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Interface Between Nonmagnetic Insulators May Be Ferromagnetic

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Interface Between Nonmagnetic Insulators May Be Ferromagnetic of e and g and therefore the proportion ing the population of a metastable state, New low pricing of atoms detected on those states. Set- they could infer jumps between two ting the detuning at just the right value other states.3 And in 1999 Steven Peil ensures that the second Ramsey cavity and Gerald Gabrielse trapped a single will rotate the superposition to e when- electron and watched it jump between ever a photon is present. Thus, detect- quantized cyclotron levels.4 4K ing an atom in g means no photon; de- When analyzed in theoretical detail, tecting an atom in e means one photon. jumps turn out to spring from the ap- ARS Displex Figure 2 shows the result of an ex- propriate Hamiltonian and the time- perimental run that lasted 2.5 s. Each dependent Schrödinger equation. Far Pneumatic vertical bar corresponds to the detec- from being awkwardly instantaneous or tion of one of the 2000 or so atoms sent unprovoked, jumps are an integral fea- Drive through the cavity. Most of the time, ture of orthodox quantum mechanics. barring noise, the atoms revealed an More recently, Wojciech Zurek of Cryocoolers empty cavity. But 1.05 s into the run, the Los Alamos National Laboratory ana- .8w @ 4.2K DE-210S cavity field jumped to its 1-photon lyzed quantum jumps by following the state, stayed there for 0.48 seconds, then progress of information from the sys- .25w @ 4.2K DE-204S jumped back down. tem to the apparatus and the environ- .1w @ 4.2K DE-202S At the cavity’s 0.8-K operating tem- ment. According to Zurek, that flow of perature, one expects the mirrors’ ther- information, along with basic assump- Research & OEM mal fluctuations to fill the cavity with tions of quantum theory, inevitably Cryostats one photon for about 5% of the time. breaks the unitary symmetry of Hilbert Averaging 500 runs, the ENS re- space and picks out the quantized states Optical & Non Optical searchers found a slightly higher value, that the system jumps to.5 True UHV (10 -11 torr) 6%, which they attribute to residual Brune, Haroche, Raimond, and their heating. team are now working on a new series Low Vibration Interface of experiments in which they fill the (5 nanometers) The meaning of jumps cavity with multiple photons. They Niels Bohr introduced the concept of hope to investigate another fundamen- quantum jumps in his model of the hy- tal quantum question: the transition to drogen atom. Their probabilistic na- classical behavior. ture—states suddenly changing with- Charles Day out cause—troubled some physicists. Quantum mechanics, they argued, is in- References trinsically statistical. Individual elec- 1. S. Gleyzes et al., Nature 446, 297 (2007). trons in atoms don’t jump. 2. M. Brune et al., Phys. Rev. Lett. 65, 976 In the 1980s physicists succeeded in (1990). 3. W. Nagourney, J. Sandberg, H. Dehmelt, trapping individual ions and looked for Phys. Rev. Lett. 56, 2797 (1986). electrons making jumps between states. 4. S. Peil, G. Gabrielse, Phys. Rev. Lett. 83, Warren Nagourney, Jon Sandberg, and 1287 (1999). Hans Dehmelt found the jumps in sin- 5. W. H. Zurek, http://arxiv.org/abs/quant- gle barium ions. By optically monitor- ph/0703160. Interface between nonmagnetic Compare insulators may be ferromagnetic our product specifications and conducting and our pricing! A ferromagnetic metal would be perhaps the first of many ARS novel electronic states that might emerge from the marriage manufactures of complex oxides. the complete cryocooler & In the past few decades, re- materials may yield even richer behav- www.arscryo.com cryostat searchers have focused much attention ior than is found in bulk. Perhaps the in- on complex oxides—oxide compounds terfaces exhibit phases that don’t exist in which the electrons interact strongly in either of the constituent compounds. with one another and with the lattice. Just look at the wide variety of elec- Such compounds exhibit a fascinating tronic devices that arise from the merg- range of properties: ferromagnetism, ing of semiconductors: One striking ex- Advanced Research ferroelectricity, high-temperature su- ample is the quantum Hall effect seen perconductivity, colossal magnetoresis- in the 2D electron gas formed at a gal- Systems, Inc. tance, and spin-glass behaviors. Not lium arsenide–aluminum gallium ar- surprisingly, these compounds are senide heterojunction. Tel 610 967 2120 being explored for myriad applications. Like semiconductors, many complex Fax 610 967 2395 e-mail; [email protected] Interfaces between complex-oxide oxides are closely lattice-matched to www.physicstoday.org June 2007 Physics Today 23 See www.pt.ims.ca/12307-14 one another and lend them- lem in different ways. The sur- selves to epitaxial growth. face