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Thermal spin power without magnetsThe spin Seebeck effect has hitherto relied on a temperature gradient in a magnetic system to generate an electric current based on the intrinsic spin of electrons. It has now been demonstrated in a non-magnetic material. See Letter p.210

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A temperature difference across a magnet gives rise to an electron spin current, which consists of electrons with oppo-

site spins moving in opposite directions. Previ-ously, this spin Seebeck effect was thought to exist only in magnetic materials. However, on page 210 of this issue, Jaworski et al.1 report the detection of a thermally driven spin cur-rent in a non-magnetic material subjected to a magnetic field.

In 1821, Thomas Seebeck gave a series of lectures in Berlin about his new finding — namely, that a temperature difference main-tained between two junctions separating dissimilar metals in a closed circuit deflects a needle made of a magnet2. Seebeck initially named this effect thermomagnetism, but shortly thereafter it was discovered that mag-nets are not needed to induce it. Rather, the temperature difference creates an electric cur-rent that produces a magnetic field, which, in turn, affects the needle. The phenomenon is now known as the Seebeck effect. If the cir-cuit is opened up, a voltage is produced that is proportional to the temperature difference. This thermoelectric Seebeck voltage is nowa-days used in temperature sensors, and there is a major effort to try to exploit it to convert waste heat from various processes into electricity.

The spin counterpart of the Seebeck effect — the spin Seebeck effect — was discovered3 just a few years ago. Here, a temperature gradi-ent applied to a magnet generates a spin cur-rent that flows transversely to the gradient. The

spin current can be converted into a measur-able voltage through a well-established phe-nomenon called the inverse spin Hall effect4. The efficiency of the effect is described by the spin Seebeck coefficient, which is the ratio of the measured voltage per unit of length to the temperature gradient. The discovery spurred a flurry of activity, and a similar effect was soon observed in magnetic insulators5 and magnetic semiconductors6.

In a turn of events curiously reminiscent of those that followed Thomas Seebeck’s initial discovery almost two centuries ago, Jaworski et al. show that magnets are not needed to observe the spin Seebeck effect. The authors produced the effect using the imbalance of ‘spin-up’ and ‘spin-down’ electrons induced by a magnetic field (Fig. 1) in an intrinsically non-magnetic semiconductor wire made of indium antimonide (InSb). The spin Seebeck coefficient was up to 1,000 times larger than that observed in previous measurements of the effect in magnetic materials. Jaworski and colleagues argue that their finding is the result of a combination of effects: spin imbalance in a magnetic field, which is especially strong in InSb; a sizeable temperature difference between the electrons and the material’s lat-tice vibrations (phonons) over macroscopic distances; and strong electron–phonon and spin–orbit interactions. The spin–orbit inter-action is an effect that couples the spin of an electron with its orbital motion — somewhat similar to the way in which the spin of a tennis ball affects its motion.

The spin Seebeck effect is peculiar because it

and bone degradation in mice7. The reason for using an acid-labile polymer is that macro-phages engulf and accumulate particulate matter in acidic membranous vesicles (lyso-somes), and so the attached drug would be released precisely within the target cells. The selective uptake of polymers by macrophages in inflamed tissues has been attributed to the cells’ high activity and to an increased permeabi lity of blood-vessel walls to large molecules at those locations8.

On the basis of Mediero and colleagues’ findings, candidate drugs that activate adeno-sine A2A receptors could be included with

other anti-osteoclastic agents — such as interleukin-10, dexamethasone and bisphos-phonates — that hold promise for prolonging the life of artificial joints. Additional experi-ments are needed to assess the effects of these compounds, alone or in combination, when added to bone cement and/or when attached to polymers that could periodically be injected intravenously. ■

Joel Linden is in the Division of Inflammation Biology, La Jolla Institute for Allergy and Immunology, La Jolla, California 92037, USA. e-mail: jlinden@liai.org

is, in essence, a finite-size effect: voltage probes ‘know’ whether they measure the hot or the cold end of the sample; the sign of the volt-age depends on this. Imagine that you are in a pool into which hot water is poured. You can feel that the water in front of you is warmer and the water behind you colder than where you are standing, but simply by probing your surroundings you cannot know whether your position is colder or warmer than the average temperature. For this you need some extra information about the whole pool. Such infor-mation can be provided by non-local correla-tions, and generally such correlations decay over a finite distance.

