arXiv subjects

Shuyun Zhou

Publications and source records attributed to Shuyun Zhou.

At least 37 records · Page 2Linked to original sources

arXiv2024

Correlated topological flat bands in rhombohedral graphite

Flat bands and nontrivial topological physics are two important topics of condensed matter physics. With a unique stacking configuration analogous to the Su-Schrieffer-Heeger (SSH) model, rhombohedral graphite (RG) is a potential candidate for realizing both flat bands and nontrivial topological physics. Here we report experimental evidence of topological flat bands (TFBs) on the surface of bulk RG, which are topologically protected by bulk helical Dirac nodal lines via the bulk-boundary correspondence. Moreover, upon {\it in situ} electron doping, the surface TFBs show a splitting with exotic doping evolution, with an order-of-magnitude increase in the bandwidth of the lower split band, and pinning of the upper band near the Fermi level. These experimental observations together with Hartree-Fock calculations suggest that correlation effects are important in this system. Our results demonstrate RG as a new platform for investigating the rich interplay between nontrivial band topology, correlation effects, and interaction-driven symmetry-broken states.

cond-mat.str-el↗

arXiv2024

Reconsidering the nonlinear emergent inductance: time-varying Joule heating and its impact on the AC electrical response

A nonlinearly enhanced electrical reactance, $\Im Z$, under a large AC current has been measured to explore emergent inductors, which constitute a new class of inductors based on the spin-transfer torque effect. A nonlinear $\Im Z$ has been observed in conducting magnets that contain noncollinear spin textures and interpreted as the realization of an inductance due to current-induced spin dynamics. However, curious behavior has concomitantly been observed. For instance, the nonlinear $\Im Z$ always has a cutoff frequency of $10^0$--$10^4$ Hz, which is much lower than the resonance frequency of a ferromagnetic domain wall, $\sim$10$^7$ Hz; furthermore, the temperature and magnetic field variations in $\Im Z$ appear to be considerably correlated with those in the temperature derivative of resistance. This behavior appears to be difficult to understand in terms of the current-induced spin dynamics, and therefore, the earlier interpretation of the nonlinear $\Im Z$ should be further verified. Here, we theoretically and experimentally show that time-varying Joule heating and its impact on the AC electrical response can naturally explain these observations. In the experimental approach, we study the nonlinear AC electrical response of two conducting materials that exhibit no magnetic order, CuIr$_2$S$_4$ and 1$T$'-MoTe$_2$. Under time-varying Joule heating, a nonlinearly enhanced $\Im Z$ with the curious behavior mentioned above is observed in both systems. Our study implies that the nonlinear $\Im Z$ previously observed in noncollinear magnets includes a considerable contribution of the Joule-heating-induced apparent AC impedance.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast Carrier Relaxation Dynamics in a Nodal-Line Semimetal PtSn$_4$

Topological Dirac nodal-line semimetals host topologically nontrivial electronic structure with nodal-line crossings around the Fermi level, which could affect the photocarrier dynamics and lead to novel relaxation mechanisms. Herein, by using time- and angle-resolved photoemission spectroscopy, we reveal the previously-inaccessible linear dispersions of the bulk conduction bands above the Fermi level in a Dirac nodal-line semimetal PtSn$_4$, as well as the momentum and temporal evolution of the gapless nodal lines. A surprisingly ultrafast relaxation dynamics within a few hundred femtoseconds is revealed for photoexcited carriers in the nodal line. Theoretical calculations suggest that such ultrafast carrier relaxation is attributed to the multichannel scatterings among the complex metallic bands of PtSn$_4$ via electron-phonon coupling. In addition, a unique dynamic relaxation mechanism contributed by the highly anisotropic Dirac nodal-line electronic structure is also identified. Our work provides a comprehensive understanding of the ultrafast carrier dynamics in a Dirac nodal-line semimetal.

cond-mat.mes-hall↗

arXiv2024

Observation of dichotomic field-tunable electronic structure in twisted monolayer-bilayer graphene

