Quasiparticle Interaction Research Articles

Spatially inhomogeneous states of charge carriers in graphene

The interaction of two-dimensional quasiparticles characterized by a linear dispersion E = ±u|p| (graphene) with impurity potentials is studied. It is shown that discrete levels corresponding to localized states are present in a one-dimensional potential well (quantum wire), whereas such states are absent in a two-dimensional well (quantum dot). The cross section for the scattering of electrons (holes) of graphene by an axially symmetric potential well is determined. It is shown that the cross section tends to a constant value in the limit of infinite particle energy. The effective Hamiltonian is derived for a curved quantum wire of graphene.

Fermi-liquid effects in the Fulde-Ferrell-Larkin-Ovchinnikov state of two-dimensionald-wave superconductors

We study the effects of Fermi-liquid interactions on quasi-two-dimensional $d$-wave superconductors in a magnetic field. The phase diagram of the superconducting state, including the periodic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in high magnetic fields, is discussed for different strengths of quasiparticle many-body interactions within Landau's theory of Fermi liquids. Decreasing the Fermi-liquid parameter ${F}_{0}^{a}$ causes the magnetic spin susceptibility of itinerant electrons to increase, which in turn leads to a reduction of the FFLO phase. It is shown that a negative ${F}_{0}^{a}$ results in a first-order phase transition from the normal to the uniform superconducting state in a finite temperature interval. Finally, we discuss the thermodynamic implications of a first-order phase transition for $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_{5}$.

Disordered d-wave superconductors with interactions

We study the localization properties of disordered d-wave superconductors by means of the fermionic replica trick method. We derive the effective non-linear σ-model describing the diffusive modes related to spin transport which we analyze by the Wilson–Polyakov renormalization group. A lot of different symmetry classes are considered within the same framework. According to the presence or the absence of certain symmetries, we provide a detailed classification for the behavior of some physical quantities, like the density of states, the spin and the quasiparticle charge conductivities. Following the original Finkel'stein approach, we finally extend the effective functional method to include residual quasiparticle interactions, at all orders in the scattering amplitudes. We consider both the superconducting and the normal phase, with and without chiral symmetry, which occurs in the so-called two-sublattice models.

Transport in the Laughlin quasiparticle interferometer: Evidence for topological protection in an anyonic qubit

We report experiments on temperature and Hall voltage bias dependence of the superperiodic conductance oscillations in the novel Laughlin quasiparticle interferometer, where quasiparticles of the 1/3 fractional quantum Hall fluid execute a closed path around an island of the 2/5 fluid. The amplitude of the oscillations fits well the quantum-coherent thermal dephasing dependence predicted for a two point-contact chiral edge channel interferometer in the full experimental temperature range 10.2<T<141 mK. The temperature dependence observed in the interferometer is clearly distinct from the behavior in single-particle resonant tunneling and Coulomb blockade devices. The 5h/e flux superperiod, originating in the anyonic statistical interaction of Laughlin quasiparticles, persists to a relatively high T~140 mK. This temperature is only an order of magnitude less than the 2/5 quantum Hall gap. Such protection of quantum logic by the topological order of fractional quantum Hall fluids is expected to facilitate fault-tolerant quantum computation with anyons.

Properties of the electron spectrum of a closed two-well spherical quantum dot and evolution of the spectrum with the outer-well width

The theory of the electron spectrum of a closed three-sphere two-well spherical quantum dot is developed and the evolution of the spectrum under variations in the width of the outer spherical well from zero (the steady-state spectrum of a simple closed spherical quantum dot) to infinity (quasi-steady-state spectrum of a simple open spherical quantum dot) is studied. The mechanism of damping of electron states in a closed two-well spherical quantum dot due to the increase in the width of the outer spherical well is considered for the first time. It is established that the physical cause of the transformation of the steady-state spectrum into the quasi-steady-state spectrum is the redistribution of the probabilities that an electron excited to the resonance state of the spherical quantum dot is found in the energy states of the quasi-steady-state band in the entire space of the nanosystem. It is shown that the basic properties of an electron in a simple open spherical quantum dot can be reproduced to any specified accuracy in the model of a closed two-well spherical quantum dot with a sufficiently large width of the outer well. The approach developed here is based on the mathematical formulation of the quantum field theory (the Green’s function method). The approach can serve as a basis for the development of the still lacking theory of quasi-steady-state spectra and the theory of interaction of quasiparticles (electrons, holes) with each other (exciton), as well as with quantum fields (photons) in open multilayered nanosystems.

