Thermopower Changes Research Articles

Electronic transport and magnetic properties of La1-xCaxMnO3 (0 < x ≤ 1)

We investigate the electronic transport and magnetic properties of La1-xCaxMnO3 across a wide range of calcium doping (x = 1/8, 1/4, 1/3, 1/2, 2/3, 3/4, and 1), unveiling distinct electronic and magnetic phase transitions. The findings can be summarized as follows. For samples with x = 1/8, 1/4, and 1/3, a transition temperature is observed at 188 K, 240 K, and 270 K, respectively. The x = 1/2 sample exhibits a charge ordering state, showing thermal hysteresis in electrical resistivity, thermopower, and magnetization. For samples with x = 2/3 and 3/4, a charge ordering state is observed at temperatures of 260 K and 225 K, respectively. However, there is no evidence of thermal hysteresis in terms of electrical resistivity, thermopower, and magnetization. Electrical resistivity and thermopower measurements follow the small polaron hopping model for x = 1/8, 1/4 and 1/3, while the variable range hopping model is applicable for x = 2/3, 3/4, and 1 at high temperatures. The sign and magnitude of the thermopower are found to be dependent on the calcium content. For samples with x less than 1/3, the thermopower is positive. In the case of the sample where x = 1/3, the thermopower changes sign, shifting from negative to positive below 101.8 K. Samples with x greater than 1/3 exhibit negative thermopower. The magnetization curves exhibit a decrease near the magnetic transition temperatures, specifically at 240 K for the x = 1/4 sample and 270 K for the x = 1/3 sample.

Tuning of thermoelectric performance by modulating vibrational properties in Ni-doped Sb2Te3

Sb2Te3, a binary chalcogenide-based 3D topological insulator, attracts significant attention for its exceptional thermoelectric performance. We report the vibrational properties of magnetically doped Sb2Te3 thermoelectric material. Ni doping induces defect/disorder in the system and plays a positive role in engineering the thermoelectric properties through tuning the vibrational phonon modes. Synchrotron powder x-ray diffraction study confirms good crystalline quality and single-phase nature of the synthesized samples. The change in structural parameters, including B iso and strain, further corroborate with structural disorder. Detailed modification of phonon modes with doping and temperature variation is analysed from temperature-dependent Raman spectroscopic measurement. Compressive lattice strain is observed from the blue shift of Raman peaks owing to Ni incorporation in Sb site. An attempt is made to extract the lattice thermal conductivity from total thermal conductivity estimated through optothermal Raman studies. Hall concentration data support the change in temperature-dependent resistivity and thermopower. Remarkable increase in thermopower is observed after Ni doping. Simulation of the Pisarenko model, indicating the convergence of the valence band, explains the observed enhancement of thermopower in Sb2−x Ni x Te3. The energy gap between the light and heavy valence band at Γ point is found to be 30 meV (for Sb2Te3), which is reduced to 3 meV (in Sb1.98Ni0.02Te3). A significant increase in thermoelectric power factor is obtained from 715 μWm−1K−2 for pristine Sb2Te3 to 2415 μWm−1K−2 for Ni-doped Sb2Te3 sample. Finally, the thermoelectric figure of merit, ZT is found to increase by four times in Sb1.98Ni0.02Te3 than that of its pristine counterpart.

Nonlinear Seebeck and Peltier effects in a Majorana nanowire coupled to leads

Abstract We theoretically study nonlinear thermoelectric transport through a topological superconductor nanowire hosting Majorana bound states (MBSs) at its two ends, a system named as Majorana nanowire (MNW). We consider that the MNW is coupled to the left and right normal metallic leads subjected to either bias voltage or temperature gradient. We focus our attention on the sign change of nonlinear Seebeck and Peltier coefficients induced by mechanisms related to the MBSs, by which the possible existence of MBSs might be proved. Our results show that for a fixed temperature difference between the two leads, the sign of the nonlinear Seebeck coefficient (thermopower) can be reversed by changing the overlap amplitude between the MBSs or the system equilibrium temperature, which are similar to the cases in linear response regime. By optimizing the MBS–MBS interaction amplitude and system equilibrium temperature, we find that the temperature difference may also induce sign change of the nonlinear thermopower. For zero temperature difference and finite bias voltage, both the sign and magnitude of nonlinear Peltier coefficient can be adjusted by changing the bias voltage or overlap amplitude between the MBSs. In the presence of both bias voltage and temperature difference, we show that the electrical current at zero Fermi level and the states induced by overlap between the MBSs keep unchanged, regardless of the amplitude of temperature difference. We also find that the direction of the heat current driven by bias voltage may be changed by weak temperature difference.

