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bands of band theory. The electrons mustoccupy the available bands sequentially, start-ing with the energetically lowest and proceed-ing upwards. Each band can accommodatetwo electrons of opposite spin. It follows that ifthe solid (more properly, one unit cell of thesolid) contains an odd number of electrons,then at least one band must be incompletelyfilled. Such a material is guaranteed to be

metallic within band theory.The experimental search for surfaces meet-

ing these conditions, but nonetheless exhibit-ing insulating behaviour, began in earnest inthe 1990s. In one classic experiment, a Mottinsulator was engineered on a silicon surfaceby first depleting its electrons using implantedboron. Then potassium was adsorbed untiljust enough electrons were supplied to meetthe BlochWilson criterion for a metal5. Butdata from photoemission spectroscopy (whichuses light to eject electrons from a material andreveal their energy distribution) showed a gapbetween filled and empty bands. This contra-

diction with band theory provided strong evi-dence, eventually confirmed theoretically6,that the potassium-covered silicon surface is aMott insulator.

A similar strategy was also applied to thesurface of germanium, but with importantdifferences. Instead of the boronpotassiumcombination, a single overlayer of lead wasused to supply the same number of electrons.When this leadgermanium system wascooled below 100 kelvin, a small energy gapappeared, but accompanied by a new feature: aperiodic, rumpling distortion of the lead over-layer7. When tin atoms (with the same number

of valence electrons as the lead atoms) wereadded instead, the same rumpling was found,but the gap was mysteriously absent8. It hasbeen persuasively argued9 that the rumplingseen at low temperatures in both systems isreally the freezing of a vibrational mode inwhich, at higher temperatures, the tin atomsundergo rapid updown oscillations. But thelarger question why the tingermaniumsystem seemed not to be a Mott insulator remained unresolved.

Corts and colleagues contribution3 to thestory is twofold. First, they establish that tin-covered germanium is indeed a Mott insulator,

but only at very low temperatures. Second,they present angle-resolved photoemissiondata that give an especially detailed pictureof the metalinsulator transition, addingweight to the earlier findings in related two-dimensional systems. Their primary evidenceconsists of photoemission spectra, taken attemperatures of 12 and 140 kelvin, that simul-taneously measure the energy and momentumdistribution of electrons. At the lower tempera-ture the spectra show the opening of a text-book insulating gap. The authors buttress thisresult with a second, surprising, finding: at12 kelvin, the rumpling distortion found in

earlier experiments

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disappears and a uniformflat surface returns.

SOLID-STATE PHYSICS

When is a metal not a metal?Steven C. Erwin

When its an insulator, of course. Materials that should in theory conduct

electricity but dont are well known, but the anomalous behaviour of

one material has caused particular head-scratching.

Every student knows the difference betweena metal and an insulator: one conducts elec-tricity and the other doesnt. Things get moreinteresting if you ask how this differencearises. Although the question is disarminglysimple, a rigorous answer was not availableuntil about 1930, when Felix Bloch and AlanWilson used the new quantum mechanics tocreate a theory that distinguished metals frominsulators1,2. The spectacular success of thisband theory of solids, as it is now known, hasmade it a cornerstone of the modern theory

of solids.In a few glaring cases, however, band theory

doesnt get it right: the materials known asMott insulators, for instance, are experi-mentally insulating, yet band theory firmlyinsists they are metallic. Writing in PhysicalReview Letters, Corts and colleagues3 take afresh look at one such material a germa-nium surface with a layer of adsorbed tinatoms that has been puzzling theorists andexperimentalists alike. Despite expectationsbased on analogy with similar materials, pre-vious experiments on this system had failed toestablish it as a Mott insulator. The results of

Corts et al. finally put this piece of the puzzleinto place.

Exceptions to band theory provide fertilesoil for testing new ideas about the behaviourof electrons in solids. Perhaps no one con-tributed more than Nevill Mott, a 1977 Nobellaureate in physics. Mott pointed out that thecentral approximation of band theory thateach electron moves independently, feeling theeffects of the others only on the average would fail badly in certain circumstances4.Armed with this realization, physicists couldfinally begin to understand those materialsthat owe their insulating nature to correlations

in the motions of different electrons. Thesecorrelations arise from nothing more than theclassical Coulomb repulsion between particlesof like charge. But they can be decisive inmaterials in which the two competing tenden-cies of electrons are already in delicate balance:the desire to be spatially localized to mini-mize Coulomb repulsion, and the need to bedelocalized to minimize the cost in kineticenergy from spatial confinement.

In bulk materials, the tendency to delocalize and so conduct usually prevails, becausethe freedom offered by three dimensions isgreater than the Coulomb penalty. Not so

at the surfaces of semiconductors, whereelectrons are effectively confined to two

dimensions (Fig. 1). Indeed, Mott insulatorshave been created over the past decade on thesurfaces of many common semiconductors,including silicon carbide, gallium arsenide andeven silicon itself. In most cases, adsorbeddopant atoms were used to tune the number ofelectrons to the point where band theorywould predict a metal. When the evidenceshowed otherwise, Motts electron correlationswere a natural suspect.

Despite the occasional failure, BlochWilson band theory still provides a useful

language for understanding even those mat-erials it gets wrong. Electrons in crystallinesolids cannot have arbitrary energies, nor canthey all have the same energy. Instead, the spe-cific arrangement of atoms within the soliddictates the ranges of allowed energies the

a

b

Figure 1 | Metalinsulator transition. a, Theregular potential wells of a normal metallic stateof a material with, on average, one electron peratom. b, If the confining potential is a littlestronger (deeper wells), electrons find it harderto delocalize and so do not conduct despitewhat the band theory of solids predicts.The material is a Mott insulator, such as the

tin-covered germanium investigated byCorts and colleagues3.

