Exotic quantum phase transitions in metals, such as the electronic nematic state, have been discovered one after another and found to be universal now. The emergence of unconventional density-wave (DW) order in frustrated kagome metal AV3Sb5 and its interplay with exotic superconductivity attract increasing attention. We find that the DW in kagome metal is the bond order, because the sizable intersite attraction is caused by the quantum interference among paramagnons. This mechanism is important in kagome metals because the geometrical frustration prohibits the freezing of paramagnons. In addition, we uncover that moderate bond-order fluctuations mediate sizable pairing glue, and this mechanism gives rise to both singlet s-wave and triplet p-wave superconductivity. Furthermore, characteristic pressure-induced phase transitions in CsV3Cb5 are naturally understood by the present theory. Thus, both the exotic density wave and the superconductivity in geometrically frustrated kagome metals are explained by the quantum interference mechanism. 

/pdf/mechanism-of-exotic-density-wave-and-beyond-migdal-2llojkht.pdf

Mechanism of exotic density-wave and beyond-Migdal unconventional superconductivity in kagome metal AV3Sb5(A = K, Rb, Cs)

The recently discovered vanadium-based kagome metals $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A$ = K, Rb, Cs) exhibit superconductivity at low temperatures and charge density wave (CDW) order at high temperatures. A prominent feature of the charge ordered state in this family is that it breaks time-reversal symmetry (TRSB), which is connected to the underlying topological nature of the band structure. In this work, a powerful combination of zero-field and high-field muon-spin rotation/relaxation is used to study the signatures of TRSB of the charge order in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$, as well as its anisotropic character. By tracking the temperature evolution of the in-plane and out-of-plane components of the muon-spin polarization, an enhancement of the internal field width sensed by the muon-spin ensemble was observed below ${T}_{\mathrm{TRSB}}={T}_{\mathrm{CDW}}\ensuremath{\simeq}95$ K. Additional increase of the internal field width, accompanied by a change of the local field direction at the muon site from the $ab$ plane to the $c$ axis, was detected below ${T}^{*}\ensuremath{\simeq}30$ K. Remarkably, this two-step feature becomes well pronounced when a magnetic field of 8 T is applied along the crystallographic $c\phantom{\rule{4.pt}{0ex}}\mathrm{axis}$, thus indicating a field-induced enhancement of the electronic response at the CDW transition. These results point to a TRSB in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ by charge order with an onset of $\ensuremath{\simeq}95$ K, followed by an enhanced electronic response below $\ensuremath{\simeq}30$ K. The observed two-step transition is discussed within the framework of different charge-order instabilities, which, in accordance with density functional theory calculations, are nearly degenerate in energy. 

/pdf/time-reversal-symmetry-broken-by-charge-order-in-mml-math-3ik0d9bl.pdf

Time-reversal symmetry broken by charge order in 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">CsV</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Sb</mml:mi><mml:mn>5</mml:mn></mml:msub></mml:math>

Following the discovery of a new family of kagomé prototypical materials with structure AV3Sb5 ( A=K, Rb, and Cs), there has been a heightened interest in studying the correlation-driven electronic phenomena in these kagomé lattice systems. The study of these materials has gone beyond magneto-transport measurements to reveal exciting features such as Dirac bands, anomalous Hall effect, bulk superconductivity with Tc∼0.9−2.5K, and the observation of charge density wave instabilities, suggesting an intertwining of topological physics and new quantum orders. Moreover, very recent works on numerous types of experiments have appeared further examining the unconventional superconductivity and the exotic electronic states found within these kagomé materials. Theories on the strong interactions that play a role in these systems have been proposed to shed light on the nature of these topological charge density waves. In this brief review, we summarize these recent experimental findings and theoretical proposals and envision the materials as new platforms to study the interplay between topological physics and strongly correlated electronic systems. 

https://aip.scitation.org/doi/10.1063/5.0079593

Electronic properties of correlated kagomé metals AV3Sb5 (A = K, Rb, and Cs): A perspective

High-performance magnesium-based thermoelectric materials: Progress and challenges

The kagome metal compounds $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A=\text{K}$, Rb, and Cs) feature a wealth of phenomena including nontrivial band topology, charge density wave (CDW), and superconductivity. One intriguing property is the time-reversal symmetry breaking in the CDW state without local moments, which leads to anomalous transport responses. Here, we report the investigation of magnetothermoelectric effects on high-quality ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ single crystals. A large anomalous Nernst effect (ANE) is observed at temperatures below 30 K and can be enhanced by the high mobility. Multiple Fermi surfaces with small effective masses are revealed by quantum oscillations in the Nernst and Seebeck signals. Furthermore, we discover a magnetic breakdown effect across the two smallest Fermi surfaces, with a gap around 20 meV between them. We propose that the two Fermi surfaces are split from a Dirac band by the CDW gap. These results indicate the large ANE originates from the CDW modulated nontrivial band structure as well as the extrinsic contributions. A second phase transition below the CDW transition temperature is also suggested by the strange temperature dependence of the ANE. 

http://link.aps.org/pdf/10.1103/PhysRevB.105.L201109

Anomalous thermoelectric effects and quantum oscillations in the kagome metal 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>CsV</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi>Sb</mml:mi><mml:mn>5</mml:mn></mml:msub></mml:mrow></mml:math>

