ESR and magnetization measurements have been performed up to 14 T in the compound CuGe03. Two kinds of magnetic transitions are observed, between excited states and from the ground state. The former result essentially from a Zeeman splitting of the S = 1 magnetic triplet state characterizing the spin Peierls gap which exists at the wave vector Q = [I, I, I] of the magnetic Brillouin zone. The latter provide the first evidence of a Zeeman splitting on a spin Peierls gap. It is observed at Q = [0,0, 0]. PACS numbers: 75.40. Gb, 75.10.Jm, 76.50.+g The recent observation of a spin Peierls (SP) transition in the inorganic compound CuGe03 [1] has renewed the interest in the study of s = 2 Heisenberg antiferromagnetic chains (2HAFC). In such isotropic spin systems, the diverging of the magnetic fluctuations as T 0 is expected to yield a dimerization of the lattice -this is the SP transition -at a well-defined temperature Tsp [2].However, such an effect is rarely observed due to the occurrence of three-dimensional ordering which, typically, stabilizes the spin chain before Tsp is reached. When the dimerized phase is realized, i.e. , below Tsp, the spin system is characterized by a S = 0 nonmagnetic singlet ground state (GS) and a S = 1 magnetic excited triplet state separated from the GS by an energy gap (the "SP gap" Esp) [2]. At this point it is worthwhile making the comparison with Haldane spin chains (HSC), since the same predictions are usually given for that spin system [3]. However, in a HSC, which is the integer-spin analog of the zHAFC, the energy gap does not result 1 from a dimerization but from the occurrence of nonlinear quantum fluctuations in the GS. Although the physics of zHAFC and of HSC are completely different, the recent experimental investigations of HSC may serve as a guide and as a point of comparison. For instance, in HSC the effect of a magnetic field has been shown to split the excited S = 1 triplet state at the "quantum" gap, which is known to open at the wave-vector value q = 1 of the one-dimensional reciprocal lattice. [In this Letter, the wave vectors refer to the magnetic Brillouin zone and are expressed in reciprocal lattice unit (rlu). ] This Zeeman effect gives rise to three distinct energy branches characterized by different magnetic states [4]. As shown by high-field ESR measurements, magnetic Am = 1 transitions can be induced directly between these split states around q = 1 [5]. The purpose of the present Letter is to report on a similar high-field ESR investigationincluding magnetization measurements -of the dimerized phase of Cuoe03. Such a high-field study will be shown to provide new insights on the dimerized phase and a bet-

The 4 K 245-GHz/8.7-T electron paramagnetic resonance spectrum of the stable tyrosyl radical in photosystem I, known as TyrD, has been measured. Illumination at 200 K enhances the signal Intensity of TyrD" by a factor of >40 compared to the signal obtained from darkadapted samples. This signal ehncement and the unusual ine shape of the TyrD' resonance result from the magnetic dipolar coupling of the radical to the manganese duster involved in oxygen evolution. The relative alar orientation of the m _ cluster with respect to TyrD-has been determined from line-shape analysis. The resonance arising from TyrD' in Tris-washed nsfree photosystem U sample is also distorted. This effect probably originates from the influence of the nonheme iron on the spin aon of the tyrosyl radical.The relative angular orientation of the nonheme iron has also been determined. Oriented samples were used to determine the angular orientation of TyrD with respect to the membrane plane. Combining angular data with pubilshed distances, we have constructed a three-dimensional picture of the relative positions of TyrD', the manganese cluster, and the noneme iron. The data suggest a more symmetrical placement of the manganese relative to TyrD-and TyrZ, the tyrosine involved in eleco transfer, than is usually assumed in current models of photosystem H.At low temperatures, the electron paramagnetic resonance (EPR) signal of the stable tyrosyl radical, known as TyrD- (1, 2), from plant photosystem II (PSIT) is known to be "relaxation enhanced" when the sample is illuminated at 200 K (3, 4). This enhancement has been attributed to the presence of a second paramagnetic species that is capable of increasing the relaxation rates of the tyrosyl radical through a dipolar interaction. The second paramagnet is thought to be the manganese cluster that is involved in charge storage and probably acts as the active site for oxygen evolution (5, 6). The dipolar spin-relaxation mechanism has been thoroughly studied (7-9). The strength of the dipolar interaction is determined by the distance between the two spins and the angle between the interspin vector and the applied magnetic field. Previous work on PSII has concentrated on the determination of the interspin distance (10-13). No attempts have been made to obtain the angular orientation of the two spins.At magnetic field strengths used in conventional EPR spectroscopy, the spectrum of the tyrosyl radical is dominated by the hyperfine interactions between the electron spin and its neighboring protons. The inhomogeneous line width of the spectrum is determined by the anisotropic parts of these hyperfine interactions and the g anisotropy, each of which has its own particular orientation dependence relative to the molecular frame. A good approximation is that all possible orientations ofthe radical with respect to the applied magnetic field contribute to a given field position on the TyrD spectrum. Thus, the determination ofthe orientation of an external relaxer is difficult. However, at magnetic fields m...

