We consider an annular superconductor-insulator-superconductor Josephson junction, with the insulator being a double layer of electron and holes at Abelian fractional quantum Hall states of identical fillings. When the two superconductors gap out the edge modes, the system has a topological ground-state degeneracy in the thermodynamic limit akin to the fractional quantum Hall degeneracy on a torus. In the quasi-one-dimensional limit, where the width of the insulator becomes small, the ground-state energies are split. We discuss several implications of the topological degeneracy that survive the crossover to the quasi-one-dimensional limit. In particular, the Josephson effect shows a $2\pi d$ periodicity, where $d$ is the ground-state degeneracy in the two-dimensional limit. We find that at special values of the relative phase between the two superconductors there are protected crossing points in which the degeneracy is not completely lifted. These features occur also if the insulator is a time-reversal-invariant fractional topological insulator. We describe the latter using a construction based on coupled wires. Furthermore, when the superconductors are replaced by systems with an appropriate magnetic order that gap the edges via a spin-flipping backscattering, the Josephson effect is replaced by a spin Josephson effect.

Recently, there has been an interest in applying the concept of weak values and weak measurements to condensed matter systems. Here a weak measurement protocol is proposed for obtaining the $Z_2$ index of a topological insulator. The setup consists of a topological insulator with a hole pierced by a time dependent Aharonov-Bohm flux. A certain weak value $\left(A_{gs}\right)$ associated with the time-integrated magnetization in the hole has a universal response to a small ambient magnetic field $\left(B\right)$, namely, $A_{gs}B=2\hslash$. This result is unaffected by disorder, interactions, and to a large extent, the speed of the flux threading. It hinges mainly on preventing the flux from leaking outside the hole, as well as being able to detect magnetization at a resolution of a few spins. A similar result may be obtained using only charge measurements, in a setup consisting of a double quantum dot weakly coupled to an $LC$ circuit. Here one obtains ${\langle \varphi \rangle }_{\mathrm{weak}}{Q}_0=2\hslash$, where ${\langle \varphi \rangle }_{\mathrm{weak}}$ is a weak value associated with the flux on the inductor and $Q_0$ is the average capacitor charging. The universality of these results suggests that they may be used as a test bed for weak values in condensed matter physics.

We combine two aspects of magnetic frustration, multiferroicity and emergent quasiparticles in spin liquids, by studying magnetoelectric monopoles. Spin ice offers to couple these emergent topological defects to external fields, and to each other, in unusual ways, making it possible to lift the degeneracy underpinning the spin liquid and to potentially stabilize novel forms of charge crystals, opening the path to a “magnetic crystallography.” In developing the general phase diagram including nearest-neighbor coupling, Zeeman energy, and electric and magnetic dipolar interactions, we uncover the emergence of a bilayered crystal of singly charged monopoles, whose stability, remarkably, is strengthened by an external [110] magnetic field. Our theory is able to account for the ordering process of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ in a large field for reasonably small electric energy scales.

Recently discovered high-temperature superconductivity in single-unit-cell (UC) FeSe films on $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) substrate has stimulated tremendous research interest, both experimental and theoretical. Whether this scenario could be extended to other superconductors is vital in both identifying the enhanced superconductivity mechanism and further raising the critical transition temperature ($T_c$). Here we successfully prepared single-UC $\mathrm{FeT}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{e}}_x(0.1\le x\le 0.6)$ films on STO substrates by molecular beam epitaxy and observed $\mathsf{U}$-shaped superconducting gaps $\left(\mathrm{\Delta }\right)$ up to $\sim 16.5\mathrm{meV}$, nearly ten times the gap value $(\mathrm{\Delta }\sim 1.7\mathrm{meV})$ of the optimally doped bulk $\mathrm{FeT}{\mathrm{e}}_{0.6}\mathrm{S}{\mathrm{e}}_{0.4}$ single crystal ($T_c\sim 14.5\mathrm{K}$). No superconducting gap has been observed on the second UC and thicker $\mathrm{FeT}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{e}}_x$ films at 5.7 K, indicating the important role of the interface. This interface-enhanced high-temperature superconductivity is further confirmed by ex situ transport measurements, which revealed an onset superconducting transition temperature above 40 K, nearly two times higher than that of the optimally doped bulk $\mathrm{FeT}{\mathrm{e}}_{0.6}\mathrm{S}{\mathrm{e}}_{0.4}$ single crystal. This work demonstrates that interface engineering is a feasible way to discover alternative superconductors with higher $T_c$.

