Collapse of Jahn-Teller Phonons in La$_{1-x}$Sr$_{x}$MnO$_3$ with Weak Magnetoresistance

Abstract

Perovskite manganites are quantum materials exhibiting competing interactions inducing colossal magnetoresistance (CMR). The prevailing theory of CMR highlights the essential role of electron-phonon coupling (EPC), but mounting evidence suggests the underlying mechanism is more complicated. Here, we investigate phonons and spin-phonon coupling in ferromagnetic CMR manganites La$_{1-x}$Sr$_x$MnO$_3$ ($x$=0.2,0.3) with relatively small CMR associated with melting of the magnetic order above room temperature. High-resolution neutron scattering experiments combined with density functional theory (DFT) show that the low-temperature ferromagnetic phase is conventional: neutron scattering from phonons agrees with DFT predictions and magnons follow sinusoidal dispersions. Fluctuating magnetic moments and low-energy phonons remain conventional in the high temperature paramagnetic phase, indicating the Mn and La/Sr sublattices are not strongly perturbed by melting of ferromagnetism. In contrast, the Jahn-Teller-active optical oxygen vibrations collapse entirely above the Curie temperature, despite low CMR in these compositions, with some of the lost spectral weight reappearing as quasielastic scattering. We attribute this highly anomalous behavior to giant EPC in the charge and/or orbital channel. It drives cooperative diffusive motion of quasistatic carrier-trapping oxygen sublattice distortions once ferromagnetism disappears. We hypothesize the magnitude of magnetoresistance correlates with the rate of diffusion rather than with the strength of Jahn-Teller EPC.

Figures (8)

• Figure 1: Phonon anomalies in La$_{0.8}$Sr$_{0.2}$MnO$_3$. Inelastic time-of-flight neutron scattering from the spin (a-b) and lattice degrees of freedom (c-d) in La$_{0.8}$Sr$_{0.2}$MnO$_3$ at 10 K (a,c) and 335 K (b,d). In (a-d), the upper/lower colormap is from $E_i=120/54$ meV. The dashed line in (a) is from a first-nearest neighbor Heisenberg model calculation using linear spin wave theory. (e-f) depict inelastic neutron scattering from phonons calculated from DFT. Line cuts through the $X$-point (in the cubic basis) half-breathing mode and the $M$-point quadrupolar mode, at $\approx 45$ meV at $\bm{Q}=(4.5,0,0)$ and $\approx 42.5$ meV at $\bm{Q}=(4.5,0,-1.5)$ respectively in (c), are compared at 10 K and 335 K in (g). The eigenvector diagrams depict the half-breathing and quadrupolar bond-stretching modes whose intensities are indicated the by the arrows in (g)
• Figure 2: Comparison of INS and DFT calculations. Comparison between TOF neutron scattering measurements from La$_{0.8}$Sr$_{0.2}$MnO$_3$ at 10 K (a) and 335 K (b) and DFT calculations in the orthorhombic (c) and rhombohedral (d) phases along $\bm{Q}=(H,0,0)$ at 0 K. The upper/lower colormaps correspond to $E_i=120/54$ meV. The $E_i=120$ meV DFT calculations were broadened in energy using a Gaussian resolution with FWHM=5 meV; the $E_i=54$ meV calculations used FWHM=2.5 meV. The strong feature in the $2\leq H\leq 3$ zone at 10 K is a magnon which becomes quasielastic at 335 K. The white circle indicates the location of a strong phonon peak expected from DFT but appearing only at 10 K in the experiments. The data are divided by the Bose factor.
• Figure 3: Phonon dispersion calculations. (a) Phonon dispersions in the orthorhombic phase of La$_{0.8}$Sr$_{0.2}$MnO$_3$. The primed labels are $\bm{q}$-points in the orthorhombic basis, while the unprimed $X$ and $M$ points in (b) correspond to the cubic basis. The orthorhombic unit cell is shown in (c) and the corresponding Brillouin zone for the same orientation is shown in (d). Panel (b) depicts the calculated inelastic scattering intensity (magenta shading) superposed with the phonon dispersions along the $\bm{Q}=(4.5,0,L)$ direction in the cubic basis (see Fig. \ref{['fig:main_fig']}e). The orientation in (b) has $L$ parallel to the orthorhombic $\bm{b}$ axis. This is the same as the orthorhombic $U'$-$R'$ direction as indicated by the blue shading in (a). In the orthorhombic basis in (b), $\bm{Q}=(4.5,2L,4.5)$. The eigenvector diagrams in (e) show the $X$-point half-breathing (HB) and $M$-point quadrupolar (QP) bond-stretching modes discussed in the text. The arrows in (b) indicate the location of the HB and QP modes in the dispersions. Only Mn-O octahedral chains along the $\bm{a}+\bm{c}$ direction are shown; the La atoms do not participate in the displacement, so are omitted.
• Figure 4: Magnon dispersions in La$_{0.8}$Sr$_{0.2}$MnO$_3$ from time-of-flight neutron scattering. Magnetic excitations in La$_{0.8}$Sr$_{0.2}$MnO$_3$ at 10 K (a,c,e,g) and at 335 K (b,d,f,h) measured by inelastic neutron scattering. The upper (lower) colormap is from $E_i=120$ (54) meV
• Figure 5: Temperature dependence of anomalous phonons in La$_{x}$Sr$_{1-x}$MnO$_3$ ($x=$0.2, 0.3). (a) TAX neutron scattering intensities from La$_{0.7}$Sr$_{0.3}$MnO$_3$ and (b) TOF and TAX neutron scattering intensities from La$_{0.8}$Sr$_{0.2}$MnO$_3$ at $\bm{Q}=(4.5,0,0)$ as a function of temperature. The data at different temperatures are offset vertically. The structural transition temperature, $T_S\approx120$ K, and the Curie temperature, $T_C\approx305$ K, of La$_{0.8}$Sr$_{0.2}$MnO$_3$ are marked on the right. The thin vertical dashed line goes through the collapsing mode; Multizone fitting shows that the other nearby peak persisting to 335 K is a different phonon.