Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Z12: Focus Session: Thermoelectric Magnetothermoelectric Magnetocaloric |
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Sponsoring Units: DMP GERA FIAP Chair: David Singh, Oak Ridge National Laboratory Room: 314 |
Friday, March 22, 2013 11:15AM - 11:27AM |
Z12.00001: Enhanced thermoelectric properties via oxygen non-stoichiometry in La$_2$NiO$_4$ and SrTiO$_3$ Victor Pardo, Antia S. Botana, Paul M. Bach, Victor Leboran, Francisco Rivadulla, Daniel Baldomir We present the results of transport properties calculations and experiments on various oxides. A large enhancement of the thermoelectric properties is predicted\footnote{PRB 86, 165114 (2012).} via ab initio calculations for La$_2$NiO$_{4+\delta}$, with electronic-only thermoelectric figure of merit ($zT$) values exceeding unity for oxygen excess $\delta$ $\leq$ 0.10. The effects of lattice strain (caused, e.g. by growth of thin films on different substrates) enhance even further the thermoelectric response. A similar result is obtained at very low electron-doping in bulk SrTiO$_3$ via oxygen removal. This is analyzed experimentally via thermal annealing that depletes oxygen ($\sim$ 1 oxygen vacancy per 10$^6$ unit cells). In both these systems, the increase in conductivity reached in the metallic limit retains a large thermopower, with the corresponding enhancement of $zT$. In the case of SrTiO$_3$, experiments indicate\footnote{arxiv:1211.1615.} that such a small oxygen vacancy level reduces drastically the thermal conductivity by introducing random scattering centers. In the talk, we will discuss the electronic structure origin of the enhancement of the thermoelectric response and how this can be tuned. Results are general and applicable to other non-stoichiometric oxides. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z12.00002: Thermometry and power sensing with SNS proximity structures Russell Lake, Joonas Govenius, Ville Pietil\"{a}, Kuan Yen Tan, Mikko M\"{o}tt\"{o}nen We present our experimental progress on thermometry employing the superconductor proximity effect in a normal-metal (N) mesoscopic wire between two superconducting (S) electrodes. We have fabricated Al/Au/Al SNS structures with junction lengths in the diffusive transport regime and performed electrical measurements between 300 K and 8 mK. Temperature dependence of the differential resistance shows sensitivity at the millikelvin level at a bath temperature of 8 mK. Specifically, the shape of the proximity effect induced dip in the differential resistance at zero current-bias serves as a direct probe of the N wire temperature. We show that the energy scale of proximity superconductivity in the N wire can be controlled by changing the wire length or by applying a perpendicular magnetic field to tune the temperature detection range. Results are discussed in terms of the temperature and resolving power noise for a thermometer and a power meter, respectively. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z12.00003: Chemical Pressure Effect and Dimer Formation in (Ba,Sr)Ni$_2$As$_2$ Solid Solutions Tyler Drye, Shanta Saha, Johnpierre Paglione Although both BaNi$_2$As$_2$ and SrNi$_2$As$_2$ form in ThCr$_2$Si$_2$ structure, these materials display very different behaviors, owing in part to an important structural difference: while the Sr compound exhibits As-As bonds between layers, the Ba compound lacks these interlayer bonds. Thus, substitution of Sr into BaNi$_2$As$_2$ produces a positive chemical pressure effect on the system that pulls the NiAs layers closer together and towards As-As dimer formation. We will present the resulting phase diagram as determined by x-ray, chemical composition, electrical resistivity and magnetization measurements. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z12.00004: Quantifying the Local Seebeck Coefficient using Scanning Thermoelectric Microscopy (SThEM) Jenna Walrath, Yen-Hsiang Lin, Kevin Pipe, Rachel Goldman Thermoelectric (TE) devices allow reliable solid-state conversion of heat to electricity. The efficiency of a TE device is determined by the figure of merit, ZT, which is sensitive to the Seebeck coefficient, S. A promising alternative to traditional macroscale measurements of S is scanning thermoelectric microscopy (SThEM), which can profile S with nm resolution [1]. In SThEM, an unheated scanning tunneling microscopy tip acts as a high-resolution voltmeter probe to measure the thermally-induced voltage, V, in a heated sample. However, the temperature (T) gradient is not localized to the sample, and the measured V is a convolution of voltages within the region of non-zero temperature gradient. Therefore we have developed a 1D Fourier heat conduction model to predict the T gradient in the tip and to deconvolute the measured V within the sample. This approach enables direct conversion between the measured V and the local S. [1] H.K. Lyeo et