Ballistic transport and thermoelectric properties of group III-VI compounds ($XY:X=\mathrm{B}$, Al, Ga, In, Tl; $Y=\mathrm{O}$, S, Se, Te, Po) are investigated based on first-principles calculations and Landauer formalism. This large family is composed of 25 compounds which stands out with their unique electronic band structures. Mexican hat shaped valence band, which exhibits quartic energy-momentum relation gives rise to a sharp peak in the density of states as well as a steplike electronic transmission spectrum near the valence band edge. The intriguing electronic band structure and transport properties motivate us to explore thermoelectric properties of group III-VI monolayers. We find that, in addition to the stepwise transmission at the band edge, flat bands, valley degeneracy, and band degeneracy are the factors that enhance thermoelectric efficiencies. For heavier compounds, better thermoelectric efficiencies are possible for both $n$-type and $p$-type carriers.

Using the standard fluctuational electrodynamics framework, we analytically calculate the radiative heat current between two thin metallic layers, separated by a vacuum gap. We analyze different contributions to the heat current (traveling or evanescent waves, transverse electric or magnetic polarization) and reveal the crucial qualitative role played by the dc conductivity of the metals as compared to the speed of light. For poorly conducting metals, the heat current may be dominated by evanescent waves even when the separation between the layers greatly exceeds the thermal photon wavelength, and the coupling is of an electrostatic nature. For well-conducting metals, the evanescent contribution dominates at separations smaller than the thermal wavelength and is mainly due to magnetostatic coupling, in agreement with earlier works on bulk metals.

We investigate transport in a superconducting nanostructure housing a Weyl point in the spectrum of Andreev bound states. A minimum magnet state is realized in the vicinity of the point. One or more normal-metal leads are tunnel-coupled to the nanostructure. We have shown that this minimum magnetic setup is suitable for realization of all common goals of spintronics: detection of a magnetic state, conversion of electric currents into spin currents, potentially reaching the absolute limit of one spin per charge transferred, and detection of spin accumulation in the leads. The peculiarity and possible advantage of the setup is the ability to switch between magnetic and nonmagnetic states by tiny changes of the control parameters: superconducting phase differences. We employ this property to demonstrate the feasibility of less common spintronic effects: spin on demand and alternative spin current.

Semiconductor quantum dots (QDs) acting as single-photon-emitters are potential building blocks for various applications in future quantum information technology. For such applications, a thorough understanding and precise control of charge states and capture/recombination dynamics of the QDs are vital. In this work, we study the dynamics of QDs spontaneously formed in GaNAsP nanowires, belonging to the dilute nitride material system. By using a random population model modified for these highly mismatched materials, we analyze the results from photoluminescence and photon correlation experiments and show a general trend of disparity in positive and negative trion populations and also a strong dependence of the capture/recombination dynamics and QD charge states on its surroundings. Specifically, we show that the presence of hole-trap defects in the proximity to some QDs facilitates formation of negative trions, which also causes a dramatic reduction of the neutral exciton lifetime. These findings underline the importance of proper understanding of the QD capture and recombination processes and demonstrate the possibility to use highly mismatched materials and defects for charge engineering of QDs.

We investigate a class of periodically driven many-body systems that allows us to extend the phenomenon of prethermalization to the vicinity of isolated intermediate-to-low drive frequencies away from the high-frequency limit. We provide numerical evidence for the formation of a parametrically long-lived prethermal plateau, captured by an effective Floquet Hamiltonian computed using the replica inverse-frequency expansion, and demonstrate its stability with respect to random perturbations in the drive period. Considering exclusively nonintegrable Floquet Hamiltonians, we find that heating rates are nonuniversal: we observe Fermi's golden rule scaling, power-law scaling inconsistent with the golden rule, and non-power-law scaling, depending on the drive. Despite the asymptotic character of the inverse-frequency expansion, we show that it describes the thermostatic properties of the state all along the evolution up to infinite temperature, with higher-order terms improving the accuracy. Our results suggest a dynamical mechanism to gradually increase the temperature in isolated quantum simulators, such as ultracold atoms, and open up an alternative possibility to investigate thermal phase transitions and the interplay between thermal and quantum criticality using Floquet drives.

Thanks to high resolution and polarization analysis, resonant inelastic x-ray scattering (RIXS) magnetic spectra of ${\mathrm{La}}_2{\mathrm{CuO}}_4$, ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{C}}_{l2}$ and ${\mathrm{CaCuO}}_2$ reveal a rich set of properties of the spin-$1/2$ antiferromagnetic square lattice of cuprates. The leading single-magnon peak energy dispersion is in excellent agreement with the corresponding inelastic neutron scattering measurements. However, the RIXS data unveil an asymmetric line shape possibly due to odd higher order terms. Moreover, a sharp bimagnon feature emerges from the continuum at $(1/2,0)$, coincident in energy with the bimagnon peak detected in optical spectroscopy. These findings show that the inherently complex spin spectra of cuprates, an exquisite manifestation of quantum magnetism, can be effectively explored by exploiting the richness of RIXS cross sections.

