We study superconductivity in a Weyl semimetal with a tilted dispersion around two Weyl points of opposite chirality. In the absence of any tilt, the state with zero momentum pairing between two Fermi sheets enclosing each Weyl point has four point nodes in the superconducting gap function. Moreover, the surface of the superconductor hosts Fermi arc states and Majorana flat bands. We show that a quantum phase transition occurs at a critical value of the tilt, at which two gap nodes disappear by merging at the center of the first Brillouin zone, or by escaping at its edges, depending on the direction of the tilt. The region in the momentum space that the Majorana flat band occupies is found to increase as the tilt parameter is made larger. Additionally, the superconducting critical temperature and electronic specific heat can be enhanced in the vicinity of the quantum phase transition due to the singularity in the electronic density of states.

Strongly correlated electrons delocalized on one-dimensional (1D) soft stacks govern the complex physics of mixed stack charge-transfer crystals, a well-known family of materials composed of electron-donor (D) and acceptor (A) molecules alternating along the 1D chain. The complex physics of these systems is well captured by a modified Hubbard model that also accounts for the coupling of electrons to molecular and lattice vibrational modes and for three-dimensional electrostatic interactions. Here we study several experimental systems to estimate relevant model parameters via density-functional theory calculations on DA units and isolated molecules and ions. Electrostatic intermolecular interactions, an important quantity not just to define the degree of charge transfer of the ground state but also to predict the propensity of the system towards multistability and hence towards discontinuous phase transitions, are also addressed. Results compare favorably with experimental data.

Using resonant angle-resolved photoemission spectroscopy and electron band-structure calculations, we explore the electronic structure and properties of Sm atoms at the surface and in the bulk of the antiferromagnet ${\mathrm{SmRh}}_2{\mathrm{Si}}_2$. We show that the Sm atoms reveal weak mixed-valent behavior both in the bulk and at the surface. Although trivalent $4f$ emission strongly dominates, a small divalent $4f$ signal near the Fermi energy can be clearly resolved for surface and bulk Sm atoms. This behavior is quite different to most other Sm-based materials which typically experience a surface valence transition to a divalent state of Sm atoms at the surface. This phenomenon is explained in analogy to the isostructural Ce compound, where strong $4f$ hybridization stabilizes mixed-valent ground state both in the bulk and at the surface, and which were described in the light of the single-impurity Anderson model. Implications for other $\mathit{RE}{\mathrm{Rh}}_2{\mathrm{Si}}_2$ $(\mathit{RE}$ = rare-earth elements) compounds are discussed.

The prediction of the quantum spin Hall effect in two-dimensional $\mathrm{ZrT}{\mathrm{e}}_5$ and $\mathrm{HfT}{\mathrm{e}}_5$ monolayers has generated considerable recent interest in these materials. In addition, Dirac points and chiral-anomaly-induced negative longitudinal magnetoresistance (MR) have been observed in $\mathrm{ZrT}{\mathrm{e}}_5$. Herein, we study the transport behavior of $\mathrm{HfT}{\mathrm{e}}_5$ single crystals in detail and find a large $(0.9\times {10}^4\%)$ out-of-plane transverse MR, which is among the highest for layered materials reported thus far. We also encounter a large linear in-plane transverse (magnetic field $\parallel c$, current $\parallel a)$ MR and discuss its possible origin. The mass anisotropy $(\gamma =5.4$ at 125 K) obtained by scaling angle-dependent MR data is consistent with the $\mathrm{HfT}{\mathrm{e}}_5$ elliptical Fermi surface.

We study the optical properties of quantum dipole emitters coupled to hyperbolic metamaterial nanoresonators using a semianalytical quasinormal mode approach. We show that coupling to metamaterial nanoresonators can lead to significant Purcell enhancements that are nearly an order of magnitude larger than those of plasmonic resonators with comparable geometry. However, the associated single-photon output $\beta$-factors are extremely low (around 10%), far smaller than those of comparably sized metallic resonators (70%). Using a quasinormal mode expansion of the photon Green function, we describe how the low $\beta$-factors are due to increased Ohmic quenching arising from redshifted resonances, larger quality factors, and stronger confinement of light within the metal. These results explain why hyperbolic metamaterial nanostructures likely make poor choices for single-photon sources.

