Our calculations, within known limitations of Ginzburg-Landau theory, are fully correct and valid for transport phenomena in asymmetric mesoscopic superconductors, deep in the superconducting state. We deemed the experiments of Burlakov et al. [JETP Lett. 86, 517 (2007)] relevant and important to mention in the general context of our paper since the observed shifts in the oscillations of different quantities are qualitatively similar, even though those measurements are performed close to the superconducting-normal state transition in the so-called Little-Parks regime.

Mazur's inequality renders statements about persistent correlations possible. We generalize it in a convenient form applicable to any set of linearly independent constants of motion. This approach is used to show rigorously that a fraction of the initial spin correlations persists indefinitely in the isotropic central spin model unless the average coupling vanishes. The central spin model describes a major mechanism of decoherence in a large class of potential realizations of quantum bits. Thus the derived results contribute significantly to the understanding of the preservation of coherence. We will show that persisting quantum correlations are not linked to the integrability of the model but are caused by a finite operator overlap with a finite set of constants of motion.

We propose a new mechanism for topological superconductivity based on an antiferromagnetically ordered chain of magnetic atoms on the surface of a conventional superconductor. In a weak Zeeman field, a supercurrent in the substrate generates a staggered spin current, which converts the preexisting topologically unprotected Shiba states into Majorana fermions (MFs). The two experimental knobs can be finely tuned providing a platform with enhanced functionality for applications. Remarkably, the electronic spin polarization of the arising edge MF wavefunctions depends solely on the parity of the number of magnetic moments, which can serve as a distinctive signature of the MFs. We introduce the basic concepts within a minimal model and make contact with experiments by a microscopic analysis based on the Shiba states.

The ferroelectric solid solution $\left(1-x\right){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3\text{−}{\mathrm{BaTiO}}_3 \left(\mathrm{NBT}\text{−}x\mathrm{BT}\right)$ is a promising material to substitute for the environmentally undesired Pb-based ferroelectrics. The strong enhancement of the dielectric permittivity and piezoelectric coefficients near the morphotropic phase boundary (MPB) in ferroelectric $AB{\mathrm{O}}_3$-type solid solutions is commonly related to the existence of a single or several long-range-ordered phases resulting in a complex nanodomain pattern. Here, $\mathrm{NBT}\text{−}x\mathrm{BT}$ is studied by Raman scattering and complementary synchrotron x-ray total elastic scattering to follow the temperature evolution of the mesoscopic-scale structural transformations for $x$ below, at, and slightly above $x_{\mathrm{MPB}}$. The most remarkable result is that at $x\sim x_{\mathrm{MPB}}$ the phonon mode involving vibrations of both off-centered A-site Bi and B-site Ti experiences strong softening and damping near the triple-point temperature in the phase diagram, in strong contrast to the compounds with $x<x_{\mathrm{MPB}}$ or $x>x_{\mathrm{MPB}}$. The chemically enhanced coupling between the Bi and Ti subsystems at $x_{\mathrm{MPB}}$ is facilitated by the subtle disturbance of the coupling processes within the Bi subsystem induced by Ba at $x_{\mathrm{MPB}}$. These mesoscopic-scale structural and dynamic phenomena seem to be significant for the enhanced response functions at the MPB in $\mathrm{NBT}\text{−}x\mathrm{BT}$.

