1,242 research outputs found
An Active Rock Glacier, Wavbal Pass, Jammu and Kashmir Himalaya, India
A currently active rock glacier with a snout elevation of 4,055 m was observed in the Jammu and Kashmir Himalaya. Its formation is ascribed to stagnation of the terminal area of a true glacier
Effects of Lifshitz Transition on Charge Transport in Magnetic Phases of Fe-Based Superconductors
The unusual temperature dependence of the resistivity and its in-plane
anisotropy observed in the Fe-based superconducting materials, particularly
Ba(FeCo)As, has been a longstanding puzzle. Here we
consider the effect of impurity scattering on the temperature dependence of the
average resistivity within a simple two-band model of a dirty spin density wave
metal. The sharp drop in resistivity below the N\'eel temperature in the
parent compound can only be understood in terms of a Lifshitz transition
following Fermi surface reconstruction upon magnetic ordering. We show that the
observed resistivity anisotropy in this phase, arising from nematic defect
structures, is affected by the Lifshitz transition as well.Comment: 8 pages, 5 figure
CrRhAs: a member of a large family of metallic kagome antiferromagnets
Kagome lattice materials are an important platform for highly frustrated
magnetism as well as for a plethora of phenomena resulting from flat bands,
Dirac cones and van Hove singularities in their electronic structures. We study
the little known metallic magnet CrRhAs, which belongs to a vast family of
materials that include , and magnetic elements, as well as
numerous nonmagnetic metals and insulators. Using noncollinear spin density
functional calculations (mostly spin spirals), we extract a model magnetic
Hamiltonian for CrRhAs. While it is dominated by an antiferromagnetic second
nearest neighbor coupling in the kagome plane, the metallic nature of the
compound leads to numerous nonzero longer range couplings and to important ring
exchange terms. We analyze this Hamiltonian and find unusual ground states
which are dominated by nearly isolated antiferromagnetic triangles that adopt
120 order either with positive or with negative vector chirality. We
discuss the connection to the few known experimental facts about CrRhAs.
Finally, we give a brief survey of other interesting magnetic members of this
family of kagome compounds.Comment: 16 page
A valence bond liquid on the honeycomb lattice
The honeycomb lattice material Li2RuO3 undergoes a dimerization of Ru4+
cations on cooling below 270C, where the magnetic susceptibility vanishes. We
use density functional theory calculations to show that this reflects the
formation of a 'valence bond crystal', with a strong bond disproportionation.
On warming, x-ray diffraction shows that discrete three-fold symmetry is
regained on average, and the dimerization apparently disappears. In contrast,
local structural measurements using high-energy x-rays, show that disordered
dimers survive at the nanoscale up to at least 650C. The high temperature phase
of Li2RuO3 is thus an example of a valence bond liquid, where thermal
fluctuations drive resonance between different dimer coverages, a classic
analogue of the resonating valence bond state often discussed in connection
with high T cuprates.Comment: 5 pages, 4 figures, References correcte
Cyclotron resonance photoconductivity of a two-dimensional electron gas in HgTe quantum wells
Far-infrared cyclotron resonance photoconductivity (CRP) is investigated in
HgTe quantum wells (QWs) of various widths grown on (013) oriented GaAs
substrates. It is shown that CRP is caused by the heating of two-dimensional
electron gas (2DEG). From the resonance magnetic field strength effective
masses and their dependence on the carrier concentration is obtained. We found
that the effective mass in each sample slightly increases from the value
(0.0260 \pm 0.0005)m_0 at N_s = 2.2x10^11 cm^(-2) to (0.0335 \pm 0.0005)m_0 at
N_s = 9.6x10^11 cm^(-2). Compared to determination of effective masses by the
temperature dependence of magnitudes of the Shubnikov-de Haas (SdH)
oscillations used so far in this material our measurements demonstrate that the
CRP provides a more accurate (about few percents) tool. Combining optical
methods with transport measurements we found that the transport time
