It is often claimed that among the strongest evidence for preformed-pair physics in the cuprates are the experimentally observed large values for the diamagnetic susceptibility and Nernst coefficient. These findings are most apparent in the underdoped regime, where a pseudogap is also evident. While the conventional (Gaussian) fluctuation picture has been applied to address these results, this preformed-pair approach omits the crucial effects of a pseudogap. In this paper we remedy this omission by computing the diamagnetic susceptibility and Nernst coefficient in the presence of a normal state gap. We find a large diamagnetic response for a range of temperatures much higher than the transition temperature. In particular, we report semiquantitative agreement with the measured diamagnetic susceptibility onset temperatures, over the entire range of hole dopings. Notable is the fact that at the lower critical doping of the superconducting dome, where the transition temperature vanishes and the pseudogap onset temperature remains large, the onset temperature for both diamagnetic and transverse thermoelectric transport coefficients tends to zero. Due to the importance attributed to the cuprate diamagnetic susceptibility and Nernst coefficient, this work helps to clarify the extent to which pairing fluctuations are a component of the cuprate pseudogap.

We report on the electronic structure of the elemental topological semimetal $\alpha -\mathrm{Sn}$ on InSb(001). High-resolution angle-resolved photoemission data allow us to observe the topological surface state (TSS) that is degenerate with the bulk band structure and show that the former is unaffected by different surface reconstructions. An unintentional $p$-type doping of the as-grown films was compensated by deposition of potassium or tellurium after the growth, thereby shifting the Dirac point of the surface state below the Fermi level. We show that, while having the potential to break time-reversal symmetry, iron impurities with a coverage of up to 0.25 monolayers do not have any further impact on the surface state beyond that of K or Te. Furthermore, we have measured the spin-momentum locking of electrons from the TSS by means of spin-resolved photoemission. Our results show that the spin vector lies fully in-plane, but it also has a finite radial component. Finally, we analyze the decay of photoholes introduced in the photoemission process, and by this gain insight into the many-body interactions in the system. Surprisingly, we extract quasiparticle lifetimes comparable to other topological materials where the TSS is located within a bulk band gap. We argue that the main decay of photoholes is caused by intraband scattering, while scattering into bulk states is suppressed due to different orbital symmetries of bulk and surface states.

The Krieger-Li-Iafrate (KLI) approximation for the exact exchange (EXX) potential of density functional theory is investigated far outside the surface of slabs. For large $z$ the Slater component of the EXX/KLI potential falls off as $-1/z$, where $z$ is the distance to the surface of a slab parallel to the $xy$ plane. The Slater potential thus reproduces the behavior of the exact EXX potential. Here it is demonstrated that the second component of the EXX/KLI potential, often called the orbital-shift term, is also proportional to $1/z$ for large $z$, at least in general. This result is obtained by an analytical evaluation of the Brillouin zone integrals involved, relying on the exponential decay of the states into the vacuum. Several situations need to be distinguished in the Brillouin zone integration, depending on the band structure of the slab. In all standard situations, including such prominent cases as graphene and Si(111) slabs, however, a $1/z$ dependence of the orbital-shift potential is obtained to leading order. The complete EXX/KLI potential therefore does not reproduce the asymptotic behavior of the exact EXX potential.

While being of persistent interest for the integration of lattice-matched laser devices with silicon circuits, the electronic structure of dilute nitride III/V-semiconductors has presented a challenge to ab initio computational approaches. The origin of the computational problems is the strong distortion exerted by the N atoms on most host materials. Here, these issues are resolved by combining density functional theory calculations based on the meta-GGA functional presented by Tran and Blaha (TB09) with a supercell approach for the dilute nitride Ga(NAs). Exploring the requirements posed to supercells, it is shown that the distortion field of a single N atom must be allowed to decrease so far that it does not overlap with its periodic images. This also prevents spurious electronic interactions between translational symmetric atoms, allowing us to compute band gaps in very good agreement with experimentally derived reference values. In addition to existing approaches, these results offer a promising ab initio avenue to the electronic structure of dilute nitride semiconductor compounds.

