10 results
Search Results
Now showing 1 - 10 of 10
- ItemRole of topology in compensated magnetic systems(Melville, NY : AIP Publ., 2024) Reichlova, Helena; Kriegner, Dominik; Mook, Alexander; Althammer, Matthias; Thomas, AndyTopology plays a crucial and multifaceted role in solid state physics, leading to a remarkable array of newly investigated materials and phenomena. In this Perspective, we provide a brief summary of well-established model materials with a particular focus on compensated magnets and highlight key phenomena that emerge due to the influence of topology in these systems. The overview covers various magneto-transport phenomena, with a particular focus on the extensively investigated anomalous magneto-transport effects. Furthermore, we look into the significance of topology in understanding elementary magnetic excitations, namely magnons, where the role of topology gained considerable attention from both theoretical and experimental perspectives. Since electrons and magnons carry energy, we explore the implications of topology in combined heat and spin transport experiments in compensated magnetic systems. At the end of each section, we highlight intriguing unanswered questions in this research direction. To finally conclude, we offer our perspective on what could be the next advancements regarding the interaction between compensated magnetism and topology.
- ItemMagnetization-driven Lifshitz transition and charge-spin coupling in the kagome metal YMn6Sn6(London : Springer Nature, 2022) Siegfried, Peter E.; Bhandari, Hari; Jones, David C.; Ghimire, Madhav P.; Dally, Rebecca L.; Poudel, Lekh; Bleuel, Markus; Lynn, Jeffrey W.; Mazin, Igor I.; Ghimire, Nirmal J.The Fermi surface (FS) is essential for understanding the properties of metals. It can change under both conventional symmetry-breaking phase transitions and Lifshitz transitions (LTs), where the FS, but not the crystal symmetry, changes abruptly. Magnetic phase transitions involving uniformly rotating spin textures are conventional in nature, requiring strong spin-orbit coupling (SOC) to influence the FS topology and generate measurable properties. LTs driven by a continuously varying magnetization are rarely discussed. Here we present two such manifestations in the magnetotransport of the kagome magnet YMn6Sn6: one caused by changes in the magnetic structure and another by a magnetization-driven LT. The former yields a 10% magnetoresistance enhancement without a strong SOC, while the latter a 45% reduction in the resistivity. These phenomena offer a unique view into the interplay of magnetism and electronic topology, and for understanding the rare-earth counterparts, such as TbMn6Sn6, recently shown to harbor correlated topological physics.
- ItemExtremely large magnetoresistance from electron-hole compensation in the nodal-loop semimetal ZrP2(Woodbury, NY : Inst., 2021) Bannies, J.; Razzoli, E.; Michiardi, M.; Kung, H.-H.; Elfimov, I.S.; Yao, M.; Fedorov, A.; Fink, J.; Jozwiak, C.; Bostwick, A.; Rotenberg, E.; Damascelli, A.; Felser, C.Several early transition metal dipnictides (TMDPs) have been found to host topological semimetal states and exhibit large magnetoresistance (MR). In this paper, we use angle-resolved photoemission spectroscopy (ARPES) and magnetotransport to study the electronic properties of a TMDP ZrP2. We find that ZrP2 exhibits an extremely large and unsaturated MR of up to 40 000% at 2 K, which originates from an almost perfect electron-hole (e-h) compensation. Our band structure calculations further show that ZrP2 hosts a topological nodal loop in proximity to the Fermi level. Based on the ARPES measurements, we confirm the results of our calculations and determine the surface band structure. This paper establishes ZrP2 as a platform to investigate near-perfect e-h compensation and its interplay with topological band structures.
