We investigate the propagation of surface-enhanced resonant Raman scattering (SERRS) light coupled with surface plasmons (SPs) by several micrometers through a one-dimensional hotspot (1D HS) located between a plasmonic nanowire dimer (NWD) by end-type excitations. The propagation exhibits properties, e.g., an effective propagation is induced by excitation and detection polarization orthogonal to the 1D HS long axis, the propagation profiles are composed of bright short and dark long propagations, SERRS spectral shapes are independent of localized plasmon (LP) resonance of NWDs, the SERRS spectra redshifts at the edges of 1D HSs, and considerable NWD-by-NWD variations appear in the propagation lengths. These properties are well reproduced by numerical calculations based on electromagnetism. These calculations reveal the following propagation mechanism: excitation light resonantly coupled with LP at the edges of 1D HSs, and the light energy is transferred to two types of junction SP modes supporting the short and long propagations; these modes are attributed to the upper and lower branches of coupled two-SP modes. This mechanism comprehensively clarifies the abovementioned properties.

In spin-orbit-coupled magnetic materials, the usually applied Curie-Weiss law can break down. This is due to potentially sharp temperature dependence of the local magnetic moments. We therefore propose a modified Curie-Weiss formula suitable for analysis of experimental susceptibility. We show for octahedrally coordinated materials of $d^5$ filling that the Weiss constant obtained from the improved formula is in excellent agreement with the calculated Weiss constant from microscopic exchange interactions. Reanalyzing the measured susceptibility of several Kitaev candidate materials with the modified formula resolves apparent discrepancies between various experiments regarding the magnitude and anisotropies of the underlying magnetic couplings.

The Raman scattering with local optical excitation from the Majorana edge modes of Kitaev spin liquids and topological superconductors is studied theoretically. Although the effective one-dimensional model is common between these two cases, the coupling to the electromagnetic field is different. It is found that the Raman spectrum at low energy scales with ${\omega }^3$ in Kitaev spin liquids while it shows the gap in topological superconductors. This is in sharp contrast to the infrared absorption, where the spectrum shows the gap in Kitaev spin liquids, while it behaves as $\sim {\omega }^2$ in topological superconductors. This indicates that the electrodynamics of Majorana edge modes depends on their higher-dimensional origins. A realistic estimate of the Raman scattering intensity is given for $\alpha \text{−}{\mathrm{RuCl}}_3$ as a candidate for a Kitaev spin liquid.

We present the detailed structural and magnetic properties of ${\mathrm{Li}}_2{\mathrm{Mn}}_3{\mathrm{O}}_7$ from powder x-ray diffraction (XRD), dc susceptibility, heat capacity, ac susceptibility, thermoremanent magnetization, magnetic memory, and exchange bias effect. Rietveld refinement of XRD data reveals that this compound has a rhombhohedral structure composed of a layered triangular lattice. Onset of spin-glass transition was confirmed by dc magnetization and ac susceptibility measurements. Dynamic scaling laws were used to analyze and classify the glassy behavior of the compound. Magnetic field dependence of irreversible temperature follows the Almeida-Thouless line, which is characteristic for an Ising spin-glass system. Fitting of the frequency-dependent freezing temperature with a power law results in $z{\nu }^{\prime }=4.06\pm 0.06$, which indicates the critical exponent of the sluggish spin dynamics and ${\tau }_0=4.2\times {10}^{-8}$ s is a characteristic time scale for a single spin-flip. Further evidence of cluster-glass behavior comes from the frequency dependence of the freezing temperature fitted with the Vogel-Fulcher law, which considers interaction between bigger magnetic entities. Values of fitting parameters are $E_a/k_B=27.62$ K and $T_0=9.57$ K, which confirm cluster-glass behavior. The presence of magnetic relaxation below freezing temperature and the magnetic memory effect confirms the nonequilibrium dynamics of the system through a number of metastable states. Moreover, observation of the exchange bias effect reflects the presence of intrinsic phase inhomogeneity. These results indicate that the triangular lattice causes a disordered ground state as a result of competing exchange interactions.

