Materials for capacitive energy storage with high energy density and low loss are desired in many fields. We investigate several polymers with urea and amide functional groups using density functional theory and classical molecular dynamics simulations. For aromatic polyurea (APU) and para-aramid (PA), we find several nearly energetically degenerate ordered structures, while meta-aromatic polyurea (mAPU) tends to be rotationally disordered along the polymer chains. Simulated annealing of APU and PA structures results in the formation of hydrogen-bonded sheets, highlighting the importance of dipole-dipole interactions. In contrast, hydrogen bonding does not play a significant role in mAPU, hence the propensity to disorder. We find that the disordered structures with misaligned chains have significantly larger dielectric constants, due to significant increase in the free volume, which leads to easier reorientation of dipolar groups in the presence of an electric field. Large segment motion is still not allowed below the glass transition temperature, which explains the experimentally observed very low loss at high field and elevated temperature. However, the degree of disorder needs to be controlled, because highly entangled structures diminish the free dipoles and decrease permittivity. Among the considered materials, mAPU is the most promising dielectric for capacitive energy storage, but the concept of increasing permittivity while maintaining low loss through disorder-induced free volume increase is generally applicable and provides an alternative pathway for the design of high-performance dielectrics for capacitive energy storage.

The critical current needed to destabilize the magnetization of a perpendicular ferromagnet via the spin Hall effect is studied. Both the dampinglike and fieldlike torques associated with the spin current generated by the spin Hall effect are included in the Landau-Lifshitz-Gilbert equation to model the system. In the absence of the fieldlike torque, the critical current is independent of the damping constant and is much larger than that of conventional spin torque switching of collinear magnetic systems, as in magnetic tunnel junctions. With the fieldlike torque included, we find that the critical current scales with the damping constant as ${\alpha }^0$ (i.e., damping independent), $\alpha$, and ${\alpha }^{1/2}$ depending on the sign of the fieldlike torque and other parameters such as the external field. Numerical and analytical results show that the critical current can be significantly reduced when the fieldlike torque possesses the appropriate sign, i.e., when the effective field associated with the fieldlike torque is pointing opposite to the spin direction of the incoming electrons. These results provide a pathway to reducing the current needed to switch magnetization using the spin Hall effect.

Neutron scattering is used to study ${\mathrm{NiCl}}_{2-2x}{\mathrm{Br}}_{2x}·4\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2,x=0.06$, a bond-disordered modification of the well-known gapped $S=1$ antiferromagnetic quantum spin system ${\mathrm{NiCl}}_2·4\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2$. The magnetic excitation spectrum throughout the Brillouin zone is mapped out at $T=60$ mK using high-resolution time-of-flight spectroscopy. It is found that the dispersion of spin excitation is renormalized, as compared to that in the parent compound. The lifetime of excitations near the bottom of the band is substantially decreased. No localized states are found below the gap energy $\mathrm{\Delta }\simeq 0.2$ meV. At the same time, localized zero wave vector states are detected above the top of the band. The results are consistent with a more or less continuous random distribution of bond strengths, and a discrete, possibly bimodal, distribution of single-ion anisotropies in the disordered material.

We report on a study of intrinsic superconductivity in a Weyl metal, i.e., a doped Weyl semimetal. Two distinct superconducting states are possible in this system in principle: a zero-momentum pairing BCS state, with point nodes in the gap function, and a finite-momentum FFLO-like state, with a full nodeless gap. We find that, in an inversion-symmetric Weyl metal, the odd-parity BCS state has a lower energy than the FFLO state, despite the nodes in the gap. The FFLO state, on the other hand, may have a lower energy in a noncentrosymmetric Weyl metal, in which Weyl nodes of opposite chirality have different energy. However, realizing the FFLO state is, in general, very difficult since the paired states are not related by any exact symmetry, which precludes a weak-coupling superconducting instability. We also discuss some of the physical properties of the nodal BCS state, in particular, Majorana and Fermi arc surface states.

Parafermions are exotic quasiparticles with non-Abelian fractional statistics that can be realized and stabilized in one-dimensional models that are generalizations of the Kitaev $p$-wave wire. We study the simplest generalization, i.e., the $Z_3$ parafermionic chain. Using a Jordan-Wigner transform we focus on the equivalent three-state chiral clock model, and study its rich phase diagram using the density matrix renormalization group technique. We perform our analyses using quantum entanglement diagnostics which allow us to determine phase boundaries, and the nature of the phase transitions. In particular, we study the transition between the topological and trivial phases, as well as to an intervening incommensurate phase which appears in a wide region of the phase diagram. The phase diagram is predicted to contain a Lifshitz type transition which we confirm using entanglement measures. We also attempt to locate and characterize a putative tricritical point in the phase diagram where the three above mentioned phases meet at a single point.

