We investigate phases of spinless fermions on the honeycomb lattice with nearest-neighbor interaction in the Hofstadter regime. The interaction induces incompressible nematic and ferrielectric phases with broken translation symmetry. Some of the transitions are accompanied by changes in the Hall conductivity. We study pair correlations and show that the quantum metric, averaged over the Brillouin zone, characterizes the shape of the pair correlation function.

The detailed temperature dependence of the infrared-active mode in ${\mathrm{Fe}}_{1.03}\mathrm{Te}$ ($T_N\simeq 68$ K) and ${\mathrm{Fe}}_{1.13}\mathrm{Te}$ ($T_N\simeq 56$ K) has been examined, and the position, width, strength, and asymmetry parameter have been determined using an asymmetric Fano profile superimposed on an electronic background. In both materials the frequency of the mode increases as the temperature is reduced; however, there is also a slight asymmetry in the line shape, indicating that the mode is coupled to either spin or charge excitations. Below $T_N$ there is an anomalous decrease in frequency, and the mode shows little temperature dependence, at the same time becoming more symmetric, suggesting a reduction in spin- or electron-phonon coupling. The frequency of the infrared-active mode and the magnitude of the shift below $T_N$ are predicted reasonably well by first-principles calculations; however, the predicted splitting of the mode is not observed. In superconducting ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ ($T_c\simeq 14$ K) the infrared-active $E_u$ mode displays asymmetric line shape at all temperatures, which is most pronounced between 100 and 200 K, indicating the presence of either spin- or electron-phonon coupling, which may be a necessary prerequisite for superconductivity in this class of materials.

We develop the stub model for the Dirac quantum dot, an electron confining device on a grapheme surface. Analytical results for the average conductance and the correlation functions are obtained and found in agreement with those found previously using semiclassical calculation. Comparison with available data is presented. The results reported here demonstrate the applicability of random matrix theory in the case of Dirac electrons confined in a stadium.

We demonstrate the capability of two molecular organic acceptors [1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile and 2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile] to tune the work function (Φ) of intrinsically doped GaN(0001) and for comparison of intrinsically doped ZnO(0001). With ultraviolet photoelectron spectroscopy we determine the accessible Φ range as 4.2–6.0 eV for GaN and 3.9–6.3 eV for ZnO. The contribution to the Φ change (ΔΦ) of acceptor-induced surface band bending within GaN was significantly smaller than in ZnO (0.35 versus 1 eV), which we attribute to surface gap states. We introduce a model that takes these surface states into account and thus allows quantifying the impact of the semiconductor bulk doping level and surface state density on ΔΦ. The lower the doping level is, the lower the surface state density needs to be to reduce the band bending contribution to ΔΦ. This limits the maximum value of Φ tuning for GaN with molecular acceptors, whereas, on the other hand, it renders the method robust against surface structural disorder.

We investigate silicene superlattices in the presence of a tunable barrier potential $U$, an exchange field $M$, and a perpendicular electric field $E_z$. The resulting miniband structure depends on the spin and valley indices and on the fields $M$ and $E_z$. These fields determine the minigaps and also affect the additional Dirac points brought about by the periodic potential $U$. In addition, we consider diffusive transport and assess its dependence on the spin and valley indices as well as on temperature. The corresponding spin and valley polarizations strongly depend on the potential $U$ and can be made almost 100% at very low temperatures at particular values of the Fermi energy.

Properties of the $\omega$ structure are investigated for 27 transition elements from the viewpoints of thermodynamical and dynamical stability based on first-principles calculations. The thermodynamical stability of the $\omega$ structure is compared with those of the body-centered-cubic, face-centered-cubic, and hexagonal-close-packed structures. Similar to the case of those popular crystal structures, the occupation number for $d$ orbitals is found to roughly determine relative energy and volume of the nonmagnetic $\omega$ structure. For the group 4 elements (Ti, Zr, and Hf), the $\omega$ structure is almost the lowest in energy among the investigated crystal structures and is also dynamically stable. The $\omega$ structure of the group 7 elements (Mn, Tc, and Re) is also dynamically stable. The $\omega$ Fe is found to exhibit a magnetic state with antiparallel magnetic moments. This magnetic state is the most favorable among the investigated magnetic states. The $\omega$ Fe in this magnetic state is also dynamically stable. Energies of binary alloys composed of the elements in the group 4 and those in the groups 5 and 6 are estimated by linear interpolation, and most of the alloys show concentration ranges where the $\omega$ structure is the lowest in energy among the investigated crystal structures.

