Ф Е Д Е Р АЛ Ь Н О Е АГ Е Н Т С Т В О П О О Б Р АЗО В АН И Ю В О Р О Н Е Ж С К И Й Г О С У Д АР С Т В Е Н Н Ы Й У Н И В Е Р С И Т Е Т
У ЧЕ Б Н О -М Е Т О Д И ЧЕ С К О Е П О С О Б И Е П О ЧТ Е Н И Ю С П Е ЦИ АЛ Ь Н О Й Л И Т Е Р АТ У Р Ы Д Л Я С ТУ Д Е Н ТО В 2 К У РС А Ф И ЗИ Ч Е С К О Г О Ф А К У Л Ь ТЕ ТА по специальн остям 010701, 010801,010803 (013800, 010400,014100) Г С Э.Ф .1.
В О РО Н Е Ж 2005
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У твержден о н аучн о-методическим советом физического факультета П ротокол № 3 от 17.03.05
С оставители: В ороб жан скаяТ.В . Д роздова И .В . И льичева Н .А .
П особ ие подготовлен о н а кафедре ан глийского язы ка факультета роман о-герман ской филологии В орон ежского государствен н ого ун иверситета. Рекомен дуетсядлястуден товвторого курса физического факультета.
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Unit I Quantum Physics Lead-in I. Use your background knowledge and agree or disagree with the statement: “ Quantum theory is the most precisely tested and most successful theory in the history of science”. II. Make a list of the most prominent scientific discoveries of the 20th century. Exchange the ideas with other students. Reading III. Compare your list of names with the one given in the lead (the opening paragraph of the article). Sum up the information. IV. Use the adjectives with prefix re – (reshape, rethink, revise… ) to describe “ scientific revolution” brought about by “ quantum mechanics”. V. Read the second paragraph and complete the chart Quantum mechanics photonics revolution
computer age
Advances
Use the chart to speak of the transformations the creation of quantum physics brought about. VI. Skim the article to trace the main steps leading to creation of “ quantum mechanics”. Fill in the fable. the name Planck Einstein
the year
the discovery
VII. Cover the rest of the text and jot down the shortcomings of “ prequantum physics” and the achievements of quantum mechanics.
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VIII. Choose the synonym and find in the text the sentence where the underlined word is used. Translate the sentence, consult the dictionary (There are words which have more than one synonym). 1. profound adj. A. proficient B. deep C. intense D. complete
5. stunning adj. A. attractive B. stupid C. immense D. shocking
9. seminal adj. A. similar B. pertaining to “ seminar” C. semicircular D. trail - blazing
2. tumultuous adj. A. noisy B. disorderly C. impressive D. essential
6. bizarre adj. A. noisy B. strange C. bionic D. biophysical
10. current adj. A. contemporary B. considerable C. visible D. flowing
3. impact n A. concept B. strong influence C. benefit D. effect
7. discrete A. separate B. discreet C. diverse D. distinct
11. genius A. power B. gene C. genesis D. talent (less strong)
4. spectacular adj. A. clear B. grand C. specific D. impressive
8. unravel v A. unveil B. become separate C. unprovoke D. replace
12. confuse v A. confront B. conclude C. mix up D. neglect
IX. Sum up the whole article using all your notes and charts. Quantum Physics An informed list of the most profound scientific developments of the 20th century is likely to include general relativity, quantum mechanics, big bang cosmology, the unraveling of the genetic code, evolutionary biology, and perhaps a few other topics of the reader’s choice. Among these, quantum mechanics is unique because of its profoundly radical quality. Quantum mechanics forced physicists to reshape their ideas of reality, to rethink the nature of things at the deepest level, and to revise their concepts of position and speed, as well as their notions of cause and effect. Although quantum mechanics was created to describe an abstract atomic world far removed from daily experience, its impact on our daily lives could hardly be greater. The spectacular advances in chemistry, biology, and medicine – and in essentially every other science – could not have occurred without the tools that quantum mechanics made possible. Without quantum mechanics there would be no
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global economy to speak of, because the electronics revolution that brought us the computer age is a child of quantum mechanics. So is the photonics revolution that brought us the Information Age. The creation of quantum physics has transformed our world, bringing with it all the benefits – and the risks – of a scientific revolution. Unlike general relativity, which grew out of a brilliant insight into the connection between gravity and geometry, or the deciphering of DNA, which unveiled a new world of biology, quantum mechanics did not spring from a single step. Rather, it was created in one of those rare concentrations of genius that occur from time to time in history. For 20 years after their introduction, quantum ideas were so confused that there was little basis for progress; then a small group of physicists created quantum mechanics in three tumultuous years. These scientists were troubled by what they were doing and in some cases distressed by what they had done. In his seminal paper on thermal radiation, Planck hypothesized that the total energy of a vibrating system cannot be changed continuously. Instead, the energy must jump from one value to another in discrete steps, or quanta of energy. The idea of energy quanta was so radical that Planck let it lay fallow. Then, Einstein, in his wonder year of 1905, recognized the implications of quantization for light. Even then the concept was so bizarre that there was little basis for progress. Twenty more years and a fresh generation of physicists were required to create modern quantum theory. To understand the revolutionary impact of quantum physics one needs only look at prequantum physics. From 1890 to 1900, physics journals were filled with papers on atomic spectra and essentially every other measurable property of matter, such as viscosity, elasticity, electrical and thermal conductivity, coefficients of expansion, indices of refraction, and thermoelastic coefficients. What is most striking to the contemporary eye, however, is that the descriptions of the properties of matter were essentially empirical. Thousands of pages of spectral data listed precise values for the wavelengths of the elements, but nobody knew why spectral lines occurred, much less what information they conveyed. Thermal and electrical conductivities were interpreted by suggestive models that fitted roughly half of the facts. There were numerous empirical laws, but they were not satisfying. For instance, the Dulong-Petit law established a simple relation between specific heat and the atomic weight of a material. Much of the time it worked; sometimes it didn’t. The masses of equal volumes of gas were in the ratios of integers – mostly. The Periodic Table, which provided a key organizing principle for the science of chemistry, had absolutely no theoretical basis. Among the greatest achievements of the revolution is this: Quantum mechanics has provided a quantitative theory of matter. We now understand essentially every detail of atomic structure; the Periodic Table has a simple and natural explanation; and spectral data fit into an elegant theoretical framework. Quantum theory permits the quantitative understanding of molecules, of solids and liquids, and of conductors and semiconductors. It explains bizarre phenomena such as superconductivity and superfluidity, and exotic forms of matter such as the stuff of neutron stars and Bose-Einstein condensates, in which all the atoms in a gas
