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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 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

Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) measurements have previously been applied to the study of the heavy-fermion system CeCoIn5 to examine the superconducting gap structure and band dispersions via quasiparticle intereference. Here we directly measure the dispersing electron bands with angle-resolved photoelectron spectroscopy (ARPES) and compare with first-principles electronic structure calculations. By autocorrelating the ARPES-resolved bands with themselves we can measure the potential q vectors and discern exactly which bands the STM is measuring. We find that the STM results are dominated by scattering associated with a cloverleaf shaped band centered at the zone corners. This same band is also a viable candidate to host the superconducting gap. The electronic structure calculations indicate that this region of the Fermi surface involves significant contributions from the Co d electrons, an indication that the superconductivity in these materials is more three dimensional than that found in the related unconventional superconductors, the cuprates and the pnictides. publishedVersion

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.

In cuprate superconductors, the doping of carriers into the parent Mott insulator induces superconductivity and various other phases whose characteristic temperatures are typically plotted versus the doping level $p$. In most materials, $p$ cannot be determined from the chemical composition, but it is derived from the superconducting transition temperature, $T_\mathrm{c}$, using the assumption that $T_\mathrm{c}$ dependence on doping is universal. Here, we present angle-resolved photoemission studies of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, cleaved and annealed in vacuum or in ozone to reduce or increase the doping from the initial value corresponding to $T_\mathrm{c}=91$ K. We show that $p$ can be determined from the underlying Fermi surfaces and that $in-situ$ annealing allows mapping of a wide doping regime, covering the superconducting dome and the non-superconducting phase on the overdoped side. Our results show a surprisingly smooth dependence of the inferred Fermi surface with doping. In the highly overdoped regime, the superconducting gap approaches the value of $2\Delta_0=(4\pm1)k_\mathrm{B}T_\mathrm{c}$, Comment: 8 pages, 4 figures

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.

Over the past two decades, advances in computational algorithms have revealed a curious property of the two-dimensional Hubbard model (and related theories) with hole doping: the presence of close-in-energy competing ground states that display very different physical properties. On the one hand, there is a complicated state exhibiting intertwined spin, charge, and pair density wave orders. We call this `type A'. On the other hand, there is a uniform d-wave superconducting state that we denote as `type B'. We advocate, with the support of both microscopic theoretical calculations and experimental data, dividing the high-temperature cuprate superconductors into two corresponding families, whose properties reflect either the type A or type B ground states at low temperatures. We review the anomalous properties of the pseudogap phase that led us to this picture, and present a modern perspective on the role that umklapp scattering plays in these phenomena in the type B materials. This reflects a consistent framework that has emerged over the last decade, in which Mott correlations at weak coupling drive the formation of the pseudogap. We discuss this development, recent theory and experiments, and open issues., 95 pages, 33 figures

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

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.

The high-${T}_{c}$ superconducting cuprates are recognized as doped Mott insulators. Several studies indicate that as a function of doping and temperature, there is a crossover from this regime into a phase characterized as a marginal Fermi liquid. Several calculations of the doped Mott insulating phase indicate that the Fermi surface defines small pockets which at the higher doping levels switch to a full closed Fermi surface, characteristic of a more metallic system. Here we use femtosecond laser-based pump-probe techniques to investigate the structure of the Fermi surface in the underdoped region of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{r}}_{2}\mathrm{CaC}{\mathrm{u}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ and compare it with that associated with the optimally doped material. We confirm the concept of a small pocket in the underdoped system consistent with theoretical predictions in this strongly correlated state.

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

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

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.

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

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.

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.

We report on a new surface phase of the Co-vicinal-Cu(111) system which exhibits self-assembled uniform Co quantum wires that are stable at 300K. STM-imaging measurements show that wires will self-assemble within a narrow range of Co coverage and, within this range, the wires increase in length as coverage is increased. The STM images show that the wires form along the leading edge of the step rise, differentiating it from previously theoretically predicted atomic-wire phases. The formation of relatively long laterally un-encapsulated one- and two-atom wires also differentiates it from past experimentally observed step-island formation. Furthermore, our experiments also show directly that the Co wires coexist with another Co phase that had been previously predicted for growth on Cu(111). Our observations allow us to comment on the formation kinetics of the atomic-wire phase and on the fit of our data to a recently developed lattice-gas model., 14 pages, 5 figures

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.