In this paper, we study a biquadratic Heisenberg model with coupled orbital degrees of freedom by using a Monte Carlo simulation to investigate the phase transitions in iron-based superconductors. The antiferroquadrupolar state, which may be related to the magnetism of FeSe [R. Yu and Q. Si, Phys. Rev. Lett. 115, 116401 (2015)], is stabilized by the anisotropic biquadratic interaction induced by a ferro-orbital-ordered state. It is revealed that the orbital and nematic transitions occur at the same temperature for all the cases, supporting the mechanism of the orbital-driven nematicity as revealed in most recent experiments [S. H. Baek, D. V. Efremov, J. M. Ok, J. S. Kim, J. van den Brink, and B. Büchner, Nat. Mater. 14, 210 (2015)]. In addition, it is suggested that the orbital interaction may lead to the separation of the structural and magnetic phase transitions, as observed in many families of iron pnictides.

There is still no general consensus on how one can describe the out-of-equilibrium phenomena in matter induced by an ultrashort light pulse. We investigate the pulse-induced dynamics in a layered Dirac semimetal ${\mathrm{SrMnBi}}_2$ by pump-and-probe photoemission spectroscopy. At $\lesssim 1$ ps, the electronic recovery slowed upon increasing the pump power. Such a bottleneck-type slowing is expected in a two-temperature model (TTM) scheme, although opposite trends have been observed to date in graphite and in cuprates. Subsequently, an unconventional power-law cooling took place at $\sim 100$ ps, indicating that spatial heat diffusion is still ill defined at $\sim 100\mathrm{ps}$. We identify that the successive dynamics before the emergence of heat diffusion is a canonical realization of a TTM scheme. Criteria for the applicability of the scheme is also provided.

We have studied the proximity-induced superconducting triplet pairing in ${\mathrm{CoO}}_x$/Py1/Cu/Py2/Cu/Pb spin-valve structure (where Py = ${\mathrm{Ni}}_{0.81}{\mathrm{Fe}}_{0.19}$). By optimizing the parameters of this structure we found a triplet channel assisted full switching between the normal and superconducting states. To observe an “isolated” triplet spin-valve effect we exploited the oscillatory feature of the magnitude of the ordinary spin-valve effect $\mathrm{\Delta }{T}_c$ in the dependence of the Py2-layer thickness $d_{Py2}$. We determined the value of $d_{Py2}$ at which $\mathrm{\Delta }{T}_c$ caused by the ordinary spin-valve effect (the difference in the superconducting transition temperature $T_c$ between the antiparallel and parallel mutual orientation of magnetizations of the Py1 and Py2 layers) is suppressed. For such a sample a “pure” triplet spin-valve effect which causes the minimum in $T_c$ at the orthogonal configuration of magnetizations has been observed.

In this article we provide a general mechanism for generating interaction-enabled fermionic topological phases. We illustrate the mechanism with crystalline symmetry-protected topological phases in one, two, and three spatial dimensions. These nontrivial phases require interactions for their existence, and in the cases we consider, the free-fermion classification yields only a trivial phase. For the one- and two-dimensional phases we consider, we provide explicit exactly solvable models which realize the interaction-enabled phases. Similar to the interpretation of the Kitaev Majorana wire as a mean-field $p$-wave superconductor Hamiltonian arising from an interacting model with quartic interactions, we show that our systems can be interpreted as “mean-field” charge-$4e$ superconductors arising, e.g., from an interacting model with eight-body interactions or through another physical mechanism. The quartet superconducting nature allows for the teleportation of full Cooper pairs and, in two dimensions, for interesting semiclassical crystalline defects with non-Abelian anyon bound states.

The formation energies and electronic structure of europium-doped zinc oxide has been determined using DFT and many-body $GW$ methods. In the absence of intrisic defects, we find that the europium-$f$ states are located in the ZnO band gap with europium possessing a formal charge of 2+. On the other hand, the presence of intrinsic defects in ZnO allows intraband $f\text{−}f$ transitions otherwise forbidden in atomic europium. This result corroborates with recently observed photoluminescence in the visible red region S. Geburt et al. [Nano Lett. 14, 4523 (2014)].

Based on the Frenkel and charge transfer mixing model of singlet fission (SF), we incorporate both the local and nonlocal phonon baths into the Hamiltonian and adopt the algorithm of the time-dependent density matrix renormalization group to simulate the SF process in tetracene and pentacene. The endergonic SF is found to be facilitated by the robust quantum coherence, which concurrently gives rise to a notable quantum beating effect. Being controlled by the nonlocal electron-phonon coupling, the SF process is accelerated and the triplet yield manifests a nonlinear relationship with the singlet density.

