Saddle-point Van Hove singularities in the topological surface states are interesting because they can provide a new pathway for accessing exotic correlated phenomena in topological materials. Here, based on first-principles calculations combined with a $\mathbf{k}·\mathbf{p}$ model Hamiltonian analysis, we show that the layered platinum mineral jacutingaite (${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$) harbors saddlelike topological surface states with associated Van Hove singularities. ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ is shown to host two distinct types of nodal lines without spin-orbit coupling (SOC), which are protected by combined inversion ($\mathcal{I}$) and time-reversal ($\mathcal{T}$) symmetries. Switching on the SOC gaps out the nodal lines and drives the system into a topological state with nonzero weak topological invariant $Z_2=(0;001)$ and mirror Chern number $n_M=-2$. Surface states on the naturally cleaved (001) surface are found to be nontrivial with a unique saddlelike energy dispersion with type II Van Hove singularities. We also discuss how modulating the crystal structure can drive ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ into a Dirac semimetal state with a pair of Dirac points. Our results indicate that ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ is an ideal candidate material for exploring the properties of topological insulators with saddlelike surface states.

Magnet-superconductor hybrid heterostructures constitute a promising candidate system for the quantum engineering of chiral topological superconductivity. Here, we investigate the stability of their topological phases in the presence of various types of potential and magnetic disorder. In particular we consider magnetic disorder in the coupling strength and spin orientation, as well as percolation-type disorder representing missing magnetic moments. We show that potential disorder leads to the weakest suppression of topological phases, while percolation disorder leads to their strongest suppression. In addition, disorder can also lead to the emergence of topological phases in part of the phase diagram that are topologically trivial in a clean system. Moreover, we demonstrate that in the case of correlated potential disorder, the spatial structure of the disorder potential is correlated not only with the particle number density, but also the Chern number density. Finally, we demonstrate how the disorder-induced destruction of topological superconductivity is reflected in the spatial structure and distribution of the Chern number density.

When two quantum systems are coupled via a mediator, their dynamics has traces of nonclassical properties of the mediator. We show how this observation can be effectively utilized to study the quantum nature of materials without well-established structure. A concrete example considered is ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$. Measurements of low-temperature magnetic and thermal properties of this compound were explained with long-range coupling of unpaired spins through dimerized spin chains. We first show that the required coupling is not provided by the spin chain alone and give alternative compact two-dimensional spin arrangements compatible with the experimental results. Then we argue that any mediator between the unpaired spins must share with them quantum correlations in the form of quantum discord and, in many cases, quantum entanglement. In conclusion, present data witness quantum mediators between unpaired spins in ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$.

We use an ab initio approach to design and study a two-dimensional material—a planar array of carbon nanotubes separated by an optimal distance defined by the van der Waals interaction. We show that the energy spectrum for an array of quasimetallic nanotubes is described by a strongly anisotropic hyperbolic dispersion and formulate a model low-energy Hamiltonian for its semianalytical treatment. Periodic-potential-induced lifting of the valley degeneracy for an array of zigzag narrow-gap nanotubes leads to the band-gap collapse. In contrast, the band gap is opened in an array of gapless armchair tubes. These unusual spectra, marked by pronounced van Hove singularities in the low-energy density of states, open the opportunity for interesting physical effects and prospective optoelectronic applications.

The electrical resistivity and thermal conductivity of a polycrystalline gold nanoribbon have been measured in an applied magnetic field up to 14 T in the temperature range from 2 to 300 K. The temperature squared dependence of the nanoribbon resistivity below 40 K indicates the electron scattering dominated by the effects of grain boundaries and surfaces. It is also found that the magnetoresistance of the gold nanoribbon increases with the magnetic field strength, and takes a maximum value at 10 K for different magnetic field strengths, which is related to the interaction of the electron scattering and the electron-phonon collision in the ribbon. The influence of the strong magnetic field on the electrical and thermal transport properties of the suspended gold nanoribbon is ultraweak. The present nanoribbon still does not follow the Wiedemann-Franz law within the experimental conditions. Furthermore, the scaling represented by Kohler's rule is still valid even for electron scattering mechanisms.

