Using terahertz and infrared spectroscopy, we obtained spectra of the complex dielectric permittivity for ${}^{6,7}\mathrm{Li}$@${\mathrm{C}}_{60}{\mathrm{PF}}_6$ and ${}^7\mathrm{Li}$@${\mathrm{C}}_{60}{\mathrm{ClO}}_4$ pressed pellets across the frequency range of 10–700 ${\mathrm{cm}}^{-1}$ and at temperatures from 5 to 300 K. A rich set of absorption lines was observed, and their origins were analyzed based on detailed temperature dependencies of lines' frequencies, intensities, and damping factors, as well as computer simulations. An isotopic frequency shift was observed for several lines and was considered in the analysis of their origins. At terahertz frequencies, we identified a temperature-dependent absorption band, for which two possible explanations are proposed: quantized rotation of the Li ion within the ${\mathrm{C}}_{60}$ cage and thermally assisted (above $\approx 100$ K) or tunnel-assisted (below $\approx 100$ K) hopping of Li over minima in the localizing potential. From the activated behavior of the frequency and intensity of this band, the depths of the corresponding potential wells were estimated to be approximately 4.3 meV above $\approx 100$ K and approximately 7 meV below $\approx 100$ K. Additionally, signatures of excitation around 6 ${\mathrm{cm}}^{-1}$ were detected, which was attributed to vibrations of the ${\mathrm{C}}_{60}$ lattice. Significant disorder among the ${}^7\mathrm{Li}$@${\mathrm{C}}_{60}{\mathrm{ClO}}_4$ molecules is observed, resulted in a broad background absorption over the entire frequency range (10–700 ${\mathrm{cm}}^{-1}$) and temperature 5–100 K interval. The results and insights gained from this study could be used in designing nanoelectronic devices based on Li@${\mathrm{C}}_{60}$ molecules.

We study superconducting (SC) phases that are naturally proximate to a spin-orbit-coupled SU(8) Dirac semimetal on a honeycomb lattice. This system, which offers enhanced low-energy symmetries, presents an interesting platform for realizing unconventional superconductivity in $j=3/2$ electrons. In particular, we find 72 superconducting charge-$2e$ fermion bilinears, which, under classification of microscopic symmetries, lead to 12 different SCs—four singlets, two doublets, and six triplets—seven of them are gapped and five are symmetry-protected nodal SCs. The strong spin-orbit coupling leads to locking of the spin of the Cooper pairs with real-space direction—as is evident from the structure of the Cooper-pair wave functions—leading to unusual nonunitary superconductors (even singlets), and with finite-momentum pairing (for the triplets). This results, in many cases, in the magnitude of multiple pairing gaps being intricately dependent on the direction of the SC order parameter. The present classification of SCs along with normal phases [Phys. Rev. B 108, 245106 (2023)] provides the complete list of naturally occurring phases in the vicinity of such a SU(8) Dirac semimetal. This study allows for understanding the global phase diagram of such systems, stimulating further experimental work on candidate materials such as metallic halides ($\mathit{MX}{}_3$ with $M=\mathrm{Zr},\mathrm{Hf}$, and $X=\mathrm{Cl},\mathrm{Br}$). Further, it provides the starting point for the exploration of unconventional phase transitions in such systems.

We study theoretically the dynamics of frustrated spin-Peierls systems after a quench from the paramagnetic state to the magnetically ordered state. By constructing and numerical simulating a minimal model, we show that it can exhibit a transient noncollinear magnetic order before settling in the equilibrium collinear magnetic order. The transient magnetic order is selected by the phonon fluctuations originated from the initial state. Our results reveal a mechanism for controlling the phases of matter by exploiting incoherent, nonequilibrium fluctuations.

