Presentation on theme: "The Independent Particle Approximation"— Presentation transcript:
1 The Independent Particle Approximation We approximate the strong electrostatic forces between e-s by treating the force on each electron independently, which includes force from nucleus and other electronsInner electrons can shield the nuclear charge, leading to “screening”+ZeelectronScreening electron cloudrThe effective potential energy felt by an electronZeff is the effective charge that the electron feels and depends on r. Note thatwhen r is inside all other electronswhen r is outside all other electronsUnlike in hydrogen, in multielectron atoms the dependence of the potential energy on r due to screening lifts the degeneracy between the n states
2 The Periodic TableColumns: groups with similar shells, similar propertiesRows: periods with elements with increasingly-full shellsClosed-shell–minus-one elements (halogens): elements with high electron affinity A (energy gained when an additional electron is added to a neutral atom); will easily form negative ions (take additional electron) in remaining p-shell state due to large nuclear charge; these elements are very reactive (e.g., F- with e.a.=3.4 eV)Closed-shell –plus one (alkali) elements: reactive due to loosely-bound outer electron in s-shell
3 The Ionic Bond: Explanation Property electrostatic force of attraction between positively and negatively charged ionsMost ionic compounds are brittle; a crystal will shatter if we try to distort it. This happens because distortion cause ions of like charges to come close together then sharply repel.BrittlenessMost ionic compounds are hard; the surfaces of their crystals are not easily scratched. This is because the ions are bound strongly to the lattice and aren't easily displaced.HardnessSolid ionic compounds do not conduct electricity when a potential is applied because there are no mobile charged particles.No free electrons causes the ions to be firmly bound and cannot carry charge by moving.ElectricalconductivityThe melting and boiling points of ionic compounds are high because a large amount of thermal energy is required to separate the ions which are bound by strong electrical forces.Melting pointand boiling pointExplanationProperty+Na+-Cl-RTotal energy of ion:The energy cost to transfer the electron from an alkali to a halogen isEffective potential2nd term is repulsion between 2 e- clouds
4 The Covalent Bonde.g.: F2, HFThe covalent bond is formed by sharing of outer shell electrons between atoms rather than by electron transfer.This lowers the energy of the system since electrons are attracted to both nuclei (stronger effective Coulomb potential)As an example, consider the H2+ molecular ion (two protons, one e-):As the distance between the atoms is decreased, significant interference between the wave functions occurIn the bonding (symmetric) y+ state electron has a larger probability of being attracted by both protons – this state is the one responsible for the molecule formation. Therefore, the bonding state has a lower energy than the antibonding (antisymmetric).
5 Comparison of Ionic and Covalent Bonding The type of bonding in a solid is determined mainly by the degree of overlap between the electronic wavefunctions of the atoms involved.
6 van der WaalsFrom charge fluctuations in atoms due to zero-point motion (from Heisenberg uncertainty principle); creates attractive dipole momentsAlways present, but significant only when other bonding not possibleTypical strength ~1% of other bonds, short range, varying as r -6To model the van der Waals interaction, considered two harmonic oscillators. Each dipole consists of a pair of opposite charges with a restoring force acting between each pair of charges.We wrote down the Hamiltonian for the oscillators. Transforming to normal coordinates decoupled the energy into a symmetric and antisymmetric contributions.Calculated the frequencies and bond energy1.11, used to model DNA
8 C2: Translational Lattice Vectors – 2D A lattice is a set of points such that a translation from any point in the lattice by a vector;Rn = n1 a + n2 blocates an exactly equivalent point, i.e. a point with the same environment as P. This is translational symmetry.The vectors a, b are known as lattice vectors and (n1, n2) is a pair of integers whose values depend on the lattice point.PPoint D (n1, n2) = (0,2)Point F (n1, n2) = (0,-1)Point P (n1, n2) = (3,2)
9 Crystal Structure = Crystal Lattice + Basis αabCBEDOAyxa) Situation of atoms at the corners of regular hexagonsb) Crystal lattice obtained by identifying all the atoms in (a)
11 Face-centered Cubic (FCC) Close-packed planes are perpendicular to cube diagonalStacking (ABCAB…) reduces symmetry to three-foldFour 3-fold rotation axes + mirror plane, therefore Oh (octahedral symmetry)Examples: Cu, Ag, Au, Ni, Pd, Pt, Al
12 Groups: Fill in this Table for Cubics SCBCCFCCVolume of conventional cella3Lattice points per cell124Volume, primitive cell½ a3¼ a3# of nearest neighbors6812Nearest-neighbor distancea½ a 3a/2# of second neighborsSecond neighbor distancea2
