Coexistence of superconductivity and charge density wave in SrPt 2As2

Article abstract of DOI:10.1143/JPSJ.79.123710

SrPt₂As₂ is a novel arsenide superconductor, which crystallizes in the CaBe₂Ge₂-type structure as a different polymorphic form of the ThCr₂Si₂-type structure. SrPt₂As₂ exhibits a charge-density-wave (CDW) ordering at about 470K and enters into a superconducting state at T_c = 5:2K. The coexistence of superconductivity and CDW refers to Peierls instability with a moderately strong electron-phonon interaction. Thus SrPt₂As ₂ can be viewed as a nonmagnetic analog of iron-based superconductors, such as doped BaFe₂As₂, in which superconductivity emerges in close proximity to spin-density-wave ordering.

Full text of DOI:10.1143/JPSJ.79.123710

Journal of the Physical Society of Japan
Vol. 79, No. 12, December, 2010, 123710
#2010 The Physical Society of Japan
LETTERS
Coexistence of Superconductivity and Charge Density Wave
in SrPt As
2
2
Kazutaka KUDO1 , Yoshihiro NISHIKUBO1;2, and Minoru NOHARA1;2
;2
ꢀ
1Department of Physics, Faculty of Science, Okayama University, Okayama 700-8530, Japan
2Transformative Research-Project on Iron Pnictides (TRIP), Japan Science and Technology Agency (JST),
Chiyoda, Tokyo 102-0075, Japan
(
Received October 19, 2010; accepted November 1, 2010; published December 10, 2010)
SrPt2As2 is a novel arsenide superconductor, which crystallizes in the CaBe2Ge2-type structure as a
different polymorphic form of the ThCr2Si2-type structure. SrPt2As2 exhibits a charge-density-wave
(
CDW) ordering at about 470 K and enters into a superconducting state at Tc ¼ 5:2 K. The coexistence of
superconductivity and CDW refers to Peierls instability with a moderately strong electron–phonon
interaction. Thus SrPt2As2 can be viewed as a nonmagnetic analog of iron-based superconductors,
such as doped BaFe2As2, in which superconductivity emerges in close proximity to spin-density-wave
ordering.
KEYWORDS: non-iron-based pnictide, superconductivity, charge-density wave, SrPt2As2
DOI: 10.1143/JPSJ.79.123710
Iron-based superconductors have been attracting consid-
erable interest since Kamihara et al. reported superconduc-
1
)
tivity in LaFeAsO1 F at T ¼ 26 K. A generic phase
ꢁx
x
c
diagram of the iron-based superconductors shows that an
antiferromagnetic (AFM) metallic and nonsuperconducting
state is driven to a paramagnetic and superconducting state
upon partial chemical substitution or by applying external
2
,3)
hydrostatic pressure.
BaFe2As2 is the compound most
studied among the iron-based families so far and is widely
thought to represent the generic features of the iron-based
2
,3)
superconductors. BaFe As exhibits AFM ordering below
2
2
4)
T ¼ 140 K. The partial chemical substitution of either Ba,
N
Fe, or As ions with a different element suppresses the AFM
5
–8)
ordering and induces superconductivity.
The maxi-
mum superconducting transition temperature, Tc ¼ 38 K
5
)
for (Ba1ꢁxKx)Fe2As2, is obtained near the critical concen-
tration where AFM ordering is completely suppressed.
It has been widely thought that AFM ordering in iron-
based superconductors originates from the spin-density-
wave (SDW) instability due to the nesting of two Fermi
pockets, a hole pocket centered at the ꢀ point and an
electron pocket centered at the M point, which are connected
Fig. 1. (Color online) Two polymorphic forms of 122: (a) CaBe2Ge2-type
structure with the space group P4=nmm and (b) ThCr Si -type structure
2
2
with the space group I4=mmm. The solid lines indicate the unit cell. The
large circles and small circles represent Ba sites and Al sites of the BaAl4-
type structure with the space group I4=mmm. The Ba sites are occupied
by Sr and the Al sites are occupied by either transition metal (TM)
elements (represented by dark-gray circles) or arsenic (represented by
yellow circles).
