DOCSLIB.ORG

The Scattering and Shrinking of a Gaussian Wave Packet by Delta Function Potentials Fei

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The Scattering and Shrinking of a Gaussian Wave Packet by Delta Function Potentials Fei The Scattering and Shrinking of a Gaussian Wave Packet by Delta Function Potentials by Fei Sun Submitted to the Department of Physics ARCHIVF in partial fulfillment of the requirements for the degree of Bachelor of Science r U at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2012 @ Massachusetts Institute of Technology 2012. All rights reserved. A u th or ........................ ..................... Department of Physics May 9, 2012 C ertified by .................. ...................... ................ Professor Edmund Bertschinger Thesis Supervisor, Department of Physics Thesis Supervisor Accepted by.................. / V Professor Nergis Mavalvala Senior Thesis Coordinator, Department of Physics The Scattering and Shrinking of a Gaussian Wave Packet by Delta Function Potentials by Fei Sun Submitted to the Department of Physics on May 9, 2012, in partial fulfillment of the requirements for the degree of Bachelor of Science Abstract In this thesis, we wish to test the hypothesis that scattering by a random potential causes localization of wave functions, and that this localization is governed by the Born postulate of quantum mechanics. We begin with a simple model system: a one-dimensional Gaussian wave packet incident from the left onto a delta function potential with a single scattering center. Then we proceed to study the more compli- cated models with double and triple scattering centers. Chapter 1 briefly describes the motivations behind this thesis and the phenomenon related to this research. Chapter 2 to Chapter 4 give the detailed calculations involved in the single, double and triple scattering cases; for each case, we work out the exact expressions of wave functions, write computer programs to numerically calculate the behavior of the wave packets, and use graphs to illustrate the results of the calculations. In Chapter 5, we study the parameters that determine how much the wave function shrinks, including the initial width, the initial position and the momentum of the Gaussian wave packet, and the strength of and the spacing between the delta functions; then we examine dif- ferent combinations of the parameters in order to find a pattern to achieve maximum shrinking. Chapter 6 concludes the thesis with the essential results of this research as well as its implications and potentials. Thesis Supervisor: Professor Edmund Bertschinger Title: Thesis Supervisor, Department of Physics 3 4 0.1 Acknowledgments I would like to express my gratitude to all those who helped me to complete this thesis. I am indebted most to my supervisor Professor Edmund Bertschinger from the Department of Physics for all his inspiring suggestions, valuable guidance and encouragement in the past two years. He was patient in all the time of research for and writing of this thesis, and he was of great help in difficult times. The research was more complicated than we intially expected, and Professor Bertschinger helped me go through many bottlenecks where patience and persistence really made a difference. I also want to thank MIT's UROP office and Professor Bertschinger for giving me funding to do the research. Professor David Vogan from the Department of Mathematics and Robyn Sanderson from the Department of Physics also helped me in my research work. Professor Vogan gave me assistance on contour integrals, and Robyn Sanderson helped me with numerical simulations. I would like to thank them for all their interest and support. 5 6 Contents 0.1 Acknowledgments ... ...... ...... ...... ...... 5 1 Introduction 15 2 Single Scattering 17 2.1 Fourier Transform . .... .... .... .... ..... .... ... 17 2.2 The 01 Integral .............. 19 2.3 Table of Notifications ... ..... ..... ..... ..... .... 25 3 Double Scattering 27 3.1 <A(x, t) and ')B(X, t) .... .... - . ---- . ... - ---. 27 3.2 Analysis of IA when fi(x) < 0 .... ...... ....... ..... 31 3.3 Table of Notifications ..... ..... ..... ..... ..... 36 4 Triple Scattering 39 4.1 Triple Scattering Solution ... .... ..... .... .... .... 39 4.2 Table of Notifications ... .... .... .... .... ..... ... 45 5 Results and Analysis 47 5.1 Verification ... ...... .... ..... ..... ..... .... 47 5.2 Spacetime Color Graphs . ...... ....... ...... ..... 49 6 Conclusion 57 7 8 List of Figures 2-1 This is a schematic graph of the position of the pole k = -io in the u-plane for (1) #0 > 0 (2) 0 > 0 > -2 (3)#o < - <0, att=0. 