Pis’ma v ZhETF, vol. 113, iss. 6, pp. 390 - 391

March 25

Evidence of the ferroelectric polarization in charge transport through

WTe₂ Weyl semimetal surface

N. N. Orlova¹⁾, N. S. Ryshkov, A. V. Timonina, N. N. Kolesnikov, E. V. Deviatov

Institute of Solid State Physics of the Russian Academy of Sciences, 142432 Chernogolovka, Russia

Submitted 19 February 2021

Resubmitted 20 February 2021

Accepted 20 February 2021

DOI: 10.31857/S1234567821060069

Recently, three-dimensional WTe₂ single crystals

were found to demonstrate coexistence of metallic con-

ductivity and ferroelectricity at room temperature [1].

The latter usually belongs to the insulators [2-6], but

it occurs in WTe₂ due to the strong anisotropy of

the non-centrosymmetric crystal structure. The spon-

taneous polarization of ferroelectric domains is found to

be bistable, it can be affected by high external electric

field [1]. Scattering of the charge carriers on the domain

walls is known to provide noticeable contribution to the

sample resistance [7]. Thus, coexistence of metallic and

ferroelectric properties should produce new physical ef-

Fig. 1. (Color online) AFM image of the 400 nm thick sam-

fects [8] for electron transport in TMDCs, and, there-

ple with Au leads, evaporated over the WTe2 flake. The

fore, it should be important for nanoelectronic applica-

leads are separated by 5 µm intervals. Inset demonstrates

tions.

the AFM scan of the flake profile between the contact

The single-crystal flakes of WTe₂ are obtained by

leads, along the white line in the image. Schematic di-

regular mechanical exfoliation, also known as scotch-

agram of the measurement circuit is also shown for the

tape technique. Next, the exfoliated samples were trans-

two-point connection scheme. For correct measurement

ferred on the insulating SiO₂ substrate. While we

of the low-resistance samples, we apply current I be-

need thick three-dimensional flakes to preserve WTe₂

tween the leads and measure the resulting voltage drop

semimetal properties, we use two different techniques

V . These measurements can be carried out in external

electric field by applying gate voltage to the silicon wafer,

for Ohmic contacts fabrication for the flakes of different

separated from the flake by 300 nm SiO2 layer. The mea-

thickness.

surements are performed at room temperature for WTe2

For the thinnest, 300-600 nm flakes, the Au leads are

samples of different thicknesses and lateral sizes, since fer-

defined over the flake surface by standard photolithog-

roelectric domains have been previously observed in WTe2

raphy and lift-off technique after thermal evaporation of

semimetal at room temperature [1]

70 nm Au, see the AFM image in Fig.1. As usual, thin

flakes are about 10-30 µm in the lateral size, so only

two or three Au leads can be placed over the flake to

substrate with pre-defined Au leads pattern, the flake is

form Ohmic contacts with 5 µm distance. These samples

slightly pressed to the leads by another oxidized silicon

are mostly suitable for the two-point transport measure-

substrate. This procedure has been verified to provide

ments.

electrically stable contacts with high quality interfaces.

The thicker (1-3 µm) flakes are about 100 µm in lat-

Also, WTe₂ surface with Au contacts is protected from

eral size, which allows different multiple contact geome-

any contamination by SiO₂ substrate in this case.

tries. However, standard 70-100 nm thick Au leads can

To our surprise, we observe small but noticeable hys-

not be formed across the 1-3 µm step, so we use different

teresis in the experimental dV/dI with current sweep

contact technique. Thick flakes are transferred to SiO₂

direction, so WTe₂ differential resistance is affected by

the sign of the current change. Differential resistance

¹⁾e-mail: honna@issp.ac.ru

dV/dI(I) is a maximum at zero bias, it falls symmetri-

390

Письма в ЖЭТФ том 113 вып. 5 - 6

2021

Evidence of the ferroelectric polarization in charge transport through WTe₂. . .

