Transition metal oxides are a rich group of materials with very interesting physical properties that arise from the interplay of the charge, spin, orbital, and lattice degrees of freedom. One interesting consequence of this, encountered in systems with orbital degeneracy, is the coexistence of long range magnetic and orbital order, and the coupling between them. In this paper we develop and study an effective spin-orbital superexchange model for $e_g^3$ systems and use it to investigate the spectral properties of a charge (hole) injected into the system, which is relevant for photoemission spectroscopy. Using an accurate, semi-analytical, magnon expansion method, we gain insight into various physical aspects of these systems and demonstrate a number of subtle effects, such as orbital to magnetic polaron crossover, the coupling between orbital and magnetic order, as well as the orbital order driving the system towards one-dimensional quantum spin liquid behavior. Our calculations also suggest a potentially simple experimental verification of the character of the orbital order in the system, something that is not easily accessible through most experimental techniques.
Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy
以$R_{1-x}$ Ca $_x$ VO $_3$钙钛矿为例，探讨了无序带电缺陷对掺杂过渡金属氧化物在Mott绝缘区光电发射谱中观察到的电子激发的影响，其中$R=$ La，$\dots$，Lu.这些具有部分填充的$t_{2g}$价轨道的钒$d^2$化合物的基本特征是自旋和轨道有序的持久性直到高掺杂，与高- $T_c$铜酸盐在低缺陷浓度下的磁有序的损失相反。我们证明了掺杂Mott-Hubbard绝缘体的无序电子结构可以得到高精度的非限制性Hartree-Fock近似。特别是：（i）在逆光电发射光谱和各种缺陷相关的状态和卫星的原子多重激发被很好地再现，（ii）一个强大的莫特间隙生存到大掺杂，和（iii）我们表明，莫特间隙内的缺陷状态发展一个软间隙在费米能量。软缺陷状态间隙可以通过从化学势附近的状态密度的威布尔统计采样中提取的形状和尺度参数来表征。我们证明，电荷缺陷触发小自旋轨道极化子，其内部动能负责软缺陷态间隙的开放。自旋轨道极化子的小尺寸推断的反参与比的分析，解释了起源的自旋和轨道秩序的鲁棒性。使用现实的参数为La $_{1-x}$ Ca $_x$ VO $_3$，我们表明，它的软带隙是很好地再现，以及边缘掺杂的化学势的位置相对于中心的较低的哈伯德带。
Magnetic properties of spin-orbital polarons in lightly doped cobaltates
本文对Na_xCoO_2在小空穴浓度（0.7 < x < 1）下的自旋轨道极化子模型进行了数值处理。我们演示了极化子如何解释这种层状化合物的特殊磁性：它们解释了大的磁化率;它们的内部自由度导致负居里-外斯温度和铁磁层内相互作用，从而解决了这些观察之间令人困惑的矛盾。我们对极化子内部自由度引起的激发的动量和能量位置进行了具体的预测，并讨论了它们对自旋波阻尼的影响。
The origin of the effect of colossal magneto-resistance (CMR) remains still unexplained. In this work we revisit the spin dynamics of the pseudo-cubic La1-xSrxMnO3 along the Mn-O-Mn bond direction at four x doping values (x < 0.5) and various temperatures and report a new lattice dynamics study at x0=0.2, representative of the optimal doping for CMR. We propose an interpretation of the spin dynamics in terms of orbital polarons. This picture is supported by the observation of a discrete magnetic energy spectrum Enmag (q) characteristic of the internal excitations of "orbital polarons" defined …
We explore the competiton of doped holes and defects that leads to the loss of orbital order in vanadate perovskites. In compounds such as La$_{1-{\sf x}}$Ca$_{\,\sf x}$VO$_3$ spin and orbital order result from super-exchange interactions described by an extended three-orbital degenerate Hubbard-Hund model for the vanadium $t_{2g}$ electrons. Long-range Coulomb potentials of charged Ca$^{2+}$ defects and $e$-$e$ interactions control the emergence of defect states inside the Mott gap. The quadrupolar components of the Coulomb fields of doped holes induce anisotropic orbital rotations of degener…
Orbital-lattice coupling and polaronic dressing of electronic states in Pr$_{0.5}$Ca$_{1.5}$MnO$_4$ revealed by ultrafast broadband spectroscopy
ICFO - Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology（巴塞罗那科学技术学院光子科学研究所（ICFO））；Aarhus University（奥胡斯大学）；ALBA Synchrotron Light Source（ALBA同步辐射光源）；Univ Rennes（雷恩大学）；CNRS（法国国家科学研究中心）；IMDEA Nanoscience（IMDEA纳米科学研究所）
The dynamics of electrons, spins and phonons induced by optical femtosecond pulses has been simulated for the polaronic crystal $\rm{Pr_{1/2}Ca_{1/2}MnO_3}$. The model used for the simulation has been derived from first-principles calculations. The simulations reproduce the experimentally observed melting of charge/orbital order with increasing fluence. The loss of charge order in the high-fluence regime induces a transition to a ferromagnetic metal. At low fluence, the dynamics is deterministic and coherent phonons are created by the repopulation of electronic orbitals, which are strongly cou…
