Antiferromagnetic Insulating Ground State Research Articles

Strongly Correlated Electronic Properties of FeO Studied by the SCAN+U Functional

The recently developed strongly constrained and appropriately normed (SCAN) meta-GGA exchange-correlation functional has been demonstrated to be able to give an accurate description of strongly correlated cuprates. However, it remains to be seen if SCAN calculations can predict the physics of compounds involving partially filled 3d orbitals. In this work, we present the first-principles electronic structure studies about the phase transition of FeO using the SCAN functional. Compared with the exchange-correlation functionals widely used in density functional theory, such as the local-density approximation (LDA) or the generalized gradient approximation (GGA), SCAN correctly predicts an antiferromagnetic insulating ground state. However, it still significantly underestimates the band gap for compounds involving partially filled 3d orbitals and, hence, predicts an incorrect phase transition pressure of FeO. Interestingly, the electronic structure and band gap properties of FeO from the SCAN calculations are similar to those from the GGA+U calculations with U0 = 0.7 eV. An on-site interelectronic interaction U is therefore still needed in order to correctly describe the phase transition physics of FeO for the SCAN calculations, predominantly as a simplified self-interaction-error reduction term that enhances the spatial compactness of 3d orbitals. The required value of U in SCAN+U is less than that required in GGA+U, and their difference is rightly equal to the value of U0. Most importantly, in comparison with GGA+U, the SCAN+U method no longer needs to use the prior-known knowledge to break orbital symmetry avoiding the metastability issue, benefiting from the spontaneous symmetry breaking it allows. The calculated properties of FeO from SCAN+U agree well with experiments.

In-situ investigation on the oxygen vacancy-driven topotactic phase transition in charge-orbital ordered Nd0.5Sr0.5MnO3 films

The topotactic phase transition from perovskite to oxygen vacancies ordered brownmillerite structure in transition metal oxides intrigues various new application possibilities. However, the in-situ observation of the oxygen vacancies formation process accompanying the phase transition is still rare. Here in this article, we report the topotactic phase transition from the perovskite to a brownmillerite phase transition in the intermediate bandwidth- (101)-Nd0.5Sr0.5MnO3-δ films. The oxygen vacancies migration process in the films was identified by the TEM technique during the in-situ high vacuum annealing process. The TEM results demonstrate a vertical brownmillerite structure with the oxygen vacancies channel perpendicular to the film plane in the (101)-oriented films. Moreover, the in-situ investigation reveals that oxygen atoms in the {010}PV plane of the perovskite phase migrate out of the film along the out-of-plane <101>PV direction at first, and the formed oxygen vacancies become ordered by elongating the high-vacuum annealing process to 42 h, transforming to the brownmillerite structure. Further studies reveal that the associated physical properties of the topotactic phases can be controlled by the phase transition. The charge-orbital ordered antiferromagnetic insulating ground state transfers to a highly insulating state with a novel magnetic phase that is distinct from that of brownmillerite structure of other manganites. The present studies thus pioneered topotactic phase transition in a new manganite system and provide useful information for developing perovskite-based functional materials by defect engineering.

Antiferromagnetic insulating state in layered nickelates at half filling

We provide a set of computational experiments based on ab initio calculations to elucidate whether a cuprate-like antiferromagnetic insulating state can be present in the phase diagram of the low-valence layered nickelate family (R_{n+1}Ni_nO_{2n+2}, R= rare-earth, n=1-infty) in proximity to half-filling. It is well established that at d^9 filling the infinite-layer (n=infty) nickelate is metallic, in contrast to cuprates wherein an antiferromagnetic insulator is expected. We show that for the Ruddlesden-Popper (RP) reduced phases of the series (finite n) an antiferromagnetic insulating ground state can naturally be obtained instead at d^9 filling, due to the spacer RO_2 fluorite slabs present in their structure that block the c-axis dispersion. In the n=infty nickelate, the same type of solution can be derived if the off-plane R-Ni coupling is suppressed. We show how this can be achieved if a structural element that cuts off the c-axis dispersion is introduced (i.e. vacuum in a monolayer of RNiO_2, or a blocking layer in multilayers formed by (RNiO_2)_1/(RNaO_2)_1).

