Direct observation of the collective modes of the charge density wave in the kagome metal CsV3Sb5

A recently discovered group of kagome metals AV[Formula: see text]Sb[Formula: see text] (A = K, Rb, Cs) exhibit a variety of intertwined unconventional electronic phases, which emerge from a puzzling charge density wave phase. Understanding of this charge-ordered parent phase is crucial for deciphering the entire phase diagram. However, the mechanism of the charge density wave is still controversial, and its primary source of fluctuations-the collective modes-has not been experimentally observed. Here, we use ultrashort laser pulses to melt the charge order in CsV[Formula: see text]Sb[Formula: see text] and record the resulting dynamics using femtosecond angle-resolved photoemission. We resolve the melting time of the charge order and directly observe its amplitude mode, imposing a fundamental limit for the fastest possible lattice rearrangement time. These observations together with ab initio calculations provide clear evidence for a structural rather than electronic mechanism of the charge density wave. Our findings pave the way for a better understanding of the unconventional phases hosted on the kagome lattice.

Unconventional specular optical rotation in the charge ordered state of Kagome metal CsV3Sb5

Kagome metals AV3Sb5 (A = K, Cs, Rb) provide a rich platform for intertwined orders, where evidence for time-reversal symmetry breaking, likely due to the long-sought loop currents, has emerged in STM and muon spin relaxation experiments. An isotropic component in the spontaneous optical rotation has also been reported and was interpreted as the magneto-optic Kerr effect. Intriguingly, the observed rotations differ by five orders of magnitude between different wavelengths and samples, suggesting more intricate physics. Here we report optical rotation and polar Kerr measurements in CsV3Sb5 crystals at the same wavelength. We observe large isotropic components of 1 milliradian in the optical rotation that do not respond to applied magnetic fields, while the spontaneous Kerr signal is less than 20 nanoradians. Our results prove unambiguously that the reported isotropic rotation is not from time-reversal symmetry breaking but represents the long-sought specular optical rotation and indicates a new intertwined order.

Anisotropic proximity-induced superconductivity and edge supercurrent in Kagome metal, K1-xV3Sb5

Materials with Kagome nets are of particular importance for their potential combination of strong correlation, exotic magnetism, and electronic topology. KV₃Sb₅ was discovered to be a layered topological metal with a Kagome net of vanadium. Here, we fabricated Josephson Junctions of K_1-xV₃Sb₅ and induced superconductivity over long junction lengths. Through magnetoresistance and current versus phase measurements, we observed a magnetic field sweeping direction-dependent magnetoresistance and an anisotropic interference pattern with a Fraunhofer pattern for in-plane magnetic field but a suppression of critical current for out-of-plane magnetic field. These results indicate an anisotropic internal magnetic field in K_1-xV₃Sb₅ that influences the superconducting coupling in the junction, possibly giving rise to spin-triplet superconductivity. In addition, the observation of long-lived fast oscillations shows evidence of spatially localized conducting channels arising from edge states. These observations pave the way for studying unconventional superconductivity and Josephson device based on Kagome metals with electron correlation and topology.

High Resolution Polar Kerr Effect Studies of CsV_{3}Sb_{5}: Tests for Time-Reversal Symmetry Breaking below the Charge-Order Transition

We report high resolution polar Kerr effect measurements on CsV_{3}Sb_{5} single crystals in search of signatures of spontaneous time-reversal symmetry breaking below the charge-order transition at T^{*}≈94  K. Utilizing two different versions of zero-area loop Sagnac interferometers operating at 1550 nm wavelength, each with the fundamental attribute that without a time-reversal symmetry breaking sample at its path, the interferometer is perfectly reciprocal, we find no observable Kerr effect to within the noise floor limit of the apparatus at 30 nanoradians. Simultaneous coherent reflection ratio measurements confirm the sharpness of the charge-order transition in the same optical volume as the Kerr measurements. At finite magnetic field we observe a sharp onset of a diamagnetic shift in the Kerr signal at T^{*}, which persists down to the lowest temperature without change in trend. Since 1550 nm is an energy that was shown to capture all features of the optical properties of the material that interact with the charge-order transition, we are led to conclude that it is highly unlikely that time-reversal symmetry is broken in the charge ordered state in CsV_{3}Sb_{5}.