Correlation between morphology and transport properties of Au/Co/Au/Si wedge ultra-thin film

We report the thermoelectric power (TEP) and electrical resistivity characterization supported by structural and surface morphological properties on a wedge shaped ultra-thin sample of Au/Co/Au/Si (100) with Co have varying thickness from 0.5 nm to 4 nm across the sample. The sample was prepared by Ion Beam Sputtering method, with an ultrathin layer of Co sandwiched between two layers of Au (3 nm thicknesses each). The observed TEP results showed that the system is semiconductor in nature and the concentration of the charge carrier is inversely proportional to the Seebeck coefficient (S) and also concentration of charge carrier modifies with the film thickness leading to a change in thermo power, which is found to be higher at lower thicknesses of Co sandwiched layer. The type of charge carrier was also determined to be electrons in this system. The observed results are in good agreement with the resistivity measurement carried out as a function of film thickness indicating a decrease in resistivity as a function of film thickness due to enhancement in charge carrier mobility. The thermal activation energy was seen to increase from ∼29.5 meV to ∼280.3 meV as the film thickness increased from the minimum to the highest. Corresponding Atomic Force Microscopy (AFM) measurement revealed island type grain growth with the introduction of a significant amount of roughness, while Magnetic Force Microscopy (MFM) experiments exhibit randomly oriented magnetic domain structures. The overall results suggest that lowest thickness film shows semiconducting nature, while variable range hopping phenomena due to some localized energy levels near the valence and conduction band is observed in the highest thickness film. This result is also well supported by XRD measurement, which revealed crystalline nature of the films. Such low cost, easy to fabricate materials may find applications in Peltier cooling or energy harvesting applications without the harmful carbon dioxide emission. For this, advance experiments will be done in future.

Thermoelectric Power and Hall Effect in Correlated Metals and Doped Mott–Hubbard Insulators: DMFT Approximation

We present comparative theoretical investigation of thermoelectric power and Hall effect in the Hubbard model for correlated metal and Mott insulator (considered as prototype cuprate superconductor) for different concentrations of current carriers. Analysis is performed within standard DMFT approximation. For Mott insulator we consider the typical case of partial filling of the lower Hubbard band (hole doping). We calculate the dependence of thermopower on doping level and determine the critical concentration of carriers corresponding to sign change of thermopower. An anomalous dependence of thermopower on temperature is obtained significantly different from linear temperature dependence typical for the usual metals. The role of disorder scattering is analyzed on qualitative level. The comparison with similar studies of the Hall effect shows, that breaking of electron-hole symmetry leads to the appearance of the relatively large interval of band-fillings (close to the half-filling) where thermopower and Hall effects have different signs. We propose a certain scheme allowing to determine the number of carriers from ARPES data and perform semi-quantitative estimate of both thermopower and Hall coefficient using the usual DFT calculations of electronic spectrum.

Polycrystalline bismuth nanowire networks for flexible longitudinal and transverse thermoelectrics.

