RESEARCH ARTICLE | JUNE 08 2023

Robustness of semimetallic transport properties of TaAs
against off-stoichiometric disorder 
A. Kawasuso ; M. Suda
M. Maekawa ; A. Miyashita; H. Murakawa ; M. Komada; C. Suzuki; H. Amada ; K. Michishio ;; N. Seko ; S. Yamamoto ; N. Oshima ; S. Seki ; N. Hanasaki

J. Appl. Phys. 133, 223903 (2023)
https://doi.org/10.1063/5.0147663

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Robustness of semimetallic transport propertiesof TaAs against off-stoichiometric disorder
Cite as: J. Appl. Phys. 133, 223903 (2023); doi: 10.1063/5.0147663
Submitted: 24 February 2023 · Accepted: 22 May 2023 ·Published Online: 8 June 2023

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CrossMarkM. Suda,2H. Murakawa,3M. Komada,3 C. Suzuki,1 H. Amada,1
K. Michishio,4A. Kawasuso,1,a)
1
1
1
1
4
2
M. Maekawa,
A. Miyashita,
N. Seko,
S. Yamamoto,
N. Oshima,
S. Seki,and N. Hanasaki3
AFFILIATIONS
1National Institute for Quantum Science and Technology, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan

2

Kyoto University, Nishikyo-ku, Kyoto-shi, Kyoto 615-8510, Japan

3

Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan

4a)Author to whom correspondence should be addressed: kawasuso.atsuo@qst.go.jp

ABSTRACT
14 September 2023 00:16:06

TaAs single crystals were grown by a standard chemical vapor transport method. The single-crystallinity and homogeneous distribution ofelements were confirmed by transmission electron microscope and x-ray diffraction observations. Positron annihilation measurements revealed
that the atomic vacancy concentration was kept below 105 at. %. However, inductively coupled plasma analysis showed an As-deficient(7–9 at. %) off-stoichiometry. First-principles calculations implied that the off-stoichiometry could be compensated for with excess Ta antisite
defects, thereby inducing metallic states. Nevertheless, excellent semimetallic transport properties of a well-suppressed carrier density(&1018 cm3 ), ultrahigh carrier mobility (*106 cm2 /V/s), and large transverse magnetoresistance (.200 000% at 9 T) with the quantum oscillation were obtained at 1.7 K. This indicated the robustness of semimetallic transport properties against the off-stoichiometric disorder and thequenching of metallic conduction associated with excess Ta atoms. The negative longitudinal magnetoresistance, which is considered evidenceof a Weyl semimetal (chiral anomaly), was not observed. These data were discussed with theoretical calculations.
Published under an exclusive license by AIP Publishing. https://doi.org/10.1063/5.0147663
I. INTRODUCTION
Some pnictide semimetals, including TaAs, have been theoretically predicted to host Weyl fermions,1 characterized by the formation of multiple Weyl node pairs of linear dispersions with
opposite chirality near the Fermi level. The inversion/time-reversal
symmetry breaking and strong spin–orbit interaction are thesources of this unusual electronic state.2 The characteristics of the
Fermi surface topology including a Fermi arc were identified for
stoichiometric TaAs by photoemission spectroscopy.3–6 Negative
longitudinal magnetoresistance (MR) is also expected to appear
due to the chiral anomaly of the Weyl system,7,8 in addition tousual semimetallic transport properties, such as ultrafast carrier
mobility and large transverse MR. These features have also been
confirmed experimentally.9–11
Although real crystals usually contain some disorder, such as
atomic exchange among different sublattices and structural andthermal equilibrium point defects, disorder has not been adequately
investigated experimentally in Weyl systems. In a disordered Weyl

J. Appl. Phys. 133, 223903 (2023); doi: 10.1063/5.0147663
Published under an exclusive license by AIP Publishingsystem, finite states may be induced near the Fermi level, changing
the system from a semimetal to a metal. The robustness of Weyl fermions against disorder needs to be explored.12,13
Structural analyses are important for examining the effects ofdisorder on Weyl fermions. In this study, we grew TaAs single crystals by the standard chemical vapor transport (CVT) method. We
performed structural and compositional analyses by transmission
electron microscopy (TEM), x-ray diffraction (XRD), positronannihilation, and inductively coupled plasma (ICP), as well as magnetotransport measurements. The results suggested that the semimetallic magnetotransport properties of TaAs are robust against
As-deficient off-stoichiomeric disorder.
II. METHODSThe TaAs bulk crystals were grown by the CVT method.14,15
Ta foil (99.95%), As grains (99.999%), and iodine (99.999%) were
put into an evacuated silica tube (2 cm in diameter and 25 cmlong). CVT growth was achieved by applying a temperature

