yangke

Scale evolution of interactions between a Weyl fermion and a heavy magnetic impurity is calculated non-perturbatively using the functional renormalization group technique. Using an expansion around the vanishing pairing gap, we derive the flow equations for all possible quartic couplings in the system. We find that contrary to conventional perturbation theory, the usual spin-spin isotropic interaction necessarily splits into two invariant parts during the scale evolution, which are fully allowed by the $SU(2)$ spin-rotation symmetry. We also find the existence of an infrared stable interacting fixed point, which can be responsible for intermediate-coupling screening effects. The calculation scheme presented here is rather general and expected to be easily applicable to various spin-spin-like interactions in fermionic systems.

Single crystals of van der Waals layered antiferroelectric and magnetic CuCrP₂S₆ show a spontaneous macroscopic polarization mediated by the defect-dipole polarization at the quasi-antipolar state. The highly tunable local ferroelectric state and the defect-dipole polarization can be achieved by a temperature specific poling procedure and survive even without the external electric field. The defect-dipole is likely related to a metastable Cu site within the van der Waals gap and is a smoking gun of a uniaxial quadruple potential well.

Abstract

Recent success in experimental and theoretical works on metal thiophosphates (MTPs) paved the way to add multiple functionalities of complex oxides, such as ferroelectricity, in 2D materials. To realize multiferroicity and magnetoelectric coupling on layered van der Waals materials, incorporating magnetic ions in the ferroelectric framework is desirable. Unfortunately, replacing the metal ion with a magnetic one in MTPs results in antiferroelectricity in which spontaneous macroscopic polarization is absent. Herein, the emergence of a tunable local ferroelectric state in antiferroelectric CuCrP₂S₆ possessing magnetic Cr³⁺ ion is reported. The spontaneous macroscopic polarization is observed, which is switchable by an external poling field through controlling a defect-dipole polarization in the quasi-antipolar state. The observations suggest that the formation of defect dipoles, which is facilitated by an order-disorder-type structural transition, is likely related to a metastable Cu site within the van der Waals gap and therefore is a smoking gun of the existence of a uniaxial quadruple potential well. The interaction between the defect-dipole polarization and dipoles in the antipolar matrix may lead to the emerging local ferroelectricity in antiferroelectric CuCrP₂S₆. The findings suggest a possibility of utilizing the local ferroelectricity of multiferroic MTPs for novel 2D applications.

Author(s): So Yeun Kim, Choong H. Kim, L. J. Sandilands, C. H. Sohn, J. Matsuno, H. Takagi, K. W. Kim, Y. S. Lee, S. J. Moon, and T. W. Noh

We investigated the electronic structure of [(SrIrO3)m,(SrTiO3)](m=1,2,and∞) superlattice (SL) thin films with optical spectroscopy and first principles calculations. Our optical results confirmed the existence of the Jeff=1/2 states in SL samples, similar to the bulk Ruddlesden-Popper series Srn+1I…

[Phys. Rev. B 94, 245113] Published Tue Dec 06, 2016

Author(s): J. J. Peng, C. Song, F. Li, Y. D. Gu, G. Y. Wang, and F. Pan

The frustration of magnetization and conductivity properties of ultrathin manganite is detrimental to their device performance, preventing their scaling down process. Here we demonstrate that the magnetism of ultrathin LaMnO3 films can be restored by a SrTiO3 capping layer, which engineers the surfa…

[Phys. Rev. B 94, 214404] Published Tue Dec 06, 2016

Animals use the neurotransmitter dopamine to encode the relationship between their responses and reward. Reinforcement learning theory (1) successfully explains the role of phasic bursts of dopamine in terms of future reward maximization. Yet, dopamine clearly plays other roles in shaping behavior that have no obvious relationship to reinforcement learning, including modulating the rate at which our subjective sense of time grows in real time. On page 1273 of this issue, Soares et al. (2) closely examine the role of dopamine in mice performing a task in which they keep track of the time between two events and make decisions about this temporal duration. The results suggest the need to reassess the leading theory of dopamine function in timing—the dopamine clock hypothesis (3). They may also help explain empirical phenomena that challenge the reinforcement learning account of dopamine function. Authors: Patrick Simen, Matthew Matell

Author(s): Fengren Fan, Zhengwei Li, Zhen Zhao, Ke Yang, and Hua Wu

The charge-spin-orbital state plays a vital role in correlated oxides with mixed transition-metal elements, and it is often a matter of debate. Here, we address this issue for the newly synthesized unusual high-valence perovskite SrFe1−xNixO3, using an analysis of crystal-field level diagrams, densi…

[Phys. Rev. B 94, 214401] Published Thu Dec 01, 2016