This paper reports on the preparation and the characterization of structural, electrical and thermoelectric properties of nanocomposite films formed from three-dimensional networks of polycrystalline bismuth (Bi) nanowires (NWs). The samples were fabricated by electrodeposition within polycarbonate (PC) templates with crossed cylindrical nanopores, yielding self-supported networks of Bi crossed nanowires (CNWs) with mean diameter values ranging from 23 nm to 230 nm. Temperature changes in electrical resistance and thermopower were studied by considering electric and thermal currents flowing in the plane of the films. While the values of the Seebeck coefficient are close to those of polycrystalline Bi for diameters greater than 100 nm, a progressive decrease in thermopower appears at smaller diameters, due to an increasing contribution of surface charge carriers as the diameter decreases. Transverse thermoelectricity based on the Nernst effect was also demonstrated on a network of Bi CNWs 230 nm in diameter. Such hierarchical architectures based on Bi CNWs are extremely robust, offering a reliable solution for the next generation of flexible thermoelectric devices.

Rare earth free oxide nano-composite of SrTi0.85Nb0.15O3 and CNT for potential n-type thermoelectrics

Despite having large electron concentration, the electrical conductivity of Nb doped SrTiO3 severely suffers from poor carrier mobility. Even after taking several attempts by different research groups, it is still challenging to improve carrier mobility without affecting the Seebeck coefficient. In this report, 1-dimensional CNT has been introduced in SrTi0.85Nb0.15O3 (STN) matrix to fabricate STN + CNT nano composites as CNT is expected to have boosted the mobility by several orders. We have reported more than 1000 % increase in electrical conductivity after CNT addition. In contrast, Seebeck coefficient remains flat with CNT loading. This remarkable enhancement of electrical conductivity accompanied by essentially no change in thermopower leads to a 380% increase in the ZT parameter. Although CNT has been previously introduced in chalcogenides, binary oxides, and oxy-chalcogenides, such a remarkable surge in electrical transport with the incorporation of a small amount of CNT has never been reported. Moreover, to the best of our knowledge, this is the first report on thermoelectric properties of oxide perovskite composites with CNT. Our results suggest that forming oxide-CNT nano-composites can be a robust strategy to improve the thermoelectric performance in bulk materials for realistic applications.

Thermoelectric properties of Rashba compounds KSnX (X = Sb, Bi)

Here, we have presented the results of the detailed theoretical study of thermoelectric properties of two Rashba compounds KSnSb and KSnBi using first principles calculations based on density functional theory and Boltzmann transport theory taking spin–orbit coupling (SOC) into account. As these compounds have layered-type crystal structures, their transport parameters are found to be highly anisotropic. For KSnBi (KSnSb), the calculated lattice thermal conductivity κl along its crystallographic c axis is found to have ultralow value of 0.49 W m−1 K−1 (0.78 W m−1 K−1) even at room temperature, whereas almost twofold larger value of κl is estimated along its crystallographic a axis. However, large values of other transport parameters like electrical conductivity σ and thermopower S desirable for a high power factor (S2σ) are found along the a axis of these compounds. For KSnSb, the optimum a axis ZT=2.6 can be reachable for an electron concentration of 3.3 × 1019 cm−3 and at a temperature of 800 K. Comparable value of optimum a axis ZT=2.5 is also noted for KSnBi despite its strong susceptibility to bipolar conduction. Both these non-centrosymmetric compounds exhibit SOC-driven Rashba spin splitting of electronic bands, which affects both thermopower and electrical conductivity of these compounds. However, such Rashba spin splitting induced change in thermopower is almost negated by the concomitant change in electrical conductivity, resulting in no appreciable impact on power factor and hence ZT of the studied compounds.

Gigantic Effect due to Phase Transition on Thermoelectric Properties of Ionic Sol–Gel Materials

AbstractSol–gel phase transition in ionic thermoelectrical (i‐TE) materials induces large rapid change in viscosity and ionic transport process and is thus expected to yield a drastic variation in thermoelectric properties, crucial in low‐grade waste heat harvesting and wearable electronic applications. In this study, four types of i‐TE materials are prepared and examined. For the first time, a large rise in the thermopower by 6.5 times during the sol–gel transition of poloxamer/LiCl system is observed, an even greater ionic figure of merit by ≈23 times. The phenomenon is found to be universal as the large variation in thermopower is confirmed in the other transitional materials. The study further reveals the mechanism and proposes a model that deals with the whole process. Finally, six factors influencing the huge variation of the thermopower during the phase‐transition are probed and light is shed on the possible gigantic changes of thermopower during the phase transition. A possible route is uncovered to design and control the desired thermoelectric performances of materials, which can lead to a new sight in tunable i‐TE devices for low‐heat energy harvesting applications.

