Pb on $\mathrm{Si}\left(111\right)\text{−}(7\times 7)$ shows surprising nucleation and mass transport dynamics at odds with standard theories. To create local imbalances on stable Pb islands we use the tip of a scanning force microscope. We enforce a short, local contact between the island and our tip. The subsequent island height growth and the local contact potential difference are studied via scanning force microscopy and Kelvin probe force microscopy. Though the island has a large volume increase after the contact, we observe that its surrounding wetting layer shows the same Pb density decrease as the global wetting layer. This indicates a collective density thinning of the wetting layer.

The realization and manipulation of topological Majorana zero modes (MZMs) have been pivotal in both theoretical and experimental research. In this paper, we present a symmetry enhanced topological model that supports stable, localized MZMs in a one-dimensional interacting fermionic system. The MZMs in our model, induced by particle-number conserving interactions, are fully decoupled from the chain's interior and remain degenerate regardless of chain length, allowing for simulations in extremely short fermionic chains. The topological phase transition is driven by changes in coupling parameters, transitioning to a trivial phase without affecting the MZMs' independence or degeneracy, only their topological protection. Additionally, local symmetries in the model enhance MZMs' stability and reveal unique odd-even effects across different chain lengths.

Manipulating quantum states in layered systems holds significant scientific value and application potential for designing novel electronic devices. Using first-principles calculations, we systematically explore the impacts of pure charge doping and interface coupling effects on charge density waves (CDWs) in monolayer $1T\text{−}{\mathrm{TiTe}}_2$ and $1T\text{−}{\mathrm{ZrTe}}_2$. While rapidly inhibited by hole doping, CDWs in these two tellurides exhibit nonmonotonic changes under increased electron doping, first intensifying and then weakening, which contrasts with the successive suppression of CDW by electron doping in monolayer ${\mathrm{TiSe}}_2$. Studies on the ${\mathrm{TiTe}}_2/\mathrm{graphene}$ heterostructures reveal a marked interlayer-interaction-potential-dependent CDW behavior, where distinct stacking configurations create varied graphene C-C bond environments underneath the ${\mathrm{TiTe}}_2$ layer either facilitating or impeding its CDW lattice distortion. In addition, the CDW gets readily suppressed in the ${\mathrm{TiTe}}_2/{\mathrm{ZrSe}}_2$ heterostructure due to interface electron transfer from ${\mathrm{TiTe}}_2$ to ${\mathrm{ZrSe}}_2$, coinciding with above hole-doping argument, and it can also be inhibited in bilayer ${\mathrm{TiTe}}_2$ owing solely to interlayer electronic coupling. For the ${\mathrm{ZrTe}}_2/{\mathrm{PtTe}}_2$ heterostructures, the effect of impairment caused by electronic coupling outweighs that of enhancement by effective electron doping to ${\mathrm{ZrTe}}_2$, thus suppressing the CDW consecutively going from a semiconducting to a semimetallic ${\mathrm{PtTe}}_2$ film substrate. We further find that after hole doping eliminates the CDW, superconductivity emerges due to strong electron-phonon coupling dominated by the suppressed soft mode, with critical temperature $T_c$ for monolayer ${\mathrm{TiTe}}_2$ and ${\mathrm{ZrTe}}_2$ reaching up to 3.1 and 0.9 K, respectively. A similar competition is also unveiled in the ${\mathrm{TiTe}}_2/{\mathrm{ZrSe}}_2$ and bilayer ${\mathrm{TiTe}}_2$ systems. These studies provide theoretical guidance for using doping and interfacial interaction to engineer CDW and superconducting properties in layered materials.

We theoretically investigate the thermoelectric response of Bogoliubov Fermi surfaces (BFSs) generated in a two-dimensional unconventional $d$-wave superconductor subjected to an external in-plane Zeeman field. These BFSs exhibiting the same dimension as the underlying normal-state Fermi surface are topologically protected by combinations of discrete symmetries. Utilizing the Blonder-Tinkham-Klapwijk formalism and considering a normal-$d$-wave superconductor hybrid junction, we compute the thermoelectric coefficients including thermal conductance, Seebeck coefficient, and figure of merit ($zT$), and examine the validation of the Widemann-Franz law in the presence of both voltage and temperature bias. Importantly, as a signature of the anisotropic nature of $d$-wave pairing, Andreev bound states (ABSs) formed at the normal-superconductor interface play a significant role in the thermoelectric response. In the presence of ABSs, we observe a substantial enhancement in the Seebeck coefficient ($\sim 200\mu \mathrm{V}/\mathrm{K}$) and $zT$ ($\sim 3.5$) due to the generation of the BFSs and thus making such setup a potential candidate for device applications. Finally, we strengthen our continuum model results by computing the thermoelectric coefficients based on a lattice-regularized version of our continuum model.

