Quasiparticle Interaction Research Articles

Effects of doping on thermally excited quasiparticles in the high-Tcsuperconducting state

The physical properties of low-energy superconducting quasiparticles in high-${T}_{c}$ superconductors are examined using magnetic penetration depth and specific-heat experimental data. We find that the low-energy density of states of quasiparticles of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ scales with $(x\ensuremath{-}{x}_{c}{)/T}_{c}$ to the leading-order approximation, where ${x}_{c}$ is the critical doping concentration below which ${T}_{c}=0.$ The linear temperature term of the superfluid density is renormalized by quasiparticle interactions and the renormalization factor times the Fermi velocity is found to be doping independent.

Ground state diagram of a 1D electron–phonon system

The ground states of a quasiparticle (electron, hole or exciton) are investigated in a one-dimensional (1D) molecular chain within the variational approach with account of quasiparticle interaction with acoustical and optical phonons. The ground state diagrams are obtained, which show the intervals of the values of dimensionless electron–phonon coupling constant, and nonadiabaticity parameter, which correspond to the regimes of an almost free electron state, small polaron, and spontaneously localized (large polaron or soliton-like) state, respectively. The creation of autolocalized states has a threshold with respect to the value of electron–phonon interaction. Such states correspond to the minimum of energy within some finite interval of the values of electron–phonon coupling constants, the critical values of which depend on the nonadiabaticity parameters of each phonon mode. It is shown that the presence of the second phonon mode enhances the stability of the autolocalized states as compared with the single mode case, and expanses the region of soliton existence in the one-mode nonadiabatic regime.

Polaron dynamics and Peierls–Nabarro barrier in a discrete molecular chain

The dynamics of an autolocalized quasiparticle in a discrete lattice is investigated analytically and numerically, taking into account the quasiparticle interaction with acoustic phonons. The dependence of the parameters of a soliton-like polaron on the carrying wave vector at large values of the latter is shown to differ from those predicted by the continuum models. We find that the saturation of the polaron velocity in a discrete system occurs below the sound velocity in the chain, a result which is in agreement with the experimental observations of the saturation of the drift velocity in some low-dimensional compounds. The potential of the Peierls–Nabarro relief caused by the lattice discreteness is calculated using perturbation theory, and pinning of a soliton by this barrier is studied numerically. For strongly localized, narrow polarons a critical value of the wave vector is needed to overcome the intersite barrier.

Quasiparticle interactions in fractional quantum Hall systems: Justification of different hierarchy schemes

The pseudopotentials describing the interactions of quasiparticles in fractional quantum Hall (FQH) states are studied. Rules for the identification of incompressible quantum fluid ground states are found, based upon the form of the pseudopotentials. States belonging to the Jain sequence $\ensuremath{\nu}{=n(1+2pn)}^{\ensuremath{-}1},$ where n and p are integers, appear to be the only incompressible states in the thermodynamic limit, although other FQH hierarchy states occur for finite size systems. This explains the success of the composite Fermion picture.

Dynamical Response of Liquid 3He Films Adsorbed on Solid Substrates

Within the general framework of linear response theory, we investigate the excitation spectrum of 3He films adsorbed on substrates. Starting from the quasiparticle spectrum and the effective interaction derived in a finite range density functional scheme, we analyze the properties of the particle-hole (ph) excitations as well as the collective modes in the Random Phase Approximation. On the one hand, the most important features of the ph spectrum, originated in the band structure of the single particle spectrum together with the fermionic character of the quasiparticles, are clearly exhibited when the free response to a probe parallel or transverse to the film is considered. On the other hand, for any film, the collective spectrum presents various branches which, in many situations, can be associated with predominantly intraband longitudinal transitions or with almost pure interband transitions. Analyzing the RPA response of a monolayer on substrates of very different adsorbing power (cesium and graphite) we can interpret the nature of the existing collective excitations and determine the influence of the geometrical features of the film on the effective quasiparticle interaction responsible of these oscillations.