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behave like a single superatom. Quantum mechanics provides essential tools for all of the sciences and for every advanced technology. Comprehension check Read the text in detail, say whether these statements are true or false. If they are false make your own true ones. T/F 1. The age of classical natural science differs from the contemporary one in that it tried to find the most simple answers to the no less complicated questions. T/F 2. In 1900 Max Planck started the quantum – mechanical snowball. T/F 3. Quantum mechanics was created on the basis of a radical renunciation of old principles. T/F 4. Bohr’s quantum postulates led to Plank’s radiation formula. T/F 5. Einstein created modern quantum theory. T/F 6. The Periodic Table has no theoretical basis. T/F 7. Quantum mechanics was created to describe an abstract atomic world. It has nothing to do with our daily lives. Discussion I. Read the passage and say why the “ string like” nature of elementary particles can be particularly useful in quantum theory. Traditionally we imagine elementary particles as something point like (точечн ы й об ъ ект). However, the new concept of “ strings” seems to be more relevant in this case. The strings are infinitesimally thin. Their length is about 10-35 m. They are much more smaller than the nuclei of atoms. The oscillations they produce are in fact the “ points” or “ particles” we observe. II. Render in English. Add what you know about “ the second super string revolution” (Use Internet or other sources). О б ъ един ен ие (the convergence) двух фун дамен тальн ы х теорий современ н ой физики – кван товой теории (quantum theory) и об щ ей теории отн осительн ости (general relativity) – в рамках (within the framework of) един ого теоретического подхода б ы ло одн ой изважн ейш их проб лем. Э ти две теории, взяты е вместе, воплощ ают почти всю сумму человеческих зн ан ий о н аиб олее фун дамен тальн ы х взаимодействиях в природе. Больш ой загадкой б ы ла н есовместимость (discrepancies between) этих двух теорий. Бы ло н епон ятн о, почему природа н а своем самом глуб оком и фун дамен тальн ом уровн е должн а треб овать двух различн ы х подходов с двумя н аб орами математических методов, двух н аб оров постулатов и двух н аб оров физических закон ов? В идеале (ideally) хотелось иметь Е дин ую теорию поля, об ъ един яющ ую эти двефун дамен тальн ы е теории. О б ъ един итьдвеэти теории удалось лиш ь в рамках теории струн и суперструн (“ the second super string revolution”). III. Discuss in teams of students the following highly disputable item. This is what Frank Wilczek, American Professor of Physics, thinks about the concept of “ force” (“ Physics Today”).
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Newton’s second law of motion, F=ma, is the soul of classical mechanics. Like other souls, it is insubstantial. The right-hand side is the product of two terms with profound meanings. Acceleration is a purely kinematical concept, defined in terms of space and time. Mass quite directly reflects basic measurable properties of bodies. The left-hand side, on the other hand, has no independent meaning. Yet clearly Newton’s second law is full of meaning, by the highest standard: It proves itself useful in demanding situations. Splendid, unlikely looking bridges, like the Erasmus Bridge (known as the Swan of Rotterdam), do bear their loads; spacecraft do reach Saturn. In fact, the concept of force is absent from our most advanced formulations of the basic laws. It doesn’t appear in Schrodinger’s equation, or in any reasonable formulation of quantum field theory, or in the foundations of general relativity. Bertrand Russell in his “ The ABC of Relativity” wrote the following: “ If people were to learn to conceive the world in the new way, without the old notion of “ force”, it would alter not only their physical imagination, but probably also their morals and politics… In the Newtonian theory of the solar system, the sun seems like a monarch whom the planets have to obey. In the Einsteinian world there is more individualism and less government than in the Newtonian”. The 14th chapter of Russell’s book is entitled “ The Abolition of Force”. For the behavior of matter we now have extremely complete and accurate laws that in principle cover the range of phenomena addressed in classical mechanics. Quantum electrodynamics (QED) and quantum chromodynamics (QCD) provide the basic laws which serve at least equally well and arguably better. The fact that changes in momentum-which correspond, by definition, to forces- are visible, whereas changes in energy often are not, is certainly a major factor. Another is that, as active participants in statics-for example, when we hold up a weight-we definitely feel we are doing something, even though no mechanical work is performed. Force is an abstraction of this sensory experience of exertion. A big part of the explanation for the continued use of the concept “ force” is no doubt (intellectual) inertia. Letters and opinions are encouraged and should be sent to Letters, Physics Today, American Center for Physics, One Physics Ellipse, College Park, MD 20740-3842 or by e-mail to
[email protected] (using your surname as “ Subject”). Unit II Pathways of Discovery Lead-in I. Quantum physics actually encompasses two entities. What are they? II. What role in science does the quantum theory of fields play? III. Use your background knowledge and say whether these statements are true or false. T/F 1. Quantum mechanics is the theory of matter at the atomic level. T/F 2. Quantum field theory reconciles quantum mechanics with special relativity.
