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This paper presents a family of new black holes with a nontrivial horizon in Einstein-$R^2$ gravity with a negative cosmological constant. The solutions are found at the critical point where the coefficient of the $R^2$ term is fixed and the phase structure of AdS black holes of the theory is reversed. These solutions possess vanishing entropy. We also analyze the case in general $F(R)$ gravity, of which the Einstein-$R^2$ gravity is a special case. We arrive at the conclustion that the solutions with arbitrary horizons mentioned above should have $0$ entropy., Comment: 6 pages, no figures

Antiferromagnets exhibiting giant anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) have recently aroused broad interest, not only for their potential applications in future electronic devices, but also because of the rich physics arising from the Berry curvature near the Fermi level. $\rm{Co_{1/3}NbS_2}$, by intercalating $\rm{Co^{2+}}$ ions between $\rm{NbS_2}$ layers, is a quasi-two-dimensional layered antiferromagnet with a chiral lattice. A large AHE has been observed in $\rm{Co_{1/3}NbS_2}$, but its origin is under debate. In this letter, we report the large AHE and ANE in exfoliated $\rm{Co_{1/3}NbS_2}$ flakes. By analyzing the thermoelectric data via the Mott relation, we determined that the observed large AHE and ANE primarily result from the intrinsic Berry curvature. We also observed the magnetic domains in $\rm{Co_{1/3}NbS_2}$ by reflective magnetic circular dichroism measurements. Combined with electrical transport measurements, we confirmed that the magnetic reversal in $\rm{Co_{1/3}NbS_2}$ is determined by domain wall motion, and the critical field ($H_c$) exhibits a memory effect of consecutive magnetic sweeps. Our work provides insight into the topological properties of $\rm{Co_{1/3}NbS_2}$ and paves the way to studying the spin configuration and magnetic domain dynamics in this fascinating antiferromagnet., Comment: 8 pages, 4 figures

We obtain new vacuum static black hole solutions with anisotropic horizons in Einstein-Gauss-Bonnet gravity with a negative cosmological constant in five dimensions. The translational invariance along one direction on the 3-dimensional horizon cross section is broken. The Gauss-Bonnet coupling {\alpha} is at the critical point where there is one single AdS vacuum. These solutions does not appear in the form of a warped product, i.e. they lack a common warping factor, and the metric contains 2 arbitrary functions, h(r) of the radial coordinate r and H(y) of the horizon coordinate y -- some degeneracy in the metric. The nontrivial horizon and the degeneracy may be closely related to the critical value of {\alpha}. We introduce the process of obtaining the solutions and some of their properties, and also prove a uniqueness theorem for the case when there is a common warping factor for the rest two directions., Comment: 8pages, no figures. arXiv admin note: text overlap with arXiv:2105.08482

Natural superlattice structures $\rm{(MnBi_2Te_4)(Bi_2Te_3)}$$_n$ ($n$ = 1, 2,...), in which magnetic $\rm{MnBi_2Te_4}$ layers are separated by nonmagnetic $\rm{Bi_2Te_3}$ layers, hold band topology, magnetism and reduced interlayer coupling, providing a promising platform for the realization of exotic topological quantum states. However, their magnetism in the two-dimensional limit, which is crucial for further exploration of quantum phenomena, remains elusive. Here, complex ferromagnetic (FM)-antiferromagnetic (AFM) coexisting ground states that persist up to the 2-septuple layers (SLs) limit are observed and comprehensively investigated in $\rm{MnBi_4Te_7}$ ($n$ = 1) and $\rm{MnBi_6Te_{10}}$ ($n$ = 2). The ubiquitous Mn-Bi site mixing modifies or even changes the sign of the subtle inter-SL magnetic interactions, yielding a spatially inhomogeneous interlayer coupling. Further, a tunable exchange bias effect is observed in $\rm{(MnBi_2Te_4)(Bi_2Te_3)}$$_n$ ($n$ = 1, 2), arising from the coupling between the FM and AFM components in the ground state. Our work highlights a new approach toward the fine-tuning of magnetism and paves the way for further study of quantum phenomena in $\rm{(MnBi_2Te_4)(Bi_2Te_3)}$$_n$ ($n$ = 1, 2,...) as well as their magnetic applications., Comment: 9 pages, 4 figures