Twisted bilayer graphene (tBLG) provides a fascinating platform for engineering flat bands and inducing correlated phenomena. By designing the stacking architecture of graphene layers, twisted multilayer graphene can exhibit different symmetries with rich tunability. For example, in twisted monolayer-bilayer graphene (tMBG) which breaks the C2z symmetry, transport measurements reveal an asymmetric phase diagram under an out-of-plane electric field, exhibiting correlated insulating state and ferromagnetic state respectively when reversing the field direction. Revealing how the electronic structure evolves with electric field is critical for providing a better understanding of such asymmetric field-tunable properties. Here we report the experimental observation of field-tunable dichotomic electronic structure of tMBG by nanospot angle-resolved photoemission spectroscopy (NanoARPES) with operando gating. Interestingly, selective enhancement of the relative spectral weight contributions from monolayer and bilayer graphene is observed when switching the polarity of the bias voltage. Combining experimental results with theoretical calculations, the origin of such field-tunable electronic structure, resembling either tBLG or twisted double-bilayer graphene (tDBG), is attributed to the selectively enhanced contribution from different stacking graphene layers with a strong electron-hole asymmetry. Our work provides electronic structure insights for understanding the rich field-tunable physics of tMBG.

cond-mat.mes-hall↗

arXiv2024

Hidden charge density wave induced shadow bands and ultrafast dynamics of CuTe investigated using time-resolved ARPES

Revealing the fine electronic structure is critical for understanding the underlying physics of low-dimensional materials. Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental technique for mapping out the experimental electronic structure. By reducing the photon energy (e.g. to 6 eV) using laser sources, a greatly improved momentum resolution can be achieved, thereby providing opportunities for ``zooming in'' the fine electronic structure and even revealing the previously unresolvable bands near the Brillouin zone center. Here, by using quasi-one-dimensional material CuTe as an example, we demonstrate the unique capability of laser-based ARPES in revealing the fine electronic structures of ``hidden'' charge density wave induced shadow bands near the Brillouin zone center, which are previously unresolvable using synchrotron sources. The observation of the shadow bands reveals the CDW phase from the aspect of band folding, and the unpredicted CDW band hybridization strongly modifies the electronic structure and Fermi surface, which suggests that such hybridization must be taken into account for studying the CDW transition. Moreover, the ultrafast non-equilibrium carrier dynamics are captured by time-resolved ARPES, revealing the relaxation dynamics through electron-phonon scattering. Our work demonstrates the advantages of laser-based ARPES in zooming in the fine electronic structures, as well as capturing the ultrafast dynamics of low-dimensional materials.

cond-mat.mes-hall↗

arXiv2024

Evolution of flat band and role of lattice relaxations in twisted bilayer graphene

Magic-angle twisted bilayer graphene (MATBG) exhibits correlated phenomena such as superconductivity and Mott insulating state related to the weakly dispersing flat band near the Fermi energy. Beyond its moiré period, such flat band is expected to be sensitive to lattice relaxations. Thus, clarifying the evolution of the electronic structure with twist angle is critical for understanding the physics of MATBG. Here, we combine nanospot angle-resolved photoemission spectroscopy and atomic force microscopy to resolve the fine electronic structure of the flat band and remote bands, and their evolution with twist angles from 1.07$^\circ$ to 2.60$^\circ$. Near the magic angle, dispersion is characterized by a flat band near the Fermi energy with a strongly reduced bandwidth. Moreover, near 1.07$^\circ$, we observe a spectral weight transfer between remote bands at higher binding energy and extract the modulated interlayer spacing near the magic angle. Our work provides direct spectroscopic information on flat band physics and highlights the role of lattice relaxations.

cond-mat.mes-hall↗

arXiv2023

Floquet engineering of black phosphorus upon below-gap pumping

Time-periodic light field can dress the electronic states and lead to light-induced emergent properties in quantum materials. While below-gap pumping is regarded favorable for Floquet engineering, so far direct experimental evidence of momentum-resolved band renormalization still remains missing. Here, we report experimental evidence of light-induced band renormalization in black phosphorus by pumping at photon energy of 160 meV which is far below the band gap, and the distinction between below-gap pumping and near-resonance pumping is revealed. Our work demonstrates light-induced band engineering upon below-gap pumping, and provides insights for extending Floquet engineering to more quantum materials.

