In a galaxy far, far away someone is enjoying this view, while
we sit here waiting for the talk to begin...
Ferromagnetic order at 298 K in
Cr0.005Sn0.995Te
C. P. Opeil1, J. C. Lashley2, R. K. Schulze2, J. L. Smith2,
J. E. Gubernatis2, B. Mihaila2, R. D. Field2, D. J. Safarik2,
T. Durakiewicz2, P. B. Littlewood3, E. Rotenberg4, A. Bostwick4
1Boston
College, Physics Department, Chestnut Hill, MA 02467 USA
2Los Alamos National Laboratory, Los Alamos, NM 87545 USA
3Cavendish Laboratory, Cambridge University, JJ Thomson Ave, Cambridge, UK
4Advanced Light Source, Lawrence Berkeley National Lab., Berkeley, CA, USA
Work sponsored by: Department of Energy and Boston College Trustees
Brief Outline:
*** Ferromagnetic ordering appears in Cr0.005Sn0.995Te at 298 K
*** Structural phase transition observed at 98 K via dilatometry
*** Phase transition at 98 K is polar & ferro-elastic not ferroelectric
*** TEM images at 298 K show modulated cubic structure, rather
than heretofore accepted rocksalt (cubic) structure.
*** ARPES on SnTe confirms metallic behavior (no gap) not
semiconductoring behavior in the low-temperature phase
We will show that SnTe is polar and the low-temperature phase is
metallic therefore it cannot be ferroelectric: one cannot observe a
dielectric hysteresis loop because of conduction arising from free
carriers. a second-order transition with strain must be accompanied
by some other internal symmetry change. One possible change is
the spontaneous development of a symmetry represented by a
polar (vector) order parameter. In the present case, this polar order
parameter is the static displacement associated with the optical
phonon.
"... structure to a low-temperature rhombohedral crystal structure.
Associated with the structural change is a relative shift of the sublattices
along the [111] direction, to produce a low-temperature phase with
ferroelectric symmetry. However, there is no direct measurement of the
ferroelectric moment, since it is evidently screened out by the free
carriers always present in these small-gap materials. A sequence of ..."
"... structure to a low-temperature rhombohedral crystal structure.
Associated with the structural change is a relative shift of the sublattices
along the [111] direction, to produce a low-temperature phase with
ferroelectric symmetry. However, there is no direct measurement of the
ferroelectric moment, since it is evidently screened out by the free
carriers always present in these small-gap materials. A sequence of ..."
!"
"
1/chi
(Oe Mol-Cr/emu)
Curie-Weiss analysis and Valence determination:
"
In this plot of 1/! vs T the Curie-Weiss region is only in the temperature range for T > 300 K.
The parameters from a fit to the data above this region gives the Weiss constant TW = 290 K
indicating ferromagnetic ordering. The effective number of Bohr magnetons from the analysis
above the ordering region is p = 3.3. For paramagnetic Cr2+ p = 4.9 and for Cr3+ p = 3.8. Thus p
= 3.3 is not unreasonable, and indicates the Cr is in the +3 valence state.
agrees with Inoue et al., J. Phys. Soc. Jpn. 50 (1981) 1222.
M (emu/mole)
QD-VSM measurements indicate ferromagnetic PT and hysteresis:
warming
cooling
diamagnetic
M vs. T: Note the two hysteretic regions, one around 98 K (upper left) supporting the
ferroelastic transition, the second ferromagnetic is shown to the right. Above 300 K
(up to 1100 K), the Cr-doped SnTe remains diamagnetic.
Dilatometry measurements:
Sn0.995Cr0.005Te
1.2
H=0T
1.0
H=9T
ΔL
! L (µm)
0.8
0.6
0.4
0.2
0.0
0
20
40
60
80
T (K)
100
120
140
160
Dilatometry measurements:
Sn0.995Cr0.005Te
1.2
H=0T
1.0
H=9T
ΔL
! L (µm)
0.8
0.6
0.4
0.2
Ferro-elastic transition
0.0
0
20
40
60
80
T (K)
100
120
140
160
Spontaneous Strain:
Red curve is Salje fit.
Quantum saturation in order
parameter in Bosonic systems
is simpler than Fermionic.
Remaining Fermions (electrons)
drive the structural transition.
Linear dependence in Tc vs n is
Dirac point.
Room Temp
SnTe single crystal
NaCl structure
a = 0.640 nm
evidence of incommensurate structure
Extra periodicity:
macroscopic view:
Cubic
microscopic view:
Incommensurate,
modulated, not cubic
Photoemission: ARPES at 20 K on SnTe, accepted by PRL 21July 2010
kx vs ky
E vs k
Data
Calculation
Conclusions:
At the L point in T = 0 limit, ARPES (at 20 K) shows no gap therefore the
low-temperature phase is a metal (this result settles an old problem). The
phase is polar (metallic) and not ferroelectric because one cannot put an
electric field on the metal.
All band calculations predict a gap there: except calculations by Rabi
et al. where fully relativistic show a gap whereas non-relativistic
calculations do not.
Can quantum fluctuations drive the truncated-cone Dirac behavior and
apparent critical behavior? One would need to look for critical scattering.
With gratitude for my collaborators:
R. K. Schulze, J. L. Smith, J. E. Gubernatis,
B. Mihaila, R. D. Field, D. J. Safarik, T. Durakiewicz,
P. B. Littlewood, E. Rotenberg, A. Bostwick, J. "Captain" Lashley
Thank you to the organizers of CALCON 2010:
Michael T. Henzl, Conference Chair
David Reme, Program Director
Gordon Kresheck, Arrangements Chair
The Board of Directors of CalCon 2010
Financial supporters of CALCON 2010:
GE Healthcare – MicroCal Products Group, USA
National Institutes of Standards and Technology
Aviv Biomedical
University of Missouri
Rutgers University of New Jersey
University of Colorado at Colorado Springs
Thank you for your kind attention.