arXiv subjects

Naoya Iwahara

Publications and source records attributed to Naoya Iwahara.

34 records · Page 2Linked to original sources

arXiv2018

Spin-orbital-lattice entangled states in cubic $d^1$ double perovskites

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 induces a strong magneto-elastic response. This multiferroic effect is at the origin of the recently reported breaking of local point symmetry accompanying the development of magnetic ordering in Ba$_2$NaOsO$_6$.

cond-mat.str-el↗

arXiv2018

Quadratic Jahn-Teller effect of fullerene anions

The quadratic Jahn-Teller effect of C$_{60}^{n-}$ ($n=$ 1-5) is investigated from the first principles. Employing the density functional theory calculations with hybrid functional, the quadratic vibronic coupling constants of C$_{60}^-$ were derived. The warping of the adiabatic potential energy surface of C$_{60}^-$ by the quadratic vibronic coupling is estimated about 2 meV, which is much smaller than the Jahn-Teller stabilization energy ($\approx$ 50 meV). Because of the selection rule and the vibronic reduction, the quadratic coupling slightly modifies the vibronic states of C$_{60}$ anions. Particularly, in the case of C$_{60}^{3-}$, parity and symmetry selection rule significantly reduces the effect of quadratic coupling on vibronic states. The present results confirm that the low-energy vibronic dynamics of C$_{60}^{n-}$ is of pseudorotational type.

cond-mat.str-el↗

arXiv2018

Dynamical Jahn-Teller effect of fullerene anions

The dynamical Jahn-Teller effect of C$_{60}^{n-}$ anions ($n = $ 1-5) is studied using the numerical diagonalization of the linear $p^n \otimes 8d$ Jahn-Teller Hamiltonian with the currently established coupling parameters. It is found that in all anions the Jahn-Teller effect stabilizes the low-spin states, resulting in the violation of Hund's rule. The energy gain due to the Jahn-Teller dynamics is found to be comparable to the static Jahn-Teller stabilization. The Jahn-Teller dynamics influences the thermodynamic properties via strong variation of the density of vibronic states with energy. Thus, the large vibronic entropy in the low-spin states enhances the effective spin gap of C$_{60}^{3-}$ quenching the spin crossover. From the calculations of the effective spin gap in function of the Hund's rule coupling, we found that the latter should amount 40 $\pm$ 5 meV in order to cope with the violation of Hund's rule and to reproduce the large spin gap. With the obtained numerical solutions the matrix elements of electronic operators for the low-lying vibronic levels and the vibronic reduction factors are calculated for all anions.

cond-mat.mtrl-sci↗

arXiv2018

Interplay of spin-dependent delocalization and magnetic anisotropy in the ground and excited states of [Gd$_2$@C$_{78}$]$^{-1}$ and [Gd$_2$@C$_{80}$]$^{-1}$

The magnetic properties and electronic structure of the ground and excited states of two recently characterized endohedral metallo-fullerenes, [Gd$_2$@C$_{78}$]$^{-}$ (1) and [Gd$_2$@C$_{80}$]$^{-}$ (2), have been studied by theoretical methods. The systems can be considered as [Gd$_2$]$^{5+}$ dimers encapsulated in a fullerene cage with the fifteen unpaired electrons ferromagnetically coupled into an $S=15/2$ high-spin configuration in the ground state. The microscopic mechanisms governing the Gd--Gd interactions leading to the ferromagnetic ground state are examined by a combination of density functional and ab initio calculations and the full energy spectrum of the ground and lowest excited states is constructed by means of ab initio model Hamiltonians. The ground state is characterized by strong electron delocalization bordering on a $σ$ type one-electron covalent bond and minor zero-field splitting (ZFS) which is successfully described as a second order spin-orbit coupling effect. We have shown that the observed ferromagnetic interaction originates from Hund's rule coupling and not from the conventional double exchange mechanism. The calculated ZFS parameters of 1 and 2 in their optimized geometries are in qualitative agreement with experimental EPR results. The higher excited states display less electron delocalization but at the same time they possess unquenched first-order angular momentum. This leads to strong spin-orbit coupling and highly anisotropic energy spectrum. The analysis of the excited states presented here constitutes the first detailed study of the effects of spin-dependent delocalization in the presence of first order orbital angular momentum and the obtained results can be applied to other mixed valence lanthanide systems.

physics.chem-ph↗

arXiv2018

Berry phase of adiabatic electronic configurations in fullerene anions

The selection rule on vibronic angular momentum of $t_{1u}^n \otimes h_g$ Jahn-Teller problem ($n = $ 1-5) is reinvestigated. It is shown that among three adiabatic orbitals only two have nonzero Berry phase. Thus, the Berry phase of adiabatic electronic configurations depends on the spin multiplicity as well as the number of electrons. On this basis, the general relation between the Berry phase and the angular momentum is described. It allows, in particular, to clarify the nature of vibronic states arising from high spin configurations. In comparison with the previous solution for the low-lying vibronic states for bimodal systems, the present solutions correctly fulfill all the symmetry requirement.

