Superconducting wires with broken time-reversal and spin-rotational symmetries can exhibit two distinct topological gapped phases and host bound Majorana states at the phase boundaries. When the wire is tuned to the transition between these two phases and the gap is closed, Majorana states become delocalized leading to a peculiar critical state of the system. We study transport properties of this critical state as a function of the length $L$ of a disordered multichannel wire. Applying a nonlinear supersymmetric sigma model of symmetry class D with two replicas, we identify the average conductance, its variance, and the third cumulant in the whole range of $L$ from the Ohmic limit of short wires to the regime of a broad conductance distribution when $L$ exceeds the correlation length of the system. In addition, we calculate the average thermal shot noise power and variance of the topological index for arbitrary $L$. The general approach developed in the paper can also be applied to study combined effects of disorder and topology in wires of other symmetries.

We introduce a family of paired-composite-fermion trial wave functions for any odd Cooper-pair angular momentum. These wave functions are parameter-free and can be efficiently projected into the lowest Landau level. We use large-scale Monte Carlo simulations to study three cases: Firstly, the Moore-Read phase, which serves us as a benchmark. Secondly, we explore the pairing associated with the anti-Pfaffian and the particle-hole-symmetric Pfaffian. Specifically, we assess whether their trial states feature exponentially decaying correlations and thus represent gapped phases of matter. For Moore-Read and anti-Pfaffian we find decay lengths of ${\xi }_{\text{Moore-Read}}=1.30\left(5\right)$ and ${\xi }_{\text{anti-Pfaffian}}=1.38\left(14\right)$, in units of the magnetic length. By contrast, for the case of PH-Pfaffian, we find no evidence of a finite length scale for up to 56 particles.

We report a systematic first-principles study on positron annihilation parameters in the $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ lattice and Ga monovacancy defects complemented with orientation-dependent experiments of the Doppler broadening of the positron-electron annihilation. We find that both the $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ lattice and the considered defects exhibit unusually strong anisotropy in their Doppler broadening signals. This anisotropy is associated with low symmetry of the $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ crystal structure that leads to unusual kind of one-dimensional confinement of positrons even in the delocalized state in the lattice. In particular, the split Ga vacancies recently observed by scanning transmission electron microscopy produce unusually anisotropic positron annihilation signals. We show that in experiments, the positron annihilation signals in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ samples seem to be often dominated by split Ga vacancies.

We study the dynamical polarization function and plasmon modes for spin-orbit coupled noncentrosymmetric metals such as ${\mathrm{Li}}_2{\left({\mathrm{Pd}}_{1-x}{\mathrm{Pt}}_x\right)}_3\mathrm{B}$. These systems have different Fermi surface topologies for Fermi energies above and below the spin degenerate point, which is also known as the band touching point (BTP). We calculate the exact dynamical polarization function numerically and also provide its analytical expression in the long wavelength limit. We obtain the plasmon dispersion within the framework of random phase approximation. In noncentrosymmetric metals, there is a finite energy gap in between intra- and interband particle-hole continuum for vanishing excitation wave vector. In the long wavelength limit, the width of the interband particle-hole continuum behaves differently for Fermi energies below and above the BTP as a clear signature of the Fermi surface topology change. We find a single undamped optical plasmon mode lying in between the intra- and interband particle-hole continuum for Fermi energies above and below the BTP within a range of parameters. The plasmon mode below the BTP has smaller velocity than that of above the BTP. It is interesting to find that as we tune the Fermi energy around the BTP, the plasmon mode becomes damped within a range of electron-electron interaction strengths. For Fermi energies above and below the BTP, we also obtain an approximate analytical result of plasma frequency and plasmon dispersion which match well with their numerical counterparts in the long wavelength limit. The plasmon dispersion is $\propto q^2$ with $q$ being the wave vector for plasmon excitation in the long wavelength limit. We find that varying the carrier density with fixed electron-electron interaction strength or vice versa does not change the number of undamped plasmon modes, although damped plasmon modes can be more in number for some values of these parameters. We demonstrate our results by calculating the loss function and optical conductivity, which can be measured in experiments.

