High-resolution Angle-resolved Photoemission Spectroscopy Study Research Articles

Electronic structure and effective mass of pristine and Cl-doped CsPbBr3

Organic–inorganic lead halide perovskites (LHPs) have attracted great interest owing to their outstanding optoelectronic properties. Typically, the underlying electronic structure would determinate the physical properties of materials. But as for now, limited studies have been done to reveal the underlying electronic structure of this material system, comparing to the huge amount of investigations on the material synthesis. The effective mass of the valance band is one of the most important physical parameters which plays a dominant role in charge transport and photovoltaic phenomena. In pristine CsPbBr3, the Fröhlich polarons associated with the Pb–Br stretching modes are proposed to be responsible for the effective mass renormalization. In this regard, it would be very interesting to explore the electronic structure in doped LHPs. Here, we report high-resolution angle-resolved photoemission spectroscopy (ARPES) studies on both pristine and Cl-doped CsPbBr3. The experimental band dispersions are extracted from ARPES spectra along both Γ¯ – M¯ – Γ¯ and X¯ – M¯ – X¯ high symmetry directions. DFT calculations are performed and directly compared with the ARPES data. Our results have revealed the band structure of Cl-doped CsPbBr3 for the first time, which have also unveiled the effective mass renormalization in the Cl-doped CsPbBr3 compound. Doping dependent measurements indicate that the chlorine doping could moderately tune the renormalization strength. These results will help understand the physical properties of LHPs as a function of doping.

Observation of momentum-dependent charge density wave gap in a layered antiferromagnet {textrm{Gd}}{textrm{Te}}_{3}

Charge density wave (CDW) ordering has been an important topic of study for a long time owing to its connection with other exotic phases such as superconductivity and magnetism. The R{textrm{Te}}_{3} (R = rare-earth elements) family of materials provides a fertile ground to study the dynamics of CDW in van der Waals layered materials, and the presence of magnetism in these materials allows to explore the interplay among CDW and long range magnetic ordering. Here, we have carried out a high-resolution angle-resolved photoemission spectroscopy (ARPES) study of a CDW material {textrm{Gd}}{textrm{Te}}_{3}, which is antiferromagnetic below sim mathrm {12~K}, along with thermodynamic, electrical transport, magnetic, and Raman measurements. Our ARPES data show a two-fold symmetric Fermi surface with both gapped and ungapped regions indicative of the partial nesting. The gap is momentum dependent, maximum along {overline{Gamma }}-mathrm{overline{Z}} and gradually decreases going towards {overline{Gamma }}-mathrm{overline{X}}. Our study provides a platform to study the dynamics of CDW and its interaction with other physical orders in two- and three-dimensions.

Flat band in hole-doped transition metal dichalcogenide observed by angle-resolved photoemission spectroscopy

Layered transition metal dichalcogenides (TMDCs) gained widespread attention because of their electron-correlation-related physics, such as charge density wave (CDW), superconductivity, etc. In this paper, we report the high-resolution angle-resolved photoemission spectroscopy (ARPES) studies on the electronic structure of Ti-doped 1T-Ti x Ta1–x S2 with different doping levels. We observe a flat band that originates from the formation of the star of David super-cell at the x = 5% sample at the low temperature. With the increasing Ti doping levels, the flat band vanishes in the x = 8% sample due to the extra hole carrier. We also find the band shift and variation of the CDW gap caused by the Ti-doping. Meanwhile, the band folding positions and the CDW vector q CDW are intact. Our ARPES results suggest that the localized flat band and the correlation effect in the 1T-TMDCs could be tuned by changing the filling factor through the doping electron or hole carriers. The Ti-doped 1T-Ti x Ta1–x S2 provides a platform to fine-tune the electronic structure evolution and a new insight into the strongly correlated physics in the TMDC materials.