Uniaxial Strain and Hydrostatic Pressure Engineering of the Hidden Magnetism in La1-xCaxMnO3 (0 ≤ x ≤ 1/2) Thin Films.

Here, using various substrates, we demonstrate that the in-plane uniaxial strain engineering can enhance the Jahn-Teller distortions and promote selective orbital occupancy to induce an emergent antiferromagnetic insulating (AFI) phase at x = 1/3 of La1-xCaxMnO3. Such an AFI phase depends not only on the magnitude of epitaxial strain but also on the symmetry of the substrates. Using the large uniaxial strain imparted by DyScO3(001) substrate, the AFI ground state is achieved in a wide range of doping levels (0 ≤ x ≤ 1/2), leaving an extended AFI phase diagram. Moreover, it is found that hydrostatic pressure can tune the AFI phase back to a hidden ferromagnetic metallic phase, accompanied by the formation of accommodation strain. The coaction of the accommodation strain, uniaxial strain, and hydrostatic pressure produces complex phase competition and evolution, and the result may shed light on phase space control of other functional perovskites with the competing magnetic interactions.

Underpinned exploration for magnetic structure, lattice dynamics, electronic properties, and disproportionation of yttrium nickelate

Magnetic behavior and disproportionation effect of nickelates are closely related to the nature of their ground state. In the present work, the magnetic structure, lattice dynamics, electronic properties, and disproportionation effect of yttrium nickelate (YNiO3) in its ground state P21/n structure were investigated by first-principles and phonon calculations based on density functional theory (DFT). The strong correlated interactions were treated by the DFT + U approach and the meta-generalized-gradient approximation approach implemented under the strongly constrained appropriately normed functional. The S-type antiferromagnetic insulating ground state of YNiO3 was captured well by both approaches. The disproportionation effect is quantitatively characterized through the Born effective charge, indicating the ligand-hole picture of Ni2+ → Ni2−δ+ Ni2+δ with δ = 0.3. The predicted phonon frequency at the Γ point agrees well with the measured value from infrared experiments, including the longitudinal and transverse optical splitting. The analysis based on stretching force constants indicated that the interaction between Ni and O atoms in the small nonmagnetic NiO6 octahedral clusters is stronger than that in the large magnetic NiO6 octahedral clusters.

Detailed X-band Studies of the π–d Molecular Conductor λ-(BETS)2FeCl4: Observation of Anomalous Angular Dependence of the g-value

The antiferromagnetic insulating ground state of the π–d molecular conductor λ-(BETS)2FeCl4 has been under intense debate for the last decades. One of the difficulties in studying this system comes...

An accurate first-principles treatment of doping-dependent electronic structure of high-temperature cuprate superconductors

A first-principles density-functional description of the electronic structures of the high-Tc cuprates has remained a long-standing problem since their discovery in 1986, with calculations failing to capture either the insulating (magnetic) state of the pristine compound or the transition from the insulating to metallic state with doping. Here, by taking lanthanum cuprate as an exemplar high-Tc cuprate, we show that the recently developed non-empirical, strongly constrained and appropriately normed density functional accurately describes both the antiferromagnetic insulating ground state of the pristine compound and the metallic state of the doped system. Our study yields new insight into the low-energy spectra of cuprates and opens up a pathway toward wide-ranging first-principles investigations of electronic structures of cuprates and other correlated materials.

