1,411 research outputs found
Non-Local Order Parameters as a Probe for Phase Transitions in the Extended Fermi-Hubbard Model
The Extended Fermi-Hubbard model is a rather studied Hamiltonian due to both
its many applications and a rich phase diagram. Here we prove that all the
phase transitions encoded in its one dimensional version are detectable via
non-local operators related to charge and spin fluctuations. The main advantage
in using them is that, in contrast to usual local operators, their asymptotic
average value is finite only in the appropriate gapped phases. This makes them
powerful and accurate probes to detect quantum phase transitions. Our results
indeed confirm that they are able to properly capture both the nature and the
location of the transitions. Relevantly, this happens also for conducting
phases with a spin gap, thus providing an order parameter for the
identification of superconducting and paired superfluid phasesComment: 7 pages, 3 figures; Submitted to EPJ Special Topics, Quantum Gases
and Quantum Coherenc
Minilaparoscopic Colorectal Resections: Technical Note
Laparoscopic colorectal resections have been shown to provide short-term advantages in terms of postoperative pain, general morbidity, recovery, and quality of life. To date, long-term results have been proved to be comparable to open surgery irrefutably only for colon cancer. Recently, new trends keep arising in the direction of minimal invasiveness to reduce surgical trauma after colorectal surgery in order to improve morbidity and cosmetic results. The few reports available in the literature on single-port technique show promising results. Natural orifices endoscopic techniques still have very limited application. We focused our efforts in standardising a minilaparoscopic technique (using 3 to 5 mm instruments) for colorectal resections since it can provide excellent cosmetic results without changing the laparoscopic approach significantly. Thus, there is no need for a new learning curve as minilaparoscopy maintains the principle of instrument triangulation. This determines an undoubted advantage in terms of feasibility and reproducibility of the procedure without increasing operative time. Some preliminary experiences confirm that minilaparoscopic colorectal surgery provides acceptable results, comparable to those reported for laparoscopic surgery with regard to operative time, morbidity, and hospital stay. Randomized controlled studies should be conducted to confirm these early encouraging results
Superconductivity in the repulsive Hubbard model: an asymptotically exact weak-coupling solution
We study the phase diagram of the Hubbard model in the limit where U, the
onsite repulsive interaction, is much smaller than the bandwidth. We present an
asymptotically exact expression for T, the superconducting transition
temperature, in terms of the correlation functions of the non-interacting
system which is valid for arbitrary densities so long as the interactions are
sufficiently small. Our strategy for computing T involves first integrating
out all degrees of freedom having energy higher than an unphysical initial
cutoff . Then, the renormalization group (RG) flows of the resulting
effective action are computed and T is obtained by determining the scale
below which the RG flows in the Cooper channel diverge. We prove that T is
independent of . Using this method, we find a variety of
unconventional superconducting ground states in two and three dimensional
lattice systems and present explicit results for T and pairing symmetries
as a function of the electron concentration.Comment: 18 pages, 17 figure
FIRST-DOSE SUCCESS OF PDE5 INHIBITORS
Summary First-dose success of phosphodiesterase type 5 (PDE5) inhibitors may be adversely affected in patients with comorbidities. This article reports first-dose success rates for vardenafil 10 mg in men with erectile dysfunction (ED) and associated comorbidities who participated in the challenge phase of the Reliability – Vardenafil for Erectile Dysfunction I study. This study involved an open-label, single-dose, 1-week challenge period where patients who achieved SEP-2 (penetration) success were randomised to vardenafil 10 mg or placebo for 12 weeks in a double-blind manner. The first-dose success rates for SEP-2 and SEP-3 (maintenance of erection to completion of intercourse) were stratified according to comorbidities. Safety was assessed using adverse events (AEs). Of 600 men who received a single 10 mg dose of vardenafil, 32% had hypertension, 16% had diabetes and 19% had dyslipidaemia. Vardenafil demonstrated overall effectiveness, including first-dose SEP-2 and SEP-3 success rates in patients with and without specific comorbidities. Initial overall success rates for SEP-2 and SEP-3 during the challenge phase were 87% and 74% respectively. First-dose SEP-2 and SEP-3 success rates were 84% and 66% in men with hypertension (n = 191); 84% and 72% in men with dyslipidaemia (n = 116); and 75% and 58% in men with diabetes (n = 95). Vardenafil was well tolerated and most AEs, including the most frequently reported flushing (3.5%), were mild to moderate in intensity. Vardenafil 10 mg is generally well tolerated and efficacious, providing first-dose success with a consistently high rate of reliability of penetration and maintenance of erection in men with ED and associated comorbidities
Spin picture of the one-dimensional Hubbard model: Two-fluid structure and phase dynamics
We propose a scheme for investigating the quantum dynamics of interacting
electron models by means of time-dependent variational principle and spin
coherent states of space lattice operators. We apply such a scheme to the
one-dimensional hubbard model, and solve the resulting equations in different
regimes. In particular, we find that at low densities the dynamics is mapped
into two coupled nonlinear Schroedinger equations, whereas near half-filling
the model is described by two coupled Josephson junction arrays. Focusing then
to the case in which only the phases of the spin variables are dynamically
active, we examine a number of different solutions corresponding to the
excitations of few macroscopic modes. Based on fixed point equation of the
simpler among them, we show that the standard one-band ground state phase space
is found.Comment: 10 pages, 1 figure, to appear on Phys. Rev.
