144 research outputs found
Square-lattice magnetism of diaboleite Pb2Cu(OH)4Cl2
We report on the quasi-two-dimensional magnetism of the natural mineral
diaboleite Pb2Cu(OH)4Cl2 with a tetragonal crystal structure, which is closely
related to that of the frustrated spin-1/2 magnet PbVO3. Magnetic
susceptibility of diaboleite is well described by a Heisenberg spin model on a
diluted square lattice with the nearest-neighbor exchange of J~35 K and about
5% of non-magnetic impurities. The dilution of the spin lattice reflects the
formation of Cu vacancies that are tolerated by the crystal structure of
diaboleite. The weak coupling between the magnetic planes triggers the
long-range antiferromagnetic order below TN~11 K. No evidence of magnetic
frustration is found. We also analyze the signatures of the long-range order in
heat-capacity data, and discuss the capability of identifying magnetic
transitions with heat-capacity measurements.Comment: 10 pages, 10 figures + Supplementary Informatio
The spin gap in malachite Cu2(OH)2CO3 and its evolution under pressure
We report on the microscopic magnetic modeling of the spin-1/2 copper mineral
malachite at ambient and elevated pressures. Despite the layered crystal
structure of this mineral, the ambient-pressure susceptibility and
magnetization data can be well described by an unfrustrated
quasi-one-dimensional magnetic model. Weakly interacting antiferromagnetic
alternating spin chains are responsible for a large spin gap of 120K. Although
the intradimer Cu-O-Cu bridging angles are considerably smaller than the
interdimer angles, density functional theory (DFT) calculations revealed that
the largest exchange coupling of 190K operates within the structural dimers.
The lack of the inversion symmetry in the exchange pathways gives rise to
sizable Dzyaloshinskii-Moriya interactions which were estimated by
full-relativistic DFT+U calculations. Based on available high-pressure crystal
structures, we investigate the exchange couplings under pressure and make
predictions for the evolution of the spin gap. The calculations evidence that
intradimer couplings are strongly pressure-dependent and their evolution
underlies the decrease of the spin gap under pressure. Finally, we assess the
accuracy of hydrogen positions determined by structural relaxation within DFT
and put forward this computational method as a viable alternative to elaborate
experiments
Frustrated spin chain physics near the Majumdar-Ghosh point in szenicsite Cu(MoO)(OH)
In this joint experimental and theoretical work magnetic properties of the
Cu mineral szenicsite Cu(MoO)(OH) are investigated. This
compound features isolated triple chains in its crystal structure, where the
central chain involves an edge-sharing geometry of the CuO plaquettes,
while the two side chains feature a corner-sharing zig-zag geometry. The
magnetism of the side chains can be described in terms of antiferromagnetic
dimers with a coupling larger than 200 K. The central chain was found to be a
realization of the frustrated antiferromagnetic - chain model with
K and a sizable second-neighbor coupling . The central and
side chains are nearly decoupled owing to interchain frustration. Therefore,
the low-temperature behavior of szenicsite should be entirely determined by the
physics of the central frustrated - chain. Our heat-capacity
measurements reveal an accumulation of entropy at low temperatures and suggest
a proximity of the system to the Majumdar-Ghosh point of the antiferromagnetic
- spin chain,
Two energy scales of spin dimers in clinoclase Cu3(AsO4)(OH)3
Magnetic susceptibility and microscopic magnetic model of the mineral
clinoclase Cu3(AsO4)(OH)3 are reported. This material can be well described as
a combination of two nonequivalent spin dimers with the sizable magnetic
couplings of J about 700 K and J(D2) about 300 K. Based on density functional
theory calculations, we pinpoint the location of dimers in the crystal
structure. Surprisingly, the largest coupling operates between the structural
Cu2O6 dimers. We investigate magnetostructural correlations in Cu2O6 structural
dimers, by considering the influence of the hydrogen position on the magnetic
coupling. Additionally, we establish the hydrogen positions that were not known
so far and analyze the pattern of hydrogen bonding
Solitonic approach to the dimerization problem in correlated one-dimensional systems
Using exact diagonalizations we consider self-consistently the lattice
distortions in odd Peierls-Hubbard and spin-Peierls periodic rings in the
adiabatic harmonic approximation. From the tails of the inherent spin soliton
the dimerization d_\infty of regular even rings is found by extrapolations to
infinite ring lengths. Considering a wide region of electron-electron onsite
interaction values U>0 compared with the band width 4t_0 at intermediately
strong electron-phonon interaction g, known relationships obtained by other
methods are reproduced and/or refined within one unified approach: such as the
maximum of d_\infty at U \simeq 3 t_0 for g \simeq 0.5 and its shift to zero
for g \to g_c \approx 0.7. The hyperbolic tangent shape of the spin soliton is
retained for any U and g <~ 0.6. In the spin-Peierls limit the d_\infty are
found to be in agreement with results of DMRG computations.Comment: 4 pages, 4 figures, Physical Review B, Rapid Communications, v. 56
(1997) accepte
Frustrated spin chain physics near the Majumdar-Ghosh point in szenicsite Cu3(MoO4)(OH)4
