98 research outputs found
Spin density wave anomaly at 140 K in the ternary iron arsenide BaFe2As2
The ternary iron arsenide BaFe2As2 with the tetragonal ThCr2Si2-type
structure exhibits a spin density wave (SDW) anomaly at 140 K, very similar to
LaFeAsO, the parent compound of the iron arsenide superconductors. BaFe2As2 is
a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase
transition at 140 K, accompanied by strong anomalies in the specific heat,
electrical resistance and magnetic susceptibility. In the course of this
transition, the space group symmetry changes from tetragonal (I4/mmm) to
orthorhombic (Fmmm). 57Fe Moessbauer spectroscopy experiments show a single
signal at room temperature and full hyperfine field splitting below the phase
transition temperature (5.2 T at 77 K). Our results suggest that BaFe2As2 can
serve as a new parent compound for oxygen-free iron arsenide superconductors.Comment: 4 pages, 6 figures, submitted to PR
Radiocarbon offsets and old world chronology as relevant to Mesopotamia, Egypt, Anatolia and Thera (Santorini)
The new IntCal20 radiocarbon record continues decades of successful practice by employing one calibration curve as an approximation for different regions across the hemisphere. Here we investigate three radiocarbon time-series of archaeological and historical importance from the Mediterranean-Anatolian region, which indicate, or may include, offsets from IntCal20 (~0–22 14C years). While modest, these differences are critical for our precise understanding of historical and environmental events across the Mediterranean Basin and Near East. Offsets towards older radiocarbon ages in Mediterranean-Anatolian wood can be explained by a divergence between high-resolution radiocarbon dates from the recent generation of accelerator mass spectrometry (AMS) versus dates from previous technologies, such as low-level gas proportional counting (LLGPC) and liquid scintillation spectrometry (LSS). However, another reason is likely differing growing season lengths and timings, which would affect the seasonal cycle of atmospheric radiocarbon concentrations recorded in different geographic zones. Understanding and correcting these offsets is key to the well-defined calendar placement of a Middle Bronze Age tree-ring chronology. This in turn resolves long-standing debate over Mesopotamian chronology in the earlier second millennium BCE. Last but not least, accurate dating is needed for any further assessment of the societal and environmental impact of the Thera/Santorini volcanic eruption.</p
Radiocarbon offsets and old world chronology as relevant to Mesopotamia, Egypt, Anatolia and Thera (Santorini)
The new IntCal20 radiocarbon record continues decades of successful practice by employing one calibration curve as an approximation for different regions across the hemisphere. Here we investigate three radiocarbon time-series of archaeological and historical importance from the Mediterranean-Anatolian region, which indicate, or may include, offsets from IntCal20 (~0–22 14C years). While modest, these differences are critical for our precise understanding of historical and environmental events across the Mediterranean Basin and Near East. Offsets towards older radiocarbon ages in Mediterranean-Anatolian wood can be explained by a divergence between high-resolution radiocarbon dates from the recent generation of accelerator mass spectrometry (AMS) versus dates from previous technologies, such as low-level gas proportional counting (LLGPC) and liquid scintillation spectrometry (LSS). However, another reason is likely differing growing season lengths and timings, which would affect the seasonal cycle of atmospheric radiocarbon concentrations recorded in different geographic zones. Understanding and correcting these offsets is key to the well-defined calendar placement of a Middle Bronze Age tree-ring chronology. This in turn resolves long-standing debate over Mesopotamian chronology in the earlier second millennium BCE. Last but not least, accurate dating is needed for any further assessment of the societal and environmental impact of the Thera/Santorini volcanic eruption.ISSN:2045-232
Magnetic Ordering in V-Layers of the Superconducting System of Sr2VFeAsO3
Results of transport, magnetic, thermal, and 75As-NMR measurements are
presented for superconducting Sr2VFeAsO3 with an alternating stack of FeAs and
perovskite-like block layers. Although apparent anomalies in magnetic and
thermal properties have been observed at ~150 K, no anomaly in transport
behaviors has been observed at around the same temperature. These results
indicate that V ions in the Sr2VO3-block layers have localized magnetic moments
and that V-electrons do not contribute to the Fermi surface. The electronic
characteristics of Sr2VFeAsO3 are considered to be common to those of other
superconducting systems with Fe-pnictogen layers.Comment: 4 pages, 4 figures, To appear in JPSJ 79 (2010) 12371
Superconductivity in SrFe2As2 with Pt Doping
We have synthesized polycrystalline samples of Pt-substituted SrFe2As2 and
measured the temperature dependence of magnetization and electrical
resistivity. We have observed the superconducting transition at Tc = 17 K with
the maximum shielding volume fraction at x = 0.125 in Sr(Fe1-xPtx)2As2. It is
found that the maximum Tc depends on the substituted element, so it is
important to substitute various elements to explore new iron-based
superconductors with higher Tc.Comment: 2 pages, 1 figure, to appear in J. Phys. Soc. Jpn. Vol. 79 No. 9
(2010
Recommended from our members
Climate Change during and after the Roman Empire: Reconstructing the Past from Scientific and Historical Evidence
