Pressure-induced superconductor-insulator transition in the spinel compound CuRh2S4.
ABSTRACT We performed resistivity measurements in CuRh2S4 under quasihydrostatic pressure of up to 8.0 GPa, and found a pressure-induced superconductor-insulator transition. Initially, with increasing pressure, the superconducting transition temperature T(c) increases from 4.7 K at ambient pressure to 6.4 K at 4.0 GPa, but decreases at higher pressures. With further compression, superconductivity in CuRh2S4 disappears abruptly at a critical pressure P(SI) between 5.0 and 5.6 GPa, when it becomes an insulator.
arXiv:cond-mat/0211232v2 [cond-mat.supr-con] 24 Jun 2003
Pressure induced superconductor-insulator transition
in the spinel compound CuRh2S4
M. Ito,∗J. Hori, H. Kurisaki, H. Okada, A. J. Perez Kuroki, N.
Ogita, M. Udagawa, H. Fujii, F. Nakamura, T. Fujita, and T. Suzuki†
Department of Quantum Matter, ADSM,
Hiroshima University, Higashi-Hiroshima 739-8530, Japan.
(Dated: July 2, 2011)
We performed resistivity measurements in CuRh2S4under quasi-hydrostatic pressure of up to
8.0 GPa, and found a pressure induced superconductor-insulator (SI) transition. Initially, with
increasing pressure, the superconducting transition temperature Tcincreases from 4.7 K at ambi-
ent pressure to 6.4 K at 4.0 GPa, but decreases at higher pressures. With further compression,
superconductivity in CuRh2S4disappears abruptly at a critical pressure PSIbetween 5.0 and 5.6
GPa, when it becomes an insulator.
PACS numbers: 71.30.+h, 74.70.-b, 74.25.Dw
∗Electronic address: firstname.lastname@example.org
†Electronic address: email@example.com
Unusual physical properties of chalcogenide-spinels have attracted current interest be-
cause of a new type of metal-insulator (MI) transition  found in the thiospinel CuIr2S4,
with a lattice parameter a = 9.847˚ A at room temperature . As far as the lattice pa-
rameter  is concerned, isomorphic CuRh2S4with a = 9.787˚ A and CuRh2Se4with a =
10.269˚ A are regarded as compressed and expanded respectively, compared to CuIr2S4. Both
compounds, however, are well known as superconductors [3, 4, 5, 6, 7] without an MI tran-
sition. In spite of the similar temperature dependence of electric resistivity in the normal
conducting state, the absolute value of the resistivity of CuRh2S4, with a smaller lattice
parameter is about 20 times larger than for CuRh2Se4, with a larger lattice parameter. This
suggests that some anomaly may occur in the transport properties of CuRh2S4as it is fur-
ther compressed. In this letter we report that CuRh2S4makes a sudden transition from a
superconductor to an insulator at a critical pressure PSIbetween 5.0 and 5.6 GPa. We are
not aware of a pressure induced superconductor-insulator transition having been previously
observed in other materials.
Polycrystalline CuRh2S4was prepared by a direct solid-state reaction. Fine powders of
Cu ( 99.999% ), Rh ( 99.9% ) and S ( 99.9999% ) were mixed in stoichiometric ratio and
were reacted in a sealed quartz tube at 850◦C for 10 days. After being pulverized, the
specimen was pressed into pellets and sintered at 1000◦C for 3 days. Sample manipulation
was always carried out in a glove box filled with purified argon to minimize oxidation of
the pellets. Electric resistivity ρ(T) was measured by a standard four-probe method with
increasing temperature T from 4.2 to 300 K under quasi-hydrostatic pressure P up to 8 GPa.
The pressure was applied by using a cubic-anvil device  cooled in a liquid4He cryostat.
