Pursuing higher-temperature superconductors under ambient pressure continues to be a prominent topic in materials discovery. Isomorphic structures like ${\mathrm{MgB}}_2$ exhibit potential for conventional BCS-type superconductivity, but their transition temperatures $T_c$ have remained below 100 K based on both experimental findings and theoretical predictions. In this study, two two-dimensional (2D) superconductors with sandwich structures, ${\mathrm{KB}}_2{\mathrm{C}}_2$, featuring BC layers in AA and AB stacking configurations, are designed, whose $T_c$ can exceed 112 K. The analyses suggest that electrons in $\sigma$-state covalent bonds and high-frequency $E$ phonon modes dominated by the in-plane vibrations of B and C atoms are predominately responsible for electron-phonon coupling (EPC). An exciting robust three-gap superconducting nature stems from the strong and evident three-region distribution characteristic of electronic EPC parameters ${\lambda }_{\mathbf{k}}^{\mathit{el}}$. When biaxial tensile strain (BTS) is applied, their $T_c$ are boosted above 153 K. The increase in $T_c$ originates from the softening of optical $E$ phonon modes around the $\mathrm{\Gamma }$ point and acoustic modes around the Q point, rather than an increase of electrons at the Fermi level $E_F$, as observed in other similar systems. Thus, phonons play a more beneficial role in the EPC of BTS cases, highlighting its significance as a medium in BCS superconductors. Moreover, we find ${\mathrm{KB}}_2{\mathrm{C}}_2$ exhibits interesting topological properties, spin antivortex, and Ising-type spin splitting. It is significantly meaningful to report that the coexistence of nontrivial topology and superconductivity with such a high $T_c$. Therefore, ${\mathrm{KB}}_2{\mathrm{C}}_2$ may offer promising sandwich structures to explore higher-$T_c$ two-dimensional superconductors, alongside present potential avenues for investigating fundamental quantum physics.

The recently realized single crystalline $\alpha$-phase tantalum (Ta) films, mainly explored for their use in quantum computing, are yet another elemental superconductor with strong spin-orbit interactions. In this work, we address the superconducting properties of the $\alpha$-Ta films from the two-dimensional (2D) limit to the 2D-3D crossover regime. We epitaxially grow $\alpha$-Ta thin films with thicknesses ranging from 6.5 nm to over 160 nm on ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrates. Interestingly, the in-plane upper critical magnetic fields $B_{c2,\parallel }$ of our Ta films always exhibit a parabolic temperature dependence—one defining character of 2D superconductivity. The extracted superconducting thickness $d_{\mathrm{sc}}$ can reach 1.7–2.4 times the in-plane coherence length ${\xi }_{ab}$ in the relatively thick films (90–160 nm), drawing a sharp contrast to the usual expectation of $d_{\mathrm{sc}}<{\xi }_{ab}$ for 2D superconductors. Furthermore, we observe deviation from the 2D Tinkham behavior in the angular dependence of the upper-critical field only in the thickest sample (about 160 nm). It suggests that Ta enters the 2D-3D crossover regime at a very large sample thickness. Finally, we analyze the enhancement of $B_{c2,\parallel }$ over the Pauli limit in the thinnest sample. The systematic evolution of $B_{c2,\parallel }$ as a function of thickness suggests the involvement of intrinsic spin-orbit interactions.

Charge density wave (CDW) orders in ${\mathrm{YNiC}}_2$ are studied by means of combined experimental and computational techniques. On the experimental side, single crystals grown by the floating-zone method were examined by means of x-ray diffraction, as well as transport and thermal techniques. Density functional theory (DFT) calculations founded on the experimentally determined parent and CDW-modified crystal structures provide details of electronic and phononic structures as well as electron-phonon coupling and resolve changes inflicted upon entering the different CDW phases. Thereby, contrasting effects of subsequently emerging CDW states characterized by incommensurate $q_{1\text{ic}}$ and commensurate $q_{2\text{c}}$ modulation vectors are revealed. The former state, on-setting below $T_{\text{1ic}}\simeq 305\mathrm{K}$, weakly modifies the electronic structure by opening an almost isotropic gap on a minor part of the Fermi surface (FS). The latter phase, which takes over below $T_{\text{2c}}\simeq 272\mathrm{K}$ has a more pronounced impact on physical properties via a decomposition of larger parts of the FS. These dissimilar behaviors are directly reflected in the electronic transport anisotropy, which is significantly weakened in the $q_{2\text{c}}$-type CDW state. As revealed by our DFT studies, CDW phases are very close in energy and their origin is directly related to the anisotropy of electron-phonon coupling, which is linked to a specific orbital character of related FS sheets. Specific heat and thermal expansion studies reveal a nearly reversible first-order phase transition at around $T_{\text{2c}}\simeq 272\mathrm{K}$, where both CDW phases coexist within a $T$ interval of about 10 K.

