We develop a Fermi-liquid based approach to investigate the power output of nano-devices in the presence of strong interactions and resonance scattering. The developed scheme is then employed to study the power output of a $\mathrm{SU}(N$) Kondo impurity at the strong-coupling regime. The interplay between Kondo resonance and the filling factors in the $\mathrm{SU}(N$) quantum systems is found to be a key to enhance output power. Such enhancement results in an output power corresponding to $\sim 50\%$ of the quantum upper bound. We demonstrate that given a proper tuning of the electron occupancy, the investigated power grows linearly with degeneracy of Kondo state ($N$). This relation can hence be exploited to obtain output power that is larger than the one in existing noninteracting setups.

We address the question of stability of protected helical edge states at the boundary of two-dimensional topological insulators upon interactions with the external bath. We study how backscattering amplitude changes when different interaction channels between the system and the environment are present. Drawing an analogy with the concept of pointer states in theory of open quantum systems, we demonstrate that in a certain regime the interplay between the Coulomb and the spin-spin interactions can make the backscattering process strictly irrelevant, and the helical modes become robust states with a well-defined momentum. This opens a possibility to use the external spin bath as a stabilizer that alleviates destructive effects and restores the helicity protection.

The discovery of infinite-layer nickelate superconductor marks the new era in the field of superconductivity. In the rare-earth ($R$) nickelates $\mathrm{RNi}{\mathrm{O}}_2$, although the Ni is also of $d^9$ electronic configuration, analogous to Cu $d^9$ in cuprates, whether electronic structures in infinite-layer nickelate are the same as cuprate and possess the single-band feature as well are still open questions. To illustrate the electronic structure of rare-earth infinite-layer nickelate, we perform first-principles calculations of $\mathrm{LaNi}{\mathrm{O}}_2$ and $\mathrm{NdNi}{\mathrm{O}}_2$ compounds and compare them with that of $\mathrm{CaCu}{\mathrm{O}}_2$ using the hybrid functional method together with Wannier projection and group symmetry analysis. Our results indicate that the Ni ${d_{x^2}}_{-y^2}$ in the $\mathrm{LaNi}{\mathrm{O}}_2$ has weak hybridization with other orbitals and exhibits the characteristic single-band feature, whereas in $\mathrm{NdNi}{\mathrm{O}}_2$, the Nd $f$ orbital hybridizes with Ni $d_{x^2-y^2}$ and is a non-negligible ingredient for transport and even high-temperature superconductivity. Given that the Cu $d_{x^2-y^2}$ in cuprate strongly hybridizes with O $2p$, the calculated band structures of nickelate imply some new band characters, which is worthy of more attention.

Optical systems that respect parity-time ($\mathit{PT}$) symmetry can be realized with proper incorporation of gain/loss materials. However, due to the absence of magnetic response at optical frequencies, the wave impedance is defined entirely by their permittivity and, hence, the $\mathit{PT}$-symmetric character is controlled solely via their refractive index. Here, we show that the separate tuning of the wave impedance enabled by metamaterials can grant access to wide control of the exceptional points, appearance of mixed phases (coexistence of PT-symmetric and PT-broken phases) and occurrence of phase reentries, not easily realizable with natural materials.

We study the finite-temperature properties of the half-filled two-band Hubbard model in the presence of Hund's rule coupling and orbital anisotropy. We use the mean-field treatment of the $Z_2$ slave-spin theory with a finite-temperature extension of the zero-temperature gauge variable previously developed by Hassan and de' Medici [Phys. Rev. B 81, 035106 (2010)]. We consider the instability of the Fermi-liquid phases and how it is enhanced by the Hund's rule. We identify paramagnetic solutions that have zero quasiparticle weight with a bad metal, and the first-order transition temperature between the bad metal and the Fermi-liquid phase as a coherence temperature that signals the crossover to the bad metallic state. When orbital anisotropy is present, we found an intermediate transition to an orbital-selective bad metal (OSBM), where the narrow band becomes a bad metal while the wide band remains a renormalized Fermi liquid. The temperatures $T_{\text{coh}}$ and $T_{\text{OSBM}}$ at which the system transitions to the bad metal phases can be orders of magnitude less than the Fermi temperature associated with the noninteracting band. The parameter dependence of the temperature at which the OSBM is destroyed can be understood in terms of a ferromagnetic Kondo-Hubbard lattice model. In general, Hund's rule coupling enhances the bad metallic phases, reduces interorbital charge fluctuations, and increases spin fluctuations. The qualitative difference found in the ground state whether the Hund's rule is present or not, related to the degeneracy of the low-energy manifold, is also maintained for finite temperatures.

