We study the effect of combining spin fluctuations and forward scattering electron-phonon (e-ph) coupling on the superconductivity in the $\mathrm{Fe}\mathrm{Se}/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ system modeled by a phenomenological two-band Hubbard model with long-range e-ph interactions. We treat the electron and phonon degrees of freedom on an equal footing using a fully self-consistent fluctuation-exchange plus Migdal-Eliashberg calculation, which includes a self-consistent determination of the spin fluctuation spectrum. Based on FeSe monolayers, we focus on the case where one of the bands lies below the Fermi level (i.e., incipient), and demonstrate that the combined interactions can enhance or suppress $T_c$, depending on their relative strength. For a suitable choice of parameters, the spin fluctuation mechanism yields a $T_c\approx 46.8$ K incipient $s_{\pm }$ superconductor, consistent with surface-doped FeSe thin films. A forward-focused e-ph interaction further enhances the $T_c$, as observed in monolayer FeSe on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$.

We investigate the effect of electron-phonon interactions (EPI) in systems exhibiting one or more flat electron bands close to the Fermi level and a comparatively large phonon energy scale. After solving the self-consistent full-bandwidth Eliashberg equations, we compute angular resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy/microscopy (STS/STM) spectra. We obtain a sequence of quasiparticle replica bands in both the normal and superconducting states that originate from frequency-dependent features of the electron mass renormalization function. We show that these replica bands can be used to extract the relevant phonon energy scale from experiments. Focusing in particular on twisted bilayer graphene, we predict replica-band formation, which, when observed, will shed light on the role of EPI in this archetypal flat-band system.

Strong local interaction in systems with nontrivial topological bands can stabilize quantum states such as magnetic topological insulators. We investigate the influence of the lattice symmetry on the possible emergence of antiferromagnetic quantum Hall states. We consider the spinful Harper-Hofstadter model extended by a next-nearest-neighbor (NNN) hopping which opens a gap at half filling and allows for the realization of a quantum Hall insulator. The quantum Hall insulator has the Chern number $\mathcal{C}=2$ as both spin components are in the same quantum Hall state. We add to the system a staggered potential $\mathrm{\Delta }$ along the $\widehat{x}$ direction favoring a normal insulator and the Hubbard interaction $U$ favoring a Mott insulator. The Mott insulator is a Néel antiferromagnet for small NNN hopping and a stripe antiferromagnet for large NNN hopping. We investigate the $U-\mathrm{\Delta }$ phase diagram of the model for both small and large NNN hoppings. We show that, while for large NNN hopping there exists a $\mathcal{C}=1$ stripe antiferromagnetic quantum Hall insulator in the phase diagram, there is no equivalent $\mathcal{C}=1$ Néel antiferromagnetic quantum Hall insulator at the small NNN hopping. We discuss that a $\mathcal{C}=1$ antiferromagnetic quantum Hall insulator can emerge only if the effect of the spin-flip transformation cannot be compensated by a space-group operation. Our findings can be used as a guideline in future investigations searching for antiferromagnetic quantum Hall states.

High-temperature superconducting cuprates exhibit an intriguing phenomenology for the low-energy elementary excitations. In particular, an unconventional temperature dependence of the coherent spectral weight (CSW) has been observed in the superconducting phase by angle-resolved photoemission spectroscopy (ARPES), both at the antinode where the $d$-wave paring gap is maximum, as well as along the gapless nodal direction. Here, we combine equilibrium and time-resolved ARPES to track the temperature-dependent meltdown of the nodal CSW in Bi-based cuprates with unprecedented sensitivity. We find the nodal suppression of CSW upon increasing temperature to be ubiquitous across single- and bi-layer Bi cuprates, and uncorrelated to superconducting and pseudogap onset temperatures. We quantitatively model both the lineshape of the nodal spectral features and the anomalous suppression of CSW within the Fermi-liquid framework, establishing the key role played by the normal state electrodynamics in the description of nodal quasiparticles in superconducting cuprates.