Exactly solvable models play a special role in condensed matter physics, serving as secure theoretical starting points for investigation of new phenomena. Changlani et al. [Phys. Rev. Lett. 120, 117202 (2018)] have discovered a limit of the XXZ model for $S=\frac{1}{2}$ spins on the kagome lattice, which is not only exactly solvable, but features a huge degeneracy of exact ground states corresponding to solutions of a three-coloring problem. This special point of the model was proposed as a parent for multiple phases in the wider phase diagram, including quantum spin liquids. Here, we show that the construction of Changlani et al. can be extended to more general forms of anisotropic exchange interaction, finding a line of parameter space in an XYZ model which maintains both the macroscopic degeneracy and the three-coloring structure of solutions. We show that the ground states along this line are partially ordered, in the sense that infinite-range correlations of some spin components coexist with a macroscopic number of undetermined degrees of freedom. We therefore propose the exactly solvable limit of the XYZ model on corner-sharing triangle-based lattices as a tractable starting point for discovery of quantum spin systems which mix ordered and spin-liquid-like properties.

Highly energetic ions traversing a two-dimensional material such as graphene produce strong electronic excitations. Electrons excited to energy states above the work function can give rise to secondary electron emission, reducing the amount of energy that remains in graphene after the ion impact. Electrons can be either emitted (kinetic energy transfer) or captured by the passing ion (potential energy transfer). To elucidate this behavior that is absent in three-dimensional materials, we simulate the electron dynamics in graphene during the first femtoseconds after ion impact. We employ two conceptually different computational methods: a Monte Carlo (MC)-based one, where electrons are treated as classical particles, and time-dependent density functional theory (TDDFT), where electrons are described quantum mechanically. We observe that the linear dependence of electron emission on deposited energy, emerging from MC simulations, becomes sublinear and closer to the TDDFT data when the electrostatic interactions of emitted electrons with graphene are taken into account via complementary particle-in-cell simulations. Our TDDFT simulations show that the probability for electron capture decreases rapidly with increasing ion velocity, whereas secondary electron emission dominates in the high-velocity regime. We estimate that these processes reduce the amount of energy deposited in the graphene layer by 15%–65%, depending on the ion and its velocity. This finding clearly shows that electron emission must be taken into consideration when modeling damage production in two-dimensional materials under ion irradiation.

Motivated by the possibility of observing the coexistence between magnetism and unconventional superconductivity in heavy-fermion ${\mathrm{Ce}}_{1-x}{\mathrm{Sm}}_x\mathrm{Co}{\mathrm{In}}_5$ alloys, we studied how the samarium substitution on the cerium site affects the magnetic field-tuned-quantum criticality of stoichiometric $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_5$ by performing specific heat and resistivity measurements. By applying an external magnetic field, we have observed Fermi-liquid to non-Fermi-liquid crossovers in the temperature dependence of the electronic specific heat normalized by temperature and of the resistivity. We obtained the magnetic-field-induced quantum critical point (QCP) by extrapolating to zero temperature the temperature-magnetic field dependence at which the crossovers take place. Furthermore, a scaling analysis of the electronic specific heat is used to confirm the existence of the QCP. We have found that the magnitude of the magnetic-field-induced QCP decreases with increasing samarium concentration. Our analysis of heat capacity and resistivity data reveals a zero-field QCP for $x_{\text{cr}}\approx 0.15$, which falls inside the region where Sm ions antiferromagnetism and superconductivity coexist.

Matrix elements of observables in eigenstates of generic Hamiltonians are described by the Srednicki ansatz within the eigenstate thermalization hypothesis (ETH). We study a quantum chaotic spin-fermion model in a one-dimensional lattice, which consists of a spin-1/2 XX chain coupled to a single itinerant fermion. In our study, we focus on translationally invariant observables including the charge and energy current, thereby also connecting the ETH with transport properties. Considering observables with a Hilbert-Schmidt norm of one, we first perform a comprehensive analysis of ETH in the model taking into account latest developments. A particular emphasis is on the analysis of the structure of the offdiagonal matrix elements $|\langle \alpha |\widehat{O}{|\beta \rangle |}^2$ in the limit of small eigenstate energy differences $\omega =E_{\beta }-E_{\alpha }$. Removing the dominant exponential suppression of $|\langle \alpha |\widehat{O}{|\beta \rangle |}^2$, we find that (1) the current matrix elements exhibit a system-size dependence that is different from other observables under investigation and (2) matrix elements of several other observables exhibit a Drude-like structure with a Lorentzian frequency dependence. We then show how this information can be extracted from the autocorrelation functions as well. Finally, our study is complemented by a numerical analysis of the fluctuation-dissipation relation for eigenstates in the bulk of the spectrum. We identify the regime of $\omega$ in which the well-known fluctuation-dissipation relation is valid with high accuracy for finite systems.

