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arXiv:2101.01189 · Physical review. B./Physical review. B

Ultrafast frustration breaking and magnetophononic driving of singlet excitations in a quantum magnet

F. Giorgianni, Björn Wehinger, Stephan Allenspach, Nicola Colonna, C. Vicario, Pascal Puphal, Ekaterina V. Pomjakushina, Bruce Normand, Christian Rüegg

Published May 25, 2023• 12 citations
arXivHTMLPDF

Abstract

Ultrafast laser probing has revolutionized the study of quantum materials. Quantum magnets realize some of the most complex and entangled many-body states in condensed matter. Here, the authors unify ultrafast physics and quantum magnetism by pump-probe experiments on the frustrated magnet SrCu${}_{2}$(BO${}_{3}$)${}_{2}$, using terahertz light pulses to create coherent driving of the lattice phonons, which in turn creates a special singlet spin excitation. Their phonon-based method introduces a universal mechanism for controlling nonequilibrium quantum many-body physics on ultrafast timescales.

DOI
10.1103/physrevb.107.184440

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Ultrafast laser probing has revolutionized the study of quantum materials. Quantum magnets realize some of the most complex and entangled many-body states in condensed matter. Here, the authors unify ultrafast physics and quantum magnetism by pump-probe experiments on the frustrated magnet SrCu2{}_{2}2​(BO3{}_{3}3​)2{}_{2}2​, using terahertz light pulses to create coherent driving of the lattice phonons, which in turn creates a special singlet spin excitation. Their phonon-based method introduces a universal mechanism for controlling nonequilibrium quantum many-body physics on ultrafast timescales.
Harnessing the most advanced capabilities of quantum technologies will require the ability to control macroscopic quantum states of matter. Quantum magnetic materials provide a valuable platform for realizing highly entangled many-body quantum systems, and have been used to investigate phenomena ranging from quantum phase transitions (QPTs) to fractionalization, topological order and the entanglement structure of the quantum wavefunction. Although multiple studies have controlled their properties by static applied pressures or magnetic fields, dynamical control at the fundamental timescales of their magnetic interactions remains completely unexplored. However, major progress in the technology of ultrafast laser pulses has enabled the dynamical modification of electronic properties, and now we demonstrate the ultrafast control of quantum magnetism. This we achieve by a magnetophononic mechanism, the driving of coherent lattice displacements to produce a resonant excitation of the quantum spin dynamics. Specifically, we apply intense terahertz laser pulses to excite a collective spin state of the quantum antiferromagnet SrCu2(BO3)2 by resonance with the nonlinear mixing frequency of the driven phonons that modulate the magnetic interactions. Our observations indicate a universal mechanism for controlling nonequilibrium quantum many-body physics on timescales many orders of magnitude faster than those achieved to date.
In the realm of open quantum systems, steady states and high-harmonic generation (HHG) existing far from equilibrium have become core pillars of ultrafast science. Most solid-state research explores charge HHG with limited investigations into spin degrees of freedom. In this study, we theoretically address spin HHG in the steady state resulting from the terahertz laser-driven spin-phonon coupling in a dissipative dimerized spin-1/2 chain. Instead of directly driving spins using time-dependent magnetic fields, we employ the magnetophononic mechanism, where the laser first drives the lattice, and then the excited lattice subsequently drives the spins. We investigate the role of various model parameters for optimizing HHG. Increasing the laser amplitude amplifies spin HHG beyond the perturbative regime, enhancing both harmonic amplitudes and orders. We find that configuring the drive frequency far below the spin band yields the highest harmonic order. Additionally, we provide a theory matching the numerical results under weak spin-phonon coupling and propose an experimental procedure to probe the emission spectrum of spin HHG.
Excited-state dynamics in condensed matter systems plays a crucial role in determining their properties. Understanding and controlling ultrafast dynamics across multiple dimensions—such as time, space, energy, momentum, and spin—has become a key emerging direction in modern condensed matter physics. In recent decades, time-resolved ultrafast laser technology based on pump-probe techniques has advanced significantly. This technology has been integrated with various experimental methods, leading to developments in ultrafast spectroscopy, time- and angle-resolved photoemission spectroscopy (TR-ARPES), ultrafast scanning tunneling microscopy (STM), ultrafast electron diffraction (UED), and ultrafast X-ray techniques, enabling the study of lattice, electron, and spin dynamics. Meanwhile, to provide a clearer physical picture of ultrafast dynamics, real-time first-principles methods have been developed by combining electronic structure methods with dynamic approaches, allowing the study of multidimensional lattice, electron, and spin behaviors. This article reviews recent progress in the field and discusses its future challenges and opportunities.
An exciting recent development is the ability to drive a quantum material into a highly excited, but still phase coherent state, using intense laser sources. Although such a state can be extremely short-lived, it can be long enough for detection using ultrafast spectroscopy. The authors here show how intense laser pulses can generate a new type of nonequilibrium superconducting phase. The authors demonstrate theoretically that the pump induces a ``twisted'' BCS state that subsequently evolves coherently after the cessation of the pulse. They show how the nonlinear coupling of the pump pulse light can induce coherent dynamics of $\mathrm{\ensuremath{\Delta}}(t)$, consistent with the experiment [R. Matsunaga et al. Phys. Rev. Lett. 111, 057002 (2013)]. Moreover, the authors show that more intense pump pulses can create a far-from-equilibrium phase of gapless superconductivity, originally predicted in the context of ultracold atomic gases. The terahertz pump is found to be much more efficient than the interaction quench at producing this gapless phase. These results open the door to engineering and observing new dynamical phases and phase transitions in quantum materials.
The advent of high-intensity ultrafast laser pulses has opened new opportunities for controlling and designing quantum materials. In particular, terahertz (THz) pulses can resonantly drive optical phonon modes, enabling dynamic manipulation of lattice degrees of freedom. In this work, we investigate the ultrafast quantum thermodynamics of optical phonon mode driven by a THz pulse by treating the phonon as an open quantum system coupled to a thermal environment within a Caldeira-Leggett-type framework. We derive the quantum heat current between the phonon and the bath and analyze its behavior under realistic pulse protocols. Our results demonstrate that ultrafast laser driving can reveal and even induce significant deviations from the commonly adopted Markovian approximation, thereby providing a pathway to probe and control non-Markovian dissipation in driven solid-state systems.
A quantum-mechanical many-particle system may exhibit nonlocal behavior in that measurements performed on one of the particles can affect a second one that is far apart. These so-called entangled states are crucial for the implementation of quantum information protocols and gates for quantum computation. Here, we use ultrafast optical pulses and coherent pump-probe techniques to create and control spin entangled states in an ensemble of up to three non-interacting electrons bound to donors in a ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}$Te quantum well. Our method, relying on the exchange interaction between optically excited excitons and the paramagnetic impurities, can in principle be applied to entangle an arbitrarily large number of electrons. A microscopic theory of impulsive stimulated Raman scattering and a model for multi-spin entanglement are presented. The signature of entanglement is the observation of overtones of donor spin-flips in the differential reflectivity of the probe pulse. Results are shown for resonant excitation of localized excitons below the gap, and above the gap where the signatures of entanglement are significantly enhanced. Data is also presented on the generation of coherent excitations of antiferromagnetically coupled manganese pairs, folded acoustic phonons, exciton Zeeman beats and entanglement involving two Mn+2{\mathrm{Mn}}^{+2}Mn+2 ions.