Molecular vibrations and their dynamics are of outstanding importance for electronic and thermal transport in nanoscale devices as well as for molecular catalysis. The vibrational dynamics of <100 molecules are studied through three-color time-resolved coherent anti-Stokes Raman spectroscopy using plasmonic nanoantennas. This isolates molecular signals from four-wave mixing (FWM) while using exceptionally low nanowatt powers to avoid molecular damage via single-photon lock-in detection. FWM is found to be strongly suppressed in nanometer-wide plasmonic gaps compared to plasmonic nanoparticles. Simultaneous time-resolved incoherent anti-Stokes Raman spectroscopy allows us to separate the contributions of vibrational population decay ($T_1$) and dephasing ($T_2$). With increasing illumination intensity, the ultrafast vibrational dephasing rates of biphenyl-4-thiol molecules are accelerated at least tenfold, while phonon population decay rates remain constant. The extreme plasmonic field enhancement within nanogaps opens up prospects for measuring single-molecule vibrationally coupled dynamics and diverse molecular optomechanics phenomena.

Disclinations play an amusing role in topological electronic states because of the ability to trap charges residing in the cores of these defects. According to the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory of two-dimensional (2D) melting, unbinding the bound pairs of disclinations (i.e., dislocations) induces the structural transition from the hexatic to liquid phase, corresponding to the destruction of quasilong range order. In this work, we elucidate the interplay between topological electronic states and the KTHNY structural transition. We verify the existence of electronic topology-induced charges that are trapped at disclinations during the 2D melting of hard disk particles. Resorting to the Ewald technique, we calculate the real-space distribution of the electrostatic potential generated by nonuniform Coulomb gas composed of these trapped charges. We found that the nonzero gradient of the electrostatic potential gives rise to additional contributions to the hydrostatic pressure. Based on the linear elastic theory, we further show that the electronic topology-induced trapped charges add extra contributions to the core energy of dislocations, increase the energy barrier of dislocation unbinding, and eventually decrease the lower critical boundary of the hexatic phase in the phase diagram. Our finding not only gives a special perspective for studying amorphous topological insulators but proposes a new impacting mechanism in the deviation of the phase boundary of structural transitions.

In the absence of disorder and interactions, fermions move coherently and their associated charge and energy exhibit ballistic spreading, even at finite energy density. In the presence of weak interactions and a finite energy density, fermion-fermion scattering leads to a crossover between early-time ballistic and late-time diffusive transport. The relevant crossover timescales and the transport coefficients are both functions of interaction strength, but the question of determining the precise functional dependence is likely impossible to answer exactly. In this work we develop a numerical method (fDAOE) which is powerful enough to provide an approximate answer to this question, and which is consistent with perturbative arguments in the limit of very weak interactions. Our algorithm, which adapts the existing dissipation-assisted operator evolution (DAOE) to fermions, is applicable to systems of interacting fermions at high temperatures. The algorithm approximates the exact dynamics by systematically discarding information from high $n$-point functions, and is tailored to capture noninteracting dynamics exactly. Applying our method to a microscopic model of interacting fermions, we numerically determine crossover timescales and diffusion constants for a wide range of interaction strengths. In the limit of weak interaction strength ($\mathrm{\Delta }$), we demonstrate that the crossover from ballistic to diffusive transport happens at a time $t_D\sim 1/{\mathrm{\Delta }}^2$ and that the diffusion constant similarly scales as $D\sim 1/{\mathrm{\Delta }}^2$. We confirm that these scalings are consistent with a perturbative Fermi's golden rule calculation, and we provide a heuristic operator-spreading picture for the crossover between ballistic and diffusive transport.

The non-Hermitian skin effects are representative phenomena intrinsic to non-Hermitian systems: the energy spectra and eigenstates under the open boundary condition (OBC) drastically differ from those under the periodic boundary condition (PBC). Whereas a nontrivial topology under the PBC characterizes the non-Hermitian skin effects, their proper measure under the OBC has not been clarified yet. This paper reveals that topological enhancement of nonnormality under the OBC accurately quantifies the non-Hermitian skin effects. Corresponding to spectrum and state changes of the skin effects, we introduce two scalar measures of nonnormality and argue that the non-Hermitian skin effects enhance both macroscopically under the OBC. We also show that the enhanced nonnormality correctly describes phase transitions causing the non-Hermitian skin effects and reveals the absence of non-Hermitian skin effects protected by average symmetry. The topological enhancement of nonnormality governs the perturbation sensitivity of the OBC spectra and the anomalous time-evolution dynamics through the Bauer-Fike theorem.

