arXiv subjects

Yigui Zhong

Publications and source records attributed to Yigui Zhong.

18 recordsLinked to original sources

arXiv2026

Topological pair density waves in kagome superconductors

The pair density wave (PDW) is an unconventional superconducting state exhibiting periodic pairing modulations due to pairing with finite momentum Q. In two dimensions, multiple Q components of the PDW can have a non-trivial relative phase, breaking time-reversal symmetry and resulting in a topological electronic structure. Here we review progress on exploring such topological PDWs (TPDWs) and discuss their potential realization in kagome superconductors. We first introduce the concept of a TPDW starting from the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state and examine the challenges toward its realization. We then discuss the possibility of a TPDW in the kagome lattice, which intertwines with chiral superconductivity and loop currents, thus connecting to models describing the quantum anomalous Hall effect. Furthermore, we review the experimental signatures of a TPDW in AV3Sb5 (A = Cs, Rb, K) superconductors and highlight related quantum effects, including switchable chiral pairing modulations, Bogoliubov Fermi states, the superconducting diode effect, and the anomalous thermal Hall effect. Finally, we project the future research opportunities of this correlated topological quantum phase and discuss its broad implications for finite-momentum pairing, topological matter, and chiral superconductivity.

cond-mat.supr-con↗

arXiv2025

Observation of Fermi-surface-dependent anisotropic Cooper pairing in kagome superconductor CsV3Sb5

In the recently discovered kagome superconductor AV3Sb5 (A = K, Rb, and Cs), superconductivity is intertwined with an unconventional charge density wave order. The pairing symmetry remains elusive owing to the lack of direct measurement of the superconducting gap in the momentum space. Here, utilizing laser-based ultra-high-resolution and low-temperature angle-resolved photoemission spectroscopy, we observe Fermi-surface-dependent anisotropic Cooper pairing in kagome superconductor CsV3Sb5. We detect a highly anisotropic superconducting gap structure with anisotropy exceeding 80% and a gap maximum along the V-V bond direction on a Fermi surface originating from the 3d-orbital electrons of the V kagome lattice. This is in stark contrast to the isotropic superconducting gap structure on the Fermi surface occupied by Sb 5p-orbital electrons. Our direct observation of the Fermi-surface-dependent anisotropic pairing in CsV3Sb5 is fundamental for understanding the intertwined orders in the ground state of kagome superconductors.

cond-mat.supr-con↗

arXiv2025

Direct observation of the surface superconducting gap in the topological superconductor candidate β-PdBi2

β-PdBi2 is one of the candidates for topological superconductors with a superconducting (SC) transition temperature (Tc) of 5.3 K, in which parity mixing of spin singlet and spin triplet has been anticipated, being crucial for the further understanding of relationship with inversion symmetry and parity mixing in the superconductivity. In this work, we measured the SC gap in high-quality single crystal of β-PdBi2 by using high-resolution laser angle-resolved photoemission spectroscopy below Tc. We found the isotropic SC gaps in momentum space for multiple bands, and observed that the difference between the SC gap of the topological surface bands and the bulk bands is about 0.1 meV, consistent with other experimental results. These direct and quantitative experimental results support the possibility of β-PdBi2 as a topological superconductor, characterized by unique crystal and electronic band structures.

cond-mat.supr-con↗

arXiv2025

Distinct amplitude mode dynamics upon resonant and off-resonant excitation across the charge density wave energy gap in LaTe3 investigated by time- and angle-resolved photoemission spectroscopy

Non-equilibrium states generated by ultrafast laser pulses are characterized by specific phenomena that are not accessible in static measurements. Previous time- and angle-resolved photoemission spectroscopy (TARPES) studies on rare-earth tritelluride materials have revealed the laser-driven melting of the charge density wave order as well as its collective amplitude mode excitation. Variation of the excess energy deposited by optical pumping in the material promises pathways to control the dynamic material response. To this end, we use an optical parametric amplifier to generate a tunable pump photon energy. Studying LaTe3 we compare the dynamics driven by pumping resonantly across the charge density wave energy gap with the effect of pumping at a twice higher photon energy in a TARPES pump-probe experiment. We clearly identify a pump photon energy dependent behavior. At the larger pump photon energy, the excess electronic energy generates lattice heating mediated by e-ph coupling and softening of the amplitude mode frequency from 3 to 2 THz. Remarkably, the resonant pumping across the CDW gap results in a time-independent amplitude mode frequency. We conclude that the resonant excitation across the energy gap excites the amplitude mode selectively while additional electronic excess energy deposited at higher pump photon energy modifies the crystal properties transiently by incoherent dissipative processes.

