We study in-plane magnetotransport for the interface of a ferromagnet insulator (FMI) and a topological insulator (TI), taking into account thermal fluctuation effect of the magnetization in the FMI. It is found that the planar Hall effect (PHE) can display complicated angular dependencies beyond the standard paradigm ${\sigma }_{xx}\sim ({\sigma }_{\parallel }-{\sigma }_{\perp }){cos}^2\theta$ and ${\sigma }_{xy}=({\sigma }_{\parallel }-{\sigma }_{\perp })sin\theta cos\theta$, where ${\sigma }_{\parallel (\perp )}$ is the longitudinal conductivity in the direction parallel (vertical) to the magnetic field and $\theta$ denotes the relative angle between the applied electric and magnetic fields. The rich structures in angular dependence of the conductivities can arise from the cooperation and competition effect between multiple coexisting mechanisms, e.g., impurity and magnon scattering, band anisotropy, as well as reconstruction of the electronic structure by magnetic fields. Consequently, in addition to the relative angle $\theta$, the actual direction of the external fields can be crucial to the PHE, dependent on the specific properties of a material. As a result, the planar Hall conductivity can be nonvanishing even for parallel construction of the applied electric and magnetic fields. Our findings are based on but not limited to the FMI/TI heterostructure, which would be helpful to understand the emergent PHEs in topological materials.

We propose that the topological Anderson insulator (TAI) can be realized by introducing special disorder into a graphene with a modified Kane-Mele model. The disorder recovers symmetry under statistical averaging, which turns a trivial insulator into a topologically nontrivial insulator. When graphene is subjected to nonmagnetic adatoms in one sublattice or global uniform magnetic adatoms, the sublattice or time-reversal (TR) symmetry is broken, respectively, making the system topologically trivial. For the former one, randomly spatial distributed adatoms result in gapless edge states and quantized transport characteristics. Such randomization preserves the sublattice symmetry on average and the system becomes a TAI phase. For the latter one, the average TR symmetry is recovered by randomizing magnetization directions and the topological phase is protected. We demonstrate the existence of gapless edge states, but the differential conductance is no longer quantized. Moreover, we construct our proposal in electric circuits and observe the disorder-induced edge state through the circuit simulation. Our work provides a more simplified scheme to realize the TAI, and deepens the understanding of the relationship between the TAI and average symmetries.

We determine the current-phase relation (CPR) of two-terminal configurations of Josephson junctions containing two-dimensional (2D) time-reversal invariant topological superconductors (TRITOPS), including TRITOPS-TRITOPS, as well as junctions between topological and nontopological superconductors (TRITOPS-S). We focus on wide junctions for which several channels intervene in the tunneling coupling. We derive effective Hamiltonians to describe the topological edge modes for different TRITOPS models, including Hamiltonians with $p$-wave pairing and Hamiltonians combining $s$-wave pairing with spin-orbit coupling. We also derive effective low-energy Hamiltonians to describe the Josephson junction. These can be solved analytically and explain the contribution of the edge states to the Josephson current as a function of the phase bias. We find that edge modes yield peculiar features to the CPR for both junction types. The primary effects occur for the response of the Majorana zero modes at half-flux quantum phase $\varphi \approx \pi$ in TRITOPS-TRITOPS junctions and for integer flux quantum phase $\varphi \approx 0$ for TRITOPS-S junctions, respectively. The former effect is particularly strong for two-component nematic superconductors. The second effect leads to a spontaneously broken time-reversal symmetry in the TRITOPS-S junction and to a breakdown of the bulk-boundary correspondence. We analyze in this case the role of the phase fluctuations. For weakly coupled junctions, we show that time-reversal symmetry is restored for large enough stiffness in these fluctuations.

According to the classic law of Da Vinci-Amontons, a friction force $f$ was found to increase macroscopically with an external normal load $N(f=\mu N)$, with a positive definite friction coefficient μ. Here we employ first-principles calculations to predict that, when sliding the ferroelectric two-dimensional ${\mathrm{In}}_2{\mathrm{Se}}_3$ over graphene, the differential friction coefficient μ, measured by the slope of the corrugation in the sliding potential energy barrier subject to load $N$, displays an overall positive feather when the dipole is aligned toward the ${\mathrm{In}}_2{\mathrm{Se}}_3$-graphene interface; however, μ exhibits intriguing negative-positive oscillation with increasing $N$ when the dipole is aligned outward from the interface. Such striking observations can be rationalized by the van der Waals and electrostatic interaction-induced competition between the downward shift of the sliding barrier by the ${\mathrm{p}}_{\mathrm{x}}$- and $p_y$-like levels and the upward shift of the barrier by the $p_{\mathrm{z}}$ states, which is accompanied by an oscillation of the ${\mathrm{In}}_2{\mathrm{Se}}_3$-graphene interfacial charge redistribution subject to the external load. The present findings are expected to play an instrumental role in the design of high-performance solid lubricants.