9
–12)
by the nesting vector Q ¼ ðꢀ; ꢀÞ.
This characteristic
electronic structure is believed to be a key ingredient of
the high-temperature superconductivity due to AFM spin
2,3,13,14)
fluctuations in iron-based families.
been theoretically proposed that the nesting determines the with those of other non-iron-based 122 arsenides such as
Moreover, it has temperature of SrPt2As2 is considerably high compared
2
1)
22)
symmetry of Cooper pairs in the iron-based superconduc- SrNi As (T ¼ 0:62 K)
and BaNi As (T ¼ 0:7 K).
2
2
c
2
2
c
tors, in which a node locates either away from the Fermi Our observation suggests that Fermi-surface nesting is a key
3,14)
1
surfaces (sꢂ-wave)
or directly at the Fermi surface ingredient of the relatively high superconducting transition
temperature of SrPt2As2.
(
d-wave).1
The charge-density-wave (CDW) instability is another
4)
SrPt2As2 crystallizes in a tetragonal CaBe2Ge2-type
23)
consequence of Fermi-surface nesting when electron–pho- structure with the space group P4=nmm (#129), as shown
non interaction remains important. CDW ordering, as well in Fig. 1(a). The structure is different from another poly-
as SDW ordering, competes with or coexists with super- morphic form of 122, a ThCr Si -type structure with the
2
2
conductivity.1
5–20)
In this letter, we demonstrate that super- space group I4=mmm (#139), in which BaFe As crystal-
2 2
4
,5)
conductivity at Tc ¼ 5:2 K coexists with CDW in platinum- lizes, as shown in Fig. 1(b). Both structures are derived
based 122 arsenide SrPt2As2. The superconducting transition from the binary BaAl4-type structure with the space group
I4=mmm. In both forms, the barium sites are occupied by
ꢀ
E-mail: kudo@science.okayama-u.ac.jp
strontium atoms and the aluminum sites are occupied by
123710-1

J. Phys. Soc. Jpn., Vol. 79, No. 12
LETTERS
K. KUDO et al.
either transition-metal (TM) atoms or arsenic atoms. The
difference between the two forms is due to the different
distributions of TM and arsenic over the aluminum sites.
These structures can be schematically visualized by plane
sequences along the c-axis: –As–TM–As–TM–As–TM– for
the CaBe2Ge2-type structure and –As–TM–As–As–TM–As–
for the ThCr2Si2-type structure. As can be seen from Fig. 1,
there exists a three-dimensional Pt–As network in SrPt As
2
2
with the CaBe Ge -type structure, whereas there is a two-
2
2
dimensional Fe–As network in BaFe2As2 with the ThCr2Si2-
type structure.
ρ
Despite the three-dimensional Pt–As network, a band
calculation indicates that the square lattice of Pt exhibits
23)
a Peierls instability in SrPt2As2. In accordance with this
prediction, SrPt2As2 exhibits a CDW transition at about
4
70 K. Imre et al. reported that a structural modulation
ꢀ
develops with a modulation vector of q ¼ 0:62a in the
–
4
As–Pt–As– layers with PtAs4 tetrahedra below about
70 K, whereas the –Pt–As–Pt– layers with Pt4As tetrahedra
Fig. 2. (Color online) Temperature dependence of electrical resistivity ꢁ
for SrPt2As2 in zero field. The inset shows temperature dependence of ꢁ
in magnetic fields up to 4 T.
remain intact. This modulation leads to a structural dis-
tortion from the high-temperature tetragonal phase with
the CaBe2Ge2-type structure to the low-temperature CDW
phase with the average structure of the orthorhombic space
group Pmmn (#59). In this phase, superconductivity
emerges, as described in the following.
