We investigated the evolution of magnetism in the $\mathrm{UI}{\mathrm{r}}_{1-x}\mathrm{R}{\mathrm{h}}_x\mathrm{Ge}$ system by the systematic study of high-quality single crystals. Lattice parameters of both parent compounds are very similar, resulting in almost identical nearest interatomic uranium distances close to the Hill limit. We established the $x\text{−}T$ phase diagram of the $\mathrm{UI}{\mathrm{r}}_{1-x}\mathrm{R}{\mathrm{h}}_x\mathrm{Ge}$ system and found a discontinuous antiferromagnetic/ferromagnetic boundary at $x_{\mathrm{crit}}=0.56$, where a local minimum in ordering temperature and maximum of the Sommerfeld coefficient $\gamma \approx 175\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$ occurs in the UCoGe-URhGe-UIrGe system, signaling an increase in magnetic fluctuation. However, a quantum critical point is not realized because of the finite ordering temperature at $x_{\mathrm{crit}}$. A magnon gap on the antiferromagnetic side abruptly suppresses magnetic fluctuations. We find a field-induced first order transition in the vicinity of the critical magnetic field along the $b$ axis ($H_{b,\mathrm{crit}}$) in the entire $\mathrm{UI}{\mathrm{r}}_{1-x}\mathrm{R}{\mathrm{h}}_x\mathrm{Ge}$ system, including the ferromagnetic region $\sim \mathrm{UC}{\mathrm{o}}_{0.6}\mathrm{R}{\mathrm{h}}_{0.4}\mathrm{Ge}\text{−}\mathrm{URhGe}$.

Lattice thermal conductivity is one of the most important thermoelectric parameters in determining the energy conversion efficiency of thermoelectric materials. However, the lattice thermal conductivity evaluation requires time-consuming first-principles (quasi)phonon calculations, which limits seeking high-performance thermoelectric materials through high-throughput computations. Here, we establish a methodology to determine the Debye temperature $\mathrm{\Theta }$, Grüneisen parameter $\gamma$, and lattice thermal conductivity $\kappa$ using computationally feasible elastic properties (the bulk and shear moduli). For 39 compounds with three different prototypes (the cubic isotropic rocksalt and zinc blende, and the noncubic anisotropic wurtzite), the theoretically calculated $\mathrm{\Theta },\gamma$, and $\kappa$ are in reasonable agreement with those determined using (quasi)harmonic phonon calculations or experimental measurements. Our results show that the methodology is an efficient tool to predict the anharmonicity and the lattice thermal conductivity.

Surface magnetism is analyzed by spin-dependent ${\mathrm{He}}^{+}$-ion neutralization (the Auger neutralization) in the vicinity of a surface using an electron spin-polarized low-energy ${\mathrm{He}}^{+}$-ion beam [spin-polarized ion scattering spectroscopy (SP-ISS)]. Recently, spin-orbit coupling (SOC) has been found to act as another mechanism of spin-dependent low-energy ${\mathrm{He}}^{+}$-ion scattering. Thus, it is crucial for surface magnetism analyses by SP-ISS to separate those two mechanisms. In the present study, we investigated the spin-induced asymmetry in scattering of low-energy ${\mathrm{He}}^{+}$ ions on ultrathin Au and Sn films as well as the oxygen adsorbate on a magnetized-Fe(100) surface where these two mechanisms may coexist. We found that the Fe surface magnetism immediately disappeared with the growth of those overlayers. On the other hand, we observed no induced spin polarization in the Au and Sn thin films even in the very initial stage of the growth. We also observed that the spin asymmetry of the O adsorbate was induced by the magnetism of the underlying Fe substrate. The present study demonstrates that the two mechanisms of the spin-asymmetric ${\mathrm{He}}^{+}$-ion scattering (the ion neutralization and SOC) can be separated by an azimuthal-angle-resolved SP-ISS measurement.

We present an analytical tight-binding theory of the optical properties of graphene nanoribbons with zigzag edges. Applying the transfer matrix technique to the nearest-neighbor tight-binding Hamiltonian, we derive analytical expressions for electron wave functions and optical transition matrix elements for incident light polarized along the structure axis. It follows from the obtained results that optical selection rules result from the wave function parity factor ${(-1)}^J$, where $J$ is the band number. These selection rules are that $\mathrm{\Delta }J$ is odd for transitions between valence and conduction subbands and that $\mathrm{\Delta }J$ is even for transitions between only valence (conduction) subbands. Although these selection rules are different from those in armchair carbon nanotubes, there is a hidden correlation between absorption spectra of the two structures that should allow one to use them interchangeably in some applications. The correlation originates from the fact that van Hove singularities in the tubes are centered between those in the ribbons if the ribbon width is about a half of the tubes circumference. The analysis of the matrix elements dependence on the electron wave vector for narrow ribbons shows a smooth nonsingular behavior at the Dirac points and the points where the bulk states meet the edge states.

