Session J06: Ultrafast Dynamics and Control of Quantum Materials

Terahertz through X-ray Control and Measurement of Quantum Material Dynamics

This is for an APS-DLS FOCUS SESSION. The focus session is "Ultrafast Dynamics and Control of Quantum Materials"

This invited talk is to be given by Prof. Keith Nelson of MIT, who is a leader in the field of dynamics in complex materials.   

Magnetic symmetry breaking driven by photoinduced piezomagnetism

Symmetries govern the macroscopic behavior of solids and dictate how their properties can be controlled by external fields. Resonantly driving optical phonons offers the possibility to coherently manipulate symmetry and induce new functional properties away from equilibrium. Here, we demonstrate magnetic symmetry breaking by light leading to an induced ferrimagnetic phase in the classical antiferromagnet CoF₂. We utilize high-intensity terahertz pulses to simultaneously excite degenerate in-plane phonon modes and observe the resulting magnetization dynamics by optical Faraday rotation and circular dichroism. We find that the excitation generates a net c-axis magnetization on the picosecond time scale, evidencing an ultrafast magnetic phase transition. First-principles calculations show that the effect is driven by a novel photoinduced piezomagnetic effect, in which anharmonic phonon dynamics uncompensate the equilibrium antiferromagnetic order via a site-selective modulation of the crystal field. This rare example of photoinduced symmetry breaking provides a new mechanism for magnetic control, which could also be used for manipulating spins at heterointerfaces and engineering magnetoelectric phenomena by light.
  

Distinguishing Intrinsic from Extrinsic Effects in Time-resolved Photoemission

Time- and angle-resolved photoemission is becoming established as a powerful tool for the study of nonequilibrium electron dynamics in quantum materials. With the proliferation of this technique it is important to develop a robust understanding of the method itself. I will discuss how electron dynamics that are intrinsic to the sample can be distinguished from generic electron dynamics that are extrinsic. This includes apparent binding energy shifts due to surface photovoltage, and shifts and broadening due to pump-induced vacuum space charge.   

Tuning time and energy resolutions in time- and angle-resolved photoemission spectroscopy

Time- and angle-resolved photoemission is a powerful probe of nonequilibrium band structures. Time and energy resolution are two important parameters which determine the range of possible measurements. We demonstrate tunable time and energy resolutions by adjusting the thickness of nonlinear BaB₂O₄ crystals commonly used to generate 6 eV pulses from a 1.5 eV fundamental. We tune the time resolution between 58 and 103 fs and obtain corresponding energy resolutions of 55 to 27 meV, maintaining a time-bandwidth product under 150% of the Fourier limit. Calculations clarify the interactions between laser bandwidth and nonlinear crystal thickness which contribute to determining experimental resolutions.   

Time-resolved ARPES study on Ta2NiSe5

Ta₂NiSe₅ has been long proposed as an excitonic insulator, in which remarkable band flattening below the structural transition temperature at 328 K, as a result of the strong Coulomb attractive interactions between the conduction and valence bands. Recently, photo-excitation has been intensively used to explore non-equilibrium phases. Our previous result also confirms that Ta₂NiSe₅ is an excitonic insulator from the characteristic dynamical behavior [1]. In this work, we have used time-resolved ARPES (TARPES) to investigate the photo-excited non-equilibrium phases in Ta₂NiSe₅. We have discovered that intense photo-excitation induce a transient semimetalic state, which could not be realized under equilibrium. In order to gain insight on the mechanism of this phase transition, we have pursued the TARPES measurements with better signal-to-noise ratio enabled by use of the higher repetition rate laser system. We have observed oscillating behaviors corresponding to the coherent phonons in the ARPES intensity maps, which allowed us to identify which phonon mode is relevant to the metallization.   

Light-driven ultrafast phonomagnetism

Over the past few decades, ultrashort pulses of light have been widely employed to control the behavior of matter in its different phases. This is a particularly interesting challenge in magnetism, where the speed, dissipation and routes for ultimately fast switching of the spin orientation often lead to proposals for novel approaches in information processing and data recording. ^[1]
In this work we control the magnetic state by resonantly pumping optical phonons, low-energy elementary vibrations of the crystalline lattice. We demonstrate that by exciting the crystal lattice of the prototypical antiferromagnetic DyFeO₃, it can be driven within picoseconds into a transient metastable magnetic state. The state is characterized by a change in the strength of magnetic anisotropy along different crystal axes. This is experimentally seen as a long-lived shift in the frequency of the spin precession driven along the corresponding axes. For sufficiently strong excitation, this promotes an instability of the initial magnetic structure and launches a spin reorientation transition within a few picoseconds.

[1] P. Němec et al., Nature Physics 14, 229 (2018).   

Absence of amplitude mode softening of intermediate states in the charge density wave material 1T-TaSe2

Recently, we uncovered new long-lived metastable charge density wave (CDW) states in 1T-TaSe₂ that are launched by an ultrafast laser pulse [1]. Moreover, the transitions from CDW to metastable to normal phases exhibit second order phase transition behaviors with continuously tunable order parameters, although it is a first order transition under thermal-equilibrium conditions. Ultrafast light-induced phase transitions thus extend the phase diagram of strongly-coupled materials, allowing us to access new states which are unreachable by chemical doping or by varying the temperature. In order to further understand the nature of these new metastable intermediate states, a second weak pump pulse is applied to probe their properties. Surprisingly, the CDW amplitude mode frequency of all intermediate states (between the CDW and normal states) is the same as that of CDW state - and vanishes only when the material reaches the normal state. This abrupt change of phonon mode frequency, reveals an exotic interatomic potential that may be intrinsic to the first-order nature of the phase transition in equilibrium. Our results provide new insight in understanding the formation and nature of ultrafast laser- induced metastable states.
[1] Science Advances 5, eaav4449 (2019).   