Pseudogap suppression by competition with superconductivity in La-based cuprates

We carried out a comprehensive high-resolution angle-resolved photoemission spectroscopy (ARPES) study of the pseudogap interplay with superconductivity in La-based cuprates. The three systems La2−xSrxCuO4, La1.6−xNd0.4SrxCuO4, and La1.8−xEu0.2SrxCuO4 display slightly different pseudogap critical points in the temperature versus doping phase diagram. We studied the pseudogap evolution into the superconducting state for doping concentrations just below the critical point. In this setting, near optimal doping for superconductivity and in the presence of the weakest possible pseudogap, we uncover how the pseudogap is partially suppressed inside the superconducting state. This conclusion is based on the direct observation of a reduced pseudogap energy scale and re-emergence of spectral weight suppressed by the pseudogap. Altogether these observations suggest that the pseudogap phenomenon in La-based cuprates is in competition with superconductivity for antinodal spectral weight.Received 27 April 2022Accepted 30 August 2022DOI:https://doi.org/10.1103/PhysRevResearch.4.043015Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectronic structurePhysical SystemsCupratesTechniquesAngle-resolved photoemission spectroscopyCondensed Matter, Materials & Applied Physics

Anomalies in the temperature evolution of Dirac states in the topological crystalline insulator SnTe

Discovery of topologically protected surface states, believed to be immune to weak disorder and thermal effects, opened up a new avenue to reveal exotic fundamental science and advanced technology. While time-reversal symmetry plays the key role in most such materials, the bulk crystalline symmetries such as mirror symmetry preserve the topological properties of topological crystalline insulators (TCIs). It is apparent that any structural change may alter the topological properties of TCIs. To investigate this relatively unexplored landscape, we study the temperature evolution of the Dirac fermion states in an archetypical mirror-symmetry protected TCI, SnTe employing high-resolution angle-resolved photoemission spectroscopy and density functional theory studies. Experimental results reveal a perplexing scenario; the bulk bands observed at 22 K move nearer to the Fermi level at 60 K and again shift back to higher binding energies at 120 K. The slope of the surface Dirac bands at 22 K becomes smaller at 60 K and changes back to a larger value at 120 K. Our results from the first-principles calculations suggest that these anomalies can be attributed to the evolution of the hybridization physics with complex structural changes induced by temperature. In addition, we discover drastically reduced intensity of the Dirac states at the Fermi level at high temperatures may be due to complex evolution of anharmonicity, strain, etc. These results address robustness of the topologically protected surface states due to thermal effects and emphasize importance of covalency and anharmonicity in the topological properties of such emerging quantum materials.

Oxide Fermi liquid universality revealed by electron spectroscopy

We present a combined soft x-ray and high-resolution vacuum-ultraviolet angle-resolved photoemission spectroscopy study of the electron-overdoped cuprate Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_4$ (PLCCO). Demonstration of its highly two-dimensional band structure enabled precise determination of the in-plane self-energy dominated by electron-electron scattering. Through analysis of this self-energy and the Fermi-liquid cut-off energy scale, we find -- in contrast to hole-doped cuprates -- a momentum isotropic and comparatively weak electron correlation in PLCCO. Yet, the self-energies extracted from multiple oxide systems combine to demonstrate a logarithmic divergent relation between the quasiparticle scattering rate and mass. This constitutes a spectroscopic version of the Kadowaki-Woods relation with an important merit -- the demonstration of Fermi liquid quasiparticle lifetime and mass being set by a single energy scale.

Temperature insensitivity of Fermi surface in electron-doped high-temperature FeSe films on SrTiO3

We have performed high-resolution angle-resolved photoemission spectroscopy study of electron-doped multilayer FeSe films on SrTiO3. We determined the evolution of the electronic structure in a wide temperature range and found temperature-insensitive nature of the Fermi surface in contrast to the strong temperature dependence observed in non-doped FeSe. Such a striking doping dependence would provide a key to understanding the origin of unconventional fermiology and physical properties in FeSe.

