Three-dimensional Electronic Structures Research Articles

Two-dimensional heavy fermions in the van der Waals metal CeSiI.

Heavy-fermion metals are prototype systems for observing emergent quantum phases driven by electronic interactions1-6. A long-standing aspiration is the dimensional reduction of these materials to exert control over their quantum phases7-11, which remains a significant challenge because traditional intermetallic heavy-fermion compounds have three-dimensional atomic and electronic structures. Here we report comprehensive thermodynamic and spectroscopic evidence of an antiferromagnetically ordered heavy-fermion ground state in CeSiI, an intermetallic comprising two-dimensional (2D) metallic sheets held together by weak interlayer van der Waals (vdW) interactions. Owing to its vdW nature, CeSiI has a quasi-2D electronic structure, and we can control its physical dimension through exfoliation. The emergence of coherenthybridization of f and conduction electrons at low temperature is supported by the temperature evolution of angle-resolved photoemission and scanning tunnelling spectra near the Fermi level andby heat capacity measurements. Electrical transport measurements on few-layer flakes reveal heavy-fermion behaviour and magnetic order down to the ultra-thin regime. Our work establishes CeSiI and related materials as a unique platform for studying dimensionally confined heavy fermions in bulk crystals and employing 2D device fabrication techniques and vdW heterostructures12 to manipulate the interplay between Kondo screening, magnetic order and proximity effects.

Nature of quasiparticle interference in three dimensions

Quasiparticle interference (QPI) imaging is a powerful tool for the study of the low-energy electronic structure of quantum materials. However, the measurement of QPI by scanning tunneling microscopy (STM) is restricted to surfaces and is thus inherently constrained to two dimensions. QPI has proved immensely successful for the study of materials that exhibit a quasi-two-dimensional electronic structure, yet it raises questions about how to interpret QPI in materials that have a highly three-dimensional electronic structure. In this paper, we address this question and establish the methodology required to simulate and understand QPI arising from three-dimensional systems as measured by STM. We calculate the continuum surface Green's function in the presence of a defect, which captures the role of the surface and the vacuum decay of the wave functions. We find that defects at different depths from the surface will produce unique sets of scattering vectors for three-dimensional systems, which nevertheless can be related to the three-dimensional electronic structure of the bulk material. We illustrate the consequences that the three-dimensionality of the electronic structure has on the measured QPI for a simple cubic nearest-neighbor tight-binding model, and then demonstrate application to a real material using a realistic model for PbS. Our method unlocks the use of QPI imaging for the study of quantum materials with three-dimensional electronic structures and introduces a framework to generically account for ${k}_{z}$ dispersions within QPI simulations.

Electronic states dressed by an out-of-plane supermodulation in the quasi-two-dimensional kagome superconductor CsV3Sb5

${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ has attracted much recent attention as the first quasi-two-dimensional (2D) kagome superconductor. While the kagome layers are 2D in nature, increasing evidence has pointed to the importance of out-of-plane correlation in this material. However, it remains unclear whether such correlation can change the fundamental electronic structure of the quasi-2D system. Here, we reveal this missing piece of information, using angle-resolved photoemission spectroscopy, complemented by scanning tunneling microscope measurements. The three-dimensional electronic structures in the high-temperature state are revealed, which agree well with density-functional theory calculations. Electron energy bands are observed in the low-temperature state that exhibit additional periodicities along the out-of-plane momentum. These results reveal a direct response to the out-of-plane electronic supermodulation in the single-particle spectral function of ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$, thus establishing an electronic platform to examine emergent phenomena beyond 2D limit in kagome superconductors.

Structural and Electronic Modulations of Imidazolium Covalent Organic Framework-Derived Electrocatalysts for Oxygen Redox Reactions in Rechargeable Zn-Air Batteries.