Dynamic equations for individual spins in materials with magnetic order: The ideal ferromagnet

The time and temperature dependence of individual spins is studied in an ideal ferromagnet. Starting from the Heisenberg Hamiltonian in a magnetic field and, building on linear-response theory, we derive the linearized microscopic analogy to the macroscopic Landau-Lifshitz equations of motion for magnetization. The dynamical equations take into account the influence of spin correlations in addition to the molecular field. In order to illustrate the validity of our approach, we calculate the temperature-dependent spin-wave spectrum and Landau damping by employing a finite-temperature perturbation theory for spin operators. The latter is adapted to suit higher-order spin-correlation functions which can be described by simple quasiparticle interactions with the spin system. Our results agree with those established on the thermodynamics of the Heisenberg model. In addition, we provide a method of how to deduce macroscopic equations of the Landau-Lifshitz type from first principles, including temperature dependence and damping.

Microscopic theory of weak pseudogap behavior in the underdoped cuprate superconductors: General theory and quasiparticle properties

We use a solution of the spin fermion model which is valid in the quasistatic limit $\ensuremath{\pi}T\ensuremath{\gg}{\ensuremath{\omega}}_{\mathrm{sf}},$ found in the intermediate (pseudoscaling) regime of the magnetic phase diagram of cuprate superconductors, to obtain results for the temperature and doping dependence of the single particle spectral density, the electron-spin fluctuation vertex function, and the low frequency dynamical spin susceptibility. The resulting strong anisotropy of the spectral density and the vertex function lead to the qualitatively different behavior of hot [around $\mathbf{k}=(\ensuremath{\pi},0)]$ and cold [around $\mathbf{k}=(\ensuremath{\pi}/2,\ensuremath{\pi}/2)]$ quasiparticles seen in ARPES experiments. We find that the broad high energy features found in ARPES measurements of the spectral density of the underdoped cuprate superconductors are determined by strong antiferromagnetic (AF) correlations and incoherent precursor effects of an SDW state, with reduced renormalized effective coupling constant. Due to this transfer of spectral weight to higher energies, the low frequency spectral weight of hot states is strongly reduced but couples very strongly to the spin excitations of the system. For realistic values of the antiferromagnetic correlation length, their Fermi surface changes its general shape only slightly but the strong scattering of hot states makes the Fermi surface crossing invisible above a pseudogap temperature ${T}_{*}.$ The electron spin-fluctuation vertex function, i.e., the effective interaction of low energy quasiparticles and spin degrees of freedom, is found to be strongly anisotropic and enhanced for hot quasiparticles; the corresponding charge-fluctuation vertex is considerably diminished. We thus demonstrate that, once established, strong AF correlations act to reduce substantially the effective electron-phonon coupling constant in cuprate superconductors.

Quasiparticle Interactions in Two-DimensionalH3eonH4eFilms

Measurements are reported of the heat capacity of ${}^{3}\mathrm{He}$ bound to an atomically layered ${}^{4}\mathrm{He}$ film on the surface of graphite. At ${}^{3}\mathrm{He}$ coverages ${n}_{3}l4{\mathrm{nm}}^{\ensuremath{-}2}$, the ${}^{3}\mathrm{He}$ forms a uniform 2D Fermi system. Thereafter with increasing ${n}_{3}$ we resolve two steps in the heat capacity, each arising from the formation of a further 2D continuum built on excited surface-normal states. We infer information on the hydrodynamic mass, quasiparticle interactions, and energetics of these states. Both short range repulsive interactions and ripplon mediated quasiparticle interactions are important.

Theory of Hall Effect and Electrical Transport in High-TcCuprates: Effects of Antiferromagnetic Spin Fluctuations

In the normal state of high-Tc cuprates, the Hall coefficient shows remarkable temperature dependence, and its absolute value is enhanced in comparison with that value simply estimated on the basis of band structure. It has been recognized that this temperature dependence of the Hall coefficient is due to highly anisotropic quasiparticle damping rate on the Fermi surface. In this paper we further take account of the vertex correction to the current vertex arising from quasiparticle interactions. Then the transport current is transformed to a large extent from the quasiparticle velocity, and is no longer proportional to the latter. As a consequence some pieces of the Fermi surface outside of the antiferromagnetic Brillouin zone make negative contribution to the Hall conductivity, even if the curvature of the Fermi surface is hole-like. The Hall coefficient is much larger at low temperatures than the estimate made without the vertex correction. Temperature dependence of the antiferromagnetic spin correlation length is also crucial to cause remarkable temperature dependence of the Hall coefficient. In our treatment the Hall coefficient of the electron-doped cuprates can be negative despite hole-like curvature of the Fermi surface.