By using ab initio calculations and symmetry analysis, we define a class of Weyl phonons: charge-four Weyl phonons (CFWPs) characterized by Chern number $\pm 4$ in their acoustic phonon spectra due to chiral point-group symmetries. Some high-symmetry points in the chiral space groups (SGs) of phononic systems behaving as quadratic Weyl points tend to form CFWPs. As enumerated in this paper, the CFWPs are located at the boundaries or the center of the three-dimensional Brillouin zone and are protected by the time-reversal symmetry $\mathcal{T}$ and the corresponding point-group symmetries. Moreover, in a realistic chiral crystal example of BiIrSe in SG 198, a monopole CFWP is confirmed at point $\mathrm{\Gamma }$ with quadratic twofold degeneracies, and in another example of ${\mathrm{Li}}_3{\mathrm{CuS}}_2$ in SG 214, the CFWPs are found at the high-symmetry points $\mathrm{\Gamma }$ and $H$. Our theoretical results not only uncover a class of Weyl phonons but also put forward an effective way to search for CFWPs in spinless systems.

Magnetic exchange couplings can be tuned by coupling to periodic light, where the frequency and amplitude are typically varied: a process known as Floquet engineering. The polarization of the light is also important, and in this paper, we show how different polarizations, including several types of unpolarized light, can tune the exchange couplings in distinct ways. Using unpolarized light, for example, it is possible to tune the material without breaking either time-reversal or any lattice symmetries. To illustrate these effects generically, we consider single-band Hubbard models at half-filling on the honeycomb, square and triangular lattices. We derive the effective Heisenberg spin models to fourth order in perturbation theory for arbitrary fixed polarizations, and several types of unpolarized light that preserve time-reversal and lattice symmetries. Coupling these models to periodic light tunes first, second, and third neighbor exchange couplings, as well as ring exchange terms on the square and triangular lattices. Circularly polarized light induces chiral fields for the honeycomb and triangular lattices, which favors noncoplanar magnetism and potential chiral spin liquids. We discuss how to maximize the enhancement of the couplings without inducing heating.

Bosonic lattice systems with nontrivial interactions represent an intriguing platform to study exotic phases of matter. Here, we study the effects of extended correlated hopping processes in a system of bosons trapped in a lattice geometry. The interplay between single particle tunneling terms, correlated hopping processes, and onsite repulsion is studied by means of a combination of exact diagonalization, strong coupling expansion, and cluster mean field theory. We identify a rich ground state phase diagram where, apart from the usual Mott and superfluid states, superfluid phases with interesting clustering properties occur.

We propose a universal method to detect the specular Andreev reflection taking the simple two-dimensional Weyl nodal-line semimetal-superconductor double-junction structure as an example. The quasiclassical quantization conditions are established for the energy levels of bound states formed in the middle semimetal along a closed path. The establishment of the conditions is completely based on the intrinsic character of the specularly reflected hole which has the same sign relation of its wave vector and group velocity as the incident electron. This brings about the periodic oscillation of conductance with the length of the middle semimetal, which is lacking for the retro-Andreev reflected hole with the opposite sign relation as the incident electron. The positions of the conductance peaks and the oscillation period can be precisely predicted by the quantization conditions. Our detection method is irrespective of the details of the materials, which may promote the experimental detection of and further research on the specular Andreev reflection as well as its applications in superconducting electronics.

We present a comprehensive study of a three-orbital lattice model suitable for the layered iridate ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. Our analysis includes various on-site interactions (including Hubbard and Hund's) as well as compressive strain, and a Zeeman magnetic field. We use a self-consistent mean-field approach with multiple order parameters to characterize the resulting phases. While in some parameter regimes the compound is well described by an effective $J=1/2$ model, in other regimes, the full multiorbital description is needed. As a function of the compressive strain, we uncover two quantum phase transitions: first a continuous metal-insulator transition, and subsequently a first-order magnetic melting of the antiferromagnetic order. Crucially, bands of both $J=1/2$ and $J=3/2$ nature play important roles in these transitions. Our results qualitatively agree with experiments of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ under strain induced by a substrate and motivate the study of higher strains.

