We report a magnetotransport study on type-II Weyl semimetal ${\mathrm{WP}}_2$ single crystals. Magnetoresistance exhibits a nonsaturating $H^n$ field dependence (14 300% at 2 K and 9 T), whereas systematic violation of Kohler's rule was observed. Quantum oscillations reveal a complex multiband electronic structure. The cyclotron effective mass close to the mass of free electron $m_e$ was observed in quantum oscillations along the $b$ axis, while a reduced effective mass of about $0.5m_e$ was observed in $a$-axis quantum oscillations, suggesting Fermi surface anisotropy. The temperature dependence of the resistivity shows a large upturn that cannot be explained by the multiband magnetoresistance of conventional metals. Even though the crystal structure of ${\mathrm{WP}}_2$ is not layered as in transition-metal dichalcogenides, quantum oscillations suggest partial two-dimensional character.

Weyl type-II fermions are massless quasiparticles that obey the Weyl equation and which are predicted to occur at the boundary between electron and hole pockets in certain semimetals, i.e., the $({\mathrm{W},\mathrm{Mo})(\mathrm{Te},\mathrm{P})}_2$ compounds. Here, we present a study of the Fermi surface of ${\mathrm{WP}}_2$ via the Shubnikov-de Haas (SdH) effect. Compared to other semimetals, ${\mathrm{WP}}_2$ exhibits a very low residual resistivity, i.e., ${\rho }_0\simeq 10\mathrm{n}\mathrm{\Omega }\mathrm{cm}$, which leads to perhaps the largest nonsaturating magnetoresistivity $\left[\rho \right(H\left)\right]$ reported for any compound. For the samples displaying the smallest ${\rho }_0,\rho \left(H\right)$ is observed to increase by a factor of $2.5\times {10}^7\%$ under ${\mu }_{0}H=35$ T at $T=0.35$ K. The angular dependence of the SdH frequencies is found to be in excellent agreement with the first-principles calculations when the electron and hole bands are shifted by 30 meV with respect to the Fermi level. This small discrepancy could have implications for the predicted topological character of this compound.

We report on Hall field-induced resistance oscillations (HIROs) in a 60-nm-wide GaAs/AlGaAs quantum well with an in situ grown back gate, which allows tuning the carrier density $n$. At low $n$, when all electrons are confined to the lowest subband (SB1), the HIRO frequency, proportional to the product of the cyclotron diameter and the Hall field, scales with $n^{-1/2}$ as expected. Remarkably, the population of the second subband (SB2) significantly enhances the HIROs, whereas their frequency now scales as $n^{-1}$. We demonstrate that in this two-subband regime HIROs still originate solely from backscattering of SB1 electrons. The unusual density dependence occurs because the population of SB2 steadily increases, whereas that of SB1 remains essentially unchanged. The enhancement of the HIROs manifests an unexpected steplike increase in the quantum lifetime of SB1 electrons, which reaches a record value of 52 ps in the two-subband regime.

We investigate thermoelectric transport through a $\text{SU}\left(N\right)$ quantum impurity in the Kondo regime. The strong-coupling fixed-point theory is described by the local Fermi-liquid paradigm. Using Keldysh technique we analyze the electric current through the quantum impurity at both finite bias voltage and finite temperature drop across it. The theory of a steady state at zero current provides a complete description of the Seebeck effect. We find pronounced nonlinear effects in temperature drop at low temperatures. We illustrate the significance of the nonlinearities for enhancement of thermopower by two examples of SU(4) symmetric regimes characterized by a filling factor $m$: (i) particle-hole symmetric at $m=2$ and (ii) particle-hole nonsymmetric at $m=1$. We analyze the effects of potential scattering and coupling asymmetry on the transport coefficients. We discuss connections between the theory and transport experiments with coupled quantum dots and carbon nanotubes.

