Seismic monitoring can greatly benefit from imaging events with a low signal-to-noise ratio (SNR) as the number of the events with a low signal grows exponentially. One way to detect weaker events is ...improvement of a SNR by migration-type stacking of waveforms from multiple stations. We have developed a new method of location of seismic events that involves stacking of seismic phases and amplitudes along diffraction traveltime curves to suppress noise and detect seismic events with a SNR lower than that on individual receivers. The stacking includes polarity correction based on a simultaneous seismic moment tensor inversion and detection algorithm on the stack function. We applied this method to locate microseismicity induced by hydraulic fracturing. First we calibrated the velocity model by locating perforation shots at known locations. Then we processed 3 d of data from microseismic monitoring of shale stimulation and benchmarked migration-type locations of the largest events that were manually located. The detected and located events induced by hydraulic fracturing in this case study are mostly shear events forming narrow bands along the maximum horizontal stress direction approximately 100 m above the injection intervals. The proposed technique is fully automated and feasible for real-time seismic monitoring.
We report the properties of an A-site spinel magnet, CoAl2-xGaxO4, and analyze its anomalous, low-temperature magnetic behavior, which is derived from inherent, magnetically frustrated interactions. ...Rietveld analysis of the x-ray diffraction profile for CoAl2-xGaxO4revealed that the metallic ions were randomly distributed in the tetrahedral (A-) and octahedral (B-) sites in the cubic spinel structure. The inversion parameterηcould be controlled by varying the gallium (Ga) composition in the range 0.055 ⩽η⩽ 0.664. The composition-induced Néel-to-spin-glass (NSG) transition occurred between 0.05 ⩽η⩽ 0.08 and was verified by measurements of DC-AC susceptibilitiesχand thermoremanent magnetization (TRM) below the Néel transition temperatureTN. The relaxation rate and derivative with respect to temperature of TRM increased at bothTNand the spin glass (SG) transition temperatureTSG. The TRM decayed rapidly above and below these transitions. TRM was highly sensitive to macroscopic magnetic transitions that occurred in both the Néel and SG phases of CoAl2-xGaxO4. In the vicinity of the NSG boundary, there was a maximum of the TRM relaxation rate atTmax<TN. With increasing inversionη(x), the anomaly atTmaxmerged with that of the Néel transition at a tricritical point (ηtc,Ttc) = (0.08, 4.0 K), where the paramagnetic, Néel, and SG states met. We successfully extracted the relaxation timeτand other characteristic parameters from the TRM isothermal temporal evolution based on the Weron function derived for a purely stochastic process. To distinguish the magnetic states, we compared our results with previously studied inversion-free A-site spinel, CoRh2O4, and CoGa2O4cluster glass. We generated an inversion-temperature phase diagram based on the comprehensive measurements of DC and AC susceptibilities, TRM, and specific heat in the range 0.055 ⩽η⩽ 0.664 for CoAl2-xGaxO4. Based on this phase diagram, we speculate that a NSG quantum critical phase transition occurred atη= 0.050(6). Our findings are consistent with suppression of the long-range order antiferromagnetic state in CoAl2O4revealed through neutron diffraction studies, even atT<<TN.
Thin layers of silicon nanocrystals (SiNC) in oxide matrix with optimized parameters are fabricated by the plasma-enhanced chemical vapor deposition. These materials with SiNC sizes of about 4.5 nm ...and the SiO
barrier thickness of 3 nm reveal external quantum yield (QY) close to 50% which is near to the best chemically synthetized colloidal SiNC. Internal QY is determined using the Purcell effect, i.e. modifying radiative decay rate by the proximity of a high index medium in a special wedge-shape sample. For the first time we performed these experiments at variable temperatures. The complete optical characterization and knowledge of both internal and external QY allow to estimate the spectral distribution of the dark and bright NC populations within the SiNC ensemble. We show that SiNCs emitting at around 1.2-1.3 eV are mostly bright with internal QY reaching 80% at room temperature and being reduced by thermally activated non-radiative processes (below 100 K internal QY approaches 100%). The mechanisms of non-radiative decay are discussed based on their temperature dependence.
Carrier multiplication by generation of two or more electron-hole pairs following the absorption of a single photon may lead to improved photovoltaic efficiencies and has been observed in ...nanocrystals made from a variety of semiconductors, including silicon. However, with few exceptions, these reports have been based on indirect ultrafast techniques. Here, we present evidence of carrier multiplication in closely spaced silicon nanocrystals contained in a silicon dioxide matrix by measuring enhanced photoluminescence quantum yield. As the photon energy increases, the quantum yield is expected to remain constant, or to decrease as a result of new trapping and recombination channels being activated. Instead, we observe a step-like increase in quantum yield for larger photon energies that is characteristic of carrier multiplication. Modelling suggests that carrier multiplication is occurring with high efficiency and close to the energy conservation limit.
•Synthesis of Ni2MnGa1−xGex Heusler alloys.•Martensitic transitions in alloys with x < 0.25.•Tentative signs of martensitic transition in Ni2MnGe.•Ferromagnetic ordering not influenced by doping up ...to x = 0.4.
