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Browsing Science & Technology by Author "Acharya, Gang Bahadur"

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    Investigation of Novel Topological Properties in Kagome Metals A2t3s4 (Where A are K, Rb & Cs and T are 3d & 4d Transition Metals
    (Institute of Science & Technology, T. U., 2024-07) Acharya, Gang Bahadur; Dr. Madhav Prasad Ghimire
    There has been significant research interest in topological materials in recent years due to their demonstration of fundamentally novel physical phenomena. A distinguishing feature of topological materials is the presence of protected surface states without energy gaps, a characteristic that emerges from the nontrivial topology of bulk wave functions. These materials could create novel quantum information technology devices and applications. Kagome lattice crystal structures are promising for exploring topological properties, including features such as Dirac, Weyl, and Nodal line characteristics in topological semimetals. The electronic structure of a kagome lattice inherently includes Dirac fermions, flat bands, and van Hove singularities. Over the past decade, the topological Weyl semimetals (WSMs) have experienced an increase in popularity in research, leading to precise theoretical predictions, controlled material synthesis, and advanced characterization techniques. These techniques include angle-resolved photoemission spectroscopy, scanning tunneling microscopy, magnetotransport measurements, and optical spectroscopy. In this thesis, we used density functional theory (DFT) simulations to investigate a series of proposed compounds’ electronic, optical, magnetic, and topological properties. Initially, we examined the electronic, magnetic, and topological properties of the gapless systems Cs2Co3S4 and Li2Fe3S4. Regarding topological features, we were especially interested in exploring Weyl semimetallic characteristics and related properties, including Berry curvature, anomalous Hall conductivity, and Fermi arc surface states. We found Cs2Co3S4 to be a ferrimagnetic half-metal with a total spin magnetic moment of about 3 𝜇B per formula unit. It shows an energy band gap of 0.36 eV in the majority-spin channel and a pseudo-gap at the Fermi level in the minority spin channel. We identified several sets of low-energy Weyl points and traced their dependence on the direction of magnetization. The intrinsic anomalous Hall conductivity is predicted to reach a magnitude of 500 Ω−1cm−1, comparable to values obtained in other celebrated Weyl semimetals. In addition, we calculate the Fermi arc surface states that connect the Weyl points with opposing chirality. Li2Fe3S4 also has Weyl semimetallic characteristics with lowenergyWeyl points near the Fermi level. For gapped systems, we found that Rb2Ni3S4, Cs2Ni3S4, Rb2Pd3S4, and Cs2Pd3S4 exhibit nonmagnetic ground state. Ni and Pd atoms constitute a kagome lattice in a two-dimensional plane. These materials possess a semiconducting nature. Interestingly, a flat band was noticed below the Fermi level, demonstrating one significant feature of the kagome lattice. Conversely, K2Mn3S4 and Rb2Mn3S4 display a magnetic ground state while maintaining a similar semiconducting character. In the case of materials with a kagome structure, we conducted further studies on their optical properties, revealing