Rare-Earth Chalcogenides:A Large Family of Triangular Lattice Spin Liquid Candidates

Frustrated quantum magnets are expected to host many exotic quantum spin states like quantum spin liquid（QSL）, and have attracted numerous interest in modern condensed matter physics. The discovery of the triangular lattice spin liquid candidate YbMgGaO_4 stimulated an increasing attention on the rare-earth-based frustrated magnets with strong spin-orbit coupling. Here we report the synthesis and characterization of a large family of rare-earth chalcogenides AReCh_2（A = alkali or monovalent ions, Re = rare earth, Ch = O,S,Se）. The family compounds share the same structure（R3 m） as YbMgGaO_4,and antiferromagnetically coupled rare-earth ions form perfect triangular layers that are well separated along the c-axis. Specific heat and magnetic susceptibility measurements on NaYbO_2,NaYbS_2 and NaYbSe_2 single crystals and polycrystals, reveal no structural or magnetic transition down to 50 mK. The family, having the simplest structure and chemical formula among the known QSL candidates, removes the issue on possible exchange disorders in YbMgGaO_4. More excitingly, the rich diversity of the family members allows tunable charge gaps, variable exchange coupling, and many other advantages.This makes the family an ideal platform for fundamental research of QSLs and its promising applications.

Gapped Spin-1/2 Spinon Excitations in a New Kagome Quantum Spin Liquid Compound Cu_3Zn(OH)_6FBr

We report a new kagome quantum spin liquid candidate CuaZn（OH）6FBr, which does not experience any phase transition down to 50inK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature （-200 K）. A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from 19F nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fraetionalization in the fractional quantum Hall state.

Some experimental schemes to identify quantum spin liquids

Despite the apparent ubiquity and variety of quantum spin liquids in theory,experimental confirmation of spin liquids remains to be a huge challenge.Motivated by the recent surge of evidences for spin liquids in a series of candidate materials,we highlight the experimental schemes,involving the thermal Hall transport and spectrum measurements,that can result in smoking-gun signatures of spin liquids beyond the usual ones.For clarity,we investigate the square lattice spin liquids and theoretically predict the possible phenomena that may emerge in the corresponding spin liquids candidates.The mechanisms for these signatures can be traced back to either the intrinsic characters of spin liquids or the external field-driven behaviors.Our conclusion does not depend on the geometry of lattices and can broadly apply to other relevant spin liquids.

Monte Carlo study of the magnetic properties of spin liquid compound NiGa_2S_4

The magnetic properties of two-dimensional antiferromagnet NiGa2S4 have attracted much attention and yet some problems are far from being solved. We investigate the magnetic properties of NiGa2S4 by Monte Carlo simulations. A new spin-interacting model is proposed to describe the system, and the specific heat together with the doping effect of nonmagnetic impurity is studied by simulations. The double peaks of the specific heat as well as other behaviors are well reproduced. We also compare our results with those of other models, and the underlying physics is discussed.

Pressure induced insulator to metal transition in quantum spin liquid candidate NaYbS_(2)

Pressure induced insulator to metal transition followed by the appearance of superconductivity has been observed recently in inorganic quantum spin liquid candidate NaYbSe_(2).In this paper,we study the properties of isostructural compound NaYbS_(2)under pressure.It is found that the resistance of Na YbS_(2)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 NaYbS_(2)under pressure are similar to those of NaYbSe_(2),suggesting that these phenomena might be the universal properties in NaLnCh_(2)(Ln=rare earth,Ch=O,S,Se)system.

Quantum Spin Liquid Phase in the Shastry–Sutherland Model Detected by an Improved Level Spectroscopic Method

We study the spin-1/2 two-dimensional Shastry–Sutherland spin model by exact diagonalization of clusters with periodic boundary conditions, developing an improved level spectroscopic technique using energy gaps between states with different quantum numbers. The crossing points of some of the relative(composite) gaps have much weaker finite-size drifts than the normally used gaps defined only with respect to the ground state, thus allowing precise determination of quantum critical points even with small clusters. Our results support the picture of a spin liquid phase intervening between the well-known plaquette-singlet and antiferromagnetic ground states, with phase boundaries in almost perfect agreement with a recent density matrix renormalization group study, where much larger cylindrical lattices were used [J. Yang et al., Phys. Rev. B 105, L060409(2022)]. The method of using composite low-energy gaps to reduce scaling corrections has potentially broad applications in numerical studies of quantum critical phenomena.