13 The Cubic Unit Cell looks like Many common semiconductors haveDiamond or Zincblende crystal structuresTetrahedral coordination: Each atom has 4 nearest-neighbors (nn). Basis set: 2 atoms. Lattice face centered cubic (fcc).Diamond or Zincblende 2 atoms per fcc lattice point.Diamond: The 2 atoms are the same. Zincblende: The 2 atoms are different.The Cubic Unit Cell looks likeFor ABCABC… stacking it is called zinc blende
14 Group: CsClThe figure shows the crystal structure of CsCl. Take the lattice constant as a, all the bonds shown have the same length. The grey atoms are Cs and the green ones are Cl.What are the primitive Bravais lattice and the associated basis for this crystal (including the locations of these atoms in terms of lattice parameter a)?What is the distance to the nearest neighbors of Cs?1V3
15 If CsCl is Simple Cubic, what is NaCl? CsCl: similar to bcc but atom at center of cube is differentNaCl: interpenetrating fcc structuresOne atom at (0,0,0)Second atom displaced by (1/2,0,0)Majority of ionic crystals prefer NaCl structure despite lower coordination (what is coordination?)Radius of cations much smaller than anionsFor very small cations, anions can not get too close in CsCl structureThis favors NaCl structure where anion contact does not limit structure as muchCsClNaCl
16 How many atoms per unit cell? PerovskitesPerovskitesA-site (Ba)OxygenB-site (Ti)BaTiO3Superconductors (YBa2Cu3O7-δ)Ferroelectrics (BaTiO3)Colossal Magnetoresistance (LaSrMnO3)Multiferroics (BiFeO3)High εr Insulators (SrTiO3)Low εr Insulators (LaAlO3)Conductors (Sr2RuO4)Thermoelectrics (doped SrTiO3)Ferromagnets (SrRuO3)Our interface work focuses in the field of complex oxides. In addition to being among the most abundant minerals on earth, complex oxides give some of the most varied and interesting properties. These include their use as dielectric and superconducting materials. Yet, only recently has the research in the field of complex oxides flourished because they were long thought to be …well complex. This complexity comes from the strong coupling with charge, spin, and lattice dynamics, which often results in very full phase diagrams.Though the coupling may lead to complex behaviors, the structure of these materials can be quite simple, such as the perovskite form shown here, where the A and B sites are typically different cations and X is an anion that bonds to both. This arrangement gives rise to a crystal field potential, hinders the free rotation of the electrons and quenches the orbital angular momentum by introducing the crystal field splitting of the d orbitals. Even among only perovskites, we can see these varied behaviors.Formula unit – ABO3A atoms (bigger) at the cornersO atoms at the face centersB atoms (smaller) at the body-centerHow many atoms per unit cell?
17 Mirror plane indicated by symbol m Reflection PlaneA plane in a cell such that, when a mirror reflection in this plane is performed (e.g., x’=-x, y’=y, z’=z), the cell remains invariant.Mirror plane indicated by symbol mExample: water molecule has 2 mirror planessv (xz)sv (yz)
18 Rotation Axes Rotation through an angle about a certain axis Trivial case is 360o rotationOrder of rotation: 2-, 3-, 4-, and 6- correspond to 180o, 120o, 90o, and 60o.These are only symmetry rotations allowed in crystals with long-range order; incompatible with translational symmetrySmall aggregates (short-range order) or molecules can also have 5-, 7-, etc. fold rotational symmetry
19 What rotation axes does a cubic perovskite have? A-site (Ba)OxygenB-site (Ti)BaTiO3
20 The density of lattice points on each plane of a set is the same and all lattice points are contained on each set of planes.z[2,3,3]Plane intercepts axes at2Reciprocal numbers are:Indices of the plane (Miller): (2 3 3)(No commas, commas are for points)2yIndices of the direction: [2,3,3]x3The vector perpendicular to the plane shares the same coordinates.Miller indices still apply for a non-cubic system
21 Distance between the (111) planes on a cubic lattice
22 Review: Reciprocal Lattice The reciprocal lattice is composed of all points lying at positions from the origin, so that there is one point in the reciprocal lattice for each set of planes (hkl) in the real-space lattice.Suppose G can be decomposed into basis vectors:(h, k, l integers)Note: a has dimensions of length, g has dimensions of length-1Ghkl is perpendicular to (hkl) planeThe basis vectors gi define a reciprocal lattice:1. for every real lattice there’s a reciprocal lattice2. reciprocal lattice vector g1 is perpendicular to plane defined by a2 and a3+ cyclic permutationsis volume of unit cella’s are not unique, but volume is
23 Constructing the Reciprocal Lattice Identify the basic planes in the direct space lattice.Draw normals to these planes from the origin.Note that distances from the origin along these normals is proportional to the inverse of the distance from the origin to the direct space planes.