23)
Polycrystalline samples of SrPt2As2 were synthesized by a
solid-state reaction. PtAs2 precursor was first synthesized by
ꢃ
heating Pt powder and As grains at 700 C in an evacuated
quartz tube. Then, stoichiometric amounts of Sr, PtAs2, and
Pt powders were mixed and ground. The resulting powder
was placed in an alumina crucible and sealed into an
ꢃ
evacuated quartz tube. The ampule was heated at 700 C for
ꢃ
10 h and then at 1100 C for 24 h. After furnace cooling,
the sample was ground, pelletized, wrapped with Ta foil and
ꢃ
heated at 700 C for 10 h in an evacuated quartz tube. The
products were characterized by powder X-ray diffraction and
confirmed to be a single phase of SrPt2As2. The lattice
˚
˚
parameters were estimated to be a ¼ 4:46 A, b ¼ 4:51 A,
˚
and c ¼ 9:81 A, which are consistent with the previous
report.2 This result indicates that the sample is indeed in the
3)
CDW phase at room temperature.
Fig. 3. Temperature dependence of magnetization divided by applied
field, M=H, of SrPt2As2 at 10 Oe under zero-field-cooling (ZFC) and
field-cooling (FC) conditions.
Magnetization M was measured with a SQUID magne-
tometer (Magnetic Property Measurement System, Quantum
Design) from 1.8 to 7 K under a magnetic field of 10 Oe.
Electrical resistivity ꢁ was measured by the standard DC
four-terminal method in the temperature range between 2
and 300 K under magnetic fields up to 4 T using the Physical a modulation vector of q ¼ 0:62aꢀ persists down to low
Property Measurement System (PPMS, Quantum Design). temperatures where superconductivity emerges.
Specific heat C was measured by the relaxation method in
the temperature range between 2 and 7 K in zero field and in transition in detail. The 10–90% transition width was about
a magnetic field of 4 T using the PPMS. 0.4 K, and the onset temperature determined from the 10%
Figure 2 shows the temperature dependence of the resistiv- rule was 5.7 K. Zero resistivity was observed at 5.2 K. The
ity for a polycrystalline sample of SrPt As . The normal-state temperature-dependent magnetization data for this sample
The inset of Fig. 2 shows the resistive superconducting
2
2
resistivity was of the order of 1 mꢁ cm at low temperatures. are shown in Fig. 3, which exhibited a diamagnetic behavior
This relatively high value indicates that part of the Fermi below 5.2 K. The shielding and flux exclusion signals
surface is depleted by CDW formation. At high temperatures, correspond to 92 and 42% of perfect diamagnetism,
the resistivity exhibited an ‘‘S’’-shaped temperature depend- respectively. These data support the emergence of bulk
ence, which is characteristic of metal. We did not observe any superconductivity at Tc ¼ 5:2 K in SrPt2As2.
anomaly in the normal-state resistivity below 300K, indicating
As shown in the inset of Fig. 2, Tc gradually decreased
the absence of transition from the incommensurate CDW to with increasing external magnetic field. The temperature
a commensurate CDW. Thus, the incommensurate CDW with dependence of the upper critical field Hc2 was determined
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J. Phys. Soc. Jpn., Vol. 79, No. 12
LETTERS
K. KUDO et al.
2
Fig. 4. Temperature dependence of upper critical field Hc2 deduced from
resistivity measurements. The solid straight line yields a slope of ꢁdHc2=
Fig. 5. Specific heat divided by temperature, C=T, as a function of T
in zero field and a magnetic field of 4 T. The solid line represents a fit
2
4
dTj
¼ 0:68 T/K. The broken line represents a curve based on the
by C=T ¼ ꢃ þ ꢄT þ ꢅT , where ꢃ is the coefficient of the electronic
specific heat, while ꢄ and ꢅ are those of phonon contributions. The broken
line represents an ideal jump at Tc, assuming entropy conservation at the
transition.
T¼Tc
Werthamer–Helfand–Hohenberg (WHH) theory.
2
4)
from the midpoint of the resistive transition, as shown in
Fig. 4. Interestingly, Hc2 increased almost linearly with
decreasing temperature down to the lowest temperature the enhanced electron–phonon interaction in SrPt As , as
2
2
measured, and deviated from the curve expected from the expected from the enhanced specific-heat jump in SrPt As ,
2
2
Werthamer–Helfand–Hohenberg (WHH) theory.2 Similar ꢃC=ꢃTc ¼ 1:67. The ‘‘S’’-shaped temperature-dependent
4)
deviation of Hc2 was reported for some multiband super- resistivity and its saturating behavior at high temperatures
conductors, such as YNi2B2C as well as doped-BaFe2- are consistent with the moderate electron–phonon coupling
As2,2 but it is unclear at present whether SrPt2As2 is a in SrPt2As2.
