Robert J. Matthys
- Published in print:
- 2004
- Published Online:
- January 2010
- ISBN:
- 9780198529712
- eISBN:
- 9780191712791
- Item type:
- book
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780198529712.001.0001
- Subject:
- Physics, History of Physics
The Shortt clock, made in the 1920s, is the most famous accurate clock pendulum ever known, having an accuracy of one second per year when kept at nearly constant temperature. Almost all ...
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The Shortt clock, made in the 1920s, is the most famous accurate clock pendulum ever known, having an accuracy of one second per year when kept at nearly constant temperature. Almost all of a pendulum clock's accuracy resides in its pendulum. If the pendulum is accurate, the clock will be accurate. This book describes many scientific aspects of pendulum design and operation in simple terms with experimental data, and little mathematics. It has been written, looking at all the different parts and aspects of the pendulum in great detail, chapter by chapter, reflecting the degree of attention necessary for making a pendulum run accurately. The topics covered include the dimensional stability of different pendulum materials, good and poor suspension spring designs, the design of mechanical joints and clamps, effect of quartz on accuracy, temperature compensation, air drag of different bob shapes and making a sinusoidal electromagnetic drive. One whole chapter is devoted to simple ways of improving the accuracy of ordinary low-cost pendulum clocks, which have a different construction compared to the more expensive designs of substantially well-made ones. This book will prove invaluable to anyone who wants to know how to make a more accurate pendulum or pendulum clock.
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The Shortt clock, made in the 1920s, is the most famous accurate clock pendulum ever known, having an accuracy of one second per year when kept at nearly constant temperature. Almost all of a pendulum clock's accuracy resides in its pendulum. If the pendulum is accurate, the clock will be accurate. This book describes many scientific aspects of pendulum design and operation in simple terms with experimental data, and little mathematics. It has been written, looking at all the different parts and aspects of the pendulum in great detail, chapter by chapter, reflecting the degree of attention necessary for making a pendulum run accurately. The topics covered include the dimensional stability of different pendulum materials, good and poor suspension spring designs, the design of mechanical joints and clamps, effect of quartz on accuracy, temperature compensation, air drag of different bob shapes and making a sinusoidal electromagnetic drive. One whole chapter is devoted to simple ways of improving the accuracy of ordinary low-cost pendulum clocks, which have a different construction compared to the more expensive designs of substantially well-made ones. This book will prove invaluable to anyone who wants to know how to make a more accurate pendulum or pendulum clock.
Andrew Briggs, Oleg Kolosov
- Published in print:
- 2009
- Published Online:
- February 2010
- ISBN:
- 9780199232734
- eISBN:
- 9780191716355
- Item type:
- book
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199232734.001.0001
- Subject:
- Physics, Condensed Matter Physics / Materials
Acoustic microscopy enables you to image and measure the elastic properties of materials with the resolution of a good microscope. By using frequencies in the microwave range, it is ...
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Acoustic microscopy enables you to image and measure the elastic properties of materials with the resolution of a good microscope. By using frequencies in the microwave range, it is possible to make the acoustic wavelength comparable with the wavelength of light, and hence to achieve a resolution comparable with an optical microscope. The contrast gives information about the elastic properties and structure of the sample. Since acoustic waves can propagate in materials, acoustic microscopy can be used for interior imaging, with high sensitivity to defects such as delaminations. Solids can support both longitudinal and transverse acoustic waves. At surfaces a combination of the two known as Rayleigh waves can propagate, and in many circumstances these dominate the contrast in acoustic microscopy. Contrast theory accounts for the variation of signal with defocus, V(z). Acoustic microscopy can image and measure properties such as anisotropy and features such as surface boundaries and cracks. A scanning probe microscope can be used to detect ultrasonic vibration of a surface with resolution in the nanometre range, thus beating the diffraction limit by operating in the extreme near‐field. This 2nd edition of Acoustic Microscopy has a major new chapter on the technique and applications of acoustically exited probe microscopy.
