We have a pleasure to invite you to attend the joint seminar of the Department of Condensed Matter Physics (DCMP) and the Materials Growth and Measurement Laboratory (MGML) http://mgml.eu.
Program
Emergent Weyl fermion excitations in Tap and NbP explored by ^{93}Nb NMR and ^{181}Ta NQR
lecture given by:
Hiroshi Yasuoka
Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany
The seminar takes place in the lecture room F2 of the Faculty of Mathematics and Physics, Ke Karlovu 5, Praha 2 on Wednesday, 6. 12. 2017 from 14:30
Vladimír Sechovský On behalf of the DCMP and MGML
Abstract
The past decade has seen an explosion of interest in the role of topology in condensed matter physics. Arguably the most topical of the new classes of material are Dirac and Weylsemi metals which are predicted to host topologically protected states in the bulk. In Dirac semimetals (DSM), (e.g. Cd_{2}As_{3} or Na_{3}Bi) each node contains fermions of two opposite chiralities, whereas in the Weyl semimetals (WSM), an even more interesting situation arises. A combination of noncentrosymmetric crystal structure and sizable spinorbit coupling (SOC) causes the nodes to split into pairs of opposite chirality (Weyl points). For WSMs such as the delectron monophosphides NbP and TaP, E_{F} does not exactly coincide with the Weyl nodes. However, if the nodes sit close enough to E_{F}, in a region of linear dispersion (E ∞ k), the Weyl physics can still be observed in the properties of very light fermions. A key issue in the study of the monophosphides is therefore to establish how close to the Fermi level the Weyl points sit, and to estimate the range of energy over which the linear dispersion exists.
The ^{93}Nb nuclear magnetic response (NMR) and ^{181}Ta nuclear quadrupole resonance (NQR) techniques have been utilized to investigate the microscopic magnetic properties of the Weyl semimetals, single crystal of NbP and powder of TaP. We made detailed measurements on the temperature dependences of line profiles K(T), n_{Q}(T) and nuclear relaxation rate (1/T_{1}T), both of which gave us a characteristic features associated with the Weyl physics. As a typical example, we show the temperature dependence of 1/T_{1}T in TaP (see Fig. (a)). One can immediately observe that there exists a characteristic temperature, T* » 30 K, where the relaxation process has a crossover from a high temperature T ^{2} behavior which is associated with the excitations in the nodal structure of Weyl points to the low temperature Korringa excitations for a parabolic bands with a weak temperature dependence. The band structure calculation of TaP tells us that besides the normal bands, two types of Weyl points appear. The first set of Weyl points, termed W1 and located at much lower energy (~40 meV) than the E_{F}. The second set of Weyl points, W2, are slightly higher in energy (~13 meV) than E_{F} [1] (see insert of light figure). From this band structure one can easily imagine that the conventional Korringa process is valid in well below temperature corresponding to the W2 energy, while increasing temperature excitations at the W2 Weyl nodes become progressively dominant. A construction of the total relaxation processes, including an anomalous orbital hyperfine coupling [2], is illustrated in Fig. (b), where we have good agreement with the experimental result.

[1] F. Arnold, et.al. Nature Communications 7, 11615 (2016),
[2] Z. Okvátovity, F. Simon and B. Dóra, PRB 92 245141 (2016)
T. K., Y. K. and H.T are appreciate the financial support from JSPS

