In this paper, the metal-insulator transition is studied in polyaniline (PANI) as the doping increases. The study incorporates a polymerization process that produces a cross-linking of the PANI chains, yielding an extended disordered branched lattice. The presence of the phenazine structure is fundamental and allows the polymer to acquire extra dimensions, which permits a transition to the metallic phase when doping is increased as the Fermi level goes through the mobility edge into a region of extended states. This PANI system is described assuming that the polymerization process gives rise to a structure that could be represented by a Bethe lattice. It is observed, as indicated by the experiments, that the conductivity increases as a function of the pH of the acid solution when agents like bipolarons are introduced.

We consider a Fermi liquid model with density-density as well as quadrupolar forward scattering interactions parametrized by the Landau parameters $F_0$ and $F_2$. Using bosonization and a decimation technique, we compute collective modes and spectral functions for a huge range of interactions, ranging from strong repulsion to strong attraction in either angular momentum channel. We present a dynamical phase diagram showing a region of parameters where the collective mode structure changes abruptly, possibly signaling a dynamical phase transition.

We study Friedel oscillations and screening effects of the impurity potential in the Hubbard model. Electronic correlations are accounted for by solving the real-space dynamical mean-field theory equations using the continuous-time quantum Monte Carlo simulations at finite temperatures and using a homogeneous self-energy approximation with the numerical renormalization group at zero temperature. We find that in the Fermi-liquid phase both the amplitudes of Friedel oscillations and the screening charge decrease with increasing the interaction and follow the behavior of the Fermi-liquid renormalization factor. Inside the Mott insulator regime, the Friedel oscillations are absent but the residual screening charge remains finite.

In this work we study the effect of light and electric field on the electrical properties of a ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ film at 300 K. Taking advantage of its charge, spin, orbital, and lattice degrees of freedom, we successfully show that light and electric field together can be used to tune the resistance states of the manganite system. Using applied gate voltage $V_g$ and light, we are able to obtain a change in resistance of about $\pm 7.5\%$. Furthermore, incorporating an ultrathin interfacial ${\mathrm{LaTiO}}_3$ layer provides valuable insight into the origin of the photoinduced effect. The observed photoinduced effect is attributed to photoexcited down-spin $e_{g\downarrow }$ electrons, interfacial charge injection, and manipulation of orbital occupancy depending on the gate voltage and wavelength of light.

We consider a model proposed before for a time-reversal-invariant topological superconductor which contains a hopping term $t$, a chemical potential $\mu$, an extended $s$-wave pairing $\mathrm{\Delta }$, and spin-orbit coupling $\lambda$. We show that for $\left|\mathrm{\Delta }\right|=\left|\lambda \right|$, $\mu =t=0$, the model has an exact analytical solution defining new fermion operators involving nearest-neighbor sites. The many-body ground state is fourfold degenerate due to the existence of two zero-energy modes localized exactly at the first and last sites of the chain. These four states show entanglement in the sense that creating or annihilating a zero-energy mode at the first site is proportional to a similar operation at the last site. By continuity, this property should persist for general parameters. Using these results, we discuss some statements related to the so-called time-reversal anomaly. The addition of a small hopping term $t$ for a chain with an even number of sites breaks the degeneracy, and the ground state becomes unique with an even number of particles. We also consider a small magnetic field $B$ applied to one end of the chain. We compare the many-body excitation energies and spin projection along the spin-orbit direction for both ends of the chains obtained treating $t$ and $B$ as small perturbations with numerical results in a short chain, obtaining good agreement.

Weyl node creation and annihilation are unique topological phenomena associated with Weyl fermions. Besides the magnetic field, high-frequency illumination can also break the time-reversal symmetry and induce the creation and annihilation of Weyl nodes from a Dirac semimetal. Experimentally, the Weyl node creation and annihilation induced by a magnetic field have been verified by the signature of negative magnetoresistance. In this work, instead of magnetoresistance signatures by application of a magnetic field, we theoretically study the Josephson signatures of Weyl node creation and annihilation in irradiated Dirac semimetals. It is found that Weyl node creation is accompanied by a two-mode oscillation and 0-$\pi$ transitions in the Josephson current. We further illustrate that two oscillation modes stem from two Andreev reflection processes: opposite-chirality and equal-chirality Andreev reflections, respectively. With Weyl node annihilation, the Josephson current is first shut down and then recovered by the activation of spin-flipping at interfaces by two ferromagnets. The recovered anomalous Josephson current has an arbitrary and tunable ground-state phase difference, other than 0 and $\pi$. These predictions are helpful for experimental observation of illumination-induced Weyl node creation and annihilation.