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T/F 3. Created at the beginning of the 20th century quantum theory of light is a “ generalization” of the wave theory, which encompasses both corpuscular and wave ideas. T/F 4. The rise of the theory of relativity led to the breakdown both of Newtonian mechanics and Maxwell – Lorentz’s electrodynamics. T/F 5. The rise of relativistic quantum mechanics is a sort of “ generalization” of nonrelativistic theory. Reading IV. The text entitled “ Pathways of Discovery” demonstrates how “ the complementarity principle”, developed by Niels Bohr and his pupils, works. Bohr highlighted the scientific endeavours to utilize the concepts of classical theory extending them beyond the framework of the factual material on which they were built up. Skim through the text to trace the “ unity” of the “ historical” and “ logical” in the evolution of knowledge. Check your answers (True/false statements, ex. III). V. Read the text more closely to make a chart of the steps to “ quantum theory”. Concisely describe contributions the scientists made. names eg. Bohr
contributions Quantitative description of the spectrum of the hydrogen atom
VI. Choose the synonym. Use the dictionary in case you need it. Find in the text the sentences where the underlined words are used. Translate them. 1. complementarity n. A. correspondence B. together making up a whole C. competence D. sufficiency
4. reluctantly adv. A. readily B. easily C. willingly D. unwillingly
7. precursor n. A. author B. protagonist C. president D. forerunner
2. desperation n. A. alarm B. reconcilement C. surprise D. recklessness
5. imbue v. A. emit B. fill C. imitate D. permeate (with opinion)
8. conundrum n. A. connection B. correspondence C. riddle D. complementarity
3. novice n. A. inventor B. innovator C. beginner D. pupil
6. incompatible adj. A. not compact B. incompetent C. incoherent D. about things which cannot coexist
9. indistinguishable adj. A. incapable B. incomplete C. incomprehensible D. unrecognizable
VII. Use the chart you have made to talk on the foundations of quantum physics.
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Pathways of Discovery The clue that triggered the quantum revolution came not from studies of matter but from a problem in radiation. The specific challenge was to understand the spectrum of light emitted by hot bodies: blackbody radiation. The phenomenon is familiar to anyone who has stared at a fire. Hot matter glows, and the hotter it becomes the brighter it glows. The spectrum of the light is broad, with a peak that shifts from red to yellow and finally to blue (although we cannot see that) as the temperature is raised. It should have been possible to understand the shape of the spectrum by combining concepts from thermodynamics and electromagnetic theory, but all attempts failed. However, by assuming that the energies of the vibrating electrons that radiate the light are quantized, Planck obtained an expression that agreed beautifully with experiment. But as he recognized all too well, the theory was physically absurd, “ an act of desperation”, as he later described it. Planck applied his quantum hypothesis to the energy of the vibrators in the walls of a radiating body. Quantum physics might have ended there if in 1905 a novice-Albert Einstein-had not reluctantly concluded that if a vibrator’s energy is quantized, then energy of the electromagnetic field that it radiates-light-must also be quantized. Einstein thus imbued light with particlelike behavior, notwithstanding that James Clerk Maxwell’s theory, and over a century of experiments, testified to light’s wave nature. Experiments on the photoelectric effect in the following decade revealed that when light is absorbed its energy actually arrives in discrete bundles, as if carried by a particle. The dual nature of light-particlelike or wavelike depending on what one looks for-was the first example of a theme that would recur throughout quantum physics. The duality constituted a theoretical conundrum for the next 20 years. The first step toward quantum theory had been precipitated by a dilemma about radiation. The second step was precipitated by a dilemma about matter. It was known that atoms contain positively and negatively charged particles. But oppositely charged particles attract. According to electromagnetic theory, therefore, they should spiral into each other, radiating light in a broad spectrum until they collapse. Once again, the door to progress was opened by a novice: Niels Bohr. In 1913, Bohr proposed a radical hypothesis: Electrons in an atom exist only in certain stationary states, including a ground state. Electrons change their energy by “ jumping” between the stationary states, emitting light whose wavelength depends on the energy difference. By combining known laws with bizarre assumptions about quantum behavior, Bohr swept away the problem of atomic stability. Bohr’s theory was full of contradictions, but it provided a quantitative description of the spectrum of the hydrogen atom. He recognized both the success and the shortcomings of his model. His vision was eventually fulfilled, although it took 12 years and a new generation of young physicists.
