Several Fe chalcogenide superconductors without hole pockets at the Fermi level display high temperature superconductivity, in apparent contradiction to naive spin fluctuation pairing arguments. Recently, scanning tunneling microscopy experiments studied the influence of impurities on some of these materials and claimed that nonmagnetic impurities do not create in-gap states, leading to the conclusion that the gap must be $s_{++}$, i.e., conventional $s$ wave with no gap sign change. Here we critique this argument, and give various ways sign-changing gaps can be consistent with the absence of such bound states. In particular, we calculate the bound states for an $s_{\pm }$ system with a hole pocket below the Fermi level, and show that the nonmagnetic impurity bound state energy generically tracks the gap edge $E_{min}$ in the system, thereby rendering it unobservable. A failure to observe a bound state in the case of a nonmagnetic impurity therefore cannot be used as an argument to exclude sign-changing pairing states.

We experimentally demonstrate a robust dynamical decoupling protocol with bounded controls using long soft pulses, eliminating a challenging requirement of strong control pulses in conventional implementations. This protocol is accomplished by designing the decoupling propagators to go through a Eulerian cycle of the coupler group [Phys. Rev. Lett. 90, 037901 (2003)]. We demonstrate that this Eulerian decoupling scheme increases the coherence time by two orders of magnitude in our experiment under either dephasing or a universal noise environment.

Collective spin excitations in magnetic materials arise from the correlated motion of electron-hole pairs with opposite spins. The pair propagation is described by the transverse magnetic susceptibility, which we calculate within many-body perturbation theory from first principles employing the full-potential linearized augmented-plane-wave formalism. Ferromagnetic materials exhibit a spontaneously broken global rotation symmetry in spin space leading to the appearance of acoustic magnons (zero gap) in the long-wavelength limit. However, due to approximations used in the numerical scheme, the acoustic magnon dispersion exhibits a small but finite gap at $\mathrm{\Gamma }$. We analyze this violation of the Goldstone mode and present an approach that implements the magnetic susceptibility using a renormalized Green function instead of the Kohn-Sham one. This much more expensive approach shows substantial improvement of the Goldstone-mode condition. In addition, we discuss a possible correction scheme, which involves an adjustment of the Kohn-Sham exchange splitting, which is motivated by the spin-wave solution of the one-band Hubbard model. The new exchange splittings turn out to be closer to experiment. We present corrected magnon spectra for the elementary ferromagnets Fe, Co, and Ni.

We introduce a method for a combined calculation of charge and vector spin transport of elastically tunneling electrons in magnetic scanning tunneling microscopy (STM). The method is based on the three-dimensional Wentzel-Kramers-Brillouin (3D-WKB) approach combined with electronic structure calculations using first-principles density functional theory. As an application, we analyze the STM contrast inversion of the charge current above the Fe/W(110) surface depending on the bias voltage, tip-sample distance, and relative magnetization orientation between the sample and an iron tip. For the spin transfer torque (STT) vector we find that its in-plane component is generally larger than the out-of-plane component, and we identify a longitudinal spin current component, which, however, does not contribute to the torque. Our results suggest that the torque-current relationship in magnetic STM junctions follows the power law rather than a linear function. Consequently, we show that the ratio between the STT and the spin-polarized charge current is not constant, and more importantly, it can be tuned by the bias voltage, tip-sample distance, and magnetization rotation. We find that the STT efficiency can be enhanced by about a factor of seven by selecting a proper bias voltage. Thus, we demonstrate the possible enhancement of the STT efficiency in magnetic STM junctions, which can be exploited in technological applications. We discuss our results in view of the indirect measurement of the STT above the Fe/W(110) surface reported by S. Krause et al. [Phys. Rev. Lett. 107, 186601 (2011)].

