High- T c Josephson Square-Law Detectors and Hilbert Spectroscopy for Security Applications
Yury Divin, Member, IEEE, Ulrich Poppe, Vladimir N. Gubankov, and Knut Urban Invited Paper
Abstract—Among various discussed ways of explosive detection, the techniques using electromagnetic radiation are considered as having great potential and research activities are recommended in this field. To identify new threats, like liquid explosives, with low rate of false alarms, fast spectral measurements are required in a broad frequency range from microwave to terahertz. We attract attention to a great potential of high- Josephson technology in security applications and present our results in developing high- Josephson junctions for Hilbert spectroscopy and detector arrays.
Index Terms—High-temperature superconductors, Hilbert transforms, Josephson radiation detectors, spectroscopy.
I. INTRODUCTION
T
HE CONVENTIONAL techniques, e.g., walk-through metal detectors for people and X-ray inspection for their luggage in airports, were found to be not sufficient to pre- vent the permanently modifying terrorist’s threats. Additional screening of people and their belongings for detection of haz- ardous explosive, chemical and biological substances might soon become a reality in airports, train terminals, theatres, sport halls, and others.Emerging technologies in this area generally fall into two basic categories:bulkandtracedetection [1]–[3]. Detection of bulkexplosives is carried out by imaging characteristics of the explosive device or by detection of the explosive itself [1], [2].
Imaging technologies can see through the clothing and produce an image of the human body and concealed items underneath [2].Tracedetection techniques include the collection of vapors and/or particles from subjects and subsequent analysis of the collected material using various spectroscopic techniques [3].
Trace detection is complicated due to the large number of dif- ferent explosive materials that have to be detected and the low vapor pressure of common explosives.
Manuscript received July 30, 2007; revised November 9, 2007; accepted Jan- uary 15, 2008. This work was supported in part by ELAN CARE Project 506395 and in part by the ISTC Project 3308. The associate editor coordinating the re- view of this paper and approving it for publication was Dr. Dwight Woolard.
Y. Divin is with the Research Center Juelich, 52425 Juelich, Germany. He is on leave from the Institute of Radio Engineering and Electronics of Russian Academy of Sciences, Moscow 125009, Russia (e-mail: Y.Divin@fz-juelich.
de).
U. Poppe and K. Urban are with the Research Center Juelich, 52425 Juelich, Germany (e-mail: U.Poppe@fz-juelich.de; K.Urban@fz-juelich.de).
V. N. Gubankov is with the Institute of Radio Engineering and Electronics of Russian Academy of Sciences, Moscow 125009, Russia (e-mail: gub@cplire.
ru).
Digital Object Identifier 10.1109/JSEN.2008.923185
Triacetone triperoxide (TATP), for example, can be produced on sitein a rather straightforward way using the readily avail- able precursor chemicals, acetone and hydrogen peroxide . Therefore, the security measures should be extended to detect not only the explosives themselves but also potentially dangerous liquids, like acetone. In the general case, one will have to deal with sealed bottles with unknown liquids where no traces of the liquid or its vapor are available outside the bottle. Therefore,bulkdetection techniques have to be ap- plied in the general case [1].
TATP detection represents a serious challenge because con- ventional detection devices in airports rely on the presence of nitro groups or metallic elements. Some techniques of trace analysis of this explosive have already been suggested based on chemical means [4], [5], but they are not applicable for fast screening. Other techniques cannot distinguish between ordinary water and hydrogen peroxide . Among various discussed ways of explosive detection, the techniques using electromagnetic radiation (i.e., microwave and terahertz imaging and spectroscopy), are considered as having great potential, and intensive research activities are recommended in this field by experts of the National Research Council of the U.S. National Academy (see [1, pp. 10, 73, 96, 121]).
In this paper, we are trying to attract attention to a great po- tential of high- Josephson technology for security applica- tions, e.g., for identification of liquids, and review our recent results in the development of high- Josephson junctions for spectroscopy and imaging.
II. DIELECTRICFUNCTION
From the point of view of electromagnetic theory, the electric displacement-field response of a substance to a rapidly varying electrical field is defined, in linear approximation, by a complex dielectric permittivity
(1) which is determined by the internal dynamics of the molecules [6]. Therefore, a substance can, in principle, be identified by measuring the dielectric function of this substance over a wide range of frequency and comparing it with available reference data.
