SIMES Publications

Abstract

Superconductivity emerges in proximity to a nematic phase in most iron-based superconductors. It is therefore important to understand the impact of nematicity on the electronic structure. Orbital assignment and tracking across the nematic phase transition prove to be challenging due to the multiband nature of iron-based superconductors and twinning effects. Here, we report a detailed study of the electronic structure of fully detwinned FeSe across the nematic phase transition using angle-resolved photoemission spectroscopy. We clearly observe a nematicity-driven band reconstruction involving d_xz, d_yz, and d_xy orbitals. The nematic energy scale between d_xz and d_yz bands reaches a maximum of 50 meV at the Brillouin zone corner. We are also able to track the d_xz electron pocket across the nematic transition and explain its absence in the nematic state. Our comprehensive data of the electronic structure provide an accurate basis for theoretical models of the superconducting pairing in FeSe.

Abstract

In normal metals, macroscopic properties are understood using the concept of quasiparticles. In the cuprate high-temperature superconductors, the metallic state above the highest transition temperature is anomalous and is known as the “strange metal.” We studied this state using angle-resolved photoemission spectroscopy. With increasing doping across a temperature-independent critical value pc ~ 0.19, we observed that near the Brillouin zone boundary, the strange metal, characterized by an incoherent spectral function, abruptly reconstructs into a more conventional metal with quasiparticles. Above the temperature of superconducting fluctuations, we found that the pseudogap also discontinuously collapses at the very same value of pc. These observations suggest that the incoherent strange metal is a distinct state and a prerequisite for the pseudogap; such findings are incompatible with existing pseudogap quantum critical point scenarios.

Abstract

The pseudogap, d-wave superconductivity and electron-boson coupling are three intertwined key ingredients in the phase diagram of the cuprates. Sr₂IrO₄ is a 5d-electron counterpart of the cuprates in which both the pseudogap and a d-wave instability have been observed. Here, we report spectroscopic evidence for the presence of the third key player in electron-doped Sr₂IrO₄: electron-boson coupling. A kink in nodal dispersion is observed with an energy scale of ∼50  meV. The strength of the kink changes with doping, but the energy scale remains the same. These results provide the first noncuprate platform for exploring the relationship between the pseudogap, d-wave instability, and electron-boson coupling in doped Mott insulators.

Abstract

The isovalent-substituted iron pnictide compound SrFe₂(As_1−xP_x)₂ exhibits multiple evidence for nodal superconductivity via various experimental probes, such as the penetration depth, nuclear magnetic resonance and specific heat measurements. The direct identification of the nodal superconducting (SC) gap structure is challenging, partly because the presence of nodes is not protected by symmetry but instead caused by an accidental sign change of the order parameter, and also because of the three-dimensionality of the electronic structure. We have studied the SC gaps of SrFe₂(As_0.65P_0.35)₂ in three-dimensional momentum space by synchrotron and laser-based angle-resolved photoemission spectroscopy. The three hole Fermi surfaces (FSs) at the zone center have SC gaps with different magnitudes, whereas the SC gaps of the electron FSs at the zone corner are almost isotropic and k_z-independent. As a possible nodal SC gap structure, we propose that the SC gap of the outer hole FS changes sign around the Z-X [(0, 0, 2π) − (π, π, 2π)] direction.

Abstract

Oxide materials are important candidates for the next generation of electronics due to a wide array of desired properties, which they can exhibit alone or when combined with other materials. While SrTiO₃ (STO) is often considered a prototypical oxide, it, too, hosts a wide array of unusual properties, including a 2-dimensional electron gas (2DEG), which can form at the surface when exposed to ultraviolet (UV) light. Using layer-by-layer growth of high-quality STO films, we show that the 2DEG only forms with the SrO termination and not with the TiO₂ termination, contrary to expectation. This dichotomy of the observed angle-resolved photoemission spectroscopy (ARPES) spectra is similarly seen in BaTiO₃ (BTO), in which the 2DEG is only observed for BaO-terminated films. These results will allow for a deeper understanding and better control of the electronic structure of titanate films, substrates, and heterostructures.

Abstract

The momentum dependence of the nematic order parameter is an important ingredient in the microscopic description of iron-based high-temperature superconductors. While recent reports on FeSe indicate that the nematic order parameter changes sign between electron and hole bands, detailed knowledge is still missing for other compounds. Combining angle-resolved photoemission spectroscopy with uniaxial strain tuning, we measure the nematic band splitting in both FeSe and  BaFe₂As₂ without interference from either twinning or magnetic order. We find that the nematic order parameter exhibits the same momentum dependence in both compounds with a sign change between the Brillouin center and the corner. This suggests that the same microscopic mechanism drives the nematic order in spite of the very different phase diagrams.

Abstract

We study the microscopic origins of photocurrent generation in the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ via time- and angle-resolved photoemission spectroscopy. We image the unoccupied band structure as it evolves following a circularly polarized optical excitation and observe an asymmetric electron population in momentum space, which is the spectroscopic signature of a photocurrent. By analyzing the rise times of the population we identify which occupied and unoccupied electronic states are coupled by the optical excitation. We conclude that photocurrents can only be excited via resonant optical transitions coupling to spin-orbital textured states. Our work provides a microscopic understanding of how to control photocurrents in systems with spin-orbit coupling and broken inversion symmetry.

Abstract

We studied the electronic structure of the heavy-fermion compound Yb(Ru_1−xRh_x)₂Ge₂ with x=0 and nominally x=0.125 using ARPES and LDA calculations. We find a valence band structure of Yb corresponding to a noninteger valence close to 3+. The three observed crystal electric field levels with a splitting of 32 and 75 meV confirm the suggested configuration with a quasiquartet ground state. The experimentally determined band structure of the conduction electrons with predominantly Ru 4d character is well reproduced by our calculations. YbRu₂Ge₂ undergoes a nonmagnetic phase transition into a ferroquadrupolar ordered state below 10.2 K and then to an antiferromagnetically ordered state below 6.5 K. A small hole Fermi surface shows nesting features in our calculated band structure and its size determined by ARPES is close to the magnetic ordering wave vector found in neutron scattering. The transitions are suppressed when YbRu₂Ge₂ is doped with 12.5% Rh. The electron doping leads to a shift of the band structure and successive Lifshitz transitions.

Abstract

We study the band structure of twinned and detwinned BaFe₂As₂ using angle-resolved photoemission spectroscopy. The combination of measurements in the ordered and normal states along four high-symmetry momentum directions Γ/Z–X/Y enables us to identify the complex reconstructed band structure in the ordered state in great detail. We clearly observe the nematic splitting of the d_xz and d_yz orbitals as well as folding due to magnetic order with a wave vector of (π,π,π). We are able to assign all observed bands. In particular we suggest an assignment of the electron bands different from previous reports. The high-quality spectra allow us to achieve a comprehensive understanding of the band structure of BaFe₂As₂.

Abstract

We study the band structure of twinned and detwinned BaFe2As2 using angle-resolved photoemission spectroscopy. The combination of measurements in the ordered and normal states along four high-symmetry momentum directions Γ/Z−X/Y enables us to identify the complex reconstructed band structure in the ordered state in great detail. We clearly observe the nematic splitting of the dxz and dyzorbitals as well as folding due to magnetic order with a wave vector of (π,π,π). We are able to assign all observed bands. In particular we suggest an assignment of the electron bands different from previous reports. The high-quality spectra allow us to achieve a comprehensive understanding of the band structure of BaFe2As2.