932 research outputs found
PT-symmetry breaking and maximal chirality in a nonuniform PT-symmetric ring
We study the properties of an N-site tight-binding ring with parity and
time-reversal (PT) symmetric, Hermitian, site-dependent tunneling and a pair of
non-Hermitian, PT-symmetric, loss and gain impurities . The
properties of such lattices with open boundary conditions have been intensely
explored over the past two years. We numerically investigate the PT-symmetric
phase in a ring with a position-dependent tunneling function
that, in an open lattice, leads to a
strengthened PT-symmetric phase, and study the evolution of the PT-symmetric
phase from the open chain to a ring. We show that, generally, periodic boundary
conditions weaken the PT-symmetric phase, although for experimentally relevant
lattice sizes , it remains easily accessible. We show that the
chirality, quantified by the (magnitude of the) average transverse momentum of
a wave packet, shows a maximum at the PT-symmetric threshold. Our results show
that although the wavepacket intensity increases monotonically across the
PT-breaking threshold, the average momentum decays monotonically on both sides
of the threshold.Comment: 11 pages, 5 figures, preprin
Radical cation salts of TTF donors with XF6 (X = Re,Ta) anions
Electrocrystallization of the dianionic Re(IV)F6 species with different organic Ï electron donors was carried out. Depending on the first oxidation potential of the TTF derivatives this crystallization technique gave rise to various radical cations salts involving Re(IV) or Re(V) anions. With tetramethyltetrathiafulvalene (TMTTF) the (TMTTF)2Re(IV)F6 Â salt was obtained. However, with tetramethyltetraselenafulvalene (TMTSF) we obtained very small needles of the (TMTSeF)2Re(V)F6 system, analogue to the so called âBechgaard saltsâ [1].
We present some characteristics of this new metallic phase. We compare to the diamagnetic (TMTSeF)2Ta(V)F6 obtained by the same strategy using TBATaF6 prepared according to the Browsteinâs method [2] and explore electrocrystallization of the tantalum based anions [3], [4].
References:
[1] K. Bechgaard, C.S. Jacobsen, K. Mortensen, H.J. Pedersen, N. Thorup, Solid State Commun. 1980, 33, 1119-1125.
[2] S. Brownstein, Inorg. Chem. 1973, 12, N°3, 584-589.
[3] C. Lenoir, K. Boubekeur, P. Batail, E. Canadell, P. Auban, O. Traetteberg, D. JĂ©rome, Synth. Met. 1991, 42, 1939-1942.
[4] F. Iwase, K. Sugiura, K. Furukawa, T. Nakamura, J. Phys. Soc. Jpn. 2009, 78, N°10, 104717:1-104717:7
An exactly solvable quantum-lattice model with a tunable degree of nonlocality
An array of N subsequent Laguerre polynomials is interpreted as an
eigenvector of a non-Hermitian tridiagonal Hamiltonian with real spectrum
or, better said, of an exactly solvable N-site-lattice cryptohermitian
Hamiltonian whose spectrum is known as equal to the set of zeros of the N-th
Laguerre polynomial. The two key problems (viz., the one of the ambiguity and
the one of the closed-form construction of all of the eligible inner products
which make Hermitian in the respective {\em ad hoc} Hilbert spaces) are
discussed. Then, for illustration, the first four simplest, parametric
definitions of inner products with and are explicitly
displayed. In mathematical terms these alternative inner products may be
perceived as alternative Hermitian conjugations of the initial N-plet of
Laguerre polynomials. In physical terms the parameter may be interpreted as
a measure of the "smearing of the lattice coordinates" in the model.Comment: 35 p
Terminal Ligand and Packing Effects on Slow Relaxation in an Isostructural Set of [Dy(Hdapp)X]+ Single Molecule Magnets**
Three new dysprosium complexes with a pentadentate ligand occupying five equatorial sites differ only in the nature of the axial ligands. These help tune the relaxation properties as judged by an analysis of the AC susceptibility data. More in depth analysis by using two recently suggested fitting equations lead to similar outcomes for all three systems. As a further contribution to the relaxation pathway involving the phonon bath it is concluded that a short nitrate-nitrate interaction between molecules helps dampen the spin phonon coupling.
