2006 APS March Meeting
Monday–Friday, March 13–17, 2006;
Baltimore, MD
Session U2: Quantum Magnets in High Magnetic Fields
8:00 AM–11:00 AM,
Thursday, March 16, 2006
Baltimore Convention Center
Room: Ballroom III
Sponsoring
Unit:
DCMP
Chair: Igor Zaliznyak, Brookhaven National Laboratory
Abstract ID: BAPS.2006.MAR.U2.4
Abstract: U2.00004 : High-field ESR and thermodynamic studies of uniform and bond-alternating $S$=1 spin chains
9:48 AM–10:24 AM
Preview Abstract
Abstract
Author:
Masayuki Hagiwara
(KYOKUGEN, Osaka University)
Recently, field-induced phenomena in quantum spin systems have
attracted
considerable interest. Gapped one-dimensional (1D) spin systems
with a spin
value$ S$=1 subject to an external magnetic field strong enough
to close the gap
($H_{c})$ are driven into a new phase. Spin excitations in this
field-induced
phase have been studied by experiments on a uniform S=1
antiferromagnetic
spin chain Ni(C$_{5}$H$_{14}$N$_{2})_{2}$N$_{3}$(PF$_{6})$, alias
NDMAP
and a bond-alternating one
Ni(C$_{9}$H$_{24}$N$_{4})$NO$_{2}$(ClO$_{4})$,
alias NTENP. We performed high-field and multi-frequency ESR
experiments at
1.5 K on these compounds and observed gapped excitations above
$H_{c}$. Two
or three excitation modes were observed depending on the field
direction in
NDMAP and only one excitation in NTENP. These results are
consistent with
those obtained by inelastic neutron scattering experiments in a
magnetic
field. Both compounds exhibit the long-range order (LRO) at a
magnetic field
above $H_{c}$ and a low temperature. Observed gapped excitations
are very
different from those expected from a conventional spin-wave
theory in the
LRO state. For NDMAP, observed branches satisfactorily agree with
those
analyzed by a phenomenological field theory. The difference of
observed
gapped excitations between NDMAP and NTENP can be explained by an
interaction with a low-lying two magnon continuum at q=$\pi $
that is
present in a bond-alternating chain but absent in a uniform one.
When an antiferromagnetic spin chain with$ S$=1 has an XY or
Heisenberg
symmetry, the phase above $H_{c}$ is critical and its low-energy
physics is
described by a Tomonaga-Luttinger liquid (TLL), which is
characterized by a
gapless $k$-linear energy dispersion with an incommensurate
$k_{0}$ and a spin
correlation having an algebraic decay. NTENP has nearly an XY
symmetry and a
linear temperature($T)$ dependence of the specific heat
($C_{mag})$ was observed
for the magnetic field parallel to the chain above $H_{c}$ in a
temperature
region above that of the LRO state. The ratio $C_{mag}$/$T$
increases as the
magnetic field approaches $H_{c}$ from above and is in good
agreement with
the prediction of the $c$=1 conformal field theory, providing a
conclusive
evidence for a TLL in a gapped quasi-1D antiferromagnet.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2006.MAR.U2.4