Recently, ${\mathrm{KFe}}_2{\mathrm{As}}_2$ was shown to exhibit a structural phase transition from a tetragonal to a collapsed tetragonal phase under an applied pressure of about 15 GPa. Surprisingly, the collapsed tetragonal phase hosts a superconducting state with $T_c\sim 12\mathrm{K}$, while the tetragonal phase is a $T_c\le 3.4\mathrm{K}$ superconductor. We show that the key difference between the previously known nonsuperconducting collapsed tetragonal phase in $A{\mathrm{Fe}}_2{\mathrm{As}}_2(A=\text{Ba},\mathrm{Ca},\mathrm{Eu},\mathrm{Sr})$ and the superconducting collapsed tetragonal phase in ${\mathrm{KFe}}_2{\mathrm{As}}_2$ is the qualitatively distinct electronic structure. While the collapsed phase in the former compounds features only electron pockets at the Brillouin zone boundary and no hole pockets are present in the Brillouin zone center, the collapsed phase in ${\mathrm{KFe}}_2{\mathrm{As}}_2$ has almost nested electron and hole pockets. Within a random phase approximation spin fluctuation approach we calculate the superconducting order parameter in the collapsed tetragonal phase. We propose that a Lifshitz transition associated with the structural collapse changes the pairing symmetry from $d$ wave (tetragonal) to $s_{\pm }$ (collapsed tetragonal). Our density functional theory combined with dynamical mean-field theory calculations show that effects of correlations on the electronic structure of the collapsed tetragonal phase are minimal. Finally, we argue that our results are compatible with a change of sign of the Hall coefficient with pressure, as observed experimentally.

Inelastic neutron scattering measurements on monoclinic zirconia $({\mathrm{ZrO}}_{\text{2}})$ and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to $1373w\mathrm{K}$. Temperature-dependent phonon densities of states (DOS) are reported, as are Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Grüneisen parameters. These mode Grüneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat more anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.

A theoretical comparison of changes in the phonon band structures and the lattice dynamics of diamond-type carbon, silicon, and germanium due to the presence of hot electrons and holes, which can be induced by a femtosecond-laser pulse, is presented. Our results indicate that lighter elements are more resilient to femtosecond-laser excitation than heavier ones. We discuss how this is consistent with a richer structural response of light solids, including the possibility of femtosecond-laser-triggered solid-to-solid structural changes.

The physical properties of metamagnetic ${\mathrm{Fe}}_3{\mathrm{Ga}}_4$ single crystals are investigated to explore the sensitivity of the magnetic states to temperature, magnetic field, and sample history. The data reveal a moderate anisotropy in the magnetization and the metamagnetic critical field along with features in the specific heat at the magnetic transitions $T_1=68$ K and $T_2=360$ K. Both $T_1$ and $T_2$ are found to be sensitive to the annealing conditions of the crystals suggesting that disorder affects the competition between the ferromagnetic (FM) and antiferromagnetic (AFM) states. Resistivity measurements reveal metallic transport with a sharp anomaly associated with the transition at $T_2$. The Hall effect is dominated by the anomalous contribution, which rivals that of magnetic semiconductors in magnitude $(-5\mu \Omega$ cm at 2 T and 350 K) and undergoes a change of sign upon cooling into the low-temperature FM state. The temperature and field dependence of the Hall effect indicate that the magnetism is likely to be highly itinerant in character and that a significant change in the electronic structure accompanies the magnetic transitions. We observe a contribution from the topological Hall effect in the AFM phase suggesting a noncoplanar contribution to the magnetism. Electronic structure calculations predict an AFM ground state with a wave vector parallel to the crystallographic $c$-axis preferred over the experimentally measured FM state by $\approx 50$ meV per unit cell. However, supercell calculations with a small density of Fe antisite defects introduced tend to stabilize the FM over the AFM state indicating that antisite defects may be the cause of the sensitivity to sample synthesis conditions.

