We report an experimental study on charge transfer properties of mixed-dimensional heterostructures formed by zero-dimensional PbS quantum dots and two-dimensional transition metal dichalcogenides. Monolayers of ${\mathrm{MoSe}}_2$ and ${\mathrm{MoS}}_2$ were fabricated by mechanical exfoliation and chemical vapor deposition techniques, respectively. PbS quantum dots with diameters of 2.3 and 5 nm were synthesized by a hot-injection method and characterized by optical absorption spectroscopy and ultraviolet photoemission spectroscopy. The quantum dots were deposited on the ${\mathrm{MoS}}_2$ and ${\mathrm{MoSe}}_2$ monolayers to form heterostructures. Photoluminescence and transient absorption measurements were performed on the heterostructures as well as individual materials to reveal their photocarrier dynamics. We found that the holes excited in ${\mathrm{MoSe}}_2$ can efficiently transfer to both 2.3- and 5-nm PbS quantum dots, while electrons in these quantum dots cannot transfer to ${\mathrm{MoSe}}_2$. Similar charge transfer properties were observed between ${\mathrm{MoS}}_2$ and the 5-nm PbS quantum dots, while no charge transfer was observed between ${\mathrm{MoS}}_2$ and the 2.3-nm quantum dots. These results provide useful information for understanding the physical mechanism of charge transfer in mixed-dimensional heterostructures and for developing PbS quantum-dot-based mixed-dimensional materials.

In heavy fermion systems, the emergence of rich phenomena, such as hidden orders and superconductivities, is made possible by multipole degrees of freedom. However, many of them remain unsolved since the origin of the higher-rank multipole interaction is not well understood. Among these issues, we focus on the quadrupole order in ${\mathrm{CeB}}_6$, which is a famous multipolar heavy fermion system that has been actively studied for decades. We analyze the multiorbital periodic Anderson model for ${\mathrm{CeB}}_6$, and find that magnetic, quadrupole, and octupole fluctuations all develop cooperatively due to the strong intermultipole coupling given by higher-order many-body effects, called vertex corrections. It is found that the antiferroquadrupole order in ${\mathrm{CeB}}_6$ is driven by the interference between magnetic-multipole fluctuations. The discovered intermultipole coupling mechanism is a potential origin of numerous hidden orders in various heavy fermion systems.

Purely amorphous germania bilayer films are grown on a reconstructed Au(111) surface. The presence of the film affects the native configuration of the Au soliton walls, as observed with scanning tunneling microscopy. They partly avoid the film islands, and partly penetrate under film patches. This behavior indicates a weaker film-substrate interaction than the one reported for other oxide films on reconstructed Au(111). Moreover, this new system highlights the impact of the metal support on the structure of ultrathin films of germania: With decreasing film-substrate interaction the amorphous phase is promoted. Density functional theory calculations confirm and rationalize the experimental observations. This work provides a useful generalization of the relationship between film structure and adhesion energy.

Polarization-dependent photon spectroscopy (dichroism) of the second-harmonic generation response is shown to reveal chiral and magnetic properties of a sample. Two dichroic signals are allowed with electric-dipole and electric-quadrupole scattering events, and both require circular polarization in the primary beam. Explicit expressions for the signals are derived using theoretical techniques from atomic physics.

We report on a spin-charge fluctuation in the all-in/all-out pyrochlore magnet ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$, where the spin fluctuation is driven by the conduction of thermally excited electrons/holes and associated fluctuation of Os valence. The fluctuation exhibits an activation energy significantly greater than the spin-charge excitation gap and a peculiar frequency range of ${10}^6–{10}^{10}{\mathrm{s}}^{-1}$. These features are attributed to the hopping motion of carriers as small polarons in the insulating phase, where the polaron state is presumably induced by the magnetoelastic coupling via the strong spin-orbit interaction. Such a coupled spin-charge-phonon fluctuation manifests as a part of the metal-insulator transition that is extended over a wide temperature range due to the modest electron correlation comparable with other interactions characteristic for $5d$-subshell systems.

Controlling spin interactions in magnetic-molecules/metal is essential for spintronic applications. Recent studies showed that using small molecule coordination, one could switch off the spin interactions between magnetic-molecules and metal substrates. However, this control should not be limited to the two-state switching. The strength of spin interaction can be reduced, but not “off” by the proper selection of small molecules. To demonstrate this, we considered two contrasting systems, $\mathrm{N}{\mathrm{H}}_3$ and $\mathrm{N}{\mathrm{O}}_2$ coordinated to Co-porphyrin/Au(111). In our scanning tunneling microscopy and spectroscopy (STM and STS), Kondo resonance was preserved with weakened spin coupling after $\mathrm{N}{\mathrm{H}}_3$ coordination. However, it disappeared after $\mathrm{N}{\mathrm{O}}_2$ coordination, implying “off” spin coupling. These observations are explained with our density functional theory calculation results. This study shows that small molecule coordination to magnetic-molecules/metal is a powerful way to control spin interactions at the single-molecule level.