Edge states in quantum spin Hall insulators have important applications in low-dissipation devices. However, the breaking of the valley degree of freedom for finite-sized nanoribbons violates the formation of gapless valley edge states. In this work, we investigate the topological and transport properties in a hybrid silicene nanoribbon, two halves of which are in different topological phases modulated by external perpendicular electric fields and antiferromagnetic exchange fields independently. By observing the inner-edge states, different band insulators, especially the quantum spin and valley Hall insulators, can be distinguished. Special inner-edge states, such as single valley or spin-valley channels, cause the valley thermal rectification effect, which can be used to design topological thermal diodes. Furthermore, we calculate the thermoelectric performance of inner-edge states with a dual scattering time model and find an enhancement compared with the original topological edge states at a higher temperature. These spin- and valley-resolved inner-edge states may facilitate more applications in topological valley electronics and thermoelectronics.

We present the theory of a new type of topological quantum order which is driven by the spin-orbit density wave order parameter, and distinguished by a $Z_2$ topological invariant. We show that when two oppositely polarized chiral bands [resulting from the Rashba-type spin-orbit coupling ${\alpha }_k$; $k$ is crystal momentum] are significantly nested by a special wave vector $\mathbf{Q}\sim (\pi ,0)/(0,\pi )$, it induces a spatially modulated inversion of the chirality (${\alpha }_{k+Q}={\alpha }_k^{*}$) between different sublattices. The resulting quantum order parameters break translational symmetry, but preserve time-reversal symmetry. It is inherently associated with a $Z_2$-topological invariant along each density wave propagation direction. Hence it gives a weak topological insulator in two dimensions, with even number of spin-polarized boundary states. This phase is analogous to the quantum spin Hall state, except here the time-reversal polarization is spatially modulated, and thus it is dubbed quantum spin Hall density wave (QSHDW) state. This order parameter can be realized or engineered in quantum wires, or quasi-two-dimensional systems, by tuning the spin-orbit coupling strength and chemical potential to achieve the special nesting condition.

The microwave-induced resistance oscillations were studied in a number of GaAs/AlGaAs quantum wells and heterojunctions of various electron sheet densities $n$. The effective mass $m^{*}$ extracted from the oscillations period demonstrated strong nonmonotonic dependence on $n$. At low densities $m^{*}$ was greatly enhanced (heavier than the cyclotron mass) and abruptly decreased with increasing $n$. Such behavior exhibited by the effective mass at low $n$ is clearly of many-particle origin. The minimal values of $m^{*}$ were observed at moderate densities and were lower than the cyclotron mass in full consistency with earlier publications by other groups. The increase of the effective mass observed at high densities can be ascribed to the nonparabolicity of the conduction band.

We present results from point-contact spectroscopy of the antiferromagnetic heavy-fermion superconductor $\mathrm{UP}{\mathrm{d}}_2\mathrm{A}{\mathrm{l}}_3$: Conductance spectra are taken from single crystals with two major surface orientations as a function of temperature and magnetic field and analyzed using a theory of cotunneling into an Anderson lattice. Spectroscopic signatures are clearly identified, including the distinct asymmetric double-peak structure arising from the opening of a hybridization gap when a coherent heavy-Fermi liquid is formed. Both the hybridization gap, found to be $7.2\pm 0.3\mathrm{meV}$ at 4 K, and the conductance enhancement above a flat background decrease upon increasing temperature. While the hybridization gap is extrapolated to remain finite up to ∼28 K, close to the temperature around which the magnetic susceptibility displays a broad peak, the conductance enhancement vanishes at ∼18 K, slightly above the antiferromagnetic transition temperature $(T_{\mathrm{N}}\approx 14\mathrm{K})$. This rapid decrease of the conductance enhancement is understood as a consequence of the junction drifting away from the ballistic regime due to increased scattering off magnons associated with the localized U $5f$ electrons. This shows that while the hybridization gap opening is not directly associated with the antiferromagnetic ordering, its visibility in the conductance is greatly affected by the temperature-dependent magnetic excitations. Our findings are not only consistent with the $5f$ dual-nature picture in the literature but also shed new light on the interplay between the itinerant and localized electrons in $\mathrm{UP}{\mathrm{d}}_2\mathrm{A}{\mathrm{l}}_3$.

