879 research outputs found
Requirements for efficient cell-type proportioning: regulatory timescales, stochasticity and lateral inhibition
The proper functioning of multicellular organisms requires the robust
establishment of precise proportions between distinct cell-types. This
developmental differentiation process typically involves intracellular
regulatory and stochastic mechanisms to generate cell-fate diversity as well as
intercellular mechanisms to coordinate cell-fate decisions at tissue level. We
thus surmise that key insights about the developmental regulation of cell-type
proportion can be captured by the modeling study of clustering dynamics in
population of inhibitory-coupled noisy bistable systems. This general class of
dynamical system is shown to exhibit a very stable two-cluster state, but also
frustrated relaxation, collective oscillations or steady-state hopping which
prevents from timely and reliably reaching a robust and well-proportioned
clustered state. To circumvent these obstacles or to avoid fine-tuning, we
highlight a general strategy based on dual-time positive feedback loops, such
as mediated through transcriptional versus epigenetic mechanisms, which
improves proportion regulation by coordinating early and flexible lineage
priming with late and firm commitment. This result sheds new light on the
respective and cooperative roles of multiple regulatory feedback, stochasticity
and lateral inhibition in developmental dynamics
Some aspects of electronic topological transition in 2D system on a square lattice. Excitonic ordered states
We study the ordered "excitonic" states which develop around the quantum
critical point (QCP) associated with the electronic topological transition
(ETT) in a 2D electron system on a square lattice. We consider the case of
hopping beyond nearest neighbors when ETT has an unusual character. We show
that the amplitude of the order parameter (OP) and of the gap in the electron
spectrum increase with increasing the distance from the QCP, \delta_c - \delta,
where \delta = 1-n and "n" is an electron concentration. Such a behavior is
different from the ordinary case when OP and the gap decrease when going away
from the point which is a motor for instability. The gap opens at "hot spots"
and extends untill the saddle points (SP) whatever is the doping concentration.
The spectrum gets a characteristic flat shape as a result of hybrydization
effect in the vicinity of two different SP's. The shape of the spectrum and the
angle dependence of the gap have a striking similarity with the features
observed in the normal state of the underdoped high-T cuprates. We discuss
also details about the phase diagram and the behaviour of the density of
states.Comment: 15 pages, 14 EPS figures included, EPJ style included, added
references, changed conten
Surface and bulk critical behaviour of the XY chain in a transverse field
The surface magnetization of the quantum XY chain in a transverse field is
found for arbitrary nearest neighbour interactions in closed form. This allows
to derive the bulk phase diagram in a simple way. The magnetic surface
behaviour and the bulk correlation length are found exactly.Comment: 5 pages, to be published in J. Phys.
Various ordered states in a 2D interacting electron system close to an electronic topological transition
We consider a 2D electron system on a square lattice with hopping beyond
nearest neighbors. The existence of the quantum critical point associated with
an electronic topological transition in the noninteracting system results in
density wave (DW) and high temperature d-wave superconducting (dSC)
instabilities in the presence of an exchange interaction J. We analyse
different
DW ordering such as isotropic Spin DW (SDW), d-wave SDW, isotropic Charge DW
(CDW) and d-wave CDW. The coexistence of dSC and SDW orders leads necessary to
the existence of a third order which is a pi triplet superconducting (PTS)
order. A new phase diagram with a mixed phase of SDW, dSC and PTS order is
found. The theory is applied to high-Tc cuprates.Comment: 2 pages, 1 figure, submitted to LT22 (Physica B
Universal scaling and quantum critical behavior of CeRhSb(1-x)Sn(x)
We propose a universal scaling rho*chi=const of the electrical resistivity
rho with the inverse magnetic susceptibility chi^(-1) below the temperature of
the quantum-coherence onset for the Ce 4f states in CeRhSb(1-x)Sn(x). In the
regime, where the Kondo gap disappears (x~0.12), the system forms a non-Fermi
liquid (NFL), which transforms into a Fermi liquid at higher temperature. The
NFL behavior is attributed to the presence of a novel quantum critical point
(QCP) at the Kondo insulator - correlated metal boundary. The divergent
behavior of the resistivity, the susceptibility, and the specific heat has been
determined when approaching QCP from the metallic side.Comment: Sent to Phys. Rev. Let
Entanglement versus Correlations in Spin Systems
We consider pure quantum states of spins or qubits and study the
average entanglement that can be \emph{localized} between two separated spins
by performing local measurements on the other individual spins. We show that
all classical correlation functions provide lower bounds to this
\emph{localizable entanglement}, which follows from the observation that
classical correlations can always be increased by doing appropriate local
measurements on the other qubits. We analyze the localizable entanglement in
familiar spin systems and illustrate the results on the hand of the Ising spin
model, in which we observe characteristic features for a quantum phase
transition such as a diverging entanglement length.Comment: 4 page
Series Expansions for Excited States of Quantum Lattice Models
We show that by means of connected-graph expansions one can effectively
generate exact high-order series expansions which are informative of low-lying
excited states for quantum many-body systems defined on a lattice. In
particular, the Fourier series coefficients of elementary excitation spectra
are directly obtained. The numerical calculations involved are straightforward
extensions of those which have already been used to calculate series expansions
for ground-state correlations and susceptibilities in a wide variety of
models. As a test, we have reproduced the known elementary excitation spectrum
of the transverse-field Ising chain in its disordered phase.Comment: 9 pages, no figures, Revtex 3.0 The revised version corrects the
incorrect (and unnecessary) statement in the original that H and H^eff are
related by a unitary transformation; in fact they are related by via a
similarity transformation. This has no implications for the calculations of
spectra, but is important for matrix element
- …