Search

Electron-pair density wave (PDW) states are now an intense focus of research in the field of cuprate correlated superconductivity. PDWs exhibit periodically modulating superconductive electron pairing that can be visualized directly using scanned Josephson tunneling microscopy (SJTM). Although from theory, intertwining the

Measuring quantum observables by grouping terms that can be rotated to sums of only products of Pauli

Electron-pair density wave (PDW) states are now an intense focus of research in the field of cuprate correlated superconductivity. PDWs exhibit periodically modulating superconductive electron pairing which can be visualized directly using scanned Josephson tunneling microscopy (SJTM). Although from theory, intertwining the d-wave superconducting (DSC) and PDW order parameters allows a plethora of global electron-pair orders to appear, which one actually occurs in the various cuprates is unknown. Here we use SJTM to visualize the interplay of PDW and DSC states in Bi2Sr2CaCu2O8+x at a carrier density where the charge density wave (CDW) modulations are virtually nonexistent. Simultaneous visualization of their amplitudes reveals that the intertwined PDW and DSC are mutually attractive states. Then, by separately imaging the electron-pair density modulations of the two orthogonal PDWs, we discover a robust nematic PDW state. Its spatial arrangement entails Ising domains of opposite nematicity, each consisting primarily of unidirectional and lattice commensurate electron-pair density modulations. Further, we demonstrate by direct imaging that the scattering resonances identifying Zn impurity atom sites occur predominantly within boundaries between these domains. This implies that the nematic PDW state is pinned by Zn atoms, as was recently proposed (Lozano et al, PHYSICAL REVIEW B 103, L020502 (2021)). Taken in combination, these data indicate that the PDW in Bi2Sr2CaCu2O8+x is a vestigial nematic pair density wave state (J. Wardh and M. Granath arXiv:2203.08250v1)., 15 pages, 4 figures

The idea of employing non-Abelian statistics for error-free quantum computing ignited interest in reports of topological surface superconductivity and Majorana zero modes (MZMs) in FeTe0.55Se0.45. However, the topological features and superconducting properties are not observed uniformly across the sample surface. The understanding and practical control of these electronic inhomogeneities present a prominent challenge for potential applications. Here, we combine neutron scattering, scanning angle-resolved photoemission spectroscopy, and microprobe composition and resistivity measurements to characterize the electronic state of Fe1+yTe1-xSex. We establish a phase diagram in which the superconductivity is observed only at sufficiently low Fe concentration, in association with distinct antiferromagnetic correlations, whereas the coexisting topological surface state occurs only at sufficiently high Te concentration. We find that FeTe0.55Se0.45 is located very close to both phase boundaries, which explains the inhomogeneity of superconducting and topological states. Our results demonstrate the compositional control required for use of topological MZMs in practical applications.

The Babai numbers and the upper chromatic number are parameters that can be assigned to any metric space. They can, therefore, be assigned to any connected simple graph. In this paper we make progress in the theory of the first Babai number and the upper chromatic number in the simple graph setting, with emphasis on graphs of diameter 2.

Recently evidence has emerged in the topological superconductor Fe-chalcogenide FeTe1-xSex for time-reversal symmetry breaking (TRSB), the nature of which has strong implications on the Majorana zero modes (MZM) discovered in this system. It remains unclear however whether the TRSB resides in the topological surface state (TSS) or in the bulk, and whether it is due to an unconventional TRSB superconducting order parameter or an intertwined order. Here by performing in superconducting FeTe1-xSex crystals both surface-magneto-optic-Kerr effect (SMOKE) measurements using a Sagnac interferometer and bulk magnetic susceptibility measurements, we pinpoint the TRSB to the TSS, where we also detect a Dirac gap. Further, we observe surface TRSB in non-superconducting FeTe1-xSex of nominally identical composition, indicating that TRSB arises from an intertwined surface ferromagnetic (FM) order. The observed surface FM bears striking similarities to the two-dimensional (2D) FM found in 2D van der Waals crystals, and is highly sensitive to the exact chemical composition, thereby providing a means for optimizing the conditions for Majorana particles that are useful for robust quantum computing., Comment: accepted version at Physical Review Letters

It is shown that whenever the edges of a connected simple graph on n vertices are colored with $$n-1$$ colors appearing so that no cycle in G is rainbow, there must be a monochromatic edge cut in G. From this it follows that such colorings of G can be represented, or ‘encoded,’ by full binary trees with n leaves, with vertices labeled by subsets of V(G), such that the leaf labels are singletons, the label of each non-leaf is the union of the labels of its children, and each label set induces a connected subgraph of G. It is also shown that $$n-1$$ is the largest integer for which the main theorem holds, for each n, although for some graphs a certain strengthening of the hypothesis makes the theorem conclusion true with $$n-1$$ replaced by $$n-2$$.