We present a systematic calculation of the effects of scattering of electrons off spin waves on electron transport properties in itinerant antiferromagnetic thin films in two and three dimensions. We study various regimes set by the parameters related to the spin-wave gap, exchange energy, as well as the exchange splitting, in addition to the scales set by temperature and disorder. We find an interaction-induced quantum correction to the conductivity linear in temperature, similar to that obtained recently for ferromagnetic systems within a certain regime of disorder, although the disorder dependence is different. In addition, we explore the phase relaxation rates and the associated weak-localization corrections for both small and large spin-wave gaps. We obtain a wide variety of temperature and disorder dependence for various parameter regimes. These results should provide an alternative way to study magnetic properties of thin antiferromagnetic films, for which neutron scattering measurements could be difficult, by direct transport measurements.

Using Raman spectroscopy, we investigate the lattice phonons, magnetic excitations, and magnetoelastic coupling in the distorted triangular-lattice Heisenberg antiferromagnet $\alpha {\text{-SrCr}}_2{\text{O}}_4$, which develops helical magnetic order below 43 K. Temperature-dependent phonon spectra are compared to predictions from density functional theory calculations which allows us to assign the observed modes and identify weak effects arising from coupled lattice and magnetic degrees of freedom. Raman scattering associated with two-magnon excitations is observed at 20 and 40 meV. These energies are in general agreement with our ab initio calculations of exchange interactions and earlier theoretical predictions of the two-magnon Raman response of triangular-lattice antiferromagnets. The temperature dependence of the two-magnon excitations indicates that spin correlations persist well above the Néel temperature.

Engineering chiral $p$-wave superconductivity in semiconductor structures offers fascinating ways to obtain and study Majorana modes in a condensed-matter context. Here, we theoretically investigate chiral $p$-wave superconductivity in quantum dots and quantum rings. Using both analytical as well as numerical methods, we calculate the quasiparticle excitation spectra in these structures and the corresponding excitation amplitudes and charge densities. In the topological regime, we can observe the chiral edge modes localized at the boundaries and possessing finite energy in quantum dots and quantum rings. By applying a magnetic field which is expelled from the quantum ring, but which creates a flux that is an odd integer multiple of ${\Phi }_0/2=\pi \hslash /e$, Majorana modes, that is (approximately), degenerate edge modes with zero energy and zero charge density, become possible in the topological regime. Furthermore, we investigate finite-size effects that split these degenerate edge modes as well as the effect of a magnetic field penetrating into the superconducting region that can under certain circumstances still support edge modes with approximately zero energy and charge.

This work provides a unified theoretical treatment of the single- and correlated double-electron emission from a general electronic system. Using Feshbach projection method, the states of interest are selected by the projection operator; the Feshbach-Schur map determines the effective Hamiltonian and the optical potential for the emitted electrons. On the other hand, the nonequilibrium Green's functions method is demonstrated to be a complementary approach, and an explicit correspondence between both methods is established. For a self-contained exposition, some results on single-electron emission are rederived using both formalisms. New insights and results are obtained for the correlated electron-pair emission: This includes the effective two-electron Hamiltonian, the explicit form of the Feshbach self-energy in terms of the many-body self-energies, and the diagrammatic expansion of the two-particle current. As an illustration of the diagrammatic technique, the process of the two-particle emission assisted by the excitation of plasmons is explicitly worked out.

For a capacitor made of a semiconducting carbon nanotube (CNT) suspended above a metallic gate, Coulomb correlations between individual electrons can lead to a capacitance that is much larger than the geometric capacitance. We argue that when the average spacing $n^{-1}$ between electrons within the low-density one-dimensional electron gas (1DEG) in the CNT is larger than the physical separation $d$ between the CNT and the gate, the enhancement of capacitance is expected to be big. A recent experiment [J. Waissman et al., Nature Nanotechnol. 8, 569 (2013)], however, has observed no obvious increase of capacitance even at very low electron density. We show that this smaller capacitance can be understood as the result of the confining potential produced by the potential difference between the source/drain electrodes and the gate, which compresses the 1DEG when the electron number decreases. We suggest that by profiling the potential with the help of multiple split gates, one can return to the case of a uniform 1DEG with anomalously large capacitance.

A lack of universality with respect to ion species has been recently established in nanostructuring of semiconductor surfaces by low-energy ion-beam bombardment. This variability affects basic properties of the pattern formation process, like the critical incidence angle for pattern formation, and has remained unaccounted for. Here, we show that nonuniform generation of stress across the damaged amorphous layer induced by the irradiation is a key factor behind the range of experimental observations, as the form of the stress field is controlled by the ion/target combination. This effect acts in synergy with the nontrivial evolution of the amorphous-crystalline interface. We reach these conclusions by contrasting a multiscale theoretical approach, which combines molecular dynamics and a continuum viscous flow model, with experiments using ${\text{Xe}}^{+}$ and ${\text{Ar}}^{+}$ ions on a Si(100) target. Our general approach can apply to a variety of semiconductor systems and conditions.

