arXiv subjects

Chenhaoping Wen

Publications and source records attributed to Chenhaoping Wen.

13 recordsLinked to original sources

arXiv2025

Direct evidence of intrinsic Mott state and its layer-parity oscillation in a breathing kagome crystal down to monolayer

We report direct spectroscopic evidence of correlation-driven Mott states in layered Nb$_3$Cl$_8$ through combining scanning tunneling microscopy (STM) and dynamical mean-field theory. The Hubbard bands persist down to monolayer, providing the definitive evidence for the Mottness in Nb$_3$Cl$_8$. While the size of the Mott gap remains almost constant across all layers, a striking layer-parity-dependent oscillation emerges in the local density of states (LDOS) between even (n = 2,4,6) and odd layers (n = 1,3,5), which arises from the dimerization and correlation modulation of the obstructed atomic states, respectively. Our conclusions are supported by a critical technical advance in atomic-scale LDOS mapping for highly insulating systems. This work provides the definitive experimental verification of correlation-driven Mott ground states in Nb3Cl8 while establishing a general protocol for investigating the interplay of electronic correlation and interlayer coupling in layered insulators by using low-temperature STM technique.

cond-mat.str-el↗

arXiv2025

Superconductivity in trilayer nickelate La4Ni3O10 under pressure

Nickelate superconductors have attracted a great deal of attention over the past few decades due to their similar crystal and electronic structures with high-temperature cuprate superconductors. Here, we report the superconductivity in a pressurized Ruddlesden-Popper phase single crystal, La4Ni3O10 (n = 3), and its interplay with the density wave order in the phase diagram. With increasing pressure, the density wave order as indicated by the anomaly in the resistivity is progressively suppressed, followed by the emergence of the superconductivity around 25 K under I4/mmm space group. The susceptibility measurements confirm bulk superconductivity with a volume fraction exceeding 80%. Moreover, theoretical analysis unveils that antiferromagnetic (AFM) super-exchange interactions can serve as the effective pairing interaction for the emergence of superconductivity (SC) in pressurized La4Ni3O10. Our research provides a new platform for the investigation of the unconventional superconductivity mechanism in Ruddlesden-Popper trilayer perovskite nickelates.

cond-mat.supr-con↗

arXiv2024

Roles of the Narrow Electronic Band near the Fermi Level in 1T-TaS$_2$-Related Layered Materials

Here we use low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) to reveal the roles of the narrow electronic band in two 1T-TaS$_2$-related materials (bulk 1T-TaS$_2$ and 4Hb-TaS$_2$). 4Hb-TaS$_2$ is a superconducting compound with alternating 1T-TaS$_2$ and 1H-TaS$_2$ layers, where the 1H-TaS$_2$ layer has weak charge density wave (CDW) pattern and reduces the CDW coupling between the adjacent 1T-TaS$_2$ layers. In the 1T-TaS$_2$ layer of 4Hb-TaS$_2$, we observe a narrow electronic band located near Fermi level, and its spatial distribution is consistent with the tight-binding calculations for two-dimensional 1T-TaS$_2$ layers. The weak electronic hybridization between the 1T-TaS$_2$ and 1H-TaS$_2$ layers in 4Hb-TaS$_2$ shifts the narrow electronic band to be slightly above the Fermi level, which suppresses the electronic correlation induced band splitting. In contrast, in bulk 1T-TaS$_2$, there is an interlayer CDW coupling induced insulating gap. In comparison with the spatial distributions of the electronic states in bulk 1T-TaS$_2$ and 4Hb-TaS$_2$, the insulating gap in bulk 1T-TaS$_2$ results from the formation of a bonding band and an antibonding band due to the overlap of the narrow electronic bands in the dimerized 1T-TaS$_2$ layers.

cond-mat.mes-hall↗

arXiv2024

Spatially Dependent in-Gap States Induced by Andreev Tunneling through a Single Electronic State

By using low-temperature scanning tunneling microscopy and spectroscopy (STM/STS), we observe in-gap states induced by Andreev tunneling through a single impurity state in a low carrier density superconductor (NaAlSi). The energy-symmetric in-gap states appear when the impurity state is located within the superconducting gap. In-gap states can cross the Fermi level, and they show X-shaped spatial variation. We interpret the in-gap states as a consequence of the Andreev tunneling through the impurity state, which involves the formation or breakup of a Cooper pair. Due to the low carrier density in NaAlSi, the in-gap state is tunable by controlling the STM tip-sample distance. Under strong external magnetic fields, the impurity state shows Zeeman splitting when it is located near the Fermi level. Our findings not only demonstrate the Andreev tunneling involving single electronic state, but also provide new insights for understanding the spatially-dependent in-gap states in low carrier density superconductors.

