We report a thorough study of Y$_{0.7}$La$_{0.3}$VO$_3$ single crystals by measuring magnetic properties, specific heat, thermal conductivity, x-ray and neutron diffraction with the motivation of revealing the lattice response to the spin-orbital entanglement in \textit{R}VO$_3$. Upon cooling from room temperature, the orbitally disordered paramagnetic state changes around T*$\sim$220\,K to spin-orbital entangled state which is then followed by a transition at T$_N$=116\,K to C-type orbital ordered (OO) and G-type antiferromagnetic ordered (AF) ground state. In the temperature interval T$_N<T<T^*$, the VO$_{6/2}$ octahedra have two comparable in-plane V-O bonds which are longer than the out-of-plane V-O1 bond. This local structural distortion supports the spin-orbital entanglement of partially filled and degenerate yz/zx orbitals. However, this distortion is incompatible with the steric octahedral site distortion intrinsic to orthorhombic perovskites. Their competition induces a second order transition from the spin-orbital entangled state to C-OO/G-AF ground state where the long range OO suppresses the spin-orbital entanglement. Our analysis suggests that the spin-orbital entangled state and G-OO are comparable in energy and compete with each other. Rare earth site disorder favors the spin-orbital entanglement rather than a cooperative Jahn-Teller distortion. The results also indicate for LaVO$_3$ a C-OO/G-AF state in T$_t$\,$\leq$\,T\,$\leq$T$_N$ and an orbital flipping transition at T$_t$.
The formation of a long-range Jahn-Teller distortion in undoped manganites is studied within the dynamical mean-field theory taking both electron-phonon and electron-electron interactions into account. We find that the observed insulating behavior at temperatures above the Jahn-Teller transition and the bond-length changes below it can most naturally by explained by assuming a strong electronic repulsion and a rather weak electron-phonon coupling.
We explore the relative strengths of spin orbit coupling and crystal field splitting in the Ir5+ compounds Ba2-xSrxYIrO6. In the case of strong spin orbit coupling and regular Ir5+ octahedra, one expects a nonmagnetic J = 0 state; in the case of distorted octahedra where crystal field effects dominate, the t2g manifold splits into a magnetic ground state. We report the results of continuously transitioning from the cubic Ba2YIrO6 double perovskite with ideal octahedra to the monoclinic Sr2YIrO6 double perovskite with distorted octahedra. We see no emergence of an enhanced Ir5+ magnetic moment …
Interplay of spin-orbit coupling and vibronic coupling on heavy $d^1$ site of cubic double perovskites is investigated by ab initio calculations. The stabilization energy of spin-orbital-lattice entangled states is found comparable to or larger than the exchange interactions, suggesting the presence of Jahn-Teller dynamics in the systems. In Ba$_2$YMoO$_6$, the pseudo JT coupling enhances the mixing of the ground and excited spin-orbit multiplet states, which results in strong temperature dependence of effective magnetic moments. The entanglement of the spin and lattice degrees of freedom indu…
The orbital order-disorder transition temperature (T_OO) versus tolerance factor (t) plot switches from monotonic to non-monotonic beyond a doping level (x) ~ 10 atom% in a family of R(1-x)A(x)MnO3 systems(R = La, Pr, Nd; A = Ca, Sr; x = 0.0-0.2). T_OO reaches maximum at a 'doping-dependent' critical tolerance factor t_C(x) (for 0.1<x<0.2)at which the orthorhombic distortion (D) also maximizes. Such an observation reflects influence of charge carriers on both 'intrinsic' octahedral-site distortion and its bias on the orbital order in doped perovskite manganites and, thus, deviation from what h…
We develop anisotropic pseudo-spin antiferromagnetic Heisenberg models for monoclinically distorted double perovskites. We focus on these A$_2$BB'O$_6$ materials that have magnetic moments on the 4d or 5d transition metal B' ions, which form a face-centered cubic lattice. In these models, we consider local z-axis distortion of B'-O octahedra, affecting relative occupancy of $t_{2g}$ orbitals, along with geometric effects of the monoclinic distortion, and spin-orbit coupling. The resulting pseudo-spin-1/2 models are solved in the saddle-point limit of the Sp(N) generalization of the Heisenberg …
We present detailed calculations of the electric field gradient (EFG) using a point charge approximation in Ba$_2$NaOsO$_6$, a Mott insulator with strong spin-orbit interaction. Recent $^{23}$Na nuclear magnetic resonance (NMR) measurements found that the onset of local point symmetry breaking, likely caused by the formation of quadrupolar order, precedes the formation of long range magnetic order in this compound. An extension of the static $^{23}$Na NMR measurements as a function of the orientation of a 15 T applied magnetic field at 8 K in the magnetically ordered phase is reported. Broken …
