We investigate the interplay between the quantum Hall (QH) effect and superconductivity in InAs surface quantum well (SQW)/NbTiN heterostructures using a quantum point contact (QPC). We use QPC to control the proximity of the edge states to the superconductor. By measuring the upstream and downstream resistances of the device, we investigate the efficiency of Andreev conversion at the InAs/NbTiN interface. Our experimental data is analyzed using the Landauer-Büttiker formalism, generalized to allow for Andreev reflection processes. We show that by varying the voltage of the QPC, $V_{\mathrm{QPC}}$, the average Andreev reflection, $A$, at the QH-SC interface can be tuned from 50% to $\sim 10\%$. The evolution of $A$ with $V_{\mathrm{QPC}}$ extracted from the measurements exhibits plateaus separated by regions for which $A$ varies continuously with $V_{\mathrm{QPC}}$. The presence of plateaus suggests that for some ranges of $V_{\mathrm{QPC}}$ the QPC might be pinching off almost completely from the QH-SC interface some of the edge modes. Our work shows an experimental setup to control and advance the understanding of the complex interplay between superconductivity and QH effect in two-dimensional gas systems.

The electronic states and topological properties of the biphenylene network (BPN) are analyzed using a tight-binding model based on the $\pi$-electron network. It is shown that tuning the hopping parameters induces topological phase transitions, leading to the emergence of edge states owing to the nontrivial topological Zak phase of the bulk BPN. Elementary band analysis clearly gives the number of edge states, which are associated with the location of Wannier centers. In addition, we present the conditions for the emergence of corner states owing to the higher-order topological nature of the BPN.

Carriers in two-dimensional semi-Dirac materials exhibit massless and massive dispersions along two perpendicular directions. Here, we investigate theoretically their properties of transmission and Goos-Hänchen (GH) shift under an electric barrier. It is found that isolated transmission zeros are allowed (prohibited) when the electric barrier aligns with the parabolic (linear) dispersion direction. Such transmission zeros require the coexistence of evanescent and propagating states in the outgoing region, whose number and position depend on the barrier width and height. Under a wide barrier, huge GH shift can be achieved at a transmission zero close to a reflection zero and at a small incident angle. The polarity of the GH shift depends on the sign of the incident angle and is controlled by the relative position between the barrier height and incident energy. The isolated transmission zeros and enhanced GH shift with tunable polarity could be helpful for potential electron-optics applications of semi-Dirac systems.

Strong correlation effects caused by Hund's coupling have been actively studied during the past decade. Hund's metal, strongly correlated while far from the Mott insulating limit, was studied as a representative example. However, recently it was revealed that a typical Mott system also exhibits a sign of Hund physics by investigating the kink structure in the spectral function of ${\mathrm{NiS}}_{2-x}{\mathrm{Se}}_x$. Therefore, to understand the Hund physics in a half-filled multiorbital system near the metal-insulator transition, we studied pressure-induced metallic states of ${\mathrm{NiS}}_2$ by using density-functional theory plus dynamical mean-field theory. Hund physics, responsible for suppressing local spin fluctuation, gives low-energy effective correlations, separated from Mott physics, which suppress charge fluctuation at higher energy. This effect is prominent when Hund's coupling J becomes comparable to the quasiparticle kinetic energy, showing apparent scaling behavior of the kink position $E_{\mathrm{kink}}\sim JZ$, where Z is the quasiparticle residue $m_e/m^{*}$. We suggest that the Hund effect can also be observed in the optical conductivity as a non-Drude-like tail with $1/\omega$ frequency dependence and nonmonotonic temperature evolution of the integrated optical spectral weight at a fixed frequency. Our study demonstrates the important role of Hund's coupling for electronic correlations even in a half-filled system.

