We perform first-principles total-energy calculations within the framework of the density-functional theory to investigate electronic properties of graphene/C-doped hexagonal boron nitride ($h\text{−}\mathrm{BN}$) heterostructures. We consider both the monolayer $h\text{−}\mathrm{BN}$ case and the trilayer $h\text{−}\mathrm{BN}$ case. From the electronic-structure analysis, it is found that substitutional doping of the C atom at the B site and that at the N site in underlying $h\text{−}\mathrm{BN}$ lead to asymmetric charge carrier concentrations in the graphene layer, indicating the importance of the impurity atom in the $h\text{−}\mathrm{BN}$ substrate and its polarity in the electronic transport properties of graphene/$h\text{−}\mathrm{BN}$ heterostructures. We also find that simulated scanning tunneling microscopy (STM) images of the graphene surface on the C-doped $h\text{−}\mathrm{BN}$ layer at the B site and that at the N site are considerably different from each other in both monolayer and trilayer $h\text{−}\mathrm{BN}$ cases. While the B site doping of the C atom in $h\text{−}\mathrm{BN}$ substrate layers essentially does not change the STM image of the graphene surface on the pristine $h\text{−}\mathrm{BN}$, the C atom doped at the N site considerably modifies the STM image of graphene even in the case of the doping in the third layer of the trilayer $h\text{−}\mathrm{BN}$. Therefore, when the graphene in used as a cover layer to the $h\text{−}\mathrm{BN}$ layered materials and thin films, the STM should be a useful tool to detect the acceptor C atoms in the $h\text{−}\mathrm{BN}$ not only at the topmost surface layer but also at second and third layers. We clarify the origin of these characteristic features of the STM images in terms of spatial distributions of the local density of states induced by the dopant C atom.

The direct experimental probing of locally varying lattice parameters and anisotropic lattice deformations in atomic multilayers is extremely challenging. Here, we develop a combined numerical and graphical method for the analysis of irregular moiré superstructures measured by scanning tunneling microscopy (STM) on a small-twist-angle ($\sim 0.6^{\circ }$) graphene on highly oriented pyrolytic graphite (gr/HOPG). We observe distorted moiré patterns with a spatially varying period in annealed gr/HOPG. The nanoscale modulation of the moiré period observed by STM reflects a locally strained (and sheared) graphene with anisotropic variation of the lattice parameters. We use a specific algorithm based on a rigid lattice Fourier method, which is able to reconstruct the irregular and distorted moiré patterns emerging from strain-induced lattice deformations. Our model is universal and can be used to study different moiré patterns occurring in two-dimensional van der Waals heterostructures. Additionally, room-temperature scanning tunneling spectroscopy measurements show electronic states at the Dirac point, localized on moiré hills, which increase significantly the apparent corrugation of the moiré pattern. The measured topography is compared to classical molecular dynamics simulations. Density functional theory calculations confirm that an AAB-stacked trilayer region itself can contribute electronic states near the Fermi level, in agreement with the measured peak in the local density of states. Furthermore, classical molecular dynamics calculations reveal direction-dependent bond alternations $(\sim 0.5\%)$ around the stacking regions, induced by shear strain, which could influence electronic properties.

Femtosecond infrared luminescence in Au was observed using the upconversion technique. The instantaneous luminescence intensity for a sample with an appropriate surface roughness was three orders of magnitudes higher than that for a flat film and comparable to the hot luminescence in InAs. The time-resolved spectra from 0 to 1 ps exhibited broad features spreading from 0.3 to 1.04 eV, indicating a continuous joint density of states around the Fermi energy. In terms of a simple model that considers nonthermal and thermal carriers for describing the hot luminescence, most of the characteristic behaviors are semiquantitatively understood.

Crystalline membranes are one of the rare examples of bidimensional systems in which long-range order can stabilize an ordered phase in the thermodynamic limit. By a careful analysis of the Goldstone mode counting, we propose a symmetry-breaking mechanism associated with the generation of the flat phase, and we show how it highlights the crucial role played by the crystalline lattice in the establishment of long-range order in these objects. Comparison with other symmetry-breaking mechanisms in membrane physics is also used to unveil the links between symmetry-breaking patterns and the physical properties of the flat phase.