The exciton spin dynamics and polarization properties of the related emission are investigated in colloidal CdSe/CdS dot-in-rod (DiR) and spherical core/shell nanocrystal (NC) ensembles by magneto-optical photoluminescence (PL) spectroscopy in magnetic fields up to 15 T. It is shown that the degree of circular polarization (DCP) of the exciton emission induced by the magnetic field is affected by the NC geometry as well as the exciton fine structure and can provide information on nanorod orientation. A theory to describe the circular and linear polarization properties of the NC emission in a magnetic field is developed. It takes into account phonon mediated coupling between the exciton fine structure states as well as the dielectric enhancement effect resulting from the anisotropic shell of DiR NCs. This theoretical approach is used to model the experimental results and allows us to explain most of the measured features. The spin dynamics of the dark excitons is investigated in magnetic fields by time-resolved photoluminescence. The results highlight the importance of confined acoustic phonons in the spin relaxation of dark excitons. The bare core surface as well as the core/shell interface give rise to an efficient spin-relaxation channel, while the surface of core/shell NCs seems to play only a minor role.

We consider a two-dimensional strongly localized system defined in a half-plane and whose transfer integral in the edge can be different than in the bulk. We predict an unbinding transition, as the edge transfer integral is varied, from a phase where conduction paths are distributed across the bulk to a bound phase where propagation is mainly along the edge. At criticality the logarithm of the conductance follows the $F_1$ Tracy-Widom distribution. We verify numerically these predictions for both the Anderson and the Nguyen, Spivak, and Shklovskii models. We also check that for a half-plane, i.e., when the edge transfer integral is equal to the bulk transfer integral, the distribution of the conductance is the $F_4$ Tracy-Widom distribution. These findings are strong indications that random sign directed polymer models and their quantum extensions belong to the Kardar-Parisi-Zhang universality class. We have analyzed finite-size corrections at criticality and for a half-plane.

We analyze the $s+is$ state that is proposed as a candidate superconducting state for strongly hole-doped ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. Such a state breaks time-reversal symmetry (TRS) but does not break any other discrete symmetry. We address the issue as to whether TRS breaking alone can generate spontaneous currents near impurity sites, which could be detected in, e.g., muon spin relaxation experiments. We argue that there are no spontaneous currents if only TRS is broken. However, supercurrents do emerge if the system is put under external strain and $C_4$ lattice rotation symmetry is externally broken.

We clarify the orbital mechanism of the $\alpha \text{−}\gamma$ transition in cerium. First we built an $sdf$ Wannier orbital basis to describe the electronic structure of cerium. Second, we use this basis to study the relative role of several orbital hoppings upon compression of cerium. Third, we use DFT + DMFT calculations to quantify the impact of these hoppings on electronic structure. Our conclusion is that upon compression of $\gamma$ cerium, the change of hybridization is due to both interatomic $ff$ and $fd$ hopping integrals. In particular, neglecting $ff$ hoppings leads to an important renormalization of both the hybridization and the quasiparticle peak. Thus, neither the Kondo volume collapse nor the Mott transition model are sufficient to describe the isostructural transition in cerium.

We report on combined neutron and resonant x-ray scattering results, identifying the nature of the spin-orbital ground state and magnetic excitations in ${\mathrm{LuVO}}_3$ as driven by the orbital parameter. In particular, we distinguish between models based on orbital-Peierls dimerization, taken as a signature of quantum effects in orbitals, and Jahn-Teller distortions, in favor of the latter. In order to solve this long-standing puzzle, polarized neutron beams were employed as a prerequisite in order to solve details of the magnetic structure, which allowed quantitative intensity analysis of extended magnetic-excitation data sets. The results of this detailed study enabled us to draw definite conclusions about the classical versus quantum behavior of orbitals in this system and to discard the previous claims about quantum effects dominating the orbital physics of ${\mathrm{LuVO}}_3$ and similar systems.