Evidence for an Unconventional Magnetic Instability in the Spin-Tetrahedra SystemCu2Te2O5Br2 (original) (raw)
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Spin liquid versus long-range magnetic order in the frustrated body-centered-tetragonal lattice
Physical Review B, 2016
We show how spin-liquid (SL) states can be stabilized in a realistic three-dimensional model as a result of frustration. SU(n)-symmetric generalization of the Heisenberg model for quantum spin S operators is used to investigate the frustrated body-centered tetragonal (BCT) lattice with antiferromagnetic interlayer coupling J 1 and intralayer first and second-neighbor couplings J 2 and J 3. By using complementary representations of the spin operators, we study the phase diagram characterizing the ground state of this system. For small n, we find that the most stable solutions correspond to four different families of long-range magnetic orders that are governed by J 1 , J 2 , and J 3. First, some possible instabilities of these phases are identified for n = 2, in large S expansions, up to the linear spin-wave corrections. Then, using a fermionic representation of the SU(n) spin operators for S = 1/2, we find that purely magnetic orders occur for n 3 while SL solutions are stabilized for n 10. The SL solution governed by J 1 breaks the lattice translation symmetry. The modulated SL is associated with a commensurate ordering wave vector (1,1,1). For 4 n 9, we show how the competition between J 1 , J 2 , and J 3 can turn the magnetically ordered ground state into a SL state. Finally, we discuss the relevance of this scenario for correlated systems with BCT crystal structure.
Magnetic ordering and ergodicity of the spin system in the Cu2Te2O5X2 family of quantum magnets
Physical Review B, 2006
We present an experimental and theoretical study of the magnetically frustrated spin system in pure and substitutionally disordered compounds from the Cu 2 Te 2 O 5 X 2 family of quantum magnets. Experimental magnetic susceptibilities and specific heats were analyzed simultaneously using models of ͑i͒ isolated tetrahedra of four antiferromagnetically coupled Cu 2+ spins and ͑ii͒ coupled tetrahedra within one-dimensional chains, in both cases involving mean-field coupling to other chains. The results show that Cu 2 Te 2 O 5 X 2 compounds are true three-dimensional systems of coupled spins. Susceptibility results are consistent with the existence of a singlet-triplet gap, whereas specific heat analysis shows that the singlet-triplet gap is filled with dense singletlike excitations that contribute to finite specific heat at temperatures far below the singlet-triplet gap, but do not contribute to a magnetic response of the system. Furthermore, measured specific heat data show excessive entropy when compared to the numerical results based on a pure spin system, which we attribute to the presence of phonons. Though Cu 2+ spins are arranged in a geometrically frustrated tetrahedral antiferromagnetic configuration and spin correlation length extends beyond the single tetrahedral cluster dimension, Cu 2 Te 2 O 5 X 2 compounds do not exhibit ergodicity breaking at low temperatures, in contrast to the related geometrically frustrated kagomé and pyrochlore antiferromagnets.
From magnetism to one-dimensional spin liquid in the anisotropic triangular lattice
We investigate the anisotropic triangular lattice that interpolates from decoupled one-dimensional chains to the isotropic triangular lattice and has been suggested to be relevant for various quasitwo-dimensional materials, such as Cs2CuCl4 or κ-(ET)2Cu2(CN)3, an organic material that shows intriguing magnetic properties. We obtain an excellent accuracy by means of a novel representation for the resonating valence bond wave function with both singlet and triplet pairing. This approach allows us to establish that the magnetic order is rapidly destroyed away from the pure triangular lattice and incommensurate spin correlations are short range. A non-magnetic spin liquid naturally emerges in a wide range of the phase diagram, with strong one-dimensional character. The relevance of the triplet pairing for κ-(ET)2Cu2(CN)3 is also discussed. PACS numbers: 75.10.-b, 71.10.Pm,75.40.Mg
Absence of Long-Range Order in a Triangular Spin System with Dipolar Interactions
Physical Review Letters, 2018
Antiferromagnetic Heisenberg model on the triangular lattice is perhaps the best known example of frustrated magnets, but it orders at low temperatures. Recent density matrix renormalization group (DMRG) calculations find that next nearest neighbor interaction J2 enhances the frustration and leads to a spin liquid for J2/J1 ∈ (0.08, 0.15). In addition, DMRG study of a dipolar Heisenberg model with longer range interactions gives evidence for a spin liquid at small dipole titling angle θ ∈ [0, 10 •). In both cases, the putative spin liquid region appears to be small. Here, we show that for the triangular lattice dipolar Heisenberg model, a robust quantum paramagnetic phase exists in a surprisingly wide region, θ ∈ [0, 54 •), for dipoles tilted along the lattice diagonal direction. We obtain the phase diagram of the model by functional renormalization group (RG) which treats all magnetic instabilities on equal footing. The quantum paramagnetic phase is characterized by a smooth continuous flow of vertex functions and spin susceptibility down to the lowest RG scale, in contrast to the apparent breakdown of RG flow in phases with stripe or spiral order. Our finding points to a promising direction to search for quantum spin liquids in ultracold dipolar molecules.
