By using a hydrostatic pressure, we have successfully tuned the normal state property and superconductivity in ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiSe}}_2$ single crystals. It is found that, with the increase of pressure, the original superconducting phase with $T_c\sim 3.5$ K can be tuned to a state with lower $T_c$, and then a new superconducting phase with $T_c\sim 6.5$ K emerges. Accompanied by this crossover, the normal state is switched from that with a low temperature resistivity upturning to a metallic one. Accordingly, the normal state resistivity also shows a nonmonotonic change with the external pressure. Furthermore, by applying a magnetic field, the new superconducting state under pressure with $T_c\sim 6.5$ K is suppressed, and the recovered normal state reveals a resistivity-upturning feature again. These results illustrate a nontrivial relationship between the normal state property and superconductivity in this newly discovered superconducting system.

We have used the combination of Fe $K$-edge and Ir $L_3$-edge x-ray absorption measurements as a function of temperature to investigate local atomic displacements in the newly discovered ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ superconducting system. We find relatively relaxed Fe-Fe atomic pair correlations with large displacements in the ${\mathrm{FeAs}}_4$ tetrahedra, revealed by Fe $K$-edge extended x-ray absorption fine structure (EXAFS) analysis. Similarly, the temperature dependence of Ir $L_3$-edge EXAFS shows nanoscale disorder in the IrAs layer that should have a significant effect on the active FeAs-layer characteristics. Furthermore, x-ray absorption near edge structure data are presented to discuss the evolution of the unoccupied electronic states revealing the marginal role of spin-orbit coupling, while the interlayer interactions and disorder should be important for describing the physics of the ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ system.

We present cavity QED experiments with an ${\mathrm{Er}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal magnetically coupled to a three-dimensional (3D) cylindrical sapphire loaded copper resonator. Such waveguide cavities are promising for the realization of a superconducting quantum processor. Here, we demonstrate the coherent integration of a rare-earth spin ensemble with the 3D architecture. The collective coupling strength of the ${\mathrm{Er}}^{3+}$ spins to the 3D cavity is 21 MHz. The cylindrical sapphire loaded resonator allowed us to explore the anisotropic collective coupling between the rare-earth doped crystal and the cavity. This work shows the potential of spin doped solids in 3D quantum circuits for application as microwave quantum memories as well as for prospective microwave to optical interfaces.

Time-reversal breaking topological superconductors are new states of matter which can support Majorana zero modes at the edge. In this Rapid Communication, we propose a different realization of one-dimensional topological superconductivity and Majorana zero modes. The proposed system consists of a monolayer of transition-metal dichalcogenides ${MX}_2$ $(M=\text{Mo},\text{W}$; $X=\text{S},\text{Se})$ on top of a superconducting substrate. Based on first-principles calculations, we show that a zigzag edge of the monolayer ${MX}_2$ terminated by a metal atom $M$ has edge states with strong spin-orbit coupling and spontaneous magnetization. By proximity coupling with a superconducting substrate, topological superconductivity can be induced at such an edge. We propose ${\mathrm{NbS}}_2$ as a natural choice of substrate, and estimate the proximity induced superconducting gap based on first-principles calculation and a low energy effective model. As an experimental consequence of our theory, we predict that Majorana zero modes can be detected at the 120° corner of a ${MX}_2$ flake in proximity to a superconducting substrate.

Combining structural and functional measurements, we have mapped the phase diagram of ${\mathrm{BiFeO}}_3/{\mathrm{LaFeO}}_3$ superlattices grown by off-axis sputtering on (110)${}_o {\mathrm{DyScO}}_3$ substrates. The phase diagram displays three distinct regions as a function of ${\mathrm{BiFeO}}_3$ fraction, with a ${\mathrm{BiFeO}}_3$-like ferroelectric phase and a ${\mathrm{LaFeO}}_3$-like paraelectric phase at its extremities, and a complex intermediate region, as supported by first-principles calculations. This intermediate region shows unusual, mixed functional behavior, most likely due to competing phases driven by substitution with a same-size central ion and the specific boundary conditions imposed by the superlattice structure. In the ${\mathrm{BiFeO}}_3$ rich superlattices, scaling of the ferroelectric-to-paraelectric transition temperature with the ${\mathrm{BiFeO}}_3$ thickness could provide an alternate route for studying ferroelectric size effects in ${\mathrm{BiFeO}}_3$.

