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Physlib.QuantumMechanics.PoschlTeller.Basic

1d Pöschl-Teller

i. Overview

The Pöschl-Teller potential, `V(x)∝−sech2(κx)V(x) \propto -\mathrm{sech}^2{(\kappa x)}`, gives rise to a one-dimensional quantum system for which the energy eigenvalues, energy eigenstates and scattering data can be computed exactly. Notably, the potential is _reflectionless_ when the parameter controlling its depth is a positive integer.

ii. Key results

iii. Table of contents

- A. Potential function - B. Hilbert space - C. Operators - C.1. Kinetic energy - C.2. Potential energy - C.3. Hamiltonian - C.4. Creation and annihilation operators - C.4.1. On Schwartz functions - C.4.2. As unbounded operators - D. As a quantum system

iv. References

  • https://arxiv.org/pdf/2411.14941

A. Potential function

B. Hilbert space

C. Operators

C.1. Kinetic energy

C.2. Potential energy

C.3. Hamiltonian

C.4. Creation and annihilation operators

#### C.4.1. On Schwartz functions

#### C.4.2. As unbounded operators

D. As a quantum system

28 declarations

definition

Pöschl-Teller potential V(x)V(x)

The Pöschl-Teller potential function maps a position xx in one-dimensional space to the real value: V(x)=−ℏ2κ2N(N+1)2mcosh⁡2(κx)V(x) = -\frac{\hbar^2 \kappa^2 N(N+1)}{2m \cosh^2(\kappa x)} where mm is the mass of the particle, ℏ\hbar is the reduced Planck's constant, κ\kappa is a parameter controlling the width of the potential, and NN is a parameter determining its depth.

abbrev

Hilbert space L2(Space 1,C)L^2(\text{Space } 1, \mathbb{C}) for the Pöschl-Teller system

For a given Pöschl-Teller system, the Hilbert space is defined as L2(Space 1,C)L^2(\text{Space } 1, \mathbb{C}), the space of square-integrable complex-valued functions on one-dimensional space.

definition

Multiplication by tanh⁡(κx)\tanh(\kappa x) as a continuous linear map

The continuous linear operator on the Schwartz space S(R,C)\mathcal{S}(\mathbb{R}, \mathbb{C}) defined by pointwise multiplication by the function x↦tanh⁡(κx)x \mapsto \tanh(\kappa x), where κ\kappa is the characteristic parameter of the Pöschl-Teller potential.

definition

Creation operator a^†\hat{a}^\dagger for the Pöschl-Teller potential

The creation operator for a one-dimensional Pöschl-Teller system is a continuous linear map on the Schwartz space S(R,C)\mathcal{S}(\mathbb{R}, \mathbb{C}) defined by the expression: 12m(p^+iℏκtanh⁡(κx)) \frac{1}{\sqrt{2m}} \left( \hat{p} + i\hbar\kappa \tanh(\kappa x) \right) where mm is the mass of the particle, κ\kappa is the characteristic parameter of the potential, p^\hat{p} is the momentum operator −iℏddx-i\hbar \frac{d}{dx}, ℏ\hbar is the reduced Planck constant, and ii is the imaginary unit. The operator involves the momentum operator and the pointwise multiplication by the hyperbolic tangent function tanh⁡(κx)\tanh(\kappa x).

definition

Annihilation operator a^\hat{a} on the Schwartz space S(R,C)\mathcal{S}(\mathbb{R}, \mathbb{C}) for the Pöschl-Teller potential

The annihilation operator a^\hat{a} for the one-dimensional Pöschl-Teller potential is defined as a continuous linear map on the Schwartz space S(R,C)\mathcal{S}(\mathbb{R}, \mathbb{C}). Given the system parameters for mass mm and the characteristic potential scale κ\kappa, the operator is expressed as: a^=12m(p^−iℏκtanh⁡(κx)) \hat{a} = \frac{1}{\sqrt{2m}} \left( \hat{p} - i\hbar\kappa \tanh(\kappa x) \right) where p^\hat{p} is the momentum operator, ℏ\hbar is the reduced Planck constant, and tanh⁡(κx)\tanh(\kappa x) denotes the operator for pointwise multiplication by the hyperbolic tangent function.