It is well known that twinning deformation plays an important role in the plastic deformation of hexagonal close-packed (HCP) metals because of the insufficient independent slip systems in HCP metals. However, recent works show twinning deformation does not happen in compression in the $c$ axis of the magnesium (Mg) single crystal, and it is not clear that how it deforms with limited dislocation glide in absence of twinning deformation. In the present work, molecular dynamics simulations with well validated modified embedded-atom method potential for Mg are performed to reveal the microscopic deformation behaviors of Mg single crystal compressed in the $c$ axis. We focus on the microstructures and the evolution during both the deformation and the relaxation of the Mg single crystal. Twinning deformation is not observed in the simulations as expected, and dislocations and the amorphous regions are found to be dominant under different loading conditions. The amorphous regions are dominant in the deformation under ultrahigh strain rate, and it results in more homogenous deformation and lower flow stress than dislocation glide. The dislocation glide is dominant under lower strain rate, and the insufficient dislocations may lead to quite inhomogeneous plastic deformation. Furthermore, it is revealed that the amorphous regions consist of irregularly distributed atoms with higher per-atom potential than those in the crystalline state. The amorphous regions thus annihilate rapidly once the high-rate deformation is stopped. The annihilated amorphous regions turn into crystalline ones with dense dislocation networks. The annihilation rate of the amorphous regions decreases with increasing pressure, while it hardly depends on temperature.

Platinum (Pt) is one of the most widely used functional materials for high-pressure and high-temperature experiments. Despite the crucial importance of its transport properties, both experimental and theoretical studies are very limited. In this study, we conducted density functional theory calculations on the electrical resistivity, the Seebeck coefficient, and the thermal conductivity of solid face-centered cubic Pt at pressures up to 200 GPa and temperatures up to 4800 K by using the Kubo-Greenwood formula. The thermal lattice displacements were treated within the alloy analogy, which is represented by means of the Korringa-Kohn-Rostoker method with the coherent potential approximation. The electrical resistivity decreases with pressure and increases with temperature. These two conflicting effects yield a constant resistivity of $\sim 70\mu \mathrm{\Omega }\mathrm{cm}$ along the melting curve. Both pressure and temperature effects enhance the thermal conductivity at low temperatures, but the temperature effect becomes weaker at high temperatures. Although the pressure dependence of the Seebeck coefficient is negligibly small at temperatures below $\sim 1500\mathrm{K}$, it becomes larger at higher temperatures. It requires a calibration of a thermocouple such as Pt-Rh in high-pressure and -temperature experiments.

A novel minimal-size pairing operator ${\mathrm{\Delta }}_{D0}^{†}$ with $d$-wave symmetry in ${\mathrm{CuO}}_2$ planes is introduced. This pairing operator creates on-site Cooper pairs at the four oxygens that surround a copper atom. Via the time evolution of ${\mathrm{\Delta }}_{D0}^{†}$, an additional interorbital pairing operator ${\mathrm{\Delta }}_{Dpd}^{†}$ with $d$-wave symmetry is generated that pairs fermions located in a Cu and its four surrounding O atoms. The subsequent time evolution of ${\mathrm{\Delta }}_{Dpd}^{†}$ generates an intraorbital $d$-wave pairing operator ${\mathrm{\Delta }}_{Dpp}^{†}$ involving the four O atoms that surround a Cu atom, as well as the $d$-wave operator ${\mathrm{\Delta }}_D^{†}$ traditionally used in single-band models for cuprates. Because we recover the larger size operators extensively used in the three-orbital Hubbard model, we suggest that long-range order using the canonical extended operators occurs together with long-range order in the new minimal operators. However, our minimal $d$-wave operators could be more practical to study $d$-wave superconductivity because in the finite-size relatively small systems accessible to computational techniques it is easier to observe long-range order using local operators. Moreover, an effective model with the usual tight-binding hopping of the ${\mathrm{CuO}}_2$ planes supplemented by an attractive potential $V$ in the $d$-wave channel is introduced. Using mean-field techniques, we show that a paired ground state is stabilized for any finite value of $V$. We observed that the values of $V$ that lead to gap sizes similar to those in the cuprates are smaller for $d$-wave pairing operators that include Cu $d$ orbitals than those that include only $p$ orbitals. In all cases the gap that opens in the spectrum has standard $d$-wave symmetry. Finally, a simpler effective model is introduced to study the phenomenology of multiorbital $d$-wave superconductors, similar to how the negative-$U$ Hubbard model is used for properties of $s$-wave superconductors.

