In this work, we use optical reflectance spectroscopy to study the plexciton formation between Ag nanoclusters and ${\mathrm{C}}_{60}$ molecules. The Ag clusters are fabricated on high-quality graphene under ultrahigh vacuum (UHV) and exhibit a strong absorption band at 3.4–3.6 eV due to the localized surface plasmons (LSPs). The plexciton formation is studied by depositing ${\mathrm{C}}_{60}$ on the Ag clusters in the same chamber under UHV. The deposition of ${\mathrm{C}}_{60}$ molecules leads to a splitting of the LSP band into multiple peaks with a systematic peak energy shift as a function of ${\mathrm{C}}_{60}$ coverage ${\theta }_{{\mathrm{C}}_{60}}$. Notably, the details of the energy shifts and intensity variation sensitively depend on the LSP energy. Model calculations in which the plexciton coupling is approximated by a point-dipole interaction predict the evolution of the plexciton eigenenergy and oscillator strength with ${\theta }_{{\mathrm{C}}_{60}}$ that reasonably explains the experiments. We demonstrate that spectral changes under the deposition of ${\mathrm{C}}_{60}$ cannot be explained by considering only one electronic transition in ${\mathrm{C}}_{60}$, as is commonly assumed. Instead, it is necessary to take into account three electronic transitions of ${\mathrm{C}}_{60}$ and their intermixing with the LSP to fully comprehend the spectral evolution. The plexciton coupling energy is estimated to exceed 0.6 eV, indicating that the system reaches a strong coupling regime. Our findings suggest the importance of simultaneously considering multiple electronic excitations of molecules to understand strongly coupled plexcitons.

Combining Floquet theory with the hierarchy of correlations (a method based on the formal expansion into inverse powers of the coordination number), we study the nonequilibrium dynamics of the Mott insulator state in the Fermi-Hubbard model under the influence of a harmonically oscillating electric field representing the pump laser. After deriving the associated Floquet exponents, we consider multiphoton resonances leading to the generation of higher harmonics. For weak electric field strengths, the strongest signal is obtained for the third harmonic when the driving frequency equals one-third of the Mott gap (because even harmonics are absent due to inversion symmetry). For stronger electric field strengths and lower driving frequencies, the higher-harmonic spectrum can also exhibit plateaulike structures extending over several harmonics (i.e., Floquet channels) below the resonance peak, which is consistent with previous findings.

We demonstrate the existence of a topological chiral spin liquid in the frustrated Shastry-Sutherland Heisenberg model with an additional spin chirality interaction, using numerically unbiased exact diagonalization and density matrix renormalization group methods. We establish a quantum phase diagram where conventional phases, including dimer singlet, plaquette singlet, Néel, and collinear phase, can be clearly identified by suitable local order parameters. Among them, an $\text{SU}{\left(2\right)}_1$ chiral spin liquid emerges in the highly frustrated region, which is unambiguously identified by two topologically degenerate ground states, a modular matrix, and characteristic level counting in an entanglement spectrum, featuring the same topological order of the $\nu =1/2$ bosonic Laughlin state. The phase boundaries among the different orders are determined by the energy level crossing analysis and wave-function fidelity susceptibility.

We investigate the antiferromagnetic heavy fermion superconductor ${\mathrm{UPd}}_2{\mathrm{Al}}_3$, employing angle-resolved photoemission spectroscopy to unravel the complex electronic structure of its U $5f$ electrons. We observe unexpected characteristics that challenge the conventional temperature-dependent behavior of heavy fermion systems, revealing unexpected characteristics. At temperatures above the anticipated coherence temperature ($T^{*}$), we observe itinerant U $5f$ electrons at temperatures higher than previously postulated. Additionally, a previously unidentified dispersionless band emerges around 600 meV below the Fermi energy, potentially linked to spin-orbit splitting within the U $5f$ states. Hybridization between the $5f$ electrons and conduction band was observed with an energy dispersion of 10 meV at low temperatures, suggesting that U $5f$ electrons near and at the Fermi surface have an itinerant nature. Temperature-dependent $5d\text{−}5f$ resonance spectra reveal that the $5f$ electron spectrum weight increases with lowering temperature and begins to decrease at temperatures significantly higher than the Néel temperature ($T_N$). We further show that the competition between the Kondo effect and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions may be responsible for the relocalization of $5f$ electrons, making relocalization a precursor to the establishment of magnetic order at lower temperatures. Our experiments also provide evidence that $5f$ electrons with the same orbital are involved in both the Kondo effect and RKKY interactions, suggesting that the two coexist at lower temperatures.

