Relativistic Heavy Quark Effective Action

We study the fermion action needed to accurately describe the low energy physics of systems including heavy quarks in lattice QCD even when the heavy fermion mass $m$ is on the order of, or larger than, the inverse lattice spacing: $m \ge 1/a$. We carry out an expansion through first order in $|\vec p| a$ (where $\vec p$ is the heavy quark momentum) and all orders in $ma$, refining the analysis of the Fermilab and Tsukuba groups. We demonstrate that the spectrum of heavy quark bound states can be determined accurately through $|\vec p| a$ and $(ma)^n$ for arbitrary exponent $n$ by using a lattice action containing only three unknown coefficients: $m_0$, $\zeta$ and $c_P$ (a generalization of $c_{SW}$), which are functions of $ma$. In a companion paper, we show how these three coefficients can be precisely determined using non-perturbative techniques.Comment: 40 pages, 1 figur

Interplay between phonon and impurity scattering in 2D hole transport

We investigate temperature dependent transport properties of two-dimensional p-GaAs systems taking into account both hole-phonon and hole-impurity scattering effects. By analyzing the hole mobility data of p-GaAs in the temperature range 10 K$<T<$100 K, we estimate the value of the appropriate deformation potential for hole-phonon coupling. Due to the interplay between hole-phonon and hole-impurity scattering the calculated temperature-dependent resistivity shows interesting nonmonotonic behavior. In particular, we find that there is a temperature range (typically 2 K$<T<$10 K) in which the calculated resistivity becomes independent of temperature due to a subtle cancellation between the temperature dependent resistive scattering contributions arising from impurities and phonons. This resistivity saturation regime appears at low carrier densities when the increasing resistivity due to phonon scattering compensates for the decreasing resistivity due to the nondegeneracy effect. This temperature-independent flat resistivity regime is experimentally accessible and may have already been observed in a recent experiment.Comment: 11 pages, 8 figure

Squeezed states of light from an optical parametric oscillator

Squeezed states of the electromagnetic field are generated by degenerate parametric downconversion in a subthreshold optical parametric oscillator. Reductions in photocurrent noise greater than 60% (-4 dB) below the limit set by the vacuum fluctuations of the field are observed in a balanced homodyne detector. A quantitative comparison with theory suggests that the observed noise reductions result from a field that in the absence of avoidable linear attenuation would be squeezed more than tenfold. A degree of squeezing of approximately fivefold is inferred for the actual field emitted through one mirror of the optical parametric oscillator. An explicit demonstration of the Heisenberg uncertainty principle for the electromagnetic field is made from the measurements, which show that the field state produced by the downconversion process is a state of minimum uncertainty

Detection of amplitude modulation with squeezed light for sensitivity beyond the shot-noise limit

An improvement in precision beyond the limit set by the vacuum-state or zero-point fluctuations of the electromagnetic field is reported for the detection of amplitude modulation encoded on a weak signal beam. The improvement is achieved by employing the squeezed light from an optical parametric oscillator to reduce the level of fluctuations below the shot-noise limit. An increase in signal-to-noise ratio of 2.5 dB relative to the shot-noise limit is demonstrated

Competing Ordered States in Bilayer Graphene

We use a perturbative renormalization group approach with short-range continuum model interactions to analyze the competition between isotropic gapped and anisotropic gapless ordered states in bilayer graphene, commenting specifically on the role of exchange and on the importance of spin and valley flavor degeneracy. By comparing the divergences of the corresponding susceptibilities, we conclude that this approach predicts gapped states for flavor numbers N=1,2,4. We also comment briefly on the related gapped states expected in chiral (ABC) trilayer graphene.Comment: 12 pages, 7 figures and 1 tabl

Electronic Structures of Antiperovskite Superconductor MgCNi3_3 and Related Compounds

Electronic structure of a newly discovered antiperovskite superconductor MgCNi$_3$ is investigated by using the LMTO band method. The main contribution to the density of states (DOS) at the Fermi energy $E_{\rm F}$ comes from Ni 3$d$ states which are hybridized with C 2$p$ states. The DOS at $E_{\rm F}$ is varied substantially by the hole or electron doping due to the very high and narrow DOS peak located just below $E_{\rm F}$. We have also explored electronic structures of C-site and Mg-site doped MgCNi$_3$ systems, and described the superconductivity in terms of the conventional phonon mechanism.Comment: 3 pages, presented at ORBITAL2001 September 11-14, 2001 (Sendai, JAPAN

dmu/dn In suspended bilayer graphene: the interplay of disorder and band gap

We present an interpretation of recent experimental measurements of dmu/dn in suspended bilayer graphene samples. We demonstrate that the data may be quantitatively described by assuming a spatially varying band gap induced by local electric fields. We demonstrate that the gap fluctuations vary amongst different samples and that the gap fluctuations are correlated with the associated charge density fluctuations, indicating that the mechanism causing this effect is likely to be an extrinsic effect. We also provide predictions for the optical conductivity and mobility of suspended bilayer graphene samples with small band gaps.Comment: 7 pages, 5 figure

Electronic structures of antiperovskite superconductors: MgXNi3_3 (X=B,C,N)

We have investigated electronic structures of a newly discovered antiperovskite superconductor MgCNi$_3$ and related compounds MgBNi$_3$ and MgNNi$_3$. In MgCNi$_3$, a peak of very narrow and high density of states is located just below $\rm E_F$, which corresponds to the $\pi^*$ antibonding state of Ni-3d and C-$2p$ but with the predominant Ni-3d character. The prominent nesting feature is observed in the $\Gamma$-centered electron Fermi surface of an octahedron-cage-like shape that originates from the 19th band. The estimated superconducting parameters based on the simple rigid-ion approximation are in reasonable agreement with experiment, suggesting that the superconductivity in MgCNi$_3$ is described well by the conventional phonon mechanism.Comment: 5 pages, 5 figure