Order-disorder charge density wave instability in the kagome metal (Cs,Rb)V3Sb5

The origin of the charge density wave phases in the kagome metal compound AV₃Sb₅ is still under great scrutiny. Here, we combine diffuse and inelastic x-ray scattering to identify a 3-dimensional precursor of the charge order at the L point that condenses into a CDW through a first order phase transition. The quasi-elastic critical scattering indicates that the dominant contribution to the diffuse precursor is the elastic central peak without phonon softening. However, the inelastic spectra show a small broadening of the Einstein-type phonon mode on approaching T_CDW. Our results point to the situation where the Fermi surface instability at the L point is of order-disorder type with critical growth of quasi-static domains. The experimental data indicate that the CDW consists on an alternating Star of David and trihexagonal distortions and its dynamics goes beyond the classical weak-coupling scenario and is discussed within strong-electron phonon coupling and non-adiabatic models.

Charge order landscape and competition with superconductivity in kagome metals

In the kagome metals AV₃Sb₅ (A = K, Rb, Cs), three-dimensional charge order is the primary instability that sets the stage for other collective orders to emerge, including unidirectional stripe order, orbital flux order, electronic nematicity and superconductivity. Here, we use high-resolution angle-resolved photoemission spectroscopy to determine the microscopic structure of three-dimensional charge order in AV₃Sb₅ and its interplay with superconductivity. Our approach is based on identifying an unusual splitting of kagome bands induced by three-dimensional charge order, which provides a sensitive way to refine the spatial charge patterns in neighbouring kagome planes. We found a marked dependence of the three-dimensional charge order structure on composition and doping. The observed difference between CsV₃Sb₅ and the other compounds potentially underpins the double-dome superconductivity in CsV₃(Sb,Sn)₅ and the suppression of T_c in KV₃Sb₅ and RbV₃Sb₅. Our results provide fresh insights into the rich phase diagram of AV₃Sb₅.

The kagomé metals RbTi3Bi5 and CsTi3Bi5

The kagomé metals RbTi3Bi5 and CsTi3Bi5 were synthesized both as polycrystalline powders by heating the elements in an argon atmosphere and as single crystals grown using a self-flux method. The compounds crystallize in the hexagonal crystal system isotypically to KV3Sb5 (P6/mmm, Z = 1, CsTi3Bi5: a = 5.7873(1), c = 9.2062(1) Å; RbTi3Bi5: a = 5.773(1), c = 9.065(1) Å). The titanium atoms form a kagomé net with bismuth atoms in the hexagons as well as above and below the triangles. The alkali metal atoms are coordinated by 12 bismuth atoms and form AlB2-like slabs between the kagomé layers. Magnetic susceptibility measurements with CsTi3Bi5 and RbTi3Bi5 single crystals reveal Pauli-paramagnetism and traces of superconductivity caused by CsBi2/RbBi2 impurities. Magnetotransport measurements reveal conventional Fermi liquid behavior and quantum oscillations indicative of a single dominant orbit at low temperature. DFT calculations show the characteristic metallic kagomé band structure similar to that of CsV3Sb5 with reduced band filling. A symmetry analysis of the band structure does not reveal an obvious and unique signature of a nontrivial topology.

Realizing Kagome Band Structure in Two-Dimensional Kagome Surface States of RV_{6}Sn_{6} (R=Gd, Ho)

We report angle resolved photoemission experiments on a newly discovered family of kagome metals RV_{6}Sn_{6} (R=Gd, Ho). Intrinsic bulk states and surface states of the vanadium kagome layer are differentiated from those of other atomic sublattices by the real-space resolution of the measurements with a small beam spot. Characteristic Dirac cone, saddle point, and flat bands of the kagome lattice are observed. Our results establish the two-dimensional (2D) kagome surface states as a new platform to investigate the intrinsic kagome physics.

Unconventional chiral charge order in kagome superconductor KV3Sb5

Intertwining quantum order and non-trivial topology is at the frontier of condensed matter physics^1-4. A charge-density-wave-like order with orbital currents has been proposed for achieving the quantum anomalous Hall effect^5,6 in topological materials and for the hidden phase in cuprate high-temperature superconductors^7,8. However, the experimental realization of such an order is challenging. Here we use high-resolution scanning tunnelling microscopy to discover an unconventional chiral 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 2 × 2 superlattice. Spectroscopically, an energy gap opens at the Fermi level, across which the 2 × 2 charge modulation exhibits an intensity reversal in real space, signalling charge ordering. At the 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 charge density wave in the frustrated kagome lattice, which can not only lead to a large anomalous Hall effect with orbital magnetism, but also be a precursor of unconventional superconductivity.

Absence of local moments in the kagome metal KV3Sb5as determined by muon spin spectroscopy

We have carried out muon spin relaxation and rotation measurements on the newly discovered kagome metal KV₃Sb₅, and find a local field dominated by weak magnetic disorder which we associate with the nuclear moments present, and a modest temperature dependence which tracks the bulk magnetic susceptibility. We find no evidence for the existence of V⁴⁺local moments, suggesting that the physics underlying the recently reported giant unconventional anomalous Hall effect in this material warrants further studies.