The orbital Hall effect (OHE) is attracting interest due to its fundamental science implications and potential applications in orbitronics and spintronics. Unlike the spin Hall effect, the connection between the OHE and band topology is not well understood. Here we present an approach for understanding the OHE based on analyzing the projected orbital angular momentum (POAM) spectrum. By considering monolayers of group IV elements, we demonstrate that the Wannier charge centers of the POAM spectrum display topologically nontrivial windings. The orbital Hall conductivity is found to form a plateau within the band gap as a direct consequence of the Chern number carried by the POAM spectrum. Importantly, we show that the topological orbital Hall phase yields a new form of bulk-boundary correspondence, which features gapless states in the POAM spectrum and induces nonzero orbital textures at the boundaries that would be amenable to experimental verification through circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES) experiments. Our study presents a systematic method for investigating the topological OHE and provides a pathway for its broader exploration in two-dimensional materials.

External electric field effects on spin relaxation dynamics in GaN/$\mathrm{A}{\mathrm{l}}_{0.25}\mathrm{G}{\mathrm{a}}_{0.75}\mathrm{N}$ heterostructures are investigated by time-resolved Kerr rotation spectroscopy. Two types of spin relaxation processes are distinguished by adjusting the excitation energy, characterized with a difference of about an order of magnitude in spin relaxation time. It is found that the electron spin transfer from the bulk GaN barrier to the triangular well occurs under the considerable built-in electric field at the GaN/AlGaN heterointerface. Significantly, a reverse bias voltage can remarkably decelerate the spin transfer process from the bulk electrons to the two-dimensional electrons gas (2DEG) by weakening the interfacial electric field, while a forward bias voltage would induce more serious spin scattering for the 2DEG leading to a short spin relaxation time of 10 ps. The electric-field controlled spin relaxation dynamics would pave a way for the development of GaN-based spintronic devices.

In semiconductor-based quantum technologies, the capability to shuttle charges between components is profoundly enabling. We numerically simulated various “conveyor-belt” shuttling scenarios for simple $\mathrm{Si}/{\mathrm{SiO}}_2$ devices, explicitly modeling the electron's wave function using grid-based split-operator methods and a time-dependent 2D potential (obtained from a Poisson solver). This allowed us to fully characterize the electron loss probability and excitation fraction. Remarkably, with as few as three independent electrodes, the process can remain near-perfectly adiabatic even in the presence of pulse imperfection, nearby charge defects, and Johnson-Nyquist noise. Only a substantial density of charge defects, or defects at “adversarial” locations, can catastrophically disrupt the charge shuttling. While we do not explicitly model the spin or valley degrees of freedom, our results from this charge propagation study support the conclusion that conveyor-belt shuttling is an excellent candidate for providing connectivity in semiconductor quantum devices.