The crystallographic and physical properties of polycrystalline and single-crystal samples of ${\mathrm{BaCo}}_2{\mathrm{As}}_2$ and K-doped ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Co}}_2{\mathrm{As}}_2$ $(x=0.06,0.22)$ are investigated by x-ray and neutron powder diffraction, magnetic susceptibility $\chi$, magnetization, heat capacity $C_{\mathrm{p}},{}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR), and electrical resistivity $\rho$ measurements versus temperature $T$. The crystals were grown using both Sn flux and CoAs self-flux, where the Sn-grown crystals contain 1.6–2.0 mol% Sn. All samples crystallize in the tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure (space group $I4/mmm)$. For ${\mathrm{BaCo}}_2{\mathrm{As}}_2$, powder neutron diffraction data show that the $c$-axis lattice parameter exhibits anomalous negative thermal expansion from 10 K to 300 K, whereas the $a$ axis lattice parameter and the unit cell volume show normal positive thermal expansion over this $T$ range. No transitions in ${\mathrm{BaCo}}_2{\mathrm{As}}_2$ were found in this $T$ range from any of the measurements. Below 40–50 K, we find $\rho \propto T^2$, indicating a Fermi liquid ground state. A large density of states at the Fermi energy $\mathcal{D}\left(E_{\mathrm{F}}\right)\approx 18$ states/(eV f.u.) for both spin directions is found from low-$T$ $C_{\mathrm{p}}\left(T\right)$ measurements, whereas the band-structure calculations give $\mathcal{D}\left(E_{\mathrm{F}}\right)=8.23$ states/(eV f.u.). The enhancement of the former value above the latter is inferred to arise from electron-electron correlations and the electron-phonon interaction. The derived intrinsic $\chi \left(T\right)$ monotonically increases with decreasing $T$, with anisotropy ${\chi }_{ab}>{\chi }_c$. The ${}^{75}\text{As-NMR}$ shift data versus $T$ have the same $T$ dependence as the derived $\chi \left(T\right)$ data, demonstrating that the derived $\chi \left(T\right)$ data are intrinsic. The observed ${}^{75}\text{As}$ nuclear spin dynamics rule out the presence of Néel-type antiferromagnetic electronic spin fluctuations, but are consistent with the presence of ferromagnetic and/or stripe-type antiferromagnetic spin fluctuations. The crystals of ${\mathrm{Ba}}_{0.78}{\mathrm{K}}_{0.22}{\mathrm{Co}}_2{\mathrm{As}}_2$ were grown in Sn flux and show properties very similar to those of undoped ${\mathrm{BaCo}}_2{\mathrm{As}}_2$. On the other hand, the crystals from two batches of ${\mathrm{Ba}}_{0.94}{\mathrm{K}}_{0.06}{\mathrm{Co}}_2{\mathrm{As}}_2$ grown in CoAs self-flux show evidence of weak ferromagnetism at $T\lesssim 10$ K with small ordered moments at 1.8 K of $\approx 0.007$ and $0.03{\mu }_{\mathrm{B}}$ per formula unit, respectively.

The solution of a generalized impurity model lies at the heart of electronic structure calculations with dynamical mean field theory. In the strongly correlated regime, the method of choice for solving the impurity model is the hybridization-expansion continuous-time quantum Monte Carlo (CT-HYB). Enhancements to the CT-HYB algorithm are critical for bringing new physical regimes within reach of current computational power. Taking advantage of the fact that the bottleneck in the algorithm is a product of hundreds of matrices, we present optimizations based on the introduction and combination of two concepts of more general applicability: (a) skip lists and (b) fast rejection of proposed configurations based on matrix bounds. Considering two very different test cases with $d$ electrons, we find speedups of $\sim 25$ up to $\sim 500$ compared to the direct evaluation of the matrix product. Even larger speedups are likely with $f$ electron systems and with clusters of correlated atoms.

We study correlated quantum impurity models that undergo a local quantum phase transition (QPT) from a strong coupling, Fermi liquid phase to a non-Fermi liquid phase with a globally doubly degenerate ground state. Our aim is to establish what can be shown exactly about such “local moment” (LM) phases, of which the permanent (zero-field) local magnetization is a hallmark, and an order parameter for the QPT. A description of the zero-field LM phase is shown to require two distinct self-energies, which reflect the broken symmetry nature of the phase and together determine the single self-energy of standard field theory. Distinct Friedel sum rules for each phase are obtained, via a Luttinger theorem embodied in the vanishing of appropriate Luttinger integrals. By contrast, the standard Luttinger integral is nonzero in the LM phase but found to have universal magnitude. A range of spin susceptibilites are also considered, including that corresponding to the local order parameter, whose exact form is shown to be RPA-like, and to diverge as the QPT is approached. Particular attention is given to the pseudogap Anderson model, including the basic physical picture of the transition, the low-energy behavior of single-particle dynamics, the quantum critical point itself, and the rather subtle effect of an applied local field. A two-level impurity model that undergoes a QPT (“singlet-triplet”) to an underscreened LM phase is also considered, for which we derive on general grounds some key results for the zero-bias conductance in both phases.

Under an appropriate symmetric extensive bipartition in a one-dimensional symmetry-protected topological (SPT) phase, a bulk critical-entanglement spectrum can be obtained, resembling the excitation spectrum of the critical point separating the SPT phase from the trivial (vacuum) state. Such a critical point is beyond the standard Landau-Ginzburg-Wilson paradigm for symmetry-breaking phase transitions. For the $S=1$ SPT (Haldane) phase with the Affleck-Kennedy-Lieb-Tasaki exact wave function, the resulting critical-entanglement spectrum has a residual entropy per lattice site $s_r=0.67602$, showing a delocalized version of the edge excitations in the SPT phase. From the wave function corresponding to the lowest entanglement energy level, the central charge of the critical point can be extracted $c\approx 1.01\pm 0.01$. The critical theory can be identified as the same effective field theory as the spin-1/2 antiferromagnetic Heisenberg chain or the spin-1/2 Haldane-Shastry model with inverse-square long-range interaction.

