We investigate the electronic dispersion of the high-$T_c$ cuprate superconductors using the fully self-consistent version of the phenomenological model, where charge planar quasiparticles are coupled to spin fluctuations. The inputs we use, the underlying (bare) band structure and the spin susceptibility $\chi$, are extracted from fits of angle-resolved photoemission and inelastic neutron scattering data of underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.6}$ by T. Dahm and coworkers [Nat. Phys. 5, 217 (2009)]. Our main results are as follows: (i) We have confirmed the finding by Dahm and coworkers that the main nodal kink is, for the present values of the input parameters, determined by the upper branch of the hourglass of $\chi$. We demonstrate that the properties of the kink depend qualitatively on the strength of the charge-spin coupling. (ii) The effect of the resonance mode of $\chi$ on the electronic dispersion strongly depends on its kurtosis in the quasimomentum space. A low (high) kurtosis implies a negligible (considerable) effect of the mode on the dispersion in the near-nodal region. (iii) The energy of the kink decreases as a function of the angle $\theta$ between the Fermi surface cut and the nodal direction, in qualitative agreement with recent experimental observations. We clarify the trend and make a specific prediction concerning the angular dependence of the kink energy in underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.6}$.

We investigated the variation in electronic, thermal, and thermoelectric properties of skutterudite ${\mathrm{IrSb}}_3$ upon filling the voids with alkaline (Na or K) or alkaline-earth atoms (Sr or Ba). Specific heat measurements revealed that the fillers donate electrons with high effective mass, which dominate the electric transport properties of the system. Vibrations associated with the filler additionally contribute to the specific heat, and also scatter phonons efficiently in conjunction with the static disorder, giving rise to the reduced thermal conductivity. The dimensionless thermoelectric figure of merit $ZT$ increases with temperature and electron concentration, and reaches to 0.44 for ${\mathrm{Ba}}_{0.4}{\mathrm{Ir}}_4{\mathrm{Sb}}_{12}$ at 685 K. The observed features for the filled ${\mathrm{IrSb}}_3$ are similar to those of filled ${\mathrm{CoSb}}_3$, indicating that $n$-type filled skutterudite antimonides have a potential as good thermoelectric materials universally.

We report on the optical properties of single InAs/GaAs quantum dots emitting near the telecommunication O band, probed via Coulomb blockade and nonresonant photoluminescence spectroscopy, in the presence of external electric and magnetic fields. We extract the physical properties of the electron and hole wave functions, including the confinement energies, interaction energies, wave-function lengths, and $g$ factors. For excitons, we measure the permanent dipole moment, polarizability, diamagnetic coefficient, and Zeeman splitting. The carriers are determined to be in the strong confinement regime. Large range electric field tunability, up to 7 meV, is demonstrated for excitons. We observe a large reduction, up to one order of magnitude, in the diamagnetic coefficient when rotating the magnetic field from Faraday to Voigt geometry due to the unique dot morphology. The complete spectroscopic characterization of the fundamental properties of long-wavelength dot-in-a-well structures provides insight for the applicability of quantum technologies based on quantum dots emitting at telecom wavelengths.

The semi-infinite jellium using the step potential model of surface potential is studied. The barrier height is found from the condition of the minimum of the surface energy. It is shown that this minimum is caused by the Coulomb interaction between electrons. The surface energy is positive in the entire domain of the Wigner-Seitz radius of metals, and it is in sufficiently good agreement with experimental data. The one-particle distribution function of electrons and the distance from the surface potential to the dividing plane within this model are calculated. Influence of the Coulomb interaction between electrons on these calculated characteristics is studied.

A key challenge in fabrication of superconductor (S)-semiconductor (Sm) hybrid devices is forming highly transparent contacts between the active electrons in the semiconductor and the superconducting metal. In this work, we show that a near perfect interface and a highly transparent contact can be achieved using epitaxial growth of aluminum on an InAs two-dimensional electron system. We demonstrate that this material system, Al-InAs, satisfies all the requirements necessary to reach into the topological superconducting regime by individual characterization of the semiconductor two-dimensional electron system, superconductivity of Al, and performance of S-Sm-S junctions. This exciting development might lead to a number of useful applications ranging from spintronics to quantum computing.

