Majorana bound states are interesting candidates for applications in topological quantum computation. Low-energy models allowing one to grasp their properties are hence conceptually important. The usual scenario in these models is that two relevant gapped phases, separated by a gapless point, exist. In one of the phases, topological boundary states are absent, while the other one supports Majorana bound states. We show that a customary model violates this paradigm. The phase that should not host Majorana fermions supports a fractional soliton exponentially localized at only one end. By varying the parameters of the model, we describe analytically the transition between the fractional soliton and two Majorana fermions. Moreover, we provide a possible physical implementation of the model. We further characterize the symmetry of the superconducting pairing, showing that the odd-frequency component is intimately related to the spatial profile of the Majorana wave functions.

Due to their band-structure and optical properties, InAs/InP quantum dots (QDs) constitute a promising system for single-photon generation at the third telecom window of silica fibers and for applications in quantum communication networks. However, obtaining the necessary low in-plane density of emitters remains a challenge. Such structures are also still less explored than their InAs/GaAs counterparts regarding optical properties of confined carriers. Here, we report on the growth via metal-organic vapor phase epitaxy and investigation of low-density InAs/InP QD-like structures, emitting in the range of $1.2–1.7\mu \mathrm{m}$, which includes the S, C, and L bands of the third optical window. We observe multiple photoluminescence (PL) peaks originating from flat QDs with the height of a few material monolayers. Temperature-dependent PL reveals a redistribution of carriers between families of QDs. Via time-resolved PL, we obtain radiative lifetimes nearly independent of emission energy in contrast to previous reports on InAs/InP QDs, which we attribute to strongly height-dependent electron-hole correlations. Additionally, we observe neutral and charged exciton emission from spatially isolated emitters. Using the eight-band $\mathit{k}·\mathit{p}$ model and configuration-interaction method, we successfully reproduce the energies of emission lines, the dispersion of exciton lifetimes, the carrier activation energies, as well as the biexciton binding energy, which allows for a detailed and comprehensive analysis of the underlying physics.

We study the electronic structure of heterostructures formed by a graphene nanoribbon (GNR) and a transition metal dichalcogenides (TMD) monolayer using first principles. We consider both semiconducting TMDs and metallic TMDs, and different stacking configurations. We find that when the TMD is semiconducting the effects on the band structure of the GNRs are small. In particular the spin splitting induced by proximity on the GNRs bands is only of the order of few meV irrespective of the stacking configuration. When the TMD is metallic, such as ${\mathrm{NbSe}}_2$, we find that the spin splitting induced in the GNRs can be very large and strongly dependent on the stacking configuration. For optimal stacking configurations the proximity-induced spin splitting is of the order of 20 meV for armchair graphene nanoribbons and as high as 40 meV for zigzag graphene nanoribbons. This results are encouraging for the prospects of using GNR-TMD heterostructures to realize quasi-one-dimensional topological superconducting states supporting Majorana modes.

We explore the spin and charge properties of electrons in wurtzite semiconductor nanowires where radial and axial confinement leads to tubular or ring-shaped quantum structures. Accounting for spin-orbit interaction induced by the wurtzite lattice as well as a radial potential gradient, we analytically derive the corresponding low-dimensional Hamiltonians. It is demonstrated that the resulting tubular spin-orbit Hamiltonian allows us to construct spin states that are persistent in time and robust against disorder. We find that these special scenarios are characterized by distinctive features in the optical conductivity spectrum, which enable an unambiguous experimental verification. In both types of quantum structures, we discuss the dependence of the occurring persistent charge and spin currents on an axial magnetic field and Fermi energy which show clear fingerprints of the electronic subband structure. Here, the spin-preserving symmetries become manifest in the vanishing of certain spin current tensor components. Our analytic description relates the distinctive features of the optical conductivity and persistent currents to band structure characteristics, which allows us to deduce spin-orbit coefficients and other band parameters from measurements.