Regime of validity of the pairing Hamiltonian in the study of Fermi gases

The ground state energy and pairing gap of the interacting Fermi gases calculated by the ab initio stochastic method are compared with those estimated from the Bardeen-Cooper-Schrieffer pairing Hamiltonian. We discuss the ingredients of this Hamiltonian in various regimes of interaction strength. In the weakly interacting $(1∕a{k}_{F}⪡0)$ regime the BCS Hamiltonian should describe Landau quasiparticle energies and interactions, on the other hand, in the strongly pairing regime, that is, $1∕a{k}_{F}\ensuremath{\gtrsim}0$, it becomes part of the bare Hamiltonian. However, the bare BCS Hamiltonian is not adequate for describing atomic gases in the regime of weak to moderate interaction strength $\ensuremath{-}\ensuremath{\infty}&lt;1∕a{k}_{F}&lt;0$ such as $a{k}_{F}\ensuremath{\sim}\ensuremath{-}1$.

Behavior of single-scale hard small-xprocesses in QCD near the black disk limit

We argue that at sufficiently small Bjorken x where pQCD amplitudes rapidly increase with energy and violate probability conservation the shadowing effects in the single-scale small x hard QCD processes can be described by an effective quantum field theory of interacting quasiparticles--perturbative QCD ladders. We find, within the WKB approximation, that the smallness of the QCD coupling constant ensures the hierarchy among many-quasiparticle interactions evaluated within the physical vacuum and, in particular, the dominance in the Lagrangian of the triple quasiparticle interaction. It is explained that the effective field theory considered near the perturbative QCD vacuum contains a tachyon relevant for the divergency of the perturbative QCD series at sufficiently small x. We solve the equations of motion of the effective field theory within the WKB approximation and find the physical vacuum and the transitions between the false (perturbative) and physical vacua. Classical solutions which dominate transitions between the false and physical vacua are kinks that cannot be decomposed into perturbative series over the powers of {alpha}{sub s}. These kinks lead to color inflation and the Bose-Einstein condensation of quasiparticles. The account of the quantum fluctuations around the WKB solution reveals the appearance of the ''massless'' particles--phonons. It is explainedmore » that phonons are relevant for the black disk behavior of cross sections of small x processes. The Bose-Einstein condensation of the ladders produces a color network occupying a ''macroscopic'' longitudinal volume. We discuss briefly the possible detection of new QCD effects. We outline albeit briefly the relationship between the small x hard QCD processes and the coherent critical phenomena.« less

Dielectric response function in [formula omitted]– [formula omitted]– [formula omitted] model

The dielectric response function ε ( q , ω ) has been derived in the frame t – J – V model. In addition to the acoustic plasmon modes, a new collective charge excitation mode has been found. We have shown, that in layered cuprates the inverse dielectric function 1 / ε ( q , ω ) may be negative in the area which partially overlaps the Fermi surface. Then, the interaction of quasiparticles via plasmon field causes the higher order harmonics in d x 2 - y 2 -wave symmetry of the superconducting gap driven mainly by the superexchange interaction.

Theory of Superconductivity in CeMIn5(M=Co, Rh, Ir) on the Basis of the Three Dimensional Periodic Anderson Model

CeMIn 5 (M=Co, Rh, Ir) are heavy fermion compounds with a HoCoGa 5 -type structure. CeCoIn 5 and CeIrIn 5 become superconducting at ambient pressure with T c =2.3 and 0.4 K, respectively. On the other hand, CeRhIn 5 is an antiferromagnet at ambient pressure, and becomes superconducting under pressures greater than 1.6 GPa. Nuclear-quadrupole-resonance (NQR), thermal conductivity, specific heat, and electrical resistivity measurements indicate that the superconductivity in CeMIn 5 has line nodes. However, the pairing symmetry between d x 2 - y 2 and d x y is controversial based on the angle-dependent measurements of thermal conductivity and specific heat under the magnetic field, respectively. Therefore, we investigate the gap structure of CeMIn 5 by a detailed calculation. In order to reproduce the band structure characteristics of CeMIn 5 , we introduce a three-dimensional (3D) periodic Anderson model (PAM) and solve the Eliashberg equations. Thus, we identify the gap structure of CeMIn 5 as the d x 2 - y 2 symmetry. In addition, we discuss the pressure dependence of T c and show that two opposing factors determine T c . One factor is the momentum dependence of quasi-particle interaction and the other factor is the wave-function renormalization factor.