We investigate point defects and defect complexes in AlN for potential applicability as single-spin centers and solid-state qubits analogous to those observed in diamond and SiC. We find that isolated anion vacancies $\left(V_{\mathrm{N}}\right)$ meet many of the criteria for an individually addressable quantum system, but their states are too close to the conduction-band edge. We therefore investigate how the properties can be tuned by complexing of the vacancy with substitutional impurities on neighboring lattice sites. Based on our comprehensive investigation, the transition-metal dopants Ti and Zr emerge as the best candidates: They favorably substitute on the Al site and form complexes with $V_{\mathrm{N}}$ that possess the desired array of electronic and optical properties. Favorable charge and spin states, binding energies, and optical excitation energies are reported. Our results indicate that implantation of Ti or Zr into single-crystal AlN substrates can lead to the formation of individually addressable solid-state qubits in this material.

The latent heat of a magnetoelastic phase transition is used as a measure of the magnetocaloric effect since it is directly proportional to the entropy change. Taking ${\mathrm{MnFeSi}}_{0.33}{\mathrm{P}}_{0.66}$ as a model magnetocaloric material, density functional theory calculations in addition to the phonon calculations based on the density functional perturbation theory were performed in order to calculate the latent heat of the magnetoelastic phase transition. The Curie temperature ($T_C$) was determined by taking into account the quasiharmonic approximation and the configurational entropy. The material exhibits a first-order magnetic transition accompanied by a large latent-heat (19.97 kJ/kg) near-room-temperature operation.

Singlet fission (SF) is a multiexciton generation process that could be harnessed to improve the efficiency of photovoltaic devices. Experimentally, systems derived from the pentacene molecule have been shown to exhibit ultrafast SF with high yields. Charge-transfer (CT) configurations are likely to play an important role as intermediates in the SF process in these systems. In molecular crystals, electrostatic screening effects and band formation can be significant in lowering the energy of CT states, enhancing their potential to effectively participate in SF. In order to simulate these, it desirable to adopt a computational approach which is acceptably accurate, relatively inexpensive, and which scales well to larger systems, thus enabling the study of screening effects. We propose an electrostatically corrected constrained density functional theory (cDFT) approach as a low-cost solution to the calculation of CT energies in molecular crystals such as pentacene. Here we consider an implementation in the context of the onetep linear-scaling DFT code, but our electrostatic correction method is in principle applicable in combination with any constrained DFT implementation, also outside the linear-scaling framework. Our newly developed method allows us to estimate CT energies in the infinite crystal limit, and with these to validate the accuracy of the cluster approximation.

Modulating topologically nontrivial states in trivial materials is of both scientific and technological interest. Using first-principles calculations, we propose a demonstration of electron-doping- (or gate-voltage-) induced multiple quantum states; namely, quantum spin Hall (QSH) and quantum anomalous Hall (QAH) states, in a single material of the organometallic framework (HTT-Pt) synthesized from triphenylene hexathiol molecules (HTT) and $\mathrm{PtC}{\mathrm{l}}_2$. At a low doping level, the trivial HTT-Pt converts to a QSH insulator protected by time-reversal symmetry (TRS). When the electronic doping concentration is further increased, TRS will be broken, making the HTT-Pt a QAH insulator. The band gaps of these topologically nontrivial states can be as large as 42.5 meV, suggesting robustness at high temperatures. The possibility of switching between the QSH and QAH states offers an intriguing platform for a different device paradigm by interfacing between QSH and QAH states.

We explore the charged-impurity-scattering-limited mobility of electrons and holes in monolayer black phosphorus (BP), a highly anisotropic material. Taking full account of the anisotropic electronic structure in effective mass approximation, the zero-temperature momentum relaxation time and the charge carrier mobility are calculated based on the Boltzmann transport equation. For carrier densities accessible in experiments, we obtain anisotropy ratios of 3–4. These results are somewhat larger than mobility anisotropy ratios determined experimentally for multilayer BP samples, but due to the complex dependence of the scattering rates on the anisotropy, they are strikingly smaller than the effective mass ratios.

The $XY$ model with four-body quenched disordered interactions and its discrete $p$-clock proxy are studied on bipartite random graphs by means of the cavity method. The phase diagrams are determined from the ordered case to the spin-glass case. Dynamic, spinodal, and thermodynamic transition lines are identified by analyzing free energy, complexity, and tree reconstruction functions as temperature and disorder are changed. The study of the convergence of the $p$-clock model to the $XY$ model is performed down to temperature low enough to determine all relevant transition points for different node connectivity.