cond-mat.mes-hall↗

arXiv2023

Ionic liquid gating induced self-intercalation of transition metal chalcogenides

Ionic liquids provide versatile pathways for controlling the structures and properties of quantum materials. Previous studies have reported electrostatic gating of nanometre-thick flakes leading to emergent superconductivity, insertion or extraction of protons and oxygen ions in perovskite oxide films enabling the control of different phases and material properties, and intercalation of large-sized organic cations into layered crystals giving access to tailored superconductivity. Here, we report an ionic-liquid gating method to form three-dimensional transition metal monochalcogenides (TMMCs) by driving the metals dissolved from layered transition metal dichalcogenides (TMDCs) into the van der Waals gap. We demonstrate the successful self-intercalation of PdTe$_2$ and NiTe$_2$, turning them into high-quality PdTe and NiTe single crystals, respectively. Moreover, the monochalcogenides exhibit distinctive properties from dichalcogenides. For instance, the self-intercalation of PdTe$_2$ leads to the emergence of superconductivity in PdTe. Our work provides a synthesis pathway for TMMCs by means of ionic liquid gating driven self-intercalation.

cond-mat.mtrl-sci↗

arXiv2023

Revealing the two-dimensional electronic structure and anisotropic superconductivity in a natural van der Waals superlattice (PbSe)$_{1.14}$NbSe$_2$

Van der Waals superlattices are important for tailoring the electronic structures and properties of layered materials. Here we report the superconducting properties and electronic structure of a natural van der Waals superlattice (PbSe)$_{1.14}$NbSe$_2$. Anisotropic superconductivity with a transition temperature $T_c$ = 5.6 $\pm$ 0.1 K, which is higher than monolayer NbSe$_2$, is revealed by transport measurements on high-quality samples. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal the two-dimensional electronic structure and a charge transfer of 0.43 electrons per NbSe$_2$ unit cell from the blocking PbSe layer. In addition, polarization-dependent ARPES measurements reveal a significant circular dichroism with opposite contrast at K and K' valleys, suggesting a significant spin-orbital coupling and distinct orbital angular momentum. Our work suggests natural van der Waals superlattice as an effective pathway for achieving intriguing properties distinct from both the bulk and monolayer samples.

cond-mat.supr-con↗

arXiv2023

Distinguishing and controlling Mottness in 1T-TaS$_2$ by ultrafast light

Distinguishing and controlling the extent of Mottness is important for materials where the energy scales of the onsite Coulomb repulsion U and the bandwidth W are comparable. Here we report the ultrafast electronic dynamics of 1T-TaS$_2$ by ultrafast time- and angle-resolved photoemission spectroscopy. A comparison of the electron dynamics for the newly-discovered intermediate phase (I-phase) as well as the low-temperature commensurate charge density wave (C-CDW) phase shows distinctive dynamics. While the I-phase is characterized by an instantaneous response and nearly time-resolution-limited fast relaxation (~200 fs), the C-CDW phase shows a delayed response and a slower relaxation (a few ps). Such distinctive dynamics refect the different relaxation mechanisms and provide nonequilibrium signatures to distinguish the Mott insulating I-phase from the C-CDW band insulating phase. Moreover, a light-induced bandwidth reduction is observed in the C-CDW phase, pushing it toward the Mott insulating phase. Our work demonstrates the power of ultrafast light-matter interaction in both distinguishing and controlling the extent of Mottness on the ultrafast timescale.

