We compute the shift-current bulk photovoltaic effect (BPVE) in bulk ${\mathrm{BaTiO}}_3$ and two-dimensional monochalcogenide SnSe considering quasiparticle corrections and exciton effects. We explore changes in shift-current peak position and magnitude reduction due to band renormalization. For BaTiO3, we demonstrate that shift current reduces near the band edge due to exciton effects. We find the calculated magnitude of shift current in ${\mathrm{BaTiO}}_3$ is only half of that obtained in experiments, indicating that mechanisms other than shift current can contribute to BPVE. Additionally, we reveal that the shift current near band gap has a very small change by exciton in two-dimensional SnSe, suggesting that thin film is a feasible way to reduce the exciton effect on the shift current. These results suggest that many-body corrections are important for accurate assessments of bulk photovoltaic materials and to understand the mechanisms behind the BPVE.

The energy scales in rare-earth-based heavy-fermion compounds are relatively small, which can be easily tuned by applying pressure, magnetic field, or chemical doping. By substituting Yb for Ce on the rare-earth site, the ground state of superconductivity can be smoothly suppressed without the appearance of an apparent quantum critical point, and a number of remarkable phenomena have been observed. The slight changes in the electronic structure are supposed to dominate the underlying physics in these compounds. In the present study, we provide an electronic structure study of ${\mathrm{Ce}}_{0.85}{\mathrm{Yb}}_{0.15}{\mathrm{CoIn}}_5$ with a superconducting state but suppressed transition temperature by angle-resolved photoemission spectroscopy, and the results are compared with ${\mathrm{CeCoIn}}_5$. We find that the $f$ electrons in ${\mathrm{Ce}}_{0.85}{\mathrm{Yb}}_{0.15}{\mathrm{CoIn}}_5$ are itinerant, forming the weakly dispersive hybridized band at low temperature. More interestingly, the hybridization strength between the $f$ electrons and conduction electrons in ${\mathrm{Ce}}_{0.85}{\mathrm{Yb}}_{0.15}{\mathrm{CoIn}}_5$ is comparable with ${\mathrm{CeCoIn}}_5$. Further temperature-dependent measurements provide direct evidence of the localized-to-itinerant crossover behavior of the $4f$ electrons in this compound.

We have investigated emission patterns and energy spectra of electrons from a tungsten nanotip induced by circularly polarized femtosecond laser pulses. Variations of emission patterns were observed for different helicities of circular polarization while the energy spectra remained almost identical. The physics behind this difference in emission patterns is the change in propagation directions of surface electromagnetic waves on the tip apex. The energy spectra showed the same spectroscopic signatures as the linearly polarized laser in a strong-field regime, which are a low-energy peak and a plateau feature. The low-energy peak is due to a delayed electron emission with respect to a prompt emission. The experimental data and plasmonic simulations support our previous conclusion, where the observed delayed emission processes originate from an inelastic rescattering process. This work demonstrates that the use of circular polarization is an easy means to add extra knobs to control the spatial and temporal emission from a nanotip at the nanometer and femtosecond scale. It could find applications as a helicity-driven subcycle optical switch.

Using the tight-binding model and the nonequilibrium Green's function method, we study Andreev reflection in a graphene-superconductor junction, where graphene has two nonequal Dirac cones split in energy and therefore time-reversal symmetry is broken. Due to the antichiral edge states of the current graphene model, an incident electron traveling along the edges makes a distinct contribution to Andreev reflections. In a two-terminal device, because Andreev retroreflection is not allowed for just the antichiral edges, in this case the mutual scattering between edge and bulk states is necessary, which leads to the coefficient of Andreev retroreflection always being symmetrical about the incident energy. In a four-terminal junction, however, the edges are parallel to the interface of superconductor and graphene, so at the interface an incident electron traveling along the edges can be retroreflected as a hole into bulk modes, or specularly reflected as a hole into antichiral edge states again. It is noted that the coefficient of specular Andreev reflection keeps symmetric as to the incident energy of electrons, which is consistent with the reported results before; however, the coefficient of Andreev retroreflection shows an unexpected asymmetrical behavior due to the presence of antichiral edge states. Our results present some ideas to study the antichiral edge modes and Andreev reflection for a graphene model with the broken time-reversal symmetry.

