Inelastic neutron scattering (INS), electron spin (ESR) and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly hole-doped La_{1-x}Sr_xCoO_3, x=0.002. Both, INS and ESR low temperature spectra show intense excitations with large effective g-factors ~10-18. NMR data indicate the creation of extended magnetic clusters. From the Q-dependence of the INS magnetic intensity we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions.
In arXiv:1108.5413v1, Lee and Lee use several comparisons to argue that the physics of the three-band model found (Phys. Rev. Lett. 106 036401 (2011)) can be explained in the one-band model's framework (Phys. Rev. Lett. 91 057001 (2003)). While superficial similarities exist between the two sets of results, for reasons discussed in this reply, we disagree that they describe the same physics.
We present a combined study of the angle-resolved-photoemission spectroscopy (ARPES) and quantum Monte Carlo simulations to propose a novel polaronic metallic state in underdoped cuprates. An approximation scheme is proposed to represent underdoped cuprates away from 1/2 filling, replacing the many-body Hamiltonian by that of a single polaron with effective electron-phonon interaction (EPI), that successfully explains many puzzles such as a large momentum-dependent dichotomy between nodal and anti-nodal directions, and an unconventional doping dependence of ARPES in the underdoped region.
We present a numerical treatment of a spin-orbital polaron model for Na_xCoO_2 at small hole concentration (0.7 < x < 1). We demonstrate how the polarons account for the peculiar magnetic properties of this layered compound: They explain the large susceptibility; their internal degrees of freedom lead both to a negative Curie-Weiss temperature and yet to a ferromagnetic intra-layer interaction, thereby resolving a puzzling contradiction between these observations. We make specific predictions on the momentum and energy location of excitations resulting from the internal degrees of freedom of t…
We device and investigate numerically a minimal yet detailed spin polaron model that describes lightly doped CuO$_2$ layers. The low-energy physics of a hole is studied by total-spin-resolved exact diagonalization on clusters of up to 32 CuO$_2$ unit cells, revealing features missed by previous studies. In particular, spin-polaron states with total spin 3/2 are the lowest eigenstates in several regions of the Brillouin zone. In these regions, and also at other points the quasiparticle weight is identically zero, indicating orthogonal states to those represented in the one electron Green's func…
By resonant x-ray scattering at the Mn K-edge on La7/8Sr1/8MnO3, we show that an orbital polaron lattice (OPL) develops at the metal-insulator transition of this compound. This orbital reordering explains consistently the unexpected coexistence of ferromagnetic and insulating properties at low temperatures, the quadrupling of the lattice structure parallel to the MnO2-planes, and the observed polarization and azimuthal dependencies. The OPL is a clear manifestation of strong orbital-hole interactions, which play a crucial role for the colossal magnetoresistance effect and the doped manganites …
We investigate the doping dependence of the nanoscale electronic and magnetic inhomogeneities in the hole-doping range 0.002<x<0.1 of cobalt based perovskites, La{1-x}Sr_xCoO_3. Using single crystal inelastic neutron scattering and magnetization measurements we show that the lightly doped system exhibits magneto-electronic phase separation in form of spin-state polarons. Higher hole doping leads to a decay of spin-state polarons in favor of larger-scale magnetic clusters, due to competing ferromagnetic correlations of Co^{3+} ions which are formed by neighboring polarons. The present data give…
We present an inelastic neutron scattering study of the low energy crystal-field excitations in the lightly doped cobalt perovskite La_0.998Sr_0.002CoO_3. In contrast to the parent compound LaCoO_3 an inelastic peak at energy transfer ~0.75 meV was found at temperatures below 30 K. This excitation apparently corresponds to a transition between a ground state orbital singlet and a higher excited orbital doublet, originating from a high-spin triplet split by a small trigonal crystal field. Another inelastic peak at an energy transfer ~0.6 meV was found at intermediate temperatures starting from …
A fully ab initio scheme based on quantum chemical wavefunction methods is used to investigate the correlated multiorbital electronic structure of a 3d-metal compound, LaCoO3. The strong short-range electron correlations, involving both Co and O orbitals, are treated by multireference techniques. The use of effective parameters like the Hubbard U and interorbital U', J terms and the problems associated with their explicit calculation are avoided with this approach. We provide new insight into the spin-state transition at about 90 K and the nature of charge carriers in the doped material. Our r…