The achievement of high-temperature superconductivity in compressed hydrides with extended lattices, e.g., ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{LaH}}_{10}$, has become a milestone in the quest for room-temperature superconductivity. For realizing room-temperature superconductivity, lattices where hydrogen adopts multicentered bonds are deemed as indispensable, while hydrides containing ${\mathrm{H}}_2$ molecular units are believed to be unfavorable. Here, we report ${\mathrm{H}}_2$ molecular type hydrides with an exceptional near room-temperature superconductivity of 270 K in compressed ${\mathrm{NaH}}_{10}$ and a $T_{\mathrm{c}}$ of 152 K in ${\mathrm{NaH}}_{12}$, where H atoms solely constitute ${\mathrm{H}}_2$ units, and Na-H forms ionic bonds. Our first-principles calculations unveil that the high $T_{\mathrm{c}}$ is mainly attributed to strong electron-phonon coupling stemming from the large electron-phonon matrix element driven by medium-frequency interatomic interactions and high-frequency H-derived phonon softening caused by Fermi surface nesting, thus scattering itinerant electrons to form Cooper pairs. Of particular note, we reveal that the unique delocalized background charges cooperate with other electrons occupying the pressure-induced $sp$-hybridized antibonding bands of molecular ${\mathrm{H}}_2$ units, acting as itinerant electrons to mediate metallic interactions and participate in electron-phonon coupling. This observation reshapes the understanding of superconductivity dominated by molecular ${\mathrm{H}}_2$ units, provides insights for elucidating phonon-mediated superconductivity, and raises broad prospects of realizing room-temperature superconductivity in molecular hydrogen-based superconductors.

The recent discovery of magnetization oscillations in a kagome spin liquid candidate motivates us to examine the origin of the gauge magnetic field term that can give rise to quantum oscillations of fermionic spinons. We find that in the presence of the Dzyaloshinskii-Moriya interaction and an average spin polarization, the spin permutation operator around the unit cell acquires an imaginary part, and a net gauge flux is generated through the unit cell of the kagome lattice. This mechanism of gauge field generation can account for the strength of the gauge magnetic field needed to explain the experiment.

We study the exciton condensation in the heterostructures where the electron layer and the hole layer formed by gapped chiral fermion (GCF) systems are separately gated. High angular momentum such as $p$- and $d$-wave-like excitonic pairing may emerge when the gap of the GCF systems is small compared to the Fermi energy, and the chiral winding numbers of the electrons and holes are the same. This is a result of the nontrivial band geometry and can be linked to the Berry curvature when projected onto the Fermi surface. In realistic systems, we propose that staggered graphene and surface states of magnetically doped topological insulators or intrinsic axion insulators are promising candidates for realizing $p$-wave exciton superfluid, and anomalous Hall conductivity can be used as a signature in experiments.

