7,783 research outputs found
Disentangling phase transitions and critical points in the proton-neutron interacting boson model by catastrophe theory
We introduce the basic concepts of catastrophe theory needed to derive
analytically the phase diagram of the proton-neutron interacting boson model
(IBM-2). Previous studies [1,2,3] were based on numerical solutions. We here
explain the whole IBM-2 phase diagram including the precise order of the phase
transitions in terms of the cusp catastrophe.Comment: To be published in Physics Letters
Integrability and Quantum Phase Transitions in Interacting Boson Models
The exact solution of the boson pairing hamiltonian given by Richardson in
the sixties is used to study the phenomena of level crossings and quantum phase
transitions in the integrable regions of the sd and sdg interacting boson
models.Comment: 5 pages, 5 fig. Erice Conferenc
Quantum Phase Transitions in the Interacting Boson Model: Integrability, level repulsion and level crossing
We study the quantum phase transition mechanisms that arise in the
Interacting Boson Model. We show that the second-order nature of the phase
transition from U(5) to O(6) may be attributed to quantum integrability,
whereas all the first-order phase transitions of the model are due to level
repulsion with one singular point of level crossing. We propose a model
Hamiltonian with a true first-order phase transition for finite systems due to
level crossings.Comment: Accepted in PR
Excited-state quantum phase transitions in a two-fluid Lipkin model
Background: Composed systems have became of great interest in the framework
of the ground state quantum phase transitions (QPTs) and many of their
properties have been studied in detail. However, in these systems the study of
the so called excited-state quantum phase transitions (ESQPTs) have not
received so much attention.
Purpose: A quantum analysis of the ESQPTs in the two-fluid Lipkin model is
presented in this work. The study is performed through the Hamiltonian
diagonalization for selected values of the control parameters in order to cover
the most interesting regions of the system phase diagram. [Method:] A
Hamiltonian that resembles the consistent-Q Hamiltonian of the interacting
boson model (IBM) is diagonalized for selected values of the parameters and
properties such as the density of states, the Peres lattices, the
nearest-neighbor spacing distribution, and the participation ratio are
analyzed.
Results: An overview of the spectrum of the two-fluid Lipkin model for
selected positions in the phase diagram has been obtained. The location of the
excited-state quantum phase transition can be easily singled out with the Peres
lattice, with the nearest-neighbor spacing distribution, with Poincar\'e
sections or with the participation ratio.
Conclusions: This study completes the analysis of QPTs for the two-fluid
Lipkin model, extending the previous study to excited states. The ESQPT
signatures in composed systems behave in the same way as in single ones,
although the evidences of their presence can be sometimes blurred. The Peres
lattice turns out to be a convenient tool to look into the position of the
ESQPT and to define the concept of phase in the excited states realm
Effect of Contrast-Enhanced Echocardiograms on the Prognosis of Infective Endocarditis
Objective - Infective endocarditis (IE) is an infectious disease of the cardiac valves where bacteria colonize the valves; typically, via the formation of vegetations. Recent research has shown that the microbubbles in a contrast-enhanced ultrasound (CEUS) examination can move and dislodge bacterial vegetations in vitro. This study investigated whether CEUS resulted in faster resolution of IE in vivo by dislodging the vegetations.
Methods - This IRB approved retrospective study reviewed 36 patients who were diagnosed with IE via echocardiography. Data was sourced from patients within the Jefferson University Hospital’s Cardiology EMR system by searching for contrast and vegetation from January 1st, 2013 – January 1st, 2018. Fifteen patients were not given contrast, whereas 21 patients were given contrast via agitated saline (n=16) or an ultrasound contrast agent (n=5). All patients received an echocardiogram after blood cultures confirmed an infection, but before resolution of infection (defined by negative blood cultures). A student’s t-test was used for analyses.
Results - The study population was heterogeneous in terms of sex (67.5% male) and race (70% Caucasian, 25% African American, and 5% Asian), with an average age of 51±20 years, and an average BMI of 29.65±7.43 in the contrast group and 27.67±3.16 in the non-contrast group (p=0.37). Following ultrasound, no patients had documented stroke, pulmonary embolism, or systemic blood clot, which physicians could have attributed to a thrombus resulting from dislodging of bacterial vegetation. Overall, blood cultures did not clear faster in patients receiving CEUS compared to those undergoing standard echocardiography, (2.63±2.69 days vs. 1.34 ±1.11 days, p=0.09). CEUS also did not shorten the admission length in patients with IE, (16.9±7.7 days vs. 19.9±12.1 days; p=0.36).
Conclusion - Based on this limited sample size, patients who underwent CEUS did not have a different prognosis when compared to patients who received a non-contrast echocardiogram
Phase diagram of an extended Agassi model
Background: The Agassi model is an extension of the Lipkin-Meshkov-Glick
model that incorporates the pairing interaction. It is a schematic model that
describes the interplay between particle-hole and pair correlations. It was
proposed in the 1960's by D. Agassi as a model to simulate the properties of
the quadrupole plus pairing model.
Purpose: The aim of this work is to extend a previous study by Davis and
Heiss generalizing the Agassi model and analyze in detail the phase diagram of
the model as well as the different regions with coexistence of several phases.
Method: We solve the model Hamiltonian through the Hartree-Fock-Bogoliubov
(HFB) approximation, introducing two variational parameters that play the role
of order parameters. We also compare the HFB calculations with the exact ones.
Results: We obtain the phase diagram of the model and classify the order of
the different quantum phase transitions appearing in the diagram. The phase
diagram presents broad regions where several phases, up to three, coexist.
Moreover, there is also a line and a point where four and five phases are
degenerated, respectively.
Conclusions: The phase diagram of the extended Agassi model presents a rich
variety of phases. Phase coexistence is present in extended areas of the
parameter space. The model could be an important tool for benchmarking novel
many-body approximations.Comment: Accepted for publication in PR
An extended Agassi model: algebraic structure, phase diagram, and large size limit
The Agassi model is a schematic two-level model that involves pairing and
monopole-monopole interactions. It is, therefore, an extension of the well
known Lipkin-Meshkov-Glick (LMG) model. In this paper we review the algebraic
formulation of an extension of the Agassi model as well as its bosonic
realization through the Schwinger representation. Moreover, a mean-field
approximation for the model is presented and its phase diagram discussed.
Finally, a analysis, with proportional to the degeneracy of each
level, is worked out to obtain the thermodynamic limit of the ground state
energy and some order parameters from the exact Hamiltonian diagonalization for
finite.Comment: Accepted in Physica Scripta. Focus on SSNET 201
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