Time- and angle-resolved photoemission spectroscopy (tr-ARPES) constitutes a powerful tool to inspect the dynamics and thermalization of hot carriers. The identification of the processes that drive the dynamics, however, is challenging even for the simplest systems owing to the coexistence of several relaxation mechanisms. Here, we devise a Green's function formalism for predicting the tr-ARPES spectral function of metals and establish the origin of carrier thermalization. The predictive power of this approach is demonstrated by an excellent agreement with experiments for graphene over timescales ranging from a few tens of femtoseconds up to several picoseconds. Our work provides compelling evidence of a nonequilibrium dynamics dominated by the establishment of a hot-phonon regime.

We exploit the high-symmetry persistent spin helix state obtained for similar Rashba and linear Dresselhaus interactions in a quantum well to revisit the weak localization problem within a perturbative approach in a Landau level formulation. We define the small parameter of the theory as the deviation from the symmetry state introduced by the mismatch of the linear terms and by the strength of the cubic Dresselhaus term. In the vicinity of the helix state, the SO field becomes uniaxial, offering a natural direction of spin quantization, thus defining the $z$ axis within the 2D plane. In contrast to previous theories, this reveals a full decoupling of the Cooperon triplet scattering modes as well as decoupled Landau levels, to lowest order in the small parameter. This makes it possible to derive a closed-form expression for the weak localization magnetoconductivity, thus providing a new paradigm of localization in the weakly-broken spin symmetry regime. We perform quantum transport experiments in GaAs quantum wells, finding very good agreement with the new theory. We present a reliable two-step method to extract the SO and transport parameters from fits of the new expression, obtaining excellent agreement with recent experiments. This is an important step towards engineering and controlling the spin-orbit interaction as a powerful resource in emerging quantum technologies.

Doping ${\mathrm{V}}_2{\mathrm{O}}_3$ with Ti stabilizes the corundum paramagnetic metallic phase and restricts the transition to the monoclinic antiferromagnetic insulating phase at lower temperatures. In this study, we investigated the dependence of phase stability on Ti concentration for ${\mathrm{V}}_2{\mathrm{O}}_3$ nanocrystals as small as 20 nm. Ti doping of several percent induces a high-temperature insulating phase preserving the magnetic transition without the crystal symmetry change. The emergence of the insulating phases, which are peculiar to this crystal size, can be attributed to the enhancement of the on-site Coulomb interaction $U$ and increased mixing between $a_{1g}$ and $e_g^{\pi }$ states accompanied by characteristic lattice parameter changes in the high Ti doping region.

Metallic nanoparticles can localize the incident light to hot spots as plasmon oscillations, where the intensity can be enhanced by up to four orders of magnitude. Even though the lifetime of plasmons is typically short, it can be increased via interactions with quantum emitters, e.g., spaser nanolasers. However, molecules can bleach in days. Here, we study the lifetime enhancement of plasmon excitations due to the coupling with longer-lifetime dark plasmon modes. We apply an analytical model based on harmonic oscillators to demonstrate that a coupled system of bright and dark plasmon modes decays more slowly than the bright mode alone. Furthermore, exact solutions of the three-dimensional Maxwell equations, i.e., finite-difference time domain, demonstrate that the lifetime of the coupled system significantly increases at the hot spot, which is not predictable by far-field response. The decay of the overall energy of such a coupled system, which can be extracted from experimental absorption measurements, is substantially different from the decay of the hot spot field. This observation enlightens the plasmonic applications in which the hot spot intensity enables the detection of the optical responses.

In several recent works it has been proposed that, due to disorder, the experimentally observed $\nu =5/2$ quantum Hall state could be microscopically composed of domains of Pfaffian order along with domains of anti-Pfaffian order. We numerically examine the energetics required for forming such domains and conclude that for the parameters appropriate for recent experiments, such domains would not occur.

We investigated, both experimentally and theoretically, the reflection phase shift (RPS) of one-dimensional plasmon polaritons. We launched one-dimensional (1D) plasmon polaritons in carbon nanotubes and probed the plasmon interference pattern using a scanning near-field optical microscopy technique, through which a nonzero phase shift was observed. We further developed a theory to understand the nonzero phase shift of 1D polaritons, and found that the RPS can be understood by considering the evanescent field beyond the nanotube end. Interestingly, our theory shows a strong dependence of RPS on polaritons wavelength and nanotube diameter, which is in stark contrast to two-dimensional plasmon polaritons in graphene where the RPS is a constant. In the short-wave region, the RPS of 1D polaritons only depends on a dimensionless variable—the ratio between polaritons wavelength and nanotube diameter. These results provide fundamental insights into the reflection of polaritons in the 1D system, and could facilitate the design of ultrasmall 1D polaritonic devices, such as resonators and interferometers.

