Density-functional theory (DFT) predictions of materials properties are becoming ever more widespread. With increased use comes the demand for estimates of the accuracy of DFT results. In view of the importance of reliable surface properties, this work calculates surface energies and work functions for a large and diverse test set of crystalline solids. They are compared to experimental values by performing a linear regression, which results in a measure of the predictable and material-specific error of the theoretical result. Two of the most prevalent functionals, the local density approximation (LDA) and the Perdew-Burke-Ernzerhof parametrization of the generalized gradient approximation (PBE-GGA), are evaluated and compared. Both LDA and GGA-PBE are found to yield accurate work functions with error bars below 0.3 eV, rivaling the experimental precision. LDA also provides satisfactory estimates for the surface energy with error bars smaller than 10%, but GGA-PBE significantly underestimates the surface energy for materials with a large correlation energy.

Intra-Landau level excitations in the fractional quantum Hall regime are not accessible via optical absorption measurements. We point out that optical probes are enabled by the periodic potentials produced by a moiré pattern. Our observation is motivated by the recent observations of fractional quantum Hall incompressible states in moiré-patterned graphene on a hexagonal boron nitride substrate, and is theoretically based on $f-\mathrm{sum}$ rule considerations supplemented by a perturbative analysis of the influence of the moiré potential on many-body states.

The growth and density dependence of the low temperature mobility of a series of two-dimensional electron systems confined to unintentionally doped, low extended defect density InAs quantum wells with $\mathrm{A}{\mathrm{l}}_{1-x}\mathrm{G}{\mathrm{a}}_x\mathrm{Sb}$ barriers are reported. The electron-mobility-limiting scattering mechanisms were determined by utilizing dual-gated devices to study the dependence of mobility on carrier density and electric field independently. Analysis of possible scattering mechanisms indicate the mobility was limited primarily by rough interfaces in narrow quantum wells and a combination of alloy disorder and interface roughness in wide wells at high carrier density within the first occupied electronic subband. At low carrier density, the functional dependence of mobility on carrier density provided evidence of Coulombic scattering from charged defects. A gate-tuned electron mobility exceeding $750000\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ was achieved at a sample temperature of 2 K.

The realization of quantum computers and quantum Internet requires not only quantum gates and quantum memories, but also transistors at single-photon levels to control the flow of information encoded on single photons. Single-photon transistor (SPT) is an optical transistor in the quantum limit, which uses a single photon to open or block a photonic channel. In sharp contrast to all previous SPT proposals which are based on single-photon nonlinearities, here I present a design for a high-gain and high-speed (up to THz) SPT based on a linear optical effect: giant circular birefringence induced by a single spin in a double-sided optical microcavity. A gate photon sets the spin state via projective measurement and controls the light propagation in the optical channel. This spin-cavity transistor can be directly configured as diodes, routers, DRAM units, switches, modulators, etc. Due to the duality as quantum gate and transistor, the spin-cavity unit provides a solid-state platform ideal for future Internet: a mixture of all-optical Internet with quantum Internet.

In addition to stable single-layer buckled honeycomb and washboard structures of group-V elements (or pnictogens P, As, Sb, and Bi) we show that these elements can also form two-dimensional, single-layer structures consisting of buckled square and octagon rings. An extensive analysis comprising the calculation of mechanical properties, vibration frequencies, and finite-temperature ab initio molecular dynamics confirms that these structures are dynamically and thermally stable and suitable for applications at room temperature and above. All these structures are semiconductors with a fundamental band gap, which is wide for P but decreases with increasing row number. The effect of the spin-orbit coupling decreases the band gap and is found to be crucial for Sb and Bi. These results are obtained from first-principles calculations based on density functional theory.

A joint experimental and theoretical study is presented for the electronic structures of copper oxides including $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$, CuO, and the metastable mixed-valence oxide $\mathrm{C}{\mathrm{u}}_4{\mathrm{O}}_3$. The optical band gap is determined by experimental optical absorption coefficient, and the electronic structure in valence and conduction bands is probed by photoemission and electron energy loss spectroscopies, respectively. The experimental results are compared with many-body $GW$ calculations utilizing an additional on-site potential for $d$-orbital energies that facilitates tractable and predictive computations. The side-by-side comparison between the three oxides, including a band insulator $\left(\mathrm{C}{\mathrm{u}}_2\mathrm{O}\right)$ and two Mott/charge-transfer insulators (CuO, $\mathrm{C}{\mathrm{u}}_4{\mathrm{O}}_3)$ leads to a consistent picture for the optical and band-structure properties of the Cu oxides, strongly supporting indirect band gaps of about 1.2 and 0.8 eV in CuO and $\mathrm{C}{\mathrm{u}}_4{\mathrm{O}}_3$, respectively. This comparison also points towards surface oxidation and reduction effects that can complicate the interpretation of the photoemission spectra.

