74 research outputs found
Disorder induced rounding of the phase transition in the large q-state Potts model
The phase transition in the q-state Potts model with homogeneous
ferromagnetic couplings is strongly first order for large q, while is rounded
in the presence of quenched disorder. Here we study this phenomenon on
different two-dimensional lattices by using the fact that the partition
function of the model is dominated by a single diagram of the high-temperature
expansion, which is calculated by an efficient combinatorial optimization
algorithm. For a given finite sample with discrete randomness the free energy
is a pice-wise linear function of the temperature, which is rounded after
averaging, however the discontinuity of the internal energy at the transition
point (i.e. the latent heat) stays finite even in the thermodynamic limit. For
a continuous disorder, instead, the latent heat vanishes. At the phase
transition point the dominant diagram percolates and the total magnetic moment
is related to the size of the percolating cluster. Its fractal dimension is
found d_f=(5+\sqrt{5})/4 and it is independent of the type of the lattice and
the form of disorder. We argue that the critical behavior is exclusively
determined by disorder and the corresponding fixed point is the isotropic
version of the so called infinite randomness fixed point, which is realized in
random quantum spin chains. From this mapping we conjecture the values of the
critical exponents as \beta=2-d_f, \beta_s=1/2 and \nu=1.Comment: 12 pages, 12 figures, version as publishe
Disorder driven phase transitions of the large q-state Potts model in 3d
Phase transitions induced by varying the strength of disorder in the large-q
state Potts model in 3d are studied by analytical and numerical methods. By
switching on the disorder the transition stays of first order, but different
thermodynamical quantities display essential singularities. Only for strong
enough disorder the transition will be soften into a second-order one, in which
case the ordered phase becomes non-homogeneous at large scales, while the
non-correlated sites percolate the sample. In the critical regime the critical
exponents are found universal: \beta/\nu=0.60(2) and \nu=0.73(1).Comment: 4 pages; 3 figure
Local field theory for disordered itinerant quantum ferromagnets
An effective field theory is derived that describes the quantum critical
behavior of itinerant ferromagnets in the presence of quenched disorder. In
contrast to previous approaches, all soft modes are kept explicitly. The
resulting effective theory is local and allows for an explicit perturbative
treatment. It is shown that previous suggestions for the critical fixed point
and the critical behavior are recovered under certain assumptions. The validity
of these assumptions is discussed in the light of the existence of two
different time scales. It is shown that, in contrast to previous suggestions,
the correct fixed point action is not Gaussian, and that the previously
proposed critical behavior was correct only up to logarithmic corrections. The
connection with other theories of disordered interacting electrons, and in
particular with the resolution of the runaway flow problem encountered in these
theories, is also discussed.Comment: 17pp., REVTeX, 5 eps figs, final version as publishe
Quantum critical behavior in disordered itinerant ferromagnets: Logarithmic corrections to scaling
The quantum critical behavior of disordered itinerant ferromagnets is
determined exactly by solving a recently developed effective field theory. It
is shown that there are logarithmic corrections to a previous calculation of
the critical behavior, and that the exact critical behavior coincides with that
found earlier for a phase transition of undetermined nature in disordered
interacting electron systems. This confirms a previous suggestion that the
unspecified transition should be identified with the ferromagnetic transition.
The behavior of the conductivity, the tunneling density of states, and the
phase and quasiparticle relaxation rates across the ferromagnetic transition is
also calculated.Comment: 15pp., REVTeX, 8 eps figs, final version as publishe
Microwave Electrodynamics of Electron-Doped Cuprate Superconductors
We report microwave cavity perturbation measurements of the temperature
dependence of the penetration depth, lambda(T), and conductivity, sigma(T) of
Pr_{2-x}Ce_{x}CuO_{4-delta} (PCCO) crystals, as well as parallel-plate
resonator measurements of lambda(T) in PCCO thin films. Penetration depth
measurements are also presented for a Nd_{2-x}Ce_{x}CuO_{4-delta} (NCCO)
crystal. We find that delta-lambda(T) has a power-law behavior for T<T_c/3, and
conclude that the electron-doped cuprate superconductors have nodes in the
superconducting gap. Furthermore, using the surface impedance, we have derived
the real part of the conductivity, sigma_1(T), below T_c and found a behavior
similar to that observed in hole-doped cuprates.Comment: 4 pages, 4 figures, 1 table. Submitted to Physical Review Letters
revised version: new figures, sample characteristics added to table, general
clarification give
Ferromagnetic/superconducting proximity effect in La0.7Ca0.3MnO3 / YBa2Cu3O7 superlattices
We study the interplay between magnetism and superconductivity in high
quality YBa2Cu3O7 (YBCO) / La0.7Ca0.3MnO3(LCMO)superlattices. We find evidence
for the YBCO superconductivity depression in presence of the LCMO layers. We
show that due to its short coherence length superconductivity survives in the
YBCO down to much smaller thickness in presence of the magnetic layer than in
low Tc superconductors. We also find that for a fixed thickness of the
superconducting layer, superconductivity is depressed over a thickness interval
of the magnetic layer in the 100 nm range. This is a much longer length scale
than that predicted by the theory of ferromagnetic/superconducting proximity
effect.Comment: 10 pages + 5 figures, submitted to Phys. Rev.