Electrical and thermal transport in van der Waals magnets 2H−MxTaS2 (M=Mn, Co)

We report a detailed study of electrical and thermal transport properties in 2H-M$_x$TaS$_2$ (M = Mn, Co) magnets where M atoms are intercalated in the van der Waals gap. The intercalation induces ferromagentism with an easy-plane anisotropy in 2H-Mn$_x$TaS$_2$, but ferromagnetism with a strong uniaxial anisotropy in 2H-Co$_{0.22}$TaS$_2$, which finally evolves into a three-dimensional antiferromagnetism in 2H-Co$_{0.34}$TaS$_2$. Temperature-dependent electrical resistivity shows metallic behavior for all samples. Thermopower is negative in the whole temperature range for 2H-Co$_x$TaS$_2$, whereas the sign changes from negative to positive with increasing Mn for 2H-Mn$_x$TaS$_2$. The diffusive thermoelectric response dominates in both high- and low-temperature ranges for all samples. A clear kink in electrical resistivity, a weak anomaly in thermal conductivity, as well as a slope change in thermopower were observed at the magnetic transitions for 2H-Mn$_{0.28}$TaS$_2$ ($T_\textrm{c}$ $\approx$ 82 K) and 2H-Co$_{0.34}$TaS$_2$ ($T_\textrm{N}$ $\approx$ 36 K), respectively, albeit weaker for lower $x$ crystals. Co-intercalation promoted ferromagnetic to antiferromagnetic transition is further confirmed by the Hall resistivity; the sign change of the ordinary Hall coefficient indicates a multi-band behavior in 2H-Co$_x$TaS$_2$.

Thermoelectric Transport in a Double-Quantum-Dot Coupled to Majorana Zero Modes

Thermoelectric transport through a double-quantum-dot (DQD) connected to the left and right leads is theoretically investigated in the framework of non-equilibrium Green’s function technique. We consider that the dots are also coupled to Majorana zero modes (MZMs) prepared at the two ends of a topological superconductor nanowire. It is found that the sign change of thermopower, which is promising in the detection of MZMs, can be realized by tuning several system’s parameters related to the MZMs, such as the coupling strength between the dots and the MZMs, the direct coupling between the MZMs, or even the magnetic flux penetrating through the structure. The above parameters also lead to significant enhancement of the thermopower and thermoelectric figure of merit (FOM), which indicates the conversion efficiency between heat and electrical energies. We also find that in this DQD system, both the thermopower and FOM are simultaneously enhanced by the MZMs around the electron-hole symmetric point, an ideal phenomenon in applications of thermoelectric effect. In addition, the thermoelectric effect is remarkably enhanced by the direct hybridization between the MZMs, which is very different from the case in single-dot structure.

Effect of Phonon Focusing on the Drag Thermopower in Single-Crystal Potassium Nanowires at Low Temperatures

The role of shear waves and the effect of phonon focusing on the anisotropy of the thermopower of the electron–phonon drag in single-crystal potassium nanowires at low temperatures are analyzed in this work. The deformation potential theory is used for the longitudinal components of elastic modes. For the shear components of vibrational modes, the electron–phonon interaction constant is used, which was determined previously from comparing the results of calculating the thermoelectric power with experimental data for bulk potassium crystals. It is shown that shear waves make a significant contribution to the drag thermopower of nanowires. In the regime of the Knudsen phonon gas flow for nanowires with a cross-section side D = 5 × 10–6 cm, the contribution of the slow transverse mode t2 with allowance for only the longitudinal component in the [111] directions turned out to be 32% smaller and, with allowance for the shear component of the t2 mode, 12% greater than the contribution of longitudinal phonons. Directions corresponding to the maximum and minimum values of the drag thermopower of nanowires are determined. It is shown that, under conditions of competition between the boundary and bulk mechanisms of phonon relaxation with an increase in the cross section of nanowires, the anisotropy of the drag thermopower changes nonmonotonically. It exceeds 30% not only in the Knudsen phonon gas flow regime, but also when the sample thickness is two orders of magnitude greater. This makes the anisotropy of thermopower available for experimental studies.