An unsaturated linear magnetoresistance (LMR) has attracted wide attention because of potential applications and fundamental interest. By controlling growth temperature, we realized a metal-to-insulator transition in ${\mathrm{Al}}_2{\mathrm{O}}_3$/$\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures. The LMR is observed in metallic samples with electron mobility varying over three orders of magnitude. The observed LMR cannot be explained by the guiding center diffusion model even in samples with very high mobility. The slope of the observed LMR is proportional to Hall mobility, and the crossover field, indicating a transition from quadratic (at low fields) to linear (at high fields) field dependence, is proportional to the inverse Hall mobility. This signifies that the classical model is valid to explain the observed LMR. More importantly, we develop an analytical expression according to the effective-medium theory that is equivalent to the classical model. And, the analytical expression describes the LMR data very well, confirming the validity of the classical model.

We demonstrate that phase transition and continuous symmetry breaking can happen in $2d$ Heisenberg ferromagnets with long range interaction in violation of the Hohenberg-Mermin-Wagner theorem. This has been possible to achieve in $2d$ van der Waals ferromagnet ${\text{Fe}}_3{\text{GeTe}}_2$ due to its exceptional tunability of magnetic properties. We argue that the variable critical exponent is a signature of phase transition in $2d$ systems with long range ordering, consistent with recent results from density functional theory and quantum Monte Carlo calculations.

To understand the multistage phase transitions in V-based kagome metals inside the charge-density-wave (CDW) phase, we focus on the impact of the “mixed-type” Fermi surface because it is intact in the CDW state on the “pure-type” Fermi surface. On the mixed-type Fermi surface, moderate spin correlations develop, and we reveal that uniform $(\mathit{q}=\mathit{0})$ bond order is caused by the paramagnon interference mechanism, which is described by the Aslamazov-Larkin vertex correction. A dominant solution is $E_{2g}$-symmetry nematic order, in which the director can be rotated arbitrarily. In addition, we obtain $A_{1g}$-symmetry order, which leads to the change in the lattice constants without symmetry breaking. The predicted $E_{2g}$ and $A_{1g}$ channel fluctuations at $\mathit{q}=\mathit{0}$ can be observed by the elastoresistance measurements. These results are useful to understand the multistage phase transitions inside the $2\times 2$ CDW phase. The present theory has a general significance because mixed-type Fermi surfaces (with multiorbital van Hove singularities) exist in various kagome lattice systems.

The generation of ultrashort light pulses is essential for the advancement of attosecond science. Here, we show that attosecond pulses approaching the Fourier limit can be generated through optimized optical driving of tunneling particles in solids. We propose an ansatz for the wave function of tunneling electron-hole pairs based on a rigorous expression for massive Dirac fermions, which enables efficient optimization of the waveform of the driving field. It is revealed that the dynamic sign change in the effective mass due to optical driving is crucial for shortening the pulse duration, which highlights a distinctive property of Bloch electrons that is not present in atomic gases, i.e., the periodic nature of crystals. These results show the potential of utilizing solid materials as a source of attosecond pulses.

Nanoparticles were predicted to exhibit unique physical properties due to quantum size effects, but their identification remains difficult. According to Kubo's theory, the gap size is inversely correlated with both the density of states at the Fermi energy and the number of atoms in the particle. Previously, we confirmed that the particle size and magnetic field dependence of NMR anomaly temperature is consistent with the estimated “Kubo” gap. Here, we investigated the density-of-states dependence in the ${\mathrm{Pt}}_{1-x}{\mathrm{Cu}}_x$ nanoparticles. While an enhancement of nuclear spin-lattice relaxation rate $1/T_1$ at low temperatures was clearly observed for the Pt-rich nanoparticles, such behavior was abruptly suppressed in the Cu-rich nanoparticles. Furthermore, the NMR anomaly temperature is nearly unchanged with varying the density of states. Our findings indicate that quantum size effect contains more profound physics than just the ones predicted by Kubo.

