10 research outputs found
Physical Response Functions of Strongly Coupled Massive Quantum Liquids
We study physical properties of strongly coupled massive quantum liquids from
their spectral functions using the AdS/CFT correspondence. The generic model
that we consider is dense, heavy fundamental matter coupled to SU(N_c) super
Yang-Mills theory at finite temperature above the deconfinement phase
transition but below the scale set by the baryon number density. In this setup,
we study the current-current correlators of the baryon number density using new
techniques that employ a scaling behavior in the dual geometry. Our results,
the AC conductivity, the quasi-particle spectrum and the Drude-limit parameters
like the relaxation time are simple temperature-independent expressions that
depend only on the mass-squared to density ratio and display a crossover
between a baryon- and meson-dominated regime. We concentrated on the
(2+1)-dimensional defect case, but in principle our results can also be
generalized straightforwardly to other cases.Comment: 21 pages, 10 figures, extra paragraph and figure are added in
response to referee's comment
Massive Quantum Liquids from Holographic Angel's Trumpets
We explore the small-temperature regime in the deconfined phase of massive
fundamental matter at finite baryon number density coupled to the 3+1
dimensional N=4 SYM theory. In this setting, we can demonstrate a new type of
non-trivial temperature-independent scaling solutions for the probe brane
embeddings. Focusing mostly on matter supported in 2+1 dimensions, the
thermodynamics indicate that there is a quantum liquid with interesting
density-dependent low-temperature physics. We also comment about 3+1 and 1+1
dimensional systems, where we further find for example a new thermodynamic
instability.Comment: 18+1 pages, 6 figures; replaced fig. 6 and comments in sec. 5.2;
minor explanations added and typos fixed, final version published in JHEP
(modulo fig. 3); factor of \sqrt{\lambda} and corresponding comments fixe
Collective Excitations of Holographic Quantum Liquids in a Magnetic Field
We use holography to study N=4 supersymmetric SU(Nc) Yang-Mills theory in the
large-Nc and large-coupling limits coupled to a number Nf << Nc of
(n+1)-dimensional massless supersymmetric hypermultiplets in the Nc
representation of SU(Nc), with n=2,3. We introduce a temperature T, a baryon
number chemical potential mu, and a baryon number magnetic field B, and work in
a regime with mu >> T,\sqrt{B}. We study the collective excitations of these
holographic quantum liquids by computing the poles in the retarded Green's
function of the baryon number charge density operator and the associated peaks
in the spectral function. We focus on the evolution of the collective
excitations as we increase the frequency relative to T, i.e. the
hydrodynamic/collisionless crossover. We find that for all B, at low
frequencies the tallest peak in the spectral function is associated with
hydrodynamic charge diffusion. At high frequencies the tallest peak is
associated with a sound mode similar to the zero sound mode in the
collisionless regime of a Landau Fermi liquid. The sound mode has a gap
proportional to B, and as a result for intermediate frequencies and for B
sufficiently large compared to T the spectral function is strongly suppressed.
We find that the hydrodynamic/collisionless crossover occurs at a frequency
that is approximately B-independent.Comment: 45 pages, 8 png and 47 pdf images in 22 figure
Holographic Roberge-Weiss Transitions II: Defect Theories and the Sakai-Sugimoto Model
We extend the work of Aarts et al., including an imaginary chemical potential
for quark number into the Sakai-Sugimoto model and codimension k defect
theories. The phase diagram of these models are a function of three parameters,
the temperature, chemical potential and the asymptotic separation of the
flavour branes, related to a mass for the quarks in the boundary theories. We
compute the phase diagrams and the pressure due to the flavours of the theories
as a function of these parameters and show that there are Roberge-Weiss
transitions in the high temperature phases, chiral symmetry restored for the
Sakai-Sugimoto model and deconfined for the defect models, while at low
temperatures there are no Roberge-Weiss transitions. In all the models we
consider the transitions between low and high temperature phases are first
order, hence the points where they meet the Roberge-Weiss lines are triple
points. The pressure for the defect theories scales in the way we expect from
dimensional analysis while the Sakai-Sugimoto model exhibits unusual scaling.
We show that the models we consider are analytic in \mu^2 when \mu^2 is small.Comment: 39 pages, 12 figures. references added, Sakai-Sugimoto section
revised, version to appear in JHE
Multicentre evaluation of MRI variability in the quantification of infarct size in experimental focal cerebral ischaemia
Ischaemic stroke is a leading cause of death and disability in the developed world.
Despite that considerable advances in experimental research enabled understanding
of the pathophysiology of the disease and identified hundreds of potential
neuroprotective drugs for treatment, no such drug has shown efficacy in humans. The
failure in the translation from bench to bedside has been partially attributed to the
poor quality and rigour of animal studies. Recently, it has been suggested that
multicentre animal studies imitating the design of randomised clinical trials could
improve the translation of experimental research. Magnetic resonance imaging (MRI)
could be pivotal in such studies due to its non-invasive nature and its high sensitivity
to ischaemic lesions, but its accuracy and concordance across centres has not yet been
evaluated.
This thesis focussed on the use of MRI for the assessment of late infarct size, the
primary outcome used in stroke models. Initially, a systematic review revealed that a
plethora of imaging protocols and data analysis methods are used for this purpose.
Using meta-analysis techniques, it was determined that T2-weighted imaging (T2WI)
was best correlated with gold standard histology for the measurement of infarctbased
treatment effects. Then, geometric accuracy in six different preclinical MRI
scanners was assessed using structural phantoms and automated data analysis tools
developed in-house. It was found that geometric accuracy varies between scanners,
particularly when centre-specific T2WI protocols are used instead of a standardised
protocol, though longitudinal stability over six months is high. Finally, a simulation
study suggested that the measured geometric errors and the different protocols are
sufficient to render infarct volumes and related group comparisons across centres
incomparable. The variability increases when both factors are taken into account and
when infarct volume is expressed as a relative estimate. Data in this study were
analysed using a custom-made semi-automated tool that was faster and more reliable
in repeated analyses than manual analysis.
Findings of this thesis support the implementation of standardised methods for the
assessment and optimisation of geometric accuracy in MRI scanners, as well as image
acquisition and analysis of in vivo data for the measurement of infarct size in
multicentre animal studies. Tools and techniques developed as part of the thesis show
great promise in the analysis of phantom and in vivo data and could be a step towards
this endeavour