In materials with strong electron-phonon ($e$-ph) interactions, the electrons carry a phonon cloud during their motion, forming quasiparticles known as polarons. Predicting charge transport and its temperature dependence in the polaron regime remains an open challenge. Here, we present first-principles calculations of charge transport in a prototypical material with large polarons, SrTiO$_{3}$. Using a cumulant diagram-resummation technique that can capture the strong $e$-ph interactions, our calculations can accurately predict the experimental electron mobility in SrTiO$_{3}$ between 150$-$300 K. They further reveal that for increasing temperature the charge transport mechanism transitions from band-like conduction, in which the scattering of renormalized quasiparticles is dominant, to a beyond-quasiparticle transport regime governed by incoherent contributions due to the interactions between the electrons and their phonon cloud. Our work reveals long-sought microscopic details of charge transport in SrTiO$_{3}$, and provides a broadly applicable method for predicting charge transport in materials with strong $e$-ph interactions and polarons.
Electrodynamic properties of La-doped SrTiO3 thin films with controlled elemental vacancies have been investigated using optical spectroscopy and thermopower measurement. In particular, we observed a correlation between the polaron formation and thermoelectric properties of the transition metal oxide (TMO) thin films. With decreasing oxygen partial pressure during the film growth (P(O2)), a systematic lattice expansion was observed along with the increased elemental vacancy and carrier density, experimentally determined using optical spectroscopy. Moreover, we observed an absorption in the mid…
The observation of substantially enhanced superconductivity of single-layer FeSe films on SrTiO3 has stimulated intensive research interest. At present, conclusive experimental data on the corresponding electron-boson interaction is still missing. Here we use inelastic electron scattering spectroscopy and angle resolved photoemission spectroscopy to show that the electrons in these systems are dressed by the strongly polarized lattice distortions of the SrTiO3, and the indispensable non-adiabatic nature of such a coupling leads to the formation of dynamic interfacial polarons. Furthermore, the…
The question whether excess electrons in SrTiO3 form free or trapped carriers is a crucial aspect for the electronic properties of this important material. This fundamental ambiguity prevents a consistent interpretation of the puzzling experimental situation, where results support one or the other scenario depending on the type of experiment that is conducted. Using density functional theory corrected with an on-site Coulomb interaction U, it is established that excess electrons form small polarons if the electron density is higher than ~ 1%. For densities below this critical value, the electr…
The cubic perovsike strontium titanate SrTiO$_3$ (STO) is one of the most studied, polarizable transition metal oxides. When excess charge is introduced to this material, e.g., through doping or atomic defects, STO tends to host polarons: Quasi-particles formed by excess charge carriers coupling with the crystal phonon field. Their presence alters the materials properties, and is a key for many applications. Considering that polarons form preferentially on or near surfaces, we study small polaron formation at the TiO$_2$ termination of the STO(001) surface via density functional theory calcula…
The experimental unexpected large Seebeck coefficient in SrTiO3 (STO) cannot be reproduced theoretically by the conventional Bloch-Boltzmann transport with electron-phonon coupling calculated perturbatively, indicating a failure of Fermi liquid picture in STO and ill-treatment of polaronic states. Starting from many-body interaction picture, the polaronic states can be precisely described by the method of cumulant expansion of retarded Green's function. By applying Kubo-Greenwood method, we found that the mysterious Seebeck coefficient can be fully described by a combination of polarons and Fe…
The hyperfine features and thermal stability of a muonium (Mu)-related paramagnetic center were investigated in the SrTiO$_3$ perovskite titanate via muon spin rotation spectroscopy. The hyperfine coupling tensor of the paramagnetic center was found to have prominent dipolar characteristics, indicating that the electron spin density is dominantly distributed on a Ti site to form a small polaron near an ionized Mu$^+$ donor. Based on a hydrogen-Mu analogy, interstitial hydrogen is also expected to form such a polaronic center in the dilute doping limit. The small activation energy of 30(3) meV …
Strontium titanate SrTiO$_3$ is one of the most studied and paradigmatic transition metal oxides. Recently, a breakthrough has been achieved with the fabrication of freestanding SrTiO$_3$ ultrathin films down to the monolayer limit. However, the many-body effects on the quasiparticle and optical properties of monolayer SrTiO$_3$ remain unexplored. Using state-of-the-art many-body perturbation theory in the GW approximation combined with the Bethe-Salpeter equation, we study the quasiparticle band structure, optical and excitonic properties of monolayer SrTiO$_3$. We show that quasiparticle cor…
Two-dimensional electron gases (2DEGs) in SrTiO$_3$ have become model systems for engineering emergent behaviour in complex transition metal oxides. Understanding the collective interactions that enable this, however, has thus far proved elusive. Here we demonstrate that angle-resolved photoemission can directly image the quasiparticle dynamics of the $d$-electron subband ladder of this complex-oxide 2DEG. Combined with realistic tight-binding supercell calculations, we uncover how quantum confinement and inversion symmetry breaking collectively tune the delicate interplay of charge, spin, orb…