Superconducting $\mathrm{C}{\mathrm{a}}_{10}\mathrm{P}{\mathrm{t}}_4\mathrm{A}{\mathrm{s}}_8{\left(\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2\right)}_5$ is rare because optimal superconducting transition temperature is achieved without chemical doping or pressure. However, the unclear crystal structure limits our ability to understand the structure-property relationship. Using atomically resolved scanning transmission electron microscopy and electron diffraction, we directly determine the structure of this superconductor: it forms a monoclinic structure (space group $P{2}_1/n$) with lattice parameters $a=b=8.76\AA$, $c=20.18\AA$, and $\gamma =90.5^{\circ }$. Compared with previously reported structures derived from diffraction experiments, the $c$-lattice constant is doubled due to alternating stacking of $\mathrm{P}{\mathrm{t}}_4\mathrm{A}{\mathrm{s}}_8$ layers, which induces a high density of stacking faults. With the establishment of the crystal structure, stacking faults, and chemical composition, the distinctive normal-state electrical and thermal transport properties of our superconducting $\mathrm{C}{\mathrm{a}}_{10}\mathrm{P}{\mathrm{t}}_4\mathrm{A}{\mathrm{s}}_8{\left({\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{P}{\mathrm{t}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2\right)}_5$ ($x\sim 0.05$) single crystals can be explained.

We report measurements of room-temperature mean free paths of long-lived THz acoustic phonons in wurtzite GaN. Longitudinal phonon wave packets are excited and probed by femtosecond laser pulses in two InGaN-GaN multiple quantum well structures separated by a GaN layer. By measuring the temperature dependence of the phonon attenuation in the range 80–300 K we isolate the intrinsic phonon mean free path at 300 K which is found to be 5.3 and 3.5 μm at 1.06 and 1.43 THz, respectively. The measurements are found to be in good agreement with ab initio calculations which show that the main channel of the acoustic phonon decay is a three-phonon scattering process involving the acoustic phonon and two high-frequency optical phonons. Our results indicate that the contribution of low-THz acoustic phonons to thermal transport in GaN is relatively smaller than in Si; thus finite size effects are expected to be less important in GaN.

The mixed spin-1 and spin-1/2 Heisenberg octahedral chain with regularly alternating monomeric spin-1 sites and square-plaquette spin-1/2 sites is investigated using a variational technique, localized-magnon approach, exact diagonalization, and density-matrix renormalization-group method. The investigated model has, in a magnetic field, an extraordinarily rich ground-state phase diagram, which includes the uniform and cluster-based Haldane phases, two ferrimagnetic phases of Lieb-Mattis type, two quantum spin liquids, two bound magnon crystals, in addition to the fully polarized ferromagnetic phase. The lowest-energy eigenstates in a highly frustrated parameter region belong to flatbands and, hence, low-temperature thermodynamics above the bound magnon-crystal ground states can be satisfactorily described within the localized-magnon approach. The variational method provides exact evidence for the magnon-crystal phase with a character of the monomer-tetramer ground state at zero field, whereas another magnon-crystal phase with a single bound magnon at each square plaquette is found in a high-field region. A diversity of quantum ground states gives rise to manifold zero-temperature magnetization curves, which may involve up to four wide intermediate plateaus at zero, one-sixth, one-third, and two-thirds of the saturation magnetization, two quantum spin-liquid regions, and two tiny plateaus at one-ninth and one-twelfth of the saturation magnetization corresponding to the fragmentized cluster-based Haldane phases.

We investigated the correlation between the crystal structure and magnetism in ${\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{Fe}}_{28}{\mathrm{O}}_{46}$ using magnetic measurements and transmission electron microscopy. With changing temperature, the magnitude of the magnetization along the $\left[2\overline{1}\overline{1}0\right]$ and $\left[01\overline{1}0\right]$ directions changed drastically at $\sim 340$ K, whereas that along the [0001] direction remained almost unchanged, suggesting a spin reorientation transition. The field dependence of the magnetization along $\left[2\overline{1}\overline{1}0\right]$ exhibited characteristic stepwise behavior below 150 K, whereas that along $\left[01\overline{1}0\right]$ did not exhibit this behavior, which suggests unusual magnetic anisotropy within the (0001) plane. Evaluation of the magnetic domain structures revealed that ${\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{Fe}}_{28}{\mathrm{O}}_{46}$ exhibits strong axial magnetocrystalline anisotropy with the axis along the direction tilted by $\sim {60}^{\circ }$ from the [0001] direction toward the $\left[01\overline{1}0\right]$ direction at low temperatures. This anisotropy along a nonunique crystallographic direction results in multiaxial anisotropy because of the crystal symmetry.