We derive an efficient method for treating renormalization contributions at two-loop level within the functional renormalization group in the one-particle irreducible formalism for fermions. It is based on a decomposition of the two-particle vertex in charge, magnetic and pairing channels. The method treats single-particle and collective excitations in all channels on equal footing, allows for the description of symmetry breaking and captures collective mode fluctuation physics in the infrared. As a first application, we study the superfluid ground state of the two-dimensional attractive Hubbard model. We obtain superfluid gaps that are reduced by fluctuations in comparison to the one-loop approximation and demonstrate that the method captures the renormalization of the amplitude mode by long-range phase fluctuations.

How the long-range ordering and local defect configurations modify the electronic structure of graphene remains an outstanding problem in nanoscience, which precludes the practical method of patterning graphene from being widely adopted for making graphene-based electronic and optoelectronic devices, because a small variation in supercell geometry could change the patterned graphene from a semimetal to a semiconductor, or vice versa. Based on the effective Hamiltonian formalism, here we reveal that a semimetal-to-semiconductor transition can be induced geometrically without breaking the sublattice symmetry. For the same patterning periodicity, however, breaking the sublattice symmetry increases the gap, while phase cancellation can lead to a semiconductor-to-semimetal transition in non-Bravais lattices. Our theory predicts the analytic relationship between long-range defect ordering and band-gap opening/closure in graphene, which is in excellent agreement with our numerical ab initio calculations of graphene nanomeshes, partially hydrogen passivated and boron-nitride-doped graphene.

Using reflection electron energy-loss spectroscopy (EELS), we unveil a direct correlation between the thickness-dependent oscillatory charge spilling of quantum-well states in epitaxial Mg(0001) films, and the linear dispersion coefficient of the monopole surface plasmon mode. In addition, the spectral intensities of the photoemission threshold excitation and multipole surface plasmon mode follow a similar but mutually anticorrelated quantum oscillatory pattern. These oscillatory features of the collective response are attributed to quantum oscillations in the ground-state charge-density profile, specifically its decay into the vacuum. Because the decay lengths can be controlled through the quantum-size effects and appropriate choice of the quantum-size boundary conditions, these insights are expected to be significant in the design of quantum-sized plasmonic devices.

We propose a system of coupled quantum dots in proximity to a superconductor and driven by separate ac potentials to realize and detect Floquet Majorana fermions. We show that the appearance of Floquet Majorana fermions can be finely controlled in the expanded parameter space of the drive frequency, amplitude, and phase difference across the two dots. While these Majorana fermions are not topologically protected, the highly tunable setup provides a realistic system for observing the exotic physics associated with Majorana fermions as well as their dynamical generation and manipulation.

We study theoretically the evolution of photoluminescence (PL) from homogeneous and inhomogeneous ensembles of a few coupled quantum dots (QDs). We discuss the relation between signals from a given QD ensemble under strong and weak excitation (full inversion and linear-response regimes): A system homogeneous enough to manifest superradiant emission when strongly inverted shows a nonexponential decay of the PL signal under spatially coherent weak excitation. In an inhomogeneous ensemble, the PL decay is always nearly exponential with a qualitatively different form of the time dependence in the two excitation regimes and with a higher rate under weak excitation.

We study intraband spectra, resulting from intraband transitions between the bound ground state of a semiconductor quantum dot and unbound continuum states in the host medium. The influence of excitonic coupling on these intraband absorption spectra and the formation of special excitons are discussed. Involving Coulomb coupled bound-to-continuum transitions, the spectra show the formation of bound excitons at the continuum edge and spectral shifts. The analyzed intraband absorption spectra exhibit different signatures for different initial carrier configurations.

Analysis of the strain-induced pseudomagnetic fields generated in graphene nanobulges under three different substrate scenarios shows that, in addition to the shape, the graphene-substrate interaction can crucially determine the overall distribution and magnitude of strain and those fields, in and outside the bulge region. We utilize a combination of classical molecular dynamics, continuum mechanics, and tight-binding electronic structure calculations as an unbiased means of studying pressure-induced deformations and the resulting pseudomagnetic field distribution in graphene nanobubbles of various geometries. The geometry is defined by inflating graphene against a rigid aperture of a specified shape in the substrate. The interplay among substrate aperture geometry, lattice orientation, internal gas pressure, and substrate type is analyzed in view of the prospect of using strain-engineered graphene nanostructures capable of confining and/or guiding electrons at low energies. Except in highly anisotropic geometries, the magnitude of the pseudomagnetic field is generally significant only near the boundaries of the aperture and rapidly decays towards the center of the bubble because under gas pressure at the scales considered here there is considerable bending at the edges and the central region of the nanobubble displays nearly isotropic strain. When the deflection conditions lead to sharp bends at the edges of the bubble, curvature and the tilting of the $p_z$ orbitals cannot be ignored and contributes substantially to the total field. The strong and localized nature of the pseudomagnetic field at the boundaries and its polarity-changing profile can be exploited as a means of trapping electrons inside the bubble region or of guiding them in channellike geometries defined by nanoblister edges. However, we establish that slippage of graphene against the substrate is an important factor in determining the degree of concentration of pseudomagnetic fields in or around the bulge since it can lead to considerable softening of the strain gradients there. The nature of the substrate emerges thus as a decisive factor determining the effectiveness of nanoscale pseudomagnetic field tailoring in graphene.