cond-mat.mes-hall↗

arXiv2023

Pseudospin-selective Floquet band engineering in black phosphorus

Time-periodic light field has emerged as a control knob for manipulating quantum states in solid-state materials, cold atoms and photonic systems via hybridization with photon-dressed Floquet states in the strong coupling limit, dubbed as Floquet engineering. Such interaction leads to tailored properties of quantum materials, for example, modifications of the topological properties of Dirac materials and modulation of the optical response. Despite extensive research interests over the past decade, there is no experimental evidence of momentum-resolved Floquet band engineering of semiconductors, which is a crucial step to extend Floquet engineering to a wide range of solid-state materials. Here, based on time- and angle-resolved photoemission spectroscopy measurements, we report experimental signatures of Floquet band engineering in a model semiconductor - black phosphorus. Upon near-resonance pumping at photon energy of 340 to 440 meV, a strong band renormalization is observed near the band edges. In particular, light-induced dynamical gap opening is resolved at the resonance points, which emerges simultaneously with the Floquet sidebands. Moreover, the band renormalization shows a strong selection rule favoring pump polarization along the armchair direction, suggesting pseudospin selectivity for the Floquet band engineering as enforced by the lattice symmetry. Our work demonstrates pseudospin-selective Floquet band engineering in black phosphorus, and provides important guiding principles for Floquet engineering of semiconductors.

cond-mat.mtrl-sci↗

arXiv2023

Light-induced emergent phenomena in 2D materials and topological materials

Light-matter interaction in 2D and topological materials provides a fascinating control knob for inducing emergent, non-equilibrium properties and achieving new functionalities in the ultrafast time scale (from fs to ps). Over the past decade, intriguing light-induced phenomena, e.g., Bloch-Floquet states and photo-induced phase transitions, have been reported experimentally, but many still await experimental realization. In this Review, we discuss recent progress on the light-induced phenomena, in which the light field could act as a time-periodic field to drive Floquet states, induce structural and topological phase transitions in quantum materials, couple with spin and various pseudospins, and induce nonlinear optical responses that are affected by the geometric phase. Perspectives on the opportunities of proposed light-induced phenomena as well as open experimental challenges are also discussed.

cond-mat.mes-hall↗

arXiv2023

A newly-designed femtosecond KBe$_2$BO$_3$F$_2$ device with pulse duration down to 55 fs for time- and angle-resolved photoemission spectroscopy

Developing a widely tunable vacuum ultraviolet (VUV) source with sub-100 femtoseconds (fs) pulse duration is critical for ultrafast pump-probe techniques such as time- and angle-resolved photoemission spectroscopy (TrARPES). While a tunable probe source with photon energy of 5.3 - 7.0 eV has been recently implemented for TrARPES by using a KBe$_2$BO$_3$F$_2$ (KBBF) device, the time resolution of 280 - 320 fs is still not ideal, which is mainly limited by the duration of the VUV probe pulse generated by the KBBF device. Here, by designing a new KBBF device which is specially optimized for fs applications, an optimum pulse duration of 55 fs is obtained after systematic diagnostics and optimization. More importantly, a high time resolution of 81 - 95 fs is achieved for TrARPES measurements covering the probe photon energy range of 5.3 - 7.0 eV, making it particularly useful for investigating the ultrafast dynamics of quantum materials. Our work extends the application of KBBF device to ultrafast pump-probe techniques with the advantages of both widely tunable VUV source and ultimate time resolution.

cond-mat.mes-hall↗

arXiv2022

Angle-resolved photoemission spectroscopy

For solid-state materials, the electronic structure, E(k), is critical in determining a crystal's physical properties. By experimentally detecting the electronic structure, the fundamental physics can be revealed. Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique for directly observing the electronic structure with energy- and momentum-resolved information. Over the past decades, major improvements in the energy and momentum resolution, alongside the extension of ARPES observables to spin (SpinARPES), micrometer or nanometer lateral dimensions (MicroARPES/NanoARPES), and femtosecond timescales (TrARPES), have led to major scientific advances. These advantages have been achieved across a wide range of quantum materials, such as high-temperature superconductors, topological materials, two-dimensional materials and heterostructures. This primer introduces key aspects of ARPES principles, instrumentation, data analysis, and representative scientific cases to demonstrate the power of the method. Perspectives and challenges on future developments are also discussed.