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At first, attempts to advance Bohr’s quantum ideas-the so-called old quantum theory-suffered one defeat after another. Then a series of developments totally changed the course of thinking. In 1923 Louis de Broglie, in his Ph.D. thesis, proposed that the particle behavior of light should have its counterpart in the wave behavior of particles. He associated a wavelength with the momentum of a particle: The higher the momentum the shorter the wavelength. The idea was intriguing, but no one knew what a particle’s wave nature might signify or how it related to atomic structure. Nevertheless, de Broglie’s hypothesis was an important precursor for events soon to take place. In the summer of 1924, there was yet another precursor. Satyendra N. Bose proposed a totally new way to explain the Planck radiation law. He treated light as if it were a gas of massless particles (now called photons) that do not obey the classical laws of Boltzmann statistics but behave according to a new type of statistics based on particles’ indistinguishable nature. Einstein immediately applied Bose’s reasoning to a real gas of massive particles and obtained a new law-to become known as the Bose-Einstein distribution-for how energy is shared by the particles in a gas. Under normal circumstances, however, the new and old theories predicted the same behavior for atoms in a gas. Einstein took no further interest, and the result lay undeveloped for more than a decade. Still, its key idea, the indistinguishability of particles, was about to become critically important. Suddenly, a tumultuous series of events occurred, culminating in a scientific revolution. In the 3-year period from January 1925 to January 1928: - Wolfgang Pauli proposed the exclusion principle, providing a theoretical basis for the Periodic Table. - Werner Heisenberg, with Max Born and Pascual Jordan, discovered matrix mechanics, the first version of quantum mechanics. The historical goal of understanding electron motion within atoms was abandoned in favor of a systematic method for organizing observable spectral lines. - Erwin Schrö dinger invented wave mechanics, a second form of quantum mechanics in which the state of a system is described by a wave function, the solution to Schrö dinger’s equation. Matrix mechanics and wave mechanics, apparently incompatible, were shown to be equivalent. - Electrons were shown to obey a new type of statistical law, Fermi-Dirac statistics. It was recognized that all particles obey either Fermi-Dirac statistics or Bose-Einstein statistics, and that the two classes have fundamentally different properties. - Heisenberg enunciated the Uncertainty Principle. - Paul A. M. Dirac developed a relativistic wave equation for the electron that explained electron spin and predicted antimatter. - Dirac laid the foundations of quantum field theory by providing a quantum description of the electromagnetic field.
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- Bohr announced the complementarity principle, a philosophical principle that helped to resolve apparent paradoxes of quantum theory, particularly waveparticle duality. Comprehension check Read the text again. Use your background knowledge and answer the questions. 1. Did quantum mechanics emerge essentially complete? Was it created in a short flurry of activity? 2. Who introduced the notion “ energy quanta”? 3. Does quantum theory owe its origin to any failure of classical physics or rather to Planck’s profound insight? 4. Did Bohr’s theory lead to the correct explanation of the spectral laws of hydrogen? 5. Did Einstein show that Bohr’s quantum postulates lead to Planck’s Radiation formula? 6. What did Lowis de Broglie in his Ph. D. thesis propose? 7. The probability of finding two electrons in the same state is zero. Who made this discovery? What is it called like? 8. One can calculate the possible values of every observable quantity from the wave functions. Who made it possible? 9. What is the Principle called like, according to which the concept of an electron having a particular location and a particular momentum loses its foundation? 10. Who laid the foundations of quantum field theory? Discussion I. Read the following anecdote. Fill in the gaps with the words from the list: advance; objection; spin; proposal; the possibility; traveled; skeptical; proposed. In 1925, the concept of electron spin had been … by Samuel Goudsmit and George Uhlenbeck. Bohr was deeply … . In December, he … to Leiden, the Netherlands, to attend the jubilee of Hendrik A. Lorentz’s doctorate. Pauli met the train at Hamburg, Germany, to find out Bohr’s opinion about the … of electron spin. Bohr said the … was “ very, very interesting”, his well-known put-down phrase. Later at Leiden, Einstein and Paul Ehrenfest met Bohr’s train, also to discuss … . There, Bohr explained his … but Einstein showed a way around it and converted Bohr into a supporter. On his return journey, Bohr met with yet more discussants. When the train passed through Göttingen, Germany, Heisenberg and Jordan were waiting at the station to ask his opinion. And at the Berlin station, Pauli was waiting, having traveled especially from Hamburg. Bohr told them all that the discovery of electron spin was a great … . Comment on the fierce debates which normally take place on the interpretation and validity of discoveries made in science. Can you give more examples?
