Impact ionization, which supports carrier multiplication, is promising for applications in single photon detection
and sharp threshold swing field effect devices
. However, initiating the impact ...ionization of avalanche breakdown requires a high applied electric field in a long active region, which hampers carrier multiplication with a high gain, low bias and superior noise performance
. Here we report the observation of ballistic avalanche phenomena in sub-mean free path (MFP) scaled vertical InSe/black phosphorus (BP)
heterostructures
. We use these heterojunctions to fabricate avalanche photodetectors (APDs) with a sensitive mid-infrared light detection (4 μm wavelength) and impact ionization transistors with a steep subthreshold swing (<0.25 mV dec
). The devices show a low avalanche threshold (<1 V), low noise figure and distinctive density spectral shape. Our transport measurements suggest that the breakdown originates from a ballistic avalanche phenomenon, where the sub-MFP BP channel support the lattice impact ionization by electrons and holes and the abrupt current amplification without scattering from the obstacles in a deterministic nature. Our results provide new strategies for the development of advanced photodetectors
via efficient carrier manipulation at the nanoscale.
The experimental manifestation of topological effects in bulk materials is attracting enormous research interest. However, direct experimental evidence of the effective k-space monopole of the Weyl ...nodes has so far been lacking. Here, signatures of the singular topology of the type-II Weyl semimetal TaIrTe
are revealed in the photoresponses, which are related to divergence of the Berry curvature. TaIrTe
exhibits a large photoresponsivity of 130.2 mA W
-with 4 μm excitation in an unbiased field-effect transistor at room temperature-arising from the third-order nonlinear optical response, approaching the performance of commercial low-temperature detectors. In addition, the circularly polarized galvanic response is enhanced at 4 μm, possibly due to the same Berry curvature singularity enhancement. Considering the optical selection rule of chiral Weyl cones, this may open the door for studying and controlling the chiral polarization of Weyl fermions with an electric field in addition to the optical helicities.
Abstract
The thermoelectric effects of topological semimetals have attracted tremendous research interest because many topological semimetals are excellent thermoelectric materials and ...thermoelectricity serves as one of their most important potential applications. In this work, we reveal the transient photothermoelectric response of Dirac semimetallic Cd
3
As
2
, namely the photo-Seebeck effect and photo-Nernst effect, by studying the terahertz (THz) emission from the transient photocurrent induced by these effects. Our excitation polarization and power dependence confirm that the observed THz emission is due to photothermoelectric effect instead of other nonlinear optical effect. Furthermore, when a weak magnetic field (~0.4 T) is applied, the response clearly indicates an order of magnitude enhancement on transient photothermoelectric current generation compared to the photo-Seebeck effect. Such enhancement supports an ambipolar transport nature of the photo-Nernst current generation in Cd
3
As
2
. These results highlight the enhancement of thermoelectric performance can be achieved in topological Dirac semimetals based on the Nernst effect, and our transient studies pave the way for thermoelectric devices applicable for high field circumstance when nonequilibrium state matters. The large THz emission due to highly efficient photothermoelectric conversion is comparable to conventional semiconductors through optical rectification and photo-Dember effect.
Photodetection with extreme performances in terms of ultrafast response time, broad detection wavelength range, and high sensitivity has a wide range of optoelectronic and photonic applications, such ...as optical communications, interconnects, imaging, and remote sensing. Graphene, a typical two-dimensional Dirac semimetal, has shown excellent potential toward a high-performance photodetector with high operation speed, broadband response, and efficient carrier multiplications benefiting from its linear dispersion band structure with a high carrier mobility and zero bandgap. As the three-dimensional analogues of graphene, Dirac semimetal Cd3As2 processes all advantages of graphene as a photosensitive material but potentially has stronger interaction with light as a bulk material and thus enhanced responsivity. In this work, we report the realization of an ultrafast broadband photodetector based on Cd3As2. The prototype metal–Cd3As2–metal photodetector exhibits a responsivity of 5.9 mA/W with a response time of about 6.9 ps without any special device optimization. Broadband responses from 532 nm to 10.6 μm are achieved with a potential detection range extendable to far-infrared and terahertz. Systematical studies indicate that the photothermoelectric effect plays an important role in photocurrent generation. Our results suggest this emerging class of exotic quantum materials can be harnessed for photodetection with a high sensitivity and high speed (∼145 GHz) over a broad wavelength range.