Near-field radiative heat transfer (NFRHT) between irregularly shaped dielectric particles made of ${\mathrm{SiO}}_2$ and morphology characterized by Gaussian random spheres is studied. Particles are modeled using the discrete system Green's function (DSGF) approach, which is a volume integral numerical method based on fluctuational electrodynamics. This method is applicable to finite, three-dimensional objects, and all system interactions are defined independent of thermal excitation by a generalized system Green's function. The DSGF method is deemed suitable to model NFRHT between irregularly shaped particles after verification against the analytical solution for chains of two and three ${\mathrm{SiO}}_2$ spheres. The NFRHT results reveal that geometric irregularity in particles leads to a reduction of the total conductance from that of comparable perfect spheres at vacuum separation distances smaller than the particle size, a regime in which NFRHT is a surface phenomenon. At vacuum separation distances larger than the particle size, NFRHT becomes a volumetric process, and the total conductance between irregularly shaped particles converges to that of comparable perfect spheres. Spectral analysis reveals, however, that particle irregularity leads to damping and broadening of resonances at all separation distances, thereby highlighting the importance of the DSGF method for spectral engineering in the near field. The reduced spectral coherence when particle size is larger than the vacuum separation distance is attributed to coupling of surface phonon-polaritons within the randomly generated, distorted particle features. For particle size smaller than the vacuum separation distance, resonance broadening and damping are linked with the multiple localized surface phonon modes supported by the composite spherical harmonic morphologies of the Gaussian random spheres. This paper has direct implications for thermal management of packed particle systems, with applications in radiative property control, electronics, energy conversion, and nanomanufacturing.

We introduce a two-dimensional Chern insulator in proximity to a $d$-wave pseudogap state of the high-$T_c$ superconducting material as an effective platform to realize the higher-order topological system. The proximity-induced $d$-density-wave (DDW) order in the Chern insulator layer serves as an effective mass. The edge states will be fully gapped by this DDW order. In the real space, the sign of the DDW order parameter changes at the system corners due to the $d$-wave factor, leading to the gapless corner states, indicating that this system may be in a higher-order topological state. The higher-order topology in this coupled system is confirmed based on the calculation of the edge polarization and the quadrupole moment. In the superconducting state where the superconducting order and the DDW order coexist, the Majorana corner states emerge.

The absorption of light by free carriers in semiconductors results in optical loss for all photon wavelengths. Since free-carrier absorption competes with optical transitions across the band gap, it also reduces the efficiency of optoelectronic devices such as solar cells because it does not generate electron-hole pairs. In this work, we develop a first-principles theory of free-carrier absorption taking into account both single-particle excitations and the collective Drude term, and we demonstrate its application to the case of doped Si. We determine the free-carrier absorption coefficient as a function of carrier concentration and we obtain excellent agreement with experimental data. We identify the dominant processes that contribute to free-carrier absorption at various photon wavelengths, and analyze the results to evaluate the impact of this loss mechanism on the efficiency of Si-based optoelectronic devices.

We study Wigner crystallization of electron systems in phosphorene quantum dots with the confinement of an electrostatic origin with both circular and elongated geometry. The large effective masses in phosphorene promote the separation of the electron charges already for quantum dots of relatively small size. The anisotropy of the effective mass allows for the formation of Wigner molecules in the laboratory frame with a confined charge density that has lower symmetry than the confinement potential. We find that in circular quantum dots separate single-electron islands are formed for two and four confined electrons but not for three trapped carriers. The spectral signatures of the Wigner crystallization to be resolved by transport spectroscopy are discussed. Systems with Wigner molecule states are characterized by a nearly degenerate ground state at $B=0$ and are easily spin-polarized by the external magnetic field. In electron systems for which the single-electron islands are not formed, a more even distribution of excited states at $B=0$ is observed, and the confined system undergoes ground-state symmetry transitions at magnetic fields of the order of 1 T. The system of five electrons in a circular quantum dot is indicated as a special case with two charge configurations that appear in the ground state as the magnetic field is changed: one with the single electron islands formed in the laboratory frame and the other where only the pair-correlation function in the inner coordinates of the system has a molecular form as for three electrons. The formation of Wigner molecules of quasi-1D form is easier for the orientation of elongated quantum dots along the zigzag direction with heavier electron mass. The smaller electron effective mass along the armchair direction allows for freezing out the transverse degree of freedom in the electron motion. Calculations are performed with a version of the configuration interaction approach that uses a single-electron basis that is preoptimized to account for the relatively large area occupied by strongly interacting electrons allowing for convergence speed-up.

