Latest Papers in Condensed Matter Physics
Mesoscale And Nanoscale Physics
Mark W. Keller, Katy S. Gerace, Monika Arora, Erna Krisztina Delczeg-Czirjak, Justin M. Shaw, T. J. Silva
2019-03-03
We report a large spin Hall effect in the 3
transition metal alloy Ni
Cu
for 
, detected via the ferromagnetic resonance of a Permalloy (Py = Ni
Fe
) film deposited in a bilayer with the alloy. A thickness series at 
= 0.6, for which the alloy is paramagnetic at room temperature, allows us to determine the spin Hall ratio 
, spin diffusion length 
, spin mixing conductance 
, and damping 
due to spin memory loss . We compare our results with similar experiments on Py/Pt bilayers measured using the same method. Ab initio band structure calculations with disorder and spin-orbit coupling suggest an intrinsic spin Hall effect in NiCu alloys, although the experiments here cannot distinguish between extrinsic and intrinsic mechanisms.
I. A. Ado, A. Qaiumzadeh, A. Brataas, M. Titov
2019-04-10
We consider the Dzyaloshinskii-Moriya interaction (DMI) in a generalized 2D Rashba ferromagnet. From electronic grand potential, we microscopically compute the micromagnetic energy density 
associated with DMI. We demonstrate that, even in the bulk of a system, cannot be expressed through Lifshitz invariants beyond the linear order in the spin-orbit coupling strength. Additional contributions to the DMI energy density arise due to the lack of spin-rotational invariance. These contributions are shown to affect the phase diagram of magnetic states and modify the spin wave dispersion. We also prove that the standard Bychkov-Rashba model leads to vanishing DMI at zero temperature when both spin sub-bands are partly occupied.
A. Martín-Ruiz, M. Cambiaso, L. F. Urrutia
2018-09-12
Weyl semimetals (WSM) are a new class of topological materials that exhibit a bulk Hall effect due to time-reversal symmetry breaking, as well as a chiral magnetic effect due to inversion symmetry breaking. These unusual electromagnetic responses can be characterized by an axion term 
with space and time dependent axion angle 
. In this paper we compute the electromagnetic fields produced by an electric charge near to a topological Weyl semimetal with two Weyl nodes in the bulk Brillouin zone. We find that, as in ordinary metals and dielectrics, outside the WSM the electric field is mainly determined by the optical properties of the material. The magnetic field is, on the contrary, of topological origin in nature due to the magnetoelectric effect of topological phases. We show that the magnetic field exhibits a particularly interesting behavior above the WSM: the field lines begin at the surface and then end at the surface (but not at the same point). This behavior is quite different from that produced by an electric charge near the surface of a topological insulator, where the magnetic field above the surface is generated by an image magnetic monopole beneath the surface, in which case, the magnetic field lines are straight rays. The unconventional behavior of the magnetic field is an experimentally observable signature of the anomalous Hall effect in the bulk of the WSM. We discuss a simple candidate material for testing our predictions, as well as two experimental setups which must be sensitive to the effects of the induced magnetic field.
Luca Ferialdi, Ashley Setter, Marko Toroš, Chris Timberlake, Hendrik Ulbricht
2019-04-10
We consider feedback cooling in a cavityless levitated optomechanics setup, and we investigate the possibility to improve the feedback implementation. We apply optimal control theory to derive the optimal feedback signal both for quadratic (parametric) and linear (electric) feedback. We numerically compare optimal feedback against the typical feedback implementation used for experiments. In order to do so, we implement a tracking scheme that takes into account the modulation of the laser intensity. We show that such a tracking implementation allows us to increase the feedback strength, leading to faster cooling rates and lower center-of-mass temperatures.
Junpeng Hou, Zhitong Li, Qing Gu, Chuanwei Zhang
2019-04-10
Parity-time (
) symmetry, originally conceived for non-Hermitian open quantum systems, has opened an excitingly new avenue for the coherent control of light. By tailoring optical gain and loss in integrated photonic structures, symmetric non-Hermitian photonics has found applications in many fields ranging from single mode lasing to novel topological matters. Here we propose a new paradigm towards non-Hermitian photonics based on the charge-parity (
) symmetry that has the potential to control the flow of light in an unprecedented way. In particular, we consider continuous dielectric chiral materials, where the charge conjugation and parity symmetries are broken individually, but preserved jointly. Surprisingly, the phase transition between real and imaginary spectra across the exceptional point is accompanied by a dramatic change of the photonic band topology from dielectric to hyperbolic. We showcase broad applications of symmetric photonics such as all-angle polarization-dependent negative refraction materials, enhanced spontaneous emission for laser engineering, and non-Hermitian topological photonics. The symmetry opens an unexplored pathway for studying non-Hermitian photonics without optical gain/loss by connecting two previously distinct material properties: chirality and hyperbolicity, therefore providing a powerful tool for engineering many promising applications in photonics and other related fields.
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