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Semimetallic ${\mathrm{MoTe}}_{2}$ has recently generated enormous attention due to its topological properties, large magnetoresistance, superconductivity, suitability for homojunction phase patterning, and, in particular, metal-insulator transition of thin layers, possibly indicating a quantum spin hall state. These observations prove the potential of ${\mathrm{MoTe}}_{2}$ for thin film applications and call for systematic investigations of the thickness dependent electronic structure. Here we apply angle-resolved photoemission spectroscopy supported by band structure calculations to elucidate the electronic structure of exfoliated $1{\mathrm{T}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoTe}}_{2}$. Electron and hole pockets of the inverted conduction and valence bands near $\mathrm{\ensuremath{\Gamma}}$ are resolved down to the monolayer. The Fermi level of exfoliated $1{\mathrm{T}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoTe}}_{2}$ monolayers lays within the electron pockets indicating intrinsic $n$-type doping. ${E}_{F}$ shifts downward with increasing thickness consistent with a surface driven mechanism. Our study provides insight on the electronic properties of semimetallic $1{\mathrm{T}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoTe}}_{2}$ as an indispensable ingredient for future thin film functionalization.

The long search for materials carrying quantum spin liquid states has recently led to $\ensuremath{\alpha}$-RuCl${}_{3}$, a layered honeycomb Mott insulator with substantial spin-orbit coupling. Often, controlled doping of materials with strong 2D character generates new exotic properties as well as a better understanding of the parent compound. This paper shows that the intercalation compound K${}_{0.5}$RuCl${}_{3}$ sustains a peculiar charge disproportionation into formally Ru${}^{2+}$ and Ru${}^{3+}$. Every Ru${}^{3+}$ with one hole in the t${}_{2g}$ shell is surrounded by Ru${}^{2+}$ where the t${}_{2g}$ level is full and magnetically inert. Thus each type of Ru sites forms a triangular lattice and nearest-neighbor interactions of the original honeycomb are blocked.

Kitaev-type interactions between neighbouring magnetic moments emerge in the honeycomb material ${\alpha}$-RuCl3. It is debated however whether these Kitaev interactions are ferromagnetic or antiferromagnetic. With electron energy loss spectroscopy (EELS) we study the lowest excitation across the Mott-Hubbard gap, which involves a d4 triplet in the final state and therefore is sensitive to nearest-neighbor spin-spin correlations. At low temperature the spectral weight of these triplets is strongly enhanced, in accordance with optical data. We show that the magnetic correlation function that determines this EELS spectral weight is directly related to a Kitaev-type spin-spin correlator and that the temperature dependence agrees very well with the results of a microscopic magnetic Hamiltonian for ${\alpha}$-RuCl3 with ferromagnetic Kitaev coupling., Comment: 5 pages, 3 figures

Novel ground states might be realized in honeycomb lattices with strong spin-orbit coupling. Here we study the electronic structure of α-RuCl_{3}, in which the Ru ions are in a d^{5} configuration and form a honeycomb lattice, by angle-resolved photoemission, x-ray photoemission, and electron energy loss spectroscopy supported by density functional theory and multiplet calculations. We find that α-RuCl_{3} is a Mott insulator with significant spin-orbit coupling, whose low energy electronic structure is naturally mapped onto J_{eff} states. This makes α-RuCl_{3} a promising candidate for the realization of Kitaev physics. Relevant electronic parameters such as the Hubbard energy U, the crystal field splitting 10 Dq, and the charge transfer energy Δ are evaluated.

Here we apply high resolution angle-resolved photoemission spectroscopy (ARPES) using a wide excitation energy range to probe the electronic structure and the Fermi surface topology of the Ba 1− x K x Fe 2 As 2 ( T c = 32 K) superconductor. We find significant deviations in the low energy band structure from that predicted in calculations. A set of Fermi surface sheets with unexpected topology is detected at the Brillouin zone boundary. At the X -symmetry point the Fermi surface is formed by a shallow electron-like pocket surrounded by four hole-like pockets elongated in Γ − X and Γ − Y directions.