Here, we investigate the ultrafast carriers, coherent acoustic phonons, and acoustic strain pulse dynamics in topological insulator ${\mathrm{BiSbTe}}_{1.5}{\mathrm{Se}}_{1.5}$ (BSTS) thin films of various thicknesses using degenerate pump-probe reflection spectroscopy. In this study, we have used sapphire as the main substrate since the BSTS-Sapphire interface has the maximum acoustic reflection in comparison to BSTS-GaAs, BSTS-Si, and BSTS-MgO interfaces. The transient reflectivity data for the films with thickness more than twice the penetration depth mainly contains the traveling acoustic strain pulses (TASP) and a single-exponential ($\sim 2$ ps) electronic background whereas the data for the films with thickness less than penetration depth is dominated by coherent acoustic phonons (CAPs) and a biexponential electronic background of ${\tau }_1\sim 2$ ps and ${\tau }_2\sim 260–380$ ps. The experimentally observed TASP are reproduced by a well-known theoretical acoustic strain model. Further, to examine the underlying physics that is responsible for the dynamics of photoexcited carriers, CAPs and strain pulses, we have carried out carrier density and temperature dependent (7–294 K) studies on 22 nm and 192 nm thick BSTS films. For the 22 nm BSTS film, the two decay constants ${\tau }_1$ and ${\tau }_2$ increase with increase in carrier density at room temperature and decrease with increase in temperature at a carrier density of 1.7 $\times {10}^{19}{\mathrm{cm}}^{-3}$, respectively. A closer inspection of the behavior of ${\tau }_1$ and ${\tau }_2$ with carrier density and temperature indicates that ${\tau }_1$ is due to the interplay of electron-phonon scattering and carrier diffusion whereas ${\tau }_2$ plausibly stems from the combination of defect-assisted and phonon-assisted recombination. Furthermore, with increasing temperature, there is an anharmonic decay induced softening of $\sim 14\%$ in the phonon frequency and $\sim 48\%$ anomalous decrease in the phonon damping parameter due to decreased scattering between the Dirac surface electrons and the acoustic phonons. Additionally, temperature-dependent studies on the 192 nm film reveal $\sim 7\%$ reduction in sound velocity, compared to $14\%$ reduction in 22 nm film.

We study quenched dynamics of fully connected spin models. The system is prepared in a ground state of the initial Hamiltonian and the Hamiltonian is suddenly changed to a different form. We apply the Krylov subspace method to map the system onto an effective tridiagonal Hamiltonian. The state is confined in a potential well and is time evolved by nonuniform hoppings. The dynamical singularities for the survival probability can occur when the state is reflected from a potential barrier. Although we do not observe any singularity in the spread complexity, we find that the entropy exhibits small dips at the singular times. We find that the presence of metastable state affects long-time behavior of the spread complexity, and physical observables. We also observe a reduction of the state-space dimension when the Hamiltonian reduces to a classical form.

Quantum geometry is a well-established framework for understanding transport and optical responses in quantum materials. In this work, I study the photon drag effect in Dirac electrons using the quantum geometric interpretation of nonvertical optical transitions. Due to the particle-hole symmetry inherent in Dirac electrons, the shift photon-drag photocurrent is dominated by dissipationless Fermi surface contributions, connected to the dipole of quantum metric tensor. I find that this dipole is significantly enhanced by a small band gap in massive Dirac electrons and remains robust in the massless limit. I demonstrate the existence of a circular shift photon-drag current in the effective Hamiltonian at the $L$ point of bismuth, where the bands exhibit trivial topology, highlighting the ubiquity of the circular photon-drag effect in centrosymmetric materials.

Recent experiments demonstrate that spin dynamics may acquire an inertial effect in a few metallic magnets, deviating from the traditional inertia-free dynamics. It remains an open question to ascertain the physical mechanisms and universality of the spin inertia across diverse magnetic systems. Here, we show that spin inertia generates nutation spin waves in the terahertz regime, which can hybridize with the surface plasmons in two-dimensional (2D) conducting materials such as graphene. By exciting hybrid spin-wave–plasmon modes and analyzing the reflection spectrum of a 2D material-magnet heterostructure, we propose a method to quantitatively determine the strength of spin inertia in magnetic layers. Our approach is universally applicable to all types of magnetic insulators and could advance the future exploration of the magnitude and physical mechanism of spin inertia.