The internal dynamics of liquids can be considered to a first approximation as an alignment of dipoles by the local electric
1530-437X/$25.00 © 2008 IEEE
Fig. 1. Real and imaginary parts of the dielectric functions of various liquids at 25 C[7]–[9].
field following a Debye relaxation process [7]. In this case, the dielectric permittivity function can be written as
(2) where is the “infinite frequency permittivity,” is the static permittivity or dielectric constant, and is a characteristic relax- ation time. In real liquids, due to the interplay of orientational, intramolecular, kinetic, H bonding, diffusional and migrational modes, the dielectric function is more complicated. This is favorable for detection purposes since, as a consequence, the fingerprint, i.e., the frequency dependence, is more specific to a particular liquid [7]. The real and imaginary parts of the dielectric functions of some liquids, calculated in Debye ap- proximation from the available data [7]–[9], are shown in Fig. 1.
The dielectric relaxation parameters, , , and of var- ious liquids, like acetone, water, ethanol, etc., were calculated from -values, measured at some discrete frequency lines in the microwave range, and summarized in reference [7].
The data for hydrogen peroxide are calculated from a few points presented in [8] and [9] with the assumption that relaxation time for this liquid is equal to that of water.
The values of the main relaxation times of different liq- uids spread over two orders of magnitude, e.g., 3.3 ps forace- tone, 8.3 ps forwater, and 163 ps forethanol. The characteristic measurement frequency , where absorption exhibits a maximum, ranges from 1 GHz for ethanol to 50 GHz for acetone. To cover a specific dispersion region in even with the main relaxation time , a frequency dynamic range of more than one order of magnitude around the characteristic fre- quency is required [7].
Additionally, when more dynamic relaxation processes are
involved, like in ethanol with and [7]
or in water with [10], the frequency range of the dielectric spectroscopy of liquids must be expanded further to hundreds of GHz (sub-terahertz) and even to the terahertz
(THz) range. It is emphasized in [7] that insufficient frequency coverage is responsible for many unsatisfactory data in the lit- erature and that an expansion of the frequency range up to the terahertz range is required for unambiguous identification and property measurements.
III. CONVENTIONALTECHNIQUES
A. Microwave Range
The techniques to measure the dielectric properties of liquids in the frequency range of up to about 100 GHz have recently been reviewed [11]. The reflection, transmission and resonator microwave techniques at fixed frequencies are the main con- ventional approaches in the field. Some technical concepts for the identification of safety-relevant liquids are known, which are based on conventional microwave measurements at fixed frequencies.
However, due to the nonmonotonous behavior of the permit- tivity of two-component solutions with increase in concentra- tion of one component [8], the measurement of the dielectric function at fixed frequency, or even at a few preset spot fre- quencies cannot distinguish unambiguously between some so- lutions (of the same or different chemicals) and the liquids of concern. This makes such approaches unreliable, and since the safety system must react in both, harmless and safety-relevant cases, this should give rise to a high frequency of false alarms.
The situation can be improved, when the dispersion is mea- sured over an extended frequency range (see Fig. 1). Such mul- tifrequency, spectroscopic approach can be, in principle, real- ized employing commercial network analyzers. However, this approach is hampered by the performance range of commercial network analyzers which, from technical reasons, is usually lim- ited to about 50 GHz maximum. Only at the cost of an exponen- tial increase of price and time required for analysis, a maximum frequency of up to 100 GHz is within reach, which, as described above, is still too low even for such “simple” liquids as acetone and water.
B. Terahertz Time-Domain Spectroscopy
To study fast dynamics in liquids, continuous measurements in the terahertz frequency range have been performed using time-domain spectroscopy [10], [12]–[14]. In time-domain spectroscopy, the shape of a short THz pulse with and without the sample is recorded and evaluated in terms of the absorption coefficient, the refractive index as well as the permittivity as a function of frequency. Particularly close to the discussed security applications is a reference [14], where transmission spectra of some inflammable liquids, stored in conventional beverage plastic bottles, have been measured in the frequency range from 300 GHz to 1.8 THz.