In the spin Seebeck effect, these correla-tions persist over distances that can exceed milli metres even at room temperature. But correlations typically encountered in electron systems such as that of InSb occur over much shorter length scales. Therefore, the long-range information has to be carried by phonons. These correlations result in a temperature dif-ference between the electron and the phonon components of the system — a difference that is maximized near the hot and cold ends. This means that a long-range ‘thermal battery’ along the InSb wire is already built into the system even in the absence of the magnetic field and the spin–orbit interaction. Jaworski et al. use the latter two ingredients to convert the elec-tron–phonon temperature difference into a measurable spin Seebeck voltage.

A striking aspect of Jaworski and colleagues’ experiment, apart from the large spin Seebeck coefficient, is the signal’s approximately even symmetry (the voltage retains its sign) under reversal of the magnetic-field direction. This observation is in stark contrast to the odd symmetry (the voltage changes sign) that has been found in previous magnetic systems. In the latter, the orientation of the thermal spin current is determined by the material’s mag-netization, which is reversed under magnetic-field reversal and so changes the sign of the voltage. In the current experiment, the nature of the signal is fundamentally distinct. In a sample that has mirror symmetry with respect to the plane formed by its length and height (Fig. 1), spatial reflection across this plane

1. Teeny, S. M., York, S. C., Mesko, J. W. & Rea, R. E. J. Arthroplasty 18, 954–962 (2003).

2. Purdue, P. E., Koulouvaris, P., Nestor, B. J. & Sculco, T. P. HSS J. 2, 102–113 (2006).

3. Mediero, A. et al. Sci. Transl. Med. 4, 135ra65 (2012).

4. Holt, G., Murnaghan, C., Reilly, J. & Meek, R. M. Clin. Orthop. Related Res. 460, 240–252 (2007).

5. Linden, J. Adv. Pharmacol. 61, 95–114 (2011).6. Murphree, L. J., Sullivan, G. W., Marshall, M. A. &

Linden, J. Biochem. J. 391, 575–580 (2005).7. Ren, K. et al. Mol. Pharmaceut. 8, 1043–1051

(2011).8. Henson, P. M. Nature Immunol. 6, 1179–1181

(2005).

The author declares competing financial interests: see go.nature.com/j3epw3 for details.

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E V O L U T I O N A R Y B I O L O G Y

Muscle’s dual originsJellyfish move using a set of muscles that look remarkably similar to striated muscles in vertebrates. However, new data show that the two muscle types contain different molecules, implying that they evolved independently. See Letter p.231

A N D R E A S H E J N O L

Jellyfish engage in a unique mode of loco-motion. They pump their umbrella-shaped bodies through a cyclical series of expan-

sions and contractions, which requires mus-culature every bit as impressive as that used by an Olympic sprinter to break the latest world record. In fact, jellyfish muscles look almost identical to the striated muscle found in ver-tebrates, and for a long time many biologists have thought that the two shared an ancient evolutionary origin1,2. On page 231 of this issue3, Steinmetz et al. present evidence that this intricate musculature in fact evolved inde-pendently, at least twice.

In the seventeenth century, zoologist Antonie van Leuvenhoek dissected a cod fish

under his newly developed microscope and noticed that the animal’s muscles consist of a complex set of interwoven fibres. His sub-sequent report of these observations became the first description of striated muscle. Modern studies have gone on to show that each striated muscle fibre is constructed of subunits called sarcomeres which, under the microscope, are demarcated by optically dense vertical discs (Z-discs). The fibres consist of an intricate arrangement of thick and thin filaments of myosin and actin proteins, respectively. The genes encoding myosin and actin are just two of more than 100 genes involved in the development of striated muscle in vertebrates.