The number of measurements demanded by hybrid quantum-classical algorithms such as the variational quantum eigensolver (VQE) is prohibitively high for many problems of practical value. For such problems, realizing quantum advantage will require methods which dramatically reduce this cost. Previous quantum algorithms that reduce the measurement cost (e.g. quantum amplitude and phase estimation) require error rates that are too low for near-term implementation. Here we propose methods that take advantage of the available quantum coherence to maximally enhance the power of sampling on noisy quantum devices, reducing measurement number and runtime compared to the standard sampling method of the variational quantum eigensolver (VQE). Our scheme derives inspiration from quantum metrology, phase estimation, and the more recent "alpha-VQE" proposal, arriving at a general formulation that is robust to error and does not require ancilla qubits. The central object of this method is what we call the "engineered likelihood function" (ELF), used for carrying out Bayesian inference. We show how the ELF formalism enhances the rate of information gain in sampling as the physical hardware transitions from the regime of noisy intermediate-scale quantum computers into that of quantum error corrected ones. This technique speeds up a central component of many quantum algorithms, with applications including chemistry, materials, finance, and beyond. Similar to VQE, we expect small-scale implementations to be realizable on today's quantum devices., 49 pages, 32 figures

The unconventional normal-state properties of the cuprates are often discussed in terms of emergent electronic order that onsets below a putative critical doping of xc = 0.19. Charge-density wave (CDW) correlations represent one such order; however, experimental evidence for such order generally spans a limited range of doping that falls short of the critical value xc, leading to questions regarding its essential relevance. Here, we use x-ray diffraction to demonstrate that CDW correlations in La2-xSrxCuO4 persist up to a doping of at least x = 0.21. The correlations show strong changes through the superconducting transition, but no obvious discontinuity through xc = 0.19, despite changes in Fermi surface topology and electronic transport at this doping. These results demonstrate the interaction between CDWs and superconductivity even in overdoped cuprates and prompt a reconsideration of the role of CDW correlations in the high-temperature cuprate phase diagram., Comment: 8 pages + 5 pages of supplemental material; accepted in npj Quantum Materials

Surface magnetism and its correlation with the electronic structure are critical to understanding the topological surface state in the intrinsic magnetic topological insulator MnBi_{2}Te_{4}. Here, using static and time resolved angle-resolved photoemission spectroscopy (ARPES), we find a significant ARPES intensity change together with a gap opening on a Rashba-like conduction band. Comparison with a model simulation strongly indicates that the surface magnetism on cleaved MnBi_{2}Te_{4} is the same as its bulk state. The inability of surface ferromagnetism to open a gap in the topological surface state uncovers the novel complexity of MnBi_{2}Te_{4} that may be responsible for the low quantum anomalous Hall temperature of exfoliated MnBi_{2}Te_{4}.

The idea of employing non-Abelian statistics for error-free quantum computing ignited interest in reports of topological surface superconductivity and Majorana zero modes (MZMs) in FeTe

The symmetries of superconducting gap functions remain an important question of iron-based superconductivity. Motivated by the recent angle-resolved photoemission spectroscopic measurements on iron-chalcogenide superconductors, we investigate the influence of pairing symmetries on the topological surface state. If the surface Dirac cone becomes gapped in the superconducting phase, it implies magnetization induced from time-reversal symmetry breaking pairing via spin-orbit coupling. Based on the crystalline symmetry constraints on the Ginzburg-Landau free energy, the gap function symmetries are among the possibilities of $A_{1g(u)}\pm iA_{2g(u)}$, $B_{1g(u)}\pm iB_{2g(u)}$, or, $E_{g(u)}\pm i E_{g(u)}$. This time-reversal symmetry breaking effect can exist in the normal state very close to $T_c$ with the relative phase between two gap functions locked at $\pm \frac{\pi}{2}$. The coupling between magnetization and superconducting gap functions is calculated based on a three-orbital model for the band structure of iron-chalcogenides. This study provides the connection between the gap function symmetries and topological properties of the surface state., Comment: 11 pages, 8 figures. Comments are welcome!

