1,673 research outputs found
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
Quantum discord andclassical correlations in the bond-charge Hubbard model: Quantum phasetransitions, off-diagonal long-range order, and violation of themonogamy property for discord
Topological Quantum Computation on Supersymmetric Spin Chains
Quantum gates built out of braid group elements form the building blocks of topological quantum computation. They have been extensively studied in SU(2)k quantum group theories, a rich source of examples of non-Abelian anyons such as the Ising (k=2), Fibonacci (k=3) and Jones-Kauffman (k=4) anyons. We show that the fusion spaces of these anyonic systems can be precisely mapped to the product state zero modes of certain Nicolai-like supersymmetric spin chains. As a result, we can realize the braid group on the product state zero modes of these supersymmetric systems. These operators kill all the other states in the Hilbert space, thus preventing the occurrence of errors while processing information, making them suitable for quantum computing
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
Clinical significance of erectile dysfunction developing after acute coronary event : exception to the rule or confirmation of the artery size hypothesis?
Erectile dysfunction (ED) has been found to frequently precedes the onset of coronary artery disease (CAD), representing an early marker of subclinical vascular disease, included CAD. Its recognition is, therefore, a "window opportunity" to prevent a coronary event by aggressive treatment of cardiovascular risk factors. The artery size hypothesis (ASH) has been proposed as a putative mechanism to explain the relationship between ED and CAD. Since atherosclerosis is a systemic disorder all major vascular beds should be affected to the same extent. However, symptoms at different points in the system rarely become evident at the same time. This is likely the result of smaller vessels (i.e. the penile artery) being able to less well tolerate the same amount of plaque when compared with larger ones (i.e. the coronary artery). If true, ED will develop before CAD. We present a case in which ED developed after a coronary event yet before a coronary recurrence potentially representing a late marker of vascular progression. Reasons for this unusual sequence are discussed as they might still fit the ASH
Apomorphine-induced brain modulation during sexual stimulation: a new look at central phenomena related to erectile dysfunction
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Repair of sphincter urethral strictures preserving urinary continence: surgical technique and outcomes
Introduction Repair of post-TURP sphincter urethral strictures represents challenging problem, due to the risk of urinary incontinence after the repair. We described a surgical technique we use to repair these strictures preserving urinary continence in patients with incompetent bladder neck. Materials and methods An observational, retrospective, study was conducted to include patients with post-TURP urethral strictures in the area of distal sphincter. We included only patients with complete clinical data and follow-up who previously underwent TURP or HOLEP or TUIP, and subsequently developed proximal bulbar urethral strictures close to the membranous urethra and the related distal urethral sphincter. Patients were included, if they were fully continent after TURP or other procedures to treat BPH. The primary outcome of the study was treatment failure, defined as the need for any post-operative instrumentation. Secondary outcome was post-urethroplasty urinary continence. Patients showing stricture recurrence or post-operative incontinence were classified as failure. Results Overall, 69 patients were included in the study. Median patient's age was 67 years; median stricture length was 4 cm. Thirty-tree patients (47.8%) underwent previous urethrotomy. Median follow-up was 52 months. Out of 69 patients, 55 (79.7%) were classified as success and 14 (20.3%) as failure. Out of the whole cohort, thus, 11/69 (16%) have a risk of recurrent strictures and 3/69 (4.3%) have incontinence. Conclusions The use of modified ventral onlay graft urethroplasty, using particular non-aggressive steps, is a suitable surgical technique for repair of sphincter urethral stricture in patients who underwent BPH transurethral surgery, using different procedures (TURP, HOLEP, TUIP)
Spin- and charge-density waves in the Hartree-Fock ground state of the two-dimensional Hubbard model
The ground states of the two-dimensional repulsive Hubbard model are studied
within the unrestricted Hartree-Fock (UHF) theory. Magnetic and charge
properties are determined by systematic, large-scale, exact numerical
calculations, and quantified as a function of electron doping . In the
solution of the self-consistent UHF equations, multiple initial configurations
and simulated annealing are used to facilitate convergence to the global
minimum. New approaches are employed to minimize finite-size effects in order
to reach the thermodynamic limit. At low to moderate interacting strengths and
low doping, the UHF ground state is a linear spin-density wave (l-SDW), with
antiferromagnetic order and a modulating wave. The wavelength of the modulating
wave is . Corresponding charge order exists but is substantially weaker
than the spin order, hence holes are mobile. As the interaction is increased,
the l-SDW states evolves into several different phases, with the holes
eventually becoming localized. A simple pairing model is presented with
analytic calculations for low interaction strength and small doping, to help
understand the numerical results and provide a physical picture for the
properties of the SDW ground state. By comparison with recent many-body
calculations, it is shown that, for intermediate interactions, the UHF solution
provides a good description of the magnetic correlations in the true ground
state of the Hubbard model.Comment: 13 pages, 17 figure, 0 table
Time dependent mean field theory of the superfluid-insulator phase transition
We develop a time-dependent mean field approach, within the time-dependent
variational principle, to describe the Superfluid-Insulator quantum phase
transition. We construct the zero temperature phase diagram both of the
Bose-Hubbard model (BHM), and of a spin-S Heisenberg model (SHM) with the XXZ
anisotropy. The phase diagram of the BHM indicates a phase transition from a
Mott insulator to a compressibile superfluid phase, and shows the expected
lobe-like structure. The SHM phase diagram displays a quantum phase transition
between a paramagnetic and a canted phases showing as well a lobe-like
structure. We show how the BHM and Quantum Phase model (QPM) can be rigorously
derived from the SHM. Based on such results, the phase boundaries of the SHM
are mapped to the BHM ones, while the phase diagram of the QPM is related to
that of the SHM. The QPM's phase diagram obtained through the application of
our approach to the SHM, describes the known onset of the macroscopic phase
coherence from the Coulomb blockade regime for increasing Josephson coupling
constant. The BHM and the QPM phase diagrams are in good agreement with Quantum
Monte Carlo results, and with the third order strong coupling perturbative
expansion.Comment: 15 pages, 8 figures. To be published in Phys. Rev.
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