cond-mat.supr-con↗

arXiv2023

Real-space Observation of Unidirectional Charge Density Wave and Complex Structural Modulation in the Pnictide Superconductor Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$

Here we use low-temperature and variable-temperature scanning tunneling microscopy to study the pnictide superconductor, Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$. In the low-temperature phase (triclinic phase) of BaNi$_2$As$_2$, we observe the unidirectional charge density wave (CDW) with $Q$ = 1/3 on both the Ba and NiAs surfaces. On the NiAs surface of the triclinic BaNi$_2$As$_2$, there are structural-modulation-induced chain-like superstructures with distinct periodicities. In the high-temperature phase (tetragonal phase) of BaNi$_2$As$_2$, the NiAs surface appears as the periodic 1 by 2 superstructure. Interestingly, in the triclinic phase of Ba$_{0.5}$Sr$_{0.5}$Ni$_2$As$_2$, the unidirectional CDW is suppressed on both the Ba/Sr and NiAs surfaces, and the Sr substitution stabilizes the periodic 1 by 2 superstructure on the NiAs surface, which enhance the superconductivity in Ba$_{0.5}$Sr$_{0.5}$Ni$_2$As$_2$. Our results provide important microscopic insights for the interplay among the unidirectional CDW, structural modulation, and superconductivity in this class of pnictide superconductors.

cond-mat.mes-hall↗

arXiv2023

Probing hidden Mott gap and incommensurate charge modulation on the polar surfaces of PdCrO$_2$

Here we report a combined study of low-temperature scanning tunneling microscopy (STM) and dynamical mean-field theory (DMFT) on PdCrO$_2$, a delafossite metal with an antiferromagnetic order below ~37.5 K. First, on the CrO$_2$-terminated polar surface we detect a gap-like feature both below and above the Néel temperature. The DMFT calculations indicate that this gap is opened due to the strong correlations of Cr-3d electrons, suggesting the hidden Mott nature of the gap. Then, we observe two kinds of Pd-terminated polar surfaces. One is a well-ordered Pd surface with the Fermi-surface-nesting-induced incommensurate charge modulation, while the other one is a reconstructed Pd surface with the individual nano-scale non-periodic domain structures. On the well-ordered Pd surface, the interference between the incommensurate charge modulation and the atomic lattice forms the periodic moiré pattern. Our results provide important microscopic information for fully understanding the correlated electronic properties of this class of materials.

cond-mat.str-el↗

arXiv2022

Fully two-dimensional incommensurate charge modulation on the Pd-terminated polar surface of PdCoO2

Here we use low-temperature scanning tunneling microscopy and spectroscopy to study the polar surfaces of PdCoO2. On the CoO2-terminated polar surface, we detect the quasiparticle interference pattern originating from the Rashba spin-split surface states. On the well-ordered Pd-terminated polar surface, we observe a regular lattice which has larger lattice constant than the atomic lattice of PdCoO2. In comparison with the shape of the hexagonal bulk Fermi surface, we identify this regular lattice as a fully two-dimensional incommensurate charge modulation that is driven by the Fermi surface nesting. More interestingly, we also find the moiré pattern induced by the interference between the two-dimensional incommensurate charge modulation in the Pd layer and the underlying CoO2 lattice. Our results not only show a new charge modulation on the Pd surface of PdCoO2, but also pave the way for fully understanding the novel electronic properties of this material.

cond-mat.str-el↗

arXiv2022

Coexistence of Quasi-two-dimensional Superconductivity and Tunable Kondo Lattice in a van der Waals Superconductor

Realization of Kondo lattice in superconducting van der Waals materials not only provides a unique opportunity for tuning the Kondo lattice behavior by electrical gating or intercalation, but also is helpful for further understanding the heavy fermion superconductivity. Here we report a low-temperature and vector-magnetic-field scanning tunneling microscopy and spectroscopy study on a superconducting compound (4Hb-TaS2) with alternate stacking of 1T-TaS2 and 1H-TaS2 layers. We observe the quasi-two-dimensional superconductivity in the 1H-TaS2 layer with anisotropic response to the in-plane and out-of-plane magnetic fields. In the 1T-TaS2 layer, we detect the Kondo resonance peak that results from the Kondo screening of the unpaired electrons in the Star-of-David clusters. We also find the intensity of the Kondo resonance peak is sensitive to its relative position with the Fermi level, and it can be significantly enhanced when it's further shifted towards the Fermi level by evaporating Pb atoms onto the 1T-TaS2 surface. Our results are not only important for fully understanding the electronic properties of 4Hb-TaS2, but also pave the way for creating tunable Kondo lattice in the superconducting van der Waals materials.

