Angle-resolved Photoemission Spectroscopy Experiments Research Articles

Hydrogen adsorption induced antiferrodistortive distortion and metallization at the (001) surface of SrTiO3

SrTiO3 (STO) is attracting lots of research interests due to the rich physics and properties presented at its surfaces and the interfaces of STO with other transition metal oxides. Based on density functional theory methods, we have investigated the influence of hydrogen (H) atoms adsorption at the (001) surface of STO on the geometrical and electronic structures of the surface. We find that H adsorption induces significant antiferrodistortive (AFD) distortion of TiO6 octahedra at the surface. By calculating H adsorption energy, we show that AFD distortion makes a significant contribution to the stability of H adsorbed STO surfaces. The calculated energy position of O-H bond states by hybrid functional method is 9.9 eV below Fermi level, in agreement with experimental value of 10 eV. The electrons donated by H atoms first occupy several dxy bands and then start to fill in degenerate dyz/dxz bands, indicating the metallization of initially insulating STO. The band splitting and occupy sequence calculated here are consistent with recent angle-resolved photoemission spectroscopy experiments. Our results reveal that H adsorption changes the atomic and electronic structures and thus induce fascinating properties at the surface of STO.

Kondo destruction in heavy fermion quantum criticality and the photoemission spectrum of YbRh2Si2

Heavy fermion metals provide a prototype setting to study quantum criticality. Experimentally, quantum critical points have been identified and studied in a growing list of heavy fermion compounds. Theoretically, Kondo destruction has provided a means to characterize a class of unconventional quantum critical points that goes beyond the Landau framework of order-parameter fluctuations. Among the prominent evidence for such local quantum criticality have been measurements in YbRh2Si2. A rapid crossover is observed at finite temperatures in the isothermal field dependence of the Hall coefficient and other transport and thermodynamic quantities, which specifies a T*B line in the temperature (T)–magnetic field (B) phase diagram. Here, we discuss what happens when temperature is raised, by analyzing the ratio of the crossover width to the crossover position. With this ratio approaching unity at T≳0.5K, YbRh2Si2 at zero magnetic field belongs to the quantum-critical fluctuation regime, where the single-particle spectral function has significant weight at both the small and large Fermi surfaces. This implies that, in this temperature range, any measurements sensitive to the Fermi surface will also see a significant spectral weight at the large Fermi surface. The angle-resolved photoemission spectroscopy experiments recently reported for YbRh2Si2 at T>1K are consistent with this expectation, and therefore support the association of the T*B line with the physics of Kondo destruction.

Pseudogap from ARPES experiment: Three gaps in cuprates and topological superconductivity (Review Article)

A term first coined by Mott back in 1968 a “pseudogap” is the depletion of the electronic density of states at the Fermi level, and pseudogaps have been observed in many systems. However, since the discovery of the high-temperature superconductors (HTSC) in 1986, the central role attributed to the pseudogap in these systems has meant that by many researchers now associate the term pseudogap exclusively with the HTSC phenomenon. Recently, the problem has got a lot of new attention with the rediscovery of two distinct energy scales (“two-gap scenario”) and charge density waves patterns in the cuprates. Despite many excellent reviews on the pseudogap phenomenon in HTSC, published from its very discovery up to now, the mechanism of the pseudogap and its relation to superconductivity are still open questions. The present review represents a contribution dealing with the pseudogap, focusing on results from angle resolved photoemission spectroscopy (ARPES) and ends up with the conclusion that the pseudogap in cuprates is a complex phenomenon which includes at least three different “intertwined” orders: spin and charge density waves and preformed pairs, which appears in different parts of the phase diagram. The density waves in cuprates are competing to superconductivity for the electronic states but, on the other hand, should drive the electronic structure to vicinity of Lifshitz transition, that could be a key similarity between the superconducting cuprates and iron-based superconductors. One may also note that since the pseudogap in cuprates has multiple origins there is no need to recoin the term suggested by Mott.

Two distinct topological phases in the mixed-valence compoundYbB6and its differences fromSmB6

We discuss the evolution of topological states and their orbital textures in the mixed valence compounds SmB6 and YbB6 within the framework of the generalized gradient approximation plus onsite Coulomb interaction (GGA+U) scheme for a wide range of values of U. In SmB6, the topological Kondo insulator (TKI) gap is found to be insensitive to the value of U, but in sharp contrast, Kondo physics in isostructural YbB6 displays a surprising sensitivity to U. In particular, as U is increased in YbB6, the correlated TKI state in the weak-coupling regime transforms into a d-p-type topological insulator phase with a band inversion between Yb-5d and B-2p orbitals in the intermediate coupling range, without closing the insulating energy gap throughout this process. Our theoretical predictions related to the TKI and non-TKI phases in SmB6 and YbB6 are in substantial accord with recent angle-resolved photoemission spectroscopy (ARPES) experiments.