A comparative study of the gaps measured in two techniques, angle-resolved photoelectron spectroscopy and spectroscopic imaging scanning tunneling microscopy, is presented. In particular the study focuses on the more overdoped region of the cuprate phase diagram in the superconducting state. While the total densities of states measured in the two techniques agree very well, it is shown that the peak in the density of states, ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{DOS}}$, is consistently displaced to higher energies relative to the maximal superconducting gap, ${\mathrm{\ensuremath{\Delta}}}_{0}$, determined in photoemission. The difference between the two gaps is more evident for the less doped samples reflecting increased normalization of bands. This observation will clearly influence the boundaries in the phase diagrams of the cuprates defined by these two techniques.

An edge-regular graph is a regular graph in which, for some $\lambda$, any two adjacent vertices have exactly $\lambda$ common neighbors. This paper is about the existence and structure of edge-regular graphs with $\lambda =1$ and about edge-regular graphs with $\lambda >1$ which have local neighborhood structure analogous to that of the edge-regular graphs with $\lambda =1$.

We report the temperature-dependent optical conductivity and ARPES studies of the iron-based superconductor (SC) Sr$_{0.67}$Na$_{0.33}$Fe$_2$As$_2$ in the high-temperature tetragonal paramagnetic phase; below the structural and magnetic transitions at $T_{\rm N}\simeq$125 K in the orthorhombic spin-density-wave (SDW)-like phase, and $T_r\simeq$42 K in the reentrant tetragonal double-Q magnetic phase where both charge and SDW order exist; and below the SC transition at $T_c\simeq$10 K. The free-carrier component in the optical conductivity is described by two Drude contributions; one strong and broad, the other weak and narrow. The broad Drude component decreases dramatically below $T_{\rm N}$ and $T_r$, with much of its strength being transferred to a bound excitation in the mid-infrared, while the narrow Drude component shows no anomalies at either of the transitions, actually increasing in strength at low temperature while narrowing dramatically. The behavior of an infrared-active mode suggests zone-folding below $T_r$. Below $T_c$ the dramatic decrease in the low-frequency optical conductivity signals the formation of a SC energy gap. ARPES reveals hole-like bands at the center of the Brillouin zone (BZ), with both electron- and hole-like bands at the corners. Below $T_{\rm N}$, the hole pockets at the center of the BZ decrease in size, consistent with the behavior of the broad Drude component; while below $T_r$ the electron-like bands shift and split, giving rise to a low-energy excitation in the optical conductivity at ~20 meV. The magnetic states, with resulting SDW and charge-SDW order, respectively, lead to a significant reconstruction of the Fermi surface that has profound implications for the transport originating from the electron and hole pockets, but appears to have relatively little impact on the SC in this material.

Practical challenges in simulating quantum systems on classical computers have been widely recognized in the quantum physics and quantum chemistry communities over the past century. Although many approximation methods have been introduced, the complexity of quantum mechanics remains hard to appease. The advent of quantum computation brings new pathways to navigate this challenging complexity landscape. By manipulating quantum states of matter and taking advantage of their unique features such as superposition and entanglement, quantum computers promise to efficiently deliver accurate results for many important problems in quantum chemistry such as the electronic structure of molecules. In the past two decades significant advances have been made in developing algorithms and physical hardware for quantum computing, heralding a revolution in simulation of quantum systems. This article is an overview of the algorithms and results that are relevant for quantum chemistry. The intended audience is both quantum chemists who seek to learn more about quantum computing, and quantum computing researchers who would like to explore applications in quantum chemistry., 194 pages, 13 figures, 5 tables and 404 references. Fixed formatting issues from the previous version. Comments welcome

When Cooper pairs are formed with finite center-of-mass momentum, the defining characteristic is a spatially modulating superconducting energy gap $\Delta(r)$. Recently, this concept has been generalized to the pair density wave (PDW) state predicted to exist in a variety of strongly correlated electronic materials such as the cuprates. Although the signature of a cuprate PDW has been detected in Cooper-pair tunnelling, the distinctive signature in single-electron tunneling of a periodic $\Delta(r)$ modulation has never been observed. Here, using a new approach, we discover strong $\Delta(r)$ modulations in Bi$_2$Sr$_2$CaCu$_2$O$_8$+$\delta$ that have eight-unit-cell periodicity or wavevectors $Q=2{\pi}/a_0(1/8,0)$; $2{\pi}/a_0(0,1/8)$. This constitutes the first energy-resolved spectroscopic evidence for the cuprate PDW state. An analysis of spatial arrangements of $\Delta(r)$ modulations then reveals that this PDW is predominantly unidirectional, but with an arrangement of nanoscale domains indicative of a vestigial PDW. Simultaneous imaging of the local-density-of-states $N(r,E)$ reveals electronic modulations with wavevectors $Q$ and $2Q$, as anticipated when the PDW coexists with superconductivity. Finally, by visualizing the topological defects in these $N(r,E)$ density waves at $2Q$, we discover them to be concentrated in areas where the PDW spatial phase changes by $\pi$, as predicted by the theory of half-vortices in a PDW state. Overall, this is a compelling demonstration, from multiple single-electron signatures, of a PDW state coexisting with superconductivity at zero magnetic field, in the canonical cuprate Bi$_2$Sr$_2$CaCu$_2$O$_8$+$\delta$., Comment: Final submitted version. arXiv admin note: text overlap with arXiv:2109.14031

