Using angle-resolved photoemission spectroscopy, we show that the recently discovered surface state on SrTiO(3) consists of nondegenerate t(2g) states with different dimensional characters. While the d(xy) bands have quasi-2D dispersions with weak k(z) dependence, the lifted d(xz)/d(yz) bands show 3D dispersions that differ significantly from bulk expectations and signal that electrons associated with those orbitals permeate the near-surface region. Like their more 2D counterparts, the size and character of the d(xz)/d(yz) Fermi surface components are essentially the same for different sample preparations. Irradiating SrTiO(3) in ultrahigh vacuum is one method observed so far to induce the "universal" surface metallic state. We reveal that during this process, changes in the oxygen valence band spectral weight that coincide with the emergence of surface conductivity are disproportionate to any change in the total intensity of the O 1s core level spectrum. This signifies that the formation of the metallic surface goes beyond a straightforward chemical doping scenario and occurs in conjunction with profound changes in the initial states and/or spatial distribution of near-E(F) electrons in the surface region.

We performed a high energy resolution ARPES investigation of over-doped Ba0.1K0.9Fe2As2 with Tc = 9 K. The Fermi surface topology of this material is similar to that of KFe2As2 and differs from that of slightly less doped Ba0.3K0.7Fe2As2, implying that a Lifshitz transition occurred between x = 0.7 and x = 0.9. Albeit for a vertical node found at the tip of the emerging off-M-centered Fermi surface pocket lobes, the superconducting gap structure is similar to that of Ba0.3K0.7Fe2As2, suggesting that the paring interaction is not driven by the Fermi surface topology.PACS numbers: 74.70. Xa, 74.25.Jb, The discovery of high-temperature superconductivity without hole Fermi surface (FS) pocket [1][2][3][4][5][6][7] in AFe 2 Se 2 (A = Tl, K, Cs, Rb) imposed severe constraints to the electron-hole quasi-nesting scenario as the main Cooper pairing force in the Fe-based superconducting (SC) materials [8] and raised fundamental questions related to the importance of their FS topology. To answer these questions, it is necessary to investigate heavily hole-doped compounds. Previous angle-resolved photoemission spectroscopy (ARPES) studies of fully hole-doped KFe 2 As 2 indicate that the FS near the M(π, 0) point is formed by small off-M-centered hole FS pocket lobes [9,10] rather than the M-centered ellipsoid-like electron FS pockets commonly observed in the other materials [8]. Interestingly, KFe 2 As 2 has a very low critical temperature (T c ) of only 3 K [11,12], which was early [9] interpreted as, and is still widely considered as, a consequence of the evolution of the FS topology. Despite their incapability to access the band structure at the M point and thus to reveal completely the SC gap structure, laser-ARPES measurements suggest a rather complicated nodal SC gap profile around the Γ point [13], which is consistent with the finite residual thermal conductivity (κ 0 /T ) of this material at low temperature [14,15].While their existence is widely accepted, the origin of the nodes in KFe 2 As 2 and their relationship with the pairing mechanism remain unclear and could possibly involve a fundamental change in the SC order parameter upon doping K from the optimal Ba 0.6 K 0.4 Fe 2 As 2 composition, for which both the ARPES [16][17][18] and thermal conductivity [19] techniques agree on a nodeless SC gap. Indeed, Tafti et al. [20] recently reported a sudden reversal in the pressure (P ) dependence of T c in KFe 2 As 2 without discontinuity in the Hall coefficient R H (P ) and in the electrical resistivity ρ(P ), which was interpreted as an evidence for a change in the order parameter incompatible with a Lifshitz transition (change in the FS topology [21]). Based on a rigid-band shift model, the Lifshitz transition corresponding to the apparition of the small off-M-centered hole FS pocket lobes was estimated to occur within the 0.8 ≤ x ≤ 0.9 doping range [22]. Determining the k-space structure of the SC gap in the vicinity of this transition is of critical importance.In this Letter, we investigate the electronic structure...

We report an angle-resolved photoemission study of the charge stripe ordered La1.6−xNd0.4SrxCuO4 system.A comparative and quantitative line shape analysis is presented as the system evolves from the overdoped regime into the charge ordered phase. On the overdoped side (x = 0.20), a normal state anti-nodal spectral gap opens upon cooling below 80 K. In this process spectral weight is preserved but redistributed to larger energies. A correlation between this spectral gap and electron scattering is found. A different lineshape is observed in the antinodal region of charge ordered Nd-LSCO x = 1/8. Significant low-energy spectral weight appears to be lost. These observations are discussed in terms of spectral weight redistribution and gapping originating from charge stripe ordering.

