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1
Pressure-induced superconductivity in ZrTe5
Yonghui Zhou1, Wei Ning2, Yongping Du3, Zhenhua Chi1,2, Xuliang Chen1,2, Xuefei
Wang1, Xiangde Zhu2, Xiangang Wan3,4, Zhaorong Yang1,2,4†, Mingliang Tian2,4† &
Yuheng Zhang2,4
1 Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese
Academy of Sciences, Hefei, Anhui, 230031, China.
2 High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, Anhui 230031,
China.
3 National Laboratory of Solid State Microstructures, College of Physics, Nanjing
University, Nanjing, 210093, China.
4 Collaborative Innovation Center of Advanced Microstructures, Nanjing University,
Nanjing, 210093, China.
†Correspondence and requests for materials should be addressed to Z.Y. (e-mail:
[email protected]); or to M.T. (e-mail: [email protected])
2
As a new topological material, ZrTe5 exhibits novel properties. Pressure is
believed to be effective for studying the topological phase transitions
experimentally. In this work, we have performed resistance and ac magnetic
susceptibility measurements for ZrTe5 single crystal at various pressures up to
68.5 GPa. We show that, accompanied by complete suppression of the high
temperature resistance anomaly, superconductivity is induced at a critical
pressure of 6.2 GPa. The superconducting transition temperature Tc initially
increases with pressure then decreases slightly with a maximum of 4.0 K at 14.6
GPa. At pressures above 21.2 GPa, a second superconducting phase with the
maximum Tc of about 6.0 K manifests and coexists with the original one. We also
performed the density functional theory calculations to explore the possibility of
topological superconductivity in the pressurized ZrTe5.
3
Dirac materials including topological insulators (TIs)1-3, Dirac semimetals4,5 and
Weyl semimetals5-7 belong to a new type of quantum matter and attract extensive
attention in the past few years. ZrTe5 is a layered material with orthorhombic crystal
structure and has been known for a long time due to its large thermoelectric power8,9,
resistivity anomaly10,11 and a large positive magnetoresistance12. However, recent
theoretical work13,14 has proposed that single-layer ZrTe5 is a large gap Quantum spin
Hall (QSH) insulator, while the bulk ZrTe5 formed by the stacking of layers are located
in the vicinity of a transition between strong and weak topological insulator. These
interesting new predictions spark the renewed interests in the research of its Dirac and
topological characters. Indeed, the magneto-transport experiments have observed the
chiral magnetic effect and the angle-resolved photoemission spectroscopy (ARPES)15
provided that the electronic structure of ZrTe5 is consistent with that of the 3D Dirac
semimetals which is similar to that in Na3Bi16-18 and Cd3As219-23. These results suggest
that ZrTe5 is a very promising system that host topological properties and might help to
pave a new way for further experimental studies of topological phase transitions.
To investigate these new quantum states, pressure is used as an important control
parameter to tune the quantum phases of the system24-26. It was well known that pressure
can effectively tune lattice structures and the corresponding electronic states in a more
systematic fashion, which avoids the complexity brought by chemical doping. In this
work, by performing resistance and ac magnetic susceptibility measurements for ZrTe5
single crystal at various pressures up to 68.5 GPa, we report the observation of pressure-
induced semimetal to metal and superconducting transition at a critical pressure of 6.2
GPa. It was found that the occurrence of the metallic phase/or superconductivity at 6.2
GPa is always accompanied by the complete suppression of the big resistance peak near
128 K by the pressure. It seems that two superconducting phases were obtained, one
4
shows a sharp resistance drop with a zero resistance near 3.8 K which is robust at
pressures up to 68.5 GPa. The second one that presents a broad resistance drop but with
a finite resistance starting from 6.0 K is visible only at a pressure above 21.2 GPa.
ZrTe5 single crystals were grown via iodine vapor transport method in a two-zone
furnace with elements Zr (99.99%) and Te (99.99%)27. The crystals show a thin
elongated rectangular shape, where the prismatic ZrTe6 chains run along the
crystallographic a-axis and linked along the c-axis via zigzag chains of Te atoms to
form two-dimensional (2D) layers, stacked along the b-axis into a crystal28. Structural
and compositional characterizations of the crystals by X-ray diffraction, scanning (or
transmission) electron microscopy, together with electron diffraction and energy
dispersive X-ray spectroscopy (EDS) studies confirm its high quality. High pressures
were generated with a screw-pressure-type diamond anvil cell (DAC) made of non-
magnetic Cu-Be alloy. Single crystals with typical dimension of 100×30×10 μm3 were
loaded without pressure-transmitting medium for run 1, while for run 2 loaded with
Daphne 7373 oil as pressure-transmitting medium. The easily cleaved crystal was
pressurized along b-axis and the standard four-probe resistance measurement was
performed along the c-axis, as illustrated schematically in the inset of Fig. 1(a). The
high-pressure ac susceptibility was measured using magnetic inductance technique.