We consider the problem of characterizing states of a multipartite quantum system from restricted, quasi-local information, with emphasis on uniquely determined pure states. By leveraging tools from dissipative quantum control theory, we show how the search for states consistent with an assigned list of reduced density matrices may be restricted to a proper subspace, which is determined solely by their supports. The existence of a quasi-local observable which attains its unique minimum over such a subspace further provides a sufficient criterion for a pure state to be uniquely determined by its reduced states. While the condition that a pure state is uniquely determined is necessary for it to arise as a non-degenerate ground state of a quasi-local Hamiltonian, we prove the opposite implication to be false in general, by exhibiting an explicit analytic counterexample. We show how the problem of determining whether a quasi-local parent Hamiltonian admitting a given pure state as its unique ground state is dual, in the sense of semidefinite programming, to the one of determining whether such a state is uniquely determined by the quasi-local information. Failure of this dual program to attain its optimal value is what prevents these two classes of states to coincide., 17 pages, 1 figure

The high-${T}_{c}$ superconductor, $\mathrm{FeT}{\mathrm{e}}_{0.55}\mathrm{S}{\mathrm{e}}_{0.45}$, has recently been shown to support a surface state with topological character. Here we use low-energy laser-based angle-resolved photoemission spectroscopy with variable light polarization, including both linear and circular polarizations, to reexamine the same material and the related $\mathrm{FeT}{\mathrm{e}}_{0.7}\mathrm{S}{\mathrm{e}}_{0.3}$, with a larger Te concentration. In both cases, we observe the presence of a surface state displaying linear dispersion in a cone-like configuration. The use of circular polarization confirms the presence of a helical spin structure. These experimental studies are compared with theoretical studies that account for the local magnetic effects related to the paramagnetism observed in this system in the normal state. In contrast to previous studies, we find that including the magnetic contributions is necessary to bring the chemical potential of the calculated electronic band structure naturally into alignment with the experimental observations.

Topological superconductivity has been sought for in a variety of heterostructure systems, the interest being that a material displaying such a phenomenon could prove to be the ideal platform to support Majorana fermions, which in turn could be the basis for advanced qubit technologies. Recently the high Tc family of superconductors, $FeSe_{x}Te_{1-x}$, have been shown to exhibit the property of topological superconductivity and further, evidence has been found for the presence of Majorana fermions. We have studied the interplay of topology, magnetism and superconductivity in the $FeSe_{x}Te_{1-x}$ family using high-resolution laser-based photoemission. At the bulk superconducting transition, a gap opens at the chemical potential as expected. However, a second gap is observed to open at the Dirac point in the topological surface state. The associated mass acquisition in the topological state points to time-reversal symmetry breaking, probably associated with the formation of ferromagnetism in the surface layer. The presence of intrinsic ferromagnetism combined with strong spin-orbit coupling provides an ideal platform for a range of exotic topological phenomena.

Parameterized quantum circuits play an essential role in the performance of many variational hybrid quantum-classical (HQC) algorithms. One challenge in implementing such algorithms is to choose an effective circuit that well represents the solution space while maintaining a low circuit depth and number of parameters. To characterize and identify expressible, yet compact, parameterized circuits, we propose several descriptors, including measures of expressibility and entangling capability, that can be statistically estimated from classical simulations of parameterized quantum circuits. We compute these descriptors for different circuit structures, varying the qubit connectivity and selection of gates. From our simulations, we identify circuit fragments that perform well with respect to the descriptors. In particular, we quantify the substantial improvement in performance of two-qubit gates in a ring or all-to-all connected arrangement compared to that of those on a line. Furthermore, we quantify the improvement in expressibility and entangling capability achieved by sequences of controlled X-rotation gates compared to sequences of controlled Z-rotation gates. In addition, we investigate how expressibility "saturates" with increased circuit depth, finding that the rate and saturated-value appear to be distinguishing features of a parameterized quantum circuit template. While the correlation between each descriptor and performance of an algorithm remains to be investigated, methods and results from this study can be useful for both algorithm development and design of experiments for general variational HQC algorithms.

