Diurnal Effect of Sub-GeV Dark Matter Boosted by Cosmic Rays.

We point out a new type of diurnal effect for the cosmic ray boosted dark matter (DM). The DM-nucleon interactions not only allow the direct detection of DM with nuclear recoils but also allow cosmic rays to scatter with and boost the nonrelativistic DM to higher energies. If the DM-nuclei scattering cross sections are sufficiently large, the DM flux is attenuated as it propagates through the Earth, leading to a strong diurnal modulation. This diurnal modulation provides another prominent signature for the direct detection of boosted sub-GeV DM, in addition to signals with higher recoil energy.

Scalar Nonstandard Interactions in Neutrino Oscillation.

The scalar nonstandard interactions (NSI) can also introduce matter effect for neutrino oscillation in a medium. Especially the recent Borexino data prefer nonzero scalar NSI, η_{ee}=-0.16. In contrast to the conventional vector NSI, the scalar type contributes as a correction to the neutrino mass matrix rather than the matter potential. Consequently, the scalar matter effect is energy independent while the vector one scales linearly with neutrino energy. This leads to significantly different phenomenological consequences in reactor, solar, atmospheric, and accelerator neutrino oscillations. A synergy of different types of experiments, especially those with matter density variation, is necessary to identify the scalar NSI and guarantee the measurement of CP violation at accelerator experiments.

Constraining Gluonic Quartic Gauge Coupling Operators with gg→γγ.

Gluon-gluon to photon-photon scattering gg→γγ offers to the LHC experiments a uniquely powerful probe of dimension-8 operators in the standard model effective field theory that are quadratic in both the electromagnetic and gluonic field-strength tensors, such as would appear in the Born-Infeld extension of the standard model. We use 13-TeV ATLAS data on the production of isolated photon pairs to set lower limits on the scales of dimension-8 operators M≳1 TeV and discuss the prospective sensitivities of possible future hadron colliders.

Ultrafast Broadband Photodetectors Based on Three-Dimensional Dirac Semimetal Cd3As2.

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.

On the Quantum Spin Hall Gap of Monolayer 1T'-WTe2.

Positive quantum spin Hall gap in mono-layer 1T'-WTe2 is consistently supported by density-functional theory calculations, ultrafast pump-probe, and electrical transport measurements. It is argued that monolayer 1T'-WTe2 , which was predicted to be a semimetallic quantum spin Hall material, is likely a truly 2D quantum spin Hall insulator with a positive quantum spin Hall gap.

Dynamical Evolution of Anisotropic Response in Black Phosphorus under Ultrafast Photoexcitation.

Black phosphorus has recently emerged as a promising material for high-performance electronic and optoelectronic device for its high mobility, tunable mid-infrared bandgap, and anisotropic electronic properties. Dynamical evolution of photoexcited carriers and the induced transient change of electronic properties are critical for materials' high-field performance but remain to be explored for black phosphorus. In this work, we perform angle-resolved transient reflection spectroscopy to study the dynamical evolution of anisotropic properties of black phosphorus under photoexcitation. We find that the anisotropy of reflectivity is enhanced in the pump-induced quasi-equilibrium state, suggesting an extraordinary enhancement of the anisotropy in dynamical conductivity in hot carrier dominated regime. These results raise attractive possibilities of creating high-field, angle-sensitive electronic, optoelectronic, and remote sensing devices exploiting the dynamical electronic anisotropy with black phosphorus.

Coherent longitudinal acoustic phonon approaching THz frequency in multilayer Molybdenum Disulphide.

Coherent longitudinal acoustic phonon is generated and detected in multilayer Molybdenum Disulphide (MoS2) with number of layers ranging from 10 to over 1300 by femtosecond laser pulse. For thin MoS2, the excited phonon frequency exhibits a standing wave nature and shows linear dependence on the sample thickness. The frequency varies from 40 GHz to 0.2 THz (10 layers), which promises possible application in THz frequency mechanical resonators. This linear thickness dependence gradually disappears in thicker samples above about 150 layers, and the oscillation period shows linear dependence on the probe wavelength. From both the oscillation period of the coherent phonon and the delay time of acoustic echo, we can deduce a consistent sound velocity of 7.11*10(3) m/s in MoS2. The generation mechanisms of the coherent acoustic phonon are also discussed through pump power dependent measurement.

Optical properties of metal-molybdenum disulfide hybrid nanosheets and their application for enhanced photocatalytic hydrogen evolution.

Limited control over charge recombination between photogenerated charge carriers largely hinders the progress in photocatalysis. Here, we introduce metal nanoparticles (Cr, Ag) to the surface of MoS2 nanosheets by simple synthetic means creating a hybrid metal-MoS2 nanosheet system with well-defined metal/semiconductor interfaces. We demonstrate that this hybrid nanosheet structure is capable of decoupling light absorption, primarily in MoS2, and carrier separation, across the metal-MoS2 heterostructure leading to drastic quenching of recombination between photogenerated carriers in MoS2, as proven by absorptance, photoluminescence, and ultrafast pump-probe spectroscopy. The photocatalytic activity in the hybrid system is also improved, which further shows excellent stability against photocorrosion.

Valley carrier dynamics in monolayer molybdenum disulfide from helicity-resolved ultrafast pump-probe spectroscopy.

We investigate the valley-related carrier dynamics in monolayer molybdenum disulfide using helicity-resolved nondegenerate ultrafast pump-probe spectroscopy at the vicinity of the high-symmetry K point under the temperature down to 78 K. Monolayer molybdenum disulfide shows remarkable transient reflection signals, in stark contrast to bilayer and bulk molybdenum disulfide due to the enhancement of many-body effect at reduced dimensionality. The helicity-resolved ultrafast time-resolved result shows that the valley polarization is preserved for only several picoseconds before the scattering process makes it undistinguishable. We suggest that the dynamical degradation of valley polarization is attributable primarily to the exciton trapping by defect states in the exfoliated molybdenum disulfide samples. Our experiment and a tight-binding model analysis also show that the perfect valley circular dichroism selectivity is fairly robust against disorder at the K point but quickly decays from the high-symmetry point in the momentum space in the presence of disorder.

Residual symmetries for neutrino mixing with a large θ13 and nearly maximal δD.

The residual Z(2)(s)(k) and Z(2)(s)(k) symmetries induce a direct and unique phenomenological relation with θx (≡ θ13) expressed in terms of the other two mixing angles θs(≡ θ12) and θa(≡ θ23) and the Dirac CP phase δD. Z(2)(s)(k) predicts a θx probability distribution centered around 3°-6° with an uncertainty of 2°-4°, while those from Z(2)(s)(k) are approximately a factor of 2 larger. Either result fits the T2K, MINOS, and Double Chooz measurements. Alternately, a prediction for the Dirac CP phase δD results in a peak at ± 74° (± 106°) for Z(2)(s)(k) or ± 123° (± 57°) for Z(2)(s)(k) which is consistent with the latest global fit. We also give a distribution for the leptonic Jarlskog invariant Jν which can provide further tests from measurements at T2K and NOνA.