In this paper, we develop the excitonic theory of the Kerr rotation angle in a two-dimensional (2D) transition metal dichalcogenide at zero magnetic field. The finite Kerr angle is induced by the interplay between spin-orbit splitting and proximity exchange coupling due to the presence of a ferromagnet. We compare the excitonic effect with the single-particle theory approach. We show that the excitonic properties of the 2D material lead to a dramatic change in the frequency dependence of the optical response function. We also find that the excitonic corrections enhance the optical response by a factor of 2 in the case of ${\mathrm{MoS}}_2$ in proximity to a cobalt thin film.

Magnetic materials in metallic nanostructures allow an external magnetic field to easily control surface plasmon polaritons and surface lattice modes (SLMs), which enhances the value of magneto-optical response. The present paper demonstrates both experimentally and numerically that the inherently different radiative damping of dark and bright SLMs in one-dimensional low-loss multilayer Au/Ni/Au magnetoplasmonic crystals can alter the resonant enhancement of the transverse magneto-optical Kerr effect (TMOKE). As the quality factor of dark SLMs goes up, the maximum TMOKE values increase as well. The spectral range of TMOKE enhancement broadens at small incident angles since dark and bright SLMs overlap each other. The Fano resonance model shows that the wavelength shift of the dark and bright modes results in the resonant enhancement of TMOKE.

A molecular heat engine is proposed based on a porphyrin single-molecule junction. The important thermoelectric transport characteristics, such as electrical conductance, thermopower, output power, and efficiency have been calculated by means of a Landauer-Büttiker approach combined with density functional theory. Different ways of engineering the heat engine performance have been investigated. (i) The efficiency of the heat engine can be considerably enhanced in special contact geometry between the molecule and electrodes that manifests a destructive quantum interference effect. (ii) Contrary to the usual length dependence of a single molecular junction, an increase of electrical conductance and thermopower with a number of porphyrin units have been found and used to optimize the molecular heat engine. (iii) The engine performance depends sensitively on the twisting angle between the molecule and graphene electrodes. Thus, a single molecular junction could be a promising candidate for a nanoscale heat engine utilizing quantum effect.

In elastic systems, the current technique to produce Dirac degeneracy is based on Bragg scattering eigenstates, which, however, suffers from operating only at relatively high frequency determined by lattice constant. Here, an elastic metamaterial plate that presents an analog to the quantum valley Hall effect (QVHE) is proposed to achieve Dirac degeneracy with local resonant states, enabling us to tune the operating frequency without altering the lattice. By introducing resonator pair into a hexagonal lattice, a local resonance states-induced Dirac cone is produced right below the local resonant band gap caused by the resonator pair. After gapping the Dirac cone with unequal masses of the resonator pair, a new local resonant band gap supporting topological edge modes immune to backscattering appears. This band gap is formed by mixing effective negative mass effect and Bragg scattering effect due to large virtual mass. These ideas are demonstrated by numerical simulation, as well as validated by the experiment on flexural wave in a textured plate. The proposed design provides a new degree of freedom to control elastic topological mode and paves the way to explore subwavelength elastic topological interface states.

Using first-principles calculations, we investigate the impact of hydrogenation on the Dzyaloshinskii-Moriya interaction (DMI) at graphene/Co interface. We find that both the magnitude and chirality of DMI can be controlled via hydrogenation absorbed on graphene surface. Our analysis using density of states combined with first-order perturbation theory reveals that the spin splitting and the occupation of Co-$d$ orbitals, especially the $d_{xz}$ and $d_{z^2}$ states, play a crucial role in defining the magnitude and the chirality of DMI. Moreover, we find that the DMI oscillates with a period of two atomic layers as a function of Co thickness what could be explained by analysis of out-of-plane of Co orbitals. Our work elucidates the underlying mechanisms of interfacial DMI origin and provides an alternative route of its control for spintronic applications.

Theoretically, the vortex melting phenomenon occurs at both low and high magnetic fields at a fixed temperature. While the high-field melting has been extensively investigated in high-$T_c$ cuprates, the low-field melting phenomena in the presence of disorder has not been well explored. Using bulk magnetization measurements and a high-sensitivity differential magneto-optical imaging technique, we detect a low-field vortex melting phenomenon in a single crystal of ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$. The low-field melting is accompanied with a significant change in local magnetization, ∼3 G, which decreases with increasing applied field. The observed vortex melting phenomenon is traced on a field-temperature phase diagram, which lies very close to theoretically predicted Lindemann criteria based low-field melting line. Our analysis shows a Lindemann number $c_L=0.14$ associated with the low-field melting. Imaging of low-field vortex melting features shows that the process nucleates via formation of extended fingerlike projections which spreads across the sample with increasing field or temperature, before entering into an interaction-dominated vortex solid phase regime. Magnetization scaling analysis and ngular dependence of bulk magnetization hysteresis loop shows extended pins naturally present in the sample. These defects create a low-field glassy vortex phase sustaining a finite critical current present in the phase diagram below the low-field liquid phase. We construct a vortex matter phase diagram, which identifies boundaries demarcating a low-field glassy vortex state from a dilute vortex liquid phase and a weakly interacting solid phase above it. All these phases are shown to be present well below the interaction dominated vortex state shown in the phase diagram.