24 Reciprocal Lattices to SC, FCC and BCC Direct lattice Reciprocal lattice Volume of RLSCBCCFCCDirectReciprocalSimple cubicbccfcc
25 DIFFRACTIONDiffraction is a wave phenomenon in which the apparent bending and spreading of waves when they meet an obstruction is measured.Light, radio, sound and water waves.Diffraction is optimally sensitive to the periodic nature of the solid’s atomic structure.Width Variable( nm)Wavelength Constant(600 nm)Distance d = Constant
26 Scattering Condition We know that G=2/dhkl =2kosin (from the figure) In a crystal, only significant contributions of this integral arise when G=K.(Reminder: G is perpendicular to plane.)DetectorNote: Real space and reciprocal space overlappedWe know that G=2/dhkl=2kosin (from the figure)Go through again. We will talk about what is today.sourcekoThus, to get diffraction:2/dhkl =2(2 /λ)sinor λ=2 dhkl sin
27 Weigner Seitz Cell: Smallest space enclosed when intersecting the midpoint to the neighboring lattice points.grapheneb1a1a2b2Real Space2-atom basisk SpaceWigner-Seitz Unit Cell of Reciprocal Lattice= First Brillouin zone, whose construction exhibits all the wavevectors k which can be Bragg-reflected by the crystalThe same perpendicular bisector logic applies in 3D
28 First Brillouin Zone of the FCC Lattice The BZ reflects lattice symmetryNote: fcc lattice in reciprocal space is a bcc latticeSCBCCFCC# of nearest neighbors6812Nearest-neighbor distancea½ a 3a/2# of second neighborsSecond neighbor distancea2FCC Primitive and Conventional Unit Cells
29 Group: Find the structure factor for FCC. Cubic form:002022220020200202000111Allowed low order reflections are:111, 200, 220, 311, 222, 400, 331, 310Forbidden reflections:100, 110, 210, 211
30 Structure Factor Ni3Al (L12) structure Simple cubic lattice, with a four atom basisAgain, since simple cubic, intensity at all points.But each point is ‘chemically sensitive’.
