Session A13: Electronic Structure: Theory and Spectra

Atomic Ordering and Electronic Structure of X2YSn Heusler Alloys: A First-Principles Study

The Heusler material class, comprised primarily of ternary intermetallic alloys, boasts a wide range of properties including half-metallic ferromagnetism, topological band structure, and band gap tunability. The competition between the “Full-Heusler” L2₁ and “Inverse-Heusler” XA space groups, complicated by antisite disorder, can disrupt material systems. Understanding the role of element-choice in phase stability and electronic structure is therefore critical to identifying materials robust against possible phase disorder.

 

Density-functional theory calculations have been performed for the X₂YSn material class (X =Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) for the both the L2₁ and XA phases, investigating the stability against antisite disorder. An XA-L2₁ phase transition is found with increasing x-site valency, accompanied by an increasing antialignment of the x-site and yttrium moments, corresponding to a spin-dependent shift in DOS peaks, indicating an asymmetric filling of electron orbitals within the systems in question. This results in movement of half-metallic gaps across the fermi-energy, indicating possible spin-polarization tuning applications.    

Logic gate analysis for spin precession and non-Abelian Berry phase in multi-subband system

In multi-subband systems lacking space inversion symmetry, the spin-orbit interaction (SOI) and inter-subband interaction produce multiple quasidegenerate points (QDPs) having characteristic non-Abelian Berry parameters, as well as time-reversal invariant momenta. In this study, we focus on spin precession and discuss spin features modulated by non-Abelian Berry phases. To extract the effects of non-Abelian Berry phases from spin precessions, we project the corresponding state vectors onto Bloch spheres, eliminating the dynamical phases. We find that the Abelian and non-Abelian Berry phases change the azimuthal and zenith angles, respectively. Furthermore, we interpret them as quantum logic operations and discuss their correspondence to the QDPs. We then apply this formalism to holes confined in SiGe quantum wells with Rashba and Dresselhaus SOIs. In the lower energy region far from the QDPs, R_z operations arise from the Abelian Berry phases. Contrary, we elucidate that, at the energies of the QDPs, the non-Abelian Berry phases cause strong inter-subband hybridizations, realizing NOT-gate operations.   

Electronic structure investigation of pyrochlore spin-ice Pr2Zr2O7

Pyrochlore Praseodymium Zirconate Pr₂Zr₂O₇ is a quantum spin-ice material at low temperatures due to its frustrated magnetic lattice. A compositional series of Pr_2+xZr_2-xO_7-x/2 crystals fabricated by four-mirror optical floating zone furnace technique¹ were characterized using Fourier-transform infrared (FTIR) spectroscopy, cathodoluminescence (CL) microscopy, and spectroscopic ellipsometry and compared to density functional theory (DFT) simulation. Ellipsometry and CL microscopy revealed a band gap of 5.25 eV for the material with a consistent set of mid-gap energy values across the compositional series. FTIR spectroscopy corroborated overlapping energy positions and determined absorption values down to 0.3 eV. Suggested assignments of defects to energy levels measured will be presented based on DFT results.

¹Koohpayeh et al, Journal of Crystal Growth 402 291-298 (2014)   

Theoretical study of the layer number and strain effects on the structural and electronic properties of Platinum diselenide (PtSe2) material

 In recent years, two-dimensional materials have attracted great interest due to their exceptional properties, which differ significantly from bulk materials. Platinum diselenide (PtSe₂) material has shown various electronic features when going from bulk to monolayer (ML).  In this work, density functional theory (DFT) plus van der Waals (vdW) corrections have been considered to study the structural and electronic properties of different PtSe₂ systems, namely ML, bilayer (BL), trilayer (TL), ourlayer (FL), and bulk. In particular, various vdW corrections, have been tested to describe the inter-layer interaction of PtSe₂ material. The accuracy of these corrections is evaluated according to the comparison with the available experimental results. Our results show that while vdW corrections have no significant effect on the electronic structure of bulk PtSe₂, they significantly alter that of PtSe₂ BL, TL and FL materials.  In fact, in the case of these three latter systems,  a correlation between the inter-layer distance and the bandgap feature was obtained. Indeed, the bandgap of PtSe₂ increases with the increase of the inter-layer distance. According to our calculations, PtSe₂ ML and BL-PtSe₂ are indirect gap semiconductors with gap energies of approximately 1.34 and 0.21eV, respectively. Furthermore, the control of the inter-layer distance via vertical strain is considered. The electronic properties of bulk can be obtained for PtSe₂ BL under -17 % vertical strain. Our work shows a deep understanding of the correlation between the structural and electronic properties, and thus a possibility to tune the band gap by  strain means.   

