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Register a new userOrigin of the Charge Density Wave in the Kagome Metal CsV$_{3}$Sb$_{5}$ as Revealed by Optical Spectroscopy
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We report on a detailed study of the optical properties of CsV$_{3}$Sb$_{5}$ at a large number of temperatures above and below the charge-density-wave (CDW) transition. Above the CDW transition, the low-frequency optical conductivity reveals two Drude components with distinct widths. An examination of the band structure allows us to ascribe the narrow Drude to multiple light and Dirac bands, and the broad Drude to the heavy bands near the $M$ points which form saddle points near the Fermi level. Upon entering the CDW state, the opening of the CDW gap is clearly observed. A large portion of the broad Drude is removed by the gap, whereas the narrow Drude is not affected. Meanwhile, an absorption peak associated with interband transitions near the saddle points shifts to higher energy and grows in weight. These observations are consistent with the scenario that the CDW in CsV$_{3}$Sb$_{5}$ is driven by nesting of Fermi surfaces near the saddle points at $M$.
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We report $^{121/123}$Sb nuclear quadrupole resonance (NQR) and $^{51}$V nuclear magnetic resonance (NMR) measurements on kagome metal CsV$_3$Sb$_5$ with $T_{rm c}=2.5$ K. Both $^{51}$V NMR spectra and $^{121/123}$Sb NQR spectra split after a charge density wave (CDW) transition, which demonstrates a commensurate CDW state. The coexistence of the high temperature phase and the CDW phase between $91$ K and $94$ K manifests that it is a first order phase transition. At low temperature, electric-field-gradient fluctuations diminish and magnetic fluctuations become dominant. Superconductivity emerges in the charge order state. Knight shift decreases and $1/T_{1}T$ shows a Hebel--Slichter coherence peak just below $T_{rm c}$, indicating that CsV$_3$Sb$_5$ is an s-wave superconductor.
Using first-principles calculations, we identify the origin of the observed charge density wave (CDW) formation in a layered kagome metal CsV$_3$Sb$_5$. It is revealed that the structural distortion of kagome lattice forming the trimeric and hexameric V atoms is accompanied by the stabilization of quasimolecular states, which gives rise to the opening of CDW gaps for the V-derived multibands lying around the Fermi level. This Jahn-Teller-like instability having the local lattice distortion and its derived quasimolecular states is a driving force of the CDW order. Specifically, the saddle points of multiple Dirac bands near the Fermi level, located at the $M$ point, are hybridized to disappear along the $k_z$ direction, therefore not supporting the widely accepted Peierls-like electronic instability due to Fermi surface nesting. It is further demonstrated that applied hydrostatic pressure significantly reduces the interlayer spacing to destabilize the quasimolecular states, leading to a disappearance of the CDW phase at a pressure of ${sim}$2 GPa. The presently proposed underlying mechanism of the CDW order in CsV$_3$Sb$_5$ can also be applicable to other isostructural kagome lattices such as KV$_3$Sb$_5$ and RbV$_3$Sb$_5$.
Recently, kagome lattice metal AV$_3$Sb$_5$ (A = K, Rb, Cs) family has received wide attention due to its presence of superconductivity, charge density wave (CDW) and peculiar properties from topological nontrivial electronic structure. With time-resolved pump-probe spectroscopy, we show that the excited quasiparticle relaxation dynamics can be explained by formation of energy gap below the phase transition being similar to a usual second-order CDW condensate, by contrast, the structure change is predominantly first order phase transition. Furthermore, no CDW amplitude mode is identified in the ordered phase. The results suggest that the CDW order is very different from the traditional CDW condensate. We also find that weak pump pulse can non-thermally melt the CDW order and drive the sample into its high temperature phase, revealing the fact that the difference in lattice potential between those phases is small.
The entanglement of charge density wave (CDW), superconductivity, and topologically nontrivial electronic structure has recently been discovered in the kagome metal $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) family. With high-resolution angle-resolved photoemission spectroscopy, we study the electronic properties of CsV$_3$Sb$_5$ deep in the CDW state. The spectra around $bar{K}$ is found to exhibit a peak-dip-hump structure associated with two separate branches of dispersion, demonstrating the isotropic CDW gap opening. The peak-dip-hump lineshape is contributed by linearly dispersive Dirac bands in the lower branch and a dispersionless flat band close to $E_{rm F}$ in the upper branch. The Fermi surface nesting scenario can account for these CDW-related features. The high density of states at $E_{rm F}$ associated with the flat band could play an essential role in the onset of superconductivity.
$A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) is a novel kagome superconductor coexisting with the charge density wave (CDW) order. Identifying the structure of the CDW order is crucial for understanding the exotic normal state and superconductivity in this system. Here, we report $^{51}$V nuclear magnetic resonance (NMR) and $^{121/123}$Sb nuclear quadrupole resonance (NQR) studies on kagome-metal CsV$_3$Sb$_5$. Below the CDW transition temperature $T_textrm{CDW} sim$ 98 K, an abrupt change of spectra was observed, indicating that the transition is of the first order. By further analysing the spectra, we find that the CDW order is commensurate. And most remarkably, we obtain the first experimental evidence that the charge modulation of the CDW order is of star-of-David pattern and accompanied by an additional charge modulation in bulk below $T^* sim$ 40 K. Our results revealing the unconventional CDW order provide new insights into $A$V$_3$Sb$_5$.
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