Electronic Fermi Surface Research Articles

Electronic structure, Fermi surface and x-ray magnetic circular dichroism in the CeAgSb2

The electronic structure, Fermi surface, angle dependence of the cyclotron masses and extremal cross sections of the Fermi surface as well as x-ray magnetic circular dichroism (XMCD) in the CeAgSb2 compound were investigated from first principles using the fully relativistic Dirac linear muffin-tin orbital method. In our calculations Ce 4f states have been considered as: 1) itinerant using the generalized gradient approximation (GGA), 2) fully localized, treating them as core states, and 3) partly localized using the GGA + U approximation. The effect of the spin-orbit (SO) interaction and Coulomb repulsion U in a frame of the GGA + U method on the Fermi surface, orbital dependence of the cyclotron masses, and extremal cross sections of the Fermi surface are examined in details. We show that the conventional GGA band calculations fail to describe the Fermi surface of the CeAgSb2 due to wrong position of Ce 4f states (too close to the EF). On the other hand, fully localized (4f states in core) and the GGA + U approach produce similar Fermi surfaces and dHvA frequencies in the CeAgSb2. A good agreement with the experimental data of XMCD spectra at the Ce M4.5 edges was achieved using the GGA + U approximation. The origin of the XMCD spectra in the compound is examined. The core hole effect in the final states has been investigated using a supercell approximation. It improves the agreement be-tween the theory and the experiment of the XAS and the XMCD spectra at the Ce M4.5 edges.

Single Crystal Growth and Magnetoresistivity of Topological Semimetal CoSi**Supported by the National Natural Science Foundation of China under Grant No 11874399, and the National Key Research and Development Program of China under Grant No 2016YFA0300602.

We report single crystal growth of CoSi, which has recently been recognized as a new type of topological semimetal hosting fourfold and sixfold degenerate nodes. The Shubnikov–de Haas quantum oscillation (QO) is observed on our crystals. There are two frequencies originating from almost isotropic bulk electron Fermi surfaces, in accordance with band structure calculations. The effective mass, scattering rate, and QO phase difference of the two frequencies are extracted and discussed.

Effective pairing interaction in a system with an incipient band

The nature and mechanism of superconductivity in the extremely electron-doped FeSe based superconductors continues to be a matter of debate. In these systems, the hole-like band has moved below the Fermi energy, and various spin-fluctuation theories involving pairing between states near the electron Fermi surface and states of this incipient band have been proposed. Here, using a dynamic cluster quantum Monte Carlo calculation for a bilayer Hubbard model we show that the pairing in these systems can be understood in terms of an effective retarded attractive interaction between electrons near the electron Fermi surface.

Study of elastic, structural, electronic, magnetic, and topological properties of η-Fe2C carbide under pressure

This paper studies the elastic, structural, electronic, magnetic, and topological properties of the η-Fe2C carbide under pressure by resorting to DFT-GGA's first principle via FPLAPW-WIEN2K and ElaStic1.0 software. Properties were calculated for strain vectors and pressures shown to maintain the orthorhombic structure of the carbide. Electronic bands, Fermi surfaces, and electron states are slightly distorted in the direction of the strain vector considered. The density of states, dominated by the 3d electrons of Fe, and to a lesser extent, by the 2p electrons of C, is lower in the pressure-borne compressed state than in the expanded one, a degree of hybridization existing between these states in certain energy regions, in tandem with a more extended first Brillouin zone and a higher Fermi level, which translates into greater structural stability in the latter state. The resulting magnetic anisotropy is a consequence of the difference in density of states between the spin-up and spin-down configurations, mainly in the orbitals of the aforesaid electrons. The pressure was also found to affect the number of bond, ring, and cage critical points (CP) found, as well as their distance from the atom.

