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Register a new userStrong-coupling charge density wave in a one-dimensional topological metal
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Scanning tunnelling microscopy and low energy electron diffraction show a dimerization-like reconstruction in the one-dimensional atomic chains on Bi(114) at low temperatures. While one-dimensional systems are generally unstable against such a distortion, its observation is not expected for this particular surface, since there are several factors that should prevent it: One is the particular spin texture of the Fermi surface, which resembles a one-dimensional topological state, and spin protection should hence prevent the formation of the reconstruction. The second is the very short nesting vector $2 k_F$, which is inconsistent with the observed lattice distortion. A nesting-driven mechanism of the reconstruction is indeed excluded by the absence of any changes in the electronic structure near the Fermi surface, as observed by angle-resolved photoemission spectroscopy. However, distinct changes in the electronic structure at higher binding energies are found to accompany the structural phase transition. This, as well as the observed short correlation length of the pairing distortion, suggest that the transition is of the strong coupling type and driven by phonon entropy rather than electronic entropy.
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The recently discovered family of vanadium-based kagome metals with topological band structures offer a new opportunity to study frustrated, correlated and topological quantum states. These layered compounds are nonmagnetic and undergo charge density wave (CDW) transitions before developing superconductivity at low temperatures. Here we report the observation of unconventional superconductivity and pair density wave (PDW) in the vanadium-based kagome lattice CsV3Sb5 using scanning tunneling microscope/spectroscopy (STM/STS) and Josephson STS. The differential conductance exhibits a V-shaped pairing gap about 0.5 meV below a transition temperature Tc about 2.3 K. Superconducting phase coherence is observed by Josephson effect and Cooper-pair tunneling to a superconducting tip. We find that CsV3Sb5 is a strong-coupling superconductor (2delta/kBTc about 5) and coexists with 4a0 unidirectional and 2x2 charge order. Remarkably, we discover a 4a0/3 bidirectional PDW accompanied by spatial modulations of the coherence peak and gap-depth in the tunneling conductance. We term the latter as a roton-PDW that can produce a commensurate vortex-antivortex lattice to account for the observed conductance modulations. Above Tc, we observe long-range ordered 4a0 unidirectional and 2a0 bidirectional CDW and a large V-shaped pseudogap in the density of state. Electron-phonon calculations attribute the 2x2 CDW to phonon softening induced structural reconstruction, but the phonon mediated pairing cannot describe the observed strong-coupling superconductor. Our findings show that electron correlations in the charge sector can drive the 4a0 unidirectional CDW, unconventional superconductivity, and roton-PDW with striking analogies to the phenomenology of cuprate high-Tc superconductors, and provide the groundwork for understanding their microscopic origins in the vanadium-based kagome superconductors.
Understanding the competition between superconductivity and other ordered states (such as antiferromagnetic or charge-density-wave (CDW) state) is a central issue in condensed matter physics. The recently discovered layered kagome metal AV3Sb5 (A = K, Rb, and Cs) provides us a new playground to study the interplay of superconductivity and CDW state by involving nontrivial topology of band structures. Here, we conduct high-pressure electrical transport and magnetic susceptibility measurements to study CsV3Sb5 with the highest Tc of 2.7 K in AV3Sb5 family. While the CDW transition is monotonically suppressed by pressure, superconductivity is enhanced with increasing pressure up to P1~0.7 GPa, then an unexpected suppression on superconductivity happens until pressure around 1.1 GPa, after that, Tc is enhanced with increasing pressure again. The CDW is completely suppressed at a critical pressure P2~2 GPa together with a maximum Tc of about 8 K. In contrast to a common dome-like behavior, the pressure-dependent Tc shows an unexpected double-peak behavior. The unusual suppression of Tc at P1 is concomitant with the rapidly damping of quantum oscillations, sudden enhancement of the residual resistivity and rapid decrease of magnetoresistance. Our discoveries indicate an unusual competition between superconductivity and CDW state in pressurized kagome lattice.
One-dimensional lattice with strong spin-orbit interactions (SOI) and Zeeman magnetic field is shown to lead to the formation of a helical charge-density wave (CDW) state near half-filling. Interplay of the magnetic field, SOI constants and the CDW gap seems to support Majorana bound states under appropriate value of the external parameters. Explicit calculation of the quasi-particles wave functions supports a formation of the localized zero-energy state, bounded to the sample end-points. Symmetry classification of the system is provided. Relative value of the density of states shows a precise zero-energy peak at the center of the band in the non-trivial topological regime.
Density functional theory and density functional perturbation theory are used to investigate the electronic and vibrational properties of TiS$_2$. Within the local density approximation the material is a semi-metal both in the bulk and in the monolayer form. Most interestingly we observe a Kohn anomaly in the bulk phonon dispersion, which turns into a charge density wave instability when TiS$_2$ is thinned to less than four monolayers. Such charge density wave phase can be tuned by compressive strain, which appears to be the control parameter of the instability.
A strong coupling between the electron spin and its motion is one of the prerequisites of spin-based data storage and electronics. A major obstacle is to find spin-orbit coupled materials where the electron spin can be probed and manipulated on macroscopic length scales, for instance across the gate channel of a spin-transistor. Here, we report on millimeter-scale edge channels with a conductance quantized at a single quantum 1 $times$ $e^2/h$ at zero magnetic field. The quantum transport is found at the lateral edges of three-dimensional topological insulators made of bismuth chalcogenides. The data are explained by a lateral, one-dimensional quantum confinement of non-topological surface states with a strong Rashba spin-orbit coupling. This edge transport can be switched on and off by an electrostatic field-effect. Our results are fundamentally different from an edge transport in quantum spin Hall insulators and quantum anomalous Hall insula-tors.
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