Here, we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi-liquid phase (LiFeAs) and an incoherent bad-metal phase (${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum-superconducting-gap and ${T}_{c}$ ratio $2{\mathrm{\ensuremath{\Delta}}}_{max}/{k}_{B}{T}_{c}$, which shows a universal scaling that is valid for all iron-based superconductors (FeSCs). Our observation excludes pairing scenarios in the BCS and the BEC limit for FeSCs and calls for a universal strong-coupling pairing mechanism for the FeSCs.

The interplay between the electronic and lattice degrees of freedom in nonequilibrium states of strongly correlated systems has been debated for decades. Although progress has been made in establishing a hierarchy of electronic interactions with the use of time-resolved techniques, the role of the phonons often remains in dispute, a situation highlighting the need for tools that directly probe the lattice. We present the first combined megaelectron volt ultrafast electron diffraction and time- and angle-resolved photoemission spectroscopy study of optimally doped Bi2Sr2CaCu2O8+δ. Quantitative analysis of the lattice and electron subsystems’ dynamics provides a unified picture of nonequilibrium electron-phonon interactions in the cuprates beyond the N-temperature model. The work provides new insights on the specific phonon branches involved in the nonequilibrium heat dissipation from the high-energy Cu–O bond stretching “hot” phonons to the lowest-energy acoustic phonons with correlated atomic motion along the crystal directions and their characteristic time scales. It reveals a highly nonthermal phonon population during the first several picoseconds after the photoexcitation. The approach, taking advantage of the distinct nature of electrons and photons as probes, is applicable for studying energy relaxation in other strongly correlated electron systems. OA gold - CA extern

One of the most fundamental properties of an interacting electron system is its frequency- and wave-vector-dependent density response function, $\chi({\bf q},\omega)$. The imaginary part, $\chi''({\bf q},\omega)$, defines the fundamental bosonic charge excitations of the system, exhibiting peaks wherever collective modes are present. $\chi$ quantifies the electronic compressibility of a material, its response to external fields, its ability to screen charge, and its tendency to form charge density waves. Unfortunately, there has never been a fully momentum-resolved means to measure $\chi({\bf q},\omega)$ at the meV energy scale relevant to modern elecronic materials. Here, we demonstrate a way to measure $\chi$ with quantitative momentum resolution by applying alignment techniques from x-ray and neutron scattering to surface high-resolution electron energy-loss spectroscopy (HR-EELS). This approach, which we refer to here as "M-EELS," allows direct measurement of $\chi''({\bf q},\omega)$ with meV resolution while controlling the momentum with an accuracy better than a percent of a typical Brillouin zone. We apply this technique to finite-q excitations in the optimally-doped high temperature superconductor, Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi2212), which exhibits several phonons potentially relevant to dispersion anomalies observed in ARPES and STM experiments. Our study defines a path to studying the long-sought collective charge modes in quantum materials at the meV scale and with full momentum control., Comment: 26 pages, 10 sections, 7 figures, and an appendix

Both electron and lattice dynamics are directly observed in the nonequilibrium state of strongly correlated Bi-2212., The interplay between the electronic and lattice degrees of freedom in nonequilibrium states of strongly correlated systems has been debated for decades. Although progress has been made in establishing a hierarchy of electronic interactions with the use of time-resolved techniques, the role of the phonons often remains in dispute, a situation highlighting the need for tools that directly probe the lattice. We present the first combined megaelectron volt ultrafast electron diffraction and time- and angle-resolved photoemission spectroscopy study of optimally doped Bi2Sr2CaCu2O8+δ. Quantitative analysis of the lattice and electron subsystems’ dynamics provides a unified picture of nonequilibrium electron-phonon interactions in the cuprates beyond the N-temperature model. The work provides new insights on the specific phonon branches involved in the nonequilibrium heat dissipation from the high-energy Cu–O bond stretching “hot” phonons to the lowest-energy acoustic phonons with correlated atomic motion along the crystal directions and their characteristic time scales. It reveals a highly nonthermal phonon population during the first several picoseconds after the photoexcitation. The approach, taking advantage of the distinct nature of electrons and photons as probes, is applicable for studying energy relaxation in other strongly correlated electron systems.