While the pronounced doping dependence of the quasiparticle spectral weight in the antinodal region of the superconducting cuprates, as seen by ARPES, unambiguously points to the magnetic origin of the strong electron-boson coupling there, the nature of the electron scattering in the nodal direction remained unclear. Here we present a short review of our recent detailed investigations of the nodal direction of Bi-2212. Our findings prove the existence of well defined quasiparticles even in the pseudogap state and show that the essential part of the quasiparticle scattering rate, which appears on top of Auger-like electron-electron interaction, also implies a magnetic origin., 6 revtex pages, short review, presented at SNS 2004

We report studies of the electronic structure and elementary excitations of doped and undoped cuprate chains, ladders and planes. Using high energy spectroscopies such as x-ray absorption, core level photoemission and angle resolved photoemission spectroscopy, important information regarding the charge distribution and hole dynamics can be obtained. The comparison of the experimental data with suitable theoretical models sets constraints on the parameters entering into the model calculations, and offers insight into the important physical quantities governing the electronic structure of these materials. Recurring themes include the importance of the dimensionality of the Cu-O network (1D->2D) and the crucial role played by the spin-background in determining the dynamics of low-lying excitations in these strongly correlated systems., J. Electron Spec. Relat. Phenom.: special issue on electron correlation, in press

We investigate the low energy electronic structure of Ba1-xKxFe2As2 (x=0; 0.3, T_{c}=32 K) single crystals by angle-resolved photoemission spectroscopy with a focus on the renormalization of the dispersion. A kink feature is detected at E approximately 25 meV for the doped compound which vanishes at T=200 K but stays virtually constant when T_{c} is crossed. Our experimental findings rule out the magnetic resonance mode as the origin of the kink and render conventional electron-phonon coupling unlikely. They put stringent restrictions on the dominant source of the electronic interaction channel.

We have investigated the low-energy electronic structure of the heavy-fermion superconductor ${\text{CeCoIn}}_{5}$ by angle-resolved photoemission and band-structure calculations. We measured the Fermi surface and energy distribution maps along the high-symmetry directions at $h\ensuremath{\nu}=100\text{ }\text{eV}$ and $T=25\text{ }\text{K}$. The compound has quasi-two-dimensional Fermi-surface sheets centered at the $M\text{\ensuremath{-}}A$ line of the Brillouin zone. The band-structure calculations have been carried out within the local-density approximation where the $4f$ electrons have been treated either localized or itinerant. We discuss the comparison to the experimental data and the implications for the nature of the $4f$ electrons at the given temperature.

Some heavy fermion materials show so-called hidden-order phases which are invisible to many characterization techniques and whose microscopic origin remained controversial for decades. Among such hidden-order compounds, CeB6 is of model character due to its simple electronic configuration and crystal structure. Apart from more conventional antiferromagnetism, it shows an elusive phase at low temperatures, which is commonly associated with multipolar order. Here we show that this phase roots in a Fermi surface instability. This conclusion is based on a full 3D tomographic sampling of the electronic structure by angle-resolved photoemission and comparison with inelastic neutron scattering data. The hidden order is mediated by itinerant electrons. Our measurements will serve as a paradigm for the investigation of hidden-order phases in f-electron systems, but also generally for situations where the itinerant electrons drive orbital or spin order., In compounds containing 4f and 5f elements, hidden-order phases exist which are undetectable by many methods, the origins of which are debated. Here, the authors use photoemission and neutron scattering methods to show how such a multipolar-ordered phase emerges due to Fermi surface instability in CeB6.