Resonant Inelastic X-Ray Scattering (RIXS) experiments on the iron-based ladder BaFe$_2$Se$_3$ unveiled an unexpected two-peak structure associated with local orbital ($dd$) excitations in a block-type antiferromagnetic phase. A mixed character between correlated band-like and localized excitations was also reported. Here, we use the density matrix renormalization group method to calculate the momentum-resolved charge- and orbital-dynamical response functions of a multi-orbital Hubbard chain. Remarkably, our results qualitatively resemble the BaFe$_2$Se$_3$ RIXS data, while also capturing the …
Fingerprints of spin-orbital entanglement in transition metal oxides
介绍了过渡金属氧化物中超交换键的自旋轨道纠缠的概念，并举例说明。结果表明，自旋-轨道纠缠在超交换模型不稳定的长程（自旋和轨道）秩序，并可能导致一个无序的自旋液体状态或新的阶段，在低温下所产生的强烈受挫的相互作用。这种新颖的基态不能在常规使用的平均场论中描述，该平均场论将自旋和轨道自由度分开。即使在基态解纠缠的情况下，自旋-轨道纠缠也发生在激发态中，并且可能对有限温度下物理性质的正确描述至关重要。作为这种行为的一个重要例子，我们提出了$R$ VO $_3$钙钛矿中的自旋轨道纠缠，其中$R$ =La，Pr，.，Yb，Lu，其中这些化合物的这种有限温度特性只能使用纠缠态来理解：（$i$）光谱权重的热演化，（$ii$）在$R$ VO $_3$钙钛矿的相图中，轨道和磁序开始的转变温度对离子半径的依赖性，和（$iii$）二聚化在磁振子谱中观察到的$C$型反铁磁相的YVO $_3$。最后，它表明，联合自旋-轨道激发在一个有序的相位与共存的反铁磁和交替的轨道顺序引入拓扑约束的空穴传播，从而从根本上改变输运性质的掺杂莫特绝缘体中的空穴运动意味着同时自旋和轨道激发。
The monopnictides TaAs and TaP are well-established Weyl semimetals. Yet, a precise assignment of Fermi arcs, accomodating the predicted chiral charge of the bulk Weyl points, has been difficult in these systems, and the topological character of different surface features in the Fermi surface is not fully understood. Here, employing a joint analysis from linear dichroism in angle-resolved photoemission and first-principles calculations, we unveil the orbital texture on the full Fermi surface of TaP(001). We observe pronounced switches in the orbital texture at the projectedWeyl nodes, and show…
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,…
Understanding the interplay between the structural, chemical and physical properties of nanomaterials is crucial for designing new devices with enhanced performance. In this regards, doping of metal oxides is a general strategy to tune size, morphology, charge, lattice, orbital and spin degrees of freedoms and has been shown to affect nanomaterials properties for photoelectrochemical water splitting, batteries, catalysis, magnetic applications and optics. Here we report the role of lattice small polaron in driving the morphological transition from nearly isotropic to nanowire crystals in Si do…
We investigate the evolution of a photoexcitation in correlated materials over a wide range of time scales. The system studied is a one-dimensional model of a manganite with correlated electron, spin, orbital, and lattice degrees of freedom, which we relate to the three-dimensional material Pr$_{1-x}$Ca$_{x}$MnO$_3$. The ground-state phases for the entire composition range are determined and rationalized by a coarse-grained polaron model. At half-doping a pattern of antiferromagnetically coupled Zener polarons is realized. Using time-dependent density-matrix renormalization group (tDMRG), we t…
We study the impact of next-nearest-neighbor (nnn) hopping on the low-energy collective excitations of strongly correlated doped antiferromagnetic cuprate spin chains. Specifically, we use exact diagonalization and the density matrix renormalization group method to study the single-particle spectral function, the dynamical spin and charge structure factors, and the Cu $L$-edge resonant inelastic x-ray scattering (RIXS) intensity of the doped $t$-$t^\prime$-$J$ model for a set of $t^\prime$ values. We find evidence for the breakdown of spin-charge separation as $|t^\prime|$ increases and identi…