Classical realization of two-site Fermi-Hubbard systems
A classical wave optics realization of the two-site Hubbard model, describing
the dynamics of interacting fermions in a double-well potential, is proposed
based on light transport in evanescently-coupled optical waveguides.Comment: 4 page
Two-Point Versus Multipartite Entanglement in Quantum Phase Transitions
We analyze correlations between subsystems for an extended Hubbard model
exactly solvable in one dimension, which exhibits a rich structure of quantum
phase transitions (QPTs). The T=0 phase diagram is exactly reproduced by
studying singularities of single-site entanglement. It is shown how comparison
of the latter quantity and quantum mutual information allows one to recognize
whether two-point or shared quantum correlations are responsible for each of
the occurring QPTs. The method works in principle for any number D of degrees
of freedom per site. As a by-product, we are providing a benchmark for direct
measures of bipartite entanglement; in particular, here we discuss the role of
negativity at the transition.Comment: 4 pages, 2 figures, 1 tabl
Band and filling controlled transitions in exactly solved electronic models
We describe a general method to study the ground state phase diagram of
electronic models on chains whose extended Hubbard hamiltonian is formed by a
generalized permutator plus a band-controlling term. The method, based on the
appropriate interpretation of Sutherland's species, yields under described
conditions a reduction of the effective Hilbert space. In particular, we derive
the phase diagrams of two new models; the first one exhibits a band-controlled
insulator-superconductor transition at half-filling for the unusually high
value ; the second one is characterized by a filling-controlled
metal-insulator transition between two finite regions of the diagram.Comment: 5 pages, REVTEX, 2 eps figure
Temperature and filling dependence of the superconducting -phase in the Penson-Kolb-Hubbard model
We investigate in the Hartree Fock approximation the temperature and filling
dependence of the superconducting -phase for the Penson-Kolb-Hubbard
model. Due to the presence of the pair-hopping term, the phase survives for
repulsive values of the on-site Coulomb interaction, exhibiting an interesting
filling and temperature dependence. The structure of the self-consistent
equations peculiar to the -phase of the model allows to explicitly solve
them for the chemical potential. The phase diagrams are shown and discussed in
dimension 2 and 3. We also show that, when a next-nearest neighbours hopping
term is included, the critical temperature of the superconducting region
increases, and the corresponding range of filling values is shifted away from
half-filling. Comparison with known exact results is also discussed.Comment: 20 pages, REVTEX, 8 eps figure
Single-site entanglement at superconductor-insulator transition in the Hirsch model
We investigate the transition to the insulating state in the one-dimensional
Hubbard model with bond-charge interaction x (Hirsch model), at half-filling
and T=0. By means of the density-matrix renormalization group algorithm the
charge gap closure is examined by both standard finite size scaling analysis
and looking at singularities in the derivatives of single-site entanglement.
The results of the two techniques show that a quantum phase transition takes
place at a finite Coulomb interaction u_c(x) for x>0.5. The region 0<u<u_c
turns out to have a superconducting nature, at least for not too large x>x_c.Comment: 5 pages, 6 figure
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