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1. Chiral and counter-propagating Majorana fermions in a p-wave superconductor

   https://doi.org/10.1088/1367-2630/ab5cad  

   Hu, Haiping ; Satija, Indubala I. ; Zhao, Erhai ( December 2019 , New Journal of Physics)

   Chiral and helical Majorana fermions are two archetypal edge excitations in two-dimensional topological superconductors. They emerge from systems of different Altland–Zirnbauer symmetries and characterized by$\mathbb{Z}$and${\mathbb{Z}}_2$topological invariants respectively. It seems improbable to tune a pair of co-propagating chiral edge modes to counter-propagate in a single system without symmetry breaking. Here, we explore the peculiar behaviors of Majorana edge modes in topological superconductors with an additional ‘mirror’ symmetry which changes the bulk topological invariant to$\mathbb{Z}\oplus \mathbb{Z}$type. A theoretical toy model describing the proximity structure of a Chern insulator and ap_x-wave superconductor is proposed and solved analytically to illustrate a direct transition between two topologically nontrivial phases. The weak pairing phase has two chiral Majorana edge modes, while the strong pairing phase is characterized by mirror-graded Chern number and hosts a pair of counter-propagating Majorana fermions protected by the mirror symmetry. The edge theory is worked out in detail, and implications to braiding of Majorana fermions are discussed.

    

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2. Reduction of PDF uncertainty in the measurement of the weak mixing angle at the ATLAS experiment *

   https://doi.org/10.1088/1674-1137/abe36d  

   Fu, Yao ; Yang, Siqi ; Liu, Minghui ; Han, Liang ; Hou, Tie-Jiun ; Schmidt, Carl ; Wang, Chen ; Yuan, C. -P. ( May 2021 , Chinese Physics C)

   We investigate the parton distribution function (PDF) uncertainty in the measurement of the effective weak mixing angleat the CERN Large Hadron Collider (LHC). The PDF-induced uncertainty is large in proton-proton collisions at the LHC due to the dilution effect. The measurement of the Drell-Yan forward-backward asymmetry () at the LHC can be used to reduce the PDF uncertainty in themeasurement. However, when including the full mass range of lepton pairs in thedata analysis, the correlation between the PDF updating procedure and theextraction leads to a sizable bias in the obtainedvalue. From our studies, we find that the bias can be significantly reduced by removing Drell-Yan events with invariant mass around theZ-pole region, while most of the sensitivity in reducing the PDF uncertainty remains. Furthermore, the lepton charge asymmetry in theWboson events as a function of the rapidity of the charged leptons,, is known to be another observable which can be used to reduce the PDF uncertainty in themeasurement. The constraint fromis complementary to that from, and thus no bias affects theextraction. The studies are performed using the error PDF Updating Method Package (ePump), which is based on Hessian updating methods. In this article, the CT14HERA2 PDF set is used as an example.

    

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3. Weyl Fermions and broken symmetry phases of laterally confined 3 He films

   https://doi.org/10.1088/1361-648X/acf42b  

   Wu, Hao ; Sauls, J. A. ( September 2023 , Journal of Physics: Condensed Matter)

   Broken symmetries in topological condensed matter systems have implications for the spectrum of Fermionic excitations confined on surfaces or topological defects. The Fermionic spectrum of confined (quasi-2D)³He-A consists of branches of chiral edge states. The negative energy states are related to the ground-state angular momentum,$L_z=(N/2)\hslash$, for$N/2$Cooper pairs. The power law suppression of the angular momentum,$L_z(T)\simeq (N/2)\hslash [1-\frac{2}{3}(\pi T/\mathrm{\Delta }{)}^2]$for$0⩽T\ll T_c$, in the fully gapped 2D chiral A-phase reflects the thermal excitation of the chiral edge Fermions. We discuss the effects of wave function overlap, and hybridization between edge states confined near opposing edge boundaries on the edge currents, ground-state angular momentum and ground-state order parameter of superfluid³He thin films. Under strong lateral confinement, the chiral A phase undergoes a sequence of phase transitions, first to a pair density wave (PDW) phase with broken translational symmetry at$D_{c2}\sim 16{\xi }_0$. The PDW phase is described by a periodic array of chiral domains with alternating chirality, separated by domain walls. The period of PDW phase diverges as the confinement length$D\to D_{c_2}$. The PDW phase breaks time-reversal symmetry, translation invariance, but is invariant under the combination of time-reversal and translation by a one-half period of the PDW. The mass current distribution of the PDW phase reflects this combined symmetry, and originates from the spectra of edge Fermions and the chiral branches bound to the domain walls. Under sufficiently strong confinement a second-order transition occurs to the non-chiral ‘polar phase’ at$D_{c1}\sim 9{\xi }_0$, in which a single p-wave orbital state of Cooper pairs is aligned along the channel.