It is shown using the method of molecular dynamics that the motion of carbon nanoparticles (rectangular graphene flakes, spherical fullerenes of size $L<10$ nm) on the surface of a thermalized graphene sheet lying on a flat substrate can be described as the motion of particles in a viscous medium with a constant coefficient of friction, the value of which depends on the temperature and particle size. It has been shown that there are two types of effective friction: diffusion and ballistic. In the ballistic regime of motion (at velocities $v>200$ m/s), deceleration occurs due to the interaction of moving nanoparticles with thermal out-of-plane bending vibrations of a graphene sheet. Because of this, with the increasing temperature, the coefficient of friction monotonically increases. In the diffusion regime of motion (at $v<20$ m/s), friction arises due to the need for the particle to overcome local energy barriers, therefore it decreases with increasing temperature. The difference between ballistic and diffusion friction is most pronounced at low temperatures, since the mobility of nanoparticles in the ballistic regime of motion decreases with increasing temperature, while in the diffusion regime it monotonously increases. Thermophoresis modeling shows that at large values of the temperature gradient along the substrate, a ballistic mode of motion of nanoparticles is realized. In this mode, the mobility of nanoparticles does not depend on their shape and size, but is determined only by the value of the temperature gradient. It is shown that the presence of a normal force pressing the nanoparticle to the substrate leads to an increase in its friction with the substrate.

Here we present a combined experimental and ab initio lattice dynamics study of the semiconducting $\beta$ phase of $\mathrm{Fe}{\mathrm{Si}}_2$. A polycrystalline $\beta \text{−}\mathrm{Fe}{\mathrm{Si}}_2$ film was prepared on Si(111) and single-crystalline, self-assembled $\beta \text{−}\mathrm{Fe}{\mathrm{Si}}_2$ nanorods were grown on Si(110) by molecular beam epitaxy. Both types of nanostructures were obtained by annealing of precursor structures, an epitaxial Fe film in the case of the film and high-aspect-ratio $\alpha \text{−}\mathrm{Fe}{\mathrm{Si}}_2$ nanowires in the case of the nanorods. The morphology and crystalline structure of the samples were investigated by reflection high-energy electron diffraction, atomic force microscopy, as well as x-ray diffraction and x-ray absorption spectroscopy. The Fe-partial phonon density of states (PDOS) was obtained from nuclear inelastic scattering. The PDOS of the film was investigated in the temperature range of 296 K down to 11 K and shows an excellent agreement with the ab initio calculations. In the PDOS of the nanorods, a shift in the number of states in the main features and an additional vibrational mode at 20 meV are observed. While the first effect can fully be explained by the specific orientation of the $\beta \text{−}\mathrm{Fe}{\mathrm{Si}}_2$ unit cell on the Si(110) surface, the second effect is attributed to the formation of an $\alpha \text{−}\mathrm{Fe}{\mathrm{Si}}_2$ interlayer at the $\beta \text{−}\mathrm{Fe}{\mathrm{Si}}_2/\mathrm{Si}$ interface. Furthermore, the thermoelastic properties of the film show a harmonic behavior in the investigated temperature range. For the nanorods, no significant deviation from the film is observed, except for a small decrease of the sound velocity.

Niobium is one of the most researched superconductors, both theoretically and experimentally. It is enormously significant in all branches of superconducting applications, from powerful magnets to quantum computing. It is therefore imperative to understand its fundamental properties in great detail. Here, we use the results of recent microscopic calculations of anisotropic electronic, phonon, and superconducting properties, and apply a thermodynamic criterion for the type of superconductivity, more accurate and straightforward than a conventional Ginzburg-Landau parameter $\kappa$-based delineation, to show that pure niobium is a type-I superconductor in the clean limit. However, disorder (impurities, defects, strain, stress) pushes it to become a type-II superconductor.

