In strong magnetic fields, massless electrons in graphene populate relativistic Landau levels with the square-root dependence of each level energy on its number and magnetic field. Interaction-induced deviations from this single-particle picture were observed in recent experiments on cyclotron resonance and magneto-Raman scattering. Previous attempts to calculate such deviations theoretically using the unscreened Coulomb interaction resulted in overestimated many-body effects. This work presents many-body calculations of cyclotron and magneto-Raman transitions in single-layer graphene in the presence of Coulomb interaction, which is statically screened in the random-phase approximation. We take into account self-energy and excitonic effects as well as Landau level mixing, and achieve good agreement of our results with the experimental data for graphene on different substrates. The important role of a self-consistent treatment of the screening is found.

We propose that topological spin currents flowing in topologically nontrivial magnetic textures, such as magnetic skyrmions and vortices, produce an intrinsic nonadiabatic torque of the form ${\mathbf{T}}_t\sim [({\partial }_x\mathbf{m}\times {\partial }_y\mathbf{m})·\mathbf{m}]{\partial }_y\mathbf{m}$. We show that this torque, which is absent in one-dimensional domain walls and/or nontopological textures, is responsible for the enhanced nonadiabaticity parameter observed in magnetic vortices compared to one-dimensional textures. The impact of this torque on the motion of magnetic skyrmions is expected to be crucial, especially to determine their robustness against defects and pinning centers.

The Néel temperature, staggered magnetization density, as well as the spin-wave velocity of a three-dimensional (3D) quantum Heisenberg model with antiferromagnetic disorder (randomness) are calculated using first-principles nonperturbative quantum Monte Carlo simulations. In particular, we examine the validity of universal scaling relations that are related to these three studied physical quantities. These relations are relevant to experimental data and are firmly established for clean (regular) 3D dimerized spin-1/2 Heisenberg models. Remarkably, our numerical results show that the considered scaling relations remain true for the investigated model with the introduced disorder. In addition, while the presence of disorder may change the physical properties of regular dimerized models, hence leading to different critical theories, both the obtained data of Néel temperature and staggered magnetization density in our study are fully compatible with the expected critical behavior for clean dimerized systems. As a result, it is persuasive to conclude that the related quantum phase transitions of the considered disordered model and its clean analogues are governed by the same critical theory, which is not always the case in general. Finally, we also find smooth scaling curves even emerging when both the data of the investigated disordered model as well as its associated clean system are taken into account concurrently. This in turn implies that, while in a restricted sense, the considered scaling relations for 3D spin-1/2 antiferromagnets are indeed universal.

Theoretical and experimental studies of a quasi-two-dimensional quantum antiferromagnet $\text{Cu}(\mathrm{en}){\left({\mathrm{H}}_2\mathrm{O}\right)}_2{\mathrm{SO}}_4$ $\left(\mathrm{en}={\mathrm{C}}_2{\mathrm{H}}_8{\mathrm{N}}_2\right)$ were performed. ab initio calculations of exchange interactions confirmed that the system represents a realization of a spatially anisotropic zigzag square lattice. Corresponding quantum Monte Carlo calculations of thermodynamic quantities were realized and the results were applied in the analysis of experimental susceptibility, magnetization, and specific heat studied at temperatures ranging from nominally 300 mK up to 8 K and magnetic fields up to 14 T. The analysis of experimental results provided the estimates of intralayer exchange couplings, $J/k_B=3.5\pm 0.2$ K and $J^{\prime }=0.35J$. Theoretical analysis of spin symmetries in $\text{Cu}(\mathrm{en}){\left({\mathrm{H}}_2\mathrm{O}\right)}_2{\mathrm{SO}}_4$ structure predicted the presence of symmetric exchange anisotropies (out-of plane and in-plane spin anisotropy) and a spin-flop transition within the easy plane induced by the magnetic field applied along the easy axis. Isothermal magnetization measurements indicated the expected transition in the field 200 mT applied along the $b$ axis which was finally identified as the easy axis lying within the easy plane $bc$. Magnetic phase diagrams with saturation fields about 6.5 T show nearly identical behavior in all studied directions. Differences appear only in weak magnetic fields as a result of the presence of weak exchange anisotropies $\approx {10}^{-3}J$. The present analysis suggests that $\text{Cu}(\mathrm{en}){\left({\mathrm{H}}_2\mathrm{O}\right)}_2{\mathrm{SO}}_4$ can be a model system for exploring the interplay of quantum fluctuations, exchange anisotropies, and magnetic field in the two-dimensional lattice space.

