Superconductivity of 2.2K under pressure in helimagnet CrAs

Article abstract of DOI:10.7566/JPSJ.83.093702

We report resistivity measurements of the helimagnet CrAs under pressures. The helimagnetic transition with T_N ～ 265K at ambient pressure is completely suppressed above a critical pressure of P_c ～ 0.7 GPa, and superconductivity is observed at ～2.2K for zero resistance, which exists in a wide pressure range extending beyond 3 GPa. Both the upper critical field H_c2 and the coeffi cient A in the resistivity increase toward P_c, suggesting that the superconductivity of CrAs is mediated by electronic correlations enhanced in the vicinity of the helimagnetic phase.

Full text of DOI:10.7566/JPSJ.83.093702

Journal of the Physical Society of Japan 83, 093702 (2014)
Letters
http://dx.doi.org/10.7566/JPSJ.83.093702
Superconductivity of 2.2 K under Pressure in Helimagnet CrAs
+
Hisashi Kotegawa , Shingo Nakahara, Hideki Tou, and Hitoshi Sugawara
Department of Physics, Kobe University, Kobe 658-8530, Japan
(
Received July 5, 2014; accepted July 11, 2014; published online August 4, 2014)
We report resistivity measurements of the helimagnet CrAs under pressures. The helimagnetic transition with
TN ³ 265 K at ambient pressure is completely suppressed above a critical pressure of Pc ³ 0.7 GPa, and super-
conductivity is observed at ³2.2 K for zero resistance, which exists in a wide pressure range extending beyond 3 GPa.
Both the upper critical field Hc2 and the coefficient A in the resistivity increase toward Pc, suggesting that the
superconductivity of CrAs is mediated by electronic correlations enhanced in the vicinity of the helimagnetic phase.
3
d electron systems can offer stages that induce intriguing pressure release from room temperature to low temperatures
9
)
superconductivity such as that realized in cuprates, Fe is ³0.3 GPa. To estimate the pressure at high TN, we
pnictides, and cobalt oxyhydrate. Superconducting (SC) assumed that pressure is unchanged below 100 K and
mechanisms and symmetries are diverse depending on the depends on temperature linearly from 100 K to room
ꢃ3
material, and thus the discovery of superconductivity in a temperature, increasing at a rate of 1:5 ꢄ 10 GPa/K.
new system is crucial for the development of the field of
Figure 1(a) shows the temperature dependence of resis-
research on superconductivity. In this paper, we report the tivity for CrAs measured at ambient pressure. A discrete
discovery of a pressure-induced superconductivity of ³2.2 K jump is observed below 263 K on cooling; however, the drop
in the helimagnet CrAs through resistivity measurements. in µ, which shows an opposite behavior to that shown in
6)
This is the first example of superconductivity in Cr-based Fig. 1(a), is expected to be intrinsic. In this setting, the
magnetic systems. Very recently, Wu and coworkers have resistivity does not return to its original value at room
independently obtained similar results on the occurrence of temperature, as shown by the red curve. This indicates that
superconductivity under high pressure in CrAs through cracks are induced in the crystal owing to the large
resistivity and susceptibility measurements.1)
magnetostriction. We measured different crystals several
CrAs has an orthorhombic MnP-type crystal structure with times for both samples #1 and #2, but a similar behavior was
the space group of Pnma and shows a first-order magnetic again observed. In fact, crystals often collapse, once they
transition at TN ꢀ 265 K. Most research studies of the enter into the ordering state. The precise estimation of the
magnetic property of CrAs have been performed in the residual resistivity ratio (RRR) = ꢂð300 KÞ=ꢂ0 at ambient
1
970s. The magnetic structure of CrAs is a double-helical pressure is difficult, because the resistivity does not return to
ꢂ
one represented by a propagation vector of 0:354 ꢁ 2ꢀc and a its original value at room temperature. If we define RRRT ¼
N
2
,3)
ꢃ
ꢃ
N
magnetic moment of ³1.7ꢁ /Cr that lies in the ab-plane.