Perfect fluids are characterized as having the smallest ratio of shear viscosity to entropy density, {\eta}/s, consistent with quantum uncertainty and causality. So far, nearly perfect fluids have only been observed in the Quark-Gluon Plasma (QGP) and in unitary atomic Fermi gases (UFG), exotic systems that are amongst the hottest and coldest objects in the known universe, respectively. We use Angle Resolve Photoemission Spectroscopy (ARPES) to measure the temperature dependence of an electronic analogue of {\eta}/s in an optimally doped cuprate high temperature superconductor, finding it too is a nearly perfect fluid around, and above, its superconducting transition temperature Tc., Comment: Article in press with Phys. Rev. B 22 pages, 9 Figures, 41 References

The electronic structure near the Fermi level of K 0.3 MoO 3 and Li 0.9 Mo 6 O 17 , two prototypical quasi-one-dimensional (1D) conductors, has been studied using very high resolution angle-resolved photoemission spectroscopy. Our results differ from earlier lower resolution studies. For Li 0.9 Mo 6 O 17 we clearly observe a quasi-1D state dispersing through the Fermi level ( E F ) and the opening up of a gap at E F as the sample is cooled through a bulk metal to non-metal transition at 24 K. For K 0.3 MoO 3 we clearly observe two quasi-1D bands cross E F , and measured a temperature dependence to the Fermi surface nesting vector that correlates well with neutron-scattering measurements of the charge density wave vector.

For optimally doped Bi{sub 2}Sr {sub 2}CaCu{sub 2}O{sub 8+{delta}} , scattering rates in the normal state are found to have a linear temperature dependence over most of the Fermi surface. In the immediate vicinity of the ({pi}, 0) point, the scattering rates are nearly constant in the normal state, consistent with models in which scattering at this point determines the c -axis transport. In the superconducting state, the scattering rates away from the nodal direction appear to level off and become temperature independent. (c) 2000 The American Physical Society.

In a high-resolution photoemission study of a Mo(110) surface state various contributions to the measured width and energy of the quasiparticle peak are investigated. Electron-phonon coupling, electron-electron interactions and scattering from defects are all identified mechanisms responsible for the finite lifetime of a valence photo-hole. The electron-phonon induced mass enhancement and rapid change of the photo-hole lifetime near the Fermi level are observed for the first time., RevTEX, 4 pages, 4 figures, to be published in PRL

The electronic properties of the quasi-one-dimensional (1D) oxide conductor Li{sub 0.9}Mo {sub 6}O{sub 17} have been investigated using high-momentum and energy resolution photoelectron spectroscopy at temperatures above and below the metal-semiconductor transition temperature. Considerable spectral intensity at the Fermi energy (E{sub F}) is observed at room temperature, and a sharp feature corresponding to a band crossing E{sub F} is clearly resolved for states along the quasi-1D axis. Below the metal-semiconductor transition at 24thinspthinspK, a gap is clearly observed to open at E{sub F} . Our data do not support the assertion that the low energy excitations of electrons in Li{sub 0.9}Mo {sub 6}O{sub 17} should be described in a Luttinger liquid model. {copyright} {ital 1999} {ital The American Physical Society }

High resolution angle-resolved photoemission is used to study the electronic structure of optimally doped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$. These studies, with an energy resolution of 8 meV, show that the sharp peak observed in the superconducting state has an intrinsic width of the order of 14 meV. Detailed temperature dependent studies reveal that this peak is fixed in binding energy at all temperatures at which it is observable and further that it exists at temperatures even above the superconducting transition temperature ${T}_{c}$. However, our analysis indicates that with the onset of long range phase coherence a gap emerges in the spectral response between the main incoherent peak and the Fermi level.

Spin-polarized photoemission studies of potassium adsorption on an Fe(001) surface provide evidence of a minority-spin surface state sitting immediately above the Fermi level on the clean surface. This state of symmetry is associated with the surface state predicted in first-principles calculations and the state utilized in chemically selective scanning tunnelling microscopy studies of the Fe(001) surface.

Spin-polarized photoemission has developed into a versatile tool for the study of surface and thin film magnetism. In this review, we examine the methodology of the technique and its application to a number of different problems, including both valence band and core level studies. After a detailed review of spin-polarization measurement techniques and the related experimental requirements we consider in detail studies of the bulk properties both above and below the Curie temperature. This section also includes a discussion of observations relating to unique metastable phases obtained via epitaxial growth. The application of the technique to the study of surfaces, both clean and adsorbate covered, is reviewed. The report then examines, in detail, studies of the spin-polarized electronic structure of thin films and the related interfacial magnetism. Finally, observations of spin-polarized quantum well states in non-magnetic thin films are discussed with particular reference to their mediation of the oscillatory exchange coupling in related magnetic multilayers.

