Ferromagnetic insulating (FMI) double perovskite oxides (DPOs) $A_{2}B{B}^{\prime }{\mathrm{O}}_6$ with near-room-temperature Curie temperatures are promising candidates for ambient-temperature spintronics applications. To realize their potential, epitaxial stabilization of DPO films and understanding the effect of multiple broken symmetries across the film/substrate interface are crucial. This study investigates ultrathin films of the FMI ${\mathrm{Nd}}_2{\mathrm{NiMnO}}_6$ grown on ${\mathrm{SrTiO}}_3$ (STO) and ${\mathrm{NdGaO}}_3$ (NGO) substrates. By comparing growth on these substrates, we examine the influence of polarity and structural symmetry mismatches, which are absent in the NGO system. The interface exhibits immeasurable resistance in both cases. Using synchrotron x-ray diffraction, we show that films have three octahedral rotational domains because of the structural symmetry mismatch with the STO substrate. Furthermore, our coherent Bragg rod analysis of specular x-ray diffraction reveals a significant modification of the out-of-plane lattice parameter within a few unit cells at the film/substrate interface and the surface. This arises from polarity compensation and surface symmetry breaking, respectively. These structural alterations influence the Mn orbital symmetry, a dependence that we further confirm through x-ray linear dichroism measurements. Since the ferromagnetism in insulating DPOs is mediated by orbital-dependent superexchange interactions [Phys. Rev. Lett. 100, 186402 (2008)], our study provides a framework for understanding the evolution of magnetism in ultrathin geometry.

We theoretically investigate the transport signature of single and multiple Floquet Majorana end modes (FMEMs), appearing in an experimentally feasible setup with Rashba nanowire (NW) placed in closed proximity to a conventional $s$-wave superconductor, in the presence of an external Zeeman field. Periodic drive causes the anomalous $\pi$-modes to emerge in addition to the regular 0-modes in the driven system where the former does not exhibit any static analog. For single 0- and/or $\pi$-FMEM, differential conductance exhibits a quantized value of $2e^2/h$ while we consider the sum over all the photon sectors, supporting Floquet sum rule. We examine the stability of this summed conductance against random onsite disorder. We further investigate the summed conductance in several cases hosting multiple (more than one) 0- or $\pi$-modes at the end of the NW. In these cases, we obtain quantized values of $n_M\times 2e^2/h$ in summed differential conductance with $n_M$ being the number of modes (0 or $\pi$) localized at one end of the NW. We repeat our analysis for another experimentally realizable model system known as helical Shiba chain. Moreover, we corroborate our results by computing the differential conductance for FMEMs using nonequilibrium Green's function method. Our work opens up the possibility of studying the transport signatures of FMEMs in these realistic models.

We study signatures of Majorana zero modes (MZMs) bound to Josephson vortices in superconductor–three-dimensional topological insulator–superconductor Josephson junctions placed in a perpendicular magnetic field. First, using semiclassical analytical as well as numerical techniques, we calculate the spatially resolved supercurrent density carried by the low-energy Caroli–de Gennes-Matricon (CdGM) states in the junction. Motivated by a recent experiment [Yue et al., Phys. Rev. B 109, 094511 (2024)], we discuss if and how the presence of vortex MZMs is reflected in supercurrent measurements, showing that Fraunhofer signatures alone are not suitable to reliably detect vortex MZMs. Next, we propose two ways in which we believe supercurrent measurements could be complemented to further verify that the junction does indeed host MZMs. Explicitly, we discuss how additional Majorana signatures could be obtained by (i) mapping out the local density of states in the junction via scanning tunneling microscopy techniques, and (ii) microwave spectroscopy of the spectrum of low-energy CdGM states in the junction.