ꢂðT Þ=ꢂ , where ꢂðT Þ is the resistivity just below T ,
B
N
0
N
The magnetic transition is accompanied by a large magneto- RRR is not affected significantly by the repetition of
TN
striction of ꢀb=b ¼ þ5:5%, ꢀa=a ¼ ꢃ0:3%, and ꢀc=c ¼ magnetic transition. However, if we evaluate the residual
4,5)
ꢃ0:9% below TN. The crystal structure in the helimagnetic resistivity ꢂ0 at ambient pressure, the crystal experiences a
phase has been reported to belong to the same space group magnetic transition at least once. This means that the ꢂ0 at
4,5)
as the paramagnetic (PM) phase,
structure is incommensurate.
although the magnetic ambient pressure is inevitably evaluated for crystals with
cracks. It is unclear whether damage to crystals extends up to
Single crystals of CrAs were prepared by the Sn-flux the microscopic level.
method similar to that described in Ref. 6. The resistivity
Figure 1(b) shows the temperature dependence of resis-
measurements under pressure were performed using samples tivity of sample #1, measured at several pressures, where the
#
1 and #2, which are from different batches. Sample #1 was resistivity is normalized by the value at 295 K. The resistivity
obtained accidentally from a mixture of Sr : Cr : As : Sn ¼ estimated actually increases by 1.8 times from ambient
: 2 : 2 : 20, which was prepared with the aim of producing pressure to 0.73 GPa owing to the occurrence of cracks. TN
SrCr As ; however, CrAs was obtained as the main product. apparently decreases with increasing pressure, and the
1
2
2
For sample #2, a mixture of Cr : As : Sn ¼ 1 : 1 : 10 was anomaly at TN is not observed above the critical pressure
prepared. For both samples, the mixture was placed in an Pc ꢀ 0:7 GPa, which is consistent with the earlier result
1
0)
alumina crucible and sealed in an evacuated quartz ampoule. obtained using a polycrystalline sample as well as with a
1)
The ampoule was heated slowly up to 1050 °C, and held there recent report. The hysteresis at TN was observed below
for 2 h, and then cooled to 600 °C at a rate of ¹5 °C/h. After ³0.7 GPa; thus, it is conjectured that the magnetic transition
centrifuging the flux, the ampoule was cooled to room is of the first order even near Pc. The inset shows the
7)
temperature. The crystals obtained grew along the a-axis.
resistivity at low temperatures, where a signature of super-
Electrical resistivity (µ) measurement at high pressures of conductivity is observed; however, zero resistance was not
8)
up to ³3 GPa was carried out using an indenter cell. The achieved in the measured temperature range. The RRR for
current was made to flow along the a-axis. We used Daphne the present sample (#1) was estimated to be ³60 at 0.73 GPa
7
474 as the pressure-transmitting medium.9) The applied just above P , where the resistivity returns to its original
c
pressure was estimated from the T of the lead manometer; value at room temperature after the measurement at low
c
however, it has been reported for Daphne 7474 that the temperatures.
0
93702-1
©2014 The Physical Society of Japan

J. Phys. Soc. Jpn. 83, 093702 (2014)
Letters
H. Kotegawa et al.
(
b)
(a)
Fig. 1. (Color online) (a) Temperature dependence of resistivity of CrAs at ambient pressure. A clear anomaly was observed at TN accompanied by
hysteresis, but the resistivity does not return to its original value, indicating that cracks are induced in the crystal owing to the large magnetostriction. This
behavior was observed regardless of the difference between the samples #1 and #2. (b) Temperature dependence of resistivity of sample #1 at several pressures.
TN is strongly suppressed under pressure. The inset shows the resistivity at low temperatures.
(a)
(c)
(b)
Fig. 2. (Color online) (a) Temperature dependence of resistivity of CrAs (sample #2) at several pressures in the PM phase. This crystal does not experience
the ordering state. RRR is estimated to be ³290 at 0.73 GPa. The arrow indicates the resistivity at ambient pressure and room temperature. (b) Resistivity at
2
low temperatures. A clear SC transition is observed with a maximum Tc of 2.17 K at 1.00GPa. (c) T vs µ plot. The slope decreases gradually with increasing
pressure. The data obeys the FL form only in the narrow temperature range.
