Observation of superconductivity in pressurized 2M WSe2 crystals

Article abstract of DOI:10.1039/c9tc02417d

In this communication, we report a new-phase 2M WSe₂ with a monoclinic space group C2/m. 2M WSe₂ presents a metallic behavior under ambient pressure and shows a superconducting transition with a maximum T_c of 7.3 K at 10.7 GPa, which arises from the enhanced density of states near the Fermi surface upon pressurization.

Full text of DOI:10.1039/c9tc02417d

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DOI: 10.1039/C9TC02417D
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Observation of Superconductivity in Pressurized 2M WSe2 Crystals
Yuqiang Fang,†ab Qing Dong,†f Jie Pan,†e Hanyu Liu,f Pan Liu,d Yiyang Sun,a Quanjun Li,f Wei
Zhao,*a Bingbing Liu*f and Fuqiang Huang*ac
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
In this communication, we reported a new-phase 2M WSe2 with a
High pressure is also an effective tool to tune the electronic
monoclinic space group C2/m. 2M WSe2 presents a metallic properties of materials and further generate superconductivity.
behavior under ambient pressure and shows superconducting The TMDs are suitable for high-pressure research, because the
transition with a maximum Tc of 7.3 K at 10.7 GPa, which arises from layered structure with weak Van der Waals force can be changed
the enhanced density of states near Fermi surface upon a lot under the high pressure.22-24 For instance, superconductivity
pressurization.
emerges in semiconducting 2H MoS2 under ultrahigh pressure of
220 GPa.25 For the type-II Weyl semimetal Td WTe2,
superconductivity emerges from the suppression of large
magnetoresistance.26 Except for sulfides and tellurides, the
structural evolution and physical properties of 1T’ selenides
under pressure have not been reported due to absence of phase-
Transition metal dichalcogenides (TMDs) constructed by the
distorted octahedrally coordinated 1T’- MX2 monolayers (M =
Mo, W; X = S, Se, Te) have attracted intensive attention due to
their intriguing physiochemical properties such as
superconductivity,1-4 quantum spin Hall effect,5-10 large
pure crystals. Recently
a direct solution-phase method
magnetoresistance11-12
and
excellent
electrocatalytic
synthesized the 1T’ WSe2 nanosheets.27 The size and purity of
the 1T’ WSe2 nanosheets are still unsuitable for the crystal
structure determination and electrical transport measurements.
Here, we successfully obtained millimeter-sized 1T’ WSe2
single crystals by a topochemical reaction. The crystal structure
of 1T’ WSe2 was determined by single crystal X-ray diffraction.
According to the nomenclature of TMDs, 1T’ WSe2 should be
called as 2M WSe2 precisely because it has a monoclinic group
and two WSe2 layers in the unit cell. The electrical transport
properties of 2M WSe2 under high pressure was performed and
performance.13-16 The physical properties of TMDs depend on
the d-orbital electronic configurations of transition metal atoms
to a great extent. Compared with semiconducting 2H phases, the
1T’-phase VIB-group TMDs are semimetallic due to the partially
occupied d orbits. Recently, there are increasing reports about
the superconductivity in 1T’ phases. For example, exfoliated
1T’-MoS2 nanosheets has
a
superconducting transition
temperature of 4.6 K.17 The 2M WS2 shows a Tc of 8.8 K with
calculated topological surface states.18 In addition,
superconductivity was induced in Td WTe2 by chemical superconductivity was observed in the resistance curves.
intercalation, electrostatic gating and the proximity effect.19-21
Superconducting transition occurs at 4.2 GPa, and Tc increases
to a maximum of 7.3 K at 10.7 GPa in the dome-shaped T-P
phase diagram. Structural evolution of 2M WSe2 was detected
through the measurements of high-pressure synchrotron XRD.
No structural transition happens in the superconducting regime.
Moreover, electronic band structure calculations reveal that the
density of states at Fermi surface increase rapidly upon
pressurization, which could ac-count for the appearance and
enhancement of Tc.