The observation of sharp atomiclike multiplet features is unexpected for individual $3d$ atoms adsorbed on transition-metal surfaces. However, we show by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism that individual Fe atoms on Cu(111) exhibit such features. They are reminiscent of a low degree of hybridization, similar to $3d$ atoms adsorbed on alkali-metal surfaces. We determine the spin, orbital, and total magnetic moments, as well as magnetic anisotropy energy for the individual Fe atoms and for small Fe clusters that we form by increasing the coverage. The multiplet features are smoothened and the orbital moment rapidly decreases with increasing cluster size. For Fe monomers, comparison with density functional theory and multiplet calculations reveals a $d^7$ electronic configuration, owing to the transfer of one electron from the $4s$ to the $3d$ states.

It was recently suggested that a novel type of phase transition may occur in the visibility of electronic Mach-Zehnder interferometers. Here, we present experimental evidence for the existence of this transition. The transition is induced by strongly non-Gaussian noise that originates from the strong coupling of a quantum point contact to the interferometer. We provide a transparent physical picture of the effect by exploiting a close analogy to the neutrino oscillations of particle physics. In addition, our experiment constitutes a probe of the singularity of the elusive full counting statistics of a quantum point contact.

We reveal the existence of a type of surface electromagnetic wave supported by hyperbolic metasurfaces, described by a conductivity tensor with an indefinite signature. We demonstrate that the spectrum of the hyperbolic metasurface waves consists of two branches corresponding to hybrid transverse electric–transverse magnetic waves with a polarization that varies from linear to elliptic or circular depending on the wave frequency and propagation direction. The shape of the equal-frequency contours drastically depends on the frequency and changes from the elliptical to hyperbolic—a topological transition takes place. We derive asymptotic formulas describing the losses of the surface waves for capacitive, inductive, and hyperbolic regimes of the metasurface. We analyze numerically the generation of surface waves by a point electric dipole placed in the vicinity of the metasurface with numerical simulations, and also reveal wave-front peculiarities in strong anisotropic hyperbolic and $\sigma$-near-zero regimes of the metasurface.

We present thermoelectric power (TEP) studies under pressure and high magnetic field in the antiferromagnet ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$ at low temperature. Under a magnetic field, large quantum oscillations are observed in the TEP, $S\left(H\right)$, in the antiferromagnetic phase. They suddenly disappear when entering in the polarized paramagnetic state at $H_c$, pointing out an important reconstruction of the Fermi surface. Under pressure, $S/T$ increases strongly at low temperature near the critical pressure $P_c$, where the antiferromagnetic (AF) order is suppressed, implying the interplay of a Fermi surface change and low-energy excitations driven by spin and valence fluctuations. The difference between the TEP signal in the polarized paramagnetic state above $H_c$ at ambient pressure and in the pressure-induced paramagnetic state above $P_c$ can be explained by different Fermi surfaces. Band-structure calculations at $P=0$ stress that in the AF phase the $4f$ contribution at the Fermi level $\left(E_F\right)$ is weak, while it is the main contribution in the paramagnetic domain. In the polarized paramagnetic phase the $4f$ contribution at $E_F$ drops. Large quantum oscillations are observed in the antiferromagnetic state while these disappear in the polarized state above $H_c$. Comparison is made to the ${\mathrm{CeRu}}_2{\mathrm{Si}}_2$ series highly studied for its $(H,T)$ phase diagram.

We present in-depth measurements of the electronic band structure of the transition-metal dichalcogenides (TMDs) ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$ using angle-resolved photoemission spectroscopy, with focus on the energy splittings in their valence bands at the $K$ point of the Brillouin zone. Experimental results are interpreted in terms of our parallel first-principles computations. We find that interlayer interaction only weakly contributes to the splitting in bulk ${\mathrm{WS}}_2$, resolving previous debates on its relative strength. We additionally find that across a range of TMDs, the band gap generally decreases with increasing magnitude of the valence-band splitting, molecular mass, or ratio of the out-of-plane to in-plane lattice constant. Our results provide an important reference for future studies of electronic properties of ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$ and their applications in spintronics and valleytronics devices.