In the solid state, a large variety of single-photon emitters present high-quality photophysical properties together with a potential for integration. However, in many cases, the host matrix induces fluctuations of the emission wavelength in time, limiting the potential applications based on indistinguishable photons. A deep understanding of the underlying spectral diffusion processes is therefore of high importance for improving the stability of the light emission. Here, we theoretically investigate the photon statistics of an emitter driven by a resonant laser, and subject to either of two qualitatively different stationary spectral diffusion processes—a continuous diffusion process and a process based on discrete spectral jumps, both of which are known to model the spectral diffusion of various solid-state emitters. We show that the statistics of light emission carries several experimentally accessible signatures that allow us to discriminate between the two classes of models, both at short times in the intensity correlation function, and at long times in the fluctuations of the integrated intensity. These results establish that resonant excitation combined with photon statistics offers a rich access to the spectral diffusion processes, yielding information that goes beyond the bare characterization of the inhomogeneous shape and noise correlation time. Incidentally, our findings shed light on recent experimental results of spectral diffusion of B centers in hexagonal boron nitride, providing more insight into their spectral diffusion mechanisms.

We theoretically study the interplay of short-range random and quasiperiodic static potentials on the low-energy properties of three-dimensional Weyl semimetals. This setting allows us to investigate the connection between the semimetal to diffusive metal “magic-angle” phase transition due to quasiperiodicity and the rare-region-induced crossover at an avoided quantum critical point (AQCP) due to disorder. We show that in the presence of both random and quasiperiodic potentials the AQCP becomes lines of crossovers, which terminate at magic-angle critical points in the quasiperiodic, disorder-free limit. We analyze the magic-angle transition by approaching it along these lines of avoided transitions, which unveils a rich miniband structure and several AQCPs. These effects can be witnessed in cold atomic experiments through potential engineering on semimetallic band structures.

Rare earth monogallide (REGa) Zintl phases are attractive for their properties in hydrogen storage and magnetic cooling. However, the magnetic effects upon hydrogen additions in REGa are not well understood. This study aims to explore the magnetic effects in ${\mathrm{REGaH}}_{\mathrm{x}}$ using SQUID magnetometry and neutron powder diffraction. To avoid challenges due to absorption and high incoherent scattering in the neutron diffraction experiments, the compound ${\mathrm{NdGaD}}_x$ ($x$ = 0, 0.9, or 1.6) was chosen for examination. It was found that NdGa exhibits two ferromagnetic structures below the Curie temperature of 42 K. Just below 42 K the magnetic moments are oriented along the crystallographic $c$ axis, and at 20 K a spin reorientation occurs where the moments turn $\sim {30}^{\circ }$ toward the $a$ axis. Upon partial deuteration ($x$ = 0.9), the magnetization decreases and two magnetic phases are observed, one intermediate incommensurate phase, and one canted ferromagnetic phase with the net magnetization aligning along the $b$ axis. For the full deuteride ($x$ = 1.6) only one incommensurate magnetic phase is observed at low temperatures. Magnetometry also reveals that there are no isotope effects when absorbing H or D. The absorption of H or D changes the Nd-Nd distances as well as the electronic structure, which results in a drastic change in the magnetic properties as compared to NdGa.

We report on the observation of negative electronic compressibility in twisted bilayer graphene for Fermi energies close to insulating states. To observe this negative compressibility, we take advantage of naturally occurring twist-angle domains that emerge during the fabrication of the samples, leading to the formation of charge islands. We accurately measure their capacitance using Coulomb oscillations, from which we infer the compressibility of the electron gas. Notably, we not only observe the negative electronic compressibility near correlated insulating states at integer filling, but also prominently near the band insulating state at full filling, located at the edges of both the flat and remote bands. Furthermore, the individual twist-angle domains yield a well-defined carrier density, enabling us to quantify the strength of electronic interactions and verify the theoretical prediction that the inverse negative capacitance contribution is proportional to the average distance between the charge carriers. A detailed analysis of our findings suggests that Wigner crystallization is the most likely explanation for the observed negative electronic compressibility.

