Low-temperature Thermopower Research Articles

Negatively enhanced thermopower near a Van Hove singularity in electron-doped Sr2RuO4

The layered perovskite Sr2RuO4 serves as a model material of the two-dimensional (2D) Fermi liquid but also exhibits various emergent phenomena including the non-Fermi-liquid (NFL) behavior under external perturbations such as uniaxial pressure and chemical substitutions. Here we present the thermoelectric transport of electron-doped system Sr2−yLayRuO4, in which a filling-induced Lifshitz transition occurs at the Van Hove singularity (VHS) point of y≈0.2. We find that the sign of the low-temperature thermopower becomes negative only near the VHS point, where the NFL behavior has been observed in the earlier study. This observation is incompatible with either a numerical calculation within a constant relaxation-time approximation or a toy-model calculation for the 2D Lifshitz transition adopting an elastic carrier scattering. As a promising origin of the observed negatively enhanced thermopower, we propose a skewed NFL state, in which an inelastic scattering with a considerable odd-frequency term plays a crucial role to negatively enhance the thermopower. Published by the American Physical Society 2024

Modified scaling of thermopower to heat capacity for Cr doped Kondo insulator FeSi near metal to insulator transition

The Low temperature thermopower (S) and heat capacity (C) measurements are investigated for Cr doped FeSi. Metals having simple parabolic band structure whose entropy of free carriers used to scale as S/T proportional to γ, where γ is Sommerfeld coefficient leading to the validity of S/T to γ scaling. However, it is shown that the entropy of free carriers of metals derived from insulators violates such a scaling. Here, we have investigated the compositions which fall in the vicinity of (metal-to-insulator) critical concentration of Fe1-xCrxSi. These compositions are found to agree well with the modified scaling (nS/T vs γ) of thermopower to heat capacity by taking the carrier concentration n into consideration in the zero temperature limit. For higher concentrations of Cr, marked deviations from the scaling are observed probably due to the magnetic contribution such as Schottky effect to the heat capacity at low temperature.

Ingredients for enhanced thermoelectric power at cryotemperatures in the correlated semiconductor CoSbS revealed by its optical response

Semiconducting CoSbS is well known for its thermoelectric performance at high temperatures but it is also of interest because of its colossal low-temperature thermopower. Here, we address the temperature dependence of its optical response over a broad spectral range, from which we reveal several ingredients determining the thermoelectric properties at cryotemperatures. We discover coherent phonon modes and with the additional support of scanning transmission electron microscopy investigations we provide evidence for in-gap impurity states driving the formation of correlated electrons in the valence band. Their implications, with respect to the high thermoelectric power at low temperatures, are discussed within the framework of the phonon-drag transport of low-mobility heavy quasiparticles.

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.

Anomalous quantum criticality in the electron-doped cuprates

In the physics of condensed matter, quantum critical phenomena and unconventional superconductivity are two major themes. In electron-doped cuprates, the low critical field (HC2) allows one to study the putative quantum critical point (QCP) at low temperature and to understand its connection to the long-standing problem of the origin of the high-TC superconductivity. Here we present measurements of the low-temperature normal-state thermopower (S) of the electron-doped cuprate superconductor La2-x Ce x CuO4 (LCCO) from x = 0.11-0.19. We observe quantum critical [Formula: see text] versus [Formula: see text] behavior over an unexpectedly wide doping range x = 0.15-0.17 above the QCP (x = 0.14), with a slope that scales monotonically with the superconducting transition temperature (TC with H = 0). The presence of quantum criticality over a wide doping range provides a window on the criticality. The thermopower behavior also suggests that the critical fluctuations are linked with TC Above the superconductivity dome, at x = 0.19, a conventional Fermi-liquid [Formula: see text] behavior is found for [Formula: see text] 40 K.

