We investigate the electronic properties and electron correlations in a three-dimensional (3D) polaron system with tunable dispersion parameter in Fermi-liquid picture. The polaronic coupling is considered as a weakly attractive bare coupling. Both the single and many-polaron, zero and finite-temperature cases are considered in this paper. The physical quantities, including the self-energy, spectral function, optical conductivity (both the planar and 3D one), free energy, and momentum distribution function, are studied and are been verified exhibit Fermi-liquid features. Both the analytic derivation and numerical calculation are done in this paper to make our results reliable.
Oxygen vacancies created in anatase TiO2 by UV photons (80 - 130 eV) provide an effective electron-doping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission (ARPES) reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.
We have investigated hybridized magnon-phonon excitation (magnon polarons) in spin Seebeck effects (SSEs) in Bi$_x$Y$_{3-x}$Fe$_{5}$O$_{12}$ (Bi$_x$Y$_{3-x}$IG; $x=0$, $0.5$, and $0.9$) films with Pt contact. We observed sharp peak structures in the magnetic field $H$ dependence of the longitudinal SSE (LSSE) voltages, which appear when the phonon dispersions are tangential to the magnon dispersion curve in Bi$_x$Y$_{3-x}$IG. By increasing the Bi amount $x$, the peak fields in the LSSE shift toward lower $H$ values due to the reduction of the sound velocities in Bi$_x$Y$_{3-x}$IG. We also meas…
Mobile impurities in cold atomic gases constitute a new platform for investigating polaron physics. Here we show that when impurity atoms interact with a two-dimensional Fermi gas with quadratic band touching the polaron picture may either hold or break down depending on the particle-hole asymmetry of the band structure. If the hole band has a smaller effective mass than the particle band, the quasiparticle is stable and its diffusion coefficient varies with temperature as $D(T) \propto \ln^2 T$. If the hole band has larger mass, the quasiparticle weight vanishes at low energies due to an emer…
We theoretically investigate magnon-phonon hybrid excitations in two-dimensional ferromagnets. The bulk bands of hybrid excitations, which are referred to as magnon-polarons, are analytically shown to be topologically nontrivial, possessing finite Chern numbers. We also show that the Chern numbers of magnon-polaron bands and the number of band-crossing lines can be manipulated by an external magnetic field. For experiments, we propose to use the thermal Hall conductivity as a probe of the finite Berry curvatures of magnon-polarons. Our results show that a simple ferromagnet on a square lattice…
We use the approximation-free Bold Diagrammatic Monte Carlo technique to study the effects of a finite dispersion of the optical phonon mode on the properties of the Holstein polaron, especially its effective mass. For weak electron-phonon coupling the effect is very small, but it becomes significant for moderate and large electron-phonon coupling. The effective mass is found to increase (decrease) if the phonon dispersion has a negative (positive) curvature at the centre of the Brillouin zone.
Dispersion of low-energy optical excitations in NiPS$_3$ explained by singlet polaron theory
Instituto de Física Rosario (CONICET) and Facultad de Ciencias Exactas, Ingeniería y Agrimensura, Universidad Nacional de Rosario（罗萨里奥物理研究所（阿根廷国家科学和技术研究委员会）及罗萨里奥国立大学精确科学、工程和测量学院）；Instituto de Nanociencia y Nanotecnología CNEA-CONICET, GAIDI, Centro Atómico Bariloche and Instituto Balseiro（CNEA-CONICET纳米科学与技术研究所、GAIDI、巴里洛切原子中心及巴尔塞罗研究所）；Physics Department, Northeastern University（东北大学物理系）
Spectral Properties of Holstein and Breathing Polarons
我们用自洽玻恩近似计算了一维Holstein极化子和呼吸极化子的光谱性质。Holstein模型的电子-声子耦合是动量无关的，而呼吸耦合随声子动量单调增加。我们发现，对于线性或紧密结合的电子色散：i）对于相同的无量纲耦合值，呼吸极化子中小动量处的准粒子重整化要小得多，ii）呼吸极化子中小动量处的准粒子重整化随着声子频率而增加，这与Holstein模型中的情况不同，在Holstein模型中，准粒子重整化随着声子频率而减少，iii）在Holstein模型中，准粒子色散在等于声子频率w 0的激发能处显示扭结和小间隙，而在呼吸模型中，它显示两个间隙，一个在激发能w 0处，另一个在2 w 0处。这些差异有两个原因：第一，相关散射声子的动量随着极化子动量的增加而增加;第二，呼吸裸耦合是声子动量的增函数。这导致呼吸模型的有效的电子-声子耦合是总极化子动量的增函数，使得小动量极化子处于弱耦合区，而大动量极化子处于强耦合区。然而，如果自由电子色散具有被大动量分离的低能态，则第一个原因不成立，例如在更高维的系统中，在这种情况下，两个模型之间的差异变得不那么显著。
