Strong Orbital Fluctuations Research Articles

Quantum Liquid with Strong Orbital Fluctuations: The Case of a Pyroxene Family.

We discuss quasi-one-dimensional magnetic Mott insulators from the pyroxene family where spin and orbital degrees of freedom remain tightly bound. We analyze their excitation spectrum and outline the conditions under which the orbital degrees of freedom become liberated so that the corresponding excitations become dispersive and the spectral weight shifts to energies much smaller than the exchange integral.

Pairing mechanism for nodal s -wave superconductivity in BaFe2(As,P)2 : Analysis beyond Migdal-Eliashberg formalism

The pairing mechanism and gap structure in Ba122 pnictides have been hotly discussed for long time as one of the central issues in Fe-based superconductors. Here, we attack this problem by taking account of the vertex corrections (VCs) for the Coulomb interaction $U$ ($U$-VCs), which are totally dropped in conventional Migdal-Eliashberg formalism. The $U$-VC in the charge susceptibility induces strong orbital fluctuations, and the $U$-VC also enlarges the orbital-fluctuation-mediated attractive interaction. By analyzing the effective multiorbital Hubbard model for Ba122 pnictides, we find that the orbital fluctuations develop in all four $d$-orbitals ($t_{2g}$- and $z^2$-orbitals), by which the FSs are composed. For this reason, nearly isotropic gap function appears on all the hole-type FSs, including the outer hole-FS around Z-point composed of $z^2$-orbital. In contrast, nodal gap structure appears on the electron-type FSs for wide parameter range. The obtained nodal $s$-wave state changes to fully-gapped $s$-wave state without sign-reversal ($s_{++}$-wave state) by introducing small amount of impurities, accompanied by small reduction in $T_c$. The present microscopic theory naturally explains the important characteristics of the gap structure of both hole- and electron-FSs in Ba122 pnictides, without introducing any phenomenological pairing interaction.

Strong orbital fluctuations in multipolar ordered states of PrV2Al20

PrT2Al20 (T=Ti, V) are ideal systems to study the quadrupole Kondo effect and quantum criticality arising from orbital degrees of freedom. The both systems have the nonmagnetic cubic Γ3 crystal electric field ground doublet with the well separated excited state. In particular, PrV2Al20 exhibits anomalous metallic behavior above and below the multipolar ordering temperatures, reflecting the even stronger hybridization between f and conduction electrons possibly due to a proximity to an orbital quantum critical point. Here we report the anomalous metallic behaviors found in a pure single crystal of PrV2Al20. Our detailed analyses revealed that the resistivity indicates power law temperature dependence proportional to T3. Furthermore, we pointed out that the 4f electron contribution to the specific heat also exhibits power law behavior proportional to T4. Both observations are in a sharp contrast to the gapped behavior found in PrTi2Al20 and indicate the strong c–f hybridization and strong orbital fluctuations in PrV2Al20. In addition, the 4f electron contribution to the entropy in PrV2Al20 reaches only 50% of Rln2 at an orbital ordering at T=0.75K, suggesting another 50% of Rln2 expected for Γ3 doublet is already released at higher temperature possibly due to quadrupole Kondo effect.

Pressure-Induced Heavy Fermion Superconductivity in the Nonmagnetic Quadrupolar SystemPrTi2Al20

We report the discovery of a pressure-induced heavy fermion superconductivity in a nonmagnetic orbital ordering state in the cubic compound PrTi(2)Al(20). In particular, we found that the transition temperature and the effective mass associated with the superconductivity are dramatically enhanced as the system approaches the putative quantum critical point of the orbital order. Our experiment indicates that the strong orbital fluctuations may provide a nonmagnetic glue for Cooper pairing.

Probing orbital fluctuations inRVO3(R=Y, Gd, or Ce) by ellipsometry

We study optical excitations across the Mott gap in the multiorbital Mott-Hubbard insulators $R$VO${}_{3}$. The multipeak structure observed in the optical conductivity can be described consistently in terms of the different $3{d}^{3}$ multiplets or upper Hubbard bands. The spectral weight is very sensitive to nearest-neighbor spin-spin and orbital-orbital correlations and thus shows a pronounced dependence on both temperature and polarization. Comparison with theoretical predictions based on either rigid orbital order or strong orbital fluctuations clearly rules out that orbital fluctuations are strong in $R$VO${}_{3}$. Both the line shape and the temperature dependence give clear evidence for the importance of excitonic effects.

Impurity-induced electronic nematic state andC2-symmetric nanostructures in iron pnictide superconductors

We propose that an impurity-induced electronic nematic state is realized above the orthorhombic structure transition temperature ${T}_{S}$ in iron-pnictide superconductors. In the presence of strong orbital fluctuations near ${T}_{S}$, it is theoretically revealed that a single impurity induces local orbital order with ${C}_{2}$ symmetry, consistently with recent Scanning Tunneling Microscopy/Spectroscopy (STM/STS) measurements. Each impurity-induced ${C}_{2}$-symmetric nanostructure aligns along the $a$ axis by applying tiny uniaxial pressure along the $b$ axis. In this impurity-induced nematic phase, the resistivity shows sizable in-plane anisotropy (${\ensuremath{\rho}}_{b}/{\ensuremath{\rho}}_{a}\ensuremath{\sim}2$) even above ${T}_{S}$, actually observed in various ``detwinned'' samples. The present study indicates the existence of strong orbital fluctuations in iron-pnictide superconductors.

