17,310 research outputs found
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
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
Effect of Electron-Electron Interactions on Rashba-like and Spin-Split Systems
The role of electron-electron interactions is analyzed for Rashba-like and
spin-split systems within a tight-binding single-band Hubbard model with
on-site and all nearest-neighbor matrix elements of the Coulomb interaction. By
Rashba-like systems we refer to the Dresselhaus and Rashba spin-orbit coupled
phases; spin-split systems have spin-up and spin-down Fermi surfaces shifted
relative to each other. Both systems break parity but preserve time-reversal
symmetry. They belong to a class of symmetry-breaking ground states that
satisfy: (i) electron crystal momentum is a good quantum number (ii) these
states have no net magnetic moment and (iii) their distribution of `polarized
spin' in momentum space breaks the lattice symmetry. In this class, the
relevant Coulomb matrix elements are found to be nearest-neighbor exchange ,
pair-hopping and nearest-neighbor repulsion . These ground states lower
their energy most effectively through , hence we name them Class states.
The competing effects of on the direct and exchange energies determine
the relative stability of Class states. We show that the spin-split and
Rashba-like phases are the most favored ground states within Class because
they have the minimum anisotropy in `polarized spin'. On a square lattice we
find that the spin-split phase is always favored for near-empty bands; above a
critical filling, we predict a transition from the paramagnetic to the
Rashba-like phase at and a second transition to the spin-split state
at . An energetic comparison with ferromagnetism highlights the
importance of the role of in the stability of Class states. We discuss
the relevance of our results to (i) the and phases proposed by
Wu and Zhang in the Fermi Liquid formalism and (ii) experimental observations
of spin-orbit splitting in \emph{Au}(111) surface states
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.
Towards an understanding of hole superconductivity
From the very beginning K. Alex M\"uller emphasized that the materials he and
George Bednorz discovered in 1986 were superconductors. Here I would
like to share with him and others what I believe to be key reason for why
high cuprates as well as all other superconductors are hole
superconductors, which I only came to understand a few months ago. This paper
is dedicated to Alex M\"uller on the occasion of his 90th birthday.Comment: Dedicated to Alex M\"uller on the Occasion of his 90th Birthday.
arXiv admin note: text overlap with arXiv:1703.0977
Collapse of Charge Gap in Random Mott Insulators
Effects of randomness on interacting fermionic systems in one dimension are
investigated by quantum Monte-Carlo techniques. At first, interacting spinless
fermions are studied whose ground state shows charge ordering. Quantum phase
transition due to randomness is observed associated with the collapse of the
charge ordering. We also treat random Hubbard model focusing on the Mott gap.
Although the randomness closes the Mott gap and low-lying states are created,
which is observed in the charge compressibility, no (quasi-) Fermi surface
singularity is formed. It implies localized nature of the low-lying states.Comment: RevTeX with 3 postscript figure
Culture, nationality and demographics in ultimatum games
We use experimental data collected in Russia and in the United States using a simple ultimatum game to evaluate two alternative hypotheses that may account for previously observed behavior in multinational experiments. One hypothesis postulates that behavioral differences observed in bargaining experiments arise from country-specific cultural environments. We submit the alternative hypothesis that different behavior in such experiments stems from differences in the demographic characteristics of the subject pools within each country. Because of its simplicity, our experimental design allows us to discriminate between these two hypotheses. Our findings support the alternative hypothesis.multinational experiments, ultimatum bargaining
Spin currents in superconductors
It is argued that experiments on rotating superconductors provide evidence
for the existence of macroscopic spin currents in superconductors in the
absence of applied external fields. Furthermore it is shown that the model of
hole superconductivity predicts the existence of such currents in all
superconductors. In addition it is pointed out that spin currents are required
within a related macroscopic (London-like) electrodynamic description of
superconductors recently proposed. The spin current arises through an intrinsic
spin Hall effect when negative charge is expelled from the interior of the
metal upon the transition to the superconducting state
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
Design of aircraft turbine fan drive gear transmission system
The following basic types of gear reduction concepts were studied as being feasible power train systems for a low-bypass-ratio, single-spool, geared turbofan engine for general aircraft use: (1) single-stage external-internal reduction, (2) gears (offset shafting), (3) multiple compound idler gear system (concentric shafting), and (4) star gear planetary system with internal ring gear final output member (concentric shafting-counterrotation). In addition, studies were made of taking the accessories drive power off both the high-speed and low-speed shafting, using either face gears or spiral bevel gears. Both antifriction and sleeve-type bearings were considered for the external-internal and star-planet reduction concepts
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