Momentum-resolved direct and inverse photoemission spectra of the $\mathrm{K}/\mathrm{Si}\left(111\right)\ensuremath{-}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}\mathrm{B}$ interface reveals the presence of strongly localized surface states. The K overlayer remains nonmetallic up to the saturation coverage. This system most likely presents the first experimental realization of a frustrated spin $1/2$ Heisenberg antiferromagnet on a two-dimensional triangular lattice.

The {ital k}-space origin of the long-period (18A) oscillation in the interlayer magnetic coupling of Fe/Cr(100) multilayers is investigated by means of angle-resolved photoemission by probing quantum well (QW) states at the Fermi energy for epitaxial Cr grown on an Fe(100) whisker at 300{degree}C. The periodicity of the intensity oscillations of the Cr QW states in the vicinity of the {ital d}-derived {open_quotes}lens{close_quote}{close_quote} feature of the Fermi surface is 17{plus_minus}2{Angstrom}. Thus the lens is identified as a prime candidate for the origin of the long period. {copyright} {ital 1997} {ital The American Physical Society}

Using low-temperature scanning tunneling microscopy, we demonstrate a one-dimensional system that undergoes a charge-density-wave (CDW) instability on a metallic substrate. For our measurements we utilize a self-assembled monatomic chain of Co atoms aligned by the steps on a vicinal Cu(111) surface. We assign the measured CDW instability to ferromagnetic interactions along the chain. We show that though the linear arrayed dimers are not electronically isolated, they are magnetically independent, and hence can potentially serve as a binary spin-memory system.

The effects of oxygen adsorption on the surface magnetization of Gd(0001) have been determined by measuring the polarization of the Gd 4{ital f} core levels and the oxygen induced states with spin-resolved photoemission. Calculations of the partial density of states of O/Gd(0001) using the LMTO-ASA method support the photoemission measurements which indicate that oxidation occurs at low coverages. The loss of surface magnetic order with oxygen exposure is explained in terms of decreasing indirect coupling. {copyright} {ital 1996 The American Physical Society.}

Spin polarized photoemission is used to study the 3s core levels of Fe and Co. The exchange splitting between the two spin components in the main 3s3p 6 3d n configuration is found to be larger in Co than Fe. The satellites corresponding to emission from the 3s 2 3p 4 3d n +1 configuration are identified in both metals. The application of a sum rule to obtain the spin moment from the complete spectrum including all configurations in investigated.

Spin-polarized photoemission has developed into a versatile tool for the study of surface and thin film magnetism. In this review, we examine the methodology of the technique and its application to a number of different problems, including both valence band and core level studies. After a detailed review of spin-polarization measurement techniques and the related experimental requirements we consider in detail studies of the bulk properties both above and below the Curie temperature. This section also includes a discussion of observations relating to unique metastable phases obtained via epitaxial growth. The application of the technique to the study of surfaces, both clean and adsorbate covered, is reviewed. The report then examines, in detail, studies of the spin-polarized electronic structure of thin films and the related interfacial magnetism. Finally, observations of spin-polarized quantum well states in non-magnetic thin films are discussed with particular reference to their mediation of the oscillatory exchange coupling in related magnetic multilayers.

Quantum confinement is studied with angle-resolved photoemission for ultrathin Cu separated from the Cu(100) substrate by an epitaxial wedge of Co (0--11 A) that serves as a finite barrier. For 2 ML of Cu, the photoemission intensities of the Cu [ital sp] quantum-well (QW) state at 1.6 eV binding energy and the corresponding [ital d]-band-derived QW state at 2.4 eV increase rapidly with the thickness of the Co barrier and indicate stronger confinement of the [ital d] QW state than of the [ital sp] state. Implications for the magnetic coupling and magnetoresistance are discussed.

A microscopic scaling relation linking the normal and superconducting states of the cuprates in the presence of a pseudogap is presented using angle-resolved photoemission spectroscopy. This scaling relation, complementary to the bulk universal scaling relation embodied by Homes' law, explicitly connects the momentum-dependent amplitude of the $d$-wave superconducting order parameter at $T\ensuremath{\sim}0$ to quasiparticle scattering mechanisms operative at $T\ensuremath{\gtrsim}{T}_{c}$. The form of the scaling is proposed to be a consequence of the marginal Fermi-liquid phenomenology and the inherently strong dissipation of the normal pseudogap state of the cuprates.