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II. What sounds like black magic is actually incredibly hard physics. With the new branch of science, eg the so-called “ Loop quantum gravity” (a unified theory of gravity and the quantum world), physicists have come close to answer the ultimate questions of physics: the mysteries of the big bang and black holes. A. Translate the following passage into Russian. Sum it up in English and say whether these ideas are new to you. Space and time are not at all fundamental, but rather built up from more basic structures, spin networks. This concept was coined by Penrose, who already in the 1970s had formulated his Twistor theory with similar motivations, and introduced spin networks as a kind of spacetime dust. Ashtekar compares the spin networks – mathematically known as graphs – to a fabric of polymer-like one-dimensional threads. If one could observe nature with maximum possible enlargement, space and time would dissolve and the granular mesh of the spin network would come to light – or more precisely: the quantum physical superposition of all possible configuration of these entities. There is ‘nothing’ between these graphs. Those entities rest only on themselves, so to speak. “ The spin networks do not exist in the space. Their structures produce the space”. “ And they are nothing but abstractly defined relations – which determine how the edges come together and interlock at the joints”. That space appears nevertheless homogeneous to us is no miracle. For, the resolution of our perception is limited, similar to observing a photograph whose individual pixels we cannot recognize from a distance. Believe it or not, 1068 quantum threads intersect the page of an ordinary physics journal. The endpoints of the graphs (i.e., open graphs) represent fermions (quarks and leptons), out of which matter is made, and the presumed Higgs boson that gives them mass. The bosons (such as photons and also gravitons), which transmit the subatomic interactions and thus evoke the forces of nature, are encoded in certain excited states of the spin network as changing colors or labels on the graphs. Our usual notion of causality does not apply here either. Even time only emerges from the variations of the excited states and the links in the networks. In a certain way time is an illusion just as space is. Ashtekar quotes Vladimir Nabokov: “ Space is a swarming in the eyes, and Time a singing in the ears”. The entire realm of reality therefore originates from the superposition of fluctuating weaves on a submicroscopic level. We, and everything we know, are like patterns or embroideries in the fabric of the spin network. B. Render the text in English. Express your own attitude towards the theory. С тру кту рнаякосмология Н аш е современ н ое представлен ие о В селен н ой и ее происхожден ии зависит н е только от фун дамен тальн ы х закон ов физики, н о и от н ачальн ы х условий во времен а Больш ого взры ва. И н огда пы таются представить этот момен т истории как взры в космической б омб ы , порождающ ей материю в уже сущ ествующ ей В селен н ой. О дн ако этот об раз н еправильн ы й. В едь когда взры вается б омб а, он а взры вается в определен н ом месте простран ства и в
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определен н ы й момен т времен и. Е е содержимое просто разлетается в разн ы е сторон ы . Больш ой взры в представляет соб ой порожден ие самого простран ства. В момен т Больш ого взры ва н еб ы ло н икакого простран ства вн е об ласти взры ва. И ли, если б ы ть точн ы м, ещ е н е б ы ло н аш его простран ства, возн икавш его как раз в процессе взры ва и ин фляцион н ого расш ирен ия. С огласн о « теории струн », в н ачальн ы й момен т сущ ествован ия В селен н ой все ее простран ствен н ы е измерен ия равн оправн ы и сверн уты в мн огомерн ы й клуб ок план ковского размера. И только в ходе ин фляции и Больш ого взры ва часть измерен ий освоб ождается изоков суперструн и разворачивается в н аш е огромн ое4-мерн оепростран ство – время. III. Debate the following items: a) So far string theory does not really agree with the world we see. It requires many complicated assumptions such as extra dimentions and super symmetry, for which there are no empirical clues. All the main problems are unsolved. It is time scientists tried something else. New approaches are welcome. b) As the French author Marcel Proust put it, “ The best discoveries are not made in new landscapes, but rather when one looks at the world with new eyes”. c) Looking to the next century, quantum theory will continue to provide fundamental concepts and essential tools for all of the sciences. Perhaps string theory – a generalization of quantum field theory – will solve the riddle of the Universe. d) The dream of ultimate understanding will continue to be a driving force for new knowledge, as it has been since the dawn of science. The routes of history are indeed unpredictable. Roleplay the debate. Unit III The History of Transistors I. Make a list of topics that you think will occur in the article entitled “ The History of Transistors”. II. Find these terms in the word puzzle: transistor, device, density, insulator, layer, voltage, channel. d e n s i t y d
t o s p n o c a
r q u u s w s v
a d l s u c o l
n e m c l p n a
s v t h a x h y
i i o a t e a e
s c k n o r f r
t e s n r d k o
o r y e t c s h
r v o l t a g e
h i z m f y a u