Crossed Andreev reflection at ferromagnetic domain walls
We investigate several factors controlling the physics of hybrid structures
involving ferromagnetic domain walls (DWs) and superconducting (S) metals. We
discuss the role of non collinear magnetizations in S/DW junctions in a spin
Nambu Keldysh formalism. We discuss transport in S/DW/N and
S/DW/S junctions in the presence of inelastic scattering in the domain wall. In
this case transport properties are similar for the S/DW/S and S/DW/N junctions
and are controlled by sequential tunneling of spatially separated Cooper pairs
across the domain wall. In the absence of inelastic scattering we find that a
Josephson current circulates only if the size of the ferromagnetic region is
smaller than the elastic mean free path meaning that the Josephson effect
associated to crossed Andreev reflection cannot be observed under usual
experimental conditions. Nevertheless a finite dc current can circulate across
the S/DW/S junction due to crossed Andreev reflection associated to sequential
tunneling.Comment: 18 pages, 8 figures, references added at the end of the introductio
The pandemic brain: Neuroinflammation in non-infected individuals during the COVID-19 pandemic
While COVID-19 research has seen an explosion in the literature, the impact of pandemic-related societal and lifestyle disruptions on brain health among the uninfected remains underexplored. However, a global increase in the prevalence of fatigue, brain fog, depression and other “sickness behavior”-like symptoms implicates a possible dysregulation in neuroimmune mechanisms even among those never infected by the virus.
We compared fifty-seven ‘Pre-Pandemic’ and fifteen ‘Pandemic’ datasets from individuals originally enrolled as control subjects for various completed, or ongoing, research studies available in our records, with a confirmed negative test for SARS-CoV-2 antibodies. We used a combination of multimodal molecular brain imaging (simultaneous positron emission tomography / magnetic resonance spectroscopy), behavioral measurements, imaging transcriptomics and serum testing to uncover links between pandemic-related stressors and neuroinflammation.
Healthy individuals examined after the enforcement of 2020 lockdown/stay-at-home measures demonstrated elevated brain levels of two independent neuroinflammatory markers (the 18 kDa translocator protein, TSPO, and myoinositol) compared to pre-lockdown subjects. The serum levels of two inflammatory markers (interleukin-16 and monocyte chemoattractant protein-1) were also elevated, although these effects did not reach statistical significance after correcting for multiple comparisons. Subjects endorsing higher symptom burden showed higher TSPO signal in the hippocampus (mood alteration, mental fatigue), intraparietal sulcus and precuneus (physical fatigue), compared to those reporting little/no symptoms. Post-lockdown TSPO signal changes were spatially aligned with the constitutive expression of several genes involved in immune/neuroimmune functions.
This work implicates neuroimmune activation as a possible mechanism underlying the non-virally-mediated symptoms experienced by many during the COVID-19 pandemic. Future studies will be needed to corroborate and further interpret these preliminary findings
Random walks and polymers in the presence of quenched disorder
After a general introduction to the field, we describe some recent results
concerning disorder effects on both `random walk models', where the random walk
is a dynamical process generated by local transition rules, and on `polymer
models', where each random walk trajectory representing the configuration of a
polymer chain is associated to a global Boltzmann weight. For random walk
models, we explain, on the specific examples of the Sinai model and of the trap
model, how disorder induces anomalous diffusion, aging behaviours and Golosov
localization, and how these properties can be understood via a strong disorder
renormalization approach. For polymer models, we discuss the critical
properties of various delocalization transitions involving random polymers. We
first summarize some recent progresses in the general theory of random critical
points : thermodynamic observables are not self-averaging at criticality
whenever disorder is relevant, and this lack of self-averaging is directly
related to the probability distribution of pseudo-critical temperatures
over the ensemble of samples of size . We describe the
results of this analysis for the bidimensional wetting and for the
Poland-Scheraga model of DNA denaturation.Comment: 17 pages, Conference Proceedings "Mathematics and Physics", I.H.E.S.,
France, November 200
COVID-19 Vaccination in Pregnancy, Paediatrics, Immunocompromised Patients, and Persons with History of Allergy or Prior SARS-CoV-2 Infection: Overview of Current Recommendations and Pre- and Post-Marketing Evidence for Vaccine Efficacy and Safety
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