391

cally at positive and negative currents by about 20 % in

are too small to align polarization of the whole WTe₂

a full current range.

flake, so they mostly affect the domain wall regions. Due

The hysteresis reflects some slow relaxation process

to the field direction, E_gate moves the position of the

in charge transport through WTe₂. To demonstrate the

wall, while E_sd changes the wall region width. Thus,

relaxation directly, we show dV/dI(t) time-dependent

any variation of electric fields leads to the additional po-

curves, which are found to depend on the sign of the cur-

larization current. The latter we observe as slow relax-

rent change. We can estimate relaxation time as about

ation in dV/dI, since polarization current is connected

300 s. This behavior well correlates with the hysteresis

with lattice deformation in ferroelectrics. The possibil-

in dV/dI(I) curves. These effects are intrinsic to bulk

ity to induce polarization current by source-drain field

WTe₂, which can be confirmed by measurements in a

variation is unique for WTe₂, since it is a direct conse-

standard four-point connection scheme, where the Au-

quence of ferroelectricity and metallic conductivity co-

WTe₂ interfaces are excluded.

existence [1].

We can also study effect of the normal-to-the-plane

The authors are grateful to V.T. Dolgopolov for

electric field on dV/dI(I) curves by using silicon sub-

fruitful discussions and S. S. Khasanov for XPS, and x-

strate as a gate electrode. Increasing the gate voltage

ray tungsten ditelluride characterization. We gratefully

value shifts dV/dI(I) curves down irrespective of the

acknowledge financial support by RF State task.

gate voltage sign, so the dV/dI level is in a maximum at

Full text of the paper is published in JETP Letters

zero gate voltage and falls symmetrically both to pos-

journal. DOI: 10.1134/S0021364021060011

itive and negative gate voltage values, in contrast to

the standard asymmetric accumulation/depletion field-

effect transistor behavior.

1. P. Sharma, F.-X. Xiang, D.-F. Shao, D. Zhang,

The hysteresis amplitude can be demonstrated di-

E. Y. Tsymbal, A. R. Hamilton, and J. Seide, Sci. Adv.

rectly by subtracting two dV/dI(I) curves for opposite

5(7), eaax5080 (2019).

current sweep directions at fixed gate voltage. The re-

2. A. N. Morozovska, E. A. Eliseev, G. I. Dovbeshko,

sulting ΔdV/dI(I) shows a maximum at the -1 mA

M. D. Glinchuk, Y. Kim, and S. V. Kalinin, Phys. Rev.

negative current and a minimum at the +1 mA posi-

B 102, 075417 (2020).

tive one. These results can be reproduced for samples of

3. A. Urru, F. Ricci, A. Filippetti, J.

Iñiguez, and

different thicknesses and lateral sizes, and, therefore, of

V. Fiorentini, Nat. Commun.

11,

4922

(2020);

different contact preparation techniques.

https://doi.org/10.1038/s41467-020-18664-6.

Even well-conducting WTe₂ single crystals demon-

4. C. Ferreyra, M. Rengifo, M. J. Sánchez, A. S. Everhard,

strate ferroelectricity at room temperature, which has

B. Noheda, and D. Rubi, Phys. Rev. Appl. 14, 044045

been shown by direct visualization of ferroelectric do-

(2020); 10.1103/PhysRevApplied.14.044045.

mains [1]. The spontaneous polarization of these do-

5. M. V. Stern, Y. Waschitz, W. Cao, I. Nevo, K. Watan-

mains is normal to the WTe₂ layers, it can be affected

abe, T. Taniguchi, E. Sela, M. Urbakh, O. Hod, and

by external electric field [1]. Due to the presence of the

M. Ben Shalom, arXiv:2010.05182 (2020).

metallic conduction, there are two possible directions

6. K. Yasuda, X. Wang, K. Watanabe, T. Taniguchi, and

of the external electric fields in our setup: (i) gate field,

P. Jarillo-Herrero, arXiv:2010.06600 (2020).

E_gate = V_g/d, where d = 300 nm is the SiO₂ oxide thick-

7. T. Hou, Ya. Ren, Yu. Quan, J. Jung, W. Ren, and

ness, E_g is directed normally to the WTe₂ surface; (ii)

Zh. Qiao, Phys. Rev. B 101, 201403(R) (2020).

source-drain field Esd = ρj, which is connected with

8. W. X. Zhou, H. J. Wu, J. Zhou et al.

the flowing current, E_sd is parallel to the WTe₂ surface.

(Collaboration), Commun. Phys.

125

(2019);

The achievable values of the fields (∼ 10⁴-10⁶ V/m)

https://doi.org/10.1038/s42005-019-0227-4.

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