cond-mat.str-el↗

arXiv2025

Phase-sensitive evidence for pair density waves in a kagome superconductor

Pair density wave (PDW) exhibits periodic amplitude and sign modulations of the superconducting order parameter. Such a pairing state has long been proposed to be highly sensitive to nonmagnetic scattering, but its experimental realization remains elusive. Here we discover a nonmagnetic PDW-breaking effect in a kagome superconductor, using designer atomic nonmagnetic impurities and high-precision scanning tunneling microscopy (STM) at a base temperature of 30mK. We detect 2x2 pair density modulations by Josephson STM with a superconducting tip and 2x2 pairing gap modulations by normal STM. We find that the pairing modulations in both cases are substantially suppressed upon doping the kagome lattice with dilute isovalent nonmagnetic impurities, whereas the charge order and uniform superconductivity remain robust. We further identify the correlation between atomic dopants and the local suppression of PDW. We attribute these findings to a nonmagnetic pair-breaking effect, arising from the phase modulation of PDW in the kagome d-orbital. Taken together with its signatures in other state-of-the-art spectroscopy and transport measurements linked by theory, our findings support the ground state of the kagome superconductor as a correlated topological phase with superconducting loop currents.

cond-mat.supr-con↗

arXiv2024

Photoemission Insights to Electronic Orders in Kagome Superconductor AV3Sb5

Kagome superconductors AV3Sb5 (A = K, Rb, and Cs) have attracted considerable attention due to their intriguing combination of unique electron correlations and nontrivial band topology. The interplay of these fundamental aspects gives rise to a diverse array of exotic electronic phenomena, including superconductivity and charge density wave (CDW) states. In this review, we present recent advancements in the study of the electronic band structure of AV3Sb5 using angle-resolved photoemission spectroscopy (ARPES), including the identification of the multiple van Hove singularities near the Fermi level and their close relationship with the CDW transition, spectroscopic features related to CDW-induced symmetry breakings, as well as direct observations of nodeless superconducting gaps and moderate electron-phonon couplings through ultrahigh-resolution ARPES, providing critical insights into the origins of CDW order and electron pairing symmetry. By synthesizing these key ARPES findings, this review aims to deepen our understanding of kagome-related physics.

cond-mat.supr-con↗

arXiv2024

Unveiling van Hove singularity modulation and fluctuated charge order in kagome superconductor $\rm{CsV_3Sb_5}$ via time-resolved ARPES

Kagome superconductor CsV3Sb5, which exhibits intertwined unconventional charge density wave (CDW) and superconductivity, has garnered significant attention recently. Despite extensive static studies, the nature of these exotic electronic orders remains elusive. In this study, we investigate the non-equilibrium electronic structure of CsV3Sb5 via time- and angle-resolved photoemission spectroscopy. Our results reveal that upon laser excitation, the van Hove singularities immediately shift towards the Fermi level and subsequently oscillate in sync with a 1.3 THz coherent phonon mode. By analyzing the coherent intensity oscillations in the energy-momentum (E-k) map, we find that this coherent phonon is strongly coupled with electronic bands from both Sb and V orbitals. While typically observable only in the CDW state, remarkably, we find that the 1.3-THz coherent phonon mode can be persistently excited at temperatures above T_CDW, suggesting the potential existence of fluctuated CDW in CsV3Sb5. These findings enhance our understanding of the unconventional CDW control of kagome superconductivity.

cond-mat.mtrl-sci↗

arXiv2024

Time- and angle-resolved photoemission spectroscopy with wavelength-tunable pump and extreme ultraviolet probe enabled by twin synchronized amplifiers

We describe a setup for time- and angle-resolved photoemission spectroscopy with wavelength-tunable excitation and extreme ultraviolet probe. It is enabled by using the 10 kHz twin Ti:sapphire amplifiers seeded by the common Ti:sapphire oscillator. The typical probe energy is 21.7 eV, and the wavelength of the pump excitation is tuned between 2400 and 1200 nm by using the optical parametric amplifier. The total energy resolution of 133 meV is achieved, and the time resolution is dependent on the wavelength for the pump, typically better than 100 fs. This system enables the pump energy to be matched with a specific interband transition and to probe a wider energy-momentum space. We present the results for the prototypical materials of highly oriented pyrolytic graphite and Bi2Se3 to show the performance of our system.

cond-mat.mtrl-sci↗

arXiv2023

Testing Electron-phonon Coupling for the Superconductivity in Kagome Metal $\rm{CsV_3Sb_5}$