Ramifications of optical pumping on the interpretation of time-resolved photoemission experiments on graphene

In pump–probe time and angle-resolved photoemission spectroscopy (TR-ARPES) experiments the presence of the pump pulse adds a new level of complexity to the photoemission process in comparison to conventional ARPES. This is evidenced by pump-induced vacuum space-charge effects and surface photovoltages, as well as multiple pump excitations due to internal reflections in the sample-substrate system. These processes can severely affect a correct interpretation of the data by masking the out-of-equilibrium electron dynamics intrinsic to the sample. In this study, we show that such effects indeed influence TR-ARPES data of graphene on a silicon carbide (SiC) substrate. In particular, we find a time- and laser fluence-dependent spectral shift and broadening of the acquired spectra, and unambiguously show the presence of a double pump excitation. The dynamics of these effects is slower than the electron dynamics in the graphene sample, thereby permitting us to deconvolve the signals in the time domain. Our results demonstrate that complex pump-related processes should always be considered in the experimental setup and data analysis.

Investigation Of Anomalies In The Electronic Dispersion Of The Hubbard Model, t–J And t–J* Models Within The Cluster Perturbation Theory In The X-Operator Representation

The electronic structure with the spectral line broadening corresponding to the angle-resolved photoemission spectroscopy resolution has been calculated for the Hubbard model, t–J and t–J* models within the norm-conserving cluster perturbation theory. The electronic dispersion anomalies similar to the high-energy kinks observed in the angle-resolved photoemission spectroscopy experiments are obtained at the energy scale of order of the hopping integral t. The doping dependence of the high-energy anomalies for different Coulomb repulsion values is discussed. The influence of the threesite correlated hopping on the results is analyzed.

G-wave pairing in BiS2 superconductors

Recent angle-resolved photoemission spectroscopy (ARPES) experiments have suggested that BiS2-based superconductors are at very low electron doping. Using random phase approximation (RPA) and functional renormalization group (FRG) methods, we find that g-wave pairing symmetry belonging to the irreducible representation is dominant at electron doping . The pairing symmetry is determined by inter-pocket nesting and orbital characters on the Fermi surfaces and is robust in a two-orbital model including both Hund's coupling J, and Hubbard-like Coulomb interactions U and with relatively small . With the increasing electron doping, the g-wave state competes with both the s-wave and d-wave states and no pairing symmetry emerges dominantly.

The thinnest high-temperature superconductor

Superconductivity is one of the most important topics in physics and materials science. The superconducting transition temperature (TC) of a quasi-2D superconductor, a superconductor with one dimension smaller than the coherence length of Cooper pairs, is normally descending with decreasing sample thickness since thermal and quantum fluctuations are expected to destroy the superconducting phase coherence even at low temperatures. The question of how thin a superconductor can be is not only crucial for understanding superconductivity in low-dimensional systems but also important for technological application in low-dissipation quantum devices. Nowadays, with the development of molecular beam epitaxy (MBE) technique, the fabrication of high-quality crystalline metallic films with thickness down to one unit-cell (1-UC) on semiconducting or insulating substrates becomes reality and makes it possible for the study of superconductivity at 2D limit. Different from traditional theoretical understanding of 2D superconductivity for free-standing films, in such film/substrate heterostructures the interface may dramatically affect the superconductivity and even possibly enhance TC compared with thicker film or bulk case. Recently, a research team led by Professor Qi-Kun Xue successfully grew 1-UC-thick and millimeterto centimeter-sized homogeneous FeSe films on SrTiO3 (STO) substrate by MBE, where a superconducting-like gap was discovered by in situ scanning tunnelingmicroscopy/spectroscopy (STM/ STS) study [1]. Surprisingly, the detected gap is 20 meV and almost 10 times larger than the gap of bulk FeSe. Soon after that, angle resolved photoemission spectroscopy experiments further revealed a nearly isotropic gap of above 15 meV, which closes at a temperature of around 65 K [2,3]. However, the question is whether the observed gap is a superconducting gap. Direct evidence of superconductivity in 1-UC FeSe films, such as zero resistance and Meissner effect, is highly desired. Very recently, for the first time Professor Jian Wang and Professor Qi-Kun Xue’s collaboration team

ARPES experiment in fermiology of quasi-2D metals (Review Article)

Angle-resolved photoemission spectroscopy (ARPES) enables direct observation of the Fermi surface and underlying electronic structure of crystals, which are the basic concepts necessary to describe all the electronic properties of solids and to reveal the nature of key electronic interactions involved. ARPES proved to be the most efficient for studies of quasi-2D metals, to which the most challenging and hence exciting compounds belong. This stimulated tremendously the development of ARPES in the recent years. The aim of this paper is to introduce the reader to the state-of-the-art ARPES experiment and to review the results of its application to such highly topical problems in solid state physics as high temperature superconductivity in cuprates and iron-based superconductors and electronic ordering in the transition metal dichalcogenides and manganites.