Emergence of novel phenomena on the border of low dimensional spin and charge order
The European Physical Journal B
Proximity to magnetic order as well as low dimensionality are both beneficial to superconductivity at elevated temperatures. Materials on the border of magnetism display a wide range of novel and potentially useful phenomena: high Tcs, heavy fermions, coexistence of magnetism and superconductivity and giant magnetoresistance. Low dimensionality is linked to enhanced fluctuations and, in the case of heavy fermions, has been experimentally shown to be beneficial for the fluctuations that are responsible for the rich abundance of novel emergent phases. This experimental strategy motivated us to explore 2D insulating magnets with a view to investigate phase evolution across metal-insulator and magnetic-non-magnetic boundaries. This has been a fruitful venture with totally novel results different to our expectations. We present results from 2 distinct systems. The MPS3 family are highly anisotropic in both their crystal and magnetic structures. FePS3 in particular is a model insulating honeycomb antiferromagnet. We find that the application of pressure to FePS3 induces an insulator to metal transition. The second system, Cs2CuCl4, is a highly-frustrated quantum spin liquid at low temperature. The competition of the 3 relevant exchange couplings is delicately balanced. It has been shown to become antiferromagnetic at very low temperatures (∼1 K). We have found that the application of pressure for 3 days or more followed by a return to ambient pressure stabilises a totally distinct magnetic ground state.
4-spin plaquette singlet state in the Shastry–Sutherland compound SrCu2(BO3)2
Nature Physics, 2017
The study of interacting spin systems is of fundamental importance for modern condensed-matter physics. On frustrated lattices, magnetic exchange interactions cannot be simultaneously satisfied, and often give rise to competing exotic ground states 1. The frustrated two-dimensional Shastry-Sutherland lattice 2 realized by SrCu 2 (BO 3) 2 (refs 3,4) is an important test case for our understanding of quantum magnetism. It was constructed to have an exactly solvable 2-spin dimer singlet ground state within a certain range of exchange parameters and frustration. While the exact dimer state and the antiferromagnetic order at both ends of the phase diagram are well known, the ground state and spin correlations in the intermediate frustration range have been widely debated 2,4-14. We report here the first experimental identification of the conjectured plaquette singlet intermediate phase in SrCu 2 (BO 3) 2. It is observed by inelastic neutron scattering after pressure tuning to 21.5 kbar. This gapped singlet state leads to a transition to long-range antiferromagnetic order above 40 kbar, consistent with the existence of a deconfined quantum critical point. In the field of quantum magnetism, geometrically frustrated lattices generally imply major difficulties in analytical and numerical studies. For very few particular topologies, however, it has been shown that the ground state, at least, can be calculated exactly as for the Majumdar-Ghosh model 15 that solves the J 1 − J 2 zigzag chain when J 1 = 2J 2. In two dimensions, the Shastry-Sutherland model 2 consisting of an orthogonal dimer network of spin S = 1/2 was developed to be exactly solvable. For an interdimer J to intra-dimer J exchange ratio α ≡ J /J ≤ 0.5 the ground state is a product of singlets on the strong bond J. Numerical calculations have further shown that this remains valid up to α ≤ ∼0.7 and for small values of three-dimensional (3D) couplings J between dimer layers. At the other end, for ∼0.9 ≤ α ≤ ∞
Physical Review B, 2005
A 1/2 magnetization plateau in magnetic fields above 23 T and antiferromagnetic (AF) long-range order (AFLRO) in low fields were found in Cu2CdB2O6. Experimental results agree with quantum Monte Carlo results for an expected spin system. There are two kinds of Cu sites [Cu(1) and Cu(2)], which are located adjacent to each other. Unexpectedly, spins on the Cu(1) and Cu(2) sites are in a nearly spin-singlet state and form AFLRO, respectively, although interactions between the Cu(1) and Cu(2) spins cannot be ignored. Cu2CdB2O6 is the first material which shows such coexistence in an atomic scale.