Kenzo Onizuka

1/3 Magnetization plateau in SrCu2(BO3)2 : Stripe order of excited triplets

High-field magnetization measurements up to 57 T have been performed at 0.08 K in a single crystal of a two-dimensional spin-gap material SrCu 2 (BO 3 ) 2 . We successfully observed the predicted plateau at 1/3 of the total magnetization around 50 T, in which the magnetic superstructure is characterized by a novel stripe order of triplets. The 1/3 plateau is much wider than the previously observed 1/4 and 1/8 plateaux.

Direct evidence for the localized single-triplet excitations and the dispersive multitriplet excitations in SrCu2(BO3)2.

We performed inelastic neutron scattering on the 2D Shastry-Sutherland system SrCu2(11BO3)2 with an exact dimer ground state. Three energy levels at around 3, 5, and 9 meV were observed at 1.7 K. The lowest excitation at 3.0 meV is almost dispersionless with a bandwidth of 0.2 meV at most, showing a significant constraint on a single-triplet hopping owing to the orthogonality of the neighboring dimers. In contrast, the correlated two-triplet excitations at 5 meV exhibit a more dispersive behavior.

Direct observation of the multiple spin gap excitations in two-dimensional dimer system SrCu2(BO3)2

Various spin-gap excitations have been observed in the two-dimensional dimer system SrCu 2 (BO 3 ) 2 by ESR. The zero-field energy gap of the lowest spin-gap excitation shows a splitting into two triplet modes and the energy splitting clearly depends on the magnetic field orientation. A zero-field splitting is also found between the S z =+1 and S z =-1 branches of each triplet. These behaviors are qualitatively explained by considering the anisotropic inter-dimer and intra-dimer exchange couplings. The averaged value of the lowest spin-gap energy is determined to be 722±2 GHz (34.7 K). We have also found a second spin-gap excitation at 1140 GHz (54.7 K), which indicates that the inter-dimer coupling is strong. Besides these modes, a number of gapped ESR absorptions are found and we propose that these excitations are caused by the localized nature of the excited state in the present system.

Collective singlet excitations and evolution of raman spectral weights in the 2D spin dimer compound SrCu2(BO3)(2)

Raman light scattering of the two-dimensional quantum spin system SrCu2(BO3)(2) shows a rich structure in the magnetic excitation spectrum, including several well-defined bound state modes at low temperature, and a scattering continuum and quasielastic light scattering contributions at high temperature. The key to the understanding of the unique features of SrCu2(BO3)(2) is the presence of strong interactions between well-localized triplet excitations in the network of orthogonal spin dimers realized in this compound.

Crystal growth of the two-dimensional spin gap system SrCu2(BO3)2

Abstract Our high-quality large single crystals of SrCu 2 (BO 3 ) 2 have been successfully grown by a travelling solvent floating zone (TSFZ) method using LiBO 2 as a solvent. We present the growth condition and the result of the temperature-dependent magnetic susceptibilities measured along the directions parallel and perpendicular to the tetragonal c -axis.

Anomalous magnetizations in single crystalline SrCu2(BO3)2

The magnetic properties of a two-dimensional spin-gap compound SrCu 2 (BO 3 ) 2 , realizing the Shastry-Sutherland model, were investigated using high-quality single crystals. The analysis of the temperature-dependent magnetic susceptibility revealed a spin gap of 34 K. The magnetization curve at 0.4 K possibly has, in addition to the already established one-quarter and one-eighth plateaux, another quantized plateau at one-tenth of the full Cu moment, which is one of the plateaux predicted by Miyahara and Ueda.

Cu nuclear spin-spin coupling in the dimer singlet state in SrCu2(BO3)2

We report results of nuclear magnetic resonance experiments in SrCu2(BO3)2, a quasi-two-dimensional spin system with a singlet ground state. When magnetic field is applied along the c-axis, each of the quadrupole-split Cu resonance lines splits further into four lines. The spin-echo intensity for some of the split lines oscillates against the separation time between π/2 and π rf pulses. These phenomena are due to strong nuclear spin-spin coupling mediated by the electronic spin system, which exists only within a pair of nuclei. Thus the results provide direct evidence for the dimer singlet ground state in this material.

Quantized magnetization plateaux in the orthogonal dimer system SrCu2(BO3)2

Abstract We carried out high-field magnetization measurements up to 57 T and down to 0.08 K on a single crystal of a two-dimensional spin-gap material SrCu2(BO3)2. We successfully observed the predicted plateau at 1/3 of the total magnetization around 50 T, in which the magnetic superstructure is characterized by a novel stripe order of excited triplets. The 1/3 plateau is stable over much wider field region than the previously observed 1/4 and 1/8 plateaux. A sharp jump in the magnetization between 1/3 and 1/4 plateaux may be associated with a first-order transition. Furthermore, several anomalous behaviors were observed in a lower-field region including field-direction dependent magnetizations around 30 T. The obtained magnetizations are compared with theoretical calculations based on a hard-core boson model and on a perturbation expansion method. The magnetization measurements were also performed on 1% zinc-doped single crystal.

Low-temperature specific heat study of SrCu2(BO3)2 with an exactly solvable ground state

The specific heat of a two-dimensional spin gap system SrCu2(BO3)2 realizing the Shastry-Suther-land model was measured between 1.3 and 25 K under various magnetic fields up to 12 T. The analysis based on an isolated dimer model in a low temperature region revealed that the value of the spin gap at zero field is Δ = 34.4 K. It turned out that Δ decreases in proportion to H due to the Zeeman splitting of the excited triplet levels. This simplest model, however, fails to reproduce the result in a high-temperature region, suggesting rather strong spin-spin correlation of the system.

Multiple singlet and triplet states in the 2D orthogonal-dimer compound SrCu2(BO3)2

Abstract The compound SrCu 2 (BO 3 ) 2 is a quantum spin system with a 2D arrangement of orthogonal spin dimers, an exact singlet ground state and a singlet–triplet gap Δ 01 =34 K. Triplet excitations in this compound are extremely localized and strongly interacting due to the strong inherent frustration. Experimentally, two triplet modes were observed both in neutron scattering and ESR investigations. In Raman scattering experiments further well-defined modes with energies close to Δ 01 exist for low temperatures ( T ⪡ Δ 01 ). The properties of these modes resemble the magnetic bound states found in dimerized spin chain and spin ladder systems.