arXiv subjects

Richard L. Hall

Publications and source records attributed to Richard L. Hall.

At least 19 recordsLinked to original sources

arXiv2020

Refining the general comparison theorem for Klein-Gordon equation

By recasting the Klein--Gordon equation as an eigen-equation in the coupling parameter $v > 0,$ the basic Klein--Gordon comparison theorem may be written $f_1\leq f_2\implies G_1(E)\leq G_2(E)$, where $f_1$ and $f_2$, are the monotone non-decreasing shapes of two central potentials $V_1(r) = v_1\,f_1(r)$ and $V_2(r) = v_2\, f_2(r)$ on $[0,\infty)$. Meanwhile $v_1 = G_1(E)$ and $v_2 = G_2(E)$ are the corresponding coupling parameters that are functions of the energy $E\in(-m,\,m)$. We weaken the sufficient condition for the ground-state spectral ordering by proving (for example in $d=1$ dimension) that if $\int_0^x\big[f_2(t) - f_1(t)\big]φ_i(t)dt\geq 0$, the couplings remain ordered $v_1 \leq v_2$ where $i = 1\, {\rm or}\, 2, $ and $\{φ_1, φ_2\}$ are the ground-states corresponding respectively to the couplings $\{v_1,\, v_2\}$ for a given $E \in (-m,\, m).$. This result is extended to spherically symmetric radial potentials in $ d > 1 $ dimensions.

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arXiv2019

General comparison theorems for the Klein-Gordon equation in d dimensions

We study bound-state solutions of the Klein-Gordon equation $φ^{\prime\prime}(x) =\big[m^2-\big(E-v\,f(x)\big)^2\big] φ(x),$ for bounded vector potentials which in one spatial dimension have the form $V(x) = v\,f(x),$ where $f(x)\le 0$ is the shape of a finite symmetric central potential that is monotone non-decreasing on $[0, \infty)$ and vanishes as $x\rightarrow\infty.$ Two principal results are reported. First, it is shown that the eigenvalue problem in the coupling parameter $v$ leads to spectral functions of the form $v= G(E)$ which are concave, and at most uni-modal with a maximum near the lower limit $E = -m$ of the eigenenergy $E \in (-m, \, m)$. This formulation of the spectral problem immediately extends to central potentials in $d > 1$ spatial dimensions. Secondly, for each of the dimension cases, $d=1$ and $d \ge 2$, a comparison theorem is proven, to the effect that if two potential shapes are ordered $f_1(r) \leq f_2(r),$ then so are the corresponding pairs of spectral functions $G_1(E) \leq G_2(E)$ for each of the existing eigenvalues. These results remove the restriction to positive eigenvalues necessitated by earlier comparison theorems for the Klein--Gordon equation.

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arXiv2019

Potential envelope theory and the local energy theorem

We consider a one--particle bound quantum mechanical system governed by a Schrödinger operator $\mathscr{H} = -Δ+ v\,f(r)$, where $f(r)$ is an attractive central potential, and $v>0$ is a coupling parameter. If $ϕ\in \mathcal{D}(\mathscr{H})$ is a `trial function', the local energy theorem tells us that the discrete energies of $\mathscr{H}$ are bounded by the extreme values of $(\mathscr{H}ϕ)/ϕ,$ as a function of $r$. We suppose that $f(r)$ is a smooth transformation of the form $f = g(h)$, where $g$ is monotone increasing with definite convexity and $h(r)$ is a potential for which the eigenvalues $H_n(u)$ of the operator $\mathcal{H}=-Δ+ u\, h(r)$, for appropriate $u >0$, are known. It is shown that the eigenfunctions of $\mathcal{H}$ provide local-energy trial functions $ϕ$ which necessarily lead to finite eigenvalue approximations that are either lower or upper bounds. This is used to extend the local energy theorem to the case of upper bounds for the excited-state energies when the trial function is chosen to be an eigenfunction of such an operator $\mathcal{H}$. Moreover, we prove that the local-energy approximations obtained are identical to `envelope bounds', which can be obtained directly from the spectral data $H_n(u)$ without explicit reference to the trial wave functions.

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arXiv2018

Exact normalized eigenfunctions for general deformed Hulthén potentials

The exact solutions of Schrödinger's equation with the deformed Hulthén potential $V_q(x)=-{μ\, e^{-δ\,x }}/({1-q\,e^{-δ\,x}}),~ δ,μ, q>0$ are given, along with a closed--form formula for the normalization constants of the eigenfunctions for arbitrary $q>0$. The Crum-Darboux transformation is then used to derive the corresponding exact solutions for the extended Hulthén potentials $V(x)= -{μ\, e^{-δ\,x }}/({1-q\,e^{-δ\,x}})+ {q\,j(j+1)\, e^{-δ\,x }}/({1-q\,e^{-δ\,x}})^2, j=0,1,2,\dots.$ A general formula for the new normalization condition is also provided.

