Energy and mass of 3D and 2D polarons in the overall range of the electron-phonon coupling strength

The ground-state characterization of the polaron problem is retrieved within the framework of a variational scheme proposed previously by Devreese et al for the bound polaron. The formulation is based on the standard canonical transformation of the strong coupling ansatz and consists of a variationally determined perturbative extension serving for the theory to interpolate in the overall range of the coupling constant. Specializing our considerations to the bulk and strict two-dimensional polaron models we see that the theory yields significantly improved energy upper bounds in the strong coupling regime and, moreover, extrapolates itself successfully towards the well-established weak coupling limits for all polaron quantities of general interest

Binding energies of excitons in II-VI compound-semiconductor based quantum well structures

We present a brief description of the calculation of the variation of the binding energy of the heavy-hole exciton as a function of well width in quantum well structures composed of II-VI compound semiconductors including the effects of exciton-optical phonon interaction as formulated by Pollmann and Büttner [J. Pollmann and H. Büttner, Phys. Rev. B 16, 4480 (1977)], and of particle masses and dielectric mismatches between the well and the barrier layers. We compare the results of our calculations with the available experimental data in ZnSe/MgS, ZnSe/Mg0.15Zn0.85Se, and ZnS/Mg0.19Z0.81S quantum well structures and find a good agreement. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A variational study of the ground Landau level of the 2D Frohlich polaron in a magnetic field

The problem of a two-dimensional polaron in a magnetic field is retrieved within the framework of an improved variational approximation which sets up a fractional admixture of the strong- and weak-coupling counterparts of the coupled electron-phonon system. The formulation is based on the usage of an adiabatic polaronic wavefunction corrected by a variationally determined perturbative extension enabling the adiabatic ansatz to be extrapolated towards the weak-coupling regime. The trial state derived here accounts for the magnetic field intensity not only in the electron part of the Hamiltonian, but also within the context of the part of the Hamiltonian describing the coupling of the electron to the phonon field

Path-integral approximation on the stability of large bipolarons in quasi-one-dimensional confinement

The stability of the singlet optical bipolaron is investigated in quasi-one-dimensional confinement with parabolic boundary potential. Under the bulk-phonon approximation the Feynman-polaron model is used to display the polaron-bipolaron phase diagram as a function of the Coulomb and phonon coupling strengths and the degree of confinement. ©2000 The American Physical Society

Path integral description of low-dimensional polarons in parabolic confinement potentials

Within the framework of the Feynman path integral theory, we provide a unified insight into ground-state properties of the Fröhlich polaron in low-dimensionally confined media. The model that we adopt consists of an electron immersed in the field of bulk LO phonons and bounded within an anisotropic parabolic potential box, whose barrier slopes can be tuned so as to yield an explicit tracking of the Fröhlich interaction encompassing the bulk and all low-dimensional geometric configurations of general interest

Strong-coupling theory of two dimensional large bipolarons in elliptical quantum dots

In the limit of strong electron-phonon coupling, we analyze the stability of two dimensional bipolarons in a two-axis elliptic potential well of harmonic boundaries. The confined two-polaron wavefunction adopted here makes the electrons to form either a bipolaronic bound state or go into a composite state of two separated polarons bounded inside the same potential well. The methodology involves the mean polaron-polaron separation treated as an adjustable parameter to be determined variationally. By tuning the barrier slopes of the confining potential we obtain an explicit tracking of the criterion for bipolaron stability encompassing the particular cases of a two dimensional circular dot or a planar strip-like quantum well wire. We observe that, while an increased degree of confinement enhances bipolaronic stability, the effect of anisotropy is to inhibit bipolaron formation

Ground-state description of quasi-one-dimensional polarons with arbitrary electron-phonon coupling strength

We consider the interaction of a confined electron with bulk polar-optical phonons in a cylindrical quantum well wire with infinite boundary potential. Expressions for the polaron self-energy and mass are derived within a variational scheme over reasonably broad ranges of the wire radius and the phonon-coupling strength. The formulation is based on the standard canonical transformation of the strong-coupling ansatz and consists of a variationally determined perturbative extension serving for the theory to interpolate in the overall range of the coupling constant. Contrary to the general trend that the electron-phonon interaction is inherently stronger in systems of lower dimensionality, our results indicate that, at weak coupling, the binding energy of the polaron can be smaller and its mass less inertial compared with the bulk case when the wire is made narrow

Quasi-two-dimensional Feynman bipolarons

We study the stability criterion for the formation of two-dimensionally confined large bipolarons. The electrons are treated as bounded within a parabolic potential well while being coupled to one another via the Fröhlich interaction Hamiltonian. Within the framework of the bulk-phonon approximation we adopt the Feynman-polaron model to derive variational results over a wide range of the Coulomb interaction and phonon coupling strengths interpolating between the bulk and the two-dimensional confinement limit. It is shown that the critical values of the electron-phonon coupling constant and the ratio of dielectric constants (η=ε∞/ε0) exhibit some nontrivial features as the effective dimensionality is tuned from 3 to 2. ©1999 The American Physical Society

On the stability of Fröhlich bipolarons in spherical quantum dots

In the strong-electron-phonon-coupling regime, we retrieve the stability criterion for bipolaron formation in a spherical quantum dot. The model that we use consists of a pair of electrons immersed in a reservoir of bulk LO phonons and confined within an isotropic parabolic potential box. In this particular quasi-zero-dimensional geometry, where the electrons do not have any free spatial direction to expand indefinitely, a plausible approach would be to treat the electrons either to form a bipolaronic bound state or enter a state of two close, but individual polarons inside the same dot. The confined two-polaron model adopted here involves the polaron-polaron separation introduced as an adjustable parameter to be determined variationally. It is found that the fundamental effect of imposing such a variational flexibility is to modify the phase diagram to a considerable extent and to sustain the bipolaron phase in a broader domain of stability

Spintronic properties of zigzag-edged triangular graphene flakes

We investigate quantum transport properties of triangular graphene flakes with zigzag edges by using first principles calculations. Triangular graphene flakes have large magnetic moments which vary with the number of hydrogen atoms terminating its edge atoms and scale with its size. Electronic transmission and current-voltage characteristics of these flakes, when contacted with metallic electrodes, reveal spin valve and remarkable rectification features. The transition from ferromagnetic to antiferromagnetic state under bias voltage can, however, terminate the spin polarizing effects for specific flakes. Geometry and size dependent transport properties of graphene flakes may be crucial for spintronic nanodevice applications. © 2010 American Institute of Physics