Giant Nernst effect and bipolarity in the quasi-one-dimensional metal, Li(0.9)Mo(6)O(17)

The Nernst coefficient for the quasi-one-dimensional metal, Li(0.9)Mo(6)O(17), is found to be among the largest known for metals (~500 microV/KT at T~20K), and is enhanced in a broad range of temperature by orders of magnitude over the value expected from Boltzmann theory for carrier diffusion. A comparatively small Seebeck coefficient implies that Li(0.9)Mo(6)O(17) is bipolar with large, partial Seebeck coefficients of opposite sign. A very large thermomagnetic figure of merit, ZT~0.5, is found at high field in the range T~35-50K.Comment: PRL in press, manuscript(5pp, 3 Fig.'s) and Supplementary Material (5pp, 7 Fig.'s

Observation of a thermoelectric Hall plateau in the extreme quantum limit

The thermoelectric Hall effect is the generation of a transverse heat current upon applying an electric field in the presence of a magnetic field. Here we demonstrate that the thermoelectric Hall conductivity $\alpha_{xy}$ in the three-dimensional Dirac semimetal ZrTe$_5$ acquires a robust plateau in the extreme quantum limit of magnetic field. The plateau value is independent of the field strength, disorder strength, carrier concentration, or carrier sign. We explain this plateau theoretically and show that it is a unique signature of three-dimensional Dirac or Weyl electrons in the extreme quantum limit. We further find that other thermoelectric coefficients, such as the thermopower and Nernst coefficient, are greatly enhanced over their zero-field values even at relatively low fields.Comment: 17+21 pages, 3+14 figures; published versio

High Temperature Characterization of Ge2Sb2Te5Thin Films for Phase Change Memory Applications

The recent proliferation of portable communication devices or data storage equipment is strongly related to the development of memory technology. Non-volatile semiconductor solid-state memories are needed for high-capacity storage media, high-speed operation and low power consumption, with stringent requirements of retention and endurance. Phase change memory (PCM) is currently seen as one of the most promising candidates for a future storage-class memory with the potential to be close to dynamic random-access memory (DRAM) in speed but with much longer retention times and as dense as flash memory. PCM devices utilize chalcogenide materials (most commonly Ge2Sb2Te5 or GST) that can be switched rapidly and reversibly between amorphous and crystalline phases with orders of magnitude difference in electrical resistivity. Since PCM devices operate at elevated (current-induced) temperatures and are significantly impacted by thermoelectric effects it is very important to determine the high temperature material properties of GST. Resistivity, carrier mobility, and carrier concentration in semiconducting materials are three key parameters indispensable for device modeling. In this work two measurement setups for high temperature thin film characterizations were developed, a Seebeck setup and a Hall setup. The Seebeck coefficient measurement setup is fully automated and uses resistive and inductive heaters to control the temperature gradient and can reach temperatures up to ~650 °C. The Hall measurement setup, developed based on the van der Paw method for characterization of semiconducting thin films, can measure thin film samples of a wide resistivity range from room temperature to ~500 °C. The resistivity, carrier concentration, and Hall carrier mobility are calculated from I-V measurements and the constant magnetic field applied in ‘up’ and ‘down’ directions. Measurement results on GST thin films with different thicknesses revealed interesting correlations between S-T and ρ-T characteristics and showed that GST behaves as a unipolar p-type semiconducting material from room temperature up to melting. The thermoelectric properties of the GST films were also correlated to the average grain sizes obtained from in-situ XRD measurements during crystallization. These studies show that the activation energy of carriers in mixed phase amorphous-fcc GST is a linear function of the Peltier coefficient. From these results and the ρ-T characteristics, the expected Seebeck coefficient of single crystal fcc GST is obtained. Using the experimental results for resistivity and Seebeck coefficient, together with a phase separation model, the temperature-dependent thermal conductivity of the mixed phase GST is extracted

Thermotronics: toward nanocircuits to manage radiative heat flux

The control of electric currents in solids is at the origin of the modern electronics revolution which has driven our daily life since the second half of 20th century. Surprisingly, to date, there is no thermal analog for a control of heat flux. Here, we summarize the very last developments carried out in this direction to control heat exchanges by radiation both in near and far-field in complex architecture networks.Comment: arXiv admin note: text overlap with arXiv:1503.0498

