In strongly correlated materials, the presence of defects may fundamentally affect both macroscopic and microscopic properties of the system. Here we investigate the effect of Gd substitution in the Kondo antiferromagnet ${\mathrm{CeRhIn}}_5$. At a concentration of only 11% Gd, the magnetic anisotropy of ${\mathrm{CeRhIn}}_5$ is drastically reduced at high temperatures, whereas its antiferromagnetic transition, $T_N^{\mathrm{Ce}}=3.8$ K, is linearly suppressed with Gd at low temperatures. The extrapolation of $T_N^{\mathrm{Ce}}$ to zero temperature occurs at a critical concentration of $x_c\sim 0.63$, which is above the two-dimensional percolation limit found in ${\mathrm{Ce}}_{1-x}{\mathrm{La}}_x{\mathrm{RhIn}}_5$ ($x_c\sim 0.4$), but very close to the three-dimensional percolation limit found in ${\mathrm{Gd}}_{1-x}{\mathrm{La}}_x{\mathrm{RhIn}}_5$ and ${\mathrm{Ce}}_{1-x}{\mathrm{La}}_x{\mathrm{In}}_3$ ($x_c\sim 0.65$), in agreement with the more isotropic magnetic response.

We have investigated the in-gap bound states (IGBS) induced by a single nonmagnetic impurity in multiband superconductors with incipient bands. Contrary to the naive expectation, we found that even if the superconducting (SC) order parameter is a sign-preserving $s$ wave on the Fermi surfaces, the incipient bands may still affect the appearance and locations of the IGBS, although the gap between the incipient bands and the Fermi level is much larger than the SC gap. Therefore in scanning tunneling microscopy experiments, the IGBS induced by a single nonmagnetic impurity are not the definitive evidence for the sign-changing order parameter on the Fermi surfaces. Our findings have special implications for the experimental determination of the pairing symmetry in the FeSe-based superconductors.

Half-metallicity and low magnetic damping are perpetually sought for spintronics materials, and full Heusler compounds in this respect provide outstanding properties. However, it is challenging to obtain the well-ordered half-metallic phase in as-deposited full Heusler compound thin films, and theory has struggled to establish a fundamental understanding of the temperature-dependent Gilbert damping in these systems. Here we present a study of the temperature-dependent Gilbert damping of differently ordered as-deposited ${\mathrm{Co}}_2\mathrm{FeAl}$ full Heusler compound thin films. The sum of inter- and intraband electron scattering in conjunction with the finite electron lifetime in Bloch states governs the Gilbert damping for the well-ordered phase, in contrast to the damping of partially ordered and disordered phases which is governed by interband electronic scattering alone. These results, especially the ultralow room-temperature intrinsic damping observed for the well-ordered phase, provide fundamental insights into the physical origin of the Gilbert damping in full Heusler compound thin films.

The Weyl semimetallic compound ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ and its hole-doped derivatives (which are achieved by substituting trivalent Eu by divalent Sr) are investigated through transport, magnetic, and calorimetric studies. The metal-insulator transition (MIT) temperature is found to get substantially reduced with hole doping, and for 10% Sr doping the composition is metallic down to temperature as low as 5 K. These doped compositions are found to violate the Mott-Ioffe-Regel condition for minimum electrical conductivity and show a distinct signature of non-Fermi liquid behavior at low temperature. The MIT in the doped compounds does not correlate with the magnetic transition point, and Anderson-Mott-type disorder-induced localization may be attributed to the ground-state insulating phase. The observed non-Fermi liquid behavior can be understood on the basis of disorder-induced distribution of the spin-orbit-coupling parameter, which is markedly different in the case of ${\mathrm{Ir}}^{4+}$ and ${\mathrm{Ir}}^{5+}$ ions.

We use low-temperature scanning tunneling microscopy in combination with angle-resolved ultraviolet and two-photon photoemission spectroscopy to investigate the interfacial electronic structure of titanyl phthalocyanine (TiOPc) on Cu(110). We show that the presence of two unique molecular adsorption configurations is crucial for a molecular-level analysis of the hybridized interfacial electronic structure. Specifically, thermally induced self-assembly exposes marked adsorbate-configuration-specific contributions to the interfacial electronic structure. The results of this work demonstrate an avenue towards understanding and controlling interfacial electronic structure in chemisorbed films even for the case of complex film structure.

The availability of large magnetic fields at international facilities and of simulated magnetic fields that can reach the flux-quantum-per-unit-area level in cold atoms calls for systematic studies of orbital effects of the magnetic field on the self-energy of interacting systems. Here we demonstrate theoretically that orbital effects of magnetic fields can be treated within single-site dynamical mean-field theory with a translationally invariant quantum impurity problem. As an example, we study the one-band Hubbard model on the square lattice using iterated perturbation theory as an impurity solver. We recover the expected quantum oscillations in the scattering rate, and we show that the magnetic fields allow the interaction-induced effective mass to be measured through the single-particle density of states accessible in tunneling experiments. The orbital effect of magnetic fields on scattering becomes particularly important in the Hofstadter butterfly regime.