    

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4. High-harmonic generation in spin and charge current pumping at ferromagnetic or antiferromagnetic resonance in the presence of spin–orbit coupling

   https://doi.org/10.1088/2515-7639/aceaad  

   Varela-Manjarres, Jalil ; Nikolić, Branislav K. ( August 2023 , Journal of Physics: Materials)

   One of the cornerstone effects in spintronics is spin pumping by dynamical magnetization that is steadily precessing (around, for example, thez-axis) with frequencyω₀due to absorption of low-power microwaves of frequencyω₀under the resonance conditions and in the absence of any applied bias voltage. The two-decades-old ‘standard model’ of this effect, based on the scattering theory of adiabatic quantum pumping, predicts that component$I^{S_z}$of spin current vector$(I^{S_x}(t),I^{S_y}(t),I^{S_z})\propto {\omega }_0$is time-independent while$I^{S_x}(t)$and$I^{S_y}(t)$oscillate harmonically in time with a single frequencyω₀whereas pumped charge current is zero$I\equiv 0$in the same adiabatic$\propto {\omega }_0$limit. Here we employ more general approaches than the ‘standard model’, namely the time-dependent nonequilibrium Green’s function (NEGF) and the Floquet NEGF, to predict unforeseen features of spin pumping: namely precessing localized magnetic moments within a ferromagnetic metal (FM) or antiferromagnetic metal (AFM), whose conduction electrons are exposed to spin–orbit coupling (SOC) of either intrinsic or proximity origin, will pump both spin$I^{S_{\alpha }}(t)$and chargeI(t) currents. All four of these functions harmonically oscillate in time at both even and odd integer multiples$N{\omega }_0$of the driving frequencyω₀. The cutoff order of such high harmonics increases with SOC strength, reaching$N_{\mathrm{m}\mathrm{a}\mathrm{x}}\simeq 11$in the one-dimensional FM or AFM models chosen for demonstration. A higher cutoff$N_{\mathrm{m}\mathrm{a}\mathrm{x}}\simeq 25$can be achieved in realistic two-dimensional (2D) FM models defined on a honeycomb lattice, and we provide a prescription of how to realize them using 2D magnets and their heterostructures.

    

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5. The Ionization and Dynamics of the Makani Galactic Wind

   https://doi.org/10.3847/1538-4357/acbfae  

   Rupke, David S. ; Coil, Alison L. ; Perrotta, Serena ; Davis, Julie D. ; Diamond-Stanic, Aleksandar M. ; Geach, James E. ; Hickox, Ryan C. ; Moustakas, John ; Petter, Grayson C. ; Rudnick, Gregory H. ; et al ( April 2023 , The Astrophysical Journal)

   The Makani galaxy hosts the poster child of a galactic wind on scales of the circumgalactic medium. It consists of a two-episode wind in which the slow, outer wind originated 400 Myr ago (Episode I;R_I= 20 − 50 kpc) and the fast, inner wind is 7 Myr old (Episode II;R_II= 0 − 20 kpc). While this wind contains ionized, neutral, and molecular gas, the physical state and mass of the most extended phase—the warm, ionized gas—are unknown. Here we present Keck optical spectra of the Makani outflow. These allow us to detect hydrogen lines out tor= 30–40 kpc and thus constrain the mass, momentum, and energy in the wind. Many collisionally excited lines are detected throughout the wind, and their line ratios are consistent with 200–400 km s⁻¹shocks that power the ionized gas, withv_shock=σ_wind. Combining shock models, density-sensitive line ratios, and mass and velocity measurements, we estimate that the ionized mass and outflow rate in the Episode II wind could be as high as those of the molecular gas:$M_{\mathrm{II}}^{\mathrm{H}\mathrm{II}}\sim M_{\mathrm{II}}^{{\mathrm{H}}_2}=(1-2)\times {10}^9{M}_{\odot }$and$\mathit{dM}/{\mathit{dt}}_{\mathrm{II}}^{\mathrm{H}\mathrm{II}}\sim \mathit{dM}/{\mathit{dt}}_{\mathrm{II}}^{{\mathrm{H}}_2}=170-250M_{\odot }$yr⁻¹. The outer wind has slowed, so that$\mathit{dM}/{\mathit{dt}}_{\mathrm{I}}^{\mathrm{H}\mathrm{II}}\sim 10M_{\odot }$yr⁻¹, but it contains more ionized gas,$M_{\mathrm{I}}^{\mathrm{H}\mathrm{II}}=5\times {10}^9$M_⊙. The momentum and energy in the recent Episode II wind imply a momentum-driven flow (p“boost” ∼7) driven by the hot ejecta and radiation pressure from the Eddington-limited, compact starburst. Much of the energy and momentum in the older Episode I wind may reside in a hotter phase, or lie further into the circumgalactic medium.

    

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