We study the spatial distribution of the density of states (DOS) at zero bias $N\left(\mathit{r}\right)$ in the mixed state of single and multigap superconductors. We provide an analytic expression for $N\left(\mathit{r}\right)$ based on deGennes' relationship between DOS and the order parameter that reproduces well scanning tunneling microscopy (STM) data in several superconducting materials. In the single gap superconductor $\beta \text{−}{\mathrm{Bi}}_2\mathrm{Pd}$, we find that $N\left(\mathit{r}\right)$ is governed by a length scale ${\xi }_H=\sqrt{{\varphi }_0/2\pi H}$, which decreases in rising fields. The vortex core size $\mathcal{C}$, defined via the slope of the order parameter at the vortex center, $\mathcal{C}\propto {(d\mathrm{\Delta }/dr{|}_{r\to 0})}^{-1}$, differs from ${\xi }_H$ by a material dependent numerical factor. The new data on the tunneling conductance and vortex lattice of the $2\mathrm{H}\text{−}{\mathrm{NbSe}}_{1.8}{\mathrm{S}}_{0.2}$ show the in-plane isotropic vortices, suggesting that substitutional scattering removes the in-plane anisotropy found in the two-gap superconductor $2\mathrm{H}\text{−}{\mathrm{NbSe}}_2$. We fit the tunneling conductance of $2\mathrm{H}\text{−}{\mathrm{NbSe}}_{1.8}{\mathrm{S}}_{0.2}$ to a two gap model and calculate the vortex core size $\mathcal{C}$ for each band. We find that $\mathcal{C}$ is field independent and has the same value for both bands. We also analyze the two-band superconductor $2\mathrm{H}\text{−}{\mathrm{NbS}}_2$ and find the same result. We conclude that, independently of the magnetic field induced variation of the order parameter values in both bands, the spatial variation of the order parameter close to the vortex core is the same for all bands.

We report measurements of the superfluid density, ${\lambda }^{-2}\left(T\right)$, in ferromagnet-on-superconductor (F/S) bilayers and ${\text{S/F/S}}^{\prime }$ trilayers comprising Nb with Ni, Py, CoFe, and NiV ferromagnets. Bilayers provide information about F/S interface transparency and the $T$ dependence of ${\lambda }^{-2}$ that inform interpretation of trilayer data. The Houzet-Meyer theory accounts well for the measured dependence of ${\lambda }^{-2}\left(0\right)$ and $T_c$ of F/S bilayers on thickness of F layer, $d_F$, except that ${\lambda }^{-2}\left(0\right)$ is slightly under expectations for CoFe/Nb bilayers. For ${\text{Nb/F/Nb}}^{\prime }$ trilayers, we are able to extract $T_c$ and and ${\lambda }^{-2}$ for both Nb layers when F is thick enough to weaken interlayer coupling. The lower “$T_c$” is actually a crossover identified by onset of superfluid in the $\text{lower-}{T}_c$ Nb layer. For ${\text{Nb/NiV/Nb}}^{\prime }$ trilayers, ${\lambda }^{-2}\left(0\right)$ versus $d_F$ for both Nb layers has a minimum followed by a recovery, suggestive of a $\pi$ junction.

Dynamics induced by a change of boundary conditions reveals rate-dependent signatures associated with topological properties in one-dimensional Kitaev chain and SSH model. While the perturbation from a change of the boundary propagates into the bulk, the density of topological edge modes in the case of transforming to open boundary condition reaches steady states. The steady-state density depends on the transformation rate of the boundary and serves as an illustration of quantum memory effects in topological systems. Moreover, while a link is physically broken as the boundary condition changes, some correlation functions can remain finite across the broken link and keep a record of the initial condition. By testing those phenomena in the nontopological regimes of the two models, none of the interesting signatures of memory effects can be observed. Our results thus contrast the importance of topological properties in boundary-induced dynamics.

We examine the structural stability and magnetization for nickel clusters containing up to 500 atoms by performing first-principles calculations based on pseudopotential in real space computed within density-functional theory. After structural relaxation, Ni clusters in this size range favor either an fcc structure, which is a crystal structure in bulk, or an icosahedral structure, which is expected for small clusters. The calculated total magnetic moments per atom of energetically stable clusters agree well with experiment, wherein the moments decrease nonmonotonically toward the bulk value as the cluster size increases. We analyze the spatial distribution of the local magnetic moment, which explains why the magnetic moments of Ni clusters are enhanced compared to their bulk value.