Figure 2(a) shows the temperature dependence of the original sample quality or the experience of the magnetic
resistivity of CrAs (sample #2) under a different setting. transition induces the difference in RRR between samples #1
Here, we applied pressure beyond Pc at room temperature as and #2. Figure 2(b) shows the resistivity at low temperatures,
a first step to avoid damage to the sample, so that this crystal measured after performing this procedure. A clear SC
does not experience the helimagnetic transition. The RRR at transition can be observed, and the maximum T of 2.17 K
c
0
.73 GPa was estimated to be ³290. It is unclear whether the for zero resistance is attained at 1.00 GPa. The T slightly
c
0
93702-2
©2014 The Physical Society of Japan

J. Phys. Soc. Jpn. 83, 093702 (2014)
Letters
H. Kotegawa et al.
the effective electron mass is enhanced by magnetic corre-
lations near Pc. n is close to 2 in the low-pressure region and
(
c)
has its minimum just above P , followed by a gradual
c
increase under higher pressures. We obtained n ¼ 1:4 ꢅ 0:1
1
)
near P , which is close to n ¼ 1:5 ꢅ 0:1 shown by Wu et al.
c
(
(
b)
a)
The highest Tc of 2.17 K is achieved at 1.00 GPa,
accompanied by the increase in A and the deviation from
the FL behavior. This behavior is reminiscent of a typical
11)
case of heavy-fermion superconductors,
which have a
quantum critical point of magnetic origin. It is conjectured
that magnetic fluctuations play a vital role in inducing
superconductivity in CrAs. However, the difference of CrAs
from heavy-fermion systems is the first-order separation of
the magnetic and PM phases in CrAs. This situation is
reminiscent of the Fe-based superconductor SrFe As , where
2
2
a Tc of 34 K is achieved in the vicinity of the magnetic phase,
but the magnetic and PM phases are separated by a first-order
1
2)
transition. In this case, the coexistence of magnetism and
superconductivity is not simple, and the hybrid state of both
1
3)
phases has been reported in SrFe As . It is an intriguing
2
2
issue whether superconductivity can coexist with the
helimagnetic state in CrAs. Another interesting similarity to
Fe-based superconductors is that the magnetic transition is
accompanied by the structural phase transition. The con-
tribution of orbital fluctuations to superconductivity might be
an interesting issue to be considered in CrAs.
Fig. 3. (Color online) (a) Pressure–temperature phase diagram of CrAs.
The helimagnetic phase disappears above Pc ꢀ 0:7 GPa, and super-
conductivity appears in the PM phase. The closed (open) squares represent
TN obtained during cooling (warming). The observation of hysteresis up to
Pc indicates that helimagnetic-paramagnetic transition is of the first order
even under pressure. (b) The pressure dependence of the coefficient A, which
is estimated from the resistivity below 4 K. Above Pc, A decreases gradually
Wu et al. have reported zero resistance below ³1.5 K,1)
whereas we observed it below ³2.2 K. The RRR range of
their samples is 240–327, and thus there is no significant
difference from that of our sample #2. The pressure-
transmitting media used are different from each other, but
we consider that the properties of the media used, i.e.,
Daphne 7474 (our study) and glycerol (Wu et al.’s study) are
similar. A possible reason for this is the experience of the
magnetic transition with a large magnetostriction, although
we do not know the experimental procedure used by Wu
et al. A systematic study is required to clarify the cause of the
2
with increasing pressure. The estimated 1=ðꢃ0TcÞ is also plotted, showing
good scaling with A. (c) Pressure dependence of n estimated from the
resistivity between Tc and 10 K. The clear deviation from the FL behavior is
confirmed. The highest Tc of 2.17 K is achieved at 1.00 GPa, accompanied by
the increase in A and the deviation from the FL behavior.