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arXiv2016

Exact and approximate solutions of Schrödinger's equation with hyperbolic double-well potentials

Analytic and approximate solutions for the energy eigenvalues generated by the hyperbolic potentials $V_m(x)=-U_0\sinh^{2m}(x/d)/\cosh^{2m+2}(x/d),\,m=0,1,2,\dots$ are constructed. A byproduct of this work is the construction of polynomial solutions for the confluent Heun equation along with necessary and sufficient conditions for the existence of such solutions based on the evaluation of a three-term recurrence relation. Very accurate approximate solutions for the general problem with arbitrary potential parameters are found by use of the {\it asymptotic iteration method}.

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arXiv2016

Sharp comparison theorems for the Klein--Gordon equation in $d$ dimensions

We establish sharp (or `refined') comparison theorems for the Klein--Gordon equation. We show that the condition $V_a\le V_b$, which leads to $E_a\le E_b$, can be replaced by the weaker assumption $U_a\le U_b$ which still implies the spectral ordering $E_a\le E_b$. In the simplest case, for $d=1$, $U_i(x)=\int_0^x V_i(t)dt$, $i=a$ or $b$, and for $d>1$, $U_i(r)=\int_0^r V_i(t) t^{d-1}dt$, $i=a$ or $b$. We also consider sharp comparison theorems in the presence of a scalar potential $S$ (a `variable mass') in addition to the vector term $V$ (the time component of a $4$-vector). The theorems are illustrated by a variety of explicit detailed examples.

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arXiv2016

Refined comparison theorems for the Dirac equation with spin and pseudo--spin symmetry in $d$ dimensions

The classic comparison theorem of quantum mechanics states that if two potentials are ordered then the corresponding energy eigenvalues are similarly ordered, that is to say if $V_a\le V_b$, then $E_a\le E_b$. Such theorems have recently been established for relativistic problems even though the discrete spectra are not easily characterized variationally. In this paper we improve on the basic comparison theorem for the Dirac equation with spin and pseudo--spin symmetry in $d\ge 1$ dimensions. The graphs of two comparison potentials may now cross each other in a prescribed manner implying that the energy values are still ordered. The refined comparison theorems are valid for the ground state in one dimension and for the bottom of an angular momentum subspace in $d>1$ dimensions. For instance in a simplest case in one dimension, the condition $V_a\le V_b$ is replaced by $U_a\le U_b$, where $U_i(x)=\int_0^x V_i(t)dt$, $x\in[0,\ \infty)$, and $i=a$ or $b$.

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arXiv2015

Refined comparison theorems for the Dirac equation in d dimensions

A single spin-$\frac{1}{2}$ particle obeys the Dirac equation in $d\ge 1$ spatial dimension and is bound by an attractive central monotone potential which vanishes at infinity (in one dimension the potential is even). This work refines the relativistic comparison theorems which were derived by Hall \cite{p75}. The new theorems allow the graphs of the two comparison potentials $V_a$ and $V_b$ to crossover in a controlled way and still imply the spectral ordering $E_a\le E_b$ for the eigenvalues at the bottom of each angular momentum subspace. More specifically in a simplest case we have: in dimension $d=1$, if $\int_0^x (V_b(t)-V_a(t)) dt\ge 0,\ x\in [0,\ \infty)$, then $E_a\le E_b$; and in $d>1$ dimensions, if $\int_0^r (V_b(t)-V_a(t))t^{2|k_d|} dt\ge 0,\ r\in [0,\ \infty)$, where $k_d=τ\left(j+\frac{d-2}{2}\right)$ and $τ=\pm 1$, then $E_a\le E_b$.

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arXiv2014

Soft and hard confinement of a two-electron quantum system

A model physical problem is studied in which a system of two electrons is subject either to soft confinement by means of attractive oscillator potentials or by entrapment within an impenetrable spherical box of finite radius $R.$ When hard confinement is present the oscillators can be switched off. Exact analytical solutions are found for special parameter sets, and highly accurate numerical solutions (18 decimal places) are obtained for general cases. Some interesting degeneracy questions are discussed at length.

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arXiv2014

Schrödinger spectrum generated by the Cornell potential

The eigenvalues $E_{n\ell}^d(a,c)$ of the $d$-dimensional Schrödinger equation with the Cornell potential $V(r)=-a/r+c\,r$, $a,c>0$ are analyzed by means of the envelope method and the asymptotic iteration method (AIM). Scaling arguments show that it is sufficient to know $E(1,λ)$, and the envelope method provides analytic bounds for the equivalent complete set of coupling functions $λ(E)$. Meanwhile the easily-implemented AIM procedure yields highly accurate numerical eigenvalues with little computational effort.

