The local density of states near dopants or impurities has recently been probed by scanning tunneling microscopy in both the parent and very lightly doped compounds of the high-$T_c$ cuprate superconductors. We use a slave-rotor representation of the Hubbard model to compute the local density of states on impurities in a Mott insulator. Our calculation accounts for the following key features of the experimental observation: (i) positions and amplitudes of the in-gap spectral weights of a single impurity; (ii) the spectral weight transfer from the upper Hubbard band to the lower Hubbard band; (iii) the difference between the cases of single and multiple impurities. For multiple impurities, our study explains the complete suppression of spectral weight observed at precisely the Fermi energy and links this property to zeros of the underlying bulk Green's function of the Mott insulating phase.

In this letter, we predict the effect of the total cross-polarization conversion of transverse-magnetic electromagnetic wave incident upon drift-current biased graphene into a reflected wave with transverse-electric polarization, which can be possible in the total reflection regime. It is shown that the degree of polarization conversion depends on the angle between the stationary drift-current direction and the plane of the wave incidence. Total polarization conversion is promoted by excitation of the hybrid electromagnetic mode (having a mixed polarization) in graphene. Remarkably, this effect is unrelated to the spatial dispersion of graphene conductivity. This is a unique feature of graphene, which does not exist in conventional two-dimensional electron systems with massive charge carriers. The hybrid electromagnetic mode in graphene can enter the self-excitation regime in the terahertz (THz) frequency range due to the negative real part of graphene conductivity induced by drift current. The studied effects can be useful for creating compact electrically and/or optically tunable polarization convertors and polarization-sensitive amplifiers of THz radiation.

In the present contribution, we discuss quantum scattering in 1D periodic finite lattices of $N$ localized potentials by means of an exact Green's function approach. By considering continued fraction techniques, we solve the resulting recurrence relations, thus being able to derive the full structure reflections $R_N$ and transmission $T_N$ amplitudes in a closed analytic form. The framework allows for dealing with extremely large arrays, in some examples for $N$ up to ${10}^{10}$ cells (or building blocks). For so great $N$'s, in practice the protocol can unveil most of the basic features of the energy band structures of the corresponding infinite systems, demanding relatively little computational effort. We further investigate general scattering properties of distinct lattices, e.g., when their cells are spatially asymmetric or composed by two or more elementary shapes, each shape commonly modeled in the literature in terms of Dirac's delta, rectangular, trapezoidal, and triangular barriers. As concrete applications, we address the problem of parameter optimization of heterostructures used to build solar cells and the identification of some transmission resonance modes, relevant in the study of band-pass transmission in superlattices.

The antisymmetric Dzyaloshinkii-Moriya interaction (DMI) arises in systems with broken inversion symmetry and strong spin-orbit coupling. In conjunction with the isotropic and symmetric exchange interaction, magnetic anisotropy, the dipolar interaction, and an externally applied magnetic field, the DMI supports and stabilizes the formation of various kinds of complex mesoscale magnetization configurations, such as helices, spin spirals, skyrmions, or hopfions. A question of importance in this context addresses the neutron scattering signature of the DMI, in particular in nanoparticle assemblies, where the related magnetic scattering signal is diffuse in character and not of the single-crystal diffraction-peak type, as it is, e.g., seen in the B20 compounds. Using micromagnetic simulations we study the effect of the DMI in spherical FeGe nanoparticles on the randomly averaged magnetic neutron scattering observables, more specifically on the spin-flip small-angle neutron scattering cross section, the related chiral function, and the pair-distance distribution function. Within the studied parameter space for the particle size ($60\mathrm{nm}\le L\le 200\mathrm{nm}$) and the applied magnetic field ($-1\mathrm{T}\le {\mu }_0{H}_0\le 1\mathrm{T}$), we find that the chiral function is only nonzero when the DMI is taken into account in the simulations. This result is discussed within the context of the symmetry properties of the magnetization Fourier components and of the involved energies under space inversion. Finally, for small applied magnetic fields, we provide an easy-to-implement phenomenological correlation function for the DMI-induced spin modulations (with wave vector $k_{\mathrm{d}}$). The corresponding randomly averaged spin-flip small-angle neutron scattering cross section reproduces the main features found in the numerical simulations.