We study the dynamics of a single qubit encoded in two pairs of Majorana modes, whereby each pair is hosted on a trijunction described by the Kitaev model extended by many-body interactions. We demonstrated that the challenging phase gate may be efficiently implemented via braiding of partially overlapping modes. Although such a qubit acquires both geometric and dynamical phases during the braiding protocol, the latter phase may be eliminated if the Majorana modes are hosted by systems with appropriate particle-hole symmetry.

We investigate many-body localization in the chain of interacting spins with a transverse power-law interaction, $J_0/r^{\alpha }$, and random on-site potentials, ${\varphi }_i\in \left(-W/2,W/2\right)$, in the long-range limit, $\alpha <3/2$, which has been recently examined experimentally on trapped ions. The many-body localization threshold is characterized by the critical disordering, $W_c$, which separates localized ($W>W_c$) and chaotic ($W<W_c$) phases. Using the analysis of the instability of localized states with respect to resonant interactions complemented by numerical finite-size scaling, we show that the critical disordering scales with the number of spins, $N$, as $W_c\approx [1.37J_0/(4/3-\alpha )]N^{4/3-\alpha }lnN$ for $0<\alpha \le 1$, and as $W_c\approx [J_0/(1-2\alpha /3)]N^{1-2\alpha /3}{ln}^{2/3}N$ for $1<\alpha <3/2$, while the transition width scales as ${\sigma }_W\propto W_c/N$. We use this result to predict the spin long-term evolution for a very large number of spins ($N=50$), inaccessible for exact diagonalization, and to suggest the rescaling of hopping interaction with the system size to attain the localization transition at finite disordering in the thermodynamic limit of infinite number of spins.

X-ray absorption fine structure (XAFS), x-ray fluorescence holography (XFH), and inelastic x-ray scattering (IXS) techniques were applied to an ${\mathrm{Fe}}_2({\mathrm{V}}_{0.95}{\mathrm{Ta}}_{0.05}$)Al Heusler-type thermoelectric material to investigate the doping effect of Ta on the local structure and the atomic dynamics. The XAFS and XFH results show that the positional correlation between Ta and surrounding Fe atoms is much more rigid than that between V and Fe. In the IXS results, we observed a broad dispersionless mode in the energy range of the acoustic phonons of undoped ${\mathrm{Fe}}_2\mathrm{VAl}$, which is regarded as the so-called resonant mode. These results revealed a clearly different feature of the dynamics of heavy dopants from that of the matrix element, which disturbs the coherency of phonons and should contribute to the enhancement of the thermal insulation.

Neutron diffraction with static and pulsed magnetic fields is used to directly probe the magnetic structures in ${\mathrm{LiNiPO}}_4$ up to $25\mathrm{T}$ and $42\mathrm{T}$, respectively. By combining these results with magnetometry and electric polarization measurements under pulsed fields, the magnetic and magnetoelectric phases are investigated up to $56\mathrm{T}$ applied along the easy $c$ axis. In addition to the already known transitions at lower fields, three new ones are reported at 37.6, 39.4, and $54\mathrm{T}$. Ordering vectors are identified with ${\mathbf{Q}}_{\mathrm{VI}}=(0,\frac{1}{3},0)$ in the interval $37.6-39.4\mathrm{T}$ and ${\mathbf{Q}}_{\mathrm{VII}}=(0,0,0)$ in the interval $39.4-54\mathrm{T}$. A quadratic magnetoelectric effect is discovered in the ${\mathbf{Q}}_{\mathrm{VII}}=(0,0,0)$ phase and the field dependence of the induced electric polarization is described using a simple mean-field model. The observed magnetic structure and magnetoelectric tensor elements point to a change in the lattice symmetry in this phase. We speculate on the possible physical mechanism responsible for the magnetoelectric effect in ${\mathrm{LiNiPO}}_4$.