Crystal growth and magnetic properties of quantum spin liquid candidate KErTe_(2)

Recently rare-earth chalcogenides have been revealed as a family of quantum spin liquid(QSL)candidates hosting a large number of members.In this paper we report the crystal growth and magnetic measurements of KErTe_(2),which is the first member of telluride in the family.Compared to its cousins of oxides,sulfides and selenides,KErTe_(2) retains the high symmetry of R3m and Er3+ions still sit on a perfect triangular lattice.The separation between adjacent magnetic layers is expectedly increased,which further enhances the two dimensionality of the spin system.Specific heat and magnetic susceptibility measurements on KErTe_(2) single crystals reveal no structural and magnetic transition down to 1.8 K.Most interestingly,the absorption spectrum shows that the charge gap of KErTe_(2) is roughly 0.93±0.35 eV,which is the smallest among all the reported members in the family.This immediately invokes the interest towards metallization even superconductivity using the compound.

Emergent symmetry in quantum phase transition:From deconfined quantum critical point to gapless quantum spin liquid

The emergence of exotic quantum phenomena in frustrated magnets is rapidly driving the development of quantum many-body physics,raising fundamental questions on the nature of quantum phase transitions.Here we unveil the behaviour of emergent symmetry involving two extraordinarily representative phenomena,i.e.,the deconfined quantum critical point(DQCP)and the quantum spin liquid(QSL)state.Via large-scale tensor network simulations,we study a spatially anisotropic spin-1/2 square-lattice frustrated antiferromagnetic(AFM)model,namely the J1x-J1y-J2 model,which contains anisotropic nearestneighbor couplings J1x,J1y and the next nearest neighbor coupling J2.For small J1y/J1x,by tuning J2,a direct continuous transition between the AFM and valence bond solid phase is observed.With growing J1y/J1x,a gapless QSL phase gradually emerges between the AFM and VBS phases.We observe an emergent O(4)symmetry along the AFM–VBS transition line,which is consistent with the prediction of DQCP theory.Most surprisingly,we find that such an emergent O(4)symmetry holds for the whole QSL–VBS transition line as well.These findings reveal the intrinsic relationship between the QSL and DQCP from categorical symmetry point of view,and strongly constrain the quantum field theory description of the QSL phase.The phase diagram and critical exponents presented in this paper are of direct relevance to future experiments on frustrated magnets and cold atom systems.

Synthesis,disorder and Ising anisotropy in a new spin liquid candidate PrMgAl_(11)O_(19)

Here we report the successful synthesis of large single crystals of triangular frustrated PrMgAl_(11)O_(19)using the optical floating zone technique.Single crystal x-ray diffraction(XRD)measurements unveiled the presence of quenched disorder within the mirror plane,specifically~7%of Pr ions deviating from the ideal 2d site towards the 6h site.Magnetic susceptibility measurements revealed an Ising anisotropy with the c-axis being the easy axis.Despite a large spin–spin interaction that develops below~10 K and considerable site disorder,the spins do not order or freeze down to at least 50 mK.The availability of large single crystals offers a distinct opportunity to investigate the exotic magnetic state on a triangular lattice with an easy axis out of the plane.

Gapless quantum spin liquid and global phase diagram of the spin-1/2 J_(1)-J_(2) square antiferromagnetic Heisenberg model

The nature of the zero-temperature phase diagram of the spin-1/2 J_(1)-J_(2)Heisenberg model on a square lattice has been debated in the past three decades,and it remains one of the fundamental problems unsettled in the study of quantum many-body theory.By using the state-of-the-art tensor network method,specifically,the finite projected entangled pair state(PEPS)algorithm,to simulate the global phase diagram of the J_(1)-J_(2)Heisenberg model up to 24×24 sites,we provide very solid evidences to show that the nature of the intermediate nonmagnetic phase is a gapless quantum spin liquid(QSL),whose spin-spin and dimer-dimer correlations both decay with a power law behavior.There also exists a valence-bond solid(VBS)phase in a very narrow region 0.56■J_(2)/J_(1)≤0.61 before the system enters the well known collinear antiferromagnetic phase.We stress that we make the first detailed comparison between the results of PEPS and the well-established density matrix renormalization group(DMRG)method through one-to-one direct benchmark for small system sizes,and thus give rise to a very solid PEPS calculation beyond DMRG.Our numerical evidences explicitly demonstrate the huge power of PEPS for highly frustrated spin systems.Finally,an effective field theory is also proposed to understand the physical nature of the discovered gapless QSL and its relation to deconfined quantum critical point(DQCP).

