Ultrafast spin-nematic and ferroelectric phase transitions induced by femtosecond light pulses

Sangeeta Rajpurohit, Liang Z. Tan, Christian Jooss, and P. E. Blöchl

Phys. Rev. B 102, 174430 – Published 18 November 2020

Abstract

Optically induced phase transitions of the manganite $\Pr_{1 / 3} {Ca}_{2 / 3} Mn O_{3}$ have been simulated by using a model Hamiltonian that captures the dynamics of strongly correlated charge, orbital, lattice, and spin degrees of freedom. Its parameters have been extracted from first-principles calculations. Beyond a critical intensity of a femtosecond light pulse, the material undergoes an ultrafast and nonthermal magnetic phase transition from a noncollinear to collinear antiferromagnetic phase. The light-pulse excites selectively either a spin-nematic or a ferroelectric phase, depending on the light polarization. The behavior can be traced to an optically induced ferromagnetic coupling between Mn trimers, i.e., polarons which are delocalized over three Mn sites. The polarization guides the polymerization of the polaronic crystal into distinct patterns of ferromagnetic chains determining the target phase.

Received 1 August 2020
Accepted 27 October 2020

DOI:https://doi.org/10.1103/PhysRevB.102.174430

Physics Subject Headings (PhySH)

Antiferromagnetism Charge order Ferroelectricity Magnetic phase transitions Nematic order Order parameters Spin state transition Stripes Ultrafast phenomena

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Sangeeta Rajpurohit ^1,*, Liang Z. Tan ¹, Christian Jooss², and P. E. Blöchl ^3,4

¹Molecular Foundry, Lawrence Berkeley National Laboratory, California 94720, USA
²Institute for Material Physics, Georg-August-Universität Göttingen, Göttingen 37077, Germany
³Institute for Theoretical physics, Clausthal University of Technology, Clausthal 38678, Germany
⁴Institute for Theoretical Physics, Georg-August-Universität Göttingen, Göttingen 37077, Germany

^*srajpurohit@lbl.gov

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Vol. 102, Iss. 17 — 1 November 2020

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Images

Figure 1
Charge, orbital, and spin order of $\Pr_{1 / 3} {Ca}_{2 / 3} Mn O_{3}$ in (a) the ground state, (b) the spin-nematic phase obtained with a polarization along $\vec{a} + \vec{b}$ , and (c) the ferroelectric phase obtained with a polarization along $\vec{a}$ or $\vec{b}$ . The three coplanar spin directions at Mn sites are shown in white, gray, and black. (d) Density of states projected on the Wannier states $| w_{1} 〉$ (white), $| w_{2} 〉$ (pink), $| w_{3} 〉$ (yellow), $| w_{4} 〉$ (black), and $| w_{5} 〉$ (gray). The right and left side shows contribution from majority- and minority-spin states, and the dashed line is the Fermi level. (e) Wannier states $| w_{i} 〉$ with $i \in {1, 2, ... 5}$ localized on a Mn trimer. The red solid arrows indicate dipole-allowed intratrimer electronic transitions. (f) $| w_{2} 〉 - | w_{5} 〉$ state hopping of adjacent orthogonal trimers, shown by red dashed arrow in panel (e).
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Figure 2
(top) Spin-correlation function $C_{S}$ for $l = 1$ of the ground state (left), the nematic phase obtained by a light pulse polarized along $\vec{a} \pm \vec{b}$ (middle) and the ferroelectric phase obtained with polarization along $\vec{a}$ or $\vec{b}$ . The correlation functions of the optically induced phases are shown for 1 ps after the pulse. Bottom: Time evolution of the spin-correlation function at $(1 / 2, 1 / 2, 1)$ characteristic for the spin-nematic phase (left). The contribution of the diffraction peaks from the immediate neighborhood of $(1 / 2, 1 / 2, 1)$ peak is taken into account. Evolution of the spin-correlation function at $(1 / 3, 1 / 2, 1)$ , which is characteristic for the ferroelectric phase (right). The full lines show the spin correlation at $(1 / 3, 1 / 3, 1)$ characteristic for the ground state. The colors refer to the increasing intensities with $A_{0} = 0.10 ℏ / e a_{o}$ (black), $0.225 ℏ / e a_{o}$ (blue), $0.45 ℏ / e a_{o}$ (green), and $0.50 ℏ / e a_{o}$ (red). The corresponding photon absorptions per Mn $D_{p}$ for polarization along $\vec{b} (\vec{a} + \vec{b})$ are $D_{p} = 0.002 (0.001)$ ph/Mn (black), 0.016(0.015) ph/Mn (blue), 0.047(0.039) ph/Mn (green), and 0.062 (0.043) ph/Mn (red).
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Figure 3
Charge correlation $C_{Q}$ at $(h, k, l) = (1 / 3, 1, 0)$ and orbital correlation $C_{O}$ at $(h, k, l) = (1 / 3, 0, 0)$ as a function of time for light pulses polarized along $\vec{b}$ (dashed lines) and $(\vec{a} + \vec{b})$ (solid lines). The intensities are color coded as described in Fig. 2.
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Figure 4
Spin angles of adjacent Mn trimers as function of time for light polarized along $\vec{a} + \vec{b}$ (left) and along $\vec{b}$ (right). The trimers are grouped according to their final spin alignment parallel or antiparallel to the global spin axis. On average, the spins deviate by less than $15^{\circ}$ from the spin axis of the trimer they reside on. The colors (black, blue, green, red) indicate increasing intensities of the light pulse as quantified in Fig. 2.
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