Quantum Spin Liquid Candidate Research Articles

Pressure-induced insulator-to-metal transition in the quantum spin liquid candidate lithium ytterbium diselenide

Pressure-induced insulator-to-metal transition in the quantum spin liquid candidate lithium ytterbium diselenide

Ba6RE2Ti4O17 (RE = Nd, Sm, Gd, Dy-Yb): A Family of Rare-Earth-Based Layered Triangular Lattice Magnets.

The exploration of new rare-earth (RE)-based triangular-lattice materials plays a significant role in motivating the discovery of exotic magnetic states. Herein, we report a family of hexagonal perovskite compounds Ba6RE2Ti4O17 (RE = Nd, Sm, Gd, Dy-Yb) with a space group of P63/mmc, where magnetic RE3+ ions are distributed on the parallel triangular-lattice layers within the ab-plane and stacked in an 'AA'-type fashion along the c-axis. The low-temperature magnetic characterizations indicate that all synthesized Ba6RE2Ti4O17 compounds exhibit dominant antiferromagnetic (AFM) interactions and the absence of magnetic order down to 1.8 K. The isothermal magnetization and electron spin resonance results reveal the distinct magnetic anisotropy for the compounds with different RE ions. Moreover, the as-grown Ba6Nd2Ti4O17 single crystals exhibit Ising-like magnetic anisotropy with a magnetic easy-axis perpendicular to the triangle-lattice plane and no long-range magnetic order down to 80 mK, as the quantum spin liquid candidate with dominant Ising-type interactions.

Synthesis, structural and magnetic characterizations of Li4Cu1-xNixTeO6 ( [formula omitted] = 0, 0.1, 0.2, 0.5, and 1)

We investigated the effect of Ni doping in a recently proposed quantum spin liquid (QSL) candidate Li4CuTeO6. We performed a comprehensive study on the structural and magnetic properties. We find that the anti-site disorder between Li+ and Cu2+ persists until 50% Ni doping in which Ni and Cu occupy different crystallographic sites. As a result, while Cu sits in both triangular and honeycomb layers in Li4CuTeO6, Ni forms only honeycomb layer in Li4NiTeO6 and Li4Cu0.5Ni0.5TeO6. Our magnetic susceptibility measurements show that the Weiss temperature decreases from -145.68 K in Li4CuTeO6 to -6.15 K in Li4NiTeO6 as Ni doping increases, and find no hint of magnetic ordering or freezing down to 1.8 K. Our analysis implies the existence of abundant low energy excitations in these materials.

Electronic, optical, and thermodynamic properties of a quantum spin liquid candidate NaRuO2: Ab-initio investigation

Quantum spin liquids (QSLs), known for their competing interactions that prevent conventional ordering, exhibit emergent phenomena and exotic properties resulting from quantum correlations. Despite these recent advancements of QSLs, a significant portion of the optical and thermodynamic properties in the Kagome lattice remains unknown. In addition, the thermodynamic phenomenology of NaRuO2 bears resemblance that of highly frustrated magnets. Here, we employed ab-initio calculations to explore the electronic, optical and thermodynamic properties of NaRuO2, a new QSL candidate. NaRuO2 was identified as a semiconductor with a small bandgap energy of 0.69 eV. Our results reveal a huge anisotropic optical properties, in which distinct refractive index within the ab-plane indicating an impressive birefringent character of the NaRuO2 system, and a significant enhancement of the optical absorption coefficient and optical conductivity in the in-plane with respect the c-axis. The investigation also examines the electronic anisotropy of the gap energy, by applying strain the gap energy displays significant variations in the ab-plane compared to the out-of-plane direction. Conversely, calculations of the thermodynamic properties reveal a low thermal conductivity (2.5–0.5 W m−1. K−1) and specific heat, which suggests the existence of strong interactions among the NaRuO2 quantum spins. The calculated linear specific heat behavior in NaRuO2 suggests the fractionalization of electrons and the presence of a spinons Fermi surface. These findings hold promising potential for future quantum applications.

Robust Spin Liquidity in 2D Metal-Organic Framework Cu3 (HHTP)2 with S=1 /2 Kagome Lattice.