definition

Multiplication operator by tanh⁡(κx)\tanh(\kappa x)

The `tanhOperator` is the unbounded linear operator on the Hilbert space L2(R,C)L^2(\mathbb{R}, \mathbb{C}) defined by the pointwise multiplication by the function f(x)=tanh⁡(κx)f(x) = \tanh(\kappa x), where κ\kappa is the characteristic parameter of the Pöschl-Teller potential. The operator maps a wave function ψ∈L2(R,C)\psi \in L^2(\mathbb{R}, \mathbb{C}) to the product (tanh⁡(κx))ψ(x)(\tanh(\kappa x)) \psi(x) on the domain where this product remains square-integrable.

definition

Creation operator a^†\hat{a}^\dagger for the Pöschl-Teller potential

For a one-dimensional Pöschl-Teller system with mass mm and characteristic parameter κ\kappa, the creation operator a^†\hat{a}^\dagger is an unbounded linear operator on the Hilbert space L2(R,C)L^2(\mathbb{R}, \mathbb{C}). It is defined as: a^†=12m(p^+iℏκtanh⁡(κx)) \hat{a}^\dagger = \frac{1}{\sqrt{2m}} \left( \hat{p} + i\hbar\kappa \tanh(\kappa x) \right) where p^\hat{p} is the momentum operator, ℏ\hbar is the reduced Planck constant, and tanh⁡(κx)\tanh(\kappa x) denotes the operator for pointwise multiplication by the hyperbolic tangent function.

definition

Annihilation operator for the Pöschl-Teller system

The annihilation operator for the Pöschl-Teller system is an unbounded linear operator on the Hilbert space L2(R,C)L^2(\mathbb{R}, \mathbb{C}) defined by the expression 12m(P−iℏκtanh⁡(κx)) \frac{1}{\sqrt{2m}} (P - i \hbar \kappa \tanh(\kappa x)) where PP is the momentum operator −iℏddx-i\hbar \frac{d}{dx}, mm is the mass of the particle, ℏ\hbar is the reduced Planck constant, and κ\kappa is the characteristic parameter of the Pöschl-Teller potential.

theorem

The Pöschl-Teller potential is even: V(−x)=V(x)V(-x) = V(x)

For the one-dimensional Pöschl-Teller potential V(x)V(x), defined as V(x)=−ℏ2κ2N(N+1)2mcosh⁡2(κx)V(x) = -\frac{\hbar^2 \kappa^2 N(N+1)}{2m \cosh^2(\kappa x)} the potential function is even, meaning that for any position xx in one-dimensional space, V(−x)=V(x)V(-x) = V(x).

theorem

The function tanh⁡(κx)\tanh(\kappa x) has temperate growth

For the one-dimensional Pöschl-Teller system with characteristic parameter κ\kappa, the function f:R→Rf: \mathbb{R} \to \mathbb{R} defined by f(x)=tanh⁡(κx)f(x) = \tanh(\kappa x) has temperate growth. (A function has temperate growth if it and its derivatives are bounded by some polynomial at infinity).

theorem

The complexified function tanh⁡(κx)\tanh(\kappa x) has temperate growth

For the parameter κ\kappa of the one-dimensional Pöschl-Teller potential, the complex-valued function defined by x↦tanh⁡(κx)x \mapsto \tanh(\kappa x) has temperate growth.

theorem

The Pöschl-Teller Potential V(x)V(x) Has Temperate Growth

The Pöschl-Teller potential function V:Space 1→RV: \text{Space } 1 \to \mathbb{R}, defined as V(x)=−ℏ2κ2N(N+1)2mcosh⁡2(κx),V(x) = -\frac{\hbar^2 \kappa^2 N(N+1)}{2m \cosh^2(\kappa x)}, has temperate growth. A function is said to have temperate growth (or be slowly increasing) if it is smooth and all its derivatives are bounded by some polynomial. Here, mm is the particle mass, ℏ\hbar is the reduced Planck constant, and κ\kappa and NN are parameters characterizing the potential's shape.