We investigate the crystal structure and magnetic properties, including spin relaxation and magnetocaloric effect, in a NASICON-type ${\mathrm{NaFe}}_2{\mathrm{PO}}_4{\left({\mathrm{SO}}_4\right)}_2$ sample. The Rietveld refinement of x-ray and neutron diffraction patterns shows a rhombohedral crystal structure with the $R\overline{3}c$ space group. The core-level spectra confirm the desired oxidation state of constituent elements. The dc-magnetic susceptibility $\left(\chi \right)$ behavior in zero field-cooled (ZFC) and field-cooled (FC) modes shows the ordering temperature $\approx 50$ K. Interestingly, the analysis of temperature-dependent neutron diffraction patterns reveal an A-type antiferromagnetic (AFM) structure with the ordered moment of 3.8 ${\mu }_B/{\mathrm{Fe}}^{3+}$ at 5 K, and a magnetostriction below $T_{\mathrm{N}}=50$ K. Further, the peak position in the ac-$\chi$ is found to be invariant with the excitation frequency, supporting the notion of a dominating AFM transition. Also, the unsaturated isothermal magnetization curve supports the AFM ordering of the moments; however, the observed coercivity suggests the presence of weak ferromagnetic (FM) correlations at 5 K. On the other hand, a clear bifurcation between ZFC and FC curves of dc-$\chi$ and the observed decrease in a peak height of ac-$\chi$ with frequency suggests complex magnetic interactions. The spin relaxation behavior in thermoremanent magnetization and aging measurements indicate the glassy states at 5 K. Moreover, the Arrott plots and magnetocaloric analysis reveal the AFM-FM interactions in the sample at lower temperatures.

We have investigated the high magnetic field properties of the Ce-based Kondo lattice system CeCoSi by means of magnetization, magnetocaloric effect, contactless resistivity, specific heat, and magnetic torque measurements under pulsed high magnetic fields up to 55 T. CeCoSi is proposed to show an unconventional ordered phase called Phase II, which appears between paramagnetic Phase I and antiferromagnetic Phase III. The magnetization, magnetocaloric effect, and contactless resistivity measurements determine the field-angle dependence of Phase II-III boundary. Magnetic torque measurements detect a fourfold symmetry of the magnetic anisotropy under high magnetic fields in all phases, implying that there are two magnetic hard axes in the (100) or (110) plane of the tetragonal structure. Through comparison between the mean-field calculation and magnetization data, we determine the crystal electric field energy scheme, which agrees with the previous inelastic neutron scattering experiment. We successfully reproduce the fourfold magnetic anisotropy through our calculation, which is attributed to hybridization of the crystal electric field wave functions. The hybridization increases multipole degrees of freedom and is the origin of the unconventional order in Phase II. Moreover, the specific heat below 4 K exhibits a nonphononic $T^3$ term in Phase II, which indicates that Phase II is not a simple polarized paramagnetic state and can be a quadrupolar ordered state.

We study few-magnon excitations in a finite-size spin-$S$ chain with ferromagnetic nearest-neighbor (NN) interaction $J>0$ and antiferromagnetic next-nearest-neighbor (NNN) interaction $J^{\prime }<0$, in the presence of the single-ion (SI) anisotropy $D$. We first reveal the condition for the emergence of zero-excitation-energy states. In the isotropic case with $\mathrm{\Delta }={\mathrm{\Delta }}^{\prime }=1$ ($\mathrm{\Delta }$ and ${\mathrm{\Delta }}^{\prime }$ are the corresponding anisotropy parameters), a threshold of $J/|J^{\prime }|$, above which the ground state is ferromagnetic, is determined by exact diagonalization for short chains up to 12 sites. Using a set of exact two-magnon Bloch states, we then map the two-magnon problem to a single-particle one on an effective open chain with both NN and NNN hoppings. The whole two-magnon excitation spectrum is calculated for large systems, and the commensurate-incommensurate transition in the lowest-lying mode is found to exhibit different behaviors between $S=1/2$ and higher spins due to the interplay of the SI anisotropy and the NNN interaction. For the commensurate momentum $k=-\pi$, the effective lattice is decoupled into two NN open chains that can be exactly solved via a plane-wave ansatz. Based on this, we analytically identify in the ${\mathrm{\Delta }}^{\prime }-D/\left|J^{\prime }\right|$ plane the regions supporting the SI or NNN exchange two-magnon bound states near the edge of the band. In particular, we prove that there always exists a lower-lying NN exchange two-magnon bound state near the band edge for arbitrary $S\ge 1/2$. Finally, we numerically calculate the $n$-magnon spectra for $S=1/2$ with $n\le 5$ by using a spin-operator matrix element method. The corresponding $n$-magnon commensurate instability regions are determined for finite chains, and consistent results with prior literature are observed.

