Absence of dynamical gap generation in suspended graphene
There is an interesting proposal that the long-range Coulomb interaction in suspended graphene can generate a dynamical gap, which leads to a semimetal–insulator phase transition. We revisit this problem by solving the self-consistent Dyson–Schwinger equations of wave function renormalization and fermion gap. In order to satisfy the Ward identity, a suitable vertex function is introduced. The impact of singular velocity renormalization and that of dynamical screening on gap generation are both included in this formalism, and prove to be very important. We obtain a critical interaction strength, 3.2 < αc < 3.3, which is larger than the physical value α = 2.16 for suspended graphene. It therefore turns out that suspended graphene is a semimetal, rather than an insulator, at zero temperature.

title: Confinement induced by fermion damping in three-dimensional QED
abstract: The three-dimensional noncompact QED is known to exhibit weak confinement when fermions acquire a finite mass via the mechanism of dynamical chiral symmetry breaking. In this paper, we study the effect of fermion damping caused by elastic scattering on the classical potential between fermions. By calculating the vacuum polarization function that incorporates the fermion damping effect, we show that the fermion damping can induce a weak confinement even when the fermions are massless and the chiral symmetry is not broken.

title: Nature of the antiferromagnetic quantum phase transition on the honeycomb lattice
abstract: We address the nature of the antiferromagnetic quantum phase transition that separates a semimetal from an antiferromagnet in the repulsive Hubbard model defined on the honeycomb lattice. At the critical point, the fermions acquire an anomalous dimension $\eta$ due to their strong coupling to the fluctuations of the order parameter $\phi$. The finite $\eta$ in turn induces a singular $\phi^{4}$ term and a non-analytical spin susceptibility signaling the breakdown of Hertz's $\phi^{4}$ theory. As a result, the continuous antiferromagnetic quantum phase transition is internally unstable and turns into a first order transition.

title: Eliashberg theory of excitonic insulating transition in graphene.
abstract: A sufficiently strong Coulomb interaction may open an excitonic fermion gap and thus drive a semi-metal-insulator transition in graphene. In this paper, we study the Eliashberg theory of excitonic transition by coupling the fermion gap equation self-consistently to the equation of the vacuum polarization function. Including the fermion gap into the polarization function increases the effective strength of the Coulomb interaction because it reduces the screening effects due to the collective particle-hole excitations. Although this procedure does not change the critical point, it leads to a significant enhancement of the dynamical fermion gap in the excitonic insulating phase. The validity of the Eliashberg theory is justified by showing that the vertex corrections are suppressed at the large N limit.

title: Fate of non-Fermi-liquid behavior in QED 3 at finite chemical potential
abstract: The damping rate of two-dimensional massless Dirac fermions exhibit non-Fermi-liquid behavior, , due to gauge field at zero temperature and zero chemical potential. We study the fate of this behavior at finite chemical potential. We first calculate explicitly the temporal and spatial components of vacuum polarization functions. The analytical expressions imply that the temporal component of gauge field develops a static screening length at finite chemical potential while the transverse component remains long-ranged owing to gauge invariance. We then calculate the fermion damping rate and show that the temporal gauge field leads to normal Fermi-liquid behavior but the transverse gauge field leads to non-Fermi-liquid behavior {proportional_to}{epsilon}{sup 2/3} at zero temperature. This energy dependence is more regular than {proportional_to}{epsilon}{sup 1/2} and does not change as chemical potential varies.

title: Non-Fermi liquid behavior due to U(1) gauge field in two dimensions
abstract: We study the damping rate of massless Dirac fermions due to the U ( 1 ) gauge field in ( 2 + 1 ) -dimensional quantum electrodynamics. In the absence of a Maxwell term for the gauge field, the fermion damping rate Im Σ ( ω , T ) is found to diverge in both perturbative and self-consistent results. In the presence of a Maxwell term, there is still divergence in the perturbative results for Im Σ ( ω , T ) . Once the Maxwell term is included into the self-consistent equations for fermion self-energy and vacuum polarization functions, the fermion damping rate is free of divergence and exhibits non-Fermi liquid behavior: Im Σ ( ω , T ) ∝ max ( ω , T ) .

title: Charge division read-out in a 1.8-m-long wire chamber
abstract: This paper reports on the design and the test results of a charge division electronic read-out system for a charged particle proportional wire detector. Electronic calibration and probable error analysis of the system are also discussed. Position resolution for the system has been measured to be 4.8-mm FWHM on a wire of 1.8-m length (0.26%, relative).

