Micrograph of a microgel

Atomic force microscopy image of microgels used for the encapsulation of retinol. © Ricarda Schroeder

Almost half a century after the first Moon landings, the next frontier of manned spaceflight—an expedition to Mars—is capturing our attention. But before the first Mars-bound astronauts can take off, many questions must be answered, including the most basic: what will the astronauts eat?

The actual journey to Mars is estimated to take a few months, but if food is to be used throughout a whole mission, it will need to stay nutritious and palatable for up to three to five years. This is far longer than the 18- to 24-month ‘shelf life’ of most food items currently consumed in space.

Essential nutrients, such as vitamins and amino acids are particularly problematic, as they degrade in long-term storage, acquiring strange tastes and, more importantly, losing their nutritional value. Ricarda Schroeder of the European Space Research and Technology Centre has now shown that it is possible to stabilise one such nutrient, vitamin A1, by packing it into a polymer microgel. Her work is published in the journal *Life Sciences in Space Research*.

Vitamin A1, also known as retinol, is important for maintaining healthy skin and blood and good vision. It is relatively unstable and can be easily broken down by oxygen, heat or ultraviolet light to form compounds that are either inactive or, worse still, toxic. Schroeder’s work has the potential to solve this problem; she found that retinol increases its stability when it is absorbed by polymer microgels.

These microgels consist of porous, colloidal particles with a crosslinked polymer network structure; the size of the pores decreases as the number of crosslinks rises. Schroeder prepared lightly-crosslinked microgels, diffused retinol into them, and then added further crosslinks, reducing the amount of space available for the bound retinol and thus reducing its opportunity to degrade. “The retinol bound into these microgel particles could be almost 100 times as chemically stable as unbound retinol,” she explains. “Vitamin A stored in this way should, therefore, remain intact for at least the length of a mission to Mars, and the method could in principle be used to stabilise other vitamins and nutrients.”

Schroeder started her career as a chemist and studied microgels during her PhD research, but she has always been interested in space and was delighted to be offered a chance to combine the two fields. “It was fascinating to apply my chemical knowledge to the requirements for nutrition in long-term space travel,” she adds. “But this research also has potential applications on Earth, for example, in storing nutrients for long periods under harsh, desert-like conditions.”

Visit elsevier.com to access content in physics, astronomy and more! Use discount code **STC317** at checkout and save up to 30%!

]]>

In early July, Google announced that it will expand its commercially available cloud computing services to include quantum computing. A similar service has been available from IBM since May. These aren’t services most regular people will have a lot of reason to use yet. But making quantum computers more accessible will help government, academic and corporate research groups around the world continue their study of the capabilities of quantum computing.

Understanding how these systems work requires exploring a different area of physics than most people are familiar with. From everyday experience we are familiar with what physicists call “classical mechanics,” which governs most of the world we can see with our own eyes, such as what happens when a car hits a building, what path a ball takes when it’s thrown and why it’s hard to drag a cooler across a sandy beach.

Quantum mechanics, however, describes the subatomic realm – the behavior of protons, electrons and photons. The laws of quantum mechanics are very different from those of classical mechanics and can lead to some unexpected and counterintuitive results, such as the idea that an object can have negative mass.

Physicists around the world – in government, academic and corporate research groups – continue to explore real-world deployments of technologies based on quantum mechanics. And computer scientists, including me, are looking to understand how these technologies can be used to advance computing and cryptography.

In our regular lives, we are used to things existing in a well-defined state: A light bulb is either on or off, for example. But in the quantum world, objects can exist in a what is called a superposition of states: A hypothetical atomic-level light bulb could simultaneously be both on and off. This strange feature has important ramifications for computing.

The smallest unit of information in classical mechanics – and, therefore, classical computers – is the bit, which can hold a value of either 0 or 1, but never both at the same time. As a result, each bit can hold just one piece of information. Such bits, which can be represented as electrical impulses, changes in magnetic fields, or even a physical on-off switch, form the basis for all calculation, storage and communication in today’s computers and information networks.