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III. RESULTS
A. Structural analyses
The picture in Fig. 1(a) shows a crystal with major facets ofthe (001), (101), and (112) planes. The typical crystal size varied
from 1 to 5 mm. Figure 1(a) also shows the x-ray diffraction intensity curves at the θ–2θ configuration for each facet. These indicateda single crystal with a body-centered tetragonal structure (spacegroup: I41 md, No. 109). Figures 1(b) and 1(c) show a TEM image
and electron diffraction pattern, respectively, that contain clear
lattice fringes and sharp diffraction spots, again confirming that asingle crystal was obtained. Stacking faults were not observed.15,26
Figure 1(d) shows the EDX profiles of Ta and As atoms, suggesting a homogeneous distribution of these elements. Generally,
EDX cannot determine the precise composition, even at the atomic
percent level. Here, we used ICP analysis for randomly selected
two pieces of TaAs crystals. The result showed 7%–9% As-deficientoff-stoichiometry, as listed in Table I. This is the largest offstoichiometry ever reported for TaAs. Such an As-deficiency wasprobably due to the high vapor pressure of As.
To reveal the inclusion of structural vacancies compensating forthe 7–9% off-stoichiometry, positron annihilation measurements
were conducted. Figure 2(a) shows the positron lifetime spectrum
after subtracting the instrumental component (510 ps, 10%).

J. Appl. Phys. 133, 223903 (2023); doi: 10.1063/5.0147663
Published under an exclusive license by AIP Publishingscitation.org/journal/jap

FIG. 1. Structural analyses of the present TaAs crystal. (a) XRD patterns fromthe (112), (101), and (001) facets, (b) TEM images, (c) electron diffraction
pattern, and (d) two-dimensional EDX maps. The inset in (a) shows a picture ofa typical crystal.

The experimental spectrum had longer lifetimes than the spectra
simulated for the perfect matrix with positron lifetimes of 117 ps(BN) and 132 ps (BPTN) calculated by DFT, indicating the inclusionof atomic P
vacancies. The spectrum was fitted with a dual-exponentialfunction [ Ii exp(  t=τ i )=τ i , where τ i and Ii (i ¼ 1, 2) are the lifetime and intensity, respectively]. The first lifetime, τ 1 , was assigned
to the reduced bulk lifetime due to the finite positron trapping rateof vacancies.16 The vacancy-related lifetime was τ 2 ¼ 181 ps with
I2 ¼ 59%, which was closer to the calculated values for Ta vacancy(163–199 ps) than to those for As vacancy (148–164 ps), where thelower and higher values were based on the BN and BPTN enhancement factors, respectively. Figure 2(b) shows the DBAR spectrum,
where the gamma-ray energy is converted to the electron momentum. The comparison with the calculation indicated the inclusion of
single vacancies. However, the vacancy concentration was below
105 at. % (10 ppm) because the intensity related to vacancies(55%–59%) was still below the saturation limit. Therefore, the mostimportant conclusion was that the 7%–9% As deficiency was notcompensated for by the formation of vacancies, and thus the presence of excess Ta antisite defects was suspected.

14 September 2023 00:16:06

gradient of 1050–980  C over 3 weeks. The sample crystallinity wasdetermined by TEM and XRD measurements. Spatial distributionsof elements were visualized by energy-dispersive x-ray (EDX) analysis in the TEM apparatus. ICP analysis was used to determine thestoichiometry precisely. The inclusion of vacancy defects was also
examined by positron annihilation measurements with monoenergetic positron beams.16–18 Finally, magnetotransport measurements
were conducted at 1.7–300 K by the four-probe method in the(001) plane. The sample dimension (widththicknesslength) was
0:51  0:66  1:78 mm.
To discuss the experimental data, density functional theory(DFT) calculations were performed by using the ABINIT code19
with the projector augmented-wave method20 within the generalized gradient approximation.21 A conventional 2  2  1 cell
(Ta16 As16 ) was constructed for a perfect TaAs crystal. Systems witha Ta single vacancy (Ta15 As16 ), As single vacancy (Ta16 As15 ), Ta
antisite (Ta17 As15 ), and As antisite (Ta15 As17 ) were also modeled.
Full structural optimization was performed starting from the experimental lattice constants (a ¼ 3:44 Å and c ¼ 11:66 Å).22 The
valence electron configurations were assumed to be 5s2 5p6 5d3 6s2(Ta) and 3d10 4s2 4p3 (As). Defect formationPenergies were calculated as E f ¼ Etot (Defect)  Etot (Perfect)  ni μi , where Etot is
the total energy, ni is the number of atoms i to be removed (,0) or
added (.0), and μi is the chemical potential. Positron wave functions
were obtained through two-component DFT.23 The Slater functions24
were used as core electron wave functions. The Borónski–Nieminen(BN)23 and Barbiellini–Puska–Torsti–Nieminen (BPTN)25 enhancement factors were used to simulate the electron-positron correlation
effect. The positron lifetime and Doppler broadening of annihilation
radiation (DBAR) spectra were calculated. The DBAR spectra wereconvoluted with the Gaussian resolution function with a full width at
half maximum of 5:488  103 m0 c.