Quasi-particle interaction in nuclear matter from chiral pion–nucleon dynamics

Based on a recent chiral approach to nuclear matter we calculate the in-medium interaction of nucleons at the Fermi surface | p → 1 , 2 | = k f . The isotropic part of this quasi-particle interaction is characterized by four density-dependent (dimensionful) Fermi-liquid parameters: f 0 ( k f ) , f 0 ′ ( k f ) , g 0 ( k f ) and g 0 ′ ( k f ) . In the approximation to 1 π-exchange and iterated 1 π-exchange (which as such leads already to a good nuclear matter equation of state) we find a spin–isospin interaction strength of g 0 ′ ( 2 m π ) = 1.14 fm 2 , compatible with existing empirical values. The consistency relations to the nuclear matter compressibility K and the spin/isospin asymmetry energies serve as a check on our perturbative calculation. In the next step we include systematically the contributions from 2 π-exchange with virtual Δ ( 1232 ) -isobar excitation which have been found important for good single-particle properties and spin-stability of nuclear matter. Without any additional short-distance terms (contributing proportional to k f 2 ) the spin-dependent Fermi-liquid parameters g 0 ( k f 0 ) and g 0 ′ ( k f 0 ) come out far too large. Estimates of these short-distance parameters from realistic NN-potentials go in the right direction, but sizeable enhancement factors are still needed to reproduce the empirical values of g 0 ( k f 0 ) and g 0 ′ ( k f 0 ) . This points towards the importance of higher order iterations subsumed in the induced interaction. We consider also the tensor part of the quasi-nucleon interaction at the Fermi surface. In comparison to the leading 1 π-exchange tensor interaction we find from the 2 π-exchange corrections almost a doubling of the isoscalar tensor strength h 0 ( k f ) , whereas the isovector tensor strength h 0 ′ ( k f ) is much less affected. These features are not changed by the inclusion of the chiral πNΔ-dynamics. The l = 1 tensor Fermi-liquid parameters h 1 ( k f ) and h 1 ′ ( k f ) follow a similar pattern.

Field dependent quasiparticles in a strongly correlated local system

We show that quasiparticles in a magnetic field of arbitrary strength $H$ can be described by field dependent parameters. We illustrate this approach in the case of an Anderson impurity model and use the numerical renormalization group (NRG) to calculate the renormalized parameters for the levels with spin $\sigma$, $\tilde\epsilon_{\mathrm{d},\sigma}(H)$, resonance width $\tilde\Delta(H)$ and the effective local quasiparticle interaction $\tilde U(H)$. In the Kondo or strong correlation limit of the model the progressive de-renormalization of the quasiparticles can be followed as the magnetic field is increased. The low temperature behaviour, including the conductivity, in arbitrary magnetic field can be calculated in terms of the field dependent parameters using the renormalized perturbation expansion. Using the NRG the field dependence of the spectral density on higher scales is also calculated.

Theory of Superconductivity in CeCoIn5on the Basis of a Three-Dimensional Periodic Anderson Model

The pairing symmetry for CeCoIn 5 is controversial between d x 2 - y 2 and d x y from the angle-dependent measurements of thermal conductivity and specific heat under the magnetic field, respectively. Therefore we investigate the gap structure of CeCoIn 5 , and identify it as d x 2 - y 2 symmetry. In addition, we show that two opponent factors determine the pressure dependence of T c : the momentum dependence of quasi-particle interaction, and the wave-function renormalization factor.

Aharonov-Bohm Superperiod in a Laughlin Quasiparticle Interferometer

We report an Aharonov-Bohm superperiod of five magnetic flux quanta (5h/e) observed in a Laughlin quasiparticle interferometer, where an edge channel of the 1/3 fractional quantum Hall fluid encircles an island of the 2/5 fluid. This result does not violate the gauge invariance argument of the Byers-Yang theorem because the magnetic flux, in addition to affecting the Aharonov-Bohm phase of the encircling 1/3 quasiparticles, creates the 2/5 quasiparticles in the island. The superperiod is accordingly understood as imposed by the anyonic statistical interaction of Laughlin quasiparticles.

Quasiparticle dephasing time in disorderedd-wave superconductors

We evaluate the low-temperature cutoff for quantum interference 1/tf induced in a d-wave superconductor by the diffusion enhanced quasiparticle interactions in the presence of disorder. We carry out our analysis in the framework of the non-linear sigma-model which allows a direct calculation of 1/tf, as the mass of the transverse modes of the theory. Only the triplet amplitude in the particle-hole channel and the Cooper amplitude with is pairing symmetry contribute to 1/tf. We discuss the possible relevance of our results to the present disagreement between thermal transport data in cuprates and the localization theory for d-wave quasiparticles.

