112 research outputs found
Divacancy clustering in neutron-irradiated and annealed n-type germanium
We have studied the annealing of vacancy defects in neutron-irradiated germanium. After irradiation, the Sb-doped samples [(Sb)=1.5×10 exp 15 cm exp −3] were annealed at 473, 673, and 773 K for 30 min. The positron lifetime was measured as a function of temperature (30–295 K). A lifetime component of 330 ps with no temperature dependence is observed in as-irradiated samples, identified as the positron lifetime in a neutral divacancy and indicating that the divacancy is stable at room temperature (RT). Annealing at 673 K resulted in an increase in the average positron lifetime, and in addition, the annealed samples further showed a larger lifetime component of 430 ps at RT, which is due to larger vacancy clusters. The average positron lifetime in the samples annealed at 473 K has a definite temperature dependence, suggesting that the divacancies become negative as the crystal recovers and the Fermi level moves upwards in the band gap. Annealing at 673 K, reduces the average lifetime and intensity of the defect component τ2 at RT, indicating that the vacancy clusters have started to anneal. Negative divacancies are still present in the samples after this anneal. Annealing at 773 K is enough to remove all observable vacancy defects.Peer reviewe
Fulde-Ferrell-Larkin-Ovchinnikov Superconducting State in CeCoIn5
We report specific heat measurements of the heavy fermion superconductor
CeCoIn5 in the vicinity of the superconducting critical field H_{c2}, with
magnetic field in the [110], [100], and [001] directions, and at temperatures
down to 50 mK. The superconducting phase transition changes from second to
first order for field above 10 T for H || [110] and H || [100]. In the same
range of magnetic field we observe a second specific heat anomaly within the
superconducting state. We interpret this anomaly as a signature of a
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting state. We
obtain similar results for H || [001], with FFLO state occupying a smaller part
of the phase diagram.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Letter
Wilson ratio in Yb-substituted CeCoIn5
We have investigated the effect of Yb substitution on the Pauli limited,
heavy fermion superconductor, CeCoIn. Yb acts as a non-magnetic divalent
substituent for Ce throughout the entire doping range, equivalent to hole
doping on the rare earth site. We found that the upper critical field in
(Ce,Yb)CoIn is Pauli limited, yet the reduced (H,T) phase diagram is
insensitive to disorder, as expected in the purely orbitally limited case. We
use the Pauli limiting field, the superconducting condensation energy and the
electronic specific heat coefficient to determine the Wilson ratio (),
the ratio of the specific heat coefficient to the Pauli susceptibility in
CeCoIn. The method is applicable to any Pauli limited superconductor in the
clean limit.Comment: 5 pages, 1 table, 4 figure
Anisotropy of Thermal Conductivity and Possible Signature of the Fulde-Ferrell-Larkin-Ovchinnikov state in CeCoIn_5
We have measured the thermal conductivity of the heavy-fermion superconductor
CeCoIn_5 in the vicinity of the upper critical field, with the magnetic field
perpendicular to the c axis. Thermal conductivity displays a discontinuous jump
at the superconducting phase boundary below critical temperature T_0 ~ 1 K,
indicating a change from a second to first order transition and confirming the
recent results of specific heat measurements on CeCoIn_5. In addition, the
thermal conductivity data as a function of field display a kink at a field H_k
below the superconducting critical field, which closely coincides with the
recently discovered anomaly in specific heat, tentatively identified with the
appearance of the spatially inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) superconducting state. Our results indicate that the thermal
conductivity is enhanced within the FFLO state, and call for further
theoretical investigations of the order parameter's real space structure (and,
in particular, the structure of vortices) and of the thermal transport within
the inhomogeneous FFLO state.Comment: 19 pages, 6 figures, submitted to Prhys. Rev.