A theoretical proposal for realizing and detecting spin supercurrent in an isotropic antiferromagnetic insulator is reported. Superfluid spin transport is achieved by inserting the antiferromagnet between two metallic reservoirs and establishing a spin accumulation in one reservoir such that a spin bias is applied across the magnet. We consider a class of bipartite antiferromagnets with Néel ground states, and temperatures well below the ordering temperature, where spin transport is mediated essentially by the condensate. Landau-Lifshitz and magnetocircuit theories are used to directly relate spin current in different parts of the heterostructure to the spin-mixing conductances characterizing the $\text{antiferromagnet}|\text{metal}$ interfaces and the antiferromagnet bulk damping parameters, quantities all obtainable from experiments. We study the efficiency of spin angular-momentum transfer at an $\text{antiferromagnet}|\text{metal}$ interface by developing a microscopic scattering theory for the interface and extracting the spin-mixing conductance for a simple model. Within the model, a quantitative comparison between the spin-mixing conductances obtained for the $\text{antiferromagnet}|\text{metal}$ and $\text{ferromagnet}|\text{metal}$ interfaces is made.

We report an experimental and theoretical investigation of the electron-boson interaction in ${\mathrm{KFe}}_2{\mathrm{As}}_2$ by point-contact (PC) spectroscopy, model, and ab initio local density approximation–based calculations for the standard electron-phonon Eliashberg function. The PC spectrum viz. the second derivative of the $I\text{−}V$ characteristic of representative PC exhibits a pronounced maximum at about 20 meV and surprisingly a featureless behavior at lower and higher energies. We discuss phonon and nonphonon (excitonic) mechanisms for the origin of this peak. Analysis of the underlying source of this peak may be important for the understanding of serious puzzles of superconductivity in this type of compound.

The physics of strongly correlated quantum particles within a flat band was originally explored as a route to itinerant ferromagnetism and, indeed, a celebrated theorem by Lieb rigorously establishes that the ground state of the repulsive Hubbard model on a bipartite lattice with an unequal number of sites in each sublattice must have nonzero spin $S$ at half filling. Recently, there has been interest in Lieb geometries due to the possibility of topological insulator, nematic, and Bose-Einstein condensed (BEC) phases. In this paper, we extend the understanding of the attractive Hubbard model on the Lieb lattice by using determinant quantum Monte Carlo to study real space charge and pair correlation functions not addressed by the Lieb theorems. Specifically, our results show unusual charge and charge transfer signatures within the flat band, and a reduction in pairing order at $\rho =2/3$ and $\rho =4/3$, the points at which the flat band is first occupied and then completely filled. We compare our results to the case of flat bands in the Kagome lattice and demonstrate that the behavior observed in the two cases is rather different.

Combined first-order reversal curve (FORC) analyses of the magnetization ($\mathrm{M}$-FORC) and magnetoresistance (MR-FORC) have been employed to provide a comprehensive study of the $\mathrm{M}$-MR correlation in two canonical systems: a NiFe/Cu/FePt pseudo spin valve (PSV) and a ${[\mathrm{Co}/\mathrm{Cu}]}_8$ multilayer. In the PSV, due to the large difference in switching fields and minimal interactions between the NiFe and the FePt layers, the $\mathrm{M}$ and MR show a simple one-to-one relationship during reversal. In the ${[\mathrm{Co}/\mathrm{Cu}]}_8$ multilayer, the correlation between the magnetization reversal and the MR evolution is more complex. This is primarily due to the similar switching fields of, and interactions between, the constituent Co layers. The FORC protocol accesses states with much higher spin disorders and larger MRs than those found along the conventional major loop field cycle. Unlike the $\mathrm{M}$-FORC measurements, which only probe changes in the macroscopic magnetization, the MR-FORCs are more sensitive to the microscopic domain configurations as those are most important in determining the resultant MR effect size. This approach is generally applicable to spintronic systems to realize the maximum spin disorder and the largest MR.