Singlet fission into a pair of triplet excitons in pentacene is envisaged to be a potential carrier multiplication route in organic photovoltaics. In order to shed light on the dissociation dynamics of the triplet states we performed electric field induced pump-probe experiments on polycrystalline films of pentacene. We find that the nascent triplet pair formed immediately after the fission process is susceptible to dissociation in the presence of the electric field. By measuring the temporal dynamics of the dissociation, we estimate its binding energy to be approximately 320 meV. After about 2 ps the triplets diffuse away from each other and the field assisted dissociation is substantially reduced. Our results suggest that if the nascent triplet pairs could be harvested at the interface in a device, the efficiency of the process would be higher.

The charge and spin injections into semiconductors are determined by the quality of the electrode/semiconductor interface. The latter is defined by the atomic penetration depth when growing metal electrodes on organic semiconductor thin films by the physical deposition method. Although the interfaces between top electrodes and a typical organic material, tris(8-hydroxyquinoline) aluminum $\left({\mathrm{Alq}}_3\right)$ thin films, have been intensively investigated by several groups, their results on the atomic penetration depth into ${\mathrm{Alq}}_3$ film diverged from each other. In this paper we study the deposition of cobalt atoms onto ${\mathrm{Alq}}_3$ thin films using the molecular dynamics method. The intermixing between Co and ${\mathrm{Alq}}_3$ is calculated to be limited to the first ${\mathrm{Alq}}_3$ layer for low Co initial kinetic energies, but spread to a few layers for high initial energies. The results of our calculation indicate that a proper deposition method with low injection energy helps to reduce the amount of intermixing at interfaces and improve the magnetoresistance of an organic-based magnetic junction.

We study the charge dynamics of a quantum dot as measured by a nearby quantum point contact probing the dot via individual single-particle wave packets. We contrast the two limiting cases of weak and strong system-detector coupling exerting vanishing and strong backaction on the system and analyze the resulting differences in the charge-charge correlator. Extending the study to multiple projective measurements modeling a continuous strong measurement, we identify a transition from a charge dynamics dominated by the system's properties to a universal dynamics governed by the measurement.

We performed density functional theory calculations on heterostructures of single layers of hexagonal BN and ${\mathrm{MoS}}_2$ to assess the effect of doping in the BN sheet and of interstitial Na atoms on the electronic properties of the adjacent ${\mathrm{MoS}}_2$ layer. Our calculations predict that $n$ doping of the boron nitride subsystem by oxygen, carbon, and sulfur impurities causes noticeable charge transfer into the conduction band of the ${\mathrm{MoS}}_2$ sheet, while $p$ doping by beryllium and carbon leaves the molybdenum disulphide layer largely unaffected. Intercalated sodium atoms lead to a significant increase of the interlayer distance in the heterostructure and to a metallic ground state of the ${\mathrm{MoS}}_2$ subsystem. The presence of such $n$ dopants leads to a distinct change of valence-band and conduction-band offsets, suggesting that doped h-BN remains a suitable substrate and gate material for applications of $n$-type ${\mathrm{MoS}}_2$.

Effects of spontaneous parity breaking by charge, spin, and orbital orders are investigated in a two-band Hubbard model on a honeycomb lattice. This is a minimal model in which the interorbital hopping, atomic spin-orbit coupling, and strong electron correlation give rise to fascinating properties, such as the magnetoelectric effects, quantum spin Hall effect, and spin or valley splitting in the band structure. We perform the symmetry analysis of possible broken-parity states and the mean-field analysis of their competition. We find that the model at 1/4 filling exhibits a spin-orbital composite ordered state and a charge ordered state, in addition to a paramagnetic quantum spin Hall insulator. We show that the composite ordered phase exhibits two types of magnetoelectric responses. The charge ordered state shows spin splitting in the band structure, while the topological nature varies depending on electron correlations.