Impact of work function induced electric fields on laser-based angle-resolved photoemission spectroscopy

We examine the effects of the electric fields caused by the difference in work function between a sample and its surroundings in laser-based angle-resolved photoemission spectroscopy (laser ARPES) experiments. To simulate these effects we created several samples and surrounding puck geometries using SimIon 8.0 modeling software, and found that in most cases the system can be approximated by a circular sample mounted on an infinite conducting plane. Experimental measurements of the cuprate superconductor Bi2Sr2CaCu2O8+δ mounted on copper, aluminum, and graphite pucks confirmed the model's accuracy. Both the model and experimental data showed that work-function-induced fields have a significant effect on the outgoing trajectories of electrons for kinetic energies up to six times the work function difference between the sample and the puck. However, with the exception of effects very close to the sample edge, all electric field effects can be taken into account using linear corrections.

Extracting the temperature of hot carriers in time- and angle-resolved photoemission

The interaction of light with a material's electronic system creates an out-of-equilibrium (non-thermal) distribution of optically excited electrons. Non-equilibrium dynamics relaxes this distribution on an ultrafast timescale to a hot Fermi-Dirac distribution with a well-defined temperature. The advent of time- and angle-resolved photoemission spectroscopy (TR-ARPES) experiments has made it possible to track the decay of the temperature of the excited hot electrons in selected states in the Brillouin zone, and to reveal their cooling in unprecedented detail in a variety of emerging materials. It is, however, not a straightforward task to determine the temperature with high accuracy. This is mainly attributable to an a priori unknown position of the Fermi level and the fact that the shape of the Fermi edge can be severely perturbed when the state in question is crossing the Fermi energy. Here, we introduce a method that circumvents these difficulties and accurately extracts both the temperature and the position of the Fermi level for a hot carrier distribution by tracking the occupation statistics of the carriers measured in a TR-ARPES experiment.

Spectral function of the two-dimensional system of massless Dirac electrons

We determine the one-particle spectral function for the two-dimensional system of massless Dirac electrons interacting via the long-range Coulomb interaction. The electron self-energy is calculated within the G0W0 approximation, paying particular attention to the contribution coming from the collective plasmon mode. We find that, due to the acoustic plasmon dispersion, the spectral function has a renormalized quasi-particle δ-peak at the chemical potential along with a side band for the Fermi wave vector together with a wide structure with a pronounced spectral weight near the Dirac point. The present approach is appropriate for the wide range of energy scales given by the plasmon energy which may be probed in angle-resolved photoemission spectroscopy (ARPES) experiments.

Observation of possible topological in-gap surface states in the Kondo insulator SmB6 by photoemission

SmB6, a well-known Kondo insulator, exhibits a transport anomaly at low temperature. This anomaly is usually attributed to states within the hybridization gap. Recent theoretical work and transport measurements suggest that these in-gap states could be ascribed to topological surface states, which would make SmB6 the first realization of topological Kondo insulator. Here by performing angle-resolved photoemission spectroscopy experiments, we directly observe several dispersive states within the hybridization gap of SmB6. These states show negligible kz dependence, which indicates their surface origin. Furthermore, we perform photoemission circular dichroism experiments, which suggest that the in-gap states possess chirality of the orbital angular momentum. These states vanish simultaneously with the hybridization gap at around 150 K. Together, these observations suggest the possible topological origin of the in-gap states.

Microscopic mechanism for asymmetric charge distribution in Rashba-type surface states and the origin of the energy splitting scale

Microscopic mechanism for the Rashba-type band splitting is examined in detail. We show how asymmetric charge distribution is formed when local orbital angular momentum (OAM) and crystal momentum get interlocked due to surface effects. An electrostatic energy term in the Hamiltonian appears when such OAM and crystal momentum dependent asymmetric charge distribution is placed in an electric field produced from an inversion symmetry breaking (ISB). Analysis by using an effective Hamiltonian shows that, as the atomic spin-orbit coupling (SOC) strength increases from weak to strong, originally OAM-quenched states evolve into well-defined chiral OAM states and then to total angular momentum J-states. In addition, the energy scale of the band splitting changes from atomic SOC energy to electrostatic energy. To confirm the validity of the model, we study OAM and spin structures of Au(111) system by using an effective Hamiltonian for the d-orbitals case. As for strong SOC regime, we choose Bi2Te2Se as a prototype system. We performed circular dichroism angle resolved photoemission spectroscopy experiments as well as first-principles calculations. We find that the effective model can explain various aspects of spin and OAM structures of the system.