Composite and janus type metallodielectric nanoparticles are increasingly considered as a means to control the spatial and temporal behavior of electromagnetic fields in diverse applications such as coupling to quantum emitters, achieving invisibility cloaks, and obtaining quantum correlations between qubits. We investigate the surface modes of a toroidal nanostructure and obtain the canonical plasmon dispersion relations and resonance modes for arbitrarily layered nanorings. Unlike particle plasmon eigenmodes in other geometries, the amplitudes of the eigenmodes of tori exhibit a distinct forward and backward coupling. We present the plasmon dispersion relations for several relevant toroidal configurations in the quasistatic limit and obtain the dominant retarded dispersion relations of a single ring for comparison, discuss mode complementarity and hybridization, and introduce two new types of toroidal particles in the form of janus nanorings. The resonance frequencies for the first few dominant modes of a ring composed of plasmon supporting materials such as gold, silver, and aluminum are provided and compared to those for a silicon ring. A generalized Green's function is obtained for multilayer tori allowing for calculation of the scattering response to interacting fields. Employing the Green's function, the scalar electric potential distribution corresponding to individual poloidal and toroidal modes in response to an arbitrarily polarized external field and the field of electrons is obtained. The results are applied to obtain the local density of states and decay rate of a dipole near the center of the torus.

Oxide films play a significant role in a wide range of industrial fields, mostly due to the thickness-dependent variation of their properties. Recently, it has been proposed based on the experimental study that carrier transport in CaO films proceeds via strong phonon excitations with a variable signal depending on the film thickness. In this paper, we report a detailed investigation in the frame of the density functional theory of structural and electronic properties of freestanding and Mo(100)-supported CaO films, as well as phonons therein, as functions of the film thickness and intensity of the external static electric field. Our calculations demonstrate that phonon frequencies negligibly depend on the external electric field. A small gradual increase of the energy of CaO phonons upon increase of the film thickness was found to be in line with earlier experimental findings. The effect of Mo support was observed in the systematic decrease of the energy of phonons. The applied electric field showed a minor effect on the structure of CaO films, whereas electronic properties of the oxide were significantly affected. In particular, the band gap of CaO films was found to gradually decrease with the growing intensity of the external electric field, while the effect is a more pronounced for thicker films. Overall, our paper provides innovative insights into the mechanism of electron transport and electronic properties of CaO films that might lead to new potential applications of oxide materials.

We describe designs for the realization of topological Majorana qubits in terms of proximitized topological insulator nanoribbons pierced by a uniform axial magnetic field. This platform holds promise for particularly robust Majorana bound states, with easily manipulable interstate couplings. We propose proof-of-principle experiments for initializing, manipulating, and reading out Majorana box qubits defined in floating devices dominated by charging effects. We argue that the platform offers design advantages which make it particularly suitable for extension to qubit network structures realizing a Majorana surface code.

Generation of topological phases of matter with SU(3) symmetry in a condensed-matter setup is challenging due to the lack of an intrinsic threefold chirality of quasiparticles. We uncover two salient ingredients required to express a three-component lattice Hamiltonian in a SU(3) format with a nontrivial topological invariant. We find that all three SU(3) components must be entangled via a gauge field, with opposite chirality between any two components, and there must be band inversions between all three components in a given eigenstate. For spinless particles, we show that such chiral states can be obtained in a tripartite lattice with three inequivalent lattice sites in which the Bloch phase associated with the nearest-neighbor hopping acts as the $k$-space gauge field. The second and a more crucial criterion is that there must also be an odd-parity Zeeman-like term, i.e., $sin\left(k\right){\sigma }_z$ term, where ${\sigma }_z$ is the third Pauli matrix defined in any two components of the SU(3) basis. Solving the electron-photon interaction term in a periodic potential with a modified tight-binding model, we show that such a term can be engineered with site-selective photon polarization. Such site-selective polarization can be obtained in multiple ways, such as using the Sisyphus cooling technique, polarizer plates, etc. With the $k$-resolved Berry curvature formalism, we delineate the relationship between the SU(3) chirality, band inversion, and $k$-space monopoles, governing the finite Chern number without breaking the time-reversal symmetry. The topological phase is affirmed by edge-state calculation, obeying the bulk-boundary correspondence.