Skeletal muscles, which make up around 50% of our body weight, are striated, but so too are cardiac muscles, which control

would reverse the voltage and the magnetic field. The fact that the voltage did not change sign upon magnetic-field reversal implies that the mirror symmetry must be broken, even at zero magnetic-field strength. This sym-metry breaking is probably associated with the particular crystallographic orientation of the semiconductor (the arrangement of the material’s atoms within the sample) chosen by the authors, although no detailed study of this orientation dependence is reported in their paper.

There is a price to pay for not using a magnet. Jaworski et al. obtained a sizeable effect only at low temperatures: the spin Seebeck voltage at a magnetic field of 2.7 tesla becomes very small at temperatures above 40 kelvin. The authors relate the maximum temperature at which the effect is observed to the Zeeman energy. This describes the energy difference between spin-up and spin-down electron states in a magnetic field. It increases with the field’s strength and is especially large in InSb. One might think, therefore, that increasing the magnetic field would increase the temperature range over which the effect can be observed. However, the magnetic-field dependence of the effect is complex, as seen in both the strength and even the sign of the measured spin Seebeck coeffi-cient. It is clear that more research is needed to obtain a plausible theoretical understanding of these measurements.

It is interesting to remark on a recurring parallel between thermoelectricity and topo-logical phenomena in condensed matter, which have been under intense scrutiny over the past several years. Curiously, recently dis-covered7 three-dimensional topological insu-lators, which insulate electric current in their bulk but conduct it on their surface, are based on alloys that have been known for decades to

properties with thermoelectricity could work to mutual advantage for elucidating the former and using the latter.

Jaworski and colleagues’ use of a magnetic field instead of a magnet shows that the spin Seebeck effect is more general than previously envisaged. It remains to be seen whether, ulti-mately, a magnetic field is needed at all: for example, linking the spin orientation to the direction of electron motion in the surfaces of topological insulators10, combined with a broken crystallographic symmetry, might be sufficient. Who will take up the challenge? ■

Tero T. Heikkilä is at the Low Temperature Laboratory (OVLL), Aalto University, FI-00076 Aalto, Finland. Yaroslav Tserkovnyak is in the Department of Physics and Astronomy, University of California, Los Angeles, Los Angeles, California 90095, USA. e-mails: tero.heikkila@aalto.fi; yaroslav@physics.ucla.edu

1. Jaworski, C. M., Myers, R. C., Johnston-Halperin, E. & Heremans, J. P. Nature 487, 210–213 (2012).

2. Seebeck, T. J. Magnetische Polarisation der Metalle und Erze durch Temperatur-Differenz Abh. Preuss.Akad. Wiss. 265–373 (1822–23).

3. Uchida, K. et al. Nature 455, 778–781 (2008).4. Kimura, T., Otani, Y., Sato, T., Takahashi, S. &

Maekawa, S. Phys. Rev. Lett. 98, 156601 (2007).5. Uchida, K. et al. Nature Mater. 9, 894–897

(2010).6. Jaworski, C. M. et al. Nature Mater. 9, 898–903

(2010).7. Chen, Y. L. et al. Science 325, 178–181 (2009).8. Wilczek, F. Nature Phys. 5, 614–618 (2009).9. Mourik, V. et al. Science 336, 1003–1007 (2012).10. Hsieh, D. et al. Nature 460, 1101–1105 (2009).

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be among the best thermoelectric materials. In a reverse twist, not long after InSb was used in the search for signatures of exotic Majorana fermions (particles that are their own anti-particles) in topological superconductors8,9, we are now informed of its remarkable spin Seebeck characteristics. The culprit in both the topological and spin Seebeck properties is the strong spin–orbit coupling. It is thus difficult to resist speculating that marrying topological

Figure 1 | Spin Seebeck effect driven by a magnetic field. Jaworski et al.1 observed the spin Seebeck effect in a non-magnetic semiconductor made of indium antimonide, InSb. The authors placed the system in a large magnetic field that has a direction parallel to an applied temperature gradient; in this illustration, the field is produced using a solenoid. Owing to the large field, more electrons have spins (black arrows) that align parallel with the field than antiparallel to it. This strong spin polarization, combined with the temperature gradient and other properties of InSb, generates a spin current that can be detected as a voltage.

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