Application of FPGA-based Lock-in amplifier for ultrasound propagation measurements using the pulse-echo technique

We describe application of a state-of-the art digital FPGA based Lock-In amplifier to measurements of ultrasound propagation and attenuation at fixed frequency in low temperatures and in high static magnetic fields. Our implementation significantly simplifies electronics required for high resolution measurements, allows to record the full echo train in single measurement and extract changes in both phase and amplitude of an arbitrary number of echa as a function of an external control parameter. The system is simple in operation requiring very little prior knowledge of electrical engineering and can bring the technique to a broad range of solid state physics laboratories. We have tested our setup measuring the magneto-acoustic quantum oscillations in the Weyl semimetal NbP. The results are directly compared with previous results obtained using standard instrumentation

Characterization of topological band structures away from the Fermi level by anomalous Nernst measurements

Resolving the structure of energy bands in transport experiments is a major challenge in condensed matter physics and material science. Sometimes, however, traditional electrical conductance or resistance measurements only provide very small signals, and thus limit the ability to obtain direct band structure information. In this case, it has been proven beneficial to employ thermoelectric measurements which are sensitive to the first derivative of the density of states with respect to energy, rather than to its value itself. Due to the large interest in topological effects these days, it is important to identify a similar concept for detecting the Berry curvature in a band structure. Nowadays, the common way to access the Berry curvature directly via measurements is the anomalous Hall effect, but the corresponding signal can be too small to be detected when the topological features of the band structure lie too far off the Fermi level. In this work we propose to investigate topological band structure features utilizing the anomalous Nernst effect which is tied to the derivative of the anomalous Hall effect with respect to energy. Thereby, also signatures become resolvable, which are elusive in anomalous Hall measurements. We demonstrate the underlying mechanisms for a minimal effective four-band model and exemplary for the real Heusler compounds Co$_2$Fe$X$ ($X$=Ge,Sn), which host topological nodal lines about 100 meV above the Fermi level. This work demonstrates that anomalous Nernst measurements can be an effective tool for the characterization of topological band structures, both at room temperature and in the quantum transport regime at cryogenic temperatures

Three-dimensional quasi-quantized Hall effect in bulk InAs

The quasi-quantized Hall effect (QQHE) is the three-dimensional (3D) counterpart of the integer quantum Hall effect(QHE),exhibited only by two-dimensional (2D) electron systems. It has recently been observed in layered materials, consisting of stacks of weakly coupled 2D platelets. Yet, it is predicted that the quasi-quantized 3D version of the 2D QHE occurs in a much broader class of bulk materials, regardless of the underlying crystal structure. Here, we report the observation of quasi-quantized plateau-like features in the Hall conductivity of the n-type bulk semiconductor InAs. InAs takes form of a cubic crystal without any low-dimensional substructure. The onset of the plateau-like feature in the Hall conductivity scales with $\sqrt{2/3}k_{F}^{z}/\pi$ in units of the conductance quantum and is accompanied by a Shubnikov-de Haas minimum in the longitudinal resistivity, consistent with the predictions for 3D QQHE for parabolic electron band structures. Our results suggest that the 3D QQHE may be a generic effect directly observable in materials with small Fermi surfaces, placed in sufficiently strong magnetic fields

Thermoelectric performance of classical topological insulator nanowires

There is currently substantial effort being invested into creating efficient thermoelectric nanowires based on topological insulator chalcogenide-type materials. A key premise of these efforts is the assumption that the generally good thermoelectric properties that these materials exhibit in bulk form will translate into similarly good or even better thermoelectric performance of the same materials in nanowire form. Here, we calculate thermoelectric performance of topological insulator nanowires based on Bi2Te3, Sb2Te3 and Bi2Se3 as a function of diameter and Fermi level. We show that the thermoelectric performance of topological insulator nanowires does not derive from the properties of the bulk material in a straightforward way. For all investigated systems the competition between surface states and bulk channel causes a significant modification of the thermoelectric transport coefficients if the diameter is reduced into the sub-10 um range. Key aspects are that the surface and bulk states are optimized at different Fermi levels or have different polarity as well as the high surface to volume ratio of the nanowires. This limits the maximum thermoelectric performance of topological insulator nanowires and thus their application in efficient thermoelectric devices

Conservation of chirality at a junction between two Weyl semimetals

In Weyl semimetals the location of linear band crossings, the Weyl cones, is not bound to any high symmetry point of the Brillouin zone, unlike the Dirac nodes in graphene. This flexibility is advantageous for valleytronics, where information is encoded in the valleys of the band structure when intervalley scattering is weak. However, if numerous Weyl cones coexist the encoded information can decohere rapidly because of band mixing. Here, we investigate how the helical iso-spin texture of Weyl cones affects valleytronics in heterojunctions of Weyl materials, and show how the chirality of this iso-spin texture can serve to encode information.Comment: 13 pages, 7 figures ; supplementary material include

Strong anisotropy of electron-phonon interaction in NbP probed by magnetoacoustic quantum oscillations

In this study, we report on the observation of de Haas-van Alphen-type quantum oscillations (QO) in the ultrasound velocity of NbP as well as `giant QO' in the ultrasound attenuation in pulsed magnetic fields. The difference of the QO amplitude for different acoustic modes reveals a strong anisotropy of the effective deformation potential, which we estimate to be as high as $9\,\mathrm{eV}$ for certain parts of the Fermi surface. Furthermore, the natural filtering of QO frequencies and the tracing of the individual Landau levels to the quantum limit allows for a more detailed investigation of the Fermi surface of NbP as was previously achieved by means of analyzing QO observed in magnetization or electrical resistivity.Comment: 5 figure

Anisotropic electrical and thermal magnetotransport in the magnetic semimetal GdPtBi

The half-Heusler rare-earth intermetallic GdPtBi has recently gained attention due to peculiar magnetotransport phenomena that have been associated with the possible existence of Weyl fermions, thought to arise from the crossings of spin-split conduction and valence bands. On the other hand, similar magnetotransport phenomena observed in other rare-earth intermetallics have often been attributed to the interaction of itinerant carriers with localized magnetic moments stemming from the $4f$-shell of the rare-earth element. In order to address the origin of the magnetotransport phenomena in GdPtBi, we performed a comprehensive study of the magnetization, electrical and thermal magnetoresistivity on two single-crystalline GdPtBi samples. In addition, we performed an analysis of the Fermi surface via Shubnikov-de Haas oscillations in one of the samples and compared the results to \emph{ab initio} band structure calculations. Our findings indicate that the electrical and thermal magnetotransport in GdPtBi cannot be solely explained by Weyl physics and is strongly influenced by the interaction of both itinerant charge carriers and phonons with localized magnetic Gd-ions and possibly also paramagnetic impurities.Comment: 11 figure