We comparably performed polarized Raman scattering studies of $\mathrm{MoT}{\mathrm{e}}_2$ and $\mathrm{WT}{\mathrm{e}}_2$. By rotating crystals to tune the angle between the principal axis of the crystals and the polarization of the incident/scattered light, we obtained the angle dependence of the intensities for all the observed modes, which is perfectly consistent with careful symmetry analysis. Combining these results with first-principles calculations, we clearly identified the observed phonon modes in the different phases of both crystals. Fifteen Raman-active phonon modes $(10A_g+5B_g)$ in the high-symmetry phase $1T^{\prime }\text{−}\mathrm{MoT}{\mathrm{e}}_2$ (300 K) were well assigned, and all the symmetry-allowed Raman modes $(11A_1+6A_2)$ in the low-symmetry phase $T_d\text{−}\mathrm{MoT}{\mathrm{e}}_2$ (10 K) and 12 Raman phonons $(8A_1+4A_2)$ in $T_d\text{−}\mathrm{WT}{\mathrm{e}}_2$ were observed and identified. The present paper provides basic information about the lattice dynamics in transition-metal dichalcogenides and may shed some light on the understanding of the extremely large magnetoresistance in this class of materials.

Direct mesoscale measurements of radiation-induced changes in the mechanical properties of bulk materials remain difficult to perform. Most widely used characterization techniques are either macro- or microscale in nature, focusing on overall properties or overly small areas for analysis. Linking the atomic structure of irradiated materials directly with their radiation-affected properties remains one of the largest unmet challenges in radiation materials science. By measuring the change in surface acoustic wave speed as a function of relative orientation on metallic single crystals, we demonstrate that transient grating (TG) spectroscopy experiments have the sensitivity necessary to detect radiation-induced material property changes. We also show that classical molecular dynamics (MD) simulations can be used to accurately simulate orientation-based changes in surface acoustic wave speed in TG experiments, by comparing with experimental measurements and theoretical predictions. The agreement between theory, simulation, and experiment gives confidence in classical MD as a predictive tool to simulate defect-based changes in elastic properties, which cannot yet be fully treated by theory. This ability is of critical importance for the informed use of TG spectroscopy to measure material property changes induced by radiation damage, which may vary by amounts formerly too small for reliable in situ detection. Finally, our MD simulation framework is used to study the effect of an imposed vacancy population on the acoustic response of several materials. The results of these studies indicate that TG experiments are well suited to the ex situ and in situ study of radiation-induced material property changes.

We report a Raman study of the effect of temperature on the self-energies of optical phonons in a number of transition metals with hexagonal-close-packed structure. Anisotropic softening of phonon energies and narrowing of phonon linewidths with increasing temperature are observed. These effects are reproduced in the calculations of phonon spectral functions based on ab initio electronic structures and with carrier scattering by phonons taken into account. The combined observations and results of simulations indicate a relation between observed anomalies and the renormalization of the electron spectrum due to electron-phonon interaction. It is emphasized that the temperature dependence of the phonon energies resembles anharmonic behavior but is actually an electron-induced effect.

Low-energy magnon bands in a two-dimensional spin-ice model become integer quantum magnon Hall bands under an out-of-plane field. By calculating the localization length and the two-terminal conductance of magnon transport, we show that the magnon bands with disorders undergo a quantum phase transition from an integer quantum magnon Hall regime to a conventional magnon localized regime. Finite size scaling analysis as well as a critical conductance distribution shows that the quantum critical point belongs to the same universality class as that in the quantum Hall transition. We characterize thermal magnon Hall conductivity in a disordered quantum magnon Hall system in terms of robust chiral edge magnon transport.

