Micro-optics for ultra-intense lasers

金沢大学先端科学・社会共創推進機構Table-top, femtosecond lasers provide the highest light intensities capable of extreme excitation of matter. A key challenge, however, is the efficient coupling of light to matter, a goal addressed by target structuring and laser pulse-shaping. Nanostructured surfaces enhance coupling but require “high contrast” (e.g., for modern ultrahigh intensity lasers, the peak to picosecond pedestal intensity ratio >1012) pulses to preserve target integrity. Here, we demonstrate a foam target that can efficiently absorb a common, low contrast 105 (in picosecond) laser at an intensity of 5 × 1018 W/cm2, giving ∼20 times enhanced relativistic hot electron flux. In addition, such foam target induced “micro-optic” function is analogous to the miniature plasma-parabolic mirror. The simplicity of the target—basically a structure with voids having a diameter of the order of a light wavelength—and the efficacy of these micro-sized voids under low contrast illumination can boost the scope of high intensity lasers for basic science and for table-top sources of high energy particles and ignition of laser fusion targets

High magneto-Seebeck effect at room temperature in Bi1.8_{1.8}Sb0.2_{0.2}Te3−y_{3-y}Sey_y crystal

We report thermoelectric and electrical transport properties of Bi$_{1.8}$Sb$_{0.2}$Te$_{3-y}$Se$_y$ by tuning y. In contrast to the reported p-type conductivity of the end compounds with y = 0 and 3, a dominant n-type conduction mechanism is observed for y = 1.5 from the Hall measurement. Intriguingly, the magneto-Seebeck consequence is enhanced up to ∼ 20 times for y = 1.5 compared to the end members. The reasonable value of magnetoresistance with an anisotropic character with respect to the direction of the magnetic field is observed at low temperature, which decreases with increasing temperature. The density of state at the Fermi level near room temperature correlates high Seebeck coefficient as well as magneto-Seebeck effect. High magneto-Seebeck effect at room temperature is promising for the application

Coexisting ferromagnetic component and negative magnetoresistance at low temperature in single crystals of the VdW material GaGeTe

We report magnetoresistance and magnetization studies of single-crystal GaGeTe, which has been proposed as a Van der Waals material. Semi-metallic character is observed in the temperature (T) variation of resistivity (ρ), following ρ(T) ​$∝$ ​T$^2$ ​at low temperature with a slope compatible with the usual spin-fluctuating system. Magnetoresistance (MR) at 2 ​K is negative and strongly dependent on the direction of the magnetic field (H) with respect to the crystallographic c-axis. MR changes sign with increasing temperature above ∼ 100 ​K, when H is applied along the c-axis. Hall measurements indicate the p-type conductivity with a considerable hole concentration of ∼ 8.7 ​× ​10$^{19}$ ​cm$^{−3}$. Angle-resolved photoemission spectroscopy reproduces the reported results and confirms a peculiar dispersion shape of the hole-like band at the bulk high-symmetry T point near the Fermi energy indicating band inversion. Magnetic hysteresis measurement at 2 ​K shows diamagnetic behaviour at high-H, whereas a ferromagnetic (FM)-like magnetic hysteresis loop is observed at low-H in between ​± ​4 ​kOe. The FM component disappears close to 3 ​K. Signature of spin-fluctuation in ρ(T), negative MR, and low-T FM component without 3d or 4f impurities in GaGeTe is attractive for the fundamental interest

Anisotropic negative-ion emission from cluster nanoplasmas

Recent experiments have shown that the Enhanced Charge Transfer by Rydberg Excited Clusters (ECTREC) reduces the highly charged ions very efficiently to neutral atoms and negative ions with little loss of momentum. Neutral-atom emission is anisotropic with respect to the laser polarization and the anisotropy is larger than that of the ion emission from Coulomb explosion of isolated single clusters. In such a scenario, it is expected that the negative-ion emission (like neutrals) should be anisotropic and have larger propensity along the laser polarization than in the perpendicular direction. Further, it may be anticipated that negative-ion emission is more anisotropic than neutral-atom emission if ECTREC is taken in to account. We demonstrate that the negative-ion emission is anisotropic. Contrary to expectations, the negative-ion emission anisotropy is not more than that of the neutral-atom emission. We show that this can be rationalized if low-energy (about 10 eV) electron collisional detachment of the negative ions is taken into account. Electron collisional detachment depletes the negative-ion yield preferentially along the laser polarization direction and reduces the negative-ion emission anisotropy

Magnetic order and surface state gap in (Sb0.95Cr0.05)2Te3(Sb_{0.95}Cr_{0.05})_{2}Te_{3}

Magnetic transition element doping in topological insulators, which breaks the time-reversal symmetry, gives rise to the diverse range of exotic consequences, though proper understanding of the magnetic order has rarely been attempted by using any microscopic experiments. We report the occurrence of the magnetic order in $(Sb_{0.95}Cr_{0.05})_{2}Te_{3}$ using the muon spin relaxation studies. The asymmetry curve at low temperature ($T$) shows an evidence of a damped oscillation, providing a clue about the internal magnetic field ($H_{int}$), which follows $H_{int}$($T$)=$H_{int}$(0)[1−$T$/$T_C$]$^β$ with ordering temperature $T_C$≈6.1 K and critical exponent $β$≈0.22. The critical exponent is close to the two-dimensional XY-type interaction. The magnetization curves at low $T$ exhibit a ferromagnetic behavior at low field ($H$) and the de Haas–van Alphen (dHvA) effect at high $H$. The analysis of the dHvA oscillation proposes the charge carrier that acts like a massive Dirac fermion. The Berry phase, as obtained from the Landau-level fan diagram, suggests a surface state gap at the Dirac point. The complex electronic structure is discussed by correlating the magnetic order attributed to the Cr doping in $Sb_2Te_3$