No terahertz transmission was found for water in bottles and some transmission, decreasing with the frequency, was found for liquids, like benzene and kerosene. From this observation, a crude inspection principle was derived following the rule“If liquids transmit terahertz radiation, it is dangerous, if not—it is water.”With this criterion in practice, acetone and hydrogen peroxide would clearly escape detection, due to their large ab- sorption in the terahertz range.
Fig. 2. SpectraS(f)of Josephson oscillations at the dc voltageV = 0:3 I R for various ratios = 2ekT=hI of thermal energykT to Josephson energy hI =2e[16].
In principle, an additional characterization at lower frequen- cies can help to identify the liquid. However, in themicrowave range, conventional characterization techniques are based on time consuming low-speed point-by-point measurements of absorption at discrete frequency lines. We note that also conventional terahertz time-domain spectroscopy involves extended measurement times, typically of a few minutes since they are involving a mechanically driven optical time-delay line for pulse screening [12].
In contrast to these conventional THz techniques, ultrafast techniques, operating in the frequency range of a few GHz to a few THz, might be suggested based on high- Josephson technology [15].
IV. HIGH- JOSEPHSONDETECTORS
A. The ac Josephson Effect and Square-Law Radiation Detection
An operation of the Josephson detector of electromagnetic ra- diation is based on the ac Josephson effect, which takes place at weak electrical contact of two superconducting electrodes, e.g., at superconducting tunnel junction [16]. A coherent tunneling of Cooper pairs through a barrier is possible without any finite voltage at the currents lower than a critical current , and, at , a coherent tunneling of Cooper pairs results in current oscillations with the frequency , where is the dc voltage bias across the junction.
In more realistic resistively shunted junction (RSJ) model [16], a quasiparticle current is added in the form of Ohmic law with the normal-state resistance and the Josephson junction is current-biased. In the RSJ model, spectra of Josephson oscillations, additionally to the fundamental frequency , contain also high-order harmonics at
the frequencies with decreasing
amplitudes [16]. When the thermal energy is much lower than the Josephson energy , these harmonics are very well separated (see Fig. 2), but with the increase of the ratio , the spectrum starts to transform from poly- chromatic to continuous spectrum with a cutoff frequency of
the order of a characteristic frequency of Josephson oscillations [16].
The difference in the spectra of Josephson oscillation in two limiting cases and is reflected in the interaction of weak external monochromatic signals with the ac currents in Josephson junctions. At and the frequency of Josephson oscillations close to the external frequency , a frequency pulling effect is observed, when the fundamental frequency of Josephson oscillations is shifted from higher (or lower) values towards the frequency of external signal. As a result, the dc square-law detector response as a function of the Josephson fre- quency in this case has an odd-symmetric form only around the frequency and no contributions from higher Josephson harmonics [16, Ch.10.3]. The square-law frequency-selective Josephson detection is actually a basis of Hilbert spectroscopy [17]. This case is usually realized with low-resistance Josephson junctions with a of several ohms or below.
At and when the frequency of external signals are as low as a fraction of the characteristic frequency , we have a case, when the signal frequency is still below the main frequen- cies, responsible for the formation of the dc curve of the Josephson junction, and the detection might be described in the terms of classical detection. In more detailed analysis the high-frequency limit for classical detection was estimated as [18]. In this case, the bias dependence of the dc response is proportional to the second derivative
versus voltage . This response is realized with high-resistance Josephson junctions with above tens of ohms.
B. High- Josephson Junctions
The central part of the Josephson detectors and Hilbert spec- trometers is a nanosize device, a Josephson tunnel junction.
Such a junction consists of two superconducting layers sepa- rated by a thin barrier layer of nonsuperconducting material.
Here, we present junctions made from the high-temperature su-
perconductor .
To operate in the terahertz range, the junctions should have characteristic voltages exceeding 1 mV (here, de- notes the critical current, at which the junction changes into the resistive state, and denotes the resistance in the normal-con- ducting state to be derived from the current-voltage character- istic of the junction). The best high- junctions are currently produced as grain-boundary junctions, where a grain boundary works as a tunnelling barrier with high critical current densi- ties [19] and low capacitances as low as [21]. A transmission electron microscopy (TEM) plane view image of a [001]-tilt grain boundary is shown in Fig. 3(a). A light-micro- scope image of an Josephson junction developed for applications in THz detectors and spectrometers is shown in Fig. 3(b).