Covalent organic frameworks (COFs) are promising candidates for the controllable design of electrocatalysts. However, bifunctional electrocatalytic activities for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) remain challenging in COFs. In this study, imidazolium-rich COFs (IMCOFs) with well-defined active sites and characteristic three-dimensional assembly structures were readily prepared, and their electronic structures were tuned by Co incorporation to elicit bifunctional electrocatalytic activities for the ORR and OER. The Co nanoparticle-incorporated spherical IMCOF-derived electrocatalyst (CoNP-s-IMCOF) exhibited lower overpotentials for the ORR and OER compared with the atomic Co-incorporated planar IMCOF-derived electrocatalyst (Co-p-IMCOF). Computational simulations revealed that the imidazole carbon sites of CoNP-s-IMCOF rather than the triazine carbons were the active sites for the ORR and OER, and its p-band center downshifted via charge transfer, facilitating the chemisorption of oxygen intermediates during the reactions. A Zn-air battery with CoNP-s-IMCOF exhibited a small voltage gap of 1.3 V with excellent durability for 935 cycles. This approach for control over the three-dimensional assembly and electronic structures of IMCOFs can be extended to the development of diverse catalytic nanomaterials for applications of interest.

Stable organic lead iodides with three-dimensional crystallographic and electronic structures showing high photoresponse

3D lead iodide perovskite analogs display both high stability and quasi-isotropic photoresponse, consistent with both 3D crystallographic and electronic structures.

Thermoelectric Properties of InA Nanowires from Full-Band Atomistic Simulations.

In this work we theoretically explore the effect of dimensionality on the thermoelectric power factor of indium arsenide (InA) nanowires by coupling atomistic tight-binding calculations to the Linearized Boltzmann transport formalism. We consider nanowires with diameters from 40 nm (bulk-like) down to 3 nm close to one-dimensional (1D), which allows for the proper exploration of the power factor within a unified large-scale atomistic description across a large diameter range. We find that as the diameter of the nanowires is reduced below d < 10 nm, the Seebeck coefficient increases substantially, as a consequence of strong subband quantization. Under phonon-limited scattering conditions, a considerable improvement of ~6× in the power factor is observed around d = 10 nm. The introduction of surface roughness scattering in the calculation reduces this power factor improvement to ~2×. As the diameter is decreased to d = 3 nm, the power factor is diminished. Our results show that, although low effective mass materials such as InAs can reach low-dimensional behavior at larger diameters and demonstrate significant thermoelectric power factor improvements, surface roughness is also stronger at larger diameters, which takes most of the anticipated power factor advantages away. However, the power factor improvement that can be observed around d = 10 nm could prove to be beneficial as both the Lorenz number and the phonon thermal conductivity are reduced at that diameter. Thus, this work, by using large-scale full-band simulations that span the corresponding length scales, clarifies properly the reasons behind power factor improvements (or degradations) in low-dimensional materials. The elaborate computational method presented can serve as a platform to develop similar schemes for two-dimensional (2D) and three-dimensional (3D) material electronic structures.

Anisotropic Transport and Quantum Oscillations in the Quasi-One-Dimensional TaNiTe5: Evidence for the Nontrivial Band Topology.

The past decade has witnessed the burgeoning discovery of a variety of topological states of matter with distinct nontrivial band topologies. Thus far, most materials that have been studied possess two-dimensional or three-dimensional electronic structures, with only a few exceptions that host quasi-one-dimensional (quasi-1D) topological electronic properties. Here we present clear-cut evidence for Dirac Fermions in the quasi-1D telluride TaNiTe5. We show that its transport behaviors are highly anisotropic, and we observe nontrivial Berry phases via quantum oscillation measurements. The nontrivial band topology is further corroborated by first-principles calculations. Our results may help to guide the future quest for topological states in this new family of quasi-1D ternary chalcogenides.

