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Home , Klaus von Klitzing, Max Planck

europhysics BULLETIN OF THE EUROPEAN PHYSICAL SOCIETY news J.A. Volume 16 Number 11/12 November/December 1985

Klaus von Klitzing 1985 IN Yoseph Imry, Tel Aviv (School of Physics and , Tel Aviv University)

An October 16, 1985, the Swedish meanwhile been surpassed by an order Academy of Sciences announced the of magnitude and the result was found award of the 1985 to be universal — independent of mate­ to of the Max-Planck rial and sample properties. The plateaux Institute in , for the discovery were also observed by Kawaji and of the Quantized Hall Effect (QHE). The Wakabayashi 2). QHE was reported in 1980 in a paper1) Besides its sheer beauty, this striking by von Klitzing in collaboration with G. result touches upon several topics of Dorda of Siemens and M. Pepper of fundamental interest, has important ap­ Cambridge. As shown in Fig. 1, the Hall plications to precision measurements Klaus von Klitzing now aged 42 began his conductance Gxy of a 2D electron gas in and metrology, has motivated and will physics studies at the Technical University an MOS was found, for finite ranges of undoubtedly lead to novel develop­ in Braunschweig and then in 1972 moved to the gate voltage in which the regular ments. Moreover, it relies on the physics the University of Wurzburg gaining his Habi­ conductance, Gxx, was severely reduc­ of devices of great industrial interest 3). litation in 1978 after spending a period at the ed, to be quantized with plateaux having Before explaining the effect briefly . He was able to conti­ Gxy = jh/e2 ≡ J. (25812.80 Ω)-1 and reviewing its applications and im­ nue with his research at Wurzburg through j integer (1) plications, we remark that this was a tru­ being awarded a Heisenberg Stipend. Then i.e. integer multiples of a basic ly experimental discovery, found as guest of the Max-Planck-Institut in Stut­ unit. The initial accuracy of ≡ 10-6 of through a measurement of, in principle, a tgart he began experiments at the joint MPI- CNRS high laboratory in this macroscopic quantum effect has routine character, except, of course, for Grenoble where he made his now celebrated measurements. Von Klitzing is presently full time with the Max-Planck-Institut fur Fest- körperforschung in Stuttgart having been for a short time Associate Professor at the Technical University in .

C ontents Klaus von Klitzing, 1985 Nobel Prize Winner 1 Message from the CMD 3 Optical Fibre Technology at STL 4 Fig. 1 — The Hall voltage, UH Cumulative Index, 1981-1985 9 and the voltage parallel to the Physics of the Ocean 13 current Upp for the device Graphite Intercalation shown in the inset as a function Compounds 16 of the gate voltage with T = 1.5 K, B = 18 T. Whenever Upp is New EPS Members 19 strongly depressed UH has a Collaboration with the APS 20 plateau with a quantized value Europhysics Letters 20 o f Gxy

Europhysics News is published monthly by the European Physical Society. © 1985. Reproduction rights reserved. ISSN 0531- 74 79