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Acoustic microscopy enables you to image and measure the elastic properties of materials with the resolution of a good microscope. By using frequencies in the microwave range, it is possible to make the acoustic wavelength comparable with the wavelength of light, and hence to achieve a resolution comparable with an optical microscope. The contrast gives information about the elastic properties and structure of the sample. Since acoustic waves can propagate in materials, acoustic microscopy can be used for interior imaging, with high sensitivity to defects such as delaminations. Solids can support both longitudinal and transverse acoustic waves. At surfaces a combination of the two known as Rayleigh waves can propagate, and in many circumstances these dominate the contrast in acoustic microscopy. Contrast theory accounts for the variation of signal with defocus, V(z). Acoustic microscopy can image and measure properties such as anisotropy and features such as surface boundaries and cracks. A scanning probe microscope can be used to detect ultrasonic vibration of a surface with resolution in the nanometre range, thus beating the diffraction limit by operating in the extreme near‐field. This 2nd edition of Acoustic Microscopy has a major new chapter on the technique and applications of acoustically exited probe microscopy.
Robert Blinc
- Published in print:
- 2011
- Published Online:
- January 2012
- ISBN:
- 9780199570942
- eISBN:
- 9780191728631
- Item type:
- book
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199570942.001.0001
- Subject:
- Physics, Condensed Matter Physics / Materials
The field of ferroelectricity has greatly expanded and changed recently. In addition to classical organic and inorganic ferroelectrics as well as composite ferroelectrics new fields and ...
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The field of ferroelectricity has greatly expanded and changed recently. In addition to classical organic and inorganic ferroelectrics as well as composite ferroelectrics new fields and materials have appeared, important for both basic science and application and showing technological promise for novel multifunctional devices. Most of these fields were unknown or inactive 20 to 40 years ago. Such new fields are multiferroic magnetoelectric systems, where the spontaneous polarization and the spontaneous magnetization are allowed to coexist, incommensurate ferroelectrics, where the periodicity of the order parameter is incommensurate to the periodicity of the underlying basic crystal lattice, ferroelectric liquid crystals, dipolar glasses, relaxor ferroelectrics, ferroelectric thin films and nanoferroelectrics. These new fields are in addition to basic physical interest also of great technological importance and allow for new memory devices, spintronic applications and electro‐optic devices. They are also important for applications in acoustics, robotics, telecommunications and medicine. New developments in relaxors allow for giant electromechanical and electrocaloric effects. The book is primarily intended for material scientists working in research or industry. It is also intended for graduate and doctoral students and can be used as a textbook in graduate courses. Finally, it should be useful for everybody following the development of modern solid‐state physics.
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The field of ferroelectricity has greatly expanded and changed recently. In addition to classical organic and inorganic ferroelectrics as well as composite ferroelectrics new fields and materials have appeared, important for both basic science and application and showing technological promise for novel multifunctional devices. Most of these fields were unknown or inactive 20 to 40 years ago. Such new fields are multiferroic magnetoelectric systems, where the spontaneous polarization and the spontaneous magnetization are allowed to coexist, incommensurate ferroelectrics, where the periodicity of the order parameter is incommensurate to the periodicity of the underlying basic crystal lattice, ferroelectric liquid crystals, dipolar glasses, relaxor ferroelectrics, ferroelectric thin films and nanoferroelectrics. These new fields are in addition to basic physical interest also of great technological importance and allow for new memory devices, spintronic applications and electro‐optic devices. They are also important for applications in acoustics, robotics, telecommunications and medicine. New developments in relaxors allow for giant electromechanical and electrocaloric effects. The book is primarily intended for material scientists working in research or industry. It is also intended for graduate and doctoral students and can be used as a textbook in graduate courses. Finally, it should be useful for everybody following the development of modern solid‐state physics.
Barry M McCoy
- Published in print:
- 2009
- Published Online:
- February 2010
- ISBN:
- 9780199556632
- eISBN:
- 9780191723278
- Item type:
- book
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199556632.001.0001
- Subject:
- Physics, Theoretical, Computational, and Statistical Physics
This book begins where elementary books and courses leave off and covers the advances made in statistical mechanics in the past fifty years. The book is divided into three parts. The ...