We study the ground state properties of the doped Hubbard model with strong interactions on the honeycomb lattice by the density matrix renormalization group (DMRG) method. At half-filling, due to the absence of minus sign problem, it is now well established by large-scale quantum Monte Carlo calculations that a Dirac semimetal to antiferromagnetic Mott insulator transition occurs with the increase of the interaction strength $U$ for the Hubbard model on the honeycomb lattice. However, an understanding of the fate of the antiferromagnetic Mott insulator when holes are doped into the system is still lacking. In this work, by calculating the local spin and charge density for width-4 cylinders with DMRG, we discover a half-filled stripe order in the doped Hubbard model on the honeycomb lattice. We also perform complementary large-scale mean-field calculations with renormalized interaction strength. We observe half-filled stripe order and find stripe states with filling close to one half are nearly degenerate in energy.

The Fano-Anderson chain consists of a linear lattice with a discrete side-unit, and it exhibits Fano-resonant scattering due to coupling between the discrete states of the side-unit and the tight-binding continuum. We study Fano-resonance-assisted transport for the case of a topologically nontrivial side unit. We find that the topology of the side-unit influences the transmission characteristics, which thus can be an effective detection tool of the topological phases of the side unit. Furthermore, we explore the role of dual links between the linear tight-binding chain and the side unit. The secondary connection between the main chain and the side-unit can modify the position or width of the Fano resonance dip in the transmission probability, and thus can yield additional control.

The pathway toward the tailored synthesis of materials starts with precise characterization of the conformational properties and dynamics of individual molecules. Electron spin resonance (ESR)-based scanning tunneling microscopy can potentially address molecular structure with unprecedented resolution. Here, we determine the fine structure and geometry of an individual titanium-hydride molecule, utilizing a combination of a newly developed millikelvin ESR scanning tunneling microscope in a vector magnetic field and ab initio approaches. We demonstrate a strikingly large anisotropy of the $g$ tensor, unusual for a spin doublet ground state, resulting from a nontrivial orbital angular momentum stemming from the molecular ground state. We quantify the relationship between the resultant fine structure, hindered rotational modes, and orbital excitations. Our model system provides avenues to determine the structure and dynamics of individual molecules.

We explore the interplay of electron-electron correlations and surface effects in the prototypical correlated insulating material, NiO. In particular, we compute the electronic structure, magnetic properties, and surface energies of the (001) and (110) surfaces of paramagnetic NiO using a fully charge self-consistent $\mathrm{DFT}+$ dynamical mean-field theory method. Our results reveal a complex interplay between electronic correlations and surface effects in NiO, with the electronic structure of the (001) and (110) NiO surfaces being significantly different from that in bulk NiO. We obtain a sizable reduction of the band gap at the surface of NiO, which is most significant for the (110) NiO surface. This suggests a higher catalytic activity of the (110) NiO surface than that of the (001) NiO one. Our results reveal a charge-transfer character of the (001) and (110) surfaces of NiO. Most notably, for the (110) NiO surface we observe a remarkable electronic state characterized by an alternating charge-transfer and Mott-Hubbard character of the band gap in the surface and subsurface NiO layers, respectively. This novel form of electronic order stabilized by strong correlations is not driven by lattice reconstructions but of purely electronic origin. We notice the importance of orbital differentiation of the Ni $e_g$ states to characterize the Mott-Hubbard insulating state of the (001) and (110) NiO surfaces. The unoccupied Ni $e_g$ surface states are seen to split from the lower edge of the conduction band to form strongly localized states in the fundamental gap of bulk NiO. Our results for the surface energies of the (001) and (110) NiO surfaces show that the (001) facet of NiO has significantly lower energy. This implies that the relative stability of different surfaces, at least from a purely energetic point of view, does not depend on the presence or absence of magnetic order in NiO.

We investigate the effect of the mass anisotropy on Friedel oscillations, Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, screening properties, and Boltzmann transport in two-dimensional (2D) metallic and doped semiconductor systems. We calculate the static polarizability and the dielectric function within the random phase approximation with the mass anisotropy fully taken into account without making any effective isotropic approximation in the theory. We find that carrier screening exhibits an isotropic behavior for small momenta despite the anisotropy of the system and becomes strongly anisotropic above a certain threshold momentum. Such an anisotropy of screening leads to anisotropic Friedel oscillations, and an anisotropic RKKY interaction characterized by a periodicity dependent on the direction between the localized magnetic moments. We also explore the disorder limited dc transport properties in the presence of mass anisotropy based on the Boltzmann transport theory. Interestingly, we find that the anisotropy ratio of the short-range disorder limited resistivity along the heavy- and light-mass directions is always the same as the mass anisotropy ratio, whereas for the long-range disorder limited resistivity the anisotropy ratio is the same as the mass ratio only in the low-density limit and saturates to the square root of the mass ratio in the high-density limit. Our theoretical work should apply to many existing and to-be-discovered anisotropic 2D systems.