We have measured the Hall effect, magnetotransport, and magnetostriction on the field-induced phases of single-crystalline ${\mathrm{UPt}}_2{\mathrm{Si}}_2$ in magnetic fields up to 60 T at temperatures down to 50 mK, firmly establishing the phase diagram for magnetic fields $B\parallel a$ and $c$ axes. Moreover, for the $B\parallel c$ axis we observe strong changes in the Hall effect at the phase boundaries. From a comparison to band structure calculations utilizing the concept of a dual nature of the uranium $5f$ electrons, we propose that these represent field-induced topological changes of the Fermi surface due to at least one Lifshitz transition. Furthermore, we find a unique history dependence of the magnetotransport and magnetostriction data, indicating that the proposed Lifshitz-type transition is of a discontinuous nature, as predicted for interacting electron systems.

We consider theoretically domain wall motion driven by spin-orbit and spin Hall torques. We find that it is possible to achieve universal absence of Walker breakdown for all spin-orbit torques using experimentally relevant spin-orbit coupling strengths. For spin-orbit torques other than the pure Rashba spin-orbit torque, this gives a linear current-velocity relation instead of a saturation of the velocity at high current densities. The effect is very robust and is found in both soft and hard magnetic materials, as well as in the presence of the Dzyaloshinskii-Moriya interaction and in coupled domain walls in synthetic antiferromagnets, where it leads to very high domain wall velocities. Moreover, recent experiments have demonstrated that the switching of a synthetic antiferromagnet does not obey the usual spin Hall angle dependence, but that domain expansion and contraction can be selectively controlled toggling only the applied in-plane magnetic field magnitude and not its sign. We show that the combination of spin Hall torques and interlayer exchange coupling produces the necessary relative velocities for this switching to occur.

We present a general method for constructing effective field theories for nonrelativistic superfluids, generalizing the previous approaches of Greiter, Witten, and Wilczek, and Son and Wingate to the case of several superfluids in solution. We investigate transport in mixtures with broken parity and find a parity-odd “Hall drag” in the presence of independent motion as well as a pinning of mass, charge, and energy to sites of nonzero relative velocity. Both effects have a simple geometric interpretation in terms of the signed volumes and directed areas of various subcomplexes of a “velocity polyhedron”: the convex hull formed by the end points of the velocity vectors of a superfluid mixture. We also provide a simple quasi-one-dimensional model that exhibits nonzero Hall drag.

A model describing the ferromagnetic resonance of multilayer structures is used to characterize the interface anisotropy constant and interlayer exchange coupling strength associated to individual components of ${[\mathrm{CoFeB}/\mathrm{Pd}]}_n$ multilayers with perpendicular magnetic anisotropy and ${[\mathrm{CoFeB}/\mathrm{Pd}]}_5$/(CoFeB or Co) exchange spring structures by comparing with ferromagnetic resonance behavior measurements. We find that the effective perpendicular anisotropy increases with the number of repetitions of the multilayer, which we could explain only after accounting for a different anisotropy at the bottom repetition of the multilayer with perpendicular anisotropy. Similarly, the characterization of the exchange coupling in our structures was only possible after accounting for individual components, thus portraying the importance of using a multilayer model to properly describe the magnetic behavior and properties of a multilayer structure. We find that the perpendicular anisotropy constant in amorphous Pd/CoFeB/Pd structures increases slightly from 0.295 to $0.315\mathrm{mJ}/{\mathrm{m}}^2$ when increasing the thickness of the CoFeB from 3 to 4 Å. Furthermore, we find that the exchange coupling in CoFeB/Pd(10 Å)/CoFeB structures decreases from 4.899 to $3.268\mathrm{mJ}/{\mathrm{m}}^2$ when increasing the thickness of the CoFeB from 3 to 4 Å. Finally, we find that the magnitude of the anisotropy at Co/Pd interfaces is 65% larger than at CoFeB/Pd interfaces, and the exchange coupling at CoFeB/Pd/Co interfaces decreases approximately 30% when compared to a CoFeB/Pd/CoFeB structure.