Dirac dispersion generates unusually large Nernst effect in Weyl semimetals

Abstract

Weyl semimetals expand research on topologically protected transport by adding bulk Berry monopoles with linearly dispersing electronic states and topologically robust, gapless surface Fermi arcs terminating on bulk node projections. Here, we show how the Nernst effect, combining entropy with charge transport, gives a unique signature for the presence of Dirac bands. The Nernst thermopower of NbP (maximum of 800 VK -1 at 9 T, 109 K) exceeds its conventional thermopower by a hundredfold and is significantly larger than the thermopower of traditional thermoelectric materials. The Nernst effect has a pronounced maximum near TM=9020 K=0/kB (0 is chemical potential at T=0 K). A self-consistent theory without adjustable parameters shows that this results from electrochemical potential pinning to the Weyl point energy at TTM, driven by charge neutrality and Dirac band symmetry. Temperature and field dependences of the Nernst effect, an even function of the charge polarity, result from the intrinsically bipolar nature of the Weyl fermions. Through this study, we offer an understanding of the temperature dependence of the position of the electrochemical potential vis-à-vis the Weyl point, and we show a direct connection between topology and the Nernst effect, a potentially robust experimental tool for investigating topological states and the chiral anomaly. EH T EH T        (1)

Large Nernst power factor over a broad temperature range in polycrystalline Weyl semimetal NbP

“…Upon cooling, a xx exhibits a sign reversal and reaches a minimum value of À20 mV K À1 near 100 K. In a previous work, Watzman et al reported a minimum a xx of about À10 mV K À1 in single crystal NbP. 22 The deviation of a xx between singlecrystalline and polycrystalline NbP might originate from the difference of the direction of measurement and slight change of the position of Fermi level induced by slightly different Nb/P View Article Online stoichiometry. Overall, the obtained a xx in both polycrystalline and single crystal NbP is about one order of magnitude lower than that of good TE materials, indicating the bad TE properties of NbP.…”

“…Both at low and high fields, N xy peaks near 136 K. Furthermore, it is worth to discuss about that the obtained absolute a xx and a xy at high fields in the polycrystalline sample are smaller than those reported in the single crystals. 22,35 As per our understanding the large difference might be due to three factors: first, the polycrystalline nature of our sample makes the direction of measurement different from that of the single crystals. The grain boundaries in the polycrystalline sample might scatter Fig.…”

Berry curvature induced thermopower in type-I and type-II Weyl semimetals

“…The anomalous Hall effect 49 predicted to exist in time reversal symmetry (TRS) broken WSMs 34,50 has been recently seen in ZrTe 5 45 . The corresponding effect in thermopower, the anomalous Nernst effect in WSM 29,33 has been demonstrated in Cd 3 As 2 42 , NbP 35 and Ti 2 MnX 47 . Chiral magnetic effect, a chiral anomaly induced phenomena 22,23,30 has been reported in ZrTe 5 51 .…”

“…This leads to the possibility of several interesting transport phenomena in isotropic and tilted WSMs [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] . Several of these have also been experimentally demonstrated 5,7,[35][36][37][38][39][40][41][42][43][44][45][46][47][48] . For instance, negative magnetoresistivity (MR) 21,24 has been observed in several WSM candidates including the TaAs family 37,38,44 and in WSMs induced magnetically from three dimensional Dirac semimetals 5,7,40 .…”

Anomalous thermoelectric phenomena in lattice models of multi-Weyl semimetals

“…[41]. In the case of Weyl materials with a broken time-reversal (TR) symmetry, it was found that, in addition to the usual magnetic-field-dependent Nernst effect, which was recently measured in NbP [42], there is also an anomalous Nernst response [41]. Similarly to the anomalous Hall effect, the anomalous Nernst effect is determined by a nonzero chiral shift.…”