Weyl fermion excitations in the ideal Weyl semimetal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>CuTlSe</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>

An ideal Weyl semimetal is characterized by a dispersion in which only Weyl cones intersect the Fermi level, with low-energy behavior being governed by Weyl fermions. Although ideal Weyl semimetals have long been anticipated, only a few are realized in nonmagnetic materials. In this study, we confirm the presence of Weyl-fermion excitations in the ideal Weyl semimetal <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>CuTlSe</a:mi><a:mn>2</a:mn></a:msub></a:math> via a combination of magnetoresistance, Hall-effect, magnetic-susceptibility, nuclear magnetic resonance (NMR), and muon-spin relaxation (<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi>µ</b:mi><b:mi>SR</b:mi></b:mrow></b:math>) experiments. Magnetoresistance measurements reveal a negative longitudinal magnetoresistance (LMR), which scales as <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msup><c:mi>B</c:mi><c:mn>2</c:mn></c:msup></c:math>, while Hall-effect results indicate a predominant contribution from Weyl fermions with a hole-type charge. Magnetic susceptibility and <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>µ</d:mi><d:mi>SR</d:mi></d:mrow></d:math> measurements indicate the lack of any intrinsic spontaneous magnetic moments down to base temperature. Finally, the NMR results can be modeled by a two-component effective Hamiltonian, which reproduces well the temperature-dependent <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mmultiscripts><e:mi>Cu</e:mi><e:mprescripts/><e:none/><e:mn>63</e:mn></e:mmultiscripts></e:math> NMR <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msup><f:mrow><f:mo>(</f:mo><f:msub><f:mi>T</f:mi><f:mn>1</f:mn></f:msub><f:mi>T</f:mi><f:mo>)</f:mo></f:mrow><f:mrow><f:mo>−</f:mo><f:mn>1</f:mn></f:mrow></f:msup></f:math> factor, shown to scale as <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:msup><g:mi>T</g:mi><g:mn>2</g:mn></g:msup></g:math> below 100 K and as <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:msup><h:mi>T</h:mi><h:mn>1</h:mn></h:msup></h:math> above 100 K. Overall, we find that the extremely low concentration (<i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:msup><i:mn>10</i:mn><i:mn>17</i:mn></i:msup><i:mspace width="4pt"/><i:msup><i:mrow><i:mi>cm</i:mi></i:mrow><i:mrow><i:mo>−</i:mo><i:mn>3</i:mn></i:mrow></i:msup></i:mrow></i:math>) of carriers in <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:msub><k:mi>CuTlSe</k:mi><k:mn>2</k:mn></k:msub></k:math> originates from an ideal nonmagnetic Weyl semimetallic state, persisting up to a thermal excitation energy of 9 meV (100 K), above which trivial electronic bands close to <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:msub><l:mi>E</l:mi><l:mi mathvariant="normal">F</l:mi></l:msub></l:math> take over. Our findings highlight <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"><n:msub><n:mi>CuTlSe</n:mi><n:mn>2</n:mn></n:msub></n:math> as a new member of the intriguing class of Weyl semimetals. Published by the American Physical Society 2024