The thermodynamic properties of the ferromagnetic perovskite YTiO$_3$ are investigated by thermal expansion, magnetostriction, specific heat, and magnetization measurements. The low-temperature spin-wave contribution to the specific heat, as well as an Arrott plot of the magnetization in the vicinity of the Curie temperature $T_C\simeq27$ K, are consistent with a three-dimensional Heisenberg model of ferromagnetism. However, a magnetic contribution to the thermal expansion persists well above $T_C$, which contrasts with typical three-dimensional Heisenberg ferromagnets, as shown by a compariso…
We present a theoretical study on the relation between the size of the rare earth ions, often known as chemical pressure, and the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites. Using a Keating model expressed in terms of atomic scale symmetry modes, we show that there exists a coupling between the uniform shear distortion and the staggered buckling distortion within the Jahn-Teller energy term. It is found that this coupling provides a mechanism by which the coherent Jahn-Teller distortion is more stabilized by smaller rare earth ions. We analyze the appear…
The evolution of spin- and orbital-ordered states has been systematically investigated for a series of insulating perovskites $R$MnO$_3$ ($R$=La,Pr,Nd,...). $R$MnO$_3$ with a large GdFeO$_3$-type distortion is regarded as a frustrated spin system having ferromagnetic nearest-neighbor and antiferromagnetic (AF) next-nearest-neighbor (NNN) interactions within a MnO$_2$ plane. The staggered orbital order associated with the GdFeO$_3$-type distortion induces the anisotropic NNN interaction, and yields the unique sinusoidal and $E$-type ("up-up-down-down") AF ordered states in the distorted perovsk…
Roles of orbital and lattice degrees of freedom in strongly correlated systems are investigated to understand electronic properties of perovskite Mn oxides such as La_{1-x}Sr_{x}MnO_{3}. An extended double-exchange model containing Coulomb interaction, doubly degenerate orbitals and Jahn-Teller coupling is derived under full polarization of spins with two-dimensional anisotropy. Quantum fluctuation effects of Coulomb interaction and orbital degrees of freedom are investigated by using the quantum Monte Carlo method. In undoped states, it is crucial to consider both the Coulomb interaction and …
Iron perovskites CaFeO_3 and La_{0.33}Sr_{0.67}FeO_3 show charge disproportionation, resulting in charge-ordered states with Fe^{3+}:Fe^{5+} =1:1 and =2:1, respectively. We have made photoemission and unrestricted Hartree-Fock band-structure calculation of CaFeO_3 and compared it with La_{0.33}Sr_{0.67}FeO_3. With decreasing temperature, a gradual decrease of the spectral weight near the Fermi level occurred in CaFeO_3 as in La_{0.33}Sr_{0.67}FeO_3 although lattice distortion occurs only in CaFeO_3. Hartree-Fock calculations have indicated that both the breathing and tilting distortions are ne…
A minimal model is proposed for the perovskite manganese oxides showing the strongly incoherent charge dynamics with a suppressed Drude weight in the ferromagnetic and metallic phase near the insulator. We investigate a generalized double-exchange model including three elements; the orbital degeneracy of $e_g$ conduction bands, the Coulomb interaction and fluctuating Jahn-Teller distortions. We demonstrate that Lancz$\ddot{\rm o}$s diagonalization calculations combined with Monte Carlo sampling of the largely fluctuating lattice distortions result in the optical conductivity which quantitative…
The usual classical behaviour of S = 3/2, B-site ordered double perovskites generally results in simple, commensurate magnetic ground states. In contrast, heat capacity and neutron powder diffraction measurements for the S = 3/2 systems La2NaB'O6 (B' = Ru, Os) reveal an incommensurate magnetic ground state for La2NaRuO6 and a drastically suppressed ordered moment for La2NaOsO6. This behaviour is attributed to the large monoclinic structural distortions of these double perovskites. The distortions have the effect of weakening the nearest neighbour superexchange interactions, presumably to an en…
The combined effect of electronic correlation and strong spin-orbit-coupling(SOC) can give rise to a variety of exotic quantum phases. Double perovskites provide a simple structure to study the spin-orbit-lattice entangled states. In this thesis, focusing on the 5d osmate double perovskite system, we conduct a combination of work including theoretical model simulation, first-principle calculation, and nuclear magnetic resonance experiments to understand the fundamental physical properties of this material system.