Superfluid stiffness ${\rho }_s$ is a defining characteristic of the superconducting state, allowing phase coherence and supercurrent. It is accessible experimentally through the penetration depth. Coexistence of $d$-wave superconductivity with other phases in underdoped cuprates, such as antiferromagnetism or charge-density waves, may drastically alter ${\rho }_s$. To shed light on this physics, the zero-temperature value of ${\rho }_s={\rho }_{zz}$ along the $c$ axis was computed for different values of Hubbard interaction $U$ and different sets of tight-binding parameters describing the high-temperature superconductors YBCO and NCCO. We used cellular dynamical mean-field theory for the one-band Hubbard model with exact diagonalization as impurity solver and state-of-the-art bath parametrization. We conclude that Mott physics plays a dominant role in determining the superfluid stiffness on the hole-doped side of the phase diagram. On the electron-doped side, antiferromagnetism wins over superconductivity near half-filling. But, upon approaching optimal electron-doping, homogeneous coexistence between superconductivity and antiferromagnetism causes the superfluid stiffness to drop sharply. Hence, on the electron-doped side, it is competition between antiferromagnetism and $d$-wave superconductivity that plays a dominant role in determining the value of ${\rho }_{zz}$ near half-filling. At large overdoping, ${\rho }_{zz}$ behaves in a more BCS-type manner in both the electron- and hole-doped cases. We comment on some qualitative implications of these results for the superconducting transition temperature.

We study subgap states induced by a single impurity in an $s$-wave superconductor with suppressed pairing. For concreteness, we consider a bulk superconductor containing a normal spherical region. We find that a point impurity in this system induces two Yu-Shiba-Rusinov states inside the minigap instead of one, which one would have in a homogeneous superconductor. Moreover, the subgap states appear even if the impurity is nonmagnetic. We prove that this result actually holds almost for any superconductor with a real and spatially inhomogeneous order parameter, if the quasiparticle spectrum is gapped.

Our joint theoretical and computer experimental study of heat-to-spin conversion reveals that noncollinear antiferromagnetic insulators are promising materials for generating magnon spin currents upon application of a temperature gradient: They exhibit spin Seebeck and spin Nernst effects. Using Kubo theory and spin dynamics simulations, we explicitly evaluate these effects in a single kagome sheet of potassium iron jarosite, ${\mathrm{KFe}}_3{\left(\mathrm{OH}\right)}_6{\left({\mathrm{SO}}_4\right)}_2$, and predict a spin Seebeck conversion factor of $0.2\mu \mathrm{V}/\mathrm{K}$ at a temperature of $20\mathrm{K}$.

We report on the lattice and spin dynamics in monopnictide CrAs using polarized Raman scattering. This system exhibits a first-order magnetostructural phase transition at $T_N\sim 265\mathrm{K}$, below which the magnetic moments of Cr form an incommensurate double helical magnetic structure while the unit-cell volume abruptly expands. Across the transition, the frequencies of the fully symmetric $A_g$ phonons strongly renormalize, which along with the results of first-principles calculations suggests the presence of a sizeable coupling of the phonons to the magnetic degrees of freedom in this compound. In addition, we observe two broad modes at around 350 and $1700\mathrm{c}{\mathrm{m}}^{-1}$ in the magnetic phase, which we associate with two-magnon Raman scattering. This work provides one of the few examples of magnetic Raman scattering from a noncollinear magnet.

The memory effect of the polarized ferroelectric domain state in a nonpolar spin-density wave phase has been reported in few multiferroics and the origin of this effect has been attributed to the presence of polar nanodomains in the nonpolar magnetic state. From magnetoelectric and dielectric-relaxation studies, we show the presence of the memory effect beyond the antiferromagnetic ordering temperature ($T_N^{Mn}=39.5\mathrm{K}$) in a single crystal of $\mathrm{G}{\mathrm{d}}_{0.5}\mathrm{D}{\mathrm{y}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$. We suggest that the Debye-like relaxation behavior, which is a manifestation of localized charge carries or polar nanodomains in the crystal, is responsible for the memory effect. Intriguingly, a correlation of dielectric relaxation behavior to the magnetic interactions is found in a wide range of temperature including noncollinear spin-induced ferroelectric, the collinear nonpolar antiferromagnetic, as well as in the paramagnetic states.