One-dimensional (1D) graphene superlattices have been predicted to exhibit extra zero-energy modes about a decade ago, but an experimental proof has remained missing. Motivated by recent experimental progresses, here we perform quantum transport simulations for 1D graphene superlattices, considering electrostatically simulated potential profiles as realistic as possible. Combined with the analysis on the corresponding miniband structures, we find that the zero modes generated by the 1D superlattice potential can be further cloned to higher energies, which are also accessible by tuning the average carrier density. Our multiterminal transverse magnetic focusing simulations further reveal the modulation-controllable ballistic miniband transport for 1D graphene superlattices. A simple idea for creating a perfectly symmetric periodic potential is proposed at the end of this work for generating perfectly aligned zero modes. Our work provides a reliable guide to how the extra zero modes and their copies at higher energies feature in transport.

In the quest to built novel metamaterials with unique optical properties, three-dimensional assemblies of metal clusters and nanoparticles are gathering significant attention. Organized geometries, such as tetrahedra and icosahedra, can be built, for example, by using DNA strands or virus capsids as templates. Here we use the jellium model and time-dependent density functional theory to study the plasmonic resonances in different arrangements of eight-electron clusters from the electronic perspective. A charge transfer ratio index based on the induced transition densities is used to quantify the charge transfer nature of the excitations at different energies. We vary the size, shape, and intercluster separation, building systems of four-cluster tetrahedra, 12-cluster icosahedra and cuboctahedra, and 20-cluster dodecahedra. All the studied systems are found to have charge transfer plasmon-type excitations at low energies. Analysis of the electron-hole transitions contributing to the transition dipole moment is further used to characterize these excitations, showing that they have significant screening contributions unlike the higher-energy excitations. The understanding gained for the optical response of these simple model systems can help in interpreting the properties of real, complex cluster systems.

Quantum wells in InAs/GaSb heterostructures can be tuned to a topological regime associated with the quantum spin Hall effect, which arises due to an inverted band gap and hybridized electron and hole states. Here, we investigate electron-hole hybridization and the fate of the quantum spin Hall effect in a quasi-one-dimensional geometry, realized in a core-shell-shell nanowire with an insulator core and InAs and GaSb shells. We calculate the band structure for an infinitely long nanowire using $\mathbf{k}·\mathbf{p}$ theory within the Kane model and the envelope function approximation, then map the result onto a Bernevig-Hughes-Zhang model which is used to investigate finite-length wires. Clearly, quantum spin Hall edge states cannot appear in the core-shell-shell nanowires which lack one-dimensional edges, but in the inverted band gap regime we find that the finite-length wires instead host localized states at the wire ends. These end states are not topologically protected; they are fourfold degenerate and split into two Kramers pairs in the presence of potential disorder along the axial direction. However, there is some remnant of the topological protection of the quantum spin Hall edge states in the sense that the end states are fully robust to (time-reversal preserving) angular disorder, as long as the bulk band gap is not closed.

Possible engineering of spin polarization through a multistranded magnetic quantum network in the presence of light irradiation is reported. Each site of the network is associated with a net magnetic moment which provides a spin-dependent scattering via spin-spin exchange interaction, yielding a finite spin polarization across the network. The quantum system is described within a tight-binding framework where the irradiation effect is incorporated through Floquet-Bloch prescription. The spin-polarization coefficient, measured by determining spin-dependent transmission probabilities following the Green's function formalism, shows several atypical features with irradiation parameters. Apart from getting a high degree of spin polarization, a complete phase reversal can be achieved. These two issues are extremely important in designing efficient spin-based electronic devices. A detailed mathematical description is given to decouple the Hamiltonian of a generalized multistranded magnetic ladder into distinct one-dimensional lattices in separate spin subspaces for any arbitrary orientation of local spin, which clearly illustrates the allowed energy spin subbands. Effects of system size and uncorrelated disorder on spin polarization are critically examined. Finally, we discuss the possible experimental realization of our magnetic quantum systemto regulate the degree and phase of spin polarization by irradiating a magnetic sample, and thus we believe that the present analysis may provide a route of engineering spin-dependent electron transfer through a spin-polarized device.