We demonstrate that Mn doping in the layered sulfides ${\mathrm{Bi}}_4{\mathrm{O}}_4{\mathrm{S}}_3$ leads to stable ${\text{Bi}}_{4-x}{\text{Mn}}_x{\text{O}}_4{\text{S}}_3$ compounds that exhibit both long-range ferromagnetism and enhanced superconductivity for $0.075\le x\le 0.15$, with a possible record superconducting transition temperature $\left(T_c\right)$ $\sim 15$ K among all ${\mathrm{BiS}}_2$-based superconductors. We conjecture that the coexistence of superconductivity and ferromagnetism may be attributed to Mn doping in the spacer ${\mathrm{Bi}}_2{\mathrm{O}}_2$ layers away from the superconducting ${\mathrm{BiS}}_2$ layers, whereas the enhancement of $T_c$ may be due to excess electron transfer to ${\mathrm{BiS}}_2$ from the ${\mathrm{Mn}}^{4+}/{\mathrm{Mn}}^{3+}$ substitutions in ${\mathrm{Bi}}_2{\mathrm{O}}_2$. This notion is empirically corroborated by the increased electron-carrier densities upon Mn doping, and by further studies of the ${\text{Bi}}_{4-x}{A}_x{\text{O}}_4{\text{S}}_3$ compounds $(A$ = Co, Ni; $x=0.1$, 0.125), where the $T_c$ values remain comparable to that of the undoped ${\mathrm{Bi}}_4{\mathrm{O}}_4{\mathrm{S}}_3$ system $(\sim 4.5$ K) due to lack of 4+ valences in either Co or Ni ions for excess electron transfer to the ${\mathrm{BiS}}_2$ layers. These findings therefore shed new light on feasible pathways to enhance the $T_c$ values of ${\mathrm{BiS}}_2$-based superconductors, although complete elucidation of the interplay between superconductivity and ferromagnetism in these anisotropic layered compounds awaits the development of single crystalline materials for further investigation.

Quasiparticle properties of quantum magnets with long-range interactions are investigated by high-order linked-cluster expansions in the thermodynamic limit. It is established that perturbative continuous unitary transformations on white graphs are a promising and flexible approach to treat long-range interactions in quantum many-body systems. We exemplify this scheme for the one-dimensional transverse-field Ising chain with long-range interactions. For this model, the elementary quasiparticle gap is determined allowing to access the quantum-critical regime including critical exponents and multiplicative logarithmic corrections for the ferro- and antiferromagnetic case.

Exotic topological states of matter, such as a Floquet topological insulator or Floquet-Weyl semimetal, can be induced by periodic driving. This work proposes a Floquet semimetal with Floquet-band holonomy. That is, the system is gapless, but as a periodic parameter viewed as the quasimomentum of a synthetic dimension completing an adiabatic cycle, each Floquet band as a whole exchanges with other Floquet bands. The dynamical manifestations of such Floquet-band holonomy are studied. Under open-boundary conditions, we discover anomalous chiral edge modes localized only at one edge, winding around the entire quasienergy Brillouin zone, well separated from bulk states, and possessing holonomy different from bulk states. These remarkable properties are further exploited to realize quantized or half-quantized edge-state pumping.

The realization of single-electron sources in integer quantum Hall systems has paved the way for exploring electronic quantum optics experiments in solid-state devices. In this paper, we characterize a single Kramers pair emitter realized by a driven antidot embedded in a two-dimensional topological insulator, where spin-momentum locked edge states can be exploited for generating entanglement. Contrary to previous proposals, the antidot is coupled to both edges of a quantum spin Hall bar, thus enabling this mesoscopic capacitor to emit an entangled two-electron state. We study the concurrence $\mathcal{C}$ of the emitted state and the efficiency $\mathcal{F}$ of its emission as a function of the different spin-preserving and spin-flipping tunnel couplings of the antidot with the edges. We show that the efficiency remains very high ($\mathcal{F}\ge 50\%$) even for maximally entangled states ($\mathcal{C}=1$). We also discuss how the entanglement can be probed by means of noise measurements and violation of the Clauser-Horne-Shimony-Holt inequality.

We study the complex electronic band structure of low In content InGaAs/GaP quantum dots. A supercell extended-basis tight-binding model is used to simulate the electronic and the optical properties of a pure GaAs/GaP quantum dot modeled at the atomic level. Transitions between hole states confined into the dots and several $X_Z$-like electronic states confined by the strain field in the GaP barrier are found to play the main role on the optical properties. Especially, the calculated radiative lifetime for such indirect transitions is in good agreement with the photoluminescence decay time measured in time-resolved photoluminescence in the µs range. Photoluminescence experiments under hydrostatic pressure are also presented. The redshift of the photoluminescence spectrum with pressure is also in good agreement with the nature of the electronic confined states simulated with the tight-binding model.

