933 research outputs found
Meron-cluster algorithms and chiral symmetry breaking in a (2+1)-d staggered fermion model
The recently developed Meron-Cluster algorithm completely solves the
exponentially difficult sign problem for a number of models previously
inaccessible to numerical simulation. We use this algorithm in a high-precision
study of a model of N=1 flavor of staggered fermions in (2+1)-dimensions with a
four-fermion interaction. This model cannot be explored using standard
algorithms. We find that the Z(2) chiral symmetry of this model is
spontaneously broken at low temperatures and that the finite-temperature chiral
phase transition is in the universality class of the 2-d Ising model, as
expected.Comment: 18 pages, LaTe
Meron-Cluster Simulation of a Chiral Phase Transition with Staggered Fermions
We examine a (3+1)-dimensional model of staggered lattice fermions with a
four-fermion interaction and Z(2) chiral symmetry using the Hamiltonian
formulation. This model cannot be simulated with standard fermion algorithms
because those suffer from a very severe sign problem. We use a new fermion
simulation technique - the meron-cluster algorithm - which solves the sign
problem and leads to high-precision numerical data. We investigate the finite
temperature chiral phase transition and verify that it is in the universality
class of the 3-d Ising model using finite-size scaling.Comment: 21 pages, 6 figure
Time-resolved imaging of guided wave phenomena
In the past decade, increasing demand and rapid developments in classical and quantum sciences resulted in advanced novel multipixel single photon detector arrays engineered on a single electronic chip. Silicon single photon avalanche detector (Si-SPAD) is one of the mainstream solution for low level light detection in visible and near-infrared wavelength region due to the dependable amplification of light signal. This thesis mainly focusses on three key experiments to showcase the potential applications of a single photon detector (Megaframe 32) consists of 32×32 square array Si-SPADs with picosecond timing circuits. With ≈ 50 ps timing resolution, each SPAD can perform time-correlated single photon counting independently. First, the concept of multiplexed single-mode wavelength-to-time mapping (WTM) of multimode light was investigated. The spacetime imaging capability of the Megaframe was then demonstrated by imaging the spatial modes emerging from a few-mode fibre enabling WTM of spatial modes. Finally, timeresolved discrete imaging in laser inscribed photonic lattices was demonstrated. By placing a photonic lattice in a linear cavity and re-injecting the output mode profile back to the lattice, the propagation of light was measured in quasi-real time manner. The experimental demonstrations using Megaframe will find applications in Raman spectroscopy, soliton imaging, quantum optics, and discrete waveguide optics
Apnea, bradycardia and desaturation spells in premature infants: impact of a protocol for the duration of 'spell-free' observation on interprovider variability and readmission rates.
ObjectiveTo study the impact of implementing a protocol to standardize the duration of observation in preterm infants with apnea/bradycardia/desaturation spells before hospital discharge on length of stay (LOS) and readmission rates.Study designA protocol to standardize the duration of in-hospital observation for preterm infants with apnea, bradycardia and desaturation spells who were otherwise ready for discharge was implemented in December 2013. We evaluated the impact of this protocol on the LOS and readmission rates of very low birth weight infants (VLBW). Data on readmission for apnea and an apparent life-threatening event (ALTE) within 30 days of discharge were collected. The pre-implementation epoch (2011 to 2013) was compared to the post-implementation period (2014 to 2016).ResultsThere were 426 and 368 VLBW discharges before and after initiation of the protocol during 2011 to 2013 and 2014 to 2016, respectively. The LOS did not change with protocol implementation (66±42 vs 64±42 days before and after implementation of the protocol, respectively). Interprovider variability on the duration of observation for apneic spells (F-8.8, P=0.04) and bradycardia spells (F-17.4, P<0.001) decreased after implementation of the protocol. The readmission rate for apnea/ALTE after the protocol decreased from 12.1 to 3.4% (P=0.01).ConclusionImplementing an institutional protocol for VLBW infants to determine the duration of apnea/bradycardia/ desaturation spell-free observation period as recommended by the American Academy of Pediatrics clinical report did not prolong the LOS but effectively reduced interprovider variability and readmission rates
FAST CHIRAL HPLC PROCEDURE FOR THE SIMULTANEOUS DETERMINATION OF DROPROPIZINE ENANTIOMERS AND ITS NONPOLAR IMPURITY IN RAW MATERIAL AND PHARMACEUTICAL FORMULATION
Objective: Levodropropizine is a novel antitussive drug, which occurs as enantiomers. They are levodropropizine (2S) [LDP] and dextrodropropizine (impurity A) (2R) [DDP]. An isocratic chiral high performance liquid chromatographic (Normal phase HPLC) method has been developed and validated for simultaneous determination of dropropizine enantiomers along with non-polar impurity-B, (1-phenyl piperazine) [1-PP] in raw material and in dosage forms.
