Quasiparticle band structure and excitonic optical response in V 2 O 5 bulk and monolayer
Not Available
Garcia, Claudio (ORCID:0009000077503536)↗
Engineering topics
Radha, Santosh Kumar
Publications and source records attributed to Radha, Santosh Kumar.
Parallel hybrid quantum-classical machine learning for kernelized time-series classification
Supervised time-series classification garners widespread interest because of its applicability throughout a broad application domain including finance, astronomy, biosensors, and many others. Here, in this work, we tackle this problem with hybrid quantum-classical machine learning, deducing pairwise temporal relationships between time-series instances using a timeseries Hamiltonian kernel (TSHK). A TSHK is constructed with a sum of inner products generated by quantum states evolved using a parameterized time evolution operator. This sum is then optimally weighted using techniques derived from multiple kernel learning. Because we treat the kernel weighting step as a differentiable convex optimization problem, our method can be regarded as an end-to-end learnable hybrid quantum-classical-convex neural network, or QCC-net, whose output is a data set-generalized kernel function suitable for use in any kernelized machine learning technique such as the support vector machine (SVM). Using our TSHK as input to a SVM, we classify univariate and multivariate time-series using quantum circuit simulators and demonstrate the efficient parallel deployment of the algorithm to 127-qubit superconducting quantum processors using quantum multi-programming.
97 MATHEMATICS AND COMPUTING↗
Optical response and band structure of LiCoO 2 including electron-hole interaction effects
The optical response functions and band structures of LiCoO 2 are studied at different levels of approximation, from density functional theory (DFT) in the generalized gradient approximation (GGA) to quasiparticle self-consistent QSGW (with G for Green's function and W for screened Coulomb interaction) without and with ladder diagrams ($QSG\hat{W}$) and the Bethe Salpeter Equation (BSE) approach. The QSGW method is found to strongly overestimate the band gap and electron-hole or excitonic effects are found to be important. They lower the quasiparticle gap by only about 11~\% but the lowest energy peaks in absorption are found to be excitonic in nature. The contributions from different band to band transitions and the relation of excitons to band-to-band transitions are analyzed. Finally, the excitons are found to be strongly localized. Finally, a comparison to experimental data is presented.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Topological obstructed atomic limit insulators by annihilating Dirac fermions
We show that annihilating a pair of Dirac fermions implies a topological transition from the critical semimetallic phase to an obstructed atomic limit insulator phase instead of a trivial insulator. This is shown to happen because of branch cuts in the phase of the wave functions, leading to nontrivial Zak phase along certain directions. To this end, we study their ${\mathbb{Z}}_{2}$ invariant and also study the phase transition using entanglement entropy. We use low-energy Hamiltonians and numerical result from model systems to show this effect. These transitions are observed in realistic materials, including strained graphene and buckled honeycomb group V (Sb/As).
36 MATERIALS SCIENCE↗
Topological quantum switch and controllable one-dimensional conducting paths in antimonene facilitated by breaking the inversion symmetry
Based on the nontrivial topology of buckled antimonene, we propose two conceptual device designs based on breaking the inversion symmetry which can be achieved by gating. The first one is a quantized topological switch obtained by breaking the degeneracy of the fractionalized charge conducting edge states, inducing a metal-insulator transition. In the second one, using local gating with different polarity in different areas, a domain wall is created at their interface, which hosts a soliton type electronic gap state in which transport could be accessed by doping and shown to be robust over a wide range of disorder.
36 MATERIALS SCIENCE↗
Buckled honeycomb antimony: Higher order topological insulator and its relation to the Kekulé lattice
Higher order topological tnsulators are d-spatial dimensional systems featuring topologically protected gapless states at their (d-n)-dimensional boundaries. With the help of ab initio calculations and tight-binding models along with symmetry considerations, we show that monolayer buckled honeycomb structures of group-V elements (Sb, As), which have already been synthesized, belong in this category and have a spinless charge fractionalization of $\frac{e}{2}$ at the corner states as well as weak topological edge states, protected by the S 6 symmetry operation, which classify this system as a quadrupole topological insulator. The robustness of these edge and corner states to perturbations is explicitly demonstrated.
36 MATERIALS SCIENCE↗
Topological band structure transitions and goniopolar transport in honeycomb antimonene as a function of buckling
The electronic band topology of monolayer β-Sb (antimonene) is studied from the flat honeycomb to the equilibrium buckled structure using first-principles calculations and analyzed using a tight-binding model and low-energy Hamiltonians. In flat monolayer Sb, the Fermi level occurs near the intersection of two warped Dirac cones, one associated with the pz orbitals, and one with the {p x , p y } orbitals. The differently oriented threefold warping of these two cones leads to an unusually shaped nodal line, which leads to anisotropic in-plane transport properties and goniopolarity. A slight buckling opens a gap along the nodal line except at six remaining Dirac points, protected by symmetry. Under increasing buckling, pairs of Dirac points of opposite winding number annihilate at a critical buckling angle. At a second critical angle, the remaining Dirac points disappear when the band structure opens a gap. Spin-orbit coupling and edge states are discussed.
36 MATERIALS SCIENCE↗
Understanding the Crystallographic Phase Relations in Alkali-Trihalogeno-Germanates in Terms of Ferroelectric or Antiferroelectric Arrangements of the Tetrahedral GeX 3 Units
The alkali-trihalogeno-germanates AGeX 3 with A: Rb or Cs and X a halogen (I, Br, Cl) along with the corresponding stannates (ASnX 3 ) and plumbates (APbX 3 ) reflect a large variety of crystal structures, some of which are of the perovskite type. These materials, better known as “halide perovskites” have recently gained worldwide attention as promising photovoltaic and more broadly opto-electronic materials. Yet, their stability problems relative to the non-perovskite phases are a major issue. It is shown that some of the phase relations in these materials can be understood in terms of the relative orientation of the GeX 3 tetrahedral units, which is ferroelectric (FE) in the perovskite phase but antiferroelectric (AFE) in the competing monoclinic phase. Electrostatic dipole interactions favor the AFE phase and the additional bond formation favors the FE phase. The change in phase is accompanied by a large change in band gap.
36 MATERIALS SCIENCE↗
Distortion modes in halide perovskites: To twist or to stretch, a matter of tolerance and lone pairs
With first-principles calculations, we show that CsBX 3 halides withB=Sn or Pb undergo octahedral rotation distortions, while for B=Ge and Si, they undergo a ferroelectric rhombohedral distortion accompanied by a rhombohedral stretching of the lattice. We demonstrate that these are mutually exclusive at their equilibrium volume although different distortions may occur as functions of lattice expansion. The choice between the two distortion modes is in part governed by the Goldschmidt tolerance factor. However, another factor explaining the difference between Sn and Pb compared with Ge and Si is the stronger lone-pair character of Ge and Si when forced to be divalent as is the case in these structures. The lone-pair chemistry is related to the off centering. While the Si-based compounds have not yet been synthesized, the Ge compounds have been established experimentally. As a final test of the importance of the tolerance factor we consider RbGeX 3 , which has smaller tolerance factor than the corresponding CsGeX 3 because Rb is smaller than Cs. We find that it can lower its energy by both rotations or rhombohedral off-centering distortions but the latter lower the energy slightly more efficiently.
36 MATERIALS SCIENCE↗