Orbital Fluctuations in the Different Phases ofLaVO3andYVO3

Abstract

We investigate the importance of quantum orbital fluctuations in the orthorhombic and monoclinic phases of the Mott insulators LaVO3 and YVO3. First, we construct ab-initio material-specific t2g Hubbard models. Then, by using dynamical mean-field theory, we calculate the spectral matrix as a function of temperature. Our Hubbard bands and Mott gaps are in very good agreement with spectroscopy. We show that in orthorhombic LaVO3, quantum orbital fluctuations are strong and that they are suppressed only in the monoclinic 140 K phase. In YVO3 the suppression happens already at 300 K. We show that Jahn-Teller and GdFeO3-type distortions are both crucial in determining the type of orbital and magnetic order in the low temperature phases. The Mott insulating t 2 2g perovskites LaVO 3 and YVO 3 exhibit an unusual series of structural and magnetic phase transitions ( Fig. 1) with temperature-induced magnetization reversal phenomena [1] and other exotic properties [2,3]. While it is now recognized that the V-t 2g orbital degrees of freedom and the strong Coulomb repulsion are the key ingredients, it is still controversial whether classical (orbital order) [1,4,5,6,7,8] or quantum (orbital fluctuations) [2, 9] effects are responsible for the rich physics of these vanadates.At 300 K, LaVO 3 and YVO 3 are orthorhombic paramagnetic Mott insulators. Their structure (Fig. 2) can be derived from the cubic perovskite ABO 3 , with A=La,Y and B=V, by tilting the VO 6 octahedra in alternating directions around the b-axis and rotating them around the c-axis. This GdFeO 3 -type distortion is driven by AO covalency which pulls a given O atom closer to one of its four nearest A-neighbors [10,11]. Since the Y 4d level is closer to the O 2p level than the La 5d level, the AO covalency increases when going from LaVO 3 to YVO 3 and, hence, the shortest AO distance decreases from being 14 to being 20 % shorter than the average, while the angle of tilt increases from 12 to 18 0 , and that of rotation from 7 to 13 0 [12,13]. Finally, the A-cube is deformed such that one or two of the ABA body-diagonals is smaller than the average by, respectively, 4 and 10 % in LaVO 3 and YVO 3 . These 300 K structures are determined mainly by the strong covalent interactions between O 2p and the empty B e g and A d orbitals, hardly by the weak interactions involving B t 2g orbitals, and are thus very similar to the structures of the t 1 2g La and Y titanates [10,11].

Orbital superexchange and crystal field simultaneously at play in YVO3: Resonant inelastic x-ray scattering at the VLedge and the OKedge

“…These are further split by the deviations from cubic symmetry. 21,27,[54][55][56] In cubic approximation, the lowest-lying two-electron state shows . 57 The cubic approximation is sufficient for the overall assignment of all RIXS peaks observed above 1 eV at the V L 3 edge.…”

“…For RVO 3 with t 2 2g electron configuration, it has been suggested that orbital quantum fluctuations may be comparatively strong because superexchange interactions between t 2g electrons are frustrated on a cubic or nearly cubic lattice. 3,5,[27][28][29] From magnetic neutron scattering, indications for highly unusual orbital correlations were found and the existence of an orbital Peierls state in the intermediate phase of YVO 3 has been proposed. 3 However, a recent study of the optical excitations across the Mott-Hubbard gap rules out that orbital fluctuations are strong in RVO 3 .…”

Multisite versus multiorbital Coulomb correlations studied within finite-temperature exact diagonalization dynamical mean-field theory

“…9 Also, the Mott transition in LaVO 3 and YVO 3 occurs before orbital polarization is complete. 10 Moreover, in a hypothetical tetragonal structure of LaTiO 3 , relevant for heterostructures, the Mott phase is reached when n xz,yz approaches 1/4 and n xy vanishes. 11 Finally, the possibility of so-called orbital-selective Mott transitions in multiband systems has been discussed extensively in the literature.…”

Unraveling Orbital Correlations with Magnetic Resonant Inelastic X-Ray Scattering

“…Yet, the precise nature of correlated orbital states, being either of ordered or liquid type, and their existence in different materials is intensely debated, which to a large part is due to the fact that orbital correlations turn out to be very difficult to detect experimentally. In fact, such experimental access would be of great help in unraveling the puzzling properties of many systems with orbital degrees of freedom, for instance the above mentioned iron-pnictide materials, where the type of the orbital ordering or its lack is heavily debated [5][6][7] or titanium and vanadium oxides where different theoretical scenarios -a rather exotic orbital liquid phase [8,9], or a classical alternating orbital-ordered state [10,11] have been proposed.…”