Time-of-flight inelastic neutron scattering measurements have been carried out on polycrystalline samples of the 4$d^3$ and 5$d^3$ double pervoskite antiferromagnets La$_2$LiRuO$_6$ and La$_2$LiOsO$_6$. These reveal the development of an inelastic spin gap in La$_2$LiRuO$_6$ and La$_2$LiOsO$_6$ of $\sim$ 1.8(8) meV and 6(1) meV, below their respective ordering temperatures, $T_N$, $\sim$ 23.8 K and 30 K. The bandwidth of the spin excitations is shown to be $\sim$ 5.7(9) to 12(1) meV, respectively, at low temperatures. Spin gaps are surprising in such magnets as the t$_{2g}$ levels of Ru$^{5+}$…
Magnetoelectric phase diagrams of the perovskite manganites, Eu1-xYxMnO3 and Gd1-xTbxMnO3, are theoretically studied. We first construct a microscopic model, and then analyze the model using the Monte-Carlo method. We reproduce the diagrams, which contain two different multiferroic states, i.e., the ab-plane spin cycloid with electric polarization P//a and the bc-plane spin cycloid with P//c. We reveal that their competition originates from a conflict between the single-ion anisotropy and the Dzyaloshinsky-Moriya interaction, which is controlled by the second-neighbor spin exchanges enhanced b…
Spin-orbit-entangled frustrated magnetism in fcc Ba$_2$(Yb,Nd)NbO$_6$ double perovskites
Shiv Nadar Institution of Eminence（Shiv Nadar卓越学院）；Bennett University（贝内特大学）；UGC-DAE Consortium for Scientific Research（UGC-DAE科学研究联盟）；Indian Institute of Science Education and Research, Thiruvananthapuram（印度科学教育与研究研究所，特里凡得琅）；Japan Atomic Energy Agency（日本原子能机构）
Structural study on the photo-switching system Pr{0.55}(Ca{1-y}Sr y){0.45}MnO3 thin films on perovskite (011) substrate has been made with synchrotron radiation diffraction experiment. The insulating phase of y=0.20 sample was found to be an antiferro-orbital ordered phase with large lattice distortion and the ferromagnetic metallic phase of y=0.40 film show no distinct lattice distortion from the paramagnetic phase, which are very similar with (Nd,Pr)0.5Sr0.5MnO3 films. A striking contrast of the orbital ordered phase of y=0.20 film to (Nd,Pr)0.5Sr0.5MnO3 films was its untwinned structure.
We study magnetism on the fcc lattice with tetragonal distortions, with general exotic directional magnetic interactions allowed by symmetry. We consider two models, corresponding to a uniform tetragonal distortion, or a two-sublattice model with a tetragonal screw axis. We establish their low temperature phase diagrams in the semi-classical limit using classical optimization and consideration of fluctuations both analytically and by Monte Carlo simulation. Both order by disorder and exchange anisotropy mechanisms favor a $\langle 110\rangle$ easy axis for magnetization. We also show that spin…
The magnetic phase diagram of the perovskite-type Ti oxides as a function of the GdFeO3-type distortion is examined by using the Hartree-Fock analysis of a multiband d-p Hamiltonian from a viewpoint of competitions of the spin-orbit interaction, the Jahn-Teller (JT) level-splitting and spin-orbital superexchange interactions. Near the antiferromagnetic (AFM)-to-ferromagnetic (FM) phase boundary, A-type AFM [AFM(A)] and FM states accompanied by a certain type of orbital ordering are lowered in energy at large JT distortion, which is in agreement with the previous strong coupling study. With inc…
By means of in situ synchrotron X-ray diffraction and Raman spectroscopy under hydrostatic pressure, we investigate the structural stability of the quadruple perovskite LaMn7O12. At 34 GPa, the data unveil a first-order structural phase transition from monoclinic I2/m to cubic Im-3 symmetry characterized by a pronounced contraction of the unit cell and by a significant modifications in the Raman phonon modes. The phase transition is also marked by the suppression of Jahn-Teller distortion which is present in the ambient monoclinic phase. In addition, above 20 GPa pressure, a sudden and simulta…
This work reports on the pressure dependence of the octahedra tilts and mean Fe-O bond lengths in RFeO3 (R=Nd, Sm, Eu, Gd, Tb and Dy), determined through synchrotron X-ray diffraction and Raman scattering, and their role on the pressure induced phase transition displayed by all of these compounds. For larger rare-earth cations (Nd-Sm), both anti- and in-phase octahedra tilting decrease as pressure increases, whereas the reverse behavior is observed for smaller ones (Gd-Dy). EuFeO3 stands at the borderline, as the tilts are pressure independent. For the compounds where the tilts increase with p…