Archetypical “push the band critical point” mechanism for peaking of the density of states in three-dimensional crystals: Theory and case study of cubic H3S

The point of zero gradient of the electronic band structure, the critical point, generally induces the singularity in the density of states (DOS), but no isolated critical point yields strict divergence of the DOS in three dimensions, differently from the lower-dimensional cases. In view of the band structure as a smooth hypersurface on the reciprocal space, we discuss the minimal deformation of the band structure that yields nondivergent but large sharp DOS peaks in three dimensions. By ``pushing down'' the energy level at the second-order saddle point (maximum), a continuous closed loop of saddle points (sphere of maxima) encircling the original position of the saddle point (maximum) emerges, with which the DOS peak is formed. Being high-dimensional features, the saddle loop and extremum shell thus formed are difficult to locate with standard band structure analysis on linear $\mathbf{k}$-point paths. The Lifshitz transition occurring over a linear or planar manifold is discussed as an indicator of such features. We also find that the celebrated DOS peak in the recently discovered superconducting hydride ${\mathrm{H}}_{3}\mathrm{S}$ originates from the saddle loop. On this basis, we successfully extract the minimal model that explains how the DOS peak is formed. Our theory characterizes a large class of DOS peaks sometimes found in the three-dimensional electronic structures, building a basis for profound understanding of their origins.

A stochastic dynamic mass spectrometric diffusion method and its application to 3D structural analysis of the analytes

Abstract There is a straightforward line in the recent development of the functional model connecting the experimental mass spectrometric variable intensity of a peak of an analyte ion with its thermodynamic, kinetic and diffusion parameters. It has been shown that the temporal behavior of the outcome intensity obeys a certain law: ${{\text{D}}_{{\text{SD}}}}{\text{ }} = {\text{ }}1.3193{\text{ }} \times {\text{ }}{10^{ - 14}}{\text{ }} \times {\text{ }}A{\text{ }} \times {\text{ }}{{(\overline {{I^2}} - {{(\bar I)}^2})} \over {{{(I - \bar I)}^2}}}.$ D SD = 1.3193 × 10 − 14 × A × ( I 2 ¯ − ( I ¯ ) 2 ) ( I − I ¯ ) 2 . This formula is universally applicable and empirically testable and verifiable. It connects the intensity with the so-called stochastic dynamic diffusion “DSD” parameter. Its application to small-scale research, so far, using soft-ionization electrospray, atmospheric pressure chemical ionization, matrix-assisted laser desorption/ionization or collision-induced dissociation methods has shown that the DSD parameter is linearly connected with the so-called quantum chemical diffusion parameter “DQC,” obtained within Arrhenius’s theory. Therefore, the DSD parameter connects experimental measurable parameters of ions with their three-dimensional (3D) molecular and electronic structures. The corroborated empirical proof, so far, has convincingly argued that the mass spectrometry appears to be not only a robust instrumentation for highly accurate, precise and selective quantification but also is capable of providing the exact 3D molecular structure of the analytes, when it is used complementary to high accuracy methods of the computational quantum chemistry.

Electrical Properties of Two-Dimensional Materials Used in Gas Sensors.

In the search for gas sensing materials, two-dimensional materials offer the possibility of designing sensors capable of tuning the electronic band structure by controlling their thickness, quantity of dopants, alloying between different materials, vertical stacking, and the presence of gases. Through materials engineering it is feasible to study the electrical properties of two-dimensional materials which are directly related to their crystalline structure, first Brillouin zone, and dispersion energy, the latter estimated through the tight-binding model. A review of the electrical properties directly related to the crystalline structure of these materials is made in this article for the two-dimensional materials used in the design of gas sensors. It was found that most 2D sensing materials have a hexagonal crystalline structure, although some materials have monoclinic, orthorhombic and triclinic structures. Through the simulation of the mathematical models of the dispersion energy, two-dimensional and three-dimensional electronic band structures were predicted for graphene, hexagonal boron nitride (h-BN) and silicene, which must be known before designing a gas sensor.