We demonstrate theoretically that nodal-line semimetals (NLSs) can be realized in an fcc lattice with orbitals belonging to the same irreducible representation, such as $\{p_x,p_y,p_z\}$ or $\{d_{xy},d_{yz},d_{zx}\}$ orbitals on every lattice site. The three orbitals are divided into two subgroups in terms of the parity with respect to the mirror reflections on high-symmetry planes of the fcc lattice, which, with rotation symmetry, endows symmetry-guaranteed NL passing through $W$ points in the Brillouin zone. Depending on the parameters, there also appears an accidental NL around the $\mathrm{\Gamma }$ point. We notice that the symmetry-guaranteed NL addressed in the present work can be found in band structures of elemental solids taking the fcc structure, such as Cu, Ag, Au, In, Ga, etc., as well as opal, which is an fcc photonic crystal of ${\mathrm{SiO}}_2$ spheres. Furthermore, we clarify that the fcc lattice of Si spheres exhibits a NL in a frequency band where no other photonic band exists, which provides a unique platform to realize topological NLSs under intensive search, and can be explored for achieving slow light.

We develop an analytical theory for edge states in monolayers of transition metal dichalcogenides based on a general boundary condition for a two-band $\mathbf{kp}$ Hamiltonian in the case of uncoupled valleys. Taking into account edge spin-orbit interaction, we reveal that edge states, in general, have linear dispersion that is determined by three real phenomenological parameters in the boundary condition. In the absence of the edge spin-orbit interaction, edge states are described by a single real parameter whose sign determines whether their spectra intersect the bulk gap or not. In the former case we show that illumination by circularly polarized light results in spin- and valley-polarized photocurrent along the edge. Flow direction and spin and valley polarization of the edge photocurrent are determined by the direction of circular polarization of the illuminated light.

We theoretically investigate the optical functionality of a semiconducting quantum ring manipulated by two electrostatic lateral gates used to induce a double quantum well along the ring. The well parameters and corresponding interlevel spacings, which lie in the THz range, are highly sensitive to the gate voltages. Our analysis shows that selection rules for interlevel dipole transitions, caused by linearly polarized excitations, depend on the polarization vector angle with respect to the gates. In striking difference from the conventional symmetric double well potential, the ring geometry permits polarization-dependent transitions between the ground and second excited states, allowing the use of this structure in a three-level lasing scheme.

We present a theoretical analysis of spin-coherent electronic transport across a mesoscopic dot-cavity system. Such spin-coherent transport has been recently demonstrated in an experiment with a dot-cavity hybrid implemented in a high-mobility two-dimensional electron gas [C. Rössler et al., Phys. Rev. Lett. 115, 166603 (2015)] and its spectroscopic signatures have been interpreted in terms of a competition between Kondo-type dot-lead and molecular-type dot-cavity singlet formation. Our analysis brings forward all the transport features observed in the experiments and supports the claim that a spin-coherent molecular singlet forms across the full extent of the dot-cavity device. Our model analysis includes (i) a single-particle numerical investigation of the two-dimensional geometry, its quantum-coral–type eigenstates, and associated spectroscopic transport features, (ii) the derivation of an effective interacting model based on the observations of the numerical and experimental studies, and (iii) the prediction of transport characteristics through the device using a combination of a master-equation approach on top of exact eigenstates of the dot-cavity system, and an equation-of-motion analysis that includes Kondo physics. The latter provides additional temperature scaling predictions for the many-body phase transition between molecular- and Kondo-singlet formation and its associated transport signatures.

The complete theory of electrical conductivity of graphene at arbitrary temperature is developed with taking into account mass-gap parameter and chemical potential. Both the in-plane and out-of-plane conductivities of graphene are expressed via the components of the polarization tensor in (2+1)-dimensional space-time analytically continued to the real frequency axis. Simple analytic expressions for both the real and imaginary parts of the conductivity of graphene are obtained at zero and nonzero temperature. They demonstrate an interesting interplay depending on the values of mass gap and chemical potential. In the local limit, several results obtained earlier using various approximate and phenomenological approaches are reproduced, refined, and generalized. The numerical computations of both the real and imaginary parts of the conductivity of graphene are performed to illustrate the obtained results. The analytic expressions for the conductivity of graphene obtained in this paper can serve as a guide in the comparison between different theoretical approaches and between experiment and theory.

Recent experiments demonstrating large spin-transfer torques in topological-insulator (TI)–ferromagnetic-metal (FM) bilayers have generated a great deal of excitement due to their potential applications in spintronics. The source of the observed spin-transfer torque, however, remains unclear. This is because the large charge transfer from the FM to the TI layer would prevent the Dirac cone at the interface from being anywhere near the Fermi level to contribute to the observed spin-transfer torque. Moreover, there is still not much understanding of the impact on the Dirac cone at the interface from the metallic bands overlapping in energy and momentum, where strong hybridization could take place. Here, we build a simple microscopic model and perform first-principles-based simulations for such a TI-FM heterostructure, considering the strong hybridization and charge-transfer effects. We find that the original Dirac cone is destroyed by hybridization, as expected. Instead, we find an interface state that we dub a “descendent state” that forms near the Fermi level due to the strong hybridization with the FM states at the same momentum. Such a descendent state carries a sizable weight of the original Dirac interface state, and thus it inherits the localization at the interface and the same Rashba-type spin-momentum locking. We propose that the descendent state may be an important source of the experimentally observed large spin-transfer torque in the TI-FM heterostructure.