We consider the polaron dynamics driven by Frohlich type, long wavelength dominated electron-phonon interaction, for three different semi-metals: single and bilayer graphene, and semi-Dirac, all grown on polar substrates such as, SiC or SiO_2. The problem of polaron has been studied by Feynman and others for ordinary polar crystals. But the study of polaron formation in the context of the above-mentioned 2D semi-metals having non-scalar effective Hamiltonians is novel. When SL and BL graphene are grown on polar substrates, their electrons can interact with the surface phonons of those polar su…
We present the first numerically exact study of a polaron with quadratic coupling to the oscillator displacement, using two alternative methodological developments. Our results cover both anti-adiabatic and adiabatic regimes and the entire range of electron-phonon coupling $g_2$, from the system's stability threshold at attractive $g_2=-1$ to arbitrary strong repulsion at $g_2 \gg 1$. Key properties of quadratic polarons prove dramatically different from their linear counterparts. They (i) are insensitive even to large quadratic coupling except in the anti-adiabatic limit near the threshold of…
The dielectric properties of La$_{1-x}$Sr$_{1+x}$MnO$_4$ single crystals with x = 0, 0.125, 0.25, and 0.5 were studied by means of electron energy-loss spectroscopy as a function of temperature and momentum transfer. A clear signature of the doped holes is observed around 1.65 eV energy loss, where spectral weight emerges with increasing x. For all $x \neq 0$, this doping-induced excitation can propagate within the ab-plane, as revealed by a clear upward dispersion of the corresponding loss peak with increasing momentum transfer. The hole-induced excitation also shifts to higher energies with …
Extending the Feynman variational principle and analytical methods to lattice polarons
Universiteit Antwerpen（安特卫普大学）；Harvard University（哈佛大学）；University of Vienna（维也纳大学）；Institute of Physics, 10000 Zagreb, Croatia（克罗地亚萨格勒布物理研究所）；Flatiron Institute（平顿研究所）；Alma Mater Studiorum – Università di Bologna（博洛尼亚大学）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所新兴物质科学中心）
Polaron spectral functions are computed for highly doped graphene-on-substrate and other atomically thin graphitic systems using the diagrammatic Monte Carlo technique. The specific aim is to investigate the effects of interaction on spectral functions when the symmetry between sub-lattices of a honeycomb lattice has been broken by the substrate or ionicity, inducing a band gap. Introduction of electron-phonon coupling leads to several polaronic features, such as band-flattening and changes in particle lifetimes. At the K point, differences between energies on each sub-lattice increase with el…
Interaction-driven spin polaron in itinerant flat-band ferromagnetism
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University（南京大学固态微结构国家重点实验室和物理学院）；School of Physics, Nanjing University of Science and Technology（南京理工大学物理学院）；Jiangsu Physical Science Research Center（江苏省物理科学研究中心）；Collaborative Innovation Center of Advanced Microstructures, Nanjing University（南京大学先进微结构协同创新中心）；Jiangsu Key Laboratory of Quantum Information Science and Technology, Nanjing University（南京大学江苏省量子信息科学与技术重点实验室）
We present an exact solution for an itinerant hole added into the oxygen orbitals of a CuO$_{3}$-like ferromagnetic chain. Using the Green's function method, the quantum polarons obtained for the Heisenberg SU(2) interaction between localized Cu spins are compared with the polarons in the Ising chain. We find that magnons with large energy are favorable towards quasiparticle existence, even in the case of relatively modest electron-magnon coupling. We observe two quasiparticle states with dispersion $\sim 2t$ each, which emerge from the incoherent continuum when the exchange coupling $J$ incre…
Interlayer excitons, comprising an electron in one material bound by Coulomb attraction to a hole in an adjacent material, are composite bosons that can assume a variety of many-body phases. The phase diagram of the bosonic system is largely determined by the dispersion relation of the bosons, which itself arises as a combination of the dispersion relations of the electron and hole separately. Here I show that in situations where either the electron or the hole has a non-monotonic, "Mexican hat-shaped" dispersion relation, the exciton dispersion relation can have a range of qualitatively diffe…