substantially exceeds the cyclotron resonance lifetime as well as the quantum
lifetime which is the shortest.Comment: 3 pages, 2 figure
Local moments and symmetry breaking in metallic PrMnSbO
We report a combined experimental and theoretical investigation of the
layered antimonide PrMnSbO which is isostructural to the parent phase of the
iron pnictide superconductors. We find linear resistivity near room temperature
and Fermi liquid-like T^{2} behaviour below 150 K. Neutron powder diffraction
shows that unfrustrated C-type Mn magnetic order develops below \sim 230 K,
followed by a spin-flop coupled to induced Pr order. At T \sim 35 K, we find a
tetragonal to orthorhombic (T-O) transition. First principles calculations show
that the large magnetic moments observed in this metallic compound are of local
origin. Our results are thus inconsistent with either the itinerant or
frustrated models proposed for symmetry breaking in the iron pnictides. We show
that PrMnSbO is instead a rare example of a metal where structural distortions
are driven by f-electron degrees of freedom
Human amniotic fluid glycoproteins expressing sialyl Lewis carbohydrate antigens stimulate progesterone production in human trophoblasts in vitro
Background: Progesterone is thought to mediate immune modulator effects by regulating uterine responsiveness. The aim of the study was to clarify the effect of transferrin and glycodelin A (former name PP14) as sialyl Lewis X-expressing glycoproteins on the release of progesterone by trophoblast cells in vitro. Methods: Cytotrophoblast cells were prepared from human term placentas by standard dispersion of villous tissue followed by a Percoll gradient centrifugation step. Trophoblasts were incubated with varying concentrations (50-300 mug/ml) of human amniotic fluid- and serum-transferrin as well as with glycodelin A. Culture supernatants were assayed for progesterone, human chorionic gonadotropin (hCG) and cortisol by enzyme immunometric methods. Results: The release of progesterone is increased in amniotic fluid transferrin- and glycodelin A-treated trophoblast cell cultures compared to untreated trophoblast cells. There is no relation between transferrin and the hCG or cortisol production of trophoblast cells. Conclusion: The results suggest that sialyl Lewis carbohydrate antigen-expressing amniotic fluid glycoproteins modulate the endocrine function of trophoblasts in culture by upregulating progesterone production. Copyright (C) 2004 S. Karger AG, Basel
Field-induced nematic-like magnetic transition in an iron pnictide superconductor, Ca(PtAs)((FePt)As)
We report a high magnetic field study up to 55 T of the nearly optimally
doped iron-pnictide superconductor Ca(PtAs)
((FePt)As) (x=0.078(6)) with a Tc 10 K using
magnetic torque, tunnel diode oscillator technique and transport measurements.
We determine the superconducting phase diagram, revealing an anisotropy of the
irreversibility field up to a factor of 10 near Tc and signatures of multiband
superconductivity. Unexpectedly, we find a spin-flop like anomaly in magnetic
torque at 22 T, when the magnetic field is applied perpendicular to the ab
planes, which becomes significantly more pronounced as the temperature is
lowered to 0.33 K. As our superconducting sample lies well outside the
antiferromagnetic region of the phase diagram, the observed field-induced
transition in torque indicates a spin-flop transition not of long-range ordered
moments, but of nematic-like antiferromagnetic fluctuations.Comment: Latex, 4 figure
Microscopic model for transitions from Mott to spin-Peierls insulator in TiOCl
On the basis of ab initio density functional theory (DFT) calculations, we
derive the underlying microscopic model Hamiltonian for TiOCl, a unique system
that shows two consecutive phase transitions from a Mott insulator to a
spin-Peierls insulator through a structurally incommensurate phase. We show
with our model that the presence of magnetic frustration in TiOCl leads to a
competition with the spin-Peierls distortion, which results in the novel
incommensurate phase. In addition, our calculations indicate that the
spin-Peierls state is triggered by adiabatic phonons, which is essential for
understanding the nature of the phase transition.Comment: extended version, accepted in PR
- …