379 research outputs found
Breakdown of the Kondo Effect in Critical Antiferromagnets
The breakdown of the Kondo effect may be the origin of the anomalous
properties of the heavy-fermion compounds at low temperatures. We study the
dynamics of one impurity embedded in an antiferromagnetic host at the quantum
critical point and show that the impurity is not screened and develops a power
law correlation function. This suggests that the breakdown of the Kondo effect
can simply be a consequence of the system's proximity to the quantum critical
point.Comment: To appear in Physical Review B (Brief Reports
Optogenetic Release of ACh Induces Rhythmic Bursts of Perisomatic IPSCs in Hippocampus
Acetylcholine (ACh) influences a vast array of phenomena in cortical systems. It alters many ionic conductances and neuronal firing behavior, often by regulating membrane potential oscillations in populations of cells. Synaptic inhibition has crucial roles in many forms of oscillation, and cholinergic mechanisms regulate both oscillations and synaptic inhibition. In vitro investigations using bath-application of cholinergic receptor agonists, or bulk tissue electrical stimulation to release endogenous ACh, have led to insights into cholinergic function, but questions remain because of the relative lack of selectivity of these forms of stimulation. To investigate the effects of selective release of ACh on interneurons and oscillations, we used an optogenetic approach in which the light-sensitive non-selective cation channel, Channelrhodopsin2 (ChR2), was virally delivered to cholinergic projection neurons in the medial septum/diagonal band of Broca (MS/DBB) of adult mice expressing Cre-recombinase under the control of the choline-acetyltransferase (ChAT) promoter. Acute hippocampal slices obtained from these animals weeks later revealed ChR2 expression in cholinergic axons. Brief trains of blue light pulses delivered to untreated slices initiated bursts of ACh-evoked, inhibitory post-synaptic currents (L-IPSCs) in CA1 pyramidal cells that lasted for 10's of seconds after the light stimulation ceased. L-IPSC occurred more reliably in slices treated with eserine and a very low concentration of 4-AP, which were therefore used in most experiments. The rhythmic, L-IPSCs were driven primarily by muscarinic ACh receptors (mAChRs), and could be suppressed by endocannabinoid release from pyramidal cells. Finally, low-frequency oscillations (LFOs) of local field potentials (LFPs) were significantly cross-correlated with the L-IPSCs, and reversal of the LFPs near s. pyramidale confirmed that the LFPs were driven by perisomatic inhibition. This optogenetic approach may be a useful complementary technique in future investigations of endogenous ACh effects
Millennium Simulation Dark Matter Haloes: Multi-fractal and Lacunarity Analysis with Homogeneity Transition
We investigate from the fractal viewpoint the way in which the dark matter is
grouped at z = 0 in the Millennium dark matter cosmological simulation. The
determination of the cross to homogeneity in the Millennium Simulation data is
described from the behaviour of the fractal dimension and the lacunarity. We
use the sliding window technique to calculate the fractal mass-radius
dimension, the pre-factor F and the lacunarity of this fractal relation.
Besides, we determinate the multi-fractal dimension and the lacunarity
spectrum, including their dependence with radial distance. This calculations
show a radial distance dependency of all the fractal quantities, with
heterogeneity clustering of dark matter haloes up to depths of 100 Mpc/h. The
dark matter haloes clustering in the Millennium Simulation shows a radial
distance dependency, with two regions clearly defined. The lacunarity spectrum
for values of the structure parameter q >= 1 shows regions with relative
maxima, revealing the formation of clusters and voids in the dark matter haloes
distribution. With the use of the multi-fractal dimension and the lacunarity
spectrum, the transition to homogeneity at depths between 100 Mpc/h and 120
Mpc/h for the Millennium Simulation dark matter haloes is detected.Comment: 13 pages, 9 figures, MNRAS - Accepte
Non-uniform phases in metals with local moments
The two-dimensional Kondo lattice model with both nearest and next-nearest
neighbor exchange interactions is studied within a mean-field approach and its
phase diagram is determined. In particular, we allow for lattice translation
symmetry breaking. We observe that the usual uniform inter-site order parameter
is never realized, being unstable towards other more complex types of order.