One-dimensional (1D) superconducting nanowires hold potential for advanced applications in quantum computing, single-photon detection, nanoscale electronic devices, and precision sensing technologies. While several 1D superconducting nanowires have been reported, the superconductivity of transition metal chalcogenide nanowires remains unexplored. Here, we use an experimentally synthesized 1D $\mathrm{M}{\mathrm{o}}_6{\mathrm{S}}_6$ nanowire as a model to investigate the superconductivity of this nanowire. Our results reveal that a 1D $\mathrm{M}{\mathrm{o}}_6{\mathrm{S}}_6$ nanowire with a diameter of around $5\AA$ exhibits unexpected partially flat-band superconductivity with a superconducting transition temperature ($T_{\mathrm{c}}$) of 3.6 K. The gradual weakening of the $d\text{−}d$ orbital electron hopping between adjacent Mo atoms within the $\mathrm{M}{\mathrm{o}}_6$ octahedral cage under tensile strain increases the density of states at the Fermi level, significantly enhancing electron-phonon coupling. Notably, under 5% tensile strain, the $T_{\mathrm{c}}$ of the 1D $\mathrm{M}{\mathrm{o}}_6{\mathrm{S}}_6$ nanowire increases to 13.6 K, accompanied by noticeable softening of the acoustic branches. These findings open different avenues for exploring the superconductivity of 1D transition metal chalcogenide nanowires and have broad implications for advancing 1D superconductors.

Two-dimensional ferrovalley systems are considered as an ideal platform to explore valley-related topological properties. To date, valley-related topological phase transitions have been mostly achieved through magnetic or optical means based on time-reversal symmetry breaking, yet electric approaches are highly desirable for topological device applications. Here, we propose that electric gating can drive valley-selective topological phase transitions assisted by valley-dependent out-of-plane electric dipole moments in ferrovalley systems without some specific symmetries. Specifically, an out-of-plane electric field induces valley-contrasted Stark shifts by coupling to the valley-dependent dipole moments, leading to field-strength-driven energy gap closing and reopening in a selected valley depending on the field's direction. Based on first-principles calculations, we present asymmetrically halogenated monolayer $1T\text{−}{\mathrm{MoSe}}_2$ as a candidate material to validate our proposal. Notably, topological phase transition from an intrinsic quantum anomalous Hall state to a quantum valley Hall state is achieved by electric gating. Our work paves the way for the electric control of topological phase transitions in ferrovalley systems.

Weyl semimetals represent a significant class of topological gapless materials in three dimensions and have been shown to exhibit three-dimensional quantum Hall effect. However, existing research mainly focuses on scenarios without interactions. Recent studies suggest that the fractional quantum Hall effect can arise in a Weyl semimetal with a one-third filled Landau level under a magnetic field. However, it remains unclear whether more exotic states, such as composite Fermi liquid and Moore-Read states, can appear at half-filling. Here we surprisingly find the existence of composite Fermi liquid, Moore-Read states, and charge density waves in the same Weyl-orbit-based Landau level of a Weyl semimetal and their transitions induced by varying the distance between Weyl points. We attribute these transitions to a significant change in the single-particle wave functions of the Landau level as the distance between Weyl points is varied. Finally, we demonstrate that a transition from composite Fermi liquid to Moore-Read states can be induced by tuning the direction of a magnetic field, a process that is more experimentally accessible.

A density matrix formalism is employed to calculate the emission of multilevel Rydberg excitonic system, highlighting picosecond-scale dynamics and coherent effects such as quantum beats. The theoretical results for one- and two-photon excitations of various Rydberg excitons and configurations providing the insight into their dynamics are compared with recent experimental data. In particular, the effect of Rydberg blockade on the quantum beat phenomenon is discussed.

We study the hydrodynamic regime of the ratchet effect, i.e., radiation-induced generation of a direct electric current $J_{\mathrm{rat}}$, in an asymmetric dual-grating gate structure without an inversion center. Recently, it was demonstrated that the frequency dependence of $J_{\mathrm{rat}}$ is essentially different within the hydrodynamic (HD) and drift-diffusion (DD) regimes of electron transport: $J_{\mathrm{rat}}^{\mathrm{HD}}\propto 1/{\omega }^6$ and $J_{\mathrm{rat}}^{\mathrm{DD}}\propto 1/{\omega }^2$ for $\omega \to \infty$. Here, we analyze the previously neglected thermoelectric contribution to the ratchet current $J_{\mathrm{th}}$, which arises due to inhomogeneous heating of the electron liquid. We demonstrate that this contribution dominates at sufficiently low electron-phonon scattering rates, yields a high-frequency asymptotic $J_{\mathrm{th}}\propto 1/{\omega }^2$ even in the HD regime, and can change the sign of the response at certain frequency intervals. We analyze the plasmonic resonance in $J_{\mathrm{rat}}$, which has the shape of an asymmetric Fano peak, and find that the asymmetry increases with increasing thermoelectric contribution. We also study the effect of finite viscosity of the electron liquid, as well as the manifestation of the plasmonic drag phenomena and inhomogeneous rectification effects in the ratchet current. We demonstrate that viscosity suppresses plasmonic resonance but unexpectedly enhances the Drude peak due to the interference nature of the ratchet effect.