Impurities, which are unavoidable in real materials, may play an important role in the magnetism of frustrated spin systems with a spin-liquid ground state. We address the impurity issue in quantum kagome antiferromagnets by investigating ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{SO}}_4$ (Zn-brochantite) by means of muon spin spectroscopy. We show that muons dominantly couple to impurities, originating from Cu-Zn intersite disorder, and that the impurity spins are highly correlated with the kagome spins, allowing us to probe the host kagome physics via a Kondo-like effect. The low-temperature plateau in the impurity susceptibility suggests that the kagome spin-liquid ground state is gapless. The corresponding spin fluctuations exhibit an unconventional spectral density and a nontrivial field dependence.

At ambient pressure $P$ and below 5.5 K, olivine-type ${\mathrm{Mn}}_2{\mathrm{GeO}}_4$ hosts a multiferroic (MF) phase where a multicomponent, i.e., multi-$k$ magnetic order generates spontaneous ferromagnetism and ferroelectricity (FE) along the c axis. Under high $P$ the FE disappears above 6 GPa, yet the $P$ evolution of the magnetic structure remained unclear based on available data. Here we report high-$P$ single crystal neutron diffraction experiments in the MF phase at $T=4.5$ K. We observe clearly that the incommensurate spiral component of the magnetic order responsible for FE varies little with $P$ up to 5.1 GPa. With support from high $P$ synchrotron x-ray diffraction measurements at room temperature $\left(T\right)$, the $P$-driven suppression of FE is proposed to occur as a consequence of a crystal structure transition away from the olivine structure. In addition, in the low $T$ neutron scattering experiments an emergent nonhydrostatic $P$ component, i.e., a uniaxial stress, leads to the selection of certain multi-$k$ domains. We use this observation to deduce a double-$k$ conical magnetic structure for the ambient $P$ ground state, this being a key ingredient for a model description of the MF phase.

We performed an angle-resolved photoemission spectroscopy study of the Ni-based superconductor ${\mathrm{SrNi}}_2{\mathrm{As}}_2$. Electron and hole Fermi surface pockets are observed, but their different shapes and sizes lead to very poor nesting conditions. The experimental electronic band structure of ${\mathrm{SrNi}}_2{\mathrm{As}}_2$ is in good agreement with first-principles calculations after a slight renormalization (by a factor 1.1), confirming the picture of Hund's exchange-dominated electronic correlations decreasing with increasing filling of the $3d$ shell in the Fe-, Co-, and Ni-based compounds. These findings emphasize the importance of Hund's coupling and $3d$-orbital filling as key tuning parameters of electronic correlations in transition-metal pnictides.

We report the synthesis, structure, and superconducting properties of single crystals of $\mathrm{SmFeAsO}{\mathrm{H}}_x$. The crystals were grown at high pressure and high temperature using a cubic anvil technique. ${}^1\mathrm{H}-\mathrm{NMR}$ studies confirm the presence of H atoms in the samples. Single crystal x-ray diffraction analyses demonstrate a remarkable disorder in the $\mathrm{S}{\mathrm{m}}_2{\mathrm{O}}_2$ layers induced by hydrogen incorporation and reveal that the H positions are compatible with a ${\mathrm{H}}_2\mathrm{O}$-like geometry inside the crystals. We have measured the magnetotransport properties of $\mathrm{SmFeAsO}{\mathrm{H}}_x$ single crystals with $x=0.07$, 0.11, and 0.16 in magnetic field up to 16 T, oriented along the two main crystallographic directions. The results show an increase of the critical temperature with hydrogen content. The zero-temperature upper critical fields and the magnetic anisotropy are calculated as a function of the hydrogen content. $\mathrm{SmFeAsO}{\mathrm{H}}_x$ crystals present significantly higher upper critical fields and magnetic anisotropies compared to $\mathrm{SmFeAs}{\mathrm{O}}_{1-x}{\mathrm{F}}_x$ compounds.