Time-dependent multistate switching of topological antiferromagnetic order in Mn3_3Sn

The manipulation of antiferromagnetic order by means of spin-orbit torques opens unprecedented opportunities to exploit the dynamics of antiferromagnets in spintronic devices. In this work, we investigate the current-induced switching of the magnetic octupole vector in the Weyl antiferromagnet Mn$_3$Sn as a function of pulse shape, field, temperature, and time. We find that the switching behavior can be either bistable or tristable depending on the temporal structure of the current pulses. Time-resolved Hall effect measurements reveal that Mn$_3$Sn switching proceeds via a two-step demagnetization-remagnetization process caused by self-heating over a timescale of tens of ns followed by cooling in the presence of spin-orbit torques. Our results shed light on the switching dynamics of Mn$_3$Sn and prove the existence of extrinsic limits on its switching speed.Comment: Rectified wrong order of MS and Supplemen

Investigation of thermo-electromagnetic materials implemented in harvesting of thermoelectric energy in electrical machines

Many researchers have tried to exploit waste heat to generate electrical power. There are two phenomena that are related to the conversion of heat into electrical power: thermoelectric (TE) and thermomagnetic (TM) phenomena. In this work the latter (TM phenomenon) deals with the conversion of waste heat to generate electrical power. TM effect began in the 1960s due to the difficulties in induction of a strong magnetic field in the past. The work presented in this thesis focuses on the preparation of polycrystalline indium antimonide (InSb) bulk materials and investigation of their TM properties. The research was motivated by their anticipated application in technologically important regions of reducing energy losses and in operating conditions of electro-magnetic machines, such as motors, generators and transformers, by incorporating such energy conversion devices into the machines at carefully chosen locations. When a thermomagnetic sample is subjected to both temperature gradient and magnetic flux density concurrently, it will produce electrical output. A high electrical power output will be produced when the sample has similar numbers of both charge carriers, and, in addition, when the sample is subject to high temperature difference and high magnetic flux density across it. A new technique has been developed in this work to make undoped and doped InSb polycrystalline bulk materials with tellurium Te, based on open quartz tube instead of the traditional method requiring sealing of the quartz tube. A modification in the raw materials ratio was adjusted to obtain pure InSb sample. The X-ray diffraction (XRD) and the inter-planar spacing analysis were carried out to check the structure of the samples and the result confirmed that the material was pure InSb. Measurements were taken under direct magnetic field, which was produced from direct current (DC) supply, and alternative magnetic field, which was induced from alternative current (AC). The design procedure involved determining the longitudinal, transverse and hybrid transverse voltages. Modifications in design of the measurement IV system have been made to minimise the effect of AC magnetic field on these parts, such as the heat sink being made of copper plate and K-type thermocouples. In addition, magnetic shielding was used for wires in the vicinity to minimise the induced voltage that affects measurements of transverse voltage. The induced voltage was still higher than the transverse voltage, even with the use of magnetic shielding. For this reason, the research performed in this work relating to the thermomagnetic parameters under AC magnetic field did not obtain appropriately good results. The thermomagnetic parameters of samples under DC magnetic field have been improved by doping them with Te at different levels of 0.1% and 0.25%. The resistivity and Seebeck coefficient of the doped InSb with 0.25% Te was lower than those for undoped InSb single crystalline, which was used as a reference sample. The resistivity was lower, around 24% and 38% at the magnetic flux density 0 T and 1 T respectively, and the Seebeck coefficient was about 8% lower for various magnetic flux densities. In contrast, the Nernst, Righi-Leduc voltages and thermomagnetic power of the doped InSb with 0.25% Te were higher than those for undoped InSb single crystalline. The Nernst voltage was around 2% and 0.5% for the magnetic flux density of 1 T and temperature difference 30 °C and 80 °C respectively, while the Righi-Leduc voltage was higher, around 2.6% and 0.9%, and thermomagnetic power was higher around, 2.9% and 1% respectively for the same magnetic field and temperature differences