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Register a new userImportance of anisotropic Coulomb interactions in the electronic and magnetic properties of Mn$_3$O$_4$
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We report the importance of anisotropic Coulomb interactions in DFT+U calculations of the electronic and magnetic properties of Mn$_3$O$_4$. The effects of anisotropic interactions in Mn$^{2+}$ and Mn$^{3+}$ are separately examined by defining two different sets of Hubbard parameters: $U^{2+}$ and $J^{2+}$ for Mn$^{2+}$ and $U^{3+}$ and $J^{3+}$ for Mn$^{3+}$. The anisotropic interactions in Mn$^{3+}$ have a significant impact on the physical properties of Mn$_3$O$_4$ including local magnetic moments, canted angle, spontaneous magnetic moment, and superexchange coupling, but those in Mn$^{2+}$ do not make any noticeable difference. Weak ferromagnetic interchain superexchange, observed in experiments, is predicted only if a sizable anisotropic interaction is considered in Mn$^{3+}$. By analyzing the eigenoccupations of the on-site Mn density matrix, we found that the spin channel involving Mn$^{3+}$ $d_{x^2-y^2}$ orbitals, which governs the 90$^circ$ correlation superexchange, is directly controlled by the anisotropic interactions. These findings demostrate that the exchange correction $J$ for the intraorbital Coulomb potential is of critical importance for first-principles description of reduced Mn oxides containing Mn$^{3+}$ or Mn$^{4+}$.
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We investigate the electronic and magnetic structures of two-dimensional transition metal tri-chalcogenide CrSiTe$_{3}$ and CrGeTe$_{3}$ materials by carrying out first-principles calculations. The single-layer CrSiTe$_3$ and CrGeTe$_3$ are found to be a ferromagnetic insulator, where the presence of the strong $dpsigma$-hybridization of Cr $e_{mathrm{g}}$-Te $p$ plays a crucial role for the ferromagnetic coupling between Cr ions. We observe that the bandgaps and the interlayer magnetic order vary notably depending on the magnitude of on-site Coulomb interaction $U$ for Cr $d$ electrons. The bandgaps are formed between the Cr $e_{mathrm{g}}$ conduction bands and the Te $p$ valence bands for both CrSiTe$_3$ and CrGeTe$_3$ in the majority-spin channel. The dominant Te $p$ antibonding character in the valence bands just below the Fermi level is related to the decrease of the bandgap for the increase of $U$. We elucidate the energy band diagram, which may serve to understand the electronic and magnetic properties of the $ABX_3$-type transition metal tri-chalcogenides in general.
Mn$_3$O$_4$ is a spin frustrated magnet that adopts a tetragonally distorted spinel structure at ambient conditions and a CaMn$_2$O$_4$-type postspinel structure at high pressure. We conducted both optical measurements and emph{ab} emph{initio} calculations, and systematically studied the electronic band structures of both the spinel and postspinel Mn$_3$O$_4$ phases. For both phases, theoretical electronic structures are consistent with the optical absorption spectra, and display characteristic band-splitting of the conduction band. The band gap obtained from the absorption spectra is 1.91(6) eV for the spinel phase, and 0.94(2) eV for the postspinel phase. Both phases are charge-transfer type insulators. The Mn 3emph{d} $t_2$$_g$ and O 2emph{p} form antibonding orbitals situated at the conduction band with higher energy.
Single crystals of CrSbSe$_3$, a structurally pseudo-one-dimensional ferromagnetic semiconductor, were grown using a high-temperature solution growth technique and were characterized by x-ray diffraction, anisotropic, temperature- and field-dependent magnetization, temperature-dependent resistivity and optical absorption measurements. A band gap of 0.7 eV was determined from both resistivity and optical measurements. At high temperatures, CrSbSe$_3$ is paramagnetic and isotropic with a Curie-Weiss temperature of $sim$145 K and an effective moment of $sim$4.1 $mu_B$/Cr. A ferromagnetic transition occurs at $T_c$ = 71 K. The $a$-axis, perpendicular to the chains in the structure, is the magnetic easy axis, while the chain axis direction, along $b$, is the hard axis. Magnetic isotherms measured around $T_c$ do not follow the behavior predicted by simple mean field critical exponents for a second order phase transition. A tentative set of critical exponents is estimated based on a modified Arrott plot analysis, giving $betasim$0.25, $gammasim$1.38 and $deltasim$6.6.
We have investigated electronic structures of La$_3$S$_4$ and Ce$_3$S$_4$ using the LSDA and LSDA+$U$ methods. Calculated density of states (DOS) are compared with the experimental DOS obtained by the valence band photoemission spectroscopy. The DOS at $E_{rm{F}}$ indicates the 5$d$ character in La$_3$S$_4$ and 4$f$ character in Ce$_3$S$_4$. It is found to be nearly half metallic in the ferromagnetic ground state of Ce$_3$S$_4$. %Ce$_3$S$_4$ has ferromagnetic ground states with spin and orbital magnetic %moments of 1.27 $mu_{rm{B}}$ and $-$2.81 $mu_{rm{B}}$ per Ce, respectively, %and shows nearly half metallic ground state. We discuss the superconductivity and structural transition in La$_3$S$_4$, and the absence of structural transition in Ce$_3$S$_4$.
The layered misfit cobaltate Ca$_3$Co$_4$O$_9$, also known as Ca$_2$CoO$_3$[CoO$_2$]$_{1.62}$, is a promising p-type thermoelectric oxide. Employing density functional theory, we study its electronic structure and determine, on the basis of Boltzmann theory within the constant-relaxation-time approximation, the thermoelectric transport coefficients. The dependence on strain and temperature is determined. In particular, we find that the $xx$-component of the thermopower is strongly enhanced, while the $yy$-component is strongly reduced, when applying 2% tensile strain. A similar anisotropy is also found in the power factor. The temperature dependence of the conductivity in the $a$-$b$ plane is found to be rather weak above 200 K, which clearly indicates that the experimentally observed transport properties are dominated by inhomogeneities arising during sample growth, i.e., are not intrinsic.
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