Localization in one-dimensional disordered or quasiperiodic noninteracting systems in the presence of power-law hopping is very different from localization in short-ranged systems. Power-law hopping leads to algebraic localization as opposed to exponential localization in short-ranged systems. Exponential localization is synonymous with insulating behavior in the thermodynamic limit. Here we show that the same is not true for algebraic localization. We show, on general grounds, that depending on the strength of the algebraic decay, the algebraically localized states can be actually either conducting or insulating in thermodynamic limit. We exemplify this statement with explicit calculations on the Aubry-André-Harper model in the presence of power-law hopping, with the power-law exponent $\alpha >1$, so that the thermodynamic limit is well defined. We find a phase of this system where there is a mobility edge separating completely delocalized and algebraically localized states, with the algebraically localized states showing signatures of superdiffusive transport. Thus, in this phase, the mobility edge separates two kinds of conducting states, ballistic and superdiffusive. We trace the occurrence of this behavior to near-resonance conditions of the on-site energies that occur due to the quasiperiodic nature of the potential.

Nonlinear magnetization dynamics excited by spin-transfer effect with feedback current is studied both numerically and analytically. The numerical simulation of the Landau-Lifshitz-Gilbert equation indicates the positive Lyapunov exponent for a certain range of the feedback rate, which identifies the existence of chaos in a nanostructured ferromagnet. Transient behavior from chaotic to steady oscillation is also observed in another range of the feedback parameter. An analytical theory is also developed, which indicates the appearance of multiple attractors in a phase space due to the feedback current. An instantaneous imbalance between the spin-transfer torque and damping torque causes a transition between the attractors, and results in the complex dynamics.

Driving superconductors out of equilibrium is a promising avenue to study their equilibrium properties as well as to control the superconducting state. Nonequilibrium superconductors are often studied using time-resolved optical conductivity measurements. Thus, the characterization of a superconducting state in a pump driven nonequilibrium state requires careful attention in the time domain. We calculate the time-resolved optical conductivity of a pumped superconducting state using a nonequilibrium Keldysh approach. Through functional derivation, the optical conductivity is obtained with full vertex corrections and used to characterize the transient superconducting state. The transient optical conductivity shows the suppression of the superconducting order parameter in the time domain. The subsequent recovery of the order parameter exhibits oscillatory behavior that corresponds to the Higgs amplitude mode, and may be seen in several parts of the spectrum.

Magnetic topological semimetals, the latest member of topological quantum materials, are attracting extensive attention as they may lead to topologically driven spintronics. Currently, magnetotransport investigations on these materials are focused on the anomalous Hall effect. Here, we report on the magnetoresistance anisotropy of topological semimetal CeBi, which has tunable magnetic structures arising from localized Ce $4f$ electrons and exhibits both negative and positive magnetoresistances, depending on the temperature. We found that the angle dependence of the negative magnetoresistance, regardless of its large variation with the magnitude of the magnetic field and with temperature, is solely dictated by the field-induced magnetization that is orientated along a primary crystalline axis and flops under the influence of a rotating magnetic field. The results reveal the strong interaction between conduction electrons and magnetization in CeBi. They also indicate that magnetoresistance anisotropy can be used to uncover the magnetic behavior and the correlation between transport phenomena and magnetism in magnetic topological semimetals.

Magnetic monopoles and toroidal order are compelling features that have long been theorized but remain elusive in real materials. Multiferroic hexagonal ferrites and manganites are an interesting realization of frustrated triangular lattice of spins, where magnetic order is interacting with ferroelectricity and trimerization. We discuss a microscopic mechanism by which trimerization leads to a modulation of magnetic exchanges, and therefore magnetic monopolar and toroidal orders emerge from the combination of ${120}^{\circ }$ antiferromagnetism and trimerization. The experimentally observable signatures of magnetic monopolar and toroidal orders are identified in the inelastic neutron scattering cross section, simulated from a microscopic model of ${\mathrm{LuFeO}}_3$. The nonreciprocal magnon propagation is demonstrated.

Standing spin waves in triangular dots (truncated pyramids) were investigated both experimentally and theoretically. Arrays of nickel triangular pyramids with the base side of 270 nm and height of 70 nm were deposited on Si (111) substrate. The spectra of ferromagnetic resonance were obtained at room temperature with an external saturating magnetic field directed perpendicular to the array plane. The theoretical approach, which allows to describe standing spin wave modes both in perpendicular magnetized regular prisms and in close to prisms truncated pyramids, was developed. Theoretically calculated resonance fields for the observed modes are in a good correlation with the experiment.

We study the relationship between the quantum Fisher information (QFI) of spin pairs and the topological quantum phase transitions (TQPTs) of the extended XY model driven by the anisotropies of the nearest-neighbor and the next-nearest-neighbor spins, the transverse magnetic field, and the three-spin interaction. We find that the first derivative of QFI can correctly characterize the TQPTs at absolute zero temperature. Meanwhile, the impacts of the thermal fluctuations and the site distance of spin pairs on the critical behaviors of the QFI are studied. It is found that the first derivative of QFI for the nearest neighbor or the long-distance spin pairs can only correctly characterize the critical points of the TQPTs at sufficiently low temperature. Remarkably, when the anisotropy of the nearest-neighbor and the next-nearest-neighbor spins are the driven parameters and the site distance $R=5$, the QFI itself can characterize the TQPTs at absolute zero temperature.