Unusually Large Thermopower Change from +330 to −185 μV K–1 of Brownmillerite SrCoO2.5

Strontium cobalt oxide (SrCoO2.5) has recently attracted increasing attention as its optoelectronic and magnetic properties can be widely controlled using electrochemical oxidation/protonation at r...

Angle-dependent magnetoresistance and its implications for Lifshitz transition in W2As3

Lifshitz transition represents a sudden reconstruction of Fermi surface structure, giving rise to anomalies in electronic properties of materials. Such a transition does not necessarily rely on symmetry-breaking and thus is topological. It holds a key to understand the origin of many exotic quantum phenomena, for example, the mechanism of extremely large magnetoresistance (MR) in topological Dirac/Weyl semimetals. Here, we report studies of the angle-dependent MR (ADMR) and the thermoelectric effect in W2As3 single crystal. The compound shows a large unsaturated MR (of about 7000% at 4.2 K and 53 T). The most striking finding is that the ADMR significantly deforms from the horizontal dumbbell-like shape above 40 K to the vertical lotus-like pattern below 30 K. The window of 30–40 K also corresponds substantial changes in Hall effect, thermopower and Nernst coefficient, implying an abrupt change of Fermi surface topology. Such a temperature-induced Lifshitz transition results in a compensation of electron-hole transport and the large MR as well. We thus suggest that the similar method can be applicable in detecting a Fermi-surface change of a variety of quantum states when a direct Fermi-surface measurement is not possible.

Giant enhancement of cryogenic thermopower by polar structural instability in the pressurized semimetal MoTe2

We found that a high mobility semimetal 1T'-MoTe2 shows a significant pressure-dependent change in the cryogenic thermopower in the vicinity of the critical pressure, where the polar structural transition disappears. With the application of a high pressure of 0.75 GPa, while the resistivity becomes as low as 10 {\mu}{\Omega}cm, thermopower reached the maximum value of 60 {\mu}VK-1 at 25 K, leading to a giant thermoelectric power factor of 300 {\mu}WK-2cm-1. Based on semiquantitative analyses, the origin of this behavior is discussed in terms of inelastic electron-phonon scattering enhanced by the softening of zone center phonon modes associated with the polar structural instability.

Low-Temperature Thermopower in CoSbS.

We report on giant thermopower of S=2.5 mV/K in CoSbS single crystals: a material that shows strong high-temperature thermoelectric performance when doped with Ni or Se. Changes of low-temperature thermopower induced by a magnetic field point to the mechanism of electronic diffusion of carriers in the heavy valence band. Intrinsic magnetic susceptibility is consistent with the Kondo-insulatorlike accumulation of electronic states around the gap edges. This suggests that giant thermopower stems from temperature-dependent renormalization of the noninteracting bands and buildup of the electronic correlations on cooling.

Thermoelectric transport through a finite-U quantum dot side-coupled to Majorana bound state

We study the thermoelectric transport through a single-level quantum dot (QD) coupled to two normal metallic leads and side-coupled to Majorana bound state (MBS). The Coulomb interaction in QD is considered. To investigate only the influence of MBS on thermoelectric transport, we focus on the relatively high temperature region ($T\gg T_K$), where Kondo effect does not appear. The electric and thermal conductance and thermopower as a function of gate voltage (i.e. QD level) are completely different whether the coupling between MBSs is zero or not. When the coupling between MBSs is finite, all thermoelectric characteristics are similar to the transport without MBS. However, for zero MBSs' coupling, the electric and thermal conductance peaks are reduced by 3/4. Especially, in the case of QD without MBS, the sign of thermopower changes three times, however, in the case of QD strongly side-coupled to ideal and isolated MBS, the sign of thermopower changes 9 or 5 times. It can be used for detecting of the signature of MBS. It has actual possibilities when the nanowire is long enough and pure without any defects.

