We studied the temperature dependence of the diagonal double-stripe spin order in 1 and 2 unit cell thick layers of FeTe grown on the topological insulator $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ via spin-polarized scanning tunneling microscopy. The spin order persists up to temperatures which are higher than the transition temperature reported for bulk $\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{Te}$ with lowest possible excess Fe content $y$. The enhanced spin order stability is assigned to a strongly decreased $y$ with respect to the lowest values achievable in bulk crystal growth, and effects due to the interface between the FeTe and the topological insulator. The result is relevant for understanding the recent observation of a coexistence of superconducting correlations and spin order in this system.

We investigated the doping dependence of magnetic excitations in the lightly doped cuprate ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ via combined studies of resonant inelastic x-ray scattering (RIXS) at the Cu $L_3$ edge and theoretical calculations. With increasing doping, the magnon dispersion is found to be essentially unchanged, but the spectral width broadens and the spectral weight varies differently at different momenta. Near the Brillouin zone center, we directly observe bimagnon excitations that possess the same energy scale and doping dependence as previously observed by Raman spectroscopy. They disperse weakly in energy-momentum space, and they are consistent with a bimagnon dispersion that is renormalized by the magnon-magnon interaction at the zone center.

Adatom systems on the Si(111) surface have recently attracted an increasing attention as strongly correlated systems with a rich phase diagram. We study these materials by a single band model on the triangular lattice, including $1/r$ long-range interaction. Employing the recently proposed TRILEX method, we find an unconventional superconducting phase of chiral $d$-wave symmetry in hole-doped systems. Contrary to usual scenarios where charge and spin fluctuations are seen to compete, here the superconductivity is driven simultaneously by both charge and spin fluctuations and crucially relies on the presence of the long-range tail of the interaction. We provide an analysis of the relevant collective bosonic modes and predict how a cumulative charge and spin paring mechanism leads to superconductivity in doped silicon adatom materials.

The pairing symmetry of interacting Dirac fermions on the $\pi$-flux lattice is studied with the determinant quantum Monte Carlo and numerical linked-cluster expansion methods. The $s^{*}$- (i.e., extended $s$-) and $d$-wave pairing symmetries, which are distinct in the conventional square lattice, are degenerate under the Landau gauge. We demonstrate that the dominant pairing channel at strong interactions is an unconventional $d{s}^{*}$-wave phase consisting of alternating stripes of $s^{*}$- and $d$-wave phases. A complementary mean-field analysis shows that while the $s^{*}$- and $d$-wave symmetries individually have nodes in the energy spectrum, the $d{s}^{*}$ channel is fully gapped. The results represent a new realization of pairing in Dirac systems, connected to the problem of chiral $d$-wave pairing on the honeycomb lattice, which might be more readily accessed by cold-atom experiments.

Alkali-metal adsorption on the surface of materials is widely used for in situ surface electron doping, particularly for observing unoccupied band structures by angle-resolved photoemission spectroscopy (ARPES). However, the effects of alkali-metal atoms on the resulting band structures have yet to be fully investigated, owing to difficulties in both experiments and calculations. Here, we combine ARPES measurements on cesium-adsorbed ultrathin bismuth films with first-principles calculations of the electronic charge densities and demonstrate a simple method to evaluate alkali-metal induced band deformation. We reveal that deformation of bismuth surface bands is directly correlated with vertical charge-density profiles at each electronic state of bismuth. In contrast, a change in the quantized bulk bands is well described by a conventional rigid-band-shift picture. We discuss these two aspects of the band deformation holistically, considering spatial distributions of the electronic states and cesium-bismuth hybridization, and provide a prescription for applying alkali-metal adsorption to a wide range of materials.

By taking account of the electric-field-induced charge screening, a self-consistent calculation within the framework of the tight-binding approach is employed to obtain the electronic band structure of gated multilayer phosphorene and the charge densities on the different phosphorene layers. We find charge density and screening anomalies in single-gated multilayer phosphorene and electron-hole bilayers in dual-gated multilayer phosphorene. Due to the unique puckered lattice structure, both intralayer and interlayer charge screenings are important in gated multilayer phosphorene. We find that the electric-field tuning of the band structure of multilayer phosphorene is distinctively different in the presence and absence of charge screening. For instance, it is shown that the unscreened band gap of multilayer phosphorene decreases dramatically with increasing electric-field strength. However, in the presence of charge screening, the magnitude of this band-gap decrease is significantly reduced and the reduction depends strongly on the number of phosphorene layers. Our theoretical results of the band-gap tuning are compared with recent experiments and good agreement is found.

