Search

Here we present YbV$_3$Sb$_4$ and EuV$_3$Sb$_4$, two new compounds exhibiting slightly distorted vanadium-based kagome nets interleaved with zig-zag chains of divalent Yb$^{2+}$ and Eu$^{2+}$ ions. Single crystal growth methods are reported alongside magnetic, electronic, and thermodynamic measurements. YbV$_3$Sb$_4$ is a nonmagnetic metal with no collective phase transitions observed between 60mK and 300K. Conversely, EuV$_3$Sb$_4$ is a magnetic kagome metal exhibiting easy-plane ferromagnetic-like order below $T_\text{C}$=32K with signatures of noncollinearity under low field. Our discovery of YbV$_3$Sb$_4$ and EuV$_3$Sb$_4$ demonstrate another direction for the discovery and development of vanadium-based kagome metals while incorporating the chemical and magnetic degrees of freedom offered by a rare-earth sublattice.

A new group of kagome metals AV$_3$Sb$_5$ (A = K, Rb, Cs) exhibit a variety of intertwined unconventional electronic phases, which emerge from a puzzling charge density wave phase. Understanding of this parent charge order 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 - have not been experimentally observed. Here, we use ultrashort laser pulses to melt the charge order in CsV$_3$Sb$_5$ 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 better understanding of the unconventional phases hosted on the kagome lattice., Comment: 17 pages, 4 figures

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 it was interpreted as the magneto-optic Kerr effect arising from time-reversal symmetry breaking. 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 but flip sign between samples, while the spontaneous Kerr signal is less than 20 nanoradian. Our results prove unambiguously that the reported isotropic rotation is not related to time-reversal symmetry breaking but instead indicates a new intertwined order that onsets at the same temperature., Comment: 15 pages, 8 figures, submitted version

Triangular lattice delafossite compounds built from magnetic lanthanide ions are a topic of recent interest due to their frustrated magnetism and realization of quantum disordered magnetic ground states. Here we report the evolution of the structure and electronic ground states of RbCe$X_2$ compounds, built from a triangular lattice of Ce$^{3+}$ ions, upon varying their anion character ($X_2$= O$_2$, S$_2$, SeS, Se$_2$, TeSe, Te$_2$). This includes the discovery of a new member of this series, RbCeO$_2$, that potentially realizes a quantum disordered ground state analogous to NaYbO$_2$. Magnetization and susceptibility measurements reveal that all compounds manifest mean-field antiferromagnetic interactions and, with the exception of the oxide, possess signatures of magnetic correlations onset below 1 K. The crystalline electric field level scheme is explored via neutron scattering and \textit{ab initio} calculations in order to model the intramultiplet splitting of the $J=5/2$ multiplet. In addition to the two excited doublets expected within the $J=5/2$ manifold, we observe one extra, local mode present across the sample series. This added mode shifts downward in energy with increasing anion mass and decreasing crystal field strength, suggesting a long-lived anomalous mode endemic to anion motion about the Ce$^{3+}$ sites., Comment: 12 pages, 7 figures

The kagome lattice provides a fascinating playground to study geometrical frustration, topology and strong correlations. The newly discovered kagome metals AV$_3$Sb$_5$ (A=K, Rb, Cs) exhibit various interesting phenomena including topological band structure and superconductivity. Nevertheless, the nature of the symmetry breaking in the CDW phase is not yet clear, despite the fact that it is crucial to understand whether the superconductivity is unconventional. In this work, we perform scanning birefringence microscopy and find that six-fold rotation symmetry is broken at the onset of the CDW transition temperature in all three compounds. Spatial imaging and angle dependence of the birefringence show a universal three nematic domains that are 120$^\circ$ to each other. We propose staggered CDW orders with a relative $\pi$ phase shift between layers as a possibility to explain the three-state nematicity in AV$_3$Sb$_5$. We also perform magneto-optical Kerr effect and circular dichroism measurements on all three compounds, and the onset of the both signals is at the CDW transition temperature, indicating broken time-reversal symmetry and the existence of the long-sought loop currents in the CDW phase. Our work strongly constrains the nature of the CDWs and sheds light on possible unconventional superconductivity in AV$_3$Sb$_5$., Comment: submitted version. extended data Fig.5 updated with the correct figure

We report high resolution polar Kerr effect measurements on CsV${}_3$Sb${}_5$ single crystals in search for signatures of spontaneous time reversal symmetry breaking below the charge order transition at $T^* \approx 94$ 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${}_5$., Comment: 11 pages, 8 figures

In this work we explore the chemical-property phase diagram of the AV$_3$Sb$_5$ family through A-site alloying. We demonstrate full miscibility of the alkali-site, highlighting that the three parent compounds are the terminal ends of a single solid-solution. Using both polycrystalline and single crystal methods, we map the dependence of the two primary electronic instabilities: (1) the onset of charge density wave (CDW) order ($T_\text{CDW}$) and (2) the onset of superconductivity ($T_\text{c}$) with alkali-site composition. We show continuous trends in both $T_\text{CDW}$ and $T_\text{c}$, including a region of enhanced CDW stability in K$_{1-x}$Cs$_{x}$V$_3$Sb$_5$ alloys. Together, our results open new routes for chemical perturbation and exploration of the chemical-property relationships in the class of AV$_3$Sb$_5$ kagome superconductors.

