The confluence of electron correlations and spin-orbit interactions is critical to realizing quantum phases in $5d$ transition metal oxides. Here, we investigate how the strength of the effective electron correlations evolve across a series of $d^5$ iridates comprised of ${\mathrm{IrO}}_6$ octahedra, ranging from the layered correlated insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, to the three-dimensional perovskite semimetal ${\mathrm{SrIrO}}_3$, to metallic rutile ${\mathrm{IrO}}_2$ in which the octahedra are arranged in a mixed edge and corner sharing network. Through a combination of reactive oxide molecular-beam epitaxy, in situ angle-resolved photoemission spectroscopy, core level photoemission, and density functional theory, we show how the effective electron correlations weaken as a function of increasing connectivity of the ${\mathrm{IrO}}_6$ network and $p\text{−}d$ hybridization. Our results demonstrate how structure and connectivity can be used to control the strength of correlations in the iridates.

We construct a tight-binding model realizing one pair of Weyl nodes and three distinct Weyl semimetals. In the type-I (type-II) Weyl semimetal, both nodes belong to type-I (type-II) Weyl nodes. In addition, there exists a third type, previously undiscovered and dubbed “hybrid Weyl semimetal”, in which one Weyl node is of type I while the other is of type II. For the hybrid Weyl semimetal, we further demonstrate the bulk Fermi surfaces and the topologically protected surface states, analyze the unique Landau-level structure and quantum oscillation, and discuss the conditions for possible material realization.

The interaction of electrons with acoustic phonons under a magnetic field leads to a remarkable kind of magnetophonon oscillation of transport coefficients, recently discovered in two-dimensional electron systems. The present study shows that similar oscillations exist in bulk conductors and provides a theory of this phenomenon for the case of spherical Fermi surfaces. The resonance peaks occur when the product of the Fermi surface diameter by the sound velocity is a multiple of the cyclotron frequency. Theoretical predictions may facilitate the experimental observation of the phenomenon.

Although bulk Bi is a prototypical semimetal with a topologically trivial electronic band structure, we show by various quantum transport measurements that epitaxial Bi(111) thin films have unexpected and nontrivial properties. Not only the top and the bottom but also the side surfaces of epitaxial Bi(111) thin films are always robustly metallic while the interior has already become insulating. We identify the coupling between the top and the bottom surface states that drives the two originally independent surface conducting channels into a single connected one. The properties of Bi(111) thin films realized could lead to promising applications in spintronics.

The magnetically ordered phase in the Hubbard model on the infinite-dimensional hyper-perovskite lattice is investigated within dynamical mean-field theory. It turns out for the infinite-dimensional hyper-perovskite lattice the self-consistent equations of dynamical mean-field theory are exactly solved, and this makes the Hubbard model exactly solvable. We find electron spins are aligned in the ferromagnetic or ferrimagnetic configuration at zero temperature and half filling of the edge-centered sites of the hyper-perovskite lattice. A ferromagnetic-ferrimagnetic phase transition driven by the energy level splitting is found and it occurs through a phase separation. The origin of ferromagnetism and ferrimagnetism arises from the band flatness and the virtual hybridization between macroscopically degenerate flat bands and dispersive ones. Based on the exact solution in the infinite-dimensional limit, a modified exact diagonalization as the impurity solver for dynamical mean-field theory on finite-dimensional perovskite lattices is also proposed and examined.

In this paper, we study $(2+1)$-dimensional quantum anomalous Hall states, i.e., band insulators with quantized Hall conductance, using exact holographic mapping. Exact holographic mapping is an approach to holographic duality which maps the quantum anomalous Hall state to a different state living in $(3+1)$-dimensional hyperbolic space. By studying topological response properties and the entanglement spectrum, we demonstrate that the holographic dual theory of a quantum anomalous Hall state is a $(3+1)$-dimensional topological insulator. The dual description enables a characterization of topological properties of a system by the quantum entanglement between degrees of freedom at different length scales.

The latest trend in studies of modern electronically and/or optically active materials is to provoke phase transformations induced by high electric fields or by short (femtosecond) powerful optical pulses. The systems of choice are cooperative electronic states whose broken symmetries give rise to topological defects. For typical quasi-one-dimensional architectures, those are the microscopic solitons taking from electrons the major roles as carriers of charge or spin. Because of the long-range ordering, the solitons experience unusual super-long-range forces leading to a sequence of phase transitions in their ensembles: the higher-temperature transition of the confinement and the lower one of aggregation into macroscopic walls. Here we present results of an extensive numerical modeling for ensembles of both neutral and charged solitons in both two- and three-dimensional systems. We suggest a specific Monte Carlo algorithm preserving the number of solitons, which substantially facilitates the calculations, allows to extend them to the three-dimensional case and to include the important long-range Coulomb interactions. The results confirm the first confinement transition, except for a very strong Coulomb repulsion, and demonstrate a pattern formation at the second transition of aggregation.