Describing the microscopic details of the interaction of magnets and spin-polarized currents is key to achieving control of such systems at the microscopic level. Here we discuss a description based on the Keldysh technique, casting the problem in the language of open quantum systems. We reveal the origin of noise in the presence of both fieldlike and dampinglike terms in the equation of motion arising from spin conductance.

We demonstrate that it is possible to use atomic manipulation techniques to adiabatically tune between one-dimensional and two-dimensional topological superconductors in magnet-superconductor hybrid (MSH) structures. This allows one to change the nature of the associated Majorana fermions between localized bound states and chiral Majorana edge modes, and provides a new approach for counting the topological invariant of the system – the Chern number – in real space. Moreover, we show that the topological nature of finite-size MSH structures can be characterized using the Chern number density which adiabatically connects MSH structures to macroscopic, translationally invariant topological superconductors characterized by an integer Chern number. These results open new possibilities for studying the nature of topological superconductivity in MSH structures using atomic manipulation techniques.

Most topological superconductors known to date suffer from low transition temperatures ($T_c$) and/or high fragility to disorder and dopant levels, which is hampering the progress in this promising field. Here, utilizing a combination of angle-resolved photoemission spectroscopy measurements and density-functional theory calculations, we show the presence of a type of topological Dirac nodal line surface state on the [010] faces of the $T_c=39$ K BCS superconductor ${\mathrm{MgB}}_2$. This surface state should be highly tolerant against disorder and inadvertent doping variations and is expected to go superconducting via the proximity effect to the bulk superconductor that this state is intimately connected to. This would represent a form of high-temperature topological superconductivity.

Realization and regulation of valley polarization is a core issue in the valleytronics fields. Here, through first-principles calculations, we find that the valley polarization (up to 39.6 meV) can be realized in stanene, by stacking it on a monolayer ferromagnetic insulator $\mathrm{Cr}{\mathrm{I}}_3$, forming the ferromagnetic stanene/$\mathrm{Cr}{\mathrm{I}}_3$ van der Waals heterostructure. The heterostructure possesses a magneto band-structure effect, whereby the spin orientation of Cr atoms can significantly influence valley polarization. When the spin orientation of Cr atoms is parallel (perpendicular) to the $c$ axis, the largest (smallest) valley polarization is achieved with the value of 71.7 meV (1.7 meV). Additionally, the spin-polarized Dirac electrons and hole doping are induced in stanene due to the magnetic proximity effect, and the Dirac cone of stanene is opened with the value of 187.6 meV. These results indicate that the stanene/$\mathrm{Cr}{\mathrm{I}}_3$ heterostructure is very promising to be applied in future spintronics and valleytronics.

We explored the possibility of realizing quantum anomalous Hall effect by placing a heavy-element atomic layer on top of monolayer ${\mathrm{CrI}}_3$ with a natural cleavage surface and broken time-reversal symmetry. We showed that ${\mathrm{CrI}}_3/X$ ($X=\text{Bi}$, Sb, or As) systems can open up a sizable bulk gap to harbor a quantum anomalous Hall effect; e.g., ${\mathrm{CrI}}_3/\mathrm{Bi}$ is a natural magnetic insulator with a bulk gap of 30 meV, which can be further enlarged via strain engineering or adjusting spin orientations. We also found that the ferromagnetic properties (magnetic anisotropic energy and Curie temperature) of pristine ${\mathrm{CrI}}_3$ can be further improved due to the presence of heavy atomic layers, and the spin orientation can be utilized as a useful knob to tune the band structure and Fermi level of the ${\mathrm{CrI}}_3/\mathrm{Bi}$ system. The topological nature, together with the enhanced ferromagnetism, can unlock potential applications for ${\mathrm{CrI}}_3$-based materials in spintronics and electronics.

The charge transport of a (Tomonaga-)Luttinger liquid with tunnel barriers exhibits universal scaling: the current-voltage curves measured at various temperatures collapse into a single curve upon rescaling. The exponent characterizing this single curve can be used to extract the strength of electron-electron interaction. Motivated by a recent experiment on InAs nanowires [Y. Sato et al., Phys. Rev. B 99, 155304 (2019)], we theoretically investigate the analogous behavior of a spin-orbit-coupled Luttinger liquid. We find that the scaling exponent differs for different impurity strengths, being weak (disorder potential) or strong (tunnel barriers), and their positions, either in the bulk or near the edge of the wire. For each case we quantify the exponent of the universal scaling and its modification due to the spin-orbit coupling. Our findings serve as a guide in the determination of the interaction strength of quasi-one-dimensional spin-orbit-coupled quantum wires from transport measurements.