optical activity in both the visible and lower ultraviolet energy ranges. Based on these findings, gapless topological Weyl semimetals may be suitable for use in high-mobility devices, spintronics, and quantum computing. Kagome-gapped materials, on the other hand, may be possible for optoelectronic device applications. cfwf/e"t ?kn] lgs} gofF / ljlzi6 u'0fx¿ k|bz{g u/]sfn] xfnsf s]xL ;dodf topological material x¿sf] cg';Gwfgdf dxTjk"0f{ cle?lr b]vfk/]sf] 5 . lagf s'g} energy gap o:tf kbfy{x¿sf] ljlzi6 u'0f;+/lIft surface states sf] pkl:ylt non-trivial topology sf] bulk wave functions af6 k|s6 x'G5 . oL material x¿ cgf}7f] quantum ;"rgf k|ljlwsf pks/0f ;[hgf tyf pkof]u ug{sf nflu k|of]u ul/G5g\ . Topological semimetal df Dirac, Weyl / nodal line ;lxtsf] ljz]iftfx¿sf] cGj]if0f ug{ kagome lattice crystal ;+/rgf ;DefAo pDd]bjf/ xf] . Kagome lattice sf] electronic :j?k leq :jfefljs ?kn] Dirac fermions, flat bands / van Hove singularities cfpF5g\ . ljutsf bzsdf topological Weyl semimetals If]qsf] nf]slk|otf ;l6s ;}4flGts eljiojf0fL, lgolGqt material ;+Zn]if0f tyf pGgt characterization k|ljlwx¿ ;lxtsf] cg';Gwfg sfo{df a9]sf] 5 . o;leq angle-resolved photoemission spectroscopy, scanning tunneling microscopy, magneto-transport dfkg, optical spectroscopy tyf cGo kb{5g\ . of] y]l;;df xfdLn] k|:tfljt of}lussf] electronic, optical, magnetic / topological u'0fx¿sf] cg';Gwfg density functional theory (DFT) k|of]u u/L u/]sf 5f}F . z'?df, Cs2Co3S4 / Li3Fe3S4 gapless system x¿sf] electronic, magnetic / topological lu'0fx¿ hfFr ul/of] . Topological ljz]iftfx¿sf] ;Gbe{df, vf;u/L Weyl semimetallic / To;Fu ;DalGwt ljz]iftfx¿ h:t} Berry curvature, anomalous Hall conductivity / Fermi arc surface df xfd|f] ?lr lyof] . xfdLn] Cs2Co3S4 df sl/a 3μ_B k|lt formula o"lg6 total spin magnetic moment ;lxt half-metallic ferrimagnetism /x]sf] kfof}F . o;n] majority spin channel sf] Fermi level df 0.36 eV sf] energy band gap / minority spin channel df pseudo-gap b]vfpFb5 . xfdLn] low-energy Weyl points sf w]/} ;d"xx¿sf] klxrfg uof}F / ltgLx¿sf] r'DasLs/0fsf] lbzfsf] lge{/tf kQf nufof}F . Intrinsic anomalous Hall conductivity clwstd 500 Ω-1cm-1 k'u]sf] kfOof] h'g rflxF k|l;4 Weyl semimetal x¿sf] value ;Fu t'ngfof]Uo 5 . o;sf cltl/St xfdLn] Fermi arc surface states h'g ljk/Lt chirality ;lxtsf Weyl points nfO{ hf]8\b5g\, To;sf] klg vf]hL uof}F . Li3Fe3S4 df klg Weyl semi-metallic u'0fx¿ 5g\ h:df Fermi level sf] glhs low-energy Weyl point x? k|fKt eP . Gapped system x¿df xfdLn] Rb2Ni3S4, Cs2Ni3S4, Rb2Pd3S4 / Cs2Pd3S4 n] non-magnetic -u}/r'DasLo_ ground state k|bz{g u/]sf] kfof}F . Ni / Pd k/df0f'x¿n] two-dimensional plane df kagome lattice lgdf{0f ub{5g\ / semi-conducting u'0f wf/0f ub{5g\ . rfvnfUbf] t s] 5 eg], oL of}lusx¿df Fermi level sf] tn flat band b]lvof] h;n] kagome lattice sf] Ps dxTjk"0f{ nIf0f bzf{pFb5 . o;sf] ljk/Lt K2Mn3S4 / Rb2Mn3S4 n] semi-conducting u'0f sfod ub}{ magnetic ground state k|bz{g ub{5g\ . xfdLn] kagome ;+/rgf ePsf material x¿df ltgLx¿sf] optical u'0fsf] yk cWoog uof}F h;n] visible / lower UV energy bfo/f optical ultljlwsf] k'li6 eof] . oL pk/f]St kl/0ffdx¿sf] cfwf/df high mobility pks/0fx¿, spintronics / quantum computing sf] nflu gapless topological Weyl semimetal x¿ pko'St x'G5g\ eGg] lgisif{ lgsfNg ;lsG5 . ;fy}, kagome gapped materials rflxF optoelectronic pks/0fx/sf] nflu pko'St / cfbz{ x'G5g\ eGg] klg lgisif{ lgsfNg ;lsG5 .