Structural and magnetic properties of the prominent magnetic shape memory alloy Ni2MnGa have been investigated in detail, as well as their evolution with tuning parameters – most frequently stoichiometry changes in the Ni-Mn-Ga system or substitution of one or more elements in the parent alloy by other d- and/or p-electron elements. Surprisingly, Ga-Ge substitution, although one of the most natural substitutions, has largely been neglected. We present our study on the preparation, phase and structural characterization, and magnetization properties of alloys within the Ni2MnGa1-xGex series. The isostructural substitution of Ga by Ge results in (i) a significant decrease of the melting point of the alloy, (ii) a contraction of the L21 crystal lattice following the Vegard’s law, (iii) a suppression of martensitic transformation of the lattice for x ≥ 0.25, and (iv) a substantial decrease of ferromagnetic Curie temperature with Ge content increasing above x = 0.4. Tentative signs of a possible martensitic transition are observed also for the other parent compound Ni2MnGe. The crucial role of sample annealing on its overall properties is demonstrated. The presented data are discussed in the frame of previous results on the parent alloy and other related doped systems.
•EuRu2P2 is a ferromagnet with a small anisotropy with a as an easy magnetization axis.•The magnetocrystalline anisotropy was confirmed by ab-initio calculations.•Full saturated magnetic moment ...confirms the Eu2 + state over all pressures.•Sudden drop of Tc at 1.5 GPa is ascribed to the lattice compressibility change.
The EuRu2P2 single crystal was investigated by means of magnetic, transport and thermodynamic studies at ambient and hydrostatic pressures. A small magnetocrystalline anisotropy with crystallographic 100 direction as an easy magnetization direction was found by experimental measurements and confirmed by first-principles calculations. We connect a previously reported change in the compressibility observed at room temperature to a rapid change of ordering temperature under applied hydrostatic pressure.
•Ni2VGa, Ni2CrGa and Ni2CrGe were prepared by arc-melting.•Non-magnetic/weakly paramagnetic behaviour of investigated samples.•Specific heat data reveal Cr ferromagnetic clusters in the ...lattice.•Measured properties are dicussed in the viewpoint of theoretical calculations.
In this report we describe the synthesis and characterization of several members of the Ni2YZ (Y = V, Cr; Z = Ga, Ge) family. While the Heusler alloy Ni2VGa was prepared phase pure, the Ni2CrGa sample consisted of a slightly Cr-deficient alloy and elemental Cr inclusions. The Ge analogues, Ni2VGe and Ni2CrGe, were also prepared as primarily Y-element deficient alloys, with minor secondary phases to balance the starting composition. Despite the minor secondary phases, all Ni2VGa, Ni2CrGa, and Ni2CrGe have diffraction patterns consistent with the cubic Heusler structure (a = 5.814(5), 5.765(3), and 5.788(8) Å, respectively). Ni2VGe was concluded to consist of at least two highly off-stoichiometric phases and was excluded from further study. Specific heat and magnetization measurements on the prepared samples demonstrated they are non-magnetic or only weakly paramagnetic down to lowest temperature. No measurable magnetic moment was found in phase pure Ni2VGa down to 2 K, while only low moments (µ7T2K = 0.027(1) μB and 0.170(2) μB) were observed in Ni2CrGa and Ni2CrGe. Low values of magnetic moments were discussed from the viewpoint of magnetic Cr impurities in a non-magnetic lattice. Such interpretation was well supported by the specific heat data revealing a strong lowest-temperature contribution ascribed to ferromagnetic Cr clusters in the material. The impact of atomic disorder is discussed, particularly with respect to previous theoretical studies and the predicted properties.
Light extraction from a thin planar layer can be increased by introducing a two-dimensional periodic pattern on its surface. This structure, the so-called photonic crystal (PhC) slab, then not only ...enhances the extraction efficiency of light but can direct the extracted emission into desired angles. Careful design of the structures is important in order to have a spectral overlap of the emission with extraction (leaky) modes. We show that by fabricating PhC slabs with optimized dimensions from silicon nanocrystals (SiNCs) active layers, the extraction efficiency of vertical light emission from SiNCs at a particular wavelength can be enhanced ∼ 11 times compared to that of uncorrugated SiNCs-rich layer. More importantly, increased light emission can be obtained in a broad spectral range and, simultaneously, the extracted light can stay confined within relatively narrow angle around the normal to the sample plane. We demonstrate experimentally and theoretically that the physical origin of the enhancement is such that light originating from SiNCs first couples to leaky modes of the PhCs and is then efficiently extracted into the surrounding.
There is an ongoing dispute in the community about the absence of a magnetic quantum critical point (QCP) in the noncentrosymmetric heavy fermion compound CeRhSi3. In order to explore this question ...we prepared single crystals of CeRh(Si1−xGex)3 with x = 0.05 and 0.15 and determined the temperature-pressure (T-p) phase diagram by means of measurements of the electrical resistivity. The substitution of isoelectronic but large Ge enforces a lattice volume increase resulting in a weakening of the Kondo interaction. As a result, the x = 0.05 and x = 0.15 compound exhibit a transition into the antiferromagnetic (AFM) at higher temperatures being TN = 4.7 K and TN1 = 19.7 K, respectively. Application of pressure suppresses TN (TN1) monotonically and pressure induced superconductivity is observed in both Ge-substituted compounds above p ⩾ 2.16 GPa (x = 0.05) and p ⩾ 2.93 GPa (x = 0.15). Extrapolation of TN(p) → 0 of CeRh(Si0.95Ge0.05)3 yields a critical pressure of pc ≈ 3.4 GPa (in CeRh(Si0.85Ge0.15)3 pc ≈ 3.5 GPa) pointing to the presence of an AFM QCP located deep inside the superconducting state.
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