A thin film superconductor $\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{L}{\mathrm{a}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ is the first electron-doped compound of $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ obtained by the substitution of Ba with La within a blocking layer. By contrast, in conventional electron-doped $\mathrm{Ba}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$, the dopant (Co) is inserted into the conduction layer. The different shapes, or geometries, of $\mathrm{FeA}{\mathrm{s}}_4$ are expected in the two compounds above. However, the structure of $\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{L}{\mathrm{a}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ is investigated here because of being a thin film. To clarify the effect of the geometry, we make use of an up-to-date method to determine the atomic positions of the $\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{L}{\mathrm{a}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ thin film using synchrotron x-ray diffraction. We established that the $\mathrm{FeA}{\mathrm{s}}_4$ geometry in $\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{L}{\mathrm{a}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ indicates a systematic variation upon doping, which is opposite to that in $\mathrm{Ba}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$. However, the superconducting transition temperatures nearly coincide with each other for the same amount of dopings. The present result contrasts with a suggestion that the $\mathrm{FeA}{\mathrm{s}}_4$ geometry strongly influences the superconductivity in iron pnictides. Hence, we propose that the carrier density is the more important parameter than the $\mathrm{FeA}{\mathrm{s}}_4$ geometry and that the electronic correlation plays a significant role in the superconductivity for the electron-doped $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$.

The electronic structure of ${\mathrm{ThRu}}_2{\mathrm{Si}}_2$ was studied using angle-resolved photoelectron spectroscopy (ARPES) with incident photon energies of $h\nu =655$–745 eV. Detailed band structure and the three-dimensional shapes of Fermi surfaces were derived experimentally, and their characteristic features were mostly explained by means of band-structure calculations based on density-functional theory. Comparison of the experimental ARPES spectra of ${\mathrm{ThRu}}_2{\mathrm{Si}}_2$ with those of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ shows that they have considerably different spectral profiles, particularly in the energy range of $1\mathrm{eV}$ from the Fermi level, suggesting that $\mathrm{U}5f$ states are substantially hybridized in these bands. The relationship between the ARPES spectra of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ and ${\mathrm{ThRu}}_2{\mathrm{Si}}_2$ is very different from the one between the ARPES spectra of ${\mathrm{CeRu}}_2{\mathrm{Si}}_2$ and ${\mathrm{LaRu}}_2{\mathrm{Si}}_2$, where the intrinsic difference in their spectra is limited only in the very vicinity of the Fermi energy. The present result suggests that the $\mathrm{U}5f$ electrons in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ have strong hybridization with ligand states and have an essentially itinerant character.

The collective electronic excitations and their dispersion behavior across both the first and second Brillouin zones in a bulk $\mathrm{Mo}{\mathrm{S}}_2$ single crystal have been investigated using nonresonant inelastic hard x-ray scattering and time-dependent density-functional theory. The experiment results revealed two main plasmons located around 8.6 and 23 eV. The $\sim 8.6\mathrm{eV}$ plasmon shows clear anisotropic behavior and was only observed in plane due to the two-dimensional geometry. In addition, theoretical calculation indicates that the dispersion behavior of this in-plane plasmon shows weak momentum dependence. The main plasmon near 23 eV demonstrates an isotropic three-dimensional collective electron excitation behavior, which suggests a weak correlation between the electron excitation and layered lattice structure. This behavior is completely different from the anisotropic dispersion behavior found in layered graphite. Our work provides clear experimental and theoretical data on the dynamic electronic behavior of excited high-energy electrons in $\mathrm{Mo}{\mathrm{S}}_2$, which not only provides guidance on device design but also furthers understanding of electronic behaviors in other similar systems.