In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome metal $\rm{CsV_3Sb_5}$, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength,$λ$, supporting an unconventional pairing mechanism in $\rm{CsV_3Sb_5}$. However, experimental determination of $λ$ is still missing, hindering a microscopic understanding of the intertwined ground state of $\rm{CsV_3Sb_5}$. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate $λ$=0.45~0.6 at T=6 K for both Sb 5p and V 3d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in $\rm{CsV_3Sb_5}$. Remarkably, the EPC on the V 3d-band enhances to $λ$~0.75 as the superconducting transition temperature elevated to 4.4 K in $\rm{Cs(V_{0.93}Nb_{0.07})_3Sb_5}$. Our results provide an important clue to understand the pairing mechanism in the Kagome superconductor $\rm{CsV_3Sb_5}$.

cond-mat.supr-con↗

arXiv2023

Nodeless electron pairing in CsV$_3$Sb$_5$-derived kagome superconductors

The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order, and lattice geometry. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive. In particular, consensus on the electron pairing symmetry has not been achieved so far, in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic, and orbital-independent superconducting gap in the momentum space of two exemplary CsV$_3$Sb$_5$-derived kagome superconductors -- Cs(V$_{0.93}$Nb$_{0.07}$)$_3$Sb$_5$ and Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$, using ultrahigh resolution and low-temperature angle-resolved photoemission spectroscopy (ARPES). Remarkably, such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Moreover, we observe a signature of the time-reversal symmetry (TRS) breaking inside the superconducting state, which extends the previous observation of TRS-breaking CDW in the kagome lattice. Our comprehensive characterizations of the superconducting state provide indispensable information on the electron pairing of kagome superconductors, and advance our understanding of unconventional superconductivity and intertwined electronic orders.

cond-mat.supr-con↗

arXiv2023

Coexistence of bulk-nodal and surface-nodeless Cooper pairings in a superconducting Dirac semimetal

The interplay of nontrivial topology and superconductivity in condensed matter physics gives rise to exotic phenomena. However, materials are extremely rare where it is possible to explore the full details of the superconducting pairing. Here, we investigate the momentum dependence of the superconducting gap distribution in a novel Dirac material PdTe. Using high resolution, low temperature photoemission spectroscopy, we establish it as a spin-orbit coupled Dirac semimetal with the topological Fermi arc crossing the Fermi level on the (010) surface. This spin-textured surface state exhibits a fully gapped superconducting Cooper pairing structure below Tc~4.5K. Moreover, we find a node in the bulk near the Brillouin zone boundary, away from the topological Fermi arc.These observations not only demonstrate the band resolved electronic correlation between topological Fermi arc states and the way it induces Cooper pairing in PdTe, but also provide a rare case where surface and bulk states host a coexistence of nodeless and nodal gap structures enforced by spin-orbit coupling.

cond-mat.supr-con↗

arXiv2022

Photo-induced nonlinear band shift and valence transition in SmS

The photo-induced band structure variation of a rare-earth-based semiconductor, samarium monosulfide (SmS), was investigated using high-harmonic-generation laser-based time-resolved photoelectron spectroscopy. A nonlinear photo-induced band shift of the Sm 4f multiplets was observed. The first one is a shift to the high-binding-energy side due to a large surface photovoltage (SPV) effect of approximately 93 meV, comparable to the size of the bulk band gap, with a much longer relaxation time than 0.1 ms. The second one is an ultrafast band shift to the low binding energy side, which is in the opposite direction to the SPV shift, suggesting an ultrafast valence transition from divalent to trivalent Sm ions due to photo-excitation. The latter energy shift was approximately 58 meV, which is consistent with the energy gap shift from ambient pressure to the boundary between the black insulator and golden metallic phase with the application of pressure. This suggests that the photo-induced valence transition can reach the phase boundary, but other effects are necessary to realize the golden metallic phase.

cond-mat.mtrl-sci↗

arXiv2022

Tunable vortex Majorana modes controlled by strain in homogeneous LiFeAs

The iron-based superconductors (FeSCs) have recently emerged as a promising single-material Majorana platform by hosting isolated Majorana zero modes (MZMs) at relatively high temperatures. To further verify its Majorana nature and move forward to build topological quantum qubit, it is highly desirable to achieve tunability for MZMs on homogeneous FeSCs. Here, with an in-situ strain device, we can controllably create MZMs on the homogeneous surface of stoichiometric superconductor LiFeAs by inducing a topological phase transition. The evolution of discrete energy modes inside a strained vortex is found to mimics exactly as the predicted topological vortex case, proving the Majorana nature of emerging zero modes of vortex. Such tunability of MZMs in a homogeneous superconductor is an important step toward their application in topological quantum computation.