We investigate the instability of layer pseudospin paramagnetic (PSP) state to the formation of pseudospin density wave (PSDW) in two-dimensional (2D) electron bilayers, analogous to the formation of Overhauser spin density wave (SDW) in a single-layer 2D electron gas (2DEG) with spin 1/2. Our comprehensive study on phase diagrams, based on the self-consistent Hartree-Fock (HF) theory, reveals that the PSDW has a lower energy than both PSP and pseudospin ferromagnetic (PSF) states near the PSP-PSF phase transition boundary. When the two layers are populated by the same number of electrons, the PSDW momentum $Q_c\sim 2k_F$ near the PSP-PSDW boundary, where $k_F={\left(2\pi n\right)}^{1/2}$ is the Fermi momentum characterized by the density in one of the two layers, and $Q_c$ decreases as the system transitions to the PSF regime. Extending the HF study to the case of unequal layer densities, the PSP phase is unstable to PSDW for small density imbalances, with momentum $Q_c\sim k_{F,t}+k_{F,b}$, where $k_{F,t}$ and $k_{F,b}$ are Fermi momenta of top and bottom layers, respectively. In PSDW regime, the ground-state stability, defined by the energy difference between PSDW and the second-lowest-energy state, is one order of magnitude lower than that in PSF regime, and decreases with increasing layer separation $d$. Furthermore, incorporating RPA static screening with the Hubbard-type local field correction leads to disappearance of both SDW and PSDW phases and pushes the phase boundaries of paramagnetic to ferromagnetic transitions to larger $r_s$ values. Our study on PSDW in 2D electron bilayers is equally applicable to 2D hole bilayers. The idea of pursuing PSDW is, in general, relevant across various 2D bilayer systems, not limited to the parabolic model that we investigate in this paper, and provides a new possibility of exploring novel coherent phases.

Altermagnetism, recently spotlighted in condensed matter physics, presents captivating physical properties and holds promise for spintronics applications. This study delves into the theoretical description and categorization of two-dimensional altermagnetism using spin group theory. Employing spin-group formalism, we establish seven distinct spin layer groups, extending beyond the conventional five spin Laue groups, to describe two-dimensional altermagnetism. Utilizing these findings, we classify previously reported two-dimensional altermagnets and identify different materials exhibiting altermagnetism. Specifically, monolayer ${\mathrm{MnTeMoO}}_6$ and ${\mathrm{VP}}_2{\mathrm{H}}_8{\left({\mathrm{NO}}_4\right)}_2$ are predicted to be two-dimensional altermagnets. Furthermore, we scrutinize their spin-momentum-locking characteristics through symmetry analysis and density functional theory calculations, substantiating their altermagnetic properties.

It has long been known that superconductors in the crossover between superconductivity types I and II exhibit an intermediate mixed state (IMS) with exotic flux configurations such as vortex clusters and chains. Achieving a comprehensive microscopic description of the vortex configurations within this intertype region has proven elusive until now due to the large computational demands involved. We have addressed this long-standing problem by presenting microscopic calculations of the IMS configurations. Our calculations, performed at zero temperature, reveal key features of the IMS, including the formation of vortex clusters and the multivortex nature of interactions between vortices. The obtained boundaries of the IT domain reproduce existing experimental observations remarkably well. Also, the results match earlier findings obtained using the perturbation theory in the vicinity of the critical temperature.

The nonequilibrium dynamics of long-range pairing Kitaev model with noiseless/noisy linear time dependent chemical potential, is investigated in the framework of dynamical quantum phase transitions (DQPTs). We have shown that for the ramp crossing a single quantum critical point, while the short-range pairing Kitaev model displays a single critical time scale, the long-range pairing induces a region with three DQPTs time scales. We have found that the region with three DQPTs time scales shrinks in the presence of the noise. In addition, we have uncovered that for a quench crossing two critical points, the critical sweep velocity above which the DQPTs disappear enhances by the long-range pairing exponent while decreases in the presence of the noise. On the basis of numerical simulations, we have shown that noise diminishes the long-range pairing inductions.