The van der Waals ferromagnet $\mathrm{F}{\mathrm{e}}_3\mathrm{GeT}{\mathrm{e}}_2$ has attracted great research attention recently due to its extraordinary properties. Here, using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the temperature evolution of the electronic structure of bulk $\mathrm{F}{\mathrm{e}}_3\mathrm{GeT}{\mathrm{e}}_2$. We observe largely dispersive energy bands with exchange splitting that are in overall agreement with our density-functional theory calculation. Interestingly, the band dispersions barely change upon heating towards the ferromagnetic transition near 225 K, except for the reduction of quasiparticle coherence, which strongly deviates from the itinerant Stoner model. We suggest that the local magnetic moments may play a crucial role in the ferromagnetic ordering and the electronic structure of $\mathrm{F}{\mathrm{e}}_3\mathrm{GeT}{\mathrm{e}}_2$, which will shed light on the generic understanding of itinerant magnetism in correlated materials.

Three types of fermions have been extensively studied in topological quantum materials: Dirac, Weyl, and Majorana fermions. Beyond the fundamental fermions in high energy physics, exotic fermions are allowed in condensed matter systems residing in three-, six- or eightfold degenerate band crossings. Here, we use angle-resolved photoemission spectroscopy to directly visualize three-doubly-degenerate bands in $\mathrm{PdS}{\mathrm{b}}_2$. The ultrahigh energy resolution we are able to achieve allows for the confirmation of all the sixfold degenerate bands at the $R$ point, in remarkable consistency with first-principles calculations. Moreover, we find that this sixfold degenerate crossing has quadratic dispersion as predicted by theory. Finally, we compare sixfold degenerate fermions with previously confirmed fermions to demonstrate the importance of this work: our study indicates a topological fermion beyond the constraints of high energy physics.

We study the electronic and transport properties of a network of domain walls between insulating domains with opposite valley Chern numbers. We find that the network is semimetallic with Dirac dispersion near the charge neutrality point and the corresponding electronic states distribute along the domain walls. Near the charge neutrality point, we find quantized conductance in a nanoribbon with sawtooth domain wall edges that propagates along the boundaries and is robust against weak disorder. For a trident edged ribbon, we find a small energy gap due to the finite-size effect making the nanoribbon an insulator. When the Fermi energy is away from the charge neutrality point, all domain walls contribute to the conduction of current. Our results provide a comprehensive analysis of the electronic transport properties in a topological domain wall network that not only agrees qualitatively with experiments on marginally twisted bilayer graphene under a perpendicular electric field, but also can provide useful insights for designing low-power topological quantum devices.

The low-temperature electron-phonon (e-ph) relaxation near the surface of noble metals, Cu and Ag, is studied by using the density-functional theory approach. The appearance of the surface phonon mode can give rise to a strong enhancement of the Eliashberg function at low frequency $\omega$. Assuming the Eliashberg function proportional to the square of $\omega$ in the low frequency limit, the e-ph relaxation time obtained from the surface calculations is shorter than that from the bulk calculation. The calculated e-ph relaxation time for the former is in agreement with a recent experiment for thin films.

The lithium manganese oxide spinel ${\mathrm{Li}}_x{\mathrm{Mn}}_2{\mathrm{O}}_4$, with $0\le x\le 2$, is an important example for cathode materials in lithium ion batteries. However, an accurate description of ${\mathrm{Li}}_x{\mathrm{Mn}}_2{\mathrm{O}}_4$ by first-principles methods like density functional theory is far from trivial due to its complex electronic structure, with a variety of energetically close electronic and magnetic states. It was found that the local density approximation as well as the generalized gradient approximation (GGA) are unable to describe ${\mathrm{Li}}_x{\mathrm{Mn}}_2{\mathrm{O}}_4$ correctly. Here, we report an extensive benchmark for different ${\mathrm{Li}}_x{\mathrm{Mn}}_y{\mathrm{O}}_z$ systems using the hybrid functionals PBE0 and HSE06, as well as the recently introduced local hybrid functional PBE0r. We find that all of these functionals yield energetic, structural, electronic, and magnetic properties in good agreement with experimental data. The notable benefit of the PBE0r functional, which relies on onsite Hartree-Fock exchange only, is a much reduced computational effort that is comparable to GGA functionals. Furthermore, the Hartree-Fock mixing factors in PBE0r are smaller than in PBE0, which improves the results for (lithium) manganese oxides. The investigation of ${\mathrm{Li}}_x{\mathrm{Mn}}_2{\mathrm{O}}_4$ shows that two Mn oxidation states, $+\mathrm{III}$ and $+\mathrm{IV}$, coexist. The ${\mathrm{Mn}}^{\text{III}}$ ions are in the high-spin state and the corresponding ${\mathrm{MnO}}_6$ octahedra are Jahn-Teller distorted. The ratio between ${\mathrm{Mn}}^{\text{III}}$ and ${\mathrm{Mn}}^{\text{IV}}$ and thus the electronic structure changes with the Li content while no major structural changes occur in the range from $x=0$ to 1. This work demonstrates that the PBE0r functional provides an equally accurate and efficient description of the investigated ${\mathrm{Li}}_x{\mathrm{Mn}}_y{\mathrm{O}}_z$ systems.

