High-field measurements [#!BrayPRB79!#,#!BlochPRL80!#,#!NorthbyPRB82!#,#!HijmansPRL85!#,#!HijmansPRL87!#,#!KiryukhinPRL95!#]

Since the first excited state of the spin Peierls phase is the triplet state, one can close the spin Peierls energy gap by applying a large magnetic field. Experimentally, high-field magnetization measurements [133,134,135] have detected this signature. At $T<T_{\rm SP}$, the magnetization M(H) remained small up to a critical field (H_c), and above H_c, it linearly increased. This experimental result indicates that one branch of the triplet states crosses levels with the singlet ground state at the critical field H_c.

  

Figure: Phase diagram of spin Peierls systems. Temperature (T) and magnetic field (H) are normalized with $T_{\rm SP}(H\!=\!0)$. Cite from Ref. [139].

High-field magnetization measurements have provided the phase diagram of spin Peierls materials as shown in Fig.51. It was found that the phase boundaries fall on universal curves, if temperature (T) and magnetic field (H) were normalized with $T_{\rm SP}(H\!=\!0)$, which is the spin-Peierls transition temperature in zero-field [135]. From the hysteresis of magnetization [135], the phase boundary between the spin-Peierls phase (SP) and the magnetic phase (M$^\ast$) was found to be first-order. The phase boundary between the uniform phase (U) and other phases (SP and M$^\ast$) is second order. This boundary is well described with a theoretical curve [140] obtained from Cross and Fisher's approach.

As a microscopic structure of the M$^\ast$ phase, a localized spin state (spin soliton) has been proposed theoretically [126]. If this localized spin state is realized, the local field at a certain position of the crystalline unit cell should have a relatively broad distribution, because the spin soliton is spatially inhomogeneous. From an investigation of the ¹H- and ¹⁹F-NMR line-shape [136] and ESR line-shift [137], the existence of spin solitons was experimentally suggested in the M$^\ast$ phase of TTF-AuBDT.

Theoretically, the spin soliton is accompanied by incommensurate lattice modulations; in the vicinity of a spin soliton, the lattice dimerization should be lifted. Recently, Kiryukhin et al. performed high-resolution X-ray diffraction measurements on TTF-CuBDT, and found the incommensurate modulations of the lattice appearing in the M$^\ast$ phase [138].