We report three structurally related single ion Dy compounds using the pentadentate ligand 2,6-bis((E)-1-(2-(pyridin-2-yl)-hydrazineylidene)ethyl)pyridine (Hdapp) [Dy(Hdapp)(NO)]NO (1), [Dy(Hdapp)(OAc)]Cl (2) and [Dy(Hdapp)(NO)]Cl(NO) (3). The (Hdapp) occupies a helical twisted pentagonal equatorial arrangement with two anionic ligands in the axial positions. Further influence on the electronic and magnetic structure is provided by a closely associated counterion interacting with the central NâH group of the (Hdapp). The slow relaxation of the magnetisation shows that the anionic acetates give the greatest slowing down of the magnetisation reversal. Further influence on the relaxation properties of compounds1 and 2 is the presence of short nitrate-nitrate intermolecular ligand contact opening further lattice relaxation pathways
Electrical manipulation of spin states in a single electrostatically gated transition-metal complex
We demonstrate an electrically controlled high-spin (S=5/2) to low-spin
(S=1/2) transition in a three-terminal device incorporating a single Mn2+ ion
coordinated by two terpyridine ligands. By adjusting the gate-voltage we reduce
the terpyridine moiety and thereby strengthen the ligand-field on the Mn-atom.
Adding a single electron thus stabilizes the low-spin configuration and the
corresponding sequential tunnelling current is suppressed by spin-blockade.
From low-temperature inelastic cotunneling spectroscopy, we infer the
magnetic excitation spectrum of the molecule and uncover also a strongly
gate-dependent singlet-triplet splitting on the low-spin side. The measured
bias-spectroscopy is shown to be consistent with an exact diagonalization of
the Mn-complex, and an interpretation of the data is given in terms of a
simplified effective model.Comment: Will appear soon in Nanoletter
Model parameterization to simulate and compare the PAR absorption potential of two competing plant species
Mountain pastures dominated by the pasture grass Setaria sphacelata in the Andes of southern Ecuador are heavily infested by southern bracken (Pteridium arachnoideum), a major problem for pasture management. Field observations suggest that bracken might outcompete the grass due to its competitive strength with regard to the absorption of photosynthetically active radiation (PAR). To understand the PAR absorption potential of both species, the aims of the current paper are to (1) parameterize a radiation scheme of a two-big-leaf model by deriving structural (LAI, leaf angle parameter) and optical (leaf albedo, transmittance) plant traits for average individuals from field surveys, (2) to initialize the properly parameterized radiation scheme with realistic global irradiation conditions of the Rio San Francisco Valley in the Andes of southern Ecuador, and (3) to compare the PAR absorption capabilities of both species under typical local weather conditions. Field data show that bracken reveals a slightly higher average leaf area index (LAI) and more horizontally oriented leaves in comparison to Setaria. Spectrometer measurements reveal that bracken and Setaria are characterized by a similar average leaf absorptance. Simulations with the average diurnal course of incoming solar radiation (1998â2005) and the mean leafâsun geometry reveal that PAR absorption is fairly equal for both species. However, the comparison of typical clear and overcast days show that two parameters, (1) the relation of incoming diffuse and direct irradiance, and (2) the leafâsun geometry play a major role for PAR absorption in the two-big-leaf approach: Under cloudy sky conditions (mainly diffuse irradiance), PAR absorption is slightly higher for Setaria while under clear sky conditions (mainly direct irradiance), the average bracken individual is characterized by a higher PAR absorption potential. (âŒ74 MJ mâ2 yearâ1). The latter situation which occurs if the maximum daily irradiance exceeds 615 W mâ2 is mainly due to the nearly orthogonal incidence of the direct solar beam onto the horizontally oriented frond area which implies a high amount of direct PAR absorption during the noon maximum of direct irradiance. Such situations of solar irradiance favoring a higher PAR absorptance of bracken occur in âŒ36% of the observation period (1998â2005). By considering the annual course of PAR irradiance in the San Francisco Valley, the clear advantage of bracken on clear days (36% of all days) is completely compensated by the slight but more frequent advantage of Setaria under overcast conditions (64% of all days). This means that neither bracken nor Setaria show a distinct advantage in PAR absorption capability under the current climatic conditions of the study area
Ferromagnetism in Co7(TeO3)4Br6: A byproduct of complex antiferromagnetic order and single-ion anisotropy
Pronounced anisotropy of magnetic properties and complex magnetic order of a
new oxi-halide compound Co7(TeO3)4Br6 has been investigated by powder and
single crystal neutron diffraction, magnetization and ac susceptibility
techniques. Anisotropy of susceptibility extends far into the paramagnetic
temperature range. A principal source of anisotropy are anisotropic properties
of the involved octahedrally coordinated single Co(2+) ions, as confirmed by
angular-overlap-model calculations presented in this work. Incommensurate
antiferromagnetic order sets in at TN=34 K. Propagation vector is strongly
temperature dependent reaching k1=(0.9458(6), 0, 0.6026(5)) at 30 K. A
transition to a ferrimagnetic structure with k2=0 takes place at TC=27 K.
