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.
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 the structural transformation of hexagonal Ba 3 YIr 2 O 9 to a cubic double perovskite form (stable in ambient conditions) under an applied pressure of 8 GPa at 1273 K. While the ambient pressure synthesized sample undergoes long-range magnetic ordering at ∼4 K, the high-pressure (HP) synthesized sample does not order down to 2 K as evidenced from our susceptibility, heat capacity, and nuclear magnetic resonance (NMR) measurements. Further, for the HP sample, our heat capacity data have the form γ T + βT 3 in the temperature (T ) range of 2-10 K with the Sommerfeld coefficient γ = 10 mJ/mol-Ir K 2 . The 89 Y NMR shift has no T dependence in the range of 4-120 K and its spin-lattice relaxation rate varies linearly with T in the range of 8-45 K (above which it is T independent). Resistance measurements of both the samples confirm that they are semiconducting. Our data provide evidence for the formation of a 5d-based, gapless, quantum spin-liquid in the cubic (HP) phase of Ba 3 YIr 2 O 9 . In this picture, the γ T term in the heat capacity and the linear variation of 89 Y 1/T 1 arises from excitations out of a spinon Fermi surface. Our findings lend credence to the theoretical suggestion [Chen, Pereira, and Balents, Phys. Rev. B 82, 174440 (2010) 9 It will be interesting to explore other iridates having a triangular lattice. Ba 3 YIr 2 O 9 has a similar chemical formula like Ba 3 IrTi 2 O 9 and it crystallizes in the hexagonal structure (P6 3 /mmc) with Ir-Ir structural dimers arranged in an edge-shared triangular fashion.10 Since all the Ir are equivalent, they should have a fractional oxidation state of +4.5 in a simple ionic picture. Our investigation of this 5d-based system is motivated by the fact that the fractional valence coupled with a geometrically frustrated lattice might lead to a spin-liquid state or possibly a heavy fermion state as in the 3d-based LiV 2 O 4 .11 This however did not turn out to be the case. Whereas we confirmed the onset of long-range order below 4 K in Ba 3 YIr 2 O 9 (in agreement with Ref. 10), we succeeded in suppressing the magnetic order with the application of pressure. In fact, when Ba 3 YIr 2 O 9 was subjected to a pressure of 8 GPa at 1273 K, it transformed to a cubic double perovskite structure as evidenced from x-ray diffraction under ambient conditions. Though the high-pressure (HP) synthesized sample remains insulating (based on our resistivity measurements), it has a metal-like linear heat capacity coefficient γ = 10 mJ/mol-Ir K 2 . Further, the 89 Y nuclear magnetic resonance (NMR) shift is found to be independent of temperature (T ) below 120 K and the 89 Y NMR spin-lattice relaxation rate crosses over from T -independent behavior at high temperature to a linear T dependence below about 45 K. These results point to the existence of low-energy excitations at low temperatures. In the absence of metallic behavior in the resistivity and the presence of local moments, the low-T data suggest the formation of an exotic ground state for the HP phase, possibly a gaple...
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