Yosuke Matsumoto

Quantum Criticality Without Tuning in the Mixed Valence Compound β-YbAlB4

A quantum phase transition is observed in a stoichiometric compound at ambient pressure and in zero magnetic field. Fermi liquid theory, the standard theory of metals, has been challenged by a number of observations of anomalous metallic behavior found in the vicinity of a quantum phase transition. The breakdown of the Fermi liquid is accomplished by fine-tuning the material to a quantum critical point by using a control parameter such as the magnetic field, pressure, or chemical composition. Our high-precision magnetization measurements of the ultrapure f-electron–based superconductor β-YbAlB4 demonstrate a scaling of its free energy that is indicative of zero-field quantum criticality without tuning in a metal. The breakdown of Fermi liquid behavior takes place in a mixed-valence state, which is in sharp contrast with other known examples of quantum critical f-electron systems that are magnetic Kondo lattice systems with integral valence.

Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2As2 Studied by 75As Nuclear Quadrupole Resonance and Specific Heat

We report the 75 As nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe 2 As 2 (superconducting transition temperature T c ≃3.5 K). The spin-lattice relaxation rate 1/ T 1 in the superconducting state exhibits a gradual temperature dependence with no coherence peak below T c . The quasiparticle specific heat C QP / T shows a small jump, which is about 30% of the electronic specific heat coefficient just below T c . The C QP / T suggests the existence of low-energy quasiparticle excitation at the lowest measurement temperature T =0.4 K≃ T c /10. The T dependences of 1/ T 1 and C QP / T can be explained by a multiple nodal superconducting gap scenario rather than by a multiple fully gapped s ± -wave scenario determined using simple gap analysis.

Heavy-fermion superconductivity in the quadrupole ordered state of PrV2Al20.

PrV2Al20 is a rare example of a heavy-fermion system based on strong hybridization between conduction electrons and nonmagnetic quadrupolar moments of the cubic Γ3 ground doublet. Here, we report that a high-quality single crystal of PrV2Al20 exhibits superconductivity at Tc=50  mK in the antiferroquadrupole-ordered state under ambient pressure. The heavy-fermion character of the superconductivity is evident from the specific heat jump of ΔC/T∼0.3  J/mol K(2) and the effective mass m*/m0∼140 estimated from the temperature dependence of the upper critical field. Furthermore, the high-quality single crystals exhibit double transitions at TQ=0.75  K and T*=0.65  K associated with quadrupole and octupole degrees of freedom of the Γ3 doublet. In the ordered state, the specific heat C/T shows a T(3) dependence, indicating the gapless mode associated with the quadrupole order, the octupole order, or both. The strong sensitivity to impurity of the superconductivity suggests unconventional character due to significant quadrupolar fluctuations.

Stochastic electron acceleration during spontaneous turbulent reconnection in a strong shock wave

Shocking! Particle accelerators in space The acceleration of charged particles to high energies has been a major mystery, with a number of competing theories based on plasma physics. Many include the concept of turbulence, but with different roles. For example, shock-based theories emphasize the importance of turbulence developed from an unstable shock layer, whereas turbulent reconnection theories emphasize interactions of multiple reconnection sites. Matsumoto et al. present results of a large particle-in-cell simulation and examine how electrons are accelerated in the transition layer of a fast nonrelativistic shock (see the Perspective by Ji and Zweibel). Surprisingly, they find that when the shock is strong enough, charged particles (electrons in this case) are efficiently accelerated by turbulent reconnection within a turbulent shock layer containing multiscale structures. Science, this issue p. 974; see also p. 944 Sufficiently powerful astrophysical shocks can yield relativistic particles via magnetic reconnection. [Also see Perspective by Ji and Zweibel] Explosive phenomena such as supernova remnant shocks and solar flares have demonstrated evidence for the production of relativistic particles. Interest has therefore been renewed in collisionless shock waves and magnetic reconnection as a means to achieve such energies. Although ions can be energized during such phenomena, the relativistic energy of the electrons remains a puzzle for theory. We present supercomputer simulations showing that efficient electron energization can occur during turbulent magnetic reconnection arising from a strong collisionless shock. Upstream electrons undergo first-order Fermi acceleration by colliding with reconnection jets and magnetic islands, giving rise to a nonthermal relativistic population downstream. These results shed new light on magnetic reconnection as an agent of energy dissipation and particle acceleration in strong shock waves.

Full electromagnetic Vlasov code simulation of the Kelvin-Helmholtz instability

Recent advancement in numerical techniques for Vlasov simulations and their application to cross-scale coupling in the plasma universe are discussed. Magnetohydrodynamic (MHD) simulations are now widely used for numerical modeling of global and macroscopic phenomena. In the framework of the MHD approximation, however, diffusion coefficients such as resistivity and adiabatic index are given from empirical models. Thus there are recent attempts to understand first-principle kinetic processes in macroscopic phenomena, such as magnetic reconnection and the Kelvin–Helmholtz (KH) instability via full kinetic particle-in-cell and Vlasov codes. In the present study, a benchmark test for a new four-dimensional full electromagnetic Vlasov code is performed. First, the computational speed of the Vlasov code is measured and a linear performance scaling is obtained on a massively parallel supercomputer with more than 12 000 cores. Second, a first-principle Vlasov simulation of the KH instability is performed in order ...

ELECTRON ACCELERATIONS AT HIGH MACH NUMBER SHOCKS: TWO-DIMENSIONAL PARTICLE-IN-CELL SIMULATIONS IN VARIOUS PARAMETER REGIMES

Electron accelerations at high Mach number collisionless shocks are investigated by means of two-dimensional electromagnetic particle-in-cell simulations with various Alfv´ en Mach numbers, ion-to-electron mass ratios, and the upstream electron βe (the ratio of the thermal pressure to the magnetic pressure). We find electrons are effectively accelerated at a super-high Mach number shock (MA ∼ 30) with a mass ratio of M/m = 100 and βe = 0.5. The electron shock surfing acceleration is an effective mechanism for accelerating the particles toward the relativistic regime even in two dimensions with a large mass ratio. Buneman instability excited at the leading edge of the foot in the super-high Mach number shock results in a coherent electrostatic potential structure. While multi-dimensionality allows the electrons to escape from the trapping region, they can interact with the strong electrostatic field several times. Simulation runs in various parameter regimes indicate that the electron shock surfing acceleration is an effective mechanism for producing relativistic particles in extremely high Mach number shocks in supernova remnants, provided that the upstream electron temperature is reasonably low.

Electron acceleration in a nonrelativistic shock with very high Alfvén Mach number.

Electron acceleration associated with various plasma kinetic instabilities in a nonrelativistic shock with very high Alfvén Mach number (M(A)~45) is revealed by means of a two-dimensional fully kinetic particle-in-cell simulation. Electromagnetic (ion Weibel) and electrostatic (ion-acoustic and Buneman) instabilities are strongly activated at the same time in different regions of the two-dimensional shock structure. Relativistic electrons are quickly produced predominantly by the shock surfing mechanism with the Buneman instability at the leading edge of the foot. The energy spectrum has a high-energy tail exceeding the upstream ion kinetic energy accompanying the main thermal population. This gives a favorable condition for the ion-acoustic instability at the shock front, which in turn results in additional energization. The large-amplitude ion Weibel instability generates current sheets in the foot, implying another dissipation mechanism via magnetic reconnection in a three-dimensional shock structure in the very-high-M(A) regime.

The secondary instability initiated by the three‐dimensional nonlinear evolution of the Kelvin‐Helmholtz instability

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Vlasov simulation of the interaction between the solar wind and a dielectric body

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