In scanning tunneling microscopy conductance curves, the superconducting gap of cuprates is sometimes accompanied by small subgap structures at very low energy. This was documented early on near vortex cores and later at zero magnetic field. Using mean-field toy models of coexisting $d$-wave superconductivity, $d$-form-factor density wave, and extended $s$-wave pair density wave ($s^{\prime }\mathrm{PDW}$), we find agreement with this phenomenon, with $s^{\prime }\mathrm{PDW}$ playing a critical role. We explore the high variability of the gap structure with changes in band structure and density wave (DW) wave vector, thus explaining why subgap structures may not be a universal feature in cuprates. In the absence of nesting, nonsuperconducting results never show signs of pseudogap, even for large density wave magnitudes, therefore reinforcing the idea of a distinct origin for the pseudogap, beyond mean-field theory. Therefore, we also briefly consider the effect of DWs on a preexisting pseudogap.

Thin amorphous bands are often thought to be the cause of drastic change of properties in boron-based ceramics. We investigate possible mechanisms of amorphization by performing a large-scale molecular dynamics study of defect formation in $\alpha$-boron. Activation of the [101]/(010) slipping system leads to the nanotwin formation. We found no evidence that the nanotwin can become amorphous; it remains stable at high pressures and temperatures. However, inherent plastic deformation leads to local increasing of density that is the precursor of so-called amorphization.

We investigate the effect of site occupancy on the chemical diffusion of hydrogen in strained vanadium. The diffusion rate is found to decrease substantially, when hydrogen is occupying octahedral sites as compared to tetrahedral sites. Profound isotope effects are observed when comparing the diffusion rate of H and D. The changes in the diffusion rate are found to be strongly influenced by the changes in the potential energy landscape, as deduced from first-principles molecular dynamics calculations.

Perpendicular magnetic anisotropy in epitaxial $\mathrm{Pd}/\mathrm{Co}/\mathrm{Pd}\left(111\right)$ trilayered films grown on Si(111) substrate was investigated. Contributions to perpendicular magnetic anisotropy from the bottom and top Co/Pd interfaces were deduced by replacement of Pd layers by Cu layers and comparative analysis of the magnetic anisotropy in the samples. Perpendicular magnetic anisotropy in Pd/Co/Pd films was induced both by interface electronic effects and by stress caused by lattice mismatch between Pd and Co. Due to asymmetry of the stress in the Co film, the contribution to magnetic anisotropy induced by the bottom Co/Pd interface was stronger than that induced by the top Pd/Co interface. The energy of the perpendicular magnetic anisotropy and asymmetrical contributions from the bottom Co/Pd and top Pd/Co interfaces to anisotropy in Pd/Co/Pd trilayers strongly depend on the thickness of the bottom and top Pd layers and may be precisely controlled. The roughness of the interfaces does not have a large influence on the energy of perpendicular magnetic anisotropy in this system.

The magnetization process of a frustrated spin-1/2 spin-dimer model on a square lattice is investigated by means of a sign-problem-free quantum Monte Carlo algorithm developed recently. Rich field-induced quantum phases are discovered. We find two spin superfluids satisfying different symmetries upon layer permutation. There exist as well two solid phases with distinct checkerboard patterns. More interestingly, a spin supersolid phase possessing both solid and superfluid long-range orders is observed over a finite regime of magnetic fields. We find that this exotic phase can be stabilized only when small but nonzero spin anisotropy is present. Various field-induced transitions among these phases are explored. Our findings may provide guidance to the search of these interesting quantum phases in real frustrated spin-dimer compounds.

