Discovery of unconventional chiral charge order in kagome superconductor KV3Sb5.
TL;DR: In this article, the authors used high-resolution scanning tunnelling microscopy (STM) to discover an unconventional charge order in a kagome material KV3Sb5, with both a topological band structure and a superconducting ground state.
Abstract: Intertwining quantum order and nontrivial topology is at the frontier of condensed matter physics. A charge density wave (CDW) like order with orbital currents has been proposed as a powerful resource for achieving the quantum anomalous Hall effect in topological materials and for the hidden phase in cuprate high-temperature superconductors. However, the experimental realization of such an order is challenging. Here we use high-resolution scanning tunnelling microscopy (STM) to discover an unconventional charge order in a kagome material KV3Sb5, with both a topological band structure and a superconducting ground state. Through both topography and spectroscopic imaging, we observe a robust 2x2 superlattice. Spectroscopically, an energy gap opens at the Fermi level, across which the 2x2 charge modulation exhibits an intensity reversal in real-space, signaling charge ordering. At impurity-pinning free region, the strength of intrinsic charge modulations further exhibits chiral anisotropy with unusual magnetic field response. Theoretical analysis of our experiments suggests a tantalizing unconventional chiral CDW in the frustrated kagome lattice, which can not only lead to large anomalous Hall effect with orbital magnetism, but also be a precursor of unconventional superconductivity.
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TL;DR: In this paper, the vanadium-based kagome lattice CsV3Sb5 was observed to exhibit a V-shaped pairing gap about 0.5 meV below a transition temperature Tc about 2.3 K.
Abstract: 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.
207 citations
TL;DR: In this paper, the anomalous Hall effect (AHE) typically occurs in ferromagnetic materials but is not expected in conventional superconductors, and the authors find a giant AHE in the kagome superconductor CsV${}_{3}$Sb${}-5}$.
Abstract: As one of the most fundamental physical phenomena, the anomalous Hall effect (AHE) typically occurs in ferromagnetic materials but is not expected in conventional superconductors. Here, the authors find a giant AHE in the kagome superconductor CsV${}_{3}$Sb${}_{5}$. Strikingly, the AHE develops spontaneously with the occurrence of a charge density wave (CDW), indicating a strong correlation between the CDW state and AHE. These discoveries make CsV${}_{3}$Sb${}_{5}$ an ideal platform to study the interplay among nontrivial band topology, CDW, and unconventional superconductivity
173 citations
TL;DR: In this paper, a temperature-dependent cascade of different symmetry-broken electronic states in a new kagome superconductor, CsV3Sb5, was discovered using spectroscopic imaging scanning tunnelling microscopy.
Abstract: The kagome lattice of transition metal atoms provides an exciting platform to study electronic correlations in the presence of geometric frustration and nontrivial band topology1–18, which continues to bear surprises. Here, using spectroscopic imaging scanning tunnelling microscopy, we discover a temperature-dependent cascade of different symmetry-broken electronic states in a new kagome superconductor, CsV3Sb5. We reveal, at a temperature far above the superconducting transition temperature Tc ~ 2.5 K, a tri-directional charge order with a 2a0 period that breaks the translation symmetry of the lattice. As the system is cooled down towards Tc, we observe a prominent V-shaped spectral gap opening at the Fermi level and an additional breaking of the six-fold rotational symmetry, which persists through the superconducting transition. This rotational symmetry breaking is observed as the emergence of an additional 4a0 unidirectional charge order and strongly anisotropic scattering in differential conductance maps. The latter can be directly attributed to the orbital-selective renormalization of the vanadium kagome bands. Our experiments reveal a complex landscape of electronic states that can coexist on a kagome lattice, and highlight intriguing parallels to high-Tc superconductors and twisted bilayer graphene. A study reveals a temperature-dependent cascade of different symmetry-broken electronic states in the kagome superconductor CsV3Sb5, and highlights intriguing parallels between vanadium-based kagome metals and materials exhibiting similar electronic phases.
168 citations
TL;DR: In this article, the authors show that the topological charge density wave phase in the quasi-2D Kagome superconductor AV3Sb5 is a chiral flux phase.
Abstract: We argue that the topological charge density wave phase in the quasi-2D Kagome superconductor AV3Sb5 is a chiral flux phase. Considering the symmetry of the Kagome lattice, we show that the chiral flux phase has the lowest energy among those states which exhibit 2 × 2 charge orders observed experimentally. This state breaks the time-reversal symmetry and displays anomalous Hall effect. The explicit pattern of the density of state in real space is calculated. These results are supported by recent experiments and suggest that these materials are new platforms to investigate the interplay between topology, superconductivity and electron–electron correlations.
