14,736 research outputs found
Superconductivity from Undressing. II. Single Particle Green's Function and Photoemission in Cuprates
Experimental evidence indicates that the superconducting transition in high
cuprates is an 'undressing' transition. Microscopic mechanisms giving
rise to this physics were discussed in the first paper of this series. Here we
discuss the calculation of the single particle Green's function and spectral
function for Hamiltonians describing undressing transitions in the normal and
superconducting states. A single parameter, , describes the strength
of the undressing process and drives the transition to superconductivity. In
the normal state, the spectral function evolves from predominantly incoherent
to partly coherent as the hole concentration increases. In the superconducting
state, the 'normal' Green's function acquires a contribution from the anomalous
Green's function when is non-zero; the resulting contribution to
the spectral function is for hole extraction and for hole
injection. It is proposed that these results explain the observation of sharp
quasiparticle states in the superconducting state of cuprates along the
direction and their absence along the direction.Comment: figures have been condensed in fewer pages for easier readin
Superconductivity from Undressing
Photoemission experiments in high cuprates indicate that quasiparticles
are heavily 'dressed' in the normal state, particularly in the low doping
regime. Furthermore these experiments show that a gradual undressing occurs
both in the normal state as the system is doped and the carrier concentration
increases, as well as at fixed carrier concentration as the temperature is
lowered and the system becomes superconducting. A similar picture can be
inferred from optical experiments. It is argued that these experiments can be
simply understood with the single assumption that the quasiparticle dressing is
a function of the local carrier concentration. Microscopic Hamiltonians
describing this physics are discussed. The undressing process manifests itself
in both the one-particle and two-particle Green's functions, hence leads to
observable consequences in photoemission and optical experiments respectively.
An essential consequence of this phenomenology is that the microscopic
Hamiltonians describing it break electron-hole symmetry: these Hamiltonians
predict that superconductivity will only occur for carriers with hole-like
character, as proposed in the theory of hole superconductivity
Compressible Flows in Fluidic Oscillators
We present qualitative observations on the internal flow characteristics of
fluidic oscillator geometries commonly referred to as sweeping jets in active
flow control applications. This is part of the fluid dynamics videos.Comment: Videos include
Proposal for a Supersymmetric Standard Model
The fact that neutrinos are massive suggests that the minimal supersymmetric
standard model (MSSM) might be extended in order to include three gauge-singlet
neutrino superfields with Yukawa couplings of the type . We
propose to use these superfields to solve the problem of the MSSM without
having to introduce an extra singlet superfield as in the case of the
next-to-MSSM (NMSSM). In particular, terms of the type in the
superpotential may carry out this task spontaneously through sneutrino vacuum
expectation values. In addition, terms of the type avoid the
presence of axions and generate effective Majorana masses for neutrinos at the
electroweak scale. On the other hand, these terms break lepton number and
R-parity explicitly implying that the phenomenology of this model is very
different from the one of the MSSM or NMSSM. For example, the usual neutralinos
are now mixed with the neutrinos. For Dirac masses of the latter of order
GeV, eigenvalues reproducing the correct scale of neutrino masses are
obtained.Comment: 9 pages, latex, title modified. Final version published in PR
Excited bands in odd-mass rare-earth nuclei
Normal parity bands are studied in 157Gd, 163Dy and 169Tm using the pseudo
SU(3) shell model. Energies and B(E2) transition strengths of states belonging
to six low-lying rotational bands with the same parity in each nuclei are
presented. The pseudo SU(3) basis includes states with pseudo-spin 0 and 1, and
1/2 and 3/2, for even and odd number of nucleons, respectively. States with
pseudo-spin 1 and 3/2 must be included for a proper description of some excited
bands.Comment: 8 pages, 6 figures, Submitted to Phys. Rev.
Quantum Monte Carlo and exact diagonalization study of a dynamic Hubbard model
A one-dimensional model of electrons locally coupled to spin-1/2 degrees of
freedom is studied by numerical techniques. The model is one in the class of
that describe the relaxation of an atomic orbital
upon double electron occupancy due to electron-electron interactions. We study
the parameter regime where pairing occurs in this model by exact
diagonalization of small clusters. World line quantum Monte Carlo simulations
support the results of exact diagonalization for larger systems and show that
kinetic energy is lowered when pairing occurs. The qualitative physics of this
model and others in its class, obtained through approximate analytic
calculations, is that superconductivity occurs through hole undressing even in
parameter regimes where the effective on-site interaction is strongly
repulsive. Our numerical results confirm the expected qualitative behavior, and
show that pairing will occur in a substantially larger parameter regime than
predicted by the approximate low energy effective Hamiltonian.Comment: Some changes made in response to referees comments. To be published
in Phys.Rev.
Exotic coloured fermions and lepton number violation at the LHC
Majorana neutrino mass models with a scale of lepton number violation (LNV)
of order TeV potentially lead to signals at the LHC. Here, we consider an
extension of the standard model with a coloured octet fermion and a scalar
leptoquark. This model generates neutrino masses at 2-loop order. We make a
detailed MonteCarlo study of the LNV signal at the LHC in this model, including
a simulation of standard model backgrounds. Our forecast predicts that the LHC
with 300/fb should be able to probe this model up to colour octet fermion
masses in the range of (2.6-2.7) TeV, depending on the lepton flavour of the
final state.Comment: 14 pages, 2 figure
Explanation of the Tao effect
In a series of experiments Tao and coworkers\cite{tao1,tao2,tao3} found that
superconducting microparticles in the presence of a strong electrostatic field
aggregate into balls of macroscopic dimensions. No explanation of this
phenomenon exists within the conventional theory of superconductivity. We show
that this effect can be understood within an alternative electrodynamic
description of superconductors recently proposed that follows from an
unconventional theory of superconductivity. Experiments to test the theory are
discussed.Comment: Submitted to Science January 2nd, declined January 6th; to Nature
January 7th, declined January 13th; to PRL January 14th, declined February
25t
The angular spin current and its physical consequences
We find that in order to completely describe the spin transport, apart from
spin current (or linear spin current), one has to introduce the angular spin
current. The two spin currents respectively describe the translational and
rotational motion of a spin. The definitions of these spin current densities
are given and their physical properties are discussed. Both spin current
densities appear naturally in the spin continuity equation. Moreover we predict
that the angular spin current can also induce an electric field , and
in particular scales as at large distance , whereas the
field generated from the linear spin current goes as .Comment: 7 pages, 2 figure
- …