2oO The integral is along the real k-axis. When 0 > 0, the contour of integral is always above the pole k = -io; when 0 < 0, the contour of integral is always below the pole k = -io. The position of the pole is given by equation (2.24), and the line Im(k)=0 accords with equation (2.26). ... .... .... .... .... .... .... .. 21 2-2 This is a schematic graph of shifting the contour of integral from the real k-axis to the real u-axis for (1) #0 > 0 (2) 0 > po > - (3) 0 < - ) < 0, at t = 0. The integral is originally along the real k-axis. When 0 > 0, the contour of integral is always above the pole k = -io; when 0 < 0, the contour of integral is always below the pole k = -iZ30 . But we want to replace the original contour of integral, the real k-axis, by the real u-axis to make the integral easier, so we close the contour at positive and negative infinities. ... ... ... 22 9 2-3 This is a schematic graph of the pole k = -io (1) moving up and crossing the real u-axis for #o > 0 (2) moving up and crossing the real u-axis for 0 > #3 > - fx (3) moving down and crossing the real u-axis for #o < - -2-T < 0, all for t > 0. The position of the pole is given by un(x, t) = -z(x, t) as in equation (2.21). In case (1), the pole crossing the real u-axis results in a negative residue added to the solution, because the contour going around the pole is clockwise. In case (2), the pole crossing the real u-axis results in the residue removed from the solution, because the pole is no longer enclosed by the contour. In case (3), the pole crossing the real u-axis results in a positive residue added to the solution, because the contour going around the pole is counterclockwise. .... .... .... .... .... .... ... 24 3-1 This is a schematic drawing for the scattering centers and the wave- function in three different regions for the double scattering case. 28 3-2 This is a schematic graph of the position of the pole k = -iO at t = 0 in the u-plane for integral IA when fi(x) < 0. The three cases are (1) 0 > - > 0, (2) 0 < - - , and (3) #o < 0. The integral is along the real k-axis. When 0 > 0, the contour of integral is always above the pole k = -i3o; when 0o < 0, the contour of integral is always below the pole k = -io. The position of the pole is given by equation (3.30), and the line Im(k)=0 always lies above the real u-axis when fi(x) < 0, according to equation (3.32). .. .... ... ... 32 10 3-3 This is a schematic graph of shifting the contour of integral from the real k-axis to the real u-axis for (1) #0 > - > 0, (2) 0 < #0 < - (3) /3o < 0, at t = 0. The integral is originally along the real k-axis. When #0 > 0, the contour of integral is always above the pole k = -i; when #o < 0, the contour of integral is always below the pole k = -io. But we want to replace the original contour of integral, the real k-axis, by the real u-axis to make the integral easier, so we close the contour at positive and negative infinities. ...... .... 34 3-4 This is a schematic graph of the pole k = -iO (1) moving up and crossing the real u-axis for 3o > -fi(x) > 0 (2) moving down and crossing the real u-axis for 0 < 0o < - 2'(T (3) moving down and crossing the real u-axis for 0 < 0, all for t > 0. The position of the pole is given by Up,A(X, t) = -zA(x, t) as in equation (3.27). In case (1), the pole crossing the real u-axis results in a negative residue added to the solution, because the contour going around the pole is clockwise. In case (2), the pole crossing the real u-axis results in the residue removed from the solution, because the pole is no longer enclosed by the contour. In case (3), the pole crossing the real u-axis results in a positive residue added to the solution, because the contour going around the pole is counterclockwise. ... .... ..... ..... 35 4-1 This is a schematic drawing for triple scattering case. ...... ... 40 5-1 The graphs on the left are made by our computer program, wpscatt.c. The graphs on the right tell us the difference between the graphs made by our computer program and the graph made by Mathematica with a delta function potential of infinite strength. The parameters are set to be: xi = 8 nm, oo = v2 nm, ko = 3 nm1, and t = 25 fs. ...... 54 11 5-2 This is the spacetime color graph of |@(x, t) 2 for single scattering; The parameters are set to be xi = 8 nm, oo = v nm, ko = 3 nm- 1 , and Vo = 0.15 * 10-19 J*nm. White indicates 1.0, purple 0.001, and black any value below 0.001. .... ........ ...... ....... 55 5-3 This is the spacetime color graph of |@(x, t) 2 for double scattering; zO = 1 nm, Vo = 0.15 * 10-19 J*nm, and all the other parameters have the same values as in the single scattering graph . White indicates 1.0 and purple 0.001. .... ..... .... .... ..... .... 55 5-4 This is the spacetime color graph of |@(x, t)12 for triple scattering; x1 = 2 nm, V1 = 0.15 * 10-19 J*nm, and all the other parameters have the same values as in the double scattering graph.