We determine the phase diagram of a polaron model with mixed breathing-mode and Su-Schrieffer-Heeger couplings and show that it has two sharp transitions, in contrast to pure models which exhibit one (for Su-Schrieffer-Heeger coupling) or no (for breathing-mode coupling) transition. Our results indicate that the physics of realistic mixed polaron models is much richer than that of simplified models. We then show that ultracold molecules trapped in optical lattices can be used to study precisely this mixed Hamiltonian, and that the relative contributions of the two couplings can be tuned with e…
We provide a comprehensive solution to the lattice dynamics problem in the two dimensional Holstein model at finite electron density and finite temperature. We work in the physically relevant adiabatic regime and vary the electron-phonon interaction from the weak coupling perturbative window to the strong coupling polaronic regime. We explore three typical electron densities, dilute - where spatial correlations between polarons is weak, intermediate - where correlations are significant, and half-filling - where there is long range checkerboard order at low temperature. We use two methods both …
Polarons with different types of electron-phonon coupling have fundamentally different properties. When the dominant interaction is between the electron density and lattice displacement, the momentum of the ground state does not change and the polaron gets exponentially heavy at strong coupling. In contrast, one-dimensional Peierls/Su-Schrieffer-Heeger (PSSH) polarons with interaction originating from displacement-modulated hopping feature a shift of the ground-state momentum to finite values and moderate values of effective mass as coupling is increased REF[Phys. Rev. Lett. 105, 266605 (2010)…
Employing a self-consistent (optimized) variational diagonalization scheme, we investigate the formation of polaronic quasiparticles in a spinless fermion-boson transport model that couples the movement of charge carriers to fluctuations and correlations of a background medium. The background is parameterized by bosonic degrees of freedom. The variational fermion-boson Hilbert space is constructed to achieve high accuracy in one to three spatial dimensions with modest computational requirements. To characterize the ground-state properties of the Edwards model in the single-particle sector, we …
Feshbach resonances are an invaluable tool in atomic physics, enabling precise control of interactions and the preparation of complex quantum phases of matter. Here, we theoretically analyze a solid-state analogue of a Feshbach resonance in two dimensional semiconductor heterostructures. In the presence of inter-layer electron tunneling, the scattering of excitons and electrons occupying different layers can be resonantly enhanced by tuning an applied electric field. The emergence of an inter-layer Feshbach molecule modifies the optical excitation spectrum, and can be understood in terms of Fe…
A comprehensive picture of polaron and bipolaron physics is essential to understand the optical absorption spectrum in many materials with electron-phonon interactions. In particular, the finite-temperature properties are of interest since they play an important role in many experiments. Here, we combine the parallel two-site time-dependent variational principle algorithm (p2TDVP) with local basis optimization (LBO) and purification to calculate time-dependent current-current correlation functions. From this information, we extract the optical conductivity for the Holstein polaron and bipolaro…
We use the Momentum Average approximation together with perturbative approaches, in the appropriate limits, to study the single polaron physics on a perovskite lattice inspired by BaBiO3. We investigate electron-phonon coupling of the Peierls type whereby the motion of ions modulates the values of the hopping integrals between sites, and show that it cannot be mapped onto the simpler one-band Holstein model in the whole parameter space. This is because the dispersion of the Peierls polaron has sharp transitions where the ground-state momentum jumps between high-symmetry points in the Brillouin…
