293 research outputs found
Finite size effects and magnetic order in the spin-1/2 honeycomb lattice compound InCu{2/3}V{1/3}O{3}
High field electron spin resonance, nuclear magnetic resonance and
magnetization studies addressing the ground state of the quasi two-dimensional
spin-1/2 honeycomb lattice compound InCu{2/3}V{1/3}O{3} are reported.
Uncorrelated finite size structural domains occurring in the honeycomb planes
are expected to inhibit long range magnetic order. Surprisingly, ESR data
reveal the development of two collinear antiferromagnetic (AFM) sublattices
below ~ 20 K whereas NMR results show the presence of the staggered internal
field. Magnetization data evidence a spin reorientation transition at ~ 5.7 T.
Quantum Monte-Carlo calculations show that switching on the coupling between
the honeycomb spin planes in a finite size cluster yields a Neel-like AFM spin
structure with a substantial staggered magnetization at finite temperatures.
This may explain the occurrence of a robust AFM state in InCu{2/3}V{1/3}O{3}
despite an unfavorable effect of structural disorder.Comment: revised version, accepted as a Rapid Communication in Phys. Rev. B
(2010
Magnetic Properties of the low dimensional spin system (VO)PO: ESR and susceptibility
Experimental results on magnetic resonance (ESR) and magnetic susceptibility
are given for single crystalline (VO)PO. The crystal growth
procedure is briefly discussed. The susceptibility is interpreted numerically
using a model with alternating spin chains. We determine =51 K and
=0.2. Furthermore we find a spin gap of meV from our ESR
measurements. Using elastic constants no indication of a phase transition
forcing the dimerization is seen below 300 K.Comment: 7 pages, REVTEX, 7 figure
Gapped Heisenberg spin chains in a field
We consider the fully anisotropic Heisenberg spin-1/2 antiferromagnet in a
uniform magnetic field, whose ground-state is characterized by broken spin
rotation symmetry and gapped spinon excitations. We expand on a recent
mean-field approach to the problem by incorporating fluctuations in a loop
expansion. Quantitative results for the magnetization, excitation gap and
specific heat are obtained. We compare our predictions with new DMRG and exact
diagonalization data and, for zero field, with the exact solution of the
spin chain from the Bethe Ansatz.Comment: 11 pages, 14 figure
Rydberg trimers and excited dimers bound by internal quantum reflection
Quantum reflection is a pure wave phenomena that predicts reflection of a
particle at a changing potential for cases where complete transmission occurs
classically. For a chemical bond, we find that this effect can lead to
non-classical vibrational turning points and bound states at extremely large
interatomic distances. Only recently has the existence of such ultralong-range
Rydberg molecules been demonstrated experimentally. Here, we identify a broad
range of molecular lines, most of which are shown to originate from two
different novel sources: a single-photon associated triatomic molecule formed
by a Rydberg atom and two ground state atoms and a series of excited dimer
states that are bound by a so far unexplored mechanism based on internal
quantum reflection at a steep potential drop. The properties of the Rydberg
molecules identified in this work qualify them as prototypes for a new type of
chemistry at ultracold temperatures.Comment: 6 pages, 3 figures, 1 tabl
Molecular evolution of the proopiomelanocortin system in Barn owl species.
Examination of genetic polymorphisms in outbred wild-living species provides insights into the evolution of complex systems. In higher vertebrates, the proopiomelanocortin (POMC) precursor gives rise to α-, β-, and γ-melanocyte-stimulating hormones (MSH), which are involved in numerous physiological aspects. Genetic defects in POMC are linked to metabolic disorders in humans and animals. In the present study, we undertook an evolutionary genetic approach complemented with biochemistry to investigate the functional consequences of genetic polymorphisms in the POMC system of free-living outbred barn owl species (family Tytonidae) at the molecular level. Our phylogenetic studies revealed a striking correlation between a loss-of-function H9P mutation in the β-MSH receptor-binding motif and an extension of a poly-serine stretch in γ3-MSH to ≥7 residues that arose in the barn owl group 6-8 MYA ago. We found that extension of the poly-serine stretches in the γ-MSH locus affects POMC precursor processing, increasing γ3-MSH production at the expense of γ2-MSH and resulting in an overall reduction of γ-MSH signaling, which may be part of a negative feedback mechanism. Extension of the γ3-MSH poly-serine stretches ≥7 further markedly increases peptide hormone stability in plasma, which is conserved in humans, and is likely relevant to its endocrine function. In sum, our phylogenetic analysis of POMC in wild living owls uncovered a H9P β-MSH mutation subsequent to serine extension in γ3-MSH to 7 residues, which was then followed by further serine extension. The linked MSH mutations highlight the genetic plasticity enabled by the modular design of the POMC gene
One-Dimensional Electron Liquid in an Antiferromagnetic Environment: Spin Gap from Magnetic Correlations
We study a one-dimensional electron liquid coupled by a weak spin-exchange
interaction to an antiferromagnetic spin-S ladder with n legs. A perturbative
renormalization group analysis in the semiclassical limit reveals the opening
of a spin gap, driven by the local magnetic correlations on the ladder. The
effect, which we argue is present for any gapful ladder or gapless ladder with
, is enhanced by the repulsive interaction among the conduction
electrons but is insensitive to the sign of the spin exchange interaction with
the ladder. Possible implications for the striped phases of the cuprates are
discussed.Comment: 5 pages, 1 figure, to appear in Phys. Rev. Let
Avoided Critical Behavior in O(n) Systems
Long-range frustrating interactions, even if their strength is infinitesimal,
can give rise to a dramatic proliferations of ground or near-ground states. As
a consequence, the ordering temperature can exhibit a discontinuous drop as a
function of the frustration. A simple model of the doped Mott insulator, where
the short-range tendency of the holes to phase separate competes with
long-range Coulomb effects, exhibits this "avoided critical" behavior. This
model may serve as a paradigm for many other systems.Comment: 4 pages, 2 figure
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