The screening effects on the light emission characteristics of wurtzite (WZ) ZnO/MgZnO quantum dots (QDs) were investigated as a function of carrier density. These results were compared with those without the screening effect. The light emission intensity is shown to be significantly affected by the screening effect. The difference of the light intensity between cases with and without screening effect gradually increases with increasing carrier density. This can be explained by the fact that the overlap between the electron and hole wave functions, and therefore the optical matrix element, is enhanced due to the increase in the screening effect with increasing carrier density. Also, the peak wavelength for the self-consistent solution is observed to be blueshifted with increasing carrier density because the optical matrix elements corresponding to transitions from the conduction subband to higher valence subbands become dominant at a higher carrier density. We observe that the screening effect is enhanced rapidly with increasing dot size. In the case of the ZnO/MgO QD grown on MgO substrate, there exist a large potential due to the strain-induced piezoelectric polarization, in addition to the spontaneous polarization. As a result, its light emission intensity is shown to be much smaller than that of ZnO/MgO QD grown on ZnO substrate.

Transport properties of helical Dirac fermions in disordered quantum wires are investigated in the large energy limit. In the quasiballistic regime, the conductance and the Fano factor are sensitive to disorder only when the Fermi energy is close to an opening of a transverse mode. In the limit of a large number of transverse modes, transport properties are insensitive to the geometry of the nanowire or the nature and strength of the disorder but, instead, are dominated by the properties of the interface between the ohmic contact and the nanowire. In the case of a heavily doped Dirac metallic contact, the conductance is proportional to the energy with an average transmission $\mathcal{T}=\pi /4$ and a Fano factor of $F\simeq 0.13$. Those results can be generalized to a much broader class of contacts, the exact values of $\mathcal{T}$ and $F$ depending on the model used for the contacts. The energy dependence of Aharonov-Bohm oscillations is determined, revealing a damped oscillatory behavior and phase shifts due to the one-dimensional subband quantization and which are not the signature of the nontrivial topology.

We develop a quantum model for the calculation of the gain of phonon-polariton intersubband lasers. The polaritonic gain arises from the interaction between electrons confined in a quantum well structure and phonons confined in one layer of the material. Our theoretical approach is based on expressing the resonant matter excitations (intersubband electrons and phonons) in terms of polarization densities in second quantization, and treating all nonresonant polarizations with an effective dielectric function. The interaction between the electronic and phononic polarizations is treated perturbatively, and gives rise to stimulated emission of polaritons in the case of inverted subbands. Our model provides a complete physical insight of the system and allows us to determine the phonon and photon fraction of the laser gain. Moreover, it can be applied and extended to any type of designs and material systems, offering a wide set of possibilities for the optimization of future phonon-polariton lasers.

We calculated the polarization-dependent Raman scattering cross-section spectra of $\mathrm{LaMn}{\mathrm{O}}_3$ below the $A$-type magnetic ordering temperature. Two strong peaks appear around the $\mathrm{Mn}{\mathrm{O}}_6$ octahedra stretching phonon frequency. One mode shows $A_{\mathrm{g}}$ symmetry, while the other mode shows $B_{\mathrm{g}}$ symmetry. We found that the $A_{\mathrm{g}}$ symmetry peak is a Jahn-Teller phonon coupled to the orbital wave and the $B_{\mathrm{g}}$ symmetry peak is an orbital wave mode coupled to a $Q_2$ phonon mode via the Jahn-Teller electron-phonon coupling.