The rare earth intermetallic system $R\mathrm{Al}X$ (R = rare earth elements, $X=\mathrm{Si} \text{and} \mathrm{Ge})$ is known to be a promising candidate of magnetic Weyl semimetal. Due to the complex interactions between the rare earth elements and surrounding atoms, as well as hybridization with itinerant electrons, this family likely possesses highly intriguing and novel magnetic structures and thus exhibits dynamic behaviors. We systematically probe polycrystalline samples of $R\mathrm{AlSi} (R=\mathrm{La}, \mathrm{Ce}, \mathrm{Pr}, \text{and} \mathrm{Nd})$ combining inelastic neutron scattering (INS), heat capacity, and magnetic susceptibility measurements. The INS measurements identify well-resolved crystalline electric field (CEF) excitations at 19.2 and 24.9 meV in CeAlSi, at 5.4 meV in PrAlSi, and at 2.5 and 4.2 meV in NdAlSi. We analyzed the INS data using the corresponding CEF models and determined the CEF parameters and ground state wave functions of $R\mathrm{AlSi} (R=\mathrm{Ce}, \mathrm{Pr}, \text{and} \mathrm{Nd})$. Our results suggest strong single-ion anisotropy in their ground states: $|\pm 3/2\rangle$ $(94.5\%)$ in CeAlSi, $|\pm 3\rangle$ $(99.2\%)$ in PrAlSi, and $|\pm 9/2\rangle$ $(76.2\%)$ in NdAlSi. Notably, the weaker anisotropy and strong exchange interactions in NdAlSi promote competing magnetic orders and CEF splitting at low temperature, contrasting with the robust CEF levels in magnetic states of CeAlSi and PrAlSi.

We identify a magnetoelectric correction that completes the theoretical description of spin splitting (SS) in magnetic systems. Derived from the Dirac equation, this term couples local magnetic moments to the scalar electric potential, providing a third fundamental mechanism—alongside Zeeman and spin-orbit coupling (SOC)—that governs SS in ferromagnets, antiferromagnets, and altermagnets. In compensated magnets, the proposed relativistic correction depends on the difference in electric potential between symmetry-inequivalent motifs, ${\mathcal{H}}_{\text{ME}}=-{\mu }_{\text{B}}{\eta }_0({\mathcal{V}}_1-{\mathcal{V}}_2)\mathit{\sigma }·\mathit{m}$, which explains how finite SS emerges in the absence of SOC and enables a complete classification of momentum dependence and motif connectivity across all 32 point groups. Through illustrative examples, we show that distinct SS behaviors—quadratic ($d$-wave altermagnets), linear ($p$-wave altermagnets or spin Zeeman effect), and $k$ independent (SS at $\mathrm{\Gamma }$ or fully compensated ferrimagnets)—are specific manifestations of the proposed magnetoelectric relativistic mechanism, each governed by electric quadrupoles, dipoles, or monopoles, respectively. The formalism naturally extends to higher-order multipoles and more complex symmetries. This work establishes a unified framework for SS in magnets and provides a predictive tool for analyzing symmetry-allowed SS in magnetic materials.

H impurities have been found to be critical for the emergence of superconductivity in nickelate superconductors, but the nature of H-induced superconductivity in nickelates remains highly controversial. In this work, using first-principles calculations and cluster expansion models with Monte Carlo simulations, we demonstrate that H impurities can effectively facilitate the charge transfer from the out-of-plane $\text{Ni-3}{d}_{z^2}$ to the in-plane $\text{Ni-3}{d}_{x^2-y^2}$ orbitals, thereby enhancing the in-plane nearest-neighbor Ni-Ni antiferromagnetic (AFM) exchange. Similar to cuprate superconductivity, this in-plane AFM correlation significantly enhances the electron-phonon (el-ph) coupling strength of the in-plane O breathing mode and consequently increases the superconducting critical temperature $T_c$. Based on these findings, we reproduce the dome-shaped variation of $T_c$ in the H-doping phase diagram with the AFM-assisted el-ph coupling mechanism. Our finding provides valuable insights into the critical role of H impurities and contributes to a fundamental understanding of H-induced superconductivity in nickelate superconductors.