For arbitrary space dimension $d$, we investigate the quantum phase transitions of two paradigmatic spin models defined on a hypercubic lattice, the coupled-dimer Heisenberg model and the transverse-field Ising model. To this end, high-order linked-cluster expansions for the ground-state energy and the one-particle gap are performed up to order 9 about the decoupled-dimer and high-field limits, respectively. Extrapolations of the high-order series yield the location of the quantum phase transition and the correlation-length exponent $\nu$ as a function of space dimension $d$. The results are complemented by $1/d$ expansions to next-to-leading order of observables across the phase diagrams. Remarkably, our analysis of the extrapolated linked-cluster expansion allows to extract the coefficients of the full $1/d$ expansion for the phase-boundary location in both models exactly in leading order and quantitatively for subleading corrections.

We propose an ab initio framework to derive the dielectric and the second-order susceptibility tensors for crystal surfaces. The single-surface response is extracted from a supercell scheme. We evaluate macroscopic quantities, taking into account the local fields. The first- and second-order susceptibilities are evaluated within time-dependent density functional theory, in the long-wavelength limit. We apply our formalism to the calculation of the second-harmonic generation for clean and hydrogenated silicon surfaces. The agreement with measured second-order susceptibility components is significantly better, illustrating the importance of local-field effects.

We report optical second-harmonic generation (SHG) in reflection from GaSe crystals of 1 to more than 100 layers using a fundamental picosecond pulsed pump at 1.58 eV and a supercontinuum white light pulsed laser with energies ranging from 0.85 to 1.4 eV. The measured reflected SHG signal is maximal in samples of $\sim 20$ layers, decreasing in thicker samples as a result of interference. The thickness- and frequency-dependence of the SHG response of samples thicker than $\sim 7$ layers can be reproduced by a second-order optical susceptibility that is the same as in bulk samples. For samples $\lesssim 7$ layers, the second-order optical susceptibility is reduced compared to that in thicker samples, which is attributed to the expected band-gap increase in mono- and few-layer GaSe.

Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed in the bulk and still stronger effects are predicted for single layers. We present here a detailed study of these properties by combining ab initio calculations and a tight-binding Wannier analysis in both real and reciprocal space. Due to the simplicity of the band structure with single valence $\left(\pi \right)$ and conduction $\left({\pi }^{*}\right)$ bands the tight-binding analysis becomes quasiquantitative with only two adjustable parameters and provides tools for a detailed analysis of the exciton properties. Strong deviations from the usual hydrogenic model are evidenced. The ground-state exciton is not a genuine Frenkel exciton, but a very localized tightly bound one. The other ones are similar to those found in transition-metal dichalcogenides and, although more localized, can be described within a Wannier-Mott scheme.

We theoretically consider superconducting proximity effect, using the Bogoliubov–de Gennes (BdG) theory, in heterostructure sandwich-type geometries involving a normal $s$-wave superconductor and a nonsuperconducting material with the proximity effect being driven by Cooper pairs tunneling from the superconducting slab to the nonsuperconducting slab. Applications of the superconducting proximity effect may rely on an induced spectral gap or induced pairing correlations without any spectral gap. We clarify that in a nonsuperconducting material the induced spectral gap and pairing correlations are independent physical quantities arising from the proximity effect. This is a crucial issue in proposals to create topological superconductivity through the proximity effect. Heterostructures of three-dimensional topological insulator (TI) slabs on conventional $s$-wave superconductor (SC) substrates provide a platform, with proximity-induced topological superconductivity expected to be observed on the “naked” top surface of a thin TI slab. We theoretically study the induced superconducting gap on this naked surface. In addition, we compare against the induced spectral gap in heterostructures of SC with a normal metal or a semiconductor with strong spin-orbit coupling and a Zeeman splitting potential (another promising platform for topological superconductivity). We find that for any model for the non-SC metal (including metallic TI) the induced spectral gap on the naked surface decays as $L^{-3}$ as the thickness ($L$) of the non-SC slab is increased in contrast to the slower $1/L$ decay of the pairing correlations. Our distinction between proximity-induced spectral gap (with its faster spatial decay) and pairing correlation (with its slower spatial decay) has important implications for the currently active search for topological superconductivity and Majorana fermions in various superconducting heterostructures.