The -values amount to 0.34 mV at 77 K and up to 2 mV at 35 K for the [001]-tilt Josephson junction.
The recently developed [100]-tilt junctions could reach the record values of the -product up to 8 mV at 4.2 K [20].
The spectral range of Josephson oscillations scales with the -values [15], [18], [21], [22]. The normalized current responses of two [001]-tilt grain-boundary
Fig. 3. (a) TEM image of anYBa Cu O grain-boundary junction and (b) micrograph of the junction (a 1m-wideYBa Cu O bridge, crossing the NdGaO bicrystal boundary) with an Ag sinuous antenna.
Fig. 4. Normalized responses1I(V )=1I of the [001]-tiltYBa Cu O Josephson junction withR = 0:5 andI R = 35 Vto monochromatic radiation with frequencies from 15 GHz to 1 THz, measured at a temperature of 85 K.
junctions to monochromatic signals with the frequencies from 17 GHz up to 5.2 THz are shown in Figs. 4 and 5.
The responses demonstrate odd-symmetric resonance’s at the voltages , due to the frequency pulling of Josephson oscillations by THz radiation. At each temperature between 30 and 85 K, the selective responses have been observed at least in one decade of frequency bandwidth. The middle frequency of this bandwidth scaled with the characteristic frequency , so the total bandwidth of selective detec- tion, which was covered by one Josephson junction at two different temperatures, was more than two orders in frequency.
At the temperature of 85 K, close to the critical temperature of junction, the values are small and the responses were detected in the frequency range from 10 GHz to 1 THz (see Fig. 4) and even can be observed as low as at 5 GHz [22]. This set of responses is actually measured in the frequency range, were liquids of interest show dispersion (see Fig. 1). This circumstance gives us the possibility to suggest the Josephson junctions developed by us in an application for the identification of liquids.
At lower temperatures, where the -values are around 1.5 mV, the response reaches terahertz frequencies for [001]-tilt junctions (Fig. 5). A high-frequency limit for the ac Josephson effect of 4.25 THz has been achieved and a spectral resolution of has been demonstrated [15]. With the new type of [100]-tilt junctions, which possess -values up to 8 mV [20], we have reached the record value of the Josephson fre- quency of 5.24 THz (inset in Fig. 5) [21].
Fig. 5. Normalized responses1I(V )=1I of the [001]-tiltYBa Cu O Josephson junction withR = 7 andI R = 1:5 mVto monochromatic radiation with frequencies from 0.4 to 4.25 THz, measured at a temperature of 35 K. Inset: Response of the [100]-tilt junction (R = 1:3 ,I R = 6:0 mV, T = 10 K) to THz radiation with a frequency of 5.24 THz.
C. Hilbert Transform Spectroscopy
It is understood that a fast technique has to be employed in re- alistic screening systems for public and air traffic security which can perform spectral measurements over the whole character- istic frequency range from gigahertz to terahertz. Only this way, an unambiguous identification of dangerous substances is pos- sible within short times.
Such a technique is Hilbert-transform spectroscopy (in the following abbreviated by HITRAS). This is a technique for spectroscopy of electromagnetic radiation in the frequency range from a few gigahertz to a few terahertz. It is based on the ac Josephson effect in superconducting tunnel junctions [17]. It is by principle a very fast technique, since the frequency scan is performed entirely electronically. In addition, it is volumewise compact, since does not involve the bulky instrumentation typical for infrared and far infrared spectroscopy.
If a Josephson junction in the resistive state is irradiated by electromagnetic radiation, the electrical response function (where denotes the voltage across the junction, is the current through the junction, and is the square-law current response of the junction to radiation) is proportional to the Hilbert transform of the spectrum of the incident radiation [17].
Applying an inverse Hilbert transformation to the measured response , the spectrum can be recovered as
(3)
where ( is Planck’s constant).