Superconducting vortex loops have so far avoided experimental detection despite being the focus of much theoretical work. We here propose a method of creating controllable vortex loops in the superconducting condensate arising in a normal metal through the proximity effect. We demonstrate both analytically and numerically that superconducting vortex loops emerge when the junction is pierced by a current-carrying insulated wire and give an analytical expression for their radii. The vortex loops can readily be tuned big enough to hit the sample surface, making them directly observable through scanning tunneling microscopy.

Andreev bound states are low-energy excitations appearing below the particle-hole gap of superconductors, and are expected to be topologically trivial. Here, we report the theoretical prediction of topologically nontrivial Andreev bound states in one-dimensional superconductors. These states correspond to another topological invariant defined in a synthetic two-dimensional space, the particle-hole Chern number, which we construct in analogy to the spin Chern number in quantum spin Hall systems. Nontrivial Andreev bound states have distinct features and are topologically nonequivalent to Majorana bound states. Yet, they can coexist in the same system, have similar spectral signatures, and materialize with the concomitant opening of the particle-hole gap. The coexistence of Majorana and nontrivial Andreev bound state is the direct consequence of “double dimensionality”, i.e., the dimensional embedding of the one-dimensional system in a synthetic two-dimensional space, which allows the definition of two distinct topological invariants (${\mathbb{Z}}_2$ and $\mathbb{Z}$) in different dimensionalities.

Motivated by the recent discovery of helical magnetic structure in ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$, we investigate interlayer ordering of magnetic moments in materials composed of spatially separated superconducting and ferromagnetically aligned layers. We consider the interplay between the normal and superconducting indirect exchange interactions mediated by tunneling between the conducting layers. We elaborate a recipe to evaluate the normal interlayer interaction via two-dimensional density of states of an isolated layer and demonstrate that for bands with small fillings, such interaction is typically ferromagnetic and short range. The nearest-layer interaction is proportional to the ratio of the interlayer hopping and in-plane bandwidth squared. On the other hand, the superconducting contribution always gives antiferromagnetic interaction and may extend over several layers when the interlayer hopping energy exceeds the superconducting gap. The frustration caused by the interplay between the normal and superconducting parts may lead to spiral ground-state magnetic configuration. The fourfold in-plane anisotropy may lock the rotation angle between the moments in the neighboring layers to ${90}^{\circ }$, as it was observed in ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$.

The monolayer FeSe on $\mathrm{SrTi}{\mathrm{O}}_{3-\delta }$ exhibits a twofold enlarged pairing gap compared with other electron-doped FeSe, which is postulated to originate from cooperative effects of interface charge transfer and electron-boson interaction that relate closely to surface oxygen vacancies in $\mathrm{SrTi}{\mathrm{O}}_3$. We create gradient oxygen vacancies on the surface of $\mathrm{SrTi}{\mathrm{O}}_{3-\delta }$ substrates using direct current heating. Combining spatially resolved spectroscopy characterizations, we disclose gradient pairing gaps but negligible doping variation in monolayer FeSe on such substrates. As oxygen vacancy concentration gradually increases from the anode to cathode region, the pairing gap increases from 12 to 17 meV, right beyond the values of electron-doped FeSe. This modulation provides a platform for spatially resolved investigations to unveil the interplay between electronic correlation and electron-phonon interaction and their cooperation to drive superconductivity.

We compute the dynamical charge susceptibility in the two-dimensional Hubbard model within the dynamical cluster approximation. In order to understand the connection between charge susceptibility and pseudogap, we investigate the momentum, doping, and temperature dependence. We find that as a function of frequency, the dynamical charge susceptibility is well represented by a single peak at a characteristic frequency. It shows little momentum or temperature dependence, while the doping dependence is more evident, and no clear signature of the pseudogap is observed. Data for the doping evolution of the static susceptibility and for fluctuation diagnostics are presented. Our susceptibilities should be directly measurable in future Momentum-resolved electron energy-loss spectroscopy experiments.

Helicity-dependent photocurrent (HDPC) has been observed in epitaxial thin films of the three-dimensional topological insulator ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ with a thickness of 7, 20, and 30 quintuple layers (QLs). By analyzing the incident angle dependence of the HDPC, we successfully extract the photocurrent induced by the circular photon drag effect (CPDE) of even parity from the HDPC. It is found that the circular photogalvanic effect (CPGE) is larger than the odd parity of CPDE. The CPGEs of the top and bottom surface states are successfully distinguished at room temperature. As the thickness of the ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ film is increased from 7 to 20 QLs, the dominant contribution of the CPGE current switches from the top surface states to the bottom surface states under the front illumination of a 1064-nm laser. Additionally, it is found that the CPGE currents of the 20- and 30-QL samples reverse sign when the temperature is increased from 77 to 300 K, which may be attributed to the HDPC current induced by the combined action of the inverse spin Hall effect and the vertical thermoelectric effect overwhelming the CPGE current at low temperature in the samples. The insights we obtained are significant for engineering optospintronic devices based on three-dimensional topological insulators with a strong thermoelectric effect.

