Session LL01: V: Light Induced Structural Control of Electronic Phases

Chiral phonon inverse Faraday and Barnett effects

The Einstein-de Haas effect conventionally describes the coupling of magnetization and mechanical rotation due to angular momentum conservation. On ultrashort timescales, such as during the sudden demagnetization of a material following the excitation with a laser pulse, the ultrafast Einstein-de Haas effect describes the coupling of spin and orbital angular momentum to the angular momentum of circularly polarized (chiral) phonons and strain waves. Here, we showcase the inverse mechanism: chiral phonon modes, driven by an ultrashort terahertz pulse, can induce a magnetization in both nonmagnetic and magnetic materials. We use a combination of phenomenological modeling and density functional theory calculations to simulate the coherent excitation of chiral phonons, which generate real and effective magnetic fields within the material, which subsequently produces a magnetization. We show that magnetizations of up to several Bohr magneton can possibly be induced. This mechanism can be seen as a phonon Barnett effect, the inverse of the Einstein-de Haas effect. At the same time, it can be considered a phonon analog of the inverse Faraday effect, in which circularly polarized (chiral) light induces a magnetization in materials. Phonon inverse Faraday/Barnett effects provide a new avenue to control the magnetic order of materials on ultrafast timescales.   

Nonlinear phononic rectification of magnons

Ultrashort electromagnetic pulses are able to control the electronic and structural properties of solids on timescales of pico- and femtoseconds. Pulses in the terahertz and mid-infrared regime enable resonant excitations of optical phonons, by coupling the electric field component of light to the electric dipole moment of IR-active phonon modes. When the vibrational amplitudes become large enough, the excited phonons couple nonlinearly to other collective excitations through ionic Raman scattering. A particular feature of ionic Raman scattering is nonlinear phononic rectification, where the coherently excited IR-active phonons act as a unidirectional force that transiently displaces the atoms along the eigenvectors of a coupled Raman-active phonon mode. Here, we show that an analog process is possible for spin waves: Coherently excited chiral IR-active phonon modes act as a unidirectional effective magnetic field that transiently displaces the spins along the eigenvectors of a coupled magnon. We use a phenomenological model based on Landau-Lifshitz-Gilbert equations and nonlinearly driven oscillators with input from first-principles calculations to describe the response of the coupled spin-lattice dynamics to the excitation with an ultrashort terahertz pulse. We show that the spins can be transiently distorted in an analog way to Raman-active phonons, which provides a novel route towards ultrafast control of magnetism.

    

Nonlinear phonon dynamics in the layered antiferromagnetic FePS3

The coherent excitation of lattice vibrations is a powerful method to manipulate the properties of quantum materials. Nonlinear phonon couplings can be used for directional control of transient structural distortions, also called nonlinear phononic rectification, where the direction of distortion can be controlled through the polarization of the incident light pulse. Here, we apply this principle to the two-dimensional layered antiferromagnet FePS3. We investigate the nonlinear lattice dynamics that follow the excitation with an ultrashort terahertz pulse and compare our results to topical experiments on phonon driving in this material. We use a combination of phenomenological models for the coherent phonon dynamics and first-principles calculations based on density functional theory for realistic input parameters to evaluate the possible nonlinear excitation pathways and structural distortions that can be induced. We in particular focus on excitations of the interlayer shear modes in FePS3 in order to determine whether control of the magnetic exchange interactions can be achieved through this mechanism.   

Witnessing nonequilibrium entanglement dynamics in a strongly correlated fermionic chain

Many-body entanglement has a profound impact on the macroscopic behavior of condensed matter quantum phases. In equilibrium, multipartite entanglement has been diagnosed from response functions through the use of entanglement witnesses together with operator-specific quantum bounds. Here, we investigate the applicability of this approach for detecting entangled states in quantum systems driven out of equilibrium. We use a multipartite entanglement witness, the quantum Fisher information, to study the dynamics of a paradigmatic fermionic chain undergoing a time-dependent ramp of the nearest-neighbor Coulomb interaction. We demonstrate that the quantum Fisher information is able to certify multipartite entanglement both near and far from equilibrium and is robust against decoherence. Our results bear implications for characterizing light-driven states without equilibrium analogues and identifying the role of quantum coherence in the absence of well-defined order parameters.   

Nonperturbative study of bulk photovoltaic effect enhanced by an optically induced phase transition

Solid systems with strong correlations and interactions under light illumination have the potential for exhibiting interesting bulk photovoltaic behavior in the nonperturbative regime, which has remained largely unexplored in past theoretical studies. We investigate the bulk photovoltaic response of a quantum material with strongly coupled electron-spin-lattice dynamics using real-time simulations performed with a tight-binding model. We demonstrate how a combination of spin- and phonon-induced processes can substantially enhance the bulk photovoltaic effect. The transient changes in the band structure and the photoinduced phase transitions, emerging from spin and phonon dynamics, result in a nonlinear current versus intensity behavior beyond the perturbative limit. The current rises sharply across a photoinduced magnetic phase transition, which later saturates at higher light intensities due to excited phonon and spin modes. We disentangle phonon-and spin-assisted components to the ballistic photocurrent, showing that they are comparable in magnitude. Our study shows that photoinduced phase transitions, which are generally ignored in perturbative theoretical methods, significantly impact photocurrent generation and its evolution. Moreover, understanding the effect of the transient spin and lattice dynamics on the dynamical nature of the band structure can be exploited for desirable photovoltaic properties by tuning the correlations and interactions in correlated systems through targeted materials design.   

Non-equilibrium electron crystals

We explore a new crystal-like non-equilibrium state of the Coulomb interacting electron gas. This state shares some similarities with the Faraday instability of classical liquids and can be created by parametrically exciting plasmons using a coherent Floquet drive with a slowly oscillating amplitude. We discuss the emergent collective modes that form as excitations on top of the time-periodic crystal-like state steady state. Possible applications include powerful plasmon sources and plasmonic time-reversal mirrors.   

Distinguishing and controlling Mottness by ultrafast light

The relative energy scales of the Coulomb repulsion U and the bandwidth W determine how important electron-electron correlation is in the material properties. In the case of strong correlation, a half-filled band becomes insulating due to the strong Coulomb repulsion for double electron occupancy, leading to Mott insulator. However, when these energy scales are similar, the effect of correlation or the extent of Mottness becomes elusive, and the material is particularly sensitive to external perturbations. Finding an effective pathway to distinguish and further control the extent of Mottness is therefore important. Here we report the ultrafast electronic dynamics of a correlated material by ultrafast time- and angle-resolved photoemission spectroscopy. Distinctive dynamics in different phases is revealed, thereby providing nonequilibrium dynamical signatures and further controlling of the Mottness.   

Mechanisms for Long-Lived, Photo-induced Superconductivity in Alkali-Doped Fullerides

Advances in the control of intense, far-infrared light has led to the striking discovery of optical signatures of long-lived, nonequilibrium superconductivity in K3C60 at 100K—five times above the equilibrium Tc in K3C60. Motivated by these experimental developments in photo-induced superconductivity, we investigate phononic mechanisms for long-lived and thermodynamically metastable photo-induced superconductivity far above Tc in a ab-initio inspired minimal model for alkali-dope fulleride superconductors. In particular, we find that optically driving specific, IR active Jahn-Teller modes in alkali-doped fullerides can induce superconductivity that lasts parametrically longer than the ring-down time of the driven phonons, providing a novel, microscopic framework for long-lived superconductivity. Furthermore, our work provides a natural explanation for the resonances in the efficiency of inducing superconducting signatures, recently discovered in K3C60.