Qubits – quantum bits – are the quantum equivalent of classical bits. One fundamental difference is that, due to superposition, qubits can simultaneously hold values of both 0 and 1. Physical realizations of qubits must inherently be at an atomic scale: for example, in the spin of an electron or the polarization of a photon.

An explanation of quantum mechanics, in terms of how well you remember someone’s name when you see him.

Another difference is that classical bits can be operated on independently of each other: Flipping a bit in one location has no effect on bits in other locations. Qubits, however, can be set up using a quantum-mechanical property called entanglement so that they are dependent on each other – even when they are far apart. This means that operations performed on one qubit by a quantum computer can affect multiple other qubits simultaneously. This property – akin to, but not the same as, parallel processing – can make quantum computation much faster than in classical systems.

Large-scale quantum computers – that is, quantum computers with hundreds of qubits – do not yet exist, and are challenging to build because they require operations and measurements to be done on a atomic scale. IBM’s quantum computer, for example, currently has 16 qubits, and Google is promising a 49-qubit quantum computer – which would be an astounding advance – by the end of the year. (In contrast, laptops currently have multiple gigabytes of RAM, with a gigabyte being eight billion classical bits.)

A physics professor untangles entanglement.

Notwithstanding the difficulty of building working quantum computers, theorists continue to explore their potential. In 1994, Peter Shor showed that quantum computers could quickly solve the complicated math problems that underlie all commonly used public-key cryptography systems, like the ones that provide secure connections for web browsers. A large-scale quantum computer would completely compromise the security of the internet as we know it. Cryptographers are actively exploring new public-key approaches that would be “quantum-resistant,” at least as far as they currently know.

Interestingly, the laws of quantum mechanics can also be used to design cryptosystems that are, in some senses, more secure than their classical analogs. For example, quantum key distribution allows two parties to share a secret no eavesdropper can recover using either classical or quantum computers. Those systems – and others based on quantum computers – may become useful in the future, either widely or in more niche applications. But a key challenge is getting them working in the real world, and over large distances.

*This article was originally published on The Conversation website under a Creative Commons Attribution 4.0 International License. Read the original article here.*

If you found this article interesting, you may be interested in browsing related content on ScienceDirect. We are pleased to offer you a free chapter from the **Quantum Inspired Computational Intelligence** book called “*Quantum computing and supervised machine learning: Training, model selection, and error estimation*.”

Visit **elsevier.com** to access computer science and more! Use discount code **STC317** at checkout and save up to 30% on your very own copy!

We are delighted to announce that our long running physics book serial ** Advances in Imaging and Electron Physics** celebrates its 200

Alongside these new and important contributions we will also publish a previously never brefore digitised chapter from the series *Advances in Optical and Election Microscopy. *As our editor Peter Hawkes explains “This series contained many articles of considerable importance, often written by major figures in the world of electron microscopy and electron optics. The fact that one of the editors, Dr V.E. Cosslett FRS, was a well-known personality and knew many of the authors personally was of course a great asset. Thus the first volume of the series contained a chapter by Ernst Ruska (Nobel Prize, 1986) who built the first electron microscope in the 1930s. This was followed by an authoritative article by Gaston Dupouy (Membre de l’Institut, former Director-General of the CNRS) on the high-voltage microscope, the first of which was built in his custom-built laboratory, and an equally scholarly article on aberration correction by Albert Septier, widely known as co-author of a book on electron optics that was the principal source of information on the subject in the 1960s and 1970s. Most of the subsequent volumes contained major chapters by very distinguished authors and the absence of AOEM on ScienceDirect means that they are read and cited much less than they deserve. They can be divided into two groups (not mutually exclusive): those that are in some sense archival, indispensable reading for anyone tracing the background of the subject (Hanszen, Erickson, Hardy, Wade, Joy, Misell, Vainshtein, Cosslett) and those that should still be consulted and used today (Dupouy, Septier, Bok. Metherell, Hawkes, Lauer, Herrmann, Colliex, Rouse, Plies, Adriaanse, Shimizu); and this does not include all the articles that deserve republication.”