Inspired by the recent experimental synthesis of ${\mathrm{Ru}}_2{\mathrm{Si}}_x{\mathrm{O}}_y$ nanosheets derived from the $I4/mmm$ parent compound ${\mathrm{YRu}}_2{\mathrm{Si}}_2$, we study the two-dimensional $\alpha$-PbO ruthenium chalcogenide ($\mathrm{Ru}Ch$) monolayers. We find that while monolayer RuO is unstable, RuS, RuSe, and RuTe exhibit robust thermodynamic and dynamic stability. The electronic structures of these $\mathrm{Ru}Ch$ monolayers closely resemble those of 11-family iron-based superconductors, featuring multiple Fermi surface sheets with hole pockets at the $\mathrm{\Gamma }$ point and electron pockets near the $M$ points, connected by $(\pi ,\pi )$ nesting vectors. Additionally, stabilizing these monolayers on a ${\mathrm{SrTiO}}_3$ (STO) substrate preserves their nesting features, providing an excellent reference for studies on FeSe/STO. By employing effective tight-binding models to explore electron correlation effects, our singular-mode functional renormalization group analysis identifies a predominant $d_{x^2-y^2}$ pairing symmetry. This symmetry, driven by spin fluctuations at the wave vector $(\pi ,\pi )$, remains robust under electron doping. These results suggest that $\mathrm{Ru}Ch$ monolayers share essential features with 11-family Fe-based superconductors and may serve as a platform for exploring RuCh-based high-temperature superconductivity.

We compute the spectra of anyon quasiparticles in all three superselection sectors of the Kitaev model (i.e., visons, fermions, and bosons), perturbed by a Zeeman field away from its exactly solvable limit, to gain insights on the competition of its non-Abelian spin-liquid with other nearby phases, such as the mysterious intermediate state observed in the antiferromagnetic model. In both ferro- and antiferromagnetic models, we find that the fermions and visons become gapless at nearly identical critical Zeeman couplings. In the ferromagnetic model, this is consistent with a direct transition into a polarized state. In the antiferromagnetic model, this implies that previous theories of the intermediate phase viewed as a spin liquid with a different fermion Chern number are inadequate, as they presume that the vison gap does not close. In the antiferromagnetic model, we also find that a bosonic quasiparticle becomes gapless at nearly the same critical field as the fermions and visons. This boson carries the quantum numbers of an antiferromagnetic order parameter, suggesting that the intermediate phase may possess spontaneously broken symmetry with this order.

With the advent of new fabrication technologies, time-varying metasurfaces have emerged as novel platforms for exotic waveform shaping in microwaves and optics, providing an additional degree of freedom to design dynamically controllable and reconfigurable scatterers. Nevertheless, inherent structural properties and material dispersion significantly complicate the computational analysis of such time-varying structures, highlighting the importance of efficient and intuitive analytical approaches that are generalizable to a wide class of meta-atom topologies and temporal modulation profiles. In this work, surface susceptibilities under generalized sheet transition conditions (GSTCs), widely used to model metasurfaces, are extended to a compact and tractable operator form for the frequency-domain analysis of metasurfaces with arbitrary temporal modulation and material dispersion. By virtue of this formalism, we derive the time-variant versions of simple surface susceptibility formulas for periodic arrays of freestanding and substrated dipolar scatterers through the use of appropriate polarizabilities. The applicability of the proposed approach to the characterization of reflective and transmissive properties of pulsed and Floquet metasurfaces is demonstrated and verified against full-wave simulations.

The magnetic ground state (MGS) and physical properties of monolayer (ML) $1T\text{−}{\mathrm{CrTe}}_2$ remain unresolved. Using first-principles calculations, we elucidate the mechanism underlying its antiferromagnetic (AFM) MGS, accompanied by structural dimerization, and reveal a unique lattice-driven topological phase transition. The MGS exhibits pronounced sensitivity to the lattice constant, governed by the competition between AFM direct exchange and ferromagnetic (FM) superexchange interactions. Below a critical lattice constant of 7.56 Å, AFM coupling dominates, inducing magnetic frustration and subsequent structural dimerization. Within the dimerized phase, the magnetic easy axis transitions from in-plane to out-of-plane orientation as the lattice constant increases. Furthermore, we identify a different AFM topological phase transition driven by lattice variations unique to the dimerized system. These findings resolve the long-standing controversy regarding the MGS and uncover a lattice-driven $\mathcal{PT}$-symmetric AFM topological insulator state.