© 2017 American Physical Society. In this joint experimental and theoretical work magnetic properties of the Cu2+ mineral szenicsite Cu3(MoO4)(OH)4 are investigated. This compound features isolated triple chains in its crystal structure, where the central chain involves an edge-sharing geometry of the CuO4 plaquettes, while the two side chains feature a corner-sharing zigzag geometry. The magnetism of the side chains can be described in terms of antiferromagnetic dimers with a coupling larger than 200 K. The central chain was found to be a realization of the frustrated antiferromagnetic J1-J2 chain model with J1≃68 K and a sizable second-neighbor coupling J2. The central and side chains are nearly decoupled owing to interchain frustration. Therefore, the low-temperature behavior of szenicsite should be entirely determined by the physics of the central frustrated J1-J2 chain. Our heat-capacity measurements reveal an accumulation of magnetic entropy at low temperatures and suggest a proximity of the system to the Majumdar-Ghosh point of the antiferromagnetic J1-J2 spin chain, J2/J1=0.5
The strength of frustration and quantum fluctuations in LiVCuO4
For the 1D-frustrated ferromagnetic J_1-J_2 model with interchain coupling
added, we analyze the dynamical and static structure factor S(k,omega), the
pitch angle phi of the magnetic structure, the magnetization curve of
edge-shared chain cuprates, and focus on LiCuVO4 for which neither a perturbed
spinon nor a spin wave approach can be applied. phi is found to be most
sensitive to the interplay of frustration and quantum fluctuations. For LiVCuO4
the obtained exchange parameters J are in accord with the results for a
realistic 5-band extended Hubbard model and LSDA + U predictions yielding
alpha=J_2/|J_1| about 0.75 in contrast to 5.5 > alpha > 1.42 suggested in the
literature. The alpha-regime of the empirical phi-values in NaCu2O2 and
linarite are considered, too.Comment: 7 pages, 7 figures, (1 figure added), improved text including also
the abstract (the present second version has been submitted to EPL
26.10.2011, so far with one missing first referee report
Heat and Charge Transport Properties of MgB2
A polycrystalline sample of the MgB_2 superconductor was investigated by
measurements of the electrical resistivity, the thermopower and the thermal
conductivity in the temperature range between 1.8K and 300K in zero magnetic
field. The electrical resistivity shows a superconducting transition at
T_c=38.7K and, similarly to borocarbides, a T^2.4 behaviour up to 200K. The
electron diffusion thermopower and its bandstructure-derived value indicate the
dominant hole character of the charge carriers. The total thermopower can be
explained by the diffusion term renormalized by a significant electron-phonon
interaction and a phonon drag term. In the thermal conductivity, for decreasing
temperature, a significant decrease below T_c is observed resulting in a T^3
behaviour below 7K. The reduced Lorenz number exhibits values smaller than 1
and a characteristic minimum which resembles the behaviour of non-magnetic
borocarbides.Comment: 7 pages, 5 figures; added references and minor changes; accepted for
publication in Physica
Raf kinase inhibitory protein suppresses a metastasis signalling cascade involving LIN28 and let-7
Raf kinase inhibitory protein (RKIP) negatively regulates the MAP kinase (MAPK), G protein-coupled receptor kinase-2, and NF-κB signalling cascades. RKIP has been implicated as a metastasis suppressor for prostate cancer, but the mechanism is not known. Here, we show that RKIP inhibits invasion by metastatic breast cancer cells and represses breast tumour cell intravasation and bone metastasis in an orthotopic murine model. The mechanism involves inhibition of MAPK, leading to decreased transcription of LIN28 by Myc. Suppression of LIN28 enables enhanced let-7 processing in breast cancer cells. Elevated let-7 expression inhibits HMGA2, a chromatin remodelling protein that activates pro-invasive and pro-metastatic genes, including Snail. LIN28 depletion and let-7 expression suppress bone metastasis, and LIN28 restores bone metastasis in mice bearing RKIP-expressing breast tumour cells. These results indicate that RKIP suppresses invasion and metastasis in part through a signalling cascade involving MAPK, Myc, LIN28, let-7, and downstream let-7 targets. RKIP regulation of two pluripotent stem cell genes, Myc and LIN28, highlights the importance of RKIP as a key metastasis suppressor and potential therapeutic agent.</p
Raf kinase inhibitory protein suppresses a metastasis signalling cascade involving LIN28 and let-7
Raf kinase inhibitory protein (RKIP) negatively regulates the MAP kinase (MAPK), G protein-coupled receptor kinase-2, and NF-κB signalling cascades. RKIP has been implicated as a metastasis suppressor for prostate cancer, but the mechanism is not known. Here, we show that RKIP inhibits invasion by metastatic breast cancer cells and represses breast tumour cell intravasation and bone metastasis in an orthotopic murine model. The mechanism involves inhibition of MAPK, leading to decreased transcription of LIN28 by Myc. Suppression of LIN28 enables enhanced let-7 processing in breast cancer cells. Elevated let-7 expression inhibits HMGA2, a chromatin remodelling protein that activates pro-invasive and pro-metastatic genes, including Snail. LIN28 depletion and let-7 expression suppress bone metastasis, and LIN28 restores bone metastasis in mice bearing RKIP-expressing breast tumour cells. These results indicate that RKIP suppresses invasion and metastasis in part through a signalling cascade involving MAPK, Myc, LIN28, let-7, and downstream let-7 targets. RKIP regulation of two pluripotent stem cell genes, Myc and LIN28, highlights the importance of RKIP as a key metastasis suppressor and potential therapeutic agent.</p
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