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Register a new userUltrafast and reversible control of the exchange interaction in Mott insulators
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The strongest interaction between microscopic spins in magnetic materials is the exchange interaction $J_text{ex}$. Therefore, ultrafast control of $J_text{ex}$ holds the promise to control spins on ultimately fast timescales. We demonstrate that time-periodic modulation of the electronic structure by electric fields can be used to reversibly control $J_text{ex}$ on ultrafast timescales in extended antiferromagnetic Mott insulators. In the regime of weak driving strength, we find that $J_text{ex}$ can be enhanced and reduced for frequencies below and above the Mott gap, respectively. Moreover, for strong driving strength, even the sign of $J_text{ex}$ can be reversed and we show that this causes time reversal of the associated quantum spin dynamics. These results suggest wide applications, not only to control magnetism in condensed matter systems, for example, via the excitation of spin resonances, but also to assess fundamental questions concerning the reversibility of the quantum many-body dynamics in cold atom systems.
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We investigate how fast and how effective photocarrier excitation can modify the exchange interaction $J_mathrm{ex}$ in the prototype Mott-Hubbard insulator. We demonstrate an ultrafast quenching of $J_mathrm{ex}$ both by evaluating exchange integrals from a time-dependent response formalism and by explicitly simulating laser-induced spin precession in an antiferromagnet that is canted by an external magnetic field. In both cases, the electron dynamics is obtained from nonequilibrium dynamical mean-field theory. We find that the modified $J_mathrm{ex}$ emerges already within a few electron hopping times after the pulse, with a reduction that is comparable to the effect of chemical doping.
In recent years, the optical control of exchange interactions has emerged as an exciting new direction in the study of the ultrafast optical control of magnetic order. Here we review recent theoretical works on antiferromagnetic systems, devoted to i) simulating the ultrafast control of exchange interactions, ii) modeling the strongly nonequilibrium response of the magnetic order and iii) the relation with relevant experimental works developed in parallel. In addition to the excitation of spin precession, we discuss examples of rapid cooling and the control of ultrafast coherent longitudinal spin dynamics in response to femtosecond optically induced perturbations of exchange interactions. These elucidate the potential for exploiting the control of exchange interactions to find new scenarios for both faster and more energy-efficient manipulation of magnetism.
In order to have a better understanding of ultrafast electrical control of exchange interactions in multi-orbital systems, we study a two-orbital Hubbard model at half filling under the action of a time-periodic electric field. Using suitable projection operators and a generalized time-dependent canonical transformation, we derive an effective Hamiltonian which describes two different regimes. First, for a wide range of non-resonant frequencies, we find a change of the bilinear Heisenberg exchange $J_{textrm{ex}}$ that is analogous to the single-orbital case. Moreover we demonstrate that also the additional biquadratic exchange interaction $B_{textrm{ex}}$ can be enhanced, reduced and even change sign depending on the electric field. Second, for special driving frequencies, we demonstrate a novel spin-charge coupling phenomenon enabling coherent transfer between spin and charge degrees of freedom of doubly ionized states. These results are confirmed by an exact time-evolution of the full two-orbital Mott-Hubbard Hamiltonian.
Most available theories for correlated electron transport are based on the Hubbard Hamiltonian. In this effective theory, renormalized hopping and interaction parameters only implicitly incorporate the coupling of correlated charge carriers to microscopic degrees of freedom. Unfortunately, no spectroscopy can individually probe such renormalizations, limiting the applicability of Hubbard models. We show here that the role of each individual degree of freedom can be made explicit by using a new experimental technique, which we term quantum modulation spectroscopy and we demonstrate here in the one-dimensional Mott insulator ET-F2TCNQ. We explore the role on the charge hopping of two localized molecular modes, which we drive with a mid infrared optical pulse. Sidebands appear in the modulated optical spectrum, and their linshape is fitted with a model based on the dynamic Hubbard Hamiltonian. A striking asymmetry between the renormalization of doublons and holons is revealed. The concept of quantum modulation spectroscopy can be used to systematically deconstruct Hubbard Hamiltonians in many materials, exposing the role of any mode, electronic or magnetic, that can be driven to large amplitude with a light field.
We observe and explain theoretically a dramatic evolution of the Dzyaloshinskii-Moriya interaction in the series of isostructural weak ferromagnets, MnCO$_3$, FeBO$_3$, CoCO$_3$ and NiCO$_3$. The sign of the interaction is encoded in the phase of x-ray magnetic diffraction amplitude, observed through interference with resonant quadrupole scattering. We find very good quantitative agreement with first-principles electronic structure calculations, reproducing both sign and magnitude through the series, and propose a simplified `toy model to explain the change in sign with 3 d shell filling. The model gives a clue for qualitative understanding of the evolution of the DMI in Mott and charge transfer insulators.
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