Quasiparticle Description Research Articles

RENORMALIZATION GROUP ANALYSIS OF QUANTUM CRITICAL POINTS IN d-WAVE SUPERCONDUCTORS

We describe a search for renormalization group fixed points which control a second-order quantum phase transition between a dx2-y2-superconductor and some other superconducting ground state. Only a few candidate fixed points are found. In the finite temperature (T) quantum-critical region of some of these fixed points, the fermion quasiparticle lifetime is very short and the spectral function has an energy width of order kBT near the Fermi points. Under the same conditions, the thermal conductivity is infinite in the scaling limit. We thus provide simple, explicit, examples of quantum theories in two dimensions for which a purely fermionic quasiparticle description of transport is badly violated.

Equation of state of deconfined matter at finite chemical potential in a quasiparticle description

A quasiparticle description of the thermodynamics of deconfined matter, reproducing both the perturbative limit and nonperturbative lattice QCD data at finite temperature, is generalized to finite chemical potential. By a flow equation resulting from Maxwell's relation, the equation of state is extended from zero to nonzero quark densities. The impact of the massive strange flavor is considered and implications for cold, charge-neutral deconfined matter in $\ensuremath{\beta}$ equilibrium in compact stars are given.

Parity breaking and phase transition induced by a magnetic field in high Tc superconductors

We suggest that the 2 + 1-dimensional Gross-Neveu model can give an effective field theory description of low-energy quasiparticles in high temperature superconductors. The magnetic catalysis of dynamical symmetry breaking is examined. The model shows that a magnetic field can induce a phase transition associated with parity breaking. In particular, it is intended to give an explanation of a second phase transition observed in a recent experiment by Krishana et al.

SPECTRAL PROPERTIES OF THE GENERALISED SPIN-FERMION MODELS

In order to account for competition and interplay of localized and itinerant magnetic behaviour in correlated many body systems with complex spectra the various types of spin-fermion models have been considered in the context of the Irreducible Green's Functions (IGF) approach. Examples are generalised d–f model and Kondo–Heisenberg model. The calculations of the quasiparticle excitation spectra with damping for these models has been performed in the framework of the equation-of-motion method for two-time temperature Green's Functions within a non-perturbative approach. A unified scheme for the construction of Generalised Mean Fields (elastic scattering corrections) and self-energy (inelastic scattering) in terms of the Dyson equation has been generalised in order to include the presence of the two interacting subsystems of localised spins and itinerant electrons. A general procedure is given to obtain the quasiparticle damping in a self-consistent way. This approach gives the complete and compact description of quasiparticles and show the flexibility and richness of the generalised spin-fermion model concept.

A NUMERICAL AND ANALYTICAL STUDY OF TWO HOLES DOPED INTO THE 2D t– J MODEL

Exact diagonalization numerical results are presented for a 32-site square cluster, with two holes propagating in an antiferromagnetic background described by the t– J model. We characterize the wave function of the lowest energy bound state found in this calculation, which has d x 2− y 2 symmetry. Analytical work is presented, based on a Lang–Firsov-type canonical transformation derived quasiparticle Hamiltonian, that accurately agrees with numerically determined values for the electron momentum distribution function and the pair correlation function. We interpret this agreement as strong support for the validity of this description of the hole quasiparticles.

The unusual normal and superconducting states of underdoped cuprates

There is increasing experimental evidence that the superconducting energy gap gD 0 in the underdoped cuprates is independent of doping concentration x while the superfluid density is linear in x. We show that under these conditions, thermal excitation of the quasiparticles is very effective in destroying the superconducting state, so that T c is proportional to xΔ 0 and part of the gap structure remains in the normal state. We then estimate H c2 and predict it to be proportional to x 2. We also discuss to what extent the assumptions that go into the quasiparticle description can be derived in the U(1) and SU(2) formulations of the t- J model.

Unusual Superconducting State of Underdoped Cuprates

There is increasing experimental evidence that the superconducting energy gap $\Delta_0$ in the underdoped cuprates is independent of doping concentration $x$ while the superfluid density is linear in $x$. We show that under these conditions, thermal excitation of the quasiparticles is very effective in destroying the superconducting state, so that $T_c$ is proportional to $x\Delta_0$ and part of the gap structure remains in the normal state. We then estimate $H_{c2}$ and predict it to be proportional to $x^2$. We also discuss to what extent the assumptions that go into the quasiparticle description can be derived in the U(1) and SU(2) formulations of the t-J model.