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Complete the sentences, using these terms: 1. A single – electron … exploits the fact that the transfer of charge through the barrier becomes quantized when the junction is made sufficiently resistive. 2. A second … of the metal shifted from the first by rotating the sample is evaporated to form the island. 3. Most early… had Coulomb energies of a few hundred microelectronvolts. 4. The difference is that electrons in the … behave as a compressible fluid with a local … that depends strongly on the electric potential at that point. 5. But as the … is increased, the electric field at the gate attracts electrons from the source and drain. 6. In contrast, the density of electrons is controlled by the gate voltage. III. The article is divided into several main parts; match each part with the most suitable heading from the following list: a) Towards single – electron devices. b) Operation of a SET transistor. c) The invention of the transistor d) A single – electron transistor. e) At the beginning. f) Perspectives on the future. g) Towards room temperature. IV. Sum up the text in 7-10 sentences according to headings given above. The History of Transistors 1. The invention of the transistor by John Bardeen and William Shockley in 1948 triggered a new era in electronics. Originally designed simply to emulate the vacuum tube, scientists soon found that this solid-state device could offer much more. The great potential of the transistor for speed, miniaturization and reliability has been fully exploited since well controlled materials such as pure single-crystal silicon became available. According to the latest “ road-map” for the microelectronics industry, microchips containing one billion transistors and operating with a clock cycle of a billionth of a second will be on the market just a few years into the new millennium. As transistors continue to shrink, a question naturally arises: will the quantum nature of electrons and atoms become important in determining how the devices are built? In other words, what will happen when a transistor is reduced to the size of a few atoms or a single molecule? 2. Two conducting electrodes, called the source and drain, are connected together by a channel of material in which the density of free electrons can be varied in practice. A voltage is applied to the semiconducting channel through the “ gate”, a third electrode that is separated from the channel by a thin insulating layer. When the gate voltage is zero, the channel does not contain any conduction
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electrons and is insulating. But as the voltage is increased, the electric field at the gate attracts electrons from the source and drain, and the channel becomes conducting. This field effect leads to an amplification mechanism in which the gate voltage can control the current flowing between the source and drain when a bias voltage is applied across these two electrodes. The sourcedrain current is determined by the conductance of the channel, which in turn depends on two factors: the density of the conduction electrons and their mobility. The mobility of electrons depends on how often the electrons collide with crystal imperfections, and is essentially independent of the gate voltage. In contrast, the density of electrons is controlled directly by the gate voltage. The transistor therefore works like a tap controlling the flow of water between two tanks, where the opening of the tap is set by the pressure of the water in a third tank. The difference is that electrons in the channel behave as a compressible fluid with a local density that depends strongly on the electric potential at that point. In other words, the electric field produced by the gate does not generate a “ hard wall” for electrons inside the channel, but a smoothly varying potential that is modified by the presence of electrons. 3. In 1985 Dmitri Averin and Konstantin Likhared, then working at the University of Moscow, proposed the idea of a new three-terminal device called a single-electron tunneling (SET) transistor. Two years later Theodore Fulton and Gerald Dolan at Bell Labs in the US fabricated such a device and demonstrated how it operates. Unlike field-effect transistors, single-electron devices are based on an intrinsically quantum phenomenon: the tunnel effect. This is observed when two metallic electrodes are separated by an insulating barrier about 1 nm thick, in other words, just 10 atoms in a row. Electrons at the Fermi energy can “ tunnel” through the insulator, even though in classical terms their energy would be too low to overcome the potential barrier. The electrical behaviour of the tunnel junction depends on how effectively the barrier transmits electron waves, which decreases exponentially with its thickness, and on the number of electron-wave modes that impinge on the barrier, which is given by the area of the tunnel junction divided by the square of the electron wavelength. A single-electron transistor exploits the fact that the transfer of charge through the barrier becomes quantized when the junction is made sufficiently resistive. 4. The SET transistor can be viewed as an electron box that has two separate junctions for the entrance and exit of single electrons. It can also be viewed as a field-effect transistor in which the channel is replaced by two tunnel junctions forming a metallic island. The voltage applied to the gate electrode affects the amount of energy needed to change the number of electrons on the island.
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The SET transistor comes in two versions that have been nicknamed “ metallic” and “ semiconducting”. These names are slightly misleading, however, since the principle of both devices is based on the use of insulating tunnel barriers to separate conducting electrodes. In the original metallic version fabricated by Fulton and Dolan, a metallic material such as a thin aluminium film is used to make all of the electrodes. The metal is first evaporated through a shadow mask to form the source, drain and gate electrodes. The tunnel junctions are then formed by introducing oxygen into the chamber so that the metal becomes coated by a thin layer of its natural oxide. Finally, a second layer of the metal shifted from the first by rotating the sample is evaporated to form the island. In the semiconducting versions, the source, drain and island are usually obtained by “ cutting” regions in a two-dimensional electron gas formed at the interface between two layers of semiconductors such as gallium aluminium arsenide and gallium arsenide. In this case the conducting regions are defined by metallic electrodes patterned on the top semiconducting layer. Negative voltages applied to these electrodes deplete the electron gas just beneath them, and the depleted regions can be made sufficiently narrow to allow tunneling between the source, island and drain. Moreover, the electrode that shapes the island can be used as the gate electrode. 