The study of the iron-based superconductor FeSe has blossomed with the availability of high-quality single crystals, obtained through flux/vapor-transport growth techniques below the structural transformation temperature of its tetragonal phase, $T\approx 450{}^{\circ }\mathrm{C}$. Here, we report on the variation of sample morphology and properties due to small modifications in the growth conditions. A considerable variation of the superconducting transition temperature $T_{\mathrm{c}}$, from 8.8 K to 3 K, which cannot be correlated with the sample composition, is observed. Instead, we point out a clear correlation between $T_{\mathrm{c}}$ and disorder, as measured by the residual resistivity ratio. Notably, the tetragonal-to-orthorhombic structural transition is also found to be quite strongly disorder dependent $(T_{\mathrm{s}}\approx 72–90\text{K})$ and linearly correlated with $T_{\mathrm{c}}$.

We report optical spectroscopy and ultrafast pump-probe measurements on the antiferromagnetic heavy-fermion compound ${\mathrm{CePt}}_2{\mathrm{In}}_7$, a member showing stronger two dimensionality than other compounds in the ${\mathrm{CeIn}}_3$-derived heavy-fermion family. We identify clear and typical hybridization spectral structures at low temperature from the two different spectroscopy probes. However, the strength and related energy scale of the hybridization are much weaker and smaller than that in the superconducting compounds ${\mathrm{CeCoIn}}_5$ and ${\mathrm{CeIrIn}}_5$. The features are more similar to observations on the antiferromagnetic compounds ${\mathrm{CeIn}}_3$ and ${\mathrm{CeRhIn}}_5$ in the same family. The results clearly indicate that the Kondo interaction and hybridizations exist in the antiferromagnetic compounds but with weaker strength.

We investigate a theoretical problem of electron-electron interactions in an inclined magnetic field in a quasi-one-dimensional (Q1D) conductor. We show that they result in strong non-Fermi-liquid corrections to a specific heat, provided that the direction of the magnetic field is far from the so-called Lebed's magic angles (LMAs). If magnetic field is directed close to one of the LMAs, the specific heat corrections become small and the Fermi-liquid picture restores. As a result, we predict Fermi-liquid–non-Fermi-liquid angular crossovers in the vicinities of the LMA directions of the field. We suggest to perform the corresponding experiment in the Q1D conductor ${\left(\mathrm{Per}\right)}_2\mathrm{Au}{\left(\mathrm{mnt}\right)}_2$ under pressure in magnetic fields of the order of $H\simeq 25\text{T}$.

It has been previously shown that two effects cause dramatic changes in the x-ray absorption and emission spectra from the N $K$ edge of the insulating crystal ammonium nitrate. First, vibrational disorder causes major changes in the absorption spectrum, originating not only from the thermal population of phonons, but, significantly, from zero-point motion as well. Second, the anomalously large broadening $(\sim 4$ eV) of the emission originating from nitrate $\sigma$ states is due to the unusually short lifetimes of quasiparticles in an otherwise extremely narrow band. In this work, we investigate the coupling of these effects to core and valence excitons that are created as the initial x-ray excitation energy is progressively reduced toward the N edge. Using a $GW$/Bethe-Salpeter approach, we show the extent to which this anomalous broadening is captured by the $GW$ approximation. The data and calculations demonstrate the importance that the complex self-energies (finite lifetimes) of the valence bands have on the interpretation of emission spectra. We produce a scheme to explain why extreme lifetimes should appear in $\sigma$ states of other similar compounds.

Using ab initio density functional calculations, we predict subatomic-resolution atomic force microscopy (AFM) and scanning tunneling microscopy (STM) images of vertical heterostructures of graphene/hexagonal boron nitride (h-BN) with an intercalated metal atom (Li, K, Cr, Mn, Co, or Cu), and study the effects of the extrinsic metal defect on the interfacial coupling. We find that the structural deformation of the graphene/h-BN layer caused by the metal defect strongly affects the AFM images, whereas orbital hybridization between the metal defect and the graphene/h-BN layer characterizes the STM images.

We investigate the influence of confinement on the positional order of a quasi-1D electron system trapped on the surface of liquid helium. We find evidence that the melting of the Wigner solid (WS) depends on the confinement strength, as well as electron density and temperature. A reentrant solid-liquid-solid transition is observed for increasing electron density under constant electrostatic confinement. As the electron row number $N_y$ changes, varying commensurability results in a modulation of the WS order, even when $N_y$ is large (several tens). This is confirmed by Monte Carlo simulations.