We investigate bilayer graphene systems with layer switching domain walls separating the two energetically equivalent Bernal stackings in the presence of an external magnetic field. To this end, we calculate quantum transport and local densities of three microscopic models for a single domain wall: a hard wall, a defect due to shear, and a defect due to tension. The quantum transport calculations are performed with a recursive Green's function method. Technically, we discuss an explicit algorithm for the separation of a system into subsystems for the recursion and we present an optimization of the well known iteration scheme for lead self-energies for sparse chain couplings. We find strong physical differences for the three different types of domain walls in the integer quantum Hall regime. For a domain wall due to shearing of the upper graphene layer, there is a plateau formation in the magnetoconductance for sufficiently wide defect regions. For wide domain walls due to tension in the upper graphene layer, there is only an approximate plateau formation with fluctuations of the order of the elementary conductance quantum ${\sigma }_0$. A direct transition between stacking regions like for the hard wall domain wall shows no plateau formation and is therefore not a good model for either of the previously mentioned extended domain walls.

We present experimental evidence of incoherent Kondo scattering as the source of resistivity minima in bulk polycrystalline and nanocrystalline ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. The temperature dependence of thermopower shows a positive maximum at high temperature followed by a negative minimum at low temperature, with the sign inversion occurring at a much higher temperature than $T_K$. Moreover, we observe little correlation between $T_K$ and intersite coupling strength given by $|{\theta }_{\text{CW}}|$. We describe the temperature dependence of thermopower and resistivity within the framework of crystal field excitation in a Kondo lattice.

Narrow and even flat energy bands reflect strong electronic correlation which brings in novel physical properties. Pursuing flatter band has recently attracted a lot of attention. Along this line, we theoretically investigate low-energy band structures and concomitant topological properties of twisted bilayer graphene at the first magic angle based on a continuum model. By considering an external moiré electric potential, in addition to the intrinsic Hartree potential, we find that the lowest bands can be further flattened by this electric potential and a superflat band structure is obtained. In such bands, the extremely small ratio between the bandwidth and the interaction strength will enhance the strong correlation. These superflat energy bands give nearly perfect nesting at the half-filling, then it benefits the formation of Cooper pairs and the rise of superconducting temperature. Moreover, combining with a perfectly aligned boron nitride substrate, the system undergoes a topological phase transition by modulating the relevant parameters of the electric potential. The topological phase transition can be well explained using simplified analytical treatments near the Dirac points.

Refinements in energy monochromation and aberration correction in state-of-the-art scanning transmission electron microscopes has opened access to the far infrared regime for spectroscopic characterization at the nanoscale. At these low energies, the dielectric environment, such as a dielectric slab adjacent to the target specimen, may no longer play a passive role in the spectrum. Instead, the environment may itself host resonances that mix with those of the target and complicate interpretation of its spectral responses. This paper explores a theoretical description of the coupling between the collective vibrational surface modes of a dielectric particle and dielectric slab of varying thickness for the purpose of elucidating the interacting phononic excitations in dielectric materials typical of inelastic electron scattering measurements in the infrared. Dynamical coordinates and a governing Hamiltonian are rigorously defined in the quasistatic limit to account for phonon mode mixing and forcing by an aloof electron probe, which travels along a grazing trajectory, parallel to the dielectric slab. As the spectral window of interrogation by fast electron probes has been extended down to thermal energies with unprecedented meV energy resolution, theoretical models like that presented herein are crucial for accurate interpretation of experimental data.

We performed first-principles density functional theory (DFT) and $GW$ calculations to investigate electronic structures of bulk and few-layer ${\mathrm{PdSe}}_2$. We obtained the quasiparticle band structure of bulk ${\mathrm{PdSe}}_2$, and the obtained energy gap agrees excellently with the reported experimental value. For monolayer and bilayer ${\mathrm{PdSe}}_2$, we obtained quasiparticle band structures with respect to the vacuum level. We analyzed DFT and $\mathit{GW}$ band structures in detail, finding $k$-space positions of valence band maxima and conduction band minima, effective masses, the quasiparticle density of states, work functions, ionization potentials, electron affinities, and $k$-space shapes of electron and hole pockets. These results provide a foundation for development of basic studies and device applications.

Two-dimensional multiferroics, combined with ferroelastic and ferroelectric orders, enable unprecedented exploration of applications, such as high-density multistate storage. However, current ferroelastic-ferroelectric-multiferroic research is based on the paradigm of unique symmetry in a single-layer lattice, which restricts ferroelastic ferroelectricity to being rarely explored. Here, going beyond the existing paradigm, we report a design principle for realizing ferroelastic ferroelectricity using the van der Waals interaction as perturbation in a bilayer lattice. Using first principles, we show that, through layer-stacking, bilayer ${\mathrm{ZrI}}_2$ not only possesses 120 ° ferroelasticity due to its crystal symmetry but also holds out-of-plane and in-plane polarizations caused by interlayer charge redistribution, thereby achieving ferroelastic-ferroelectric multiferroics in a bilayer lattice. The switch of out-of-plane polarization relates to interlayer sliding, while in-plane polarization reversal correlates with 120 ° ferroelastic switching. Based on this fact, we propose the six-logic-state multiferroicity. This paper illustrates a tantalizing scheme for achieving and developing two-dimensional ferroelastic ferroelectricity.