We are pleased to offer you a complimentary chapter called **“Past and Present Attempts to Attain the Resolution Limit of the Transmission Electron Microscope”** by Ernst Ruska.

If you would like to view more chapters from Advance in Electron Physics, visit ScienceDirect today.

]]>As seen in the figure below, the protein chain is gene-encoded and initially has no structure (below left) and its intricate structure (below right), with every atom in its unique position, results from spontaneous folding.

This is as amazing as if a multicolored thread could produce a shirt itself!

The chain spontaneously finds its stable fold (Figure 1: from left to right) within minutes or faster (both in vitro and in living cells), though much more than the entire life-span of the Universe would have been required to sample all possible chain structures in search for the most stable one. This is called “the Levinthal’s paradox”. To resolve it, various models of folding were proposed during decades.

However, these models fail to overcome the Levinthal’s paradox *when* the globular structure stability is close to that of the unfolded chain and provide for no estimate of the folding rates (spaning over 11 orders of magnitude, Figure 2).

The folding rate problem was solved using *unfolding *(not folding!) as the starting point, i.e., when the free-energy barrier between the globular and unfolded state (Figure 1, middle) was viewed “from the globule side” (Figure 1: from right to left).

The trick is that, firstly, the rates of the forward and reverse reactions coincide when the globular structure stability equals to that of the unfolded chain (according to the “principle of detailed balance” well-known in physics). Secondly, it is much easier to imagine – and investigate – how the thread unfolds than how it obtains a certain fold.

The validity of this theory (proposed two decades ago, when the experimental data were scarce) has been recently confirmed by all currently available experimental data (Figure 2).

However, dissatisfaction felt because the *folding* problem has not been solved yet “from the viewpoint of the folding chain” (Figure 1: from left to right) underlay further efforts made to estimate a volume of the necessary sampling in search for the most stable chain fold.

Recently, it has been shown that this volume, when considered at the level of formation and packing of the most strongly interacting protein structure elements (helices and strands, Figure 1) is by many orders of magnitude smaller than at the Levintal-considered level of separate chain links (beads in Figure 1), and the rate of its sampling, at *folding*, become physically and biologically reasonable (and close to the unfolding-derived estimates, Figure 2).

Thus, the protein folding puzzle is solved by viewing on it from two sides: the side of unfolding *and* the side of folding.

*Protein Physics: A Course of Lectures* covers the most general problems of protein structure, folding and function; it describes key experimental facts and introduces concepts and theories. It deals with fibrous, membrane and especially water-soluble globular proteins, in both their native and denatured states. The book summarizes and presents in a systematic way the results of several decades of worldwide fundamental research on protein physics, structure and folding. It describes many simple physical models aimed to help a reader to estimate and predict of physical processes occurring in and with proteins.

The author, Alexei V. Finkelstein, has been invited to speak at the 4th International Conference on Integrative Biology on July 18-20, 2016 in Berlin, Germany. The book, *Protein Physics: A Course of Lectures *is scheduled to publish in July. If you would like to pre-order a copy at 15% off the list price, visit the Elsevier Store today. The author, Alexei V. Finkelstein, will be speaking at the 4th International Conference on Integrative Biology on July 18-20, 2016 in Berlin, Germany.

One of the great short stories of the 20th century is Nobel Laureate Isaac Bashevis Singer’s The Spinoza of Market Street. It tells of an aged scholar who has devoted his life to the study of Spinoza’s great work, Ethics. Protagonist Dr Fischelson has lost his library job and, like his hero, been expelled from his religious community for his heretical views. Looking down from his garret with disdain at the crowded street below him, he devotes his days to solitary scholarship. At night he gazes up through his telescope at the heavens, where he finds verification of his master’s wisdom.

Then one day Dr Fischelson falls ill. A neighbor, an uneducated “old maid,” nurses him back to health. Eventually, though the good doctor never understands exactly how or why, they are married. On the night of the wedding, after the unlikeliest of passionate consummations, the old man gazes up at the stars and murmurs, “Divine Spinoza, forgive me. I have become a fool.” He has learned that there is more to life than the theoretical speculations that have preoccupied him for decades.