By using the variational Monte Carlo method, we study the magnetization process of the Kitaev honeycomb model in a magnetic field. Our trial wave function is a generalized Bardeen-Cooper-Schrieffer wave function with the Jastrow correlation factor, which exactly describes the ground state of the Kitaev model at zero magnetic field using the Jordan-Wigner (JW) transformation. We find that two phase transitions occur for the antiferromagnetic Kitaev coupling, while only one phase transition occurs for the ferromagnetic Kitaev coupling. For the antiferromagnetic Kitaev coupling, we also find that the topology of the momentum distribution of the JW fermions changes at the transition point from the Kitaev spin liquid to an intermediate state. Our numerical results indicate that the intermediate state between the Kitaev spin liquid and the fully polarized phases stably exists in the bulk system on two dimensions for the antiferromagnetic Kitaev coupling against many-body correlations.

Optical fields enhanced by surface plasmon polaritons (SPPs) on metal-dielectric interfaces are useful for increasing several light-matter interactions, and application to nonlinear optics (NLO) is one of the most important uses of this type of structure. Most previous studies on this subject took advantage of the nonlinear susceptibilities of metal surfaces. However, the limited NLO light-matter interaction lengths prevented the transfer of the technologies to practical use. In the present paper, we tried to overcome this problem by growing NLO polymer thin films on metal surfaces. The NLO responses of the system were characterized by second-harmonic generation (SHG) spectroscopy. Our experimental results suggested that SPP-enhanced optical fields induced not only surface nonlinearities in Ag but also bulk nonlinearities in NLO polymer. There was an optimal polymer thickness for the SHG conversions, and a greater thickness did not always result in higher conversion. The maximum conversion efficiency was approximately 40 times higher than that of the nonpolymer-coated bare Ag surface. The growth and propagation of the SHG waves were addressed with a numerical approach combining the transfer matrix method and Green's function analysis. The SHG conversion efficiencies were determined by three factors, the SPP-field enhancement factor, the nonlinear light-matter interaction length, and the degree of interference between the forward- and backward-propagating SHG waves. The latter two factors predominantly determined the optimal SHG conversion efficiencies. The proposed strategy of hybridizing NLO polymers brings us closer to practical uses for nonlinear plasmonics.

Thermal transport by phonons in films with thicknesses of less than 10 nm is investigated in a soft system (Lennard-Jones argon) and a stiff system (Tersoff silicon) using two-dimensional lattice dynamics calculations and the Boltzmann transport equation. This approach uses a unit cell that spans the film thickness, which removes approximations related to the finite cross-plane dimension required in typical three-dimensional-based approaches. Molecular dynamics simulations, which make no assumptions about the nature of the thermal transport, are performed to obtain finite-temperature structures for the lattice dynamics calculations and to predict thermal conductivity benchmarks. Thermal conductivity decreases with decreasing film thickness for both the two-dimensional lattice dynamics calculations and the MD simulations, until the thickness reaches four unit cells (2.1 nm) for argon and three unit cells (1.6 nm) for silicon. With a further decrease in film thickness, thermal conductivity plateaus in argon while it increases in silicon. This unexpected behavior, which we identify as a signature of phonon confinement, is a result of an increased contribution from low-frequency phonons, whose density of states increases as the film thickness decreases. Phonon mode-level analysis suggests that confinement effects emerge below thicknesses of ten unit cells (5.3 nm) for argon and six unit cells (3.2 nm) for silicon. These transition points both correspond to approximately twenty atomic layers. Thermal conductivity predictions based on the bulk (i.e., three-dimensional) phonon properties combined with a boundary scattering model do not capture the low thickness behavior. To match the two-dimensional lattice dynamics and molecular dynamics predictions for larger thicknesses, the three-dimensional lattice dynamics calculations require a finite specularity parameter that in some cases approaches unity. These findings point to the challenges associated with interpreting experimental thermal conductivity measurements of ultrathin silicon films, where surface roughness and a native oxide layer impact phonon transport.

