13,331 research outputs found
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.
Correcting 100 years of misunderstanding: electric fields in superconductors, hole superconductivity, and the Meissner effect
From the outset of superconductivity research it was assumed that no
electrostatic fields could exist inside superconductors, and this assumption
was incorporated into conventional London electrodynamics. Yet the London
brothers themselves initially (in 1935) had proposed an electrodynamic theory
of superconductors that allowed for static electric fields in their interior,
which they unfortunately discarded a year later. I argue that the Meissner
effect in superconductors necessitates the existence of an electrostatic field
in their interior, originating in the expulsion of negative charge from the
interior to the surface when a metal becomes superconducting. The theory of
hole superconductivity predicts this physics, and associated with it a
macroscopic spin current in the ground state of superconductors ("Spin Meissner
effect"), qualitatively different from what is predicted by conventional
BCS-London theory. A new London-like electrodynamic description of
superconductors is proposed to describe this physics. Within this theory
superconductivity is driven by lowering of quantum kinetic energy, the fact
that the Coulomb repulsion strongly depends on the character of the charge
carriers, namely whether electron- or hole-like, and the spin-orbit
interaction. The electron-phonon interaction does not play a significant role,
yet the existence of an isotope effect in many superconductors is easily
understood. In the strong coupling regime the theory appears to favor local
charge inhomogeneity. The theory is proposed to apply to all superconducting
materials, from the elements to the high cuprates and pnictides, is
highly falsifiable, and explains a wide variety of experimental observations.Comment: Proceedings of the conference "Quantum phenomena in complex matter
2011 - Stripes 2011", Rome, 10 July -16 July 2011, to be published in J.
Supercond. Nov. Mag
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
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
Determining R-parity violating parameters from neutrino and LHC data
In supersymmetric models neutrino data can be explained by R-parity violating
operators which violate lepton number by one unit. The so called bilinear model
can account for the observed neutrino data and predicts at the same time
several decay properties of the lightest supersymmetric particle. In this paper
we discuss the expected precision to determine these parameters by combining
neutrino and LHC data and discuss the most important observables. We show that
one can expect a rather accurate determination of the underlying R-parity
parameters assuming mSUGRA relations between the R-parity conserving ones and
discuss briefly also the general MSSM as well as the expected accuracies in
case of a prospective e+ e- linear collider. An important observation is that
several parameters can only be determined up to relative signs or more
generally relative phases.Comment: 13 pages, 13 figure
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
Clinical potential of necitumumab in non-small cell lung carcinoma
Despite significant progress, new therapeutic approaches for advanced non-small cell lung cancer (NSCLC) are highly needed, particularly for the treatment of patients with squamous cell carcinoma. The epidermal growth factor receptor (EGFR) is often overexpressed in NSCLC and represents a relevant target for specific treatments. Although EGFR mutations are more frequent in non-squamous histology, the receptor itself is more often overexpressed in squamous NSCLC. Necitumumab is a human monoclonal antibody that is able to inhibit the EGFR pathway and cause antibody-dependent cell cytotoxicity. This drug has been studied in combination with first-line chemotherapy for advanced NSCLC in two Phase III trials, and a significant survival benefit was reported in squamous NSCLC (SQUIRE trial); by contrast, necitumumab did not prove itself beneficial in non-squamous histotype (INSPIRE trial). On the basis of the SQUIRE results, necitumumab was approved in combination with cisplatin and gemcitabine as a first-line treatment for advanced squamous NSCLC, both in the US and Europe, where its availability is limited to patients with EGFRexpressing tumors. The aim of this review is to describe the tolerability and the efficacy of necitumumab by searching the available published data and define its potential role in the current landscape of NSCLC treatment
R-parity Conserving Supersymmetry, Neutrino Mass and Neutrinoless Double Beta Decay
We consider contributions of R-parity conserving softly broken supersymmetry
(SUSY) to neutrinoless double beta (\znbb) decay via the (B-L)-violating
sneutrino mass term. The latter is a generic ingredient of any weak-scale SUSY
model with a Majorana neutrino mass. The new R-parity conserving SUSY
contributions to \znbb are realized at the level of box diagrams. We derive
the effective Lagrangian describing the SUSY-box mechanism of \znbb-decay and
the corresponding nuclear matrix elements. The 1-loop sneutrino contribution to
the Majorana neutrino mass is also derived.
Given the data on the \znbb-decay half-life of Ge and the neutrino
mass we obtain constraints on the (B-L)-violating sneutrino mass. These
constraints leave room for accelerator searches for certain manifestations of
the 2nd and 3rd generation (B-L)-violating sneutrino mass term, but are most
probably too tight for first generation (B-L)-violating sneutrino masses to be
searched for directly.Comment: LATEX, 29 pages + 4 (uuencoded) figures appende
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