We investigate one- and two-photon absorption in twisted bilayer graphene (TBLG) by examining the effects of tuning the twist angle $\theta$ and the excitation energy $E_l$ on its absorption coefficients ${\alpha }_{i=1,2}$. We find that ${\alpha }_1$ as a function of $E_l$ for TBLG exhibits distinct peaks corresponding to its van Hove singularities. For small twist angles, such as $\theta \sim 1.8^{\circ }$, the magnitude of the resonant peak for ${\alpha }_1$ is roughly twice that of bilayer graphene (BLG). This enhanced response, compared to BLG, can be attributed to the increased density of states in the twisted structure. However, as the twist angle increases, the magnitude of the resonant peak approaches that of two decoupled single-layer graphene (SLG) sheets. On the other hand, the two-photon absorption coefficient ${\alpha }_2$ for TBLG at low twist angles displays an enhancement of about one order of magnitude compared to SLG at the energies corresponding to the resonant peak, as well as a small but notable increase relative to BLG. As the twist angle decreases from $8^{\circ }$ to $2.5^{\circ }$, the resonant peak intensifies by three orders of magnitude. Interestingly, as $\theta$ increases, the resonant features exhibited by ${\alpha }_{i=1,2}$ vs $E_l$ shift progressively from the infrared to the visible. On doping TBLG, both ${\alpha }_1$ and ${\alpha }_2$ vs $E_l$ remain essentially unchanged, but with a slight redshift in their resonant peaks. Additionally, we explore various polarization configurations for two-photon absorption and determine the conditions under which ${\alpha }_2$ becomes extremal.

Despite the presence of the exact solution and well-established recipe for its realization, the quest for the Kitaev spin liquid in real materials remains exceptionally challenging. Among many magnets, $\alpha \text{−}{\mathrm{RuCl}}_3$ emerges as a prime candidate, albeit the hallmarks of the spin liquid manifest only within the specific region where the zigzag-type antiferromagnetic order is suppressed by an applied magnetic field. Here, we propose the possible realization of the Kitaev spin liquid at zero field by making van der Waals heterostructures of $\alpha \text{−}{\mathrm{RuCl}}_3$ and a ferromagnet $\mathrm{Cr}{X}_3$ ($X=\mathrm{Cl}$ and I). Using ab initio calculations, we find that in the case of $X=\mathrm{Cl}$ the zigzag order is suppressed by the proximity effect of the ferromagnetic ${\mathrm{CrCl}}_3$ layer, while the Kitaev interaction is still relevant in the $\alpha \text{−}{\mathrm{RuCl}}_3$ layer. Notably, the induced Ru moment is close to the value observed in the spin liquid region of the bulk material, signifying the possibility of the Kitaev spin liquid at zero field. In contrast, in the case of $X=\mathrm{I}$, the system is on the verge of an insulator-metal transition by carrier doping through interlayer hybridization. Our results indicate that van der Waals heterostructures provide a platform for studying not only the magnetic but also the electronic properties of Kitaev magnets.

Quantum spin liquids realized in the Kitaev model offer a platform for fractionalization of spin into two quasiparticles: itinerant Majoranas and localized visons. Introducing a uniform weak magnetic field associates a Majorana zero mode with each vison excitation. The vison accompanied by a Majorana zero mode is known to behave as a non-Abelian anyon, which has garnered significant attention for its potential applications in topological quantum computing. Although spatial and temporal control of these anyons is essential for exploring their applicability in quantum computing, numerical simulations of creating, moving, and annihilating non-Abelian anyons by an external field remain challenging as this field violates the exact solvability of the Kitaev model. Moreover, such a field to control non-Abelian anyons may disturb the quantum state due to the energy injection it causes. In this study, by introducing energy dissipation phenomenologically in real-time simulations, we demonstrate that the generation, movement, and annihilation of vison excitations can be achieved while maintaining their localization. In particular, we find that a vison can be moved in a desired direction by using time-dependent local magnetic fields or gradient magnetic fields, and it remains accompanied by a Majorana zero mode even after its movement. We also reveal that a larger spatial extent of the Majorana zero modes bound to a vison facilitates the movement of the vison with smaller field gradients. Furthermore, our numerical simulations demonstrate pair creation and annihilation of visons, triggered by time-dependent magnetic fields. The results obtained in this study highlight the significance of energy dissipation in controlling non-Abelian anyons, which will stimulate further investigations into the nonequilibrium dynamics of fractional quasiparticles in strongly correlated electron systems, as well as studies for applications in topological quantum computation.