Transition metal dichalcogenide (TMD) bilayers have recently emerged as a robust and tunable moiré system for studying and designing correlated electron physics. In this Rapid Communication, by combining a large-scale first-principles calculation and continuum model approach, we provide an electronic structure theory that maps long-period TMD heterobilayer superlattices onto diatomic crystals with cations and anions. We find that the interplay between the moiré potential and Coulomb interaction leads to filling-dependent charge transfer between different moiré superlattice regions. We show that the insulating state at half filling found in recent experiments on $\mathrm{W}{\mathrm{Se}}_2/\mathrm{W}{\mathrm{S}}_2$ is a charge-transfer insulator rather than a Mott-Hubbard insulator. Our work reveals the richness of simplicity in moiré quantum chemistry.

We suggest the possibility that the two-dimensional $\mathrm{SU}{\left(2\right)}_k$ Wess-Zumino-Witten (WZW) theory, which has global $\mathrm{SO}\left(4\right)$ symmetry, can be continued to $2+\epsilon$ dimensions by enlarging the symmetry to $\mathrm{SO}(4+\epsilon )$. This is motivated by the three-dimensional sigma model with $\mathrm{SO}\left(5\right)$ symmetry and a WZW term, which is relevant to deconfined criticality. If such a continuation exists, the structure of the renormalization group flows at small $\epsilon$ may be fixed by assuming analyticity in $\epsilon$. This leads to the conjecture that the WZW fixed point annihilates with a new, unstable fixed point at a critical dimensionality $d_c>2$. We suggest that $d_c<3$ for all $k$, and we compute $d_c$ in the limit of large $k$. The flows support the conjecture that the deconfined phase transition in $\mathrm{SU}\left(2\right)$ magnets is a “pseudocritical” point with approximate $\mathrm{SO}\left(5\right)$, controlled by a fixed point slightly outside the physical parameter space.

The validity of the particlelike phonon picture in describing the thermal transport physics in strongly anharmonic crystalline materials is a subject of recent debate. On the one hand, the Peierls-Boltzmann transport equation-based particlelike treatment of phonons was found to be insufficient in explaining the experimentally observed ultralow thermal conductivity of ${\text{Tl}}_3\text{V}{\text{Se}}_4$, and subsequently another thermal transport theory based on an additional thermal transport channel was proposed. On the other hand, the validity of this multichannel transport was questioned through a higher-level theory accounting for higher-order terms. Here, using the highest level of thermal transport theory with contributions from a temperature-dependent potential energy surface, lattice thermal expansion, force constant renormalization, long-range Coulombic interactions, and higher-order quartic phonon scattering processes, the validity of multichannel thermal transport is tested for ${\text{Tl}}_3\text{V}{\text{Se}}_4$. The particlelike transport channel is found to severely underpredict the experimentally measured thermal conductivity by a factor of 3 at a temperature of 300 K. The contribution of the wavelike coherent transport channel is found to be twice that of the particlelike channel, and the total thermal conductivity from multichannel transport theory is found to be in agreement with experimental measurements.

We have investigated electronic structure evolution by cation substitution in ${\mathrm{Ba}}_{3-x}{\mathrm{Sr}}_x{\mathrm{Nb}}_5{\mathrm{O}}_{15}$ by means of hard x-ray photoemission spectroscopy. Localization of Nb $4d$ electrons manifests as spectral weight transfer from a coherent component at the Fermi level to an incoherent one at 1–2 eV below it. This behavior is similar to that of electron-doped ${\mathrm{SrTiO}}_3$. On the other hand, Nb $3d$ and $4p$ core level spectra exhibit a screening effect with an energy scale of 1–2 eV by the coherent Nb $4d$ electrons similar to $4d$ electron systems near Mott transitions. The energy scale indicates that electron correlation is involved in the metal to insulator transition in the present system, although the Nb $4d$ band is about 1/10 filled. The present results suggest a mechanism of electron localization due to atomic disorder and electron correlation.

