We report the direct observation of 30-fs period Rabi oscillations between excitons and cavity photons in a strongly coupled $J$-aggregate microcavity by means of time-resolved up-conversion and spectral interferometry measurements. The time structure of the transmitted electric field, measured by linear spectral interferometry, shows pronounced ultrafast beats. Its spectral phase reveals a distinct signature caused by destructive interference between the coherent drive and the field radiated by the exciton. This antiresonance selectively probes the uncoupled exciton excitation, and its observation uncovers the coherent and ultrafast exchange of energy between the optically excited cavity and the $J$-aggregate excitons, as confirmed by transfer matrix calculations.

In semiconducting armchair graphene ribbons a tensile strain can induce pairs of topological gap states with opposite energies. Near the critical value of the deformation potential these kink and antikink states become almost degenerate with zero energy and have a fractional charge of one-half. Such a semiconducting armchair ribbon represents a one-dimensional topological insulator with nearly zero energy end states. Using data collapse of numerical results we find that the shape of the kink displays an anomalous power-law dependence on the width of the local lattice deformation. We suggest that these gap states may be probed in optical measurements. However, “metallic” armchair graphene ribbons with a gap induced by many-electron interactions have no gap states and are not topological insulators.

We have performed an ab initio theoretical study of the energetic stability and the electronic properties of pristine and hydrogen-adsorbed grain boundaries (GBs) in silicene. We find that GBs in silicene present lower formation energy when compared with their counterparts in graphene. Removing the inversion symmetry, by applying an external electric field perpendicular to the silicene sheet, we verify the formation of valley-indexed metallic states lying along the GBs, characterizing the quantum valley Hall effect (QVHE). Here, we find the maintenance of the QVHE upon the presence of disordered and asymmetric geometries along the GBs. Those metallic states are suppressed upon the adsorption of H adatoms along the GBs. The H adatoms promote an unbalance on the electronic occupation of the unsaturated $\pi$ electrons beside the hydrogenated GB rows, giving rise to (i) a net magnetic moment on the Si atoms along the edge sites of the hydrogenated GBs and (ii) an electronic band structure characterized by spin-polarized valley states protected against backscattering processes.

Isoelectronic differential force measurements provide a unique opportunity to probe controversial features of the thermal Casimir effect that are still much debated in the current literature. Isolectronic setups offer two major advantages over conventional Casimir setups. On the one hand, they are immune from electrostatic forces caused by potential patches on the plates surfaces that plague present Casimir experiments, especially for separations in the micron range. On the other hand, they can strongly enhance the discrepancy between alternative theoretical models that have been proposed to estimate the thermal Casimir force for metallic and magnetic surfaces. Thanks to these two features, isoelectronic differential experiments should allow one to establish conclusively which among these models correctly describes the thermal Casimir force.

Structural stabilities of the Wadsley-type bronzes $\beta \text{−}{\mathrm{Ag}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ and $\beta \text{−}{\mathrm{Li}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ (both C2/m, Z = 6) have been studied with single-crystal x-ray diffraction in diamond anvil cells at room temperature to 8 and 13 GPa, respectively. $\beta \text{−}{\mathrm{Ag}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ is stable at least to 8 GPa. $\beta \text{−}{\mathrm{Li}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ undergoes two phase transitions at about 9 and 11 GPa due to relative displacements of the adjacent octahedral vanadate layers. In the intermediate phase between 9 and 11 GPa, very strong one-dimensional diffuse scattering is observed, indicating the presence of stacking faults. The structural refinements (C2/m, Z = 6) of the data above 11 GPa reveal that the layers of ${\mathrm{VO}}_6$ octahedra remain essentially intact. However, the relative position of the chains of edge-sharing ${\mathrm{VO}}_5$ tetragonal pyramids with respect to the octahedral layers is changed. As a result, the tunnels populated by the ${\mathrm{Li}}^1+$ cations collapse on compression. The distribution of the ${\mathrm{Li}}^1+$ cations in the vanadate framework is fully ordered in the polymorph above 11 GPa. The present structural data could be used to better understand the pressure-induced superconductivity in all the Wadsley-type bronzes $\beta \text{−}{\mathrm{A}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ (A = Li, Na, Ag).