In situ and real-time monitoring reveals the dynamic reactions, changes, and even unexpected constructions occurring during the growth process, which is crucial for understanding the fundamental growth mechanism and control of film quality. The solution-based growth approach is widely applied for organic semiconductors because of its easy implementation, low costs, and large-scale growth, whereas its in situ monitoring is yet less studied. Here, we developed a fast differential reflectance spectroscopy (DRS) system for real-time monitoring of drop-casting growth of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene). The results demonstrate that the extremely low-drift DRS could successfully capture the growth process of TIPS-pentacene thin film, including the ultrafast solvent evaporation, transient liquid-solid transition, and eventually formation of the TIPS-pentacene film. A four-phase optical model was developed to fit the in situ collected DRS spectra, which revealed the dynamic changes in the volume fraction of TIPS-pentacene solution and the effective thickness of TIPS-pentacene film. This work demonstrates an effective approach to in situ monitoring of physical and even chemical reactions at the liquid-solid interface.

We investigate spin dynamics in $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ (Py) heterostructures, uncovering a robust mechanism for designing the ferromagnetic resonance (FMR) frequency through control of crystal orientation, temperature, and applied magnetic field. Employing cryogenic ferromagnetic resonance spectroscopy, we demonstrate that the relative orientation of the Néel vector of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ and the magnetization of the Py layer is highly tunable across the Morin transition temperature ($T_{\text{M}}$). Central to our findings is the pivotal role of crystal orientation in governing the spin dynamics, with the reorientation of the Néel vector serving as a complementary mechanism that further modulates the system's behavior. Our experiments and corroborating theoretical model reveal distinct resonance behavior for different crystal orientations, highlighting the pivotal role of the mutual Néel vector and magnetization geometry in dictating FMR frequencies. In this way, we can tune the FMR frequencies up to a tenfold increase by manipulating the mutual Néel vector/magnetization configuration. Our study comprises all common $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ crystal orientations and develops a unified theoretical description of the ensuing spin dynamics.

We investigate the effects of temperature on higher-order topological insulators (HOTIs). The finite-temperature topological invariants for HOTIs can be constructed by generalizing the Resta's polarization for the ground state to the ensemble geometric phase (EGP) for the mixed states, [C.-E. Bardyn, L. Wawer, A. Altland, M. Fleischhauer, and S. Diehl, Phys. Rev. X 8, 011035 (2018)]. The EGP is consistent with the Resta's polarization both at zero temperature and at finite temperatures in the thermodynamic limit. We find that the temperature can change the critical point and thus induce a phase transition from a topologically trivial phase to a nontrivial phase in a finite-size system, manifesting changes in the winding of the EGP.

Altermagnets are crystallographic rotational symmetry breaking spin-ordered states, possessing a net zero magnetization despite manifesting Kramer's nondegenerate bands. Here, we show that momentum-independent local spin-nematic orders in monolayer, Bernal bilayer, and rhombohedral trilayer graphene give rise to $p$-wave, $d$-wave, and $f$-wave altermagnets, respectively, thereby inheriting the topology of linear, quadratic, and cubic free fermion band dispersions that are also described in terms of angular momentum $\ell =1,2$, and 3 harmonics in the reciprocal space. The same conclusions also hold inside a spin-triplet nematic superconductor, featuring Majorana altermagnets. Altogether, these findings highlight the importance of electronic band structure in identifying such exotic magnetic orders in quantum materials. We depict the effects of in-plane magnetic fields on altermagnets, and propose spin-disordered alter-valley magnets in these systems.