Accurately determining the thermal transport properties of the two-dimensional material ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ is essential for its application in micro- and nanoscale semiconductor devices. To date, substantial discrepancy persists between theoretical prediction and experimental measurement of the thermal conductivity ${\kappa }_L$ of ${\mathrm{MoSi}}_2{\mathrm{N}}_4$, with the underlying physical mechanisms remaining unclear. In this work, the intrinsic ${\kappa }_L$ of ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ is accurately determined by using the neuroevolutionary potential combined with the phonon Boltzmann transport equation (BTE). Taking the temperature-dependent interatomic force constants extracted from effective harmonic model as the inputs, the room-temperature ${\kappa }_L$ of ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ calculated by the BTE method (approximately $351.84\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$) exhibits remarkable consistency with results obtained from a homogeneous nonequilibrium molecular-dynamics method (approximately $343.04\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$). These results underscore the critical influence of higher-order phonon scattering and temperature renormalization of force constants on the intrinsic ${\kappa }_{\iota }$ of ${\mathrm{MoSi}}_2{\mathrm{N}}_4$. Our calculations also demonstrate that the thermal transport performance of ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ could be evidently suppressed by nitrogen vacancy. As for the ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ sample with 0.02% randomly distributed N vacancy, the ${\kappa }_L$ could be decreased to approximately $175\pm 8.96\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, which provides a plausible explanation for the discrepancy between theoretical predictions and experimental measurements (approximately $173\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$). Interestingly, compared to the internal N vacancy, the surface N vacancy exhibits particularly pronounced effect in suppressing the ${\kappa }_L$ of ${\mathrm{MoSi}}_2{\mathrm{N}}_4$. Such a phenomenon is primarily attributed to significant modifications of the symmetry of the charge density distribution and the geometric environment induced by the surface N vacancy, which weaken Si-N bond interactions and amplify the vibrational motion of Si atoms, thereby enhancing the phonon anharmonicity. This work offers valuable insights into the thermal transport properties of the ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ family and serves as a case study for exemplifying the use of machine-learning interatomic potentials to investigate the impact of low-concentration disordered vacancy defects on the thermal conductivity of materials.

We explore the phenomena of prethermalization in a many-body classical system of rotors under aperiodic drives characterized by waiting time distribution (WTD), where the waiting time is defined as the time between two consecutive kicks. We consider here two types of aperiodic drives: random and quasiperiodic. We observe a short-lived pseudothermal regime with algebraic suppression of heating for the random drive where WTD has an infinite tail, as observed for Poisson and binomial kick sequences. On the other hand, quasiperiodic drive characterized by a WTD with a sharp cutoff, as observed for the Thue-Morse sequence of kicks, lead to a prethermal region where heating is exponentially suppressed. However, according to our observations, the exponential suppression of the heating rate can not be associated with the boundedness of the WTD. The kinetic energy growth is analyzed using an average surprise associated with the WTD quantifying the randomness of the drive. In all aperiodic drives we obtain the chaotic heating regime for late times; however, the diffusion constant gets renormalized by the average surprise of the WTD in comparison to the periodic case.

Antiferromagnets have emerged as promising spintronic materials due to their ultrafast spin dynamics and zero stray field. While these materials exhibit fascinating magnetoresistance effects, the inherent lack of stray field presents a fundamental duality-enabling high density integration but impeding low-field operation, which has remained a key challenge in the field. In this work, we report enhanced low-field anisotropic magnetoresistance (AMR) in lightly hole-doped $J_{\mathrm{eff}}=1/2$ antiferromagnets ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Zn}}_x{\mathrm{O}}_4$, which meanwhile exhibits exceptional angular sensitivity. The large low-field AMR persists across wide temperature ranges and diverse current-direction configurations. This essentially relies on the intrinsic rigidity of the antiferromagnetic phase to hole-doping, evidenced by a slight decrease in the Néel temperature $T_{\mathrm{N}}$ over a broad doping range. The AMR is relevant entirely to crystal symmetry, e.g., a crystalline type effect depending on the magnetocrystalline anisotropy, which is more essentially associated with the low valance state of ${\mathrm{Zn}}^{2+}$.