The diamond culet was 800 μm in size and a Be-Cu gasket was pre-indented from 450
μm to 200 μm. Pressure was calibrated by using the ruby fluorescence shift at room
temperature for all experiments29. The electronic band structure calculations have been
carried out by using the full potential linearized augmented plane wave method as
implemented in WIEN2K package30. Local-density approximation (LDA) has been
adopted as the exchange-correlation potential. 24×24×6 and 30×30×8 k-mesh are used
to sample the Brillouin zone for self-consistent calculation and Fermi surface,
5
respectively. Spin-orbit coupling for all elements was taken into account by a second-
variational method.
Figure 1(a) and 1(c) shows the evolution of the resistance as a function of
temperature for ZrTe5 single crystal at various pressures. Upon cooling at 0.5 GPa from
300 K down to 1.8 K, the overall behavior of resistance displays a typical
semiconducting-like feature above 128 K, then decreases with the decrease of
temperature followed by a slight upturn below 20 K. The large resistance anomaly
around 128 K is quite similar to those observed under ambient pressure12,15,31 and was
generally correlated to the sign change of charge carriers although the origin still
remains elusive31. With increasing pressure, the peak temperature increases initially up
to 150 K and then shifts back towards lower temperatures accompanied by the
broadening of the peak and the decrease of the peak resistance. Since it was reported
that the resistance anomaly could be strongly enhanced by an application of magnetic
field, we investigated the magnetoresistance (MR) at the peak temperature for different
pressures. As shown in Fig. 1(b), the magnetotransport properties at each pressure show
positive MR behavior with the increase of magnetic field but the magnitude of the MR
decreases monotonically with the increase of pressure no matter where the peak
temperature locates.
Surprisingly, with further increasing pressure, accompanied by complete
suppression of the resistance anomaly peak, a metallic transport behavior with an
almost constant normal state resistance within 5% is obtained and a small drop of
resistance is observed at ~2.5 K and 6.2 GPa. At 6.7 GPa, the resistance drops to zero
at 1.8 K, signaling the appearance of superconductivity (Fig. 1(c)). To make sure that
the drop of the resistance was indeed a superconducting transition, we carried out ac
magnetic susceptibility measurements on ZrTe5 at several pressures up to 9.0 GPa. As
6
seen from Fig. 1(d), diamagnetic signal is observed at 7.6 GPa and 9.0 GPa, which is
in agreement with the resistance measurements.
Figure 2(a) and 2(b) are the blow-up of the R-T curves at different pressures near
the superconducting transition, it was clearly seen that the superconducting transition
is quite sharp between 8.3 GPa and 21.2 GPa, indicating the bulk superconductivity.
However, when the pressure is applied up to 30.0 GPa, the onset temperature of the
resistance drop occurs from ~6.0 K, accompanied by a gradual decrease of resistance
down to 3.9 K, at which the sharp superconducting transition presents, as shown in Fig.
2(b). In fact, such a two-step like transition can be recognized from 25.3 GPa, implying
two superconducting phases coexist in the sample. With further increasing pressure, the
higher superconducting phase (SC-II) is suppressed gradually and the lower
superconducting transition (SC-I) becomes more and more sharp. Clearly, the SC-I
exists in a broad pressure region from 6.2 GPa to 68.5 GPa, the maximum pressure we
achieved in this work, while the SC-II manifests only at pressures above 21.2 GPa. It is
interesting to note that similar two-step like transition behavior has also been reported
in putative topological superconductor LuPdBi32.
Figure 3 displays temperature dependence of resistance under external magnetic
field aligned along b-axis of ZrTe5 at 14.6 GPa and 30.0 GPa, respectively. For both
cases, a finite resistive tail at low temperatures were clearly seen and gradually lifted
with the increase of magnetic field. A magnetic field of 1.5 T almost smears out the
superconducting transition completely. By defining Tc with two different resistance
criteria, we constructed the magnetic field (H) - temperature (T) phase diagrams, as
shown in Fig. 3(b) and 3(d). For P = 14.6 GPa, Tc decreases monotonically with
increasing magnetic field. The upper critical field μ0Hc2(0) is estimated to be about 1.54
T for the resistance criterion of Rcri = 90%Rn (Rn is the normal state resistance) and 1.26
7
T for the resistance criterion of Rcri = 50%Rn according to the Werthamer-Helfand-
Hohenberg (WHH) formula33. However, for P = 30.0 GPa, being associated with the
coexistence of SC-I and SC-II, the H-T phase diagram for the resistance criterion of Rcri
= 90%Rn is clearly divided into two parts. The Tc is depressed more pronounced at fields
below 0.2 T, which indicates that the SC-II is sensitive to the magnetic field. At fields
above 0.2 T, the superconductivity is dominated by the SC-I, where the R-T curves are
similar to those measured at 14.6 GPa. The Tc-H relationship can be described with the
WHH equation both at fields below and above 0.2 T, which gives μ0Hc2(0) about 0.46
T for SC-II and 1.26 T for SC-I. The large difference of μ0Hc2(0) indicates that the SC-
I and SC-II might have different origins.