Strong electronic correlations and Fermi surface reconstruction in the quasi-one-dimensional iron superconductor BaFe2S3

BaFe2S3 is a special iron superconductor with two-leg ladder structure which can help to unravel the role played by the electronic correlations in high-Tc superconductivity. At zero pressure it is insulating with stripe antiferromagnetic (AF) order and superconductivity emerges under pressure. We use a slave-spin technique to analyze the strength of the local correlations in BaFe2S3. We find that at the pressure at which the superconductivity appears the electronic correlations in BaFe2S3 are similar to the ones measured in other iron superconductors. However, at zero pressure the strength of the correlations is strongly enhanced, being particularly severe for the two orbitals with the largest weight at the Fermi level what invalidates nesting as the mechanism for AF. The system is not a Mott insulator at zero temperature, but these two orbitals with mass enhancements m* ~ 12-15 will become incoherent at higher temperatures. Different from what happens in other iron superconductors, at both pressures, the Fermi surface is reconstructed by the electronic correlations.

Atomic and electronic structure of domains walls in a polar metal

Polar metals counterintuitively bring two well-known phenomena into coexistence, namely, bulk polar displacements, and an electronic Fermi surface giving rise to metallic conduction. However, little is known about the polar domains or domain walls in such materials. Using atomic resolution electron microscopy imaging combined with first principles density functional theory, we show that uncharged head-to-tail walls, and ``charged'' head-to-head and tail-to-tail walls can exist in the bulk of such crystals of polar metals ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$, where both structural changes at the wall as well as electrostatic considerations define the wall nature. Significant built-in potentials of 30--170 meV are predicted at such walls.

Disorder Quenching of the Charge Density Wave in ZrTe_{3}.

The charge density wave (CDW) in ZrTe_{3} is quenched in samples with a small amount of Te isoelectronically substituted by Se. Using angle-resolved photoemission spectroscopy we observe subtle changes in the electronic band dispersions and Fermi surfaces upon Se substitution. The scattering rates are substantially increased, in particular for the large three-dimensional Fermi surface sheet. The quasi-one-dimensional band is unaffected by the substitution and still shows a gap at low temperature, which starts to open from room temperature. Long-range order is, however, absent in the electronic states as in the periodic lattice distortion. The competition between superconductivity and the CDW is thus linked to the suppression of long-range order of the CDW.

Scaling of the superconducting gap with orbital character in FeSe

We use high-resolution angle-resolved photoemission spectroscopy to map the three-dimensional momentum dependence of the superconducting gap in FeSe. We find that on both the hole and electron Fermi surfaces, the magnitude of the gap follows the distribution of $d_{yz}$ orbital weight. Furthermore, we theoretically determine the momentum dependence of the superconducting gap by solving the linearized gap equation using a tight binding model which quantitatively describes both the experimental band dispersions and orbital characters. By considering a Fermi surface only including one electron pocket, as observed spectroscopically, we obtain excellent agreement with the experimental gap structure. Our finding of a scaling between the superconducting gap and the $d_{yz}$ orbital weight supports the interpretation of superconductivity mediated by spin-fluctuations in FeSe.

Superconducting-state properties and electronic band structure calculations of a noncentrosymmetric Th7Ni3 compound

Superconducting-state properties of a noncentrosymmetric Th7Ni3 compound have been investigated using magnetic, electrical resistivity and specific heat measurements as well as by electronic band structure calculations. The study reveals that the studied compound is a dirty type-II superconductor with K and a weak electron–phonon coupling . Moreover, in contrast to an exotic superconductivity observed previously in Th7Fe3 and Th7Co3, data reported in this paper give clear evidence that Th7Ni3 is a conventional single-gap superconductor. The experimental results are supported by DFT calculations of the electronic band structure, density of states, electron localization functions and Fermi surfaces which were performed using the full-potential linear muffin-tin-orbital and full-potential linearized augmented plane wave methods. Theoretical data show that asymmetric spin–orbit coupling in Th7Ni3 is quite small; hence the electronic band structure of this compound is weakly affected by the spin–orbit effects. We have determined fundamental parameters of Th7Ni3 and compared the superconducting properties with other Th7T3 (T = Fe, Co and Ru) compounds.