154 citations
TL;DR: In this article, tunneling microscopy of a kagome superconductor was used to confirm a number of previously hinted at electronic states, including the existence of a triangular lattice of atoms.
Abstract: Scanning tunneling microscopy of a kagome superconductor---an unusual metal with a triangular lattice of atoms---confirms a number of previously hinted at electronic states.
144 citations
References
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TL;DR: A two-dimensional condensed-matter lattice model is presented which exhibits a nonzero quantization of the Hall conductance in the absence of an external magnetic field, and exhibits the so-called "parity anomaly" of (2+1)-dimensional field theories.
Abstract: A two-dimensional condensed-matter lattice model is presented which exhibits a nonzero quantization of the Hall conductance ${\ensuremath{\sigma}}^{\mathrm{xy}}$ in the absence of an external magnetic field. Massless fermions without spectral doubling occur at critical values of the model parameters, and exhibit the so-called "parity anomaly" of (2+1)-dimensional field theories.
4,606 citations
Book•
01 Jan 1994
TL;DR: In this article, the theoretical and experimental state of affairs of two novel types of broken symmetry ground states of metals, charge, and spin density waves are discussed, as the consequence of electron-phonon and electron-electron interactions in low-dimensional metals.
Abstract: ?Density Waves in Solids is written for graduate students and scientists interested in solid-state sciences. It discusses the theoretical and experimental state of affairs of two novel types of broken symmetry ground states of metals, charge, and spin density waves. These states arise as the consequence of electron-phonon and electron-electron interactions in low-dimensional metals.Some fundamental aspects of the one-dimensional electron gas, and of the materials with anisotropic properties, are discussed first. This is followed by the mean field theory of the phases transitions?discussed using second quantized formalism?together with the various experimental observations on the transition and on the ground states. Fluctuation effects and the collective excitations are reviewed next, using the Ginzburg-Landau formalism, followed by the review of the interaction of these states with the underlying lattice and with impurities. The final chapters are devoted to the response of the ground states to external perturbations.
1,179 citations
TL;DR: In this paper, the enigmatic pseudogap phase of cuprate superconductors is characterized by a hidden broken symmetry of ${d}_{{x}^{2}\ensuremath{-}{y}^{ 2}}$-type.
Abstract: We propose that the enigmatic pseudogap phase of cuprate superconductors is characterized by a hidden broken symmetry of ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-type. The transition to this state is rounded by disorder, but in the limit that the disorder is made sufficiently small, the pseudogap crossover should reveal itself to be such a transition. The ordered state breaks time-reversal, translational, and rotational symmetries, but it is invariant under the combination of any two. We discuss these ideas in the context of ten specific experimental properties of the cuprates, and make several predictions, including the existence of an as-yet undetected metal-metal transition under the superconducting dome.
854 citations
TL;DR: In this article, the interplay between different order parameters in high temperature superconductors is discussed, and the intertwining of these orders leads to new experimentally observable consequences, shedding new light into the physics of these fascinating materials.
Abstract: Understanding high temperature superconductors is a central problem in condensed matter physics. Many experiments have uncovered ordering tendencies which are responsible for the complex phase diagram of high temperature superconductors. This Colloquium discusses the interplay between different order parameters in these materials. Considering the intertwining of these orders leads to new experimentally observable consequences, shedding new light into the physics of these fascinating materials.
837 citations
TL;DR: It is found that the predictions of the standard mean-field model agree only semi-quantitatively with the experimental data and that there is not one generally dominant factor driving charge-density-wave formation in this family of layer compounds.
Abstract: The occurrence of charge-density waves in three selected layered transition-metal dichalcogenides-1T-TaS(2), 2H-TaSe(2) and 1T-TiSe(2)-is discussed from an experimentalist's point of view with a particular focus on the implications of recent angle-resolved photoelectron spectroscopy results. The basic models behind charge-density-wave formation in low-dimensional solids are recapitulated, the experimental and theoretical results for the three selected compounds are reviewed, and their band structures and spectral weight distributions in the commensurate charge-density-wave phases are calculated using an empirical tight-binding model. It is explored whether the origin of charge-density waves in the layered transition-metal dichalcogenides can be understood in a unified way on the basis of a few measured and calculated parameters characterizing the interacting electron-lattice system. It is found that the predictions of the standard mean-field model agree only semi-quantitatively with the experimental data and that there is not one generally dominant factor driving charge-density-wave formation in this family of layer compounds. The need for further experimental and theoretical scrutiny is emphasized.
592 citations