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In magnetic crystals, despite the explicit breaking of time-reversal symmetry, two equilibrium states related by time reversal are always energetically degenerate. In ferromagnets such time-reversal degeneracy can be manifested by hysteresis loops in the magnetic field dependence of the magnetization and, if metallic, in the anomalous Hall effect. Importantly, both quantities simply change signs but not their absolute sizes under time reversal, which follows from their fundamental definitions. Our integral experimental and theoretical study shows that in the metallic kagome spin ice HoAgGe subject to finite magnetic fields parallel to the kagome plane, an emergent time-reversal-like degeneracy appears between magnetic states that have the same energy and net magnetization, but different sizes of the anomalous Hall effect. These degeneracies are unraveled by finite hysteresis in the field-dependent anomalous Hall effect contrasted with the vanishing hysteresis in the magnetization, which appears only at low-temperatures T$<$4K when the kagome spin ice is fully ordered into $\sqrt{3}$$\times$$\sqrt{3}$ state. By explicitly determining the degenerate states and calculating the corresponding physical properties using a tight-binding model, we nailed down the time-reversal-like operation that transforms these degenerate states into each other. The operation is related to the nontrivial distortion of the kagome lattice in HoAgGe and is effective only because of the richness of degenerate states unique to kagome spin ice. Our work points to the powerful role of the anomalous Hall effect to diagnose hidden symmetries in frustrated spin systems., Comment: This is the initial submission version during first part of 2022, and the recent version after several round significant modifications will appear on Nature Physics later

Interplay between magnetism and electronic band topology is of central current interest in topological matter research. We use quantum oscillations as powerful tool to probe the evolution of band topology in the Dirac semimetal EuMnSb$_2$. The Eu local 4f magnetic moments display different antiferromagnetic states below 25 K and a field-polarized phase above 16 T. Upon cooling from 65 K into the field-polarized state, an exotic temperature dependent shift of oscillation peaks arises, accompanied by the development of non-zero Berry phase and a huge unconventional splitting of the oscillations. Band-structure calculations confirm the change from trivial to non-trivial band topology induced by the ferromagnetic Eu state, classifying EuMnSb$_2$ as unique magnetic topological semimetal., Comment: Spin degeneracy splitting of the Dirac band evidenced by high field SdH oscillations; Accepted for publication as Letter in PRB

We demonstrate that the new single crystal of YCu$_3$[OH(D)]$_{6.5}$Br$_{2.5}$ (YCOB) is a kagome Heisenberg antiferromagnet (KHA) without evident orphan spins ($\ll$ 0.8\%). The site mixing between polar OH$^-$ and non-polar Br$^-$ causes local distortions of Cu-O-Cu exchange paths, and gives rise to 70(2)\% of randomly distributed hexagons of alternate bonds ($\sim$ $J_1-\Delta J$ and $J_1+\Delta J$) and the rest of almost uniform hexagons ($\sim$ $J_1$) on the kagome lattice. Simulations of the random exchange model with $\Delta J$/$J_1$ = 0.7(1) show good agreement with the experimental observations, including the weak upturn seen in susceptibility and the slight polarization in magnetization. Despite the average antiferromagnetic coupling of $J_1$ $\sim$ 60 K, no conventional freezing is observed down to $T$ $\sim$ 0.001$J_1$, and the raw specific heat exhibits a nearly quadratic temperature dependence below 1 K $\sim$ 0.02$J_1$, phenomenologically consistent with a gapless (spin gap $\leq$ 0.025$J_1$) Dirac quantum spin liquid (QSL). Our result sheds new light on the theoretical understanding of the randomness-relevant gapless QSL behavior in YCOB, as well as in other relevant materials., Comment: 18 pages, 17 figures

Perpendicular magnetic anisotropy of the new ferromagnetic semiconductor (Ba,K)(Zn,Mn)$_{2}$As$_{2}$ is studied by angle-dependent x-ray magnetic circular dichroism measurements. The large magnetic anisotropy with the anisotropy field of 0.85 T is deduced by fitting the Stoner-Wohlfarth model to the magnetic-field-angle dependence of the projected magnetic moment. Transverse XMCD spectra highlights the anisotropic distribution of Mn 3$d$ electrons, where the $d_{xz}$ and $d_{yz}$ orbitals are less populated than the $d_{xy}$ state because of the $D_{2d}$ splitting arising from the elongated MnAs$_{4}$ tetrahedra. It is suggested that the magnetic anisotropy originates from the degeneracy lifting of $p$-$d_{xz}$, $d_{yz}$ hybridized states at the Fermi level and resulting energy gain due to spin-orbit coupling when spins are aligned along the $z$ direction.

Spin ices are exotic phases of matter characterized by frustrated spins obeying local ice rules, in analogy with the electric dipoles in water ice. In two dimensions, one can similarly define ice rules for in-plane Ising-like spins arranged on a kagome lattice. These ice rules require each triangle plaquette to have a single monopole, and can lead to various unique orders and excitations. Using experimental and theoretical approaches including magnetometry, thermodynamic measurements, neutron scattering and Monte Carlo simulations, we establish HoAgGe as a crystalline (i.e. non-artificial) system that realizes the kagome spin ice state. The system features a variety of partially and fully ordered states and a sequence of field-induced phases at low temperatures, all consistent with the kagome ice rule., Comment: 4 figures and 2 tables, with the figures slightly different from the formal online version

Several neutrino observatories have searched for coincident neutrino signals associated with gravitational waves induced by the merging of two black holes.No statistically significant neutrino signal in excess of background level was observed.These experiments use different neutrino detection technologies and are sensitive to various neutrino types.A combined analysis was performed on the KamLAND, Super-Kamiokande and Borexino experimental data with a frequentist statistical approach to achieve a global picture of the associated neutrino fluence.Both monochromatic and Fermi-Dirac neutrino spectra were assumed in the calculation.The final results are consistent with null neutrino signals associated with a binary of black holes merging process.The derived 90\% confidence level upper limits on the fluence and luminosity of various neutrino types are presented for neutrino energy less than 110 MeV., Comment: 9 pages, 3 figures