5. So how does a SET transistor work? The key point is that charge passes through the island in quantized units. For an electron to hop onto the island, its energy must equal the Coulomb energy e2/2C. When both the gate and bias voltages are zero, electrons do not have enough energy to enter the island and current does not flow. As the bias voltage between the source and drain is increased, an electron can pass through the island when the energy in the system reaches the Coulomb energy. This effect is known as the Coulomb blockade, and the critical voltage needed to transfer an electron onto the island, equal to e/C, is called the Coulomb gap voltage. 6. Until recently single-electron transistors had to be kept at temperatures of a few hundred millikelvin to maintain the thermal energy of the electrons below the Coulomb energy of the device. Most early devices had Coulomb energies of a few hundred microelectronvolts because they were fabricated using conventional electron-beam lithography, and the size and capacitance of the island were relatively large. For a SET transistor to work at room temperature, the capacitance of the island must be less than 1017 F and therefore its size must be smaller than 10 nm. 7. Researchers have long considered whether SET transistors could be used for digital electronics. Although the current varies periodically with gate voltage in
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contrast to the threshold behaviour of the field-effect transistor a SET could still form a compact and efficient memory device. However, even the latest SET transistors suffer from ‘offset charges’, which means that the gate voltage needed to achieve maximum current varies randomly from device to device. Such fluctuations make it impossible to build complex circuits. Comprehension check Reading for details. True or false? T/F 1. A voltage is applied to the semiconducting channel through the drain. T/F 2. Conductance of the channel depends on two factors the density of the conduction electrons and their mobility. T/F 3. Single-electron devices are based on an intrinsically quantum phenomenon: the channel effect. T/F 4. A single electron transistor exploits the fact that the transfer of charge through the barrier becomes quantized when the junction is made sufficiently resistive. T/F 5. The single-electron transistor can be viewed as a field-effect transistor in which the channel is replaced by three tunnel junctions forming a metallic island. T/F 6. A metallic material such as a thin silver film is used to make all the electrodes. T/F 7. The electrode that shapes the island can be used as the source electrode. T/F 8. For an electron to hop onto the island its energy must equal the Coulomb energy e2/2C. T/F 9. For a SET transistor to work at room temperature, the capacitance of the island must be more than 1017 F. T/F 10. The latest SET transistors don’t suffer from “ offset charges”. Discussion 1. Distinguish between a diode and a transistor. 2. Explain what is meant by an integrated circuit and a chip, and give some examples of applications. Unit IV Photonic crystals I. What vocabulary items can you expect based on the title? Optical fiber, impurity, diamond-lattice structure… - continue the list. II. Make up sentences using the following word combinations: a) / in a diamond-lattice structure / for example / the atoms are arranged / in a silicon crystal. b) / in an energy range / for pure and perfect silicon crystals / called the band gap / no electrons can be found /. c) / to create energy levels / it is possible / by changing the size of a few of the air holes in the material / in the photonic band gap. d) a material is opal / example of such / A naturally occurring.
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III. Match the sentence beginnings on the left with the sentence endings on the right. 1. These materials have highly a) because it is more favourable for a photonic periodic structures that band gap to form in dielectrics with a high refractive index 2. The situation is different for real materials:
b) to find suitable materials and processing techniques to fabricate special structures
3. The main challenge has been
c) electrons can have an energy within the band gap if the periodicity of the lattice is broken by a missing silicon atom
4. The problem in making d) can be designed to control and manipulate small structures is the propagation of light compounded IV. Skim through the text consisting of two main parts. Choose two suitable headings from the list: Silicon Photonics Basics of photonic band gaps Fibre Lasers Optical Communications V. Sum up the introductory part; the first part and the second one. VI. Write down an abstract of the whole of the article. Photonic crystals Artificial structures with the optical equivalent of the energy gap in semiconductors promise a wealth of new devices that could satisfy the demand for ever-faster computers and optical communications. We can now buy personal computers that operate at 1 GHz (109 Hz), which is very impressive, but what is the likelihood of a 100 GHz desktop computer appearing on the market? Indeed, given our current understanding of semiconductor technology, even producing a 10 GHz personal computer would seem to be difficult. However, by transmitting signals with light rather than electrons, it might be possible to build a computer that operates at hundreds of terahertz (1012 Hz). Researchers now believe that such an awesome processing engine could be built from optical components made from so-called photonic crystals and quasicrystals. These materials have highly periodic structures that can be designed to control and manipulate the propagation of light.