Recent experimental data on the optical conductivity of niobium doped SrTiO$_{3}$ are interpreted in terms of a gas of large polarons with effective coupling constant $α_{eff}\approx2$. The {theoretical approach takes into account} many-body effects, the electron-phonon interaction with multiple LO-phonon branches, and the degeneracy and the anisotropy of the Ti t$_{2g}$ conduction band. {Based on the Fröhlich interaction, the many-body large-polaron theory} provides an interpretation for the essential characteristics, except -- interestingly -- for the unexpectedly large intensity of a peak a…
We generalize the momentum average approximation to study the properties of single polarons in models with boson affected hopping, where the fermion-boson scattering depends explicitly on both the fermion's and the boson's momentum. As a specific example, we investigate the Edwards fermion-boson model in both one and two dimensions. In one dimension, this allows us to compare our results with exact diagonalization results, to validate the accuracy of our approximation. The generalization to two-dimensional lattices allows us to calculate the polaron's quasiparticle weight and dispersion throug…
The parity-preserving $U(1)\times U(1)$ massless QED$_3$ is proposed as a pristine graphene-like planar quantum electrodynamics model. The spectrum content, the degrees of freedom, spin, masses and charges of the quasiparticles (electron-polaron, hole-polaron, photon and Néel quasiparticles) which emerge from the model are discussed. The four-fold broken degeneracy of the Landau levels, similar as the one experimentally observed in pristine graphene submitted to high applied external magnetic fields, is obtained. Furthermore, the model exhibits zero-energy Landau level indicating a kind of ano…
We examine generation dynamics of coherent phonons (CPs) in both of polar and non-polar semiconductors -- such as GaAs and Si -- based on a polaronic-quasiparticle (PQ) model. In the model concerned, the PQ operator is composed of two kinds of operators. One is a quasiboson operator -- defined as a linear combination of a set of pairs of electron operators -- and the other is a longitudinal optical (LO) phonon operator. The problem of transient and non-linear Fano resonance (FR) is tackled in particular; the vestige of this quantum interference effect was observed exclusively in lightly $n$-do…
Emergent quasiparticles that arise from the fractionalization of the microscopic degrees of freedom have been one of the central themes in modern condensed matter physics. The notion of magnetic monopoles, freely moving quasiparticles fragmented from local dipole excitations, has enjoyed much success in understanding the thermodynamic, static, and transport properties of the so-called spin-ice materials. The artificial version of spin ice, where a lattice of nanoscale magnetic dipoles is sculpted out of a ferromagnetic film, provides a unique opportunity to study these unusual quasiparticles i…
We study the spectral function of the homogeneous electron gas using many-body perturbation theory and the cumulant expansion. We compute the angle-resolved spectral function based on the GW approximation and the `GW plus cumulant' approach. In agreement with previous studies, the GW spectral function exhibits a spurious plasmaron peak at energies 1.5$ω_{\rm pl}$ below the quasiparticle peak, $ω_{\rm pl}$ being the plasma energy. The GW plus cumulant approach, on the other hand, reduces significantly the intensity of the plasmon-induced spectral features and renormalizes their energy relative …
We study a two-dimensional electron gas exchanged-coupled to a system of classical magnetic ions. For large Rashba spin-orbit coupling a single electron can become self-trapped in a skyrmion spin texture self-induced in the magnetic ions system. This new quasiparticle carries electrical and topological charge as well as a large spin, and we named it as magnetic skyrmionic polaron. We study the range of parameters; temperature, exchange coupling, Rashba coupling and magnetic field, for which the magnetic skyrmionic polaron is the fundamental state in the system. The dynamics of this quasipartic…
Universität Heidelberg（海德堡大学）；University of Warsaw（华沙大学）；National University of Singapore（新加坡国立大学）；Max-Planck-Institut für Quantenoptik（马克斯·普朗克量子光学研究所）
We report experimental and theoretical evidence of strong electron-plasmon interaction in n-doped single-layer MoS2. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal the emergence of distinctive signatures of polaronic coupling in the electron spectral function. Calculations based on many-body perturbation theory illustrate that electronic coupling to two-dimensional (2D) carrier plasmons provides an exhaustive explanation of the experimental spectral features and their energies. These results constitute compelling evidence of the formation of plasmon-induced polaronic qua…
With infrared ellipsometry and transport measurements we investigated the electrons at the interface between LaAlO3 and SrTiO3. We obtained a sheet carrier density of Ns~5-9x 10E13 cm^-2, an effective mass of m*~3m_e, and a strongly frequency dependent mobility. The latter are similar as in bulk SrTi1-xNbxO3 and therefore suggestive of polaronic correlations of the confined carriers. We also determined the vertical density profile which has a strongly asymmetric shape with a rapid initial decay over the first 2 nm and a pronounced tail that extends to about 11 nm.
