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Register a new userObservation of a charge-neutral muon-polaron complex in antiferromagnetic Cr$_2$O$_3$
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We report a comprehensive muon spin rotation ($mu$SR) study of the prototypical magnetoelectric antiferromagnet Cr$_2$O$_3$. We find the positively charged muon ($mu^+$) occupies several distinct interstitial sites, and displays a rich dynamic behavior involving local hopping, thermally activated site transitions and the formation of a charge-neutral complex composed of a muon and an electron polaron. The discovery of such a complex has implications for the interpretation of $mu$SR spectra in a wide range of magnetic oxides, and opens a route to study the dopant characteristics of interstitial hydrogen impurities in such materials. We address implications arising from implanting a $mu^+$ into a linear magnetoelectric, and discuss the challenges of observing a local magnetoelectric effect generated by the charge of the muon.
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We perform detailed muon spin rotation ($mu$SR) measurements in the classic antiferromagnet Fe$_2$O$_3$ and explain the spectra by considering dynamic population and dissociation of charge-neutral muon-polaron complexes. We show that charge-neutral muon states in Fe$_2$O$_3$, despite lacking the signatures typical of charge-neutral muonium centers in nonmagnetic materials, have a significant impact on the measured $mu$SR frequencies and relaxation rates. Our identification of such polaronic muon centers in Fe$_2$O$_3$ suggests that isolated hydrogen (H) impurities form analogous complexes, and that H interstitials may be a source of charge carrier density in Fe$_2$O$_3$.
The electronic structure and magnetic properties of the strongly correlated material La$_2$O$_3$Fe$_2$Se$_2$ are studied by using both the density function theory plus $U$ (DFT+$U$) method and the DFT plus Gutzwiller (DFT+G) variational method. The ground-state magnetic structure of this material obtained with DFT+$U$ is consistent with recent experiments, but its band gap is significantly overestimated by DFT+$U$, even with a small Hubbard $U$ value. In contrast, the DFT+G method yields a band gap of 0.1 - 0.2 eV, in excellent agreement with experiment. Detailed analysis shows that the electronic and magnetic properties of of La$_2$O$_3$Fe$_2$Se$_2$ are strongly affected by charge and spin fluctuations which are missing in the DFT+$U$ method.
I use first principles calculations to investigate the thermal conductivity of $beta$-In$_2$O$_3$ and compare the results with that of $alpha$-Al$_2$O$_3$, $beta$-Ga$_2$O$_3$, and KTaO$_3$. The calculated thermal conductivity of $beta$-In$_2$O$_3$ agrees well with the experimental data obtain recently, which found that the low-temperature thermal conductivity in this material can reach values above 1000 W/mK. I find that the calculated thermal conductivity of $beta$-Ga$_2$O$_3$ is larger than that of $beta$-In$_2$O$_3$ at all temperatures, which implies that $beta$-Ga$_2$O$_3$ should also exhibit high values of thermal conductivity at low temperatures. The thermal conductivity of KTaO$_3$ calculated ignoring the temperature-dependent phonon softening of low-frequency modes give high-temperature values similar that of $beta$-Ga$_2$O$_3$. However, the calculated thermal conductivity of KTaO$_3$ does not increase as steeply as that of the binary compounds at low temperatures, which results in KTaO$_3$ having the lowest low-temperature thermal conductivity despite having acoustic phonon velocities larger than that of $beta$-Ga$_2$O$_3$ and $beta$-In$_2$O$_3$. I attribute this to the fact that the acoustic phonon velocities at low frequencies in KTaO$_3$ is less uniformly distributed because its acoustic phonon branches are more dispersive compared to the binary oxides, which causes enhanced momentum loss even during the normal phonon-phonon scattering processes. I also calculate thermal diffusivity using the theoretically obtained thermal conductivity and heat capacity and find that all four materials exhibit the expected $T^{-1}$ behavior at high temperatures. Additionally, the calculated ratio of the average phonon scattering time to Planckian time is larger than the lower bound of 1 that has been observed empirically in numerous other materials.
Cr$_2$O$_3$ is the archetypal magnetoelectric (ME) material, which has a linear coupling between electric and magnetic polarizations. Quadratic ME effects are forbidden for the magnetic point group of Cr$_2$O$_3$, due to space-time inversion symmetry. In Cr$_2$O$_3$ films grown by sputtering, we find a signature of a quadratic ME effect that is not found in bulk single crystals. We use Raman spectroscopy and magetization measurements to deduce the removal of space-time symmetry, and corroborate the emergence of the quadratic ME effect. We propose that meta-stable site-selective trace dopants remove the space, time, and space-time inversion symmetries from the original magnetic point group of bulk Cr$_2$O$_3$. We include the quadratic ME effect in a model describing the switching process during ME field cooling, and estimate the effective quadratic susceptibility value. The quadratic magnetoelectric effect in a uniaxial antiferromagnet is promising for multifunctional antiferromagnetic and magnetoelectric devices that can incorporate optical, strain-induced, and multiferroic effects.
The perovskite TbFe$_{0.5}$Cr$_{0.5}$O$_3$ shows two anomalies in the magnetic susceptibility at $T_N$ = 257K and $T_{SR}$ = 190K which are respectively, the antiferromagnetic and spin reorientation transition that occur in the Fe/Cr sublattice. Analysis of the magnetic susceptibility reveals signatures of Griffiths-like phase in this compound. Neutron diffraction analysis confirms that, as the temperature is reduced from 350K, a spin reorientation transition from $Gamma_2$ (F$_x$, C$_y$, G$_z$) to $Gamma_4$ (G$_x$, A$_y$, F$_z$) occurs at $T_N$ = 257K and subsequently, a second spin reorientation takes place from $Gamma_4$ (G$_x$, A$_y$, F$_z$) to $Gamma_2$ (F$_x$, C$_y$, G$_z$) at $T_{SR}$ = 190K. The $Gamma_2$ (F$_x$, C$_y$, G$_z$) structure is stable until 7.7K where an ordered moment of 7.74(1)$mu_mathrm B$/Fe$^{3+}$(Cr$^{3+}$) is obtained from neutron data refinement. In addition to the long-range order of the magnetic structure, indication of diffuse magnetic scattering at 7.7K is evident, thereby lending support to the Griffiths-like phase observed in susceptibility. At 7.7K, Tb develops a ferromagnetic component along the crystallographic $a$ axis. Thermal conductivity, and spin-phonon coupling of TbFe$_{0.5}$Cr$_{0.5}$O$_3$ through Raman spectroscopy are studied in the present work. An antiferromagnetic structure with ($uparrow downarrow uparrow downarrow$) arrangement of Fe/Cr spins is found in the ground state through first-principles energy calculations which supports the experimental magnetic structure at 7.7K. The spin-resolved total and partial density of states are determined showing that TbFe$_{0.5}$Cr$_{0.5}$O$_3$ is insulating with a band gap of $sim 0.12$ (2.4) eV within GGA (GGA+$U$) functionals.
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