Hiroshi Oike

Robust metastable skyrmions and their triangular–square lattice structural transition in a high-temperature chiral magnet

Skyrmions, topologically protected nanometric spin vortices, are being investigated extensively in various magnets. Among them, many structurally chiral cubic magnets host the triangular-lattice skyrmion crystal (SkX) as the thermodynamic equilibrium state. However, this state exists only in a narrow temperature and magnetic-field region just below the magnetic transition temperature Tc, while a helical or conical magnetic state prevails at lower temperatures. Here we describe that for a room-temperature skyrmion material, β-Mn-type Co 8Zn 8Mn 4, a field-cooling via the equilibrium SkX state can suppress the transition to the helical or conical state, instead realizing robust metastable SkX states that survive over a very wide temperature and magnetic-field region. Furthermore, the lattice form of the metastable SkX is found to undergo reversible transitions between a conventional triangular lattice and a novel square lattice upon varying the temperature and magnetic field. These findings exemplify the topological robustness of the once-created skyrmions, and establish metastable skyrmion phases as a fertile ground for technological applications.

Phase-change memory function of correlated electrons in organic conductors

Phase-change memory (PCM), a promising candidate for next-generation non-volatile memories, exploits quenched glassy and thermodynamically stable crystalline states as reversibly switchable state variables. We demonstrate PCM functions emerging from a charge-configuration degree of freedom in strongly correlated electron systems. Non-volatile reversible switching between a high-resistivity charge-crystalline (or charge-ordered) state and a low-resistivity quenched state, charge glass, is achieved experimentally via heat pulses supplied by optical or electrical means in organic conductors

Quenching of Charge and Spin Degrees of Freedom in Condensed Matter

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Skyrmion lattice structural transition in MnSi

-(BEDT-TTF)

Current‐Induced Nucleation and Annihilation of Magnetic Skyrmions at Room Temperature in a Chiral Magnet

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Anomalous metallic behaviour in the doped spin liquid candidate κ -(ET) 4 Hg 2.89 Br 8

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Kinetic approach to superconductivity hidden behind a competing order

. Switching that is one order of magnitude faster is observed in another isostructural material that requires faster cooling to kinetically avoid charge crystallization, indicating that the materials critical cooling rate can be useful guidelines for pursuing a faster correlated-electron PCM function.

Interplay between topological and thermodynamic stability in a metastable magnetic skyrmion lattice

Electrons in condensed matter have internal degrees of freedom, such as charge, spin, and orbital, leading to various forms of ordered states through phase transitions. However, in individual materials, a charge/spin/orbital ordered state of the lowest temperature is normally uniquely determined in terms of the lowest-energy state, i.e., the ground state. Here, recent results are summarized showing that under rapid cooling, this principle does not necessarily hold, and thus, the cooling rate is a control parameter of the lowest-temperature state beyond the framework of the thermoequilibrium phase diagram. Although the cooling rate utilized in low-temperature experiments is typically 2 × 10-3 to 4 × 10-1 K s-1 , the use of optical/electronic pulses facilitates rapid cooling, such as 102 -103 K s-1 . Such an unconventionally high cooling rate allows some systems to kinetically avoid a first-order phase transition, resulting in a quenched charge/spin state that differs from the ground state. It is also demonstrated that quenched states can be exploited as a non-volatile state variable when designing phase-change memory functions. The present findings suggest that rapid cooling is useful for exploring and controlling the metastable electronic/magnetic state, which is potentially hidden behind the ground state.

Pressure-Induced Mott Transition in an Organic Superconductor with a Finite Doping Level

A triangular-to-square lattice transition of topological magnetic particles (skyrmions) was observed in a chiral magnet MnSi. Magnetic skyrmions exhibit particle-like properties owing to the topology of their swirling spin texture, providing opportunities to study crystallization of topological particles. However, they mostly end up with a triangular lattice, and thus, the packing degree of freedom in the skyrmion particles has been overlooked so far. We report a structural transition of the skyrmion lattice in MnSi. By use of small-angle neutron scattering, we explore a metastable skyrmion state spreading over a wide temperature and magnetic field region, after thermal quenching. The quenched skyrmions undergo a triangular-to-square lattice transition with decreasing magnetic field at low temperatures. Our study suggests that various skyrmion lattices can emerge at low temperatures, where the skyrmions exhibit distinct topological nature and high sensitivity to the local magnetic anisotropy arising from the underlying chemical lattice.