With electron filling $n=1$ in the $\rm Sr_2VO_4$ compound, the octahedrally coordinated $t_{\rm 2g}$ orbitals are strongly active due to tetragonal distortion induced crystal field tuning by external agent such as pressure. Considering the full range of crystal field induced tetragonal splitting in a realistic three-orbital model, collective spin-orbital excitations are investigated using the generalized self consistent and fluctuation approach. The variety of self consistent states obtained including orbital entangled ferromagnetic and antiferromagnetic orders reflects the rich spin-orbital …
We discuss Mott insulating and metallic phases of a model with $e_g$ orbital degeneracy to understand physics of Mn perovskite compounds. Quantum Monte Carlo and Lanczos diagonalization results are discussed in this model. To reproduce experimental results on charge gap and Jahn-Teller distortions, we show that a synergy between the strong correlation effects and the Jahn-Teller coupling is important. The incoherent charge dynamics and strong charge fluctuations are characteristic of the metallic phase accompanied with critical enhancement of short-ranged orbital correlation near the insulator…
The origin of the antiferromagnetic order and puzzling properties of LaTiO_3 as well as the magnetic phase diagram of the perovskite titanates are studied theoretically. We show that in LaTiO_3, the t_{2g} degeneracy is eventually lifted by the La cations in the GdFeO_3-type structure, which generates a crystal field with nearly trigonal symmetry. This allows the description of the low-energy structure of LaTiO_3 by a single-band Hubbard model as a good starting point. The lowest-orbital occupation in this crystal field stabilizes the AFM(G) state, and well explains the spin-wave spectrum of L…
We present a detailed de Haas van Alphen effect study of the perovskite CaVO3, offering an unprecedented test of electronic structure calculations in a 3d transition metal oxide. Our experimental and calculated Fermi surfaces are in good agreement -- but only if we ignore large orthorhombic distortions of the cubic perovskite structure. Subtle discrepancies may shed light on an apparent conflict between the low energy properties of CaVO3, which are those of a simple metal, and high energy probes which reveal strong correlations that place CaVO3 on the verge of a metal-insulator transition.
We use temperature dependent neutron powder diffraction (NPD) to investigate the structural changes and magnetic interactions in double perovskite Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0.4 and 0.6). A structural phase transition from tetragonal ($I4/m$) to monoclinic ($P2_1/n$) is observed between $x=$ 0.4 and 0.6 samples. Interestingly, temperature evolution of the unit cell parameters follow the Grüneisen approximation, and the analysis suggest the isotropic thermal expansion in case of the $x=$ 0.4 sample, whereas the $x=$ 0.6 sample shows the anisotropy where the thermal expansion along the $c$-…
Selenium and tellurium are among the few elements that form $AB$O$_3$ perovskite structures with a four valent ion in the $A$ site. This leads to highly distorted structures and unusual magnetic behavior. Here we investigate the Co and Ni selenite and tellurite compounds, CoSeO$_3$, CoTeO$_3$, NiSeO$_3$ and NiTeO$_3$ using first principles calculations. We find an interplay of crystal field and Jahn-Teller distortions that underpin the electronic and magnetic properties. While all compounds are predicted to show an insulating G-type antiferromagnetic ground state, there is a considerable diffe…
We theoretically study the anomalous Hall effect (AHE) in perovskites with antiferromagnetic (AFM) orderings. By studying the multiorbital Hubbard model for $d$ electrons in perovskite transition metal oxides under the GdFeO$_3$-type distortion within the Hartree-Fock approximation, we investigate the behavior of intrinsic AHE owing to the atomic spin-orbit coupling via the linear response theory. We consider the cases where there exist two ($d^2$) and three ($d^3$) electrons in the $t_{2g}$ orbitals, and show that AFM ordered states can exhibit AHE. In the $d^2$ case, $C$-type AFM states give…