Soft-X-ray ARPES (SX-ARPES) with its enhanced probing depth and chemical specificity allows access to fundamental electronic structure characteristics - momentum-resolved spectral function, band structure, Fermi surface - of systems difficult and even impossible for the conventional ARPES such as three-dimensional materials, buried interfaces and impurities. After a recap of the spectroscopic abilities of SX-ARPES, we review its applications to oxide interfaces, focusing on the paradigm LaAlO3-SrTiO3 interface. Resonant SX-ARPES at the Ti L-edge accentuates photoemission response of the mobile…
A recent muon spin rotation ($μ^+$SR) study on a paramagnetic defect complex formed upon implantation of $μ^+$ pseudo-proton into SrTiO$_3$ is reviewed with a specific focus on the relation with experimental signatures of coexisting delocalized and localized electrons in hydrogen-irradiated metallic SrTiO$_3$ films. The paramagnetic defect complex, composed of interstitial $μ^+$ and Ti$^{3+}$ small polaron, is characterized by a small dissociation energy of about 30 meV. Density functional theory (DFT) calculations in the generalized gradient approximation (GGA) +$U$ scheme for a corresponding…
The polaron is the archetypal example of a quasiparticle emerging from the interaction between fermionic and bosonic fields in quantum field theory. In crystalline solids, polarons are formed when electrons and holes become dressed by the quanta of lattice vibrations. While experimental signatures of polarons in bulk three-dimensional materials abound, only rarely have polarons been observed in two-dimensional atomic crystals. Here, we shed light on this asymmetry by developing a quantitative ab initio theory of polarons in atomically-thin crystals. Using this conceptual framework, we unravel …
We report on an infrared spectroscopy study of mobile holes in the accumulation layer of organic field-effect transistors based on rubrene single crystals. Our data indicate that both transport and infrared properties of these transistors at room temperature are governed by light quasiparticles in molecular orbital bands with the effective masses m* comparable to free electron mass. Furthermore, the m* values inferred from our experiments are in agreement with those determined from band structure calculations. These findings reveal no evidence for prominent polaronic effects, which is at varia…
A novel type of exciton-phonon bound state -- interlayer polaron -- in a double-layer two-dimensional semiconductor with transition metal dichalcogenides as an example, is predicted. In these systems the interaction of the interlayer exciton with the soft modes of out-of-plane lattice vibrations caused by van der Waals forces and flexural rigidity gives rise to a bound quasiparticle. The energy and effective mass of the formed polaron for weak and strong exciton-phonon coupling regimes are calculated and analyzed. Possible manifestations of these effects in transport- and spectroscopy-related …
When mobile impurities are introduced and coupled to a Fermi sea, new quasiparticles known as Fermi polarons are formed. There are two interesting, yet drastically different regimes of the Fermi polaron problem: (I) the attractive polaron (AP) branch, connected to pairing phenomena spanning the crossover from BCS superfluidity to the Bose-Einstein condensation of molecules; and (II) the repulsive branch (RP), which underlies the physics responsible for Stoner's itinerant ferromagnetism. Here, we study Fermi polarons in two dimensional systems, where many questions and debates regarding their n…
The effect of a variety of intrinsic defects and defect clusters in bulk and thin films of SrTiO$_3$ on ferroelectric polarization and switching mechanism is investigated by means of density-functional-theory (DFT) based calculations and the Berry phase approach. Our results show that both the titanium Ti$_\mathrm{Sr}^{\bullet \bullet}$ and strontium Sr$_\mathrm{Ti}^{"}$ antisite defects induce ferroelectric polarization in SrTiO$_3$, with the Ti$_\mathrm{Sr}^{\bullet \bullet}$ defect causing a more pronounced spontaneous polarization and higher activation barriers of polarization reversal tha…
