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Abstract The microscopic origins of emergent behaviours in condensed matter systems are encoded in their excitations. In ordinary magnetic materials, single spin-flips give rise to collective dipolar magnetic excitations called magnons. Likewise, multiple spin-flips can give rise to multipolar magnetic excitations in magnetic materials with spin S ≥ 1. Unfortunately, since most experimental probes are governed by dipolar selection rules, collective multipolar excitations have generally remained elusive. For instance, only dipolar magnetic excitations have been observed in isotropic S = 1 Haldane spin systems. Here, we unveil a hidden quadrupolar constituent of the spin dynamics in antiferromagnetic S = 1 Haldane chain material Y 2 BaNiO 5 using Ni L 3 -edge resonant inelastic x-ray scattering. Our results demonstrate that pure quadrupolar magnetic excitations can be probed without direct interactions with dipolar excitations or anisotropic perturbations. Originating from on-site double spin-flip processes, the quadrupolar magnetic excitations in Y 2 BaNiO 5 show a remarkable dual nature of collective dispersion. While one component propagates as non-interacting entities, the other behaves as a bound quadrupolar magnetic wave. This result highlights the rich and largely unexplored physics of higher-order magnetic excitations. 

Abstract Optical manipulation of coherent phonon frequency in two-dimensional (2D) materials could advance the development of ultrafast phononics in atomic-thin platforms. However, conventional approaches for such control are limited to doping, strain, structural or thermal engineering. Here, we report the experimental observation of strong laser-polarization control of coherent phonon frequency through time-resolved pump-probe spectroscopic study of van der Waals (vdW) materials Fe 3 GeTe 2 . When the polarization of the pumping laser with tilted incidence is swept between in-plane and out-of-plane orientations, the frequencies of excited phonons can be monotonically tuned by as large as 3% (~100 GHz). Our first-principles calculations suggest the strong planar and vertical inter-atomic interaction asymmetry in layered materials accounts for the observed polarization-dependent phonon frequencies, as in-plane/out-of-plane polarization modifies the restoring force of the lattice vibration differently. Our work provides insightful understanding of the coherent phonon dynamics in layered vdW materials and opens up new avenues to optically manipulating coherent phonons. 

The layered perovskite Ca₃Mn₂O₇(CMO) is a hybrid improper ferroelectric candidate proposed for room temperature multiferroicity, which also displays negative thermal expansion behavior due to a competition between coexisting polar and nonpolar phases. However, little is known about the atomic-scale structure of the polar/nonpolar phase coexistence or the underlying physics of its formation and transition. In this work, we report the direct observation of double bilayer polar nanoregions (db-PNRs) in Ca_2.9Sr_0.1Mn₂O₇using aberration-corrected scanning transmission electron microscopy (S/TEM). In-situ TEM heating experiments show that the db-PNRs can exist up to 650 °C. Electron energy loss spectroscopy (EELS) studies coupled with first-principles calculations demonstrate that the stabilization mechanism of the db-PNRs is directly related to an Mn oxidation state change (from 4+ to 2+), which is linked to the presence of Mn antisite defects. These findings open the door to manipulating phase coexistence and achieving exotic properties in hybrid improper ferroelectric.

 

Spin chains in solid state materials are quintessential quantum systems with potential applications in spin-based logic, memory, quantum communication, and computation. A critical challenge is the experimental determination of spin lifetimes with the ultimate goal of increasing it. Local measurements by scanning tunneling microscopy (STM) have demonstrated the importance of decoupling spins from their environment, with markedly improved lifetimes in spin chains on the surfaces of band insulators. In this work we use low-temperature scanning tunneling microscopy to reveal long-lifetime excitations in a chain of spin-1/2 electrons embedded in a charge density wave Mott insulator, 1T-TaS 2 . Naturally occurring domain walls trap chains of localized spin-1/2 electrons in nearby sites, whose energies lie inside the Mott gap. Spin-polarized measurements on these sites show distinct two-level switching noise, as well as negative differential resistance in the dI/dV spectra, typically associated with spin fluctuations. The excitations show exceptionally long lifetimes of a few seconds at 300 mK. Our work suggests that layered Mott insulators in the chalcogenide family, which are amenable to exfoliation and lithography, may provide a viable platform for quantum applications. 

Ferroelectric hafnium and zirconium oxides have undergone rapid scientific development over the last decade, pushing them to the forefront of ultralow-power electronic systems. Maximizing the potential application in memory devices or supercapacitors of these materials requires a combined effort by the scientific community to address technical limitations, which still hinder their application. Besides their favorable intrinsic material properties, HfO2–ZrO2 materials face challenges regarding their endurance, retention, wake-up effect, and high switching voltages. In this Roadmap, we intend to combine the expertise of chemistry, physics, material, and device engineers from leading experts in the ferroelectrics research community to set the direction of travel for these binary ferroelectric oxides. Here, we present a comprehensive overview of the current state of the art and offer readers an informed perspective of where this field is heading, what challenges need to be addressed, and possible applications and prospects for further development.

 

Data files for the manuscript "Quadrupolar magnetic excitations in an isotropic spin-1 antiferromagnet".

Reference: A. Nag, A. Nocera, S. Agrestini, M. Garcia-Fernandez, A. C. Walters, Sang-Wook Cheong, S. Johnston, and Ke-Jin Zhou, "Quadrupolar magnetic excitations in an isotropic spin-1 antiferromagnet". arXiv:2111.03625 (2021).

Preprint: arXiv:2111.03625 (2021), URL: https://arxiv.org/abs/2111.03625