The181 Ta quadrupole resonance (NQR) technique has been utilized to investigate the microscopic magnetic properties of the Weyl semi-metal TaP. We found three zero-field NQR signals associated with the transition between the quadrupole split levels for Ta with I=7/2 nuclear spin. A quadrupole coupling constant, νQ =19.250 MHz, and an asymmetric parameter of the electric field gradient, η = 0.423 were extracted, in good agreement with band structure calculations. In order to examine the magnetic excitations, the temperature dependence of the spin lattice relaxation rate (1/T1T ) has been measured for the f2-line (±5/2 ↔ ±3/2 transition). We found that there exists two regimes with quite different relaxation processes. Above T * ≈ 30 K, a pronounced (1/T1T ) ∝ T 2 behavior was found, which is attributed to the magnetic excitations at the Weyl nodes with temperature dependent orbital hyperfine coupling. Below T *, the relaxation is mainly governed by Korringa process with 1/T1T = constant, accompanied by an additional T −1/2 type dependence to fit our experimental data. We show that Ta-NQR is a novel probe for the bulk Weyl fermions and their excitations.PACS numbers: 02.40. Pc, 76.60.Gv, 31.30.Gs The past decade has seen an explosion of interest in the role of topology in condensed matter physics. Major discoveries have included the two dimensional graphene [1] and the topological insulators (TI) (e.g. HgTe or Bi 2 Se 3 ), [2][3][4] whose topological properties require the existence of gapless surface states. Many of the new materials host exotic excitations whose observation can be regarded as direct experimental evidence for the existence of quasiparticles. Arguably, the most topical of the new classes of materials are Dirac-and Weyl-semi metals which are predicted to host topologically protected states in the bulk. [5] In Dirac semimetals (DSM), [5][6][7] (e.g. Cd 2 As 3 or Na 3 Bi) each node contains fermions of two opposite chiralities, whereas in the Weyl semimetals (WSM), [8][9][10][11][12] an even more interesting situation arises. A combination of non-centrosymmetric crystal structure and sizable spin-orbit coupling (SOC) causes the nodes to split into pairs of opposite chirality (Weyl points). In the ideal case, there would be exactly half filling of the relevant bands, such that the Weyl points would sit at the Fermi level (E F ) and the Weyl fermions would be massless. In actuality, Weyl semimetals such as the d-electron monophosphides NbP and TaP, E F does not exactly coincide with the Weyl nodes. [9,10,12] 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 excitations in the energy window k B T . 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. This presents a considerable experimental challenge. The nodes appear in the electronic structure of the bulk, and the mate...

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11 B-NMR measurements have been carried out for UNi 4 B showing a ferroic toroidal ordering at H ≈ 1 T. 11 B FFT-NMR spectrum has nuclear quadrupole satellites that consists of four kinds of quadrupole split lines. Furthermore, field angle dependence of the resonance frequency in the aplane (hexagonal [0001]-plane) obeys the twofold rotational symmetry, evidencing that the crystal structure is orhorombic. The present NMR study demonstrates that the crystal structure of UNi 4 B is not the hexagonal but the orthorhombic crystal structure with the space group of Cmcm.

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