Point-contact spectroscopy of DyNi 5

We have studied the electron—quasiparticle interaction function in the oriented hexagonal single crystal DyNi 5 using the point-contact (PC) spectroscopy above and below T N = 11.6 K. The observed peaks in the low-energy range up to ≈ 10 meV have the crystalline electric field (CEF) origin (the excitations of 4f electrons from ground CEF level to higher excited levels). The broad maximum at about 18 meV is connected with phonons DyNi 5.

Doping dependence of the pseudogap inLa2−xSrxCuO4

We report the results of Raman scattering experiments on single crystals of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ that span the range from underdoped $(x=0.10)$ to overdoped $(x=0.22).$ The spectra are consistent with the existence of a strong anisotropic quasiparticle interaction that results in a normal state depletion of spectral weight from regions of the Fermi surface located near the zone axes. The strength of the interaction decreases rapidly with increasing hole concentration and the spectral evidence for the pseudogap vanishes when the optimum doping level is reached. The results suggest that the pseudogap and superconducting gap arise from different mechanisms.

Observation of thermally activated quasiparticle interaction by ballistic electron transport and electron focusing

Electron focusing patterns originating from ballistic electron transport in single crystalline materials are observed in transmission. Spatially resolved excitation by scanning electron microscopy and detection by a point contact is employed for imaging. The effects of electron–phonon and electron–electron interactions are demonstrated for bismuth by observation of the thermal dependence of the spatially resolved transport properties of quasiparticles in the range of 5–100 K.

Imaging nonequilibrium phonons in Nb

Phonon imaging techniques are applied to superconducting Nb. Due to the superconducting energy gap, the electronic scattering of phonons in metals is greatly reduced and ballistic phonon transport is observed. The non-equilibrium phonons which are generated in the experiment are accompanied by non-equilibrium quasiparticles. These quasiparticles modify the observed phonon propagation and also recombine to generate additional phonons which are detected. Thus, our experiments provide information not only about phonon propagation in a Nb lattice, but also about the phonon–quasiparticle interactions and the dynamics of the non-equilibrium quasiparticles.

Point-contact spectroscopy of the electron–phonon interaction in RENi 5 (RE-rare earths)

The comparison of the characteristic point-contact spectra of heterocontacts between RENi 5 (RE=La, Ce, Pr, Nd, Sm, Dy, Y) and Cu is presented. It is shown that in all samples the contribution of the electron–phonon interaction to the electron–quasiparticle interaction function is qualitatively similar. This observation reflects the very similar lattice constants for all RENi 5 and supports the manner of subtracting the phonon part of electrical resistivity of bulk RENi 5 samples.

Long-wavelength collective excitations in a two-dimensional spin-polarized electron gas: A phenomenological theory

The dispersion relations of the collective excitations induced in a 2D spin-polarized electron gas are derived within the framework of the Landau-Silin theory applied to a two-component Fermi liquid. The quasiparticle interaction is described by spin-dependent coefficients, which are parametric functions of the initial degree of polarization. The motion of the low-energy excitations satisfies a semiclassical transport equation solved consistently with the Maxwell equations for the electromagnetic field produced by spin and charge fluctuations. The long-wavelength limit of the self-sustained oscillations is analyzed as a function of the initial degree of polarization of the system, preset by a dc magnetic induction. thinsp {copyright} {ital 1998} {ital The American Physical Society}

Collective excitations in a 2D electron system: Canted field geometry

We investigate the charge and spin collective modes induced in a 2D electron gas by a weak electromagnetic perturbation in the presence of a dc magnetic field which makes an angle θ with the electron layer. The excitation frequencies are determined within the framework of the Landau–Silin theory of Fermi liquids by solving a semi-classical equation for transport in the self-consistent electromagnetic field associated with particle density fluctuations. The quasiparticle interaction is spin dependent and varies parametrically with the degree of spin polarization. In the long wavelength limit, we obtain analytic results for the frequencies of the collective modes as functions of θ.

