Magnetic susceptibility, NMR, muon spin relaxation, and inelastic neutron scattering measurements show that kapellasite, Cu3Zn(OH)6Cl2, a geometrically frustrated spin-1/2 kagome antiferromagnet polymorphic with herbertsmithite, is a gapless spin liquid showing unusual dynamic short-range correlations of noncoplanar cuboc2 type which persist down to 20 mK. The Hamiltonian is determined from a fit of a high-temperature series expansion to bulk susceptibility data and possesses competing exchange interactions. The magnetic specific heat calculated from these exchange couplings is in good agreement with experiment. The temperature dependence of the magnetic structure factor and the muon relaxation rate are calculated in a Schwinger-boson approach and compared to experimental results.

We have studied the effect of nonmagnetic Zn impurities in the coupled spin ladder Bi(Cu_{1-x}Zn_{x})_{2}PO_{6} using ;{31}P NMR, muon spin resonance (microSR), and quantum Monte Carlo simulations. Our results show that the impurities induce in their vicinity antiferromagnetic polarizations, extending over a few unit cells. At low temperature, these extended moments freeze in a process which is found universal among various other spin-gapped compounds: isolated ladders, Haldane, or spin-Peierls chains. This allows us to propose a simple common framework to explain the generic low-temperature impurity-induced freezings observed in low-dimensional spin-gapped materials.

Ba3IrTi2O9 crystallizes in a hexagonal structure consisting of a layered triangular arrangement of Ir 4+ (J ef f = 1/2). Magnetic susceptibility and heat capacity data show no magnetic ordering down to 0.35 K inspite of a strong magnetic coupling as evidenced by a large Curie-Weiss temperature θCW ∼ −130 K. The magnetic heat capacity follows a power law at low temperature. Our measurements suggest that Ba3IrTi2O9 is a 5d, Ir-based (J ef f = 1/2), quantum spin liquid on a 2D triangular lattice. [2] in geometrically frustrated magnets. In such materials, incompatibility of local interactions, called frustration, leads to a strong enhancement of quantum spin fluctuations and effectively suppresses the long range magnetic ordering. As a result, the material remains paramagnetic down to very low temperature compared to the Curie-Weiss (CW) temperature θ CW . The frustration in these materials often arises from some special geometries like triangular, kagomé, pyrochlore, garnet etc.[3].

We report magnetic susceptibility (χ) and heat capacity (C p ) measurements along with ab-initio electronic structure calculations on PbCuTe 2 O 6 , a compound made up of a three dimensional 3D network of corner-shared triangular units. The presence of antiferromagnetic interactions is inferred from a Curie-Weiss temperature (θ CW ) of about −22 K from the χ(T ) data. The magnetic heat capacity C m data show a broad maximum at T max ≃ 1.15 K (i.e.T max /θ CW ≃ 0.05), which is analogous to the the observed broad maximum in the C m /T data of a hyper-Kagome system, Na 4 Ir 3 O 8 . In addition, C m data exhibit a weak kink at T * ≃ 0.87 K. While the T max is nearly unchanged, the T * is systematically suppressed in an increasing magnetic field (H) up to 80 kOe. For H ≥ 80 kOe, the C m data at low temperatures exhibit a characteristic power-law (T α ) behavior with an exponent α slightly less than 2. Hopping integrals obtained from the electronic structure calculations show the presence of strongly frustrated 3D spin interactions along with non-negligible unfrustrated couplings. Our results suggest that PbCuTe 2 O 6 is a candidate material for realizing a 3D quantum spin liquid state at high magnetic fields.