A new pyrochlore oxide $\mathrm{C}{\mathrm{d}}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ with an $\mathrm{I}{\mathrm{r}}^{5+}$ charge state was prepared by high-pressure techniques. Although strong spin-orbit coupling (SOC) dominates the electronic states in most iridates so that a SOC-Mott state is proposed in $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ in the assumption of an undistorted $\mathrm{Ir}{\mathrm{O}}_6$ octahedral crystalline field, the strongly distorted one in the current $\mathrm{C}{\mathrm{d}}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ exhibits a competing interaction with the SOC. Unexpected from a strong SOC limit, $\mathrm{C}{\mathrm{d}}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ deviates from a nonmagnetic and insulating $J=0$ ground state. It displays short-range ferromagnetic correlations and metallic electrical transport properties. First-principles calculations well reproduce the experimental observation, revealing the large mixture between the $j_{\mathrm{eff}}=1/2$ and $j_{\mathrm{eff}}=3/2$ bands near the Fermi surface due to the significant distortion of $\mathrm{Ir}{\mathrm{O}}_6$ octahedra. This work sheds light on the critical role of a noncubic crystalline field in electronic properties which has been ignored in past studies of $5d$-electron systems.

Scandium Nitride (ScN) is an emerging rocksalt semiconductor with octahedral coordination and an indirect bandgap. ScN has attracted significant attention in recent years for its potential thermoelectric applications, as a component material in epitaxial metal/semiconductor superlattices, and as a substrate for defect-free GaN growth. Sputter-deposited ScN thin films are highly degenerate $n$-type semiconductors and exhibit a large thermoelectric power factor of $\sim 3.5\times {10}^{-3}\mathrm{W}/\mathrm{m}\text{−}{\mathrm{K}}^2$ at 600–800 K. Since practical thermoelectric devices require both n- and p-type materials with high thermoelectric figures-of-merit, development and demonstration of highly efficient p-type ScN is extremely important. Recently, the authors have demonstrated p-type $\mathrm{S}{\mathrm{c}}_{1-x}\mathrm{M}{\mathrm{g}}_x\mathrm{N}$ thin film alloys with low $\mathrm{M}{\mathrm{g}}_{\mathrm{x}}{\mathrm{N}}_{\mathrm{y}}$ mole-fractions within the ScN matrix. In this article, we demonstrate temperature dependent thermal and thermoelectric transport properties, including large thermoelectric power factors in both n- and p-type $\mathrm{S}{\mathrm{c}}_{1-x}\mathrm{M}{\mathrm{g}}_x\mathrm{N}$ thin film alloys at high temperatures (up to 850 K). Employing a combination of temperature-dependent Seebeck coefficient, electrical conductivity, and thermal conductivity measurements, as well as detailed Boltzmann transport-based modeling analyses of the transport properties, we demonstrate that p-type $\mathrm{S}{\mathrm{c}}_{1-x}\mathrm{M}{\mathrm{g}}_x\mathrm{N}$ thin film alloys exhibit a maximum thermoelectric power factor of $\sim 0.8\times {10}^{-3}\mathrm{W}/\mathrm{m}\text{−}{\mathrm{K}}^2$ at 850 K. The thermoelectric properties are tunable by adjusting the $\mathrm{M}{\mathrm{g}}_{\mathrm{x}}{\mathrm{N}}_{\mathrm{y}}$ mole-fraction inside the ScN matrix, thereby shifting the Fermi energy in the alloy films from inside the conduction band in case of undoped $n$-type ScN to inside the valence band in highly hole-doped $p$-type $\mathrm{S}{\mathrm{c}}_{1-x}\mathrm{M}{\mathrm{g}}_x\mathrm{N}$ thin film alloys. The thermal conductivities of both the n- and p-type films were found to be undesirably large for thermoelectric applications. Thus, future work should address strategies to reduce the thermal conductivity of $\mathrm{S}{\mathrm{c}}_{1-x}\mathrm{M}{\mathrm{g}}_x\mathrm{N}$ thin-film alloys, without affecting the power factor for improved thermoelectric performance.