Floating Surface Band in WO2I2

WO₂I₂ is a long-known member of the tungsten oxyhalide family of compounds (WO₂X₂ X = halide) that first garnered interest in the late 1960s due to its role in the chemical vapor transport of elemental tungsten in halogen lamps. Since its discovery, however, the electronic and structural properties of WO₂I₂ remain unresolved. Based on recent single crystal x-ray diffraction experiments and density functional theory (DFT) analysis, WO₂I₂ was reported to be a metal with an orthorhombic unit cell (#71: Immm). Here, we present a combination of transport, scanning tunneling spectroscopy (STS), angle-resolved photoemission spectroscopy (ARPES), and optical conductivity measurements which demonstrate that WO₂I₂ is in fact a semiconductor with a 0.5V gap, in direct contrast with the metallic DFT band structures calculated using the orthorhombic structure. Interestingly, we also observe a surface band that lies within the gap of the semiconductor. We show that this comes from the unique three-dimensional hybridization in WO₂I₂ and its modification at the surface. We will also discuss new structural refinements which suggest that WO₂I₂ has a disordered, monoclinic unit cell.   

A systematic study to investigate the effects of X-ray exposure on electrical properties of silicon dioxide thin films using X-ray photoelectron spectroscopy

X-ray Photoelectron Spectroscopy (XPS) is generally used for chemical analysis of surfaces and interfaces. This method involves the analysis of changes in binding energies and peak shapes of elements under consideration. It is also possible to use XPS to study the effect of X-ray radiation on the electrical properties of thin films. We measured the Si 2p peak using X-ray powers of and on approximately silicon dioxide (SiO₂) thin films grown on both n- and p-type substrates while applying DC or AC external biases. Using the shifts in the binding energy of the Si 2p peak, we calculated the resistances and the capacitances of the SiO₂ thin film. The way that the binding energies of the Si 2p peak and the capacitance of the thin film change as a function of the type of Si substrate and the power of the X-ray are explained using band bending.   

Application of Full-Potential DFT Method Based on Multiple Scattering Theory on Group IV Alloys

The cubic diamond Group IV Si-Ge, Se-Ge-Sn alloys have been a topic of intense research since their inception due to a plethora of applications including logic circuits, cell phones, global positioning systems (GPS), wireless communications (WiFi), optoelectronic devices, and so on. Though extensively studied, it is still challenging to experimentally synthesize Si-Ge-Sn alloy, particularly with high concentrations of Sn and henceforward further computational investigations on this material can greatly facilitate such experimental discovery. In this work, we apply the full potential multiple scattering theory KKR, KKR LSMS, and KKR CPA methods to these Group-IV alloys. Our computed structural and electronic properties are in good agreement with experimental observations and the results obtained from the pseudopotential calculations. We also investigate the possibility of short-range ordering on this Si-Ge-Sn ternary alloy and its effect on engineering material properties. Our work establishes the multiple scattering theory (MuST) open-source code MuST(https://github.com/mstsuite/MuST) for semiconducting materials for the first time.    

Periodic coupled-cluster theory for the ground and excited states with atom-centered basis functions

Periodic coupled-cluster (CC) theory promises to be a reliable, highly accurate electronic structure method in materials science [1]. The all-electron code FHI-aims [2], which employs numeric atom-centered orbitals (NAOs), has recently been interfaced to the CC theory for solids (Cc4s) code [3,4], making CC theory for both the ground state and excited states (in the equation-of-motion CC theory (EOM-CC)) accessible to FHI-aims. For molecules, EOM-CC predicts quasi-particle energies more accurately and reliably than the GW approximation[5], and we expect that this will also hold for bandstructures and gaps of solids. Like most correlated wave function methods, CC methods exhibit excessively slow convergence with the size of the super cell. While in the plane wave basis framework and in the case of the CC ground state, an efficient approach has been suggested for solving this problem [6], no analogous technique exists for NAOs, so far. We present the current state of the CC theory framework available in FHI-aims and possible avenues to address the finite-size error.

[1] G. Booth et al., Nature 493, 365–370 (2013)

[2] The FHI-aims web page, https://fhi-aims.org

[3] F. Hummel et al., JCP, 146, 124105 (2017)

[4] E. Moerman et al., JOSS, 7, 4 (2022)

[5] M. Lange et al.,JCTC, 14, 4224-4236 (2018)

[6] K. Liao, A. Grüneis, JCP, 145, 141102 (2016)   

Electronic structure and thermoelectric performance of SnSe with controlled vacancy population

We report the electronic structure of thermoelectric material SnSe studied by angle-resolved photoemission spectroscopy and first principles calculations. We found that the thermoelectric performance of SnSe strongly depends on its low‑energy electron band structure that provides high density of states in a narrow energy window due to the multi‑valley valence band maximum. Moreover, the binding energy of the valence band maximum can be tuned by the population of Sn vacancy, which can be controlled by the cooling rate during the sample growth. The shift in the valence band maximum follows precisely the behavior of the thermoelectric power factor, while the effective mass is barely modified upon changing the population of Sn vacancies. These findings indicate that the low‑energy electronic structure is closely related with the high thermoelectric performance of hole‑doped SnSe, providing a viable route toward engineering the intrinsic defect‑enhanced thermoelectric performance.   