The principle of HITRAS is similar to that of Fourier-trans- form spectroscopy (FTRAS) or time-domain spectroscopy (TDS), because in all these techniques the spectrum of elec- tromagnetic radiation and the detected electrical signal as a function of a variable parameter are related by an integral
Fig. 6. Two demonstrators of Hilbert transform spectrometers with (a) a Stir- ling cooler and (b) a liquid-nitrogen cryostat.
Fig. 7. Emission spectra of harmonic multiplier at input microwave frequency f equal to (a) 12.6 GHz and (b) 19.1 GHz, measured by Hilbert spectroscopy.
transformation. The important distinction, however, is that a direct transformation of the spectrum into an electrical signal in HITRAS is achieved by a nanoelectronic device, the Josephson junction, while in FTRAS or TDS this procedure requires a bulky optical-mechanical device, an interferometer or optical delay line, together with a broadband detector.
Several prototypes of Josephson detectors and Hilbert transform spectrometers have been developed and tested with Josephson junctions cooled by a Stirling cooler or a liquid-nitrogen cryostat (Fig. 6) [15]. Power dy- namic ranges of the square-law response up to 53 dB (selective) and 63 dB (broadband) have been demonstrated with a noise
equivalent power NEP in the sub-THz
range and at liquid nitrogen temperatures [15], [23]. These demonstrators have been successfully used for the measurement of Lorentz spectra of Josephson oscillations, the high-harmonic contents of commercial millimeter-wave oscillators (Fig. 7), polychromatic radiation from optically-pumped far-infrared gas lasers (see Fig. 8), and spectra of coherent transition ra- diation from relativistic electron bunches at the TESLA Test Facility at DESY (Hamburg) [15].
Fast measurements of a pulsed Gunn oscillator with Hilbert spectroscopy was reported with a pulse duration of 200 ns and repetition rate of 1 MHz [24]. A power dynamic range up to 28 dB has been reached in the measurements [Fig. 9(a)]. The
Fig. 8. Emission spectra of aCH OHlaser pumped by the 9P36 line of aCO pump laser, measured by Hilbert spectroscopy (below). The responseH(V )of the Hilbert spectrometer to laser radiation is shown above. Inset: energy diagram forCH OH. The [100]-tilt junction was cooled to 56 K.
Fig. 9. (a) Power dynamic range and (b) spectrum of Gunn oscillator, measured by a Hilbert spectrometer in pulsed mode with a pulse duration of 200 ns and a repetition rate of 1 MHz. The [001]-tilt junction had a temperature of 80 K.
spectrum of the output radiation of a Gunn oscillator, consisting of 512 points, has been measured within 7 ms [Fig. 9(b)]. The measurements were performed with a [001]-tilt junctions at
. With this experience, Hilbert spectrometers might be also applied for the spectral analysis of pulsed THz sources and sam- ples (e.g., liquids) illuminated by pulsed sources.
In addition, demonstrators of Hilbert transform spectrometers were developed to measure the absorption spectra of a number of substances including carbon monoxide, and the vapors of liquids such as methanol and acetone (Fig. 10) [25], [26]. The measurements were performed with the [001]-tilt junction, with
and at . The absorp-
tion spectrum of methanol consists of large variety of closely spaced spectral lines, which were not resolved with a resolution of around 10 GHz achieved by the junction used during these measurements. However, these absorption spectra, averaged by
Fig. 10. Absorption spectra of (a) methanol vapor at 150 mbar and (b) acetone vapor at 200 mbar, measured by Hilbert-transform spectroscopy with a broad- band mercury arc lamp.
Fig. 11. Voltage responses 1V (V ) of the [100]-tilt YBa Cu O Josephson junction to electromagnetic radiation with the frequenciesfof (1) 0.145 THz, (2) 0.404 THz, (3) 0.762 THz, and differential resistanceR (V ). I R = 2:8 mV,R = 23 atT = 40 K.
a finite resolution, showed very large difference when methanol vapor was replaced by acetone.
The fact that the Josephson junctions have to be cooled does not pose any problem nowadays since the cooling can be achieved by means of highly reliable and practically mainte- nance-free Stirling coolers.
