Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2021"

Higher-Order Topological Band Structures

Abstract: The interplay of topology and symmetry in a material's band structure may result in various patterns of topological states of different dimensionality on the boundary of a crystal. The protection of these “higher‐order” boundary states comes from topology, with constraints imposed by symmetry. Herein, the bulk–boundary correspondence of topological crystalline band structures, which relates the topology of the bulk band structure to the pattern of the boundary states, is reviewed. Furthermore, recent advances in the K‐theoretic classification of topological crystalline band structures are discussed.

Origin of Red Emission in β‐Ga 2 O 3 Analyzed by Cathodoluminescence and Photoluminescence Spectroscopy

Abstract: The spectroscopic techniques of cathodoluminescence (CL) and photoluminescence (PL) are used to study the origin of red emission in β-Ga 2O 3 grown using the edge-defined film-fed grown (EFG) method and hydride vapor phase epitaxy. Room-temperature CL shows red emission peaks from samples doped with Fe, Sn, and Si and from unintentionally doped (UID) samples. Narrow emission lines around 690 nm are seen strongly in the Fe and UID samples. Temperature-dependent PL analysis of the two prominent red emission lines reveals properties similar to the R lines in sapphire for all samples but with different levels of emission intensities. These lines are attributed to Cr 3+ ionic transitions rather than Fe 3+, as reported previously. The most likely origin of the unintentional Cr incorporation is the source material used in the EFG method.

Strong and Weak 3D Topological Insulators Probed by Surface Science Methods

Abstract: We review the contributions of surface science methods to discover and improve 3D topological insulator materials, while illustrating with examples from our own work. In particular, we demonstrate that spin-polarized angular-resolved photoelectron spectroscopy is instrumental to evidence the spin-helical surface Dirac cone, to tune its Dirac point energy towards the Fermi level, and to discover novel types of topological insulators such as dual ones or switchable ones in phase change materials. Moreover, we introduce procedures to spatially map potential fluctuations by scanning tunneling spectroscopy and to identify topological edge states in weak topological insulators.

Ab Initio Theory of Fourier‐Transformed Quasiparticle Interference Maps and Application to the Topological Insulator Bi2Te3

Abstract: The quasiparticle interference (QPI) technique is a powerful tool that allows to uncover the structure and properties of electronic structure of a material combined with scattering properties of defects at surfaces. Recently this technique has been pivotal in proving the unique properties of the surface state of topological insulators which manifests itself in the absence of backscattering. In this work we derive a Green function based formalism for the ab initio computation of Fourier-transformed QPI images. We show the efficiency of our new implementation at the examples of QPI that forms around magnetic and non-magnetic defects at the Bi$_2$Te$_3$ surface. This method allows a deepened understanding of the scattering properties of topologically protected electrons off defects and can be a useful tool in the study of quantum materials in the future.

Nodal Semimetals: A Survey on Optical Conductivity

Abstract: Among different topological and related phases of condensed matter, nodal semimetals occupy a special place - the electronic band topology in these materials is related to three-dimensional bulk, rather than to surface, states. A great variety of different realizations of electronic band crossings (the nodes) leads to a plethora of different electronic properties, ranging from the chiral anomaly to solid-state realizations of a black-hole horizon. The different nodal phases have similar low-energy band structure and quasiparticle dynamics, which both can be accessed experimentally by a number of methods. Optical measurements with their large penetration depth and high energy resolution are ideally suited as such a bulk probe; especially at low energies where other spectroscopic methods often lack the required resolution. In this contribution, we review recent optical-conductivity studies of different nodal semimetals, discuss possible limitations of such measurements, and provide a comparison between the experimental results, simple theoretical models, and band-structure-based calculations.

Space‐Projected Conductivity and Spectral Properties of the Conduction Matrix

Abstract: In this paper, we utilize the Kubo-Greenwood formula to spatially project the electronic conductivity into real space, and discuss a Hermitian positive semidefinite matrix Γ , which we call the conduction matrix, that reduces the computation of spatial conduction activity to a diagonalization. We show that for low density amorphous carbon, connected sp rings the sp chains are conduction-active sites in the network. In amorphous silicon, transport involves hopping through tail states mediated by the defects near the Fermi-level. We find that for liquid silicon, thermal fluctuations induce spatial and temporal conductivity fluctuations in the material. We also study the frequency-dependent absorption of light as a function of wavelength in an amorphous silicon suboxide (a-SiO1.3). We show that the absorption is strongly frequency dependent and selects out different oxygen vacancy subnetworks depending on the frequency. We diagonalize Γ to obtain conduction eigenvalues and eigenvectors and show that the density of states of the eigenvalues for FCC aluminum has an extended spectral tail that distinguishes metals from insulators and semiconductors. The method is easy to implement with any electronic structure code providing suitable estimates for single-particle electronic states and energies.