We demonstrate that topological invariants of band insulators can be derived efficiently from the eigenvalues of the local-orbital (LO) based embedding potential, called also the contact (lead) self-energy. The LO based embedding potential is a bulk quantity. Given the tight-binding Hamiltonian describing the bulk valence and conduction bands, it is constructed straightforwardly from the bulk wave functions satisfying the generalized Bloch condition. When the one-electron energy $\varepsilon$ is located within a projected bulk band gap at a given planar wave vector $\mathbf{k}$, the embedding potential becomes Hermitian. Its real eigenvalues exhibit distinctly different behavior depending on the topological properties of the valence bands, thus enabling unambiguous identification of bulk topological invariants. We consider the Bernevig-Hughes-Zhang model as an example of a time-reversal invariant topological insulator and tin telluride (SnTe) crystallized in a rock-salt structure as an example of a topological crystalline insulator.

We investigate the spectrum of finite-length carbon nanotubes in the presence of onsite and nearest-neighbor superconducting pairing terms. A one-dimensional ladder-type lattice model is developed to explore the low-energy spectrum and the nature of the electronic states. We find that zero energy edge states can emerge in zigzag class carbon nanotubes as a combined effect of curvature-induced Dirac point shift and strong superconducting coupling between nearest-neighbor sites. The chiral symmetry of the system is exploited to define a winding number topological invariant. The associated topological phase diagram shows regions with nontrivial winding number in the plane of chemical potential and superconducting nearest-neighbor pair potential (relative to the onsite pair potential). A one-dimensional continuum model reveals the topological origin of the zero energy edge states: a bulk-edge correspondence is proven, which shows that the condition for nontrivial winding number and that for the emergence of edge states are identical. For armchair class nanotubes, the presence of edge states in the superconducting gap depends on the nanotube's boundary shape. For the minimal boundary condition, the emergence of the subgap states can also be deduced from the winding number.

We present experimental evidence of transverse magnetic domains, previously observed only in nanostrips, in CoNi cylindrical nanowires with designed crystal symmetry and tailored magnetic anisotropy. The transverse domains are found together with more conventional vortex domains along the same cylindrical nanowire, denoting a bistable system with similar energies. The surface and the inner magnetization distribution in both types of domains are analyzed by photoemission electron microscopy with x-ray magnetic circular dichroism contrast, and hysteresis loop in individual nanowires are measured by magneto-optical Kerr effect. These experimental data are understood and compared with complementary micromagnetic simulations.

We present the generalized quasiclassical theory of the long-range superconducting proximity effect in heterostructures with strong ferromagnets, where the exchange splitting is of the order of Fermi energy. In the ferromagnet the propagation of equal-spin Cooper pairs residing on the spin-split Fermi surfaces is shown to be governed by the spin-dependent Abelian gauge field which results either from the spin-orbital coupling or from the magnetic texture. This additional gauge field enters into the quasiclassical equations in superposition with the usual electromagnetic vector potential and results in the generation of spontaneous superconducting currents and phase shifts in various geometries which provide the sources of long-range spin-triplet correlations. We derive the Usadel equations and boundary conditions for the strong ferromagnet and consider several generic examples of the Josephson systems supporting spontaneous currents.

By numerical integration, we study the relaxation dynamics of degenerate harmonic oscillator modes dispersively coupled to particle positions. Depending on whether the effective inertial potential induced by the oscillators keeps the particles confined or if the particle trajectories traverse the system, the local oscillator energy dissipation rate changes drastically. The inertial trapping, release, and retrapping of particles result in a characteristic stepwise relaxation process, with alternating regions of fast and slow dissipation. To demonstrate this phenomenon we consider first a one-dimensional minimal prototype model which displays these characteristics. We then treat the effect of dispersive interaction in a model corresponding to an adsorbate diffusing on a circular membrane interacting with its three lowest vibrational modes. In the latter model, stepwise relaxation appears only in the presence of thermal noise, which also causes a slow-in-time stochastic precession of the mixing angle between the degenerate eigenmodes.