We study the concurrence of entanglement between two quantum dots in contact to Majorana bound states on a floating superconducting island. The distance between the Majorana states, the charging energy of the island, and the average island charge are shown to be decisive parameters for the efficiency of entanglement generation. We find that long-range entanglement with basically distance-independent concurrence is possible over wide parameter regions, where the proposed setup realizes a “Majorana entanglement bridge.” We also study the time-dependent concurrence obtained after one of the tunnel couplings is suddenly switched on, which reveals the time scales for generating entanglement. Accurate analytical expressions for the concurrence are derived both for the static and the time-dependent cases. Our results indicate that entanglement formation in interacting Majorana devices can be fully understood in terms of an interplay of elastic cotunneling (also referred to as “teleportation”) and crossed Andreev reflection processes.

We study the impurity entanglement entropy $S_e$ in quantum impurity models that feature a Kondo-destruction quantum critical point (QCP) arising from a pseudogap in the conduction-band density of states or from coupling to a bosonic bath. On the local-moment (Kondo-destroyed) side of the QCP, the entanglement entropy contains a critical component that can be related to the order parameter characterizing the quantum phase transition. In Kondo models describing a spin-$S_{\mathrm{imp}},S_e$ assumes its maximal value of $ln(2S_{\mathrm{imp}}+1)$ at the QCP and throughout the Kondo phase, independent of features such as particle-hole symmetry and under- or overscreening. In Anderson models, $S_e$ is nonuniversal at the QCP and, at particle-hole symmetry, rises monotonically on passage from the local-moment phase to the Kondo phase; breaking this symmetry can lead to a cusp peak in $S_e$ due to a divergent charge susceptibility at the QCP. Implications of these results for quantum critical systems and quantum dots are discussed.

The magnetic state of a quantum dot attached to superconducting leads is experimentally shown to be controlled by the superconducting phase difference across the dot. This is done by probing the relation between the Josephson current and the superconducting phase difference of a carbon nanotube junction whose Kondo energy and superconducting gap are of comparable size. It exhibits distinctively anharmonic behavior, revealing a phase-mediated singlet-to-doublet transition. We obtain an excellent quantitative agreement with numerically exact quantum Monte Carlo calculations. This provides strong support that we indeed observed the finite-temperature signatures of the phase-controlled zero temperature level crossing transition originating from strong local electronic correlations.

We construct the generic phase diagrams encoding the topologically distinct localized and delocalized phases of noninteracting fermionic quasiparticles for any symmetry class from the tenfold way in one, two, and three dimensions. To this end, we start from a massive Dirac Hamiltonian perturbed by a generic disorder for any dimension of space and for any one of the ten symmetry classes from the tenfold way. The physics of Anderson localization is then encoded by a two-dimensional phase diagram that we deduce from the topology of the space of normalized Dirac masses. This approach agrees with previously known results and gives an alternative explanation for the even-odd effect in the one-dimensional chiral symmetry classes. We also give a qualitative explanation for the Gade singularity and Griffiths effects in the density of states using the first homotopy group of the normalized Dirac masses in two dimensions. Finally, this approach is used to analyze the stability of massless Dirac fermions on the surface of three-dimensional topological crystalline insulators.

Recent concrete proposals suggest it is possible to engineer a two-dimensional bulk phase supporting non-Abelian Fibonacci anyons out of Abelian fractional quantum Hall systems. The low-energy degrees of freedom of such setups can be modeled as ${\mathbb{Z}}_3$ parafermions “hopping” on a two-dimensional lattice. We use the density matrix renormalization group to study a model of this type interpolating between the decoupled-chain, triangular-lattice, and square-lattice limits. The results show clear evidence of the Fibonacci phase over a wide region of the phase diagram, most notably including the isotropic triangular-lattice point. We also study the broader phase diagram of this model and show that elsewhere it supports an Abelian state with semionic excitations.

We report two first-order Raman modes in the spectra of few-layer ${\mathrm{MoS}}_2$ at 286 and 471 ${\mathrm{cm}}^{-1}$ that are only observed at excitation energies above 2.4 eV. We show that these normally not observed modes are interlayer modes related to symmetry-forbidden modes of the single layer. Based on group theory, we provide a general treatment and systematic classification of all phonon modes in few-layer crystals with inversion symmetry and/or horizontal reflection symmetry. The results can thus be applied to different materials like few-layer graphene, transition-metal dichalcogenides, or BN. Moreover, the few-layer specific Raman modes are strongly resonant with the $C$ optical transition in ${\mathrm{MoS}}_2$. We conclude that the corresponding exciton wave function is extended over all layers of the few-layer ${\mathrm{MoS}}_2$, in contrast to the $A$ and $B$ excitons.

