814 research outputs found
Bounding and approximating parabolas for the spectrum of Heisenberg spin systems
We prove that for a wide class of quantum spin systems with isotropic
Heisenberg coupling the energy eigenvalues which belong to a total spin quantum
number S have upper and lower bounds depending at most quadratically on S. The
only assumption adopted is that the mean coupling strength of any spin w.r.t.
its neighbours is constant for all N spins. The coefficients of the bounding
parabolas are given in terms of special eigenvalues of the N times N coupling
matrix which are usually easily evaluated. In addition we show that the
bounding parabolas, if properly shifted, provide very good approximations of
the true boundaries of the spectrum. We present numerical examples of
frustrated rings, a cube, and an icosahedron.Comment: 8 pages, 3 figures. Submitted to Europhysics Letter
Magnetic ground state and 2D behavior in pseudo-Kagome layered system Cu3Bi(SeO3)2O2Br
Anisotropic magnetic properties of a layered kagome-like system
Cu3Bi(SeO3)2O2Br have been studied by bulk magnetization and magnetic
susceptibility measurements as well as powder and single-crystal neutron
diffraction. At T_N = 27.4 K the system develops an alternating
antiferromagnetic order of (ab) layers, which individually exhibit canted
ferrimagnetic moment arrangement, resulting from the competing ferro- and
antiferro-magnetic intralayer exchange interactions. A magnetic field B_C ~ 0.8
T applied along the c axis (perpendicular to the layers) triggers a
metamagnetic transition, when every second layer flips, i.e., resulting in a
ferrimagnetic structure. Significantly higher fields are required to rotate the
ferromagnetic component towards the b axis (~7 T) or towards the a axis (~15
T). The estimates of the exchange coupling constants and features indicative of
an XY character of this quasi-2D system are presented.Comment: 7 pages, 6 figures, final versio
Many-spin effects in inelastic neutron scattering and electron paramagnetic resonance of molecular nanomagnets
Many molecular magnetic clusters, such as single-molecule magnets, are
characterized by spin ground states with defined total spin S exhibiting
zero-field-splittings. In this work, the spectroscopic intensities of the
transitions within the ground-state multiplet are analyzed. In particular, the
effects of a mixing with higher-lying spin multiplets, which is produced by
anisotropic interactions and is neglected in the standard single-spin
description, are investigated systematically for the two experimental
techniques of inelastic neutron scattering (INS) and electron paramagnetic
resonance (EPR), with emphasis on the former technique. The spectroscopic
transition intensities are calculated analytically by constructing
corresponding effective spin operators perturbationally up to second order and
consequently using irreducible tensor operator techniques. Three main effects
of spin mixing are observed. Firstly, a pronounced dependence of the INS
intensities on the momentum transfer Q, with a typical oscillatory behavior,
emerges in first order, signaling the many-spin nature of the wave functions in
exchange-coupled clusters. Secondly, as compared to the results of a
first-order calculation, the intensities of the transitions within the spin
multiplet are affected differently by spin mixing. This allows one, thirdly, to
differentiate the higher-order contributions to the cluster magnetic anisotropy
which come from the single-ion ligand-field terms and spin mixing,
respectively. The analytical results are illustrated by means of the three
examples of an antiferromagnetic heteronuclear dimer, the Mn-[3 x 3] grid
molecule, and the single-molecule magnet Mn12.Comment: 18 pages, 3 figures, REVTEX4, to appear in PR
Encapsulation of cations and anions by azacrowns: thermodynamic and structural aspects
Abstract -The ligands in the series [3k]aneNk (k=6-12), when neutral or partially protonated, can form complexes with many cations, and when extensively protonated they can form adducts with polycharged anions. We shall report some general trends in both cation and anion coordination. In particular thermodynamic and structural parameters obtained for cation and anion complexes will be considered in this paper. The analysis of such parameters enables some characteristics in the molecular recognition of both metal cations and anions to be outlined
Exchange-coupling constants, spin density map, and Q dependence of the inelastic neutron scattering intensity in single-molecule magnets
The Q dependence of the inelastic neutron scattering (INS) intensity of
transitions within the ground-state spin multiplet of single-molecule magnets
(SMMs) is considered. For these transitions, the Q dependence is related to the
spin density map in the ground state, which in turn is governed by the
Heisenberg exchange interactions in the cluster. This provides the possibility
to infer the exchange-coupling constants from the Q dependence of the INS
transitions within the spin ground state. The potential of this strategy is
explored for the M = +-10 -> +- 9 transition within the S = 10 multiplet of the
molecule Mn12 as an example. The Q dependence is calculated for powder as well
as single-crystal Mn12 samples for various exchange-coupling situations
discussed in the literature. The results are compared to literature data on a
powder sample of Mn12 and to measurements on an oriented array of about 500
single-crystals of Mn12. The calculated Q dependence exhibits significant
variation with the exchange-coupling constants, in particular for a