The head on quenching of statistically planar turbulent premixed flames by an isothermal inert wall has been analysed using three-dimensional Direct Numerical Simulation (DNS) data for different ...values of global Lewis number
Le
(0.8, 1.0 and 1.2) and turbulent Reynolds number
R
e
t
. The statistics of head on quenching have been analysed in terms of the wall Peclet number
P
e
(i.e. distance of the flame from the wall normalised by the Zel’dovich flame thickness) and the normalised wall heat flux Φ. It has been found that the maximum (minimum) value of Φ(
P
e
) for the turbulent
L
e
=0.8 cases are greater (smaller) than the corresponding laminar value, whereas both
P
e
and Φ in turbulent cases remain comparable to the corresponding laminar values for
L
e
=1.0 and 1.2. Detailed physical explanations are provided for the observed
Le
dependences of
P
e
and Φ. The existing closure of mean reaction rate
ω
̇
¯
using the scalar dissipation rate (SDR) in the near wall region has been assessed based on
a-priori
analysis of DNS data and modifications to the existing closures of mean reaction rate and SDR have been suggested to account for the wall effects in such a manner that the modified closures perform well both near to and away from the wall.
Photodetectors based on Weyl semimetal promise extreme performance in terms of highly sensitive, broadband and self‐powered operation owing to its extraordinary material properties. Layered Type‐II ...Weyl semimetal that break Lorentz invariance can be further integrated with other two‐dimensional materials to form van der Waals heterostructures and realize multiple functionalities inheriting the advantages of other two‐dimensional materials. Herein, we report the realization of a broadband self‐powered photodetector based on Type‐II Weyl semimetal Td‐MoTe2. The prototype metal–MoTe2–metal photodetector exhibits a responsivity of 0.40 mA W−1 and specific directivity of 1.07 × 108 Jones with 43 μs response time at 532 nm. Broadband responses from 532 nm to 10.6 μm are experimentally tested with a potential detection range extendable to far‐infrared and terahertz. Furthermore, we identify the response of the detector is polarization angle sensitive due to the anisotropic response of MoTe2. The anisotropy is found to be wavelength dependent, and the degree of anisotropy increases as the excitation wavelength gets closer to the Weyl nodes. In addition, with power and temperature dependent photoresponse measurements, the photocurrent generation mechanisms are investigated. Our results suggest this emerging class of materials can be harnessed for broadband angle sensitive, self‐powered photodetection with decent responsivities.
A broadband self‐powered photodetector based on the layered Type‐II Weyl semimetal Td‐MoTe2 exhibits a responsivity of 0.40 mA W‐1 and specific directivity of 1.07 × 108 Jones with 43 μs response time at 532 nm. The response of the detector is polarization‐angle sensitive and the degree of anisotropic response increases as the excitation wavelength gets closer to the Weyl nodes.
Fe2P-type compounds exhibit a giant magnetocaloric effect (MCE) and are extensively studied for room temperature applications. The reduction of their transition temperature below 77 K can pave the ...way for the potential application of these materials for hydrogen liquefaction using cryogenic magnetic refrigeration. Most of the known magnetocaloric materials with a giant MCE below 77 K are rare-earth-based compounds. In order to explore the possibility of developing rare-earth-free compounds with cryogenic MCE, we collected a dataset by conducting data mining on published experimental results on Fe2P-type magnetocaloric compounds and used machine learning for composition optimization aiming at lowering the transition temperature below 77 K. Guided by the predictions of an artificial neural network, we found a promising composition of Mn1.70Fe0.30P0.63Si0.37 with a transition temperature of 97 K at 1 T magnetic field which was lowered to 73 K by the minor substitution of Fe with Co. The developed rare-earth-free compounds exhibit a large magnetocaloric performance in isothermal magnetic entropy change (∆SM) of 7.5–11.5 J/kgK at the temperatures below 100 K. This study demonstrates that data-driven development of magnetocaloric materials can efficiently boost the optimization of their properties, thus aiding the practical applicability of magnetic refrigeration technology.
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