quadratic free energy invariant 12 . The primary order parameter must necessarily be symmetry-breaking according to Landau's theory of second-order phase transitions 12 . Furthermore, the anisotropic thermal parameter  33 , which describes Li displacements along the c-axis, increases markedly below T s (Fig. 1c). This indicates that the primary structural instability involves the position of the Li ions along the c-axis (Fig. 1d).Given that the phase transition involves a change in symmetry, we find from representation theory 13 that there are three isotropy subgroups, R-3, R32 and R3c, which maintain the translational invariance of the R-3c space group and allow the transition to be continuous. These space groups were tested by refinement against the neutron diffraction data at 10 K. Note that R-3 and R32 should generate additional reflections below T s which were not observed in the experiment. The refinement in the non-centrosymmetric R3c space group gave the best de...

The recent discovery of large magnetoresistance in tungsten ditelluride provides a unique playground to find new phenomena and significant perspective for potential applications. The large magnetoresistance effect originates from a perfect balance of hole and electron carriers, which is sensitive to external pressure. Here we report the suppression of the large magnetoresistance and emergence of superconductivity in pressurized tungsten ditelluride via high-pressure synchrotron X-ray diffraction, electrical resistance, magnetoresistance and alternating current magnetic susceptibility measurements. Upon increasing pressure, the positive large magnetoresistance effect is gradually suppressed and turned off at a critical pressure of 10.5 GPa, where superconductivity accordingly emerges. No structural phase transition is observed under the pressure investigated. In situ high-pressure Hall coefficient measurements at low temperatures demonstrate that elevating pressure decreases the population of hole carriers but increases that of the electron ones. Significantly, at the critical pressure, a sign change of the Hall coefficient is observed.

Two-dimensional materials provide extraordinary opportunities for exploring phenomena arising in atomically thin crystals. Beginning with the first isolation of graphene, mechanical exfoliation has been a key to provide high-quality two-dimensional materials, but despite improvements it is still limited in yield, lateral size and contamination. Here we introduce a contamination-free, one-step and universal Au-assisted mechanical exfoliation method and demonstrate its effectiveness by isolating 40 types of single-crystalline monolayers, including elemental two-dimensional crystals, metal-dichalcogenides, magnets and superconductors. Most of them are of millimeter-size and high-quality, as shown by transfer-free measurements of electron microscopy, photo spectroscopies and electrical transport. Large suspended two-dimensional crystals and heterojunctions were also prepared with high-yield. Enhanced adhesion between the crystals and the substrates enables such efficient exfoliation, for which we identify a gold-assisted exfoliation method that underpins a universal route for producing large-area monolayers and thus supports studies of fundamental properties and potential application of two-dimensional materials.