Optical excitation of chiral phonons plays a vital role in studying the phonon-driven magnetic phenomena in solids. Transition metal dichalcogenides host chiral phonons at high symmetry points of the Brillouin zone, providing an ideal platform to explore the interplay between chiral phonons and valley degree of freedom. Here, we investigate the helicity-resolved magneto-Raman response of monolayer ${\mathrm{MoS}}_2$ and identify a doubly degenerate Brillouin-zone-center chiral phonon mode at $\sim 270{\mathrm{cm}}^{-1}$. Our wavelength- and temperature-dependent measurements show that this chiral phonon is activated through the resonant excitation of $A$ exciton. Under an out-of-plane magnetic field, the chiral phonon exhibits giant Zeeman splitting, which corresponds to an effective magnetic moment of $\sim 2.5{\mu }_B$. Moreover, we carry out theoretical calculations based on the morphic effects in nonmagnetic crystals, which reproduce the linear Zeeman splitting and Raman cross section of the chiral phonon. Our study provides important insights into lifting the chiral phonon degeneracy in an achiral covalent material, paving a route to excite and control chiral phonons.

Topological band theory has achieved great success in the high-throughput search for topological band structures both in paramagnetic and magnetic crystal materials. However, a significant proportion of materials are topologically trivial insulators at the Fermi level. In this paper, we show that, remarkably, for a subset of the topologically trivial insulators, knowing only their electron number and the Wyckoff positions of the atoms we can separate them into two groups: the obstructed atomic insulator (OAI) and the atomic insulator (AI). The interesting group, the OAI, have a center of charge not localized on the atoms. Using the theory of topological quantum chemistry, in this work we first derive the necessary and sufficient conditions for a topologically trivial insulator to be a filling enforced obstructed atomic insulator (feOAI) in the 1651 Shubnikov space groups. Remarkably, the filling enforced criteria enable the identification of obstructed atomic bands without knowing the representations of the band structures. Hence, no ab initio calculations are needed for the filling enforced criteria, although they are needed to obtain the band gaps. With the help of the Topological Quantum Chemistry website, we have performed a high-throughput search for feOAIs and have found that 957 ICSD entries (638 unique materials) are paramagnetic feOAIs, among which 738 (475) materials have an indirect gap. The metallic obstructed surface states of feOAIs are also showcased by several material examples.

We report on Landau-level spectroscopy of an epitaxially grown thin film of the topological insulator ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, complemented by ellipsometry and magnetotransport measurements. The observed response suggests that ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ is a direct-gap semiconductor with the fundamental band gap located at the $\mathrm{\Gamma }$ point or along the trigonal axis, and its width reaches $E_g=(190\pm 10)$ meV at low temperatures. Our data also indicate the presence of other low-energy extrema with a higher multiplicity in both the conduction and valence bands. The conclusions based on our experimental data are confronted with and to a great extent corroborated by the electronic band structure calculated using the $GW$ method.

Altermagnetism is a topic that has lately been gaining attention and the ${\mathrm{RuO}}_2$ compound is among one of the most studied altermagnetic candidates. However, the survey of available literature on ${\mathrm{RuO}}_2$ properties suggests that there is no consensus about the magnetism of this material. By performing density functional theory (DFT) calculations, we show that the electronic properties of stoichiometric ${\mathrm{RuO}}_2$ are described in terms of a Hubbard $U$, within $\mathrm{DFT}+U$, smaller than the value required to have magnetism. We further argue that Ru vacancies can actually aid the formation of a magnetic state in ${\mathrm{RuO}}_2$. This in turn suggests that a characterization of the amount of Ru vacancies in experimental samples might help the resolution of the controversy between the different experimental results.