D. Quasi-Classical THz Detection and Imaging
Now, consider the case of high-resistance Josephson junc- tions, where separate Josephson spectral lines at
with the thermally broadened linewidths , start to overlap. At lower frequencies, the spectrum of Josephson oscil- lation should demonstrate a continuous form (see Fig. 2) so the detection process for low frequencies up to will not be of a frequency-selective type. This type of detection has been re- cently studied in our high-resistance [100]-tilted junctions with high - values [21].
The experimental results are presented in Fig. 11. The voltage responses of the [100]-tilt junction to radiation with fre- quencies of 0.145 and 0.404 THz are practically the same and proportional to the voltage dependence of the second deriva- tive of the curve. This behavior is a feature of classical detection. Only the response to 0.762 THz follows the Josephson frequency-selective mechanism. The estimated -value was around 0.450 THz and this value might be in- creased for optimized high-resistance junctions.
A numerical simulation of radiation detection by high-re- sistive Josephson junctions with high -values has been recently made, using the RSJ model [21]. The voltage response
Fig. 12. Theoretical inverse noise-equivalent power(NEP )of a Josephson detector (R = 300 Ohm,I R = 5 mVatT = 30 K) as a function of frequency and voltage. A zero imaginary part of junction impedance(Im Z = 0)is also shown.
, the impedance , and the noise voltage were cal- culated from the solution of the Fokker–Plank equation for the Fourier-coefficients of the probability distribution of the Josephson phase [16] and, from this, the responsivity and the noise-equivalent power (NEP) were derived. The results of a computer simulation for the NEP of a Josephson detector are shown in Fig. 12. The parameters of the Josephson detector are close to those of our best [100]-tilt bicrystal Josephson junctions.
It follows from Fig. 12, that this Josephson detector, when biased at the voltage , might have values of NEP
better than in the frequency range from
0 to 1.8 THz. So, numerical simulations gave us more opti- mistic values of the spectral range of broadband detection by Josephson junctions.
This broadband and high-sensitive detector might be useful in passive imaging applications. It is also attractive to produce a 1D array, as the quasiclassical response, in contrast to the frequency-selective Josephson response of low-resistance junc- tions, is less sensitive to a variation in biasing (see Fig. 12).
This quasiclassical terahertz Josephson detector requires an optimized operational temperature of 30–40 K, which can be easily realized with a cryocooler. We think that this broadband detector will find an application in terahertz imaging for medical and security screening.
V. CONCLUSION
Hilbert-transform spectroscopy is a well-characterized new technique for ultra-fast spectroscopy in the frequency range from a few gigahertz to a few terahertz.
It seems possible to apply Hilbert spectroscopy with high- Josephson junctions for chemical identification of liquids.
Among the different types of spectroscopic measurements (transmission or reflection), the reflection measurements are indicated as better suited for the identification of highly ab- sorbing liquids [10], [12]. Using pulsed broadband sources and reflection type measurements, it might be possible to realize
a fast demonstrator of Hilbert-transform spectroscopy for identification of liquids.
An additional contribution of Josephson technology to se- curity screening may come from quasiclassical detection with high-resistance high- Josephson junctions. This technique might lead to a realization of a high-sensitive 1D detector arrays for active and even passive imaging.
We see future applications of high- Josephson detectors in spectroscopy and imaging with pulsed microwave and terahertz sources.
ACKNOWLEDGMENT
The authors are thankful to V. V. Pavlovskii, O. Y. Volkov, V. V. Shirotov and M. V. Lyatti.
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Yury Y. Divin(M’97) was born in Chernyakhovsk, USSR, in 1948. He studied physics at the Moscow Institute of Physics and Technology and received the Diploma degree in physics in 1972 and the Ph.D.
degree from the Institute of Radio Engineering and Electronics of Russian Academy of Sciences (IRE RAS), in 1979.
Since 1986, he has been a Senior Research Scien- tist at the IRE RAS and a Group Leader. In 1983, 1984, 1991, and 1992, he was a Guest Scientist at the Physics Department of the Technical University of Denmark, and from 1993 to 1998, he was a Guest Scientist at the Research Centre Juelich, Germany. Since 1998, he has been employed as a member of scientific staff at the Institute for Solid State Research at the Research Centre Juelich. Since 1970, his scientific interests were in the field of superconducting electronics, especially in the applications of the ac Josephson effect for detec- tion, spectroscopy, and imaging of electromagnetic radiation. Current research interests are within physics, technology and terahertz applications of high-T Josephson junctions.