Dirac state in a centrosymmetric superconductor α−PdBi2

Topological superconductor (TSC) hosting Majorana fermions has been established as a milestone that may shift our scientific trajectory from research to applications in topological quantum computing. Recently, superconducting Pd-Bi binaries have attracted great attention as a possible medium for the TSC phase as a result of their large spin-orbit coupling strength. Here, we report a systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) study on the normal state electronic structure of superconducting $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{PdBi}}_{2}\phantom{\rule{4pt}{0ex}}({T}_{c}=1.7$ K). Our results show the presence of Dirac states at higher-binding energy with the location of the Dirac point at 1.26 eV below the chemical potential at the zone center. Furthermore, the ARPES data indicate multiple band crossings at the chemical potential, consistent with the metallic behavior of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{PdBi}}_{2}$. Our detailed experimental studies are complemented by first-principles calculations, which reveal the presence of surface Rashba states residing in the vicinity of the chemical potential. The obtained results provide an opportunity to investigate the relationship between superconductivity, topology, and the Majorana fermion, as well as explore pathways to possible future platforms for topological quantum computing.

Electronic structure of FeS

Here we report the electronic structure of FeS, a recently identified iron-based superconductor. Our high-resolution angle-resolved photoemission spectroscopy studies show two hole-like ($\alpha$ and $\beta$) and two electron-like ($\eta$ and $\delta$) Fermi pockets around the Brillouin zone center and corner, respectively, all of which exhibit moderate dispersion along $k_z$. However, a third hole-like band ($\gamma$) is not observed, which is expected around the zone center from band calculations and is common in iron-based superconductors. Since this band has the highest renormalization factor and is known to be the most vulnerable to defects, its absence in our data is likely due to defect scattering --- and yet superconductivity can exist without coherent quasiparticles in the $\gamma$ band. This may help resolve the current controversy on the superconducting gap structure of FeS. Moreover, by comparing the $\beta$ bandwidths of various iron chalcogenides, including FeS, FeSe$_{1-x}$S$_x$, FeSe, and FeSe$_{1-x}$ Te$_x$, we find that the $\beta$ bandwidth of FeS is the broadest. However, the band renormalization factor of FeS is still quite large, when compared with the band calculations, which indicates sizable electron correlations. This explains why the unconventional superconductivity can persist over such a broad range of isovalent substitution in FeSe$_{1-x}$Te$_{x}$ and FeSe$_{1-x}$S$_{x}$.

Shifts and Splittings of the Hole Bands in the Nematic Phase of FeSe

We report a high-resolution laser-based angle-resolved photoemission spectroscopy (laser-ARPES) study of single crystals of FeSe, focusing on the temperature-dependence of the hole-like bands around the ${\rm \Gamma}$ point. As the system cools through the tetragonal-orthorhombic "nematic" structural transition at 90~K, the splitting of the $d_{xz}$/$d_{yz}$ bands is observed to increase by a magnitude of 13 meV. Moreover, the onset of a $\sim$10 meV downward shift of the $d_{xy}$ band is also at 90~K. These measurements provide clarity on the nature, magnitude and temperature-dependence of the band shifts at the ${\rm \Gamma}$ point in the nematic phase of FeSe.

Observation of the spin-polarized surface state in a noncentrosymmetric superconductor BiPd.

Recently, noncentrosymmetric superconductor BiPd has attracted considerable research interest due to the possibility of hosting topological superconductivity. Here we report a systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES study of the normal state electronic and spin properties of BiPd. Our experimental results show the presence of a surface state at higher-binding energy with the location of Dirac point at around 700 meV below the Fermi level. The detailed photon energy, temperature-dependent and spin-resolved ARPES measurements complemented by our first-principles calculations demonstrate the existence of the spin-polarized surface states at high-binding energy. The absence of such spin-polarized surface states near the Fermi level negates the possibility of a topological superconducting behaviour on the surface. Our direct experimental observation of spin-polarized surface states in BiPd provides critical information that will guide the future search for topological superconductivity in noncentrosymmetric materials.