Grain boundaries in two-dimensional transition-metal dichalcogenides can strongly affect the transport properties by reducing the electron mobility or allowing gap conduction through extended grain boundary states. Here, by combining advanced modeling tools–density-functional-theory-calibrated tight-binding Hamiltonians and Green's function techniques–we investigate transport along and across mirror twin grain boundaries in ${\mathrm{MoS}}_2$. Our results show that the grain boundary conductive channels are strongly affected by sulfur vacancies, while short-range Anderson disorder has a moderate impact, which we quantitatively analyze, and long-range disorder has a very weak effect. As for transport across the grain boundaries, the system conductance turns out to be less than half that for the pristine system, and the spin-orbit coupling and intervalley scattering are found to play an important role. Our findings are beneficial to the understanding and the prediction of the impact of mirror twin grain boundaries in the transport phenomena and could be of help in designing electronic devices based on transition metal dichalcogenides.

Gate patterning on semiconductors is routinely used to electrostatically restrict electron movement into reduced dimensions. At cryogenic temperatures, where most studies are carried out, differential thermal contraction between the patterned gate and the semiconductor often lead to an appreciable strain modulation. The impact of such modulated strain to the conductive channel buried in a semiconductor has long been recognized, but measuring its magnitude and variation are rather challenging. Here we present a way to measure that modulation in a gate-defined GaAs-based one-dimensional channel by applying resistively detected NMR with in situ electrons coupled to quadrupole nuclei. The detected strain magnitude, deduced from the quadrupole-split resonance, varies spatially on the order of ${10}^{-4}$, which is consistent with the predicted variation based on an elastic strain model. We estimate the initial lateral strain ${\epsilon }_{xx}$ and ${\epsilon }_{yy}$ developed at the interface to be about $5.0\times {10}^{-3}$.

We experimentally demonstrate hot exciton transport in hexagonal boron nitride encapsulated $\mathrm{WS}{\mathrm{e}}_2$ monolayers via spatially and temporally resolved photoluminescence measurements at room temperature. We show that the nonlinear evolution of the mean-squared displacement of the nonresonantly excited hot exciton gas is primarily due to the relaxation of its excess kinetic energy and is characterized by a density-dependent fast expansion that converges to a slower, constant rate expansion. We also observe saturation of the hot exciton gas's expansion rate at high excitation densities due to the balance between Auger-assisted hot exciton generation and the phonon-assisted hot exciton relaxation processes. These measurements provide insight into a process that is ubiquitous in exciton transport measurements where nonresonant optical excitation is typically employed.

In addition to its promising potential for applications, $\mathrm{Ga}{\mathrm{V}}_4{\mathrm{S}}_8$ shows very interesting physical properties with temperature and magnetic field. These properties can be tuned by applying hydrostatic pressure in order to reveal and understand the physics of these materials. Not only pressure induces an insulator-to-metal transition in $\mathrm{Ga}{\mathrm{V}}_4{\mathrm{S}}_8$ but it also has an interesting effect on the structural and magnetic transitions. Using a combination of AC calorimetry, capacitance, and resistivity measurements under pressure, we determine the evolution of the structural and magnetic transitions with pressure and thus establish the $T\text{−}P$ phase diagram of $\mathrm{Ga}{\mathrm{V}}_4{\mathrm{S}}_8$. To detect the insulator-to-metal transition (IMT), we use optical conductivity and DC resistivity measurements and we follow the evolution of the Mott gap under pressure. The structural transition temperature increases with pressure and a second transition appears above 6 GPa indicating a possible new phase with a very small gap. Pressure has a surprisingly very weak effect on the ferromagnetic transition that persists even very close to the IMT that occurs at around 14 GPa, implying that the metallic state may also be magnetic.

