Quantum magnetism in spin, charge and orbital systems
2003.10.1-2003.10.4 at Paris
Sponsored by CNRS/DRI, CNRS/DRI, JSPS and French ambassy at Tokyo
Quantum magnetism is a frontier of magnetism. In the old view, magnetism is
discussed with semi-classical concepts as ordered states,
(sub)-lattice magnetization as the order parameter, spin-wave
excitations. As it has been noticed since many years and by many
authors, this modelization is insufficient to describe many
experimental situations, specifically in low dimensional
During the last fifteen years, a considerable amount of
results both experimental and theoretical has been obtained for
one-dimensional systems (1d). These remarkable progresses have been
supported by major efforts in three directions:
From these coordinated efforts simple and beautiful pictures have emerged.
Simplifying a bit, 1-d insulating quantum systems may:
In theory: use of theoretical field approaches, conformal field theory,
and results on integrable systems.
In experiments: considerable progresses
in NMR and ESR, neutron spectroscopies and X-ray spectroscopies.
In material engineering: inventive
realizations of materials from oxides to organic compounds.
In two dimensions (2d), the variety of possible phases seems larger:
Have gapless excitations. In such a case,
they are critical systems, they have fractionalized excitations
forming continuum (spinons) and not only spin wave modes (magnons).
Understanding the non-linear excitations as well as the role of the
magnetic field in inducing incommensurability have been crucial stone
Have gapped excitations. In this case,
where quantum effects are emphasized, it has been speculated
theoretically (Haldane) and shown experimentally that half integer
and integer spin systems behave differently. It has also been shown
that the coupling of the spins to the lattice degrees of freedom
(spin-Peierls mechanism) is essential. The magnetic field can
transform this gapped phase in a gapless magnetized one. Impurities
can also be at the origin of various interesting effects (ordering
On this way to a deeper understanding of what is going on in real
materials, other issues appear as both timely and central:
Semi-classical Néel phases are
theoretically possible at T=0 and experimentally seen due to
stabilization through 3-dimensional couplings,
Gapped phases with long-range order in
dimers exist in 2d as well as in 1d. The first most spectacular
results on SrCu2(BO3)2 are very
recent. Both the experimental realization of new materials with these
properties and the study of their excitation spectra are exciting and
numerous new enlightments are expected,
Gapped phases without long-range
order in any local order parameter (Resonating Valence Bond Spin
Liquids) have been predicted but not observed yet. This is a great
challenge. In both countries, teams are looking for this new magnetic
quantum ground-state in 2d solid 3He, but the Wigner crystal phase in
silicon MOSFETs may be a good candidate too as well as many oxides.
These last quantum phases are suspected to support fractionalized
non-confined excitations (as the spinons of the 1d chains but with
some differences, still to be completely elucidated), and there
should be qualitative differences between spin-1/2 and spin-1
systems. A critical study of different experimental works done in our
two countries during the last fifteen year should be undertaken, as
well as some prospective work.
Critical phases are equally possible, and
there is a priori a larger variety of critical phases than in one
dimension. This is largely a terra incognita both from the
experimental point of view and from the theoretical one.
In between one and two-dimensional
systems the inhomogeneous situations with stripes may be of the
New experimental probe such as real space
resolved spectroscopy (microscope, STM, micro-SQUID) and the probe
for real time dynamics are necessary.
Magnetic field is an important part
of the studies; new quantum phases, tuning of quantum critical point
and incommensurability control are very important.
To answer these different questions, it is important to have good
samples of old and new materials, and study their low energy
excitations (far infra red and micro wave) as well as the high energy
scale behaviors (X- rays, THz, high fields) and to encourage and
initiate more and more collaborations between scientists creating the
materials, experimentalists and theoreticians.
The spins, we have been talking of up to
now, are in many cases complex objects resulting of a modelization, a
simplification of reality. In fact, they come from different degrees
of freedom, true spin and orbital ones, coupled in various ways in
the crystal fields. This richer reality gives rise to a variety of
phenomena of great importance: orbital ordering is one issue,
Dyaloshinski Moryia interaction another one etc…
Many of these quantum magnets display
intriguing and paradoxical features as the coexistence of
spin-glass like behavior: hysteresis in high field and zero field
cooling, divergence of non linear magnetic susceptibility and very
low dynamics, absence of freezing of a very large number of degrees
of freedom at very low temperatures…
Spin chirality is very important in
frustrated spin systems; an anomalous Hall effect due to the
chirality is a good example. It indicates that a Berry phase should
be considered seriously in the motion of electron as well as the
rotation of spin in quantum tunneling of magnetization in
Orbital degeneracy and electron-lattice
coupling in diagonal and in off diagonal terms are very important.
New concepts such as “orbital spin –Peierls transition”,
orbital liquids and orbital-spin coupled to low energy mode should be
examined experimentally and theoretically.
Collaborations have been made so far on non-linear excitations in the
copper benzoates, Spin Peierls and Haldane compounds, magnetization
plateaus, molecular magnetism, solid-state chemistry, NMR, neutron
and ESR spectroscopy. We expect an extension of the collaborations in
theory, NMR, ESR and neutrons, collaborations on new materials, and
new techniques such as X-Ray (including high field experiments).
We aim to tighten existing collaborations, try
to launch an open and creative discussion on some of the challenging
questions listed above, and initiate if possible new coordinated