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Squeezing the magnetism out of CaFe

1
Squeezing the magnetism out of CaFe
2
As
2
a volume and
moment collapsed superconducting phase
A. Kreyssig
1, 2
, M. A. Green
3, 4
, Y. Lee
1, 2
, G. D. Samolyuk
1, 2
, P. Zajdel
3, 5
, J. W. Lynn
3
,
S. L. Budko
1, 2
, M. S. Torikachvili
6
, N. Ni
1, 2
, S. Nandi
1, 2
, J. Leão
3
, S. J. Poulton
3, 4
,
D. N. Argyriou
7
, B. N. Harmon
1, 2
, P. C. Canfield
1, 2
, R. J. McQueeney
1, 2
&
A. I. Goldman
1, 2

1
Ames Laboratory, US DOE, Iowa State University, Ames, IA 50011, USA;
2
Department
of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA;
3
NIST Center
for Neutron Research, National Institute of Standards and Technology, Gaithersburg,
MD 20899, USA;
4
Department of Materials Science and Engineering, University of
Maryland, College Park, MD 20742, USA;
5
Department of Chemistry, University College
of London, 20 Gordon Street, London, W1X 0AJ, UK;
6
Department of Physics, San Diego
State University, San Diego, CA 92182, USA;
7
Helmholtz-Zentrum Berlin für Materialien
und Energie, Glienicker Str. 100, 14109 Berlin, Germany
Two recently discovered
1,2,3,4
series of high transition temperature (high-T
C
)
superconductors originate from the parent systems R</i>FeAsO (R = rare earth) and
A</i>Fe
2
As
2
(A = alkaline earth metal), which are tetragonal at room temperature but
undergo an orthorhombic distortion in the range 100-220 K that is associated with
the onset of antiferromagnetic order
5,6,7,8,9,10,11
. Tuning the system via element
substitution
2,3,4,12,13,14
or oxygen deficiency
15,16
suppresses the magnetic order and
structural distortion in favour of superconductivity (T
C
s up to 55 K), with an
overall behaviour strikingly similar to the high-T
C
copper oxide family of
superconductors. However, the recent report of pressure-induced
superconductivity in the parent A</i>Fe
2
As
217,18
opens an alternative path to
superconductivity. Here we report that CaFe
2
As
2
undergoes a pressure-induced
2
transition to a non-magnetic, volume collapsed tetragonal phase, which becomes
superconducting at lower temperature. Spin-polarized total energy calculations on
the collapsed structure find that the magnetic Fe moment itself collapses, consistent
with the absence of magnetic order in neutron diffraction and the loss of spin-
disorder scattering in resistivity measurements
17
. The loss of the Fe moment raises
new questions concerning the role of spin fluctuations as a pairing mechanism for
pressure-induced superconductivity in CaFe
2
As
2
, as proposed for the doped iron
arsenids
5
.
Recent investigations of the superconducting iron-arsenide families have
highlighted the role of pressure, be it chemical or mechanical, in fostering
superconductivity. Perhaps most intriguing is the discovery that, under quite modest
applied pressure, the distinct resistivity signature of the first-order tetragonal-to-
orthorhombic and magnetic phase transition
19
in CaFe
2
As
2
is suppressed
17
and
superconductivity emerges at lower temperature for pressures between 0.35 GPa and
0.86 GPa
17
. The pressure-induced superconductivity in CaFe
2
As
2
was confirmed
20
and
followed by observations of superconductivity for BaFe
2
As
2
and SrFe
2
As
2
at significantly
higher pressures
18
. In this respect the iron arsenides bear a similarity to other exotic
superconductors, such as CeRhIn
521
, where the appearance of superconductivity with
pressure is associated with proximity to a quantum critical point. In CaFe
2
As
2
, a second
high-temperature phase transition, associated with the loss of spin-disorder scattering, is
observed above 0.55 GPa
17
by anomalies in resistivity. However, the nature of the phase
at temperatures below this transition and its relation to the ambient-pressure tetragonal
and orthorhombic phases are as yet unknown. Neutron scattering experiments on
polycrystalline CaFe
2
As
2
were performed to enlighten these issues.
3
Figure 1 shows neutron diffraction scans taken through the nuclear (0

0

2), (2

2

0)
T
,
and magnetic (1

2

1)
OR, magnetic
diffraction peaks at selected temperatures and pressures.
At 50 K and ambient pressure (A), the splitting of the (2

2

0)
T
into the orthorhombic
(4

0

0)
OR
/(0

4

0)
OR
preaks signals the existence of the orthorhombic phase (Fig. 1(b)).
This, together with the observation of the (1

