Unusual multipolar order in the unconventional superconductor CeRh2As2

最近发现的超导体CeRh2As2有望提供独特的机会来研究由Ce独特的局部环境提供的物质的非传统状态。当T_c=0.26K时，超导电性表现出前所未有的从低场自旋单态到高场自旋三重态的场致相变。这很可能是由于Ce位置缺乏局域反转对称性和伴随的自旋-轨道耦合所致。此外，我们在T0≈0.4K处发现了一个几乎不具有磁性的额外相变。T0随面内磁场的增加而升高，在低于T0的9T以上出现新相。我们假设T0级起源于四次或更高阶矩，通过近藤相互作用的两个低位双星的混合例外地允许这样的情况。然而，具体的机制仍有待澄清。理解T_0序是很重要的，因为它不仅是一种新的序，而且还影响到非常规超导的类型。这一建议的主要目的是生长高质量的、部分取代的单晶及其详细的高场表征，以便研究T0级的性质。我们计划建立完整的磁相图，这需要20特斯拉以上的高磁场。通过化学取代来调整相关参数将导致T0顺序和超导电性的变化，这应该会揭示潜在的物理意义。为此，我们将：(1)在水平温度梯度下，优化(Ce，La)Rh_2As_2，Ce(Rh，Ir)_2As_2和CeRh_2(As，P)_2单晶的合成。使用这些晶体，我们将(2)在MPI CPfS和德累斯顿霍奇菲尔德磁工(HLD)上进行高达70特斯拉的热容、磁化和传输测量。

The recently discovered superconductor CeRh2As2 promises unique opportunities to study unconventional states of matter given by the distinctive local environment of Ce. Superconductivity with Tc = 0.26 K shows an unprecedented field-induced transition between a low-field spin-singlet to a high-field spin-triplet state. This most probably is due to by the lack of local inversion symmetry at the Ce site and accompanying spin-orbit coupling. Moreover, we identified an additional transition at T0 ≈ 0.4 K that has a nearly non-magnetic nature. T0 rises with increasing in-plane magnetic fields and a new phase appears above 9 T below T0. We suppose the T0 order to originate from quadruple or higher-order moments, exceptionally allowed by a mixing of two low-lying doublets through Kondo interactions. The detailed mechanism remains, however, to be clarified. Understanding the T0 order is important because not only it is a new type of order but also it affects the type of unconventional superconductivity. The main objects of this proposal are the growth of high-quality, partially substituted single crystals and their detailed high-field characterization in order to study the nature of the T0 order. We plan to establish complete magnetic phase diagrams, which require high magnetic fields above 20 tesla. Tuning relevant parameters by chemical substitution will introduce a change in the T0 order and superconductivity that should shed light on the underlying physics. For this purpose, we will (1) optimize the synthesis of quality-improved pristine and chemically doped (Ce,La)Rh2As2, Ce(Rh,Ir)2As2, and CeRh2(As,P)2 single crystals under a horizontal temperature gradient. Using these crystals, we will (2) perform heat-capacity, magnetization, and transport measurements both at the MPI CPfS and at the Hochfeld-Magnetlabor Dresden (HLD) up to 70 tesla.