The iron-pnictide superconductors have a layered structure formed by stacks of FeAs planes from which the superconductivity originates. Given the multiband and quasi three-dimensional 1 (3D) electronic structure of these hightemperature superconductors, knowledge of the quasi-3D superconducting (SC) gap is essential for understanding the superconducting mechanism. By using the k z capability of angle-resolved photoemission, we completely determined the SC gap on all five Fermi surfaces (FSs) in three dimensions on Ba 0.6 K 0.4 Fe 2 As 2 samples. We found a marked k z dispersion of the SC gap, which can derive only from interlayer pairing. Remarkably, the SC energy gaps can be described by a single 3D gap function with two energy scales characterizing the strengths of intralayer ∆ 1 and interlayer ∆ 2 pairing. The anisotropy ratio ∆ 1 /∆ 2 , determined from the gap function, is close to the c-axis anisotropy ratio of the magnetic exchange coupling J c /J ab in the parent compound 2 . The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrains the possible pairing force in the pnictides. A suitable candidate could arise from short-range antiferromagnetic fluctuations.Angle-resolved photoemission spectroscopy (ARPES) has played an important role in revealing the electronic structure of the pnictides. These measurements have typically been carried out at a fixed incident photon energy (hν) and varying incident angles that map out the planar band dispersion as a function of k x and k y . Thus far, four FS sheets have been observed with two hole pockets centred around the (0, 0) point and two electron pockets around the M (π,0) point in the unfolded 2D Brillouin zone. Below the superconducting transition temperature T c , nodeless SC gaps open everywhere on the FS sheets 3-6 , pointing to a pairing order parameter with an s-wave symmetry in the a-b plane, in agreement with a number of theoretical results 7-11 . However, there are other experiments that have indicated possible nodes in the superconducting gap of some pnictides, either line nodes in the a-b plane or nodes along the c axis [12][13][14] . It is well known that on tuning the incident photon energy hν, the allowed direct transitions will shift in energy and, consequently, in the momentum perpendicular to the a-b plane (k z ), which enables the determination of the electronic dispersion along the c axis. spectral intensity measured at 10 K plotted on a false-colour scale as a function of the in-plane momentum (k ) and binding energy along -X using 46-eV photons, which corresponds to k z = 0. Two hole-like bands (α (inner) and β (outer)) are observed. b, Second derivative of the spectral intensity plot as shown in a. c, A set of EDCs within the E-k range indicated by the red rectangle in a. The red EDC is at k F of the α band. d, Second derivative plot of the dispersion along Z-R (k z = π) measured using 32-eV photons. Three hole-like bands (α (inner), α (middle) and β (outer)) are observed. 198NATURE PHYSICS |...

Using angle-resolved photoemission spectroscopy (ARPES), it is revealed that the low-energy electronic excitation spectra of highly underdoped superconducting and nonsuperconducting La(2-x)Sr(x)CuO(4) cuprates are gapped along the entire underlying Fermi surface at low temperatures. We show how the gap function evolves to a d(x(2)-y(2)) form with increasing temperature or doping, consistent with the vast majority of ARPES studies of cuprates. Our results provide essential information for uncovering the symmetry of the order parameter(s) in strongly underdoped cuprates, which is a prerequisite for understanding the pairing mechanism and how superconductivity emerges from a Mott insulator.

Ultrafast spectroscopies have become an important tool for elucidating the microscopic description and dynamical properties of quantum materials. In particular, by tracking the dynamics of non-thermal electrons, a material's dominant scattering processes -and thus the many-body interactions between electrons and collective excitations -can be revealed. Here we present a new method for extracting the electron-phonon coupling strength in the time domain, by means of time and angle-resolved photoemission spectroscopy (TR-ARPES). This method is demonstrated in graphite, where we investigate the 1 arXiv:1902.05572v3 [cond-mat.str-el] 1 Aug 2019 dynamics of photo-injected electrons at the K point, detecting quantized energyloss processes that correspond to the emission of strongly-coupled optical phonons.We show that the observed characteristic timescale for spectral-weight-transfer mediated by phonon-scattering processes allows for the direct quantitative extraction of electron-phonon matrix elements, for specific modes, and with unprecedented sensitivity.The concept of the electronic quasiparticle as proposed by Landau, i.e. the dressing of an electron with many-body and collective excitations (1), is essential to the modern understanding of condensed matter physics. Among the plethora of interactions relevant to solid-state systems, electron-phonon coupling (EPC) has been a persistent subject of great interest, related as it is to the emergence of disparate physical phenomena, from resistivity in normal metals to conventional (BCS) superconductivity and charge-ordered phases (2,3). While strong EPC is desirable in systems like BCS superconductors (4, 5), it is deleterious for conductivity in normal metals, curtailing the application of several compounds as room-temperature electronic devices (6).Given the important role of the electron-phonon interaction in relation to both conventional and quantum materials, extensive theoretical and experimental efforts have been devoted towards determining the strength and anisotropy of EPC. While ab-initio calculations are powerful, they rely on complex approximations which require precise experimental data to benchmark their validity (7). Inelastic scattering experiments -such as Raman spectroscopy (8), electron energy loss spectroscopy (9), inelastic x-ray (10), and neutron scattering (11) -are able to access EPC for specific phonon modes, yet integrated over all electronic states. Angle-resolved photoemission spectroscopy (ARPES), on the converse, can access the strength of EPC via phononmediated renormalization effects for specific momentum-resolved electronic states, as revealed by "kinks" in the electronic band dispersion (12-16). However, extraction of EPC strength from these kinks requires accurate modelling of the bare band dispersion and of the electronic