We theoretically study the spectrum induced by one and two magnetic impurities near the boundary of a one-dimensional nanowire in proximity to a conventional $s$-wave superconductor and extract the ground-state magnetic configuration. We show that the energies of the subgap states, supported by the magnetic impurities, are strongly affected by the boundary for distances less than the superconducting coherence length. In particular, when the impurity is moved towards the boundary, multiple quantum phase transitions periodically occur in which the parity of the superconducting condensate oscillates between even and odd. We find that the magnetic ground-state configuration of two magnetic impurities depends not only on the distance between them, but also explicitly on their distance away from the boundary of the nanowire. As a consequence, the magnetic ground state can switch from ferromagnetic to antiferromagnetic while keeping the interimpurity distance unaltered by simultaneously moving both impurities away from the boundary. The ground-state magnetic configuration of two impurities is found analytically in the weak coupling regime and exactly for an arbitrary impurity coupling strength using numerical tight-binding simulations.

We explore here the decisive role of film thickness on Casimir and electrostatic forces from topological insulating ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films. The forces were measured for the Au-${\mathrm{Bi}}_2{\mathrm{Se}}_3$ system under ambient conditions to simulate realistic applications. It is shown that the electrostatic forces associated with a nonzero contact potential of $V_{\mathrm{o}}\sim 200\mathrm{mV}$ played a dominant role, and could not be compensated for thinner films ($\le 20\mathrm{nm}$ thick). However, for thicker films the measured force, after voltage compensation, started to approach the genuine Casimir force in very good agreement with Lifshitz theory. Our results point out that any gradual deviation from theory for thinner films underlies the presence of strong electrostatic effects.

Using density functional theory (DFT) methods, we have calculated x-ray absorption spectroscopy and x-ray circular dichroism spectra in bulk and thin films of ${\mathrm{Fe}}_3\mathrm{Ge}{\mathrm{Te}}_2$, ${\mathrm{CrI}}_3$, and $\mathrm{Cr}\mathrm{Ge}{\mathrm{Te}}_3$. $\mathrm{DFT}+U$ methods are employed for better handling of correlation effects of $3d$ electrons of transition metals. We discuss relations between the density of states, radial matrix elements, and the corresponding spectra. By comparing the calculated spectra with previously measured spectra, we discuss the reliability of $\mathrm{DFT}+U$ methods to describe the electronic structures of these materials and determine the corresponding optimal $U$ and $J$ parameters.

Over the past six years, superconductivity at high temperatures has been reported in a variety of hydrogen-rich compounds under high pressure. That high temperature superconductivity should exist in these materials is expected according to the conventional theory of superconductivity, as shown by detailed calculations. However, here we argue that experimental observations rule out conventional superconductivity in these materials. Our results indicate that either these materials are unconventional superconductors of a novel kind, which we term “nonstandard superconductors,” or alternatively, that they are not superconductors. If the first is true, we point out that the critical current in these materials should be several orders of magnitude larger than in standard superconductors, potentially opening up the way to important technological applications. If the second is the case, which we believe is more likely, we suggest that the signals interpreted as superconductivity are either experimental artifacts or they signal other interesting physics but not superconductivity.

We performed a temperature-modulated first-order reversal curve (FORC) study on a Pt/Co/Fe/Ir magnetic stack that exhibited a magnetic phase transition from isolated skyrmions to skyrmion lattice with increasing temperature. Using in situ magneto-optical Kerr imaging, a generalized description of domain transformations associated with the FORC distribution peaks at both their reversal and sweeping field are derived to allow for direct analysis from the FORC diagram. The sweeping field of the peak, which is commonly ignored in analysis, is identified as the process of domain propagation or nucleation towards terminal domain separation. This process is found to be essential in inducing magnetization irreversibility to reveal domain transformations. In addition, a model characterized by the FORC distribution peaks was developed to describe the transition from the isolated skyrmion to skyrmion lattice phase as well as to identify important field ranges for the transformations. This study establishes an intuitive form of analysis for the otherwise abstract data of FORC distribution for the characterization of magnetic skyrmions in the active field of skyrmionics.