Unlike the linear anomalous Nernst effect, the nonlinear anomalous Nernst effect (NLANE) can survive in an inversion symmetry broken system even in the presence of time-reversal symmetry. Using semiclassical Boltzmann transport theory, we study the nonlinear anomalous Nernst effect that arises as the second-order response function to the applied temperature gradient. We find that the nonlinear Nernst current, which flows perpendicular to the temperature gradient even in the absence of a magnetic field, arises due to the Berry curvature of the states near the Fermi surface, and thus is associated with purely a Fermi surface contribution. We apply these results to bilayer ${\mathrm{WTe}}_2$, which is an inversion broken but time-reversal symmetric type-II Weyl semimetal supporting chiral Weyl fermions. By tuning the spin-orbit coupling, we show that the sign of the NLANE can change in this system. Together with the angular dependence, we calculate the temperature and chemical potential dependencies of NLANE in bilayer ${\mathrm{WTe}}_2$, and predict specific experimental signatures that can be checked in experiments.

We design a set of classical macroscopic electric circuits in which charge exhibits the mobility restrictions of fracton quasiparticles. The crucial ingredient in these circuits is a transformer, which induces currents between pairs of adjacent wires. For an appropriately designed geometry, this induction serves to enforce conservation of the dipole moment. We show that a network of capacitors connected via ideal transformers will forever remember the dipole moment of its initial charge configuration. Relaxation of the dipole moment in realistic systems can only occur via flux leakage in the transformers, which will lead to violations of fracton physics at the longest times. We propose a concrete diagnostic for these “fractolectric” circuits in the form of their characteristic equilibrium charge configurations, which we verify using simple circuit simulation software. These circuits not only provide an experimental testing ground for fracton physics, but also serve as DC filters. We outline extensions of these ideas to circuits featuring other types of higher moment conservation laws, as well as to higher-dimensional circuits which act as fracton “current ice.” While our focus is on classical circuits, we discuss how these ideas can be straightforwardly extended to realize quantized fractons in superconducting circuits.

Plasmonic crystals are well known to have band structure including a band gap, enabling the control of surface plasmon propagation and confinement. The band dispersion relation of bulk crystals has been generally measured by momentum-resolved spectroscopy using far field optical techniques while the defects introduced in the crystals have separately been investigated by near field imaging techniques so far. Particularly, defect related energy levels introduced in the plasmonic band gap have not been observed experimentally. In order to investigate such a localized mode, we performed electron energy-loss spectroscopy (EELS) on a point defect introduced in a plasmonic crystal made up of flat cylinders protruding out of a metal film and arranged on a triangular lattice. The energy level of the defect mode was observed to lie within the full band-gap energy range. This was confirmed by a momentum-resolved EELS measurement of the band gap performed on the same plasmonic crystal. Furthermore, we experimentally and theoretically investigated the emergence of the defect states by starting with a corral of flat cylinders protrusions and adding sequentially additional shells of those in order to eventually form a plasmonic band-gap crystal encompassing a single point defect. It is demonstrated that a defectlike state already forms with a crystal made up of only two shells.

The Purcell factor and the spontaneous emission spectrum of a quantum emitter (QE) placed close to the edge of a graphene half-space nanostructure is investigated, using semianalytical methods at the electrostatic regime. The half-space geometry supports an edge and a bulk surface plasmon (SP) mode. The Purcell factor of the QE is enhanced over eight orders of magnitude when its emission energy matches the resonance energy modes, for a specific value of the in-plane wave vector, at a separation distance of 10 nm. The different transition dipole moment orientations influence differently the enhancement factor of a QE, leading to large anisotropic behavior when positioned at different places above the half-space geometry. The field distribution is presented, showing clearly the excitation of the SP modes at the edge of the nanostructures. Also, we present the spontaneous emission spectrum of the QE near the half-space graphene nanostructure, and we show that strong light-matter coupling may emerge. When a QE with a free-space lifetime of 10 ns is placed at a distance of 10 nm from the edge of the graphene half-space, a Rabi splitting of 109 meV is found for emission energy of 0.15 eV. Our contribution can be used for designing future quantum applications using a combination of QEs and graphene nanostructures.

