Charge-density-waves, which occur mainly in low-dimensional systems, have a macroscopic wave function similar to superfluids and superconductors. Kosterlitz-Thouless transition is observed in superfluids and superconductors, but the presence of Kosterlitz-Thouless transition in ultra-thin charge-density-waves systems has been an open problem. We report the direct realspace observation of charge-density-waves with new order states in mono-layer, bi-layer, and tri-layer 1T-TaS 2 crystals using a low voltage scanning-transmission-electron-microscopy without a substrate. This method is ideal to observe local atomic structures and possible defects. We clearly observed that the mono-layer crystal has a new triclinic stripe charge-density-waves order without satisfying the triple q condition q 1 + q 2 + q 3 = 0. A strong electron-phonon interaction gives rise to new crevasse (line) type defects instead of disclination (point) type defects due to the Kosterlitz-Thouless transition. These results reaffirm the importance of the electron-phonon interaction in mono-layer nanophysics.npj Quantum Materials (2017) 2:22 ; doi:10.1038/s41535-017-0025-8 INTRODUCTION Dimensionality and topology are the most important parameters characterizing physical systems. For example, the integral and fractional quantum Hall effects (QHE) 1, 2 are observed only in two-dimensional systems such as metal-oxide-semiconductor field-effect transistors, GaAs/AlGaAs interfaces, and graphene.
We report the coexistence of the Kondo effect and spin glass behavior in Fe-doped NbS 2 single crystals. The Fe x NbS 2 shows the resistance minimum and negative magnetoresistance due to the Kondo effect, and exhibits no superconducting behavior at low temperatures. The resistance curve follows a numerical renormalization-group theory using the Kondo temperature T K = 12.3 K for x = 0.01 as evidence of Kondo effect. Scanning tunneling microscope/spectroscopy (STM/STS) revealed the presence of Fe atoms near sulfur atoms and asymmetric spectra. The magnetic susceptibility exhibits a feature of spin glass. The static critical exponents determined by the universal scaling of the nonlinear part of the susceptibility suggest a three-dimensional Heisenberg spin glass. The doped-Fe atoms in the intra-and inter-layers revealed by the X-ray result can realize the coexistence of the Kondo effect and spin glass.
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