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This book begins where elementary books and courses leave off and covers the advances made in statistical mechanics in the past fifty years. The book is divided into three parts. The first part is on general theory which includes a summary of the basic principles of statistical mechanics; a presentation of the physical phenomena covered and the models used to discuss them; theorems on the existence and uniqueness of partition functions; theorems on order; and critical phenomena and scaling theory. The second part is on series and numerical methods which includes derivations of the Mayer and Ree–Hoover expansions of the low density virial equation of state; Groeneveld's theorems; the application to hard spheres and discs; a summary of numerical studies of systems at high density; and the use of high temperature series expansions to estimate critical exponents for magnets. The third part covers exactly solvable models which includes a detailed presentation of the Pfaffian methods of computing the Ising partition function, magnetization, correlation functions, and susceptibility; the star-triangle (Yang–Baxter equation); functional equations and the free energy for the eight-vertex model; and the hard hexagon and chiral Potts models. All needed mathematics is developed in detail and many open questions are discussed. The goal is to guide the reader to the current forefront of research.
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This book begins where elementary books and courses leave off and covers the advances made in statistical mechanics in the past fifty years. The book is divided into three parts. The first part is on general theory which includes a summary of the basic principles of statistical mechanics; a presentation of the physical phenomena covered and the models used to discuss them; theorems on the existence and uniqueness of partition functions; theorems on order; and critical phenomena and scaling theory. The second part is on series and numerical methods which includes derivations of the Mayer and Ree–Hoover expansions of the low density virial equation of state; Groeneveld's theorems; the application to hard spheres and discs; a summary of numerical studies of systems at high density; and the use of high temperature series expansions to estimate critical exponents for magnets. The third part covers exactly solvable models which includes a detailed presentation of the Pfaffian methods of computing the Ising partition function, magnetization, correlation functions, and susceptibility; the star-triangle (Yang–Baxter equation); functional equations and the free energy for the eight-vertex model; and the hard hexagon and chiral Potts models. All needed mathematics is developed in detail and many open questions are discussed. The goal is to guide the reader to the current forefront of research.
Wai-Kee Li, Gong-Du Zhou, Thomas Mak
- Published in print:
- 2008
- Published Online:
- May 2008
- ISBN:
- 9780199216949
- eISBN:
- 9780191711992
- Item type:
- book
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199216949.001.0001
- Subject:
- Physics, Crystallography
This text is an updated English version of a class-tested textbook originally published in Chinese in 2006. Its contents are based on the lecture notes of several courses taught by the ...
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This text is an updated English version of a class-tested textbook originally published in Chinese in 2006. Its contents are based on the lecture notes of several courses taught by the authors at The Chinese University of Hong Kong and Peking University. These courses include Chemical Bonding, Structure and Properties of Matter, Advanced Inorganic Chemistry, Quantum Chemistry, Group Theory, and Chemical Crystallography. This book consists of three parts. Part I reviews the basic theories of chemical bonding, with chapters on elementary quantum theory, atomic structure, bonding in molecules, bonding in solids, and computational chemistry. Part II introduces point groups and space groups, and their applications to the study of discrete molecules and crystals. A large number of worked examples are provided in order to illustrate the usefulness and elegance of the symmetry concept. Part III constitutes about half of the book and it gives a succinct description of the structural chemistry of the elements in the Periodic Table. The main-group elements are covered in seven chapters and three other chapters deal with the rare-earth elements, transition-metal clusters and supramolecular systems. The selected systems, many of them from recent literature, are used to elucidate various aspects of structure and bonding presented in Parts I and II, and to expound the current research trends in structural inorganic chemistry
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This text is an updated English version of a class-tested textbook originally published in Chinese in 2006. Its contents are based on the lecture notes of several courses taught by the authors at The Chinese University of Hong Kong and Peking University. These courses include Chemical Bonding, Structure and Properties of Matter, Advanced Inorganic Chemistry, Quantum Chemistry, Group Theory, and Chemical Crystallography. This book consists of three parts. Part I reviews the basic theories of chemical bonding, with chapters on elementary quantum theory, atomic structure, bonding in molecules, bonding in solids, and computational chemistry. Part II introduces point groups and space groups, and their applications to the study of discrete molecules and crystals. A large number of worked examples are provided in order to illustrate the usefulness and elegance of the symmetry concept. Part III constitutes about half of the book and it gives a succinct description of the structural chemistry of the elements in the Periodic Table. The main-group elements are covered in seven chapters and three other chapters deal with the rare-earth elements, transition-metal clusters and supramolecular systems. The selected systems, many of them from recent literature, are used to elucidate various aspects of structure and bonding presented in Parts I and II, and to expound the current research trends in structural inorganic chemistry
John Banhart (ed.)