The history of modern physics boasts its own version of Fischelson. His name was Paul Dirac. I first encountered Dirac in physics courses, but was moved to revisit his life and legacy through my service on the board of the Kinsey Institute for the Study of Human Sexuality and teaching an undergraduate course on sexuality and love.

Born in Bristol, England, in 1902, Dirac became, after Einstein, the second most important theoretical physicist of the 20th century. He studied at Cambridge, where he wrote the first-ever dissertation on quantum mechanics. Shortly thereafter he produced one of physics’ most famous theories, the Dirac equation, which correctly predicted the existence of antimatter. Dirac did more than any other scientist to reconcile Einstein’s general theory of relativity to quantum mechanics. In 1933 he received the Nobel Prize in Physics, the youngest theoretical physicist ever to do so.

At the time Dirac received the Nobel Prize, he was leading a remarkably drab and, to most eyes, unappealing existence. As detailed in Graham Farmelo’s wonderful biography, The Strangest Man: The Hidden Life of Paul Dirac, Mystic of the Atom, on which I rely heavily in this article, Dirac was an incredibly taciturn individual. Getting him to utter even a word could prove nearly impossible, leading his mischievous colleagues to introduce a new unit of measure for the rate of human speech, the Dirac, which amounted to one word per hour.

Dirac was the kind of man who would “never utter a word when no word would do.” Farmelo describes him as a human being completely absorbed in his work, with absolutely no interest in other people or their feelings, and utterly devoid of empathy. He attributes this in part to Dirac’s tyrannical upbringing. His father ruthlessly punished him for every error in speech, and the young Dirac adopted the strategy of saying as little as possible.

Dirac was socially awkward and showed no interest in the opposite sex. Some of his colleagues suspected that he might be utterly devoid of such feelings. Once, Farmelo recounts, Dirac found himself on a two-week cruise from California to Japan with the eminent physicist Werner Heisenberg. The gregarious Heisenberg made the most of the trip’s opportunities for fraternization with the opposite sex, dancing with the flapper girls. Dirac found Heisenberg’s conduct perplexing, asking him, “Why do you dance?” Heisenberg replied, “When there are nice girls, it is always a pleasure to dance.” Dirac pondered this for some minutes before responding, “But Heisenberg, how do you know beforehand that the girls are nice?”

Then one day, something remarkable entered Dirac’s life. Her name was Margit Wigner, the sister of a Hungarian physicist and recently divorced mother of two. She was visiting her brother at the Institute for Advanced Study in Princeton, New Jersey, where Dirac had just arrived.

Known to friends and family as “Manci,” one day she was dining with her brother when she observed a frail, lost-looking young man walk into the restaurant. “Who is that?” she asked. “Why that is Paul Dirac, one of last year’s Nobel laureates,” replied her brother. To which she replied, “Why don’t you ask him to join us?”

Thus began an acquaintance that eventually transformed Dirac’s life. Writes Farmelo:

*His personality could scarcely have contrasted more with hers: to the same extent that he was reticent, measured, objective, and cold, she was talkative, impulsive, subjective, and passionate.*

A self-described “scientific zero,” Manci embodied many things that were missing in Dirac’s life. After their first meeting, the two dined together occasionally, but Dirac, whose office was two doors down from Einstein, remained largely focused on his work.

After Manci returned to Europe, they maintained a lopsided correspondence. Manci wrote letters that ran to multiple pages every few days, to which Dirac responded with a few sentences every few weeks. But Manci was far more attuned than Dirac to a “universally acknowledged truth” best expressed by Jane Austen: “A single man in possession of a good fortune must be in want of a wife.”

She persisted despite stern warnings from Dirac:

*I am afraid I cannot write such nice letters to you – perhaps because my feelings are so weak and my life is mainly concerned with facts and not feelings.*

When she complained that many of her queries about his daily life and feelings were going unanswered, Dirac drew up a table, placing her questions in the left column, paired with his responses on the right. To her question, “Whom else should I love?” Dirac responded, “You should not expect me to answer this question. You would say I was cruel if I tried.” To her question, “Are there any feelings for me?” Dirac answered only, “Yes, some.”