Since the metastable 2M WSe2 crystals cannot be synthesized
directly from high-temperature solid state reactions, we firstly
synthesized the KxWSe2 crystals as precursor from the reaction
of the K and 2H WSe2 at 800 ⁰C. Then, the KxWSe2 crystals were
soaked in an oxidizing solution to extract K+ ions to obtain 2M
WSe2 crystals, as depicted in the text of Supporting Information
a. State Key Laboratory of High Performance Ceramics and Superfine
Microstructure Shanghai Institute of Ceramics, Chinese Academy of Sciences,
Shanghai, 200050, P. R. China. E-mail: huangfq@mail.sic.ac.cn
b. University of Chinese Academy of Sciences, Beijing 100049, China
c. State Key Laboratory of Rare Earth Materials Chemistry and Applications,
College of Chemistry and Molecular Engineering, Peking University, Beijing
100871, China
d. State Key Laboratory of Metal Matrix Composites, School of Materials Science
and Engineering, Shanghai Jiao Tong University, Shanghai 200030, P. R. China
e. Department of Materials Science and Engineering, City University of Hong Kong
f. State Key Laboratory of Superhard Materials, Jilin University, Changchun
120012, China
† Y. Fang, Q. Dong and J, Pan contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Raman spectra of 2M WSe2. Different from 2H WVSieew2,ArtthicelerOenalirnee
seven peaks located at 106 cm-1, 146 cm-D1O, 1I:5104.1c0m39/C, 91T7C702c4m17D,
218 cm-1,236 cm-1 and 259 cm-1 in the Raman spectrum, which
match well with the calculated data.
(SI). The crystal structure and refinement information of 2M
WSe2 are shown in Table S1-S4.
-1
-1
Figure 1. (a) The schematic structure of 2M WSe2 from b axis.
(b) High-resolution HADDF-STEM image of 2M WSe2. Inset: the
FFT of Figure b, which shows a  2a superstructure clearly. (c)
Powder X-ray diffraction pattern of the sample, where the red
curve was theoretically calculated from the structure of 2M
WSe2. (d) Experimental and calculated Raman spectra of 2M
WSe2.
Figure 2. (a) The resistivity of 2M WSe2 measured at ambient
pressure. (b) Temperature dependence of electrical resistance
measured at the pressures from 0.8 to 12.8 GPa. (c) The
superconductivity transition of the 2M WSe2 crystals at 10.7
GPa under the magnetic fields of up to 3.9 T. (d) Temperature
dependence of the upper critical field for 2M WSe2. The blue
line represents fitting by the WHH formula.
Figure 1a displays the schematic structure of 2M WSe2, which
crystallizes in a monoclinic space group of C2/m, with the lattice
parameters, a = 13.838(3)Å, b = 3.291(7)Å, c = 5.912(2)Å and β
= 111.468(8)⁰. The unit cell of 2M WSe2 consists of one
independent W site and two independent Se sites. The W atom
in 2M WSe2 is coordinated to six Se atoms in a distorted
octahedron, which buckles the Se atom layers. The coordination
number of W atoms in 2M WSe2 is eight due to the formation of
W-W bonds.
The electrical transport properties of 2M WSe2 were measured
using normal four-point method on a single-crystal sample as
shown in Figure S2. Figure 2a presents the temperature
dependence of resistivity for 2M WSe2 at ambient pressure. The
resistivity decreases from 0.59 mΩ cm at 300 K to 6.18 μΩ cm
at
2
K, suggesting
a
metallic behavior. No sign of
K.
