S O L I D - S TAT E P H Y S I C S

A new spin on spintronicsBy harnessing the way charge carriers move in a magnetic field, computing blocks based on semiconductor junctions have been made that are reconfigurable and can be interconnected to perform complex logic functions. See Letter p.72

S A Y E E F S A L A H U D D I N

Conventional computers rely on the controlled motion of charge in tran-sistors, the basic building blocks of

integrated circuits. It has been pointed out, however, that if one could devise electronic devices based on the spin of particles — a quantum-mechanical property responsible for magnetism — completely new computing technologies might emerge that could out-perform their conventional analogues. Such ‘spintronic’ devices have attracted much attention among researchers worldwide1. On page 72 of this issue, Joo et al.2 describe a computing scheme that combines the phys-ics of charge motion in a magnetic field with conventional electronic devices in a way that promises to provide a significant step towards a spintronic computing technology*.

Joo and colleagues’ scheme hinges on the physics of the Lorentz force on charged parti-cles. When a charged particle moves in a mag-netic field, it experiences a force that deflects it in a specific direction. The direction of this force depends on the sign of the charge, the direction of the magnetic field and the direc-tion of motion. It can be reversed by changing the sign of any of these three quantities. So, for example, if carriers move from left to right in a magnetic field, the force experienced by the negatively charged carriers will act in the opposite direction to that experienced by the positively charged carriers.

This property can be exploited in a semiconductor, in which mobile charges that are predominantly nega-tive or positive can be created using a technique called ‘doping’. Doping consists of adding a relatively small number of atoms of a dif-ferent material from that of the host semiconductor to increase its negative (electrons) or positive (‘holes’) charges. A semi conductor that has mostly mobile electrons is said to be of the n-type category, whereas one that has mainly holes

is dubbed a p-type semiconductor. By combining n-type and p-type, many dif-ferent p–n junctions can be made. These junctions are the basic building blocks of all semiconductor devices.

One such device is an avalanche diode, in which highly energetic carriers are injected beyond a specific electrical voltage from one side of the junction to the other. Owing to their high kinetic energy, these carriers can gener-ate additional carriers by breaking chemical bonds as they collide with atoms near the junc-tion. In turn, the additional carriers produce more carriers in a chain reaction that leads to an ‘avalanche’ of carriers (Fig. 1). Avalanche diodes are commercially available and are routinely used to provide voltage reference and electrical-surge protection in integrated circuits. Now, if an avalanche diode is placed in a magnetic field, it is conceivable that, owing to the Lorentz force, the energetic carriers will be deflected and will therefore need a larger voltage to reach the energy threshold above which they can multiply and produce a carrier avalanche. As a result, the current in the diode will decrease3–5.

Joo and colleagues’ computing scheme

builds on these concepts. The key idea comes from the fact that, by introducing an asym-metry in the physical geometry of the diode, asymmetry can be created in the device’s cur-rent–voltage behaviour such that the same voltage will yield a different amount of current for two opposite directions of the magnetic field. Take, for example, the case of a planar device that has two vertically separated p- and n-type semiconductor layers lying in a mag-netic field whose direction is completely in the same plane as the device. Because of this in-plane field, charge carriers will be deflected more in one direction in the planar device than in the other. But if an out-of-plane field is applied in the same planar device, carriers will be deflected in all directions equally. For the in-plane field, the asymmetry in carrier deflec-tion dictates that if the avalanche diode is in the ‘on’ state for a particular direction of the field, it will be ‘off ’ when the field is reversed. Therefore, two avalanche diodes under two opposite directions of the applied field can pro-vide a complementary pair that can be used, almost like conventional transistors, to build up computing blocks.

One interesting aspect of Joo and colleagues’ work is the exploitation of mirror symme-try. Because oppositely charged carriers are deflected in opposite directions in the mag-netic field, the behaviour of a device that has a top n/bottom p-layer configuration will be exactly opposite to a top p/bottom n one. Thus, the requirement to reverse the direction of the magnetic field to create the complementary pair can be replaced by simply interchanging the ordering of n and p layers. This strategy provides a significant advantage in terms of flexibility in designing complicated and inter-connected computing blocks by leveraging existing expertise on semiconductor process-

ing. Also, the fact that the direction of the magnetic field does not have to be reversed is highly advantageous for the operation of the device at low power and for overall scaling up of the technology.