Sign-reversal superconducting gaps revealed by phase-referenced quasiparticle interference of impurity-induced bound states in (Li1−xFex)OHFe1−yZnySe

By measuring the spatial distribution of differential conductance near impurities on Fe sites, we have obtained the quasi-particle interference (QPI) patterns in the (Li$_{1-x}$Fe$_x$)OHFe$_{1-y}$Zn$_y$Se superconductor with only electron Fermi surfaces. By taking the Fourier transform on these patterns, we investigate the scattering features between the two circles of electron pockets formed by folding or hybridization. We treat the data by using the recent theoretical approach [arXiv:1710.09089] which is specially designed for the impurity induced bound states. It is found that the superconducting gap sign is reversed on the two electron pockets, which can be directly visualized by the phase-referenced QPI technique, indicating that the Cooper pairing is induced by the repulsive interaction. Our results further show that this method is also applicable for data measured for multiple impurities, which provides an easy and feasible way for detecting the gap function of unconventional superconductors.

Electronic properties of [formula omitted]Al20 (R = La, Ce, Yb, Th, U; X = Ti, V, Cr and Mn) cage compounds

Non-spin-polarized electronic structures and Fermi surface properties of RX2Al20 (R = La, Ce, Yb, Th, U; X = Ti, V, Cr, Mn) intermetallic compounds were calculated using the full potential all-electron local orbital (FPLO) approach in the framework of the local density approximation (LDA). Trends of the magnetism are discussed in terms of the characteristics of X-3d bands with a quantitative analysis of the relationship between band electron filling and crystal electric field splitting. Since coordination icosahedra of X-atoms have small trigonal distortion, crystal electric field splits the fivefold degenerate X-3d state into low-energy singlet a1g and two higher-energy doublets eg. In RTi2Al20 and RV2Al20 the population of the related 3d sub-band is not sufficient to cause energetically favorable spin polarization, whereas magnetic instabilities develop in the RCr2Al20 series. Finally, a manifestation of strong repulsive interactions between itinerant Mn-d electrons become most pronounced in ferromagnetic UMn2Al20. The influence of non-magnetic R-f states on magnetic and thermodynamic properties is discussed with special emphasis on the role of the f–p and f–d hybridization. For LaTi2Al20 and LaV2Al20 the calculated quantum oscillation frequencies are in accord with experimental reports.

Electronic structure of V2AlC

We report an experimental and theoretical study of the electronic band structure and Fermi surfaces (FSs) of ${\mathrm{V}}_{2}\mathrm{AlC}$ single crystals using angle-resolved photoemission spectroscopy and density functional theory (DFT) calculations. We provide evidence of the existence of equivalent, complex hole FSs with a nearly tubular form along the $c$ axis, and of an intricate electron FS exhibiting tubes parallel to $c$ connected to one another by smaller tubes with axes parallel to the $ab$ plane. The electron FS thus exhibits a small delocalization along the $c$ axis. The local orbital character of each observed band is experimentally and theoretically assessed. DFT calculations show an excellent agreement with experiment. We also report the observation of an unstable surface state.

K-space imaging of anisotropic 2D electron gas in GaN/GaAlN high-electron-mobility transistor heterostructures

Nanostructures based on buried interfaces and heterostructures are at the heart of modern semiconductor electronics as well as future devices utilizing spintronics, multiferroics, topological effects, and other novel operational principles. Knowledge of electronic structure of these systems resolved in electron momentum k delivers unprecedented insights into their physics. Here we explore 2D electron gas formed in GaN/AlGaN high-electron-mobility transistor heterostructures with an ultrathin barrier layer, key elements in current high-frequency and high-power electronics. Its electronic structure is accessed with angle-resolved photoelectron spectroscopy whose probing depth is pushed to a few nanometers using soft-X-ray synchrotron radiation. The experiment yields direct k-space images of the electronic structure fundamentals of this system—the Fermi surface, band dispersions and occupancy, and the Fourier composition of wavefunctions encoded in the k-dependent photoemission intensity. We discover significant planar anisotropy of the electron Fermi surface and effective mass connected with relaxation of the interfacial atomic positions, which translates into nonlinear (high-field) transport properties of the GaN/AlGaN heterostructures as an anisotropy of the saturation drift velocity of the 2D electrons.