Electronic structure and spin dynamics of ACo2As2(A=Ba,Sr,Ca)

The electronic structures, charge and spin dynamics of the cobalt pnictide compounds ACo2As2 (A=Ba, Sr, Ca) in the paramagnetic state are investigated by using density functional theory combined with dynamical mean-field theory. In contrast to their iron counterparts, these cobalt pnictide compounds have three-dimensional electronic structures and strong ferromagnetic low-energy spin excitations. The Co 3d eg orbitals dominate the electronic states around the Fermi level and have stronger electronic correlation strength than the Co 3d t2g orbitals. The overall electronic correlation strength is much weaker than that in the iron arsenides, however, the most strongly correlated Co 3d x2-y2 orbital, especially in CaCo2As2, has electronic correlation strength comparable to Fe 3d t2g orbitals in iron arsenides. ACo2As2 (A=Ba, Sr, Ca) shows similar electronic structures where a conduction band of primarily Co 3d x2-y2 orbital character is close to a Van Hove singularity around the Brillouin-zone corner, which promotes ferromagnetic low-energy spin excitations. Originated from its increased nearest-neighbor Co-Co distance and significantly reduced As height from the Co plane, the strong electronic correlation strength and close proximity to the Van Hove singularity of the Co 3d x2-y2 orbital in CaCo2As2 is responsible for its unique A-type antiferromagnetic order observed in experiments. In comparison, despite substantial ferromagnetic low-energy spin excitations, BaCo2As2 and SrCo2As2 remain paramagnetic down to very low temperature because the Co 3d x2-y2 orbital has weaker electronic correlation strength and is further away from the Van Hove singularity.

Single crystalline electronic structure and growth mechanism of aligned square graphene sheets

Recently, commercially available copper foil has become an efficient and inexpensive catalytic substrate for scalable growth of large-area graphene films for fundamental research and applications. Interestingly, despite its hexagonal honeycomb lattice, graphene can be grown into large aligned square-shaped sheets on copper foils. Here, by applying angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES) to study the three-dimensional electronic structures of square graphene sheets grown on copper foils, we verified the high quality of individual square graphene sheets as well as their merged regions (with aligned orientation). Furthermore, by simultaneously measuring the graphene sheets and their substrate copper foil, we not only established the (001) copper surface structure but also discovered that the square graphene sheets’ sides align with the ⟨110⟩ copper direction, suggesting an important role of copper substrate in the growth of square graphene sheets—which will help the development of effective methods to synthesize high-quality large-size regularly shaped graphene sheets for future applications. This work also demonstrates the effectiveness of micro-ARPES in exploring low-dimensional materials down to atomic thickness and sub-micron lateral size (e.g., besides graphene, it can also be applied to transition metal dichalcogenides and various van der Waals heterostructures)

Anisotropic thermoelectric properties in layered complex nitrides with α-NaFeO2-type structure

Electronic structures and thermoelectric transport properties of α-NaFeO2-type d0-electron layered complex nitrides AMN2 (A = Sr or Na; M = Zr, Hf, Nb, Ta) were evaluated using density-functional theory and Boltzmann theory calculations. Despite the layered crystal structure, all materials had three-dimensional electronic structures. Sr(Zr, Hf)N2 exhibited isotropic electronic transport properties because of the contribution of the Sr 4d orbitals to the conduction band minimums (CBMs) in addition to that of the Zr 4d (Hf 5d) orbitals. Na(Nb,Ta)N2 showed weak anisotropic electronic transport properties due to the main contribution of the Nb 4d (Ta 5d) and N 2p orbitals to the CBMs and no contribution of the Na orbitals.

Three-dimensional electronic structures and the metal-insulator transition in Ruddlesden-Popper iridates