The Fermi surface topologies of underdoped samples of the high-T(c) superconductor Bi2Sr2CaCu2O(8+δ) have been measured with angle resolved photoemission. By examining thermally excited states above the Fermi level, we show that the observed Fermi surfaces in the pseudogap phase are actually components of fully enclosed hole pockets. The spectral weight of these pockets is vanishingly small at the magnetic zone boundary, creating the illusion of Fermi "arcs." The area of the pockets as measured in this study is consistent with the doping level, and hence carrier density, of the samples measured. Furthermore, the shape and area of the pockets is well reproduced by phenomenological models of the pseudogap phase as a spin liquid.

The high upper critical field characteristic of the recently discovered iron-based superconducting chalcogenides opens the possibility of developing a new type of non-oxide high-field superconducting wires. In this work, we utilize a buffered metal template on which we grow a textured FeSe$_{0.5}$Te$_{0.5}$ layer, an approach developed originally for high temperature superconducting coated conductors. These tapes carry high critical current densities (>1$\times10^{4}$A/cm$^{2}$) at about 4.2K under magnetic field as high as 25 T, which are nearly isotropic to the field direction. This demonstrates a very promising future for iron chalcogenides for high field applications at liquid helium temperatures. Flux pinning force analysis indicates a point defect pinning mechanism, creating prospects for a straightforward approach to conductor optimization., Accepted for publication in Appl. Phys. Lett

We report angle-resolved photoemission spectroscopy (ARPES) and a set of in situ scanning tunneling microscopy (STM) measurements on a narrow-terrace-width vicinal Cu(111) crystal surface, Cu(775), whose vicinal cut lies close to the transition between terrace and step modulation. These measurements show sharp zone-folding (or umklapp) features with a periodicity in ${k}_{\ensuremath{\parallel}}$, indicating that the predominant reference plane is that of Cu(775), i.e., that the surface is predominately step-modulated. Our measurements also show variation in umklapp intensity with photon energy, which is consistent with prior ARPES experiments on other vicinal Cu(111) surfaces and in agreement with our designation of the state as being step-modulated. The measurements also show a weak terrace-modulated state, which, based on several characteristics, we attribute to the presence of terrace widths larger than the ideal terrace width. By measuring the intensity ratio of the two distinct surface-state modulations from photoemission and the terrace-width distribution from STM, we derive a value for the terrace width at which the surface state switches between the two modulations.

We present a combined analysis of neutron scattering and photoemission measurements on superconducting FeSe{sub 0.5}Te{sub 0.5}. The low-energy magnetic excitations disperse only in the direction transverse to the characteristic wave vector (1/2,0,0) whereas the electronic Fermi surface near (1/2,0,0) appears to consist of four incommensurate pockets. While the spin resonance occurs at an incommensurate wave vector compatible with nesting, neither spin-wave nor Fermi-surface-nesting models can describe the magnetic dispersion. We propose that a coupling of spin and orbital correlations is key to explaining this behavior. If correct, it follows that these nematic fluctuations are involved in the resonance and could be relevant to the pairing mechanism.

Superconducting thin films of ${\text{Fe}}_{1.08}\text{Te}:{\text{O}}_{x}$ have been epitaxially grown on ${\text{SrTiO}}_{3}$ substrates by pulsed-laser deposition in controlled oxygen atmosphere. Although the bulk ${\text{Fe}}_{1.08}\text{Te}$ is not superconducting, thin films with oxygen are superconducting with an onset and a zero resistance transition temperature around 12 and 8 K, respectively. Oxygen was found to be crucial to the superconducting properties of these films, suggesting that the oxygen incorporation can induce superconductivity in FeTe thin films. A metal-insulator transition is found at about 60 K, lower than that of bulk $(\ensuremath{\sim}70\text{ }\text{K})$. From magnetoresistive measurements, we obtained the irreversibility line and the upper critical field.

Laser-based photoemission with photons of energy 6 eV is used to examine the fine details of the very low-energy electron dispersion and associated dynamics in the nodal region of optimally doped Bi2212. A ``kink'' in the dispersion in the immediate vicinity of the Fermi energy is associated with scattering from an optical phonon previously identified in Raman studies. The identification of this phonon as the appropriate mode is confirmed by comparing the scattering rates observed experimentally with the results of calculated scattering rates based on the properties of the phonon mode.

