Electronic Fermi Surface Research Articles

Transverse Thermoelectric Conversion in the Mixed-Dimensional Semimetal WSi 2

Materials with axis-dependent conduction polarity are known as p×n-type or goniopolar conductors that can be used for transverse thermoelectric devices, allowing the longitudinal thermal current to be converted into the transverse electrical current. Here, we have performed experimental and computational studies on the transport properties of WSi2 single crystals, in which such axis-dependent conduction polarity of the thermopower and the Hall coefficient have recently been reported, and demonstrated the transverse thermoelectric effect by applying the temperature gradient in a direction rotated 45∘ from the crystallographic axis. We have observed strongly sample-dependent transport properties, which have also been observed in previous studies, and together with first-principles calculations we show that such sample-dependent transport properties originate from the band-dependent scattering rates of carriers. The calculated band-resolved Peltier conductivity shows that the mixed-dimensional electronic structure consisting of a quasi-one-dimensional electron Fermi surface and a quasi-two-dimensional hole surface is a key property for the axis-dependent conduction polarity. The directly obtained transverse thermoelectric figure of merit is comparable to that of the anomalous Nernst materials, implying that the present material is a potential candidate for transverse thermoelectric conversion. Published by the American Physical Society 2024

The Superconducting Mechanism in BiS2-Based Superconductors: A Comprehensive Review with Focus on Point-Contact Spectroscopy.

The family of BiS2-based superconductors has attracted considerable attention since their discovery in 2012 due to the unique structural and electronic properties of these materials. Several experimental and theoretical studies have been performed to explore the basic properties and the underlying mechanism for superconductivity. In this review, we discuss the current understanding of pairing symmetry in BiS2-based superconductors and particularly the role of point-contact spectroscopy in unravelling the mechanism underlying the superconducting state. We also review experimental results obtained with different techniques including angle-resolved photoemission spectroscopy, scanning tunnelling spectroscopy, specific heat measurements, and nuclear magnetic resonance spectroscopy. The integration of experimental results and theoretical predictions sheds light on the complex interplay between electronic correlations, spin fluctuations, and Fermi surface topology in determining the coupling mechanism. Finally, we highlight recent advances and future directions in the field of BiS2-based superconductors, underlining the potential technological applications.

Evidence for large thermodynamic signatures of in-gap fermionic quasiparticle states in a Kondo insulator

The mixed-valence compound YbB12 displays paradoxical quantum oscillations in electrical resistivity and magnetic torque in a regime with a well-developed insulating charge gap and in the absence of an electronic Fermi surface. However, signatures of such unusual fermionic quasiparticles in other bulk thermodynamic observables have been missing. Here we report the observation of a series of sharp double-peak features in the specific heat as a function of the magnetic field. The measured Hall resistivity evolves smoothly across the field values at which the characteristic anomalies appear in the thermodynamic response and rules out the possibility of conventional electrons as their origin. Our observations of thermodynamic anomalies in a bulk three-dimensional electrical insulator provide the evidence for the presence of emergent dispersing fermionic excitations within the insulating bulk, which sets the stage for further investigation of electron fractionalization in other correlated mixed-valence compounds.

Superconductivity in Monolayer Carbon Allotropes with High Thermal Stability.

Intrinsic superconductivity is rarely discovered in sp2-hybridized monolayer carbon allotropes. Here we design a carbon monolayer configured of pentagon, heptagon, and hexagon rings with p2 plane group symmetry. Full-sp2 hybridization is proposed to favor thermal metastability on a low Gibbs free energy. The extremely small thermal expansion coefficient is predicted to the turn negative value to positive with elevating temperature. Carbon polygon structures remain intact at a high thermal temperature of 3,000 K. The high specific surface area is found to approach 2,700 m2/g, with O2-adsorption being advantageous over pristine graphene. We reveal electronic Fermi surfaces mediated by phonon modes of carbon out-of-plane vibrations. By calculating the Eliashberg equation, we evaluate intrinsic superconductivity with a large electron-phonon coupling coefficient. The superconducting transition temperature is estimated to reach 20 K under a high logarithmic average frequency. These first-principles calculations shall stimulate experimentalists' interest in exploring low-dimensional carbon superconductors with gas sensitivity.