Elemental sulfur shows a superconducting phase at high pressure (above 100 GPa), with critical temperatures that rise up to 20 K [Phys. Rev. B 65, 064504 (2002); Nature (London) 525, 73 (2015)] and presenting a jump at about 160 GPa, close to a structural phase transition to the $\beta$-Po phase. In this work we present a theoretical and fully ab initio characterization of sulfur based on superconducting density functional theory (SCDFT), focusing in the pressure range from 100 to 200 GPa. Calculations result in very good agreement with available experiments and point out that the origin of the critical temperature discontinuity is not related to the structural phase transition but induced by an electronic Lifshitz transition. This brings a strongly (interband) coupled electron pocket available for the superconducting condensation.

The spin-galvanic (inverse Edelstein) and inverse spin-Hall effects are calculated for a hybrid system that combines thin superconductor and Rashba metal layers. These effects are produced by a nonequilibrium spin polarization that is injected into the normal metal layer. This polarization gives rise to an electric potential that relaxes within some characteristic length, which is determined by Andreev reflection. Within this length, the dissipative electric current of quasiparticles in the normal layer converts into the supercurrent. This process involves only subgap states, and at low temperature the inelastic electron-phonon interactions are not important. It is discussed how such a hybrid system can be integrated into a SQUID, where it produces an effect similar to a magnetic flux.

We analyze the ground-state localization properties of an array of identical interacting spinless fermionic chains with quasirandom disorder, using nonperturbative renormalization-group methods. In the single- or two-chain case localization persists, while for a larger number of chains a different qualitative behavior is generically expected, unless the many-body interaction is vanishing. This is due to number-theoretical properties of the frequency, similar to the ones assumed in Kolmogorov-Arnold-Moser theory, and cancellations due to the Pauli principle, which in the single- or two-chain case imply that all the effective interactions are irrelevant; in contrast, for a larger number of chains, relevant effective interactions are present.

We present a simple scheme to evaluate linear response functions including quantum fluctuation corrections on top of the Gutzwiller approximation. The method is derived for a generic multiband lattice Hamiltonian without any assumption about the dynamics of the variational correlation parameters that define the Gutzwiller wave function, and which thus behave as genuine dynamical degrees of freedom that add on those of the variational uncorrelated Slater determinant. We apply the method to the standard half-filled single-band Hubbard model. We are able to recover known results, but, as a by-product, we also obtain a few other results. In particular, we show that quantum fluctuations can reproduce, almost quantitatively, the behavior of the uniform magnetic susceptibility uncovered by dynamical mean-field theory, which, though enhanced by correlations, is found to be smooth across the paramagnetic Mott transition. By contrast, the simple Gutzwiller approximation predicts that susceptibility to diverge at the transition.

Planar composite structures formed from the stripes of transition metal dichalcogenides joined commensurately along their zigzag or armchair edges can attain different states in a two-dimensional (2D), single-layer, such as a half metal, 2D or one-dimensional (1D) nonmagnetic metal and semiconductor. Widening of stripes induces metal-insulator transition through the confinements of electronic states to adjacent stripes, that results in the metal-semiconductor junction with a well-defined band lineup. Linear bending of the band edges of the semiconductor to form a Schottky barrier at the boundary between the metal and semiconductor is revealed. Unexpectedly, strictly 1D metallic states develop in a 2D system along the boundaries between stripes, which pins the Fermi level. Through the $\delta$ doping of a narrow metallic stripe one attains a nanowire in the 2D semiconducting sheet or narrow band semiconductor. A diverse combination of constituent stripes in either periodically repeating or finite-size heterostructures can acquire critical fundamental features and offer device capacities, such as Schottky junctions, nanocapacitors, resonant tunneling double barriers, and spin valves. These predictions are obtained from first-principles calculations performed in the framework of density functional theory.