We describe a solvable model of a quantum transition in a single band model involving a change in the size of the electron Fermi surface without any symmetry breaking. In a model with electron density $1-p$, we find a “large” Fermi surface state with the conventional Luttinger volume $1-p$ of electrons for $p>p_c$, and a first-order transition to a “small” Fermi surface state with a non-Luttinger volume $p$ of holes for $p<p_c$. As required by extended Luttinger theorems, the small Fermi surface state also has fractionalized spinon excitations. The model has electrons with strong local interactions in a single band; after a canonical transformation, the interactions are transferred to a coupling to two layers of ancilla qubits, as proposed by Zhang and Sachdev [Phys. Rev. Research 2, 023172 (2020)]. Solvability is achieved by employing random exchange interactions within the ancilla layers, and taking the large $M$ limit with $\mathrm{SU}\left(M\right)$ spin symmetry, as in the Sachdev-Ye-Kitaev models. The local electron spectral function of the small Fermi surface phase displays a particle-hole asymmetric pseudogap, and maps onto the spectral function of a lightly doped Kondo insulator of a Kondo-Heisenberg lattice model. We discuss connections to the physics of the hole-doped cuprates: the asymmetric pseudogap observed in STM, and the sudden change from incoherent to coherent antinodal spectra observed recently in photoemission. A holographic analogy to wormhole transitions between multiple black holes is briefly noted.

We present studies of neutron diffraction, electrical resistivity, magnetic susceptibility, magnetization, and specific heat of noncentrosymmetric $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_2$. At normal pressure, $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_2$ is a moderate heavy-fermion compound with Kondo lattice. At 16 K we observe an anomaly in the temperature dependence of specific heat which is ascribed to an abrupt valence change of Yb ions. At pressures above 7 GPa, the valence change and the Kondo lattice state are suppressed, and near a temperature of 10 K we detect the appearance of magnetic order. Above 5 GPa, the temperature dependence of the resistivity behaves similarly to other compounds of the $\mathrm{Ce}\mathrm{Ni}{\mathrm{C}}_2$-type family, indicating the formation of charge density waves. This is attributed to the nesting properties of the Fermi surface (FS) found within the density functional $\mathrm{theory}+\mathrm{dynamical}$ mean field theory treatment. Our ab initio calculations also show that the valence of Yb in $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_2$ at normal conditions is 2.85, which increases with temperature and pressure. The FS of $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_2$ is anisotropic in shape in comparison with the 3D surface of $\mathrm{Yb}\mathrm{Co}{\mathrm{C}}_2$.

In this work the above-band-gap absorption spectrum in two-dimensional Dirac materials is calculated with account for the interaction between the photocarriers. Both the screened Rytova-Keldysh and pure Coulomb attraction potentials between the electron and hole are used in the study. We find that in the materials under consideration, the interaction enhances the absorbance in the narrow interband edge region, in sharp contrast to the band model with the parabolic free-carrier energy dispersion. We develop an approximation of the weak interaction, which allows us to reproduce the main features of the exactly calculated Sommerfeld factor. We show a substantial reduction of this factor at higher frequencies due to the single-particle energy renormalization.

In this work, we report for the first time the crucial role of topological anomalies like Stone-Wales (SW)–type bond rotations in tuning the optical properties of graphene quantum dots (GQDs). By means of first-principles calculations, we first show that the structural stability of GQDs strongly depends on the position of SW defects. Optical absorption spectra are then computed using electron-correlated methodology to demonstrate that SW-type reconstruction is responsible for the appearance of new defect-induced peaks below the optical gap and dramatically modifies the optical absorption profile. In addition, our investigations signify that electron correlation effects become more dominant for SW-defected GQDs. We finally establish that the introduction of SW defects at specific locations strongly enhances light absorption in the visible range, which is of prime importance for designing light harvesting, photocatalytic, and optoelectronic devices.

