Excitations across the equilibrium and photoinduced `hidden' states of magnetoresistive manganites

Abstract

"Hidden" phases, generated using ultrafast laser pulses (few hundred femtoseconds), with properties distinct from thermodynamic equilibrium, are appealing for technologies because they can be long-lived, with lifetimes of hours or weeks, and reversible with temperature sweeping or extra pulses. In this regard, La$_{2/3}$Ca$_{1/3}$MnO$_3$ (LCMO) stands out due to its tunability through epitaxial strain, which can drive the bulk ferromagnetic metal (FMM) into an antiferromagnetic insulator (AFI), and its susceptibility to photo-induced transitions. Indeed, AFI LCMO displays a long-lived photo-induced transition into a putative 'hidden' phase whose exact nature and excitations are still largely unknown. Here, we combine ultrafast photo-excitation in the near infrared with in situ transport, x-ray absorption (XAS), and Resonant Inelastic X-ray Scattering (RIXS) to investigate the excitations (polarons, phonons, and orbital) of the photo-excited phase of LCMO and contrast them with the thermodynamic phases achieved through strain and temperature. In the thermodynamic regime, we establish the correlation between polarons and transport, placing them in the 'strong coupling' regime of the Holstein model. Upon photo-excitation of LCMO-AFI, we uncover a long-lived phase characterized by the softening of the polaron excitations, the partial suppression of the Jahn-Teller distortion, and nearly unchanged phonons, showing the emergence of a photo-excited state absent in the equilibrium phase diagram. Finally, by varying temperature, epitaxial strain, and photo-excitation fluence, we construct a polaron phase diagram and identify the key spectroscopic signatures of each phase. Our laser-RIXS approach establishes a versatile platform for exploring photo-induced 'hidden' phases in quantum materials in non-stroboscopic conditions.

Figures (8)

• Figure 1: (a) Schematic view of the in-situ laser-transport-RIXS experimental setup. The laser pulses are 250 fs and 1030 nm center wavelength. The wave-vector of the incident x-ray ($k_i$) is perpendicular to the ultrafast laser beam. $\sigma$ and $\pi$ refer to the polarization of the incident x-ray as perpendicular or parallel to the scattering plane, respectively. All RIXS spectra are taken with the $\sigma$-polarization. The laser polarization is linearly horizontal, parallel to the scattering plane defined by $k_i$ and $k_s$. The laser beam has a $\sim$ 40 degrees incident angle with respect to the film surface. The RIXS data are collected after photo-excitation in a 'non-stroboscopic' manner. (b) Sample holder for the laser-transport-RIXS measurement. The top and bottom films correspond to the AFI LCMO/NGO (100) and LCMO/NGO (001), respectively. A temperature sensor is mounted next to the films to monitor temperature change upon laser irradiation. On the right there is a close-up view of the electrodes deposited on the LCMO/NGO (100) film. The laser spot size is about 120 $\mu$m and the distance between electrodes is 500 $\mu$m. (c) Relative resistance change with respect to the resistance of the initial AFI state ($R_{initial}$) at different recovering times. $\Delta R$ is defined as the resistance difference between the photo-induced and the initial AFI phase. (d) $\Delta R$/$R_{initial}$ as a function of laser fluence of the LCMO - AFI (100) and LCMO - AFI (001) films.
• Figure 2: (a) RIXS spectrum of the AFI phase of LCMO/NGO (100) at the Mn $L_3$ edge ($E_i$ = 643.2 eV). We indicate the pump photon energies used here (1030 nm, 1.2 eV) and in Refs. McLeod2019Zhang2016 (800 nm, 1.55 eV). (b) Schematic view of a possible photo-excitation mechanism. While the laser wavelength is on resonance to the Mn$^{3+}$ on-site $d$ - $d$ excitation, the 1.2 - 1.6 eV energy window also overlaps with the Mn$^{3+}$$\rightarrow$ Mn$^{4+}$ intersite charge-transfer (CT) channel.
• Figure 3: (a) O-$K$ edge XAS of the FMM and AFI phases at 100 K. The peaks between 528-531 eV are related to the Mn-3$d$ and O-2$p$ hybridization. (b) O-$K$ edge XAS of the AFI and photo-induced phases at 100 K. (c) O-$K$ edge XAS of the AFI phase at 100 K and the PMI phase at 320 K. Purple curves in (a-c): XAS difference plots of the FMM-AFI, photo-induced-AFI, and PMI-AFI phases, respectively. (d,e) Schematic view of the spin-projected partial density of states (DOS) of the FMM and AFI phases, respectively, along with the O-$K$ XAS process. The DOS of O-2$p$, Mn e$_g$, and Mn t$_{2g}$ states are filled with yellow, purple, and green colors. $E_i$ refers to the incident x-ray energy. The view of the Mn 3$d$ orbital splitting based on the DOS is included for better visualization of the weak and strong $Q_2$ Jahn-Teller distortion for the FMM and AFI phases, respectively. The DOS of the AFI phase is calculated based on an CE-type antiferromagnetic structure Zhou2011.
• Figure 4: (a) RIXS intensity map of the FMM phase as a function of incident photon energy and energy loss. The XAS of FMM at 100 K is included on the left panel. (b) RIXS spectra of the FMM phase at two incident energies: 529.4 and 530.2 eV. The peaks assigned to phonon, polaron, and Mn $dd$ excitations are labeled. (c) RIXS spectra of the FMM and AFI phases at 100 K. The intensity of the RIXS spectra of the AFI phase is enlarged by 1.3 times for better visualization. (d) RIXS spectra of the AFI and photo-induced phases at 100 K. The spectrum of the photo-induced phase is offset along the vertical axis for better visualization. (e) RIXS spectra of the AFI and PMI phase at 100 K and 320 K, respectively. Solid lines in (c-e): Fitting of the polaron peak. Vertical lines at the bottom of each figure indicate the polaron energies. All RIXS spectra in (c-e) are taken with $E_i$ = 529.4 eV.
• Figure 5: (a) Low-energy RIXS spectra of the FMM and AFI (100) phases at 100 K. (b) Low-energy RIXS spectra of the AFI and photo-induced phases at 100 K. (c) Low-energy RIXS spectra of the AFI and the PMI phases at 100 K and 320 K, respectively. The phonon and bi-phonon are labeled in each figure. The insets show the phonon at $\sim$ 60 meV. Vertical lines at the bottom of each figure indicate the phonon energies. All RIXS spectra are taken with $E_i$ = 529.4 eV.