may roughen as pyramids (AlO )– Only in the past decade, how- 2 e form there, so that other crystal ever, have experimenters /2 facets than the problematic (LaO)+ honed the techniques to grow e (001) plane present themselves high-quality oxide crystals and /2 to the outside. Or the ions might fabricate atomically abrupt (AlO )– rearrange themselves on the 2 e and uniform surfaces. /2 surface to compensate for the (LaO)+ One particular complex- e charge imbalance. / oxide interface has recently 0 2 What happens at the inter- shown some promise of fulfill- (TiO2 ) face when you try to grow a ing the high expectations. That positively charged plane of LaO (SrO)0 is the interface between lan- on top of the neutral TiO2 layer thanum aluminate (LaAlO3 or 0 of STO? To screen the charge LAO) and strontium titanate (TiO2 ) from the LaO layer, some of the (SrTiO3 or STO). Although 0 cations might shift position, or both are insulators in their (SrO) oxygen vacancies might form at bulk form, Akira Ohtomo and the interface. It’s also possible, Harold Hwang found two Figure 1. Layered structure of the interface however, for the electrons to re- years ago that the interface be- between lanthanum aluminate and strontium arrange themselves there. For tween them was conducting. titanate, showing the alternating planes of atoms. example, electrons might be The electron carriers had a La, Al, Sr, Ti, and O atoms are shown as orange, transferred from the LaO layer 2 very high mobility, 1 m /(V-s), yellow, purple, green, and gray, respectively. to the TiO2 layer, with a net ad- with a scattering rate compara- Arrows labeled e/2 indicate the charge transfer dition of half an electron, e/2, ble to that of gallium ar- per unit cell. Some of these elec- between LaO and AlO2 layers because of ionic senide.1 Ohtomo is at Tohoku bonding. When the LaO layer is grown atop the trons might enter the previ- University and Hwang is at the ously unoccupied d shell of the originally neutral TiO2 layer, there is a net charge University of Tokyo. transfer of e/2 per unit cell to that layer. (Adapted Ti atoms, changing their va- Now it seems that the from refs. 3 and 10.) lence from 4+ to 3+ and giving LAO/STO heterojunction might them local moments because of be ferromagnetic as well as the unpaired d-shell electrons. metallic, even though neither of the important depending on growth (Ti, like most transition metals, can constituent compounds is magnetic. circumstances. have several different valences.) This This month a team from the University charge transfer is what gives rise to the of Twente in the Netherlands reported Electronic reconstruction possibility of both conductive and fer- evidence for such magnetic behavior as How can the electrons, which are local- romagnetic behavior. a large negative magnetoresistance at ized in the constituent insulators, gain To grow the LAO/STO interfaces, the LAO/STO junction and, below 0.3 mobility at the interface? A key factor is experimenters use single crystals of K, hysteresis in its sheet resistance— the electric field that is created when STO as a substrate and etch atomically that is, resistance in a 2D plane.2 you terminate a complex oxide crystal flat terraces on top of that, terminating Twente’s Alexander Brinkman ac- along a (001) plane. To understand the in a TiO2 layer. The LAO layers are de- knowledges that more evidence may be origin of this field, consider the struc- posited on this substrate by pulsed needed to nail down the existence and ture of the compounds that make up the laser deposition in a low-pressure nature of ferromagnetism. Still, ob- apparent metallic ferromagnet: STO background of oxygen. Atomic force serves Andrew Millis of Columbia Uni- and LAO. Each one is a type of oxide microscopy can confirm that the step- versity, it’s exciting just to see some type known as a perovskite, having the gen- and-terrace structure of the substrate of ferromagnetic signature at the inter- eral form ABO3, where A and B are has been maintained as the LAO film is face when none is seen in the bulk. cations. In perovskites, (001) planes of deposited. The findings of conductivity and AO alternate with planes of BO2. As il- In their 2004 experiment, Ohtomo ferromagnetism are not without some lustrated in figure 1, the layers of STO and Hwang found that the charge car- reservations. The largest question con- are both neutral. But in LAO, positively riers at the LaO/TiO2 interface were cerns the origin of the charge and spin charged planes of LaO alternate with negative, as expected from either the carriers. Does the conductivity arise negatively charged planes of AlO2. The polar discontinuity or the presence of from an intrinsic effect, specifically lanthanum ions are sharing their elec- oxygen vacancies, and had high mobil- electronic rearrangement at the sur- trons with the aluminum ions in the ad- ity.
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