Session B43: New Development in Understanding and Controlling Excited States in Quantum Materials

Novel Excited States in 2D van der Waals Structures and Moiré Superlattices

Recent experiments revealed spectroscopic signatures of novel excited states and intriguing pump-probe responses in 2D van der Waals structures. The nature of many of these phenomena remains to be fully understood. Here, we present results on the photophysics of these systems based on an ab initio interacting Green's function approach. We show that there is a rich diversity of excitons in transition metal dichalcogenide (TMD) moiré superlattices, including unforeseen novel intralayer charge-transfer moiré excitons. In pump-probe calculations, we discovered a self-driven exciton-Floquet effect in the time-resolved ARPES of 2D materials, wherein prominent satellite bands and renormalization of the quasiparticle bands are induced by excitons, analogously to the optical Floquet effect driven by photons. We demonstrated a new exciton mechanism (direct coupling of intralayer with interlayer excitons) in the ultrafast optical response of TMD heterobilayers. Moreover, we showed that strong excitonic physics in 2D materials can greatly enhances their nonlinear optical responses (e.g., shift currents and SHG). This has led to the discovery of a striking phenomenon of formation of light-induced shift current vortex crystals in TMD moiré systems – i.e., 2D periodic arrays of moiré-scale current vortices and associated magnetic fields with remarkable intensity under laboratory laser setup. Our studies are made possible with the development of two new methods that allow for the ab initio calculations of excitonic physics and photo responses of systems with thousands of atoms in the unit cell and in the time domain.   

Division of Laser Science Invited Symposium: New Development in Understanding and Controlling Excited States in Quantum MaterialsKeshav DaniExcitons in Momentum Space

 

Optical techniques have provided us with rich information about the exciton – a two-particle photoexcited state in semiconductors and insulators. Yet, they have left a fundamental degree of freedom of the exciton inaccessible – it’s momentum!

In this talk, I will discuss the application of time-resolved photoemission techniques to access the momentum coordinate of excitons in 2D semiconductors, thereby providing us with the formation pathways of momentum-forbidden dark excitons [1], an image of the electron around the hole in the exciton [2], the observation of the long-predicted anomalous dispersion of the exciton-bound electron [2], the momentum distribution of the exciton-bound hole [3], the confinement of the interlayer exciton in a moiré cell [3], the impact of excitons in the dense limit [4], and the dynamics of the spin-dark excitons [5].

References ( ^*equal authors)

[1] J. Madeo^*, M. K. L. Man^*, et al. Science 370, 1199 (2020).

[2] M. K. L. Man^*, J. Madeo^*, et al. Science Advances 7, eabg0192 (2021).

[3] O Karni^*, E. Barr^*, V. Pareek^*, J. D. Georgaras^*, M. K. L. Man^*, C. Sahoo^*, et al. Nature 603, 247 (2022).

[4] V. Pareek^*, D. Bacon^*, X. Zhu^*, et al. in prep

[5] D. Bacon^*, X. Zhu^*, V. Pareek^*, et al, in prep   

Controlling quantum materials with time-periodic drive

Time-periodic light-field can dress the electronic states of quantum materials, providing a fascinating controlling knob for transient modifications of the electronic structure with light-induced emergent phenomena [1]. In this talk, I will present our recent experimental progress on the Floquet engineering of two-dimensional materials using time- and angle-resolved photoemission spectroscopy (TrARPES). Using black phosphorus as an example, I will present the light-induced manipulation of the transient electronic structure in two different pumping regimes: near-resonance pumping and far below-gap pumping. A modification of the valence band from parabolic to a Mexican-hat like dispersion is observed upon near-resonance pumping with the band gap, and intriguing light polarization dependence is observed, reflecting the intriguing pseudospin selectivity in the Floquet engineering [2]. By further pushing the pump beam to even lower photon energy [3], we found that transient manipulation of the electronic structure can also be achieved by far below-gap pumping, yet with distinctive behavior near the top of the valence band. Based on what we have learned from these works, insights for extending Floquet engineering to more materials will also be discussed.

 

[1] Changhua Bao et al., “Light-induced emergent phenomena in two-dimensional materials and topological materials”, Nat. Rev. Phys. 4, 33 (2022)

[2] Shaohua Zhou⁺, Changhua Bao⁺ et al., “Pseudospin-selective Floquet band engineering in black phosphorus”, Nature 614, 75 (2023)

[3] Shaohua Zhou⁺, Changhua Bao⁺, et al., “Floquet engineering of black phosphorus by below-gap pumping”, Phys. Rev. Lett. 131, 116401 (2023)   

Terahertz field induced metastable magnetization in a van der Waals antiferromagnet

Controlling the functional properties of quantum materials with light has emerged as a frontier of condensed matter physics, leading to discoveries of various light-induced phases of matter, such as superconductivity, ferroelectricity, magnetism, and charge density waves. However, in most cases, the photoinduced phases return to equilibrium on ultrafast timescales after the light is turned off, limiting their practical applications. In this study, we use intense terahertz pulses to induce a metastable magnetization with a remarkably long lifetime of over 2.5 milliseconds in a van der Waals antiferromagnet, FePS₃. The metastable state becomes increasingly robust as the temperature approaches the transition point, suggesting a significant role played by critical fluctuations in facilitating extended lifetimes. By combining first principles calculations with classical Monte Carlo and spin dynamics simulations, we find that the displacement of a specific phonon mode modulates the exchange couplings in a manner that favors a ground state with finite magnetization close to the Néel temperature. This analysis also clarifies how critical fluctuations amplify the magnitude and the lifetime of the new magnetic state. Our discovery demonstrates the efficient manipulation of the magnetic ground state in layered magnets through non-thermal pathways using terahertz light, and establishes the regions near critical points with enhanced fluctuations as promising areas to search for metastable hidden quantum states.   

Lightwave electronics in quantum materials – from Floquet band engineering to attoclocking

The concept of ‘lightwave electronics’ has pushed quantum control of condensed matter to unprecedented time scales: By harness­ing the carrier wave of intense light pulses as an alternating voltage, electrons can be driven faster than a cycle of light, unlocking a fascinating coherent quantum world. In the unique environ­­ment of the topological surface state on bulk Bi₂Te₃, terahertz light fields can accelerate electrons like relativistic particles to cover large distan­ces without scattering and heating. This motion leads to a new quality of non-integer high-harmonic generation, whose polarization reveals topologically non-trivial electron trajectories. By advancing angle-resolved photoelectron spectroscopy (ARPES) to subcycle time scales, we can now even visualize the lightwave-driven acceleration of Dirac electrons, the transient formation of Floquet-Bloch states and the non-perturbative interplay of inter- and intraband dynamics in actual subcycle band-structure movies. Our results shed new light on the process of high-harmonic generation and open novel possibilities for ultrafast band-structure engineering.

Also electron–hole pairs in atomically thin quantum materials can be accelerated and collided by strong lightwaves, giving rise to high-order sideband generation. By clocking electron-hole recollisions with attosecond precision, we can proceed beyond the single-particle picture and directly capture how many-body correlations affect the motion of Bloch electrons. Strong Coulomb correlations in atomically thin WSe₂ are found to shift the optimal timing of recollisions by up to 1.2 fs compared to the bulk material. Attosecond chronoscopy of delocalized electrons could become a powerful tool in exploring unexpected phase transitions and emergent many-body quantum-dynamic phenomena.