Crystal symmetry has long been regarded as a crucial factor in generating various topological quasiparticle states. Constrained by all possible symmetries in materials, the Chern number $\mathcal{C}$ of a twofold Weyl point (WP) is believed to have a maximum value of $\left|\mathcal{C}\right|=4$. In this study, we discover that when considering the coupling between two quadratic crossing bands and their neighboring bands in momentum space, the crossing point may transform into a twofold quadratic WP. Due to its partial crystal symmetry constraint, the Chern number of this twofold WP surpasses the maximum Chern number constraint, attaining a value of $\left|\mathcal{C}\right|=6$. Its pseudospin structure differs from those of all previously reported WPs and the sextuple-helicoid surface arc state is well produced, thereby firmly confirming its nontrivial topological nature. Notably, this twofold WP with $\left|\mathcal{C}\right|=6$ is well established in an acoustic crystal in space group no. 168. Our theoretical discovery paves another way for exploring unconventional topological phenomena.

When magnetic skyrmions decay, their size in real space decreases in a finite time before they eventually collapse. We construct an effective continuum model and use its dynamics to describe the shrinking behavior of skyrmions before they collapse. Using the Landau-Lifshitz-Gilbert equation and the time derivative of the vector field, we find a set of coupled nonlinear ordinary differential equations for the time dependent effective skyrmion radius and its helicity. In particular, we use a triangular-shaped skyrmion profile of its polar angle. Contrary to the commonly expected simple exponential decrease in size, we reveal a more complicated time dependence, in which the time-dependent radius crosses over from an exponential decay towards a square root decrease, $\sim {(t-t_c)}^{1/2}$, near a critical time $t_c$ at which it collapses. This critical time is found to depend logarithmically on the lattice constant. In addition, we examine the interplay between the shrinking dynamics and an accompanying transformation through different skyrmion configurations, depending on the various system parameters. The findings are verified by numerical studies on the lattice, supporting the predictions from the theoretical continuum model.

Using density functional theory (DFT), we calculate the magnetic short-ranged exchange forces between a magnetic tip and an adatom adsorbed on the antiferromagnetic Mn monolayer on the W(110) surface [Mn/W(110)]. These exchange forces can be measured in magnetic exchange force microscopy allowing atomic-scale imaging of spin structures on insulating and conducting surfaces. We consider two types of $3d$ transition-metal atoms with intrinsic magnetic moments: Co and Mn and Ir as an example of a $5d$ transition-metal atom exhibiting an induced magnetic moment on Mn/W(110). The tips are modeled by Fe pyramids and terminated either with an Fe or a Mn apex atom. From our total energy DFT calculations for a parallel and antiparallel alignment between tip and adatom magnetic moments we obtain the exchange energy $E_{\mathrm{ex}}\left(d\right)$ as a function of tip-adatom distance $d$. The exchange forces, $F_{\mathrm{ex}}\left(d\right)$, are calculated based on the Hellmann-Feynman theorem. We show that structural relaxations of tip and sample due to their interaction need to be taken into account. Due to the exchange interaction the relaxations depend on the alignment between tip and adatom magnetization—an effect which will affect the tunneling magnetoresistance that can be measured by a spin-polarized scanning tunneling microscope. A maximum in the exchange energy and force curves is obtained for magnetic adatoms at tip-adatom separations of about 3 to 4 Å. The exchange forces with an Fe terminated tip reach a maximum value of up to 0.2 and 0.6 nN for Co and Mn adatoms, respectively, and prefer an antiferromagnetic coupling. Surprisingly, we also find an exchange force of up to 0.2 nN for Ir adatoms. We analyze the exchange interaction between tip and adatom based on the spin-polarized electronic structure of the coupled system. A competition occurs between long-range Zener-type indirect double exchange favoring ferromagnetic coupling and short-range direct $d\text{−}d$ antiferromagnetic exchange. For the Ir adatom the interaction can be explained from the spin-dependent hybridization with the tip apex atom. Our results show that magnetic adatoms on Mn/W(110) are a promising system to study exchange forces at the single-atom level via magnetic exchange force microscopy.