title: Excitonic Effects in Two-Dimensional Massless Dirac Fermions
abstract: We study excitonic effects in two-dimensional massless Dirac fermions with Coulomb interactions by solving the ladder approximation to the Bethe-Salpeter equation. It is found that the general four-leg vertex has a power-law behavior with the exponent going from real to complex as the coupling constant is increased. This change of behavior is manifested in the antisymmetric response, which displays power-law behavior at small wave vectors reminiscent of a critical state, and a change in this power law from real to complex that is accompanied by poles in the response function for finite-size systems, suggesting a phase transition for strong enough interactions. The density-density response is also calculated, for which no critical behavior is found. We demonstrate that exciton correlations enhance the cusp in the irreducible polarizability at $2{k}_{F}$, leading to a strong increase in the amplitude of Friedel oscillations around a charged impurity.

title: Schrödinger Fermi liquids
abstract: A class of strongly interacting many-body fermionic systems in 2+1D non-relativistic conformal field theory is examined via the gauge-gravity duality correspondence. The 5D charged black hole with asymptotic Schrodinger isometry in the bulk gravity side introduces parameters of background density and finite particle number into the boundary field theory. We propose the holographic dictionary, and realize a quantum phase transition of this fermionic liquid with fixed particle number by tuning the background density $\beta$ at zero temperature. On the larger $\beta$ side, we find the signal of a sharp quasiparticle pole on the spectral function A(k,w), indicating a well-defined Fermi surface. On the smaller $\beta$ side, we find only a hump with no sharp peak for A(k,w), indicating the disappearance of Fermi surface. The dynamical exponent $z$ of quasiparticle dispersion goes from being Fermi-liquid-like $z\simeq1$ scaling at larger $\beta$ to a non-Fermi-liquid scaling $z\simeq 3/2$ at smaller $\beta$. By comparing the structure of Green's function with Landau Fermi liquid theory and Senthil's scaling ansatz, we further investigate the behavior of this quantum phase transition.

title: The Properties of Horizontal Magnetic Elements in Quiet Solar Intranetwork
abstract: Using the data observed by the Solar Optical Telescope/Spectropolarimeter aboard the Hinode satellite, the horizontal and vertical fields are derived from the wavelength-integrated measures of Zeeman-induced linear and circular polarizations. The quiet intranetwork regions are pervaded by horizontal magnetic elements. We categorize the horizontal intranetwork magnetic elements into two types: one is the nonisolated element which is accompanied by the vertical magnetic elements during its evolution; another is the isolated element which is not accompanied by the vertical magnetic elements. Their properties, such as lifetime, size, and magnetic flux density, are studied, and the relationships among various magnetic parameters are investigated. We identify 446 horizontal intranetwork magnetic elements among them 87 elements are isolated and 359 are nonisolated. Quantitative measurements reveal that the isolated elements have relatively weaker horizontal magnetic fields, almost equal size, and shorter lifetime compared with the nonisolated elements. Most nonisolated horizontal intranetwork magnetic elements are identified to associate with the emergence of {omega}-shaped flux loops. A few nonisolated elements seem to indicate scenarios of submergence of {omega} loops or emergence of U-like loops. There is a positive correlation between the lifetime and the size for both the isolated and nonisolated horizontal intranetwork field elements (HIFs). Positivemore » correlation between the lifetime and the magnetic flux density for nonisolated HIFs is also found, but no correlation for isolated HIFs. Even though the horizontal elements show lower magnetic flux density, they could carry the total magnetic flux in the order of magnitude close to 10{sup 25} Mx to the solar surface each day.« less

title: Minimizing the Energy Spread within a Single Bunch by Shaping Its Charge Distribution
abstract: When electrons or positrons in a bunch pass through the periodic structure of a linear accelerator, they leave behind them energy in the form of longitudinal wake fields. The wakefields thus induced by early particles in a bunch offset the energy of later particles. For a linear collider, the energy spread introduced within the bunches by this beam loading effect must be minimized because it limits the degree to which the particles can be focused to a small spot due to chromatic effects in the final focus system. For example, for the SLC, the maximum allowable energy spread is + or - 0.5%. It has been known for some time that partial compensation of the longitudinal wake field effects can be obtained for any bunch by placing it ahead of the accelerating crest (in space), thereby letting the positive rising sinusoidal field offset the negative beam loading field. The work presented in this paper shows that it is possible to obtain complete compensation, i.e., to reduce the energy spread essentially to zero by properly shaping the longitudinal charge distribution of the bunch and by placing it at the correct position on the wave.