A high quality factor and a small modal volume for compact, integrable optical devices have always been in great demand. Although metamaterials may be designed to achieve selected mode confinement at a subwavelength scale, the structures themselves have retained a large size. We address the issue of subwavelength mode confinement in structures of reduced size by utilizing unique properties of spoof surface plasmon polaritons (SSPPs). While SSPP modes are commonly considered in the context of periodic structures, we demonstrate that SSPP formation does not depend crucially on periodicity, and therefore metastructures of minimal length can support a well-formed spoof plasmon state. This general property is explicated through our study of the transmission spectrum of a three-cell waveguide with the spoof plasma frequency of the middle cell (defect cell) different from that of the remaining cells (host). The SSPP state in the defect cell supports resonant tunneling manifested by a narrow transmission resonance inside the band gap of the host structure. Despite the minimal length of the structure, the localized defect SSPP mode is characterized by very high quality $\left(\sim {10}^5\right)$ and Purcell $\left(\sim {10}^3\right)$ factors. The proposed concept can be a promising alternative for making miniaturized sources, information storage, and sensing devices at low terahertz frequencies.

We calculate an electron-phonon scattering and intrinsic transport properties of black phosphorus monolayer using tight-binding and Boltzmann treatments as a function of temperature, carrier density, and electric field. The low-field mobility shows weak dependence on density and, at room temperature, falls in the range of 300–1000 ${\mathrm{cm}}^2$/Vs in the armchair direction and 50–$120{\mathrm{cm}}^2$/Vs in the zigzag direction with an anisotropy due to the effective mass difference. At high fields, drift velocity is linear with field up to 1–2 V/$\mu \mathrm{m}$ reaching values of ${10}^7$ cm/s in the armchair direction, unless self-heating effects are included.

We predict UV reflectance anisotropy spectra (RAS) of the organically functionalized silicon (001) surface covered by pyrimidinic uracil-like nucleobases. First-principles results based on density functional theory show characteristic spectral features appearing in the UV range between 3 and 7 eV, besides the expected quench in the well-known two-minima RAS signal of clean Si(001). Nucleobase adsorption in the energetically favored “dimer bridge” configuration gives rise to a characteristic RAS line shape, common to thymine, uracil, and 5-fluorouracil. We trace back the origin of such spectral features by singling out RAS structures induced by relaxation and passivation effects on the Si surface, and those directly associated with molecular excitations. The former turn out to be the same for the three nucleobases, and are totally unaffected by molecular tilting. The sign and position of the latter RAS peaks at higher energy exhibit a moderate nucleobase dependence, and can be fully rationalized in terms of the molecular orbitals involved. The present theoretical results call for a RAS experimental study in the UV region extending up to $\simeq 6–7$ eV.

It is generally believed that a Wigner crystal in single layer graphene cannot form because the magnitudes of the Coulomb interaction and the kinetic energy scale similarly with decreasing electron density. However, this scaling argument does not hold for the low energy states in bilayer graphene. We consider the formation of a Wigner crystal in weakly doped bilayer graphene with an energy gap opened by a perpendicular electric field. We argue that in this system the formation of the Wigner crystal is not only possible, but different phases of the crystal with very peculiar properties may exist here depending on the system parameters.

In this paper, we present a three-dimensional one-step photoemission model that can be used to calculate the quantum efficiency and momentum distributions of electrons photoemitted from ordered single-crystal surfaces close to the photoemission threshold. Using Ag(111) as an example, we show that the model can not only calculate the quantum efficiency from the surface state accurately without using any ad hoc parameters, but also provides a theoretical quantitative explanation of the vectorial photoelectric effect. This model in conjunction with other band structure and wave function calculation techniques can be effectively used to screen single-crystal photoemitters for use as electron sources for particle accelerator and ultrafast electron diffraction applications.