3,162 research outputs found
The biogeochemistry of microbial mats, stromatolites and the ancient biosphere
Stromatolites offer an unparalleled geologic record of early life, because they constitute the oldest and most abundant recognizable remains of microbial ecosystems. Microbial mats are living homologs of stromatolites; thus, the physiology of the microbiota as well as the processes which create those features of mats (e.g., biomarker organic compounds, elemental and stable isotopic compositions) which are preserved in the ancient record. Observations of the carbon isotopic composition (delta C-13) of stromatolites and microbial mats were made and are consistent with the hypothesis that atmospheric CO2 concentrations have declined by at least one to two orders of magnitude during the past 2.5 Ga. Whereas delta C-13 values of carbonate carbon average about 0 permil during both the early and mid-Proterozoic, the delta C-13 values of stromatolitic organic matter increase from an average of -35 between 2.0 and 2.6 Ga ago to an average of about -28 about 1.0 Ga ago. Modern microbial mats in hypersaline environments have delta C-13 values typically in the range of -5 to -9, relative to an inorganic bicarbonate source at 0 permil. Both microbial mats and pur cultures of cyanobacteria grown in waters in near equilibrium with current atmospheric CO2 levels exhibit minimal discrimination against C-13. In contrast, hot spring cyanobacterial mats or cyanobacterial cultures grown under higher CO2 levels exhibit substantially greater discrimination. If care is taken to compare modern mats with stromatolites from comparable environments, it might be possible to estimate ancient levels of atmospheric CO2. In modern microbial mats, a tight coupling exists between photosynthetic organic carbon production and subsequent carbon oxidation, mostly by sulfate reduction. The rate of one process fuels a high rate of the other, with much of the sulfate reduction occurring within the same depth interval as oxygenic photosynthesis. Other aspects of this study are presented
Angular dependent planar metamagnetism in the hexagonal compounds TbPtIn and TmAgGe
Detailed magnetization measurements, M(T,H,theta), were performed on single
crystals of TbPtIn and TmAgGe (both members of the hexagonal Fe_2P/ZrNiAl
structure type), for the magnetic field H applied perpendicular to the
crystallographic c axis. These data allowed us to identify, for each compound,
the easy-axes for the magnetization, which coincided with high symmetry
directions ([120] for TbPtIn and [110] for TmAgGe). For fixed orientations of
the field along each of the two six-fold symmetry axes, a number of
magnetically ordered phases is being revealed by M(H,T) measurements below T_N.
Moreover, T ~ 2 K, M(H)|_theta measurements for both compounds (with H applied
parallel to the basal plane), as well as T = 20 K data for TbPtIn, reveal five
metamagnetic transitions with simple angular dependencies: H_{ci,j} ~
1/cos(theta +/- phi), where phi = 0^0 or 60^0. The high field magnetization
state varies with theta like 2/3*mu_{sat}(R^{3+})*cos(theta), and corresponds
to a crystal field limited saturated paramagnetic, CL-SPM, state. Analysis of
these data allowed us to model the angular dependence of the locally saturated
magnetizations M_{sat} and critical fields H_c with a three coplanar Ising-like
model, in which the magnetic moments are assumed to be parallel to three
adjacent easy axes. Furthermore, net distributions of moments were inferred
based on the measured data and the proposed model
Asymptotic enumeration of dense 0-1 matrices with specified line sums
Let S=(s_1,s_2,..., s_m) and T = (t_1,t_2,..., t_n) be vectors of
non-negative integers with sum_{i=1}^{m} s_i = sum_{j=1}^n t_j. Let B(S,T) be
the number of m*n matrices over {0,1} with j-th row sum equal to s_j for 1 <= j
<= m and k-th column sum equal to t_k for 1 <= k <= n. Equivalently, B(S,T) is
the number of bipartite graphs with m vertices in one part with degrees given
by S, and n vertices in the other part with degrees given by T. Most research
on the asymptotics of B(S,T) has focused on the sparse case, where the best
result is that of Greenhill, McKay and Wang (2006). In the case of dense
matrices, the only precise result is for the case of equal row sums and equal
column sums (Canfield and McKay, 2005). This paper extends the analytic methods
used by the latter paper to the case where the row and column sums can vary
within certain limits. Interestingly, the result can be expressed by the same
formula which holds in the sparse case.Comment: Multiple minor adjustments. Accepted by JCT-
Phase diagram of CeVSb3 under pressure and its dependence on pressure conditions
We present temperature dependent resistivity and ac-calorimetry measurements
of CeVSb3 under pressure up to 8 GPa in a Bridgman anvil cell modified to use a
liquid medium and in a diamond anvil cell using argon as a pressure medium,
respectively. We observe an initial increase of the ferromagnetic transition
temperature Tc with pressures up to 4.5 GPa, followed by decrease of Tc on
further increase of pressure and finally its disappearance, in agreement with
the Doniach model. We infer a ferromagnetic quantum critical point around 7 GPa
under hydrostatic pressure conditions from the extrapolation to 0 K of Tc and
the maximum of the A coefficient from low temperature fits of the resistivity
\rho (T)=\rho_{0}+AT^{n}. No superconductivity under pressure was observed down
to 0.35 K for this compound. In addition, differences in the Tc(P) behavior
when a slight uniaxial component is present are noticed and discussed and
correlated to choice of pressure medium
Regularly spaced subsums of integer partitions
For integer partitions , where , we study the sum of the parts of odd index. We show
that the average of this sum, over all partitions of , is of the
form More
generally, we study the sum of the parts whose
indices lie in a given arithmetic progression and we show that the average of
this sum, over all partitions of , is of the form
, with explicitly given
constants . Interestingly, for odd and we have
, so in this case the error term is of lower order. The methods used
involve asymptotic formulas for the behavior of Lambert series and the Zeta
function of Hurwitz.