LaTiO$_{3+δ/2}$, NdTiO$_{3+δ/2}$, and Nd$_{1-x}$Sr$_{x}$TiO$_{3+δ/2}$ thin films have been epitaxially grown on (100)SrTiO$_{3}$ and (100)LaAlO$_{3}$ single crystal substrates by using the pulsed laser deposition technique. The oxygen pressure during deposition has been carefully controlled to ensure that the films are lightly doped in the metal side near the metal-Mott-insulator boundary. A steep drop of the effective carrier number at low temperatures has been observed in some of the films, which may correspond to a gradually opening of the Spin Density Wave (SDW) gap due to the antiferromag…
We extend the methods recently introduced in Phys. Rev. Lett. 106 036401 (2011) to investigate correlations between two spin-polarons in a quasi-two-dimensional CuO2 layer. The low-energy wavefunctions for two doped holes introduced in a half-filled CuO2 plane with 32 copper and 64 oxygen sites are calculated explicitly using an efficient yet accurate truncation scheme to model the antiferromagnet background. The energetics and wavefucntions show that the charges form three-spin polarons and the spin is carried by a disturbance around the three-spin polaron core. The low-energy band results fr…
In a recent Letter (Phys. Rev. Lett. 106, 036401 (2011)), Lau, et al. investigated the single hole problem in an effective model containing Cu $d$ and O $2p$ orbitals by exact diagonalization on clusters composed of 32 CuO$_2$ unit cells. With full quantum fluctuations due to the antiferromagnetic (AFM) backgroud, they found that spin-polaron solutions with spin 3/2 become lowest energy solutions in certain region of Brillouin zone (BZ), which they claimed no similiar solutions were obtained in other models or approximations before. In this Comment, we would like to point out that such high-sp…
BaBiO3 is a prototype ``charge ordering system'' forming interpenetrating sublattices with nominal valence Bi(3+) and Bi(5+). It can also be regarded as a three-dimensional version of a Peierls insulator, the insulating gap being a consequence of an ordered distortion of oxygen atoms. When holes are added to BaBiO3 by doping, it remains insulating until a very large hole concentration is reached, at which point it becomes superconducting. The mechanism for insulating behavior of more lightly-doped samples is formation of small polarons or bipolarons. These are self-organized point defects in t…
Self-localization of holes in the Holstein t-J model is studied in the adiabatic limit using exact diagonalization and the retraceable path approximation. It is shown that the critical electron-phonon coupling λ_c decreases with increasing J and that this behavior is determined mainly by the incoherent rather than by the coherent motion of the hole. The obtained spin correlation functions in the localized region can be understood within a percolation picture where antiferromagnetic order can persist up to a substantial hole doping. These results restrict the possibility of self-localization of…
We study the one-dimensional t-J-J'-model in the limit of small hole doping $x$ and small $J/t$, $J'/t$. Special emphasis is put on the regime $J'/J \approx .5$ where a spin gap is present at small doping and the undoped spin chain is strongly dimerized. Using a perturbative approach and Luttinger liquid arguments, we demonstrate for this non-integrable class of models that the charge degrees of freedom behave as non-interacting spinless solitons in the dilute hole limit. Our approach is also used to evaluate the energy and mass renormalization of a single hole. Interestingly, the corrections …
The electrical resistivity, Hall coefficient, and thermopower as functions of temperature are reported for lightly electron-doped Ca(1-x)La(x)MnO(3)(0 <= x <= 0.10). Unlike the case of hole-doped ferromagnetic manganites, the magnitude and temperature dependence of the Hall mobility for these compounds is found to be inconsistent with small-polaron theory. The transport data are better described by the Feynman polaron theory and imply intermediate coupling (alpha \~ 5.4) with a band effective mass, m*~4.3 m_0, and a polaron mass, m_p ~ 10 m_0.
We argue that in lightly hole doped perovskite-type Mn oxides the holes (Mn$^{4+}$ sites) are surrounded by nearest neighbor Mn$^{3+}$ sites in which the occupied $3d$ orbitals have their lobes directed towards the central hole (Mn$^{4+}$) site and with spins coupled ferromagnetically to the central spin. This composite object, which can be viewed as a combined orbital-spin-lattice polaron, is accompanied by the breathing type (Mn$^{4+}$) and Jahn-Teller type (Mn$^{3+}$) local lattice distortions. We present calculations which indicate that for certain doping levels these orbital polarons may …
Distortions of the oxygen sublattice couple to $e_g$ orbitals of Mn$^{3+}$ and drive a cooperative Jahn-Teller (orbital ordering) transition in LaMnO$_3$. A simple model for this transition is studied. Without further adjustment, the model predicts the shape and stability of small (anti-Jahn-Teller) polarons which form when holes are doped into the material. This leads to a new description of the lightly doped insulator, the anti-ferromagnetic to ferromagnetic transition, and the metal-insulator transition.