Starting from the Bogolubov-de Gennes theory describing the induced p-wave superconductivity in the Majorana wire of an arbitrary shape, we predict a number of intriguing phenomena such as the geometry-dependent phase battery (or a phi-Josephson junction with the spontaneous superconducting phase difference) and generation of additional quasiparticle modes at the Fermi level with the spatial position tuned by the external magnetic field direction. This tuning can be used to extend the capabilities of the braiding protocols in Majorana networks.

Recently, a local-Ising-type magnetic order was inferred from neutron diffraction of the antiferromagnetic ${\mathrm{Er}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ (pg-ErGO) with an applied magnetic field. Here, we use neutron spectroscopy to investigate the energetics of pg-ErGO and the isostructural ${\mathrm{Yb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ (pg-YbGO) to evaluate the adequacy of the local-Ising description. To begin, we generate a model of the magnetic structure of pg-YbGO using neutron diffraction and find a net ferromagnetic moment. While pg-ErGO and pg-YbGO have highly symmetric crystal structures ($P{4}_1{2}_{1}2$ tetragonal space group 92) with only one trivalent rare-earth magnetic site, the point symmetry of the rare-earth site is low with only a single symmetry element (point group ${\mathrm{C}}_1$). For both compounds, the energy scale of the first excited state is large compared to the magnetic ordering temperature, suggesting Ising character. The ground-state Kramer's doublet of both compounds is dominated by the maximal $m_{\mathrm{J}}$ component. However, the low point group symmetry of the rare-earth site leads to finite mixing of all other $m_{\mathrm{J}}$'s, which suggests potential deviations from Ising behavior. Moreover, quasielastic scattering is observed deep in the ordered state of pg-ErGO and pg-YbGO that may be due to non-Ising behavior. The dominant magnetic interaction in both compounds is found to be magnetostatic by considering the magnetic excitations in the ordered state. From consideration of these data, the pg-YbGO is more Ising-like than pg-ErGO. Also, quantum multicritical points are anticipated with applied magnetic field in both compounds.

Nematic order is an exotic property observed in several strongly correlated systems, such as iron-based superconductors. Using large-scale density matrix renormalization group (DMRG) techniques, we study at zero temperature the nematic spin liquid that competes with spin dipolar and quadrupolar orders. We use these nematic orders to characterize different quantum phases and quantum phase transitions. More specifically, we study a spin-1 bilinear-biquadratic Heisenberg model on the square lattice with couplings beyond nearest neighbors. We focus on parameter regions around the highly symmetric $SU\left(3\right)$ point where the bilinear and biquadratic interactions are equal. With growing further-neighbor biquadratic interactions, we identify different spin dipolar and quadrupolar orders. We find that the DMRG results for cylindrical geometries correctly detect nematicity in different quantum states and accurately characterize the quantum phase transitions among them. Therefore, spin-driven nematicity, here defined as the spontaneous breaking of the lattice invariance under a ${90}^{\circ }$ rotation, is an order parameter which can be studied directly in DMRG calculations in two dimensions in different quantum states.

Magnetic insulating and exchange bias (EB) are two basic magnetic states with actual and potential applications in spintronic and magnonic devices. However, ferromagnetic insulators (FMIs) are rare, and it is even more challenging to obtain a FMI with large EB. In this paper, high performance of FMIs and EB in $\mathrm{LaMn}{\mathrm{O}}_3$:NiO nanocomposite epitaxial thin films is reported. The antiferromagnetic NiO nanoparticles, randomly embedded in a ferromagnetic $\mathrm{LaMn}{\mathrm{O}}_3$ matrix, enhance the robustness of the insulating state and result in an EB field as large as 1100 Oe. The simultaneously enhanced resistivity and EB are attributed to localized oxygen vacancy, preserved strain state, and increased ferromagnetic-antiferromagnetic interface ratio. This paper provides not only a material candidate for spintronics but also a referential strategy to design artificial materials with multifunctionality.