We propose an effective model to describe the $A_{2g}$ signal in Raman scattering experiments in the ${\mathrm{URu}}_2{\mathrm{Si}}_2$ compound. We follow the scheme proposed earlier by D. V. Khveshchenko and P. B. Wiegmann [Phys. Rev. Lett. 73, 500 (1994)] to calculate the $A_{2g}$ scattering vertex. We extract the imaginary part of the two-point current-current correlation function and compare it directly with the Raman response. We obtain an inelastic peak at the $A_{2g}$ channel owing to the interplay between a local staggered ordering and a possible quantum spin liquid behavior. Our results offer an explanation for the electronic Raman scattering experiments at the hidden order phase in the ${\mathrm{URu}}_2{\mathrm{Si}}_2$ compound [Buhot et al., Phys. Rev. Lett. 113, 266405 (2014)].

We report diffusion quantum Monte Carlo (DMC) and many-body $GW$ calculations of the electronic band gaps of monolayer and bulk hexagonal boron nitride (hBN). We find the monolayer band gap to be indirect. $GW$ predicts much smaller quasiparticle gaps at both the single-shot $G_0{W}_0$ and the partially self-consistent $G{W}_0$ levels. In contrast, solving the Bethe-Salpeter equation on top of the $G{W}_0$ calculation yields an exciton binding energy for the direct exciton at the $K$ point in close agreement with the DMC value. Vibrational renormalization of the electronic band gap is found to be significant in both the monolayer and the bulk. Taking vibrational effects into account, DMC overestimates the band gap of bulk hBN, while $GW$ theory underestimates it.

We study the effect of an in-plane magnetic field on the noninteracting dispersion of twisted bilayer graphene. Our analysis is rooted in the chirally symmetric continuum model, whose zero-field band structure hosts exactly flat bands and large energy gaps at the magic angles. At the first magic angle, the central bands respond to a parallel field by forming a quadratic band crossing point (QBCP) at the moiré Brillouin zone center. Over a large range of fields, the dispersion is invariant with an overall scale set by the magnetic field strength. For deviations from the magic angle and for realistic interlayer couplings, the motion and merging of the Dirac points lying near charge neutrality are discussed in the context of the symmetries, and we show that small magnetic fields are able to induce a qualitative change in the energy spectrum. We conclude with a discussion on the possible ramifications of our study to the interacting ground states of twisted bilayer graphene systems.

This study aims at a quantitative understanding of the optical spectra taken from bound excitons weakly coupled to phonons in indirect semiconductors, which had been missing for decades. Insights on the properties of excitons bound at neutral acceptor impurities are obtained by analyzing the spectra reported for diamond with appropriate boron doping [Y. Kubo et al., Appl. Phys. Lett. 114, 132104 (2019)]. We focus on the mirror symmetry holding between phonon-assisted absorption and luminescence simultaneously regarding energies, linewidths, and intensities. New analytic expressions are proposed to reproduce the spectra of phonon-assisted recombination lines. The detailed analysis reveals the contribution from excited bound-exciton states whose origin is discussed and modeled. Taking them into account improves simulation of the bound-exciton optical spectra in indirect band-gap semiconductors up to a quantitative level.

Optically generated excitons dictate the absorption and emission spectrum of doped semiconductor transition-metal dichalcogenide monolayers. We show that upon increasing the electron density, the elementary optical excitations develop a rotonlike dispersion, evidenced by a shift of the lowest-energy state to a finite momentum on the order of the Fermi momentum. This effect emerges due to Pauli exclusion between excitons and the electron Fermi sea, but the robustness of the roton minimum in these systems is a direct consequence of the long-range nature of the Coulomb interaction and the nonlocal dielectric screening characteristic of monolayers. Finally, we show that the emergence of rotons could be related to certain features of photoluminescence spectra in doped transition-metal dichalcogenide monolayers.

