Deep Traps in AlGaN/GaN Heterostructures Studied by Deep Level Transient Spectroscopy: Effect of Carbon Concentration in GaN Buffer Layers

Electrical properties, including leakage currents, threshold voltages, and deep traps, of AlGaN/GaN heterostructure wafers with different concentrations of carbon in the GaN buffer layer, have been investigated by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy (DLTS), using Schottky barrier diodes (SBDs). It is found that (i) SBDs fabricated on the wafers with GaN buffer layers containing a low concentration of carbon (low-[C] SBD) or a high concentration of carbon (high-[C] SBD) have similar low leakage currents even at 500 K; and (ii) the low-[C] SBD exhibits a larger (negative) threshold voltage than the high-[C] SBD. Detailed DLTS measurements on the two SBDs show that (i) different trap species are seen in the two SBDs: electron traps Ax (0.9 eV), A1 (0.99 eV), and A2 (1.2 eV), and a holelike trap H1 (1.24 eV) in the low-[C] SBD; and electron traps A1, A2, and A3 ( ∼ 1.3 eV), and a holelike trap H2 (\u3e1.3 eV) in the high-[C] SBD; (ii) for both SDBs, in the region close to GaN buffer layer, only electron traps can be detected, while in the AlGaN/GaN interface region, significant holelike traps appear; and iii) all of the deep traps show a strong dependence of the DLTS signal on filling pulse width, which indicates they are associated with extended defects, such as threading dislocations. However, the overall density of electron traps is lower in the low-[C] SBD than in the high-[C] SBD. The different traps observed in the two SBDs are thought to be mainly related to differences in microstructure (grain size and threading dislocation density) of GaN buffer layers grown at different pressures

Constraints on Cardassian Scenario from the Expansion Turnaround Redshift and the Sunyaev-Zeldovich/X-ray Data

Cosmic acceleration is one of the most remarkable cosmological findings of recent years. Although a dark energy component has usually been invoked as the mechanism for the acceleration, A modification of Friedmann equation from various higher dimensional models provides a feasible alternative. Cardassian expansion is one of these scenarios. In this work, we first consider the constraints on the parameter space from the turnaround redshift, z_{q=0}, at which the universe switches from deceleration to acceleration. We show that, for every Omega_m, there exist a unique n_{peak} (Omega_m), which makes z_{q=0} reach its maximum value, [z_{q=0}]_{max} = exp[1/ (2-3n_{peak})] -1, which is unlinearly inverse to Omega_m. If the acceleration happans earlier than z_{q=0} = 0.6, suggested by Type Ia supernovae measurements, we have Omega_m < 0.328 no matter what the power index is, and moreover, for reasonable matter density, Omega_m ~ 0.3, it is found n ~ (-0.45,0.25). We next test this scenario using the Sunyaev-Zeldovich/X-ray data of a sample of 18 galaxy clusters with 0.14 < z < 0.83 compiled by Reese et al. (2002). The constraints from the current SZ/X-ray data is weak, though a model with lower matter density is prefered. A certain range of the model parameters is also consistent with the data.Comment: 18 pages, 4 figures, accepted for publication in Ap

The thermal conductivity of the spin-1/2 XXZ chain at arbitrary temperature

Motivated by recent investigations of transport properties of strongly correlated 1d models and thermal conductivity measurements of quasi 1d magnetic systems we present results for the integrable spin-1/2 $XXZ$ chain. The thermal conductivity $\kappa(\omega)$ of this model has $\Re\kappa(\omega)=\tilde\kappa \delta(\omega)$, i.e. it is infinite for zero frequency $\omega$. The weight $\tilde\kappa$ of the delta peak is calculated exactly by a lattice path integral formulation. Numerical results for wide ranges of temperature and anisotropy are presented. The low and high temperature limits are studied analytically.Comment: 12 page

Determining the Cosmic Equation of State Using Future Gravitational Wave Detectors

The expected chirp mass distribution of observed events for future gravitational wave detectors is extensively investigated in the presence of an exotic fluid component with an arbitrary equation of state, $-1 \leq \omega_x \equiv p_x/\rho_x < 0$, i.e., the so-called dark energy component. The results for a flat model dominated by a dark energy are compared to those for the standard flat model dominated by cold dark matter. It is found that for a flat universe the chirp mass distribution shows a sensitive dependence on $\omega_x$, which may provide an independent and robust constraint on the cosmic equation of state.Comment: 5 pages, four figures, aa.sty LaTex fil