The polarized Raman spectra of orthorhombic RMnO3 series (R=La,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Y) were studied at room temperature. The variation of phonon frequencies with R ionic radius rR as a whole confirms the commonly accepted Raman line assignments with two noticeable exceptions: (1) with decreasing rR the stretching Ag(1) and bending Ag(3) modes strongly mix for R=Sm to Tb, while for further decrease or rR (R=Dy,Ho,Y) the Ag(3) mode is observed at higher frequency than Ag(1) mode; (2) similar distortion-dependent mode mixing takes place for the rotational Ag(2) and O1(x) [Ag(7)] modes. The mod…
KCrF3 represents another prototypical orbital-ordered perovskite, where Cr2+ possesses the same electronic configuration of 3d4 as that of strongly Jahn-Teller distorted Mn3+ in many CMR manganites. The crystal and magnetic structures of KCrF3 compound are investigated by using polarized and unpolarized neutron powder diffraction methods. The results show that the KCrF3 compound crystallizes in tetragonal structure at room temperature and undergoes a monoclinic distortion with the decrease in temperature. The distortion of the crystal structure indicates the presence of cooperative Jahn-Teller…
We compute the zero temperature phase diagram and the Hall response of the doped perovskite manganites within the model of double exchange and Jahn-Teller coupling, employing the large dimensionality approximation proposed by Millis et al. We find that in this scenario a ``hole - like'' $R_H$ for the hole doped manganites, as observed at low temperature by Matl et al., can be obtained only when lattice distortions persist in the metallic state at zero temperature. The calculated temperature dependence of $R_H$ seems to be consistent with the measured ``normal'' Hall coefficient in thin films.
Several puzzling aspects of interplay of the experimental lattice distortion and the the magnetic properties of four narrow $t_{2g}$-band perovskite oxides (YTiO$_3$, LaTiO$_3$, YVO$_3$, and LaVO$_3$) are clarified using results of first-principles electronic structure calculations. First, we derive parameters of the effective Hubbard-type Hamiltonian for the isolated $t_{2g}$ bands using newly developed downfolding method for the kinetic-energy part and a hybrid approach, based on the combination of the random-phase approximation and the constraint local-density approximation, for the screene…
We introduce an effective model for $e_g$ electrons to describe three-dimensional perovskite (La$_{1-x}$Sr$_{x}$MnO$_3$ and La$_{1-x}$Ca$_{x}$MnO$_3$) manganites and study the magnetic and orbital order on $4\times 4\times 4$ cluster using correlated wave functions. The model includes the kinetic energy, and on-site Coulomb interactions for $e_g$ electrons, antiferromagnetic superexchange interaction between $S=3/2$ core spins, and the coupling between $e_g$ electrons and Jahn-Teller modes. The model reproduces the experimentally observed magnetic order: ($i$) $A$-type antiferromagnetic phase …
The temperature dependence of the O K-edge pre-edge structure in the x-ray absorption spectra of the perovskites La(1-x)A(x)MnO(3), (A = Ca, Sr; x = 0.3, 0.4) reveals a correlation between the disappearance of the splitting in the pre-edge region and the presence of Jahn-Teller distortions. The different magnitudes of the distortions for different compounds is proposed to explain some dissimilarity in the line shape of the spectra taken above the Curie temperature.
We introduce a microscopic model which unravels the physical mechanisms responsible for the observed phase diagram of the $R$VO$_3$ perovskites. It reveals a nontrivial interplay between superexchange, the orbital-lattice coupling due to the GdFeO$_3$-like rotations of the VO$_6$ octahedra, and orthorhombic lattice distortions. We find that the lattice strain affects the onset of the magnetic and orbital order by partial suppression of orbital fluctuations. The present approach provides also a natural explanation of the observed reduction of magnon energies from LaVO$_3$ to YVO$_3$.