al., Science \textbf{303}, 816 (2004). [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z12.00005: Profiling the Local Seebeck Coefficient with Nanometer Resolution Using Scanning Thermoelectric Microscopy (SThEM) Yen-Hsiang Lin, Jenna Walrath, Rachel Goldman Thermoelectric (TE) devices offer a method of recovering waste heat through solid state conversion of heat to electricity. Nanostructured thermoelectric materials may provide the key to increased efficiencies, which are sensitive to the Seebeck coefficients (S) However, traditional bulk measurement techniques can only provide a spatially averaged measurement of S over the whole sample, which can hardly investigate the effects of nanostructures on S on the nanoscale. A novel technique known as scanning thermoelectric microscopy (SThEM) has recently been developed to measure induced thermal voltages with nanometer resolution In SThEM, an unheated scanning tunneling microscopy tip acts as a high-resolution voltmeter probe to measure the thermally-induced voltage, V, in a heated sample. Here we present a local S measurement using SThEM across an InGaAs P-N junction. The thermovoltage shows an abrupt change of sign within 10 nanometers, which reveals nanometer spatial resolution. We will discuss local S measurements of AlAs/GaAs superlattices (SLs) with various SL periods and compare the local S with scanning tunneling spectroscopy measurements, which will reveal how local electronic states influence thermoelectric properties. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z12.00006: Scaling of electrical and thermal conductivities in an almost integrable chain Joel Moore, Christoph Karrasch, Roni Ilan Many low-dimensional materials are well described by integrable one-dimensional models such as the Hubbard model of electrons or the Heisenberg model of spins. However, the small perturbations to these models required to describe real materials are expected to have singular effects on transport quantities: integrable models often support dissipationless transport, while weak non-integrable terms lead to finite conductivities. We use translation-invariant matrix-product-state methods to obtain quantitative values of electrical and thermal conductivities in an almost integrable chain (an XXZ spin chain with staggered fields, or equivalently a spinless fermion chain with staggered on-site potentials). [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z12.00007: Flexible thermoelectric films using the spin Seebeck effect Akihiro Kirihara, Masahiko Ishida, Hiroko Someya, Koichi Kondo, Naoharu Yamamoto, Ken-ichi Uchida, Eiji Saitoh, Shigeru Kohmoto, Tomoo Murakami Thermoelectric (TE) technologies have been of great interest, since they can directly generate electricity from thermal energy that is available in various places. For making full use of such omnipresent heat, TE devices using the spin Seebeck effect (SSE) potentially open opportunities for large-area TE applications, because of their favorable features such as a simple film structure and convenient scaling capability [1]. In this work, we show a SSE-based flexible TE device, which consists of metallic and magnetic-insulator films on a 25-um-thick polyimide substrate. Novel fabrication processes enabled us to form the magnetic insulator, having a good spin-current-conduction property for the SSE, on the highly flexible organic film. Such flexible TE sheets are readily implementable on various curved or uneven surfaces, leading to versatile energy-harvesting and heat-sensing applications. [1] A. Kirihara, et al., Nature Mat. 11, 686 (2012). [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z12.00008: Nonlocal thermoelectric effects and nonlocal Onsager relations in a three-terminal superconducter/ferromagnet proximity system Matthias Eschrig, Peter Machon, Wolfgang Belzig Heterostructures of ferromagnets and superconductors are presently subject of intense study since they show interesting phenomena based on the singlet-triplet conversion of pairing amplitudes at the interfaces, and the resulting spin-dependent proximity effect. Spectacular examples are long-range triplet Josephson currents due to inhomogeneous magnetic order, or due to the spin-dependence of the interface reflection and transmission amplitudes, which were confirmed in a set of pivotal experiments in 2010. Here, we study thermal and charge transport in a three-terminal setup consisting of a superconducting and two ferromagnetic contacts. We predict that the simultaneous presence of spin-filtering and of spin-dependent scattering phase shifts at each of the two interfaces will lead to giant non-local thermoelectric effects both in clean and in disordered systems. The symmetries of thermal and electric transport coefficients are related to fundamental thermodynamic principles by the Onsager reciprocity. Our results show that a non-local version of the Onsager relations for thermoelectric currents holds in a three terminal quantum coherent ferromagnet-superconductor heterostructure including spin-dependent crossed Andreev reflection and coherent electron transfer processes. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z12.00009: Magneto Themoelectric Generator with Carbon Nanotube Thermal Interfaces Patrick T. McCarthy, Timothy S. Fisher, Ernesto E. Marinero We report the thermal behavior of Gd foils used in a magneto thermoelectric generator cells. The device exploits the ferromagnetic phase transition of gadolinium to drive the movement of a diaphragm ``shuttle'' whose mechanical energy is converted to electrical form and which enhances heat transfer through both conduction and convection. Efficient heat transfer at mechanical interfaces is critical to increase shuttle speed and the commensurate rate of heat transfer. The synthesis and characterization of carbon nanotube thermal interfaces for the Gd foils are described. The samples generated in this study were consistently measured with total thermal interface resistances in the range of 65--105 mm$^{2}$ K/W, a reduction of 55--70{\%} compared to bare Gd (R$_{\mathrm{int}}$ $\sim$ 230 mm$^{2}$ K/W). The addition of carbon nanotube arrays did not alter the magnetic properties of the gadolinium foils and only a slight decrease in the magnetic moment of the gadolinium samples (8--13{\%}) was measured after growth. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z12.00010: Thermomagnetic effects in elemental rare-earth single crystals Audrey M. Chamoire, Joseph P. Heremans Thermomagnetic properties and magnetothermal conductivity of elemental rare-earth (R-E) metals are for the first time systematically presented from 80 to 400 K. Measurements are given with heat flux applied along the [100] and the [111] directions since R-E present mainly a hexagonal symmetry at room temperature. This work is motivated by the complex Fermi surfaces of the R-E's and by their magnon contributions to the thermal conductivity. Elemental rare-earths are multicarrier systems involving electron ($e)$ and hole ($h)$ pockets and have a very small thermopowers ($S)$, which can result in large Nernst coefficients. This would be suitable for transverse Nernst cooler since they could be used as a single material with a particular design, then resolving the problems of contact resistances of actual Peltier coolers where materials need to be cascaded. Magnetic field dependent thermal conductivity is used to extract magnon heat conduction. Magnons are bosons, but unlike acoustic phonons they can have energy gaps. Taken together, these two properties should theoretically lead to a non-linear thermal conductivity in the presence of a magnetic field gradient. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z12.00011: Magnetic refrigeration capabilities of magnetocaloric Ni2Mn:75Cu:25Ga S.K. Mishra, C.A. Jenkins, I. Dubenko, T. Samanta, N. Ali, S. Roy Doping-driven competition between energetically similar ground states leads to many exciting materials phenomena such as the emergence of high-\emph{T$_{c}$} superconductivity, diluted magnetic semiconductors, and colossal magnetoresistance. Doped Ni$_{2}$MnGa Heusler alloy, which is a multifunctional ferromagnetic alloy with various exotic physical properties demonstrates this notion of rich phenomenology \emph{via} modified ground spin states. Adopting this generic concept, here we will present a novel doped Ni$_{2}$Mn$_{.75}$Cu$_{.25}$Ga alloy that offers unprecedented co-existence of the magnetocaloric effect and fully controlled ferromagnetism at room temperature. Application of site engineering enables us to manipulate the ground spin state that leads to the decrease in magnetic transition temperature and also increases the delocalization of the Mn magnetism. SQUID magnetometery suggests that Cu doping enhances the saturation magnetization, coercive field and clarity of magnetic hysteresis loops. By exploiting x-ray absorption techniques and measuring element specific magnetic hysteresis loops, here we will describe the microscopic origin of enhnaced magnetocaloric properties and \emph{d-d} interaction driven charge transfer effects in Ni$_{2}$Mn$_{.75}$Cu$_{.25}$Ga [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z12.00012: First successful growth of magnetic thin films of meta-stable monoclinic Gd$_{5}$(Si$_{x}$Ge$_{1-x)4}$ David C. Jiles, Ravi L. Hadimani, Ikenna C. Nlebedim, Yevgen Melikhov We report on the first successful growth of magnetic thin films of the giant magnetocaloric material Gd$_{5}$(Si$_{x}$Ge$_{1-x})_{4}$. This material has been widely studied for its unusual properties including the coupled magnetic-structural phase transition. We report on the successful growth of films of Gd$_{5}$Si$_{2.09}$Ge$_{1.91}$ that can be used in micro-cooling applications. The film was grown by Pulsed Laser Deposition (PLD) on a (001) silicon wafer deposited at 200$^{\circ}$C from