Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

Fourier amplitude (arbit. units)
1.0
0.5
Different mechanisms have been proposed for this FS reconstruction, involving d-DW order [11],
incommensurate antiferromagnetic
data
F1
F2
F3
oscillatory torque (a.u.)
1
0
-1
T=0.7 �0.2 K
y=6.54
y=6.51
30 40
B (T)
50 60
0.0
0 500 1000 1500 2000 2500
F (T)
Fig 4 : Fourier analysis of the oscillatory torque (see
inset) for underdoped YBa2Cu3O6.54.
17
order [8] or stripe order [12]. Each
scenario predicts a different FS
topology and the observation of an
additional frequency would provide
a stringent test. In particular, recent
dHvA measurements in the same
compound than the present work
found evidence of a larger pocket
which impose strong constraint on
the ordering wave vector for the
reconstruction [8].
In order to determine whether there
are other closed FS, which is of
fundamental importance to clarify
the FS of HTSC in the pseudogap
phase, we have performed highprecision
measurements of the
dHvA effect in underdoped
YBa2Cu3Oy [13]. Raw data of torque
for two slightly different
compositions (y=6.51 and y=6.54)
are shown in the inset of Fig. 4.
Solid lines are fits of the Lifshitz-Kosevich theory, assuming that four frequencies are involved in the data. The
main figure presents the Fourier analysis of the oscillatory torque for underdoped YBa 2Cu 3O 6.54. A broad peak
with a maximum around 535 T is observed, in agreement with previous results. Our results do not support the
existence of a larger pocket reported recently in the same compound [8]. The main frequency, so far believed to
be a single frequency, is in fact composed of three closely spaced frequencies: F1=540 T (black line in Fig. 4)
and two satellites F 2=450 T (green line) and F 3=630 T (purple line). We attribute these frequencies to bilayer
effect and corrugation of the FS, which point out for the first time in an underdoped cuprate the coherence of the
quasiparticle along the interplane direction at low temperature.
Heavy-fermion systems
Activity on heavy-fermion physics was developed during the last years at the LNCMI-Toulouse. Among recent
works, two studies performed at the LNCMI-Toulouse on the “hidden-ordered” paramagnet URu 2Si 2 and on the
antiferromagnet CeRh 2Si 2 are summarized below.
Fig 5 : Nernst signal in URu 2Si 2 at different temperatures. The
inset compares the phase diagram (T,B) obtained from the MR
(square) and Nernst effect (triangles) studies in pulsed fields
with steady field measurements of the MR (circles).
Transport properties of the heavy-fermion
URu 2Si2 have been studied in pulsed
magnetic fields [14]. Combination of
magnetoresistance, Hall effect and Nernst
effect measurements (see Fig. 5) permitted to
outline the reconstruction of the Fermi
surface in the magnetic field-temperature
phase diagram. The zero-field ground state
(called ‘hidden order state’) is a compensated
heavy-electron semi-metal, which is
destroyed by magnetic fields through a
cascade of field-induced transitions. A large
Nernst signal emerges in the hidden order
state. A finite Nernst signal is indeed
expected in a multi-band metal with different
types of carriers. In addition, as illustrated by
the case of elemental bismuth, the Nernst
effect, which tracks the ratio of mobility to
the Fermi energy, becomes particularly large
in a clean semimetal. This signal was found
to be suppressed above 35 T, when the
magnetic field destabilizes the hidden order.