The recent experiment of Stettenheim et al. showed that, contrary to conventional belief, the coupling of a quantum electronic device to its substrate can have important effects on the noise power spectrum, since the substrate functions as a mechanical oscillator. We carry out a theoretical analysis of this coupling in the case of a quantum point contact (QPC). First we derive the noise power spectrum from the Hamiltonian without making the Markovian approximation, and obtain numerical results that reproduce the experimental data. Next we investigate the nature of the coupling. In most previous analyses, the coupling of an electronic device to a mechanical oscillator has been modeled as a position coupling. We model it both as a position coupling and as a momentum coupling and compare the results. We find that, as long as one includes backaction between position and momentum, the assumed mode of coupling makes little difference, since the backaction transmits momentum fluctuations to position fluctuations and vice versa. Finally, we ask whether the salient features of the model persist in the Markovian approximation. We find that a Markovian analysis confirms the QPC-substrate coupling, but underestimates the noise floor and leads to excessively sharp and narrow noise peaks around the resonant frequencies.

Commensurate magnetoresistance periodic oscillations generated by transversal electron snake orbits are found experimentally. A two-dimensional electron gas is exposed to a magnetic field that changes sign along the current longitudinal direction and is homogeneous in the transverse direction. The change in sign of the magnetic field directs the electron flow along the transversal direction, in snake orbits. This generates resistance oscillations with a predictable periodicity that is commensurate with the width of the electron gas. Numerical simulations are used to reveal the character of the oscillations.

Extrinsic optical chirality in a form of the asymmetric transmission of circularly polarized light at the oblique light incidence has been observed in three-dimensional opal photonic crystals assembled from monodisperse polymer spheres in the closely packed face-centered-cubic lattice. This effect has been assigned to the translation-rotation symmetry of the lattice. The cross-polarization conversion of the circularly polarized light and the related asymmetry of the transmission of the inverted circularly polarized light have been demonstrated.

Multi-Weyl semimetals are new types of Weyl semimetals which have anisotropic nonlinear energy dispersion and a topological charge larger than one, thus exhibiting a unique quantum response. Using a unified lattice model, we calculate the optical conductivity numerically in the multi-Weyl semimetal phase and in its neighboring gapped states, and obtain the characteristic frequency dependence of each phase analytically using a low-energy continuum model. The frequency dependence of longitudinal and transverse optical conductivities obeys scaling relations that are derived from the winding number of the parent multi-Weyl semimetal phase and can be used to distinguish these electronic states of matter.

Multi-Weyl semimetals (m-WSMs) are a new type of Weyl semimetal that have linear dispersion along one symmetry direction but anisotropic nonlinear dispersion along the two transverse directions with a topological charge larger than one. Using the Boltzmann transport theory and fully incorporating the anisotropy of the system, we study the dc conductivity as a function of carrier density and temperature. We find that the characteristic density and temperature dependence of the transport coefficients at the level of Boltzmann theory are controlled by the topological charge of the multi-Weyl point and distinguish m-WSMs from their linear Weyl counterparts.

Using density matrix renormalization group numerical calculations, we study the phase diagram of the half filled Bose-Hubbard system in the sawtooth lattice with strong frustration in the kinetic energy term. We focus in particular on values of the hopping terms which produce a flat band and show that, in the presence of contact and near neighbor repulsion, three phases exist: Mott insulator (MI), charge density wave (CDW), and the topological Haldane insulating (HI) phase which displays edge states and particle imbalance between the two ends of the system. We find that, even though the entanglement spectrum in the Haldane phase is not doubly degenerate, it is in excellent agreement with the entanglement spectrum of the Affleck-Kennedy-Lieb-Tasaki (AKLT) state built in the Wannier basis associated with the flat band. This emphasizes that the absence of degeneracy in the entanglement spectrum is not necessarily a signature of a nontopological phase, but rather that the (hidden) protecting symmetry involves nonlocal states. Finally, we also show that the HI phase is stable against small departure from flatness of the band but is destroyed for larger ones.

First-principles calculations are performed to investigate the properties of defect-free $\mathrm{LaBaC}{\mathrm{o}}_2{\mathrm{O}}_6$ materials as well as $\mathrm{LaBaC}{\mathrm{o}}_2{\mathrm{O}}_5$ films possessing ordered oxygen vacancy structures. These calculations predict that the defect-free $\mathrm{LaBaC}{\mathrm{o}}_2{\mathrm{O}}_6$ crystallizes in a tetragonal state with an axial ratio smaller than unity, and is metallic and ferromagnetic as consistent with the literature. In contrast, the $\mathrm{LaBaC}{\mathrm{o}}_2{\mathrm{O}}_5$ film adopts a low-symmetry monoclinic state having an axial ratio larger than 1, is insulating and even ferroelectric, as well as exhibits a G-type antiferromagnetic ordering. Our calculations further reveal the origin of these defect-induced changes in properties. Moreover, further experiments we conducted demonstrate the possibility to grow highly epitaxial $\mathrm{LaBaC}{\mathrm{o}}_2{\mathrm{O}}_5$ films with ordered oxygen vacancies, via reduction treatments, and confirm some of our predictions. These findings can open a new avenue for the design and synthesis of multiferroics via defect engineering.

