The axion insulator is believed to host half-quantized chiral currents running antiparallelly on its top and bottom surfaces. However, the experimental detection of the half quantization in axion insulators remains elusive. In this paper, we propose a mechanism to explain why the half quantization is hard to be probed by showing that the half-quantized counterpropagating currents in axion insulator thin films are strongly suppressed due to the hybridization mediated by the massless side-surface states. This side-surface-mediated hybridization leads to a different type of finite-size effect, which features a power-law decay with the increasing film thickness, different from the exponential decay in topological insulators. Moreover, we show that the half quantization can be extracted in the axion insulator phase by adopting the nonlocal transport measurement. 

Side-surface-mediated hybridization in axion insulators


 Charge-changing cross section ($\sigma_{\text{cc}}$) measurements via the transmission method have made important progress recently aiming to determine the charge radii of exotic nuclei. In this work, we report a new $\sigma_{\text{cc}}$ measurement of 299(9) MeV/nucleon $^{28}$Si on carbon at the second Radioactive Ion Beam Line in Lanzhou (RIBLL2) and describe the data analysis procedure in detail. This procedure is essential to evaluate the systematic uncertainty in the transmission method. The determined $\sigma_{\mathrm{cc}}$ of 1125(11) mb is found to be consistent with the existing data at similar energies. The present work will serve as a reference in the $\sigma_{\text{cc}}$ determinations at RIBLL2.

Charge-changing cross section measurements of 300 MeV/nucleon 28Si on carbon and data analysis

Topological insulators offer significant potential to revolutionize diverse fields driven by nontrivial manifestations of their topological electronic band structures. However, the realization of superior integration between exotic topological states and superconductivity for practical applications remains a challenge, necessitating a profound understanding of intricate mechanisms. Here, we report experimental observations for a novel superconducting phase in the pressurized second-order topological insulator candidate Ta2Pd3Te5, and the high-pressure phase maintains its original ambient pressure lattice symmetry up to 45 GPa. Our in situ high-pressure synchrotron X-ray diffraction, electrical transport, infrared reflectance, and Raman spectroscopy measurements, in combination with rigorous theoretical calculations, provide compelling evidence for the association between the superconducting behavior and the densified phase. The electronic state change around 20 GPa was found to modify the topology of the Fermi surface directly, which synergistically fosters the emergence of robust superconductivity. In-depth comprehension of the fascinating properties exhibited by the compressed Ta2Pd3Te5 phase is achieved, highlighting the extraordinary potential of topological insulators for exploring and investigating high-performance electronic advanced devices under extreme conditions. 

Observation of Emergent Superconductivity in the Topological Insulator Ta2Pd3Te5 via Pressure Manipulation.

Using first-principle calculations, we investigate the electronic, topological and superconducting properties of Nb$_3$X (X = Ge, Sn, Sb) and Ta$_3$Y (Y = As, Sb, Bi) A15 compounds. We demonstrate that these compounds host Dirac surface states which are related to a nontrivial Z$_2$ topological value. The spin-orbit coupling (SOC) splits the eightfold degenerate R point close to the Fermi level enhancing the amplitude of the spin Hall conductance. Indeed, despite the moderate spin-orbit of the Nb-compounds, a large spin Hall effect is also obtained in Nb$_3$Ge and Nb$_3$Sn compounds. We show that the Coulomb interaction opens the gap at the R point thus making more evident the occurrence of Dirac surface states. We then investigate the superconducting properties by determining the strength of the electron-phonon BCS coupling. The evolution of the critical temperature is tracked down to the 2D limit indicating a reduction of the transition temperature which mainly arises from the suppression of the density of states at the Fermi level. Finally, we propose a minimal tight-binding model based on three coupled Su-Schrieffer-Heeger chains with t$_{2g}$ Ta- and Nb-orbitals reproducing the spin-orbit splittings at the R point among the $\pi$-bond bands in this class of compounds. We separate the kinetic parameters in $\pi$ and $\delta$-bonds, in intradimer and interdimer hoppings and discuss their relevance for the topological electronic structure. We point out that Nb$_3$Ge might represent a Z$_2$ topological metal with the highest superconducting temperature ever recorded.

pdf/dirac-surface-states-multiorbital-dimerization-and-1hi091u92w.pdf

Dirac surface states, multiorbital dimerization, and superconductivity in Nb- and Ta-based A15 compounds