Load more

Recommended publications
  • Dirac -Function Potential in Quasiposition Representation of a Minimal-Length Scenario
    Eur. Phys. J. C (2018) 78:179 https://doi.org/10.1140/epjc/s10052-018-5659-6 Regular Article - Theoretical Physics Dirac δ-function potential in quasiposition representation of a minimal-length scenario M. F. Gusson, A. Oakes O. Gonçalves, R. O. Francisco, R. G. Furtado, J. C. Fabrisa, J. A. Nogueirab Departamento de Física, Universidade Federal do Espírito Santo, Vitória, ES 29075-910, Brazil Received: 26 April 2017 / Accepted: 22 February 2018 / Published online: 3 March 2018 © The Author(s) 2018. This article is an open access publication Abstract A minimal-length scenario can be considered as Although the first proposals for the existence of a mini- an effective description of quantum gravity effects. In quan- mal length were done by the beginning of 1930s [1–3], they tum mechanics the introduction of a minimal length can were not connected with quantum gravity, but instead with be accomplished through a generalization of Heisenberg’s a cut-off in nature that would remedy cumbersome diver- uncertainty principle. In this scenario, state eigenvectors of gences arising from quantization of systems with an infinite the position operator are no longer physical states and the number of degrees of freedom. The relevant role that grav- representation in momentum space or a representation in a ity plays in trying to probe a smaller and smaller region of quasiposition space must be used. In this work, we solve the the space-time was recognized by Bronstein [4] already in Schroedinger equation with a Dirac δ-function potential in 1936; however, his work did not attract a lot of attention.
    [Show full text]
  • Scattering from the Delta Potential
    Phys 341 Quantum Mechanics Day 10 Wed. 9/24 2.5 Scattering from the Delta Potential (Q7.1, Q11) Computational: Time-Dependent Discrete Schro Daily 4.W 4 Science Poster Session: Hedco7pm~9pm Fri., 9/26 2.6 The Finite Square Well (Q 11.1-.4) beginning Daily 4.F Mon. 9/29 2.6 The Finite Square Well (Q 11.1-.4) continuing Daily 5.M Tues 9/30 Weekly 5 5 Wed. 10/1 Review Ch 1 & 2 Daily 5.W Fri. 10/3 Exam 1 (Ch 1 & 2) Equipment Load our full Python package on computer Comp 5: discrete Time-Dependent Schro Griffith’s text Moore’s text Printout of roster with what pictures I have Check dailies Announcements: Daily 4.W Wednesday 9/24 Griffiths 2.5 Scattering from the Delta Potential (Q7.1, Q11) 1. Conceptual: State the rules from Q11.4 in terms of mathematical equations. Can you match the rules to equations in Griffiths? If you can, give equation numbers. 7. Starting Weekly, Computational: Follow the instruction in the handout “Discrete Time- Dependent Schrodinger” to simulate a Gaussian packet’s interacting with a delta-well. 2.5 The Delta-Function Potential 2.5.1 Bound States and Scattering States 1. Conceptual: Compare Griffith’s definition of a bound state with Q7.1. 2. Conceptual: Compare Griffith’s definition of tunneling with Q11.3. 3. Conceptual: Possible energy levels are quantized for what kind of states (bound, and/or unbound)? Why / why not? Griffiths seems to bring up scattering states out of nowhere. By scattering does he just mean transmission and reflection?" Spencer 2.5.2 The Delta-Function Recall from a few days ago that we’d encountered sink k a 0 for k ko 2 o k ko 2a for k ko 1 Phys 341 Quantum Mechanics Day 10 When we were dealing with the free particle, and we were planning on eventually sending the width of our finite well to infinity to arrive at the solution for the infinite well.
