536 research outputs found
Anomalous NMR Spin-Lattice Relaxation in SrB_{6} and Ca_{1-x}La_{x}B_{6}
We report the results of {11}B nuclear magnetic resonance (NMR) measurements
of SrB_{6} and Ca_{0.995}La_{0.05}B_{6} below room temperature. Although the
electrical resistivities of these two materials differ substantially, their
{11}B-NMR responses exhibit some strikingly common features. Both materials
exhibit ferromagnetic order, but their {11}B-NMR spectra reveal very small
hyperfine fields at the Boron sites. The spin lattice relaxation T_{1}^{-1}
varies considerably with external field but changes with temperature only below
a few K. We discuss these unusual results by considering various different
scenarios for the electronic structure of these materials.Comment: Accepted for publication in Phys. Rev. B Rapid communication, 4
pages, 3 figures. This manuscript replaces an earlier version and includes
some minor changes in the text and in Fig.
Fermi Surface Measurements on the Low Carrier Density Ferromagnet Ca1-xLaxB6 and SrB6
Recently it has been discovered that weak ferromagnetism of a dilute 3D
electron gas develops on the energy scale of the Fermi temperature in some of
the hexaborides; that is, the Curie temperature approximately equals the Fermi
temperature. We report the results of de Haas-van Alphen experiments on two
concentrations of La-doped CaB6 as well as Ca-deficient Ca1-dB6 and
Sr-deficient Sr1-dB6. The results show that a Fermi surface exists in each case
and that there are significant electron-electron interactions in the low
density electron gas.Comment: 4 pages, 5 figures, submitted to PR
Theory of High \tc Ferromagnetism in family: A case of Doped Spin-1 Mott insulator in a Valence Bond Solid Phase
Doped divalent hexaborides such as exhibit high \tc
ferromagnetism. We isolate a degenerate pair of -orbitals of boron with two
valence electrons, invoke electron correlation and Hund coupling, to suggest
that the undoped state is better viewed as a spin-1 Mott insulator; it is
predicted to be a type of 3d Haldane gap phase with a spin gap ,
much smaller than the charge gap of seen in ARPES. The
experimentally seen high \tc `ferromagnetism' is argued to be a complex
magnetic order in disguise - either a canted 6-sublattice AFM ()
order or its quantum melted version, a chiral spin liquid state, arising from a
type of double exchange mechanism.Comment: 4 pages, 2 figures; minor corrections, references adde
Charge dynamics and "ferromagnetism" of A1-xLaxB6 (A=Ca and Sr)
Ferromagnetism has been reported recently in La-doped alkaline-earth
hexaborides, A1-xLaxB6 (A=Ca, Sr, and Ba). We have performed the reflectivity,
Hall resistivity, and magnetization measurements of A1-xLaxB6. The results
indicate that A1-xLaxB6 can be regarded as a simple doped semimetal, with no
signature of an excitonic state as suggested by several theories. It is also
found that the surface of as-grown samples (10 micrometer in thickness) has a
different electronic structure from a bulk one, and a fairly large number of
paramagnetic moments are confined in this region. After eliminating these
paramagnetic moments at the surface, we could not find any evidence of an
intrinsic ferromagnetic moment in our samples, implying the possibility that
the ferromagnetism of A1-xLaxB6 reported so far is neither intrinsic.Comment: 7 pages, 8 figure
Time--delay autosynchronization of the spatio-temporal dynamics in resonant tunneling diodes
The double barrier resonant tunneling diode exhibits complex spatio-temporal
patterns including low-dimensional chaos when operated in an active external
circuit. We demonstrate how autosynchronization by time--delayed feedback
control can be used to select and stabilize specific current density patterns
in a noninvasive way. We compare the efficiency of different control schemes
involving feedback in either local spatial or global degrees of freedom. The
numerically obtained Floquet exponents are explained by analytical results from
linear stability analysis.Comment: 10 pages, 16 figure
Application of direct bioautography and SPME-GC-MS for the study of antibacterial chamomile ingredients
The isolation and characterization of antibacterial chamomile components were performed by the use of direct bioautography and solid phase microextraction (SPME)-GC-MS. Four ingredients, active against Vibrio fischeri, were identified as the polyacetylene geometric isomers cis- and trans-spiroethers, the coumarin related herniarin, and the sesquiterpene alcohol (-)-alpha-bisabolol
Ferromagnetism in the Two-Dimensional Periodic Anderson Model
Using the constrained-path Monte Carlo method, we studied the magnetic
properties of the two-dimensional periodic Anderson model for electron fillings
between 1/4 and 1/2. We also derived two effective low energy theories to
assist in interpreting the numerical results. For 1/4 filling we found that the
system can be a Mott or a charge transfer insulator, depending on the relative
values of the Coulomb interaction and the charge transfer gap between the two
non-interacting bands. The insulator may be a paramagnet or antiferromagnet. We
concentrated on the effect of electron doping on these insulating phases. Upon
doping we obtained a partially saturated ferromagnetic phase for low
concentrations of conduction electrons. If the system were a charge transfer
insulator, we would find that the ferromagnetism is induced by the well-known
RKKY interaction. However, we found a novel correlated hopping mechanism
inducing the ferromagnetism in the region where the non-doped system is a Mott
insulator. Our regions of ferromagnetism spanned a much smaller doping range
than suggested by recent slave boson and dynamical mean field theory
calculations, but they were consistent with that obtained by density matrix
renormalization group calculations of the one-dimensional periodic Anderson
model
Sorbents with immobilized glycopeptide antibiotics for separating optical isomers by high-performance liquid chromatography
Îą-intercalated cells defend the urinary system from bacterial infection
{alpha}âIntercalated cells (A-ICs) within the collecting duct of the kidney are critical for acid-base homeostasis. Here, we have shown that A-ICs also serve as both sentinels and effectors in the defense against urinary infections. In a murine urinary tract infection model, A-ICs bound uropathogenic E. coli and responded by acidifying the urine and secreting the bacteriostatic protein lipocalin 2 (LCN2; also known as NGAL). A-ICâdependent LCN2 secretion required TLR4, as mice expressing an LPS-insensitive form of TLR4 expressed reduced levels of LCN2. The presence of LCN2 in urine was both necessary and sufficient to control the urinary tract infection through iron sequestration, even in the harsh condition of urine acidification. In mice lacking A-ICs, both urinary LCN2 and urinary acidification were reduced, and consequently bacterial clearance was limited. Together these results indicate that A-ICs, which are known to regulate acid-base metabolism, are also critical for urinary defense against pathogenic bacteria. They respond to both cystitis and pyelonephritis by delivering bacteriostatic chemical agents to the lower urinary system
Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV
Results are presented from a search for a W' boson using a dataset
corresponding to 5.0 inverse femtobarns of integrated luminosity collected
during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV.
The W' boson is modeled as a heavy W boson, but different scenarios for the
couplings to fermions are considered, involving both left-handed and
right-handed chiral projections of the fermions, as well as an arbitrary
mixture of the two. The search is performed in the decay channel W' to t b,
leading to a final state signature with a single lepton (e, mu), missing
transverse energy, and jets, at least one of which is tagged as a b-jet. A W'
boson that couples to fermions with the same coupling constant as the W, but to
the right-handed rather than left-handed chiral projections, is excluded for
masses below 1.85 TeV at the 95% confidence level. For the first time using LHC
data, constraints on the W' gauge coupling for a set of left- and right-handed
coupling combinations have been placed. These results represent a significant
improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe
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