ARTICLEB. Magnetotransport measurements
Figure 3(a) shows the temperature dependence of the longitudinal resistivity (ρxx ). At a zero field, ρxx increased upon heating,suggesting that phonon scattering was the major cause of resistivity.
The residual resistance ratio of 1.7 K vs 300 K was 31.
Figure 3(b) (upper panels) shows the Hall resistivity (ρxy ) as afunction of magnetic field at different temperatures. Below 50 K, theslopes were negative overall with a quantum oscillation. AboveTABLE I. ICP compositional analysis of two TaAs crystals in the same batch.

No. 1
No. 2

Ta:
Ta:

59.00%
57.14%

As:
As:

41.00%
42.86%

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FIG. 2. (a) Positron annihilation lifetime spectrum obtained for the present TaAs
crystal at an incident positron energy of Eþ ¼ 23 keV (open circles). Simulated
spectra with the positron bulk (perfect matrix) lifetimes calculated by DFT with
the BN (broken line) and BPTN (short-broken line) enhancement factors. Fitted
curve assuming a dual-exponential function (solid line) and the individual components (one- and two-dotted chain lines denoted as 1 and 2, respectively) with
their lifetimes (τ 1 and τ 2 ) and intensities are shown. (b) DBAR positron spectrum obtained for the present TaAs crystal summed over Eþ ¼ 16–20 keV(open circles). Spectra for the bulk (broken line) and bulk with a Ta single
vacancy (solid line) calculated by DFT are shown.

J. Appl. Phys. 133, 223903 (2023); doi: 10.1063/5.0147663
Published under an exclusive license by AIP Publishing

14 September 2023 00:16:06

100 K, the slopes were positive and the absolute values were twoorders of magnitude smaller than those above 50 K. These results
suggested that the electrons were the main carriers at low temperatures, whereas the holes took over at high temperatures. Focusing on
the lowest temperature (1.7 K), the analysis based on the two-carriermodel assuming both electrons and holes (σ xy ¼ ρxy =(ρ2xy þ ρ2xx )¼ eB½nh μ2h =f1 þ (μh B)2 g  ne μ2e =f1 þ (μe B)2 g, where ne(h) , μe(h) , B
and e denote the electron (hole) density and mobility, magnetic fieldand elementary charge, respectively) gave ne ¼ 4:0  1017 cm3 ,nh ¼ 2:5  1017 cm3 ,andμh ¼ 5μe ¼ 1  106 cm2 /V/s,
104 cm2 /V/s as in the previous works.9–11
Figure 3(b) (lower left panel) shows the transverse MR values([ρxx (B)  ρxx (0)]/ρxx (0)) at ~B ?~I, where ~B and ~
I denote the magnetic field and current vectors, respectively, and at different temperatures. Below 20 K, the transverse MR values were over 200 000% at
9 T, and the quantum oscillation again appeared. Figure 3(b) (lower
right panel) shows the longitudinal MR values at around ~B k~I.
Although the MR values were largely suppressed from ~
B ?~
I to
~
B k~
I, the positive sign still remained.
Figure 3(c) shows the oscillatory spectrum of transverse MR(Δρxx ) at 1.7 K by subtracting the MR curve at 20 K as a function of
inverse of magnetic field (B1 ). As indicated by arrows, two oscillation frequencies [F ¼ 7:0 T (A) and 13.2 T (B)] could be specified.
The maximal cross sections of the Fermi surfaces were estimated to
be AF ¼ 6:7  104 Å2 for A and AF ¼ 1:26  103 Å2 for B
with AF ¼ 2πeF=h, where h is the Planck constant. These might correspond to the theoretical values for one of the Weyl-type electron
pockets and the hole poket, respectively.27 As shown in Fig. 3(c), theoscillatory spectrum could also be fitted with the Lifshits–Kosevich