It was recently proposed that the stress state of a material can also be altered via electron or hole doping, a concept termed electronic stress (ES), which is different from the traditional mechanical stress (MS) due to lattice contraction or expansion. Here we demonstrate the equivalence of ES and MS in structural stabilization, using In wires on Si(111) as a prototypical example. Our systematic density-functional theory calculations reveal that, first, for the same degrees of carrier doping into the In wires, the ES of the high-temperature metallic $4\times 1$ structure is only slightly compressive, while that of the low-temperature insulating $8\times 2$ structure is much larger and highly anisotropic. As a consequence, the intrinsic energy difference between the two phases is significantly reduced towards electronically phase-separated ground states. Our calculations further demonstrate quantitatively that such intriguing phase tunabilities can be achieved equivalently via lattice-contraction induced MS in the absence of charge doping. We also validate the equivalence through our detailed scanning tunneling microscopy experiments. The present findings have important implications for understanding the underlying driving forces involved in various phase transitions of simple and complex systems alike.

The antiferromagnetically ordered state of the monoclinic quasi-one-dimensional $S=1/2$ Heisenberg antiferromagnet ${\mathrm{CuSb}}_2{\mathrm{O}}_6$ was studied combining torque magnetometry with a phenomenological approach to magnetic anisotropy. This system is known to have a number of different twins in the monoclinic $\beta$ phase, which differ in the orientation of the two ${\mathrm{CuO}}_6$ octahedra in the unit cell resulting in different orientation of magnetic axes with respect to crystal axes for each twin. We performed torque measurements in magnetic fields $H\le 0.8\mathrm{T}$ on a sample where a certain type of twin was shown to be dominant by ESR spectroscopy. The measured data reveal that the easy axis is the crystallographic $b$ axis for this sample. Phenomenological magnetocrystalline anisotropy energy invariant to crystal symmetry operations was used to model the spin axis direction in zero and finite magnetic fields. Our model reproduces the value of the spin-flop field $H_{\mathrm{SF}}=1.25\mathrm{T}$ found in literature. A combination of this approach with our torque results shows that the spin axis will flop in the direction of the maximal value of measured $\mathbf{g}$ tensor when the magnetic field $H>H_{\mathrm{SF}}$ is applied along the easy axis direction. Our analysis of magnetocrystalline anisotropy energy predicts two possibilities for the easy axis direction in this system, $b$ or $a$, connected to different crystallographic twins that can be realized in ${\mathrm{CuSb}}_2{\mathrm{O}}_6$. These results offer a possibility to reconcile the different reports of easy axis direction found in literature for this system and also nicely demonstrate how a combination of torque magnetometry and a phenomenological approach to magnetic anisotropy can be used to determine the value of the spin-flop field and the direction of spin axis in antiferromagnets in both $H<H_{\mathrm{SF}}$ and $H>H_{\mathrm{SF}}$ by performing measurements in fields significantly smaller than $H_{\mathrm{SF}}$.

Effects of thermal spin disorder and excess Mn on the electronic spectrum of half-metallic NiMnSb are studied using first-principles calculations. Temperature-dependent spin disorder, introduced within the vector disordered local moment model, causes the valence band at the $\mathrm{\Gamma }$ point to broaden and shift upwards, crossing the Fermi level and thereby closing the half-metallic gap above room temperature. The spectroscopic signatures of excess Mn on the Ni, Sb, and empty sites (${\mathrm{Mn}}_{\mathrm{Ni}}, {\mathrm{Mn}}_{\mathrm{Sb}}$, and ${\mathrm{Mn}}_{\mathrm{E}}$) are analyzed. ${\mathrm{Mn}}_{\mathrm{Ni}}$ is spectroscopically invisible. The relatively weak coupling of ${\mathrm{Mn}}_{\mathrm{Sb}}$ and ${\mathrm{Mn}}_{\mathrm{E}}$ spins to the host strongly deviates from the Heisenberg model, and the spin of ${\mathrm{Mn}}_{\mathrm{E}}$ is canted in the ground state. While the half-metallic gap is preserved in the collinear ground state of ${\mathrm{Mn}}_{\mathrm{Sb}}$, thermal spin disorder of the weakly coupled ${\mathrm{Mn}}_{\mathrm{Sb}}$ spins destroys it at low temperatures. This property of ${\mathrm{Mn}}_{\mathrm{Sb}}$ may be the source of the observed low-temperature transport anomalies.

