Dirac dispersion generates unusually large Nernst effect in Weyl semimetals (original) (raw)
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Thermopower and thermal conductivity in the Weyl semimetal NbP
Journal of physics. Condensed matter : an Institute of Physics journal, 2017
The Weyl semimetal NbP exhibits an extremely large magnetoresistance and an ultra-high mobility. The large magnetoresistance originates from a combination of the nearly perfect compensation between electron- and hole-type charge carriers and the high mobility, which is relevant to the topological band structure. In this work we report on temperature- and field-dependent thermopower and thermal conductivity experiments on NbP. Additionally, we carried out complementary heat capacity, magnetization, and electrical resistivity measurements. We found a giant adiabatic magnetothermopower with a maximum of [Formula: see text] at 50 K in a field of 9 T. Such large effects have been observed rarely in bulk materials. We further observe pronounced quantum oscillations in both thermal conductivity and thermopower. The obtained frequencies compare well with our heat capacity and magnetization data.
Thermoelectric properties of Weyl and Dirac semimetals
Physical Review B, 2014
We study the electronic contribution to the thermal conductivity and the thermopower of Weyl and Dirac semimetals using a semiclassical Boltzmann approach. We investigate the effect of various relaxation processes including disorder and interactions on the thermoelectric properties, and also consider doping away from the Weyl or Dirac point. We find that the thermal conductivity and thermopower have an interesting dependence on the chemical potential that is characteristic of the linear electronic dispersion, and that the electron-electron interactions modify the Lorenz number. For the interacting system, we also use the Kubo formalism to obtain the transport coefficients. We find exact agreement between the Kubo and Boltzmann approaches at high temperatures. We also consider the effect of electric and magnetic fields on the thermal conductivity in various orientations with respect to the temperature gradient. Notably, when the temperature gradient and magnetic field are parallel, we find a large contribution to the longitudinal thermal conductivity that is quadratic in the magnetic field strength, similar to the magnetic field dependence of the longitudinal electrical conductivity due to the presence of the chiral anomaly when no thermal gradient is present.
Charge Transport in Weyl Semimetals
Physical Review Letters, 2012
We study transport in three dimensional Weyl semimetals with N isotropic Weyl nodes in the presence of Coulomb interactions or disorder at temperature T . In the interacting clean limit, we determine the conductivity by solving a quantum Boltzmann equation within a 'leading log' approximation and find it to be proportional to T , upto logarithmic factors arising from the flow of couplings. In the noninteracting disordered case, we compute the finite-frequency Kubo conductivity and show that it exhibits distinct behaviors for ω ≪ T and ω ≫ T : in the former regime we recover the results of a previous analysis, of a finite conductivity and a Drude width that vanishes as N T 2 ; in the latter, we find a conductivity that vanishes linearly with ω whose leading contribution as T → 0 is the same as that of the clean, non-interacting system σ(ω, T = 0) = N e 2 12h |ω| v F . We compare our results to experimental data on Y2Ir2O7 and also comment on the possible relevance to recent transport data on Eu2Ir2O7.
Arxiv preprint arXiv: …, 2011
We study transport in three dimensional Weyl semimetals in the presence of Coulomb interactions or disorder. We consider N Weyl nodes with isotropic dispersion at temperature T. In the clean limit, including Coulomb interactions, we determine the conductivity by solving a quantum Boltzmann equation within a 'leading log' approximation. The conductivity is found to be proportional to T , upto logarithmic factors arising from the flow of couplings. In the disordered case, we use the Kubo formula to compute conductivity of non-interacting electrons in the presence of impurities. Here, the finite-frequency conductivity exhibits distinct behaviors, depending on whether ω ≪ T or ω ≫ T : in the former regime we recover the results of a previous analysis, of a finite conductivity and a Drude width that vanishes as N T 2 ; however, in the latter case, we find a conductivity that vanishes linearly with ω whose leading contribution as T → 0 is the same as that of the clean, non-interacting system σ(ω, T = 0) = N e 2 12h |ω| v F. A comparison is made with existing dc transport data in a pyrochlore iridate, which is predicted to have N = 24 Weyl nodes.
Recent developments in transport phenomena in Weyl semimetals
Comptes Rendus Physique, 2013
The last decade has witnessed great advancements in the science and engineering of systems with unconventional band structures, seeded by studies of graphene and topological insulators. While the band structure of graphene simulates massless relativistic electrons in two dimensions, topological insulators have bands that wind non-trivially over momentum space in a certain abstract sense. Over the last couple of years, enthusiasm has been burgeoning in another unconventional and topological (although, not quite in the same sense as topological insulators) phase -the Weyl Semimetal. In this phase, electrons mimic Weyl fermions that are well-known in high-energy physics, and inherit many of their properties, including an apparent violation of charge conservation known as the Chiral Anomaly. In this review, we recap some of the unusual transport properties of Weyl semimetals discussed in the literature so far, focusing on signatures whose roots lie in the anomaly. We also mention several proposed realizations of this phase in condensed matter systems, since they were what arguably precipitated activity on Weyl semimetals in the first place.