However, improvement is needed before this computing scheme can be viable for practical applications. Even the need to generate a unidirectional magnetic field in an integrated micro-chip could consume significant energy. One possibility, which the authors suggest, would be to develop tech-nologies in which the required field is produced by thin-film perma-nent magnets rather than by pass-ing a current through a wire. However, integrating the materi-als needed for this purpose with conventional semiconductors poses a considerable challenge.

An attractive aspect of this tech-nology is the reconfigurability of the computing blocks such that the

Carrier

Carrierde�ection

Avalanche

Conduction band

Valence band

Figure 1 | Bending avalanche carriers. Charge carriers going through an avalanche multiplication process in a p–n semiconductor junction can be deflected by an applied magnetic field. Here, the junction is depicted through its energy-band diagram, which consists of the valence and conduction bands separated by an energy gap. The deflection leads to a reduction in the electrical current running through the junction. Joo et al.2 have exploited this effect to develop basic computing blocks that can be interconnected to perform complex logic operations.

*This article and the paper under discussion2 were published online on 30 January 2013.

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N E U R O S C I E N C E

Salty sensations Salt is important in health and disease, yet how mammals sense it is not completely clear. Evidence in worms suggests that TMC proteins, which are implicated in human hearing, are salt receptors involved in taste. See Letter p.95

B E R T R A N D C O S T E & A R D E M P A T A P O U T I A N

Salt is one of the oldest and most common food seasonings. And, together with sweet, bitter, sour and the savoury taste

umami, it forms the five modalities of taste1. Salt is essential for survival, but its intake has to be tightly controlled because overconsumption impairs ion balance and regulation of blood pressure. Indeed, depending on its concen-tration, salt can be attractive or repulsive in many vertebrates and invertebrates. Little is known about how organisms sense the main component of salt, sodium chloride. In mice, epithelial sodium channels are required for the creatures to exhibit attractive responses to the taste associated with low salt concentrations, but not for them to show aversion to high salt concentrations2. It is exciting, therefore, that on page 95 of this issue, Chatzigeorgiou et al.3 report a promising candidate for a salt sensor*.

The authors explored the function of trans-membrane channel-like protein-1 (TMC-1) in the nematode worm Caenorhabditis elegans. The TMC family of multipass transmem-brane proteins has eight known members in humans and two (TMC-1 and TMC-2) in C. elegans. TMC-1 mutations cause deafness in humans and mice, and in the absence of both TMC-1 and TMC-2 the hair cells of the mouse auditory system cannot show mecha-nosensory responses4. However, it is not clear whether TMCs are bona fide ion channels and/or a component of the elusive channel

complex of the hair-cell mechanotransducer.Chatzigeorgiou et al. report that TMC-1 in

C. elegans is expressed in a few sensory neu-rons, including the ASH neurons, which are important for avoidance of noxious stimuli such as nose touch (the worms reverse their direction when they bump into an object with their nose), hyperosmolarity, heavy metals, acids and high salt concentrations5. Notably, TMC-1 is detected in the sensory cilia of ASH neurons, the site of sensory transduction in these cells — consistent with a role for TMC-1 in sensory detection.

The researchers’ behavioural studies following deletion of the tmc-1 gene demonstrated the relevance of TMC-1 in sensing salt, but not other repulsive chemical or mechan-

ical stimuli such as glycerol, copper and nose touch. They also showed that TMC-1-medi-ated aversion to salt is specific to sodium ions. In addition, the inability of tmc-1-mutant worms to avoid sodium chloride (NaCl) was overcome by inducing the expression of exogenous TMC-1 in ASH neurons.

These results establish that TMC-1 contrib-utes specifically to sodium sensing in ASH neurons. But is TMC-1 sufficient to account for salt sensitivity? To address this question, Chatzigeorgiou et al. induced TMC-1 expres-sion specifically in salt-insensitive ASK neu-rons and found that this protein confers salt

sensitivity in vivo. Remarkably, expression of C. elegans TMC-1 in four distinct mammalian cell lines induced sodium-activated conduct-ance. The threshold of activation was about 140 millimolar NaCl, which is consistent with the NaCl concentrations that induce aversive responses in worms, mice6 and flies7 .