It is noted that in both resistance and magnetic susceptibility measurements, we did
not use pressure-transmitting medium. To examine if the pressure induced
superconductivity is only related to the b-axis compression, a comparative experiment
of resistance measurement by using Daphne 7373 oil as the pressure-transmitting
medium was also carried out with pressure up to 20.3 GPa. In this run, a trace of
superconductivity was first observed at 7.2 GPa and a zero resistance was achieved at
10.8 GPa. All the characteristic temperatures in the above experimental results are
summarized in a T-P phase diagram shown in Fig. 4. It can be seen that, with increasing
pressure, the peak temperature of the resistance anomaly initially increases up to 150 K
and then decreases abruptly. When the peak anomaly disappears at the critical pressure
of 6.2 GPa, superconducting phase emerges immediately, indicating a possible quantum
critical point (QCP) near 6.2 GPa, below which the sample is semimetallic with a weak
topological character. However, if we carefully check the normal state resistance at 300
K and 10 K (see the inset of Fig. 4), both pressure-induced variations of normal state
resistance intuitively follows the T-P phase diagram. The slight enhancement of the
8
normal state resistance by the application of pressure up to 2.0 GPa is probably an
indication of the pressure-induced competition of the multiband carriers of the sample.
With further increasing pressure above 6.2 GPa, Tc increases monotonically until it
reaches a maximum of about 4.0 K at 14.6 GPa for the SC-I. The SC-II emerges only
above 21.2 GPa with the highest Tc of about 6.0 K. It sounds very clear that the pressure-
induced SC-I is a bulk superconductor, while the SC-II without zero resistance is not.
There are a number of possibilities that may be responsible for this tiny resistance drop
near 6.0 K. One of them is from the impurity phase in the sample. Because the resistance
drop reaches about 20%Rn, such a speculation is almost unlikely in our single crystal
sample. The second possibility is due to the pressure-induced fluctuation of
superconductivity, which is closely related to the SC-I. This suggestion cannot interpret
the linear-like decay of the resistance over a large temperature range of ~2 K and only
appears at a pressure larger than 21.2 GPa. A possible origin is that it is an independent
superconducting phase induced by the pressure but this phase is not a bulk phase, such
as the filamentary superconductivity which might be related to the edges/or surfaces of
the crystal34.
To have a comprehensive understanding of the physical properties of ZrTe5, we also
perform the density functional theory calculations for the electronic band structures.
Our result agrees well with the previous study, and confirms that this compound is a
weak topological insulator at ambient pressure13. We also perform calculations to
investigate the effect of high pressure. The total energies are calculated for a number of
different volumes ranging from 0.9V0 to 1.2V0, where V0 is the equilibrium unit-cell.
For each volume, the relaxations of cell geometry and atomic positions are carried out
using a conjugate gradient algorithm until the Hellman-Feynman force on each of the
unconstrained atoms is less than 0.001 eV/Å. The obtained volume versus the total
9
energy was found to be in good agreement with the Murnaghan equation of state35. Our
numerical bulk modulus at equilibrium is about 67.7 GPa, slightly larger than that of
WTe236 and MoS2
37.
Our calculations show that the pressure will change the band structure dramatically,
and at around 5.0 GPa, this compound becomes a metal. As shown in Fig. 5, at 7.0 GPa,
considerable states appear at the Fermi level, which agrees with the emergence of the
superconductivity at 6.2 GPa. As shown in Fig. 5(a), at 7.0 GPa there are two bands
cross the Fermi energy, forming a electron-like pocket and hole-like pocket around Γ
and Z point, respectively. The electron pockets denoted by the green color in Fig. 5(c)
are small and exhibit three-dimension character. While the hole pockets, i.e., red sheets
in Fig. 5(c), are much larger and display strong two-dimension feature. A time-reversal-
invariant topological insulator requires odd-parity symmetry and the Fermi surface
enclosing an odd number of time-reversal-invariant momenta (TRIM)38. The states at
Fermi surface are contributed to Zr-4d and Te-5p. Since these bands are spatially
extended, the electronic correlation should quite small due to the strong screening effect.