Orbital fluctuations, spin-orbital coupling, and anomalous magnon softening in an orbitally degenerate ferromagnet

The correlated motion of electrons in the presence of strong orbital fluctuations and correlations is investigated with respect to magnetic couplings and excitations in an orbitally degenerate ferromagnet. Introduction of the orbital degree of freedom results in a class of diagrams representing spin-orbital coupling which become particularly important near the orbital ordering instability. Low-energy staggered orbital fluctuation modes, particularly with momentum near (\pi/2,\pi/2,0) (corresponding to CE-type orbital correlations), are shown to generically yield strong intrinsically non-Heisenberg (1-\cos q)^2 magnon self energy correction, resulting in no spin stiffness reduction, but strongly suppressed zone-boundary magnon energies in the Gamma-X direction. The zone-boundary magnon softening is found to be strongly enhanced with increasing hole doping and for narrow-band materials, which provides insight into the origin of zone-boundary anomalies observed in ferromagnetic manganites.

Theory of magnetic resonance as an orbital state probe

It is demonstrated that magnetic resonance in a magnetically ordered state is a sensitive indirect method for the investigation of the orbital ground state. This idea is illustrated for two perovskite titanates: ${\text{LaTiO}}_{3}$ and ${\text{YTiO}}_{3}$. In contrast to the spin-wave energy spectra, antiferromagnetic resonance spectra in an external magnetic field reveal clear dependence on the orbital state and it can distinguish the state with strong orbital fluctuations from the static orbital order. Our theoretical analysis is based on the model, which explicitly takes into consideration the strong correlation among lattice, orbital, and magnetic degrees of freedom.

Competition between Jahn-Teller coupling and orbital fluctuations inHoVO3

We have carried out a detailed study of the structural properties of ${\text{HoVO}}_{3}$ perovskite using a combination of single-crystal neutron diffraction and synchrotron x-ray and neutron powder diffraction. We focus on the competition between one-dimensional fluctuations of the occupied vanadium $d$ orbitals and coherent Jahn-Teller distortion due to long-range orbital ordering. At room temperature orbital fluctuations are dominant. Below 188 K a structural phase transition from orthorhombic $Pbnm$ to monoclinic $Pb11$ symmetry takes place, corresponding to a state where strong orbital fluctuations are superimposed on an underlying orbitally ordered state. However, the fluctuations are not strong enough to give rise to a long-range orbitally dimerized state as theoretically predicted. Ordering of the vanadium spins at 114 K has little effect on the orbital fluctuations, but the orbital ordering becomes coherent below a first-order transition to an orthorhombic $Pbnm$ phase at 40 K.

Subband Filling and Mott Transition inCa2−xSrxRuO4

A new concept is proposed for the paramagnetic metal insulator transition in the layer perovskite Ca2-xSrxRuO4. Whereas the pure Sr compound is metallic up to large Coulomb energies due to strong orbital fluctuations, structural changes induced by doping with Ca give rise to an interorbital charge transfer which makes the material extremely sensitive to local correlations. Using dynamical mean field theory based on finite temperature multiband exact diagonalization, it is shown that the combination of crystal field splitting and on-site Coulomb interactions leads to complete filling of the d(xy) band and to a Mott transition in the half-filled d(xz,yz) bands.

Zero-Field Incommensurate Spin-Peierls Phase with Interchain Frustration in TiOCl

We report on the magnetic, thermodynamic, and optical properties of the quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been discussed as spin-Peierls compounds. The observed deviations from canonical spin-Peierls behavior, e.g., the existence of two distinct phase transitions, have been attributed previously to strong orbital fluctuations. This can be ruled out by our optical data of the orbital excitations. We show that the frustration of the interchain interactions in the bilayer structure gives rise to incommensurate order with a subsequent lock-in transition to a commensurate dimerized state. In this way, a single driving force, the spin-Peierls mechanism, induces two separate transitions.

NMR as a local probe of magnetic anisotropy: The possibility of orbital ordering and orbital liquid states in colossal magnetoresistance manganites

${}^{139}\mathrm{La}$ rf enhancement experiments are shown to be an excellent tool for detecting changes of the local magnetic anisotropy in colossal magnetoresistance manganites. Based on this, we discuss the possibility of orbital ordering in ferromagnetic (FM) ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3},$ and provide evidence about the formation of an unusual kind of phase separation in the low temperature regime of the FM conductive (FMC) phase for $0.25l~xl0.5.$ We show that by lowering temperature, an extra NMR signal is formed in the FMC phase, which originates from regions with vanishingly small magnetic anisotropy. We anticipate that this feature is due to the formation of regions with strong orbital fluctuations within an orbitally frozen matrix.

Spin dynamics and orbital state in LaTiO3

A neutron scattering study of the Mott-Hubbard insulator LaTiO3 ( T(N) = 132 K) reveals a spin wave spectrum that is well described by a nearest-neighbor superexchange constant J = 15.5 meV and a small Dzyaloshinskii-Moriya interaction ( D = 1.1 meV). The nearly isotropic spin wave spectrum is surprising in view of the absence of a static Jahn-Teller distortion that could quench the orbital angular momentum, and it may indicate strong orbital fluctuations. A resonant x-ray scattering study has uncovered no evidence of orbital order in LaTiO3.