Origin of the pseudogap and its influence on superconducting state

When holes move in the background of strong antiferromagnetic correlation, two effects with different spatial scale emerge, leading to a much reduced hopping integral with an additional phase factor. An effective Hamiltonian is then proposed to investigate the underdoped cuprates. We argue that the pseudogap is the consequence of dressed hole moving in the antiferromagnetic background and has nothing to do with the superconductivity. The momentum distributions of the gap are qualitatively consistent with the recent ARPES measurements both in the pseudogap and superconducting state. Two thermal qualities are further calculated to justify our model. A two-gap scenario is concluded to describe the relation between the two gaps.Comment: 7 pages, 5 figure

Thermodynamic properties and shear viscosity over entropy density ratio of nuclear fireball in a quantum-molecular dynamics model

Thermodynamic and transport properties of nuclear fireball created in the central region of heavy-ion collisions below 400 MeV/nucleon are investigated within the isospin-dependent quantum molecular dynamic (IQMD) model. These properties including the density, temperature, chemical potential, entropy density ($s$) and shear viscosity ($\eta$), are calculated by a generalized hot Thomas Fermi formulism and a parameterized function, which was developed by Danielewicz. As the collision goes on, a transient minimal $\eta/s=5/4\pi-10/4\pi$ occurs in the largest compression stage. Besides, the relationship of $\eta/s$ to temperature ($T$) in the freeze-out stage displays a local minimum which is about 9-20 times $1/4\pi$ around $T$ = 8-12 MeV, which can be argued as indicative of a liquid gas phase transition. In addition, the influences of nucleon-nucleon (NN) cross section ($\sigma_{NN}$) and symmetry energy coefficient ($C_{s}$) are also discussed, and it is found that the results are sensitive to $\sigma_{NN}$ but not to $C_{s}$.Comment: 10 pages, 13 figures; Phys. Rev. C (in press) (x-axis of Fig.1 is corrected

Pentaquarks in semileptonic Λb\Lambda_b decays

In terms of ${\cal P}_{c1,c2}={\cal P}_c(4380,4450)^+$ as the hidden charm pentaquark states to consist of $c\bar c uud$, we study the semileptonic $\Lambda_b\to ({\cal P}_{c1,c2}\to)J/\psi p \ell^-\bar \nu_\ell$ decays. In our discussion, while the main contribution to $\Lambda_b\to K^-J/\psi p$ is from the non-perturbative process via the doubly charmful $b\to c\bar cs$ transition, we propose that the $\Lambda_b\to {\cal P}_{c1,c2}\to J/\psi p$ transitions are partly contribute to the $\Lambda_b\to K^-({\cal P}_{c1,c2}\to)J/\psi p$ decays, in which the required $c\bar c$ pair is formed by the sea quarks, intrinsic charm, or both. We predict that ${\cal B}(\Lambda_b\to J/\psi p \ell^-\bar \nu_\ell)=(2.04^{+4.82}_{-1.57},1.75^{+4.14}_{-1.35})\times 10^{-6}$ for $\ell=(e,\mu)$, which are about two orders of magnitude smaller than the observed decay of $\Lambda_b\to p\mu\bar \nu_\mu$. We also explore the angular correlations for the $J/\psi p$ and $\ell^-\bar \nu_\ell$ pairs. Our results of the decay branching ratios and angular asymmetries in $\Lambda_b\to J/\psi p \ell^-\bar \nu_\ell$, accessible to the ongoing experiments at the LHCb, can be used to improve the understanding of the hidden charm pentaquark states.Comment: 11 pages, 3 figures, accepted by Annals of Physic

Magnetism out of disorder in a J=0 compound Ba2YIrO6

We systematically investigate the magnetic properties and local structure of Ba2YIrO6 to demonstrate that Y and Ir lattice defects in the form of antiphase boundary or clusters of antisite disorder affect the magnetism observed in this $d^4$ compound. We compare the magnetic properties and atomic imaging of (1) a slow cooled crystal, (2) a crystal quenched from 900\degree C after growth, and (3) a crystal grown using a faster cooling rate than the slow cooled one. Atomic imaging by scanning transmission electron microscopy (STEM) shows that quenching from 900oC introduces antiphase boundary to the crystals, and a faster cooling rate during crystal growth leads to clusters of Y and Ir antisite disorder. STEM study suggests the antiphase boundary region is Ir-rich with a composition of Ba2YIrO6. The magnetic measurements show that Ba2YIrO6 crystals with clusters of antisite defects have a larger effective moment and a larger saturation moment than the slow-cooled crystals. Quenched crystals with Ir-rich antiphase boundary shows a slightly suppressed saturation moment than the slow cooled crystals, and this seems to suggest that antiphase boundary is detrimental to the moment formation. Our DFT calculations suggest magnetic condensation is unlikely as the energy to be gained from superexchange is small compared to the spin-orbit gap. However, once Y is replaced by Ir in the antisite disordered region, the picture of local non-magnetic singlets breaks down and magnetism can be induced. This is because of (a) enhanced interactions due to increased overlap of orbitals between sites, and, (b) increased number of orbitals mediating the interactions. Our work highlights the importance of lattice defects in understanding the experimentally observed magnetism in Ba2YIrO6 and other J=0 systems

