Tao Wu

A new frontier in condensed matter physics is to stack atomically thin layered materials with different properties and create intriguing phenomena which do not exist in any of the constituent layers. Transition metal dichalcogenide 4Hb-TaS _2, with an alternating stacking of a spin liquid candidate 1T-TaS ₂ and a superconductor 1H-TaS ₂ , is a natural heterostructure for such a purpose. Recently, rare phenomena were indeed observed, including chiral superconductivity, two-component nematic superconductivity, topological surface superconductivity, and enigmatic magnetic memory. A widely proposed starting point to understand such a mysterious heterostructure requires strong electronic correlation, presumably provided by 1T-TaS ₂ layers with a narrow flat band near the Fermi level (E _F ). Here, by using angle-resolved photoemission spectroscopy, we reveal the theoretically expected flat band near E _F in the energy-momentum space. However, this flat band only exists on the 1T-TaS ₂ terminated surface layer with broken translational symmetry, but not on the 1T-TaS ₂ subsurface layers buried in the bulk. These results directly challenge the foundation of the current theoretical paradigm. On the 1T-TaS ₂ terminated surface layer, we further revealed a pseudogap and an anomalous doping effect. These phenomena and the dichotomy between surface and bulk layers also shed light on the unusual coexistence of distinct electronic orders in this mysterious heterostructure.

Since the discovery of charge density wave (CDW) and superconductivity, kagome metal AV 3 Sb 5 ( A = K , Rb, Cs) provides a new platform for exploring novel many-body quantum phenomena. In CsV 3 Sb 5 , a stripelike CDW with commensurate wave vector q = 3 / 8 was observed under moderate pressures, which leads to a peculiar superconducting double-dome behavior in pressure-dependent phase diagram. Previous density functional theory (DFT) calculations indicate that the pressure-induced stripelike CDW is beyond conventional phonon softening scenario, suggesting a nontrivial role of electronic correlations. However, an in-depth understanding for the pressure-induced unconventional CDW remains elusive. Here, we performed pressure-dependent V 51 nuclear magnetic resonance (NMR) measurements on KV 3 Sb 5 and RbV 3 Sb 5 . Although the superconducting double-dome behavior is absent in pressurized KV 3 Sb 5 and RbV 3 Sb 5 , a pressure-induced CDW phase, ascribed to a possible incommensurate triple- Q CDW, is identified by NMR spectra in both materials, indicating that the pressure-induced unconventional CDW beyond DFT calculations is a common feature for kagome metal AV 3 Sb 5 . In contrast to the stripelike CDW, the pressure-induced incommensurate triple- Q CDW does not strongly suppress the superconducting temperature ( T c ) but coincides with an almost plateau behavior at intermediate pressure regime in the pressure-dependent superconducting phase diagram. Furthermore, by systematically analyzing the Korringa relation between Knight shift and nuclear spin-lattice relaxation rate in AV 3 Sb 5 , Van Hove singularity-driven electronic fluctuations are revealed as an effective knob for the pressure-induced unconventional CDW. Finally, our present findings underscore the pressure-induced unconventional CDW as a novel correlated quantum state in kagome metal AV 3 Sb 5 .

Mott physics lies at the heart of many intriguing quantum phenomena. Recently, van der Waals materials emerge as a powerful platform to explore these correlated electronic states, yet tracking how such a Mott state develops and interacts with neighboring phases remains an experimental challenge. Here we use scanning tunnelling spectroscopy to study a filling-controlled Mott state and its interplay with superconductivity in 4Hb-TaS₂, which interleaves strongly correlated 1T-TaS₂ layers and superconducting 1H-TaS₂ layers. By resolving the electronic spectral function across different filling levels driven by interlayer charge transfer, we quantitatively capture the continuous evolution of the Mott state, characterized by the emergence of Mott-Hubbard bands and the systematic suppression of the central quasiparticle peak. Moreover, while the 1H layers exhibit a uniform, fully opened superconducting gap, the heavily correlated electrons in the 1T layers act destructively against pairing, creating nanoscale, non-superconducting paramagnetic puddles. Our findings provide a quintessential visualization of the iconic Hubbard model and establish layered heterostructures as an ideal playground for designing exotic correlated quantum phases.