First Principle Calculations of Electronic Properties of Bulk and 2D Hexagonal Boron Nitride (hex-BN)

We present results from ab-initio, self-consistent density functional theory (DFT) calculations of electronic properties of hexagonal boron nitride (h-BN), for the bulk and a 2D sample. The bulk h-BN belongs to the <!--[if gte msEquation 12]> style='font-family:"Cambria Math",serif;mso-ascii-font-family:"Cambria Math";
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normal'>D _{style='mso-bidi-font-style:normal'>6h} ^{style='mso-bidi-font-style:normal'>4} space group and is in the hexagonal structure with the space group P6₃/mmc and Pearson symbol hP6 (#194); the 2D sample is also in the hexagonal structure with the space group P-6m2. We utilized a local density approximation (LDA) potential and the linear combination of atomic orbital (LCAO) formalism. Our calculations performed a generalized minimization of the energy to reach the ground state, as required by the second DFT theorem. This process ensures the full, physical content of our findings that include electronic energy bands, total and partial densities of states, and electron and hole effective masses. Our calculated band gap for room temperature lattice constants of a = 2.504 Å and c = 6.661 Å is indirect band gap of 4.37 eV, for the bulk. Our preliminary results for the 2D sample is a direct band gap of 4.7 eV.   

Ab-Initio Computations of Electronic and Related Properties of Hexagonal ZrS2

We have performed ab-initio, self-consistent calculations of electronic, transport, and bulk properties of hexagonal ZrS₂. Our computations employed the local density approximation (LDA) potential of Ceperley and Alder and the linear combination of atomic orbital (LCAO) formalism. We performed a generalized minimization of the energy using successive, self-consistent calculations with augmented basis sets to reach the ground state of the material, without employing over-complete basis sets. The bulk h-ZrS₂ is in the hexagonal structure with the space group Pm1 and Pearson symbol hP3 (#164).  Calculations are performed for a room temperature lattice constant of a = 3.69 Å and c = 6.04 Å. We discuss the band gap, the total and partial densities of states, electron and hole effective masses, and the bulk modulus.   

Interband and exciton related optical properties of ZnGeN2 in comparison to GaN

In this work, the optical dielectric function of ZnGeN2 is calculated from the interband transitions using the energy bands calculated in the quasiparticle self-consistent (QS)GWˆ method with two different levels of approximation: the independent particle approximation (IPA) and the Bethe-Salpeter Equation (BSE) approach. While the IPA allows us to relate peaks in ε2(ω) to specific bands and k-points, it does not include electron-hole interaction effects. The second approximation includes electron-hole interaction or excitonic effects. The corresponding changes due to these excitonic effects in the shape of ε2(ω) are found to be similar to those in GaN. The screened Coulomb interaction Wˆ is here calculated including electron-hole interactions in the polarization function and gives a band structure already going beyond the random phase approximation. The convergence of the bound excitons with the density of the k-mesh included in the BSE is studied. The excitons are related to each of the three-fold split valence bands and the splittings of the latter are also studied as a function of strain. 

This work was supported by the U.S. Department of Energy Basic Energy Sciences (DOE-BES) under grant No. DE-SC0008933.   

Energy gap and interband transitions in black phosphorus studied with momentum-resolved EELS

Black phosphorus has been intensively studied because of its narrow energy gap and large in-plane anisotropy, giving it great potential for micro electronic and optical sensors. However, few studies of the band gap at finite momentum transfer have been reported. Using momentum-resolved electron energy-loss spectroscopy (M-EELS), we measured the interband transitions of black phosphorus as a function of temperature and momentum transfer.

We found that, as the temperature is lowered, the energy gap in bulk black phosphorus decreases for all momenta, from near the optical limit to the Brillouin boundary. We also observed, for T=30K, that a new peak emerges near the band edge, for momenta near the zone boundary. This peak may correspond to a dark exciton from the band structure changes at low temperature.   

Quantum thermal avalanche: Spectral characteristics of size-invariant variations of energy landscapes

Size-invariant shape transformation is a technique of altering the shape of a domain by preserving its sizes. It leads to quantum shape effects in the thermodynamic and transport properties of quantum-confined systems. In this work, we investigate interesting spectral properties of systems involving size-invariant variations in their Hamiltonians. In particular, we show that the geometric couplings between levels generated by the size-invariant shape transformations cause nonuniform scaling in the spectra. The defining characteristics of such transformations are ground state reduction and differential modifications in the spectral gaps. Moreover, we find that the ground state reduction leads to so-called quantum thermal avalanche causing an unexpected swapping in the thermal occupation probabilities. Thermal occupation of the ground state abruptly increases while the that of excited states decreases. This transition has intriguing non-classical implications, including spontaneous shifts to lower-entropy states in systems that exhibit the quantum shape effect. We also identify avoided crossing effects in the spectra which are important in determining the peculiar behaviors in the thermodynamic properties. The implications of this study could be generalized to any potential energy profile with an appropriate variable causing size-invariant modifications of the energy landscape.