- Published in print:
- 2008
- Published Online:
- May 2008
- ISBN:
- 9780199213245
- eISBN:
- 9780191707582
- Item type:
- book
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199213245.001.0001
- Subject:
- Physics, Condensed Matter Physics / Materials
Tomography provides three-dimensional images of heterogeneous materials or engineering components, and offers an unprecedented insight into their internal structure. By using X-rays ...
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Tomography provides three-dimensional images of heterogeneous materials or engineering components, and offers an unprecedented insight into their internal structure. By using X-rays generated by synchrotrons, neutrons from nuclear reactors, or electrons provided by transmission electron microscopes, hitherto invisible structures can be revealed which are not accessible to conventional tomography based on X-ray tubes. This book provides detailed descriptions of the recent developments in this field, especially the extension of tomography to materials research and engineering. The book is grouped into four parts: a general introduction into the principles of tomography, image analysis and the interactions between radiation and matter, and one part each for synchrotron X-ray tomography, neutron tomography, and electron tomography. Within these parts, individual chapters written by different authors describe important versions of tomography, and also provide examples of applications to demonstrate the capacity of the methods.
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Tomography provides three-dimensional images of heterogeneous materials or engineering components, and offers an unprecedented insight into their internal structure. By using X-rays generated by synchrotrons, neutrons from nuclear reactors, or electrons provided by transmission electron microscopes, hitherto invisible structures can be revealed which are not accessible to conventional tomography based on X-ray tubes. This book provides detailed descriptions of the recent developments in this field, especially the extension of tomography to materials research and engineering. The book is grouped into four parts: a general introduction into the principles of tomography, image analysis and the interactions between radiation and matter, and one part each for synchrotron X-ray tomography, neutron tomography, and electron tomography. Within these parts, individual chapters written by different authors describe important versions of tomography, and also provide examples of applications to demonstrate the capacity of the methods.
Michael Bordag, Galina Leonidovna Klimchitskaya, Umar Mohideen, Vladimir Mikhaylovich Mostepanenko
- Published in print:
- 2009
- Published Online:
- September 2009
- ISBN:
- 9780199238743
- eISBN:
- 9780191716461
- Item type:
- book
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199238743.001.0001
- Subject:
- Physics, Condensed Matter Physics / Materials, Atomic, Laser, and Optical Physics
The subject of this book is the Casimir effect, i.e., a manifestation of zero-point oscillations of the quantum vacuum in the form of forces acting between closely spaced bodies. It is a ...
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The subject of this book is the Casimir effect, i.e., a manifestation of zero-point oscillations of the quantum vacuum in the form of forces acting between closely spaced bodies. It is a purely quantum effect. There is no force acting between neutral bodies in classical electrodynamics. The Casimir effect has become an interdisciplinary subject. It plays an important role in various fields of physics such as condensed matter physics, quantum field theory, atomic and molecular physics, gravitation and cosmology, and mathematical physics. Most recently, the Casimir effect has been applied to nanotechnology and for obtaining constraints on the predictions of unification theories beyond the Standard Model. The book assembles together the field-theoretical foundations of this phenomenon, the application of the general theory to real materials, and a comprehensive description of all recently performed measurements of the Casimir force, including the comparison between experiment and theory. There is increasing interest in forces of vacuum origin. Numerous new results have been obtained during the last few years which are not reflected in the literature, but are very promising for fundamental science and nanotechnology. The book provides a source of information which presents a critical assessment of all of the main results and approaches contained in published journal papers. It also proposes new ideas which are not yet universally accepted but are finding increasing support from experiment.