Magneto-Seebeck effect and ambipolar Nernst effect in the CsV 3 Sb 5 superconductor

We report a comprehensive magnetization and magnetotransport study on a metallic-type of ${\mathrm{EuMnSb}}_{2}$ single crystals. A large Hall signal up to room temperature is found in our metallic ${\mathrm{EuMnSb}}_{2}$, which is dominated by hole carriers with very low carrier density and high mobility. Clear Shubnikov--de Haas oscillations are observed at low temperatures (T $l$ 30 K), from which a dominant tiny hole pocket with small effective mass is deduced. The interplay of carrier density and Mn and Eu moments results in complex magnetic ground states with two successive antiferromagnetic (AFM) transitions appearing at ${T}_{N1}=24 \mathrm{K}$ and ${T}_{N2}=9 \mathrm{K}$. In line with the rich magnetic phases, the transport properties, including magnetoresistance, quantum oscillation, and Berry phase, experience abrupt disturbances when crossing the magnetic phase boundaries. The Dirac state is likely quenched once the AFM orders set in, indicating a strong coupling between the AFM magnetic order and the band topology. Our results suggest that metallic-type ${\mathrm{EuMnSb}}_{2}$ is an ideal platform for the study of the interplay between magnetism, charge transport, and band topology.

Strong coupling between magnetic order and band topology in the antiferromagnet EuMnSb 2

We studied the effects of multiband electronic structure on the thermoelectric and electrical transport properties in the normal state of kagome superconductors AV3Sb5 (A = K, Rb, Cs). In all three members, the multiband nature is manifested by sign changes in the temperature dependence of the Seebeck and Hall resistivity, together with sublinear response of the isothermal Nernst and Hall effects to external magnetic fields in the charge ordered state. Moreover, ambipolar transport effects appear ubiquitously in all three systems, giving rise to sizable Nernst signal. Finally, possible origins of the sign reversal in the temperature dependence of the Hall effect are discussed.

Multiband effects in thermoelectric and electrical transport properties of kagome superconductors AV3Sb5 (A = K, Rb, Cs)

We report electrical and thermal transport properties of single crystalline kagome metals $A$Ti$_3$Bi$_5$ ($A$ = Rb, Cs). Different from the structural similar kagome superconductors $A$V$_3$Sb$_5$, no charge density wave instabilities are found in $A$Ti$_3$Bi$_5$. At low temperatures below 5 K, signatures of superconductivity appear in $A$Ti$_3$Bi$_5$ as seen in magnetization measurements. However, bulk superconductivity is not evidenced by specific heat results. Similar to $A$V$_3$Sb$_5$, $A$Ti$_3$Bi$_5$ show nonlinear magnetic field dependence of the Hall effect below about 70 K, pointing to a multiband nature. Unlike $A$V$_3$Sb$_5$ in which phonons and electron-phonon coupling play important roles in thermal transport, the thermal conductivity in $A$Ti$_3$Bi$_5$ is dominated by electronic contributions. Moreover, our calculated electronic structure suggests that van Hove singularities are sitting well above the Fermi energy. Compared with $A$V$_3$Sb$_5$, the absence of charge orders in $A$Ti$_3$Bi$_5$ is closely associated with minor contributions from electron-phonon coupling and/or van Hove singularities.

Electrical and thermal transport properties of kagome metals $A$Ti$_3$Bi$_5$ ($A$ = Rb, Cs)

Achieving high thermoelectric power factor in thin film heterostructures is essential for integrated and miniatured thermoelectric device applications. In this work, we demonstrate a mechanism and device performance of enhanced thermoelectric power factor through coupling the interfacial confined twodimensional electron gas (2DEG) with thin film conductivity in a transition metal dichalcogenidesSrTiO3 heterostructure. Owing to the formed conductive interface with two-dimensional electron confinement effect and the elevated conductivity, the ZrTe2/SrTiO3 (STO) heterostructure presents enormous thermoelectric power factor as high as 4 × 10 μWcmK at 20 K and 4800 μWcmK at room temperature. Formation of quasi-two-dimensional electrons gas at the interface is attributed to the giant Seebeck coefficient, and enhanced electrical conductivity is suggested to be originated from charge transfer induced doping in the ZrTe2, which leads to extremely large thermoelectric power factor. By taking the thermal conductivity of STO substrate as a reference, the effective zT value of this heterostructure can reach 1.5 at 300 K. This high thermoelectric figure of merit is demonstrated by a prototype device based on this heterostructure which results in 3K temperature cooling by passing through a current of 100 mA. This superior thermoelectric property makes this heterostructure a promising candidate for future thermoelectric device, and more importantly, paves a new pathway to design promising high-performance thermoelectric systems.

/pdf/excellent-thermoelectric-performance-of-zrte2-thin-films-and-2j8fs9ga7m.pdf

Long Zhang

Papers

Magneto-Seebeck effect and ambipolar Nernst effect in the CsV 3 Sb 5 superconductor

Strong coupling between magnetic order and band topology in the antiferromagnet EuMnSb 2

Multiband effects in thermoelectric and electrical transport properties of kagome superconductors AV3Sb5 (A = K, Rb, Cs)

Electrical and thermal transport properties of kagome metals $A$Ti$_3$Bi$_5$ ($A$ = Rb, Cs)

Excellent thermoelectric performance of ZrTe2 thin films and device