On one hand electron or hole doping of quantum spin liquid (QSL) may unlock high-temperature superconductivity and on the other hand it can disrupt the spin liquidity, giving rise to a magnetically ordered ground state. Recently, a 2D MOF, Cu3 (HHTP)2 (HHTP - 2,3,6,7,10,11-hexahydroxytriphenylene), containing Cu(II) S= frustrated spins in the Kagome lattice is emerging as a promising QSL candidate. Herein, we present an elegant in situ redox-chemistry strategy of anchoring Cu3 (HHTP)2 crystallites onto diamagnetic reduced graphene oxide (rGO) sheets, resulting in the formation of electron-doped Cu3 (HHTP)2 -rGO composite which exhibited a characteristic semiconducting behavior (5 K to 300 K) with high electrical conductivity of 70 S ⋅ m-1 and a carrier density of ~1.1×1018 cm-3 at 300 K. Remarkably, no magnetic transition in the Cu3 (HHTP)2 -rGO composite was observed down to 1.5 K endorsing the robust spin liquidity of the 2D MOF Cu3 (HHTP)2 . Specific heat capacity measurements led to the estimation of the residual entropy values of 28 % and 34 % of the theoretically expected value for the pristine Cu3 (HHTP)2 and Cu3 (HHTP)2 -rGO composite, establishing the presence of strong quantum fluctuations down to 1.5 K (two times smaller than the value of the exchange interaction J).

Angular-Momentum Transfer Mediated by a Vibronic-Bound-State.

The notion that phonons can carry pseudo-angular momentum has many major consequences, including topologically protected phonon chirality, Berry curvature of phonon band structure, and the phonon Hall effect. When a phonon is resonantly coupled to an orbital state split by its crystal field environment, a so-called vibronic bound state forms. Here, a vibronic bound state is observed in NaYbSe2 , a quantum spin liquid candidate. In addition, field and polarization dependent Raman microscopy is used to probe an angular momentum transfer of ΔJz = ±ℏ between phonons and the crystalline electric field mediated by the vibronic bound stat. This angular momentum transfer between electronic and lattice subsystems provides new pathways for selective optical addressability of phononic angular momentum via electronic ancillary states.

Ba9RE2(SiO4)6 (RE = Ho-Yb): A Family of Rare-Earth-Based Honeycomb-Lattice Magnets.

Rare-earth (RE)-based honeycomb-lattice materials with strong spin-orbit coupled Jeff = 1/2 moments have attracted great interest as a platform to realize the Kitaev quantum spin liquid (QSL) state. Herein, we report the discovery of a family of RE-based honeycomb-lattice magnets Ba9RE2(SiO4)6 (RE = Ho-Yb), which crystallize into the rhombohedral structure with the space group R3̅. In these serial compounds, magnetic RE3+ ions are arranged on a perfect honeycomb lattice within the ab-plane and stacked in the "ABCABC"-type fashion along the c-axis. All synthesized Ba9RE2(SiO4)6 (RE = Ho-Yb) polycrystals exhibit the dominant antiferromagnetic interaction and absence of magnetic order down to 2 K. In combination with the magnetization and electron spin resonance results, magnetic behaviors are discussed for the compounds with different RE ions. Moreover, the as-grown Ba9Yb2(SiO4)6 single crystals show large magnetic frustration with frustration index f = θCW/TN > 8 and no long-range magnetic ordering down to 0.15 K, being a possible QSL candidate material. These series of compounds are attractive for exploring the exotic magnetic phases of Kitaev materials with 4f electrons.

Fluctuating magnetic droplets immersed in a sea of quantum spin liquid

The search of quantum spin liquid (QSL), an exotic magnetic state with strongly fluctuating and highly entangled spins down to zero temperature, is a main theme in current condensed matter physics. However, there is no smoking gun evidence for deconfined spinons in any QSL candidate so far. The disorders and competing exchange interactions may prevent the formation of an ideal QSL state on frustrated spin lattices. Here we report comprehensive and systematic measurements of the magnetic susceptibility, ultralow-temperature specific heat, muon spin relaxation (μSR), nuclear magnetic resonance (NMR), and thermal conductivity for NaYbSe2 single crystals, in which Yb3+ ions with effective spin-1/2 form a perfect triangular lattice. All these complementary techniques find no evidence of long-range magnetic order down to their respective base temperatures. Instead, specific heat, μSR, and NMR measurements suggest the coexistence of quasi-static and dynamic spins in NaYbSe2. The scattering from these quasi-static spins may cause the absence of magnetic thermal conductivity. Thus, we propose a scenario of fluctuating ferrimagnetic droplets immersed in a sea of QSL. This may be quite common on the way pursuing an ideal QSL, and provides a brand new platform to study how a QSL state survives impurities and coexists with other magnetically ordered states.

Combinatorial exploration of quantum spin liquid candidates in the herbertsmithite material family

Geometric frustration of magnetic ions can lead to a quantum spin liquid ground state where long range magnetic order is avoided despite strong exchange interactions. The physical realization of quantum spin liquids comprises a major unresolved area of contemporary materials science. One prominent magnetically-frustrated structure is the kagome lattice. The naturally occurring minerals herbertsmithite [ZnCu$_3$(OH)$_6$Cl$_2$] and Zn-substituted barlowite [ZnCu$_3$(OH)$_6$BrF] both feature perfect kagome layers of spin-$1/2$ copper ions and display experimental signatures consistent with a quantum spin liquid state at low temperatures. To investigate other possible candidates within this material family, we perform a systematic first-principles combinatorial exploration of structurally related compounds [$A$Cu$_3$(OH)$_6B_2$ and $A$Cu$_3$(OH)$_6BC$] by substituting non-magnetic divalent cations ($A$) and halide anions ($B$, $C$). After optimizing such structures using density functional theory, we compare various structural and thermodynamic parameters to determine which compounds are most likely to favor a quantum spin liquid state. Convex hull calculations using binary compounds are performed to determine feasibility of synthesis. We also estimate the likelihood of interlayer substitutional disorder and spontaneous distortions of the kagome layers. After considering all of these factors as a whole, we select several promising candidate materials that we believe deserve further attention.

A three-dimensional uranium-fluorine spin-frustrated CaB6-type lattice as potential conductive and quantum spin liquid candidate

A three-dimensional uranium-fluorine spin-frustrated CaB6-type lattice as potential conductive and quantum spin liquid candidate

Tunneling spectroscopic signatures of charge doping and associated Mott transition in α−RuCl3 in proximity to graphite

The layered Mott insulator $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ has been extensively studied as a potential Kitaev quantum spin liquid candidate. Here, by constructing heterostructures with graphite, we employed electron tunneling measurements on few-layer $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ using scanning tunneling microscopy/spectroscopy. Characteristic tunneling spectra were detected on $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ layers in proximity to graphite. In the single-layer $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ in direct contact with graphite, distinct states in the Mott gap regime were observed. The in-gap states are demonstrated to be closely related to the electron orbitals in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ and graphite, and to be sensitive to interfacial coupling, where a hybridization at the heterointerface is hypothesized. The in-gap states are also thought of as a charge reservoir for weakly doping the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ upper layers. It demonstrated that the weak doping effect causes a considerable decrease in the Mott gap within the upper layers, suggesting that an unconventional Mott transition is occurring in these layers. The results show that the heterostructure comprising $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ and graphite is a good platform for investigating the doping physics in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$. Therefore, tunneling into such a doped system is a useful probe for studying otherwise insulating spin liquid candidates.

Spectral function of the J1−J2 Heisenberg model on the triangular lattice

Triangular lattice compounds have received extensive attention recently as quantum spin liquid candidates. Here, the authors look at the ${J}_{1}$-${J}_{2}$ Heisenberg model on the triangular lattice, and calculate the spectral function relevant for neutron scattering experiments. They find a high-energy spectrum consistent with reports of a spinon Fermi surface state in triangular lattice compounds, although the low-energy spectrum rules out such a phase, and points towards a U(1) Dirac spin liquid. They provide clear signatures to look for in future neutron scattering experiments to distinguish these two phases.

Pressure induced insulator to metal transition in quantum spin liquid candidate NaYbS2

Pressure induced insulator to metal transition followed by the appearance of superconductivity has been observed recently in inorganic quantum spin liquid candidate NaYbSe2. In this paper, we study the properties of isostructural compound NaYbS2 under pressure. It is found that the resistance of NaYbS2 single crystal exhibits an insulating state below 82.9 GPa, but with a drop of more than six orders of magnitude at room temperature. Then a minimum of resistance is observed at about 100.1 GPa and it moves to lower temperature with further compression. Finally, a metallic state in the whole temperature range is observed at about 130.3 GPa accompanied by a non-Fermi liquid behavior below 100 K. The insulator to metal transition, non-monotonic resistance feature and non-Fermi liquid behavior of NaYbS2 under pressure are similar to those of NaYbSe2, suggesting that these phenomena might be the universal properties in NaLnCh 2 (Ln = rare earth, Ch = O, S, Se) system.

First principles investigation of screened Coulomb interaction and electronic structure of low-temperature phase TaS2

By means of ab initio computation schemes, we examine the electronic screening, Coulomb interaction strength, and the electronic structure of a quantum spin liquid candidate monolayer TaS2 in its low-temperature commensurate charge-density-wave phase. Not only local () but non-local () correlations are estimated within random phase approximation based on two different screening models. Using GW+ EDMFT (GW plus extended dynamical mean-field theory) method, we investigate the detailed electronic structure by increasing the level of non-local approximation from DMFT () to EDMFT and GW+ EDMFT.

Behavior under magnetic field of resonance at the edge of the upper Hubbard band in 1T−TaS2

Recent theoretical investigations of quantum spin liquids have described phenomenology amenable to experimental observation using scanning tunneling microscopy. This includes characteristic resonances found at the edge of the upper Hubbard band of the host Mott insulator, that under certain conditions shift into the Mott gap under external magnetic field [W.-Y. He and P. A. Lee, arXiv:2212.08767]. In light of this we report scanning tunneling microscopy observations, in samples of the quantum spin liquid candidate 1T-TaS2, of a conductance peak at the upper Hubbard band edge and its magnetic field dependent behavior. These observations potentially represent evidence for the existence of a quantum spin liquid in 1T TaS2. We also observe samples in which such field dependence is absent, but with no observed correlate for the presence or absence of field dependence. This suggests one or more material properties controlling electronic behavior that are yet to be understood, and should help to motivate renewed investigation of the microscopic degrees of freedom in play in 1T-TaS2, as well as the possible realization of a quantum spin liquid phase.

High-energy spin excitations in the quantum spin liquid candidate Zn-substituted barlowite probed by resonant inelastic x-ray scattering

A quantum spin liquid is a novel ground state that can support long-range entanglement between magnetic moments, resulting in exotic spin excitations involving fractionalized $S=\frac{1}{2}$ spinons. Here, we measure the excitations in single crystals of the spin liquid candidate Zn-barlowite using resonant inelastic x-ray scattering. By analyzing the incident polarization and temperature dependences, we deduce a clear magnetic scattering contribution forming a broad continuum that surprisingly extends up to $\ensuremath{\sim}200$ meV ($\ensuremath{\sim}14J$, where $J$ is the magnetic exchange). The excitation spectrum indicates that significant contributions may arise from multiple pairs of spinons and/or antispinons at high energies. Furthermore, the observation of similar scattering in the hexagonal barlowite parent material above its ordering temperature suggests the presence of similar high-energy excitations, irrespective of the low-energy physics.