cond-mat.supr-con↗

arXiv2022

Evolution of the strange-metal scattering in momentum space of electron-doped ${\rm La}_{2-x}{\rm Ce}_x{\rm CuO}_4$

The linear-in-temperature resistivity is one of the important mysteries in the strange metal state of high-temperature cuprate superconductors. To uncover this anomalous property, the energy-momentum-dependent imaginary part of the self-energy Im ${\rm Σ}(k, ω)$ holds the key information. Here we perform systematic doping, momentum, and temperature-dependent angle-resolved photoemission spectroscopy measurements of electron-doped cuprate ${\rm La}_{2-x}{\rm Ce}_x{\rm CuO}_4$ and extract the evolution of the strange metal scattering in momentum space. At low doping levels and low temperatures, Im ${\rmΣ} \propto ω$ dependence dominates the whole momentum space. For high doping levels and high temperatures, Im ${\rmΣ} \propto ω^2$ shows up, starting from the antinodal region. By comparing with the hole-doped cuprates ${\rm La}_{2-x}{\rm Sr}_x{\rm CuO}_4$ and ${\rm Bi}_2{\rm Sr}_2{\rm CaCu}_2{\rm O}_8$, we find a dichotomy of the scattering rate exists along the nodal and antinodal direction, which is ubiquitous in the cuprate family. Our work provides new insight into the strange metal state in cuprates.

cond-mat.supr-con↗

arXiv2020

Creation of a novel inverted charge density wave state

Charge density wave (CDW) order is an emergent quantum phase that is characterized by a periodic lattice distortion and charge density modulation, often present near superconducting transitions. Here we uncover a novel inverted CDW state by using a femtosecond laser to coherently over-drive the unique star-of-David lattice distortion in 1T-TaSe$_2$. We track the signature of this novel CDW state using time- and angle-resolved photoemission spectroscopy and time-dependent density functional theory, and validate that it is associated with a unique lattice and charge arrangement never before realized. The dynamic electronic structure further reveals its novel properties, that are characterized by an increased density of states near the Fermi level, high metallicity, and altered electron-phonon couplings. Our results demonstrate how ultrafast lasers can be used to create unique states in materials, by manipulating charge-lattice orders and couplings.

cond-mat.mtrl-sci↗

arXiv2019

Phenomenological Single-Particle Green's Function for the Pseudogap and Superconducting Phases of High-T$_c$ Cuprates

We present a phenomenological Green's function to characterize the superconducting and pseudogap phases of the cuprates based on a microscopic theory of doped Mott insulators. In this framework, the "Fermi arc" and "kink" phenomena observed by angle-resolved photoemission spectroscopy (ARPES) experiments in the pseudogap phase can be systematically explained as a function of doping, which is further connected to the two-gap feature in the superconducting phase with dichotomy between the nodal and antinodal physics. We demonstrate that a phase-string-induced fractionalization plays the key role in giving rise to such a peculiar Green's function with a unique two-component structure.

cond-mat.str-el↗

arXiv2019

Anomalous doping evolution of nodal dispersion revealed by in-situ ARPES on continuously doped cuprates

We study the systematic doping evolution of nodal dispersions by in-situ angle-resolved photoemission spectroscopy on the continuously doped surface of a high-temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$. We reveal that the nodal dispersion has three segments separated by two kinks, located at ~10 meV and roughly 70 meV, respectively. The three segments have different band velocities and different doping dependence. In particular, the velocity of the high-energy segment increases monotonically as the doping level decreases and can even surpass the bare band velocity. We propose that electron fractionalization is a possible cause for this anomalous nodal dispersion and may even play a key role in the understanding of exotic properties of cuprates.

cond-mat.supr-con↗

arXiv2019

Coherent modulation of the electron temperature and electron-phonon couplings in a 2D material

Ultrashort light pulses can selectively excite charges, spins and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge density wave (CDW) material 1T-TaSe$_2$. After exciting the material with a short light pulse, spatial smearing of the electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This indicates a bi-directional energy exchange between the electrons and the strongly-coupled phonons. By tuning the laser excitation fluence, we can control the magnitude of the electron temperature modulation, from ~ 200 K in the case of weak excitation, to ~ 1000 K for strong laser excitation. This is accompanied by a switching of the dominant mechanism from anharmonic phonon-phonon coupling to coherent electron-phonon coupling, as manifested by a phase change of $π$ in the electron temperature modulation. Our approach thus opens up possibilities for coherently manipulating the interactions and properties of quasi-2D and other quantum materials using light.

cond-mat.mtrl-sci↗