Kagome magnets are believed to have numerous exotic physical properties due to the possible interplay between lattice geometry, electron correlation and band topology. Here, we report the large anomalous Hall effect in the kagome ferromagnet ${\mathrm{LiMn}}_6{\mathrm{Sn}}_6$, which has a Curie temperature of 382 K and easy plane along with the kagome lattice. At low temperatures, unsaturated positive magnetoresistance and opposite signs of ordinary Hall coefficient for ${\rho }_{xz}$ and ${\rho }_{yx}$ indicate the coexistence of electrons and holes in the system. A large intrinsic anomalous Hall conductivity of $380{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$, or $0.44e^2/h$ per Mn layer, is observed in ${\sigma }_{xy}^A$. This value is significantly larger than those in other $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ ($R$ = rare earth elements) kagome compounds. Band structure calculations show several band crossings, including a spin-polarized Dirac point at the K point, close to the Fermi energy. The calculated intrinsic Hall conductivity agrees well with the experimental value, and shows a maximum peak near the Fermi energy. We attribute the large anomalous Hall effect in ${\mathrm{LiMn}}_6{\mathrm{Sn}}_6$ to the band crossings closely located near the Fermi energy.

Experiments and numerical simulations show that quantum turbulence exists in two distinct limiting regimes: Kolmogorov turbulence (which shares with classical turbulence the important property of a cascade of kinetic energy from large eddies to small eddies) and Vinen turbulence (which is more similar to a random flow). In this work, we define a mesoscale helicity for the superfluid, which, tested in numerical experiments, distinguishes the two turbulent regimes, quantifying the amount of nonlocal vortex interactions and the orientation of the vortex lines.

Exploiting novel aspects of the quantum geometry of charged particles in a magnetic field via gauge-invariant variables, we provide tangible connections between the response of quantum Hall fluids to nonuniform electric fields and the characteristic geometry of electronic motion in the presence of magnetic and electric fields. The geometric picture we provide motivates the following ansatz: nonuniform electric fields mimic the presence of spatial curvature. Consequently, the gravitational coupling constant also appears in the charge response to nonuniform electric fields.

Twisted two-dimensional bilayer anisotropy materials exhibit many exotic physical phenomena. Manipulating the “twist angle” between the two layers enables the hybridization phenomenon of polaritons, resulting in fine control of the dispersion engineering of the polaritons in these structures. Here, combined with the hybridization phenomenon of anisotropy polaritons, we study theoretically the near-field radiative heat transfer (NFRHT) between two twisted hyperbolic systems. These two twisted hyperbolic systems are mirror images of each other. Each twisted hyperbolic system is composed of two graphene gratings, where there is an angle φ between these two graphene gratings. By analyzing the photonic transmission coefficient as well as the plasmon dispersion relation of the twisted hyperbolic system, we prove the enhancement effect of the topological transitions of the surface state at a special angle [from open (hyperbolic) to closed (elliptical) contours] on radiative heat transfer. Meanwhile the role of the thickness of dielectric spacer and vacuum gap on the manipulating the topological transitions of the surface state and the NFRHT are also discussed. We predict the hysteresis effect of topological transitions at a larger vacuum gap, and demonstrate that as the thickness of the dielectric spacer increases, the transition from the enhancement effect of heat transfer caused by the twisted hyperbolic system to a suppression.

Fermi surface topology and Berry phase of square-net-layered materials with P4/nmm space group were classified based on the first-principles analysis of Berry phases in each materials. The variation of electron-hole (e-h) asymmetry, driven by the third-nearest-neighbor hoppings of pnictogen $p_{x,y}$ orbitals in square-net, is essential to yield various topological Fermi surfaces. The change of chemical potential too yields various topological Fermi surfaces. By determining e-h asymmetry and chemical potential of the reported square-net-layered materials, we construct the phase diagram of the Fermi surface topologies. In addition, we adopt interlayer interaction and investigate three-dimensional Fermi surface. We classify the three-dimensional Fermi surface topologies based on the phase diagram, and our classification reproduces the quantum oscillation and spectroscopy results for various square-net compounds.