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arXiv:1407.4761 · Nature Communications

Ultrafast and reversible control of the exchange interaction in Mott insulators

Johan H. Mentink, Karsten Balzer, Martin Eckstein

Published Mar 30, 2015• 241 citations
arXivHTMLPDF

Abstract

The strongest interaction between microscopic spins in magnetic materials is the exchange interaction Jex. Therefore, ultrafast control of Jex 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 Jex on ultrafast timescales in extended antiferromagnetic Mott insulators. In the regime of weak driving strength, we find that Jex can be enhanced and reduced for frequencies below and above the Mott gap, respectively. Moreover, for strong driving strength, even the sign of Jex 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.

DOI
10.1038/ncomms7708

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The strongest interaction between microscopic spins in magnetic materials is the exchange interaction Jex. Therefore, ultrafast control of Jex 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 Jex on ultrafast timescales in extended antiferromagnetic Mott insulators. In the regime of weak driving strength, we find that Jex can be enhanced and reduced for frequencies below and above the Mott gap, respectively. Moreover, for strong driving strength, even the sign of Jex 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.
The manipulation of electronic structure through periodic electric fields enables the reversible control of effective interactions in extended antiferromagnetic Mott insulators on ultrafast timescales. A careful analytical examination of the modulated effective interactions is conducted, accurately characterising it through the use of exact summation formulas and Bessel functions. As a result, time reversals are analytically determined in terms of Bessel zeroes. We discuss the half-filled Hubbard model, as well as multi-orbital models, various characteristics of the Kitaev-Heisenberg model, and the emergence of chiral spin terms.
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}} J ex that is analogous to the single-orbital case. Moreover we demonstrate that also the additional biquadratic exchange interaction B_{\textrm{ex}} B 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.
Quasi-one-dimensional systems exhibit many-body effects elusive in higher dimensions. A prime example is spin-orbital separation, which has been measured by resonant inelastic x-ray scattering (RIXS) in Sr2CuO3{\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}Sr2​CuO3​. Here, we theoretically analyze the time-resolved RIXS spectrum of Sr2CuO3{\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}Sr2​CuO3​ under the action of a time-dependent electric field. We show that the external field can reversibly modify the parameters in the effective $t\text{\ensuremath{-}}J$ model used to describe spinon and orbiton dynamics in the material. For strong driving amplitudes, we find that the spectrum changes qualitatively as a result of reversing the relative spinon to orbiton velocity. The analysis shows that in general, the spin-orbital dynamics in Mott insulators in combination with time-resolved RIXS should provide a suitable platform to explore the reversible control of many-body physics in the solid with strong laser fields.
We theoretically propose a route to control magnetic and topological orders in a broad class of insulating magnets with a DC electric field. We show from the strong-coupling expansion that magnetic exchange interactions along the electric field direction are generally enhanced in Mott insulators. Employing this DC field effect, we demonstrate that several magnetic or topological ordered phases such as quantum spin liquids and Haldane-gap states can be induced if we apply a strong enough DC electric field to typical frustrated or low-dimensional magnets. Our proposal is effective especially for magnets in the vicinity of magnetic phase transition points, and would also be applicable for magnets under low-frequency AC electric fields such as terahertz laser pulses. A similar strategy of controlling exchange interactions can also be utilized in cold atomic systems.
We investigate how fast and how effective photocarrier excitation can modify the exchange interaction J_{ex} in the prototype Mott-Hubbard insulator. We demonstrate an ultrafast quenching of J_{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_{ex} emerges already within a few electron hopping times after the pulse, with a reduction that is comparable to the effect of chemical doping.
Abstract Motivated by cold atom and ultra‐fast pump‐probe experiments we study the melting of long‐range antiferromagnetic order of a perfect Néel state in a periodically driven repulsive Hubbard model. The dynamics is calculated for a Bethe lattice in infinite dimensions with non‐equilibrium dynamical mean‐field theory. In the absence of driving melting proceeds differently depending on the quench of the interactions to hopping ratio from the atomic limit. For decay occurs due to mobile charge‐excitations transferring energy to the spin sector, while for it is governed by the dynamics of residual quasi‐particles. Here we explore the rich effects that strong periodic driving has on this relaxation process spanning three frequency ω regimes: (i) high‐frequency , (ii) resonant with integer l, and (iii) in‐gap away from resonance. In case (i) we can quickly switch the decay from quasi‐particle to charge‐excitation mechanism through the suppression of ν0. For (ii) the interaction can be engineered, even allowing an effective regime to be reached, giving the reverse switch from a charge‐excitation to quasi‐particle decay mechanism. For (iii) the exchange interaction can be controlled with little effect on the decay. By combining these regimes we show how periodic driving could be a potential pathway for controlling magnetism in antiferromagnetic materials. Finally, our numerical results demonstrate the accuracy and applicability of matrix product state techniques to the Hamiltonian DMFT impurity problem subjected to strong periodic driving.