1 the high magnetic field (~ 15T) and the that direction for an electric field E||x high quality MOS used. The experiment even in a uniform isotropic system. [In has not required "big physics" equip­ the usual experiments, with an open cir­ ment or machinery. Also, while there cuit Iy = 0 an electric field E related to had been a theoretical calculation yield­ the "Hall voltage" is generated.] Thus, ing approximately such an effect 4), the the relation between the current density conditions under which it was really and electric field vectors is tensorial and observed were outside the range of vali­ the xy component of the conductivity dity of that theory. Thus, the QHE disco­ tensor is called the Hal! conductivity. very (as well as subsequent ones 5), did The xy component of the inverse resisti­ take theoreticians by surprise! vity tensor is the Hall resistivity which is shown in the classical case to be given Explanations by ρxy = - B/nec, n being the density of The initial discovery employed a carriers and e their charge. Thus, in the MOSFET (metal-oxide-semiconductor- Fig. 2 — Schematic of the MOSFET show­ simplest case the Hall effect provides a ing the different field configurations. field-effect-transistor), where a 2D elec­ useful measure of the carrier density. In tronic layer 6) is formed at the interface real experiments, one is measuring con­ have a quantitative understanding of the between, e.g. p-type silicon and a 103 Å ductance, G, not conductivity, σ. The 2D extent of the quantized Hall steps. Nor layer of SiO2, in a strong electric field case is special in that σxy= Gxy. do we have a proper microscopic quan­ perpendicular to the interface. The latter The regular conductance Gxx of a tum theory of the Hall effect for strong is controlled by a gate voltage applied MOSFET can be made very small in a disorder. between the Si and a metallic layer large magnetic field at low deposited on top of the SiO2. It was first 6) by changing the gate voltage so that Developments and their Significance demonstrated in Ref. 7 and is now well the Fermi level is between Landau levels. Since the velocity of is a fixed established both experimentally and This means that the j lowest Landau quantity and is known with very good theoretically 6) that owing to the quan­ levels are full and all those from the accuracy, the QHE means that one can tization of the electronic levels perpen­ (j + 1)th on are empty. Thus, one has an now accurately measure the fine struc­ dicular to the interface and the possibili­ effective gap of hωc in the excitation ture constant "with an ammeter and a ty of controlling the electron density by spectrum and the system is insulating or voltmeter". Alternatively, knowing e one the gate voltage, conditions where all dissipationless in the sense that Gxx is can likewise measure Planck's cons­ electrons reside in the lowest such level exponentially activated (hωc ≡ 6 meV tant ! can be achieved. The electronic and T = 1.5K in the experiment of Ref. The metrological consequences of is then predominantly 2D, parallel to the 1). Indeed, in the experiment conducted this were fully realized in Ref. 1. By now, interface. One interesting aspect of an by von Klitzing at the Grenoble high field the method has already given a theory- ideal 2D system is that the motion in laboratory, Gxx ≤ 10-10 (Ω)-1 was achiev­ independent determination of a which is these parallel dimensions can be fully ed over significant ranges of gate vol­ better than previous such determina­ quantized in a strong magnetic field, B, tage V g, accompanied by observable tions by almost an order of magnitude. into discrete Landau levels having ener­ quantized plateaux in G xy. This disco­ For producing resistance standards it is gies (n + 1/2)hωc, with hωc = eB/m*c, very was not sheer luck ; it took an alert in principle absolute, time independent m* being the effective mass of the 2D and astute observer, with a good deal of and relatively available. motion. Other 2D electron systems are intuition and understanding of the phy­ Of course, such applications rest on available, notably in quantum wells for­ sics involved, to grasp that this was a the accuracy of Eq. 1. Does it have small med at the interface between two ap­ real and an extremely significant effect! corrections arising from state ef­ propriately grown semiconductors 5,6). The effect was then interpreted in fects, higher order QED terms and relati­ The QHE has by now been observed in terms of the classical result for ρxy (ex­ vistic effects? The last of these should several of those as well. cluding spin and valley degeneracy ef­ not be very important because of the The quantum unit of conductance, fects). One recalls 1) that the degene­ small v/c(< 10-9) values usually used; h/e2, is quite interesting in itself. In clas­ racy per unit area of each Landau level is solid state effects could be expected to sical physics, a natural unit for conduc­ p = H/ϕϕ0 , ϕ0 being the (single electron) be much larger than the measured ac­ tance is provided by the velocity of light flux quantum, ϕo = hc/e ≡ 4 x 10-15 curacy of Eq. 1, so the experiments sug­ in vacuum, c, as the impedance of free Tm2. Thus, the surface electron density gest that they are not relevant. Higher- space is 4 π/c ≡ 376.7 Ω (here we is n = j p, which yields Eq. 1. This ex­ order QED terms present an interesting employ cgs units but give resistances in planation gives the essence of the ef­ open problem. Clearly, one needs a SI units). In quantum physics, the pro­ fect. However, to understand the finite general theoretical argument, perhaps duct of the fine structure constant α = plateaux, one has to understand why similar to that given by Bloch 8) for the e2/hc = 1/137.04 and c gives the con­ the Fermi level remains pinned between accuracy of determining h/e from the ductance e2/h (which is 2 π times the Landau levels over finite ranges of elec­ Josephson effect, to explain the accu­ quantum unit for the Hall conductance). tron density (or Vg ). As recognized al­ racy of Eq. 1. The usual Hall effect, originally disco­ ready in Ref. 1, this has to do with the Such an argument was indeed put for­ vered in 1879 and by now a standard existence of localized electronic states ward by Laughlin 9). In a simplified ver­ characterization technique for semicon­ (due to unavoidable disorder). This in­ sion the following Gedanken experiment ductors, occurs when a magnetic field is troduces the fundamental problem of is considered : The 2D system with the applied, say, in the z direction perpen­ understanding localization and delocali­ large magnetic field always perpendicu­ dicular to the current / (parallel to x), in a zation of states in 2D for large magnetic lar to the layer is wrapped on a cylinder conductor. A Lorentz force operates in fields. In spite of many interesting sub­ and the small Hall voltage is generated the y direction and leads to a current in sequent developments, we still do not by applying a Aharonov-Bohm flux, 0,