Consequently, one can expect that the superconductivity discovered in this work is
mediated by the electron-phonon interaction. Although the intra-pocket phonon
mediated pairing, which may has singular behaviour of the electron–phonon interaction
at long wavelengths, can possess odd-parity symmetry39. Unfortunately as shown in Fig.
5(c), the Fermi surface does not enclose any TRIM. Thus, this compound is unlikely
the topological superconductor at least at low-pressure region (less than 10.0 GPa). We
also perform band structure calculations for high-pressure region. As one expects that
increasing the pressure will increase the band width, consequently, there are many
bands cross the Fermi level and the Fermi surface becomes quite complicated as shown
in Fig. 5(b) and Fig. 5(d), and the possibility of topological superconductor at high
10
pressure cannot be exclude, which remains an open question.
In conclusions, by combining experimental and theoretical investigations, we
demonstrated the pressure-induced superconductivity on a weak topological insulator
ZrTe5. The appearance of superconductivity at the critical pressure is accompanied by
the complete suppression of the high temperature resistance anomaly around 128 K.
While our theoretical study rules out the possibility of topological superconductivity at
low pressure, at high pressure (above 20.0 GPa) the system has complicate Fermi
surface and a possible second superconducting phase, thus deserving further study.
11
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Acknowledgements
This research was financially supported by the National Key Basic Research of
China Grants No. 2011CBA00111 and the National Natural Science Foundation of
China Grants (Nos. U1332143, 11374137).
13
Figure 1. Experimental evidence for the pressure-induced superconductivity in
ZrTe5 single crystal. (a) Temperature-dependent resistance R(T) at pressures up to 3.9
GPa in the whole temperature range. (b) Magnetoresistance at the peak temperature of
resistance anomaly at different pressures. (c) The emergence of pressure-induced
superconducting transition at higher pressures from 6.2 GPa to 68.5 GPa. (d) The real
part of ac magnetic susceptibility as a function of temperature at different pressures.
The inset of Fig. 1(a) illustrates how the pressure, magnetic field and current are applied.
The inset of Fig. 1(d) is the image of experimental setup for ac magnetic susceptibility
measurement.
14
Figure 2. Temperature dependence of resistance at low temperatures in the whole
investigated pressure range. (a) The first sharp resistance drop can be seen as the
manifestation of superconductivity at 6.7 GPa. The superconducting transition
temperature Tc is defined by the 50% drop of the normal state resistance. With
increasing pressure, the Tc increases monotonously towards 17.8 GPa. (b) Above 21.2
GPa, a higher transition emerges around 6.0 K, suggesting a novel superconducting
phase. Here, the Tc symbols the onset of the superconducting transition (see the text).
15
Figure 3. Temperature dependence of upper critical field μ0Hc2(T) with magnetic
field parallel to the b-axis. (a,c) Temperature dependence of resistance under different
magnetic fields up to 1.5 T at 14.6 GPa and up to 1.0 T at 30.0 GPa, respectively. Here,
superconducting transition temperature Tc at different magnetic fields are determined
by 50% and 90% drop of the normal state resistance, respectively. (b,d)
Superconducting transition temperature dependence of the upper critical field μ0Hc2 at
14.6 GPa and 30.0 GPa, respectively. The dashed lines represent the fitting lines based
on the Werthamer-Helfand-Hohenberg (WHH) formula.
16
Figure 4. Temperature-pressure phase diagram of ZrTe5 single crystal. T* denotes
the peak temperature of resistance anomaly. The magenta region corresponds to SC-I
phase where Tc is defined as 50% drop of the normal state resistance. The green region
corresponds to SC-II phase where the Tc is determined as the onset temperature of
resistance drop. The pentagram represents the onset temperature of Meissner effect in
the ac magnetic susceptibility measurements. The rhomb and down triangle represent
the peak temperature of resistance anomaly and onset temperature of resistance drop at
pressures with Daphne 7373 oil as the pressure-transmitting medium. The inset shows
the specific resistance as a function of applied pressure at 10 K and 300 K, respectively.
17
Figure 5. The band structure and Fermi surface of ZrTe5 at 7.0 GPa and 20.0 GPa.
(a) and (c) are the band structure and Fermi surface of ZrTe5 at 7.0 GPa, respectively.
(b) and (d) are the band structure and Fermi surface of ZrTe5 at 20.0 GPa, respectively.