Panchanana Khuntia

The structure of K2Ni2(MoO4)3 consists of S=1 tetramers formed by Ni2+ ions. The magnetic susceptibility χ(T) and specific heat CP(T) data on a single crystal show a broad maximum due to the low dimensionality of the system with short-range spin correlations. A sharp peak is seen in χ(T) and CP(T) at about 1.13 K, well below the broad maximum. This is an indication of magnetic long-range order, i.e., the absence of spin gap in the ground state. Interestingly, the application of a small magnetic field (H>0.1 T) induces magnetic behavior akin to the Bose-Einstein condensation (BEC) of triplon excitations observed in some spin-gap materials. Our results demonstrate that the temperature-field (T-H) phase boundary follows a power law (T-TN) 1/α with the exponent 1/α close to 23, as predicted for the BEC scenario. The observation of BEC of triplon excitations in small H infers that K2Ni2(MoO4)3 is located in the proximity of a quantum critical point, which separates the magnetically ordered and spin-gap regions of the phase diagram.

Dimerized quantum magnets provide a unique possibility to investigate the Bose-Einstein condensation of magnetic excitations in crystalline systems at low temperature. Here, we model the low-temperature magnetic properties of the recently synthesized spin S=1 dimer system K2Ni(MoO4)2 and propose it as a candidate material for triplon and quintuplon condensation. Based on a first-principles analysis of its electronic structure, we derive an effective spin dimer model that we first solve within a mean-field approximation to refine its parameters in comparison to experiment. Finally, the model is solved by employing a numerically exact quantum Monte Carlo technique which leads to magnetic properties in good agreement with experimental magnetization and thermodynamic results. We discuss the emergent spin model of K2Ni(MoO4)2 in view of the condensation of magnetic excitations in a broad parameter regime. Finally, we comment on a geometrical peculiarity of the proposed model and discuss how it could host a supersolid phase upon structural distortions.

Competing magnetic interactions in low-dimensional quantum magnets can lead to the exotic ground state with fractionalized excitations. Herein, we present our results on an S=52 quasi-one-dimensional spin system Bi3FeMo2O12. The structure of Bi3FeMo2O12 consists of very well separated, infinite zigzag S=52 spin chains. The observation of a broad maximum around 10 K in the magnetic susceptibility χ(T) suggests the presence of short-range spin correlations. χ(T) data do not fit the S=52 uniform spin chain model due to the presence of second-nearest-neighbor coupling (J2) along with the first-nearest-neighbor coupling (J1) of the zigzag chain. The electronic structure calculations infer that the value of J1 is comparable with J2 (J2/J1≈1.1) with a negligible interchain interaction (J′/J≈0.01) implying that Bi3FeMo2O12 is a highly frustrated triangular chain system. The absence of magnetic long-range ordering down to 0.2 K is seen in the heat-capacity data, despite a relatively large antiferromagnetic Curie-Weiss temperature θCW≈-40 K. The magnetic heat capacity follows nearly a linear behavior at low temperatures indicating that the S=52 anisotropic triangular chain exhibits the gapless excitations.

Spin liquids are exotic phases of quantum matter that challenge Landau’s paradigm of symmetry-breaking phase transitions. Despite strong exchange interactions, spins do not order or freeze down to zero temperature. Although well established for one-dimensional quantum antiferromagnets, in higher dimensions where quantum fluctuations are less acute, realizing and understanding such states is a major issue, both theoretically and experimentally. In this regard, the simplest nearest-neighbour Heisenberg antiferromagnet Hamiltonian on the highly frustrated kagome lattice has proven to be a fascinating and inspiring model. The exact nature of its ground state remains elusive and the existence of a spin-gap is the first key issue to be addressed to discriminate between the various classes of proposed spin liquids. Here, through low-temperature NMR contrast experiments on high-quality single crystals, we single out the kagome susceptibility and the corresponding dynamics in the kagome archetype, the mineral herbertsmithite, ZnCu3(OH)6Cl2. We firmly conclude that this material does not harbour any spin-gap, which restores a convergence with recent numerical results promoting a gapless Dirac spin liquid as the ground state of the Heisenberg kagome antiferromagnet.