Giant in-plane vibrational and transport anisotropy in van der Waals Ta2Ni3Te5

The unique electronic and crystal structures driven by external pressure in transition metal chalcogenides (TMCs) can host emergent quantum states. Here we report pressure-induced metallization, nontrivial ${Z}_{2}$ band topology, and superconductivity in TMC ${\mathrm{Ta}}_{2}{\mathrm{Ni}}_{3}{\mathrm{Te}}_{5}$. Our electrical transport measurements show that the metallization emerges at 3.3 GPa, followed by appearance of the superconductivity at ${P}_{\mathrm{c}}=21.3\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ with ${T}_{\mathrm{c}}\ensuremath{\sim}0.4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Room-temperature synchrotron x-ray-diffraction experiments demonstrate the stability of the pristine orthorhombic structure upon compression. Our first-principles calculations further reveal a topological phase transition (from ${Z}_{2}=0$ to ${Z}_{2}=1$), which occurs after ${\mathrm{Ta}}_{2}{\mathrm{Ni}}_{3}{\mathrm{Te}}_{5}$ is turned into an electron-hole compensated semimetal by pressure. The pressure-induced superconductivity at ${P}_{\mathrm{c}}$ could be attributed to the abruptly enhanced density of states at the Fermi level. These findings demonstrate that ${\mathrm{Ta}}_{2}{\mathrm{Ni}}_{3}{\mathrm{Te}}_{5}$ is a new platform for realizing exotic quantum phenomena in TMCs as well as exploring the interplay between topological property and superconductivity. 

/pdf/pressure-induced-nontrivial-mml-math-xmlns-mml-http-www-w3-3czit9n3.pdf

Pressure-induced nontrivial 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>Z</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>
 band topology and superconductivity in the transition metal chalcogenide 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mm

Elemental fragmentation cross sections (EFCSs) of stable and unstable nuclides have been investigated with various projectile-target combinations at a wide range of incident energies. These data are critical to constrain and develop the theoretical reaction models and to study the propagation of galactic cosmic rays (GCR). In this work, we present a new EFCS measurement for $^{28}$Si on carbon at 218~MeV/nucleon performed at the Heavy Ion Research Facility (HIRFL-CSR) complex in Lanzhou. The impact of the target thickness has been well corrected to derive an accurate EFCS. Our present results with charge changes $\Delta Z$ = 1-6 are compared to the previous measurements and to the predictions from the models modified EPAX2, EPAX3, FRACS, ABRABLA07, NUCFRG2, and IQMD coupled with GEMINI (IQMD+GEMINI). All the models fail to describe the odd-even staggering strength in the elemental distribution, with the exception of the IQMD+GEMINI model, which can reproduce the EFCSs with an accuracy of better than 3.5\% for $\Delta Z\leq5$. The IQMD+GEMINI analysis shows that the odd-even staggering in EFCSs occurs in the sequential statistical decay stage rather than in the initial dynamical collision stage. This offers a reasonable approach to understand the underlying mechanism of fragmentation reactions. 

/pdf/new-measurement-of-the-elemental-fragmentation-cross-390pt4h2.pdf

New measurement of the elemental fragmentation cross sections of 218 MeV/nucleon 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Si</mml:mi><mml:mprescripts /><mml:none /><mml:mn>28</mml:mn></mml:mmultiscripts></mml:math>
 on a carbon target

The unoccupied part of the band structure in the magnetic topological insulator MnBi$_2$Te$_4$ is studied by first-principles calculations. We find a second, unoccupied topological surface state with similar electronic structure to the celebrated occupied topological surface state. This state is energetically located approximate $1.6$ eV above the occupied Dirac surface state around $\Gamma$ point, which permit it to be directly observed by the two-photon angle-resolved photoemission spectroscopy. We propose a unified effective model for the occupied and unoccupied surface states. Due to the direct optical coupling between these two surface states, we further propose two optical effects to detect the unoccupied surface state. One is the polar Kerr effect in odd layer from nonvanishing ac Hall conductance $\sigma_{xy}(\omega)$, and the other is higher-order terahertz-sideband generation in even layer, where the non-vanishining Berry curvature of the unoccupied surface state is directly observed from the giant Faraday rotation of optical emission. 

/pdf/unoccupied-topological-surface-state-in-mml-math-xmlns-mml-259gmdi7.pdf

Unoccupied topological surface state in 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>MnBi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Te</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>

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Pressure-induced nontrivial <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>Z</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math> band topology and superconductivity in the transition metal chalcogenide <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mm

New measurement of the elemental fragmentation cross sections of 218 MeV/nucleon <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Si</mml:mi><mml:mprescripts /><mml:none /><mml:mn>28</mml:mn></mml:mmultiscripts></mml:math> on a carbon target

Unoccupied topological surface state in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>MnBi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Te</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>