Ultrafast Separation of Photodoped Carriers in Mott Antiferromagnets

Martin Eckstein and Philipp Werner

Phys. Rev. Lett. 113, 076405 – Published 13 August 2014

Abstract

We use inhomogeneous nonequilibrium dynamical mean-field theory to investigate the spreading of photoexcited carriers in Mott insulating heterostructures with strong internal fields. Antiferromagnetic correlations are found to affect the carrier dynamics in a crucial manner: An antiferromagnetic spin background can absorb energy from photoexcited carriers on an ultrafast time scale, thus enabling fast transport between different layers and the separation of electron and holelike carriers, whereas in the paramagnetic state, carriers become localized in strong fields. This interplay between charge and spin degrees of freedom can be exploited to control the functionality of devices based on Mott insulating heterostructures with polar layers, e.g., for photovoltaic applications.

Received 6 March 2014

DOI:https://doi.org/10.1103/PhysRevLett.113.076405

Authors & Affiliations

Martin Eckstein^1,* and Philipp Werner²

¹Max Planck Research Department for Structural Dynamics, University of Hamburg-CFEL, 22761 Hamburg, Germany
²Department of Physics, University of Fribourg, 1700 Fribourg, Switzerland

^*martin.eckstein@mpsd.cfel.de

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Vol. 113, Iss. 7 — 15 August 2014

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Images

Figure 1
Equilibrium configuration of the heterostructure and nonequilibrium setup. (a) Layer and frequency resolved spectral function $A (ω, z)$ in the Mott heterostructure with leads, for temperature $T = 1 / 3$ (paramagnetic phase) and internal field $Δ E = 1.5$ ; $ω = 0$ is the Fermi level of the external leads. (b) Layer-dependent density for various gradients $Δ E$ . (c) Layer-dependent antiferromagnetic order for various temperatures up to the Néel temperature. (d) Setup used to study the doublon and hole diffusion to the leads. (e) Setup without leads, which allows us to study the spreading of doublons over a longer distance.
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Figure 2
(a)–(d) Time and layer-dependent doublon and hole densities for two different internal fields. The dashed lines indicate the mean ${\bar{z}}_{d / h} (t)$ of the distributions in space. The heterostructure is excited at layers $z = 3$ , 4. (e),(f) Current to the leads for various $Δ E$ [see key in panel (h)]. Here, the heterostructure is excited at all layers. (g),(h) Time-integrated current $Q = \int_{0}^{t} d t^{'} J_{lead} (t^{'})$ , compared to the total photoexcited charge, for the same parameters as in (e) and (f). The inset in (g) shows the layer-averaged antiferromagnetic order parameter as a function of time.
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Figure 3
Temperature dependence of carrier motion. (a),(b) Layer and time-resolved doublon and hole densities for two temperatures, $T = 1 / 3.5$ (paramagnetic) and $T = 1 / 8$ (antiferromagnetic). To study the carrier motion over a longer distance, we consider a heterostructure which is not coupled to leads, apply the excitation at layer $z = 1$ , and focus on the doublon dynamics (holes remain trapped at the boundary); see sketch in Fig. 1. The internal field is $Δ E = 1$ . The dotted line shows the spatial mean ${\bar{z}}_{d / h} (t)$ . (c) Velocity $v_{d} (t) = d {\bar{z}}_{d} (t) / d t$ at intermediate time $t = 6$ , plotted for various temperatures against the square $m^{2}$ of the layer-averaged staggered magnetization.
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Figure 4
Layer-resolved, spin-integrated photoemission spectrum $I (Ω, t_{p}, z)$ of the upper Hubbard band, for the same geometry as in Fig. 3. (a)–(f) Antiferromagnetic structure at $T = 1 / 8$ . (g)–(l) Paramagnetic structure at $T = 1 / 3$ . The dashed lines indicate the center of the upper Hubbard band.
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Figure 5
Diffusion of carriers in zero field: Photoemission spectrum of the upper Hubbard band shortly after the excitation ( $t_{p} = 2.1$ ) and at time $t_{p} = 7.5$ . The setup is identical to Fig. 3, but for $Δ E = 0$ . (a) $t_{p} = 2.1$ , $T = 1 / 8$ . (b) $t_{p} = 7.5$ , $T = 1 / 8$ . (c) $t_{p} = 2.1$ , $T = 1 / 3$ . (d) $t_{p} = 7.5$ , $T = 1 / 3$ . Vertical arrows in (a) and (c) indicate the mean of the distribution in the spectral region $10 < ω < 16$ . Insets in (b) and (d): density $n_{d} (z, t)$ for the respective temperatures. In zero field, the result for $n_{h} (z, t)$ is identical.
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