We demonstrate that the coupling between light emitters in extended polymer layers and modes supported by arrays of plasmonic particles can be selectively enhanced by accurate positioning of the emitters in regions where the electric field intensity of a given mode is maximized. The enhancement, which we measure to reach up to 70%, is due to the improved spatial overlap and coupling between the optical mode and emitters. This improvement of the coupling leads to a modification of the emission spectrum and the luminous efficacy of the sample.

Motivated by a recent experiment [Nature (London) 531, 206 (2016)], we consider charging of a nanowire which is proximitized by a superconductor and connected to a normal-state lead by a single-channel junction. The charge $Q$ of the nanowire is controlled by gate voltage $e{\mathcal{N}}_g/C$. A finite conductance of the contact allows for quantum charge fluctuations, making the function $Q\left({\mathcal{N}}_g\right)$ continuous. It depends on the relation between the superconducting gap $\mathrm{\Delta }$ and the effective charging energy $E_C^{*}$. The latter is determined by the junction conductance in addition to the geometrical capacitance of the proximitized nanowire. We investigate $Q\left({\mathcal{N}}_g\right)$ at zero magnetic field $B$ and at fields exceeding the critical value $B_c$ corresponding to the topological phase transition [Phys. Rev. Lett. 105, 077001 (2010); Phys. Rev. Lett. 105, 177002 (2010)]. Unlike the case of $\mathrm{\Delta }=0$, the function $Q\left({\mathcal{N}}_g\right)$ is analytic even in the limit of negligible level spacing in the nanowire. At $B=0$ and $\mathrm{\Delta }>E_C^{*}$, the maxima of $dQ/d{\mathcal{N}}_g$ are smeared by $2e$ fluctuations described by a single-channel “charge Kondo” physics, whereas the $B=0,\mathrm{\Delta }<E_C^{*}$ case is described by a crossover between the Kondo and the mixed-valence regimes of the Anderson impurity model. In the topological phase, $Q\left({\mathcal{N}}_g\right)$ is an analytic function of the gate voltage with $e$-periodic steps. In the weak-tunneling limit, $dQ/d{\mathcal{N}}_g$ has peaks corresponding to Breit-Wigner resonances, whereas in the strong-tunneling limit (i.e., small reflection amplitude $r$) these resonances are broadened, and $dQ/d{\mathcal{N}}_g-e\propto rcos\left(2\pi {\mathcal{N}}_g\right)$.

Low-dimensional quantum magnets at finite temperatures present a complex interplay of quantum and thermal fluctuation effects in a restricted phase space. While some information about dynamical response functions is available from theoretical studies of the one-triplet dispersion in unfrustrated chains and ladders, little is known about the finite-temperature dynamics of frustrated systems. Experimentally, inelastic neutron scattering studies of the highly frustrated two-dimensional material ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ show an almost complete destruction of the one-triplet excitation band at a temperature only 1/3 of its gap energy, accompanied by strong scattering intensities for apparent multi-triplet excitations. We investigate these questions in the frustrated spin ladder and present numerical results from exact diagonalization for the dynamical structure factor as a function of temperature. We find anomalously rapid transfer of spectral weight out of the one-triplet band and into both broad and sharp spectral features at a wide range of energies, including below the zero-temperature gap of this excitation. These features are multi-triplet bound states, which develop particularly strongly near the quantum phase transition, fall to particularly low energies there, and persist all the way to infinite temperature. Our results offer valuable insight into the physics of finite-temperature spectral functions in ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ and many other highly frustrated spin systems.

We use inelastic neutron scattering to study the temperature and energy dependence of spin excitations in an optimally P-doped ${\mathrm{BaFe}}_2\left({\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}\right){}_2$ superconductor ($T_c=30$ K) throughout the Brillouin zone. In the undoped state, spin waves and paramagnetic spin excitations of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ stem from an antiferromagnetic (AF) ordering wave vector ${\mathbf{Q}}_{\text{AF}}=(\pm 1,0)$, and peak near the zone boundary at $(\pm 1,\pm 1)$ around 180 meV. Replacing 30% As by smaller P to induce superconductivity, low-energy spin excitations of ${\mathrm{BaFe}}_2\left({\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}\right){}_2$ form a resonance in the superconducting state and high-energy spin excitations now peak around 220 meV near $(\pm 1,\pm 1)$. These results are consistent with calculations from a combined density functional theory and dynamical mean field theory, and suggest that the decreased average pnictogen height in ${\mathrm{BaFe}}_2\left({\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}\right){}_2$ reduces the strength of electron correlations and increases the effective bandwidth of magnetic excitations.