Gadolinium hexaboride $\left({\mathrm{GdB}}_6\right)$ is a well-known field emitter material that has been investigated for more than three decades. We perform a systematical density-functional theory study of ${\mathrm{GdB}}_6$ by using the generalized-gradient approximation and considering the electron interaction parameter $U$. The basic structural and electronic properties are carefully revised, as well as a strong $U$-value dependence in determining the antiferromagnetic (AFM) magnetic structures of Gd $4f$ electronic states. We found a small $U(0\sim 3$ eV) showing the most consistent experimental ground-state properties, which gives rise to a magnetic structure with a ground state of C-AFM and a second stable E-AFM. Moreover, we find the distortion modes of boron octahedron play an important role in the interaction between spin and lattice structures in this system. These results will deepen our understanding of the boron-based correlated rare earth compounds.

We determine the mean-field ground state of the three-dimensional rotationally symmetric $d$-wave $\left(\ell =2\right)$ superconductor at weak coupling. It is a noninert state, invariant under the symmetry $C_2$ only, which breaks time-reversal symmetry almost maximally, and features a high but again less-than-maximal average magnetization. The state obtained by minimization of the expanded sixth-order Ginzburg–Landau free energy is found to be an excellent approximation to the true ground state. The coupling to a parasitic $s$-wave component has only a minuscule quantitative and no qualitative effect on the ground state.

Electronic correlation is believed to play an important role in exotic phenomena such as insulator-metal transition, colossal magnetoresistance, and high-temperature superconductivity in correlated electron systems. Recently, it has been shown that electronic correlation may also be responsible for the formation of unconventional plasmons. Herewith, using a combination of angle-dependent spectroscopic ellipsometry, angle resolved photoemission spectroscopy, and Hall measurements, all as a function of temperature supported by first-principles calculations, the existence of low-loss high-energy correlated plasmons accompanied by spectral weight transfer, a fingerprint of electronic correlation, in topological insulator ${\left(\mathrm{B}{\mathrm{i}}_{0.8}\mathrm{S}{\mathrm{b}}_{0.2}\right)}_2\mathrm{S}{\mathrm{e}}_3$ is revealed. Upon cooling, the density of free charge carriers in the surface states decreases whereas that in the bulk states increases, and the recently reported correlated plasmons are key to explaining this phenomenon. Our result shows the importance of electronic correlation in determining correlated plasmons and opens an alternative path in engineering plasmonic-based topologically insulating devices.

We study the optical properties of coupled quantum dot-microcavity systems with an elliptical cross section. First, we develop an analytic model that describes the spectrum of the cavity modes that are split due to the reduced symmetry of the resonator. By coupling the quantum dot (QD) emission to the polarized fundamental cavity modes, we observe the vectorial nature of the Purcell enhancement, which depends on the intrinsic polarization of the quantum dot and its relative alignment with respect to the cavity axis. The variable interaction strength of the QD with the polarized cavity modes leads to the observation of strong and weak coupling. Finally, we demonstrate the capability of elliptical micropillars to emit single and highly indistinguishable photons (visibility of $87\%$).

Electric currents flowing transverse to the electric field in the absence of magnetic fields in systems with broken time-reversal symmetry lead to the anomalous Hall effect observed in DC transport experiments. When the system is in equilibrium and there is no transport, anomalous currents still exist, inducing measurable magnetic fields or magnetic moments. In this paper, we employ the two-dimensional massive Dirac equation to find the exact current flowing along a potential step of arbitrary shape. The current is universal, i.e., it depends only on the asymptotic value of the potential drop. For a spatially slowly varying potential we find the current density $\mathbf{j}\left(\mathbf{r}\right)$ and the energy distribution of the current density ${\mathbf{j}}^{\varepsilon }\left(\mathbf{r}\right)$. The latter turns out to be unexpectedly nonuniform, behaving like a $\delta$ function at the border of the classically accessible area at energy $\varepsilon$. Consequently, even in a weak electric field the transverse current density cannot be described semiclassically. To demonstrate explicitly the difference between the magnetization and transport currents we consider the transverse shift of an electron ray in an electric field.

We investigate the Andreev reflection in a parallel double quantum dot structure, by considering one metallic lead coupled to one $s$-wave superconductor through the quantum dots. It is found that if an arm of this system provides the reference channel for the Andreev reflection, the Fano line shapes will have opportunities to appear in the linear conductance spectra and can be reversed by adjusting the dot level or local magnetic flux. To present the underlying physics, we obtain the Fano form of the linear conductance expression. Despite the complicated structural parameters, the property of the Fano effect is clearly shown. We believe that the results of this work will help us to understand the Fano interference in the Andreev reflection process.