We study theoretically the resonant absorption of terahertz electromagnetic waves of TM polarization in a slab of layered superconductor enclosed within twofold dielectric layers. Using the transfer-matrix method and assuming weak dissipation of waves in the sample, we analyze the absorption for both symmetric and antisymmetric bilateral incident radiation. The excitation of electromagnetic waves localized on layered superconductors gives rise to resonant absorption, a phenomenon known as Wood's anomaly, providing for suppression of the specular wave reflection. This phenomenon is essentially modified in the layered superconductors as compared with usual conductors. The dispersion curves of localized modes are nonmonotonic, thus resulting in the specific dependence of the absorptance $A$ on the incidence angle $\theta$. We reveal the conditions for emergence of the resonant absorption and derive the asymptotic expressions for the resonant absorptance. A specific non-Lorentzian line shape of $A\left(\theta \right)$ dependence, like twin resonant peaks or a merged single broad peak, appears when the wave frequency $\omega$ is tuned close to the top point on the dispersion curve $\omega \left(\theta \right)$. Experimental observation of such resonant line shapes can serve as a verification of the existence of localized waves with nonmonotonic dispersion in the layered superconductors.

We present a first-principles-based multiscale simulation framework for quantitative predictions of the high-energy part of the ultraviolet photoelectron spectroscopy (UPS) spectra of amorphous molecular solids. The approach combines a deposition simulation, many-body Green's function theory, polarizable film embedding, and multimode electron-vibrational coupling and provides a molecular-level view on the interactions and processes giving rise to spectral features. This insight helps bridging the current gap between experimental UPS and theoretical models as accurate analyses are hampered by the energetic disorder, surface sensitivity of the measurement, and the complexity of excitation processes. In particular, this is relevant for the unambiguous determination the highest occupied molecular orbital energy (HOMO) of organic semiconductors, a key quantity for tailoring and engineering new optoelectronic devices. We demonstrate the capabilities of the simulation approach studying the spectrum of two isomers of 2-methyl-9,10-bis(naphthalen-2-yl)anthracene as archetypical materials showing a clearly separated HOMO peak in experiment. The agreement with experiment is excellent, suggesting that our approach provides a route for determining the HOMO energy with an accuracy better than 0.1 eV.

We investigate plasmon-emitter interactions in a nanoparticle-on-mirror cavity. We consider two different sorts of emitters: Those that sustain dipolar transitions and those hosting only quadrupolar, dipole-inactive excitons. By means of a fully analytical two-dimensional transformation optics approach, we calculate the light-matter coupling strengths for the full plasmonic spectrum supported by the nanocavity. We reveal the impact of finite-size effects in the exciton charge distribution and describe the population dynamics in a spontaneous emission configuration. Pushing our model beyond the quasistatic approximation, we extract the plasmonic dipole moments, which enables us to calculate the far-field scattering spectrum of the hybrid plasmon-emitter system. Our findings, tested against fully numerical simulations, reveal the similarities and differences between the light-matter coupling phenomenology for bright and dark excitons in nanocavities.

Subband occupancy (i.e., the number of occupied confinement-induced subbands of the semiconductor wire) is a key physical parameter determining the topological properties of superconductor-semiconductor hybrid systems investigated in the context of the search for non-Abelian Majorana zero modes. We theoretically study the subband occupation of semiconductor nanowire devices as a function of the applied gate potential, the semiconductor-superconductor (SM-SC) work function difference, and the surface charge density by solving self-consistently the Schrödinger-Poisson equations for the conduction electrons of the semiconductor nanowire. Realistic surface charge densities, which are responsible for band bending, are shown to significantly increase the number of occupied subbands, making it difficult or impossible to reach a regime where only a few subbands are occupied. We also show that the energy separation between subbands is significantly reduced in the regime of many occupied subbands, with highly detrimental consequences for the realization and observation of robust Majorana zero modes. As a consequence, the requirements for the realization of robust topological superconductivity and Majorana zero modes should include a low value of the chemical potential, consistent with the occupation of only a few subbands. Finally, we show that the local density of states on the exposed nanowire facets provides a powerful tool for identifying a regime with many occupied subbands and is capable of providing additional critical information regarding the feasibility of Majorana physics in semiconductor-superconductor devices. In our work we address both InAs/Al and InSb/Al superconductor-nanowire hybrid systems of current experimental interest.