Ulrich Poppe was born in Leipzig, Germany, in 1948. He studied physics at the University of Cologne and received the Diploma degree in the- oretical solid-state physics in 1976 and the Ph.D.
degree from the University of Cologne and the Research Centre Juelich, Juelich, Germany, in 1979, for “Tunneling Experiments on Chevrelphase-Su- perconductors.”
Since 1979, he has been with the Institute for Solid State Research at the Research Centre Juelich.
Until 1987, he worked in the low-temperature and superconductivity group on tunneling experiments on magnetic-, organic- and heavy fermion-superconductors. Later, he became a Group Leader in the Institute for Microstructure Research at the Research Centre Juelich. Since 1987, his scientific interests and research areas included the following topics:
scanning tunneling microscopy and studies on dislocations on GaAs, InP and Si surfaces, development of anin situsputtering deposition method at high oxygen pressures for epitaxial high-T superconductor and other oxide thin films, studies on high-T superconducting quantum interference devices (SQUIDs) for magnetometers and gradiometers on the basis of ramp and grain boundary type Josephson junctions, high-T-SQUID-microscopy, development of high-T Josephson junctions suitable for detection and spectroscopy of millimeter- and submillimeter-wave radiation, deposition and study of epitaxial heterostructures of ferroelectric and magnetic oxides.
Dr. Poppe is a member of the German Physics Society (DPG).
Vladimir N. Gubankov was born in Moscow, USSR, in 1941. He graduated in physics from Moscow State University, Moscow, in 1964.
He joined the Institute of Radio Engineering and Electronics of Russian Academy of Sciences (IRE RAS), Moscow. Since that time he has been working in this Institute, now as Head of the Department of Physical Electronics (1986) and Professor (1985). At the same time (since 1988), he has been a Professor of Physics at the Moscow Institute of Physics and Tech- nology. He is known as a specialist in the field of cryogenic radiophysics, superconducting electronics, and weak link supercon- ductivity.
Prof. Gubankov is a member of the Russian National Committee of URSI.
Knut Urbanwas born in Stuttgart, Germany, in 1941. He studied physics at the Technical University of Stuttgart, Stuttgart, Germany, where he also received the Doctoral degree in 1972.
He then joined the Max-Planck-Institute for Metals Research, Stuttgart, and became Head of the High- Voltage Electron Microscopy Group in 1975. From 1980 to 1981, and in 1982, he was Guest Scientist at the Metallurgy Section of Saclay Nuclear Research Center, Paris, France, and at the Bhabha Atomic Re- search Centre Mumbai, Bombay, India. In 1986, he
became Professor for Materials Sciences at the University of Erlangen-Nurem- berg. Since 1987, he is holding a Chair in Experimental Physics at RWTH Aachen Technical University and is Director of the Department of Microstruc- ture Research at the Institute of Solid State Research at Research Centre Juelich.
In 1997, he was a JSPS Fellow at Tohoku University, Sendai, Japan. In 2003, he founded the Jülich/AachenErnst Ruska Centerfor Microscopy and Spec- troscopy with Electrons as a German national user facility for atomic resolu- tion transmission electron microscopy and becoming one of the two Directors of this Center. Since 2005, he has been the Scientific Coordinator of the EC Network of Excellence “Complex Metallic Alloys.” His research interests are broad: physical properties of complex metallic alloys and quasicrystals, defects in perovskitic oxides, Josephson effects in high-temperature superconductors and their application to SQUID-systems, and Hilbert transform spectroscopy.
In 1991, together with colleagues from EMBL Heidelberg and Darmstadt Tech- nical University, he started the VW-Foundation funded project, which let to the first spherical-aberration corrected transmission electron microscope in 1997.
Since that time, he has pioneered the application of this technique developing a number of novel approaches to ultra-high-resolution in the sub-Angstrom range.
Dr. Urban, who has received a number of national and international awards, was Chairman of the Physics Section of the Society of German Natural Scien- tists and Physicians, and from 2004 to 2006 President of the German Physical Society.