The phase diagram associated with the high Tc superconductors is complicated by an array of different ground states. The parent material represents an antiferromagnetic insulator but with doping superconductivity becomes possible with transition temperatures previously thought unattainable. The underdoped region of the phase diagram is dominated by the so-called pseudogap phenomena whereby in the normal state the system mimics superconductivity in its spectra response but does not show the complete loss of resistivity associated with the superconducting state. An understanding of this regime presents one of the great challenges for the field. In the present study we revisit the structure of the phase diagram as determined in photoemission studies. By careful analysis of the role of nanoscale inhomogeneities in the overdoped region we are able to more carefully separate out the gaps due to the pseudogap phenomena from the gaps due to the superconducting transition. Within a mean field description we are thus able to link the magnitude of the gap directly to the Heisenberg exchange interaction term, $J\sum{s_i \cdot s_j}$, contained in the $t-J$ model. This approach provides a clear indication that the pseudogap is that associated with spin singlet formation., Comment: revised to include supplementary section and correct references in Fig. 5

In complex materials various interactions have important roles in determining electronic properties. Angle-resolved photoelectron spectroscopy (ARPES) is used to study these processes by resolving the complex single-particle self-energy and quantifying how quantum interactions modify bare electronic states. However, ambiguities in the measurement of the real part of the self-energy and an intrinsic inability to disentangle various contributions to the imaginary part of the self-energy can leave the implications of such measurements open to debate. Here we employ a combined theoretical and experimental treatment of femtosecond time-resolved ARPES (tr-ARPES) show how population dynamics measured using tr-ARPES can be used to separate electron–boson interactions from electron–electron interactions. We demonstrate a quantitative analysis of a well-defined electron–boson interaction in the unoccupied spectrum of the cuprate Bi2Sr2CaCu2O8+x characterized by an excited population decay time that maps directly to a discrete component of the equilibrium self-energy not readily isolated by static ARPES experiments., Differentiation of quantum interactions in correlated materials is ambiguous in measurements of the single particle self-energy. Here, Rameau et al. employ a combined theoretical and experimental time domain treatment to separate electron-boson interactions from electron-electron interactions in Bi2Sr2CaCu2O8+x .

We study sequences (both cyclic and randomized) of idempotent completely-positive trace-preserving quantum maps, and show how they asymptotically converge to the intersection of their fixed point sets via alternating projection methods. We characterize the robustness features of the protocol against randomization and provide basic bounds on its convergence speed. The general results are then specialized to stabilizing en- tangled states in finite-dimensional multipartite quantum systems subject to a resource constraint, a problem of key interest for quantum information applications. We conclude by suggesting further developments, including techniques to enlarge the set of stabilizable states and ensure efficient, finite-time preparation., Comment: 12 pages, no figures

The connected hub number hc(G) of a connected graph G is the smallest order of a connected subgraph H of G such that any two nonadjacent vertices of G − H are joined in G by a path with all internal vertices in H. Letting γc(G) denote the connected domination number of G, it is easy to see that hc(G) ≤ γc(G) ≤ hc(G) + 1 for every connected graph G. Here we characterize the graphs G for which γc(G) = hc(G) + 1. Our result contributes to the search for the solution of an extremal problem of (Newman-Wolfe et al., Congressus Numerantium 67 (1988), 67–76). © 2011 Wiley Periodicals, Inc. NETWORKS, 2011 © 2011 Wiley Periodicals, Inc.

Angle-resolved photoemission has developed into one of the leading probes of the electronic structure and associated dynamics of condensed matter systems. As with any experimental technique the ability to resolve features in the spectra is ultimately limited by the resolution of the instrumentation used in the measurement. Previously developed for sharpening astronomical images, the Lucy–Richardson deconvolution technique proves to be a useful tool for improving the photoemission spectra obtained in modern hemispherical electron spectrometers where the photoelectron spectrum is displayed as a 2D image in energy and momentum space.

Let @g"f denote the fractional chromatic number and @r the Hall ratio, and let the lexicographic product of G and H be denoted GlexH. Main results: (i) @r(GlexH)@?@g"f(G)@r(H); (ii) if @r(G)=@g"f(G) then @r(GlexH)=@r(G)@r(H) for all H; (iii) @g"f-@r is unbounded. In addition, the question of how big @g"f/@r can be is discussed.