The possibility of driving phase transitions in low-density condensates through the loss of phase coherence alone has far-reaching implications for the study of quantum phases of matter. This has inspired the development of tools to control and explore the collective properties of condensate phases via phase fluctuations. Electrically gated oxide interfaces, ultracold Fermi atoms and cuprate superconductors, which are characterized by an intrinsically small phase stiffness, are paradigmatic examples where these tools are having a dramatic impact. Here we use light pulses shorter than the internal thermalization time to drive and probe the phase fragility of the BiSrCaCuO cuprate superconductor, completely melting the superconducting condensate without affecting the pairing strength. The resulting ultrafast dynamics of phase fluctuations and charge excitations are captured and disentangled by time-resolved photoemission spectroscopy. This work demonstrates the dominant role of phase coherence in the superconductor-to-normal state phase transition and offers a benchmark for non-equilibrium spectroscopic investigations of the cuprate phase diagram.

Spin-and angle-resolved photoemission spectroscopy is used to reveal that a large spin polarization is observable in the bulk centrosymmetric transition metal dichalcogenide MoS 2 . It is found that the measured spin polarization can be reversed by changing the handedness of incident circularly polarized light. Calculations based on a three-step model of photoemission show that the valley and layer-locked spinpolarized electronic states can be selectively addressed by circularly polarized light, therefore providing a novel route to probe these hidden spin-polarized states in inversion-symmetric systems as predicted by Zhang et al. [Nat. Phys. 10, 387 (2014).]. DOI: 10.1103/PhysRevLett.118.086402 Transition metal dichalcogenide (TMDC) monolayers have been heavily investigated due to the locking of the spin with valley pseudospins and the presence of a direct gap, which makes them ideal candidates for valleytronic devices [1]. Thanks to the lack of inversion symmetry and the non-negligible spin-orbit coupling, TMDC monolayers also feature well-defined spin-polarized ground states [2], which can, in principle, be investigated by spin-and angleresolved photoemission spectroscopy (spin-ARPES). However, while few ARPES studies clearly observed the indirect to direct band gap transition going from the bulk crystal to the monolayer [3-6], spin-ARPES investigation of TMDC monolayers is more challenging, given the low cross section of photoemission from single layers.ARPES measurements on the bulk system are instead less demanding, but early studies detected spin-resolved signals only from TMDCs with broken inversion symmetry [7]. Interestingly, a recent theoretical study [8] suggested that the spin texture of the TMDC could be probed by photoemission, even in the inversion symmetric bulk TMDC crystals, as a result of the localization of two spin-degenerated valence band maxima on different layers of the unit cell and of the finite penetration depth of the photoemission process probing preferentially the uppermost layer. Experimentally this effect has been observed for WSe 2 , where the spin orbit (SO) coupling is the strongest one among the TMDCs [9].Here we show that a large out-of-plane spin-polarization is observable in the bulk dichalcogenide MoS 2 , and more importantly, that its sign depends on the handedness of the incident circularly polarized light. Our calculations, based on a three step model of the photoemission process demonstrate that the observed spin reversal is an initial state effect. Using left-(C L ) and right-handed (C R ) circularly polarized light results in selecting different initial states that present a positive or negative out-of-plane spin polarization depending on their localization on the S-Mo-S layers of the 2H-stacked MoS 2 unit cell. Our findings not only highlight the locking of the spin with layer and valley pseudospins in MoS 2 but also provide a novel and improved route, other than taking profit of the inelastic mean free path (IMFP) [8], for selectively probing hidden spin-polari...