a polycrystalline target. PLD was achieved using a femtosecond laser with a repetition rate of 1kHz, pulse energy of up to 3.5mJ. The deposited film thickness was $\sim$ 400nm measured using Scanning Electron Microscopy and the composition of the film was analyzed using Energy Dispersive Spectroscopy and found to be close to the target composition. Magnetic measurements were carried out in a SQUID magnetometer. Magnetic moment vs. magnetic field measurement confirmed that the film was ferromagnetic at 200K. The transition temperature of the film was measured from magnetic moment vs. temperature measurements using inflection point. Transition temperature was measured at 280K which was close to the 1$^{st}$ order phase transition temperature of bulk material. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z12.00013: Strain induced ferromagnetism and magnetocaloric effect in LaFe2Si2 thin film Guixin Cao, German Samolyuk, Siwei Tang, Liang Qiao, Wenbin Wang, Jieyu Yi, Thomas Zac Ward, Michael Biegalski, Wolter Siemons, David Mandrus, Malcolm Stocks, Zheng Gai Great interest in magnetic refrigeration techniques based on the magnetocaloric effect (MCE) has grown recently due to its high efficiency and environmental friendliness. Although the thin film form of the materials is very important in both application and fundamental research, as the properties of films can be tailored by parameters like epitaxial strain, studies on MCE in single crystal films are limited by the difficulty of the growth. In this work, LaFe2Si2 thin films are successfully tuned from Pauli paramagnetic to ferromagnetic, and MCEs are observed around 50K. The ferromagnetic transition is a first order transition, and the magnetic entropy $\Delta $S $\approx $-8.5 J/Kg K is obtained under a magnetic field of 7T. The magnetocaloric effect is characterized by a 14 K hysteresis in the field cooling and field warming process. Our temperature dependent X-ray measurements exclude the correlation between the striking MCE of the thin film and structural transition. Density functional theory (DFT) calculations indicate that the strain induced distance variations of Si-Fe bonds control the magnitude of the magnetic moment and MCE. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z12.00014: Entropy changes and the caloric effects in R$_{5}$Si$_{2}$Ge$_{2}$ (R$=$Gd and Tb) Nilson de Oliveira It has been experimentally shown that at ambient pressure, the compound Gd$_{5}$Si$_{2}$Ge$_{2}$ undergoes a first order transition with giant magnetocaloric effect around this room temperature. Experimental data also show that an applied pressure increases the critical temperature of this compound and keeps the first order phase transition. On the other hand, experimental data show that the compound Tb$_{5}$Si$_{2}$Ge$_{2}$ undergoes a second order phase transition with a normal magnetocaloric effect around 100 K. It has also been shown that an applied pressure increases its critical temperature without changing the order of the phase transition. In this work, we calculate the magnetocaloric and barocaloric effects in and Gd$_{5}$Si$_{2}$Ge$_{2}$ and Tb$_{5}$Si$_{2}$Ge$_{2}$. For this purpose, we use a model of localized magnetic moments including the magnetoelastic interaction. In the model, the order of the phase transition is controlled by the ratio between the exchange interaction and the magnetoelastic coupling parameter. Our calculations show that these compounds exhibit large values of the entropy changes upon pressure variation in good agreement with the available experimental data. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z12.00015: Energy-harvesting at the Nanoscale Andrew Jordan, Bj\"orn Sothmann, Rafael S\'anchez, Markus B\"uttiker Energy harvesting is the process by which energy is taken from the environment and transformed to provide power for electronics. Specifically, the conversion of thermal energy into electrical power, or thermoelectrics, can play a crucial role in future developments of alternative sources of energy. Unfortunately, present thermoelectrics have low efficiency. Therefore, an important task in condensed matter physics is to find new ways to harvest ambient thermal energy, particularly at the smallest length scales where electronics operate. To achieve this goal, there is on one hand the miniaturizing of electrical devices, and on the other, the maximization of either efficiency or power the devices produce. We will present the theory of nano heat engines able to efficiently convert heat into electrical power. We propose a resonant tunneling quantum dot engine that can be operated either in the Carnot efficient mode, or maximal power mode. The ability to scale the power by putting many such engines in a ``Swiss cheese sandwich'' geometry gives a paradigmatic system for harvesting thermal energy at the nanoscale. [Preview Abstract] |
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