We study magneto-transport in heterostructures composed of the van der Waals antiferromagnet CrPS$_{4}$ and the heavy metals Pt and Pd. The transverse resistance (R$_{xy}$) signal reveals the spin-flop transition of CrPS$_{4}$ and a strongly enhanced magnetic ordering temperature (>300 K), which might originate from a strong spin-orbit coupling at the interface. While CrPS$_{4}$/Pt devices allow for easy detection of the spin-flop transition, CrPS$_{4}$/Pd devices show a more substantial enhancement in magnetic ordering temperature and exhibit a topological Hall effect signal, possibly related to chiral spin structures at the interface. The longitudinal magnetoresistance (R$_{xx}$) results from a combination of spin-Hall magnetoresistance and the negative magnetoresistance that can be explained by a field-induced change of the electronic band structure of CrPS$_{4}$., Comment: 19 pages, 4 figures

We investigate vacuum static black hole solutions of Einstein-Gauss-Bonnet gravity with a negative cosmological constant in five dimensions. These are solutions with horizons of nontrivial topologies. The first one possesses a horizon with the topology $S^1 \times H^2$, and a varying Gauss-Bonnet coupling constant $\alpha$. By looking into its thermodynamic properties, we find that its specific heat capacity with fixed volume is negative, therefore it is thermodynamically unstable. The second one is equipped with a so-called "Sol-manifold" as its horizon, and interestingly, the product of the Gauss-Bonnet coupling constant $\alpha$ and the cosmological constant $\Lambda$ is fixed. For the second solution, the total energy and entropy vanish. These results enlarge our knowledge of both topological black holes in higher dimensions and the property of higher curvature corrections of gravitational theories., Comment: 6pages, no figures, some references added in the 2nd version

The intrinsic magnetic layered topological insulator MnBi2Te4 with nontrivial topological properties and magnetic order has become a promising system for exploring exotic quantum phenomena such as quantum anomalous Hall effect. However, the layer-dependent magnetism of MnBi2Te4, which is fundamental and crucial for further exploration of quantum phenomena in this system, remains elusive. Here, we use polar reflective magnetic circular dichroism spectroscopy, combined with theoretical calculations, to obtain an in-depth understanding of the layer-dependent magnetic properties in MnBi2Te4. The magnetic behavior of MnBi2Te4 exhibits evident odd-even layer-number effect, i.e. the oscillations of the coercivity of the hysteresis loop (at {\mu}0Hc) and the spin-flop transition (at {\mu}0H1), concerning the Zeeman energy and magnetic anisotropy energy. In the even-number septuple layers, an anomalous magnetic hysteresis loop is observed, which is attributed to the thickness-independent surface-related magnetization. Through the linear-chain model, we can clarify the odd-even effect of the spin-flop field and determine the evolution of magnetic states under the external magnetic field. The mean-field method also allows us to trace the experimentally observed magnetic phase diagrams to the magnetic fields, layer numbers and especially, temperature. Overall, by harnessing the unusual layer-dependent magnetic properties, our work paves the way for further study of quantum properties of MnBi2Te4., Comment: 46 pages, 16 figures

The effect of interface roughness of ferromagnetic and antiferromagnetic layers on exchange bias is still not well understood. In this report we have investigated the effect of surface roughness in (111)-oriented antiferromagnetic CoO films on exchange bias with ferromagnetic Fe grown on top. The surface roughness is controlled at the atomic scale, over a range below ~ 0.35 nm, by varying layer thickness of the CoO films. It is observed that both exchange bias field ($H_{E}$) and coercivity ($H_{C}$) extensively depend on the atomic scale roughness of the CoO (111) at the interface with Fe film. An opposite dependence of $H_{E}$ and $H_{C}$ on interface roughness was found, which was ascribed to partially compensated spin states induced by the atomic roughness at the fully uncompensated CoO (111) surfaces and was corroborated using the Monte Carlo simulations. Moreover, the onset temperature for $H_{C}$ is found to be up to ~ 80 K below the blocking temperature ($T_{B}$) and the temperature dependence of $H_{C}$ follows the power law with a critical exponent equal to one, which indicates that, in this system, $H_{C}$ is more of an interface-related property than $H_{E}$.