31 Atomic Scattering Factor f (aka Structure or Form Factor) Atoms are of a comparable size to the wavelength of the x-rays and so the scattering is not point like. There is a small path difference between waves scattered at either side of the electron cloud. Increases with For x-rays, scattering strength depends on electron densityCore electrons localized around nucleus, so density profile ~sphericalOnly at 2=0 does f=Z
32 Diffraction MethodsAny particle will scatter and create a diffraction patternBeams are selected by experimentalists depending on sensitivityX-rays not sensitive to low Z elements, but neutrons areElectrons sensitive to surface structure if energy is lowAtoms (e.g., helium) sensitive to surface only
33 Lattice VibrationsWhen a wave propagates along one direction, 1D problem.Use harmonic oscillator approx., meaning amplitude vibration small.Atoms are tied via bonds, so they can't vibrate independently. The vibrations take the form of collective modes which propagate. Phonons are quanta of lattice vibrations.Longitudinal WavesTransverse Waves
34 Monatomic Linear Chain The force on the nth atom;aaThe force to the right;The force to the left;Un-1UnUn+1The total force = Force to the right – Force to the leftThus, Newton’s equation for the nth atom isDiatomic benzetEqn’s of motion of all atoms are of this form, only the value of ‘n’ varies
35 Brillouin Zones of the Reciprocal Lattice Reciprocal Space Lattice:2p/a3rd Brillouin Zone2nd Brillouin Zone1st Brillouin Zone (BZ=WS)Each BZ contains identical information about the latticeThere is no point in saying that 2 adjacent atoms are out of phase by more than (e.g., 1.2 =-0.8 )Modes outside first Brillouin zone can be mapped to first BZ
37 Standing wave at the boundaries of the BZ (λ=2a) Wave velocityGROUP VELOCITY is velocity of energy transferIf vphase > vgroup, wave is dispersivevphase=k/kThe slope of the dispersion curve gives the group velocity.Near the origin k = 0 the phase and group velocity must be the same (dispersionless)The edges of the FBZ correspond to neighboring atoms moving in opposite directions. The energy cannot propagate along the crystal.Standing wave at the boundaries of the BZ (λ=2a)
38 Diatomic Chain(2 atoms in primitive basis) 2 different types of atoms of masses m1 and m2 are connected by identical springs(n-2) (n-1) (n) (n+1) (n+2)KKKKm2a)m1m1mm2ab)Un-1UnUn+1Un+2Un-2Since a is the repeat distance, the nearest neighbors separations is a/2Two equations of motion must be written;One for mass m1, and One for mass m2.
39 A when the two atoms oscillate in antiphase As there are two values of ω for each value of k, the dispersion relation is said to have two branchesOptical Branchл/a2–лkwABCUpper branch is due to thepositive sign of the root.Acoustical BranchNegative sign: k for small k. Dispersion-free propagation of sound wavesAt C, M oscillates and m is at rest.At B, m oscillates and M is at rest.This result remains valid for a chain containing an arbitrary number of atoms per unit cell.A when the two atoms oscillate in antiphase
41 3D Dispersion curvesEvery crystal has 3 acoustic branches, 1 longitudinal and 2 transverseEvery additional atom in the primitive basis contributes 3 further optical branches (again 2 transverse and 1 longitudinal)P atoms/primitive unit cell means 3 acoustic branches and 3(p-1) optical branches=3p branchesOne for each degree of freedom
42 Stress Tensor Forces divided by an area are called stresses. The stresses/tractions tk (or k) along axis k are... in components we can write this aswhere ij is the stress tensor and nj is a surface normal.The stress tensor describes the forces acting on planes within a body. Due to the symmetry conditionthere are only six independent elements.The vector normal to the corresponding surfaceThe direction of the force vector acting on that surface
43 We can therefore write: Similarly, the strain tensor can be written as:Additional simplification of the stress-strain relationship can be realized through simplifying the matrix notationfor stresses and strains.We can replace the indices as follows:Voigt’s notation:
44 For the generalized case, Hooke’s law may be expressed as: where, Both Sijkl and Cijkl are fourth-rank tensor quantities.The consequence of the symmetry in the stress and strain tensors is that only 36 components of the compliance and stiffness tensors are independent and distinct terms.But only one-half of the non-diagonal terms are independent constants since Cij = CjiIndependent terms
45 The cubic axes are equivalent, so the diagonal components for normal and shear distortions must be equal.And cubic is not elastically isotropic because a deformation along a cubic axis differs from the stress arising from a deformation along the diagonal.e.g.,  vs. Zener Anisotropy Ratio:x=(a-b)/2 or
46 These quantized normal modes of vibration are called PHONONSPHONONS are massless quantum mechanical particles which have no classical analogue.They behave like particles in momentum space or k space.Phonons are one example of many like this in many different areas of physics. Such quantum mechanical particles are often called“Quasiparticles”Examples of other Quasiparticles:Photons: Quantized Normal Modes of electromagnetic waves.Magnons: Quantized Normal Modes of magnetic excitations in magnetic solidsExcitons: Quantized Normal Modes of electron-hole pairsPolaritons: Quantized Normal Modes of electric polarization excitations in solids+ Many Others!!!
47 Conditions for: elastic scattering Phonon spectroscopy =Conditions for: elastic scatteringinConstraints:Conservation laws ofMomentumEnergyIn all interactions involving phonons, energy must be conserved and crystal momentum must be conserved to within a reciprocal lattice vector.