A Raman probe of phonons and electron-phonon interactions in the Weyl semimetal NbIrTe4

There is tremendous interest in measuring the strong electron-phonon interactions seen in topological Weyl semimetals. The semimetal NbIrTe4 has been proposed to be a Type-II Weyl semimetal with 8 pairs of opposite Chirality Weyl nodes which are very close to the Fermi energy. We show using polarized angular-resolved micro-Raman scattering at two excitation energies that we can extract the phonon mode dependence of the Raman tensor elements from the shape of the scattering efficiency versus angle. This van der Waals semimetal with broken inversion symmetry and 24 atoms per unit cell has 69 possible phonon modes of which we measure 19 modes with frequencies and symmetries consistent with Density Functional Theory calculations. We show that these tensor elements vary substantially in a small energy range which reflects a strong variation of the electron-phonon coupling for these modes.

CsV_{3}Sb_{5}: A Z_{2} Topological Kagome Metal with a Superconducting Ground State

Recently discovered alongside its sister compounds KV_{3}Sb_{5} and RbV_{3}Sb_{5}, CsV_{3}Sb_{5} crystallizes with an ideal kagome network of vanadium and antimonene layers separated by alkali metal ions. This work presents the electronic properties of CsV_{3}Sb_{5}, demonstrating bulk superconductivity in single crystals with a T_{c}=2.5  K. The normal state electronic structure is studied via angle-resolved photoemission spectroscopy and density-functional theory, which categorize CsV_{3}Sb_{5} as a Z_{2} topological metal. Multiple protected Dirac crossings are predicted in close proximity to the Fermi level (E_{F}), and signatures of normal state correlation effects are also suggested by a high-temperature charge density wavelike instability. The implications for the formation of unconventional superconductivity in this material are discussed.

Giant, unconventional anomalous Hall effect in the metallic frustrated magnet candidate, KV3Sb5

The anomalous Hall effect (AHE) is one of the most fundamental phenomena in physics. In the highly conductive regime, ferromagnetic metals have been the focus of past research. Here, we report a giant extrinsic AHE in KV3Sb5, an exfoliable, highly conductive semimetal with Dirac quasiparticles and a vanadium Kagome net. Even without report of long range magnetic order, the anomalous Hall conductivity reaches 15,507 Ω-1 cm-1 with an anomalous Hall ratio of ≈ 1.8%; an order of magnitude larger than Fe. Defying theoretical expectations, KV3Sb5 shows enhanced skew scattering that scales quadratically, not linearly, with the longitudinal conductivity, possibly arising from the combination of highly conductive Dirac quasiparticles with a frustrated magnetic sublattice. This allows the possibility of reaching an anomalous Hall angle of 90° in metals. This observation raises fundamental questions about AHEs and opens new frontiers for AHE and spin Hall effect exploration, particularly in metallic frustrated magnets.

Experimental and computational phase boundary mapping of Co4Sn6Te6

Binary Co4Sb12 skutterudite (also known as CoSb3) has been extensively studied; however, its mixed-anion counterparts remain largely unexplored in terms of their phase stability and thermoelectric properties. In the search for complex anionic analogs of the binary skutterudite, we begin by investigating the Co4Sb12-Co4Sn6Te6 pseudo-binary phase diagram. We observe no quaternary skutterudite phases and as such, focus our investigations on the ternary Co4Sn6Te6 via experimental phase boundary mapping, transport measurements, and first-principles calculations. Phase boundary mapping using traditional bulk syntheses reveals that the Co4Sn6Te6 exhibits electronic properties ranging from a degenerate p-type behavior to an intrinsic behavior. Under Sn-rich conditions, Hall measurements indicate degenerate p-type carrier concentrations and high hole mobility. The acceptor defect SnTe, and donor defects TeSn and Coi are the predominant defects and rationally correspond to regions of high Sn, Te, and Co, respectively. Consideration of the defect energetics indicates that p-type extrinsic doping is plausible; however, SnTe is likely a killer defect that limits n-type dopability. We find that the hole carrier concentration in Co4Sn6Te6 can be further optimized by extrinsic p-type doping under Sn-rich growth conditions.

Solubility limits in quaternary SnTe-based alloys

A combined theoretical and experimental approach was used to determine the equilibrium as well as non-equilibrium solubility lines in the quaternary Sn_1−yMn_yTe_1−xSe_x alloy space, revealing a large area of accessible metastable phase space.

Effect of extended strain fields on point defect phonon scattering in thermoelectric materials

Inexpensive computational descriptors for point defect scattering in alloyed thermoelectric systems developed through a combination of ab initio computation and experimental validation.