Bain Deformation Mechanism and Lifshitz Transition in Magnesium under High Pressure

Abstract: The phase transitions of alkaline-earth metals (AEMs) under extreme pressure have revealed orbital electron transfer from lowto high-pressure phases. The electrons are anticipated to play an important role in describing the electronic structures of AEMs. Calculations in terms of structural energy differences have revealed a hexagonal close-packed (hcp) structure of magnesium (Mg). The crystal structure of Mg has been observed at high pressure by using energy-dispersive X-ray diffraction (XRD) at room temperature, with it transforming into the body-centered cubic (bcc) structure at 50GPa. Moriarty and McMahan predicted the conditions with which the hcp structure of Mg transformed into the bcc structure and then transformed into the facecentered cubic (fcc) structure by linear muffin-tin orbital calculations. In addition, Errandonea et al. revealed the behavior of Mg using in situ energy-dispersive XRD in a multianvil apparatus. The results of the experimental observations indicated that the hcp structure transformed to a double hexagonal close-packed (dhcp) structure above 9.6 GPa, with no hcp to bcc transition found. Moreover, Li et al. extensively explored crystal prediction evidence of the fcc structure for Mg at 456GPa. To investigate the possibility of the stabilized fcc structure, the high-pressure phase transitions of Mg were considered by Yao and Klug, who explored the characterized phase-transition mechanisms of the dhcp transition, as well as the stabilized fcc structure, by first-principles calculations. The calculations indicated that the bcc structure transformed into the fcc structure above 450 GPa. Recently, the phase diagram of Mg has been investigated using a combination of XRD and resistive and laser heating at high temperatures and pressures ranging from 211 GPa at 300 K to 105 GPa at 4500 K. It was also reported that the hcp structure transformed to the bcc structure at 45 GPa. The melting of Mg has been investigated in the bcc structure by density functional theory. The calculations predicted that the maximum melting temperature and pressure of the bcc structure are 4500 K and 300 GPa, respectively. Several AMEs have been investigated under high pressure for metallization, including Ca, Sr, and Ba. Ca possesses a high superconductivity temperature of 29 K under a pressure of 216 GPa and also has the highest superconducting critical temperature among metal elements. In the case of Mg, the high-pressure phase of Mg has revealed that the fcc structure remained metallic and it is thus anticipated that Mg could transform to a superconducting phase at very high pressure. The structural phase transitions in AEMs have been previously investigated. A theoretical study of Sr, as indicated by the screened exchange local density approximation, was able to predict the transition nature of Sr and in turn reported agreement with experimental observations. Moreover, the structures in Dr. P. Tsuppayakorn-aek, Dr. T. Bovornratanaraks Extreme Conditions Physics Research Laboratory (ECPRL) and Physics of Energy Materials Research Unit Department of Physics Faculty of Science Chulalongkorn University 10330 Bangkok, Thailand E-mail: thiti.b@chula.ac.th Dr. P. Tsuppayakorn-aek, Dr. T. Bovornratanaraks Thailand Center of Excellence in Physics Commission on Higher Education 328 Si Ayutthaya Road, Bangkok 10400, Thailand Dr. W. Luo, Prof. R. Ahuja Condensed Matter Theory Group Department of Physics and Astronomy Uppsala University Box 516, S-751 20 Uppsala, Sweden E-mail: wei.luo@physics.uu.se Dr. J. Zhang, Dr. Y. Ding Center for High Pressure Science and Technology Advanced Research Beijing 100094, People’s Republic of China E-mail: yang.ding@hpstar.ac.cn Prof. R. Ahuja Applied Materials Physics Department of Materials and Engineering Royal Institute of Technology (KTH) S-100 44 Stockholm, Sweden