Delta well + infinite barrier -> bound state

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Discussion Overview

The discussion revolves around the quantum mechanical analysis of a potential defined by a delta function well and an infinite barrier. Participants explore the conditions for the existence of bound states, focusing on the parameters of the potential, specifically the strength of the delta function (\lambda) and its position (d). The conversation includes theoretical considerations, mathematical formulations, and interpretations of bound states in the context of quantum mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents the potential and seeks to understand the conditions for bound states, noting the wavefunction forms in different regions.
  • Another participant suggests that there may not be an explicit relation between \lambda and d, but mentions a derivative condition related to the delta function.
  • Several participants discuss the requirement for the wavefunction to decay exponentially for x > d, indicating that k must become imaginary.
  • There is a debate about the definition of a bound state, with one participant questioning how E > 0 solutions can be considered bound states given the infinite barrier at x = 0.
  • Another participant clarifies that normalizable solutions are considered bound states, while non-normalizable solutions belong to the continuum.
  • One participant provides a working equation and attempts to derive a condition relating \lambda and d, but expresses uncertainty about the implications of E > 0 and E < 0.
  • Discussions arise regarding the use of sinh and cosh functions versus sin and cos functions for bound states, particularly in relation to the behavior of the wavefunction as x approaches infinity.
  • Corrections and refinements are made to earlier claims, with one participant ultimately deriving a condition for the existence of bound states as \frac{2m\lambda d}{\hbar^2} > 1.

Areas of Agreement / Disagreement

Participants exhibit a mix of agreement and disagreement. While some points, such as the requirement for wavefunction decay and the nature of bound states, are acknowledged, there remains uncertainty regarding the explicit relationship between \lambda and d, as well as the interpretation of energy levels in this context.

Contextual Notes

Limitations include unresolved assumptions about the nature of bound states and the dependence on the definitions of normalizable versus non-normalizable solutions. The discussion also reflects varying interpretations of the mathematical conditions derived from the Schrödinger equation.

  • #31
reilly said:
Think of the delta function, with positive lambda, as a very narrow, deep potential well. Use your common sense and experience with the potential well to get the appropriate general form of the solution.
Regards,
Reilly Atkinson

Reilly, thanks for your reply, but which part are you referring to?

As I stated in an earlier post, I was able to get the bound state solution. But in post #18, I posed the second part of the problem, which deals with the asymptotic nature of the wavefunction in case of a scattering problem for the same V(x). I solved that and got an equation for the phase shift.

But as far as I can tell, the phase shift can be determined exactly from that equation and unless I have made a mistake in the computation, there is no need to assume that \phi \approx ak and determine a as the question demands.
 

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