We study the relation between the models commonly used to describe the dynamics of nonresonantly pumped exciton-polariton condensates, namely the ones described by the complex Ginzburg-Landau equation, and by the open-dissipative Gross-Pitaevskii equation including a separate equation for the reservoir density. In particular, we focus on the validity of the adiabatic approximation and small density fluctuations approximation that allow one to reduce the coupled condensate-reservoir dynamics to a single partial differential equation. We find that the adiabatic approximation consists of three independent analytical conditions that have to be fulfilled simultaneously. By investigating stochastic versions of the two corresponding models, we verify that the breakdown of these approximations can lead to discrepancies in correlation lengths and distributions of fluctuations. Additionally, we consider the phase diffusion and number fluctuations of a condensate in a box, and show that self-consistent description requires treatment beyond the typical Bogoliubov approximation.

Based on first-principles density-functional theory calculations, we present a comparative study of the electronic structures of ultranarrow zigzag graphene nanoribbons (ZGNRs) embedded in a hexagonal boron nitride (BN) sheet and fully hydrogenated graphene (graphane) as a function of their width $N$ (the number of zigzag C chains composing the ZGNRs). We find that ZGNRs/BN have the nonmagnetic ground state except at $N=5$ and 6 that weakly stabilize as a half-semimetallic state, whereas ZGNRs/graphane with $N$ $\ge 2$ exhibit a strong antiferromagnetic coupling between ferromagnetically ordered edge states on each edge. It is revealed that the disparate magnetic properties of the two classes of ZGNRs are attributed to the contrasting interedge superexchange interactions arising from different interface structures: that is, the asymmetric interface structure of ZGNRs/BN gives a relatively short-range and weak superexchange interaction between the two inequivalent edge states, while the symmetric interface structure of ZGNRs/graphane gives a long-range, strong interedge superexchange interaction.

We have employed ab initio density functional theory (DFT) to study the adsorption, dissociation, diffusion, and vibration of oxygen on $\mathrm{Ag}\left(110\right)$. We find that the fourfold-hollow site is the preferred site for ${\mathrm{O}}_2$ adsorption and that the ${\mathrm{O}}_2$ molecular axis marginally prefers to align along the $\left[1\overline{1}0\right]$ direction (for which the adsorption energy is $-0.41\mathrm{eV})$ rather than along the $\left[001\right]$ (for which it is $-0.37\mathrm{eV})$. By weakening the O-O bond, the net charge transfer of $\sim 0.9e$ from Ag to the antibonding orbital $\left(2{\pi }^{*}\right)$ of ${\mathrm{O}}_2$ facilitates dissociation of ${\mathrm{O}}_2$ on $\mathrm{Ag}\left(110\right)$. Contrary to a previous theoretical suggestion (subsequently incorporated into interpretation of an experiment), our DFT calculations for adsorption energies and dissociation energy barriers, taken together with findings concerning vibrational frequencies and charge transfer, indicate that, when adsorbing, ${\mathrm{O}}_2$ prefers to align along the $\left[1\overline{1}0\right]$ direction, and when dissociating, does align along the $\left[001\right]$ direction with the dissociated O atoms adsorbing onto threefold-hollow sites rather than short-bridge sites. Importantly, our calculations clearly show that dissociation of ${\mathrm{O}}_2$ on $\mathrm{Ag}\left(110\right)$ is coupled with the surface Rayleigh mode $(\sim 4\mathrm{meV})$ and ${\mathrm{O}}_2$ dissociation along the $\left[001\right]$ direction is more strongly coupled with the substrate motion than ${\mathrm{O}}_2$ dissociation along the $\left[1\overline{1}0\right]$ direction indicating that ${\mathrm{O}}_2$ dissociation (particularly, ${\mathrm{O}}_2\left[001\right]$) can be activated by surface phonon even at quite low temperature. Once thus dissociated, the O atoms can more easily diffuse in the $\left[1\overline{1}0\right]$ direction, from one fourfold-hollow site to the next owing to a smaller diffusion barrier in the latter.