Anisotropic magneto-thermoelectric properties of single-layer dilute charged impurity-infected black phosphorus

The continuum model, the Born approximation, and the Green's function approach are used to investigate thermoelectric properties of impurity-infected phosphorene subjected to a Zeeman magnetic field. We found that both disordered electrical and electronic thermal conductivities decrease (increase) with the magnetic field with (without) a threshold value in x−(y−)direction, stemming from the distributed charge with new potential. Furthermore, we found an anisotropic Drude weight at fairly low temperatures in the electrical conductivity. The opposite sign for low-temperature thermopower is captured by strong magnetic fields. Moreover, this information for the thermoelectric efficiency of disordered phosphorene tells us that the efficiency of the disordered phosphorene oscillates (increases) with the magnetic field in x−(y−)direction. Investigation of impurity configuration effects results in the significant differences. The efficiency regardless of the direction oscillates with high impurity concentrations and vanishes at low temperatures, whereas it does not change (oscillates) in x−(y−)direction with impurity scattering potential.

Structural and Electrical Properties Characterization of Sb1.52Bi0.48Te3.0 Melt-Spun Ribbons

Melt-spinning (MS) has been reported as a promising tool to tailor the microstructure of bulk thermoelectric materials leading to enhanced thermoelectric performances. Here, we report on a detailed characterization of p-type Bi0.48Sb1.52Te3 ribbons produced by melt-spinning. The microstructure of the melt-spun ribbons has been studied by means of X-ray diffraction, scanning and transmission electron microscopy (TEM). The analyses indicate that the ribbons are highly-textured with a very good chemical homogeneity. TEM reveals clear differences in the microstructure at large and short-range scales between the surface that was in contact with the copper wheel and the free surface. These analyses further evidence the absence of amorphous regions in the melt-spun ribbons and the precipitation of elemental Te at the grain boundaries. Low-temperature electrical resistivity and thermopower measurements (20–300 K) carried out on several randomly-selected ribbons confirm the excellent reproducibility of the MS process. However, the comparison of the transport properties of the ribbons with those of bulk polycrystalline samples of the same initial composition shows that MS leads to a more pronounced metallic character. This difference is likely tied to changes in deviations from stoichiometry due to the out-of-equilibrium conditions imposed by MS.

Structure, magnetic, electrical and thermal transport properties of Dy-doped Ca3Co2O6 ceramics

The effects of Dy doping on the structure, magnetic, electrical and thermal transport properties of Ca3−xDyxCo2O6 (x=0–0.4) ceramics have been investigated. All samples show the rhombohedral structure with space group R3̅c. The lattice parameter c is found to increase whereas a decreases in the Dy-doped samples by the Rietveld refinement on the powder x-ray diffraction patterns. The magnetization at high temperatures increases slightly with increasing Dy-doping content, whereas the low-temperature magnetization decreases. The variations of the magnetization in the high- and low-temperature range are considered to be related to the paramagnetism of the doped Dy3+ ions and the weakened ferromagnetic (FM) interaction along the spin–chains, respectively. The resistivity decreases initially and then increases slightly with increasing Dy-doping level. The competition between the increased carrier concentration and the enhanced disorder in the Dy-doped samples may be the origin of the observed phenomenon. The low-temperature thermopower increases considerably in the Dy-doped samples, although its value has a decreasing trend at high temperatures. The result shows that the magnetic ion doping may be an effective method to tune the low-temperature thermoelectric properties of these spin–chain compounds.

Phonon-drag thermopower in 3D Dirac semimetals

A theory of low-temperature phonon-drag thermopower Sg in three-dimensional (3D) Dirac semimetals has been developed considering screened electron–phonon deformation potential coupling. Numerical investigations of Sg, in the boundary scattering regime for phonons, are made in 3D Dirac semimetal Cd3As2, as a function of temperature T and electron concentration ne. Sg is found to increase rapidly for about T < 1 K and nearly levels off for higher T. It is also seen that Sg increases (decreases) with decreasing ne at lower (higher) T (<2 K). A screening effect is found to be very significant, strongly affecting T and ne dependence for about <1 K and becoming negligible at higher temperature. In the Bloch–Gruneisen (BG) regime the power laws Sg ~ T8 (T4) and Sg ~ with (without) screening are obtained. These laws with respect to T and ne are, respectively, characteristics of 3D phonons and Dirac 3D electrons. Comparison with diffusion thermopower Sd shows that Sg dominates (and is much greater than) Sd for about T > 0.2 K. Herring’s law Sgμp ~ T −1, relating phonon limited mobility μp and Sg in the BG regime, is shown to be valid in 3D Dirac semimetals. The results obtained here are compared with those in 3D semiconductors, low-dimensional semiconductor heterojunctions and graphene. We conclude that ne-dependent measurements, rather than T-dependent ones, provide a clearer signature of the 3D Dirac semimetal phase.