Methods: The compounds were separated on chiral stationary phase (CSP) Chiralpak AD-H column, with a mixture of n-hexane, anhydrous ethanol, diethyl amine (DEA) in the ratio of 55:45:0.1 v/v as mobile phase at a flow rate of 1.4 ml/min. UV detection was performed at 254 nm. The method was validated for accuracy, precision, specificity, linearity, and sensitivity. The developed and validated method was successfully used for quantitative analysis of commercially available Tablets.
Results: Total chromatographic analysis time per sample was ~5 min. with 1-PP, levodrpropizne, dextropropizine eluting with retention times of 2.5 min., 3.05 min., and 3.66 min., respectively. Validation studies revealed the method is specific, rapid, reliable and reproducible for levodropropizne and its impurity A and non chiral impurity B. Calibration plots were linear over the concentration ranges 0.5-5 µg/ml and 0.5-5 µg/ml for levodropropizine and dextrodropropizine respectively.
Conclusion: The high recovery and low relative standard deviation confirm the suitability of the method for determination of dropropizine compounds in commercial tablets
Spectroscopy of the Kondo Problem in a Box
Motivated by experiments on double quantum dots, we study the problem of a
single magnetic impurity confined in a finite metallic host. We prove an exact
theorem for the ground state spin, and use analytic and numerical arguments to
map out the spin structure of the excitation spectrum of the many-body
Kondo-correlated state, throughout the weak to strong coupling crossover. These
excitations can be probed in a simple tunneling-spectroscopy transport
experiment; for that situation we solve rate equations for the conductance.Comment: 4 pages, 4 figure
Ground State and Excitations of Quantum Dots with "Magnetic Impurities"
We consider an "impurity" with a spin degree of freedom coupled to a finite
reservoir of non-interacting electrons, a system which may be realized by
either a true impurity in a metallic nano-particle or a small quantum dot
coupled to a large one. We show how the physics of such a spin impurity is
revealed in the many-body spectrum of the entire finite-size system; in
particular, the evolution of the spectrum with the strength of the
impurity-reservoir coupling reflects the fundamental many-body correlations
present. Explicit calculation in the strong and weak coupling limits shows that
the spectrum and its evolution are sensitive to the nature of the impurity and
the parity of electrons in the reservoir. The effect of the finite size
spectrum on two experimental observables is considered. First, we propose an
experimental setup in which the spectrum may be conveniently measured using
tunneling spectroscopy. A rate equation calculation of the differential
conductance suggests how the many-body spectral features may be observed.
Second, the finite-temperature magnetic susceptibility is presented, both the
impurity susceptibility and the local susceptibility. Extensive quantum
Monte-Carlo calculations show that the local susceptibility deviates from its
bulk scaling form. Nevertheless, for special assumptions about the reservoir --
the "clean Kondo box" model -- we demonstrate that finite-size scaling is
recovered. Explicit numerical evaluations of these scaling functions are given,
both for even and odd parity and for the canonical and grand-canonical
ensembles.Comment: 16 pages; published version, corrections to figure and equation,
clarification
State-recycling and time-resolved imaging in topological photonic lattices
Photonic lattices - arrays of optical waveguides - are powerful platforms for
simulating a range of phenomena, including topological phases. While probing
dynamics is possible in these systems, by reinterpreting the propagation
direction as "time," accessing long timescales constitutes a severe
experimental challenge. Here, we overcome this limitation by placing the
photonic lattice in a cavity, which allows the optical state to evolve through
the lattice multiple times. The accompanying detection method, which exploits a
multi-pixel single-photon detector array, offers quasi-real time-resolved
measurements after each round trip. We apply the state-recycling scheme to
intriguing photonic lattices emulating Dirac fermions and Floquet topological
phases. In this new platform, we also realise a synthetic pulsed electric
field, which can be used to drive transport within photonic lattices. This work
opens a new route towards the detection of long timescale effects in engineered
photonic lattices and the realization of hybrid analogue-digital simulators.Comment: Comments are welcom
Kosterlitz-Thouless Universality in a Fermionic System
A new extension of the attractive Hubbard model is constructed to study the
critical behavior near a finite temperature superconducting phase transition in
two dimensions using the recently developed meron-cluster algorithm. Unlike
previous calculations in the attractive Hubbard model which were limited to
small lattices, the new algorithm is used to study the critical behavior on
lattices as large as . These precise results for the first time
show that a fermionic system can undergo a finite temperature phase transition
whose critical behavior is well described by the predictions of Kosterlitz and
Thouless almost three decades ago. In particular it is confirmed that the
spatial winding number susceptibility obeys the well known predictions of
finite size scaling for and up to logarithmic corrections the pair
susceptibility scales as at large volumes with for .Comment: Revtex format; 4 pages, 2 figure
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