We report on resonant magnetic x-ray scattering and absorption spectroscopy studies of exchange-coupled antiferromagnetic ordering of ${\mathrm{Eu}}^{3+}$ magnetic moments in multiferroic ${\mathrm{Eu}}_{1-x}{\mathrm{Y}}_x{\mathrm{MnO}}_3$ in the absence of an external magnetic field. The observed resonant spectrum is characteristic of a magnetically ordered ${}^7{F}_1$ state that mirrors the Mn magnetic ordering, due to exchange coupling between the Eu $4f$ and Mn $3d$ spins. Here, we observe long-range magnetic order generated by exchange coupling of magnetic moments of formally nonmagnetic Van Vleck ions, which is a step further towards the realization of exotic phases induced by exchange coupling in systems entirely composed of non magnetic ions.

${\mathrm{Sb}}_2{\mathrm{O}}_3$-based materials are of broad interest in materials science and industry. High-pressure study using diamond anvil cells shows promise in obtaining new crystal and electronic structures different from their pristine states. Here, we conducted in situ angle dispersive synchrotron x-ray-diffraction and Raman spectroscopy experiments on $\alpha \text{−}{\mathrm{Sb}}_2{\mathrm{O}}_3$ up to 50 GPa with neon as the pressure transmitting medium. A first-order structural transition was observed in between 15 and 20 GPa, where the cubic phase I gradually transformed into a layered tetragonal phase II through structural distortion and symmetry breaking. To explain the dramatic changes in sample color and transparency, we performed first-principles calculations to track the evolution of its density of states and electronic structure under pressure. At higher pressure, a sluggish amorphization was observed. Our results highlight the structural connections among the sesquioxides, where the lone electron pair plays an important role in determining the local structures.

Single crystals of ${\mathrm{Na}}_{1-\delta }{\mathrm{Fe}}_{1-x}{\mathrm{T}}_x\mathrm{As}$ with T = Co, Rh have been grown using a self-flux technique. The crystals were thoroughly characterized by powder x-ray diffraction, magnetic susceptibility, and electronic transport with particular focus on the Rh-doped samples. Measurements of the specific heat and ARPES were conducted exemplarily for the optimally doped compositions. The spin-density wave transition (SDW) observed for samples with low Rh concentration $\left(0\le x\le 0.013\right)$ is fully suppressed in the optimally doped sample. The superconducting transition temperature $\left(T_c\right)$ is enhanced from 10 K in ${\mathrm{Na}}_{1-\delta }\mathrm{FeAs}$ to 21 K in the optimally doped sample $\left(x=0.019\right)$ of the ${\mathrm{Na}}_{1-\delta }{\mathrm{Fe}}_{1-x}{\mathrm{Rh}}_x\mathrm{As}$ series and decreases for the overdoped compounds, revealing a typical shape for the superconducting part of the electronic phase diagram. Remarkably, the phase diagram is almost identical to that of Co-doped ${\mathrm{Na}}_{1-\delta }\mathrm{FeAs}$, suggesting a generic phase diagram for both dopants.

Metal-insulator transitions in a three-component Falicov-Kimball model are investigated within the Kotliar-Ruckenstein slave boson mean-field approach. The model describes a mixture of two interacting fermion atom species loaded into an optical lattice at ultralow temperature. One species is two-component atoms, which can hop in the optical lattice, and the other is single-component atoms, which are localized. Different correlation-driven metal-insulator transitions are observed depending on the atom filling conditions and local interactions. These metal-insulator transitions are classified by the band renormalization factors and the double occupancies of the atom species. The filling conditions and the critical value of the local interactions for these metal-insulator transitions are also analytically established. The obtained results not only are in good agreement with the dynamical mean-field theory for the three-component Falicov-Kimball model but also clarify the nature and properties of the metal-insulator transitions in a simple physics picture.

Interfaces in oxide heterostructures always provide a fertile ground for emergent properties. Charge transfer from a high energy band to a low energy opponent is naturally expected, as occurring in semiconductor $p\text{−}n$ junctions. In this study, several exceptional physical phenomena have been predicted in $\left({\mathrm{YFeO}}_3\right){}_n/\left({\mathrm{YTiO}}_3\right){}_n$ superlattices. First, the charge transfer between these Mott insulators is in opposition to the intuitive band alignment scenario. Second, hybrid ferroelectricity with a moderate polarization is generated in the $n=2$ magnetic superlattice. Furthermore, the ferroelectric-type distortion can persist even if the $\left(AB{\mathrm{O}}_3\right){}_2/\left(A{B}^{\prime }{\mathrm{O}}_3\right){}_2$ system turns out to be metallic, rending possible metallic ferroelectricity.