increases from 0.73 GPa close to Pc, reaches its maximum difference in Tc.
at approximately 1.00 GPa, and decreases with increasing
Figures 4(a)–4(c) show the temperature dependence of the
pressure. At 3.06 GPa, the onset of superconductivity can be resistivity under different magnetic fields along the a-axis.
confirmed in the measured temperature range. The Tc for zero For 1.00, 1.40, and 1.88 GPa, the field dependences of Tc
resistance is significantly higher than that for sample #1, are obtained and displayed in Fig. 4(d). The initial slope of
orb
orb
where the onset of superconductivity is observed below ðꢃdH =dTÞ gives the orbital field Hc2 through H ð0Þ ¼
c2
Tc
c2
1
4)
³
1.6 K at 0.73 GPa, as shown in the inset of Fig. 1(b). 0:727ðꢃdH =dTÞ T in the clean limit. The slopes are
c2
T
c
c
2
Figure 2(c) shows the T vs µ plot. The resistivity obeys the estimated to be 0.91 T/K (1.00 GPa), 0.80 T/K (1.40 GPa),
2
orb
Fermi liquid (FL) form of ꢂðTÞ ¼ ꢂ0 þ AT in a narrow and 0.62 T/K (1.88 GPa), giving Hc2 ð0Þ of 1.44 T (1.00
temperature range, and its temperature range is likely to GPa), 1.22 T (1.40 GPa), and 0.85 T (1.88 GPa), respectively.
extend slightly under higher pressures. The slope corresponds These values correspond to the Ginzburg–Landau coherence
orb
2
0
to the coefficient A, which represents the inelastic scattering length ꢃ0, through Hc2 ð0Þ ¼ ꢁ0=2ꢀꢃ where ꢁ0 is the
between electrons and is generally proportional to the square quantum fluxoid. ꢃ0 along the bc-plane is estimated to be
of the effective electron mass. A clearly decreases with 151 Å (1.00 GPa), 164 Å (1.40 GPa), and 197 Å (1.88 GPa).
increasing pressure.
We perform a simple comparison between the pressure
The pressure–temperature phase diagram of CrAs up to dependences of Hc2 and A, which corresponds to the effective
2
ꢂ
³
3 GPa is shown in Fig. 3(a). The helimagnetic phase electron mass. ꢃ0 is given by ꢃ0 ’ hꢂ vF=ꢀꢀ0 ¼ hꢂ kF=ꢀꢀ0m ,
ꢂ
disappears above Pc ꢀ 0:7 GPa. The SC phase appears in the where vF, kF, ꢀ0, and m are Fermi velocity, Fermi wave
PM state, extending to relatively high pressures. Figures 3(b) number, SC gap size, and effective electron mass, respec-
and 3(c) show the pressure dependence of A, which is tively. If we assume for simplicity that kF and ꢀ0=kBTc
2
estimated using ꢂðTÞ ¼ ꢂ0 þ AT for the data between Tc are independent of pressure, we obtain the relation
2
and 4 K, and the pressure dependence of n, which is A / 1=ðꢃ T Þ , which is plotted in Fig. 3(b). The good
0
c
0
n
2
evaluated using ꢂðTÞ ¼ ꢂ þ A T between T and 10 K. A scaling between A and 1=ðꢃ T Þ strongly suggests that
0
c
0 c
decreases gradually with increasing pressure, indicating that the superconductivity in CrAs is mediated by electronic
0
93702-3
©2014 The Physical Society of Japan

J. Phys. Soc. Jpn. 83, 093702 (2014)
Letters
H. Kotegawa et al.
(
d)
(
a)
(
b)
(
c)
Fig. 4. (Color online) (a–c) Temperature dependences of the resistivity under different magnetic fields along the a-axis. At 1.88 GPa, superconductivity is
suppressed by the small magnetic field. (d) Temperature dependence of Hc2. The magnitude of the slope decreases with increasing pressure. From the slopes,
orb
we obtained the orbital field Hc2 ð0Þ to be 1.37 T (1.00 GPa), 1.16 T (1.40 GPa), and 0.79 T (1.88 GPa).