Current-Carrying Ground States in Mesoscopic and Macroscopic Systems

We extend a theorem of Bloch, which concerns the net orbital current carried by an interacting electron system in equilibrium, to include mesoscopic effects. We obtain a rigorous upper bound to the allowed ground-state current in a ring or disc, for an interacting electron system in the presence of static but otherwise arbitrary electric and magnetic fields. We also investigate the effects of spin-orbit and current-current interactions on the upper bound. Current-current interactions, caused by the magnetic field produced at a point r by a moving electron at r, are found to reduce the upper bound by an amount that is determined by the self-inductance of the system. A solvable model of an electron system that includes current-current interactions is shown to realize our upper bound, and the upper bound is compared with measurements of the persistent current in a single ring.Comment: 7 pager, Revtex, 1 figure available from [email protected]

The Monte Carlo Program KoralW version 1.51 and The Concurrent Monte Carlo KoralW&YFSWW3 with All Background Graphs and First Order Corrections to W-Pair Production

The version 1.51 of the Monte Carlo (MC) program KoralW for all $e^+e^-\to f_1\bar f_2 f_3\bar f_4$ processes is presented. The most important change since the previous version 1.42 is the facility for writing MC events on the mass storage device and re-processing them later on. In the re-processing one may modify parameters of the Standard Model in order to fit them to experimental data. Another important new feature is a possibility of including complete ${\cal O}(\alpha)$ corrections to double-resonant W-pair component-processes in addition to all background (non-WW) graphs. The inclusion is done with the help of the YFSWW3 MC event generator for fully exclusive differential distributions (event-per-event). Technically, it is done in such a way that YFSWW3 runs concurrently with KoralW as a separate slave process, reading momenta of the MC event generated by KoralW and returning the correction weight to KoralW. KoralW introduces the ${\cal O}(\alpha)$ correction using this weight, and finishes processing the event (rejection due to total MC weight, hadronization, etc.). The communication between KoralW and YFSWW3 is done with the help of the FIFO facility of the UNIX/Linux operating system. This does not require any modifications of the FORTRAN source codes. The resulting Concurrent MC event generator KoralW&YFSWW3 looks from the user's point of view as a regular single MC event generator with all the standard features.Comment: 8 figures, 5 tables, submitted to Comput. Phys. Commu

Upper critical field in superconductors near ferromagnetic quantum critical points; UCoGe

We study the strong-coupling superconductivity near ferromagnetic quantum critical points, mainly focusing on the upper critical fields $H_{c2}$. Based on our simple model calculations, we discuss experimentally observed unusual behaviors of $H_{c2}$ in a recently discovered ferromagnetic superconductor UCoGe. Especially, the large anisotropy between $H_{c2}\parallel a$-axis and $H_{c2}\parallel c$-axis, and the strong-coupling behaviors in $H_{c2}^{\parallel a}$ are investigated. We also examine effects of non-analytic corrections in the spin susceptibility on the superconductivity, which can arise from effective long range interactions due to particle-hole excitations.Comment: Proceedings of ICHE2010, Toky

Asymptotically Exact Solution for Superconductivity near Ferromagnetic Criticality

We analyze an asymptotically exact solution for the transition temperature of p-wave superconductivity near ferromagnetic criticality on the basis of the three-dimensional electron systems in which scattering processes are dominated by exchange interactions with small momentum transfers. Taking into account all Feynman diagrams in the gap equation, we show that vertex corrections neglected in the conventional Eliashberg's formalism enhance the dynamical retarded effect of the pairing interaction, and raise the superconducting transition temperature significantly, though they just give subleading corrections to properties of the normal state.Comment: 6 pages, 2 figures, published final versio

One-Dimensional Electron Liquid in an Antiferromagnetic Environment: Spin Gap from Magnetic Correlations

We study a one-dimensional electron liquid coupled by a weak spin-exchange interaction to an antiferromagnetic spin-S ladder with n legs. A perturbative renormalization group analysis in the semiclassical limit reveals the opening of a spin gap, driven by the local magnetic correlations on the ladder. The effect, which we argue is present for any gapful ladder or gapless ladder with $nS\gg 1$, is enhanced by the repulsive interaction among the conduction electrons but is insensitive to the sign of the spin exchange interaction with the ladder. Possible implications for the striped phases of the cuprates are discussed.Comment: 5 pages, 1 figure, to appear in Phys. Rev. Let

Luminescent properties of Bi-doped polycrystalline KAlCl4

We observed an intensive near-infrared luminescence in Bi-doped KAlCl4 polycrystalline material. Luminescence dependence on the excitation wavelength and temperature of the sample was studied. Our experimental results allow asserting that the luminescence peaked near 1 um belongs solely to Bi+ ion which isomorphically substitutes potassium in the crystal. It was also demonstrated that Bi+ luminescence features strongly depend on the local ion surroundings