FIG. 3. (a) Longitudinal resistivity as a function of temperature, (b) Hall resistivity (upper two panels), transverse (~B ? ~I) magnetoresistances (lower left panel)and longitudinal (~B k ~I) magnetoresistances (lower right panel) in the magnetic
field of 9 to þ9 T at different temperatures and (c) the oscillatory spectrum oftransverse MR (open circles) at 1.7 K as a function of B1 obtained for thepresent TaAs. In (c), arrows indicate the peak positions of two oscillation components designated by their frequencies, A (7.0 T) and B (13.2 T), and the Landaulevel indices(n). The solid line is the fitting by the Lifshitz–Kosevich formula, andthe inset shows the peak position (1=B) vs n plots.

formula,9,10,28 MR / exp(  α)β=sinh(β) cosf2π(F=B þ γ)g withα ¼ 2π 2 kB TD mcyc =(eBh) and β ¼ 2π 2 kB Tmcyc =(eBh), where kB , TD ,γ, mcyc , and T denote the Boltzmann constant, the Dingle temperature, the Onsager phase factor, cyclotron mass and temperature,
respectively. The peak position vs Landau-level index plot in theinset of Fig. 3(c) suggested that the Onsager phase factor may be 0
or 1. From the temperature-dependent amplitude of n ¼ 1 for A at
1.7–10 K, the cyclotron mass was determined to be mcyc ¼ 0:096 m0 ,where m0 is the electron rest mass. Subsequently, using this value,
the Dingle temperature was determined to be TD ¼ 8:0 K at 1.7 K.

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The Fermi vector, the Fermi velocity, the scattering time constantand eventually the mean free path were estimated to bevF ¼ hkF =mcyc ¼ 1:7  107 cm/s,
kF ¼ (AF =π)1=2 ¼ 0:015 Å1 ,andl ¼ vF τ tr ¼ 9:2 μm,
τ tr ¼ μhkF =(evF ) ¼ 5:4  1011 s,
respectively.
The above-observed ultrahigh mobility, well-suppressed
carrier density, giant transverse MR, and the deduced physicalquantities through the analyses were comparable to those obtained
for stoichiometric TaAs in the previous works,9,10 suggesting thepresence of Weyl semimetallicity even in the off-stoichiometriccase. The one missing observation was of the negative longitudinalMR, which is considered to be the signature of Weyl characteristics
due to chiral anomaly.
IV. DISCUSSION

J. Appl. Phys. 133, 223903 (2023); doi: 10.1063/5.0147663
Published under an exclusive license by AIP Publishing

14 September 2023 00:16:06

The present TaAs crystals were highly homogeneous single
crystals with 7%–9% As-deficient off-stoichiometry and few structural vacancies. The present DFT calculation gave the formationenergies of vacancies and antisite defects as 3.7 eV (Ta vacancy),
4.0 eV (As vacancy), 3.5 eV (Ta antisite), and 4.4 eV (As antisite).
The value for As vacancy agreed with that calculated in the previous study.26 These high formation energies implied that TaAs contains few point defects even at 1000  C, as long as it is
stoichiometric. According to the grand canonical theory on point
defect formation in binary alloys,29 Ta antisite defects are probably
formed in As-deficient conditions. The present TaAs may correspond to this case.
The modification of the electronic states of TaAs by point
defects may be a concern. Figure 4 shows the DFT calculation of
the electron density of states (DOS) for pristine TaAs and withsingle vacancies and antisite defects on both sublattices. The concentration of these point defects was 3.125 at. % (=1/32). The
minimum DOS point (Weyl node) for the pristine TaAs was
located at the Fermi level (0 eV). The DOS profile around the
Fermi level exhibited linear dispersion. The minimum DOS points
for the As vacancies and Ta antisites were slightly below the Fermi
level, whereas those for Ta vacancies and As antisites were shiftedabove the Fermi level. These tendencies were explained qualitatively
by considering the difference in the electronegativities of Ta (1.5)
and As (2.18) atoms, and hence the ionicity in the crystal bonding.
That is, As (Ta) vacancies and Ta (As) antisite defects have a positive (negative) effective charge and dope electrons (holes) to maintain the charge neutrality of the system. Therefore, the Fermi level
tended to shift upwards (As vacancy and Ta antisite) or downwards(Ta vacancy and As antisite). In all cases, finite states were createdat the Fermi level, as seen in Fig. 4, and thus metallic conduction
could appear.
However, the present transport measurements (Fig. 3) showedno metallic conduction. The off-stoichiometry seemed to cause noserious degradation of the semimetallic transport properties compared with TaAs crystals reported to be stoichiometric.9,10 Therefore,
even though the off-stoichiometry created states near the Fermi level,
they hardly generated metallic conduction (i.e., quenched), forinstance, due to the Anderson localization below the mobility edge.
Furthermore, the semimetallic transport properties of TaAs wererobust against the disorder created by excess Ta antisite defects. The