We study the spin texture of a generic helical liquid, the edge modes of a two-dimensional topological insulator with broken axial spin symmetry. By considering honeycomb and square-lattice realizations of topological insulators, we show that in all cases the generic behavior of a momentum-dependent rotation of the spin quantization axis is realized. Here we establish this mechanism also for disk geometries with continuous rotational symmetry. Finally, we demonstrate that the rotation of spin-quantization axis remains intact for arbitrary geometries, i.e., in the absence of any continuous symmetry. We also calculate the dependence of this rotation on the model and material parameters. Finally, we propose a spectroscopy measurement which should directly reveal the rotation of the spin-quantization axis of the helical edge states.

We demonstrate a controllable enhancement in the electrostrictive properties of a medium using dilute composite artificial materials. Analytical expressions for the composite electrostriction are derived and used to show that enhancement, tunability, and suppression can be achieved through a careful choice of constituent materials. Numerical examples with Ag, ${\mathrm{As}}_2{\mathrm{S}}_3$, Si, and ${\mathrm{SiO}}_2$ demonstrate that even in a nonresonant regime, artificial materials can bring more than a threefold enhancement in the electrostriction.

Ejection of an electron by absorption of an extreme ultraviolet (xuv) photon probes the many-electron response of a solid well beyond the single-particle picture. Photoemission spectra feature complex correlation satellite structures signifying the simultaneous excitation of single or multiple plasmons. The time delay of the plasmon satellites relative to the main line can be resolved in attosecond streaking experiments. Time-resolved photoemission thus provides the key to discriminate between intrinsic and extrinsic plasmon excitation. We demonstrate the determination of the branching ratio between intrinsic and extrinsic plasmon generation for simple metals.

We present a neutron scattering study of the chromate spinel ${\mathrm{LiInCr}}_4{\mathrm{O}}_8$, which realizes a “breathing” pyrochlore lattice of $S=\frac{3}{2}$ spins. High-resolution neutron diffraction reveals a phase separation into a mixture of minority cubic and majority tetragonal phases below 18 K, followed by apparent magnetic order in a complex two-up two-down pattern in the latter around 12 K. The spin-spin correlation length appears to remain small down to $1.7$ K. Above both transitions, time-of-flight neutron spectroscopy shows that the response is inelastic rather than quasielastic, consistent with the relatively large alternation in sizes of adjacent Cr tetrahedra. Finally, a remarkable softening of the inelastic mode is observed preceding the structural transition, implying proximity to a critical point.

Nonradiative carrier recombination is of both great applied and fundamental importance, but the correct ab initio approaches to calculate it remain to be inconclusive. Here we used five different approximations to calculate the nonradiative carrier recombinations of two complex defect structures $\mathrm{GaP}:\mathrm{Z}{\mathrm{n}}_{\mathrm{Ga}}\text{−}{\mathrm{O}}_{\mathrm{P}}$ and $\mathrm{GaN}:\mathrm{Z}{\mathrm{n}}_{\mathrm{Ga}}\text{−}{\mathrm{V}}_{\mathrm{N}}$, and compared the results with experiments. In order to apply different multiphonon assisted electron transition formalisms, we have calculated the electron-phonon coupling constants by ab initio density functional theory for all phonon modes. Compared with different methods, the capture coefficients calculated by the static coupling theory are $4.30\times {10}^{-8}$ and $1.46\times {10}^{-7}\mathrm{c}{\mathrm{m}}^3/\mathrm{s}$ for $\mathrm{GaP}:\mathrm{Z}{\mathrm{n}}_{\mathrm{Ga}}\text{−}{\mathrm{O}}_{\mathrm{P}}$ and $\mathrm{GaN}:\mathrm{Z}{\mathrm{n}}_{\mathrm{Ga}}\text{−}{\mathrm{V}}_{\mathrm{N}}$, which are in good agreement with the experiment results, $\left(4_{-1}^{+2}\right)\times {10}^{-8}$ and $3.0\times {10}^{-7}\mathrm{c}{\mathrm{m}}^3/\mathrm{s}$, respectively. We also provided arguments for why the static coupling theory should be used to calculate the nonradiative decays of semiconductors.