Direct Observation of the Quantum Phase Transition of SrCu₂(BO₃)₂ by High-Pressure and Terahertz Electron Spin Resonance

Abstract

High-pressure and high-field electron spin resonance (ESR) measurements have been performed on a single crystal of the orthogonal-dimer spin system SrCu 2 (BO 3 ) 2 . With frequencies below 1 THz, ESR signals associated with transitions from the singlet ground state to the one-triplet excited states and the two-triplet bound state were observed at pressures up to 2.1 GPa. We obtained directly the pressure dependence of the gap energies, finding a clear firstorder phase transition at P c = 1.85 ± 0.05 GPa. By comparing this pressure dependence with the calculated excitation energies obtained from an exact diagonalization, we determined the precise pressure dependence for inter-(J ′ ) and intra-dimer (J) exchange interactions considering the Dzyaloshinski-Moriya interaction. Thus this system undergoes a first-order quantum phase transition from the dimer singlet phase to a plaquette singlet phase above the ratio (J ′ /J) c = 0.660 ± 0.003.

Sign switching of dimer correlations in SrCu2(BO3)2 under hydrostatic pressure

“…At higher pressures they become undetectably weak or shift outside our measurement window (T 1 ). The observed softening of the triplet modes is fully consistent with previous Electron spin resonance(ESR) [27] and inelastic neutron scattering [23] studies, shown as triangles and squares in the figure. The softening of the singlet mode is a new result, since neither ESR nor neutrons are sensitive to singlet-singlet transitions.…”

“…It can only be driven by a subtle change in the J /J ratio, as assumed in most theoretical studies [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. It takes place long before the exchange constants themselves can be expected to switch sign [27]. It also seems not to be associated with any structural transition, of which we do not observe any obvious signs such as splitting of phonon lines or the appearance of new modes.…”

“…As shown by ESR experiments [27], J decreases by only about 12% from 0 to 15 kBar. This allows us to assume that the 2nd derivative in Eq.…”

Development and application of 2.5 GPa–25 T high-pressure high-field electron spin resonance system using a cryogen-free superconducting magnet

“…As the high-field ESR measurements observed the excited states very clearly for this system [15], such measurements at high pressure offer a means to investigate this phase transition. Recently, within the field range of 10 T, we revealed that the system undergoes a quantum phase transition at 1.85 GPa [16]. On the other hand, the first excited triplet states cross the ground state at around 25 T at 0 GPa [15].…”

Development of High-Field and High-Pressure ESR System and Application to Triangular Antiferromagnet $$\hbox {CsCuCl}_{3}$$

“…We use a transmission technique in our ESR systems and we have developed specially designed piston-cylinder type pressure cells using ceramics inner parts which have good transmittance to millimeter and submillimeter waves. It turned out that the high-pressure and high-field ESR measurement is a good means to explore the pressure-induced phase transition of quantum magnet like SrCu 2 (BO 3 ) 2 [7].…”