${\mathrm{Eu}}_5{\mathrm{In}}_2{\mathrm{Sb}}_6$ is a member of a family of orthorhombic nonsymmorphic rare-earth intermetallics that combines large localized magnetic moments and itinerant exchange with a low carrier density and perpendicular glide planes. This may result in special topological crystalline (wallpaper fermion) or axion insulating phases. Recent studies of ${\mathrm{Eu}}_5{\mathrm{In}}_2{\mathrm{Sb}}_6$ single crystals have revealed colossal negative magnetoresistance and multiple magnetic phase transitions. Here, we clarify this ordering process using neutron scattering, resonant elastic x-ray scattering, muon spin-rotation, and magnetometry. The nonsymmorphic and multisite character of ${\mathrm{Eu}}_5{\mathrm{In}}_2{\mathrm{Sb}}_6$ results in coplanar noncollinear magnetic structures with an Ising-like net magnetization along the $a$ axis. A reordering transition, attributable to competing ferro- and antiferromagnetic couplings, manifests as the onset of a second commensurate Fourier component. In the absence of spatially resolved probes, the experimental evidence for this low-temperature state can be interpreted either as an unusual double-$q$ structure or in a phase separation scenario. The net magnetization produces variable anisotropic hysteretic effects which also couple to charge transport. The implied potential for functional domain physics and topological transport suggests that this structural family may be a promising platform to implement concepts of topological antiferromagnetic spintronics.

Van der Waals (vdW) assembly could efficiently modulate the symmetry of two-dimensional (2D) materials that ultimately governs their physical properties. Of particular interest is the ferroelectric polarization being introduced by proper vdW assembly that enables the realization of novel electronic, magnetic, and transport properties of 2D materials. Four-layer bulklike stacking antiferromagnetic $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ ($\mathit{FB}$-MBT) offers an excellent platform to explore ferroelectric polarization effects on magnetic order and topological transport properties of nanomaterials. Here, by applying symmetry analyses and density-functional-theory calculations, the ferroelectric interface effects on magnetic order, anomalous Hall effect (AHE) or even quantum AHE (QAHE) on the $\mathit{FB}$-MBT are analyzed. Interlayer ferroelectric polarization in $\mathit{FB}$-MBT efficiently violates the $\widehat{P}\widehat{T}$ symmetry [the combination of central inversion ($\widehat{P}$) and time reverse ($\widehat{T}$) symmetry] of the $\mathit{FB}$-MBT by conferring magnetoelectric couplings, and stabilizes a specific antiferromagnetic order encompassing a ferromagnetic interface in the $\mathit{FB}$-MBT. We predict that engineering an interlayer polarization in the top or bottom interface of $\mathit{FB}$-MBT allows converting $\mathit{FB}$-MBT from a trivial insulator to a Chern insulator. The switching of ferroelectric polarization at the middle interfaces results in a direction reversal of the quantum anomalous Hall current. Additionally, the interlayer polarization of the top and bottom interfaces can be aligned in the same direction, and the switching of polarization direction also reverses the direction of anomalous Hall currents. Overall, our work highlights the occurrence of quantum-transport phenomena in 2D vdW four-layer antiferromagnets through vdW assembly. These phenomena are absent in the bulk or thin-film in bulklike stacking forms of $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$.

We have measured the longitudinal and transverse resistivity, ${\rho }_{xx}$ and ${\rho }_{xy}$ on the optimally doped ${\mathrm{Bi}}_{2+x}{\mathrm{Sr}}_{2-x}{\mathrm{CuO}}_{6+\delta }$ single crystals with a well-shaped Hall bar structure. It is found that the Hall resistance in the flux flow region exhibits a clear sign-reversal effect. The strongest reversal Hall signal appears in the region of about the half-transition way, either on the temperature dependence of ${\rho }_{xy}$ at a fixed field or the field dependence of ${\rho }_{xy}$ at a fixed temperature. Detailed analysis shows that the theoretically proposed scaling law ${\rho }_{xy}=A{\rho }_{xx}^{\beta }$ with $1.5<\beta <2$ in the small dissipation limit is not obeyed very well. The Hall resistivity is plotted in a $H-T$ phase diagram, and the boundary for the Hall reversal effect to occur follows the theoretical prediction of Feigel'man et al. [JETP Lett. 62, 834 (1995)], which suggests that the anomalous Hall effect may be induced by the properties of a single vortex, rather than by the collective flux motion with pinning and strong thermal fluctuations.

