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Register a new userDouble-dome superconductivity under pressure in the V-based Kagome metals AV3Sb5 (A = Rb and K)
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We present high-pressure electrical transport measurements on the newly discovered V-based superconductors $A$V$_3$Sb$_5$ ($A$ = Rb and K), which have an ideal Kagome lattice of vanadium. Two superconducting domes under pressure are observed in both compounds, as previously observed in their sister compound CsV$_3$Sb$_5$. For RbV$_3$Sb$_5$, the $T_c$ increases from 0.93 K at ambient pressure to the maximum of 4.15 K at 0.38 GPa in the first dome. The second superconducting dome has the highest $T_c$ of 1.57 K at 28.8 GPa. KV$_3$Sb$_5$ displays a similar double-dome phase diagram, however, its two maximum $T_c$s are lower, and the $T_c$ drops faster in the second dome than RbV$_3$Sb$_5$. An integrated temperature-pressure phase diagram of $A$V$_3$Sb$_5$ ($A$ = Cs, Rb and K) is constructed, showing that the ionic radius of the intercalated alkali-metal atoms has a significant effect. Our work demonstrates that double-dome superconductivity under pressure is a common feature of these V-based Kagome metals.
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Recent high pressure experiments discovered abnormal double-dome superconductivities in the newly-synthesized kagome materials $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs), which also host abundant emergent quantum phenomena such as charge density wave (CDW), anomalous Hall effect, nontrivial topological property, etc. In this work, by using first-principles electronic structure calculations, we have studied the CDW state, superconductivity, and topological property in CsV$_3$Sb$_5$ under pressures ($<$ 50 GPa). Based on the electron-phonon coupling theory, our calculated superconducting $T_text{c}$s are consistent with the observed ones in the second superconducting dome at high pressure, but are much higher than the measured values at low pressure. The further calculations including the Hubbard U indicate that with modest electron-electron correlation the magnetism on the V atoms exists at low pressure and diminishes gradually at high pressure. We thus propose that the experimentally observed superconductivity in CsV$_3$Sb$_5$ at ambient/low pressures may still belong to the conventional Bardeen-Cooper-Schrieffer (BCS) type but is partially suppressed by the V magnetism, while the superconductivity under high pressure is fully conventional without invoking the magnetism. We also predict that there are a second weak CDW state and topological phase transitions in CsV$_3$Sb$_5$ under pressures. Our theoretical assertion calls for future experimental examination.
Exotic quantum phase transitions in metals, such as the nematic and smectic states, were discovered one after another and found to be universal now. The emergence of unconventional density-wave order in frustrated kagome metal AV$_3$Sb$_5$ and its interplay with exotic superconductivity attract increasing attention. We reveal that the smectic bond-density-wave is naturally caused by the paramagnon interference mechanism, because strong scatterings among different van-Hove singularity points are induced. In addition, the fluctuations of the bond-order induce sizable beyond-Migdal pairing glue, and therefore both singlet nodal $s$-wave pairing and triplet $p$-wave pairing states are expected to emerge. The coexistence of both states would explain exotic superconducting states. Unexpected similarities between kagome metal and some Fe-based superconductors are discussed. This study enables us to understand the exotic density wave, superconductivity and their interplay in kagome metals based on the interference mechanism.
Unconventional superconductivity from heavy fermion (HF) is always observed in f-electron systems, in which Kondo physics between localized f-electrons and itinerant electrons plays an essential role. Whether HF superconductivity could be achieved in other systems without f electrons, especially for d-electron systems, is still elusive. Here, we experimentally study the origin of d-electron HF behavior in iron-based superconductors (FeSCs) AFe2As2 (A = K, Rb, Cs). Nuclear magnetic resonance on 75As reveals a universal coherent-incoherent crossover with a characteristic temperature T*. Below T*, a so-called Knight shift anomaly is first observed in FeSCs, which exhibits a scaling behavior similar to f-electron HF materials. Furthermore, the scaling rule also regulates the manifestation of magnetic fluctuation. These results undoubtedly support an emergent Kondo scenario for the d-electron HF behavior, which suggests the AFe2As2 (A = K, Rb, Cs) as the first material realization of d-electron HF superconductors.
Superconductivity in topological kagome metals has recently received great research interests. Here, charge density wave (CDW) orders and the evolution of superconductivity under various pressures in CsV3Sb5 single crystal with V kagome lattice are investigated. By using high-resolution scanning tunnelling microscopy /spectroscopy (STM/STS), two CDW orders in CsV3Sb5 are observed which correspond to 4a*1a and 2a*2a superlattices. By applying pressure, the superconducting transition temperature Tc is significantly enhanced and reaches a maximum value of 8.2 K at around 1 GPa. Accordingly, CDW state is gradually declined as increasing the pressure, which indicates the competing interplay between CDW and superconducting state in this material. The broad superconducting transitions around 0.4 - 0.8 GPa can be related to the strong competition relation among two CDW states and superconductivity. These results demonstrate that CsV3Sb5 is a new platform for exploring the interplay between superconductivity and CDW in topological kagome metals.
Recently superconductivity was discovered in the Kagome metal AV3Sb5 (A = K, Rb, and Cs), which has an ideal Kagome lattice of vanadium. These V-based superconductors also host charge density wave (CDW) and topological nontrivial band structure. Here we report the ultralow-temperature thermal conductivity and high pressure resistance measurements on CsV3Sb5 with Tc = 2.5 K, the highest among AV3Sb5. A finite residual linear term of thermal conductivity at zero magnetic field and its rapid increase in fields suggest nodal superconductivity. By applying pressure, the Tc of CsV3Sb5 increases first, then decreases to lower than 0.3 K at 11.4 GPa, showing a clear first superconducting dome peaked around 0.8 GPa. Above 11.4 GPa, superconductivity re-emerges, suggesting a second superconducting dome. Both nodal superconductivity and superconducting domes point to unconventional superconductivity in this V-based superconductor. While our finding of nodal superconductivity puts a strong constrain on the pairing state of the first dome, which should be related to the CDW instability, the superconductivity of the second dome may present another exotic pairing state in this ideal Kagome lattice of vanadium.
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