NEWS & VIEWS

Superconductivity Beyond convention

For high-temperature superconductors, results from more refined experiments on better-quality samples are issuing fresh challenges to theorists. It could be that a new state of matter is at play, with unconventional excitations.

Didier Poilblanc antiferromagnetic insulator changes to as a function of an applied magnetic is in the Laboratoire de Physique Théorique, a superconductor — two very distinct field) in the magnetic-field-induced CNRS and Université Paul Sabatier, F-31062 states of matter. Their theory aims, in normal phase5, a fingerprint of small Toulouse, France. particular, to reconcile two seemingly closed Fermi surfaces named ‘pockets’. e-mail: [email protected] conflicting experimental observations. A valid theoretical description should Angular-resolved photoemission then account simultaneously for the lthough copper-oxide spectroscopy (ARPES) provides a unique destruction of the Fermi surface seen superconductors share with experimental set-up to map the locus in ARPES and for the SdH quantum A conventional superconductors of the zero-energy in oscillations. It should also explain the the remarkable property of offering no momentum space. Instead of showing a apparent violation of the Luttinger resistance to the flow of electricity, their large Fermi surface as most metals would, sum rule, the observed area of the SdH characteristic critical temperatures (Tc) above Tc low-carrier-density cuprates pockets being significantly smaller than below which this happens can be as show enigmatic ‘Fermi arcs’ — small the doping level (in appropriate units). high as 135 K. In fact, these materials disconnected segments in momentum A key feature of Kaul and colleagues’ belong to the emerging class of so-called space that continuously evolve into nodal theory1 is the emergence of a fractional strongly correlated systems — a class of points at Tc (ref. 2). This suggests that spinless particle of Fermi statistics materials whose list is rapidly growing. the disappearance of Landau–Fermi carrying charge e (the opposite of the Evidence has accumulated that such quasiparticles in the unconventional charge −e), bearing strong systems cannot be described simply by ‘normal’ (or ‘pseudo-gap’) phase is similarities to the original holon the celebrated Landau–Fermi theory, a a precursor of the superconducting introduced by Anderson. This arises paradigm of condensed-matter physics phase. In a pioneering 1987 paper3, naturally in the vicinity of a special for almost 50 years, which works well Phil Anderson proposed that the critical point (that is, a zero-temperature for most metals. Landau–Fermi theory capacity to become a superconductor phase transition) called a ‘deconfined ascribes a one-to-one correspondence is actually ‘hidden’ in the Mott critical point’, referring to the fact between the low-energy excitations of insulator itself. Here the are that the original confinement that ties the system, named ‘quasiparticles’, and already paired up in (resonating) local electron and charge close together those of a free-electron gas carrying bond singlets. This predicts electron vanishes spontaneously at this point. the usual spin and charge quantum into separate charge The SdH oscillations could then be numbers (a feature that has recently and spin components — the holon and attributed to the small elliptical Fermi been seriously questioned for systems in the — and provides a natural pockets of a ‘holon metal’. However, in which the screened Coulomb repulsion explanation of nodal quasiparticles. such a state, a finite broadening of the between conduction electrons is large). However, this type of Mott insulator ARPES spectrum is expected, to reflect Indeed, it seems that the beautiful does not arise in the undoped cuprates, the decay of the electrons into spin complexity of the physics of cuprate which are quantum antiferromagnets. and charge components, and hence the superconductors, and of a number In contrast, the treatment by Kaul et al.1 spectrum lacks the sharp of other strongly correlated systems, takes the presence of antiferromagnetic features of the Fermi arcs. In contrast, the is rooted rather in the fundamental order seriously, but still aims to capture ‘holon–hole’ metal — in which a fraction nature of the parent compound: the some of the unconventional features of of the holons binds to — provides electrons in the highest d-orbitals Anderson’s proposal to account for the extra hole Fermi pockets resembling experience a Coulomb interaction anomalous properties of the normal the mysterious ARPES Fermi arcs, and strong enough to localize them, yielding phase. Putting the antiferromagnetic is therefore a good candidate for the a commensurate (one-electron-per- nature of the Mott insulator back into normal phase of the low-carrier-density site) Mott insulator. At low enough play is important as it is known that a cuprates. In this scenario, the hole-like temperature, this antiferromagnetic small number of carriers moving in an character of the carriers implies a positive system is then stabilized by the weaker antiferromagnet background do not sign for the Hall constant — in contrast exchange interaction between the naturally fractionalize4. to some recent measurements6 whose spins of the localized electrons. The The synthesis of higher-purity interpretation is still actively debated. key question addressed by Kaul and low-carrier-density cuprates has Last but not least, the low-temperature colleagues1 on page 28 of this issue is recently enabled the observation of superconductor originates from a (rather how, by doping itinerant holes (defined Shubnikov–de Haas (SdH) oscillations conventional) pairing instability of the as moving electron vacancies), a Mott (oscillations of the electrical resistance holon–hole metal.