Shear viscosity of hot nuclear matter by the mean free path method

The shear viscosity of hot nuclear matter is investigated by using the mean free path method within the framework of IQMD model. Finite size nuclear sources at different density and temperature are initialized based on the Fermi-Dirac distribution. The results show that shear viscosity to entropy density ratio decreases with the increase of temperature and tends toward a constant value for $\rho\sim\rho_0$, which is consistent with the previous studies on nuclear matter formed during heavy-ion collisions. At $\rho\sim\frac{1}{2}\rho_0$, a minimum of $\eta/s$ is seen at around $T=10$ MeV and a maximum of the multiplicity of intermediate mass fragment ($M_{\text{IMF}}$) is also observed at the same temperature which is an indication of the liquid-gas phase transition.Comment: 5 figs and 5 pages; accepted by Physical Review

Measurement of Change of 7Be Decay Rate in Be and Au

We have measured the possible change of the decay rate of 7Be implanted into hosts of natural beryllium and natural gold. No difference between the 7Be decay rates in the two hosts is observed within the experimental precision of 0.12%. This result implies that change of the decay rate of 7Be implanted in different materials cannot be simply expected from the electron affinity difference consideration lonely and the lattice structure of the host materials should be taken into account.Comment: 5 pages, 2 figure

Diophantine Approach to Blind Interference Alignment of Homogeneous K-user 2x1 MISO Broadcast Channels

Although the sufficient condition for a blindly interference-aligned (BIA) 2-user 2x1 broadcast channel (BC) in homogeneous fading to achieve its maximal 4/3 DoF is well understood, its counterpart for the general K-user 2x1 MISO BC in homogeneous block fading to achieve the corresponding 2k/(2+K-1) (DoF) remains unsolved and is, thus, the focus of this paper. An interference channel is said BIA-feasible if it achieves its maximal DoF only via BIA. In this paper, we cast this general feasibility problem in the framework of finding integer solutions for a system of linear Diophantine equations. By assuming independent user links each of the same coherence time and by studying the solvability of the Diophantine system, we derive the sufficient and necessary conditions on the K users' fading block offsets to ensure the BIA feasibility of the K-user BC. If the K offsets are independent and uniformly distributed over a coherence block, we can further prove that 11 users are enough for one to find, with certainty of 95%, 3 users among them to form a BIA-feasible 3-user 2x1 BC.Comment: 16 page

Implement Blind Interference Alignment over Homogeneous 3-user 2x1 Broadcast Channel

This paper first studies the homogeneous 3-user 2x1 broadcast channel (BC) with no CSIT. We show a sufficient condition for it to achieve the optimal 3/2 degrees of freedom (DoF) by using Blind Interference Alignment (BIA). BIA refers to the interference alignment method without the need of CSIT. It further studies the 2x1 broadcast network in which there are K>=3 homogeneous single-antenna users, and their coherence time offsets are independently and uniformly distributed. We show that, if K>=11, the two-antenna transmitter can find, with more than 95% certainty, three users to form a BIA-feasible 3-user BC and achieve the optimal 3/2 DoF.Comment: The proof of Theorem 3 is revised. arXiv admin note: text overlap with arXiv:1209.313

Superconducting Resonance and paring symmetry in electron-doped cuprates

The magnetic excitations in the superconducting electron-doped cuprates are studied in the framework of spin-density-wave description. The superconducting resonance is a natural product of the superconductivity due to the opening of d-wave gap. Its resonance energy exhibits well linear scaling with superconducting gap as $E_{res}/2\Delta\sim 0.6$, quantitatively consisting with the experimental discovery. This ratio is insensitive to the selected parameters, manifesting its universality. Another lower-energy peak below resonance energy is predicted when the hole pocket emerges due to suppression of spin-density wave. We further verify that the ratio of linear scales is intimately related to the pairing symmetry. Distinct ratio can be found with respective pairing symmetry. In comparison with the inelastic neutron scattering data, the monotonic d-wave superconductivity is the most likely candidate in the electron-doped cuprates. Furthermore, we proposed a new method to check the pairing symmetry by the inelastic neutron scattering measurements.Comment: 4 Pages, 4 Figure

Raman scattering, X-ray photoemission spectra and superconductivity of a tiny Ag diffusion to MgCNi3

The tiny Ag diffusion to MgCNi3 has been prepared by solid states reaction. Its structure was characterized by X-ray diffractometer (XRD). The results show that a small mount of Ag substitute for Ni sites, and much of Ag are in vacancy sites of the MgCNi3. We have further studied the surface properties of Ag-MgCNi3 using Raman scattering spectra, and X-ray photoemission spectra (XPS). Raman spectroscopy shows that the Ag-MgCNi3 has a special Raman peak around 842.1 cm-1 compared to that of C. The superconductor transition temperature (about 6.6 K) of Ag-MgCNi3 was lower than that of pure MgCNi3. It was interpreted properly by the conventional BCS phonon mechanism.Comment: 12 pages, 7 figure