In this study, we systematically investigate 3D momentum($\hbar k$)-resolved electronic structures of Ruddlesden-Popper-type iridium oxides Sr$_{n+1}$Ir$_n$O$_{3n+1}$ using soft-x-ray (SX) angle-resolved photoemission spectroscopy (ARPES). Our results provide direct evidence of an insulator-to-metal transition that occurs upon increasing the dimensionality of the IrO$_2$-plane structure. This transition occurs when the spin-orbit-coupled $j_{\rm eff}$=1/2 band changes its behavior in the dispersion relation and moves across the Fermi energy. In addition, an emerging band along the $\Gamma$(0,0,0)-R($\pi$,$\pi$,$\pi$) direction is found to play a crucial role in the metallic characteristics of SrIrO$_3$. By scanning the photon energy over 350 eV, we reveal the 3D Fermi surface in SrIrO$_3$ and $k_z$-dependent oscillations of photoelectron intensity in Sr$_3$Ir$_2$O$_7$. In contrast to previously reported results obtained using low-energy photons, folded bands derived from lattice distortions and/or magnetic ordering make significantly weak (but finite) contributions to the $k$-resolved photoemission spectrum. At the first glance, this leads to the ambiguous result that the observed $k$-space topology is consistent with the unfolded Brillouin zone (BZ) picture derived from a non-realistic simple square or cubic Ir lattice. Through careful analysis, we determine that a superposition of the folded and unfolded band structures has been observed in the ARPES spectra obtained using photons in both ultraviolet and SX regions. To corroborate the physics deduced using low-energy ARPES studies, we propose to utilize SX-ARPES as a powerful complementary technique, as this method surveys more than one whole BZ and provides a panoramic view of electronic structures.

Three-dimensionality of electronic structures and thermoelectric transport in SrZrN₂ and SrHfN₂ layered complex metal nitrides.

Layered materials have several properties that make them suitable as high-performance thermoelectric materials. In this study, we focus on the complex metal nitrides SrZrN2 and SrHfN2, which have an α-NaFeO2 layered crystal structure. To determine their electronic band structure features and potential thermoelectric transport properties, we calculated the electronic band structures and electronic transport coefficients for SrZrN2 and SrHfN2 using density-functional theory and Boltzmann transport theory, respectively. Despite the layered crystal structure, SrZrN2 and SrHfN2 both had three-dimensional electronic structures and isotropic electronic transport because of the contribution of the Sr 4d(x(2)-y(2)) + 4d(xy) orbitals to the bottom of the conduction bands in addition to that of the Zr 4d(z)(2) (Hf 5d(z)(2)) orbital. The three-dimensional electronic structures predict the appearance of large Seebeck coefficients (-145 μV K(-1) at 300 K, -370 μV K(-1) at 1200 K) and large electronic thermoelectric figures of merit.

Single-Component Molecular Conductor [Cu(tmdt)2] Containing an Antiferromagnetic Heisenberg Chain

Traditional molecular conductors are composed of more than two chemical species and are characterized by low-dimensional electronic band structures. By contrast, the single-component molecular metals [M(tmdt)(2)] (M = Ni, Pt, Au; tmdt = trimethylenetetrathiafulvalenedithiolate) possess three-dimensional electronic structures that can be widely tuned by exchanging the central transition metal atom (M). In this study, the Cu atom was used to realize a new magnetic single-component molecular conductor exhibiting strong pi-d interactions. The crystal structure of [Cu(tmdt)(2)] was found to be essentially the same as those of the Ni, Pt, or Au-based systems with metallic states down to low temperature, but different from the structure of [Cu(dmdt)(2)] (dmdt = dimethyltetrathiafulvalenedithiolate) with its tetrahedrally coordinated dmdt ligands. A compressed pellet of microcrystals exhibited fairly high room-temperature conductivity (sigma(RT) approximately 7 S.cm(-1)), which increased almost linearly with pressure, reaching 110 S.cm(-1) at 15 kbar. This strongly suggests that the single crystal of [Cu(tmdt)(2)] is metallic at high pressure. Magnetic susceptibility measurements indicated one-dimensional Heisenberg behavior with |J| = 117 cm(-1) and an antiferromagnetic transition at 13 K. Density functional theory molecular orbital calculations revealed that the alpha-spin orbital of pdsigma(-) is distributed at the central part of the complex (CuS(4)), and alpha- and beta-sym-Lpi orbitals have almost the same energies and their spins are distributed mainly in the pdsigma(-) orbital. This is in contrast to the first single-component molecular metal [Ni(tmdt)(2)], which has stable metal bands formed from an almost degenerated sym-Lpi orbital (the highest occupied molecular orbital) and asym-Lpi(d) orbital (the lowest unoccupied molecular orbital). These results suggest that the alpha-pdsigma(-) state of [Cu(tmdt)(2)] exists just around the Fermi energy of the virtual metal band formed from the asym-Lpi(d) and sym-Lpi states. Thus, as expected, [Cu(tmdt)(2)] is a non-trivial single-component molecular conductor with pi-d multifrontier orbitals. In addition, ((n)Bu(4)N)(2)[Cu(tmdt)(2)] was synthesized, and its crystal structure was determined. Its Curie behavior (chi(rt) = 1.2 x 10(-3) emu mol(-1); C = 0.36 emu.K mol(-1)) indicates the existence of an isolated S = 1/2 spin on each dianionic molecule.