- ItemCorrelation induced magnetic topological phases in the mixed-valence compound SmB6(College Park, MD : APS, 2023) Liu, Huimei; Hirschmann, Moritz M.; Sawatzky, George A.; Khaliullin, Giniyat; Schnyder, Andreas P.SmB6 is a mixed-valence compound with flat f-electron bands that have a propensity to magnetism. Here, using a realistic Γ8 quartet model, we investigate the dynamical spin susceptibility and describe the in-gap collective mode observed in neutron scattering experiments. We show that as the Sm valence increases with pressure, the magnetic correlations enhance and SmB6 undergoes a first-order phase transition into a metallic antiferromagnetic state, whose symmetry depends on the model parameters. The magnetic orderings give rise to distinct band topologies: while the A-type order leads to an overlap between valence and conduction bands in the form of Dirac nodal lines, the G-type order has a negative indirect gap with weak Z2 indices. We also consider the spin polarized phase under a strong magnetic field, and find that it exhibits Weyl points as well as nodal lines close to the Fermi level. The magnetic phases show markedly different surface states and tunable bulk transport properties, with important implications for experiments. Our theory predicts that a magnetic order can be stabilized also by lifting the Γ8 cubic symmetry, thus explaining the surface magnetism reported in SmB6.
- ItemTopological boundaries between helical domains as a nucleation source of skyrmions in the bulk cubic helimagnet Cu2OSeO3(College Park, MD : APS, 2022) Leonov, A.O.; Pappas, C.Cu2OSeO3 represents a unique example in the family of B20 cubic helimagnets with a tilted spiral and a low-temperature skyrmion phase arising for magnetic fields applied along the easy crystallographic (100) axes. Although the stabilization mechanism of these phases can be accounted for by cubic magnetic anisotropy, the skyrmion nucleation process is still an open question, since the stability region of the skyrmion phase displays strongly hysteretic behavior with different phase boundaries for increasing and decreasing magnetic fields. Here, we address this important point using micromagnetic simulations and come to the conclusion that skyrmion nucleation is underpinned by the reorientation of spiral domains occurring near the critical magnetic fields of the phase diagrams: HC1, the critical field of the transition between the helical and conical/tiled spiral phase, and HC2, the critical field between the conical/tiled spiral and the homogenous phase. By studying a wide variety of cases we show that domain walls may have a 3D structure. Moreover, they can carry a finite topological charge stemming from half-skyrmions (merons) also permitting along-the-field and perpendicular-to-the-field orientation. Thus, domain walls may be envisioned as nucleation source of skyrmions that can form thermodynamically stable and metastable lattices as well as skyrmion networks with misaligned skyrmion tubes. The results of numerical simulations are discussed in view of recent experimental data on chiral magnets, in particular, for the bulk cubic helimagnet Cu2OSeO3.
- ItemShort range order and topology of binary Ge-S glasses(Lausanne : Elsevier, 2022) Pethes, I.; Jóvári, P.; Michalik, S.; Wagner, T.; Prokop, V.; Kaban, I.; Száraz, D.; Hannon, A.; Krbal, M.Short range order and topology of GexS100-x glasses over a broad composition range (20 ≤ x ≤ 42 in at%) was investigated by neutron diffraction, X-ray diffraction, and Ge K-edge extended X-ray absorption fine structure (EXAFS) measurements. The experimental data sets were fitted simultaneously in the framework of the reverse Monte Carlo simulation method. It was found that both constituents (Ge and S) satisfy the Mott-rule in all investigated glasses: Ge and S atoms have 4 and 2 neighbours, respectively. The structure of these glasses can be described with the chemically ordered network model: Ge-S bonds are preferred; S-S bonds are present only in S-rich glasses. Dedicated simulations showed that Ge-Ge bonds are necessary in Ge-rich glasses. Connections between Ge atoms (such as edge-sharing GeS4/2 tetrahedra) in stoichiometric and S-rich glasses were analysed. The frequency of primitive rings was also calculated.
- ItemComprehensive scan for nonmagnetic Weyl semimetals with nonlinear optical response(London : Nature Publishing Group, 2020) Xu, Q.; Zhang, Y.; Koepernik, K.; Shi, W.; van den Brink, J.; Felser, C.; Sun, Y.First-principles calculations have recently been used to develop comprehensive databases of nonmagnetic topological materials that are protected by time-reversal or crystalline symmetry. However, owing to the low symmetry requirement of Weyl points, a symmetry-based approach to identifying topological states cannot be applied to Weyl semimetals (WSMs). To date, WSMs with Weyl points in arbitrary positions are absent from the well-known databases. In this work, we develop an efficient algorithm to search for Weyl points automatically and establish a database of nonmagnetic WSMs with Weyl points near the Fermi level based on the experimental non-centrosymmetric crystal structures in the Inorganic Crystal Structure Database (ICSD). In total, 46 Weyl semimetals were discovered to have nearly clean Fermi surfaces and Weyl points within 300 meV of the Fermi level. Nine of them are chiral structures which may exhibit the quantized circular photogalvanic effect. In addition, the nonlinear optical response is studied and the giant shift current is explored. Besides nonmagnetic WSMs, our powerful tools can also be used in the discovery of magnetic topological materials.