In a class of systems, there are gapped boundary-localized states described by a boundary Hamiltonian. The topological classification of gapped boundary Hamiltonians, same as the standard tenfold way for gapped bulk states, can lead to the emergence of boundary topological insulators (TIs) and superconductors (TSCs). In this work, we present a theoretical study of boundary TIs and TSCs of the full Altland-Zirnbauer tenfold symmetry classes. Based on the boundary projection analyses for a $d$-dimensional Dirac continuum model, we demonstrate that nontrivial boundary topology can arise at a $(d-n)$-dimensional boundary if the Dirac model incorporates ($n+1$) mass terms with $0<n<d$ although its bulk and $(d-1)\mathrm{D}, \cdots , (d-n+1)\mathrm{D}$ boundaries are topologically trivial. Furthermore, we present a unified criterion for the emergence of nontrivial boundary topology by extending bulk classification within the context of the Dirac model, which provides a unified framework for nontrivial bulk and boundary topology. Inspired by the Dirac continuum model analysis, we further construct bulk lattice Hamiltonians for realizing boundary TIs and TSCs of the full Altland-Zirnbauer tenfold symmetry classes, which enables the realization of higher-order TIs and TSCs in arbitrary dimensions with arbitrary orders. We analyze some typical examples of the constructed boundary TIs and TSCs in physical dimensions.

A bang-bang (BB) algorithm prepares the ground state of a lattice quantum many-body Hamiltonian $H=H_1+H_2$ by evolving an initial product state alternating between $H_1$ and $H_2$. We optimize the algorithm with tensor networks in one and two dimensions. The optimization has two stages. In stage one, a shallow translationally-invariant circuit is optimized in an infinite lattice. In stage two, the infinite-lattice gate sequence is used as a starting point for a finite lattice where it remains optimal in the bulk. The prepared state requires optimization only at its boundary, within a healing length from lattice edges, and the gate sequence needs to be modified only within the causal cone of the boundary. We test the procedure in the 1D and 2D quantum Ising model near its quantum critical point employing, respectively, the matrix product state (MPS) and the pair-entangled projected state (PEPS). At the boundary already the infinite-lattice sequence turns out to provide a more accurate state than in the bulk.

High spin-Chern-number topological phases provide a promising low-dimensional platform for realizing double-helical edge states. Here we show how these edge states can host a variety of phases driven by electron interaction effects, including multichannel helical Luttinger liquid, spin density wave, superconducting phases, and a new type of $\pi$-junction analog of the latter two, where the transitions between the phases can be controlled. The superconducting phase in the interacting system is shown to be adiabatically connected to a time-reversal-symmetric topological superconductor in the noninteracting DIII class. This connection stabilizes Majorana Kramers pairs as domain wall states at the interface between the superconducting and $\pi$-spin-density wave phases, with the latter exhibiting a time-reversal-symmetric spin-density wave phase. We discuss the possibility of realizing our proposed scheme for generating Majorana Kramers pairs in a cold-atom-based platform with existing techniques, and how it could offer potential advantages over other approaches.