Electronic Origin of Optically-Induced Sub-Picosecond Lattice Dynamics in MoSe2 Monolayer.

Atomically thin layers of transition metal dichalcogenide (TMDC) semiconductors exhibit outstanding electronic and optical properties, with numerous applications such as valleytronics. While unusually rapid and efficient transfer of photoexcitation energy to atomic vibrations was found in recent experiments, its electronic origin remains unknown. Here, we study the lattice dynamics induced by electronic excitation in a model TMDC monolayer, MoSe2, using nonadiabatic quantum molecular dynamics simulations. Simulation results show sub-picosecond disordering of the lattice upon photoexcitation, as measured by the Debye-Waller factor, as well as increasing disorder for higher densities of photogenerated electron-hole pairs. Detailed analysis shows that the rapid, photoinduced lattice dynamics are due to phonon-mode softening, which in turn arises from electronic Fermi surface nesting. Such mechanistic understanding can help guide optical control of material properties for functionalizing TMDC layers, enabling emerging applications such as phase change memories and neuromorphic computing.

Interplay between charge order and superconductivity in cuprate superconductors

One of the central issues in the recent study of cuprate superconductors is the interplay of charge order with superconductivity. Here the interplay of charge order with superconductivity in cuprate superconductors is studied based on the kinetic-energy-driven superconducting (SC) mechanism by taking into account the intertwining between the pseudogap and SC gap. It is shown that the appearance of the Fermi pockets is closely associated with the emergence of the pseudogap. However, the distribution of the spectral weight of the SC-state quasiparticle spectrum on the Fermi arc, or equivalently the front side of the Fermi pocket, and back side of Fermi pocket is extremely anisotropic, where the most part of the spectral weight is located around the tips of the Fermi arcs, which in this case coincide with the hot spots on the electron Fermi surface (EFS). In particular, as charge order in the normal-state, this EFS instability drives charge order in the SC-state, with the charge-order wave vector that is well consistent with the wave vector connecting the hot spots on the straight Fermi arcs. Furthermore, this charge-order state is doping dependent, with the charge-order wave vector that decreases in magnitude with the increase of doping. Although there is a coexistence of charge order and superconductivity, this charge order antagonizes superconductivity. The results from the SC-state dynamical charge structure factor indicate the existence of a quantitative connection between the low-energy electronic structure and collective response of the electron density. The theory also shows that the pseudogap and charge order have a root in common, they and superconductivity are a natural consequence of the strong electron correlation.

Evolution of crystal structures and electronic properties for TiS2 at high pressure

Using the swarm structure searching technique in combination with density functional theory, we have extensively investigated the crystal structures of TiS2 in a wide pressure range. Our prediction identifies the hexagonal close packed cadmium iodide structure with P-3m1 symmetry is most stable at ambient conditions. An orthorhombic structure with Immm space group is predicted to be energetically much superior to previously proposed orthorhombic cotunnite-type phase in the pressure range from 9.5 to 52.1 GPa, and then I4/mmm is the thermodynamically stable structure above 52.1 GPa. Our calculated results of phonon dispersions have confirmed the dynamical stability of the current predicted Immm and I4/mmm phases. The theoretical electronic band structures and Fermi surfaces reveal pressure-induced electronic topological transitions in P-3m1 and I4/mmm phases, while it is absent in Immm structure.

