Khác biệt giữa bản sửa đổi của “Vật lý vật chất ngưng tụ”
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{{main|Sự chuyển pha}}
Nghiên cứu [[hiện tượng tới hạn]] và [[sự chuyển pha]] (hay chuyển tiếp pha) là một phần quan trọng trong vật lý vật chất ngưng tụ hiện đại.<ref>{{cite book |title=Physics Through the 1990s|publisher=National Research Council|year=1986|chapter=Chapter 3: Phase Transitions and Critical Phenomena|url=http://books.google.com/books?id=bVq5_t9YwhYC&pg=PA7|isbn=0-309-03577-5}}</ref> Sự chuyển pha liên quan tới sự thay đổi pha của một hệ, dẫn tới sự thay đổi của những tham số bên ngoài như [[nhiệt độ]], dẫn điện... Đặc biệt, [[sự chuyển pha lượng tử]] là sự chuyển tiếp khi nhiệt độ của hệ tiến về 0, và pha của hệ coi như phân biệt với [[trạng thái nền]] của [[ma trận Hamilton]]. Các hệ trải qua sự chuyển pha thể hiện hành xử tới hạn, khi mà một số tính chất của chúng như tương quan độ dài (correlation length), nhiệt dung riêng và [[độ cảm từ]] trở lên phân kỳ. Sự chuyển pha liên tục được miêu tả trong lý thuyết Ginzburg–Landau, mà hoạt động trong xấp xỉ trường trung bình. Tuy nhiên, một vài tính chất quan trọng của sự chuyển pha, như chuyển pha [[cách điện Mott]]–[[siêu chảy]], được biết là không tuân theo mô hình Ginzburg–Landau.<ref name=balents-2005>{{cite journal|last=Balents|first=Leon|coauthors=Bartosch, Lorenz; Burkov, Anton; Sachdev, Subir and Sengupta, Krishnendu |title=Competing Orders and Non-Landau–Ginzburg–Wilson Criticality in (Bose) Mott Transitions|journal=Progress of Theoretical Physics|year=2005|volume=Supplement|issue=160|doi=10.1143/PTPS.160.314|arxiv = cond-mat/0504692 |bibcode = 2005PThPS.160..314B|pages=314 }}</ref> Nghiên cứu sự chuyển tiếp pha trong những hệ tương quan mạnh là một lĩnh vực nghiên cứu năng động.<ref name=Sachdev-LG-2010>{{cite journal|last=Sachdev|first=Subir|coauthors=Yin, Xi|title=Quantum phase transitions beyond the Landau–Ginzburg paradigm and supersymmetry|journal=Annals of Physics|year=2010|volume=325|issue=1|arxiv=0808.0191v2.pdf|doi=10.1016/j.aop.2009.08.003|pages=2|bibcode = 2010AnPhy.325....2S }}</ref>
==Experimental==
Experimental condensed matter physics involves the use of experimental probes to try to discover new properties of materials. Experimental probes include effects of electric and [[magnetic field]]s, measurement of [[response function]]s, [[transport theory (statistical physics)|transport properties]] and [[thermometry]].<ref name=exptcm>{{cite book|last=Richardson|first=Robert C.|title=Experimental Techniques in Condensed Matter Physics at Low Temperatures|year=1988|publisher=Addison-Wesley|isbn=0-201-15002-6}}</ref> Commonly used experimental techniques include [[spectroscopy]], with probes such as [[X-ray spectroscopy|X-rays]], [[infrared spectroscopy|infrared light]] and [[inelastic neutron scattering]]; study of thermal response, such as [[specific heat]] and measurement of transport via thermal and heat [[conduction (heat)|conduction]].
[[File:Lysozym diffraction.png|thumb|upright|Image of X-ray diffraction pattern from a [[protein]] crystal.]]
===Scattering===
{{main|Scattering}}
Several condensed matter experiments involve scattering of an experimental probe, such as [[X-ray]], optical [[photon]]s, [[neutron]]s, etc., on constituents of a material. The choice of scattering probe depends on the observation energy scale of interest.<ref name=chaikin-lubensky>{{cite book|last=Chaikin|first=P. M.|last2=Lubensky|first2=T. C.|title=Principles of condensed matter physics|year=1995|publisher=Cambridge University Press|isbn=0-521-43224-3}}</ref> [[Visible light]] has energy on the scale of 1 [[electron volt|eV]] and is used as a scattering probe to measure variations in material properties such as [[dielectric constant]] and [[refractive index]]. X-rays have energies of the order of 10 [[electron volt|keV]] and hence are able to probe atomic length scales, and are used to measure variations in electron charge density. [[Neutron]]s can also probe atomic length scales and are used to study scattering off nuclei and electron [[Spin (physics)|spins]] and magnetization (as neutrons themselves have spin but no charge).<ref name=chaikin-lubensky/> Coulomb and [[Mott scattering]] measurements can be made by using electron beams as scattering probes,<ref>{{cite book|last=Riseborough|first=Peter S.|title=Condensed Matter Physics I|year=2002|url=http://www.scribd.com/doc/74216869/179/Electron-Scattering-Experiments}}</ref> and similarly, [[positron]] annihilation can be used as an indirect measurement of local electron density.<ref name=siegel-1980>{{cite journal|last=Siegel|first=R. W.|title=Positron Annihilation Spectroscopy|journal=Annual Review of Materials Science|year=1980|volume=10|pages=393–425|doi=10.1146/annurev.ms.10.080180.002141|bibcode = 1980AnRMS..10..393S }}</ref> [[Laser spectroscopy]] is used as a tool for studying phenomena with energy in the range of [[visible light]], for example, to study [[non-linear optics]] and [[forbidden transition]]s in media.<ref name=nap-cmp />