Exploiting the entanglement concept within a matrix-product-state based infinite density-matrix renormalization group approach, we show that the spin-density-wave and bond-order-wave ground states of the one-dimensional half-filled extended Hubbard model give way to a symmetry-protected topological Haldane state in case an additional alternating ferromagnetic spin interaction is added. In the Haldane insulator, the lowest entanglement level features a characteristic twofold degeneracy. Increasing the ratio between the nearest-neighbor and local Coulomb interaction $V/U$, the enhancement of the entanglement entropy, the variation of the charge, spin, and neutral gaps, and the dynamical spin and density response signal a quantum phase transition to a charge-ordered state. Below a critical point, which belongs to the universality class of the tricritical Ising model with central charge 7/10, the model is critical with $c=1/2$ along the transition line. Above this point, the transition between the Haldane insulator and charge-density-wave phases becomes first order.

In light of the new experimental and theoretical important developments in high-$T_c$ superconductivity, we revisit the fermionic hot-spot model relevant to the phenomenology of the cuprates. We extend previous results by means of a complete two-loop order renormalization group (RG) framework. Here, we explicitly study the effect of the charge-density-wave (CDW) order parameter with a $d$-wave form factor with the experimentally observed modulation $(\pm Q_0,0)$ and $(0,\pm Q_0)$ at the infrared-stable nontrivial fixed point obtained previously for this model. Additionally, we proceed to investigate also the so-called pair-density-wave (PDW) order that was recently proposed in the literature as a possible candidate for the “hidden” order to describe the pseudogap phase observed in underdoped cuprates. We confirm that although the above two ordering tendencies are also found to be nearly degenerate both at one-loop and two-loop RG orders and linked by an emergent SU(2) pseudospin symmetry, they turn out to be subleading for weaker couplings in the present model to antiferromagnetism, $d$-wave bond-density wave (BDW) order with modulation along Brillouin zone diagonals $(\pm Q_0,\pm Q_0)$, and $d$-wave singlet superconductivity (SSC). However, as we increase the strength of the initial coupling towards moderate values, we do capture a tendency for the entangled PDW/CDW order to become leading compared to BDW/SSC in the model, which suggests that the former composite order might be indeed a viable concept to describe some cuprate superconductors at high temperatures in the underdoped regime, as has been recently alluded to by many authors in the literature.

We propose a general, yet simple model for describing the weak field magnetotransport properties of nearly-free electrons in two-dimensional hexagonal metals. We modify this model so as to apply it to the magnetotransport properties of the $M_{n+1}A{X}_n$ phases, a particular class of nanolamellar carbides and nitrides. We argue that the values of the in-plane Hall coefficient and the in-plane parabolic magnetoresistance are due to the specific shape of the Fermi surface of almost two-dimensional hole and electron bands. If the contribution of the electron pockets to in-plane resistivity is often (but not always) predicted to be a minor one, in contrast, both holes and electrons should substantially contribute to the overall value of the in-plane Hall coefficient. The relevance of our model is supported by elementary considerations and a set of experimental data obtained from single crystals of ${\mathrm{V}}_2\mathrm{AlC}$ and $\mathrm{C}{\mathrm{r}}_2\mathrm{AlC}$. In particular, we obtain a high ratio between the in-plane $\left({\rho }_{ab}\right)$ and parallel to the $c$ axis $\left({\rho }_c\right)$ resistivities.

We report the bulk and surface electronic properties and spin polarization of a rich family of Sn-based ternary topological insulators studied by means of first-principles calculations within the framework of density functional theory. These compounds exist with the following stoichiometries: $\mathrm{Sn}{X}_2{\mathrm{Te}}_4,\mathrm{Sn}{X}_4{\mathrm{Te}}_7$, and ${\mathrm{SnBi}}_6{\mathrm{Te}}_{10}$ ($X$ = Sb and Bi). Where a septuple layer or a quintuple layer and septuple layer blocks alternate along the hexagonal axis. We reveal that the bulk band gap in these compounds is about 100 meV and recognize a strong dependence of the spin polarization on the cleavage surface. The calculated spin polarization reaches 85% in some cases, that is one of the highest predicted values hitherto. Since the electron spin polarization is a relevant parameter for spintronics technology, this new family is suitable for applications within this field.

The electronic band structure of heavily boron doped diamond was investigated by angle-resolved photoemission spectroscopy on (100)-oriented epilayers. A unique set of Luttinger parameters was deduced from a comparison of the experimental band structure of metallic diamond along the $\mathrm{\Delta }\left(\mathrm{\Gamma }X\right)$ and $\mathrm{\Sigma }\left(\mathrm{\Gamma }K\right)$ high-symmetry directions of the reciprocal space, with theoretical band structure calculations performed both within the local density approximation and by an analytical $\mathbf{k}·\mathbf{p}$ approach. In this way, we were able to describe the experimental band structure over a large three-dimensional region of the reciprocal space and to estimate hole effective masses in agreement with previous theoretical and experimental papers.