It is shown that the Shields-Harary index of vulnerability of the complete bipartite graph K"m","n, with respect to the cost function f(x)=1-x, 0==m+2m, and [email protected]?(n+m)^[email protected]?, if m==m+2m, and between [email protected]?(n+m)^[email protected]?f(0) and mf(0), if m=

In 1988, A. J. W. Hilton and P. D. Johnson Jr. found a natural generalization of the condition in Philip Hall's celebrated theorem on systems of distinct representatives. This generalization was formed in the relatively new theory of list colorings of graphs. Here we give an account of a strand of development arising from this generalization, concentrating on extensions of Hall's theorem. New results are presented concerning list colorings of independence systems and colorings of graphs with nonnegative measurable functions on positive measure spaces.

A series of LiFe$_{1-x}$Co$_{x}$As compounds with different Co concentrations have been studied by transport, optical spectroscopy, angle-resolved photoemission spectroscopy and nuclear magnetic resonance. We observed a Fermi liquid to non-Fermi liquid to Fermi liquid (FL-NFL-FL) crossover alongside a monotonic suppression of the superconductivity with increasing Co content. In parallel to the FL-NFL-FL crossover, we found that both the low-energy spin fluctuations and Fermi surface nesting are enhanced and then diminished, strongly suggesting that the NFL behavior in LiFe$_{1-x}$Co$_{x}$As is induced by low-energy spin fluctuations which are very likely tuned by Fermi surface nesting. Our study reveals a unique phase diagram of LiFe$_{1-x}$Co$_{x}$As where the region of NFL is moved to the boundary of the superconducting phase, implying that they are probably governed by different mechanisms., Comment: 10 pages, 11 figures

The scalar field will be either the real or complex numbers. Suppose that lambda is a solid sequence space over the scalar field and A is an infinite lower triangular matrix with non-negative entries and positive entries on the main diagonal such that each of its columns is in lambda. For each positive integer k, the kth predecessor of lambda with respect to A is the solid vector space of scalar sequences x such that A(k)vertical bar x vertical bar is an element of lambda. We denote this space by Lambda(k) and lambda itself will be denoted by Lambda(0). Under reasonable assumptions, these spaces inherit some topological properties from lambda. We are interested in a projective limit of the infinite product of the Lambda(k) consisting of sequences of sequences (x((k))) satisfying Ax((k)) = x((k-1)) for each k > 0. We show that for interesting classes of situations including the cases when lambda = l(p) for some p > 1 and A is the Cesaro matrix, the space of our interest can be non-trivial.

We investigate under which conditions a mixed state on a finite-dimensional multipartite quantum system may be the unique, globally stable fixed point of frustration-free semigroup dynamics subject to specified quasi-locality constraints. Our central result is a linear-algebraic necessary and sufficient condition for a generic (full-rank) target state to be frustration-free quasi-locally stabilizable, along with an explicit procedure for constructing Markovian dynamics that achieve stabilization. If the target state is not full-rank, we establish sufficiency under an additional condition, which is naturally motivated by consistency with pure-state stabilization results yet provably not necessary in general. Several applications are discussed, of relevance to both dissipative quantum engineering and information processing, and non-equilibrium quantum statistical mechanics. In particular, we show that a large class of graph product states (including arbitrary thermal graph states) as well as Gibbs states of commuting Hamiltonians are frustration-free stabilizable relative to natural quasi-locality constraints. Likewise, we provide explicit examples of non-commuting Gibbs states and non-trivially entangled mixed states that are stabilizable despite the lack of an underlying commuting structure, albeit scalability to arbitrary system size remains in this case an open question., Comment: 44 pages, main results are improved, several proofs are more streamlined, application section is refined

Part I Materials: Synthesis, structural properties, and phase diagrams.- Bulk.- Film.- Part II Experiments: Characterization of Electronic and Magnetic Properties.- Electron spectroscopy - ARPES.- Magnetic order and dynamics - neutron scattering.- Scanning Tunneling Spectroscopy.- X-ray scattering and diffraction.- Optics and transport.- Other techniques.- Properties under extreme conditions.- Part III Theories.- First-principles band calculation.- Many-body computation.- Itinerant electron model.- t-J-like model with localized moments.- Coexisting itinerant and localized electrons.- Orbital-selective Mott transition.

Described are general protocols for the rapid construction of various C-15-substituted analogues of vindoline using palladium-catalyzed cross-coupling reactions. The required bromo- and iodovindolines were prepared in high yield by the reaction of vindoline with N-bromosuccinimide or N-iodosuccinimide, respectively. The study not only led to the preparation of a number of structurally novel vindoline analogues but also opens the door to new strategies for the synthesis of vinblastine, vincristine, and related anticancer agents. Also described is the conversion of ent-tabersonine to ent-vindoline.

