Using unicode to input symbols in math mode with unicode-math

Question

I want to input a lambda with a bar (ƛ) with font STIX Two Math in math mode. I've checked in the unicode table that it had a code U+019B, and I'm sure STIX Two Math contains this character. I've tried

\documentclass{article}
\usepackage{amsmath,unicode-math}
\setmathfont{STIX Two Math}
\begin{document}
    $ \symbol{"019B} $, $ ƛ $
\end{document}

Both of them didn't output anything. My question is how can I type it in math mode?

Answer 1

By default unicode-math does not assign a mathematical definition or ƛ, but yu can give it a mathcode yourself:

You have to decide which class, family and codepoint it should have. Since our font is Unicode encoded, the codepoint is 0x019B. Since ƛ behaves like a greek letter, we can treat is similar to other greek letters and assign it class 7 (variable ordinary) in the "operators" family:

\Umathcode `ƛ 7 0 "019B

This leads to the document

\documentclass{article}
\usepackage{amsmath,unicode-math}
\setmathfont{STIX Two Math}
\Umathcode `ƛ 7 \symoperators "019B
\begin{document}
    $ \symbol{"019B} $, $ ƛ $
\end{document}

which works correctly:

Answer 2

You can enter it as a text-mode command within \text or \textnormal. The Unicode character is not defined for math mode by default, but you can set it active yourself with newunicodechar.

\documentclass{article}
\tracinglostchars=2
\usepackage{amsmath,unicode-math}
\usepackage{newunicodechar}

\newfontface\symbolfont{STIX Two Math}
\setmathfont{STIX Two Math}

\newcommand\textlambdabar{{\symbolfont\symbol{"019B}}}
\newunicodechar{^^^^019b}{%
  \ifmmode\mathalpha{\textnormal{\textlambdabar}}%
  \else\textlambdabar%
  \fi}

\begin{document}
    $^^^^019b$, $ ƛ $
\end{document}