Recently, PdTe was identified as a spin-orbit-coupled topological Dirac semimetal and was claimed to exhibit both bulk-nodal and surface-nodeless superconducting gaps. Here we report on ultra-low-temperature thermal-conductivity measurements on PdTe single crystals with $T_c$ = 4.5 K to investigate its superconducting gap structure. It is found that the residual linear term ${\kappa }_0/T$ is negligible in zero magnetic field. Furthermore, the field dependence of ${\kappa }_0\left(H\right)/T$ exhibits an $\mathsf{S}$-shaped curve. These results suggest that PdTe has multiple nodeless superconducting gaps, which is at odds with the claimed bulk-nodal gap. The reason for the discrepancy is likely that previous angle-resolved photoemission spectroscopy measurements were only performed down to 2 K and cannot observe the smaller nodeless gap. The fully gapped superconducting state in PdTe is compatible with it being a topological superconductor candidate.

We develop a time-dependent Ginzburg-Landau theory of the vortex spin Hall effect, i.e., a spin Hall effect that is driven by the motion of superconducting vortices. For the direct vortex spin Hall effect in which an input charge current drives the transverse spin current accompanying the vortex motion, we start from the well-known Schmid-Caroli-Maki solution for the time-dependent Ginzburg-Landau equation under the applied electric field, and find out the expression of the induced spin current. For the inverse vortex spin Hall effect in which an input spin current drives the longitudinal vortex motion and produces the transverse charge current, we microscopically construct the time-dependent Ginzburg-Landau equation under the applied spin accumulation gradient, and calculate the induced transverse charge current as well as the open circuit voltage. The time-dependent Ginzburg-Landau equation and its analytical solution developed here can be a basis for more quantitative numerical simulations of the vortex spin Hall effect.

Iron-based superconductors display a large degree of variability in electronic structure at the Fermi surface, resulting in superconducting gap structures, $T_c$'s, and other properties that vary considerably from family to family. Recently, it was noted that across many different families of Fe-based systems, the ratio ${\mathrm{\Delta }}_{\text{max}}/T_c$ is found to be quasiuniversal with a large value of $\sim 3.5$ compared to the one-band BCS weak-coupling result of 1.76. Here, ${\mathrm{\Delta }}_{\text{max}}$ is the measured maximum gap across the Fermi surface. This remarkable fact was attributed to strong-coupling effects arising from Hund's metal physics. Here, we perform a “high-throughput” scan across band masses, Fermi energies, and interaction parameters in a weak-coupling Suhl-Matthias-Walker model. We find that unexpectedly large values of ${\mathrm{\Delta }}_{\text{max}}/T_c$ can be achieved within weak coupling, and that quasiuniversal behavior similar to experiment emerges for those systems where interband interactions dominate intraband ones. However, within the current framework, a large mass contrast between bands is required.

We report the dynamics of liquid crystal vortices in layered dilute Fe-doped ${\mathrm{NbS}}_2$ superconductors. We observed enhanced superconductivity with Fe atom doping. The thermally activated vortex flow causes a broad resistive transition. The temperature dependence of the melting field $H_m$ on the applied magnetic field exhibits an upturn behavior as the temperature decreases. The result indicates that the vortex has transitioned from solid to liquid crystal phase. The in-plane anisotropy with twofold symmetry results from the different number of vortex pairs that stably exist on conduction planes when liquid crystal vortices move between neighboring layers. We suggest that under high magnetic fields, the resistance is suppressed due to the pinning of three-dimensional vortices formed by aligned vortex loops.