Electronic structure of the titanium-based oxypnictide superconductorBa0.95Na0.05Ti2Sb2Oand direct observation of its charge density wave order

${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$ is a titanium-based oxypnictide superconductor with possible density wave order. We have performed high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy (STM) studies on ${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$. Our STM studies find a charge density wave (CDW) order with the wave vector of ($\ensuremath{\pi}, \ensuremath{\pi}$). The electronic structure shows both multiorbital and three-dimensional nature, consistent with the theoretical calculations. The observed Fermi surfaces are well nested along the ($\ensuremath{\pi}, \ensuremath{\pi}$) direction, which might drive the CDW transition. Likely due to the weak strength of the CDW order, we do not observe a leading-edge gap. Instead, we observe an overall spectral weight enhancement with decreasing temperature, but the rate of this enhancement is abruptly reduced or diminished in the CDW state, suggesting the suppression of spectral weight or opening of a partial CDW gap. Our results give a comprehensive picture of the electronic structure and direct observation of the CDW order in ${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$.

Fermi surface topology and hot spot distribution in the Kondo lattice systemCeB6

We present high-resolution angle-resolved photoemission spectroscopy studies of trivalent CeB6 and divalent BaB6 rare-earth hexaborides. We find that the Fermi surface electronic structure of CeB6 consists of large oval-shape pockets around the X points of the Brillouin zone, while the states around the zone centre 'Gamma' point are strongly renormalized. Our first-principles calculations agree with data around the X points, but not at the 'Gamma' points, indicating areas of strong renormalization located around 'Gamma'. The Ce quasi-particle states participate in formation of hotspots at the Fermi surface, while the incoherent f states hybridize and lead to the emergence of dispersive features absent in non-f counterpart BaB6. These experimental and theoretical results provide a new understanding of rare-earth hexaboride materials.

Suppression of orbital ordering by chemical pressure inFeSe1−xSx

We report a high-resolution angle-resolved photo-emission spectroscopy study of the evolution of the electronic structure of FeSe1-xSx single crystals. Isovalent S substitution onto the Se site constitutes a chemical pressure which subtly modifies the electronic structure of FeSe at high temperatures and induces a suppression of the tetragonal-symmetry-breaking structural transition temperature from 87K to 58K for x=0.15. With increasing S substitution, we find smaller splitting between bands with dyz and dxz orbital character and weaker anisotropic distortions of the low temperature Fermi surfaces. These effects evolve systematically as a function of both S substitution and temperature, providing strong evidence that an orbital ordering is the underlying order parameter of the structural transition in FeSe1-xSx. Finally, we detect the small inner hole pocket for x=0.12, which is pushed below the Fermi level in the orbitally-ordered low temperature Fermi surface of FeSe.

Observation of strong electron pairing on bands without Fermi surfaces in LiFe(1-x)CoxAs.

In conventional BCS superconductors, the quantum condensation of superconducting electron pairs is understood as a Fermi surface instability, in which the low-energy electrons are paired by attractive interactions. Whether this explanation is still valid in high-Tc superconductors such as cuprates and iron-based superconductors remains an open question. In particular, a fundamentally different picture of the electron pairs, which are believed to be formed locally by repulsive interactions, may prevail. Here we report a high-resolution angle-resolved photoemission spectroscopy study on LiFe(1-x)CoxAs. We reveal a large and robust superconducting gap on a band sinking below the Fermi level on Co substitution. The observed Fermi-surface-free superconducting order is also the largest over the momentum space, which rules out a proximity effect origin and indicates that the order parameter is not tied to the Fermi surface as a result of a surface instability.

Ytterbium-driven strong enhancement of electron-phonon coupling in graphene

We present high-resolution angle-resolved photoemission spectroscopy study in conjunction with first principles calculations to investigate how the interaction of electrons with phonons in graphene is modified by the presence of Yb. We find that the transferred charges from Yb to the graphene layer hybridize with the graphene $\pi$ bands, leading to a strong enhancement of the electron-phonon interaction. Specifically, the electron-phonon coupling constant is increased by as much as a factor of 10 upon the introduction of Yb with respect to as grown graphene ($\leq$0.05). The observed coupling constant constitutes the highest value ever measured for graphene and suggests that the hybridization between graphene and the adatoms might be a critical parameter in realizing superconducting graphene.