Weak antilocalization signatures in magnetotransport measurements provide specific information about the spin-orbit coupling at oxide interfaces. However, for many of these two-dimensional systems, the commonly used theory for single-band electrons is not appropriate. The atomic spin-orbit coupling, jointly with inversion symmetry breaking terms, mixes states of different bands, making a multiband treatment of the Cooperon equation unavoidable. Here we consider an effective $t_{2g}$ multiband model that allows a decomposition into a pseudospin representation $1/2\oplus 3/2$. For the spin-$3/2$ sector, supervening quintet and septet channels from the Cooperon $3/2\otimes 3/2$ representations alter the quantum correction considerably. We study the impact of the spin winding number at the Fermi surface and report about a new experimentally accessible structure in the magnetoconductivity that is sensitive to the ratio of single and triple spin winding contributions.

We study a two-terminal graphene Josephson junction with contacts shaped to form a narrow constriction, less than $100\mathrm{nm}$ in length. The contacts are made from type-II superconducting contacts and able to withstand magnetic fields high enough to reach the quantum Hall regime in graphene. In this regime, the device conductance is determined by edge states, plus the contribution from the constricted region. In particular, the constriction area can support supercurrents up to fields of $\sim 2.5\mathrm{T}$. Additionally, enhanced conductance is observed through a wide range of magnetic fields and gate voltages. This additional conductance and the appearance of supercurrent is attributed to the tunneling between counterpropagating quantum Hall edge states along opposite superconducting contacts.

Weak antilocalization offers an experimental tool to address spin-orbit coupling of two-dimensional oxide surfaces and interfaces via magnetotransport. To overcome the shortcomings of the formulation for single-band spin-$\frac{1}{2}$ electrons, we consider an effective three-band model that allows a decomposition into a pseudospin representation $\frac{1}{2}\oplus \frac{3}{2}$. Whereas the well-established spin-$\frac{1}{2}$ transport signature results from singlet and triplet channels in the Cooperon equation, a new structure originates from the quintet and septet channels generated by the spin $\frac{3}{2}\otimes \frac{3}{2}$ representations.

We study the magnetic susceptibility in the normal state of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ using dynamical mean-field theory including dynamical vertex corrections. Besides the well-known incommensurate response, our calculations yield quasi-local spin fluctuations which are broad in momentum and centered around the $\mathrm{\Gamma }$ point, in agreement with recent inelastic neutron-scattering experiments [Steffens et al., Phys. Rev. Lett. 122, 047004 (2019)]. We show that these quasi-local fluctuations are controlled by the Hund's coupling and account for the dominant contribution to the momentum-integrated response. Although all orbitals contribute equally to the incommensurate response, the enhanced $\mathrm{\Gamma }$-point response originates from the planar $xy$ orbital.

We have extended the canonical tree tensor network (TTN) method, which was initially introduced to simulate the zero-temperature properties of quantum lattice models on the Bethe lattice, to finite temperature simulations. By representing the thermal density matrix with a canonicalized tree tensor product operator, we optimize the TTN and compute accurately the thermodynamic quantities, including the internal energy, specific heat, spontaneous magnetization, etc., at various temperatures. By varying the anisotropic coupling constant $\mathrm{\Delta }$, we obtain the phase diagram of the spin-1/2 Heisenberg XXZ model on the Bethe lattice, where three magnetic ordered phases, namely the ferromagnetic, XY, and antiferromagnetic phases, are found in low temperatures and separated from the high-$T$ paramagnetic phase by a continuous thermal phase transition at $T_c$. The XY phase is separated from the other two phases by two first-order phase transition lines at the symmetric coupling points $\mathrm{\Delta }=\pm 1$. We have also carried out a linear spin wave calculation on the Bethe lattice, showing that the low-energy magnetic excitations are always gapped, and find the obtained magnon gaps in very good agreement with those estimated from the TTN simulations. Despite the gapped excitation spectrum, Goldstone-like transverse fluctuation modes, as a manifestation of spontaneous continuous symmetry breaking, are observed in the ordered magnetic phases with $\left|\mathrm{\Delta }\right|\le 1$. One remarkable feature there is that the transverse correlation length reaches ${\xi }_c=1/ln(z-1)$ for $T\le T_c$, the maximal value allowed on a $z$-coordinated Bethe lattice.