Effective model and electron correlations in trilayer nickelate superconductor La4Ni3O10

Recently, signatures of superconductivity with critical temperature from 20 to 30 K have been reported in pressured trilayer nickelate La4Ni3O10 through a pressure-induced structure transition. Here we explore the evolution of electronic structures and electronic correlations in different phases of La4Ni3O10 under corresponding pressure regions, by using density functional theory (DFT) combined with dynamical mean-field theory (DMFT). Similar to bilayer superconductor La3Ni2O7, the electronic bands in superconducting La4Ni3O10 are dominated by Ni-3 dx2−y2 and 3 dz2 orbits near the Fermi level, in contrast, the inner Ni–O plane in La4Ni3O10 generates a doublet hole-pocket Fermi surfaces around the Brillouin-zone corner, meanwhile one branch of the Ni- 3dz2 bands is pushed very close above the Fermi level, which can induce an electron pocket through small electron doping. The DFT+DMFT simulations suggest that the electronic correlations only give minor modification to the Fermi surfaces, meanwhile the Ni- 3dz2 and 3 dx2−y2 states on outer Ni–O layers have considerable greater mass enhancements than on the inner layer. The sensitiveness of electronic structure under doping and unique layer dependence of correlation suggest a distinct superconducting mechanism with respect to bilayer La3Ni2O7. Based on the DFT and DFT+DMFT simulations, we eventually derive a trilayer effective tight-binding model, which can produce rather precise electronic bands and Fermi surfaces, hence can serve as an appropriate model to further study the superconducting mechanism and paring symmetry in trilayer La4Ni3O10.

Electronic Band Structure and Fermi Surface Analysis of Ce2 Ni1-Xcoxge2: A Pseudo Potential Study

This study uses pseudopotential calculations to investigate the electronic band structure and Fermi surface properties of Ce2Ni1-xCoxGe2 alloys. The research problem centers on understanding the effects of Co substitution on the electronic properties of these alloys, particularly regarding potential high-temperature superconducting behavior. The methodology involves density of states (DOS) analysis and Fermi surface calculations for Ce2Ni1-xCoxGe2 alloys with varying Co concentrations. Results reveal a notable increase in DOS around the Fermi level with increasing Co substitution, indicative of enhanced metallic behavior and potential superconducting characteristics. Additionally, band structure analyses demonstrate the significant role of hybridization between Ce 4f, Ni/Co 3d, and Ge 4s/4p orbitals in determining electronic behavior. Fermi surface calculations unveil a transition from near-spherical to complex 2D and 3D shapes with higher Co concentrations, suggesting modifications in Fermi energy surface nestings or electron scattering. This study's conclusions emphasized the intricate electronic structure of Ce2Ni1-xCoxGe2 alloys and highlighted the potential for further exploration in high-temperature super conductivity research.

Theoretical Investigation of Mechanical and Electronic Properties of Hexagonal BaB2

A comprehensive investigation of the electronic and mechanical properties in the hexagonal BaB2 binary system using state of the art first-principles computational techniques is critical for an in-depth understanding of the fundamental properties unique to this binary system. In this context, we derived elastic constants using the metric-tensor formulation, which allowed us to find important mechanical properties such as Bulk Modulus, Shear Modulus, and Vickers's hardness which are fundamental mechanical quantities. Also, this research includes a detailed analysis of the electronic band structures and a study comparison of Fermi surface topologies. The charge density at the Fermi level (N(EF)), which is very important in superconductivity theories, was found to be 1.43 states/eV.uc. Furthermore, we have explored whether there exists a close relationship between these properties and the superconducting behavior of the BaB2 material. Nevertheless, our calculations unequivocally demonstrate that the information derived from electronic band structures and Fermi surfaces alone is insufficient for a comprehensive explanation of the superconductivity phenomenon observed in such materials.

Competing itinerant and local spin interactions in kagome metal FeGe

The combination of a geometrically frustrated lattice, and similar energy scales between degrees of freedom endows two-dimensional Kagome metals with a rich array of quantum phases and renders them ideal for studying strong electron correlations and band topology. The Kagome metal, FeGe is a noted example of this, exhibiting A-type collinear antiferromagnetic (AFM) order at TN ≈ 400 K, then establishes a charge density wave (CDW) phase coupled with AFM ordered moment below TCDW ≈ 110 K, and finally forms a c-axis double cone AFM structure around TCanting ≈ 60 K. Here we use neutron scattering to demonstrate the presence of gapless incommensurate spin excitations associated with the double cone AFM structure of FeGe at temperatures well above TCanting and TCDW that merge into gapped commensurate spin waves from the A-type AFM order. Commensurate spin waves follow the Bose factor and fit the Heisenberg Hamiltonian, while the incommensurate spin excitations, emerging below TN where AFM order is commensurate, start to deviate from the Bose factor around TCDW, and peaks at TCanting. This is consistent with a critical scattering of a second order magnetic phase transition with decreasing temperature. By comparing these results with density functional theory calculations, we conclude that the incommensurate magnetic structure arises from the nested Fermi surfaces of itinerant electrons and the formation of a spin density wave order.

