642 research outputs found
High-T_{c} Superconductors with AF Order: Limitations on Spin-Fluctuation Pairing Mechanism
The very intriguing antagonistic interplay of antiferromagnetism (AF) and
superconductivity (SC), recently discovered in high-temperature
superconductors, is studied in the framework of a microscopic theory. We
explain the surprisingly large increase of the magnetic Bragg peak intensity
at in the magnetic field at low
temperatures in . Good agreement
with experimental results is found. The theory predicts large anisotropy of the
relative intensity %, i.e.
. The quantum (T=0) phase
diagram at H=0 is constructed. The theory also predicts: (i) the magnetic field
induced AF order in the SC state; (ii) small value for the spin-fluctuation
coupling constant . The latter gives very small SC
critical temperature , thus questioning the spin-fluctuation
mechanism of pairing in HTS oxides.Comment: Linguistic changes, improved readabilty, changed titl
Hitchhiking Through the Cytoplasm
We propose an alternative mechanism for intracellular cargo transport which
results from motor induced longitudinal fluctuations of cytoskeletal
microtubules (MT). The longitudinal fluctuations combined with transient cargo
binding to the MTs lead to long range transport even for cargos and vesicles
having no molecular motors on them. The proposed transport mechanism, which we
call ``hitchhiking'', provides a consistent explanation for the broadly
observed yet still mysterious phenomenon of bidirectional transport along MTs.
We show that cells exploiting the hitchhiking mechanism can effectively up- and
down-regulate the transport of different vesicles by tuning their binding
kinetics to characteristic MT oscillation frequencies
A Solvable Model for Polymorphic Dynamics of Biofilaments
We investigate an analytically tractable toy model for thermally induced
polymorphic dynamics of cooperatively rearranging biofilaments - like
microtubules. The proposed 4 -block model, which can be seen as a
coarse-grained approximation of the full polymorphic tube model, permits a
complete analytical treatment of all thermodynamic properties including
correlation functions and angular fourier mode distributions. Due to its
mathematical tractability the model straightforwardly leads to some physical
insights in recently discussed phenomena like the "length dependent persistence
length". We show that a polymorphic filament can disguise itself as a classical
worm like chain on small and on large scales and yet display distinct anomalous
tell-tale features indicating an inner switching dynamics on intermediate
length scales
Why Microtubules run in Circles - Mechanical Hysteresis of the Tubulin Lattice
The fate of every eukaryotic cell subtly relies on the exceptional mechanical
properties of microtubules. Despite significant efforts, understanding their
unusual mechanics remains elusive. One persistent, unresolved mystery is the
formation of long-lived arcs and rings, e.g. in kinesin-driven gliding assays.
To elucidate their physical origin we develop a model of the inner workings of
the microtubule's lattice, based on recent experimental evidence for a
conformational switch of the tubulin dimer. We show that the microtubule
lattice itself coexists in discrete polymorphic states. Curved states can be
induced via a mechanical hysteresis involving torques and forces typical of few
molecular motors acting in unison. This lattice switch renders microtubules not
only virtually unbreakable under typical cellular forces, but moreover provides
them with a tunable response integrating mechanical and chemical stimuli.Comment: 5 pages, 4 Movies in the Supplemen
Statics and dynamics of charge fluctuations in the t-J model
The equation for the charge vertex of the model is derived and
solved in leading order of an 1/N expansion, working directly in terms of
Hubbard operators. Various quantities which depend crucially on are
then calculated, such as the life time and the transport life time of electrons
due to a charge coupling to other degrees of freedom and the charge-charge
correlation function. Our results show that the static screening of charges and
the dynamics of charge fluctuations depend only weakly on and are mainly
determined by the constraint of having no double occupancies of sites.Comment: 10 latex pages, 4 figures as post-script file
Conventional Magnetic Superconductors: Coexistence of Singlet Superconductivity and Magnetic Order
The basic physics of bulk magnetic superconductors (MS) related to the
problem of the coexistence of singlet superconductivity (SC) and magnetic order
is reviewed. The interplay between exchange (EX) and electromagnetic (EM)
interaction is discussed and argued that the singlet SC and uniform
ferromagnetic (F) order practically never coexist. In case of their mutual
coexistence the F order is modified into a domain-like or spiral structure
depending on magnetic anisotropy. It turns out that this situation is realized
in several superconductors such as , ,
with electronic and in with nuclear magnetic order.