We performed angle-resolved photoemission spectroscopy (ARPES) on CeBi which undergoes a two-step antiferromagnetic (AF) transition with temperature. Soft-x-ray ARPES revealed the inverted band structure at the $X$ point of bulk Brillouin zone for CeBi (and also for LaBi) as opposed to LaSb with noninverted band structure. Low-energy ARPES on CeBi revealed the Dirac-cone band at the $\overline{\mathrm{\Gamma }}$ point in the paramagnetic phase associated with the bulk band inversion. On the other hand, a double Dirac-cone band appears on entering the first AF phase at $T$ = 25 K, whereas a single Dirac-cone band recovers below the second AF transition at $T$ = 14 K. The present result suggests an intricate interplay between antiferromagnetism and topological surface states in CeBi.

Electron transport in a graphene quantum well can be analogous to photon transmission in an optical fiber. In this work, we present a detailed theoretical analysis to study the transport characteristics of graphene waveguides under the influence of different edge orientations. The non-equilibrium Green's function approach in combination with the tight-binding Hamiltonian has been utilized to investigate the conductance properties of straight armchair and zigzag oriented graphene waveguides. Conductance plateaus at integer steps of $4e^2/h$ have been observed in both orientations while the zigzag oriented waveguides present a wider first quantized plateau compared to that in the armchair oriented ones. Using various geometric and physical parameters, including side-barrier and waveguide width, and the metallic properties of terminals, we investigate the conductance profile of waveguides. In addition to the observation of valley symmetry in both edge orientations, this article explores the critical influence of drain contacts on waveguide conductance. Furthermore, we extended our transport study to three different highly bent waveguide configurations, such as U-shape, L-shape, and split-shape waveguides, in order to explore their applications in graphene-based ballistic integrated circuit devices. In the end, we also calculated the conductance of larger graphene waveguides using the scalable tight-binding model, in order to compare the results obtained from the original model.

Spectral ordering between dark and bright excitons in transition-metal dichalcogenides is of increasing interest for optoelectronic applications. However, little is known about dark exciton energies and their binding energies. We report the exciton landscape including momentum-forbidden dark excitons of the ${\mathrm{MoS}}_2$ monolayer using single-shot GW–Bethe Salpeter equation (${\mathrm{G}}_0{\mathrm{W}}_0$-BSE) calculations. We find the lowest-energy exciton to be indirect at ($K_v^{\prime }\to K_c$), in agreement with recent GdW-BSE calculations [T. Deilmann and K. S. Thygesen, 2D Mater. 6, 035003 (2019)]. We also find that by and large, the lowest-energy dark exciton binding energies ($E_b$) scale with the quasiparticle energies ($E_g$) according to the empirical $E_b/E_g=0.25$ rule. Differences in exciton binding energies are explained using an orbital theory.

We show that skyrmions on the surface of a magnetic topological insulator may experience an attractive interaction that leads to the formation of a skyrmion-skyrmion bound state. This is in contrast to the case of skyrmions in a conventional chiral ferromagnet, for which the intrinsic interaction is repulsive. The origin of skyrmion binding in our model is the molecular hybridization of topologically protected electronic orbitals associated with each skyrmion. Attraction between the skyrmions can therefore be controlled by tuning a chemical potential that populates/depopulates the lowest-energy molecular orbital. We find that the skyrmion-skyrmion bound state can be made stable, unstable, or metastable depending on the chemical potential, magnetic field, and easy-axis anisotropy of the underlying ferromagnet, resulting in a rich phase diagram. Finally, we discuss the possibility of realizing this effect in a recently synthesized Cr-doped ${\left({\mathrm{Bi}}_{2-y}{\mathrm{Sb}}_y\right)}_2{\mathrm{Te}}_3$ heterostructure.

Hybrid spin-mechanical systems are a promising platform for future quantum technologies. Usually they require the application of additional microwave fields to project integer spin to a readable state. We develop a theory of optically detected spin-mechanical resonance associated with half-integer spin defects in silicon carbide (SiC) membranes. It occurs when a spin resonance frequency matches a resonance frequency of a mechanical mode, resulting in a shortening of the spin-relaxation time through resonantly enhanced spin-phonon coupling. The effect can be detected as an abrupt reduction of the photoluminescence intensity under optical pumping without application of microwave fields. We propose all-optical protocols based on such spin-mechanical resonance to detect external magnetic fields and mass with ultrahigh sensitivity. We also discuss room-temperature nonlinear effects under strong optical pumping, including spin-mediated cooling and heating of mechanical modes. Our approach suggests a concept for quantum sensing using spin-optomechanics.