3 Recent developments in transport phenomena in Weyl semimetals
2016
The last decade has witnessed great advancements in the science and engineering of systems with unconventional band structures, seeded by studies of graphene and topological insulators. While the band structure of graphene simulates massless relativistic electrons in two dimensions, topological insulators have bands that wind non-trivially over momentum space in a certain abstract sense. Over the last couple of years, enthusiasm has been burgeoning in another unconventional and topological (although, not quite in the same sense as topological insulators) phase-the Weyl Semimetal. In this phase, electrons mimic Weyl fermions that are well-known in high-energy physics, and inherit many of their properties, including an apparent violation of charge conservation known as the Chiral Anomaly. In this review, we recap some of the unusual transport properties of Weyl semimetals discussed in the literature so far, focusing on signatures whose roots lie in the anomaly. We also mention several proposed realizations of this phase in condensed matter systems, since they were what arguably precipitated activity on Weyl semimetals in the first place.
Transverse thermopower in Dirac and Weyl semimetals
Physical Review B, 2019
Dirac semimetals (DSM) and Weyl semimetal (WSM) fall under the generic class of three-dimensional solids, which follow relativistic energy-momentum relation k = vF |k| at low energies. Such a linear dispersion when regularized on a lattice can lead to remarkable properties such as the anomalous Hall effect, presence of Fermi surface arcs, positive longitudinal magnetoconductance, and dynamic chiral magnetic effect. The last two properties arise due to the manifestation of chiral anomaly in these semimetals, which refers to the nonconservation of chiral charge in the presence of electromagnetic gauge fields. Here, we propose the planar Nernst effect, or transverse thermopower, as another consequence of chiral anomaly, which should occur in both Dirac and Weyl semimetals. We analytically calculate the planar Nernst coefficient for DSMs (type-I and type-II) and also WSMs (type-I and type-II), using a quasi-classical Boltzmann formalism. The planar Nernst effect manifests in a configuration when the applied temperature gradient, magnetic field, and the measured voltage are all co-planar, and is of distinct origin when compared to the anomalous and conventional Nernst effects. Our findings, specifically a 3D map of the planar Nernst coefficient in type-I Dirac semimetals (Na3Bi, Cd3As2 etc) and type-II DSM (PdTe2, VAI3 etc), can be verified experimentally by an in-situ 3D double-axis rotation extracting the full 4π solid angular dependence of the Nernst coefficient.
Nernst and magnetothermal conductivity in a lattice model of Weyl fermions
Physical Review B, 2016
Weyl semimetals (WSM) are topologically protected three dimensional materials whose low energy excitations are linearly dispersing massless Dirac fermions, possessing a non-trivial Berry curvature. Using semiclassical Boltzmann dynamics in the relaxation time approximation for a lattice model of time reversal (TR) symmetry broken WSM, we compute both magnetic field dependent and anomalous contributions to the Nernst coefficient. In addition to the magnetic field dependent Nernst response, which is present in both Dirac and Weyl semimetals, we show that, contrary to previous reports, the TR-broken WSM also has an anomalous Nernst response due to a non-vanishing Berry curvature. We also compute the thermal conductivities of a WSM in the Nernst (∇T ⊥ B) and the longitudinal (∇T B) setup and confirm from our lattice model that in the parallel setup , the Wiedemann-Franz law is violated between the longitudinal thermal and electrical conductivities due to chiral anomaly.
Chiral anomaly induced nonlinear Nernst and thermal Hall effects in Weyl semimetals
Physical Review B, 2022
Chiral anomaly or Adler-Bell-Jackiw anomaly in Weyl semimetals (WSMs) has a significant impact on the electron transport behaviors, leading to remarkable longitudinal or planar electrical and thermoelectric transport phenomena in the presence of electromagnetic gauge fields. These phenomena are consequences of the imbalanced chiral charge and energy induced by chiral anomaly in the presence of parallel electric (E) and magnetic (B) fields (E • B = 0) or (B • ∇T = 0) ((∇T) is the thermal gradient). We here propose another two fascinating transport properties, namely, the nonlinear planar Nernst effect and nonlinear planar thermal Hall effect induced by chiral anomaly in the presence of B • ∇T = 0 in WSMs. Using the semiclassical Boltzmann transport theory, we derive the analytical expressions for the chiral anomaly induced nonlinear Nernst and thermal Hall transport coefficients and also evaluate the fundamental mathematical relations among them in the nonlinear regime. The formulas we find in this current work are consistent with that predicted for the nonlinear anomalous electrical and thermoelectric effects induced by Berry curvature dipole recently. Additionally, in contrast to the recent work, by utilizing the lattice Weyl Hamiltonian with intrinsic chiral chemical potential, we find that the chiral anomaly induced nonlinear planar effects can exist even for a pair of oppositely tilted or non-tilted Weyl cones in both time reversal and inversion broken WSMs. The chiral anomaly induced nonlinear planar effects predicted here along with the related parameter dependencies are hence possible to be realized in realistic WSMs in experiment.
Helicity protected ultrahigh mobility Weyl fermions in NbP
Non-centrosymmetric transition metal monopnictides are promising Weyl semimetals (WSMs) with exotic physical properties. Although chiral WSM states have been observed in TaAs and NbAs, there is no conclusive evidence on the existence of Weyl fermions in NbP. Here, we use angle dependent quantum oscillations to reveal that NbP has four pairs of unusually large Weyl fermion pockets in the kz=0 plane near the high symmetry points Sigma, dominating over the coexisting massive hole pockets and the previous reported WSM pockets in the kz=1.18pi/c plane. Such dominant WSM pockets are highly anisotropic in k-space and approaching the parabolic band top along the internode direction. The corresponding Fermi surface is consisting of helical Weyl fermions with unprecedented mobility of 1*10E7 cm2V-1s-1 at 1.5 K, well protected from defect backscattering by real spin conservation associated to the chiral Weyl nodes. Inter-pocket pumping of Weyl fermions with opposite helicity becomes feasible w...