The growth and electronic structures of about 7 monolayer-thick Fe thin films on a vicinal Cu(001) surface have been investigated by scanning tunneling microscopy. We find from atomically resolved observations that the Fe epitaxial fcc(001) lattice is stabilized near the step edges, while several kinds of reconstructed surfaces exist on the regions relatively far from the step edges. Statistical analyses of the surface lattices reveal the importance of high-density Cu steps serving as strain relievers to preserve epitaxially stabilized Fe fcc(001) lattice against transforming bulk stable bcc(110) lattice. Spectroscopic measurements further clarify the intrinsic electronic structures of the fcc Fe thin film in real space, implying the electronic differences between the topmost layers in 6 and 7 monolayer-thick films induced by the expansion of the lateral lattice constant.

Numerical calculations of excitonic properties of novel nanostructures, such as nanowire and crystal phase quantum dots, must combine atomistic accuracy with an approachable computational complexity. The key difficulty comes from the fact that excitonic spectra details arise from atomic-scale contributions that must be integrated over a large spatial domain containing a million and more atoms. In this work we present a step-by-step solution to this problem: a combined empirical tight-binding and configuration interaction scheme that unites linearly scaling computational time with the essentials of the atomistic modeling. We benchmark our method on the example of well-studied self-assembled InAs/GaAs quantum dots. Next, we apply our atomistic approach to crystal phase quantum dots containing more than 10 million atoms.

Radiation-induced coupling between A and B excitons in ZnO is theoretically studied. Considering the center-of-mass motion of excitons in bulk and thin-film structures, we calculate the eigenmodes of an exciton–radiation coupled system and reveal the ratio of each excitonic component in the respective eigenmodes, which is determined from diagonalization of the self-consistent equation between the polarization and the Maxwell electric field. In particular, in a nano-to-bulk crossover size regime, the large interaction volume between multipole-type excitonic waves and radiation waves causes radiative coupling between excitons from different valence bands, which leads to an enhancement of the radiative correction. The results presented in this study are in striking contrast with the conventional view of the optical response of excitons in ZnO, where A and B excitons are independently assigned to their respective spectral structures. We demonstrate an alternative spectral assignment of nonlinear optical signals by focusing on the degenerate four-wave mixing.

We investigate the magnetic minibands of a heterostructure consisting of bilayer graphene (BLG) and hexagonal boron nitride (hBN) by numerically diagonalizing a two-band Hamiltonian that describes electrons in BLG in the presence of a moiré potential. Due to inversion-symmetry breaking characteristic for the moiré potential, the valley symmetry of the spectrum is broken, but despite this, the zero-energy Landau level in BLG survives, albeit with reduced degeneracy. In addition, we derive effective models for the low-energy features in the magnetic minibands and demonstrate the appearance of secondary Dirac points in the valence band, which we confirm by numerical simulations. Then, we analyze how single-particle gaps in the fractal energy spectrum produce a sequence of incompressible states observable under a variation of carrier density and magnetic field.

We propose versatile materials based on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in a zigzag silicene nanoribbon (ZSNR) on half filling in the presence of an out-of-plane electric field. We show that the topological phase transition in the band dispersion of ZSNR can be probed by using the RKKY interaction. We find that, due to the zero-energy edge states of the ZSNR, the exchange coupling is significantly enhanced when the impurities are located on the zigzag edges, and also explore that the strength of the interaction in the topological insulator phase is much greater than that when the system is in the band insulator region. We present a model to investigate the phase of a system of two magnetic impurities located on the edge of the ZSNR and find that three different magnetic phases, spiral, ferromagnetic, and antiferromagnetic, are possible for different values of the electric field. This electrical tunability of the magnetic phases in silicene can be explored by using current experimental techniques and can be of interest in the field of spintronics.

Ground-state phases of a generalized XY model with magnetic and generalized nematic couplings on a nonbipartite triangular lattice are investigated in the exchange interactions parameter space. We demonstrate that the model displays a number of ordered and quasiordered phases as a result of geometrical frustration and/or competition between the magnetic and the generalized nematic interactions. The nature and the extent of the respective phases depend on the parameter $q$ that characterizes the higher-order harmonics term in the Hamiltonian. Motivated by a recent discovery of the experimental realization of the model with $q=2$ in a seemingly unrelated field of the system chemistry [A. B. Cairns, M. J. Cliffe, J. A. M. Paddison, D. Daisenberger, M. G. Tucker, F.-X. Coudert, and A. L. Goodwin, Nature Chemistry 8, 442 (2016)], the model with $q\ge 2$ is discussed in the context of the prediction of structural phases of a class of bimetallic cyanides based on a mapping between the two systems.