cond-mat.mtrl-sci↗

arXiv2022

Coexistence of extended flat band and Kekulé order in Li-intercalated graphene

Doping graphene near the 1/4 filling to shift the extended flat band and van Hove singularity below E$_F$ has been highly desirable. Here we report the experimental observation of an extended flat band below E$_F$ in Li-intercalated graphene. Strong electron-phonon interaction is clearly identified by notable kinks in the band dispersion. Moreover, the evolution of the band structure upon Li intercalation shows that the extended flat band and the Kekulé order emerge simultaneously. Our work provides opportunities for investigating flat band related instabilities and its interplay with the Kekulé order

cond-mat.mes-hall↗

arXiv2022

Ultrafast photothermoelectric effect in Dirac semimetallic Cd3As2 revealed by terahertz emission

The thermoelectric effects of topological semimetals have attracted tremendous research interest because many topological semimetals are excellent thermoelectric materials and thermoelectricity serves as one of their most important potential applications. In this work, we reveal the transient photothermoelectric response of Dirac semimetallic Cd3As2, namely the photo-Seebeck effect and photo-Nernst effect, by studying the terahertz (THz) emission from the transient photocurrent induced by these effects. Our excitation polarization and power dependence confirm that the observed THz emission is due to photothermoelectric effect instead of other nonlinear optical effect. Furthermore, when a weak magnetic field (~0.4 T) is applied, the response clearly indicates an order of magnitude enhancement on transient photothermoelectric current generation compared to the photo-Seebeck effect. Such enhancement supports an ambipolar transport nature of the photo-Nernst current generation in Cd3As2. These results highlight the enhancement of thermoelectric performance can be achieved in topological Dirac semimetals based on the Nernst effect, and our transient studies pave the way for thermoelectric devices applicable for high field circumstance when nonequilibrium state matters. The large THz emission due to highly efficient photothermoelectric conversion is comparable to conventional semiconductors through optical rectification and photo-Dember effect.

cond-mat.mtrl-sci↗

arXiv2021

Population inversion and Dirac fermion cooling in 3D Dirac semimetal Cd$_3$As$_2$

Revealing the ultrafast dynamics of three-dimensional (3D) Dirac fermions upon photoexcitation is critical for both fundamental science and device applications. So far, how the cooling of 3D Dirac fermions differs from that of two-dimensional (2D) Dirac fermions and whether there is population inversion are fundamental questions that remain to be answered. Here we reveal the ultrafast dynamics of Dirac fermions in a model 3D Dirac semimetal Cd$_3$As$_2$ by ultrafast time- and angle-resolved photoemission spectroscopy (TrARPES) with a tunable probe photon energy from 5.3 - 6.9 eV. The energy- and momentum-resolved relaxation rate shows a linear dependence on the energy, suggesting Dirac fermion cooling through intraband relaxation. Moreover, a population inversion is reported based on the observation of accumulated photoexcited carriers in the conduction band with a lifetime of $τ_n$ = 3.0 ps. Our work provides direct experimental evidence for a long-lived population inversion in a 3D Dirac semimetal, which is in contrast to 2D graphene where the interband relaxation occurs on a much faster timescale.

cond-mat.mes-hall↗

arXiv2021

Ultrafast time- and angle-resolved photoemission spectroscopy with widely tunable probe photon energy of 5.3-7.0 eV for investigating dynamics of three-dimensional materials

Time- and angle-resolved photoemission spectroscopy (TrARPES) is a powerful technique for capturing the ultrafast dynamics of charge carriers and revealing photo-induced phase transitions in quantum materials. However, the lack of widely tunable probe photon energy, which is critical for accessing the dispersions at different out-of-plane momentum $k_z$ in TrARPES measurements, has hindered the ultrafast dynamics investigation of 3D quantum materials such as Dirac or Weyl semimetals. Here we report the development of a TrARPES system with a highly tunable probe photon energy from 5.3 to 7.0 eV. The tunable probe photon energy is generated by the fourth harmonic generation of a tunable wavelength femtosecond laser source by combining a $β$-BaB$_2$O$_4$ (BBO) crystal and a KBe$_2$BO$_3$F$_2$ (KBBF) crystal. High energy resolution of 29 - 48 meV and time resolution of 280 - 320 fs are demonstrated on 3D topological materials ZrTe$_5$ and Sb$_2$Te$_3$. Our work opens up new opportunities for exploring ultrafast dynamics in 3D quantum materials.

cond-mat.mes-hall↗