We investigated the electronic states of the quasi-one-dimensional organic salts ${\delta }_P^{\prime }-{\left(\text{BPDT-TTF}\right)}_2{\mathrm{ICl}}_2$ and ${\delta }_C^{\prime }-{\left(\text{BPDT-TTF}\right)}_2{\mathrm{ICl}}_2$, both of which are insulating at room temperature owing to strong electron correlations. Through measurements of electrical resistivity, optical conductivity, and magnetic susceptibility, as well as band-structure calculations, we have revealed that the two materials possess completely different ground states, even though they have the same chemical composition and stacking configuration of the donor molecules. We have found that the ${\delta }_P^{\prime }$-type salt with an effective half-filled band behaves as a dimer-Mott insulator and exhibits a phase transition to a nonmagnetic state at 25 K, whereas the ${\delta }_C^{\prime }$-type salt with a 3/4-filled band shows a charge ordering transition just above room temperature and becomes nonmagnetic below 20 K. The optical spectra of the ${\delta }_P^{\prime }$-type salt are composed of two characteristic bands due to intra- and interdimer charge transfers, supporting the dimer-Mott insulating behavior arising from the strong on-site Coulomb interaction. By contrast, in the ${\delta }_C^{\prime }$-type salt, a single band characterizing the formation of charge ordering arising from the off-site Coulomb interactions is observed. Upon lowering the temperature, the shape of the optical spectra in the ${\delta }_C^{\prime }$-type salt becomes asymmetric and shifts to much lower frequencies, suggesting the emergence of domain-wall excitations with fractional charges expected in a one-dimensional charge-ordered chain. The temperature dependence of the magnetic susceptibility of the ${\delta }_P^{\prime }$-type salt is well described by a two-dimensional (2D) spin-1/2 Heisenberg antiferromagnetic model on an anisotropic square lattice in the dimerized picture, while in the ${\delta }_C^{\prime }$-type salt, it can be explained by a 2D spin-1/2 Heisenberg antiferromagnetic model on an anisotropic honeycomb lattice formed in the charge-ordered state. These completely different ground states between the ${\delta }_P^{\prime }$- and ${\delta }_C^{\prime }$-type salts come from the difference of degree of dimerization of two face-to-face BPDT-TTF molecules.

In spite of their intrinsic one-dimensional nature, matrix product states have been systematically used to obtain remarkably accurate results for two-dimensional systems. Motivated by basic entropic arguments favoring projected entangled-pair states as the method of choice, we assess the relative performance of infinite matrix product states and infinite projected entangled-pair states on cylindrical geometries. By considering the Heisenberg and half-filled Hubbard models on the square lattice as our benchmark cases, we evaluate their variational energies as a function of both bond dimension and cylinder width. In both examples, we find crossovers at moderate cylinder widths, i.e., for the largest bond dimensions considered, we find an improvement on the variational energies for the Heisenberg model by using projected entangled-pair states at a width of about eleven sites, whereas for the half-filled Hubbard model, this crossover occurs at about seven sites.

The cluster slave-spin method is introduced to systematically investigate the solutions of the Hubbard model including the symmetry-broken phases. In this method, the electron operator is factorized into a fermionic spinon describing the physical spin and a slave-spin describing the charge fluctuations. Following the $U\left(1\right)$ formalism derived by Yu and Si [Phys. Rev. B 86, 085104 (2012)], it is shown that the self-consistent equations to explore various symmetry-broken density wave states can be constructed in general with a cluster of multiple slave-spin sites. We employ this method to study the antiferromagnetic (AFM) state in the single band Hubbard model with the two- and four-site clusters of slave spins. While the Hubbard gap, the charge gap due to the doubly occupied states, scales with the Hubbard interaction $U$ as expected, the AFM gap $\mathrm{\Delta }$, the gap in the spinon dispersion in the AFM state, exhibits a crossover from the weak- to strong-coupling behaviors as $U$ increases. Our cluster slave-spin method reproduces not only the traditional mean-field behavior of $\mathrm{\Delta }\sim U$ in the weak-coupling limit, but also the behavior of $\mathrm{\Delta }\sim t^2/U$ predicted by the superexchange mechanism in the strong-coupling limit. In addition, the holon-doublon correlator as functions of $U$ and doping $x$ is also computed, which exhibits a strong tendency toward the holon-doublon binding in the strong coupling regime. We further show that the quasiparticle weight obtained by the cluster slave-spin method is in a good agreement with the generalized Gutzwiller approximation in both AFM and paramagnetic states, and the results can be improved beyond the generalized Gutzwiller approximation as the cluster is enlarged from a single site to four sites. Our results demonstrate that the cluster slave-spin method can be a powerful tool to systematically investigate the strongly correlated system.