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The subject of this book is the Casimir effect, i.e., a manifestation of zero-point oscillations of the quantum vacuum in the form of forces acting between closely spaced bodies. It is a purely quantum effect. There is no force acting between neutral bodies in classical electrodynamics. The Casimir effect has become an interdisciplinary subject. It plays an important role in various fields of physics such as condensed matter physics, quantum field theory, atomic and molecular physics, gravitation and cosmology, and mathematical physics. Most recently, the Casimir effect has been applied to nanotechnology and for obtaining constraints on the predictions of unification theories beyond the Standard Model. The book assembles together the field-theoretical foundations of this phenomenon, the application of the general theory to real materials, and a comprehensive description of all recently performed measurements of the Casimir force, including the comparison between experiment and theory. There is increasing interest in forces of vacuum origin. Numerous new results have been obtained during the last few years which are not reflected in the literature, but are very promising for fundamental science and nanotechnology. The book provides a source of information which presents a critical assessment of all of the main results and approaches contained in published journal papers. It also proposes new ideas which are not yet universally accepted but are finding increasing support from experiment.
Oliver Johns
- Published in print:
- 2005
- Published Online:
- January 2010
- ISBN:
- 9780198567264
- eISBN:
- 9780191717987
- Item type:
- book
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780198567264.001.0001
- Subject:
- Physics, Atomic, Laser, and Optical Physics
This book provides an innovative and mathematically sound treatment of the foundations of analytical mechanics and the relation of classical mechanics to relativity and quantum theory. A ...
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This book provides an innovative and mathematically sound treatment of the foundations of analytical mechanics and the relation of classical mechanics to relativity and quantum theory. A distinguishing feature of the book is its integration of special relativity into teaching of classical mechanics. After a thorough review of the traditional theory, the book introduces extended Lagrangian and Hamiltonian methods that treat time as a transformable coordinate rather than the fixed parameter of Newtonian physics. Advanced topics such as covariant Langrangians and Hamiltonians, canonical transformations, and Hamilton-Jacobi methods are simplified by the use of this extended theory. And the definition of canonical transformation no longer excludes the Lorenz transformation of special relativity. This is also a book for those who study analytical mechanics to prepare for a critical exploration of quantum mechanics. Comparisons to quantum mechanics appear throughout the text. The extended Hamiltonian theory with time as a coordinate is compared to Dirac’s formalism of primary phase space constraints. The chapter on relativistic mechanics shows how to use covariant Hamiltonian theory to write the Klein-Gordon and Dirac equations. The chapter on Hamilton-Jacobi theory includes a discussion of the closely related Bohm hidden variable model of quantum mechanics. Classical mechanics itself is presented with an emphasis on methods, such as linear vector operators and dyadics, that will familiarise the student with similar techniques in quantum theory. Several of the current fundamental problems in theoretical physics, such as the development of quantum information technology and the problem of quantising the gravitational field, require a rethinking of the quantum-classical connection.
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This book provides an innovative and mathematically sound treatment of the foundations of analytical mechanics and the relation of classical mechanics to relativity and quantum theory. A distinguishing feature of the book is its integration of special relativity into teaching of classical mechanics. After a thorough review of the traditional theory, the book introduces extended Lagrangian and Hamiltonian methods that treat time as a transformable coordinate rather than the fixed parameter of Newtonian physics. Advanced topics such as covariant Langrangians and Hamiltonians, canonical transformations, and Hamilton-Jacobi methods are simplified by the use of this extended theory. And the definition of canonical transformation no longer excludes the Lorenz transformation of special relativity. This is also a book for those who study analytical mechanics to prepare for a critical exploration of quantum mechanics. Comparisons to quantum mechanics appear throughout the text. The extended Hamiltonian theory with time as a coordinate is compared to Dirac’s formalism of primary phase space constraints. The chapter on relativistic mechanics shows how to use covariant Hamiltonian theory to write the Klein-Gordon and Dirac equations. The chapter on Hamilton-Jacobi theory includes a discussion of the closely related Bohm hidden variable model of quantum mechanics. Classical mechanics itself is presented with an emphasis on methods, such as linear vector operators and dyadics, that will familiarise the student with similar techniques in quantum theory. Several of the current fundamental problems in theoretical physics, such as the development of quantum information technology and the problem of quantising the gravitational field, require a rethinking of the quantum-classical connection.
