Takahiro Matsuoka

Direct observation of a pressure-induced metal-to-semiconductor transition in lithium

Lithium, the lightest metal, has long been considered to have a ‘simple’ electronic structure that can be well explained within the nearly-free-electron model. But lithium does not stay ‘simple’ under compression: rather than becoming more free-electron-like as pressure is increased, first-principles calculations suggest that it transforms into a semi-metal or semiconductor. Experimentally, it has been shown that dense lithium adopts low-symmetry structures; there is also evidence that its resistivity increases with pressure. However, the electronic transport properties of lithium have so far not been directly monitored as a function of increasing static pressure. Here we report electrical resistance measurements on lithium in a diamond anvil cell up to pressures of 105 GPa, which reveal a significant increase in electrical resistivity and a change in its temperature dependence near 80 GPa. Our data thus provide unambiguous experimental evidence for a pressure-induced metal-to-semiconductor transition in a ‘simple’ metallic element.

Plasma devices to guide and collimate a high density of MeV electrons

The development of ultra-intense lasers has facilitated new studies in laboratory astrophysics and high-density nuclear science, including laser fusion. Such research relies on the efficient generation of enormous numbers of high-energy charged particles. For example, laser–matter interactions at petawatt (1015 W) power levels can create pulses of MeV electrons with current densities as large as 1012 A cm-2. However, the divergence of these particle beams usually reduces the current density to a few times 106 A cm-2 at distances of the order of centimetres from the source. The invention of devices that can direct such intense, pulsed energetic beams will revolutionize their applications. Here we report high-conductivity devices consisting of transient plasmas that increase the energy density of MeV electrons generated in laser–matter interactions by more than one order of magnitude. A plasma fibre created on a hollow-cone target guides and collimates electrons in a manner akin to the control of light by an optical fibre and collimator. Such plasma devices hold promise for applications using high energy-density particles and should trigger growth in charged particle optics.

Superconductivity of Ca Exceeding 25 K at Megabar Pressures

The pressure dependence of the superconducting transition temperature T c in calcium was measured up to 161 GPa. T c increased significantly with pressure and reached 25 K at 161 GPa, which is the highest observed T c for all elements. Compared with the result obtained in a recent structural experiment, T c increases within the simple cubic structure phase and becomes rather stable but still increases in the Ca-IV and Ca-V phases.

On the behavior of ultraintense laser produced hot electrons in self-excited fields

A large number of hot electrons exceeding the Alfven current can be produced when an ultraintense laser pulse irradiates a solid target. Self-excited extreme electrostatic and magnetic fields at the target rear could influence the electron trajectory. In order to investigate the influence, we measure the hot electrons when a plasma was created on the target rear surface in advance and observe an increase of the electron number by a factor of 2. This increase may be due to changes in the electrostatic potential formation process with the rear plasma. Using a one-dimensional particle-in-cell simulation, it is shown that the retardation in the electrostatic potential formation lengthens the gate time when electrons can escape from the target. The electron number escaping within the lengthened time window appears to be much smaller than the net produced number and is consistent with our estimation using the Alfven limit.

Generation of Multi-Megabar Pressure Using Nano-Polycrystalline Diamond Anvils

A nano-polycrystalline diamond was synthesized from graphite by direct conversion under high pressure. The nano-polycrystalline diamond consists of nanosized diamond grains oriented in random directions and has higher toughness and more isotropic mechanical properties than the single-crystal diamond. We generated the pressure using a pair of anvils composed of nano-polycrystalline diamond particles. The highest generated pressure achieved was 210 GPa. The generated maximum pressure was almost the same as that achieved by the single-crystal diamond anvils.

Note: high-pressure generation using nano-polycrystalline diamonds as anvil materials.

Nano-polycrystalline diamonds (NPDs) consist of nanosized diamond grains oriented in random directions. They have high toughness and isotropic mechanical properties. A NPD has neither the cleavage feature nor the anisotropy of hardness peculiar to single-crystal diamonds. Therefore, it is thought to be useful as a diamond anvil. We previously reported the usefulness of a NPD as an anvil for high-pressure development. In this study, some additional high-pressure generating tests using diamond anvils of various shapes prepared from NPDs were conducted to investigate the advantage of using NPDs for anvil applications. The results revealed that the achievable pressure value of a NPD anvil with a culet size of more than 300 μm is about 1.5 to 2 times higher than that of single-crystal diamond anvils, indicating that NPD anvils have considerable potential for large-volume diamond anvils with large culet sizes.

Phase boundary of hot dense fluid hydrogen

We investigated the phase transformation of hot dense fluid hydrogen using static high-pressure laser-heating experiments in a laser-heated diamond anvil cell. The results show anomalies in the heating efficiency that are likely to be attributed to the phase transition from a diatomic to monoatomic fluid hydrogen (plasma phase transition) in the pressure range between 82 and 106 GPa. This study imposes tighter constraints on the location of the hydrogen plasma phase transition boundary and suggests higher critical point than that predicted by the theoretical calculations.

Study of ultraintense laser propagation in overdense plasmas for fast ignition

Laser plasma interactions in a relativistic regime relevant to the fast ignition in inertial confinement fusion have been investigated. Ultraintense laser propagation in preformed plasmas and hot electron generation are studied. The experiments are performed using a 100 TW 0.6 ps laser and a 20 TW 0.6 ps laser synchronized by a long pulse laser. In the study, a self-focused ultraintense laser beam propagates along its axis into an overdense plasma with peak density 1022/cm3. Channel formation in the plasma is observed. The laser transmission in the overdense plasma depends on the position of its focus and can take place in plasmas with peak densities as high as 5×1022/cm3. The hot electron beams produced by the laser-plasma interaction have a divergence angle of ∼30°, which is smaller than that from laser-solid interactions. For deeper penetration of the laser light into the plasma, the use of multiple short pulse lasers is proposed. The latter scheme is investigated using particle-in-cell simulation. It ...

Pressure-induced superconducting state in crystalline boron nanowires

We report high-pressure induced superconductivity in boron nanowires (BNWs) with rhombohedral crystal structure. Obviously different from bulk rhombohedral boron (beta-r-B), these BNWs show a semiconductor-metal transition at much lower pressure than bulk beta-r-B. Also, we found that these BNWs become superconductors with T(c)=1.5 K at 84 GPa, at the pressure of which bulk beta-r-B is still a semiconductor, via in situ resistance measurements in a diamond-anvil cell. With increasing pressure, T(c) of the BNWs increases. The occurrence of superconductivity in the BNWs at a pressure as low as 84 GPa probably arises from the size effect.

Large amplitude fluxional behaviour of elemental calcium under high pressure

Experimental evidences are presented showing unusually large and highly anisotropic vibrations in the “simple cubic” (SC) unit cell adopted by calcium over a broad pressure ranging from 30–90 GPa and at temperature as low as 40 K. X-ray diffraction patterns show a preferential broadening of the (110) Bragg reflection indicating that the atomic displacements are not isotropic but restricted to the [110] plane. The unusual observation can be rationalized invoking a simple chemical perspective. As the result of pressure-induced s → d transition, Ca atoms situated in the octahedral environment of the simple cubic structure are subjected to Jahn-Teller distortions. First-principles molecular dynamics calculations confirm this suggestion and show that the distortion is of dynamical nature as the cubic unit cell undergoes large amplitude tetragonal fluctuations. The present results show that, even under extreme compression, the atomic configuration is highly fluxional as it constantly changes.