Experimental arguments in favor of refining model ideas of the cascade gamma decay of a compound state of a complex nucleus

An analysis of the entire body of data on the intensities of two-step gamma cascades studied in thermal-neutron capture for more than 50 nuclei from the range 27 ≤ A ≤ 199 suggests that such processes should be described in terms of model concepts that are much more involved than those currently adopted by experimentalists. According to the results of this analysis, models of relevant parameters, such as the density of excited levels and radiative strength functions for dipole gamma transitions, should take into account more explicitly the coexistence and interaction of quasiparticle and phonon interactions. A direct inclusion of the idea that a second-order phase transition occurs and affects not only level densities but also radiative strength functions for dipole transitions may prove to be necessary. These conclusions concern primarily the excitation-energy region below a value of about 0.5Bn.

Dynamics of Cooper pair formation inBi2Sr2CaCu2O8+δ

We report ultrafast measurements of the transient terahertz conductivity in the high-${T}_{C}$ superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$. After depletion of the superconducting condensate by optical excitation, the ensuing dynamics is tracked with high sensitivity at various excitation densities and temperatures. We measure low-energy spectra and shapes of decay that demonstrate a bimolecular kinetics of condensate formation on a picosecond time scale and yield a measure of quasiparticle interactions hidden in linear spectroscopy.

Quasiparticles in transition metals with strong local correlations: band formation and collective effects

AbstractDedicated to Bernhard Mühlschlegel on the occasion of his 80th birthdayWe address the question, whether low lying one‐particle excitations in the Fermi‐liquid phase of highly correlated electron systems can form well defined bands of nearly stable quasiparticles, and comment on features of a universal band picture for these systems. We outline how to derive a description of instabilities to magnetic, charged ordered or superconducting phases, which bears a close analogy to Stoner theory and lends itself to an interpretation in terms of quasiparticle bands and residual interactions at low temperatures. Concepts and problems are illustrated via calculations for some standard models of solid‐state theory using modern many‐body techniques like NRG, NCA and DMFT. Differences to conventional band structure theory are pointed out. We shortly comment on the relevance of these questions to the physics of inhomogeneous systems and small particles.

The Bose-Hubbard model: from Josephson junction arrays to optical lattices

Dedicated to Bernhard Mühlschlegel on the occasion of his 80th birthday We address the question, whether low lying one-particle excitations in the Fermi-liquid phase of highly correlated electron systems can form well defined bands of nearly stable quasiparticles, and comment on features of a universal band picture for these systems. We outline how to derive a description of instabilities to magnetic, charged ordered or superconducting phases, which bears a close analogy to Stoner theory and lends itself to an interpretation in terms of quasiparticle bands and residual interactions at low temperatures. Concepts and problems are illustrated via calculations for some standard models of solid-state theory using modern many-body techniques like NRG, NCA and DMFT. Differences to conventional band structure theory are pointed out. We shortly comment on the relevance of these questions to the physics of inhomogeneous systems and small particles.

Composite-Fermion Spin Excitations asνApproaches1/2: Interactions in the Fermi Sea

Measurements of low-lying spin excitations by inelastic light scattering unveil a delicate balance between spin reversal and Fermi energies in the Fermi sea of composite fermions that emerges in the limit of nu --> 1/2. The interplays between these two fundamental quasiparticle interactions are uncovered in lowest spin-flip excitations in which the spin orientation and Landau level index of composite fermions change simultaneously. A collapse of the spin-flip excitation gap as nu --> 1/2 is linked to vanishing quasiparticle energy level spacings and loss of full spin polarization.

Short range correlations and spectral functions in asymmetric nuclear matter

Dynamical correlations in asymmetric infinite nuclear matter are investigated in a transport theoretical approach. Self-energies due to short range correlations and their influence on the nucleon spectral functions are described in an approach accounting for a realistic treatment of mean-field dynamics and a self-consistently derived quasiparticle interaction. Landau–Migdal theory is used to derived the short range interaction from a phenomenological Skyrme energy density functional. The spectral functions in asymmetric nuclear matter are found to follow in their gross features closely the patterns observed previously in symmetric nuclear matter. An interesting sensitivity of dynamical self-energies and spectral functions on the momentum structure of the underlying interactions is found.