Growing scientific evidence from modern climate science is loaded with implications for the environmental history of the Roman Empire and its successor societies. The written and archaeological evidence, although richer than commonly realized, is unevenly distributed over time and space. A first synthesis of what the written records and multiple natural archives (multi-proxy data) indicate about climate change and variability across western Eurasia from c. 100 b.c. to 800 a.d. confirms that the Roman Empire rose during a period of stable and favorable climatic conditions, which deteriorated during the Empire's third-century crisis. A second, briefer period of favorable conditions coincided with the Empire's recovery in the fourth century; regional differences in climate conditions parallel the diverging fates of the eastern and western Empires in subsequent centuries. Climate conditions beyond the Empire's boundaries also played an important role by affecting food production in the Nile valley, and by encouraging two major migrations and invasions of pastoral peoples from Central Asia.Earth and Planetary SciencesHistor
Synthesis, crystal structure and spin-density-wave anomaly of the iron arsenide-fluoride SrFeAsF
The new quaternary iron arsenide-fluoride SrFeAsF with the tetragonal
ZrCuSiAs-type structure was synthesized and the crystal structure was
determined by X-ray powder diffraction (P4/nmm, a = 399.30(1), c = 895.46(1)
pm). SrFeAsF undergoes a structural and magnetic phase transition at 175 K,
accompanied by strong anomalies in the specific heat, electrical resistance and
magnetic susceptibility. In the course of this transition, the space group
symmetry changes from tetragonal (P4/nmm) to orthorhombic (Cmme). 57Fe
Moessbauer spectroscopy experiments show a single signal at room temperature at
an isomer shift of 0.30(1) mm/s and magnetic hyperfine-field splitting below
the phase transition temperature. Our results clearly show that SrFeAsF
exhibits a spin density wave (SDW) anomaly at 175 K very similar to LaFeAsO,
the parent compound of the iron arsenide-oxide superconductors and thus SrFeAsF
may serve as a further parent compound for oxygen-free iron arsenide
superconductors.Comment: 5 pages, 7 figure
Radiocarbon offsets and old world chronology as relevant to Mesopotamia, Egypt, Anatolia and Thera (Santorini)
Abstract: The new IntCal20 radiocarbon record continues decades of successful practice by employing one calibration curve as an approximation for different regions across the hemisphere. Here we investigate three radiocarbon time-series of archaeological and historical importance from the Mediterranean-Anatolian region, which indicate, or may include, offsets from IntCal20 (~0–22 14C years). While modest, these differences are critical for our precise understanding of historical and environmental events across the Mediterranean Basin and Near East. Offsets towards older radiocarbon ages in Mediterranean-Anatolian wood can be explained by a divergence between high-resolution radiocarbon dates from the recent generation of accelerator mass spectrometry (AMS) versus dates from previous technologies, such as low-level gas proportional counting (LLGPC) and liquid scintillation spectrometry (LSS). However, another reason is likely differing growing season lengths and timings, which would affect the seasonal cycle of atmospheric radiocarbon concentrations recorded in different geographic zones. Understanding and correcting these offsets is key to the well-defined calendar placement of a Middle Bronze Age tree-ring chronology. This in turn resolves long-standing debate over Mesopotamian chronology in the earlier second millennium BCE. Last but not least, accurate dating is needed for any further assessment of the societal and environmental impact of the Thera/Santorini volcanic eruption
Pressure Dependence of Superconducting Transition Temperature on Perovskite-Type Fe-Based Superconductors and NMR Study of Sr2VFeAsO3
We report the pressure dependences of the superconducting transition
temperature (T_c) in several perovskite-type Fe-based superconductors through
the resistivity measurements up to ~4 GPa. In Ca_4(Mg,Ti)_3Fe_2As_2O_y with the
highest T_c of 47 K in the present study, the T_c keeps almost constant up to
~1 GPa, and starts to decrease above it. From the comparison among several
systems, we obtained a tendency that low T_c with the longer a-axis length at
ambient pressure increases under pressure, but high T_c with the shorter a-axis
length at ambient pressure hardly increases. We also report the ^75As-NMR
results on Sr_2VFeAsO_3. NMR spectrum suggests that the magnetic ordering
occurs at low temperatures accompanied by some inhomogeneity. In the
superconducting state, we confirmed the anomaly by the occurrence of
superconductivity in the nuclear spin lattice relaxation rate 1/T_1, but the
spin fluctuations unrelated with the superconductivity are dominant. It is
conjectured that the localized V-3d moments are magnetically ordered and their
electrons do not contribute largely to the Fermi surface and the
superconductivity in Sr_2VFeAsO_3.Comment: 7 pages, 9 figure
Distorted magnetic orders and electronic structures of tetragonal FeSe from first-principles
We use the state-of-the-arts density-functional-theory method to study
various magnetic orders and their effects on the electronic structures of the
FeSe. Our calculated results show that, for the spins of the single Fe layer,
the striped antiferromagnetic orders with distortion are more favorable in
total energy than the checkerboard antiferromagnetic orders with tetragonal
symmetry, which is consistent with known experimental data, and the inter-layer
magnetic interaction is very weak. We investigate the electronic structures and
magnetic property of the distorted phases. We also present our calculated spin
coupling constants and discuss the reduction of the Fe magnetic moment by
quantum many-body effects. These results are useful to understand the
structural, magnetic, and electronic properties of FeSe, and may have some
helpful implications to other FeAs-based materials
- …