Chiral spin liquids (CSL) based on spin-1/2 fermionic projected entangled pair states (fPEPS) are considered on the square lattice. First, fPEPS approximants of Gutzwiller-projected Chern insulators (GPCI) are investigated by variational Monte Carlo (VMC) techniques on finite-size tori. We show that such fPEPS of finite bond dimension can correctly capture the topological properties of the chiral spin liquid, as the exact GPCI, with the correct topological ground-state degeneracy on the torus. Further, more general fPEPS are considered and optimized (on the infinite plane) to describe the CSL phase of a chiral frustrated Heisenberg antiferromagnet. The chiral modes are computed on the edge of a semi-infinite cylinder (of finite circumference) and shown to follow the predictions from conformal field theory. In contrast to their bosonic analogs the (optimized) fPEPS do not suffer from the replication of the chiral edge mode in the odd topological sector.

In this study, we combine the ab initio Migdal-Eliashberg approach with the intermediate representation of the Green's function, enabling accurate and efficient calculations of the momentum-dependent superconducting gap function while fully considering the effect of the Coulomb retardation. Unlike the conventional scheme that relies on a uniform sampling across Matsubara frequencies, demanding hundreds to thousands of points, the intermediate representation works with fewer than 100 sampled Matsubara Green's functions. The developed methodology is applied to investigate the superconducting properties of three representative low-temperature elemental metals: aluminum, lead, and niobium. The results demonstrate the power and reliability of our computational technique to accurately solve the ab initio anisotropic Migdal-Eliashberg equations even at extremely low temperatures below 1 K.

A higher-order topological superconductor can experience topological phase transitions driven by variations in a bulk parameter without closing the bulk gap. This presents a challenge in establishing a direct bulk-boundary correspondence, as conventional bulk invariants change only upon the closure of the bulk gap. Our study of two-dimensional higher-order phases in the DIII and BDI symmetry classes, both characterized by chiral symmetry, demonstrates that zero-energy crossings facilitate a direct connection between the bulk Hamiltonian and Majorana zero modes at corners. These crossings, emerging as boundary conditions vary, can be identified from the bulk Hamiltonian. For both classes, we introduce a pair of topological invariants derived from these zero-energy crossings to characterize the higher-order topology. Phases in which at least one invariant assumes a nonzero value are anticipated to host Majorana corner modes. Moreover, these invariants may change with the closure of either bulk or edge gaps, thereby providing a clear and direct demonstration of bulk-boundary correspondence in higher-order phases. Our findings offer a promising framework for systematically exploring higher-order topology through boundary condition modulation.

In this paper, we investigate the spectral thermal transport properties of silicon nanomeshes (SNMs) with large and small holes arranged periodically, aperiodically in the $x$ direction (ap-x), and fully aperiodically (ap-xy). Our simulations reveal a significant reduction in thermal conductivity $\left(\kappa \right)$ of small-hole SNMs due to ap-x aperiodicity, in contrast with large-hole SNMs. Spectral phonon analysis indicates that low-frequency phonons in small-hole SNMs are notably suppressed by ap-x aperiodicity, like one-dimensional superlattices. Spectral energy density analysis shows the existence of well-defined coherent phonon modes in small-hole SNMs in the 0–3 THz range; spectral thermal conductivity analysis shows that these modes transport ballistically in the SNM. In contrast, ap-x aperiodicity has negligible impact on large-hole SNMs, attributed to hindered coherent phonon formation. Surprisingly, low-frequency phonons in small-hole SNMs demonstrate enhanced heat transfer capabilities compared with large-hole counterparts, highlighting coherent phonon behavior. Finally, while the heat flux splitting and shape factors effectively categorize $\kappa$ for large-hole SNMs, their utility is limited for small-hole SNMs, emphasizing the importance of coherent phonon considerations in thermal transport analysis.