The lanthanide pyrochlore oxides Ln$_2$B$_2$O$_7$ are one of the most intensely studied classes of materials within condensed matter physics, firmly centered as one of the pillars of frustrated magnetism. The extensive chemical diversity of the pyrochlores, coupled with their innate geometric frustration, enables realization of a wide array of exotic and complex magnetic ground states. Thus, the discovery of new pyrochlore compositions has been a persistent theme that continues to drive the field in exciting directions. The recent focus on the mixed B-site pyrochlores offers a unique route towards tuning both local coordination chemistry and sterics, while maintaining a nominally pristine magnetic sublattice. Here, we present a broad overview of the pyrochlore stability field, integrating recent synthetic efforts in mixed B-site systems with the historically established Ln$_2$B$_2$O$_7$ families. In parallel, we present the discovery and synthesis of the entire Ln$_2$InSbO$_7$ family (Ln: La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) located near the boundary of the pyrochlore stability field using a rapid, hybrid mechanicochemical/microwave-assisted synthesis technique. Magnetic characterization on the entire class of compounds draws striking parallels to the stannate analogs, suggesting that these compounds may host a breadth of exotic magnetic ground states.

The family of AV$_3$Sb$_5$ (A = K, Rb, Cs) kagome metals exhibit charge density wave (CDW) order, proposed to be chiral, followed by a lower temperature superconducting state. Recent studies have proposed the importance of band structure saddle points proximal to the Fermi energy in governing these two transitions. Here we show the effects of hole-doping achieved via chemical substitution of Sn for Sb on the CDW and superconducting states in both KV$_3$Sb$_5$ and RbV$_3$Sb$_5$, and generate a phase diagram. Hole-doping lifts the $\Gamma$ pocket and van Hove singularities (vHs) toward $E_F$ causing the superconducting $T_C$ in both systems to increase to about 4.5 K, while rapidly suppressing the CDW state., Comment: 6 pages, 4 figures

The kagomé metals RbTi$_3$Bi$_5$ and CsTi$_3$Bi$_5$ were synthesized both as polycrystalline powders by heating the elements an argon atmosphere and as single crystals grown using a self-flux method. The compounds crystallize in the hexagonal crystal system isotypically to KV$_3$Sb$_5$ (P6/mmm, Z = 1, CsTi3Bi5: a = 5.7873(1) Å, c = 9.2062(1) Å; RbTi3Bi5: a = 5.773(1) Å, c = 9.065(1) Å). 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 AlB$_2$-like slabs between the kagomé layers. Magnetic susceptibility measurements with CsTi$_3$Bi$_5$ and RbTi$_3$Bi$_5$ single crystals reveal Pauli-paramagnetism and traces of superconductivity caused by CsBi$_2$/RbBi$_2$ 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 CsV$_3$Sb$_5$ with reduced band filling. A symmetry analysis of the band structure does not reveal an obvious and unique signature of a nontrivial topology., 20 pages, 5 Figures, submitted

We employ polarization-resolved electronic Raman spectroscopy and density functional theory to study the primary and secondary order parameters, as well as their interplay, in the charge density wave (CDW) state of the kagome metal AV3Sb5. Previous x-ray diffraction data at 15K established that the CDW order in CsV3Sb5 comprises of a 2x2x4 structure: one layer of inverse-star-of-David and three consecutive layers of star-of-David pattern. We analyze the lattice distortions based the 2x2x4 structure at 15K, and find that U lattice distortion is the primary order parameter while M and L distortions are secondary order parameters for Vanadium displacements. This conclusion is confirmed by the calculation of bare susceptibility that shows a broad peak at around qz=0.25 along the hexagonal Brillouin zone face central line (U-line). We also identify several phonon modes emerging in the CDW state, which are lattice vibration modes related to V and Sb atoms as well as alkali atoms. The detailed temperature evolution of these modes' frequencies, HWHM, and integrated intensities support a phase diagram with two successive structural phase transitions in CsV3Sb5: the first one with a primary order parameter appearing at TS=94K and the second isostructural one appearing at around 70K.