- ItemFate of density waves in the presence of a higher-order van Hove singularity(College Park, MD : APS, 2023) Zervou, Alkistis; Efremov, Dmitry V.; Betouras, Joseph J.Topological transitions in electronic band structures, resulting in van Hove singularities in the density of states, can considerably affect various types of orderings in quantum materials. Regular topological transitions (of neck formation or collapse) lead to a logarithmic divergence of the electronic density of states (DOS) as a function of energy in two dimensions. In addition to the regular van Hove singularities, there are higher-order van Hove singularities (HOVHS) with a power-law divergence in DOS. By employing renormalization group techniques, we study the fate of a spin-density wave phase formed by nested parts of the Fermi surface, when a HOVHS appears in parallel. We find that the phase formation can be boosted by the presence of the singularity, with the critical temperature increasing by orders of magnitude, under certain conditions. We discuss possible applications of our findings to a range of quantum materials such as Sr3Ru2O7, Sr2RuO4, and transition metal dichalcogenides.
- ItemAbsence of induced magnetic monopoles in Maxwellian magnetoelectrics(College Park, MD : APS, 2022) Nogueira, Flavio S.; van den Brink, JeroenThe electromagnetic response of topological insulators is governed by axion electrodynamics, which features a topological magnetoelectric term in the Maxwell equations. As a consequence magnetic fields become the source of electric fields and vice versa, a phenomenon that is general for any material exhibiting a linear magnetoelectric effect. Axion electrodynamics has been associated with the possibility to create magnetic monopoles, in particular, by an electrical charge that is screened above the surface of a magnetoelectric material. Here we explicitly solve for the electromagnetic fields in this geometry and show that while vortexlike magnetic screening fields are generated by the electrical charge their divergence is identically zero at every point in space, which implies an absence of induced magnetic monopoles. Nevertheless magnetic image charges can be made explicit in the problem, and even if no bound state with electric charges yielding a dyon arises, a dyonlike angular momentum follows from our analysis. Because of its dependence on the dielectric constant this angular momentum is not quantized, which is consistent with a general argument that precludes magnetic monopoles to be generated in Maxwell magnetoelectrics. We also solve for topologically protected zero modes in the Dirac equation induced by the point charge. Since the induced topological defect on the topological insulator's surface carries an electric charge as a result of the axion term, these zero modes are not self-conjugated.
- ItemPhononic-magnetic dichotomy of the thermal Hall effect in the Kitaev material Na2 Co2 TeO6(College Park, MD : APS, 2023) Gillig, Matthias; Hong, Xiaochen; Wellm, Christoph; Kataev, Vladislav; Yao, Weiliang; Li, Yuan; Büchner, Bernd; Hess, ChristianThe quest for a half-quantized thermal Hall effect of a Kitaev system represents an important tool to probe topological edge currents of emergent Majorana fermions. Pertinent experimental findings for α-RuCl3 are, however, strongly debated, and it has been argued that the thermal Hall signal stems from phonons or magnons rather than from Majorana fermions. Here, we investigate the thermal Hall effect of the Kitaev candidate material Na2Co2TeO6, and we show that the measured signal emerges from at least two components, phonons and magnetic excitations. This dichotomy results from our discovery that the longitudinal and transversal heat conductivities share clear phononic signatures, while the transversal signal changes sign upon entering the low-temperature, magnetically ordered phase. Our results demonstrate that uncovering a genuinely quantized magnetic thermal Hall effect in Kitaev topological quantum spin liquids such as α-RuCl3 and Na2Co2TeO6 requires disentangling phonon vs magnetic contributions, including potentially fractionalized excitations such as the expected Majorana fermions.