First-Principles Study of NaFeAs, NaCoAs, and NaNiAs

To investigate the material dependence of the electronic structure of arsenide superconductors, the chemical trend of the Kohn–Sham band structures of a series of compounds, i.e., NaFeAs, NaCoAs, and NaNiAs, is studied by first-principles calculation based on generalized gradient approximation. Hypothetical structures of NaCoAs and NaNiAs in P 4/ n m m are found to be stable by structural optimization simulation. Results on the electronic states suggest that a characteristic two-dimensional electronic structure appearing as rodlike Fermi pockets is clearly found only in NaFeAs, while three-dimensional electronic structures are found in NaCoAs and NaNiAs with larger density of states than NaFeAs at the Fermi level, when paramagnetic electronic states are assumed.

バルク敏感角度分解光電子分光による固体３次元電子構造とフェルミ面の観測

High-resolution and high-excitation energy photoemission spectroscopy has an advantage in probing the bulk electronic states in solids whereas the surface electronic states deviated from the bulk states are strongly reflected in low-excitation energy photoemission spectra which have so far extensively measured for many materials. We have succeeded in realization of the high-energy bulk-sensitive angle-resolved photoemission in order to probe the three-dimensional electronic structures and the Fermi surfaces. After introducing the angle-integrated photoemission spectra of 3d1-electron system Sr1-xCaxVO3 as an example for the bulk (surface) sensitivity of the high-(low-)energy photoemission, we show the recently obtained results of high-energy angle-resolved photoemission for strongly correlated electron systems such as Sr2RuO4 and ferromagnet CeRu2Ge2 in the paramagnetic phase, revealing their bulk Fermi surfaces. It is found that the Fermi surfaces of CeRu2Ge2 in the paramagnetic phase are substantially deviated from those in the ferromagnetic phase. Although our results for CeRu2Ge2 are roughly explained by the band-structure calculation for LaRu2Ge2, a few qualitative discrepancies are seen.

Stereo-PEEM for three-dimensional atomic and electronic structures of microscopic materials

The principle and design of a new photoemission electron microscope (PEEM), which is called Stereo-PEEM, is described here. Stereo-PEEM can display not only the image of microscopic materials but also the angular distribution of high-energy photoelectrons up to about ±60°, which is about 100-fold the acceptance angle of usual PEEM. This wide angle acceptance for high-energy photoelectrons enables the three-dimensional (3D) display of atomic structure as well as the 3D electronic structure of individual micromaterials. The 3D atomic structure of a sample can be observed directly by taking stereophotographs using photons with angular momentum.

Three-dimensional electronic structures in the Si inversion layer of nanoscale metal-oxide-semiconductor field-effect transistors

The three-dimensional electronic structure in the Si inversion layer of nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs) were calculated by using a self-consistent method. The electronic energy states and the probability density functions in a three-dimensionally confined quantum structure were determined. The energy states strongly depended on the thickness of the thin oxide layer and the applied gate voltage. The few electrons occupying the Si inversion layer significantly affected the electric potential profile of the inversion layer, and a small variation in the oxide thickness dramatically changed the electronic properties in the Si inversion layer. These results can help in understanding the electronic structures in Si inversion layers of nanoscale MOSFETs.