Theoretical modeling of the electronic structure and Fermi surfaces of Gd4Sb3 and GdSb2 compounds

Our theoretical modeling of the electronic structure in the intermetallic Gd4Sb3 and GdSb2 compounds has been done in the framework of density functional theory accounting for spin-orbit coupling. It revealed the metallic character of the summed total density of electronic states for both materials. The complicated Fermi surfaces were found in the half-metallic Gd4Sb3 compound which corresponds to the band structure. The GdSb2 material is obtained to be a bad metal with the low density of states near the Fermi energy, the Fermi surfaces of GdSb2 are found to have the cylindrical shape.

A first principle +U correction to the investigation of intrinsic vacancy and zinc vacancy-cluster induced magnetism in w-ZnO

Pure ZnO shows the probable origin of the magnetic nature. Microscopic correlation to point defect VO and the cluster VZn is obtained by first-principle calculation and +U interaction energy of the p-d orbital subjected to improve band and DOS. Hydrogen contribution to the magnetic moment (MM) due to the coupling of vacancy and 1 s electron correlates with the experimental outcomes. Intrinsic vacancy modulates Fermi surface electron density, leading to the stoner criterion. A higher-order magnetic moment is associated with the cluster vacancy, and the change in the MM has accounted for the +U adjustment to the relevant orbital impact. Nonlinear relations are due to cluster vacancy complexes VZn-x/Oi-x (x = H, 2H), and the FM and AFM exchange has been analyzed. The material encloses large grains; hence, it is advantageous for VZn to develop near the grain edge, and in O-rich environments, chemisorbed O2 is formed near the grain surface.

Anisotropic positive linear and sub-linear magnetoresistivity in the cubic type-II Dirac metal Pd3In7

We report a transport study on Pd3In7 which displays multiple Dirac type-II nodes in its electronic dispersion. Pd3In7 is characterized by low residual resistivities and high mobilities, which are consistent with Dirac-like quasiparticles. For an applied magnetic field (μ0H) having a non-zero component along the electrical current, we find a large, positive, and linear in μ0H longitudinal magnetoresistivity (LMR). The sign of the LMR and its linear dependence deviate from the behavior reported for the chiral-anomaly-driven LMR in Weyl semimetals. Interestingly, such anomalous LMR is consistent with predictions for the role of the anomaly in type-II Weyl semimetals. In contrast, the transverse or conventional magnetoresistivity (CMR for electric fields E⊥μ0H) is large and positive, increasing by 103−104 % as a function of μ0H while following an anomalous, angle-dependent power law {rho }_{{{{rm{xx}}}}}propto {({mu }_{0}H)}^{n} with n(θ) ≤ 1. The order of magnitude of the CMR, and its anomalous power-law, is explained in terms of uncompensated electron and hole-like Fermi surfaces characterized by anisotropic carrier scattering likely due to the lack of Lorentz invariance.

Superlattices, Bonding-Antibonding, Fermi Surface Nesting, and Superconductivity

Raman and synchrotron THz absorption spectral measurements on MgB2 provide experimental evidence for electron orbital superlattices. In earlier work, we have detected THz spectra that show superlattice absorption peaks with low wavenumbers, for which spectral density evolves and intensifies after cooling below the superconducting transition temperature for MgB2. In this work, we show how these observations indicate a direct connection to superconducting properties and mechanisms. Bonding–antibonding orbital character is identified in calculated electronic band structures and Fermi surfaces consistent with superlattice structures along the c-axis. DFT calculations show that superlattice folding of reciprocal space generates Brillouin zone boundary reflections, Umklapp processes, and substantially enhances nesting relationships. Tight binding equations are compared with expected charge density waves from nesting relationships and adjusted to explicitly accommodate these linked processes. Systematic analysis of electronic band structures and Fermi surfaces allows for direct identification of Cooper pairing and the superconducting gap, particularly when the k-grid resolution of a calculation is suitably calibrated to structural parameters. Thus, we detail a robust and accurate DFT re-interpretation of BCS superconductivity for MgB2.

