Emerging chiral edge states from the confinement of a magnetic Weyl semimetal in Co3_3Sn2_2S2_2

The quantum anomalous Hall effect (QAHE) and magnetic Weyl semimetals (WSMs) are topological states induced by intrinsic magnetic moments and spin-orbit coupling. Their similarity suggests the possibility of achieving the QAHE by dimensional confinement of a magnetic WSM along one direction. In this study, we investigate the emergence of the QAHE in the two-dimensional (2D) limit of magnetic WSMs due to finite size effects in thin films and step-edges. We demonstrate the feasibility of this approach with effective models and real materials. To this end, we have chosen the layered magnetic WSM Co$_3$Sn$_2$S$_2$, which features a large anomalous Hall conductivity and anomalous Hall angle in its 3D bulk, as our material candidate. In the 2D limit of Co$_3$Sn$_2$S$_2$ two QAHE states exist depending on the stoichiometry of the 2D layer. One is a semimetal with a Chern number of 6, and the other is an insulator with a Chern number of 3. The latter has a band gap of 0.05 eV, which is much larger than that in magnetically doped topological insulators. Our findings naturally explain the existence of chiral states in step edges of bulk Co$_3$Sn$_2$S$_2$ which habe been reported in a recent experiment at $T = 4K$ and present a realistic avenue to realize QAH states in thin films of magnetic WSMs.Comment: Revised 3rd version of the manuscrip

Dzyaloshinkii-Moriya Interaction and Hall Effects in Bulk Chiral Magnets from First Principle Calculations

In this dissertation I will discuss the effect of real, momentum, and mixed space Berry phases in B20 compounds: MnSi, Mn1-xFexGe, and Fe1-yCoyGe. Recently there has been a tremendous experimental effort in stabilizing skyrmion crystal phases in these systems. We calculate, from state of the art first principle calculations, the Dzyaloshiniskii-Moriya interaction (DMI), the anomalous Hall effect (AHE), and the topological Hall effect (THE). These three effects are intimately related through Berry phase physics, where I test how the strength of the exchange interactions and spin-orbit coupling play a role in the underlying physics for these systems. In this dissertation, I compare the strength of different first principle methods in calculating magnetic ground state properties in B20 compounds. In this, I see that Full Potential Linearized Augmented Plane Wave Method treats different magnetic states most accurately. Calculations of spin-spiral states are performed in these B20 compounds showing long wavelength spin-spirals due to the interaction of the exchange stiffness and the DMI field. The DMI in these materials reaches maxima and minima with alloying concentration due the hybridization of d-states, which I complement with an intuitive tight-binding model. The AHE is also calculated in these materials and shows remarkable agreement with experimental measurements. Whereas the THE agrees in sign for these materials and quantitatively in the FeGe, the values in MnGe predict smaller values. This discrepancy, where the DMI is also smaller than expected, is attributed to breakdown of the adiabatic theorem, where in MnGe, the magnetic texture rotates too quickly to capture the real space Berry phase physics. The work of this dissertation is compared with computational results that have followed and ongoing experimental studies

Topological Hall effect in thin films of Mn1.5_{1.5}PtSn

Spin chirality in metallic materials with non-coplanar magnetic order can give rise to a Berry phase induced topological Hall effect. Here, we report the observation of a large topological Hall effect in high-quality films of Mn$_{1.5}$PtSn that were grown by means of magnetron sputtering on MgO(001). The topological Hall resistivity is present up to $\mu_{0}H \approx 4~$T below the spin reorientation transition temperature, $T_{s}=185$~K. We find, that the maximum topological Hall resistivity is of comparable magnitude as the anomalous Hall resistivity. Owing to the size, the topological Hall effect is directly evident prior to the customarily performed subtraction of magnetometry data. Further, we underline the robustness of the topological Hall effect in Mn\textsubscript{2-x}PtSn by extracting the effect for multiple stoichiometries (x~=~0.5, 0.25, 0.1) and film thicknesses (t = 104, 52, 35~nm) with maximum topological Hall resistivities between $0.76~\mu\Omega$cm and $1.55~\mu\Omega$cm at 150~K.Comment: 6 pages, 5 figure