Angle-resolved photoemission spectroscopy (ARPES) provides a detailed view of the renormalized band structure and, consequently, is a key to the self-energy and the single-particle Green's function. Here, we summarize the ARPES data accumulated over the whole Brillouin zone for the optimally doped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8\ensuremath{-}\ensuremath{\delta}}$ into a parametric model of the Green's function, which we use for calculating the itinerant component of the dynamic spin susceptibility in absolute units with many-body effects taken into account. By comparison with inelastic neutron scattering (INS) data, we show that the itinerant component of the spin response can account for the integral intensity of the experimental INS spectrum. Taking into account the bilayer splitting, we explain the magnetic resonances in the acoustic (odd) and optic (even) INS channels.

In this contribution we first present a short introduction into the lattice structure, the phase diagram and the electronic structure of high-Tc superconduc- tors. Then we explain the principles of angle-resolved photoemission spectroscopy (ARPES) and the influence of the dressing of the charge carriers, which is normally described by the complex self-energy function, on the spectral function in the nor- mal and the superconducting state. Finally we review our recent ARPES results on high-Tc superconductors at various k-points in the Brillouin zone near the Fermi surface. Information on the dressing of the charge carriers, i.e., on the effective mass and the scattering length, is obtained as a function of doping concentration, tem- perature, momentum and energy. The strong renormalization of the bandstructure due to the dressing can be explained in terms of a coupling to a continuum of spin fluctuations and in the superconducting state by an additional coupling to a triplet exciton excitation. Possibly, this dressing is related to the glue for the pair formation in cuprate superconductors.

We show that in the angle resolved photoemission spectroscopy (ARPES) near-surface induced fields can be useful for disentangling the surface and bulk related emission. The jump of the dielectric function at the interface results in a nonzero term $\operatorname{div}\textbf{A}$ in the photoemission matrix element. The term happens to be significant approximately within the first unit cell and leads to the circular dichroism for the states localized therein. As an example we use ARPES spectra of an YBa$_2$Cu$_3$O$_{7-\delta}$ crystal to distinguish between the overdoped surface related component and its bulk counterparts., Comment: 4 pages, 2 figures

Angle-resolved photoemission spectroscopy (ARPES) is used to study the spectral function of the optimally doped high-${T}_{c}$ superconductor $(\mathrm{Bi},\mathrm{Pb}{)}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ in the vicinity of the antinodal point in the superconducting state. Using a parametrized self-energy function, it was possible to describe both the coherent and the incoherent spectral weight of the bonding and the antibonding band. The renormalization effects can be assigned to a very strong coupling to the magnetic resonance mode and at higher energies to a bandwidth renormalization by a factor of two, probably caused by a coupling to a continuum. The present reevaluation of the ARPES data allows one to come to a more reliable determination of the value of the coupling strength of the charge carriers to the mode. The experimental results for the dressing of the charge carriers are compared to theoretical models.

Angle resolved photoemission spectroscopy (ARPES) provides a detailed view of the renormalized band structure in cuprates and, consequently, is a key to the self-energy and the quasiparticle Green's function. Such information gives a clue to the comparison of ARPES with scanning tunneling microscopy, inelastic neutron scattering (INS), and Raman scattering data. Here we touch on a potential possibility of such a comparison with the dynamical magnetic susceptibility measured in INS experiments. Calculations based on the experimentally measured quasiparticle self-energies in cuprates lead to the estimated magnetic susceptibility response with many-body effects taken into account., Will be presented at the M2S-HTSC-VIII conference in Dresden

We apply the new-generation angle-resolved photoemission spectroscopy methodology to the most widely studied cuprate superconductor YBa2Cu3O(6+x). Considering the nodal direction, we found noticeable renormalization effects known as kinks both in the quasiparticle dispersion and scattering rate, the bilayer splitting, and evidence for strong interband scattering--all the characteristic features of the nodal quasiparticles detected earlier in Bi2Sr2CaCu2O(8+delta). The typical energy scale and the doping dependence of the kinks clearly point to their intimate relation with the spin-1 resonance seen in the neutron scattering experiments. Our findings strongly suggest a universality of the electron dynamics in the bilayer superconducting cuprates and a dominating role of the spin fluctuations in the formation of the quasiparticles along the nodal direction.