De Haas–van Alphen Oscillations for the Field Along c-axis in UTe2

We performed de Haas-van Alphen (dHvA) experiments in the spin-triplet superconductor UTe2 for magnetic field along the c-axis above 15T. Three fundamental dHvA frequencies, named alpha1, alpha2 and beta corresponding to the cross sections of cylindrical Fermi surfaces (FSs) with large cyclotron effective masses (33-43 m0) were detected. No other fundamental dHvA frequencies were detected at high frequency range, suggesting a cylindrical-shaped electron FS without connecting at the Z point of the Brillouin zone. However, the existence of small pocket FSs associated with extremely heavy masses cannot be fully excluded.

Collective density fluctuations of strange metals with critical Fermi surfaces

Recent spectroscopic measurements in a number of strongly correlated metals that exhibit non-Fermi liquid like properties have observed evidence of anomalous frequency and momentum-dependent charge-density fluctuations. Specifically, in the strange metallic regime of the cuprate superconductors, there is a featureless particle-hole continuum exhibiting unusual power-laws, and experiments suggest that the plasmon mode decays into this continuum in a manner that is distinct from the expectations of conventional Fermi liquid theory. Inspired by these new experimental developments, we address the nature of low-energy collective modes and the particle-hole continua for different "solvable" lattice models of non-Fermi liquids that host a critical Fermi surface -- a sharp electronic Fermi surface without any low-energy electronic quasiparticles. We scrutinize theoretically the possible existence of a long-lived zero-sound mode, which is renormalized to the plasma frequency in the presence of long-ranged coulomb interactions, and its decay into the continuum over a wide range of frequencies and momenta. Quite remarkably, some of the models analyzed here can account for certain aspects of the universal experimental observations, that clearly lie beyond the purview of standard Fermi liquid theory.

Effect of Confinement and Coulomb Interactions on the Electronic Structure of the (111) LaAlO3/SrTiO3 Interface.

A tight binding supercell approach is used for the calculation of the electronic structure of the (111) LaAlO3/SrTiO3 interface. The confinement potential at the interface is evaluated solving a discrete Poisson equation by means of an iterative method. In addition to the effect of the confinement, local Hubbard electron-electron terms are included at the mean-field level within a fully self-consistent procedure. The calculation carefully describes how the two-dimensional electron gas arises from the quantum confinement of electrons near the interface due to the band bending potential. The resulting electronic sub-bands and Fermi surfaces show full agreement with the electronic structure determined by angle-resolved photoelectron spectroscopy experiments. In particular, we analyse how the effect of local Hubbard interactions change the density distribution over the layers from the interface to the bulk. Interestingly, the two-dimensional electron gas at the interface is not depleted by local Hubbard interactions which indeed induce an enhancement of the electron density between the first layers and the bulk.

Field-induced Lifshitz transition in the magnetic Weyl semimetal candidate PrAlSi

Lifshitz transition (LT) refers to an abrupt change in the electronic structure and Fermi surface and is associated to a variety of emergent quantum phenomena. Amongst the LTs observed in known materials, the field-induced LT has been rare and its origin remains elusive. To understand the origin of field-induced LT, it is important to extend the material basis beyond the usual setting of heavy fermion metals. Here, we report on a field-induced LT in PrAlSi, a magnetic Weyl semimetal candidate with localized 4f electrons, through a study of magnetotransport up to 55 T. The quantum oscillation analysis reveals that across a threshold field B* ≈ 14.5 T the oscillation frequency (F1 = 43 T) is replaced by two new frequencies (F2 = 62 T and F3 = 103 T). Strikingly, the LT occurs well below the quantum limit, with obvious temperature-dependent oscillation frequency and field-dependent cyclotron mass. Our work not only enriches the rare examples of field-induced LTs but also paves the way for further investigation of the interplay among topology, magnetism, and electronic correlation.