Spectral Function of Superconducting Cuprates near Optimal Doping

We argue that the unusual peak-dip-hump features observed in photoemission experiments on Bi2212 at $T\ensuremath{\ll}{T}_{c}$ can be explained by the interaction of the fermionic quasiparticles with overdamped spin fluctuations. We show that the strong spin-fermion interaction combined with the feedback effect on the spin damping due to superconductivity yields the fermionic spectral function $A({\mathbf{k}}_{F},\ensuremath{\omega})$ which simultaneously displays a quasiparticle peak at $\ensuremath{\omega}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{\Delta}$ and a broad maximum (hump) at $\ensuremath{\omega}\ensuremath{\gg}\ensuremath{\Delta}$. In between the two regimes, the spectral function has a dip at $\ensuremath{\omega}\ensuremath{\sim}2\ensuremath{\Delta}$. We argue that our theory also explains the tunneling data.

Surface-State Lifetime Measured by Scanning Tunneling Spectroscopy

Quasiparticle interactions broaden spectral features at surfaces, which can be measured using the scanning tunneling microscope (STM). We report the first study of lifetime effects on Shockley surface-state electrons using low-temperature STM spectroscopy. Data taken from Ag(111) are analyzed using detailed tunneling calculations and a simple model, and are found to correspond to a self-energy of $\ensuremath{\Sigma}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4.9\ifmmode\pm\else\textpm\fi{}0.6\mathrm{meV}$. This is considerably below values determined by angle-resolved photoemission, but remains higher than theoretical predictions.

Comprehensive numerical and analytical study of two holes doped into the two-dimensionalt−Jmodel

We report on a detailed examination of numerical results and analytical calculations devoted to a study of two holes doped into a two-dimensional, square lattice described by the t-J model. Our exact diagonalization numerical results represent the first solution of the exact ground state of 2 holes in a 32-site lattice. Using this wave function, we have calculated several important correlation functions, notably the electron momentum distribution function and the hole-hole spatial correlation function. Further, by studying similar quantities on smaller lattices, we have managed to perform a finite-size scaling analysis. We have augmented this work by endeavouring to compare these results to the predictions of analytical work for two holes moving in an infinite lattice. This analysis relies on the canonical transformation approach formulated recently for the t-J model. From this comparison we find excellent correspondence between our numerical data and our analytical calculations. We believe that this agreement is an important step helping to justify the quasiparticle Hamiltonian, and in particular, the quasiparticle interactions, that result from the canonical transformation approach. Also, the analytical work allows us to critique the finite-size scaling ansatzes used in our analysis of the numerical data. One important feature that we can infer from this successful comparison involves the role of higher harmonics in the two-particle, d-wave symmetry bound state -- the conventional (\cos(k_x) - \cos(k_y)) term is only one of many important contributions to the d-wave symmetry pair wave function.

ANISOTROPIC STRONG COUPLING CALCULATION OF THE LOCAL ELECTROMAGNETIC RESPONSE OF HIGH- Tc SUPERCONDUCTORS

The electromagnetic response of the CuO 2-planes is calculated within a strong coupling theory using model tight binding bands and momentum dependent pairing interactions representing spin fluctuation and phonon exchange. The superconducting state resulting from these interactions has d-wave symmetry. With phonon exchange included the order parameter amplitude grows rapidly below T c at elevated frequencies which leads to improved agreement with the observed temperature dependence of the penetration depth. Good agreement between theory and experiment can only be achieved if it is assumed that the strength of the quasiparticle interaction decreases with temperature in the superconducting state. The amount of this reduction depends sensitively on the momentum dependence of the interactions, the energy dispersion and the position of the Fermi line.