Superconductors with kagome lattices have been identified for over 40 years, with a superconducting transition temperature TC up to 7K. Recently, certain kagome superconductors have been found to exhibit an exotic charge order, which intertwines with superconductivity and persists to a temperature being one order of magnitude higher than TC. In this work, we use scanning tunneling microscopy (STM) to study the charge order in kagome superconductor RbV3Sb5. We observe both a 2x2 chiral charge order and nematic surface superlattices (predominantly 1x4). We find that the 2x2 charge order exhibits intrinsic chirality with magnetic field tunability. Defects can scatter electrons to introduce standing waves, which couple with the charge order to cause extrinsic effects. While the chiral charge order resembles that discovered in KV3Sb5, it further interacts with the nematic surface superlattices that are absent in KV3Sb5 but exist in CsV3Sb5.

The recently reported \textit{A}V$_3$Sb$_5$ (\textit{A} = K, Rb, Cs) family of kagome metals are candidates for unconventional superconductivity and chiral charge density wave (CDW) order; both potentially arise from nested saddle points in their band structures close to the Fermi energy. Here we use chemical substitution to introduce holes into CsV$_3$Sb$_{5}$ and unveil an unconventionalcoupling of the CDW and superconducting states. Specifically, we generate a phase diagram for CsV$_3$Sb$_{5-x}$Sn$_{x}$ that illustrates the impact of hole-doping the system and lifting the nearest vHs toward and above $E_F$. Superconductivity exhibits a non-monotonic evolution with the introduction of holes, resulting in two "domes" peaked at 3.6\,K and 4.1\,K and the rapid suppression of three-dimensional CDW order. The evolution of CDW and superconducting order is compared with the evolution of the electronic band structure of CsV$_3$Sb$_{5-x}$Sn$_x$, where the complete suppression of superconductivity seemingly coincides with an electron-like band comprised of Sb $p_z$ orbitals pushed above E$_F$., Comment: 6 pages, 4 figures

The new two-dimensional (2D) kagome superconductor CsV$_3$Sb$_5$ has attracted much recent attention due to the coexistence of superconductivity, charge order, topology and kagome physics. A key issue in this field is to unveil the unique reconstructed electronic structure, which successfully accommodates different orders and interactions to form a fertile ground for emergent phenomena. Here, we report angle-resolved photoemission spectroscopy (ARPES) evidence for two distinct band reconstructions in CsV$_3$Sb$_5$. The first one is characterized by the appearance of new electron energy band at low temperature. The new band is theoretically reproduced when the three dimensionality of the charge order is considered for a band-folding along the out-of-plane direction. The second reconstruction is identified as a surface induced orbital-selective shift of the electron energy band. Our results provide the first evidence for the three dimensionality of the charge order in single-particle spectral function, highlighting the importance of long-range out-of-plane electronic correlations in this layered kagome superconductor. They also point to the feasibility of orbital-selective control of the band structure via surface modification, which would open a new avenue for manipulating exotic phenomena in this system, including superconductivity.

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., Comment: Authors' version of the manuscript

Pressure evolution of the superconducting kagome metal CsV$_3$Sb$_5$ is studied with single-crystal x-ray diffraction and density-functional band-structure calculations. A highly anisotropic compression observed up to 5 GPa is ascribed to the fast shrinkage of the Cs-Sb distances and suppression of Cs rattling motion. This prevents Sb displacements required to stabilize the three-dimensional charge-density-wave (CDW) order and elucidates the disappearance of the CDW already at 2 GPa despite only minor changes in the electronic structure of the normal state. At higher pressures, vanadium bands still change only marginally, whereas antimony bands undergo a major reconstruction caused by the gradual formation of the interlayer Sb-Sb bonds. Our results exclude pressure tuning of vanadium kagome bands as the main mechanism for the non-trivial evolution of superconductivity in real-world kagome metals. Concurrently, we establish the central role of Sb atoms in the stabilization of a three-dimensional CDW and Fermi surface reconstruction., Comment: published version

The kagome metals $A$V$_3$Sb$_5$ ($A=$ K, Rb, Cs) under ambient pressure exhibit an unusual charge order, from which superconductivity emerges. In this work, by applying hydrostatic pressure using a liquid pressure medium and carrying out electrical resistance measurements for RbV$_3$Sb$_5$, we find the charge order becomes suppressed under a modest pressure $p_{\rm c}$ ($1.4, Comment: intended for a special issue on vanadium-based kagome materials in Chinese Physics B, updated with additional measurements on a second sample