We investigate a thermoelectric nanoengine whose properties are steered by Coulomb interaction. The device whose design decouples charge and energy currents is made up of two interacting quantum dots connected to three different reservoirs. We show that, by tailoring the tunnel couplings, this setup can be made very attractive for energy-harvesting prospects, due to a delivered power that can be of the order of the quantum bound [R. S. Whitney, Phys. Rev. Lett. 112, 130601 (2014); Entropy 18, 208 (2016)], with a concomitant fair efficiency. To unveil its properties beyond the sequential quantum master equation, we apply a nonequilibrium noncrossing approximation in the Keldysh Green's function formalism, and a quantum master equation that includes cotunneling processes. Both approaches are rather qualitatively similar in a large operating regime where sequential tunneling alone fails.

We study thermoelectric effects in Coulomb-coupled quantum-dot (CCQD) systems beyond lowest-order tunneling processes and the often applied wide-band approximation. To this end, we present a master-equation (ME) approach based on a perturbative $T$-matrix calculation of the charge and heat tunneling rates and transport currents. Applying the method to transport through a noninteracting single-level QD, we demonstrate excellent agreement with the Landauer-Büttiker theory when higher-order (cotunneling) processes are included in the ME. Next, we study the effect of cotunneling and energy-dependent lead couplings on the heat currents in a system of two CCQDs. We find that cotunneling processes (i) can dominate the off-resonant heat currents at low temperature and bias compared to the interdot interaction, and (ii) give rise to a pronounced reduction of the cooling power achievable with the recently demonstrated Maxwell's demon cooling mechanism. Furthermore, we demonstrate that the cooling power can be boosted significantly by carefully engineering the energy dependence of the lead couplings to filter out undesired transport processes. Our findings emphasize the importance of higher-order cotunneling processes as well as engineered energy-dependent lead couplings in the optimization of the thermoelectric performance of CCQD systems.

We present a recipe that leads to Weyl semimetals with extended topologically protected features. We show that compounds in a family that possess time-reversal symmetry and share a noncentrosymmetric cubic structure with the space group $F\text{−}43m$ (no. 216) host robust Weyl fermions with extended and easily measurable protected features. The candidates in this family are compounds with different chemical formulas, $\mathrm{A}{\mathrm{B}}_2$, ABC, $\mathrm{AB}{\mathrm{C}}_2$, and ABCD, and their Fermi levels are predominantly populated by nontrivial Weyl fermions. Symmetry of the system requires that the Weyl nodes with opposite chirality are well separated in momentum space. Adjacent Weyl points have the same chirality; thus these Weyl nodes would not annihilate each other with respect to lattice perturbations. As Fermi arcs and surface states connect Weyl nodes with opposite chirality, the large separation of the latter in momentum space guarantees the appearance of very long arcs and surface states. This work demonstrates that the use of system symmetry by first-principles calculations is a powerful approach for discovering new Weyl semimetals with attractive features whose protected fermions may be candidates of many applications.

It was recently shown that an interacting Kitaev topological superconductor model is exactly solvable based on two-step Jordan-Wigner transformations together with one spin rotation. We generalize this model by including the dimerization, which is shown also to be exactly solvable. We analytically determine the topological phase diagram containing seven distinct phases. It is argued that the system is topological when a fermionic many-body Majorana zero-energy edge state emerges. It is intriguing that there are two tetracritical points, at each of which four distinct phases touch.