Dirac and Weyl point- and line-node semimetals are characterized by a zero band gap with simultaneously vanishing density of states. Given a sufficient interaction strength, such materials can undergo an interaction instability, e.g., into an excitonic insulator phase. Due to generically flatbands, organic crystals represent a promising materials class in this regard. We combine machine learning, density functional theory, and effective models to identify specific example materials. Without taking into account the effect of many-body interactions, we found the organic charge transfer salts [bis(3,4-diiodo-3′,4′-ethyleneditio- tetrathiafulvalene), 2,3-dichloro-5,6-dicyanobenzoquinone, acetenitrile] $\left[{\left(\mathrm{EDT}\text{−}\mathrm{TTF}\text{−}{\mathrm{I}}_2\right)}_2\right]\left(\mathrm{DDQ}\right)·\left({\mathrm{CH}}_3\mathrm{CN}\right)$ and $2,2^{\prime },5,5^{\prime }\text{−}\mathrm{tetraselenafulvalene}–7,7,8,8\text{−}\mathrm{tetracyano}\text{−}p\text{−}\mathrm{quinodimethane}$ (TSeF-TCNQ) and a bis-1,2,3-dithiazolyl radical conductor to exhibit a semimetallic phase in our ab initio calculations. Adding the effect of strong particle-hole interactions for ${\left(\mathrm{EDT}\text{−}\mathrm{TTF}\text{−}{\mathrm{I}}_2\right)}_2\left(\mathrm{DDQ}\right)·\left({\mathrm{CH}}_3\mathrm{CN}\right)$ and TSeF-TCNQ opens an excitonic gap on the order of 60 and 100 meV, which is in good agreement with previous experiments on these materials.

$4f$ intermediate valence (IV) compounds are canonical hosts of correlated electron physics and can contribute to our understanding of the larger class of correlated electron materials. Here we study the prototype IV compound $\mathrm{Yb}{\mathrm{Al}}_3$ which exhibits a nonintegral valence with a moderately heavy fermion ground state and a large Kondo temperature ($T_{\mathrm{K}}\sim 500–600\mathrm{K}$). To better characterize the correlated physics of $\mathrm{Yb}{\mathrm{Al}}_3$, we have measured the phonon and the magnetic excitation spectra on single crystals of this material by time-of-flight inelastic neutron scattering and inelastic x-ray scattering. We have also performed theoretical calculations of the phonon spectra. We present three findings of these measurements. First, we observe that the measured phonon spectra can be described adequately by a calculation based on standard DFT+$U$ density functional theory. The calculated energies, however, are 10% too low compared to the measured energies. This discrepancy may reflect a hardening of the phonons due to dynamic $4f$ correlations. Second, the low-temperature spin fluctuations on the Kondo energy scale $k_{\mathrm{B}}{T}_{\mathrm{K}}$ have a momentum ($\mathit{Q}$) dependence similar to that seen recently in the IV compound $\mathrm{CeP}{\mathrm{d}}_3$. For that system, the $\mathit{Q}$ dependence has been attributed to particle-hole excitations in a coherent itinerant $4f$ correlated ground state. We suggest a similar origin for the momentum dependence seen in $\mathrm{Yb}{\mathrm{Al}}_3$. This $\mathit{Q}$ dependence disappears as the temperature is raised towards room temperature and the $4f$ electron band states become increasingly incoherent. Such a coherent/incoherent crossover is expected to be generic for correlated electron systems. Third, a low-temperature magnetic peak observed in the neutron scattering near 30 meV shows dispersion identical to a particular optic-phonon branch. This $4f/\mathrm{phonon}$ resonance disappears for $T\ge 150\mathrm{K}$. The phonon spectrum appears to be unaffected by the resonance. We discuss several possibilities for the origin of this unusual excitation, which may be unique to $\mathrm{Yb}{\mathrm{Al}}_3$. We suggest that the excitation may arise from the large amplitude beating of the light Al atoms against the heavy Yb atoms, resulting in a dynamic $4f/3p$ hybridization.

Lee, Rice, and Anderson, in their monumental paper [Solid State Commun. 14, 703 (1974)], proved the existence of a collective mode describing the coupled motion of electron density and phonons in a one-dimensional incommensurate charge density wave in the Peierls state. This mode, which represents the coherent sliding motion of electrons and lattice distortions and affects low-energy transport properties, is described by the phase of the complex order parameter of the Peierls condensate, leading to Fröhlich superconductivity in pure systems. Once spatial disorder is present, however, the phason is pinned, and the system is transformed into an insulating ground state: a dramatic change. Since phasons can be considered an ultimate phonon drag effect, it is of interest to see its effects on thermoelectricity, which is studied in the present paper based on linear response theory of Kubo and Luttinger. The result indicates that a large absolute value of the Seebeck coefficient proportional to the square root of the resistivity is expected at low temperatures $k_{\mathrm{B}}T/\mathrm{\Delta }\ll 1$ ($\mathrm{\Delta }$ is the Peierls gap) with a sign opposite to the electronic contributions in the absence of the Peierls gap.