2 confined to the cylinder's opening. If 0 varies linearly (adiabatically, for small enough V): ϕ = - cVt + const, an azi­ muthal EMF, V is induced. We also con­ sider the system to behave as a perfect insulator with Gxx = 0, so it is in equili­ brium at zero . By very gene­ ral arguments 8), the states of the sys­ tem are invariant to changing ϕ by a flux quantum ϕo (which is equivalent to changing the phase of the ground state around the cylinder by 2π). Thus each time ϕ changes byϕ o the only consequence is that an integral number of electrons, j, is transferred from the lower to the upper contact of the cylinder. This yields a current I = j crucial, in other branches of physics. It is e2V/h, consistent with Eq. 1! While not safe to predict that we shall be hearing CMD Applauds being able to calculate j (except for free even more on 2D electron systems and It is with great joy that the members of electrons) this argument establishes the the QHE and related phenomena in the the EPS Condensed Matter Division generality and exactness of Eq. 1 within years to come. learned the news that the 1985 Nobel non-relativistic quantum . REFERENCES Prize for Physics had been awarded to The current / is a sort of supercurrent 1. Von Klitzing K., Dorda G. and Pepper M., Dr. Klaus von Klitzing. (since it flows perpendicular to V, there Phys. Rev. Lett. 45 (1980) 494. Von Klitzing is well known to EPS is no dissipation). The QHE is a macro­ 2. Kawaji S. and Wakabayashi J., in Physics as he was awarded the EPS Hewlett- scopic quantum phenomenon related to in High Magnetic Fields, Eds. S. Chikazumi Packard Europhysics Prize* for 1981 and the fundamental role of the phase in and N. Miura (Springer, Heidelberg) 1981. was presented with this award at the and its sensitivity 3. Pepper M., Europhysics News, 16 (1985) 2nd General Conference of the CMD to the vector potential (the Aharonov- 7/8. 4. Ando T., Matsumoto Y. and Uemura Y., J. held in Manchester in March 1982. His Bohm effect). In fact, the QHE has sti­ Phys. Soc. Japan, 39 (1975) 279. research and subsequent have mulated a recent revival of the interest in 5. Tsui D.C., Störmer H.L. and Gossard A.C., been regularly followed up at the CMD such effects in small normal conductor Phys. Rev. Lett. 48 (1982) 1559. General Conferences and he was a ple­ rings, where impressive results have re- 6. Ando T., Fowler A.B. and Stern F., Revs. nary speaker at the 6th General Confe­ centy been reported. Mod. Phys. 54 (1982) 437. rence held in Prague in August 1984 A new surprise was in store for the 7. Fowler A.B., Sang S.S., Howard N.E. and (when the above photo was taken). theoreticians when experiments repor­ Steines N.P.A. , Phys. Rev. Lett. 16 (1966) The QHE (von Klitzing's and the frac­ 901. ted 5) in 1982 in high quality quantum 8. Bloch F., Phys. Rev. Lett. 21 (1968) 1241 ; tional), despite much progress over the well samples first revealed a new Frac­ Byers N. and Yang C.N., Phys. Rev. Lett. 7 past few years presents a major chal­ tional Quantized Hall Effect (FQHE) in (1961) 46. lenge to . which the integer in Eq. 1 is replaced by 9. Laughlin R.B., Phys. Rev. B23 (1981) * Europhysics News 13 (1982) 4. a simple rational fraction such as Mm, m 5632; Halperin B.I., Phys. Rev. B25 (1981) being an odd integer, as well as e.g. 2/3, 2185; Imry Y„ J. Phys. C16 (1983) 3501. 2/5, 2/7, 3/7, 4/5, with more to come. 10. Laughlin R.B., Phys. Rev. Lett. 50 (1983) J.T. Devreese This requires extra stabilization of the 1395. (Chairman of the Condensed Matter Division) 2D electron gas at those fractional fill­ ings of a Landau level, with gaps in the excitation spectrum. The relevant quasi­ THE PENNSYLVANIA STATE UNIVERSITY particles have fractional charges (a sub­ Experimental Low Temperature Physics ject of current excitement in both solid state physics and ) The Department of Physics is seeking candidates for a tenure-track to agree with the above general argu­ position of Assistant or Associate Professor in experimental low ment. In fact, in a seminal theoretical temperature physics starting in the 1985-86 or 1986-87 academic paper, Laughlin 10) showed how elec­ years. Candidates should have an established record of research tron-electron Coulomb interactions can accomplishments and may expect to work in conjunction with the lead to such effects. The study of the 2D ongoing research effort on the main campus at University Park. quantum plasma, has since become an Minimal requirements include a Ph.D. degree in this field and some important field of many body theory. postdoctoral experience. A desire and aptitude for teaching under­ The universal significance of the QHE graduate and graduate students is essential. is further demonstrated by the useful­ Send applications, including a curriculum vitae and names of at ness of the concepts originating in it in least four references, to high-energy physics. This is yet another Gerald A. Smith, Head, Department of Physics, demonstration (following the renormali­ Box QQ, 104 Davey Laboratory, University Park, PA 16802, USA, zation group method — 1982 Nobel by February 1, 1986 or until a suitable pool of applicants is iden­ Prize to K. Wilson) of how concepts tified. developed in condensed matter con­ An affirmative /equal opportunity employer. texts can become relevant, and even 3