superconducting transition is observed down to
2
The neighboring W atoms form linked zigzag chains with an
Subsequently we investigated the electrical behavior of 2M
WSe2 under the quasi-hydrostatic pressure. Figure 2b shows
temperature dependence of the resistance at the pressures from
2.9 GPa to 12.8 GPa. From the inset of Figure 2b, the
superconducting sign of a slight drop in the resistance curves
emerges at the pressure of 4.2 GPa with a critical temperature Tc
of 4.3 K. Here Tc is defined as the temperature at which the
resistance starts to deviate from the normal state. With increasing
pressure, Tc shifts to higher temperature, and reaches a maximum
of 7.3 K at 10.7 GPa. Zero resistance cannot been observed in all
the resistance curves, which results from that the pressure
gradients of the pressurized 2M WSe2 broaden the
equal bond length of 2.803(9) Å along the b axis. Besides, the
W-Se bond lengths are equal to 2.600(1)Å, 2.630(2)Å 2.511(2)Å,
and 2.536(2)Å respectively, compared with 2.526(1)Å in 2H
WSe2. The crystal structure of 2M WSe2 is different from the
known Td WTe2 owing to a different stacking pattern of 1T’ MX2
monolayers. The adjacent layers in Td WTe2 were rotated by
180 along the c axis, while the Se-W-Se sandwiched layers
stack through transition along the a axis to build up 2M WSe2
crystals.
The prepared 2M WSe2 crystals are ribbon-shaped with
hundreds of micrometers in length due to the preferred growth
direction along the zigzag chains (along the b axis), as shown in
scanning electron microscopy (SEM) image (Figure S1). The
atomic structure of 2M WSe2 was characterized by the high-
angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM). Figure 1b shows the W-W
zigzag chains clearly, which conforms to the schematic structure
of 2M WSe2. The Fourier transform (FFT) pattern (see the inset
of Figure 1b) indicates a 1  2a superstructure. The PXRD
pattern of the sample is shown in Figure 1c, where all the
diffraction peaks fit well with the simulated diffraction data of
2M WSe2. Figure 1d displays experimental and calculated
superconducting
transition.27
Above
10.7
GPa,
superconductivity vanishes gradually. The pressure-temperature
(P-T) phase diagram of 2M WSe2 in Figure S3 shows a dome-
shaped superconductivity behavior, similar to that of Td WTe2
and 1T TiSe2.29-30
To further confirm that the resistance drop in Figure 2a is
superconducting transition, external magnetic field was applied
perpendicular to the plane of 2M WSe2 single crystal. Figure 2c
shows the temperature dependence of the resistance under
various magnetic fields at 10.7 GPa. The superconducting
2 | J. Name., 2012, 00, 1-3
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transition was gradually suppressed with increasing field and a bonding. Moreover, it is interesting that the crystal can be
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magnetic field of 4T almost smear out the superconductivity. compressed more hardly in the direction oDfObI:a1x0i.s10t3h9a/nC9cTaCx0i2s4d17uDe
This provides an important evidence of the presence of to the existence of W-W metal bonding along b axis. Figure S4
superconductivity in the pressurized 2M WSe2 crystals. shows the phonon calculation calculated at 13.4 GPa. It can be
Furthermore, the superconductivity of 2M WSe2 can also be seen that the phonon spectrum and DOS do not show any
demonstrated by the measurements of magnetic susceptibility imaginary modes, suggesting that under this pressure the 2M
under high pressure (see the Figure S3). Figure 2d shows the structure can be kinetically stable. Combining the HPXRD data
critical temperature Tc dependence of the upper critical field and calculated phonon spectrum, the superconductivity in the
μ0Hc2, which can be fitted by the one-band Werthamer-Helfand- pressurized 2M WSe2 does not arise from the structure transition.
Hohenberg (WHH) formula Hc2 (T) = Hc2* (1 - T/Tc)1 + α 31
. The
zero-temperature Hc2 (0) is roughly estimated to be 6.79 T. In
addition, the estimated value of Hc2 (0) is lower than the Pauli
paramagnetic limit of μ0Hp = 1.84 Tc = 13.4 T for Tc = 7.3 K,
which suggests no Pauli pair breaking. The nearly linear
temperature dependence of Hc2 may arise from an
unconventional superconducting state or a multi-band Fermi
surface topology. Usually, such linear temperature dependence
of Hc2 has also occurred in pressurized topological insulators
Bi2Se3, pressurized Dirac semimetal Cd3As2 and unconventional
superconductor YPtBi. 32-33
Figure 4. (a-b) The electronic band structure of 2M WSe2 at
pressures of 0 GPa and 8.1 GPa respectively.