We show that for a particular choice of gauge the vector potential of any nonradiating source is spatially localized along with its electric and magnetic fields. Important on its own, this special property of nonradiating sources dramatically simplifies the analysis of their quantitative aspects, and enables the interpretation of nonradiating sources as distributions of the elementary dynamic anapoles. Using the developed approach we identify and discuss a possible scenario for observing the time-dependent version of the Aharonov-Bohm effect in such systems.

We present a theory of magnetoconductivity for general three-dimensional nonmagnetic metals within the Berry-curvature-corrected semiclassical and Boltzmann framework. We find a contribution, which is intrinsic in the sense that its ratio to the zero-magnetic-field conductivity is fully determined by the intrinsic band properties, independent of the transport relaxation time, showing a clear violation of Kohler's rule. Remarkably, this contribution can generally be positive for the longitudinal configuration, providing a mechanism for the appearance of positive longitudinal magnetoconductivity besides the chiral anomaly effect.

We show that the out-of-time-order correlation (OTOC) $\langle W{\left(t\right)}^{†}V{\left(0\right)}^{†}W\left(t\right)V\left(0\right)\rangle$ in many-body localized (MBL) and marginal MBL systems can be efficiently calculated by the spectrum bifurcation renormalization group (SBRG). We find that in marginal MBL systems, the scrambling time $t_{\text{scr}}$ follows a stretched exponential scaling with the distance $d_{WV}$ between the operators $W$ and $V$: $t_{\text{scr}}\sim exp\left(\sqrt{d_{WV}/l_0}\right)$, which demonstrates Sinai diffusion of quantum information and the enhanced scrambling by the quantum criticality in nonchaotic systems.

We address the problem of superconductivity for non-Fermi liquids using two commonly adopted, yet apparently distinct, methods: (1) the renormalization group (RG) and (2) Eliashberg theory. The extent to which both methods yield consistent solutions for the low-energy behavior of quantum metals has remained unclear. We show that the perturbative RG beta function for the 4-Fermi coupling can be explicitly derived from the linearized Eliashberg equations, under the assumption that quantum corrections are approximately local across energy scales. We apply our analysis to the test case of phonon-mediated superconductivity and show the consistency of both the Eliashberg and RG treatments. We next study superconductivity near a class of quantum critical points and find a transition between superconductivity and a “naked” metallic quantum critical point with finite, critical BCS couplings. We speculate on the applications of our theory to the phenomenology of unconventional metals.

Using angle-resolved photoemission spectroscopy (ARPES), we studied bulk and surface electronic band structures of narrow-gap semiconductor lead telluride (PbTe) thin films grown by molecular beam epitaxy both perpendicular and parallel to the $\mathrm{\Gamma }\text{−}L$ direction. The comparison of ARPES data with the first-principles calculation reveals the details of band structures, orbital characters, spin-orbit splitting energies, and surface states. The photon-energy-dependent spectra show the bulk character. Both the $L$ and Σ valence bands are observed and their energy difference is determined. The spin-orbit splitting energies at $L$ and Γ points are 0.62 eV and 0.88 eV, respectively. The surface states below and close to the valence band maximum are identified. The valence bands are composed of a mixture of Pb $6s$ and Te 5 $p_z$ orbitals with dominant in-plane even parity, which is attributed to the layered distortion in the vicinity of the PbTe (111) surface. These findings provide insights into PbTe fundamental properties and shall benefit relevant thermoelectric and optoelectronic applications.

Working within the nonequilibrium Green's function formalism, a formula for the two-time current correlation function is derived for the case of transport through a nanojunction in response to an arbitrary time-dependent bias. The one-particle Hamiltonian and the wide-band limit approximation are assumed, enabling us to extract all necessary Green's functions and self-energies for the system, extending the analytic work presented previously [Ridley et al., Phys. Rev. B 91, 125433 (2015)]. We show that our expression for the two-time correlation function generalizes the Büttiker theory of shot and thermal noise on the current through a nanojunction to the time-dependent bias case including the transient regime following the switch-on. Transient terms in the correlation function arise from an initial state that does not assume (as is usually done) that the system is initially uncoupled, i.e., our approach is partition free. We show that when the bias loses its time dependence, the long-time limit of the current correlation function depends on the time difference only, as in this case an ideal steady state is reached. This enables derivation of known results for the single-frequency power spectrum and for the zero-frequency limit of this power spectrum. In addition, we present a technique which facilitates fast calculations of the transient quantum noise, valid for arbitrary temperature, time, and voltage scales. We apply this formalism to a molecular wire system for both dc and ac biases, and find a signature of the traversal time for electrons crossing the wire in the time-dependent cross-lead current correlations.