NMR in an electric field: A bulk probe of the hidden spin and orbital polarizations

Abstract

Recent theoretical work has established the presence of hidden spin and orbital textures in nonmagnetic materials with inversion symmetry. Here, we propose that these textures can be detected by nuclear magnetic resonance (NMR) measurements carried out in the presence of an electric field. In crystals with hidden polarizations, a uniform electric field produces a staggered magnetic field that points to opposite directions at atomic sites related by spatial inversion. As a result, the NMR resonance peak corresponding to inversion partner nuclei is split into two peaks. The magnitude of the splitting is proportional to the electric field and depends on the orientation of the electric field with respect to the crystallographic axes and the external magnetic field. As a case study, we present a theory of electric-field-induced splitting of NMR peaks for 77 Se, 125 Te and 209 Bi in Bi2Se3 and Bi2Te3. In conducting samples with current densities of 10 6 A/cm 2 , the splitting for Bi can reach 100 kHz, which is comparable to or larger than the intrinsic width of the NMR lines. In order to observe the effect experimentally, the peak splitting must also exceed the linewidth produced by the Oersted field. In Bi2Se3, this requires narrow wires of radius 1 µm. We also discuss other potentially more promising candidate materials, such as SrRuO3 and BaIr2Ge2, whose crystal symmetry enables strategies to suppress the linewidth produced by the Oersted field. arXiv:1709.02376v2 [cond-mat.mtrl-sci]

Electronic properties of the topological insulators Bi₂Se₃ and Bi₂Te₃

“…The result is very similar to what was found for 77 Se NMR in Bi 2 Se 3 [23]. Furthermore, the model calculations that associate the indirect coupling in Bi 2 Se 3 with a slowly decaying Bloembergen-Rowland (BR) susceptibility, also hold for Bi 2 Te 3 [74].…”

“…The large values, Λ 0 ≈ 50(10) kHz, are nonetheless in good agreement with an unusual strong indirect coupling mechanism as suggested by Georgieva et al [23] and Ramírez-Ruiz et al [74]. However, due to large errors in Λ 0 , the present results cannot strictly be taken as an additional proof of a Bloembergen-Rowland (BR) coupling.…”

“…With a similar observation of extensive, field and carrier independent NMR line broadening of the 77 Se NMR in Bi 2 Se 3 , Georgieva et al [23] argued that a surprisingly slowly decaying Bloembergen-Rowland (BR) interaction governs the indirect dipole coupling among nuclear moments. In the present study, it is not possible to confirm a Bloembergen-Rowland mechanism as the source for internuclear coupling as proposed by the same theoretical model used for the 77 Se NMR in Bi 2 Se 3 [74].…”

“…In the context of the magnetism exhibited by Mn doped Bi 2 Te 3 , the underlaying coupling mechanism remains an open question, with experimental data that seems to be in disagreement with the theoretically proposed RKKY coupling. The NMR of Bi 2 Te 3 may therefore offer valuable insight as an indirect coupling of Bloembergen-Rowland (BR) type was predicted to give rise to an extensive NMR line broadening [74], similar to what was found with an interaction of the nuclear magnetic moments with free charge carriers, while 77 Se NMR shows multicomponent spin-lattice relaxtaion (T 1 ). These results are related to their thermoelectricity as well as topological properties of the samples.…”

“…Using 77 Se NMR in Bi 2 Se 3 single crystals, Georgieva et al [23] reported extensive NMR line broadening that was attributed to an exotic indirect internuclear dipole coupling of Bloembergen-Rowland type. The same calculations imply a similar effect for 125 Te nuclei in Bi 2 Te 3 [74]. The NMR linewidths of Te in and Te out have therefore been measured at different magnetic fields in order to separate the field independent NMR linewidth that accounts for dipole interactions.…”