Anisotropic Topological Hall Effect with Real and Momentum Space Berry Curvature in the Antiskrymion Hosting Heusler Compound Mn1.4_{1.4}PtSn

The topological Hall effect (THE) is one of the key signatures of topologically non-trivial magnetic spin textures, wherein electrons feel an additional transverse voltage to the applied current. The magnitude of THE is often small compared to the anomalous Hall effect. Here, we find a large THE of 0.9 $\mu\Omega$cm that is of the same order of the anomalous Hall effect in the single crystalline antiskyrmion hosting Heusler compound Mn$_{1.4}$PtSn, a non-centrosymmetric tetragonal compound. The THE is highly anisotropic and survives in the whole temperature range where the spin structure is noncoplanar (<170 K). The THE is zero above the spin reorientation transition temperature of 170 K, where the magnetization will have a collinear and ferromagnetic alignment. The large value of the THE entails a significant contribution from the momentum space Berry curvature along with real space Berry curvature, which has never been observed earlier

Aviram-Ratner rectifying mechanism for DNA base pair sequencing through graphene nanogaps

We demonstrate that biological molecules such as Watson-Crick DNA base pairs can behave as biological Aviram-Ratner electrical rectifiers because of the spatial separation and weak hydrogen bonding between the nucleobases. We have performed a parallel computational implementation of the ab-initio non-equilibrium Green's function (NEGF) theory to determine the electrical response of graphene---base-pair---graphene junctions. The results show an asymmetric (rectifying) current-voltage response for the Cytosine-Guanine base pair adsorbed on a graphene nanogap. In sharp contrast we find a symmetric response for the Thymine-Adenine case. We propose applying the asymmetry of the current-voltage response as a sensing criterion to the technological challenge of rapid DNA sequencing via graphene nanogaps

Thickness dependence of the anomalous Hall effect in thin films of the topological semimetal Co2_2MnGa

Topological magnetic semimetals promise large Berry curvature through the distribution of the topological Weyl nodes or nodal lines and further novel physics with exotic transport phenomena. We present a systematic study of the structural and magnetotransport properties of Co$_2$MnGa films from thin (20 nm) to bulk like behavior (80 nm), in order to understand the underlying mechanisms and the role on the topology. The magnetron sputtered Co$_2$MnGa films are $L$$2_{\mathrm {1}}$-ordered showing very good heteroepitaxy and a strain-induced tetragonal distortion. The anomalous Hall conductivity was found to be maximum at a value of 1138 S/cm, with a corresponding anomalous Hall angle of 13 %, which is comparatively larger than topologically trivial metals. There is a good agreement between the theoretical calculations and the Hall conductivity observed for the 80 nm film, which suggest that the effect is intrinsic. Thus, the Co$_2$MnGa compound manifests as a promising material towards topologically-driven spintronic applications.Comment: 7 pages, 5 figures, 1 tabl

Topological Hall effect in thin films of Mn1.5PtSn

Spin chirality in metallic materials with noncoplanar magnetic order can give rise to a Berry phase induced topological Hall effect. Here, we report the observation of a large topological Hall effect in high-quality films of Mn1.5PtSn that were grown by means of magnetron sputtering on MgO(001). The topological Hall resistivity is present up to μ0H≈4T below the spin reorientation transition temperature, Ts=185 K. We find that the maximum topological Hall resistivity is of comparable magnitude as the anomalous Hall resistivity. Owing to the size, the topological Hall effect is directly evident prior to the customarily performed subtraction of magnetometry data. Further, we underline the robustness of the topological Hall effect in Mn2-xPtSn by extracting the effect for multiple stoichiometries (x=0.5,0.25,0.1) and film thicknesses (t=104,52,35 nm) with maximum topological Hall resistivities between 0.76 and 1.55μΩcm at 150 K. © 2019 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI