Session F56: Quantum Spin Liquid Materials I: Geometric Frustration

Quantum spin liquid behavior and ferroelectricity in PbCuTe2O6

The quantum spin liquid is a highly entangled magnetic state characterized by the absence of static magnetism in its ground state, instead, the spins fluctuate in a highly correlated way down to the lowest temperatures. Three-dimensional spin liquids are rare and have mostly been sought in materials where the magnetic ions form pyrochlore or hyperkagome lattices. Here we study the compound PbCuTe₂O₆ which is a rare example of the highly frustrated, three-dimensional defect windmill or hyper-hyperkagome lattice. Using a combination of experiment and theory we show that this system exhibits features consistent with being a quantum spin liquid with no detectable static magnetism together with the presence of diffuse continua in the magnetic spectrum suggestive of fractional spinon excitations. More recently ferroelectric order was discovered in single crystal samples below T_FE≈1K, which is suppressed by small grain size in powder samples. This transition is accompanied by a lattice distortion and a modified magnetic response which however is still consistent with a quantum spin liquid ground state.   

Local Magnetic Susceptibility Measurements of Thin Films of Spin Liquid Candidate TbInO3

Spin liquids are characterized by the presence of magnetic fluctuations and the absence of long-range order, even at the lowest temperatures where magnetic interactions typically lead to spin ordering or freezing. Recently, TbInO­₃ has been reported as a candidate to realize a highly frustrated spin liquid^[1],[2]. A transition to a magnetically ordered state typically results in distinct features in the temperature and frequency dependence of the magnetic susceptibility. Conversely, the absence of such features in the magnetic susceptibility suggests that no magnetic order is present. Previous measurements^[1],[2] of the magnetic susceptibility in bulk TbInO­₃ crystals report a lack of magnetic ordering down to 450mK. In this work, we use scanning superconducting quantum interference device (SQUID) susceptometry as a local probe to measure the temperature dependent magnetic susceptibility of TbInO­₃ thin films grown by molecular beam epitaxy (MBE) down to 40mK.

[1] Nat. Phys. 15, 262–268 (2019).

[2] Phys. Rev. X. 9, 031005 (2019).   

Magnetic field induced Quantum Criticalities in the triangular antiferromagnet KYbSe2

Quantum spin liquid (QSL) is a state of matter characterized by fractionalized quasiparticle excitations, quantum entanglement, and a lack of long-range magnetic order. QSL so far has evaded a definitive experimental observation. There have been several Yb³⁺ based materials (S = ½) suggested as QSL candidates, but thus far, their studies have not provided unambiguous proof of a QSL. Here we investigate the Yb³⁺ delafossite material KYbSe₂, a promising QSL candidate, via specific heat and magnetocaloric measurements of a single crystal of KYbSe₂ down to 30 mK. The heat capacity C shows an anomaly at T_N = 0.29 K in zero field, consistent with magnetic ordering demonstrated at this temperature by neutron scattering studies. Analysis of the low-temperature specific heat at 0 T indicates the presence of a T-linear component of C/T in the proximate QSL region above T_N suggested by neutron scattering experiments at low temperature. When magnetic field is applied along the a-axis, both T-linear and T-logarithmic behavior at higher temperature are suppressed, and we observe multiple magnetically ordered phases in the H-T phase diagram.

    

High Field Magnetization of NaYbxLu1-xSe2: Tracking the Evolution from Single Ion Effects to Many-Body Correlations

NaYbSe₂ belongs to a recently discovered class of delafossites, a number of which exhibit the hallmarks typical of a quantum spin liquid. These include the absence of long-range magnetic order, persistent spin fluctuations [Bordelon et al., Nature Physics (2019)], and a large anomalous heat capacity [Ranjith et al., PRB (2019)], all at the lowest measurable temperatures. In a previous study, we were successful in growing the doping series NaYb_xLu_1-xSe₂, substituting Yb³⁺ for non-magnetic Lu³⁺. This allowed us to demonstrate the evolution of magnetic interactions from single ion effects at low Yb doping to highly-correlated many-body entanglement in NaYbSe₂ [Pritchard Cairns et al., PRB (2022)]. Seeking a more quantitative analysis, we have performed pulsed field magnetization measurements in fields of up to 60 T. By modeling the results obtained for different dopings and temperatures, we are able to shed light on the correlations in the full magnetic compound and on this broader class of quantum spin liquid candidate materials.   

Anisotropic magnetic response of BaCo2(AsO4)2

Quantum spin liquid (QSL) is an exotic state of matter predicted to refrain from forming long-range magnetic order down to absolute zero temperature. However, the non-Kitaev interactions in the explored candidate materials lead to the presence of an unconventional magnetically ordered state, which can be suppressed by applying an external magnetic field to push the system towards a QSL state. Recently, theoretical studies suggested that Co-based compounds, such as BaCo₂(AsO₄)₂, have a high potential to be Kitaev QSLs. The required field needed to suppress the magnetic order in BaCo₂(AsO₄)₂ is about 0.5T for the in-plane fields. Above this field, the magnetic state that emerges is not well understood, as is also the case for RuCl₃.

 

Our main objective is to investigate the magnetic anisotropy of BaCo₂(AsO₄)₂ to clarify the magnetic phases both within the AFM phase, as well as nearby. By vibrating small single crystals of BaCo₂(AsO₄)₂ in a magnetic field, we study the rotational robustness of its magnetism- magnetotropic coefficient [1,2]. Our preliminary data illustrates the field-angle variation of the AFM suppression field, as well as the angle dependence of the low-field spin textures. Studies beyond the AFM phase will be compared to similar measurements in RuCl₃ to determine the likelihood that BaCo₂(AsO₄)₂ hosts a QSL.

    

Magnetism in a Disordered Co-honeycomb

The search for magnetic materials displaying physics associated with Kitaev’s exactly solvable S=1/2 honeycomb lattice model is a focal point of contemporary research due to the model’s unique quantum spin liquid ground state and application towards fault tolerant quantum computing. Initial candidate materials focused on 4d/5d Mott insulators, but a more recent extension to 3d materials has led to a large body of work focused mostly on high-spin Co²⁺ systems realizing a Kramer’s doublet j_eff = 1/2 ground state. One promising candidate material is Na₂Co₂TeO₆. While the nature of the magnetism in Na₂Co₂TeO₆ remains an open topic, the onset of zig-zag magnetic order below T_N = 27 K clearly indicates non-Kitaev terms present in the low-energy Hamiltonian. In the Kitaev candidate material α-RuCl₃, substituting nonmagnetic Ir³⁺ for Ru³⁺ suppressed conventional long-range magnetic order while keeping the signatures of fractionalized excitations, suggesting magnetic dilution as a promising route.  Here we explore substitution of nonmagnetic ions for Co²⁺.  We present magnetometry and specific heat measurements on synthesized powders of disordered Na₂Co_2-xR_xTeO₆ (R = Mg, Zn) and discuss the effect of magnetic dilution on the complex magnetism found in the parent compound.

    

Thermodynamics and neutron scattering studies of on a new Tm-based triangular lattice system

Motivated by the recent discovery of the quantum spin liquid (QSL) candidate material YbMgGaO4 and possible Kosterlitz-Thouless (KT) phase in TmMgGaO4, we successfully synthesized the Tm-based triangular lattice sister compound. The key characteristic of this compound is that compared to the previously proposed QSL candidates, it has a more two-dimensional magnetic structure in which the role of chemical disorder is minimized. Our thermodynamics measurements measured down to 50 mK rule out any signature of long-range magnetic ordering in this system. Furthermore, we employed neutron scattering techniques to probe the static and dynamic magnetic properties of this system.   

Experimental Study of the Dipolar-Octupolar Pyrochlore Quantum Spin Liquid Candidate Ce2Hf2O7

We present new experimental results on powder and single crystal samples of the new magnetic pyrochlore Ce₂Hf₂O₇. High-energy neutron spectroscopy confirms a dipole-octupole crystal electric field ground state for this rare-earth pyrochlore magnet, consistent with previous measurements and corresponding to two components of pseudospin-1/2 transforming as dipoles and the other component transforming as an octupole. Higher energy-resolution neutron spectroscopy and neutron diffraction show a disordered ground state, which is also consistent with magnetic susceptibility and heat capacity measurements.

    

Spiral spin liquid on a honeycomb lattice

The spiral spin liquid state initially proposed by Bergman D. et al. in Nat. Phys. 3, 487 (2007) has become a new paradigm to achieve exotic spin correlations through competing interactions instead of the conventional geometrical frustration. In the spiral spin liquid state, spins fluctuate collectively as spirals, and their propagation vectors form a continuous ring or surface in reciprocal space. Due to this enormous degeneracy, spiral spin liquids present a novel route to realize quantum spin liquids and topological spin textures. Experimental realization of spiral spin liquids, however, is challenging as the competition between interactions is often too weak in real materials. Until now, the diamond-lattice compound MnSc₂S₄ studied in Nat. Phys. 13, 157 (2017) has remained as the only host of a spiral spin liquid. According to theoretical calculations, spiral spin liquids should be generally existent on the bipartite lattices, which include the prototype bipartite honeycomb lattice. Realizing the spiral spin liquid state on the honeycomb lattice is highly desirable as the reduced dimension may enhance quantum fluctuation in favor of the quantum spin liquid state and allow more convenient manipulation of the topological textures for spintronics applications.  We present evidence for the existence of the spiral spin liquid state in the honeycomb magnet FeCl₃.  Using diffuse neutron scattering, we observed a continuous ring of scattering in reciprocal space, which provides evidence for the existence of a spiral spin liquid state. The spiral correlations can be ascribed to the competition between the exchange interactions within the honeycomb layer, which is further corroborated through inelastic neutron scattering in the long-range ordered phase.

    

Magnetic properties of layered CsNdSe2 with triangular lattice

The family of rare-earth triangular lattice materials ARCh₂ (A: alkali metal, Cu, Ag; R: rare earth, Ch: chalcogen) attracts great attention due to its rich magnetism. We have successfully synthesized CsNdSe₂ single crystals by the salt flux. The crystal structure consists of perfect Nd triangular lattice without distortions. Its AC and DC magnetic susceptibility has been measured with a field applied along different directions down to 0.05 K. By fitting the magnetic susceptibility from 200 K to 300 K using the Curie-Weiss law, we obtain Curie-Weiss temperature -4.5 K and -88 K, revealing weak antiferromagnetic interactions in CsNdSe₂. The AC susceptibility at zero DC field shows a downturn below 0.5 K, suggesting short-range AFM correlations. The absence of long-range magnetic ordering suggests possible spin liquid formation in CsNdSe₂.

    

Neutron Scattering studies of a new Yb-based triangular lattice compound

In two-dimensional magnetic systems, the triangular lattice has long been an interesting topic due to its potential to host exotic ground states such as quantum spin liquid (QSL). However, most of the real-world compounds of this class process chemical disorders which makes the interpretation of the ground state difficult. We have grown the first single crystal of a new Yb-based triangular lattice compound without the chemical disorder. In this talk, we are going to present our latest results on this new compound including thermodynamics and neutron scattering measurements.