Nesting-induced local electronic patterns around a single As (Te, Se) vacancy in iron-based superconductors

The local electronic states around a single As (Te, Se) vacancy are investigated in order to shed light on the role of ligands in a series of iron-based superconductors. Such a vacancy can produce a local hopping correction ranging from −0.22 to 0.12 eV and always induce two in-gap resonance peaks in the local density of states (LDOS) at the fixed symmetrical bias voltages, which are rather robust and irrelevant to the phase of superconducting order parameter. The LDOS images near the defect predominantly possess 0° and 45° stripes. These energy-dependent charge modulations created by quasiparticle interference are originated in the nesting effect between the inner (outer) hole Fermi surface around Γ point and the inner (outer) electron Fermi surface around M point.

Spin–orbit coupling, minimal model and potential Cooper-pairing from repulsion in BiS2-superconductors

We develop the realistic minimal electronic model for recently discovered BiS2 superconductors including the spin–orbit (SO) coupling based on the first-principles band structure calculations. Due to strong SO coupling, characteristic for the Bi-based systems, the tight-binding low-energy model necessarily includes px, py, and pz orbitals. We analyze a potential Cooper-pairing instability from purely repulsive interaction for the moderate electronic correlations using the so-called leading angular harmonics approximation. For small and intermediate doping concentrations we find the dominant instabilities to be -wave, and s±-wave symmetries, respectively. At the same time, in the absence of the sizable spin fluctuations the intra and interband Coulomb repulsions are of the same strength, which yield the strongly anisotropic behavior of the superconducting gaps on the Fermi surface. This agrees with recent angle resolved photoemission spectroscopy findings. In addition, we find that the Fermi surface topology for BiS2 layered systems at large electron doping can resemble the doped iron-based pnictide superconductors with electron and hole Fermi surfaces maintaining sufficient nesting between them. This could provide further boost to increase Tc in these systems.

Fermi surfaces in Kondo insulators

We report magnetic quantum oscillations measured using torque magnetisation in the Kondo insulator YbB12 and discuss the potential origin of the underlying Fermi surface. Observed quantum oscillations as well as complementary quantities such as a finite linear specific heat capacity in YbB12 exhibit similarities with the Kondo insulator SmB6, yet also crucial differences. Small heavy Fermi sections are observed in YbB12 with similarities to the neighbouring heavy fermion semimetallic Fermi surface, in contrast to large light Fermi surface sections in SmB6 which are more similar to the conduction electron Fermi surface. A rich spectrum of theoretical models is suggested to explain the origin across different Kondo insulating families of a bulk Fermi surface potentially from novel itinerant quasiparticles that couple to magnetic fields, yet do not couple to weak DC electric fields.

First-principles study of electronic structure and Fermi surface in semimetallic YAs

In the course of searching for new systems, which exhibit nonsaturating and extremely large positive magnetoresistance, electronic structure, Fermi surface, and de Haas-van Alphen characteristics of the semimetallic YAs compound were studied using the all-electron full-potential linearized augmented-plane wave (FP–LAPW) approach in the framework of the generalized gradient approximation (GGA). In the scalar-relativistic calculation, the cubic symmetry splits fivefold degenerate Y-d orbital into low-energy threefold-degenerate t2g and twofold degenerate doublet e1g states at Γ point around the Fermi energy. One of them, together with the threefold degenerate t1u character of As-p orbital, render the YAs semimetal with a topologically trivial band order and fairly low density of states at the Fermi level. Including spin–orbit (SO) coupling into the calculation leads to pronounced splitting of the t1u state and shifting the bands in the energy scale. Consequently, the determined four different 3-dimensional Fermi surface sheets of YAs consists of three concentric hole-like bands at Γ and one ellipsoidal electron-like sheet centred at the X points. In full accordance with the previous first-principles calculations for isostructural YSb and YBi, the calculated Fermi surface of YAs originates from fairly compensated multi-band electronic structures.