    [Show full text]
  • Revisiting Double Dirac Delta Potential
    Revisiting double Dirac delta potential Zafar Ahmed1, Sachin Kumar2, Mayank Sharma3, Vibhu Sharma3;4 1Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India 2Theoretical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India 3;4Amity Institute of Applied Sciences, Amity University, Noida, UP, 201313, India∗ (Dated: June 14, 2016) Abstract We study a general double Dirac delta potential to show that this is the simplest yet versatile solvable potential to introduce double wells, avoided crossings, resonances and perfect transmission (T = 1). Perfect transmission energies turn out to be the critical property of symmetric and anti- symmetric cases wherein these discrete energies are found to correspond to the eigenvalues of Dirac delta potential placed symmetrically between two rigid walls. For well(s) or barrier(s), perfect transmission [or zero reflectivity, R(E)] at energy E = 0 is non-intuitive. However, earlier this has been found and called \threshold anomaly". Here we show that it is a critical phenomenon and we can have 0 ≤ R(0) < 1 when the parameters of the double delta potential satisfy an interesting condition. We also invoke zero-energy and zero curvature eigenstate ( (x) = Ax + B) of delta well between two symmetric rigid walls for R(0) = 0. We resolve that the resonant energies and the perfect transmission energies are different and they arise differently. arXiv:1603.07726v4 [quant-ph] 13 Jun 2016 ∗Electronic address: 1:[email protected], 2: [email protected], 3: [email protected], 4:[email protected] 1 I. INTRODUCTION The general one-dimensional Double Dirac Delta Potential (DDDP) is written as [see Fig.
    [Show full text]
  • Paradoxical Quantum Scattering Off a Time Dependent Potential?
    Paradoxical Quantum Scattering off a Time Dependent Potential? Ori Reinhardt and Moshe Schwartz Department of Physics, Raymond and Beverly Sackler Faculty Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel We consider the quantum scattering off a time dependent barrier in one dimension. Our initial state is a right going eigenstate of the Hamiltonian at time t=0. It consists of a plane wave incoming from the left, a reflected plane wave on the left of the barrier and a transmitted wave on its right. We find that at later times, the evolving wave function has a finite overlap with left going eigenstates of the Hamiltonian at time t=0. For simplicity we present an exact result for a time dependent delta function potential. Then we show that our result is not an artifact of that specific choice of the potential. This surprising result does not agree with our interpretation of the eigenstates of the Hamiltonian at time t=0. A numerical study of evolving wave packets, does not find any corresponding real effect. Namely, we do not see on the right hand side of the barrier any evidence for a left going packet. Our conclusion is thus that the intriguing result mentioned above is intriguing only due to the semantics of the interpretation. PACS numbers: 03.65.-w, 03.65.Nk, 03.65.Xp, 66.35.+a The quantum problem of an incoming plain wave The eigenstates of the Hamiltonian (Eq. 1) can be encountering a static potential barrier in one dimension is classified according to the absolute value of incoming an old canonical text book example in which the reflection momentum and to whether they are right or left going.