Magnetic phase diagram for the mixed-valence Eu oxide EuTi1−xAlxO3 (0≤x≤1)

We have investigated the magnetic properties of ${\mathrm{EuTi}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}{\mathrm{O}}_{3}$ with $0\ensuremath{\le}x\ensuremath{\le}1$ and established a detailed magnetic phase diagram. Substitution of ${\mathrm{Ti}}^{4+}$ with ${\mathrm{Al}}^{3+}$, which oxidizes magnetic ${\mathrm{Eu}}^{2+}$ $(J=7/2)$ into nonmagnetic ${\mathrm{Eu}}^{3+}$ $(J=0)$, turns the antiferromagnetic insulating ground state $(x=0)$ into the ferromagnetic (FM) insulating one $(0.10\ensuremath{\le}x\ensuremath{\le}0.50)$. The ${\mathrm{Eu}}^{2+}/{\mathrm{Eu}}^{3+}$ mixed valence is indispensable for the FM behavior. In $0.10\ensuremath{\le}x\ensuremath{\le}0.33$, the Curie temperature ${T}_{\mathrm{C}}$ has a plateau at $\ensuremath{\sim}4$ K, and the FM correlation is most enhanced around $x=0.25$. In $0.33\ensuremath{\le}x\ensuremath{\le}1,{\mathrm{Al}}^{3+}$ substitution suppresses the FM correlation due to the dilution of magnetic ${\mathrm{Eu}}^{2+}$ ions. Our present results would provide useful information for magnetoelectric phase control of ${\mathrm{EuTiO}}_{3}$-based compounds.

Electronic structure of CuTeO4 and its relationship to cuprates

Based on first principles calculations, the electronic structure of CuTeO$_4$ is discussed in the context of superconducting cuprates. Despite some significant crystallographic differences, we find that CuTeO$_4$ is similar to these cuprates, exhibiting a quasi two dimensional electronic structure that involves hybridized Cu-$d$ and O-$p$ states in the vicinity of the Fermi level, along with an antiferromagnetic insulating ground state. Hole doping this material by substituting Te$^{6+}$ with Sb$^{5+}$ would be of significant interest.

Antiferromagnetic Insulating Ground State of Molecular π-d System Λ-(BETS)2FeCl4 (BETS = Bis(ethylenedithio)tetraselenafulvalene): A Theoretical and Experimental Review

The πd molecular conductor λ‐(BETS)2FeCl4, where BETS is bis(ethylenedithio) tetraselenafulvalene, has attracted considerable interest for the discovery of its field induced superconducting state. A mystery of this system is its antiferromagnetic insulating ground state. The point still under strong debate is whether the d spins in Fe3+ are ordered or not. Here, we review experimental and theoretical studies on the antiferromagnetic insulating phase in λ‐(BETS)2FeCl4 and mention our perspective based on our ESR measurements for λ‐(BETS)2FexGa1−xCl4. Our ESR results indicate that the π‐d interaction in the system is very strong and there is no sign of paramagnetic Fe spins in the antiferromagnetic ground state.

Annealing assisted substrate coherency and high-temperature antiferromagnetic insulating transition in epitaxial La0.67Ca0.33MnO3/NdGaO3(001) films

Bulk La0.67Ca0.33MnO3 (LCMO) and NdGaO3 (NGO) have the same Pbnm symmetry but different orthorhombic lattice distortions, yielding an anisotropic strain state in the LCMO epitaxial film grown on the NGO(001) substrate. The films are optimally doped in a ferromagnetic-metal ground state, after being ex-situ annealed in oxygen atmosphere, however, they show strikingly an antiferromagnetic-insulating (AFI) transition near 250 K, leading to a phase separation state with tunable phase instability at the temperatures below. To explain this drastic strain effect, the films with various thicknesses were ex-situ annealed under various annealing parameters. We demonstrate that the ex-situ annealing can surprisingly improve the epitaxial quality, resulting in the films with true substrate coherency and the AFI ground state. And the close linkage between the film morphology and electronic phase evolution implies that the strain-mediated octahedral deformation and rotation could be assisted by ex-situ annealing, and moreover, play a key role in controlling the properties of oxide heterostructures.