We present bulk property measurements of NpIr, a newly synthesized member of the Np-Ir binary phase diagram, which is isostructural to the noncentrosymmetric pressure-induced ferromagnetic superconductor UIr. Magnetic susceptibility, electronic transport properties at ambient and high pressure, and heat capacity measurements have been performed for temperatures $T=0.55\text{–}300$ K in a range of magnetic fields up to 14 T and under pressure up to $17.3$ GPa. These reveal that NpIr is a moderately heavy fermion Kondo system with strong antiferromagnetic interactions, but there is no evidence of any phase transition down to $0.55$ K or at the highest pressure achieved. Experimental results are compared with ab initio calculations of the electronic band structure and lattice heat capacity. An extremely low lattice thermal conductivity is predicted for NpIr at temperatures above 300 K.

The optical nonlocality in symmetric metal-dielectric multilayer metamaterials is theoretically and experimentally investigated with respect to transverse-magnetic-polarized incident light. A nonlocal effective medium theory is derived from the transfer-matrix method to determine the nonlocal effective permittivity depending on both the frequency and wave vector in a symmetric metal-dielectric multilayer stack. In contrast to the local effective medium theory, our proposed nonlocal effective medium theory can accurately predict measured incident angle-dependent reflection spectra from a fabricated multilayer stack and provide nonlocal dispersion relations. Moreover, the bulk plasmon polaritons with large wave vectors supported in the multilayer stack are also investigated with the nonlocal effective medium theory through the analysis of the dispersion relation and eigenmode.

We demonstrate absorption imaging of $1s$ paraexcitons in a strain-induced trap potential in bulk semiconductor ${\mathrm{Cu}}_2\mathrm{O}$. Using excitonic $1s\text{−}2p$ induced absorption in the midinfrared region, clear images of exciton ensembles are taken at liquid-helium temperatures. The observed spatial spread and redshift of the $1s\text{−}2p$ resonance prove that the images are of a thermalized gas of trapped $1s$ paraexcitons. With its high sensitivity, midinfrared absorption imaging is especially important for characterizing quantum degenerate paraexcitons including Bose-Einstein condensates at near 100 mK temperatures, as they do not luminesce because of the principle of momentum conservation.

Using the $k·p$ theory and first-principles simulations, we predict that applying a moderate uniaxial or hydrostatic pressure $(>0.6\mathrm{GPa})$ on bulk or multilayer black phosphorus (BP) can diminish its bandgap and produce one-dimensional and even two-dimensional (2D) Dirac cones. Similar to topological insulators, these 2D Dirac cones result from two competing mechanisms: the unique linear band dispersion tends to open a gap via a “pseudo-spin-orbit” coupling, while the band symmetries preserve the material's gapless spectrum. In particular, these Dirac cones in BP are bulk states that do not require time-reversal symmetry, thus they can keep the high carrier mobility even in the presence of surface or magnetic perturbations. Finally, our predictions can be detected by the material's unusual Landau levels.

Single-layer graphene supported on a metal surface has shown remarkable properties relevant for novel electronic and optoelectronic devices. However, the nature of the electronic states derived from unoccupied surface states and quantum well states, lying in the real-space gap between the graphene and the solid surface, has not been explored and exploited yet. Herein, we use ultraviolet nonlinear angle-resolved photoemission spectroscopy to unveil the coexistence at the graphene/Cu(111) interface of a highest occupied Shockley surface state (HOSS) and the two lowest unoccupied surface states (LUSS). The experimental results and electronic structure calculations, based on one-dimensional model potential, indicate that the two unoccupied states originate from the hybridization of an $n=1$ image potential state with a quantum well state. The hybridized nature of these unoccupied states is benchmarked by a similar experiment done on single-layer graphene grown on copper polycrystalline foil where only the image state survives being the quantum well state at this interface inhibited.

By combining scanning tunneling microscopy with density functional theory it is shown that the $\mathrm{Bi}\left(111\right)$ surface provides a well-defined incorporation site in the first bilayer that traps highly coordinating atoms such as transition metals (TMs) or noble metals. All deposited atoms assume exactly the same specific sevenfold coordinated subsurface interstitial site while the surface topography remains nearly unchanged. Notably, $3d$ TMs show a barrier-free incorporation. The observed surface modification by barrier-free subsorption helps to suppress aggregation in clusters. It allows a tuning of the electronic properties not only for the pure $\mathrm{Bi}\left(111\right)$ surface, but may also be observed for topological insulators formed by substrate-stabilized Bi bilayers.