    [Show full text]
  • Casimir Effect for Impurity in Periodic Background in One Dimension
    Journal of Physics A: Mathematical and Theoretical J. Phys. A: Math. Theor. 53 (2020) 325401 (24pp) https://doi.org/10.1088/1751-8121/ab9463 Casimir effect for impurity in periodic background in one dimension M Bordag Institute for Theoretical Physics, Universität Leipzig Brüderstraße 16 04103 Leipzig, Germany E-mail: [email protected] Received 2 April 2020, revised 7 May 2020 Accepted for publication 19 May 2020 Published 27 July 2020 Abstract We consider a Bose gas in a one-dimensional periodic background formed by generalized delta function potentials with one and two impurities. We inves- tigate the scattering off these impurities and their bound state levels. Besides expected features, we observe a kind of long-range correlation between the bound state levels of two impurities. Further, we define and calculate the vac- uum energy of the impurity. It causes a force acting on the impurity relative to the background. We define the vacuum energy as a mode sum. In order toget a discrete spectrum we start from a finite lattice and use Chebychev polynomi- als to get a general expression. These allow also for quite easy investigation of impurities in finite lattices. Keywords: quantum vacuum, Casimir effect (theory), periodic potential (Some figures may appear in colour only in the online journal) 1. Introduction One-dimensional systems provide a large number of examples to study quantum effects. These allow frequently for quite explicit and instructive formulas, but at the same time there are many one-dimensional and quasi-one-dimensional systems in physics which are frequently also of practical interest like nanowires and carbon nanotubes.
    [Show full text]
  • Tunneling in Fractional Quantum Mechanics
    Tunneling in Fractional Quantum Mechanics E. Capelas de Oliveira1 and Jayme Vaz Jr.2 Departamento de Matem´atica Aplicada - IMECC Universidade Estadual de Campinas 13083-859 Campinas, SP, Brazil Abstract We study the tunneling through delta and double delta po- tentials in fractional quantum mechanics. After solving the fractional Schr¨odinger equation for these potentials, we cal- culate the corresponding reflection and transmission coeffi- cients. These coefficients have a very interesting behaviour. In particular, we can have zero energy tunneling when the order of the Riesz fractional derivative is different from 2. For both potentials, the zero energy limit of the transmis- sion coefficient is given by = cos2 (π/α), where α is the T0 order of the derivative (1 < α 2). ≤ 1. Introduction In recent years the study of fractional integrodifferential equations applied to physics and other areas has grown. Some examples are [1, 2, 3], among many others. More recently, the fractional generalized Langevin equation is proposed to discuss the anoma- lous diffusive behavior of a harmonic oscillator driven by a two-parameter Mittag-Leffler noise [4]. Fractional Quantum Mechanics (FQM) is the theory of quantum mechanics based on the fractional Schr¨odinger equation (FSE). In this paper we consider the FSE as introduced by Laskin in [5, 6]. It was obtained in the context of the path integral approach to quantum mechanics. In this approach, path integrals are defined over L´evy flight paths, which is a natural generalization of the Brownian motion [7]. arXiv:1011.1948v2 [math-ph] 15 Mar 2011 There are some papers in the literature studying solutions of FSE.
    [Show full text]
  • Some Exact Results for the Schršdinger Wave Equation with a Time Dependent Potential Joel Campbell NASA Langley Research Center
    Some Exact Results for the Schršdinger Wave Equation with a Time Dependent Potential Joel Campbell NASA Langley Research Center, MS 488 Hampton, VA 23681 [email protected] Abstract The time dependent Schršdinger equation with a time dependent delta function potential is solved exactly for many special cases. In all other cases the problem can be reduced to an integral equation of the Volterra type. It is shown that by knowing the wave function at the origin, one may derive the wave function everywhere. Thus, the problem is reduced from a PDE in two variables to an integral equation in one. These results are used to compare adiabatic versus sudden changes in the potential. It is shown that adiabatic changes in the p otential lead to conservation of the normalization of the probability density. 02.30Em, 02.60-x, 03.65-w, 61.50-f Introduction Very few cases of the time dependent Schršdinger wave equation with a time dependent potential can be solved exactly. The cases that are known include the time dependent harmonic oscillator,[ 1-3] an example of an infinite potential well with a moving boundary,[4-6] and various other special cases. There remain, however, a large number of time dependent problems that, at least in principal, can be solved exactly. One case that has been looked at over the years is the delta function potential. The author first investigated this problem in the mid 1980s [7] as an unpublished work. This was later included as part of a PhD thesis dissertation [8] but not published in a journal.