Imaging the Three-Dimensional Fermi-Surface Pairing near the Hidden-Order Transition inURu2Si2Using Angle-Resolved Photoemission Spectroscopy

We report angle-resolved photoemission spectroscopy experiments probing deep into the hidden-order state of URu(2)Si(2), utilizing tunable photon energies with sufficient energy and momentum resolution to detect the near Fermi-surface (FS) behavior. Our results reveal (i) the full itinerancy of the 5f electrons, (ii) the crucial three-dimensional k-space nature of the FS and its critical nesting vectors, in good comparison with density-functional theory calculations, and (iii) the existence of hot-spot lines and pairing of states at the FS, leading to FS gapping in the hidden-order phase.

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.

Spin-Orbital Texture in Topological Insulators

For three-dimensional topological insulators in the Bi2Se3 family, topological surface states with p(z) orbitals have a left-handed spin texture for the upper Dirac cone and a right-handed spin texture for the lower Dirac cone. In this work, we predict a new form of the spin-orbital texture associated with the p(x) and p(y) orbitals. For the upper Dirac cone, a left-handed (right-handed) spin texture is coupled to the "radial" ("tangential") orbital texture, whereas for the lower Dirac cone, the coupling of spin and orbital textures is the exact opposite. The "tangential" ("radial") orbital texture is dominant for the upper (lower) Dirac cone, leading to the right-handed spin texture for the in-plane orbitals of both the upper and lower Dirac cones. A spin-resolved and photon polarized angle-resolved photoemission spectroscopy experiment is proposed to observe this novel spin-orbital texture.

Tuning of magnetic and transport properties in Bi2Te3by divalent Fe doping

We report the doping effect of a magnetic Fe ion in Bi${}_{2}$Te${}_{3}$, that is, Bi${}_{2\ensuremath{-}x}$Fe${}_{x}$Te${}_{3}$ ($x$ $=$ 0, 0.08, 0.15, 0.2, 0.25, and 0.3). The paramagnetic magnetization data reveal that the Fe ions are doped in the divalent form. The Fe${}^{2+}$ state substituted for Bi${}^{3+}$ can create hole donors, which compensate for the electron dopants of Bi${}_{2}$Te${}_{3}$ with a small amount of Te excess. This causes the $n$-type carrier density to be decreased with increasing $x$, and finally be changed to $p$ type at $x$ $=$ 0.3, where the carrier mobility suddenly drops and the electrical resistivity abruptly increases. These results are consistent with angle-resolved photoemission spectroscopy experiments. The Fermi level shifts downward with increasing $x$. Furthermore, we find a larger spin polarization for the Fe-doped Bi${}_{2}$Te${}_{3}$ samples, which is crucial for future spintronics applications.

Compositional evolution of the anti-phase stripe superstructure in Bi2Sr2−xLaxCuO6 (0 < x ≤ 1.1) revealed by transmission electron microscopy

The detailed structural properties of La-doped Bi2Sr2−xLaxCuO6 (La–Bi2201, 0 ≤ x ≤ 1.1) have been studied by transmission electron microscopy (TEM). The well-known incommensurate supermodulation q1 in the Bi-based cuprates evolves from a monoclinic superstructure in the pristine Bi2201 phase to an orthogonal one in the La–Bi2201 (x = 0.73) phase. The b-component of the modulation vector (q1) for the x = 0.25 sample is about 0.24b* and increases slightly to 0.246b* in the x = 0.84 sample, while it increases significantly to 0.286b* for the x = 1.10 sample. We have revealed a new anti-phase stripe superstructure in all the La-doped Bi2201 samples, giving rise to a new modulation with a vector q2. This q2 modulation is directly evolved from the orthogonal modulation q1 through an addition of the anti-phase structure so that its vector q2 is equal to q1b/2. We also discussed the implication of these structural studies on the electronic structure by angle-resolved photoemission spectroscopy (ARPES) experiments in La–Bi2201.

Rashba Effect on the Structure of the Bi One-Bilayer Film: Fully Relativistic First-Principles Calculation

Using first-principles calculations, we study the spin–orbit interactions and spin textures of a Bi one-bilayer film, which attracts scientific interest because of the topological insulator and so on. The substrate effect is successfully mimicked by applying on electric field in the perpendicular direction of the film, which breaks the inversion symmetry. We study the highest occupied band around the Γ point. Although the vortex of the in-plane spin component is well explained on the basis of the conventional Rashba effect, we find a substantial out-of-plane component which cannot be explained by the conventional Rashba model. This spin texture is similar to that of a multi-bilayer Bi film, which has recently been observed using a spin-resolved angle-resolved photoemission spectroscopy experiment. We also find a spin vortex around the K point although this point has no time-reversal symmetry. We expect that a similar vortex appears in materials having the p3m1 symmetry, whose spin–orbit interactions have recently attracted scientific interest.