Invited: In Situ Characterization of Plasma Assisted Hydrogenation/Fluorination of Graphene and CFx Battery Applications

Monolayer graphene synthesized by chemical vapor deposition was subjected to controlled and sequential hydrogenation using RF plasma while monitoring its electrical properties in-situ. Low temperature transport properties, viz., electrical resistance (R), thermopower (S), Hall mobility (m), and magneto-resistance (MR) were measured for each sample and correlated with ex-situ Raman scattering and X-ray photoemission (XPS) characteristics. For weak-hydrogenation, the transport is seen to be governed by electron diffusion and low temperature transport properties show metallic behavior (conductance G remains non-zero as 0). For strong-hydrogenation, the transport is found to be describable by variable range hopping (VRH) and the low T conductance shows insulating behavior (0 as 0). Weak localization (WL) behavior is seen with a negative MR for weakly-hydrogenated graphene and this WL effects are seen to diminish as the hydrogenation progresses. A clear transition to strong localization (SL) is evident with the emergence of pronounced negative MR for strongly-hydrogenated graphene. Multi-walled carbon nanotubes synthesized using fluidized bed chemical vapor deposition technique were fluorinated sequentially to prepare a series of CFx battery electrodes. Primary battery performance was tested using CFx as a cathode against Li. Fully fluorinated MWNTs showed capacity exceeding 815 mAh/g while partially fluorinated samples showed systematically lowered capacity with decreasing x (in CFx). However, fully fluorinated MWCNTs showed distinctly low rechargeable capacity compared to the subfluorinated samples when used as an anode against Li. Mildly fluorinated MWNTs show high capacity and better stability during charge/discharge cycles. High concentrations of fluorine seem to affect capacity retention due to the increased defect densities and reduced electronic conduction. These defects of nanotubes will provide additional pathways for lithium ions to diffuse within the core of the fluorinated structure and to access the electrochemically active C-F sites. XRD, XPS, and Raman spectroscopy were utilized to characterize the samples. Finally the electrochemical performance of fluorinated MWNTs was compared with that of Natural Chinese Graphite (NCG).

Low temperature thermopower and electrical conductivity in highly conductive CuInO2thin films

This is the first report on the detailed study on low temperature (6–300 K) electrical conductivity and Seebeck coefficient on copper indium oxide polycrystalline thin films deposited on soda lime glass substrates by a reactive evaporation method. These films, characterized using various experimental techniques, show the highest electrical conductivity (20 to 125 S cm−1) reported to date as well as n- and p- type conductivity. The electrical conductivity mechanisms, based on Mott's, Seto's and Arrhenius models, in different temperature regimes, comprise of variable range hopping, grain boundary effect and activated thermal conduction. Most interestingly, the thin films manifest a high thermoelectric power factor and absorption in the UV region, indicating their potential in thermoelectric and UV energy conversion device fabrication, respectively.

Thermopower studies of rare earth doped lanthanum barium manganites

Influence of rare earth doping on electrical, magnetic and thermopower studies of La0.34Re0.33Ba0.33MnO3 compound was investigated. Ferro to paramagnetic transition and metal to insulator transition temperatures decrease with decreasing ionic radius of the dopant ion. Electrical resistivity in the entire temperature range is explained by phase separation model. The magnitude of Seebeck coefficient increases with increasing dopant ionic radius. A cross over from negative to positive sign has also been observed in thermopower data with decreasing A site ionic radius (〈rA〉). The low temperature thermopower data has been explained using a qualitative model containing diffusion; magnon drag and phonon drag effects while the paramagnetic insulating part has been analyzed using small polaron hopping mechanism.