theorem

The Complexified Pöschl-Teller Potential Has Temperate Growth

Let V:Space 1→RV: \text{Space } 1 \to \mathbb{R} be the Pöschl-Teller potential defined by V(x)=−ℏ2κ2N(N+1)2mcosh⁡2(κx),V(x) = -\frac{\hbar^2 \kappa^2 N(N+1)}{2m \cosh^2(\kappa x)}, where mm is the mass of the particle, ℏ\hbar is the reduced Planck constant, and κ\kappa and NN are parameters characterizing the potential. Then the complex-valued function f:Space 1→Cf: \text{Space } 1 \to \mathbb{C} defined by the composition of the real-to-complex inclusion ofReal\text{ofReal} and VV (i.e., f(x)=V(x)+0if(x) = V(x) + 0i) has temperate growth.

theorem

The Pöschl-Teller potential V(x)V(x) is continuous

The Pöschl-Teller potential function V:Space 1→RV: \text{Space } 1 \to \mathbb{R}, defined as V(x)=−ℏ2κ2N(N+1)2mcosh⁡2(κx),V(x) = -\frac{\hbar^2 \kappa^2 N(N+1)}{2m \cosh^2(\kappa x)}, is continuous, where mm is the mass of the particle, ℏ\hbar is the reduced Planck constant, and κ\kappa and NN are parameters characterizing the potential.

abbrev

1D Pöschl-Teller quantum system

The Pöschl-Teller system is defined as a one-dimensional quantum mechanical system (a `SpaceDQuantumSystem` with dimension d=1d=1). It describes a particle of mass mm subject to the Pöschl-Teller potential: V(x)=−ℏ2κ2N(N+1)2mcosh⁡2(κx)V(x) = -\frac{\hbar^2 \kappa^2 N(N+1)}{2m \cosh^2(\kappa x)} where ℏ\hbar is the reduced Planck's constant, and κ\kappa and NN are parameters determining the width and depth of the potential respectively. The structure includes the definition of the Hilbert space, the kinetic energy operator, the potential energy operator, and the total Hamiltonian H^=T^+V^\hat{H} = \hat{T} + \hat{V}.

theorem

The Dimension of the Pöschl-Teller System is 11

For a Pöschl-Teller quantum system QQ, the spatial dimension dd of its underlying dd-dimensional quantum system structure is equal to 11.

theorem

The mass of the 1D Pöschl-Teller system equals mm

For a 1-dimensional Pöschl-Teller quantum system QQ, the mass mm of the associated quantum mechanical system is equal to the mass mm defined within the Pöschl-Teller configuration.

theorem

The potential of the 1D Pöschl-Teller system is V(x)V(x)

For a one-dimensional Pöschl-Teller quantum system QQ, the potential energy function is the Pöschl-Teller potential V(x)V(x), which is defined as: V(x)=−ℏ2κ2N(N+1)2mcosh⁡2(κx)V(x) = -\frac{\hbar^2 \kappa^2 N(N+1)}{2m \cosh^2(\kappa x)} where mm is the mass of the particle, ℏ\hbar is the reduced Planck's constant, κ\kappa is the width parameter, and NN is the depth parameter.

theorem

Action of the 1D Kinetic Energy Operator T^=12mp^2\hat{T} = \frac{1}{2m} \hat{p}^2

For a one-dimensional Pöschl-Teller quantum system QQ with particle mass mm, the kinetic energy operator T^\hat{T} (viewed as a continuous linear map on the Schwartz space S(R,C)\mathcal{S}(\mathbb{R}, \mathbb{C})) acts on a Schwartz function ψ\psi according to: T^ψ=12mp^(p^ψ) \hat{T} \psi = \frac{1}{2m} \hat{p}(\hat{p} \psi) where p^\hat{p} is the momentum operator (the 00-th component in one dimension).