We also show that if is the number of partitions of the sum of
whose parts of even index is , then for every , agrees with a
certain universal sequence, Sloane's sequence \texttt{#A000712}, for
but not for any larger
Crystal growth and annealing study of fragile, non-bulk superconductivity in YFeGe
We investigated the occurrence and nature of superconductivity in single
crystals of YFeGe grown out of Sn flux by employing x-ray diffraction,
electrical resistivity, and specific heat measurements. We found that the
residual resistivity ratio (RRR) of single crystals can be greatly improved,
reaching as high as 60, by decanting the crystals from the molten Sn at
350C and/or by annealing at temperatures between 550C and
600C. We found that samples with RRR 34 showed resistive
signatures of superconductivity with the onset of the superconducting
transition K. RRR values vary between 35 and 65 with, on
average, no systematic change in value, indicating that systematic
changes in RRR do not lead to comparable changes in . Specific heat
measurements on samples that showed clear resistive signatures of a
superconducting transition did not show any signature of a superconducting
phase transition, which suggests that the superconductivity observed in this
compound is either some sort of filamentary, strain stabilized
superconductivity associated with small amounts of stressed YFeGe
(perhaps at twin boundaries or dislocations) or is a second crystallographic
phase present at levels below detection capability of conventional powder x-ray
techniques.Comment: 8 pages, 11 figure
Anisotropic transport and magnetic properties, and magnetic-field tuned states of CeZn11 single crystals
We present detailed temperature and field dependent data obtained from
magnetization, resistivity, heat capacity, Hall resistivity and thermoelectric
power measurements performed on single crystals of CeZn11. The compounds orders
antiferromagnetically at 2 K. The zero-field resistivity and TEP data
show features characteristic of a Ce-based intermetallic with crystal electric
field splitting and possible correlated, Kondo lattice effects. We constructed
the T-H phase diagram for the magnetic field applied along the easy, [110],
direction which shows that the magnetic field required to suppress T_N below
0.4 K is in the range of 45-47.5 kOe. A linear behavior of the rho(T) data,
H||[110], was observed only for H=45 kOe for 0.46 K<T<1.96 K followed by the
Landau-Fermi-liquid regime for a limited range of fields, 47.5 kOe< H<60 kOe.
From the analysis of our data, it appears that CeZn11 is a weakly to moderately
correlated local moment compound with rather small Kondo temperature. The
thermoelectric and transport properties of CeZn11 are mostly governed by the
CEF effects. Given the very high quality of our single crystals, quantum
oscillations are found for both CeZn11 and its non-magnetic analogue, LaZn11
Understanding School Genres Using Systemic Functional Linguistics: A Study of Science and Narrative Texts
The purpose of this study is to examine elementary level textbooks (grades 2-4; Houghton Mifflin Harcourt Publishing; The Trophies Collection) using Systemic Functional Linguistics as the theoretical framework to study the different types of lexical choice and grammatical options made in the textbooks. The two genres examined are science and narrative, which are significantly different from each other. Science texts are “information based,” and narrative texts, “story based.” It is very important for teachers to understand how the genres are different so that they can convey those differences to their students.
The two school genres, science and narrative, differ from each other in their lexicogrammatical features. These features can be analyzed and evaluated and then taught. An appraisal analysis identifies items that display the author’s attitude in the text, and a grammatical metaphor analysis identifies modes of expression displaying incongruency between the two levels of semantics and lexicogrammar. Evaluating appraisal items and understanding how grammatical metaphors are arranged within these texts can help differentiate some of the discourse semantic features of science or “informational” texts and narratives or “story” texts. The results of this analysis may help teachers during class instruction
Anisotropy Reversal of the Upper Critical Field at Low Temperatures and Spin-Locked Superconductivity in K2Cr3As3
We report the first measurements of the anisotropic upper critical field
for KCrAs single crystals up to 60 T and K. Our results show that the upper critical field parallel to the Cr
chains, , exhibits a paramagnetically-limited behavior,
whereas the shape of the curve (perpendicular to the Cr
chains) has no evidence of paramagnetic effects. As a result, the curves
and cross at K, so that
the anisotropy parameter
increases from near to at 0.6 K. This behavior of is inconsistent with triplet
superconductivity but suggests a form of singlet superconductivity with the
electron spins locked onto the direction of Cr chains
- …