single-crystal sample, but the experimental findings did not permit an
unambiguous determination. However, although challenging, suitable experiments
are within the reach of today's instruments.Comment: 11 pages, 6 figures, REVTEX4, to appear in PR
Field dependent anisotropy change in a supramolecular Mn(II)-[3x3] grid
The magnetic anisotropy of a novel Mn(II)-[3x3] grid complex was investigated
by means of high-field torque magnetometry. Torque vs. field curves at low
temperatures demonstrate a ground state with S > 0 and exhibit a torque step
due to a field induced level-crossing at B* \approx 7.5 T, accompanied by an
abrupt change of magnetic anisotropy from easy-axis to hard-axis type. These
observations are discussed in terms of a spin Hamiltonian formalism.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let
Automatic Quantum Error Correction
Criteria are given by which dissipative evolution can transfer populations
and coherences between quantum subspaces, without a loss of coherence. This
results in a form of quantum error correction that is implemented by the joint
evolution of a system and a cold bath. It requires no external intervention
and, in principal, no ancilla. An example of a system that protects a qubit
against spin-flip errors is proposed. It consists of three spin 1/2 magnetic
particles and three modes of a resonator. The qubit is the triple quantum
coherence of the spins, and the photons act as ancilla.Comment: 16 pages 12 fig LaTex uses multicol, graphicx expanded version of
letter submitted to Phys Rev Let
Spin dynamics in the low-dimensional magnet TiOCl
We present detailed ESR investigations on single crystals of the
low-dimensional quantum magnet TiOCl. The anisotropy of the g-factor indicates
a stable orbital configuration below room temperature, and allows to estimate
the energy of the first excited state as 0.3(1) eV ruling out a possible
degeneracy of the orbital ground state. Moreover, we discuss the possible spin
relaxation mechanisms in TiOCl and analyze the angular and temperature
dependence of the linewidth up to 250 K in terms of anisotropic exchange
interactions. Towards higher temperatures an exponential increase of the
linewidth is observed, indicating an additional relaxation mechanism.Comment: 10 pages, 8 figures; accepted for publication in Phys. Rev.
The anti-ageing and whitening potential of a cosmetic serum containing 3-o-ethyl-l-ascorbic acid
Skin ageing has many manifestations such as wrinkles, dryness, hyperpigmentation, and uneven skin tone. Extrinsic and intrinsic factors, especially solar ultraviolet light (UVB), contribute to skin ageing; its main features are brown spots, alterations in melanin pigmentation, and a decrease in collagen and hyaluronic acid linked to oxidative stress. Several studies showed that topical products containing ingredients with antioxidant activity can reduce oxidative damage; to provide a maximum anti-ageing effect to the skin, topical products can combine various ingredients. C-SHOT SERUM contains a combination of two molecules with a proven anti-ageing activity: a high percentage (30%) of a more stable vitamin C derivative, 3-O-ethyl-L-ascorbic acid, and lactic acid (1%). The product showed a high biocompatibility, assessed through an MTT assay on keratinocytes and on Reconstructed Human Epidermis (RHE, SkinEthic); the anti-ageing activity was demonstrated on human dermal fibroblasts and keratinocytes by a statistically significant increase in collagen production and a reduction of a UVB-induced DNA damage marker (\u3b3-H2AX histone), indicating DNA protection. Moreover, a depigmenting activity, shown by a highly significant decrease in melanin content on treated Reconstructed Human Pigmented Epidermis (RHPE), was assessed. According to the data of our study, the tested product contrasts the effect of skin ageing and irregular pigmentation due to the physiological decline of the skin
Biophysical and Biological Tools to Better Characterize the Stability, Safety and Efficacy of a Cosmeceutical for Acne-Prone Skin
(1) Background: Acne is a widespread skin disease, especially among adolescents. Following the COVID-19 pandemic and the use of masks, the problem has been affecting a greater number of people, and the attention of the skin care beauty routine cosmetics has been focused on the \u201cMaskne\u201d, caused by the sebum excretion rate (SER) that stimulates microbial proliferation. (2) Methods: the present study was focused on the rheological characterization and quality assurance of the preservative system of an anti-acne serum. The biological effectiveness (cytotoxicity\u2014skin and eye irritation\u2014antimicrobial, biofilm eradication and anti-inflammatory activity) was evaluated in a monolayer cell line of keratinocytes (HaCaT) and on 3D models (reconstructed human epidermis, RHE and human reconstructed corneal epithelium, HCE). The Cutibacterium acnes, as the most relevant acne-inducing bacterium, is chosen as a pro-inflammatory stimulus and to evaluate the antimicrobial activity of the serum. (3) Results and Conclusions: Rheology allows to simulate serum behavior at rest, extrusion and application, so the serum could be defined as having a solid-like behavior and being pseudoplastic. The preservative system is in compliance with the criteria of the reference standard. Biological effectiveness evaluation shows non-cytotoxic and irritant behavior with a good antimicrobial and anti-inflammatory activity of the formulation, supporting the effectiveness of the serum for acne-prone skin treatment
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