The detailed electronic superlattice structure of the Remeika phase compound ${\mathrm{Nd}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ was investigated. The material undergoes a structural phase transition at $T_{\mathrm{D}}\simeq 338$ K between the high temperature phase ($Pm\overline{3}n$) and the low temperature chiral structure phase ($I{2}_{1}3$), which is the same symmetry change as that of the La- and Ce-based Remeika phase compounds. The inequivalent Nd-ion sites in the chiral phase are occupied by different crystalline electronic field splitting schemes of the $4f^3$ electronic configuration with the total angular moment $J=9/2$, which results in two-sublattice antiferromagnetic ordering below $T_{\mathrm{N}}=1.65$ K. The one-dimensional Nd-ion chains in which the magnetic moments are alternately arranged are connected via the second-neighbor triangular lattice in the chiral phase, which suggests a magnetic frustration effect.

Recently, it has been shown that transmon qubit architectures experience a transition between many-body localized and quantum chaotic phases. While it is crucial for quantum computation that the system remains in the localized regime, the most common way to achieve this has been relying on disorder in Josephson junction parameters. Here, we propose a quasiperiodic patterning of parameters as a substitute for random disorder. We demonstrate, using the Walsh-Hadamard diagnostic, that quasiperiodicity is more effective than disorder for achieving localization. To study the localizing properties of our Hamiltonian for large, experimentally relevant system sizes, we use two complementary perturbation theory schemes, one with respect to the many-body interactions and one with respect to the hopping parameter of the free Hamiltonian.

Reduced thickness could have dramatic impact on the dynamic ordering kinetics and the electronic structures in two-dimensional materials. Here we focus on ${\mathrm{ZrTe}}_3$, a quasi-one-dimensional charge-density-wave system that can transform into a mixed bulk-filament anisotropic superconducting state with the superconducting transition temperature ($T_{\mathrm{c}}^{\mathrm{zero}}$) of 1.6 K. We have successfully exfoliated ${\mathrm{ZrTe}}_3$ crystal from bulk crystal to bilayer (2 nm) flake. By examining the anisotropic transport properties, we reveal a competitive relationship between superconductivity and charge density wave (CDW) in the thickness range down to 8 nm. With decreasing thickness, superconductivity is enhanced (yielding a maximum $T_{\mathrm{c}}^{\mathrm{zero}}$ of about 5 K at 18 nm) while CDW is suppressed. Upon further decreasing thickness, an unexpected enhancement on CDW transition temperature was observed accompanied with the superconductivity rapidly suppressing and finally vanishing. Shifting of Fermi level upon thinning, anisotropic strain from the substrates, and the increase of disorder scattering as approaching the two-dimensional limit might be taken into account for understanding these behaviors. The nonmonotonic evolution with a reduction of thickness is rare and may indicate an intricate mechanism of CDW and SC in ${\mathrm{ZrTe}}_3$.

We report on the role of light's angular momentum in magnetic all-optical switching (AOS), which entails magnetization reversal resulting from ultrafast laser pulses applied to a magnetic structure. The magnetic system is comprised of a ferromagnetic thin film of Co/Pt, excited by femtosecond vortex beams of light carrying spin, orbital angular momentum, or a superposition of both. We find that AOS is predominantly driven by the spin angular momentum of light. The experimental findings suggest that the magnitude of topological charge and handedness of the orbital angular momentum of light do not play a significant role. These results improve our understanding of the basic interactions between magnetism and light, paving the way for further investigation and the creation of all-optical magnetic switching technologies.

We study the XXZ model with a random magnetic field in contact with a weakly disordered spin chain, acting as a finite thermal bath. We revise Fermi's golden rule description of the interaction between the thermal bath and the XXZ spin chain, contrasting it with a nonperturbative quantum avalanche scenario for the thermalization of the system. We employ two-point correlation functions to define the extent ${\xi }_d$ of the thermalized region next to the bath. Unbounded growth of ${\xi }_d$ proportional to the logarithm of time or faster is a signature of an avalanche. Such a behavior signifies the thermalization of the system, as we confirm numerically for a generic initial state in the ergodic and critical regimes of the XXZ spin chain. In the many-body localized regime, a clear termination of avalanches is observed for specifically prepared initial states and, surprisingly, is not visible for generic initial product states. Additionally, we extract the localization length of the local integrals of motion and show that a bath made out of a weakly disordered XXZ chain has a similar effect on the system as a bath modeled by a Hamiltonian from a Gaussian orthogonal ensemble of random matrices. We also comment on the result of the earlier study [Phys. Rev. B 108, L020201 (2023)], arguing that the observed thermalization is due to external driving of the system and does not occur in the autonomous model. Our work reveals experimentally accessible signatures of quantum avalanches and identifies conditions under which termination of the avalanches may be observed.