Motivated by recent experiments, we investigate the excitation energy of a proximitized Rashba wire in the presence of a position dependent pairing. In particular, we focus on the spectroscopic pattern produced by the overlap between two Majorana bound states that appear for values of the Zeeman field smaller than the value necessary for reaching the bulk topological superconducting phase. The two Majorana bound states can arise because locally the wire is in the topological regime. We find three parameter ranges with different spectral properties: crossings, anticrossings, and asymptotic reduction of the energy as a function of the applied Zeeman field. Interestingly, all these cases have already been observed experimentally. Moreover, since an increment of the magnetic field implies the increase of the distance between the Majorana bound states, the amplitude of the energy oscillations, when present, gets reduced. The existence of the different Majorana scenarios crucially relies on the fact that the two Majorana bound states have distinct $k$-space structures. We develop analytical models that clearly explain the microscopic origin of the predicted behavior.

We present an infinite lattice density matrix renormalization group sweeping procedure which can be used as a replacement for the standard infinite lattice blocking schemes. Although the scheme is generally applicable to any system, its main advantages are the correct representation of commensurability issues and the treatment of degenerate systems. As an example we apply the method to a spin chain featuring a highly degenerate ground-state space where the new sweeping scheme provides an increase in performance as well as accuracy by many orders of magnitude compared to a recently published work.

We derive the expressions for configurational forces in Kohn-Sham density functional theory, which correspond to the generalized variational force computed as the derivative of the Kohn-Sham energy functional with respect to the position of a material point $\mathbf{x}$. These configurational forces that result from the inner variations of the Kohn-Sham energy functional provide a unified framework to compute atomic forces as well as stress tensor for geometry optimization. Importantly, owing to the variational nature of the formulation, these configurational forces inherently account for the Pulay corrections. The formulation presented in this work treats both pseudopotential and all-electron calculations in a single framework, and employs a local variational real-space formulation of Kohn-Sham density functional theory (DFT) expressed in terms of the nonorthogonal wave functions that is amenable to reduced-order scaling techniques. We demonstrate the accuracy and performance of the proposed configurational force approach on benchmark all-electron and pseudopotential calculations conducted using higher-order finite-element discretization. To this end, we examine the rates of convergence of the finite-element discretization in the computed forces and stresses for various materials systems, and, further, verify the accuracy from finite differencing the energy. Wherever applicable, we also compare the forces and stresses with those obtained from Kohn-Sham DFT calculations employing plane-wave basis (pseudopotential calculations) and Gaussian basis (all-electron calculations). Finally, we verify the accuracy of the forces on large materials systems involving a metallic aluminum nanocluster containing 666 atoms and an alkane chain containing 902 atoms, where the Kohn-Sham electronic ground state is computed using a reduced-order scaling subspace projection technique [P. Motamarri and V. Gavini, Phys. Rev. B 90, 115127 (2014)].

Resta proposed a definition of the electric polarization in one-dimensional systems in terms of the ground-state expectation value of the large gauge transformation operator. Vanishing of the expectation value in the thermodynamic limit implies that the system is a conductor. We study Resta's polarization amplitude (expectation value) in the $S=1/2$ XXZ chain and its several generalizations, in the gapless conducting Tomonaga-Luttinger liquid phase. We obtain an analytical expression in the lowest-order perturbation theory about the free fermion point (XY chain) and an exact result for the Haldane-Shastry model with long-range interactions. We also obtain numerical results, mostly using the exact diagonalization method. We find that the amplitude exhibits a power-law scaling in the system size (chain length) and vanishes in the thermodynamic limit. On the other hand, the exponent depends on the model even when the low-energy limit is described by the Tomonaga-Luttinger liquid with the same Luttinger parameter. We find that a change in the exponent occurs when the Umklapp term(s) are eliminated, suggesting the importance of the Umklapp terms.

Monolayer staggered materials of the graphene family present intrinsic spin-orbit coupling and can be driven through several topological phase transitions using external circularly polarized lasers and static electric or magnetic fields. We show how topological features arising from photoinduced phase transitions and the magnetic-field-induced quantum Hall effect coexist in these materials and simultaneously impact their Hall conductivity through their corresponding charge Chern numbers. We also show that the spectral response of the longitudinal conductivity contains signatures of the various phase-transition boundaries, that the transverse conductivity encodes information about the topology of the band structure, and that both present resonant peaks which can be unequivocally associated with one of the four inequivalent Dirac cones present in these materials. This complex optoelectronic response can be probed with straightforward Faraday rotation experiments, allowing the study of the crossroads between quantum Hall physics, spintronics, and valleytronics.