We use angle-resolved photoemission with circularly polarized excitation to demonstrate that in the 5x1 superstructure-free Pb-Bi2212 material there are no signatures of time-reversal symmetry breaking in the sense of the criteria developed earlier (Kaminski et al. Nature {\bf 416}, 610 (2002)). In addition to the existing technique, we suggest and apply an independent experimental approach to prove the absence of the effect in the studied compounds. The dichroic signal retains reflection antisymmetry as a function of temperature and doping and in all mirror planes, precisely defined by the experimental dispersion at low energies. The obtained results demonstrate that the signatures of time-reversal symmetry violation in pristine Bi2212, as determined by ARPES, are not a universal feature of all cuprate superconductors., 4.5 RevTEX pages, 4 figures, after referees

Angle-resolved photoemission spectroscopy (ARPES) is used to study the doping dependence of the lifetime and the mass renormalization of the low-energy excitations in high-${T}_{c}$ cuprate $(\mathrm{B}\mathrm{i},\mathrm{P}\mathrm{b}{)}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ along the zone diagonal. We find a linear energy dependence of the scattering rate for the underdoped samples and a quadratic energy dependence for the overdoped case. The mass enhancement of the quasiparticles due to the many body effects at the Fermi energy is found to be in the order of 2 and the renormalization extends over a large energy range for both the normal and the superconducting state. The much discussed kink in the dispersion around 70 meV is interpreted as a small additional effect at low temperatures.

One of the most remarkable properties of the high-temperature superconductors is a pseudogap regime appearing in the underdoped cuprates above the superconducting transition temperature T_c. The pseudogap continously develops out of the superconducting gap. In this paper, we demonstrate by means of a detailed comparison between theory and experiment that the characteristic change of quasiparticle dispersion in crossing T_c in the underdoped cuprates can be understood as being due to phase fluctuations of the superconducting order parameter. In particular, we show that within a phase fluctuation model the characteristic back-turning BCS bands disappear above T_c whereas the gap remains open. Furthermore, the pseudogap rather has a U-shape instead of the characteristic V-shape of a d_{x^2-y^2}-wave pairing symmetry and starts closing from the nodal k=(pi/2,pi/2) directions, whereas it rather fills in at the anti-nodal k=(pi,0) regions, yielding further support to the phase fluctuation scenario., 6 pages, 4 eps-figures

Studying the nodal quasiparticles in superconducting cuprates by photoemission with highly improved momentum resolution, we show that a new "kink" feature in the scattering rate is a key to uncover the nature of electron correlations in these compounds. Our data provide evidence that the main doping independent contribution to the scattering can be well understood in terms of the conventional Fermi liquid model, while the additional doping dependent contribution has a magnetic origin. This sheds doubt on applicability of a phonon-mediated pairing mechanism to high temperature superconductors., Comment: rextex, 4 pages, 4 figures

We show that the photoelectron spectrum of YBCO-123 generally includes two components. The one, which is the most pronounced, is characterized by unusually high hole doping level and exhibits no superconducting gap, while the other component retains superconductivity. We believe, the occurrence of these two components in ARPES spectra was one of the reasons that caused difficulties with the interpretation of earlier ARPES data of this compound.

The Cu substitution by Zn and Ni impurities and its influence on the mass renormalization effects in angle resolved photoelectron spectra (ARPES) of Bi 2 Sr 2 CaCu 2 O 8+ δ is addressed. We show that the nonmagnetic Zn atoms have much stronger effect both in nodal and antinodal parts of the Brillouin zone than magnetic Ni. The observed changes are consistent with the behaviour of the spin resonance mode as seen by inelastic neutron scattering in YBCO-123. This strongly suggests that the “peak-dip-hump” and the “kink” in ARPES on the one side and neutron resonance on the other are closely related features.