Pressure-induced superconductivity in triangular lattice spin liquid candidate NaYbSe<sub>2</sub>

Quantum spin liquid is an exotic state without magnetic order down to zero-temperature due to spin frustration, which is closely related to high temperature superconductivity. Therefore, an important issue arises whether the quantum spin liquid can be adjusted into a superconductor, even high-<i>T</i><sub>c</sub> superconductor, by using pressure or chemical doping. Rear-earth chalcogenides NaYbCh<sub>2</sub> (Ch = O, S, Se), consisting of planar triangular lattice, exhibit no long-range magnetic order down to the lowest measured temperatures in specific heat, nuclear magnetic resonance, and neutron scattering, and are considered as a quantum spin liquid candidate. Here we investigate the electrical transport properties of NaYbCh<sub>2</sub> (Ch = O, S, Se) under high pressures. For NaYbSe<sub>2</sub>, zero-resistance behavior is observed at 26.9 GPa, showing that the superconductivity comes into being. The superconducting transition temperature (<i>T</i><sub>c</sub>) is around 5.6 K at 26.9 GPa and robust against pressure till 45 GPa. The phase diagram of <i>T</i><sub>c</sub> versus pressure for NaYbSe<sub>2</sub> is constructed. For NaYbS<sub>2</sub>, the room temperature resistance decreases from the order of 10<sup>11</sup> Ω at 10 GPa to 10 Ω at 67 GPa. However, neither superconductivity nor insulator-metal transition is observed. Additionally, the NaYbO<sub>2</sub> keeps insulating and the resistance is too large to be detected in a pressure range of 0–60 GPa.

A quantum spin liquid candidate isolated in a two-dimensional CoIIRhIII bimetallic oxalate network.

A quantum spin liquid (QSL) is an elusive state of matter characterized by the absence of long-range magnetic order, even at zero temperature, and by the presence of exotic quasiparticle excitations. In spite of their relevance for quantum communication, topological quantum computation and the understanding of strongly correlated systems, like high-temperature superconductors, the unequivocal experimental identification of materials behaving as QSLs remains challenging. Here, we present a novel 2D heterometallic oxalate complex formed by high-spin Co(ii) ions alternating with diamagnetic Rh(iii) in a honeycomb lattice. This complex meets the key requirements to become a QSL: a spin ½ ground state for Co(ii), determined by spin-orbit coupling and crystal field, a magnetically-frustrated triangular lattice due to the presence of antiferromagnetic correlations, strongly suppressed direct exchange interactions and the presence of equivalent interfering superexchange paths between Co centres. A combination of electronic paramagnetic resonance, specific heat and ac magnetic susceptibility measurements in a wide range of frequencies and temperatures shows the presence of strong antiferromagnetic correlations concomitant with no signs of magnetic ordering down to 15 mK. These results show that bimetallic oxalates are appealing QSL candidates as well as versatile systems to chemically fine tune key aspects of a QSL, like magnetic frustration and superexchange path geometries.

Phonon renormalization effects accompanying the 6 K anomaly in the quantum spin liquid candidate κ−(BEDT-TTF)2Cu2(CN)3

Inelastic neutron scattering measurements have revealed an abrupt change in the phonon damping at the 6 K anomaly in the quantum spin liquid candidate $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu${}_{2}$(CN)${}_{3}$. This is interpreted as a phase transition to the valence bond solid state on the basis of an effective model describing the spin-charge coupling in the dimer-Mott system.

Universal scaling of specific heat in the S=12 quantum kagome antiferromagnet herbertsmithite

Despite tremendous investigations, a quantum spin liquid (QSL) state realized in a spin-1/2 kagome Heisenberg antiferromagnet remains largely elusive. In herbertsmithite $\mathrm{Zn}{\mathrm{Cu}}_{3}{(\mathrm{O}\mathrm{H})}_{6}{\mathrm{Cl}}_{2}$, a quantum spin liquid candidate on the perfect kagome lattice, precisely characterizing the intrinsic physics of the kagome layers is extremely challenging due to the presence of interlayer Cu/Zn antisite disorder within its crystal structure. Here we measured the specific heat and thermal conductivity of single-crystal herbertsmithite in magnetic fields with high resolution. Strikingly, intrinsic magnetic specific heat contribution arising from the kagome layers exhibits excellent scaling collapse as a function of $T/H$ (temperature/magnetic field). In addition, no residual linear term in the thermal conductivity $\ensuremath{\kappa}/T(T\ensuremath{\rightarrow}0)$ is observed in zero and applied magnetic fields, indicating the absence of itinerant gapless excitations. These results capture a new essential feature of the QSL state of the kagome layers; localized orphan spins are induced by exchange bond randomness, surrounded by a nonitinerant QSL.