Our detailed temperature dependent synchrotron powder x-ray diffraction studies along with first-principles density functional perturbation theory calculations, enable us to shed light on the origin of ferroelectricity in GdCrO3. The actual lattice symmetry is found to be noncentrosymmetric orthorhombic Pna21 structure, sup- porting polar nature of the system. Polar distortion is driven by local symmetry breaking and by local distortions dominated by Gd off-centering. Our study reveals an intimate analogy between GdCrO3 and YCrO3. However, a distinctive difference exists that Gd is less displa…
Using $t_{2g}$ Wannier-functions, a low-energy Hamiltonian is derived for orthorhombic $3d^{1}$ transition-metal oxides. Electronic correlations are treated with a new implementation of dynamical mean-field theory for non-cubic systems. Good agreement with photoemission data is obtained. The interplay of correlation effects and cation covalency (GdFeO$_{3}$-type distortions) is found to suppress orbital fluctuations in LaTiO$_{3},$ and even more in YTiO$_{3}$, and to favor the transition to the insulating state.
We use the LDA+U method to study the possibility of exciton condensation in perovskites of transition metals with $d^6$ electronic configuration such as LaCoO$_3$. For realistic interaction parameters we find several distinct solutions exhibiting spin-triplet exciton condensate, which gives rise to a local spin density distribution while the ordered moments are vanishingly small. Rhombohedral distortion from the ideal cubic structure suppresses the ordered state, contrary to the spin-orbit coupling which enhances the excitonic condensation energy. We explain the trends observed in the numerica…
Properties and mechanism of the magnetic phase transition of the perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle distortion, are studied theoretically by using the effective spin and pseudo-spin Hamiltonian with strong Coulomb repulsion. It is shown that the A-type antiferromagnetic(AFM(A)) to ferromagnetic(FM) phase transition occurs as the Ti-O-Ti bond angle is decreased. Through this phase transition, the orbital state is hardly changed so that the spin-exchange coupling along the c-axis changes nearly continuously from positive to negative and takes approximately zero …
In this work, we investigate the local structural distortion of the double perovskite Ba$_2$YMoO$_6$ by means of infrared transmission spectroscopy. At 300 K, three bands are observed at $\sim$ 255.1 cm$^{-1}$, $\sim$ 343.4 cm$^{-1}$, and $\sim$ 561.5 cm$^{-1}$, which are related to the motion between the cation Ba$^{2+}$ and the anion YMO$_6^{-2}$, the Y-O stretching motion and the stretching vibration of the MoO$_6$ octahedron, respectively. These modes continue to harden upon cooling owing to the shrink of the lattice constant. When the temperature decreases to $T \leq$ 130 K around which t…
We theoretically show that materials with perovskite-type crystal structures provide a platform for spin current generation, taking advantage of a mechanism requiring neither the spin-orbit coupling nor a ferromagnetic moment, but is based on spin-split band structures in certain kinds of collinear antiferromagnetic states. By investigating a multiband Hubbard model for transition metal compounds by means of the Hartree-Fock approximation and the Boltzmann transport theory, we find that a pure spin current is induced by an electric field applied to a C-type antiferromagnetic metallic phase. Th…
Normally $4d$ or $5d$ transition metals are in a low-spin state. Here using first-principles calculations, we report on a rare case of a high-spin $S$=1 magnetic state for the Ag$^{3+}$ ion in the double perovskite Cs$_{2}$KAgF$_{6}$. We also explored a possibility of a conventional low-spin $S$=0 ground state and find an associated tetragonal distortion to be 0.29 Å. However, the lattice elastic energy cost and the Hund exchange loss exceed the e$_{g}$ crystal-field energy gain, thus making the low-spin tetragonal structure less favorable than the high-spin cubic structure. We conclude that t…
Using results of the band structure calculations in the local-spin-density approximation we demonstrate how the crystal distortions affect the magnetic structure of orthorhombically distorted perovskites leading to a non-collinear spin arrangement. Our results suggest that the non-collinearity of the spin magnetic moments, being generally small in La$M$O$_3$ series with $M$=Cr-Fe, is large in SrRuO$_3$.