Magnetically ordered phase is characterized by very unusual anisotropy as well:
while M-H scans along b-axis reveals spectacularly rectangular but otherwise
standard ferromagnetic hysteresis loops, M-H studies along other two principal
axes are perfectly reversible, revealing very sharp spin flop (or spin flip)
transitions, like in a standard antiferromagnet (or metamagnet). Altogether,
the observed magnetic phenomenology is interpreted as an evidence of competing
magnetic interactions permeating the system, first of all of the single ion
anisotropy energy and the exchange interactions. Different coordinations of the
Co(2+)-ions involved in the low-symmetry C2/c structure of Co7(TeO3)4Br6 render
the exchange-interaction network very complex by itself. Temperature dependent
changes in the magnetic structure, together with an abrupt emergence of a
ferromagnetic component, are ascribed to continual spin reorientations
described by a multi-component, but yet unknown, spin Hamiltonian.Comment: 12 pages, 13 figures; submitted to PR
Do MRI findings identify patients with chronic low back pain and Modic changes who respond best to rest or exercise: A subgroup analysis of a randomised controlled trial
Background: No previous clinical trials have investigated MRI findings as effect modifiers for conservative treatment of low back pain. This hypothesis-setting study investigated if MRI findings modified response to rest compared with exercise in patients with chronic low back pain and Modic changes. Methods: This study is a secondary analysis of a randomised controlled trial comparing rest with exercise. Patients were recruited from a specialised outpatient spine clinic and included in a clinical trial if they had chronic low back pain and an MRI showing Modic changes. All patients received conservative treatment while participating in the trial. Five baseline MRI findings were investigated as effect modifiers: Modic changes Type 1 (any size), large Modic changes (any type), large Modic changes Type 1, severe disc degeneration and large disc herniation. The outcome measure was change in low back pain intensity measured on a 0-10 point numerical rating scale at 14-month follow-up (n = 96). An interaction = 1.0 point (0-10 scale) between treatment group and MRI findings in linear regression was considered clinically important. Results: The interactions for Modic Type 1, with large Modic changes or with large Modic changes Type 1 were all potentially important in size (-0.99 (95% CI -3.28 to 1.29), -1.49 (-3.73 to 0.75), -1.49 (-3.57 to 0.58), respectively) but the direction of the effect was the opposite to what we had hypothesized-that people with these findings would benefit more from rest than from exercise. The interactions for severe disc degeneration (0.74 (-1.40 to 2.88)) and large disc herniation (-0.92 (3.15 to 1.31)) were less than the 1.0-point threshold for clinical importance. As expected, because of the lack of statistical power, no interaction term for any of the MRI findings was statistically significant. Conclusions: Three of the five MRI predictors showed potentially important effect modification, although the direction of the effect was surprising and confidence intervals were wide so very cautious interpretation is required. Further studies with adequate power are warranted to study these and additional MRI findings as potential effect modifiers for common interventions
Mixing and Circulation in the Tropical Atlantic Cruise No. M98
July 01 â July 28, 2013
Fortaleza (Brazil) â Walvis Bay (Namibia
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