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Register a new userOptical conductivity of the Weyl semimetal NbP
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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 further 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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Weyl Semimetals (WSMs), a recently discovered topological state of matter, exhibit an electronic structure governed by linear band dispersions and degeneracy (Weyl) points leading to rich physical phenomena, which are yet to be exploited in thin film devices. While WSMs were established in the monopnictide compound family several years ago, the growth of thin films has remained a challenge. Here, we report the growth of epitaxial thin films of NbP and TaP by means of molecular beam epitaxy. Single crystalline films are grown on MgO (001) substrates using thin Nb (Ta) buffer layers, and are found to be tensile strained (1%) and with slightly P-rich stoichiometry with respect to the bulk crystals. The resulting electronic structure exhibits topological surface states characteristic of a P-terminated surface and linear dispersion bands in agreement with the calculated band structure, and a Fermi-level shift of -0.2 eV with respect to the Weyl points. Consequently, the electronic transport is dominated by both holes and electrons with carrier mobilities close to 10^3 cm2/Vs at room-temperature. The growth of epitaxial thin films opens up the use of strain and controlled doping to access and tune the electronic structure of Weyl Semimetals on demand, paving the way for the rational design and fabrication of electronic devices ruled by topology.
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.
We present an optical conductivity study of the multifold semimetal PdGa, performed in a broad spectral range (100 - 20000 cm-1; 12 meV - 2.5 eV) down to T = 10 K. The conductivity at frequencies below 4000 cm-1 is dominated by free carriers while at higher frequencies interband transitions provide the major contribution. The spectra do not demonstrate a significant temperature evolution: only the intraband part changes as a function of temperature with the plasma frequency remaining constant. The interband contribution to the conductivity exhibits a broad peak at around 5500 cm-1 and increases basically monotonously at frequencies above 9000 cm-1. The band-structure-based computations reproduce these features of the interband conductivity and predict its linear-in-frequency behavior as frequency diminishes.
We measured the optical reflectivity of [001]-oriented $n$-doped Cd$_{3}$As$_{2}$ in a broad frequency range (50 - 22000 cm$^{-1}$) for temperatures from 10 to 300 K. The optical conductivity, $sigma(omega) = sigma_{1}(omega) + {rm i}sigma_{2}(omega)$, is isotropic within the (001) plane; its real part follows a power law, $sigma_{1}(omega) propto omega^{1.65}$, in a large interval from 2000 to 8000 cm$^{-1}$. This behavior is caused by interband transitions between two Dirac bands, which are effectively described by a sublinear dispersion relation, $E(k) propto lvert k rvert ^{0.6}$. The momentum-averaged Fermi velocity of the carriers in these bands is energy dependent and ranges from $1.2 times 10^{5}$ to $3 times 10^{5}$ m/s, depending on the distance from the Dirac points. We detect a gaplike feature in $sigma_{1}(omega)$ and associate it with the Fermi level positioned around $100$ meV above the Dirac points.
The possibility of inducing superconductivity in type-I Weyl semimetal through coupling its surface to a superconductor was investigated. A single crystal of NbP, grown by chemical vapor transport method, was carefully characterized by XRD, EDX, SEM, ARPES techniques and by electron transport measurements. The mobility spectrum of the carriers was determined. For the studies of interface transmission, the (001) surface of the crystal was covered by several hundred nm thick metallic layers of either Pb, or Nb, or In. DC current-voltage characteristics and AC differential conductance through the interfaces as a function of the DC bias were investigated. When the metals become superconducting, all three types of junctions show conductance increase, pointing out the Andreev reflection as a prevalent contribution to the subgap conductance. In the case of Pb-NbP and Nb-NbP junctions, the effect is satisfactorily described by modified Blonder-Tinkham-Klapwijk model. The absolute value of the conductance is much smaller than that for the bulk crystal, indicating that the transmission occurs through only a small part of the contact area. An opposite situation occurs in In-NbP junction, where the conductance at the peak reaches the bulk value indicating that almost whole contact area is transmitting and, additionally, a superconducting proximity phase is formed in the material. We interpret this as a result of indium diffusion into NbP, where the metal atoms penetrate the surface barrier and form very transparent superconductor-Weyl semimetal contact inside. However, further diffusion occurring already at room temperature leads to degradation of the effect, so it is observed only in the pristine structures. Despite of this, our observation directly demonstrates possibility of inducing superconductivity in a type-I Weyl semimetal.
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