"Realistic modeling" is a new direction of electronic structure calculations, where the main emphasis is made on the construction of some effective low-energy model entirely within a first-principle framework. Ideally, it is a model in form, but with all the parameters derived rigorously, on the basis of first-principles electronic structure calculations. The method is especially suit for transition-metal oxides and other strongly correlated systems, whose electronic and magnetic properties are predetermined by the behavior of some limited number of states located near the Fermi level. After r…
Orthorhombically distorted perovskite manganites, RMnO3 with R being a trivalent rare-earth ion, exhibit a variety of magnetic and electric phases including multiferroic (i.e. concurrently magnetic and ferroelectric) phases and fascinating magnetoelectric phenomena. We theoretically study the phase diagram of RMnO3 by constructing a microscopic spin model, which includes not only the superexchange interaction but also the single-ion anisotropy (SIA) and the Dzyaloshinsky-Moriya interaction (DMI). Analysis of this model using the Monte-Carlo method reproduces the experimental phase diagrams as …
The properties of Mott insulators with orbital degrees of freedom are described by spin-orbital superexchange models, which provide a theoretical framework for understanding their magnetic and optical properties. We introduce such a model derived for $(xy)^1(yz/zx)^1$ configuration of V$^{3+}$ ions in the $R$VO$_3$ perovskites, $R$=Lu,Yb,$\cdots$,La, and demonstrate that $\{yz,zx\}$ orbital fluctuations along the $c$ axis are responsible for the huge magnetic and optical anisotropies observed in the almost perfectly cubic compound LaVO$_3$. We argue that the GdFeO$_3$ distortion and the large …
High pressure x-ray diffraction (XRD) measurements on RMnO3 (R=Dy, Ho and Lu) reveals that varying structural changes occurs for different R ions. Large lattice changes (orthorhombic strain) occur in DyMnO3 and HoMnO3 while the Jahn-Teller (JT) distortion remains stable. On the other hand, in LuMnO3, Mn-O bond distortions are observed in the region 4-8 GPa with the broad minimum in the JT distortion. High pressure IR measurements indicate that a phonon near 390 cm-1 corresponding to the complex motion of the Mn and O ions changes anomalously for LuMnO3. It softens in the 4-8 GPa region, which …
The Jahn-Teller (JT) distortion that can remove electronic degeneracies in partially occupied states and results in systematic atomic displacements is a common underlying feature to many of the intriguing phenomena observed in 3d perovskites, encompassing magnetism, superconductivity, orbital ordering and colossal magnetoresistance. Although the seminal Jahn and Teller theorem has been postulated almost a century ago, the origins of this effect in perovskite materials are still debated, including propositions such as super exchange, spin-phonon coupling, sterically induced lattice distortions,…
Ferromagnetism and its evolution in the orthorhombic perovskite system Sr1-xCaxRuO3 have been widely believed to correlate with the structural distortion. The recent development of high-pressure synthesis of the Ba substituted Sr1-yBayRuO3 makes it possible to study ferromagnetism over a broader phase diagram, which includes new orthorhombic Imma and the cubic phases. However, the chemical substitutions introduce the A-site disorder effect on Tc, which complicates determination of the relationship between ferromagnetism and the structural distortion. By clarifying the site disorder effect on T…
Using density-functional theory calculations, we investigated the electronic structure and magnetic exchange interactions of the ordered 3d-5d double perovskite Sr2FeOsO6, which has recently drawn attention for interesting antiferromagnetic transitions. Our study reveals the vital role played by long-range magnetic exchange interactions in this compound. The competition between the ferromagnetic nearest neighbor Os-O-Fe interaction and antiferromagnetic next nearest neighbor Os-O-Fe-O-Os interaction induces strong frustration in this system, which explains the lattice distortion and magnetic p…
We have studied the interplay between orbital ordering, Jahn-Teller and GdFeO3-type lattice distortions in perovskite-type transition-metal oxides using model Hartree-Fock calculations. It has been found that the covalency between A-site cations and oxygens causes interaction between the Jahn-Teller and GdFeO3-type distortions. The present calculations explain why the d-type Jahn-Teller distortion and orbital ordering compatible with it are realized in LaMnO3, YVO3 and YTiO3
LaMnO$_3$ is considered as a prototypical Jahn-Teller perovskite compound, exhibiting a metal to insulator transition at $T_{JT} = 750K$ related to the joint appearance of an electronic orbital ordering and a large lattice Jahn-Teller distortion. From first-principles, we revisit the behavior of LaMnO$_3$ and show that it is not only prone to orbital ordering but also to charge ordering. Both charge and orbital orderings appear to be enabled by rotations of the oxygen octahedra and the subtle competition between them is monitored by a large tetragonal compressive strain, that is itself a Jahn-…