Graphene-based heterostructures are a promising material system for designing the topologically nontrivial Chern insulating devices. Recently, a two-dimensional monolayer ferromagnetic insulator ${\mathrm{CrI}}_3$ was successfully synthesized in experiments [B. Huang et al., Nature (London) 546, 270 (2017)]. Here, these two interesting materials are proposed to build a heterostructure $\left(\mathrm{Gr}\text{/}{\mathrm{CrI}}_3\right)$. Our first-principles calculations show that the system forms a van der Waals (vdW) heterostructure, which is relatively facilely fabricated in experiments. A Chern insulating state is acquired in the $\mathrm{Gr}\text{/}{\mathrm{CrI}}_3$ heterostructure if the vdW gap is compressed to a distance between about 3.3 and 2.4 Å, corresponding to a required external pressure between about 1.4 and 18.3 GPa. Amazingly, very strong magnetization (about 150 meV) is found in graphene, induced by the substrate ${\mathrm{CrI}}_3$, despite the vdW interactions between them. A low-energy effective model is employed to understand the mechanism. The work functions, contact types, and band alignments of the $\mathrm{Gr}\text{/}{\mathrm{CrI}}_3$ heterostructure system are also studied. Our work demonstrates that the $\mathrm{Gr}\text{/}{\mathrm{CrI}}_3$ heterostructure is a promising system to observe the quantum anomalous Hall effect at high temperatures (up to 45 K) in experiments.

We use femtosecond x-ray diffraction to study the structural response of charge and orbitally ordered ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ thin films across a phase transition induced by 800 nm laser pulses. By investigating the dynamics of both superlattice reflections and regular Bragg peaks, we disentangle the different structural contributions and analyze their relevant timescales. The dynamics of the structural and charge order response are qualitatively different when excited above and below a critical fluence $f_c$. For excitations below $f_c$ the charge order and the superlattice is only partially suppressed and the ground state recovers within a few tens of nanosecond via diffusive cooling. When exciting above the critical fluence the superlattice vanishes within approximately half a picosecond followed by a change of the unit cell parameters on a 10 picoseconds timescale. At this point all memory from the symmetry breaking is lost and the recovery time increases by many order of magnitudes due to the first order character of the structural phase transition.

Using polarized neutron reflectometry, we measured the influence of elastic bending stress on the magnetization depth profile of a $\mathrm{L}{\mathrm{a}}_{0.8}\mathrm{S}{\mathrm{r}}_{0.2}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) epitaxial film grown on a $\mathrm{SrTi}{\mathrm{O}}_3$ substrate. The elastic bending strain of $\pm 0.03\%$ has no obvious effect on the magnetization depth profile at saturation. This result is in stark contrast to that of $(\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{P}{\mathrm{r}}_x{)}_{1-y}\mathrm{C}{\mathrm{a}}_y\mathrm{Mn}{\mathrm{O}}_3$ (LPCMO) films for which strain of $\pm 0.01\%$ produced dramatic changes in the magnetization profile and Curie temperature. We attribute the difference between the influence of strain on the saturation magnetization in LSMO (weak or none) and LPCMO (strong) to a difference in the ability of LSMO (weak or none) and LPCMO (strong) to phase separate. Our observation provides an upper limit of tuning LSMO saturation magnetization via elastic strain effect.

We investigate the laser-induced spin precession dynamics in ferromagnetic $\mathrm{N}{\mathrm{i}}_{0.8}\mathrm{F}{\mathrm{e}}_{0.2}$ thin films by using a time-resolved magneto-optical Kerr effect (TR-MOKE) measurement with two pump pulses. By changing the time interval between the pump pulses, the amplitude of the TR-MOKE signal is strongly modulated according to the phase of the TR-MOKE signal at the time when the second pulse arrives. The change in the oscillation amplitude can be explained with the temporal evolution of the spin precession induced by laser heating, as shown with a numerical simulation. We conclude that the ultrafast control of coherent spin precession dynamics is possible by multiple pump pulses via thermal excitation.