Recent progress on magnetic-field studies on quantum-spin-liquid candidates

Quantum spin liquids（QSLs） represent a novel state of matter in which quantum fluctuations prevent the conventional magnetic order from being established, and the spins remain disordered even at zero temperature. There have been many theoretical developments proposing various QSL states. On the other hand, experimental movement was relatively slow largely due to limitations on the candidate materials and difficulties in the measurements. In recent years, the experimental progress has been accelerated. In this topical review, we give a brief summary of experiments on the QSL candidates under magnetic fields. We arrange our discussions by two categories： i） Geometrically-frustrated systems, including triangularlattice compounds YbMgGaO4 and YbZnGaO4, κ-（BEDT-TTF）2 Cu2（CN）3, and EtMe3 Sb[Pd（dmit）2]2, and the kagom′e system ZnCu3（OH）6 Cl2; ii） the Kitaev material α-RuCl3. Among these, we will pay special attention to α-RuCl3, which has been intensively studied by ours and other groups recently. We will present evidence that both supports and rejects the QSL ground state for these materials, based on which we give several perspectives to stimulate further research activities.

Possible quantum paraelectric state in Kitaev spin liquid candidate H3LiIr2O6

A new quantum spin liquid(QSL)candidate material H3LiIr2O6 was synthesized recently and was found not to show any magnetic order or phase transition down to low temperatures.In this work,we study the quantum dynamics of the hydrogen ions,i.e.,protons,in this material by combining first-principles calculations and theoretical analysis.We show that each proton and its adjacent oxygen ions form an electric dipole.The dipole interactions and the proton tunneling are captured by a transverse-field Ising model with a quantum disordered paraelectric ground state.The dipole excitations have an energy gap△d=60 meV,and can be probed by the infrared optical spectroscopy and the dielectric response.We argue that the electric dipole fluctuations renormalize the magnetic interactions in H3LiIr2O6 and lead to a Kitaev QSL state.

Synthesis and Magnetic Properties of Spin-Liquid Tb_2Ti_2O_7

Polycrystalline samples of a novel spin-liquid compound Tb2Ti2O7 were prepared by a standard solid-state reaction. X-ray diffraction at room temperature confirms that the synthesized compound of Tb2Ti2O7 is single phase with cubic unit cell constant a0 of 1.015 44 nm. Magnetic susceptibility measurements in the temperature range between 100 and 300 K give an effective moment of 9.44 μB and Curie-Weiss temperature of 12.68 K, respectively, indicating the dominance of antiferromagnetic interactions. However, below 50 K, the magnetic behavior of Tb2Ti2O7 deviates from Curie-Weiss law, whose origin remains suspicion.

Generic spiral spin liquids

Spiral spin liquids are unique classical spin liquids that occur in many frustrated spin systems,but do not comprise a new phase of matter.Owing to extensive classical ground-state degeneracy,the spins in a spiral spin liquid thermally fluctuate cooperatively from a collection of spiral configurations at low temperatures.These spiral propagation wavevectors form a continuous manifold in reciprocal space,i.e.,a spiral contour or a spiral surface,that strongly governs the low-temperature thermal fluctuations and magnetic physics.In this paper,the relevant spin models conveying the spiral spin liquid physics are systematically explored and the geometric origin of the spiral manifold is clarified in the model construction.The spiral spin liquids based on the dimension and the codimension of the spiral manifold are further clarified.For each class,the physical properties are studied both generally and for specific examples.The results are relevant to a wide range of frustrated magnets.A survey of materials is given and future experiments are suggested.