High-resolution angle-resolved photoemission spectroscopy study of monolayer and bilayer graphene on the C-face of SiC

High-energy and $k$-space resolution angle-resolved photoemission spectroscopy experiments were achieved on nominally single and bilayer graphene grown by Si-flux assisted molecular beam epitaxy (MBE) on the C-face of SiC. This material shows the same structure as the graphene grown by standard high-temperature annealing of SiC, noticeably the rotational disorder and the very weak electronic coupling between stacked layers. The SiC substrate induces a strong doping by charge transfer, with a Dirac point located 320 meV below the Fermi level for monolayer graphene. The efficient screening by the successive graphene layers results in a reduction of this value to 190 meV for bilayer graphene. The opening of an energy band gap, whose width is inversely dependent on the thickness, is also reported. These measurements emphasize the potentialities of the Si-flux assisted MBE technique, more particularly for homogeneous low thickness graphene growth on the C-face of SiC.

Observation of an isotropic superconducting gap at the Brillouin zone centre of Tl0.63K0.37Fe1.78Se2

We performed a high-resolution angle-resolved photoemission spectroscopy study on superconducting (SC) Tl0.63K0.37Fe1.78Se2 (Tc = 29 K) in the whole Brillouin zone (BZ). In addition to a nearly isotropic ∼8.2 meV 2-dimensional (2D) SC gap (2Δ/kBTc ∼ 7) on quasi-2D electron Fermi surfaces (FSs) located around M(π,0,0)-A(π,0,π), we observe a ∼6.2 meV isotropic SC gap (2Δ/kBTc ∼ 5) on the Z-centred electron FS that rules out any d-wave pairing symmetry and rather favors an s-wave symmetry. All isotropic SC gap amplitudes can be fit by a single-gap function derived from a local strong-coupling approach suggesting an enhancement of the next-nearest neighbor exchange interaction in the ferrochalcogenide superconductors.

Coupling parameters of many-body interactions for the Al(100) surface state: A high-resolution angle-resolved photoemission spectroscopy study

We examined the dimensionless coupling parameters of many-body interactions for a free-electron-like surface-derived state in Al(100) by means of high-resolution angle-resolved photoemission spectroscopy. A kink structure was found to exist in the energy-band dispersion near the Fermi level (${E}_{F}$), which was attributed to the electron-phonon interaction. At 50 K, the coupling parameters of the electron-phonon and electron-electron interactions were estimated as ${\ensuremath{\lambda}}_{ep}=0.67\ifmmode\pm\else\textpm\fi{}0.05$ and ${\ensuremath{\lambda}}_{ee}\ensuremath{\sim}0.003$, respectively, indicating that the effective mass enhancement was mainly derived from the electron-phonon interaction. The temperature dependence of the kink structure, as measured by ${\ensuremath{\lambda}}_{ep}(T)$, was consistent with a theoretical calculation based on the Eliashberg function. A quasiparticle peak with a width of $15--20$ meV was found near ${E}_{F}$, which was explained well by the simulated spectral function incorporating the self-energy evaluated in this study. We found that the electrons at the surface were strongly scattered by the defects at the surface and that the linewidth was significantly broadened (${\ensuremath{\Gamma}}_{0}=0.238\ifmmode\pm\else\textpm\fi{}0.006$ eV).

High-resolution angle-resolved photoemission spectroscopy study of the electronic structure ofEuFe2As2

We report the high-resolution angle-resolved photoemission spectroscopy studies of electronic structure of ${\text{EuFe}}_{2}{\text{As}}_{2}$. The paramagnetic state data are found to be consistent with density-functional calculations. In the antiferromagnetic ordering state of Fe, our results show that the band splitting, folding, and hybridization evolve with temperature, which cannot be explained by a simple folding picture. Detailed measurements reveal that a tiny electron Fermi pocket and a tiny hole pocket are formed near $(\ensuremath{\pi},\ensuremath{\pi})$ in the $(0,0)\ensuremath{-}(\ensuremath{\pi},\ensuremath{\pi})$ direction, which qualitatively agree with the results of quantum oscillations, considering ${k}_{z}$ variation in Fermi surface. Furthermore, no noticeable change within the energy resolution is observed across the antiferromagnetic transition of ${\text{Eu}}^{2+}$ ordering, suggesting weak coupling between Eu sublattice and FeAs sublattice.