25)
6)
multiband superconductor or not. Thus the WHH theory The present compound, SrPt2As2, and the iron-based
gives an estimate of the lower limit of the upper critical superconductors share a common electronic structure with
field at 0 K, H ð0Þ ¼ 2:5 T, from the slope of H at Tc, Fermi-surface nesting, which leads to Peierls instability.
c2
c2
ꢁ
dH =dTj
¼ 0:68 T/K. The Ginzburg–Landau coher- SrPt As shows superconductivity in close proximity to
c2
T¼T
2
2
c
˚
ence length, ꢂ0, was estimated to be 115 A from ꢂ0 ¼ ½ꢂ0= CDW ordering due to electron–phonon coupling, while the
1=2
2
ꢀHc2ð0Þꢄ , where ꢂ0 is the magnetic flux quantum.
iron-based families show superconductivity in close prox-
Further support of the bulk superconductivity was imity to SDW/AFM ordering due to the electron correla-
2
,3,13,14)
obtained from the specific heat CðTÞ, where a clear jump tions. Because of the extremely wide diversity of
at the superconducting transition was observed, as shown materials in transition-metal pnictides, we may have a
in Fig. 5. In order to accurately determine bulk Tc in zero chance to develop a compound that possesses the character-
magnetic field, an ideal jump at T was assumed to satisfy istics of both platinum- and iron-based pnictides. Such
c
the entropy conservation at the transition. This yielded compounds will provide us an opportunity to realize novel
2
estimates of Tc ¼ 5:0 K and ꢃC=Tc ¼ 16:2 mJ/(molꢅK ). superconductors with higher transition temperatures, since
The superconductivity was completely suppressed in a the high-Tc copper oxide superconductors show supercon-
magnetic field of 4 T. A standard analysis on the normal- ductivity closely related to both SDW/AFM and CDW/
3
0–32)
state specific heat yielded the T-linear specific heat charge-ordered phases.
2
coefficient ꢃ ¼ 9:72 mJ/(molꢅK ) and Debye temperature
In summary, SrPt2As2 is a novel pnictide superconductor
ꢄD ¼ 211 K. Using these values, we estimated ꢃC=ꢃTc to undergoing both a charge-density-wave transition at 470 K
be 1.67, which is larger than the value expected from the and a superconducting transition at T ¼ 5:2 K. The super-
c
BCS weak-coupling limit (ꢃC=ꢃT ¼ 1:43).
conducting transition temperature of SrPt As is consider-
2 2
c
The ꢃ value of SrPt2As2 is comparable to those reported ably high among non-iron-based pnictide superconductors.
for other non-iron-based 122 superconductors.21,22,27–29) For The present superconductor, SrPt2As2, is analogous to iron-
the Ni-based arsenides, which are isoelectronic to SrPt2As2, based arsenide superconductors in that the superconductivity
ꢃ ¼ 8:7 and 10.8 mJ/(molꢅK ) for SrNi2As2 and BaNi2- emerges in close proximity to the Peierls instability. We
As2,2 respectively. However, Tc is almost one order of suggest that Fermi-surface nesting is a key ingredient for
magnitude different between SrPt2As2 and the Ni-based realizing high transition temperature.
2
21)
2)
1
0
22 system: T ¼ 5:2 K for SrPt As , while T ¼ 0:62 and
c
2
2
c
21)
22)
Acknowledgements
.7 K for SrNi As
2
and BaNi As , respectively. The
2 2
2
difference in Debye temperatures, ꢄD ¼ 211 and 244 K for
We would like to thank Dr. T. Nishizaki and Dr. N.
21)
SrPt2As2 and SrNi2As2, respectively, is unable to account Yoneyama for fruitful discussions. A part of this work
for the difference in Tc between the two classes of materials. was performed at the Advanced Science Research Center,
We suggest that the relatively high Tc is most likely due to Okayama University.
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J. Phys. Soc. Jpn., Vol. 79, No. 12
LETTERS
K. KUDO et al.
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