The interplay of disorder and short finite wire length is the crucial physics hindering progress in the semiconductor-superconductor nanowire platform for realizing non-Abelian Majorana zero modes. Disorder effectively segments the nanowire into isolated patches of quantum dots which act as subgap Andreev bound states often mimicking Majorana zero modes. In this work, we propose and develop a new theoretical approach to model the disorder, effectively a spatially varying effective mass model, which does not rely on incorporating the unknown microscopic details of disorder into the Hamiltonian. This model effectively segments the wire into multiple quantum dots, characterized by highly enhanced effective mass at impurity sites leading to the segmentation of the wire into effective random quantum dots. We find that this model can qualitatively and quantitatively reproduce the disorder physics, providing a crystal clear way to understand the effects of disorder by comparing the mean free path to the superconducting coherence length. In addition, this model allows precise control over the disorder regime, enabling us to evaluate the reliability of topological invariants in predicting Majorana zero modes. We find that topological invariants alone may yield a significant false-positive rate as indicators for topology in the actual wire with increasing disorder strength. Therefore, we propose a set of new indicators to characterize the spatial distribution of the zero-energy state, emphasizing the key necessity for isolated Majorana zero modes localized at the wire ends. Employing this set of new indicators for stringent characterizations, we explore their experimental relevance to the measured differential conductance spectra. Our findings highlight the critical role of isolated localized states, beyond the topological invariant, in identifying topological Majorana zero modes. We believe that this approach is a powerful tool for studying realistic Majorana nanowires where disorder and short wire length obfuscate the underlying topological physics.

We demonstrate reliable machine-learned tuning of quantum-dot-based artificial Kitaev chains to Majorana sweet spots, using the covariance matrix adaptation algorithm. We show that a loss function based on local tunneling spectroscopy features of a chain with two additional sensor dots added at its ends provides a reliable metric to navigate parameter space and find points where crossed Andreev reflection and elastic cotunneling between neighboring sites balance in such a way to yield near-zero-energy modes with very high Majorana quality. We simulate tuning of two- and three-site Kitaev chains, where the loss function is found from calculating the low-energy spectrum of a model Hamiltonian that includes Coulomb interactions and finite Zeeman splitting. In both cases, the algorithm consistently converges towards high-quality sweet spots. Since tunneling spectroscopy provides one global metric for tuning all on-site potentials simultaneously, this presents a promising way towards tuning longer Kitaev chains, which are required for achieving topological protection of the Majorana modes.

We investigate the steady-state energy flow of the periodically driven nonequilibrium spin-boson model by means of combining counter-rotating-hybridized rotating-wave (CHRW) method with full counting statistics. In the case of weak driving $(A<0.1\mathrm{\Delta })$, where $A$ is the driving amplitude and $\mathrm{\Delta }$ is the energy gap of the two-level system, our results are consistent with those obtained by traditional rotating-wave approximation (RWA) approach; in the case of high-frequency driving, our results are in agreement with the ones of the secular Floquet-Redfield method. As the driving amplitude increases, the steady-state flow exhibits nonlinear behavior: it reaches a maximum at medium driving, and then decreases to zero, which means that the coherent destruction of tunneling (CDT) appears, and then continues to grow slowly. We also find that, when the steady-state energy flow varies with driving frequency ${\omega }_d$, comparing with RWA results, the counter-rotating (CR) terms increase the energy flow at low-frequency driving $({\omega }_d<\mathrm{\Delta })$, where ${\omega }_d$ is the driving frequency, and decreases it at high-frequency driving $({\omega }_d>\mathrm{\Delta })$. In the case of strong driving, when the zero-order Bessel function of the first kind satisfies $J_0\left(\frac{A}{{\omega }_d}\zeta \right)=0$, instead of $J_0\left(\frac{A}{{\omega }_d}\right)=0$, the steady-state energy flow is 0, which corresponds to the CDT of the energy flow. The modified parameter $\zeta$ mainly defines the contribution of the CR terms. At high-frequency driving, the modified parameter $\zeta$ tends to $1,$ our result is consistent with previous results $J_0\left(\frac{A}{{\omega }_d}\right)=0$. We find that the CDT of flow only occurs when both CR terms of the driving and the dissipation are considered simultaneously. Our results show the influence of the CR terms on the energy flow and the modification of CDT condition and give theoretical guidance for energy transport of small quantum devices.