We put forth a theoretical framework for engineering a two-dimensional (2D) second-order topological superconductor (SOTSC) by utilizing a heterostructure: incorporating noncollinear magnetic textures between an $s$-wave superconductor and a 2D quantum spin Hall insulator. It stabilizes the higher order topological superconducting phase, resulting in Majorana corner modes (MCMs) at four corners of a 2D domain. The calculated nonzero quadrupole moment characterizes the bulk topology. Subsequently, through a unitary transformation, an effective low-energy Hamiltonian reveals the effects of magnetic textures, resulting in an effective in-plane Zeeman field and spin-orbit coupling. This approach provides a qualitative depiction of the topological phase, substantiated by numerical validation within an exact real-space model. Analytically calculated effective pairings in the bulk illuminate the microscopic behavior of the SOTSC. The comprehension of MCM emergence is supported by a low-energy edge theory, which is attributed to the interplay between effective pairings of $(p_x+p_y)$-type and $(p_x+i{p}_y)$-type. Our extensive study paves the way for practically attaining the SOTSC phase by integrating noncollinear magnetic textures.

In the substituted ${\mathrm{Nd}}_{1-x}{\mathrm{Tb}}_x{\mathrm{Fe}}_3{\left({\mathrm{BO}}_3\right)}_4$ $(x=0.1 \text{and} x=0.2)$, possessing almost easy-axis magnetic structure at low temperatures, an unusual two-step transition in fields along the trigonal $c$ axis was observed by magnetization and single-crystal neutron diffraction studies. At the first step, only part of the Tb Ising-type moments flip to the $c$ axis, which is accompanied by a significant deviation of the antiferromagnetic Fe spins from the $c$ axis. At the second step, the remaining Tb moments flip and the Fe moments flop into the basal plane. The ob- served evolution is qualitatively explained by a model assuming small deviations of Tb moments from the trigonal axis due to local environment distortions, which leads to nonequivalence of the Tb ions with respect to effective Tb-Fe exchange and external field. Thus, an intrinsic “hidden” instability of the magnetic system in the magnetic field occurs.

As an intriguing and useful nonlinear behavior, chaos has recently received increasing attentions in the field of spintronics with the aim of designing novel devices for practical applications. In the present work, we demonstrate a chaotic oscillation of a ferrimagnetic skyrmion through numerical simulations. In contrast to the previous proposals of magnetic-soliton-based chaotic oscillators, the chaos of our ferrimagnetic skyrmion is driven solely by a constant current, instead of a time-dependent one. The occurrence of the chaotic oscillation is found to benefit from the strong nonlinearity of the magnetization dynamics in a ferrimagnet with a relatively weak antiferromagnetic exchange coupling and a low magnetic damping. Our results reveal the potentials of ferrimagnetic skyrmion oscillators in studying chaotic physics and related applications.

We investigate electronic properties of solid solution $\mathrm{Ru}{\left({\mathrm{Br}}_{1-x}{\mathrm{I}}_x\right)}_3$ with honeycomb structure which bridges the spin-orbit-entangled Mott insulator ${\mathrm{RuBr}}_3$ and strongly electron-correlated semimetal ${\mathrm{RuI}}_3$. We find a first-order insulator-metal transition at $x\sim 0.85$ triggered by increased $d\text{−}p$ hybridization and formation of interlayer I–I bonds. We also observe switching of the magnetic structure at $x=0.30$. In the metallic phase, we find critical mass enhancement leading to the carrier localization. This study furthers understanding of the Mott transition in a strongly spin-orbit-entangled system.

Quantum entanglement, lacking any classical counterpart, provides a fundamental new route to characterize the quantum nature of many-body states. In this work, we discuss an implementation of a new path integral method [Phys. Rev. Res. 2, 033505 (2020)] for fermions to compute entanglement for extended subsystems in the Hubbard model within dynamical mean-field theory (DMFT) in one and two dimensions. The new path integral formulation measures entanglement by applying a “kick” to the underlying interacting fermions. We show that the Rényi entanglement entropy can be extracted efficiently within the DMFT framework by integrating over the strength of the kick term. Using this method, we compute the second Rényi entropy as a function of subsystem size for metallic and Mott insulating phases of the Hubbard model. We explore the thermal entropy to entanglement crossover in the subsystem Rényi entropy in the correlated metallic phase. We show that the subsystem-size scaling of the second Rényi entropy is well described by the crossover formula which interpolates between the volume-law thermal Rényi entropy and the universal boundary-law Rényi entanglement entropy with logarithmic violation, as predicted by conformal field theory. We also study the mutual information across the Mott metal-insulator transition.