Magnetic, thermodynamic, and electrical transport properties of the noncentrosymmetric B20 germanides MnGe and CoGe
We present magnetization, specific heat, resistivity, and Hall effect
measurements on the cubic B20 phase of MnGe and CoGe and compare to
measurements of isostructural FeGe and electronic structure calculations. In
MnGe, we observe a transition to a magnetic state at K as identified
by a sharp peak in the ac magnetic susceptibility, as well as second phase
transition at lower temperature that becomes apparent only at finite magnetic
field. We discover two phase transitions in the specific heat at temperatures
much below the Curie temperature one of which we associate with changes to the
magnetic structure. A magnetic field reduces the temperature of this transition
which corresponds closely to the sharp peak observed in the ac susceptibility
at fields above 5 kOe. The second of these transitions is not affected by the
application of field and has no signature in the magnetic properties or our
crystal structure parameters. Transport measurements indicate that MnGe is
metal with a negative magnetoresistance similar to that seen in isostructural
FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about
0.5 carriers per formula unit also similar to that found in FeGe and MnSi. CoGe
is shown to be a low carrier density metal with a very small, nearly
temperature independent diamagnetic susceptibility.Comment: 16 pages 23 figure
Fermi-surface topology of the iron pnictide LaFeP
We report on a comprehensive de Haas--van Alphen (dHvA) study of the iron
pnictide LaFeP. Our extensive density-functional band-structure
calculations can well explain the measured angular-dependent dHvA frequencies.
As salient feature, we observe only one quasi-two-dimensional Fermi-surface
sheet, i.e., a hole-like Fermi-surface cylinder around , essential for
pairing, is missing. In spite of considerable mass enhancements due to
many-body effects, LaFeP shows no superconductivity. This is likely
caused by the absence of any nesting between electron and hole bands.Comment: 5 pages, 4 figure
Fermi surface evolution through a heavy fermion superconductor-to-antiferromagnet transition: de Haas-van Alphen effect in Cd-substituted CeCoIn
We report the results of de-Haas-van-Alphen (dHvA) measurements in Cd doped
CeCoIn and LaCoIn. Cd doping is known to induce an antiferromagnetic
order in the heavy fermion superconductor CeCoIn, whose effect can be
reversed with applied pressure. We find a slight but systematic change of the
dHvA frequencies with Cd doping in both compounds, reflecting the chemical
potential shift due to the addition of holes. The frequencies and effective
masses are close to those found in the nominally pure compounds with similar
changes apparent in the Ce and La compounds with Cd substitution. We observe no
abrupt changes to the Fermi surface in the high field paramagnetic state for corresponding to the onset of antiferromagnetic ordering at H=0 in
CeCo(InCd). Our results rule out electron localization as
the mechanism for the tuning of the ground state in CeCoIn with Cd doping
Superconducting fluctuations and the Nernst effect: A diagrammatic approach
We calculate the contribution of superconducting fluctuations above the
critical temperature to the transverse thermoelectric response
, the quantity central to the analysis of the Nernst effect. The
calculation is carried out within the microscopic picture of BCS, and to linear
order in magnetic field. We find that as , the dominant contribution
to arises from the Aslamazov-Larkin diagrams, and is equal to the
result previously obtained from a stochastic time-dependent Ginzburg-Landau
equation [Ussishkin, Sondhi, and Huse, arXiv:cond-mat/0204484]. We present an
argument which establishes this correspondence for the heat current. Other
microscopic contributions, which generalize the Maki-Thompson and density of
states terms for the conductivity, are less divergent as .Comment: 11 pages, 5 figure
Apache Mahout: Machine Learning on Distributed Dataflow Systems
Apache Mahout is a library for scalable machine learning (ML) on distributed dataflow systems, offering various implementations of classification, clustering, dimensionality reduction and recommendation algorithms. Mahout was a pioneer in large-scale machine learning in 2008, when it started and targeted MapReduce, which was the predominant abstraction for scalable computing in industry at that time. Mahout has been widely used by leading web companies and is part of several commercial cloud offerings. In recent years, Mahout migrated to a general framework enabling a mix of dataflow programming and linear algebraic computations on backends such as Apache Spark and Apache Flink. This design allows users to execute data preprocessing and model training in a single, unified dataflow system, instead of requiring a complex integration of several specialized systems. Mahout is maintained as a community-driven open source project at the Apache Software Foundation, and is available under https://mahout.apache.org
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