In order to design next-generation ferroelectrics, a microscopic understanding of their macroscopic properties is critical. One means to achieve an atomistic description of ferroelectric and dielectric phenomena is classical molecular dynamics simulations. Previously, we have shown that interatomic potentials based on the bond valence molecular dynamics method can be used to study structural phase transitions, ferroelectric domain nucleation, and domain wall migration in several perovskite oxides and fixed-composition binary and ternary alloys. Most modern devices, however, use variable-composition perovskite oxide alloys such as ${\mathrm{Ba}}_x{\mathrm{Sr}}_{1-x}{\mathrm{TiO}}_3$ (BST). In this paper, we extend our bond valence approach to BST solid solutions and, in so doing, show that the potential parameters for each element are transferable between materials with different $x$. Using this potential, we perform bond valence molecular dynamics simulations investigating the temperature and composition dependence of the lattice constants, Ti displacements, and ferroelectric polarization of BST and find that our predictions match experiments and first-principles theory. Additionally, based on a detailed analysis of local dipole distributions in rhombohedral BST, we demonstrate that substitution of Sr for Ba scrambles dipoles, reduces global polarization, and enhances the order-disorder character of the ferroelectric-paraelectric phase transition.

The distribution of dopants has significant influence on the electronic property of semiconductors. However, in a homogeneous crystalline material, the occupation of dopants is usually randomly distributed. Using the generalized Bloch theorem, we show that a twisting deformation has a pronounced impact on the distribution of dopants in single-crystalline Si nanowires (SiNWs). With systematic calculations, we find that the dopant atoms with smaller sizes than the host Si atom prefer atomic sites near the NW core, while dopants of larger sizes are prone to staying around the NW surface. The underlying mechanism of this intriguing phenomenon is related to the inhomogeneous shear strain along the NW radial direction. Such a trend is nearly independent of dopants with different numbers of valence electrons and also independent of the detailed crystal structures of NWs. Our findings provide an effective approach to control the dopant distribution in semiconductors, which is critical for the design of devices with reproducible electronic properties.

We develop an approach of calculating the many-body path integral based on the linked cluster expansion method. First, we derive a linked cluster expansion and we give the diagrammatic rules for calculating the free energy and the pair distribution function $g\left(r\right)$ as a systematic power-series expansion in the particle density. We also present a structured Padé approximation scheme in the momentum space to determine $g\left(r\right)$. The calculated $g\left(r\right)$ for distinguishable particles interacting with Lennard-Jones and hard-sphere potential in various attempted schemes of approximation of the diagrammatic series compares very well with the results of path-integral Monte Carlo simulation. Our method is applicable to a wide range of problems of current general interest and may be extended to the case of identical particles and, in particular, to the case of the many-fermion problem.

A computational method has been developed to compute the electron-phonon interactions using the projector augmented wave method and finite displacements. Aluminium and diamond crystals are considered to test our strategy. It is found that the potential derivatives are quickly convergent with respect to the supercell size. Moreover the method is found to be efficient enough to densely sample the Brillouin zone and compute the Eliashberg function and phonon lifetime without requiring the use to interpolations.

Bulk properties of quantum phases should be independent of a specific choice of boundary conditions as long as the boundary respects the symmetries. Based on this physically reasonable requirement, we discuss the Lieb-Schultz-Mattis-type ingappability in two-dimensional quantum magnets under a boundary condition that makes evident a quantum anomaly underlying the lattice system. In particular, we direct our attention to those on the checkerboard lattice, which are closely related to frustrated quantum magnets on the square lattice and on the Shastry-Sutherland lattice. Our discussion is focused on the adiabatic U(1) flux insertion through a closed path in a boundary condition twisted by a spatial rotation and a reflection. Two-dimensional systems in this boundary condition are effectively put on a nonorientable space, namely the Klein bottle. We show that the translation symmetry on the Klein-bottle space excludes the possibility of the unique and gapped ground state. Taking advantage of the flux insertion argument, we also discuss the ground-state degeneracy on magnetization plateaus of the Heisenberg antiferromagnet on the checkerboard lattice.