Realizing that Dirac lacked the insight to see that many of her questions were rhetorical, she informed him that “most of them were not meant to be answered.” Eventually, exasperated by Dirac’s lack of feeling, Manci wrote to him that he should “get a second Nobel Prize in cruelty.” Dirac wrote back:

*You should know that I am not in love with you. It would be wrong for me to pretend that I am, as I have never been in love I cannot understand fine feelings.*

Yet with time, Dirac’s outlook began to change. After returning from a visit with her in Budapest, Dirac wrote, “I felt very sad leaving you and still feel that I miss you very much. I do not understand why this should be, as I do not usually miss people when I leave them.” The man whose mathematical brilliance had unlocked new truths about the fundamental nature of the universe was, through his relationship with Manci, discovering truths about human life that he had never before recognized.

Soon thereafter, when she returned for a visit, he asked her to marry him, and she accepted immediately. The couple went on two honeymoons little more than month apart. Later he wrote to her:

*Manci, my darling, you are very dear to me. You have made a wonderful alteration in my life. You have made me human… I feel that life for me is worth living if I just make you happy and do nothing else.*

A Soviet colleague of Dirac corroborated his friend’s self-assessment: “It is fun to see Dirac married, it makes him so much more human.”

In Dirac, a thoroughly theoretical existence acquired a surprisingly welcome practical dimension. A man who had been thoroughly engrossed in the life of the mind discovered the life of the heart. And a human being whose greatest contributions had been guided by the pursuit of mathematical beauty discovered something beautiful in humanity whose existence he had never before suspected.

In short, a brilliant but lonely man found something new and wonderful that had been missing his entire life: love. As my students and I discover in the course on sexuality and love, science can reveal a great deal, but there are some aspects of reality – among them, love – that remain largely outside its ambit.

Paul Dirac introduced some useful formal tools (such as his notation for integrals and operators). One of them is the Dirac delta function δ(x), an object then unknown to mathematicians, which turned out to be very useful in physics. The book *Ideas of Quantum Chemistry, 2nd Edition*, has an excellent appendix that describes the Dirac Delta Function. We are pleased to offer you a download of this sample from the book.

If you are interested in more from the *Ideas of Quantum Chemistry, 2nd Edition* you can order a print copy at up to 30% off the list price and free global shipping via the Elsevier Store. Enter discount code STC215 at checkout.

Upon arriving in France, he stepped out of the airport arrivals hall into a crowd of excited journalists. One of whom informed Amano that he had been jointly awarded the Nobel prize in physics with his former supervisor Isamu Akasaki and another Japanese scientist Shuji Nakamura. “It was really unexpected,” he says. In fact, he was so surprised that at first he thought it must be a joke or a mistake.

The third volume of the *Handbook of Crystal Growth* describes Amano’s and others’ recent work to improve our fundamental understanding of the growth of nitrides by MOCVD and MOVPE for a range of nitride-based devices.

The Nobel committee had awarded the prize to these three researchers for inventing efficient blue light-emitting diodes (LEDs) which enabled the development of white LEDs. Today, white LEDs are the most energy efficient and longest lasting bulbs on the market, and can be found lighting our homes and business and inside our TVs, computers and mobile phones. In 2012, more than 210 billion LEDs packages were reportedly produced worldwide – this is approximately 30 for each person on Earth.

“It was really unexpected,” said Hiroshi Amano . In fact, he was so surprised that at first he thought it must be a joke or a mistake.

To create white light, LEDs that produce all three of the primary colours of light are needed. By the end of the 1960s, red and green LEDs had been successfully made but LEDs that produced blue light were to prove elusive. Akasaki had identified that gallium nitride was the most likely candidate, but his group were struggling to grow crystals of the material of a high enough quality. Additionally, while n-type gallium nitride semiconductors were proving fairly easy to make, the p-type counterpart was not. Amano overcame both those hurdles whilst working under Akasaki’s supervision in the late 1980s. “My contribution was showing that the high quality GaN can be grown on a sapphire substrate by depositing low temperature AlN buffer layer before the growth of GaN and also that p-type GaN can be made by Mg doping followed by low energy electron beam irradiation treatment,” he explains.