In this work, we discuss a method to control the polarization anisotropy of spontaneous emission from neutral excitons confined in quantum-dot-like nanostructures, namely single epitaxial InAs quantum dashes emitting at telecom wavelengths. The nanostructures are embedded inside lithographically defined, in-plane asymmetric photonic mesa structures, which generate polarization-dependent photonic confinement. First, we study the influence of the photonic confinement on the polarization anisotropy of the emission by photoluminescence spectroscopy, and we find evidence of different contributions to a degree of linear polarization (DOLP), i.e., from the quantum dash and the photonic mesa, in total giving rise to $\mathrm{DOLP}=0.85$. Then, we perform finite-difference time-domain simulations of photonic confinement, and we calculate the DOLP in a dipole approximation showing well-matched results for the established model. Furthermore, by using numerical calculations, we demonstrate several types of photonic confinements where highly linearly polarized emission with DOLP of about 0.9 is possible by controlling the position of a quantum emitter inside the photonic structure. Then, we elaborate on anisotropic quantum emitters allowing for exceeding $\mathrm{DOLP}=0.95$ in an optimized case, and we discuss the ways towards efficient linearly polarized single photon source at telecom bands.

Because of the important role of electron-boson interactions in conventional superconductivity, it has long been asked whether any similar mechanism is at play in high-temperature cuprate superconductors. Evidence for strong electron-boson coupling is observed in cuprates with angle-resolved photoemission spectroscopy (ARPES), in the form of a dispersion kink and peak-dip-hump structure. What is missing is evidence of a causal relation to superconductivity. Here we revisit the problem using the technique of time-resolved ARPES on ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$. We focus on the peak-dip-hump structure, and show that laser pulses shift spectral weight into the dip as superconductivity is destroyed on picosecond time scales. We compare our results to simulations of Eliashberg theory in a superconductor with an Einstein boson, and find that the magnitude of the shift in spectral weight depends on the degree to which the bosonic mode contributes to superconductivity. Further study could address one of the longstanding mysteries of high-temperature superconductivity.

This work reports the changes in structure and lattice dynamics induced by substituting the Jahn-Teller-active $\mathrm{M}{\mathrm{n}}^{3+}$ ion by the Jahn-Teller-inactive $\mathrm{F}{\mathrm{e}}^{3+}$ in $\mathrm{TbM}{\mathrm{n}}_{1\text{−}x}\mathrm{F}{\mathrm{e}}_x{\mathrm{O}}_3$ over the full composition range. The structural analysis reveals that the amplitude of the cooperative Jahn-Teller distortion decreases linearly from $x=0$ (pure $\mathrm{TbMn}{\mathrm{O}}_3)$ to $x=0.5$, where it is completely suppressed. We then correlate this evolution with the behavior of the Raman modes across the solid solution. In particular, we show that the Raman modes associated with the rotation of octahedra, whose wave number is commonly considered to scale linearly with the tilt angles in orthorhombic Pnma perovskites, are also sensitive to the amplitude of the Jahn-Teller distortion.

In this work we present a detailed NMR and ${\mu }^{+}\mathrm{SR}$ investigation of the spin dynamics in the new hydrated sodium salt containing the single-ion magnet ${\left[\mathrm{Er}{\left({\mathrm{W}}_5{\mathrm{O}}_{18}\right)}_2\right]}^{9-}$. The ${}^1\mathrm{H}\mathrm{NMR}$ absorption spectra at various applied magnetic fields present a line broadening on decreasing temperature which indicates a progressive spin freezing of the single-molecule magnetic moments. The onset of quasistatic local magnetic fields, due to spin freezing, is observed also in the muon relaxation curves at low temperature. Both techniques yield a local field distribution of the order of 0.1–0.2 T, which appears to be of dipolar origin. On decreasing the temperature, a gradual loss of the ${}^1\mathrm{H}\mathrm{NMR}$ signal intensity is observed, a phenomenon known as wipe-out effect. The effect is analyzed quantitatively on the basis of a simple model which relies on the enhancement of the NMR spin-spin, $T_2^{-1}$, relaxation rate due to the slowing down of the magnetic fluctuations. Measurements of spin-lattice relaxation rate $T_1^{-1}$ for ${}^1\mathrm{H}\mathrm{NMR}$ and of the muon longitudinal relaxation rate λ show an increase as the temperature is lowered. However, while for the NMR case the signal is lost before reaching the very slow fluctuation region, the muon spin-lattice relaxation λ can be followed until very low temperatures and the characteristic maximum, reached when the electronic spin fluctuation frequency becomes of the order of the muon Larmor frequency, can be observed. At high temperatures, the data can be well reproduced with a simple model based on a single correlation time $\tau ={\tau }_0\exp \left(\mathrm{\Delta }/\mathrm{T}\right)$ for the magnetic fluctuations. However, to fit the relaxation data for both NMR and ${\mu }^{+}\mathrm{SR}$ over the whole temperature and magnetic field range, one has to use a more detailed model that takes into account spin-phonon transitions among the $\mathrm{E}{\mathrm{r}}^{3+}$ magnetic sublevels. A good agreement for both proton NMR and ${\mu }^{+}\mathrm{SR}$ relaxation is obtained, which confirms the validity of the energy level scheme previously calculated from an effective crystal field Hamiltonian.