The later problem is also discussed here.
The coexistence of SC and antiferromagnetism is more favorable than with the
modified F order. Very interesting physics is realized in systems with SC and
weak-ferromagnetism which results in an very reach phase diagram.
The properties of magnetic superconductors in magnetic field are very
peculiar, especially near the (ferro)magnetic transition temperature where the
upper critical field becomes smaller than the thermodynamical critical field.
The extremely interesting physics of Josephson junctions based on MS with
spiral magnetic order is also discussed. The existence of the triplet pairing
amplitude () in MS with
rotating magnetization (the effect recently rediscovered in SFS junctions)
gives rise to the so called -contact. Furthermore, the interplay of the
superconducting and magnetic phase in such a contact renders possibilities for
a new type of coupled Josephson-qubits.Comment: 28 pages, 3 figures; submitted for the Special Issue Comptes de
l'Academie des Sciences: Problems of the Coexistence of Magnetism and
Superconductivity, edited by A. Buzdi
Stable Gaussian Process based Tracking Control of Lagrangian Systems
High performance tracking control can only be achieved if a good model of the
dynamics is available. However, such a model is often difficult to obtain from
first order physics only. In this paper, we develop a data-driven control law
that ensures closed loop stability of Lagrangian systems. For this purpose, we
use Gaussian Process regression for the feed-forward compensation of the
unknown dynamics of the system. The gains of the feedback part are adapted
based on the uncertainty of the learned model. Thus, the feedback gains are
kept low as long as the learned model describes the true system sufficiently
precisely. We show how to select a suitable gain adaption law that incorporates
the uncertainty of the model to guarantee a globally bounded tracking error. A
simulation with a robot manipulator demonstrates the efficacy of the proposed
control law.Comment: Please cite the conference paper. arXiv admin note: text overlap with
arXiv:1806.0719
Populist communication in the post-truth age: A comparative analysis of treatment of journalists by Donald Trump and Aleksandar Vučić
The populist communication model of perceiving journalists as "the enemies of the state", presented in the political communication of the 45th US President Donald Trump, seems to be not only reserved for the US but has become a trend for populist leaders worldwide. Such rhetoric has been revitalized by the recent rise of populism in Western Europe, fostered in Central and Eastern Europe and recognized in the local media landscape in Serbia. Tis paper compares the communication models of US President Donald Trump and the Serbian President Aleksandar Vučić as directed toward journalists. The results show that these leaders use a similar communication pattern: they favour divisive issues, stressing a friendly vs. unpatriotic media, alienating the media outlets and contributing to the further polarisation in media and society. This research is based on a qualitative analysis of 12 press conferences, including media statements chosen for their unique interactions with media representatives
Why is d-wave pairing in HTS robust in the presence of impurities?
In the recent theory of strong correlations by Kulic and Zeyher it has been
shown that by lowering doping concentration a forward peak in the charge
scattering channel is developed. Accordingly, near the optimal doping the
nonmagnetic scattering is pronounced in the d-channel and its effect on d-wave
pairing is reduced. As a consequence, d-wave pairing is robust against defects
and impurities, the order parameter keeps its d-wave shape for any scattering
rate and the density of states becomes finite at the Fermi surface. For large
doping scattering anisotropy parameter is small and d-wave loses its
robustness. The theory is generally formulated for the bi-layer model by
including: 1) intra- and inter-plane pairing; 2) intra- and inter-plane
impurities.Comment: Complete revision, 4 pages with 2 PS figures, RevTeX, submitted to
Phys. Rev. Let
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