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Lecture Notes on Quantum Mechanics

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Lecture Notes on Quantum Mechanics Lecture Notes on Quantum Mechanics J¨orgSchmalian, Karlsruhe Institute of Technology Summer Semester 2015 2 Preface These lecture notes summarize the main content of the course Quantum Me- chanics I (Theory D), taught at the Karlsruhe Institute of Technology during the summer semester 2015. They are based on the graduate course Quantum Physics, taught at Iowa State University during Fall 2006, 2007 and 2008. 3 4 Contents 1 The Schr¨odingerequation 7 1.1 De Broglie's matter waves . 7 1.2 Interpretation of the Schr¨odingerequation . 9 1.3 Stationary Schr¨odinger equation . 13 1.4 Particle in a box . 14 1.5 Continuity of probability . 17 2 Measurement and uncertainty 19 2.1 Hermitian operators . 19 2.2 Dirac notation . 23 2.3 The momentum representation . 27 2.3.1 Particle in a homogeneous field . 28 2.4 The Uncertainty principle . 30 3 The harmonic oscillator 33 4 Additional one-dimensional problems 43 4.1 One dimensional barriers . 43 4.1.1 The step potential . 43 4.1.2 Rectangular barrier and tunneling . 46 4.2 Bound and extended states . 51 4.2.1 Rectangular box . 51 5 Angular momentum and spin 55 5.1 Particle on a circular orbit . 55 5.2 angular momentum operator . 56 5.3 General properties of angular momentum operators . 59 5.4 Eigenfunctions of the angular momentum . 62 5.5 The spin . 67 5.5.1 Precession of a spin in a magnetic field . 69 5.6 Addition of angular momentum . 70 5.7 Interacting spins . 71 5 6 CONTENTS 6 Particles in an external magnetic field 73 6.1 Gauge invariance . 73 6.2 Landau levels in a magnetic field . 74 6.2.1 Landau levels with spin . 76 6.3 Atom in a magnetic field . 77 6.4 Magnetic Monopoles . 78 6.5 The Aharonov-Bohm effect . 80 7 Pictures in quantum mechanics 83 8 Particle in a central potential 89 8.1 The hydrogen atom . 90 9 Time independent Perturbation theory 95 9.1 Non-degenerate perturbation theory . 96 9.1.1 Example: anharmonic oscillator . 98 9.2 Degenerate perturbation theory . 99 9.2.1 Example 1: two fold degenerate state . 100 9.2.2 Example 2: Stark Effect . 101 10 Variational principle 103 11 Path integral formulation of quantum mechanics 109 11.1 Path integral of a free particle . 113 12 Scattering Theory 117 Chapter 1 The Schr¨odingerequation 1.1 De Broglie's matter waves The beginning of the 20th century was characterized by an increasing accumula- tion of experimental data that could not be understood anymore using classical mechanics, electrodynamics or classical statistical mechanics, even though these approaches proved highly successful for a broad range of problems. These de- velopments include: 1898 Marja Sklodowska (Mdm. Curie) Radioactive polonium and radium 1901 Max Planck Unification of blackbody radiation 1905 Albert Einstein Photoelectric effect 1911 Ernest Rutherford Internal structure of the atom 1913 Niels Bohr Quantum theory of spectra 1922 Compton Scattering photons off electrons 1927 Davisson-Germer electron interference measurement These observations led to Planck's analysis of the black-body radiation and Einstein's postulate that light should be understood as a superposition of single quanta whose energy E and frequency ν are related by E = hν: (1.1) The proportionality factor is Planck's constant h = 6:6260755 × 10−34Js (1.2) and has dimension energy×time, just like an action. In his 1900 publication Planck already estimates the value h ' 6:55 × 10−34Js: The momentum of the photon is E hν p = = : (1.3) c c 7 8 CHAPTER 1. THE SCHRODINGER¨ EQUATION Using the wave length λ = c/ν and, what is often more convenient, the wave number 2π k = (1.4) λ it then follows p = ~k (1.5) with h = = 1:05457266 × 10−34Js: (1.6) ~ 2π Similarly this yields E = ~! (1.7) with angular frequency ! = 2πν. The idea that there is a particle character in what was accepted to be a wave had a complement in case of electrons. Those were believed to be particles, yet they displayed interference phenomena and thus behaves as waves. Louis de Broglie then made the radical assumption that not only photons have a particle- wave duality. The same is true for electrons and other quantum particles. He assumed similarly that there are waves obeying p = ~k and E = ~!. However, the ! (k) dependence must be consistent with the energy momentum dispersion relation p2 E = : (1.8) 2m It is said that