Planar normal-state resistivity data from three families of hole-doped single-layer cuprate superconductors, i.e., Bi2201 (${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CuO}}_{6+x}$), Tl2201 (${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CuO}}_{6+x}$), and Hg1201 (${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+x}$), are calculated using the extremely correlated Fermi liquid theory (ECFL). This theory was recently employed by us to compute the resistivity of three families of single-layer cuprate superconductors, i.e., ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO), ${\mathrm{Bi}}_2{\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CuO}}_{6+\delta }$ (BSLCO), and ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ (NCCO), followed by a detailed comparison. Adding the three systems studied here accounts for all the remaining single-layer compounds, where data are available for a range of densities and temperatures, thereby providing a comprehensive study of one class of important cuprate superconductors. The added study of the material Bi2201 is of particular interest since it is the system where the almost linear in temperature resistivity was first reported in 1990. Only recently, in 2022, has a systematic doping analysis become available. The Tl2201 system has two distinct sets of band parameters that fit the same Fermi surface, providing new challenges and insights into the ECFL theory.

Monolayer ${\mathrm{WSe}}_2$ (ML ${\mathrm{WSe}}_2$) exhibits orders of magnitude enhancement in second-harmonic-generation efficiency under single one-photon (1-p) or two-photon (2-p) resonant excitation. Here, we propose a strain engineering approach to greatly boost the in-plane second-order nonlinear susceptibility (${\chi }_{yyy}$) of ML ${\mathrm{WSe}}_2$ by strain shifting the $K$-valley A-exciton and a high-lying exciton (HX) into double resonance. We first identify the A-exciton and HX from the two-dimensional (2D) Mott-Wannier model for pristine ML ${\mathrm{WSe}}_2$ and calculate the ${\chi }_{yyy}$ for 1-p and 2-p resonance excitations, and observe a $\sim 40$-fold ${\chi }_{yyy}$ enhancement arising from the 2-p HX resonance state (compared with the A-exciton case). By applying a small uniform biaxial strain (0.16%), we observe an exciton double-resonance state ($E_{\mathrm{HX}}=2E_{\mathrm{A}}$; $E_{\mathrm{HX}}$ and $E_{\mathrm{A}}$ are the exciton absorption energies), which yields up to an additional $\sim 50$-fold enhancement in ${\chi }_{yyy}$ (compared to the 2-p HX resonance state), indicating an overall $\sim 2000$-fold enhancement (compared with the single 2-p A-exciton resonance case). Further exploration of the strain-engineered exciton states (for a $\sim 0.16$% biaxial strain) reveals that double resonance also occurs at other wave vectors near the $K$ valley, leading to additional enhancements in ${\chi }_{yyy}$. This confirms that strain engineering is an effective approach for enhancing ${\chi }_{yyy}$. Our findings suggest different avenues for strain engineering optical properties of 2D materials for novel nonlinear optical and optoelectronic applications.

It has recently been shown that signatures of non-Hermitian topology can be realized in a conventional quantum Hall device connected to multiple current sources. These signatures manifest as robust current-voltage characteristics, dictated by the presence of a nontrivial, non-Hermitian topological invariant of the conductance matrix. Chiral edge states are believed to be responsible for this non-Hermitian response, similar to how they lead to a quantized Hall conductivity in the presence of a single current source. Here, we go beyond this paradigm, showing that multiterminal conductance matrices can exhibit non-Hermitian topological phase transitions that cannot be traced back to the presence and directionality of a boundary-localized chiral mode. By performing quantum transport simulations in the quantum Hall regime of monolayer graphene, we find that when the chemical potential is swept across the zeroth Landau level, unavoidable device imperfections cause the appearance of an additional non-Hermitian phase of the conductance matrix. This highlights graphene as an ideal platform for the study of non-Hermitian topological phase transitions, and is a first step towards exploring how the geometry of quantum devices can be harnessed to produce robust, topologically-protected transport characteristics.