INFLUENCE OF POLYMER CHARACTERISTICS AND MELT-SPINNING CONDITIONS ON THE PRODUCTION OF HIGH RANDOMNESS 60PHB/PET THERMOTROPIC LIQUID CRYSTAL COPOLYESTER FIBER

Six samples of linear high randomness 60PHB/ PET thermotropic liquid crystal copolyesters are made by melt copolymerization at 290℃ , whose randomness about 0.955 is measured by the discernible ’H-NMR spectrometer. High tenacity, high module fiber is prepared by melt spinning in liquid crystal phase. The effect of molecular weight, shear rate, temperature as well as spinning drawn ratio on the mechanical behavior of 60PHB / PET copolyester fiber are shown that, lower shear rate (2 10 s-1), higher temperature melting （300℃ ）, lower temperature spinning （280℃ ） and higher molecular weight are favourable to the increase of the fiber mechanical properties. With the variance of drawn ratio, fiber mechanical property has a transition point due to traversion from shear-orientation to drawn-orientation. The copolyester fiber has high crystallinity, high orientation at the crystalline region, high chain orientation and high regular fibrillar structure.

On the Ground State of Spin Systems with Orbital Degeneracy

In order to understand the properties of the spin system with orbital degeneracy, we first study the ground state of the SU(4) spin-orbital model on a square lattice. The mean-field results suggest that for a small Hund's interaction, the flavor liquid state is stable against the solid state, but with sufficient deviation from the SU(4) limit the long-range order may be attained in 2D system. Furthermore, we employ a variational approach to calculate the phase diagram of the ground state and the temperature-dependent susceptibility by taking into account the Hund's interaction and the anisotropy in orbital wavefunctions. Finally, the implications for the experimental observations on the material, , are discussed.

Wet-spun poly(ionic liquid)-graphene hybrid fibers for high performance all-solid-state flexible supercapacitors

It is crucial to develop flexible and wearable electronic devices that have attracted tremendous interest due to their merits on compactness,flexibility and high capacitive properties.Herein we report the continuously ordered macroscopic poly(ionic liquid)-graphene fibers by wet spinning method via liquid crystal assembly for supercapacitor application.The fabricated all-solid-state supercapacitors exhibited a high areal capacitance(268.2 mF cm 2)and volumetric capacitance(204.6 F cm 3)with an outstanding areal energy density(9.31μWh cm-2)and volumetric energy density(8.28 mWh cm-3).The fiber supercapacitors demonstrated exceptional cycle life for straight electrodes of about 10,000 cycles(94.2%capacitance retention)and flexibility at different angles(0°,45°,90°,180°)along with a good flexible cycling stability after 6000 cycles(92.7%capacitance retention).To date,such a novel poly(ionic liquid)-graphene fiber supercapacitors would be a new platform in real-time flexible electronics.

Liquid phase separation of Cu-Cr alloys during rapid cooling

The ribbons of Cu-Cr alloys with high Cr content (15%- 35%, mass fraction) were prepared by rapid solidification. The microstructures of solidified samples were analyzed by scanning electron microscopy and transmission electron microscopy. The results reveal that a representative liquid phase separation microstructures are observed in Cu75Cr25 ribbons solidified at a cooling rate of about 104K/s. The liquid phase separation is not restrained when the cooling rate is enhanced to about 107K/s. However, the size of Cr particles solidified from Cr-rich liquid or Cr-rich regions in alloy melts could be refined by increasing the cooling rates. The size of Cr particles increases with increasing Cr contents when the ribbons contain 15% to 35%Cr.

Electronic Properties of Nanopore Edges of Ferromagnetic Graphene Nanomeshes at High Carrier Densities under Ionic-Liquid Gating

Graphene edges with a zigzag-type atomic structure can theoretically produce spontaneous spin polarization despite being a critical-metal-free material. We have demonstrated this in graphene nanomeshes (GNMs) with honeycomb-like arrays of low-defect hexagonal nanopores by observing room-temperature ferromagnetism and spin-based phenomena arising from the zigzag-pore edges. Here, we apply extremely high electric fields to the ferromagnetic (FM) GNMs using an ionic-liquid gate. A large on/off-ratio for hole current is observed for even small applied ionic-liquid gate voltages (Vig). Observations of the magnetoresistance behavior reveal high carrier densities of ~1013 cm-2 at large Vig values. We find a maximum conductance peak in the high -Vig region and its separation into two peaks upon applying a side-gate (in-plane external) voltage (Vex). It is discussed that localized edge-π band with excess-density electrons induced by Vig and its spin splitting for majority and minority of spins by Vex (half-metallicity model) lead to these phenomena. The results must realize critical-element-free novel spintronic devices.