Several algorithms have been proposed to calculate the spatial entanglement spectrum from high order Rényi entropies and maximum entropy techniques. We present an alternative approach for computing the entanglement spectrum with quantum Monte Carlo for both continuum and lattice Hamiltonians. This method provides direct access to the matrix elements of the spatially reduced density matrix and we determine an estimator that can be used in variational Monte Carlo as well as other Monte Carlo methods. The algorithm is based on using a generalization of the swap operator, which can be extended to calculate a general class of density matrices that can include combinations of spin, space, particle, and even momentum degrees of freedom. We demonstrate the method by applying it to the ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$ molecules and describe how the spatial entanglement spectrum encodes a covalent bond that includes all the many body correlations.

We investigate equilibrium and nonequilibrium charge-transfer processes by performing high-resolution transport spectroscopy. Using electrostatically defined quantum dots for energy-selective emission and detection, we achieved very high spectral resolution and a high degree of tunability of relevant experimental parameters. Most importantly, we observe that the spectral width of elastically transferred electrons can be substantially smaller than the linewidth of a thermally broadened Coulomb peak. This finding indicates that the charge-transfer process is fast compared to the electron-phonon interaction time. By drawing an analogy to double quantum dots, we argue that the spectral width of the elastic resonance is determined by the lifetime broadening $h\mathit{\Gamma }$ of the emitter and detector states. Good agreement with the model is found also in an experiment in which the charge transfer is in the regime $h\mathit{\Gamma }\gg k_{\mathrm{B}}T$. By performing spectroscopy below the Fermi energy, we furthermore observe elastic and inelastic transfer of holes.

${\mathrm{SrAl}}_2{\mathrm{O}}_4:\mathrm{Eu},\mathrm{Dy}$ is presumably the best known persistent luminescent phosphor. At room temperature, its green emission remains visible for hours after switching off the excitation. It is known that upon lowering the temperature of the phosphor a second photoluminescence emission band arises in the blue part of the visible spectrum, although its origin is still the subject of discussion. In this paper we thoroughly study the origin of both emission bands in ${\mathrm{SrAl}}_2{\mathrm{O}}_4:\mathrm{Eu},\mathrm{Dy}$ and we attribute this to europium ions substituting for the two different Sr sites in the phosphor's monoclinic host lattice. The photoluminescence properties, the thermal quenching behavior, and photoluminescence lifetime of both emission bands are investigated. A lanthanide energy level scheme is constructed for both sites. Using an integrated approach, i.e., combining charging, afterglow, and thermoluminescence measurements in the same run, we study the charging or trap filling processes in ${\mathrm{SrAl}}_2{\mathrm{O}}_4:\mathrm{Eu},\mathrm{Dy}$ upon excitation with site selective excitation wavelengths and at different temperatures. We show that trap filling is a thermally activated process when the green emitting center is excited at 435 nm. Furthermore, we also demonstrate that the distribution of filled traps after charging depends strongly on the excitation wavelength and thus on which ${\mathrm{Eu}}^{2+}$ center has been excited. This suggests trapping of the electron close to the ionized ${\mathrm{Eu}}^{2+}$ ion, without full delocalization to the conduction band during the trapping process. Finally, the quantum efficiency of the persistent luminescence is estimated at 65 (+/−10)%.

Temperature dependence of the electronic structure of ${\mathrm{SmB}}_6$ is studied by high-resolution angle-resolved photoemission spectroscopy (ARPES) down to 1 K. We demonstrate that there is no essential difference for the dispersions of the surface states below and above the resistivity saturating anomaly ($\sim 3.5\mathrm{K}$). Quantitative analyses of the surface states indicate that the quasiparticle scattering rate increases linearly as a function of temperature and binding energy, which differs from Fermi-liquid behavior. Most intriguingly, we observe that the hybridization between the $d$ and $f$ states builds gradually over a wide temperature region $(30\mathrm{K}<\mathrm{T}<110\mathrm{K})$. The surface states appear when the hybridization starts to develop. Our detailed temperature-dependence results give a complete interpretation of the exotic resistivity result of ${\mathrm{SmB}}_6$, as well as the discrepancies among experimental results concerning the temperature regions in which the topological surface states emerge and the Kondo gap opens, and give insights into the exotic Kondo crossover and its relationship with the topological surface states in the topological Kondo insulator ${\mathrm{SmB}}_6$.

We use the momentum average approximation to study the ground-state properties of strongly bound bipolarons in the double-well electron-phonon (el-ph) coupling model, which describes certain intercalated lattices where the linear term in the el-ph coupling vanishes due to symmetry. We show that this model predicts the existence of strongly bound yet lightweight bipolarons in some regions of the parameter space. This provides an alternative mechanism for the appearance of such bipolarons, in addition to long-range el-ph coupling and special lattice geometries.