Since those discoveries, Amano has gone on to set up his own successful research team currently based at Nagoya University – which is also where he did his Nobel prize-worthy research in the 1980s. His team works broadly on growing novel crystals of semiconducting group 3 nitrides with the aim of enabling the development of other sustainable devices. These crystals are grown either by MOCVD (metalorganic chemical vapor deposition) or the related technique MOVPE (metalorganic vapour phase epitaxy).

*Read more on SciTech Connect – Ice Crystals Give Up Their Secrets in Microgravity*

The design of LEDs that produce deep UV light has been one of Amano’s team’s most significant recent developments. Photons of deep UV light interact with a huge variety of different chemical and biological molecules and these types of LEDs are expected to find use in applications ranging from sensing to cleaning up pollutants. “The most exciting research carried out in my group recently was realising high efficiency deep UV LEDs by a high temperature MOVPE growth method,” Amano says. The team are also working on designing improved nitrides for powering more energy-efficient heterojunction field-effect transistors and laser diodes.

To achieve the atomic-level control needed to grow nitrides suitable for these applications; Amano’s team spend much of their time studying how the growth processes occur. They are currently developing a method to observe the growth of InGaN and related semiconductors in almost real-time inside an x-ray diffractometer. “A fundamental understanding of the growth process is essential for realizing new types of devices,” he says.

Since winning the Nobel prize, Amano says he has been inundated with invitations to give talks. “By the end of this year, I will have given more than 200 lectures since the prize was announced. Of course it is busy, but I am enjoying these unexpected encounters with researchers in the different fields to my own,” he says. “I learn a lot through discussions with researchers with different specialties.” It is also talking with others that he credits for his success so far: “I have got many of my inspirations though discussions with my colleagues.”

The third volume of the *Handbook of Crystal Growth*, published by Elsevier and available on ScienceDirect, describes Amano’s and others’ recent work to improve our fundamental understanding of the growth of nitrides by MOCVD and MOVPE for a range of nitride-based devices.

I had the opportunity to attend this meeting and found it a great pleasure to meet our authors and editors face-to-face and to receive feedback on how we can continue to meet their needs. Elsevier was present with a booth staffed by colleagues from our books and journals groups.

One of books on display was the recently published second edition of the *Handbook of Crystal Growth* which attracted much attention. These newly revised and updated handbooks together provide a comprehensive compendium on crystal growth, documenting authoritatively in separate self-contained volumes, or as one complete publication, the fundamentals and application of the subject. The work consists of Volume 1: Fundamentals; Volume 2: Bulk Crystal Growth; and Volume 3: Thin Films and Epitaxy.

In 2015 we continue to expand our physics books list with titles that will enable scientists in academia and industry and students to perform their research and build on existing knowledge by providing specific, reliable, and authoritative information in all physics areas.

In particular I would like to bring to your attention the recently published second edition of John Morrison’s *Modern Physics for Scientists and Engineers*. This book gives a brief, focused account of what led to modern quantum theory, then discussing its underlying physics.

I am very interested in growing our physics books program with titles in all physics areas so if you are interested in authoring or editing a book I would love to hear from you.

These titles and many more are available for purchase on the Elsevier Store. Use discount code “STC215” at checkout and save up to 30% on all books and ebooks.

Read more from SciTech Connect:

Ice Crystals Give Up Their Secrets in Microgravity

Elsevier Attends the APS Conference for Women in Physics

**Anita Koch, Acquisitions Editor, Physics & Astronomy**

Anita joined Elsevier in 1991 with a master’s degree in Physics. Since then she held many positions in the Science & Technology journals and books organization. In 2011 she had the opportunity to move to books acquisition, initially in Chemical Engineering and since 2014 also in Physics & Astronomy.