We present a novel approach to electron-lattice interaction beyond the linear-coupling regime. Based on the solution of a Holstein-Peierls-type model, we derive explicit analytical expressions for the eigenvalue spectrum of the Hamiltonian, resulting in a narrowing of bands as a function of temperature. Our approach enables the intuitive interpretation in terms of quasiparticles, i.e. polaron bands and dressed-phonon frequencies. Being nonperturbative, the formalism also applies in the strong-coupling case. We apply it to the organic crystal naphthalene, with the coupling strengths obtained by…
Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel（基尔大学理论与天体物理研究所）；Oden Institute for Computational Engineering and Sciences, The University of Texas at Austin（德克萨斯大学奥斯汀分校奥登计算工程与科学研究所）；Department of Physics, University of the Basque Country UPV/EHU（巴斯克大学物理系）；Department of Physics, The University of Texas at Austin（德克萨斯大学奥斯汀分校物理系）
The electronic band structure of SrTiO$_3$ is investigated in the all-electron QS$GW$ approximation. Unlike previous pseudopotential based QS$GW$ or single-shot $G_0W_0$ calculations, the gap is found to be significantly overestimated compared to experiment. After putting in a correction for the underestimate of the screening by the random phase approximation in terms of a 0.8$Σ$ approach, the gap is still overestimated. The 0.8$Σ$ approach is discussed and justified in terms of various recent literature results including electron-hole corrections. Adding a lattice polarization correction (LPC…
The formation of polarons is a pervasive phenomenon in transition metal oxide compounds, with a strong impact on the physical properties and functionalities of the hosting materials. In its original formulation the polaron problem considers a single charge carrier in a polar crystal interacting with its surrounding lattice. Depending on the spatial extension of the polaron quasiparticle, originating from the coupling between the excess charge and the phonon field, one speaks of small or large polarons. This chapter discusses the modeling of small polarons in real materials, with a particular f…
This review will deal with several types of free charge localisation in oxides and their consequences on the effective dielectric spectra of such materials. The first one is the polaronic localisation at the unit cell scale on residual impurities in ferroelectric networks. The second one is the collective localisation of free charge at macroscopic interfaces like surfaces, electrodes and grain boundaries in ceramics. Polarons have been observed in many oxide perovskites mostly when cations having several stable electronic configurations are present. In manganites, the density of such polarons …
A new variational approach is proposed at zero temperature for a finite density of charge carriers in order to study ground state features of the Frohlich model including electron-electron and electron-phonon interactions. Within the intermediate electron-phonon coupling regime characteristic of large polarons, the approach takes into account on the same footing polaron formation and polaron-polaron correlations which play a relevant role going from low to high charge densities. Including fluctuations on top of the variational approach, the electronic spectral function is calculated from the w…
The electron-phonon interaction causes thermal and zero-point motion shifts of electron quasiparticle (QP) energies $ε_k(T)$. Other consequences of interactions, visible in angle-resolved photoemission spectroscopy (ARPES) experiments, are broadening of QP peaks and appearance of sidebands, contained in the electron spectral function $A(k,ω)=-{\Im m}G_R(k,ω) /π$, where $G_R$ is the retarded Green's function. Electronic structure codes (e.g. using density-functional theory) are now available that compute the shifts and start to address broadening and sidebands. Here we consider MgO and LiF, and…
We use determinant quantum Monte Carlo to study the single particle properties of quasiparticles and phonons in a variant of the two-dimensional Holstein model that includes an additional non-linear electron-phonon (e-ph) interaction. We find that a small positive non-linear interaction reduces the effective coupling between the electrons and the lattice, suppresses charge-density wave (CDW) correlations, and hardens the effective phonon frequency. Conversely, a small negative non-linear interaction can enhance the e-ph coupling resulting in heavier quasiparticles, an increased tendency toward…