PbCuTe 2 O 6 is a rare example of a spin liquid candidate featuring a three dimensional magnetic lattice. Strong geometric frustration arises from the dominant antiferromagnetic interaction which generates a hyperkagome network of Cu 2+ ions although additional interactions enhance the magnetic lattice connectivity. Through a combination of magnetization measurements and local probe investigation by NMR and µSR down to 20 mK, we provide a robust evidence for the absence of magnetic freezing in the ground state. The local spin susceptibility probed by the NMR shift hardly deviates from the macroscopic one down to 1 K pointing to a homogeneous magnetic system with a low defect concentration. The saturation of the NMR shift and the sublinear power law temperature (T) evolution of the 1/T 1 NMR relaxation rate at low T point to a non-singlet ground state favoring a gapless fermionic description of the magnetic excitations. Below 1 K a pronounced slowing down of the spin dynamics is witnessed, which may signal a reconstruction of spinon Fermi surface. Nonetheless, the compound remains in a fluctuating spin liquid state down to the lowest temperature of the present investigation.

We present magnetic susceptibility and heat capacity data on a new S =1/2 two-leg spin ladder compound BiCu 2 PO 6 . From our susceptibility analysis, we find that the leg coupling J 1 / k B is ϳ80 K and the ratio of the rung-to-leg coupling J 2 / J 1 ϳ 0.9. We present the magnetic contribution to the heat capacity of a two-leg ladder. The spin-gap ⌬ / k B = 34 K obtained from the heat capacity agrees very well with that obtained from the magnetic susceptibility. Significant interladder coupling is suggested from the susceptibility analysis. The hopping integrals determined using the Nth order muffin-tin-orbital based downfolding method lead to ratios of various exchange couplings in agreement with our experimental data. Based on our band structure analysis, we find the interladder coupling in the bc plane J 3 to be about 0.75J 1 placing the compound presumably close to the quantum critical limit.

We investigate magnetic, thermal, and dielectric properties of SrCuTe2O6, which is isostructural to PbCuTe2O6, a recently found, Cu-based 3D frustrated magnet with a corner-sharing triangular spin network having dominant first and second nearest neighbor (nn) couplings [B. Koteswararao, et al. Phys. Rev. B 90, 035141 (2014)]. Although SrCuTe2O6 has a structurally similar spin network, but the magnetic data exhibit the characteristic features of a typical quasi-one-dimensional magnet, which mainly resulted from the magnetically dominant third nn coupling, uniform chains. The magnetic properties of this system are studied via magnetization (M), heat capacity (Cp), dielectric constant (ε'), measurements along with ab-initio band structure calculations. Magnetic susceptibility (T) data show a broad maximum at 32 K and the system orders at low temperatures TN1  5.5 K and TN2  4.5 K, respectively. The analysis of (T) data gives an intra-chain coupling, J3/kB, to be about  -42 K with non-negligible frustrated inter-chain couplings (J1/kB and J2/kB). The hopping parameters obtained from LDA band structure calculations also suggest the presence of coupled uniform chains. The observation of simultaneous anomalies in ε'(T) at TN1 and TN2 suggests the presence of magneto-dielectric effect in SrCuTe2O6. A magnetic phase diagram is also built based on M, Cp, and ε' results.

We report 35 Cl NMR, ESR, μSR, and specific-heat measurements on the S = 1 2 frustrated kagome magnet kapellasite α-Cu 3 Zn(OH) 6 Cl 2 , where a gapless spin-liquid phase is stabilized by a set of competing exchange interactions. Our measurements confirm the ferromagnetic character of the nearest-neighbor exchange interaction J 1 and give an energy scale for the competing interactions |J | ∼ 10 K. The study of the temperature-dependent ESR line shift reveals a moderate symmetric exchange anisotropy term D, with |D/J | ∼ 3%. These findings validate a posteriori the use of the J 1 -J 2 -J d Heisenberg model to describe the magnetic properties of kapellasite [Bernu et al., Phys. Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this model is the severe random depletion of the magnetic kagome lattice by 27%, due to Cu/Zn site mixing, and specifically address the effect of this disorder by 35 Cl NMR, performed on an oriented polycrystalline sample. Surprisingly, while being very sensitive to local structural deformations, our NMR measurements demonstrate that the system remains homogeneous with a unique spin susceptibility at high temperature, despite a variety of magnetic environments. Unconventional spin dynamics is further revealed by NMR and μSR in the low-T , correlated, spin-liquid regime, where a broad distribution of spin-lattice relaxation times is observed. We ascribe this to the presence of local low-energy modes.