She feels privileged to work with dedicated and outstanding authors and editors in developing high-quality content for students and researchers in academia and industry. Also the fast going technological developments in publishing from print books to electronically available content fascinates her.

Anita lives in the Netherlands. Her hobbies are classical music, literature, cooking, wine, and exploring long distance footpaths.

]]>This panel will provide students with insight into career tracks they may be interested in once they’ve received their degrees. She has shared her professional experiences before in the form of blog posts she wrote three years ago about her time working at the Large Hadron Collider at CERN in Geneva:

Just before the panel, Senior Acquisitions Editor Katey Birtcher and Editorial Project Manager Marisa LaFleur will host a booth at the conference’s Graduate School and Career Fair, providing more information to students interested in the field of academic publishing.

We are excited to be a part of encouraging young women to consider career options involving academic publishing, whether as authors, editors, or journals managers. The options available to them are wide and diverse, and we look forward to welcoming them to the work force in the coming years.

For more information on STEM careers for women, see our blog posts: Success Strategies from Women in STEM “Book in Press” Party and Women In STEM: Q&A with Dr. Denise Faustman of MGH

To celebrate this event, we are offering **up to 25% off Physics titles** when you use discount code “PHYSICS215” at checkout. Be sure to take a look at some of our top Physics books written by leading women in the field:

*Tensors, Relativity, and Cosmology*by Mirjana Dalarsson*Physics in the Arts*by P.U.P.A Gilbert*Ultrarelativistic Heavy-Ion Collisions*by Ramona Vogt*Linear Ray and Wave Optics in Phase Space*by Amalia Torre*Nanophysics: Coherence and Transport*by Hélène Bouchiat

]]>

At present, the general problem of anharmonic vibrations commands a significant place among scientific investigations in various branches of experimental and theoretical physics. The phenomena of anharmonicity are displayed in the vibrations of molecules and crystals, in the mechanics of molecular rotations and librations, in the resonance interaction of vibrational levels, in electro-optical effects, in non-linear spin interaction, and so on.

Behind the scenes of various phenomena, a unique essence is hidden. Through the absence of a perfect harmony, the various physical effects become possible. A harmony fails to be a tendency to a simple ideal, but it is a capacity of the nature to order an anharmonicity. The understanding of the natural beauty leads us to the anharmonic world. Under the constraint of physical laws, the anharmonicity bears the harmony…

The simplest choice to describe the vibrations of an arbitrary system in quantum mechanics is a model of harmonic oscillators. A harmonic case is certainly only an idealization of vibrations of a real system. The potential energy of vibrations, as in classical physics, is generally written in a form of an expansion in terms of normal coordinates; in this expansion, the linear terms disappear through the fact that the first derivative of the potential equals zero at the equilibrium condition. A set of harmonic oscillators corresponds to a first approximation. This model is only qualitatively correct: vibrations, failing to conform to a harmonic law, are anharmonic. To describe correctly the vibrations, apart from the quadratic part of the potential energy, one must therefore take into account the normal coordinates to greater than quadratic powers in an expansion of the potential. These terms additional to the harmonic Hamiltonian are defined by anharmonicity coefficients and characterize the interactions among various vibrational modes. The calculation of the corresponding corrections is generally performed with a perturbation theory for stationary states of a perturbed Hamiltonian, in which the perturbation function represents an expansion in powers of a small parameter. A formalism in terms of the polynomials of quantum numbers might serve as one example of the perturbation methods. In the book ‘Uncommon Paths in Quantum Physics’, we consider the polynomial formalism in detail.

First, the polynomials form, with the required accuracy, all necessary physical observables of the anharmonicity problem. The desired quantities are obtained immediately on solving or opening the recurrence equations or relations avoiding conventional intermediate manipulations. We compare two schemes to construct the stationary perturbation theory:

1) Schrödinger equation → eigenfunctions and eigenvalues →

→ matrix elements;

2) recurrence equations → eigenvalues and matrix elements.

