AHMF& Cadet Program Joint Lectures and seminar
先端強磁場科学研究センター＆カデットプログラム共催講義とセミナー

講師 (Speaker): Dr. Alexander Smirnov (P. L. Kapitza Institute for Physical Problems of
the Russian Academy of Sciences, Russia)
場所 (Venue): 講義(Lectures) 2F Meeting room in the old KYOKUGEN center
極限科学研究棟２階会議室
セミナー(seminar) H601 meeting room (tentative)
理学研究科 H 棟 601 会議室
Lecture 1: Low frequency dynamics of spin-gap magnets. (November 20, 13:0014:30)
Abstract: Magnetic dielectric crystals with antiferromagnetic exchange usually exhibit
antiferromagnetic ordering at a temperature of the order of the exchange integral.
Nevertheless, crystals with a special arrangement of antiferromagnetic exchange bonds(spin chains, dimer networks, frustrated lattices) demonstrate a strong delay of themagnetic ordering till very low temperatures, or do not order even at absolute zero. The
ordered spin component in the ground state may be equal to zero (in this case wespeak about spin-liquids) or is strongly reduced in comparison with the nominal value.
Quantum fluctuations destroy the ordered state, proposed for these systems from theclassical point of view. In this sense, quantum magnets are analogous to liquid helium,
which does not crystallize or solidified at absolute zero. However, the spin-disorderedground state appears to be strongly correlated and its excitations are quantized modesof spin motion, which are drastically different from conventional magnons. Magnons inconventional magnets are oscillations of the order parameter, while ordering is absent inquantum magnets. We shall discuss quantum magnets demonstrating excitation spectrawith a spin gap, which stabilizes a spin liquid state. In spin-gap quantum magnets theexcitations are quantized and carry spin S=1, they may be excited by transmitting
quantized energy and momentum from neutrons or photons, enabling the experimental
study. This study resolves the general view of the spectrum, as well as the fine structure.
We will discuss some of these experiments.

Lecture 2: Spin dynamics of gapless spin liquids (December4, 13:00-14:30)
Abstract. Solving the Heisenberg problem for S=1/2 antiferromagnetic spin chain
resulted in a set of new concepts, challenging experimentalists to check the theoreticalpredictions. It was shown theoretically, that excitations of S=1/2 antiferromagnetic chain
are fractionalized, i.e., carry spin S=1/2, in contrast to triplet excitations of Haldane(S=1) chains. The spin structure corresponding to fractionalized excitations (spinons)corresponds rather to a delocalized domain wall than to a flipped spin, as in aconventional antiferromagnet or in the spin gap magnet. The concept of unboundfermions within the one-dimensional chain has resulted in several unexpectedpredictions in spin dynamics. At first, for neutron scattering or photon absorption, theseexcitations provide a continuum of the transmitted energy for a fixed wavevector, sincefermions are excited by pairs, having the total spin S=1. Further, the fermion characterof excitations presumes incommensurate soft modes at distinct field-dependent
momentum values. The presence of well defined quasiparticles implies high heat
conductivity along the chains. And finally, uniform Dzyaloshinsky-Moriya interaction waspredicted to provide a fine structure of the energy spectrum in the middle of the Brillouinzone due to a modification of the continuum. We shall discuss the experimental proofsof these predictions.

Seminar: Thermodynamics and spin dynamics of selected triangular lattice
antiferromagnets. (December18, 13:00-14:30)
Abstract. Triangular antiferromagnets provide one of the simplest models of frustrationof the exchange interaction. One of the features of frustration is the impossibility of theminimization of the energy of interaction for pairs of magnetic ions in the ground state of
the whole multi-spin system. Besides, frustrated systems demonstrate a strong
degeneration of the states, which minimize the molecular-field energy. In case of 2Dtriangular lattice antiferromagnet this degeneracy is lifted by the so-called order-by
disorder mechanism, where ground state is selected due to fluctuations, which givedifferent contributions to the free energy for different degenerate configurations. We
shall discuss a quasiclassical S=5/2 triangular lattice antiferromagnet RbFe(MoO4)2 with
the ideal triangular lattice and S=1/2 antiferromagnet Cs2CuCl4 with a distorted
triangular lattice. First we will consider experiments with the first compound, which
demonstrate a “1/3”- plateau of magnetization in the correspondence with the scenarioof the selection between the degenerate states by means of the fluctuations. Further,
we shall analyze the quasi 2D S=1/2 model with distorted triangular lattice, which
demonstrates completely different behavior. This behavior includes continuum of
fractionalized excitations, cycloid–type ordering and magnetic phases controlled by tiny
residual interactions. Experiments testing these features will be described. In particular,
experiments uncover an exotic coexistence of magnon and spinon modes.