physics.chem-ph↗

arXiv2017

Zeeman interaction and Jahn-Teller effect in $Γ_8$ multiplet

We present a thorough analysis of the interplay of magnetic moment and the Jahn-Teller effect in the $Γ_8$ cubic multiplet. We find that in the presence of dynamical Jahn-Teller effect, the Zeeman interaction remains isotropic, whereas the $g$ and $G$ factors can change their signs. The static Jahn-Teller distortion also can change the sign of these $g$ factors as well as the nature of the magnetic anisotropy. Combining the theory with state-of-the-art {\it ab initio} calculations, we analyzed the magnetic properties of Np$^{4+}$ and Ir$^{4+}$ impurity ions in cubic environment. The calculated $g$ factors of Np$^{4+}$ impurity agree well with experimental data. The {\it ab initio} calculation predicts strong Jahn-Teller effect in Ir$^{4+}$ ion in cubic environment and the strong vibronic reduction of $g$ and $G$ factors.

cond-mat.mtrl-sci↗

arXiv2017

Vibronic coupling in C$_{60}^-$ anion revisited: Precise derivations from photoelectron spectra and DFT calculations

The vibronic coupling constants of C$_{60}^-$ are derived from the photoelectron spectrum measured by Wang {\it et al}. [X. B. Wang, H. K. Woo, and L. S. Wang, J. Chem. Phys., {\bf 123}, 051106 (2005).] at low temperature with high-resolutions. We find that the couplings of the Jahn-Teller modes of C$_{60}^-$ are weaker than the couplings reported by Gunnarsson {\it et al}. [O. Gunnarsson, H. Handschuh, P. S. Bechthold, B. Kessler, G. Gantef{ö}r, and W. Eberhardt, Phys. Rev. Lett., {\bf 74}, 1875 (1995).]. The total stabilization energy after $h_g$ and $a_g$ modes is reduced with respect to the previous derivation of Gunnarsson {\it et al}. by 30 \%. The computed vibronic coupling constants using DFT with B3LYP functional agree well with the new experimental constants, so the discrepancy between theory and experiment persistent in the previous studies is basically solved.

cond-mat.mtrl-sci↗

arXiv2017

Mechanisms of localization in isotope-substituted dynamical Jahn-Teller systems

The mechanisms of localization of Jahn-Teller deformations and vibronic wavefunctions in isotope substituted dynamical Jahn-Teller systems are elucidated. It is found that the localization in the trough is of potential type in the case of strong vibronic coupling, while it becomes of kinetic type in the case of intermediate and weak coupling. It is shown that the vibronic levels in the linear $E\otimes e$-problem remain double degenerate upon arbitrary isotope substitution on the reasons similar to time reversal symmetry in which the role of spin is played by orbital pseudospin.

physics.chem-ph↗

arXiv2016

Orbital disproportionation of electronic density - a universal feature of alkali-doped fullerides

Alkali-doped fullerides A$_n$C$_{60}$ show a remarkably wide range of electronic phases in function of A= Li, Na, K, Rb, Cs and the degree of doping, $n=1-5$. While the presence of strong electron correlations is well established, recent investigations give also evidence for dynamical Jahn-Teller instability in the insulating and the metallic phase of A$_3$C$_{60}$. To reveal the interplay of these interactions in fullerides with even $n$, we address the electronic phase of A$_4$C$_{60}$ with accurate many-body calculations within a realistic electronic model including all basic interactions extracted from first principles. We find that the Jahn-Teller instability is always realized in these materials too. More remarkably, in sharp contrast to strongly correlated A$_3$C$_{60}$, A$_4$C$_{60}$ displays uncorrelated band-insulating state despite pretty similar interactions present in both fullerides. Our results show that the Jahn-Teller instability and the accompanying orbital disproportionation of electronic density in the degenerate LUMO band is a universal feature of fullerides.

cond-mat.str-el↗

arXiv2016

Giant exchange interaction in mixed lanthanides

Combining strong magnetic anisotropy with strong exchange interaction is a long standing goal in the design of quantum magnets. The lanthanide complexes, while exhibiting a very strong ionic anisotropy, usually display a weak exchange coupling, amounting to only a few wavenumbers. Recently, an isostructural series of mixed Ln$^{3+}$-N$_2^{3-}$-Ln$^{3+}$ (Ln $=$ Gd, Tb, Dy, Ho, Er) have been reported, in which the exchange splitting is estimated to reach hundreds wavenumbers. The microscopic mechanism governing the unusual exchange interaction in these compounds is revealed here by combining detailed modeling with density-functional theory and {\it ab initio} calculations. We find it to be basically kinetic and highly complex, involving non-negligible contributions up to seventh power of total angular momentum of each lanthanide site. The performed analysis also elucidates the origin of magnetization blocking in these compounds. Contrary to general expectations the latter is not always favored by strong exchange interaction.