Ferrimagnetic ${\mathrm{Mn}}_4\mathrm{N}$ thin films have low magnetization and perpendicular magnetic anisotropy (PMA), as required for fast and low current switching spintronics devices. However, there need to be improvements in the growth of epitaxial quality films with fast spin dynamics for such spintronic applications. Here we prepared ${\mathrm{Mn}}_4\mathrm{N}$ thin films with a Pd buffer layer and investigated the crystalline structure and magnetic properties. We demonstrated that both crystalline quality and PMA of ${\mathrm{Mn}}_4\mathrm{N}$ thin films are enhanced significantly due to the relaxation of tensile stress induced by the Pd buffer layer. We also demonstrated a fast magnetization precession at room temperature, almost 100 GHz. With the characteristics of high thermal stability, enhanced PMA by buffer layer, and fast magnetization precession, ${\mathrm{Mn}}_4\mathrm{N}$ thin films are a promising material for spintronic applications.

The topological Hall effect is the result of spin-asymmetric deflection of charge carriers flowing through a noncollinear spin system. Effective manipulation of the topological Hall conductivity (THC) in skyrmions is currently a vigorous area of research with an eye towards potential spintronics application. Here, we show that the band topology and the THC in a skyrmion crystal can be tuned by changing the strength of the Rashba spin-orbit coupling (SOC), which can be accomplished via a perpendicular electric field. This results in the change of the subband Chern numbers and a transition between ordinary insulator and Chern insulator as the Rashba SOC is varied. For partially filled subbands, the Rashba SOC can tune the THC and reverse its sign, so that the direction of the Hall current is flipped. The critical Rashba strength for this depends on the skyrmion type and the carrier density. We extend our analysis to the cases of Dresselhaus and Weyl SOC as well and show that they can be directly mapped to the Rashba SOC case and therefore lead to similar results. Our work opens up the scope to go beyond the existing avenues for the control of charge transport in skyrmion crystals.

Weyl semimetals host topologically protected surface states, the so-called Fermi arcs, that have a penetration depth into the bulk that depends on surface-momentum, and diverges at the Weyl points. It has recently been observed in ${\mathrm{PtBi}}_2$ that such Fermi arc states can become superconducting, with a critical temperature larger than that of the bulk. Here we introduce a general variational method that captures the interplay between surface and bulk superconductivity for any bulk Hamiltonian that harbors (topological) surface states with varying penetration depth. From the self-consistent solutions, we establish that the surface state localization length of Weyl semimetals leads to characteristic features in the surface superconductivity, with a gap depending on surface momentum and a penetration length for the order parameter that is temperature-dependent due to competition with the bulk superconductivity.

The existence of $\eta$-pairing eigenstates in the fermionic Hubbard model is fundamentally rooted in the $\eta$-pairing symmetry, which may hold for systems with nonuniform Hubbard interaction $U$. In this work, we present a generalized Hubbard model containing a variety of pseudospin terms that break the ${\mathrm{SO}}_4$ symmetry but retain the $\eta$-pairing symmetry. This allows us to construct a variety of correlated systems possessing $\eta$-pairing eigenstates. We exemplify our findings by considering a modified Hubbard model associated with alternative magnetic fields and on-site repulsion. We find that the same quasi-$\eta$-pairing eigenstate exhibits two distinct dynamic behaviors in the two models. Numerical results of the time evolution driven by several typical Hamiltonians accord with the analytic predictions and provide a method for the control of an $\eta$-pairing wave packet with the aid of a time-dependent Hamiltonian.