The spin current density functional theory (SCDFT) is the generalization of the standard DFT to treat a fermionic system embedded in the effective external field produced by the spin-orbit coupling interaction. Even in the absence of a spin polarization, the SCDFT requires the electron-electron potential to depend on the spin currents ${\mathbf{J}}^x, {\mathbf{J}}^y$, and ${\mathbf{J}}^z$, which only recently was made possible for practical relativistic quantum-mechanical simulations, through the inclusion of a fraction of nonlocal Fock exchange into local-density or generalized-gradient density functional approximations [J. K. Desmarais, J.-P. Flament, and A. Erba, Phys. Rev. B 102, 235118 (2020)]. Here, we apply the SCDFT to the quantum spin Hall phase and show how it improves (even qualitatively) the description of its electronic features relative to the DFT. We study the Bi (001) 2D bilayer and its band insulator to topological insulator phase transition (via $s+p_z\leftrightarrow p_x+i{p}_y$ band inversion) as a function of mechanical strain. We show that the explicit account of spin currents in the electron-electron potential of the SCDFT is key to the appearance of a Dirac cone at the $\mathrm{\Gamma }$ point in the valence band structure at the onset of the topological phase transition. Finally, the valence band structure of this system is rationalized using a simple first-order $\mathbf{k}·\mathbf{p}$ quasidegenerate perturbation theory model.

We numerically investigate heating-and-cooling quenches taking place entirely in the nonordered phase of the discrete Gross-Pitaevskii equation on a three-dimensional cubic lattice. In equilibrium, this system exhibits a $U\left(1\right)$-ordering phase transition at an energy density which is significantly lower than the minimum one during the quench. Yet, we observe that the postquench relaxation is accompanied by a transient $U\left(1\right)$ ordering, namely, the correlation length of $U\left(1\right)$ fluctuations significantly exceeds its equilibrium prequench value. The longer and the stronger the heating stage of the quench, the stronger is the $U\left(1\right)$ transient ordering. We identify the origin of this ordering with the emergence of a small group of slowly relaxing lattice sites accumulating a large fraction of the total energy of the system. Our findings suggest that the transient ordering may be a robust feature of a broad class of physical systems. This premise is consistent with the growing experimental evidence of the transient $U\left(1\right)$ order in rather dissimilar settings.

Electrons' multiple quantum degrees of freedom can lead to rich physics, including a competition between various exotic ground states, as well as novel applications such as spintronics and valleytronics. Here we report magnetotransport experiments demonstrating how the valley degree of freedom impacts the fractional quantum states (FQHSs), and the related magnetic-flux-electron composite fermions (CFs), at very high magnetic fields in the extreme quantum limit when only the lowest Landau level is occupied. Unlike in other multivalley two-dimensional electron systems such as Si or monolayer graphene and transition-metal dichalcogenides, in our AlAs sample we can continuously tune the valley polarization via the application of in situ strain. We find that the FQHSs remain exceptionally strong even as they make valley polarization transitions, revealing a surprisingly robust ferromagnetism of the FQHSs and the underlying CFs. Our observation implies that the CFs are strongly interacting in our system. We are also able to obtain a phase diagram for the FQHS and CF valley polarization in the extreme quantum limit as we monitor transitions of the FHQSs with different valley polarizations.

Many intriguing physical phenomena have emerged from the Bi-containing materials due to the interplay of band topology and strong spin-orbit coupling (SOC). In this work, we report anisotropic spin-to-charge conversion in Bi, a semimetal endowed with highly anisotropic Fermi surfaces in addition to strong SOC. The value of spin Hall angle ${\theta }_{\mathrm{SH}}$ in Bi, that measures the spin-to-charge conversion efficiency, varies from ${\theta }_{\mathrm{SH}}\approx 0$ to ${\theta }_{\mathrm{SH}}\approx 2.6\%$ for two predominant crystal orientations. In addition, the propensity of oxidation and interface quality also affect the spin-to-charge conversion in Bi. The highly anisotropic spin-to-charge conversion between Bi(003) and Bi(012) provide significant insights into the exploration and development of Bi-based topological quantum materials and spintronics.