In contrast to the first-order correlation-driven Mott metal-insulator transition, continuous disorder-driven transitions are intrinsically quantum critical. Here, we investigate transport quantum criticality in the Falicov-Kimball model, a representative of the latter class in the strong disorder category. Employing cluster-dynamical mean-field theory, we find clear and anomalous quantum critical scaling behavior manifesting as perfect mirror symmetry of scaling curves on both sides of the MIT. Surprisingly, we find that the beta function $\beta \left(g\right)$ scales as $log\left(g\right)$ deep into the bad-metallic phase as well, providing a sound unified basis for these findings. We argue that such strong localization quantum criticality may manifest in real three-dimensional systems where disorder effects are more important than electron-electron interactions.

We study a bilayer Kane-Mele-Hubbard model with lattice distortion and interlayer spin exchange interaction under cylinder geometry. Our analysis based on real-space dynamical mean field theory with continuous-time quantum Monte Carlo demonstrates the emergence of a topological edge Mott insulating (TEMI) state which hosts gapless edge modes only in collective spin excitations. This is confirmed by the numerical calculations at finite temperatures for the spin-Hall conductivity and the single-particle excitation spectrum; the spin-Hall conductivity is almost quantized, ${\sigma }_{\mathrm{spin}}^{xy}\sim 2(e/2\pi )$, predicting gapless edge modes carrying the spin current, while the helical edge modes in the single-particle spectrum are gapped out with respecting symmetry. It is clarified how the TEMI state evolves from the ordinary spin-Hall insulating state with increasing the Hubbard interaction at a given temperature and then undergoes a phase transition to a trivial Mott insulating state. With a bosonization approach at zero temperature, we further address which collective modes host gapless edge modes in the TEMI state.

We theoretically investigate the behavior of Andreev levels in a single-orbital interacting quantum dot in contact with superconducting leads, focusing on the effect of electrostatic gating and applied magnetic field, as relevant for recent experimental spectroscopic studies. In order to account reliably for spin-polarization effects in the presence of correlations, we extend here two simple and complementary approaches that are tailored to capture effective Andreev levels: the static functional renormalization group (fRG) and the self-consistent Andreev bound states (SCABS) theory. We provide benchmarks against the exact large-gap solution as well as renormalization group (NRG) calculations and find good quantitative agreement in the range of validity. The large flexibility of the implemented approaches then allows us to analyze a sizable parameter space, allowing us to get a deeper physical understanding into the Zeeman field, electrostatic gate, and flux dependence of Andreev levels in interacting nanostructures.

We study the interplay of nematic and superconducting order in the two-dimensional Hubbard model and show that they can coexist, especially when superconductivity is not the energetically dominant phase. Due to a breaking of the $C_4$ symmetry, the coexisting phase inherently contains admixture of the $s$-wave pairing components. As a result, the superconducting gap exhibits nonstandard features including changed nodal directions. Our results also show that in the optimally doped regime the pure superconducting phase is typically unstable towards developing nematicity (breaking of the $C_4$ symmetry). This has implications for the cuprate high-$T_c$ superconductors, for which in this regime the so-called intertwined orders have recently been observed. Namely, the coexisting phase may be viewed as a precursor to such more involved patterns of symmetry breaking.

The monoclinic (M) phases in high-performance relaxor-based ferroelectric single crystals have been recognized to be a vital structural factor for the outstanding piezoelectric property. However, due to the complexity of the structure in M phases, the understanding about it is still limited. In this paper, the local twin domains and tip-voltage-induced domain switching of the ${\mathrm{M}}_C$ phase in $\mathrm{Pb}\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right){\mathrm{O}}_3-0.34\mathrm{PbTi}{\mathrm{O}}_3$ (PMN-0.34PT) single crystal have been intensively investigated by piezoresponse force microscopy (PFM). By theoretically analyzing the experimental patterns of domain walls on the ${\left(001\right)}_C$ face, the specific ${\mathrm{M}}_C$ twin domains in the initial annealed state of a selected area have been clarified, and the polarization orientation of the ${\mathrm{M}}_C$ phase in this sample is determined to be at an angle of ${29}^{\circ }$ to the ${\langle 001\rangle }_C$ directions. In addition, based on the evolution of domains and the motion of domain walls under the step-increased PFM tip dc voltage ($V_{\mathrm{dc}}$), the switching process and features of different types of ${\mathrm{M}}_C$ domain variants are visually revealed.