The electrical Hall conductivities of any anisotropic interacting system with reflection symmetry obey ${\sigma }_{xy}=-{\sigma }_{yx}$. In contrast, we show that the analogous relation between the transverse thermoelectric Peltier coefficients, ${\alpha }_{xy}=-{\alpha }_{yx}$, does not generally hold in the same system. This fact may be traced to interaction contributions to the heat current operator and the mixed nature of the thermoelectric response functions. Remarkably, however, it appears that emergence of quasiparticles at low temperatures forces ${\alpha }_{xy}=-{\alpha }_{yx}$. This suggests that quasiparticle-free ground states (so-called non-Fermi liquids) may be detected by examining the relationship between ${\alpha }_{xy}$ and ${\alpha }_{yx}$ in the presence of reflection symmetry and microscopic anisotropy. These conclusions are based on the following results. (i) The relation between the Peltier coefficients is exact for elastically scattered noninteracting particles. (ii) It holds approximately within Boltzmann theory for interacting particles when elastic scattering dominates over inelastic processes. In a disordered Fermi liquid, the latter lead to deviations that vanish as $T^3$. (iii) We calculate the thermoelectric response in a model of weakly coupled spin-gapped Luttinger liquids and obtain strong breakdown of antisymmetry between the off-diagonal components of $\widehat{\alpha }$. We also find that the Nernst signal in this model is enhanced by interactions and can change sign as function of magnetic field and temperature.

A rich temperature-field phase diagram has been constructed for the geometrically frustrated, Ising-like heavy-fermion compound CePdAl. In the limit of zero temperature, the magnetic field applied along the Ising axis ($B\parallel c$) induces a suppression of the antiferromagnetism, resulting in a sequence of phase transitions/crossovers, rather than a single critical point, in a critical region of field 3–5 T. In the lowest-temperature region, a power-law description of resistivity yields $A·T^n$ with $n>2$ for all applied fields up to at least 14 T. A quadratic-in-temperature resistivity term, however, can be extracted when assuming an additional electron-scattering channel from magnonlike excitations, with $A$ tending to be divergent upon approaching the critical regime. Our observations suggest CePdAl to be a new playground for exotic physics arising from the competition between Kondo screening and geometrical frustration.

Using exact diagonalization and quantum Monte Carlo calculations we investigate the effects of disorder on the phase diagram of both noninteracting and interacting models of two-dimensional topological insulators. In the fermion sign problem-free interacting models we study, electron-electron interactions are described by an on-site repulsive Hubbard interaction and disorder is included via the one-body hopping operators. In both the noninteracting and interacting models we make use of recent advances in highly accurate real-space numerical evaluation of topological invariants to compute phase boundaries and in the noninteracting models determine critical exponents of the transitions. We find different models exhibit distinct stability conditions of the topological phase with respect to interactions and disorder. We provide a general analytical theory that accurately predicts these trends.

We suggest the possibility of a linear magnetochiral effect in time reversal breaking Weyl semimetals. The magnetochiral effect consists of the simultaneous linear dependence of the magnetotransport coefficients with a magnetic field and a momentum vector. This simultaneous dependence is allowed by the Onsager reciprocity relations, being the separation vector between the Weyl nodes that plays such a role. The presence of band-bending terms in the dispersion relation of generalized Weyl fermions is responsible for the magnetochiral effect. This linear magnetochiral effect constitutes a transport effect associated with the topological structures linked to time reversal breaking Weyl semimetals.

Proximity effects resulting from depositing a graphene layer on a transition-metal dichalcogenide substrate layer change the dynamics of the electronic states in graphene, inducing spin-orbit coupling and staggered potential effects. An effective Hamiltonian that describes different symmetry-breaking terms in graphene, while preserving time-reversal invariance, shows that an inverted mass band-gap regime is possible. The competition of different perturbation terms causes a transition from an inverted mass phase to a staggered gap in the bilayer heterostructure as seen in its phase diagram. A tight-binding calculation of the bilayer validates the effective model parameters. A relative gate voltage between the layers may produce such a phase transition in experimentally accessible systems. The phases are characterized in terms of Berry curvature and valley Chern numbers, demonstrating that the system may exhibit quantum spin Hall and valley Hall effects.

We theoretically investigated the dependence of higher-order harmonic generation (HHG) in solid-state materials on the ellipticity of the electric field. We found that in the multiphoton absorption and ac Zener regimes, the intensity of HHG monotonically decreases with increasing ellipticity of the incident electric field, while in the semimetal regime, the intensity reaches a maximum for finite values of ellipticity. Moreover, the characteristics of the polarization of the emitted HHG change depending on the field intensity; only parallel emissions with respect to the major axis exist in the multiphoton absorption and ac Zener regimes, while both parallel and perpendicular emissions exist in the semimetal regime. These peculiar characteristics of the semimetal regime can be understood on the basis of changes in the HHG mechanism and may be able to be identified in experiments utilizing solid-state materials such as narrow-gap semiconductors.