TMC-1-induced cationic currents occur mainly in response to sodium and are insensi-tive to amiloride — a blocker of the epi thelial sodium channels1. Intriguingly, low NaCl concentrations (10–150 mM) are attrac-tive to mice, and this attraction is blocked by amiloride. However, mice exposed to higher concentrations of NaCl show aversive responses that are insensitive to amiloride2. In light of Chatzigeorgiou and colleagues’ data, it seems that TMC-1 constitutes an amiloride-insensi-tive ion-channel component that, in worms, is activated by high sodium concentrations. The role of TMC proteins in the mammalian taste system remains to be explored.

Can the present data, obtained by genetic studies in worms, be reconciled with the pro-posal that TMC-1 and TMC-2 are part of the mechanotransduction apparatus in mamma-lian auditory hair cells4? Chatzigeorgiou and co-workers’ findings suggest that TMC-1 is not involved in worm mechanotransduction. As for TMC-2, the authors show that tmc-2 muta-tions do not exacerbate the effects of tmc-1 mutations on the response to sodium, which suggests that TMC-2 is not required for salt detection. Nonetheless, as tmc-2 transcripts are enriched in mechanoreceptors8, the role of TMC-2 in worm mechanotransduction — as well as other sensory systems — should be investigated.

By demonstrating that TMC-1 encodes a putative ion channel that is activated by extra-cellular sodium ions, Chatzigeorgiou et al. highlight another role for TMC proteins in sensory transduction. Given that the intro-duction of worm TMC-1 into human cells can induce sodium-activated currents, similarly cloning and characterizing mammalian TMC proteins could provide further insight into their function. Follow-up studies should con-firm whether TMC proteins are pore-forming subunits of ion channels or whether they are modulators of as yet unidentified ion channels, as well as address their physiological role in various species. ■

Bertrand Coste is in the Ion Channels and Sensory Transduction Group, CRN2M, CNRS–Aix-Marseille University, Marseille 13015, France. Ardem Patapoutian is in the Molecular and Cellular Neuroscience Department, Dorris Neuroscience Center, The Scripps Research Institute, La Jolla, California, and at the Genomics Institute of the Novartis Research Foundation, San Diego, California 92121, USA. e-mails: bertrand.coste@univ-amu.fr; apatapoutian@gnf.org

same block can perform different logic func-tions depending on an external signal. But this can be achieved only if the magnetic field is reversed. The authors suggest using an effect called spin-transfer torque6,7 to attain this reversal. However, such an effect can work only on small magnets, and so this approach may not be scalable. Additionally, the use of energetic carriers in the avalanche diode requires a large amount of energy to be supplied to the diode from external cir-cuits, which is then translated into the kinetic energy of the carriers. This may prove to be a bottleneck for low-power device operation. Nevertheless, none of these difficulties seems to be of a fundamental nature, and greater insight into the capabilities of this technology

may lead to completely new ways of doing computation. ■

Sayeef Salahuddin is in the Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, California 94720, USA. e-mail: sayeef@berkeley.edu

1. Insight: Spintronics Nature Mater. 11, 367–416 (2012); available at go.nature.com/sarlr9

2. Joo, S. et al. Nature 494, 72–76 (2013).3. Delmo, M. P., Yamamoto, S., Kasai, S., Ono, T. &

Kobayashi, K. Nature 457, 1112–1115 (2009).4. Wan, C., Zhang, X., Gao, X., Wang, J. & Tan, X. Nature

477, 304–307 (2011).5. Lee, J. et al. Appl. Phys. Lett. 97, 253505 (2010).6. Slonczewski, J. C. J. Magn. Magn. Mater. 159, L1–L7

(1996).7. Berger, L. Phys. Rev. B 54, 9353–9358 (1996).

“These results establish that the TMC-1 protein contributes specifically to sodium sensing.”

*This article and the paper under discussion3 were published online on 30 January 2013.

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