The synthesis and characterization of vanadium-based kagome metals YV$_6$Sn$_6$ and GdV$_6$Sn$_6$ are presented. X-ray diffraction, magnetization, magnetotransport, and heat capacity measurements reveal an ideal kagome network of V-ions coordinated by Sn and separated by triangular lattice planes of rare-earth ions. The onset of low-temperature, likely noncollinear, magnetic order of Gd spins is detected in GdV$_6$Sn$_6$, while V-ions in both compounds remain nonmagnetic. Density functional theory calculations are presented modeling the band structures of both compounds, which can be classified as $\mathbb{Z}_2$ topological metals in the paramagnetic state. Both compounds exhibit high mobility, multiband transport and present an interesting platform for controlling the interplay between magnetic order associated with the $R$-site sublattice and nontrivial band topology associated with the V-based kagome network. Our results invite future exploration of other $R$V$_6$Sn$_6$ ($R$=rare earth) variants where this interplay can be tuned via $R$-site substitution., Comment: 11 pages, 10 figures

The kagome lattice is host to flat bands, topological electronic structures, Van Hove singularities and diverse electronic instabilities, providing an ideal platform for realizing highly tunable electronic states. Here, we report soft- and mechanical- point-contact spectroscopy (SPCS and MPCS) studies of the kagome superconductors KV$_3$Sb$_5$ and CsV$_3$Sb$_5$. Compared to the superconducting transition temperature $T_{\rm c}$ from specific heat measurements (2.8~K for CsV$_3$Sb$_5$ and 1.0~K for KV$_3$Sb$_5$), significantly enhanced values of $T_{\rm c}$ are observed via the zero-bias conductance of SPCS ($\sim$4.2~K for CsV$_3$Sb$_5$ and $\sim$1.8~K for KV$_3$Sb$_5$), which become further enhanced in MPCS measurements ($\sim$5.0~K for CsV$_3$Sb$_5$ and $\sim$3.1~K for KV$_3$Sb$_5$). While the differential conductance curves from SPCS are described by a two-gap $s$-wave model, a single $s$-wave gap reasonably captures the MPCS data, likely due to a diminishing spectral weight of the other gap. The enhanced superconductivity probably arises from local strain caused by the point-contact, which also leads to the evolution from two-gap to single-gap behaviors in different point-contacts. Our results demonstrate highly strain-sensitive superconductivity in kagome metals CsV$_3$Sb$_5$ and KV$_3$Sb$_5$, which may be harnessed in the manipulation of possible Majorana zero modes.

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. This topological electronic structure is made possible because of the broken inversion symmetry of NbIrTe4 which is an orthorhombic crystal with Td symmetry. Using micro-Raman scattering as a probe, we observe the frequencies and symmetries of 19 phonon modes (ranging from 40 to 260 cm-1) in this material and compare to Density Functional Theory calculations. Using angular and polarization resolved Raman scattering for green (514 nm) and red (633 nm) laser excitation, we show that it is possible to extract the excitation energy dependence of the Raman tensor elements associated with each measurable phonon mode. 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., Comment: 36 pages, 12 figures

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 (ARPES) and density functional theory (DFT), which categorize CsV$_3$Sb$_5$ as a $\mathbb{Z}_2$ 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 wave-like instability. The implications for the formation of unconventional superconductivity in this material are discussed., Comment: 6 pages, 4 figures

The layered vanadium antimonides AV3Sb5 (A = K, Rb, Cs) are a recently discovered family of topological kagome metals with a rich phenomenology of strongly correlated electronic phases including charge order and superconductivity. Understanding how the singularities inherent to the kagome electronic structure are linked to the observed many-body phases is a topic of great interest and relevance. Here, we combine angle-resolved photoemission spectroscopy and density functional theory to reveal multiple kagome-derived van Hove singularities (vHs) coexisting near the Fermi level of CsV3Sb5 and analyze their contribution to electronic symmetry breaking. Intriguingly, the vHs in CsV3Sb5 have two distinct flavors - p-type and m-type - which originate from their pure and mixed sublattice characters, respectively. This twofold vHs is unique property of the kagome lattice, and its flavor critically determines the pairing symmetry and ground states emerging in AV3Sb5 series. We establish that, among the multiple vHs in CsV3Sb5, the m-type vHs of the dxz/dyz kagome band and the p-type vHs of the dxy/dx2-y2 kagome band cross the Fermi level to set the stage for electronic symmetry breaking. The former band exhibits pronounced Fermi surface nesting, while the latter contributes via higher-order vHs. Our work reveals the essential role of kagome-derived vHs for the collective phenomena realized in the AV3Sb5 family, paving the way to a deeper understanding of strongly correlated topological kagome systems., Comment: Submitted to Nature Physics on May 28th, 2021. A revised version will appear in Nature Physics