Thermoelectric misfit-layered cobalt oxides with interlayers of hydroxide and peroxide species

Among the thermoelectric misfit-layered cobalt oxides, [MmA2Om+2]qCoO2, the parent m=0 phases exhibit divergent chemical features but are less understood than the more common m>0 members of the series. Here we synthesize Sr-for-Ca substituted [(Ca1−xSrx)z(O,OH)2]qCoO2 zero phases up to x=0.2 through low-temperature hydrothermal conversion of precursor powders of the m=1 misfit system, [Co(Ca1−xSrx)2O3]qCoO2. In the zero-phase [(Ca1−xSrx)z(O,OH)2]qCoO2 system, as the Sr content x increases the lattice expands anisotropically along the c axis such that the ab-plane dimension and the misfit parameter q remain essentially constant. X-ray absorption spectroscopy data suggest the presence of peroxide-type oxygen species in the (Ca1−xSrx)z(O,OH)2 rock-salt block and together with infrared spectroscopy, thermogravimetric and low-temperature resistivity and thermopower measurements evidence that the isovalent Sr-for-Ca substitution controls the balance between the peroxide and hydroxide species in the (Ca1−xSrx)z(O,OH)2 block but leaves the valence of Co essentially intact in the CoO2 block. The higher electrical conductivity of the Sr-substituted phases is explained as a consequence of increased carrier mobility.

In Situ Study of Hydrogenation of Graphene and New Phases of Localization between Metal–Insulator Transitions

Monolayer graphene synthesized by chemical vapor deposition was subjected to controlled and sequential hydrogenation using RF plasma while monitoring its electrical properties in situ. Low-temperature transport properties, namely, electrical resistance (R), thermopower (S), Hall mobility (μ), and magnetoresistance (MR), were measured for each sample and correlated with ex situ Raman scattering and X-ray photoemission (XPS) characteristics. For weak hydrogenation, the transport is seen to be governed by electron diffusion, and low-temperature transport properties show metallic behavior (conductance G remains nonzero as T → 0). For strong hydrogenation, the transport is found to be describable by variable range hopping (VRH) and the low T conductance shows insulating behavior (G → 0 as T → 0). Weak localization (WL) behavior is seen with a negative MR for weakly hydrogenated graphene, and these WL effects are seen to diminish as the hydrogenation progresses. A clear transition to strong localization (SL) is evident with the emergence of pronounced negative MR for strongly hydrogenated graphene.

Renormalized bands and low-temperature colossal thermopower induced by Ir doping in Ca3Co4O9 system

The effects of Ir doping on the low-temperature thermal and electrical transport properties of Ca3Co4−xIrxO9 (0 ≤ x ≤ 0.4) have been investigated. As Ir ions are doped into the system, a low-temperature thermopower upturn is introduced, and the temperature corresponding to the minimum thermopower TSmin increases monotonously with increasing x. Moreover, a low-temperature colossal thermopower is observed in the Ir-doped samples, which is beneficial to the low-temperature thermoelectric refrigeration in superconducting devices. For the x = 0.4 sample, its maximum thermopower Smax at the lowest measured temperature ∼24 K reaches 1500 μV/K. The appearance of such a colossal thermopower phenomenon is suggested to be closely related to the variations of the transport mechanism and the band structure induced by 5d-Ir doping.

Thermopower Studies of Polycrystalline Ag Doped LaMnO3 Manganites

The effects of silver doping on the magnetotransport and thermopower of La1−x Ag x MnO3 (0.05≤x≤0.30) have been investigated. For the sample with x=0.05, temperature dependent resistivity exhibits an insulating behavior, while thermopower is found to be large and negative over the measured temperature range. An increase in the Ag doping enhances the conductivity and shifts the metal-insulator transition temperature toward high temperature side. The low temperature thermopower data has been explained in terms of diffusion, magnon drag, and phonon drag effects and found that the magnon drag effect dominates in this region. Finally, the electrical transport in the high temperature region has been analyzed by using adiabatic small polaron hopping mechanism.