We introduce a class of “chromatic” graph parameters that include the chromatic number, the circular chromatic number, the fractional chromatic number, and an uncountable horde of others. We prove some basic results about this class and pose some problems.

An easy algorithm is described a special case of which will generate a list of all n-ary Huffman sequences of length m, for any given integers m ≥ 1, n ≥ 2.

A graph is (t,r)-regular iff it has at least one independent t-set of vertices and the open neighborhood of any such set contains exactly r vertices. Our goal is to show that when t⩾3 and the order is sufficiently large, then the structure of (t,r)-regular graphs is similar to, but not exactly the same as the structure of (2,r)-regular graphs as derived by Faudree and Knisley. That is, there is an “almost” complete kernel of order at most r surrounded by satellite cliques, all of the same order, which are “mostly” joined to the kernel.

Objectives: Many groups are developing computer-interpretable clinical guidelines (CIGs) for use during clinical encounters. CIGs use “Task-Network Models” for representation but differ in their approaches to addressing particular modeling challenges. We have studied similarities and differences between CIGs in order to identify issues that must be resolved before a consensus on a set of common components can be developed. Design: We compared six models: Asbru, EON, GLIF, GUIDE, PRODIGY, and PRO form a. Collaborators from groups that created these models represented, in their own formalisms, portions of two guidelines: the American College of Physicians–American Society of Internal Medicine's guideline for managing chronic cough and the Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure. Measurements: We compared the models according to eight components that capture the structure of CIGs. The components enable modelers to encode guidelines as plans that organize decision and action tasks in networks. They also enable the encoded guidelines to be linked with patient data—a key requirement for enabling patient-specific decision support. Results: We found consensus on many components, including plan organization, expression language, conceptual medical record model, medical concept model, and data abstractions. Differences were most apparent in underlying decision models, goal representation, use of scenarios, and structured medical actions. Conclusion: We identified guideline components that the CIG community could adopt as standards. Some of the participants are pursuing standardization of these components under the auspices of HL7.

The Shields-Harary numbers are a class of graph parameters that measure a certain kind of robustness of a graph, thought of as a network of fortified reservoirs, with reference to a given cost function. We prove a result about the Shields-Harary numbers with respect to concave continuous cost functions which will simplify the calculation of these numbers for certain classes of graphs, including graphs formed by two intersecting cliques, and complete multipartite graphs.

The development of new capabilities in high resolution photoemission is allowing detailed studies of the role of collective many-body excitations in the decay of a photohole. This in turn provides new insights into the physics of condensed matter systems, in particular the strongly-correlated systems, which exhibit a rich variety of exotic phenomena including high Tc superconductivity.

High resolution angle-resolved photoemission spectroscopy of highly overdoped Bi 2 Sr 2 CaCu 2 O 8+δ with Tc = 51 K indicates that the basic transport processes in this material are fundamentally different from both the lesser-doped cuprates and model metallic compounds. The overdoped sample has sharp ARPES peaks at the Fermi energy throughout the Brillouin zone even in the normal state, unlike the lesser-doped compounds. In particular, the spectra near the (π, 0) point show the presence of a sharp peak well above Tc. The ARPES line shapes, and thus the self-energy, at a given energy are almost independent of k. Further, the quasiparticle scattering rate at the Fermi energy seems to be closely tied to direct resistivity measurements. This leads us to the conclusion that overdoped Bi 2 Sr 2 CaCu 2 O 8+δ is best described as a quasiparticle liquid. However, the energy dependence of the scattering rates is quite similar to that found in the lesser-doped compounds and quite different from that seen in a typical metal.

The dressing of quasiparticles in solids is investigated by changes in the electronic structure $E$($k$) driven by femtosecond laser pulses. Employing time- and angle-resolved photoemission on an optimally doped cuprate above ${T}_{\mathrm{c}}$, we observe two effects with different characteristic temporal evolutions and, therefore, different microscopic origins. First, a marked change in the effective mass due to the 70-meV kink in $E$($k$) is found to occur during the experiment's 100-fs temporal resolution and is assigned to laser-driven perturbation of an electronic interaction dressing the bare band. Second, a change in ${k}_{\mathrm{F}}$ is explained by effective photodoping due to particle-hole asymmetry and offers opportunities for ultrafast optoelectronic switches based on an optically driven insulator-superconductor transition.