When the nearest neighbor exchange J_1 is ferromagnetic the flux phase is
always the most stable state, irrespective of the value of the
next-nearest-neighbor interaction J_2. For antiferromagnetic J_1, however,
either a columnar or a flux phase is realized, depending on conduction electron
filling and the value of J_2.Comment: 5 pages, 7 figure
Overexpression of miRNA-25-3p inhibits Notch1 signaling and TGF-β-induced collagen expression in hepatic stellate cells
During chronic liver injury hepatic stellate cells (HSCs), the principal source of extracellular matrix in the fibrotic liver, transdifferentiate into pro-fibrotic myofibroblast-like cells - a process potentially regulated by microRNAs (miRNAs). Recently, we found serum miRNA-25-3p (miR-25) levels were upregulated in children with Cystic Fibrosis (CF) without liver disease, compared to children with CF-associated liver disease and healthy individuals. Here we examine the role of miR-25 in HSC biology. MiR-25 was detected in the human HSC cell line LX-2 and in primary murine HSCs, and increased with culture-induced activation. Transient overexpression of miR-25 inhibited TGF-β and its type 1 receptor (TGFBR1) mRNA expression, TGF-β-induced Smad2 phosphorylation and subsequent collagen1α1 induction in LX-2 cells. Pull-down experiments with biotinylated miR-25 revealed Notch signaling (co-)activators ADAM-17 and FKBP14 as miR-25 targets in HSCs. NanoString analysis confirmed miR-25 regulation of Notch- and Wnt-signaling pathways. Expression of Notch signaling pathway components and endogenous Notch1 signaling was downregulated in miR-25 overexpressing LX-2 cells, as were components of Wnt signaling such as Wnt5a. We propose that miR-25 acts as a negative feedback anti-fibrotic control during HSC activation by reducing the reactivity of HSCs to TGF-β-induced collagen expression and modulating the cross-talk between Notch, Wnt and TGF-β signaling
Odd Frequency Pairing in the Kondo Lattice
We discuss the possibility that heavy fermion superconductors involve
odd-frequency pairing of the kind first considered by Berezinskii. Using a toy
model for odd frequency triplet pairing in the Kondo lattice we are able to
examine key properties of this new type of paired state. To make progress
treating the strong constraint in the Kondo lattice model we use the
technical trick of a Majorana representation of the local moments, which
permits variational treatments of the model without a Gutzwiller approximation.
The simplest mean field theory involves the development of bound states between
the local moments and conduction electrons, characterized by a spinor order
parameter. We show that this state is a stable realization of odd frequency
triplet superconductivity with surfaces of gapless excitations whose spin and
charge coherence factors vanish linearly in the quasiparticle energy. A
NMR relaxation rate coexists with a linear specific heat. We discuss possible
extensions of our toy model to describe heavy fermion superconductivity.Comment: 67 page
Mean field theory of the Mott-Anderson transition
We present a theory for disordered interacting electrons that can describe
both the Mott and the Anderson transition in the respective limits of zero
disorder and zero interaction. We use it to investigate the T=0 Mott-Anderson
transition at a fixed electron density, as a the disorder strength is
increased. Surprisingly, we find two critical values of disorder W_{nfl} and
W_c. For W > W_{nfl}, the system enters a ``Griffiths'' phase, displaying
metallic non-Fermi liquid behavior. At even stronger disorder, W=W_c > W_{nfl}
the system undergoes a metal insulator transition, characterized by the linear
vanishing of both the typical density of states and the typical quasiparticle
weight.Comment: 4 pages, 2 figures, REVTEX, eps
Local fluctuations in quantum critical metals
We show that spatially local, yet low-energy, fluctuations can play an
essential role in the physics of strongly correlated electron systems tuned to
a quantum critical point. A detailed microscopic analysis of the Kondo lattice
model is carried out within an extended dynamical mean-field approach. The
correlation functions for the lattice model are calculated through a
self-consistent Bose-Fermi Kondo problem, in which a local moment is coupled
both to a fermionic bath and to a bosonic bath (a fluctuating magnetic field).
A renormalization-group treatment of this impurity problem--perturbative in
, where is an exponent characterizing the spectrum
of the bosonic bath--shows that competition between the two couplings can drive
the local-moment fluctuations critical. As a result, two distinct types of
quantum critical point emerge in the Kondo lattice, one being of the usual
spin-density-wave type, the other ``locally critical.'' Near the locally
critical point, the dynamical spin susceptibility exhibits scaling
with a fractional exponent. While the spin-density-wave critical point is
Gaussian, the locally critical point is an interacting fixed point at which
long-wavelength and spatially local critical modes coexist. A Ginzburg-Landau
description for the locally critical point is discussed. It is argued that
these results are robust, that local criticality provides a natural description
of the quantum critical behavior seen in a number of heavy-fermion metals, and
that this picture may also be relevant to other strongly correlated metals.Comment: 20 pages, 12 figures; typos in figure 3 and in the main text
corrected, version as publishe
X-boson cumulant approach to the periodic Anderson model
The Periodic Anderson Model (PAM) can be studied in the infinite U limit by
employing the Hubbard X operators to project out the unwanted states. We have
already studied this problem employing the cumulant expansion with the
hybridization as perturbation, but the probability conservation of the local
states (completeness) is not usually satisfied when partial expansions like the
Chain Approximation (CHA) are employed. Here we treat the problem by a
technique inspired in the mean field approximation of Coleman's slave-bosons
method, and we obtain a description that avoids the unwanted phase transition
that appears in the mean-field slave-boson method both when the chemical
potential is greater than the localized level Ef at low temperatures (T) and
for all parameters at intermediate T.Comment: Submited to Physical Review B 14 pages, 17 eps figures inserted in
the tex
- …