cond-mat.str-el↗

arXiv2016

New mechanism of kinetic exchange interaction induced by strong magnetic anisotropy

It is well known that the kinetic exchange interaction between single-occupied magnetic orbitals (s-s) is always antiferromagnetic, while between single- and double-occupied orbitals (s-d) is always ferromagnetic and much weaker. Here we show that the exchange interaction between strongly anisotropic doublets of lanthanides, actinides and transition metal ions with unquenched orbital momentum contains a new s-d kinetic contribution equal in strength with the s-s one. In non-collinear magnetic systems, this s-d kinetic mechanism can cause an overall ferromagnetic exchange interaction which can become very strong for transition metal ions. These findings are fully confirmed by DFT based analysis of exchange interaction in several Ln$^{3+}$ complexes.

cond-mat.mtrl-sci↗

arXiv2015

Ising exchange interaction in lanthanides and actinides

The Ising exchange interaction is a limiting case of strong exchange anisotropy and represents a key property of many magnetic materials. Here we find necessary and sufficient conditions to achieve Ising exchange interaction for metal sites with unquenched orbital moments. Contrary to current views, the rules established here narrow much the range of lanthanide and actinide ions which can exhibit Ising exchange interaction. It is shown that the arising Ising interaction can be of two distinct types: (i) coaxial, with magnetic moments directed along the anisotropy axes on the metal sites and (ii) non-coaxial, with arbitrary orientation of one of magnetic moments. These findings will contribute to purposeful design of lanthanide and actinide based materials.

cond-mat.str-el↗

arXiv2015

Exchange interaction between $J$-multiplets

Analytical expressions for the exchange interaction between $J$-multiplets of interacting metallic centers are derived on the basis of a complete electronic model. A common belief that this interaction can be approximated by an isotropic form $\propto {\mathbf{J}}_1\cdot{\mathbf{J}}_2$ (or $\propto {\mathbf{J}}_1\cdot{\mathbf{S}}_2$ in the case of interaction with an isotropic spin) is found to be ungrounded. It is also shown that the often used "1/U approximation" for the description of the kinetic contribution of the exchange interaction is not valid in the case of $J$-multiplets. The developed theory can be used for microscopic description of exchange interaction in materials containing lanthanides, actinides and some transition metal ions.

cond-mat.str-el↗

arXiv2015

Dynamical Jahn--Teller instability in metallic fullerides

Dynamical Jahn-Teller effect has escaped so far direct observation in metallic systems. It is particularly believed to be quenched also in correlated conductors with orbitally degenerate sites such as cubic fullerides. Here the Gutzwiller approach is extended to treat electron correlation over metals with Jahn-Teller active sites and applied to the investigation of the ground state of K$_3$C$_{60}$. It is shown that dynamical Jahn-Teller instability fully develops in this material when the interelectron repulsion $U$ on C$_{60}$ sites exceeds some critical value. The latter is found to be lower than the current estimates of $U$, meaning that dynamical Jahn-Teller effect takes place in all cubic fullerides. This leads to strong splitting of LUMO orbitals on C$_{60}$ sites and calls for reconsideration of the role of orbital degeneracy in the Mott-Hubbard transition in fullerides.

cond-mat.str-el↗

arXiv2015

Dynamical Jahn--Teller effect and antiferromagnetism in Cs$_3$C$_{60}$

The dynamical Jahn--Teller effect on fullerene sites in insulating Cs$_3$C$_{60}$ is investigated fully ab initio. The vibronic excitations of rotational type are at $\ge$65 cm$^{-1}$ while the net kinetic contribution to the Jahn--Teller stabilization energy constitutes ca 90 meV. This means that no localization of distortions by intermolecular interactions is possible in these fullerides, therefore, free rotations of deformations take place independently on each C$_{60}$. The latter destroy the orbital ordering and establish a conventional exchange interaction between $S=1/2$ on fullerene sites. The corresponding exchange model is derived and predicts the Néel temperature for A15 Cs$_3$C$_{60}$ close to experiment.

cond-mat.str-el↗

arXiv2012

Critical Reinvestigation on Vibronic Couplings in Picene from View of Vibronic Coupling Density Analysis

Vibronic coupling constants in the monoanionic, trianionic, and excited states of picene are evaluated from the total energy gradients using the density functional theory. Employing the calculated vibronic coupling constants in the excited state of the neutral molecule, electron energy loss spectrum (EELS) is simulated to be compared with the experimental spectrum. The calculated vibronic coupling constants are analyzed in terms of the vibronic coupling density which enables us to analyze vibronic couplings based on the relation between the electronic and vibrational structures. The vibronic coupling constants reported by Kato et al. [J. Chem. Phys. 116, 3420 (2002) and Phys. Rev. Lett. 107, 077001 (2011)] are critically discussed based on the vibronic coupling density analysis.

physics.chem-ph↗