001048443 001__ 1048443
001048443 005__ 20251125202202.0
001048443 0247_ $$2doi$$a10.18154/RWTH-2025-06556
001048443 037__ $$aFZJ-2025-04649
001048443 041__ $$aEnglish
001048443 1001_ $$0P:(DE-Juel1)192445$$aHofmann, Jonathan Karl$$b0$$ufzj
001048443 245__ $$aElectrical anisotropy and shear-resistant topology in the quasi one-dimensional van-der-Waals material α-Bi$_{4}$Br$_{4}$$$f - 2025-03-10
001048443 260__ $$bRWTH Aachen University$$c2025
001048443 300__ $$apages 1 Online-Ressource : Illustrationen
001048443 3367_ $$2DataCite$$aOutput Types/Dissertation
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001048443 3367_ $$2BibTeX$$aPHDTHESIS
001048443 3367_ $$02$$2EndNote$$aThesis
001048443 3367_ $$0PUB:(DE-HGF)11$$2PUB:(DE-HGF)$$aDissertation / PhD Thesis$$bphd$$mphd$$s1764067691_23058
001048443 3367_ $$2DRIVER$$adoctoralThesis
001048443 502__ $$aDissertation, RWTH Aachen, 2025$$bDissertation$$cRWTH Aachen$$d2025
001048443 520__ $$aThe quasi-one-dimensional van-der-Waals material α-Bi4Br4 crystallizes in a monoclinic crystal structure consisting of covalently bonded Bi4Br4 chains parallel to the lattice vector b. The van-der-Waals interaction connects these chains to form 2D layers. These layers are then stacked in c-direction. α-Bi4Br4 features AB stacking. In contrast to well-known van-der-Waals materials such as WTe2 or MoS2, α-Bi4Br4 features two van-der-Waals gaps. A monolayer of α-Bi4Br4 is a quantum spin Hall insulator. α-Bi4Br4 bulk crystals readily cleaves to expose the (001) surface. Furthermore, flakes of α-Bi4Br4 showing the same surface can be prepared by mechanical exfoliation. Electrical transport measurements are preformed using a four-tip scanning tunnelling microscope (STM) to investigate the anisotropy of the resistivity of α-Bi4Br4. A four-tip STM integrates four individual STMs into a tight unit, to enable transport measurements on surfaces. The piezo drives of the individual STMs allow flexible tip configurations to be set up as needed for a transport measurement. Furthermore, a four-tip STM still can image the surface by scanning a single tip and perform scanning tunnelling microscopy. Due to the small resistances measured here, the exact calibration of the voltage measurement in the four-tip STM became a major issue for the measurement. This calibration is therefore addressed in chapter 3. Chapter 5 presents a modified surface structure of the α-Bi4Br4(001) surface. Atomically resolved STM images show that the parallel Bi4Br4 chains exhibit a mutual shift different from the one expected for this surface. Density functional theory calculations by Mingqian Zheng and Jin-Jian Zhou indicate that a monolayer of this new structure is also a quantum spin Hall insulator. The modified structure arises due to shear stress which is able to shift the parallel chains with respect to each other because neighbouring chains are only connected by weak van-der-Waals forces. Two different methods to disentangle the resistivity tensor ρ of α-Bi4Br4 are implemented: In chapter 6, the in-plane anisotropy is first measured on the (001) surface of a bulk α-Bi4Br4 crystal. For this, two measurements of the resistance in a square tip configuration are used. Then, the value of resistivity in b-direction is determined using a distance-dependent measurement on a thin flake. Assuming that the influence of the off-diagonal element of the resistivity tensor can be neglected, an in-plane anisotropy of A= ρ_{a} / ρ_{b} = 6.4(5) is obtained at room temperature. Furthermore, the anisotropy normal to the ab plane is found to be A_{z} = ρ_{z} / ρ_{b} = 1300. Thus, the resistivity in b-direction, parallel to the chains, is the smallest, as expected from the crystal structure. At 77 K, A = 5.0(3) and A_{z} = 6500 were measured. Chapter 7 demonstrates an alternative approach to disentangle the three elements on the main diagonal of the resistivity tensor ρ when the off-diagonal element is neglected. Here, the tips are positioned in the corners of a large, rectangular flake. The anisotropy can then be obtained by the Bierwagen-Simon method. While it is possible to demonstrate the disentanglement of the three components of the resistivity tensor, the in-plane anisotropy A measured with the second method was substantially smaller than the result obtained before. The origin of this discrepancy is traced back to imperfections of the flake.
001048443 536__ $$0G:(DE-HGF)POF4-5213$$a5213 - Quantum Nanoscience (POF4-521)$$cPOF4-521$$fPOF IV$$x0
001048443 588__ $$aDataset connected to DataCite
001048443 650_7 $$2Other$$aHochschulschrift
001048443 650_7 $$2Other$$aRastertunnelmikroskopie ; scanning tunneling microscopy ; Vierspitzen-Rastertunnelmikroskopie ;  four-tip scanning tunneling microscopy ; Ladungstransport ; charge transport ; topologische Isolatoren ; topological insulators ; quasi-one-dimensional crystals ; quasi-eindimensionale Kristalle ; higher-order topological insulators ; Anisotropie ; anisotropy ; Resistivitätstensor ; resistivity tensor
001048443 7001_ $$0P:(DE-Juel1)128794$$aVoigtländer, Bert$$b1$$eSupervisor$$ufzj
001048443 7001_ $$0P:(DE-HGF)0$$aMorgenstern, Markus$$b2$$eSupervisor
001048443 773__ $$a10.18154/RWTH-2025-06556
001048443 8564_ $$uhttps://publications.rwth-aachen.de/record/1015733
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001048443 9141_ $$y2025
001048443 9201_ $$0I:(DE-Juel1)PGI-3-20110106$$kPGI-3$$lQuantum Nanoscience$$x0
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001048443 980__ $$aI:(DE-Juel1)PGI-3-20110106
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