Klaus Von Klitzing 1985 NOBEL PRIZE in PHYSICS Yoseph Imry, Tel Aviv (School of Physics and Astronomy, Tel Aviv University)
Total Page:16
File Type:pdf, Size:1020Kb
europhysics BULLETIN OF THE EUROPEAN PHYSICAL SOCIETY news J.A. Volume 16 Number 11/12 November/December 1985 Klaus von Klitzing 1985 NOBEL PRIZE IN PHYSICS Yoseph Imry, Tel Aviv (School of Physics and Astronomy, 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 Nobel Prize in Physics to be universal — independent of mate to Klaus von Klitzing of the Max-Planck rial and sample properties. The plateaux Institute in Stuttgart, 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 University of Oxford. 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 quantum 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 magnetic field 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 Munich. 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 temperatures 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 light 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 motion 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 solid 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.