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 systems.

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:

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.

From these coordinated efforts simple and beautiful pictures have emerged. Simplifying a bit, 1-d insulating quantum systems may:

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 marks.
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 etc…).

In two dimensions (2d), the variety of possible phases seems larger:

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 utmost interest…

On this way to a deeper understanding of what is going on in real materials, other issues appear as both timely and central:

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 nano-magnets.
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.

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.

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 researches.