Anomalous Nernst Effect in the Dirac Semimetal Cd3As2

Abstract

Dirac and Weyl semimetals display a host of novel properties. In Cd3As2, the Dirac nodes lead to a protection mechanism that strongly suppresses backscattering in zero magnetic field, resulting in ultrahigh mobility (∼ 10 7 cm 2 V −1 s −1 ). In applied magnetic field, an anomalous Nernst effect is predicted to arise from the Berry curvature associated with the Weyl nodes. We report observation of a large anomalous Nernst effect in Cd3As2. Both the anomalous Nernst signal and transport relaxation time τtr begin to increase rapidly at ∼ 50 K. This suggests a close relation between the protection mechanism and the anomalous Nernst effect. In a field, the quantum oscillations of bulk states display a beating effect, suggesting that the Dirac nodes split into Weyl states, allowing the Berry curvature to be observed as an anomalous Nernst effect.The field of topological quantum materials has recently expanded to include the Dirac (and Weyl) semimetals, which feature 3D bulk Dirac states with nodes that are protected by symmetry [1][2][3][4]. In Dirac semimetals, each Dirac cone is the superposition of two Weyl nodes which have opposite chiralities (χ = ±1). The Weyl nodes are prevented from hybridizing by the combination of point group symmetry, inversion symmetry and time-reversal symmetry (TRS) [4]. In the presence of a magnetic field B, the breaking of TRS leads to separation of the Weyl nodes and the appearance of a Berry curvature Ω(k).Because Ω(k) acts like an intense magnetic field, it exerts a strong force on charge carriers [5,6]. The first examples of Dirac semimetals, Na 3 Bi and Cd 3 As 2 , were identified by Wang et al. [7,8] Quite distinct from the chiral anomaly, the Berry curvature arising from separation of the Weyl nodes leads to other unusual transport effects, particularly the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) [24,25]. Unlike conventional system, no ferromagnetism is required for the AHE and ANE in Dirac semimetals because of the strong Berry curvature emanated by Weyl nodes. The anomalous Hall conductivity is expressed as [3,26],where ∆k i is the distance between the i th pair of Weyl nodes. The thermopower and Nernst effect in Weyl semimetals has been calculated in the Boltzmann equation approach [27][28][29][30].We report measurements of the thermoelectric tensor S ij of Cd 3 As 2 in two samples (A4, A5) in "set A" and two samples (B10, B20) in "set B" with the applied thermal gradient −∇T ||x and magnetic field B||ẑ (see Ref. [20] for details of the electrical transport measurements in set A and set B samples). We obtain S xx and S xy aswhere α ij is the thermoelectric linear response tensor, and ρ ij is the resistivity tensor (see Supplement for the details). In Dirac semimetals, the AHE and ANE arise because the Berry curvature Ω(k) imparts to the carriers an anomalous velocity v A = Ω(k) × k , i.e. Ω(k) acts like an effective magnetic field in k space (k is the rate of change of the wavevector k) [25]. Previously, the AHE was observed in Cd 3 As 2 as a wea...

Holographic calculation of the magneto-transport coefficients in Dirac semimetals

“…There have been some experimental measurements of magnetic field dependence of the transport parameters of this material. It is interesting to note that the holographically calculated elements of the conductivity and thermopower tensor show resemblance with the experimental data measured for Cd 3 As 2 system [36,37]. Furthermore, the dependence of the conductivity tensor on the magnetic field features quantitative similarity to the measurement.…”

“…The same is true for the interference effects conjectured due to the Berry phases from electron and hole sheets, at Fermi energy. These are the reasons why our results compare only qualitatively with real data [37].…”

“…Also in three dimensional systems the back-scattering in a zero magnetic field is strongly suppressed. This ensues high mobility [36][37][38] of carriers. The above features authorize holographic examination of the interaction limited transport phenomena in DSM.…”

“…According to the earlier discussion, this condition is expected to be valid relatively close to the Dirac point. In [37] both Hall and Nernst effects were analyzed in terms of anomalous contributions arising solely from the Berry phase. Our approach can not quantitatively describe the Berry phase induced contribution to the transport.…”

Thermoelectric transport coefficients of a Dirac electron gas in high magnetic fields

“…Under an external magnetic field some of these are the chiral anomaly 22 and as a consequence negative magnetoresistance 23 and a non-saturating linear magnetoresistance 15,[24][25][26][27] . Furthermore, several thermoelectric experiments were carried out in an external magnetic field [28][29][30][31][32] . The thermopower was found to be increasing linearly with the magnetic field 29 .…”