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تسجيل مستخدم جديدThermopower and thermal conductivity in the Weyl semimetal NbP
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The Weyl semimetal NbP exhibits an extremely large magnetoresistance (MR) and an ultra-high mobility. The large MR 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 800 $mu$V/K at 50 K in a field of 9 T. Such large effects have been observed rarely in bulk materials. We suggest that the origin of this effect might be related to the high charge-carrier mobility. We further observe pronounced quantum oscillations in both thermal conductivity and thermopower. The obtained frequencies compare well with our heat capacity and magnetization data.
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The optical properties of (001)-oriented NbP single crystals have been studied in a wide spectral range from 6 meV to 3 eV from room temperature down to 10 K. The itinerant carriers lead to a Drude-like contribution to the optical response; we can fu
rther identify two pronounced phonon modes and interband transitions starting already at rather low frequencies. By comparing our experimental findings to the calculated interband optical conductivity, we can assign the features observed in the measured conductivity to certain interband transitions. In particular, we find that transitions between the electronic bands spilt by spin-orbit coupling dominate the interband conductivity of NbP below 100 meV. At low temperatures, the momentum-relaxing scattering rate of the itinerant carriers in NbP is very small, leading to macroscopic characteristic length scales of the momentum relaxation of approximately 0.5 $mu$m.
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vation law by quantum fluctuations. For instance, the number of Weyl fermions of each chirality is not independently conserved in parallel electric and magnetic field, a phenomenon known as the chiral anomaly. In addition, an underlying curved spacetime provides a distinct contribution to a chiral imbalance, an effect known as the mixed axial-gravitational anomaly, which remains experimentally elusive. However, the presence of a mixed gauge-gravitational anomaly has recently been tied to thermoelectrical transport in a magnetic field, even in flat spacetime, opening the door to experimentally probe such type of anomalies in Weyl semimetals. Using a temperature gradient, we experimentally observe a positive longitudinal magnetothermoelectric conductance (PMTC) in the Weyl semimetal NbP for collinear temperature gradients and magnetic fields (DT || B) that vanishes in the ultra quantum limit. This observation is consistent with the presence of a mixed axial-gravitational anomaly. Our work provides clear experimental evidence for the existence of a mixed axial-gravitational anomaly of Weyl fermions, an outstanding theoretical concept that has so far eluded experimental detection.
As one of Weyl semimetals discovered recently, NbP exhibits two groups of Weyl points with one group lying inside the $k_z=0$ plane and the other group staying away from this plane. All Weyl points have been assumed to be type-I, for which the Fermi
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p$, similar to that observed in graphite where it was attributed to local superconductivity. The $T_p(H)$ relationship follows a power-law dependence $T_psim(H-H_c)^{1/v}$ where $v$ can be derived from the temperature dependence of the zero-field resistivity $rho_0(T) sim T^v$. From concurrent measurements of the transverse $rho_{xx}(T)$ and Hall $rho_{xy}(T)$ magnetoresistivities, we reveal a clear correlation between the rapidly increasing $rho_{xy}(T)$ and the occurrence of a peak in the $rho_{xx}(T)$ curve. Quantitative analysis indicates that the reentrant metallic behavior arises from the competition of the magneto conductivity $sigma_{xx}(T)$ with an additional component $Deltasigma_{xx}(T)=kappa_Hsigma_{xx}(T)$ where $kappa_H=[rho_{xy}(T)/rho_{xx}(T)]^2$ is the Hall factor. We find that the Hall factor ($kappa_H approx 0.4$) at peak temperature $T_p$ is nearly field-independent, leading to the observed $T_p(H)$ relationship. Furthermore, the reentrant metallic behavior in $rho_{xx}(T)$ also is reflected in the behavior of $rho_{xx}(H)$ that ranges from non-saturating at $T>70$ K to saturation at liquid helium temperatures. The latter can be explained with the magnetic field dependence of the Hall factor $kappa_H(H)$. Our studies demonstrate that a semiclassical theory can account for the anomalies in the magnetotransport phenomena of NbP without invoking an exotic mechanism.
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