Orbital Ordering Phenomena in D‐ and F‐Electron Systems

Hotta, Takashi

doi:10.1002/chin.200719205

Cited by 9 publications

(10 citation statements)

References 253 publications

(415 reference statements)

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“…Also in 4d materials spin-orbital physics plays a role [61], as for instance in Ca 2−x Sr x RuO 4 systems with Ru 4+ ions in 4d 4 configuration [62][63][64][65][66]. Recently it has been shown that unconventional magnetism is possible for Ru 4+ and similar ions where spin-orbit coupling plaus a role [67,68].…”

Section: Introductionmentioning

confidence: 99%

Spin-Orbital Order Modified by Orbital Dilution in Transition-Metal Oxides: From Spin Defects to Frustrated Spins Polarizing Host Orbitals

2015

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We investigate the changes in spin and orbital patterns induced by magnetic transition metal ions without an orbital degree of freedom doped in a strongly correlated insulator with spin-orbital order. In this context we study the 3d ion substitution in 4d transition metal oxides in the case of 3d 3 doping at either 3d 2 or 4d 4 sites which realizes orbital dilution in a Mott insulator. Although we concentrate on this doping case as it is known experimentally and more challenging than other oxides due to finite spin-orbit coupling, the conclusions are more general. We derive the effective 3d−4d (or 3d − 3d) superexchange in a Mott insulator with different ionic valencies, underlining the emerging structure of the spin-orbital coupling between the impurity and the host sites and demonstrate that it is qualitatively different from that encountered in the host itself. This derivation shows that the interaction between the host and the impurity depends in a crucial way on the type of doubly occupied t2g orbital. One finds that in some cases, due to the quench of the orbital degree of freedom at the 3d impurity, the spin and orbital order within the host is drastically modified by doping. The impurity acts either as a spin defect accompanied by an orbital vacancy in the spinorbital structure when the host-impurity coupling is weak, or it favors doubly occupied active orbitals (orbital polarons) along the 3d−4d bond leading to antiferromagnetic or ferromagnetic spin coupling. This competition between different magnetic couplings leads to quite different ground states. In particular, for the case of a finite and periodic 3d atom substitution, it leads to striped patterns either with alternating ferromagnetic/antiferromagnetic domains or with islands of saturated ferromagnetic order. We find that magnetic frustration and spin degeneracy can be lifted by the quantum orbital flips of the host but they are robust in special regions of the incommensurate phase diagram. Orbital quantum fluctuations modify quantitatively spin-orbital order imposed by superexchange. In contrast, the spin-orbit coupling can lead to anisotropic spin and orbital patterns along the symmetry directions and cause a radical modification of the order imposed by the spin-orbital superexchange. Our findings are expected to be of importance for future theoretical understanding of experimental results for 4d transition metal oxides doped with 3d 3 ions. We suggest how the local or global changes of the spin-orbital order induced by such impurities could be detected experimentally.

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Section: Introductionmentioning

confidence: 99%

Spin-Orbital Order Modified by Orbital Dilution in Transition-Metal Oxides: From Spin Defects to Frustrated Spins Polarizing Host Orbitals

2015

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Fermi-surface instability at the ‘hidden-order’ transition of URu2Si2