We report the observation of local superconductivity induced at the point contact formed between a normal metal tip and tungsten carbide—a triple point topological semimetal with superhardness. Remarkably, the maximum critical temperature is up to near 12 K but insensitive to the tip's magnetism. The lateral dimensions of the superconducting puddles were evaluated and the temperature dependencies of the superconducting gap and upper critical field were also obtained. These results put constraints on the explanation of the induced superconductivity and pave a pathway for exploring topological superconductivity.

Layered materials are presently under intense study, and most applications require knowledge about their defects. It has been shown earlier that the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE), with parameters tuned to reproduce the relative position of the band edges and to satisfy the generalized Koopmans’ theorem in the given material, is capable of providing defect properties very accurately in traditional bulk semiconductors. This success is thought to be connected to the proper description of electronic screening. In this paper we investigate whether such a functional can be optimized for layered compounds, such as GaSe and hexagonal BN (hBN). While we find this to be possible in the bulk materials, as expected, the optimal parameters change significantly in the monolayers (ML), due to the effect of the surface on the electronic screening. For ML GaSe (thickness $\sim 5\AA$), where the dielectric constant does not change much within the layer, a reoptimization is possible, while for the atomically thin ML hBN the optimization criteria can only be met approximately. In other words, the accuracy which can be achieved for the electronic structure of defects by a HSE functional will always be less in atomically thin monolayers than in the bulk built from such layers. We also show that the accuracy of HSE functionals, with parameters optimized for the bulk in traditional (nonlayered) bulk semiconductors, also diminishes in surface calculations, even for thick slabs, if the dielectric constant $\left({\varepsilon }^{\infty }\right)$ is low. Generally, the accuracy of bulk and surface calculation by any functional can be very different.

We investigate the charge and spin structures associated with arbitrary smooth polarization textures in Ising (integer) quantum Hall ferromagnets. We consider the case where the two polarizations (denoted “pseudospin” up and down) correspond to states with opposite physical spin and different Landau level indices, $n\uparrow$ and $m\downarrow$. We derive analytic expressions for the charge and spin densities, as functions of the underlying pseudospin texture, and use these results to investigate different types of linear domain walls, both analytically and numerically. We find that any smooth domain wall between two oppositely polarized domains carries a universal quantized charge dipole density proportional to the difference of Landau level indices, $n-m$. Additionally, nonuniformities in the domain wall may give rise to excess net charge localized at the domain wall. Interestingly, the physical spin density associated with the domain wall generally exhibits a much more complex multipolar structure than that of the pseudospin texture. These results should for example help to elucidate the mechanisms underlying nuclear electric resonance and nuclear polarization oscillations in Ising quantum Hall systems.

Chiral Majorana edge modes are theoretically proposed to perform braiding operations for the potential quantum computation. Here we suggest a scheme to regulate trajectories of the chiral Majorana fermion based on a quantum anomalous Hall insulator (QAHI)–topological superconductor heterostructure. An applied external gate voltage to the QAHI region introduces a dynamical phase so that the outgoing Majorana fermions can be prominently tuned to different leads. The trajectory is mechanically analyzed and the electrical manipulation is represented by the oscillating transmission coefficients versus the gate voltage. Through the optimization of devices, the conductance is likewise detectable to be periodically oscillating, which means an experimental control of chiral Majorana edge modes. Besides, this oscillating period which is robust against disorder also provides an attainable method of observing the energy dispersion relation of the edge mode of the QAHI. Furthermore, the oscillating behavior of conductance serves as smoking-gun evidence of the existence of the chiral Majorana fermion, which could be experimentally confirmed.

Anisotropic crystals have recently attracted considerable attention because of their ability to support polaritons with a variety of unique properties, such as hyperbolic dispersion, negative phase velocity, or extreme confinement. Particularly, the biaxial crystal $\alpha -{\mathrm{MoO}}_3$ has been demonstrated to support phonon polaritons, light coupled to lattice vibrations, with in-plane anisotropic propagation and unusually long lifetime. However, the lack of theoretical studies on electromagnetic modes in biaxial crystal slabs impedes a complete interpretation of the experimental data, as well as an efficient design of nanostructures supporting such highly anisotropic polaritons. Here, we derive the dispersion relation of electromagnetic modes in biaxial slabs surrounded by semi-infinite isotropic dielectric half-spaces with arbitrary dielectric permittivities. Apart from a general dispersion relation, we provide very simple analytical expressions in typical experiments in nano-optics: the limits of short polaritonic wavelength and/or very thin slabs. The results of our study will allow for an in-depth analysis of anisotropic polaritons in novel biaxial van der Waals materials.