Electron vortex beams in a magnetic field: A new twist on Landau levels and Aharonov-Bohm states

We examine the propagation of the recently-discovered electron vortex beams in a longitudinal magnetic field. We consider both the Aharonov-Bohm configuration with a single flux line and the Landau case of a uniform magnetic field. While stationary Aharonov-Bohm modes represent Bessel beams with flux- and vortex-dependent probability distributions, stationary Landau states manifest themselves as non-diffracting Laguerre-Gaussian beams. Furthermore, the Landau-state beams possess field- and vortex-dependent phases: (i) the Zeeman phase from coupling the quantized angular momentum to the magnetic field and (ii) the Gouy phase, known from optical Laguerre-Gaussian beams. Remarkably, together these phases determine the structure of Landau energy levels. This unified Zeeman-Landau-Gouy phase manifests itself in a nontrivial evolution of images formed by various superpositions of modes. We demonstrate that, depending on the chosen superposition, the image can rotate in a magnetic field with either (i) Larmor, (ii) cyclotron (double-Larmor), or (iii) zero frequency. At the same time, its centroid always follows the classical cyclotron trajectory, in agreement with the Ehrenfest theorem. Remarkably, the non-rotating superpositions reproduce stable multi-vortex configurations that appear in rotating superfluids. Our results open up an avenue for the direct electron-microscopy observation of fundamental properties of free quantum electron states in magnetic fields.Comment: 21 pages, 10 figures, 1 table, to appear in Phys. Rev.

Magnetic transitions and magnetodielectric effect in the antiferromagnet SrNdFeO4_4

We investigated the magnetic phase diagram of single crystals of SrNdFeO$_{4}$ by measuring the magnetic properties, the specific heat and the dielectric permittivity. The system has two magnetically active ions, Fe$^{3+}$ and Nd$^{3+}$. The Fe$^{3+}$ spins are antiferromagnetically ordered below 360 K with the moments lying in the ab-plane, and undergo a reorientation transition at about 35-37 K to an antiferromagnetic order with the moments along the c-axis. A short-range, antiferromagnetic ordering of Nd$^{3+}$ along the c-axis was attributed to the reorientation of Fe$^{3+}$ followed by a long-range ordering at lower temperature [S. Oyama {\it et al.} J. Phys.: Condens. Matter. {\bf 16}, 1823 (2004)]. At low temperatures and magnetic fields above 8 T, the Nd$^{3+}$ moments are completely spin-polarized. The dielectric permittivity also shows anomalies associated with spin configuration changes, indicating that this compound has considerable coupling between spin and lattice. A possible magnetic structure is proposed to explain the results.Comment: 8 pages, 10 figures, submitted to PR

Dissipationless Spin Current between Two Coupled Ferromagnets

We demonstrate the general principle which states that a dissipationless spin current flows between two coupled ferromagnets if their magnetic orders are misaligned. This principle applies regardless the two ferromagnets are metallic or insulating, and also generally applies to bulk magnetic insulators. On a phenomenological level, this principle is analogous to Josephson effect, and yields a dissipationless spin current that is independent from scattering. The microscopic mechanisms for the dissipationless spin current depend on the systems, which are elaborated in details. A uniform, static magnetic field is further proposed to be an efficient handle to create the misaligned configuration and stabilize the dissipationless spin current.Comment: 10 pages, 5 figure

A visual tool for assessing tension-resolving models in the H0H_0-σ8\sigma_8 plane

Beyond-$\Lambda$CDM models, which were proposed to resolve the "Hubble tension", often have an impact on the discrepancy in the amplitude of matter clustering, the "$\sigma_8$-tension". To explore the interplay between the two tensions, we propose a simple method to visualize the relation between the two parameters $H_0$ and $\sigma_8$: For a given extension of the $\Lambda$CDM model and data set, we plot the relation between $H_0$ and $\sigma_8$ for different amplitudes of the beyond-$\Lambda$CDM physics. We use this visualization method to illustrate the trend of selected cosmological models, including non-minimal Higgs-like inflation, early dark energy, a varying effective electron mass, an extra number of relativistic species and modified dark energy models. We envision that the proposed method could be a useful diagnostic tool to illustrate the behaviour of complex cosmological models with many parameters in the context of the $H_0$ and $\sigma_8$ tensions.Comment: 19 pages, 3 figures, 5 table

Two-photon Lithography for 3D Magnetic Nanostructure Fabrication

Ferromagnetic materials have been utilised as recording media within data storage devices for many decades. Confinement of the material to a two dimensional plane is a significant bottleneck in achieving ultra-high recording densities and this has led to the proposition of three dimensional (3D) racetrack memories that utilise domain wall propagation along nanowires. However, the fabrication of 3D magnetic nanostructures of complex geometry is highly challenging and not easily achievable with standard lithography techniques. Here, by using a combination of two-photon lithography and electrochemical deposition, we show a new approach to construct 3D magnetic nanostructures of complex geometry. The magnetic properties are found to be intimately related to the 3D geometry of the structure and magnetic imaging experiments provide evidence of domain wall pinning at a 3D nanostructured junction