Current-Driven Nonvolatile Phase Change of Electronic/Magnetic/Superconducting States

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. In this presentation, I summarize our results showing that under rapid cooling based on electric pulse, 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 thermal equilibrium phase diagram [1]. Although the cooling rate utilized in low-temperature experiments is typically 2×10⁻³ to 4×10⁻¹ K/s, the use of optical/electronic pulses facilitates rapid cooling, such as 10²–10⁷ K/s. Such an unconventionally high cooling rate allows some systems to kinetically avoid a first-order phase transition, resulting in a quenched charge/spin/superconducting 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 function, such as charge [2] and spin [3, 4] degrees of freedom and even superconductivity [5]. In a microfabricated sample, we found that the application of DC current can trigger a nonthermal magnetic transition from the skyrmion phase to nonskyrmion phases [6], implying that a tantalizing variety of novel nonequilibrium phenomena and their applications in quantum materials should be explored with attention to the system size. On the other hand, real-space observations using scanning Raman microscopy revealed that complex behaviors in the phase evolution may be involved in a small-sized specimen [7]; therefore, it may be necessary to confront these complexities when constructing nanodevices based on first-order transitions.<br/><br/>[1] FK and H. Oike, Adv. Mat. <b>29</b>, 1601979 (2017). [2] H. Oike et al., Phys. Rev. B. <b>91</b>, 041101(R) (2015). [3] H. Oike, et al., Nat. Phys. <b>12</b>, 62 (2016). [4] K. Matsuura et al., Phys. Rev. B. <b>103</b>, L041106 (2021). [5] H. Oike et al., Sci. Adv. <b>4</b>, eaau3489 (2018). [6] T. Sato et al., Phys. Rev. B. 106, 144425 (2022). [7] H. Oike et al., Phys. Rev. Lett. <b>127</b>, 145701 (2021).