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The observation of both phase transitions electronic and structural makes the pic-ture particularly convincing. One final experi-ment measuring the inverse photoemissionspectrum (which reveals the distribution ofempty states just above the gap) would clinchthe case. For this, however, we must awaitimprovements in resolution. In the meantime,the results of Corts and colleagues restore a

sense of order to our limited, but growing,understanding of Mott insulators on surfaces.Such an understanding may help to unravelother mysteries, from high-temperature super-conductivity to ultracold atoms. And as thedimensions of electronic devices continue toshrink, Motts theory may itself become the

new cornerstone for describing how electronsbehave at the very smallest scales. Steven C. Erwin is at the Center for

Computational Materials Science,

Naval Research Laboratory,

Washington DC 20375, USA.

e-mail: erwin@dave.nrl.navy.mil

1. Bloch, F. Z. Phys.52,555600 (1928).

2. Wilson, A. H.Proc. R. Soc. Lond. A133,

458491 (1931).3. Corts, R. et al. Phys. Rev. Lett. 96, 126103 (2006).4. Mott, N. F. Metal-Insulator Transitions (Taylor and Francis,

London, 1974).5. Weitering, H. H. et al. Phys. Rev. Lett. 78,13311334 (1997).6. Hellberg, C. S. & Erwin, S. C. Phys. Rev. Lett.83, 10031006

(1999).7. Carpinelli, J. M. et al. Nature381,398400 (1996).8. Carpinelli, J. M. et al. Phys. Rev. Lett. 79,28592862 (1997).9. Avila, J. et al. Phys. Rev. Lett. 82,442445 (1999).

CELL BIOLOGY

Skin care by keratinsM. Bishr Omary and Nam-On Ku

Keratin proteins perform several functions in skin cells, including those of

providing mechanical support and protection against injury. But it seems

they also have a more active part to play in healing wounds.

Figure 1 | The skin injury response. Injury to keratinocyte cells of the uppermost skin layer(epidermis), or injury to other tissues, triggers an elaborate repair response that includes possiblebleeding and aggregation of platelet cells from the blood, scab formation, infiltration of inflammatorycells into the wound, and release of growth factors. The size, shape and adhesive properties of thekeratinocytes surrounding the wound also change. These alterations result in part from changes ingene expression, including the induction of genes encoding the K6, K16 and K17 keratins. K17probably undergoes phosphorylation, which results in binding of cytoplasmic K17 to the adaptorprotein 14-3-3 and movement of nuclear 14-3-3 to the cytoplasm. As demonstrated by Kim et al.4, K17

induction and binding to 14-3-3 are key events for the activation of the mTOR signalling pathway andthe ensuing stimulation of cell growth and migration.

Like the musculoskeletal framework ofhumans or the steel-girder scaffolds of build-ings, the intermediate filaments of thecytoskeleton shield cells from mechanicalforms of injury13. Intermediate filaments aremade up of a large family of tissue-specificproteins, including desmin in muscle, neuro-

filaments in neurons and keratins in theepithelial cells that line organs such as theskin13. Although intermediate filaments arewell known for their protective properties, itseems that they may also have a role in damagerepair. On page 362 of this issue, Kim et al.4

provide a direct mechanistic link between anincrease in the expression of the gene forkeratin K17 and the characteristic increase inprotein synthesis and cell growth seen in thecells around wounds.

Wound healing involves the orchestration ofnumerous events in the recovering cells andthose adjacent to them5,6, such as cell growth,

cell division and migration, and the upregula-tion of many genes including those encod-ing several intermediate filament proteins(Fig. 1). In response to skin injury, for exam-ple, the cells surrounding the wound ramp upprotein synthesis and enlarge to help seal thewound. In these cells, there is a rapid increasein the expression of several keratins (K6, K16and K17)6.

Kim et al. began their study by pursuingtheir previous observation that injuries inmouse embryos lacking K17 show a strikingdelay in wound closure4,6. They noted that thewound-edge cells from such embryos did not

swell up as usual, and were about 40% smallerthan those of normal embryos. Moreover,

these cells did not show the full protein-synthetic activity that typically accompaniescell growth. This initial link with cell growthled Kim et al. to explore the role of a regu-latory enzyme called mammalian target of

rapamycin (mTOR). This enzyme can regulatecell growth and proliferation in other systemsby controlling protein synthesis7. Kim et al.found that the activation of mTOR seen innormal cells is reduced in cells that lack K17.

So how does K17 affect mTOR signalling?The authors found that K17 binds to an adap-tor protein called 14-3-3, which belongs tothe 14-3-3 protein family4. The 14-3-3 proteins

bind to more than 100 other proteins, prim-arily at particular phosphoserine/phospho-threonine residues that is, serine (Ser) andthreonine (Thr) residues that have a phos-phate group attached. One of their many func-tions is to regulate where their bindingpartners can reside in the cell8. Mutation oftwo sites on the K17 protein that looked likelyto act as 14-3-3 binding motifs (Thr 9 andSer 44) not only blocked the binding of K17 to14-3-3, but also prevented the normal activa-tion of mTOR and translocation of 14-3-3from the nucleus to the cytoplasm in culturedcells. The investigators then went full circle by

showing that reintroducing K17 into kera-tinocytes lacking K17 restored the movementof 14-3-3 from the nucleus to the cytoplasm,leading to stimulation of mTOR signallingand increased protein synthesis and cell size.By contrast, introducing K17 that was mutatedat its 14-3-3-binding sites had no signifi-cant effect.

These findings directly implicate the keratincytoskeleton in the regulation of protein syn-thesis and cell size, highlighting an overlookednon-mechanical function for skin keratins

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