===External magnetic fields===
In experimental condensed matter physics, external [[magnetic field]]s act as [[thermodynamic variable]]s that control the state, phase transitions and properties of material systems.<ref name=iupap-report>{{cite web|last=Committee on Facilities for Condensed Matter Physics|title=Report of the IUPAP working group on Facilities for Condensed Matter Physics : High Magnetic Fields|url=http://www.iupap.org/wg/wg3/hmff/file_50963.pdf|publisher=International Union of Pure and Applied Physics|year= 2004}}</ref> [[Nuclear magnetic resonance]] (NMR) is a technique by which external magnetic fields can be used to find resonance modes of individual electrons, thus giving information about the atomic, molecular and bond structure of their neighborhood. NMR experiments can be made in magnetic fields with strengths up to 65 [[Tesla (unit)|Tesla]].<ref>{{cite book|title=High Magnetic Fields|chapter=Nuclear Magnetic Resonance in Solids at very high magnetic fields|author=Moulton, W. G. and Reyes, A. P. |editor=Herlach, Fritz |series=Science and Technology|publisher=World Scientific|year=2006|url=http://books.google.com/books?id=tN8CbCHzBmcC&pg=PA185|isbn=9789812774880}}</ref> [[Quantum oscillations (experimental technique)|Quantum oscillations]] is another experimental technique where high magnetic fields are used to study material properties such as the geometry of the [[Fermi surface]].<ref name=doiron-leyraud2007>{{cite journal|last=Doiron-Leyraud|first=Nicolas|coauthors=et al.|title=Quantum oscillations and the Fermi surface in an underdoped high-Tc superconductor|journal=Nature|year=2007|volume=447|pages=565–568|doi=10.1038/nature05872|arxiv = 0801.1281 |bibcode = 2007Natur.447..565D|issue=7144|pmid=17538614 }}</ref> The [[quantum hall effect]] is another example of measurements with high magnetic fields where topological properties such as [[Chern–Simons theory|Chern–Simons angle]] can be measured experimentally.<ref name=nap-cmp />
[[File:Bose Einstein condensate.png|thumb|left|The first [[Bose–Einstein condensate]] observed in a gas of ultracold [[rubidium]] atoms. The blue and white areas represent higher density.]]
===Cold atomic gases===
{{main|Optical lattice}}
[[Ion trap|Cold ion trapping]] in optical lattices is an experimental tool commonly used in condensed matter as well as [[atomic, molecular, and optical physics]].<ref name=schmeid-iontrap2008>{{cite journal|last=Schmeid|first=R.|coauthors=Roscilde, T.; Murg, V.; Porras, D. and Cirac, J. I.|title=Quantum phases of trapped ions in an optical lattice|journal=New Journal of Physics|year=2008|volume=10|doi=10.1088/1367-2630/10/4/045017|arxiv = 0712.4073 |bibcode = 2008NJPh...10d5017S|issue=4|pages=045017 }}</ref> The technique involves using optical lasers to create an [[interference (wave propagation)|interference pattern]], which acts as a "lattice", in which ions or atoms can be placed at very low temperatures.<ref name=greiner-nature2008>{{cite journal|last=Greiner|first=Markus|coauthors=Fölling, Simon |title=Condensed-matter physics: Optical lattices|journal=Nature|year=2008|volume=453|pages=736–738|doi=10.1038/453736a|bibcode = 2008Natur.453..736G|issue=7196|pmid=18528388 }}</ref> Cold atoms in optical lattices are used as "quantum simulators", that is, they act as controllable systems that can model behavior of more complicated systems, such as [[Geometrical frustration|frustrated magnets]].<ref name=buluta-science2009>{{cite journal|last=Buluta|first=Iulia|coauthors=Nori, Franco |title=Quantum Simulators|journal=Science|year=2009|volume=326|issue=5949|doi=10.1126/science.1177838|bibcode = 2009Sci...326..108B|pages=108–11|pmid=19797653 }}</ref> In particular, they are used to engineer one, two and three dimensional lattices for a [[Hubbard model]] with pre-specified parameters.<ref name=jaksch-aop2005>{{cite journal|last=Jaksch|first=D.|coauthors=Zoller, P. |title=The cold atom Hubbard toolbox|journal=Annals of Physics|year=2005|volume=315|issue=1|pages=52–79|doi=10.1016/j.aop.2004.09.010|arxiv = cond-mat/0410614 |bibcode = 2005AnPhy.315...52J }}</ref> and to study phase transitions for [[Néel temperature|Néel]] and [[spin liquid]] ordering.<ref name=schmeid-iontrap2008 />
In 1995, a gas of [[rubidium]] atoms cooled down to a temperature of 170 [[Kelvin|nK]] was used to experimentally realize the [[Bose–Einstein condensate]], a novel state of matter originally predicted by [[S. N. Bose]] and [[Albert Einstein]], wherein a large number of atoms occupy a single [[quantum state]].<ref name=nytimes-BEC>{{cite news|last=Glanz|first=James|title=3 Researchers Based in U.S. Win Nobel Prize in Physics|url=http://www.nytimes.com/2001/10/10/us/3-researchers-based-in-us-win-nobel-prize-in-physics.html|accessdate=23 May 2012|newspaper=The New York Times|date=October 10, 2001}}</ref>
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