Physikalisches hstitut. UniversitZt F r d m t , D-6000 F t d u r t am Main, Federal Republic of Germany $ Institut fiu Kernphysik, UniversitHt E t d u r t , D-6000 Frankfurt a m Main, Federal Republic of Germany

We present a high-frequency electron spin resonance study of the magnetic transitions occurring between excited states in a Haldane spin system. We show that these transitions are induced between the low-energy magnon states, which develop at the boundary iq ^ n) of the Brillouin zone. Our study provides therefore the first evidence that, in the Haldane phase, the excitations associated with the quantum gaps correspond actually to a 5 == 1 magnetic state.PACS numbers: 75.10.Jm, 75.40.Gb, 76.50.+g According to the Haldane conjecture [l], the ground state (GS) of isotropic antiferromagnetic (AF) chains of integer spins is disordered and nonmagnetic (the GS is actually defined as an ^ =0 state). The low-energy spectrum of such a system exhibits very peculiar properties. The corresponding excitations result from a triplet state and, unlike the half-integer spin case, a quantum gap is expected to open in the dispersion curve. This gap should occur at the boundary of the Brillouin zone q "^nla (hereafter the lattice parameter is a = 1). All these properties have been proved by experimental investigations, in particular on the compound NENP [Ni(C2H8N2)N02-CIO4], which is a good realization of a Haldane spin system [2]. However, concerning these excitations around q-n^ there remains an important prediction to be checked: Since the triplet state is defined as an ^ = 1 state, magnetic "Am = r' transitions should be allowed between the different components of the triplet state. At the zone center {q =0), the fluctuation spectrum results from two-particle excitations [3]. The gap at ^=0 is therefore predicted to be twice the value of the quantum gap [4].Magnetic transitions can be probed by the electron spin resonance (ESR) technique. In that case, the absorption of energy is realized without momentum {q) transfer: The required condition is therefore t^q =0. As a consequence, if the transitions are induced from the GS, they correspond to ^ =0 excitations. However, if they are induced between excited states they can be produced anywhere in the Brillouin zone. In Haldane spins chains, ESR measurements were carried out for the first time by Date and Kindo [5], at the fixed frequency 6>=47 GHz. In NENP an ESR signal was clearly observed for H applied perpendicular to the chain axis b. However, no signal was detected for H parallel to b. It was suggested [5] that for that direction, no resonance can be seen because it is dramatically broadened by a spin-wave continuum occurring, at ^ =0, in the same frequency range. To explain the data, an energy-level diagram (ELD) describing the field dependence of the resulting excitations at q =0 was given. Surprisingly, it appears to be comparable -although not identical-to the q^n ELD determined by neutron inelastic scattering (NIS) measurements [6]. This analysis contradicts the theoretical prediction that the gap at q =0 should be twice the gap at ^ ==;r [3].In the present work, we report an ESR investigation on NENP performed in a wide frequency range between 160 and 1000 GHz and in hi...