Valence-band structure of highly efficientp-type thermoelectric PbTe-PbS alloys

Experimental evidence is given relevant to the temperature dependence of the valence band structure of PbTe and PbTe${}_{1\ensuremath{-}x}$S${}_{x}$ alloys ($0.04\ensuremath{\le}x\ensuremath{\le}0.12$), and its effect on the thermoelectric figure of merit $ZT$. The $x=0.08$ sample has $ZT\ensuremath{\sim}1.55$ at 773 K. The magnetic field dependence of the high-temperature Hall resistivity of heavily $p$-type ($&gt;{10}^{19}$ cm${}^{\ensuremath{-}3}$) Na-doped PbTe${}_{1\ensuremath{-}x}$S${}_{x}$ reveals the presence of high-mobility electrons. This casts doubts on prior analyses of the Hall coefficient suggesting that temperature induces a rapid rise in energy of the ``heavy'' hole relative to the ``light'' hole bands. The electron-like behavior is likely induced by the topology of the Fermi surface when the L- and \ensuremath{\Sigma}-bands merge. Negative values for the low-temperature thermopower are also observed. The data show that PbTe continues to be a direct-gap semiconductor at temperatures where the $ZT$ and ${S}^{2}\ensuremath{\sigma}$ of $p$-type PbTe are optimal, e.g., 700--800 K.

Thermoelectric properties and Kondo behavior in indium incorporated p-type Ce0.9Fe3.5Ni0.5Sb12 skutterudites

Herein, we report the effects of indium (In) incorporation upon the thermoelectric and magnetic properties of Ni doped Fe-based filled skutterudites (InxCe0.9Fe3.5Ni0.5Sb12 with x = 0, 0.1, and 0.5). We find that secondary phases (such as InSb) can be formed upon surpassing the filling fraction limit and these in turn result in improved thermoelectric properties. A maximum dimensionless figure of merit, ZT ≈ 0.9 at ∼650 K was obtained for the sample with the nominal composition In0.1Ce0.9Fe3.5Ni0.5Sb12. Interestingly, we also observe Kondo-like behavior and evidence of the crystal field effect in these samples. The low-temperature (T &amp;lt; 100 K) thermopower and electrical resistivity of our samples exhibit Kondo-like behavior while their corresponding magnetic susceptibility suggests that the Ce3+ ions are influenced by the cubic crystal symmetry of the skutterudites class of materials, thus resulting in the crystal field effect. Lastly, the magnetic susceptibility data can be interpreted in the context of a second-order perturbation model.

Optoelectronic and thermoelectric properties in Ga doped β- PbS2 nanostructured thin films

Lead sulphide nanostructured thin films were grown on soda lime glass substrates by chemical bath deposition. The films were then doped with gallium using vacuum evaporation technique. X-ray diffraction (XRD) established the structural type of the host films to be tetragonal β-PbS2 with average grain size of the order of 15nm. The nanostructure of films was further confirmed from scanning electron and atomic force micrographs. The shift in the binding energies of the 4f and 4d states of lead, 2p state of sulphur and the 2p states of Ga from their elemental binding energy values, determined from X-ray photoelectron spectroscopy (XPS), indicated intact chemical bonding in the compound. Compositional analysis showed about 0.01% doping of Ga into PbS2. Low temperature thermopower measurements indicated p-type conductivity for the films with Fermi level positioned at about 0.017eV above the maxima of valence band. Optical absorption studies in conjunction with photo sensitivity measurements established its pertinence in junction formation in photovoltaic applications due to the blue shift in the band gap to 2.37eV and the increased photoconductivity of the films.

Zero-field thermopower of a thin heterostructure membrane with a two-dimensional electron gas

We study the low-temperature thermopower of micron sized, free-standing membranes containing a two-dimensional electron system. Suspended membranes of 320 nm thickness including a high electron mobility structure in Hall bar geometry of 34 {\mu}m length are prepared from GaAs/AlGaAs heterostructures grown by molecular beam epitaxy. Joule heating on the central region of the membrane generates a thermal gradient with respect to the suspension points where the membrane is attached to cold reservoirs. Temperature measurements on the membrane reveal strong thermal gradients due to the low thermal conductivity. We measure the zero-field thermopower and find that the phonon-drag contribution is suppressed at low temperatures up to 7 K.