We consider the isotropic $S=1$ Heisenberg chain with a finite Haldane gap $\mathrm{\Delta }$ and use state-of-the-art numerical techniques to investigate its dynamical properties at finite temperature, focusing on the nuclear spin-lattice relaxation rate $1/T_1$ measured in nuclear magnetic resonance (NMR) experiments, for instance. In particular, we analyze the contributions from modes with momenta close to $q\approx 0$ and $q\approx \pi$ as a function of temperature. At high-temperature we observe spin diffusion, while at low-temperature we argue that a simple activated behavior $1/T_1\propto exp(-\mathrm{\Delta }/T)$ can be observed only at temperatures much smaller than the gap $\mathrm{\Delta }$.

We show that Pb and Bi adatoms and dimers have a large tunneling anisotropic magnetoresistance (TAMR) of up to 60% when adsorbed on a magnetic transition-metal surface due to strong spin-orbit coupling and the hybridization of $6p$ orbitals with $3d$ states of the magnetic layer. Using density functional theory, we have explored the TAMR effect of Pb and Bi adatoms and dimers adsorbed on a Mn monolayer on W(110). This surface exhibits a noncollinear cycloidal spin-spiral ground state with an angle of ${173}^{\circ }$ between neighboring spins which allows to rotate the spin-quantization axis of an adatom or dimer quasicontinuously and is ideally suited to explore the angular dependence of TAMR using scanning tunneling microscopy. We find that the induced magnetic moments of Pb and Bi adatoms and dimers are small, however, the spin polarization of the local density of states (LDOS) is still very large. The TAMR obtained from the anisotropy of the vacuum LDOS is up to 50%–60% for adatoms. For dimers the TAMR depends sensitively on the dimer orientation with respect to the crystallographic directions of the surface due to the formation of bonds between the adatoms with the Mn surface atoms and the symmetry of the spin-orbit coupling induced mixing. Dimers oriented along the spin-spiral direction of the Mn monolayer display the largest TAMR of 60% which is due to hybrid $6p-3d$ states of the dimers and the Mn layer.

We conducted a systematic angular dependence study of nonlinear magnetoresistance in NiFe/Pt bilayers at variable temperatures and fields using the Wheatstone bridge method. We successfully disentangled magnon magnetoresistance from other types of magnetoresistances based on their different temperature and field dependences. Both the spin Hall / anisotropic and magnon magnetoresistances contain $sin{\phi }_m$ and $sin3{\phi }_m$ components, with ${\phi }_m$ the angle between current and magnetization, but they exhibit different field and temperature dependence. The competition between different types of magnetoresistances leads to a sign reversal of the $sin3{\phi }_m$ component at a specific magnetic field, which was not reported previously. The phenomenological model developed is able to account for the experimental results for both NiFe/Pt and NiFe/Ta samples with different layer thicknesses. Our results demonstrate the importance of disentangling different types of magnetoresistances when characterizing the charge-spin interconversion process in magnetic heterostructures.

We report combined experimental and theoretical analysis of superconductivity in $\mathrm{CaK}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_4{\mathrm{As}}_4$ (CaK1144) for $x=0$, 0.017, and 0.034. To obtain the superfluid density $\rho ={\left[1+\mathrm{\Delta }{\lambda }_L\left(T\right)/{\lambda }_L\left(0\right)\right]}^{-2}$, the temperature dependence of the London penetration depth $\mathrm{\Delta }{\lambda }_L\left(T\right)$ was measured by using a tunnel-diode resonator (TDR) and the results agreed with the microwave coplanar resonator (MWR) with the small differences accounted for by considering a three orders of magnitude higher frequency of MWR. The absolute value of ${\lambda }_L(T\ll T_c)\approx {\lambda }_L\left(0\right)$ was measured by using MWR, ${\lambda }_L\left(5\mathrm{K}\right)\approx 170\pm 20$ nm, which agreed well with the NV centers in diamond optical magnetometry that gave ${\lambda }_L\left(5\mathrm{K}\right)\approx 196\pm 12$ nm. The experimental results are analyzed within the Eliashberg theory, showing that the superconductivity of CaK1144 is well described by the nodeless $s_{\pm }$ order parameter and that upon Ni doping the interband interaction increases.