New Aspects of Non-Fermi-Liquid Behaviour in Q1D Metals

The second order corrections to magnetic moment, M, magnetic susceptibility, χ, and specific heat, C, due to electron-electron (”-e”) interactions are calculated in a Q1 D conductor (TMTSF) 2 ClO 4 in a metallic phase. Strong magnetic oscillations of these thermodynamical quantities are found to exist due to the oscillations of matrix elements of ”e-e” interactions inthe Peierls channel under the condition of magnetic breakdown. We note that the frequency of the oscillations corresponds to open (”interference”) orbits that contradicts Fermi liquid description of quasiparticles. We demonstrate that the relative magnitudes of these forbidden oscillations are anomalously large: δɛ/ɛ 0 ~ 1–10, δM/M 0 ~ 10 −1, and δC/Co ~ 10 −2.

Phase string effect in the t-J model: General theory

We reexamine the problem of a hole moving in an antiferromagnetic spin background and find that the injected hole will always pick up a sequence of nontrivial phases from the spin degrees of freedom. Previously unnoticed, such a stringlike phase originates from the hidden Marshall signs which are scrambled by the hopping of the hole. We can rigorously show that this phase string is nonrepairable at low energy and give a general proof that the spectral weight Z must vanish at the ground-state energy due to the phase-string effect. Thus, the quasiparticle description fails here and the quantum interference effect of the phase string dramatically affects the long-distance behavior of the injected hole. We introduce a so-called phase-string formulation of the t-J model for a general number of holes in which the phase-string effect can be explicitly tracked. As an example, by applying this new mathematical formulation in one dimension, we reproduce the well-known Luttinger-liquid behaviors of the asymptotic single-electron Green's function and the spin-spin correlation function. We can also use the present phase-string theory to justify previously developed spin-charge separation theory in two dimensions, which offers a systematic explanation for the transport and magnetic anomalies in the high-${\mathrm{T}}_{\mathrm{c}}$ cuprates.

Quasiparticle invariants and energy transfer between particles and fields

The second-order approximation in a quasiparticle description of magnetized plasmas is studied. The second-order adiabatic invariant of quasiparticle motion is found. Global adiabatic invariants for a magnetized plasma are revealed, and their possible role in energy exchange between particles and fields, nonlinear mode cascades and global plasma stability is discussed.

The independent-pair model for one-dimensional fermions interacting through a repulsive delta-potential

The single-particle energies and the ground-state energy for a one-dimensional system of interacting fermions via a repulsive δ-function potential are self-consistently calculated using the Brueckner and Gammel method. The breakdown of the usual quasi-particle description is signaled in this approach by the occurrence of a negative gap at ± k F in the excitation spectrum. A new ground-state configuration of the system emerges: a reduced Fermi sea and an assembly of bosons at ± k F. The ground-state energy computed within this scheme is in remarkable agreement with the Bethe-ansatz results at all coupling constants.

Magnetic Oscillations in a Normal State of Organic Conductors: Many-Body Approach

We argue that magnetic and angular oscillations observed in quasi-one-dimensional (Q1D) organic conductors represent a new type of many-body phenomena. The physical nature of such effects as magic angles in (TMTSF) 2 ClO 4 , (TMTSF) 2 PF 6 , and (DMET-TSeF) 2 AuCl 2 as well as rapid magnetic oscillations in (TMTSF) 2 ClO 4 is shown to be beyond the standard theory of metals. Below we discuss an explanation of these phenomena which utilizes unusual many-body effect - a change of an effective dimensionality of electron-electron (e-e) interactions with changing both a magnitude and a direction of a magnetic field. We show that some exotic transport properties of a metallic state can be interpreted in terms of these dimensional crossovers. We also demonstrate that magnetic field dependence of e-e interactions has to break Fermi liquid description of quasiparticles at high magnetic fields, H ≥ 25 - 30 T. This leads to the appearance of strong forbidden oscillations of magnetic susceptibility, δ x / xo ∼1-10, and magnetic moment, δM/M o ∼-0.1. All of the above mentioned unique properties of a metallic phase in (TMTSF) 2 X and (DMET-TSeF)AuCl 2 allow us to call it an anomalous metallic phase.