Strongly correlated itinerant magnetism near superconductivity in NiTa4Se8

Metallic ferromagnets with strongly interacting electrons often exhibit remarkable electronic phases such as ferromagnetic superconductivity, complex spin textures, and nontrivial topology. In this report, we discuss the synthesis of a layered magnetic metal $\mathrm{Ni}{\mathrm{Ta}}_{4}{\mathrm{Se}}_{8}$ (or ${\mathrm{Ni}}_{1/4}\mathrm{Ta}{\mathrm{Se}}_{2}$) with a Curie temperature of 58 Kelvin. Magnetization data and density functional theory calculations indicate that the nickel atoms host uniaxial ferromagnetic order of about $0.7{\ensuremath{\mu}}_{B}$ per atom, while an even smaller moment is generated in the itinerant tantalum conduction electrons. Strong correlations are evident in flat bands near the Fermi level, a high heat capacity coefficient, and a high Kadowaki-Woods ratio. Density functional theory calculations suggest that electron and hole Fermi surfaces in the ferromagnetic phase are associated with opposite spin polarization. When the system is diluted of magnetic ions, the samples become superconducting below about 2 Kelvin. We discuss possible mechanisms for superconductivity in this family.

Evidence of pressure-induced multiple electronic topological transitions in BiSe

BiSe, an exciting member of homologous family (Bi2)m (Bi2Se3)n [where, m = 1; n = 2], has recently gained attention in thermoelectric research due to its intrinsically ultralow lattice thermal conductivity and promising n-type thermoelectric performance near room temperature. It is experimentally observed to be a weak topological insulator and possesses a fascinating crystal structure where Bi2 bilayer is sandwiched between two Bi2Se3 quintuple layers, thus the structure resembles natural van der Waals heterostructure (Bi2Se3–Bi2–Bi2Se3). Herein, we have studied the pressure induced electronic topological transition (ETT) in BiSe through high pressure (0–10 GPa) synchrotron X-ray diffraction, Raman spectroscopy and first-principles density functional theory (DFT). We have observed clear anomalies at ∼1 and ∼2.2 GPa in the pressure dependent lattice parameters (a, c, and c/a ratio), cell volume, Raman mode shift and line width data suggesting the presence of two ETTs which is further verified by electronic structures and Fermi surface calculations under pressure. The origin of these ETTs is associated with the two different vibrational modes arising from Bi2 bilayer and Bi2Se3 quintuple layers of BiSe, respectively.

Anomalous metallic oxygen band in the potential superconductor KCa[formula omitted]Fe[formula omitted]As[formula omitted]O[formula omitted]: A DFT study

The electronic structure, magnetism and Fermi surface of a hole self-doped potential iron-based superconductor KCa2Fe4As4O2 (12442) were studied by a first-principles DFT/GGA approach. The self-doping value is obtained from DFT is 0.75 hole on Fe-3d states for KCa2Fe4As4O2. Surprisingly, for the KCa2Fe4As4O2 it was found that except the Fe-3d bands the O-2p bands also cross the Fermi level, which is quite anomalous behavior for O-2p states. In general, the O-2p states are usually fully occupied in transition metal oxides. Here it is not the case due to the fact that CaO layer gives some of the electrons to the FeAs layer, thus leaving O-2p states partially occupied. The O-2p states form additional Fermi surface sheet with propeller-like shape around Γ-point and modify the Fe-3d Fermi surface sheets due to the hybridization. To prove the presence of anomalous metallic O-2p band in KCa2Fe4As4O2 the comparative study with the same family systems KCa2Fe4As4F2 and RbGd2Fe4As4O2 (where it is not the case) was performed. Finally, we discuss the main differences between the emergence of the metallic O-2p band in case of KCa2Fe4As4O2 and those in case of cuprate superconductors LaBaCuO4 and YBa2Cu3O7. Therefore, we expect that the KCa2Fe4As4O2 might be a potential superconductor with unusual properties.

Theory of a continuous bandwidth-tuned Wigner-Mott transition

We develop a theory for a continuous bandwidth-tuned transition at fixed \textit{fractional} electron filling from a metal with a generic Fermi surface to a `Wigner-Mott' insulator that spontaneously breaks crystalline space-group symmetries. Across the quantum critical point, (i) the entire electronic Fermi surface disappears abruptly upon approaching from the metallic side, and (ii) the insulating charge gap and various order parameters associated with the spontaneously broken space-group symmetries vanish continuously upon approaching from the insulating side. Additionally, the insulating side hosts a Fermi surface of neutral spinons. We present a framework for describing such continuous metal-insulator transitions (MITs) and analyze the example of a bandwidth-tuned transition at a filling, $\nu=1/6$, for spinful electrons on the triangular lattice. By extending the theory to a certain large-$N$ limit, we provide a concrete example of such a continuous MIT and discuss numerous experimental signatures near the critical point. We place our results in the context of recent experiments in moir\'e transition metal dichalcogenide materials.