A Majorana box—two topological superconducting nanowires coupled via a trivial superconductor—is a building block in devices aiming to demonstrate non-Abelian physics, as well as for topological quantum computer architectures. We theoretically investigate charge transport through a Majorana box and show that current can be blocked when two Majoranas couple to the same lead, fixing their parity. In direct analogy to a Pauli spin blockade in spin qubits, this parity blockade can be used for fast and high-fidelity qubit initialization and readout, as well as for current-based measurements of decoherence times. Furthermore, we demonstrate that transport can distinguish between a clean Majorana box and a disordered box with additional unwanted Majorana or Andreev bound states.

We propose and theoretically investigate an alternative way to create the poor man's Majorana bound states (MBSs) introduced in Phys. Rev. B 86, 134528 (2012). Our proposal is based on two quantum dots (QDs) with strong electron-electron interactions that couple via a central QD with proximity-induced superconductivity. In the presence of spin-orbit coupling and a magnetic field, gate control of all three QDs allows tuning the system into sweet spots with one MBS localized on each outer dot. We quantify the quality of these MBSs and show how it depends on the Zeeman energy and interaction strength. We also show how nonlocal transport spectroscopy can be used to identify sweet spots with high MBS quality. Our results provide a path for investigating MBS physics in a setting that is free of many of the doubts and uncertainties that plague other platforms.

Polaritons are quasiparticles describing the coupling between light and matter. In two-dimensional materials, polaritonic phenomena are abundant and unique, owing to their low dimensionality and the extraordinary properties of the supported quasiparticles. In this work, we predict the existence of hyperbolic exciton polaritons (HEPs) in few-layer transition-metal dichalcogenides (TMDs) at visible frequencies. We show that hyperbolicity can be induced in the TMD under certain conditions owing to the resonant behavior of the supported excitons, leading to the existence of HEPs. We derive the HEPs’ dispersion relation under these conditions and analyze their confinement and loss properties, incorporating nonlocal corrections stemming from the high momentum of the modes. In addition, we show that owing to the valley properties of TMDs, the HEPs are coupled to the valley degree of freedom, leading to a hyperbolic spin-valley Hall effect. Such highly confined and valley-polarized HEPs provide opportunities to control strong light-matter interaction at the atomic scale.

There is an apparent mismatch between electron paramagnetic resonance and Mössbauer spectroscopy results on the charge and spin states of dilute Fe impurities in NaCl; Mössbauer spectroscopy data have been interpreted in terms of high-spin ${\mathrm{Fe}}^{2+}$, while electron paramagnetic resonance studies suggest low-spin ${\mathrm{Fe}}^{1+}$. In the present study, the charge and spin states of dilute substitutional Fe impurities in NaCl and LiF have been investigated with ${}^{57}\mathrm{Mn}\to {}^{57}\mathrm{Fe}$ emission Mössbauer spectroscopy. A scheme is proposed which takes into account the effects of nearest-neighbor distances and electronegativity difference of the host atoms on the Mössbauer isomer shift and allows for the unequivocal differentiation between high-spin ${\mathrm{Fe}}^{2+}$ and high/low-spin ${\mathrm{Fe}}^{1+}$ in Mössbauer spectroscopy. From these considerations, the Mössbauer results are found to be consistent with dilute Fe impurities in NaCl and LiF in a low-spin ${\mathrm{Fe}}^{1+}$ state. These conclusions are supported by theoretical calculations of isomer shifts and formation energies based on the density-functional theory. The experimental results furthermore suggest that charge compensation of dilute ${\mathrm{Mn}}^{2+}$ dopants in NaCl and LiF is achieved by Na vacancies and ${\mathrm{F}}^{-}$ interstitials, respectively.

Based on the PXP model adapted for Rydberg-blockaded chains, we investigate the dynamics of topological domain walls between different quantum many-body scar states of ${\mathbb{Z}}_2$ symmetry. It is found that the domain walls not only possess oscillating features of scars but also manifest longstanding bipartite entanglement with exactly unchanged flip-flop phase difference, suggesting their potential as a quantum information resource. A periodically driven field is exerted and the high-frequency drive gives rise to a crossover from prethermalization to Floquet localization. In order to investigate the stability of domain walls acting as information carriers, we further simulate the collision between them and find negligible influence on each other. Subsequently, the quench dynamics with domain walls reveals exotic physics and applicable potentials of nonthermalized scar states.