Starting from an ab initio-derived two-site dimer Hubbard Hamiltonian on a triangular lattice, we calculate the superconducting gap functions and critical temperatures for representative $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_2\mathrm{X}$ superconductors by solving the linearized Eliashberg equation using the two-particle self-consistent approach (TPSC) extended to multisite problems. Such an extension allows for the inclusion of molecule degrees of freedom in the description of these systems. We present both benchmarking results for the half-filled dimer model as well as detailed investigations for the 3/4-filled molecule model. Remarkably, we find in the latter model that the phase boundary between the two most competing gap symmetries discussed in the context of these materials—$d_{xy}$ and the recently proposed eight-node $s+d_{x^2-y^2}$ gap symmetry—is located within the regime of realistic model parameters and is especially sensitive to the degree of in-plane anisotropy in the materials as well as to the value of the on-site Hubbard repulsion. We show that these results provide a more complete and accurate description of the superconducting properties of $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_2\mathrm{X}$ than previous random phase approximation (RPA) calculations and, in particular, we discuss predicted critical temperatures in comparison to experiments. Finally, our findings suggest that it may be even easier to experimentally switch between the two pairing symmetries as previously anticipated by invoking pressure, chemical doping, or disorder effects.

Single-crystal x-ray diffraction studies with synchrotron radiation on the honeycomb iridate $\alpha -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ reveal a pressure-induced structural phase transition with symmetry lowering from monoclinic to triclinic at a critical pressure of $P_c=3.8$ GPa. According to the evolution of the lattice parameters with pressure, the transition mainly affects the $ab$ plane and thereby the Ir hexagon network, leading to the formation of Ir-Ir dimers. These observations are independently predicted and corroborated by our ab initio density functional theory calculations where we find that the appearance of Ir-Ir dimers at finite pressure is a consequence of a subtle interplay between magnetism, correlation, spin-orbit coupling, and covalent bonding. Our results further suggest that at $P_c$ the system undergoes a magnetic collapse. Finally we provide a general picture of competing interactions for the honeycomb lattices $A_{2}M{\mathrm{O}}_3$ with $A=\text{Li}$, Na and $M=\text{Ir}$, Ru.

A well-known feature of the ${\mathrm{CaFe}}_2{\mathrm{As}}_2$-based superconductors is the pressure-induced collapsed tetragonal phase that is commonly ascribed to the formation of an interlayer As–As bond. Using detailed x-ray scattering and spectroscopy, we find that Rh-doped ${\mathrm{Ca}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{Fe}}_2{\mathrm{As}}_2$ does not undergo a first-order phase transition and that local Fe moments persist despite the formation of interlayer As–As bonds. Our density functional theory calculations reveal that the Fe–As bond geometry is critical for stabilizing magnetism and the pressure-induced drop in the $c$ lattice parameter observed in pure ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ is mostly due to a constriction within the FeAs planes. The collapsed tetragonal phase emerges when covalent bonding of strongly hybridized Fe $3d$ and As $4p$ states completely wins out over their exchange splitting. Thus the collapsed tetragonal phase is properly understood as a strong covalent phase that is fully nonmagnetic with the As–As bond forming as a by-product.

We use a combination of symmetry analysis and high-throughput density functional theory calculations to search for new ferroelectric materials. We use two search strategies to identify candidate materials. In the first strategy, we start with nonpolar materials and look for unrecognized energy-lowering polar distortions. In the second strategy, we consider polar materials and look for related higher symmetry structures. In both cases, if we find new structures with the correct symmetries that are also close in energy to experimentally known structures, then the material is likely to be switchable in an external electric field, making it a candidate ferroelectric. We find 16 candidate materials, with a variety of properties that are rare in typical ferroelectrics, including large polarization, hyperferroelectricity, antiferroelectricity, and multiferroism.