Spin-orbit torques in ferromagnet/nonmagnet/ferromagnet trilayers are studied using a combination of symmetry analysis, circuit theory, semiclassical simulations, and first-principles calculations using the nonequilibrium Green's function method with supercell disorder averaging. We focus on unconventional processes involving the interplay between the two ferromagnetic layers, which are classified into direct and indirect mechanisms. The direct mechanism involves spin current generation by one ferromagnetic layer and its subsequent absorption by the other. In the indirect mechanism, the in-plane spin-polarized current from one ferromagnetic layer “leaks” into the other layer, where it is converted into an out-of-plane spin current and reabsorbed by the original layer. The direct mechanism results in a predominantly dampinglike torque, which damps the magnetization towards a certain direction ${\mathbf{s}}_d^{}$. The indirect mechanism results in a predominantly fieldlike torque with respect to a generally different direction ${\mathbf{s}}_f^{}$. Similarly to the current-in-plane giant magnetoresistance, the indirect mechanism is only active if the thickness of the nonmagnetic spacer is smaller than or comparable to the mean free path. Numerical calculations for a semiclassical model based on the Boltzmann equation confirm the presence of both direct and indirect mechanisms of spin current generation. First-principles calculations reveal sizable unconventional spin-orbit torques in Co/Cu/Co, Py/Cu/Py, and Co/Pt/Co trilayers and provide strong evidence of indirect spin current generation.

A basic feature of superconductors is flux quantization, which leads to periodicity of superconducting parameters with magnetic field. This periodicity is crucial for understanding basic concepts, such as elementary charge, symmetry of the order parameter, etc. In quantum circuit applications the periodicity is utilized for maximizing design performance. These applications rely on the fact that the periodicity is well defined for a given superconducting structure. We use scanning SQUID imaging and numerical simulations to show that, in realistic nanoscale devices, the periodicity depends on the temperature and the actual geometric details of the structure, specifically, the width of the wires that define the superconducting network. This should be taken into account in any experiment or application based on complex superconducting structures.

We propose an experimentally feasible dynamic coupling of ferromagnets mediated by a charge current in a nonmagnetic heavy metal. Dynamic magnetic moments generate a spin current via spin pumping, subsequently converted into charge current through the inverse spin Hall effect. This charge current, in turn, generates another spin current via the spin Hall effect, exerting an anti-damping-like torque on the adjacent ferromagnet. Our theoretical framework reveals the dissipative nature of this dynamic coupling, characterized by energy-level attraction that promotes synchronization of frequency, phase, and amplitude information. Experimental validation is realized on submicrometer-sized Py rectangles interconnected by Pt strips using spin-torque ferromagnetic resonance techniques. The standing spin waves excited in Py arrays exhibit high spin pumping efficiency, enabling the demonstration of the proposed dynamic coupling. A notably long coupling length exceeding 2 µm is achieved. These findings underscore a promising pathway for integrated magnonic devices, particularly in applications related to spin synchronization and transmission.

In this study, we investigate the electronic structure and topological properties of the compound $\mathrm{SrS}{\mathrm{i}}_2$ using angle-resolved photoemission spectroscopy (ARPES), electrical transport measurements, and ab initio calculations. In contrast to recent theoretical predictions that $\mathrm{SrS}{\mathrm{i}}_2$ is a Weyl semimetal with robust Weyl nodes and Fermi arcs, our ARPES measurements on undoped and Ca-doped $\mathrm{SrS}{\mathrm{i}}_2$ single crystals show no evidence of the predicted Weyl fermions or Fermi arcs at ambient pressure. Instead, ARPES and transport data show that $\mathrm{SrS}{\mathrm{i}}_2$ is a narrow-gap semiconductor at ambient conditions. Our hybrid functional calculations also find a small band gap, in agreement with experiments. However, by applying external pressure, we induce a topological phase transition where the electronic bands overlap and Weyl nodes appear, as confirmed by transport measurements and calculations. Interestingly, we also observe a pressure-induced superconducting transition above 20 GPa. Our results establish $\mathrm{SrS}{\mathrm{i}}_2$ as a platform to study the interplay between topological states and superconductivity driven by external pressure. Importantly, our results challenge the previously predicted intrinsic Weyl semimetallic nature of $\mathrm{SrS}{\mathrm{i}}_2$ and highlight the need for combined experimental and advanced computational approaches to correctly describe the complex electronic structures in topological materials.