Cu chalcopyrites exhibit excellent thermoelectric performance because of their high thermopower and low thermal conductivity. Conversely, despite that Ag chalcopyrites have even lower thermal conductivity than the Cu-based ones, they generally display lower thermoelectric performance. The underlying physics for the thermoelectric disparity between Cu- and Ag-based materials remains unclear. In this work we investigate thermal transport and thermopower of ternary $AM{\mathrm{Se}}_2$ chalcopyrites $(A=\mathrm{Cu}/\mathrm{Ag}; M=\mathrm{Ga}/\mathrm{In})$ using first-principles methods. We reveal that strong anharmonicity from $s-d$ coupling leads to low thermal conductivity in Cu as well as Ag chalcopyrites, whereas weaker bond strength and heavier atomic mass contribute to even lower thermal conductivity in Ag chalcopyrites than the Cu ones. On the other hand, Cu chalcopyrites show superior thermopower owing to their higher band degeneracy and stronger $p-d$ coupling, which leads to enhanced mobility. We conclude that if Ag chalcopyrites could reach comparable hole concentrations as in Cu chalcopyrites, high thermoelectric performance in Ag chalcopyrites can also be achieved because of their low thermal conductivity. Understanding the disparity between Cu and Ag chalcogenides from a mechanistic perspective is important for future design of superior thermoelectric materials.

Spin carriers subject to Dresselhaus [110] (D110) spin-orbit coupling (SOC) gather null spin phases in closed circuits, contrary to usual Rashba and Dresselhaus [001] SOC. We show that D110 spin phases can be activated in square circuits by introducing an in-plane Zeeman field, where localized field inhomogeneities act as effective spin-scattering centers. Our simulations show rich interference patterns in the quantum conductance, which work as maps for a geometric classification of the propagating spin states. We also find that disorder facilitates low-field implementations.

This paper proposes a method for generating and independently controlling multiple modes of surface modes using a domino-ring structure. The domino structure, which forms a waveguide, gives a reference potential to the ring structure to realize a waveguide in which multiple surface modes are formed. The two-dimensional periodic structure of the ring mimics the helical edge state, one of the characteristic properties of topological materials, and the number of modes is independent of the shape. The quasi-two-dimensional periodic structure obtained by stacking the ring structure and combining it with the domino structure multiplexes the surface modes and produces coupling modes as the complimentary modes. Eigenvalue analyses are performed to calculate the dispersion relations for the domino, ring, and domino-ring structures, and the results were compared with the theoretical curve for the domino structure and the fitting curve for the ring structure, and the mode controllability and skin depth of the modes are discussed. The results show good agreement between the theoretical and fitting curves and the analytical results and also confirmed that the modes are generated according to the number of rings in the unit cell. And it confirmed that the skin depth of the multiple surface modes on the domino-ring structure varies with each coupling point and can be controlled by the exciting condition.

We show that constraints imposed by strong Hilbert space fragmentation (HSF) along with the presence of certain global symmetries can ensure the reality of eigenspectra of non-Hermitian quantum systems; such a reality cannot be guaranteed by global symmetries alone. We demonstrate this insight for two interacting finite chains, namely the fermionic Nelson-Hatano and the Su-Schrieffer-Heeger models, none of which has a $\mathcal{PT}$ symmetry. We show analytically that strong HSF and real spectrum are both consequences of the same dynamical constraints in the limit of large interaction, provided the systems have sufficient global symmetries. We also show that a local equal-time correlation function can detect the many-body exceptional point at a finite critical interaction strength above which the eigenspectrum is real.