Frustrated magnets based on oxide double perovskites offer a viable ground wherein competing magnetic interactions, macroscopic ground state degeneracy and complex interplay between emergent degrees of freedom can lead to correlated quantum phenomena with exotic excitations highly relevant for potential technological applications. By local-probe muon spin relaxation (μSR) and complementary thermodynamic measurements accompanied by first-principles calculations, we here demonstrate novel electronic structure and magnetic phases of Ba2MnTeO6, where Mn2 + ions with S = 5/2 spins constitute a perfect triangular lattice. Magnetization results evidence the presence of strong antiferromagnetic interactions between Mn2 + spins and a phase transition at TN = 20 K. Below TN, the specific heat data show antiferromagnetic magnon excitations with a gap of 1.4 K, which is due to magnetic anisotropy. μSR reveals the presence of static internal fields in the ordered state and short-range spin correlations high above TN. It further unveils critical slowing-down of spin dynamics at TN and the persistence of spin dynamics even in the magnetically ordered state. Theoretical studies infer that Heisenberg interactions govern the inter- and intra-layer spin-frustration in this compound. Our results establish that the combined effect of a weak third-nearest-neighbour ferromagnetic inter-layer interaction (owing to double-exchange) and intra-layer interactions stabilizes a three-dimensional magnetic ordering in this frustrated magnet.

The subtle interplay between low-dimensionality and spin correlations can lead to exotic ground states with unconventional excitations in two-dimensional honeycomb-lattice-based quantum magnets. Herein, we present the structural, magnetic, and heat capacity measurements; density functional theory + Hubbard U (DFT+U) based electronic structure calculations; and quantum Monte Carlo simulations for NaCuIn(PO4)2. The structure of NaCuIn(PO4)2 consists of a well-separated, S=12 distorted J1-J2 honeycomb layer which is a combination of the magnetic couplings J1 (forming spin dimers) and J2 (constituting spin chains). At high temperatures, the magnetic susceptibility χ(T) follows paramagnetic behavior with a Curie-Weiss temperature θCW≈-16 K, implying the presence of antiferromagnetic interactions. A broad maximum is observed at about 13K in χ(T), indicating the presence of short-range spin correlations. The quantum Monte Carlo simulations using the S=12J1-J2 Heisenberg model on a distorted honeycomb lattice are in good agreement with the measured magnetic susceptibility data. The obtained ratio of the exchange couplings (J2J1) is 2.63, which is consistent with the value obtained from our DFT+U calculations. The title material undergoes a magnetic long-range order at 0.4 K in the heat capacity, which is suppressed with an applied magnetic field of 10 kOe. The magnetic heat capacity data follow a linear temperature-dependent behavior well above the transition temperature, suggesting the presence of gapless excitations. The observed behavior can be attributed to the presence of low connectivity and weak magnetic frustration in this two-dimensional distorted honeycomb lattice.

Spin correlation, competing interactions and subtle interplay between various degrees of freedom can lead to novel quantum states in correlated electron systems. Insights into the intrinsic magnetic susceptibility and the origin of complex magnetic ordering on a microscopic scale set an attractive setting in correlated quantum matter. In this context, Nuclear Magnetic Resonance (NMR) probe offers an unprecedented approach to explore the underlying physical mechanism of correlated quantum phenomena in condensed matter physics. NMR is a site selective microscopic technique extremely sensitive to low energy spin dynamics defining the ground state properties and is useful in mapping the tomography of correlated electron materials. NMR is a powerful microscopic probe to extract the intrinsic magnetic susceptibility and to probe topological order and elucidate the nature of enigmatic elementary excitations in correlated quantum matter. NMR shares an interface with muon spin relaxation (µSR) and neutron scattering and the combination of these techniques on a complementary scale provides an outstanding track to shed insights into the dynamics of dressed quasi-particles in the ground state of correlated electron materials. In this short review, we focus on some of the emergent physical phenomena in a few correlated electron materials sharing common experimental signatures in magnetization, specific heat, and NMR spin susceptibility and spin lattice relaxation rates.