Universal dynamics of order parameter fluctuations in pump-probe experiments

Upon excitation by a laser pulse, broken-symmetry phases of a wide variety of solids demonstrate similar order parameter dynamics characterized by a dramatic slowing down of relaxation for stronger pump fluences. Motivated by this recurrent phenomenology, we develop a simple non-perturbative effective model of dynamics of collective bosonic excitations in pump-probe experiments. We find that as the system recovers after photoexcitation, it shows universal prethermalized dynamics manifesting a power-law, as opposed to exponential, relaxation, explaining the slowing down of the recovery process. For strong quenches, long-wavelength over-populated transverse modes dominate the long-time dynamics; their distribution function exhibits universal scaling in time and space, whose universal exponents can be computed analytically. Our model offers a unifying description of order parameter fluctuations in a regime far from equilibrium, and our predictions can be tested with available time-resolved techniques.   

Ultrafast nonlinear excitation of collective modes in iron pnictides by intense terahertz pulses

Nonlinear terahertz (THz) spectroscopy has been a promising tool for the study of collective excitations in superconductors, as demonstrated by the observation of Higgs mode and Leggett mode in BCS superconductors with phonon-assisted pairing. The nonlinear nature of the excitation is essential as these modes arise from exotic symmetries and do not linearly couple to the electromagnetic field. Unconventional superconductors with strong inter-band pairing channels and multiple spin/charge fluctuations are expected to host new and rich collective excitations which have not yet been explored. Here we report the observation of collective modes in iron pnictides excited by intense THz pulses. The modes diminishes at both high temperatures and field strengths when there is strong quenching of the superconducting coherence. Our results clearly show out of equilibrium driving of multiband superconducting systems can induce nonlinear couplings between collective excitations of order parameters, leading to the formation of novel phase.   

Experimental observation of phonon- and band-specific electron-phonon coupling in a topological semi-metal

Light excitation of quantum materials can drive phase transitions such as insulator-to-metal transitions or the melting of magnetic orders. It can also lead to ultrafast topological phase transitions, as it was demonstrated recently in the topological semi-metal WTe₂ using ultrafast electron diffraction [1]. The photoinduced transition in this system occurs when a shear phonon mode is driven sufficiently strongly to recover the inversion symmetry of the lattice. In this context we studied the electron dynamics of WTe₂ using time- and angle-resolved photoemission spectroscopy. We report the observation of band oscillations at four different frequencies that are due to A_1g coherent phonon modes and include the shear mode causing the photoinduced topological transition. By isolating the effect of each phonon on the different electronic bands, we reveal the complexity of electron-phonon coupling in the electronic band structure.

[1] E. J. Sie, et al. Nature 565, 61 (2019).   

Light-induced coherent modulation of the electrons and lattice in a charge density wave material

Ultrashort light pulses can selectively excite charges, spins and phonons in materials, providing a powerful approach for manipulating their properties. In this work, we use time- and angle-resolved photoemission spectroscopy to show that a femtosecond laser pulse can coherently modulate the electrons and lattice in the charge density wave (CDW) material 1T-TaSe₂ [1, 2]. These intertwined electron-phonon dynamics are launched by displacive excitation of the CDW amplitude mode, and provide unique opportunities to capture mode-specific electron-phonon couplings, interatomic potentials, and hidden phases that are inaccessible using equilibrium excitation. Surprisingly, we observe a coherent modulation of the electron temperature/occupation at the amplitude mode frequency, that is synchronized to the modulation of the Ta 5d band (CDW order). Then, as we increase the laser fluence to drive the material into a metastable state mediated by mode-selective electron-phonon coupling, this oscillation exhibits a phase change of p, indicating a competition between different interactions. This approach can be extended to other complex materials, to steer strongly-coupled quantum materials towards a desired state using light.
[1] Science Advances 5, eaav4449 (2019).
[2] arXiv:1906.09545.   

Towards a Many Body Theory for Fourier Transform Inelastic X-Ray Scattering Experiments in Correlated Materials

In recent years new ultrafast x-ray sources have enabled the probing of materials at previously inaccessible time and length scales, allowing for new insights into phonon populations and dynamics far from equilibrium through direct time domain measurements. As these experiments improve, classical descriptions of phonons, particularly in correlated materials, will be pushed to their limits. With this in mind we seek to develop a fully quantum mechanical, many-body theory for ultrafast pump-probe scattering experiments that accounts for electron-phonon coupling and electronic correlations that can inform and inspire experiments. A new theory could help us gain better insight into exotic nonequilibrium states in correlated materials, superconductivity, as well as lead to a better understanding of charge and heat transport under normal operating conditions in current and new devices.   

Ultrafast Nonlinear Optical Spectroscopy of the Charge Density Wave in 1T-TaS2

Nonlinear optics has recently emerged as an appealing symmetry-sensitive probe for revealing and investigating exotic quantum phases of matter, in addition to its more traditional application in determining crystalline structures. Here, we report using temperature-dependent and time-resolved nonlinear optical techniques to study the 1T polytype of TaS₂, an intensively-studied prototypical system hosting the charge density wave (CDW) order. We show that an enhancement of broken mirror symmetry is identified across the nearly commensurate to commensurate CDW transition by rotation anisotropy second harmonic generation (RA SHG). We then report the ultrafast responses of commensurate CDW to optical excitations, using both time-resolved reflectivity and time-resolved RA SHG. The excitation fluence dependence of the commensurate CDW phase will further be discussed.