Hexagonal CeRhSn with paramagnetic $4f$ moments on a distorted kagome lattice displays zero-field quantum critical behavior related to geometrical frustration. We report high-resolution thermal expansion and magnetostriction measurements under multiextreme conditions such as uniaxial stress up to 200 MPa, temperatures down to 0.1 K, and magnetic fields up to 10 T. Under uniaxial stress along the $a$ direction, quantum criticality disappears and a complex magnetic phase diagram arises with a sequence of phases below 1.2 K and fields between 0 and 3 T ($\parallel a$). Since the Kondo coupling increases with stress, which alone would stabilize paramagnetic behavior in CeRhSn, the observed order arises from the release of geometrical frustration by in-plane stress.

We study interaction effects, including both long-ranged Coulomb and short-range interactions, in three-dimensional topological triple-Weyl semimetals whose triple-Weyl points are protected by crystal symmetries. By performing Wilsonian renormalization group analysis of the low-energy effective field theory of the minimal model with triple-Weyl nodes, we find that the fixed point of noninteracting triple-Weyl fermions is unstable in the presence of Coulomb interactions and flows to a nontrivial stable fixed point representing marginal Fermi liquids with anisotropic screening effects. We further discuss relevant unusual physical consequences due to the novel behavior of correlation effects in this system.

The degree of freedom (DOF) of the $K$ ($K^{\prime }$) valley in transition-metal dichalcogenides, especially molybdenum disulfide (${\mathrm{MoS}}_2$), offers an opportunity for next-generation valleytronics devices. In this work, the $K$ ($K^{\prime }$) valley DOF of multilayer ${\mathrm{MoS}}_2$ is studied by means of the photon wavelength dependent circular photogalvanic effect (CPGE) at room temperature upon a strong external out-of-plane electric field induced by an ionic liquid (IL) gate, which breaks the spatial-inversion symmetry. It is demonstrated that only on resonant excitations in the $K$ ($K^{\prime }$) valley can the valley-related CPGE signals in multilayer ${\mathrm{MoS}}_2$ with an IL gate be detected, indicating that the valley contrast is indeed regenerated between the $K$ and $K^{\prime }$ valleys when the electric field is applied. As expected, it can also be seen that the $K$ ($K^{\prime }$) valley DOF in multilayer ${\mathrm{MoS}}_2$ can be modulated by the external electric field. The observation of photon wavelength dependent valley photocurrent in multilayer ${\mathrm{MoS}}_2$, with the help of better Ohmic contacts, may pave a way for optoelectronic applications of valleytronics in the future.

Inelastic electron tunneling (IET) accompanying nuclear motion is not only of fundamental physical interest but also has strong impacts on chemical and biological processes in nature. Although excitation of rotational motion plays an important role in enhancing electric conductance at a low bias, the mechanism of rotational excitation remains veiled. Here, we present a basic theoretical framework of IET that explicitly takes into consideration quantum angular momentum, focusing on a molecular ${\mathrm{H}}_2$ rotator trapped in a nanocavity between two metallic electrodes as a model system. It is shown that orientationally anisotropic electrode-rotator coupling is the origin of angular-momentum exchange between the electron and molecule; we found that the anisotropic coupling imposes rigorous selection rules in rotational excitation. In addition, rotational symmetry breaking induced by the anisotropic potential lifts the degeneracy of the energy level of the degenerated rotational state of the quantum rotator and tunes the threshold bias voltage that triggers rotational IET. Our theoretical results provide a paradigm for physical understanding of the rotational IET process and spectroscopy, as well as molecular-level design of electron-rotation coupling in nanoelectronics.

Intervalley scattering by atomic defects in monolayer transition-metal dichalcogenides (TMDs; $M{X}_2$) presents a serious obstacle for applications exploiting their unique valley-contrasting properties. Here, we show that the symmetry of the atomic defects can give rise to an unconventional protection mechanism against intervalley scattering in monolayer TMDs. The predicted defect-dependent selection rules for intervalley scattering can be verified via Fourier transform scanning tunneling spectroscopy (FT-STS), and provide a unique identification of, e.g., atomic vacancy defects ($M$ vs $X$). Our findings put the absence of the intervalley FT-STS peak in recent experiments in a different perspective.

We studied the physical properties of ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{V}}_{13}{\mathrm{O}}_{18}$ single crystals, which exhibit three different phases caused by the orbital and charge degrees of freedom in V $t_{2g}$ states. We found from the magnetic susceptibility and the NMR measurement that the low-temperature phase of this series of compounds is characterized by the spin-singlet state. We also found from the electrical resistivity and optical conductivity spectra that these compounds become heavily anisotropic in the low-temperature phase with V trimerization, indicating that the orbital ordering of V $t_{2g}$ states causes the anisotropy of the electronic structure of this series of compounds.