We present an advanced ab initio quantum Monte Carlo (QMC) calculation of the ground state of undoped ${\mathrm{CaCuO}}_2$. We extend the traditional single-determinant Slater-Jastrow approach to include multideterminant wave functions, inhomogeneous Jastrow factors, and orbital optimization. Our results demonstrate not only an improvement in the variational bound of the ground state energy—2.3 eV per formula unit lower than previous state-of-the-art techniques—but also confirm the presence of spin fluctuations in multideterminant wave functions in a strongly correlated cuprate system, which is integral to understanding high-$T_c$ superconductivity. This is a demonstration of capturing spin fluctuations in QMC wave functions on a cuprate, establishing the groundwork for studies on doped cuprates in the superconducting state, where spin fluctuations require more accurate characterization.

Quantum scars correspond to enhanced probability densities along unstable classical periodic orbits. In recent years, research on quantum scars has extended to various systems including the many-body regime. In this work we focus on the formation, prevalence, and stability of linear bouncing-ball (BB) scars in two-dimensional (2D) quantum wells. These scars have relevance as effective and controllable channels in quantum transport. We utilize imaginary time propagation to solve the 2D Schrödinger equation within an arbitrary external confining potential, specifically the quantum well model with external perturbations. We show how BB scars begin to emerge with a single perturbative peak, such as a repulsive bump or attractive dip that simulates the effect of a charged nanotip in the system. We then identify the optimal size of the perturbative peak to maximize the prevalence of these scars. Finally, we investigate the stability of BB scars against external noise and find that some of them are remarkably robust. This suggests promising opportunities for further applications of BB scars in quantum transport.

The quantum cellular automata (QCA) effect is a transition in which multiple electron move coordinately by Coulomb interactions and observed in multiple quantum dots. This effect will be useful for realizing and improving quantum cellular automata and information transfer using multiple electron transfer. In this paper, we investigate the real-time dynamics of the QCA charge transitions in a triple quantum dot by using fast charge-state readout realized by rf reflectometry. We observe real-time charge transitions and analyze the tunneling rate comparing with the first-order tunneling processes. We also measure the gate voltage dependence of the QCA transition and show that it can be controlled by the voltage.

The structural, electronic, optical, and vibrational properties of the antidepressant drugs duloxetine and sertraline were investigated through first-principles calculations using the density functional theory approach; in addition, the Hirshfeld surface was analyzed. Duloxetine had an indirect band gap of 3.37 eV, while for sertraline, a direct band gap of 4.27 eV was obtained. Duloxetine and sertraline have optical absorption in the UV region and static dielectric constants of 1.86 and 1.51, respectively. The IR and Raman spectra were analyzed, and the vibration modes were assigned to the functional groups, showing good agreement with the results in the literature.

Highly disordered NbN thin films exhibit promising superconducting and optical properties. Despite extensive study, discrepancies in its basic electronic properties persist. Analysis of the optical conductivity of disordered ultrathin NbN films, obtained from spectroscopic ellipsometry by the standard Drude-Lorentz model, provides inconsistent parameters. We argue that this discrepancy arises from neglecting the presence of quantum corrections to conductivity in the IR range. To resolve this matter, we suggest a modification to the Drude-Lorentz model, incorporating quantum corrections. The parameters obtained from the modified model are consistent with transport and superconducting measurements. The revisited values describing conduction electrons, which differ significantly from commonly adopted ones, are the electron relaxation rate $\mathrm{\Gamma }\approx 1.8\text{eV}/\hslash$, the Fermi velocity $v_F\approx 0.7\times {10}^6{\text{ms}}^{-1}$, and the electron density of states $N\left(E_F\right)=2$ states of both spins/eV/$V_{\text{f.u.}}$.