By frequency-time spectroscopy, microscopy, and electronic structure theory, we investigate the low coverage adsorption and electronic structure of Ca on an Ag(111) surface. The chemisorption of alkaline-earth Ca atoms starting from their closed $4s^2$ shell electronic configuration differs from that of the open $n{s}^1$ shell alkali-metal atoms because they retain some $4s$ electron charge and can involve hybridization with low-lying unoccupied $3d$ orbitals. Spectroscopic investigation of the unoccupied electronic structure of Ca on Ag(111) by three-photon photoemission (3PP) reveals the electronic states with dominant 3d-orbital character, as confirmed by electronic structure theory. Coverage-specific spectral features enable attribution to two distinct chemisorption phases at submonolayer coverages, which are confirmed at the atomic scale by scanning tunneling microscopy and spectroscopy measurements. Furthermore, time-resolved 3PP spectroscopy reveals a resonant two-photon transition between occupied $(\sim -0.3 \mathrm{eV})$ and unoccupied $(\sim 2.7 \mathrm{eV})$ states with dominant s- and $d$-orbital character. Interferometric time-resolved measurements probing this resonance reveal a dephasing time of $\sim 21$ fs and population lifetime of $\sim 52$ fs for the transiently populated $d$-like state at $\sim 2.7$ eV above $E_F$. Contrary to alkali-metal atom chemisorption, the characteristic development of the chemisorption and electronic structure of Ca reveals the importance of Ca adsorbate and Ag substrate interactions that cause a low-coverage atomic condensation into compact islands. Discovery of the long-lived $d$-character state informs on the catalytic and optoelectronic applications of the alkaline-earth element Ca.

In the absence of parity and time-reversal symmetries, insulators can exhibit magnetoelectric responses in which applied magnetic fields induce charge polarization and, conversely, applied electric fields induce magnetization. While there is a long history of the study of magnetoelectric response in fermionic insulators, the same for bosonic insulators has been limited. We consider the magnetoelectric response in lattice insulators built out of charged bosonic degrees of freedom and derive a bulk formula for the corresponding linear response tensor. The resulting formulas feature several contributions, including a Chern-Simons integral over the bands of the bosonic excitations. We construct several minimal microscopic models that illustrate the ingredients required to obtain a sizable bosonic magnetoelectric response. Our formalism can be applied to bosonic Mott insulators subject to synthetic gauge fields and/or tilted potentials as well as to the spinon sector in the Coulomb phase of a $U\left(1\right)$ quantum spin liquid.

The electronic band structure of monolayer phosphorene is thoroughly studied by considering the presence of spin-orbit interaction. We employ a multiorbital Slater-Koster tight-binding approach to derive effective $\mathit{k}·\mathit{p}$-type Hamiltonians that describes the dominant spin-orbit coupling (SOC) effects of the Rashba and intrinsic origin at the high $\mathrm{\Gamma }$ and $\mathrm{S}$ high symmetry points in phosphorene. In the absence of SOC effects a minimal admixture of $p_z$ and $p_y$ atomic orbitals suffices to reproduce the well-known anisotropy of highest valence and the lowest conduction bands at the $\mathrm{\Gamma }$ point, consistent with density functional theory (DFT) and $\mathit{k}·\mathit{p}$ methods. In contrast, the inclusion of the $p_x$ and $s$ atomic orbitals are rather crucial for an adequate description of the SOC effects in phosphorene at low energies, particularly at the $\mathrm{S}$ point. We introduce useful analytical expressions for the Rashba and intrinsic SOC parameters in terms of the relevant Slater-Koster integrals. In addition, we report simple formulas for the interband dipole strengths, revealing the nature of the strong anisotropic behavior of its lower bands. Our findings can be useful for further studies of electronic and spin transport properties in monolayer phosphorene and its nanoribbons.

Current-carrying chiral edge states in quantum Hall systems exhibit fascinating properties typically studied using electron spectroscopy and interferometry. Here we demonstrate that electron occupation, current, and coherence in the chiral edge states can be selectively probed and controlled by low-energy electromagnetic radiation in the microwave to infrared range, without affecting electron states in the bulk or disrupting the quantum Hall effect conditions within the sample. Both linear and nonlinear optical controls are feasible due to the inevitable violation of adiabaticity and inversion symmetry breaking for electron states near the edge. This opens up new pathways for frequency- and polarization-selective spectroscopy and the control of individual edge states.