Weakly ferromagnetic metallic state in heavily doped Ba1−xKxMn2As2

Heavily doped Ba${}_{1\ensuremath{-}x}$K${}_{x}$Mn${}_{2}$As${}_{2}$ ($x=0.19$ and 0.26) single crystals were successfully grown and investigated by the measurements of resistivity and anisotropic magnetization. In contrast to the antiferromagnetic insulating ground state of the undoped BaMn${}_{2}$As${}_{2}$, the K-doped crystals show metallic conduction with weak ferromagnetism below $\ensuremath{\sim}$50 K and Curie--Weiss-like in-plane magnetic susceptibility above $\ensuremath{\sim}$50 K. Under high pressures up to 6 G Pa, the low-temperature metallicity changes into a state characterized by a Kondo-like resistivity minimum. Electronic structure calculations for $x=0.25$ using a $2\ifmmode\times\else\texttimes\fi{}2\ifmmode\times\else\texttimes\fi{}1$ supercell reproduce the hole-doped metallic state. The density of states at Fermi energy has significant As-4$p$ components, indicating that the 4$p$ holes are mainly responsible for the metallic conduction. Our results suggest that the interplay between the itinerant 4$p$ holes and the local 3$d$ moments is mostly responsible for the novel metallic state.

Doped electron localization: Electron paramagnetic resonance probing of La0.3Ca0.7MnO3 compound

The X-band electron paramagnetic resonance (EPR) was studied in the electron-doped ceramic La0.3Ca0.7MnO3. The model fittings of measured temperature dependences of the EPR linewidth and intensity were employed to probe the paramagnetic spin correlations and spin dynamics. These fittings revealed notable ferromagnetic short-range correlations and pure ion–ion spin relaxation in the paramagnetic state of La0.3Ca0.7MnO3. The latter fact supports the local nature of ferromagnetic double exchange coupling in this very case. It is suggested that such character of ferromagnetic exchange interactions results in the localization of carriers and establishing of charge ordered antiferromagnetic insulating ground state. This finding may be considered a strong prerequisite for the electron–hole doping asymmetry observed in the phase diagram of La–Ca manganites.

Electronic structure calculations with the Tran-Blaha modified Becke-Johnson density functional

We report a series of calculations testing the predictions of the Tran-Blaha functional for the electronic structure and magnetic properties of condensed systems. We find a general improvement in the properties of semiconducting and insulating systems, relative to calculations with standard generalized gradient approximations, although this is not always by the same mechanism as other approaches such as the quasiparticle GW method. In ZnO the valence bands are narrowed and the band gap is increased to a value in much better agreement with experiment. The $\text{Zn}\text{ }d$ states do not move to higher binding energy as they do in local-density $\text{approximation}+U$ calculations. The functional is effective for systems with hydride anions, where correcting self-interaction errors in the $1s$ state is important. Similarly, it correctly opens semiconducting gaps in the alkaline-earth hexaborides. It correctly stabilizes an antiferromagnetic insulating ground state for the undoped cuprate parent ${\text{CaCuO}}_{2}$, but seriously degrades the agreement with experiment for ferromagnetic Gd relative to the standard local-spin-density approximation and generalized gradient approximations. This is due to positioning of the minority-spin $4f$ states at too low an energy. Conversely, the position of the $\text{La}\text{ }4f$ conduction bands of ${\text{La}}_{2}{\text{O}}_{3}$ is in reasonable accord with experiment as it is with standard functionals. The functional narrows the $\text{Fe}\text{ }d$ bands of the parent compound LaFeAsO of the iron high-temperature superconductors while maintaining the high Fe spectral weight near the Fermi energy.

Metallicity inLaTiO3thin films induced by lattice deformation

Orthorhombic bulk ${\text{LaTiO}}_{3}$ is a correlated Mott insulator whose delicate antiferromagnetic insulating ground state can be easily destroyed by cation vacancies, excess oxygen, or alkaline-earth dopants, resulting in metallicity. Here we show that metallic conduction can also be realized in epitaxial ${\text{LaTiO}}_{3}$ thin films on (001) ${\text{SrTiO}}_{3}$ substrates, while films grown on (110) ${\text{DyScO}}_{3}$ and ${\text{GdScO}}_{3}$ substrates are insulating. These results illustrate the sensitivity of electrical transport to lattice effects and demonstrate the tunability of correlated oxide thin-film properties via substrate-induced deformation.

