Bold slanted \lambda symbol not bold enough in Stix font family

Question

1. Important boundary conditions

1. pdflatex should be used. No xelatex please.
2. The font should not be completely different from Computer Modern, Times New Roman, Stix.
3. Stix2 is a completely differently looking font.

Why?

• Because manuscripts upon submission to Physical Review journals are compiled by pdflatex.
• Because many people never heard of xelatex

2. Actual question

I need to use the STIX math italic (second row) and math bold italic (third row) in the same text. I found that the distinction for all small greek letters except lambda is visible enough. The shape of lambda seems to be inconsistent (too thin). Since I have both typefaces in the same text, the two lambdas can hardly be distinguished. What could be the solution in this case?

3. Description of the figure.

• It is generated with Adobe Illustrator. To the best of my knowledge, the stix package provides the first row (upright text) for the text mode and 2nd and 3rd rows for the math mode.
• Rows 4 and 5 illustrate proper visual differences between a) the upright font (1) and the serif-less upright font (4) and b) the italic font (2) and the serif-less slanted font.
• Row 3 illustrates the problematic bold italic font. To the right, I superimposed the bold italic over the italic font to demonstrate the tiny difference for lambda.

4. Copy-pastable latex code

Currently, I am defining the serif-less bold italic small greek math letters

\documentclass{article}
\usepackage{amsmath}
\usepackage{stix}
\usepackage{bm}
\DeclareMathSymbol{\alphaSF}{\mathalpha}{arrows2}{"0B}
\DeclareMathSymbol{\lambdaSF}{\mathalpha}{arrows2}{"15}

\newcommand{\lambdaBold}{{\bm{\lambdaSF}}}
\newcommand{\alphaBold}{{\bm \alphaSF}}
\begin{document}
\begin{align*}
  \alpha&=\lambda\\
  \bm{\alpha}&=\bm{\lambda}\\
  \alphaBold&=\lambdaBold
 \end{align*} 
\end{document}

and using them instead of bold italic, but serif-less fonts stand out too much.

Answer 1

Consider the following screenshot. In the upper half, the math-mode greek letters are shown first in normal, i.e., slanted and non-bold, form, followed by bold, upright, and bold-upright, for the XITS Math math font. The normal and bold slanted variants of \lambda do look very similar.

In the lower half, the same set of four math-greek letters is shown, but now for the Stix Two Math math font. You'll notice right away that the regular and bold slanted-lambda glyphs look quite different.

\documentclass{article}
\usepackage{unicode-math}
\newcommand\blurb{\alpha\beta\gamma\delta\epsilon\zeta\eta%
                  \theta\iota\kappa\lambda\mu\nu\xi\pi\rho%
                  \sigma\tau\upsilon\phi\chi\psi\omega}
\begin{document}
\obeylines
\setmainfont{XITS}
\setmathfont{XITS Math}
XITS Math glyphs
$\blurb$
$\symbf{\blurb}$
$\symup{\blurb}$
$\symbfup{\blurb}$

\bigskip
\setmainfont{Stix Two Text}
\setmathfont{Stix Two Math}
Stix Two Math glyphs
$\blurb$
$\symbf{\blurb}$
$\symup{\blurb}$
$\symbfup{\blurb}$
\end{document}

Answer 2

There is no need to load bm with stix; the fonts come with bold letters that works with the standard \mathversion{bold} and \boldsymbol from amsmath.

With PDFLaTeX, you can load a bold sans-serif math alphabet with isomath. There are several options, including Computer Modern Bold and Arev.

\documentclass{article}
\usepackage{iftex}

\ifTUTeX
  \usepackage{unicode-math}
  \setmainfont{XITS}[Scale=1.0]
  \setmathfont{XITS Math}[Scale=MatchLowercase] % Also loads STIX Math Bold.
\else
  \usepackage{amsmath}
  \usepackage{stix}
  % sfdefault=zavm is Arev. sfdefault=cmbr is Computer Modern Bright.
  \usepackage[sfdefault=cmbr]{isomath} % For \mathsfbfit

  \newcommand{\mbfitsansalpha}{\mathsfbfit{\alpha}}
  \newcommand{\mbfitsanslambda}{\mathsfbfit{\lambda}}
\fi

\pagestyle{empty} % For convenient cropping of the MWE.

\begin{document}
\begin{align*}
  \alpha              &= \lambda \\
  \boldsymbol{\alpha} &= \boldsymbol{\lambda} \\
  \mbfitsansalpha     &= \mbfitsanslambda
 \end{align*} 
\end{document}

Compiled with PDFLaTeX, this gives:

With unicode-math, this is even simpler: the package supports both \symbfit and \symbfsfit alphabets out of the box. The XITS Math font by Khaled Hosny additionally comes in a bold version.

The same MWE compiled with LuaLaTeX or XeLaTeX produces:

The range= option of unicode-math allows you to substitute the sans-serif math alphabet from any other math or Greek font. Here is an example of how to substitute the Greek sans-serif italic alphabet from STIX Two Math.

Unfortunately, as of 2020, range= is not compatible with math versions, so you would have problems with \boldsymbol. I therefore switch to \symbfit.

\documentclass{article}

\usepackage{unicode-math}
\defaultfontfeatures{Scale=MatchLowercase}
\setmainfont{XITS}[Scale=1.0]
\setmathfont{XITS Math}
\setmathfont{STIX Two Math}[range=bfsfit/{Greek,greek}]

\pagestyle{empty} % For convenient cropping of the MWE.

\begin{document}
\begin{align*}
  \alpha              &= \lambda \\
  \symbfit{\alpha}    &= \symbfit{\lambda} \\
  \symbfsfit{\alpha}  &= \symbfsfit{\lambda}
 \end{align*} 
\end{document}

You're more likely to want to substitute a different bold italic lambda (𝝀) symbol here, though, with range=\mbfitlambda. This example adds a bit of FakeBold to this one symbol:

\documentclass{article}

\usepackage{unicode-math}
\defaultfontfeatures{Scale=MatchLowercase}
\setmainfont{XITS}[Scale=1.0]
\setmathfont{XITS Math}
\setmathfont{XITS Math}[range=\mbfitlambda, FakeBold=1.1]

\pagestyle{empty} % For convenient cropping of the MWE.

\begin{document}
\begin{align*}
  \alpha              &= \lambda \\
  \symbfit{\alpha}    &= \symbfit{\lambda} \\
  \symbfsfit{\alpha}  &= \symbfsfit{\lambda}
 \end{align*} 
\end{document}

If you genuinely care about portability between isomath and unicode-math, \mathbfit works in both packages if you give unicode-math the option bfit=sym, but the equivalent of \symbfsfit in unicode-math is \mathsfbfit in isomath.