cond-mat.supr-con↗

arXiv2022

Inducing and tuning Kondo screening in a narrow-electronic-band system

Although the single-impurity Kondo physics has already been well understood, the understanding of the Kondo lattice problem where a dense array of local moments couples to the conduction electrons is still far from complete. The ability of creating and tuning the Kondo lattice in non-f-electron systems will be great helpful for further understanding the Kondo lattice behavior. Here we show that the Pb intercalation in the charge-density-wave-driven narrow-electronic-band system 1T-TaS2 induces a transition from the insulating gap to a sharp Kondo resonance in the scanning tunneling microscopy measurements. It results from the Kondo screening of the localized moment in the 13-site Star-of-David clusters of 1T-TaS2, and thus confirms the cluster Mott localization of the unpaired electrons and local moment formation in the 1T-TaS2 layer. As increasing the Pb concentration, the narrow electronic band derived from the localized electrons shifts away from the Fermi level and the Kondo resonance peak is gradually suppressed. Our results pave a way for creating and tuning many-body electronic states in layered narrow-electronic-band materials.

cond-mat.str-el↗

arXiv2021

Charge density wave orders and enhanced superconductivity under pressure in the kagome metal CsV3Sb5

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.

cond-mat.supr-con↗

arXiv2020

Orbital-selective Dirac fermions and extremely flat bands in frustrated kagome-lattice metal CoSn

Layered kagome-lattice 3d transition metals are emerging as an exciting platform to explore the frustrated lattice geometry and quantum topology. However, the typical kagome electronic bands, characterized by sets of the Dirac-like band capped by a phase-destructive flat band, have not been clearly observed, and their orbital physics are even less well investigated. Here, we present close-to-textbook kagome bands with orbital differentiation physics in CoSn, which can be well described by a minimal tight-binding model with single-orbital hopping in Co kagome lattice. The capping flat bands with bandwidth less than 0.2 eV run through the whole Brillouin zone, especially the bandwidth of the flat band of out-of-plane orbitals is less than 0.02 eV along G-M. The energy gap induced by spin-orbit interaction at the Dirac cone of out-of-plane orbitals is much smaller than that of in-plane orbitals, suggesting orbital-selective character of the Dirac fermions.

cond-mat.mtrl-sci↗

arXiv2020

Impurity-pinned incommensurate charge density wave and local phonon excitations in 2H-NbS2

Here we report a scanning tunneling microscopy (STM) and spectroscopy (STS) study in the superconducting state of 2H-NbS2. We directly visualize the existence of incommensurate charge density wave (CDW) that is pinned by atomic impurities. In strong tunneling conditions, the incommensurate CDW is de-pinned from impurities by the electric field from STM tip. We perform STM-based inelastic tunneling spectroscopy (IETS) to detect phonon excitations in 2H-NbS2 and measure the influence of atomic impurities on local phonon excitations. In comparison with the calculated vibrational density of states in 2H-NbS2, we find two branches of phonon excitations which correspond to the vibrations of Nb ions and S ions, and the strength of the local phonon excitations is insensitive to the atomic impurities. Our results demonstrate the coexistence of incommensurate CDW and superconductivity in 2H-NbS2, and open the way of detecting atomic-scale phonon excitations in transition metal dichalcogenides with STM-based IETS.

cond-mat.mes-hall↗

arXiv2019

Magnetic and Electronic Properties of Single-Crystalline BaCoSO

Doped BaCoSO was recently predicted to be a high-temperature superconductor in a new class based on Co and Ni. Using a Co-S self flux method, we synthesized single crystals of the antiferromagnetic insulator BaCoSO. Our magnetic and specific heat measurements and neutron diffraction provide details of its magnetic anisotropy and order. Its band gap was determined to be about 1.3 eV by our measurements of its photoemission spectrum and infrared optical conductivity. Our results can pave the way to exploring the predicted superconductivity in this Co-based material.

cond-mat.supr-con↗