Magnetotransport in nearly antiferromagnetic Fermi liquids

Abstract The temperature dependence of the normal-state dc magnetotransport in high-temperature superconductors is investigated using the nearly anti-ferromagnetic Fermi-liquid description of planar quasiparticles. A direct numerical solution of the Boltzmann equation is obtained in a finite magnetic field. Highly anisotropic quasiparticle scattering is found at different regions of the Fermi surface, giving rise to the measured anomalous temperature dependences of the resistivity, the Hall coefficient and the orbital magnetoresistance.

Second-order adiabatic invariant of particle motion in electromagnetic fields

The second-order adiabatic invariant of particle motion in electromagnetic fields is found within the context of the quasi-particle description. Macroscopic invariants for the magnetized plasma are derived, and the constraints on stable plasma states which follow from them are discussed.

Anomalous Hall effect in YBa2Cu3O7.

The temperature dependence of the normal state Hall effect and magnetoresistance in YBa$_2$Cu$_3$O$_7$ is investigated using the Nearly Antiferromagnetic Fermi Liquid description of planar quasiparticles. We find that highly anisotropic scattering at different regions of the Fermi surface gives rise to the measured anomalous temperature dependence of the resistivity and Hall coefficient while yielding the universal temperature dependence of the Hall angle observed for both clean and dirty samples. This universality is shown to arise from the limited momentum transfers available for the anomalous, spin fluctuation scattering and is preserved for any system with strong antiferromagnetic correlations.

Constraints on energy transfer and temperature gradients in magnetized plasma

Generic constraints on energy transfer in inhomogeneous magnetized plasmas and on plasma stability are derived within the context of the quasiparticle description. Their relation to the problem of turbulent fluctuations driven by temperature gradients is discussed in regard to temperature gradient thresholds and sufficient conditions for stability.

Quasiparticle pair creation in unstable superflow.

Landau`s instability mechanism in superflow is considered with special attention given to the role of nonuniformity in the flow. Linear stability analysis applied to the first in a series of approximate microscopic equations for the superfluid reveals a growth rate for Landau`s instability proportional to the shear in the flow. In a quasiparticle description, the shear acts as a source of particle pair creation. The observation of roton-pair creation in experiments with electron bubbles in helium is offered as evidence of this phenomenon.

Quasi-particles in magnetized plasmas: second-order approximation

A second-order approximation is formulated and studied within the context of the quasi-particle description of magnetized plasmas. The general case of relativistic particles in non-uniform but stationary magnetic fields, and in additional force fields that are strongly non-uniform but slowly evolving in time compared with particle gyrations with the cyclotron frequency, is considered. In order to reveal the physical significance of the second-order approximation, the mean (reduced) particle velocity is calculated up to second order, when polarization particle drift as well as the renormalization of the lower-order result become equally significant. A general expression for the velocity of particle polarization drift is obtained in terms of quasi-particle properties, and with account taken of finite-Larmor-radius effects and non-uniformity of magnetic fields. A guiding-centre transformation is found that makes it possible to achieve equal mean velocities of particle, guiding centre and quasi-particle up to second order. Then polarization drifts enter the particle, guiding-centre and quasi-particle equations of motion.

ON STATE COUNTING AND CHARACTERS

We outline the relationship between the thermodynamic densities and quasiparticle descriptions of spectra of RSOS models with an underlying Bethe equation. We use this to prove completeness of states in some cases and then give an algorithm for the construction of branching functions of their emergent conformal field theories. Starting from the Bethe equations of Dn type, we discuss some aspects of the Zn lattice models.

Spin bags in the doped t-J model.

We present a nonperturbative method for deriving a quasiparticle description of the low-energy excitations in the t-J model for strongly correlated electrons. Using the exact diagonalization technique we evaluated exactly the spectral functions of composite operators, which describe an electron or hole dressed by antiferromagnetic spin fluctuations as expected in the string or spin bag picture. For hole doping up to 1/8, use of the composite operators leads to a drastic simplification of the single-particle spectral function: at half-filling it takes free-particle form, for the doped case it resembles a system of weakly interacting fermions corresponding to the doped holes. We conclude that for all doping levels studied, the elementary electronic excitations next to the Fermi level are adequately described by the antiferromagnetic spin-fluctuation picture.