We demonstrate angle-resolved, tunable, two-photon photoemission (2PPE) to map a bulk unoccupied band, viz. the Cu $sp$ band 0 to 1 eV below the vacuum level, in the vicinity of the $L$ point. This short-lived bulk band is seen due to the strong optical pump rate, and the observed transition energies and their dispersion with photon energy $\ensuremath{\hbar}\ensuremath{\omega}$, are in excellent agreement with tight-binding band-structure calculations. The variation of the final-state energy with $\ensuremath{\hbar}\ensuremath{\omega}$ has a measured slope of $\ensuremath{\sim}1.64$ in contrast to values of 1 or 2 observed for 2PPE from two-dimensional states. This unique variation illustrates the significant role of the perpendicular momentum $\ensuremath{\hbar}{k}_{\ensuremath{\perp}}$ in 2PPE.

The combination of spin‐polarization detection with core‐level photoemission to give spin‐polarized electron spectroscopy for chemical analysis provides new information on the oxidation of an iron surface. During the initial stage of oxidation the predominant phase is of a ferrimagnetic γ‐Fe2O3 form. The observed antiferromagnetic coupling of this oxide with the iron substrate is attributed to the superexchange interaction mediated by an oxygen ion. This provides evidence for a site exchange mechanism during the initial oxidation. Annealing transforms this oxide into an antiferromagnetic FexO form which is also present at the initial stage.

We measure the electronic structure of low-coverage Co on a Cu(775) stepped substrate using high-resolution photoemission spectroscopy with the particular goal of relating the electronic dispersion to the coverage-dependent surface structure. In particular, we follow the evolution of the electronic dispersion of the $sp$-like Cu surface state and the position of the band minimum as a function of Co coverage. On the bare Cu(775) surface, we observe band folding of this state due to the stepped surface-superlattice array. In addition, we determine that the reference plane, as measured by the position of the band minimum of this state, changes dramatically after addition of just 0.03 ML Co. At 0.06 ML, we observe the formation of a second surface state at a binding energy of $0.68\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. This feature is attributed to a quantum-well state hybridized with the substrate.

Angle-resolved photoemission and optical spectroscopy are used to probe the electronic structure of the one-dimensional Mott insulator ${\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{3+\ensuremath{\delta}}$, both at half-filling and with small concentrations of excess oxygen doping. Spin-charge separation can be seen as evidenced by the existence of spinon and holon branches in the photoemission spectra. Optical studies reveal no significant doping dependence, while photoemission studies show a large energy shift in the spinon and holon states with respect to the main valence band states which remain nearly unaffected. The results suggest the excess dopant carriers are incorporated solely within the one-dimensional Hubbard band which is electronically isolated from the rest of the states in the system.

Recent angle resolved photoemission \cite{yang-nature-08} and scanning tunneling microscopy \cite{kohsaka-nature-08} measurements on underdoped cuprates have yielded new spectroscopic information on quasiparticles in the pseudogap phase. New features of the normal state such as particle-hole asymmetry, maxima in the energy dispersion and accompanying drops in the spectral weight of quasiparticles agree with the ansatz of Yang \textit{et al.} for the single particle propagator in the pseudogap phase. The coherent quasiparticle dispersion and reduced asymmetry in the tunneling density of states in the superconducting state can also be described by this propagator., Comment: updated version, 6 pages, 7 figures, 1 table, EPL 86 (2009) 37002 (https://www.epletters.net)

Spin polarized photoemission studies of different adsorbates on an Fe (001) substrate show the interaction between the adsorbate and the substrate d bands. Studies of the adsorption of oxygen reveal exchange split adsorbate bands indicative of both a magnetic moment on the adsorbate atom and also a strong adsorbate p z substrate d z 2 bond. The oxygen p z bands show a large variation in exchange splitting across the surface zone, an observation that is not predicted in first principles calculations of this system. Spin Polarised Auger Electron Spectroscopy studies of sulfur adsorbed on the same surface provide some indication of the magnetic moment on the adsorbate. It is found to be of the order of 0.1 μB. Studies of carbon monoxide adsorption in two different configurations, vertical and tilted, reveal little or no exchange splitting in the molecular 5σ orbital suggesting that the interaction of this orbital with the substrate d bands is much weaker.

Spin-polarized photoelectron spectroscopy has developed into a versatile tool for the study of surface and thin film magnetism. In this chapter, we examine the methodology of the technique and its recent application to a number of different problems. We first examine the photoemission process itself followed by a detailed review of spin-polarization measurement techniques and the related experimental requirements. We review studies of spin polarized surface states, interface states and quantum well states followed by studies of the technologically important oxide systems including half-metallic transition metal oxides, ferromagnet/oxide interfaces and the antiferromagnetic cuprates that exhibit high Tc Superconductivity. We also discuss the application of high-resolution photoemission with spin resolving capabilities to the study of spin dependent self energy effects.