The alternating-sign magnetoresistance of polycrystalline manganese chalcogenide films

The correlation between the dc and ac electrical resistance and the structural, magnetic, and thermoelectric properties of polycrystalline MnSe1−XTeХ (0.3 ≤ X ≤ 0.4) manganese chalcogenide films in the temperature range of 80–400 K has been investigated. Inhomogeneous electronic states and transitions between them accompanied by structural lattice deformations have been found at temperatures including the Neel temperature region. The magnetic susceptibility maximum above the Neel temperature in a magnetic field of 8.6 kOe has been observed. Temperature ranges of the coexistence of two types of electrically inhomogeneous states have been established by impedance spectroscopy in the frequency range of 0.1–1000 kHz and the change of the hopping conductivity for diffusion one accompanied by the magnetic susceptibility minimum has been found. The magnetoresistance in magnetic fields of up to 12 kОе has been established. It has been revealed that the thermopower and magnetoresistance change its sign upon heating. The experimental data are explained using a spin polaron model with the localization of polarons and formation of the electron phase-separation. The alternation of magnetoresistance in sign is attributed to the ferromagnetic orbital ordering of electrons and the negative magnetoresistance is explained by suppression of spin fluctuations in a magnetic field.

Colossal change in thermopower with temperature-driven p–n-type conduction switching in LaxSr2−xTiFeO6 double perovskites

Double perovskite materials have been studied in detail by many researchers, as their magnetic and electronic properties can be controlled by the substitution of alkaline earth metals or lanthanides in the A site and transition metals in the B site. Here we report the temperature-driven, p–n-type conduction switching assisted, large change in thermopower in La3+-doped Sr2TiFeO6-based double perovskites. Stoichiometric compositions of LaxSr2−xTiFeO6 (LSTF) with 0 ⩽ x ⩽ 0.25 were synthesized by the solid-state reaction method. Rietveld refinement of room-temperature XRD data confirmed a single-phase solid solution with cubic crystal structure and space group. From temperature-dependent electrical conductivity and Seebeck coefficient (S) studies it is evident that all the compositions underwent an intermediate semiconductor-to-metal transition before the semiconductor phase reappeared at higher temperature. In the process of semiconductor–metal–semiconductor transition, LSTF compositions demonstrated temperature-driven p–n-type conduction switching behavior. The electronic restructuring which occurs due to the intermediate metallic phase between semiconductor phases leads to the colossal change in S for LSTF oxides. The maximum drop in thermopower (ΔS ~ 2516 µV K−1) was observed for LSTF with x = 0.1 composition. Owing to their enormous change in thermopower of the order of millivolts per kelvin, integrated with p–n-type resistance switching, these double perovskites can be used for various high-temperature multifunctional device applications such as diodes, sensors, switches, thermistors, thyristors, thermal runaway monitors etc. Furthermore, the conduction mechanisms of these oxides were explained by the small polaron hopping model.

Geometric Shape Induced Small Change of Seebeck Coefficient in Bulky Metallic Wires.

In this paper, we report the results of slight changes in the thermopower of long W, Mo, Zn, Cu, brass, and Ti wires, that resulted from changes in the wire’s diameter or cross-sectional area. The samples used in the tests had a round shape with a diameter that ranged from tens of micron to 2 mm, which was much larger than the corresponding mean free paths of these materials. Nevertheless, a small change in thermopower, at the order of 1–10 nV/K, was repeatedly observed when the wire diameter was changed, or when the cross-sectional area of the wire was altered by mechanical methods, such as grinding or splitting. The results are consistent with previous observations showing that the thermopower in metallic thin film stripes changes with their width, from 100 μm to as little as 70 nm, implying a universal, geometric-boundary-related size effect of thermopower in metal materials, that occurs at the nanometer scale and continuously decreases all the way to the millimeter scale. This effect could be applied in the manufacturing of high-temperature sensors with simple structures.