The first scheme is conventional, whereas we proposed the second scheme. The main disadvantage of the conventional scheme is that, at each stage, one must return virtually to the beginning — to the Schrödinger equation — to improve the eigenfunctions by increasing the order of the perturbation calculation. Only after these calculations is one in a position to evaluate the matrix elements. In our method, intermediate calculations are performed on an equal footing; i.e., the procedures to calculate the eigenvalues and arbitrary matrix elements are performed simultaneously.

Second, the proposed theory automatically keeps track of non-zero contributions of the total perturbation to the result sought, and takes into account the history of the calculations, i.e., the intermediate calculations. This advantage is achieved on expanding, in a small parameter, the derivatives of the energies and their wave functions, rather than by expanding the eigenfunctions and eigenvalues as is done traditionally. In this sense, the expansion in exact eigenvectors plays a principal role [K. V. Kazakov, Electro-optics of molecules, *Opt. Spectrosc.*, **97**, 725—734, 2004],

because it ensures a full use of the history of the calculations and, consequently, significantly simplifies the general solution algorithm. If the expansion is performed in terms of the exact eigenvectors, rather than in terms of zero-order basis functions, it is assumed that the former functions exist and are expressible algebraically, for example, with recurrence relations. In addition, one might avoid the renormalization of the function; this problem presents considerable difficulties in the traditional approach in which the function should be renormalized upon passing from one perturbation order to the next.

Other advantages of this method appear in various applications of this perturbation theory [K. V. Kazakov, *Quantum Theory of Anharmonic Effects in Molecules*, 2012, Elsevier]. For example, in a framework of the polynomial formalism, one might consider the problem of electro-optical anharmonicity; this problem involves an electric dipolar-moment function in a non-linear form, and its solution requires evaluation of matrix elements. The absolute values of dipolar-moment derivatives might be unknown beforehand, which complicates the problem. In the traditional formalism, the consideration proceeds, as a rule, from the wave function of a definite order, which leads to the loss of significant contributions. In the polynomial formalism, we consider separately each term in an expansion of the dipolar-moment function, and, consequently, calculate the entire matrix element in a given order in a small parameter.

** Beyond the Predictable Trend**

A prospectively fruitful direction for further investigation is to proceed beyond solutions with perturbation theory. We assume that the effective internuclear potential is a real function that is represented as an expansion in a power series in terms of the normal coordinates. In this case, the procedure of quantization, i.e., the calculation of matrix elements of an arbitrary coordinate function, taking into account the influence of anharmonicity, is reduced to the sum of polynomials multiplied by factor √g [K. V. Kazakov, *Uncommon Paths in Quantum Physics*, 2014, Elsevier]:

Expanding here the polynomials in terms of quantum numbers, we obtain this intriguing formula,

for the one-dimensional case, and this one,

for the many-dimensional case.

The derived expansions in terms of quantum numbers hold for the matrix elements of an arbitrary physical function that is represented as an expansion in a power series in terms of creation and destruction operators. This consequence of perturbation-theory calculations is trivial. The values of energy *E _{n}* are expressible also from the formula for (

in which Ω-coefficients_{ }are the mechanical anharmonicity parameters. To generalize our theory, we assume that quantity *E _{n}* is a function of quantum numbers

We can determine heuristically a function Φ for the matrix element of a particular physical quantity *ƒ*(*ξ*), for instance, the dipolar moment, as a dependence on quantum number *n+k*/2+1/2 :

Functions Φ* _{k}* are here arbitrarily expressible, for example,

with parameters θ_{k }_{ }and ϕ* _{k }*determined from experiment. In the present formalism, one might also construct phenomenologically function Φ for a system with

Konstantin’s books *Uncommon Paths in Quantum Physics* and *Quantum Theory of Anharmonic Effects in Molecules* are both available on the Elsevier Store. Use discount code “STC215” and **save up to 30%** on your very own copies!

**About the Author**

Konstantin V. Kazakov obtained a Dr. Sc. in Physics and Mathematics at the St. Petersburg State University. He has published papers in internationally scientific journals, communications at scientific symposia and congresses, as well as 3 books.

]]>