    [Show full text]
  • The Attractive Nonlinear Delta-Function Potential
    The Attractive Nonlinear Delta-Function Potential M. I. Molina and C. A. Bustamante Facultad de Ciencias, Departamento de F´ısica, Universidad de Chile Casilla 653, Las Palmeras 3425, Santiago, Chile. arXiv:physics/0102053v1 [physics.ed-ph] 16 Feb 2001 1 Abstract We solve the continuous one-dimensional Schr¨odinger equation for the case of an inverted nonlinear delta–function potential located at the origin, ob- taining the bound state in closed form as a function of the nonlinear expo- nent. The bound state probability profile decays exponentially away from the origin, with a profile width that increases monotonically with the non- linear exponent, becoming an almost completely extended state when this approaches two. At an exponent value of two, the bound state suffers a discontinuous change to a delta–like profile. Further increase of the expo- nent increases again the width of the probability profile, although the bound state is proven to be stable only for exponents below two. The transmission of plane waves across the nonlinear delta potential increases monotonically with the nonlinearity exponent and is insensitive to the sign of its opacity. 2 The delta-function potential δ(x x ) has become a familiar sight in the − 0 landscape of most elementary courses on quantum mechanics, where it serves to illustrate the basic techniques in simple form. As a physical model, it has been used to represent a localized potential whose energy scale is greater than any other in the problem at hand and whose spatial extension is smaller than other relevant length scales of the problem. Arrays of delta-function potentials have been used to illustrate Bloch’s theorem in solid state physics and also in optics, where in the scalar approximation, wave propagation in a periodic medium resembles the dynamics of an electron in a crystal lattice.
    [Show full text]
  • Green Function for Delta Potential and One Loop Correction to Quantum Electrodynamic Scattering Process
    GREEN FUNCTION FOR DELTA POTENTIAL AND ONE LOOP CORRECTION TO QUANTUM ELECTRODYNAMIC SCATTERING PROCESS A project report for partial fulfillment of requirements for degree of Integrated Master of Science in Physics Submitted by Raj Kishore 410PH5084 Supervised by Dr. Biplab Ganguli Department of Physics and Astronomy National Institute of Technology Rourkela Abstract Greens function a technique used to solve in general non homogeneous dif- ferential equations. It is basically a correlation function. Its application in high energy physics in finding the propagators has discussed here. In this paper, its application in quantum physics to find Greens functions for quan- tum operators and its solutions has the main focus. By knowing the Greens function we can calculate density of states. In this paper, contains a detail calculations to find the Greens function for single and double delta function potential and then analysis of the bound state. In particle physics one of the main observable concern for the experiment is the scattering cross-section. Physicists used the perturbation theory to approximate results of the scattering process, as we do not have the ex- act knowledge of scattering process. In the higher order corrections, due to loops, the amplitude expression for the Feynman diagram diverges. We apply a theory of renormalization to get a physically possible results. This paper start with the basic introduction to renormalization theory then its application to quantum electrodynamics scattering process. I have briefly discussed all three types of loops found in QED and focused mainly on the vacuum polarization (photon self energy) corrections which give the charge renormalization.
    [Show full text]
  • Solution of the Quantum Harmonic Oscillator Plus a Delta-Function Potential at the Origin: the Oddness of Its Even-Parity Solutions
    Home Search Collections Journals About Contact us My IOPscience Solution of the quantum harmonic oscillator plus a delta-function potential at the origin: the oddness of its even-parity solutions This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 Eur. J. Phys. 32 1377 (http://iopscience.iop.org/0143-0807/32/5/025) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 186.105.181.147 The article was downloaded on 09/09/2011 at 03:47 Please note that terms and conditions apply. IOP PUBLISHING EUROPEAN JOURNAL OF PHYSICS Eur. J. Phys. 32 (2011) 1377–1384 doi:10.1088/0143-0807/32/5/025 Solution of the quantum harmonic oscillator plus a delta-function potential at the origin: the oddness of its even-parity solutions J Viana-Gomes and N M R Peres Physics Department, University of Minho, CFUM, P-4710-057 Braga, Portugal E-mail: zgomes@fisica.uminho.pt Received 3 May 2011, in final form 11 July 2011 Published 12 August 2011 Online at stacks.iop.org/EJP/32/1377 Abstract We derive the energy levels associated with the even-parity wavefunctions of the harmonic oscillator with an additional delta-function potential at the origin. Our results bring to the attention of students a non-trivial and analytical example of a modification of the usual harmonic oscillator potential, with emphasis on the modification of the boundary conditions at the origin. This problem calls the attention of the students to an inaccurate statement in quantum mechanics textbooks often found in the context of the solution of the harmonic oscillator problem.