The temperature dependence of the optical phonons and high-frequency microwave excitations of morphotropic $\mathrm{Pb}\left(\mathrm{Z}{\mathrm{r}}_{1-x}\mathrm{T}{\mathrm{i}}_x\right){\mathrm{O}}_3$ (PZT) ceramics with compositions PZT 52/48 and 53/47 were studied by a broadband spectroscopy approach, using Raman, far-infrared, time-domain terahertz (THz), and microwave spectroscopies. A careful evaluation of the evolution of phonon parameters with temperature clearly revealed only two macroscopic phase transitions of intrinsic origin, driven by phonons: the ferroelectric one at high temperatures near 650 K (driven by an anharmonic soft mode at THz frequencies coupled to a weak component at higher frequencies, $\omega \sim 70\mathrm{c}{\mathrm{m}}^{-1}$) and a transition to an antiferrodistortive phase around 400 K, signaled by a new peak $\left(\omega \sim 60\mathrm{c}{\mathrm{m}}^{-1}\right)$ that appeared in the infrared and Raman spectra. This peak confirmed the onset of the antiphase tilt of the oxygen octahedra and the doubling of the unit cell, as well as the splitting of the $B_1\text{−}E$ doublet in the Raman spectra. No indications of an additional phase transition were found by these techniques down to 20 K. On the other hand, dielectric measurements below the phonon frequencies showed anomalies at lower temperatures; however, they had extrinsic character and were not related to the atomic vibrations. An excitation in the gigahertz range displayed a softening towards 270 K, in agreement with a maximum in the low-frequency dielectric loss spectra. The quantitative analysis of this excitation revealed the presence of two contributions assigned to piezoelectric resonances in grains and domain wall oscillations, which, together, satisfactorily explain the dielectric anomaly below room temperature, without taking into account another transition to a lower-symmetry phase.

We performed high-resolution magneto-optical spectroscopy of the hyperfine transitions from ${}^{4}I_{15/2}$ to the ${}^{4}I_{13/2}$ and ${}^{4}I_{9/2}$ multiplets of ${}^{167}\mathrm{Er}^{3+}$ and ${}^{166}\mathrm{Er}^{3+}$ in an isotopically purified ${}^7\mathrm{LiYF}_4$ crystal in various external magnetic fields up to 0.7 T. The obtained experimental results are interpreted in the framework of the generalized theoretical approach. The derived model successfully explains all the experimentally observed optical hyperfine transitions by using a single set of basic parameters found for the crystal-field interaction, magnetic dipole and electric quadrupole hyperfine interactions, together with Zeeman interactions at different orientations of the external magnetic field. A number of the studied quantum transitions appears to be promising for use in Raman quantum storage at optical telecommunication wavelengths.

We propose a generic scenario for realizing gapful topological superconductors (TSCs) from gapless spin-singlet superconductors (SCs). Noncentrosymmetric nodal SCs in two dimensions are shown to be gapful under a Zeeman field, as a result of the cooperation of inversion-symmetry breaking and time-reversal-symmetry breaking. In particular, non-$s$-wave SCs acquire a large excitation gap. Such paramagnetically induced gapful SCs may be classified into TSCs in the symmetry class D specified by the Chern number. We show nontrivial Chern numbers over a wide parameter range for spin-singlet SCs. A variety of the paramagnetically induced gapful TSCs are demonstrated, including $D+p$-wave TSC, extended $S+p$-wave TSC, $p+D+f$-wave TSC, and $s+P$-wave TSC. Natural extension toward three-dimensional Weyl SCs is also discussed.

Weak magnetic fields significantly affect the properties of strongly disordered superconductors through diamagnetic orbital shrinking as well as geometric phase frustration effects. The relative importance of these orbital effects is analyzed for thin films in physical regimes dominated by a Coulomb blockade, thermal phase fluctuations, and phase frustration, respectively, using phenomenological and replica field theory approaches. As an important outcome, we obtain the field dependences of superfluid stiffness and resistance, and we show that they can be used to distinguish between phase frustration and diamagnetic shrinking effects. We also find that even though highly disordered films may never become superconducting, their inhomogeneous structure leaves a distinct signature on the magnetoresistance. The predicted low-field magnetoresistance is in very good accord with experiment.