ELFT HÍRADÓ

[tcsaba <tcsaba AT eik.bme.hu>]

                 M E G H Í V Ó - I N V I T A T I O N

    Seminar Series of the Department of Theoretical Physics at the
           Budapest University of Technology and Economics


                           Hiroshi Yasuoka
       (Max Planck Institute for Chemical Physics of Solids)

              Microscopic magnetic properties of the Weyl
                         semi-metals NbP and TaP

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 Weyl-semi metals which are 
predicted to host topologically protected states in the bulk. In Dirac 
semimetals (DSM), (e.g. Cd2As3 or Na3Bi) each node contains fermions of 
two opposite chiralities, whereas in the Weyl semimetals (WSM), 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). For WSMs such 
as the d-electron monophosphides NbP and TaP, EF does not exactly 
coincide with the Weyl nodes. However, if the nodes sit close enough to 
EF, 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 93Nb nuclear magnetic response (NMR) and 181Ta nuclear quadrupole 
resonance (NQR) techniques have been utilized to investigate the 
microscopic magnetic properties of the Weyl semi-metals, single crystal 
of NbP and powder of TaP. We made detailed measurements on the 
temperature dependences of line profiles K(T), Q(T) and nuclear 
relaxation rate (1/T1T), both of which gave us a characteristic feature 
associated with the Weyl physics. As a typical example, we show the 
temperature dependence of 1/T1T 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 band 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 EF. The 
second set of Weyl points, W2, are slightly higher in energy (~13 meV) 
than EF [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. Okvatovity, F. Simon and B. Dora, PRB 92 245141 (2016)


Időpont: 2017. november 3. péntek, 10:15
Helyszín: BME Fizikai Intézet, Elméleti Fizika Tanszék,
Budafoki út 8. F-épület, III lépcsőház, szemináriumi szoba