We investigate signatures of the spin and orbital states of $R{\mathrm{VO}}_3$ ($R=\text{Y}$ and La) in optical conductivity using density functional theory plus dynamical mean-field theory (DFT+DMFT). From the assignment of multiplet state configurations to optical transitions, the DFT+DMFT reproduces experimental temperature-dependent evolutions of optical conductivity for both ${\mathrm{YVO}}_3$ and ${\mathrm{LaVO}}_3$. We also show that the optical conductivity is a useful quantity to probe the evolution of the orbital state even in the absence of spin order. The result provides a reference to investigate the spin and orbital states of ${t_{2g}}^2$ vanadate systems, which is an important issue for both fundamental physics on spin and orbital states and applications of vanadates by means of orbital state control.

The electronic properties of guest Cs atoms in a deformable lattice are investigated at various densities $n$. Low values of $n$ show optical absorptions of small bipolarons. At intermediate $n$ values, new bands appear in the midinfrared (MIR) and high-frequency regions, which coexist with the small bipolaron bands. With a further increase in $n$, the small bipolaron bands become less discernible and subsequently disappear, resulting in the appearance of a Drude component superimposed on a MIR sideband suggesting a phase transition to a polaronic metal. In this itinerant phase, an approximately twofold mass enhancement is observed. This continuous transition of a gas of small bipolarons to a polaronic metal characterized by a Fröhlich polaron reveals an important part of the complex phase diagram of the metal-insulator transition in a deformable lattice.

The equilibrium van der Waals binding energy is an important factor in the design of materials and devices. However, it presents great computational challenges for materials built up from nanostructures. Here we investigate the binding-energy scaling behavior from first-principles calculations. We show that the equilibrium binding energy per atom between identical nanostructures can scale up or down with nanostructure size, but can be parametrized for large $\mathcal{N}$ with an analytical formula (in meV/atom), $E_b/\mathcal{N}=a+b/\mathcal{N}+c/{\mathcal{N}}^2+d/{\mathcal{N}}^3$, where $\mathcal{N}$ is the number of atoms in a nanostructure and $a$, $b$, $c$, and $d$ are fitting parameters, depending on the properties of a nanostructure. The formula is consistent with a finite large-size limit of binding energy per atom. We find that there are two competing factors in the determination of the binding energy: Nonadditivities of van der Waals coefficients and center-to-center distance between nanostructures. To decode the detail, the nonadditivity of the static multipole polarizability is investigated from an accurate spherical-shell model. We find that the higher-order multipole polarizability displays ultrastrong intrinsic nonadditivity, no matter if the dipole polarizability is additive or not.

The vibrations of gold nanowires and nanorods are investigated numerically in the framework of continuum elasticity using the Rayleigh-Ritz variational method. Special attention is paid to identify the vibrations relevant in Raman scattering experiments. A comprehensive description of the vibrations of nanorods is proposed by determining their symmetry, comparing with standing waves in the corresponding nanowires, and estimating their Raman intensity. The role of experimentally relevant parameters such as the anisotropic cubic lattice structure, the presence of faceted lateral surfaces, and the shape of the ends of the nanorods is evaluated. Elastic anisotropy is shown to play a significant role contrarily to the presence of facets. Localized vibrations are found for nanorods with flat ends. Their evolution as the shape of the ends is changed to half-spheres is discussed.