We consider the generic problem of a two-level quantum emitter near a two-dimensional Chern insulator in the dipole approximation, and study how the frequency-dependent response and electronic density of states of the insulator modifies the transition rate of the emitter between the ground and excited levels. To this end, we obtain the full real-frequency behavior of the conductivity tensor by performing a tight-binding calculation based on the Qi-Wu-Zhang model and using a Kubo formula, and derive the full electromagnetic Green tensor of the system, which breaks Onsager reciprocity. This enables us to find that for frequencies smaller than the maximum band gap, the system is sensitive to time-reversal symmetry breaking, whereas for much larger frequencies the system becomes insensitive, with implications for the discrimination of the state of a circularly polarized dipole emitter. We also study the impact of a Van Hove singularity on the surface-induced correction to the transition rate, finding that it can enhance its amplitude by a few orders of magnitude compared to the case where the conductivity is set to its static value. By considering configurations in which the dipole is circularly polarized or parallel with the surface of the Chern insulator, we find that the surface correction to the transition rate can exhibit a novel decay with sine-integral-like oscillations.

We perform a theoretical study of the near-field heat transfer between two atomically thin metallic layers, isolated galvanically but coupled by the Coulomb interaction, within the framework of fluctuational electrodynamics in the Coulomb limit. We clarify the role of disorder and spatial dispersion, and identify several distinct regimes of the heat transfer. We find that the plasmon contribution to the heat current is suppressed in both the clean and diffusive limits, but dominates in a parametrically wide crossover region at sufficiently high temperatures.

The effects of spin-orbit (SOC) and electron-phonon coupling on the collective excitation of doped monolayer ${\mathrm{Sb}}_2$ are investigated using density functional and many-body perturbation theories. The spin-orbit coupling is exclusively important for the monolayer ${\mathrm{Sb}}_2$ and it leads to the modification of the electronic band structure. In particular, plasmon modes of monolayer ${\mathrm{Sb}}_2$ are quite sensitive to the SOC and are characterized by very low damping rates, owing to small electron-phonon scatterings. Our results show how plasmons in antimonene are significantly less damped compared to monolayer graphene when plasmon energies are larger than 0.2 eV due to smaller plasmon-phonon coupling in the former material.

Metallicity and ferroelectriclike polar distortion are mutually noncompatible, and their coexistence in the same system is an intriguing subject of fundamental interest in the field of structure phase transition. However, it is unclear what mechanism may extend the limit of metallicity that allows polar distortion. We investigate the polar instability and soft modes in electron-doped ${\mathrm{PbTiO}}_3$ using linear-response density functional calculations. We find that ferroelectric instability can remarkably sustain up to an electron concentration of $n_e=0.7$ per unit cell, which is beyond the limit that causes the polar catastrophe in ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$. Our study further reveals two discoveries: (i) electron doping can turn nonsoft mode into soft mode, which leads to different microscopic mechanism for ferroelectricity when the system is strongly metallic; (ii) the frequency change $\mathrm{\Delta }\omega /\mathrm{\Delta }{n}_e$ is surprisingly flat at large $n_e$, which is pivotal for the persistence of soft mode and polar distortion at high metallicity. We also provide an interesting physical origin—which is caused by the strong mode-mode interaction—to explain these phenomena, and the finding of this origin may further extend the limit where metallicity and polar distortion coexist.

Upon excitation by a laser pulse, broken-symmetry phases of a wide variety of solids demonstrate similar order parameter dynamics characterized by a dramatic slowing down of relaxation for stronger pump fluences. Motivated by this recurrent phenomenology, we develop a simple nonperturbative effective model for photoinduced dynamics of collective bosonic excitations. We find that as the system recovers after photoexcitation, it shows universal prethermalized dynamics manifesting a power-law, as opposed to exponential, relaxation, explaining the slowing down of the recovery process. For strong quenches, long-wavelength overpopulated transverse modes dominate the long-time dynamics; their distribution function exhibits universal scaling in time and space, whose universal exponents can be computed analytically. Our model offers a unifying description of order parameter fluctuations in a regime far from equilibrium, and our predictions can be tested with available time-resolved techniques.