Polar discontinuity at interfaces plays deterministic roles in charge transport, magnetism, and even superconductivity of functional oxides. To date, most polar discontinuity problems have been explored in heterointerfaces between two dissimilar materials. Here, we show that charged domain walls (CDWs) in epitaxial thin films of ferroelectric $\mathrm{PbZ}{\mathrm{r}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.8}{\mathrm{O}}_3$ are strongly coupled to polar interfaces through the formation of $\frac{1}{2}\langle 101\rangle \left\{h0l\right\}\text{−}$ type crystallographic shear planes (CSPs). Using atomic resolution imaging and spectroscopy we illustrate that the CSPs consist of both conservative and nonconservative segments when coupled to the CDWs where necessary compensating charges for stabilizing the CDWs are associated with vacancies at the CSPs. The CDW/CSP coupling yields an atomically narrow domain wall, consisting of a single atomic layer of oxygen. This study shows that the CDW/CSP coupling is a fascinating venue to develop emergent material properties.

Two-dimensional (2D) materials have attracted considerable interest due to their remarkable properties and potential applications for nanoelectronics, electrodes, energy storage devices, among others. However, many well-studied 2D materials lack appreciable conductivity and tunable mechanical strength, limiting their applications in flexible devices. Newly developed MXenes open up the opportunity to design novel flexible conductive electronic materials. Here, using density functional theory (DFT), we investigate systematically the effects of several functional groups on the stabilization, mechanical properties, and electronic structures of a representative MXene. It is found that oxygen possesses the largest adsorption energy as compared to other functional groups, indicating its good thermodynamic stabilization. In comparison with bare and other functionalized titanium carbides, the oxygen functionalized one exhibits the most superior ideal strength; however, the premature softening of the long-wave phonon modes might limit the intrinsic strength for $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{O}}_2$. Furthermore, the introduction of functional groups can induce a strong anisotropy under tensile loading. By analyzing the deformation paths and the electronic instability under various loadings, we demonstrate that the unique strengthening by oxygen functional groups is attributed to a significant charge transfer from inner bonds to outer surface ones after functionalization. Our results shed a novel view into exploring a variety of MXenes for their potential applications in flexible electronic and energy storage devices.

Heat flux exchanged between two hot bodies at subwavelength separation distances can exceed the limit predicted by the blackbody theory. However, this super-Planckian transfer is restricted to these separation distances. Here we demonstrate the possible existence of a super-Planckian transfer at arbitrary large separation distances if the interacting bodies are connected in the near field with weakly dissipating hyperbolic waveguides. This result opens the way to long-distance transport of near-field thermal energy.

In this study, we systematically investigate three-dimensional (3D) momentum ($\hslash k$)-resolved electronic structures of Ruddlesden-Popper-type iridium oxides ${\mathrm{Sr}}_{n+1}{\mathrm{Ir}}_n{\mathrm{O}}_{3n+1}$ using soft-x-ray (SX) angle-resolved photoemission spectroscopy (ARPES). Our results provide direct evidence of an insulator-to-metal transition that occurs upon increasing the dimensionality of the ${\mathrm{IrO}}_2$-plane structure. This transition occurs when the spin-orbit-coupled $j_{\mathrm{eff}}=1/2$ band changes its behavior in the dispersion relation and moves across the Fermi energy. In addition, an emerging band along the $\mathrm{\Gamma }$(0,0,0)-$\mathrm{R}(\pi ,\pi ,\pi$) direction is found to play a crucial role in the metallic characteristics of ${\mathrm{SrIrO}}_3$. By scanning the photon energy over 350 eV, we reveal the 3D Fermi surface in ${\mathrm{SrIrO}}_3$ and $k_z$-dependent oscillations of photoelectron intensity in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$. In contrast to previously reported results obtained using low-energy photons, folded bands derived from lattice distortions and/or magnetic ordering make significantly weak (but finite) contributions to the $k$-resolved photoemission spectrum. At the first glance, this leads to the ambiguous result that the observed $k$-space topology is consistent with the unfolded Brillouin zone (BZ) picture derived from a nonrealistic simple square or cubic Ir lattice. Through careful analysis, we determine that a superposition of the folded and unfolded band structures has been observed in the ARPES spectra obtained using photons in both ultraviolet and SX regions. To corroborate the physics deduced using low-energy ARPES studies, we propose to utilize SX-ARPES as a powerful complementary technique, as this method surveys more than one whole BZ and provides a panoramic view of electronic structures.