Schr¨odingeronly wanted to put the de Broglie relationship on a formally more satisfying level and searched for a wave equation that reproduces the proper dispersion relation. Let us try to guess how such a wave equation could look like. To obtain the correct dispersion relation we start from the equation: @n @m a = b : (1.9) @tn @xm We want to be a wave, i.e. a solution of the kind / exp (ikx − i!t) : (1.10) @n n n @m m m It holds @tn = (−i) ! and @xm = i k and we find a (−i)n !n = bimkm : (1.11) Since we want our wave equation to yield E p2= (2m) k2 ! = = = ~ (1.12) ~ ~ 2m we can insert this and find k2 n a (−i)n ~ = bimkm (1.13) 2m 1.2. INTERPRETATION OF THE SCHRODINGER¨ EQUATION 9 This requires a fixed relation between the temporal and spatial derivatives: m n = : (1.14) 2 The number of time derivatives is half the number of space derivatives. This p2 result follows directly from the classical dispersion relation E = 2m , where the energy is the square of momentum. In addition it holds for the pre-factors n a (−i)n ~ = bim (1.15) 2m This only determines (for given n) the ratio a=b. The simplest choice (but by no means a unique choice) is to start with n = 1. This leads with a = i~ to: @ 2 @2 i = − ~ : (1.16) ~ @t 2m @x2 This is the Schr¨odingerequation for a single non-interacting non-relativistic particle. 1.2 Interpretation of the Schr¨odingerequation The generalization of the Schr¨odingerequation to more than one spatial dimen- sion is obvious: @ 2 i = − ~ r2 : (1.17) ~ @t 2m Having arrived at this new equation of motion a number of questions arise: 1. What is the physical interpretation of (x; t)? 2. How to go beyond the limit of a particle on free space, i.e. how does this equation look like in case of a finite potential? 3. How can one make contact to Newton's equation of motion that proved to be so successful for the mechanical motion of macroscopic bodies? etc. etc. A proposal that addresses the first question was made very early on by Max Born. He realized that (x; t) has not only an arbitrary sign (after all it is a wave). In general it can also be complex. The latter is due to the fact that time and space derivatives enter differently, leading to the imaginary unit i in the wave equation. It makes therefore no sense to talk about a large or a small wave function (x; t). On the other hand j (x; t)j2 can be large or small. Since it is positive definite it seems natural to call j (x; t)j2the density of the quantum particle. However, since the wave function is supposed to describe the properties of individual elementary particles, it makes strictly no sense to call j (x; t)j2 the particle density, a notion that requires that some fraction of the particle is 10 CHAPTER 1. THE SCHRODINGER¨ EQUATION located at one position and another fraction elsewhere. Born refined this and called ρ (r; t) = j (x; t)j2 = ∗ (x; t) (x; t) (1.18) the probability density. Thus, knowing j (x; t)j2 determines the probability to find the electron at a given time, t, at position x. This makes of course only sense if the probability distribution is properly normalized, i.e. that: d3rρ (r; t) = d3x ∗ (x; t) (x; t) = 1: (1.19) ˆ ˆ Since the Schr¨odingerequation is a linear equation it holds that λψ (x; t) is a solution if (x; t) is a solution, where λ is a time and coordinate independent complex number. Thus, we can always fix λ to ensure Eq.1.19. From probability we know that the expectation value of x, (i.e. the mean value of the position) is given by hxi = d3xxρ (x; t) t ˆ = d3x ∗ (x; t) x (x; t) : (1.20) ˆ Similarly, the mean square of the position is x2 = d3xx2ρ (x; t) t ˆ = d3x ∗ (x; t) x2 (x; t) : (1.21) ˆ The velocity (i.e. the change of the mean particle with time) is then @ hvi = hxi t @t t @ @ = d3x ∗ (x; t) x (x; t) + d3x ∗ (x; t) x (x; t) ˆ @t ˆ @t = ~ d3x r2 ∗ x (x; t) − ∗ (x; t) xr2 (x; t) 2mi ˆ = ~ d3x ∗r2x (x; t) − ∗ (x; t) xr2 (x; t) 2mi ˆ = ~ d3x ∗r ((rx) (x; t) + xr (x; t)) − ∗ (x; t) xr2 (x; t) 2mi ˆ = ~ d3x ∗ (rx) r (x; t) + (rx) r (x; t) + xr2 (x; t) 2mi ˆ − ∗ (x; t) xr2 (x; t) 1 = ~ d3x ∗ (x; t) r (x; t) = ~r mi ˆ m i t 1.2. INTERPRETATION OF THE SCHRODINGER¨ EQUATION 11 Classically we would expect that 1 hvi = hpi (1.22) t m t and we therefore realize that in order to determine the mean value of the mo- mentum we have to evaluate the average of p=~r (1.23) b i ~ The physical quantity momentum is therefore represented by the operator i r.
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