We use scanning tunneling microscopy (STM) and quasiparticle interference (QPI) imaging to investigate the low-energy orbital texture of single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$. We develop a $T$-matrix model of multiorbital QPI to disentangle scattering intensities from Fe $3d_{xz}$ and $3d_{yz}$ bands, enabling the use of STM as a nanoscale detection tool of nematicity. By sampling multiple spatial regions of a single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ film, we quantitatively exclude static $xz/yz$ orbital ordering with domain size larger than $\delta r^2=20\mathrm{nm}\times 20\mathrm{nm}$, $xz/yz$ Fermi wave vector difference larger than $\delta k=0.014\pi$, and energy splitting larger than $\delta E=3.5\mathrm{meV}$. The lack of detectable ordering pinned around defects places qualitative constraints on models of fluctuating nematicity.

We report on the observation of the quantum Hall effect at high temperatures in HgTe quantum wells with a finite band gap and a thickness below and above the critical thickness $d_c$ that separates a conventional semiconductor from a two-dimensional topological insulator. At high carrier concentrations, we observe a quantized Hall conductivity up to 60 K with energy gaps between Landau levels of the order of 25 meV, in good agreement with the Landau level spectrum obtained from $\mathbf{k}·\mathbf{p}$ calculations. Using the scaling approach for the plateau-plateau transition at $\nu =2\to 1$, we find the scaling coefficient $\kappa =0.45\pm 0.04$ to be consistent with the universality of scaling theory, and we do not find signs of increased electron-phonon interaction to alter the scaling even at these elevated temperatures. Comparing the high-temperature limit of the quantized Hall resistance in HgTe quantum wells with a finite band gap with room-temperature experiment in graphene, we find that the energy gaps at the breakdown of the quantization exceed the thermal energy by the same order of magnitude.

We report on microscopic measurements of the low-energy electronic structures both at the zigzag and armchair edges of bilayer graphene using scanning tunneling microscopy and spectroscopy (STM and STS). We have found that, both in the absence and in the presence of a magnetic field, an almost zero-energy peak in the density of states was localized at the zigzag edges, as expected for the surface states at the zigzag edges of bilayer graphene. In the quantum Hall regime, we have clearly observed Landau levels bending away from the charge neutrality point near both the zigzag and armchair edges. Such a result is direct evidence for the evolution of Landau levels into quantum Hall edge states in graphene bilayers. Our experiment indicates that it is possible to explore rich quantum Hall physics in graphene systems using STM and STS.

The stochastic variational method is applied to excitonic formations within semiconducting transition metal dichalcogenides using a correlated Gaussian basis. The energy and structure of two- to six-particle systems are investigated along with their dependence on the effective screening length of the two-dimensional Keldysh potential and the electron-hole effective mass ratio. Excited state biexcitons are shown to be bound, with binding energies of the $L=0$ state showing good agreement with experimental measurements of biexciton binding energies. Ground and newly discussed excited state exciton-trions are predicted to be bound and their structures are investigated.

The spin-resolved density of states of Co atoms on a noncollinear magnetic support displays a distinct shape contrast, which is superimposed on the regular height contrast in spin-polarized scanning tunneling microscopy. The apparent atom height follows the well-known cosine dependence on the angle formed by the tip and adatom local magnetization directions, whereas the shape contrast exhibits a sine dependence. We explain this effect in terms of a noncollinear spin density induced by the substrate, which in our case is the spin spiral of the Mn monolayer on W(110). The two independent contrast channels, apparent height and shape, are identified with the Co magnetization projections onto two orthogonal axes. As a result, all components of the overall atom magnetic moment vector can be determined with a single spin-sensitive tip in the absence of an external magnetic field. This result should be general for any atom deposited on noncollinear magnetic layers.

We describe how ab initio molecular dynamics can be used to determine the Hugoniot locus (states accessible by a shock wave) for materials with a number of stable phases, and with an approximate treatment of plasticity and yield, without having to simulate these phenomena directly. We consider the case of bulk silicon, with forces from density-functional theory, up to 70 GPa. The fact that shock waves can split into multiple waves due to phase transitions or yielding is taken into account here by specifying the strength of any preceding waves explicitly based on their yield strain. Points corresponding to uniaxial elastic compression along three crystal axes and a number of postshock phases are given, including a plastically yielded state, approximated by an isotropic stress configuration following an elastic wave of predetermined strength. The results compare well to existing experimental data for shocked silicon.