To examine whether a pressure-induced structural transition occurs, Raman spectra under
quasi-hydrostatic pressure up to 11.7 GPa were measured by a micro-Raman spectrometer
with a diamond-anvil cell. An Ar laser with the wave length of 514.5 nm was used as the
incident beam. A mixture of methanol and ethanol was used as the pressure-transmitting
medium. The pressure inside the cell was determined from the shift of the fluorescence line
Figure 1 shows the T-dependence of ρ between 4.2 and 300 K at various pressures. The
response of resistivity to pressure is quite unusual. At ambient pressure, ρ(T) shows metallic
T-dependence with ∂ρ(T)/∂T > 0 above the superconducting transition temperature Tc
= 4.7 K defined by the onset of the resistance drop. As was pointed out by Hagino et
al. , ρ(T) in the normal state of CuRh2S4is similar to that for A-15 compounds. The
T-dependence can be fitted by a phenomenological equation [9, 10]:
ρ(T) = ρ0+ ρ1T + ρ2exp(−T0/T),(1)
where, ρ0, ρ1, ρ2 and T0 are T-independent fitting parameters. The variation of ρ(T) is
monotonic under low pressure. However as P increases above 3.0 GPa, a broad peak appears
in ρ(T) at around a characteristic temperature T∗, which is indicated by arrows in Fig. 1.
T∗initially decreases with increasing P and then starts to increase above 5.0 GPa. In the
range 3.0 ≤ P ≤ 5.0 GPa, ∂ρ(T)/∂T changes its sign from negative to positive at T∗as T
decreases. ρ(T) shows metallic T-dependence (∂ρ(T)/∂T > 0) at low temperatures, with
the superconducting transition occurring above 4.2 K. The metallic variation in the range
Tc< T < T∗can be fitted by the equation (1) as shown by the solid lines in Fig. 1b, with the
parameters listed in Table 1. Application of pressure enhances ρ0drastically to a 250 times
larger value. A more important point is that superconductivity completely disappears for
P ≥ 6.5 GPa, where ρ(T) shows non-metallic T-dependence (∂ρ(T)/∂T < 0) over the whole
temperature range measured. As shown in Fig. 2, it is also remarkable that ρ(10 K) is greatly
enhanced, by over 7 orders of magnitude, with increasing pressure from 0.1 MPa to 8.0 GPa.
This enhancement is highly anomalous. Compression usually gives rise to a reduction in
resistivity in many materials and in some even to superconductivity , because overlapping
of electronic wave functions among neighboring atoms is promoted by compression. Well-
known examples of the pressure effect due to this electronic origin are found in the Mott
transition  or Wilson transition [13, 14]. Even in random systems, a transition from an
insulator to a metal was found by applying uniaxial compressive stress . The effects of
compression on Tcand T∗are summarized in Fig. 3. The Tcvalue increases with increasing
P, which is consistent with the previous work  carried out under pressures lower than
2.0 GPa. With further compression, Tcstarts to decrease after having a maximum value of
6.4 K at 4.0 GPa. Around 5.3 GPa, where T∗has a minimum value, the low-temperature
transport properties of CuRh2S4 change from superconducting to insulating. We believe
that this SI transition is a bulk property of CuRh2S4. The slight depression of Tcobserved
above 4.0 GPa might be due to a precursor or fluctuation of the SI transition. To our
knowledge, this is the first report on a pressure induced SI transition.
The P-variation of ρ(T) over the whole temperature range shown in Fig. 1 further
TABLE I: Fitting parameters ρ0, ρ1, ρ2and T0evaluated for the low temperature part of ρ(T)
under pressures from 0.1 MPa to 5.0 GPa.
P ( GPa )
ρ0( Ωcm )
ρ1( Ωcm/K )
ρ2( Ωcm )
suggests that the SI transition occurs at the same P that the sample changes from a metal
to an insulator. The SI transition presumably results from the disappearance of carriers at
the MI transition between 5.0 and 5.6 GPa. The mechanism of the MI transition is still
unclear in the present case, and one may consider other origins for the transition besides
the localization of carriers .