The revised Chen's derivative rule for electron tunneling is implemented to enable computationally efficient first-principles-based calculations of the differential conductance $dI/dV$ for scanning tunneling spectroscopy simulations. The probing tip is included through a single tip apex atom, and its electronic structure can be modeled as a linear combination of electron orbitals of various symmetries or can be directly transferred from first-principles electronic structure calculations. By taking pristine and boron- or nitrogen-doped graphene sheets as sample surfaces, the reliability of our implementation is demonstrated by comparing its results to those obtained by the Tersoff-Hamann and Bardeen's electron tunneling models. It is highlighted that the energy-resolved direct and interference contributions to $dI/dV$ arising from the tip's electron orbitals result in a fingerprint of the particular combined surface-tip system. The significant difference between the electron acceptor boron and donor nitrogen dopants in graphene is reflected in their $dI/dV$ fingerprints. The presented theoretical method allows for an unprecedented physical understanding of the electron tunneling process in terms of tip-orbital-resolved energy-dependent $dI/dV$ maps that is anticipated to be extremely useful for investigating the local electronic properties of novel material surfaces in the future.

We consider the effects of Coulomb interactions on the transport properties of a one-dimensional topological superconductor nanowire hosting Majorana bound states at its ends. The analytic results are derived for the temperature-dependent differential conductance of a grounded nanowire as a function of the Ohmic circuit impedance. For a gate-controlled nanowire, analytic expressions are derived for the rates of single-electron tunneling, local and nonlocal Andreev reflections, and elastic cotunneling. These rates are used in numerical calculations of current-voltage characteristics and differential conductance. The obtained results will be helpful in experimental searches of Majorana signatures in one-dimensional topological superconductors.

Characterizing materials and devices at the nanoscale demands the innovation of new techniques for measurement and analysis. Scattering-type scanning near-field infrared microscopy (S-SNIM) has proven to be an indispensable nano-optical probe of quantum materials. It has been most effective in measuring local phononic and electronic phenomena at infrared frequencies with nanometer-scale spatial resolution. However, realizing the full potential of experimental access to the nanoscale via S-SNIM requires an intricate understanding of the complex light-probe-sample interaction. In this work, through numerical modeling, we drastically expand our knowledge of the complex light-probe-sample interaction in S-SNIM to analyze and describe experimental measurements on anisotropic, bilayered, and nanostructured systems. This work is focused toward a universal S-SNIM analysis method based on numerical modeling.

We consider a generalization of the Su-Schrieffer-Heeger (SSH) model by including next-nearest-neighbour (NNN) interaction and balanced loss-gain (BLG), and subjecting the whole system to an external uniform magnetic field. We study the band structure, edge states, and persistent current in this extended SSH model under general boundary condition (GBC) of which the periodic, antiperiodic, and open boundary conditions appear as special cases. It is shown that the point band gap decreases with increasing strength of the NNN interaction and vanishes beyond a critical value for both topologically trivial and nontrivial phases. Further, the line gap exhibits closed-loop-like structures for nonvanishing NNN interaction under the periodic boundary condition (PBC). The Zak phase receives no contribution from the NNN interaction under the PBC. We show that the NNN interaction has no effect on the persistent current in the half-filled limit for the case of PBC, while for other fillings less than the half-filling, it enhances the magnitude of the current significantly. We numerically study the variation of the persistent current with respect to the system parameters under the GBC, for the case in which the Hamiltonian admits entirely real spectra, and show that its magnitude increases with the increasing strength of the BLG. We show that the model without the NNN interaction is exactly solvable for a class of GBC, of which PBC, antiperiodic boundary condition (APBC) and anti-Hermitian boundary condition (AHBC) arise as special cases. We obtain analytic expressions for the edge states in the case of open boundary condition (OBC) and AHBC for vanishing NNN interaction. We show numerically for OBC that edge states in the topologically trivial phase appear for nonvanishing NNN interaction only when the strength of the loss-gain term is greater than the modulus of the difference between the intercell and intracell hopping strengths. In the topologically nontrivial phase, the edge states under OBC exist only up to a critical value of the NNN strength and vanish beyond this critical value. The bulk-boundary correspondence (BBC) for unbroken $\mathcal{PT}$ phase is similar to Hermitian SSH model, while the non-Hermitian skin effect (NHSE) is observed for broken $\mathcal{PT}$ phase.