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Part I
The easiest way to understand the behaviour of light in a photonic crystal is to compare it to the movement of electrons and holes in a semiconductor. In a silicon crystal, for example, the atoms are arranged in a diamond-lattice structure, and electrons moving through this lattice experience a periodic potential as they interact with the silicon nuclei via the Coulomb force. This interaction results in the formation or allowed and forbidden energy states. For pure and perfect silicon crystals, no electrons will be found in an energy range called the forbidden energy gap or simply the band gap. However, the situation is different for real materials: electrons can have an energy within the band gap if the periodicity of the lattice is broken by a missing silicon atom or by an impurity atom occupying a silicon site, or if the material contains interstitial impurities (additional atoms located at non-lattice sites). Now consider photons moving through a block of transparent dielectric material that contains a number of tiny air holes arranged in a lattice pattern. The photons will pass through regions of high refractive index – the dielectric – interspersed with regions of low refractive index – the air holes. To a photon, this contrast in refractive index looks just like the periodic potential that an electron experiences travelling through a silicon crystal. Indeed, if there is large contrast in refractive index between the two regions then most of the light will be confined either within the dielectric material or the air holes. This confinement results in the formation of allowed energy regions separated by a forbidden region – the so-called photonic band gap. Since the wavelength of the photons is inversely proportional to their energy, the patterned dielectric material will block light with wavelengths in the photonic band gap, while allowing other wavelengths to pass freely. It is possible to create energy levels in the photonic band gap by changing the size of a few of the air holes in the material. This is the photonic equivalent to breaking the perfect periodicity of the silicon-crystal lattice. In this case, the diameter of the air holes is a critical parameter, together with the contrast in refractive index throughout the material. Photonic band gap structures can also be made from a lattice of highrefractive-index material embedded within a medium with a lower refractive index. A naturally occurring example of such a material is opal. However, the contrast in the refractive index in opal is rather small, which results in a rather small band gap. In spite of this success, it has taken over a decade to fabricate photonic crystals that work in the near-infrared (780-3000 nm) and visible (450-750 nm) regions of the spectrum. The main challenge has been to find suitable materials and processing techniques to fabricate structures that are about a thousandth the size of microwave crystals. A rough estimate of the spacing between the air holes (or the lattice size) is given by the wavelength of the light divided by the refractive index of the dielectric material. The problem in making small structures is compounded because it is more
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favourable for a photonic band gap to form in dielectrics with a high refractive index, which reduces the size of the lattice spacing even further. To be able to create photonic crystals for optical devices, we need to use state-of-the-art semiconductor-microfabrication techniques with their associated high production costs and investment. For this reason computer modeling of prospective photonic-crystal structures is also a very important area of research, as it may prevent expensive fabrication errors later. Part II
So why are photonic crystals generating so much interest? Again it is useful to draw analogies with silicon. The current explosion in information technology has been derived from our ability to control the flow of electrons in a semiconductor in the most intricate ways. Photonic crystals promise to give us similar control over photons – with even greater flexibility because we have far more control over the properties of photonic crystals than we do over the electronic properties of semiconductors. Given the impact that semiconductor materials have had on every sector of society, photonic crystals could play an even greater role in the 21st century, particularly in the optical-communications industry. In current communications systems, audio or video signals are encoded as streams of digital data packets. These voltage pulses are applied to a light-emitting diode (LED) or a semiconductor laser, which converts them into short pulses of light that are then sent along an optical-fibre network. Many different conversations or video signals can be transmitted using a single wavelength of light by interweaving the data packets from different sources, a technique known as timedivision multiplexing. The optical data pulses are sorted at the receiving end of the fibre, where they are converted by photodetectors into continuous analogue electrical signals that are then transmitted along copper wires. A simple way of increasing the amount of data that can be transmitted by a single optical fibre is to make the incoming electronic pulses as short as possible. Current optical systems have achieved data rates in excess of 40 gigabits per second. But to increase this value significantly will require cheap, bright LEDs that can be switched on and off at very high speed. Another way to increase the capacity of the fibre is to add new signals at other optical wavelengths, a method known as dense wavelength division multiplexing (DWDM). However, the optical fibre is only transparent over a small range of wavelengths, so the number of separate conversations that can be transmitted depends on the linewidth of neighbouring optical channels (which is currently at the sub-nanometre level). Encoding and sorting thousands of such channels poses quite a problem: at the transmitting end of the system, each wavelength channel requires a very stable light source that only emits in a very narrow range of wavelengths. Although LEDs offer high switching speeds, they emit light over a wide range of wavelengths, which makes them less suitable than lasers for DWDM systems.
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At the receiving end, very narrow linewidth filters and optical switches are required to separate individual channels and then route them to the appropriate destinations. Due to the large number of individual components in a DWDM system, it makes sense to combine as many of them as possible onto an “ integrated” optical chip. This would reduce both the amount of costly manual work that is needed to build the system and the number of possible points where it could fail. Such integration, however, raises other problems. First we need to develop small-scale optical “ interconnects” – tiny planar waveguides that can steer light round tight corners. While it is straightforward to route electrons round sharp bends in microchips, it is impossible to direct light in the same way using conventional glass waveguides because the losses at the bends are so large. Photonic crystals could address many of the problems that currently limit the speed and capacity of optical-communications networks. For example, these structures could be used to create novel LEDs and laser that emit light in a very narrow wavelength range, together with highly selective optical filters that could be integrated on a chip. Reading comprehension. Answer the questions: 1. What kind of materials should be used to control and manipulate the propagation of light? 2. What is the difference between perfect silicon crystals and real materials? 3. What happens if there is a large contrast in refractive index between the two regions. 4. What sort of techniques do engineers need to use in order to create photonic crystals for optical devices? Why? 5. Why will photonic crystals play greater role in the 21st century? 6. What is special about the optical fibre? Discussion 1. How do Polaroid sunglasses work? 2. From the equation for the index of refraction, determine the speed of light in a diamond.
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О сн овн аялитература 1. Shipman J.T. An Introduction to Physical Science / J.T. Shipman, J.D. Wilson. – Massachusetts Toronto: D.C. Heath and company, 1990. – 630 p. 2. Haughes N. North Star: Focus on Reading and writing, basic / N. Haughes, B. Maher. – NY: London, 1998. – 208 p. Д ополн ительн аялитература 1. Э лектрон н ы й каталог Н аучн ой б иб лиотеки В орон ежского государствен н ого ун иверситета. – (http://www.lib.vsu.ru/).
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С оставители ст. преп. Д роздова И .В . ст.преп. И льичева Н .А . преп. В ороб жан скаяТ.В . Рецен зен т д.ф.н ., проф. Баб уш кин А .П . Редактор Бун ин а Т.Д .
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