Novel Strongly Spin-Orbit Coupled Quantum Dimer Magnet: Yb2Si2O7

Abstract

The quantum dimer magnet (QDM) is the canonical example of quantum magnetism. The QDM state consists of entangled nearest-neighbor spin dimers and often exhibits a field-induced triplon Bose-Einstein condensate (BEC) phase. We report on a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb2Si2O7. Our single crystal neutron scattering, specific heat, and ultrasound velocity measurements reveal a gapped singlet ground state at zero field with sharp, dispersive excitations. We find a field-induced magnetically ordered phase reminiscent of a BEC phase, with exceptionally low critical fields of Hc1 ∼ 0.4 T and Hc2 ∼ 1.4 T. Using inelastic neutron scattering in an applied magnetic field we observe a Goldstone mode (gapless to within δE = 0.037 meV) that persists throughout the entire field-induced magnetically ordered phase, suggestive of the spontaneous breaking of U(1) symmetry expected for a triplon BEC. However, in contrast to other well-known cases of this phase, the high-field (µ0H ≥ 1.2T) part of the phase diagram in Yb2Si2O7 is interrupted by an unusual regime signaled by a change in the field dependence of the ultrasound velocity and magnetization, as well as the disappearance of a sharp anomaly in the specific heat. These measurements raise the question of how anisotropy in strongly spin-orbit coupled materials modifies the field induced phases of QDMs.Quantum dimer magnets (QDMs) represent the simplest cases of quantum magnetism, where entanglement is a required ingredient for even a qualitative understanding of the phase. In a QDM, entangled pairs of spins form S tot = 0 dimers and result in a non-magnetic ground state. The excited states of these entangled spins can be treated as bosons, called triplons, which can undergo Bose-Einstein condensation (BEC) as their density is tuned by an applied magnetic field. This BEC state is a magnetic field-induced long range ordered phase, which occupies a symmetric "dome" in the field vs. temperature phase diagram with two temperature-dependent critical fields, H c1 (T ) and H c2 (T ). The vast majority of the previously studied QDMs are based on 3d transition metal ions with "bare" (spin-only) S = 1/2 or S = 1 angular momentum, resulting in simple Heisenberg or XXZ spin interaction Hamiltonians, and high critical fields set by the relatively high energy scale of exchange interactions [1-6].Lanthanide-based magnetic materials with spin-orbit coupled pseudo-spin 1/2 (S eff = 1/2) angular momenta can also exhibit quantum phases, and these are often directly analogous to their traditional 3d transition metal ion counterparts. However, entirely new phases are possible due to the anisotropic exchange in these materials [7][8][9][10][11][12]. In the lanthanide series, Yb 3+ has been of particular interest as it can generically host interactions leading to quantum fluctuations irrespective of the Crystal Electric Field (CEF) ground state doublet composition [13]. Indeed, various quantum phases have been discovered in Yb-based systems [14][15][...

Crystal Growth of Quantum Magnets in the Rare-Earth Pyrosilicate Family R2Si2O7 (R = Yb, Er) Using the Optical Floating Zone Method

“…Note that these data, as well as data collected at additional field strengths, are also shown and discussed in detail in Ref. [13].…”

“…Large, clear single crystal pieces (≈15 mm) of C-Yb 2 Si 2 O 7 were prepared from micro-pulling-down (µ-PD) technique to fabricate components for near-infra red scintillators in medical applications [12]. While studies on magnetic properties of Yb 2 Si 2 O 7 were non-existent until the recent discovery of quantum dimer magnet by our group [13], the electronic structure of C-Yb 2 Si 2 O 7 has been studied in brief using X-ray photoemission spectroscopy [14] and valence electron energy-loss spectroscopy [15]. The existence of an isolated energy level in the band gap of Yb 2 Si 2 O 7 was identified in the latter report.…”

“…We also demonstrate the magnetic properties of the samples via their low-temperature specific heats. The crystals obtained by this method are suitable for single crystal neutron scattering investigations, the first of which have recently been carried out by our group [13].…”

Low-temperature spin dynamics in the TmFeO3 orthoferrite with a non-Kramers ion

“…However, this is usually in the context of single ion physics, and the crystalline environments may change the electronic properties of the rare-earth ions considerably. Experimental hallmarks of exotic quantum states have been recently reported in a number of Yb-based (4 f 13 ) compounds by the observation of Tomonaga-Luttinger liquid behavior and spinon confinement-deconfinement transitions in YbAlO 3 [13]; spinon continuum and spinon confinement in Yb 2 Pt 2 Pb [14,15] and in the triangular-lattice systems YbMgGaO 4 [16,17]; the quantum dimer magnet state in Yb 2 Si 2 O 7 [18] and a Higgs transition from a magnetic Coulomb liquid to a ferromagnet in Yb 2 Ti 2 O 7 [19]. In the rare-earth orthoferrite YbFeO 3 the Yb magnetic sublattice also exhibits rich quantum spin dynamics [20].…”

Synthesis, structure, and electronic properties of the Li11RbGd4Te6O30 single crystal

“…[3][4][5] BaCuSi 2 O 6 is a quasi-2D square lattice magnetic insulator with a gapped spin dimer ground state and is supposed to be an ideal candidate for studying the BEC of triplons as a function of the external magnetic eld. 6 Recently, Hester et al 7 reported a new Yb 3+ based QDM in the distorted honeycomb-lattice material Yb 2 Si 2 O 7 with strong spin-orbit coupling (SOC). The intradimer bond length of two Yb ions is about 3% shorter than that of the inter-dimer ones, resulting in contrastive exchange interactions J intra ¼ 0.236 meV and J inter ¼ 0.06 meV.…”