We propose a systematic approach that can empirically correct three major errors typically found in a density functional theory (DFT) calculation within the local density approximation (LDA) simultaneously for a set of common cation binary semiconductors, such as III-V compounds, (Ga or In)$X$ with $X=\mathrm{N},\mathrm{P},\mathrm{As},\mathrm{Sb},$ and II-VI compounds, (Zn or Cd)$X$, with $X=\mathrm{O},\mathrm{S},\mathrm{Se},\mathrm{Te}$. By correcting (1) the binary band gaps at high-symmetry points $\mathrm{\Gamma }, L, X$, (2) the separation of $p$-and $d$-orbital-derived valence bands, and (3) conduction band effective masses to experimental values and doing so simultaneously for common cation binaries, the resulting DFT-LDA-based quasi-first-principles method can be used to predict the electronic structure of complex materials involving multiple binaries with comparable accuracy but much less computational cost than a GW level theory. This approach provides an efficient way to evaluate the electronic structures and other material properties of complex systems, much needed for material discovery and design.

We present a systematic study of the electronic, transport, and optical properties of disordered graphene, including the next-nearest-neighbor hopping. We show that this hopping has a nonnegligible effect on resonant scattering but is of minor importance for long-range disorder such as charged impurities, random potentials, or hoppings induced by strain fluctuations. Different types of disorders can be recognized by their fingerprints appearing in the profiles of dc conductivity, carrier mobility, optical spectroscopy, and Landau level spectrum. The minimum conductivity $4e^2/h$ found in the experiments is dominated by long-range disorder and the value of $4e^2/\pi h$ is due to resonant scatterers only.

We study the scattering of electrically excited surface plasmon polaritons (SPP) from individual nanostructures. The tunneling electrons from a scanning tunneling microscope (STM) are used to excite an out-going, circular SPP wave on a thin (50-nm) gold film on which isolated gold nanoparticles (NPs) have been deposited. Interaction of the excited SPPs with the NPs leads to both in-plane (SPP-to-SPP) and out-of-plane (SPP-to-photon) scattering. We use SPP leakage radiation microscopy to monitor the interference between the incident and in-plane scattered SPP waves in the image plane. By changing the location of the STM tip, the distance of the pointlike SPP source to the scatterers can be varied at will, which constitutes a key advantage over other existing techniques. As well, the out-of-plane scattered radiation interferes with the direct light emission from the STM tip in the back focal plane (Fourier plane). This confirms the mutual coherence of the light and SPP emission resulting from the inelastic tunneling of an electron in the STM junction. We use this effect to demonstrate that SPP-to-photon scattering at NPs is highly directional.

We propose a scheme that the material dispersion can be immune in a kind of photonic topological metacrystal with uniform permittivity and permeability but staggered bianisotropy. The topological behaviors can be maintained even though the metamaterials are intrinsically dispersive. A nontrivial topological band gap with a large gap spin Chern number is confirmed in such dispersion-immune photonic topological metacrystals. Two proposals of a robust pseudospin-polarized power splitter and a slow-light waveguide show evidence on the manipulation of the pseudospin-momentum locked states in a backscattering-free manner. In addition, a realistic design of a nontrivial dispersion-immune photonic metacrystal is also issued and discussed.

In inhomogeneous bulk ferromagnets, the dominating sources of spin disorder are related to spatial variations of (i) the magnitude of the local saturation magnetization and of (ii) the magnitude and/or direction of the magnetic anisotropy field. For the particular example of a porous ferromagnet, where the magnetization inhomogeneity is at maximum, we demonstrate, by means of experimental neutron scattering data and micromagnetic simulations, the anisotropic character of magnetization fluctuations induced by the dipolar interaction.

Resonant inelastic x-ray scattering (RIXS) tuned for the Cu $L$ edge is a possible tool to detect charge excitations in cuprate superconductors. We theoretically investigate the possibility for observing a collective charge excitation by the RIXS. The RIXS process via the intermediate state inevitably makes the spectral weight of charge excitation stronger in electron doping than in hole doping. Electron-hole asymmetry also appears in the dynamical charge structure factor, showing a new enhanced small-momentum low-energy mode in electron doping. These facts indicate a possibility of detecting the new charge mode by RIXS in electron-doped systems.