theorem

Pointwise Action of the Pöschl-Teller Potential Operator: (V^ψ)(x)=V(x)ψ(x)(\hat{V} \psi)(x) = V(x) \psi(x)

For a one-dimensional Pöschl-Teller quantum system QQ, the potential operator V^\hat{V} (represented as a continuous linear map on the Schwartz space S(R,C)\mathcal{S}(\mathbb{R}, \mathbb{C})) acts by pointwise multiplication. Specifically, for any Schwartz function ψ∈S(R,C)\psi \in \mathcal{S}(\mathbb{R}, \mathbb{C}) and any position x∈Rx \in \mathbb{R}, the value of the resulting function at xx is: (V^ψ)(x)=V(x)ψ(x) (\hat{V}\psi)(x) = V(x) \psi(x) where V(x)V(x) is the Pöschl-Teller potential defined by V(x)=−ℏ2κ2N(N+1)2mcosh⁡2(κx)V(x) = -\frac{\hbar^2 \kappa^2 N(N+1)}{2m \cosh^2(\kappa x)}.

theorem

The Pöschl-Teller potential operator V^\hat{V} is self-adjoint

In the one-dimensional Pöschl-Teller quantum system QQ, the potential operator V^\hat{V} (the multiplication operator on the Hilbert space L2(R,C)L^2(\mathbb{R}, \mathbb{C}) defined by the potential function V(x)=−ℏ2κ2N(N+1)2mcosh⁡2(κx)V(x) = -\frac{\hbar^2 \kappa^2 N(N+1)}{2m \cosh^2(\kappa x)}) is self-adjoint.

theorem

The Schwartz space S(R,C)\mathcal{S}(\mathbb{R}, \mathbb{C}) is contained in the domain of the Pöschl-Teller potential operator V^\hat{V}

For a one-dimensional Pöschl-Teller quantum system QQ, let dom(V^)\text{dom}(\hat{V}) denote the domain of the potential operator V^\hat{V} acting on the Hilbert space L2(R,C)L^2(\mathbb{R}, \mathbb{C}). The space of Schwartz functions S(R,C)\mathcal{S}(\mathbb{R}, \mathbb{C}) is contained within the domain of the potential operator: S(R,C)⊆dom(V^). \mathcal{S}(\mathbb{R}, \mathbb{C}) \subseteq \text{dom}(\hat{V}). This implies that for any Schwartz function ψ\psi, the product VψV \psi is square-integrable, where V(x)=−ℏ2κ2N(N+1)2mcosh⁡2(κx)V(x) = -\frac{\hbar^2 \kappa^2 N(N+1)}{2m \cosh^2(\kappa x)}.

theorem

Pointwise action of the 1D Pöschl-Teller Hamiltonian (H^ψ)(x)=12m(p^2ψ)(x)+V(x)ψ(x)(\hat{H}\psi)(x) = \frac{1}{2m}(\hat{p}^2\psi)(x) + V(x)\psi(x)

For a one-dimensional Pöschl-Teller quantum system with mass mm, the action of the Hamiltonian operator H^\hat{H} on a Schwartz function ψ∈S(R,C)\psi \in \mathcal{S}(\mathbb{R}, \mathbb{C}) at a point x∈Rx \in \mathbb{R} is defined by the sum of the kinetic and potential energy components: (H^ψ)(x)=12m(p^(p^ψ))(x)+V(x)ψ(x) (\hat{H}\psi)(x) = \frac{1}{2m} (\hat{p}(\hat{p}\psi))(x) + V(x)\psi(x) where p^\hat{p} is the momentum operator and V(x)V(x) is the Pöschl-Teller potential function evaluated at xx.

theorem

(Q.tanhCLM ψ)(x)=tanh⁡(κx)ψ(x)(Q.\text{tanhCLM} \, \psi)(x) = \tanh(\kappa x) \psi(x)