We study correlations and magnetic textures of localized spins, doped in three-dimensional Dirac semimetals. An effective field theory for magnetic moments is constructed by integrating out the fermionic degrees of freedom. The spin correlation shows a strong anisotropy, originating from spin-momentum locking of Dirac electrons, in addition to the conventional Heisenberg-like ferromagnetic correlation. The anisotropic spin correlation allows topologically nontrivial magnetic excitation textures such as a transient hedgehog state, as well as the ferromagnetic ground state. The spin-wave dispersion in ferromagnetic Weyl semimetal also becomes anisotropic, being less dispersed perpendicular to the magnetization.

One of the interesting open questions for the high-transition-temperature $\left(T_{\mathrm{c}}\right)$ superconductivity in sulfur hydrides is why high-pressure phases of ${\mathrm{H}}_3\mathrm{S}$ have extremely high $T_{\mathrm{c}}$'s. Recently, it has been pointed out that the presence of the Van Hove singularities (VHS) around the Fermi level is crucial. However, while there have been quantitative estimates of $T_{\mathrm{c}}$ based on the Migdal-Eliashberg theory, the energy dependence of the density of states (DOS) has been neglected to simplify the Eliashberg equation. In this study, we go beyond the constant DOS approximation and explicitly consider the electronic structure over 40 eV around the Fermi level. In contrast with the previous conventional calculations, this approach with a sufficiently large number of Matsubara frequencies enables us to calculate $T_{\mathrm{c}}$ without introducing the empirical pseudo Coulomb potential. We show that while ${\mathrm{H}}_3\mathrm{S}$ has much higher $T_{\mathrm{c}}$ than ${\mathrm{H}}_2\mathrm{S}$ for which the VHS is absent, the constant DOS approximation employed so far seriously overestimates (underestimates) $T_{\mathrm{c}}$ by $\sim 60$ K $(\sim 10$ K) for ${\mathrm{H}}_3\mathrm{S}$ $\left({\mathrm{H}}_2\mathrm{S}\right)$. We then discuss the impact of the strong electron-phonon coupling on the electronic structure with and without the VHS and how it affects the superconductivity. In particular, we focus on (1) the feedback effect in the self-consistent calculation of the self-energy, (2) the effect of the energy shift due to the zero-point motion, and (3) the effect of the changes in the phonon frequencies due to strong anharmonicity. We show that the effect of (1)–(3) on $T_{\mathrm{c}}$ is about 10–30 K for both ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{H}}_2\mathrm{S}$. Eventually, $T_{\mathrm{c}}$ is estimated to be 181 K for ${\mathrm{H}}_3\mathrm{S}$ at 250 GPa and 34 K for ${\mathrm{H}}_2\mathrm{S}$ at 140 GPa, which explains the pressure dependence of $T_{\mathrm{c}}$ observed in the experiment. In addition, we evaluate the lowest-order vertex correction beyond the Migdal-Eliashberg theory and discuss the validity of the Migdal approximation for sulfur hydrides.

We use ab initio calculations to estimate the formation energies of cation (transition-metal) antisite defects at oxide interfaces and to understand the basic physical effects that drive or suppress the formation of these defects. Antisite defects are found to be favored in systems with substantial charge transfer across the interface, while Jahn-Teller distortions and itinerant ferromagnetism can prevent antisite defects and help stabilize atomically sharp interfaces. Our results enable identification of classes of systems that may be more and less susceptible to the formation of antisite defects, and they motivate experimental studies and further theoretical calculations to elucidate the local structure and stability of oxide interface systems.

We introduce the spectrum bifurcation renormalization group (SBRG) as an improved application of the excited-state real-space renormalization group (RSRG-X) for a class of qubit models. Starting from a disordered many-body Hamiltonian in the full many-body localized (MBL) phase, SBRG flows to the MBL fixed-point Hamiltonian, and generates the local integrals of motion and the matrix product state representations for all eigenstates. The method is applicable to many interacting spin and fermion models in the full MBL phase. As a Hilbert-space preserving RG, SBRG also generates an entanglement holographic mapping, which duals the MBL state to a fragmented holographic space.

We investigate valence-bond-solid (VBS) phases in one-dimensional spin systems by an effective field theory developed by Schulz [Phys. Rev. B 34, 6372 (1986)]. While the distinction among the VBS phases is often understood in terms of different entanglement structures protected by certain symmetries, we adopt a different but more fundamental point of view, that is, different VBS phases are separated by a gap closing under certain symmetries. In this way, the effective field theory reproduces the known three symmetries: time reversal, bond-centered inversion, and dihedral group of $\pi$ spin rotations. It also predicts that there exists another symmetry: site-centered inversion combined with a spin rotation by $\pi$. We demonstrate that the last symmetry gives distinct trivial phases, which cannot be characterized by their entanglement structure, in terms of a simple perturbative analysis in a spin chain. We also discuss several applications of the effective field theory to the phase transitions among VBS phases in microscopic models and an extension of the Lieb-Schultz-Mattis theorem to non-translational-invariant systems.