The recent discovery of superconductivity in Ruddlesden‒Popper (RP) nickelates R_n+1Ni_nO_3n+1 (R = rare earth) under high pressure provides a new platform to understand the underlying physics of high-temperature superconductivity. Previous transport measurements suggest a notable correlation between pressure-induced high-temperature superconductivity and a density-wave (DW) state. Therefore, identifying the nature of the DW state is a prerequisite for decoding the superconducting mechanism in the new family of high-temperature superconductors. Here, we report a comprehensive investigation of the ambient-pressure DW transition in high-quality La₄Ni₃O₁₀ single crystals using ¹³⁹La (I = 7/2) nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR). Our findings reveal a two-stage evolution of the DW order. Below T* ≈ 150 K, a short-range charge order develops in the inner Ni-O layer, accompanied by a dramatic enhancement of spin fluctuations. This is followed by a DW transition at T_DW ≈ 133 K, establishing fully developed charge and spin orders across all Ni-O planes. The layer-dependent behaviour highlights that the mechanism of DW transitions in La₄Ni₃O₁₀ may involve both the interlayer coupling and the electronic structure disparities between the inner and outer layers. These findings provide a new framework for understanding the complex DW state in RP nickelates and their potential role in high-temperature superconductivity.

In materials with broken inversion symmetry, nonreciprocal magneto-transport manifests as a bilinear dependence of charge conductivity on electric and magnetic fields. This phenomenon is rooted in symmetry and electronic quantum geometry and is relevant for rectification and detection technologies. Experimental studies generally attribute nonreciprocal magneto-transport to Zeeman-driven mechanisms and exhibit quadratic scaling with conductivity. Here, we report a microscopic mechanism based on Lorentz skew scattering in BiTeBr, arising from the cooperation of classical Lorentz force and quantum skew scattering, exhibiting a quartic scaling of the nonreciprocal response. Systematic measurements on samples with different mobilities reveal a crossover between Zeeman-related and Lorentz-skew scattering-dominated regimes, uncovering the mobility plays a central role in determining the dominant mechanism. Our finding unveils the leading mechanism in high-mobility systems and suggests a universal principle towards strong nonreciprocal response by enhancing electronic relaxation time in topological materials, rendering guidance for low-dissipation rectifiers and high-performance quantum electronics.

Owing to strong electronic correlations, high-temperature superconductivity always exhibits intricate intertwinement with various competing electronic orders in phase diagrams, such as spin/charge density waves (S/CDWs). In cuprate superconductors,the intertwinement of superconductivity and CDW order could strongly affect the fundamental properties of superconductivity, such as the critical temperature(Tc) and critical magnetic field(Hc). Recent high-field transport measurements indicate that when quantum fluctuations become important at low temperatures and high magnetic fields, the CDW order also reshapes the vortex states, which leads to fragile superconductivity with extremely low critical current(Jc). Here, by performing comprehensive high-field transport measurements, the H-T phase diagram of vortex states is mapped to H = 33 T in a quasi-two-dimensional FeSe-based superconductor (TBA+)xFeSe with a zero-resistivity transition temperature above 40 K. Our results indicate that (TBA+)xFeSe is an extremely type II superconductor with significant thermal fluctuations.At low temperatures, high magnetic fields cause the vortex solid state to exhibit similar current-dependent zero-resistance behavior as the fragile superconductivity in cuprate superconductors with CDW order. When the vortex solid state is melted with increasing temperature, a superconducting regime with vortex-like phase fluctuations emerges as an intermediate state, which features finite longitudinal resistance and vanishing Hall resistance. At higher temperatures, a vortex liquid state with finite Hall resistance eventually appears due to thermal fluctuations. All these observations suggest exotic vortex states beyond the classical paradigm of vortex matter.

A central mystery in high-temperature cuprate superconductors is the coexistence of multiple exotic orders, which is presumably associated with strong electronic correlation. The ongoing interest in this enigmatic phenomenon is further energized when similar electronic orders and states emerge and coexist in less correlated kagome superconductors. Here, by utilizing angle-resolved photoemission spectroscopy (ARPES), nuclear magnetic resonance (NMR) spectroscopy, scanning tunneling microscopy (STM) and first-principles calculations, we reveal the sudden emergence of a distinct short-range charge stripe order in Sn-doped CsV 3 Sb 5 kagome superconductors when the long-range 2 × 2 charge density wave order in pristine CsV 3 Sb 5 is suppressed. This short-range stripe order features a modulation vector of approximately 1 / 3 along one of the three lattice directions, induces remarkable quasiparticle scattering between the original quasi-1D kagome d bands and their replica of folding, and clearly suppresses the electron density of states at the Fermi level. Our first-principles calculations reveal that 3 × 1 supermodulation represents a hidden secondary instability in pristine CsV 3 Sb 5 . This instability is further enhanced by in-plane chemical pressure induced by Sn substitution, and coupled to the electronic correlation, leading to a unique charge stripe order in the system. As such, our results reveal a new route toward emergent electronic orders via cooperative interactions between the lattice and electronic degrees of freedom.