16 nature physics | VOL 4 | JANUARY 2008 | www.nature.com/naturephysics © 2008 Nature Publishing Group

NEWS & VIEWS

Most theorists’ favourite microscopic simulations of microscopic models of ideas put forward by Kaul et al. to more lattice models of strongly correlated correlated electrons4. It would indeed be of rigorous tests. electrons (Hubbard, t-J and so on) are great interest to know whether these lattice References usually written in terms of the original models can, by themselves, exhibit such a 1. Kaul, R. K., Kim, Y. B., Sachdev, S. & Senthil, T. Nature Phys. ‘bare’ electrons, in contrast to effective rich phase diagram. On the experimental 4, 28–31 (2008). actions like that of Kaul et al.1, constructed side, ongoing effort in sample preparation 2. Kanigel, A. et al. Nature Phys. 2, 447–451 (2006). from holon and spinon degrees of freedom. and signal resolution will steadily reinforce 3. Anderson, P. W. Science 235, 1196–1198 (1987). 4. Dagotto, E. Rev. Mod. Phys. 66, 763–840 (1994). Interestingly, Kaul et al. offer simple the constraints on theories of strongly 5. Doiron-Leyraud, N. et al. Nature 447, 565–568 (2007). predictions that can be tested by numerical correlated electrons, subjecting the 6. LeBoeuf, D. et al. Nature 450, 533–536 (2007).

MAGNETIC TUNNEL JUNCTIONS Spin-torque measured up

The ‘spin-transfer torque effect’ could provide a powerful means of controlling the orientation of spins with electric currents rather than magnetic fields in future spintronic devices. Quantitative measurements of this effect represent an important next step.

Maxim Tsoi Magnetization When an unpolarized current passes of ‘free’ layer is in the Department of Physics, University of through a magnetized ferromagnetic Texas at Austin, 1 University Station, C1600, material, it becomes spin-polarized in the Austin, Texas 78712-0264, USA. direction of the materials’ magnetization. e-mail: [email protected] Similarly, when a spin-polarized current ‘Free’ ferromagnet is injected into a ferromagnetic material s conventional electronic circuits whose magnetization direction is different, shrink to smaller and smaller Non-magnetic spacer it will cause the polarization of injected dimensions, not only do they current to change. The resulting rotation A ‘Hard’ ferromagnet become more difficult to build, but of the injected current generates a torque require more power to run, which in on the magnet — a spin-transfer torque — turn generates more heat that must Magnetization which in turn can cause its magnetization be dissipated to avoid damage. This of ‘hard’ layer Current to precess and potentially even reverse represents a serious challenge to direction completely1,2. This process continuing the rate of improvement typically takes place in a nanometre-sized in computer power and cost that has Figure 1 Excitation of a ferromagnet by pillar composed of two ferromagnetic been maintained over the past forty spin-transfer torque. The passage of a current layers separated by a non-magnetic layer years, and which is responsible for through a ‘hard’ magnetic layer causes it to become (see Fig. 1). The first layer is usually a hard much of the economic prosperity seen spin-polarized. When this current is subsequently magnetic layer whose magnetization is over this period. Spintronics, in which injected across a thin non-magnetic spacer layer resistant to change, which serves to spin- information is encoded in electronic and into a ‘free’ magnetic layer, the realignment of polarize any current that passes along the spin rather than electronic charge, is one its polarization induces a torque on the magnetic pillar. And the second layer, also known technology that could enable us to exceed moment of the free layer, causing it to precess and as the ‘free’ layer, is one that is more the limitations faced by the electronics even reverse. susceptible to the torque generated by an industry. Reorienting an electron’s spin injected current. The ability to change the is in principle faster to implement and magnetization of this free layer with an requires less power than moving it from electric current has obvious implications one place to another. As well as offering for improving the speed and reducing a potentially faster and more energy in a magnetic material is influenced by the size of magnetic random access efficient means of processing information a spin-polarized electrical current. But memories (MRAMs). Another possible than conventional electronics, the use of although the occurrence of this effect has application is based on the spin-transfer- spin could enable qualitatively different been unambiguously identified, its precise induced precession of magnetization, sorts of functions to be performed, such details have remained unclear. Two studies which converts a d.c. current input into as quantum computing. But for this to be in this issue — one on page 67 by Sankey an a.c. voltage output. The frequency of realized, ways of manipulating spin that do and colleagues3, and the other on page 37 such a precession can be tuned from a not rely on magnetic fields, the generation by Kubota and colleagues4 — describe few GHz to over 100 GHz by changing of which is usually energy inefficient, must the first quantitative measurements of the applied magnetic field and/or the d.c. be developed. One promising approach both the magnitude and direction of current, effectively resulting in a current- exploits the so-called spin-transfer torque the spin-transfer torque in a magnetic controlled oscillator to be used in practical effect1,2, in which the distribution of spins tunnel junction. microwave circuits. nature physics | VOL 4 | JANUARY 2008 | www.nature.com/naturephysics 17 © 2008 Nature Publishing Group