Let S(R,C)\mathcal{S}(\mathbb{R}, \mathbb{C}) denote the Schwartz space of complex-valued functions on the real line. For any function ψ∈S(R,C)\psi \in \mathcal{S}(\mathbb{R}, \mathbb{C}) and any point x∈Rx \in \mathbb{R}, the continuous linear operator Q.tanhCLMQ.\text{tanhCLM} acts on ψ\psi via pointwise multiplication by the hyperbolic tangent function: (Q.tanhCLM ψ)(x)=tanh⁡(κx)ψ(x) (Q.\text{tanhCLM} \, \psi)(x) = \tanh(\kappa x) \psi(x) where κ\kappa is the characteristic parameter associated with the Pöschl-Teller potential QQ.

theorem

Pointwise action of the Pöschl-Teller creation operator a^†\hat{a}^\dagger

For any Schwartz function ψ∈S(R,C)\psi \in \mathcal{S}(\mathbb{R}, \mathbb{C}) and any point x∈Rx \in \mathbb{R}, the pointwise action of the Pöschl-Teller creation operator a^†\hat{a}^\dagger is given by: (a^†ψ)(x)=12m(p^ψ)(x)+iℏκ2mtanh⁡(κx)ψ(x) (\hat{a}^\dagger \psi)(x) = \frac{1}{\sqrt{2m}} (\hat{p} \psi)(x) + \frac{i\hbar\kappa}{\sqrt{2m}} \tanh(\kappa x) \psi(x) where mm is the mass of the particle, κ\kappa is the characteristic parameter of the Pöschl-Teller potential, p^\hat{p} is the momentum operator, ℏ\hbar is the reduced Planck constant, and ii is the imaginary unit.

theorem

Pointwise Action of the Pöschl-Teller Annihilation Operator a^\hat{a}

For a Schwartz function ψ∈S(R,C)\psi \in \mathcal{S}(\mathbb{R}, \mathbb{C}) and a point x∈Rx \in \mathbb{R}, the pointwise action of the annihilation operator a^\hat{a} for the one-dimensional Pöschl-Teller potential with mass mm and characteristic scale κ\kappa is given by: (a^ψ)(x)=12m(p^ψ)(x)−iℏκ2mtanh⁡(κx)ψ(x) (\hat{a}\psi)(x) = \frac{1}{\sqrt{2m}} (\hat{p}\psi)(x) - \frac{i\hbar\kappa}{\sqrt{2m}} \tanh(\kappa x) \psi(x) where p^=−iℏddx\hat{p} = -i\hbar \frac{d}{dx} is the momentum operator, ℏ\hbar is the reduced Planck constant, and ii is the imaginary unit.

theorem

a^†+a^=22mp^\hat{a}^\dagger + \hat{a} = \frac{2}{\sqrt{2m}} \hat{p}

For a one-dimensional Pöschl-Teller system with mass mm, the creation operator a^†\hat{a}^\dagger and the annihilation operator a^\hat{a} satisfy the following relation involving the momentum operator p^\hat{p}: a^†+a^=22mp^ \hat{a}^\dagger + \hat{a} = \frac{2}{\sqrt{2m}} \hat{p} This means that the sum of the ladder operators is equal to the momentum operator scaled by the factor 2/2m2 / \sqrt{2m}.

theorem

a^†−a^=2iℏκ2mtanh⁡(κx)\hat{a}^\dagger - \hat{a} = \frac{2 i \hbar \kappa}{\sqrt{2m}} \tanh(\kappa x) for the Pöschl-Teller Potential

For a one-dimensional Pöschl-Teller system with mass mm and potential scale parameter κ\kappa, the difference between the creation operator a^†\hat{a}^\dagger and the annihilation operator a^\hat{a} acting on the Schwartz space S(R,C)\mathcal{S}(\mathbb{R}, \mathbb{C}) is given by: a^†−a^=2iℏκ2mtanh⁡(κx) \hat{a}^\dagger - \hat{a} = \frac{2 i \hbar \kappa}{\sqrt{2m}} \tanh(\kappa x) where ii is the imaginary unit, ℏ\hbar is the reduced Planck constant, and tanh⁡(κx)\tanh(\kappa x) denotes the operator for pointwise multiplication by the hyperbolic tangent function.