Reinhold A. Bertlmann
- Published in print:
- 2000
- Published Online:
- January 2010
- ISBN:
- 9780198507628
- eISBN:
- 9780191706400
- Item type:
- book
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780198507628.001.0001
- Subject:
- Physics, Theoretical, Computational, and Statistical Physics
The anomaly, which forms the central part of this book, is the failure of classical symmetry to survive the process of quantization and regularization. The study of anomalies is the key ...
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The anomaly, which forms the central part of this book, is the failure of classical symmetry to survive the process of quantization and regularization. The study of anomalies is the key to a deeper understanding of quantum field theory and has played an increasingly important role in the theory over the past twenty years. This book presents all the different aspects of the study of anomalies in an accessible and self-contained way. Much emphasis is now being placed on the formulation of the theory using the mathematical ideas of differential geometry and topology. This approach is followed here, and the derivations and calculations are given explicitly. Topics discussed include the relevant ideas from differential geometry and topology and the application of these paths (path integrals, differential forms, homotopy operators, etc.) to the study of anomalies. Chapters are devoted to abelian and nonabelian anomalies, consistent and covariant anomalies, and gravitational anomalies.
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The anomaly, which forms the central part of this book, is the failure of classical symmetry to survive the process of quantization and regularization. The study of anomalies is the key to a deeper understanding of quantum field theory and has played an increasingly important role in the theory over the past twenty years. This book presents all the different aspects of the study of anomalies in an accessible and self-contained way. Much emphasis is now being placed on the formulation of the theory using the mathematical ideas of differential geometry and topology. This approach is followed here, and the derivations and calculations are given explicitly. Topics discussed include the relevant ideas from differential geometry and topology and the application of these paths (path integrals, differential forms, homotopy operators, etc.) to the study of anomalies. Chapters are devoted to abelian and nonabelian anomalies, consistent and covariant anomalies, and gravitational anomalies.
Ted Janssen, Gervais Chapuis, Marc de Boissieu
- Published in print:
- 2007
- Published Online:
- September 2007
- ISBN:
- 9780198567776
- eISBN:
- 9780191718335
- Item type:
- book
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780198567776.001.0001
- Subject:
- Physics, Crystallography
Until the 1970s, all materials studied consisted of periodic arrays of unit cells, or were amorphous. In the last decades a new class of solid state matter, called aperiodic crystals, ...
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Until the 1970s, all materials studied consisted of periodic arrays of unit cells, or were amorphous. In the last decades a new class of solid state matter, called aperiodic crystals, has been found. It is a long range ordered structure, but without lattice periodicity. It is found in a wide range of materials: organic and anorganic compounds, minerals (including a substantial portion of the earths crust), and metallic alloys, under various pressures and temperatures. Because of the lack of periodicity, the usual techniques for the study of structure and physical properties no longer work, and new techniques have to be developed. This book deals with the characterization of the structure, the structure determination, and the study of the physical properties, especially dynamical and electronic properties of aperiodic crystals. The treatment is based on a description in a space with more dimensions than three, the so-called superspace. This allows us to generalise the standard crystallography and to look differently at the dynamics. The three main classes of aperiodic crystals, modulated phases, incommensurate composites, and quasicrystals are treated from a unified point of view, which stresses the similarities of the various systems.
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Until the 1970s, all materials studied consisted of periodic arrays of unit cells, or were amorphous. In the last decades a new class of solid state matter, called aperiodic crystals, has been found. It is a long range ordered structure, but without lattice periodicity. It is found in a wide range of materials: organic and anorganic compounds, minerals (including a substantial portion of the earths crust), and metallic alloys, under various pressures and temperatures. Because of the lack of periodicity, the usual techniques for the study of structure and physical properties no longer work, and new techniques have to be developed. This book deals with the characterization of the structure, the structure determination, and the study of the physical properties, especially dynamical and electronic properties of aperiodic crystals. The treatment is based on a description in a space with more dimensions than three, the so-called superspace. This allows us to generalise the standard crystallography and to look differently at the dynamics. The three main classes of aperiodic crystals, modulated phases, incommensurate composites, and quasicrystals are treated from a unified point of view, which stresses the similarities of the various systems.