2

1)
OR, magnetic
peak (Fig. 1(c)), is consistent
with previous x-ray and neutron diffraction measurements at ambient pressure
10,19
. Upon
increasing pressure at T = 50 K, the structure remains orthorhombic and
antiferromagnetic up to approximately 0.24 GPa. Between 0.24 and 0.35 GPa, dramatic
changes take place in the measured diffraction patterns. At pressures above 0.35 GPa
(Fig. 1(c)), the magnetic peak is absent and the orthorhombic structure has transformed to
a tetragonal phase, similar to the high temperature ambient pressure structure, but with
extraordinarily different lattice parameters. This is most evident from the strong shift in
position of the (0

0

2) peak at (B) in Fig. 1(a). Upon cooling to 4 K, within the
superconducting regime
17
, the collapsed tetragonal structure remains unchanged.
The central region (shown in yellow) of Figs. 2(b) and (c) shows the results of
Rietveld refinements of the lattice parameters and volume for this collapsed tetragonal
phase. We find an astonishing 9.5% reduction in the c</i>-lattice parameter with respect to
the orthorhombic phase and a nearly 5% decrease in the unit cell volume. Even more
striking is the reduction of the c/a ratio, a key parameter for bond geometries in the iron
arsenides, by nearly 11%. As a consequence, the As-Fe-As bond angles change strongly
as illustrated in Fig. 2(d).
With the pressure maintained at 0.63 GPa, the temperature was raised in 50 K steps
(right panels of Fig. 2). Between 150 and 200 K an isostructural transition between the
low-temperature collapsed tetragonal phase and the high temperature tetragonal
structure is observed. Upon release of the pressure at 250 K, the curves labelled (D) in
4
Fig. 1 show only small changes in the lattice parameters between 0.63 GPa and ambient
pressure, providing a measure of the modest, but strongly anisotropic compressibility of
the high temperature phase. We note that there is a difference of about 50 K between the
temperature of the isostructural transition at 0.63 GPa measured here and that reported in
transport measurements
17
. However, as pointed out in Ref. 17, the resistive anomalies
are rather broad in applied pressure, and different criteria for the definition of transition
temperatures can shift temperature assignments. In addition, the data in Ref. 17 were
taken with decreasing temperature whereas here the temperature was stepwise increased.
With these uncertainties understood, the tetragonal-to-collapsed tetragonal transition
appears to be responsible for the loss of resistivity whose locus defines the high
temperature, high-pressure phase line found in Ref. 17 and shown in Fig. 2(a).
In order to relate the volume change to relative changes in the unit cell dimensions,
and to verify the stability of this phase, spin-polarized total energy calculations were
performed for volume changes of V/V = 0% (for ambient pressure) and V/V = -5% (for
the collapsed phase). From the blue curves in Fig. 3(a) we see that, for ambient
pressure, the orthorhombic magnetic phase is lowest in energy, consistent with our
ambient pressure low temperature measurement. The red curves in Fig. 3(a) show that
the tetragonal phase is lowest in total energy for the 5% volume reduction. The minimum

energy of this collapsed tetragonal phase is found at c/a 2.65, close to the
experimental value of 2.67 (Fig. 2(c)). The c/a</i>-dependence of the spin-polarized total
energy calculations for the collapsed phase can be correlated with a loss of the Fe
magnetic moment, as shown in Fig. 3(b). The astonishing result of a quenched magnetic
moment ground state is consistent with our observation of the loss of magnetic order in
the collapsed tetragonal phase as well as the loss of spin-disorder scattering in
resistivity measurements, and further confirmed by the additional non-spin-polarized total
5
energy calculations (see red stars in Fig. 3(a)). The band structure calculations also
indicate that several bands cross the Fermi level at the pressure-induced transition.
The principal result of these neutron diffraction measurements is the discovery of a
transition from the magnetically ordered orthorhombic phase to a non-magnetically
ordered collapsed tetragonal phase preceding the onset of superconductivity. Further,
the second, higher pressure, transition noted in transport measurements
17
has been
identified as an isostructural transition between the pressure-induced collapsed phase
and the high-temperature tetragonal structure. The loss of spin disorder scattering
postulated in Ref. 17 follows from the loss of magnetic moment in the collapsed
tetragonal phase. We note that the observed volume reduction can also serve to increase
the charge carrier density. The schematic phase diagram in Fig. 3(c) summarizes our
findings. The structure of the pressure-induced collapsed phase is unchanged in the
superconduct