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arXiv2014

Schrödinger Models for Solutions of the Bethe-Salpeter Equation in Minkowski Space. II. Fermionic Bound-State Constituents

In view of the obstacles encountered in any attempts to solve the Minkowski-space Bethe-Salpeter equation for bound states of two fermions, we study the possibility to model the bound-state features, at least at a qualitative level, by a Schrödinger description. Such a nonrelativistic potential model can be constructed by applying, to any given Bethe-Salpeter spectral data, "geometric spectral inversion" in its recently extended form, which tolerates also singular potentials. This leads to the adaptation of explicit models that provide an overview accounting for the Bethe-Salpeter formalism's complexities.

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arXiv2013

Polynomial solutions for a class of second-order linear differential equations

We analyze the polynomial solutions of the linear differential equation $p_2(x)y''+p_1(x)y'+p_0(x)y=0$ where $p_j(x)$ is a $j^{\rm th}$-degree polynomial. We discuss all the possible polynomial solutions and their dependence on the parameters of the polynomials $p_j(x)$. Special cases are related to known differential equations of mathematical physics. Classes of new soluble problems are exhibited. General results are obtained for weight functions and orthogonality relations.

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arXiv2013

Nodal theorems for the Dirac equation in d >= 1 dimensions

A single particle obeys the Dirac equation in $d \ge 1$ spatial dimensions and is bound by an attractive central monotone potential that vanishes at infinity. In one dimension, the potential is even, and monotone for $x\ge 0.$ The asymptotic behavior of the wave functions near the origin and at infinity are discussed. Nodal theorems are proven for the cases $d=1$ and $d > 1$, which specify the relationship between the numbers of nodes $n_1$ and $n_2$ in the upper and lower components of the Dirac spinor. For $d=1$, $n_2 = n_1 + 1,$ whereas for $d >1,$ $n_2 = n_1 +1$ if $k_d > 0,$ and $n_2 = n_1$ if $k_d < 0,$ where $k_d = τ(j + \frac{d-2}{2}),$ and $τ= \pm 1.$ This work generalizes the classic results of Rose and Newton in 1951 for the case $d=3.$ Specific examples are presented with graphs, including Dirac spinor orbits $(ψ_1(r), ψ_2(r)), r \ge 0.$

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arXiv2012

Wide effectiveness of a sine basis for quantum-mechanical problems in d dimensions

It is shown that the spanning set for L^2([0, 1]) provided by the eigenfunctions {sqrt{2} sin(nπx)}_{n=1}^{\infty} of the particle-in-a-box in quantum mechanics provide a very effective variational basis for more general problems. The basis is scaled to [a,b], where a and b are then used as variational parameters. What is perhaps a natural basis for quantum systems confined to a spherical box in R^d, turns out to be appropriate also for problems that are softly confined by U-shaped potentials, including those with strong singularities at r=0. Specific examples are discussed in detail, along with some bound N-boson systems

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arXiv2012

Exact and approximate solutions of Schroedinger's equation for a class of trigonometric potentials

The asymptotic iteration method is used to find exact and approximate solutions of Schroedinger's equation for a number of one-dimensional trigonometric potentials (sine-squared, double-cosine, tangent-squared, and complex cotangent). Analytic and approximate solutions are obtained by first using a coordinate transformation to reduce the Schroedinger equation to a second-order differential equation with an appropriate form. The asymptotic iteration method is also employed indirectly to obtain the terms in perturbation expansions, both for the energies and for the corresponding eigenfunctions.

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arXiv2012

Geometric Spectral Inversion

A discrete eigenvalue E_n of a Schroedinger operator H = -Δ+ vf(r) is given, as a function F_n(v) of the coupling parameter v\ge v_c. It is shown how the potential shape f(x) can be reconstructed from F_n(v). A constructive inversion algorithm and a functional inversion sequence are both discussed.

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arXiv2012

Schroedinger models for solutions of the Bethe-Salpeter equation in Minkowski space

By application of the 'geometric spectral inversion' technique, which we have recently generalized to accommodate also singular interaction potentials, we construct from spectral data emerging from the solution of the Minkowski-space formulation of the homogeneous Bethe-Salpeter equation describing bound states of two spinless particles a Schroedinger approach to such states in terms of nonrelativistic potential models. This spectrally equivalent modeling of bound states yields their qualitative features (masses, form factors, etc.) without having to deal with the more involved Bethe-Salpeter formalism.

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arXiv2012

Dirac eigenvalues for a softcore Coulomb potential in d dimensions

A single fermion is bound by a softcore central Coulomb potential V(r) = -v/(r^q + b^q)^(1/q), v>0, b>0, q \ge 1, in d>1 spatial dimensions. Envelope theory is used to construct analytic lower bounds for the discrete Dirac energy spectrum. The results are compared to accurate eigenvalues obtained numerically.

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