+kotegawa@crystal.kobe-u.ac.jp
correlations enhanced near the helimagnetic phase. Thus, the
1
)
W. Wu, J. Cheng, K. Matsubayashi, P. Kong, F. Lin, C. Jin, N. Wang,
Y. Uwatoko, and J. Luo, arXiv:1406.6431.
elucidations of the underlying correlations in the PM phase
and the pressure evolution of the magnetic structure are
important for understanding superconductivity in CrAs.
In summary, we have completed the pressure–temperature
phase diagram of CrAs up to ³3 GPa. The helimagnetic
phase is suppressed at ³0.7 GPa, and superconductivity is
realized in a wide pressure range in the PM state with a
2) H. Watanabe, N. Kazama, Y. Yamaguchi, and M. Ohashi, J. Appl. Phys.
0, 1128 (1969).
4
3
)
K. Selte, A. Kjekshus, W. E. Jamison, A. Andresen, and J. E.
Engebretsen, Acta Chem. Scand. 25, 1703 (1971).
4
5
)
)
H. Boller and A. Kallelt, Solid State Commun. 9, 1699 (1971).
T. Suzuki and H. Ido, J. Appl. Phys. 73, 5686 (1993).
6) W. Wu, X. Zhang, Z. Yin, P. Zheng, N. Wang, and J. Luo, Sci. China
Phys. Mech. Astron. 53, 1207 (2010).
maximum T ꢀ 2:2 K at 1.0 GPa. The experimental results
c
7
)
In Ref. 6, it has been described that the crystal is grown along the
b-axis. In our Laue measurement, the crystal was grown along the
a-axis.
indicate that the superconductivity of CrAs is likely to be
sensitive to the sample quality and experimental conditions.
The pressure dependences of A and Hc2 suggest that
superconductivity is mediated by electronic correlations
enhanced in the vicinity of the helimagnetic phase. The
phase diagram of and the behavior observed in CrAs have
similarities to those of heavy-fermion superconductors and
some Fe-based superconductors. CrAs is an interesting
material for investigating how SC symmetry is realized in
the vicinity of the helimagnetic phase with a large magneto-
8
)
)
T. C. Kobayashi, H. Hidaka, H. Kotegawa, K. Fujiwara, and M. I.
Eremets, Rev. Sci. Instrum. 78, 023909 (2007).
K. Murata, K. Yokogawa, H. Yoshino, S. Klotz, P. Munsch, A. Irizawa,
M. Nishiyama, K. Iizuka, T. Nanba, T. Okada, Y. Shiraga, and S.
Aoyama, Rev. Sci. Instrum. 79, 085101 (2008).
9
1
0) É. A. Zavadskil and I. A. Sibarova, Sov. Phys. JETP 51, 542 (1980).
11) A. Demuer, D. Jaccard, I. Sheikin, S. Raymond, B. Salce, J.
Thomasson, D. Braithwaite, and J. Flouquet, J. Phys.: Condens.
Matter 13, 9335 (2001).
1
2) H. Kotegawa, H. Sugawara, and H. Tou, J. Phys. Soc. Jpn. 78, 013709
striction.
(
2009).
Acknowledgements We thank Hisatomo Harima for helpful discussions.
This work has been partially supported by Grants-in-Aid for Scientific Research
1
3) K. Kitagawa, N. Katayama, H. Gotou, T. Yagi, K. Ohgushi, T.
Matsumoto, Y. Uwatoko, and M. Takigawa, Phys. Rev. Lett. 103,
(Nos. 22740231, 20102005, and 24340085) from the Ministry of Education,
2
57002 (2009).
Culture, Sports, Science and Technology of Japan (MEXT).
1
4) E. Helfand and N. R. Werthamer, Phys. Rev. 147, 288 (1966).
0
93702-4
©2014 The Physical Society of Japan