Motivated by the experimental realization of a multi-channel charge Kondo device [Iftikhar et al., Nature (London) 526, 233 (2015)], we study generic charge and heat transport properties of the charge two-channel Kondo model. We present a comprehensive discussion of the out-of-equilibrium and time-dependent charge transport, as well as thermal transport within linear response theory. The transport properties are calculated at, and also in the vicinity of, the exactly solvable Emery-Kivelson point, which has the form of a Majorana fermion resonant level model. We focus on regimes where our solution gives exact results for the physical quantum dot device, and highlight new predictions relevant to future experiments.

We demonstrate a first order metal-insulator phase transition in the repulsive, fully frustrated, single-band Hubbard model as a function of the coupling to a fermion bath. Time dependent manipulation of the bath coupling allows switching between metallic and insulating states both across the phase transition and within the coexistence region. We propose a simple nanoelectronic device for experimentally realizing dynamic control of the bath coupling. Analysis of the device characteristics shows that it can act as a two-terminal memristor.

We investigate carrier and collective mode dynamics in $2H\text{−}\mathrm{Nb}{\mathrm{Se}}_2$ using time-resolved optical pump-probe spectroscopy and compare the results with first-principles calculations. Broadband ultrafast reflectivity studies of $2H\text{−}\mathrm{Nb}{\mathrm{Se}}_2$ in a wide temperature interval covering the normal, charge density wave (CDW) and superconducting phase were performed. Spectral features observed in the transient reflectivity experiment were associated with specific optical transitions obtained from band structure calculations. Displacive excitation of coherent phonons showed CDW-associated coherent oscillations of the soft phonon mode across the whole spectral range. Temperature evolution of this coherent phonon mode in the low-excitation linear regime shows softening of the mode down to the CDW transition temperature $T_{\mathrm{CDW}}$ with subsequent hardening below $T_{\mathrm{CDW}}$. The global fit of the broadband probe data reveals four different relaxation times associated with characteristic electron-electron, electron-phonon, and phonon-phonon relaxation processes. From first-principles calculations of electron-phonon coupling we associate the few picosecond electron-phonon relaxation time ${\tau }_2$ with a specific group of phonons with frequencies around 20 meV. On the other hand, the anomalously long relaxation time of ${\tau }_3\sim 100\mathrm{ps}$ is associated with anharmonicity-driven phonon-phonon scattering. All relaxation processes result from anomalies near the second order CDW phase transition that are reflected in the temperature dependencies of the characteristic relaxation times and amplitudes of coherent oscillations. At the highest fluences we observe electronic melting of the CDW and disappearance of the mode hardening below $T_{\mathrm{CDW}}$.

Schottky barrier formation at metal/insulating oxide interfaces relies on complex mechanisms which are difficult to unravel. We propose a detailed numerical study of the atomic, magnetic, and electronic properties of the $\mathrm{Fe}/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3\left(001\right)$ interface, in which we focused our discussion on different parameters which can affect the Schottky barrier height (SBH). The interface termination appears to be the most critical aspect to be controlled for this interface: While an ideal $\mathrm{Ti}{\mathrm{O}}_2$-terminated interface would guarantee a $n$-type barrier of about 1.2–1.6 eV, the presence of a SrO termination can drastically decrease its value down to few meV. The oxidation state of the interface is also an important criterium to maintain a high barrier value. Oxygen vacancies are always cited as the source of a deterioration of the SBH. For a $\mathrm{Ti}{\mathrm{O}}_2$-terminated interface, we found that in their most stable position, i.e., in the interface layer, the oxygen vacancies do not affect the value of the SBH; when moving some atomic layers away from the interface, the SBH on the contrary decreases regularly. We propose that oxidizing the interface would allow us to improve the $n$-type SBH by healing the oxygen vacancies and forming an interfacial FeO layer, which seems favorable to the formation of a higher SBH.