We study the spectral function of a single hole doped into the (a,b) plane of the Mott insulator LaVO$_3$, with antiferromagnetic (AF) spin order of S=1 spins accompanied by alternating orbital (AO) order of active $\{d_{yz},d_{zx}\}$ orbitals. Starting from the respective t-J model, with spin-orbital superexchange and effective three-site hopping terms, we derive the polaron Hamiltonian and show that a hole couples simultaneously to the collective excitations of the AF/AO phase, magnons and orbitons. Next, we solve this polaron problem using the self-consistent Born approximation and find a s…
I give a brief overview of a polaron formation in three distinct transition metal oxides: (i) spin polaron when a hole is added to the antiferromagnetic (AF) ordered plane in La$_2$CuO$_4$, (ii) orbital polaron when a hole is added to the alternating orbital (AO) ordered plane in LaMnO$_3$, and (iii) spin-orbital polaron when a hole is added to the AF and AO ordered plane in LaVO$_3$. Comparison of the distinct features of the above polarons can shed some light on the basic differences between the experimental phase diagrams of the lightly doped transition metal oxides La$_{2-x}$Sr$_x$CuO$_4$,…
Much progress has been made recently in the study of the effects of electron-phonon (el-ph) coupling in doped insulators using angle resolved photoemission (ARPES), yielding evidence for the dominant role of el-ph interactions in underdoped cuprates. As these studies have been limited to doped Mott insulators, the important question arises how this compares with doped band insulators where similar el-ph couplings should be at work. The archetypical case is the perovskite SrTiO$_3$ (STO), well known for its giant dielectric constant of 10000 at low temperature, exceeding that of La$_2$CuO$_4$ b…
Two-channel physics in a lightly doped antiferromagnetic Mott insulator revealed by two-hole spectroscopy
Ludwig-Maximilians-Universität München（慕尼黑路德维希-马克西米利安大学）；Arnold Sommerfeld Center for Theoretical Physics (ASC)（阿诺德·索末菲理论物理中心）；Munich Center for Quantum Science and Technology (MCQST)（慕尼黑量子科学与技术中心）；University of Colorado, Boulder（科罗拉多大学博尔德分校）；Center for Theory of Quantum Matter, University of Colorado, Boulder（科罗拉多大学博尔德分校量子物质理论中心）
Recent realization of well-controlled quantum simulators of the triangular-lattice Fermi-Hubbard model, including the triangular optical lattices loaded with ultracold Fermions and the heterostructures of the transition-metal dichalcogenides, as well as the more advanced techniques to probe them, pave the way for studying frustrated Fermi-Hubbard physics. Here, we theoretically predict asymmetric photoemission and tunneling spectroscopies for a lightly hole-doped and electron-doped triangular Mott antiferromagnet, and reveal two distinct types of magnetic polarons: a \emph{lightly} renormalize…
Recent studies exposed many remarkable properties of layered cobaltates NaxCoO2. Surprisingly, many-body effects have been found to increase at sodium-rich compositions of NaxCoO2 where one expects a simple, nearly free motion of the dilute S=1/2 holes doped into a band insulator NaCoO2. Here we discuss the origin of enigmatic correlations that turn a doped NaCoO2 into a strongly correlated electronic system. A minimal model including orbital degeneracy is proposed and its predictions are discussed. The model is based on a key property of cobalt oxides - the spin-state quasidegeneracy of CoO6 …
We explore with exact diagonalization the propagation of a single hole in four magnetic phases of the t-J-like Kitaev-Heisenberg model on a honeycomb lattice: the Neel antiferromagnetic, stripe, zigzag and Kitaev spin-liquid phase. We find coherent propagation of spin-polaron quasiparticles in the antiferromagnetic phase by a similar mechanism as in the $t$-$J$ model for high-$T_c$ cuprates. In the stripe and zigzag phases clear quasiparticles features appear in spectral functions of those propagators where holes are created and annihilated on one sublattice, while they remain largely {\it hid…
Room-temperature metallicity of lightly doped SrTiO$_3$ is puzzling, because the combination of mobility and the effective mass would imply a mean-free-path (mfp) below the Mott Ioffe Regel (MIR) limit and a scattering time shorter than the Planckian time ($τ_P=\hbar/k_BT$). We present a study of electric resistivity, Seebeck coefficient and inelastic neutron scattering extended to very high temperatures, which deepens the puzzle. Metallic resistivity persists up to 900 K and is accompanied by a large Seebeck coefficient whose magnitude (as well as its temperature and doping dependence) indica…
