We demonstrate that the plasmon frequency and Drude weight of the electron liquid in a doped graphene sheet are strongly renormalized by electron-electron interactions even in the longwavelength limit. This effect is not captured by the Random Phase Approximation (RPA), commonly used to describe electron fluids and is due to coupling between the center of mass motion and the pseudospin degree of freedom of the graphene's massless Dirac fermions. Making use of diagrammatic perturbation theory to first order in the electron-electron interaction, we show that this coupling enhances both the plasmon frequency and the Drude weight relative to the RPA value. We also show that interactions are responsible for a significant enhancement of the optical conductivity at frequencies just above the absorption threshold. Our predictions can be checked by far-infrared spectroscopy or inelastic light scattering.

We study the ground-state properties of a two-dimensional spin-polarized fluid of dipolar fermions within the Euler-Lagrange Fermi-hypernetted-chain approximation. Our method is based on the solution of a scattering Schrödinger equation for the "pair amplitude" g(r), where g ( r) is the pair distribution function. A key ingredient in our theory is the effective pair potential, which includes a bosonic term from Jastrow-Feenberg correlations and a fermionic contribution from kinetic energy and exchange, which is tailored to reproduce the Hartree-Fock limit at weak coupling. Very good agreement with recent results based on quantum Monte Carlo simulations is achieved over a wide range of coupling constants up to the liquid-to-crystal quantum phase transition. Using the fluctuation-dissipation theorem and a static approximation for the effective inter-particle interactions, we calculate the dynamical density-density response function, and furthermore demonstrate that an undamped zero-sound mode exists for any value of the interaction strength, down to infinitesimally weak couplings. © 2013 Elsevier Inc

A Bethe-Ansatz spin-density functional approach is developed to evaluate the ground-state density profile in a system of repulsively interacting spin-1/2 fermions inside a quasi-one-dimensional harmonic well. The approach allows for the formation of antiferromagnetic quasi-order with increasing coupling strength and reproduces with high accuracy the exact solution that is available for the two-fermion system.

We study the Ruderman-Kittle-Kasuya-Yosida (RKKY) interaction in the presence of spin polarized two dimensional Dirac fermions. We show that a spin polarization along the z-axis mediates an anisotropic interaction which corresponds to a XXZ model interaction between two magnetic moments. For undoped graphene, while the x part of interaction keeps its constant ferromagnetic sign, its z part oscillates with the distance of magnetic impurities, R. A finite doping causes that both parts of the interaction oscillate with R. We explore a beating pattern of oscillations of the RKKY interaction along armchair and zigzag lattice directions, which occurs for some certain values of the chemical potential. The two characteristic periods of the beating are determined by inverse of the difference and the sum of the chemical potential and the spin polarization.

The peculiar shape of the Fermi surface of topological nodal line semimetals at low carrier concentrations results in their unusual optical and transport properties. We analytically investigate the linear optical responses of three and two-dimensional nodal line semimetals using the Kubo formula. The optical conductivity of a three-dimensional nodal line semimetal is anisotropic. Along the axial direction (i.e., the direction perpendicular to the nodal-ring plane), the Drude weight has a linear dependence on the chemical potential at both low and high carrier dopings. For the radial direction (i.e., the direction parallel to the nodal-ring plane), this dependence changes from linear into quadratic in the transition from low into high carrier concentration. The interband contribution into optical conductivity is also anisotropic. In particular, at large frequencies, it saturates to a constant value for the axial direction and linearly increases with frequency along the radial direction. In two-dimensional nodal line semimetals, no interband optical transition could be induced and the only contribution to the optical conductivity arises from the intraband excitations. The corresponding Drude weight is independent of the carrier density at low carrier concentrations and linearly increases with chemical potential at high carrier doping.

The pseudospin degree of freedom in a semiconductor bilayer gives rise to a collective mode analogous to the ferromagnetic-resonance mode of a ferromagnet. We present a many-body theory of the dependence of the energy and the damping of this mode on layer separation d. Based on these results, we discuss the possibilities of realizing transport-current driven pseudospin-transfer oscillators in semiconductors, and of using the pseudospin-transfer effect as an experimental probe of intersubband plasmons. DOI: 10.1103/PhysRevLett.99.206802 PACS numbers: 73.21.ÿb, 71.10.Ca, 76.50.+g, 85.75.ÿd Introduction.-The layer degree of freedom in semiconductor bilayers is often regarded [1] as an effective spin-1=2 pseudospin degree of freedom in which electrons in the top layer are assigned one pseudospin state, and electrons in the other layer the opposite one. In the quantum Hall regime [2] (high magnetic field), or, possibly, at zero magnetic field but extremely low densities [3], electron bilayers are sometimes pseudospin ferromagnets. The appearance of these broken-symmetry states has motivated a long-standing interest in phenomena which are pseudospin analogs of the very robust magnetoelectric effects which underpin spintronics in ferromagnetic metals. Although spontaneous pseudospin polarization does not usually occur at zero field, tunneling between the two layers acts as an effective magnetic field which leads to a finite pseudospin magnetization and to a pseudospin resonance analogous to the ferromagnetic resonance of conventional magnetized materials. This resonance-better known in the semiconductor literature as the transverse or intersubband plasmon [4]-is of fundamental interest because both its frequency and its linewidth depend sensitively on many-body effects which cannot be completely described in the random phase approximation (RPA) [5].This Letter develops the theory of the pseudospin resonance in three ways. First of all, we apply a new theoretical approach which is distinctly superior to the RPA [6] and its extensions [7], becoming exact in the limit in which the difference V ÿ between interlayer and intralayer electron-electron interaction is small. In particular, the calculation of the linewidth to second order in V ÿ is exact and equivalent to the calculation of the Gilbert damping [8,9] in real spin dynamics. Second, we point out the feasibility of a new semiconductor device, which is analogous to the spin-transfer oscillator [10] of ordinary spintronics. In a spin-transfer oscillator, spin-polarized currents drive ferromagnetic-resonance collective spindynamics in the presence of applied fields strong enough to oppose hysteretic switching. In a semiconductor bilayer pseudospin-polarized currents (corresponding to an inter-

Motivated by current interest in strongly correlated quasi-one-dimensional (1D) Luttinger liquids subject to axial confinement, we present a novel density-functional study of few-electron systems confined by power-low external potentials inside a short portion of a thin quantum wire. The theory employs the 1D homogeneous Coulomb liquid as the reference system for a Kohn-Sham treatment and transfers the Luttinger ground-state correlations to the inhomogeneous electron system by means of a suitable local-density approximation (LDA) to the exchange-correlation energy functional. We show that such 1D-adapted LDA is appropriate for fluid-like states at weak coupling, but fails to account for the transition to a "Wigner molecules" regime of electron localization as observed in thin quantum wires at very strong coupling. A detailed analyzes is given for the two-electron problem under axial harmonic confinement.

Ground-state properties of a two-dimensional fluid of bosons with repulsive dipole-dipole interactions are studied by means of the Euler-Lagrange hypernetted-chain approximation. We present a self-consistent semi-analytical theory of the pair distribution function g(r) and ground-state energy of this system. Our approach is based on the solution of a zero-energy scattering Schrödinger equation for the "pair amplitude" g(r) with an effective potential from Jastrow-Feenberg correlations. We find excellent agreement with quantum Monte Carlo results over a wide range of coupling strength, nearly up to the critical coupling for the liquid-to-crystal quantum phase transition. We also calculate the one-body density matrix and related quantities, such as the momentum distribution function and the condensate fraction.