Quantum fluctuations enhanced by frustration and subtle interplay between competing degrees of freedom offer an ideal ground to realize novel states with fractional quantum numbers in quantum materials that defy standard theoretical paradigms. Quantum spin liquid (QSL) is a highly entangled state wherein frustration-induced strong quantum fluctuations preclude symmetry-breaking phase transitions down to zero temperature without any order parameter. Experimental realizations of QSL in quantum materials with spin dimensionality greater than one is very rare. Here, we present our thermodynamic, nuclear magnetic resonance, muon spin relaxation, and inelastic neutron scattering studies of a rare-earth hyperkagome compound Li3Yb3Te2O12 in which Yb3+ ions constitute a three-dimensional spin lattice without any detectable disorder. Our comprehensive experiments evince neither signature of magnetic ordering nor spin freezing down to 38 mK that suggest the realization of dynamic liquid-like ground state in this antiferromagnet. The ground state of this material is interpreted by a low energy Jeff=1/2 degrees of freedom with short-range spin correlations. The present results demonstrate a viable basis to explore spin-orbit driven enigmatic correlated quantum states in a class of rare-earth-based three-dimensional frustrated magnets that may open avenues in theoretical and experimental search for spin liquids.

We report magnetization, heat capacity, Li7 nuclear magnetic resonance (NMR), and muon-spin rotation (μSR) measurements on the honeycomb 4d5 spin liquid candidate Li2RhO3. The magnetization in small magnetic fields provides evidence of the partial spin-freezing of a small fraction of Rh4+ moments at 6 K, whereas the Curie-Weiss behavior above 100 K suggests a pseudo-spin-1/2 paramagnet with a moment of about 2.2μB. The magnetic specific heat (Cm) exhibits no field dependence and demonstrates the absence of long-range magnetic order down to 0.35 K. Cm/T passes through a broad maximum at about 10 K and CmT2 at low temperatures. Measurements of the spin-lattice relaxation rate (1/T1) reveal a gapless slowing-down of spin fluctuations upon cooling with 1/T1∼T2.2. The results from NMR and μSR are consistent with a scenario in which a minority of Rh4+ moments are in a short-range correlated frozen state and coexist with a majority of moments in a liquid-like state that continue to fluctuate at low temperatures.

Double-perovskite-based magnets wherein frustration and competition between emergent degrees of freedom are at play can lead to novel electronic and magnetic phenomena. In this paper, we report the electronic structure and magnetic properties of an ordered double perovskite material, Ho2CoMnO6. In the double perovskites with general class A2BB′O6 (A = rare-earth ions; B, B′ = transition metal ions), the octahedral B and B′ sites have a distinct crystallographic site. The Rietveld refinement of x-ray diffraction data reveals that Ho2CoMnO6 crystallizes in the monoclinic P21/n space group. X-ray photoelectron spectroscopy confirms the charge state of cations present in this material. The temperature dependence of magnetization and specific heat exhibits a long-range ferromagnetic ordering at Tc∼76 K owing to superexchange interaction between Co2+ and Mn4+ moments. Furthermore, the magnetization isotherm at 5 K shows a hysteresis curve that confirms the ferromagnetic behavior of this double perovskite. We observed a reentrant glassy state in the intermediate-temperature regime, which is attributed to inherent antisite disorder and competing interactions. A large magnetocaloric effect has been observed much below the ferromagnetic transition temperature. Temperature-dependent Raman spectroscopy studies support the presence of spin-phonon coupling and short-range order above Tc in this double perovskite. The stabilization of magnetic ordering and charge states is further analyzed through electronic structure calculations. The latter also infer the compound to be a narrow-band-gap insulator with the gap arising between the lower and upper Hubbard Co d subbands. Our results demonstrate that antisite disorder and complex 3d-4f exchange interactions in the spin lattice account for the observed electronic and magnetic properties in this promising double perovskite material.