FIG. 4. Electron DOS for pristine TaAs crystal and TaAs crystals containing
3.125 at. % As, Ta vacancies (VAs, VTa ), and antisites (TaAs , AsTa ) calculated by
DFT using 2  2  1 conventional cells. The Fermi level is located at 0 eV.

above scenario agreed with the theoretical prediction of the conservation of ballistic transport of Weyl fermions in disorderedsystems.12,13 The theory suggests that linear dispersion plays anessential role. The robustness of ultrahigh mobility against impurities
has also been proposed in intentionally doped NbP.30 To pursue therobustness of semimetallic transport properties of TaAs against offstoichiometric disorder more in detail, the Ta/As ratio should becontrolled more systematically in future.
As for the absence of negative longitudinal MR in the present
TaAs, a technical problem is first considered. In TaAs, an extremely
large positive transverse MR may disturb the relatively weak negative longitudinal MR, even for a small misalignment of current andfield directions.9 This explanation has also been proposed in thecontext of the current jet effect.2 Excess Ta anitisite defects may
also degrade weak negative longitudinal MR through carrier

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scattering among valleys of opposite chirality.31,32 Lastly, we discuss
the negative longitudinal MR from a general viewpoint. Although
this effect is thought to be evidence for the chiral anomaly of Weyl
fermions, there are other channels that can produce negative longitudinal MR. For example, negative longitudinal MR can be induced
through the unentanglement of weakly localized disorder (i.e.,
quenched metallic states) with the magnetic field.33 Anisotropic
DOS can also induce negative longitudinal MR in magnetic
systems.34 Considering that negative longitudinal MR is widely
observed in magnetic Weyl systems (Co2 MnGa35 and Mn3 Sn36),
and also in non-Weyl systems, such as TaAs2 ,37,38 it is still uncertain whether this effect could be evidence of the chiral anomaly of
Weyl fermions.2
V. CONCLUSION
We investigated CVT-grown TaAs single crystals via structural
analyses and magnetotransport measurements. The crystals hadhigh single-crystalline order and 7%–9% As-deficient offstoichiometry. Excess Ta atoms due to the off-stoichiometry wereconsidered to form Ta antisite defects rather than As vacancies. No
metallic conduction was caused by the off-stoichiometric disorder;that is, metallic states might be quenched for some reason. The
robustness of the excellent semimetallic transport properties against
off-stoichiometric disorder may be promising in future device
applications.

The authors thank K. Kobayashi of Kyushu University for
fruitful discussion. This work was financially supported by a JSPS
KAKENHI Grant-in-Aid for Scientific Research (B) (No.
JP21H03748) and partly supported by MEXT ARIM (No.
JPMXP1222AT5020).
DATA AVAILABILITY
The data that support the findings of this study are available
within the article.
AUTHOR DECLARATIONSConflict of Interest
The authors have no conflicts to disclose.
Author Contributions
A. Kawasuso: Conceptualization (equal); Data curation (equal);
Formal analysis (equal); Funding acquisition (equal); Investigation(equal); Methodology (equal); Project administration (equal);
Resources (equal); Software (equal); Supervision (equal);
Validation (equal); Visualization (equal); Writing – original draft(equal); Writing – review & editing (equal). M. Suda: Data curation (equal); Formal analysis (equal); Investigation (equal);
Methodology (equal); Software (equal); Validation (equal).
H. Murakawa: Data curation (equal); Investigation (equal);
Methodology (equal); Validation (equal). M. Komada:
Methodology (lead). C. Suzuki: Formal analysis (lead). H. Amada:
Formal analysis (lead). K. Michishio: Data curation (lead); Formal

J. Appl. Phys. 133, 223903 (2023); doi: 10.1063/5.0147663
Published under an exclusive license by AIP Publishingscitation.org/journal/jap

analysis (lead). M. Maekawa: Funding acquisition (equal);
Methodology (equal). A. Miyashita: Formal analysis (equal);
Methodology (equal). N. Seko: Methodology (equal); Supervision(equal). S. Yamamoto: Formal analysis (equal); Methodology(equal). N. Oshima: Methodology (equal); Supervision (equal).
S. Seki: Methodology (supporting); Supervision (equal).
N. Hanasaki: Methodology (supporting); Supervision (equal).
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ACKNOWLEDGMENTS

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J. Appl. Phys. 133, 223903 (2023); doi: 10.1063/5.0147663
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