Over the past two decades, the magnetic ground states of all rare earth titanate pyrochlores have been extensively studied, with the exception of Sm2Ti2O7. This is, in large part, due to the very high absorption cross-section of naturally-occurring samarium, which renders neutron scattering infeasible. To combat this, we have grown a large, isotopically-enriched single crystal of Sm2Ti2O7. Using inelastic neutron scattering, we determine that the crystal field ground state for Sm 3+ is a dipolar-octupolar doublet with Ising anisotropy. Neutron diffraction experiments reveal that Sm2Ti2O7 orders into the all-in, all-out magnetic structure with an ordered moment of 0.44(7) µB below TN = 0.35 K, consistent with expectations for antiferromagnetically-coupled Ising spins on the pyrochlore lattice. Zero-field muon spin relaxation measurements reveal an absence of spontaneous oscillations and persistent spin fluctuations down to 0.03 K. The combination of the dipolar-octupolar nature of the Sm 3+ moment, the all-in, all-out ordered state, and the low-temperature persistent spin dynamics make this material an intriguing candidate for moment fragmentation physics.Rare earth titanate pyrochlores of the form R 2 Ti 2 O 7 have long been a centerpiece in the study of geometricallyfrustrated magnetism [1]. In this family of materials, the magnetism is carried by the R 3+ rare earth ions, which decorate a network of corner-sharing tetrahedra.The study of this family has led to the discovery of a range of fascinating ground states such as the dipolar spin ice state, which was first observed in Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 [2-4]. Here local Ising anisotropy combines with dominant dipolar interactions, which are ferromagnetic at the nearest neighbour level on the pyrochlore lattice [5]. The spin ice state is characterized by individual tetrahedra obeying two-in, two-out "ice rules", wherein two spins point directly towards the tetrahedron's center and the other two spins point outwards (left inset of Fig. 1). This configuration can be achieved in six equivalent ways for a single tetrahedron, giving rise to a macroscopic degeneracy for the lattice as a whole. In other titanates, where the rare earth moments are smaller than in Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 , dipolar interactions become less important and exchange interactions tend to dominate. This is exactly the case when R = Sm 3+ (∼ 1 µ B ), where the magnetic moment is reduced by a factor of ten from R = Ho 3+ and Dy 3+ (∼ 10 µ B ), corresponding to dipolar interactions that are weaker by two orders of magnitude.In this letter we show that anitferromagnetically coupled Ising spins with negligible dipolar interactions give rise to an all-in, all-out (AIAO) magnetic ground state in Sm 2 Ti 2 O 7 . The AIAO structure is characterized by adjacent tetrahedra alternating between all spins pointing inwards and all spins pointing outwards (right inset of Fig. 1). Unlike the ferromagnetic spin ice state, the antiferromagnetic AIAO state does not give rise to a macroscopic degenerac...
We have performed magnetic susceptibility, heat capacity, muon spin relaxation, and neutron scattering measurements on three members of the family Ba3M Ru2O9, where M = In, Y and Lu. These systems consist of mixed-valence Ru dimers on a triangular lattice with antiferromagnetic interdimer exchange. Although previous work has argued that charge order within the dimers or intradimer double exchange plays an important role in determining the magnetic properties, our results suggest that the dimers are better described as molecular units due to significant orbital hybridization, resulting in one spin-1/2 moment distributed equally over the two Ru sites. These molecular building blocks form a frustrated, quasi-two-dimensional triangular lattice. Our zero and longitudinal field µSR results indicate that the molecular moments develop a collective, static magnetic ground state, with oscillations of the zero field muon spin polarization indicative of longrange magnetic order in the Lu sample. The static magnetism is much more disordered in the Y and In samples, but they do not appear to be conventional spin glasses.
We present measurements on a series of materials, Li_{2}In_{1-x}Sc_{x}Mo_{3}O_{8}, that can be described as a 1/6th-filled breathing kagome lattice. Substituting Sc for In generates chemical pressure which alters the breathing parameter nonmonotonically. Muon spin rotation experiments show that this chemical pressure tunes the system from antiferromagnetic long range order to a quantum spin liquid phase. A strong correlation with the breathing parameter implies that it is the dominant parameter controlling the level of magnetic frustration, with increased kagome symmetry generating the quantum spin liquid phase. Magnetic susceptibility measurements suggest that this is related to distinct types of charge order induced by changes in lattice symmetry, in line with the theory of Chen et al. [Phys. Rev. B 93, 245134 (2016)PRBMDO2469-995010.1103/PhysRevB.93.245134]. The specific heat for samples at intermediate Sc concentration, which have the minimum breathing parameter, show consistency with the predicted U(1) quantum spin liquid.
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][...
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