Here, let us discuss the pressure induced MI transition from the viewpoint of change in
the band structure both with and without change in the crystalline lattice. In the first case,
one can consider a transition driven by charge ordering, as for the new type MI transition
reported in CuIr2S4 at TMI= 226 K . As mentioned above, CuRh2S4is a compressed
version of CuIr2S4and therefore has a similar electronic structure to CuIr2S4. The Rh ions
in CuRh2S4occupy the octahedral sites and have valences Rh3+(electronic configuration
of 4dǫ6dγ0) and Rh4+(4dǫ5dγ0) , analogous to Ir3+(5dǫ6dγ0) and Ir4+(5dǫ5dγ0) in
CuIr2S4. As the temperature decreases, CuIr2S4undergoes a transition from a metal to an
insulator at TMI, which is accompanied by a structural change from cubic to tetragonal with
a volume contraction [1, 2] of 0.7%. In this case, compression stabilizes the insulating phase
with charge ordering as was evidenced by the elevation of TMIwith increasing pressure .
The absolute value of resistivity above TMI is also enhanced substantially by this charge
ordering. If we assume the same mechanism, we may easily predict that as a consequence
of the charge ordering of Rh3+and Rh4+, CuRh2S4undergoes an MI transition above 300
K by compression, and a discontinuity should be detected in the P-dependence of ρ(300 K).
However, no sign of such a jump is found in ρ(300 K) shown in Fig. 2. Consequently,
the MI transition mechanism of CuIr2S4 type is ruled out for CuRh2S4. To confirm the
discussion, we performed the structure-sensitive micro-Raman spectroscopy under pressure
in the energy range between 200 and 600 cm−1at 300 K. The laser beam was focused on one
of the small single-crystals in the polycrystalline sample. Figure 4 shows Raman spectra
for selected pressures. For a spinel structure, five phonons (A1g, Eg and three T2g) are
Raman active . One of the T2gphonons usually has the lowest energy with around 100
cm−1. We observed all Raman active phonons of CuRh2S4except for the T2gphonon
with the lowest energy as shown in Fig. 4. By compression, the position of the phonon peak
monotonically shifts to the higher energy, suggesting simple hardening of the lattice. There
is neither appearance of a new phonon peak nor indication of peak splitting. Therefore,
the micro-Raman spectroscopy reveals that there is no structural change across the MI
Another possible origin, which is not accompanied by a lattice change, is a modification of
the band structure by pressurization, as reported for a divalent fcc-Yb crystal . The fcc-
Yb is known to be a material in which compression induces a semimetal-insulator transition
without change of lattice symmetry. The transition is ascribed to the formation of an energy
gap between two bands, both of which originally cross the Fermi level in the semimetallic
phase . According to the band calculation by Oguchi , the metallic conduction in
CuRh2S4is ascribable to the two holes in two bands, each of which crosses the Fermi level
as the two bands of the fcc-Yb. ρ(300 K) of CuRh2S4increases gradually from 4.6×10−4to
1.3×10−3cm with compression of up to 8.0 GPa. This P-dependence is quite similar to that
of fcc-Yb in which ρ(300 K) also increases gradually from 3×10−5cm at ambient pressure
to 3×10−4cm at 3.7 GPa. Although CuRh2S4is a good metal, an MI transition of the kind
observed in the semimetal fcc-Yb, may be expected if the two bands near the Fermi level
are sensitive to pressure. This scenario, based on the modification of the band structure, is
likely to explain the pressure dependence of T∗as well as the decrease of Tcprior to the SI
transition. The pressure-induced MI transition in non-superconducting CuIr2Se4 may
be understood by the same scenario.
In summary, we have found a pressure induced SI transition in a chalcogenide-spinel
CuRh2S4from resistivity measurements under quasi-hydrostatic pressure of up to 8.0 GPa.
As pressure increases, the Tcvalue initially increases from 4.7 to 6.4 K and starts to slightly
decrease after a broad maximum is reached. With further compression, superconductivity
vanishes suddenly at PSI around 5.3 GPa. The SI transition occurs in concurrence with