Specific heat and magnetic order of La0.2Ca0.8MnO3

Measurements of specific heat of the electron-doped La0.2Ca0.8MnO3 manganite are reported for 2&amp;lt;T&amp;lt;400 K. It was found that La0.2Ca0.8MnO3 exhibits an orbital ordering at TOO=204 K and a long-range antiferromagnetic order below TN=178 K. The entropy changes (ΔS) around both TOO[ΔS(TOO)≈0.38 J/mol K] and TN[ΔS(TN)≈3.58 J/mol K] were determined. The entropy change associated with the orbital ordering was found to be much smaller than that for charge ordered manganites. The low temperature specific heat indicates that La0.2Ca0.8MnO3 has rather high Debye temperature ΘD=685 K in comparison with existing results for electron-doped La1−xCaxMnO3 manganites. Moreover, the presence of a large 1/T2 hyperfine contribution and of a considerable linear term, unexpected for La0.2Ca0.8MnO3 with antiferromagnetic insulating ground state, was revealed. The possible relationship between the behavior of the heat capacity and the phase separation is discussed.

First-principles study of electronic and magnetic properties of TlNiO 3

First-principles calculations on the electronic and magnetic properties of TlNiO 3 reveal that the antiferromagnetic structure with the insulating ground state is more stable than other possible configurations in the monoclinic structure. The band gap of the antiferromagnetic insulating ground state is predicted to be 0.18 eV. The spin magnetic moments of Ni(1) and Ni(2) ions are about ±1.62 and ±0.92μ B, respectively. The differing chemical environments between the two Ni positions as well as charge disproportionation effect result in unequal magnetic moments for adjacent Ni atoms in the monoclinic TlNiO 3.

First-principles study of the electronic structure and optical properties of UO2

The electronic structure and optical properties of UO2 are investigated using the first principles density functional method within local-spin density approximation (LSDA), and the Coulomb correlation energy (U) is used to calculate the lattice constant, energy band structure and optical properties of UO2. The calculated lattice constant is 5.40 ?, and the band gap is 1.82 eV. We succeeded in predicting the correct anti-ferromagnetic insulating ground state of uranium dioxide. By analyzing the energy structure and dielectric function, we find that in uranium ions the 6d band splits into two sub-bands, which is in agreement with experimental results.

From trans-polyacetylene to zigzag-edged graphene nanoribbons

A zigzag-edged graphene nanoribbon (ZGNR) can be indexed by a width, N, which is the number of parallel trans-polyacetylene-like chains comprising the ribbon. Although ZGNRs with N = 4 or greater have been found to have an antiferromagnetic ground state and a ferromagnetic metastable state, how the ribbon makes the transition from N = 1 (namely, trans-polyacetylene – a one-dimensional, nonmagnetic semiconductor) to the wider ones is unclear. Using hybrid density functional theory, we show that N = 2 (polyacene) can be considered the narrowest ZGNR with an antiferromagnetic insulating ground state and a metastable, metallic ferromagnetic state.

Electronic Properties, Band Structure, and Fermi Surface Instabilities ofNi1+/Ni2+NickelateLa3Ni2O6, Isoelectronic with Superconducting Cuprates

Electronic structure calculations were performed for the mixed-valent Ni(1+)/Ni(2+) nickelate La3Ni2O6, which exhibits electronic instabilities of the Fermi surface similar to that of the isostructural superconducting La2CaCu2O6 cuprate. La3Ni2O6 shows activated hopping, which fits to Mott's variable-range-hopping model with localized states near the Fermi level. However, a simple local spin density approximation calculation leads to a metallic ground state. The calculations including local density approximation+Hubbard U and hybrid functionals indicate a multiply degenerate magnetic ground state. For electron-doped La2ZrNi2O6, which is isoelectronic with La2CaCu2O6, an antiferromagnetic insulating ground state is found when correlations are included. The nickelates are thus ideal model systems for a deeper understanding of correlated transition metal compounds, magnetism, and superconductivity.