Quantum anomalous Hall effect (QAHE) has been experimentally realized in magnetically doped topological insulators or intrinsic magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ by applying an external magnetic field. However, either the low observation temperature or the unexpected external magnetic field (tuning all ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ layers to be ferromagnetic) still hinders further application of QAHE. Here, we theoretically demonstrate that proper stacking of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ layers is able to produce intrinsically ferromagnetic van der Waals heterostructures to realize the high-temperature QAHE. We find that interlayer ferromagnetic transition can happen at $T_C=42\mathrm{K}$ when a five-quintuple-layer ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ topological insulator is inserted into two septuple-layer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ with interlayer antiferromagnetic coupling. Band structure and topological property calculations show that ${\mathrm{MnBi}}_2{\mathrm{Te}}_4/{\mathrm{Sb}}_2{\mathrm{Te}}_3/{\mathrm{MnBi}}_2{\mathrm{Te}}_4$ heterostructure exhibits a topologically nontrivial band gap around 26 meV, that hosts a QAHE with a Chern number of $\mathcal{C}=1$. In addition, our proposed materials system should be considered as an ideal platform to explore high-temperature QAHE due to the fact of natural charge compensation, originating from the intrinsic $n$-type defects in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and $p$-type defects in ${\mathrm{Sb}}_2{\mathrm{Te}}_3$.

Heusler compounds having $D_{2d}$ crystal symmetry gained much attention recently due to the stabilization of a vortexlike spin texture called antiskyrmions in thin lamellae of $\mathrm{M}{\mathrm{n}}_{1.4}\mathrm{P}{\mathrm{t}}_{0.9}\mathrm{P}{\mathrm{d}}_{0.1}\mathrm{Sn}$ as reported in the work of Nayak et al. [Nature (London) 548, 561 (2017)]. Here we show that bulk $\mathrm{M}{\mathrm{n}}_{1.4}\mathrm{P}{\mathrm{t}}_{0.9}\mathrm{P}{\mathrm{d}}_{0.1}\mathrm{Sn}$ undergoes a spin-reorientation transition from a collinear ferromagnetic to a noncollinear configuration of Mn moments below 135 K, which is accompanied by the emergence of a topological Hall effect. We tune the topological Hall effect in Pd and Rh substituted $\mathrm{M}{\mathrm{n}}_{1.4}\mathrm{PtSn}$ Heusler compounds by changing the intrinsic magnetic properties and spin textures. A unique feature of the present system is the observation of a zero-field topological Hall resistivity with a sign change which indicates the robust formation of antiskyrmions.

The effect of interface roughness of ferromagnetic and antiferromagnetic layers on exchange bias is still not well understood. In this report we have investigated the effect of surface roughness in (111)-oriented antiferromagnetic CoO films on exchange bias with ferromagnetic Fe grown on top. The surface roughness is controlled at the atomic scale, over a range below ∼0.35 nm, by varying layer thickness of the CoO films. It is observed that both exchange-bias field $\left(H_E\right)$ and coercivity $\left(H_C\right)$ extensively depend on the atomic scale roughness of the CoO (111) at the interface with Fe film. An opposite dependence of $H_E$ and $H_C$ on interface roughness was found, which was ascribed to partially compensated spin states induced by the atomic roughness at the fully uncompensated CoO (111) surfaces and was corroborated using Monte Carlo simulations. Moreover, the onset temperature for $H_C$ is found to be up to ∼80 K below the blocking temperature $\left(T_B\right)$ and the temperature dependence of $H_C$ follows the power law with a critical exponent equal to one, which indicates that, in this system, $H_C$ is more of an interface-related property than $H_E$.

We study the thermodynamic and high-magnetic-field properties of the magnetic insulator ${\mathrm{Ba}}_5{\mathrm{CuIr}}_3{\mathrm{O}}_{12}$, which shows no magnetic order down to 2 K, consistent with a spin-liquid ground state. While the temperature dependence of the magnetic susceptibility and the specific heat shows only weak antiferromagnetic correlations, we find that the magnetization does not saturate up to a field of 59 T, leading to an apparent contradiction. We demonstrate that the paradox can be resolved, and all of the experimental data can be consistently described within the framework of random singlet states. We demonstrate a generic procedure to derive the exchange coupling distribution $P\left(J\right)$ from the magnetization measurements and use it to show that the experimental data are consistent with the power-law form $P\left(J\right)\sim J^{-\alpha }$ with $\alpha \approx 0.6$. Thus, we reveal that high-magnetic-field measurements can be essential to discern quantum spin-liquid candidates from disorder dominated states that do not exhibit long-range order.