We uncover high-order harmonics and the reverse of the squaring-up process, in terms of analyzing the evolution of magnetic orders in a centrosymmetric layered triangular-lattice magnet ${\mathrm{HoPdAl}}_4{\mathrm{Ge}}_2$, based on a detailed study of the crystal structure, magnetic susceptibility, magnetization, heat capacity, and magnetic structure. Temperature dependencies of magnetic susceptibility and heat capacity show two magnetic transitions at $T_N=10.5$ K and $T_{\mathit{t}}=5.5$ K. Below $T_N, {\mathrm{Ho}}^{3+}$ spins order antiferromagnetically as a transverse spin-density wave with the propagation vector ${\mathit{k}}_{\mathit{1}}=(001.5-\delta )$ with $\delta \approx 0.18$. Upon further cooling through $T_{\mathit{t}}$, the high-order harmonics with ${\mathit{k}}_{\mathit{n}}=(001.5-n\delta ), n=3$, 5, 7 develop, suggesting a squaring-up process. It is surprising that the squaring-up process does not continue down to 0 K but reverses the trend below 3 K. Magnetic-field-induced metastable transitions were observed in $M\left(H\right)$ curves with the fields applied both parallel and perpendicular to the triangular-lattice plane. Neutron-diffraction results suggest that the magnetization process in ${\mathrm{HoPdAl}}_4{\mathrm{Ge}}_2$ involves the conversion of ${\mathit{k}}_{\mathit{n}}=(001.5-n\delta )$ with $n=1$, 3, 5, 7, to a ferromagnetic ${\mathit{k}}_{\mathit{F}}=\left(000\right)$ component. It is worth noting that the already weak seventh harmonic magnetic peak is enhanced by applying a small magnetic field in the $ab$ plane at 1.5 K or warming up to 3 K, accompanied by a slight decrease of $\delta$.

Single crystals of ${\mathrm{DyRh}}_2{\mathrm{Si}}_2$ and ${\mathrm{HoRh}}_2{\mathrm{Si}}_2$ were investigated by thermal expansion and magnetostriction. The different types of magnetic order can clearly be seen in these measurements, particularly the canting of the moments away from the crystallographic $c$ direction below about 12 K and the spin flip for magnetic field applied along the $c$ direction. For ${\mathrm{HoRh}}_2{\mathrm{Si}}_2$, an additional transition just below $T_{\mathrm{N}}$ is analyzed by means of the Grüneisen ratio and is likely caused by a change in the magnetic structure. Our results nicely corroborate findings from other magnetic and thermodynamic measurements on these materials and provide further evidence suggesting the formation of magnetic domains.

In this work we consider the superfluid stiffness of a generically non-Galilean-invariant interacting system and investigate under what conditions the stiffness may nonetheless approach the Galilean-invariant value $n/m$. Within Eliashberg theory we find that the renormalized stiffness is approximately given by $n/m$ in the case when the $l=0$ and 1 components of the effective Fermi-surface projected interaction are approximately equal over a range of frequencies. This holds, in particular, when the interaction is peaked at zero momentum transfer. We examine this result through three complementary lenses: the $\delta \left(\omega \right)$ term in the conductivity, the phase dependence of the Luttinger-Ward free energy, and the coupling of the amplitude and phase sectors in the Hubbard-Stratonovich collective mode action. From these considerations we show that the value of the stiffness is determined by the strength of renormalization of the current vertex and that the latter can be interpreted as the shift of the self-consistent solution due to flow of the condensate, or alternatively as coupling of the phase mode to $l=1$ fluctuations of the order parameter. We highlight that even though the superfluid stiffness in some non-Galilean systems approaches the Galilean value, this is not enforced by symmetry, and in general the stiffness may be strongly suppressed from its BCS value. As a corollary we obtain the generic form of the phase action within Eliashberg theory and charge and spin Ward identities for a superconductor with frequency-dependent gap function.