Trigonal tellurium with helical structure exhibits intriguing properties due to its inherent chirality, which has attracted considerable research interest recently. Using cryogenic scanning tunneling microscopy/spectroscopy and density functional theory calculations, we investigated quasiperiodic defects at particular edges of Te nanoribbons grown on graphene/SiC(0001) substrate. The detailed topography of edge defects reveals the chirality of Te nanoribbon, and the defects induce a quasiperiodic modulation of the electronic states along the edge. The average period of the edge defects varies with the twist angle between Te nanoribbon and substrate, implying the electronic modulation can be tuned by the interfacial interaction. Based on the morphology analyses and modeled calculations, we further propose the growth mechanism of the Te nanoribbons.

The ultraconfined light of plasmonic modes puts their effective wavelength close to the mean-free path of electrons inside the metal electron gas. The Drude model, which can not take the repulsive interactions of electrons into account, then clearly begins to show its limits. In an intermediate length scale where a full quantum treatment is computationally prohibitive, the semiclassical hydrodynamic model, instrinsically nonlocal, has proven successful. Here we generalize the expression for the absorption volume density and the reciprocity theorem in the framework of this hydrodynamic model. We validate numerically these generalized theorems and show that using classical expressions instead leads to large discrepancies.

Many conductors, including recently studied Dirac materials, show saturation of coherence length on decreasing temperature. This surprising phenomenon is assigned to external noise, residual magnetic impurities, or two-level systems specific to noncrystalline solids. Here, by considering the SnTe-class of compounds as an example, we show theoretically that breaking of mirror symmetry deteriorates Berry's phase quantization, leading to additional dephasing in weak-antilocalization magnetoresistance (WAL-MR). Our experimental studies of WAL-MR corroborate these theoretical expectations in (111) ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Se}$ thin film with Sn contents $x$ corresponding to both topological crystalline insulator and topologically trivial phases. In particular, we find the shortening of the phase coherence length in samples with intentionally broken mirror symmetry. Our results indicate that the classification of quantum transport phenomena into universality classes should encompass, in addition to time-reversal and spin-rotation invariances, spatial symmetries in specific systems.

We study theoretically spin relaxation during phonon-assisted tunneling of a single electron in self-assembled InAs/GaAs quantum-dot molecules formed by vertically stacked dots. We find that the spin-flip tunneling rate may be as high as $1\%$ of the spin-conserving one. By studying the dependence of spin-relaxation rate on external fields, we show that the process is active at a considerable rate even without the magnetic field, and scales with the latter differently than the relaxation in a Zeeman doublet. Utilizing a multiband $\mathit{k}·\mathit{p}$ theory, we selectively investigate the impact of various spin-mixing terms in the electron energy and carrier-phonon interaction Hamiltonians. As a result, we identify the main contribution to come from the Dresselhaus spin-orbit interaction, which is responsible for the zero-field effect. At magnetic fields above $\sim 15\mathrm{T}$, this is surpassed by other contributions due to the structural shear strain. We also study the impact of the sample morphology and determine that the misalignment of the dots may enhance relaxation rate by over an order of magnitude. Finally, via virtual tunneling at nonzero temperature, the process in question also affects stationary electrons in tunnel-coupled structures and provides a Zeeman-doublet spin-relaxation channel even without the magnetic field.

We apply an intense infrared laser pulse in order to perturb the electronic and vibrational states in the three-dimensional charge density wave material $1T\text{−}{\mathrm{VSe}}_2$. Ultrafast snapshots of the light-induced hot carrier dynamics and nonequilibrium quasiparticle spectral function are collected using time- and angle-resolved photoemission spectroscopy. The hot carrier temperature and time-dependent electronic self-energy are extracted from the time-dependent spectral function, revealing that incoherent electron-phonon interactions heat the lattice above the charge density wave critical temperature on a timescale of $(200\pm 40)$ fs. Density functional perturbation theory calculations establish that the presence of hot carriers alters the overall phonon dispersion and quenches efficient low-energy acoustic phonon scattering channels, which results in a new quasiequilibrium state that is experimentally observed.