et al. 2009

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1 Solids with strong electron correlations generally develop exotic phases of electron matter at low temperatures [1,2,3,4,5]. Among such systems, the heavyfermion semi-metal URu 2 Si 2 presents an enigmatic transition at T o = 17.5 K to a 'hidden order' state whose order parameter remains unknown after 23 years of intense research [6,7]. Various experiments point to the reconstruction and partial gapping of the Fermi surface when the hidden-order establishes [8,9,10,11,12,13,14,15,16,17,18]. However, up to now, the question of how this transition affects the electronic spectrum at the Fermi surface has not been directly addressed by a spectroscopic probe. Here we show, using angleresolved photoemission spectroscopy, that a band of heavy quasi-particles drops below the Fermi level upon the transition to the hidden-order state. Our data provide the first direct evidence of a large reorganization of the electronic structure across the Fermi surface of URu 2 Si 2 occurring during this transition, and unveil a new kind of Fermi-surface instability in correlated electron systems.Earlier angle-resolved photoemission spectroscopy (ARPES) experiments mapped the basic band structure of URu 2 Si 2 in the paramagnetic state (above T o ), establishing the existence of hole-pockets at the Γ, Z and X points of the Brillouin zone [19,20,21]. These experiments revealed strong disagreements with the calculations for the electronic structure and Fermi surface of URu 2 Si 2 . It was speculated that this was due to the presence of narrow features from the U-5f states, not taken into account by the calculations, and difficult to characterize experimentally with the resolutions available at the time [21]. To date, no reports exist of high-resolution ARPES experiments below or across T o . The pressing question is to determine experimentally the electronic structure near the Fermi level (E F ), inlcuding the heavy 5f states, above and below T o . Figure 1 summarizes our findings for the temperature evolution of the electronic structure near E F . Figure 1a shows the angle-integrated spectra of electrons with k , the momentum component parallel to the sample surface, along the (110) direction at two temperatures across the transition. At T = 26 K, the only apparent feature is a surface state (SS) at binding energies E B < −35 meV, observed at all the investigated temperatures (see Supple-mentary Material). In contrast, at 13 K a narrow peak at E B ≈ −7 meV appears, signaling the presence of a quasi-particle (QP) band. The temperature dependence of this QP band was systematically studied, and is shown in Figures 1b-d. In these figures we normalized 2 the spectra by the Fermi-Dirac distribution, following a well established procedure [22], to reveal the thermally occupied part of the spectral function up to energies ∼ 5k B T above E F .The angle-integrated data of Fig. 1b shows that at 26 K the QP band lies at E B ≈ 5 meV, at 18 K ≈ T o it appears right at E F , and below T o the band shifts to energies below E F . At 10 K the QP peak is ...

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“…As pointed in many papers on oxides 38 , the effective Hamiltonian for obtaining the phase diagram is given by: In the above equation, the electron-phonon coupling term contains breathing mode and Jahn-Teller modes (i.e., modes corresponding to optical phonons) because they correspond to the dominant electron-phonon interactions. Here, since we deal with a single e g electron, the electron-electron-interaction term is not relevant.…”

Section: Discussionmentioning

confidence: 99%

Temperature dependence of long coherence times of oxide charge qubits

Dey

Yarlagadda

2018

Sci Rep

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The ability to maintain coherence and control in a qubit is a major requirement for quantum computation. We show theoretically that long coherence times can be achieved at easily accessible temperatures (such as boiling point of liquid helium) in small (i.e., ~10 nanometers) charge qubits of oxide double quantum dots when only optical phonons are the source of decoherence. In the regime of strong electron-phonon coupling and in the non-adiabatic region, we employ a duality transformation to make the problem tractable and analyze the dynamics through a non-Markovian quantum master equation. We find that the system decoheres after a long time, despite the fact that no energy is exchanged with the bath. Detuning the dots to a fraction of the optical phonon energy, increasing the electron-phonon coupling, reducing the adiabaticity, or decreasing the temperature enhances the coherence time.

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Orbital Ordering Phenomena in D‐ and F‐Electron Systems

Abstract: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Cited by 9 publications

References 253 publications

Spin-Orbital Order Modified by Orbital Dilution in Transition-Metal Oxides: From Spin Defects to Frustrated Spins Polarizing Host Orbitals

Spin-Orbital Order Modified by Orbital Dilution in Transition-Metal Oxides: From Spin Defects to Frustrated Spins Polarizing Host Orbitals

Fermi-surface instability at the ‘hidden-order’ transition of URu2Si2

Temperature dependence of long coherence times of oxide charge qubits

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