We study the scaling properties of the ground-state entanglement between finite subsystems of infinite two-dimensional free lattice models, as measured by the logarithmic negativity. For adjacent regions with a common boundary, we observe that the negativity follows a strict area law for a lattice of harmonic oscillators, whereas for fermionic hopping models the numerical results indicate a multiplicative logarithmic correction. In this latter case we conjecture a formula for the prefactor of the area-law violating term, which is entirely determined by the geometries of the Fermi surface and the boundary between the subsystems. The conjecture is tested against numerical results and a good agreement is found.

Rare $d$-electron-derived heavy-fermion properties of the solid-solution series ${\mathrm{LaCu}}_3{\mathrm{Ru}}_x{\mathrm{Ti}}_{4-x}{\mathrm{O}}_{12}$ were studied for $1\le x\le 4$ by resistivity, susceptibility, specific-heat measurements, and magnetic-resonance techniques. Our results suggest the existence of a coherent Kondo lattice formed by localized Ru $4d$ electrons leading to strongly enhanced effective electron masses. Pure ruthenate $\left(x=4\right)$ is a heavy-fermion metal characterized by a resistivity proportional to $T^2$ at low temperatures $T$. By increasing titanium substitution the coherent Fermi-liquid state is disturbed, yielding single-ion Kondo-type properties as in the paradigm $4f$-based heavy-fermion compound ${\mathrm{Ce}}_x{\mathrm{La}}_{1-x}{\mathrm{Cu}}_{2.05}{\mathrm{Si}}_2$ [M. Ocko et al., Phys. Rev. B 64, 195106 (2001)]. In ${\mathrm{LaCu}}_3{\mathrm{Ru}}_x{\mathrm{Ti}}_{4-x}{\mathrm{O}}_{12}$ the heavy-fermion behavior finally breaks down upon crossing the metal-to-insulator transition close to $x=2$.

It is expected that the interplay between nontrivial band topology and strong electron correlation will lead to very rich physics. Thus a controlled study of the competition between topology and correlation is of great interest. Here, employing large-scale quantum Monte Carlo simulations, we provide a concrete example of the Kane-Mele-Hubbard model on an AA-stacking bilayer honeycomb lattice with interlayer antiferromagnetic interaction. Our simulation identified several different phases: a quantum spin Hall insulator (QSH), an $xy\text{-plane}$ antiferromagnetic Mott insulator, and an interlayer dimer-singlet insulator. Most importantly, a bona fide topological phase transition between the QSH and the dimer-singlet insulators, purely driven by the interlayer antiferromagnetic interaction, is found. At the transition, the spin and charge gap of the system close while the single-particle excitations remain gapped, which means that this transition has no mean-field analog and it can be viewed as a transition between bosonic symmetry-protected topological (SPT) states. At one special point, this transition is described by a $(2+1)dO\left(4\right)$ nonlinear sigma model with exact $SO\left(4\right)$ symmetry and a topological term at exactly $\mathrm{\Theta }=\pi$. The relevance of this work towards more general interacting SPT states is discussed.

We demonstrate a high-throughput density functional perturbation theory (DFPT) methodology capable of screening compounds for their dielectric properties. The electronic and ionic dielectric tensors are calculated for 88 compounds, where the eigenvalues of the total dielectric tensors are compared with single crystal and polycrystalline experimental values reported in the literature. We find that GGA/PBE has a smaller mean average deviation from experiments ($\text{MARD}=16.2$%) when compared to LDA. The prediction accuracy of DFPT is lowest for compounds that exhibit complex structural relaxation effects (e.g., octahedra rotation in perovskites) and/or strong anharmonicity. Despite some discrepancies between DFPT results and reported experimental values, the high-throughput methodology is found to be useful in identifying interesting compounds by ranking. This is demonstrated by the high Spearman correlation factor $\left(\rho =0.92\right)$. Finally, we demonstrate that DFPT provides a good estimate for the refractive index of a compound without calculating the frequency dependence of the dielectric matrix ($\text{MARD}=5.7$%).