The pressure-dependent electronic band structure of 2M WSe2
has been calculated using density-function theory (DFT)
calculations to understand the emergency of superconductivity
in WSe2 under high pressure. Figure 4a-b show the band
structure at ambient pressure and 8.1 GPa respectively. The
applied pressures can tune the electronic conditions near the
Fermi surfaces. Compared to band structure at 0 GPa, there is a
nearly flat band crossing the Fermi level at 8.1 GPa that enlarges
the amount of carriers. It is this flat band that is responsible for
the large DOS value at high pressure. Figure S5 shows the
calculated density of states at the Fermi surface under different
pressures. DOS increases to a maximum and decreases gradually
with increasing pressure, which is similar to the tendency of the
observed Tc. For the Td WTe2, the adjacent Te-W-Te layers
happen to slide upon compression resulting in a phase transition
from Td to 1T’. The structural transition leads to the emergency
of superconductivity.34 Although Se-W-Selayers in 2M WSe2 do
not slide below 11.85 GPa, the interlayer distance is closer and
the interaction of Se 4p orbitals is enhanced, which can affect the
band structure. Since no structure transition happens as entering
the superconductivity states in high pressure, we suppose that the
emergence and improvement of superconductivity in 2M WSe2
results from enhanced DOS near Fermi surface under high
pressure.
Figure 3. Structural information of 2M WSe2 at high pressure.
(a) XRD patterns of 2M WSe2 collected at different pressure (λ=
0.6199 Å). (b-c) Pressure dependence of crystal lattice
constants. (d) Pressure-dependent volume of unit cell.
The structural evolution of 2M WSe2 crystals upon
pressurization was measured through high-pressure synchrotron
XRD (HPXRD) measurements at room temperature. Figure 3a
shows the XRD patterns collected under the pressures from 0.39
to 11.85 GPa. The peaks move to higher angles with increasing
pressure, which corresponds to the decrease of lattice
parameters. Meanwhile, no extra peaks appeared, illustrating
structure phase transition did not happen in this pressure regime.
The lattice constants a, b and c dependence of pressure was
shown in Figure 3b-c. The value of a, b and c decreases from
13.9 Å to 12.7 Å, 3.3 Å to 3.25 Å and 5.95 Å to 5.8 Å,
respectively. The out-of-plane lattice can be compressed to a
larger extent than the in-plane lattice due to the weak interlayer
van der Waals interaction and strong intralayer chemical
Conclusions
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Hashimoto, D. Lu, R. G. Moore, C.-C. Hwang, C. Hwang, Z.
In summary, we successfully prepared 2M WSe2 crystals and
observed superconductivity in the pressurized sample. The T-P
Hussain, Y. Chen, M. M. Ugeda, Z. Liu, X. Xie, T.VPie.wDAertvicelereOanulinxe,
DOI: 10.1039/C9TC02417D
M. F. Crommie, S.-K. Mo, Z.-X. Shen, Nat. Phys. 2017, 13, 683-
689.
phase diagram exhibits
a dome-shaped superconducting
behavior that shows a maximum Tc of 7.3 K. Different from Td
WTe2, no structural transition occur in the superconductivity
region, confirmed by the high pressure XRD. According to the
DFT calculations, N (EF) at Fermi surface is enhanced with
increasing pressure, accounting for the emergence of
superconductivity and enhancement of Tc. Our work provides a
new platform for the investigation of superconductivity in
topological 1T’-phase TMDs.
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Shanghai (Grant 16JC1401700), National Science
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