Quantum interference gives rise to the asymmetric Fano resonance line shape when the final states of an electronic transition follows within a continuum of states and a discrete state, which has significant applications in optical switching and sensing. The resonant optical phenomena associated with Fano resonance have been observed by absorption spectra, Raman spectra, transmission spectra, etc., but have rarely been reported in photoluminescence (PL) spectroscopy. In this work, we performed spectroscopic studies on layered chromium thiophosphate (CrPS4), a promising ternary antiferromagnetic semiconductor with PL in near-infrared wavelength region and observed Fano resonance when CrPS4 experiences phase transition into the antiferromagnetic state below N\'eel temperature (38 K). The photoluminescence of the continuum states results from the d band transitions localized at Cr3+ ions, while the discrete state reaches saturation at high excitation power and can be enhanced by the external magnetic field, suggesting it is formed by an impurity level from extra atomic phosphorus. Our findings provide insights into the electronic transitions of CrPS4 and their connection to its intrinsic magnetic properties., Comment: 16 pages, 4 figures

We investigate the holographic complexity growth rate of a conformal field theory in a FLRW universe. We consider two ways to realize a FLRW spacetime from an Anti-de Sitter Schwarzschild geometry. The first one is obtained by introducing a new foliation of the Schwarzschild geometry such that the conformal boundary takes the FLRW form. The other one is to consider a brane universe moving in the Schwarzschild background. For each case, we compute the complexity growth rate in a closed universe and a flat universe by using both the complexity-volume and complexity-action dualities. We find that there are two kinds of contributions to the growth rate: one is from the interaction among the degrees of freedom, while the other one from the change of the spatial volume of the universe. The behaviors of the growth rate depend on the details to realize the FLRW universe as well as the holographic conjecture for the complexity. For the realization of the FLRW universe on the asymptotic boundary, the leading divergent term for the complexity growth rate obeys a volume law which is natural from the field theory viewpoint. For the brane universe scenario, the complexity-volume and complexity-action conjectures give different results for the closed universe case. A possible explanation of the inconsistency when the brane crosses the black hole horizon is given based on the Lloyd bound., Comment: double columns, 14 pages and 11 figures, minor revision and reference added

Recently, a practical approach to holographic renormalization has been developed based on the Hamilton-Jacobi formulation. Using a simple Einstein-scalar theory, we clarify that this approach does not conflict with the Hamiltonian constraint as it seems. Then we apply it to the holographic renormalization of massive gravity. We assume that the shift vector is falling off fast enough asymptotically. We derive the counterterms up to the boundary dimension d=4. Interestingly, we find that the conformal anomaly can even occur in odd dimensions, which is different from the Einstein gravity. We check that the counterterms cancel the divergent part of the on-shell action at the background level. At the perturbation level, they are also applicable in several time-dependent cases., Comment: 30 pages. Revised version accepted for publication in JHEP

We study the effect of the Gauss-Bonnet term on the complexity growth rate of dual field theory using the "Complexity--Volume" (CV) and CV2.0 conjectures. We investigate the late time value and full time evolution of the complexity growth rate of the Gauss-Bonnet black holes with horizons with zero curvature ($k=0$), positive curvature ($k=1$) and negative curvature ($k=-1$) respectively. For the $k=0$ and $k=1$ cases we find that the Gauss-Bonnet term suppresses the growth rate as expected, while in the $k=-1$ case the effect of the Gauss-Bonnet term may be opposite to what is expected. The reason for it is briefly discussed, and the comparison of our results to the result obtained by using the "Complexity--Action" (CA) conjecture is also presented. We also briefly investigate two proposals applying some generalized volume functionals dual to the complexity in higher curvature gravity theories, and find their behaviors are different for $k=0$ at late times., Comment: 18 pages, 20 figures, 1 table, A new section (section V) and some reference added in the third version

Based on the definition of the apparent horizon in a general two-dimensional dilaton gravity theory, we analyze the tunnelling phenomenon of the apparent horizon by using Hamilton-Jacobi method. In this theory the definition of the horizon is very different from those in higher-dimensional gravity theories. The spectrum of the radiation is obtained and the temperature of the radiation is read out from this spectrum and it satisfies the usual relationship with the surface gravity. Besides, the calculation with Parikh's null geodesic method for a simple example conforms to our result in general stationary cases., Comment: revtex4, 8 pages, no figure