    [Show full text]
  • 3. Some One-Dimensional Potentials These Notes Are a Supplement to Sections 3.1, 3.3 and 3.5 in Hemmer’S Book
    TFY4215/FY1006 | Lecture notes 3 1 Lecture notes 3 3. Some one-dimensional potentials These notes are a supplement to sections 3.1, 3.3 and 3.5 in Hemmer's book. 3.1 General properties of energy eigenfunctions (Hemmer 3.1, B&J 3.6) For a particle moving in a one-dimensional potential V (x), the energy eigenfunctions are the acceptable solutions of the time-independent Schr¨odingerequation Hc = E : h¯2 @2 ! d2 2m − + V (x) = E ; or = [V (x) − E] : (T3.1) 2m @x2 dx2 h¯2 3.1.a Energy eigenfunctions can be chosen real Locally, this second-order differential equation has two independent solutions. Since V (x) and E both are real, we can notice that if a solution (x) (with energy E) of this equation is complex, then both the real and the imaginary parts of this solution, 1 1 <e[ (x)] = [ (x) + ∗(x)] and =m[ (x)] = [ (x) − ∗(x)]; 2 2i will satisfy (T3.1), for the energy E. This means that we can choose to work with two independent real solutions, if we wish (instead of the complex solutions (x) and ∗(x)). An example: For a free particle (V (x) = 0), 1 p (x) = eikx; with k = 2mE h¯ is a solution with energy E. But then also ∗(x) = exp(−ikx) is a solution with the same energy. If we wish, we can therefore choose to work with the real and imaginary parts of (x), which are respectively cos kx and sin kx, cf particle in a box. Working with real solutions is an advantage e.g.
    [Show full text]
  • Self-Adjoint Extensions of Operators and the Teaching of Quantum
    Self-adjoint extensions of operators and the teaching of quantum mechanics Guy BONNEAU ∗ Jacques FARAUT † Galliano VALENT ∗ Abstract For the example of the infinitely deep well potential, we point out some paradoxes which are solved by a careful analysis of what is a truly self- adjoint operator. We then describe the self-adjoint extensions and their spectra for the momentum and the Hamiltonian operators in different physical situations. Some consequences are worked out, which could lead to experimental checks. arXiv:quant-ph/0103153v1 28 Mar 2001 PAR/LPTHE/99-43 December 1999 ∗Laboratoire de Physique Th´eorique et des Hautes Energies, Unit´eassoci´ee au CNRS UMR 7589, Universit´e Paris 7-Denis Diderot, 2 Place Jussieu, 75251 Paris Cedex 05. †Laboratoire d’Analyse Alg´ebrique, Universit´eParis 6-Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05. 2 Bonneau, Faraut, Valent 1 Introduction In most French universities, quantum mechanics is usually taught in the third year courses, separately from its applications to atomic, molecular and subnuclear physics, which are dealt with during the fourth year. In such “first contact” lectures, many mathematical subtleties are necessarily left aside. However, even in such commonly used examples as infinitely deep potential wells, overlooking the mathematical problems leads to contradictions which may be detected by a careful student and which have to do with a precise definition of the “observables” i.e. the self-adjoint operators. Of course, experts in the mathematical theory of unbounded operators in Hilbert spaces know the correct answer to these questions, but we think it could be useful to popularize these concepts among the teaching community and the more mature students of fourth year courses.
    [Show full text]