Photoinduced excitonic magnetism in a multiorbital Hubbard system

Abstract

Multiorbital Hubbard models with Hund coupling and crystal-field splitting exhibit an instability toward spin-triplet excitonic order in the parameter regime characterized by strong local spin fluctuations. Upon chemical doping, two distinct types of excitonic ferromagnetism have been reported. Using steady-state nonequilibrium dynamical mean-field theory, we demonstrate that photo-doped half-filled systems can host nonthermal counterparts of these excitonic phases and exhibit a rich phase diagram in the space of photo-doping and crystal field splitting. Photo-doping a spin-triplet excitonic insulator provides a route towards nonequilibrium control of magnetic order.

Figures (7)

• Figure 1: (a) Order parameter $|\text{Im}\phi^y|$ as a function of $\Delta$ for paramagnetic equilibrium systems at $T_b=0.02$ ($U=14$, $J=0.5$). Also shown are the orbital polarization $(n_1-n_2)/2$ and the probability of high-spin configurations, $p_{\uparrow\uparrow}+p_{\downarrow\downarrow}$. The equilibrium excitonic (EX) phase appears between the high-spin insulating (HS) and low-spin orbitally polarized insulating (LS) phases. The yellow stars indicate $|\text{Im}\phi^y|$ obtained from the OCA calculation. (b) Orbital polarization, high-spin probability, and antiferromagnetic order parameter $|n_{\uparrow}-n_{\downarrow}|/2$ computed for the same parameters as in (a), but without allowing for excitonic order. If both instabilities are considered, the dominant one switches from antiferromagnetic to excitonic order at $\Delta=1.61$ (red dashed line).
• Figure 2: (a) Paramagnetic (PM) equilibrium spectral functions at $T_b=0.02$ for $\Delta=1.3$, 1.6, and 1.9, corresponding to the HS, EX, and LS solutions ($U=14$, $J=0.5$). Solid (dashed) lines represent the spectral functions of orbital 1 (2). (b) Antiferromagnetic spectral functions for the same parameters at $\Delta=1.6$. (c) Comparison of NCA and OCA paramagnetic spectral functions for orbital 1 at $\Delta=1.6$.
• Figure 3: (a) Excitonic order parameters $|\mathrm{Im}\,\phi^y|$, $|\mathrm{Re}\,\phi^x|$, and the magnetization $|m|$ as functions of the bath chemical potential $\mu_b$ for $T_b=0.02$ and $\Delta=1.6$ ($U=14$, $J=0.5$). L, E, and C denote linear, elliptical, and circular polarizations, respectively, following the nomenclature of Ref. Kunes2014b. The yellow hatched region exhibits FM order. (b) Same quantities calculated as a function of the chemical potential $\mu$ in a chemically doped equilibrium system without baths.
• Figure 4: Nonequilibrium phase diagram of the photo-doped system in the $\Delta$-$\delta$ plane for $U=14$, $J=0.5$ and $T_b=0.02$. The red, purple, and blue shaded regions represent the ranges of the L, E, and C phases, respectively, and the yellow hatched region indicates FM order. The green shaded area shows the extent of the AFM phase in a calculation without excitonic orders.
• Figure 5: Total spectral functions of photo-doped systems for $T_b=0.02$ and $\mu_b=4.8$, 5.0, 5.2, and 5.35 ($\Delta=1.6$, $U=14$, $J=0.5$). The solid line represents the spin- and orbital-summed spectral function $A(\omega)$, while the dashed line shows the corresponding occupation function $A^<(\omega)$. The negative-frequency part is symmetric and corresponds to hole occupation rather than electron occupation. To highlight the small peak around $\omega = 2$, the region marked by the black dashed rectangle is magnified and shown in the upper-left inset.